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An Experimental Study of CO2 Separation Using a Silica Based Composite Membrane
0957–5820/03/$23.50+0.00 # Institution of Chemical Engineers Trans IChemE, Vol 81, Part B, July 2003
www.ingentaselect.com=titles=09575820.htm
AN EXPERIMENTAL STUDY OF CO2 SEPARATION USING A SILICA BASED COMPOSITE MEMBRANE Y. YILDIRIM 1 and R. HUGHES2 1
Engineering Faculty, Zonguldak Karaelmas University, Zonguldak, Turkey 2 Chemical Engineering Unit, The University of Salford, Salford, UK
I
n this investigation, the preparation and gas selectivity characteristics of an ‘ultra ne’ composite ceramic membrane are reported. A dip-coating technique was used to prepare a thin selective membrane on a commercially available ceramic macroporous lter (SCT, France). The permeabilities of H2, N2 and CO2 were measured at temperatures of 25–470 C and average pressures of 1–2 bar. The separation of CO2=N2 binary mixtures was also performed. The separation factors for CO2 were found to be higher than that of the Knudsen separation mechanism at room and high temperature. The potential applications of these membranes to CO2 separation at these temperatures are discussed on the basis of measured selectivity values. Keywords: gas separation; CO2; silica membrane; membrane preparation.
Brinker et al., 1993; Yildirim et al., 1997; Yildirim and Hughes, in press). Most of the separation applications have concentrated on the Knudsen region but the pore sizes still need to be modi ed for application of gas separation of composite membranes due to the low gas selectivity in the Knudsen region. In this study, the preparation and gas selectivity characteristics of an ultra ne composite ceramic membrane were investigated as a new separation tool for CO2 at room and high temperature.
INTRODUCTION The international agreement to reduce greenhouse gases, such as CO2, under the Kyoto protocol, signed by over 160 countries in December 1997, presents signi cant challenges. The excessive discharge of CO2 into the atmosphere due to the consumption of large amounts of fossil fuels has become one of the most serious global environment problems. The increase of CO2 concentration in the atmosphere is considered to be the major cause for global warming. The principal anthropogenic sources of CO2 are combustion of fossil fuels, cement manufacture and deforestation. Fossil fuels are used for power generation, transport, heating and many other purposes. The amounts of greenhouse gases, such as CO2, CH4, in the atmosphere can be reduced either by controlling emissions or by increasing the rate at which they are removed. CO2 handling technology is divided into four categories: CO2 separation and collection processes; CO2 capture processes; CO2 utilization processes; and CO2 disposal processes. CO2 separation and collection processes consist of absorption, adsorption, membrane separation and the other separation processes such as cryogenic (Go¨ttlicher and Pruschek, 1998; Pruschek et al., 1998). CO2 separation by absorption into a reactive solvent such as monoethanolamine (MEA) is considered to be prohibitively expensive. The cryogenic separation of CO2 consumes considerable energy due to heating and cooling (Go¨ttlicher and Pruschek, 1996). Membrane separation of CO2 offers in situ separation without changing the ambient temperature. The increasing interest in membrane-based gas separations and=or puri cation and catalytic application is evident by the large number of publications on the subject in recent years (Keizer et al., 1988; DeLange et al., 1995;
THEORY OF GAS SEPARATION The performance of a gas separation membrane system is largely determined by three parameters. The rst parameter is its permselectivity or selectivity towards the gases to be separated. Permselectivity affects the percentage recovery of the valuable gas in the feed. The second issue is the permeate ux or permeability which is related to productivity and determines the membrane area required. The third parameter is related to the membrane stability or service life which has a strong impact on the replacement and maintenance cost of the system. A frequently used indicator of how much of two gases in a multicomponent gaseous mixture are separated with respect to each other through a membrane is called the separation factor. It is de ned as am,n ˆ
ym =xm yn =xn
(1)
where y and x represent the mole fractions of the gas components feed side and permeate side of the membrane, respectively. It is essentially determined by their relative 257
258
YILDIRIM and HUGHES
permeabilities through the membrane. For a binary gas mixture, the separation factor becomes y1 =x1 a1,2 ˆ (2) (1 ¡ y1 )=(1 ¡ x1 )
It can be shown that a1,2 ˆ a¤1,2
1 ¡ … pf y1 †=… pp x1 † 1 ¡ … pf (1 ¡ y1 ) †=… pp (1 ¡ x1 ) †
(3)
where pf and pp are the feed side and permeate side pressures, respectively, and a¤1,2 is the so called ideal separation factor which is the ratio of the pure component permeabilities (P1 and P2): a¤1,2 ˆ
P1 P2
(4)
The ideal separation factor is the theoretical overall selectivity of a membrane and is often referenced in the literature (Ulhorn and Burggraaf, 1991). METHODS AND MATERIALS Membrane Preparation 10 ml of a silicone elastomer base (Slygard1 ) was added to 90 ml isopentane contained in a clean glass tube of 150 ml volume and the solution mixed thoroughly to obtain a clear, colourless sol. After this, 1 ml of curing agent (Slygard1 ) was added to the resulting solution and remixed thoroughly at room temperature. After aging, the prepared sols were then deposited onto a tubular membrane substrate (supplied by Membralox1 ). These supports were a-alumina and had a mean pore diameter of 200 nm. The wall thickness was 1.5 mm and outside diameter was approximately 10 mm. The opposite side of the bore of the substrates were completely plugged with rubber before being immersed into the sol. The sol diffused into the membrane structure and formed a layer on the support surface. After 20 minutes immersion, the substrate was drained, prior to the controlled evaporation process. This was carried out in an oven at 65¯ C for 24 hours under air circulation. This ve-stage process
resulted in a single dip-coating membrane (Ulhorn et al., 1992). The same procedure as detailed above was repeated for subsequent coatings. In this study, up to three dip-coated membranes have been prepared and evaluated. The stability of the deposited membranes and lms for high temperature applications was assessed by calcining in air at a heating rate of approximately 1¯ C min¡1 to 450¯ C. This temperature was maintained for 2 hours and the tube then cooled down to room temperature at a rate of 0.5¯ C min 7 1. Experimental Rig and Leakage Test The experimental rig essentially consisted of the gas delivery system, the permeation cell and gas analytical system as seen in Figure 1. The permeation cell consisted of three main parts including the composite membrane tube, an outer stainless steel tubular shell and inlet and outlet ports in the tube and shell sides respectively. Sealing of the membrane tube to the stainless steel reactor was achieved by moulded graphite rings (density 1.6 g cc¡1, 98% purity) of dimensions 30 mm OD, 10 mm ID and 8 mm thickness (supplied by Geegraf Limited, UK). All the pipelines and the connections were checked for leakage by pressurizing with pure nitrogen at twice the highest pressure to be used in the system and observing any drop in pressure along the pipe connections. In addition, all connections in the system were tested for leakage by means of a soap solution. Two sets of experiments were performed for each dipcoated membrane. In the rst set, ‘pure’ gases of N2, CO2, and H2 (BOC pure, UK) were admitted into the shell-side and allowed to permeate through the coated membranes at predetermined differential pressures. In the second set of experiments the separation of binary gaseous mixtures consisting of N2 ‡ CO2 was performed. The permeate was analysed on-line using a CO2 infrared analyser (The Analytical Development Co. Ltd., Hoddeston, England). The ‘pure’ gases were at least 99.99% pure and were supplied from high-pressure cylinders. No further puri cation was carried out. The gaseous mixtures of 97.5% N2=2.5 vol.% CO2, were supplied premixed (BOC mixtures, UK).
Figure 1. Experimental rig.
Trans IChemE, Vol 81, Part B, July 2003
EXPERIMENTAL STUDY OF CO2 SEPARATION
RESULTS AND DISCUSSION Single and Binary Gas Separation Factors at Room Temperature Figure 2 shows the effect of pressure ratio, de ned as the permeate pressure divided by the feed pressure, on the separation factor of CO2 for both pure and binary gas mixtures of N2=CO2. It also shows the value of nonseparative viscous ow and the ideal Knudsen separation. The pure gas separation factor for CO2 is obtained by the ratio of the permeability of CO2 to that of N2 at the same pressure and temperature. The ideal Knudsen value is obtained from the reciprocal of the square root of the ratio of the respective molecular weights. The CO2 selectivity or separation factor from a gas mixture, aCO2 ,i was obtained by equation (5); (CCO2 =CN2 )p aCO2 ,N2 ˆ (5) (CCO2 =CN2 )f
259
gas permeance. All three gases follow a Knudsen type ow mechanism up to 300¯ C. However, after that temperature, only hydrogen increases with increasing temperature, indicating that an active ow mechanism is the dominant ow mechanism for hydrogen. This is the well known mechanism and is supported by many investigators (Brinker et al., 1993; Ilias and Govind, 1989). Figure 4 represents the dependency of permeation of hydrogen on temperature for the membrane coated with silica. The activation energy was calculatedas 3.01 kJ mol K¡1 from the gure (Yildirim, 1998). The ideal gas selectivity was described by equation (4) above. According to this equation, the separation factors were found as 6.6 and 4.2 for H2=CO2 and H2=N2 respectively at a temperature of 470¯ C. Separation of H2 and CO2 from Coal Gasi cation
In order to measure single gas permeabilities at high temperature, an a-Al 2O3 ceramic tube with 10 mm OD, 7 mm ID and 250 mm length, was modi ed twice from the outside using silica sol as described above. The silicamodi ed tube was tested for gas permeation of CO2, H2 and N2. Figure 3 shows the relationship between temperature and
Hydrogen is increasingly becoming a high-value product with a variety of potential uses. It can be applied to upgrade coal and oil shale-derived liquids, as a chemical feedstock and as an alternative fuel. Therefore, in an integrated coal gasi cation combined cycle unit, the process economics can be improved if H2 can be ef ciently separated from the gasi cation stream. An approximate composition of a dry gasi cation stream is given in Table 1. It contains about 20% (v=v) H2, 16% (v=v) CO2 and 13% (v=v) CO. The remaining components are 47% (v=v) N2 and only 4% (v=v) CH4. A ow diagram of a process for production and separation of H2 and CO2 from coal gasi cation (air blown) is shown in Figure 5. CO2 separation from H2 is important in the separation of the water gas shift reaction products. After the gas shift reaction where CO is converted to CO2, the H2 and CO2 so obtained can be separated by an inorganic membrane to concentrate H2 as a valuable product. Comparisons of the membrane permeability and CO2 separation factor with other membrane related studies are presented in Table 2. In this study, a separation factor and a permeability of 6.5 and 5.9 £ 105 barrer respectively, were obtained which are higher than those of membranes quoted in literature. In brief, Table 2 and Figure 4 show that H2 permeation through the silica membrane is 6.6 times higher than CO2 permeation. The separation factor of hydrogen over carbon dioxide is high enough to separate CO2=H2 in
Figure 2. CO2 separation factor as a function of pressure ratio at room temperature.
Figure 3. Permeance of CO2, N2 and H2 gases in the membrane as a function of temperature.
Where CCO2 and CN2 are the respective percentage concentrations of CO2 and N2 in the feed and permeate streams. The general trend is that the CO2 selectivity decreases with pressure ratio. However, there are important differences. The rst difference is between pure gas separation factors and that of the mixture. Higher separation values are obtained for the mixture than for pure gases. The second difference is the fact that separation factor values signi cantly higher than the ideal Knudsen values are obtained for the N2=CO2 mixture. If the Knudsen mechanism is dominant, then the CO2 separation = factor is equal to (MN2=MCO2)1 2 where M is the relevant molecular weight of the gas. This should give values of 0.798 for the N2=CO2 binary systems. However, values approaching 0.9 for the N2=CO2 were observed in the membranes indicative of the contribution of a more CO2 selective mechanism, which was attributed to surface diffusion. This is also supported by other researchers (Keizer et al., 1998; Chao et al., 1995). Single Gas Separation Factors at High Temperature
Trans IChemE, Vol 81, Part B, July 2003
260
YILDIRIM and HUGHES Table 1. Approximate gas composition of an air blown gasi er (McMohan et al., 1990).
Figure 4. Permeability of H2 gas in the membrane as a function of inverse temperature.
the last stage of the coal gasi cation process shown in Figure 5. CONCLUSIONS The main mechanism of gas transport through a silica coated a-Al2O3 membrane was by Knudsen diffusion. Binary gas separation factors for the N2=CO2 mixtures were higher than the predicted ideal Knudsen selectivity indicating the existence of a more selective separative
Gas
Concentration (vol.%)
Nitrogen (N2) Hydrogen (H2) Carbon dioxide (CO2) Carbon monoxide (CO) Methane (CH4)
47 20 16 13 4
mechanism which is speculated to be due to surface diffusion of CO2. Such a separation factor is high enough to form the basis of a small-scale CO2 removal unit for land ll gas and for natural gas treatment. The membranes could be used for other applications such as removal of CO2 from other systems including life support, i.e. breathing systems. The thermo-stability of the membranes was con rmed by a relatively constant value of the separation factor following a 2-hour calcination at 450¯ C. In the high-temperature study, the separation factors were found to be 6.6 and 4.2 for H2=CO2 and H2=N2 respectively at a temperature of 470¯ C. This shows that H2 permeation through the silica membrane is 6.6 times higher than CO2 permeation. Performances of this level could have signi cant applications in effecting gas separations at high temperature and have potential application in the separation of CO2=H2 binary mixture from the coal gasi cation process.
Figure 5. Schematic representation diagram of coal gasi cation process (air blown gasi er) to produce hydrogen from coal (Pellegrino et al., 1988).
Table 2. Separation of carbon dioxide and hydrogen by inorganic membranes. Membrane material (pore diameter, nm) Al2O3 (10–20) Al2O3 (10–20) Al2O3 (4) SiO2 SiO2
Temperature ( C) 196–97 20 104–445 25–70 400–470
Note: S.F. Separation factor, P permeate side
TMP (bar) 0.23–1.05 0.03–0.21 21.00 1.00
Permeability (barrer or 1010
S.F. 5.0 5.0 4–8 4.7–5.4 6.5
P (barrer) 3.2 2.5
105 105
0.2–2.8 5.9
102 105
References
Note
Itaya et al., 1984 Toyo Soda, 1985 Lee et al., 1994 Way and Roberts, 1992 In this study
Anodic alumina Anodic alumina Alumina=Pd imp. Hollow ber Silica=alumina
(cm3(STP)-cm=s-cm2-cm Hg)), TMP
pressure differences between feed side and
Trans IChemE, Vol 81, Part B, July 2003
EXPERIMENTAL STUDY OF CO2 SEPARATION REFERENCES Brinker, C.J., Ward, T.L., Sehgal, R., Raman, N.K., Hietala, S.L., Smith, D.M., Hua, D.W. and Headley, T.J., 1993, J Membrane Sci, 77: 165. Chao, Y.K., Han, K. and Lee, K.H., 1995, J Membrane Sci, 104: 219. DeLange, R.S.D., Hekking, J.H.A., Keizer, K. and Burgraaf, A.J., 1995, Microporous Materials, 4: 169. Go¨ttlicher, G. and Pruschek, R., 1996, International Conference on Carbon Dioxide Removal, Boston. Go¨ttlicher, G. and Pruschek, R., 1998, Proc 4th International Conference on Greenhouse Gas Control Technologies, Interlaken. Ilias, S. and Govind, R., 1989, AIChE Symposium Series no. 268, 85: 18. Itaya, K., Sugawara, S., Arai, K. and Saito, S., 1984, Journal of Chem Eng Japan, 17: 514. Keizer, K., Ulhorn, R.J.R., van Vuren, R.J. and Burggraaf, A.J., 1988, J Membrane Sci, 39: 285. Lee, S.-J., Yang, S.-M. and Park, S.B., 1994, J Membrane Sci, 96: 223. McMohan, T.J., Gasper, L. and Hsu, P., 1990, in Proc 10th Annual Gasi cation Gas Stream Cleanup Systems Contractors Review Meeting, Kothari, V.P. and Beeson, J.L. (eds) (Morgantown, WV, USA), p 279. Pellegrino, J.J., Nassimbene, R., Kirkkopru, A., Noble, R.D. and Way, J.D., 1988, Proceeding U.S. Department of Energy Contractors Review Meeting on Gasi cation Gas Stream Cleanup System (Morgantown, WV, USA).
Trans IChemE, Vol 81, Part B, July 2003
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Pruschek, R., Go¨ttlicher, G., Oeljeklaus, G., Haupt, G. and Zimmermann, G., 1998, Proc Power-Gen Europe’98, Milano. Toyo Soda Manufacturing, 1985, Japanese Patent Application 60,187,320. Ulhorn, R.J.R. and Burggraaf, A.J., 1991, in Bhave, R.R. (ed) (Van Nostrand Reinhold, New York), pp 155–176. Uhlhorn, R.J.R., Keizer, K. and Burggraaf, A.J., 1992, J Membrane Sci, 66: 271. Way, J.D. and Roberts, D.L., 1992, Separation Science and Technology, 27: 29. Yildirim, Y., 1998, Ph.D. Thesis, The University of Salford, Salford, UK. Yildirim, Y. and Hughes, R., 2002, The effective combustion of o-xylene in a Knudsen controlled catalylic membrane reactor, Trans IChemE, Part B, Proc Safe Env Prot, 80(B3): 159–165. Yildirim, Y., Gobina, E. and Hughes, R., 1997, J Membrane Sci, 135: 107.
ADDRESS Correspondence concerning this paper should be addressed to Dr Y. Yildirim, Zonguldak Karaelmas University, Engineering Faculty, 67100 Zonguldak, Turkey. E-mail: [email protected] The manuscript was received 29 April 2002 and accepted for publication after revision 9 June 2003.
www.ingentaselect.com=titles=09575820.htm
AN EXPERIMENTAL STUDY OF CO2 SEPARATION USING A SILICA BASED COMPOSITE MEMBRANE Y. YILDIRIM 1 and R. HUGHES2 1
Engineering Faculty, Zonguldak Karaelmas University, Zonguldak, Turkey 2 Chemical Engineering Unit, The University of Salford, Salford, UK
I
n this investigation, the preparation and gas selectivity characteristics of an ‘ultra ne’ composite ceramic membrane are reported. A dip-coating technique was used to prepare a thin selective membrane on a commercially available ceramic macroporous lter (SCT, France). The permeabilities of H2, N2 and CO2 were measured at temperatures of 25–470 C and average pressures of 1–2 bar. The separation of CO2=N2 binary mixtures was also performed. The separation factors for CO2 were found to be higher than that of the Knudsen separation mechanism at room and high temperature. The potential applications of these membranes to CO2 separation at these temperatures are discussed on the basis of measured selectivity values. Keywords: gas separation; CO2; silica membrane; membrane preparation.
Brinker et al., 1993; Yildirim et al., 1997; Yildirim and Hughes, in press). Most of the separation applications have concentrated on the Knudsen region but the pore sizes still need to be modi ed for application of gas separation of composite membranes due to the low gas selectivity in the Knudsen region. In this study, the preparation and gas selectivity characteristics of an ultra ne composite ceramic membrane were investigated as a new separation tool for CO2 at room and high temperature.
INTRODUCTION The international agreement to reduce greenhouse gases, such as CO2, under the Kyoto protocol, signed by over 160 countries in December 1997, presents signi cant challenges. The excessive discharge of CO2 into the atmosphere due to the consumption of large amounts of fossil fuels has become one of the most serious global environment problems. The increase of CO2 concentration in the atmosphere is considered to be the major cause for global warming. The principal anthropogenic sources of CO2 are combustion of fossil fuels, cement manufacture and deforestation. Fossil fuels are used for power generation, transport, heating and many other purposes. The amounts of greenhouse gases, such as CO2, CH4, in the atmosphere can be reduced either by controlling emissions or by increasing the rate at which they are removed. CO2 handling technology is divided into four categories: CO2 separation and collection processes; CO2 capture processes; CO2 utilization processes; and CO2 disposal processes. CO2 separation and collection processes consist of absorption, adsorption, membrane separation and the other separation processes such as cryogenic (Go¨ttlicher and Pruschek, 1998; Pruschek et al., 1998). CO2 separation by absorption into a reactive solvent such as monoethanolamine (MEA) is considered to be prohibitively expensive. The cryogenic separation of CO2 consumes considerable energy due to heating and cooling (Go¨ttlicher and Pruschek, 1996). Membrane separation of CO2 offers in situ separation without changing the ambient temperature. The increasing interest in membrane-based gas separations and=or puri cation and catalytic application is evident by the large number of publications on the subject in recent years (Keizer et al., 1988; DeLange et al., 1995;
THEORY OF GAS SEPARATION The performance of a gas separation membrane system is largely determined by three parameters. The rst parameter is its permselectivity or selectivity towards the gases to be separated. Permselectivity affects the percentage recovery of the valuable gas in the feed. The second issue is the permeate ux or permeability which is related to productivity and determines the membrane area required. The third parameter is related to the membrane stability or service life which has a strong impact on the replacement and maintenance cost of the system. A frequently used indicator of how much of two gases in a multicomponent gaseous mixture are separated with respect to each other through a membrane is called the separation factor. It is de ned as am,n ˆ
ym =xm yn =xn
(1)
where y and x represent the mole fractions of the gas components feed side and permeate side of the membrane, respectively. It is essentially determined by their relative 257
258
YILDIRIM and HUGHES
permeabilities through the membrane. For a binary gas mixture, the separation factor becomes y1 =x1 a1,2 ˆ (2) (1 ¡ y1 )=(1 ¡ x1 )
It can be shown that a1,2 ˆ a¤1,2
1 ¡ … pf y1 †=… pp x1 † 1 ¡ … pf (1 ¡ y1 ) †=… pp (1 ¡ x1 ) †
(3)
where pf and pp are the feed side and permeate side pressures, respectively, and a¤1,2 is the so called ideal separation factor which is the ratio of the pure component permeabilities (P1 and P2): a¤1,2 ˆ
P1 P2
(4)
The ideal separation factor is the theoretical overall selectivity of a membrane and is often referenced in the literature (Ulhorn and Burggraaf, 1991). METHODS AND MATERIALS Membrane Preparation 10 ml of a silicone elastomer base (Slygard1 ) was added to 90 ml isopentane contained in a clean glass tube of 150 ml volume and the solution mixed thoroughly to obtain a clear, colourless sol. After this, 1 ml of curing agent (Slygard1 ) was added to the resulting solution and remixed thoroughly at room temperature. After aging, the prepared sols were then deposited onto a tubular membrane substrate (supplied by Membralox1 ). These supports were a-alumina and had a mean pore diameter of 200 nm. The wall thickness was 1.5 mm and outside diameter was approximately 10 mm. The opposite side of the bore of the substrates were completely plugged with rubber before being immersed into the sol. The sol diffused into the membrane structure and formed a layer on the support surface. After 20 minutes immersion, the substrate was drained, prior to the controlled evaporation process. This was carried out in an oven at 65¯ C for 24 hours under air circulation. This ve-stage process
resulted in a single dip-coating membrane (Ulhorn et al., 1992). The same procedure as detailed above was repeated for subsequent coatings. In this study, up to three dip-coated membranes have been prepared and evaluated. The stability of the deposited membranes and lms for high temperature applications was assessed by calcining in air at a heating rate of approximately 1¯ C min¡1 to 450¯ C. This temperature was maintained for 2 hours and the tube then cooled down to room temperature at a rate of 0.5¯ C min 7 1. Experimental Rig and Leakage Test The experimental rig essentially consisted of the gas delivery system, the permeation cell and gas analytical system as seen in Figure 1. The permeation cell consisted of three main parts including the composite membrane tube, an outer stainless steel tubular shell and inlet and outlet ports in the tube and shell sides respectively. Sealing of the membrane tube to the stainless steel reactor was achieved by moulded graphite rings (density 1.6 g cc¡1, 98% purity) of dimensions 30 mm OD, 10 mm ID and 8 mm thickness (supplied by Geegraf Limited, UK). All the pipelines and the connections were checked for leakage by pressurizing with pure nitrogen at twice the highest pressure to be used in the system and observing any drop in pressure along the pipe connections. In addition, all connections in the system were tested for leakage by means of a soap solution. Two sets of experiments were performed for each dipcoated membrane. In the rst set, ‘pure’ gases of N2, CO2, and H2 (BOC pure, UK) were admitted into the shell-side and allowed to permeate through the coated membranes at predetermined differential pressures. In the second set of experiments the separation of binary gaseous mixtures consisting of N2 ‡ CO2 was performed. The permeate was analysed on-line using a CO2 infrared analyser (The Analytical Development Co. Ltd., Hoddeston, England). The ‘pure’ gases were at least 99.99% pure and were supplied from high-pressure cylinders. No further puri cation was carried out. The gaseous mixtures of 97.5% N2=2.5 vol.% CO2, were supplied premixed (BOC mixtures, UK).
Figure 1. Experimental rig.
Trans IChemE, Vol 81, Part B, July 2003
EXPERIMENTAL STUDY OF CO2 SEPARATION
RESULTS AND DISCUSSION Single and Binary Gas Separation Factors at Room Temperature Figure 2 shows the effect of pressure ratio, de ned as the permeate pressure divided by the feed pressure, on the separation factor of CO2 for both pure and binary gas mixtures of N2=CO2. It also shows the value of nonseparative viscous ow and the ideal Knudsen separation. The pure gas separation factor for CO2 is obtained by the ratio of the permeability of CO2 to that of N2 at the same pressure and temperature. The ideal Knudsen value is obtained from the reciprocal of the square root of the ratio of the respective molecular weights. The CO2 selectivity or separation factor from a gas mixture, aCO2 ,i was obtained by equation (5); (CCO2 =CN2 )p aCO2 ,N2 ˆ (5) (CCO2 =CN2 )f
259
gas permeance. All three gases follow a Knudsen type ow mechanism up to 300¯ C. However, after that temperature, only hydrogen increases with increasing temperature, indicating that an active ow mechanism is the dominant ow mechanism for hydrogen. This is the well known mechanism and is supported by many investigators (Brinker et al., 1993; Ilias and Govind, 1989). Figure 4 represents the dependency of permeation of hydrogen on temperature for the membrane coated with silica. The activation energy was calculatedas 3.01 kJ mol K¡1 from the gure (Yildirim, 1998). The ideal gas selectivity was described by equation (4) above. According to this equation, the separation factors were found as 6.6 and 4.2 for H2=CO2 and H2=N2 respectively at a temperature of 470¯ C. Separation of H2 and CO2 from Coal Gasi cation
In order to measure single gas permeabilities at high temperature, an a-Al 2O3 ceramic tube with 10 mm OD, 7 mm ID and 250 mm length, was modi ed twice from the outside using silica sol as described above. The silicamodi ed tube was tested for gas permeation of CO2, H2 and N2. Figure 3 shows the relationship between temperature and
Hydrogen is increasingly becoming a high-value product with a variety of potential uses. It can be applied to upgrade coal and oil shale-derived liquids, as a chemical feedstock and as an alternative fuel. Therefore, in an integrated coal gasi cation combined cycle unit, the process economics can be improved if H2 can be ef ciently separated from the gasi cation stream. An approximate composition of a dry gasi cation stream is given in Table 1. It contains about 20% (v=v) H2, 16% (v=v) CO2 and 13% (v=v) CO. The remaining components are 47% (v=v) N2 and only 4% (v=v) CH4. A ow diagram of a process for production and separation of H2 and CO2 from coal gasi cation (air blown) is shown in Figure 5. CO2 separation from H2 is important in the separation of the water gas shift reaction products. After the gas shift reaction where CO is converted to CO2, the H2 and CO2 so obtained can be separated by an inorganic membrane to concentrate H2 as a valuable product. Comparisons of the membrane permeability and CO2 separation factor with other membrane related studies are presented in Table 2. In this study, a separation factor and a permeability of 6.5 and 5.9 £ 105 barrer respectively, were obtained which are higher than those of membranes quoted in literature. In brief, Table 2 and Figure 4 show that H2 permeation through the silica membrane is 6.6 times higher than CO2 permeation. The separation factor of hydrogen over carbon dioxide is high enough to separate CO2=H2 in
Figure 2. CO2 separation factor as a function of pressure ratio at room temperature.
Figure 3. Permeance of CO2, N2 and H2 gases in the membrane as a function of temperature.
Where CCO2 and CN2 are the respective percentage concentrations of CO2 and N2 in the feed and permeate streams. The general trend is that the CO2 selectivity decreases with pressure ratio. However, there are important differences. The rst difference is between pure gas separation factors and that of the mixture. Higher separation values are obtained for the mixture than for pure gases. The second difference is the fact that separation factor values signi cantly higher than the ideal Knudsen values are obtained for the N2=CO2 mixture. If the Knudsen mechanism is dominant, then the CO2 separation = factor is equal to (MN2=MCO2)1 2 where M is the relevant molecular weight of the gas. This should give values of 0.798 for the N2=CO2 binary systems. However, values approaching 0.9 for the N2=CO2 were observed in the membranes indicative of the contribution of a more CO2 selective mechanism, which was attributed to surface diffusion. This is also supported by other researchers (Keizer et al., 1998; Chao et al., 1995). Single Gas Separation Factors at High Temperature
Trans IChemE, Vol 81, Part B, July 2003
260
YILDIRIM and HUGHES Table 1. Approximate gas composition of an air blown gasi er (McMohan et al., 1990).
Figure 4. Permeability of H2 gas in the membrane as a function of inverse temperature.
the last stage of the coal gasi cation process shown in Figure 5. CONCLUSIONS The main mechanism of gas transport through a silica coated a-Al2O3 membrane was by Knudsen diffusion. Binary gas separation factors for the N2=CO2 mixtures were higher than the predicted ideal Knudsen selectivity indicating the existence of a more selective separative
Gas
Concentration (vol.%)
Nitrogen (N2) Hydrogen (H2) Carbon dioxide (CO2) Carbon monoxide (CO) Methane (CH4)
47 20 16 13 4
mechanism which is speculated to be due to surface diffusion of CO2. Such a separation factor is high enough to form the basis of a small-scale CO2 removal unit for land ll gas and for natural gas treatment. The membranes could be used for other applications such as removal of CO2 from other systems including life support, i.e. breathing systems. The thermo-stability of the membranes was con rmed by a relatively constant value of the separation factor following a 2-hour calcination at 450¯ C. In the high-temperature study, the separation factors were found to be 6.6 and 4.2 for H2=CO2 and H2=N2 respectively at a temperature of 470¯ C. This shows that H2 permeation through the silica membrane is 6.6 times higher than CO2 permeation. Performances of this level could have signi cant applications in effecting gas separations at high temperature and have potential application in the separation of CO2=H2 binary mixture from the coal gasi cation process.
Figure 5. Schematic representation diagram of coal gasi cation process (air blown gasi er) to produce hydrogen from coal (Pellegrino et al., 1988).
Table 2. Separation of carbon dioxide and hydrogen by inorganic membranes. Membrane material (pore diameter, nm) Al2O3 (10–20) Al2O3 (10–20) Al2O3 (4) SiO2 SiO2
Temperature ( C) 196–97 20 104–445 25–70 400–470
Note: S.F. Separation factor, P permeate side
TMP (bar) 0.23–1.05 0.03–0.21 21.00 1.00
Permeability (barrer or 1010
S.F. 5.0 5.0 4–8 4.7–5.4 6.5
P (barrer) 3.2 2.5
105 105
0.2–2.8 5.9
102 105
References
Note
Itaya et al., 1984 Toyo Soda, 1985 Lee et al., 1994 Way and Roberts, 1992 In this study
Anodic alumina Anodic alumina Alumina=Pd imp. Hollow ber Silica=alumina
(cm3(STP)-cm=s-cm2-cm Hg)), TMP
pressure differences between feed side and
Trans IChemE, Vol 81, Part B, July 2003
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ADDRESS Correspondence concerning this paper should be addressed to Dr Y. Yildirim, Zonguldak Karaelmas University, Engineering Faculty, 67100 Zonguldak, Turkey. E-mail: [email protected] The manuscript was received 29 April 2002 and accepted for publication after revision 9 June 2003.