Zhang Weiqi

7/26/2019 Zhang Weiqi

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Experimental Investigation on Gas Separation

Using Porous Membranes

vorgelegt von

Master-Ing.

Weiqi ZHANG

von der Fakultt III - Prozesswissenschaften

der Technischen Universitt Berlin

zur Erlangung des akademischen Grades

Doktorin der Ingenieurwissenschaften

Dr.-Ing.

genehmigte Dissertation

Promotionsausschuss:

Vorsitzender: Prof. Dr.-Ing. Felix ZieglerBerichter: Prof. Dr. Frank Behrendt

Berichter: Prof. Dr.-Ing. Bernd Hillemeier

Tag der wissenschaftlichen Aussprache: 03. December 2010

Berlin 2011

D 83


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Ich erklre hiermit, dass ich die vorliegende Arbeit selbstndig verfasst und keine an-

deren als die angegebenen Quellen und Hilfsmittel verwendet habe.

Berlin, den 03. December 2010


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Acknowledgment

I am deeply thankful to colleagues and advisors, who helped me complete for this

project, firstly to Univ.-Prof. Dr. Frank Behrendt, who gave me the opportunity to

do this Ph.D., made this work possible; Prof. Dr.-Ing. Bernd Hillemeier and Prof.

Dr.-Ing. Felix Ziegler, who took over the supervision of my thesis.

Maria Gaggl, who helped me with the practicalities of living in Germany, and even

shared with me her flat for two weeks when I first started my Ph.D.. Gregor Gluth,

for making all the membranes used in this project, but also for his patience. Dr.-

Ing. York Neubauer andDr.-Ing. Nico Zobel, for their competence; if you encounter

any problems, either theoretical or experimental status, you can turn to them and

certainly get a reasonable answer. Horst Lochner and Uwe Rhr, who made membrane

cell and many other small parts patiently for me, and helped me with all sorts oftechnicalities. Susanne Hoffmann who gave me lots of suggestions over operations with

gas chromatograph (GC). Fang He, Gregor Drenkelfort, Birgit Packeiser, Renhui sun,

etc., my special thanks also go to for their patience and advice.

I would also like to acknowledge the on going financial support provided by Federal

Ministry of Food, Agriculture and Consumer Protection (BMELV),Agency for Renew-

able Resources (FNR), and the scholarship from Womens central officeto finish my

thesis.

Last but not least, I would like to thank all the helpful persons that I have forgotten

to mention by name. This thesis could not have been written without the support of

my parents, my husband Jingqun Song and my friends.


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Contents

Abstract XIII

Nomenclature XV

1 Introduction 1

2 State of the Art 5

2.1 An introduction to gas separation using membranes . . . . . . . . . . . 5

2.2 Inorganic membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.1 Dense inorganic membranes . . . . . . . . . . . . . . . . . . . . 7

2.2.2 Porous inorganic membranes. . . . . . . . . . . . . . . . . . . . 8

2.3 Porous cement membranes . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.4 Separation and process design . . . . . . . . . . . . . . . . . . . . . . . 10

2.4.1 Possible flow patterns. . . . . . . . . . . . . . . . . . . . . . . . 10

2.4.2 Number of stages . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.4.3 Known influence of operating parameters . . . . . . . . . . . . . 14

3 Experimental Setup 17

3.1 Flow chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.2 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.3 Operating parameters and procedure . . . . . . . . . . . . . . . . . . . 26


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Contents VII

4 Summary of Equations 31

4.1 Basic assumptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.2 Gas equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.2.1 The fundamental equations for ideal gases . . . . . . . . . . . . 32

4.2.2 Balances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.3 Equations for transport mechanisms through porous membranes . . . . 33

4.4 Equations for the experimental setup . . . . . . . . . . . . . . . . . . . 35

4.4.1 LabVIEW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.4.2 Soap film flowmeter. . . . . . . . . . . . . . . . . . . . . . . . . 36

4.4.3 Mass flow controller. . . . . . . . . . . . . . . . . . . . . . . . . 36

4.4.4 Gas chromatograph . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.5 Efficiency of gas separation through membrane. . . . . . . . . . . . . . 37

5 Experimental Results and Discussion 41

5.1 Controlling equipment and corresponding special procedures, calibration 41

5.1.1 Bubble flow-meter . . . . . . . . . . . . . . . . . . . . . . . . . 41

5.1.2 Data correction of mass flow controller . . . . . . . . . . . . . . 42

5.1.3 Calibration of gas chromatograph (GC) . . . . . . . . . . . . . 43

5.2 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

5.2.1 First set of experiments with Gaggls membranes . . . . . . . . 55

5.2.2 Second set of experiments with modified cell . . . . . . . . . . . 62

5.2.3 Third set of experiments with tubular membrane and cell. . . . 83

6 Summary and Outlook 91

6.1 Summary of results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

6.2 Observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

6.3 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Bibliography 97

Mitteilungen 107


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List of Figures

2.1 Schematic representation of membrane separation . . . . . . . . . . . . 6

2.2 Transport mechanisms in porous membranes [1] . . . . . . . . . . . . . 9

2.3 Schematics of possible flow patterns[2, 3] . . . . . . . . . . . . . . . . . 10

2.4 Flow pattern in presence of sweep gas[2] . . . . . . . . . . . . . . . . . 11

2.5 Schemes of commercial two-stage separation [2, 3] . . . . . . . . . . . . 12

2.6 Schemes of commercial three-stage separation[2, 3] . . . . . . . . . . . 13

2.7 Novel single-stage separation with recycling [2,3]. . . . . . . . . . . . . 13

3.1 Process schematic of gas separation . . . . . . . . . . . . . . . . . . . . 173.2 Process schematic of reference measurements . . . . . . . . . . . . . . . 18

3.3 Gas chromatographic system. . . . . . . . . . . . . . . . . . . . . . . . 22

3.4 Chromatogram of five-component gas . . . . . . . . . . . . . . . . . . . 23

3.5 LabVIEW controlling system . . . . . . . . . . . . . . . . . . . . . . . 25

4.1 Chromatograms of two-component gas and pure standard-gases . . . . 37

5.1 Flow rate of two-component gas at 2.4 bar . . . . . . . . . . . . . . . . 43

5.2 Flow rate of 2 % to 4 % . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.3 Flow rate of two-component gas at different pressures . . . . . . . . . . 45

5.4 GC measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.5 Base line of chromatogram . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.6 Area of H2in two-component gas measurements with different run times 475.7 Area of pure H2measurements at different temperatures . . . . . . . . 48


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X List of Figures

5.8 GC measurements of different reference-flow rate. . . . . . . . . . . . . 50

5.9 N2amount and flow rates in automatic injection . . . . . . . . . . . . . 51

5.10 Flow rate calculation of standard-gases . . . . . . . . . . . . . . . . . . 52

5.11 Pure H2peak area for calibration of2Mmeasurement. . . . . . . . . . 53

5.12 Calibration curve for H2of2M measurement. . . . . . . . . . . . . . . 54

5.13 Schematic of the first idea . . . . . . . . . . . . . . . . . . . . . . . . . 55

5.14 Schematic of the first membrane cell . . . . . . . . . . . . . . . . . . . 56

5.15 The first membrane cell and holders . . . . . . . . . . . . . . . . . . . . 56

5.16 Pore size distribution of the first membranes . . . . . . . . . . . . . . . 57

5.17 Gaskets for the first membrane cell . . . . . . . . . . . . . . . . . . . . 58

5.18 Flow rate influence at different temperatures . . . . . . . . . . . . . . . 58

5.19 Gas separation with different volume flows . . . . . . . . . . . . . . . . 59

5.20 Gas separation with different feed gases. . . . . . . . . . . . . . . . . . 60

5.21 Experimental and theoretic selectivity. . . . . . . . . . . . . . . . . . . 61

5.22 Problem of the first membrane cell . . . . . . . . . . . . . . . . . . . . 625.23 First version of the secondary membrane cell . . . . . . . . . . . . . . . 63

5.24 Final design of the modified membrane cell . . . . . . . . . . . . . . . . 63

5.25 Axial section view of the modified membrane cell . . . . . . . . . . . . 64

5.26 Pore distribution of PZ-2. . . . . . . . . . . . . . . . . . . . . . . . . . 67

5.27 Graphite gaskets around membrane . . . . . . . . . . . . . . . . . . . . 68

5.28 Performance of membrane cells in . . . . . . . . . . . . . . . 695.29 Influence of temperature in . . . . . . . . . . . . . . . . . . . 70

5.30 Influence of temperature in . . . . . . . . . . . . . . . . . . 70

5.31 Influence of equivalent water to cement ratio in . . . . . . . 71

5.32 Influence of pore size in . . . . . . . . . . . . . . . . . . . . 72

5.33 Effect of different sample thickness on diffusion in . . . . . . 73

5.34 Membranes after heating . . . . . . . . . . . . . . . . . . . . . . . . . . 745.35 Comparison of compositions in . . . . . . . . . . . . . . . . 75


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List of Figures XI

5.36 Permeabilities of H2using different feed gases . . . . . . . . . . . . . . 76

5.37 Influence of pressure difference in . . . . . . . . . . . . . . . 76

5.38 Influence of sweeping gas in . . . . . . . . . . . . . . . . . . . 77

5.39 Measurements of different sweeping gases in and 78

5.40 Influence of adhesives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

5.41 SEM images of the PZ-2+MS . . . . . . . . . . . . . . . . . . . . . . . 80

5.42 Knudsen number of H2and CO2. . . . . . . . . . . . . . . . . . . . . . 81

5.43 Diffusion coefficients of H2 . . . . . . . . . . . . . . . . . . . . . . . . . 82

5.44 Schematic of transport in tubular membrane cell [1] . . . . . . . . . . . 83

5.45 Tubular membrane cell . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

5.46 Heating system for tubular membrane. . . . . . . . . . . . . . . . . . . 84

5.47 Design of tubular membrane cells . . . . . . . . . . . . . . . . . . . . . 85

5.48 Tubular membrane and gaskets . . . . . . . . . . . . . . . . . . . . . . 86

5.49 Components in permeate-gas in . . . . . . . . . . . . . . . . 87

5.50 Separation factors of H2to CO2 . . . . . . . . . . . . . . . . . . . . . . 87

5.51 Water from tubular membrane. . . . . . . . . . . . . . . . . . . . . . . 88

5.52 Chromatogram in at 200 C . . . . . . . . . . . . . . . . . . 88

5.53 Large cracks after heating . . . . . . . . . . . . . . . . . . . . . . . . . 89

6.1 Process schematic of using CO2as sweeping gas . . . . . . . . . . . . . 95

6.2 Process schematic of using steam as sweeping gas . . . . . . . . . . . . 95


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List of Tables

2.1 Applications of gas separation using membranes . . . . . . . . . . . . . 6

2.2 Classification of inorganic materials on pore size[4] . . . . . . . . . . . 7

2.3 Separation factor of some typical gas mixtures . . . . . . . . . . . . . . 15

3.1 Operating conditions of Gas Chromatograph . . . . . . . . . . . . . . . 23

5.1 Flow rates of two-component gas controlled by MFC . . . . . . . . . . 42

5.2 Gases compositions using N2as reference . . . . . . . . . . . . . . . . . 48

5.3 Gases compositions using He as reference . . . . . . . . . . . . . . . . . 49

5.4 Volume flows of permeate-gases . . . . . . . . . . . . . . . . . . . . . . 50

5.5 Corresponding points of standard-gases . . . . . . . . . . . . . . . . . . 51

5.6 Characteristic peak area of H2 in permeate-gas and standard gases. . . 52

5.7 H2concentration in permeate-gases . . . . . . . . . . . . . . . . . . . . 53

5.8 Physical and geometrical data of the first test membranes. . . . . . . . 56

5.9 Cement membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

5.10 Code name of cement membrane. . . . . . . . . . . . . . . . . . . . . . 66

5.11 Porosity of cement membrane . . . . . . . . . . . . . . . . . . . . . . . 665.12 Permeation fluxes usingPZ-2+MS in . . . . . . . . . . . . . 73

5.13 Separation factors usingHOZ+MS in . . . . . . . . . . . . . 75

5.14 Separation factors using 5 mm membranes in . . . . . . . . 80

5.15 Data usingPZ-2+MS(5 mm,(w/c)eq0.25) in . . . . . . . 81

5.16 Permeation ability usingPZ-2+MS(5 mm,(w/c)eq0.25) in 82

5.17 Permeation ability using tubular-PZ-2+MS(5 mm, 0.25) in 89

5.18 Permeation ability using tubular-PZ-2+MS(5 mm, 0.25) in 90


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Abstract

Membranes have been long utilized in industry for separation of gas mixtures [5].

Thanks to their chemical, physical, and thermodynamic stability, as well as for their

high durability at elevated temperatures and high permeation flux, ceramic membraneshave become especially popular in the field. Cement is looked at as a valid alternative

for the future, as in addition to being stable, it would bring the advantage of lower costs

and longer lifespan. Research is still necessary to access the performance and reliability

of cement membranes and the present thesis wants to contribute to this topic.

More specifically, purpose of this work was to investigate the influence of gas molecules

properties on material transport, and to explore the influence of operating conditions

and membrane composition on separation efficiency. To this aim, a series of experiments

were performed.

In more detail, an experimental setup was manufactured and tested. Fifty types of

membranes were produced. Several membrane cells were designed into a module with

counter current flow pattern, where gases on two sides of the membrane flow in contrary

directions. Pure H2, CO2, CO, CH4, two-component gas (49.8 % H2and 50.2 % CO2),

and five-component gas (13 % H2, 16 % CO, 13 % CO2, 53 % N2, and 5 % CH4) were

used as feed gases, while N2and CO were used as sweeping gases.

A new method was introduced to calibrate the automatic injection of sample gases

into gas chromatography. Experiments were conducted from high to low temperatures.

Chromatograms obtained byGCcould then be used to determine the amount of each

component in both permeate and retentate gas. New calibration formulas, which offer

more accurate quantification methods, are also presented in this work. On this base,

permeation rate and efficiency of gas separation could be calculated and the influence

of operating condition and membrane shape and composition could be studied.

Main results of this work cement are: the influence of all the above parameters iscollected, the best conditions and membrane type are found, cementitious material has


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XIV Abstract

the ability to separate gas mixtures, and new designs considering purification of the

product gases are provided.


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Nomenclature

Abbreviations

ngstrm =1010 m

bar 0.9869 atmosphere or 100 kPaC degrees Celsius, K-273.15cm centimeter

g gram

GC gas chromatography

J joule

K degrees Kelvin

kg kilogram

m meter

mA milliampereMF C mass flow controller

min minute

ml milliliter

mm millimeter

mol gram-mole

m micron

V microvolt=106 v

nm nanometer

s second

V ol volume

VOL volatile organic liquids

Latin Letters

A surface area of the membrane [cm2]

C molar density of the fluid mixture [mol/ml]

CH2 partial molar density of component H2 [mol/ml]


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XVI Nomenclature

CCO2 partial molar density of component CO2 [mol/ml]

D diffusion coefficient [cm2/min] or [m2/s]

Di molecular diffusion coefficient [m2/s]

Di,K Knudsen diffusion coefficient [m2

/s]DPI value of pressure difference meter [bar]

DS surface diffusion coefficient [cm2/min] or [m2/s]

D transition diffusion coefficient [m2/s]

e base of the natural log [-]

ED activation energy of diffusion [kJ/mol]

F driving force parameters [-]

h Plancks constant [-]

I ampere values [A]j setpoint ofMFC [-]

J permeation rate [molmin1cm2]Jn permeation rate in mole [mol]

k phenomenological coefficient [-]

kB Boltzmann constant [-]

L permeability coefficient [mols1m1Pa1]LKn permeability coefficient of Knudsen diffusion [mols1m1Pa1]

L permeability coefficient of viscous flow [kmolm

2s

1Pa

1]Mi molecular weight of componenti [kg/mol]

n amount of gas present [mol]

NKn Knudsen number [-]

p absolute pressure [Pa]

PI value of pressure meter [bar]

pm average pressure in the membrane [Pa]

R universal gas constant 8.314472 [Jmol1K1]rp pore radius [m]S Separation factor [-]

SD activation entropy of diffusion [kJmol1K1]SH2,CO2 Separation factor of H2to CO2 [-]

t time [min]

T thermodynamic temperature [K]


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Nomenclature XVII

V volume [m3]

v volume flow rate [m3/min]

v average volume flow rate [ml/min]

vn volume flow rate under standard condition [mln/min]x mole fraction of certain component [100 %]

z compressibility factor [-]

Z (average) value of peak area [25 Vs]

Greek Letters

selectivity [-]

ideal selectivity [-]

permeability [-]

coordinate dimension [mm]

thickness of the membrane [mm]

fluid viscosity [kgm1s1] mean free path of molecular [nm]

density of the membrane [kg/m3]

pore tortuosity [-]

S pore tortuosity of the surface [-]

p porosity of the membrane [-]

Superscripts

2M two-component gas

5M five-component gas

F feed gas

P permeate gas

R retentate gas

Subscripts

i one component

j setpoint value

m mass

o objective component


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Chapter

1Introduction

Gas separation is any operation that separates a mixture into two or more gases which

differ in composition. This result can be achieved by either removing a single component

from a mixture, as in purification or concentration, or by separating a mixture into

almost pure gases, as in fractionation [6].

Gas separation is a key issue in various industrial fields. For example, hydrogen has

the potential for application in clean fuel technologies and in the fertilizer and refinery

industry [7], hydrogen separation and purification is an important research subject.

Separating CO2 in chemical processes would allow to capture it rather than releasingit in the atmosphere, where it would contribute to global warming.

A number of membrane materials have already been used for various gaseous separa-

tion and membrane process applications. Compared with traditional processes on gas

separation, using membranes enables higher energy efficiency, reduces cost, makes in-

stallation, operation and scalability easier [2]. It is already reported that gas mixtures

can be effectively separated by membranes [813]. Recently, research on membrane-

based gas separation has especially focused on high temperature and low humidityconditions, low-cost membranes, data collection for membrane transport characteri-

zation. Interpretation of the results is still controversial. A large breakthrough in

applications has not yet been realized due to limited reproducibility of prototypes, lack

of fundamental transport data, and cost perspectives. says Mottern [14], implying the

current state of membrane technology.

Membrane materials are classified as organic and inorganic. Nowadays, there is al-

ready a wide variety of commercial inorganic membranes. Compared with organic

membranes, inorganic membranes can work at higher pressures up to 10 MPa and canbe cleaned with steam, which is impossible with organic membranes[15]. Inorganic


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2 Chapter 1 Introduction

membranes can be categorized on the base of their pore size, which is usually criti-

cal pore diameter. There are porous membrane and non-porous membrane, which

is also referred to dense membrane and is commercially available. Porous membranes

are especially promising for gas separation, as they have the advantage of large gaspermeation which is the penetration of a permeate through a membrane.

Generally four types of transport mechanisms are used to explain gas traveling in

porous membranes: Knudsen diffusion, surface diffusion, capillary condensation flow

and molecular diffusion [2, 1520]. According to T. C. Merkel [21], it has been ob-

served that, when the pressure on the permeation side is low, the diffusion coef-

ficients for both rubbery and glassy polymers manifest in the following sequence:

H2> O2> N2> CO2> CH4. That is, the larger the molecule size, the lower the

diffusion coefficient [22]. The size of permeation molecules seems especially important

in determining diffusion coefficients [21]. In the present work, the authors employed

two-gas mixtures which contained H2and CO2. In principle, if cementitious materials

themselves have no driving selectivity between H2 and CO2, for example, if there is

no reaction or solution, the difference between the two gases is just different molecular

sizes, so membrane separation can be presumed size-sieving. The molecular diameter

of H2and CO2are 0.74 and 3.87 respectively, H2molecule is much smaller. Further-

more, the kinetic diameter of H2is 0.29 nm, smaller than the 0.33 nm of CO2[23, 24].

Thus, H2was set as the preferential permeation component for the cement membranes.

If the pore size of the membrane is small enough and driving forces for components are

the same, H2should be better separated from CO2.

As introduced above, membranes used for gas separation are better to be cheap porous

inorganic membrane, and membranes should guarantee a maximum discharge of hydro-

gen. That is because high permeation flux, is another important advantage of porous

membranes, they can then be used as substrates for dense or selective materials during

gas separation. So far, porous ceramic membranes are widely utilized, thanks to theirchemical, physical, and thermodynamic stability. However, cement represents a valid

attraction. It has low thermal expansion coefficients (0.9 to 1.2105 K1), relativelyhigh compressive strengths (thousands pounds per square inch), and can be processed

in various way to meet, for example the desired density or strength. Cement also offers

good thermal resistance properties. Moreover, cement is cheap, has a longer service

life and is recyclable. At present, no report on gas separation using porous cement

membrane exists. The porous cement membranes used in this study were designed

and produced in cooperation with Institut fr Bauingenieurwesen (Institute of CivilEngineering). Different kinds of membranes with different base materials, additions,


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3

pore-sizes, equivalent water to cement ratios, shapes, areas and thickness were man-

ufactured by Gregor Gluth, a researcher working in the department ofBaustoffe und

Baustoffprfung (Building Materials and Construction Materials Testing),Technische

Universitt Berlin.Typically, membranes are used in gas separation without sweeping gas [25]. In this

study, N2 was used as the sweep gas to carry the permeations away. Next, CO was

used for comparison. To the purpose of reproducibility, experimental setup and pro-

cedures will be described in detail in Chapter2and3. A better understanding on the

inherent structures of membranes and their effects on membrane properties is crucial

in improving the predictability of material performance[26]. Main objectives of this

study are: (1) to select a simple but reliable model which is single membrane cell for

detailed study; (2) to determine the influence of ambient parameters, such as pres-sure, temperature, sweeping gas, and flow rate on the separation process; (3) to study

structure-property (e.g., membrane thickness, equivalent water to cement ratio, pore

size, and additions) influence on permeation transport; (4) to develop a mathematical

model of permeation transport, which incorporates independent constitutive relations

to interpret the mass transfer data; (5) to identify the best conditions and for optimiz-

ing gas separation; (6) to collect data for transport characterization and gas separation

through membranes.

This study is organised as follows.

1. The general aspects of membrane technology are discussed in Chapter 2. First,

importance and application of membranes in the field of gas separation are intro-

duced. Then, material and transport mechanisms of dense and porous inorganic

membranes are described, followed by the discussion on the cementitious material,

technology review, and process design.

2. In Chapter3, the experiments are described. This chapter also details the appa-ratus, methods for reducing error, operating parameters, and the experimental

procedure.

3. Chapter 4 presents common assumptions and general equations for transport

mechanism, and basic processing methods for the experimental operations. In the

last part of this chapter, parameters in order to assess the ability of separation

are illustrated.

4. Data correction of controlling equipment, calibration of measuring instrumentsare firstly introduced in Chapter5. Then based on the melioration order of the


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4 Chapter 1 Introduction

membrane cells and membrane shapes, Chapter 5lists the different membrane

stages, membrane cells, gaskets, and corresponding data.

5. Lastly, Chapter6presents the results in contrast to the best selection method

for the membrane and its parameters. This chapter also lists the achieved effi-ciency and problems encountered (e.g., diffusion in the bubble flow meter) during

the study, as well as suggestions for further research, solutions to improve the

technique, and areas for new applications are discussed.


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Chapter

2State of the Art

2.1 An introduction to gas separation using membranes

Thomas Graham did the first study on gas permeation through polymer in 1829 [2729].

The first membrane for hydrogen recovering was patented in 1980, and the technology of

hydrogen separation and purification by membranes is in constant development [30,31].

Nowadays, membrane processes are effective with unlimited selectivity which is one

parameter to determine efficiency of separation, and play an important role in the

so called green chemistry. As Koltuniewicz and Drioli pointed out, Based on therecent definition of clean technologies, almost all attributes may be fulfilled by using

membrane processes[5].

Membrane processes have encountered many applications in the field of gas separation.

The processes of applications are usually denominated from the target they can achieve,

such as separation, recovery, enrichment, removal of undesired components, desiccation,

purification, recycling and reuse of specific substances. Most of the applications where

membranes are employed to separate gases are listed in table 2.1[2, 12, 30, 32]:

The membrane is the central part of the membrane separation, and a schematic rep-

resentation is given in Figure2.1. A mixture gas is used as feed gas, and flows along

one side of the membrane surface. During this process, some components permeate

through the membrane, and are carried out of the membrane cell by one sweeping gas.

The residual gas on the feed side is called retentate. The other side of the membrane

is permeate side, and the outgoing gas on this side is permeate gas.

To the purpose of gas separation, the permeability coefficient and separation factor

of membranes are especially relevant. The permeability coefficient measures the gaspermeation volume, while separation factor expresses the membrane performance as


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6 Chapter 2 State of the Art

Table 2.1: Applications of gas separation using membranes

Process Object Source

Separation H2 H2/N2[33]

Separation H2 H2/CH4[34]Recovery H2 Product streams of ammonia plants[35, 36]

Recovery H2 In oil refinery processes [37, 38]

Separation H2 Biogas [39, 40]

Enrichment O2 Air for medical or metallurgical purposes [41,42]

Removal Water vapor Natural gas [4345]

Removal CO2 Natural gas[31, 44, 45]

Separation CO2 Landfill gas (CH450 %, CO2 40 %) [30]

Removal H2S Natural gas[44, 45]Recovery He Natural gas [46]

Removal VOL Air of exhaust streams [47]

Figure 2.1: Schematic representation of membrane separation

ratio of percentages that components can be permeated [48]. Materials are not yet

available for the separation of all existing gaseous mixtures[49].

2.2 Inorganic membranes

Membrane materials are classified as organic and inorganic. Since 1980s scientists en-

visioned the possibility of gas separation or purification using ceramic membranes, and

a lot of studies have been done. Nowadays, there is already a wide variety of commer-

cial inorganic membranes. Compared with organic membranes, inorganic membranes

can work at higher pressures (up to 10 MPa) and can be cleaned with steam, whichis impossible with organic membranes [15]. The most common inorganic membrane


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2.2 Inorganic membranes 7

materials are silica[5052], carbon-silica [53], zeolites [5456], glass [57], metal[58,59],

alumina [8, 5961], and ceramic [62,63] recently.

As introduced above, inorganic membranes can be categorized on the base of their

critical pore diameters. There are porous membrane, micro-porous membrane andnon-porous membrane, which is also referred as dense membrane and is commercially

available. There are slight differences in the definition and recognition of micro-porous

membrane, e.g. 0.52 nm by M.L. Mottern [14,15].

Table 2.2: Classification of inorganic materials on pore size[4]

Membrane Pore diameter

Porous membrane 0.005 - 1 m

Micro-porous membrane 0.001 - 0.005 m

Dense membrane


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Balachandran et al. studied dense ceramic metal composite membranes. These are

thermodynamically and mechanically stable with reasonable flux for hydrogen per-

meation[64, 65]. Peinemann achieved hydrogen separation from nitrogen with dense

ceramic hydrogen membranes using hydrogen partial pressure gradient as the drivingforce[66]. Palladium and its alloys are usually the first choice for dense metal mem-

branes to separate hydrogen [62]. Palladium membrane allows transport of hydrogen

solely, but are very expensive and have low durability.

2.2.2 Porous inorganic membranes

Porous inorganic membranes, such as porous polymeric or ceramic membranes, areused for ultrafiltration and gas separation. Ultrafiltration is one of the pressure driven

membrane processes, and the pore sizes of the membranes are from 1 to 50 nm [ 3]. In

these membranes, the pore diameter is smaller than the mean free path of the product-

gas molecules. Usually the greater the difference between properties of molecules, such

as molecular weights, sizes or shapes, the more effective the separation.

Although dense inorganic membranes can achieve high separation factors, gas fluxes

through porous membranes are much higher. Therefore, many researchers [6771] useda porous structure material as substrate, and covered it with a thin dense inorganic

membrane layer to increase the separation factor.

Porous membranes are made of microporous media, such as graphite, sintering metal

[72], metal oxide[72], ceramic, polymer and hollow-fiber [73], and might be symmetrical

or asymmetric, charged or uncharged [64]. Cement membranes are also porous.

Gas separation generally results from four types of transport mechanisms: Knudsen

diffusion, surface diffusion, capillary condensation flow and molecular diffusion. Asshown in Figure2.2a, Knudsen diffusion suits to larger molecular weight ratios. Its

separation is in inverse proportion to molecular weight. surface diffusion in Figure

2.2b is more useful for vapor separation, and usually happens when membrane pore

diameter(dp)< 10nm. The surface concentration gradient forces the transport. When

the membrane pores are extremely fine usually dp < 10 nm, and one component in gas

mixture is condensible while others not, it is capillary condensation flow or partial

diffusion as shown in Figure2.2c. In molecular diffusion or molecular sieving in Figure

2.2d, the pressure difference forces the transport of different-sized molecules, and thegas molecule larger than the pore diameter is screened[2, 1520].


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2.3 Porous cement membranes 9

Figure 2.2: Transport mechanisms in porous membranes [1]

2.3 Porous cement membranes

The potential applications of a porous inorganic membrane depend on the physical

properties of the membrane, include thermodynamical and mechanical stability which

determine the conditions and duration the membrane can be employed, membrane

thickness which influences the permeation rate and separation factor, pore size and

pore size distribution which relate to the transport mechanisms [1, 64].

Cement is chemically and physically stable. Firstly, it has a very low coefficient of ther-mal expansion (105 K1), so when temperature rises, no distortion happens. Secondly,

cement has relatively high compressive strength (107 Pa). As a result, a cement mem-

brane installed into a membrane cell will not be easily destroyed by rational gaskets

setting for gas-tightness. Thirdly, cementitious material with a tiny porosity can be

obtained. Fourthly, the finished product can cover a wide range of physical properties

and strengths, or chemical, volumetric and thermal resistance properties.

Due to its high compressive strength, cement has been widely applied in the field ofconstruction. Cementitious material is mainly cement, but also of water, aggregate,

and chemical admixtures. There are many sorts of cements, such as Portland cement,

Natural cement, Pozzolanic and slag cements, Masonry cement and Generic cement

[74]. Different denominations correspond different additivesalso referred as cement

ingredients, such as fly ash and slag cement. Cement can also have different equivalent

water to cement ratios. By varying the proportions of materials and the production

processes, we can get different types of porous cement membranes.

Cementitious material is much cheaper than many other membrane materials, has lowmoisture permeability and high durability, and can be recycled. No report on gas


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separation with porous cement membranes exists to date, which arose the necessity of

the present work.

2.4 Separation and process design

Process design focuses on either recovery or purity of the product, depending on

whether the product of a separation program is the permeate-gas or retentate.

2.4.1 Possible flow patterns

A module is defined as the smallest unit plugged with a membrane. Therefore, inthe single-stage process, membrane cell with the membrane in the center is a module.

Many different flow pattern operations through a module are possible, as shown in

Figure2.3. The existing flow patterns includes the skew flow, the parallel flow and

cross flow (Figure2.3D [31, 75]). And the parallel flow could be co-current (Figure

2.3A), counter-current (Figure 2.3B [13]) or both (Figure 2.3C). In dead-end type

(Figure2.3E), there is no way for retentate outgoing. Gas-mixture enters the module

Figure 2.3: Schematics of possible flow patterns [2, 3]

as feed gas, some of the components permeate through the membrane and exit via thepermeate-pipe, and others will stay in the feed side. As has been pointed out in a


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2.4 Separation and process design 11

host of studies, the type of flow patterns impacts the degree of separation significantly.

Counter-current flow pattern usually performs separation much better. It is the most

efficient type among these flow patterns [2,76, 77].

Contrary to common practice, sweeping gas was also fetched in the module. Sweepinggas carries the permeation components away instantly and continuously, which reduces

the permeate partial pressure and enhances the driving force. Furthermore, by ad-

justing the permeate-side pressure, sweeping gas could also be used to influence two

other factors on permeation, those are permeate pressure and the flow rate of carrying

gas. Knudsen diffusion was expected to be achieved without pressure difference. In

order to obtain better performance, the counter-current flow pattern was adopted in

our experiments, as shown in Figure2.4. Experiments without sweeping gas were also

performed to confirm the impact of carrying gas on gas separation.

Figure 2.4: Flow pattern in presence of sweep gas [2]

In order to analyze the effects of gas types, N2 and CO have been used as carrying

gases. Since the sweeping gas itself has some permeability through some materials, it

will generally appear in the retentate [2].

2.4.2 Number of stages

In order to enhance operational efficiency, meet process criteria and reduce operating

cost, most commercial applications of gas separation through membranes are designed

as multistage and recycling processes (see Figure 2.5two-stage separation schemes,

and Figure2.6three-stage separation schemes). Different membrane process designs

exist. If target separation purity is too high, decreasing flow recycling might help

achieve the target. Increase in membrane surface does not always obtain the corre-

sponding progress in separation. However the high economic cost makes huge mem-

brane surface not suitable for commercial applications. In a word, the membrane unitis necessary to be multistage. Due to economic constraints, the recycling and cascade


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Figure 2.5: Schemes of commercial two-stage separation[2, 3]


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Figure 2.6: Schemes of commercial three-stage separation[2, 3]

are essential, and in some design they are used for the final product purification after

membrane separation.

There are also recycling processes for single-stage, as illustrated in Figure 2.7. Since

sometimes increasing membrane area will not improve separation, a single-stage mem-

brane separation is often insufficient to match the separation target in the industry.

Furthermore, the higher product purity results in lower product recovery, which is noteconomical. However, single-stage is the basic building block of gas permeation pro-

Figure 2.7: Novel single-stage separation with recycling[2, 3]


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cesses. A detailed knowledge is required for the development of such processes. Hence,

in order to study the influence from operating conditions and the inherent properties of

membranes on the operation of the permeation stage directly and simply, single-stage

module without recycling is used in this project. If it is proved to be successful, themultistage-research might be a further topic in this area.

2.4.3 Known influence of operating parameters

Relevant parameters include: temperature, pressure difference between two sides of

a membrane, pressure, feed-flow rate, feed composition, sweep-flow rate, sweep-gas

composition, membrane area, equivalent water to cement ratio, membrane thickness,

pore size, pore distribution, membrane shape, permeabilities and selectivity. So far,the following relationships have been characterized:

1. T. C. Merkel [21], using size-sieving rubbery polymers without pressure difference

between two sides of the membranes, found out that more soluble penetrants are more

permeable, and that the permeability coefficients increase with increasing pressure.

Penetrants are the permeated components. However, H2, N2 and O2 are essentially

independent of pressure [22]. Furthermore, at low permeate-pressure, for both rub-

bery and glassy polymers, permeation components with large molecular size give lowdiffusion coefficients.

2. Each model has a maximum operational temperature limit. In the affordable tem-

perature range, the higher the temperature, the better membrane permeability, but

the smaller separation factor and selectivity [2].

3. Permeate to feed-flow pressure ratio, and their difference are both important for

membrane separation[2, 32].

4. Increasing feed-pressure and decreasing permeate-pressure both increase permeationrate and purity of the retentate. However, the higher the feed pressure, the higher the

costs[2, 32].

5. Larger membrane areas result in higher retentate purities and lower purities of

permeate gas [2,32].

6. Generally speaking, high product recovery is associated with low product purity

[2, 32].

7. The larger the membrane surface the higher the membrane selectivity, though thismight increase the overall membrane process costs[2].


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8. Given the membrane, the permeate-flow rate will not be significantly affected by

the feed-flow rate. However, the purity of the retentate will be reversely affected [2].

9. The counter-current flow pattern usually performs separation much better than other

patterns in the parallel flow models. For an asymmetric membrane, the separation willbe better performed by the cross flow rather than the counter-current flow[2].

10. Increasing difference between molecules in molecular weights, sizes or shapes will

raise separation efficiency[2].

11. In the permeation of non-condensible gases in membranes, diffusion coefficients are

independent of permeation concentrations [26].

12. Permeation rate is inversely proportional to membranes thickness [1].

13. Some Knudsen separation factors obtained by Klaas Keizer [15] at room tempera-

ture are listed in Table2.3:

Table 2.3: Separation factor of some typical gas mixtures

Gas mixture Knudsen separation factor

O2/N2 0.94

H2/CH4 2.83

H2/N2 3.74

CO2/CH4 0.60

And Renate M. de Vos et. al. obtained the selectivities of H2/CO2 3.9 at 100 Cand

6.8 at 200 C, when the average pressure was 1.5 bar, and pressure difference between

feed-flow and permeate-flow was 1 bar [51]; Koros and Mahajan have experimented with

similar membranes for gas separation and got a separation factor of 6.75 for H2/CO2at low temperature [78].


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Chapter

3Experimental Setup

3.1 Flow chart

Figure 3.1: Process schematic of gas separation

The flow chart of H2 permeation measurement through a cement membrane included

a heating system is shown in Figure3.1. Separation experiments were performed with

H2or mixture gas containing H2as feed gas, and with N2or CO as sweeping gas. Thesweeping gas also ensured pressure balance between two sides of the membrane. The


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18 Chapter 3 Experimental Setup

gases were stored in high pressure cylinders and reducers were used for pressure control

(2.4 bar).

Before gases entered the membrane cell, their flow rate was set at the wanted level

by mass flow controllers (MFC). Pressure difference between the two membrane exitswas measured as well as the absolute pressure of permeate-gas. Finely regulated valve

could change the pressure difference between the two sides of the membrane. In the

measurements, the membrane cell was heated in a gas chromatography (GC) oven and

temperatures were measured with type-K thermocouples.

The permeate-gas was detected and analyzed by aGC. The flow rates were measured

by two soap film flow meters located after the GC. The amount of H2in the effluents of

sweep-side was calculated from the character spectrum peak of the product and pure

H2 as reference gas. The residual gases went directly through the flow meter to the

outside at atmospheric pressure.

Figure 3.2: Process schematic of reference measurements

Qualitative and quantitive analysis of gas separation through porous membranes were

both needed. Therefore, so were the spectrum peaks of the reference gases in order

to calculate their percentage. The process of reference gas measurements is shown inFigure3.2. Pure gases of feed-components were usually injected into the front column


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3.2 Experimental setup 19

ofGC, while sweeping gas was analyzed by the back detector. In addition to hydrogen

and nitrogen, the amount of other gases could be measured by the detectors using He

or N2as reference gases.

3.2 Experimental setup

Membranes

Three different membrane main-cells were employed in this project, whose settings

progressively allowed to solve problems of heat-stability, air leak, gas turbulence. In

the experiments, membranes with different mineral composition and additives wereemployed. Membranes are classified depending on shape, thickness, base materials,

additives, equivalent water to cement ratio, pore size.

The disc-shaped porous cement membranes from Maria Gaggls Diploma project were

tested in the first part of ours study. These membranes had a 50 mm diameter, 10

mm thickness, and 0.6 equivalent water to cement ratio. Using the method of mercury

intrusion porosimetry (MIP) which is a widely used technique to characterize the dis-

tribution of pore sizes in cement-based materials [79], the average pore diameter is 110

nm.

At a second stage, fifty disc-shaped and one tubular cement membranes made by Gregor

Gluth, from theInstitut fr BauingenieurwesenofTechnische Universitt Berlinwere

used. The disc-shaped membranes had thickness of 5, 10 and 20 mm. There were seven

different kinds of base materials, and two additives. Equivalent water to cement ratio

was 0.25, 0.30, 0.35 or 0.45. Most of the pore diameters in these materials were from

8 to 100 nm. Details will be introduced.

The scientific literature reports about hydrogen purification achieved using large poredsubstrate materials coated with a selective surface layer. Accordingly, 7 nm-surface-

layer sample membranes were used in the 3rd part of this study. The membranes

produced byKERAFOL-Keramische Folien GmbHas alpha stage products were made

of ceramic. The substrate had pore diameter of 2 m and thickness of 6.0 mm.

Sealings

For the primary membrane cell, the seal rings between membrane and metal shell weremade of Teflon. However, as Teflon is only suitable for temperatures lower than 250


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C, graphite gaskets were used to perform experiments of gas separation at higher

temperature.

Gases

In the experiments, at least eight kinds of gases were used. N2 and He were reference

and sweeping gases for the gas chromatography. In the measurements, feed gas could

be H2, CO, CO2, CH4, two-component gas (49.8 % volume percentage H2 and 50.2

% CO2), and five-component mixture gas (the volume percentage reported on the

cylinder were 13% H2, 16% CO, 13% CO2, 53% N2, and 5% CH4). Sweeping gas

were mostly N2 and sometimes CO. All gas cylinders were bought from the company

ALPHAGAZTM1Ar.

Oven

Three heaters were employed in this project.

The first one was a drying machine manufactured by Heraeusin Hanau, type T6060.

It was employed to dry the membranes and remove the excess moisture. The dried

membranes were then put into the desiccator.

In Figure3.1,the oven for membrane cell heating was aHEWLETT PACKARD(hp)

GC, type 5890A, seriesII, with maximum and minimum temperature of 400 C and

-80 C, respectively.

TheGC-oven was not large enough for the cell of the tubular membrane. Therefore,

aTemperature Controller-HT MC1 made byHORST GmbHin Lorsch was employed,

with maximum temperature 800 Cand minimum temperature 0 C. Serial interface

wasRS-232; Series-number wasRD1051000000. There was a special heating asbestinemat designed for tubular-membrane cell. The probes used in the heater to measure

temperature were made ofNiCr Ni, typeK. The heating mat covered the membranecell so closely that, the heat loss can be compensated when it is used for heating.

Pipe system

Chromatographic Service GmbH(CS, Artifical number198004 ) copper pipes were used.

Outside diameter was signed as 1/83.2 mm and inside diameter was 2.0 mm. Themaximum pressure it could withstand was 45bar.


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In order to avoid the effect caused by pipe loss, the corresponding pipelines of the

primary and secondary side had same length. The enter pipelines were much longer,

almost 2.3 m, to make sure that the gases were heated enough before reaching the

membrane.The pipes connected with the reducer valves and MFCs were made of Teflon and had

6mm length.

Mass flow controllers

Both mass flow meters/controllers (MFC) wereBronkhorst Select. One was for 750

mln/min (100 % range) N2, series number: m7211047B; F.201CV-1K0-ABD-33-V;the other mass flow controller was for 750 mln/min CO2(series number: m7211047A;

F.201CV-1K0-ABD-33-Z). Both working pressure should be between 1 and 4 bar,

which was measured at the normal condition (20 C), with 8bar He by the company

Bronkhorst Mttig GmbH.

This company also designed a special program for the project, which incorporated the

densities of gases. As a result, the N2 type can be used directly for N2, He, air, Ar,

and the CO2one can be used directly for all the feed gas mentioned above.

Gas chromatograph

Our gas chromatograph (GC) measures parts-per-billion concentrations in gaseous

samples. One of the equipment used was made byAgilent Technologies(type 6890N

and Serial number: US10149120).

ThisGC (Figure3.3[26]), plugged with the G2613Ainjector andG2614Atray, iden-

tified gas composite with thermal conductivity detectors (TCD), and had the 7683automatic liquid sampler. Analysis time was approximately 15minutes. Two sample-

source switching systems (ten-port switching valve) are used in the GCanalysis. Upon

switching, the contents of the sample loop (0.6 ml Scott Gas Mix) are inserted auto-

matically into the inlets. Better reproducibility and time-optimization were realized

by using automatic injection. Column A and column B shown in Figure3.3are both

metal tube packed columns (12392U,15 1/8 stainless steel), inside which are thestationary phase of60/80 Carboxen-1000. At the end of both columns are the TCD

detectors. The inside covered phase has the ability to adsorb and desorb some gasesin various time periods. Therefore, different components in a sample are separated by


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Figure 3.3: Gas chromatographic system

the stationary phase inside the columns, expelled from the columns at different times

which is defined as retention time, and detected by the detectors. The chromatograms

with peaks at different time were thus achieved. Each gas has a different, characteristic

peak spectrum. Both flow rate of carrier gas and the temperature can slightly alter the

retention time.

The choice of reference gases is significant. The TCD is a concentration sensitive

detector in that it responds to all solutes and determines by the thermal conductivity of

gases [80]. An inert gas helium and an unreactive gas nitrogen are used as the referencegases in this project. He is non-flammable and has relatively high thermal conductivity,

that can obtain a large measurable range of gas types. However, employing helium as

the carrier gas in a gas Chromatograph for quantitative determination of hydrogen was

difficult, because the thermal conductivity of hydrogen-helium mixtures with hydrogen

at low concentrations is anomalous [81,82]. The representative peak of H2with He as

reference will be very small. In order to avoid large calculation errors, N 2 was used as

a carrier gas with a respective channel for H2. The reference gas flow rate was set at

20 ml/min.

Both permeate-gas and retentate were measured synchronously. The concentrations of

gas components easily change with time, and are sensitive to temperature and pressure.

That is also the reason for using two columns as this allows to have both He and N2as reference gases for the measurements.

We chose the parameters at fixed values reported in Table3.1.

A chromatogram represents each component by a diagram of peaks, and corresponding

different retention times. The gas amount can be calculated from the area under itsrepresentative peak through a calibration curve. For instance, the chromatogram of


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Table 3.1: Operating conditions of Gas Chromatograph

inlet pressure/[kPa] 200.0

Columns outlet pressure Ambient

max temperature/[

C] 230pressure/[kPa] 200.0

Reference flow front (back) gas N2(He)

flow rate/[ml/min] 20

heater/[C] 230

Detectors front det. N2-Negative Polarity

back det. He

set point/[C] 60 (1 min)

maximum/[C] 225Oven heating rate/[C/min] 20.00

target value/[C] 205 (2.25 min)

whole run time/[min] 10.5

valve1 (front) on/[min] 0.00

Runtime valve2 (back) on/[min] 0.01

valve1 (front) off/[min] 0.10

valve2 (back) off/[min] 0.11

Thermal Aux #1 heater/[C] 80Signals data rate/[Hz] 20

minimum peak width/[min] 0.01

the five-component gas is shown in Figure 3.4. Here the x-coordinate is retention

time, where retention time zero means injection time. Thus, gas-1# represents H2

Figure 3.4: Chromatogram of five-component gas


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with a retention time of 1.316 min, gas-2# is N2 of 3.544 min,gas-3#is CO of 4.437

min,gas-4# is CH4 of 7.318 min, and gas-5# is CO2 with a character peak at 9.495

min. The area of each peak under the curve is the peak area mentioned above. The

chromatograms have good reproducibility.

Thermocouples

Thermocouples were employed to measure the temperature near the surface of the

membrane. One thermocouple was used with the disc-shape membranes. With tubular

membrane, four thermocouples were placed at different positions around the membrane,

to check whether the membrane was heated uniformly. All thermocouples were made

byElectronic Sensor, typeK, with diameter of 1.0 mm and length of 1000 mm. Their

measurable range of temperature went from -200 to 1000 C, with tolerances of0.4%. The experiments with disc-membranes were run at 350 C, those with the tubular

membrane at 800 C.

Pressure meter

A pressure meter was installed to obtain the absolute pressure of the permeate agent,at the exit of the sweeping side. The KELLERpressure meter had a precision of 0.05

%F S(0.003% as linearity error at 25 C). This typePAA-33X/3bar/80794(Serialnumber: 106276) a measurement-range of 0 to 3 bar, and can work at temperature

between -10 C and 80 C. Connected with theBinder 723 and multibus NI 9203,

PAA-33X sends the signal to computer. The output signal is converted from the

ampere value (4 20mA ) to pressure value (0 3bar ) via aLabVIEW program.

Pressure difference meter

A pressure difference meter is connected directly with both exits of the membrane cell

via two triple-connections. The finely regulated valves, on outgoing pipes of the cell,

adjust the pressure of gases at both sides by screwing. Thus, the pressure difference

between two sides of the membrane could be measured by the pressure difference meter.

This meter was also made by KELLER, type ofPD-33X/-0.1...0.1bar/80920(Serial

number:102764 ). It worked at the temperature range of -10 Cto 80 Cand measuredthe pressure difference between -0.1 and 0.1 bar with a linearity error of0.002%FS.


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Same as the pressure meter, PD-33Xshows us the digital value of pressure difference

with the help ofBinder 723, multibusNI 9203 andLabVIEW.

National Instruments Lab View

National Instruments Lab Viewwas the graphical program in the project. It controlled

the running ofGC and MFCs and measured most of the operation-parameters syn-

chronously. For instance, the parameters included the absolute pressure of permeate-

gas, pressure difference between the two sides of the membrane (NI 9203 8-channel

20 mA, 16-Bit analog input module), the chromatogram fromGC(from which theamount of certain component in the permeate-gas was possible to be calculated ), the

flow rate (RS232, value ofMFC), and the temperatures at different positions of the

cell or ambient (NI 9211 4-channel thermocouple input module, see Figure3.5). With

the module box-NI cDAQ-9172,LabVIEWbecomes an instrument for various devices

administration and numerous data acquisition. It realizes multi-processing in one pro-

gram. This multi to one is of easy controlment, fast collection and accurate analysis.

Figure 3.5: LabVIEW controlling system


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Soap film flowmeters

Soap film flowmeter is a glass tube marked with volume lines. It is suitable for checking

flow rates. Since the flow rate had changed after diffusion and permeation through the

membrane from both sides, two HEWLETT-PACKARD 1-10-100ml 0101-0113-soap

film flowmeters were used after each GCdetector, before the gas going out to vent-

pipe. In theory, every timeGCinjects the same amount (0.6 ml) of test gases, and the

loop temperature and pressures are invariable, so the changing of the flow rate should

not affect the GCmeasurements. In order to make sure the impact of flow rates on

GCmeasurements as well as verify the accuracy ofMFC, it is necessary to measure

the volumetric flow rate again.

Since the soap film flowmeter has four volume marks of 0, 1 ml, 10 ml, 100 ml, sixmeasuring volumes are available. Those are 1 ml, 10 ml, 100ml, 10 1 = 9 ml,100 1 = 99ml, 100 10 = 90ml depending one bubble speed. After interposed intothe flow path by the pump, a flat soap film moves from one volume mark to another. A

stop watch is used to record the travel time. Then the flow rate can easily be calculated

as ratio of volume to the travel time.

Pipe cooling system

The suitable working environment for pressure meters should be no higher than 80 C;

the temperature of sample-gases injected intoGChad to be lower than 60 Cand GC

analyses the samples by increasing the gas temperature from 60 to 230 C. In that

way, the gases had to be cooled down enough before reaching the devices.

The gas-pipes were very thin and the flow rates for measurements were lower than 100

ml/min. Therefore, pipes of half meter were chosen. Wet paper ensured safe operation

of instruments and the precision of the results (keeping the samples almost at the same

temperature lower than 60 C). In the pipe cooling system, paper were dewed, then

wrapped over the pipe, and later re-wetted every twenty minutes with distilled water.

3.3 Operating parameters and procedure

Errors control

In this work, each permeate-sample was obtained as average of three stable measure-ments. The first GCmeasurements, either as the membrane vessel or pipeline still


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3.3 Operating parameters and procedure 27

had residue gases from last measurements, or because of incomplete warm up. Mea-

surements usually became stable after one hour ofGC operation. In order to avoid

experimental errors arising from atmosphere, the gas-samples of comparison were done

in a continuous way and at the same conditions.When using the soap film flowmeters, a flat soap film was made to measure the flow

rate of gases. Actually one film is not enough, we made more bubbles and chose the

sixth or seventh film for calculation. Furthermore, taking the accuracy into account,

measure range should be between 10 to 200 ml/min.

Operational steps

1 . Open N2and He gas for GC.

2 . Switch on computer and the power of theNational Instruments(NI cDAQ-9172);

Write down the time.

3 . Connect theGCmachine (6890N (G1530N)) to the computer and open it. When

theGCwas ready, run the Instrument 1 Online program on the desktop.

4 . Activate the two FlowDDEs (MFC, V4.58 (MBC)); and change the gas types in

the program ofFlowView (V1.15).

5 . When the temperature of the GC reached 80 C at both detectors, change the

Methodto FNBHE.M, and record the time. After the temperature rising upto 230C, run the detectors and choose the Negative for the front detector. Record the

time (Denoted as the time A).

6 . Change the paper on the pipes over the oven to wet paper, then power on the oven

and change the setpoint of temperature to 350 C. Record the time (Denoted as the

time B).

7 . Open the gases for measurements in two minutes of time A(Here take H2 and N2for example).

8 . Opentest LabVIEW Instrumentfile, which could not only controlMFCs but also

display temperatures, pressure and pressure difference. Run it and set the flow rate to

3 %. Record the time.

9 . Thirty minutes later than time A, press Start onGC, reference-gases (See Figure

3.2) were measured. During this period, the membranes were dried fully. The programwas regulated to stop automatically in 10.5 minutes. Wait until theGCinterface was


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ready, that meanGCwas well prepared for the next measurement. Then make sure

the active base line are stable, then Start again. Record the values into the form.

10 . When the detectors stop injecting gases at the last reference-value measurement,

set the flow rates of gases in thetest LabVIEW Instrument fileto 0 first, then changethe pipe-connections to the experimental stateadd the main cell as shown in Figure

3.1.

11 . Set the flow rates of gases in the test LabV Instrument fileto 30 %. Use soap

bubble to test the gas-tightness of the connections. If there was no air leak, change the

flow rates in the test LabVIEW Instrument fileback to 0 again.

12 . Sixty minutes later than the time B, set the flow rates back to 3 % again, and

record the time.

13 . Ninety minutes later than the time B, begin the experiments.Press the button

Start on GCand begin measurement. The program stopped automatically in 10.5

minutes. Wait for ready and make sure the base-line is stable, then Start again

as above. Measurements of five temperature groups were done: 350 C, 350 C(II)20

minutes later, 200 C, 100 C and ambient temperature. Each group contained four

measures. Generally, the first value could not be used. When a lower temperature

was required, setpoint was better to be 20 Cor even 50 C lower than the standard

to save time.

14 . Record all the data into the form.

15 . When all the measurements are finished, switch off the detectors first. Then

change themethodof the Instrument 1 Online program to STBY1.M.

16 . Set the flow in thetest LabVIEW Instrument fileto zero.

17 . Change the connections of the pipes back to the reference way as shown in Figure

3.2.

18 . Set the flow rates in the test LabVIEW Instrument file to 30 %. Use soap

bubbles to test the gas-tightness of the connections. If the tightness was good, close

the gas bottles of H2and N2.

19 . Take off the paper over the pipes, and put them together.

20 . When the flow rates in the test LabVIEW Instrument filedecreased to zero,

change the setpoints to zero and close the testfile.21 . Press F4 to halt running ofDDEs. Power off the National Instruments.


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3.3 Operating parameters and procedure 29

22 . When the temperature ofGCdetectors is below 80 C, turn off the window of

Instrument 1 Onlineprogram, shut down the computer and GC machine.

23 . Close the bottles of N2an He. Shut down the fuming cupboard.

24 . The last task is data processing.


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Chapter

4Summary of Equations

Cement separation ability and gas transport mechanisms are still object of research.

Corresponding equations are reported in this chapter. The equations used for data

acquisition and processing are also listed. Finally, the expressions are introduced,

which will be used to quantify the membrane performance in gas separation.

4.1 Basic assumptions

The assumptions employed in analysis and design are as follows [2]:

1. No pressure drop and end effects caused by the pipes and connections or GC, hence,

the flow rate is unaltered between the outgoing section of the membrane cell and the

outgoing section of theGC;

2. No influence on gas viscosities from pressure;

3. Uniform distribution of gas;

4. Steady-going permeation at the same condition;

5. No physical deformation of the membranes;

6. No gas concentration gradients at cross section of the module;

7. No fouling in the pores of the membrane, so the pore sizes are invariable;

8. No pressure drop on the shell side of the membrane cell;9. Negligible pressure drop on the transverse section of the module.


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32 Chapter 4 Summary of Equations

4.2 Gas equations

4.2.1 The fundamental equations for ideal gases

The ideal-gas equation says that [49]:

pV = (z)nRT (4.1)

where,

pis the absolute pressure of the gas (Pa);

Vis the volume of the gas (m3);

zis the compressibility factor, depending on conditions and phase of the compo-nents. For an ideal gas zis constant;

nis the amount of gas present, normally expressed in moles;

Ris the universal gas constant (8.314472 Jmol1K1);

andT is the thermodynamic temperature (K).

Since gases were flowing all the time in our experiments, we introduced the time into

Equation (4.1). The volumetric flow rate v (m3/min) is equal to the volume V (m3)

divided by timet(min):v=

V

t. (4.2)

By replacing V, we get:

pvt = nRT. (4.3)

That can also be written as :

pv=n

tRT. (4.4)

At room temperature, when the MFCcontrols the gas flow at a fixed value, the mo-

lar amount of gas flowed per unit time remains unchanged, which means that n/t isconstant. Meanwhile, both ends of two gas channels are opened to the atmosphere,

so p can be considered equal to atmospheric pressure, which is also constant. From

Equation (4.4) v is directly proportional to temperature T. Therefore, when the im-

pact of temperature on gas separation is studied, whether the flow rate affects GC

measurements should also be taken into account.

4.2.2 Balances

At ideal conditions, the total amount of materials stays invariant [49].


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4.3 Equations for transport mechanisms through porous membranes 33

Material balances

For counter-current flow in our experiments, the overall material balance is expressed

as follows,nF +nS =nP +nR (4.5)

For the i component,

nFi =nPi +n

Ri (4.6)

nFxFi =nPxPi +n

RxRi (4.7)

wherex is the mole fraction of certain component, Fis the feed gas, Pis the permeate

gas,Ris the retentate gas, and Sis the sweeping gas.

From Equation4.3,we get,

n=P V

RT =

P vt

RT (4.8)

Therefore, the Equation (4.7) could be written as:

P vFt

RT xFi =

P vPt

RT xPi +

P vRt

RT xRi (4.9)

vF

xF

i =vP

xP

i +vR

xR

i (4.10)

Membrane rate balance

For each componenti,

nPi = (Jn)i (4.11)

nPxPi = (Jn)i (4.12)

where,(Jn)i(mol) means molar permeation rate of component i.

4.3 Equations for transport mechanisms through porous mem-

branes

As described above, there are generally four gas transport mechanisms in porous

membranes: Knudsen diffusion, viscous flow, surface diffusion and molecular diffu-sion [5, 14, 17,83].


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Knudsen diffusion

When the pore size is much smaller than the mean free path of the molecule (expressed

in Equation (4.13)) [84] and molecular weight ratios are larger, Knudsen diffusion

occurs. It can be described as Equation (4.14) [83]:

= kBT

2pm2(4.13)

Di,K=4rp

3

2RT

Mi(4.14)

here,kB is the Boltzmann constant, is collision diameter, Di,Kis the Knudsen diffu-

sion coefficient (m2/s), means the fluid viscosity (kgm1

s1), rp is the pore radius

(m), Mi is the molecular weight of component i, and pm is the average pressure in

the membrane (Pa). Usually researchers reckon that the transport is mainly Knudsen

diffusion (Equation (4.43)), ifrpis between 530, and /dp > 1 [5]. The expression/dpis called the Knudsen number (NKn). It is also reported that, Knudsen diffusion

predominates when the Knudsen number is far larger than one; when it is far smaller

than one, the transport mechanism is mainly molecular diffusion and when it nears one

the transport is transition diffusion [83]. The diffusivity of the transient region can be

expressed in the following formula[85].1

D=

1

Di,K+

1

Di(4.15)

where D is transition diffusion coefficient, Di is molecular diffusion coefficient, p =

Ap/Athe porosity of the membrane, Ap is the pore area, and is the pore tortuosity,

is the thickness of the membrane.

Viscous Flow

Viscous flow (also referred to laminar flow) is one of the transport mechanism in porous

membranesthe capillary condensation flow or partial diffusion as discussed in Chapter

2. According to Professor Koltuniewicz [5],

L =r2p p P

8 R T (4.16)

For gas:

i= 1

N d2m

Mi R T

3 (4.17)


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4.4 Equations for the experimental setup 35

Surface diffusion

Surface diffusion can occur in parallel with Knudsen diffusion. Some components can

be adsorbed onto the pore walls and move along the surface. Thus the more adsorbable

components can permeate further using surface diffusion[1].

JS= S DS (1 p) dC

d (4.18)

here, is the density of the membrane, Sis the tortuosity of the surface, DS is the

surface diffusion coefficient, andCis the molar density (mol/ml). At high temperature,

adsorbability can be restrained, so Knudson diffusion will be more ascendant.

Molecular diffusion

Molecular diffusion (also referred as molecular sieving) dominates the transport mecha-

nism when membrane pores are of similar size to the molecule. It is a highly restrictive

diffusion. The smaller the permeation components, the faster to diffuse.

The molecular diffusion obeys the following relation[17, 86]:

Di = e2 kB T

h exp(

SDR

) exp(EDR T) =

18.58T3/2 [(Mi+Mj )/MiMj ]1/2p2i,j

(4.19)

where e is the base of the natural log, h is Plancks constant, SD is the activationentropy of diffusion (kJmol1K1),EDis the activation energy of diffusion (kJ/mol),andis the collision integral.

4.4 Equations for the experimental setup

4.4.1 LabVIEW

National Instruments Lab Viewcontrols the running ofGC andMFCs and measures

synchronously most of the operation-parameters such as absolute pressure of permeate-

gas, pressure difference between the two sides of the membrane. MFC and GC can

be controlled directly, but the pressure values must be converted first. The signal

from pressure meters are in the form of ampere values I(A). The outcome range of

the pressure meter is 4 20 mA, and the measurable pressure range is 0 3 bar.Furthermore, the pressure difference meter should also convert the signal from 4 20

mA to0.1 0.1bar. Hence, the conversion Equation (4.20) is shown bellow:PI= 187.5 I 0.75 DPI= 12.5 I 0.15 (4.20)


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4.4.2 Soap film flowmeter

The soap film flowmeter has four volume marks of 0, 1 ml, 10 ml, 100 ml, and six

measuring volumes are available. After selecting two mark line, a stop watch is used

to record the travel time t (min). Then the flow rate can be calculated easily, using

the measuring volume divided by the travel time. Equation (4.21) shows the example

of the marks of 10 ml and 100 ml.

v=100 10

t (4.21)

4.4.3 Mass flow controller

MFCcan be regulated byLabVIEWprogram. The set value is in the form of percent-age. BothMFCs ranges are 750 mln/min (100%). The setted flow rates are actually

equal to 750 mln/min multiplying by the percentages.

vn= 750 j

100 (4.22)

here, natural number j (0


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4.5 Efficiency of gas separation through membrane 37

Figure 4.1: Chromatograms of two-component gas and pure standard-gases

4.5 Efficiency of gas separation through membrane

Factors affecting separation ability of a membrane include: diffusivity of species in the

membrane, group complexity, crystalline, free volume, orientation, fillers, humidity.

There are four parameters to determine the performance of a given membrane, i.e. the

efficiency of gas separation.

Separation factor

One of the parameters to indicate the ability of separation is the separation factor,

which is also referred to as the relative split ratio and the separation power. Therewere two key components in the multicomponent feed gas, H2 and CO2. Separation

factor,S, shows the relatively sharp separation between these two key components. It

is defined in Equation (4.25) [3], by the compositions of the two products in feed and

permeate-gas (or retentate and permeate gas Equation (4.26)) [87]:

SH2,CO2 = CPH2/C

FH2

CPCO2/CFCO2

(4.25)

orSH2,CO2 =

CPH2/CRH2

CPCO2/CRCO2

(4.26)


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here,Cis the molar density (mol/ml) of the fluid mixture; CH2andCCO2is the partial

molar density (mol/ml) of component H2 and CO2. For a binary system, we have

CPH2 +CPCO2

= 1and CRH2 +CRCO2

= 1, then the separation factor is readily converted

into the following forms[15]:

SH2,CO2 =CPH2/(1 CRCO2)(1 CPH2)/CRCO2

= CPH2C

RCO2

(1 CPH2)(1 CRCO2) (4.27)

Using the GCmeasurements results, it is easy to calculate mole fraction of certain

components. Thus, the Equation (4.25) could also be written as follows:

SH2,CO2 = (CPxPH2)/(C

FxFH2)

(CPxPCO2)/(CFxFCO2)

= xPH2/x

FH2

xPCO2/xFCO2

(4.28)

In general, when two key components are selected, Sis better to be much larger than1.0 or far lower than 1.0. In the case ofSH2,CO2 < 1, we can consider the opposite of

the representation, i.e.,SCO2,H2. Then, when the separation factor is in the expression

of greater than 1, the larger value corresponds to the higher separation effectivity, and

a value close to 1.0 spells a low degree of separation [88]. As S the membranetends towards super selectivity.

Permeability

The permeability of certain component is also used in several articles. It has already

been formulated in the previous equations. Some researchers named it as selectivity

[89]. Here we use the former-permeability ().

i=CPiCRi

(4.29)

Thus, Equation (4.25) could also be written as:

SH2,CO2 = H2CO2

(4.30)

Permeation rate

Another parameter that can indicate the ability of separation is the permeation rate [3].

The rate of molar transfer of certain component is usually expressed as the molar

amount of the component passing through unit area of the interface per unit time,

thus,Ji=

vP CPiA

(4.31)


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4.5 Efficiency of gas separation through membrane 39

here,A is the surface area of the membrane (cm2),J(molmin1cm2) means perme-ation rate.

According to Koichis conclusion[90], we have

Ni=1

Ji 0 (4.32)

These should also be verified in the experiments.

Separation could take place as a result of the different permeation-ability of a given

membrane for dissimilar gasesone or some components are able to permeate a mem-

brane easier than others. The permeation ability depends on not only the different

properties between the membrane and components but also the driving force between

both sides of the membrane.

The driving force comes from the gradient or difference in some generalized quantity,

such as concentration, pressure, or temperature between both sides of the membrane.

Furthermore, the permeation rate, which sometimes is also called flux or absolute

activity, is proportional to the driving force [49]. The relationship can be expressed by

the following equation:

Ji= k dF

d (4.33)

whereFrepresents the driving force that causes the trend of transporting between bothsides of the membrane, k is the phenomenological coefficient, and is the coordinate

dimension in the membrane.

Although there are many different means of expression, the most common expression

is the mass transport recently, which is referred toFicks lawof diffusion in Equation

4.34, and volume flux results from pressure difference, which is depicted as permeation

inDarcys lawin Equation (4.35) [3, 5,90, 91].

Jm= D dCd (4.34)

JV = LdP

d (4.35)

where, the diffusion coefficient Dis, the diffusivity in dimensions of m2s1. As intro-duced above, it depends on diffusing species, membrane characters, temperature and

sometimes concentration. Lis the permeability coefficient (mols1m1Pa1).

TakeDarcys lawas an example, after integration the relationship forms the Equation

(4.36) [49, 62]:(JV)i = JV xPi = L

PPxPi PRxRi

(4.36)


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Therefore, Equation (4.12) could also be expressed as:

nPxPi = LPPxPi PRxRi

At (4.37)

PPvp

RT xPi = LPPxPi

PRxRi

A (4.38)Using Henrys Law for equilibrium of molecules, Equation (4.35) could be written as:

Ji=LPRi PPi

=L

PFxFi +x

Ri

2 Patm xPi

(4.39)

here,Patmrepresents the pressure of atmosphere (Pa).

Selectivity

The last parameter is the selectivity of the membrane , defined as the ratio of thepermeabilities between components[26, 62,92].

H2,CO2 = LH2LCO2

(4.40)

Substituting Equations (4.36) into (4.40), we get:

H2,CO2 =

xPH2

xPCO2

(PPxPH2

PRxRH2

)

(PPxPCO2

PRxRCO2

)

(4.41)

When PP approaches to zero,

H2,CO2xPH2

xPCO2

xRH2

xRCO2

=SH2,CO2 (4.42)

Hence, the selectivity of the membrane is approximately equal to the separation factor

of the membrane.

In the transport mechanism of Knudsen diffusion[5],

Li,Kn =

8

rp

p3

12RTMi (4.43)

where,Li,Knis the permeability coefficient in Knudsen diffusion. Substituting Equation

(4.43) into Equation (4.40), then we get[1, 15]:

H2,CO2 = DH2,KDCO2,K

= (MCO2MH2

)1/2 (4.44)

Therefore, molecular weight ratios should be larger as mention above. Low selectivity

based on Knudsen diffusion will be got with a low weight ratio. The high value of

the permeability is one of the advantages of Knudsen diffusion. Using an extra trans-

port mechanism (surface diffusion, for instance) for one of the components will mostlyincrease the separation factor.


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Chapter

5Experimental Results and Discussion

This chapter includes data correction of controlling equipment, calibration of mea-suring instruments, and the results of all measurements, which are worked out with

the methods and formulas mentioned in Chapter4. In the project, the main model

employed was the single-module of counter-current flow pattern with purge gas. The

introductions of membranes, membrane cell, gaskets, detail procedures and results will

be listed according to the order of the melioration.

5.1 Controlling equipment and corresponding special procedures,

calibration

5.1.1 Bubble flow-meter

Measurements using soap film flowmeters are quick and results are reproducible. How-

ever, when measuring very low-rate gas flow, diffusion problems of soap-film flowmeter

arise. Especially with hydrogen at low speed, the flat soap bubbles move too slow formeasurement to be reliable. After tens of tests, I found it was not so accurate enough

if just one single soap film was tested in the glass tube, maybe because of the diffusion

problems mentioned by Jia Guo[93]. If six or seven bubbles were interposed regular

at intervals, e.g., every 10 ml, and the penultimate bubble was taken as the measure

target, then the results were reliable and reproducible. With the sixth or seventh soap

film, both sides were the pure sample gas, gases were far away from the air and no

pressure drop occurred, so that n

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Zhang Weiqi