new techniques for continuous analysis in the pulp & paper ...8895/fulltext01.pdf · new...

New Techniques for Continuous

Chemical Analysis in the

Pulp & Paper Industry

Matthew Rice

Department of Chemistry Division of Analytical Chemistry

Royal Institute of Technology SE-100 44 Stockholm

Sweden

Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framlägges till granskning för avläggande av teknologie doktorsexamen, fredagen den 4 maj 2001, kl 10.00 i Kollegiesalen

Administrationsbyggnaden, Vallhallavägen 79, KTH, Stockholm. Avhandlingen försvaras på engelska.

ii

New Techniques for Continuous Chemical Analysis in the Pulp & Paper Industry PhD Thesis © Matthew Rice, 2001 ISBN 91-7283-063-8

Royal Institute of Technology Department of Chemistry Division of Analytical Chemistry SE-100 44 Stockholm Sweden

Printed by Universitetsservice US AB, Stockholm, 2001.

iii

An expert is a man who has made all the mistakes

which can be made, in a narrow field.

Niels Bohr (1885-1962)

This doctoral thesis is dedicated to the pursuit of a better

understanding of papermaking chemistry gained from new

analytical techniques for continuous chemical analysis.

v

Abstract

New Techniques for Continuous Chemical Analysis in the Pulp & Paper Industry Matthew Rice, Royal Institute of Technology, Department of Chemistry, Division of Analytical Chemistry, SE-100 44 Stockholm, Sweden.

This thesis presents some new techniques that were developed for continuous chemical analysis of a paper furnish. First, a general background is presented, covering topics from the origins of papermaking to present day. A short introduction to papermaking chemistry and a variety of presently available on-line chemical analysers and measurement strategies are also discussed.

A method is described for the continuous fractionation of a paper furnish containing coarse fibres (>10µm) in order to obtain a sample for analytical purposes (Paper I). A consistent sample, containing a representative fraction of the dissolved and colloidal substances (DCS) present in the bulk furnish, was achieved by preventing cake formation on a filter surface. A combination of turbulent flow above a membrane filter, while continuously withdrawing a relatively low sample volume, were key factors in the prevention of filter fouling.

For the continuous flow-extraction of DCS, a technique is described whereby the extracting solvent was injected at a high velocity into a continuous flow of analyte (Paper II). Comparison with conventional flow extraction showed an extraction enhancement of up to 9 times for colloidal triglycerides.

To achieve a continuous determination of chemical substances, a real-time fully automated colorimetric titration apparatus was developed (Paper III & IV). This was achieved by using a series of micro-machined mixing channels for the continuous flow of analyte, with a sequence of detection units and titrant addition points along the flowpath (Paper III). A fuzzy logic controller was implemented to continuously adapt for changes in the sample concentration, providing the possibility of titrating over two orders of magnitude in sample concentration with minimal loss of accuracy (Paper IV).

Also, a system is presented whereby the filtration apparatus (Paper I) is combined with the titration device (Paper III & IV) in order to continuously determine total charge (or colloidal charge) of a paper furnish in real-time (Paper V). This was achieved by utilising a back-titration approach and selected examples are presented showing the dynamic interactions between wood fibres and polyelectrolyte adsorption at various conditions of pH and polyelectrolyte molecular weight.

Finally, some suggestions for a more comprehensive wet-end chemical monitoring platform are discussed and the role of the present work in evaluated in this context.

Keywords: Chemical monitoring, continuous flow extraction, cross-flow filtration, dissolved and colloidal components, fuzzy-logic control, on-line system, pitch analysis, polyelectrolyte titration, process control, sample work-up, titrimetric analysis.

© Matthew Rice, 2001 ISBN 91-7283-063-8

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List of Publications

This thesis is based on the following publications, which are referred to in the text by their Roman numerals.

I. Rice, M., Roeraade, J. and Holmbom, B. “High-shear turbulent flow filtration for chemical monitoring of pulp and paper process waters” Nord. Pulp Pap. Res. J., 14(4) 1999, 292-299.

II. Rice, M., Roeraade, J. and Holmbom, B. “Continuous-flow extraction of colloidal components in aqueous samples” Anal. Chem. 69(17) 1999, 3565-3569.

III. Rice, M. and Roeraade, J. “Continuous titration device for process analytical applications” Anal. Chim. Acta (submitted 2001).

IV. Rice, M. and Roeraade, J. “Fuzzy-logic to fully automate end-point control of a continuous titrating apparatus for process analytical applications” Anal. Chim. Acta (submitted 2001).

V. Rice, M. and Roeraade, J. “Continuous filtration and titration apparatus for real time monitoring of polyelectrolyte concentration and cationic demand of a paper furnish” Nord. Pulp Pap. Res. J. (submitted 2001).

Reprints are published with the kind permission of the journals concerned.

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Contents

1. Introduction .............................................................................................................................1 2. Papermaking.............................................................................................................................3

2.1. A Historical Perspective ...............................................................................................3 2.2. Modern Papermaking: A Global Industry ................................................................5 2.3. Papermaking Chemistry................................................................................................7

2.3.1. Papermaking Fibres ...............................................................................................7 2.3.2. Wood Fibre Chemistry ..........................................................................................8 2.3.3. Chemical Additives ............................................................................................. 10 2.3.4. Detrimental Substances...................................................................................... 12

2.4. Current Trends & Problems ..................................................................................... 13 2.4.1. Recent Changes in the Papermaking Chemical Process ................................ 13 2.4.2. Recent Developments of On-Line Instrumentation...................................... 14

3. Sample Work-Up.................................................................................................................. 17 3.1. Primary Separation Techniques [Paper I]................................................................ 17

3.1.1. Centrifugation...................................................................................................... 18 3.1.2. Filtration ............................................................................................................... 18

3.2. Continuous Flow Extraction [Paper II] .................................................................. 21 4. Continuous Dynamic Titration .......................................................................................... 23

4.1. New Titration Device [Paper III]............................................................................. 23 4.1.1. General Principle & Construction .................................................................... 24 4.1.2. End-Point Determination .................................................................................. 26 4.1.3. System Characteristics ........................................................................................ 28

4.2. Automation of Titration Device [Paper IV] ........................................................... 29 4.2.1. Fuzzy Logic Controller....................................................................................... 29 4.2.2. Continuous Titration .......................................................................................... 33

4.3. LabVIEW® Program.................................................................................................. 34 5. Applications .......................................................................................................................... 35

5.1. Continuous Back Titration on a Paper Furnish [Paper V] ................................... 35 5.1.1. Polyelectrolyte titration ...................................................................................... 36 5.1.2. Experimental Results .......................................................................................... 39

6. Conclusions & Future Outlooks ........................................................................................ 43 7. Acknowledgements .............................................................................................................. 47 8. References ............................................................................................................................. 49

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List of Abbreviations

A-D analogue-digital CFE continuous flow extraction COD chemical oxygen demand DCS dissolved and colloidal substances FIA flow injection analysis FLC fuzzy logic controller GC gas chromatography GUI graphical user interface HPLC high performance liquid chromatography HVIFE high velocity injection flow extraction IR infra red LED light emitting diode LWC lightweight coated m.w. molecluar weight MSE mean squared error MTBE methyl tert-butyl ether NTU nephelometric turbidity unit oTB o-toluidine blue P&B paper and board PCMCIA personal computer memory card international association pDADMAC poly-(diallyldimethylammonium chloride) PTFE polytetrafluoroethylene (Teflon®) PVSA poly-(vinylsulfonic acid, sodium salt) SC super-calendered TOC total organic carbon TMP thermomechanical pulp UV ultra violet Vdc volts (direct current) vis visible (spectra)

xi

List of Symbols

a0 curve-fit coefficient a1 curve-fit coefficient a2 curve-fit coefficient A absorbance e error (end-point) i photocell number k cycle step K1 equilibrium constant K2 equilibrium constant N photocell calibration constant t time T transmittance V voltage (signal) V0 voltage (calibration constant) λ wavelength µ degree of membership

1

1. Introduction

he origin of chemical analysis dates back to the advent of chemistry, the first chemists requiring knowledge towards the identity and quantitative composition of

the materials with which they were working. In 1788, the Swedish chemist Torbern Olof Bergman published his De Analysi Aquarum (“On Water Analysis”), a comprehensive account introducing devised analytical methods for the analysis of mineral waters1-4. This was generally accredited as the birth of Analytical chemistry as a discipline5. Towards the end of the 19th century, new analytical techniques such as electrogravimetric analysis and photometers were introduced to complement the wealth of classical assays that were available at the time, however instrumental chemical analysis did not flourish until well into the 20th century5. Since the advent of the microprocessor during the 1960s, analytical chemistry has undergone a rapid growth, strongly influenced by advances in automation and miniaturization. The rapid transformation of classical to instrumental analysis has overseen the automation of many laborious analytical techniques, providing greater throughput, accuracy and reproducibility.

Parallel to the development of laboratory instrumental analysis, process analytical chemistry developed in response to the needs of industry to obtain real-time measurements. New analysis tools evolved to provide direct chemical and physical information describing the composition of a process stream, enabling optimisation and control to be implemented in real-time. Today, industrial processes ranging from petrochemical to biotechnology often rely upon process analytical measurements for such benefits as enhanced product and process quality management, environmental control and monitoring, and crisis alerting6-9.

More recently, the pulp and paper industry has also realised the essential benefits gained from a real-time monitoring of the wet-end chemistry. Primarily instigated from strengthening environmental legalisation and increasing product demands stipulated by the consumer, the past decades have seen substantial change with the chemistry of the papermaking process10-19. In addition, the variety of papermaking chemical additives is ever increasing to suit the specific requirements of the papermaker20-23. Such recent changes have emphasised the urgent requirement for a better understanding of the papermaking chemistry, and as a consequence, the necessity of more specific on-line chemical sensors and monitoring devices24-34.

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2 New Techniques for Continuous Chemical Analysis in the Pulp & Paper Industry

Prior to the mid 1980s, attempts to monitor and control the paper machine wet-end were still scarce35. Although computer-based online process control systems have been widely used in the pulp and paper industry, progress has been slow for wet-end chemistry system development (currently only pH, thickness, consistency and additive flow rate are routinely measured on-line)36-39. In order to develop computer based wet-end chemistry process control systems, new robust on-line sensors must be developed. Concentrating on the most important variables and their interrelationship is the key to improving performance on a paper machine. A wet-end chemistry process control system would reduce variability by detecting chemical parameters that influence paper quality at an early stage. A stable papermaking process is the basis for higher quality, lower costs, higher recycled content, substantially less waste, faster grade changes and the development of higher value-added products.

This thesis presents some new techniques that were developed with the aim of dedicated use in continuous “real-time” chemical analysis of a paper furnish. The dissolved and colloidal components of the furnish were the key constituents chosen for analysis as these materials are generally almost always detrimental in nature and it is clear that a better understanding of the dynamic interactions of such substances and their impact on the process is of great importance to the papermaker.

Initial work focuses on the development of suitable sample work-up procedures in order to continuously remove extraneous materials from a bulk paper furnish that are not required for subsequent analysis (Paper I), and to continuously extract colloidally dispersed pitch from an analyte (Paper II). Subsequently, a continuous “real-time” colorimetric titration apparatus was developed in order to allow a quantitative determination of the amount of chemical substances present in an analyte (Paper III & IV). The final work (Paper V) demonstrates a combination of the apparatus and techniques developed in papers I, III and IV, in order to perform a continuous colloidal titration directly on a paper furnish in real time. While the bulk of this work was carried out with the objective to be able to monitor the wet-end of the papermachine, it should also be possible to employ the apparatus and techniques developed for use in pulping and bleaching plant, as well as for effluent monitoring.

3

2. Papermaking

lthough modern papermaking is a highly mechanised process, the basic procedure has remained essentially unchanged for nearly two thousand years. Fibres, most

commonly obtained from wood nowadays, are first separated and wetted to produce a paper pulp or stock, then filtered on a woven screen to form a sheet, pressed to squeeze out most of the water, and finally dried to evaporate the remaining water. The dry sheet is often further compressed, coated and possibly impregnated with other substances to obtain the desired properties for its intended use.

2.1. A Historical Perspective40-48

From the dawn of civilization, the human race has had the need to communicate in the form of writing, whatever the physical medium. The first records of written language were clay pictographs used by the Sumerians as early as the fifth millennium BC. As written languages developed throughout other civilizations, many different materials were utilized to record the written word. Stone, clay, parchment, vellum, bark and tree leaves have all filled this role at one time or another.

The ancient Egyptians, as discovered in tombs dating back to 3500 BC, made the written word more portable with the advent of papyrus. Papyrus, the word paper is derived from, consists of fibrous layers from the reedy plant papyrus placed side-by-side and perpendicular in alternate layers. The structure was dampened, pressed and upon drying, the glue-like sap of the plant fastened the layers together.

Complete defibring, a central requirement in modern papermaking, did not occur with the manufacture of papyrus. The first account of papermaking was ca. AD 105 when Ts’ai Lun, an official attached to the imperial court of China, created a sheet of paper from the bark of the mulberry tree and other fibres made on a mold of bamboo strips. However, the discovery and art of papermaking remained in China for over 500 years.

From China, trade caravans imported paper to Japan around AD 610, Central Asia and Persia ca. 750. In its slow migration westward, the Moors brought paper to Europe with the first papermaking mill established in Spain around 1150. During the subsequent centuries, papermaking spread throughout most of the European countries.

A


However, it was not until the introduction of the printing press in 1450 by the German ironworker Johann Gutenberg that papermaking experienced significant growth - for the first time in history the written word could be mass-produced and paper was in demand.

The ever-increasing need for paper resulted in shortages of raw materials during the 17th and 18th centuries. Up to this period, European papermaking primarily utilized linen and cotton rag as raw materials and it was evident that a new and more abundant substitute was required. During the same period, attempts were made to further reduce the cost of paper by developing machinery to replace the hand-molding process. Nicholas-Louis Robert in France introduced the first papermaking machine in 1798 whereby a continuous flow of stock was delivered to a moving screen belt, delivering an unbroken sheet of wet paper to a pair of squeeze rolls. Roberts’s machine was not realized however until the British stationers Henry and Sealy Fourdrinier developed an improved version in 1803, followed two years later with a cylinder papermaking machine by the English Papermaker John Dickinson. A cheap and abundant raw material substitute was first achieved by a German weaver, Friedrich Gottlob Keller, in 1840 by the introduction of the groundwood pulp process – a mechanical method of defibring wood fibre. Hugh Bergess and Charles Watt developed the first chemical defibring process, soda pulp, in England approximately 10 years later. From these crude beginnings, modern papermaking evolved.

Figure 2.1. Water paper mill circa 1662. From Georg Andreæ Böckler, Theatrum Machinarum Novum(Nuremberg 1662)45.

Papermaking 5

2.2. Modern Papermaking: A Global Industry

At the turn of the new millennium, there were 8873 paper and paperboard (P&B) mills and 5946 wood pulp mills, producing a combined 316 million tons of P&B and 179 million tons of pulp annually, in over 103 countries spanning the globe49. Despite all the hype of how new technologies such as the “paperless office” and the Internet would strangle paper output, the last year of the 20th century saw a staggering increase of over 4.6 % in production on 1998 figures50-52. Figure 2.2 illustrates the production and consumption statistics representing the global market.

Annual growth is projected at the average rate of ca. 1.8 % for P&B and 1.5 % for pulp, with the most significant production increases expected for printing and writing papers53-55. The major driving force behind this growth is the enormous population of Asia. For example, the Peoples Republic of China, with a current population of over 1.2 billion has a per-capita consumption increase of ca. 1 kg per year56.

The past decades have also witnessed the introduction of huge and fast paper machines57,58. The fastest newsprint machine today was recorded in June 1999 at the Braviken paper mill (Sweden), with a record speed of 1 780 m.min-1 (107 km.hr-1)59. Every day this machine produces a continuous 8 m wide web long enough to cover a road from Norrkoping in Sweden to Gibraltar. An even faster machine has recently been installed at the Golbey paper mill in France, officially inaugurated on April 30, 199960. This newsprint machine is over 1.2 km in length, with a design speed of 1 800 m.min-1 and capacity of 330 000 ton.yr-1. The fastest tissue machines today operate up to 2 200 m.min-1, the speed limited by the drying capacity57.

Presently, over 400 specific types of papers are produced, indicating the diversity of the pulp and paper industry61. Each grade is based upon broad categories such as the type of fibre used, the basis weight of the

Europe

North

America Asia

Austra

lasia

Latin

America

Africa

Prod

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onsu

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(1 0

00 0

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ns)

20

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Global P&B Production/Consumption(1999)

Capacity

Production

Consumption

Europe

North

America Asia

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lasia

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Africa

Prod

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(1 0

00 0

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20

40

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Global Pulp Production/Consumption(1999)

Capacity

Production

Consumption

Figure 2.2. Global Paper & Board (P&B) and Pulp manufacturing and consumption statistics for 199949.


material and specific properties62. Some descriptions of commodity type P&B grades are provided in Table 2.1.

Table 2.1. Common paper and board grades with typical applications62.

SC papers - low-end magazines, cataloguesNewsprint - newspapers, telephone directories, inserts & flyers

Coated - magazines, catalogues, inserts, commercial printing

Coated - magazines, catalogues, brochures, books and manuals, timetablesUncoated - magazines, catalogues, books, envelopes, business forms, bibles

Special - copying, non-impact printing, low-volume paperbacks & hard cover books

Mechanical pulp

Chemical pulp (fine papers)

Printing & Writing Papers

coffee filter paper, cigarette filters, padding for meat packaging

Hygene products

Other

Tissue

Kitchen towels - as above however generally greater basis weightBathroom - smooth or embossed, unprinted or patterned, pure white, off-white or tinted

Facial tissue - low-basis weight, smooth (calendered), generally two-plyTable napkins - from small coffe cup napkins to large table napkins

White lined chipboard - as above except for food (due to recycled fibre content)Folding boxboard - packaging of cosmetics, cigarettes, pharmaceticals, food

Solid bleached board - chocolate and cigarettes (no problems with odor or taint)

CartonboardsPaperboards

Solid unbleached board - beverage (can) packaging (high tear strength and stiffness)Liquid Packaging board - milk and juice packaging

Corrugating medium - used in corrugated board (the "wave" between the two liners)Linerboard - linear for corrugated board

Container boards (cardboard boxes)

Wallpaper base - wallpapersCore based - cores for paper rolls (such as toilet paper)

Special boards

Plaster board- liners for gypsum board (a popular wall covering material)Others - book binding board, beer mat board (used in restaurants under beer glasses)

Specialtypapers

Transformer board - for uses where an insulation structure is required Cable paper - used for insulation between electrical cables

Electrical papers

furniture, decoration panels, floors (normally laminated onto a particle- or fibreboardAbsorbant lamination papers

to provide a good-looking resistant surface.

Laboaratory & diagnosis - filters, test papers (impregnated with chemical reagents)Automotive - air and oil filters

Household - coffee filters, tea bags, vacuum cleaner dust bag

Filter papers

strong papers (base paper for grinding belts and sandpaper), masking tape, photographicOther

paper, carbonless copy paper, map paper, security paper (banknotes), cigarette papers etc.

Papermaking 7

2.3. Papermaking Chemistry63-71

One of the objectives of papermaking chemistry is to gain a better understanding of the complex chemical and physio-chemical properties and interactions between the papermaking process and raw materials. An understanding of papermaking chemistry makes it possible to overcome a number of limitations and deficiencies inherent in the raw materials in order to provide specific paper property specifications, to meet a customer’s requirements more closely.

A paper furnish is the combination of all materials used to make paper, and generally consists of fibres, fibre fines, functional additives, chemical process aids and a variety of substances referred to as “detrimental substances” which interfere with the action of the chemical additives in an adverse way. A typical paper furnish is schematically illustrated in Figure 2.3.

2.3.1. Papermaking Fibres

Many different types of fibres have been used to make paper, including mineral, animal, synthetic, and vegetable fibres. Vegetable fibres are by far the most important and, of these, wood fibres find the greatest use.

Softwoods. Softwood fibres, which are approximately 3-7 mm long and 20-50 µm in diameter, have a high specific surface area and tend to flocculate when suspended in water, providing an ideal matrix for papermaking. Typical softwoods include conifers or cone bearing trees, such as pine, fur and cedar.

Figure 2.3. Typical components present in a paper furnish.

Hardwood fiberwidth = 22µm

length = 1200µm

Softwood fiberwidth = 36µm

length = 3500µm

Ti02

0,25µmCationic

Coagulant

Polyelectrolytes

Ground CaCO3

1 - 5µm

Precipitated CaCO3 0.2 - 1µm

Pitch0.1-10 µm


Hardwoods. Hardwood fibres, which are approximately 1-2 mm long and 10-40 µm in diameter, while shorter than softwood fibres, still have a strong tendency to flocculate when suspended in water. Unlike softwoods, hardwoods contain a large proportion of non-fibrous elements, including vessel segments, ray cells and parenchyma cells. Typical hardwoods include deciduous or annuals, such as birch and eucalyptus.

Other Fibres. Although virgin wood fibre is the foremost source of fibre for papermaking today, increasing quantities of secondary fibres (recycled wastepaper) and paperboard are being used to supplement the raw supply. Rag fibres are still used to produce paper of maximum strength, durability and performance and are used extensively for bank notes and security certificates. Other fibres include bagasse (residue from crushed sugarcane), bamboo, esparto, flax, hemp, jute, kenaf and straw, while some speciality papers are produced from synthetic fibres.

2.3.2. Wood Fibre Chemistry

Wood pulp fibres contain cellulose, hemicellulose, lignin as well as organic solvent-soluble resins and fatty materials (extractives) that adversely contribute to the papermaking process. The relative amounts of such materials are shown in Table 2.2.

Cellulose. Cellulose is the principal structural component of a wood fibre. Chemically, it is a polysaccharide composed of glucose sugar units bonded end-to-end through carbons 1 and 4 by an acetyl β-linkage (Figure 2.4). In native cellulose, the chains are typically 10 000 units long, however a shortening of the chains often occurs through hydrolysis and oxidation during the pulping and bleaching processes. Cellulose chains at the fibre surface contain hydroxyl groups that contribute to the fibre-to-fibre bonding in paper.

Hemicelluloses. Hemicelluloses are also polysaccharides, but differ from cellulose in that they contain a variety of sugars such as xylose, galactose, mannose and arabinose (Figure 2.4). They are often branched and have a lower degree of polymerisation compared to cellulose, typically containing 200-400 sugar units. Hemicelluloses interact strongly with water and contribute greatly to the swelling of wood fibres during beating, and promote the development of fibre-to-fibre bonding in paper. The carboxyl groups on xylan glucuronic acid groups are the principal source of negative fibre surface charge.

Table 2.2. Chemical composition (typical) of wood69.

Cellulose Hemicellulose Lignin Extractives

Softwood 42% 27% 28% 3%

Hardwood 44% 33% 20% 3%

Papermaking 9

Lignin. Lignin is concentrated in the cell walls of wood, holding the cellulose and hemicellulose wood fibres together, providing mechanical strength. It is a mixture of complex, polymeric compounds, phenylpropane units for the most part, containing many inter-unit ether and carbon-carbon bonds (Figure 2.4). Lignin has a network structure and a very high molecular weight. Residual lignin in papermaking fibres is detrimental to both the papermaking process as well as the finished product. It inhibits interfibre bonding, reduces swelling of the fibres in water, contributes to a rapid draining mat on the paper machine and produces finished products with poor aging properties (yellowing). The free phenolic groups on the surface contribute to fibre negative surface charge when they ionise.

Extractives. Wood extractives include a wide range of non-polymeric organic compounds that do not belong to the cell wall, typically fatty acids, resin acids, lignans, sitoststerols, steryl esters and triglycerides (Figure 2.5). Extractives play important roles in the life of a tree, providing food and energy storage and protection from microbial attack, and offer valuable by-products in the kraft pulping process. However, their effects on papermaking are always detrimental, contributing to such problems as foaming, deposits, desizing, strength losses, and decreased paper quality.

Figure 2.4. Chemical structures of primary materials present in a typical wood furnish.

Two glucose molecules

Prominant structures of softwood lignin64

OHOCH3

HOH2C C CH O

HO

HCOH

HC O

CH2OH

H3CO

H2C CHO

HC

HC

CH

CH2

O

HCOH

HC

CH2OH

OCH3

OCH3

O

HCOH

CH

CH2OH

OH

HCOH

CH

CH2OH

H3CO

O

C

C

H3CO

C

O

OCH3

O

O

HCOH

CH

CH2OH

H3CO

OCH3

HO

HCOH

HC

H2COH

OH3CO

OHOCH3

C

HC

H2COH

OHCOH

OH[O C]H3CO

CH

H2COH

O

HC

HC

H2COH

H3CO

OCH3

O

HC

CH

H2COH

H3COO

HCOH

CH

CH2OH

O

CH

HC

HC O[CH2OH]

H3CO

Structure of softwood aribinoglucoronoxylan. The structure is branced and contains xylose, glucuronicacid, and arabinose sugars.

OH

OH

OOH

O OOH

OHO

OHO

O

OH

O

OH

COOH

H3CO

O

OH

O

OHHOH2C

O

CH2OH

H

H OH

HOH

H

OH

OCH2OH

H

OH

HH

O

HO

HH

n

O

OOO


2.3.3. Chemical Additives

Papermaking chemical additives are utilised to satisfy two primary objectives, namely function and control, and are usually added to the stock at various locations prior to the head-box. Functional additives are used to achieve desired paper properties and product specifications, ultimately imposed by the customer, while control additives are used to enhance paper machine runnability, a key factor for mill profitability.

Functional additives, such as dyes, internal sizing agents, wet and/or dry strength agents and fillers etc. are often added to improve or impart certain properties to the finished paper product. For such additives to be effective and the process to run efficiently, these must be retained on the sheet. The majority of paper properties that are affected by wet end chemistry can be summarised in the following five categories:

1. Structural properties (basis weight, porosity, roughness, directionality, two-sidedness etc.);

2. Mechanical properties (tensile strength, bursting strength, tearing resistance, stiffness, folding endurance, internal bonding strength etc.);

3. Appearance properties (opacity, brightness, colour, gloss etc.);

4. Permanence properties (durability, chemical stability, colour reversion etc.);

5. Barrier and resistance properties (sizing).

Control additives, such as biocides, drainage aids, retention aids, pitch control agents and defoamers etc. are added to improve the papermaking process and do not directly affect

Figure 2.5. Naturally occurring extractives present in a typical wood furnish.

HO

COOH

Resin acid:abietic acid

Sterylester:sitosteryllinolat

Sterol:campesterol

Lignan:hydroxymatairesinol

Fatty acid:linoleic acid

O

O

O

O

O

O

O

O

COOH

O

OHO

HO

OH

O

O

Triglyceride:1,2-dilinolen-3-linoleninglycerol

Papermaking 11

the finished paper product or are even necessarily retained on the finished product. Some additives can also impart several effects at the same time, such as alum, which is used for sizing under acid conditions as well as a drainage aid. Table 2.3 is a list of some of the more common chemical additives used in papermaking with their intended effects.

FUNCTIONAL ADDITIVES fillers opacity, brightness and basis weight

- clay, calcium carbonate, talc, titanium dioxide dyes and brighteners imparts colour/brightness

- basic/acid dyes, direct dyes, fluorescent brightening agents internal size resistance to water penetration

- rosin sizing with alum, alkylketone dimer (AKD), alkyl succinic anhydride (ASA) dry strength additives increases hydrogen bonding

- polyacrylamides, starch, guar gum, (carboxy)methyl cellulose wet strength resins promotes covalent fibre-fibre bonding

- urea formaldehyde (UF), melamine-formaldehyde (MF), glyoxal-polyacrylamide copolymers, epoxidized polyamine-polyamide

specialty chemicals flame retardants, anti-tarnish chemicals etc.

CONTROL ADDITIVES keeps material on the sheet & increases water removal on the wire retention &

drainage aids - high charge density polyelectrolytes (polyethylenimine, poly(diallyldimethyl ammonium chloride)), high m.w. polyacrylamides, polyethyleneoxide, starches, gums, alum, aluminium polymers

formation aids promotes dispersion of fibres - water-soluble linear polyelectrolytes of ultra high m.w. (anionic polyacrylamides), guar gum

defoamers used to control entrapped air - alkylpolyesthers, polydimethylsiloxanes, oligomers of ethylene oxide (EO) or polypropylene oxide (PO), hydrocarbon or polyethylene waxes, fatty alcohols, fatty acids, fatty esters, ethylenebisstearamide (EBS)

deposit control agents used to control inorganic or organic deposits - chelants (EDTA, NTA, DTPA), polyphosphates, polyacrylates, phosphonates, non-ionic surfactants, polypropylene

biocides reduces slime from microorganisms - quaternary ammonium salts, methylene bis-thiocyanate, dibromonitrile- propionamide, glutaraldehyde, isothiazolin

other additives PH control agents, corrosion inhibitors

Table 2.3. Common papermaking chemical additives71-72.


2.3.4. Detrimental Substances

As the short circulation water system is closed, dissolved and colloidal substances (DCS) accumulate within the white water system according to the degree of retention of such material within the paper sheet itself73. These substances are introduced into the system at various production locations by raw materials, broke, carryover from the pulping process as well as hydrodynamic shear applied to the wood fibres during the various pumping stages, which transfers residual material from the wood fibres e.g. lignins, carbohydrates and extractives, to the water phase74-76. The organic materials with poor solubility often form colloidally dispersed pitch droplets, which are anionic.

At high levels, DCS interfere with the action of chemical additives, such as retention aids and wet strength resins, as well as effecting drainage, flocculation, and dryer efficiency77-80. Additionally, DCS accumulation contributes to pitch deposition problems on the papermaking machine and stock preparation equipment, which reduces runnability by causing breaks, down-time for “clean-ups”, as well as paper defects, such as holes, dirt and reduced paper strength81-83.

The increased use of recycled wastepaper, in combination with system closure and neutral papermaking, has also resulted in other types of detrimental substances, which originate from microbiological activity in the wet-end of a papermachine84-91. Uncontrolled growth of freshwater microorganisms frequently results in a multitude of undesired effects, such as biomass formation and slime deposition, which can impact paper machine runnability and cause problems with heat transfer and equipment life throughout the mill water system.

Papermaking 13

2.4. Current Trends & Problems

For most of last century, the majority of papermaking advances were focused upon the mechanics of the paper machines – the demand for larger and faster machines required challenging engineering solutions. Such advancements include high-speed papermaking (efficient sheet threading and drying), twin wire forming and hydraulic head-boxes to name just a few92-98. Surprisingly, the material the new machines were built around, the paper furnish, remained essentially unchanged, uncontrolled, and poorly understood99-103. Despite the significance wet-end chemistry (for this discussion, the wet end of a paper machine extends from the pulp mill to the machine press section) has on many aspects of paper production, this sector has suffered from poor understanding more than any other area of the papermaking system104.

2.4.1. Recent Changes in the Papermaking Chemical Process

Over the past decades changes in the papermaking chemical process have become necessary, primarily due to factors including environmental and associated legislative issues (e.g. reduced effluent discharge and water intake), the increasing cost of raw product (e.g. wood fibre) and the ever-increasing demands imposed by the customer10-19. It is evident that the “bottle neck” of the papermaking process is slowly shifting towards the wet-end of the paper machine. The most significant and recent papermaking trends that have a bearing upon the wet-end chemistry include:

• Conversion from acidic to neutral/alkaline conditions. Benefits include improved strength, energy conservation, decreased corrosion, less complex systems, enhanced permanence, and decreased costs105-108. A major aspect of this conversion is the replacement of alum109,110 and clay and titanium dioxide fillers to calcium carbonate111-113. In addition, new retention aids114,115, sizing116-121 and wet strength resins122,123 have become necessary. Problems reported include poor drainage, decreased dryer efficiency and sheet quality, with an increase of holes, web breaks and pitch deposit problems124-131.

• Increased use of recycled wastepaper. Recycled fibre is a cheap and readily available virgin fibre substitute132-134 - worldwide use is expected to reach 130 million tons, or 40 % of total P&B fibre, by 2001 (ca. 75 million tons or 32 % was used in 1988)135. However, recycled fibre consists of increased amounts of short fibre lengths (fines) and is often contaminated with de-inking chemicals, glue and other substances that result in increased deposit problems, such as slime and stickies136-139. In addition, the contaminants can lead to an increased consumption of chemical additives and require the addition of retention aids to maintain stable paper quality140-142.


• Closure of water systems. The advantages of closure include lower treatment costs, more stable operating conditions, increased system temperature and lower losses of fibres, fillers and chemicals143-146 – the Canadian Pulp and Paper Association reported that the amount of water required to make 1 ton of paper decreased by 50 % over the last 20 years147. However, higher concentrations of dissolved and colloidal materials accumulate within the system, leading to pitch deposit problems, as well as an increase of suspended solids, thermal energy build-up, microbial activity and corrosion148-152. Such problems have shown to affect both product quality and machine performance, and can result in an increased consumption and/or poor utilisation of chemical additives153-157.

• Increased chemical additive use. In order to meet the requirements of improved paper quality and higher productivity, taking into consideration the increased use of recycled fibres, closed water systems and the conversion to neutral/alkaline papermaking, new chemical strategies have become essential for efficient and effective management of the wet-end process158-160. A variety of chemical products are being increasingly used for such purposes as retention, distribution of fines, sizing, microbiological and deposit control20-23. Wet end chemicals, namely processing aids and functional additives, account for up to 20 % of world pulp and paper producers' total raw material expenditures161.

Such process modifications have emphasised the urgent requirement for a more fundamental understanding, control and optimisation of the wet-end chemistry in order to achieve profitability and long-term viability24-34. In addition to the above-mentioned problems, it is evident that the lack of wet-end monitoring also results in poor utilisation of chemical additives with over-dosage a common scenario162. It is estimated that between 3 % and 10 % of sub quality and up to 5 % of lost production, is directly related to poor wet end chemistry control163.

2.4.2. Recent Developments of On-Line Instrumentation

The first dissolved solids, or true “chemical measurement” introduced to the papermachine was pH, and today is by far the most frequent on-line chemical parameter monitored164-171. During the 1970s, research focused upon the development of on-line sensors for the pulping and bleaching plant, measuring such parameters as alkali, sulphur and peroxide concentration172-180. Commercial devices soon became available (e.g. the Technicon Auto-Analyzer181 and Kemotron Alkali Analyzer182) and were readily adopted by the papermaker due to the ease of relating the on-line measurement data to the chemistry of the process181-185. The 1980s saw the further development of on-line Kappa number sensors (e.g. the Kajaani Kappa-Bright analyser186, STFI OPTI-

Papermaking 15

Kappa187-190 and Honeywell Pulpstar191) that measured the dissolved lignin content of pulps and provided further control to the bleaching process172,186-193.

As far as the wet-end is concerned, the use of devices to monitor and control the chemistry prior to the mid 1980s were very scarce162. The first wet-end analysers developed were retention and stock consistency instrumentation – although not directly monitoring the wet-end chemistry, such sensors provide invaluable information on key parameters reflecting wet-end variability24,31,194-214. Commercial devices include the Chemtronics 4000 retention monitor211,212 and the Kajaani LC-100 and RM-200 consistency sensors201-208.

It was soon recognised that furnish charge determination in the wet-end of the papermachine was one of the most critical on-line measurements required – if the charge of detrimental substances can be measured, then efficient neutralisation would be possible, and the elimination of these substances is feasible215-221. Various charge measurement techniques, such as zeta-potential222-224 using microelectrophoresis225-227 or streaming potential228-233 detection and colloidal titration234-236 using streaming current237-244 detection have been developed, however the implementation of such devices for measurements on-line has not been straightforward245,246. Thus, besides commercial devices such as the Paper Chemistry Laboratory Zeta Data System (zeta potential)247-253, Mutek PCT10 (particle charge titrator)254-257 and the Innomatic Fibre Charge Analyzer (based on streaming potential technique)258, there is still a considerable need for further developments in this area.

The increase of microbiological activity in the wet-end of a papermachine has necessitated more stringent preventive, treatment and control measures, resulting in a demand for on-line sensors relating the microbiological pollution load of process-water flows (parameters as oxygen consumption rate, total organic carbon and bacteria count have been shown to be effective)259-263. Commercial on-line devices include the Nalco Quick Count263 and the Nalco Optical Fouling Monitor262.

While sensor developments and techniques have advanced considerably in recent years, due to the complexity of the wet-end system the number of devices that are readily applicable to the papermaker still remain few35. In fact, the majority of papermachines only utilise flow, pressure and temperature sensors for wet-end monitoring on a routine basis; such parameters relating more to the physical state of the process than that of the chemistry36-39. In an attempt to compensate for the lack of dedicated chemical sensors, considerable effort has been spent to utilise computer-based diagnostic systems for analysing wet-end control data using data from conventional sensors35,36.

Multivariate statistical time-series analysis and artificial neural networks are two computer-based systems currently being developed264-274. Such systems utilize adaptive


algorithms to build non-linear process models based on enormous historical data sets, to make use of all available wet-end data (both on-line and off-line), in order to find short- and long-term relationships between variations in process parameters275-277. The chemometrics program WEDGE® (Wet End Diagnostics Genius), developed by the Finnish Pulp Paper Research Institute, is an example of such a system and results indicate that significant cost savings and increased product consistencies may be achieved278-281.

Another approach to overcome the lack of wet-end chemical sensor development is the recent introduction of the WIC® (Wet-end Information Centre) developed by Rasio Chemicals163,282-285. This system is an at-line “robotic laboratory” that utilises a continuous sample work-up procedure and standard automated laboratory techniques, such as titration, to determine charge, alkalinity or calcium etc. on discrete samples163,286. It has been reported that the use of this system may help correlate wet-end parameters with papermachine performance and improve runnability, however at the expense of a substantial capital investment and installation procedure286,287.

In summary, it can be concluded that there still is a substantial need for improvements in wet-end chemical monitoring. However, it must be considered that the implementation of new sensor technology into the papermaking domain, a process that remains as much of a craft as a science, may be met with a general reluctance from mill operators that often have limited knowledge in papermaking chemistry. In an attempt to remove some of the hysteria behind the use of complex chemical analysis systems (both in terms of the apparatus itself and the derived results), the systems developed should be simple, robust and most importantly, reliable. In addition, the results should either relate directly to machine runnability or to the concentration of chemical substances that can be controlled. The work presented in this thesis introduces some new techniques and ideas that may help in achieving this goal.

17

3. Sample Work-Up

n order to analyse a complex matrix, sample work-up techniques are often required in order to remove, wherever possible, the materials that are not required to be analysed

and would otherwise interfere with the subsequent analysis and/or analysis equipment. Manual laboratory sample work-up techniques have standardised procedures such as sieving (separation based on particle size), filtration (complete solids removal), centrifugation, extraction and distillation etc., but these are generally not applicable to systems where continuous unattended operation is desired.

As depicted in Figure 2.3, a paper furnish is a heterogeneous mixture comprised of wood fibres, filler materials (clay, calcium carbonate etc.), various dissolved and colloidal substances (DCS) and chemical additives. As pointed out earlier (see section 2.3.4), it is of particular interest to monitor the levels of DCS at the wet-end, since their contribution to papermaking are generally detrimental in nature.

The sample work-up methodology developed in this study focuses on the separation of the DCS in order to facilitate the use of further analysis techniques, such as spectrophotometry, titration and/or gas chromatography etc.

3.1. Primary Separation Techniques [Paper I]

The primary aim of the initial sample workup was to either utilise or design an apparatus to produce a continuous stream of analytical sample not containing extraneous fibrous materials, while containing a representative fraction of the DCS present within the bulk furnish. The wood fibres do not contribute to the subsequent analysis of the DCS, and could tend to hinder measurements or even damage sensitive analysis equipment.

Wood fibre can be separated from the bulk furnish either on the basis of density or size. To exploit such physical properties, either centrifugation of filtration can be employed.

I


3.1.1. Centrifugation

Separation utilising centrifugation is achieved through differences in phase density between the solids and the liquid matrix, providing an enhanced field force over that provided by gravity to cause particle or liquid motion288. Batch operated centrifugal separation for the separation of DCS has shown to be a successful approach, whereby a laboratory centrifuge was utilised (1500 min-1, 30 min.)289,290.

For the development of a continuous on-line analytical system, several methods of continuous centrifugation were investigated. Basket and disc-stack centrifuges were found unsuitable due to the fact that these are solids retaining and have no cleaning mechanism other than the manual removal of such materials (this would not be acceptable in a mill environment). Hydrocyclones were also found unsuitable as the g-values obtained were too low and the wood fibres tended to upset the vortex formed within the apparatus, resulting in poor separation efficiency. A decanting centrifuge, which utilises an archimedean screw within the centrifuge bowl to continuously remove solids from the analyte, has shown to be successful for the removal of large wood fibres from a paper furnish291, however commercial decanting centrifuges are large, expensive and inconvenient.

3.1.2. Filtration

Separation utilising filtration is achieved through differences in the particle size of solids that are generally suspended in a liquid matrix288,292. However, traditional filtration techniques, whereby the solid-liquid mixture is directed towards a filter where the solids are retained, has shown to remove lipophilic droplets and colloidal particles in the fibre mat (cake) formed on the filter293.

Cross-flow filtration was initially investigated, a filtration method whereby the sample is usually pumped at a high velocity, tangential to the surface of a membrane filter, in order to limit cake formation294-298. The mechanism responsible for cake formation in cross-flow filtration has

Figure 3.1. Suggested mechanisms for filtration: (a) Cross-flow filtration with cake formation. (b) High shear turbulent flow filtration without cake formation (from Paper I).

a)

b)

colloidal droplets

fines

wood fibres

filtration medium

a)

b)

Sample Work-Up 19

been suggested as equilibrium between the forces acting tangential and across the filtration medium299,300. Cake formation reaches a stable thickness when the hydrodynamic forces of the bulk flow acting tangential to the filter overcome the forces acting to drag particles across the filtration medium (Figure 3.1(a)). It has been reported that a critical condition exists whereupon zero cake formation can be achieved299,301,302.

Initial experiments investigated this possibility; utilizing a cylindrical filtration cartridge containing a 22 µm woven polyester cloth. The paper furnish was pumped through the unit, tangential to the filtration medium, while the filtrate was withdrawn using a peristaltic pump303. The pressure on the filtrate line was monitored, as the presence of a vacuum, or a decrease in pressure from initial conditions, indicates a pressure drop across the filtration medium that is indicative of cake formation298. Experiments showed that it was possible to eliminate cake formation by significantly increasing the cross-flow velocity relative to the filtrate velocity. As previously mentioned, these findings appear to be in agreement with a balancing of the equilibrium between the shear forces acting at the filter surface (cross-flow velocity) with respect to the drag forces pulling fibres into the pores of the filter (filtrate flowrate)299,300. The filtration device was evaluated both in the laboratory and at a paper mill, where NTU (Nephelometric turbidity unit) measurements were routinely taken from the centrifuged filtrate and compared to the corresponding centrifuged raw sample (such NTU measurements have been shown to correlate well with the concentration of lipophilic extractives289). The filtration device was operated continuously for several days with no signs of sample deterioration and with good correlation between the NTU measurements (Figure 3.2), however final inspection of the filtration cloth revealed a partial blockage, observed as a random “stapling”304 of fibres between adjacent filter pores.

Figure 3.2. Turbidity measurement (NTU) vs. time for the 22 µm cross-flow filter (from Paper I).

time of experiment (h)

turb

idity

(NTU

)

40

35

30

25

200 10 20 30 40 50 60 70 80 90 100

centrifuged raw pulp slurrycentrifuged filtratemill shutdown


Alternative filtration mediums were evaluated in an attempt to prevent stapling and the possibility of eventual filter blockage. A filtration medium that was found most suitable for this application was a track etched polycarbonate membrane, notably Whatman Cyclopore™ and Millipore Isopore™ filters295. These filters are advantageous as they are very thin with no voids within the membrane for attachment of fibre or filler materials, and are available in a range of discrete pore sizes (12 – 0.05 µm). A new design was constructed to house the membrane, which were supplied as flat discs, whereby an impeller was utilised to generate a high shear turbulent flow

regime directly above the membrane which was glued to a flat perforated disc (Figure 3.3). This filter/filtration-mode is referred to as a “dynamic membrane filter” or “high-shear cross-flow filtration” respectively305-307 (Figure 3.1(b)).

Using a 10 µm membrane filter, laboratory experiments showed that continuous filtration, without cake formation, was possible for a high fibre (5 w/w %) with CaCO3 filler material (5 w/w %) furnish over a one week period of constant operation. An impeller speed fast enough to generate turbulence within the tank (ca. 750 rpm) in combination with a low sample flowrate (ca. 4 L.h-1) were key factors required for the prevention of a filter cake. Mill trials were also conducted and the samples analysed did not indicate any pitch retention. Visual inspection of the membrane indicated no sign of blockage, fibre stapling, or deterioration. Use of the same membrane for a period of intermittent continuous operation over several months (subsequent to Paper I) also showed no sign of blockage, fibre stapling or deterioration.

Unfortunately, utilising filtration for sample pre-treatment does not discriminate against filler materials that may also hinder measurements. Various post-filtration methods of filler removal were investigated, e.g. gravity settling. Some encouraging results were obtained, however the problem needs to be addressed in more detail.

Figure 3.3. Schematic of filtration device: (1)cylindrical tank; (2) baffle; (3) stirrer; (4) silicon rubbergasket; (5) base plate; (6) peristaltic pump; (7) vacuumgauge; (8) filter membrane; (9) filtrate chamber; (10)stirring device (from Paper I).

1

2

39

10

6

8

7

5

4

sampleinlet

samplereturn

analyticalsample

Sample Work-Up 21

3.2. Continuous Flow Extraction [Paper II]

The primary aim of this work was to develop a system for continuous-flow extraction of samples containing DCS present in an aqueous matrix. A laboratory procedure has been reported for the analysis of the colloidally dispersed lipophilic and water-soluble extractives present in a paper furnish, using a liquid-liquid extraction with methyl tert-butyl ether (MTBE) solvent of the centrifuged sample, followed by gas chromatography (GC) analysis308. Extraction with a non-polar solvent, such as MTBE or hexane, is required to transfer the hydrophobic colloidal material into the organic phase. In a subsequent step the solvent is evaporated to concentrate the sample prior to GC analysis. It was found that strong mechanical forces were required (i.e. vigorous shaking for several minutes) to provide sufficient contact between the colloidal droplets and the solvent during the extraction308.

Liquid-liquid extraction is a commonly used technique in the chemical laboratory to achieve pre-concentration, cleanup, or matrix exchange in order to facilitate and/or improve chemical analysis of a liquid sample. The principle and theory of liquid-liquid extraction is based upon heterogeneous equilibrium309, influenced by the diffusion of analyte through a phase boundary as well as concentration equalising within the continuous phases310. The rate of diffusion can be enhanced by increasing the interfacial surface area of the phases, such as distributing the phases into small droplets311 and/or by the addition of mechanical energy312 e.g. vigorous shaking.

Continuous flow extraction (CFE) was the first method evaluated313-318, whereby solvent was continuously pumped into the aqueous sample stream, forming a segmented flow, followed by a length of PTFE tubing (5 m) to provide a region of substantial contact between the segmented phases (Figure 3.4 (a)). Dissolved components (such as lignans) extracted well, however poor extraction yields were experienced for the more hydrophobic “colloid-forming”

Figure 3.4. (a) Conventional continuous flowextraction segmentor. (b) High-velocity injection flowextraction unit (from Paper II).

a)

b)

solvent

sample

solvent

sampleb)

a)


components (Table 3.1). The poor extraction yield of such components is likely to be related to an insufficient contact between the colloids and the extractant308,319.

In order to increase the mechanical energy added to the system and the interfacial surface area of the phases, the solvent was brought into forced contact with the sample by high-velocity injection through a 10 µm capillary nozzle (Figure 3.4 (b)). Greatly improved extraction yields of the hydrophobic components were obtained over traditional CFE, with an extraction enhancement of up to 9 times experienced with the triglycerides (Table 3.1). This new mode of continuous extraction was paraphrased “high velocity injection flow extraction” (HVIFE).

High-speed photographs were taken of the solvent inlet region to provide some insight into the mechanism of extraction utilising HVIFE. The solvent jet remained continuous until collision with the adjacent wall of the T-piece, after which the jet disintegrated resulting in a high degree of turbulence with a large amount of solvent droplets that varied considerably in size. Shortly after the solvent inlet region, coalescence of the solvent droplets occurred forming a segmented flow similar to that observed in CFE. Experiments were also undertaken whereby the downstream PTFE tube was omitted (used for extraction in CFE), the results showing there was no significant variation in extraction yields. From these findings, the conclusion is that HVIFE occurs almost instantaneously due to the intense contact achieved within the T-piece.

Choice of nozzle diameter, providing some control over the interfacial area of the resulting solvent droplets, was also investigated. The use of a 5 µm nozzle, while resulting in smaller solvent droplets, resulted in an excessive entrainment of solvent within the water phase, with an emulsion-like appearance, and with a slight decrease in extraction yields. The addition of chemicals to break this emulsion was also investigated, with promising results using multi-valent salts, notably ammonium dihydrogen phosphate, however the results from this approach remain inconclusive.

fatty acids

resin acids

lignans sito-

sterols steryl esters

triglyc-erides

manual extraction average 0.31 2.14 8.05 0.37 1.95 2.70std dev 7 4 2 3 2 4

CFE average 0.29 1.05 8.34 0.40 0.50 0.25std dev 69 25 6 18 8 15

yield (%) 94 49 104 108 26 9

HVIFE average 0.27 1.94 8.67 0.35 1.60 2.12std dev 14 3 2 7 2 3

yield (%) 87 91 108 95 82 79

Table 3.1. Average amount (mg.L-1), standard deviation (% of average) and yield (compared to manual extraction with vigorous shaking which is set at 100%) of thecomponent groups obtained by extraction of the sampleby manual extraction, CFE and HVIFE (10 µm i.d. nozzle) (from Paper II).

23

4. Continuous Dynamic Titration

itration is one of the most accurate and selective wet chemical analytical methods for the quantitative determination of soluble analytes320. Since the advent of the

microprocessor and the subsequent development of modern instrumental analysis, automating the laborious procedure of titration has been a key topic, with a rapid development of automatic titration devices321-323 and computerised data handling procedures324-326.

Many automated titration devices are commercially available today, both for use in the laboratory and apparatus for the semi-continuous monitoring of discrete samples and/or industrial process streams327-331. Considering the advancements made, it is surprising that there are only a few reported apparatus than can titrate continuously in real-time332,333. The majority of methods presented, while reported as being continuous, require a periodic sampling to provide a succession of discrete results. Continuous methods should ideally provide a readout or indication of the state of a streaming sample in real time.

4.1. New Titration Device [Paper III]

The primary aim of this part of the work was to develop a simple and robust device that could titrate a continuous stream of analytical sample in real time, based upon the well-established colorimetric neutralisation titration technique334-337. The titration apparatus developed is referred to as a “continuous dynamic titration” device, in order to distinguish the apparatus from other “continuous” titrators that do not titrate streaming samples in real-time.

In titrimetry, substances are quantified by measuring the volume of known concentration reagent solution that is required for a defined, complete chemical conversion with the substance being analysed. The addition of a reagent, referred to as the titrant, until one can recognise the end-point of the reaction (with the aid of an indicator that undergoes an easily recognisable colour change at this point) is known as colorimetric neutralization titration. Knowing the volume of sample and titrant added, the concentration of the titrant, and the stoichiometric relationship between the sample and the titrant, the concentration of the sample can be calculated.

T


Classical manual titrations are carried out using a burette to accurately deliver the titrant to a known volume of sample with indicator. Titrant is added drop-wise to the sample, mixing between additions, until a permanent colour change of the sample solution occurs, at which point the volume of titrant consumed is read from the burette. As with most analytical procedures, more than one titration is often required to reduce the chance of error. Consecutive titrations can be carried out faster by the continuous addition of titrant up to an amount just prior to the neutralisation volume (determined either by the first titration or more commonly a rough titration), followed by precise drop-wise additions of titrant up to the end-point.

4.1.1. General Principle & Construction

The general operating principle for the new titration apparatus presented here is, in a sense, analogous to the manual titration procedure described above. An initial rough titration, referred to as the “pre-titration”, is first performed by combining a continuous stream of sample with a continuous stream of titrant up to an amount just prior to the neutralisation volume, followed by further, considerably smaller, continuous stepwise additions of titrant to accurately resolve the end-point. A simplified schematic of the operating principle is given in Figure 4.1.

The experimental device developed in this work primarily consists of a machined Plexiglas board with channels for the passage of the working fluids and integrated electronic photometric detection cells for a monitoring of the end-point. Between each successive titrant addition and subsequent detection cell, static mixing channels (zigzag structures machined into the Plexiglas board) were employed to ensure a complete

Figure 4.1 Operating principle for the continuous dynamic titration device.

Pump 2pre-titrant

Pump 1sample

Pump 3titrant

sample

titrant +indicator

waste

11

6

78 9 10 11 12

1 2 3 4

5Abs

orba

nce

111 2 3 4 5 6 7 8 9 10 11 12

Continuous Dynamic Titration 25

reaction of sample, titrant and indicator338,339. The photometric cell construction was simplified using solid-state electronic components (light emitting diodes (LEDs) and photodiodes with on-chip amplifiers), mounted directly on the titration unit with no lenses, mirrors or sophisticated optical or mechanically moving parts340-347. The light paths were holes drilled through the Plexiglas plate (black Plexiglas was used), to provide a fixed light path and to reduce interference from internal reflections348. The Plexiglas board was sandwiched between two metal plates containing the electronic components required for the photometric cells, threaded holes to facilitate convenient connection of ancillary fluidics and gaskets to prevent leakage of the fluids. The titration unit was comprised of a sample inlet, 11 titrant addition points and 12 mixing channels with subsequent detection cells. The number of channels and addition points were arbitrarily chosen.

Three HPLC pumps were utilized to accurately deliver the fluids to the titration device; a sample pump (pump 1), pre-titrant pump (pump 2) and titrant pump (pump 3). To provide 11 titrant addition points using one pump, capillary tubes were utilized to split the titrant flow; the large pressure drop incurred across each narrow bore capillary (50 µm i.d.) ensured a consistent delivery of titrant with the same flowrate. The hardware control and data acquisition was fully computer automated using a program written in the LabVIEW® graphical programming language349, utilizing an A-D converter to control and acquire data from the detection cells and RS-232 communication to control the pumps. A simplified schematic of the continuous dynamic titration device is given in Figure 4.2.

Figure 4.2. Simplified schematic of the continuous dynamic titration device (from Paper IV).

waste

titrant +indicator

pump 1

pump 2

pump 3

sample

4

11

2

12

10

3

1

computercontrol &data handling

LEDs

photodetector

zigzag mixing channelplexiglas plate

capillary tube


The total combined flowrate of the titration apparatus (waste flowrate) was set at a constant value of 4.00 mL.min-1, as higher flowrates resulted in leakage of fluids (the upper flowrate limit was ca. 6 mL.min-1). In addition, the titrant flowrate (pump 3) was also kept constant, thus resulting in a constant combined flowrate of sample and pre-titrant (pump 1 + pump 2 = 4.00 - pump 3 = const.). Control of the pre-titration was established by varying the ratio of sample to pre-titrant, referred to as the “pre-titration ratio”.

4.1.2. End-Point Determination

In order to monitor the end-point of the titration, an indicator is added to the titrant that undergoes an easily recognisable colour change at this point. The titration unit was evaluated using a standard acid-base titration, using bromothymol blue (sodium salt) as the indicator which provides a distinct colour change from yellow (acidic) to blue (basic), with an absorbance maximum around 615 nm when basic. The absorbance spectrum for bromothymol blue is given in Figure 4.3.

Red-orange LEDs were utilized as the light source to monitor indicator colour change, providing a spectral emission centring around 615 nm and a bandwidth of 17 nm (shown in Figure 4.3). In addition, 950 nm LEDs were utilized as a reference light source since the indicator does not adsorb at this wavelength and therefore could be used to monitor changes in the samples turbidity350,351. Photodiodes were used to detect the intensity of both the 615 and 950 nm incident light that passed through each light path, the acquisition of the voltage outputs were synchronised with the LEDs which were switched on and off alternatively, providing a sampling rate of ca. 20 Hz (for both wavelengths).

Figure 4.3. Absorbance vs. wavelength spectra for the indicator dye bromothymol blue (sodium salt) inboth acid and basic form, and relative intensity vs. wavelength (spectral emission) of the 615 nm red-orange LED light source.

615 nm

Abso

rban

ce /

Rel

ativ

e In

tens

ity

200 300 400 500 600 700 800

wavelength (nm)

basic (blue)acidic (yellow) LED output


After de-multiplexing the voltage signals (Vi) obtained from each photodiode (i) for the wavelengths monitored (λ), transmittance ((Ti)λ) or absorbance ((Ai)λ) was calculated according to equations 4.1 and 4.2 respectively321,335,348.

( ) ii

ii N

VVT ⋅

=

λλ )( 0

Eqn. 4.1

( ) ii

ii N

VV

A ⋅

=

λλ

)(log 0 Eqn. 4.2

Prior to operation, it was necessary to calibrate each photocell for the voltage corresponding to maximum light transmittance (V0) and to standardise the photocell output values (Ni) in order to accommodate for minor variations between optical path length and/or LED/photodiode alignment321,348. The calibration procedure was computer automated, achieved by making the flow regime entirely yellow/acidic (for determining V0), or entirely blue/basic (for determining ki). For the present application using bromothymol blue indicator, absorbance values were used to determine the end point, producing a “S-shaped” set of data points.

To obtain a numerical value for the end-point, the twelve absorbance values corresponding to the indicator colour change were fitted to an inverse hyperbolic sine function, according to equation 4.3, where a0 = y scale offset (x-inflexion), a1 = x scale offset (y-inflexion), a2 = height (max-min), and slope = slope of the curve.

102 ))asinh(()( aslopeaxax,af +⋅−⋅= Eqn. 4.3

To fit the absorbance data to this function, the non-linear least-squares Levenberg-Marquardt method was utilized350,351, requiring both initial guess coefficients and the Jacobian to be specified. The initial guess coefficients were estimated using the following approximations: a0 = 6.5 (mid point of the twelve photocells); a1 = 50 % of max. absorbance; a2 = max. – min. absorbance. The Jacobian function (∂ƒ/∂A) was specified from the partial derivatives (equations 4.4 – 4.6).

))(1(

),(22

0

20 slopeax

slopeaaxfa ⋅−+

⋅−=∂∂ Eqn. 4.4

11

=∂∂ )a,x(fa

Eqn. 4.5

))asinh(( 02

slopeax)a,x(fa

⋅−=∂∂ Eqn. 4.6


Other methods of end-point determination were also evaluated, notably directly processing the raw data using techniques such as first and second derivatives, Gran’s plotting and Sevitsky-Golay curve smoothing etc., however the present method appeared superior for very small data sets352-362. A further advantage of using a general non-implicit mathematical expression to model the data is that different titration assays could be readily adopted so long as the general appearance of the end-point region is S-shaped. The slope of the curve (slope) was left as a variable in order to tune the fit for different titration conditions, while the model directly yields an approximation for the end-point (a0), reducing the requirement of further data processing. Figure 4.4 demonstrates absorbance data fitted with the asinh function.

4.1.3. System Characteristics

The titrant and pre-titrant concentration, as well as the titrant flowrate, are three parameters that must be chosen prior to operation. The concentration of pre-titrant determines the titratable concentration range, while the concentration of titrant and the titrant flowrate determines the sensitivity. To reduce the work of preparing standards, the pre-titrant and titrant were taken from the same solution. Two fixed parameters that effect the magnitude of the titratable concentration range is the maximum combined flowrate (4.00 mL.min-1) and the flowrate precision (minimum adjustable flowrate) of the pumps utilised. The effects of these parameters are discussed in detail in Paper IV.

From both theoretical and experimental work, it was shown that an acetic acid sample, using sodium hydroxide (10 mM) as the pre-titrant/titrant, could be titrated from ca. 0.5 mM to 150 mM (titrant flowrate was 0.5 mL.min-1), by adjusting the pre-titration ratio accordingly. At very low sample concentrations (<0.5 mM), insufficient absorbance data was available for a good curve fit (S-curve was far left, end-point < 3), while at high sample concentrations (>150 mM), minor flow instabilities, such as pulsed flow from the pump heads, resulted in chaotic end-points (the concentration range of sequential titrant addition was low compared to sample concentration fluctuations).

The accuracy and reproducibility of the titration apparatus was primarily dependent upon the characteristics of the three pumps utilised. The flowrate accuracy of the pumps was ± 2 % or ± 0.002 mL.min-1 (whichever is greater), with a reproducibility of ± 0.3 % or

Figure 4.4. Typical strong acid-base titration data withcorresponding fit (slope = 2000) (from Paper III).

80

60

40

20

0

-20

100

1 2 3 4 5 6 7 8 9 10 11 12

abso

rban

ce

photocell #


± 0.003 mL.min-1 (whichever is greater). Based on these specifications, it was calculated that the determination of the sample concentration has a subsequent error of ca. ± 4 % for pump flowrates > 0.1 mL.min-1 (lower flowrates introduces increased error as per the pump specifications).

The error incurred from the curve fit appeared to be low – for an acid/base system where indicator colour change was abrupt, the end point could be estimated within ± 0.5 photocell/titrant addition points. Intermediate absorbance values helped assist with the estimation of the end-point, however such points were not always present. In addition, the mean squared error (MSE) between the fitted values and the observed values was calculated in order to automatically monitor the “goodness of fit”. This was used to reject poorly fitted data, sometimes resulting from transient flowrate fluctuations, concentration spikes, or small air bubbles.

4.2. Automation of Titration Device [Paper IV]

The primary objective of this work was to develop and implement a control methodology in order to fully automate end-point detection and subsequent control. This was achieved using a fuzzy logic controller (FLC) to automatically maintain the pre-titration ratio based on the numerical value obtained for the end-point. A schematic of the controller implementation is provided in Figure 4.5.

An FLC is a problem solving, control system methodology using rule-based decision-making, replicating the decision based thought process used by human beings363-365. Linguistic control terms that would be used to manually describe how the system should be controlled were directly translated into the rule base of the FLC366-368.

4.2.1. Fuzzy Logic Controller369-373

The FLC is comprised of three main decision making stages; fuzzification, fuzzy inference and defuzzification. The input parameters used for the FLC are end-point error (e) and change of end-point error (∆e). To simplify the programming of the FLC, the input parameter error at each cycle step (k) is transformed into a range of –1 to 1 according to equation 4.7, where zero represents the centre point of the 12 photocells (set point). The change of error is the difference between the error at cycle step k and the previous step (k-1), according to equation 4.8.


The first stage of the FLC (fuzzification) is to translate the input parameters into linguistic terms. The input parameters were modified by adjectives to describe both the degree of error in relation to the set point (ek ⇒ left full, left, left center, center, right center, right, right full) and the change of error with time (∆e ⇒ fast neg(ative), medium neg, slow neg, zero, slow pos(itive), medium pos and fast pos). The input membership functions that were used to translate the input parameters are shown in Figure 4.6 (a) and (b). To avoid abrupt controller response, the membership functions were distributed symmetrically and in such a way that the overlap between adjacent functions was 50%.

111

2)1()( −×−= EndPointeerror k Eqn. 4.7

1)( −−=∆ kkk eeeerrorofchange Eqn. 4.8

The second stage of the FLC (fuzzy inference) generates consequent linguistic variables from a linguistic rule base that represents the control strategy, and imparts the most significant and primary influence upon the function of the FLC. The complete rule base is given in Table 4.1, and consists of 49 rules based on expert knowledge provided from experience gained through prior manual operation and control of the titration apparatus. A gradual variation was imposed to consecutive rules to ensure continuous and smooth controller response, in addition to a region of “no change” around the set-point region to allow minor changes in the sample concentration to be tolerated.

FuzzificationModule

DefuzzificationModule

Fuzzy InferenceEngine

FuzzyRule-Base

FuzzyLogicController

Pump 2

Pump 1

Pump Ratio Control(pump 1 + pump 2 = const.)

Process

Control Factor

(RS-232)

ErrorCalculator

(Vdc A/D card)ek ∆ek

outk

(ek= 0)

Existing ManualTitrator Control Software

Proposed Controller to AutomateEnd-Point / Pump Ratio Control

Observation

��

Response

��

End-Point Determination(1 ≤ end-point ≤ 12)

Set-Point

Figure 4.5. Schematic of the fuzzy-logic controller implementation (from Paper IV).


Fuzzy inference consists of two operations, firstly “aggregation”, an evaluation of the “IF” condition to each rule, and secondly “composition”, an evaluation of the “THEN” conclusion of each rule. Aggregation determines how adequately each rule describes the present situation, calculating a degree of truth for each “IF” condition specified in the rule base. For each possible pair of antecedent elements obtained from the fuzzy input sets (µ(e) and µ(∆e)), there is a corresponding output fuzzy set (µ(out)), defined by the rule base, and logically related by the operator “AND”. Since the linguistic terms utilised by fuzzy logic impart partial truth, the Boolean “AND” from conventional dual logic cannot be used. In the present work, the membership degree of any output fuzzy set is obtained from the minimum of the two truth-values for each rule premise, according to equation 4.9.

change of error (∆∆∆∆e) AND fast neg medium neg slow neg zero slow pos medium pos fast pos

left full large pos large pos medium pos medium pos small pos small pos no change

left large pos medium pos medium pos small pos small pos no change small neg

left center Medium pos medium pos small pos no change no change small neg small neg

center Medium pos small pos no change no change no change small neg medium neg

right center small pos small pos no change no change small neg medium neg medium neg

right small pos no change small neg small neg medium neg medium neg large neg

erro

r (e)

right full no change small neg small neg medium neg medium neg large neg large neg

Table 4.1. Complete linguistic "IF-THEN" rule base (from Paper IV).

Figure 4.6. Membership functions for FLC inputvariables, a) error (e), b) change of error (∆e), and c) theFLC output variable (out) (from Paper IV).

a)

b)

c)

fastneg zero fast

pos

1.00

0.75

0.50

0.25

0.00-1.0 -0.5 0.0 +0.5 +1.0

change of error (∆e)

Deg

ree

of M

embe

rshi

p µ(

∆e)

mediumneg

slowneg

slowpos

mediumpos

leftfull center right

centerrightfull

1.00

0.75

0.50

0.25

0.00-1.0 -0.5 0.0 +0.5 +1.0

error (e)

Deg

ree

of M

embe

rshi

p µ(

e)

leftcenterleft right

1.00

0.75

0.50

0.25

0.00-1.0 -0.5 0.0 +0.5 +1.0

smallpos

mediumpos

larg

e po

soutput (out)

Deg

ree

of M

embe

rshi

p µ(

out)

larg

e ne

g

smallneg

mediumneg

nochange


))e(),e(()out( kkk ∆= µµµ min Eqn. 4.9

Composition determines what action should be taken for the THEN condition, taking into consideration the differently weighted conclusions of the active rules. Since the rules given in the rule base are defined separately and are logically related by the operator “OR”, difficulties again arise, as the conventional Boolean OR operator cannot be used. In the present work, if any output fuzzy set is activated more than once, its final µ(out) is the maximum membership function of this element, according to equation 4.10, where i denotes each unique output fuzzy set combination.

),..)(),((max)( )( kkik outoutout µµµ ′′′= Eqn. 4.10

The third and final stage of the FLC (defuzzification) determines a crisp numerical value from a fuzzy linguistic conclusion that best represent the control strategy to be implemented. The relationship between the output linguistic values and the corresponding numerical value (out) is provided from a set of output membership functions, shown in Figure 4.6 (c). In order to derive a crisp numerical value, a numerical compromise is required to arrive at the “best” final conclusion as a result of combining the individual inferences. In the present work, the centre of maximum (CoM) defuzzification method is utilized, according to equation 4.11, where n is the number of unique output linguistic terms activated.

∑

∑

=

=

⋅= n

iik

n

iikik

k

out

outoutout

1)(

1)()(

)(

)()(median

µ

µ Eqn. 4.11

To transform the FLC output value (outk) into a real value suitable for controlling the pump ratio, a control factor was utilised, according to equations 4.12 and 4.13. The Control Factor corresponds to the maximum allowable pump flow-rate change per cycle step (k), and affects the stability of the controller, especially at higher sample concentrations. For the present application, a Control Factor of 0.05 was found suitable.

1)1 pump( )1 pump( −+⋅= kkk FactorControlout Eqn. 4.12

kk )1 pump()3 pump(00.4)2 pump( −−= Eqn. 4.13

A graphical example of how the FLC operates is provided in publication IV, Figure 8.


4.2.2. Continuous Titration

In order to evaluate the FLC, a continuous titration was performed on an acetic acid sample with 0.01 M sodium hydroxide as the titrant/pre-titrant. The sample concentration could be either kept constant or varied, either gradually or suddenly, the operator manually determining the rate and extent of the sample concentration change. The result of the continuous titration, performed over three successive days of ca. 8 hr continuous operation, is given in Figure 4.7. During this time interval, 49 297 consecutive titrations were logged.

The stability of the FLC and its ability to maintain accurate and fast control was outstanding, even for large and abrupt changes in concentration where the end-point suddenly shifted, requiring large changes in the pre-titration ratio in order to re-establish control. The results showed that it was possible to automatically titrate from ca. 0.5 to 150 mM, with good stability of the end-point and subsequent FLC at both extremes.

The titration apparatus showed no apparent memory effects, such as leaching of sample into the apparatus, possibly assisted by the fact the system operates at near neutral pH conditions (around the end-point), whereby no extremes of sample or reagent are in contact with the apparatus for extended periods of time. In addition, the sample and reagents are completely sealed from the environment, minimising titration errors from airborne contaminants such as CO2.

For the work undertaken with acid-base systems, the samples did not exhibit any turbidity and the 950 nm wavelength was monitored only as a reference for flow instabilities such as turbulence. Further work presented in Paper V does use dual wavelength detection to compensate for turbidity.

Con

cent

ratio

n (m

M)

time (hr)

1000

100

10

1

0.10 2 4 6 8 10 12 14 16 18 20 22 24

Figure 4.7. Continuous dynamic titration of acetic acid over 24 hrs (from Paper IV).


4.3. LabVIEW® Program

Data acquisition, control and calibration of the titration apparatus was fully computer automated using a program written in LabVIEW® 5.0 graphical programming language349, operated under Microsoft Windows® 98. A 200 MHz Pentium II laptop computer was used to run the program, using a 16 bit PCMCIA multifunction A-D converter to control and acquire data from the detection cells and RS-232 serial communication to control the pumps.

The sampling frequency was ca. 20 Hz (both LEDs) and was dependent upon the speed of the computer processor. To reduce the overwhelming amount of titration data collected, 25 absorbance data were collected and averaged prior to curve-fitting and subsequent control, providing an effective titration rate of ca. 0.4 Hz or ca. 1400 sequential titrations per hour.

A graphical user interface (GUI) was developed to display the status of the pump flowrates, curve-fit parameters (MSE Fit and inflexion), calibrated concentration, as well as a graphical plot of the titration data (with corresponding fit and end-point), and a concentration history profile. In addition, auxiliary parameters, such as temperature (ambient and sample) and pH (sample), were also displayed. The GUI is shown in Figure 4.8. The required output data was automatically logged to the hard disk and/or sent directly to other computers on a local network.

Figure 4.8. Graphical user interface of the titrator control program written in LabVIEW®.

35

5. Applications

he equipment and techniques presented thus far should be very suitable as a foundation for a comprehensive wet chemical analysis system. The filtration unit

performs an initial sample work-up, allowing the possibility to analyse the filtrate using analytical apparatus, such as a turbidity meter, or of particular interest to this work, the continuous titration apparatus. In addition, the continuous liquid extraction apparatus could be utilised to extract the colloidal components from the filtrate in order to perform spectrophotometry or automated GC analysis.

The continuous titration apparatus, since it utilises the well established technique of colorimetric titration, should have numerous possible applications for the pulp and paper industry, such as determination of total alkalinity, peroxide, ion concentration (chloride, magnesium, calcium), hydrogen sulphide and colloidal/furnish charge.

5.1. Continuous Back Titration on a Paper Furnish [Paper V]

The primary aim of the work described in paper V was to utilise the high shear cross flow filtration apparatus in combination with the continuous dynamic titration apparatus, in order to perform a continuous titrimetric analysis on a paper furnish.

Back titration involves adding excess reagent to a sample, e.g. a polyelectrolyte to a paper furnish, and titrating the excess reagent. A sample clean-up stage is required prior to analysis to remove extraneous material that would otherwise hinder or prevent the subsequent analysis; in the present application the high shear cross flow filtration apparatus was utilised to separate excess reagent from the wood fibres. Alternatively, excess reagent could be added directly to the filtrate line containing the DCS in order to specifically monitor these substances (see Figure 5.1). Using either strategy, it was possible to directly titrate the excess reagent contained in the filtrate using the continuous dynamic titration apparatus.

A paper furnish is a complex heterogeneous mixture of charged substances, that if out of control, can result in a myriad of process disorders and reduced product quality (see chapter 2). Complex chemical additives are often used to abate known problems, however poor utilisation and over dosage is a common scenario experienced162,163. The importance of furnish charge determination in the wet-end of a paper machine has been recognised as one of the most critical on-line measurements required215. Of particular

T


interest is the continuous charge determination of detrimental substances; if monitored, a more efficient neutralisation and subsequent elimination could be undertaken216-221. This work presents a method for a continuous quantitative determination of the total number of charges in a volume of paper furnish. The method utilised is colloidal titration, or polyelectrolyte back tiration, using a cationic dye to colorimetrically resolve the end-point374,375.

5.1.1. Polyelectrolyte titration

Polyelectrolyte titration involves titrating a sample with standard anionic or cationic polyelectrolyte that will react with the oppositely charged counterpart in the sample, forming a one-to-one charge association. For this work, the anionic polyelectrolyte used was poly(vinylsulfonic acid, sodium salt) (PVSA) with poly (diallyldimethylammonium chloride) (pDADMAC) as the cationic poly-electrolyte374,376-378. The indicator used to resolve the end-point was the cationic dye o-toluidine blue (oTB), which changes colour from blue with excess cationic polyelectrolyte, to pink with excess anionic polyelectrolyte, according to relationship below374.

Figure 5.1. Back titration procedure using the high-shear turbulent flow filtration device. (a) Excess polyelectrolyte added to the bulk furnish, containing fibres and DCS. (b) Excess polyelectrolyte added to the filtrate containing DCS only (from paper V).

a)

b)

fibre

colloidal material

cationic polyelectrolyte

filter (10 µm)

anionicpolyelectrolyte

cationicpolyelectrolyte+ K1

anionic / cationicpolyelectrolyte

complex

anionicpolyelectrolyte

cationicindicator+ K2

anionic polyelectrolyte /cationic indicator

complex

* where K1 >> K2

*

*

Applications 37

A necessary prerequisite is that K1 should be significantly greater than K2 to ensure the cationic polyelectrolyte complexes prior to the indicator and subsequent colour change. As illustrated in Figure 5.2, the sample initially contains an excess of cationic pDADMAC so the less charged oTB does not interact and the uncomplexed dye gives a blue colour to the solution. As anionic PVSA is added, the free pDADMAC binds until fully complexed, at which point oTB begins to complex. When oTB has fully complexed, the dye gives a pink colour to the solution.

Since it is difficult to identify the exact end-point of titration due to the fact an unknown amount of oTB must complex with PVSA prior to any significant colour change occurs, the titration of oTB must be included within the calibration procedure377. A charge balance on the polyelectrolyte/oTB system is given in equation 5.1.

)(

conc.PVSA

).(indicator

titratetoPVSA volume

).(sample


)(conc.

pDADMAC

)(added

pDADMACvolume

111

11

−−−

−−

×

−=×mEq.LminmLminmLmEq.LmL.min

Eqn. 5.1

In order to use the continuous dynamic titration apparatus, the LED light source was modified according to the absorbance spectrum of the oTB indicator (626 nm primary and 525 nm reference), in addition to subtracting the reference from primary transmittance values to compensate for turbidity. Transmittance values were used in order to preserve the S-shape of the titration curve, preventing the requirement of modifying the fitting algorithm. A further amendment was an additional pump for dosing the indicator, since oTB participates in the titration. The filtrate stream from the high shear turbulent flow filtration apparatus, after a further dilution or reagent addition stage, was pumped to the sample inlet of the titration apparatus via a 7 µm filter. This filter was included to safeguard the titration apparatus from particulate material that could cause a clogging within the mixing channels. A schematic of the experimental configuration is given in Figure 5.3.

Figure 5.2. Colorimetric polyelectrolyte titration mechanism. Continual addition of PVSA (titrant) topDADMAC (sample) using oTB as the end-point indicator (from Paper V).

PVSA(titrant)

o-toluidine blue(indicator)

indicatorcomplex

excessPVSA

polyelectrolytecomplex

indicatorcomplex

polyelectrolytecomplex

BLUE PINK

IndicatorIndicator

Indicator

IndicatorIndicatorIndicator

Indicator

excess pDADMAC

(back titrant)


The system was calibrated by prepar-ing a pDADMAC standard that was used to determine the concentration of the PVSA titrant/pre-titrant. Rearranging equation 5.1, the slope of the calibration curve (known concentration of pDADMAC plotted against the corrected flowrate of PVSA divided by the flowrate of pDADMAC) provides the concentra-tion of the unknown PVSA titrant, according to equation 5.2. Figure 5.4 shows such a calibration, using 9 consecutive concentrations of pDADMAC from 0.05 to 5 mEq.L-1. The fitted calibration curve was linear with a slope of 0.359 (mEq.L-1), the proportion of variation in the data (r2) was 0.99994.

)(conc.PVSA

).(added

pDADMACvolume

).(indicator


).(sample titrateto

PVSA volume

)(conc.

pDADMAC

1

1

11

1 −

−

−−

−×

−

=mEq.L

minmL

minmLminmL

mEq.L

Eqn. 5.2

10

pDAD

MAC

con

c.(m

Eq.L

-1) 1

0.1

0.01

[PVSA/pDADMAC] polyelectrolyte flowrates

0.01 0.1 1 10

y = 0.35944xR2 = 0.99994

Figure 5.4. Manual calibration using high m.w. pDADMAC. oTB conc. was ca. 6.25×10-6 M. Solid linerepresents corrected calibration taking oTB participation into account. The dashed line representsuncorrected calibration (volume PVSA required to titrate oTB not included) (from Paper V).

Figure 5.3. Simplified schematic of the back-titrationapparatus for the continuous charge determination of apaper furnish (from Paper V).

waste

waste

continuous dynamictitration apparatus

pre-titrant

analyte

indicator

titrant

HPLCpumps

pH

high shear turbulentflow filtrationapparatus

peristalticpump

water / reagent

pH electrode

PVSA

oTB

pDADMAC

filter

Applications 39

5.1.2. Experimental Results

In order to demonstrate the usefulness of a continuous polyelectrolyte titration, three selected examples from paper V are presented. The first two examples utilise the filtration tank in batch mode (as a stirred tank only) in order to monitor polyelectrolyte adsorption kinetics and phenomena. Such experiments are valuable for determining the effectiveness of different polyelectrolytes for differing paper furnish conditions prior to deployment in the actual process. The third example demonstrates an on-line monitoring of polyelectrolyte/DCS content within a paper furnish. The experiment demonstrates the potential of the apparatus for continuous charge or cationic demand determination on a raw furnish, such as in a paper mill.

i) Polyelectrolyte adsorption: different molecular weight

Due to the porosity of cellulose fibres, polyelectrolyte m.w. has a significant effect upon the amount of polyelectrolyte adsorbed. It is expected that low m.w. polyelectrolytes will penetrate the fibre pores, essentially covering the entire surface both inside and out, while higher m.w. polyelectrolytes may not significantly penetrate the fibre pores, providing a better indication of the fibre surface charge378-381.

Figure 5.5 shows the results of the effect of polyelectrolyte adsorption, using the same concentration although differing m.w. solutions of the same polyelectrolyte (pDADMAC). pDADMAC was added to the tank containing a 0.5 wt % pulp furnish while the filtrate was continuously titrated. Temperature, pH and salt concentration were kept constant (same raw furnish used).

Figure 5.5. Polyelectrolyte adsorption isotherms for different m.w. pDADMAC. (1) very low (5 000-20 000); (2) low (100 000-200 000); (3) medium (200 000-300 000); (4) high (400 000-500 000). 0.5 wt %pulp furnish, pH = 6.0±0.1. Ca. 800 consecutive titrations (from Paper V).

6.00 5 10 15 20 25 30 35 40 45 50

8.0

10.0

12.0

14.0

16.0

18.0

20.0

Abso

rbed

pD

ADM

AC (µ

Eq.g

-1)

time (min)

(1)

(2)

(3)(4)


From Figure 5.5, polyelectrolyte adsorption of very low m.w. pDADMAC was almost twice that of high m.w. under the same conditions. In addition, the absorption of very low m.w. pDADMAC showed a considerable time dependency, possibly related to the kinetics of polyelectrolyte transport into the inner fibre pores. The initial time delay was a result of both the system dead time (time for tank filtrate to reach the titration unit) and controller lag (time taken for the FLC to establish the initial end-point location), neither affecting the subsequent accuracy of the titrations.

ii) Polyelectrolyte adsorption: influence of pH

The pH of a paper furnish has profound influence upon the ionisation state of functional groups on the fibre surface which directly influences fibre charge379,382. For example, at pH values below ca. 2.75, most of the carboxyl groups are protonated and the charge becomes essentially zero, while at more neutral pH conditions the majority of carboxyl groups are ionised and the surface charge is more negative. Additionally, at pH values above ca. 9, phenolic OH groups become ionised, further contributing to a greater negative surface charge67.

Figure 5.6 shows the results of the effect of polyelectrolyte adsorption while continuously increasing the pH from 4 to 10 by a continuous addition of sodium hydroxide. From Figure 5.6, the greatest change in the amount of absorbed polyelectrolyte, or change in surface charge, was between pH 4-6, corresponding to the ionisation of the carboxyl groups present on the fibre’s surface. The spikes in the titrated values were related to the FLC, each corresponding to a change in the pre-titration ratio, and were transient in nature

Figure 5.6. Polyelectrolyte adsorption with respect to pH value. Constant addition of 0.02 M NaOH (3 mL.min-1) to 5 L of a 0.5 wt % pulp furnish with high m.w. pDADMAC (0.250 mEq.L-1). Ca. 800 consecutive titrations (from Paper V).

4

Abso

rbed

pD

ADM

AC

(µEq

.g-1)

6.0

7.0

8.0

9.0

10.0

11.0

12.0

0

150

25

50

75

100

125

NaO

H 0

.02M

(mL)

0

150

25

50

75

100

125

1095 6 7 8

pH

Absorbed pDADMAC

NaOH added

Applications 41

iii) Continuous polyelectrolyte titration

The continuous on-line monitoring of furnish charge and/or cationic demand is of great interest to the papermaker. The continuous filtration device was designed to operate on a continuous stream of pulp and therefore should be ideally suited for direct measurements in combination with the titration apparatus. However, it must be taken into consideration that a furnish charge could range from both anionic (e.g. presence of anionic colloidal components) to cationic (e.g. presence of cationic polyelectrolyte additives), and this must be taken into consideration prior to configuring the back-titration apparatus.

To allow both anionic and cationic samples to be continuously monitored, additional cationic polyelectrolyte (0.500 mEq.L-1 pDADMAC) was added continuously to the filtrate line to ensure the combined flowrate remained cationic, even for anionic samples. The additional pDADMAC is depicted in Figure 5.3 as the “reagent” stream, which was added at the same flow-rate as the tank filtrate. Using this configuration, a neutral sample (deionised water) titrated as 0.250 mEq.L-1. Adjusting for this “zero” offset, the system could titrate from –0.250 mEq.L-1 (anionic) to ca. +0.250 mEq.L-1 (cationic) (the range of the titration unit was calibrated from 0 - 0.500 mEq.L-1 pDADMAC, see publication V, Figure 11(a) for details).

Figure 5.7 shows the results of a continuous titration performed over a ca. 9 hr period. During this interval, various random concentrations and volumes of PVSA, pDADMAC or water were either pumped or manually added to the filtration tank in order to provide a sample with continuously varying charge characteristics (both positive and negative).

Figure 5.7. Continuous polyelectrolyte/DCS titration. Continuous run over 9hr period. Randomamounts of PVSA and pDADMAC were manually added (or pumped) into the tank (with a ca. 0.5 wt % pulp furnish) during this period. Ca. 9 000 consecutive titrations (from Paper V).

0.00

+0.25

-0.250 1 2 3 4 5 6 7 8 9

time (hr)

Con

cent

ratio

n (m

Eq.L

-1)


While the actual charge in the filtration tank was unknown for the duration of the experiment, the calibrated results from the titration apparatus followed the expected trends.

The data presented in Figure 5.7 would hopefully never represent a “real life” situation in a paper mill, however the sudden drastic changes demonstrate the robustness of the titration apparatus and the fuzzy control used for determining the location of the end-point. Such a charge monitoring could be useful in a mill environment for the control of detrimental substances through the addition of polyelectrolyte as a charge neutralising agent, and to help optimise the most efficient dosage of polyelectrolyte required.

43

6. Conclusions & Future Outlooks

he work presented in this thesis can be considered as a possible platform for a continuous wet chemical analysis system and was initiated from the clear demand

for process analytical instrumentation for use in the pulp and paper industry. The work consists of two continuous sample work-up techniques; a continuous filtration apparatus that was successful for removing extraneous wood fibres from the dissolved and colloidal components required for subsequent analysis, and a liquid-extraction apparatus that continuously extracts hydrophobic components from a centrifuged paper furnish sample, providing a significantly greater yield than obtainable from conventional flow extraction techniques (over nine times extraction yield for triglycerides). A continuous real-time colorimetric titration apparatus was developed in order to automate the procedure of titration – providing virtually continuous and adaptive titrations with a greatly increased dynamic titration range compared to existing titration techniques (can titrate over two orders of magnitude of sample concentration). In order to demonstrate the potential for a wet chemical analysis system, whereby a paper furnish could be directly chemically monitored in real time, the filtration device was combined with the titration apparatus, utilizing a back-titration approach. The example explored was continuous charge titration (by choosing the relevant reagents and indicator for the titration apparatus), and it was possible to continuously monitor either total charge or colloidal charge directly on a paper furnish in real-time.

As mentioned previously, the work undertaken for this thesis forms a basic platform for a wet chemical analysis system. As schematically depicted in Figure 6.1, by combining some of the techniques and apparatus developed in this work with existing sensor and automated laboratory techniques, it should be feasible to design a powerful analysis system, that could be tailor made to the papermakers individual needs.

The filtration apparatus, or primary sample work-up, is an essential operation where the presence of wood fibres does not contribute to subsequent analysis as the presence of such extraneous material may result in hindrance or even fouling of sensitive analysis equipment. Measurements that could be made directly on the filtrate include electrochemical measurements such as pH and conductivity, which can be used for a monitoring of effective alkali383,384, corrosion385,386 and foam control387. In addition, direct spectrophotometic methods can be used to monitor turbidity (which has shown to correlate well with the concentration of lipophilic extractives289) and for the

T


determination of Kappa number (measurements in the UV range have been shown useful for determining the concentration of lignin388-393).

Figure 6.1. A possible future platform for continuous analysis.

Sample clean-uphigh shear crossflow filtration

Liquid extractionhigh velocity injectionflow extraction

Titrimetric analysiscontinuous dynamictitration

Paper mill process water

organic phase

water phase(not used)

Spectrophotometricmethods • vis •

UV •

IR

Ion selectiveelectrodes

direct

indirect

addition ofa reagent

Pitch dropletcounter

ElectrochemicalpH, conductivity

alkalinty/acidity(Paper III&IV), ammonia413, calcium414,415, chemical oxygen demand417,418, chlorine416, colloidal charge(Paper V), Kappa # (lignin)172,428, magnesium419, starch427, sulfur/sulfide172,420-426

Colloidally dispersed pitch droplet size distribution and relative amount438-441

gas chromatography: sulfur394-395, lignin396-398, volatile organic compounds and extractives394,399-412

turbidity (correlates to lipophilic extractives)289

Kappa # (lignin absorbes in UV region)388-393

ammonia429, chemical oxygen demand430, chlorine431, hydrogen peroxide432, nitrogen433, pitch434, phosphates435, resin acids404, silicon436, sulfur437

aluminium443-450, ammonia451, calcium452, chlorine453-457, nitrate451,458, sodium459-461 and sulfur454,462-469

usefull for a monitoring of: effective alkali383,384, corrosion385,386 and for foam control387

Conclusions & Future Outlooks 45

The continuous flow extraction apparatus provides a further sample work-up that has potential for further development, particularly suited as a sample work-up for gas chromatography (GC) or high-performance liquid chromatography (HPLC) systems. If further sample manipulation is required (such as chemical derivatisation), a robotic sample preparation station could be employed. GC has been a used in the laboratory to determine such wood chemistry parameters as sulphur394,395, lignin396-398 (using pyrolysis), volatile organic compounds and extractives394,399-412 (the more hydrophobic extractives, such as triglycerides, have been shown to contribute to process disorders and poor runnability). GC could be feasible in an on-line monitoring set-up where specific and detailed analysis are sought, however the increased specificity obtainable must be weighted against the often-greater complexity of the apparatus.

The continuous dynamic titration apparatus has the potential to perform numerous on-line measurements on the wet-end of the papermachine and bleaching plant by simply choosing a suitable reagent (or reagents) and corresponding indicator required for the colorimetric determination required. Other than the colorimetric measurements presented in this thesis (alkalinity/acidity and colloidal charge), numerous laboratory methods with applications for papermaking chemistry have been established and include ammonia413, calcium414,415, chlorine416, chemical oxygen demand417,418, magnesium419, sulphur/sulphide172,420-426, starch427 and lignin172,428 determinations. It should be possible to adapt such methods to the continuous dynamic titration apparatus with only minor modifications to the titrator hardware and control software.

Another continuous analytical technique is colorimetric determination using spectrophotometic detection (after the addition of a chemical reagent that can be colorimetrically determined – similar to colorimetric titration although less sensitive). Such measurements that have potential for papermaking chemistry include methods for the determination of ammonia429, chemical oxygen demand430, chlorine431, hydrogen peroxide432, nitrogen433, pitch434, phosphates435, resin acids404, silicon436 and sulphur437.

The filtration apparatus would also be particularly suited as a sample work-up stage in conjunction with an optical particle counting device to determine the concentration and size distribution of colloidally dispersed pitch438-441. Such a measurement would greatly assist in developing cause and effect relationships between chemical parameters (such as pH and colloidal charge) with pitch deposition problems, in addition to assisting the implementation of effective pitch control strategies (e.g. addition of pitch scavengers such as low m.w. polyelectrolytes).

A further area of sensor development for papermaking chemistry is the implementation of ion-selective electrodes. Such electrodes are beneficial in that they are in-line and non-intrusive (they do not destroy the sample and introduce negligible contamination) and


can often be placed directly into the paper furnish. A good calibration is critical for obtaining meaningful results, which is not a trivial issue with such sensors442. Possible applications include the determination of aluminium443-450, ammonia451, calcium452, chlorine453-457, nitrate451,458, sodium459-461 and sulphur454,462-469. Mainly due to the mentioned calibration problems, in some instances, the use of the continuous titration apparatus may be more reliable.

While the focus of this work deals with dissolved and colloidal substances in the wet-end of the paper machine, the same techniques and apparatus could be also adopted for a continuous monitoring of substances in the pulp and bleach plant, as well for effluent and emission monitoring. The portability and ease of changing the sampling location for the filtration unit is beneficial as one unit could be used to sequentially monitor various locations on the papermachine e.g. machine chest, headbox, white water etc. or before and after chemical addition points. This would provide an invaluable diagnostic tool for tracking the source of process disorders and instabilities, as well as for optimising chemical dosage. In addition, the apparatus can be utilised off-line in laboratory environments for conducting dynamic batch experiments e.g. the experiments presented in paper V on polyelectrolyte titration.

In summary, I believe that this thesis has presented a number of new approaches for the on-line and continuous monitoring of chemical parameters with applications for the pulp and paper industry. It is my hope that further development of such an analysis platform will contribute significantly to a better understanding of the complex chemical and physio-chemical properties and interactions between the papermaking process and raw materials.

47

7. Acknowledgements

his work has been carried out at the Department of Chemistry, Division of Analytical Chemistry, Royal Institute of Technology, Stockholm, Sweden. This

thesis would not have come about if it were not for the help of the following people, to whom I would like to express my deep gratitude.

• Prof. Johan Roeraade my principal supervisor, for his support, guidance and unparalleled enthusiasm. I appreciate the freedom he provided, allowing me to develop and mature without (too many) constraints.

• Prof. Bjarne Holmbom – for his expertise and many good suggestions.

• Prof. Per Claesson and Dr. Andra Dèdinaitè, for their invaluable help and wealth of enthusiasm with surface chemistry.

• Dr. Johnny Gustafsson for his assistance with the laser fluorescence photography – I think we broke the world speed record in photography!

• Jarl Hemming and Ralf Degerth for help with the GC analysis.

• Dr. Agne Swerin for providing me with mill samples and kappa analysis.

• Dr. James Crawford – a true friend. Thanks for all your help and support, both academic and otherwise.

• Mark Tazewell, thanks for helping me out one long night with the titrator!

• Dr. Paul Dickman, for his help with statistics and introducing me to his “little bit drunk… but Ok” Finnish colleague.

• Mark Booth, for accompanying me on a voyage into the remote depths of the Finnish forests - a mill trial I certainly will never forget.

• Isabella Gruenauer – my friend and chauffer on many an occasion.

• All the people at the department, both present and past… thanks for putting up with me and listening to all my problems!

• To all my friends… thanks for all the laughs and good times!

• My dear mother… I know I was only supposed to be in Sweden for one and a half years… I love you very much and know you understand (although I am still not sure you know what it is I am doing here!).

• To my beloved Catha. Your love and kindness has brought out the best in me.

T

49

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