Ind. Eng. Chem. Res. 1997, 36, 3815-3820
3815
Recovery of Volatile Organic Solvent Compounds from Air by Ceramic Membranes Pei Huang,† Nanping Xu,*,† Jun Shi,† and Y. S. Lin‡ Membrane and Science Research Center, Nanjing University of Chemical Technology, Nanjing 210009, People’s Republic of China, and Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221
Defect-free γ-Al2O3 and La2O3-modified γ-Al2O3 membranes were prepared for recovery of acetone from nitrogen by multilayer diffusion and capillary condensation mechanisms. There existed an optimum range for acetone permeance (∼8 × 10-7mol/m2‚s‚Pa) and acetone-nitrogen separation factor (∼1000) with respect to membrane temperature or feed composition. Hysteresises of acetone permeability and separation factor were also found between desorption and adsorption. The La2O3-modified γ-Al2O3 membrane appears to be the most effective for use in the VOC recovery process as compared to polymeric, Vycor, and unmodified γ-Al2O3 membranes. Introduction Many industrial processes such as printing, metal cleaning, or painting produce waste air streams containing low concentrations of volatile organic compounds (VOCs), such as acetone. The volume of VOCs involved in these processes is very large and represents a significant reuse opportunity. Total USA emissions in 1975 estimated by the Environmental Protection Agency (EPA) were about 31 million tons, and the total value of the compounds lost with the waste air was considerable. In addition, these VOCs represent a significant pollution problem. In past years several procedures for dealing with these problems, such as carbon adsorption, incineration, etc., were introduced in the industry. However, most of them are still not economically practical (Baker et al., 1987; Kimmerle et al., 1988; Paul et al., 1988; Larsson and Wimmerstedt, 1993). The membrane process has recently emerged as an attractive alternative to the conventional methods. In some preliminary investigations, polymeric membranes have been used, such as a hollow fiber composite membrane of silicone rubber. Although their selectivities are relatively high, the permeabilities for solvents are very small. Moreover, polymeric membranes cannot withstand high temperature and harsh chemical environments. It is known that the ceramic porous membranes not only have adequate thermal and chemical resistance but also have higher permeabilities (Qiu and Hwang, 1991). Knudsen diffusion and surface diffusion are common mechanisms for membrane separation of gas mixtures. However, these two mechanisms give rise to relatively low selectivities. Multilayer diffusion and capillary condensation are considered to be more effective for separation of vapor mixtures. Uhlhorn et al. (1990, 1992) used γ-Al2O3 membranes with 2.5 nm diameter pores and MgO-modified membranes with a decreased pore size to separate C3H6/N2 mixtures. C3H6 preferentially permeated through the membranes, and separation factors and permeabilities of C3H6 obtained were higher than those for Knudsen diffusion. Asaeda and Du (1986) separated light alcohol from water by con* Corresponding author. Telephone: 0086-025-33167553024. Fax: 0086-025-3211316. Email:
[email protected]. † Nanjing University of Chemical Technology. ‡ University of Cincinnati. S0888-5885(96)00760-9 CCC: $14.00
densate flow through a silica/alumina membrane with 3 nm pores. The azeotropic points in distillation were bypassed. Sperry et al. (1991) separated CH3OH/H2 mixtures using the alumina membrane with 2.5 nm pores. CH3OH also preferentially permeated through the membrane by a capillary condensation mechanism. Qiu and Hwang (1991) recovered acetone and ethanol vapors from the mixtures with nitrogen using a porous glass membrane by capillary condensation flow. It is obvious that these two separation mechanisms offer a great potential for separation of organic vapors from air and recover them for reuse. Lin et al. (1994) reported modification of γ-alumina membrane using La2O3 to change its pore structure and surface properties. In their work, doping La2O3 was done in order to improve the thermal stability of the γ-alumina membrane. In the present work, we report gas permeation properties of acetone and nitrogen for pure γ-alumina and La2O3-modified γ-alumina membranes. Acetone and nitrogen are selected to represent industrial VOC’s air. Experimental Section Membranes and Their Characterization. γ-Alumina membranes studied in this work were prepared by the sol-gel technique reported in the literature (Huang, 1996; Lin et al., 1994). Boehmite (γ-AlOOH) sol with pH ) 3.9 was synthesized by hydrolysis and condensation of aluminum butoxide. Tubular support used in this work was made of two R-alumina layers with respectively a pore size of 0.83 µm (base support) and 0.15 µm (intermediate layer). A 0.5 mol/L boehmite (γ-AlOOH) sol with 1.5 wt % PVA (Mn ) 7200) was made for preparation of the top layer. γ-alumina-supported membrane was prepared by a multiple sol coating/ drying/calcination process in order to obtain the top layer without pinholes or defects. The La2O3-modified alumina membrane was prepared by the common wetimpregnation method. In experiment, the gel membrane on the support was immersed in a 0.2mol/L La(NO3)3 solution for several seconds. The La(NO3)3coated alumina membrane was dried and calcined under controlled conditions. La(NO3)3 was converted to La2O3 in the calcination step. This preparation and modification process was repeated several times to minimize pinholes or defects in the top layer and to increase the amount of La2O3 coated on the surface of γ-alumina © 1997 American Chemical Society
3816 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
Figure 1. Schematic apparatus for solvent recovery: (1) gas cylinder, (2) dry tube, (3) needle valves, (4) gas flowmeter, (5) saturators, (6) pressure gauge, (7) preheat tube, (8) permeator, (9) manometer, (10) six-way valves, (11) massmeter, (12) buffer vessel, (13) gas chromatograph, (14) vacuum pump.
Figure 2. Cross-sectional SEM of asymmetric alumina membrane.
particles. The unsupported membranes were prepared by a similar process, in which a glass plate was used instead of the support tube. Pore-size distributions of supported and unsupported membranes were respectively determined by the modified permporometry method (Huang et al., 1996) and the nitrogen adsorption-desorption technique. The permeation test apparatus shown schematically in Figure 1 was used to measure nitrogen and acetone permeabilities and separation factors. The composition of the feed gas was adjusted by varying of the two saturators’ temperatures. The permeate stream pressure was kept at a low value by a vacuum pump to avoid the permeate condensing in the lines. The compositions of feed and permeate were measured by a gas chromatograph using hydrogen as the carrier gas. The permeabilities of acetone and nitrogen were calculated from the pressure difference through membrane, the composition, and flow rates of the feed and permeate flows. Results and Discussion Permeation Properties. Figure 2 shows the cross section of an asymmetric γ-Al2O3 membrane presented in this work. Pure N2 permeance data for γ-Al2O3 and La2O3-modified γ-Al2O3 membranes are plotted in Figure 3 versus the average pressure across the membrane. Doping La2O3 resulted in a 300-fold reduction in gas
Figure 3. Nitrogen permeability as a function of the mean pressure at room temperature.
Figure 4. Pore-size distributions of supported membranes by modified permporometry.
permeance, indicating a considerable amount of La2O3 is coated in the pores of the γ-Al2O3 membrane layer. The pore-size distributions of supported and unsupported γ-Al2O3 membranes before and after modification with La2O3 are shown in Figures 4 and Figure 5. The microstructural properties of both pure γ-Al2O3 and La2O3-modified γ-Al2O3 membranes are summarized in Table 1. As shown, modification by La2O3 appears as a reduction in the pore size of γ-Al2O3 membrane. Binary vapor permeation through the pure γ-Al2O3 and La2O3-modified ceramic membranes is examined in
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3817
Figure 5. Pore-size distributions of unsupported membranes by nitrogen adsorption and desorption.
Figure 6. Permeabilities of acetone and nitrogen as a function of membrane temperature for a γ-Al2O3 membrane.
Figure 7. Permeabilities of acetone and nitrogen as a function of membrane temperature for a La2O3-modified γ-Al2O3 membrane.
Figure 8. Separation factor of acetone-nitrogen mixtures as a function of membrane temperature for a γ-Al2O3 membrane.
Table 1. Microstructural Parameters of γ-Al2O3 and La2O3-Modified γ-Al2O3 Membranes membrane unsupported supported
γ-Al2O3
La2O3-modified γ-Al2O3
425 52.7 3.1 3.1
126 26.1 2.9 2.4
specific area (m2/g) porosity (%) mean pore size (nm) mean pore size (nm)
terms of the permeance and the separation factor, defined as follows:
Qi ) R12 )
Ji/A , ∆Pi
y2/y1 , x2/x1
∆P ) PF,i - PP,i yi )
PP,i , PP
xi )
PF,i PF
(i ) 1, 2)
(1)
(i ) 1, 2) (2)
where Qi is the permeance of species i (mol/m2‚s‚Pa), J is the permeation rate (mol/s), A is the membrane area (m2), PF and PP are the total pressure in the feed stream and permeation stream (Pa), respectively, and 1 and 2 represent nitrogen and acetone, respectively. The permeabilities and separation factors at different membrane temperatures were measured, and the results are shown in Figures 6-9. In these measurements acetone concentration in the feed was kept at 12 mol % for the γ-Al2O3 membrane and 8 mol % for the La2O3modified γ-Al2O3 membrane. The membrane temperature increased stepwise starting from the temperature at which the acetone saturation vapor pressure is equal to its absolute vapor pressure in the feed. The results
Figure 9. Separation factor of acetone-nitrogen mixtures as a function of membrane temperature for a La2O3-modified γ-Al2O3 membrane.
in Figures 6 and 7 show that, with increasing membrane temperature, acetone permeance reaches a maximum. It is also found that nitrogen permeance changes with membrane temperature. Before the maximum acetone permeance, capillary condensation takes place and the pores in the top layer are filled with condensate of acetone liquid. After that, capillary condensation disappears in the pores. It is known that multilayer diffusion coexisting with the capillary condensation may change the transport behavior (Uhlhorn et al., 1992). As a result, when membrane temperature increases, i.e., a decrease of the relative vapor pressure, the acetone
3818 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
Figure 10. Permeabilities of acetone and nitrogen as a function of acetone concentration for a La2O3-modified γ-Al2O3 membrane.
Figure 11. Separation factor of acetone-nitrogen mixtures as a function of acetone concentration for a La2O3-modified γ-Al2O3 membrane.
permeability increases due to the increase of pore area for multilayer diffusion. As soon as capillary condensation disappears in the pores, monolayer diffusion and/ or gas flow (such as Knudsen diffusion) may appear subsequently instead of multilayer diffusion. This is the reason that acetone permeability decreases again and nitrogen permeability increases with a further increase of the membrane temperature. The transport mechanism of nitrogen as a noncondensable gas in the pores is different from that of vapors when capillary condensation takes place. In the case of capillary condensation, nitrogen dissolves in and permeates through the condensate within pores (Elkamel and Noble, 1992). Nitrogen solubility decreases with an increase of temperature (Reid et al., 1987). However, nitrogen permeability increases rapidly as the capillary condensation disappears. The relative change of the gas permeability for acetone and nitrogen at different temperatures is better shown in Figures 8 and 9 in terms of the separation factor. There exists a maximum separation factor corresponding to the maximum acetone permeability and the minimum nitrogen permeability. The influence of feed composition on permeabilities and separation factors was determined at 5 °C, and the results are shown in Figures 10 and 11. The adsorption branch in the figures was measured by a stepwise increase of the acetone concentration in the feed. As opposed to adsorption, the desorption branch was measured by a stepwise decrease of the acetone concentration in the feed. It is found in Figure 10 that
there also exists a maximum of acetone permeability in both adsorption and desorption, and acetone permeabilities show hysteresis between adsorption and desorption. According to Uhlhorn et al. (1992), in the desorption case the condensate has already formed in the pores and might be slowly emptied by a decrease of the acetone concentration (i.e., a decrease of the acetone relative vapor pressure). Although the adsorbate layer thickness becomes smaller than the pore radius, there exists a meniscus, contrary to the adsorption case. Therefore, the liquid in the pore may disappear and the maximum of acetone permeability in the desorption would appear at lower acetone concentration than in the adsorption. The separation factor also shows hysteresis between adsorption and desorption in Figure 11 and can be explained in the same line of reasoning as given above. For all measurements, acetone permeability reaches a maximum by change of the membrane temperature or feed composition. That is due to the influence of the relative vapor pressure (defined as the ratio of the actual vapor pressure to its saturated pressure) on the VOC’s permeability. The present results are in agreement with previous results reported by Uhlhron et al. (1992) and described by Rhim and Hwang (1975), both reporting that the condensable vapor (C3H6, C2H6) permeability also reaches a maximum with a change of the relative vapor pressure. However, the relative vapor pressure influence on the separation factor found in this work is different from the reported results of Uhlhorn et al. (1992), who showed that an increase of the relative vapor pressure increases the separation factor of propylene and nitrogen for both the γ-Al2O3 membrane and MgO-modified γ-Al2O3 membrane. It is probable that the pore-size distribution of the membranes prepared without any defects and used in this work is narrower than that of membranes reported in the literature. Therefore, at the same relative vapor pressure, most of the pores can be filled with condensate and the transport (gas phase) of nitrogen is decreased considerably due to the blockage of the condensate in the pores. This is also the reason that the separation factor observed here is far higher than the previously reported results. Assessment for VOC Removal. Polymeric membranes have been applied for recovery of organic compounds (Baker et al., 1987; Kimmerle et al., 1988; Paul et al., 1988; Larsson and Wimmerstedt, 1993). Based on the permeation data of polymeric membranes, the process economics has been evaluated (Kimmerle et al., 1988; Paul et al., 1988). The result shows that the polymeric membrane process is indeed an attractive alternative to conventional air cleaning techniques. Due to several advantages of ceramic membranes over the polymeric ones, it is interesting to evaluate economics of recovering VOCs by ceramic membranes. The permeation properties of polymeric membrane, Vycor glass membrane, and the alumina membranes used in this work are compared in Table 2. It is remarkable that acetone permeabilities and separation factors of the γ-Al2O3 membrane and the La2O3-modified γ-Al2O3 membrane are higher than those of both the polymeric membrane and Vycor glass membrane. The Vycor glass membrane has a narrow pore-size distribution. However, it has a symmetrical structure. Therefore, its resistance for gas permeation is larger than the asymmetric porous ceramic membranes. For comparison of ceramic membranes process economics with those of other membranes, the membrane
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3819 Table 2. Comparison of Acetone Permeance and Acetone-Nitrogen Separation Factors of Four Membranes acetone Qacetone (×108 mol/m2‚s‚Pa)/Rnitrogen
acetone content (mol %)
acetone relative pressure
0.4 1 5 10
0.03 0.12 0.68 0.99
polymeric (Kimmerle et al., 1988)
8/9.5 12/26.7
1.22/25
area is chosen and the area of the La2O3-modified γ-Al2O3 membrane is chosen as a base. According to eqs 1 and 2, the following equation can be obtained for calculating each membrane area of recovering acetone from nitrogen
A)
J2 Q2∆P2 J2
[
Q2 x2PF -
R12x2PP 1 - x2 + R12x2
(3)
]
(4)
where
∆P2 ) PF,2 - PP,2 ) x2PF - y2PP
(5)
x1 + x2 ) 1
(6)
y1 + y2 ) 1
(7)
y2 )
γ-Al2O3 (this work)
γ-Al2O3/La2O3 (this work)
70/720 40/650
12/390 33/650 21/600 16/170
0.06/11
Figure 12. Comparison of areas Al and MAl indicating a γ-Al2O3 membrane and a La2O3-modified γ-Al2O3 membrane, respectively.
A)
Vycor (Qiu and Huang, 1991)
R12x2 1 - x2 + R12x2
(8)
PF (feed stream pressure) ) 120 kPa and PP (permeation stream pressure) ) 10 kPa are assumed. When the acetone permeation rate (J2) is assumed as a constant value, the membrane area with the acetone concentration in the feed can be calculated. The area ratios to polymeric membrane, Vycor glass membrane, and γ-Al2O3 membrane are calculated and shown in Figure 12. It is clear that the area of the La2O3-modified γ-Al2O3 membrane is considerably smaller than that of the polymeric membrane, especially in the range of lower acetone concentration. It is also found that, for recovery of acetone at lower concentration, the area of the La2O3-modified γ-Al2O3 membrane
is smaller than that of the γ-Al2O3 membrane and that of the Vycor glass membrane. However, the area is larger than that of the γ-Al2O3 membrane and the Vycor glass membrane in the range of higher acetone concentration. This is because the pore size of the La2O3modified γ-Al2O3 membrane is smaller than that of the γ-Al2O3 membrane and the Vycor glass membrane. Therefore, capillary condensation will take place at lower acetone concentration. Ceramic membranes have a disadvantage: higher module price (about 10-15 times that of polymeric membranes). However, by the above-calculated results, the ceramic membrane area requited for recovery of acetone from nitrogen is over 10 times smaller than that of polymeric membrane. Moreover, ceramic membranes have an advantage of long application life ranging from 3 to 5 yr, compared to that of polymeric membranes of about several months. This indicates that the ceramic membrane process is an attractive alternative to the polymeric membrane process and other conventional techniques for recovery of VOCs from air. Conclusions Defect-free asymmetric γ-Al2O3 membrane with narrow pore-size distribution was prepared by a sol-gel technique, and the alumina membrane was modified by La2O3 using the wet-impregnation method and its pore size was decreased. Multilayer diffusion and capillary condensation mechanisms were used to separate acetone-nitrogen mixtures which represented the industrial waste streams. The influences of membrane temperature and feed composition on acetone and nitrogen permeabilities and separation factors were investigated. Acetone permeability appeared a maximum as membrane temperature or feed composition increased, and so did the separation factor. There existed an optimum area for recovery of organic vapor by multilayer diffusion and capillary condensation mechanisms. Decrease of the pore size would also increase the solvent permeability and separation factor at lower acetone content. The hysteresises of acetone permeability and separation factor were also found between adsorption and desorption. Comparing with polymeric and Vycor glass membranes, asymmetric alumina membranes exhibit higher acetone permeability and separation. This shows that the alumina membranes could be effectively used to recover organics from waste gas stream. Acknowledgment Authors are very grateful for the financial support by National Foundation of Natural Science of China and the Ministry of Chemical Industry of China. Y.S.L. acknowledges the support of U.S. WSF for the collaboration.
3820 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
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Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill Book Company: New York, 1987. Rhim, H.; Hwang, S. T. Transport of Capillary Condensate. J. Colloid Interface Sci. 1975, 52, 174-181. Sperry, D. P.; Fabconer, J. L.; Noble, R. D. Methanol-Hydrogen Separation by Capillary Condensation in Inorganic Membranes. J. Membr. Sci. 1991, 60, 185-193. Uhlhorn, R. J. R.; Keizer, K.; Burggraaf, A. J. Gas Transport and Separation Properties of Ceramic Membranes with Pores of Molecular Dimensions. Paper presented at the International Congress on membranes and membrane processes (ICOM’ 90), Chicago, IL, Aug 1990. Uhlhorn, R. J. R.; Keizer, K.; Burggraaf, A. J. Gas Transport and Separation with Ceramic Membranes. Part I. Multilayer Diffusion and Capillary Condensation. J. Membr. Sci. 1992, 66, 259-269.
Received for review December 2, 1996 Revised manuscript received March 19, 1997 Accepted March 24, 1997X IE960760B
X Abstract published in Advance ACS Abstracts, May 1, 1997.
