US3471382A

US3471382A - Method for improving the operation of chloro-alkali diaphragm cells and apparatus therefor

Info

Publication number: US3471382A
Application number: US598243A
Authority: US
Inventors: Morris P Grotheer
Original assignee: Hooker Chemical Corp
Current assignee: Occidental Chemical Corp
Priority date: 1966-12-01
Filing date: 1966-12-01
Publication date: 1969-10-07
Anticipated expiration: 1986-10-07

Description

1969 M. P. GROTHEER 3,471,382

METHOD FOR IMPROVING THE OPERATION OF CHLORO-ALKALI DIAPHRAGM CELLS AND APPARATUS THEREFOR Filed Dec. 1, 1966 United States Patent 3,471,382 METHOD FOR IMPROVING THE OPERATION OF CHLORO-ALKALI DIAPHRAGM CELLS AND AP- PARATUS THEREFOR Morris P. Grotheer, Lewiston, N.Y., assignor to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York Filed Dec. 1, 1966, Ser. No. 598,243 Int. Cl. C0111 1/06; C(llb 11/26; B01k 3/00 U.S. Cl. 204-98 8 Claims This invention relates to an apparatus and method for improving the operation of chlor-alkali diaphragm cells. More particularly, this invention relates to an apparatus and method for increasing the effective life of the diaphragm used in diaphragm type chlor-alkali electrolytic cells and for maintaining an uninterrupted constant flow of electrolyte through the electrolytic cell while maintaining the current efiiciency of the cell.

Chlor-alkali diaphragm cells are used for the production of large quantities of chlorine, hydrogen and caustic, particularly caustic soda. These basic chemicals are produced by electrolyzing an aqueous solution of an alkali metal chloride in a cell having a diaphragm separating the anode from the cathode. The diaphragm serves to maintain the chlorine and hydrogen produced by the electrolysis separate from each other. The diaphragm is of a material having a suflicient porosity to permit the desired flow of brine from the anolyte compartment to the catholyte compartment while keeping the hydrogen and chlorine gas separate from each other.

One of the most suitable diaphragm materials used in chlor-alkali cells is asbestos. Although other materials such as woven polypropylene, Teflon, and the like synthetics can also be used. However, in the operation of such electrolytic cells, the diaphragm eventually becomes restricted by deposition of impurities contained in the brine and solubilized from the concrete cell structures which are normally employed, and thus the flow of electrolyte through the diaphragm is reduced. The height of the brine within the cell increases as the diaphragm loses its permeability thus requiring the lessening of the flow of brine to the electrolytic cell to compensate for the decreased flow through the diaphragm. Eventually, the diaphragm becomes so restricted that it is desirable to replace it to maintain a desirable cell efliciency. Normally, as the diaphragm becomes restricted, and the brine feed rate decreased to avoid overflowing the cell, the efiiciency of the cell decreases because of the reduced flow and the corresponding increase in caustic concentration within the catholyte compartment. The increased caustic concentration increases the back migration of hydroxyl ions into the anolyte compartment thereby reducing the anode efiiciency of the cell.

The need for replacement of the diaphragm normally occurs two or three times during the life of the graphite anodes. The replacement of the diaphragm requires the shutdown of the electrolytic cell, the disassembly thereof, the removal of the cathode section on which the diaphragm is positioned and the replacement of the diaphragm with a new diaphragm followed by reassembly of the cell. Because of the frequency of replacing the diaphragm, the

labor involved in disassembling the cell, the production time loss and the decrease in electrical efliciencies prior to the replacement of the diaphragm, it is particularly desirable to eliminate or lessen the number of occasions during which the diaphragm is replaced.

It is an object of this invention to provide a method for maintaining the current efliciency of chlor-alkali diaphragm cells while extending the useful life of the diaphragm. It is another object of this invention to provide an apparatus particularly suited for maintaining the current efiiciency of the chlor-alkali diaphragm cells while reducing the frequency of diaphragm renewal. It is a further object of this invention to provide a method whereby maintenance and adjustments of brine flow through the electrolytic cell can be largely eliminated. These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

In accordance with the invention, an electrolytic diaphragm cell for the production of chlorine and caustic is provided having an anode compartment and a cathode compartment separated by a porous diaphragm, the improvement comprising feeding brine solution to the anolyte compartment of said cell while feeding a supplemental brine solution to the catholyte compartment of said cell in excess of the flow through the diaphragm. In a more specific embodiment, the cell is operated by feeding brine solution to the anolyte compartment of said cell at a rate in excess of the flow through the diaphragm, withdrawing the excess feed solution from the anolyte compartment, circumventing the diaphragm with said excess feed solution and passing said excess solution to the catholyte compartment of an electrolytic cell. In addition to the method of operating the electrolytic cell, an apparatus is provided which is particularly suited for maintaining a constant level within the electrolytic cell while withdrawing the excess anolyte solution from the anolyte compartment and feeding it to the catholyte compartment of the electrolytic cell.

The present invention has numerous advantages in the cell operation both in maintaining high current efliciencies over the entire life of the anode and the lesser attention required to the operation of the cell. Using the present method, the flow through the cell is maintained independent of the porosity of the diaphragm. Thus, as the diaphragm becomes restricted and loses some of its permeability, the level within the anolyte compartment of the cell is controlled so that feed solution in excess of that flowing through the diaphragm is circumvented around the diaphragm and fed to the catholyte compartment. This is particularly advantageous in maintaining the current efficiency by keeping the salt-caustic ratio of the catholyte liquor within the most preferred concentrations commensurable with the most desirable current efliciencies. Previously, increases in the concentration of caustic in the catholyte compartment, due to the slower flow rate, reduced the anode current efficiency. The present method is also particularly useful in high current density cells, that is, cells operating at about 1 to 5 amperes per square inch of cathode surface. Also, the present invention is particularly useful in chlorate production methods which utilize c'hlor-alkali diaphragm cells and an electrolyte which contains chlorate as well as chloride.

The invention will be described more particularly with reference to the drawing which is a partial sectional view of a chlor-alkali diaphragm cell illustrating an apparatus particularly useful in the present invention.

A chlor-alkali cell is composed of an anode 12 and a cathode 14, separated by a porous diaphragm 16 to form an anolyte compartment 22 and a catholyte compartment 26. The diaphragm 16 is normally applied to the outer surface of a foraminous cathode. Electrolyte 18, which is normally a concentrated brine solution, is fed to the chlor-alkali cell 10 through inlet into the anolyte compartment 22. The electrolyte level 24 within the anolyte compartment is maintained above the cathode 14 so that a constant hydrostatic pressure is applied against diaphragm 16 as the electrolyte flows into the catholyte compartment 26. In the catholyte compartment 26, a liquid level 28 is maintained by means of an overflow (not shown) which draws cell liquor out of the catholyte compartment.

During the operation of the electrolytic cell, the porosity of the diaphragm decreases as impurities begin to restrict the flow of electrolyte from the anolyte compartment into the catholyte compartment. As the flow of electrolyte through the diaphragm lessens, the electrolyte level 24 in the anolyte compartment 22 increases if the feed rate of electrolyte to the cell is maintained constant. Eventually, the flow of electrolyte to the cell is lessened or the excess brine will overflow the sightglass or otherwise be expelled from the anolyte compartment and wasted.

In the present invention, an overflow means 30 is positioned within the anolyte compartment 22 to withdraw excess electrolyte from the anolyte compartment or alternatively, brine is fed in the desired amount of about 0.005 to about 1.5 and more preferably 0.01 to about 0.5 times the flow through the diaphragm directly to the catholyte compartment. Overflow means 30 can conveniently feed the excess liquor to hydrogen gas withdrawal means 32 which is in communication with catholyte compartment 26. When overflow means 30 is connected to hydrogen gas withdrawal means 32 or the like, gas trap 34 is preferably provided to eliminate the danger of hydrogen gas back feeding into the anolyte compartment 22 wherein it could mix with the chlorine gas to form an explosive mixture. Overflow means 30 is preferably constructed in a manner whereby the electrolyte level 24 in anolyte compartment 22 can be readily adjusted by raising or lowering the intake end 31 of overflow means 30. A convenient means of effecting such an adjustment is by curving a section of overflow means 30 in the manner illustrated in the drawing, thereby providing for a means of readily increasing or decreasing the electrolyte level 24 by merely rotating overflow means 30 to raise and lower intake end 31.

Alternatively, other means for withdrawing electrolyte from the anolyte compartment can be used with correspondingly efifective results. For instance, an internal overflow pipe of adjustable length can be placed to extend upwardly from the bottom of the catholyte compartment 26 through the cathode 14 and upwardly into the anolyte compartment 22 to terminate with a liquid intake at the electrolyte level desired. In another embodiment, an orifice of adjustable size can be used to control the flow of electrolyte from the anode compartment through the diaphragm into the cathode compartment.

The chlor-alkali cell of the present invention is preferably operated utilizing an alkali metal chloride brine, particularly sodium chloride. Other alkali metal chlorides such as lithium chloride, potassium chloride, cesium chloride, rubidium chloride and the like are used to produce the corresponding caustic material. Because of the plentiful supply of sodium chloride and relatively low cost thereof, and the higher demand for caustic soda, sodium chloride is normally the brine feed material. Because of this, the invention will be described more particularly with reference to sodium chloride. However, when sodium chloride is described as the alkali metal chloride, it is to be considered that other alkali metal chlorides can be substituted therefor with correspondingly good results.

The present invention is operated by feeding a brine solution to the electrolytic cell at a rate in excess of the rate at which the brine flows from the anolyte compartment through the diaphragm into the catholyte compartment. Normally, this occurs after the cell has gone high level due to the restriction of the diaphragm, but it can also be produced by using a higher than normal feed rate. The brine solution fed to the cell is a chloride solution containing about to 330 grams per liter of sodium chloride. Preferably, the brine is near the saturation point at the feed temperature. Thus, in a preferred method of operation, the aqueous brine solution contains about 260 to 330 grams per liter.

Under the decomposition voltage applied to the elec' trodes of the electrolytic cell, chlorine is evolved at the anodes and hydrogen is evolved at the cathode along with the production of caustic. The sodium hydroxide concentration in the catholyte compartment of a conventional diaphragm cell is about 9 to 12 percent sodium hydroxide. This corresponds to about 110 to about grams per liter of sodium hydroxide. The exact concentration depends upon a number of variables, particularly the rate of electrolyte flow through the cell and the current applied with respect to the electrolyte flow rate. Thus, at a constant current, a lesser flow rate through the diaphragm increases the sodium hydroxide content of the catholyte liquor. With an increased concentration of hydroxyl ion and a decrease in salt concentration, the anode current efliciency is adversely affected due to the back migration of hydroxyl ions into the anolyte compartment. Therefore, as an embodiment of the present invention, it is preferred to regulate the anolyte pH within a certain desirable pH range to produce the most desirable anode current efliciency.

It has been found that the anode current efficiency is the highest at a pH in the range of about 1 to 4. Therefore, the brine fed to the cell is preferably acidified with hydrochloric acid in either gaseous or liquid form in an amount suflicient to provide the most desired acidity within the anolyte compartment. Normally, unacidified feed brine has a pH of about 9. The chlorine evolved from the electrolytic cell in the anode compartment contributes to the reduction of the pH and therefore, the brine feed solution need not necessarily be reduced in acidity to a pH of 4 prior to entering the anolyte compartment but rather acidification to below a pH of about 6 to 7 is often sufiicient to result in the pH being within the desired 1 to 4 range during cell operation.

To compensate for the back migration of hydroxyl ions, and the corresponding adverse affect on the anolyte pH, the present invention retains and/or regulates the concentration of caustic in the catholyte compartment to within the most desirable range independent of the porosity of the diaphragm. The excess brine withdrawn from the anolyte compartment and added to the catholyte compartment dilutes the catholyte liquor and retains the salt-caustic ratio in the catholyte liquor to within the most desirable range thereby retaining higher anode and cathode current efliciencies. Therefore, in accordance with the present invention the salt-caustic ratio in the normal production of chlorine and caustic is retained within the range of about 800 to 2,500, and more preferably within the range of 1,200 to 1,700 wherein the salt caustic ratio is determined by the formula Grams per liter of salt Grams per liter of caustic 1000 favorable voltage drop. In addition to asbestos, certain inert organic materials can be used with correspondingly good results. Materials such as chlorinated polyvinyl chloride, polyvinylidene chloride, polyvinyl fluoride, polyvinylidene fluoride, polypropylene and the like in woven or fabric form can be used with correspondingly good results.

The following examples illustrate certain preferred embodiments of the present invention. Unless otherwise indicated, all parts and percentages used herein are by weight and all temperatures are in degrees centigrade.

EXAMPLE 1 The present invention was operated utilizing a Hooker type S-l cell having a deposited asbestos diaphragm. The electrolytic cell had been operating in the usual manner for 142 days on the same diaphragm with initially new graphite anodes. As of the l42nd day, the diaphragm had become restricted so that the original feed rate had been reduced by about 50 percent. The current efliciency of the cell averaged 93.5 percent. The catholyte liquor was comprised of 166 grams per liter of sodium hydroxide and the salt caustic ratio was 1,078 and the chlorate concentration was one pound per 1,000 pounds of caustic. These ratios were obtained at a brine feed concentration of about 320 grams per liter of sodium chloride. Normally, a cell operating in this manner would have had the diaphragm replaced.

In accordance with the invention, the cell was modified by the installation of an overflow device in communication with'the anolyte compartment to withdraw excess electrolyte from the anolyte comparment and pass it into the hydrogen gas withdrawal pipe and subsequently into the catholyte chamber in accordance with the drawing. The flow rate of brine to the electrolytic cell was then increased to the normal flow of about 0.7 gallon per minute. Analysis of electrolysis products indicated that the current efficiency had increased to an 8 day average of 95.0 percent and that the sodium hydroxide concentration in the catholyte compartment had been reduced to an average of 125 grams per liter. Also, a more desirable saltcaustic ratio of about 1,680 was obtained. The increased efliciency continued for the life of the anodes. In addition to the increased efiiciency and a more desirable salt-caustic ratio, the requirement for renewal of the diaphragm was eliminated. After 212 days of operation, the sodium hydroxide concentration and salt-caustic ratio in the catholyte liquor was substantially unchanged.

EXAMPLE 2 The present invention was again operated utilizing a Hooker type S-l cell having a deposited asbestos diaphragm. The electrolytic cell had been operating for an extended period of time and the current efficiency had dropped to 92.7 percent. The flow rate of feed brine to the anolyte compartment had been reduced due to the lessening of the diaphragm porosity. The salt-caustic ratio was about 1,110 and the caustic concentration averaged 154 grams per liter of sodium hydroxide in the catholyte liquor. The cell was modified in accordance with the invention by the installation of an overflow device in communication with the catholyte compartment to thereby withdraw a portion of the electrolyte from the anolyte compartment and passing it into the hydrogen gas withdrawal pipe from whence it passed into the catholyte chamber of the electrolytic cell. The flow rate of the brine to the anolyte compartment in the electrolytic cell was then returned to the normal flow rate of about 0.7 gallon per minute. Initially, about 10 percent of this flow was withdrawn from the anolyte compartment and passed into the catholyte compartment of the cell thereby circumventing the diaphragm. On continued operation, the percentage of bypassing electrolyte increased with the age of the diaphragm. Analysis of the current efficiency for the next 14 days of cell operation averaged 95.3 percent. The sodium hydroxide concentration in the catholyte liquor was reduced to an average of about grams per liter and the salt-caustic ratio returned to a more favorable level of an average of about 1,500. The cell continued to operate at improved current efficiencies for the remaining life of the anodes. By this method, the current efficiency of the cell was returned to the desired operating range without the requirement for the renewal of the diaphragm and the associated expense of renewal and shutdown time previously required.

In the same manner, the invention is operated in high current densities chlor-alkali cells having current densities in the range of about 1 to 5 amperes per square inch of cathode surface by feeding separate streams of brine feed solution to both the anolyte and catholyte compartment. The catholyte feed rate is adjusted to provide the most preferred sodium hydroxide concentration of about grams per liter.

While there have been described various embodiments of the present invention, the method and apparatus described are not intended to be understood as limiting the scope of the invention as it is realized that changes therein are possible. It is intended that each element recited in any of the following claims is intended to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner. It is intended to cover the invention broadly in whatever form its principles may be utilized.

What is claimed is:

1. In a method for the production of chlorine and caustic using an electrolytic diaphragm cell having an anolyte compartment and a catholyte compartment separated by a porous diaphragm, the improvement comprising feeding brine solution to the anolyte compartment of said cell while feeding a supplemental brine solution from the anolyte compartment through a hydrogen gas withdrawal means to the catholyte compartment of said cell in excess of the flow through the diaphragm.

2. The process of claim 1 wherein the first said brine solution is fed to the anolyte compartment of said cell at a rate in excess of the flow through the diaphragm, withdrawing the excess feed solution from the anolyte compartment, circumventing the diaphragm with said excess feed solution and passing said excess solution to the catholyte compartment of an electrolytic cell.

3. The process of claim 1 wherein the electrolyte fed to the anolyte compartment is a sodium chloride solution of a concentration of about 120 to 330 grams per liter.

4. The process of claim 1 wherein the electrolyte solution in the anolyte compartment is maintained at a pH of about 1 to 4 by the addition of HCl.

5. The process of claim 1 wherein the flow of electrolyte to the catholyte compartment is in an amount of about 0.005 to about 1.5 times the flow of the electrolyte through the diaphragm.

6. A chlor-alkali diaphragm cell comprising an anode and a cathode separated by a porous diaphragm thereby forming an anolyte compartment and a catholyte compartment, a hydrogen gas withdrawal means in communication with said catholyte compartment, a liquid passage means communicating between said anolyte compartment and said hydrogen gas withdrawal means for the passage of anolyte liquor to the catholyte compartment without passing through said diaphragm.

7. The apparatus of claim 6 wherein the liquid passage means comprises a tubular passage.

8. The apparatus of claim 7 wherein the tubular passage has a curved portion which extends into the anolyte compartment.

References Cited UNITED STATES PATENTS Gaus 20498 Gibbs 204-98 Dow 204-98 Heiskell et al. 20498 Kircher 204-98 10 Inove et al 20499 X Cox 20498 Cooper 20498 Currey et al. 20498 8 FOREIGN PATENTS 3/1929 France.

OTHER REFERENCES Hooker Chemical Corp., Bulletin 20-A Hooker Type S Cells, Niagara Falls, N.Y., 1959, 8 pgs.

Mantel], C. L., Electrochemical Engineering (4th Ed.) McGraw-Hill, New York, 1960, pp. v and 280-285. WINSTON A. DOUGLAS, Primary Examiner A. BEKELMAN, Assistant Examiner US. Cl. X.R. 204263, 266