Ind. Eng. Chem. Res. 1997, 36, 137-143
137
SEPARATIONS Separations of Cyclic, Branched, and Linear Hydrocarbon Mixtures through Silicalite Membranes Hans H. Funke, Andrew M. Argo, John L. Falconer,* and Richard D. Noble Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424
Binary and ternary mixtures of organic vapors were separated at elevated temperatures with a silicalite zeolite membrane on a porous, tubular, γ-alumina support. Linear alkanes (C5-C9), branched alkanes, aromatics, and saturated ring compounds were used as feeds, and permeances of pure compounds and mixtures were measured between ∼360 and 510 K. Pure compound permeances of the linear alkanes strongly decrease with increasing chain length, whereas the branched and cyclic compounds permeate at rates similar to those of n-hexane and n-heptane. Almost all permeances increase with increasing temperature. Mixtures of branched or cyclic molecules with small linear alkanes were readily separated with high selectivities (over 200 for n-hexane/benzene), even though the ratios of pure component permeances were small. The separation behavior is not due to molecular sieving but instead appears to be due to preferential adsorption (adsorption on external surface, pore entering, adsorption in pores) of one species, which prevents the other organics from adsorbing and transporting through the membrane. Mixtures of cyclic or branched molecules showed small or no separations. For all systems, separations factors decrease as temperature increases apparently because preferential adsorption becomes less important at elevated temperatures. For mixtures of benzene or methylyclohexane with 2,2,4-trimethylpentane and for mixtures of 2,2-dimethylbutane with 3-methylpentane, both compounds permeated at similar rates and no separations were obtained. Single-file transport in the zeolite channels is suspected to limit transport. The membranes have intercrystalline regions in parallel with the zeolite pores that may also permeate the organics. Introduction Zeolites, which are inorganic crystalline structures with pores of the same size as small molecules, can operate at high temperatures and are resistant to chemical degradation. When zeolites are prepared as continuous membrane films, they provide an opportunity to greatly expand the use of membrane technology. The large number of zeolite structures offers the potential for many new membrane materials, but only a few types of zeolite membranes have been fabricated. The most studied membranes use MFI-type zeolites such as silicalite, which is a pure silica zeolite with a threedimensional channel system. According to Flanigen et al. (1978), silicalite has elliptical pore openings of 0.51 × 0.57 nm interconnected by zigzag channels with a nearly circular cross section of about 0.54 nm diameter as determined by X-ray diffraction. Adsorption studies by Wu et al. (1986) with hydrocarbon molecules of different sizes and shapes have yielded larger effective pore sizes of 0.7 × 0.55 nm, which are based on kinetic diameters calculated from Lennard-Jones potential functions. Apparently kinetic diameters provide only rough estimates for predicting the ability of molecules to enter silicalite pores if the pore dimensions determined by X-ray diffraction are used as a basis. Silicalite is hydrophobic and preferentially adsorbs organic molecules that are small enough to enter the pore openings. Several recent publications report permeation of higher boiling organic compounds (i.e., those that are liquids at room temperature) through silicalite or ZSM-5 membranes on flat supports, but separation properties of the membranes were significantly different S0888-5885(96)00472-1 CCC: $14.00
for each membrane even thought the same zeolite was used. Xiang and Ma (1994) used silicalite membranes on R-Al2O3 supports to separate liquid mixtures of higher hydrocarbons by pervaporation. For equal concentrations of p-xylene, m-xylene, and triisopropylbenzene in a ternary liquid mixture, the bulky triisopropylbenzene molecules (kinetic diameter of 0.93 nm) did not permeate through the membrane and the permeate contained 93.8% p-xylene and 6.2% m-xylene. Vroon (1995) separated p-xylene from o-xylene with selectivities greater than 200 at 400 K using an R-aluminasupported silicalite membrane at organic feed concentrations below 0.3 mol % and a p-xylene flux of 4.2 × 10-6 mol/(m2 s). The o-xylene fluxes through Vroon’s membrane, however, were close to the detection limit of his permeation setup, and thus the selectivities could not be determined very accurately. Bakker et al. (1993) measured the permeation of mixtures of butane isomers and 2,2,4-trimethylpentane (i-octane) with CH4 and found selectivities as high as 380 for a binary n-butane/ CH4 mixture through metal-supported silicalite membranes at 300 K. Methane permeated 75 times faster than i-octane in a 95/5 binary mixture. Tsikoyiannis and Haag (1992) fabricated silicalite membranes without a support and obtained a separation factor of 17.2 for n-hexane/2,2-dimethylbutane vapor mixtures at 422 K. The partial pressure of n-hexane and 2,2-dimethylbutane in the feed was 9 and 17 kPa, respectively. For similar feed concentrations, Vroon (1995) found that n-hexane permeated more than 2000 times faster than 2,2-dimethylbutane at 473 K through his R-aluminasupported silicalite membrane. The fluxes of n-hexane © 1997 American Chemical Society
138 Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997
through Vroon’s and Tsikoyiannis and Haag’s membranes were comparable, which indicates that Vroon’s membrane had a significantly smaller number of nonzeolitic pores or defects. The fluxes of 2,2-dimethylbutane through Vroon’s membrane were close to the detection limit of his system, and thus the selectivities could not be determined very precisely. Yan et al. (1995) found that triisopropylbenzene vapor did not permeate through the pores of their ZSM-5 membrane and concluded it was defect-free. Similarly, Sano et al. (1994) used organic molecules of different sizes to determine the extent of defects in their silicalite membranes. The largest molecules that were able to permeate relatively easily through the membranes were mand p-diisopropylbenzene (0.71 nm kinetic diameter for the para isomer). Bigger molecules such as 1,3,5triisopropylbenzene and perfluoro-n-butylamine (1.02 nm) hardly passed through the membrane. These membranes were prepared by hydrothermally synthesizing the zeolite from a gel onto a support. In contrast, Ko¨lsch et al. (1994) embedded silicalite crystals into a metallic grid by galvanic techniques. Their membranes were able to separate binary vapor mixtures of nheptane and toluene with separation factors of 5.4 in favor of toluene at 413 K. The transient time to reach steady state was rather long, however (up to several days), and initially n-heptane was the faster permeating compound. In contrast to the above listed membranes, which were prepared on flat supports, we grow zeolite layers on tubular supports, which are geometrically more favorable for the fabrication of modules with large membrane areas. Our silicalite membranes were able to separate methanol vapor from mixtures with CH4 and H2 at elevated temperatures because CH3OH blocked the silicalite pores and prevented the gases from permeating (Jia et al., 1994). Methanol permeated up to 190 times faster than CH4 and more than 1000 times faster than H2, so effective separations were obtained. These silicalite membranes can separate hydrocarbons that have similar size, molecular weight, or boiling point (Funke et al., 1996a,b). Vapor mixtures of n-octane and i-octane were separated with a maximum selectivity of 40 for n-octane over i-octane at 413 K when hexane was present. Selectivities dropped above and below this temperature. The permeation of components in the mixtures cannot be predicted from pure compound permeances since pure i-octane permeated slightly faster than pure n-octane. The i-octane permeance strongly decreased in the mixture, whereas the permeances of the linear compounds changed only slightly. Similar behavior with a selectivity maximum as a function of temperature and strong differences between pure and mixed compound permeances were also found for heptane/i-octane/n-hexane mixtures (Funke et al., 1996a). The highest selectivity for heptane/i-octane in a ternary mixture with n-hexane was over 130. Preferential adsorption was suspected to cause this behavior. In contrast to alkane separations, aromatics could not be separated in our silicalite membranes (Baertsch et al., 1996). For example, pure m-xylene permeated faster than ethylbenzene with a permeance ratio over 12. In mixtures, however, all components were limited by permeance of the slowest component as if single-file diffusion controlled the permanence. The permeances of each component in the mixtures were similar to the permeances of the slowest permeating pure component.
To obtain a more complete understanding of the processes that govern separations in zeolite membranes, we studied 12 pure hydrocarbons and some of their binary and ternary mixtures. Molecules with different sizes and shapes were used to determine the separation potential of the membranes, and single-gas behavior does not predict behavior for mixtures. All separations were done at 363 K or higher (the highest temperature studied was 509 K) so that condensation in the membrane or in the system was not a concern. Since permeances were still high at 363 K and the best separation factors were obtained at the lowest temperature used, separation selectivity may increase at lower temperatures. Experimental Procedures Membrane Preparation. Silicalite layers were synthesized on the inside of commercially-available, porous γ-alumina tubes (US Filter, 5-nm pores). The zeolite synthesis was performed hydrothermally from a gel at 453 K in an autoclave. After two zeolite layers were synthesized, the membranes were washed with deionzied water for 2 h, dried for 1 h at 453 K, and calcined at 733 K to remove the organic template. The detailed synthesis procedure is described elsewhere (Funke et al., 1996b). Some alumina from the support may dissolve and incorporate into the zeolite during synthesis so that the zeolite in the interfacial region contains aluminum (ZSM-5 zeolite). The ZSM-5 zeolite has a similar framework to silicalite but slightly different pore sizes due to counterions that compensate for charge deficits associated with aluminum atoms. The membrane consists of an apparently continuous layer (within SEM resolution) with zeolite crystals growing out of it. The maximum thickness of the continuous layer is 2-10 µm, on the basis of SEM measurements (Jia et al., 1994), but the actual separation layer is probably much thinner; thus, aluminum that is preferentially in the separating layer could affect the membrane properties even if present in low concentrations. The membranes were characterized by pure gas permeation at room temperature. The ideal selectivity for N2 (kinetic diameter of 0.36 nm) over SF6 (0.55 nm) was used as a measure of the quality of the membrane. The membrane used in this study had a N2/SF6 permeance ratio of 86 and a N2 permeance of 2.0 × 10-6 mol/(m2 s Pa). Separation Measurements. The separation experiments were performed in a continuous flow system with He carrier gas on both feed and permeate sides. The membrane was sealed with graphite gaskets. The organic compounds were fed with a syringe pump as liquids, which were completely evaporated and mixed with He prior to entering the module. Retentate and permeate streams were analyzed on-line with a GC equipped with a FID. All experiments were performed at ambient pressure on both feed and permeate sides. The total organic feed concentration was maintained between 10 and 20 mol % for the pure and mixed feed studies. Concentrations of individual compounds were thus lower in the mixtures than when used alone. Material balances were obtained within 5%, and reproducibility of permeances at steady state was better than 3%. The experimental setup and procedures are described in detail elsewhere (Jia et al., 1994; Funke et al., 1996b). The hydrocarbons were all Alltech analytical grades, and their boiling points and kinetic diameters are listed in Table 1.
Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 139 Table 1. Kinetic Diameters, Boiling Points, Permeances, and Activation Energies for Pure Compounds (Organic Feed Concentration ∼ 20 mol %, He as Carrier)
compound n-pentane n-hexane n-heptane n-octane n-nonane benzene toluene cyclohexane methylcyclohexane 3-methylpentane 2,2-dimethylbutane i-octane
permeance × 109 (mol/(m2 s Pa)) activation kinetic T T energy diameter Tboil (K) ∼ 363 K ∼ 408 K (kJ/mol) (nm) 0.43 0.43 0.43 0.43 0.43 0.59 0.59 0.60 0.60 0.50 0.62 0.62
309 342 371 399 424 353 384 354 374 333 343 372
531 178 170 63 14 161 189 118 164 200 136 110
1140 269 154 107 42 223 227 193 194 227 139 201
11.5 ( 4.9 14.0 ( 2.0 n/a 21.4 ( 2.5 46.0 ( 0.9 3.2 (0.7 5.0 ( 0.4 7.1 ( 2.2 5.2 ( 0.5 3.5 ( 0.9 13.3 ( 9.2 12.1 ( 3
Membrane Stability. The separation performance of the membranes remains stable for several weeks of experimental runs, but occasionally large drops in fluxes and a total loss in selectivity were observed, especially after changing the feed compounds. Coke formation or strongly adsorbed impurities that block the membrane pores are suspected, but no systematic trends were observed. Heating the membrane above 673 K in an air stream overnight restored the original properties. Results and Discussion Single-Component Feeds. The permeances of pure n-alkanes monotonically decrease with increasing chain length; as shown in Table 1, n-pentane permeated 38 times faster than n-nonane at 363 K. Because of their similar molecular structures, n-alkanes are likely to occupy similar adsorption sites, and because the initial heats of adsorption increase with alkane chain length (Thamm et al., 1984), the longer chains might be expected to be less mobile, as is observed. Additionally, the longer, flexible hydrocarbon chains might be statistically more hindered while entering the zeolite pores. The kinetic diameters of all the other molecules studied are larger than the alkanes, but their permeances, shown in Table 1, are similar to those of n-hexane or n-heptane. That is, a larger kinetic diameter did not result in a slower permeance; only npentane had a significantly higher permeance than the other molecules studied. The shapes of the aromatics and the branched alkanes are significantly different, but, as shown in Table 1, their permeances differ by less than a factor of 2 at 363 and 408 K. The addition of a methyl group to a cyclic compound (benzene, cyclohexane), either increased the permeance (363 K) or had no effect (408 K). In contrast, adding a methyl group to pentane decreased the permeance by a factor of 2.5-5, but the permeance of 3-methylpentane was similar to that of the corresponding linear paraffin. The bulkier branched and cyclic molecules are expected to permeate more slowly than the linear molecules in the zeolite pores, but differences in adsorption behavior might compensate for the size differences if less strongly adsorbed compounds diffuse faster. The larger molecules can occupy different adsorption sites from the linear hydrocarbons in the geometrically restricted zeolite channels (June et al., 1990), resulting in different heats of adsorption and mobilities. These trends cannot be quantified since heats of adsorption (Thamm, 1987) and diffusivities (Ruthven and Ka¨rger, 1992) in zeolites are strong functions of the amount
adsorbed, especially when the molecular size approaches the pore size as it does for the cyclic and branched compounds. Moreover, the concentration of the permeating species in the membrane layer decreases from the feed side the to permeate side, and the adsorption strength and mobility across the membrane are expected to change accordingly. The high permeances of molecules that are of similar size to the zeolite pores may result because: (i) the effective pore size is larger in the presence of adsorbed compounds than that determined by X-ray diffraction (Wu et al., 1986), (ii) pore openings increase with temperature, or (iii) a significant fraction of the total flux is through pores that are in parallel with the zeolite pores. These pores could be the regions between zeolite crystals. The intercrystalline pores cannot be much larger than the zeolite pores since high ideal selectivities are obtained for N2/SF6 at room temperature, but at elevated temperatures these pathways might become more important. Activation energies for permeation were calculated from measurements at four or more temperatures and are listed in Table 1. These values are the same magnitude as diffusion activation energies in crystals (Ka¨rger and Ruthven, 1992), but they don’t correlate with molecular size, except for a monotonic increase with carbon chain length for four of the linear alkanes. Smaller molecules usually have lower diffusion activation energies in crystals (Ka¨rger and Ruthven, 1992). The temperature dependence for the permeance of heptane could not be fit by an Arrhenius expression, and instead it had a minimum near 400 K (Funke et al., 1996a). Other transport pathways, such as the intercrystalline regions, might contribute to this permeation behavior. Separations of Mixed Feeds. The permeation of eleven binary mixtures and one ternary mixture of the hydrocarbons listed in Table 1 were measured at total organic feed concentrations between 10 and 20 mol %. Three types of mixtures were used: (1) alkane/aromatic mixtures, where the alkane chain length was varied in mixtures with benzene, (2) an alkane (n-hexane) mixed with cyclic and branched molecules, and (3) mixtures of cyclic or branched molecules. The ternary mixture of n-heptane/benzene/i-octane was studied to determine if benzene and i-octane would both be affected by the presence of n-heptane. Alkane/Benzene Mixtures. As shown in Table 1, the permeances of the pure n-alkanes decreased significantly as the carbon number increased. Similar behavior was observed for alkanes in alkane/benzene mixtures (Figure 1). The n-alkane permeances in the mixtures (solid line in Figure 1) were almost identical to those for the pure components (dashed line in Figure 1). The only exception was n-pentane, whose permeance in the mixture was only 2/3 of its pure component permeance. In contrast, Figure 1 shows that the permeances of benzene in mixtures with n-alkanes were much lower than the pure component values. The most dramatic effect was observed for mixtures with nhexane; the benzene permeance decreased by a factor of 180, whereas the n-hexane permeance was almost unchanged from its pure component value. The benzene permeance decreased in all the mixtures with n-alkanes but by smaller amounts than for n-hexane (a factor of 15 for n-pentane, 58 for n-heptane, 3 for n-octane, and 11 for n-nonane). As a result, the largest separations selectivity was obtained for n-hexane/benzene mixtures,
140 Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997
Figure 1. Permeances of pure compounds and binary n-alkane/ benzene mixtures at 363 K (feed: 50 mol % n-alkane, 50 mol % benzene, organic feed concentration is 20 mol % in He).
Figure 2. Pure compound permeance ratios and mixed feed selectivities of binary n-alkane/benzene mixtures at 363 K (Feed: 50 mol % n-alkane, 50 mol % benzene, organic feed concentration is 20 mol % in He).
as shown in Figure 2. Whereas the separation selectivity is 219 for n-hexane/benzene at 363 K, the pure compound permeance ratio (ideal selectivity) at the same temperature is only 1.1. Permeances of benzene are similar to those of n-octane or n-nonane in binary mixtures, and thus the selectivities for these mixtures are close to 1. The ideal selectivities for n-octane/ benzene (0.4) and n-nonane/benzene (0.09) are smaller than 1, however, because pure benzene permeates faster than these n-alkanes. The n-alkanes selectively permeate apparently because they inhibit benzene from entering the membrane pores. The longer alkanes are less effective at blocking benzene, and thus the C8/benzene and C9/benzene selectivities are not as high as selectivities for lower alkanes, but they are higher than the ideal selectivities. That is, all the alkanes decrease the benzene permeance, as shown in Figure 1. A combination of increasing heats of adsorption with increasing chain length and increasing entropic effects that hinder the longer chains from entering and blocking the pores might cause these trends. Smit and Measen (1995) reported that n-hexane and n-heptane have significantly different adsorption isotherms from shorter or longer chains. They attributed this behavior to a particular dense and rigid packing of the hydrocarbon chains. This packing might enable n-hexane to efficiently inhibit benzene permeation. Heptane/Toluene Mixtures. Similar to the results for the alkane/benzene mixtures, n-heptane permeated
Figure 3. Permeances of pure compounds and binary n-heptane/ toluene mixtures as a function of temperature (50 mol % nheptane, 50 mol % toluene, organic feed concentration is 18 mol % in He).
Figure 4. Pure compound permeance ratios and mixed feed selectivities of binary n-heptane/toluene mixtures as a function of temperature (50 mol % n-heptane, 50 mol % toluene, organic feed concentration is 18 mol % in He).
faster than toluene over the entire temperature range studied (Figure 3) even though pure toluene permeated faster than pure heptane. That is, n-heptane blocked toluene from entering the pores; the toluene permeance in the presence of n-heptane was a factor of 54 lower than pure toluene at 363 K. As a result the separations selectivity was 40 at 363 K, but it decreased significantly as the temperature increased, as shown in Figure 4. The decrease in selectivity was due to both an increase in the toluene permeance with temperature and a decrease in the n-heptane permeance with temperature, as seen in Figure 3. These high selectivities are in contract to the results reported by Ko¨lsch et al. (1994) for a silicalite membrane fabricated by embedding silicalite crystals into a metal matrix. For binary vapor mixtures at steady state, toluene permeated approximately three to five times faster than n-heptane through their membrane at feed conditions similar to ours. Their thicker membrane (30 µm) only reached steady state after several days, and initially heptane was the faster permeating compound. The crystals in the metal-embedded membrane are aligned and diffusion is only possible along one of their axes, whereas the crystals in our membranes are randomly oriented and molecules can diffuse through pores in three dimensions. Differences in permeation behavior might also be due to influence of the support structure or diffusion through intercrystalline pores.
Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 141 Table 2. Permeances of a Binary n-Hexane/2,2-Dimethylbutane Mixture as a Function of Temperature (50 mol % n-hexane, 50 mol % 2,2-Dimethylbutane, Organic Feed Concentration ∼ 18 mol %, He as Carrier); Pure Compound Results in Parentheses
Table 3. Permeances of a Binary n-Hexane/ 3-Methylpentane Mixture as a Function of Temperature (50 mol % n-Hexane, 50 mol % 3-Methylpentane, Organic Feed Concentration ∼ 18 mol %, He as Carrier); Pure Compound Results in Parentheses
permeance × 109 (mol/(m2 s Pa))
permeance × 109 (mol/(m2 s Pa))
temp (K)
n-hexane
2,2-dimethylbutane
selectivity
temp (K)
n-hexane
3-methylpentane
selectivity
363 370 407 448
184 (178) 191 210 (269) 190
8.2 (136) 9.1 99.8 (139) 170
22.5 (1.3) 21.1 2.1 (1.9) 1.1
362 389 405 443
46 (179) 89 147 (269) 208
1.9 (136) 47 123 (139) 196
24 (1.3) 1.9 1.2 (1.9) 1.1
Mixtures of Aliphatic C6-Hydrocarbons. Mixtures of branched or cyclic hexanes with linear n-hexane separate with high selectivities for n-hexane at lower temperatures. The high separation selectivities resulted because the permeances of the branched or cyclic compounds dramatically decrease in the presence of a linear alkane. This is the same behavior observed for the n-alkane/aromatic mixtures. As also observed with the aromatics, in all cases separation selectivities decrease at higher temperatures due to strongly increasing permeances of the branched and cyclic molecules, whereas the n-alkane permeance changes only slightly in this temperature range. Table 2 shows the permeances and selectivities obtained for mixtures of n-hexane and one of its isomers, 2,2-dimethylbutane. In this and subsequent tables, pure component permeances are indicated in parentheses in the tables. Note that at 363 K the 2,2-dimethylbutane permeance in the mixture is a factor of 17 lower than the pure component permeance, and this results in a separation factor of 22.5. The n-hexane permeances in the mixture are close to the pure compound permeances at 363 K but deviate more at higher temperatures, whereas the permeance of the 2,2-dimethylbutane increases significantly with temperature and approaches the pure component permeances. Our results for tubular membranes are similar to observations by Tsikoyiannis and Haag (1992), who obtained a separation factor of 17.2 for the same mixture at 422 K with an unsupported, silicalite membrane prepared as a flat disk. Vroon (1995) reported selectivities over 2000 at 473 K with an R-alumina-supported silicalite membrane, which was prepared on a disk and apparently had less defects or intercrystalline pores. The fluxes of hexane through Vroon’s and Tsikoyiannis and Haag’s membranes were approximately 1/5 of the fluxes through our membrane. The fluxes of 2,2dimethylbutane through Vroon’s membrane were close to his detection limit. Apparently the dense separation layer of our membrane is thinner or the diffusion pathway is less tortuous, and this is partially responsible for the lower selectivity than that seen by Vroon. In addition to the difference in membrane thickness, Vroon’s membranes differ in two other ways from ours. First, the preparation procedures for zeolite membranes on the inside of tubes are different from those used for preparing membranes on flat disks, and second, the pore sizes of the supports are different. It is also interesting that Vroon reported pure gas permeance ratios for H2 and SF6 of approximately 10, whereas pure H2 permeates more than 200 times faster than SF6 through our membranes. These numbers indicate that compared to our membranes, Vroon had a larger number of defects that were permeation pathways for the large SF6 molecules (kinetic diameter ∼ 0.55 nm), even though he was able to separate some hydrocarbon mixtures with very high selectivities. Pure gas permeances
Table 4. Permeances of a Binary n-Hexane/Cyclohexane Mixture as a Function of Temperature (50 mol % n-Hexane, 50 mol % Cyclohexane, Organic Feed Concentration ∼ 18 mol %, He as Carrier); Pure Compound Data in Parentheses permeance × 109 (mol/(m2 s Pa)) temp (K)
n-hexane
cyclohexane
selectivity
369 386 389 413 443
80 (178) 102 86 119 (269) 177
1.4 (118) 2.1 2.6 62 (193) 150
55 (1.5) 49 33 1.9 (1.4) 1.2
Table 5. Permeance of a Binary 2,2-Dimethylbutane/ 3-Methylpentane Mixture (50 mol % 2,2-Dimethylbutane, 50 mol % 3-Methylpentane, Organic Feed Concentration ∼ 14 mol %); Pure Compound Data in Parentheses permeance × 109 (mol/(m2 s Pa)) temp (K)
2,2-dimethylbutane
3-methylpentane
selectivity
362 443
163 (136) 276
163 (200) 276
1.0 (0.7) 1.0
apparently cannot be used to predict if a membrane can separate hydrocarbon mixtures. Similar separation selectivities were obtained when n-hexane was separated from another of its isomers, 3-methylpentane. As shown in Table 3, the n-hexane permeance decreased by approximately a factor of 4 in mixtures with 3-methylpentane, but the 3-methylpentane permeance decreased a factor of 71 from its pure compound permeance. Thus a separation selectivity of 24 was obtained at 362 K, but as the temperature increased the 3-methylpentane permeance increased by a factor of ∼24 over a temperature range of only 25 K whereas the n-hexane permeance increased by a factor of only 2. Therefore selectivities rapidly decreased close to 1. Apparently 3-methylpentane, which has only one branch and thus has a structure close to n-hexane, hinders the permeation of n-hexane even though its permeation is hindered by n-hexane. Mixtures of n-hexane with cyclohexane (Table 4) exhibited the same behavior as the other C6 mixtures, and highest selectivity was 55, which was obtained at 369 K The n-hexane permeance was approximately a factor of 2 lower and the cyclohexane permeance was a factor of 84 lower in the mixture at 369 K. Increasing the temperature strongly decreased the selectivity so that almost no separations were seen at 443 K. Mixtures of branched hexane isomers showed no separations, however, as shown in Table 5. Over the temperature range studied the separation selectivity was 1.0, as the 2,2-dimethylbutane permeance was higher and the 3-methylpentane permeance was lower in the mixture compared to the pure compounds. Ternary Mixture. One ternary mixture of benzene, i-octane, and n-heptane was studied to investigate the effect of a third compound. As shown in Table 6, the mixture behaved similarly to the binary systems; n-
142 Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 Table 6. Permeance of a Ternary n-Heptane/i-Octane/ Benzene Mixture (40% n-C7/40% i-C8/20% Benzene, Organic Concentration ∼ 20 mol %, He Carrier Gas); Pure Compound Data in Parentheses 9 2 selectivity temp permeance × 10 (mol/(m s Pa)) (K) benzene n-heptane i-octane n-C7/i-C8 n-C7/benzene
359 364
2.5 4.7 (161)
170 162 (170)
2.4 4.8 (110)
72 34 (1.6)
68 35 (1.1)
Table 7. Permeance of a Binary i-Octane/Benzene Mixture (50 mol % i-Octane, 50 mol % Benzene, Organic Feed Concentration ∼ 18 mol %, He as Carrier); Pure Compound Data in Parentheses permeance × 109 (mol/(m2 s Pa)) temp (K)
benzene
i-octane
selectivity
360 371
146 (161) 178
134 (110) 161
1.1 (1.5) 1.1
Table 8. Permeance of a Binary i-Octane/ Methylcyclohexane Mixture (50 mol % i-Octane, 50 mol % Methylcyclohexane, Organic Feed Concentration ∼ 14 mol %); Pure Compound Data in Parentheses permeance × 109 (mol/(m2 s Pa)) temp (K)
methylcyclohexane
i-octane
selectivity
367 506
120 (164) 189
112 (110) 178
1.1 (1.5) 1.1
heptane permeated at almost the same rate in the mixture as for the pure compound, whereas the permeances of the two larger compounds decreased dramatically in the mixture. The benzene permeance was a factor of 34 lower and the i-octane permeance was a factor of 23 lower in the mixture at 364 K. Thus, high selectivities were obtained for n-heptane over i-octane (72 at 359 K) and benzene (68 at 359 K), but selectivities dropped a factor of 2 when the temperature was increased only 5 K. This behavior is similar to the results obtained with n-hexane/3-methylpentane mixture where the permeance of the slower permeating molecule increased more rapidly with temperature. Note that benzene and i-octane were not separated from each other, and instead their permeances were essentially identical. Mixtures of Larger Molecules. Binary mixtures of molecules, both of which have large kinetic diameters, showed the same behavior observed for mixtures of the branched isomers of hexane. No separation was obtained, and instead both components permeated at similar rates (Tables 7 and 8). Benzene and methylcyclohexane have similar permeation properties whether alone or mixed with i-octane. Apparently they don’t compete for adsorption sites, or single-file diffusion where the channels are not large enough for molecules to pass each other may limit transport for these molecules (e.g., Baertsch et al., 1996; Gupta et al., 1995). Transport Processes. When any of the molecules with larger kinetic diameters (Table 1) were fed as binary or ternary mixtures with linear alkanes, the permeances of the branched and cyclic compounds were significantly lower than the corresponding pure compounds at low temperatures (∼365 K). In contrast, the permeances of the linear alkanes were similar to those of the pure compounds. Thus the alkanes permeated faster than the larger molecules in most cases at low temperatures, and high separation selectivities were obtained. The high separation selectivities indicate that fluxes through larger, nonselective pathways parallel to the zeolite pores, such as gaps between crystals, are small
even at elevated temperatures. Nonzeolitic pores may make a significant contribution to the total permeance if the pores are similar in size to the silicalite pores. Note that despite their significantly larger size, permeances of the faster permeating hydrocarbons are high and only about a factor of 3 smaller than the permeance of Ar (kinetic diameter ) 0.34 nm) at 440 K (Bai et al., 1995). Since the membrane used in this study showed relatively high fluxes compared to other studies with thicker membranes, even higher selectivities and different separation behavior might be possible for thicker membranes. Indeed, as mentioned above, Vroon4 obtained very high selectivities at lower fluxes for some of the same hydrocarbon mixtures. The lower fluxes of thicker membranes, however, might offset the benefits of higher selectivity in an application. The highest separation selectivities were obtained at the lowest temperature used in this study, apparently because the adsorbed amounts are highest at the lowest temperature, and thus the more strongly adsorbed species can better block adsorption sites. We are lumping together adsorption on the external surface, pore-entering processes, and adsorption within the zeolite pores as adsorption since we cannot distinguish these processes. Lowering the temperature would likely increase the separation selectivities further with a corresponding loss in flux, but the selectivity might go through a maximum and then decrease, as observed previously (Funke et al., 1996a,b). Since the fluxes are high for many of the molecules at the lowest temperature used (363 K), lower temperatures can be used, but this was not done in the current study to avoid condensation in the flow system. The effect of organic feed concentration was not studied because previous results obtained with a mixture of n-hexane, n-octane, and i-octane (Funke et al., 1996b) indicated that the hydrocarbon permeances are independent of the feed concentration between ∼10 and 23 mol % at temperatures around 373 K. These results are surprising since the strong inhibition of permeances of larger molecules by alkanes would be expected to take place when adsorption coverages were high, and thus permeances would not increase linearly with pressure. However, a combination of preferential coverage on the external surface, pore-entering statistics, and adsorption inside the zeolite pores determines how one species inhibits another. For example, one species might exhibit an energetically favorable packing in the pores and exclude other compounds. These processes do not appear to be sufficiently understood to allow us to determine their pressure dependence. Additionally, concentration may play a role at other temperatures and for systems other than i-octane/n-octane/n-hexane mixtures. Separation might still be obtained at higher temperature if the preferentially permeating species is present in higher concentration. Summary n-Alkanes can be separated efficiently from mixtures with branched and cyclic compounds. Selectivities decreased significantly as the temperature increased so that the highest selectivities were obtained at the lowest temperature used. The n-alkanes apparently selectively block the pores and prevent the larger compounds from permeating. Molecular sieving is not responsible for separations since molecules that separate well in mixtures have similar permeances when pure. Mixtures of branched and cyclic molecules do not separate be-
Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 143
cause either they have similar adsorption behavior or single-file transport prevents the faster molecules from passing the slower species in the narrow pores. Transport through intercrystalline regions might contribute to the overall transport behavior in these membranes. Acknowledgment We gratefully acknowledge support by NSF Grant EEC-9528068 and the NSF/IU Center for Separations Using Thin Films and partial support by Chevron Research and Technology Company. Literature Cited Baertsch, C. D.; Funke, H. H.; Falconer, J. L.; Noble, R. D. Permeation of aromatic hydrocarbon vapors through silicalitezeolite membranes. J. Phys. Chem. 1996, 100, 7676. Bai, C.; Jia, M.; Falconer, J. L.; Noble, R. D. Preparation and separation properties of silicalite composite membranes. J. Membr. Sci. 1995, 105, 79. Bakker, W. J. W.; Zheng, G.; Kapteijn, F.; Makkee, M.; Moulijn, J A.; Geus, E. R.; van Bekkum, H. Single and multi-component transport through metal supported MFI zeolite membranes. Precision Process Technology; Weijnen, M. P. C., Drinkenburg, A. A. H., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1993; p 425. Flanigen, E. M.; Bennett, J. M.; Grose, R. W.; Cohen, J. P.; Patton, R. L.; Kirchner, R. M.; Smith, J. V. Silicalite, a new hydrophobic crystalline silica molecular sieve. Nature 1978, 271, 512. Funke, H. H.; Argo, A. M.; Baertsch, C. D.; Falconer, J. L.; Noble, R. D. Separation of close boiling hydrocarbons with silicalite zeolite membranes. J. Chem. Soc., Faraday Trans. 1996a, 92, 2499. Funke, H. H.; Kovalchick, M. G.; Falconer, J. L.; Noble, R. D. Separation of hydrocarbon isomer vapors with silicalite zeolite membranes. Ind. Eng. Chem. Res. 1996b, 35, 1575. Gupta, V.; Nivarthi, S. S.; McCormick, A. V.; Davis, H. T. Evidence for single file diffusion in the molecular sieve AlPO4-5. Chem. Phys. Lett. 1995, 596. Jia, M.; Chen, B.; Noble, R. D.; Falconer, J. L. Ceramic-zeolite composite membranes and their application for separation of vapor/gas mixtures. J. Membr. Sci. 1994, 90, 1.
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Received for review July 31, 1996 Revised manuscript received October 7, 1996 Accepted October 8, 1996X IE960472F X Abstract published in Advance ACS Abstracts, November 15, 1996.
