I n d . E n g . Chem. Res. 1995,34, 4494-4500
4494
Study of Volatile Hydrocarbon Emission Control by an Aromatic Poly(ether imide) Membrane S. Deng, A. Sourirajan, and T. Matsuura* Industrial Membrane Research Institute, Department of Chemical Engineering, University of Ottawa, Ottawa, Canada K I N 6N5
B. Farnand Natural Resources Canada, CANMET-ERL Division, 555 Booth Street, Ottawa, Canada K1A OG1
The separation of the mixtures of hydrocarbon vapors from nitrogen was studied by using a n asymmetric aromatic poly(ether imide) membrane prepared by the phase-inversion technique. The effect of the presence of water in the feed stream on the separation of hydrocarbon vapors from nitrogen and on the separation of gasoline vapor from nitrogen was also investigated. It was found that the asymmetric membrane is effective for the separation of hydrocarbon vapors from nitrogen without silicone rubber coating. It was also found that the permeability of water is far greater than those of hydrocarbon vapors.
Introduction Many industrial processes handling organic solvents produce solvent containing air exhaust streams. These streams cause not only severe air pollution problems but also a significant economic loss. In the past, when air pollution regulations were lax and the solvents were not expensive, these organic solvent vapors were simply discharged into the atmosphere. Since the 19709, however, the need for air pollution control was recognized. Industries that produce organic solvent contaminated air streams have come under increasing economic and regulatory pressure. The volatile organic compounds produce a significant emission annually (Theodore and Buonicore, 1988). From an environmental point of view, it is necessary to limit and control vapor emissions because they affect the change of climate, the growth and decay of plants, and the health of human beings and all animals. For example, according t o a report of the National Academy of Sciences, the release of chlorofluoromethanes and other chlorine-containing compounds in the atmosphere increases the absorption and emission of infrared radiation (1976). The heat loss from the earth is retarded, and thus the earth’s temperature and climate are affected. Studies on carcinogenicity of certain classes of hydrocarbons indicate that some cancers appear to be caused by exposure to aromatic hydrocarbons found in soot and tars. Hydrocarbons in combination with NO,, in the presence of sunlight, undergo photochemical oxidation, producing a photochemical smog that is environmentally hazardous. Regulations on controlling organic vapor pollutants in air have been issued worldwide. In the Ambient Air Quality Standards, produced by the US. Environmental Protection Agency, the maximum 3-hour concentration (from 6 to 9 a.m.) of hydrocarbon content (corrected for methane) is 160 pg/m3 (0.24 ppm), not to be exceeded more than a year (Theodore and Buonicore, 1988). From an economic point of view, it is desirable to recover and reuse the organic solvents that are released t o the air. Membrane separation processes are a potentially useful technology for this purpose. The amount of solvents emitted to the atmosphere is extremely large. It is estimated that solvent emissions in the U.S. are on the order of 10-15 million tondyear,
which represents an annual loss of two billion dollars, based on their fuel value alone (Peinemann et al., 1986). There are many sources of organic solvent emissions. According t o a report from U.S. Environmental Protection Agency, the synthetic organic chemicals manufacturing industry (SOCMI) is the most significant contributor to air pollution (1984a). Petroleum industries and petroleum storagehransfer units with a total storage capacity exceeding 30000 bbl are two of the major sources that have the potential to emit hazardous pollutants a t a rate of more than 100 tons/year (Theodore and Buonicore, 1988). In the U.S.A., petroleum refining is the largest single-emissionsource, producing approximately 10%of the total emissions (Baker et al., 1987). In processes such as gluing, painting, coating, and dry cleaning, effluent gas streams are loaded with solvents. The US.Environmental Protection Agency (EPA) estimates that approximately 2 million tons of organic solvents are annually emitted to the atmosphere from solvent-based coating facilities (198413). From a variety of organic solvent emission sources, a homogeneous exhaust emission is formed. Naphtha, toluene, xylene, perchloroethylene, trichloroethane, ethyl alcohol, methyl alcohol, and acetone together represent almost 80% of the total solvent emissions (Baker et al., 1987). Gasoline and other light oil hydrocarbon emissions are of considerable importance. Gasoline is generally a mixture of CS to CS components. The recovery of evaporated gasoline from loading, unloading, and other handling operations has been under investigation. Methanol and ethanol may also be used as an octane value enhancer, particularly in lead-free grades of gasoline. Although air and vapor permeation experiments were conducted for various polymeric films by Baker et al. (19871, most of the experimental works reported so far on membrane vapor recovery are concentrated on composite silicone rubber membranes. Pinnau et al. (19881, Kimmerle et al. (19881, Strathmann et al.(19861, and Paul et al. (1988)tested silicone rubber membranes coated on a porous polysulfone substrate. Behling (1986) and Behling et al. (1989) chose poly(ether imide) as the supporting material because it is much more stable to organic vapors than polysulfone. Buys et al. (1990)used polyhydantoine and polyimide as the porous support t o the silicone rubber coating layer in their study. A membrane system for the treatment of low-volume,
0888-5885/95/2634-4494$09.00/0 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4495 high-vapor concentration gas streams was tested, but no information was disclosed concerning membrane materials (Wijmans and Helm, 1989). The resistance of silicone rubber to some organic vapors, for example, gasoline, is, however, poor (Billmeyer, 1984). An attempt was made, therefore, by the authors to prepare membranes from a single polymeric material of high organic resistance. In our previous studies, asymmetric aromatic polyimide membranes were investigated for this purpose (Feng et al., 1991). It was shown that membranes of both high selectivity and reasonably high permeability could be produced by controlling the conditions of the membrane preparation properly. Membranes were also prepared from aromatic poly(ether imide) material and the membrane transport studied (Feng et al., 1993). The objective of this work is to study the separation of organic vapor mixtures from nitrogen using the aromatic poly(ether imide) membrane of the highest selectivity achieved in the previous work. The effect of the presence of water vapor in the feed stream is also investigated, since water permeability through polymeric membranes is extremely high. Finally, attempts are made to separate gasoline vapor from nitrogen.
Theoretical Section The permeability of nitrogen gas through the membrane, JN(mol/m2wPa),is obtained by
where V is the volumetric permeation rate (mumin) of nitrogen gas, A is the effective film area (m2),T is the absolute temperature (K), PO is the upstream (feed) pressure (Pa),P3 is the downstream (permeate)pressure (Pa), and R is the gas constant (8.314 JK-mol). The following transport equations are used for the analysis of membrane permeation data when the feed is a mixture of nitrogen gas and n different vapors.
(2)
(4) where i = 1, 2, 3, ..., n. Qs are permeation fluxes; P's are pressures, and Ys are mole fractions of vapors. Subscript N and the first subscript i represent nitrogen and the ith vapor, respectively. The second subscripts 1and 3 represent the feed and the permeate, respectively. It is assumed in eq 2 that the permeability of nitrogen gas is unaffected by the presence of organic vapors in the feed. The justification for the above assumption was also made in our previous paper (Feng et al., 1993). Looking into eqs 2 and 4, there are a total of 2n 1 equations in which QL(i = 1, 2, 3, ...,n), PI, PB, Yi,l(i = 1, 2, 3, ..., n), and JNare known quantities from the permeation experiment for pure nitrogen and from the permeation experiment for a feed mixture of nitrogen 1 gas and n number of vapors. Consequently, 2n unknowns, QN and Yi,3 (i = 1,2, 3, ..., n), and Ji (i = 1, 2, 3, ..., n), can be calculated from the above 2n 1 equations.
+
+ +
Experimental Section Materials. Aromatic poly(ether imide) (PEI) (1000/ 2000 grade) was supplied by General Electric Co. in a
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a
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b Figure 1. (a) Flow diagram of the experimental apparatus. 1, 7 = nitrogen cylinder; 2, 8 = valve; 3, 9 = flowmeter; 4, 10 = pressure gauge; 5, 11 = water bath; 6 = saturator for organic vapor; 12 = saturator,for water vapor; 13 = mixer for two gas lines; 14 = membrane separator; 15 = isothermal chamber; 16 = GC; 17 = cold trap; 18 = vacuum gauge; 19 = vacuum pump. (b) Diagram of the nitrogen permeation test.
pellet form. The pellets were dried at 150 "Cfor 4 h in an oven with forced-air circulation as suggested by the supplier. Lithium nitrate (LiN03)from Fisher Scientific Co. was dried at 140 "C for 4 h before being used as additives in membrane preparation. All organic chemicals were supplied by BDH Inc. and were of reagent grade. Nitrogen gas with a purity of 99.997% was obtained from Air Products. Regular unleaded gasoline was supplied from PIONEER Co.. Membrane Preparation. Membranes were prepared by a phase-inversion solvent-exchange method. The casting solution was prepared by dissolving PEI polymer and LiN03 in a dimethylacetamide (DMAc) solvent. The composition of the casting solution is as follows: PEI, 25 wt %; DMAc, 74 wt %; LiN03,l wt %. The solution was cast on a clean glass plate to a thickness of 250 pm. The temperature of the casting solution and that of the casting atmosphere was ambient. Then, the solvent in the cast film was partially evaporated, leaving the film, together with the glass plate, in an oven with forced-air circulation at 110 "C for 1.8 min. The polymer film on the glass plate was further immersed into ice-cold water (2-4 "C) in which gelation took place, and the film stood apart from the glass plate spontaneously. The film was then immersed into an ethanol bath so that the water in the film was replaced by ethanol before the film was dried in air. Permeation Experiments. The flow diagram of the experimental setup is shown schematically in Figure la. A mixture of organic vapor and nitrogen gas was produced by bubbling nitrogen gas from a porous sintered ball immersed in a chosen organic liquid at room temperature. The permeation cell, whose structure is the same as that used in our earlier study (Feng et al., 19911, was housed in an isothermal chamber whose temperature was controlled within f0.5 "C. If necessary, the feed gas mixture was preheated to the experimental temperature in a heating coil before it was introduced to the feed side of the membrane. The permeate side of the membrane was connected to two
4496 Ind. Eng. Chem. Res., Vol. 34,No. 12,1995 cold traps, followed by a DuoSeal vacuum pump (Model 1400). The permeation of the organic vapor was induced by maintaining its partial pressure on the permeate side lower than the feed side. The membrane-permeated organic vapor was condensed and collected initially in one of the cold traps, and then the cold trap was switched to the other after the steady state was reached. The permeation rate was determined gravimetrically by weighing the sample collected for a predetermined period. The feed gas stream was connected to a Varian gas chromatograph (Model 3400) both at the inlet and at the outlet of the feed chamber of the permeation cell through bypass valves to determine the composition of the feed mixture. The feed flow rate was so high that the concentration change from the inlet to the outlet of the feed chamber was negligible. In order to change the feed vapor concentration, nitrogen gas supplied from a cylinder was divided into two streams in such a way that only one stream was bubbled through organic liquid t o be saturated by vapor (the first nitrogen stream), while the other (the second nitrogen stream) was used to dilute the vapor-saturated stream. By changing the flow rate of the two nitrogen streams, different feed vapor concentrations were obtained. When the permeation experiment of hydrocarbonslwater mixtures was performed, the second nitrogen stream was bubbled through water before being mixed with the first nitrogen stream. The permeation rate of pure nitrogen was determined in the following way. A bubble flow meter was connected to the feed chamber outlet of the permeation cell, and the inlet valve was closed as illustrated in Figure lb. When the permeate side of the permeation cell was evacuated by a vacuum pump, nitrogen was sucked from the bottom of the bubble flow meter into the feed chamber, pushing a soap film in the buret upward. The nitrogen flow rate was determined from the speed of the movement of the soap film. The compositions of the feed gas mixture and the permeate sample collected in the cold trap were determined by a gas chromatograph (Varian 3400) using a chromosorb (CHROM 102)column with a length of 2 m and a diameter of 0.318cm.
Results and Discussion Separation of the Mixtures of Hydrocarbon Vapors from Nitrogen. The separation of mixtures of hydrocarbon vapors from nitrogen was studied by bubbling the feed nitrogen stream through a liquid mixture of n-pentane (36 wt %), n-heptane (36 w t %), toluene (22wt %), and cyclohexane (5 w t %I before being introduced to the permeation cell. The above hydrocarbon concentrations correspond to those of major components in gasoline. The typical mole fractions of individual components in the feed stream so prepared were nitrogen (0.7071,n-pentane (0.2571,n-heptane (0.021),toluene (0.003),and cyclohexane (0.012).The flow rate of the feed stream was 60 mumin when undiluted. Hydrocarbons in the feed stream were diluted at two levels by adding 30 and 60 mumin of nitrogen into the undiluted stream. Consequently, the mole fractions of hydrocarbons became 111.5and 112 of those in the undiluted stream. These three feed streams are hereafter called feed streams without dilution, with 1.5 times dilution, and with 2 times dilution. The permeation studies were made at 24 and 80 “C and a t pressures on the permeate side, 1.33,4.00,6.67, and 10.67 kPa (10,30,50,and 80 mmHg).
0.86
.
0.82
.
h-m
0
2
6
4
8
10
12
(kP4
p3
Figure 2. Effect of the total permeate pressure on the total mole fraction of hydrocarbons in the permeate: (0,O)without dilution; (.,0) 1.5 times dilution: (+,O) 2 times dilution; (-1 at 24 “C: (---) at 80 “C.
n-. 7.”n
1 0.10
0.15
0.20
0.25
0.30
Ypentane, 1
Figure 3. Mole fraction of n-pentane in the permeate versus that in the feed: (0,O) 1.33kPa; (.,0) 4.00 kPa; (+,O) 6.67 kPa; (A,v) 10.67 kPa; (-) at 24 “C; (---I a t 80 “C.
The experimental results are summarized in Figures 2-7. Figure 2 correlates the total mole fraction of hydrocarbons in the permeate, ZYi,3, as a function of the total permeate pressure, PB.The experiments were conducted at 24 and 80 “C and with feed streams without and with dilutions. The figure shows that the total mole fraction of hydrocarbons was always more than 0.88,even when the feed stream was diluted by a nitrogen stream. The figure also shows that the total mole fraction of hydrocarbons was lower at a higher temperature. Figure 3 correlates the mole fraction of n-pentane in the permeate versus the mole fraction of n-pentane in the feed. The figure shows that the n-pentane mole fraction in the permeate increases with an increase in the feed and the permeate is predominantly n-pentane. Figure 4 correlates the total molar flux CQi as a function of the total permeate pressure. The total molar flux decreases as the permeate pressure increases, and the flux is lower at a higher temperature. Similar trends were observed in the molar fluxes of
Ind. Eng. Chem. Res., Val. 34, No. 12, 1995 4497 20
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Figure 5. Molar flux versus total permeate pressure (a) for n-pentane and (b) for n-heptane: ( 0 , O )without dilution; (.,El) 1.5 tims dihtim; !*,O) 2 tims &!ut% (-) at 24 T; (---)ateo T!.
individual hydrocarbons, which are illustrated in Figures 5 (a, n-pentane; b, n-heptane) and 6 (a, toluene; b, cyclohexane) as a function of the total permeate pres-
(kPa)
Figure 8. Molar flux versus total permeate pressure (a) for toluene; (b) for cyclohexane (+,O) 2 times dilution (-) at 24 "C; (---I a t 80 "C.
sure. Figures 5 and 6 show that the order in the hydrocarbon flux is n-pentane > n-heptane > toluene > cyclohexane Finally, Figure 7 shows the effect of the n-pentane mole fraction in the feed on the n-pentane flux. While the flux continues to increase with an increase in the feed mole fraction a t 24 "C,the flux levels off a t the high feed mole fraction of 80 "C. Separation of the Mixtures of Hydrocarbon Vapors from Nitrogen in the Presence of Water. As described in the Experimental Section, the first nitrogen stream was bubbled through a liquid containing n-pentane (36 wt %), n-heptane (36 wt %), toluene (23 wt %), and cyclohexane (5 wt %) at a flow rate of 58-83 d m i n . The second nitrogen stream was bubbled through distilled water at a flow rate of 62-90 mumin, and both streams were introduced to a mixer where they were blended. The composition of the feed stream was controlled by adjusting the temperatures of the two saturators. Compositions of five feed streams used for the experiments are summarized in Table 1. Because of the immiscibility of hydmca&ons and water, the permeate vapors were separated into two phases when they were condensed in the cold trap. The upper layer was a hydrocarbon phase containing less than 0.05 wt
4498 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
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Figure 7. Flux of n-pentane versus mole fraction of n-pentane in feed (0,O) 1.33kPa; (.,0) 4.00 kPa; (4,O) 6.67 kPa; (A,v)10.67 kPa; (-1 at 24 "C;(-1 at 80 "C.
Figure 9. n-Heptane(.), toluene(.), and cyclohexane (0) permeabilities versus their mole fractions in the feed. 1 -
Table 1. Composition of Feed Streams in Mole Fractions feed 1
feed2
feed3
feed4
0.912 0.069 0.012 0.002 0.004 0.001
0.847 0.130 0.013 0.003 0.006
0.850 0.125 0.014 0.003 0.006 0.002
0.903 0.077 0.012 0.002 0.004 0.002
~
nitrogen n-pentane n-heptane toluene cyclohexane water
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0.741 0.226 0.019 0.003 0.008 0.003
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Yi,1 Figure 10. Mole fraction in permeate versus mole fraction in feed for water and n-pentane. (0)water; (4)n-pentane. 500
400 0.000
0.001
0.0 02
0.003
0.004
Yweter, 1
Figure 8. Water permeability versus water mole fraction in the feed.
% of water, while the lower layer was a water phase with more than 99.95 wt % of water. Therefore, two layers were isolated carefully from each other, weighed, and analyzed separately. The experiments were carried out at 24 "C and at a permeate side pressure of 1.33 kPa (10 mmHg). The experimental results are shown in Figures 8 and 9 where the permeation fluxes are correlated to the mole fraction in the feed for each component. Note that the range of feed mole fraction of water is the lowest amongst all vapor components but the range of the permeation flux is the highest, indicating an extremely
high permeability of water. For each component the permeation flux increases with an increase in the feed mole fraction. Regarding n-pentane, the flux versus feed mole fraction plot was the same in both the presence and absence of water, indicating that the membrane permeation of water and that of n-pentane are mutually independent. This may also apply to other hydrocarbons. Finally, Figure 10 shows the correlation of the mole fraction in the permeate versus that in the feed for water and n-pentane. The mole fraction of water in the permeate reached 0.68-0.82, although the feed water mole fraction was only 0.001-0.003. This reflects extremely high water permeability compared with the permeabilities of hydrocarbons. As a result, the n-pentane mole fraction, which constitutes the major component in the permeate, could not be more than 0.25 even when the feed mole fraction of n-pentane was as high as 0.225. Separation of Gasoline Vapor from Air. Gasoline is a very complex mixture with hundreds of components. A maximum of 23 peaks were obtained from the liquid gasoline sample by the chromosorb (CROM 102) column used in this experiment, while 16 peaks were obtained
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4499 Table 2. Area Percent in the Feed and Permeate for Gasoline Permeation feed stream G l a
feed stream G2b
feed stream G3‘
peak a r e a % a r e a % i n area % area % i n a r e a % a r e a % i n no. in feed permeate in feed permeate in feed permeate 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
53.80 0.90 7.39 18.55 0.07 7.96 5.30 0 0.30 0.38 2.31 1.17 0.50 0.29 0 0.75 0 0 0.33 0 0 0 0
3.28 0.01 0.75 5.36 0.02 20.86 16.35 0.28 1.09 2.21 15.06 13.98 3.77 2.17 1.02 8.86 0.67 0.37 2.12 0.33 1.44 0 0
69.11 0.04 3.20 10.65 0.04 6.80 4.53 0 0.33 0.36 1.63 1.43 0.51 0.34 0 0.59 0.08 0 0.36 0 0 0 0
5.77 0.01 2.01 12.55 0.04 21.94 17.24 0.24 0.87 1.62 10.43 9.69 2.53 2.05 0.65 7.06 0.34 0 3.08 0 1.88 0 0
82.27 0 0.73 3.43 0 4.07 2.66 0 0.21 0.23 1.35 1.48 0.51 0.45 0 1.04 0 0 1.29 0 0.28 0 0
9.19 0.01 0.92 6.71 0.02 20.06 13.37 0.24 0.89 1.52 11.90 9.86 2.94 2.54 0.80 9.12 0 0.61 5.13 0 2.23 1.49 0.43
a Feed stream G1-feed stream without dilution. Total gasoline flux = 0.0346 g/s.mz. Feed stream G2-feed stream with 2 times dilution. Total gasoline flux = 0.0248 g/s-mz. Feed stream G3-feed stream with 3 times dilution. Total gasoline flux = 0.0180 g/s*mz.
bons since peak 6 corresponds to pentane. Probably, these volatile hydrocarbons escaped from the sample during gas chromatographic analysis. Interestingly, some peaks for less volatile components were obtained only in the permeate. It is also found from Table 2 that the percents of the peak areas in the permeate do not change very much even when the composition of the feed stream is changed by dilution. The total vapor permeation flux, CQi, decreases with an increase in the dilution ratio. GKSS of Germany disclosed operational details of a demonstration plant to treat 300 Nm3/h of air containing 37 vol % of hydrocarbons using 80 m2 of a silicone rubber coated poly(ether imide) membrane. The plant was operated a t a feed pressure of 2 bar and a permeate pressure of 0.2 bar. Almost 100% of hydrocarbons in the feed were collected in the permeate, and, as a result, the hydrocarbon concentration was increased from 37 vol % in the feed to 51.1 vol % in the permeate. The permeate was compressed to 10 bar to liquefy the hydrocarbons (Behling et al., 1989). The permeabilities of hydrocarbons and air could be calculated from the process flow sheet to be 5 and 0.9 Nm3/m2*h.bar, respectively. Apparently, they used a membrane of high permeability to reduce the membrane area, while the selectivity of the membrane was kept low. According to our latest experimental data, permeabilities for some typical vapors and gases are as follows: Nm3/m2h bar
from the gasoline vapor in air. Obviously, the chromatographic retention time increases with a decrease in the volatility of the component, and the first peak is considered as that of nitrogen in both chromatograms. Identification of each peak was not done in this work. As well, the fraction of the area of each peak in the sum of the peak areas is reported instead of the mole fraction of each peak. Such a representation is considered adequate for practical purposes since each chromatographic peak includes several unisolated components. The separation experiments were carried out at 24 “C and at the permeate pressure of 1.33 kPa (10 mmHg). The total vapor mole fraction in the feed was maintained at about 0.5, by bubbling a nitrogen stream at a flow rate of 45 mIJmin through liquid gasoline that was maintained a t 6-8 “C. This undiluted feed flow was further diluted a t two levels by adding 45 and 90 mIJ min of nitrogen, respectively. These three feed streams with different gasoline contents are called hereafter feed streams G1, G2, and G3, respectively. Table 2 shows the percent of the area of each peak to the sum of the areas of all peaks for feed streams G1, G2, and G3, respectively. Similar data are also given for the permeate corresponding t o each feed stream. As mentioned earlier, the first peak is that of nitrogen. Looking into the data for the feed, the percent of the first peak area increases from 53.8 t o 82.2, whereas the percents of all other peaks decrease as the dilution ratio increases from G1 to G3. Looking into the data for the permeate, the percent of the first peak increases from 3.28 to 9.19 as the dilution ratio increases. These values are extremely low compared with those in the feed, reflecting the low permeability of nitrogen. The percent of the peak area for other peaks increases, in most cases, from the feed to the permeate. There are, however, some exceptional cases. The percent of the peak area decreases from the feed t o the permeate for peaks 2-5 of feed stream G1 and peaks 2 and 3 of feed stream G2. These peaks seem to correspond t o very light hydrocar-
pentane (representing hydrocarbons) pentanol water nitrogen (representing air)
0.02 0.5 2.9 0.0009
Obviously, the permeability of hydrocarbons is too small for the membrane to be useful for the above process, since the membrane area required would be too large. The permeabilities of pentanol and water are in a reasonable range. Moreover, the nitrogen permeability of our membrane is 3 orders of magnitude lower than that of the commercial membrane. This will have two effects. First, the capacity of the vacuum pump can be smaller, since there is no “air leak” through the membrane. Second, the permeate stream does not need to be compressed very much for liquefaction, since the solvent concentration in the permeate is very high. Thus, the newly developed membranes have several advantages over the commercial one, because of their extremely high selectivity values. Furthermore, our membranes do not require silicone rubber coating, which reduces the cost involved in the membrane preparation. Silicone rubber is also susceptible to damage by many organic solvents. Therefore, our membranes have a practical usefulness in the removal of hydrophilic solvents from a relatively small amount of air. Proper adjustment of the proe size would be desirable to optimize the membrane performance for the removal of hydrocarbons from air.
Conclusions The following conclusions can be drawn from the above experimental results. (1)Hydrocarbon mixtures can be effectively separated from nitrogen by an asymmetric aromatic poly(ether imide) membrane without silicone rubber coating. (2) Water permeability is higher than hydrocarbon permeabilities.
4500 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
(3) The presence of water vapor in the feed does not affect the hydrocarbon permeability. In other words, water and hydrocarbon molecules permeate through the membrane independently. (4) Gasoline vapor can be separated effectively by a n asymmetric aromatic poly(ether imide) membrane prepared in our laboratory. ( 5 ) The newly developed membranes may be of commercial value to remove volatile organic compounds when the amount of air to be treated is relatively small.
Acknowledgment The support of the project by Energy, Mines and Resources Canada under DSS Contract No. 23440-99237101 SS is gratefully acknowledged.
Nomenclature A: effective membrane area, m2 Ji: permeability of the ith vapor component, mol/m2wPa JN: permeability of nitrogen gas, mol/m2wPa P: pressure, Pa Q: permeation flux, mol/m2-s R: gas constant, 8.314 J/K.mol T absolute temperature, K V: volumetric permeation rate, mumin Y: mole fraction Subscripts
i: ith vapor component N nitrogen 1: feed 3: permeate
Literature Cited Baker, R. W.; Yoshioka, N.; Mohr, J. M.; Khan, A. J. Separation of Organic Vapors from Air, J. Membr. Sci. 1987,31,259. Behling, R. D. Separation of Hydrocarbon from Air; Proceedings of the 6th Annual Membrane Technology Planning Conference, Session V-4, Cambridge, 1986. Behling, R. D.; Ohlrogge, K.; Peinemann, K. V.; Kyburz, E. The Separation of Hydrocarbons from Waste Vapor Streams; AIChE Symposium Series 272; American Institute of Chemical Engineers: New York, 1989; Vol. 85, p 68. Billmeyer, F. W., Jr. Textbook of Polymer Science, 3rd ed.; John Wiley & Sons: New York, 1984.
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Received for review January 25, 1995 Revised manuscript received July 5, 1995 Accepted July 24, 1995@ IE950077+
@
Abstract published in Advance A C S Abstracts, October 15,
1995.
