Effect of Operating Conditions and Membrane Quality on the

Ind. Eng. Chem. Res. 1998, 37, 4071-4083

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SEPARATIONS Effect of Operating Conditions and Membrane Quality on the Separation Performance of Composite Silicalite-1 Membranes Jolinde M. van de Graaf,* Elbert van der Bijl, Arnoud Stol, Freek Kapteijn, and Jacob A. Moulijn Industrial Catalysis, Waterman Institute for Precision Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

The separation capacity of silicalite-1 membranes for various hydrocarbon mixtures is determined as a function of membrane quality, operating conditions, and orientation of the composite membrane with respect to the feed side. The quality of the membranes is judged on the basis of the n-butane/i-butane permselectivity. Membranes with a different n-butane/i-butane permselectivity showed an identical separation capacity for ethane/methane mixtures, but the quality difference was affecting separation of hydrogen from the butane isomers. The selectivity of the membrane is significantly affected by the operating conditions, such as mixture composition, temperature, and absolute pressure. These effects are shown for ethane/methane, propene/ethene, and n-butane/i-butane mixtures. The selectivity for ethane in ethane/methane mixtures, found when the zeolite layer is facing the feed side, is completely lost when the orientation of the composite membrane is reversed, due to concentration polarization. Depending on the membrane orientation, the major resistance of the composite is in the support layer or in the zeolite layer. Introduction Over the past decade increasing attention has been paid to the development of zeolite membranes. The molecular sieving properties of these kinds of membranes are expected to have great potential for separation and reactor applications. For the successful implementation of (zeolite) membranes in industrial processes, both the selectivity and the permeability of the membrane are important.1 Depending on the application and the desired purity, selectivities on the order of 1540 already yield substantial improvements in, for example, CO2 emission control in the water-gas shift reaction.1 For practical applications it is convenient if the selectivity of the membrane is independent of the composition of the feed since this composition changes continuously during operation. For all zeolite membranes reported so far, the selectivity of mixture permeation deviated considerably from the so-called, permselectivity or ideal selectivity, calculated from the singlecomponent permeation fluxes. This feature is typical for microporous membranes, in which molecules are significantly hindered by the presence of other components in the pores. The dominant separation mechanisms are selective sorption in the membrane and shape selectivity.2 In general, the membrane is selective toward the component that adsorbs more strongly on the zeolite, resulting in pore blocking for the weaker adsorbing * To whom correspondence should be addressed. Current address: Shell Treating Services, Shell Global Solutions, P.O. Box 38000, 1030 BN Amsterdam, The Netherlands. Phone: +31-20-630 3858. Fax: +31-(0)20-630 3079. E-mail: Jolinde. [email protected].

component. Examples are hydrogen/n-butane at temperatures up to 450 K2 and methane/n-butane.3,4 This is also found for other microporous membranes such as carbon molecular sieve membranes, where hydrocarbons are separated selectively from hydrogen/hydrocarbon mixtures.5,6 The shape selectivity of the zeolite is demonstrated by several studies, mainly applied to n-butane/i-butane mixtures2,4,7-9 and other hydrocarbon isomers.10-12 In those cases the less bulky compound was always separated from the mixture, as can be expected for microporous membranes. In the case of n-butane/i-butane separations, widely varying selectivities are obtained, ranging from 5 to 90 depending on the temperature and quality of the membrane. For a single-crystal ferrierite membrane, selectivities as high as 116 at 383 K were found,13 which can be viewed as the upper limit for ferrierite, since this is without the possibility of some permeation through intercrystalline defects. Yan et al.14 used a postsynthetic coking treatment to modify a ZSM-5 membrane and studied the single-component permeation of n-butane and i-butane. They found that the ratio between the permeances increased from 45 to 320 at 458 K and attributed this to closing of intercrystalline defects. This significant increase in the selectivity was accompanied by a large decrease in the permeation flux. This indicates that even high-quality membranes still contain some microdefects which can decrease the selectivity considerably. It cannot be excluded that part of the increase in the selectivity can be attributed to the reduction of the pore mouth of the zeolite as a result of the coke treatment. For the application of zeolite membranes in industrial processes, an accurate model for the description of

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4072 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998

multicomponent permeation through these membranes is indispensable. It is important to know the membrane performance as a function of temperature, pressure, composition, and membrane configuration. In the previously mentioned studies, attention is primarily paid to the temperature dependence of the selectivity. Surprisingly, only a little is known about its pressure and concentration dependence. The composition dependence of the selectivity of zeolite membranes is almost exclusively studied for pervaporation studies.15-18 The composition dependence of the selectivity is an important parameter in separation processes because, during the separation of a mixture into more or less pure components, the composition on the feed side of the membrane changes continuously. Assuming that the selectivity of the membrane is independent of composition will lead to an over- or underdesign of the separation unit. In this study the effect of operating conditions on the separation performance of the membrane is analyzed for several types of separation. The effect of membrane quality on the separation performance is also evaluated. Experimental Section Membrane Synthesis. The membranes used in this study were synthesized under autogenous pressure on a stainless steel support (Krebso¨ge Sika-RF). The synthesis mixture was composed of SiO2 (Aerosil 200, Degussa), TPA-OH (40% CFZ, Zaltbommel, The Netherlands), TPA-Br (CFZ, Zaltbommel, The Netherlands), and H2O. The molar ratio of these components was 100:65:65:14 000. The synthesis mixture was aged for 6 h at ambient temperature. Crystallization took place at 453 K for 15 h. After crystallization the layer was rinsed with water and ethanol and dried in air. Calcination took place in the permeation equipment at 673 K for at least 10 h (heating rate 1 K/ min). The thickness of the membranes prepared in this way was about 30-50 µm, as determined from SEM pictures. The zeolite layer was grown on an ultra high vacuum (UHV) flange with a spherical, porous stainless steel disk clamped in the middle. The zeolite layer was grown over the porous center and the nonporous periphery of the flange and thus provided the seal between porous and nonporous parts in the flange. The permeable area of the flange was between 2.0 × 10-4 and 3.1 × 10-4 m2. The support used in this study was a 3-mmthick porous stainless steel disk (porosity 0.2) with a 200-µm-thick top layer of metal wool (porosity 0.7, average pore size 7-8 µm). Before synthesis this support was cleaned with a 0.5 M KOH solution for 3 h at 453 K and overnight with water at 453 K. In some cases an extra cleaning step with toluene or ethylbenzene was employed after the KOH solution. Permeation Experiments. Permeation experiments were performed according to the Wicke-Kallenbach (WK) method. The membrane flange was placed between two holders for gas supply and removal. At the permeate side helium was used as a sweep gas. At the feed side mixtures of up to four different gases, including helium, were composed. Standard experiments were performed with flow rates of 100 mL‚min-1 (STP) on each side of the membrane. In normal operation, the zeolite layer was facing the feed side. Feed, retentate, and permeate streams were analyzed with a mass spectrometer (Ledamass quadrupole analyzer) or

a GC (Chrompack CP-9001, equipped with a PLOT fused-silica Al2O3/KCl column and a FID detector). The membrane module was placed in a ventilation oven. The pressure at each side of the membrane was controlled by means of backpressure controllers. Separation experiments were performed between 273 and 650 K at feed pressures up to 425 kPa, and the permeate pressure was kept at atmospheric (101 kPa). Feed partial pressures below 101 kPa were obtained by mixing the component under study with helium, up to 101 kPa. The effect of operating conditions on the separation of binary mixtures by the membrane was studied systematically. Three types of experiments were conducted. 1. The composition of the binary mixture was varied at constant total feed pressure and constant temperature. 2. The total hydrocarbon pressure on the feed side was raised, leaving the composition and the temperature constant. 3. The effect of temperature was determined from experiments under constant feed pressure and constant feed composition. The feed composition and pressure can be set independently in the experimental setup, but the retentate composition is dependent on the performance of the membrane. In our membrane module the flows on each side of the membrane are completely mixed. The concentrations at both sides of the membrane can be calculated from the retentate and permeate compositions. The influence of the orientation of the composite membrane on the separation performance was also investigated. In most experiments the zeolite layer was facing the feed side. However, in some experiments the membrane was turned around and the zeolite layer faced the permeate side. At the start of each set of experiments, the membrane was heated overnight to 623 K under flowing helium, to remove adsorbed components. This treatment gave reproducible separation results. Definitions. Throughout this paper, a distinction is made between the permselectivity of the membrane and the selectivity of the membrane. The permselectivity ( ), also called the ideal selectivity, is referring to Rperm ij the ratio of the single-component fluxes (Ni) at identical feed composition:

Rperm ij

)

Nsingle i Nsingle j

(1)

The selectivity of the membrane toward component i in a mixture containing i and j (Rij) is determined from the retentate and permeate compositions during mixture permeation.

Rij )

xi,p/xj,p xi,r/xj,r

(2)

In this equation xi,p and xj,p refer to the mole fractions of the components in the permeate and xi,r and xj,r refer to the mole fractions in the retentate. The overall selectivity of the membrane is given, that is, the selectivity of the zeolite layer in combination with the support, unless stated otherwise. When the selectivity

Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 4073 Table 1. Reproducibility of Silicalite-1 Membrane Synthesis Using the n-Butane/i-Butane Permselectivity at 303 K as a Standard Test membrane

n-butane flux [mmol‚m-2‚s-1]

i-butane flux [mmol‚m-2‚s-1]

Rperm ij

A1a E1a G1 H1 B2 G2 N1a M1 K2 M2 H2a G4

6.0 3.3 5.5 7.5 7.1 9.4 7.5 6.7 16.1 4.5 6.2 6.8

0.64 0.13 3.3 3.2 3.0 4.9 0.32 2.8 7.0 2.2 0.57 1.0

9.4 25.4 1.7 2.3 2.4 1.9 23.3 2.4 2.3 2.0 10.9 6.8

a These membranes were used for further separation experiments.

of the membrane and the flux of the fastest component (i) are high, the retentate is enriched in the retained component (j) and depleted in the separated component (i). The selectivity determined from the retentate mole fractions is thus higher than that obtained from the corresponding feed mole fractions. In tubular membranes the composition of the retentate changes along the length of the tube, which results in an overestimation of the selectivity based on the outlet composition of the retentate and an underestimation of the selectivity based on the feed composition. In this study a flat sheet membrane is used and the feed and sweep streams are provided through spray heads, ensuring complete mixing. This means that the concentrations in the retentate and permeate volumes are equal to the outlet composition of these streams. This justifies the use of the retentate mole fractions for the calculation of the selectivity. Results Reproducibility of Membrane Synthesis. Almost all synthesized membranes were gastight after synthesis. Only when crystallization was not complete, due to, for example, leaking of the autoclave, was a small amount of krypton permeation observed before calcination of the membrane. In those cases gel between crystals was clearly visible with a light microscope. The quality of the membranes after calcination was tested using the ratio between the single-component fluxes of n-butane and i-butane. This ratio is listed in Table 1 for a number of different membranes. A membrane was considered to be of good quality when the permselectivity was about 10 or higher. It can be seen that the synthesis method has limited reproducibility, only 4 out of the 12 membranes listed here had an acceptable quality according to our definition, viz., membrane A1, E1, N1, and H2. The n-butane fluxes obtained for different membranes are comparable (Table 1) while the i-butane fluxes varied considerably among the tested membranes. The quality of the membrane seems to be related directly to the magnitude of the i-butane flux. When this flux is higher than 1 mmol‚m-2‚s-1, the quality of the membrane is poor. Long-Term Stability. The membranes that were of good quality showed an excellent long-term stability. Membrane N1 was synthesized in Nov 1995 and still shows good separation behavior at the time of writing

Figure 1. Flux of ethane (a) and methane (b) as a function of their partial pressure on the feed side. Key: closed symbols, single component (balance helium); open symbols, binary mixture (balance other hydrocarbon). ptot ) 101 kPa. T ) 303 K. Membrane N1.

this paper. During these years the membrane was subjected to temperatures between 200 and 675 K and to pressure differences up to 500 kPa. Only brute mechanical force was found to destroy membrane performance, as was observed for membrane E1, which was accidentally scratched with a screwdriver. Single-Component vs Mixture Permeation. Parts a and b of Figure 1 show the permeation results for ethane and methane through a silicalite-1 membrane at 303 K, respectively. It can be seen that although methane is slightly faster in the unary system, its flux is significantly reduced in the presence of ethane. The flux of ethane is hardly influenced by the presence of methane. This is a result of the fact that ethane adsorbs more strongly on silicalite-1 than methane and, in the mixture, ethane blocks the pores for methane. The temperature dependence of the permeation of ethane and methane is given in Figure 2. As the temperature increases, adsorption becomes weaker and the retarding effect of ethane on methane permeation is reduced. In Figure 3 the results for the n-butane/i-butane system are given as a function of temperature. In the mixture both n-butane and i-butane are hindered by the presence of the other component. A strange dip in the

4074 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 Table 2. Separation Performance Silicalite-1 Membranes (50:50 Mixtures, 303 K) and Selectivity Range under Varying Operating Conditions

membrane A1 N1

E1 H2

Figure 2. Fluxes of methane (2) and ethane (9) as a function of temperature. Key: closed symbols, single component (pi,feed ) 50 kPa, balance helium); open symbols, binary mixture (pethane,feed ) pmethane,feed ) 50 kPa). Membrane N1.

mixture i/j

Rij

ethane/methane 5.4 n-butane/hydrogen 4.1 hydrogen/i-butane 4.1 ethane/methane 6.1 propane/methane 40.5 n-butane/hydrogen 46.5 hydrogen/i-butane 1.6 propene/ethene 7.0 trans-2-butene/ 1.6 cis-2-butene trans-2-butene/propene 5.2 n-butane/i-butane 23.3 n-butane/neopentanea 102 ethane/methane 10.4 ethane/ethene 1.5 propene/ethene 6.4 trans-2-butene/ 1.5 cis-2-butene trans-2-butene/propeneb 5.0

selectivity range (200 < T < 675 K, 5 < pi < 450 kPa) 1.9-6.2 0.3-5.9 4.1-6.2 0.8-11.0 16.6-40.5 0.3-112 1.6-8.7 7.0-1.0 1.6-1.9 1.9-5.5 1.3-49.2 3.8-200 2.7-10.4 1.3-1.7 1.7-6.5 1.5-1.9 4.6-5.0

a

b

Mixture of 50% n-butane and 7% neopentane (balance helium). Mixture of 75% trans-2-butene and 25% propene.

Table 3. Comparison of Membrane Selectivity for Each Binary Pair in Ternary Mixtures to Membrane Selectivity in Binary Mixtures (303 K) Rij membrane

mixture i/j

N1

n-butane (30%)/i-butane (30%)/ neopentane (6%) n-butane/neopentane n-butane/i-butane i-butane/neopentane trans-2-butene (17.5%)/ propene (65%)/ethene (17.5%) trans-2-butene/propene trans-2-butene/ethene propene/ethene

H2

Figure 3. Fluxes of n-butane (2) and i-butane (9) as a function of temperature. Key: closed symbols, single component (pi,feed ) 50 kPa, balance helium); open symbols, binary mixture (pn-butane,feed ) pi-butane,feed ) 50 kPa). Membrane N1.

n-butane flux is observed around 450 K, just after i-butane starts to permeate through the membrane. From experiments with mixtures of different compositions, it was found that while for a 75/25 n-butane/ibutane mixture the dip was absent, it became increasingly significant with increasing i-butane content. At high temperature the fluxes of both components in the mixture approach their single-component fluxes. Separation Performance. Table 2 lists some typical separation results obtained for different membranes. The separation selectivity of a 50:50 mixture of the components at 303 K is listed, but it should be noticed that the selectivity is highly dependent on operating conditions and membrane orientation as will be shown later. In some cases the selectivity of the membrane is reversed at higher temperature. For example, the membrane preferentially separates n-butane from hydrogen at low temperature, whereas at higher temperature hydrogen is selectively removed from the mixture. The range of the obtained selectivities found for different operating conditions such as temperature and pressure is also given in Table 2.

a

ternary binary mixture mixture

43.2 53.0 0.8

102a 30.4 n.a.

5.1 26.6 5.3

5.0 n.a. 6.4

Mixture of 50% n-butane and 7% neopentane (balance helium).

High separation selectivities are obtained for mixtures of normal and branched alkanes, such as n-butane/ i-butane and n-butane/neopentane, and for mixtures of components that have distinctly differing adsorption characteristics, for example, n-butane/H2. The selectivity of the membranes for each binary pair in ternary mixtures is compared to the separation performance of the membranes in a binary mixture at similar concentration ratios between the components in Table 3. The n-butane/i-butane selectivity increases in the presence of neopentane, while the n-butane/neopentane selectivity decreases in the presence of i-butane. The separation performance of the silicalite-1 membrane for each binary pair in a ternary mixture composed of trans-2-butene/propene/ethene is similar to the selectivity found in binary mixtures. Figure 4 shows the selectivity toward ethane in the separation of methane/ethane mixtures as a function of composition for different membranes. Although membrane A1 is expected to have a lower quality on the basis of the n-butane/i-butane permselectivity, the selectivity toward ethane is the same as that of membrane N1. Membrane E1 exhibits a slightly higher selectivity for this system, but the composition-dependent behavior is the same.

Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 4075

Figure 4. Selectivity toward ethane for ethane/methane mixtures with different compositions (pmethane ) 101 kPa - pethane, ptot ) 101 kPa, 303 K). Key: 9, N1; 2, E1, [, A1.

Figure 5. Selectivity toward n-butane for an equimolar mixture of n-butane and hydrogen as a function of temperature (ptot ) 101 kPa). Key: 9, N1; [, A1.

The n-butane/H2 selectivities of membranes A1 and N1 are compared in Figure 5. For this system an order of magnitude difference is observed between the maximum selectivity achieved as a function of temperature for the different membranes. Membrane A1, with the lowest n-butane/i-butane permselectivity also has the lowest n-butane/H2 selectivity. The difference in quality between membranes A1 and N1 is also reflected in the separation of a H2/i-butane mixture. Figure 6a shows the development of the fluxes of hydrogen and i-butane as a function of time after supply of the feed to the membrane. Just after exposure of the feed to the membrane, hydrogen starts to permeate. The flux of hydrogen as a function of time passes through a maximum, and it starts to decrease at the point where i-butane starts to permeate. The steadystate fluxes of both i-butane and hydrogen are much lower for membrane N1. Moreover, the hydrogen flux is more efficiently blocked by i-butane in membrane N1. The selectivity of both membranes toward hydrogen is plotted as a function of temperature in Figure 6b. At low temperature the efficient blocking of the zeolite pores by i-butane results in a lower selectivity of membrane N1 compared to that of membrane A1.

Figure 6. (a) Breakthrough of an equimolar hydrogen/i-butane mixture through two different membranes (ptot ) 101 kPa, 303 K). Open symbols: N1. Closed symbols: A1. Key: 9, hydrogen; [, i-butane. (b) Selectivity toward hydrogen of different membranes for an equimolar mixture of i-butane and hydrogen as a function of temperature (ptot ) 101 kPa). Key: 9, N1; 2, A1. Table 4. Comparison between Permselectivity and Selectivity at 303 K and 473 K 303 K membrane

components i/j

Rperm ij

N1 H2 N1 N1 A1 N1

ethane/methane propene/ethene n-butane/hydrogen hydrogen/i-butane hydrogen/i-butane n-butane/i-butane

1.0 0.5 0.3 72 37 27

473 K

Rij

Rperm ij

Rij

7.0 7.1 47 1.6 4.2 21

1.2 0.9a,b 0.6 9.4 9.6 4.1

1.5 2.9a,b 1.1 6.9 5.2 4.5

a Propene/ethene data at 373 K. b interpolated between 40:60 propene/ethene and 60:40 propene/ethene.

However, at high temperature the selectivity of membrane N1 exceeds that of membrane A1. Separation as a Function of Operating Conditions. In Figures 1-3 it has been shown that the fluxes of components in a mixture can be altered significantly by the presence of the other component. In Table 4 the permselectivity obtained from the comparison of singlecomponent fluxes is compared to the selectivity determined from mixture permeation. At low temperature real selectivities can deviate considerably from permselectivities, especially when separation is governed by


Figure 7. Selectivity toward ethane (membrane N1) for ethane/methane mixtures as a function of (a) composition (pmethane ) 101 kPa -pethane, ptot ) 101 kPa), (b) temperature (equimolar mixture, ptot ) 101 kPa), and (c) total hydrocarbon pressure (equimolar mixtures). Key: 9, 303 K; [, 340 K; 2, 373 K.

Figure 8. Selectivity toward propene (membrane H2) for propene/ ethene mixtures as a function of composition (pethene ) 101 kPa ppropene, ptot ) 101 kPa). Key: 9, 303 K; 2, 325 K; b, 345 K; [, 373 K.

selective sorption. At higher temperatures these selectivities approach each other. Factors such as feed composition, temperature, and absolute pressure influence the separation performance of the membrane, as will be shown in the next paragraphs. Composition. Figure 7a gives the composition dependence of the selectivity toward ethane for ethane/ methane mixtures. It can be seen that the selectivity increases when the content of ethane, the component that preferentially permeated, increases. Similar results have been obtained for propene/ethene mixtures as a function of composition. In these systems the selectivity increases with increasing propene content in the feed (Figure 8). The composition-dependent n-butane/i-butane selectivity is shown in Figure 9a. Again it is observed that the selectivity increases with an increase in the content of the component that has the highest flux in the mixture. In this case this is the less bulky n-butane. Temperature. With increasing temperature the selectivity for ethane in 50:50 ethane/methane mixtures decreases and even reverses. The ethane/methane selectivity starts at 10.8 (273 K) in favor of ethane, while at 620 K it is 1.2 in favor of methane (Figure 7b). Apparently, the separation mechanism changes from adsorption controlled to mobility controlled. The same

decreasing trend in the selectivity with an increase in temperature was found for the separation of propene/ ethene mixtures (Figure 8). The selectivity of a 50:50 n-butane/i-butane mixture goes through a maximum with temperature. At 377 K the selectivity is the highest, 33.4 toward n-butane (Figure 9b). At the highest temperature of the experiment, the selectivity is close to unity. For other mixtures, for example, trans-2-butene/cis-2-butene and n-butane/H2 (Figure 5), a maximum in the selectivity with temperature was also observed. Pressure. The selectivity for an equimolar mixture of ethane/methane was determined as a function of the total hydrocarbon pressure on the feed side. Selectivities around 7 were obtained at 303 K (Figure 7c). A slight maximum in the selectivity was observed around 200 kPa. This maximum disappears as the temperature increases. At higher temperature, the selectivity becomes lower. For equimolar mixtures of n-butane/i-butane, a maximum in the selectivity is observed as a function of the total hydrocarbon pressure on the feed side (Figure 9c). For this system selectivities as high as 52 are obtained, depending on the gas-phase composition. The maximum in the selectivity shifts to higher pressure as the temperature increases. Experimental Configuration. The separation of ethane and methane mixtures at 303 K was performed in two experimental configurations. In most of experiments the zeolite layer was facing the feed side and the support layer was facing the permeate side. The separation results obtained in this way are compared with the results obtained when the composite membrane was reversed; e.g., the zeolite layer was facing the permeate side (WK perm), in Figure 10. Surprisingly, the selectivity of the membrane was lost in this case. Discussion The separation performance of the membranes used in this study can be compared with the essential features of adsorption and diffusion in zeolites. Uptake of mixtures of components in zeolites often shows a maximum as a function of time for the fast and weakly adsorbing component, while under steady-state conditions (t ) ∞), the slow, strongly adsorbing component is dominating the adsorbed phase. These characteristics are observed for, for example, uptake of N2/CH4 mixtures in zeolite 4A19 and uptake of n-heptane/benzene


Figure 9. Selectivity toward n-butane (membrane N1) for n-butane/i-butane mixtures as a function of (a) composition (pi-butane ) 101 kPa - pn-butane, ptot ) 101 kPa), (b) temperature (equimolar mixture, ptot ) 101 kPa), and (c) total hydrocarbon pressure (equimolar mixtures). Key: 9, 303 K; 2, 376 K; b, 474 K.

Figure 10. Selectivity toward ethane (membrane N1) for ethane/ methane mixtures as a function of composition (pmethane ) 101 kPa - pethane, ptot ) 101 kPa, 303 K). Key: 9, zeolite layer facing feed side, [, zeolite layer facing permeated side (closed symbols, calculated overall performance of the membrane; open symbols, performance of the zeolite layer, corrected for local boundary conditions).

mixtures in NaX zeolite.20 The transient permeation results presented for the membranes used in this study (Figure 6a) exhibit the same behavior. A maximum in the permeation flux as a function of time is measured for the fast, weakly adsorbing component, indicating a significant blocking of this component as soon as the slow, strongly adsorbing component starts to permeate. Diffusion in molecular sieves such as zeolites is a strong function of the size and shape of the molecules.21 When the kinetic diameter of the molecules approaches the pore aperture of the zeolite, diffusion becomes very slow. This difference in mobility between components of different size is also observed for the membranes in this study; see, for example, the separation of n-butane and i-butane (Figure 3). It can be concluded that the performance of the membranes can be attributed to micropore diffusion. Reproducibility of Membrane Synthesis and Long-Term Stability. The reproducibility of the singlestep membrane synthesis procedure is poor. About 80% of all attempts to synthesize a membrane failed according to our selection criterion, without an obvious cause. Clearly, the synthesis procedure is not under control.

Also, the quality of the support may vary among the synthesized membranes, since this is a commercial filter material that differs from batch to batch. Several studies in the literature report multiple synthesis steps before good-quality membranes are obtained.4,8 Coronas et al. repeated crystallization two or three times, until the membrane was impermeable for nitrogen before calcination.8 In this study all membranes were gastight before calcination, and the origin of low n-butane to i-butane permselectivities must be found in defects that are opened up or formed upon calcination. Lack of reproducibility is a general problem and more knowledge about controlled nucleation and growth of continuous zeolite films is required.22,23 The C4 isomers both are strongly adsorbing components24 and have the same molecular weight. It is expected that their permeation behavior through defects, when it occurs by surface or Knudsen diffusion, is similar. Only for pores in the microporous range can a significant difference in permeation behavior of nbutane and i-butane be expected due to the difference in kinetic diameter between the molecules 0.43 and 0.5 nm, respectively. The ratio between the fluxes of n-butane and i-butane is therefore a good quality criterion for the membrane. In literature, the permselectivity of a specific pair of components is usually taken as a quality criterion for zeolite membranes. Noble and co-workers used the flux ratio of N2 and SF6 as a quality criterion.8,10-12,18 Others used the ratio between n-butane and i-butane permeation fluxes.2,9 Striking differences are observed upon comparison of literature data. Coronas et al.8 reported a N2/SF6 permselectivity of 138 for a ZSM-5 membrane at 303 K and a n-butane to i-butane flux ratio of 14. These data were measured with a pressure drop method, without the use of a sweep gas, and the permeate pressure was atmospheric (at Boulder, CO, ∼82 kPa). Kusakabe9 reported an opposite result: the N2/SF6 permselectivity was 3.3, and the n-butane/i-butane permselectivity was 10. Bakker et al. also found a higher n-butane/i-butane permselectivity (∼60) than the N2/SF6 permselectivity (∼1.6).25 In the latter two studies the Wicke-Kallenbach method was used, employing a sweep gas to keep the permeate partial pressure low. Parts a and b of Figure 11 show the effect of the permeate conditions on the n-butane/i-butane permselectivity and the N2/SF6 permselectivity, respectively. In these calculations the feed pressure was kept at 100 kPa and the permeate pressure was increased from 0


Figure 11. Model calculation of the effect of permeate conditions on single-component permeation fluxes and permselectivity. Feed pressure was 100 kPa, and permeate pressure increased from 0 to 100 kPa, 303 K: (a) n-butane and i-butane and (b) nitrogen and SF6. Flux calculations are based on eq 3, permselectivity is calculated from eq 1. Parameters used: n-butane Di ) 1.3 × 10-11 m2‚s-1 (this work), K1 ) 16.9 kPa-1,24 qsat,1 ) 1.2 mmol‚g-1,24 K2 ) 0.2 kPa-1,24 qsat,2 ) 0.2 mmol‚g-1,24 i-butane Di ) 0.11 × 10-11 m2‚s-1 (this work), K1 ) 13.0 kPa-1,24 qsat,1 ) 0.6 mmol‚g-1,24 K2 ) 0.02 kPa-1,24 qsat,2 ) 0.9 mmol‚g-1;24 nitrogen Di ) 146 × 10-11 m2‚s-1,25 K1 ) 0.0008 kPa-1,27 qsat,1 ) 2.4 mmol‚g-1;27 SF6 Di ) 0.8 × 10-11 m2‚s-1,25 K1 ) 0.11 kPa-1,28 qsat,1 ) 2.0 mmol‚g-1.28

to 100 kPa. A permeate pressure of 0 kPa is representative of the Wicke-Kallenbach method, using a sweep gas, while a high permeate pressure is comparable to the pressure drop method. It is intended to show the qualitative difference between the two experimental methods in the determination of the permselectivity of a membrane for a given pair of components. The calculations are made for an isolated zeolite layer, without the interference of support effects. For the flux calculations the well-known Darken equation (eq 3) was used to describe surface diffusion in the zeolite layer.26

Nsi ) Fqsat,iDi∇ ln(1 - θi)

(3)

The diffusivities for n-butane and i-butane were obtained from the permeation data presented in this work, using the adsorption parameters of Zhu et al.24 Adsorption of these components is best described with the double Langmuir model, with two locations for adsorption in the zeolite. Adsorption on each site is described with an equilibrium constant (K1 and K2, respectively) and a saturation capacity (qsat,1 and qsat,2). The diffusivities of nitrogen and SF6 in a silicalite-1 membrane were taken from the literature,25 as were the Langmuir parameters for these components.27,28 The values of all parameters are given in the legend of Figure 11. The predicted n-butane/i-butane permselectivity decreases with an increase in the permeate pressure (Figure 11a), whereas the N2/SF6 permselectivity increases as the permeate pressure increases (Figure 11b). Permeation is significantly retarded by the permeate concentration; all fluxes decrease as the permeate pressure increases. For strongly adsorbing components even low partial pressures in the permeate yield high adsorbed phase concentrations in the zeolite. Strongly adsorbing components are therefore more affected by nonzero permeate concentrations than weakly adsorbing components. In the n-butane/i-butane system the fastest component is also the strongest adsorbing component. Increasing the permeate pressure reduces the n-butane flux more than the i-butane flux, so the permselectivity decreases with permeate pressure. For

N2/SF6 the opposite is true: the slowest component adsorbs the strongest. Increasing the permeate pressure reduces the flux of SF6 more than it reduces the nitrogen flux. In this case high permeate pressures enhance the permselectivity. This explains why opposite trends are found in the literature for n-butane/ i-butane permselectivities and N2/SF6 permselectivities, depending on the experimental method used. The synthesized membranes have an excellent stability, judged from their performance over long periods of time and exposure to a wide variety of conditions. Separation Performance and Characteristics. Two factors determine the separation capacity of the membrane, which are differences in affinity and differences in shape. From examples given Figures 1-3 and Table 4, it is clear that mixture permeation is quite different from the permeation of single components at low temperatures. At 303 K the lower mobility of ethane in the zeolitic pores is largely compensated by its high concentration, resulting in almost identical single-component fluxes of ethane and methane through the membrane. The methane flux is significantly reduced by the presence of ethane, while the ethane flux is not influenced by the presence of methane. This is entirely dominated by the stronger adsorption of ethane. As the temperature increases, adsorption becomes weaker and the components behave more independent of each other. Shape selectivity is an important factor in the separation of isomers, with comparable adsorption characteristics, or in the case when there is a large difference between the size of the molecules. Examples are the separation of n-butane/i-butane, trans-2-butene/cis-2butene, and H2/i-butane mixtures. The dip in the n-butane flux in the presence of i-butane around 450 K is remarkable. Studies on the adsorption of large molecules in ZSM-5 revealed that when the size of the molecule becomes close to the dimensions of the zeolite channels, kinks or steps in the adsorption isotherms or isobars can be observed.24,28-31 These kinks have been attributed to the preferential packing of these molecules in the zeolite.24,31 The bulky i-butane molecules preferentially reside in the channel


intersections. Adsorption in the channels of the zeolite is possible, but it is accompanied by a loss in adsorption entropy.24 This explains the kink in the adsorption isotherms and isobars. It is possible that considerations such as preferential packing and selective blocking of diffusion pathways in the zeolite are also responsible for the dip in the n-butane flux as a function of temperature. Data on mixture adsorption are needed to unravel these phenomena further. In many cases the adsorption and shape selectivity counteract each other. A good example is the separation of H2/i-butane mixtures. Hydrogen is the smaller molecule that has a higher mobility in the zeolite. However, in a mixture i-butane blocks the pores for hydrogen permeation, leading to a very low selectivity toward hydrogen. This is counterintuitive to what could be expected based on the difference in the molecular diameter between those molecules, 0.27 nm for hydrogen and 0.5 nm for i-butane, only. In some cases, for example, n-butane/i-butane separation, mobility and adsorption difference cooperate: n-butane has the highest mobility and the highest adsorbability. Upon comparison of the membrane performance in binary and ternary mixtures, it can be concluded that the qualitative separation capacity of the membrane is not altered by the presence of a third component. However, for shape-selective separations, it was found that the third component in a mixture can influence the absolute value for the selectivity of each binary pair. Funke et al.10 also found that the separation selectivity of a silicalite-1 membrane for an n-heptane/i-octane mixture changed by the presence of n-hexane. For adsorption selective separation the effect of a third component on the absolute value of the binary selectivity was negligible (Table 3). Membrane Quality vs Separation Performance. The data given in Table 1 show the lower n-butane/ibutane ratio of some of the membranes can always be attributed to high i-butane fluxes. Measurements on the diffusion of n-butane and i-butane in silicalite-1 with the zero length column (ZLC) method indicate that the diffusivity of i-butane in silicalite-1 is at least five times slower at 348 K than that of n-butane.32 Permeation of the two isomers through defects in the membrane is expected to be similar. If defects exist, they will have a much larger effect on the i-butane flux than on the n-butane flux, since the flux of i-butane through the zeolite crystals is low. A high i-butane flux is, therefore, a strong indication for the poor quality of the membrane. Membrane A1, which has a n-butane/i-butane permselectivity of 9.4, possesses the same ethane/methane selectivity as membrane N1, which has an average n-butane/i-butane permselectivity of 23 (Figure 4). Apparently, the separation capacity of the membrane for ethane/methane mixtures is not related to the n-butane/i-butane permselectivity. Based on the ibutane flux through membrane A1, this membrane must have more or larger defects than membrane N1. Since some selectivity is still observed, it is expected that the size of the defects is not larger than a few times the diameter of the zeolitic pores. In these pores, surface diffusion of adsorbed species is prevailing. Although the blocking of these pores by ethane might be less efficient, the selectivity of these pores is still largely determined by adsorption differences, which is essentially the same as in the zeolitic pores. Obviously,

for a system where separation is dominated by differences in adsorption, the quality of the membrane is less important. Larger effects of differences in quality are observed for separation of n-butane/H2 and H2/i-butane mixtures (Figures 5 and 6). For both systems the flux of H2 is more efficiently blocked with membrane N1 (high quality) compared to membrane A1 (moderate quality) at low temperature. This is clearly observable in Figure 6a. For membrane N1 the steady-state flux of H2 is 20 times lower than its maximum flux before blocking of the zeolitic pores by i-butane starts. For membrane A1 the ratio between the maximum observed flux and the flux under steady state is only 2.5. At 303 K, the selectivity toward hydrogen for membrane N1 (1.7) is significantly lower than the value of 4.2 found for membrane A1. Hydrogen adsorbs weakly on silicalite-1, but because of its low molecular mass, it has a high mobility in nonzeolitic pores where Knudsen-type behavior is expected for this component. At low temperature, the zeolitic pores are blocked by the presence of strongly adsorbing components. Blocking of the defects by the butane isomers is apparently less effective for membrane A1 than for membrane N1. This is consistent with the lower n-butane/i-butane permselectivity of membrane A1. Less efficient blocking of the nonzeolitic pores results in a higher selectivity toward H2 in H2/i-butane mixtures for membrane A1 at low temperature. At high temperature the selectivity toward H2 in H2/i-butane mixtures of membrane N1 is higher than that of membrane A1 (8.3 and 5.9 at 500 K, respectively). At high temperature the pore blocking effect is less, due to the weaker adsorption of i-butane, and differences in mobility in the zeolitic pores are more important. Under these conditions it can be expected that the selectivity is higher when the contribution of permeation through defects is low, which is in accordance with the experimental results. For these kinds of separations a small amount of defects can have a huge impact on the separation performance of the membrane. Separation as a Function of Operating Conditions. Composition. For all membrane/mixture combinations presented in this study, it was found that the selectivity of the membrane increased with an increase in the feed content of the preferentially separated component. For ethane/methane and propene/ethene, this increase levels off at high feed concentrations, while for n-butane/i-butane, the selectivity increases almost linearly with an increase in the n-butane content in the feed. The composition dependence of the separation of the butane isomers found in this study is in good agreement with work of Kusakabe et al.,9 who found a selectivity increasing from 17 for a 1:3 n-butane/i-butane mixture to 27 for a 3:1 n-butane/i-butane mixture. This composition dependence reflects that the blocking of the zeolite pores is more efficient when the feed concentration of the preferentially permeating component increases relative to that of the retained component. In Figure 12a,b schematic representations are given of the concentration profiles of ethane and methane in the membrane during separation of a 1:1 mixture of these components, under steady-state conditions. The pressure gradient in the support layer is calculated by assuming molecular diffusion in the support layer,33 and the concentration profile in the zeolite layer is calculated based on the complete Maxwell-Stefan equations for binary surface diffusion in microporous materials.34 The


Figure 12. Schematic representation of the concentration profiles in a composite membrane during the separation of ethane/methane mixtures. (a) Zeolite layer facing the feed side, 1:1 mixture. (b) Zeolite layer facing the permeate side, 1:1 mixture. Numbers at the bottom indicate the selectivity toward ethane at different positions in the membrane. Solid line: ethane. Dotted line: methane. ptot ) 100 kPa, 303 K.

selectivity at each interface in the membrane is also given in Figure 11a. At the gas/zeolite interface at the feed side, the adsorption selectivity of ethane over methane amounts to 25, assuming that the extended Langmuir equation can be used to describe binary adsorption. The adsorption data are taken from the work of Zhu et al.24 At the zeolite/support interface the selectivity is reduced because of the lower mobility of ethane in the zeolitic pores. The permeation flux of the components is significantly retarded by the fact that the permeate pressure is not equal to zero.33 The concentration gradient, which is the apparent driving force for permeation, is reduced by this boundary condition. This effect is enlarged by the support because there exists a significant partial pressure gradient across the support in the Wicke-Kallenbach experiments.33 In a mixture the fastest component is the most retarded by the nonzero boundary conditions, since it has the highest concentration in the permeate. This reduces the selectivity of the membrane. At the support/gas interface, the selectivity is further reduced. Molecular diffusion of ethane in helium (4.8 × 10-5 m2‚s-1 35) is slower than molecular diffusion of methane in helium (6.7 × 10-5 m2‚s-1 35). This means that the support has a higher resistance for ethane than for methane, explaining the decrease in the selectivity. On the basis of the above considerations one can expect that the more efficient removal of the permeating components increases selectivity. Attempts were made to verify this experimentally, but for the propene/ethene mixtures, no significant increase in the selectivity could be obtained when increasing the sweep gas flow rate from 100 to 200 mL/min (STP). However, model prediction of the selectivity as a function of the sweep gas flow rate shows the effect of more efficient removal of the permeating component (Figure 13). Calculations were made for a mixture of ethane and methane. Diffusion in the zeolite layer was described by the MaxwellStefan equations for binary surface diffusion assuming Langmuir adsorption,34 and diffusion in the support layer was described by the molecular diffusion.33 The experimental data are well-described by this model. More details on the calculations are given elsewhere.36 The highest selectivity is obtained for high sweep gas flow rates, when the permeate pressure is almost equal to zero. The ethane flux benefits much more from the decrease in the permeate partial pressure as the sweep gas flow rate is increased. This illustrates that the strongest adsorbing component is the most affected by the permeate conditions, especially at low sweep gas

Figure 13. Model prediction of the effect of the sweep gas flow rate on the flux and on the selectivity toward ethane (1:1 mixture of ethane and methane, ptot ) 101 kPa). Lines: model prediction (solid line, ethane flux; dashed line, methane flux; dotted line, selectivity toward ethane). Symbols: experimental data (9, ethane; 2, methane; b, selectivity toward ethane).

flow rates. Increasing the sweep gas flow rate above 100 mL/min has little effect on the selectivity. Temperature. Selectivity based on adsorption decreases with temperature. As the temperature increases, the zeolitic pores are less efficiently blocked by the strongest adsorbing component. Differences in mobility, reflected in the activation energy for diffusion, also become less significant as the temperature increases. At higher temperature permeation through the zeolite membrane proceeds by activated gaseous diffusion.25 The preexponential term for the activated gaseous diffusivity is related to 1/Mi)1/2, which means that at high temperature, in the absence of an adsorbed phase, the selectivity approaches that of Knudsen diffusion. This is indeed observed for n-butane/hydrogen and ethane/methane mixtures. For all the separations reported here, it was found that selectivity is lost at high temperature. This is also observed in other studies on the separation of hydrocarbon vapors using a silicalite-1 or ZSM-5 membrane.10-12 Only for mixtures of components with a large difference in activation energy or for separations based on complete exclusion of one of the components from the pores can it be expected that the selectivity of the membrane is still high at high temperatures.


Pressure. The effect of raising the total hydrocarbon pressure while keeping the molar ratio between the components constant is expected to have the same effect as increasing the concentration of the preferentially removed component from the mixture. For ethane/ methane mixtures, raising the absolute pressure or changing the composition indeed affects the selectivity of the membrane in the same way. At high feed pressures, high fluxes are obtained for ethane. This results in an enrichment in methane at the feed side, so for comparison of results, care must be taken to calculate the selectivity based on the retentate composition and not the feed composition, as this is significantly different from the feed composition when the supply of the feed is limited. The results found for equimolar n-butane/i-butane mixtures show a peculiar behavior. The maximum found as a function of the total hydrocarbon pressure cannot be explained on the basis of operational effects such as the permeate conditions. Again it seems that separation of this mixture is very complex. Experimental Configuration. For single-component permeation, it was found that reversal of the membrane led to an increase in the permeance of that component.33 In this configuration the component was more effectively removed at the permeate side, thus increasing the concentration gradient across the zeolite layer. It was, therefore, expected that reversal of the composite membrane would lead to an increase in the selectivity of the membrane. Surprisingly, it turned out that upon reversal of the membrane the selectivity was completely lost. This observation can be explained as follows. When the support layer faces the feed side, it is predominantly filled with ethane and methane. Diffusion of ethane in methane is 3 times slower than diffusion in helium, which takes place when the support faces the permeate side. Methane diffusion in ethane is 4 times slower than the diffusion in helium. This means that turning around the membrane increases the resistance of the support layer by a factor of 3-4. As an indication, the maximum flux of ethane through the support layer, based on molecular diffusion in methane at 50 kPa partial pressure difference, is 22 mmol‚m-2‚s-1. This is very close to the fluxes observed during the experiments with the zeolite layer facing the permeate side, which are around 20 mmol‚m-2‚s-1 at the same partial pressure gradient. This means that the highest resistance in the membrane is no longer the zeolite layer, but it is shifted to the support layer. Of course, diffusion in the zeolite layer is much slower than diffusion in the support layer, but the support layer is about 2 orders of magnitude thicker and it has a low porosity (0.2). The concentration profile in the composite membrane with the zeolite layer facing the permeate side is schematically shown in Figure 12b for a 1:1 mixture of ethane and methane. The composition at the zeolite/ support interface is calculated back from the permeate concentrations using the Maxwell-Stefan equations for binary surface diffusion.34 These calculations revealed that at the support/zeolite interface the retentate must be enriched in methane. The ethane/methane composition of the retentate changes from approximately 1:1 in the gas phase to about 1:6 at the support/zeolite interface. Apparently, the relatively high flux of ethane through the zeolite layer compared to the flux of ethane through the support layer is causing ethane depletion. This results in concentration polarization at the zeolite/

support interface. If the separation selectivity of the zeolite layer is determined from the calculated boundary conditions of the zeolite layer alone, this selectivity is comparable to the selectivity of the membrane in the other configuration (Figure 10). Thus, the zeolite layer still exhibits the same selectivity, but the overall selectivity is to a large extent determined by the resistance of the support layer. According to the calculations presented here, methane is transported against its own concentration gradient. Due to depletion at the support/zeolite interface, a small absolute pressure drop exists across the support, resulting in a contribution of viscous flow to overall permeation. The direction of the absolute pressure drop is opposite to that of the direction of the partial pressure gradient. This results in methane transport against its own partial pressure gradient. A few millibar is already high enough to obtain the fluxes found for methane. The effect of the support layer on the separation performance of the composite membrane will strongly depend on the resistance of the support in a given configuration compared to that of the zeolite layer. The balance between the two is determined by the mobility of the components under study in the zeolite layer relative to that in the support layer. In Figure 12b the selectivity at each position in the membrane is given. The support is selective toward methane, whereas the zeolite layer still is selective for ethane, based on the feed composition. Keizer et al.37 suggested that the selectivity of a composite membrane is the product of the selectivity of the separating layer and the support layer. However, the selectivity of each layer is largely determined by the presence of the other layer. In the absence of the zeolite layer and an absolute pressure gradient, the selectivity of the support would be about 1, as only molecular diffusion of ethane and methane would occur. In the presence of the zeolite layer, the resistance for exiting the support layer is higher for methane than for ethane, since ethane blocks the pores of the zeolite layer for methane. This results in a higher apparent ‘selectivity’ of the support toward methane. So, the separate determination of the selectivity of each layer cannot be used to predict the overall selectivity of the composite. Implications for Applications. The use of membranes in separation processes is an interesting alternative to conventional separation processes such as distillation and pressure-swing adsorption. Depending on the desired purity, the value of the permeate and retentate streams, and the membrane selectivity, a singlestage membrane process or a membrane cascade is needed to achieve the separation wanted. In both cases the composition of the mixtures varies along the length of the membrane unit, which implies that the selectivity and the permeance of the membrane varies as well. For the design of a separation unit, the variation of the selectivity with composition has to be taken into account. Neglecting these effects can lead to the over- or underdesign of membrane separation units.38 The data shown here reveal that the permeate and retentate boundary conditions largely determine the absolute value of the selectivity. Lowering the resistance of the support or efficient removal of permeating components from the permeate might improve the membrane performance significantly. For the separation of ethane/methane mixtures, the experimental configuration in the membrane setup was a much more


important parameter than membrane quality. This is an indication that a lot can be gained by careful analysis of the optimal process conditions. The considerations outlined above are a first attempt to quantify and understand the important factors in zeolite membrane separation. It has been shown that membrane performance is determined not only by intrinsic zeolite membrane features, like adsorption and diffusion, but also by operating conditions and the way the experiments are conducted. Conclusions (a) The determination of the permselectivity for a given pair of components is highly dependent on the permeate (partial) pressure and thus on the experimental method employed. When literature values on the permselectivity are compared as a quality criterion for zeolite membranes, experimental conditions should be considered. (b) The separation results obtained with a number of stainless steel supported silicalite-1 membranes are consistent and reflect micropore diffusion phenomena. Separations are based on differences in affinity for the zeolite between components or differences in mobility. In most cases these effects counteract each other. High selectivities are obtained for the separation of molecules that differ largely in size (n-butane/i-butane, up to 55; n-butane/neopentane, 102) or in adsorption strength (nbutane/hydrogen, up to 60). (c) The silicalite-1 membranes used in this study have an excellent stability. (d) A third component in a mixture can affect the selectivity for each binary pair in shape-selective separations. (e) Variation in membrane quality, based on the ratio between the single-component fluxes of n-butane and i-butane, is not too critical for selectivity of the membrane for mixtures of components that mainly differ in adsorption strength such as ethane/methane mixtures. However, membrane quality is important for separations where one of the components is expected to have a significantly higher mobility in defects in the membrane (n-butane/hydrogen, hydrogen/i-butane). (f) The operating conditions such as feed composition, feed pressure, temperature, and membrane orientation have a large impact on the performance of the membrane. This should be taken into account when comparing separation results reported in the literature. The nonzero partial pressure at the permeate side lowers the overall selectivity of the membrane. The support also affects the selectivity of the composite membrane. Depending on the membrane orientation, it reduces the selectivity or the selectivity of the membrane is completely lost. These are important considerations for the design of separation modules with microporous membranes. Nomenclature Di ) diffusivity of component i, m2‚s-1 Nsingle ) single-component flux of component i, mol‚m-2‚s-1 i Nsi ) surface flux of component i, mol‚m-2‚s-1 K ) adsorption constant, Pa-1 pi ) partial pressure of component i, Pa qsat,i ) saturation capacity, mmol‚g-1 xi,p ) mole fraction in the permeate xi,r ) mole fraction in the retentate

Rij ) selectivity toward component i in a mixture containing i and j Rperm ) permselectivity toward component i ij θi ) occupancy of component i F ) density, g‚m-3

Literature Cited (1) Van Veen, H. M.; Bracht, M.; Hamoen, E.; Anderliesten, P. T. Feasibility of the application of porous inorganic gas separation membranes in some large-scale chemical processes. Chapter 14 In Fundamentals of Inorganic Membrane Science and Technology; Burggraaf, A. J., Cot, L., Eds.; Elsevier Science B.V.: Amsterdam, The Netherlands, 1996. (2) Bakker, W. J. W.; Kapteijn, F.; Poppe, J.; Moulijn, J. A. Permeation characteristics of a metal supported silicalite-1 zeolite membrane. J. Membr. Sci. 1996, 117, 57. (3) Bakker, W. J. W.; Zheng, G.; Kapteijn, F.; Makkee, M.; Moulijn, J. A.; Geus, E. R.; van Bekkum, H. Single and multicomponent transport through metal-supported MFI zeolite membranes. In Precision process technology; Weijnen, M. P. C., Drinkenburg, A. A. H., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1993. (4) Vroon, Z. A. E. P.; Keizer, K.; Gilde, M. J.; Verweij, H.; Burggraaf, A. J. Transport properties of alkanes through ceramic thin zeolite MFI membranes. J. Membr. Sci. 1996, 113, 293. (5) Anand, M.; Langsam, M.; Rao, M. B.; Sircar, S. Multicomponent gas separation by selective surface flow (SSF) and polytrimethylsilylpropyne (PTMSP) membranes. J. Membr. Sci. 1997, 123, 17. (6) Rao, M. B.; Sircar, S. Nanoporous carbon membranes for separation of gas mixtures by selective surface flow. J. Membr. Sci. 1993, 85, 253. (7) Yan, Y.; Davis, M. E.; Gavalas, G. R. Preparation of zeolite ZSM-5 membranes by in-situ crystallization on porous R-Al2O3. Ind. Eng. Chem. Res. 1995, 34, 1652. (8) Coronas, J.; Falconer, J. L.; Noble, R. D. Characterization and permeation properties of ZSM-5 tubular membranes. AIChE J. 1997, 43 (7), 1797. (9) Kusakabe, K.; Murata, A.; Kuroda, T.; Morooka, S. Preparation of MFI-type zeolite membranes and their use in separation n-butane and i-butane. J. Chem. Eng. Jpn. 1997, 30 (1), 72. (10) 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. 1996, 92 (13), 2499. (11) 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. 1996, 35, 1575. (12) Funke, H. H.; Argo, A. M.; Falconer, J. L.; Noble, R. D. Separations of cyclic, branched and linear hydrocarbon mixtures silicalite membranes. Ind. Eng. Chem. Res. 1997, 36 (1), 137. (13) Lewis, J. E., Jr.; Gavalas, G. R.; Davis, M. E. Permeation studies on oriented single-crystal ferrierite membranes. AIChE J. 1997, 43 (1), 83. (14) 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 (1), 5. (15) Sano, T.; Ejiri, S.; Yamada, K.; Kawakami, Y.; Yanagishita, H. Separation of acetic acid-water mixtures by pervaporation through silicalite membrane. J. Membr. Sci. 1997, 123 (2), 225. (16) Sano, T.; Yanagishita, H.; Kiyozumi, Y.; Mizukami, F.; Haraya, K. Separation of ethanol/water mixture by silicalite membrane on pervaporation. J. Membr. Sci. 1994, 95, 221. (17) Smetana, J. F.; Falconer, J. L.; Noble, R. D. Separation of methyl ethyl ketone from water by pervaporation using a silicalite membrane. J. Membr. Sci. 1996, 114, 127. (18) Liu, Q.; Noble, R. D.; Falconer, J. L.; Funke, H. H. Organics/water separation by pervaporation with a zeolite membrane. J. Membr. Sci. 1996, 117, 163. (19) Habgood, H. W. The kinetics of molecular sieve action. Sorption of nitrogen-methane mixtures by Linde molecular sieve 4A. Can. J. Chem. 1958, 36, 1384. (20) Ka¨rger, J.; Bu¨low, M. Theoretical prediction of uptake behavior in adsorption kinetics of binary gas mixtures using irreversible thermodynamics. Chem. Eng. Sci. 1975, 30, 893.

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(31) Smit, B.; Maesen, T. L. M. Commensurate ‘freezing’ of alkanes in the channels of a zeolite. Nature 1995, 374, 42. (32) Hufton, J. R.; Ruthven, D. M. Diffusion of light alkanes in silicalite studied by the zero length column method. Ind. Eng. Chem. Res. 1993, 32, 2379. (33) Van de Graaf, J. M.; Kapteijn, F.; Moulijn, J. A. Methodological and operational aspects of permeation measurements on silicalite-1 membranes. J. Membr. Sci. 1998, 144, 87. (34) Krishna, R. A unified approach to the modeling of intraparticle diffusion in adsorption processes. Gas Sep. Purif. 1993, 7 (2), 92. (35) Fu¨ller, E. N.; Schettler, P. D.; Giddings, J. C. A New method for the prediction of gas-phase diffusion coefficients. Ind. Chem. Eng. 1996, 58, 19. (36) Van de Graaf, J. M.; Kapteijn, F.; Moulijn, J. A. Modeling permeation of binary mixtures through zeolite membrane. Submitted for publication. (37) Keizer, K.; Uhlhorn, R. J. R.; Van Vuren, R. J.; Burggraaf, A. J. Gas separation mechanisms in microporous modified γ-Al2O3 membranes. J. Membr. Sci. 1988, 39, 285. (38) Keurentjes, J. T. F.; Linders, L. J. M.; Beverloo, W. A.; Van’t Riet, K. Membrane cascades for the separation of binary mixtures. Chem. Eng. Sci. 1992, 47 (7), 1561.

Received for review April 23, 1998 Revised manuscript received July 13, 1998 Accepted July 20, 1998 IE980250C

Effect of Operating Conditions and Membrane Quality on the

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