US3496077A - Electrolyzing of salt solutions - Google Patents

Description

7, 1970 H. a. H. COOPER 3496077 ELECTROLYZING OF SALT SOLUTIONS Filed Dec. 18, 1967 2 Sheets-Shet 1 FEED BRINE CATHODE FEED WATER 22 (,so

6 CATHODIC GASES EXI BRINEQ s AN 0 GASES FIGURE-I I NI ha! l l CATHOLYTE EFFLUENT FEED BRINE v 'ANODIC GA n- CATHODI C GASES INVENTOR.

H.B.H. COOPER 2 Sheets-Sheet 2 -CATHOD|C GASES FIGURE- 3 CATHOLYTE EFFLUENT 76 r-CATHOD|C GASES FIGURE-4 -CATHOLYTE EFFLUENT INVENTOR.

H. B.H. COOPER CATHODE FEED WATER 9 .D 211 w I m E C b 1= 2 CATHODE FEED WATER Z =IE LUTION :LW I Q H. B. H. COOPER Feb. 17, 1970 ELECTROLYZING OF SALT SOLUTIONS Filed Dec. 18, 1967 EXIT SOLUTION 8| ANODIC GASES EXIT SOLUTION 8| ANODIC GASES United States Patent US. Cl. 204-98 6 Claims ABSTRACT OF THE DISCLOSURE A method of converting an electrolyte salt solution into its corresponding anodic and cathodic products in an electrolytic cell having a terminal anode compartment separated from an adjacent anode feed compartment by a fluid-permeable porous diaphragm, and a cathode compartment separated by a cationic-selective ion-exchange membrane from the anode feed compartment, the steps comprising:

(1) providing an open transfer passageway apart from the porous diaphragm between the anode feed compartment and the terminal anode compartment;

(2) introducing the salt solution into the anode feed compartment at a distance spaced from the passagey;

(3) flowing a preponderance of the introduced salt solution through the anode feed compartment in general parallel flow to the ion-exchange membrane and transferring said preponderance of the salt solution from the anode feed compartment to the terminal anode compartment through the open transfer passageway;

(4) introducing an electrolyte solution into the cathode compartment;

(5) passing a direct current transversely through said compartments, ion-exchange membrane, and porous diaphragm to cause migration of cations from the anode feed compartment into the cathode compartment, and

(6) removing anodic and cathodic products from the anode and cathode compartments, respectively.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to electrolytic processes for the electrochemical conversion of aqueous electrolyte solutions and more particularly relates to processes for converting an electrolyte salt solution into its corresponding anodic and cathodic products.

Description of the prior art It has been recognized in the prior art, as discussed in Tirrell and Parsi U.S. Patents Nos. 3,135,673 and 3,222,267, that it is important to restrain and minimize the transfer or migration of various of the products of electrolysis within their respective anode and cathode compartments, for example, the migration of hydrogen ions formed at the anode of an electrolytic cell toward the cathode compartment and migration of hydroxyl ions formed at the cathode toward the anode. The process of the invention is concerned with the operation and design of an electrolytic cell having a terminal anode compartment separated from an adjacent anode feed compartment by a fluid-permeable porous diaphragm with the cathode compartment of the cell being separated from the anode feed compartment by a cationic-selective ion exchange membrane. An electrolyte salt feed solutino is supplied to the anode feed compartment. In the operation of such a typical cell, the electrolyte salt solution is decomposed into its corresponding acidic and basic components. Typically, in the operation of such an electrolyte cell, tap water or other electrolyte is passed into the cathode compartment and an electric current is impressed upon the cell causing migration of metal cationic ions of the electrolyte salt solution through the cationic selective membrane into the cathode compartment where the cationic ions combine with the hydroxyl ions produced by the electrolysis at the cathode, to produce a corresponding metal hydroxide. It has been recognized that since hydrogen ions are also formed at the anode and are positively charged and have a high mobility rate, they also tend to migrate from the anode compartment through the cationic selective membrane into the cathode compartment, and in so doing represent a loss in electrical efiiciency. Because of the high mobility of the hydrogen ion, as compared to the typical cation, for example, sodium ions in the instance of a sodium chloride brine electrolyte solution, the hydrogen ion will transfer through the cationic selective membrane preferentially to that of the sodium or other cation. Thus, the current impressed upon the cell can convey to a substantial degree hydrogen ions through the cationic membrane into the cathode compartment. To the extent that such hydrogen ion migration occurs there will be a corresponding reduction of current efficiency of the cell.

Tirrell et al. in their Patents 3,135,673 and 3,222,267 limit hydrogen ion migration by introducing the electrolyte solution being processed into a terminal feed compartment adjacent to thecation permselective membrane and force all of the introduced solution through a porous diaphragm into an adjacent anode terminal compartment by providing only an inlet to the anode feed compartment. All of the feed solution thus passes through the porous diaphragm into tthe terminal anode compartment. It is reported in the Tirrell et al. patents that this arrangement minimizes migration of hydrogen ions to the cathode by the counter-current flow of the salt feed solution to and through the porous membrane.

The approach employed in the Tirrell et al. patents causes the entering feed solution to pass through the porous diaphragm over its full area. There is, therefore, transfer of a considerable amount of the liquid feed in the area or section adjacent to the spent brine removal port of the cell without contact with the anode and therefore a short circuiting of a significant amount of the electrolyte solution occurs. To the extent this occurs, an excess of electrolyte is therefore circulated through the cell and, as a result, the brine removed from the cell has an unnecessarily high concentration of salt. In order to conserve the salt, it becomes imperative to resaturate and recycle the stream to the cell. There is a tendency in the Tirrell et al. cell, which has only an inlet to the anode feed compartment, in which all the electrolyte feed brine passes through the porous diaphragm for sediment to collect, resulting in earlier fouling of the porous diaphragm and the feed compartment.

Summary of the invention In the cell of the instant invention, short circuiting of the feed solution introduced to the anode feed compartment and fouling of the porous diaphragm and sediment buildup is minimized by providing an open, unobstructed passageway apart from the porous membrane between the anode feed compartment and the terminal anode compartment. Thus, this arrangement contrasts to that of the Tirrell et al. cell in having both an inlet and an outlet to the anode feed compartment. The feed solution is introduced into the anode feed compartment at some distance from the transfer passageway connecting the feed and terminal anode compartments and is passed in general parallel flow to the face of the ion exchange membrane with a preponderance of the feed solution being transferred from the anode feed compartment to the terminal anode compartment through the foregoing passageway. That is to say, a lesser portion of the feed solution passes directly through the fluid-permeable porous diaphragm into the terminal anode compartment. With this arrangement, the major portion of the feed brine flows parallel and adjacent to the surface of the cationexchange membrane, thus maintaining a high concentration of the electrolyte cations, for example, sodium ions, at the interface. This minimizes their depletion at that liquid-ion exchange membrane boundary, thereby insuring more opportunity for migration directly through the permselective membrane into the cathode compartment. The feed solution in its parallel flow through the anode feed compartment serves as a flushing stream for any anodic products (including the hydrogen ions and anodic products harmful to the cationic membrane) that have entered the feed compartment through the porous diaphragm and is effective in returning the major portion of such anode products back to the anode terminal compartment.

It will be appreciated that in providing both an inlet and an outlet to the anode feed compartment the feed solution passing through the open transfer passageway will remove fouling materials and prevent the buildup of objectionable sediment, in the feed compartment, and the fouling of the fine openings of the porous membrane. The parallel flow of the feed solution through the anode feed compartment together with the minor portion permeating the porous diaphragm is adequate to curtail substantially the migration of hydrogen ions from the anode towards the cathode, and at the same time avoids fouling of the porous diaphragm and the anode feed compartment, protects the ion-exchange membrane from injurious anodic products, and provides means for more complete electrolysis of the feed solution passing therethrough. It will be appreciated that in some instances, it may be desirable to utilize two or more transfer passageways for connecting the anode feed compartment and the terminal anode compartment in order to obtain more efiicient and uniform flow through the anode feed compartment. The connecting passageways are preferably positioned with respect to the feed inlet to the anode feed compartment so as to expose substantially the full eifective length and area of the cationic membrane to the parallel flow of the feed solution.

FIG. 1 is a diagrammatic representation of the electrolytic cell of the invention in vertical cross section;

FIG. 2 is a variation of the cell of FIG. 1 wherein both the anode and cathode sections are provided With feed compartments defined by fluid-permeable porous diaphragms which are closely spaced to a cationic selective ion-exchange membrane;

FIG. 3 is a further variation of the cell of FIG. 2 again illustrated in vertical cross section, employing a pair of fluid-permeable porous diaphragms in each of the anode and cathode sections; and

FIG. 4 is a further variation having general similarities to the cell of FIG. 2 but wherein the streams passing through the anode and cathode feed compartments are passed to their respective terminal compartments via ex ternal lines.

For descriptive purposes the cells in FIGS. 1 and 2 are illustrated as having feeds of a sodium chloride brine solution. It will be appreciated that various other electrolyte feed salts solutions, for example, solutions containing sodium sulfite or sodium sulfate, may be processed.

In cell of FIG. 1, a cationic selective ion-exchange membrane 12 divides the structure into a cathode section 16 and an anode section 17. The anode section 17 is further subdivided by a tight, fluid-permeable porous membrane 18 into an anode feed compartment 20 and an anode terminal compartment 21. The feed and terminal compartments are interconnected through an open transfer passageway 23. The anode terminal compartment 21 contains an acid resistant anode 25 and is provided with an outlet 26 for the exit brine and anodic gases, such as chlorine and oxygen. The cathode section 16 contains a cathode 27 and is provided with a conduit 30 through which feed water is introduced into the cathode section. Catholyte efiluent is withdrawn through a line 33 from the base of the cathode section 16 and cathodic gases such as hydrogen from an upper portion of the section in a line 36. Feed brine is introduced through a conduit 22 into the feed compartment 20 of the anode section.

In the operation of the cells of the invention, a solution of an electrolyte, for example a near-saturated water solution of sodium chloride is introduced into the cell through conduit 22 at a rate sufiicient to cause most (typically in excess of 60%, usually about of the introduced salt solution to pass in general parallel flow to the ion exchange membrane 12 through the anode feed compartment 20. The major portion of the brine solution is removed from the anode feed compartment 20 through the passageway 23 into the adjoining terminal anode compartment 21. The flow of the brine solution through the porous diaphragm 18 is at a rate sufficient to curtail substantial migration of hydrogen ions from the anode into the cathode section. Additionally, the brine solution in its parallel flow past the cationic-selective ion-exchange membrane 12 returns to the anode terminal compartment anolyte products that may have entered the anode feed compartment which tend to damage the cationic selective membrane.

Simultaneously, an electrolyte solution, typically tap water, is passed via line 30 into the cathode section 16 at a rate corresponding to the concentration of the alkali metal hydroxide desired in the catholyte effluent 33. A direct electric current is impressed across the anode 25 and cathode 27. Under the influence of the electric current, cationic constituents of the electrolyte feed solution, for example, sodium ions in the instance of a sodium chloride brine solution, pass through the cationic selective ion-exchange membrane 12 into the cathode compartment 16 Where they migrate to the cathode 27, are discharged and combine with the hydroxyl ions formed to produce sodium hydroxide which is withdrawn as the catholyte effluent 33 in a concentration dependent upon the rate of water flow into the cathode compartment through conduit 30.

The brine is introduced through conduit 22 into the anode feed compartment 20 and is partially depleted of its sodium ions and chloride ions in its parallel flow therethrough, with the positive sodium ions migrating through the cationic-selective ion-exchange membrane 12 toward the cathode 27 and negatively charged chloride ions transferring through the porous membrane 18 along with the minor portion of the feed brine permeating the porous membrane and not passing through the open transfer passageway 23. The electrolysis produces an anolyte efliuent of hydrochloric acid, chlorine, secondary reaction products such as hypochlorous acid and oxygen, and unre'acted excess sodium chloride.

Another embodiment of the cell of the invention is illustrated in FIG. 2 wherein the anode 40 and the cathode section 42 are respectively divided into two compartments. As in the previous cell of FIG. 1, the two sec tions 40 and 42 are separated by cationic-selective ionexchange membrane 44. In order to minimize attack of the ion-exchange membrane 44 by constituents of the anolyte and catholyte and to obtain high current efficiencies, the electrode products should be restricted to the regions of their respective electrodes, for example, in the case of the latter, the hydrogen ions from the anode and hydroxyl ions from the cathode should be precluded from mixing together due to back migration. The four compartment cell of FIG. 2 minimizes this back migration of the ions by placing a porous, acid resistant, fluidpermeable diaphragm 46 between anode fed compartment 47 and an anode terminal compartment 49, and similarly, by inserting a caustic-resistant, fluid-permeable diaphragm 50 between a cathode feed compartment 51 and a cathode terminal compartment 53. Thus, the ionexchange membrane 44 divides the cell into two distinct sections, with each section being a mirror image of the other.

The respective feed solutions pass in a general parallel flow to the ion-exchange membrane 44 at rates sufficient that migrating hydrogen and hydroxyl ions entering the respective fed compartments from the adjoining electrode terminal compartments are returned to the terminal compartments of origin, by being continuously flushed back and returned to their respective electrode chambers.

The cell of FIG. 2 operates in the following fashion. During electrolysis, negatively charged hydroxyl ions produced at cathode 55 are attracted through the hydraulically permeable porous cathode diaphragm 50 in the direction of anode 57. Because of the cation-selective ion-exchange membrane 44, the negative ions are substantially prevented from further migration into the anode section 40. However, it is known that when the concentration of these hydroxyl ions becomes sufiiciently great there is a tendency for some of the hydroxyl ions to pass through the ion-exchange membrane 44 because of the inherent inefficiency of the membrane and due to diffusion phenomenom. Hence, the higher the concentration of hydroxyl ions in contact with the ion-exchange membrane 44 the greater is the rate of loss of such ions from the cathode section 42 into the anode section 40. In the cell of FIG. 2 this tendency is counteracted by the water entering and flowing through the cathode feed compartment 51 which stream serves to dilute and flush back into the cathode terminal compartment 53 those ions permeating the diaphragm 50. Thereby, the current which is wasted by the hydroxyl ions migrating in the direction of the anode 57 is minimized, thus improving the cell efliciency and production rate. The manner of operation of the cathode feed compartment 51 of the cell of FIG. 2 avoids sediment formation within the feed compartment and fouling of the porous diaphragm which could occur if all of the cathode feed water was passed through the porous diaphragm into the cathode terminal compartment. This is achieved in the cell of FIG. 2 by providing an open transfer passageway between the cathode feed compartment 51 and the cathode terminal compartment 53, thus assuring that a preponderance (typically, in excess of 60%, usually about 80%) of the cathode feed water will pass in general parallel fiow to the ion-exchange membrane 44 and through the open passageway connecting the two cathode compartments rather than through the diaphragm 50 itself. The maintenance of a low alkali metal hydroxide concentration solution in contact with the cation exchange membrane reduces the degrading effect of strong alkali on the membrane and extends its useful life.

The cell of FIG. 3 is similar to that of FIG. 2, differing in that the cathode section 60 and the anode section 62 are each divided into three compartments, namely an inner fed compartment, an intermediate compartment, and a terminal compartment. The anode compartment 62 is segmented by two acid resistant, fluid- permeable diaphragms 64 and 65 into an anode fed compartment 62-a, an anode intermediate compartment 62-b and an anode terminal compartment 62c. The feed solution introduced into the anode fed compartment 62a passes in general parallel flow to the cationic ion-exchange membrane 61 and is transferred in a preponderant portion through an open passageway into the anode intermediate compartment 62-11. In a similar fashion the feed solution passing through the intermediate anode compartment 62-!) moves therethrough in general parallel flow to the two porous diaphragms 64 and 65. In passage through compartments 62-a and 6211 the feed solution passes in each instance into the adjoining compartment, principally through the open transfer passageway connecting such compartments rather than through the permeable membranes 64 and 65, that is to say, a lesser portion of the feed solution passes into the adjoining compartment by direct permeation of the respective diaphragm.

In a similar fashion, the cathode section 60 is divided by two spaced diaphragms 68 and 69 into a cathode feed compartment 60-a, an intermediate cathode compartment 60-h, and a terminal cathode compartment 60c. In a manner similar to that described for the anode compartment, the catholytic feed water or other conducting solution passes in general parallel flow to the porous diaphragms 68 and 69 through open transfer passageways into the adjoining compartments, that is to say, a lesser amount of the cathode feed water passes directly through the respective diaphragms 68 and 69 into the adjoining cathode compartment.

The cell of FIG. 4 is generally like that of FIG. 2, differing in that fed compartments of the respective anode and cathode sections are connected by an exterior passageway or conduit to the adjoining terminal compartments. The anode section 70 is divided by a porous diaphragm 71 into an anode feed compartment 70-a and an anode terminal compartment 70-11. The two anode compartments are interconnected through an exterior transfer line 73. This arrangement permits the feed solution to be introduced into the base of the cell at the lower end of the feed compartment 70-a and removal of the exit solution and anodic gases at the upper end of the cell terminal anode compartment 70-12. In the cell in FIG. 2 the feed brine and the exit brine are respectively introduced and removed at upper levels of the cell. In a. similar fashion the cathode compartment is divided by a porous diaphragm 75 int-o a cathode fed compartment 74-0: and a cathode terminal compartment 74b. The two cathode compartments are interconnected through an exterior line 76. In the operation of the cell of FIG. 4 the respective feed streams pass mainly in parallel flow through the compartments rather passing through the respective porous diaphragms, that is to say. only a minor portion of the respective streams pass into an adjoining compartment by diaphragm permeation.

The non-permselective porous diaphragms used for subdividing the anode and cathode sections into their respective compartments are formed preferably in a tight mesh and of such suitable microporous material such as rubber, ceramic, glass cloth, polyethylene, polypropylene, polyvinylchloride, asbestos, polytetrafluoroethylone, or other synthetic fabrics with suitable resistance to the local environment. The diaphragm employed in the anode section will be acid resistant, while that utilized in the cathode compartment will withstand the attack of the alkaline hydroxide formed therein.

The cationic selective ion-exchange membrane is formed of cation ion-exchange resin prepared in a thin sheet. The membrane is substantially impervious to water and to ions carrying a negative charge, but permeable to ions having a positive charge. Permselectivity towards cations is defined as the membrane characteristics of a higher transport number for cations than that of the solution in which it performs. The membrane desirably has as high a cation transport number as possible and is desirably low in permeability to anions. Suitable cation eX- change materials are well known in the art. Typically, the type of cation exchange membrane ordinarily aifording the highest permselectivity towards cations is that in which the carboxylic acid groups are fixed into a polymeric matrix. U.S. Patent 2,731,408 discloses a method for preparing a suitable membrane. The resinous cationic exchange material is typically incorporated into a sheetlike reinforcing matrix in order to increase the mechanical strength and heat resistance of the membrane. Suitable reinforcing materials include woven or felted ma terials, such as glass filter cloth, polyv inylidene chloride,

. l polypropylene, cellulose paper, asbestos, and similar porous materials of adequate strength.

Although exemplary embodiments of the invention have been disclosed herein for purposes of illustration, it will be understood that various changes, modifications and substitutions may be incorporated in such embodiment without departing from the spirit of the invention as defined by the claims which follow.

I claim:

1. In a method of converting an electrolyte salt solution into its corresponding anodic and cathodic products in an electrolytic cell having a terminal anode compartment separated from an adjacent anode feed compartment by a fluid-permeable porous diaphragm, and a cathode compartment separated by a cationic-selective ionexchange membrane from the anode feed compartment, the improvement comprising:

(1) providing an open transfer passageway apart from the porous diaphragm between the anode feed com partment and the terminal anode compartment;

(2) introducing the salt solution into the anode feed compartment at a distance spaced from the passagey;

(3) flowing a preponderance of the introduced salt solution through the anode feed compartment in general parallel flow to the ion-exchange membrane and transferring said preponderance of the salt solution from the anode feed compartment to the terminal anode compartment through the open transfer passageway;

(4) introducing an electrolyte solution into the cathode compartment;

(5) passing a direct current transversely through said compartments, ion-exchange membrane, and porous diaphragm to cause migration of cations from. the anode feed compartment into the cathode compartment; and

(6) removing anodic and cathodic products from the anode and cathode compartments, respectively.

2. A method in accordance with claim 1 wherein the electrolyte salt solution is a water solution of sodium chloride.

3. A method in accordance with claim 1 wherein the electrolyte salt solution contains sodium sulfite.

4. A method in accordance with claim 1 wherein the electrolyte salt solution contains sodium sulfate.

5. A method in accordance with claim 1 wherein the electrolyte solution introduced into the cathode compartment in water.

6. A method in accordance with claim 1 wherein the cathode compartment is divided by a fluid permeable porous membrane into a cathode feed compartment adjacent to the ion-exchange membrane and a terminal cathode compartment and said method includes providing an open transfer passageway between the cathode feed compartment and the terminal cathode compartment apart from the porous membrane, and wherein the electrolyte solution is introduced into the cathode feed compartment at a distance spaced from the open transfer passageway and a preponderance of the electrolyte solution flows in a general parallel fashion to the ion-exchange membrane and passes from the cathode feed compartment to the terminal cathode compartment through said transfer passageway.

References Cited UNITED STATES PATENTS 2,967,807 1/1961 Osborne et al. 20498 3,017,338 1/1962 Butler et al. 204-98 3,135,673 6/ 1964 Tirrell et al. 204-30l 3,222,267 12/1965 Tirrell et al. 204-98 FOREIGN PATENTS 1,084,1966 9/1967 Great Britain.

T. TUNG, Primary Examiner US. Cl. X.R. 204-180, 301

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