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Ind. Eng. Chem. Res. 1999, 38, 2292-2298
Cuprous-Chloride-Modified Nanoporous Alumina Membranes for Ethylene-Ethane Separation Y. S. Lin,*,† W. Ji,‡,§ Y. Wang,† and R. J. Higgins‡ Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221, and CeraMem Corporation, 12 Clematis Avenue, Waltham, Massachusetts 02154
This paper reports an attempt to synthesize a CuCl-modified γ-alumina membrane for separation of ethylene from ethane. CuCl was effectively coated in the 4 nm pore γ-alumina top layers of disk-shaped and tubular alumina membranes by the reservoir method. Permeation of a single gas and binary mixture of ethylene and ethane was measured to characterize separation properties of the modified membranes. Pure ethylene permeance of the CuCl-modified membrane is 10-40% lower than that predicted from the pure ethane permeance by the Knudsen theory. This result is explained by a model based on the adsorbed layer of ethylene via π-complexation. Such an adsorbed layer hinders the diffusion of ethylene in the nanopores of CuCl-modified γ-alumina. Multiple gas permeation measurements on the CuCl-modified membranes show a separation factor for ethylene over ethane larger than the Knudsen value. This confirms a positive contribution of the surface flow of ethylene to the permeance of ethylene in the multiple gas permeation system. A maximum separation factor for ethylene over ethane of 1.4 is obtained for the CuCl-modified membrane at 60 °C. Introduction Ethylene is traditionally produced by a method that combines high-temperature cracking with low-temperature cryogenic distillation, which is energy-intensive.1 Among a number of alternative methods, separation based on π-complexation appears most promising.2 The π-complexation pertains to the main group (or d-block) transition metals (such as Cu and Ag) whose ions can form weak chemical bonds with ethylene. Some efforts have been reported to develop the adsorption systems with Cu+ or Ag+ as the active site for the selective adsorption of ethylene. Recent work in this area includes preparation and adsorption property study of ethylene on Cu+ supported on zeolite, alumina, silica, activated carbon, and polymer resin.3-6 Separation of ethylene by membranes, if available, is more desirable because of the simplicity and energy efficiency of the membrane separation processes. Several efforts have been made to develop a separation system for ethylene based on the supported liquid membrane which contains Ag+ or Cu+ ions as the active sites for selective permeation. Immobilized liquid membranes containing mobilized reactive species (Ag+) have been applied to the separation of ethylene and other olefins from hydrocarbon mixtures.7-11 Cuprous (Cu+) ions also have been applied as carriers for the facilitated transportation of carbon monoxide in liquid membranes.12,13 However, the immobilized liquid membranes suffer from the instability problem which limits its industrial applications. Sol-gel-derived CuCl/γ-alumina adsorbents have large surface areas and nanopore sizes with uniform * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Cincinnati. ‡ CeraMem Corp. § Current address: BOC Gases, R&D, 100 Mountain Avenue, Murray Hill, NJ 07954.
pore size distributions.14 Preferential adsorption of ethylene and fast sorption kinetics on the CuCl-modified γ-alumina thin films and granular particles (2-3 mm) were reported recently by Wang and Lin.15 The methods used to make these thin films and granular sorbents in these studies were similar to that for synthesis of supported γ-alumina membranes.16 These results show feasibility in preparing solid inorganic membranes with selective surface flow of ethylene over ethane. This paper reports the synthesis of the CuCl-coated γ-alumina membranes and their permeation separation properties for ethylene over ethane. Experimental Section CuCl was coated on the inner pore surface of the top layer of both disk-shaped and tubular alumina membranes. The disk-shaped alumina membranes consisted of a thin (4-5 µm) nanopoporus γ-alumina top layer and a thick (2-3 mm) macroporous R-alumina support. These membranes of 20 mm in diameter were prepared by the sol-gel method according to the procedure described in our earlier publications.16,17 In brief, 1 M boehmite sol was prepared by hydrolysis and condensation of aluminum butoxide, followed by peptization with HNO3. Thin boehmite films were coated on the R-alumina support by the slipcasting process (dip-coating). Boehmite was converted to γ-alumina upon calcination in air at 450 °C. Unsupported γ-alumina layers of about 0.1 mm thick were prepared by drying a given amount of the boehmite sol in Petri dishes, followed by the same calcination process.15 These samples were prepared for characterizing the membrane pore structure and for adsorption study. Tubular alumina membranes of 7 mm inner diameter and 25 cm in length were supplied from U.S. Filter. The membrane consisted of a thin (2-3 µm) nanoporous γ-alumina top layer and an asymmetric coarse pore R-alumina support. The average pore diameter of the γ-alumina layers in both disk-shaped and tubular
10.1021/ie980662l CCC: $18.00 © 1999 American Chemical Society Published on Web 04/30/1999
Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2293
Figure 1. Schematic diagram of gas permeation setup for pure gas permeation measurement.
membranes was about 4 nm, with a narrow pore size distribution.14,15 The R-alumina support of the diskshaped membranes had a symmetric structure with a pore diameter of about 0.2 µm, but the support of the tubular membranes was made of multilayer R-alumina with a pore diameter changing from about 0.2 to 5 µm for various layers.18 Therefore, the support of the tubular membrane offers a significantly lower masstransfer resistance than that of the disk-shaped membranes. The reservoir method was used to coat CuCl on the membrane γ-alumina top layer. This is an effective coating method for modifying the surface chemical properties of the membrane top layer by impregnation and subsequent controlled drying.19,20 In this method, the whole supported membrane is initially filled with an impregnation solution and then dried from the toplayer side. During the drying stage, the support acts as a continuous reservoir which supplies the top layer with impregnation solution. In this way high loading of the active species could be achieved within the membrane top layer. The cuprous solution was prepared from cuprous chloride (CuCl) (Reagent ACS, Matheson), ammonium hydroxide (30 NH3 wt %) (Reagent ACS, Fisher), ammonium citrate (Reagent ACS, Merck), and deionized water with the composition of 1.98 g/8.8 mL/0.4 g/20 mL. Supported γ-alumina membranes were put into the cuprous solution for more than 16 h under a nitrogen atmosphere. The membranes were then dried from the top-layer side in a vacuum oven at 40 °C under a small flow of nitrogen (normally for 4 h). CuCl was also coated in the unsupported γ-alumina layers by the conventional wet-impregnation method.15 The coated membranes were then calcined at 200 °C in a nitrogen flow for 10 h with a heating and cooling ramp of 30 °C/h. X-ray photoelectron spectroscopy (XPS) analysis showed that the CuCl-coated samples prepared by this method did not contain the nitrogen element.14 This indicated that NH4+ was decomposed from the membrane samples upon calcination at 200 °C. Prior to permeation measurement, a membrane was further activated in the permeation cell at 250 °C in ethylene flow for 4 h (to reduce possible Cu2+ to Cu+). The modified membranes were characterized by XRD (Cu KRl) and energy-dispersive analysis for X-rays (EDAX) for phase and chemical composition. Pore structure and pore size distribution of unsupported membranes were characterized by nitrogen adsorption porosimetry (Micromeritics ASAP2000). Adsorption equilibrium of ethylene on CuCl-modified unsupported γ-alumina layers was measured by the gravimetric method with an electronic microbalance (Cahn 1000).15 Single-gas permeation through tubular membranes was measured on the permeation setup shown in Figure
1. The permeation flow from the shell side was measured by a bubble flowmeter while the pure gas was passing through the tube side at a constant pressure. After measurement at one condition, the system was purged with nitrogen for 15 min prior to testing at the next condition. Typically, three measurements were performed at each condition in order to determine that steady state had been achieved. The permeance was calculated from the permeation flow rate, Q, and membrane upstream and downstream pressures, Ph and Pl by
F)
Q S(Ph - Pl)
(1)
where S is the membrane surface area. The ideal separation factor is the ratio of the pure ethylene permeance to pure ethane permeance. Multiple gas permeation on the disk-shaped membranes was performed on a permeation apparatus shown in Figure 2. This permeation measurement setup included mass flow controllers to set the feed composition and a gas chromatograph (GC) for composition analyses. The feed was a mixture of ethylene, ethane, and nitrogen. Nitrogen was also used as the sweep gas for the permeation cell and carrier gas for gas chromatography equipment. A four-way sampling valve in the system was used as a switch between the feed gas and permeate gas for GC sampling. A computer was used for acquisition and processing of the data obtained from the GC. The gas permeance and separation factor for species i in multiple-gas permeation were calculated by
Fi )
Qi S(XiPh - YiPl)
(2)
and
Ri/j )
Yi/Yj Xi/Xj
(3)
where Ph and Pl are the total pressure upstream and downstream (both being 1 atm in this work) and Xi and Yi are the molar fractions of species i and j in the effluents (outlets) upstream and downstream. Results and Discussion The membranes prepared with careful calcination in an oxygen-free environment (in the permeation cell) exhibited a pale green color, the same as that of the cuprous chloride powder from the commercial source. The XRD pattern of such a CuCl-coated membrane is given in Figure 3, which shows XRD peaks of CuCl in addition to those of R-alumina (γ-alumina layer is X-ray amorphous). However, samples calcined in a regular vacuum oven (under nitrogen flow) had a very dark black color. XRD patterns given in Figure 4 show that CuCl has been converted to CuO. As we know, the Cu+ ion is very sensitive to the oxidizing environment and exposure to even a small amount of air (oxygen) may cause its oxidization, especially at high temperatures. So, when the CuCl-coated membrane was thermally treated in a normal vacuum oven without a complete gastight feature, a small leakage of air into the oven may oxidize Cu+ ions to form CuO. All the gas perme-
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Figure 2. Schematic representation of the permeation apparatus for multiple-gas permeation measurement.
Figure 3. XRD pattern of a CuCl-coated γ-alumina supported membrane.
ation data to be present were measured on the membranes calcined in the permeation cell. Figure 5 is the result of EDAX analysis showing Al and Cu profiles along the cross section of the CuClmodified γ-alumina layer of a tubular membrane. The presence of copper in the entire modified γ-alumina layer (about 2-3 µm) is clearly shown in the figure. Cu diminishes around 7 µm from the top-layer surface. This indicates that there is essentially no CuCl in the membrane support (about 2 mm in thickness). The analysis gives more Cu than Al in the top layer. Since EDAX does not measure the bulk concentration of the elements, this result does not necessarily indicate that the top layer contains more Cu than Al. However, it clearly shows that a considerable amount of CuCl was coated on the membrane top layer. Unsupported γ-alumina membranes were characterized by nitrogen adsorption porosimetry to provide the pore structure of the membranes studied in this work. Figure 6 shows pore size distributions of the unmodified and CuCl- (about 30 wt %) modified γ-alumina membranes. Both membranes have a very narrow pore size distribution (2-6 nm), with an average pore diameter of about 3-4 nm. The CuCl-modified membrane has a
smaller pore size than the unmodified one. The surface area and pore volume of the CuCl-modified membrane are 305 m2/g and 0.27 cm3/g, also smaller than those of the unmodified membrane (322 m2/g and 0.36 cm3/g). These results show that coating CuCl has changed the pore structure of the membrane. Single-gas permeance of ethane and ethylene was measured for the unmodified and modified tubular membranes (with a transmembrane pressure of about 2 atm and downstream pressure of 1 atm). Table 1 lists the permeance and ideal separation factor of ethylene to ethane for the unmodified and CuCl-modified tubular alumna membranes. For the unmodified membranes, the ethylene-to-ethane separation factor is 0.9. The CuCl-coated membranes exhibited 3-10-fold reduction in gas permeance, indicating that a sufficient amount of CuCl was coated in the membrane top layer, as also verified by the EDAX data shown in Figure 5. Coating CuCl in the membranes caused more reduction in the permeance of ethylene than ethane. The higher the CuCl loading, the more reduction in the ethylene permeance relative to ethane. For the four-time CuClcoated membrane, the ideal separation factor for eth-
Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2295
Figure 4. XRD pattern of a CuCl-coated γ-alumina supported membrane showing formation of CuO due to oxidation.
Figure 5. Composition profiles from EDAX through a CuClmodified tubular γ-Al2O3 membrane.
ylene to ethane is 0.65, as compared to 0.90 for the unmodified membrane. For the tubular membranes, the mass-transfer resistance of the support is negligible. The permeance data given in Table 1 can be considered as those for the γ-alumina top layer. For permeation of pure gas through the 4 nm pore γ-alumina layer where viscous flow can be neglected, the permeance is correlated to the contribution of the Knudsen and surface flow as20
2µkrv Ds dq F) + F(1 - ) 2 3RTL k L dP
(4)
s
where is the porosity of the membrane system, µk the shape factor (reciprocal of the tortuosity) of the membrane, r the mean pore radius of the membrane, v the average molecule velocity [equal to (8RT/πM)1/2], F the true density of the membrane, Ds the surface diffusion coefficient, ks the tortuosity of the surface, and (dq/dP) the average slope of the adsorption isotherm (amount adsorbed versus pressure). Equation 4 indicates that the surface flow is determined by the adsorption properties and the surface diffusivity of the adsorbed species. Figure 7 shows adsorption isotherms of ethylene on the unsupported CuCl- (30 wt %) modified γ-alumina membrane. The Henry constant for ethylene, defined as the slope of the adsorption isotherm at zero pressure, is 35.3, 11.6, and 6.5 mmol/g‚atm at 25, 60, and 100 °C, respectively. Both CuCl-modified and unmodified γ-alu-
Figure 6. Pore size distribution of unmodified and CuCl-modified γ-alumina membranes. Table 1. Permeance and Ideal Separation Factor for Unmodified and CuCl-Modified Tubular γ-Al2O3 Membranes C2H4 C2H6 permeance permeance mem- CuCl permeation (10-6 mol/ (10-6 mol/ C2H4/C2H6 brane coats temp (°C) m2‚s‚Pa) m2‚s‚Pa) selectivity A B B C D D E
0 2 2 2 4 4 4
20 20 250 20 20 250 20
8.61 3.76 2.05 3.43 1.39 0.79 0.86
9.54 4.65 2.16 4.29 2.11 0.79 1.09
0.90 0.81 0.95 0.80 0.65 1.01 0.78
mina films have a much lower adsorption capacity for ethane.14,15 For example, the Henry constant for the adsorption of ethane in the CuCl-modified membrane is 2.6 mmol/g‚atm at 25 °C. These results show that the concentration of ethylene on the internal pore surface of the γ-alumina membrane is significantly higher than that in the gas phase. Therefore, a considerable surface flow was expected to give a larger permeance for ethylene than for ethane. The experimental results listed in Table 1 are, however, the opposite. The ideal separation factor for species i to j based on the Knudsen permeation mechanism is equal to the ratio of the square root of the molecular weight for species j to that for species i. This value for ethylene
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Figure 7. C2H4 adsorption isotherms on an unsupported CuCl/ γ-AL2O3 membrane.
over ethane is 1.04. On the other hand, the viscous flow (through possible membrane defects) under a total pressure gradient is inversely proportional to the viscosity of the gas. If the permeation is governed by the viscous flow, the ideal separation factor for ethylene over ethane is 0.91 at 20 °C from the viscosity data of ethylene (0.0101 cp) and ethane (0.0092 cp).21 The ideal separation factor for the permeation of ethane and ethylene through the 4 nm pore membrane should be between of 0.91 (lower limit) and 1.04 (higher limit) at 20 °C. Additional surface flow of the ethylene is expected to give an ideal separation factor larger than the upper limit. The experimentally measured separation factors shown in Table 1 are all smaller than the lower limit, indicating that adsorption of ethylene on the CuCl-modified layer in fact suppresses the total flow of ethylene. The reduction in ethylene permeance with respect to ethane is more significant for membranes with a higher loading of CuCl and at lower temperatures. These results are different from what was expected, that the adsorbed layer would provide additional surface flow, increasing the total permeance of ethylene. Such seemingly unusual experimental results can be explained by considering the change of the pore size affecting the Knudsen flow term because of the adsorbed layer, as shown in eq 4. Figure 8a shows a crosssectional view of an idealized straight pore with its length representing the thickness of the γ-alumina membrane layer. The sol-gel-derived γ-alumina has slit-shaped pores 22,23 with the slit height (about 4 nm) being the pore diameter, as shown in Figure 8b. The slit length is represented by b in Figure 8b. The shaded area represents CuCl coated on the γ-alumina surface and the dark area the adsorbed ethylene layer on the surface of CuCl. During permeation of pure ethylene, the adsorbed layer of ethylene could effectively narrow the pore size of the γ-alumina layer. This reduces the Knudsen flow (first term in eq 4) in an amount exceeding the additional flow provided by the surface diffusion (second term in eq 4). Such an adsorbed layer is negligible during permeation of pure ethane. A quantitative analysis is presented next to support this argument. Deposition of a solid (e.g., CuCl) in the slit-shaped pore of the γ-alumina layer occurs in such
Figure 8. Proposed model for ethylene permeation through a CuCl-modified γ-Al2O3 membrane.
a manner as to first transform the pore from the slit shape to the circular one and then to reduce the radius of the circular pore,20,22,23 as shown in Figure 8b. Thus, according to eq 4 and the geometry shown in Figure 8b, the reduction in ethane permeance due to modification of CuCl could be correlated to initial R0 and final pore radius R as23
φ ) 1.57λ(R/R0)3
(5)
where φ is the ratio of the gas permeance of the modified membrane to that of the unmodified membrane and λ is the ratio of the slit radius to the length (R0/b) with b being the slit length. With an adsorbed layer of ethylene of t in thickness, the effective pore radius for ethylene diffusion becomes (R - t), smaller than that for ethane diffusion. Considering the difference in the effective pore radius and neglecting the contribution of the surface flow, the modified separation factor for ethylene over ethane is
Rm ) (1 - t/R)3Rk
(6)
where Rk is the Knudsen separation factor ()1.04). Combining eqs 5 and 6 gives
Rm ) [1 - (1.57λ/φ)1/3(t/R0)]3Rk
(7)
Equation 7 clearly shows that the ideal separation factor decreases (deviates more from the Knudsen value) as the adsorbed layer thickness increases (higher t, corresponding to a lowering temperature) or the permeance reduction by modification decreases (smaller φ, corresponding to more coats of CuCl). The results of the theoretical analysis are consistent with the experimental findings. For membranes with larger initial pore sizes (larger R0), such a suppression effect of the adsorbed layer cannot be observed, as shown by eq 7. A more quantitative comparison of eq 7 with experimental data is given next for the last membrane sample listed in Table 1. For this CuCl-modified membrane, φ is 0.11 and R0 ) 2 nm. The value of λ of 0.06 can be used for the sol-gel-derived γ-alumina membranes.23
Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2297 Table 2. Gas Permeation Properties of a C2H4 and C2H6 Mixture (in Nitrogen) through a CuCl-Modified Alumina Membrane at Different Temperatures (N2 Flow in the Upstream and Downstream Were Respectively about 5.5 and 5.8 cm3/min) C2H4%
upstream (outlet) C2H6% C2H4/C2H6
downstream (outlet) C2H4% C2H6%
separation factor RC2H4/C2H6
permeance (mol/m2‚Pa‚s) C2H4 C2H6
0.041 0.056 0.127
0.324 0.296 0.189
0.127 0.189 0.675
0.028 0.027 0.068
25 °C 0.197 0.125 0.093
1.11 1.16 1.08
3.57 × 10-7 1.51 × 10-7 1.83 × 10-7
2.63 × 10-7 1.14 × 10-7 1.55 × 10-7
0.048 0.050 0.143
0.413 0.326 0.243
0.117 0.154 0.591
0.024 0.020 0.052
60 °C 0.150 0.115 0.074
1.36 1.14 1.20
2.04 × 10-7 1.36 × 10-7 1.13 × 10-7
1.19 × 10-7 1.10 × 10-7 8.62 × 10-8
0.039 0.047 0.136
0.323 0.274 0.184
0.120 0.170 0.739
0.018 0.023 0.055
100 °C 0.139 0.111 0.073
1.09 1.22 1.01
1.81 × 10-7 1.93 × 10-7 1.31 × 10-7
1.53 × 10-7 1.35 × 10-7 1.29 × 10-7
Assuming 0.2 nm for the thickness of the adsorbed layer, Rm predicted from eq 7 is 0.77. This is close to the experimental data. It should be pointed out that accurate prediction of the effect of the adsorbed layer on the permeation flux is very difficult, if not impossible. This requires an accurate quantification of the microstructure of the pores and the arrangement of the adsorbed molecules. Nevertheless, the above analysis is sufficient to support the argument that the adsorbed layer hinders gas diffusion in the nanopores of the γ-alumina layer. In the multiple-gas permeation, the effect of the adsorbed layer of ethylene on the permeance of both ethylene and ethane should be same. It was expected that a more selective permeation of ethylene over ethane on the CuCl-modified membranes would be observed in the multiple-gas permeation experiments. To simulate more closely a differential permeation reactor, multiplegas permeation was performed on the disk-shaped CuClmodified membranes. Single-gas permeation measurements show that the ethane permeance for the modified membrane disks is about 70-80% of that for the unmodified membranes. Since the mass-transfer resistance of the support in the disk-shaped membranes is 1/3-1/2 of the total resistance of the unmodified membranes,16 the modification resulted in about 1.5-2-fold reduction in the gas permeance for the top layer where CuCl was coated. The ideal separation factor for ethylene over ethane is again lower than the Knudsen value (0.91 at 20 °C), consistent with the results of the tubular membranes. Table 2 shows the permeance and separation factor for the multiple-gas permeation through the CuClmodified disk-shaped membranes at different temperatures and gas composition. The experimental results show a separation factor ranging from 1.1∼1.4 for the multiple-gas separation in the composition and temperature ranges studied. These values are larger than the Knudsen separation factor. Obviously, the surface flow has appreciable contribution to the ethylene permeance. The maximum separation factor is found at 60 °C. Since the surface diffusivity (Ds) increases and adsorption amount (dq/dP) decreases with increasing temperature, as to be shown in the next paragraph, this indicates that the product of Ds (dq/dP) is maximum at 60 °C. The gas permeance of the disk-shaped membranes is much lower than that of the tubular membranes (Table 1). This is mainly due to the fact that the mass-transfer resistance of the support used in the tubular membrane is negligible. Furthermore, the top layer of the disk-shaped
Table 3. Estimation of Surface Diffusivity Data for Ethylene in CuCl-Coated γ-Alumina Membranes temperature (°C) Ke (mol/kg/atm) feed C2H4% for Fs Fs (10-7 mol/m2‚Pa‚s) × 10 Ds (10-7 cm2/s)
25 35.3 4.1 0.83 0.28
60 11.6 4.8 0.80 0.83
100 6.5 4.7 0.52 0.97
membranes is also thicker than that of the tubular membranes. The average slope of the adsorption isotherm (dq/dP) at low ethylene partial pressures (
