Gas and water vapor permeation through highly hydrophilic membranes

Transcription

1 Gas and water vapor permeation through highly hydrophilic membranes Bachelor Thesis Jeroen Jansen s September 2007 Supervisor: Dipl.-Ing. J. Potreck Membrane Technology Group / Chemical Engineering Faculty of Science & Technology University of Twente

2 Preface This report is the result of my bachelor assignment for Advanced Technology. About three months have been spent for research on membranes for flue gas treatment. At first the aim of the assignment was to systematically investigate the effect of the addition of carbon nanotubes into the polymer membrane matrix on the mass transport behavior of these membranes for the simultaneous separation of CO 2 and water from flue gasses. Removal of CO 2 and water from flue gasses is important due to environmental aspects. Due to some delivery problems with the gas chromatograph column which is suitable with CO 2 detection, the subject of the assignment changed to investigation of mass transport behavior of nitrogen (N 2 ) and water vapor. I would like to thank my supervisor, Jens Potreck, for his support in my research. I also want to thank Sander Reijerkerk for his support in the preparation of the membranes. Also a big thank to Joop van der Linden for continuing the measurements during my absence. At last I would like to thank all the members and students of the Membrane Technology Group at the University of Twente for all their assistance when needed and the great time I had during this assignment. 1

3 Abstract The effect of multi walled carbon nanotubes (MWNT) in Pebax 1074 membranes has been investigated. For this purpose membranes with carbon nanotubes dispersed in have been prepared. The membranes are characterized with a mixed permeation setup to determine water vapor and nitrogen permeabilities. In comparison with pure Pebax 1074 membranes the nitrogen and water vapor permeability increases when MWNTs are added to the membrane. There are some possible reasons for these observations, but it is not yet clear if these statements are correct. One of the possibilities is the creation of more free volume in the membrane by addition of MWNTs, which improves the permeability of nitrogen and water vapor. It is recommenced to perform some sorption measurements to obtain more data about the mass transport in the membranes. 2

4 Contents Preface... 1 Abstract... 2 Contents Introduction Scope of this report CO 2 and water vapor removal Membranes Theoretical background Introduction Solution-diffusion model Mass transport Concentration polarization Permeability Water vapor/gas selectivity Experimental Introduction Membranes Pebax Carbon nanotubes Membrane preparation Permeation setup Gas Chromatograph Gas injector Thermal Conductivity Detector Gas Chromatograph calibration Cooled mirror dew point meter Water vapor permeability

5 3.4.1 Determination of k m Determination of the water vapor permeability Gas permeability Experimental conditions Results and Discussion Introduction Gas chromatograph calibration Membrane Water vapor permeability Nitrogen permeability Selectivity (P H2O /P N2 ) Conclusions and recommendations List of symbols and abbreviations Symbols Abbreviations References

6 1 Introduction 1.1 Scope of this report The aim of this work is to investigate the influence of carbon nanotubes in Pebax 1074 membranes on the permeability of water vapor and nitrogen at different temperatures and water vapor activities. This report will start with an introduction about the necessity and methods to remove gas and water vapor from flue gasses and a short introduction about membranes. In the next chapter a theoretical background about mass transport through membranes, concentration polarization, permeability and selectivity is given. The preparation of the membranes and the working principle of the mixed permeation setup are described in the experimental chapter. The obtained results are presented in the results and discussion part of this thesis. The thesis will end with conclusions and recommendations about the effect of carbon nanotubes in Pebax 1074 membranes. 1.2 CO 2 and water vapor removal Nowadays about 85% of our energy is produced by fossil fuel burning power plants [1]. One of the disadvantages of this method is the generation of tremendous amounts of flue gasses. These gasses consist mostly of nitrogen (75-85 vol.%), oxygen (5-7 vol.%), carbon dioxide (10-14 vol.%) and water vapor (6-8 vol.%) [2]. With respect to environmental aspects it is desired to remove as much carbon dioxide and water vapor from these flue gasses. Carbon dioxide (CO 2 ) is a greenhouse gas and one of the main contributors to the global warming and climate changes. The removal of water vapor will prevent condensation of the water vapor in the chimney. Condensation in the chimney is a problem because this can form sulfuric acids in combination with sulfur dioxides [3]. Nowadays this is prevented by reheating the gas and the water vapor is emitted to the air. Avoidance of the additional heating saves energy. Furthermore the product water can be used for the steam generation again. There are a few methods to remove CO 2 from flue gasses. Figure 1.1 shows some general methods to capture CO 2 from gas streams: adsorption, absorption, cryogenic cooling and by use of membranes [4]. 5

7 CO 2 capture Adsorption Absorption Cryogenic Membranes PSA TSA ESA Physical Absorption Chemical Absorption Amine Inorganic Figure 1.1: Several methods to capture CO 2 from gasses [4]. The removal of CO 2 is mostly achieved with chemical absorption [5]. The gas is in direct contact with a liquid or solid absorbent that is capable of capturing the CO 2 by a chemical reaction as shown in Figure 1.2. Figure 1.2: CO 2 removal with sorbents. The gas with CO 2 flows trough a vessel in which the CO 2 is captured by sorbents. The sorbent which is loaded with CO 2 will be regenerated by transferring it to another vessel and the CO 2 is stripped from the sorbents. The method of stripping depends on the type of sorbent. The cleaned and regenerated sorbent will be transferred back to the capture vessel. The sorbent flow between the vessels can be very large because it has to remove a lot of CO 2 in a power plant. This results in very large installations and energy consumptions, which decrease the efficiency of the power plant. Therefore there is still a lot of research activity in optimizing sorbent performances. Organic amine based sorbents are mostly used to capture CO 2, but also ammonia based sorbents are capable to separate CO 2. With physical absorption, the CO 2 is captured by non-chemical surface forces. The CO 2 sticks to the sorbent less strongly in comparison with chemical absorption. The advantage of this is 6

8 that CO 2 can be very easily stripped from the sorbent. After changing conditions, like heating the sorbent, the CO 2 will be released. Pressure swing adsorption (PSA), temperature swing adsorption (TSA) and electrical swing adsorption (ESA) are more examples of adsorption processes to separate CO 2 from flue gasses. The method of regeneration is basically the same as with (chemical) absorption. But in this case the CO 2 is removed by changing some process conditions like temperature and pressure. Another method to remove CO 2 is by use of cryogenic coolers as shown in Figure 1.3. Figure 1.3: Cryogenic cooling. The gas is converted into a liquid by compressing and cooling. The liquid CO 2 can be separated from the mixture by distillation. This technology is mostly used to separate CO 2 from streams with a high CO 2 concentration. Normally it is not used to remove CO 2 from dilute streams such as flue gasses, because this takes too much energy ergy to cool down the gas [4]. The use of membranes to capture CO 2 from flue gasses is an upcoming technology with a lot of advantages. Membrane modules are smaller and cheaper than sorption and cryogenic modules. Furthermore, the use of membranes avoid operational problems like foaming and flooding [5]. Another advantage of membrane processes is the absence of regeneration steps. But there is still a lot of research required before membranes are applicable in industrial applications. One of the disadvantages of membrane usage is they easily can get damaged in some processes for example when acids are present in the process. Another disadvantage is the difficulty to scale up membrane processes, which leads to complex systems, higher energy [4]. consumptions and extra costs [4] 1.3 Membranes Membrane technology is developing very quickly. Membranes have become a very important and powerful separating agent in the industry. They are used in a lot of applications, for example for wastewater treatment, beer breweries, fuel cells etc. [6, 7] 7

9 A membrane can be defined as a permselective barrier between two phases [8]. This means that the membrane is selective for some components of a mixture and let them permeate through, while other components will not permeate through the membrane (Figure 1.4) [9]. When a mixture of liquids, vapors or gases is fed to the membrane a part of the mixture will permeate through the membrane, which is the permeate stream. The remaining part, which does not permeate, is the retentate stream. Feed Retentate Membrane Permeate Figure 1.4: Principle of a membrane; a feed stream is separated into a retentate and permeate stream [9]. A difference can be made between porous and nonporous membranes. Porous membranes have pores which can act as a sieve. Processes in which membranes with pores between 0.1 and 10 µm are used, are called microfiltration. When the pores size is between 2 and 100 nm the process is called ultrafiltration. The molecules permeate through the membrane via its pores and channels. Properties like size and hydrophilicity of the components are the most important factors which determine the transport through the membrane. Nonporous or dense membranes have no fixed pores. The transport is determined by a solution and diffusion mechanism, which consists of three steps: sorption of the molecule into the membrane, diffusion through the membrane and desorption from the membrane to the permeate stream. 8

10 2 Theoretical background 2.1 Introduction In order to be able to calculate the permeability and selectivity of dense membranes, equations have to be derived which relate measured data to the water vapor and gas selectivity and permeability of the membranes. 2.2 Solution-diffusion model Mass transport through a dense membrane can be described with the solution-diffusion model [10]. According to this model the molecules first dissolve in the membrane. Because of a driving force, for example a concentration or pressure gradient, the molecules diffuse through the material. Separation can be achieved due to differences in solution and diffusion rates of the permeants. 2.3 Mass transport According to the solution diffusion model, the permeability through a dense membrane is defined by the product of diffusion and solubility (Equation (1) [8] ). with P the permeability (Barrer (1 Barrer = P = D S (1) coefficient (cm 2 /s) and S the solubility coefficient (cm 3 (STP)/(cm 3 cmhg)). (cm 3 cm)/(cm 2 s cmhg))), D the diffusion The simplest description of gas diffusion through a dense membrane is Fick s Law [8] : J dc = D (2) dx with J the flux through the membrane at standard temperature and pressure (cm 3 (STP) /(cm 2 s)), D the diffusion coefficient (cm 2 /s) and dc/dx the concentration gradient over the membrane. Equation (2) can be integrated, assuming steady state conditions: x= l c= i, l Jidx = Didc (3) x= 0 c= ci,0 J i (,0, ) Di ci ci l = (4) l with c i,0 and c i,l the concentrations on the feed side and the permeate side of the membrane (mol/cm 3 ), respectively, and l the thickness of the membrane (cm). 9

11 At low pressures Henry s Law directly relates the concentration c i to the partial pressure p i [8] : ci = Si pi (5) with S i the solubility coefficient (cm 3 (STP)/(cm 3 cmhg)) and p i the partial pressure (cmhg). By combining Equations (1), (4) and (5) the flux through the membrane can be written as: Pi Ji = ( pi, feed pi, permeate ) (6) l 2.4 Concentration polarization The flux through the membrane can be described as a product of the concentration difference between the feed stream and the sweep stream as driving force and the overall resistance over the membrane, like in Equation (7): ( ) J = k c c (7) H2O ov p f where J H2O is the flux through the membrane (mol/(m 2 s)), k ov the overall resistance of the membrane (m/s) and c p the bulk water vapor concentration (mol/m 3 ) at the permeate and c f the bulk water vapor concentration (mol/m 3 ) at the feed side of the membrane. Within the system, the membrane resistance is not the only resistance that influences the permeability. Due to concentration polarization at the feed and permeate side of the membrane, the overall resistance k ov exists of several resistances. Concentration polarization can occur in gas/vapor mixtures, if there are stagnant gas/vapor layers present at the feed or permeate side of a membrane. In these stagnant layers, gas or vapor transport can only take place through diffusion, resulting in a concentration gradient. The concentration of the vapor or gas at the surface of the membrane is therefore not the same as the concentration in the bulk of the gas. This concentration polarization causes a profile for the chemical potential over the membrane as well as resistance (Figure 2.1) [11]. 10

12 µ H2O,feed k f k m k p Membrane Feed Permeate µ H2O,permeate Stagnant boundary layers Figure 2.1: Schematic representation of the resistances in the stagnant boundary layers at the feed side, the membrane and the permeate side. The overall resistance can thus be described using the resistance-in-series model: = + + (8) kov k f km k p where k ov is the overall resistance over the membrane, k f is the resistance in the boundary layer at the feed side of the membrane, k m the resistance of the membrane itself and k p is the resistance in the boundary layer at the permeate side of the membrane. When k ov, k f and k p are known, the real resistance k m can be calculated. 2.5 Permeability When the flux of a component trough a membrane and the pressure difference as driving force are known, the permeability can be calculated: P i = J i l ( pi, feed pi, permeate ) (9) where P i is the permeability (Barrer), l the thickness of the membrane (cm) and p i, feed p i, permeate the pressure difference of component i over the membrane (cmhg). 2.6 Water vapor/gas selectivity If the permeability of the gas and water vapor are known, the selectivity can be calculated with: PH 2O α = (10) P gas with α the water vapor/gas selectivity. 11

13 3 Experimental 3.1 Introduction Membranes have been prepared to investigate the effect of carbon nanotubes in Pebax 1074 on nitrogen and water vapor permeation. A mixed permeation setup is used to determine the permeation of nitrogen and water vapor for these membranes at different temperatures and water vapor activities. 3.2 Membranes Pebax 1074 The membranes are made of Pebax 1074 which is a hydrophilic block copolymer consisting of 45% of hard polyamide-block (PA12 or Nylon 12) and of 55% of a soft polyethylene oxide (PEO)-block (Figure 3.1) [12]. Diffusion takes place in the (soft) amorphous PEO parts of the polymer. The harder crystalline PA blocks give the membrane its mechanical stability. O O HO C (PA) C O (PE) O H n Figure 3.1: Structure of PEBAX Carbon nanotubes [13] The membranes are functionalized with multi walled carbon nanotubes (MWNT). A MWNT can be represented as several single walled carbon nanotubes in each other as in Figure 3.2. Figure 3.2: Artist impression of a multi walled carbon nanotube [14]. The nanotubes are produced by catalytic carbon vapor deposition (CCVD) with catalysts containing 2.5 wt.% iron and 2.5 wt.% cobalt on an alumina support. The synthesis is carried out in a fixed-bed flow reactor of 1 m at 700 C wi th ethylene as carbon source and nitrogen as 12

14 gas carrier. After the synthesis of the MWNT the crude mix still contains some unreacted carbons and catalyst materials, the MWNT amount is about 80 till 90 wt.%. After purification the purity is higher than 95 wt.%, but it still contains impurities consisting of alcohol and acid groups. These impurities can be removed by heating. It is possible to attach some amine (NH 2 ) groups to the MWNTs, which might enhance the CO 2 selectivity of membranes. There are several types of MWNTs created for testing which are listed in Table 3.1. Table 3.1: Different types of multi walled carbon nanotubes. Type Treatment Modifications Impurities AF 61.2 Crude Unreacted carbon/catalyst AF 62.6 Purified Alcohol and acid groups AF 62.3 Purified Heated AF 63.3 Crude Amine groups Unreacted carbon/catalyst AF 64.3 Purified Amine groups Alcohol and acid groups The MWNTs have a length of about 0.5 µm. They consist of an average of 8 tubes placed in each other. The average outer diameter is about 8 till 10 nm and the inner diameter between 4 and 5 nm Membrane preparation To create polymeric membranes, solutions with functionalized multi walled carbon nanotubes and 7 wt.% Pebax 1074 were created in N-Methyl-2-Pyrrolidone (NMP) at 100 C. Membranes have been made with 0.25, 0.50, 0.75 and 1.00 wt.% MWNTs. The hot solutions were cast on a preheated glass plate (80 C) with a 0.47 mm casting knife. To ensure a controlled atmosphere to evaporate the NMP and solidify the membrane, the plates were put into a closed box with a constant nitrogen stream. To evaporate the rest of the solvent, the plates were put into an oven at 80 C for one week. The membranes were removed from the glass by putting it in a water reservoir. To ensure all the solvents are removed, the film was rinsed for three days with water. The film was dried on a nonwoven paper and stored in a vacuum oven at 30 C. [9, 11] 3.3 Permeation setup Water vapor permeation and gas permeation through a membrane are usually measured separately to calculate the selectivity of a membrane. But this method gives no information about the influence of water vapor and gas on each other. Some polymer membranes swell under the influence of water vapor, which can have effect on the gas permeability. To measure the effect of these phenomena, a setup was designed to measure the gas and water vapor 13

15 permeabilities through polymer membranes simultaneously. The main part of this setup, the permeation cell, is schematically depicted in Figure 3.3. n gas + m H 2O (feed) (n - dn) gas + (m - dm) H 2O (retentate) H 2O gas He He + dn gas + dm H 2O (permeate) He (sweep gas) Figure 3.3: Schematic setup of the permeation cell. The feed stream containing a certain gas (for example CO 2 or N 2 ) and water vapor is fed into the permeation cell and flows radially over a dense membrane. The sweep gas (helium) enters the permeation cell from the down side and flows radially to the center. In this way the feed stream and the sweep stream establish a counter-current flow pattern (Figure 3.4).The water and gas permeate through the membrane and are removed by the sweep gas. By removing the permeate, a constant concentration difference as driving force is achieved. Figure 3.4: Design of the permeation cell. The concentration of permeated gas is measured by a gas chromatograph. A sample of the permeate gas is injected into the gas chromatograph. The concentration of the gas can be 14

16 calculated from the area of the graph from the gas chromatograph, which is calibrated with some predefined calibration gasses. To measure the permeability of water vapor through the membrane, the amount of water that has permeated through the membrane has to be measured. The water vapor concentration of the feed and the permeate stream is related to the water vapor activity, which can be determined by measuring the dew point temperature of the feed and permeate stream. This parameter indicates the temperature at which water vapor condenses and is related to the vapor pressure of the water in the gas mixture. The dew point temperature is measured by dew point mirrors, where water vapor condenses at a cooled mirror. By measuring the dew point, the water vapor activity (p H2O /p H2O ) of the feed and the permeate stream can be calculated, which allows to determine the permeability of water vapor. A flow chart of the water vapor and gas permeation setup is depicted in Figure 3.5. Retentate MFC = Mass Flow Controller PI = Pressure Indicator FI = Flux Indicator TD = Temperature Detector DM = Dew point Mirror GC = Gas Chromatograph Permeate Figure 3.5: Flow chart for the setup to measure water vapor and gas permeability in membranes. The gas stream is split into two different streams. By leading one of the streams through a water reservoir, the gas becomes saturated with water. The wet stream is lead through a demister to avoid water droplets in the stream. The water vapor activity of the final feed stream can be adjusted by two mass flow controllers (MFC 1 & 2, Brooks 0154), which allows mixing the dry gas and the saturated gas. The precise activity of the feed is measured by a dew point meter (DM, Dewmet Cooled Mirror Dew point meter, Michell Instruments), a thermometer (TD, thermocouple) and a manometer (PI, Druck PTX) which is placed before the permeation cell. The feed is led into the permeation cell, which separates he stream into a retentate and permeate stream. The water vapor activity of the permeate stream is measured with a dew point meter, a thermometer and a manometer. A gas chromatograph (GC, Varian 3400, Packed 15

17 Column: Molecular Sieve 13X, Alltech Associates Inc., Thermal Conductivity Detector) is used to analyze the permeate gas stream. The system pressure is regulated with a backpressure valve behind the permeation cell Gas Chromatograph To determine the concentration of a certain component in the permeate gas, a gas chromatograph (Varian 3400, Packed Column: Molecular Sieve 13X, Alltech Associates Inc.) with a Thermal Conductivity Detector (TCD) is used Gas injector The gas is injected into the gas chromatograph by a sample loop injector. This is a 10-port valve which is able to take two positions (Figure 3.6). In the loading position the valve connects port 1 with port 2, port 3 with port 4 etc. and the sample gas from the setup flows through sample loop 2. This loop has a volume of 1 ml. The incoming helium stream flows via loop 1 to the column in the gas chromatograph. When the valve is switched into the injecting position, the inner part of the valve rotates 36 which conne cts port 2 with port 3, port 4 with port 5 and so on. In this way the helium stream will push the 1 ml sample gas in sample loop 2 into the column. After injecting, the valve will rotate 36 and load again. Figure 3.6: Gas chromatograph injection valve. The left side is the valve in the loading position where sample loop 1 fills with gas and on the right in the injecting position where the analyzing gas is injected from sample loop 1 into the GC Thermal Conductivity Detector The Thermal Conductivity Detector (TCD) is used in gas chromatographs to detect the different molecules in a gas stream. Basically, a TCD is a Wheatstone bridge (Figure 3.7) which measures the voltages between some temperature dependent resistances. 16

18 Reference R R Sample Power Supply Sample R R Reference Figure 3.7: Circuit of a Wheatstone bridge [15]. The resistances are placed separated in two chambers, one with the sample and one as reference with the carrier gas. The gas streams flow through a heating device to heat up and then into the chambers with the temperature dependent resistances. With different specific heats of sample and reference gas, the temperature of gasses changes differently with the distance they pass. The different temperatures causes different resistances, which create a voltage difference which can be measured. This measured change of voltage can be related to the amount of injected sample in the carrier gas Gas Chromatograph calibration To relate the output of the gas chromatograph to a certain concentration of gas, the gas chromatograph is calibrated with calibration gasses for N 2 in He for three different concentrations (100, 1000 and 2500 volume PPM N 2 in He). The calibration is achieved by injecting 1.0 ml gas at 100 C into the gas chromat ograph column (40 C) with a flow rate of ml/min Cooled mirror dew point meter [16] The dew point temperature is measured by a cooled mirror dew point meter as schematically shown in Figure 3.8. In the dew point meter a polished metal mirror is cooled down until it reaches the dew point of the gas/vapor mixture flowing through. When this temperature is reached the vapor will condensate on the mirror, which can be detected by an electro-optical circuit which consists of a red light emitting diode in combination with a high gain photo detector. When a vapor condensates, a reduction in intensity of reflecting light will be detected. The mirror is controlled until an equilibrium is reached in which evaporation and condensation occurs at the same time. In this situation the temperature of the mirror is equal to the dew point temperature of the sample gas. 17

19 Figure 3.8: Schematic view of a cooled mirror dew point meter. 3.4 Water vapor permeability To calculate the water vapor permeability, the flux trough the membrane and pressure difference over the membrane have to be determined. The dew point of the gas stream, which is measured with the dew point meters, is related to the water vapor pressure in the gas mixture. To calculate the vapor pressure from the dew point temperature the Antoine equation is used [11] : log10 ( ph ) O = T dew (11) with p H20 the water vapor pressure (bar) and T dew the dew point temperature ( C). The water vapor flux permeating through the membrane can be determined if the flow rate of the sweep gas is known: J H 2O φ = p V v, tot H2O m R T A γ (12) with J H2O the water vapor flux (cm 3 /(cm 2 s)), Φ v,tot the volume flow of the sweep gas (m 3 /s), which contains the permeated water, p H2O the water vapor pressure (Pa), R the gas constant (8.314 J/(mol K)), T the temperature (K), V m the volume of one mole of gas at STP (22414 cm 3 /mol), A the membrane area (cm 2 ) and γ the activity coefficient (-). The activity coefficient is considered to be one, because the sweep gas is mainly helium at 1 bar and behaves like an ideal gas Determination of k m Due to concentration polarization, the measured pressure difference is not the real driving force over the membrane. To calculate the real driving force, k m should be determined with use of equation (8) in which k ov, k f and k p should be known. 18

20 With the dew points of the water vapor on the feed side and permeate side, it is possible to calculate the partial pressure difference of water vapor over the membrane. With this result the concentration difference can be calculated with the ideal gas law. c = p R + ( T ) dew (13) With c the difference in concentration on the feed and permeate side of the membrane (c H2O, feed c H2O, permeate, (mol/m 3 )), p the difference in partial water vapor pressure over the membrane (p H2O, feed p H2O, permeate, (Pa)) and R the gas constant (8.314 J/mol K). The overall resistance k ov can now be calculated using Equation (7), as J H2O and c are known. The resistance in the boundary layer at the feed side of the membrane 1/k f is small compared to the overall resistance at high feed flow rates. The relative contribution from the feed side resistance 1/k f is therefore negligible at relative high feed flow rates, so 1/k f is assumed to equal 0 at infinity. The resistance at the permeate side of the membrane, 1/k p, can be determined by plotting the overall resistance 1/k ov as a function of the membrane thickness [17]. A linear increase of the overall resistance as function of the membrane thickness was found. The intercept at the y-axis indicates, that the transport resistance is not only located in the membrane but also in a stagnant permeate boundary layer. Repeating this for various temperatures, water vapor activities and membrane thicknesses makes it possible to determine the needed resistances of the system at the permeate side. With this information, k m can be determined using Equation (14): k m = k k ov p (14) Determination of the water vapor permeability When J H2O and k m are known, the real driving force over the membrane can be calculated by using Equation (15): 2 ( ch O, membrane, feed ch O, membrane, permeate ) c 2 2 J H O = = (15) k It is useful to express the driving force as a pressure difference, by converting the concentrations to the effective water vapor pressures by use of the ideal gas law: 19 m

21 ( ) p = c R T perm + (16) The real water vapor permeability of the membrane can now be calculated by rewriting Equation (6) to: P H2O = J H O l 2 ( ph O, feed ph O, permeate ) 2 2 (17) 3.5 Gas permeability If the feed pressure of a gas is known, the flux can be determined with Equation (18): J gas φ c ν,tot gas = (18) A in which J gas is the gas flux through the membrane (cm 3 (STP)/(cm 2 s)), Ф v,tot the flow rate of the sweep gas (cm 3 (STP)/s), c gas the concentration of the gas in the sweep gas and A the membrane area (cm 2 ). Assuming that the pressure in the sweep gas is negligible compared to the pressure in the feed, the permeability can be calculated by rewriting equation (6) to Equation (19): P gas J gas = (19) p gas l where P gas is the gas permeability (Barrer), l the thickness of the membrane (cm) and p gas the gas pressure (cmhg) in the feed gas. 3.6 Experimental conditions Two different membranes have been used in the measurements. One with 0.25 wt% and one with 1.0 wt.% MWNTs. The membranes contain AF 61.2 MWNTs, which are crude nanotubes with some impurities left (Table 3.1). The experiments were performed at different temperatures and water vapor activities to investigate the effect of these parameters on the membranes. The permeation data is measured at 30, 50 and 70 C at the activities 0, 0.4, 0.8, 0.9 and No measurements have been performed between activities 0 and 0.4, because the dew point mirrors are not able to measure the dew point of these low activities [9]. During the experiments the total feed pressure is kept constant a 2.5 bar. 20

22 4 Results and Discussion 4.1 Introduction The mixed gas permeation setup is used to measure and calculate the nitrogen and water vapor permeability for two different membranes at different temperatures and water vapor activities. To investigate the effect of multi walled carbon nanotubes (MWNT), two types of membranes have been used; one with 0.25 wt.% MWNTs and one with 1 wt.% MWNTs in Pebax These results are compared with pure Pebax 1074 data [15]. 4.2 Gas chromatograph calibration Before the gas chromatograph could be use to analyze the nitrogen concentration in the permeate gas stream a calibration had to be performed. The calibration results for nitrogen in helium are shown in Figure volume N 2 in He [%] Y =2.22x x10-8 X R 2 = x x x x x x10 6 GC Area counts [-] Figure 4.1: Calibration curve of the gas chromatograph for the nitrogen concentration in helium as a function of the gas chromatograph area counts. The data points are achieved by injecting 1.0 ml calibration gas at 100 C into the gas chromatograph column (40 C) with a flow rate of ml/min. Linear regression is used to obtain a relation between the volume of nitrogen in helium as a function of the GC area counts. 4.3 Membrane Figure 4.2 shows a TEM image of a Pebax 1074 membrane with 0.5 wt.% MWNTs which is created by FUNDP [13]. 21

23 Figure 4.2: TEM picture [13] of a Pebax 1074 membrane containing 0.5 wt.% MWNTs. The black particles are the nanotubes and some little impurities. The picture shows how the carbon nanotubes are dispersed in the polymer. The carbon nanotubes show some clustering, which might create some free volume in the membrane. 4.4 Water vapor permeability The water vapor permeability is measured at 30 C, 50 C and 70 C for different water vapor activities. The results are shown in Figure 4.3 for Pebax 1074 with 0.25 wt.% MWNTs and for Pebax 1074 with 1 wt.% MWNTs C 30 C Permeability (H 2 O) 10 5 [Barrer] C 70 C 30 C Permeability (H 2 O) 10 5 [Barrer] C Activity (H 2 O) [-] Activity (H 2 O) [-] Figure 4.3: Water vapor permeability for Pebax 1074 with 0.25% MWNT (left) and 1% MWNT (right) at different temperatures as a function of the water vapor activity. 22

24 The permeation data show an exponential increase in water vapor permeability with increasing water vapor activity. Because permeability is a product of solubility and diffusion one of these factors influences this exponential increase. Probably this is related to the solubility which increased exponentially with water vapor activity like in other rubbery polymers [18]. The water vapor permeability for Pebax with 1 wt.% MWNTs at 50 C is very high for activities above 0.8. It is not yet clear why this value is so high in comparison with 30 C and 70 C. Differential Scanning Calorimetry [19] did not show any remarkable observations for Pebax with 1 wt.% MWNTs. The melting temperature of the PEO part is determined to be about 8.5 C and 157 C for the PA part of the polymer, this is in a greement with pure Pebax 1074 where the melting temperature is 11 C and 156 C respectivel y [12]. The glass transition temperature is with -57 C a little bit lower than the glass trans ition temperature for pure Pebax 1074 which is about -55 C [12]. This indicates that these material properties did not change significantly. In Figure 4.4, Figure 4.5 and Figure 4.6 a comparison in water vapor permeabilities is made between pure Pebax 1074 and Pebax 1074 with respectively 0.25 wt.% and 1 wt.% MWNTs for 30 C, 50 C and 70 C Permeability (H 2 O) 10 6 [Barrer] % MWNT 1% MWNT pure Activity (H 2 O) [-] Figure 4.4: Water vapor permeabilities for different membranes at 30 C as a function of the water vap or activity. 23

25 1.4 Permeability (H 2 O) 10 6 [Barrer] % MWNT 0.25% MWNT pure Activity (H 2 O) [-] Figure 4.5: Water vapor permeabilities for different membranes at 50 C as a function of the water vap or activity Permeability (H 2 O) 10 6 [Barrer] % MWNT 1% MWNT pure Activity (H 2 O) [-] Figure 4.6: Water vapor permeabilities for different membranes at 70 C as a function of the water vap or activity. Again the permeabilities for 50 C differ from the data for 30 C and 70 C, the permeability for Pebax with 1 wt.% MWNTs with high activities is very high in comparison with the other polymers. Work of Majumder et al. [20] suggest that water transport through carbon nanotubes is four to five orders of magnitude faster than would be predicted from conventional fluid-flow theory. Another reason can be the increase of free volume in the membrane by addition of MWNTs. An increase in free volume can speed up the diffusion and permeability because the 24

26 molecules do not have to diffuse through the polymer. This can be an explanation for the increased permeability when MWNTs are added to the polymer. It is remarkable that the permeability of Pebax with 0.25 wt.% MWNTs is lower than the permeability of pure Pebax for this temperature. The reason for this is not well-known yet, but might be a result of different water sorption behavior at 50 C in comparison with 30 C and 70 C. 4.5 Nitrogen permeability Figure 4.7 show the nitrogen permeability at different temperatures and different water vapor activities for Pebax 1074 with 0.25 wt.% and 1 wt.% MWNTs C Permeability (N 2 ) [Barrer] C 50 C Permeability (N 2 ) [Barrer] C 2 30 C 2 30 C Activity (H 2 O) [-] Activity (H 2 O) [-] Figure 4.7: Nitrogen permeability for Pebax 1074 with 0.25% MWNT (left) and 1% MWNT (right) at different temperatures as a function of the water vapor activity. The graphs show that the permeability increases with increasing temperature. This might be attributed to a diffusion coefficient which increases relatively more with temperature than the sorption coefficient which decreases with increasing temperature. It is also visible that the water vapor activity influences the nitrogen permeability. The permeability decreases with increasing water vapor activity. The decrease in nitrogen permeability with increasing water vapor activity can possibly be described to a change in free volume of the membrane. It is possible that some spaces in the membrane get filled with water molecules. In these regions the diffusion of nitrogen can be different from diffusion through the polymer without water. Additional to this can be the forming of water clusters inside the polymer when the water vapor activity is high [21]. These clusters might form a barrier for nitrogen which has to find a way around the clusters. This increases the length of the total path a molecule has to pass. Another reason can be the swelling of the polymer by absorption of water, which increases the thickness of the membrane. This elongates the way a nitrogen molecule has to pass through the polymer. 25

27 Figure 4.8, Figure 4.9 and Figure 4.10 show the comparison of three different membranes at different temperatures and water vapor activities. A comparison is made between pure Pebax 1074 and Pebax 1074 with respectively 0.25 wt.% and 1 wt.% MWNTs Permeability (N 2 ) [Barrer] % MWNT 0.25% MWNT 2 pure Activity (H 2 O) [-] Figure 4.8: Nitrogen permeabilities for different membranes at 30 C as a function of the water vapor activity Permeability (N 2 ) [Barrer] % MWNT 0.25% MWNT pure Activity (H 2 O) [-] Figure 4.9: Nitrogen permeabilities for different membranes at 50 C as a function of the water vapor activity. 26

28 20 Permeability (N 2 ) [Barrer] % MWNT 0.25% MWNT pure Activity (H 2 O) [-] Figure 4.10: Nitrogen permeabilities for different membranes at 70 C as a function of the water vapor activity. The graphs show an increase in nitrogen permeability when MWNTs are added to Pebax One of the reasons for this increase can be the creation of free volume in the membrane by addition of MWNTs. An increase in free volume can speed up the diffusion and permeability because the molecules do not have to diffuse through the polymer. Another reason can be found in gas transport properties of MWNTs. Studies [22, 23] show that nitrogen transport through MWNTs is very fast in comparison with transport through polymers. 4.6 Selectivity (P H2O /P N2 ) Figure 4.11 show the selectivities calculated from the mixed permeation data for different temperatures for Pebax 1074 with respectively 0.25 wt.% and 1 wt.% MWNTs Selectivity α (P H2O / P N2 ) 10 5 [-] C 50 C Selectivity α (P H2O / P N2 ) 10 5 [-] C 30 C 70 C 70 C Activity (H 2 O) [-] Activity (H 2 O) [-] Figure 4.11: Selectivity for Pebax 1074 with 0.25% MWNT (left) and 1% MWNT (right) at different temperatures as a function of the water vapor activity. 27

29 Figure 4.12, Figure 4.13 and Figure 4.14 compare the selectivities of pure Pebax 1074 and Pebax 1074 with respectively 0.25 wt.% and 1 wt.% MWNTs at 30 C, 50 C and 70 C % MWNT Selectivity α (P H2O / P N2 ) 10 5 [-] % MWNT pure Activity (H 2 O) [-] Figure 4.12: Selectivity for different membranes at 30 C as a function of the water vapor activity. 1.6 Selectivity α (P H2O / P N2 ) 10 5 [-] % MWNT pure 0.25% MWNT Activity (H 2 O) [-] Figure 4.13: Selectivity for different membranes at 50 C as a function of the water vapor activity. 28

30 Selectivity α (P H2O / P N2 ) 10 5 [-] pure 0.25% MWNT 1% MWNT Activity (H 2 O) [-] Figure 4.14: Selectivity for different membranes at 70 C as a function of the water vapor activity. The selectivity for water over nitrogen of Pebax 1074 is about the same when 0.25 wt.% is added to the polymer, but the selectivity improves by addition of 1 wt.% of MWNTs. Especially for high activities at 50 C the selectivity become s very high, which can be explained by the high water vapor permeability at 50 C (Figure 4.5). 29

31 5 Conclusions and recommendations The selectivity and permeability of water vapor for Pebax 1074 with 1 wt.% multi walled carbon nanotubes (MWNTs) is relatively high in comparison with pure Pebax 1074 and Pebax 1074 with 0.25 wt.% MWNTs. The addition of MWNTs to Pebax 1074 increases the nitrogen permeability significantly. It is not yet clear why the addition of MWNTs increases the permeabilities. It might be the result of introducing more free volume into the polymer by the addition of MWNTs. The water vapor permeability increases with increasing the water vapor activity, while the nitrogen permeability decreases with increasing activity. This might be attributed to clustering of water molecules in the membrane, which makes it harder for nitrogen molecules to permeate through the polymer. Permeability can be described as a product of diffusion and solubility, which are both dependent of temperature. Diffusion will increase with temperature and solubility will decrease by increasing the temperature. Therefore it can be interesting to investigate the effects of water absorption by performing some sorption measurements with different temperatures and water vapor activities to investigate the effect of water sorption on pure Pebax 1074 films and Pebax 1074 with MWNTs. Especially the water sorption properties for high activities at 50 C can give some interesting results and may explain the high water vapor permeability for this temperature. Sorption measurements can also give more details about water molecule clustering in Pebax 1074 polymers. 30

32 6 List of symbols and abbreviations 6.1 Symbols A Area cm 2 c Concentration mol/cm 3 D Diffusion coefficient cm 2 /s J Flux cm 3 (STP)/(cm 2 s) k f Feed side mass transfer rate m/s k m Membrane mass transfer rate m/s k ov Overall mass transfer rate m/s k p Permeate side mass transfer rate m/s l Membrane thickness cm P Permeability Barrer 1 Barrer = (cm 3 cm)/(cm 2 s cmhg) p Pressure bar, Pa or cmhg p i Partial pressure of i bar, Pa or cmhg R Gas constant J/(mol K) S Solubility coefficient cm 3 (STP)/(cm 3 cmhg) T Temperature C, K V m Molar volume cm 3 /mol α Selectivity - γ Activity coefficient - φ Volume flow cm 3 /s 6.2 Abbreviations GC Gas chromatograph MWNT Multi Walled Carbon Nanotubes NMP 1-methyl-2-pyrrolodinone Pebax Trademark for Polyether Block Amide polymer resins STP Standard Temperature and Pressure ( K; 1 bar) TCD Thermal Conductivity Detector 31

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