Experimental Investigation on Gas Separation Using Porous Membranes

vorgelegt von Master-Ing. Weiqi ZHANG

von der Fakultät III - Prozesswissenschaften der Technischen Universität Berlin zur Erlangung des akademischen Grades

Doktorin der Ingenieurwissenschaften – Dr.-Ing. –

genehmigte Dissertation

Promotionsausschuss:

Vorsitzender: Prof. Dr.-Ing. Felix Ziegler Berichter: Prof. Dr. Frank Behrendt Berichter: Prof. Dr.-Ing. Bernd Hillemeier

Tag der wissenschaftlichen Aussprache: 03. December 2010

Berlin 2011 D 83

Ich erkläre hiermit, dass ich die vorliegende Arbeit selbständig verfasst und keine an- deren als die angegebenen Quellen und Hilfsmittel verwendet habe.

Berlin, den 03. December 2010

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 and Dr.-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 Röhr, who made membrane cell and many other small parts patiently for me, and helped me with all sorts of technicalities. 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 Women’s central office to 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. 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 ...... 19

3.3 Operating parameters and procedure ...... 26 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 Gaggl’s 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

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 ...... 17

3.2 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 H2 in two-component gas measurements with diﬀerent run times 47

5.7 Area of pure H2 measurements at diﬀerent temperatures ...... 48 X List of Figures

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

5.9 N2 amount and ﬂow rates in automatic injection ...... 51

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

5.11 Pure H2 peak area for calibration of 2M measurement ...... 53

5.12 Calibration curve for H2 of 2M 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 ...... 62

5.23 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 ...... 69

5.29 Inﬂuence of temperature in ...... 70

5.30 Inﬂuence of temperature in <2M;N2> ...... 70

5.31 Inﬂuence of equivalent water to cement ratio in <2M;N2> ...... 71

5.32 Inﬂuence of pore size in <2M;N2> ...... 72

5.33 Effect of different sample thickness on diffusion in <2M;N2> ...... 73

5.34 Membranes after heating ...... 74

5.35 Comparison of compositions in <5M;N2> ...... 75 List of Figures XI

5.36 Permeabilities of H2 using diﬀerent feed gases ...... 76

5.37 Inﬂuence of pressure diﬀerence in <2M;N2>...... 76

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

5.39 Measurements of diﬀerent sweeping gases in <2M;N2> and <2M;CO> 78

5.40 Inﬂuence of adhesives ...... 79

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

5.42 Knudsen number of H2 and 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 <2M;N2>...... 87

5.50 Separation factors of H2 to CO2 ...... 87

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

◦ 5.52 Chromatogram in <5M;N2> at 200 C ...... 88

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

6.1 Process schematic of using CO2 as sweeping gas ...... 95

6.2 Process schematic of using steam as sweeping gas ...... 95 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 N2 as 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 H2 concentration 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 ...... 66

5.12 Permeation ﬂuxes using PZ-2+MS in <2M;N2>...... 73

5.13 Separation factors using HOZ+MS in <5M;N2>...... 75

5.14 Separation factors using 5 mm membranes in <2M;N2> ...... 80

5.15 Data using PZ-2+MS (5 mm, (w/c)eq 0.25) in <2M;N2> ...... 81

5.16 Permeation ability using PZ-2+MS (5 mm, (w/c)eq 0.25) in <2M;N2> 82

5.17 Permeation ability using tubular-PZ-2+MS (5 mm, 0.25) in <2M;N2> 89

5.18 Permeation ability using tubular-PZ-2+MS (5 mm, 0.25) in <5M;N2> 90 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 membranes have 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 % H2 and 50.2 % CO2), and five-component gas (13 % H2, 16 % CO, 13 % CO2, 53 % N2, and 5 % CH4) were used as feed gases, while N2 and 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 by GC could 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 is collected, the best conditions and membrane type are found, cementitious material has XIV Abstract the ability to separate gas mixtures, and new designs considering puriﬁcation of the product gases are provided. Nomenclature

Abbreviations Å ångström = 10−10 m bar 0.9869 atmosphere or 100 kPa ◦C degrees Celsius, K-273.15 cm centimeter g gram GC gas chromatography J joule K degrees Kelvin kg kilogram m meter mA milliampere MFC mass ﬂow controller min minute ml milliliter mm millimeter mol gram-mole µm micron µV microvolt=10−6 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] XVI Nomenclature

CCO2 partial molar density of component CO2 [mol/ml] D diffusion coefficient [cm2/min] or [m2/s] 2 Di molecular diffusion coefficient [m /s] 2 Di,K Knudsen diffusion coefficient [m /s]

DPI value of pressure difference meter [bar] 2 2 DS surface diffusion coefficient [cm /min] or [m /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 Planck’s constant [-] I ampere values [A] j setpoint of MFC [-] J permeation rate [mol·min−1·cm−2]

Jn permeation rate in mole [mol] k phenomenological coeﬃcient [-]

kB Boltzmann constant [-] L permeability coefficient [mol·s−1·m−1·Pa−1] −1 −1 −1 LKn permeability coefficient of Knudsen diffusion [mol·s ·m ·Pa ] −2 −1 −1 Lη permeability coefficient of viscous flow [kmol·m ·s ·Pa ]

Mi molecular weight of component i [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 [J·mol−1·K−1]

rp pore radius [m] S Separation factor [-] −1 −1 SD activation entropy of diﬀusion [kJ·mol ·K ]

SH2,CO2 Separation factor of H2 to CO2 [-] t time [min] T thermodynamic temperature [K] 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 µV·s]

Greek Letters α selectivity [-] α˜ ideal selectivity [-] β permeability [-] γ coordinate dimension [mm] δ thickness of the membrane [mm] η ﬂuid viscosity [kg·m−1·s−1] λ mean free path of molecular [nm] ρ density of the membrane [kg/m3] τ pore tortuosity [-]

τS pore tortuosity of the surface [-]

εp 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

Chapter 1 Introduction

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 releasing it in the atmosphere, where it would contribute to global warming.

A number of membrane materials have already been used for various gaseous separation 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 [8–13]. Recently, research on membrane- based gas separation has especially focused on high temperature and low humidity conditions, low-cost membranes, data collection for membrane transport characterization. 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 already a wide variety of commercial inorganic membranes. Compared with organic membranes, inorganic membranes can work at higher pressures up to 10 MPa and can be cleaned with steam, which is impossible with organic membranes [15]. Inorganic 2 Chapter 1 Introduction membranes can be categorized on the base of their pore size, which is usually critical 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 gas permeation 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, 15–20]. According to T. C. Merkel [21], it has been observed that, when the pressure on the permeation side is low, the diffusion coefficients 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 H2 and 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 H2 and CO2 are 0.74 and 3.87 Å respectively, H2 molecule is much smaller. Further- more, the kinetic diameter of H2 is 0.29 nm, smaller than the 0.33 nm of CO2 [23,24].

Thus, H2 was 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, H2 should 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 hydrogen. 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 their chemical, physical, and thermodynamic stability. However, cement represents a valid attraction. It has low thermal expansion coefficients (0.9 to 1.2×10−5 K−1), relatively high 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 für Bauingenieurwesen (Institute of Civil Engineering). Different kinds of membranes with different base materials, additions, 3 pore-sizes, equivalent water to cement ratios, shapes, areas and thickness were manufactured by Gregor Gluth, a researcher working in the department of Baustoffe und Baustoffprüfung (Building Materials and Construction Materials Testing), Technische Universität 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 procedures will be described in detail in Chapter 2 and 3. 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 pressure, 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 introduced. 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 Chapter 3, the experiments are described. This chapter also details the apparatus, 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 instruments are ﬁrstly introduced in Chapter 5. Then based on the melioration order of the 4 Chapter 1 Introduction

membrane cells and membrane shapes, Chapter 5 lists the diﬀerent membrane stages, membrane cells, gaskets, and corresponding data.

5. Lastly, Chapter 6 presents the results in contrast to the best selection method for the membrane and its parameters. This chapter also lists the achieved efficiency 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. Chapter 2 State 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 [27–29]. 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 the recent 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 representation is given in Figure 2.1. 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 gas permeation volume, while separation factor expresses the membrane performance as 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 [43–45]

Removal CO2 Natural gas [31,44,45]

Separation CO2 Landﬁll gas (CH4 50 %, 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 commercial inorganic membranes. Compared with organic membranes, inorganic membranes can work at higher pressures (up to 10 MPa) and can be cleaned with steam, which is impossible with organic membranes [15]. The most common inorganic membrane 2.2 Inorganic membranes 7 materials are silica [50–52], carbon-silica [53], zeolites [54–56], glass [57], metal [58,59], alumina [8,59–61], 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 and “non-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. 0.5–2 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 µm Micro-porous membrane 0.001 - 0.005 µm Dense membrane < 0.001 µm

Membranes can also be classiﬁed as symmetric or asymmetric. Symmetric membranes, also called “isotropic”, have uniform structure and character. In asymmetric membranes, also called “anisotropic”, chemical and physical features vary along the axial direction.

Both porous and non-porous inorganic membranes are applicable for gas separations [48, 64]. However, their transport mechanisms are diﬀerent [16]. Before discussing membranes for gas separation, the transport mechanisms through a membrane are introduced.

2.2.1 Dense inorganic membranes

Dense inorganic membranes are usually made of metals and their alloys, and are especially suitable for separating hydrogen at high temperatures. Gas separation through dense inorganic membranes is based on different solubility and diffusivity of different gases through the membrane [31]. Solubility is the ability of a gas to dissolve in a membrane. Diffusivity is the property of gas molecules to diffuse through a membrane. In turn, diffusivity is determined by the molecule size.

The transport mechanism is usually described via a solution-diﬀusion model, in which components adsorbed by the membrane surface at the feed side, are then diﬀused through the membrane and desorption on the permeate side [18,27–29]. 8 Chapter 2 State of the Art

Balachandran et al. studied dense ceramic metal composite membranes. These are thermodynamically and mechanically stable with reasonable ﬂux for hydrogen permeation [64, 65]. Peinemann achieved hydrogen separation from nitrogen with dense ceramic hydrogen membranes using hydrogen partial pressure gradient as the driving force [66]. Palladium and its alloys are usually the ﬁrst choice for dense metal membranes 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, are used 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 [67–71] used a 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. As shown in Figure 2.2a, 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) < 10 nm. 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 Figure 2.2c. In molecular diffusion or molecular sieving in Figure 2.2d, the pressure difference forces the transport of different-sized molecules, and the gas molecule larger than the pore diameter is screened [2,15–20]. 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 thermal expansion (10−5 K−1), so when temperature rises, no distortion happens. Secondly, cement has relatively high compressive strength (107 Pa). As a result, a cement membrane 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 of construction. 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 additives—also 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 low moisture permeability and high durability, and can be recycled. No report on gas 10 Chapter 2 State of the Art 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, in the 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 Figure 2.3. The existing flow patterns includes the skew flow, the parallel flow and cross flow (Figure 2.3D [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 (Figure 2.3E), 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 the permeate-pipe, and others will stay in the feed side. As has been pointed out in a 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. Sweeping gas 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 Figure 2.4. 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 2.5—two-stage separation schemes, and Figure 2.6—three-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 corresponding progress in separation. However the high economic cost makes huge membrane surface not suitable for commercial applications. In a word, the membrane unit is necessary to be multistage. Due to economic constraints, the recycling and cascade 12 Chapter 2 State of the Art

Figure 2.5: Schemes of commercial two-stage separation [2, 3] 2.4 Separation and process design 13

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 membrane separation is often insuﬃcient to match the separation target in the industry. Furthermore, the higher product purity results in lower product recovery, which is not economical. However, single-stage is the basic building block of gas permeation pro-

Figure 2.7: Novel single-stage separation with recycling [2, 3] 14 Chapter 2 State of the Art 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, the multistage-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 rubbery and glassy polymers, permeation components with large molecular size give low diﬀusion coeﬃcients.

2. Each model has a maximum operational temperature limit. In the aﬀordable temperature 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 permeation rate 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 this might increase the overall membrane process costs [2]. 2.4 Separation and process design 15

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 will be 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 temperature are listed in Table 2.3:

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 C and 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/CO2 at low temperature [78].

Chapter 3 Experimental 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 Figure 3.1. Separation experiments were performed with

H2 or mixture gas containing H2 as feed gas, and with N2 or CO as sweeping gas. The sweeping gas also ensured pressure balance between two sides of the membrane. The 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 exits was 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 a GC. The flow rates were measured by two soap film flow meters located after the GC. The amount of H2 in 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 in Figure 3.2. Pure gases of feed-components were usually injected into the front column 3.2 Experimental setup 19 of GC, 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 N2 as 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 were employed. 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 Gaggl’s 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 distribution 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 the Institut für Bauingenieurwesen of Technische Universität Berlin were 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 pored substrate 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 by KERAFOL-Keramische Folien GmbH as alpha stage products were made of ceramic. The substrate had pore diameter of 2 µm and thickness of 6.0 mm.

Sealings

For the primary membrane cell, the seal rings between membrane and metal shell were made of Teﬂon. However, as Teﬂon is only suitable for temperatures lower than 250 20 Chapter 3 Experimental Setup

◦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 ALPHAGAZ TM 1Ar.

Oven

Three heaters were employed in this project.

The ﬁrst one was a drying machine manufactured by Heraeus in 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 Figure 3.1, the oven for membrane cell heating was a HEWLETT PACKARD (hp) GC, type 5890A, series II, with maximum and minimum temperature of 400 ◦C and -80 ◦C, respectively.

The GC -oven was not large enough for the cell of the tubular membrane. Therefore, a Temperature Controller-HT MC1 made by HORST GmbH in Lorsch was employed, with maximum temperature 800 ◦C and minimum temperature 0 ◦C. Serial interface was RS-232 ; Series-number was RD1051000000. There was a special heating asbestine mat designed for tubular-membrane cell. The probes used in the heater to measure temperature were made of NiCr−Ni, type K. The heating mat covered the membrane cell so closely that, the heat loss can be compensated when it is used for heating.

Pipe system

Chromatographic Service GmbH (CS, Artiﬁcal number 198004 ) copper pipes were used. Outside diameter was signed as 1/8”–3.2 mm and inside diameter was 2.0 mm. The maximum pressure it could withstand was 45 bar. 3.2 Experimental setup 21

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 MFC s were made of Teﬂon and had 6 mm length.

Mass ﬂow controllers

Both mass ﬂow meters/controllers (MFC ) were Bronkhorstr 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 8 bar He by the company Bronkhorst Mättig 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 CO2 one 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 by Agilent Technologies (type 6890N and Serial number: US10149120 ).

This GC (Figure 3.3 [26]), plugged with the G2613A injector and G2614A tray, iden- tiﬁed gas composite with thermal conductivity detectors (TCD), and had the 7683 automatic liquid sampler. Analysis time was approximately 15 minutes. Two sample- source switching systems (ten-port switching valve) are used in the GC analysis. Upon switching, the contents of the sample loop (0.6 ml Scott Gas Mix) are inserted automatically into the inlets. Better reproducibility and time-optimization were realized by using automatic injection. Column A and column B shown in Figure 3.3 are both metal tube packed columns (12392U,150 × 1/800 stainless steel), inside which are the stationary phase of 60/80 Carboxen-1000. At the end of both columns are the TCD detectors. The inside covered phase has the ability to adsorb and desorb some gases in various time periods. Therefore, diﬀerent components in a sample are separated by 22 Chapter 3 Experimental Setup

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 reference gases 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 H2 with He as reference will be very small. In order to avoid large calculation errors, N2 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 N2 as reference gases for the measurements.

We chose the parameters at ﬁxed values reported in Table 3.1.

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 its representative peak through a calibration curve. For instance, the chromatogram of 3.2 Experimental setup 23

Table 3.1: Operating conditions of Gas Chromatograph inlet pressure/[kPa] 200.0 Columns outlet pressure Ambient max temperature/[◦C] 230 pressure/[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] 225 Oven 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] 80 Signals 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 24 Chapter 3 Experimental Setup

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 by Electronic Sensor, type K, with diameter of 1.0 mm and length of 1000 mm. Their measurable range of temperature went from -200 to 1000 ◦C, with tolerances of ±0.4 %. 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 KELLER pressure meter had a precision of 0.05 %FS (±0.003 % as linearity error at 25 ◦C). This type–PAA-33X/3bar/80794 (Serial number: 106276 ) a measurement-range of 0 to 3 bar, and can work at temperature between -10 ◦C and 80 ◦C. Connected with the Binder 723 and multibus NI 9203, PAA-33X sends the signal to computer. The output signal is converted from the ampere value (4 ∼ 20 mA ) to pressure value (0 ∼ 3 bar ) via a LabVIEW 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 of PD-33X/-0.1. . . 0.1bar/80920 (Serial number: 102764 ). It worked at the temperature range of -10 ◦C to 80 ◦C and measured the pressure diﬀerence between -0.1 and 0.1 bar with a linearity error of ±0.002 %FS. 3.2 Experimental setup 25

Same as the pressure meter, PD-33X shows us the digital value of pressure diﬀerence with the help of Binder 723, multibus NI 9203 and LabVIEW.

National Instruments Lab View

National Instruments Lab View was the graphical program in the project. It controlled the running of GC and MFC s and measured most of the operation-parameters synchronously. 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 from GC (from which the amount of certain component in the permeate-gas was possible to be calculated ), the flow rate (RS232, value of MFC ), and the temperatures at different positions of the cell or ambient (NI 9211 4-channel thermocouple input module, see Figure 3.5). With the module box-NI cDAQ-9172, LabVIEW becomes an instrument for various devices administration and numerous data acquisition. It realizes multi-processing in one program. This “multi to one” is of easy controlment, fast collection and accurate analysis.

Figure 3.5: LabVIEW controlling system 26 Chapter 3 Experimental Setup

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 GC detector, before the gas going out to vent- pipe. In theory, every time GC injects 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 GC measurements. In order to make sure the impact of flow rates on GC measurements as well as verify the accuracy of MFC, 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, six measuring volumes are available. Those are 1 ml, 10 ml, 100ml, 10 − 1 = 9 ml, 100 − 1 = 99 ml, 100 − 10 = 90 ml depending one bubble speed. After interposed into the 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 into GC had to be lower than 60 ◦C and 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 measurements. The ﬁrst GC measurements, either as the membrane vessel or pipeline still 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 of GC 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 N2 and He gas for GC.

2 . Switch on computer and the power of the “National Instruments” (NI cDAQ-9172); Write down the time.

3 . Connect the GC machine (6890N (G1530N)) to the computer and open it. When the GC was 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 of FlowView (V1.15).

5 . When the temperature of the GC reached 80 ◦C at both detectors, change the Method to “FNBHE.M”, and record the time. After the temperature rising upto 230 ◦C, 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 N2 for example).

8 . Open “test” LabVIEW Instrument file, which could not only control MFC s 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” on GC, reference-gases (See Figure 3.2) were measured. During this period, the membranes were dried fully. The program was regulated to stop automatically in 10.5 minutes. Wait until the GC interface was 28 Chapter 3 Experimental Setup

“ready”, that mean GC was 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 the “test” LabVIEW Instrument file to 0 first, then change the pipe-connections to the experimental state—add the main cell as shown in Figure 3.1.

11 . Set the flow rates of gases in the “test” LabV Instrument file to 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 file back 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 GC and 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 ◦C or 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 the method of the “Instrument 1 Online” program to “STBY1.M”.

16 . Set the ﬂow in the “test” LabVIEW Instrument ﬁle to 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 H2 and N2.

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

20 . When the flow rates in the “test” LabVIEW Instrument file decreased to zero, change the setpoints to zero and close the test file.

21 . Press F4 to halt running of DDEs. Power oﬀ the “National Instruments”. 3.3 Operating parameters and procedure 29

22 . When the temperature of GC detectors is below 80 ◦C, turn oﬀ the window of “Instrument 1 Online” program, shut down the computer and GC machine.

23 . Close the bottles of N2 an He. Shut down the fuming cupboard.

24 . The last task is data processing.

Chapter 4 Summary 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 the GC ;

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. 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,

p is the absolute pressure of the gas (Pa);

V is the volume of the gas (m3);

z is the compressibility factor, depending on conditions and phase of the components. For an ideal gas z is constant;

n is the amount of gas present, normally expressed in moles;

R is the universal gas constant (8.314472 J·mol−1·K−1);

and T 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 time t (min): V v = . (4.2) t By replacing V , we get: pvt = nRT. (4.3) That can also be written as : n pv = RT. (4.4) t At room temperature, when the MFC controls the gas flow at a fixed value, the molar amount of gas flowed per unit time remains unchanged, which means that n/t is constant. 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 impact 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]. 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, F P R ni = ni + ni (4.6)

F F P P R R n xi = n xi + n xi (4.7) where x is the mole fraction of certain component, F is the feed gas, P is the permeate gas, R is the retentate gas, and S is the sweeping gas.

From Equation 4.3, we get, PV P vt n = = (4.8) RT RT Therefore, the Equation (4.7) could be written as:

P vF t P vP t P vRt xF = xP + xR (4.9) RT i RT i RT i

F F P P R R v xi = v xi + v xi (4.10)

Membrane rate balance

For each component i, P ni = (Jn)i (4.11)

P P n xi = (Jn)i (4.12) where, (Jn)i (mol) means molar permeation rate of component i.

4.3 Equations for transport mechanisms through porous membranes

As described above, there are generally four gas transport mechanisms in porous membranes: Knudsen diffusion, viscous flow, surface diffusion and molecular diffusion [5,14,17,83]. 34 Chapter 4 Summary of Equations

Knudsen diﬀusion

When the pore size is much smaller than the mean free path of the molecule (λ expressed in Equation (4.13)) [84] and molecular weight ratios are larger, Knudsen diﬀusion occurs. It can be described as Equation (4.14) [83]:

k T λ = √ B (4.13) 2 2πpmσ r 4rp 2RT Di,K = (4.14) 3 πMi here, kB is the Boltzmann constant, σ is collision diameter, Di,K is the Knudsen diffu- 2 −1 −1 sion coefficient (m /s), η means the fluid viscosity (kg·m ·s ), 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)), if rp is between 5∼30 Å, and λ/dp > 1 [5]. The expression

λ/dp is 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 1 1 ∗ = + (4.15) D Di,K Di

∗ where D is transition diffusion coefficient, Di is molecular diffusion coefficient, εp =

Ap/A the porosity of the membrane, Ap is the pore area, and τ is the pore tortuosity, δ is the thickness of the membrane.

Viscous Flow

Viscous flow (also referred to laminar flow) is one of the transport mechanism in porous membranes—the capillary condensation flow or partial diffusion as discussed in Chapter 2. According to Professor Koltuniewicz [5],

2 r · εp · P L = p (4.16) η 8η · δ · τ · R · T For gas: r 1 Mi · R · T ηi = 2 3 (4.17) N · dm π 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]. dC J = −ρ · τ · D · (1 − ε ) · (4.18) S S S p dγ here, ρ is the density of the membrane, τS is the tortuosity of the surface, DS is the surface diffusion coefficient, and C is 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 mechanism 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]: 3/2 1/2 2 kB · T SD −ED 18.58T · [(Mi + Mj)/MiMj] Di = eλ exp( ) exp( ) = 2 (4.19) h R R · T pσi,jΩ where e is the “base of the natural log”, h is Planck’s constant, SD is the activation −1 −1 entropy of diffusion (kJ·mol ·K ), ED is the activation energy of diffusion (kJ/mol), and Ω is the collision integral.

4.4 Equations for the experimental setup

4.4.1 LabVIEW

National Instruments Lab View controls the running of GC and MFC s 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 to −0.1 ∼ 0.1 bar. Hence, the conversion Equation (4.20) is shown bellow:

PI = 187.5 · I − 0.75 DPI = 12.5 · I − 0.15 (4.20) 36 Chapter 4 Summary of Equations

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. 100 − 10 v = (4.21) t

4.4.3 Mass ﬂow controller

MFC can be regulated by LabVIEW program. The set value is in the form of percentage. Both MFC s’ ranges are 750 mln/min (100 %). The setted ﬂow rates are actually equal to 750 mln/min multiplying by the percentages. j v = 750 · (4.22) n 100 here, natural number j (0

ﬂow rate under standard condition (T0 = 273.5 K). For example, setpoint of 3 % means the ﬂow rate of 22.5 mln/min and 24.56 ml/min at room temperature. T j T v = vn · = 750 · · (4.23) T0 100 T0

4.4.4 Gas chromatograph

GC can detect and measure gas components by chromatograms, which are diagrams of characteristic peaks with diﬀerent retention times. The amount of components can be calculated from the area under its representative peak and a calibration curve. From a theoretical point of view, the content of certain component is equal to the ratio of its peak areas of objective gas and pure standard-gas. The chromatograms of two- component gas and pure H2 and pure CO2 are put into one ﬁgure. Thus, the principle is easily to be understood, as shown in Figure 4.1 and Equation (4.24). The green line shapes into peaks of H2 (left) and CO2 (right) in two-component gas (2M). The blue peak is pure H2, and red peak is pure CO2.

ZH2−2M ZCO2−2M CH2−2M = CCO2−2M = (4.24) ZP ure−H2 ZP ure−CO2 here, CH2−2M is the molar percentage (100%) of H2 in two-component gas, ZH2−2M

(25µV·s) is the characteristic peak area of H2 in chromatogram of 2M. CO2 is expressed similarly. 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. There were 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]:

CP /CF S = H2 H2 (4.25) H2,CO2 CP /CF CO2 CO2 or CP /CR S = H2 H2 (4.26) H2,CO2 CP /CR CO2 CO2 38 Chapter 4 Summary of Equations

here, C is the molar density (mol/ml) of the ﬂuid mixture; CH2 and CCO2 is the partial molar density (mol/ml) of component H2 and CO2. For a binary system, we have P P R R CH2 + CCO2 = 1 and CH2 + CCO2 = 1, then the separation factor is readily converted into the following forms [15]:

CP /(1 − CR ) CP CR S = H2 CO2 = H2 CO2 (4.27) H2,CO2 (1 − CP )/CR (1 − CP )(1 − CR ) H2 CO2 H2 CO2 Using the GC measurements results, it is easy to calculate mole fraction of certain components. Thus, the Equation (4.25) could also be written as follows:

(CP xP )/(CF xF ) xP /xF S = H2 H2 = H2 H2 (4.28) H2,CO2 (CP xP )/(CF xF ) xP /xF CO2 CO2 CO2 CO2 In general, when two key components are selected, S is better to be much larger than

1.0 or far lower than 1.0. In the case of SH2,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 membrane tends 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 (β).

P Ci βi = R (4.29) Ci Thus, Equation (4.25) could also be written as:

βH2 SH2,CO2 = (4.30) βCO2

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, vP · CP J = i (4.31) i A 4.5 Eﬃciency of gas separation through membrane 39 here, A is the surface area of the membrane (cm2), J (mol·min−1·cm−2) means permeation rate.

According to Koichi’s conclusion [90], we have

N X Ji ≡ 0 (4.32) i=1 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 gases—one or some components are able to permeate a membrane 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: dF J = −k × (4.33) i dγ where F represents the driving force that causes the trend of transporting between both sides of the membrane, k is the phenomenological coefficient, and γ 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 to Fick’s law of diffusion in Equation 4.34, and volume flux results from pressure difference, which is depicted as permeation in Darcy’s law in Equation (4.35) [3,5,90,91]. dC J = −D (4.34) m dγ dP J = −L (4.35) V dγ where, the diffusion coefficient D is, the diffusivity in dimensions of m2·s−1. As introduced above, it depends on diffusing species, membrane characters, temperature and sometimes concentration. L is the permeability coefficient (mol·s−1·m−1·Pa−1).

Take Darcy’s law as an example, after integration the relationship forms the Equation (4.36) [49,62]: P P xP − P RxR (J ) = J × xP = −L i i (4.36) V i V i δ 40 Chapter 4 Summary of Equations

Therefore, Equation (4.12) could also be expressed as: P P xP − P RxR nP xP = −L i i At (4.37) i δ P P vp P P xP − P RxR xP = −L i i A (4.38) RT i δ Using Henry’s Law for equilibrium of molecules, Equation (4.35) could be written as: xF +xR P R − P P P F i i − P · xP J = L i i = L 2 atm i (4.39) i δ δ here, Patm represents the pressure of atmosphere (Pa).

Selectivity

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

LH2 αH2,CO2 = (4.40) LCO2 Substituting Equations (4.36) into (4.40), we get: xP H2 xP α = CO2 (4.41) H2,CO2 (P P xP −P RxR ) H2 H2 (P P xP −P RxR ) CO2 CO2 When P P approaches to zero, xP H2 xP α ' CO2 = S (4.42) H2,CO2 xR H2,CO2 H2 xR CO2 Hence, the selectivity of the membrane is approximately equal to the separation factor of the membrane.

In the transport mechanism of Knudsen diﬀusion [5],

8 · rp · εp 1 Li,Kn = √ (4.43) 3 · τ · δ 2πRT Mi where, Li,Kn is the permeability coeﬃcient in Knudsen diﬀusion. Substituting Equation (4.43) into Equation (4.40), then we get [1,15]:

DH2,K MCO2 1/2 αH2,CO2 = = ( ) (4.44) DCO2,K MH2 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 transport mechanism (surface diffusion, for instance) for one of the components will mostly increase the separation factor. Chapter 5 Experimental Results and Discussion

This chapter includes data correction of controlling equipment, calibration of measuring instruments, and the results of all measurements, which are worked out with the methods and formulas mentioned in Chapter 4. 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 for measurement 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 no diffusion occurred.

If the gas flow rate was very slow, the bubble wormed in the glass tube, and it became difficult for the counter to gauge the begin and stop time. When the gas flowed fleetly, 42 Chapter 5 Experimental Results and Discussion it was too fast to count. Therefore, though it was reported that the 100 ml bubble flowmeter had a kit range of lower than 300 ml/min, we chose measure the gas flow between 10 and 200 ml/min, taking the accuracy into account.

5.1.2 Data correction of mass ﬂow controller

As introduced in Chapter 4, both MFC s’ ranges are 750 mln/min (100 %). The setted

ﬂow rates are equal to the percentages multipying by 750 mln/min. For example, setpoint of 3 % means the ﬂow rate of 22.5 mln/min. One MFC in this project was for direct control of nitrogen, air, etc., and the other was for direct control of the feed gases such as hydrogen, carbon dioxide, methane, and mixed gases. Their allowable working pressure range was from 1 bar to 4 bar.

In actual operation, the gas pressure was controlled by the reducer at 2.4 bar. The set ﬂow rates and the actual ﬂow rates of two-component gas are shown in Table 5.1 and Figure 5.1. In Figure 5.1, dashed line represents the estimative value of MFC,

Table 5.1: Flow rates of two-component gas controlled by MFC Setpoint [%] 1 10 20 30 50 100

Ideal estimative value [mln/min] 7.50 75.00 150.00 225.00 375.00 750.00 Estimative value [ml/min] 8.19 81.86 163.71 245.57 409.28 818.56 Actual flow rate [ml/min] 12.84 89.53 173.36 256.90 433.97 855.13 and the real line is the actual volume flow measured by soap film flowmeter at 25 ◦C. The results from 1 % to 50 % answer for linear-relationship. The whole-range value in Table 5.1 is deduced from this liner-relationship of measured points. The real outcome of MFC is a little larger than its setpoints. The flow rate of 55 % or higher was too fast to be measured by a stop watch.

Other gases were all measured and results were similar to two-component gas. Since most of the experiments were done with low-velocity gas (22.5 mln/min), the comparison of other gases’ flow rates is shown in Figure 5.2. For different kinds of gases, the MFC s operated their flow rates in similar liner-relationship but of different slopes. Therefore, though feed gas and sweeping gas were both set at 3 % in experiments, the actual volume flows were sightly different.

In order to ensure the absence of control errors under small gas pressure, the actual ﬂow rates of two-component gas at 3.0 bar and 4.0 bar were also measured (see Figure 5.3). The results show that, in the allowable range of work pressure, MFC can control gases 5.1 Controlling equipment and corresponding special procedures, calibration 43

Figure 5.1: Flow rate of two-component gas at 2.4 bar stably. Different work pressures did not influence the volume flow of MFC. Equipment error of MFC existed, but the relationship of estimative points and real values were stable. Therefore, the MFC s could be calibrated and were believable.

However, to ascertain the actual control regulations of MFC s, even more data were required. They were not the main object of the project, and the duration of project was restricted, so the regulations were not totally clear. It can be included in the further research. Thus, the results were calculated only by the linear-relationship of the small area around the experimental ﬂow rate instantaneously. Details were introduced in the next section.

5.1.3 Calibration of gas chromatograph (GC)

The basic principle and the theoretic method of calculation were introduced in Chapter 3 and Chapter 4 respectively. For quantification with the 6890N, calibration became more essential. Since the injection was automatic, it was difficult to use some standard- gases or to calibrate with the manual-injection-reference measurements. Though automatic injection was much more convenient and seemed more accurate without operator errors, lots of parameters can influence the measure of GC. The problems which appeared in the experiments and new methods for calibration are described. 44 Chapter 5 Experimental Results and Discussion

Figure 5.2: Flow rate of 2 % to 4 %

Measurements of warming up

When GC started to measure samples, the first two or three data gathered were usually not stable. As shown in Figure 5.4, the first two data at room-temperature (RT-blue line) are obviously not normal. That might because the membrane vessel or pipeline were not swept clean enough, so there were some residue gases from last measurements. That also might be caused by the incomplete “warming up” of GC. However, generally since the fourth or fifth measurement, data become stable and normal. Furthermore, the first measurements of all the posterior processes are unusual, see data at 100 ◦C in Figure5.4. This is likely caused by the pipeline-residual gas. Therefore, in the following processes, three or four measurements were carried out for the average value (except the first one).

Base line of chromatogram

Program of GC makes the base line and calculates peak height and areas automatically. Usually operators prefer using the automatic data, since there are no man-made operating errors. However, sometimes the automatic base line is actually obviously bad, then the base line should be redrew. Figure 5.5 shows one chromatogram with both automatic and manual base line. The rose line is the base line made by GC automatically that shapes a much larger area than the peak itself; and the orange one made manually with mouse pointer is necessary to get the right area value. The manual line 5.1 Controlling equipment and corresponding special procedures, calibration 45

Figure 5.3: Flow rate of two-component gas at diﬀerent pressures should be made in a ﬁxed range for certain gas characteristic peak, as then the data are comparable with each other.

Inﬂuence of run time

In the measurements with GC, one problem appeared, that time duration affects the area of spectrum peaks. Because of different retention times of different components, at the beginning of the project, run time was not constant. For example, when H2 was the object to be observed, three minutes were long enough for appearance of its characteristic peaks. Later when the time duration was set longer for other gases, the area of H2 peak varied. Then tests of the same gas sample with different time duration were done. Outcomes revealed that, the run time of GC-Method could affect the chromatograms slightly. Longer time duration resulted in lower area value. For instance, H2 in two-component gas is shown in Figure 5.6. Though the differences were small (just 2.94 %), in order to obtain much more accurate quantification, fixed run-time in the later experiments was applied. Furthermore, CO2 was the component of the longest rentention time (9.495 min). Hence, measure method was set to run 10.5 min. 46 Chapter 5 Experimental Results and Discussion

Figure 5.4: GC measurements

Figure 5.5: Base line of chromatogram

Inﬂuence of gas temperature on automatic-injection

According to the ideal-gas equation (Equation (4.4)), temperature can change the volume flow or density of gas. Measure method of GC used a basic temperature of 60 ◦C to avoid such influence. Gases at different temperatures were still tested for confirmation.

Figure 5.7 shows the characteristic peak areas of pure H2 at higher temperatures with pipe cooling system, which has been introduced in Chapter 3. The results show that, with the pipe cooling system and 60 ◦C basic temperature, GC measurements are not impacted by temperature. Therefore, compositions of permeate-gases outgoing from 5.1 Controlling equipment and corresponding special procedures, calibration 47

Figure 5.6: Area of H2 in two-component gas measurements with diﬀerent run times membrane cell at higher temperature can also be calculated with calibration gases at room temperature.

Inﬂuence of reference gases for GC measurements

As has been pointed out in Chapter 3, both N2 and He were required as reference gases for GC. If just He was used, the representative peak of H2 was very small. Thus, N2 was used as a carrier gas at an individual channel for H2 to avoid large calculation errors. If just N2 was used, the composition of N2 in mixture was impossible to be measured, and the measurement of other gases was inaccurate. The feed gases used in this project were mainly H2, CO2, two-component gas (2M—contains 49.8 % volume percentage H2 and 50.2 % CO2), and ﬁve-component mixture gas (5M—13 %H2, 16

% CO, 13 % CO2, 53 %N2, and 5 % CH4). Compositions of feed gases measured by

GC with N2 or He as reference gas are shown respectively in Table 5.2 and Table 5.3.

As can be observed, using N2 as reference gas, content of H2 can be measured accurately (12.35 % is close to 13 % and 49.88 % is close to 49.8 %). When He is used as reference gas, CO2 and N2 are in the right range, which are 13.17 %, 50.3 % and 56.20 % corresponding to 13 %, 50.2 % and 53 %. But the contents of H2 are just 4.97 % in the ﬁve-component gas and 34.57 % in the two-component gas, which are far away from the right levels. The measurements of CO2 using N2 are also of large errors. Therefore, 48 Chapter 5 Experimental Results and Discussion

Figure 5.7: Area of pure H2 measurements at diﬀerent temperatures

Table 5.2: Gases compositions using N2 as reference

Peak area H2 CO2 Gas type [25µV·s] [25µV·s] 5M 10289.70 1691.83 2M 41550.75 5297.60

H2 83302.05 –

CO2 – 9209.05 Percentage [%] in 5M 12.35 % 18.37 % Percentage [%] in 2M 49.88 % 57.53 %

the front detector of N2 ought to monitor H2 only. He can be used as reference gas for all the other gas components in the project.

Inﬂuence of ﬂow rate on automatic-injection

Upon switching the gas sampling valve of alternate streams, the contents of sample loops (0.6 ml) were inserted automatically into the inlets. Since the volumes of the sample loops and injection-time (0.1 min) were all fixed, the flow rate determined the injected amount of samples at room temperature and under ambient pressure. The influence of flow rate (H2) on automatic injection is shown in Figure 5.8. 5.1 Controlling equipment and corresponding special procedures, calibration 49

Table 5.3: Gases compositions using He as reference

Peak area H2 CO2 N2 Gas type [25µV·s] [25µV·s] [25µV·s] 5M 93.40 20894.60 52502.9 2M 649.75 79804.00 –

H2 1879.65 – –

CO2 – 158648.20 –

N2 – – 93429.6 Percentage [%] in 5M 4.97 % 13.17 % 56.20 % Percentage [%] in 2M 34.57 % 50.30 % –

This result testifies the influence of flow rate on automatic-injection amount. Higher volume flow causes larger amount in automatic injection. N2 changes more obviously (see Figure 5.9). As a result, there are only two ways to realize the believable quantification. One is accurately controlling, and keeping the same for all the sample-flows rates. Another is promptly measuring the flow rate of sample-gas, and designing a serial experiments for exact calibrations. Automatic injection is not convenient or accurate unless the problems of flow rates have been well solved.

New method for calibration

The membranes employed were made of porous material and gas separation depended mainly on the sizes of molecules. There was no special selectivity over the gas types. Thus, gas components in both feed gas and sweeping gas, can permeate through membrane. Though the flow rates of feed gas and sweeping gas can be controlled by the MFC, those of permeate-gas and retentate actually depended on the membrane and conditions. The controlling of the fluxes can not be easily realized. Thus, the method of prompt measuring and reference-design was the best choice in this situation. Actual flow rate and characteristic peak areas were supposed to be linear in the smallest setting area of volume flow (each 1 % of MFC ). Take H2 as an example, this project had three H2 contained gas cylinders—pure H2, two-component gas, five-component gas. They were feed gases for experiments as mentioned above, but they could also be used as standerd-gased for H2 calibration.

For instance, in the experiments of gas separation using porous cement membrane ◦ at 200 C, feed gases and sweeping gas (N2) were set to 3 % (22.5 mln/min). Five- component gas, two-component gas and H2 were used as feed gases respectively, and 50 Chapter 5 Experimental Results and Discussion

Figure 5.8: GC measurements of diﬀerent reference-ﬂow rate

the three kinds of measurements can be expressed as <5M;N2>, <2M;N2>, in short. During the experiments, volume flows of permeate-gases and retentates were obtained with the help of soap film flowmeter. Flow rates of permeate-gases for the last three feed gases are listed in Table 5.4.

Table 5.4: Volume ﬂows of permeate-gases

Measurement <5M;N2> <2M;N2> Volume ﬂow [ml/min] [ml/min] [ml/min] Permeate-gas 33.18 34.14 35.16

After measurements with membrane, standard-gases were connected directly to GC, and measured orderly with the flow rates around the small setting area of permeate-gas volume flow (3 % and 4 %). Then all the chromatograms and flow rates of permeate- gas, retentate and standard-gases were gathered. The deductive process of calculation is depicted as follows.

Figure 5.10 shows the actual flow rates of standard-gases with the setpoint of MFC between 3 % and 4 %, and this range covers the volume flows of permeate-gases. Because of the linear relationships, corresponding points are easily found in this figure. For example, a, b and c are the corresponding points in sequence of standard five- component gas, two-component gas and pure H2 in <2M;N2> measurement. All the corresponding relationships are listed in Table 5.5. 5.1 Controlling equipment and corresponding special procedures, calibration 51

Figure 5.9: N2 amount and ﬂow rates in automatic injection

Table 5.5: Corresponding points of standard-gases

Measurement <5M;N2> <2M;N2> Standard gas [%] [%] [%] 5M 3.22 3.33 3.45 2M 3.29 3.40 3.52

H2 3.20 3.31 3.42

Peak areas of chromatograms were also collected at 3 % and 4 % by GC. Figure 5.11 illustrates area values for pure H2 standard-gas. In the assumptive linear setting area, the standard value can be found through the corresponding point. The area point marked with C is the standard area value of 100 % H2 in <2M;N2> measurement metioned in Figure 5.10 and Table 5.5. Other standard values for calibration are ﬁgured out likewise (see Table 5.6).

The volume-content of H2 in ﬁve-component gas, two-component gas and pure H2 are 13 %, 49.8 % and 100 % respectively. Then the calibration curves are achievable. The calibration curve for <2M;N2> measurement is shown in Figure 5.12-A.

Though there are just three points, curve obeys linear relationship quite well. Ex- tending the line, and low concentration range is zoomed in to ﬁgure B in Figure 5.12. There is a small error here, so three points are far not enough for the calibration curve. All the concentration range should be covered for a more believable calibration, from zero to 100 %. However, the project was lack of standard-gases for automatic injection 52 Chapter 5 Experimental Results and Discussion

Figure 5.10: Flow rate calculation of standard-gases

Table 5.6: Characteristic peak area of H2 in permeate-gas and standard gases

Measurement <5M;N2> <2M;N2>

Peak area of H2 [25µV·s] [25µV·s] [25µV·s] Permeate-gas 6152.900 21240.800 43383.750 5M 10802.42 10829.70 10858.48 2M 43513.98 43668.43 43831.37

H2 84381.87 84409.17 84437.96

and calibration curve was not stable. When GC was restarted or its detectors began to work, the calibration curve should be drawn again. There was not so much time for the detail calibration curve. Acctually, every experiment took thirteen to fourteen hours daily already, just with these three standard-gases. Therefore, I added another standard point into the calibration curve, that is (0, 0). When there is no hydrogen, chromatogram contains no characteristic peak for H2. Using the linear relationship in small ranges, the contents of H2 were calculated more reasonable (Permeate-gas from experiment of <5M;N2> used the range of 0-13 %; <2M;N2> used 13 %-50 %; and

used 13 %-50 % or 13 %-100 % range). Results for the permeate-gases are shown in Table 5.7. 5.1 Controlling equipment and corresponding special procedures, calibration 53

Figure 5.11: Pure H2 peak area for calibration of 2M measurement

Table 5.7: H2 concentration in permeate-gases

Permeate-gas <5M;N2> <2M;N2>

Content of H2 7.40 % 25.50 % 51.94 %

Use natural number j (0

P Zo Co = 5M · 13% (5.1) 5M 5M vo−vj 5M (Zj+1 − Zj ) · 5M 5M + Zj vj+1−vj

here Z is the average value of peak area (25µV·s), v is the average value of volume flow 5M (ml/min). Superscript 5M means standard five-component gas. So vj is the average value of actual flow rates of five-component gas, when the MFC is set to be j %. The Equation (5.1) can be reduced to Equation (5.2) as follows.

5M 5M P Zo · (vj+1 − vj ) Co = 5M 5M 5M 5M · 13% (5.2) Zj+1 · (vo − vj ) + Zj · (vj+1 − vo) 54 Chapter 5 Experimental Results and Discussion

Figure 5.12: Calibration curve for H2 of 2M measurement

Similarly, in the range of 13 % to 50 %, the molar percentage of objective component is presented in Equation (5.3). And the mole fraction larger than 50 % is formulated in Equation (5.4).

5M 5M 5M vo−vj 5M Zo − [(Zj+1 − Zj ) · 5M 5M + Zj ] P vj+1−vj Co = 2M 5M ·36.8%+13% 2M 2M vo−vj 2M 5M 5M vo−vj 5M [(Zj+1 − Zj ) · 2M 2M + Zj ] − [(Zj+1 − Zj ) · 5M 5M + Zj ] vj+1−vj vj+1−vj (5.3)

2M 2M 2M vo−vj 2M Zo − [(Zj+1 − Zj ) · 2M 2M + Zj ] P vj+1−vj Co = H ·49.8%+49.8% v −v 2 v −v2M [(ZH2 − ZH2 ) · o j + ZH2 ] − [(Z2M − Z2M ) · o j + Z2M ] j+1 j H2 H2 j j+1 j v2M −v2M j vj+1−vj j+1 j (5.4)

When calculating CO2 percentage-concentration in permeate-gas, the balance equations in Chapter 4 can be used, as shown in Equation (5.5):

CF · vF − CR · vR CP = CO2 CO2 CO2 CO2 (5.5) CO2 vP CO2

Other gas components and experiments were also calculated in the same way. The calibration methods were found during the long period of experiments, so some early data were just calculated in the original theoretic way. 5.2 Experiments 55

5.2 Experiments

5.2.1 First set of experiments with Gaggl’s membranes

Maria Gaggl did her diploma thesis over gas separation with dead-end membrane module, and designed the ﬁrst membrane cell for the project at the early beginning. The membranes used for testing were from her diploma project. Maria suggested Wicke- Kallenbach cell with a co-current ﬂow pattern (see Figure 5.13).

Figure 5.13: Schematic of the ﬁrst idea

As mentioned in Chapter 2 the counter-current ﬂow pattern usually performs separation much better. Hence, the whole experiments were actually operated with the counter-current ﬂow pattern in my proposal. The thermal couple was also added on the secondary side (sweeping gas side) to monitor the instantaneous temperature of permeate-gas. Although the membrane cell had changed greatly after several improve- ments, the primal idea still played an important role in model design as reference.

First membrane cell

The first membrane cell is shown in Figure 5.14 and Figure 5.15. In Figure 5.14, the center part marked with “M ” is the cement membrane. The white “T ” parts, made of Teflon, is holders for membrane. Besides Teflon, there are also two metal holders covered Teflon parts (see details in the right picture of Figure 5.15). Other parts, including module shell and side covers, are all made of stainless steel. The cavum of this membrane cell is a columniform space of 8 cm length and 4 cm diameter. Then, spaces on two sides of membrane are 43.98 cm3 for 10 mm membrane, 47.12 cm3 for 5 mm membrane, and 37.70 cm3 for 20 mm membrane. 56 Chapter 5 Experimental Results and Discussion

Figure 5.14: Schematic of the ﬁrst membrane cell

Figure 5.15: The ﬁrst membrane cell and holders

First membranes

The specimens were one type of the micro-porous cement slices in Maria’s experiments. In Table 5.8 are the physical and geometric data of the specimens. Pore size distribution of this membranes is shown in Figure 5.16.

Table 5.8: Physical and geometrical data of the ﬁrst test membranes Water Total Total Pellet Solid cement pore pore density/ density/ ratio volume volume Density True density [-] [mm3/g] [V ol.− %] [g/cm3] [g/cm3] 0.6 65.97 19.81 2.10 2.43 Total Average Thickness Surface Water Speciﬁc porosity pore of the area absorption surface diameter specimen [nm] [mm] [mm2] [mol- %] [m2/g] 13.8343 % 110 10 1963 10.5 3.77 5.2 Experiments 57

Figure 5.16: Pore size distribution of the ﬁrst membranes

Teﬂon materials for gas-tightness

The Teflon holders introduced above, played the role of not only fixation and protection of samples, but also gas-tightness between the slices and metal parts. This Teflon was hard enough and was virtually chemically inert, so it was good at supporting and protecting membranes. Furthermore, Teflon was flexible. Teflon holders contacted tightly with the membrane surface. Therefore, gases were not easy to leak, and there was no damage to the membrane.

It is also reported that Teflon has non-contaminating, non-adhesive and thermal properties, thus it can be used at higher temperatures. However, actually, in this project, Teflon turned into yellow and became harder after being heated. Because of flexibility loss, Teflon holders were not easy to be detached with the membrane. If they were forced to be dismantled, the membrane would be damaged. Membrane might be difficult to insert back into the Teflon holders, and joints of the reassembled system could hardly be airtight.

Four different materials (see Figure 5.17) were compared as gaskets between the main shell and the side covers. In Figure 5.17, the top left one is made of copper, top right one stainless gasket, lower left one Teflon and lower right one rubber ring. Testing results show that, the first two annular gaskets were of no effect, regardless of the smooth surface and the identical thickness. Airtightness of the latter two were much 58 Chapter 5 Experimental Results and Discussion better, especially at low temperature. However, the heating problems existed with Teflon, as mentioned above. Furthermore, rubber materials can not withstand higher temperature (more than 300 ◦C) either. Therefore, searching for new gasket materials which can work at higher temperature was imperative.

Figure 5.17: Gaskets for the ﬁrst membrane cell

Figure 5.18: Flow rate inﬂuence at diﬀerent temperatures 5.2 Experiments 59

Results

When this membrane cell was working, just one detector was active. H2 in permeate- gas is the important study object, so N2 is used as the reference gas of GC. At the beginning, the influence of volume flow was studied. In order to minimize the effect from pressure difference, sweeping gas was controlled at the same setpoint as feed gas.

As shown in Figure 5.18, volume ﬂow (pure H2 as feed gas and pure N2 as sweeping gas) of 112.5 mln/min (setpoint of 15 %), 150 mln/min (20 %) and 225 mln/min (30

%) were measured for comparison. When the volume ﬂow is larger than 150 mln/min, the air will leak from the joints of the membrane-cell. The experiments were executed at room temperature and 100 ◦C, respectively.

Figure 5.19: Gas separation with diﬀerent volume ﬂows

The permeability decreases as flow rate increases. Hence, lower flow rate is preferred for the higher permeation efficiency. Figure 5.19 shows the contents of H2 in permeate-gas with even lower flow rates, which were 22.5 mln/min (setpoint of 3 %), 75 mln/min

(10 %) and 112.5 mln/min (15 %). Gas permeation at higher temperature was also studied. For all the temperature conditions, the permeation rate of 22.5 mln/min was obviously better than larger volume flows. Considering the measurable range of the soap film flowmeters, 3 % was the lowest setpoint of MFC s. Hence, both feed gas and sweeping gas were controlled basically at 22.5 mln/min. 60 Chapter 5 Experimental Results and Discussion

Then four pure gases were used as feed gas with ﬂow rate of 22.5 mln/min. They were all swept by 22.5 mln/min N2, and the results are shown in Figure 5.20. It can be observed that, that molar percentage of hydrogen, methane, carbon monoxide, carbon dioxide are inverse to their molecular weight. As the smallest molecule among all the components, H2 permeates best. Therefore, we concluded that the larger the molecule is, the less the gas can permeate through membranes.

Figure 5.20: Gas separation with diﬀerent feed gases

According to Equation 4.44, the selectivity of two gases is inversely proportional to the square root of their molecular weight. Figure 5.21 lists out the theoretical value (dashed line) and experimental results (solid line) together. The experimental selectivity of H2 from CO2 matches the theoretic value quite well. The other two gases performed the same trend. Since just one detector was employed, measurements of permeate-gas and retentate can not be done at the same time. Thus, temperature and pressure might influence the permeation process and results, and mall difference between experimental results and theoretic value was reasonable. Figure 5.20 and Figure 5.21 verify the theory in Chapter 4 very well, where the porous materials can separate gases with different molecular sizes. The increasing difference between molecular weights, sizes or shapes will raise the efficiency of separation. Therefore, H2 and CO2 mixture gas is used as feed gas for comparison of other factors. 5.2 Experiments 61

Figure 5.21: Experimental and theoretic selectivity

In summary, the ﬁrst module was an important prototype of this project. Larger ﬂow rates resulted in lower permeation rate and less content of object in the product.

Henceforth, most of the experiments were carried on with volume flow of 22.5 mln/min (setpoint). The permeation component with smaller molecular size resulted in the higher permeation flux and then higher diffusion coefficient, theoretically supported by T. C. Merkel [21]. The experimental selectivity was close to the theory—inversely proportional to the square root of the ratio of molecular weights, which was derived from Knudsen diffusion. Teflon holders and gaskets were flexible and airtight, but became harder at temperature higher than 200 ◦C. Hence, the material of gaskets should be replaced.

Furthermore, this membrane cell had large spaces on both side. With long distance, gases may stay longer but the problem of turbulent flow appeared. Some gases flowed out of the reactor directly, and the gas in the corner may stay in the “dead angle” (see Figure 5.22-A). In theory, gases should flow along the membrane in the permeation process, as shown in Figure 5.22-B. That is, the vertical length of the spaces of membrane should be much shorter than its diameter. Then to facilitate the refitting operation, the membrane was first designed as plan C in Figure 5.22. In this way, it is convenient to add two steel plate on both sides of the membrane cell, and all the gas molecules would have the chance to touch the membrane, and then to permeate freely. 62 Chapter 5 Experimental Results and Discussion

Figure 5.22: Problem of the ﬁrst membrane cell

5.2.2 Second set of experiments with modiﬁed cell

After consulting machinists for profession suggestions, and considering the gas-tightness and soundness of the added parts, some cylinders made of stainless steel and more graphite paper were installed into the membrane cell. The initial model of the secondary membrane cell was produced for testing. And the axial section diagram of this initial model is shown in Figure 5.23. Membrane is placed firmly in the center of the module. Right blue part is the space on the primary side with a cubage of 9.94 cm3, and left sweeping volume is 5.03 cm3. Black parts are the seals made of graphite. However, left cylinders are not so well pressed closely with each other, so the cylinder A is not fixed, which is bad for experiments. As a result, membrane cell was modified again in 2009.

Membrane cell

The design of the membrane cell was ﬁnalized as shown in Figure 5.24 (for 10 mm membranes). The spaces on the two sides of membrane are both 5.03 cm3 (with a diameter of 4 cm and thickness of 4 mm). Both axial section view of the center membrane (5 mm) subassembly (left) and the whole module (right) are exhibited in Figure 5.25. 5.2 Experiments 63

Figure 5.23: First version of the secondary membrane cell

Figure 5.24: Final design of the modiﬁed membrane cell

Membranes

Cement membrane Old membranes used for the ﬁrst membrane cell were also tested to prove the advantage of new membrane cell. As introduced above, microporous cementitious slice of made in Institut für Bauingenieurwesen of Technische Universität Berlin were the main objective material in the project. Usually the thinner the membrane is, the larger permeation rate is, but the membrane must be thick enough to endure mechanical stresses 64 Chapter 5 Experimental Results and Discussion

Figure 5.25: Axial section view of the modiﬁed membrane cell and prevent rupture. Thus, the thickness of the membrane should be no less than 5 mm. Membranes employed had thicknesses of 5, 10 and 20 mm.

Cementitious materials is mainly cement, but also water, aggregate, and chemical admixtures. The detail basic materials, additives and equivalent ratios of water to cement ((w/c)eq) of the membranes, are listed in Table 5.9.

There are short code names defined for these membranes shown in Table 5.10. Here, PZ is Portland cement, SFA is additive Coal fly ash-“EFA-building mineral filler KM/C ”, MS is additive micro silica-“Elkem Micro silica 971-U ”, HOZ is Blast furnace cement, TZ is Alumina cement, F-PZ is Micro-adhensive based on Portland cement, F-HÜS is Micro-adhensive based on slag, and number 1 and 2 are used to distinguish different types of the same cement. 5.2 Experiments 65

Table 5.9: Cement membranes Cement and adhesive Adhesive (w/c)eq Aalborg White I CEM 52.5 R-HS/EA/≤2 none 0.45 Aalborg White I CEM 52.5 R-HS/EA/≤2 none 0.35 Aalborg White I CEM 52.5 R-HS/EA/≤2 none 0.25 Cemex Rüdersdorf CEMI 32.5R none 0.45 Cemex Rüdersdorf CEMI 32.5R none 0.35 Cemex Rüdersdorf CEMI 32.5R none 0.25 Zementwerk Berlin CEM III/B 32.5N-LH/HS/NA none 0.45 Zementwerk Berlin CEM III/B 32.5N-LH/HS/NA none 0.35 Zementwerk Berlin CEM III/B 32.5N-LH/HS/NA none 0.25 Zementwerk Berlin CEM III/B 32.5N-LH/HS/NA Fly ash 0.45 Zementwerk Berlin CEM III/B 32.5N-LH/HS/NA Fly ash 0.35 Zementwerk Berlin CEM III/B 32.5N-LH/HS/NA Fly ash 0.25 Zementwerk Berlin CEM III/B 32.5N-LH/HS/NA Micro silica 0.45 Zementwerk Berlin CEM III/B 32.5N-LH/HS/NA Micro silica 0.35 Zementwerk Berlin CEM III/B 32.5N-LH/HS/NA Micro silica 0.25 Cemex Rüdersdorf CEMI 32.5R Fly ash 0.45 Cemex Rüdersdorf CEMI 32.5R Fly ash 0.35 Cemex Rüdersdorf CEMI 32.5R Fly ash 0.25 Cemex Rüdersdorf CEMI 32.5R Micro silica 0.45 Cemex Rüdersdorf CEMI 32.5R Micro silica 0.35 Cemex Rüdersdorf CEMI 32.5R Micro silica 0.25 Aalborg White CEM I 52.5 R-HS/EA/≤2 Micro silica 0.25 Calucem Istra 40 none 0.35 Calucem Istra 40 none 0.25 Kerneos Ciment Fondu EN 14647 CAC none 0.35 Kerneos Ciment Fondu EN 14647 CAC none 0.25 Dyckerhoﬀ MIKRODUR P-U none 0.30 Dyckerhoﬀ MIKRODUR R-U none 0.30

Table 5.11 gives the porosity informations of these membranes. 66 Chapter 5 Experimental Results and Discussion

Table 5.10: Code name of cement membrane Cement and adhesive Additive Code name Aalborg White I CEM 52.5 R-HS/EA/≤2 none PZ-2 Aalborg White CEM I 52.5 R-HS/EA/≤2 Micro silica PZ-2+MS Cemex Rüdersdorf CEMI 32.5R none PZ-1 Zementwerk Berlin CEM III/B 32.5N-LH/HS/NA none HOZ Zementwerk Berlin CEM III/B 32.5N-LH/HS/NA Fly ash HOZ+SFA Zementwerk Berlin CEM III/B 32.5N-LH/HS/NA Micro silica HOZ+MS Cemex Rüdersdorf CEMI 32.5R Fly ash PZ-1+SFA Cemex Rüdersdorf CEMI 32.5R Micro silica PZ-1+MS Calucem Istra 40 none TZ-2 Kerneos Ciment Fondu EN 14647 CAC none TZ-1 Dyckerhoﬀ MIKRODUR P-U none F-PZ Dyckerhoﬀ MIKRODUR R-U none F-HÜS

Table 5.11: Porosity of cement membrane Average Critical Mercury- Total Code name (w/z)eq pore diameter pore diameter porosity porosity [nm] [nm] [V ol.− %] [V ol.− %] PZ-2 0.45 98 138 25.3 28.1 PZ-2 0.25 66 80 16.3 18.6 PZ-1 0.25 86 96 20.5 24.1 HOZ 0.25 63 100 13.8 21.7 HOZ+SFA 0.45 58 90 22.9 37.6 HOZ+SFA 0.25 18 8 15.4 21.4 HOZ+MS 0.25 12 8 11.6 23.0 PZ-1+SFA 0.25 53 59 16.6 23.7 PZ-1+MS 0.45 66 110 24.0 33.0 PZ-1+MS 0.25 37 16 14.0 21.4 PZ-2+MS 0.25 32 8 6.5 17.5 TZ-2 0.35 1189 1734 26.0 29.5 F-PZ 0.30 30 28 18.0 23.9 F-HÜS 0.30 19 8 2.6 19.7

To be more in detail, the pore size distributions of membrane PZ-2 with equivalent water to cement ratio of 0.25 and 0.45 are shown in Figure 5.26. 5.2 Experiments 67

Figure 5.26: Pore distribution of PZ-2

S938H One kind of multi-layer membrane named S938H was also used for comparison. Sam- ples of the multi-layer membrane were made by KERAFOL -Ceramic Films GmbH. The membrane (plate-diameter 50.5 mm) had a ceramic substrate (pore diameter of 2 µm and thickness of 6.0 mm) and a 7 nm-thickness coating (pore diameter of 30 nm) on one side.

Gaskets

Graphite has lots of advantages. First of all, the graphite seals used in the modified membrane cell had self-lubricating properties and low friction coefficient. They were much softer than Teflon. Graphite paper can be pressed thinner, so the small surface defect can be remedied in this way. Thus, the joints were well airtight. The smooth material could even protect membrane against excessive press rather than destroying it. Secondly, graphite paper was cheap and the gaskets were shaped easily. When the membrane cell was installed again, new graphite gaskets were used to keep the air-tightness. The last but not least, other advantages of graphite were high thermal conductivity, high chemical stability, low thermal expansion coefficient, and high thermal stability (thousands degree), which were much better than Teflon (250 ◦C).

Black colored parts in Figure 5.25 are the graphite seals. The detail placements of graphite gaskets around membrane (take the 5 mm as an example) are shown in Figure 5.27. The two parts marked with “S” are the holders for 5 mm membranes. They are made of stainless steel. The numbered dark rings are gaskets used in the center of the membrane cell. Gaskets 2 and 4 are placed between the plate surface and membrane holders. Gasket 3 is set to cover side surface of the membrane, thus, the permeated gas 68 Chapter 5 Experimental Results and Discussion

Figure 5.27: Graphite gaskets around membrane components does not leak here. Gaskets 1 and 5 are used to maintain the airtightness between membrane holders and their neighboring stainless steel cylinders.

Results

The experiments with the old membrane were repeated for comparison. Then tens of the membranes mentioned above were measured with different gas-pairs, to investigate the influence of different parameters. Performance of the modified membrane cell Since membrane cell was modified, the comparison of the old and modified membrane cells was the primal task. In the experiments with first membrane cell, pure gas measurements using earlier membrane type (thickness 10 mm, (w/c)eq 0.6) were carried out. Among those, results of measurement with H2 as feed gas and N2 as purge gas

() are shown in Figure 5.28.

Here, blue points represent the value of H2 molar content in permeate-gas obtained at diﬀerent temperatures with modiﬁed membrane cell. Points colored with rosiness shows

H2 molar percentage of permeate gas using the old membrane cell. The membrane cell spaces were changed from 43.98 cm3 to 5.03 cm3 on both sides. The change of internal volume leads to an alteration of residence time and improvement of the ﬂow conditions. It is obvious that using the new cell, permeation rates are much higher, and smooth curve can be got instead of 100 ◦C maximum point with old cell. 5.2 Experiments 69

Figure 5.28: Performance of membrane cells in

Influence of temperature At the beginning of each daily experiment, in order to drive vapor and residual gases out of membrane cell, the membrane was heated at 350 ◦C for 90 min. Flow rates were both set on the setpoint of 22.5 mln/min, almost the minimum flow rate controlled by MFC. Furthermore, gases ran 30 min before measurements. This was aimed to drive residual gases away from the pipelines and pores in the membrane, as well as to achieve the equilibrium state of the gases diffusion on both sides. Experiments were conducted from high to low temperatures, in which each temperature group (350 ◦C, 200 ◦C, 100 ◦C and room temperature) were carried out four times. Since the first value was always inaccurate, the other values were used to obtain the average value. The results for different membranes are shown in Figure 5.29. The curves of different color refer to different membrane type. Membranes in this figure all have the thickness of 5 mm. The equivalent ratio of water to cement ((w/c)eq) is 0.45 for PZ-2 and 0.25 for others. For all of them, when temperature raised up, the permeation rates of H2 will also increase. Separation factors of H2/CO2 achieved at different temperature using these membranes are shown in Figure 5.30. Results reveal that in experiments using two-component mixture as feed gas, separation factor of H2/CO2 decreases with the rising temperature.

When the operating temperature raises up, the diﬀusion rates of isolated permeating molecules and associated permeating molecules are high, and as a result, the total 70 Chapter 5 Experimental Results and Discussion

Figure 5.29: Inﬂuence of temperature in

Figure 5.30: Inﬂuence of temperature in <2M;N2>

permeation rate is high and the separation factor is low [94,95]. Therefore, using membranes without selectivity, the increase in temperature leads to better the permeability of H2 but the separation factor and selectivity will become smaller accordingly. At room temperature, we can get much higher separation factors of H2 from CO2. 5.2 Experiments 71

Influence of equivalent water to cement ratio In Table 5.9, all kinds of membranes produced for the project are listed. There are seven basic cements, and membranes can be classified into twelve types with different adhesives. Each type has also different specimens with a variety of equivalent water to cement ratios. Figure 5.31 shows the influence of equivalent water to cement ratio on gas separation. PZ-2 samples with (w/c)eq of 0.45 (sky blue) and 0.25 (dark blue) are compared here. Thicknesses of both membranes are 5 mm. Since the sample with (w/c)eq of 0.35 is 10 mm thick, 0.35 comparison is not considered. Samples of 0.45 and

0.25 were measured in the experiment using two-component mixture as feed gas and N2 as purge gas. Separation factors of H2 to CO2 were proved by experiments to decrease with an increase in temperature. Furthermore, membrane of lower equivalent water to cement ratio got higher separation of H2 from CO2. Hence, the 0.25 equivalent water to cement ratio membranes attracted more attention later.

Figure 5.31: Inﬂuence of equivalent water to cement ratio in <2M;N2>

Influence of pore size From Figure 5.26 we learn that most of the pore diameter of cement PZ-2 with equivalent water to cement ratio 0.25 is smaller than that of the cement PZ-2 with equivalent water to cement ratio 0.45. Table 5.11 also shows that material of lower equivalent water to cement ratio also has a little lower pore sizes. The advantage of lower equivalent water to cement ratio might be caused by the variation of pore size. Therefore, another material of maximum pore diameter (8 nm) was brought into comparison. PZ-2 (thickness of 5 mm, (w/c)eq 0.25) has the average pore diameter of 66 nm, and most of pore 72 Chapter 5 Experimental Results and Discussion diameter is 80 nm. The average and maximum-quantity pore diameters of PZ-2+MS (thickness of 5 mm, (w/c)eq 0.25) are 32 nm and 8 nm respectively. The permeate-gas components are shown in Figure 5.32-A, and the PZ-2 (dark blue line) achieved larger permeation flux of CO2 and slightly more H2 than PZ-2+MS. However, Figure 5.32-B reveals that the latter obtains much better separation factor of H2/CO2. When the pore diameter becomes smaller, molecules with small size can still permeate but not for the large molecules. Therefore, in particular, the pore size distribution influences the separation properties of the membrane. A relative small pore size of the membrane resulted in better separation ability.

Figure 5.32: Inﬂuence of pore size in <2M;N2>

Inﬂuence of thickness The cylinder products of cement were cut into disc-shaped specimens with the thickness of 5, 10 and 20 mm. Since the products were friable, not all membrane types had these three thicknesses. Basically, most of the membranes had the thickness of 5 mm, so the membranes with the thickness of 5 mm were mainly used in comparative experiments. Both permeation and separation factor using 5 mm membrane were much better than those using 10 and 20 mm. Figure 5.33 shows the separation factor of H2 to CO2 in the measurements <2M;N2> with specimens PZ-2+MS ((w/c)eq 0.25) of 5 mm, 10 mm, and 20 mm. Using 5 mm thick specimen performs much higher separation of H2 from

CO2. It verify the theory that, permeation rate is inversely proportional to thickness of membranes [1, 96]. The membranes employed were not thinner than 5 mm, since 5.2 Experiments 73 thinner membrane can not endure mechanical stresses. Thus, the thickness of 5 mm is a good choice for other comparisons.

Figure 5.33: Effect of different sample thickness on diffusion in <2M;N2>

One problem came out in the experiments of 20 mm specimen. In general, the thinner the membrane is, the larger permeation rate is. However, as shown in Table 5.12 (examples at 350 ◦C and room temperature), permeation ﬂuxes using 20 mm specimen are unreasonable. Values of 20 mm are larger than those of 10 mm, and the permeation

ﬂuxes of CO2 are even larger than those of 5 mm.

Table 5.12: Permeation ﬂuxes using PZ-2+MS in <2M;N2> RT 350 ◦C Thickness of membrane 5 mm 10 mm 20 mm 5 mm 10 mm 20 mm

H2 in permeate-gas 13.24 % 4.84 % 10.02 % 16.49 % 6.37 % 11.42 %

CO2 in permeate-gas 3.90 % 3.45 % 7.24 % 9.45 % 3.45 % 5.27 %

Figure 5.34 shows those membranes after heating. 20 mm specimen-PZ-2+MS (top right) was already broken after the heating. Lots of deep cracks appeared on this membrane. The surface of 5 mm-PZ-2+MS (top left) looks better with a bit of minor cracks which are not obvious. There is another membrane-TZ-2 (10 mm, (w/c)eq 0.25) with additive iron shown at the bottom of Figure 5.34. The surface layer became darker because of heating, and fall oﬀ after the experiments. 74 Chapter 5 Experimental Results and Discussion

Figure 5.34: Membranes after heating

The cracks might be caused by water loss while heating or reaction between CO2 and cement. When Gregor produced the tube membrane, he also found that the last drying step of making membrane was hard to ﬁnish. Water lost might lead to the unuseful membranes, which had several cracks on their surfaces. Thus, the usable membranes should be dried slowly for 6 weeks. Among them 20 mm was the thickest one, so water loss must be more obvious than others. That might also be the reason for the large permeation ﬂuxes.

Inﬂuence of diﬀerent feed concentration

Besides pure gases and two-component mixture, ﬁve-component mixture (13 % H2 and 13 % CO2 included) was also measured as feed gas. One sample with specimen- HOZ+MS (thickness of 5 mm, (w/c)eq=0.25) is shown in Table 5.13. It lists the separation factors of H2 from other components. Compared with the other three gases, hydrogen has much better degree of separation, which reveals the possibility to separate hydrogen from the mixtures. Figure 5.35 shows the situation more intuitionisticly. A and B are the compositions of feed gas and permeate-gas respectively without purge 5.2 Experiments 75

gas N2.H2 can permeate more through a cement membrane, so concentration of H2 increases in this way. This result is just from the single stage. If the mixture was separated with cement membranes using multiple stages, recycle design or both, results will be much better and puriﬁcation of H2 can be realized then.

Table 5.13: Separation factors using HOZ+MS in <5M;N2> Temperature Separation factor

H2/CO H2/CH4 H2/CO2 RT 3.365 2.081 2.180 100 ◦C 3.104 1.912 1.950 200 ◦C 3.006 1.760 1.705 350 ◦C 2.782 1.639 1.548

Figure 5.35: Comparison of compositions in <5M;N2>

Permeabilities of different feed gases using specimen-PZ-2+MS (5 mm, (w/c)eq 0.25) are summarized in Figure 5.36. When pure H2, two-component mixture and five- component mixture are used as feed gases, and the concentrations of H2 component are 100 %, 49.8 % and 13 %, respectively. Lower concentration of feed gas (<5M;N2>) corresponds to higher permeability. Therefore, when using real gas from a biomass gasified, even very low concentrations of target components, this technology is still operational. Furthermore, the application of cement membranes and circulatory system will be helpful to reduce costs.

Influence of pressure difference Pressure on both side of the membrane can affect separation. Pressure difference is one of the driving forces that determine permeation ability. Membrane-PZ-2+MS (5 mm, (w/c)eq 0.25) was measured in experiment <2M;N2> with variational pressure difference, while flow rates and temperature kept constant (see Figure 5.37).

The value of abscissa is the pressure diﬀerence of retentate to permeate side of the membrane. The results prove that, larger pressure diﬀerence leads to higher permeation 76 Chapter 5 Experimental Results and Discussion

Figure 5.36: Permeabilities of H2 using diﬀerent feed gases

Figure 5.37: Inﬂuence of pressure diﬀerence in <2M;N2>

rate. Moreover, larger pressure difference corresponds to lower separation ability of H2 from CO2. When the pressure difference is larger than 0.005 bar, the separation factor will keep constant of 1; thus, membrane could not separate H2 from CO2. The lower pressure in the feed side of the membrane is, the larger separation factor is. However, when the pressure is too low, just little gas could permeate, and slow separation is of poor usability. Thus, the range of 0.002 to 0 bar is the optimal pressure difference in the experiments. 5.2 Experiments 77

Inﬂuence of sweeping gas

Figure 5.38 shows data in experiments of <2M;N2> with the decreasing ﬂow rate of

N2. The setpoint of feed gas was ﬁxed at 75 mln/min, and that of sweeping gas was conducted from 75 mln/min to zero. Membrane-HOZ+SFA (5 mm, (w/c)eq 0.25) are used here. Both permeation ability and separation factor became larger as ﬂow rate of sweeping gas decreases. Sweeping the permeation components away instantly reduced the permeate partial pressure and enhanced the driving force. Hence, sweeping gas was necessary. Moreover, sweeping gas could also be used to adjust the permeate-side pressure and then to obtain another factor on permeation — permeate pressure.

Figure 5.38: Inﬂuence of sweeping gas in

The majority of the measurements were performed with nitrogen as purge gas. Alter- natively, carbon monoxide was also used. Figure 5.39 shows the mole fraction of H2 in permeate-gas in six experiments. Membranes used here are PZ-2 (5 mm, (w/c)eq 0.45), HOZ+SFA (5 mm, (w/c)eq 0.25), and HZ-2+MS (5 mm, (w/c)eq 0.25). Lines with foursquare points are data from the measurements , and points round in shape are results from . Values of the same membrane are of slight difference between the experiments using different sweeping gas. Especially for HZ-2+MS, results from and are much more similar. Therefore, different kind of sweeping gas did not influence the process of gas separation. The choice of optimal sweeping gas is determined by the different application. In most cases, the product gas is required to be with a good separability. There is no theorem to select the purge gas until now, so the sweeping gas is usually determined by the actual requirement.

In further research, CO can be studied more, since there are already lots of relatively mature technologies in pre-combustion ﬁeld to separate CO. Researches can also be 78 Chapter 5 Experimental Results and Discussion

Figure 5.39: Measurements of diﬀerent sweeping gases in <2M;N2> and <2M;CO>

done on CO2 and steam. CO2 can be adsorbed easily by water or lime water, and it is easy to be released and recycled. Advantages of steam are well-known. Product gas can be simply puriﬁed by a drying system. There are three disadvantages. First it requires higher temperature. Second, product gas must be dried enough before cooling down, otherwise water contained will block the pipes. Third, water may react with some components in cement and change the membrane structure, which should be totally avoided.

Influence of adhesives Influence of material with different adhesives were also considered. Usually membranes containing Microsilica displays better separation factor. As shown in Figure 5.40 with base cement PZ-2 and PZ-1. However, take PZ-2 as example, with Microsilica corresponding a small critical pore diameter. The critical pore diameter of PZ-2 0.25 is 80 nm. The critical pore diameter of PZ-2+MS 0.25 is 8 nm, the latter obtains much better separation factor of H2 to CO2. Therefore, advantages of the separation ability of PZ-2+MS might also be caused by pore size distribution. This can be studied further. 5.2 Experiments 79

Figure 5.40: Inﬂuence of adhesives

Separation factor Figure 5.29 and Figure 5.30 show that higher separation factors and lower permeation rates at low temperatures are obtained than those at higher temperature regardless of the feed composition. At higher temperature, the diﬀusion rates of isolated permeating molecules and associated permeating molecules are higher, so that total permeation rate is higher and the separation factor low. Thus, separation factors are more comparable at room temperature. Table 5.14 shows the separation factors for all 5 mm’s membranes that have been measured.

Membranes are in descending order according to separation factor. The separation factor of S938H is near 1, which means S938H can not separate H2 from CO2 eﬀectively [88]. Other cement membranes are better than this multilayer ceramic membrane. PZ- 2+MS ((w/c)eq 0.25) in the ﬁrst row performs the best separation ability. Therefore, it is selected as the material for tubular membrane in further research. Images of PZ-2+MS made by Center for Electron Microscopy of Technische Universität Berlin (ZELMI) is shown in 5.41. 80 Chapter 5 Experimental Results and Discussion

Table 5.14: Separation factors using 5 mm membranes in <2M;N2> Membranes (w/c)eq Mole fraction in permeate-gas Separation factor

code name H2[%] CO2[%] of H2/CO2 PZ-2+MS 0.25 13.24 3.9 3.425 HOZ+SFA 0.25 14.20 5.64 2.537 HOZ 0.25 13.08 5.20 2.535 F-HÜS 0.25 10.30 4.16 2.496 PZ-1+MS 0.25 14.73 5.98 2.483 PZ-1+SFA 0.25 9.60 4.09 2.367 PZ-2 0.25 13.84 5.95 2.346 HOZ+MS 0.25 11.98 5.34 2.260 PZ-1+MS 0.45 12.92 6.39 2.036 F-PZ 0.25 13.45 7.15 1.895 PZ-1 0.25 16.69 9.40 1.790 PZ-2 0.45 18.29 11.43 1.613 HOZ+SFA 0.45 14.24 9.87 1.454 S938H unknown 27.18 30.84 0.942

Figure 5.41: SEM images of the PZ-2+MS

Permeation ability and diffusion coefficient Membranes we used are mostly with pore size between 8 to 100 nm. The Knudsen numbers of H2 and CO2 using membranes with pore diameter 8 nm and 100 nm are shown in Figure 5.42. We can see that the Knudsen numbers are around one, and those of membrane with pore diameter 8 nm are larger but the differences are small. Therefore, according to the theory above the transport mechanism using these porous cement membranes should be transition diffusion. 5.2 Experiments 81

Figure 5.42: Knudsen number of H2 and CO2

Among the membranes, PZ-2+MS (5 mm, (w/c)eq 0.25) was the best observed object.

The data from experiment <2M;N2> using this specimen are listed in Table 5.15. According to Equation(4.29), Equation(4.34) and Equation(4.31) in Chapter 4, the

Table 5.15: Data using PZ-2+MS (5 mm, (w/c)eq 0.25) in <2M;N2> Temperature [◦C] 27.65 101.08 199.79 349.68 Flow rate of feed gas [ml/min] 31.41 31.41 31.41 31.41

Mole fraction of H2 in feed [%] 49.8 49.8 49.8 49.8

Mole fraction of CO2 in feed [%] 50.2 50.2 50.2 50.2 Flow rate of purge gas [ml/min] 30.34 30.34 30.34 30.34 Flow rate of permeate gas [ml/min] 32.02 32.30 32.57 32.30

Mole fraction of H2 in permeate-gas [%] 13.24 14.31 15.26 16.49

Mole fraction of CO2 in permeate-gas [%] 3.90 5.75 7.42 9.45 Flow rate retentate [ml/min] 26.56 27.07 27.43 27.40

Mole fraction of H2 in retentate [%] 31.48 30.10 28.48 26.48

Mole fraction of N2 in retentate [%] 19.5 21.44 23.21 25.12

Mole fraction of CO2 in retentate [%] 45.50 45.06 44.18 42.66 82 Chapter 5 Experimental Results and Discussion permeability, diffusion coefficients (Equation(5.6)) and permeation rate at different temperature are carried out in Table 5.16.

Table 5.16: Permeation ability using PZ-2+MS (5 mm, (w/c)eq 0.25) in <2M;N2> Temperature 27.65 ◦C 101.08 ◦C 199.79 ◦C 349.68 ◦C

SH2,CO2 3.425 2.510 2.072 1.759

βH2 42.05 % 47.55 % 53.58 % 62.26 % 2 −3 −3 −3 −2 DH2 [cm /s] 7.42 × 10 8.34 × 10 9.21 × 10 1.02 × 10 2 −3 −3 −3 −3 DCO2 [cm /s] 1.72 × 10 2.67 × 10 3.62 × 10 4.80 × 10

αH2,CO2 4.30 3.12 2.55 2.13 −1 −2 −5 −5 −5 −6 JH2 [mol·min ·cm ] 1.37 × 10 1.20 × 10 1.02 × 10 8.30 × 10

P P Ji 3 273.5 · R · Ci · V Di = F P · δ = δ · 10 · F P (5.6) Ci − Ci 60 · 22.41 · A · (Pi − Pi )

Moreover, as mentioned in chapter 2, the transport is reckoned mainly as Knudsen diffusion, if the rp is between 5∼30 Å [5]. Most pore diameters of the membranes employed in the project were between 8 to 100 nm, so the gas transport mechanism might not be dominated by Knudsen diffusion. Furthermore, the theoretic values of the Knudsen diffusion coefficient according to Equation(4.14) and the molecular diffusion coefficient (Equation(4.19)) with an increase in temperature from room temperature to 350 ◦C are shown in Figure 5.43. We find actual diffusion coefficients are not

Figure 5.43: Diffusion coefficients of H2 the same with molecular diffusion theory, not with the Knudsen diffusion theory, but 5.2 Experiments 83 between their theoretic ranges. Therefore, the gas transport mechanism using the cement membranes in the project should combine Knudsen diffusion with molecular diffusion. The diffusion coefficient should be express as in Equation(5.7). Details can be studied further. 1 a b = + (5.7) D Di,K Di where, a and b are two constants.

5.2.3 Third set of experiments with tubular membrane and cell

As the performance of specimen-PZ-2+MS (5 mm, (w/c)eq 0.25) was excellent, it was chosen as the material for columnar membranes. Using tubular membrane, the gases can stay a longer time for permeation, larger area can be created for gas diffusion, and a larger extract rate is hopefully achieved. Falconer illustrated the transport of gases in a tubular membrane cell [1]. Since counter-current flow pattern performs better than co-current, it is mainly employed in the project and the co-current flow pattern in Falconer’s schematic diagram has been replaced with counter-current here (see Figure 5.44).

Figure 5.44: Schematic of transport in tubular membrane cell [1]

Membrane cell

Figure 5.45 shows an axial cut-away section of the new reactor. The blue-gray cir- cularity cylinder is the tubular porous membrane-PZ-2+MS (5 mm, (w/c)eq 0.25), and the dark-gray parts are made of non-permeable stainless steel. The spaces beside membrane are routes for gases. Feed gas ﬂows from left to right inside membrane-tube. Some components permeate through the porous membrane meanwhile. Sweeping gas 84 Chapter 5 Experimental Results and Discussion enters membrane cell from right, and carries the permeation components out along the outside-route. The center nonpermeable stainless steel cylinder reduces the space inside membrane, so gases will travel along membrane surface. Furthermore, this center cylinder can protect membrane from pressing of side covers. However, the length of the center cylinder must be controlled ﬁtly. If it is too long, then gases will leak from the spacing between membrane and side covers; on the other hand, center cylinder can not work as protection, and it will move or even crush the fragile wall of the membrane from inside. The tubular membrane cell with the special designed asbestine heater is shown in Figure 5.46.

Figure 5.45: Tubular membrane cell

Figure 5.46: Heating system for tubular membrane

The two side covers were much more complicated. In Figure 5.47, top left picture is the left view of the whole part. Eight screws ﬁx the left side cover onto the middle metal shell. The center hole-A (permeate-gas exit) will be separated to four (top 5.2 Experiments 85 middle), connected with the outside space of the membrane. The other large hole- B (entrance of feed gas) on the left view, will be oriented to the center-C (bottom left) then separated to three (bottom middle), and lead to the inside space of the membrane. Two other holes marked with T1 and T2 in the above four diagrams are designed for thermocouples. They are located at the same radial position but two sides of membrane. Right picture in Figure 5.47 is the left view of the inside structure (without left side cover). Spaces inside and outside membranes are clearer to observe. T3 and T4 are thermocouple holes settled on the right side cover, in the opposite direction with T2 and T1, which are planed on measuring temperature roundly. The length of the tubular-shaped membrane is 18 cm, and diameters of two cirques are 1.72 cm, 2.00 cm, 2.50 cm and 2.70 cm in sequence from inside to outside. Hence, volumes of inside and outside membrane are 58.90 and 58.81 cm3 respectively.

Figure 5.47: Design of tubular membrane cells

Tubular membrane and gaskets

Photos of tubular membrane and one side cover are shown in Figure 5.48. Membrane is 18 cm long and 5 mm thick. This was the only measurable tubular membrane, since membranes were easily destroyed, sometimes cracks came out from fast water loss, or even holes were present on the surface in the drying process. One of the plane is not smooth (Figure 5.48-left), which should be avoided in the further research. 86 Chapter 5 Experimental Results and Discussion

This membrane is just a tentative step, more membranes and measurements should be carried out for a universal study. Figure 5.48 shows graphite gaskets in a side cover. 1, 2, 3 are marked at the position for graphite ring-plates. 1 is between the side cover and metal shell; 2 is between side cover and membrane; and 3 is between side cover and the center metal cylinder. Those of the other side of the membrane cell are same. An nonpermeable copper tubular membrane, was used to measure the gas-tightness of the joints. All the connections were veriﬁed to be well airtight.

Figure 5.48: Tubular membrane and gaskets

Results and comparison

One test tubular membrane was heated at 350 ◦C for three hours before measurements, and a longitudinal crack appeared. In order to avoid the errors from large cracks that might be caused by heating, the experiment procedures were modified. It was proved to be feasible to make measurements of all the gas groups at temperature from low to high. All the gas groups were done at room temperature first; then 100 ◦C; later ◦ membrane was blew with N2 for 16 hours at 200 C and measurements were finished ◦ ◦ at 200 C and 350 C, respectively. Standard-gases of 2M, 5M,H2, CO2 and N2 were measured each day as references. Each group was conducted three times for a average value.

Mole fractions of H2 and CO2 using tubular and plate membranes in <2M;N2> are compared in Figure 5.49. Separation factors of H2 to CO2 in <2M;N2> and <5M;N2> are shown in Figure 5.50. The results are not so positive. There are more permeations 5.2 Experiments 87

Figure 5.49: Components in permeate-gas in <2M;N2>

Figure 5.50: Separation factors of H2 to CO2

for both H2 and CO2 using tubular membrane than plate one (the other gas pairs show the same trend), but the separation factors are a little smaller. As discussed above, there was just one tubular membrane, one of whose planes was not smooth. Therefore, this membrane was just a tentative step, and more membranes should be produced and investigated to obtain representative values for tubular-shaped cement membrane. 88 Chapter 5 Experimental Results and Discussion

When the temperature increased from 100 ◦C to 200 ◦C for the ﬁrst time, the experiments stopped, as something blocked the pipes and gases could not move further. When the connections were opened 10 cm away from the outlet of permeate-gas, clear liquid was observed (see Figure 5.51).

Figure 5.51: Water from tubular membrane

The liquid was smelly and it might be water mixed with the solvent (adhesives in the ◦ cements). That was the reason for 16 hours heating with N2 at 200 C—to drive all the liquid out of the membrane cell. After that measurements at 200 ◦C went well. When the membrane was heated from 200 ◦C to 350 ◦C, same phenomena came out. Thus, the standard-gases were measured ﬁrst for seven hours, and experiments at 350 ◦C was measured successfully.

One block-chromatograms is shown in Figure 5.52. One more peak appears (gray) at 7.5

◦ Figure 5.52: Chromatogram in <5M;N2> at 200 C min, which can be steam (up to parked materials in columns). The water-loss problem might also happen at 100 ◦C. A small amount of water can not easily be found. Under normal circumstances, the water vaporizes at 100 ◦C. In the experiments, vaporizing also occurred at 200 ◦C. That may because that, water in micropores gasiﬁed at higher 5.2 Experiments 89 temperature, when the pore pressure increased large enough, vapors can be pressed out. In much smaller pores, water loss did not happen at 200 ◦C, but it was achieved by 350 ◦C. At last, large crack occurred on the tubular membrane (Figure 5.53).

Figure 5.53: Large cracks after heating

Vapors flowed out of the heating membrane cell, into the pipes at room temperature. Then they were cooled, condense and accumulated until the pipelines were blocked. Inner diameter of the pipes was small, only 2 mm, as long as a drop of water came into being, the problem became obvious. Pipe blocking baffled drawing gases for chromatograph-measurements, while also changed pressures on both sides of membrane, which then influenced the gas separation. As mentioned above, water loss can result in larger holes and cracks, which changed the characteristics of membrane itself, and also influenced its gas separation performance. Hence, the methods of manufacture and experiments need to be improved.

Diﬀusion coeﬃcients and other parameters calculated are listed in Table 5.17 and Table 5.18.

Table 5.17: Permeation ability using tubular-PZ-2+MS (5 mm, 0.25) in <2M;N2> Temperature 23.2 ◦C 101.5 ◦C 190.6 ◦C 358.65 ◦C

SH2,CO2 1.465 1.844 1.747 1.842

βH2 81.89 % 82.27 % 60.37 % 47.75 % 2 −4 −4 −3 −3 DH2 [cm /s] 7.59 × 10 8.00 × 10 1.27 × 10 1.69 × 10 2 −4 −4 −4 −4 DCO2 [cm /s] 4.42 × 10 3.43 × 10 5.03 × 10 5.63 × 10

αH2,CO2 1.72 2.33 2.53 2.99 −1 −2 −6 −6 −6 −7 JH2 [mol·min ·cm ] 1.25 × 10 1.02 × 10 1.01 × 10 8.50 × 10 90 Chapter 5 Experimental Results and Discussion

Table 5.18: Permeation ability using tubular-PZ-2+MS (5 mm, 0.25) in <5M;N2> Temperature 23.2 ◦C 101.5 ◦C 190.6 ◦C 358.65 ◦C

SH2,CO2 1.140 1.406 1.724 2.302

βH2 81.99% 81.01 % 54.32 % 39.45 % 2 −4 −4 −3 −3 DH2 [cm /s] 8.51 × 10 8.91 × 10 1.56 × 10 2.14 × 10 2 −4 −4 −4 −4 DCO2 [cm /s] 6.95 × 10 5.32 × 10 5.82 × 10 4.78 × 10

αH2,CO2 1.22 1.67 2.68 4.47 −1 −2 −7 −7 −7 −7 JH2 [mol·min ·cm ] 3.53 × 10 2.83 × 10 2.86 × 10 2.50 × 10 Chapter 6 Summary and Outlook

6.1 Summary of results

Three membrane cells were designed to form a the single-stage module with the counter- current ﬂow pattern. 25 cement membranes were tested. Furthermore, experiments were conducted in order to characterized the following points: 1. gas molecules properties related to their transport in materials; 2. impact of operating conditions on the separation process; 3. inﬂuence of membrane properties of the membrane structure on gas permeation; 4. the relative parameters necessary to determine the membrane performance.

1. Gas molecules properties related to their transport in materials.

Permeation component with a relatively small molecular size corresponded to a relatively high permeation flux and diffusion coefficient. Compared with the other three gases, hydrogen had a considerably good separation factor. The experimen-

tal selectivity of H2 to CO2 matched well the theoretical value. The selectivity of the two components was with good approximation inversely proportional to the

square root of their molecular weight. Use of CO as a sweeping gas instead of N2 did not aﬀect the results. In most cases, good separability of the permeated gas determined the choice of the optimal sweeping gas. A lower feed concentration value led to a higher value of permeability.

2. Impact of operating conditions on the separation process.

Permeation ﬂuxes increased, while the separation factors of H2 to CO2 decreased with increasing temperature. A large pressure diﬀerence resulted in a high per-

meation rate and a low separation ability of H2 from CO2. The permeability 92 Chapter 6 Summary and Outlook

decreased with an increase in the ﬂow rate of feed gas. Both the permeation ability and the separation factor increased with a decrease in the ﬂow rate of the sweeping gas. Sweeping the permeation components away instantly reduced the permeate partial pressure and enhanced the driving force. Thus, the sweeping gas was necessary.

3. Inﬂuence of membrane properties of the membrane structure on gas permeation.

Membranes with a thickness of 5 mm show a considerably higher separation of H2

from CO2, than membranes with thicknesses of 10 mm and 20 mm. A membrane with low equivalent water to cement ratio could achieve a high separation of

H2 from CO2. In particular, the pore size distribution inﬂuenced the separation properties of the membrane. A relative small pore size of the membrane resulted

in better separation ability. Relatively large permeation ﬂuxes of both H2 and

CO2 were obtained when a tubular membrane was used than when a plate one was used, but the separation factors were slightly smaller. More membranes should be produced and investigated to obtain representative values for the tubular-shaped membranes.

4. The relative parameters necessary to determine the membrane performance.

The separation factors of all the disk-shaped membranes at room temperature were calculated. The multilayer ceramic membrane S938H was the worst with

an H2/CO2 separation factor of almost 1.0. Furthermore, PZ-2+MS (5 mm (w/c)eq 0.25) has the best separation ability. The permeation ability and diﬀusion coeﬃcients of a disc-shaped membrane were successfully estimated, as shown in Table 5.16. These values were slightly lower than those of other inorganic

membranes (e.g., a separation factor of 6.75 for H2/CO2 at low temperature obtained by Koros and Mahajan [78]). However, as a cheap new material, PZ- 2+MS has potential, and the relevant researches are in progress.

6.2 Observations

Provided below are the observations pertaining to the sealing materials and the operating apparatus:

1. Teﬂon could not withstand a higher than 250 ◦C. It became darker and harder when it was heated. Furthermore, the airtightness became poor, especially at 6.3 Future work 93

the connections between the side covers and the metal shell. Graphite showed an excellent capability as an optional gasket.

2. The measurement of the volumetric flow rate using soap film flowmeters was a quick process and the results were reproducible. However, a large quantity of flat soap was required during the measurements because of the diffusion problem. Furthermore, the range of measurement improved from 10 to 200 ml/min.

3. The flow rates controlled by the mass flow controllers (MFC s) were generally larger than the set values. Fortunately, the actual flow rates and setpoints had a linear relationship. Therefore, the observed linear relationship of a small range around the experimental flow rate was considered data processing.

4. The front detector of a gas chromatograph (GC ) using N2 as a reference, should

be used to monitor only H2. He could be used as a reference gas for all the other gas components used in the project. A high volumetric flow rate led to the automatic injection of a large amount of the sample. However, the use of new methods of data processing helped overcome such problems during calibration. Reliable measurements were taken by using a manual base line with a fixed run time of 10.5 min. The ambient temperature did not affect the chromatograph.

6.3 Future work

More membranes in diﬀerent shapes should be produced and investigated. The following are the suggestions for further researches, and some ideas over puriﬁcation of product-gas.

1. Incongruous flow rates caused by MFC can easily be solved by the linear - relationship for laboratory experiments. However, when the measurements are ex- tended to a larger range of flow rates, more standard-gases measurements would be required. Therefore, new equipments or methods are necessary to control the flow rates strictly for further measuring. Experiments on real gas from a biomass gasified are necessary, since they are fundamental for practical applications.

2. Influence of flow rates on GC measurements can also be amplified in further experiments, the same as that of MFC. Therefore, standard-gases with more different mole fractions of target components are required to calibrate GC more accurately, which is the most essential for quantification. A new method for faster measuring can also be considered, e.g., laser. 94 Chapter 6 Summary and Outlook

3. Graphite has high thermal stability. The material is soft, so it can protect membranes and remedy the small surface defect. However, graphite is also fragile. When the membrane cell is installed, graphite gaskets pasted on the membrane cell should be cleared and new gaskets are required. Handling the thicknesses of graphite around the columnar surface of membranes is also diﬃcult. Gaskets of new material which should be much harder, airtight and reusable, e.g., Carbon graphite, are demanded to replace graphite paper.

4. Water dissolved with adhesives was lost from membrane, which brought new components into product gas, changed the structure of membrane, embarrassed measurements with GC, and influenced gas separation. Moreover, water loss stunted cement membranes using at higher temperature, which was the notable advantage of inorganic membranes. Better methods or bind to produce membrane is required to avoid this problem. In addition, because the effectivity of membrane with a equivalent ratio of water to cement ((w/c)eq) of 0.25 is better than that of 0.45, we can search for new methods to reduce the equivalent water to cement ratio further. Hence, the membrane itself can be more refined and more stable, and better separation ability can be achieved. With good results, more membranes can be manufactured, and different shapes membranes can then be studied at even higher temperatures.

5. Provided below are the ideas to use a diﬀerent sweeping gas for the maneuverable puriﬁcation of permeation-gas.

As pointed out above, there are already lots of relatively mature technologies in pre-combustion ﬁeld to separate CO, so using CO as feed gas can be studied

further. Other possibilities of feed gas are CO2 and steam. Firstly, CO2 can be adsorbed by water or lime water easily, and it is also easy to be released and recycled. Possible schematic process is shown in Figure 6.1.

Secondly, using steam as sweeping gas, product gas can be simply puriﬁed by a drying system (Figure 6.2). The precondition is that water should not react with some components in the material, otherwise it will change the membrane structure.

6. The investigation of relationship among diffusion coefficient and all the other parameters, such as temperature, pressure difference, thickness of membranes is necessary. An expression can be built on the investigation. 6.3 Future work 95

Figure 6.1: Process schematic of using CO2 as sweeping gas

Figure 6.2: Process schematic of using steam as sweeping gas

7. More experiments on using cement membranes to separate gases from a biomass gasified are required. With good result, this material can be applied on gas separation or purification in the industrial field.

Bibliography

[1] Falconer, John L. ; Noble, Richard D. ; Sperry, David P.: Catalytic membrane reactors. In: Noble, Richard D. (Hrsg.) ; Stern, S. A. (Hrsg.): Mem- brane Separations Technology-Principles and Applications. (Membrane Science and Technology Series, Volume 2). Amsterdam, Netherlands : Elsevier Science B.V., 1995, Kapitel Chapter 14, S. 669–712

[2] Sengupta, Amitava ; Sirkar, Kamalesh K.: Analysis and design of membrane permeatiors for gas separation. In: Noble, Richard D. (Hrsg.) ; Stern, S. A. (Hrsg.): Membrane Separations Technology-Principles and Applications. (Mem- brane Science and Technology Series, Volume 2). Amsterdam, Netherlands : El- sevier Science B.V., 1995, Kapitel Chapter 11, S. 499–552

[3] Mulder, Marcel: Basic Principles of Membrane Technology. 2nd ed. The Nether- lands : Kluwer Academic Publishers, 2000

[4] Sutherland, Ken: Filters and ﬁltration handbook. 5th ed. Butterworth- Heinemann, 2008. – 68 S

[5] Koltuniewicz, Andrzej B. ; Drioli, Enrico: Membranes in Clean Technologies: Volume 1 - Theory and Practice. Weinheim, Germany : WILEY-VCH Verlag GmbH & Co. KGaA, 2008

[6] Noble, Richard D. ; Terry, Patricia A. ; Varma, Arvind (Hrsg.): Princi- ples of Chemical Separations with Environmental Applications. United Kingdom : Cambridge University Press, 2004

[7] Service, Robert F.: HYDROGEN POWER: Bringing Fuel Cells Down to Earth. In: Science 285 (1999), July, Nr. 5428, S. 682 – 685

[8] Sea, Bongkuk ; Lee, Kew-Ho: Modiﬁcation of Mesoporous γ-Alumina with Silica and Application for Hydrogen Separation at Elevated TemperSea2001ature. In: Journal Industrial & Engineering Chemistry Research 7 (2001), September, Nr. 6, S. 417–423

[9] Ismail, Ahmad F. ; Ridzuan, Norida ; Rahman, Sunariti A.: Latest development on the membrane formation for gas separation. In: Songklanakarin Journal of Science and Technology 24 (2002), April, S. 1025–1043 98 Bibliography

[10] Gavalas, George R.: Diﬀusion in Microporous Membranes: Measurements and Modeling. In: Industrial & Engineering Chemistry Research 47 (2008), July, Nr. 16, S. 5797–5811

[11] Meinema, Harry A. ; Dirrix, Ruud. W. ; Brinkman, H.w. ; Terpstra, R.A. ; Jekerle, Jiri ; Kösters, P.H.: Ceramic Membranes for Gas Separation - Recent Developments and State of the Art. In: Interceram 54 (2005), Nr. 2, S. 86–91

[12] Bernardo, P. ; Drioli, E. ; Golemme, G.: Membrane Gas Separation: A Review/State of the Art. In: Industrial & Engineering Chemistry Research 48 (2009), April, Nr. 10, S. 4638–4663

[13] Al-Rabiah, A.A.: Membrane Technology for Hydrogen Separation in Ethylene Plants. In: 4th Ibero-American Congress on Membrane Science and Technology (CITEM), 2003

[14] Mottern, M. L. ; Shi, J. Y. ; Shqau, K. ; Yu, D. ; Verweij, Henk: Mi- crostructural Optimization of Thin Supported Inorganic Membranes for Gas and Water Puriﬁcation. In: Li, Norman N. (Hrsg.) ; Fane, Anthony G. (Hrsg.) ; Ho, W. S. W. (Hrsg.) ; Matsuura, T. (Hrsg.): Advanced Membrane Technology and Applications. New Jersey, the United states of America : John Wiley & Sons, Inc., 2008. – 903, Kapitel Chapter 34, S. 899–928

[15] Keizer, Klaas ; Uhlhorn, Robert J. ; Burggraaf, Ton J.: Gas separation using inorganic membranes. In: Noble, Richard D. (Hrsg.) ; Stern, S. A. (Hrsg.): Membrane Separations Technology-Principles and Applications. (Membrane Sci- ence and Technology Series, Volume 2). Amsterdam, Netherlands : Elsevier Sci- ence B.V., 1995, Kapitel Chapter 12, S. 553–588

[16] Cen, Y. ; Lichtenthaler, R.N.: Vapor permeation. In: Noble, Richard D. (Hrsg.) ; Stern, S. A. (Hrsg.): Membrane Separations Technology-Principles and Applications. (Membrane Science and Technology Series, Volume 2). Amsterdam, Netherlands : Elsevier Science B.V., 1995, Kapitel Chapter 3, S. 85–112

[17] Williams, P. J. ; Koros, William J.: Gas Separation by Carbon Membranes. In: Li, Norman N. (Hrsg.) ; Fane, Anthony G. (Hrsg.) ; Ho, W. S. W. (Hrsg.) ; Matsuura, T. (Hrsg.): Advanced Membrane Technology and Applications. New Jersey, the United states of America : John Wiley & Sons, Inc., 2008. – 601, Kapitel Chapter 23, S. 599–631

[18] Javaid, Asad: Membranes for solubility-based gas separation applications. In: Chemical Engineering Journal 112 (2005), September, Nr. 1-3, S. 219–226

[19] Adhikari, Sushil ; Fernando, Sandun: Hydrogen Membrane Separation Tech- niques. In: Industrial & Engineering Chemistry Research 45 (2006), January, Nr. 3, S. 875–881 Bibliography 99

[20] Wu, Jeﬀrey Chi-Sheng: Feasibility of Manufacturing Hydrogen and Styrene through the Use of Porous Ceramic Membranes. In: Industrial & Engineering Chemistry Research 38 (1999), September, Nr. 11, S. 4491–4495

[21] Merkel, T. C. ; Bondar, V. I. ; Nagai, K. ; Freeman, B. D. ; Pinnau, I.: Gas Sorption, Diﬀusion, and Permeation in Poly(dimethylsiloxane). In: Journal of Polymer Science: Part B: Polymer Physics 38 (2000), S. 415–434

[22] Kusuma, Victor A. ; Freeman, Benny D. ; Jose-Yacaman, Miguel: Struc- ture/Property Characteristics of Polar Rubbery Membrane for Carbon Dioxide Removal. In: Li, Norman N. (Hrsg.) ; Fane, Anthony G. (Hrsg.) ; Ho, W. S. W. (Hrsg.) ; Matsuura, T. (Hrsg.): Advanced Membrane Technology and Applica- tions. New Jersey, the United states of America : John Wiley & Sons, Inc., 2008. – 934, Kapitel Chapter 35, S. 929–953

[23] Li, Yan-Shuo ; Liang, Fang-Yi ; Bux, Helge ; Feldhoff, Armin ; Yang, Wei- Shen ; Caro, Jürgen: Molecular Sieve Membrane: Supported Metal-Organic Framework with High Hydrogen Selectivity. In: Angewandte Chemie International Edition 49 (2010), S. 548 –551

[24] Itta, Arun K. ; Tseng, Hui-Hsin ; Weya, Ming-Yen: Eﬀect of dry/wet-phase

inversion method on fabricating polyetherimide-derived CMS membrane for H2/N2 separation. In: international journal of hydrogen energy 35 (2010), S. 1650–1658

[25] Krantz, William B.: Membrane Characterization by Ultrasonic Time-Domain Reﬂectometry. In: Li, Norman N. (Hrsg.) ; Fane, Anthony G. (Hrsg.) ; Ho, W. S. W. (Hrsg.) ; Matsuura, T. (Hrsg.): Advanced Membrane Technology and Applications. New Jersey, the United states of America : John Wiley & Sons, Inc., 2008. – 889, Kapitel Chapter 33, S. 879–897

[26] Vieth, Wolf R.: Diﬀusion in and through polymers: principles and applications. Munich : Hanser, 1991

[27] Suloff, Eric C.: Sorption Behavior of an Aliphatic Series of Aldehydes in the Presence of Poly (ethylene terephthalate) Blends Containing Aldehyde Scavenging Agents, Food Science and Technology, Diss., 2002

[28] Dhingra, Sukhtej S.: Mixed Gas Transport Study Through Polymeric Mem- branes: a Novel Technique, Virginia Polytechnic Institute, Diss., 1997

[29] Pandey, Pratibha ; Chauhan, R. S.: Membranes for gas separation. In: Progress in Polymer Science 26 (2001), S. 853–893

[30] Kase, Yoji: Gas Separation by Polyimide Membranes. In: Li, Norman N. (Hrsg.) ; Fane, Anthony G. (Hrsg.) ; Ho, W. S. W. (Hrsg.) ; Matsuura, T. (Hrsg.): Advanced Membrane Technology and Applications. New Jersey, the United states of America : John Wiley & Sons, Inc., 2008, Kapitel Chapter 22, S. 581–598 100 Bibliography

[31] Baker, Richard W.: Membrane Technology and Applications. 2nd ed. Wiley, New York : John Wiley & Sons, Ltd., 2004

[32] Spillman, Robert: Economics of gas separation membrane processes. In: Noble, Richard D. (Hrsg.) ; Stern, S. A. (Hrsg.): Membrane Separations Technology- Principles and Applications. (Membrane Science and Technology Series, Volume 2). Amsterdam, Netherlands : Elsevier Science B.V., 1995, Kapitel Chapter 13, S. 589–667

[33] Sebastián, Víctor ; Lin, Zhi ; Rocha, Joao ; Téllez, Carlos ; Santamaría, Jesús ; Coronas, Joaquín: A new titanosilicate umbite membrane for the sepa-

ration of H2. In: Chemical Communications 24 (2005), April, S. 3036–3037

[34] Hayes, Richard A.: Polyimide Gas Separation Membrne. Nov. 1989. – Appl. No.: 136846

[35] Graham, Tommy E. ; MacLean, Donald L. Process for Hydrogen Recovery from Ammonla Purge Gases. Dec. 1979

[36] Thomas, George ; Parks, George: Potential Roles of Ammonia in a Hydrogen Economy-A Study of Issues Related to the Use Ammonia for On-Board Vehicular Hydrogen Storage / U.S. Department of Energy. 2006. – Forschungsbericht

[37] Whysall, Michael ; Picioccio, Kathy W. Selection and Revamp of Hydrogen Puriﬁcation Processes. March 1999

[38] Yamashiro, H. ; Hirajo, M. ; Maitland, C.F. ; Schell, W.J.: Plant uses membrane separation. In: Hydrocarbon Process 64 (1985), Feb., Nr. 2, S. 87–89

[39] Olsson, David: Comparisson of Reforming Process Between Diﬀerent Types of Biogas Reforming Reactors / Dept. of Energy Sciences, Faculty of Engineering, Lund University,. Sweden, May 2008. – Forschungsbericht

[40] Hedström, L. ; Wallmark, C. ; Alvfors, P. ; Rissanen, M. ; Stridh, B. ; Ekman, J.: Description and modelling of the solar-hydrogen-biogas-fuel cell system in GlashusEtt / Royal Institute of Technology, KTH Chemical Engineering and Technology. 2003. – Forschungsbericht

[41] Bisio, Giacomo ; Bosio, Alessandro ; Rubatto, Giuseppe: Thermodynamics applied to oxygen enrichment of combustion air. In: Energy Conversion and Management 43 (2002), November, S. 2589–2600

[42] Clark, G.A. ; Rowan, M.J.: A Feasibility Study of Using Oxygen Enrichment for Fuel Cell Air Independent Propulsion / Aeronautical and Maritime Research Laboratory. Melbourne, August 1996. – Forschungsbericht

[43] Metz, S.J. ; van de Ven, W.J.C. ; Potreck, J. ; Mulder, M.H.V. ; Wessling, M.: Transport of water vapor and inert gas mixtures through highly Bibliography 101

selective and highly permeable polymer membranes. In: Journal of Membrane Science 251 (2005), April, S. 29–41

[44] TABE-MOHAMMADI, ABDULREZA: A Review of the Applications of Mem- brane Separation Technology in Natural Gas Treatment. In: SEPARATION SCI- ENCE AND TECHNOLOGY 34 (1999), Nov., Nr. 10, S. 2095–2111

[45] Baker, Richard W. ; Lokhandwala, Kaaeid: Natural Gas Processing with Membranes: An Overview. In: Industrial & Engineering Chemistry Research 47 (2008), Nr. 7, S. 2109–2121

[46] Pan, C. Y.: Gas separation by permeators with high-ﬂux asymmetric membranes. In: AIChE Journal 29 (1982), August, Nr. 4, S. 545 – 552

[47] Majumdar, S. ; Bhaumik, D. ; Sirkar, K. K. ; Simes, G.: A pilot-scale demonstration of a membrane-based absorption- stripping process for removal and recovery of volatile organic compounds. In: Environmental Progress 20 (2004), April, Nr. 1, S. 27–35

[48] Dickenson, T. C.: Filters and Filtration Handbook. 4th ed. United Kingdom : Elsevier A.T., 1997

[49] Hoffman, E. J.: Membrane Separations Technology: Single-Stage, Multistage, and Diﬀerential Permeation. United States of America : Gulf Professional Pub- lishing, 2003

[50] Kim, Young-Seok ; Kusakabe, Katsuki ; Morooka, Shigeharu ; Yang, Seung- Man: Preparation of Microporous Silica Membranes for Gas Separation. In: Korean Journal of Chemical Engineering 18 (2001), Nr. 1, S. 106–112

[51] de Vos, Renate M. ; Verweij, Henk: High-Selectivity, High-Flux Silica Mem- branes for Gas Separation. In: Science 279 (1998), Nr. 5357, S. 1710–1711

[52] Petersa, T.A. ; Fontalvoa, J. ; Vorstmana, M.A.G. ; Benesa, N.E. ; van Damb, R.A. ; Vroonb, Z.A.E.P. ; van Soest-Vercammenc, E.L.J. ; Keuren- tjesa, J.T.F.: Hollow ﬁbre microporous silica membranes for gas separation and pervaporation: Synthesis, performance and stability. In: Journal of Membrane Science 248 (2005), S. 73–80

[53] Park, Ho B. ; Lee, Young M.: Pyrolytic carbon-silica membrane: a promising membrane material for improved gas separation. In: Journal of Membrane Science 213 (2003), March, Nr. 1-2, S. 263–272

[54] Casanave, D. ; Ciavarella, P. ; Fiaty, K. ; Dalmon, J.-A.: Zeolite membrane reactor for isobutane dehydrogenation: Experimental results and theoretical modelling. In: Chemical Engineering Science 54 (1999), S. 2807–2815 102 Bibliography

[55] Vankelecom, I. F. J. ; Dotremont, C. ; Morobe, M. ; Uytterhoeven, J.-B. ; ; Vandecasteele, C.: Zeolite-Filled PDMS Membranes: 1. Sorption of Halogenated Hydrocarbons. In: The Journal of Physical Chemistry B 101 (1997), Nr. 12, S. 2154–2159

[56] Vankelecom, Ivo F. J. ; Merckx, Edouard ; Luts, Mark ; Uytterhoeven, Jan B.: Incorporation of zeolites in polyimide membranes. In: The Journal of Physical Chemistry 99 (1995), Nr. 35, S. 13187–13192

[57] Yang, Jianhua ; Čermáková, Jiřina ; Uchytil, Petr ; Hamel, Christof ; Seidel-Morgenstern, Andreas: Gas phase transport, adsorption and surface diﬀusion in a porous glass membrane. In: Catalysis Today 104 (2005), S. 344–351

[58] Jirage, Kshama B. ; Martin, Charles R.: New developments in membrane-based separations. In: TIBTECH 17 (1999), May, S. 197–200

[59] Mottern, M.L. ; Shqau, K. ; Shi, J.Y. ; Yu, D. ; Verweij, Henk: Thin supported inorganic membranes for energy-related gas and water puriﬁcation. In: International Journal of Hydrogen Energy 32 (2007), S. 3713–3723

[60] Miller, James R. ; Koros, William J.: The formation of chemically modiﬁed γ -alumina microporous membranes. In: SEPARATION SCIENCE AND TECH- NOLOGY 25 (1990), Nr. 13-15, S. 1257–1280

[61] Wei, Qi ; Wang, Fei ; Nie, Zuo-Ren ; Song, Chun-Lin ; Wang, Yan-Li ; Qun- Yan: Highly Hydrothermally Stable Microporous Silica Membranes for Hydrogen Separation. In: The Journal of Physical Chemistry B 112 (2008), July, Nr. 31, S. 9354–9359

[62] Li, Kang: Ceramic Membranes for Separation and Reaction. West Sussex, England : John Wiley & Sons, Ltd., 2007

[63] Zhang, Yuwen ; Li, Qian ; Shen, Peijun ; Liu, Yong ; Yang, Zhibin ; Ding, Weizhong ; Lu, Xionggang: Hydrogen ampliﬁcation of coke oven gas by reforming of methane in a ceramic membrane reactor. In: INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 33 (2008), S. 3311–3319

[64] Ma, Yi H.: Hydrogen Separation Membranes. In: Li, Norman N. (Hrsg.) ; Fane, Anthony G. (Hrsg.) ; Ho, W. S. W. (Hrsg.) ; Matsuura, T. (Hrsg.): Advanced Membrane Technology and Applications. New Jersey, the United states of America : John Wiley & Sons, Inc., 2008, Kapitel Chapter 25, S. 671–684

[65] Balachandran, U. ; Lee, T.H. ; Chen, S.J.and Picciolo J. ; Dorris, S.E.: Hydrogen permeation and chemical stability of dense hydrogen separation membranes. In: Proceedings of the 22nd Annual International Pittsburgh Coal Confer- ence. PA, Pittsburgh, Sept. 2005 Bibliography 103

[66] Peinemann, Klaus-Viktor ; Numes, Suzana P.: Membrane Technology: Volume 2 - Membranes for Energy Conversion. Weinheim : Wiley-VCH Verlag GmbH & C. KGaA, 2008

[67] Ayturk, M. E. ; Engwall, Erik E. ; Ma, Y. H.: Microstructure Analysis of the Intermetallic Diﬀusion Induced Alloy Phases in Composite P d/Ag/Porous Stain- less Steel (PSS) Membranes. In: Industrial & Engineering Chemistry Research (ACS Publications) 46 (2007), Nr. 12, S. 4295–4306

[68] Li, Anwu ; Liang, Weiqiang ; Hughes, Ronald: Fabrication of Defect-free P d/α-

Al2O3 composite membranes for hydrogen separation. In: Thin Solid Films 350 (1999), March, S. 106–112

[69] Zhao, H.-B. ; Xiong, G.-X. ; Baron, G.V.: Preparation and Characterization of Palladium-based Composite Membranes by Electroless Plating and Magnetron Sputtering. In: Catalysis Today 56 (2000), S. 89–96

[70] Wang, Li ; Bao, Shiguo ; Yi, Jianhua ; He, Fei ; Mi, Zhentao: Preparation and Properties of P d/Ag Composite Membrane for Direct Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen. In: Applied Catalysis B: Environmental 79 (2008), S. 157–162

[71] Wang, Dan ; Tong, Jianhua ; Xub, Hengyong ; Matsumura, Yasuyuki: Prepa- ration of Palladium Membrane over Porous Stainless Steel Tube Modiﬁed with Zirconium Oxide. In: Catalysis Today 93-95 (2004), S. 689–693

[72] Kleinlogel, C. ; Gauckler, L.J.: Sintering and properties of nanosized ceria solid solutions. In: Solid State Ionics 135 (2000), S. 567–573

[73] Cao, Chun ; Chung, Tai-Shung ; Chen, Shing B. ; Dong, ZhengJun: The study of elongation and shear rates in spinning process and its eﬀect on gas separation performance of Poly(ether sulfone) (PES) hollow ﬁber membranes. In: Chemical Engineering Science 59 (2004), S. 1053–1062

[74] van Oss, Hendrik G.: Background Facts and Issues Concerning Cement and Cement Data / U.S. Department of the Interior and U.S. Geological Survey. 2005 ( 2005-1152). – Forschungsbericht

[75] Baker, Richard W.: Future Directions of Membrane Gas Separation Technology. In: Industrial & Engineering Chemistry Research 41 (2002), February, Nr. 6, S. 1393–1411

[76] Walawender, W. P. ; Stern, S. A.: Analysis of Membrane Separation Param- eters. II. Countercurrent and Cocurrent Flow in a Single Permeation Stage. In: Separation Science and Technology 7(5) (1972), S. 553–584

[77] Gallucci, Fausto ; Basile, Angelo: Co-current and counter-current modes for Methanol Steam Reforming Membrane Reactor. In: Proceedings International Hydrogen Energy Congress and Exhibition. Istanbul, Turkey, July 2005 104 Bibliography

[78] Koros, W. J. ; Mahajan, R.: Pushing the Limits on Possibilities for Large Scale gas Separation: Which Strategies? In: Journal of Membrane Science 175 (2000), Nr. 2, S. 181–196

[79] Abell, A. B. ; Willis, K. L. ; Lange, D. A.: Mercury Intrusion Porosimetry and Image Analysis of Cement-Based Materials. In: Journal of Colloid and Interface Science 211 (1999), S. 39–44

[80] Gupta, Pardeep K. ; Troy, David B. (Hrsg.): Remington: the science and practice of pharmacy. 21st ed. Lippincott Williams & Wilkins, 2005. – 608 S

[81] Minter, Clarke C.: Thermal Conductivity of Binary Mixtures of Gases. In: The Journal of Physical Chemietry 72 (1968), S. 1924–1926

[82] Mukhopadhyay, P. ; Barua, A. K.: Thermal conductivity of hydrogen-helium gas mixtures. In: British Journal of Applied Physics 18 (1967), S. 635

[83] Reinecke, Stefan A.: The Eﬀect of Water Content on Knudsen Diﬀusion in Unconsolidated Porous Media, Applied Science Graduate Department of Civil En- gineering University of Toronto, Diplomarbeit, 2000

[84] Khayet, Mohamed: Membrane Distillation. In: Li, Norman N. (Hrsg.) ; Fane, Anthony G. (Hrsg.) ; Ho, W. S. W. (Hrsg.) ; Matsuura, T. (Hrsg.): Advanced Membrane Technology and Applications. New Jersey, the United states of America : John Wiley & Sons, Inc., 2008, Kapitel Chapter 12, S. 297–370

[85] Chiang, Hung-Lung ; Tsai, Jiun-Horng ; Chang, Dai-Huang ; Jeng, Fu-Teng: Diﬀusion of hydrogen sulﬁde and methyl mercaptan onto microporous alkaline activated carbon. In: Chemosphere 41 (2000), S. 1227–1232

[86] Pavlin, Tina: Hyperpolarized Gas Polarimetry and Imaging at Low Magnetic Field, California Institute of Technology, Diss., 2003

[87] Bernal, M. P. ; Coronas, J. ; Menéndez, M. ; Santamaría, J.: Separation

of CO2/N2 Mixtures Using MFI-Type Zeolite Membranes. In: American Institute of Chemical Engineers 50 (2004), S. 127–135

[88] Seader, J. D. ; Henley, Ernest J. ; Welter, Jennifer (Hrsg.) ; McFadden, Patricia (Hrsg.): Separation Process Principles. 2nd ed. Hoboken, United States of America : John Wiley & Sons, Inc., 2006

[89] Néel, J.: Pervaporation. In: Noble, Richard D. (Hrsg.) ; Stern, S. A. (Hrsg.): Membrane Separations Technology-Principles and Applications. (Membrane Sci- ence and Technology Series, Volume 2). Amsterdam, Netherlands : Elsevier Sci- ence B.V., 1995, Kapitel Chapter 5, S. 143–211

[90] Asano, Koichi: Mass Transfer–From Fundamentals to Modern Industrial Appli- cations. Weinheim, Germany : WILEY-VCH Verlag GmbH & Co. KGaA, 2006 Bibliography 105

[91] Nelson, Philip: Biological Physics: Energy, Information, Life. New York : W. H. Freeman and Company, 2008

[92] Baker, Richard W.: Vapor and Gas Separation by Membranes. In: Li, Nor- man N. (Hrsg.) ; Fane, Anthony G. (Hrsg.) ; Ho, W. S. W. (Hrsg.) ; Matsuura, T. (Hrsg.): Advanced Membrane Technology and Applications. New Jersey, the United states of America : John Wiley & Sons, Inc., 2008, Kapitel Chapter 21, S. 559–580

[93] Guo, Jia ; Heslop, Mark J.: Diﬀusion Problems of Soap-Film Flowmeter When Measuring Very Low-Rate Gas Flow. In: Flow Measurement and Instrumentation 15 (2004), S. 331–334

[94] Huang, R. Y. M. ; Yeom, C. K.: Pervaporation separation of aqueous mixtures using crosslinked polyvinyl alcohol membranes. III. Permeation of acetic acid-water mixtures. In: Journal of Membrane Science 58 (1991), S. 33–47

[95] Kim, Sang-Gyun ; Lee, Ki-Sub ; Lee, Kew-Ho: Pervaporation Separation of Sodium Alginate/Chitosan Polyelectrolyte Complex Composite Membranes for the Separation of Water/Alcohol Mixtures: Characterization of the Permeation Behavior with Molecular Modeling Techniques. In: Journal of Applied Polymer Science 103 (2007), S. 2634–2641

[96] Lin, Jerry Y. ; Cheng, Scott ; Gupta, Vineet: Proton-Conducting Dense Ce- ramic Membranes for Hydrogen Separation / Department of Chemical Engineer- ing, University of Cincinnati. 2003. – Forschungsbericht

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