the removal of oil from oil-water mixtures using selective oil ...selective filtration of oil across...

June 1988

THE REMOVAL OF OIL FROM OIL-WATER MIXTURES USING SELECTIVE OIL FILTRATION

Paula Magdich Dr. Michael Semmens

University o f Minnesota Department of Civil and Mineral Engineering

Minneapolis, MN 55455

Project Officer

James S . Bridges Office of Environmental Engineering and Technology Demonstration

Hazardous Waste Engineering Research Laboratory Cincinnati, OH 45268

This study was conducted through

Minnesota Waste Management Board St. Paul, MN 5 5 1 0 8

and the

Minnesota Technical Assistance Program University of Minnesota Minneapolis, MN 55455

HAZARDOUS WASTE ENGINEERING RESEARCH LABORATORY OFFICE OF RESEARCH AND DEVELOPMENT

U.S. ENVIRONMENTAL PROTECTION AGENCY CINCINNATI, OH 45268

This project was partially supported with a United States Environmental Protection Agency cooperative agreement through the Minnesota Waste Management Board and the Minnesota Technical Assistance Program.

Although the research described i n this report has been funded in part by the United States Environmental Protection Agency through a cooperative agreement, it has not been subjected to Agency review, and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred.

PROJECT SUMMARY

MlNNESOtA q!kHNICAL A!k%STANCE PROGRAd UNfVERSlTY OF MINNFSOTA --- ...

BOX 197 MAYO, 420 DELAWARE ST. S.E. MINNEAPCLIS, MINNESOTA 55455

The Removal of Oil from Oil-Water Mixtures Using Selective Oil Filtration

Paula Magdich and Michael Semmens

Based on the findings of this study it appears that the selective oil filtration (SOF) process has potential as an alternative method for removing oil from oily wastewaters or from other oil-containing wastes. The results indicate that the SOF process is capable of removing free, mechanically dispersed and chemically emulsified oil from oil-water mixtures. SOF is not suitable for'the removal of oil- wet solids, however, and prior to filtration, the suspended solids must be removed to prevent membrane fouling.

Introduction

Oil-containing wastewaters are generated at an estimated rate of more than one billion gallons per year in the United States. Genera- tors .of these wastestreams are extremely varied and may include petroleum refineries, metal fabrication plants, rolling mills, chemical processing plants, machine shops, and vehicle maintenance shops. Many different types of oils may be present in oily wastewater such as diesel fuel, cutting and grinding oils, lubricating oils, water soluble coolants, natural animal or vegetable fats or any other organic immiscible in water. The removal of these oily wastes from wastewater is of importance in preventing pollution and meeting environmental compliance standards. Oily waste removal may also be beneficial for water and oil recovery and reuse.

Several different methods are currently available for separating oil from oily wastewaters, but they are generally limited in the forms of oil that they can remove. Oily wastewaters containing chemically emulsified oil are particularly difficult to separate, and the options available for treating them are few in number.

I n this study, a novel membrane separation process, termed selective oil filtration (SOF) was studied as an alternative separation technique for removing emulsified oil from oil-water mixtures. SOF is similar to ultrafiltration in that microporous membranes are used. It is unlike ultrafiltration, however, in that oil rather than water is selectively removed by applying a pressure across the membrane (Figure 1).

The purpose of this research was to derive some understanding of how and why the SOF process works and to determine some of its limitations. More specifically the objectives o f this research were as follows: (1) demonstrate that oil could be removed from oil-water mixtures using selective oil filtration; (2) determine the effects of various operating parameters such as pressure, feed flowrate, oil viscosity and oil concentration on process performance, ( 3 ) determine the effect of emulsion stability and the presence of emulsifying agents on the ability of the membrane to selectively remove oil, and (4) determine the mechanism of oil transport from the bulk solution t o the permeate stream.

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Procedures

A comprehensive literature search was conducted on oil-water separation techniques and on the theory of emulsions. In the laboratory study, microporous hollow-fiber membranes were used to separate oil from oil-water mixtures using SOF. This research was carried out in two stages. Stage one concerned itself with qualitatively assessing the SOF process. Several experiments were conducted in which a naphthenic base engine oil was removed from different oil-water mixtures and the SOF process was evaluated both in the presence and absence of emulsifying agents. Two types of membranes were tested and compared in these experiments. From this work some inferences about the SOF process were made and narrower research goals were defined. The aim of stage two of this project was to achieve these research goals.

During stage two the performance of the SOF process was assessed under more carefully defined conditions in which dodecane was employed as a model oil. This was done by studying the effects of various operating parameters. Dodecane was removed from dodecane-water mixtures to which sodium dodecyl sulfate was added as an emulsifying agent. An assessment of emulsion stability as a function of emulsifier concentration was also made and correlated to the observed results.

Results and Discussion

This study demonstrated that oil can be selectively removed from oil-water mixtures using microporous polypropylene fibers, in both the presence and absence of emulsifying agents. The effect of various operating parameters on oil removal was investigated, and the controlling mechanisms of oil transport were identified under different operating conditions. The impact of surfactants on process performance was examined, and the role of surfactants in the SOF process was determined.

Influence of Oil Viscosity: While both oils were effectively removed by the membrane process, the response of the oil flux rates to changes in operating conditions showed significant differences in behavior due to viscosity. The high viscosity naphthenic oil caused low permeate fluxes to be obtained. The low viscosity dodecane passed through the membrane more easily, and higher permeate fluxes were obtained.

Influence of Oil Concentration: Changes in oil concentration appeared to have a similar effect on the removal of both the naphthenic oil and dodecane. As the oil concentration increased, the rate of oil removal also increased. This increase was not found to be linear, however, over the oil concentration ranges tested. In general it appeared that the effect of oil concentration was more marked at low oil concentrations and it became less significant at higher oil concentrations. There appeared to be a critical concentration beyond which further increases in oil concentration had little effect on the permeate flux and the value of this critical oil concentration decreased with increasing feed flowrate.

Influence of Surfactants: The removal of both dodecane and naphthenic oil was adversely affected by the presence of surfactants

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and the permeate fluxes decreased as a result. The naphthenic oil experiments demonstrated this behavior qualitatively for anionic and nonionic emulsifying agents, while the dodecane experiments quantified the effect of a single anionic surfactant by measuring the permeate flux as a function of surfactant concentration. The permeate flux decreased with increasing surfactant concentration, although the flux decline was more significant for small additions of surfactant and it became less dramatic with further surfactant additions.

Conclusions and Recommendations

The following conclusions were drawn from this study on the selective filtration of oil across microporous fibers o f polypropylene.

1. Selective oil filtration can be used to recover a water-free oil product from oil-water mixtures in both the presence and absence of emulsifying agents.

2 . Low viscosity oils, such as dodecane, are more efficiently removed than high viscosity oils such as naphthenic oil. This difference in removal efficiency is due to differences in the controlling mechanism of oil transport.

3 . In this study, the transport of high viscosity oils was membrane limited whereas the transport of low viscosity oils was limited by transport and attachment to the membrane and droplet collapse.

4. The removal of low viscosity oils is strongly affected by system hydrodynamics. Oil removal rates increase with increasing feed flowrate as the opportunity for contact between the dispersed droplets and the fibers is improved.

5. The rate of oil removal is affected by the feed oil concentration. As the oil concentration increases, the rate of oil removal also increases. This study showed that this increase is not linear over a broad oil concentration range and that oil removal is limited at low oil concentrations (<l%).

6. Both anionic and nonionic emulsifying agents have a deleterious effect on oil removal. The rate of oil removal decreases markedly with the addition of these types of emulsifying agents.

7. The exact role of emulsifying agents in selective oil filtration is not clear. These studies indicated that emulsifying agents may affect oil removal by altering the electrical and/or mechanical properties of the droplets and the fibers. Further studies are required to determine the effect of mechanical stability.

8. Electrostatic interactions between the droplets and the fibers play an important role in the oil removal process. Repulsive electrostatic interactions between the negatively charged oil droplets and the membrane, which is also negatively charged, hinder droplet approach and attachment.

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9. The oil removal process can be improved by encouraging attractive electrostatic interactions between the droplets and the fibers. One way of accomplishing this is to apply a positive coating to the membrane surface.

10. The selective oil filtration process has potential as an alternative oil-water separation technique. This process is better suited than ultrafiltration to the treatment o f oily wastewaters with a high oil concentration or for the treatment of water-in-oil emulsions. It cannot replace ultrafiltration, however, for the removal o f oil from dilute o i l y wastewaters.

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Figure 1: The SOF Process

mbrane

P

(Oil + Water)

Bulk Oil Concentration

(C)

Oil Permeate

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THE REMOVAL OF OIL FROM OIL-WATER MIXTURES USING SELECTIVE OIL FILTRATION

A THESIS SUBMllTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF MINNESOTA

BY PAULA MAGDICH

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE IN CIVIL ENGINEERING

JULY, 1988

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ABSTRACT

A novel membrane separation process, called selective oil filtration (SOF), was studied as a separation technique for removing oil from oil- water mixtures. SOF is similar to ultrafiltration (UF) in that microporous membranes are used; it is unlike UF, however, in that oil rather than water is selectively removed by applying a pressure across the membrane. The main advantage foreseen in using SOF is that a water- free oil product can be obtained.

Research was undertaken with the following objectives in mind: (1 ) demonstrate that oil can be separated from oil-water mixtures using SOF; (2 ) determine the effects of various operating parameters such as pressure, feed flowrate, oil concentration and module geometry on the process performance; (3) determine the effect of emulsion stability and emulsifying agents and (4) determine the mechanism by which oil is transported from the bulk solution across the membrane.

.

Hydrophobic microporous hollow-fibre membranes were employed. The oil-water mixtures were fed on the outside of the fibres and the oil permeate was collected from the inside of the fibres. Emulsions containing a light engine oil and dodecane were used to study the behavior of the process. Mechanically and chemically stabilized emulsions were tested. Surfactant concentrations up to 10% by weight of oil were examined for ABS, Triton X-102 and sodium dodecyl sulfate.

The oil permeate flux was measured as a function of various operating parameters (pressure, oil concentration, feed flowrate, surfactant concentration) and the rate controlling mechanisms for oil transport were identified. Emulsion stability was assessed by measuring the zeta potential of the dispersed oil droplets and by measuring the droplet size distribution. These data were correlated to the results to determine the role of emulsion stability in the SOF process. Finally, the fibres were coated with N( p-aminoet hyl)-y-ami nopropyl-t rimet hoxysi lane

(AEAPTMS), which is a cationic polymer, to impart a positive charge to the fibres. This enabled the importance of electrostatic interactions between the dispersed oil droplets and the fibres in the SOF process to be determined.

It was shown that oil can be selectively removed from water in the presence or absence of surfactants. Viscous oils are limited by the permeability of the fibres and as such the permeate rate is pressure dependent. With low viscosity oils (or high temperature operation) the separation process is transport limited and as such there is a strong influence of emulsion flowrate and fibre coating.

The presence of surfactants slows the rate of oil separation but separation is still feasible. The process is most effective at higher oil concentrations (Le. >2% oil in water) and as such it may be used in conjunction with UF for the concentration and separation of oil from dilute oily wastewaters. The behavior of the process suggests that it also has potential for application in separating oil from water-in-oil emulsions.

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ACKNOWLEDGEMENTS

There are several people whom I wish to acknowledge for helping to make this research project possible. First and foremost, I would like to thank my advisor, Dr. Semmens, for his technical guidance and assistance, and also for his friendship and patience. 1 would like to thank my other committee members, Dr. Brezonik and Dr. lwasaki for taking the time to read my thesis and for offering their advice.

Many of the other graduate students in the Environmental Engineering program have contributed to this thesis indirectly, by offering their encouragement and friendship. Specifically, I would like to thank Amy Zander and Janice Tacconi for helping me in this way.

This research project was funded by the Minnesota Technical Assistance Program (MNTAP) and The Donaldson Company, Inc. I would like to express my thanks to Cindy McComas of MNTAP for devoting her time to this project, and for her cooperation and flexibility.

Finally, I would like to thank Andrew Keith for his encouragement and support, particularly during the final writing stages.

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TABLE OF CONTENTS

Page

Abstract ....................................................................................................................... i ... Acknowledgements .................................................................................................... II i

Table of Contents ........................................................................................................ iv

List of Tables ................................................................................................................ vi

List of Figures .............................................................................................................. vii

1 .

2 .

3 .

4 .

I n t roductio n ................................................................................................. 1 1 . 1 Scope of the Oily Wastewater Problem ........................................ 1 1.2 Oily Wastewater Characteristics ..................................................... 2 1.3 The Concept of Selective Oil Filtration .......................................... 3 1.4 Research Objectives ......................................................................... 4 1.5 Scope of This Research ................................................................... 5

Literature Review ....................................................................................... 6

(a) Primary Treatment ...................................................................... 6 (b) Secondary Treatment ............................................................... 10 (c) Tertiary Treatment ...................................................................... 17

(a) Relevant Aspects of Surface Chemistry ................................ 21 (b) Emulsion Formation .................................................................. 25 (c) Physical Properties of Emulsions ........................................... 28 (d) Emulsion Stability ...................................................................... 30

2.1 Oily Wastewater Treatment Methods ............................................. 6

2.2 Theory of Emulsions ........................................................................ 18

. .

(e) Assessing Emulsion Stability .................................................. 37

Theory Relevant to Selective Oil Filtration ........................................... 40 3.1 Droplet Coalescence on Fibres ..................................................... 40 3.2 Proposed Mechanism of Oil Transport in the SOF Process ..... 46

Materials and Methods ............................................................................ 49 4.1 Oil-Water Mixtures ............................................................................ 49

(a) Oils ............................................................................................... 49 (b) Water ............................................................................................ 50 (c) Emulsifying Agents .................................................................... 50

4.2 Hollow-Fibre Modules ..................................................................... 53 (a) Membrane Selection ................................................................ 53

(d) Preparation of Oil-Water Mixtures .......................................... 52

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Page

(b) Module Design ........................................................................... 54 (c) Module Cleaning ....................................................................... 60

4.3 Experimental Apparatus and Method ........................................... 60 4.4 Zeta Potential Measurements ........................................................ 62 4.5 Droplet Size Distribution Determination ...................................... 63

5 . Experiments and Results ........................................................................ 64 5.1 Naphthenic Oil Experiments .......................................................... 64

(a) AMT Experiments ...................................................................... 64

(c) Celgard Experiments ................................................................ 76 (d) Additional Observations ........................................................... 88

5.2 Dodecane Experiments .................................................................. 89 (a) Preliminary Experiments .......................................................... 89 (b) Countercurrent M0dule-5O/~ Dodecane Experiments ......... 91 (c) Cross Flow Module-1 0% Dodecane Experiments .............. 95 (d) Cross Flow Module-5% Dodecane Experiments ............... 100

(b) AMT Vs Celgard Experiments ................................................. 71

(e) AEAPTMS Coated Fibre Experiment .................................... 107

6 . Discussion ................................................................................................ 115 6.1 Discussion of Results ..................................................................... 115 6.2 Potential Uses and Limitations of SO .......................................... 126 6.3 future Research Work .................................................................... 130

7 . Conclusions ............................................................................................. 132

References ................................................................................................................. 134 Appendices ................................................................................................................ 139

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LIST OF TABLES Page

Table 1 : Szmmary of Oily Wastewater Treatment Methods .............................. 19

Table 2: HLB Method of Emulsifier Selection ...................................................... 28

Table 3: Properties of Celgard Hollow-Fibres X-10 and X-20 .......................... 55

Table 4: Properties of the Celgard Countercurrent Hollow-Fibre Modules ............................................................................. 57

Table 5: AMT Module: 100% Oil; Baseline Data ................................................ 65

Table 6: Zeta Potential As a Function of SDS Concentration ......................... 104

Table 7: A Comparison of Permeate Fluxes for a 5% Dodecane-Water Mixture on the AEAPTMS Coated and Uncoated Cross Flow Modules ...................................................................................................... 109

Table 8: Flux Ratios for a 5% Dodecane-Water Mixture on the AEAPTMS Coated and Uncoated Cross Flow Modules ................... 1 10

Table 9: Removal Ratios for the 5% Dodecane-Water Mixture on the Uncoated Cross Flow Module ............................................................... 120

Table 10: Correlation of the Data for the 5% Dodecane-Water Mixture on the Uncoated Cross Flow Module With Empirical Expressions for Coalescence Efficiency ............................................ 1 47

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LIST OF FIGURES

Page

Figure 1 : The SOF Process ...................................................................................... 3

Figure 2: The Ultrafiltration Process ...................................................................... 14

Figure 3: The Oriented Wedge Theory ................................................................. 32 (a) An O M Emulsion Stabilized By a Monovalent Soap (b) A W/O Emulsion Stabilized By a Bivalent Soap

Figure 4: Stern Double Layer Theory ................................................................... 36 (a) Distribution of Counter Ions (b) Potential as a Function of Distance From the Droplet

Surface

Figure 5: Potential Diagram For the Interaction of Two Charged Droplets .... 36

Figure 6: Proposed Mechanism of Oil Transport for the SOF Process ........... 46

Figure 7: General Structure of ABS ....................................................................... 51

Figure 8: General Structure of Triton Surfactants ............................................... 51

Figure 9: Celgard Countercurrent Hollow-Fibre Module ................................... 56

Figure 10: AMT Countercurrent Hollow-Fibre Module ........................................ 58

Figure 11 : Celgard Cross Flow Hollow-Fibre Module ....................................... 59

Figure 12: Experimental Apparatus ....................................................................... 61

Figure 13: AMT Module; Flux vs Pressure ........................................................... 66

Figure 14: AMT Module; Flux vs Feed Flowrate .................................................. 67

Figure 15: AMT Module; Flux vs Feed Flowrate for a 19'0 Oil-Water Mixture .................................................................................................... 68

Figure 16: AMT Module; Flux vs Oil Concentration ........................................... 68

Figure 17: AMT Module Regeneration: Pure Oil Flux vs Time ......................... 71

Figure 18: AMT vs Celgard 1 ; Flux vs Pressure for Pure Oil ............................. 72

Figure 19: AMT vs Celgard 2; Flux vs Time for a 1 o/o Oil-Water Mixture ........ 75

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Page

Figure 20: AMT vs Celgard 2; Flux vs Time for a 10% Oil-Water Mixture ...... 75

Figure 21 : Celgard 2 Module; Flux vs Time for a 10% Oil-Water Mixture Containing 1% Pet Mix #9 ................................................ 76

Figure 22: Celgard 2 Module; Flux vs Time for a 10% Oil-Water Mixture Containing 1 o/o ABS ............................................................... 79

Figure 23: Celgard 2 Module: Flux vs '10 Oil Remaining for a 10% Oil-Water Mixture Containing 1 Yo ABS ..................................... 79

Figure 24: Celgard 2 and C-#3 Modules; Flux vs Time for a 10%

Figure 25 Celgard 2 and C-#3 Modules; Flux vs O/O Oil Remaining for a

Oil-Water Mixture Containing 1 o/o ABS ............................................. 80

10% Oil-Water Mixture Containing 1 Ol0 ABS ..................................... 8 1 .

Figure 26: Celgard 2 Module; Flux vs Time for a 10% Oil-Water Mixture

Figure 27: Celgard 2 Module; Flux vs 240 Oil Remaining for a 10% Oil-

Containing 1 Yo ABS at High and Low pH ....................................... 83

Water Mixture Containing 1% ABS at High and Low pH .............. 84

Figure 28: P-#l Module; Flux vs Time for a 10% Oil-Water Mixture Containing 1 O h Triton X-1 02 ................................................................. 87

Figure 29: P-#l Module; Flux Vs YO Oil Remaining for a 10% Oil- Water Mixture Containing 1 YO Triton X-102 ..................................... 87

Figure 30: Countercurrent Module; 5% Dodecane-Water Mixture Pressure Effect ...................................................................................... 92

Figure 31 : Countercurrent Module; 5% Dodecane-Water Mixture Feed Flowrate Effect............................................................................. 93

Figure 32: Cross Flow Module; 10% Dodecane-Water Mixture Pressure Effect ...................................................................................... 96

Figure 33: Cross Flow Module; 10% Dodecane-Water Mixture Feed Flowrate Effe ............................................................................... 96

Figure 34: Cross Flow Module: Oil Concentration Effect for the Diluted 10% Dodecane-Water Mixture .............................................. 99

Figure 35: Cross Flow Module; 5% Dodecane-Water Mixture Feed Flowrate Effect ............................................................................ 1 01

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Page

Figure 36: Cross Flow Module; Oil Concentration Effect at Different Feed Flowrates .................................................................... 103

Figure 37: Droplet Size Distribution Data ........................................................... 105

Figure 38: AEAPTMS Coated Cross Flow Module; 5% Dodecane-Water Mixture; Feed Flowrate Effect ............................................................ 1 08

Figure 39: AEAPTMS Coated vs Uncoated Fibres: Flux vs Feed Flowrate for the 5% Dodecane-Water Mixture Containing No SDS and 100 mg/l SDS ............................................................... 1 12

Figure 40: AEAPTMS Coated vs Uncoated Fibres: Flux vs Feed Flowrate for the 5% Dodecane-Water Mixture Containing 50 mg/l and 200 mg/l SDS ................................................................. 1 13

1 Figure 41 : Oil Removal Efficiency vs 2(2-ln(Re)) for the 5%

Dodecane-Water Mixture on the Uncoated Fibres ........................ 122

Figure 42: Oil Removal Efficiency vs (A - 0.87A3) for the 5% Dodecane-Water Mixture on the Uncoated Fibres ......................... 1 23

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1. INTRODUCTION

1.1 Scope of the Oily Wastewater Problem Oil containing wastewaters are generated at an estimated rate of more

than one billion gallons per year in the United States alone (Tabakin et al., 1978). Generators of these wastestreams are extremely varied and they may be of industrial, commercial or domestic origin. The major contributors however, are petroleum refineries, metal fabrication plants, rolling mills, chemical processing plants, food processing plants and dischargers of bilge and ballast waters from ships (Tabakin et al., 1978; Wang et al., 1975).

Many different types of oils may be present in an oily wastewater, such as diesel fuel, cutting and grinding oils, lubricating oils, water soluble coolants, natural animal or vegetable fats or any other organic immiscible in water. The removal of these oily wastes from wastewater is of paramount importance in preventing pollution and it is required to meet the increasingly stringent environmental regulations. Oily waste removal may also be beneficial from the point of view of water and oil recovery and re-use.

Oily wastes can have many adverse effects if discharged to a receiving stream. Among these are: (1) formation of a noticeable film on the water surface which also has potential for burning and creating a safety hazard; (2) exertion of an oxygen demand; (3) prevention of natural water reaeration; (4) toxicity to aquatic life and (5 ) objectionable taste and odor in fish and the water. Oily wastes may also interfere with municipal wastewater treatment or water purification operations (Tabakin et al., 1978; Nalco Technifax, 1985).

Current legislation requires that effluents discharged directly to receiving streams be free of any visible floating oil and that the "oil and grease" content be limited to levels as low as 5-15 mg/l (Nalco Technifax, 1985). This limit varies depending upon local environmental regulations. For discharge to municipal sewer systems the "oil and grease" content may have to be on the order of 50-100 mg/l. The exact value depends on the pretreatment standards required by individual wastewater treatment facilities. "Oil and grease" refers to

-.

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any substance Methods, 1985).

ext facta b le by t ric h lo rot ri f luo ro et hane (f reon) (Standard

1.2 Oily Wastewater Characteristics Oily wastewaters vary widely in composition and character depending

upon their source. Oil contents for example, may be as low as 50 ppm or as high as 50%; values between 1% and 15% are more typical, however (Nalco Technifax, 1985). In addition there are five different ways in which oil may exist in water: (1) free; (2) mechanically dispersed; (3) chemically emulsified: (4) "dissolved" or (5) adhered to particle surfaces (Freestone and Tabakin, 1975).

Free oil is that which readily separates from water under quiescent conditions. It is generally the easiest type of oil to remove from water. Both mechanical and chemically emulsified dispersions contain stabilized oil droplets with diameters ranging from microns to fractions of a millimeter. The difference between them is that mechanical dispersions are stabilized only by electrical forces, whereas chemical emulsions are stabilized by emulsifying agents as well. As a result, oil is more difficult to separate from chemical emulsions. "Dissolved" oil includes that which is truly dissolved in a chemical sense plus that oil dispersed in such fine droplets that removal by physical means is impossible (Freestone and Tabakin, 1975). Removal of "dissolved" oil requires more sophisticated techniques, and it is often considered an advanced treatment step. When oil adheres to particle surfaces, the product is commonly called oil-wet solids and the removal of this type of oil from water often occurs with the removal of suspended solids.

The degree of difficulty in separating oil from an oily wastewater is strongly affected by the form(s) of oil that are present. Other wastestream characteristics that affect the separation process include the suspended solids concentration and particle size distribution, oil and bulk fluid densities, the presence or absence of various chemicals, pH and temperature (Nalco Technifax, 1985; Tabakin et al., 1978).

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1.3 The Concept of Selective Oil Filtration Several different methods are currently available for separating oil from

oily wastewaters but they are generally limited in the forms of oil that they can remove. Oily wastewaters containing chemically emulsified oil are particularly problematic and the options available for treating them are not only few in number, but they all have certain disadvantages associated with them. A novel membrane separation process, called selective oil filtration (SOF), was studied as an alternative separation technique for removing emulsified oil from oil- water mixtures.

SOF is similar to ultrafiltration in that microporous membranes are used. It is unlike ultrafiltration however, in that oil rather than water is selectively removed by applying a pressure across the membrane. Figure 1 shows a schematic diagram of the SOF process.

Figure 1: The SOF Process

P

Qf c (Oil + Water)

Bulk Oil Concentration

(C)

Oil Permeate

1

4

An oil-water mixture with an oil concentration, C, flows along one side of an oil-saturated membrane at a feed flowrate, Qf. A pressure, P, is applied across the membrane, which causes the oil to pass through the membrane pores while water is retained. Water-free oil is collected from the other side of the membrane at a Permeate rate, Qp, while the water retentate slowly becomes depleted of oil. For this process to work the membrane must be hydrophobic in nature. If the membrane loses its hydrophobicity during the course of operation, water will wet the surface and the separation process will fail.

The SOF process is intended to remove free, mechanically or chemically emulsified and perhaps some "dissolved" oil from wastewaters. Oil-wet solids can not be removed and they would have to be separated by conventional means prior to SOF.

The critical criterion for operating an SOF system is the oil permeate flux which is the volume of oil passed per unit membrane area per unit time (ml/min-A2). The purpose of this study was to evaluate the dependence of the permeate flux upon the operating conditions, the chemistry of the emulsion and the system design.

The main advantage foreseen in using SOF is that a water-free oil product can be obtained rather than an oil-water concentrate or an oily sludge which is usually recovered with the currently used separation methods. This is important from the point of view of oil recycling and reuse, and waste minimization. Possible limitations of the SOF process are loss of membrane hydrophobicity due to the presence of chemical emulsifying agents and low permeate fluxes at low bulk oil concentrations.

1.4 Research Objectives No evidence was found in the literature to indicate that previous work had

been done on the SOF process. Consequently, much of the research work conducted in this study was fundamental in nature. The intent of this research was to derive some understanding of how and why the SOF process works and

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to determine some of its limitations. More specifically the objectives of this research were as follows: (1) demonstrate that oil could be removed from oil- water mixtures using selective oil filtration: (2) determine the effects of various operating parameters such as pressure, feed flowrate, oil viscosity and oil concentration on process performance, (3) determine the effect of emulsion stability and the presence of emulsifying agents on the ability of the membrane to selectively remove oil, and (4) determine the mechanism of oil transport from the bulk solution to the permeate stream.

A hypothesis for the mechanism of oil transport was derived and experiments were carried out to determine what the limiting step in the overall transport process is under various operating conditions. In doing this it was hoped that a better understanding of the process could be achieved.

1.5 Scope of This Research In this study microporous hollow-fibre membranes were used to separate

oil from oil-water mixtures using SOF. This research was carried out in two stages and will be presented as such in this thesis. Stage one concerned itself mainly with achieving objective (1) above and with qualitatively assessing the SOF process. Several experiments were conducted in which a naphthenic base engine oil was removed from different oil-water mixtures and the SOF process was evaluated both in the presence and absence of emulsifying agents. Two types of membranes were tested and compared in these experiments. From this work some inferences about the SOF process were made and narrower research goals were defined. The aim of stage two of this project was to achieve these research goals.

During stage two the performance of the SOF process was assessed under more carefully defined conditions in which dodecane was employed as a model oil. This was done by studying the effects of various operating parameters. Dodecane was removed from dodecane-water mixtures to which sodium dodecyl sulfate was added as an emulsifying agent. An assessment of emulsion stability as a function of emulsifier concentration also was made and correlated to the observed results.

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2. LITERATURE REVIEW

2.1 Oily Wastewater Treatment Techniques Oily wastewater treatment techniques may be physical, chemical,

physicochemical, electrical, mechanical or biological in nature, and they may be used singly or in combination depending upon the wastestream characteristics and the objectives of the wastewater treatment. A given wastewater may be subjected to one of three levels of treatment. Primary treatment is aimed at removing free oil, mechanically dispersed oil and easily settleable solids. It is normally achieved by using physical separation methods that take advantage of differences in specific gravities. Secondary treatment is used to treat mechanical or chemically emulsified dispersions, and this often entails emulsion breaking. Tertiary treatment is used primarily to remove low concentrations of soluble or finely dispersed oil and it is practiced when a very clean effluent is desired.

A review of several different oil-water separation techniques is given below. A brief description of each method is given along with a discussion of the advantages, disadvantages and applicabilities of each.

(a) Primary Treatment

Gravity Separation

Gravity separation is the most widely used oily wastewater treatment technique. The main objective is to separate free oil and suspended solids from the wastewater by utilizing differences in specific gravity; a small amount of dispersed oil may also be removed. The size of oil globule or particle that can be removed by gravity separation is determined by the time provided for separation and the rise velocity. The latter is calculated from Stokes' Law: (Manual on Disposal of Refinery Waste, 1969)

where, Vr = rise velocity g = acceleration due to gravity d = oil globule or particle diameter pw = density of water pp = density of oil or particle p = viscosity of water

Free oil droplets rise to the water surface where they can be removed by skimming while the settleable solids sink. Most gravity separators are effective at removing oil globules larger than 0.015 cm in diameter (Ford and Elton, 1977). The effectiveness depends, however, on the hydraulic design and the retention time employed.

The most basic form of gravity separator is the so-called API unit which consists of a rectangular or circular basin in which the wastewater flows horizontally. A variation on this design is the addition of extended plate surfaces. Two units of this type are the corrugated-plate interceptor (CPI) and the parallel-plate interceptor (PPI). The purpose of the plates is to coalesce the smaller oil droplets into larger drops that move up the plate to form a floating layer on the water surface. The plates effectively reduce the distance that the oil droplets must rise to be collected. Consequently, CPI and PPI units require less space than API units (Tabakin et al., 1978).

Effluent oil concentrations that can be achieved with gravity separators typically range from 20-100 mg/l (Ford and Elton, 1977; Tabakin et al., 1978); higher or lower values may be obtained depending upon the predominant forms of oil present in the wastewater. The main advantages of gravity separators are that they are economical and relatively simple to operate. They are limited however, in that they cannot remove emulsified or soluble oil.

a

Centr i fugat ion Like gravity separators, centrifugal separators exploit differences in

specific gravity. Oil-water separations are achieved by moving the wastewater in a circular path and imposing a centrifugal force. Denser phases move to the outside while lighter phases remain nearer the axis of rotation. In centrifuges the applied centrifugal force is many times larger than the force of gravity, up to 6000 G's, and as a result they are more effective than gravity separators.

Centrifuges are efficient at removing oil-wet solids and free oil from water and depending on the centrifuge design these may be removed singly or simultaneously. In most centrifuges the oily wastewater enters from the bottom and exits from the top. Given sufficient residence time for adequate separation to occur the solids are forced to the outside of the unit while the oil moves towards the center.

Centrifuges have been found to produce effluent oil concentrations of 50- 70 mg/l, which is substantially better than gravity separators (Tabakin et al., 1978). The cost associated with centrifugation depends primarily on the volumetric flow that must be treated. Higher flowrates are more expensive because more individual units are required.

Flotation Flotation techniques are used to treat gravity separator effluents to reduce

the oil and suspended solids contents to lower levels. They utilize the same principles as gravity separation but they are much more effective at removing dispersed oil and oil-wet solids that have specific gravities very close to that of water. Buoyancy differences are enhanced by introducing fine air bubbles, typically 30-120 JJ in diameter, which attach to the oil droplets or oil-wet solids and increase their rise velocities (Ford and Elton, 1977).

Two methods of air flotation are used and they differ only in the method in which air bubbles are generated. In Dissolved Air Flotation (DAF) a portion of the wastestream is saturated with air under pressure and when the pressure is released bubbles are formed. In Induced Air Flotation (IAF) air is self-

-

induced by a rotor disperser mechanism. Chemical coagulants may be added to agglomerate smaller oil droplets into larger flocs which are easier to remove. This may greatly improve the effluent quality but it has the disadvantage of producing a chemical sludge that must be disposed. For optimum clarification it is necessary to remove the buik free oil and settleable solids prior to flotation.

High quality effluents can be produced using flotation; values as low as 1-

20 mg/l oil have been reported (Tabakin et a1.,1978; Ryan, 1986). Flotation systems are capable of handling high suspended solids contents and shock loads but they cannot remove soluble oil or emulsified oil unless an emulsion breaking technique is employed.

Coalescence Coalescers cause fine oil droplets to grow into larger droplets that are

more easily removed by gravity methods. For coalescence to occur, the oil droplets must be forced into physical contact with one another to encourage agglomeration and reduce their surface energies. Coalescers rely on a variety of different physical and surface chemical mechanisms to accomplish this.

Coalescence is used primarily to remove free and mechanically dispersed oil from water; chemically emulsified oil droplets are normally too stable to be forced together. Three different types of coalescers are used for oil-water separations: fibrous-media, loose-media and plate coalescers (Ford and Elton, 1977). Plate coalescers which use gravity separation and parallel plates have already been discussed.

Fibrous-media coalescers have a fixed filter-like media of finely intewoven fibres that provide a tortuous path for the dispersed oil droplets. As the wastewater filters through the media, the oil droplets collide with the fibres which are hydrophobic. Agglomeration of the droplets subsequently occurs at the fibre surface. Once the drops have grown to a certain size they are sheared off by the passing water and new drops begin to grow. These systems are sensitive to the presence of particulate matter and clogging of the fibres may occur, The presence of surface-active chemicals can also have an

10

adverse effect since they may alter the hydrophobic nature of the fibres. Loose-media coalescers operate in much the same way as fibrous-media coalescers except the media is loose rather than fixed. Multi-grade sand or other loose media may be used.

Wide variations in oil removal efficiencies have been found for fibrous- media and loose-media coalescers. These variations are due largely to fluctuations in the characteristics of the wastewater. Effluent oil concentrations as low as 1-50 mg/l have been reported, however, indicating a potentially high efficiency (Tabakin et al.. 1978).

(b) Secondary Treatment: Removal of Chemically Emulsified Oil Oily wastewaters to which chemical emulsifying agents have been added

are more difficult to treat because of the electrical and mechanical barriers that prevent the oil droplets from agglomerating. Traditionally this form of oil has been separated from oily wastewaters by first breaking the oil-in-water emulsion and then removing the "freed" oil by flocculation and gravity separation. To break an emulsion the repulsive electrical forces on the oil droplets must be neutralized and/or the effectiveness of the emulsifying agent must be destroyed (Gambhir, 1983). Several different techniques can be used to break oil-water emulsions and separate emulsified oil from water. These may be chemical, physical or electrical in nature (Tabakin et al., 1978).

Chemical Treatment Chemical demulsification methods are the most widely used and they

usually include acidification and/or coagulation followed by flocculation (Tabakin et al., 1978; Berne, 1982). The conventional means of separating oil- water emulsions is the acid-alum process. In this process the pH is lowered into the 2-4 range by the addition of acid. (usually sulfuric acid). This causes most of the oil droplets to destabilize and separate out, and the freed oil is subsequently removed by skimming. Aluminum sulfate (alum) is added as a

-

I

1 1

coagulant and lime or caustic is added to raise the pH into the neutral range which causes the aluminum to hydrolyze and form an insoluble aluminum hydroxide precipitate. Other inorganic salts such as ferric chloride may be used in place of alum. The hydroxide precipitate entraps the residual oil which is then removed by dissolved air flotation or other gravity separation techniques (Bauer, 1976; Harlow et al., 1982).

This process works well but it has several disadvantages: oily wastes are normally alkaline and they require large amounts of acid to drop the pH below 4; acid corrosion and handling problems prevail; pH adjustments with lime and caustic increase the total dissolved solids content in the effluent and high alum feed rates create large volumes of sludge which require dewatering and disposal (Harlow et al., 1982).

Another chemical approach to oil-water emulsion separations is the polymer-alum method. In this case, a cationic polymer is used in place of acid for neutralizing the surface charge on the stable oil droplets. Cationic polymers are effective over a wide pH range which minimizes the need for pH adjustments. Alum is usually added after polymer addition to remove residual oil droplets as in the acid-alum process. The polymer-alum process offers the following advantages over the acid-alum process: (1) reduced use of acid and alum; (2) lower total dissolved solids in the effluent; (3) reduced corrosion problems and (4) reduced sludge production. A major advantage of both these techniques is their ability to handle high solids contents (Gambhir, 1983).

The disadvantages associated with chemical emulsion breaking techniques have led to the development of a variety of non-chemical oil-water emulsion separation methods. These include electrolytic treatment, various physical treatment methods and membrane separation techniques. These are discussed in turn below.

D

12

EIec t rol yt ic Treat men t Several different electrolytic methods have been investigated for

separating oil-water emulsions over the years (Snyder and Willihnganz, 1 976; Kramer et al., 1979; Oblinger, 1984). Electroflotation was one of the first processes developed in which oil was removed from emulsions that had been previously broken by chemical additives. More recent efforts have focused on the application of electrochemical techniques to break emulsions and separate destabilized oil without the addition of chemicals. The key process involved in most of these methods is electrocoagulation which can be considered a two step process: (1) aluminum or iron ions are introduced electrolytically to reduce the repulsive forces on the negatively charged oil droplets and break the emulsion and (2) a DC voltage is applied across the emulsion to cause the charged droplets to migrate and coalesce. The feasibility of these electrochemical methods for separating oil-water emulsions has been demonstrated in laboratory and pilot plant tests. Their applicability to large scale operations is questionable, however. Electrolytic methods are most useful for treating smaller volumes of wastewater with a relatively constant composition and character.

Physical Treatment Methods

are available for treating oil-water emulsions, including heating, high-speed centrifugation and magnetization (Wang et at., 1975). Oil-water emulsion breaking by heating is technically feasible but not economically practical because of the large amount of energy that is required to vaporize the water before the oil can be removed. In the case of high-speed centrifugation the maximum benefit of the centrifugal force is realized at the outer extremities of the centrifuge where the denser phases accumulate. It is easier therefore, to separate a small amount of dispersed water from a continuous oil phase, such as an oily sludge, rather than separate a small amount of dispersed oil from a continuous water phase (Wang et al., 1975; Tabakin et al., 1978).

A variety of physical methods

I

13

Researchers have found a magnetization technique that is efficient at separating oil-water emulsions (Wang et al., 1975). By adding a ferrofluid to a wastewater the oil droplets become magnetically responsive and when the emulsion is passed through a packed bed of magnetic particles the droplets can be collected. This process has been carried out only on a laboratory scale and its feasibility on an industrial scale is not known.

A number of other non-chemical oil-water separation processes have been reviewed by Wang et al. (1975). These include solvent extraction, layer filtration, crystallization and freezing and adsorption flotation. An emerging technology which is discussed in greater detail below is ultrafiltration.

UI ttaf iltration Ultrafiltration is a membrane separation technique that is becoming

increasingly more important in treating oily wastewaters. It is a physical separation and concentration method in which free, emulsified and finely dispersed oil are removed from a wastestream by forcing the water through a membrane under low pressure. An essentially oil-free effluent can be produced along with an oil-rich concentrate (Pinto, 1978).

An ultrafiltration membrane is a molecular filter which makes separations based on size; small solutes and water are allowed to pass through the membrane while larger oil droplets and suspended solids are retained. Typical pore sizes range from 0.001 to 0.02 microns in diameter which corresponds to molecular weight cut-off values of 1000 to 100,000 (Applegate, 1984).

A schematic diagram of an ultrafiltration process is shown in Figure 2 (Dhawan, 1978). The oily wastewater enters the membrane unit under an applied hydrostatic pressure (20-60 psi), and flows parallel to the membrane surface. The free oil, emulsified oil and suspended solids are retained and concentrated inside the unit while water, and dissolved solids having a molecular weight smaller than the molecular weight cut-off, pass through the membrane. The pores of the membrane are much smaller than the particles

-_

14

that are retained and this prevents the particles from entering and plugging the membrane structure. The membrane also has no depth to its pore structure and this minimizes plugging.

Figure 2: The Ultrafiltration Process

*. . "E.

o(t FEED I/

011 e-

The critical criterion for designing and operating an ultrafiltration system is the capacity of the membrane to pass water. This is called the membrane or permeate flux and is defined as the volume of water passed per unit membrane area per unit time. To minimize both capital and operating costs it is desirable to maximize the membrane flux. For most ultrafiltration applications the following relation holds: (Goldsmith et al., 1974: Michaels, 1968)

J - K ' I n T C*

15

where, J = membrane flux (gal/ft2/min) C* = a constant C = feed concentration of oil K = aqueous phase mass transfer coefficient

The membrane flux is inversely related to the feed concentration and directly related to the mass transfer coefficient. Temperature and feed flowrate also affect the flux as the value of K depends on them. Increasing pressure increases the flux up to certain point beyond which membrane compaction occurs and the permeate rate is reduced (Pinto, 1978). pH also exerts an effect on membrane flux in that higher rates have been obtained for acidified waters containing anionic surfactants (Goldsmith et al., 1974).

The equation given above assumes that reductions in permeate flux are due to the formation of a concentrated boundary layer at the membrane sutface that has a concentration C*. This phenomena is known as concentration polarization, and it is inherent in all membrane separation processes. Concentration polarization cannot be eliminated but it can be minimized by increasing the flowrate, which reduces the boundary layer thickness. A compromise must be made, however, between pumping costs and increased permeate flux.

Membrane fouling, which may be caused by the build up of oil or suspended solids at the membrane surface, also reduces the permeate flux by creating a hydrodynamic resistance. This effect tends to be more pronounced than concentration polarization at low oil concentrations, less than 10-1 5% (Pinto, 1978). Membrane fouling can be minimized by increasing the flow velocity. Wastewater pretreatment and frequent membrane cleaning also help to prevent fouling.

Ultrafiltration membranes are made from a variety of different polymeric materials such as cellulose acetate, polyvinyl chloride, polysulfone or poypropylene. Important criteria in selecting a membrane is compatibility with

16

the feed solution and the cleaning agents. Polysulfone membranes are widely used because they can tolerate temperatures as high as 93OC, pH's between 0.3 and 13 and they are compatible with many different cleaning agents (Applegate, 1984).

Three different ultrafiltration configurations are gene rally used. T h e se include tubular, spiral-wound and hollow-fibre units. Tubular systems are easy to clean by backwashing and they are best used to treat low flowrate wastewaters in which large amounts of fouling are expected. Hollow-fibre and spiral-wound membranes offer a greater membrane surface area per unit volume and therefore they are capable of treating larger volumetric flowrates. They cannot be cleaned by backwashing, however, and they require the use of

. cleaning agents (Applegate, 1984).

The separation of oil from water is usually carried out as a semi-batch operation. The oil concentrate is recirculated until the desired degree of concentration or dewatering has been achieved. An oil concentration of about 50% can be attained quite easily; concentrations above this are increasingly more difficult to achieve as the membrane flux decreases due to membrane fouling (Nordstrom, 1974; Hockenberry, 1977). Up to 90% of the water is removed from an oily wastewater, however, by the time a 60% oil concentrate is produced (assuming a typical oily wastewater contains 1-1 0% oil).

The permeate, which is drawn off continuously, normally has a suspended solids content less than 10 mg/l and an oil and grease content less than 100 mg/l (Pinto, 1978). The oil content of the effluent is directly related to the soluble, low molecular weight organic content of the feed, and if this is low an essentially oil-free effluent can be produced. For optimum performance the wastewater should be pretreated prior to ultrafiltration to remove the bulk free oil and suspended solids. Suspended solids may damage the membrane surface and free oil may enhance membrane fouling.

The major application of ultrafiltration in treating oily wastewater is for separating oil-water emulsions. It offers many advantages over conventional techniques such as: (1) consistently high permeate quality can be achieved

17

because the membrane acts as a physical barrier and it is not as susceptible to operational upset; (2) no chemical additions are required; (3) the oil concentrate, which may be reduced to less than 10% of its original volume, has a higher heating value, and in addition, the sludge hauling and disposal costs are reduced and (4) capital and operating costs are relatively low. The major disadvantage is the risk of membrane fouling which adversely affects the permeate flux and membrane life. Ultrafiltration systems are also incapable of recovering a directly reusable oil product. The oil-water concentrate obtained requires further processing to recover the oil (Goldsmith et al., 1974; Nordstrom, 1974; Pinto, 1978).

(c ) Tertiary Treatment Biological treatment and carbon adsorption are two of the most common

tertiary treatment methods for separating oil from water. Both are aimed at removing dissolved oil and any emulsified oil which was not removed by previous treatment steps. These methods are limited in their application but they can produce extremely clean effluents when properly used.

Biological organisms are efficient at oxidizing soluble oil but they are prone to upsets. Suitable pretreatment and dilution are nearly always required prior to biological treatment. Free oil concentrations in excess of 0.01 Ib/lb MLVSS (mixed liquor volatile suspended solids) cannot be tolerated as the free oil tends to coat the biological flocs and hinder oxygen transfer. It also reduces sludge settleability. Trickling filters can treat wastestreams with oil concentrations as high as 100 mg/l while activated sludge systems can only handle about 25 mg/l. Effluents from biological treatment typically contain less than 10 mg/l oil (Ford and Elton, 1977; Tabakin et ai., 1978).

Soluble oil is also efficiently removed by carbon adsorption once the wastewater has been adequately pretreated. Free oil and solids may clog and coat the activated carbon, which reduces its effectiveness. If this occurs backwashing is required. A suitable means of regenerating the activated carbon is also necessary. Effluents from carbon adsorption treatment may have oil contents as low as 2-10 mg/l (Ford and Elton, 1977). A major

I

i a

drawback to this method is the expense involved as both capital and operating costs are high. Consequently, it is used only for very specialized applications (Ryan, 1986).

Several different oil-water separation techniques have been discussed here. A summary of the advantages, disadvantages and applicabilities of each is given in Table 1. The information given in this table is fairly general and should be regarded as such. The selection of one oil-water separation process over another requires the consideration of many factors. The technique that is ultimately chosen must not only be technically feasible but it must be economic as well.

2.2 Theory of Emulsions An emulsion is a colloidal system consisting of two immiscible liquids, one

of which is dispersed in the other as fine droplets. The droplets, which normally have diameters between 0.1 and 20 p, possess a minimum stability due to electrical forces and this stability may be enhanced by the addition of emulsifying agents (Becher, 1965). With respect to the oil-water system two types of emulsions may exist: a water-in-oil (W/O) emulsion in which water is the dispersed phase and oil is the continuous phase or an oil-in-water ( O M ) emulsion in which oil is the dispersed phase and water is the continuous phase. Both types are widely used in a number of different applications.

A "stable" emulsion is one that will not separate into its two phases in a "reasonable" period of time. This may range from minutes to years depending on the emulsion in question. Surface-active or other agents may be added to an emulsion to increase its stability and these are known as emulsifiers or e mu lsi fy i ng agents.

Given here is a brief review of the theory of emulsions. Some relevant aspects of surface chemistry are discussed, followed by a description of the formation of emulsions and their properties. A few of the theories explaining emulsion stability are then described. Finally, the methods that are used to

assess emulsion stability are addressed.

19

Table 1: Summary of Oil-Water Separation Techniques

7

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-can lrrndlr h i & s u r - pendud rolldm -auld o w n L I t I u Ian r r y u l r r d III L r r y r

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-can handle f l u c t w - -curroulon p r o b l e m

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-nu c h m l c a l u taqulrrd - ryy l l c*b l l l ry on a0 Ludur11l*l E l a c r r o l y t Lc Prucabecu -r-vaa awlslf LOP 011 tram water e c a h l a q u r r l u a b l e

-reduced r l d y a p r o d u c t l a - c w p o l hradlr rbuck lurdu

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- ramVal rltlclrncleu of W-WZ b.w b r a obbervcd In r h Iab- oratory or p l l o i p h o t

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-used y r l m r l l y to r m n v r -w c l m r l c s l r r r q u l r r d -all c o w e n i t d r c a w u i h e r u l b l t l r d 011 re-ubrJ d l r c c t l y

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Irbr t lwn iu() ry/l U l l and i 0 r u than 10 rg/l I O U - r r l r t l v e l y luu c a p l t d and

fou l lny ulrlcb lovarr thr tlur

oparar lry corcr -mum1 p r r t r r r t L O ~ O W V L bulk f r e e 011 and n o l l d r

- p r a t r e a t u n t r r y u l r r d L o r e w v a f r u 011

Curbun Adwrp t lun -remuem ulublr 011 -blab po tan t 1.1 a t f Iclancy -expans1 va

- p r a r r e a r u u t r r q u l r r d

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I

21

(a) Relevant Aspects of Surface Chemistry

Surface Tension When a pure liquid is in contact with air, short-range attractive forces

(London van der Waals) exist between all the molecules in the liquid. In the bulk liquid these forces tend to balance out but at the surface they do not because the molecules at the surface are not completely surrounded by other liquid molecules. This results in a net downward force which causes the surface to contract and this behavior is called surface tension.

Because of surface tension, molecules at the surface are at a higher potential energy level than those in the bulk liquid. Consequently, work must be done to bring a molecule from the interior of the liquid to the surface to create new surface area. This work can be expressed as follows: (Adamson, 1982)

dG- Y*dA

where, dG = work required to create a unit of surface area dA = a unit of surface area Y = surface tension

The surface tension is defined as the work required to create one square centimeter of surface. It can be seen from the above equation that to minimize the surface energy of the system the surface area must be kept as small as possible.

Interfacial Tension Interfacial tension is similar to surface tension except it exists between two

liquids rather than a liquid and a gas. It is also of much more importance in understanding the theory of emulsions. When two pure liquids are in contact

I

22

with one another the forces acting on the molecules in the bulk liquids are balanced while those at the interface are not. The net imbalance of forces on the interfacial molecules is different from that of a simple air-liquid surface however, because of van der Waals' interactions with molecules of the second liquid. The interfacial tension between two liquids normally lies between the surface tensions of the individual liquids (Becher, 1965).

As with simple surfaces the work required to create new interfacial area can be expressed in terms of the interfacial tension, Y as follows: dG = Y 'dA. To form an emulsion, which consists of a vast number of dispersed droplets with a correspondingly high surface area, a very large input of energy is required. An effective means of reducing this energy requirement is to reduce the interfacial tension by adding a surface-active agent, or surfactant (Becher, 1965).

-

Surface Adsorption Compounds that lower the interfacial tension between two liquids do so

by adsorbing and accumulating at the interface. The driving force for adsorption is the minimization of surface free energy. This occurs when the concentration of a species with a low interaction energy is greater at the interface than in the bulk solution. Gibbs derived an equation based on thermodynamic considerations which relates the concentration of adsorbed species at the interface to the interfacial tension. He defined a quantity r, called the surface excess, which is the excess concentration of solute per unit area of interface over that present in the bulk solution. The most general form of this equation is: (Rosen, 1978)

where, dY = change in interfacial tension of the solvent Ti = interfacial excess concentration of component i dpi = change in chemical potential of component i

23

For a two phase system and dilute solution this equation reduces to:

where, C = solute concentration in the bulk liquid R = universal gas constant T = absolute temperature

For negative excess concentrations or negatively adsorbed solutes the interfacial tension increases with increasing solute concentration which is undesirable from the point of view of emulsification. For positive excess concentrations or positively adsorbed solutes the interfacial tension decreases with increasing solute concentration. Surface-active agents or surfactants fall under this category (Becher, 1965).

Gibbs' equation can be used to indirectly determine the amount of surfactant adsorbed per unit area of liquid-liquid interface. This is done by making interfacial tension measurements and plotting interfacial tension as a function of surfactant concentration. The slope of this plot is the surface excess concentration or surfactant adsorbed per unit area (Rosen, 1978).

Surf ace- Ac t i ve Agents

Surface-active agents as mentioned above, are substances which when present in low concentration have the property of adsorbing onto surfaces, or at interfaces, and of reducing the surface or interfacial energy. Surfactants have a characteristic molecular structure consisting of a lyophobic (solvent hating) group and a lyophilic (solvent loving) group. If the solvent is aqueous these

-.

24

terms may be replaced by hydrophobic and hydrophilic, respectively. This type of structure is known as amphipathic.

The hydrophilic group of a surfactant is ionic or highly polar, and depending on the nature of this group surfactants may be classified as anionic, cationic or nonionic. The surface-active portion of an anionic surfactant bears a negative charge, and a cationic surfactant has a positive charge. The hydrophobic groups of each of these classes are similar in that they are long- chain hydrocarbons (Rosen, 1978).

When a surfactant dissolves in an aqueous solution, the hydrophobic portions of the molecules tend to cause an increased degree of structure in the water, which increases the overall free energy of the system. To minimize the free energy, the surfactant molecules migrate to an interface where they orient themselves so that the hydrophobic groups extend away from the water and into the oil phase while the hydrophilic groups remain dissolved in the water. As surfactant molecules continue to concentrate at the interface to reduce the overall free energy of the system they also reduce the interfacial tension, which allows for more interfacial area to be created. This in turn allows more surfactant to be adsorbed and the process continues as such.

An alternative method of removing the hydrophobic groups from contact with water is to form organized clusters of surfactant molecules called micelles. In a micelle the hydrophobic groups are oriented away from the water and the hydrophilic groups are directed towards the water. The exact shape of a micelle is still a debated issue and there is evidence supporting both spherical and laminar structures (Becher, 1965; Adamson, 1982).

The efficiency of a surfactant in reducing interfacial tension depends on several factors. Most important is the nature and concentration of the surfactant. Generally, as the concentration of a surfactant increases the surface or interfacial tension decreases steadily up to a certain point, termed the Critical Micelle Concentration (CMC), beyond which it remains relatively constant. At this concentration micelle formation occurs. Above the CMC any new surfactant entering solution will aggregate to form micelles rather than

25

migrate to an interface. Thus, from the point of view of emulsification it is of no benefit to add surfactants in amounts greater than the CMC.

Other factors that affect the ability of a surfactant to reduce interfacial tension include the solubility limit of the surfactant, temperature, pH and additions of organics which alter the amphipathic nature of the surfactant.

(b) Emulsion Formation To form a stable emulsion two conditions must be satisfied. Sufficient

mechanical mixing must be provided to disperse one phase in the other as fine droplets and once these droplets have been formed they must be stabilized by the addition of an emulsifying agent.

Most methods for preparing emulsions use "brute force" to break the interface into fine droplets. Studies have shown that droplets in the 50-100 p range may be produced by hand shaking (Becher, 1965). To get smaller droplets, however, more vigorous agitation must be applied and most commercial methods are designed to provide a very large velocity gradient to achieve an appropriate droplet size.

Emulsification techniques can be divided into three broad categories: (1 ) mixing; (2) colloid milling and (3) homogenization (Sherman, 1968). A mixer consists simply of a stirrer that rotates in a cylindrical vessel. Turbulent flow is required for effective mixing and this is best achieved by using vertical baffles near the walls and a propeller shaped stirrer. Colloid mills emulsify liquids under strong shearing flow in a narrow gap between a high speed rotor and a stator surface. The rotor can rotate at speeds of 1000-20,000 rpm. This, together with the narrow gap, sets up very strong shear flows which tears the liquid interface apart and droplet diameters of about 2 p can be obtained. Homogenizers cause liquids to disperse in one another by forcing them through an orifice under high pressure. Finely dispersed systems can be achieved by homogenization with droplet diameters of 1 p or less. Mixers, homogenizers and colloid mills are the standard methods of producing emulsions; several other methods are available for special uses, however.

26

Sherman (1968) gave a detailed description of both the standard and less commonly used emulsifying techniques.

Other aspects of emulsion formation must also be considered aside from the "brute force" techniques involved in breaking up the interface. Among these are the character and concentration of the emulsifying agent , the mode of addition of emulsifier, the mode of adding the two phases, the time and intensity of agitation and temperature. These parameters greatly affect many of the properties of emulsions and they can be manipulated to produce an emulsion with particular characteristics (Becher, 1965).

Emulsifying Agents Emulsifying agents are added to an emulsion to ensure a certain stability.

Three types of emulsifying agents are used: (1) surface-active agents; (2) naturally occurring compounds and (3) finely-divided solids (Becher, 1965). Surface-active compounds which were discussed previously are by far the most commonly used emulsifiers. They may be ionic or nonionic and they stabilize dispersed droplets by adsorbing strongly at the oil-water interface.

Naturally occurring emulsifying agents, which include proteins, gums, starches and derivatives of these substances, also stabilize emulsions by adsorbing onto the oil-water interface. Because of their macromolecular nature and their multiplicity of hydrophobic and hydrophilic groups they can be very strongly held and produce very stable emulsions.

Finely divided solids stick to the oil-water interface by surface tension forces and they help stabilize emulsions by forming a protective monolayer around the dispersed droplets. The requirements for sufficient stabilization are that the solids have a particle size much smaller than the oil droplets and that they have a substantial contact angle at the oil-water-solid boundary. The latter is to ensure that the solids accumulate at the oil-water interface and do not enter the oil or water phases. A variety of different materials may be use including clays, powdered silica and basic salts of metals.

I

27

Sherman (1 968) summarized the desirable characteristics of an emulsifying agent as follows: (1) it must sufficiently reduce the interfacial tension; (2) it must adsorb quickly onto the dispersed droplets to form a film which will not thin out when two drops collide; (3) it must have a specific molecular structure with the polar end attracted to water and the non-polar end attracted to oil: (4) it must be more soluble in the continuous phase so as to be readily available for adsorption; ( 5 ) it must have adequate electrokinetic potential and (6) it must affect the viscosity of the emulsion. Further, it should be efficient at low concentration and it should be relatively inexpensive.

Numerous emulsifying agents exist and the selection of one over another for a particular application is not a simple task. Different agents may be more effective depending upon the emulsion in question and the existing conditions. The best means of selecting an emulsifier is to test a number of agents to determine which yields a stable emulsion with desirable physical properties at a reasonable cost. This method is not always feasible, however, as it is time consuming and labor intensive, and an alternative means of emulsifier selection is the hydrophile-lipophile balance (HLB) method (Griffin, 1949).

In the HLB method a number is assigned to an emulsifying agent, based on its emulsifying behavior, which is related to the balance between the hydrophilic and lipophilic portions of the molecule. The concept upon which this method is based is that any emulsifier contains both hydrophobic and hydrophilic groups and the ratio of their respective weights should influence emulsification behavior. The assigned numbers are related to a scale of suitable application as shown in Table 2 (Becher, 1965). It can be seen that surfactants with high HLB numbers tend to form O M emulsions while those with low HLB numbers form W/O emulsions. The HLB system only indicates the type of emulsion that will be produced and it does not give an indication of the efficiency of emulsification.

28

Table 2: The HLB Method of Selecting an Emulsifying Agent

HLB Number Oispersibility in Water A m l i c a t i o n

1-4 nil

3-6 poor W/O emulsifier

6-8 milky dispersion Wetting agent

8-1 0 stable dispersion Wetting agent O M emulsifier

10-1 3 translucent dispersion OMI emulsifier

13 clear dispersion O N emulsifier Solubilizing agent

(c) Physical Properties of Emulsions Emulsions have a number. of physical properties which are used to

characterize them. These include emulsion type, volume ratio of inner to outer phase, droplet size distribution, viscosity, electrical conductivity and dielectric constant. These properties are largely dependent on the composition of the emulsion and its mode of preparation. In addition, many of them are not entirely independent and changes in one may affect another.

As mentioned previously an oil-water emulsion may be one of two types: ONV or W/O. The type of emulsion that forms is dependent both on the relative amounts of the two phases and the nature and amount of emulsifying agent present. There are several ways of determining what the type of a given emulsion is. One can distinguish between the two by diluting an emulsion with one of the two phases; the emulsion can be readily diluted by its continuous phase. Alternately, a dye which is soluble in only one of the phases may be added and color will be imparted to the emulsion if the dye is soluble in the

29

continuous phase. Electrical conductivity measurements may also be used since O N emulsions conduct electricity much better than W/O emulsions

An important parameter used to describe an emulsion is the volume ratio of the inner to outer phase, @. As the volume of the dispersed phase exceeds that of the continuous phase the emulsion will become less stable and it will have a tendency to invert (change types). As shown later, a value of @ = 0.74 represents the closest packing of spheres and inversion should theoretically occur beyond this value.

By definition an emulsion has a droplet diameter greater than 0.1 p (Becher, 1965). In practice the droplet size may be as large as 10-20 p, and the droplets do not occur as a uniform size. Instead they are distributed over a size range and the droplet sjze distribution will change with time as the emulsion stability changes. Droplet size is also related to the mode of preparation and the nature and concentration of emulsifying agent. More intense mixing and higher concentrations of emulsifying agents for example, will favor the formation of smaller droplets. The size of the dispersed droplets determines the appearance of the emulsion to the naked eye due to the scattering of light. Most emulsions are opaque and milky colored; some may be transparent if the droplets are fine enough or if the two phases have similar refractive indices.

Viscosity is an important property of an emulsion from both a practical and theoretical point of view. To render an emulsion useful for a given application a specific viscosity is usually required. The viscosity however, affects the stability of an emulsion and other physical properties so it may be difficult to meet the conflicting criteria of emulsion stability and desired physical properties. Six factors have been found to affect the rheology of an emulsion: (1) viscosity of the internal phase; (2) viscosity of the external phase; (3) volume ratio of inner to outer phases; (4) nature of the emulsifying agent and the interfacial film; (5) electroviscous effect and (6) particle size distribution. Becher (1965) and Sherman (1968) describe these in detail.

30

Electrical conductivity, as previously mentioned, is one means of determining emulsion type. Emulsions in which water is the dispersed phase show little or no conductance while those with water as the continuous phases show high conductance. The dielectric constant of an emulsion was thought to be linearly related to the dielectric constants of the individual phases but studies have demonstrated that this is not the case and in fact the dielectric constant is quite different from that expected (Becher, 1965).

(d) Emulsion Stability

When two immiscible pure liquids are mixed vigorously they will form a dispersion. When the mixing stops however, the dispersed droplets have a natural tendency to recombine due to the thermodynamic instability of the system. To form an emulsion an emulsifying agent is required to provide stability. Despite this, an emulsion is never completely stable in a thermodynamic sense and the dispersed droplets stili tend to recombine. The "stability" of an emulsion therefore, may be defined as its ability to overcome the forces which cause the dispersed droplets to recombine (Becher, 1965).

-

Two processes are involved in the recombination of dispersed droplets. These include: (1) flocculation in which the droplets come together and form aggregates but still maintain their identity and (2) coalescence in which the droplet aggregates fuse together to form single drops. Flocculation is a transport step involving long-range forces and/or Brownian motion and coalescence is a transport and destabilization step involving short-range forces and film-film interactions (Weber, 1972).

Both Becher (1965) and Rosen (1978) described several theories which have been proposed over the years to explain emulsion stability and the role of emulsifying agents. Originally it was thought that the most important factor leading to emulsion stability was the lowering of interfacial tension caused by the adsorption of an emulsifying agent at the oil-water interface. It has long since been realized however, that although this reduces the amount of energy required to form an emulsion, it does not play a significant role in the stabilization of an emulsion. Of primary importance is the oriented interfacial

31

film which is formed by an emulsifying agent at the oil-water interface. This monolayer film decreases the rate of agglomeration and coalescence of the dispersed phase by presenting mechanical and/or electrical barriers. Discussed below are some of the theories of emulsion stability. A brief discussion of the earlier theories is given followed by a more detailed description of the modern theories which consider the role of the interfacial film.

Earlier Theories of Emulsion Stability

The early theories of emulsions concerned themselves more with explaining the formation of particular types of emulsions rather than focusing on the cause of emulsion stability. Bancroft (1913) recognized that the type of emulsion depends on the nature of the emulsifying agent. He formulated a general rule which still has some validity: The phase in which the emulsifying agent is more soluble will be the continuous phase . Later, Bancroft (1926) developed a more elaborate form of this theory which rationalized emulsion type in terms of interfacial tensions. He proposed that the interfacial film was duplex in nature consisting of an inner and outer interfacial tension and that the film curves toward the phase with the higher interfacial tension.

Becher (1 965) described a theory called the "oriented wedge", which was proposed to explain emulsion type based on the orientation of a monolayer of emulsifying agent adsorbed at the interface. This theory assumes that the interfacial film must be oriented so that the hydrophilic groups extend into the aqueous phase and the hydrophobic groups into the oil phase and it requires that the monomolecular film be fairly closed-packed. It was concluded that due to geometrical considerations the bulkier molecular groups would have to be on the outside of the droplet. This is demonstrated in Figure 3 (Becher, 1965). Figure 3(a) shows the stabilization of an OM/ emulsion by a monovalent soap and Figure 3(b) shows the stabilization of a W/O emulsion by the soap of a bivalent metal. The hydrophilic group of the monovalent soap is bulkier than the hydrophobic group and thus is outside the droplet; the reverse situation is true for the bivalent metal soap.

I

32

Figure 3: The Oriented Wedge Theory

(a) O/W Emulsion Stabilized (b) W/O Emulsion Stabilized By a Monovalent Soap By a Bivalent Soap

Becher (1965) explained the "volume phase theory" which was proposed by Ostwald in an attempt to explain emulsion inversion. It can easily be calculated that the phase volume ratio for the most densely packed arrangement of solid spheres is 0.74. Ostwald concluded that this value also corresponds to the most densely packed emulsion and that if t$ > 0.74, an emulsion would be more densely packed than possible and therefore invert or break. Theoretically, this behavior would be expected but in reality it is not observed. The reasons for this are that the dispersed droplets are not necessarily spherical in shape or of a uniform size distribution and in addition they are deformable so they can be more closely packed than t$ =0.74.

33

Modern Theories of Emulsion Stability

emulsion stability: (i) mechanical and (ii) electrical. The modern theories of emulsions have dealt with two aspects of

(i) Mechanical Aspects of the Interfacial Film The more modern theories of emulsions focus directly on emulsion

stability and in particular they consider the effect of the interfacial film. King (1941), who was one of the first investigators to realize the importance of the interfacial film, considered the mechanical aspects of the film. He felt that the most important factor controlling emulsion stability was the strength and compactness of this film. He also thought that the quantitative source of the film strength was the nature and concentration of emulsifying agent. The reduction in. interfacial tension due to the addition of an emulsifying agent was considered to play a minor role in emulsion stability. Subsequent workers have drawn similar conclusions with respect to the importance of the physical properties of the interfacial film (Becher, 1965; Horder, 1977).

To impart stability to an emulsion the mechanical properties of the interfacial film must be such that colliding droplets resist rupture. The film must be capable, therefore, of withstanding compaction and shearing forces. For maximum mechanical stability the interfacial film should be condensed with strong intermolecular forces and it should exhibit high film elasticity. A highly viscous film may also contribute to the formation of a mechanical barrier; the displacement of emulsifier molecules is required for coalescence and high film viscosities hinder this process.

Highly purified surface-active agents generally do not produce close- packed interfacial films and good emulsification is generally achieved using a mixture of two or more surfactants. It is thought that complex formation occurs at the interface which results in an interfacial film with greater strength. Densest packing appears to be realized for an emulsion system containing both a water soluble and an oil soluble surfactant. (Rosen, 1978).

34

(ii) Electrical Aspects of the Interfacial Film Electrical theories describing emulsion stability are based on the fact that

dispersed droplets carry an electrical charge that leads to electrostatic repulsion between droplets. How closely two droplets may approach one another depends on the magnitude of the repulsive force, and to keep an emulsion stabilized the repulsive forces between droplets must be greater than the attractive forces.

Droplets can become charged in one of three ways: ionization, adsorption or frictional contact (Becher, 1965). A surface-active agent with an ionized hy :-ophilic group for example, may adsorb onto a dispersed droplet. Thus, droplets in emulsions stabilized by anionic surfactants will possess a negative surface charge such as an O M emulsion stabilized by a soap. For nonionic emulsifiers, a general rule for predicting the surface charge is that the phase having the higher dielectric constant will be positively charged. Regardless of how these charges arise they result in the formation of an electrical double layer which gives rise to the repulsive forces.

In the case of an anionic surfactant the oil-water interface has a net negative charge and to maintain electrical neutrality this charge must be balanced by a net positive charge on the water side of the interface. This results in the formation of a potential across the interface and the rate of change of this potential with distance away from the surface is determined by the distribution of counter charges in the water phase.

The electrical double layer theory proposed by Stern suggests that the distribution of counter ions is as shown in Figure 4(a). Figure 4(b) shows the corresponding potential drop as a function of distance away from the interface (Rosen, 1978). Adjacent to the interface is a fixed layer of counter ions of approximately a single ion thickness; in this layer the potential drops rapidly. Beyond this is a diffuse layer which extends into the bulk aqueous phase and within this layer the potential drops more gradually. Two competing processes lead to this distribution of counter ions; an electrostatic attraction towards the interfacial area and Fickian diffusion away from the interface. The former is

35

due to the charge on the interface and the latter is due to a concentration gradient of counter ions in the water phase.

The effective thickness of the electrical double layer is the distance from the charged surface into the aqueous phase within which the surface charge is neutralized. This has been found to be directly proportional to the surface potential, wo , and the square root of the temperature, and inversely related to the valence and concentration of ions in solution (Adamson, 1982). Electrical effects therefore, have a shorter range in the presence of high concentrations of electrolytes and at lower temperatures under which conditions the double layer is compressed.

A term associated with the electrical double layer and one which is related to various electrokinetic effects is the zeta potential, t; . This is the potential of the charged surface at the plane of shear between the droplet and the surrounding solution. This plane, as indicated in Figure 4(b), is in the diffuse layer beyond the Stern layer boundary. Zeta potentials are conveniently measured and they can be related to the surface potential (Adamson, 1982).

Becher (1 965) described the interaction of charged particles using the D.L.V.O. theory, which considers emulsion stability in terms of the attractive and repulsive forces acting on dispersed droplets. The repulsive force between droplets is due to the overlapping of electrical double layers and the magnitude of this force depends on the double layer thickness and the surface potential, vo. The attractive force is due to van der Waals' forces as discussed previously. The net interaction between two droplets is the sum of the repulsive forces and the attractive forces and if the repulsive force is greater than the attractive force an energy barrier will exist as shown by the total energy curve (2) in Figure 5 (Rosen, 1978). To form a stable emulsion it is required that this energy barrier be formed. In curve (1) in Figure 5 an energy barrier does not exist and a stable emulsion would not be formed.

36

Figure 4: Stern Electrical Double Layer

(a) Distribution of Counter Ions (b) Potential versus Distance From the Interface

+ m:+ Droplet 1 1 + w-- I + ++

+ + + + + + + + +

/Stern Layer

Shear Plane

4 i

Distance From Surface x - 0

Figure 5: Potential Energy Diagram For Two Dispersed Droplets

I

37

In summary, one can say that the flocculation of droplets in an emulsion is prevented by the electrical barriers formed by the interfacial film of an emulsifying agent while coalescence is prevented by the mechanical barriers formed. The classic D.L.V.O. theory can be used to describe the process of flocculation and any factors minimizing the electrostatic repulsion between droplets will favor flocculation. Coalescence cannot be described by the D.L.V.O. theory, however, as it involves the rupturing of the interfacial film. Whether or not coalescence occurs depends primarily on the physical nature of this film and the methods used to destroy it.

(e) Assessing Emulsion Stability To assess emulsion stability one must find a means of measuring droplet

resistance to flocculation and coalescence. According to Horder (1 977) this may be done in one of three ways: (1) directly by measuring the change of state of the emulsion with time; (2) indirectly by measuring an emulsion property and relating it to emulsion stability or (3) by directly determining the stability under an applied stress. The latter of these methods is sometimes referred to as accelerated aging (Becher, 1965).

One can directly assess emulsion stability simply by observing the change in appearance of the emulsion with time. As the emulsion becomes more unstable the two phases will begin to cream and/or separate out. It may take a long time however, for noticeable changes to be observed and for this reason it is not always practical to apply this technique. Alternatively, the droplet size distribution can be used to define the stability of an emulsion. An emulsion with a small mean diameter and a narrow size distribution represents a situation of maximum stability. As an emulsion becomes more unstable with time the mean diameter will increase and the size distribution will broaden.

The droplet size distribution may be measured microscopically or by using a particle counter. Microscopic measurement, though extremely laborious, allows for differentiation between spherical and polyhedral droplets and between aggregates of droplets and single coalesced drops. Particle counting

I

3a

methods are generally much quicker and less tedious, but they assume all drops to be spherical and they cannot differentiate between aggregated and coalesced droplets. Turbidity measurements have also been used to determine the average droplet size of an emulsion (Reddy and Fogler, 1981). This method is relatively easy and inexpensive, but it yields an average drop diameter rather than a size distribution, and it also requires a significant difference in the refractive indices of the two phases.

Accelerated aging tests are used to speed up the rate of coalescence so that a direct assessment of emulsion stability may be made more rapidly. Two kinds of stresses are generally applied to accelerate droplet coalescence: abnormal temperatures and centrifugation. Centrifugation accelerates the effect of gravity separation. It also subjects the droplets to very close packing compared with normal conditions, and therefore centrifugation provides information on the final stage of coalescence: droplet rupture. Elevated temperatures accelerate the rate of droplet collision by increasing the kinetic energy due to Brownian motion. In using accelerated aging tests one must take care to ensure that the applied stress does not alter the mechanism of coalescence or introduce additional mechanisms; some researchers criticize this method because there is often little direct correlation between accelerated tests and behavior under normal conditions (Horder, 1977).

The stability of an emulsion has been shown to be a function of the electrical and mechanical properties of the interfacial film. As a result, the measurement of these properties may be used to assess emulsion stability. It must be borne in mind, however, that the relationship between an emulsion property and emulsion stability depends on the mode of action of the emulsifying agent. If the mechanical aspects of the interfacial film are principally responsible for emulsion stability, then interfacial rheometry should be measured: if the emulsion owes its stability mostly to electrostatic repulsion, however, the electrical properties of the emulsion should be measured (Horder, 1977).

Methods of measuring the interfacial rheometry of an emulsion have been reviewed by Sherman (1953). The kinds of measurements that are made

I

39

include interfacial viscosity, interfacial tension and spreading coefficients. In addition, the response of an emulsion to an applied stress may also be measured to determine film strength.

The electrical properties of an emulsion can be determined using electrophoresis techniques. In this method a known D.C. voltage is applied across a sample of emulsion which causes the charged droplets to migrate towards the electrode of opposite charge. The electrophoretic mobility of a droplet depends on the magnitude of the charge density at the droplet surface. Zeta potentials are calculated from the measured droplet velocity and from knowledge of other emulsion properties.

The zeta potential can be calculated from the measurement of the electrophoretic velocity using the Helmholtz-Smoluchouski equation as follows (Sherman, 1968):

where,

4mv c= - &E

6 = zeta potential q = viscosity v = electrophoretic velocity e = dielectric constant of the continuous medium E = applied voltage

A more detailed discussion of electrophoretic techniques was given by both Sherman (1 968) and Becher (1 965).

40

3. THEORY RELEVANT TO SOF

3.1 Droplet Coalescence on Fibres A review of the literature addressing droplet coalescence on fibres was

made in an attempt to gain a better understanding of the possible role of oil droplet coalescence on fibres in the SOF process. In the context of this discussion, "coalescence" refers to both the aggregation and collapse of dispersed droplets. It was hoped that some of the observations and theories presented in the literature would be applicable to the SOF process in order that predictions could be made about the effects of various system parameters on the SOF performance.

Numerous studies of oil and water droplet coalescence on fibres have been conducted. Most of the studies reported were aimed ultimately at understanding the mechanism of coalescence in fibrous filter beds. Fibrous filters differ fundamentally from the SOF process in that the dispersed droplets coalesce as they pass through the filter bed, and a final separation of the two phases is made following the filtration process so that the dispersed phase remains in contact with the continuous phase. In the SOF process the coalesced or collapsed oil droplets are separated from the water phase immediately by forcing them through the membrane pores. Despite this difference, some aspects of the reported studies were found to be relevant. The findings of several studies are reported below.

Hazlett (1965) examined the steps involved in the coalescence of water droplets in a fibrous filter bed. Three steps were considered: (1 ) approach of a droplet to a fibre; (2) attachment of a droplet to a fibre and (3) release of an enlarged droplet from a fibre surface. He found that interception is the predominant mechanism of approach and the limiting step in overall coalescence. He described the interception mechanism by an equation derived by Langmuir (1 942):

4 4 I [2(1+R)ln(l+R) - (1+R) +i$$ Es = 2(2-ln(Re))

41

where, Es = collection efficiency by a single isolated fibre - Oilfeed'OiJoroduct -

Oilfeed drvp Re = Reynolds number =- P

R = interception parameter =

d, = particle diameter df = fibre diameter v = superficial velocity p = oil-water mixture viscosity p = oil-water mixture density

9 2 df

This equation indicates that the efficiency of interception on a single fibre is affected by both the fibre and droplet diameters and the flow velocity. The collection efficiency increases as flow velocity increases, fibre diameter decreases and drop diameter increases. Based on this equation Hazlett concluded that hydrodynamic factors control interception.

The attachment process was considered to involve droplet film drainage and rupture prior to attachment. Hazlett speculated that surfactants may affect the attachment of droplets by reducing the rate of film drainage and by lowering the interfacial tension which increases the deformability of the droplets. He also suggested that surfactants decreased the wettability of the fibres which contributed to reduced coalescer performance. In addition, he predicted improved coalescence with increasing surface energy of the fibre.

The extent of growth of coalesced droplets and their release from the fibre was found to depend on the interfacial forces between the continuous oil phase and the water. Small amounts of sodium petroleum sulfonate were found to have a deleterious effect on the coalescer efficiency and it was thought to be due to changes in interfacial forces which tend to keep the coalesced droplets small in size and causes the droplets to redisperse after coalescence.

.

42

Bitten (1970) conducted a visual study of the coalescence of water droplets from fuel oil on five different kinds of fibres. Single fibres were used and they were placed perpendicular to the flow of the oil-water mixture. He found that a minimum velocity existed below which little or no coalescence occurred and that the maximum size droplet that could be held by the fibre varied considerable among the different fibres. He also found that small amounts of sodium sulfonate greatly inhibited coalescence.

Moses and Ng (1985) also conducted a visual study of droplet coalescence, but they looked at the coalescence of silicon oil droplets from water. They considered the effects of wettability, emulsion and collector zeta potentials and emulsion droplet size on the coalescence process. Larger droplets were found to enhance the collection efficiency through inertial impaction and interception. Globule formation was encouraged when the outlet of the coalescer was non-wetting to the dispersed phase. Moses and Ng found that the relative signs and magnitudes of the zeta potentials of the droplets and the fibres were important; improved coalescence of the negatively charged oil droplets occurred when a less negative or positive collector was employed.

Several theoretical analyses have been made to predict coalescence efficiencies under different conditions. Spielman and Goren (1 970) developed transport equations for coalescence on a cylindrical collector in which they accounted for colloidal and hydrodynamic particle-collector interactions at low Reynolds numbers. Later Spielman and Su (1 977) presented another theoretical analysis focusing on the effects of droplet release and transport through the filter bed.

Adamczyk and van de Ven (1981) predicted deposition rates of Brownian particles flowing past an isolated cylindrical collector and a fibrous filter. They developed complete transport equations taking into account surface interactions and external forces. Their numerical calculations indicated that particle deposition rates increase significantly when strong attractive double layer forces exist between the particles and the collector.

43

Weber and Paddock (1 983) predicted collision efficiencies for spherical particles on spherical and cylindrical collectors. They considered only gravitational and interception mechanisms of approach, and they did not consider electrical or surface forces. Their equations were developed for Reynolds numbers up to 100, however, and the solutions to these equations predict an increase in collision efficiency with decreasing fibre diameter, increasing drop diameter and increasing flow velocity. These results are in agreement with those found by Hazlett.

Albery et al. (1985) studied the kinetics of colloid deposition on microscopic slides. They measured the rate of deposition of negatively charged carbon particles onto clean glass and onto clean glass treated with N ( p - a m i n o et h y I ) - y-a m i n o p ro p y I - t ri m et h o x y s i I an e (A E A P TM S ) w h i c h g av e t h e surface a positive charge. The kinetics of deposition on the clean glass slides was found to be about an order of magnitude slower than that for the surface- coated slides. This implied the existence of a kinetic barrier due to electrostatic repulsion between the similarly charged particles and collector surface. The coated surface carried a charge of opposite sign to the particles and thus a kinetic barrier was not observed.

Clayfield et al. (1985) conducted a comprehensive study to elucidate the mechanisms of coalescence and to determine the roles of surface chemistry and eiectrostatic interactions between the droplets and the collector surface. They measured the coalescence efficiency of kerosine droplets from water onto glass fibres that had been surface treated with a variety of polymeric coatings. Their results indicated that coalescence was not correlated with surface energy or wettability as predicted by previous investigators. They did find a strong influence of surface charge effects, however, in that coalescence improved when the dispersed droplets and the fibres were of opposite charge.

Clayfield et al. derived a model based on the three step mechanism proposed by Hazlett. They considered droplet approach to be primarily an interception process and that this step plays a critical role in the overall efficiency of coalescence. The probability of interception depends on the hydrodynamics of the system and the electrostatic interaction between the

44

droplets and fibres. Clayfield et at. proposed that coalescence will take place at a hydrodynamic limiting rate when there is no electrostatic repulsion and at a rate less than this when there is an electrostatic effect, They also speculated the effect of surfactants, suggesting that they modifiy the electrical properties of the droplets and the fibres, and that they affect droplet film drainage and interfacial tension .

Hughes and Foulds (1986) studied the coalescence of kerosine, Nigerian light crude oil and Brienenard crude oil on clean polypropylene fibres and fibres coated with AEAPTMS. Electrokinetic measurements were made to assess the effect of electrostatic interactions. The effect of surfactants, pH and ionic strength on droplet interception were also examined. A theoretical model was developed to predict coalescence efficiency on a filter bed as a function of system parameters.

-

As observed previously, the electrostatic interaction between oil droplets and the fibre surface was significant. Fibres to which AEAPTMS had been applied had a highly positive zeta potential, in contrast to the highly negative zeta potential on the oil droplets. Consequently, high coalescence efficiencies were obtained. Surfactants affected the electrostatic interaction by adsorbing onto both the droplets and the fibre surfaces. Dissolved salts compressed the electrical double layer and at higher salt concentrations the electrokinetic properties were less important. Surface potential and the double layer thickness were both affected by pH.

A theoretical model derived by Hughes and Foulds is based on the assumption that interception is the determining step in coalescence. They began the development of his theory with Langmuir's equation (Eq. 3.1) and they then added a factor 0 which includes the effects of electrostatic i n teractions so that:

where,

Est = oEs

o = 1 .O for no repulsive interaction

45

CT e 1 .O for same charge on particles and fibres

They extended equation (3.2) from application to an isolated single fibre to a bed of fibres so that:

where, Eb = coalescence efficiency for a bed of fibres n = number of fibres encountered by a droplet

The value of n is calculated as follows:

where,

d n = 2(1 -E)T

d = bed depth df = fibre diameter e = bed porosity

(3.4)

From the studies described above the following statements can be made about the coalescence of droplets on fibres: (1) the interception of droplets by fibres is of primaty importance in determining the overall coalescence efficiency; (2) the hydrodynamics of the system determine the limiting coalescence efficiency; (3) repulsive electrostatic interactions reduce the coalescence efficiency; (4) surfactants may alter the electrostatic interaction between the droplets and fibres, and they may reduce the growth of coalesced droplets and affect their release; (5) pH and ionic strength also affect the

46

electrostatic interaction by modifying surface charges and double layer interactions; (6) surface energies and wettabilities may or may not play a role in the coalescence process and (7) several theoretical analyses have been made to predict coalescence efficiencies and care must be taken in selecting one of these models to ensure that the appropriate operating conditions and coalescing mechanisms have been taken into account.

3.2 Proposed Mechanism of Oil Transport in the SOF Process

A hypothesis for the transport of oil from the bulk solution to the permeate stream using SOF was derived. A schematic diagram of this conceptual model is shown in Figure 6. Three steps are considered in the overall transport process: (1) transport to the membrane surface, J; (2) attachment and collapse at the membrane surface, A, and (3) transport through the membrane pores,

J m.

Figure 6: Proposed Mechanism of Oil Transport in the SOF Process

Bulk Solution

(C)

c J .------..

A cap

47

Transport to the membrane surface, J, is a convective transport phenomena, and therefore it should depend on the hydrodynamics of the system. The greater the contact opportunity between the oil droplets and the membrane, the greater J should be. Thus, one would expect J to be a function of feed flowrate, bulk oil concentration and module geometry. In addition, the electrostatic interaction between the oil droplets and the membrane is important. An attractive electrostatic interaction would increase J.

Transport of oil across the membrane, Jm, depends on the properties of the membrane and the emulsion. J m can be described by the following equation (Michaels, 1968):

K*P

where, K = membrane permeability P = pressure differential across the membrane p = oil viscosity 6 = membrane thickness

Transport through the membrane should increase linearly with pressure up to a point, beyond which membrane compaction occurs. Increasing oil viscosity or decreasing temperature should cause Jm to decrease.

The rate of attachment and collapse of the oil droplets at the membrane surface depends on two factors: (1) the intimacy of contact between the dispersed droplets and the membrane and (2) the ability of the droplets to adhere to and collapse at the membrane surface. Of importance are the relative sign and magnitude of the electrical charge on both the droplets and the membrane; a repulsive electrical interaction would hinder the attachment step. The mechanical strength of the oil droplets would influence droplet rupture and collapse. The hydrodynamics of the system would also affect droplet attachment and collapse as it effects t h e extent of the droplet- membrane contact.

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4a

It was the intent of this study to determine the conditions under which each of these three steps limits the overall transport process. This information is valuable as it allows for appropriate measures to be taken to increase the permeate rate and improve the process performance. This was determined by studying the influence of various operating parameters on the permeate flux.

Oil transport to the membrane would be expected to limit under the following conditions: low bulk oil concentration and low feed flowrate. The transport step may also limit if there is a strong electrical repulsion between the oil droplets and the membrane, as may be the case in the presence of an ionic emulsifying agent. Transport through the membrane should limit at low operating pressures and for high viscosity oils. Low operating temperatures would also promote this situation since viscosity is inversely related to temperature. The attachment and collapse step should limit if the oil droplets are electrically and/or mechanically stable. This would be expected in the presence of high concentrations of emulsifying agents.

.

If transport to the membrane surface is limiting, one would expect the permeate flux to be dependent on the hydrodynamics of the system and relatively independent of pressure, oil viscosity or temperature. If transport across the membrane were limiting, the reverse would be true, and i f the attachment step limits, the flux should depend on surfactant type and concentration and/or emulsions stability.

49

4. MATERIALS AND METHODS

4.1 Oil-Water Mixtures

A variety of oil-water mixtures were tested with different oils and emulsifying agents. The materials used to make these mixtures along with a description of the method of preparation.are described below.

(a) Oils

(ii) dodecane. Two different oils were used in these experiments: (i) naphthenic oil and

(i) Naphthenic Oil : A naphthenic lube base stock engine oil was supplied by Farm Oyl (St. Paul, MN.) and used in the first stage of this research. Lube oils are not pure compounds but rather made up of a number of different compounds such as paraffins, naphthenes and aromatics. The exact nature and character of a lube oil varies with the selection of crude from which it is derived and the type and seventy of processing. Typically, naphthenic lube oils contain 50-60% naphthenes and 4040% paraffins and aromatics. The naphthenic oil received was light brown in color and it had a viscosity of 100 SUS at 100' F or 53 CP at 20' C. The density of this oil was calculated to be 0.88 g/cm3- The naphthenic oil was selected for use as it represents one type of oil typically found in oily wastewaters and it emulsifies easily with many different emulsifying agents.

(ii) Dodecane: Dodecane used in stage two of this research is a straight chain n-alkane. Its chemical formula is CH3(CH2)10CH3 and it has chemical and physical properties as follows:

Molecular Weight: 170.34 g/mole Density: 0.749 g/cm3 Melting Point: -9.6' C

50

Boiling Point: 216.3' C Index of Refraction: 1.421 6 Heat of Vaporization: 12,588 cal/mol Heat of Fusion: 51.3 cal/g Viscosity: 1.27 CP at 32OC Soluble in: alcohol, ether, acetone, chloroform Color: clear

Dodecane was selected for use as it is a pure compound, it is immiscible with water, it has a low viscosity and it is non-volatile.

(b) Water

was used during stage two. Tap water was used during stage one of this research and distilled water

(c) Emulsifying Agents Four different emulsifying agents were used in these experiments. Three

of these were employed in stage one of the research including Pet Mix#9, alkylbenzene sulfonic acid and Triton X-102. The fourth emulsifier which was used in stage two was sodium dodecyl sulfate.

(a) Pet Mix#9: Pet Mix#9 which was supplied by Witco Chemical Corporation (New York, NY) is an anionic emulsifier package. It is composed primarily of sodium petroleum sulfonates and soaps of tall oil fatty acids. Pet Mix#9 emulsifies naphthenic lube oils very well and it is commonly used to prepare metal working lubricant-coolant solutions. This emulsifier was received in the form of a dark brown viscous liquid.

(b) Alkylbenzene Sulfonic Acid: Alkylbenzene Sulfonic which is an anionic surfactant was supplied by Fremont

Acid (ABS), Industries

51

(Minneapolis, MN). It has a random distribution of benzene rings and an average alkyl chain length of about 12. The anionic functional group is SO3- which results from the disassociation of the sulfonic acid. The general structure of ABS may be represented as shown in Figure 7. Because of the acidic nature of this surfactant emulsions prepared with it have a very low pH.

Figure 7: General Structure of Alkylbenzene Sulfonic Acid

(c) Triton X-102: Triton X-702 is a nonionic surfactant supplied by Rohm and Haas (Chicago, IL). It is prepared by the reaction of octylphenol with ethylene oxide and the product is often referred to as an alkylaryl polyether alcohol. The general structure of this class of surfactants is as shown in Figure 8 (Rohm and Haas Information Bulletin, 1977).

Figure 8: General Structure of Triton Surfactants

52

Triton X-102 has an x value of 12-13, where x is the average number of ethylene oxide units in the ether side chain. This surfactant was received in the form of a clear viscous liquid and it has the following properties: (Rohm and Haas Information Bulletin, 1977)

Density: 8.9 Ib/gal (5.1 7 g/cm3) Viscosity: 330 cps Pour Point: 6OoC Flash Point: >300° C Calculated HLB: 14.6

(c) Sodium Dodecyl Sulfate: Sodium dodecyl sulfate (SDS) which is an anionic surfactant was supplied by Aldrich Chemical Company (Milwaukee, WI). It is derived from the reduction of natural fatty acids and it has the molecular formula CHa(CH2)11OSO3Na. The SDS was received in the form of a white powder and some of its properties are given below:

Molecular Weight: 288.38 g/mole Melting Point: 204-207OC CMC: 9.7 x 1 Oe3 M at 40' C (Rosen, 1978) HLB: 40 (Becher, 1965)

SDS dissolves readily in water but remains undissolved in dodecane. It was selected as it is a pure compound and it is a widely used detergent and therefore typical of surfactants one might encounter in practice.

(d) Preparation of Oil-Water Mixtures Oil-water mixtures ranging from 1-100% oil by weight were tested.

Emulsifying agents were employed for some experiments and omitted in others. In all cases however, a high-speed rotary vane type pump was used to

53

disperse the oil in water. The pump is made by Procon Products (Murfreesboro, TN) and it has a capacity of 15-1 00 GPM. A reducer connector was placed on the outlet tubing from the pump to constrict the flow and produce a high velocity jet of solution which provided sufficient mixing conditions.

In most experiments, the surfactants were mixed with the oil phase and then the oil phase and then the oil+ emulsifier mixture was mixed with the water. However, SDS was added directly to the oil-water mixture due to its insolubility with oil , Mixing was carried out using the pump in recirculating mode for about five minutes initially and during the course of an experiment the oil was kept dispersed by either magnetic stirring or intermittent mixing with the

Pump.

4.2 Hollow-Fibre Modules Selective oil filtration was carried out using a hollow-fibre membrane

system. In all the experiments the oil-water mixtures were made to flow on the outside of the fibres and the oil permeate was collected inside the fibres. Many different modules were used in this study, and these are described below. Also given is a description of the membranes used and the method by which the modules were made.

(a) Membrane Selection The most important criteria in the selection of a membrane for the SOF

process is that the membrane be hydrophobic in nature. In addition to this, the membrane should be resistant to chemical attack by a variety of acids, bases and organic solvents. Finally, the membrane configuration should provide a high surface area to volume ratio to promote efficient mass transport and this can be accomplished through the use of hollow membrane fibres.

D’Elia (1985) and Dahuron (1987) found that only a few hydrophobic microporous hollow-fibre membranes were available. These included a polypropylene membrane made by Questar Corporation (Charlotte, NC) , a

54

Teflon membrane made by W.L. Gore & Associates (Elkton, MD) and a polypropylene membrane made by Membrana (Pleasonton, CA). Of these, the polypropylene fibres made by Questar were found to be the most suitable. They have a strong resistance to acids, organic solvents and surfactants and cleaning agents, and they also have geometric characteristics which encourage mass transport.

The Questar membranes specifically chosen for this study have the trade name Celgard. Two variations of this fibre were used: (1) uncoated and (2) coated. Two types of uncoated fibres were received directly from Questar called X-10 and X-20. These fibres differ only in their porosities as shown in Table 3 which lists some of their properties. Celgard fibres swell in the presence of hydrocarbons but the swelling is reversible and the membranes will return to their original size and shape after rinsing with alcohol and drying thoroughly. The fibres can be rendered hydrophilic by wetting with alcohol; this is not desirable, however, for the SOF process.

The coated fibres which were supplied by Applied Membrane Technology, Inc. (AMT) (Bloomington, MN), are Celgard fibres to which a fluorocarbon coating had been applied. The fibres were of the X-20 type with an outside diameter of 240 p. The coating, which is hydrophobic in nature, was applied to the outside of the fibres; more specific information about the coating could not be obtained as this is proprietary information. It was expected, however, that this coating would enhance the oil separation process.

(b) Module Design Three types of hollow-fibre modules were used; ( i ) Celgard countercurrent; (ii) AMT countercurrent and (iii) Celgard cross flow. In the countercurrent modules the oil-water mixture flows parallel to the direction of the fibres and in the cross flow modules the oil-water mixture flows perpendicular to the fibres. The AMT countercurrent module was constructed by AMT while both the countercurrent and cross flow Celgard modules were constructed in the lab using module designs developed by Yang (1987). .

55

Table 3: Properties of Celgard Hollow-Fibres X-10 and X-20

- . - -

~ - s i

56

(i) Celgard Countercurrent Modules Figure 9 shows a schematic diagram of a Celgard countercurrent hollow-

fibre module. The module looks like a small tube heat exchanger and it consists of a glass tube or shell with an inside diameter of 0.8 cm and an outside diameter of 1 .O cm. The length of the tube is about 20 cm.

Figure 9: Celgard Countercurrent Hollow-Fibre Module

:o::b, Fibres

I I Threaded Epoxy End

To prepare these modules a desirable number of hollow fibres are bundled together and threaded through the glass shell. The fibres are then glued in place at either end with a potting compound. The potting compound used was an epoxy (FE5045) by H.B. Fuller (St. Paul, MN). Dahuron (1987) and D'Elia (1 985) examined the properties of several different potting compounds and they found FE5045 to be the best. It adheres well to the fibres and the glass shell, it has excellent resistance to chemical attack and it has a low viscosity and long curing time which provides sufficient time for module asse m b ly .

The fibres were potted into a threaded Teflon mould so that upon removal of the mould end the epoxy caps were threaded. After the epoxy had hardened for several hours at room temperature the fibres protruding from the epoxy

57

were cut with a razor. A more detailed description of the module assembly technique is given by Dahuron (1987). For the SOF experiments one end of the module was closed off with epoxy to allow oil to flow in only one direction within the fibres.

Several different Celgard countercurrent modules were used in this study and they varied in the type of membrane and number of fibres used. The properties of each of these modules are given in Table 4 and they will be referred to subsequently by the names given in this table.

Table 4: Properties of the Celgard Countercurrent Modules

Module Membrane Type # of Fibres Membrane Area

Ce Igard- 1 x-20 60 0.14 ft2 400p 1.0. 130 cm2

Ce lg a rd-2 x-20 60 0.14 ft2 400~ 1.0. 130 cm2

c-#3 X-1 0 or X-20 120 0.28 ft2 400p I.D. 260 cm2

P-#l x-10 120 0.20 ft2 200 p 1.0. 186 cm2

P-#5 x-20 72 0.181 ft2 400 p 1.0. 168 cm2

The surface area available for transport was calculated based on the outside diameter of the fibres and it was assumed that all the fibres were contacting the oil-water mixture and that they were all available for transport (i.e. none were plugged).

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(ii) AMT Countercurrent Module A schematic diagram of the AMT module is shown in Figure 10. As can be

seen this design is not truly countercurrent as the fibres enter and exit from only one end. The fibres were looped to provide sufficient space for swelling when wetted with oil. The flow of the oil-water mixture is still along the length of the fibres, however. This module, which was constructed by AMT, has a PVC shell that is approximately 30 cm long. 1100 coated fibres were glued in place with a polyeurothane potting compound.and the surface area available for transport is 2 ft2 (1938 cm2). The module had been used previously by AMT, for about one year, to separate a lubricating oil from an oil-glycol mixture. The effect of t h i s on the nature of the fibre was not known.

Figure 10: AMT Countercurrent Module

Oil

Coated Hollow Fibres

\ Feed Outlet

t

Permeate . T

Feed Inlet

(iii) Celgard Cross Flow Module

of this module was made with plexiglass and it had the following dimensions: Figure 11 shows a schematic diagram of a cross flow module. The shell

59

length, L = 10.2 cm width, W = 1 .O cm height, H = 1.8 cm

The fibres were threaded parallel to the length of the module while the flow of oil-water mixture was perpendicular to the length.

Figure 1 1 : Celgard Cross Flow Module

/ \ I- Closed End

Oil Hollow Fibres

Glass Beads E

t Feed Inlet

Only one cross flow module was used in this study and the module contained 720, X-20 fibres having an inside diameter of 400 p. As with the countercurrent modules the fibre bundle was threaded through the module shell and potted at either end with FE5045 epoxy. Glass beads were placed in the inlet of the module as shown in Figure 11 to help disperse the oil-water mixture flow evenly along the length of the fibres. One end of the fibre bundle

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60

was closed off with epoxy. The surface area available for transport was calculated to be 1.1 5 ft2 (1 068 cm2).

(c) Module Cleaning Module cleaning was carried out occasionally between experiments. The

method of cleaning was as follows. The module was flushed with warm soapy water for 10-1 5 minutes under an applied pressure of about 5-1 0 psi; tap water and Dawn dishsoap were used. This removed oil from the system. The module was rinsed with warm tap water for 10-15 minutes to remove the detergent and then rinsed with distilled water for another 5-10 minutes. Isopropyl alcohol was flushed through the m o d u I e to remove any water in the membrane pores and finally the module was rinsed with pentane and dried thoroughly. After cleaning the membranes retained their original size and shape and their hydrophobic nature.

4.3 Experimental Apparatus and Method Figure 12 shows a schematic diagram of the experimental apparatus used

in this study. The oil-water reservoir was mixed continuously using a magnetic stirrer during stage one of the research. During stage two, the Procon pump was used to mix the resewoir intermittently. The pump was programmed to run on a 0.5 min on/ 2.5 min off cycle.

The oil-water mixture was pumped through the hollow-fibre module using one of two kinds of pumps. In stage one a positive displacement pump made by Fluid Metering, Inc. (Oyster Bay, NY) was used and in stage two a Master- Flex pump by Cole Parmer Instrument Company (Chicago, IL) was used. For the dodecane experiments special tubing that would not react with the dodecane was required. The tubing used, called Tygon Special was supplied by Cole Parmer Instrument Company (Chicago, IL). Regular Tygon tubing was employed for the naphthenic oil experiments.

61

In P

L

Figure 12: Experimental Apparatus

3 I

Valve

Hollow Fibre

Pout t

Reservoir + Oil Permeate

A needle valve was used to control the flow and pressure differential across the hollow-fibres. The pressure inside the fibres was assumed to be atmospheric. The pressure applied on the outside of the fibres was monitored at both the inlet and outlet of the module. Test gages with an accuracy of 0.25% were used. By measuring the inlet and outlet pressures an average applied pressure could be determined and the pressure drop across the module could also be measured. After the oil-water mixture passed through the module it was recycled back to the oil-water reservoir.

The naphthenic oil experiments were carried out at room temperature, which vaned between 23-32' C. The temperature of the reservoir increased due to heat inputs from the pumps. For the dodecane experiments, this problem was eliminated by using an aquarium glass rod heater with a temperature controller to keep the temperature constant.

The hollow-fibre module shown in Figure 12 is countercurrent in design. With both the countercurrent or cross flow module however, the oil-water mixture flowed outside the fibres. This flow configuration was chosen to avoid excessive pressure drops across the module and for ease of permeate

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62

collection. Prior to each experiment with a new module, the fibres were wetted with oil to facilitate oil transport. The permeate rate of oil passing through the fibres was measured using a 10 ml or 20 ml graduated cylinder and a digital stopwatch. Depending on the type of experiment being conducted the oil permeate was either removed continuously from the system or recycled back to the reservoir to maintain a constant oil concentration.

4.4 Zeta Potential Measurements Zeta potential measurements were made using a Laser Zee Meter, Model

501 made by Penkem Incorporated (Bedford Hills. NY). The method of measurement used with this instrument is microelectrophoresis as described in Section 2.1.

Zeta potential measurements were made as outlined in the instruction manual; these are described briefly here. The cell chamber was rinsed with distilled water and then filled with the oil-water sample using a syringe. All oil- water samples measured were diluted approximately 1000 times to allow for easier viewing of the droplets. The chamber was placed on the microscope stage and the microscope was focused on the top outer surface of the cell. Using the fine focus, the microscope objective lens was lowered until the stationary layer was reached. The distance from the top surface to the stationary layer was previously calculated using a technique outlined in the manual to be 1230 p. Once at the stationary level a voltage of 150 V was applied and the rotating prism was adjusted until the focused droplets appeared stationary. At this time an average zeta potential was calculated by the meter. The temperature correction factor and the specific conductance were measured. After calculating these values the cell chamber was removed and cleaned with a solution provided by Penkem Incorporated. The cell was then rinsed well with distilled water and stored with water in it.

63

4.5 Droplet Size Distribution Determination A Hiac/Royco Particle Counter, Model 4100A equipped with a model HR-

60HC sensor was used to determine the droplet size distribution of the oil- water mixture. This microprocessor based counter acquires count data on colloids dispersed in a liquid medium in six different particle size ranges which can be set independently. The overall particle size range that could be measured was 1 to 60 p.

The counter was operated in a constant volume mode. A volume sampler was used which issues stadstop counting signals to the counter at a lower and upper detector which are set by the operator in a metering tube. The volume of sample analyzed in these experiments was 30 mls.

The oil-water mixtures were diluted to an the appropriate concentration range prior to counting. This required dilutions of at least lo6 times and a dilution of 2 x lo6 was used. Dilution water consisted of distilled water containing 10 mg/l SDS filtered through a 0.2 p filter. Prior to each run baseline data was collected on the filtered water. These counts were then subtracted from the counts obtained for the samples prepared with the water. In this way the true count data for a sample was obtained.

The cell chamber was cleaned after running a sample, by flushing with filtered distilled water at a higher feed flowrate (40-50 m Wmin). Occasionally the sensor cell required more vigorous cleaning. This involved rinsing with warm soapy water, distilled water and then freon to remove any remaining oil.

64

5. EXPERIMENTS AND RESULTS

A description of the experiments conducted in this study and the results obtained are given below. The experimental work, as mentioned previously, was carried out in two stages: (1) naphthenic oil experiments and (2) dodecane experiments.

5.1 Naphthenic Oil Experiments

The naphthenic oil experiments were generally quite qualitative in nature, and they were aimed primarily at identifying the feasibility of the SOF process, and testing process performance under a wide variety of conditions. Three sets of experiments were conducted. Initially, work was begun with only the coated AMT fibres, and studies were made to determine whether or not oil could be separated from oil-water mixtures. These tests determined the effect of various operating parameters on oil separation in the absence of surfactants. In the second set of experiments, a comparative study was made between the coated AMT fibres and the uncoated Celgard fibres in both the presence and absence of surfactants. Finally, research was directed towards a detailed examination of the behavior of the uncoated Celgard fibres in the presence of surfactants.

(a) AMT Experiments The hollow-fibre module supplied by AMT was used for all of these

experiments. Preliminary experiments were carried out using pure naphthenic oil to obtain baseline data and to determine the ability of the fibres to pass oil in the absence of water or surfactants. The effect of oil viscosity was assessed qualitatively by varying the operating temperature. It was found that as the temperature increased the permeate rate also increased. The effects of pressure and feed flowrate were determined and these results are given in Table 5. It can be seen that while pressure exerted a near linear effect, feed flowrate exerted little if any effect. These results were expected and they agree

65

with Equation (3.5) which expresses t h e membrane flux as a function of pressure, oil viscosity, membrane 2ermeability and membrane thickness.

Table 5: AMT Module: 100% Oil; Baseline Data

01 (mllmin) p (Psi) FIUX (ml/min-ft2)

30 5 0.65 30 13 1.49 30 21 2.36 75 5 0.69 75 13 1.51 75 21 2.46 150 5 0.69 150 11 1.29 150 21 2.27 190 5 0.81 190 9 1.23 190 21 2.47

The removal of oil from oil-water mixtures of varying oil concentration was assessed under the following experimental conditions:

Oil Concentration: 1 %, 1 O%, 50% (w/w) Pressure: 5-25 psi Feed Flowrate: 30-330 mI/min Temperature: 32-38' C Surfactants: NONE

Oil was removed from the 1% and 10% oil-water mixtures over a relatively long time period of 92 and 50 hours, respectively, and from the 50% oil-water

66

mixture for a shorter time period of only 4 hours. For each of these experiments the permeate rate was measured as a function of pressure and feed flowrate. In all cases, the oil permeate was continuously recycled to the oil-water reservoir to maintain a constant oil concentration. The results of these experiments are summarized in Appendix I.

The effect of pressure is shown graphically in Figure 13. For oil concentrations between 1% and 100% the effect of pressure appears to be linear over the experimental range. This suggests that transport across the membrane, Jm, is limiting under these conditions (Eq. 3.5).

. Figure 13: AMT Module; Flux vs Pressure

n (Y

7

3 1 p5-I 100% Oil

1 4

7

2- / /

0 LLszzi 0 5 1 0 1 5 2 0 2 5

Pressure (psi)

Qf =300 mllmln

The e d c t of feed flowrate is shown graphically in Figure 14 for 1 %,10%,50% and 100% oil concentrations. Feed flowrate appears to exert very little effect on the permeate flux except at low oil concentrations. Figure 15 shows the significance of this effect for the 1 Yo oil-water mixture. These results

67

suggest that the overall transport of oil is not limited by transport to the membrane, J, for oil concentrations of 10% or greater but that it is limited by J for oil concentrations of 1% or less. Further experiments in the range of 1%- 10% oil would be required to determine at what oil concentration this shift in the transport limiting step occurs.

The effect of oil concentration on permeate flux is shown in Figure 16. It can be seen that the the fluxes obtained for the oil-water mixtures are well below that for pure oil. A plateau in the permeate flux is observed between 10% and 50% oil which implies that oil is being transported to the membrane faster than it is passing through the membrane in this oil concentration range. The plateau flux value is approached more rapidly at higher feed flowrates as the oil droplets are brought into better contact with the hollow-fibres. These results suggest that the overall transport of oil is membrane limited at higher oil concentrations and higher feed flowrates.

Figure 14: AMT Module; Flux vs Feed Flowrate

0 100 200 3 0 0 400

Q. (ml/min)

P=12 PSI

I '

68

0 . 0 0 -

Figure 15:

- ' * ' - ' . I - I 1 .

CY-

c U - X a i i

0.1 6

0.1 2

0.08

0.04

AMT Module; Flux vs Feed Flowrate for 1 % Oil-Water Mixture

P=12 PSI

Figure 16: AMT Module; Flux vs Oil Concentration

2

el-

0'

Flux Corresponding to 100% 011 c I - -a

. - , - I - , - , - , - , . , -

0 5 1 5 2 5 35 4 5 5 5

Oil Concentraion (%)

P=12 PSI

A 75 ml/mln

0 300 mi/min

69

Based on the above results, the removal of naphthenic oil from oil-water mixtures using the AMT module can be explained by the proposed mechanism of oil transport as follows. Oil transport is membrane limited (Jm) except at low oil concentrations and at low feed flowrates. This is not surprising as the naphthenic oil has a relatively high viscosity which hinders oil transport through the membrane pores. At low oil concentrations the overall transport of oil is limited by both transport to the membrane (J) and transport across the membrane (Jm); this is evidenced by the effect of feed flowrate and pressure on the permeate flux in the 1 oil-water mixture experiment. Little information regarding the importance of the attachment mechanism can be drawn from the above experiments. It is likely however, that droplet attachment and collapse is encouraged at high feed flowrates as the droplets are brought into better contact with the fibres.

The fact that the permeate flux for the oil-water mixtures was much lower than for pure oil indicates that oil dispersed in water is more difficult to remove than pure oil. This result was expected as there is obviously less probability of dispersed oil droplets contacting the fibres than pure oil. The probability of contact should also diminish as the concentration of oil droplets is reduced and this was observed as the permeate flux decreased with decreasing oil concentration below 10% oil as shown in Figures 14 and 16. Another contributing factor to the reduced rate of transport for the oil-water mixtures may be that the oil droplets must first attach to the fibre surface and collapse before they can pass through the membrane. In this case the attachment mechanism clearly becomes important.

One other possible explanation for the difference between the permeate flux for pure oil and the oil-water mixtures is that water molecules may adhere to the membrane surface and blind the membrane pores making it more difficult for oil to pass through the membrane. Murkes (1986), in his study of water removal from water-in-oil emulsions using microporous filters also observed water blinding. At no time in the experiments, however, did water pass through the fibres.

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Evidence supporting the water blinding explanation was found in the 1% oil-water mixture experiment. After running the oil-water mixture for several days the module was cleaned with isopropyl alcohol and a pentane-water mixture and then replaced by pure oil. In spite of the extensive cleaning procedure the pure oil became cloudy as it passed through the module, suggesting that water may have been sheared off the membrane surface or removed from inside the membrane pores.

An unexpected result in these experiments was a decline in permeate flux with time. This was observed in all of the experiments, and in some cases this decline was quite marked. In the 1% oil-water experiment, for example, the permeate flux dropped by an order of magnitude over a four day period. This flux decline implies that steady state was not reached, and this may have been due to the build up of water in the membrane pores or at the membrane surface. Murkes (1 986) also observed a decline in oil permeate flux which he attributed to water binding. To reach steady state more quickly he suggested using higher feed flowrates.

This variation in permeate flux with time made the analysis of the data more difficult. It was found that over a short time period consistent results could be obtained and the trends that are shown in Figures 14 and 15 were observed. Over longer time periods, however, the flux values changed quite markedly, and this must be taken into consideration when interpreting these results. To compound this effect, the AMT module was not cleaned or "regenerated" between experiments for fear of altering the fluorocarbon coating. The condition of the module therefore, may have changed with time, which in turn could affect the membrane's ability to pass oil.

Following these experiments, the AMT module was "regenerated" with isopropyl alcohol and an oil-pentane mixture, which was run through the fibres for several hours. The pentane was added to reduce the oil viscosity and facilitate oil transport. The oil-pentane mixture was replaced by pure oil and the pure oil flux was measured as a function of time. As mentioned previously, water appeared in the permeate initially. Shown in Figure 17 is the oil flux for the water-free oil permeate. It can be seen that several hours were required to

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71

"regenerate" the AMT module; a permeate flux similar to that previously observed however, was eventually obtained.

It was decided that a comparative study should be made between the coated and uncoated fibres to determine whether or not the coating significantly improved the ability of the membrane to pass oil.

Figure 17: AMT Module Regeneration; Pure Oil Flux vs Time

1.6

1.4

1.2

1 .o

P=13 PSI

Q =140 ml/mln f

0.8 0 2 0 4 0 6 0 8 0

Time (hr)

(b) AMT vs Celgard Experiments

The performance of the coated AMT fibres was compared with that of the uncoated Celgard fibres. Preliminary experiments were carried out on the Celgard 1 module to compare its behavior with that of the AMT module. Oil- water mixtures containing 1%, 10% and 100% oil by weight were tested, and

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72

the results are summarized in Section 1 of Appendix II. As with the AMT module, feed flowrate exerted little or no effect on the permeate flux for the 10% and 100% oil-water mixtures and pressure exerted a near linear effect on the permeate flux for the 10% oil concentration. A decline in permeate flux with time also was observed with the Celgard 1 module, and this decline was significant for the 1 O/O oil-water mixture.

Before comparative studies were conducted, both the Celgard 1 and AMT modules were "regenerated" with pure oil. The permeate flux for pure oil was measured as a function of pressure and these results are shown graphically in Figure 18.

Figure 18: AMT vs Celgard 1; Flux vs Pressure For Pure Oil

X 3 c

6 -

5 -

4 -

3 -

2 '

0 I I i

0 1 0 2 0

Pressure (psi)

Q =200 ml/min

The uncoated Celgard fibres yielded higher permeate fluxes at all pressures. This could be due to differences in membrane porosity or a reduction in membrane permeability in the AMT module caused by the

73

fluorocarbon coating. (The coating may have reduced the pore size and thus the overall porosity of the membrane.) The plot for the Celgard module suggests some curvature at higher pressures. This type of behavior would not be unexpected as membrane compaction could occur at these pressures. Theoretically these plots should pass through the origin as pressure is the driving force for oil transport through the membrane.

Oil-water mixtures containing 1% and 10% oil, and no surfactants were run on the two modules in parallel and permeate rates were collected over a 25 hour and 49 hour period, respectively. The following operating conditions were used:

Pressure: 12-13 psi Feed Flowrate; 200 ml/min Temperature: 32-38' C Surfactants: NONE

The temperature varied due to heat inputs from the mixing devices: both experiments were conducted over similar temperature ranges, however.

The data obtained are given in Section 2 of Appendix II. Between runs the modules were "regenerated" with an oil-pentane mixture and then pure oil. The pure oil fluxes were measured and the flux values recorded at time zero in Appendix II correspond to those immediately after pure oil regeneration. It can be seen that the pure oil flux dropped slightly between runs.

Figure 19 shows a plot of permeate flux versus time for the 1% oil-water mixture and Figure 20 for the 10% oil-water mixture. The uncoated fibres yielded higher permeate fluxes than the coated fibres for both these oil concentrations over the time period tested. Again, this could have been due to differences in membrane porosity. It appears from Figure 19, that the uncoated fibres took somewhat longer to reach a steady state flux than the coated AMT fibres. The Celgard 1 module and the AMT module both showed an initial drop

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in flux over the first 10 hours of operation. However, the flux in the Celgard 1 module declined dramatically between 10 and 24 hours. This flux decline may have been due to water blinding. Following this result it was decided that the 10% oil-water mixture should be tested over a longer period of time.

The results shown in Figure 20 indicate a very similar decline in flux for the two modules. In this case, the dramatic decline in flux in the Celgard 1 module after 10 hours was not observed. Interestingly, however, the absolute drop in the flux measured in the Celgard module was greater than that observed in the AMT module; the percent drop in flux was 49% and 74% for the Celgard 1 and AMT modules, respectively.

These comparative studies demonstrated that Celgard fibres are capable of selectively removing oil from oil-water mixtures without the application of a fluorocarbon coating. Further, the uncoated fibres were found to be more efficient at removing oil from surfactant-free oil-water mixtures.

At this point a decision was made to continue experimental work with only the uncoated Celgard fibres. The reasons for this decision were as follows: (1) the exact nature of the AMT fluorocarbon coating was proprietary ; (2) the AMT module had been used previously and was received in "unknown" condition; (3) the coated fibre made by AMT was over a year old and changes in the surface character of the coating may have occurred; (4) AMT could not provide sufficient technical support to continue working with the AMT module, and they had no intention of making new coated fibre in the near future; and (5) the uncoated Celgard fibres were capable of selectively removing oil from oil-water mixtures.

Subsequent experiments with the uncoated Celgard fibres were aimed at characterizing their behavior in the presence of different surfactants.

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Figure 19: AMT vs Celgard 1; Flux vs Time for a 1% Oil-Water Mixture

I I

0 1 0 2 0 30

Figure 20:

Time (hr)

AMT vs Celgard 1; Flux vs Time for a 10% Oil-Water Mixture

3 l?

n . I I I I I

u - 0 10 20 30 4 0

Time (hr)

5 0

c

76

(c) Celgard Experiments Preliminary experiments were carried out to determine the effect of

emulsifying agents on the permeate flux for the Celgard fibres. An experiment was conducted in which a 10% oil-water mixture containing 1% Pet Mix#9 by weight of oil was tested on the Celgard 2 module. The Celgard 2 module was used for these experiments as the Celgard 1 module was damaged. The results of this experiment are shown graphically in Figure 21.

Figure 21: Celgard 2 Module; Flux vs Time for a 10% Oil-Water Mixture Containing 1% Pet Mix#9

4.0

3.0

2.0

Water in Permeate

3

"' ,l 1.0 -

0.0 0 1 2

Time (hr)

The permeate flux declined rapidly and water appeared in the permeate of the Celgard 2 module after only 2 1/2 hours. The experiment on this module was terminated at this time. The module was cleaned with detergent, isopropyl alcohol and pentane, and then dried thoroughly. After cleaning, pure oil fluxes comparable to those previously observed were obtained. This demonstrated that the uncoated fibres can be regenerated after the use of a surfactant.

77

The effect of surfactants was further tested by running a 10% oil-water mixture containing 1% ABS by weight of oil on the Celgard 2 module. Water appeared in the permeate stream after only about 5 minutes, and the experiment was terminated shortly thereafter.

It was clear that water could not be prevented from passing through the uncoated membrane in the presence of a surfactant. It was observed, however, that the permeate separated quite readily into two distinct phases, oil and water, respectively. Although this behavior is not ideal, it is favorable since the resulting permeate could easily be treated using gravity separation techniques to recover the oil from the water. Additional Celgard experiments were conducted therefore, to determine whether or not this behavior is observed over longer periods of time and in the presence of different surfactants.

Tests were initiated with the Celgard 2 module. The module was cleaned and dried as described above and pure oil was run through the module to check the pure oil flux. A value similar to that achieved previously was obtained (4.2 ml/min-ft2 at 12 psi). A 10% oil-water mixture containing 1 YO ABS by weight of oil was then prepared and run on this module.

This emulsion was tested over a 70 hour period at a feed flowrate of 240 mumin and a pressure of 12 psi. Permeate samples were collected and the permeate rate was measured at different time intervals. In determining the permeate rates, permeate samples were collected for roughly 5 minutes. Oil was removed continuously from the system to determine the permeate flux as a function of oil concentration and to determine to what practical oil concentration the oil-water mixture could be reduced. The data collected from this experiment is summarized in Section 1 of Appendix 111.

Water passed through the membrane almost immediately after beginning the experiment. At this time the permeate was water rich and it separated readily into two phases. As time passed however, the permeate became depleted in water and enriched in oil until a cloudy oil permeate was obtained from which water would not separate. Finally, after 3 or 4 hours, a pure oil

permeate was obtained and it continued for the remainder of this experiment. No water passed through the membrane after about 3 hours.

Figure 22 shows a plot of flux versus time for this experiment. The flux at time zero corresponds to that immediately after pure oil was run and so the sharp initial drop in flux was anticipated. Thereafter, the flux increased slightly due to a rise in temperature from 32Oto 38' C, but remained relatively constant for about 18 hours (from 4 to 22 hours). During this time the oil concentration dropped from about 10.4% to 2.8% by volume. This is shown in Figure 23 which presents a plot of permeate flux versus the residual oil concentration. (The Yo oil remaining was determined using a mass balance rather than by direct oil analysis. The volume of oil collected was subtracted from that initially

* present and the oil remaining was calculated.) At oil concentrations less than about 2% the permeate flux decreased dramatically. After about 40 hours an oil concentration of less than 1% remained and at this point the permeate flux was extremely low (< 0.01 ml/min-ft2). This suggests that there is a lower practical limit beyond which SOF is not feasible for oil removal.

After running this experiment for about 20 hours it was observed that the permeate oil had become darker in color, and it continued to do so for the remainder of the experiment. After 40 hours the permeate was very dark brown, which implies that ABS, which is also dark colored, was passing through the fibres. No specific tests were made at the time to verify this.

After 70 hours the reversibility of the separation process was tested by adding fresh oil back to the emulsion to increase the oil concentration to that initially used (1 0% by weight or 11.2% by volume). Following this, a permeate flux similar to that previously measured was obtained. This is shown in Figure 22. The flux rates are therefore concentration dependent and reversible.

79

Figure 22: Celgard 2 Module; Flux vs Time for a 10% Oil-Water Mixture Containing 1% ABS

2.5 '"1 More Oil Added

Q f =240 ml/mln

P=12 PSI

0 2 0 4 0 6 0 8 0

Time (hr)

Figure 23: Celgard 2 Module; Flux vs % Oil Remaining for a 10% Oil-Water Mixture Containing 1% ABS

n

w t 'E Y z

0 2 4 6 8 1 0 1 2

Yo Oil Remaining

Q, =240 ml/mln

P+12 psi

The above experiment was repeated on the C-#3 module to check the reproducibility of the results. This module had been made by a previous researcher. The results of this reproducibility study are given in Section 2 of Appendix I l l .

The same general trends were observed in this experiment in that water passed through the fibres initially, and after a few hours only oil passed through. However, lower permeate fluxes were obtained and this may have been due to differences in membrane porosity. (It was not known for sure if the fibres were of the X-10 or X-20 type). The results of the two experiments are shown graphically in Figures 24 and 25, which give permeate flux as a function of time and O h oil remaining, respectively. Although the absolute fluxes are somewhat different the system response was similar.

Figure 24: Celgard 2 and C-#3 Modules; Flux vs Time for a 10% Oil-Water Mixture Containing 1% ABS

3.5

3.0

2.5

2.0

1.5

1 .o

0.5

0.0

- Celgard 2 Q =200 ml/min

P=12 PSI f

0 1 0 2 0 3 0 4 0 5 0 Time (hr)

81

Figure 25: Celgard 2 and C-#3 Modules; Flux vs % Oil Remaining for a 10% Oil-Water Mixture Containing 1% ABS

21 I Q =200 mllmln

f P=12 PSI

- 0 2 4 6 8 1 0 1 2 70 Oil Remaining

The permeate flux for the C-#3 module remained relatively constant for about 22 hours (4 to 26 hours). Beyond this the permeate flux declined. This behavior was different from that obsewed for the Celgard 2 module in that the flux decline began at a higher oil concentration (5.3%) and it occurred more gradually. As a result a much longer time was required to obtain an oil concentration of 2.0% (46 hours as opposed to 25 hours). Again, the SOF process appears to be limited at low oil concentrations.

The behavior of the Celgard membrane appeared to be reproducible for the ABS naphthenic oil experiments. In both cases water permeation occurred initially followed by a reversal to oil permeation only. The reasons for this behavior are unclear. The system is complicated with a complex engine oil and a commercial surfactant blend of alky benzene sulfonates. The exact

-_

mechanisms involved in the membrane separation processes are difficult to identify.

Murkes (1986) reported a similar behavior. He found that upon passing an oil-in-water emulsion through a microporous filter that both oil and water appeared in the permeate and that two distinct phases were formed. In his case, oil droplets finer than the membrane pores passed through the membrane and in so doing they were coalesced to form large, easily separable droplets. This phenomena may have occurred initially in these experiments, although Murkes' observation was made for a surfactant-free emulsion.

Another explanation for the initial passage of water is that a local inversion of emulsion type at or near the membrane surface may have occurred. If this were the case, dispersed water droplets surrounded by surfactant would appear hydrophobic to the membrane and thus pass through the membrane pores. In so doing, they may also have coalesced. Murkes (1986) did not observe water coalescence within the membrane pores in his experiments, however. The subsequent change in membrane character which we observed after 3 hours is still difficult to explain.

.

In an attempt to develop a better understanding of this separation process, three sets of experiments were conducted to evaluate the influence of emulsion stability. The stability of the ABS oil-water emulsion was modified by: (i) pH adjustment; (ii) increased ABS concentration and (iii) salt addition.

(i) pH Adjustment A 10% oil-water mixture containing 1% ABS by weight of oil was tested on

the Celgard 2 module. The pH of the emulsion was increased from 2.4 to 10.1 with the addition of approximately 4 x M NaOH. Since ABS is an anionic surfactant increasing the pH above the pK value, which is approximately 2, deprotonates more of the hydrophilic functional groups of the surfactant molecules making it a more effective emulsifying agent. The results of this experiments are given in Section 3 of Appendix 111.

.

83

Figure 26 presents a plot of permeate flux versus time for both the high and low pH experiments. The permeate flux for the low pH experiment is greater than that for the high pH experiment over the entire time period tested (except where additional oil was added to check reversibility). This implies that oil removal is adversely affected as emulsion stability increases and that the coalescence and attachment mechanism is important in the overall transport process. Figure 27, which is a plot of permeate flux versus Ol0 oil remaining, also shows higher fluxes for the low pH emulsion except at a few of the higher oil concentrations. A plateau in the flux was not observed for the oil-water mixture at high pH and oil removal efficiency dropped more rapidly with time. The permeate was free of water, however.

Figure 26: Celgard 2 Module; Flux vs Time for a 10% Oil-Water Mixture Containing 1% ABS at High and Low pH

Q ~ 2 0 0 ml/min P=12 PSI

f

0 1 0 2 0 3 0 4 0 5 0

Time (hr)

84

Figure 27: Flux vs O h Oil Remaining for a 10% Oil -Water Mixture Containing 1% ABS at High pH and Low pH

3.0 1 Ir Qf=200 mI/min

P=12 PSI

0 2 4 6 a 1 0 1 2

% Oil Remaining

(ii) Increased ABS Concentration

The emulsion stability of the 10% oil-water ABS emulsion was altered by increasing the surfactant concentration to 10% by weight of oil. A new emulsion was prepared, and the Celgard 2 module was regenerated with pure oil prior to this experiment. The oil permeate flux dropped rapidly to an extremely low value and continued to decrease with time. Water, however, did not pass through the membrane at any time. These results imply that the SOF process is limited by the concentration of surfactant present. The mechanism by which the surfactant reduces the permeate flux could be due to either an increase in emulsion stability or a coating of the fibre surface which prevents the passage of oil, or both.

85

( i i i ) Salt Addition A final experiment on emulsion stability was carried out by adding 0.1 M

KCI to the 10% ABS emulsion. The addition of salt should result in a compression of the electrical double layer surrounding the oil droplets and therefore cause a reduction in emulsion stability. It was anticipated that this would lead to an increase in permeate flux as observed for the low pH tests. The results were unexpected. After the addition of KCI the membrane immediately began to pass water and retain oil. In other words, the membrane became an ultrafilter. This behavior continued for a few hours after which time the test was terminated.

A hypothesis for this behavior is that the salt addition may have caused an apparent depression in the pK of the sulfonic groups attached to the membrane surface (Helfferich, 1962). At an ABS concentration of 10% one would expect a large amount of surfactant to be present at the membrane surface and oriented in such a way that the hydrophilic heads of the surfactant (RSO3-H+) extend out from the surface. The K+ ions could displace some of the H+ ions and cause a shift in the dissociation equilibrium of the sulfonic acid groups which would effectively lower the pK and result in an increase in the amount of unprotonated RSO5 functional groups. Thus, the fibres would acquire a more negative charge which would not only repel the negatively charged dispersed oil droplets but the RSOf ions would attract water molecules and become hydrated. In so doing, the surface would appear strongly hydrophilic and the passage of water would be encouraged.

The separation of oil from oil-in water emulsions appears to be strongly influenced by the presence of emulsifying agents and the stability of the oil droplets. The experiments with increased pH and with increased ABS concentrations clearly demonstrate this effect. The influence of KCI raises additional questions about the mechanism, however.

86

Nonionic Surfactant Effect A final test was conducted on a nonionic surfactant, Triton X-102.

Communications with R. Callahan (Questar) suggested that this surfactant has a high affinity for polypropylene membranes and as such it was anticipated that the Celgard fibres would be ineffective in removing oil from emulsions stabilized with this surfactant .

An experiment similar to those described above was carried out on a 10% oil-water mixture containing 1% Triton X-102 by weight of oil to determine the effect of a nonionic surfactant. The P-#l Module, which contained X-10 fibres with a 200 p diameter, was used for this test. The experiment was run for 29 hours during which permeate rates were measured. The results are shown in Section 4 of Appendix 111.

The results of the Triton X-102 experiment differed from those of the ABS experiments in that water did not pass through the membrane initially and a pure oil permeate was collected throughout the experiment. The permeate flux was also found to be lower than that for the ABS experiments over the entire time tested. This effect which appears to be due to the presence of a nonionic surfactant may also be due to the differences in the membrane types used in the modules. The results of this experiment are shown graphically in Figures 28 and 29 which give plots of the permeate flux versus time and Yo oil remaining.

These plots show a continuous decrease in permeate flux with time and o/o oil remaining for the emulsion stabilized with Triton X-102 rather than a plateau region as observed in Figures 24 and 25 for the ABS emulsions. The oil flux was not measured however, between 6 and 20 hours for the Triton X-102 experiment, and so the flux may have remained relatively constant for some time and not been observed. Nevertheless, the flux fell dramatically below an oil concentration of about 2%, which suggests that the process is limited for nonionic surfactants as well. When the concentration of oil in the emulsion was increased it was observed that the flux of oil rebounded to its initial values as shown in Figure 28. Thus the process appears to be reproducible and reversible.

a7

Figure 28:

n el F E E C .- \

W

x a i i

Figure 29:

P-#l Module; Flux vs Time for a 10% Oil-Water Mixture Containing 1% Triton X-102

More Oil Added P=12 psi

5 1 0 1 5 2 0 2 5 3 0 3 5 0

Time (hr)

P-#l Module; Flux vs YO Oil Remaining for a 10% Oil-Water Mixture Containing 1% Triton X-102

1.00 1

o.*o/

0.60

0.40 -

0.20 -

- f

0 2 4 6 a 1 0 1 2

% Oil Remaining

P=12 psi

Qf=200 ml/mln

(d) Additional Observations An interesting observation made during all of the Celgard experiments

was that the permeate flux tended to increase whenever the Procon pump was used in conjunction with the magnetic stirrer. The pump, which was switched on occasionally to provide additional mixing, resulted in the formation of a significant amount of air bubbles in the oil-water mixture. The air bubbles may have enhanced oil transport by increasing the opportunity for contact between the dispersed oil droplets and the membrane surface by providing more turbulent flow conditions. Also, the air bubbles may have altered the contact angle between the oil droplets and the fibres in such a way that droplet attachment and coalescence was encouraged.

The Celgard experiments described above demonstrated that oil can be selectively removed from chemically emulsified oil-water mixtures using SOF. The efficiency of the separation however, appears to be dependent on the type and concentration of emulsifying agent and perhaps on emulsion stability. The SOF process also seems to be limited at low oil concentrations.

These studies raised many basic questions regarding the mechanism of oil separation and the role of emulsion stability. Unfortunately, the practical systems studied above were too complicated to extract answers. The base engine oil was a blend of hydrocarbons and the surfactants were also mixtures of products. In an attempt to simplify the system additional tests were conducted with pure model compounds.

89

5.2 Dodecane Experiments The dodecane experiments were carried out to assess the SOF process

under more carefully controlled conditions. Purer compounds were employed in order to minimize experimental uncertainties. Dodecane was used as a model oil and sodium dodecyl sulfate (SDS) was employed as a surfactant.

The experiments on dodecane-water mixtures stabilized with SDS were intended initially to duplicate some of the preliminary studies with naphthenic oil: the influence of operating pressure, feed flowrate, dodecane concentration and system design were tested. The permeate flux was measured as a function of these operating parameters, and an attempt was made to characterize the mechanism of oil transport under different conditions and to determine some of the controlling factors in the SOF process. Following these studies a more careful examination of emulsion stability and its influence on dodecane separation was conducted.

(a) Preliminary Experiments Preliminary experiments were carried out to verify that dodecane could be

emulsified using SDS as an emulsifying agent and to obtain some baseline data on the removal of pure dodecane using the SOF process. SDS was found to emulsify dodecane very easily in distilled water. A 5% dodecane- water mixture was made and tested on the Celgard 2 module which had been saturated with dodecane. Only dodecane passed through the fibres under an applied pressure of 10 psi and a feed flowrate of 200 ml/min. It was concluded that dodecane-water mixtures stabilized by SDS were suitable for use in these experiments.

The fibres of the P-#5 module were wetted with pure dodecane, and the dodecane flux was measured. Fluxes of approximately 58 ml/min-ft2 and 120 ml/min-ft2 were obtained at operating pressures of 5 and 10 psi, respectively. Though the data are limited, it appears that pressure exerted a linear effect on the flux for pure dodecane, as predicted by equation (3.5) which describes membrane transport. The flux values for dodecane were considerably higher,

90

however, than those obtained for pure naphthenic oil on the Celgard fibres, which were about 2.10 ml/min-ft2 and 4.30 mI/min-ft2 at 6 and 13 psi, respectively. This is due most probably to the large viscosity difference between the two oils.

As shown in equation (3.5), the rate of oil transport through a membrane

may be defined as, Jm-- . For a given membrane one might assume that the

membrane permeability, K, and the membrane thickness, 6, remain constant

under different operating conditions, and thus one would expect J m a-. If the

same operating pressure was used for two different oils with varying viscosities, one should find that (Jm*p)l = (Jm*p)2, where 1 and 2 denote oils 1

and 2, respectively. To determine whether or not this relationship holds true in the SOF process, a comparison of the fluxes and viscosities for the pure naphthenic oil and the pure dodecane was made.

KAP

P6

A? CI

The comparison was made on the basis of pure oil flux measurements taken at room temperature, which was assumed to be about 25OC. The viscosity of dodecane at this temperature is 1.35 CP (CRC Handbook of Chemistry and Physics, 1982-1983). The viscosity for the naphthenic oil was measured to be 52.6 CP at 2OoC (by AMT), and thus an extrapolation had to be made to 25OC. Based on viscosity data given in the CRC Handbook for a light machine oil, the naphthenic oil viscosity was estimated to be 40 cP. Pure oil fluxes at an operating pressure of 5 psi were used in this calculation. The flux values for the naphthenic oil were extrapolated to 5 psi assuming a linear flux- pressure relationship for pure oil. A value of 1.8 ml/min-ft2 was used for the naphthenic oil and a value of 58 ml/min-ft* was used for dodecane..

It was found that (Jm*p)~=72 and (Jm*p)o=78 under the conditions described above, for the naphthenic oil and dodecane, respectively . These values are quite similar which suggests that the properties of the membrane did not change significantly for the two different oil. One might have anticipated a change in membrane permeability in the presence of different hydrocarbons

91

due to differences in solvent-membrane interactions. This does not appear to be the case here. however.

Following these preliminary tests, four suites of experiments, which are described in turn below, were conducted under the following operating conditions:

Oil Concentration = 5%, 10% dodecane (w/w) Pressure = 5-20 psi Feed Flowrate = 100-2400 ml/min Temperature = 36-38' C SDS Concentration = 0-500 mg/l Module Geometry = Countercurrent and Cross Flow

A constant temperature was maintained to prevent changes in oil viscosity, which could cause inconsistencies in the experimental results. Two different module geometries were tested to compare the influence of hydraulic design on the process performance; it was anticipated that the cross flow module would yield higher permeate fluxes as it provides better contact between the oil droplets and the fibres.

For consistency in measuring the permeate rates the following procedure was adopted. After changing either the operating pressure or feed flowrate, 15 minutes was allowed for equilibration. The permeate rate was then measured several times over a half hour period. In any given measurement roughly 10 ml of dodecane were collected. The average of the permeate rate values obtained over this time was used. After changing the SDS concentration, several hours were allowed for equilibration; usually the system was left running overnight.

(b) Countercurrent Module; 5% Dodecane Experiments Initially a 5% dodecane-water mixture was tested on the countercurrent

module P-#5. SDS concentrations of 50, 100 and 500 mg/l were tested. At

92

each of these surfactant concentrations the effect of pressure and feed flowrate on permeate flux was determined over the ranges of 5-20 psi and 100-750 ml/min, respectively. The results of these experiments are given in Appendix IV.

Figures 30 and 31 show the effect of pressure and feed flowrate respectively, on permeate flux. The flux did not increase linearly with pressure as previously demonstrated in the naphthenic oil-water experiments. Unexpectedly, the permeate flux tended to decrease with increasing pressure. One possible explanation for this is that membrane compaction occurred at the higher pressures; this would tend to reduce the membrane porosity. The permeate flux appears to have increased linearly with feed flowrate, however, over the experimental range tested.

Figure 30: Countercurrent Module; 5% Dodecane Pressure Effect

4.0

3.5

3.0

2.5

2.0

1.5

1 .o 0.5

0.0

- Qf =300

T=37 C 0

mllmin

0 1 0 2 0

Pressure (psi)

93

Figure 31 : Countercurrent Module; 5% Dodecane Feed Flowrate Effect

8 -

6 '

4 -

2 -

10 I

50 mg/l SDS o 100 mg/i SDS 4500 mg/l SDS

0 200 400 600 80

P=10 psl 0

T=37 C

IO

Flowrate (ml/min)

These results imply that the SOF process is not membrane limited for the transport of dodecane from a 5% dodecane-water mixture and that dodecane passes through the membrane pores faster than it can be supplied to the membrane surface. This is not surprising since dodecane has a low viscosity, which facilitates membrane transport. Increasing the feed flowrate not only increases the rate of oil transport to the membrane but it may also enhance the attachment and coalescence step as the dodecane droplets are brought into more intimate contact with the fibres.

It would be of interest to study the effect of feed flowrate over a broader range to determine at what flowrate the permeate flux stops increasing. One would expect the permeate flux to level out at some point which corresponds to a shift in the rate controlling process from transport to the membrane surface to

94

transport through the membrane. Beyond this flowrate the permeate flux should become dependent on pressure, oil viscosity and membrane characteristics.

figures 30 and 31 both show a decrease in permeate flux with increasing SDS concentration. The exact cause for this effect was not clear and it may be due either to an increase in emulsion stability which is accompanied by reduced coalescence and attachment, or to a build up of surfactant at the membrane surface which presents a mechanical and/or electrical barrier to membrane transport. If the membrane were to become coated with SDS, it would acquire a negative charge and thus have a tendency to repel the similarly charged oil droplets.

The linear regressions shown in Figure 31 for the flux versus feed flowrate data show that the plots do not pass through the origin. The positive intercept on the absicissa indicates that there is some critical feed flowrate below which oil cannot be transported across the membrane. This suggests that a minimum amount of kinetic energy is required for the dodecane droplets to collide with, coalesce and attach to the fibres. One would expect the value of this critical flowrate to vary with dodecane concentration and SDS concentration. At higher oil concentrations the critical flowrate should decrease as there are more dodecane droplets available for transport to the fibre surface and thus a higher probability of contact. At higher SDS concentrations the critical flowrate should increase as more kinetic energy is required to force the stable oil droplets into intimate contact with the fibres. The plots in Figure 31 show no increase in the critical flowrate between 50 mg/l and 100 mg/l SDS and only a slight increase at 500 mg/l SDS.

Permeate samples were taken at different SDS concentrations and qualitatively tested for the presence of surfactant. This was done by adding water to the samples, hand shaking them and observing whether or not the dodecane readily re-emulsified. SDS was found to be present in all the permeate samples. In practice, this may or may not be desirable depending upon the intended use of the recovered oil.

95

(c) Cross Flow Module; 10% Dodecane Experiments In the second suite of experiments a 10% dodecane-water mixture was

tested on a cross flow module. This module, as described previously, had a surface area of 1.1 5 ft2. SDS concentrations of 0, 50, 100 and 200 mg/l were tested and the effects of pressure and feed flowrate on the permeate flux were determined over the ranges of 5-20 psi and 100-750 mlhin, respectively. The results of these experiments are given in Appendix V.

Before running these experiments the module was wetted with dodecane and the permeate flux for pure dodecane was measured. The permeate flux was found to be similar to that obtained for the countercurrent module (60 ml/min-ft2 at 5 psi). This was expected as system design should have no effect on membrane transport, Jm.

Figures 32 and 33 present the effects of pressure and feed flowrate, respectively. The same general trends were observed for the 10% dodecane- water mixture on the cross flow module as for the 5% dodecane-water mixture on the countercurrent module. Increasing pressure caused a decrease in permeate flux and increasing feed flowrate caused an increase in permeate flux. These results indicate that the SOF process is not membrane limited even at a dodecane concentration of 10% and suggests that dodecane is not reaching the membrane surface as fast as it can be transported through the membrane. The permeate flux decreased with increasing SDS concentration and the decline was not linear. The first addition of SDS caused a dramatic drop in permeate flux whereas subsequent additions of SDS had a less marked effect.

96

Figure 32: Cross Flow Module; 10% Dodecane Pressure Effect

3 w

LL 2 1 1

8 1 0 1 2 1 4 1 6 1 8 2 0 O ! . ' . ' . ' . 1 - 1 . I .

Pressure (psi)

Qf=300 mllmln T=37 0 C

Figure 33: Cross Flow Module; 10% Dodecane Feed Flowrate Effect

3 1 . - I I No SDS I A I

50 mg/l SDS a 100 mg/l SDS

2 2

100 mg/l SDS

0 200 400 600 800

Flowrate (ml/min)

P=10 psi 0

T=37 C

97

As shown in Figure 33 the increase in permeate flux with feed flowrate was linear except in the absence of SDS. In this case, the permeate flux declined more gradually at higher feed flowrates. At a dodecane concentration of 10% the opportunity for contact between dodecane droplets and the fibres is improved by the presence of more dodecane droplets. The transport of oil to the membrane therefore, is not as strongly enhanced at high feed flowrates compared to a 5% dodecane-water mixture which contains fewer dodecane droplets, At a permeate flux of about 2-3 ml/min-ft* one could expect a shift in the transport limiting step for the No SDS case, to membrane control; this should occur at some feed flowrate beyond 750 ml/min.

As with the 5% dodecane experiment on the countercurrent module, there appears to be a critical feed flowrate below which oil transport cannot occur. (The plots in Figure 33 do not pass through the origin.) Figure 33 shows a slight increase in the critical flowrate with increasing SDS concentration and this may be compared to Figure 31, which shows a similar trend. A comparison of Figures 31 and 33 'also indicates a slight decrease in the critical feed flowrate with increasing dodecane concentration. Different module geometries were used however, which would affect the hydraulics of the system and thus affect the efficiency of oil transport to the membrane surface.

A direct comparison of the data for the two module geometries indicated that the countercurrent design was more effective at removing dodecane from dodecane-water mixtures. For the same pressures and feed flowrates the countercurrent module yielded higher permeate fluxes, even though the oil- water mixtures contained only half as much oil as those run on the cross flow module. This type of comparison is misleading, however, as the Reynolds numbers corresponding to a given feed flowrate are very different for the two modules. The Reynolds number may be defined as follows (Hughes and Foulds, 1986):

VSPdf Re =- P

98

where, vs = superficial velocity p = oil-water mixture density df = fibre diameter p = oil-water mixture viscosity

The cross sectional area of the countercurrent module was much smaller than that of the cross flow module (0.785 cm2 versus 17.85 cm'). As a result the superficial velocities, and hence the Reynolds numbers, were much greater for the countercurrent module for a given feed flowrate. This is the reason for the higher permeate fluxes observed for the countercurrent module. It was not possible to compare permeate fluxes of the two modules at similar Reynolds numbers because this would require the use of feed flowrates greater than 3 Vmin for the cross flow module; these flowrates could not be achieved without producing excessive head losses across the module (> 10 psi). No further attempts were made therefore, to characterize the effect of module geometry and all subsequent experiments were carried out on the cross flow module.

To determine the influence of oil concentration on permeate flux the 10% dodecane-water mixture containing 200 mg/l SDS was diluted with distilled water to half its original dodecane concentration. The SDS concentration of the 5% dodecane-water mixture was 100 mg/l as the SDS concentration was also reduced upon dilution. The permeate flux was measured as a function of feed flowrate and the results were compared to those obtained for the 10% dodecane-water mixture. This is shown graphically in Figure 34.

Figure 34 shows that the permeate flux was lower for the 5% dodecane- water mixture even though the SDS concentration was half that of the 10% dodecane-water mixture. This result was expected as the opportunity for contact between the oil droplets and the fibres should decrease as the concentration of oil droplets decreases. This result also supports the hypothesis that the dodecane flux is transport controlled within this oil concentration range.

I

/

99

Figure 34: Cross Flow Module; Oil Concentration Effect

0.6 -

0.4 -

0.2 -

0.8

0.0 *<' ' I I 8

0 200 400 600 8 0 0 1000

P=10 psl 0

T=37 C

perme te flux v

Flowrate (ml/min)

lues were on average only about 3 times as 1

for the 10% dodecane-water mixture rather than twice as high. This suggests that the permeate flux is not linearly related to dodecane concentration in this concentration range. The 5% dodecane values shown here however, may be high since less SDS was present than with the 10% dodecane mixture. Figure 34 also shows that the critical feed flowrate below which oil transport cannot occur is higher for the 5% dodecane mixture, as expected

These tests for the countercurrent and cross flow modules agreed well, and the data indicate that the SOF process appears to be controlled by the transport and attachment mechanisms. In the next experiments, additional studies were conducted to characterize the emulsion in order that performance could be related to emulsion stability as well as normal operating parameters.

100

(d) Cross Flow Module; 5% Dodecane Experiments In these experiments, which were carried out on the cross flow module

using a 5% dodecane-water mixture, the effects of feed flowrate and pressure were determined as they were previously. SDS concentrations of 0, 50, 100, and 200 mg/l were tested, and the permeate flux was measured over a slightly broader feed flowrate range of 300-2400 ml/min. An attempt was made to assess emulsion stability as a function of SDS concentration by measuring the droplet size distribution and zeta potential. These data then were correlated to the observed effect of increasing SDS concentration on the permeate flux with the intent of clarifying the role of SDS in the process. The results of these experiments are given in Appendix VI.

Before running these experiments the cross flow module was flushed with distilled water for about 30 minutes to remove any residual SDS from the system. Dodecane was then run through the module and the permeate flux for pure dodecane was measured. The flux was found to be considerably lower than that obtained prior to the 10% dodecane-water experiments (36 ml/min-ft2 versus 60 ml/min-ft2 at 5 psi). This implied that the fibres had lost some of their membrane permeability during operation. Possible causes of this are water blinding or surfactant coating of the membrane.

Figure 35 shows a plot of permeate flux versus feed flowrate for the 5% dodecane-water mixture at the various SDS concentrations. As with the 10% dodecane-water experiments, the flux decreased with increasing SDS concentration. The flux decrease was marked initially and then it became more gradual with subsequent SDS additions. Unlike the 10% dodecane-water mixture, the permeate flux increased linearly with feed flowrate in both the presence and absence of SDS even up to 2400 ml/min. (No leveling off was observed for the No SDS flux at higher feed flowrates.)

101

Figure 35: Cross Flow Module; 5% Dodecane Feed Flowrate Effect

J

a- 8 - ?

E 6 - E = 4 '

C

Y

X

u,

2 -

n 50 mgll SDS 100 mgll SDS

o ! I I

P=10 PSI

T=37 C 0

- 0 1000 2000 3000

Flowrate (ml/min)

This linear increase in permeate flux would be expected since the probability of droplet-fibre contact is lower at low dodecane concentrations. Feed flowrate should have an effect over a broader range therefore, as more kinetic energy is required to result in droplet attachment and collapse.

A comparison of Figures 33 and 35 shows however, that the permeate flux for the 5% doecane-water mixture at feed flowrates above 1000 mI/min was greater than that obtained for the 10% dodecane-water mixture. This implies that the limiting transport step does not shift to membrane control at a flux of about 2-3 ml/min-ft2 as suggested by the 10% dodecane-water results. The transport controlling mechanism should shift at the same permeate flux regardless of the oil concentraiton, although higher feed flowrates may be required at lower oil concentrations to obtain this critical flux. These data lead

102

one to conclude that the value of point A in Figure 33 is low, and that a plot of flux versus flowrate for the 10% dodecane-water mixture should be linear up to a flux of at least 9 ml/min-ft*, as shown in Figure 35 for the 5% dodecane-water mixture.

The linear regression data given in Figure 35 shows that a critical feed flowrate exists, below which oil cannot be transported across the membrane, for the 5% dodecane-water mixture at all SDS concentrations. Comparing this data to that shown in Figure 33, it appears that the critical flowrate values are higher for the 5% dodecane mixture than for the 10% dodecane mixture.

A comparison of the data for the 5% dodecane-water experiments and the 10% dodecane-water experiments on the cross flow module show that oil concentration has an effect on the permeate flux. It is not possible to quantify the exact nature of this effect as only two dodecane concentrations were tested. However, it appears that the permeate flux increased with increasing dodecane concentration and that this effect became more pronounced at higher feed flowrates. This is illustrated in Figure 36, which shows a plot of flux versus dodecane concentration for two different SDS concentrations at two different f lowrates.

The slopes of the lines at a feed flowrate of 500 ml/min are steeper than those at 300 ml/min. The slopes of the lines for the higher SDS concentration also appear shallower, which further suggests that oil transport is more difficult as the SDS concentration increases. The oil concentration effect may have been accentuated somewhat by the loss in membrane permeability for the 5% dodecane-water experiments, as indicated by the decline in pure dodecane flux.

103

Figure 36: Cross Flow Module; Oil Concentration Effect at Different Feed Flowrates

1 .oo

a 300 mllmin

0.60 -

0.00 I . , . , . , . , . , .

P=10 psi 0 T=37 C

4 5 6 7 8 9 1 0 1 1

YO Oil

A comparison of Figures 34 and 35 shows that the fluxes obtained for the diluted 10% dodecane-water mixture were slightly higher than those obtained in these experiments. This could be due to a loss of membrane permeability as discussed above.

Table 6 shows the zeta potential data that were obtained in these experiments. The values ranged from about -56 mV to -70 mV for no SDS and 200 mg/l SDS, respectively. Hughes and Foulds (1986) conducted zeta potential measurements on a Nigerian light crude oil and kerosine and they found the zeta potential of the oil droplets to be between -40 mV and -70 mV in the absence of surfactant and at a pH of about 6.0-7.0. The pH of the 5% dodecane-water mixture was measured to be 6.4. Although the zeta potential for the dodecane-water mixture containing no SDS fell within this range, it is possible that this zeta potential value may have been low as a result of contamination of the system with SDS from previous experiments. (Even

104

though the system was flushed with SDS it is possible that a very small amount of SDS still remained which could have been sufficient to impart some stability to the oil droplets.)

Table 6: Zeta Potential as a Function of SDS Concentration

~~

SDS Concentration (mgll) Zeta Potential (mV)

0 -56.7 50 -65.7 100 -66.1

The zeta potential of the dodecane droplets became only slightly more negative with increasing SDS concentration; a larger decrease in zeta potential was anticipated. The change in zeta potential with increasing SDS concentration however, corresponded quite well to the observed changes in the permeate flux values since the largest decrease in zeta potential occurred between 0 and 50 mg/l SDS and then the decrease was more gradual. (See Figure 35.)

This apparently small effect of SDS concentration on emulsion stability may have been sufficient to significantly limit the transport of oil across the membrane by increasing the electrical repulsion between the oil droplets themselves and/or between the droplets and the fibres which would reduce the rates of droplet attachment coalescence and collapse. Hughes and FouIds (1 986) made zeta potential measurements on polypropylene fibres in an aqueous medium as a function of pH and found the fibres to be strongly negatively charged, with a zeta potential of about -100 mV at a pH of 6-7. This suggests that the electrostatic repulsion between the fibres and the dodecane droplets may be very strong.

I

105

The droplet size distribution data is given in Figure 37. The percentages of the total oil volume within each of the different droplet size ranges are shown for the different SDS concentrations. It can be seen that the droplet size distribution did not change significantly with increasing SDS concentration. In all cases, the largest percentage (on a volume basis) of the droplets was in the 10-20 p range and the smallest percentage was in the 1-3 p range. A slight increase in emulsion stability was indicated by a small increase in the percentage of oil in the 1-3 p range and a small decrease in the percentage in the 10-20 p range. This suggests that the smaller, more stable droplets are more difficult to remove by SOF because of a reduced tendency for these droplets to coalesce and attach to the fibres, and collapse. This is in agreement with the theory of droplet coalescence on fibres (Hazlett, 1969).

Figure 37:

O h Total

Volume

" 1-3 3-5 5-7 7-10 10-20 >20

Droplet Diameter (p)

106

The nearly constant shape of the droplet size distribution curves implies that the stability of the dodecane-water emulsions was not significantly increased by the addition of SDS. This seems unlikely, however, and it is possible that the droplet size distribution remained constant due to limitations imposed by the mixing pump. The pump, for example, may not have been capable of producing a finer emulsion even though the interfacial tension was sufficiently reduced by the addition of SDS. Alternatively, it is possible that SDS was migrating to the fibre surface and therefore, unavailable for droplet stabilization.

Together, Table 6 and Figure 37 show that there is little increase in emulsion stability with increasing SDS concentration, which in turn implies that emulsion stability has little effect on the permeate flux. The mechanical aspects of emulsion stability may be important however, and they may change with increasing SDS concentration.

.

The most likely explanation for the lack of correlation between the change in permeate flux and emulsion stability with increasing SDS concentration is that some of the SDS migrates to the fibre surface and forms electrical and/or mechanical barriers which oppose the transport of oil. As the polypropylene fibres are already negatively charged (Hughes and Foulds, 1986), an accumulation of anionic surfactant at the surface should result in an even stronger negative charge and one would expect a reduction in oil transport due to electrical repulsion. To verify this hypothesis, the electrical charge on the fibre surface would have to be measured as a function of SDS concentration in solution to determine the electrostatic repulsion between the dispersed droplets and the fibres.

The exact effect of SDS on the SOF process was still not clear after the completion of these experiments. Subsequent experiments were conducted to further investigate the importance of electrostatic interactions between the dispersed droplets and the fibres.

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(e) AEAPTMS Coated Fibre Experiments The cross flow module was coated with N(P-aminoethy1)-y-aminopropyl-

trimethoxysilane (AEAPTMS), which is a cationic polymer, to impart a positive charge to the fibre surface. It was anticipated that if the fibres were positively charged, the negative dodecane droplets would have a stronger attraction for them, and hence a higher permeate flux could be achieved. The fibres were coated by immersing the cross flow module into a 1% solution (v/v) of AEAPTMS for about 10 minutes and then drying overnight at 55-60' C. This procedure was similar to that outlined by Hughes and Foulds (1986). Before the AEAPTMS coating was applied, the module was thoroughly cleaned and dried.

The coated fibres were saturated with dodecane and the pure dodecane flux was measured at a pressure of 5 psi. The permeate flux was found to be 54 ml/min-ft2, which was comparable to that obtained initially on the uncoated module before it had been used. This suggested that the cleaning process was effective in "regenerating" the fibres.

A 5% dodecane-water mixture with SDS concentrations of 0, 50, 100 and 200 mg/l was tested on the coated module. The permeate flux was measured at a pressure of 10 psi and at feed flowrates of 300, 500, and 1000 ml/min. The data from these experiments is given in Appendix VII.

Figure 38 shows the effect of feed flowrate on the permeate flux for the coated fibres. As with the 5%' dodecane-water mixture on the uncoated fibres the permeate flux increased linearly with increasing feed flowrate. The flux also decreased with increasing SDS concentration but the rate of decline was unlike that observed for the uncoated fibres (See Figure 35). With the coated fibres, the flux decreased markedly from both 0 to 50 mg/l SDS and from 100 to 200 mg/l SDS. These data compared very well with the zeta potential data shown in Table 6 in that a small change in zeta potential occurred between 50 mg/l and 100 mg/l SDS.

I

108

Figure 38: AEAPTMS Coated Module; O h 5 Dodecane-Water Mixture; Feed Flowrate Effect

n N .. U c

K 3 is

P=10 psi 0

T=37 C J

c

I I I I I I

0 200 400 600 8 0 0 1000 1200

9 (mllmin)

In contrast to the uncoated fibres there does not appear to be a critical feed flowrate below which oil transport is limited, except for an SDS concentration of 200 mg/l. At this concentration, the effect of SDS must be significant enough to severely limit the transport and attachment mechanisms. The positive intercepts on the ordinate axis in Figure 38 suggest that under stagnant flow conditions oil transport is still possible. This is probably due to a strong attractive interaction between the dodecane droplets and the coated fibres.

A comparison of the permeate fluxes for the coated and uncoated modules is given in Table 7. The AEAPTMS coating had a significant effect on the permeate flux particularly at low feed flowrates. Flux values up to an order of magnitude greater were obtained for the coated fibres. A contributing factor to this may have been the increase in membrane permeability due to module

.

109

cleaning but the major effect is thought to be the attractive electrostatic interactions between the droplets and the fibres.

Table 7: Permeate Fluxes for the AEAPTMS Coated and Uncoated Cross Flow Modules for a 5% Dodecane-Water Mixture

FIUX (ml/min-ft21

Of (ml/min) No SDS 50 mg/l 100 mg/l 200mg/l SDS S D S S D S

C U c u C U c U

300 2.08 0.87 1.44 0.16 1.20 0.11 0.26 0.09

500 2.90 1.31 2.03 0.23 1.76 0.19 0.44 0.15

1000 5.34 3.63 3.12 1.12 2.78 0.59 1.00 0.59

C = AEAPTMS coated module; U = uncoated module P = 10 psi; T = 37%

The increase in dodecane removal caused by the application of AEAPTMS was not consistent over the SDS concentration and feed flowrate ranges tested. In fact, the observed effect of the coating vaned dramatically, as seen in Table 8, which gives the ratio of the permeate fluxes for the coated and uncoated fibres, respectively. The improvement in dodecane removal varied from 1.7 times for an SDS concentration of 200 mg/l and a feed flowrate of 1000 mI/min to 10.9 times for an SDS concentration of 100 mg/l and a feed flowrate of 300 ml/min.

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Table 8: Permeate Flux Ratios of the Coated and Uncoated Fibres for a 5% Dodecane-Water Mixture

Coated Flux Ratio (Uncoated)

Qf (ml/min) No SDS 50 mg/l 100 mg/l 200 mg/l S D S S D S S D S

300 2.4 9.0 10.9 3.1

500 2.9 8.8 9.3 2.9

1000 1.5 2.8 4.7 1.7

In general, it appears that the improvement in dodecane removal is most pronounced at lower feed flowrates. This is expected because there is less opportunity for contact between the dodecane droplets and the fibres at low feed flowrates. Under these conditions, for the uncoated fibres, the electrostatic repulsion between the droplets and the fibres is more difficult to overcome. At higher feed flowrates the repulsive forces still exist but the mechanism of capture shifts more to interception. The higher impact energy of the droplets can better overcome the repulsive forces, and electroistatic interaction is less important.

The effect of the coating on the permeate flux varied quite dramatically with SDS concentration. Very large improvements in the rate of dodecane removal were observed for the intermediate SDS concentrations (50 mg/l and 100 mg/l) while smaller increases were observed for the No SDS case and for 200 mg/l SDS. This may be explained as follows.

When there is no SDS present, the dodecane droplets are less negatively charged and the electrostatic interactions with the fibres are weaker as a result. There is a tendency for the droplets to be repelled by the uncoated fibres, which are negatively charged under these conditions, and to be attracted by the coated fibres which are positively charged. The rate of droplet approach and attachment should therefore, be strongly affected by the coating, and this was observed for dodecane.

111

When SDS is added to the dodecane-water mixtures the dodecane droplets become more negatively charged and their mechanical stability increases. At the same time some of the SDS may migrate to the fibre.surface and cause the uncoated fibres to become more negative and the coated fibres to become less positive. The very low fluxes shown in Table 7 suggest that at intermediate SDS concentrations of 50 and 100 mg/l a strong electrostatic repulsion is observed between the droplets and the uncoated fibres. The data also indicate a strong attractive interaction for the coated fibres. This suggests that the migration of SDS to the coated fibre surface may not have been sufficient to significantly reduce the positive charge at these SDS concentrations.

At an SDS concentration of 200 mg/l the ability of the AEAPTMS coating to improve dodecane removal appears to have diminished. One explanation for this is that sufficient SDS has migrated to the fibre surface to significantly reduce the positive charge on the fibres. This could be verified by measuring the zeta potential of the fibre surface. With further additions of SDS, one would expect the ratio of flux values to approach unity as the coated fibres become more negatively charged; in other words the benefit of AEAPTMS may be negated at high surfactant concentrations.

The effect of the AEAPTMS coating is shown graphically in Figures 39 and 40, which plot the dodecane flux versus feed flowrate for the coated and uncoated fibres at the different SDS concentrations. The AEAPTMS coating clearly had a dramatic effect at low feed flowrates as shown by the differences in the y-intercepts for the coated and uncoated fibres. The vertical distance between the plots for the coated and uncoated fibres reflects the increase in flux attributable to the AEAPTMS coating. The improvement varied with SDS concentration, with the greatest improvement in dodecane removal being observed for SDS concentrations of 50 and 100 mg/l as discussed.

I

112

Figures 39 and 40 show that the slopes of the plots for the coated fibres are only slighlty greater than those for the uncoated fibres. This similarity in slopes may reflect the separate role of droplet transport to the membrane and droplet attachment and collapse at the membrane surface in the SOF process.

Feed fiowrate strongly affects both the transport and attachment mechanisms, but the application of the coating primarily affects droplet transport and attachment, and perhaps collapse. The observed differences in the y-intercepts and the vertical distances between the plots appear therefore, to be caused by the change in the electrostatic interactions between the droplets and fibres for the coated and uncoated fibres. The similarity of the slopes on the other hand, may be due to the insensitivity of the droplet collapse mechanism to the application of the AEAPTMS coating. a

Figure 39 AEAPTMS Coated Fibres vs Uncoated Fibres; Flux vs Feed Flowrate for the 5% Dodecane-Water Mixture Containing No SDS and 100 mg/l SDS

4 '

100 mg/l

P = 10 PSI

0 2 0 0 400 600 800 1000 1 2 0 0

q (ml/min)

113

Figure 40: AEAPTMS Coated Fibres vs Uncoated Fibres: Flux vs Feed Flowrate for a 5% Dodecane-Water Mixture Containing 50 mg/l and 200 mg/l SDS

x 3 ii

1

0 I I I

0 200 4 0 0 600 800 1000 120

Qf (mllmin)

P = 10 psi

200 mgll SDS

IO

Hughes and Foulds ( 986) conducted streaming potentia, measurements on AEAPTMS coated polypropylene fibres in aqueous solution and found the zeta potential to be about +40 mV at a pH of 6-7. He did not conduct measurements in the presence of surfactants, however. Without zeta potential data on the coated and uncoated fibres at different SDS concentrations it is difficult to quantify the magnitude of the electrostatic interactions between the dodecane droplets and the coated fibres and their importance on the SOF process. Streaming potential measurements should be done to verify the hypothesis described above.

Zeta potential measurements were made on the dodecane-water mixtures at SDS concentrations of 50 mg/l and 100 mg/l. Values similar to those reported earlier for the uncoated fibre experiments were obtained. The observed increase in dodecane removal therefore, can be attributed to the

114

application of the AEAPTMS coating on the fibres rather than differences in draplet stability.

These experiment demonstrated the importance of electrostatic interactions between the oil droplets and the fibres on the SOF process. In effect, they confirmed what was suggested by much of the work done by previous researchers on droplet coalescence on fibres. The results of these experiments also imply that surfactants affect the SOF process primarily by coating the fibre surfaces. In so doing, they present electrical, and possibly mechanical barriers, which oppose droplet approach and attachment. The role of emulsion stability on the SOF process appears to be secondary. Further tests must be done to verify this hypothesis and to quantify the effect of SDS on the SOF process.

115

6. DISCUSSION

6.1 Discussion of Results The fundamental research conducted in this study demonstrated that oil

can be selectively removed from oil-water mixtures using microporous polypropylene fibres, in both the presence and absence of emulsifying agents. The effect of various operating parameters on oil removal was investigated, and inferences as to the controlling mechanisms of oil transport were identified under different operating conditions. The impact of surfactants on process performance was examined and the role of surfactants in the SOF process was determined. The principal findings of the study are discussed below.

Influence of Oil Viscosity

Two oils were selected for study: a naphthenic oil with a viscosity of 52.6 CP at 2OoC and dodecane with a viscosity of 1.35 at 25OC. Both oils were effectively removed by the membrane process; however, the response of the oil flux rates to changes in operating conditions showed marked differences in behavior. Removal of the naphthenic oil appeared to be limited primarily by the membrane since the permeate flux was dependent on operating pressure and insensitive to feed flowrate (except at low oil concentrations). This behavior may be attributed to the high viscosity of the naphthenic oil which caused low permeate fluxes to be obtained. By comparison the low viscosity dodecane passed through the membrane more easily, and thus higher permeate fluxes were obtained. The strong dependence of dodecane flux rate on feed flowrate and its insensitivity to operating pressure clearly indicate that the dodecane was not membrane limited but rather limited by the transport and attachment mechanisms responsible for bringing the oil to the membrane surface.

Direct comparison of dodecane and naphthenic oil to evaluate the influence of viscosity requires an assumption that any interaction between the oil and membrane is essentially equal for both oils in spite of their structural differences. This assumption is questionable. The oil transport through the

,

I

116

membrane is not directly analogous to ultrafiltration, where membrane-solvent interactions are generally unimportant. In the SOF process the membrane swells and dissolves in the oil phase. The degree of swelling and the affinity of the oil for the membrane may have an important effect on separation rates. In this sense the membrane is more analogous to reverse osmosis.

An effective way of assessing the role of oil-membrane interaction would be to compare oils having very different structures and/or molecular weight but similar viscosities.

Influence of Oil Concentration Changes in oil concentration appeared to have a similar effect on the

removal of both the naphthenic oil and dodecane. As the oil concentration increased the rate of oil removal also increased. This increase was not found to be linear, however, over the oil concentration ranges tested. In general it appeared that the effect of oil concentration was more marked at low oil concentrations and became less significant at higher oil concentrations. This is shown in Figure 16 for the naphthenic oil. There appeared to be a critical concentration beyond which further increases in oil concentration had little effect on the permeate flux and the value of this critical oil concentration decreased with increasing feed flowrate. In the dodecane experiments a similar effect of feed flowrate was observed. For both oils the permeate flux dropped dramatically at low oil concentrations, which suggests that there is a limiting oil concentration below which SOF is not feasible.

Influence of Surfactants

The removal of both dodecane and naphthenic oil was adversely affected by the presence of surfactants, and the permeate fluxes decreased as a result. The naphthenic oil experiments demonstrated this behavior qualitatively for anionic and nonionic emulsifying agents, while the dodecane experiments quantified the effect of a single anionic surfactant by measuring the permeate flux as a function of SDS concentration. The permeate flux decreased with increasing SDS concentration, although the flux decline was more significant

117

for small additions of surfactant and became less dramatic with further SDS additions.

These findings agree with those of other researchers who studied the effect of surfactants on oil and water droplet coalescence on fibres (Hazlett, 1969; Bitten, 1970; Clayfield et al., 1985; Hughes and Foulds, 1986). They found that the presence of surfactants greatly reduced the coalescence efficiency. Clayfield et al. (1 985) speculated that surfactants modify the electrical properties of the droplets and the fibres and also that they affect droplet film drainage and rupture. If this were the case in these experiments, then both the transport and attachment mechanisms would be adversely affected. The zeta potential measurements which were made in our study indicated a slight increase in the charge on the dodecane droplets with increasing SDS concent ration.

The dramatic influence of SDS is clearly not solely attributable to the small change in the charge on the oil droplets. The zeta potential dropped only from -56.7 mV to -70.4 mV when the SDS concentration was changed from 0 to 200 mg/l SDS. In addition, measurements of the droplet size distribution showed little difference in the presence of SDS. These measurements suggested that the emulsion stability had not increased dramatically in the presence of SDS; however it was apparent that the oil permeate rate declined dramatically. The mechanical aspects of emulsion stability may have been overlooked, and their role in preventing droplet attachment and collapse should be assessed before conclusions as to the predominant role of surfactants on the SOF process are made.

It is possible that the dramatic impact that SDS had on oil flux across the fibres resulted from some modification of the fibre surface. SDS may adsorb onto the fibres and render the fibres more negatively charged. Hughes and Foulds (1 986) showed that polypropylene fibres are negatively charged in water with a zeta potential of about -80 to -100 mV in the neutral pH range, and the adsorption of SDS to the fibres could be expected to depress the zeta potential to more negative values. The combined effect of SDS on the droplets and membrane surface may explain the observed behavior.

118

Interestingly, it was apparent from the results that although the flux of oil was reduced by the presence of surfactants, it was still possible to effect a separation of the oil. The results suggest that the decrease in oil flux caused by surfactant addition could be overcome by increasing the flowrate past the fibres.

This result was somewhat surprising. It was anticipated that the addition of a substantial surfactant concentration would stabilize the oil droplets and tend to extract the oil from the membrane in a manner analogous to the cleaning action of detergents. This action, together with adsorption of surfactant molecules onto the membrane itself, would serve to make the membrane hydrophilic and cause the membrane to act as a water permeable ultrafilter.

This wetting process appears to have occurred in at least two tests. When ABS was employed as a surfactant (1% ABS by weight of oil) in a 10% naphthenic oil-water test the membrane passed both water and oil initially. However, after approximately two hours of operation the membrane passed only oil and water permeation of the membrane was no longer occurring. Inspection of the oil-water permeate at the beginning of the test showed complete phase separation and the test was continued only for this reason. Murkes (1986) showed that stable oil-water emulsions may be separated in a similar manner by forcing the emulsion through a variety of microporous filters and ultrafilters. However, the reversion of the permeate to oil only after two hours of operation is difficult to explain. Even when the ABS concentration was increased by an order of magnitude, no water permeation occurred. Further research is needed to evaluate the factors that cause water, water and oil, and pure oil to pass through the membrane.

Studies with other surfactants gave different results. No water permeation was observed at the highest SDS concentration tested although at 200 mg/l it was a factor of five lower that the ABS dose. (0.2% SDS by weight in 10 Yo dodecane). When tests were conducted with Triton X-102 at the same concentration as the ABS (lYo by weight in 10% naphthenic oil), no water permeation was observed either. Discussion with the engineers at Questar led

119

me to believe that the nonionic surfactant molecules employed in this Triton formulation would adhere strongly to the polypropylene fibres. It was expected that these molecules would cause water wetting of the membrane and convert the membrane to an ultrafilter. This was not observed.

Oil Removal Efficiency

In an effort to characterize the efficiency of the SOF process, oil removal efficiencies were calculated for the 5% dodecane-water experiments on the uncoated cross flow module. The removal efficiency is defined as follows:

n

Where, Q, = rate of oil permeation Qd = rate of oil delivery to the module

= (Qf) (Volume Yo of oil in feed)

Table 9 gives the oil removal efficiency as a function of feed flowrate and SDS concentration. Obviously as the feed flowrate increases, the rate of oil delivery to the membrane also increases. (The rate of oil delivery was calculated as Qf (ml/min)x6.6 vel.% dodecane.) The results in Table 9 show that as the feed flowrate increases the efficiency of oil removal also increases; this increase does not appear to be linear, however. As the SDS concentration increases the efficiency of oil removal decreases. These results were already suggested by the flux versus flowrate data for this experiment. (See Section 5.2 (c).)

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Table 9: Oil Removal Efficiencies for the 5% Dodecane-Water Mixture on the Uncoated Cross Flow Module

Oil Remova I Efficiencv (16) 0

Qf Qd No 50 mg/l 100 mg/l 200 mg/l (ml/min) (ml/min) S D S S D S S D S S D S

300 19.8 5.1 0.9 0.640 0.52 500 33.0 4.6 0.8 0.66 0.52 1000 66.0 6.3 2.0 1 .o 1 .o 2000 132.0 6.5 2.9 1.7 1.7 2400 158.4 7.0 3.4 1.9 1.5

Overall, the removal efficiency values were very low for the dodecane- water experiments. Coalescence efficiencies much greater than this are normally achieved (Hughes and Foulds, 1986). This result is not surprising, however, as the fibres used in these experiments had a much larger diameter than those typically used in oil coalescence. The fibres used here had an outside diameter of 425 p whereas those used by Hughes and Foulds (1986) had a diameter of 7.5 p.

Removal efficiencies as calculated above are valid only for oil separation processes limited by the transport and attachment mechanisms. Oil removal that is limited by the membrane, such as for the naphthenic oil studied in these experiments, cannot be described in this way since the oil flux is insensitive to changes in feed flowrate. Another means of assessing the effectiveness of the SOF process is needed to include this type of transport control so that comparisons can be made with other oil-water separation methods.

The Influence of System Hydrodynamics

The importance of system hydrodynamics on droplet coalescence on fibres has been demonstrated by several researchers, and it is generally accepted that the interception mechanism of droplet approach is of primary

121

importance in promoting droplet coalescence. Langmuir (1 942) derived an equation to describe droplet coalescence on a single fibre strictly in terms of hydrodynamic considerations. This equation was described in Section 3.1. Subsequent workers have used this equation and made modifications to account for electrostatic interactions and to consider a bed of fibres (Hughes and Foulds, 1986), and still others have developed their own empirical relations (Weber and Paddock, 1983). All these empirical correlations, however, relate the coalescence efficiency to the Reynolds number and the ratio of droplet to fibre diameters. These correlations indicate an increase in droplet coalescence with increasing Reynolds number and increasing droplet to fibre diameter ratio.

All but Weber and Paddock's correlation were given in Section 3.1; this equation is as follows:

where

E, = (A - 0.87A3)* 3

E, = coalescence efficiency for a single fibre A = (2.022 - In(Re))''

droplet diameter = fibre diameter

An attempt was made to correlate some of the data with the equations derived by Langmuir (1942), Hughes and Foulds (1986), and Weber and Paddock (1983). The data from the naphthenic oil experiments were not amenable to this because the removal of naphthenic oil is membrane controlled. Data from the 5% dodecane-water experiments on the uncoated cross flow module were selected to make these correlations. Details of the

assumptions and calculations which were made in correlating this data are given in Appendix VIII.

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Figures 41 and 42 give plots of the oil removal efficiency, ER, versus 1

and (A - 0.87A3), respectively, for the different SDS concentrations. 2 (2-ln( Re)) As shown in Appendix Vlil, the bed coalescence efficiency, Eb, is equivalent to the oil removal efficiency, ER, which is related to the single fibre efficiency, E, by the relationship E, = 7 ER where n is the number of droplet-fibre contacts.

Figures 41 and 42 show that the equations developed by Langmuir (1942) and Weber and Paddock (1983) yield similar correlations. Hughes and Foulds (1986) pointed out this similarity between the two expressions, and they found that despite their apparent difference these equations gave similar E, values. Langmuirs' equation, however, is limited to very low Reynolds numbers, while Weber and Paddock's equation was developed for a broader range of Reynolds numbers (up to 100).

for the 5% Figure 41: Removal Efficiency vs 2(2-,n(Re))

Dodecane-Water Mixture on the Uncoated Fibres

1

5% Dodecane

o 50 mg/l SDS

A 200 mgll SDS A 100 mg/l SDS

0.07 0.08 0.09 0.10 0.11 0.12

1

2( 2-1 n ( Re))

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Figure 42: Removal Efficiency vs (A - 0.87A3) for the 5% Oodecane-Water Mixture on the Uncoated Fibres

8 1 1

6

R E

4 1

5% Dodecane

100 mg/l SDS 200 mg/l SDS

2

I I I 0.14 0.1 6 0.1 8 0.20 0.22

3 (A-0.87A )

The data correlated well with the expressions developed by Langmuir (1942), Hughes and Foulds (1986) and Weber and Paddock (1983). This is illustrated by the linearity of the plots shown in Figures 41 and 42. The data fits the general form of these equations, suggesting that the behavior of selective oil filtration is compatible to the theories developed for droplet coalescence on fibres. This provides further evidence that oil removal is transport and attach me nt li mi t ed.

In the dodecane experiments on the uncoated fibres a critical feed flowrate was identified below which no dodecane was transported across the module. The value of this critical flowrate appeared to decrease with increasing dodecane concentration and decreasing SDS concentration. Bitten (1970) also determined that there was a critical feed flowrate below which no

I

124

coalescence occurred. No critical flowrate was exhibited for the AEAPTMS experiments except at high SDS concentrations because of the reduction in electrostatic repulsion of the dodecane droplets and the fibres. These data suggested that dodecane removal was possible even under stagnant flow conditions.

The effect of fibre and droplet diameter on the removal of oil using SOF was not examined in this study. The theory of droplet coalescence on fibres

suggests however, that as decreases droplet coalescence increases. With

respect to the SOF process one would expect an improvement in performance for large oil droplets which are less stable. The effect of decreasing fibre diameter, however, is not obvious. While the efficiency of coalescence may increase with a decrease in fibre diameter, it is possible that the rate of oil transport within the fibre lumen may be reduced. There may be a tradeoff between improved coalescence and oil removal from the lumen in which case an optimum fibre diameter should exist for a given oil.

df

Water Blinding

The membrane was always wetted with the pure oil to be separated at the beginning of a test, and it was observed that the flux of oil declined over the first hours of exposure to the oil-water mixture. This decline in flux was not solely attributable to the removal of excess oil from the membrane during the first hours of operation. It appears that the water adheres to the membrane and occludes a fraction of the surface. Murkes (1986) discussed "water blinding" in his studies. Microscopic examination of the fibre surface may be useful in further characterizing this phenomena.

Air Entrainment

The oil flux through the membrane was always observed to increase in the presence of entrained air. This may result from a modification of the oil- membrane contact angle. This effect is worthy of additional study.

125

Influence of Surface Coatings

The AMT experiments initially conducted did not show an improvement in oil removal for fibres to which a hydrophobic fluorocarbon coating had been applied. Theoretically, an increase in the hydrophobicity of the membrane should result in a more effective oil-water separation. My results did not demonstrate this, and this behavior was attributed to a loss of membrane permeability due to the application of the coating.

The importance of electrical interactions between the dodecane droplets and the fibres was demonstrated by the AEAPTMS experiments. The positive coating on the fibre surface greatly increased the dodecane flux. Clayfield et al. (1985) and Hughes and Foulds (1986) both found an improvement in the coalescence efficiency of oil droplets on AEAPTMS coated surfaces, and they stressed the importance of electrostatic interactions. These results suggest that the application of a positively charged coating, such as AEAPTMS, is beneficial to selective oil removal.

-

pH Effect Hughes and Foulds (1986) illustrated the importance of pH on the

coalescence of oil droplets. The magnitude of the charge on both the dispersed oil droplets and the fibres that they examined varied significantly with pH. In our study pH was not a variable except in one experiment with the ABSnaphthenic oil-water mixture. This experiment indicated a decline in permeate flux at high pH.

Hughes and Foulds (1986) found that both the oil droplets and the polypropylene fibres became more negatively charged with increasing pH. The decrease in flux which was observed therefore, may be the result of stronger repulsive electrostatic interactions between the droplets and the fibres. In addition, the flux decline may be due to an increase in emulsion stability as discussed previously. Further experiments should be conducted to study the effect of pH in more detail. The effect of pH will vary depending upon the nature of the surfactant , the oil and the membrane surface character. .

126

Controlling Mechanisms of Oil Transport

Based on the results of this study the following observations were made as to the conditions under which the different mechanisms of oil transport control the overall process. Transport to the membrane, Jm, appears to limit for high viscosity oils and at low operating temperatures. Oil transport through the membrane may also limit for membranes with low porosity.

For low viscosity oils, Jm is not likely to limit, but rather transport to the membrane, J, or attachment and collapse at the membrane, A. If either the oil concentration or feed flowrate is low the probability of contact between the oil droplets and the fibres is reduced and the rate of oil removal is decreased. Electrical interactions between the droplets and the fibres may also affect how closely the droplets may approach the fibres and repulsive interactions will deleteriously affect the transport of oil to the membrane. The attachment and collapse step requires that the droplets have a strong affinity for the fibres and that they readily collapse. Repulsive electrical interactions will hinder droplet attachment while increased mechanical strength of the droplets will affect the rate of droplet rupture and collapse.

6.2 Potential Uses and Limitations of the SOF Process

Based on the findings of this study it appears that the SOF process has potential as an alternative method for removing oil from oily wastewaters or frcm other oil-containing wastes. My results indicate that the SOF process is capable of removing free, mechanically dispersed and chemically emulsified oil from oil-water mixtures. SOF is not suitable for the removal of oil-wet solids, however, and prior to filtration the suspended solids must be removed to prevent membrane fouling.

The results of this study indicate that the SOF process is most effective at removing oil from wastewaters containing high oil concentrations. The ultrafiltration (UF) process, which is conventionally used to treat wastewaters containing chemically emulsified oils, is limited in that it cannot handle high oil concentrations. As mentioned previously, UF performance deteriorates as the

# 127

feed oil concentration increases, and 3eyond about 30% oil the water permeation rate begins to drop dramatically. The SOF process, on the other hand, improves in performance as the oil concentration increases. Thus, the SOF process could replace UF in certain applications. Also, the UF process requires the removal of free oil as a pretreatment step; with SOF this step is not necessary.

SOF would not be a viable alternative to UF for the removal of oil from dilute oily wastewaters (<5-10% oil). As shown in this study, the rate of oil removal is limited in the SOF process at low oil concentrations whereas UF is most efficient under these conditions.

A comparison of the relative efficiencies of SOF and UF can be made by comparing the membrane areas required to treat the same oily wastewater. Given below is a rough calculation estimating the membrane areas required to treat a typical oily wastewater:

Considered here is a 1 liter volume of a typical oily wastewater containing 5% oil by volume.

If Selective Oil Filtration were used to reduce the oil content to el%, approximately 45 ml of oil would have to be removed. Assuming an average oil removal rate of 2 ml/min-ft2, the membrane area required to treat 1 Vmin of oily wastewater is,

If Ultrafiltration were used to concentrate the oil to 50%, approximately 900 ml of water would have to be removed. Assuming an average water

128

permeation rate of 100 ml/min-ft2 (Pinto, 1978), the membrane area required to treat 1 I/min of oily wastewater is,

This calculation suggests that UF is more efficient at treating an oily wastewater with an oil concentration of 5%. It must be borne in mind however, that higher operating pressures are required with UF and in addition a pure oil

. product cannot be obtained.

Another option for treating dilute oily wastewaters is to combine UF and SOF. Ultrafiltration could be used to concentrate the oily wastewater and produce an oil-free effluent, while SOF could be used to recover a water-free oil product.

An application for which the SOF process appears to be very promising is the removal of water from water-in-oil emulsions. Fuel oils, for example, are often contaminated with water droplets that must be removed prior to use. UF cannot be used for this separation as the water permeate flux is virtually zero under these conditions. Presently, coalescing filters and centrifuges are the methods of treatment. It is thought that a pure oil product could readily be obtained using SOF; the oil fluxes should be high with oil as the continuous phase.

The SOF process may be limited in its ability to remove oil from wastewaters containing high concentrations of surfactants. The results of this study indicate that as the concentration of surfactant increases, the rate of oil removal decreases. The results also indicate that some of the surfactant passes through the membrane with the oil. This may limit the process if surfactants are undesirable in the oil. In some applications, however, the presence of surfactants may be desirable; one such application is in the area of metal working fluids which are purchased with surfactants in them.

129

Water blinding of the membrane is another possible limitation of the SOF process. In some of the experiments a dramatic decline in oil removal with time was observed, which was attributed to water blinding. This behavior would be expected to be significant at lower oil concentrations. Further studies are necessary to characterize this phenomena and to determine methods of minimizing its effect.

The SOF process may also be limited in the types of oils that it can remove. This study showed that high viscosity oils were more difficult to remove as transport through the membrane controlled the overall process. Oil viscosity is not a critical parameter in UF, although it may affect the degree of concentration polarization and/or membrane fouling.

The main advantages foreseen in using SOF are that a water-free oil product can be recovered and that high concentrations of oil can be handled. The main disadvantages are that the process appears to be limited for high viscosity oils, low concentrations of oil and it is adversely affected by the presence of surfactants and water blinding.

The SOF process can be optimized in one of several ways depending upon the oily wastewater in question and the controlling mechanism of oil transport. The controlling mechanism, which is dependent on the operating conditions, will dictate the degree to which the process can be improved. The most difficult case is for membrane limited transport, Jm. The only way to improve oil removal is to increase the operating pressure and/or temperature, and the membrane is restricted in the pressures and temperatures that it can withstand (typically 30-50 psi and 90°C, respectively). In addition, one might experiment with membranes having different pore size distributions to find one with a high permeability for a given oil.

The removal of oils that are limited by transport to the membrane, J, and attachment and collapse at the membrane, A, can be optimized more easily by improving the system hydraulics and encouraging droplet-membrane contact. The feed flowrate could be increased or the module geometry could be adjusted. The stability of the dispersed droplets could be reduced by varying

.

130

the pH or ionic strength of the oil-water mixture. In addition, a coating could be applied to the membrane to enhance the attractive electrostatic interactions between the droplets and the fibres.

6.3 Future Research It is evident that a considerable amount of research has yet to be done,

both fundamentally and on a more practical level, to better characterize and understand the SOF process and to better assess its potential as an alternative oil separation technique.

As a continuation of the dodecane experiments conducted in this study, the role of electrostatic interactions between the dispersed dodecane droplets and the fibres should be quantified. This would require streaming potential measurements to be made on the coated fibres at different SDS concentrations. Also, the role of SDS in the SOF process must be clarified further; in particular, the importance of droplet stability must be determined. The mechanical stability of the dodecane droplets should be assessed along with further studies of the electrical stability. It would be worthwhile to look more closely at other means of varying emulsion stability besides surfactant concentration, to separate the effects of emulsion stability from those of surfactant adsorption at the fibre surface. Both pH adjustment and salt additions could be made. Also, the anomalous results observed in these experiments should be verified, and the removal of surfactant must be quantified.

In a broader sense, much more fundamental research could be done on the SOF process. Different oils and surfactants could be tested, different module geometries could be assessed and the effect of alternative membrane configurations could be examined (i.e. flat sheets as opposed to fibres). In particular, the effect of hydrocarbon character and nonionic surfactants should be studied. The influence of fibre diameter and system hydraulics should be further investigated, and an empirical expression relating SOF efficiency to the system parameters should be formulated.

-

131

The influence of membrane characteristics is important and needs to be examined carefully. For example, the influence of membrane porosity and pore size distribution should be assessed. Membrane coatings or surface functionality may be important in modifying oil droplet attachment and rupture. Also, selected model organic compounds might be useful to determine whether the membrane exhibits any selectivity beyond the influence of viscosity.

From a practical point of view, tests should be run on representative oily wastewater samples, and methods of optimizing the oil separation process should be considered. Also, it would be informative to interface an SOF system with a UF system to determine the feasibility of this process combination, and carry out studies to compare the performance of the SOF process to that of other oil-water separation techniques.

132

7. CONCLUSIONS

filtration of oil across microporous fibres of polypropylene. The following conclusions were drawn from this study on the selective

1. Selective oil filtration can be used to recover a water-free oil product from oil-water mixtures in both the presence and absence of emulsifying agents.

2. Low viscosity oils, such as dodecane, are more efficiently removed than high viscosity oils such as naphthenic oil. This difference in removal efficiency is due to differences in the controlling mechanism of oil transport.

3. In this study, the transport of high viscosity oils was membrane limited whereas the transport of low viscosity oils was limited by transport and attachment to the membrane and droplet collapse.

4. The removal of low viscosity oils is strongly affected by system hydrodynamics. Oil removal rates increase with increasing feed flowrate as the opportunity for contact between the dispersed droplets and the fibres is improved.

5. The rate of oil removal is affected by the feed oil concentration. As the oil concentration increases the rate of oil removal also increases. This study showed that this increase is not linear over a broad oil concentration range and that oil removal is limited at low oil concentrations (4%).

6. Both anionic and nonionic emulsifying agents have a deleterious effect on oil removal. The rate of oil removal decreases markedly with the addition of these types of emulsifying agents.

7. The exact role of emulsifying agents in selective oil filtration is not clear. These studies indicated that emulsifying agents may affect oil removal by altering the electrical and/or mechanical properties of the droplets and the fibres. Further studies are required to determine the effect of mechanical stability.

133

8,

9.

Electrostatic interactions between the droplets and the fibres play an important role in the oil removal process. Repulsive electrostatic interactions between the negatively charged oil droplets and the membrane, which is also negatively charged, hinder droplet approach and attachment.

The oil removal process can be improved by encouraging attractive electrostatic interactions between the droplets and the fibres. One way of accomplishing this is to apply a positive coating to the membrane surface, such as the cationic polymer, AEAPTMS.

10. The selective oil filtration process has potential as an alternative oil-water separation technique. This process is better suited than ultrafiltration to the treatment of oily wastewaters with a high oil concentration or for the treatment of water-in-oil emulsions. It cannot replace ultrafiltration however, for the removal of oil from dilute oily wastewaters.

134

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139

A P P E N D I C E S

140

Appendix I

Naphthenic Oil Experiments AMT Module

141

1 O h Oi I- Wa t er Mi x t u re

Time (hr) Qf (ml/min) P (psi) FIUX (ml/min-ft2) 0.038 0 78 12

II

77 20 0.027 200 12 0.1 0 3

I1

205 20 0.1 4 200 6 0.07 310 12 0.1 6

I*

I1

11 31 0 6 0.1 1 300 20 0.22

18 76 12 0.02 31 0 12 0.12

I1

4 78 12 0.05

It

N 31 0 20 0.1 6 20 305 6 0.05

300 12 0.1 6 31 0 20 0.18

26 300 25 0.1 6

27 455 12 0.16

24 31 0 6 0.1 0 N

N

11 31 0 12 0.1 2

42 460 12 0.1 1 45 20 0.01 3

46 220 12 0.05 205 20 0.06

51 305 12 0.06 ll 31 0 20 0.08

53 75 12 0.01 1

73 205 12 0.02 92 205 12 0.02

I*

75 20 0.01 5

T = 32-38' C

142

10% Oil-Water Mixture

Time (hr) Qt (ml/min) P (psi) FIUX (mt/min-ft2)

0 75 12 0.35 19

1 II

8 10 24 27 28 30

I9

n

I9

32

75 75 75 75 75 75 75

330 330 31 0 31 0 320 30

6 12 20 12 12 12 12 12 12 20 6 12 20

0.20 0.40 0.62 0.38 0.38 0.38 0.36 0.39 0.36 0.58 0.20 0.37 0.28

30 12 0.33 50 T = 32-38' C

50% Oil-Wa ter Mixture

Time(hr) Qf (ml/min) P (psi) FIUX (ml/min-ft2)

0 200 13 0.57 0.5 200 11 0.47 2.0 200 12 0.49 2.5 75 12 0.44

75 11 0.45 75 13 0.46

3.5 30 13 0.42

n

I1

4.0 200 13 0.47 T = 32-38' C

I

143

Appendix il

Naphthenic Oil Experiments AMT Versus Celgard

144

Section 1: Baseline Data For Celaard 1 Module

Pure Oil Data

Time (hr) Of (mllmin) P (psi) FIUX (mt/min-ftZ)

0 78 12.5 4.31 0.5 78 12.5 4.37 1.5 155 12.5 4.33

11

155 13.5 4.63 305 13.0 4.40 305 14.0 4.70

2.0 305 12.0 4.46 7.0 305 12.5 4.23

1V

1,

24.0 303 12.5 3.74 T = 32-38' C


Time (hr) Qf (mllmin) P (psi) FIUX (ml/min-ftZ) 0

0.5 1 .o 2.0 3.0 4.5 5.5 22.0

303 160 160 31 0 80 80

400 155

12 12 7 12 12 12 12 12

3.66 3.06 1.42 2.74 2.56 2.54 2.50 1 .85

23.0 390 12 1.83 T = 32-38' c

145

Section 1 continued ....


Time (hr) Q f (mllmin) P (psi) FIUX (mt/min-ft*)

0 280 12 2.92 0.25 1.25 1.50 2.0

17.25

280 280 280 280 280

12 12 12 12 12

2.32 1.73 1.19 1.49 0.52

17.75 280 12 0.41 T = 32-38' C

Section 2: AMT Vs Celaard 1

ter Mixtura

FIUX (mi/min-ft2) Time (hr) AMT Flux Celgard 1 Flux

0.0 1.22 3.03 0.5 1 .o 2.0 3.0 4.0 5.0 6.0 7.0 8.0

47.5

1.02

0.60 0.64 0.63 0.62 0.58 0.57 0.59 0.33

0.78 2.82 2.86 2.78 2.52 2.36 2.42 2.28 2.50 2.02 1.74

48.75 0.32 1.54 T = 32-38' C

I

146

Section 2 Continued ...


FIUX (ml/min-ft2)

Time (hr) AMT Flux Celgard 1 Flux 0 1.19 2.38

0.25 0.80 2.32 1 0.43 2.24 2 0.26 1.72 3 0.24 2.06 4 0.23 1.92 5 0.20 2.10 6 0.1 9 2.06 7 0.20 2.02 8 0.1 9 2.02 9 0.18 2.02 24 0.1 3 1.34 25 0.1 3 1.36

T = 32-38' C

I

147

Appendix 111

Naphthenic Oil Experiments Surfactant Effects

Cel g ard Ex per i me n t s

I

148

Section 1: Celaard 2 Module

10% O/W Emulsion Containina 1% ABS

Time Flux O h Oil (hr) (mllmin oft2) Remaining ( vh ) Comments

0 0.25

1 .o 2.0

3.0 4.0 5.0 6.0 7.0

17.0 18.0 19.0 22.0 24.0 25.0 26.0 41 .O 70.0 70.5 71 .O 72.0

4.21 2.57

0.93 0.86

0.93 1.21 1.29 0.93 1.29 1 S O 1.50 1.43 1.64 0.86 0.64 0.50 0.01 0.0001 1.71 1.86 1.92

100.0 11.1

---- 10.8

10.6 10.4 10.2 9.9 9.6 4.8 4.4 3.9 2.4 2.1 1.9 1.7

0.44 ------ 11.2 9.5 7.9

pure oil flux water in permeate; two distinct phases

50% OMI permeate phases not separating

pure oil permeate

It ,I It

I t It II

I t It I t

I t I I I t

I t I t I t

flux still high overnight permeate darker in color

flux starting to decrease

flux very low overnight flux virtually zero more oil added

permeate dark brown T = 32-38' C; P = 12 psi; Qf = 200 mI/min

149

Section 2: C -#3 Module

ReDroducibilitv Study

10% OIW Emulsion Containina 1% ABS

~

Ti m e Flux YO Oil (hr) (ml/min-ft2) Remaining (vlv) Comments 0 5.32 100.0 pure oil flux 0.25 0.56 11.1 water in permeate;

two distinct phases 1 .o 0.31 1 1 .o 2.0 2.5 3.5 4.0 5.5 6.0 7.5 8.0 22.0 23.0 24.0

0.44 0.48 0.53 0.54 0.68 0.69 0.65 0.63 0.52 0.41 0.37

pure oil permeate

flux lower overnight

26.0 0.47 ---- 27.5 0.32 3.8 29.0 0.31 ----

flux decreasing rapidly

31.5 0.25 3.3 46.0 0.1 4 1 .a 47.5 0.06 ----

T = 32-38' C; P = 12 psi; Qf = 200 ml/min

150

Section 3: Ce laard 2 Module: pH Effect

10% Oil-Water Mixture Containina 134, ABS

Time (hr)Low pH Flux High pH Flux 06 Oil Low pH O h Oil High pH

0 4.21 3.01 11.2 11.2 0.25 1 .o 2.0 3.0 4.0 5.0 6.0 7.0

17.0 22.0 24.0 25.0 26.0 29.0 30.0 41 70.0 70.5

2.57 0.93 0.86 0.93 1.21 1.29 0.93 1.29 1.50 1.64 0.86 0.64 0.5 ..... .....

0.01 1.71 1.86

2.39 1.80 0.91 .....

0.55 0.66 0.73 0.55 .....

0.07 .....

0.06 0.04 1.21 1.31 0.66 ..... .....

11.1 .....

10.8 10.6 10.4 10.2 9.9 9.6 4.8 2.4 .....

1.9 1.7 ..... .....

0.44 11.1 9.5

11.1

9.5 8.6 ...,.

7.1 6.5 6.0 5.3 .....

4.0 .....

3.9 3.8

10.9 10.6 9.7 ..... .....

7.9 ..... ...... 71 .O 1.93 T = 32-38' C; p = 12 psi; Qf = 200 mI/min

151

10% Oil-Water Emulsion Containina 1% Triton X-102

Time (hr) FIUX (ml/min-ft*) Oh Oil Remaining 0 1.79 11.2 0.25 1 .o 2.0 4.0 5.0 6.0

20.0 22.0 24.0 25.0

0.93 0.87 0.60 0.64 0.58 0.53 0.08 0.08 0.05 0.04

11.1 10.0 9.0 8.0 7.6 6.2 1.5 .....

1.1 .....

0.04 ..... 29.0 T = 32-38' C; P = 12 psi; Q:

152

Appendix IV:

Dodecane Experiments Countercurrent Module: 5% Oodecane

153

Pressu re Effect o n Permeate Flux

P (psi) FIUX (ml/min-ft2) 50 mg/l 100 mg/l 500 mg/l

S D S S D S S D S

5 ..... 3.342 0.188 10 2.652 2.61 3 0.249 20 ..... 2.1 10 0.1 71

T = 37' C; Qf = 300 ml/min

F eed Flowrate mfect on Per meate F lux

Qf (mllmin) Flux (ml/min-ft2) 50 mg/l 100 mg/l 500 mg/l

S D S S D S S D S

0.243 0.028 100 0.243

300 2.641 1.597 0.334 500 5.535 2.220 0.699 750 9.130 - 3.920 1.21 0

T = 370 C; P = 10 psi

I

154

Appendix V:

Dodecane Experiments Cross Flow Module: 10% Dodecane

155

Pressu re Effect on Permeate Flux

P (psi) FIUX (ml/min-ft2)

No SDS 50 mg/l 100 mg/l 200 mg/l S D S SDS S D S

1 2.10 ..... 5 2.30 0.77 ..... 0.36

20 3.68 0.44 0.13 0.1 7

5 2.39 0.75 0.63 0.25

..... .....

10 2.78 0.54 0.36 0.22

10 1.46 0.63 0.35 0.1 9

T = 37' C; Qr = 300 ml/min

Feed Flowrate E ffect o n Permeate Flu3

Q f (ml/min) FIUX (ml/min-ft2)

No SDS 50 mg/l 100 mg/l 200 mg/l SDS SDS SDS

300 1.37 0.61 0.35 0.21 100 0.38 0.14 0.09 0.036 300 1.22 0.58 0.31 0.23 500 1.99 1.01 0.67 0.50 750 2.51 1.51 1 .oo 0.72 500 1.42 0.91 0.71 0.50

T = 370 C; P = 10 psi

156

Oil Concentration Effect Dilution of 10% Dodeca ne-Water Mixture

Qf (ml/min) Flux (ml/min0ft2) 5% Oil 10% Oil

100 ..... 0.036 300 0.1 8 0.22 500 0.32 0.50 750 0.56 0.71

1250 1.15 ..... T = 370 C; P = 10 psi; 100 mg/l SDS

157

Appendix VI

Dodecane Experiments Cross Flow Module: 5% Dodecane

158

Feed Flowrate Effect o n Permeate Rate

Q f (ml/min) FIUX (ml/min-ft*) 50 mg/l 100 mg/l 200 mg/l

S D S S D S S D S 300 0.87 0.16 0.1 1 0.089 500 1.31 0.23 0.19 0.1 5 1000 3.63 1.12 0.59 0.50 2000 7.49 3.28 1.99 1.90 2400 9.59 4.73 2.62 2.04

No SDS

. T = 370 C; P = 10 psi

Zeta Potential Values

Zeta Potential (mV) No SDS 50 mg/l 100 mgll 200 mg/l

S D S S D S S D S Run 1 -55.1 -67.8 -66.5 -71.6

-58.3 -64.6 -66.1 -69.2 Ave. -56.7 -66.2 -66.3 -70.4

Run 2 ..... -68.2 -68.4 -69.9 ..... -62.2 -63.4 -67.2

Ave ..... -65.2 -65.9 -68.5

Overall Ave -56.7 -65.7 -66.1 -69.5

I

159

DroDlet S ize Data

~~

Size (p) O/O Total Oil Volume

No SDS 50 mg/l 100 mg/l 200 mg/l S D S S D S S D S

1-3 78.1 76.0 78.7 78.9 3-5 12.6 16.1 13.8 14.9 5-7 4.1 5.8 4.4 4.4

7-1 0 3.4 2.5 1.7 1.1 10-20 2.4 1.3 1.3 0.7 >20 0.33 0.07 0.09 0.03

I

160

Appendix VII:

AEAPTMS Coated Fibres Cross Flow Module: 5% Dodecane

161

Effect o f Feed Flowrate o n Permeate Flux

No SDS 50 mg/l 100 mg/l 200 mgll SDS SDS S D S

300 2.08 1.44 1.20 0.26 500 2.90 2.03 1.76 0.44

1000 5.34 3.1 2 2.78 1 .oo T = 370 C; P = 10 psi

162

Appendix Vlll

Correlation of the Data From the 5% Dodecane Experiments on the Uncoated Cross Flow Module

to Empirical Expressions for Coalescence Efficiency

c

163

EmDirical ExDressionS

Where,

Hughes and FOuldS: Eb = 1 - (l-oEs)" (ii)

Weber and Paddock: Es = ( ~ - 0 . 8 7 A ~ ) * ? (iii)

Es = coalescence efficiency of a single fibre Et, = coalescence efficiency for a bed of fibres

vdrp Re = Reynolds number = - CL

CJ = electrostatic interaction effect d n = 2( 1 -e)- df

d = bed depth e = bed porosity df = fibre diameter

dp = droplet diameter A = (2.022 - In(Re))-'

In our calculations it was assumed that the oil removal efficiency, ER was equivalent to the bed coalescence efficiency, Eb, and proportional to the single fibre efficiency E,. This assumption was made based on the following argument :

Hughes and Foulds(l986) defined the bed coalescence as,

1

164

where, Q, = oil permeate rate Qf = oil feed rate C, = feed oil concentration

The equation presented by Hughes and Foulds (Eq. (ii) above) can be simplified by using a binomial expansion since E,' values cc 1 for our experiments. This expansion yields,

Eb = 1 -( 1 -nE,' )

Where higher order terms are considered to be negligible; the above equation becomes,

Thus,

Reynolds numbers were calculated for feed flowrates ranging from 300 ml/min to 2400 mVmin assuming a flow area of 17.85 cm2. The value of n was calculated to be 30.83 assuming a porosity of 0.82, a bed depth of 3.6 cm and a fibre diameter of 0.0425 cm.

The empirical equations (i), (ii) and (iii) can be simplified as follows:

Lang m ui r: K ER a 2(2-Ln(Re)) ' K = f (r, n)

165

K' Hughes: ER a 2(2-Ln(Re)) ;

Weber: ER 01 K ' (A-0.87A3);

Plots of the relationships given above

K' = f (r, o,n)

K ' = f(r, n)

hould be linear if our data fits 1 i e empirical expressions. Table 10 summarizes the calculations for the 5% dodecane experiments.

Table 10 : Correlation of the Data From the 5% Dodecane-Water Mixture on the Uncoated Fibres to the Empirical Expressions for Coalescence Efficiency

Removal Efficiency, ER ( O h )

( A - 0 . 8 7 A 3 ) NO 50 mg/l 100 mg/l 200 mg/l 1

2( 2-Ln( Re)) S D S S D S S D S S D S

0.0758 0.1 48 5.1 0.9 0.64 0.52 0.0822 0.160 4.6 0.8 0.66 0.52 0.0928 0.1 79 6.3 2.0 1 .o 1.0 0.1064 0.204 6.5 2.9 1.7 1.7 0.1 107 0.21 1 7.0 3.4 1.9 1.5

the removal of oil from oil-water mixtures using selective oil ...selective filtration of oil across...

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