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Use of polar solvents in an electroconvective liquid
Journal of Membrane Science, 84 (1993) 191-196 Elsevier Science Publishers B.V., Amsterdam
Rapid Communication
Use of polar solvents in an electroconvective liquid membrane for gas separation Chunrong Wan”, Richard D. Noble”, * and Noel A. Clarkb “Department of Chemical Engineering, University of Colorado, Boulder, CO 80309 (USA) bPhysics Department, University of Colorado, Boulder, CO 80309 (USA) (Received March 25,1993; accepted in revised form June 25,1993)
Abstract An electroconvective liquid crystal membrane was previously investigated. The gas permeation rate through a N- (4-methoxybenzylidine)-4-butylaniline (MBBA) membrane was increased by a factor of 7 when an a.c. electric field of 100 volts/mm was applied. Recently, polar solvents such as 2-ethylhexanol (2EH) and diethyl phthalate (DEP) were studied as new electroconvective liquid membrane candidates. They are much cheaper than MBBA, in air chemically more stable, and may be used to replace the liquid crystal. A de-ionization procedure was used to decrease the conductivity of the polar solvents below 10 -lo ohm-‘-cm-’ so they would exhibit convectivity. Key words: liquid membranes; electroconvective transport
gas and vapor permeation;
Introduction We previously demonstrated a composite electroconvective liquid crystal membrane [ 11. N- (4-methoxybenxylidine) -4-butylaniline (MBBA ) was contained between two electrodes forming a sandwich-like membrane structure, which contained the liquid crystal but allows the passage of gas through the membrane. An a.c. electric field induced convective motion of the liquid crystal layer and facilitated the transport of gases across the membrane. To whom correspondence should be addressed.
0376-7388/93/$06.00
facilitated
transport;
polar solvents;
MBBA has a high cost and chemical stability problems arise with respect to free charges in the liquid. Also, dissociated water can hydrogen bond with the polar core of MBBA and cause the molecule to break. Therefore, it is reasonable to search for electroconvective and chemically stable candidates to replace the liquid crystal. The requirements for this liquid phase are: (1) It must be electroconvective; (2) It must be much cheaper in price than a liquid crystal; (3) It must be stable in air; (4) The selectivity for gas separation should be as high as possible;
0 1993 Elsevier Science Publishers B.V. All rights reserved.
Ch. Wan et al. /J. Membrane Sci. 84 (1993) 191-196
192
(5 ) It must be compatible with the barrier film between the fluid and the electrodes. In accordance with these objectives, three polar solvents [ 2-ethylhexanol (2EH), l-octanol and diethyl phthalate (DEP ) ] have been selected as candidates. The acid gas separation results published by S.L. Matson [ 31 were used in the selection of these solvents. The use of electrohydrodynamic stirring to facilitate mass transfer across a fluid membrane, other than a liquid crystal membrane, has been demonstrated by Plonski et al. [ 41. In these experiments, the ion flux through a non conducting (l-octanol) film separating aqueous ionic solutions was controlled over a factor of 10 by an applied electrical field. However, the electric field required for convection made the films unstable. The reasons for the instability were not discussed. In fact it has been known since Faraday’s era that an electric field exerted upon an insulating liquid dielectric could produce numerous hydrodynamic effects, such as swirling, turbulence, etc., as reviewed by Pickard [ 51. Tobason [6] summarized the developments on the physical mechanism and mathematical formulations of electrohydrodynamic instability and electroconvection in the transient and a.c. regime in insulating liquids. His experiments proved that the Coulomb force acting on free changes in the fluid is far greater than the dielectric force exerted on the dipoles. As an example, a water molecule is highly polar; its dielectric constant being 73.5. It can be transported using a piezo-electric pump driven by a flexural progressive wave [ 71, but no convective motion was observed under an a.c. electric field. The dielectric constant of a solvent does not appear to be the dominant characteristic parameter for convectivity. An electroconvective solvent must essentially be an electrically insulating liquid [ 51. The dielectric constant and conductivity of some polar solvents under an a.c. voltage have
been determined. An equivalent R-C electric circuit model and its mathematical representation are proposed to describe the conductivity for the sandwich composite membrane configuration. Permeation measurements as a function of the a.c. field strength were performed to confirm the fact that convectivity could be induced. Theory An equivalent circuit containing the minimal number of elements which is reasonable for the sandwich composite membrane is shown in Fig. 1. The parallel circuit composed of the lumped resistance, RI, and capacitor, C1, is intended to represent the conductivity of the liquid layer. The impedance of the liquid layer, 2, can be estimated by the equations: Z=l/(R,
+l/&)
(I)
2, = l/27&
(2)
Ci = E,iE,AJdi
(3)
where f is the frequency of the alternating current voltage, E, is the dielectric permittivity of vacuum space (8.854 x lo-l2 C/V-m), E,i is the relative dielectric permittivity or dielectric
-
AC-
Fig. 1. The equivalent R-C electric circuit model for the sandwich composite membrane configuration. The parallel circuit composed of R, and C, is intended to represent the conductivity of the liquid layer, the conductivities of the two barrier layers are analogous to C2 and C,.
193
Ch. Wan et al. /J. Membrane Sci. 84 (1993) 191-196
constant of the liquid medium, A and d are the electrode surface area and inter-electrode distance respectively. The d.c. conductivity of C, should he nil. 2 and eri can be determined experimentally and the resistance for direct current, RI, can he estimated indirectly. A barrier layer is required between each electrode and the liquid membrane layer to prevent wetting of the electrode. The two barrier layers are made of a polymer such as silicone rubber (SR). The thickness of each layer is about 25 ,um. The electric conductivity of the polymer is less than 1 x lo-l4 ohm-l-cm-‘. Although its thickness is only about l/100 of that of the liquid layer, the d.c. conductivity is still much lower than that of the liquid layer. Therefore, its a.c. conductivity is dominated by its capacitance which can be estimated by eqn. (3 ) . The two barrier layers are represented by two capacitors (C, and C, ) in the equivalent circuit. For the operation of the sandwich membrane, the impedance of the liquid layer must be higher than that of the barrier layers: 2,/22,
>1
cBdL/2dB cL > 1
(4) (5)
The dielectric constant of SR is about 3.0 to 3.3 [ 81. So assuming tg = 3, the ratio of d L to dB is 100, so: EL< 150
(6)
Equation (6) means that if the dielectric constant of the liquid medium is about 150, half of the applied a.c. voltage will be applied to the barrier layers. If we want 90% of the a.c. voltage on the liquid layer, the dielectric constant should be less than 15, while the d.c. conductivity of the liquid barrier layer could be thinner, the dielectric constant of the liquid medium could be higher. When the a.c. conductivity of a polar solvent at 60 Hz frequency is more than lx lo-” ohm-‘-cm-l, the resistance of the liquid layer
for direct current (RI ) must be taken into consideration. This is not a constant. It is dependent on time when a d.c. voltage is applied [ 91. When an a.c. voltage is applied, the time per cycle is very short. For example, it is 0.02 set for an a.c. square wave at 50 Hz frequency. Under these conditions, we can assume that it is a constant. For a polar solvent to exhibit convectivity, the conductivity must be on the order of lo-” ohm-l-cm-’ [lo]. Fortunately, an ion exchange technique has been developed for the de-ionization of a polar liquid. Felici [ 11-131 proved that the electric conductivity of some strongly polar solvents, such as acetone and nitrobenzene, could be reduced to the 10-11-10-12 ohm-‘-cm-’ range. When the strongest acidic and basic ion exchangers were employed for deionization purposes, and some molecular sieve material was used for the dehydration of the liquid as well, the results were very good. The residual conductivity was no longer due to impurities, but to natural dissociation of the liquid molecules themselves. Experimental procedures The zero differential pressure system using a liquid piston and the single component gas permeation system described previously [ 1] were both employed for the gas permeation experiments. Gas permeability measurements with each solvent were performed as described in our previous work [ 11. A silicon rubber film cast on a Celgard 2500 film was used as the barrier layer to prevent the liquid from contacting and wetting the porous electrodes. It was found that the silicone rubber paste made by Dow Corning Company is a little too thick to make a defect-free silicone rubber thin film supported by a Celgard 2500 membrane. It was much better to use a diluent such as toluene for the silicone rubber paste. The determination of the conductivity and
194
Ch. Wan et al. /J. Membrane Sci. 84 (1993) 191-196
dielectric constant was conducted with a cell consisting of two parallel electrodes. The contact area of the liquid layer was equal to that of electrodes. The conductivity was measured directly and the dielectric constant was estimated by the ratio of the capacitance of the system filled with solvent to that filled with air. The flowsheet of the de-ionization process to remove the ionized impurity from polar solvents is shown in Fig. 2. Strong acidic and basic ion exchangers, Amberlyst XN-1010 and A27 ion-exchange resins supplied by Aldrich Chemical Company, Inc., were employed for de-ionization purposes, and molecular sieves, 3A l/ ION BXCHANGE
BEADS
t MOLECULAR
SIEVE
DEHYDRATION
DlBTlLlATlON
MEASUREMENT
DE IONIZRD SOLVENT
Fig. 2. Process for de-ionization of polar solvents. TABLE 1 Swelling of SR in various liquids Solvent
Swelling of SR (o/o)
Water MBBA 2EH 1-0ctanol DEP
0 4.1 15.7 2.9 0
16-inch beads supplied by the same company, were used for dehydration of the liquid. Dehydration of the ion exchange beads was conducted by washing the adsorbed water from the beads using a hydrophilic agent such as ether or dioxane, and then drying the beads in vacuum for 4 hr or more. De-ionization and dehydration were conducted by contacting the solvents with mixed exchangers and molecular sieve beads respectively in a erlenmyer flask using a magnetic stirrer; 1 g cation exchange resin, 1 g anion exchange and 2 g molecular sieve beads were used for 50 ml solvent, and the contacting time was not less than 6 hr for de-ionization and dehydration respectively. Distillation was used to remove any particles in the solvent. It was conducted in a Pyrex glass system using a 100 flask for 50 ml solvent. The polymer SR film samples were connected with MBBA and the three solvents for two weeks, and the degree of swelling was measured to characterize the stability of the SR film in the solvents. The results are listed in Table 1. No swelling of SR in DEP was observed. Results and discussion The permeability and selectivity of all the candidate solvents are shown in Table 2. The selectivity of 2EH and 1-octanol for separation of CO, from Hz is in the same order of magnitude as that of MBBA, while DEP has a higher selectivity than MBBA. DEP has the highest boiling point, but the conductivity of the commercial product is only in the order of lo-’ ohm-‘-cm-’ which is too large for electroconvective motion. The conductivity of polar solvents is primarily due to electrolytic impurities. De-ionization experiments for DEP have been conducted according to the process flowsheet shown in Fig. 2, and the results are listed in Table 3. The results show that the conductivity of polar solvents such as DEP can successfully be reduced
195
Ch. Wan et al. /J. Membrane Sci. 84 (1993) 191-196 TABLE 2 Permeability and selectivity of candidate solvents Solvent
P N2
P HZ
PC02
%OdHz
%OdNz
MBBA 2EH l-O&an01 DEP
0.9 6.3 3.4 2.7
4.0 14.0 17.4 8.8
19.2 58.6 70.7 54.5
4.8 4.2 4.1 6.2
21.3 9.3 20.9 20.9
“Unit of permeability: 1O-8 cm3 (STP)-cm/see-cm2-cmHg.
TABLE 3 Properties of candidate solvents Solvent
b.p. (“C)
Conductivity a.c., 60 Hz, 50 V
Dielectric constant (1000 Hz)
2EH l-O&an01 DEP DEPb
183 196 298 298
4.4x 1.8x 1.6x 8.8x
6.7 9.4 8.0 7.2
lo-‘0 lo-’ lo-’ lo-*’
AC
“Unit of conductivity: ohm-‘-cm-‘. bDe-ionized.
voltage
Fig. 4. Effect of a.c. electric field on the permeability of diethyl phthalate.
0 ACvoltage
Fig. 3. Effect of a.c. electric field on gas permeation across a diethyl phthalate membrane.
using ion exchange based technology, and that the dielectric constant can be reduced as well. The electroconvectivity of de-ionized DEP has been demonstrated by a gas permeation experiment using permeation of individual gases. The results are shown in Figs. 3 and 4. The
permeability was estimated by the method proposed previously [ 11. The selectivity remains high, but the enhancement, about 6 times at 1200 volts, is not as great as for MBBA. That is because the permeability of DEP in the rest state is much higher than that of MBBA and its viscosity is also higher. If the viscosity is higher, the resistance for convective motion will be increased. Consequently the voltage required to create turbulence must be increased. Conclusions A polar solvent to be used to replace the liquid crystal MBBA in a electroconvective liquid membrane basically must be an insulating solvent. This work has demonstrated that if the conductivity is on the order of lo-” ohm-‘cm-‘, the solvent can exhibit a stable electro-
196
convectivity. According to the analysis using the equivalent R-C electric current model, when two pieces of 25 pm thick silicon rubber (SR) film are employed as barrier layers, the dielectric constant of the polar solvent should be less than 15 to apply 90% of the electric voltage on the liquid layer. When the conductivity of a polar solvent for an a.c. of 60 Hz frequency is more than 1 x 10-l' ohm-‘-cm-‘, it is primarily controlled by ionized impurities in the solvent. An ion exchange based technique can be used for the de-ionization of polar solvents. In this technology, a mixed bed composed of strongly acid and basic ion exchange resins is employed for the removal of the ionized impurities and molecular sieve beads are useful for dehydration of the polar solvent. The dielectric constant which depends not only on the molecular polarity but also partly on the ionized impurities can be reduced as well. Diethyl phthalate (DEP) has been selected as one of the candidate electroconvective solvents; its dielectric constant is in the 5-15 range, boiling point is 298”C, selectivity for separations of CO, from H2 is better than that of MBBA, the retail price is less than l/100 of that for MBBA, and the SR barrier film is very stable in DEP. The conductivity of the commercial product supplied by Aldrich Chemical Company, Inc. is in the order of lo-’ ohm-lcm-‘, this can be reduced to the order of lo-” ohm-l-cm-’ using ion exchange based technology, and then DEP exhibited stable electroconvectivity. Gas permeation through the DEP membrane can be facilitated by inducing an a.c. electric field.
Ch. Wan et al. /J. Membrane Sci. 84 (1993) 191-196
Acknowledgment This research was funded by the NSF I/U CRC Center for Separations Using Thin Films. The authors gratefully acknowledge their support. References 1
2
3 4
5 6
7
8 9 10
11
12
13
Ch. Wan, R.D. Noble and N.A. Clark, Control of gas permeation via electrohydrodynamic convection in a liquid crystal membrane, J. Membrane Sci., 74 (1992) 223-231. D.H.V. Winkle, J. Gurung and R. Biggers, Electrohydrodynamic flow in thick liquid crystal cells, Mat. Res. Sot., Symp. Proc., 177 (1990) 311-316. S.L. Matson, Acid gas scrubbing by composite solvent-swollen membranes, US Patent 4,737,166 (1988). J.W. Plonski, J.F. Hoburg, D.F. Evans and E.L. Cussler, Mixing liquid membranes with electric fields, J. Membrane Sci., 5 (1979) 371-374. W. Pickard, Process in Dielectrics, Vol. 6, Heywood and Co. Ltd., London, 1965. R. Tobason, Electrohydrodynamic instabilities and electroconvection in the transient and a.c. regime of unipolar injection in insulating liquids: A review, J. Electrostat., 15 (1984) 359-384. S. Miyazaki, T. Kawai and M. Araragi, A piezo-electric pump driven by a flexural progressive wave, IEEE Workshop on micro electro mechanical systems, (1991) 283-288. D.R. Lide, Handbook of Physics and Chemistry, CRC Press, Boca Raton, FL, 1992/1993, pp. 6-126. J.C. Lacroix, Ph.D. Thesis, University of Grenoble, France, 1976. A.I. Zhakin, I.E. Taranov and A.I. Faedonenko, Experimental study of the conduction mechanism in polar liquid dielectrics, Elektgron. Obrab. Mater., 5 (1983) 37-41. N.J. Felici, D.C. conduction in liquid dielectrics, A survey of recent progress (Part l), Direct Current, 2 (3) (1971) 90-99. N.J. Felici, D.C. conduction in liquid dielectrics (Part 2)) Electrohydrodynamic phenomena, Direct Current, 2 (4) (1971) 147-165. N.J. Felici, The de-ionization of strongly polar solvents, Brit. J. Appl. Phys., 15 (1964) 801-805.
Rapid Communication
Use of polar solvents in an electroconvective liquid membrane for gas separation Chunrong Wan”, Richard D. Noble”, * and Noel A. Clarkb “Department of Chemical Engineering, University of Colorado, Boulder, CO 80309 (USA) bPhysics Department, University of Colorado, Boulder, CO 80309 (USA) (Received March 25,1993; accepted in revised form June 25,1993)
Abstract An electroconvective liquid crystal membrane was previously investigated. The gas permeation rate through a N- (4-methoxybenzylidine)-4-butylaniline (MBBA) membrane was increased by a factor of 7 when an a.c. electric field of 100 volts/mm was applied. Recently, polar solvents such as 2-ethylhexanol (2EH) and diethyl phthalate (DEP) were studied as new electroconvective liquid membrane candidates. They are much cheaper than MBBA, in air chemically more stable, and may be used to replace the liquid crystal. A de-ionization procedure was used to decrease the conductivity of the polar solvents below 10 -lo ohm-‘-cm-’ so they would exhibit convectivity. Key words: liquid membranes; electroconvective transport
gas and vapor permeation;
Introduction We previously demonstrated a composite electroconvective liquid crystal membrane [ 11. N- (4-methoxybenxylidine) -4-butylaniline (MBBA ) was contained between two electrodes forming a sandwich-like membrane structure, which contained the liquid crystal but allows the passage of gas through the membrane. An a.c. electric field induced convective motion of the liquid crystal layer and facilitated the transport of gases across the membrane. To whom correspondence should be addressed.
0376-7388/93/$06.00
facilitated
transport;
polar solvents;
MBBA has a high cost and chemical stability problems arise with respect to free charges in the liquid. Also, dissociated water can hydrogen bond with the polar core of MBBA and cause the molecule to break. Therefore, it is reasonable to search for electroconvective and chemically stable candidates to replace the liquid crystal. The requirements for this liquid phase are: (1) It must be electroconvective; (2) It must be much cheaper in price than a liquid crystal; (3) It must be stable in air; (4) The selectivity for gas separation should be as high as possible;
0 1993 Elsevier Science Publishers B.V. All rights reserved.
Ch. Wan et al. /J. Membrane Sci. 84 (1993) 191-196
192
(5 ) It must be compatible with the barrier film between the fluid and the electrodes. In accordance with these objectives, three polar solvents [ 2-ethylhexanol (2EH), l-octanol and diethyl phthalate (DEP ) ] have been selected as candidates. The acid gas separation results published by S.L. Matson [ 31 were used in the selection of these solvents. The use of electrohydrodynamic stirring to facilitate mass transfer across a fluid membrane, other than a liquid crystal membrane, has been demonstrated by Plonski et al. [ 41. In these experiments, the ion flux through a non conducting (l-octanol) film separating aqueous ionic solutions was controlled over a factor of 10 by an applied electrical field. However, the electric field required for convection made the films unstable. The reasons for the instability were not discussed. In fact it has been known since Faraday’s era that an electric field exerted upon an insulating liquid dielectric could produce numerous hydrodynamic effects, such as swirling, turbulence, etc., as reviewed by Pickard [ 51. Tobason [6] summarized the developments on the physical mechanism and mathematical formulations of electrohydrodynamic instability and electroconvection in the transient and a.c. regime in insulating liquids. His experiments proved that the Coulomb force acting on free changes in the fluid is far greater than the dielectric force exerted on the dipoles. As an example, a water molecule is highly polar; its dielectric constant being 73.5. It can be transported using a piezo-electric pump driven by a flexural progressive wave [ 71, but no convective motion was observed under an a.c. electric field. The dielectric constant of a solvent does not appear to be the dominant characteristic parameter for convectivity. An electroconvective solvent must essentially be an electrically insulating liquid [ 51. The dielectric constant and conductivity of some polar solvents under an a.c. voltage have
been determined. An equivalent R-C electric circuit model and its mathematical representation are proposed to describe the conductivity for the sandwich composite membrane configuration. Permeation measurements as a function of the a.c. field strength were performed to confirm the fact that convectivity could be induced. Theory An equivalent circuit containing the minimal number of elements which is reasonable for the sandwich composite membrane is shown in Fig. 1. The parallel circuit composed of the lumped resistance, RI, and capacitor, C1, is intended to represent the conductivity of the liquid layer. The impedance of the liquid layer, 2, can be estimated by the equations: Z=l/(R,
+l/&)
(I)
2, = l/27&
(2)
Ci = E,iE,AJdi
(3)
where f is the frequency of the alternating current voltage, E, is the dielectric permittivity of vacuum space (8.854 x lo-l2 C/V-m), E,i is the relative dielectric permittivity or dielectric
-
AC-
Fig. 1. The equivalent R-C electric circuit model for the sandwich composite membrane configuration. The parallel circuit composed of R, and C, is intended to represent the conductivity of the liquid layer, the conductivities of the two barrier layers are analogous to C2 and C,.
193
Ch. Wan et al. /J. Membrane Sci. 84 (1993) 191-196
constant of the liquid medium, A and d are the electrode surface area and inter-electrode distance respectively. The d.c. conductivity of C, should he nil. 2 and eri can be determined experimentally and the resistance for direct current, RI, can he estimated indirectly. A barrier layer is required between each electrode and the liquid membrane layer to prevent wetting of the electrode. The two barrier layers are made of a polymer such as silicone rubber (SR). The thickness of each layer is about 25 ,um. The electric conductivity of the polymer is less than 1 x lo-l4 ohm-l-cm-‘. Although its thickness is only about l/100 of that of the liquid layer, the d.c. conductivity is still much lower than that of the liquid layer. Therefore, its a.c. conductivity is dominated by its capacitance which can be estimated by eqn. (3 ) . The two barrier layers are represented by two capacitors (C, and C, ) in the equivalent circuit. For the operation of the sandwich membrane, the impedance of the liquid layer must be higher than that of the barrier layers: 2,/22,
>1
cBdL/2dB cL > 1
(4) (5)
The dielectric constant of SR is about 3.0 to 3.3 [ 81. So assuming tg = 3, the ratio of d L to dB is 100, so: EL< 150
(6)
Equation (6) means that if the dielectric constant of the liquid medium is about 150, half of the applied a.c. voltage will be applied to the barrier layers. If we want 90% of the a.c. voltage on the liquid layer, the dielectric constant should be less than 15, while the d.c. conductivity of the liquid barrier layer could be thinner, the dielectric constant of the liquid medium could be higher. When the a.c. conductivity of a polar solvent at 60 Hz frequency is more than lx lo-” ohm-‘-cm-l, the resistance of the liquid layer
for direct current (RI ) must be taken into consideration. This is not a constant. It is dependent on time when a d.c. voltage is applied [ 91. When an a.c. voltage is applied, the time per cycle is very short. For example, it is 0.02 set for an a.c. square wave at 50 Hz frequency. Under these conditions, we can assume that it is a constant. For a polar solvent to exhibit convectivity, the conductivity must be on the order of lo-” ohm-l-cm-’ [lo]. Fortunately, an ion exchange technique has been developed for the de-ionization of a polar liquid. Felici [ 11-131 proved that the electric conductivity of some strongly polar solvents, such as acetone and nitrobenzene, could be reduced to the 10-11-10-12 ohm-‘-cm-’ range. When the strongest acidic and basic ion exchangers were employed for deionization purposes, and some molecular sieve material was used for the dehydration of the liquid as well, the results were very good. The residual conductivity was no longer due to impurities, but to natural dissociation of the liquid molecules themselves. Experimental procedures The zero differential pressure system using a liquid piston and the single component gas permeation system described previously [ 1] were both employed for the gas permeation experiments. Gas permeability measurements with each solvent were performed as described in our previous work [ 11. A silicon rubber film cast on a Celgard 2500 film was used as the barrier layer to prevent the liquid from contacting and wetting the porous electrodes. It was found that the silicone rubber paste made by Dow Corning Company is a little too thick to make a defect-free silicone rubber thin film supported by a Celgard 2500 membrane. It was much better to use a diluent such as toluene for the silicone rubber paste. The determination of the conductivity and
194
Ch. Wan et al. /J. Membrane Sci. 84 (1993) 191-196
dielectric constant was conducted with a cell consisting of two parallel electrodes. The contact area of the liquid layer was equal to that of electrodes. The conductivity was measured directly and the dielectric constant was estimated by the ratio of the capacitance of the system filled with solvent to that filled with air. The flowsheet of the de-ionization process to remove the ionized impurity from polar solvents is shown in Fig. 2. Strong acidic and basic ion exchangers, Amberlyst XN-1010 and A27 ion-exchange resins supplied by Aldrich Chemical Company, Inc., were employed for de-ionization purposes, and molecular sieves, 3A l/ ION BXCHANGE
BEADS
t MOLECULAR
SIEVE
DEHYDRATION
DlBTlLlATlON
MEASUREMENT
DE IONIZRD SOLVENT
Fig. 2. Process for de-ionization of polar solvents. TABLE 1 Swelling of SR in various liquids Solvent
Swelling of SR (o/o)
Water MBBA 2EH 1-0ctanol DEP
0 4.1 15.7 2.9 0
16-inch beads supplied by the same company, were used for dehydration of the liquid. Dehydration of the ion exchange beads was conducted by washing the adsorbed water from the beads using a hydrophilic agent such as ether or dioxane, and then drying the beads in vacuum for 4 hr or more. De-ionization and dehydration were conducted by contacting the solvents with mixed exchangers and molecular sieve beads respectively in a erlenmyer flask using a magnetic stirrer; 1 g cation exchange resin, 1 g anion exchange and 2 g molecular sieve beads were used for 50 ml solvent, and the contacting time was not less than 6 hr for de-ionization and dehydration respectively. Distillation was used to remove any particles in the solvent. It was conducted in a Pyrex glass system using a 100 flask for 50 ml solvent. The polymer SR film samples were connected with MBBA and the three solvents for two weeks, and the degree of swelling was measured to characterize the stability of the SR film in the solvents. The results are listed in Table 1. No swelling of SR in DEP was observed. Results and discussion The permeability and selectivity of all the candidate solvents are shown in Table 2. The selectivity of 2EH and 1-octanol for separation of CO, from Hz is in the same order of magnitude as that of MBBA, while DEP has a higher selectivity than MBBA. DEP has the highest boiling point, but the conductivity of the commercial product is only in the order of lo-’ ohm-‘-cm-’ which is too large for electroconvective motion. The conductivity of polar solvents is primarily due to electrolytic impurities. De-ionization experiments for DEP have been conducted according to the process flowsheet shown in Fig. 2, and the results are listed in Table 3. The results show that the conductivity of polar solvents such as DEP can successfully be reduced
195
Ch. Wan et al. /J. Membrane Sci. 84 (1993) 191-196 TABLE 2 Permeability and selectivity of candidate solvents Solvent
P N2
P HZ
PC02
%OdHz
%OdNz
MBBA 2EH l-O&an01 DEP
0.9 6.3 3.4 2.7
4.0 14.0 17.4 8.8
19.2 58.6 70.7 54.5
4.8 4.2 4.1 6.2
21.3 9.3 20.9 20.9
“Unit of permeability: 1O-8 cm3 (STP)-cm/see-cm2-cmHg.
TABLE 3 Properties of candidate solvents Solvent
b.p. (“C)
Conductivity a.c., 60 Hz, 50 V
Dielectric constant (1000 Hz)
2EH l-O&an01 DEP DEPb
183 196 298 298
4.4x 1.8x 1.6x 8.8x
6.7 9.4 8.0 7.2
lo-‘0 lo-’ lo-’ lo-*’
AC
“Unit of conductivity: ohm-‘-cm-‘. bDe-ionized.
voltage
Fig. 4. Effect of a.c. electric field on the permeability of diethyl phthalate.
0 ACvoltage
Fig. 3. Effect of a.c. electric field on gas permeation across a diethyl phthalate membrane.
using ion exchange based technology, and that the dielectric constant can be reduced as well. The electroconvectivity of de-ionized DEP has been demonstrated by a gas permeation experiment using permeation of individual gases. The results are shown in Figs. 3 and 4. The
permeability was estimated by the method proposed previously [ 11. The selectivity remains high, but the enhancement, about 6 times at 1200 volts, is not as great as for MBBA. That is because the permeability of DEP in the rest state is much higher than that of MBBA and its viscosity is also higher. If the viscosity is higher, the resistance for convective motion will be increased. Consequently the voltage required to create turbulence must be increased. Conclusions A polar solvent to be used to replace the liquid crystal MBBA in a electroconvective liquid membrane basically must be an insulating solvent. This work has demonstrated that if the conductivity is on the order of lo-” ohm-‘cm-‘, the solvent can exhibit a stable electro-
196
convectivity. According to the analysis using the equivalent R-C electric current model, when two pieces of 25 pm thick silicon rubber (SR) film are employed as barrier layers, the dielectric constant of the polar solvent should be less than 15 to apply 90% of the electric voltage on the liquid layer. When the conductivity of a polar solvent for an a.c. of 60 Hz frequency is more than 1 x 10-l' ohm-‘-cm-‘, it is primarily controlled by ionized impurities in the solvent. An ion exchange based technique can be used for the de-ionization of polar solvents. In this technology, a mixed bed composed of strongly acid and basic ion exchange resins is employed for the removal of the ionized impurities and molecular sieve beads are useful for dehydration of the polar solvent. The dielectric constant which depends not only on the molecular polarity but also partly on the ionized impurities can be reduced as well. Diethyl phthalate (DEP) has been selected as one of the candidate electroconvective solvents; its dielectric constant is in the 5-15 range, boiling point is 298”C, selectivity for separations of CO, from H2 is better than that of MBBA, the retail price is less than l/100 of that for MBBA, and the SR barrier film is very stable in DEP. The conductivity of the commercial product supplied by Aldrich Chemical Company, Inc. is in the order of lo-’ ohm-lcm-‘, this can be reduced to the order of lo-” ohm-l-cm-’ using ion exchange based technology, and then DEP exhibited stable electroconvectivity. Gas permeation through the DEP membrane can be facilitated by inducing an a.c. electric field.
Ch. Wan et al. /J. Membrane Sci. 84 (1993) 191-196
Acknowledgment This research was funded by the NSF I/U CRC Center for Separations Using Thin Films. The authors gratefully acknowledge their support. References 1
2
3 4
5 6
7
8 9 10
11
12
13
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