III

PRODUCTION OF MIXED-MATRIX MEMBRANE

(PSF/ZEOLITE) FOR CO2 / N2 SEPARATION: SCREENING STUDY OF POLYMER

CONCENTRATION

CHEN CHEE LEK

Thesis submitted in partial fulfilment of the requirements for the award of the degree of

Bachelor of Chemical Engineering (Gas Technology)

Faculty of Chemical & Natural Resources Engineering UNIVERSITI MALAYSIA PAHANG

JANUARY 2014

©CHEN CHEE LEK (2014)

VIII

ABSTRACT

This research investigated the permeability of CO2 and N2 as well as selectivity of CO2

over N2 of polysulfone (PSF) mixed matrix membranes filled with zeolite 4Å particles.

The mixed matrix membrane was prepared by immersion precipitation method with

zeolite and PSf were ranged from 10 to 30 W/V% dissolved in the NMP varied from 75

to 100 cm3. The mixture was then mixed homogenously and utilized to determine the

permeation rates of N2 and CO2. It is characterized by FTIR and the gas separation

performance is analysed by Design of Expert (DOE) method. In accordance to our

expectation, FTIR results revealed that the intensity at 3596.17 cm−1 described the

existence of an interaction between polymer and zeolite. The permeability of CO2

through the membrane was in higher range from 9.873 to 11.641 GPU. At lower

operating pressure, the greater solubility of gas encourages a higher permeation rate.

The single concentration variable has low effect, however the interaction between PSf

and NMP (BC) has considerable effect on the permeability of CO2 with the highest F

value of 0.46. NMP exhibited a high degree of polarity and hydrogen bonding which led

to effect of selective skin and permeation rate. The more solvent evaporated, the thicker

the concentrated polymer region which then leads to a thicker selective skin and a

reduction in permeation rate. The interaction of zeolite and PSf had the least significant

effect on the selectivity of CO2 over N2 due to the lower F value with 0.023, however

the interaction had higher permeability of CO2 with the F value of 0.45. The cause of

low permeability of CO2 was poor association between the zeolite and the polymer

leading to an increased in free volume within the membrane. Although it enhanced the

permeability of CO2, but it limited the sieving mechanism which resulted low selectivity

of CO2 over N2. The model regression equations were evaluated by F-test ANOVA

which revealed that these regressions are statistically significant at 95% confidence level

to screening the permeability of CO2 and N2 as well as selectivity of CO2/N2. In future

research, optimisation is proposed based on the screening study of PSf and zeolite

concentration ranged from 22.30 to 27.45 W/V% to stress on the high permeability and

selectivity CO2/N2 separation membranes.

IX

ABSTRAK

Kajian ini disiasat tentang kebolehtelapan CO2 dan N2 serta pemilihan CO2 bagi

membran matriks Polisulfon (PSF) yang dipenuhi dengan zeolite 4A zarah. Membran

matriks telah disediakan dengan kaedah rendaman pemendakan. Kandungan bagi zeolite

dan PSF adalah antara 10 hingga 30 W/V % telah dibubarkan dalam pelarut NMP

antara 75 hingga 100 cm3. Membran tersebut dikaji untuk menentukan kadar

penyerapan N2 dan CO2. Ia telah dicirikan dengan kaedah FTIR dan prestasi pemisahan

gas dianalisis oleh Design of Experiment (DOE). Selaras dengan jangkaan kami,

keputusan FTIR mendedahkan bahawa 3596.17 cm-1 menyifatkan kewujudan interaksi

antara PSf dengan zeolite. Kebolehtelapan CO2 melalui membran adalah antara

daripada 9.873 GPU kepada 11.641 GPU. Kepekatan tunggal mempunyai kesan yang

rendah, namun interaksi antara PSf dan NMP (BC) mempunyai kesan yang tinggi

kepada kebolehtelapan CO2 dengan nilai F tertinggi sebanyak 0.46. Kekutuban NMP

yang tinggi membawa kesan kepada pemilihan CO2 dan kadar penyerapan.

Pengurangan kadar penyerapan CO2 disebabkan oleh kecairan pelarut NMP yang

rendah. Interaksi antara zeolite dengan PSF membawa impak minimum pada kepilihan

CO2 dengan nilai F 0.023, manakala interaksi tersebut memainkan peranan penting

dalam kebolehtelapan CO2 dengan nilai F 0.45. Punca minimum ketelapan CO2 adalah

peningkatkan kelantangan zeolite pada permukaan PSf turut melemahkan ikatan antara

zeolite dan PSf, Walaupun kebolehtelapan CO2 dipertingkatkan, tetapi ia terhad

mekanisme penapisan zeolite yang menyebabkan pemilihan CO2 lebih N2 rendah.

Model persamaan regresi dinilai dengan analisis ANOVA, ia mendedahkan bahawa

regresi ini mempunyai statistik yang signifikan tinggi pada 95% bagi keputusan

kebolehtelapan CO2 dan N2 serta pemilihan CO2/N2. Dalam kajian masa depan,

pengoptimuman membran akan disiasat berdasarkan kajian ini yang dianalisis dengan

kandungan PSf dan zeolite antara 22.30 kepada 27.45 W/V% supaya dapat menekankan

maksimum kebolehtelapan dan pemilihan CO2/N2.

X

TABLE OF CONTENTS

Page

SUPERVISOR’S DECLARATION IV

STUDENT’S DECLARATION V

DEDICATION VI

ACKNOWLEDGEMENT VII

ABSTRACT VIII

ABSTRAK IX

TABLE OF CONTENTS X

LIST OF FIGURES XIII

LIST OF TABLES XV

LIST OF SYMBOLS XVI

LIST OF ABBREVIATIONS XVII

CHAPTER 1 INTRODUCTION

1.1 Motivation and Statement of Problem 1

1.2 Objectives 2

1.3 Scopes of this Research 3

1.4 Main Contributions of this Work 3

1.5 Organisations of this Thesis 3

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction of Gas Separation Membrane 6

2.2 Membrane for Gas Separation 8

2.2.1 Scientific Milestones 9 2.2.2 Gas Separation Technologies 12

XI

2.2.2.1 Cryogenic Distillation 13 2.2.2.2 Absorption 14 2.2.2.3 Adsorption 14 2.2.2.4 Membrane 15

2.2.3 Advantages of Gas Separation Membrane 16

2.3 Membrane Classification 17

2.3.1 Symmetrical Membrane 17 2.3.1.1 Microporous Membrane 17 2.3.1.2 Non-porous Membrane 18 2.3.1.3 Dense Membrane 19 2.3.1.4 Electrically-charged Membrane 20

2.3.2 Asymmetrical Membrane 20 2.3.2.1 Thin Film Composite Membrane 21 2.3.2.2 Liquid Membrane 21 2.3.2.3 Mixed Matrix Membrane 22

2.4 Polymeric Material for Membrane Fabrication 24

2.5 Membrane Module 27

2.5.1 Plate and Frame Module 27 2.5.2 Spiral Wound Module 28 2.5.3 Tubular Module 30 2.5.4 Hollow Fiber Module 30 2.5.5 Application of Membrane Modules 31

2.6 Phase Inversion Technique 32

2.6.1 Precipitation by Controlled Evaporation 32 2.6.2 Precipitation from the Vapour Phase 33 2.6.3 Thermally-Induced Phase Separation (TIPS) 33 2.6.4 Immersion Precipitation 33

2.7 Gas Transport Mechanism Through Membrane 34

2.7.1 Poiseuille Flow 35 2.7.2 Knudsen Diffusion 36 2.7.3 Molecular Sieving 37 2.7.4 Solution-diffusion 37

2.8 Terminology of gas transport 38

2.8.1 Permeability 38 2.8.2 Selectivity 39

CHAPTER 3 MATERIALS AND METHODS

3.1 Overview 41

3.2 Chemicals 41

XII

3.3 Preparation of Mixed-Matrix Membrane 42

3.4 Gas Permeation Testing for CO2/N2 43

3.5 Characterization of Membrane for CO2/N2 Separation 43

CHAPTER 4 RESULT AND DISCUSSION

4.1 Overview 46

4.2 Material Selection 46

4.2.1 Polysulfone 46 4.2.2 Zeolite 4A 47 4.2.3 N-methyl-pyrrolidinone (NMP) 47

4.3 Characterisation of Polysulfone-Zeolite based Membrane 47

4.4 Gas Performance Analysis 49

4.5 ANOVA Analysis 51

4.6 Empirical Model Analysis 52

4.7 Verification on Statistical Models and Diagnostic Statistic 55

4.8 Model Equations based on Screening Effect 61

CHAPTER 5 CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion 63

5.2 Recommendations 63

REFERENCES 65

XIII

LIST OF FIGURES

Figure No. Title Page 2.1 Schematic diagram of gas separation membrane (Tin, 2004) 9 2.2 Milestones in the development of membrane gas separations 11 2.3 Schematic diagram of the spevific volume of polymer as a function of

temperature (Koros & Hellums, 1989) 26 2.4 Plate and frame membrane module (Parimal et. al., 2009) 28 2.5 Typical spiral wound membrane module (Parimal et. al., 2009) 29 2.6 Tubular module (Cheng & et. al., 2011) 30 2.7 Hollow fiber membrane module (Parimal &et. al., 2009) 31 2.8 Phase diagram of a ternary system showing a one-phase region and a 32

two-phase region (shaded area) (Ying, 2004) 2.9 Schematic presentation of gas separation mechanisms (a) poiseuille 35

flow, (b) knudsen diffusion, (c) molecular sieving and (d) solution diffusion. (Koros, 1991)

3.1 Repeating unit of polysulfone 42 3.2 Structure of N-methyl-2-pyrrolidone, NMP 42 3.3 Schematic diagram of gas permeation apparatus (Kong & Li, 2001) 43 3.4 Flow diagram of the membrane preparation method 45 4.1 FTIR spectra of polysulfone based membrane with higher permeability 48

of CO2 and higher selectivity of CO2/N2 4.2 Interaction via concentration of PSf and zeolite for permeability of 55

CO2 4.3 Interaction via concentration of PSf and zeolite for permeability of N2 56 4.4 Interaction via concentration of PSf and zeolite for selectivity of CO2/N2 56 4.5 Normal probability plot of residual for permeability of CO2 57 4.6 Normal probability plot of residual for permeability of N2 57

XIV

4.7 Normal probability plot of residual for selectivity of CO2/N2 58 4.8 Plot of residual versus predicted response for permeability of CO2 58 4.9 Plot of residual versus predicted response for permeability of N2 59 4.10 Plot of residual versus predicted response for selectivity of CO2/N2 59 4.11 Predicted vs. actual values plot for permeability of CO2 60 4.12 Predicted vs. actual values plot for permeability of N2 60 4.13 Predicted vs. actual values plot for selectivity of CO2/N2 61

XV

LIST OF TABLES

Table No. Title Page 2.1 Development of membrane processes market (Yampolskii et al., 2002) 7 2.2 Future market of membrane gas separation (Baker, 2001) 7 2.3 Scientific developments of membrane gas transport 10 2.4 Membrane separation processes for non-porous membrane (Sukhtej, 19

1997) 2.5 Materials for gas separation membrane (Chung & Kafchinski, 2002) 25 2.6 Qualitative comparison of various membrane modules (Mulder, 1996) 31 4.1 Functional group of membrane sample with selectivity of CO2/N2 of 49

1.222 in comparison of characteristics adsorption (Silverstein et al., 1981)

4.2 Experimental design matrix and response results 50 4.3 ANOVA and R-squared (R2) statistics for the fitted models 51 4.4 ANOVA for the regression model equation and coefficients of 53

permeability of CO2, permeability of N2 and selectivity of CO2/N2 4.5 The model equations for permeability of CO2, permeability of N2 and 62

selectivity of CO2/N2

XVI

LIST OF SYMBOLS

A Effective membrane area (cm2) dp/dt Molar flow rate (mol/s) dn/dt Pressure increase J Flux (cm3/cm2.s) K Adjustable parameter M Molecular weight of the gas P Permeability (GPU) pf Feed side pressure (cmHg) pp Permeate side pressure (cmHg) R Ideal gas constant T Temperature (°C) Tg Glass transition temperature (°C) Tga and Tgp Vd

Glass transition temperatures of additive and polymer Dead volume (cm3)

wa and wp Weight fractions of additive and polymer xi Mole fraction of gas component i in the feed side yi Mole fraction of gas component i in the permeate side

δ

Membrane thickness (m)

α Selectivity Β and λ Empirical parameters ρ Density of the gas ν Volumetric flow rate (cm3/s) Δp Trans membrane pressure difference (cmHg)

XVII

LIST OF ABBREVIATIONS

CA Cellulose acetate CMS Carbon molecular sieve DOE Design of Experiment DPE Diphenyl ether DSC Differential scanning calorimetry FTIR Fourier transform infrared GC Gas chromatograph MMM Mixed matrix membrane NMP N-Methyl-2-pyrrolidone PAN Polyacrylnitrile PC Polycarbonate PEI Polyetherimide PES Polyethersulfone PI Polyimide PP Polypropylene PSF Polysulfone PVDF Polyvinyldenefluoride SEM Scanning electron microscopy SLM Supported liquid membrane TFC Thin film composite membrane TGA Thermal gravimetric analyzer TIPS Thermal inversion phase separation XRD X-Ray diffractometer

1

CHAPTER 1

INTRODUCTION

1.1 Motivation and Statement of Problem

Mixed matrix membranes (MMM) have recently emerged as a promising alternative

morphology to overcome the performance limitation of conventional polymeric

membranes for gas separation. They are obtained by embedding a filler material, such

as zeolites (Suer et al., 1994;), silica (Moaddeb & Koros W.J., 1997), carbon molecular

sieves (Duval et al., 1993) or conductive polymers (Hacarlıoğlu, 2001) into a polymer

matrix. A significant effort has been devoted to prepare MMMs using zeolites as filler

due to their molecular sieving properties and glassy polymers as the polymer matrix due

to their rigidities and higher intrinsic selectivity (Mahajan & Koros, 2002).

According to Moore & Koros (2005) and Li et al. (2005), most MMMs were

investigated to suffer from lack of interaction between zeolite particles and glassy

polymer chains, thus it is the factor of insufficient improvement of membrane due to

non-selective voids at the polymer-zeolite membrane interface. Several methods have

been introduced and examined to modify the interaction between polymer and zeolite

(Suer et al., 1994). These methods can be classified into two categories which are to

promote flexibility of polymer during membrane formation and improve the

compatibility between zeolite and polymer. Based on the research, the polyimides,

polyesters, polysulfones and polyamides resulted higher gas separation performance

(Aroon et al., 2010). According to Sridhar et al. (2007), the factors of affecting

membrane preparation are the mixture concentration of polymer and solvent, type of

2

solvent, and effect of solvent evaporation on the permeability and selectivity of

membranes were investigated in detail .

To date, many researchers have tried to enhance and optimize the membrane

morphology (Chan & Tsao, 2003; Guthrie & Lin, 1995) and performances (Idris et al.,

2006) through a statistical approach. In the research, the further investigation on the

interaction of mixed matrix membrane variables was statistically designed experiment

with minimum experimental runs to locate ideal setting of mixture concentration factors

for improving permeability of CO2 and N2 as well selectivity of CO2 over N2.

According to Montgomery (2001), statistical approaches such as ANOVA analysis,

fitted model equations and factorial design were identified to calculate the complex

interaction between the independent process factors Therefore, both the applicability of

these membranes for the separation of industrially important gas pairs could be

determined, and the transport mechanism through each membrane type could be

enlightened by mechanistic explanations on deviations from single gas permeability

measurements of the membranes.

This research aimed to synthesize and investigate the flat sheet mixed-matrix membrane

of polysulfone (PSf) and zeolite for CO2 and N2 gas separation by screening different

concentration of mixture. The membranes were fabricated and characterized for their

permeability and selectivity. In order to investigate permeability of CO2 and selectivity

of CO2 over N2, this current study is aimed at illustrating the interaction between

concentration of PSf, zeolite and NMP in diagnostic analysis using Design of Expert

(DOE). Final regression models obtained from DOE are expected to be able to predict

the optimum mixture concentration parameters in producing high permeability of CO2.

1.2 Objectives

The objectives of this research to synthesize and investigate the flat sheet mixed-matrix

membrane of Polysulfone (PSf) and zeolite for CO2 / N2 gas separation by screening

polymer concentration.

3

1.3 Scopes of this Research

The following are the scopes of this research:

1. To fabricate the membrane from the mixture of polysulfone, zeolite and solvent

through the application of immersion precipitation method. The concentration of

zeolite and PSf are ranged from 10 to 30 W/V% that dissolved in the NMP

varied from 75 to 100 cm3.

2. To characterize the membranes for their permeability, selectivity and structure

using instruments such as single gas permeation test rig and Fourier Transform

Infrared Spectroscopy (FTIR). Subsequently, comprehensive gas separation of

carbon dioxide and nitrogen will be studied.

3. To screen the operating variables via application of Design of Experiment

(DOE) software, Version 7.1. It is capable of analyzing the effect of mixture

concentration in order to locate ideal process settings for top performance.

1.4 Main Contributions of this Work The following works are the contributions:

1. To implement the method of immersion precipitation to fabricate the

polysulfone membrane with zeolites and NMP.

2. To investigate the screening of single and interaction effect of polymer

concentration to improve the gas separation performance based on the ANOVA

analysis and the characteristics of the membrane.

3. To locate the final regression models to synthesis the mixed-matrix membrane

with different polymer concentration effect in producing higher permeability of

CO2 and selectivity of CO2/N2.

1.5 Organisations of this Thesis

 This dissertation is organized and structured into six chapters and four appendices.

Chapter One is an introductory chapter of this dissertation. The research objective and

outline of this dissertation are also presented in this chapter.

4

The introduction and fundamental concepts of polymeric gas separation membranes are

given in Chapter Two. This chapter highlights the background and basic theories of gas

transport. It provides the introduction of membrane-based gas separation, which covers

the historical survey and advantages of gas separation membranes, industrial

applications and transport mechanisms of gas separation membranes. A brief

introductory description on the engineering principles of membrane gas separation,

including membrane material selection, membrane preparation and modification,

membrane characterization and evaluation, as well as the membrane module design are

presented. Finally, the membrane materials and advanced modification of polymeric

membranes are depicted.

Chapter 3 gives a review of the method of membrane fabrication and screening of

permeability of CO2 and N2 as well as selectivity of CO2/N2 through the polysulfone-

zeolite based membrane. The fabrication method of membrane of mixed matrix is

introduced through the application of immersion precipitation method. The

characterisation of membrane is based on their permeability, selectivity and structure

using instruments such as single gas permeation test rig and FTIR. Subsequently,

comprehensive gas separation of carbon dioxide and nitrogen is studied. The

concentration variables are studied based on the ANOVA analysis by the application of

Design of Experiment (DOE).

Chapter 4 is devoted to analyse the performance of gas separation such as permeability

and selectivity of carbon dioxide and nitrogen in comparison of the prediction result

obtained from several researches. A brief review of the common method for achieving

higher permeability and selectivity is also presented. A detailed description of the

statistical data used to optimise the operating variables is also outlined. It analyses the

single and interaction effect of polymer concentration to improve the gas separation

performance based on the ANOVA analysis and the characteristics of the membrane. It

also presents the final regression models based the effect of polymer concentrations in

order to locate ideal process settings for higher permeability of CO2 and selectivity of

CO2/N2.

Chapter 5 presents the conclusion of research, which ties together and integrates the

ANOVA analysis of gas separation performance together with its implication of

5

resulting from the discussion on the single and interaction effect of polymer

concentration, final screening regression models and recommendations for future work.

6

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction of Gas Separation Membrane

Over the last 40 years, membrane separation process was developed following the

discovery of asymmetric membranes, which first applied in reverse osmosis (Paul and

Yampol’skii, 1994) and rapidly became a major interest in separation applications

(Rousseau, 1987). Membrane separation has been widely adopted by process industries

and became more and more important. Compared to other conventional mass separation

technologies which are already at their old age, this is relatively new unit operation. The

membrane acts as a semipermeable barrier and separation occurs when membrane

controls the rate of movement of various molecules between two liquid phases, two gas

phases, or a liquid and gas phase.

In recent years, membrane separation process has been utilized as separation tool and

supplemented the conventional mass separation techniques such as distillation,

crystallization, absorption, adsorption, solvent extraction, etc (Scott, 1990;

Mohammadi, 1999). It is recognized as an energy efficient and economical tool in

solving many mass separation tasks.

The market of membrane separation is extremely heterogeneous and growing fast,

where requires different membrane structures and processes for specific application.

The membranes and module sales is growing at a rate of 10 %/year to US$ 4.8 billion at

year 2000 (Yampolskii et al., 2002). The development of membrane processes in the

end of century is reviewed as shown in Table 2.1.

7

Table 2.1 Development of membrane processes market (Yampolskii et al., 2002)

Membrane Process Sales (US$ Million)

Growth per year (%) 1996 2000

Dialysis 1300 2100 10

Microfiltration 600 960 10

Ultrafiltration 350 560 10

Reverse Osmosis 250 450 12

Gas Exchange 160 260 10

Gas Separation 120 250 15

Electrodialysis 80 130 10

Miscellaneous >5 >10 7

Total 1940 4810 >10

Table 2.2 Future market of membrane gas separation (Baker, 2001)

Separation Sales (US$ Million)

2000 2010 2020

Nitrogen from air 75 100 125

Oxygen from air <1 10 30

Hydrogen 25 60 150

Natural gas

Carbon dioxide 30 60 100

Natural gas liquefaction <1 20 50

Nitrogen & water 0 10 25

Vapor/nitrogen 10 30 60

Vapor/vapor 0 20 100

Air dehydration/Other 15 30 100

Total 155 340 760

Conclusively, although gas separation is a relatively young technology, it accounts for

about US$ 250 million/year and is growing relatively fast with a rate of 15 %/year.

With the development of new membranes with enhanced separation properties and

stability, the importance of membrane-based gas separation to solve difficult mass

8

separation problems will certainly increase in the future, especially in responding to the

market demand for industrial applications. Currently, the major application of gas

membrane separation is the separation of non-condensable gases, such as nitrogen from

air, carbon dioxide from methane, and hydrogen from nitrogen, methane. Nevertheless,

membrane gas separation technology in refinery, petrochemical and natural gas

industries are predicted to be a great potential market for membrane technology. Table

1.2 illustrates the prediction of future membrane gas separation market made by Baker

(2001).

2.2 Membrane for Gas Separation

 Membrane technology for gas mixtures separation has rapidly grown from being a

laboratory curiosity to becoming a commercially viable separation approach within the

last two decades (Nunes and Peinemann, 2001). Membrane gas separation has emerged

as one of the most significant new unit operations in the chemical industries in the past

25 years. At least 20 companies worldwide offer membrane-based gas separation

systems for a variety of industrial applications. Membrane gas separation plays an

increasing important role in the separation industry with over $125 million in module

sales with an annual growth rate of around 10 % over the next decade (Crull, 1997).

Thus, there is a large potential for this separation technology to capture a significant

slice of the separation market. Membrane gas separation is an area of considerable

current research interest as the number of applications is expected to expand rapidly

over the next decade.

Membrane in a gas permeation process act as a selective barrier, usually thin, interposed

between two phases, which obstructs gross mass movement between the phases but

permits passage of certain species from one phase to the other with various degrees of

restriction (Koros and Fleming, 1993; Mulder, 1996). Generally, in membrane gas

separation processes, the bulk phases are gas mixtures. Gas Permeation is a physical

phenomenon where certain gas components selectively pass through a membrane. The

membrane is selective to one of the gas species, where one of the species in the mixture

is allowed to be exchanged in preference to others. One bulk phase is enriched in one of

the species while the other is depleted of it. Separation of a gas mixture occurs since

each type of molecules diffuses at a different rate through the membrane (Geankoplis,

9

1995). This movement of any species across the membrane is caused by one or more

driving forces. These driving forces arise from a gradient of chemical potential due to

concentration gradient or pressure gradient or both. Figure 2.1 shows schematic diagram

of a two-phase gas separation system separated by a membrane.

Figure 2.1 Schematic diagram of gas separation membrane (Tin, 2004)

2.2.1 Scientific Milestones

The membrane-based gas separation was discovered and documented since mid-

nineteenth century. In 1829, Thomas Graham, father of colloid chemistry made the first

scientific discovery related to membrane separation (Graham, 1833). He observed

gaseous osmosis through a wet animal bladder for an air-carbon dioxide system. Two

years later in 1831, J.K. Mitchell perceived the different deflation rates by 10 gases

through natural rubber balloons (Mitchell, 1830; 1833). At approximately the same

time, A. Fick, an outstanding physiologist postulated the concept of diffusion and

formulated the well-known Fick’s first law by studying the gas transport through

nitrocellulose membranes (Fick, 1855).

However, many significant scientific observations about membrane separation, such as

the first quantitative measurement of the rate of gas permeation were accomplished by

Sir Thomas Graham, the discoverer of Graham’s law of gas effusion. He proposed

“solution-diffusion” mechanism for gas permeation through a membrane by repeating

Mitchell’s experiments with the films of natural rubber in 1866 (Graham, 1866).

Approximately 13 years later in 1879, Von Wroblewski quantified Graham’s model and

defined the permeability coefficient as the permeation flux multiplied by the membrane

10

thickness divided by the transmembrane pressure (Wrobleski, 1879). He also

characterized the permeability coefficient as a product of diffusivity and solubility

Table 2.3 Scientific Developments of Membrane Gas Transport (Kesting & Fritzsche, 1993)

Researchers Achievement

Graham (1829) First recorded observation

Mitchell (1931) Gas permeation through natural rubbers

Fick (1855) Law of mass diffusion

Von Wroblewski (1879) Permeability coefficient product of diffusion and absorption coefficient

Kayser (1891) Demonstration of validity of Henry’s Law for the absorption of carbon dioxide in rubber

Lord Rayleigh (1990) Determination of relative permeabilities of oxygen, nitrogen and argon in rubber

Knudsen (1908) Knudsen diffusion defined

Daynes (1920) Developed time lag method to determine diffusion and solubility coefficient

Barrer (1939-1943) Permeabilities and diffusivities followed Arrhenius equation

Matthes (1944) Combined Langmuir and Henry’s law sorption for water in cellulose

Meares (1954) Observed break in Arrhenius plots at glass transition temperature and speculated about two modes of solution in glassy polymers

Michaels, Vieth and Barrie (1963)

Demonstrated and quantified dual mode sorption concept

Vieth and Sladek (1965) Model for diffusion in glassy polymers

Paul (1969) Effect of dual mode sorption on time lag and permeability

Petropoulos (1970) Proposed partial immobilization of sorption

Paul and Koros (1976) Defined effect of partial immobilizing sorption on permeability and diffusion time lag

11

coefficients, which soon became an important model in membrane permeation. A

decade later in 1891, H. Kayser demonstrated the validity of Henry’s law for the

absorption of carbon dioxide in rubber (Kayser, 1891).

The progress of membrane separation techniques was very slow in the early stage.

Nevertheless, many fundamental scientific works and contributions related to gas

separation membranes were carried out in the twentieth century, as summarized in

Table 2.3 (Kesting & Fritzsche, 1993). Partucularly, Daynes (1920) developed the time

lag method from nonsteady-state transport behavior of gases via a membrane to

determine diffusion coefficient.

The above fundamental works provide the foundation in membrane processes, which

conduce to the commercialization of membrane separation technology in industrial

applications. Following the first breakthrough of asymmetric phase- inverted

Figure 2.2 Milestones in the development of membrane gas separations. (Baker, 2001)

12

membranes made of cellulose acetate for reverse osmosis by Loeb and Sourirajan in

1960 (Loeb and Sourirajan, 1960; 1962; 1964), membrane gas separation appeared to be

a competitive separation tool for industry processes in the 1970’s. The first

commercially viable gas separation membrane, Prism® was produced at 1980

subsequent upon the method of repairing pinhole size defects in the thin layer of

asymmetric membranes by Henis and Tripodi (Henis & Tripodi, 1980). As a

consequence, the successful application of the first commercial gas separation

membrane has accelerated the development of novel membrane materials as it offer an

attractive alternative for specific separation applications. Figure 2.2 displays the

important milestones in the history and scientific development of membrane gas

separation technology (Baker, 2002).

2.2.2 Gas Separation Technologies

The focus of this research is to investigate the potential of distinctively membrane

processes for the energy-efficient and effective separation of CO2 and N2. Traditional

methods used to separate CO2 from gas mixtures are pressure swing adsorption,

cryogenic distillation and the most frequently used method amine absorption (Khoo &

Tan, 2006; Freni, et al., 2004). Also membrane processes are frequently used for gas

separation. The main limitation of currently existing membranes is the occurrence of

severe plasticization of the membrane in the presence of high pressure CO2. Due to

excessive swelling of the polymer membrane upon exposure to CO2, the performance of

selectivity decreases significantly, thus reducing the purity of the CO2 and consequently

reducing the possibilities for reuse of the gas. Energy requirements on the other hand

significantly benefit the use of membrane technology over other technologies:

membrane technology uses 70-75 kWh per ton of recovered CO2 compared to

significantly higher values for pressure swing adsorption (160-180 kWh), cryogenic

distillation (600-800 kWh) or amine absorption (330-340 kWh) (Khoo & Tan, 2006),

making membrane technology an attractive alternative.

13

2.2.2.1 Cryogenic Distillation

Pure gases can be separated from air by first cooling it until it liquifies, then selectively

distilling the components at their various boiling temperatures. The process can produce

high purity gases but is energy-intensive. This process was pioneered by Dr. Carl von

Linde in the early 20th century and is still used today to produce high purity gases

(Zhang et al., 2006). The cryogenic separation process requires a very tight integration

of heat exchangers and separation columns to obtain a good efficiency and all the

energy for refrigeration is provided by the compression of the air at the inlet of the unit

(Borden et al., 2002; Bates et al., 2002; Ma’mun et al., 2007).

To achieve the low distillation temperatures an air separation unit requires a

refrigeration cycle that operates by means of the Joule–Thomson effect, and the cold

equipment has to be kept within an insulated enclosure (commonly called a "cold box").

The cooling of the gases requires a large amount of energy to make this refrigeration

cycle work and is delivered by an air compressor. Modern ASUs use expansion turbines

for cooling; the output of the expander helps drive the air compressor, for improved

efficiency.

The process consists of the several steps is before compression the air is pre-filtered of

dust. Air is compressed where the final delivery pressure is determined by recoveries

and the fluid state (gas or liquid) of the products. Typical pressures range between 5 and

10 bar gauge (Ma'mun et al., 2007). The air stream may also be compressed to different

pressures to enhance the efficiency of the ASU. During compression water is condensed

out in inter-stage coolers.

The process air is generally passed through a molecular sieve bed, which removes any

remaining water vapour, as well as carbon dioxide, which would freeze and plug the

cryogenic equipment. Molecular sieves are often designed to remove any gaseous

hydrocarbons from the air, since these can be a problem in the subsequent air distillation

that could lead to explosions (Ma'mun et al., 2007). The molecular sieves bed must be

regenerated. This is done by installing multiple units operating in alternating mode and

using the dry co-produced waste gas to desorb the water.

Process air is passed through an integrated heat exchanger (usually a plate fin heat

exchanger) and cooled against product (and waste) cryogenic streams. Part of the air