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Gas permeation in polycarbonate membranes prepared by the wet-phase inversion method
Chemical Enginewring Science, Printsd in Great Britain.
Vol. 48. NO. 24. pp. 4069-la74,
1993. 0
OCO%25G9/93 S56.00 + O.a, 1993 Pergamon Press Ltd
GAS PERMEATION IN POLYCARBONATE MEMBRANES PREPARED BY THE WET-PHASE INVERSION METHOD
Department
Department
of Chemical
Engineering,
of Chemical
J. Y. LA1 and J. M. JEN Chung Yuan University, Chung
Engineering,
(Received
12 March
Li, Taoyuan
S. H. LIN’ Yuan Ze Institute of Technology, ROC 1993; accepted for publication
320, Taiwan,
Neili, Taoyuan
ROC
320, Taiwan,
17 June 1993)
Abstract~Polycarbonate (PC) membranes prepared by the wet-phase inversion process was investigated in the present study. Varying amounts of a non-solvent (methanol) were added to the casting solution during membrane preparation and was found to increase significantly membrane porosity. The gas permeability of the PC membranes was increased from 1.68 and 0.26 barrers, respectively, for oxygen and nitrogen without the non-solvent, to 35.1 and 3.86 barrers with 3 ml of the nonsolvent added. The drastic increase in gas permeability was accompanied by only a slight decrease in selectivity. Hence, the PC membranes prepared with addition of methanol have a great potential for industrial oxygen enrichment applications. A simple gas diffusion model with an effective gas diffusion coefficient was found to describe reasonably well the gas permeation process. The estimated effective gas diffusion coefficient can closely reflect the increase in gas permeability of the membranes due to the addition of methanol. INTRODUCTION Gas permeation through a membrane is a science of practical interest to academics and industries. In contrast to other separation technologies, membrane processes have the distinct advantage of lower energy requirement (Belfort, 1984; Rautenbach and Albrecht, 1989; Mulder, 1991). As energy cost is bound to increase on a long-term basis, application of membrane processes can be expected to expand significantly in the near future as recognised by most industrial observers. Many works have been performed in the past, investigating gas permeation through various membranes (Koros et al., 1976, 1977; Chan et al., 1978; Lai et al., 1986; Toi er al., 1989; Hachisuka et al., 1991). Mathematical models were proposed for describing gas sorption and permeation processes (Vieth and Sladek, 1965; Toi et al., 1983; Barrer, 1984; Lin, 1992). One primary aspect common to most of the previous investigations is that gas permeation through the membrane was rather low. The dual sorption model as advanced by Vieth et al. (1965) and several investigators (Toi et al., 1983; Barrer, 1984; Lin, 1992) has been shown to describe well such a gas permeation process. In order to render the membranes to be worthy of any practical industrial applications, its gas permeability needs to be considerably enhanced. However, increase in gas permeability must be achieved without great compromise in the selectivity of the membrane. It has been well recognised that such kinds of membranes with both good gas permeability and selectivity are very difficult to obtain.
+Author to whom correspondence
should be addressed.
This accounts for the little attention that has been paid to address simultaneously gas permeability and selectivity of a membrane in the past. The purpose of this work is to investigate the gas permeation and selectivity of a special kind of polycarbonate (PC) membrane which was prepared by a unique wet-phase inversion process with incorporation of varying amounts of a nonsolvent (methanol) for permeability enhancement. The wet method has the advantages of rapid membrane preparation, ease of control of the preparation conditions and increase in membrane porosity. The PC membranes so prepared achieve very high gas permeability and also retain reasonably good selectivity. A simple Fick’s diffusion model with an effective gas diffusion coefficient has been adapted to characterize the gas diffusion and sorption processes. EXPERIMENTAL STUDIES
PC membrane preparation method
by the wet-phase
inversion
Two grams of polycarbonate (Mitsubishi Gas Chemical Co., Inc., Japan) and 17 ml of dichloromethane (Merck, Inc., Germany) were placed in a 50 ml flask and well mixed by a magnetic stirrer until the polycarbonate resin was completely dissolved to form a 8 wt % casting solution. The mixing continued for a total of 16 h from the beginning. As soon as the first stage of mixing was finished, varying amounts of the nonsolvent (CH,OH), ranging from 1 to 3 ml, were added and mixing was resumed for another 6 h. The mixture was placed in a constant-temperature bath for 12 h for degasification. A piece of 3 mm-thick glass plate was thoroughly washed with distilled water and acetone and dried by an air blower. The PC casting
4070
J. Y.
LA1 et a/.
solution was uniformly coated on the clean glass plate with a garner’s knife to a desired thickness. The coated glass plate was then immersed immediately in a pure methanol bath. The PC casting solution gelled due to phase separation in just a few minutes to form a thin membrane_ The membrane was placed in a vacuum drier, which was maintained at 3O”C, for at least 6 h to remove the residual gelation nonsolvent
(CH,OH). Gas sorption
in the PC
membrane
The experimental setup for gas sorption measurements is shown in Fig. 1. A microbalance (Cahn Model 1000 Electrobalance) was enclosed in a constant-temperature box which was set at 35°C. Four pieces of 3 cm x 2.5 cm PC membrane were put in the microbalance system. The system pressure was then lowered to about 4 x lo-’ Torr. The vacuum condition was maintained for another hour. The gas to be tested (99.99% pure oxygen or nitrogen) was let in until the system pressure reached 1 atm. The system was maintained under this condition until sorption equilibrium was reached. It was reported by Koros et al. (1977) that nitrogen has a diffusion coefficient of 1.76 x 10-a cm’/s in a pure PC membrane. Accord-
ingly, the time to reach 99.999% sorption equilibrium can be computed from (McCabe et al., 1985)
teq ==jj(~>‘ln($XlO’) where S is the membrane thickness, z the operating pressure and t_, is the time to reach sorption equilibrium. For the present PC membrane of a uniform thickness of 0.003 cm and with the gas diffusion coefficient quoted above, it would need only 585 s to reach sorption equilibrium. For the present experiments, 3 h were allowed which could be deemed more than sufficient. After gas sorption equilibrium was reached, the total weight of the membrane was measured. The same procedure was-repeated for other gas absorption experiments. From the weight change of the PC membrane, the weight of the gas adsorbed can be calculated according to weight of gas absorbed by change of membrane by the PC membrane weight of gas displaced = volume of membrane
C
CP
II3 %
Go3ltRd
unit
p:
Mp: machpic~ pump OR oil dtffudon pm&l PO: Pirud plgo
IC: inmalltcd ombiuet
T:
fan OR: *aa mmsrvoir H: heeta
Sl-s7:
#toPcocks
th-uple
Tl and T2:
liquid N, tmpr
Fig. 1. Schematic of sorption apparatus.
PC membrane = weight weight of gas displaced by the PC membrane x density of gas
Polycarbonate
membranes
prepared
by wet-phase
inversion
PC membranes nitrogen. Membrane
method
4071
are listed in Table 1 for oxygen and
permeability test
A circular PC membrane of 3 cm in diameter was placed in the diffusion cell of the gas permeability testing apparatus (Yanaco Model GTR-IO, Japan) which is shown in Fig. 3. The system pressure was then lowered until it was less than 5 x lo-’ Torr. The gas to be tested (99.99 % quality air) was then let in on the upper side of the diffusion cell. The gas composition of the underside of the diffusion cell was measured periodically by a gas chromatograph (Shimadzu GC-14A, J&pan). The gas permeability was calculated from the following equation: Fig. 2. Gas
concentration vs pressure at equilibrium determining the sorption constant.
Table 1. Effect of added nonsolvent in casting on the sorption constant Sorption
constant
qKL p = (P‘ - P,)At
for
The separation as
factor (selectivity) of a gas was defined
a=*.P
solution
rNI k+
Nonsolvent
CH,OH (ml)
02
N,
1
0.2362 0.3506 0.4186
0.3106 0.4442 0.5375
2 3
(3)
DISCUSSION
‘k in cm3 gas (STP)/cm3polymer-atm.
where the volume of the membrane was measured and the gas density was determined by the PengRobinson equation (Sandler, 1989). According to Kamiya et al. (1986), gas sorption by the membrane at 1 atm pressure or below can be represented by C = kP
(2)
where C and P are, respectively, the gas concentration and pressure at equilibrium and k the sorption constant determined from the linear C vs P plot by the least-squares method. Figure 2 displays a typical plot for nitrogen. The calculated sorption constants for the
Ga8 pameability analyzer
Tompemtorecontroller
Fig. 3. Schematic
OF RESULTS
The amount of the nonsolvent (methanol) employed for the present membrane preparation was varied from 1 to 3 ml. The scanning electron micrographs (SEM) (Hitachi S-570, Hitachi, Inc., Japan) of the four membranes with and without addition of the nonsolvent are shown in Fig. 4. The four SEM micrographs on the left were those taken at the surface of the membrane, while those on the right were the cross-sectional images taken by cutting across the thin membrane. The amount of the nonsolvent added was 0 ml for the top SEM images and 3 ml for the bottom ones. These micrographs clearly indicate that the pore size of the membrane increases with increasing the amount of the nonsolvent. The skin layer on the membrane surface was observed to become increasingly thinner as the amount of nonsolvent increases. This skin layer was generally considered as a dense layer in contrast to the underlying porous structure inside the membrane resulting in an asymmetricity of the membrane (Strathmann et al., 1975;
of permeation
apparatus.
4072
J. Y. LAIet af.
.L
-
Model Htcing
/
(b)
Fig. 4. SEM images of PC membranes prepared by the wet method from casting solution of 8% PC/CH&l,. Nonsolvent added: (a) 0 mf; (b) 1 mt; (e) 2 ml; (d) 3 ml. Broens et al., 1980). The pore size and the thickness of the dense skin layer have considerable influence on the gas liability of the membrane. The permeability and selectivity determined according to eqs (2) and (3) for the three membranes are given in Table 2. This table further shows that the gas permeabilities for oxygen and nitrogen were drasticaIly enhanced as the amount of the nonsolvent
Time (I) Fig. 5. Com~~son of model predictions and measured data of gas permeation through membrane prepared with addition of CH,OH in the casting solution: (a) 1 ml; (b) 2 ml; (c) 3 ml. was increased, and that the drastic increase in gas ~~eabil~ty however is accompanied by only a slight decrease in selectivity. As is widely recognized, a membrane must have the characteristics of high gas permeability and reasonable selectivity for it ta he af any commercial value. These requirements are met by
Polycarbonate
4073
membranes prepared by wet-phase inversion method
Table 2. Effect of added nonsolvent in casting solution on the gas permeability and selectivity Permeability (barrer)
P lit
P 01
Selectivity
Nonsolvent CH,OH (ml)
eq. (2)
eq. (3)
eq. (2)
eq. (3)
0 :
1.68 6.06 3.29
0.09 0.20 0.16
0.26 0.56 1.10
0.07 0.07 0.10
6.45 5.87 5.50
0.21 0.15 0.39
3
35.10
0.19
16.04
0.24
2.18
0.02
PoJP,,
Notes: (1) Gel&on nonsolvent: pure CH,OH. (2) Casting. solution composition: PC 2 g/CH,Cl, 17ml. (3) Gelat& temperature: 20°C.
PC membrane prepared with an appropriate amount of a nonsolvent. These PC membranes have therefore a great potential for industrial applications in oxygen enrichment. As observed earlier, the present PC membranes have a thin, dense skin layer on the surface which renders the membrane asymmetric. The gas diffusion characteristics in this layer would be different from that inside the porous layer. For simplicity, however. we assume that an effective diffusion coefficient can be employed to characterize gas diffusion in such an asymmetric system. The diffusion process of any gas component is assumed to follow the simple Fick’s equation, the present
subject to the initial and boundary conditions c=o
@aI
x = 0,
C = kP,
WI
x = L,
C = kP,.
(64
t = 0,
The cumulative downstream gas pressure P, is determined from
Analytical solution to eq. (4) subject to coupled eqs (5aH5c) and (6) is not available. An iterative Crank-Nicolson implicit finite-difference scheme (Crank, 1975) was employed for integrating these equations. The only unknown constant in the above equation is the effective diffusion coefficient D,,, which was determined by best fitting the computed amount of gas passing through the membrane with the measured data. Comparison of the measured and predicted gas fluxes for the PC membranes with 1, 2 and 3 ml of CH,OH are demonstrated in Fig. 5. The agreement between the predictions and the observed data is deemed very good. The effective gas diffusion coefficients for oxygen and nitrogen determined are listed in Table 3. There appears to be a drastic increase in the effective gas diffusion coefficient as the amount of the nonsolvent used in the membrane preparation is CES48:24-c
Table 3. Effect of added nonsolvent in casting solution on the effective gas diffusion coefficient (D&
Nonsolvent CH,OH 1 2 3
(ml)
Diffusion coefficient, 10-s cm2/s 02
N2
12.233 12.823 63.772
2.104 2.673 25.992
increased from 2 to 3 ml. This is also reflected in the correspondingly high gas flux as shown in Fig. 5. The gas fluxes of the PC membranes prepared by the present wet-phase inversion process are considerably higher than those of other membranes (like cellulose acetates or polyimides). The time lag which appears in those cases of low gas permeation (Koros et al., 1976, 1977; Toi et al., 1983; Hachisuka et al., 1991; Lin, 1992) does not appear to exist, as are shown in Fig. 5. Hence, the method of determination of the effective diffusion coefficient utilizing time lag is no longer applicable for the high gas flux cases. Instead, the method adopted here utilizing the proposed mathematical model provides the only alternative. CONCLUSIONS
A polycarbonate membrane was prepared by a wetphase inversion process in which varying amounts of a nonsolvent (CH,OH) were added during the preparation period in an attempt to increase the porosity of the membrane. The PC membranes so formed were found to possess considerably higher gas permeability and yet still retain rather good selectivity toward oxygen/nitrogen mixtures. Hence, these PC membranes show great potentials for industrial oxygen enrichment. Although the PC membrane tends to have an asymmetrical structure because of the existence of a dense layer on the membrane surface, a simple one layer gas diffusion model with an effective gas diffusion coefficient was found to be adequate for representing the gas permeation process. The increased gas permeability of the PC membrane with increasing amount of
J. Y. LAI et al.
4074 CH,OH
is fully reflected
by the effective gas diffusion
coefficient. Acknowledgment-The authors wish to sincerely thank the National Science Council of Taiwan, ROC (NSC81-0405E033-503). for the financial support of this project. NOTATION
surface area of membrane gas concentration effective gas diffusion coefficient gas sorption constant correction coefficient in eq. (2) membrane thickness equilibrium gas pressure gas permeability upstream pressure
A c D err k K L P P PI P, P N2
downstream pressure permeability of nitrogen Permeability of oxygen cumulative gas pressure volume of gas passing through
PO, P, 4 f
t=a V X Greek a
the mem-
brane time time to reach sorption equilibrium volume of downside test cell spatial coordinate letter separation
factor
(or selectivity)
REFERENCES
Barrer, R. M., 1984, Diffusivities of glassy membrane for the dual-mode sorption model. J. Membrane Sci. 18, 25. Belfort, G. (Ed.), 1984, Synthetic Membrane Processes. Academic Press, New York.
Broens, L., Altena, F. W. and Smolders, C. A., 1980, Asymmetric membrane structure as a result of phase separation phenomena. Desalination 32. 33. Chan, A. H., Koros, W. J. and Paul, D. R., 1978, An analysis of hydrocarbon gas sorption and transport in ethyl cellulose using the dual sorption partial immobilized models. J. Membrane Sci. 3, 117. Crank, J., 1975, Mathematics of Diflision. Clarendon Press, Oxford. Hachisuka, H., Tsujita, Y., Takizawa, A. and Kinoshita, T., 1991, Gas transport properties of annealed polyimide films. J. Polym. Sci.-Polym. Phys. Edn 29, 11. Kamiya, Y., Mizoguchi, K., Naito, Y. and Hirose, T., 1986, Gas sorption in poly(viny1 benzoate). J. Polym. Sci.Polym. Phys. Edn 24, 535. Koros. W. J., Chan, A. H. and Paul, D. R., 1977, Sorption and transport of various gases in polycarbonates. J. Membrane Sci. 2, 165. Koros, W. J., Paul, D. R. and Rocha, A. A., 1976, Carbon dioxide sorption and transport in polycarbonate. J. Poiym. Sci.-Polym. Phys. Edn 14,687. Lai, J. Y., Yamada, S., Kamide, K., Manabe, S. and Kawai, T., 1986, Gas permeability of composite membranes. J. oppl. Polym. Sci. 32,4625. Lin, S. H., 1992, Solute permeation and sorption in a membrane separating two finite compartments. Sep. Technol. 2, 73. McCabe, W. L., Smith, J. C. and Harriott, P., 1985, Unit Operations of Chemical Engineering, 4th Edition. McGraw-Hill, New York, Mulder, R. L., 1991, Basic Principles of Membrane Technology. Kluwer, Netherlands. Rautenbach, R. and Albrecht, R. (Eds), 1989, Membrane Processes. Wiley, New York. Sandler, S. I., 1989, Chemical and Engineering 7’hermodynamits. Wiley, New York. Strathmann, H., Kock, K., Amar, 0. and Baker, R. W., 1975, The formation mechanism of asymmetric membrane. Des&nation 16, 179. Toi, K., Maeda, Y. and Tokuda, T., 1983, Numerical solution of the time-lag diffusion incorporating the dual sorption model. .I. appl. Polym. Sci. 28, 3589. Vieth, W. R. and Sladek, K. J., 1965, A model for diffusion in a glassy polymer. J. Colloid Sci. 20, 1014.
Vol. 48. NO. 24. pp. 4069-la74,
1993. 0
OCO%25G9/93 S56.00 + O.a, 1993 Pergamon Press Ltd
GAS PERMEATION IN POLYCARBONATE MEMBRANES PREPARED BY THE WET-PHASE INVERSION METHOD
Department
Department
of Chemical
Engineering,
of Chemical
J. Y. LA1 and J. M. JEN Chung Yuan University, Chung
Engineering,
(Received
12 March
Li, Taoyuan
S. H. LIN’ Yuan Ze Institute of Technology, ROC 1993; accepted for publication
320, Taiwan,
Neili, Taoyuan
ROC
320, Taiwan,
17 June 1993)
Abstract~Polycarbonate (PC) membranes prepared by the wet-phase inversion process was investigated in the present study. Varying amounts of a non-solvent (methanol) were added to the casting solution during membrane preparation and was found to increase significantly membrane porosity. The gas permeability of the PC membranes was increased from 1.68 and 0.26 barrers, respectively, for oxygen and nitrogen without the non-solvent, to 35.1 and 3.86 barrers with 3 ml of the nonsolvent added. The drastic increase in gas permeability was accompanied by only a slight decrease in selectivity. Hence, the PC membranes prepared with addition of methanol have a great potential for industrial oxygen enrichment applications. A simple gas diffusion model with an effective gas diffusion coefficient was found to describe reasonably well the gas permeation process. The estimated effective gas diffusion coefficient can closely reflect the increase in gas permeability of the membranes due to the addition of methanol. INTRODUCTION Gas permeation through a membrane is a science of practical interest to academics and industries. In contrast to other separation technologies, membrane processes have the distinct advantage of lower energy requirement (Belfort, 1984; Rautenbach and Albrecht, 1989; Mulder, 1991). As energy cost is bound to increase on a long-term basis, application of membrane processes can be expected to expand significantly in the near future as recognised by most industrial observers. Many works have been performed in the past, investigating gas permeation through various membranes (Koros et al., 1976, 1977; Chan et al., 1978; Lai et al., 1986; Toi er al., 1989; Hachisuka et al., 1991). Mathematical models were proposed for describing gas sorption and permeation processes (Vieth and Sladek, 1965; Toi et al., 1983; Barrer, 1984; Lin, 1992). One primary aspect common to most of the previous investigations is that gas permeation through the membrane was rather low. The dual sorption model as advanced by Vieth et al. (1965) and several investigators (Toi et al., 1983; Barrer, 1984; Lin, 1992) has been shown to describe well such a gas permeation process. In order to render the membranes to be worthy of any practical industrial applications, its gas permeability needs to be considerably enhanced. However, increase in gas permeability must be achieved without great compromise in the selectivity of the membrane. It has been well recognised that such kinds of membranes with both good gas permeability and selectivity are very difficult to obtain.
+Author to whom correspondence
should be addressed.
This accounts for the little attention that has been paid to address simultaneously gas permeability and selectivity of a membrane in the past. The purpose of this work is to investigate the gas permeation and selectivity of a special kind of polycarbonate (PC) membrane which was prepared by a unique wet-phase inversion process with incorporation of varying amounts of a nonsolvent (methanol) for permeability enhancement. The wet method has the advantages of rapid membrane preparation, ease of control of the preparation conditions and increase in membrane porosity. The PC membranes so prepared achieve very high gas permeability and also retain reasonably good selectivity. A simple Fick’s diffusion model with an effective gas diffusion coefficient has been adapted to characterize the gas diffusion and sorption processes. EXPERIMENTAL STUDIES
PC membrane preparation method
by the wet-phase
inversion
Two grams of polycarbonate (Mitsubishi Gas Chemical Co., Inc., Japan) and 17 ml of dichloromethane (Merck, Inc., Germany) were placed in a 50 ml flask and well mixed by a magnetic stirrer until the polycarbonate resin was completely dissolved to form a 8 wt % casting solution. The mixing continued for a total of 16 h from the beginning. As soon as the first stage of mixing was finished, varying amounts of the nonsolvent (CH,OH), ranging from 1 to 3 ml, were added and mixing was resumed for another 6 h. The mixture was placed in a constant-temperature bath for 12 h for degasification. A piece of 3 mm-thick glass plate was thoroughly washed with distilled water and acetone and dried by an air blower. The PC casting
4070
J. Y.
LA1 et a/.
solution was uniformly coated on the clean glass plate with a garner’s knife to a desired thickness. The coated glass plate was then immersed immediately in a pure methanol bath. The PC casting solution gelled due to phase separation in just a few minutes to form a thin membrane_ The membrane was placed in a vacuum drier, which was maintained at 3O”C, for at least 6 h to remove the residual gelation nonsolvent
(CH,OH). Gas sorption
in the PC
membrane
The experimental setup for gas sorption measurements is shown in Fig. 1. A microbalance (Cahn Model 1000 Electrobalance) was enclosed in a constant-temperature box which was set at 35°C. Four pieces of 3 cm x 2.5 cm PC membrane were put in the microbalance system. The system pressure was then lowered to about 4 x lo-’ Torr. The vacuum condition was maintained for another hour. The gas to be tested (99.99% pure oxygen or nitrogen) was let in until the system pressure reached 1 atm. The system was maintained under this condition until sorption equilibrium was reached. It was reported by Koros et al. (1977) that nitrogen has a diffusion coefficient of 1.76 x 10-a cm’/s in a pure PC membrane. Accord-
ingly, the time to reach 99.999% sorption equilibrium can be computed from (McCabe et al., 1985)
teq ==jj(~>‘ln($XlO’) where S is the membrane thickness, z the operating pressure and t_, is the time to reach sorption equilibrium. For the present PC membrane of a uniform thickness of 0.003 cm and with the gas diffusion coefficient quoted above, it would need only 585 s to reach sorption equilibrium. For the present experiments, 3 h were allowed which could be deemed more than sufficient. After gas sorption equilibrium was reached, the total weight of the membrane was measured. The same procedure was-repeated for other gas absorption experiments. From the weight change of the PC membrane, the weight of the gas adsorbed can be calculated according to weight of gas absorbed by change of membrane by the PC membrane weight of gas displaced = volume of membrane
C
CP
II3 %
Go3ltRd
unit
p:
Mp: machpic~ pump OR oil dtffudon pm&l PO: Pirud plgo
IC: inmalltcd ombiuet
T:
fan OR: *aa mmsrvoir H: heeta
Sl-s7:
#toPcocks
th-uple
Tl and T2:
liquid N, tmpr
Fig. 1. Schematic of sorption apparatus.
PC membrane = weight weight of gas displaced by the PC membrane x density of gas
Polycarbonate
membranes
prepared
by wet-phase
inversion
PC membranes nitrogen. Membrane
method
4071
are listed in Table 1 for oxygen and
permeability test
A circular PC membrane of 3 cm in diameter was placed in the diffusion cell of the gas permeability testing apparatus (Yanaco Model GTR-IO, Japan) which is shown in Fig. 3. The system pressure was then lowered until it was less than 5 x lo-’ Torr. The gas to be tested (99.99 % quality air) was then let in on the upper side of the diffusion cell. The gas composition of the underside of the diffusion cell was measured periodically by a gas chromatograph (Shimadzu GC-14A, J&pan). The gas permeability was calculated from the following equation: Fig. 2. Gas
concentration vs pressure at equilibrium determining the sorption constant.
Table 1. Effect of added nonsolvent in casting on the sorption constant Sorption
constant
qKL p = (P‘ - P,)At
for
The separation as
factor (selectivity) of a gas was defined
a=*.P
solution
rNI k+
Nonsolvent
CH,OH (ml)
02
N,
1
0.2362 0.3506 0.4186
0.3106 0.4442 0.5375
2 3
(3)
DISCUSSION
‘k in cm3 gas (STP)/cm3polymer-atm.
where the volume of the membrane was measured and the gas density was determined by the PengRobinson equation (Sandler, 1989). According to Kamiya et al. (1986), gas sorption by the membrane at 1 atm pressure or below can be represented by C = kP
(2)
where C and P are, respectively, the gas concentration and pressure at equilibrium and k the sorption constant determined from the linear C vs P plot by the least-squares method. Figure 2 displays a typical plot for nitrogen. The calculated sorption constants for the
Ga8 pameability analyzer
Tompemtorecontroller
Fig. 3. Schematic
OF RESULTS
The amount of the nonsolvent (methanol) employed for the present membrane preparation was varied from 1 to 3 ml. The scanning electron micrographs (SEM) (Hitachi S-570, Hitachi, Inc., Japan) of the four membranes with and without addition of the nonsolvent are shown in Fig. 4. The four SEM micrographs on the left were those taken at the surface of the membrane, while those on the right were the cross-sectional images taken by cutting across the thin membrane. The amount of the nonsolvent added was 0 ml for the top SEM images and 3 ml for the bottom ones. These micrographs clearly indicate that the pore size of the membrane increases with increasing the amount of the nonsolvent. The skin layer on the membrane surface was observed to become increasingly thinner as the amount of nonsolvent increases. This skin layer was generally considered as a dense layer in contrast to the underlying porous structure inside the membrane resulting in an asymmetricity of the membrane (Strathmann et al., 1975;
of permeation
apparatus.
4072
J. Y. LAIet af.
.L
-
Model Htcing
/
(b)
Fig. 4. SEM images of PC membranes prepared by the wet method from casting solution of 8% PC/CH&l,. Nonsolvent added: (a) 0 mf; (b) 1 mt; (e) 2 ml; (d) 3 ml. Broens et al., 1980). The pore size and the thickness of the dense skin layer have considerable influence on the gas liability of the membrane. The permeability and selectivity determined according to eqs (2) and (3) for the three membranes are given in Table 2. This table further shows that the gas permeabilities for oxygen and nitrogen were drasticaIly enhanced as the amount of the nonsolvent
Time (I) Fig. 5. Com~~son of model predictions and measured data of gas permeation through membrane prepared with addition of CH,OH in the casting solution: (a) 1 ml; (b) 2 ml; (c) 3 ml. was increased, and that the drastic increase in gas ~~eabil~ty however is accompanied by only a slight decrease in selectivity. As is widely recognized, a membrane must have the characteristics of high gas permeability and reasonable selectivity for it ta he af any commercial value. These requirements are met by
Polycarbonate
4073
membranes prepared by wet-phase inversion method
Table 2. Effect of added nonsolvent in casting solution on the gas permeability and selectivity Permeability (barrer)
P lit
P 01
Selectivity
Nonsolvent CH,OH (ml)
eq. (2)
eq. (3)
eq. (2)
eq. (3)
0 :
1.68 6.06 3.29
0.09 0.20 0.16
0.26 0.56 1.10
0.07 0.07 0.10
6.45 5.87 5.50
0.21 0.15 0.39
3
35.10
0.19
16.04
0.24
2.18
0.02
PoJP,,
Notes: (1) Gel&on nonsolvent: pure CH,OH. (2) Casting. solution composition: PC 2 g/CH,Cl, 17ml. (3) Gelat& temperature: 20°C.
PC membrane prepared with an appropriate amount of a nonsolvent. These PC membranes have therefore a great potential for industrial applications in oxygen enrichment. As observed earlier, the present PC membranes have a thin, dense skin layer on the surface which renders the membrane asymmetric. The gas diffusion characteristics in this layer would be different from that inside the porous layer. For simplicity, however. we assume that an effective diffusion coefficient can be employed to characterize gas diffusion in such an asymmetric system. The diffusion process of any gas component is assumed to follow the simple Fick’s equation, the present
subject to the initial and boundary conditions c=o
@aI
x = 0,
C = kP,
WI
x = L,
C = kP,.
(64
t = 0,
The cumulative downstream gas pressure P, is determined from
Analytical solution to eq. (4) subject to coupled eqs (5aH5c) and (6) is not available. An iterative Crank-Nicolson implicit finite-difference scheme (Crank, 1975) was employed for integrating these equations. The only unknown constant in the above equation is the effective diffusion coefficient D,,, which was determined by best fitting the computed amount of gas passing through the membrane with the measured data. Comparison of the measured and predicted gas fluxes for the PC membranes with 1, 2 and 3 ml of CH,OH are demonstrated in Fig. 5. The agreement between the predictions and the observed data is deemed very good. The effective gas diffusion coefficients for oxygen and nitrogen determined are listed in Table 3. There appears to be a drastic increase in the effective gas diffusion coefficient as the amount of the nonsolvent used in the membrane preparation is CES48:24-c
Table 3. Effect of added nonsolvent in casting solution on the effective gas diffusion coefficient (D&
Nonsolvent CH,OH 1 2 3
(ml)
Diffusion coefficient, 10-s cm2/s 02
N2
12.233 12.823 63.772
2.104 2.673 25.992
increased from 2 to 3 ml. This is also reflected in the correspondingly high gas flux as shown in Fig. 5. The gas fluxes of the PC membranes prepared by the present wet-phase inversion process are considerably higher than those of other membranes (like cellulose acetates or polyimides). The time lag which appears in those cases of low gas permeation (Koros et al., 1976, 1977; Toi et al., 1983; Hachisuka et al., 1991; Lin, 1992) does not appear to exist, as are shown in Fig. 5. Hence, the method of determination of the effective diffusion coefficient utilizing time lag is no longer applicable for the high gas flux cases. Instead, the method adopted here utilizing the proposed mathematical model provides the only alternative. CONCLUSIONS
A polycarbonate membrane was prepared by a wetphase inversion process in which varying amounts of a nonsolvent (CH,OH) were added during the preparation period in an attempt to increase the porosity of the membrane. The PC membranes so formed were found to possess considerably higher gas permeability and yet still retain rather good selectivity toward oxygen/nitrogen mixtures. Hence, these PC membranes show great potentials for industrial oxygen enrichment. Although the PC membrane tends to have an asymmetrical structure because of the existence of a dense layer on the membrane surface, a simple one layer gas diffusion model with an effective gas diffusion coefficient was found to be adequate for representing the gas permeation process. The increased gas permeability of the PC membrane with increasing amount of
J. Y. LAI et al.
4074 CH,OH
is fully reflected
by the effective gas diffusion
coefficient. Acknowledgment-The authors wish to sincerely thank the National Science Council of Taiwan, ROC (NSC81-0405E033-503). for the financial support of this project. NOTATION
surface area of membrane gas concentration effective gas diffusion coefficient gas sorption constant correction coefficient in eq. (2) membrane thickness equilibrium gas pressure gas permeability upstream pressure
A c D err k K L P P PI P, P N2
downstream pressure permeability of nitrogen Permeability of oxygen cumulative gas pressure volume of gas passing through
PO, P, 4 f
t=a V X Greek a
the mem-
brane time time to reach sorption equilibrium volume of downside test cell spatial coordinate letter separation
factor
(or selectivity)
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