Ind. Eng. Chem. Res. 1997, 36, 2487-2489
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New Possibilities of Electroinduced Membrane Gas and Vapor Separation Dmitri G. Bessarabov,*,† Ron D. Sanderson,† Viacheslav V. Valuev,‡ Yuri M. Popkov,‡ and Serge F. Timashev‡ Institute for Polymer Science, University of Stellenbosch, Private Bag X1, Matieland 7602, Stellenbosch, South Africa, and Karpov Institute of Physical Chemistry, 10 Vorontsovo pole, Moscow 103064, Russia
A novel membrane technique to effect electroinduced facilitated transport of neutral molecules in ion-exchange membranes was suggested. Experiments have been carried out with platinumcoated Nafion membranes in Cu2+/Cu1+ form. This may be a potential technique for the separation of olefin/paraffin mixtures. It was shown that by applying an electric current to the membrane the permeability of ethylene increased 6-fold, compared to the permeability of the initial Pt-coated membrane without current. 1. Introduction Electrochemically driven separation processes are an interesting research area which has, to date, received relatively little attention. One of the first examples of the membrane-based “electrically-induced carrier transport” was demonstrated by Ward III (1970). The experiment included pumping nitric oxide through a liquid film containing Fe2+ and Fe3+ ions. The film was bounded by platinum gauze electrodes. There was no pressure difference in the nitric oxide across the liquid film, but there was a flux of nitric oxide through the film when external dc power was supplied to the electrodes. There are electroinduced membrane gas-separation processes in which certain chemical compounds (for example, CO2) are transformed by electrolysis into a new chemical compound (for example, CO32-) and then transferred through a membrane. Polymeric membranes and supported liquid membranes are used in these cases. Winnick et al. (1974) and Winnick (1990) described several examples of such processes, including the removal of pure oxygen, CO2, SO2, and H2S from gas mixtures by means of supported liquid membranes. New possibilities in using of an electric current as an intensification factor for the membrane separation of neutral gases and vapor components were proposed by Timashev (1991a) and achieved by Timashev et al. (1994). The key mechanism required to effect such active transport comprises the entry of a target neutral component into the solvate shell of a counterion, and its transfer by means of electric current through a membrane, by exchange with counterions localized near adjacent ion-exchange groups. The process takes place only when water electrolysis occurs and the H+-ions serve as carriers for ammonia molecules in the form of NH4+. The removal of carbon monoxide from gas mixtures containing it has been recently demonstrated by Terry et al. (1995), who used the principle of the electrochemically-modulated complexation of some transitional metal ions (with varying valences) with certain gas or vapor components. Terry et al. (1995) devised a continuous process for selectively removing gas from one phase * To whom correspondence should be addressed. Fax: +27(21)808-4967. Telephone: +27(21)808-3172. E-mail: db2@ maties.sun.ac.za. † University of Stellenbosch. ‡ Karpov Institute of Physical Chemistry. S0888-5885(96)00191-1 CCC: $14.00
and concentrating it into another by an electrochemically-modulated mass-separating agent. A copper(II,I) aqueous HCl/KCl complexing system was used with carbon monoxide as a model solute/complexing agent system to demonstrate the general concepts of the separation process. Terry et al. (1995) pointed out that this process could be used to transport gases such as CO2, CO, and H2S and selectively separate olefins from paraffins when an appropriate complexing agent is used. New designs and possibilities for effecting electroinduced membrane gas and/or vapor separation, on the basis of the above-mentioned ideas, are further discussed below. 2. Proposed Technique To Effect an Electrically-Induced Facilitated-Carrier Transport of an Olefin in Cation-Exchange Membranes for Possible Olefin/Paraffin Separations One of the new ways of doing the above comprises the introduction of transition metals (for example, copper), as counterions or as fragments of ion-exchange groups, into ion-exchange membranes. The electric current will maintain a gradient of reduced forms (for example, Cu+) and oxidized forms (for example, Cu2+) of the metal in the membrane. The establishment of different concentration profiles across the membrane for Cu+ and Cu2+ is possible without water electrolysis if the membrane used is not only ion-conductive but also electron-conductive. It is possible to achieve electron conductivity in Nafion-like membranes if fine microscopic clusters of a metal (for example, Pt) are thinly distributed throughout the membrane. The concentration of the reduced form will fall from the cathode to the anode, whereas the oxidized form will have the opposite gradient. If copper in the Cu+ and Cu2+ states is introduced, as a counterion, to a cation-exchange membrane with metallic conductivity (such as a Nafion membrane impregnated with Pt clusters) and if target components (gas or vapor molecules) form complexes with the reduced Cu+ ions (i.e., olefin-Cu+ complexes), the concentration of the component in the form of a complex in the membrane will increase when the gas-vapor mixture, containing this component, contacts the cathode side of the membrane. As a result, a gradient of the target component concentration will be formed in the membrane. In this case, the target component may © 1997 American Chemical Society
2488 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
Figure 1. Schematic diagram of an experimental cell: 1, ionexchange membrane in Cu+/Cu2+ form; 2, feed (i.e., ethylene) inlet; 3, sweep-gas inlet; 4, thin layers of platinum on the membrane; 5, electric power supply; 6, O-rings; 7, porous Pt-coated titanium electrodes.
hop, using the counterion system, to the anode side of the membrane. It should be particularly emphasized that the strength of a bond between the target component and copper ions becomes weaker as the degree of ion oxidation increases. This means that the mobility of the target component will be promoted as the component approaches the anode side. In other words, there may be effective diffusion of the target components in the membrane, and a steady diffusion flux of the target components from a “near-cathode“ feed phase to a “near-anode” receiving phase will be formed.
Figure 2. Volt-current characteristic of the membrane used in the experiments.
3. Experimental Section 3.1. Membrane Cell. In order to evaluate the permeation rate of ethylene through a platinum-coated Nafion membrane, a special membrane cell shown in Figure 1 was used. The membrane samples (1) coated with platinum (Pt) layers (4) were placed between the upper and lower sections of the membrane cell. An O-ring (6) ensured leak-free conditions. Porous Ptcoated titanium electrodes (7) were used to supply direct current (5) to the membrane cell. Humidified ethylene was fed to the upper side of the membrane cell under a pressure of 1 atm (2) and humidified sweep-gas was applied to the lower section of the membrane cell under a pressure of 1 atm (3). The rate of ethylene permeation through the membrane was determined by gas chromatography. 3.2. Membrane Preparation and characterization. Flat-sheet membranes were prepared from a perfluorinated Nafion-like copolymer. The thickness of the membranes varied in the range of 200-260 µm. The equivalent mass of the polymer varied from 1130 to 1290. Membranes were hydrolyzed in 6 N NaOH and converted into the H+-ionic form in an HCl solution. A thin metallic layer of Pt was deposited on both sides of the membranes by the well-known technique of autocatalytic Pt reduction (see Enea et al., 1995), based on the chemical reaction: NaOH
PtCl62- + N2H4 98 N2 + Pt + 6Cl- + 4H+ After an ion-exchange process in an aqueous CuSO4 solution the membranes were converted into the Cu2+form. The effective surface area of the membranes in the membrane cell was 4.5 cm2. The thickness of the Pt layer was 2-3 µm as measured by scanning electron microscopy. Fine particles of Pt were detected within the membrane by scanning electron microscopic analysis with an electron backscattering detector and by use of the particle-induced X-ray emission (PIXE) technique. An important feature of the electrically-induced process was the occurrence of electron conductivity of the membrane due to the presence of finely dispersed Pt. The specific resistance of the
Figure 3. Dependence of the ethylene permeability upon the current density.
membrane exposed at ambient temperature and humidity was ≈5 × 10-3 Ω cm. The permeability of ethylene in the Cu2+-form membrane, without Pt layers, was 3.6 × 10-9 cm3 cm/cm2 s cmHg. Permeability of ethylene in the Cu2+-form membrane, with Pt layers, was 1.2 × 10-9 cm3 cm/cm2 s cmHg. It is therefore possible to conclude that the two platinum layers, on both sides of the membrane, can cause diffusional resistance to mass transfer. 4. Results Figure 2 shows the volt-current characteristic of the membrane used in the experiments. The dependence of the steady-state ethylene permeability upon the current density (µA/cm2) is shown in Figure 3. The permeability becomes steady after a certain current is applied. It is also seen that by applying an electric current to the membrane the permeability of ethylene increased 6-fold, compared with the permeability of ethylene through the initial Pt-coated membrane without current. When a 2 V current was applied to the electrodes of the membrane in the initial Cu2+-form, as shown in Figure 1, an increase in permeability was observed as shown in Figure 4. Figure 4 shows that a “transformation” process (seen as a maximum in the curve) occurs within the membrane when current is applied. 5. Discussion The original membrane was in a Cu2+-ionic form. Under the presence of electric current, ions of Cu+ occur near the cathode. In other words, profiles of Cu2+ and Cu+ concentrations are formed within the membrane.
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It is suggested that the mechanism of ethylene transfer in such an electroinduced process includes diffusion of ethylene (in the form of a complex with Cu+) through Cu+ ions incorporated within the membrane. It is also proposed that the direct current causes the distribution of copper ions in two oxidation states across the membrane in the steady state, that is, the concentration of the reduced form falls from the cathode to the anode, whereas the oxidized form has the opposite gradient. Under such conditions the electroneutrality remains constant and there is no significant flow of copper ions through the membrane. 6. Conclusions Figure 4. Dependence of the ethylene permeability upon the time when a 2 V current was applied to the electrodes of the membrane in the initial Cu2+-form.
Figure 5. Schematic idealized distribution of Cu+ (1), H+ (2), and Cu2+ (3) ions within the membrane.
After the transformation processes are completed, the concentration profiles of copper ions in Cu2+ and Cu+ ionic forms and of protons H+ are formed in the membrane for each value of membrane potential. The profiles can be computed by solving the local equation of the electric neutrality with the systems of equations describing the local electron fluxes that occur between nanoparticles of Pt and copper ions in the Cu2+, Cu+, and Cu0 forms. Figure 5 shows one of the probable concentration profiles of Cu2+, Cu+, and H+ in the membrane. Under the conditions of electric polarization, ions of Cu2+ are partially reduced near the cathode. The ions are usually transformed into the Cu+-ionic form and may be reduced to Cu0. In this case, electric neutrality is achieved by H+ ions in the membrane, formed by the heterolytic dissociation of H2O molecules. The latter takes place in the strong local electric fields which occur during the transformation of Cu2+ to Cu+ (Timashev, 1991 b). In this case, OH- ions are oxidized on Pt particles distributed within the membranes, as follows:
1 2OH- f O2 + H2O + 2e2 The concentration of each of the forms of copper ions, as well as that of H+, depends upon the local potential in the membrane. This dependence is reflected by taking into account the probability of electron transfer between nanoparticles of Pt and copper ions, as well as between copper ions (for example, Cu2+ T Cu+, Cu+ T Cu0), with the probability depending on the electric potential. The relevant system of equations and the results of the analysis of the concentration profiles of copper in various ionic forms will be presented in a forthcoming paper.
A novel process to effect an electroinduced facilitated transport of neutral components (with ethylene as an example) in ion-exchange membranes containing ions of metals with variable valences is suggested. Preliminary results have shown that it is possible to obtain a considerable increase in ethylene flux through a membrane by applying an electric current to both sides of a Cu2+/Cu+-form Nafion membrane, which contains fine platinum clusters, with Pt-containing layers on both sides of the membrane. However, the actual mechanism of ethylene transport in such a system needs to be verified, and more results (for example, on actual selectivity of olefin/paraffin separations) are desirable. At this stage, it is very difficult to estimate the potential real cost-effectiveness of such a separation process as that described here since the purpose of the study, to date, was only to demonstrate new possibilities in olefin/paraffin separation. Research is continuing. Acknowledgment S.F.T. expresses his deep gratitude to the Ernest Oppenheimer Memorial Trust (South Africa) for financially supporting his stay at Institute for Polymer Science, University of Stellenbosch, Stellenbosch, South Africa. Literature Cited Enea, O.; Duprez, D.; Amadelli, R. Gas phase electrocatalysis on metal/Nafion membranes. Catal. Today 1995, 25, 271. Terry, P. A.; Walls, H. J.; Noble, R. D.; Koval, C. A. Electrochemically modulated complexation process for gas removal and concentration. AIChE J. 1995, 41, 2556. Timashev, S. F. Membrane gas-separation processes induced by an electric current. Russ. J. Phys. Chem. 1991a, 65, 1741. Timashev, S. F. Physical Chemistry of Membrane Processes. In Ellis Horwood Series in Physical Chemistry; Kemp, T. J., Series Ed.; Ellis Horwood, Ltd.: New York, 1991b. Timashev, S. F.; Valuev, V. V.; Vorobiev, A. V.; Salem, R. R.; Strugatskaya, A. J. Pervaporation induced by electric current. J. Membr. Sci. 1994, 91, 249. Ward, W. J., III Electrically induced carrier transport. Nature 1970, 227, 162. Winnick, J. Electrochemical membrane gas separation. Chem. Eng. Prog. 1990, 86, 41. Winnick, J.; Marshall, R. D.; Schubert, F. H. An electrochemical device for carbon dioxide concentration. I: system design and performance. Ind. Eng. Chem. Process Des. Dev. 1974, 13, 59.
Received for review April 2, 1996 Revised manuscript received March 21, 1997 Accepted March 21, 1997X IE9601918
X Abstract published in Advance ACS Abstracts, May 1, 1997.
