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RESEARCH NOTES Separation of Propylene/Propane Mixtures Using Faujasite-Type Zeolite Membranes Ioannis G. Giannakopoulos†,‡ and Vladimiros Nikolakis*,† Foundation for Research and Technology Hellas, Institute of Chemical Engineering and High-Temperature Chemical Processes, P.O. Box 1414, GR 265 04 Patras, Greece, and Department of Chemical Engineering, University of Patras, GR 265 00 Patras, Greece
Faujasite-type zeolite membranes, having a thickness of approximately 20 µm, were synthesized on porous R-alumina supports using the secondary growth method. For the first time, a detailed investigation of the faujasite membrane’s ability to separate propylene/propane mixtures has been performed. Single-component and binary mixture permeation experiments indicated that the membranes are propylene permselective. The maximum separation factor (13.7 ( 1 at 100 °C) and the maximum ideal selectivity (28 at 35 °C) indicate that the membranes have improved performance, compared to previously reported results. Membrane permselective behavior was maintained over the entire range of feed compositions examined (Ptotal ) 101 kPa and Ppropylene ) 12-93 kPa). The temperature dependence differences between the single-component and binary mixture permeation fluxes indicate that the presence of propylene enhances propane transport through the membrane. Introduction The separation of olefin and paraffin gas mixtures, an important process for the petrochemical industry, is usually performed by cryogenic distillation, which is an energy-intensive separation process.1 The high capital and operating costs of a cryogenic distillation unit provide the motivation for research toward the development of alternative separation methods such as membrane separation processes. The synthesis of a membrane that can successfully separate olefin/paraffin mixtures can be used to replace cryogenic distillation, as well as to treat small gas streams for which separation is not currently economically attractive. During the past decade, many attempts have been made to separate olefin/paraffin mixtures using polymeric2,3 or carbon membranes.4 Details about the work based on those materials can be found in recently published review papers and in the references mentioned therein.2,4 Membranes synthesized from zeolite materials are also promising candidates for the separation of olefin/paraffin gas mixtures. Zeolites have welldefined pore structures with sizes of molecular dimensions that enable them to discriminate between subnanometer molecules. Several research groups have reported the synthesis of a large number of zeolite membranes and have demonstrated their ability to separate gas or liquid mixtures.5-13 The synthesis of faujasite membranes on porous alumina supports has already been reported by several * To whom correspondence should be addressed. Tel.: ++302610965242. Fax: ++30-2610965223. E-mail: vnikolak@ iceht.forth.gr. † Institute of Chemical Engineering and High-Temperature Chemical Processes. ‡ University of Patras.
research groups.14-19 The synthesized membranes have shown promising results in the separation of a large number of mixtures such as CO2/N2,17 CO2/CH4,17 benzene/cyclohexane,14-16 C4-C7 hydrocarbons,15 1,3propanediol/glycerol,18 methanol/methyl tert-butyl ether,16 and H2O/ethanol.16 However, only a limited amount of data regarding the permeation behavior of light hydrocarbons such as propane or propylene is available in the literature.14,19 In the present paper, we report the separation of propane/propylene gas mixtures using faujasite membranes prepared on porous R-alumina supports. Additionally, the effects of permeation temperature and membrane feed composition in permeation fluxes and separation factors are examined. Experimental Methods The zeolite membranes were synthesized using the secondary growth method on homemade porous R-Al2O3 disk supports prepared by pressing commercial R-Al2O3 powder (Baiwkowski CR-1) and firing for 30 h at 1100 °C. The alumina disks were ∼2 mm thick and had a diameter of 14 mm and an average pore size of 150200 nm. A more detailed description of the preparation procedure can be found in the literature.20 Prior to any use, supports were polished using a silicon carbide paper (grit size 600). Before hydrothermal treatment, a layer of faujasite crystals (Na-Y; Aldrich) was deposited on the polished side of the support using a dip-casting technique. The seeded supports were secured in Teflon holders that were placed vertically in polypropylene bottles and were hydrothermally treated at 85 °C with a mixture of a molar composition of 4.17:1.0:10:1.87:460 Na2O/Al2O3/ triethanolamine (TEA)/SiO2/H2O. This molar composition is known to result in nucleation and growth of both
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Figure 1. Experimental setup used in the permeation measurements: (1) rotameter; (2) bubble flowmeter; (3) permeation cell; (4) gas chromatograph; (5) mass flow controller; (6) furnace; (7) control needle valve.
Na-X faujasite crystals21 and Na-X membranes.14 For its preparation, tetraethyl orthosilicate (98%; Aldrich) was dissolved in a NaOH and TEA (95%; Aldrich) solution at room temperature, under stirring until a clear mixture was obtained (2-3 h); at the same time, aluminum foil (Al; 99.8%, 0.05 mm thick; Aldrich) was dissolved in a NaOH solution, which was then slowly added to the silica mixture under stirring. After hydrothermal growth, the membranes were cooled, washed several times with distilled water, and then heat-treated in air at 420 °C overnight to free the zeolite pores from the remaining TEA or water molecules. The heating and cooling rate was 2 °C‚min-1. The membranes were characterized with X-ray diffraction (XRD) and scanning electron microscopy (SEM). The X-ray patterns were collected on a Philips system using Cu KR radiation. SEM images were obtained using a LEO SUPRA 35V microscope operated at 20 kV in the variable-pressure mode. Permeation measurements were performed as a function of temperature and feed composition in a WickeKallenbach setup (Figure 1). The membrane was sealed in a custom-made brass permeation cell with Viton O-rings, and the cell was then placed in a tubular furnace. The feed-side flow rates of propane and propylene were adjusted using a variable-area rotameter with a built-in control valve (Omega). The permeate side was flushed with a helium sweep stream, the flow rate of which was controlled by a mass flow controller (Aera FC-7700C). The total feed flow rate was equal to 80 mL‚min-1, while the helium flow rate at the permeate side was equal to 55 mL‚min-1. The pressure on both sides of the membrane was atmospheric. The permeate gas analysis composition was performed in a Shimadzu (GC-14B) gas chromatograph equipped with a flame ionization detector. The separation factor for a binary mixture is calculated as follows:
SF )
C perm /C perm 1 2 feed C feed 1 /C 2
(1)
where C1 is the concentration of propylene and C2 the concentration of propane on the permeate (C perm ) or i ) side of the membrane (i ) 1 and 2). feed (C feed i Results and Discussion The films grown on the alumina supports were examined by SEM and XRD. SEM images of the
Figure 2. Top-view (a) and cross-sectional (b) images of the supported faujasite membranes, after (7 days) hydrothermal treatment at 85 °C.
Figure 3. XRD pattern of the polycrystalline membrane synthesized on an R-Al2O3 support. (The marked peak corresponds to R-Al2O3 of the support.)
membrane top view and a typical cross section after 7 days of hydrothermal growth are presented in Figure 2. The SEM images reveal that a dense polycrystalline membrane layer, having a thickness of ∼20 µm, is formed on both sides of the support. The membrane XRD pattern shown in Figure 3 indicates that faujasite zeolite crystals have been formed.
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Ind. Eng. Chem. Res., Vol. 44, No. 1, 2005 Table 1. Propylene/Propane Mixture Permeation Fluxes and Maximum Separation Factors through Different Faujasite Membranes membrane
separation factora
C3H6 flux (mmol‚m-2‚s-1)
temperature (°C)
M1 M2 M3 M4
13.7 ( 1 10.7 ( 0.9 9.3 ( 0.8 8.5 ( 0.7
0.392 0.685 0.672 0.304
100 90 90 100
a The error bars in the separation factors correspond to the uncertainty of the feed composition.
Figure 4. Propane (2, 4) and propylene (9, 0) permeation fluxes for single-component (filled symbols) and binary mixtures (open symbols) as a function of the temperature. (The feed-side and permeate-side pressure was 101 kPa in all experiments. In the binary mixture, the feed partial pressures were Ppropylene ) 55 kPa and Ppropane ) 46 kPa. In the single-component experiments, the feed partial pressures were Ppropylene ) 55 kPa, Phelium ) 46 kPa and Ppropane ) 46 kPa, Phelium ) 55 kPa.)
Figure 5. Effect of the permeation temperature on the propylene/ propane separation factor (b) and selectivity (O). (The error bars correspond to the uncertainty of the feed composition.)
Figure 4 shows the permeation fluxes of propane and propylene single components and mixtures as a function of temperature from 35 to 150 °C. To make a meaningful comparison between them, the single-component feed stream of each component was diluted with helium to the corresponding feed partial pressure in the binary mixture experiment. Propylene’s single-component flux was higher than propane’s flux over the entire range of temperatures examined. Nevertheless, the permeation flux of each compound exhibited different temperature dependences. Propylene’s flux passed through a minimum at approximately 80 °C, whereas the permeation flux of propane increased with temperature over the entire temperature range examined (Figure 4). The optimum ideal selectivity was approximately 28 and had been observed at 35 °C (Figure 5). Similar trends of fluxes as a function of temperature have been observed for the transport of hydrocarbons through silicalite-1 membranes.22,23 The decrease in the permeation flux of propylene with temperature, at the lowest temperatures examined, can be attributed to a decrease in the adsorbed-phase concentration. Moreover, the permeation flux minimum can be attributed to a change in the transport mechanism from surface-
activated diffusion to gaseous-activated transport, which usually prevails at higher temperature.23,24 When a binary mixture was fed, the permeation flux of propylene remained unchanged and almost equal to the one measured in the single-component experiment. On the other hand, as shown in Figure 4, the permeation flux of propane passed through a minimum with increasing temperature, having now a completely different behavior from the single-component measurement. In more detail, at permeation temperatures of less than 70 °C, the permeation flux of propane was larger than the corresponding one for the single-component feed. This behavior was reversed at higher permeation temperatures, in which case the propane flux was reduced almost an order of magnitude from the singlecomponent value. As a result, the effect of the temperature on the separation factor was the opposite of its effect on ideal selectivity. As shown in Figure 5, the measured separation factor of the propylene/propane mixtures was small at low temperatures and increased with the permeation temperature, reaching a maximum value that has been observed at 90 °C. A further increase of the permeation temperature up to 150 °C resulted in a slight decrease of the separation factor. The error bars shown in Figure 5 were calculated from the estimated error in the feed mixture composition. The reproducibility of the membrane’s ability to separate propylene/propane mixtures has been examined by testing several synthesized membranes (M1M4). The measured separation factors and propylene permeation fluxes are presented in Table 1. The maximum separation factor measured was 13.7 ( 1 at 100 °C, and the corresponding propylene flux was 0.39 mmol‚m-2‚s-1. The results presented in Table 1 show that the maximum separation factor of different membranes has been observed either at 90 °C or at 100 °C. Despite the variation in the temperature of the maximum separation factor, all membranes examined were able to separate the propane/propylene mixtures, exhibiting higher separation factors than those previously reported. The experimental observations of the permeation flux in the case of a binary mixture appear to be counterintuitive. To explain the observed increase of the propane flux in the presence of propylene, we considered previously reported data, by Huang et al.,25 on intracrystalline diffusion coefficients of propane and propylene, estimated by fitting binary mixture adsorption and desorption rates on faujasite crystals. These results indicate that propane diffusion through faujasite crystals is significantly enhanced when adsorption of propylene takes place simultaneously. It is not easy to explain why propane’s flux has a more pronounced minimum than propylene’s flux when a binary mixture is fed. Multicomponent transport through zeolite membranes is a complex process that depends on both
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Figure 6. Propylene (0) and propane (4) permeation fluxes and separation factors (b) at 90 °C as a function of the propylene feed partial pressure. (The error bars in the separation factors correspond to the uncertainty of the feed composition.)
adsorption and diffusion. For each compound, the amount adsorbed in the crystal depends on the corresponding partial pressure in the gas phase and its intracrystalline diffusion coefficient depends on crystal loading as well as on host-guest and guest-guest interactions, which are difficult to quantify. Furthermore, the setup (Wicke-Kallenbach) used for measuring permeation fluxes uses helium as the sweep gas, which might counterdiffuse to the feed side. However, the effect of helium counterdiffusion on the measured permeation fluxes of propane and propylene is not expected to be significant because both hydrocarbons are strongly adsorbed components while helium is a weakly adsorbed component. van de Graaf et al.23,26 have shown that in such a case and at low permeation temperatures the counterdiffusion of helium through zeolite membranes (silicalite-1) is much lower than the permeances of the strongly adsorbed compounds. For the single-component permeation measurements, the effect of helium counterdiffusion is expected to be even smaller because the concentration difference between the two sides of the membranes is smaller. To determine the effect of the feed mixture composition on the separation, permeation measurements were performed on membrane M2 as a function of the feed composition. The measurements were performed at the temperature where the maximum separation factor had been observed. By adjustment of the ratios of the two flows, the propylene feed partial pressure was varied between 12 and 93 kPa. Permeation fluxes, presented in Figure 6, show that the increase of the propylene mole fraction in the feed caused minor changes in its permeation flux, whereas it caused the propane flux to decrease. The maximum separation factor (11.5 ( 0.9) has been observed when the propylene feed partial pressure was 25 kPa, while the lowest value (7.1 ( 0.6) has been observed in the case of a propylene-rich feed mixture (Ppropylene ) 93 kPa). These results indicate that the membranes are selective over a wide range of compositions and can be successfully used even when small amounts of propylene are present in the mixture. Finally, it is desired to compare the separation performance of the faujasite membranes prepared in the present work to the performances of carbon4,27,28 and polymeric2,3,29 membranes. This is not an easy task because quite often the permeation experiments were performed under different conditions (i.e., cross-mem-
Figure 7. Propylene/propane separation factor as a function of the propylene permeability for different membrane materials: carbon membranes (9; refs 4, 27, and 28); polymeric membranes (2; refs 2, 3, and 29); this work (f). The solid line is the separation upper bound curve plotted using the equation derived by Burns and Koros.3 The parameters of the equation were estimated3 by fitting permeation data through 6FDA-DDBT polyimide membranes [separation factor ) 25.2/(permeability coefficient)0.244].
brane pressure difference) or with use of membranes having different thicknesses. To normalize the effect of those parameters, the permeation fluxes are converted to permeability coefficients by multiplying them with the membrane thickness and dividing with the pressure difference between the feed and permeate sides. The effective membrane thickness of the faujasite membranes was assumed to be 15 µm. A comparison between the faujasite, polymeric, and carbon membranes’ experimentally observed propylene/propane separation factors as a function of propylene’s permeability coefficient is presented in Figure 7. Details about the types of membrane materials used, the membrane fabrication/ synthesis methods, and the conditions of the permeation experiments can be found in the corresponding references.2-4,27-29 In the same figure, the propylene/propane separation upper bound curve for 6FDA-DDBT polyimide membranes, which is a representation of the experimentally observed tradeoff between the separation factor and permeation flux, is shown. It has been plotted using the equation derived by Burns and Koros,3 together with the parameters estimated by fitting permeation data through 6FDA-DDBT polyimide membranes. It is clear from Figure 7 that the separation performance of membranes made from molecular-sieving materials, such as faujasite and carbon molecular sieves, lies above the upper bound polymeric tradeoff curve. It is important to note that the results shown in Figure 7 provide only an indication of how the type of the membrane material affects the separation performance. This is due to the fact that the permeability coefficient units are normalized for the membrane thickness. Nevertheless, the results in Figure 7 indicate that thin, defect-free faujasite membranes will be expected to have better separation performances than membranes made by many other types of materials. Conclusions In the present paper, the separation of propylene/ propane mixtures using a faujasite-type zeolite membrane was demonstrated. The synthesized membranes exhibited separation factors much higher than any
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others previously reported for zeolite membranes. The maximum separation factor was 13.7 ( 1 and was observed at temperatures of around 90-100 °C. At that temperature, the membranes were able to separate the propylene/propane mixture over a wide range of feed compositions (Ptotal ) 101 kPa and Ppropylene ) 12-93 kPa). Furthermore, the maximum measured ideal selectivity was approximately 28 and was higher than the optimum separation factor, indicating that the presence of propylene enhances propane transport through the membrane. Acknowledgment The authors acknowledge financial support from FORTH/ICEHT. They also acknowledge Dr. Th. Ioannides for providing access to the gas chromatography facility, Prof. C. Katagas for acquisition of the XRD patterns, and Dr. V. Drakopoulos for acquisition of the SEM images. Literature Cited (1) Baker, R. W. Future directions of membrane gas separation technology. Ind. Eng. Chem. Res. 2002, 41, 1393. (2) Semenova, S. I. Polymer membranes for hydrocarbon separation and removal. J. Membr. Sci. 2004, 231, 189. (3) Burns, R. L.; Koros, W. J. Defining the challenges for C3H6/ C3H8 separation using polymeric membranes. J. Membr. Sci. 2003, 211, 299. (4) Ismail, A. F.; David, L. I. B. A review of the latest development of carbon membranes for gas separation. J. Membr. Sci. 2001, 193, 1. (5) Bernal, M. P.; Coronas, J.; Menendez, I.; Santamaria, J. Separation of CO2/N2 mixtures using MFI type membranes. AIChE J. 2004, 50, 127. (6) Caro, J.; Noack, M.; Kolsch, P.; Schafer, R. Zeolite membranes - state of their development and perspective. Microporous Mesoporous Mater. 2000, 38, 3. (7) Jansen, K.; Maschmeyer, T. Progress in zeolitic membranes. Top. Catal. 1999, 9, 113. (8) Lai, Z. P.; Bonilla, G.; Diaz, I.; Nery, J. G.; Sujaoti, K.; Amat, M. A.; Kokkoli, E.; Terasaki, O.; Thompson, R. W.; Tsapatsis, M.; Vlachos, D. G. Microstructural optimization of a zeolite membrane for organic vapor separation. Science 2003, 300, 456. (9) Lin, Y. S. Microporous and dense inorganic membranes: current status and prospective. Sep. Purif. Technol. 2001, 25, 39. (10) Noack, M.; Kolsch, P.; Schafer, R.; Toussaint, P.; Caro, J. Molecular sieve membranes for industrial application: Problems, progess, solutions. Chem. Eng. Technol. 2002, 25, 221. (11) Tavolaro, A.; Drioli, E. Zeolite membranes. Adv. Mater. 1999, 11, 975. (12) Yan, Y.; Davis, M. E.; Gavalas, G. R. Preparation of highly selective zeolite ZSM-5 membranes by a post-synthetic coking treatment. J. Membr. Sci. 1997, 123. (13) Baertsch, C. D.; Funke, H. H.; Falkoner, J. L.; Noble, R. D. Permeation of aromatic hydrocarbon vapors through silicalitezeolite membranes. J. Phys. Chem. 1996, 100, 7676.
(14) Nikolakis, V.; Xomeritakis, G.; Abibi, A.; Dickson, M.; Tsapatsis, M.; Vlachos, D. G. Growth of a faujasite-type zeolite membrane and its application in the separation of saturated/ unsaturated hydrocarbon mixtures. J. Membr. Sci. 2001, 184, 209. (15) Jeong, B. H.; Hasegawa, Y.; Sotowa, K. I.; Kusakabe, K.; Morooka, S. Permeation of binary mixtures of benzene and saturated C4-C7 hydrocarbons through an FAU-type zeolite membrane. J. Membr. Sci. 2003, 213, 115. (16) Kita, H.; Fuchida, K.; Horita, T.; Asamura, H.; Okamoto, K. Preparation of faujasite membranes and their permeation properties. Sep. Purif. Technol. 2001, 25, 261. (17) Hasegawa, Y.; Tanaka, T.; Watanabe, K.; Jeong, B. H.; Kusakabe, K.; Morooka, S. Separation of CO2-CH4 and CO2-N2 systems using ion-exchanged FAU-type zeolite membranes with different Si/Al ratios. Korean J. Chem. Eng. 2002, 19, 309. (18) Li, S.; Tuan, V. A.; Falconer, J. L.; Noble, R. D. X-type zeolite membranes: preparation, characterization, and pervaporation performance. Microporous Mesoporous Mater. 2002, 53, 59. (19) Nair, S.; Lai, Z.; Nikolakis, V.; Bonilla, G.; Tsapatsis, M. Separation of close boiling hydrocarbon mixtures by MFI and FAU membranes made by secondary growth. Microporous Mesoporous Mater. 2001, 48, 219. (20) Xomeritakis, G.; Nair, S.; Tsapatsis, M. Transport properties of alumina-supported MFI membranes made by secondary(seeded) growth. Microporous Mesoporous Mater. 2000, 38, 61. (21) Qiu, S.; Yu, J.; Zhu, G.; Terasaki, O.; Nozue, Y.; Pang, W.; Xu, R. Strategies for the synthesis of large zeolite single crystals. Microporous Mesoporous Mater. 1998, 21, 245. (22) van den Broeke, L. J. P.; Kapteijn, F.; Moulijn, J. A. Binary permeation through a silicalite-1 membrane. AIChE J. 1999, 45, 976. (23) van de Graaf, J. M.; Kapteijn, F.; Moulijn, J. A. Methological and operational aspects of permeation measurements on Silicalite-1 membranes. J. Membr. Sci. 1998, 144, 87. (24) Xiao, J. R.; Wei, J. Diffusion Mechanism of Hydrocarbons in Zeolites. 1. Theory. Chem. Eng. Sci. 1992, 47, 1123. (25) Huang, Y. H.; Liapis, A. I.; Xu, Y.; Crosser, O. K.; Johnson, J. W. Binary Adsorption and Desorption Rates of Propylene Propane Mixtures on 13X Molecular-Sieves. Sep. Technol. 1995, 5, 1. (26) van de Graaf, J. M.; Kapteijn, F.; Moulijn, J. A. Permeation of weakly adsorbing components through a silicalite-1 membrane. Chem. Eng. Sci. 1999, 54, 1081. (27) Hayashi, J.; Mizuta, H.; Yamamoto, M.; Kusakabe, K.; Morooka, S.; Suh, S. H. Separation of ethane/ethylene and propane/propylene systems with a carbonized BPDA-pp′ODA polyimide membrane. Ind. Eng. Chem. Res. 1996, 35, 4176. (28) Okamoto, K.; Kawamura, S.; Yoshino, M.; Kita, H.; Hirayama, Y.; Tanihara, N.; Kusuki, Y. Olefin/paraffin separation through carbonized membranes derived from an asymmetric polyimide hollow fiber membrane. Ind. Eng. Chem. Res. 1999, 38, 4424. (29) Tanaka, K.; Taguchi, A.; Hao, J. Q.; Kita, H.; Okamoto, K. Permeation and separation properties of polyimide membranes to olefins and paraffins. J. Membr. Sci. 1996, 121, 197.
Received for review June 7, 2004 Revised manuscript received November 8, 2004 Accepted November 19, 2004 IE049508R
