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Effect of toluene adsorption on permeation through SAPO-34 membranes
Journal of Membrane Science 560 (2018) 108–114
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Effect of toluene adsorption on permeation through SAPO-34 membranes Nicholas O. Chisholm, Hans H. Funke, Richard D. Noble, John L. Falconer
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Department of Chemical and Biological Engineering, University of Colorado Boulder, 3415 Colorado Avenue, Boulder, CO 80309-0596, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: SAPO-34 Zeolite membrane Toluene Competitive adsorption
When 0.65 mol% toluene was added to SAPO-34 membrane feeds (172-kPa feed pressure), the CO2 and N2 permeances decreased by approximately 50%, even though toluene is too large to diffuse into the SAPO-34 pores. Single-component adsorption isotherms for CO2 and toluene were used to fit a dual-site Langmuir model to estimate external surface coverages. The lower CO2 permeances qualitatively correlated with the reduced CO2 surface coverages. Changes in permeances for 10-kPa feed pressures were also measured so that surface coverages could be estimated more accurately using ideal adsorbed solution theory. The estimated decreases in CO2 coverages on the membrane surface due to competitively-adsorbed toluene reduced the trans-membrane CO2 driving force and accounted for the observed decreases in permeance.
1. Introduction Zeolite membranes selectively permeate gases based on differences in molecular size, diffusivity, and loading. Because CO2 has both higher loading and a higher diffusivity than CH4 in SAPO-34 pores (0.38-nm diameter), SAPO-34 membranes have high selectivities for separating mixtures of CO2 (0.33-nm kinetic diameter) and CH4 (0.38 nm) [1]. The membranes are also selective for N2 (0.36 nm) over CH4, but that selectivity is lower because N2 is bigger and adsorbs more weakly than CO2 [2]. Since natural gas can have high concentrations of CO2 and/or N2, separating these gases from CH4 is necessary because they decrease the gas heating value, and CO2 can corrode pipelines. Both CO2/CH4 and N2/CH4 separation selectivities in membranes are affected by other impurities in natural gas, such as water and higher hydrocarbons. Even low concentrations of impurities can lower CO2 and N2 permeation by competing for adsorption sites and/or by decreasing diffusivities. For example, 5% propane decreased CO2 and CH4 permeances and CO2/ CH4 separation selectivities in both SAPO-34 and SSZ-13 membranes at 270-kPa feed pressure [2]. However, at 1.73-MPa feed pressure, C3H8 increased both CO2 permeance and CO2/CH4 separation selectivity in SSZ-13 membranes [2]. Propane also decreased H2 and N2 permeances in SAPO-34 membranes at 350-kPa feed pressure, but it increased H2/ N2 separation selectivity [3]. The effect of C3H8 was reversible, and heating the membranes to 473 K overnight desorbed propane and restored the original permeances. Similarly, water decreased permeances through SAPO-34 membranes, even at ppm levels, and heating to 573 K desorbed the water and restored the initial permeances [4,5]. Propane and water are small enough to enter SAPO-34 and SSZ-13 pores and ⁎
compete for adsorption sites, and their presence inside the pores inhibits permeation of other gases. Natural gas also contains impurity molecules that are too large to enter the SAPO-34 pores, but these molecules may adsorb on the external surface of SAPO-34 crystals and affect CO2/CH4 separations. Keizer et al. showed that CH4 flux through a silicalite-1 membrane at 298 K decreased by two orders of magnitude when 0.6% p-xylene, which is too large to enter the zeolite pores, was added to the feed at 100 kPa [6]. Large molecules can also decrease permeation of smaller molecules due to concentration polarization; i.e., the larger molecules have higher concentrations near the membrane surface and thus decrease driving force for transport by decreasing the partial pressures of smaller molecules near the membrane surface [7]. In this study, CO2 and N2 permeances were measured in the presence of toluene, which can be found at trace levels in some natural gas streams [8]. Even though toluene, which has a kinetic diameter of 0.59 nm, is too large to enter SAPO-34 pores, it decreased CO2 and N2 permeances through SAPO-34 membranes at low concentrations. The permeances did not decrease because of concentration polarization since CO2 and N2 permeances only decreased slightly in the presence of SF6 (0.58-nm kinetic diameter), which adsorbs more weakly than toluene but has a similar gas-phase diffusivity. This suggest that toluene decreases permeances by competitively adsorbing on the external surface of SAPO-34 crystals and decreasing the driving force for the CO2 and N2 permeation. The amounts of CO2 and toluene adsorbed on the external membrane surface were estimated with a binary Langmuir isotherm, but this required extrapolation to pressures exceeding available isotherm data; the CO2 partial pressure on the feed side of the membrane was about
Corresponding author. E-mail addresses: [email protected] (N.O. Chisholm), [email protected] (H.H. Funke), [email protected] (R.D. Noble), [email protected] (J.L. Falconer). https://doi.org/10.1016/j.memsci.2018.05.016 Received 4 April 2018; Received in revised form 6 May 2018; Accepted 8 May 2018 Available online 09 May 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.
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2.3. Gas permeation measurements at 172 kPa
170 kPa during the permeation experiments, but the CO2 adsorption isotherm could only be measured to about 100 kPa. Moreover, the binary Langmuir isotherm is not thermodynamically consistent with the Gibbs adsorption isotherm for an ideal adsorbed solution [9]. Thus, permeances were also measured at feed pressures below 10 kPa, where toluene adsorption isotherms are available, and the thermodynamically-consistent ideal adsorbed solution theory (IAST) could be used. The decreased surface coverage of CO2 in the presence of toluene predicted by IAST accounted for the decrease in CO2 permeance, assuming that the driving force for permeation was proportional to the CO2 loading on the external membrane surface.
Single-component and mixed-gas permeances were measured in a flow system equipped with computer-controlled mass flow controllers. The membranes were sealed in stainless steel modules with Viton orings. Toluene was added to saturate a feed stream by bubbling CO2 or N2 through two toluene-filled glass bubblers in-series. A thermostat maintained the bubbler temperature slightly below ambient (290 K) to prevent toluene condensation in the lines. The toluene-saturated streams were then mixed with CO2 or N2 streams to obtain the desired toluene feed concentration. Retentate and permeate flow rates were measured with soap film flow meters. No sweep gas was used. The toluene feed compositions were calculated from the toluene vapor pressure and the gas flow rates, and these calculated compositions were consistent with GC analysis. To determine the effect of toluene or SF6 on permeation through the membrane, initially single-component CO2 or N2 (both AirGas UHP) permeances were measured, and then up to 0.65% toluene (Sigma Aldrich) or up to 1.5% SF6 (AirGas) was added to the feed. The total feed flow rate was 300 sccm, the feed pressure was 172 kPa, and the permeate pressure was 85 kPa. An adsorption trap filled with 3A zeolite was placed at the inlet of the membrane module to remove water or other impurities from the feed. This trap only slightly affected equilibration times after addition of toluene to the feed because toluene is too large to enter the 3 A pores.
2. Experimental methods 2.1. Preparation and characterization of SAPO-34 membranes SAPO-34 membranes were prepared using hydrothermal synthesis, as reported previously [1]. Briefly, tubular alumina supports (Inopor) with 100-nm pores were cleaned in boiling DI water, and then seeded by rubbing SAPO-34 crystals onto the inner surface. The seed crystals were prepared in a Teflon autoclave by microwave heating a gel to 453 K for 7 h. The gel contained Al(O-i-Pr)3, tetraethyl ammonium hydroxide (TEAOH), deionized water, Ludox AS-40 colloidal silica, and H3PO4 (all chemicals from Sigma-Aldrich) with a composition of 1.0 Al2O3: 2.0 P2O5: 0.6 SiO2: 4.0 TEAOH: 75 H2O. The membrane layer was grown by placing seeded supports into an autoclave with a synthesis gel containing the same precursors as the seed gel, but with a composition of 0.85 Al2O3: 1.0 P2O5: 0.3 SiO2: 2 TEAOH: 155 H2O. The autoclave was then held at 483 K for 6 h. After synthesis, the membranes were cooled, rinsed for 24 h in DI water to remove synthesis gel residue, calcined in vacuum at 823 K for 8 h with heating and cooling rates of 1 K/min, and then stored in an oven at 473 K in air. Scanning electron microscope images of a membrane prepared previously using the same procedures [1] showed a 3-μm thick SAPO-34 layer and 1–3 µm cubic crystals on the surface. The flux through defects in the SAPO-34 membrane was estimated from the CO2/isobutane ideal selectivity. Isobutane (0.50-nm kinetic diameter) is too large to enter the SAPO-34 pores and thus only permeates through defects. The membrane used for measurements at 172-kPa feed pressure had a CO2/isobutane ideal separation selectivity of about 80 at room temperature and 140-kPa feed pressure. The percentage of CO2 flux through defects for this membrane was estimated to be less than 1.5%, but as much as 30% of the N2 flux was through defects if Knudsen diffusion dominated the defect permeation, because N2 permeated more slowly than CO2. We have prepared SAPO-34 membranes with much higher CO2/isobutane selectivities previously [2], but the small defect contribution to the overall CO2 flux for this membrane does not affect the conclusions. A second SAPO-34 membrane was used for permeation measurements at 10-kPa feed pressure. This membrane had a CO2/isobutane ideal separation selectivity of 2200 at room temperature and 125-kPa feed pressure. Carbon dioxide flux through defects was estimated to be 0.05% of the total CO2 flux for this membrane.
2.4. Gas permeation measurements at 10 kPa The same system was used for permeation measurements at lower pressures. Feed and permeate pressures of 10 and 2.1 kPa, respectively, were obtained with mechanical vacuum pumps and controlled with backpressure regulators. The toluene feed compositions were calculated from the toluene vapor pressure and the gas flow rates. The toluene concentrations were confirmed by adsorbing toluene onto a packed bed of activated carbon from a flowing helium stream at the CO2/toluene experimental pressures and measuring the weight gain of the carbon bed. Helium was used as the carrier gas because it does not adsorb significantly on activated carbon. The permeate flux was measured with soap film flow meters at the exhaust of the vacuum pump. 3. Results and discussion 3.1. Effect of toluene on CO2 and N2 permeances at 172 kPa The SAPO-34 membrane had single-gas permeances of 2.0 × 10−6 mol/(m2 Pa s) for CO2 and 1.6 × 10−7 mol/(m2 s Pa) for N2 at 293 K and a feed pressure of 172 kPa (82-kPa permeate pressure). As shown in Fig. 1, when 1.1 kPa (0.65%) toluene was added to the feed, the CO2 permeance decreased by about 45% within a few minutes; it reached steady state after about 30 min. At lower toluene concentrations, permeances reached steady state faster (not shown). In contrast, as previously reported, CO2 permeances reached steady state over 2 d when propane was added to a CO2 feed because propane slowly diffuses into SAPO-34 pores [2]. Steady state was reached much more quickly when toluene was added to the feed because toluene only adsorbs on the external surface of SAPO-34 crystals. When toluene was removed from the feed, the CO2 permeance increased to within 10% of its initial permeance over several hours. The CO2 permeance fully recovered after heating the membrane to 473 K overnight in air. Some toluene may strongly adsorb on SAPO-34 at room temperature; Nagao et al. found that toluene was not completely removed from dehydroxylated TiO2 by evacuation for 4 h at 298 K [12]. They concluded from IR measurements that surface Ti atoms interact strongly with the π-electrons of the aromatic ring to prevent complete desorption, and toluene may have similar strong π-bonding on the SAPO-34 surface. Fig. 2 shows that the steady-state CO2 permeances decreased more
2.2. Characterization of SAPO-34 crystals The same hydrothermal synthesis used to prepare the membranes was also used to prepare SAPO-34 crystals for adsorption measurements. These crystals were cubic with sizes ranging from 0.4–3 µm, as determined by SEM. The XRD peaks were consistent with the SAPO-34 structure, and no other peaks were detected. Their BET surface area was 580 m2/g, which is in the range of values reported in the literature (483–677 m2/g) [10,11]. Toluene adsorption isotherms and BET surface areas were measured with a Quantachrome AS1-C volumetric adsorption system. 109
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Fig. 4. Steady-state CO2 permeances as a function of toluene fraction on feed side (log-mean feed/retentate fraction) at a feed pressure of 10 kPa and a permeate pressure of 2.1 kPa.
Fig. 1. Carbon dioxide permeance through a SAPO-34 membrane at 298 K and a total feed pressure of 172 kPa. Toluene at a partial pressure of 1.1 kPa (0.65 mol%) was added to the feed after 50 min, and it was removed after another 45 min.
1.1-kPa toluene was added to the feed, the N2 permeance decreased by 50%. 3.2. Effect of toluene on CO2 permeance at 10 kPa The CO2 permeance of a second SAPO-34 membrane was measured in the presence of toluene at a feed pressure of 10 kPa. The permeance measurements are in the pressure range where a toluene adsorption isotherm was measured so that IAST could be applied. This membrane had a CO2 single-gas permeance of 3.4 × 10−6 mol/(m2 Pa s) at 293 K and a feed pressure of 125 kPa (82-kPa permeate pressure), and a CO2 permeance of 4 × 10−6 mol/(m2 Pa s) at 10-kPa feed pressure and 2.1kPa permeate pressure. As shown in Fig. 4, when toluene was added to the feed, the CO2 permeance decreased. Steady-state was reached in 30 min after the toluene concentration increased. At the highest toluene concentration (0.87-kPa toluene, 8.5 mol% toluene in feed), the CO2 permeance decreased by 19%. When toluene was removed from the feed, the single-gas CO2 permeance recovered within 60 min.
Fig. 2. Carbon dioxide permeance through a SAPO-34 membrane at 298 K and a total feed pressure of 172 kPa. The upper line is the single-gas CO2 permeance, and the arrows indicate the changes in steady-state permeance after toluene was added.
3.3. Toluene adsorption on SAPO-34 crystals The decreases in CO2 and N2 permeances in the presence of toluene suggest that toluene either blocks access to surface pores or decreases coverage of the permeating species by competitive adsorption. Multicomponent models can estimate adsorption equilibria based on single-component adsorption isotherms, and thus toluene and CO2 adsorption on SAPO-34 crystals was measured. The BET surface area of uncalcined SAPO-34 crystals was 37 m2/g, and this was assumed to approximate the external surface area of calcined SAPO-34 crystals since N2 could not adsorb in the pores of the uncalcined SAPO-34 because they were filled with the structure-directing template, tetraethylammoniumhydroxide. Toluene and CO2 adsorption isotherms were also measured on the uncalcined crystals as a way to approximate adsorption on the external surface of crystals. The toluene isotherms were measured at three temperatures, as shown in Fig. 5, and toluene isotherms on calcined crystals were similar because toluene cannot enter SAPO-34 pores. The toluene isotherms at 296 K show type-II multilayer adsorption well below saturation pressure (3.4 kPa at 296 K). The desorption isotherm exhibits hysteresis, which may be due to capillary condensation between the crystals or stronger π-bonding with the OH-groups on the surface if adsorbed toluene reorganized at higher loading. Similarly, Nagao et al. measured type-II isotherms for toluene adsorption at 298 K on rutile TiO2 particles [12]. They observed multilayer adsorption at pressures that were above 5% of saturation, and concluded, based on BET analysis, that a toluene molecule occupied 0.602 nm2 on rutile TiO2. Molecular models predict that a toluene molecule adsorbed flat on a surface occupies about 0.47 nm2 [13], and
Fig. 3. Nitrogen permeance through a SAPO-34 membrane at 298 K and a total feed pressure of 172 kPa. The upper line is the single-gas N2 permeance, and the arrows indicate the changes in steady-state permeances after toluene was added.
as the toluene partial pressure increased. After each measurement at a given toluene partial pressure, the membrane was heated overnight to remove toluene. Pure-component CO2 permeances measured at the start of each experiment show that the CO2 permeance in the absence of toluene did not change over the course of the experiments (Fig. 2). Toluene had a similar effect on N2 permeance, as shown in Fig. 3. When 110
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Fig. 5. Toluene adsorption isotherms on uncalcined SAPO-34 crystals at 296, 313, and 333 K.
Fig. 7. Carbon dioxide adsorption isotherm on uncalcined SAPO-34 crystals (diamonds) at 296 K: dashed line - Langmuir fit; solid line - polynomial fit.
thus, Nagao et al. suggested that toluene packs loosely on the rutile surface. The toluene adsorption isotherm at 296 K, when fit to the BET model, yielded a monolayer coverage of 80 μmol/g, which was calculated to occur around 0.8 kPa, and is less than 20% of the toluene saturation pressure at 296 K (note the y-axis scale in Fig. 5). This coverage corresponds to an area of 0.77 nm2 per toluene molecule. Even though SAPO-34 is crystalline, its porosity and non-ideal termination of the crystals might make the external surface irregular, which could prevent dense packing of adsorbed molecules [14]. Thus, a toluene molecule may occupy more area on the SAPO-34 surface than the 0.602 nm2 reported for TiO2 crystals by Nagao et al. [12]. Nitrogen, which was used to measure the SAPO-34 external surface area, is much smaller than toluene and thus can pack more densely on a rough surface.
Table 1 Binary Langmuir isotherm parameters.
CO2 toluene
Keq [kPa−1]
Q [mmol/g]
0.024 1.6
0.68 0.16
The Langmuir parameters for toluene were determined from a fit between 0 and 1 kPa, and the parameters for CO2 were calculated from the isotherms measured between 0 and 100 kPa (Table 1). The model qualitatively accounts for the difference in surface areas of the different-sized molecules. The CO2 external surface coverages calculated from the binary Langmuir isotherm qualitatively correlate with the CO2 permeances (Fig. 8); as the CO2 surface coverage decreased, the CO2 permeance decreased. The surface coverage predictions at 172-kPa feed pressure may not be accurate because they were extrapolated from the Langmuir isotherm that was fit to the CO2 adsorption data, which was only obtained up to 100 kPa (Fig. 7). Also, the binary Langmuir isotherm has been criticized because it is not thermodynamically consistent and does not account for spreading pressure, which is the two-dimensional equivalent of partial pressures in gas-phase mixtures that is often used to describe adsorption equilibria on surfaces [17]. Ideal adsorbed solution theory is generally more accurate than the Langmuir model and is also thermodynamically consistent. Crittenden et al. showed that IAST reliably predicted adsorbed amounts in mixtures of up to six volatile organic chemicals in aqueous solution [18]. Sakuth et al. measured toluene and 1-propanol adsorption on Y-zeolites and reported that the IAST adequately predicted binary adsorption for a
3.4. Modeling of competitive adsorption on the external surface The Maxwell-Stefan model for transport through zeolite membranes assumes molecules adsorb on the external surface of the zeolite crystals before entering the pores [15,16], and thus a lower CO2 external surface concentration due to competition with adsorbed toluene reduces the driving force for CO2 permeation. The surface coverages of CO2 and toluene at a feed pressure of 172 kPa were estimated with a binary Langmuir model:
ni Ki Pi = Qi 1+Ki Pi + Kj Pj The total coverage Qi and equilibrium constants Ki for the Langmuir isotherms were estimated from single-component toluene and CO2 adsorption isotherms (Figs. 6 and 7). The loading for component i is ni.
Fig. 6. Toluene adsorption isotherm on uncalcined SAPO-34 crystals (diamonds) at 296 K: dashed line - Langmuir fit; solid line - polynomial fit.
Fig. 8. Carbon dioxide permeance at 172-kPa feed pressure versus CO2 surface coverage (from Langmuir isotherm). 111
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Fig. 10. Log-mean feed/retentate CO2 coverage of external membrane surface as a function of toluene vapor fraction on feed side (log-mean feed/retentate fraction).
Fig. 9. Single-component spreading pressure of toluene and CO2 as a function of pressure. Each set of points with the same spreading pressure is a solution for specific experimental parameters, and the unpaired point represents pure CO2, before any toluene was added.
where xi is the mole fraction of component i on the surface, yi is the mole fraction of component i in the vapor, and P is the total pressure. Fig. 9 shows the IAST solutions for each feed composition from Fig. 4. The spreading pressures of toluene and CO2 must be equal in the mixture, and the surface composition is determined by the lever rule around the total pressure, 10 kPa [17]. At the constant feed pressure of 10 kPa, increasing the toluene vapor concentration increased the toluene surface coverage, increased the spreading pressure (and total surface coverage), and decreased the CO2 surface coverage. The decreases in CO2 coverages estimated by IAST as a result of competitive adsorption with toluene are sufficient to explain the reduced CO2 permeances. The analysis assumes that the CO2 concentration in the pores near the membrane surface is directly proportional to the CO2 surface coverage. Adding toluene lowers the CO2 surface coverage, which lowers the CO2 concentration in the membrane, and thus the driving force for permeation decreases. Fig. 10 shows that the CO2 external surface coverage decreases as the toluene feed concentration increases; 8.5 mol% toluene in the feed stream reduced the CO2 surface coverage by about 20%. Fig. 11 shows the correlation between the average CO2 permeance for each toluene concentration in Fig. 4, and the estimated, corrected driving force for CO2 transport using the reduced surface coverage of CO2 (log-mean difference between the calculated CO2 external surface coverage on the feed/retentate and permeate side). Changes in the CO2 concentration in the membrane were calculated from CO2 fluxes using Fick's law, and the CO2 diffusion coefficient in the membrane was assumed independent of CO2 loading because loading was low at 10-kPa
zeolite with Si/Al ratio of 100, but that a predictive real adsorbed solution theory (PRAST) model was necessary to describe the non-ideal adsorption behavior on Y-zeolites with lower Si/Al ratios [19]. Furmaniak et al. simulated adsorption of CO2, CH4, and N2 on porous carbon and reported that the IAST model predicted surface coverages reasonably well. Simulated amounts adsorbed deviated from ideal behavior as the total pressure increased from 0.1 to 1.0 MPa, and deviation from ideal behavior was greater for more microporous systems [20]. The co-adsorption of CO2 and toluene on the SAPO-34 external surface at pressures below 10 kPa was modeled using IAST. Toluene adsorption isotherm data were used from 0 to 2.3 kPa; the polynomial fitting function becomes non-physical at higher pressures. The CO2 permeances were measured in a mixed feed with toluene partial pressures below 1 kPa so that the spreading pressures calculated in Fig. 9 stayed within the available data range. As discussed by Myers and Prausnitz [17], polynomials were fit to the CO2 and toluene singlecomponent adsorption data (Figs. 6 and 7):
ni° = Fi (Pi°) where ni° is the surface loading of pure component i, Fi(Pi°) is the numeric function for loading (polynomial fit with intercept at 0), and Pi° is the pressure of pure component i. Then πi°, the spreading pressure of pure component i on the SAPO-34 surface, was determined by numerical integration of this equation:
πi° (Pi°) =
RT A
P = P′
∫P=0
ni° (P ) dP = ψi (Pi°) P
where R is the gas constant, T is temperature, A is the area, and ψi(Pi°) is the numeric function for spreading pressure (polynomial fit with intercept at 0). The following equations were then solved simultaneously using the log-mean, vapor-phase composition for the feed side and the total feed pressure: ° ° πtol = ψtol (Ptol ) ° ° πCO = ψCO2 (PCO ) 2 2
° P ytol = Ptol x tol
° P yCO2 = PCO x 2 CO2
° ° πtol = πCO 2
Fig. 11. Normalized CO2 permeance versus normalized effective driving force for two measurements. Carbon dioxide permeance was normalized to the pureCO2 permeance, and effective driving force was normalized to the pure-CO2 trans-membrane driving force. The line indicates a 1-to-1 correlation.
x tol + x CO2 = 1 ytol + yCO2 = 1 112
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was determined from the membrane dimensions, the gas velocity (v = 0.15 m/s), and the gas-phase diffusivity of toluene (D = 8.7 × 10−6 m2/s) [24]:
feed pressure. At the highest toluene concentrations, the CO2 permeance decreased by 20% whereas the predicted drop is 30%. This discrepancy may be due to non-ideal adsorption on the external surface. Since the measured decrease in permeance was less than calculated, other processes (concentration polarization, blocking of the external zeolite pores by adsorbed toluene, toluene condensation) cannot account for the discrepancy since they would further decrease the permeance.
δ=
1 ⎛ vd ⎞−1/3 1/3 2/3 L d A⎝ D ⎠
where d is the membrane inner diameter (6 mm), L is the membrane length (30 mm), and A is dependent on the Graetz number (Gz = (v/D) d2/L ≈ 19), which is greater than 10, resulting in A = 2.43 [25]. The boundary layer thickness was about 1.2 mm. The Peclet number (Pe = v δ/D ≈ 0.029) was used to determine the polarization index, I, which is the ratio of the concentration near the membrane surface to the bulk concentration. The polarization index is [7,25]:
3.5. Effect of concentration polarization Replacing toluene in the feed with SF6 only lowered CO2 permeances slightly. This result demonstrates that concentration polarization does not cause the decrease in CO2 permeance when toluene was added to the feed at 172-kPa feed pressure. Concentration polarization occurs when the slower-permeating (rejected) species accumulates near the membrane surface because these only slowly diffuse away from the membrane surface. Thus, concentration polarization decreases the concentration and the driving force of the faster permeating species [7]. The saturation pressure of SF6 (1.2 MPa) is orders of magnitude higher than the saturation pressure of toluene (2.9 kPa) at 293 K, and thus SF6 adsorbs much less strongly than toluene (e.g., ΔHads for SF6 on NaX zeolite ~ 28 kJ/mol; ΔHads for toluene on Na-faujasite ~ 90 kJ/mol [21,22]). Like toluene, SF6 is too large to enter the SAPO-34 pores. Therefore, SF6, when used at the same concentrations as toluene, should have a small effect on CO2 surface loadings. As a result, the decrease in CO2 permeance when SF6 was added to the CO2 feed should be due mostly to concentration polarization, and this decrease should be an upper limit on how much concentration polarization decreases CO2 permeation in the presence of toluene. Fig. 12 shows that 1.2-kPa toluene decreased the CO2 permeance 10 times as much as SF6 at the same partial pressure. The CO2 permeance decreased by the same amount, 18%, for an SF6 pressure of 4 kPa and a toluene pressure of 0.1 kPa. Moreover, the bulk diffusivity of SF6 (MW of 146) is about 20% lower than that of toluene (MW of 96) [23]. Thus, concentration polarization should be slightly larger for CO2/SF6 mixtures than for CO2/toluene mixtures, and thus concentration polarization should account for less than 10% of the measured decrease in CO2 permeance at 1.2-kPa toluene pressure. To more accurately determine toluene surface concentrations, the increase in toluene partial pressure near the membrane surface due to concentration polarization was estimated from a transport model, and these concentrations were used in the determination of surface coverages discussed earlier. With a Reynolds number of about 100 (laminar-flow), the boundary layer thickness (δ) at the membrane surface
I = e Pe + β (1−e Pe ) where β is the enrichment factor, which is the ratio of the permeate concentration to the retentate concentration. The calculated values of I (0.9996 for CO2, 0.99999 for N2) show toluene does not decrease CO2 permeation because of concentration polarization. For the CO2/toluene feed, the toluene concentration near the surface increased by about 6% over the bulk concentration, for all toluene concentrations, but for the N2/toluene feed, the toluene concentration near the surface only increased by about 0.2% over the bulk concentration, because N2 fluxes are lower than CO2 fluxes. 3.6. Transport resistance of condensed toluene Even though the SF6 measurements and transport calculations suggest that concentration polarization is not significant for toluene/ CO2 membrane separations, toluene could condense if its partial pressure near the membrane surface increased enough, and a liquid film would decrease CO2 and N2 permeances. The adsorption isotherms in Fig. 5 indicate that toluene multilayers form well below its saturation pressure, and thus, even a moderate increase in toluene concentration could potentially cause condensation. The thickness of a liquid layer that would cause the observed decreases in CO2 permeances, neglecting any changes in CO2 surface coverage due to surface effects, was estimated using a resistance in series model [26]:
Rtot = R1 + R2 where Ri is the resistance to flow though layer i and Ri = Pi/(li ΔCi), Pi is the layer permeance, li is the layer thickness (3 µm for the zeolite layer), and ΔCi is the concentration change across the layer. Fick's Law was used to estimate the toluene film thickness. The interfacial resistance between the liquid film and the zeolite surface was assumed to be zero, and the added resistance was assumed to be due solely to CO2 diffusion through a condensed toluene film. The diffusion coefficient of CO2 in toluene is 4.9 × 10−9 m2/s [27], and Henry's Law constant for CO2 in toluene is 965 (Pa m3)/mol [28]. For the observed changes in permeance, the toluene film would have to be between 0.5 µm and 2.1 µm thick for toluene concentrations used. The toluene flowrates during experiments supply enough toluene for a 1-µm thick film in a few minutes. While a liquid layer several molecules thick could form on the surface below saturation pressure, a µm-thick film would have bulk liquid properties at the vapor interface and would evaporate, so this amount of liquid toluene is thermodynamically inconsistent. Therefore a liquid film would be too thin to completely account for the decrease in CO2 permeance, and surface resistances must be considered to account for the decreased CO2 permeances in the presence of toluene. The competitive adsorption model discussed above assumes a gas/solid interface and more than accounts for the decreased permeances, so a liquid-film resistance is unlikely.
Fig. 12. Normalized CO2 and N2 permeances through SAPO-34 membrane as a function of toluene partial pressure, and normalized CO2 permeance as a function of SF6 partial pressure. All measurements were at 298 K and a total feed pressure of 172 kPa. 113
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4. Conclusions
adsorbed solutions, J. Phys. Chem. 85 (1981) 3247–3250. [10] S. Li, J.L. Falconer, R.D. Noble, SAPO-34 membranes for CO2/CH4 separations: effect of Si/Al ratio, Microporous Mesoporous Mater. 110 (2008) 310–317. [11] M. Salmasi, S. Fatemi, S.J. Hashemi, MTO reaction over SAPO-34 catalysts synthesized by combination of TEAOH and morpholine templates and different silica sources, Sci. Iran. 19 (2012) 1632–1637. [12] M. Nagao, Y. Suda, Adsorption of benzene, toluene, and chlorobenzene on titanium dioxide, Langmuir 5 (1989) 42–47. [13] T. Morimoto, M. Nagao, Y. Suda, Heat of immersion of zinc oxide in organic liquids. 3. Immersion in benzene, toluene, and chlorobenzene, J. Phys. Chem. 89 (1985) 4881–4883. [14] J. Garcia-Martinez, K. Li, Mesoporous Zeolites: Preparation, Characterization and Applications, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2015. [15] R. Krishna, L.J.P. van den Broeke, The Maxwell-Stefan description of mass transport across zeolite membranes, Chem. Eng. J. Biochem. Eng. J. 57 (1995) 155–162. [16] F. Kapteijn, J.A. Moulijn, R. Krishna, The generalized Maxwell–Stefan model for diffusion in zeolites, Chem. Eng. Sci. 55 (2000) 2923–2930. [17] A.L. Myers, J.M. Prausnitz, Thermodynamics of mixed-gas adsorption, AIChE J. 11 (1965) 121–127. [18] D.W. Hand, S. Loper, M. Ari, J.C. Crittenden, Prediction of multicomponent adsorption equilibria using ideal adsorbed solution theory, Environ. Sci. Technol. 19 (1985) 1037–1043. [19] M. Sakuth, J. Meyer, J. Gmehling, Measurement and prediction of binary adsorption equilibria of vapors on dealuminated Y-zeolites (DAY), Chem. Eng. Process. Process Intensif. 37 (1998) 267–277. [20] S. Furmaniak, S. Koter, A.P. Terzyk, P.A. Gauden, P. Kowalczyk, G. Rychlicki, New insights into the ideal adsorbed solution theory, Phys. Chem. Chem. Phys. 17 (2015) 7232–7247. [21] D.V. Cao, S. Sircar, Heat of Adsorption of Pure Sulfur Hexafluoride on MicroMesoporous Adsorbents, 2001st ed., Kluwer Academic Publishers, 2001, pp. 73–80. [22] R. Yoshimoto, K. Hara, K. Okumura, N. Katada, M. Niwa, Analysis of toluene adsorption on Na-form zeolite with a temperature-programmed desorption method, J. Phys. Chem. C 111 (2007) 1474–1479. [23] E.L. Cussler, Diffusion: Mass Transfer in Fluid Systems, Cambridge University Press, 2009. [24] H.Y. Erbil, Y. Avci, Simultaneous determination of toluene diffusion coefficient in air from thin tube evaporation and sessile drop evaporation on a solid surface, Langmuir 18 (2002) 5113–5119. [25] L. Mi, S.-T. Hwang, Correlation of concentration polarization and hydrodynamic parameters in hollow fiber modules, J. Membr. Sci. 159 (1999) 143–165. [26] J.M.S. Henis, M.K. Tripodi, Composite hollow fiber membranes for gas separation: the resistance model approach, J. Membr. Sci. 8 (1981) 233–246. [27] E.D. Snijder, M.J.M. Te Riele, G.F. Versteeg, W.P.M. Van Swaaij, Diffusion coefficients of CO, CO2, N2O, and N2 in ethanol and toluene, J. Chem. Eng. Data 40 (1996) 37–39. [28] P. Luhring, A. Schumpe, Gas solubilities (H2, He, N2, CO, O2, Ar, CO2) in organic liquids at 293.2 K, J. Chem. Eng. Data 34 (1989) 250–252.
Adding small amounts of toluene to the feed significantly decreased CO2 and N2 permeances through SAPO-34 membranes, even though toluene is too large to enter the SAPO-34 pores. Steady state was reached within 30 min after toluene addition. Through extrapolation, a Langmuir binary adsorption model gave a qualitative correlation between reduced surface coverage and reduced permeances. Analysis with IAST of permeation measurements at lower pressures also indicated that CO2 permeances decreased because CO2 concentrations were lower on the external surface of the membrane as a result of competitive adsorption with toluene. The small effect of SF6 on CO2 permeances demonstrated that concentration polarization was not the main reason that toluene decreased CO2 and N2 permeances. Acknowledgments We gratefully acknowledge support by the National Science Foundation, Grant CBET 1263130. References [1] R. Zhou, E.W. Ping, H.H. Funke, J.L. Falconer, R.D. Noble, Improving SAPO-34 membrane synthesis, J. Membr. Sci. 444 (2013) 384–393. [2] T. Wu, M.C. Diaz, Y. Zheng, R. Zhou, H.H. Funke, J.L. Falconer, et al., Influence of propane on CO2/CH4 and N2/CH4 separations in CHA zeolite membranes, J. Membr. Sci. 473 (2015) 201–209. [3] N.O. Chisholm, G.C. Anderson, J.F. McNally, H.H. Funke, R.D. Noble, J.L. Falconer, Increasing H2/N2 separation selectivity in CHA zeolite membranes by adding a third gas, J. Membr. Sci. 496 (2015) 118–124. [4] S. Li, G. Alvarado, R.D. Noble, J.L. Falconer, Effects of impurities on CO2/CH4 separations through SAPO-34 membranes, J. Membr. Sci. 251 (2005) 59–66. [5] J.C. Poshusta, R.D. Noble, J.L. Falconer, Characterization of SAPO-34 membranes by water adsorption, J. Membr. Sci. 186 (2001) 25–40. [6] K. Keizer, A.J. Burggraafa, Z.A.E.P. Vroon, H. Verweij, Two component permeation through thin zeolite MFI membranes, J. Membr. Sci. 147 (1998) 159–172. [7] A.M. Avila, H.H. Funke, Y. Zhang, J.L. Falconer, R.D. Noble, Concentration polarization in SAPO-34 membranes at high pressures, J. Membr. Sci. 335 (2009) 32–36. [8] S. Mokhatab, W.A. Poe, J.Y. Mak, Handbook of Natural Gas Transmission and Processing, 3rd edition, Elsevier, Amsterdam, 2015. [9] M.D. LeVan, T. Vermeulen, Binary Langmuir and Freundlich isotherms for ideal
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Effect of toluene adsorption on permeation through SAPO-34 membranes Nicholas O. Chisholm, Hans H. Funke, Richard D. Noble, John L. Falconer
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T
Department of Chemical and Biological Engineering, University of Colorado Boulder, 3415 Colorado Avenue, Boulder, CO 80309-0596, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: SAPO-34 Zeolite membrane Toluene Competitive adsorption
When 0.65 mol% toluene was added to SAPO-34 membrane feeds (172-kPa feed pressure), the CO2 and N2 permeances decreased by approximately 50%, even though toluene is too large to diffuse into the SAPO-34 pores. Single-component adsorption isotherms for CO2 and toluene were used to fit a dual-site Langmuir model to estimate external surface coverages. The lower CO2 permeances qualitatively correlated with the reduced CO2 surface coverages. Changes in permeances for 10-kPa feed pressures were also measured so that surface coverages could be estimated more accurately using ideal adsorbed solution theory. The estimated decreases in CO2 coverages on the membrane surface due to competitively-adsorbed toluene reduced the trans-membrane CO2 driving force and accounted for the observed decreases in permeance.
1. Introduction Zeolite membranes selectively permeate gases based on differences in molecular size, diffusivity, and loading. Because CO2 has both higher loading and a higher diffusivity than CH4 in SAPO-34 pores (0.38-nm diameter), SAPO-34 membranes have high selectivities for separating mixtures of CO2 (0.33-nm kinetic diameter) and CH4 (0.38 nm) [1]. The membranes are also selective for N2 (0.36 nm) over CH4, but that selectivity is lower because N2 is bigger and adsorbs more weakly than CO2 [2]. Since natural gas can have high concentrations of CO2 and/or N2, separating these gases from CH4 is necessary because they decrease the gas heating value, and CO2 can corrode pipelines. Both CO2/CH4 and N2/CH4 separation selectivities in membranes are affected by other impurities in natural gas, such as water and higher hydrocarbons. Even low concentrations of impurities can lower CO2 and N2 permeation by competing for adsorption sites and/or by decreasing diffusivities. For example, 5% propane decreased CO2 and CH4 permeances and CO2/ CH4 separation selectivities in both SAPO-34 and SSZ-13 membranes at 270-kPa feed pressure [2]. However, at 1.73-MPa feed pressure, C3H8 increased both CO2 permeance and CO2/CH4 separation selectivity in SSZ-13 membranes [2]. Propane also decreased H2 and N2 permeances in SAPO-34 membranes at 350-kPa feed pressure, but it increased H2/ N2 separation selectivity [3]. The effect of C3H8 was reversible, and heating the membranes to 473 K overnight desorbed propane and restored the original permeances. Similarly, water decreased permeances through SAPO-34 membranes, even at ppm levels, and heating to 573 K desorbed the water and restored the initial permeances [4,5]. Propane and water are small enough to enter SAPO-34 and SSZ-13 pores and ⁎
compete for adsorption sites, and their presence inside the pores inhibits permeation of other gases. Natural gas also contains impurity molecules that are too large to enter the SAPO-34 pores, but these molecules may adsorb on the external surface of SAPO-34 crystals and affect CO2/CH4 separations. Keizer et al. showed that CH4 flux through a silicalite-1 membrane at 298 K decreased by two orders of magnitude when 0.6% p-xylene, which is too large to enter the zeolite pores, was added to the feed at 100 kPa [6]. Large molecules can also decrease permeation of smaller molecules due to concentration polarization; i.e., the larger molecules have higher concentrations near the membrane surface and thus decrease driving force for transport by decreasing the partial pressures of smaller molecules near the membrane surface [7]. In this study, CO2 and N2 permeances were measured in the presence of toluene, which can be found at trace levels in some natural gas streams [8]. Even though toluene, which has a kinetic diameter of 0.59 nm, is too large to enter SAPO-34 pores, it decreased CO2 and N2 permeances through SAPO-34 membranes at low concentrations. The permeances did not decrease because of concentration polarization since CO2 and N2 permeances only decreased slightly in the presence of SF6 (0.58-nm kinetic diameter), which adsorbs more weakly than toluene but has a similar gas-phase diffusivity. This suggest that toluene decreases permeances by competitively adsorbing on the external surface of SAPO-34 crystals and decreasing the driving force for the CO2 and N2 permeation. The amounts of CO2 and toluene adsorbed on the external membrane surface were estimated with a binary Langmuir isotherm, but this required extrapolation to pressures exceeding available isotherm data; the CO2 partial pressure on the feed side of the membrane was about
Corresponding author. E-mail addresses: [email protected] (N.O. Chisholm), [email protected] (H.H. Funke), [email protected] (R.D. Noble), [email protected] (J.L. Falconer). https://doi.org/10.1016/j.memsci.2018.05.016 Received 4 April 2018; Received in revised form 6 May 2018; Accepted 8 May 2018 Available online 09 May 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.
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2.3. Gas permeation measurements at 172 kPa
170 kPa during the permeation experiments, but the CO2 adsorption isotherm could only be measured to about 100 kPa. Moreover, the binary Langmuir isotherm is not thermodynamically consistent with the Gibbs adsorption isotherm for an ideal adsorbed solution [9]. Thus, permeances were also measured at feed pressures below 10 kPa, where toluene adsorption isotherms are available, and the thermodynamically-consistent ideal adsorbed solution theory (IAST) could be used. The decreased surface coverage of CO2 in the presence of toluene predicted by IAST accounted for the decrease in CO2 permeance, assuming that the driving force for permeation was proportional to the CO2 loading on the external membrane surface.
Single-component and mixed-gas permeances were measured in a flow system equipped with computer-controlled mass flow controllers. The membranes were sealed in stainless steel modules with Viton orings. Toluene was added to saturate a feed stream by bubbling CO2 or N2 through two toluene-filled glass bubblers in-series. A thermostat maintained the bubbler temperature slightly below ambient (290 K) to prevent toluene condensation in the lines. The toluene-saturated streams were then mixed with CO2 or N2 streams to obtain the desired toluene feed concentration. Retentate and permeate flow rates were measured with soap film flow meters. No sweep gas was used. The toluene feed compositions were calculated from the toluene vapor pressure and the gas flow rates, and these calculated compositions were consistent with GC analysis. To determine the effect of toluene or SF6 on permeation through the membrane, initially single-component CO2 or N2 (both AirGas UHP) permeances were measured, and then up to 0.65% toluene (Sigma Aldrich) or up to 1.5% SF6 (AirGas) was added to the feed. The total feed flow rate was 300 sccm, the feed pressure was 172 kPa, and the permeate pressure was 85 kPa. An adsorption trap filled with 3A zeolite was placed at the inlet of the membrane module to remove water or other impurities from the feed. This trap only slightly affected equilibration times after addition of toluene to the feed because toluene is too large to enter the 3 A pores.
2. Experimental methods 2.1. Preparation and characterization of SAPO-34 membranes SAPO-34 membranes were prepared using hydrothermal synthesis, as reported previously [1]. Briefly, tubular alumina supports (Inopor) with 100-nm pores were cleaned in boiling DI water, and then seeded by rubbing SAPO-34 crystals onto the inner surface. The seed crystals were prepared in a Teflon autoclave by microwave heating a gel to 453 K for 7 h. The gel contained Al(O-i-Pr)3, tetraethyl ammonium hydroxide (TEAOH), deionized water, Ludox AS-40 colloidal silica, and H3PO4 (all chemicals from Sigma-Aldrich) with a composition of 1.0 Al2O3: 2.0 P2O5: 0.6 SiO2: 4.0 TEAOH: 75 H2O. The membrane layer was grown by placing seeded supports into an autoclave with a synthesis gel containing the same precursors as the seed gel, but with a composition of 0.85 Al2O3: 1.0 P2O5: 0.3 SiO2: 2 TEAOH: 155 H2O. The autoclave was then held at 483 K for 6 h. After synthesis, the membranes were cooled, rinsed for 24 h in DI water to remove synthesis gel residue, calcined in vacuum at 823 K for 8 h with heating and cooling rates of 1 K/min, and then stored in an oven at 473 K in air. Scanning electron microscope images of a membrane prepared previously using the same procedures [1] showed a 3-μm thick SAPO-34 layer and 1–3 µm cubic crystals on the surface. The flux through defects in the SAPO-34 membrane was estimated from the CO2/isobutane ideal selectivity. Isobutane (0.50-nm kinetic diameter) is too large to enter the SAPO-34 pores and thus only permeates through defects. The membrane used for measurements at 172-kPa feed pressure had a CO2/isobutane ideal separation selectivity of about 80 at room temperature and 140-kPa feed pressure. The percentage of CO2 flux through defects for this membrane was estimated to be less than 1.5%, but as much as 30% of the N2 flux was through defects if Knudsen diffusion dominated the defect permeation, because N2 permeated more slowly than CO2. We have prepared SAPO-34 membranes with much higher CO2/isobutane selectivities previously [2], but the small defect contribution to the overall CO2 flux for this membrane does not affect the conclusions. A second SAPO-34 membrane was used for permeation measurements at 10-kPa feed pressure. This membrane had a CO2/isobutane ideal separation selectivity of 2200 at room temperature and 125-kPa feed pressure. Carbon dioxide flux through defects was estimated to be 0.05% of the total CO2 flux for this membrane.
2.4. Gas permeation measurements at 10 kPa The same system was used for permeation measurements at lower pressures. Feed and permeate pressures of 10 and 2.1 kPa, respectively, were obtained with mechanical vacuum pumps and controlled with backpressure regulators. The toluene feed compositions were calculated from the toluene vapor pressure and the gas flow rates. The toluene concentrations were confirmed by adsorbing toluene onto a packed bed of activated carbon from a flowing helium stream at the CO2/toluene experimental pressures and measuring the weight gain of the carbon bed. Helium was used as the carrier gas because it does not adsorb significantly on activated carbon. The permeate flux was measured with soap film flow meters at the exhaust of the vacuum pump. 3. Results and discussion 3.1. Effect of toluene on CO2 and N2 permeances at 172 kPa The SAPO-34 membrane had single-gas permeances of 2.0 × 10−6 mol/(m2 Pa s) for CO2 and 1.6 × 10−7 mol/(m2 s Pa) for N2 at 293 K and a feed pressure of 172 kPa (82-kPa permeate pressure). As shown in Fig. 1, when 1.1 kPa (0.65%) toluene was added to the feed, the CO2 permeance decreased by about 45% within a few minutes; it reached steady state after about 30 min. At lower toluene concentrations, permeances reached steady state faster (not shown). In contrast, as previously reported, CO2 permeances reached steady state over 2 d when propane was added to a CO2 feed because propane slowly diffuses into SAPO-34 pores [2]. Steady state was reached much more quickly when toluene was added to the feed because toluene only adsorbs on the external surface of SAPO-34 crystals. When toluene was removed from the feed, the CO2 permeance increased to within 10% of its initial permeance over several hours. The CO2 permeance fully recovered after heating the membrane to 473 K overnight in air. Some toluene may strongly adsorb on SAPO-34 at room temperature; Nagao et al. found that toluene was not completely removed from dehydroxylated TiO2 by evacuation for 4 h at 298 K [12]. They concluded from IR measurements that surface Ti atoms interact strongly with the π-electrons of the aromatic ring to prevent complete desorption, and toluene may have similar strong π-bonding on the SAPO-34 surface. Fig. 2 shows that the steady-state CO2 permeances decreased more
2.2. Characterization of SAPO-34 crystals The same hydrothermal synthesis used to prepare the membranes was also used to prepare SAPO-34 crystals for adsorption measurements. These crystals were cubic with sizes ranging from 0.4–3 µm, as determined by SEM. The XRD peaks were consistent with the SAPO-34 structure, and no other peaks were detected. Their BET surface area was 580 m2/g, which is in the range of values reported in the literature (483–677 m2/g) [10,11]. Toluene adsorption isotherms and BET surface areas were measured with a Quantachrome AS1-C volumetric adsorption system. 109
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Fig. 4. Steady-state CO2 permeances as a function of toluene fraction on feed side (log-mean feed/retentate fraction) at a feed pressure of 10 kPa and a permeate pressure of 2.1 kPa.
Fig. 1. Carbon dioxide permeance through a SAPO-34 membrane at 298 K and a total feed pressure of 172 kPa. Toluene at a partial pressure of 1.1 kPa (0.65 mol%) was added to the feed after 50 min, and it was removed after another 45 min.
1.1-kPa toluene was added to the feed, the N2 permeance decreased by 50%. 3.2. Effect of toluene on CO2 permeance at 10 kPa The CO2 permeance of a second SAPO-34 membrane was measured in the presence of toluene at a feed pressure of 10 kPa. The permeance measurements are in the pressure range where a toluene adsorption isotherm was measured so that IAST could be applied. This membrane had a CO2 single-gas permeance of 3.4 × 10−6 mol/(m2 Pa s) at 293 K and a feed pressure of 125 kPa (82-kPa permeate pressure), and a CO2 permeance of 4 × 10−6 mol/(m2 Pa s) at 10-kPa feed pressure and 2.1kPa permeate pressure. As shown in Fig. 4, when toluene was added to the feed, the CO2 permeance decreased. Steady-state was reached in 30 min after the toluene concentration increased. At the highest toluene concentration (0.87-kPa toluene, 8.5 mol% toluene in feed), the CO2 permeance decreased by 19%. When toluene was removed from the feed, the single-gas CO2 permeance recovered within 60 min.
Fig. 2. Carbon dioxide permeance through a SAPO-34 membrane at 298 K and a total feed pressure of 172 kPa. The upper line is the single-gas CO2 permeance, and the arrows indicate the changes in steady-state permeance after toluene was added.
3.3. Toluene adsorption on SAPO-34 crystals The decreases in CO2 and N2 permeances in the presence of toluene suggest that toluene either blocks access to surface pores or decreases coverage of the permeating species by competitive adsorption. Multicomponent models can estimate adsorption equilibria based on single-component adsorption isotherms, and thus toluene and CO2 adsorption on SAPO-34 crystals was measured. The BET surface area of uncalcined SAPO-34 crystals was 37 m2/g, and this was assumed to approximate the external surface area of calcined SAPO-34 crystals since N2 could not adsorb in the pores of the uncalcined SAPO-34 because they were filled with the structure-directing template, tetraethylammoniumhydroxide. Toluene and CO2 adsorption isotherms were also measured on the uncalcined crystals as a way to approximate adsorption on the external surface of crystals. The toluene isotherms were measured at three temperatures, as shown in Fig. 5, and toluene isotherms on calcined crystals were similar because toluene cannot enter SAPO-34 pores. The toluene isotherms at 296 K show type-II multilayer adsorption well below saturation pressure (3.4 kPa at 296 K). The desorption isotherm exhibits hysteresis, which may be due to capillary condensation between the crystals or stronger π-bonding with the OH-groups on the surface if adsorbed toluene reorganized at higher loading. Similarly, Nagao et al. measured type-II isotherms for toluene adsorption at 298 K on rutile TiO2 particles [12]. They observed multilayer adsorption at pressures that were above 5% of saturation, and concluded, based on BET analysis, that a toluene molecule occupied 0.602 nm2 on rutile TiO2. Molecular models predict that a toluene molecule adsorbed flat on a surface occupies about 0.47 nm2 [13], and
Fig. 3. Nitrogen permeance through a SAPO-34 membrane at 298 K and a total feed pressure of 172 kPa. The upper line is the single-gas N2 permeance, and the arrows indicate the changes in steady-state permeances after toluene was added.
as the toluene partial pressure increased. After each measurement at a given toluene partial pressure, the membrane was heated overnight to remove toluene. Pure-component CO2 permeances measured at the start of each experiment show that the CO2 permeance in the absence of toluene did not change over the course of the experiments (Fig. 2). Toluene had a similar effect on N2 permeance, as shown in Fig. 3. When 110
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Fig. 5. Toluene adsorption isotherms on uncalcined SAPO-34 crystals at 296, 313, and 333 K.
Fig. 7. Carbon dioxide adsorption isotherm on uncalcined SAPO-34 crystals (diamonds) at 296 K: dashed line - Langmuir fit; solid line - polynomial fit.
thus, Nagao et al. suggested that toluene packs loosely on the rutile surface. The toluene adsorption isotherm at 296 K, when fit to the BET model, yielded a monolayer coverage of 80 μmol/g, which was calculated to occur around 0.8 kPa, and is less than 20% of the toluene saturation pressure at 296 K (note the y-axis scale in Fig. 5). This coverage corresponds to an area of 0.77 nm2 per toluene molecule. Even though SAPO-34 is crystalline, its porosity and non-ideal termination of the crystals might make the external surface irregular, which could prevent dense packing of adsorbed molecules [14]. Thus, a toluene molecule may occupy more area on the SAPO-34 surface than the 0.602 nm2 reported for TiO2 crystals by Nagao et al. [12]. Nitrogen, which was used to measure the SAPO-34 external surface area, is much smaller than toluene and thus can pack more densely on a rough surface.
Table 1 Binary Langmuir isotherm parameters.
CO2 toluene
Keq [kPa−1]
Q [mmol/g]
0.024 1.6
0.68 0.16
The Langmuir parameters for toluene were determined from a fit between 0 and 1 kPa, and the parameters for CO2 were calculated from the isotherms measured between 0 and 100 kPa (Table 1). The model qualitatively accounts for the difference in surface areas of the different-sized molecules. The CO2 external surface coverages calculated from the binary Langmuir isotherm qualitatively correlate with the CO2 permeances (Fig. 8); as the CO2 surface coverage decreased, the CO2 permeance decreased. The surface coverage predictions at 172-kPa feed pressure may not be accurate because they were extrapolated from the Langmuir isotherm that was fit to the CO2 adsorption data, which was only obtained up to 100 kPa (Fig. 7). Also, the binary Langmuir isotherm has been criticized because it is not thermodynamically consistent and does not account for spreading pressure, which is the two-dimensional equivalent of partial pressures in gas-phase mixtures that is often used to describe adsorption equilibria on surfaces [17]. Ideal adsorbed solution theory is generally more accurate than the Langmuir model and is also thermodynamically consistent. Crittenden et al. showed that IAST reliably predicted adsorbed amounts in mixtures of up to six volatile organic chemicals in aqueous solution [18]. Sakuth et al. measured toluene and 1-propanol adsorption on Y-zeolites and reported that the IAST adequately predicted binary adsorption for a
3.4. Modeling of competitive adsorption on the external surface The Maxwell-Stefan model for transport through zeolite membranes assumes molecules adsorb on the external surface of the zeolite crystals before entering the pores [15,16], and thus a lower CO2 external surface concentration due to competition with adsorbed toluene reduces the driving force for CO2 permeation. The surface coverages of CO2 and toluene at a feed pressure of 172 kPa were estimated with a binary Langmuir model:
ni Ki Pi = Qi 1+Ki Pi + Kj Pj The total coverage Qi and equilibrium constants Ki for the Langmuir isotherms were estimated from single-component toluene and CO2 adsorption isotherms (Figs. 6 and 7). The loading for component i is ni.
Fig. 6. Toluene adsorption isotherm on uncalcined SAPO-34 crystals (diamonds) at 296 K: dashed line - Langmuir fit; solid line - polynomial fit.
Fig. 8. Carbon dioxide permeance at 172-kPa feed pressure versus CO2 surface coverage (from Langmuir isotherm). 111
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Fig. 10. Log-mean feed/retentate CO2 coverage of external membrane surface as a function of toluene vapor fraction on feed side (log-mean feed/retentate fraction).
Fig. 9. Single-component spreading pressure of toluene and CO2 as a function of pressure. Each set of points with the same spreading pressure is a solution for specific experimental parameters, and the unpaired point represents pure CO2, before any toluene was added.
where xi is the mole fraction of component i on the surface, yi is the mole fraction of component i in the vapor, and P is the total pressure. Fig. 9 shows the IAST solutions for each feed composition from Fig. 4. The spreading pressures of toluene and CO2 must be equal in the mixture, and the surface composition is determined by the lever rule around the total pressure, 10 kPa [17]. At the constant feed pressure of 10 kPa, increasing the toluene vapor concentration increased the toluene surface coverage, increased the spreading pressure (and total surface coverage), and decreased the CO2 surface coverage. The decreases in CO2 coverages estimated by IAST as a result of competitive adsorption with toluene are sufficient to explain the reduced CO2 permeances. The analysis assumes that the CO2 concentration in the pores near the membrane surface is directly proportional to the CO2 surface coverage. Adding toluene lowers the CO2 surface coverage, which lowers the CO2 concentration in the membrane, and thus the driving force for permeation decreases. Fig. 10 shows that the CO2 external surface coverage decreases as the toluene feed concentration increases; 8.5 mol% toluene in the feed stream reduced the CO2 surface coverage by about 20%. Fig. 11 shows the correlation between the average CO2 permeance for each toluene concentration in Fig. 4, and the estimated, corrected driving force for CO2 transport using the reduced surface coverage of CO2 (log-mean difference between the calculated CO2 external surface coverage on the feed/retentate and permeate side). Changes in the CO2 concentration in the membrane were calculated from CO2 fluxes using Fick's law, and the CO2 diffusion coefficient in the membrane was assumed independent of CO2 loading because loading was low at 10-kPa
zeolite with Si/Al ratio of 100, but that a predictive real adsorbed solution theory (PRAST) model was necessary to describe the non-ideal adsorption behavior on Y-zeolites with lower Si/Al ratios [19]. Furmaniak et al. simulated adsorption of CO2, CH4, and N2 on porous carbon and reported that the IAST model predicted surface coverages reasonably well. Simulated amounts adsorbed deviated from ideal behavior as the total pressure increased from 0.1 to 1.0 MPa, and deviation from ideal behavior was greater for more microporous systems [20]. The co-adsorption of CO2 and toluene on the SAPO-34 external surface at pressures below 10 kPa was modeled using IAST. Toluene adsorption isotherm data were used from 0 to 2.3 kPa; the polynomial fitting function becomes non-physical at higher pressures. The CO2 permeances were measured in a mixed feed with toluene partial pressures below 1 kPa so that the spreading pressures calculated in Fig. 9 stayed within the available data range. As discussed by Myers and Prausnitz [17], polynomials were fit to the CO2 and toluene singlecomponent adsorption data (Figs. 6 and 7):
ni° = Fi (Pi°) where ni° is the surface loading of pure component i, Fi(Pi°) is the numeric function for loading (polynomial fit with intercept at 0), and Pi° is the pressure of pure component i. Then πi°, the spreading pressure of pure component i on the SAPO-34 surface, was determined by numerical integration of this equation:
πi° (Pi°) =
RT A
P = P′
∫P=0
ni° (P ) dP = ψi (Pi°) P
where R is the gas constant, T is temperature, A is the area, and ψi(Pi°) is the numeric function for spreading pressure (polynomial fit with intercept at 0). The following equations were then solved simultaneously using the log-mean, vapor-phase composition for the feed side and the total feed pressure: ° ° πtol = ψtol (Ptol ) ° ° πCO = ψCO2 (PCO ) 2 2
° P ytol = Ptol x tol
° P yCO2 = PCO x 2 CO2
° ° πtol = πCO 2
Fig. 11. Normalized CO2 permeance versus normalized effective driving force for two measurements. Carbon dioxide permeance was normalized to the pureCO2 permeance, and effective driving force was normalized to the pure-CO2 trans-membrane driving force. The line indicates a 1-to-1 correlation.
x tol + x CO2 = 1 ytol + yCO2 = 1 112
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was determined from the membrane dimensions, the gas velocity (v = 0.15 m/s), and the gas-phase diffusivity of toluene (D = 8.7 × 10−6 m2/s) [24]:
feed pressure. At the highest toluene concentrations, the CO2 permeance decreased by 20% whereas the predicted drop is 30%. This discrepancy may be due to non-ideal adsorption on the external surface. Since the measured decrease in permeance was less than calculated, other processes (concentration polarization, blocking of the external zeolite pores by adsorbed toluene, toluene condensation) cannot account for the discrepancy since they would further decrease the permeance.
δ=
1 ⎛ vd ⎞−1/3 1/3 2/3 L d A⎝ D ⎠
where d is the membrane inner diameter (6 mm), L is the membrane length (30 mm), and A is dependent on the Graetz number (Gz = (v/D) d2/L ≈ 19), which is greater than 10, resulting in A = 2.43 [25]. The boundary layer thickness was about 1.2 mm. The Peclet number (Pe = v δ/D ≈ 0.029) was used to determine the polarization index, I, which is the ratio of the concentration near the membrane surface to the bulk concentration. The polarization index is [7,25]:
3.5. Effect of concentration polarization Replacing toluene in the feed with SF6 only lowered CO2 permeances slightly. This result demonstrates that concentration polarization does not cause the decrease in CO2 permeance when toluene was added to the feed at 172-kPa feed pressure. Concentration polarization occurs when the slower-permeating (rejected) species accumulates near the membrane surface because these only slowly diffuse away from the membrane surface. Thus, concentration polarization decreases the concentration and the driving force of the faster permeating species [7]. The saturation pressure of SF6 (1.2 MPa) is orders of magnitude higher than the saturation pressure of toluene (2.9 kPa) at 293 K, and thus SF6 adsorbs much less strongly than toluene (e.g., ΔHads for SF6 on NaX zeolite ~ 28 kJ/mol; ΔHads for toluene on Na-faujasite ~ 90 kJ/mol [21,22]). Like toluene, SF6 is too large to enter the SAPO-34 pores. Therefore, SF6, when used at the same concentrations as toluene, should have a small effect on CO2 surface loadings. As a result, the decrease in CO2 permeance when SF6 was added to the CO2 feed should be due mostly to concentration polarization, and this decrease should be an upper limit on how much concentration polarization decreases CO2 permeation in the presence of toluene. Fig. 12 shows that 1.2-kPa toluene decreased the CO2 permeance 10 times as much as SF6 at the same partial pressure. The CO2 permeance decreased by the same amount, 18%, for an SF6 pressure of 4 kPa and a toluene pressure of 0.1 kPa. Moreover, the bulk diffusivity of SF6 (MW of 146) is about 20% lower than that of toluene (MW of 96) [23]. Thus, concentration polarization should be slightly larger for CO2/SF6 mixtures than for CO2/toluene mixtures, and thus concentration polarization should account for less than 10% of the measured decrease in CO2 permeance at 1.2-kPa toluene pressure. To more accurately determine toluene surface concentrations, the increase in toluene partial pressure near the membrane surface due to concentration polarization was estimated from a transport model, and these concentrations were used in the determination of surface coverages discussed earlier. With a Reynolds number of about 100 (laminar-flow), the boundary layer thickness (δ) at the membrane surface
I = e Pe + β (1−e Pe ) where β is the enrichment factor, which is the ratio of the permeate concentration to the retentate concentration. The calculated values of I (0.9996 for CO2, 0.99999 for N2) show toluene does not decrease CO2 permeation because of concentration polarization. For the CO2/toluene feed, the toluene concentration near the surface increased by about 6% over the bulk concentration, for all toluene concentrations, but for the N2/toluene feed, the toluene concentration near the surface only increased by about 0.2% over the bulk concentration, because N2 fluxes are lower than CO2 fluxes. 3.6. Transport resistance of condensed toluene Even though the SF6 measurements and transport calculations suggest that concentration polarization is not significant for toluene/ CO2 membrane separations, toluene could condense if its partial pressure near the membrane surface increased enough, and a liquid film would decrease CO2 and N2 permeances. The adsorption isotherms in Fig. 5 indicate that toluene multilayers form well below its saturation pressure, and thus, even a moderate increase in toluene concentration could potentially cause condensation. The thickness of a liquid layer that would cause the observed decreases in CO2 permeances, neglecting any changes in CO2 surface coverage due to surface effects, was estimated using a resistance in series model [26]:
Rtot = R1 + R2 where Ri is the resistance to flow though layer i and Ri = Pi/(li ΔCi), Pi is the layer permeance, li is the layer thickness (3 µm for the zeolite layer), and ΔCi is the concentration change across the layer. Fick's Law was used to estimate the toluene film thickness. The interfacial resistance between the liquid film and the zeolite surface was assumed to be zero, and the added resistance was assumed to be due solely to CO2 diffusion through a condensed toluene film. The diffusion coefficient of CO2 in toluene is 4.9 × 10−9 m2/s [27], and Henry's Law constant for CO2 in toluene is 965 (Pa m3)/mol [28]. For the observed changes in permeance, the toluene film would have to be between 0.5 µm and 2.1 µm thick for toluene concentrations used. The toluene flowrates during experiments supply enough toluene for a 1-µm thick film in a few minutes. While a liquid layer several molecules thick could form on the surface below saturation pressure, a µm-thick film would have bulk liquid properties at the vapor interface and would evaporate, so this amount of liquid toluene is thermodynamically inconsistent. Therefore a liquid film would be too thin to completely account for the decrease in CO2 permeance, and surface resistances must be considered to account for the decreased CO2 permeances in the presence of toluene. The competitive adsorption model discussed above assumes a gas/solid interface and more than accounts for the decreased permeances, so a liquid-film resistance is unlikely.
Fig. 12. Normalized CO2 and N2 permeances through SAPO-34 membrane as a function of toluene partial pressure, and normalized CO2 permeance as a function of SF6 partial pressure. All measurements were at 298 K and a total feed pressure of 172 kPa. 113
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4. Conclusions
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Adding small amounts of toluene to the feed significantly decreased CO2 and N2 permeances through SAPO-34 membranes, even though toluene is too large to enter the SAPO-34 pores. Steady state was reached within 30 min after toluene addition. Through extrapolation, a Langmuir binary adsorption model gave a qualitative correlation between reduced surface coverage and reduced permeances. Analysis with IAST of permeation measurements at lower pressures also indicated that CO2 permeances decreased because CO2 concentrations were lower on the external surface of the membrane as a result of competitive adsorption with toluene. The small effect of SF6 on CO2 permeances demonstrated that concentration polarization was not the main reason that toluene decreased CO2 and N2 permeances. Acknowledgments We gratefully acknowledge support by the National Science Foundation, Grant CBET 1263130. References [1] R. Zhou, E.W. Ping, H.H. Funke, J.L. Falconer, R.D. Noble, Improving SAPO-34 membrane synthesis, J. Membr. Sci. 444 (2013) 384–393. [2] T. Wu, M.C. Diaz, Y. Zheng, R. Zhou, H.H. Funke, J.L. Falconer, et al., Influence of propane on CO2/CH4 and N2/CH4 separations in CHA zeolite membranes, J. Membr. Sci. 473 (2015) 201–209. [3] N.O. Chisholm, G.C. Anderson, J.F. McNally, H.H. Funke, R.D. Noble, J.L. Falconer, Increasing H2/N2 separation selectivity in CHA zeolite membranes by adding a third gas, J. Membr. Sci. 496 (2015) 118–124. [4] S. Li, G. Alvarado, R.D. Noble, J.L. Falconer, Effects of impurities on CO2/CH4 separations through SAPO-34 membranes, J. Membr. Sci. 251 (2005) 59–66. [5] J.C. Poshusta, R.D. Noble, J.L. Falconer, Characterization of SAPO-34 membranes by water adsorption, J. Membr. Sci. 186 (2001) 25–40. [6] K. Keizer, A.J. Burggraafa, Z.A.E.P. Vroon, H. Verweij, Two component permeation through thin zeolite MFI membranes, J. Membr. Sci. 147 (1998) 159–172. [7] A.M. Avila, H.H. Funke, Y. Zhang, J.L. Falconer, R.D. Noble, Concentration polarization in SAPO-34 membranes at high pressures, J. Membr. Sci. 335 (2009) 32–36. [8] S. Mokhatab, W.A. Poe, J.Y. Mak, Handbook of Natural Gas Transmission and Processing, 3rd edition, Elsevier, Amsterdam, 2015. [9] M.D. LeVan, T. Vermeulen, Binary Langmuir and Freundlich isotherms for ideal
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