IL46555A - Process and cell for electrolysis of aqueous alkali metal halide solutions - Google Patents

Abstract

1480538 Diaphragm electrolysis of alkali metal halides DIAMOND SHAMROCK CORP 3 Feb 1975 [4 Feb 1974] 4585/75 Heading C7B The electrolysis is conducted in a cell wherein the anode and cathode are separated by a single upright hydraulically impervious cationic membrane positioned nearer the anode than the cathode. This is achieved either using pressure to flex a thin membrane towards the anode as by adjustment of the liquid heads or application of a gas-pressure or vacuum, or using spacers on the electrodes. Conventional membrances based on e.g. divinyl benzene/acrylic acid polymer, polyethylene, poly (divinylbenzene-styrene) or polyvinylfluorocarbons, having sulphonic, phosphonic or carboxylic groups and containing 15-40% gel water are specified. The membrane may be on the anode. Significant reductions in operating voltage are achieved. Conventional anodes and cathodes are specified. [GB1480538A]

Description

Process and cell for electrolysis of aqueous alkali metal halide solutions DIAMOND SHAMROCK CORPORATION C. 44450 \ ; FIELD OF THE INVENTION The present inventio is directed to the electrolytic production of alkali metal hydroxides and halogens. More particularly, the invention is concerned with the production of halogens and alkali metal hydroxides in electrolytic cells of the type employing electrol tically conductive, hydr aulically impervious, cation permselective membranes. In its specific aspects the invention relates to an improved method of operating membrane-type electrolytic cells for the production of chlorine and caustic soda.

BACKGROUND OF THE INVENTION Halogens and alkali metal hydroxides have been conventionally produced by the electrolysis of aqueous alkali metal halide solutions in diaphragm-type cells. ' Such cells generally have an opposed anode and cathode separated by a fluid permeable diaphragm, usually of asbestos, forming separate anode and cathode compartments. In operation, brine is fed to the anode compartment where halogen gas is generated at the anode, and the brine then percolates through the diaphragm into the cathode compartment where alkali metal hydroxide is produced. The alkali metal hydroxide thus produced contains large amounts of alkali metal halide, which must be removed by further processing to obtain the desired product.

Recently, electrolytic cells have been developed which utilize a permselective cation-exchange membrane in place of the conventional diaphragm. Such membranes, while e le c t ro ly i ca lly conductive under cell conditions, are substantially impervious to the flow of liquids and gases. In operation, brine is introduced into the anode compartment where halogen gas is formed at the anode.

The alkali metal ions are then selectively transported, often in hydrated form, through the membrane into the cathode compartment. The alkali metal ions then combine with hydroxide ions generated at the cathode by the electrolysis of water to form the alkali metal hydroxide.

Among the advantages of the membrane-type cell are the production of relatively pure alkali metal hydroxide in high concentrations, the production of more halogen per unit size, and the ability. o operate the cell at higher current densities. The operating parameters of membrane-type cells differ considerably from those of diaphragm-type cells. Thus, methods of operation of membrane-type cells which allow realization of the economics and advantages theoretically obtainable are in demand.

SUMMARY OF THE INVENTION Briefly, in accordance with the invention it has been discovered that the operation of electrolytic cells of the type having anodes and cathodes in compartments separated by an electrolytically conductive, hydraulically impervious, cation pe rms e le c t ive membrane is significantly improved by positioning and maintaining the membrane nearer to the anode than to the cathode. It has been discovered that for a given anode to cathode gap, reduction of the anode to membrane gap results in a marked reduction in cell operating voltage.

One approach found desirable for positioning the membrane close to, or directly upon, the anode is by maintaining a net positive pressure on the cathode side of the membrane. Operation of the cell in this manner has the effect of displacing the somewhat flexible membrane closer to the anode. A convenient method for establishing such a net positive pressure on the cathode side of the membrane is adjustment of the relative head heights of the anolyte and catholyte. Defending upon the specific gravities of the anolyte (brine solution) and catholyte (alkali metal hydroxide solution), such adjustment of relative head heights may be used to achieve a net positive hydrostatic pressure across the entire surface of the membrane from the catholyte side, positively displacing substantially all of the surface area of the membrane toward the anode.

It will be appreciated that it was often conventional practice to maintain a significant differential of anolyte over catholyte head height in the operation of diaphragm-type cells. This was done to force the percolation of alkali metal halide solution through the fluid-permeable diaphragm and to promote circulation of electrolyte throughout the cell. Additionally, a positive pressure on the anolyte side of the diaphragm had the effect of reducing the back-migration of hydroxide ions from the catholyte to the anolyte. In membrane- type cells, operational advantages resulting from the regulation of relative electrolyte head heights are unexpected, since the membranes are hydraulically impervious and typical operating pressures can be demonstrated to have no significant effect upon the flow of hydrated cations through the membrane. Additionally, the common practice in diaphragm-type cell operation was either to deposit the diaphragm directly upon the cathode or to position it relatively close to the cathode for most efficient operation. Thus, from the prior diaphragm cell practice, cell operation with substantially equal pressures on both sides of a membrane located relatively close to the cathode would be expected.

A diaphragm-type cell for the production of peroxides in which, instead of positioning the diaphragm close to the cathode or depositing it thereon, the anode is positioned in supporting contact with the diaphragm, is known from U.S. Patent Specification No. 3^_607,687. This known arrangement this relates to the variation of the gap between the two electrodes , while the present invention is concerned with variation of the membrane to anode gap, the distance between the electrodes in a given cell remaining unchanged.

It is an object of the present invention to provide a membrane-type electrolytic cell and an improved method of operation for the electrolysis of alkali metal halide solutions.

It is a further object of the invention to provide an improved method for the operation of membrane-type cells at reduced voltages.

It is a still further object to provide an improved method for the operation of membrane- type electrolytic cells wherein the membrane is positioned close, to or directly upon the anode .

Another object is to provide an improved method for the operation of membrane-type electrolytic cells wherein a net positive pressure is maintained on the cathode side of the membrane.

Further objects will be apparent from the following description of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is generally applicable to membrane-type cells for the electrolysis of aqueous solutions of alkali metal halides such as sodium chloride and potassium chloride. .Such cells typically comprise two compartments, -separated by the membrane, one compartment containing the anode and the other the cathode. Both compartments are provided with the appropriate gas vents and solution inlets and outlets. The electrodes and intervening membrane are disposed in a substantially vertical orientation to facilitate gas release and collection. The membrane is held in place by attachment about its perimeter.

In operation an alkali metal halide solution, typically an aqueous alkali metal chloride solution, is fed at a predetermined rate and concentration to the anode compartment. Upon passage of an electroly zing current between the opposed electrodes, chlorine gas is liberated at the anode and rises through the solution for collection.

At the same time sodium ions are selectively transported through the cation-exchange membrane into the cathode compartment. Initially the cathode compartment is charged with water or an aqueous hydroxide solution. In the cathode compartment the sodium ions combine with hydroxide ions released at the cathode by the electrolysis of water, hydrogen gas also being liberated. Finally, depleted brine is removed from the anode compartment while a high purity sodium hydroxide solution flows from the cathode compartment.

Typical operating parameters, not critical to the practice of the present invention, include feed brine concentrations ranging from about 100 to 325 grams per liter (gpl) sodium chloride, anode current densities of from e 1 to 5 amperes per square inch (asi), operating temperatures of 65 to 105°C. and brine feed rates on the order of 0.05-2 milliliters per ampere minute.

In general, any hydraulically impervious perm-selective cation-exchange membrane material is suitable for use. For economic and practical reasons, these materials are used in relatively thin sheets (e.g. , 3 to 20 mils) and are thus inherently somewhat flexible. In general, these membrane materials are composed of a polymer backbone chemically and mechanically resistant to the environment of an electrolytic cell and carrying active cation-exchange sites, such as sulfonic, phosphonic, or carboxylic acid groups. Typical polymer backbones of the prior art include divinylbenzene/acrylic acid copolymers, polyethylene, divinylbenzene/polystyrene copolymers, polyvlnylfluorocarbons , and the like most often in the sulfonated form. The preferred membranes are those which incorporate within their structure from 15-40% gel watej, since this gel water assists in the ionic transport of hydrated alkali metal ions and therefore renders the membrane more electroly tically conductive. An exemplary and highly desirable membrane of this type is manufactured and sold by E. I. duPont deNetnours and Compan under the trademark of Naf ion perfluorosulfonic acid membranes and is based upon a completely fluorinated vinyl ether polymer containing pendent sulfonyl groups (typically in the sulfonic acid, sulfonate, etc. form) and having an equivalent weight within the range of 1000-2200 (grams of polymer per equivalent of proton) and a gel water content within the above-stated range. Typical membranes will be found and further described by reference to U.S. Patent Nos. 2,636,851; 3,017,338; 3.496,077; 3,041,317; 3,624,053; 3,560,568; 2, '967 , 807; 3 , 282 , 875 ; Dutch published application 72/12249 ; and British Patent 1,184,321.

The invention relies on the advantages obtained by positioning the membrane closer to the anode than to the cathode. In some instances it has been found desirable to place the membrane in direct contact with the anode. A significant reduction results in the cell voltage during electrolysis. This desirable result is believed to stem from two factors. First, with a given electrode spacing, displacement of the membrane toward the anode increases the current path through the alkali metal hydroxide electrolyte with a corresponding decrease in the current path through the brine electrolyte. The greater electrical conductivity of the hydroxide solution under normal conditions of cell operation contributes to the decreased voltage. Secondly, the increase in catholyte volume which occurs with displacement of the membrane results in a reduction of hydrogen bubble concentration in the hydroxide catholyte. Such a reduction also contributes to the decreased voltage.

Such positioning of the membrane closer to the anode is achieved in a number of ways. One convenient method entails establishing and maintaining a sufficient positive pressure on the catholyte side of the membrane to physically displace, or bow, the membrane toward the anode. This is generally achieved through adjustment of the relative heights of the anolyte and catholyte heads, i.e., the relative liquid "levels in the anode and cathode compartments. The relative levels are adjusted to exert a net positive pressure on the cathode side of the membrane on the order of 0.1 to 20 pounds per square inch. The actual head differential required to obtain this pressure will depend somewhat upon the specific gravities of the electrolyte solutions and the cell geometry, but may readily be determined by one skilled in the art. The exact pressure which must be exerted on the membrane to achieve the desired displacement will depend in some measure upon the physical characteristics of the particular membrane and the design of the particular cell. For example, membranes useful in the process will have varying elasticity, thickness, surface area, and the. like and the cells used . may be diverse sizes with various anode to cathode gaps. Since a reasonable excess of pressure is of no harm, the pressure employed to obtain the desired result may be readily determined in each case.

Alternative methods for positioning of the membrane include imposing a positive gas pressure over the catholyte or a negative gas pressure over the anolyte, the use' of spacers on the electrode faces, and physical affixing of the membrane on or very near the anode. . However, due to the flexible nature of the membranes and their observed tendency to "float" under the aqueous conditions existing in the cell, adjustment of the electrolyte head differentials to effect a positive catholyte pressure and achieve the desired displacement of the membrane is an expedient method for regulating the anode to membrane gap.

In order that those skilled in the art may more readily understand the present invention and certain preferrec embodiments by which it may be carried into effect, the following specific examples are afforded.

Example I An aqueous brine solution having a sodium chloride concentration of about 304 grams per liter was fed to the anode compartment of an electrolytic cell employing a flat-rolled expanded steel cathode, a d imens ional ly stable anode of expanded titanium bearing an electrolytically active coating on the surface thereof, and a duPont Naf ion membrane as described above. The membrane had a thickness of seven mils, a surface area of 302 square inches, and an average gel^water content of about 25 percent. The anode to cathode gap was set at 0.53 inches and the membrane was fixed about its perimeter equidistant between the electrodes. The catholyte initially consisted of a 36-37 percent sodium hydroxide solution. An anode current density of 2.0 amperes per square inch was employed, a cell temperature in the range of 90-97°C,. was maintained, and brine was fed to the anolyte compartment at the rate of 0.124 milliliters per ampere minute. The cell was operated for a number of days and allowed to reach equilibrium r at which time the following conditions had been established: Anolyte concentration - 130-140 gpl Catholyte concentration - 498-520 gpl Brine feed concentration - 304 gpl Current density - 2.0 asi Voltage - 4.91v During this period the relative height of the anolyte had been maintained at 12 inches above the catholyte The anolyte level was then decreased in one inch increments, gradually increasing the hydrostatic pressure on the catholyte side of the membrane. As the membrane was deflected toward the anode, a steady decrease in cell voltage was observed. At the point when the anolyte level had been reduced to 2 inches above the catholyte level, · the voltage reached a minimum of 4.02 volts, a decrease of 0.89 volts (18.5%) from the original. Further reduction of the anolyte head had no effect on voltage ( indicating the net positive pressure on the cathode side of the membrane was sufficient to displace the membrane to the minimum possible distance from the anode.

The anolyte was then progressively raised to reduce the net pressure on the cathode side of the membrane. The voltage increased at the same rate as it had decreased with the raising of the hydrostatic pressure.

Example II The following test was run to establish a voltage profile for- various anode to membrane gaps. A similar, but somewhat smaller, electrolytic cell to that described in.. Example I was employed. The cathode material was flat-rolled expanded steel, plated with one mil of nickel, and the anode material was the same as in Example I. The membrane was a 10 mil, 25 square inch duPont Nafion type containing about 25% gel water. The cell was operated with a sodium chloride brine feed until the following conditions were established: Cell temperature - 90-92°C.

Anolyte concentration - 170-220 gpl Catholyte concentration - 490-530 gpl Current density - 1.75 as i The anode to membrane gap was then varied from 400 mils to zero by use of adjustable electrodes. The cathode and membrane were fixed in position, and the electrolyte heads were maintained constant. The electrodes were designed so that they could be adjusted to achieve predetermined electrode to membrane gaps from outside of the' cell. Typical results are shown in Table I.

TABLE I Anode-Membrane Gap (mils) Voltage 400 4.06 350 4.01 300 3.96 250 3.92 200 3.86 150 3.78 100 3.67 50 3.55 0 3.40 Reduction of the anode-membrane gap from 400 mils to zero resulted in a voltage reduction of 0.66 volts or 16.2 percent. To demonstrate that this voltage reduction was not merely a function of the reduction in overall electrode gap, the test was repeated using an adjustable cathode and varying the cathode-membrane gap over the same range. Typical results are shown in Table II.

TABLE II Cathode-Membrane (mils) Voltage 400 4.10 300 4.08 200 4.05 100 4.01 0 3.92 Reduction of the cathode-membrane gap from 400 mils to zero resulted in a voltage reduction of 0.18 volts, or only 4.4 percent. This demonstrates that the change in voltage per membrane-electrode unit distance is greatest on the anode side of the membrane. The desirable voltage reduction is directly related to the position of the membrane with respect to the anode, rather than to variation in the overall electrode spacing or membrane-cathode gap. Thus for a given electrode spacing it is clear that lower voltage is realized by operating with the membrane closer to the anode.

Thus it is apparent that there has been provided an improved process that satisfies the objects and advantages set forth above. While the invention has been described with particular reference to specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to encompass all such alternatives, modifications, and variations as fall within the spirit and scope of the appended claims.

Claims (6)

1. WHAT IS CLAIMED IS: 1. In a process for the electrolysis of aqueous alkali metal halide solutions in an electrolytic cell having an anode and cathode separated by an electrolytically conductive, hydraulically impervious, cation permselective membrane, the improvement which comprises conducting the electrolysis with said membrane positioned nearer to the anode than to the cathode.

2. A process as recited in claim 1, wherein the alkali metal halide solution is sodium chloride.

3. A process as recited in claim 1, wherein the membrane is positioned directly on the surface of the anode.

4. A process as recited in claim 1, wherein the membrane is positioned by maintaining a net positive pressure on the cathode side of the membrane.

5. A process as recited in claim 4, wherein the membrane is positioned by adjusting the relative liquid levels in the anode and cathode compartments to maintain a net positive pressure on the cathode side of the membrane.

6. A process as recited in claim' 1, wherein the membrane is positioned by the use of spacers on the electrode surfaces . 46555/2 7* An improved process for the electrolysis of aqueous sodium chloride solution in an electrolytic cell* utilizing at least one cation permsslective membrane to separate the anode and cathode, which comprises operating the cell with said membrane positioned nearer to the anode than to the cathode. 8. A process as recited in Claim 7, wherein the membrane is positioned directly o tho anode surface. 9i A process as recited in Claim V wherein the membrane is positioned by maintaining a net positive pressure on the cathode side of the membrane. 10. In an electrolytic cell of the type for the electrolysis f an aqueous alkali metal halide solution, having ah anode and cathode in compartments separated by an electrolytically conductive, hydraulleally impervious catio permselective membrane, the improvement which comprises positioning the membrane nearer to the anode than to the cathode. S -