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Ind. Eng. Chem. Res. 1999, 38, 4424-4432

Olefin/Paraffin Separation through Carbonized Membranes Derived from an Asymmetric Polyimide Hollow Fiber Membrane Ken-ichi Okamoto,* Shigeo Kawamura, Makoto Yoshino, and Hidetoshi Kita Faculty of Engineering, Yamaguchi University, Ube, Yamaguchi, 755-8611 Japan

Yusei Hirayama, Nozomu Tanihara, and Yoshihiro Kusuki Polymer Laboratory (Chiba), Ube Industries Ltd., Ichihara, Chiba, 290-0045 Japan

Carbonized hollow fiber membranes were prepared by pyrolyzing an asymmetric hollow fiber membrane of a polyimide from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and aromatic diamines at temperatures of 500-700 °C under a nitrogen stream. The precursor membrane was treated in air at 400 °C for 0.5 h before the pyrolysis. This pretreatment was effective for improvement of gas permeance of the carbonized membranes. The carbonized membranes had an asymmetric structure with a skin layer of around 200 nm in thickness. They had the characteristics of larger permeance and lower permselectivity for inorganic gas pairs such as O2/N2, but this was rather preferable to the separation of olefin/paraffin. The membranes pyrolyzed at 600-630 °C displayed good stability and excellent performances of propylene/ propane and 1,3-butadiene/n-butane separation based on the molecular sieving. Introduction Separation of olefins and paraffins is one of the most important processes in petrochemical industries and currently performed by low-temperature distillation with large energy consumption. The separation of olefins from paraffins has been ranked at a high level in evaluation of the potential impact of membrane separations in petroleum refining.1 In previous papers, we investigated permeation and separation properties of various polymer membranes for ethylene/ethane, propylene/propane, and 1,3-butadiene/n-butane systems.2,3 Polyimides, especially based on 4,4′-(hexafluoroisopropylidene) diphtalic anhydride (6FDA), have much better performance compared with other polymers. However, their permeability to olefin is not high enough, and their selectivity is significantly reduced in a mixed gas system because of the plasticization effect of olefins. The degree of the plasticization effect strongly depends on the experimental conditions such as upstream and downstream pressure, feed gas composition, and temperature. This may make it difficult to apply them to practical uses. Carbon molecular sieve (CMS) membranes prepared by pyrolyzing precursor polymer membranes have been reported to display high gas separation performance compared with the precursor ones.4-22 Hayashi et al. prepared carbonized membranes by the pyrolysis of composite membranes of polyimide from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and oxydianiline (ODA) coated on porous alumina substrate tubes at temperatures of 600-900 °C in a nitrogen stream,12 and reported good performances of ethylene/ethane and propylene/propane separation, that is, a permeance of propylene, RC3H6, of 9 GPU (gas permeation units) (1 GPU ) 10-6 cm3(STP)/(cm2 s cmHg)) and selectivity of propylene over propane, RC3H6/C3H8, of 33 at 100 °C for the membranes pyrolyzed at 700 °C.13 Suda and Haraya * To whom correspondence should be addressed. E-mail: [email protected]. Fax: 81-836-35-9965.

reported that a CMS membrane prepared by the vacuum pyrolysis of Kapton polyimide film at 1000 °C was impermeable to hydrocarbons, but upon mild activation with water vapor at 600 °C displayed excellent propylene/propane permselectivity.10 We have also found that carbonized membranes from 6FDA-based polyimide and polypyrrolone films have much higher permeability and higher selectivity compared with those from the precursor polymers.18,22 The morphological structures of the CMS membranes investigated by now are dense films, composite tubes or plates, and hollow fibers. Asymmetric hollow fiber membranes are the most interesting from the viewpoint of practical uses. Jones and Koros prepared asymmetric CMS hollow fiber membranes by vacuum pyrolysis of an asymmetric hollow fiber of polyimide from 6FDA/BPDA and 2,4,6-trimethyl-1,3-phenylene diamine (TrMPD).5-7 The membranes pyrolyzed at 500 or 550 °C displayed excellent performances for gas separations such as O2/N2 and CO2/N2, but were impermeable to propylene and propane.6 Geiszler and Koros investigated effects of the pyrolysis atmosphere on the membrane performance and reported that inert purge pyrolysis produced CMS membranes with higher permeances and lower selectivities compared with vacuum pyrolysis.7 Kusuki et al. have continuously prepared asymmetric CMS hollow fiber membranes by pyrolyzing an asymmetric BPDA-based polyimide hollow fiber membrane in a nitrogen stream within a short residence time of 3.6 min in a furnace.15,16 The membranes pyrolyzed at temperatures of 700-850 °C displayed excellent H2/CH4 separation performances. As mentioned above, by now, investigations of olefin/ paraffin separation through CMS membranes were limited to dense flat or composite tube membranes, which did not have high enough permeances to olefins. In this study, we report on the preparation of carbonized membranes by pyrolysis of an asymmetric BPDA-based polyimide hollow fiber membrane and their olefin/ paraffin separation properties.

10.1021/ie990209p CCC: $18.00 © 1999 American Chemical Society Published on Web 10/07/1999

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Experimental Section A precursor polyimide was prepared by condensation of BPDA with aromatic diamines in a p-chlorophenol solution at 180 °C. The major component (85 wt %) of the diamines was dimethyl-3,7-diaminodiphenyl-thiophene-5,5-dioxide (DDBT) and the minor ones were 3,5diaminobenzoic acid and a CF3-containing diamine. DDBT is a mixture of isomers having two methyl groups of 63% at the 2,8-positions, 33% at the 2,6-positions, and 4% at the 4,6-positions of the aromatic rings. An asymmetric hollow fiber membrane was prepared from a p-chlorophenol solution of the polyimide by a similar method described previously.15,16 The outer and inner diameters were 400 and 200 µm, respectively. The hollow fiber membrane was finally dried at 320 °C in nitrogen for 1 h. A few pieces of the precursor hollow fiber of 10 cm in length were held on a small stainless steel stand and heated up to 400 °C in atmospheric air at a heating rate of 5 °C/min and kept at 400 °C for 0.5 h. We called this pretreatment thermostabilization. Then they were heated up to a pyrolysis temperature (500-700 °C) at a heating rate of 5 °C/min under a nitrogen stream of 100 cm3/min (a superficial velocity of 7.2 cm/min) and then cooled down. Elementary analysis was performed on a PerkinElmer 240-C element analyzer. Thermogravimetry with gas chromatography and mass spectrometry (TG-MS) was measured with a Seikodenshi TG-MS system 220 at a heating rate of 5 °C/min in helium. Observation of a cross section and a surface was performed by field emission scanning electron microscopy (FESEM), using a Nihon Denshi JSM-840A. ATR-FTIR spectra of the precursor and carbonized hollow fibers were measured with a Perkin-Elmer FTIR spectrometer SPECTRUM 1000. A sorption cell equipped with a Sartorius S3D-P model electronic microbalance was used to measure the sorption amount of hydrocarbon gases in carbon hollow fiber membranes. One-end-opened type membrane modules were made of a few pieces of the carbon hollow fiber; one end of the fiber was sealed with epoxy resin and the other was potted in a stainless steel pipe of 3.5-cm length and 6-mm diameter with the epoxy resin. The active length of the fiber is about 3.5 cm, and the effective membrane area of the modules is 0.3-1.0 cm2. The single-gas permeation experiments were carried out by a vacuum time-lag method. The mixed gas permeation experiments were also carried out by a vacuum method followed by gas chromatograph analysis of the permeate. The permeation flux (permeance) is given in GPU:

GPU ) 10-6 cm3(STP)/(cm2 s cmHg) ) 3.35 × 10-3 kmol/(m2 s kPa) Results and Discussion Membrane Characterization. DSC analysis of the polyimide showed only an exothermic drift of the DSC line around 400 °C, indicating its glass-transition temperature was above 400 °C. As shown in Figure 1, TGMS analysis indicated that SO2 (MS No. 64), SO (48), and CO2 (44) began to evolve around 400 °C. The evolution of SO2 and SO for the thermostabilized fiber was smaller and shifted to a higher temperature compared with the untreated one. After the thermostabilization (heat treatment in air at 400 °C for 30 min), the length of the fiber was unchanged, but the outer and

Figure 1. TG-MS spectra of evolved gases with pyrolysis of (a) the precursor and (b) the thermostabilized polyimide hollow fibers. MS No. 44, 48, and 64 correspond to CO2, SO, and SO2, respectively.

inner diameter shrunk a little to 370 and 190 µm, respectively, the weight decreased by 8.9%, and the fiber became insoluble in the casting solvent, p-chlorophenol. Table 1 lists the elemental analysis results of the precursor, thermostabilized and pyrolyzed hollow fibers. The total amounts of elements detected were less than 100%, especially for the carbonized membranes, but the precursor polyimide did not contain the other elements. By thermostabilization, the C, H, N, and S contents decreased, whereas the O content increased. These results indicate that the thermostabilization caused oxidation and cross-linking of the polyimide via decomposition of sulfonyl groups. Weight loss occurred significantly at temperatures from 500 to 650 °C and amounted up to 45% at 700 °C. With increasing pyrolysis temperature, the O and S contents decreased and the C content increased, whereas the N and H contents changed rather slightly. The membranes pyrolyzed at 600-630 °C, which displayed the best performance for propylene/propane separation as will be mentioned below, had rather low C contents of less than 75% and high O and N contents of around 10 and 5%, respectively. This is similar to the cases of the membranes prepared by the inert purge pyrolysis of BPDA-ODA polyimide composite membranes,14 an asymmetric BPDA-based polyimide hollow fiber membrane16 and 6FDA-based polyimide and polypyrrolone films,22 but quite different from the case of asymmetric CMS hollow fiber membranes prepared by the vacuum pyrolysis of 6FDA/BPDA-based polyimide at temperatures of 500 and 550 °C for 2 h, where the C contents were more than 95%.5,6 Figure 2 shows ATR-FTIR spectra of the precursor polyimide and carbonized hollow fiber membranes. The intensities of the peaks assigned to a symmetric CdO stretch (1776 cm-1), asymmetric CdO stretch (1720 cm-1), C-N stretch (1353 cm-1), and bending of CdO (741 cm-1) were reduced at higher pyrolysis temperature. For the membranes pyrolyzed at 610 °C, the absorbances of these bands were less than a half of those of the precursor, but these bands were still clearly

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Table 1. Results of Elementary Analysis of the Precursor, Thermostabilized, and Pyrolyzed Hollow Fibers fiber

Ta (°C)

weight loss (%)

C

H

N

O

S

F

total

precursor thermostabilized pyrolyzed

320 400 550 590 650

64.13 62.76 67.60 70.68 75.05

3.04 2.95 3.18 3.30 3.15

5.52 4.80 5.31 5.30 4.89

19.73 22.61 15.31 12.39 7.37

4.30 3.03 1.53 1.47 1.41

0.36

8.9 25.3 36.1 43.1

97.08 96.15 92.93 93.14 91.87

a

Heat treatment temperature.

Figure 2. ATR-FTIR spectra of the asymmetric hollow fiber membranes of (a) the precursor polyimide and (b) the pyrolyzed one at 610 °C.

observed, indicating that the structure of the imide ring still remained. These peaks were slightly observed at 650 °C and completely disappeared at 700 °C. The peaks assigned to the sulfonyl group (1307, 1172, and 1149 cm-1) disappeared at 550 °C. The intensity of the peak assigned to the CdC stretch (1611 cm-1) increased at higher pyrolysis temperature, and the peak assigned to the nitrile group (2226 cm-1) appeared at 550 °C. Judging from the results of the IR spectra and the elemental analysis, the membranes pyrolyzed at temperatures below 650 °C are not a true “carbon” membrane. They might contain subdomains where the structure of the polymer precursor can be recognized in part, as pointed out by Tostis et al.20 For the pyrolysis at 600 °C, the weight loss amounted to 40%, the outer and inner diameters shrank to 330 and 170 µm, respectively, and the length also shrank by 7%. The membranes pyrolyzed at 600 °C were flexible enough to form rings with a diameter of 2 cm or less. Figure 3 shows FESEM photographs of cross sections of the thermostabilized and carbonized hollow fibers. The precursor and thermostabilized hollow fibers were composed of an aggregate of nodules of 30-50 nm in diameter. Many macropores of several tens of nanometers in diameter were observed in the bulk of 500 nm in depth from the surface. On going to the surface the macropores became smaller. The skin layer without macropores was estimated to be 100 nm or less in

thickness. The asymmetric structure was held, even for the carbonized hollow fibers, although fusion of the nodules was observed and as a result the macropores became less cylindrical. The skin layer was estimated to be around 200 nm in thickness, being a little thicker. Gas Permeation and Separation Properties. The typical gas separation performances are listed in Table 2. Figure 4 shows the relationship between the permeances of several gases and their kinetic diameters, dkt23 for the precursor and carbonized hollow fiber membranes. The thermostabilized membrane displayed rather small changes in gas permeance compared with the precursor. Pyrolysis at 500 °C hardly increased the gas permeance. Pyrolysis at 560 °C increased the gas permeance 4 times for He, about 10 times for CO2, O2, N2, and C3H6, but only 2 times for C3H8. As a result, the permselectivity decreased to 21 for He/N2 and increased to 14 for C3H6/C3H8. Pyrolysis at 600-630 °C further increased the gas permeance and decreased a little the selectivity of inorganic gas pairs, whereas the selectivity of C3H6/C3H8 hardly decreased. At 650 °C, the gas permeance started to decrease, especially for larger gases. The membrane pyrolyzed at 700 °C displayed much lower gas permeances compared with the precursor membrane. For C3H6/C3H8 separation, the precursor and thermostabilized membranes displayed very low performance, whereas the pyrolysis increased both the permeance of C3H6 and the permselectivity.

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Figure 3. FESEM photographs of cross sections of the asymmetric hollow fiber membranes (a) thermostabilized and (b) pyrolyzed at 600 °C.

The optimum pyrolysis temperature of the present hollow fiber was 600-630 °C for the C3H6/C3H8 separation. As mentioned above, the drastic change in gas permeance occurred at temperature ranges of 500-550 °C and 650-700 °C. The weight loss and chemical decomposition continuously occurred from around 450 °C. The FTIR spectra changed drastically at temperatures of 500-650 °C rather than above 650 °C. From these results together with the results reported for dense and flat carbonized membranes,9,22 the following is suggested. Penetrating micropores were formed by the

pyrolysis of polyimide. As a result, the gas permeation mechanism changed from solution diffusion for the precursor and thermostabilized membranes to molecular sieving or adsorption-activated diffusion for the membranes pyrolyzed above 550 °C. At a temperature range of 550-650 °C, the gas permeance changed only a little, in spite of the significant change in chemical structure, suggesting that such a change in chemical structure took a less important role in the gas permeation of the carbonized membranes. Above 650 °C, the size distribution of the micropores began to shift to smaller sizes. The membrane pyrolyzed at 700 °C was

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Table 2. Gas Separation Performances of the Precursor, Thermostabilized, and Pyrolyzed Hollow Fiber Membranes at 100 °C and 1 atm R (GPU)

pyrolysis temp. (°C)

He

CO2

O2

N2

C3H6

C3H8

He/N2

a 400b 560 580 600 625 650 700

310 210 790 1100 1100 1200 1100 180

59 56 370 490 590 680 420 6.8

21 19 140 220 280 320 170 2.4

4.7 4.3 37 60 87 97 42 0.65

1.8 1.8 18 30 51 47 14 0.20

0.50 0.59 1.3 2.2 4.5 3.5 1.5 0.18

66 49 21 18 13 12 26 280

a

selectivity CO2/N2 O2/N2 13 13 10 8.3 6.8 7.0 9.9 10

4.5 4.3 3.8 3.7 3.2 3.3 3.9 3.7

C3H6/C3H8c 3.7 3.0 14 14 12 13 9.8 1.1

Precursor. b Thermostabilized at 400 °C. c Mixed-component (C3H6/C3H8 ) 50/50).

Figure 4. Plots of permeances of single-component gases and a mixed C3H6/C3H8 (50/50%) gas at 100 °C and 1 atm against their kinetic diameters for the precursor, thermostabilized, and carbonized hollow fiber membranes. The superscript “a)” indicates data at 25 °C and 1 atm and cited from ref 5.

impermeable to C3H6 and C3H8, suggesting that most of the micropores were less than 0.40 nm in diameter. Figure 5 shows the influence of thermostabilization on the performance of the membranes pyrolyzed at 600 °C. The thermostabilization at 400 °C gave larger permeances of CO2 and O2 with a little smaller permeance ratios of CO2/N2 and O2/N2 compared with that at 350 °C or the nonthermostabilization. For C3H6/C3H8 separation, the thermostabilization at 400 °C gave larger permeance of C3H6 and similar permselectivity. The thermostabilization at 450 °C gave a rather smaller permeance and a smaller selectivity compared with that at 400 °C. In this study, the hollow fiber membranes were thermostabilized at 400 °C before pyrolysis, unless otherwise noted. The cross-linking of polyimide caused by this pretreatment might reduce the fusion of nodules, and as a result the asymmetric structure of the precursor hollow fibers was well held. As shown in Figure 6, for the O2/N2 and CO2/N2 separation, both the permeance and selectivity of the present precursor hollow fiber membrane were lower than those of the asymmetric hollow fiber membrane of the similar BPDA-based polyimide reported by Kusu-

Figure 5. Influence of thermostabilization on the performance of (a) O2/N2, (b) CO2/N2, and (c) C3H6/C3H8 separation at 100 °C and 1 atm for the hollow fiber membranes pyrolyzed at 600 °C. Nonthermostabilization (2) and thermostabilization at 350 °C ([), 400 °C (O), and 450 °C (0). The performances for the precursor (9) and thermostabilized (×) polyimide hollow fiber membranes are also shown for comparison.

ki et al.16 On the other hand, the present carbonized membranes had larger gas permeances and similar or a little lower selectivities, compared with the carbonized membranes continuously prepared (700 °C, 4 min) by Kusuki et al. The carbon membrane prepared (550 °C, 2 h) by Jones and Koros from an asymmetric hollow fiber of 6FDA/BPDA-TrMPD polyimide displayed much smaller gas permeances and much higher permselectivities of O2/N2 and CO2/N2,5 compared with the present carbonized membranes. Their membrane displayed very sharp dependence of gas permeance on dkt in the range of 0.33-0.38 nm, as shown in Figure 4. Their high permselectivity of O2/N2 and CO2/N2 was due to a sharp molecular sieving effect. As a result, their asymmetric CMS membrane was impermeable to propylene and propane.6 The asymmetric carbonized hollow fiber membranes in this study have the characteristics of larger permeance and lower selectivity for inorganic gas pairs, probably because of relatively larger sizes of the mi-

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Figure 7. Temperature dependence of permeance and selectivity of C3H6/C3H8 for the hollow fiber membrane pyrolyzed at 625 °C. After the first measurement (2), the membrane was subjected to desorption in vacuum at 100 °C, and then the first heating (4), the first cooling (O), and the second heating (0) runs were done successively.

Figure 6. Comparison of the performances of O2/N2 and CO2/N2 separation at 35 °C and 1 atm for the present precursor hollow fiber membrane (2) and the membranes pyrolyzed at 600 °C (b) with that (room temperature ∼50 °C) for other polymeric (4) and carbonized (O) membranes. The superscripts “a)” and “b)” indicate dated cited from refs 5 and 16, respectively.

cropores. This is rather preferable to the separation of larger gas pairs such as C3H6/C3H8. The carbonized membranes displayed similar performances of C3H6/C3H8 separation between single-component and mixed-components experiments. The permeance and selectivity were independent of feed composition and feed pressure up to 5 atm. Figure 7 shows temperature dependence of R and R of C3H6/C3H8 for the carbonized membrane. The permeance became a little larger after the first heating run, probably because of desorption of some impurity such as water vapor, but the selectivity hardly changed. The activation energies of RC3H6 and RC3H8 were 6.5 and 11 kJ/mol, respectively. RC3H6/C3H8 decreased a little with increasing temperature. Figure 8 and Table 3 show a comparison of C3H6/C3H8 separation performances of the present membranes with the reference data. The membranes pyrolyzed at 600630 °C displayed much better performances compared with the asymmetric hollow fiber membrane of BPDAbased polyimide,24 that is, RC3H6 was maximally 30 times larger and the RC3H6/C3H8 was on a similar level. They displayed higher RC3H6 and lower selectivity, compared with the composite carbonized membrane from the BPDA-ODA polyimide13 and the dense and flat carbonized membranes.18,22 However, their RC3H6 values were not large enough to be expected from the very thin skin layer of around 200 nm; the values were as large as those of the dense and flat carbonized (500 °C, 1 h) membrane from the 6FDA-m-phenylenediamine (mPD) polyimide.22 Judging from the SEM observation, it is reasonable to consider that the skin layer mainly

Figure 8. Comparison of the performances of C3H6/C3H8 separation at 100 °C and 1 atm for the present precursor, thermostabilized, and carbonized hollow fiber membranes with those for other carbonized membranes and BPDA-based polyimide hollow fiber membranes. The superscripts “a)”, “b)”, and “c)” indicate data cited from refs 11, 22, and 24, respectively. “b)” and “c)” indicate data that were measured at 35 and 70 °C, respectively. The open and closed symbols are for mixed-components and single-component permeation, respectively.

controls the gas permeation process and the bulk substrate has no significant resistance to it. The situation seems similar to that of the composite carbonized membrane from the BPDA-ODA polyimide;13 its RC3H6 values are comparable with those of the dense and flat carbonized membranes of which the thicknesses are 10 times larger. Furthermore, the present asymmetric carbonized hollow fiber membranes and the composite

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Table 3. Performances of Carbonized Membranes for C3H6/C3H8 Separationa precursor 6FDA-mPD

morphology dense and flat

Kapton 6FDA-DABZc 6FDA-mPD BPDA-ODA

dense and flat dense and flat composite tubular composite tubular

6FDA/BPDA-TrMPD BPDA-DDBT/DABA

asymmetric hollow fiber asymmetric hollow fiber

thickness (µm)

pyrolysis conditions

57

500 °C, 1 h

50

700 °C, 1 h

110 55 5 5-6 0.2

1000 °Cb 500 °C, 1 h 500 °C, 0.25 h 700 °C, 0 h 550 °C, 2 h 600 °C, 0 h 625 °C, 0 h

temp. (°C)

RC3H6 (GPU)

selectivity

35 100 35 100 100 35 35 35 100 25 35 100 100

19 (28) 35 0.5 (1.1) 1.2 (0.09) 1.7 (3.1) 6.1 (5.6) 2.4 (2.6) 8.7 (9.3) (0.16) 26 (20) 43 (32) 32 (34)

13 (25) 11 130 (530) 60 (20) 44 (78) 19 (27) 46 (54) 33 (29) (3) 11 (14) 11 (12) 15 (17)

ref 22 22 10 18, 22 25 13 6 this work this work

a

1 atm and feed composition of 50/50%. The data in parentheses were obtained for a single-component system. b After mild activation with water vapor at 400 °C. c Polypyrrolone from 6FDA and 3,3′-diaminobenzidine (DABZ). Table 4. Performance of Olefin/Paraffin Separation for the Hollow Fiber Membrane Pyrolyzed at 600 °Ca RC2H4

RC2H6

R

RC3H6

RC3H8

R

RC4H6

RC4H10

R

110

35

3.1

51

4.5

12

78

1.5

51

a

Figure 9. Adsorption isotherms of hydrocarbon gases in the hollow fiber membrane pyrolyzed at 600 °C. The closed and open symbols are for sorption and desorption, respectively. The density of the carbon fiber was assumed to be 1.5 g/cm3.

carbonized membranes displayed similar performances between the single-component and the mixed-components systems. On the other hand, for the dense and flat carbonized membranes, both RC3H6 and the selectivity are smaller for the mixed components system than for the single-component one. Recently, we have found similar differences in the permeation and separation behavior between the composite carbonized membranes and the dense and flat ones from 6FDA-mPD polyimide, of which the carbonized layers are 5 and 50 µm in thickness, respectively.25 The differences in the permeation and separation behavior between these carbonized membranes might originate from the difference in the morphological structure of the membranes rather than the difference in the chemical structure of the precursor polyimide. Thus, the results from thick and dense films could not be used to predict the behavior of asymmetric carbonized hollow fiber or composite carbonized membranes. As shown in Figure 9, adsorption and desorption isotherms of hydrocarbon gases for the asymmetric carbonized hollow fiber membrane belonged to Type 1 sorption isotherms, without hysteresis in the desorption isotherms, indicating no capillary condensation. The adsorption equilibrium of hydrocarbon gases was achieved within an experimental response time of 90 s. We could not determine the adsorption rate because of

R: (GPU), at 100 °C, 1 atm, and feed composition of 50/50%.

the very thin skin layer. The performance of olefin/ paraffin separation for the asymmetric carbonized hollow fiber membrane is listed in Table 4. It has been reported in some papers that a difference in the adsorption affinity between mixed gas components to a carbon membrane play an important role in determining the separation efficiency, especially at lower temperatures.17,20 In such a case, the permselectivity becomes reverse or significantly increases for the mixed-components system compared with that for the singlecomponent one. This is not the present case. The adsorption was a little higher for olefins than for the corresponding paraffins. Both the permeances to olefins and the separation factors of olefins over paraffins for the asymmetric carbonized hollow fiber membranes and the composite carbonized membranes13,22 were similar between the mixed-components system and the singlecomponent one. As one possible explanation of this behavior, Kusakabe et al. have proposed slit-shape molecular-sieving pores of the carbon membranes in which permeating molecules could pass one another by moving to the wider side of the slit.26 Their permselectivity was determined by the narrow width of the slitshaped micropores. The olefin/paraffin separation is considered as being a result of molecular sieving based on the difference in the molecular size. An effective diameter of a gas for diffusion in polymer membranes, either the collision diameter of LennardJones potential dLJ27,28 or the kinetic diameter dkt given by Breck23 has been often used.2,3,29 As shown in Figure 10a, for the plots of R versus dLJ, the permeances of linear and planar molecules such as CO2, C2H4, C3H6, and C4H6 are larger than those estimated from the correlation between R and dLJ for other gases, indicating that the minimum cross section of such a molecule significantly affects its permeation. Breck used the dkt values calculated from minimum cross-sectional diameters in consideration of molecular-sieving behavior in the gas adsorption of zeolites.23 We evaluated the dkt value of C4H6 as 0.48 nm. As shown in Figure 10b, the permeances of C2H4, C3H6, and C4H6 are larger than those estimated from the correlation between R and dkt for other gases. Thus, both dLJ and dkt are not good

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Figure 10. Plots of R at 100 °C and 1 atm versus dLJ, dkt, and deff of inorganic and hydrocarbon gases for the hollow fiber membrane pyrolyzed at 600 °C. Table 5. Membrane Stability Tests of the Hollow Fiber Membranes Pyrolyzed at 600 °Ca R (GPU)

selectivity

membrane

He

CO2

O2

C3H6

CO2/N2

O2/N2

C3H6/C3H8

(1) original after measurements at 130 °C after kept in C3H6/C3H8 mixture for a week at room temp.

1200

680

320

(47) (54) 34 (32)

7.0

3.3

(13) (13) 17 (15)

(2) original after kept in a desiccator for 2 months after heated at 200 °C in vacuum for 24 h

1200 1000 1200

600 500 650

280 210 320

(48) (25) (63)

7.0 8.8 6.0

3.3 3.7 3.0

(12) (9.3) (10)

(3) original after measurement run for 80 h at 100 °C after kept in C3H6/C3H8 mixture for a week at room temp. a

(53) (55) (54)

(10) (10) (9.5)

At 1 atm and 100 °C. The data in parentheses were obtained for mixed-components system with a feed composition of 50/50%.

measures of effective diameter for the permeation of rigid and planar molecules such as olefins in carbon or carbonized membranes. A detailed discussion is impossible at present because not only the penetrant size but also the interaction between the penetrant and surface of micropores must be considered in understanding the permeation behavior. However, adopting the diameter corresponding to the minimum cross-sectional area as the effective diameter of CO2, C2H4, C3H6, and C4H6 instead of dLJ in Figure 10a, we can get a fairly smooth correlation between ln R and effective diameter for permeation deff, as shown in Figure 10c. The effective diameter of C4H6 for diffusion in glassy polymer membranes was evaluated to be 0.44 nm based on good correlation between the logarithm of the diffusion coefficient and dLJ for other gases.3 The effective diameter of C4H6 is similar between the glassy polymer membranes and the carbonized ones, irrespective of different permeation mechanisms. The high performance of the carbonized membranes for C4H6/C4H10 separation is also explained based on the difference in the molecular size. Table 5 shows the results of membrane stability tests of the carbonized hollow fiber membranes. The propylene/propane separation performance hardly changed during the continuous measurements for 80 h at 100 °C. After kept in contact with the feed mixture at room temperature for a week, one test membrane showed small reductions in RC3H6 with small increases in RC3H6/C3H8, but another did not. After a membrane was kept in a desiccator for 2 months, the propylene/propane separation performance decreased a little, but it was

recovered by heating the membrane at 200 °C in vacuum for 24 h. Conclusions Asymmetric polyimide hollow fiber membranes were pretreated in atmospheric air at 400 °C for 30 min and then pyrolyzed in a nitrogen stream at a heating rate of 5 °C/min up to 600-630 °C. The carbonized hollow fiber membranes held the asymmetric structure and displayed good stability and excellent propylene/propane and 1,3-butadiene/n-butane separation performances: RC3H6 and RC4H6 were 50 and 80 GPU and RC3H6/C3H8 and RC4H6/C4H10 were 13 and 50, respectively, at 100 °C. Acknowledgment This work was supported partly by a Grant-in-Aid for Development Scientific Research (No. 10450296) from the Ministry of Education, Science, and Culture of Japan, and also by the Petroleum Energy Center (PEC) subsidized from the Ministry of International Trade and Industry. Literature Cited (1) Wilson, R. B.; Bhown, A. Evaluation of Separation Process in Petroleum Refining; SRI Report; SRI International, 1994. (2) Tanaka, K.; Taguchi, A.; Hao, J.; Kita, H.; Okamoto, K. Permeation and Separation Properties of Polyimide Membranes to Olefins and Paraffins. J. Membr. Sci. 1996, 121, 197. (3) Okamoto, K.; Noborio, K.; Hao, J.; Tanaka, K.; Kita, H. Permeation and Separation Properties of Polyimide Membranes to 1,3-Butadiene and n-Butane. J. Membr. Sci. 1997, 134, 171.

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Received for review March 22, 1999 Revised manuscript received July 29, 1999 Accepted July 30, 1999 IE990209P