Abstract:
Alkali metal hydroxide solutions are purified and concentrated by electrolysis of such solutions in a hybrid cell comprising an anode compartment and a central compartment separated from the anode compartment by a cation permeable diffusion barrier, and a cathode compartment in flow communication with the central compartment and separated from the central compartment by a diaphragm. To enable operation, gaseous hydrogen is supplied to the anode, oxygen to the cathode, an aqueous solution of at least one alkali metal hydroxide to the anode compartment, and an aqueous media receptive to alkali metal ions to the central compartment. A plurality of the hybrid cells may be operated in hydrodynamic series.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of copending application Ser. No. 107,688, filed Dec. 27, 1979, which is a continuation of application Ser. No. 32,151, filed Apr. 23, 1979, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to electrochemical processes for production of chlorine and caustic from brine and to the simultaneous production of electrical energy. More particularly, the invention is directed to the treatment of cell liquor from a chloralkali cell to separate the sodium ions from the cell liquor and concentrate them in another liquor to form a sodium hydroxide solution. 
     The production of chlorine and crude caustic solutions by electrolysis of brine is a major industry. Two types of electrolysis cells are used in the production of chlorine and caustic. They are the diaphragm cell and the mercury cell. Membrane cells are also used to a minor but growing extent. Considerable quantities of energy are required for electrolysis of the brine to produce chlorine and subsequent treatment of the cell liquor resulting from electrolysis in diaphragm cells is necessary to obtain caustic solutions of the desired purity and concentration. A 50 weight percent aqueous caustic solution of low sodium chloride content is a commercially desired product. 
     Known processes for electrolysis of brine in diaphram, mercury and membrane cells produce cathode cell liquors having a caustic content of from about 10 to as high as about 40 percent by weight in membrane cells and 50 percent by weight in mercury cells. The sodium chloride content of the liquor is up to about 15 percent by weight for diaphragm cells and virtually none for membrane cells and mercury cells. Mercury cells have environmental problems and are no longer the technology of choice in industrialized countries. The cathode cell liquor produced by a diaphragm cell typically contains about 10 to 12 percent by weight caustic (NaOH) and 15 percent by weight sodium chloride (NaCl). 
     In the diaphragm cell, brine is continuously fed to an anode compartment, where chlorine is produced, and then flows through a diaphragm, usually made of asbestos, to a cathode compartment. Hydrogen gas is discharged from the solution at the cathode, with attendant generation of hydroxyl ions. To minimize back-migration of hydroxide ions from the cathode compartment to the anode compartment, a positive flow rate is from the anode compartment to the cathode compartment always maintained; that is, a flow in excess of the conversion rate. As a consequence, the resulting catholyte solution, i.e., the cathode cell liquor as the term is used herein, has unconsumed sodium chloride in addition to product solium hydroxide. The cathode cell liquor containing the sodium hydroxide and sodium chloride must be purified and must be concentrated to obtain a saleable caustic solution. 
     A membrane cell, which employs a membrane selectively permeable to certain cations in place of a diaphragm, yields a catholyte of low salt content and having a caustic content of up to about 40 percent by weight. The highly corrosive chlorine medium, however, is harsh on membrane materials. Accordingly, specifications for the membrane must be rigid and membranes useful in the presence of chlorine are quite expensive. In addition, voltage drop within the membrane cell is relatively high which increases consumption of electricity. In sum, membrane cells are costly in regard to investment and operating costs. 
     Typical processes for concentrating cell liquor and separating the sodium chloride from the caustic involve evaporation and crystallization with the consumption of large amounts of steam and, consequently, fuel required to generate steam. Investment in such processes is considerable. 
     One solution to the problem of obtaining cell liquor having high caustic concentration is described in U.S. Pat. No. 3,899,403 to Cook, Jr., et al. A three-compartment electrolytic cell produces solutions of high and low caustic concentration. A two-compartment cell then concentrates the solution of low caustic concentration. Current efficiency in the two-compartment cell is lower than that in the three-compartment cell. 
     U.S. Pat. No. 4,036,717 to Babinsky et al describes a three-chamber electrolytic cell for concentrating and purifying cell liquor containing sodium or potassium hydroxide. The Babinsky cell has a porous catalytic anode, a porous asbestos diaphragm between the anode chamber and a central chamber, and a cation-permselective membrane between the central chamber and the cathode chamber. Cell liquor is passed through the central chamber and concentrated caustic is withdrawn from the cathode chamber. Hydrogen gas generated by the electrolysis is supplied to the anode to decrease the potential across the cell below the evolution potential for chlorine and coincidentally reduce the power requirements for the cell. 
     The use of hydrogen-air fuel cells to consume hydrogen gas from chloralkali cells and to produce electricity for a powering a portion of the chloralkali cells is described in Canadian Pat. No. 642,449. In the process described, hydrogen formed by the electrolysis of brine in a chloralkali cell is fed to the anode of a fuel cell and air is fed to the cathode. The fuel cell employs an aqueous electrolyte of sodium or potassium hydroxide having a concentration of 5 to 50 weight percent. There is no suggestion in the patent that the cell liquor from the chloralkali cells can be purified or concentrated by using such cell liquor as the electrolyte in the fuel cell. 
     U.S. Pat. No. 3,511,712 to Giner describes a process for removing carbon dioxide from gas streams using a fuel cell. An alkali metal carbonate solution formed by absorption of carbon dioxide is introduced to the anode compartment of a fuel cell operating by consumption of an oxidant and a fuel to generate hydrogen ions, consume hydroxyl ions and generate electricity. Operation decreases the pH of the electrolyte in the vicinity of the anode to a point where carbon dioxide is evolved with a simultaneous increase of pH. This restores the carbon dioxide absorptive capacity of the solution. The patent does not suggest the use of a fuel cell to purify and concentrate chloralkali cell liquor and, in fact, the Giner process would not be suitable for the treatment of cell liquor because the required lowering of the anolyte pH to 9 results in polarization of the anode and a severe lowering of the current efficiency of the cell. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a process for the simultaneous production of alkali metal hydroxide and the generation of electrical energy using a hybrid cell. The invention is particularly directed to forming a purified and concentrated caustic solution from the effluent of a chloralkali cell with attendant generation of electrical energy for use by the chloralkali cell. 
     In the process of the invention, an aqueous solution of at least one alkali metal hydroxide is introduced, as anolyte, between a first surface of a gas diffusion anode and a first surface of a diffusion barrier and an aqueous fluid medium receptive to alkali metal ions is introduced, as catholyte between a second surface of the diffusion barrier and a first surface of a diaphram permeable to cations and anions. 
     Catholyte is withdrawn from between the diffusion barrier second surface and the diaphram first surface and introduced between a second surface of the diaphram and a first surface of a gas diffusion cathode. 
     Hydrogen gas and oxygen-containing gas are supplied to a second surface of the gas diffusion anode and a second surface of the gas diffusion cathode, respectively, and alkali metal ions are caused to pass from the anolyte through the diffusion barrier and into the catholyte flowing between the anode first surface and the diffusion barrier first surface. Such alkali metal ion movement is caused by electrically interconnecting the gas diffusion anode and the gas diffusion cathode through an external load thereby generating electrical energy by oxidation of the hydrogen gas supplied to the gas diffusion anode and the reduction of the oxygen gas supplied to the gas diffusion cathode. 
     A catholyte is withdrawn from between the gas diffusion cathode first surface and the diaphram second surface which is more concentrated in alkali metal hydroxide than the aqueous solution of alkali metal hydroxide introduced between the gas diffusion anode first surface and the diffusion barrier first surface. Further, an anolyte is withdrawn from between the gas diffusion anode first surface and the diffusion barrier first surface which is less concentrated in alkali metal hydroxide than the aqueous solution of alkali metal hydroxide introduced between the gas diffusion anode first surface and the diffusion barrier first surface. 
     In the process of the invention, flow of an aqueous solution of at least one alkali metal hydroxide is introduced, as anolyte, to the anode compartment of at least one hybrid cell which is comprised of a gas diffusion anode, a gas diffusion cathode, a diffusion barrier selectively permeable to cations, and a diaphragm permeable to cations and anions. The anode, cathode, diffusion barrier, and the diaphragm have first and second surfaces. The anode compartment is defined by the first surface of the gas diffusion anode and the first surface of the diffusion barrier. A central compartment is defined by the second surface of a diffusion barrier and the first surface of the diaphragm. A cathode compartment is defined by the second surface of the diaphragm and the first surface of the gas diffusion cathode. The anode, central and cathode compartments have both an inlet and an outlet, with the outlet of the central compartment being in flow communication with the inlet of the cathode compartment. The anolyte flows from the inlet to the outlet of the anode compartment. A flow of catholyte, which is an aqueous fluid medium receptive to alkali metal ions, is introduced into the inlet of the central compartment. Flow of the catholyte is withdrawn from the central compartment outlet and introduced into the inlet of the cathode compartment. A flow of current passing from the cathode through an external load to the anode is generated by oxidation of hydrogen supplied to the second surface of the anode and by generation of hydroxide ions by reduction of an oxygen-containing gas supplied to the second surface of the cathode. Under conditions of current flow, some alkali metal ions selectively pass from the anolyte, through the diffusion barrier, and into the catholyte flowing through the central and cathode compartments. The metal ions form, with cathode generated hydroxide ions, an alkali metal hydroxide solution. Catholyte, which is more concentrated in alkali metal hydroxide solution than the aqueous fluid medium introduced to the central compartment, is withdrawn from the outlet of the cathode compartment. Anolyte, depleted in alkali metal hydroxide, is withdrawn from the outlet of the anode compartment. 
     Concentration of the alkali metal hydroxide in the receptive aqueous media occurs as a consequence of cation transfer, electrolytic consumption of water with reduction of oxygen at the cathode to form hydroxyl ions, and evaporation of water through the cathode into air. Air, the most economical supply of oxygen, is passed over the cathode surface opposite to the cathode surface in contact with the catholyte, i.e., the second cathode surface. 
     In sum, separation of alkali metal ions from the solution introduced to the anode compartment occurs by alkali metal ions migrating through the diffusion barrier and concentration of caustic in the central and cathode compartments occurs by diffusion and electrolytic consumption of water with reduction of oxygen and evaporation of water. 
     The net result of the process is to cause the aqueous solution of alkali metal hydroxide, as it passes through the anode compartment, to be continuously depleted of hydroxide ions, and alkali metal ions for ionic neutrality, resulting in an alkali metal ion and hydroxide concentration gradient existing between the inlet and outlet of the anode compartment. As a consequence of acceptance of alkali metal ions by the aqueous media passing through the central and cathode compartments and generation of hydroxide ions by reduction of oxygen, the aqueous media becomes more concentrated in alkali metal hydroxide as it progresses through the central and cathode compartments. The catholyte leaves the cathode compartment as a solution more concentrated in alkali metal hydroxide than the aqueous media introduced to the central compartment. 
     The present invention, in its preferred application, is for the treatment of a chloralkali cell liquor to produce a purified and concentrated aqueous caustic solution. In this application, chloralkali cell liquor containing sodium hydroxide and sodium chloride is introduced into the anode compartment of a three compartment hybrid cell which preferably employs thin compartments. Hydrogen, generated by the chloralkali cell, is contacted with one surface of the gas diffusion anode of the hybrid cell. The hydrogen is oxidized at the anode with the production of water. The anode of the hybrid cell is separated from the central compartment by a diffusion barrier selectively permeable to cations, preferably a membrane. A diaphragm is interposed between the diffusion barrier and a gas diffusion cathode which is in contact with a flow of air. The central compartment is in flow communication with the cathode compartment. An aqueous medium, typically water or a dilute solution of at least one alkali metal hydroxide, is introduced into the central compartment and flows through the central compartment to the cathode compartment and from the cathode compartment to an outlet. 
     Although the electrical energy generated as a consequence of the electrochemical oxidation and reduction reactions which occur in the hybrid cell may be fed to any load, it is advantageous to combine a number of hybrid cells in a circuit and feed the power output to a chloralkali cell to provide part of the electrical energy required to operate the chloralkali cell. 
     The following occurs in the hybrid cell when operated on chloralkali cell liquor: Sodium ions from the introduced chloralkali cell liquor pass transverse to the flow of the anolyte chloralkali cell liquor in the anode compartment through the diffusion barrier and into the aqueous medium flowing in the central compartment. Sodium ions enter the cathode compartment as part of the aqueous medium flowing from the central compartment to the cathode compartment and by passage through the diaphragm. 
     Hydroxyl ions generated as a consequence of reduction of oxygen at the cathode combine with the transferred sodium ions to form sodium hydroxide. Consumption of water by generation of hydroxyl ions also serves to concentrate the sodium hydroxide solution being formed in the central and cathode compartments. Additional concentration occurs by evaporation of water through the cathode into air passing over the surface of the cathode opposite to the surface in contact with the catholyte. This water evaporation also serves to cool the hybrid cell. 
     A plurality of hybrid cells may be staged in a cascade. The cells are operated hydrodynamically in series. The anolyte flows sequentially through the anode compartments of the hybrid cells. The catholyte flows through both the central and cathode compartments of each of the hybrid cells in sequence. Each hybrid cell is operated under conditions which are effective for removing only a fraction of the alkali metal from the anoalyte and concentrating it in the catholyte. The anolyte flow may be cocurrent or counter-current to the catholyte flow. The fluids may either ascend or descend the cascade. The cascade may include a bypass connecting a cathode compartment outlet with the initial central compartment inlet to improve the conductivity of the catholyte by addition of some of the produced alkali metal hydroxide to the catholyte feed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more clearly understood by reference to the drawings, wherein: 
     FIG. 1 is a schematic illustration of a hybrid cell used in practice of this invention; 
     FIG. 2 is a partial cross-sectional view of a hybrid cell having a plurality of thin cell units; 
     FIG. 3 is a flow diagram showing a chloralkali cell and a hybrid cell being operated in combination; 
     FIG. 4A is a schematic illustration of a cocurrent cascade of individual hybrid cells showing the sequence and arrangement of the cells in the cascade; and 
     FIG. 4B is a schematic illustration of a counter-current cascade of individual hybrid cells. 
    
    
     DETAILED DESCRIPTION 
     Alkali metal hydroxide solutions, especially solutions containing alkali metal halides, can be treated in accordance with this invention. For convenience of explanation, the feed solution typically has an alkali metal concentration between about 5 and 30 weight percent, calculated as the alkali metal hydroxide. Preferably, the solution is a chloralkali cell liquor, having a sodium hydroxide concentration of up to about 25 weight percent, preferably between about 10 and 25 weight percent sodium hydroxide, and up to about 26 weight percent sodium chloride, preferably up to about 15 percent by weight sodium chloride. Solutions of other alkali metal hydroxides, such as potassium hydroxide and lithium hydroxide, can also be treated. The cell liquor can also contain other alkali metal salts, such as sodium bromide, potassium iodide, and the like. The invention is adaptive to treating liquors from chloralkali cells, including diaphragm cells, membrane cells, and the like. Methods of operating such cells and the nature of the cell liquor produced in their operation are well known to those skilled in the art and have been described in an extensive body of technical publications and patents. Accordingly, the ensuing description will be directed primarily to the operation of the hybrid cells alone and in combination with chloralkali cells. 
     FIG. 1 schematically depicts the operation of a three compartment cell of this invention. A chloralkali cell liquor, containing about 10 weight percent NaOH and about 15 weight percent NaCl is introduced, as anolyte, into the anode compartment of the hybrid cell. The compartments of the hybrid cell are designed to provide flow substantially in one direction from inlet to outlet without appreciable mixing, back convection, or diffusion parallel to the electrodes of molecules and ions in the compartments. Preferably a condition of plug flow is maintained. This is more easily achieved when the average distance (d) between anode and diffusion barrier, diffusion barrier and diaphragm, and diaphragm and cathode are respectively about 1 mm or less, typically about 0.1 mm to about 1 mm. 
     The cell liquor contacts a gas diffusion anode. Hydrogen gas from any source, and preferably from a chloralkali cell, contacts the opposite side of the anode. The anode provides a surface for intimate contact between the hydrogen gas and the anolyte. 
     Hydrogen gas undergoes an oxidation reaction with the anolyte hydroxide ion at the anode which may be schematically represented as: 
     
         H.sub.2 +2OH.sup.- →2H.sub.2 O+2e.sup.- 
    
     As the anolyte flows through the anode compartment, its hydroxide ion content is progressively reduced and its water content progressively increased. 
     Separating the anode compartment from the central cathode compartment (hereinafter referred to as the central compartment) of FIG. 1 is a cation-permselective diffusion barrier, preferably a membrane. This is a barrier which is permeable to cations such as a sodium ion, but is relatively impermeable to anions such as the chloride ions. To maintain electroneutrality and to account for depletion of hydroxide ion from the anolyte, sodium ions, under condition of current flow through an external load, separate from the anolyte and pass through the cation-permselective barrier into a catholyte passing through the central compartment. Substantially all of the chloride ions remain in the anolyte, along with sufficient sodium ions to electrically balance the chloride ions. 
     The central compartment is separated from the anode compartment by the above-mentioned cation-permselective diffusion barrier. The central compartment is separated from the cathode compartment by a diaphragm which is permeable both to anions and cations, such as a semi-permeable asbestos diaphragm. An aqueous medium such as water or a dilute ionic solution, which may be part of the solution drawn from the anode compartment, is introduced as catholyte into the central compartment and progressively picks up sodium ions moving through the cation-permselective barrier. 
     Catholyte in the cathode compartment contacts one surface of a gas diffusion cathode where oxygen gas, preferably from air, is reduced by the schematic equation: 
     
         H.sub.2 O+1/2O.sub.2 +2e.sup.- →20H.sup.- 
    
     Some of the hydroxide ions pass from the cathode compartment to the central compartment. The net effect is that the sodium hydroxide content of the catholyte increases as it flows through the central compartment. 
     A catholyte, now of intermediate sodium hydroxide concentration, is withdrawn from the central compartment and introduced into the cathode compartment of the three-compartment hybrid cell. A proportion of the sodium ions entering the central compartment through the cation permselective barrier continues on through the ion-permeable barrier or diaphragm into the cathode compartment. When sodium hydroxide solution from the central compartment is introduced into the cathode compartment, the sodium ions which pass through the ion permeable barrier accumulate in the catholyte contacting the gas diffusion cathode. Oxygen from the air is reduced, forming hydroxide ions to balance the sodium ions and consume water of the catholyte; thus partially concentrating the sodium hydroxide solution. 
     Contemporaneously, in the operation of the cell, some water from the catholyte evaporates at the opposite surface of the cathode into the air stream. For a given cathode surface area and permeability, the flow of air may be regulated to control evaporation of water from the surface of the cathode to modify the concentration of sodium hydroxide in the catholyte. In practice, the rate of addition of water to the cathode compartment, the rate of transport of sodium ions through the cation-permselective diffusion barrier into the catholyte, the rate of consumption of water at the cathode, and the rate of evaporation of water from the cathode, are all correlated so as to provide a product catholyte of desired caustic concentration. 
     Thus, when the cell liquor and the water introduced to the hybrid cell flow through their respective compartments concurrently, as shown in FIG. 1, the sodium hydroxide concentration of the relevant anolyte decreases from about 10% by weight at the appropriate inlet and approaches 0.5% or less at the outlet. The sodium hydroxide concentration of the relevant catholyte, by contrast, increases from about 0% at the appropriate inlet to about 40% or more at the outlet. High concentration differences are achievable with currently available diffusion barriers and diaphragms. 
     As indicated, the anolyte withdrawn from the anode compartment is substantially depleted of sodium hydroxide. However, even when the effluent from the anode compartment contains as little as 0.1 weight percent or 0.01 weight percent of sodium hydroxide, the pH of the effluent is high, i.e., above 12. The high pH of the effluent from the anode compartment is advantageous in that polarization and loss of current efficiency which can be associated with a change from an alkaline to a neutral or acid pH within the cell is minimized. 
     The process and hybrid cell illustrated in FIG. 1 can, of course, be used to treat cell liquors having differing concentrations of alkali metal hydroxide and alkali metal halide. By regulating the flow of water or dilute aqueous alkali hydroxide into the central compartment of FIG. 1, and by evaporation of water from the porous cathode, the concentration of the product flowing from the cathode compartment can be varied over a wide range. Thus, the range of concentrations of product alkali metal hydroxide up to the point of solubility in water can be achieved at will. 
     The hybrid cell can be arranged in a filter press type structure with a multitude of elementary hybrid cells connected in series forming a net hybrid cell. 
     FIG. 2 is a partial cross-sectional view of a portion of a filter press hybrid cell unit showing the sequence and arrangement of elements in the cell. There is provided gas diffusion cathodes 10 and electrically conductive gas separator and current collectors 12 which helps to define air channels 14 and hydrogen channels 16; gas diffusion type anodes 18; an anolyte compartment 20; a central compartment 22; a catholyte compartment 24; membrane 26, and diaphragm 28. The following conduits are formed by insulating ported spacers 32. Conduit 30 serves hydrogen channels 16; conduit 34 is for the anolyte liquor to be processed; conduit 36 is for water; conduit 38 for fluid flow to cathode compartment 24; while conduit 40 is for feed of air to channels 14. 
     Given the sequence of elements, such variables as the thickness and spacing of elements, the shape of the air and hydrogen channels are subject to wide variation. In addition, many different materials of construction may be employed because the process of this invention is practiced under relatively mild conditions, particularly when compared with the highly oxidative and corrosive conditions found in a chloralkali cell. Thus, any material stable to alkali hydroxide and cell operating temperature may be used. 
     Materials of construction and cell construction arrangements are described, for instance, in U.S. Pat. Nos. 3,098,762; 3,196,048; 3,296,025; 3,511,712; 3,516,866; 3,530,003; 3,764,391; 3,899,403; 3,901,731; 3,957,535; 4,036,717 and 4,051,002 and British Patent Specifications Nos. 1,211,593 and 1,212,387, each incorporated herein by reference. 
     The cation permselective diffusion barries may be perfluorosulfonic acid polymers manufactured by du Pont under the trade name Nafion and perfluorocarboxylic acid polymers manufactured by Asahi Chemical Co. Other low cost membranes prepared from sulfonated polymers, carboxylated hydrocarbon polymers, phenolic resins, polyolefins and the like, may also be used. 
     Whatever the selected material, the membrane should preferably have a permselectivity in 40% NaOH of at least about 0.95, an ohmic resistance not more than about 3 ohm-cm and an electrosmotic coefficient of not more than about 74 gms of water per Faraday. 
     The gas diffusion anodes and cathodes currently employed in the fuel cell art may be used in the construction of the hybrid cells and are semi hydrophobic. They generally consist of a gas diffusion layer which may be catalytic per se or have catalytic properties induced or promoted by a noble metal and the ike. A suitable gas diffusion type cathode and/or anode may be formed of activated carbon which may be promoted by a noble metal and combined with a support material such as Teflon.sup.™. 
     The porous diaphragms can be made of fuel cell grade asbestos films, porous rubber battery separators, or ion exchange membranes which are permeable to both anions and cations. 
     It is contemplated that the catholyte can be transferred from the central compartment of the hybrid cell to the cathode compartment in either or both of two ways. First, the catholyte can be withdrawn from an outlet of the central compartment and introduced into an inlet of the cathode compartment. Second, by establishing a pressure differential across a porous diaphragm, the catholyte from the central compartment can be made to flow through the diaphragm into the cathode compartment. Both means of transferring catholyte from the central compartment to the cathode compartment can be employed simultaneously. Liquid permeable polymeric films and woven or non-woven fabrics may also be used as materials of construction for the porous diaphragm. 
     The hybrid cell can be operated at any temperature which maintains the electrolytes in a liquid state and avoids the precipitation of dissolved constituents such as alkali metal halide or alkali metal hydroxide. Temperatures of from about 20° C. to 100° C., more preferably 40° C. to 70° C., may be employed. Because the cell liquor from a chloralkali cell is warm and because heat is generated within the fuel cell during its operation, it is necessary to cool the cell to maintain a desired operating temperature. The cell is conveniently cooled as an incidence of evaporation of water from the catholyte through the gas diffusion cathode into the stream of air which is passed across the surface of the cathode opposite to the surface in contact with the catholyte to supply oxygen to the cathode. In a filter press type of construction, the individual cells are sufficiently thin so that there is excellent heat transfer between the anode, cathode, and fluid compartments. 
     To achieve effective cooling through the cathode by evaporation, it may be desirable to continuously introduce fresh, dry air into the hybrid cell at a point removed from the air intake which supplies the hybrid cell. Air can be dried conveniently by passing it over cooling coils or through desiccant such as silica gel in accordance with known methods. The flow of electrolytes through the compartments of the fuel cell can be cocurrent as shown in FIG. 1 or countercurrent. Cocurrent flow is presently preferred. 
     Air is the lowest cost source of oxygen required for the cathode and serves to carry off evaporated water. Other oxygen-containing gases as well as oxygen enriched air can also be used but at greater expense. 
     FIG. 3 shows the interrelationships between chloralkali cells and hybrid cells used to treat the cell liquor from the chloralkali cells in accordance with this invention. Brine is introduced to the chloralkali cell 42 by line 44. Chlorine is generated at anode 46 and hydrogen released at cathode 48. Diaphragm 50 separates the compartments. Hydrogen generated in the chloralkali cells is supplied to gas diffusion anode 50 of the hybrid cell 52 and cell liquor fed to anode compartment 54 by line 56. Air is supplied to gas diffusion cathode 58 and water to central compartment 60. With current flow, as induced by reduction of oxygen at the cathode and oxidation of hydrogen at the anode, sodium ions pass through diffusion barrier 68 to the central and cathode compartments. Catholyte is drawn from compartment 62 by line 64. Line 66 connects the central compartment with the cathode compartment. The diaphragm is shown as 70. 
     The hybrid cell of FIG. 3 is in series with the chloralkali cell and will produce a fraction of the power consumed by the chloralkali cell. Thus, while additional electric current from an outside source is required to operate the chloralkali cell and is shown as &#34;power supply&#34;, the external energy required to operate the chloralkali cell is reduced. 
     In a typical operation of the hybrid cell, a cell liquor containing about 12 percent by weight NaOH and 15  percent by weight NaCl is supplied to anode compartment 50. Water is introduced to cathode compartment 54. The products withdrawn from the hybrid cell may be an approximately 15 to 22 percent by weight NaCl solution containing a small amount of NaOH from the anode compartment and a purified, substantially chlorine-free 40 percent by weight NaOH solution from the cathode compartment. 
     Although the hybrid cell may be operated in such a manner as to remove substantially all of the alkali metal hydroxide from the cell liquor, as indicated, it can be more efficient to operate the cell so as to leave a small proportion of the sodium hydroxide in the effluent from the anode compartment. This is done to improve the efficiency of the hybrid cell. 
     The effluent from the anode compartment of a hybrid cell run at high efficiencies can be depleted down to an anolyte strength of around 2% alkali metal hydroxide and can then be fed to the anode compartment of another finishing hybrid cell operated under conditions which are effective for removing the last few percent of the alkali metal from the anolyte. The conditions in such a finishing cell can be regulated to produce concentrated alkali metal hydroxide product even though the feed to the anode compartment is relatively dilute. However, because the concentration of alkali metal hydroxide from such a dilute feed is accomplished at lower current efficiency than the concentration from more concentrated feed, the cell produces little or no useable power. In the latter case, the cell can be run essentially under short circuited conditions which are effective for concentrating the alkali metal hydroxide but produce no useable current. However, because the bulk of the separation was accomplished in the main hybrid cells under conditions providing higher current efficiency, the overall current efficiency of the process is not seriously diminished. 
     Although a main hybrid cell and a finishing cell may be operated in such a manner as to remove first the bulk and then the remainder of substantially all of the alkali metal hydroxide from the cell liquor, it can be more efficient to operate a cascade of hybrid cells hydrodynamically in series so as to remove only a small proportion of the sodium hydroxide in each anode compartment. When this is done, the current efficiently of the individual hybrid cells can be kept high to maximize the yield of electric current in each stage of the cascade. 
     FIG. 4A is a schematic illustration of a cocurrent cascade of individual hybrid cells showing the sequence and arrangement of the cells in the cascade while FIG. 4B illustrates countercurrent flow. There are provided individual hybrid cells 72 which may be either the three-component hybrid cells or the filter press type cells discussed previously. The individual cells include an anode compartment 74 separated from a central compartment 76 by a diffusion barrier 78, which in turn is separated from a cathode compartment 80 by a diaphragm 86, are preferably stacked in a vertical cascade and are operated hydrodynamically in series. Cell liquor from a chloralkali cell is introduced as anolyte to the inlet of the anode compartment of a hybrid cell 88 at one end of the cascade. The anolyte flows through the anode compartment and is partially depleted of alkali metal hydroxide. The effluent from the anode compartment is withdrawn from the outlet and is introduced as anolyte into the anode compartment inlet of a second hybrid cell 90. The anolyte passes through the remainder of the cascade in this manner. The effluent withdrawn from the anode compartment of the hybrid cell 92 at the other end of the cascade is substantially depleted of alkali metal hydroxide. 
     The catholyte also flows in series through the cells. The catholyte is introduced into the central compartment inlet of a hybrid cell 86 or 94 at one of the ends of the cascade. The catholyte is made to flow through both the central and cathode compartments of an individual cell before passing to the next stage of the cascade and is partially enriched in ion alkali metal hydroxide during each stage of the cascade. The flow of catholyte is preferably cocurrent through the central and cathode compartments, although it need not be cocurrent to the flow of the anolyte. 
     The catholyte withdrawn from the appropriate cathode compartment outlet at the opposite end of the cascade contains a purified solution of caustic concentrated alkali metal hydroxide. 
     Each hybrid cell is operated under conditions which are effective for removing only a fraction of the alkali metal from the anolyte and concentrating it in the catholyte. The fraction may be determined by the number of cells operated in the cascade. Any number of stages can be employed in the cascade. There is no upper limit except for economies of cost and size required by the user. In the presently preferred embodiments, eight to ten stages are used. 
     Consequently, in the cascade, as distinguished from a single hybrid cell or the filter press hybrid cell systems previously described, each individual anode operates under a proportionately smaller anolyte concentration gradient between the inlet and outlet of the anode compartment. As a consequence voltage efficiency of the individual cells, and of the cascade as a whole, may be increased to its practical maximum using diffusion anodes commercially available. For hybrid cell cascades using these anodes, the greater the number of stages, the smaller the concentration gradient of alkali metal hydroxide in each stage, and the higher the voltage efficiency of the individual cells. 
     As depicted in FIG. 4A, both the anolyte and the catholyte enter the cascade in the same stage. Cocurrent flow maintains a condition of plug flow in the compartments to minimize any cross diffusion of caustic related to membrane imperfections such as holes, for example. 
     FIG. 4B depicts a countercurrent cascade of individual hybrid cells. As depicted in FIG. 4B, the anolyte and the catholyte enter at opposite ends of the cascade. Hybrid cell 94 at one end of the cascade serves both as the final stage for the anolyte and as the initial stage for the catholyte. The concentrations of sodium hydroxide in this cell are at their minimum values: e.g. anolyte at 0.5% NaOH or less, catholyte at about 10% NaOH. 
     In hybrid cell 96 at the opposite end of the cascade, which serves as the initial stage for the anolyte and as the final stage for the catholyte, sodium hydroxide concentrations are maximized: anolyte at about 10% NaOH, catholyte at about 40% NaOH. However, within a countercurrent cascade, as compared to a cocurrent cascade, there is the least possible average difference in caustic concentration across the diffusion barriers. Countercurrent circulation may be practiced to minimize concentration differences of the caustic across the diffusion barrier of each cell. 
     As concentration gradients increase across the diffusion barrier, chemical driving forces are thought to promote back-diffusion of the caustic product from a high strength catholyte to the lower strength anolyte, which reduces the concentration of sodium hydroxide in the product and the overall efficiency of the process. Many commercially available diffusion barriers, such as the membranes discussed previously, exhibit a decrease in permselectivity at concentration differences across the membrane above about 30% by weight caustic which affects efficiency. A countercurrent circulation may increase efficiency and product purity by minimizing the average concentration differential of sodium hydroxide throughout the cascade. 
     The cascade may be operated with either ascending or descending electrolytes for cocurrent circulation modes. A countercurrent cascade may be operated either with an ascending anolyte or an ascending catholyte. 
     A bypass, shown in both FIG. 4A and FIG. 4B, may be included to provide flow of product catholyte from the last catholyte stage of the cascade to the central compartment feed of the first catholyte stage. It may be desirable to add a small portion of product catholyte to the catholyte entering the cascade, which may be pure water, to increase its conductivity. Feed strengths ranging from around 0 to around 25 percent by weight NaOH, preferably between about 10 and about 15 percent by weight, are representative of bypass caustic provided for efficient operation. 
     Although the present invention has been described with reference to particular details and embodiments thereof, these details are intended to illustrate the invention, the scope of which is defined in the following claims.