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H2 gas mixture at elevated pressures
Journal of Membrane Science 162 (1999) 1–8
Permselectivity of a PA6 membrane for the separation of a compressed CO2 /H2 gas mixture at elevated pressures G. Härtel ∗ , Th. Püschel Lehrstuhl für Umweltverfahrenstechnik der TU Bergakademie Freiberg, Leipziger Straße 28, D-09596 Freiberg, Germany Received 13 July 1998; received in revised form 27 January 1999; accepted 11 February 1999
Abstract Since recently, compressed gases can substitute for hazardous solvents in the chemical industry. To meet economic and ecological demands, the reuse of compressed gas has become very desirable. This creates the need to remove, in advance, unwanted components (e.g. unconverted H2 from hydrogenation reactions). This study examines a gas permeation process to achieve the separation of CO2 /H2 mixture under high pressure. For hydrogen separation from the mixture, a homogeneous PA6 membrane is applied at a temperature of 35◦ C and at a pressure range between 20 and 380 bar. Due to the competitive impact of membrane compaction and swelling on the transport properties, significant changes in permeability coefficients and selectivity with feed gas composition and pressure can be observed. As reported in this paper, a substantial increase in selectivity can be gained at elevated pressures. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Gas permeation; Polyamide (PA); Symmetric membrane; Compaction; Selectivity
1. Introduction In the last few years, the use of compressed gases or gas mixtures in a sub- or a supercritical state has widely increased. Due to their specific properties, these fluids can be used as solvents for organic compounds [1]. By changing the temperature and the pressure, the solvent power of the compressed fluid can be controlled. Low viscosity, and compared to conventional liquids, higher diffusion coefficients effect higher conversion rates and greater selectivities of chemical reactions carried out in supercritical fluids.
∗ Corresponding author. Tel.: +49-3731-392376; +49-3731-393652 E-mail address: [email protected] (G. Härtel)
fax:
Besides, compressed gases are physiologically safe and can substitute for hazardous organic solvents [2]. In order to lead the supercritical gas in a circuit, the products have to be separated from the supercritical solvent. Depending on the technical solution, this step consumes a large amount of energy [3,4]. A common way of separating the product/solvent mixture is the expansion to lower pressures so that the fluid loses its solvent power. Dissolved organic compounds are precipitated and can then be separated from the gaseous phase [5]. Reuse of the gas may meet desired ecological and economic demands. Therefore, the removal of undesired components is a pre-requisite for the recycling of the gas. In order to reach the supercritcal state, high energy and capital costs are required. Therefore, a quasi-isobaric separation process of the solutes contained in the supercritical solvent is highly desirable.
0376-7388/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 0 6 6 - 6
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High pressure membrane based separations (gas permeation) may offer one possible solution. For high pressure procedures, e.g. extraction steps in supercritical fluids or chemical synthesis in compressed gases, the feed has already been brought to a high pressure level. So the configuration of the membrane systems can be implemented easily because the supercritical fluid can be treated directly without further pressurization. Unconverted or accumulated gas components can be separated economically while keeping the supercritical solvent itself at an elevated pressure. Conventional purification methods such as adsorption, scrubbing steps or condensation of single components are energy and hardware cost extensive or they cannot be applied due to the specific properties of supercritical fluids. Membrane systems, on the contrary, are easy to control and can be extended to meet the demands of the separation process. A concrete application deriving from a hydrogenation process is the separation of a carbon dioxide/hydrogen mixture [6]. The surplus unconverted hydrogen has to be removed to avoid the phase transition of the mixture, which can result in problems with pumps and heat exchangers.
2. Theory Polymer materials consist of large macromolecules which may be linked together in different ways. Molecular structure, which also is influenced by intermolecular attraction, dictates the mechanical properties of a polymer. Thermoplastic polymers have comparatively weak attractive forces. These can be overcome by an increase in the temperature, and at the penetration of the polymer matrix, these can be overcome by molecules acting as plasticizers [7]. The first case leads to a significant loss of rigidity when reaching a certain temperature which is characteristic for each polymer. Due to thermic motion of molecules, the bonds within highly ordered regions that contribute to the rigidity of the polymer will be broken. This temperature is addressed as glass transition temperature Tg . Polymers used as membrane material are known to show a considerable loss in selectivity when the operating temperature reaches or exceeds the glass transition point [8].
The influence of penetrant molecules can have an impact on the polymer’s properties too. The penetrants are located within free volume between the polymer chains. At higher concentrations, the cohesion between the polymer chains is weakened, and therefore, the amount of free volume will increase [9]. This process leads to a softening of the material and an increase in volume which is referred to as swelling. Compressed carbon dioxide can have a high solubility in polymers and can, therefore, act as a swelling agent. The solubility of CO2 in polymers depends on the latter’s polar character. Concerning the aptitude of a polymer film as membrane material, the swollen state results in an increase in permeability of the swelling component. For this reason, the process of separating a CO2 /H2 mixture discussed here will show a loss of selectivity [10]. On the other hand, the exposure of the membrane to a high differential pressure will lead to a compaction of the polymer structure, which will decrease the amount of free volume. This will reduce the mobility of the penetrating molecules, and therefore, counteract the swelling of the polymer [11]. The processes described have opposite effects on the permeability of the membrane material. When operating the membrane at very high pressures, the influence of compaction, therefore, may exceed the swelling effect of compressed carbon dioxide. This shall be determined in Section 3 of this study.
3. Experimental For the separation of the compressed gas mixture, a membrane device (Fig. 1) was employed. The apparatus consisted of a gas supply, a membrane cell with a temperature controlling feature and an expansion module to route the permeate and retentate lines to a gas chromatograph to determine the concentration of each gas. The gas mixture (50 mol% CO2 /50 mol% H2 ) was provided by a compressed gas cylinder through a pneumatic compressor to increase the pressure. Using a regulating valve, the feed gas pressure was controlled. A tubular membrane cell with an effective membrane surface of 180 cm2 was used. As the active layer, dense, homogeneous polyamide (PA6) sheets were chosen which are known to have
G. Härtel, Th. Püschel / Journal of Membrane Science 162 (1999) 1–8
3
Fig. 1. Schematic drawing of the membrane test device.
quite a high selectivity for separating a CO2 /H2 mixture [12]. These are standard, commercially available polymers, with the following properties: thickness 12 m, glass transition temperature 10◦ C (depending on the humidity), crystallinity 40%, specific weight 1.14 g/cm3 , melting temperature 228◦ C [13]. During the manufacturing process, the polymer is axially stretched to gain better mechanical properties. In order to bear the high differential pressure, the membrane was supported by a sintered stainless steel porous tube. The membrane was attached to this support structure on the upstream side and sealed by a mixed adhesive. In contrast to asymmetric membranes, this construction will not suffer from a compaction of the porous supporting layer if high differential pressures will be applied. The retentate and permeate streams from the membrane cell were expanded before entry into the gas analyzer. This was achieved by a two-stage expansion module with integrated mass flow controller. Gas analysis was performed by an automated gas chromatograph with a thermal conductivity detector. With the above-described membrane device, the pressure-dependence of permeability and selectivity of the PA foils can be measured. As described in [12], the highest selectivity could be achieved at temperatures between 30◦ C and 40◦ C; so the effect of temperature was not a subject of this study. The permeate flux and the composition of the retentate and permeate are used to calculate the permeabilities of both gas components. Due to the applied pressure, the non-ideal behavior of the gas mixture
was taken into account using fugacities, rather than the pressures, of the components to determine permeabilities. Therefore, the following equation was applied to calculate the permeabilities of the gas components: Pi = J
δ φpi00 − φpi0
(1)
where Pi is the permeability of component i (cm3 cm/cm2 s bar), J the flux (cm3 /cm2 s), δ the membrane thickness (cm), φ the fugacity coefficient 0 (dimensionless), pi the partial pressure of compo00 nent i, upstream (bar) and pi the partial pressure of component i, downstream (bar). The calculation of the fugacity coefficient is carried out with the method of Hougen/Watson/Ragatz by determination of the reduced temperature Tr and the reduced pressure pr of the gas mixture. Tr =
T Tc
T i yi Tc,i
=P
pr =
p p =P pc i yi pc,i
(2)
where T is the operating temperature (K), p the operating pressure (bar), yi the molar fraction of component i (dimensionless), Tc,i the critical temperature of component i (K) and pc,i the critical pressure of component i (bar). The resulting values of Tr and pr can be used to determine the fugacity coefficient φ which is listed in spread-sheets [14]. The critical data of the gas components involved are listed in Table 1.
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Fig. 2. Effect of pressure on the permeability of hydrogen and carbon dioxide and the selectivity of their separation with a PA6 membrane.
Table 1 Critical data of the gas components [15] Component
Critical pressure pc (bar)
Carbon dioxide Hydrogen
73.82 13.16
Critical temperature Tc (K) 304.19 33.23
4. Results and discussion As mentioned already, the scope of work was to determine the influence of a high feed gas pressure on the permeability and selectivity of a polymer membrane. Within two different experimental set-ups, the influence of absolute pressure and composition of the feed gas on the separation process were determined. 4.1. Effect of feed pressure on permeability The membrane module was operated at a pressure between 20 and 380 bar. As can be seen in Fig. 2, the components H2 and CO2 show differing responses to the increasing pressure. While the permeability of carbon dioxide diminishes slightly, a significant growth of hydrogen permeability can be observed. The selec-
tivity, which is defined as the quotient PH2 /PCO2 also increases. In the subsequent diagram, the selectivity is shown by arrows. While increasing the partial pressure of carbon dioxide, the amount of CO2 absorbed in the polymer increases. This can be proved by sorption measurements in high-pressure CO2 . According to the solution–diffusion model, for the transportation of gases in non-porous polymers [16], the permeability will increase under these circumstances. However, the polymer sheet used in the membrane cell described is compacted mechanically according to the applied differential pressure. By this process, the free volume within the polymer is reduced and the mobility of the CO2 molecule decreases significantly. The two effects compete with each other and the influence on membrane selectivity can be explained by their impact on the permeability of the gas components involved. For the CO2 transport membrane, compaction is predominant, while the transport of hydrogen is mainly affected by the swelling of the membrane. With increasing pressure and resulting compaction, the permeability of carbon dioxide will be reduced. On the other hand, the rising CO2 partial pressure leads
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Fig. 3. Retentate CO2 concentration vs. operating time.
to an extended plasticization of the polymer matrix. This enables the hydrogen molecules to permeate the membrane more easily. Due to the above-described competition, the separation process shows a dramatic change in selectivity, as can be seen in Fig. 2. 4.2. Effect of feed gas composition In these experiments, the effect of feed gas composition on the selectivity of a PA6 membrane shall be determined. Due to technical limitations of the gas supply system, it was not possible to feed gas mixtures of different compositions at high pressures. Therefore, it was required that the membrane module be operated in a dead-end mode in order to achieve a slow but continuous change in the CO2 concentration on the upstream side of the membrane. No retentate stream was drawn from the high pressure side of the module and only the permeate was able to leave the membrane cell. The effluent gas was replaced by the equimolar feed gas mixture to maintain constant operating pressure. Due to the selectivity of the membrane, this mode of operation leads to an enrichment of CO2 on the high pressure side. The volume and the composition of the permeate were monitored continuously. By balancing the gas components, the upstream concentration of CO2 were determined and used for the cal-
culation of permeabilities. The gas chromatographic analysis of the upstream gas concentration confirmed the calculation carried out by balancing. Fig. 3 shows the retentate composition as dependent on the operating time of the membrane module. As Fig. 3 shows, the dead-end operation mode is capable of providing a wide ranging concentration of CO2 in the membrane cell. Furthermore, the change in concentration in this experiment is very slow so that the whole set-up (membrane, permeate and analytic device) has time to adapt to the altered conditions. During the operating time of the dead-end experiment described in Fig. 3, the upstream carbon dioxide concentration started at 50 vol% and ended at 98 vol%. The aim of this set-up was to observe the influence of CO2 concentration on the separation process. As mentioned before, the membrane selectivity will be affected by the amount of carbon dioxide on the upstream side. Increasing partial pressure causes CO2 molecules to enrich within the polymer structure, resulting in plasticization of the polymer matrix. This is a well known effect that has been observed at moderate pressures, e.g. at the separation of natural gas from carbon dioxide with polymer membranes [17], where a growth in CO2 permeability results. In this case, the influence of CO2 reduces the performance of the membrane separation process.
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Fig. 4. Effect of the CO2 mole fraction in the retentate on the permeability of hydrogen and carbon dioxide.
However, at the high pressure application of separation of a CO2 /H2 mixture, an enhanced compaction of the membrane polymer occurs while the membrane is exposed to very high differential pressures. As stated in Section 4.1, this mechanical compaction will mainly reduce the permeability of carbon dioxide. In addition to that, the retentate concentration of carbon dioxide, which grows during the experiment, leads to an even higher degree of plasticization. This will result in growing hydrogen permeability. For this reason, the difference in permeabilities of CO2 and H2 results in an enhanced selectivity which is indicated by arrows in Fig. 4. Fig. 4 clearly indicates the influence of CO2 concentration on the separating efficiency of the PA6 membrane. At high amounts of carbon dioxide in the retentate, a significant increase in hydrogen permeability can be observed. This can be interpreted as a synergism between the compaction of the membrane by the mechanical load and the influence of the penetrating CO2 on the polymer structure. The data presented are given as an example for these effects. Within the scope of this work, different conditions of pressure and feed gas composition were tested by the above-specified dead-end method. It can
be stated that the membrane selectivity increases both with rising feed pressure and with higher CO2 concentration on the upstream side.
5. Conclusions As has been described above, the selectivity of a polymer membrane for the separation of a compressed CO2 /H2 gas mixture can be enhanced. The main reasons for this are the mechanical compaction of the membrane under the influence of an extreme differential pressure and structural changes in the polymer due to the swelling ability of CO2 . While separating a gas mixture by means of a polymer membrane, this drastic improvement in selectivity will enable the economic operation within a wide range of feed gas concentrations. Even at a low H2 concentration on the upstream side of the membrane, the hydrogen content in the permeate will be comparatively high, as demonstrated in Fig. 5. Therefore, CO2 can be enriched efficiently at high pressures and the permeate composition allows the reuse of hydrogen in the process.
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Fig. 5. Permeate quality vs. fugacity of hydrogen in the retentate.
In further work, the main emphasis will be put on the investigation of the influence that by-products, e.g. those of a chemical synthesis, could have on the separation process, with the focus on organic solvents acting as plasticizers on the membrane material and how it may influence its separating performance.
[3]
[4]
Acknowledgements [5]
This AiF-project number 10296 B was granted by the Ministry of Economy (BMWi). We also want to acknowledge CAFFARO S.p.A. and Rhône-Poulenc for supporting the investigated polymer foils. References [1] E. Kiran, J.F. Brennecke, Current state of supercritical fluid science and technology, in: Proceedings of the Annual Meeting in Los Angeles, CA, 17–22 November 1991, American Chemical Society Symposium Series 514, 1993, pp. 1–8. [2] C.L.J. Adkins, E.M. Russick, H.M. Smith, R.B. Olson, Development and design of a high pressure carbon dioxide
[6]
[7] [8]
[9]
system for the separation of hazardous contaminants from non-hazardous debris, in: Proceedings of 27th International SAMPE Technical Conference, 9–12 October 1995, pp. 55–61. A. Birtigh, G. Brunner, Regeneration of loaded CO2 by means of isobaric processes, in: Proceedings of the 3rd international Symposium on Supercritical Fluids, Tome 2, Strasbourg, 17–19 October 1994, pp. 41–46. U. Sievers, R. Eggers, Heat recovery in a supercritical extraction process with separation at subcritical pressure, Chem. Eng. Processing 35 (1996) 239–246. E. Weidner, Z. Knez, Z. Novak, PGSS (particles from gas saturated solutions) — a new process for powder generation, in: Proceedings of the 3rd International Symposium on Supercritical Fluids, Tome 3, Strasbourg (France), 17–19 October 1994, pp. 229–234. P.G. Jessop, Y. Hsiao, T. Ikariya, R. Noyori, Catalytic production of dimethylformamide from supercritical carbon dioxide, J. Am. Chem. Soc. 116 (1994) 8851–8852. G.R. Strobl, The Physics of Polymers, Springer, Berlin (Heidelberg), 1996, pp. 217–223. S. Egli, A. Ruf, A. Buck, Gastrennung mittels Membranen — ein Ueberblick, Swiss review for the chemical industry (Swiss Chem) 6, vol. 9, Verlag Dr. Felix Wuest AG, Zuerich, 1984, pp. 89–126. S.A. Stern, S.M. Fang, H.L. Frisch, Effect of pressure on gas permeability coefficients. A new application of free volume theory, Polym. Sci. A-2 10 (1972) 210–219.
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[10] M. Wessling, Relaxation phenomena in dense gas separation membranes, Ph.D. Thesis, University of Twente (NL), 1993, pp. 91–126. [11] R.E. Kesting, A.K. Fritzsche, Polymeric Gas Separation Membranes, Wiley, New York, 1993. [12] G. Härtel, F. Rompf, Th. Püschel, Separation of a CO2 /H2 gas mixture under high pressure with polyethylene terephthalate membranes, J. Membr. Sci. 113 (1996) 115–120. [13] C.P. MacDermott, Selecting Thermoplastics for Engineering Applications, Dekker, New York, 1984.
[14] O. Hougen, K. Watson, R. Ragatz, Chemical Process Principles, vol. 2, Ed. Reverte, Barcelona, pp. 102–105. [15] R.C. Reid, J.M. Prausnitz, T.K. Sherwood, The Properties of Gases and Liquids, McGraw-Hill, New York, 1977. [16] J.G. Wijmans, R.W. Baker, The solution–diffusion model: a review, J. Membr. Sci. 107 (1995) 1–21. [17] A. Bos, High pressure CO2 /CH4 separation with glassy polymer membranes — aspects of CO2 induced plasticization, Ph.D. Thesis, University of Twente (NL), 1996, pp. 41– 66.
Permselectivity of a PA6 membrane for the separation of a compressed CO2 /H2 gas mixture at elevated pressures G. Härtel ∗ , Th. Püschel Lehrstuhl für Umweltverfahrenstechnik der TU Bergakademie Freiberg, Leipziger Straße 28, D-09596 Freiberg, Germany Received 13 July 1998; received in revised form 27 January 1999; accepted 11 February 1999
Abstract Since recently, compressed gases can substitute for hazardous solvents in the chemical industry. To meet economic and ecological demands, the reuse of compressed gas has become very desirable. This creates the need to remove, in advance, unwanted components (e.g. unconverted H2 from hydrogenation reactions). This study examines a gas permeation process to achieve the separation of CO2 /H2 mixture under high pressure. For hydrogen separation from the mixture, a homogeneous PA6 membrane is applied at a temperature of 35◦ C and at a pressure range between 20 and 380 bar. Due to the competitive impact of membrane compaction and swelling on the transport properties, significant changes in permeability coefficients and selectivity with feed gas composition and pressure can be observed. As reported in this paper, a substantial increase in selectivity can be gained at elevated pressures. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Gas permeation; Polyamide (PA); Symmetric membrane; Compaction; Selectivity
1. Introduction In the last few years, the use of compressed gases or gas mixtures in a sub- or a supercritical state has widely increased. Due to their specific properties, these fluids can be used as solvents for organic compounds [1]. By changing the temperature and the pressure, the solvent power of the compressed fluid can be controlled. Low viscosity, and compared to conventional liquids, higher diffusion coefficients effect higher conversion rates and greater selectivities of chemical reactions carried out in supercritical fluids.
∗ Corresponding author. Tel.: +49-3731-392376; +49-3731-393652 E-mail address: [email protected] (G. Härtel)
fax:
Besides, compressed gases are physiologically safe and can substitute for hazardous organic solvents [2]. In order to lead the supercritical gas in a circuit, the products have to be separated from the supercritical solvent. Depending on the technical solution, this step consumes a large amount of energy [3,4]. A common way of separating the product/solvent mixture is the expansion to lower pressures so that the fluid loses its solvent power. Dissolved organic compounds are precipitated and can then be separated from the gaseous phase [5]. Reuse of the gas may meet desired ecological and economic demands. Therefore, the removal of undesired components is a pre-requisite for the recycling of the gas. In order to reach the supercritcal state, high energy and capital costs are required. Therefore, a quasi-isobaric separation process of the solutes contained in the supercritical solvent is highly desirable.
0376-7388/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 0 6 6 - 6
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G. Härtel, Th. Püschel / Journal of Membrane Science 162 (1999) 1–8
High pressure membrane based separations (gas permeation) may offer one possible solution. For high pressure procedures, e.g. extraction steps in supercritical fluids or chemical synthesis in compressed gases, the feed has already been brought to a high pressure level. So the configuration of the membrane systems can be implemented easily because the supercritical fluid can be treated directly without further pressurization. Unconverted or accumulated gas components can be separated economically while keeping the supercritical solvent itself at an elevated pressure. Conventional purification methods such as adsorption, scrubbing steps or condensation of single components are energy and hardware cost extensive or they cannot be applied due to the specific properties of supercritical fluids. Membrane systems, on the contrary, are easy to control and can be extended to meet the demands of the separation process. A concrete application deriving from a hydrogenation process is the separation of a carbon dioxide/hydrogen mixture [6]. The surplus unconverted hydrogen has to be removed to avoid the phase transition of the mixture, which can result in problems with pumps and heat exchangers.
2. Theory Polymer materials consist of large macromolecules which may be linked together in different ways. Molecular structure, which also is influenced by intermolecular attraction, dictates the mechanical properties of a polymer. Thermoplastic polymers have comparatively weak attractive forces. These can be overcome by an increase in the temperature, and at the penetration of the polymer matrix, these can be overcome by molecules acting as plasticizers [7]. The first case leads to a significant loss of rigidity when reaching a certain temperature which is characteristic for each polymer. Due to thermic motion of molecules, the bonds within highly ordered regions that contribute to the rigidity of the polymer will be broken. This temperature is addressed as glass transition temperature Tg . Polymers used as membrane material are known to show a considerable loss in selectivity when the operating temperature reaches or exceeds the glass transition point [8].
The influence of penetrant molecules can have an impact on the polymer’s properties too. The penetrants are located within free volume between the polymer chains. At higher concentrations, the cohesion between the polymer chains is weakened, and therefore, the amount of free volume will increase [9]. This process leads to a softening of the material and an increase in volume which is referred to as swelling. Compressed carbon dioxide can have a high solubility in polymers and can, therefore, act as a swelling agent. The solubility of CO2 in polymers depends on the latter’s polar character. Concerning the aptitude of a polymer film as membrane material, the swollen state results in an increase in permeability of the swelling component. For this reason, the process of separating a CO2 /H2 mixture discussed here will show a loss of selectivity [10]. On the other hand, the exposure of the membrane to a high differential pressure will lead to a compaction of the polymer structure, which will decrease the amount of free volume. This will reduce the mobility of the penetrating molecules, and therefore, counteract the swelling of the polymer [11]. The processes described have opposite effects on the permeability of the membrane material. When operating the membrane at very high pressures, the influence of compaction, therefore, may exceed the swelling effect of compressed carbon dioxide. This shall be determined in Section 3 of this study.
3. Experimental For the separation of the compressed gas mixture, a membrane device (Fig. 1) was employed. The apparatus consisted of a gas supply, a membrane cell with a temperature controlling feature and an expansion module to route the permeate and retentate lines to a gas chromatograph to determine the concentration of each gas. The gas mixture (50 mol% CO2 /50 mol% H2 ) was provided by a compressed gas cylinder through a pneumatic compressor to increase the pressure. Using a regulating valve, the feed gas pressure was controlled. A tubular membrane cell with an effective membrane surface of 180 cm2 was used. As the active layer, dense, homogeneous polyamide (PA6) sheets were chosen which are known to have
G. Härtel, Th. Püschel / Journal of Membrane Science 162 (1999) 1–8
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Fig. 1. Schematic drawing of the membrane test device.
quite a high selectivity for separating a CO2 /H2 mixture [12]. These are standard, commercially available polymers, with the following properties: thickness 12 m, glass transition temperature 10◦ C (depending on the humidity), crystallinity 40%, specific weight 1.14 g/cm3 , melting temperature 228◦ C [13]. During the manufacturing process, the polymer is axially stretched to gain better mechanical properties. In order to bear the high differential pressure, the membrane was supported by a sintered stainless steel porous tube. The membrane was attached to this support structure on the upstream side and sealed by a mixed adhesive. In contrast to asymmetric membranes, this construction will not suffer from a compaction of the porous supporting layer if high differential pressures will be applied. The retentate and permeate streams from the membrane cell were expanded before entry into the gas analyzer. This was achieved by a two-stage expansion module with integrated mass flow controller. Gas analysis was performed by an automated gas chromatograph with a thermal conductivity detector. With the above-described membrane device, the pressure-dependence of permeability and selectivity of the PA foils can be measured. As described in [12], the highest selectivity could be achieved at temperatures between 30◦ C and 40◦ C; so the effect of temperature was not a subject of this study. The permeate flux and the composition of the retentate and permeate are used to calculate the permeabilities of both gas components. Due to the applied pressure, the non-ideal behavior of the gas mixture
was taken into account using fugacities, rather than the pressures, of the components to determine permeabilities. Therefore, the following equation was applied to calculate the permeabilities of the gas components: Pi = J
δ φpi00 − φpi0
(1)
where Pi is the permeability of component i (cm3 cm/cm2 s bar), J the flux (cm3 /cm2 s), δ the membrane thickness (cm), φ the fugacity coefficient 0 (dimensionless), pi the partial pressure of compo00 nent i, upstream (bar) and pi the partial pressure of component i, downstream (bar). The calculation of the fugacity coefficient is carried out with the method of Hougen/Watson/Ragatz by determination of the reduced temperature Tr and the reduced pressure pr of the gas mixture. Tr =
T Tc
T i yi Tc,i
=P
pr =
p p =P pc i yi pc,i
(2)
where T is the operating temperature (K), p the operating pressure (bar), yi the molar fraction of component i (dimensionless), Tc,i the critical temperature of component i (K) and pc,i the critical pressure of component i (bar). The resulting values of Tr and pr can be used to determine the fugacity coefficient φ which is listed in spread-sheets [14]. The critical data of the gas components involved are listed in Table 1.
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G. Härtel, Th. Püschel / Journal of Membrane Science 162 (1999) 1–8
Fig. 2. Effect of pressure on the permeability of hydrogen and carbon dioxide and the selectivity of their separation with a PA6 membrane.
Table 1 Critical data of the gas components [15] Component
Critical pressure pc (bar)
Carbon dioxide Hydrogen
73.82 13.16
Critical temperature Tc (K) 304.19 33.23
4. Results and discussion As mentioned already, the scope of work was to determine the influence of a high feed gas pressure on the permeability and selectivity of a polymer membrane. Within two different experimental set-ups, the influence of absolute pressure and composition of the feed gas on the separation process were determined. 4.1. Effect of feed pressure on permeability The membrane module was operated at a pressure between 20 and 380 bar. As can be seen in Fig. 2, the components H2 and CO2 show differing responses to the increasing pressure. While the permeability of carbon dioxide diminishes slightly, a significant growth of hydrogen permeability can be observed. The selec-
tivity, which is defined as the quotient PH2 /PCO2 also increases. In the subsequent diagram, the selectivity is shown by arrows. While increasing the partial pressure of carbon dioxide, the amount of CO2 absorbed in the polymer increases. This can be proved by sorption measurements in high-pressure CO2 . According to the solution–diffusion model, for the transportation of gases in non-porous polymers [16], the permeability will increase under these circumstances. However, the polymer sheet used in the membrane cell described is compacted mechanically according to the applied differential pressure. By this process, the free volume within the polymer is reduced and the mobility of the CO2 molecule decreases significantly. The two effects compete with each other and the influence on membrane selectivity can be explained by their impact on the permeability of the gas components involved. For the CO2 transport membrane, compaction is predominant, while the transport of hydrogen is mainly affected by the swelling of the membrane. With increasing pressure and resulting compaction, the permeability of carbon dioxide will be reduced. On the other hand, the rising CO2 partial pressure leads
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Fig. 3. Retentate CO2 concentration vs. operating time.
to an extended plasticization of the polymer matrix. This enables the hydrogen molecules to permeate the membrane more easily. Due to the above-described competition, the separation process shows a dramatic change in selectivity, as can be seen in Fig. 2. 4.2. Effect of feed gas composition In these experiments, the effect of feed gas composition on the selectivity of a PA6 membrane shall be determined. Due to technical limitations of the gas supply system, it was not possible to feed gas mixtures of different compositions at high pressures. Therefore, it was required that the membrane module be operated in a dead-end mode in order to achieve a slow but continuous change in the CO2 concentration on the upstream side of the membrane. No retentate stream was drawn from the high pressure side of the module and only the permeate was able to leave the membrane cell. The effluent gas was replaced by the equimolar feed gas mixture to maintain constant operating pressure. Due to the selectivity of the membrane, this mode of operation leads to an enrichment of CO2 on the high pressure side. The volume and the composition of the permeate were monitored continuously. By balancing the gas components, the upstream concentration of CO2 were determined and used for the cal-
culation of permeabilities. The gas chromatographic analysis of the upstream gas concentration confirmed the calculation carried out by balancing. Fig. 3 shows the retentate composition as dependent on the operating time of the membrane module. As Fig. 3 shows, the dead-end operation mode is capable of providing a wide ranging concentration of CO2 in the membrane cell. Furthermore, the change in concentration in this experiment is very slow so that the whole set-up (membrane, permeate and analytic device) has time to adapt to the altered conditions. During the operating time of the dead-end experiment described in Fig. 3, the upstream carbon dioxide concentration started at 50 vol% and ended at 98 vol%. The aim of this set-up was to observe the influence of CO2 concentration on the separation process. As mentioned before, the membrane selectivity will be affected by the amount of carbon dioxide on the upstream side. Increasing partial pressure causes CO2 molecules to enrich within the polymer structure, resulting in plasticization of the polymer matrix. This is a well known effect that has been observed at moderate pressures, e.g. at the separation of natural gas from carbon dioxide with polymer membranes [17], where a growth in CO2 permeability results. In this case, the influence of CO2 reduces the performance of the membrane separation process.
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Fig. 4. Effect of the CO2 mole fraction in the retentate on the permeability of hydrogen and carbon dioxide.
However, at the high pressure application of separation of a CO2 /H2 mixture, an enhanced compaction of the membrane polymer occurs while the membrane is exposed to very high differential pressures. As stated in Section 4.1, this mechanical compaction will mainly reduce the permeability of carbon dioxide. In addition to that, the retentate concentration of carbon dioxide, which grows during the experiment, leads to an even higher degree of plasticization. This will result in growing hydrogen permeability. For this reason, the difference in permeabilities of CO2 and H2 results in an enhanced selectivity which is indicated by arrows in Fig. 4. Fig. 4 clearly indicates the influence of CO2 concentration on the separating efficiency of the PA6 membrane. At high amounts of carbon dioxide in the retentate, a significant increase in hydrogen permeability can be observed. This can be interpreted as a synergism between the compaction of the membrane by the mechanical load and the influence of the penetrating CO2 on the polymer structure. The data presented are given as an example for these effects. Within the scope of this work, different conditions of pressure and feed gas composition were tested by the above-specified dead-end method. It can
be stated that the membrane selectivity increases both with rising feed pressure and with higher CO2 concentration on the upstream side.
5. Conclusions As has been described above, the selectivity of a polymer membrane for the separation of a compressed CO2 /H2 gas mixture can be enhanced. The main reasons for this are the mechanical compaction of the membrane under the influence of an extreme differential pressure and structural changes in the polymer due to the swelling ability of CO2 . While separating a gas mixture by means of a polymer membrane, this drastic improvement in selectivity will enable the economic operation within a wide range of feed gas concentrations. Even at a low H2 concentration on the upstream side of the membrane, the hydrogen content in the permeate will be comparatively high, as demonstrated in Fig. 5. Therefore, CO2 can be enriched efficiently at high pressures and the permeate composition allows the reuse of hydrogen in the process.
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Fig. 5. Permeate quality vs. fugacity of hydrogen in the retentate.
In further work, the main emphasis will be put on the investigation of the influence that by-products, e.g. those of a chemical synthesis, could have on the separation process, with the focus on organic solvents acting as plasticizers on the membrane material and how it may influence its separating performance.
[3]
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Acknowledgements [5]
This AiF-project number 10296 B was granted by the Ministry of Economy (BMWi). We also want to acknowledge CAFFARO S.p.A. and Rhône-Poulenc for supporting the investigated polymer foils. References [1] E. Kiran, J.F. Brennecke, Current state of supercritical fluid science and technology, in: Proceedings of the Annual Meeting in Los Angeles, CA, 17–22 November 1991, American Chemical Society Symposium Series 514, 1993, pp. 1–8. [2] C.L.J. Adkins, E.M. Russick, H.M. Smith, R.B. Olson, Development and design of a high pressure carbon dioxide
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