GB2056494A - Bipolar electrolyzer having synthetic separator - Google Patents

Abstract

In a bipolar electrolyzer having fingered, interleaved electrodes, where the electrodes of one polarity are hollow and bear synthetic separators thereon, the hollow electrodes are individually removable and have individual synthetic separators thereon. This design provides an electrolyte seal while avoiding complex post-assembly seaming and joining of the separator.

Description

SPECIFICATION Bipolar electrolyzer having synthetic separator In one commercial manufacture of chlorine and alkali metal hydroxides, e.g., sodium hydroxide and potassium hydroxide, an electrolytic cell having an anolyte compartment separated from the catholyte compartment by an ion permeable separator is utilized. In an electrolytic cell having an ion permeable separator the anolyte compartment has an acidic anolyte liquor at a pH from about 2.5 to about 5.5, containing from about 125 to about 250 grams per liter of alkali metal chloride, and with chlorine being evolved at the anode therein. The catholyte compartment has an alkaline catholyte having an alkali metal hydroxide content in excess of about one mole per liter, with hydrogen being evolved at the cathode therein.

The synthetic separator separates the acidic anolyte from the alkaline catholyte, maintaining differences in pH and concentration therebetween.

The synthetic separator may be a microporous diaphragm, or it may be a permionic membrane.

Microporous diaphragms, e.g., microporous fluorocarbon films, allow chloride ions to diffuse through the separator, thereby providing a cell liquor of about 10 to about 1 5 weight percent alkali metal hydroxide and about 1 5 to about 25 weight percent alkali metal chloride.

According to an alternative exemplification, the synthetic separator may be a permionic membrane, as a cation selective permionic membrane. Cation selective permionic membranes useful for chlor alkali electrolysis are fluorocarbon resins with acid groups thereon such as carboxylic acid groups, sulfonic acid groups, phosphonic acid groups, phosphoric acid groups, derivatives thereof, and precursors thereof. Cation selective permionic membranes are substantially impermeable to the flow of chloride ions, thereby providing a substantially chloride free cell liquor containing from about 50 percent alkali metal hydroxide.

The anticipated long life and stable performance of synthetic separators encourages the use of bipolar electrolyzers, which also are reported to offer significant economy of construction and operation. Bipolar electrolyzers are characterized by a backplate, also known as a bipolar unit, or as a bipolar electrode, as will be described more fully hereinafter. The bipolar unit including the bipolar backplate, serves as a common structural member, supporting cathodes of one cell of a bipolar electrolyzer and the anodes of the next adjacent cell of the electrolyzer.

An individual cell of a bipolar electrolyzer is formed by the anode element of one bipolar electrode or bipolar unit, and the cathode element of the next adjacent bipolar electrode or bipolar unit. The cathodes are electrolyte permeable and covered with an ion permeable separator as described hereinabove.

In the operation of the bipolar electrolyzer, brine is fed into each of the separate cells, and an electric potential is imposed across the electrolyzer. The electrolytic potential causes current to flow from a power supply to an anodic end unit, and from the anodic unit of the electrolyzer to the individual cells of the electrolyzer, in series, and finally to a cathodic end unit, and thence back to the power supply or to an adjacent bipolar electrolyzer.

Typically brine, for example concentrated or even saturated brine containing from about 300 to about 325 grams per liter of sodium chloride or from about 400 to about 450 grams per liter of potassium chloride, is fed to the anolyte compartments of the individual electrolytic cells, and chlorine is recovered from the anolyte chambers of the individual cells, while hydrogen gas and caustic cell liquor is recovered from the individual catholyte compartments of the electrolyzer.

Where the synthetic separator is an microporous diaphragm, the catholyte liquor typically contains about 1 20 to about 220 grams per liter of sodium chloride and from about 110 to about 1 50 grams per liter of sodium hydroxide, or from about 160 to about 300 grams per liter potassium chloride and from about 1 60 to about 220 grams per liter of potassium hydroxide.

Alternatively, where the synthetic separator is a permionic membrane rather than a microporous diaphragm, the catholyte liquor may contain up to 300 or more grams per liter of sodium hydroxide and considerably lesser amounts of sodium chloride, for example less than about 10 grams per liter of sodium chloride and preferably less than one gram per liter of sodium chloride.

Alternatively, the catholyte liquor may contain up to about 450 or more grams per liter of potassiurr hydroxide and considerably lesser amounts, e.g., less than about 10 grams per liter of potassium chloride and preferably less than one gram per liter of potassium chloride.

While bipolar electrolyzers offer significant economies of construction and operation, especially with synthetic separators interposed between an anolyte compartment and a catholyte compartment of an individual cell, the fluorocarbon material useful in forming the synthetic separators are difficult to form into the shapes necessary for the banks of fingered electrodes in a narrow pitch fingered electrode bipolar electrolytic cell. The provision of the joints, seams, and convolutions requires high temperatures, strong reagents, high pressures or combinations thereof, all of which have a deleterious effect on the electrodes.

A particularly satisfactory design for a bipolar electrolyzer utilizing a synthetic separator between the anolyte compartment and catholyte compartment of the individual cells should be one providing an electrolyte tight seal while avoiding complex post-assembly seaming and joining of the permionic membrane or microporous diaphragm.

It has now been found that a particularly satisfactory design is one where the cathode fingers are independently or individually removable from the cathode backscreen, and where each of the independently removable individual cathode fingers bears a separate ion permeable synthetic separator sheet. The ion permeable synthetic separator sheet should be one surrounding the individual cathode finger, being perforate between the cathode and the cathodic backscreen, whereby to allow the flow of catholyte therebetween, and being jointed upon itself at a location remote from the cathodic backscreen.

According to the present invention therefore a bipolar electrolyzer is provided having a plurality of bipolar elements electrically and mechanically in series, each of said bipolar elements comprising: a. a backplate having an anolyte resistant surface on one side and a catholyte resistant surface on the opposite side, said backplate separating the anolyte compartment of one cell from the catholyte compartment of the prior adjacent cell in the electrolyzer; b. anodic electrodes extending from the anolyte resistant surface of the backplate; c. cathodic electrodes extending from the catholyte resistant surface of the backplate;; d. at least one of said sets of electrodes including a backscreen parallel to and spaced from the surface of the backplate, and hollow electrode fingers mounted on, in fluid communication with, and perpendicularly extending from the backscreen, whereby the volume within said hollow electrode fingers and between said backscreen and the backplate defines an electrolyte volume; and e. an ion permeable synthetic separator on said hollow electrode fingers; wherein (i) said hollow electrode fingers are independently removable from said backscreen; (ii) each of said hollow electrode fingers bears an ion permeable synthetic separator sheet; and (iii) said ion permeable separator sheet surrounds said individual hollow electrode finger being perforate between the electrode finger and the backscreen whereby to allow the flow of electrolyte therebetween, and being jointed upon itself remote from the backscreen.

In the accompanying drawings, which illustrate an embodiment of this invention, FIG. 1 is an isometric view of a bipolar electrolyzer which may have the electrode structure and synthetic separator combination herein contemplated.

FIG. 2 is a partial cutaway plan view of bipolar electrolyzer shown in FIG. 1.

FIG. 3 is a partial cutaway side elevation of the bipolar electrolyzer shown in FIGS. 1 and 2.

FIG. 4 is a partial cutaway of an individual cathode element of the bipolar electrolyzer shown in FIGS. 1,2 and 3.

FIG. 5 is a partially exploded isometric view of an individual cathode element of bipolar electrolyzer shown in FIGS. 1 through 4.

FIG. 1 shows a bipolar electrolyzer having a plurality of individual electrolytic cells 11, 12, 13, and 14 electrically and mechanically in series through bipolar elements 21, 23, 25 and 27 which are also electrically and mechanically in series.

The bipolar electrolyzer 1 has brine feed header 91 through brine-chlorine tank 95 and brine return line 99 into the anolyte compartment of each individual cell, with chlorine coming up through chlorine and brine line 97, to the brine tank 95 where the chlorine and brine froth is separated with the chlorine being recovered through brine header 91. In this way a circulatory motion is provided to the anolyte liquor especially when the brine return line 99 extends into the anolyte compartment, e.g., below the level of electrolyte therein. Additionally, depleted brine is recovered from the anolyte compartments through brine recovery line 1 11, and acid is fed to the individual anolyte compartments through acid feed header 101 to provide an acidified anolyte liquor.

Water is fed to the catholyte compartments of the individual electrolytic cells 11, 12, 13, 14 through water header 103 and individual water iines 105, with hydrogen being recovered through hydrogen recovery line 109 to hydrogen header 107. Cell liquor is recovered through cell liquor recovery line 1 15 to cell liquor header 113.

The individual bipolar units 21, 23, 25 and 27 include backplates 31 which separate the anolyte compartments of one cell, for example cell 11, from the catholyte compartments of the prior cell, for example cell 12, in the electrolyzer. The backplate 31 includes an anolyte resistant surface 41 on one side and a catholyte resistant surface 61 on the opposite side. The anolyte resistant surface 41 is fabricated of a valve metal and may be a sheet, plate, coating or lining. Valve metals are those metals which form an oxide upon exposure to acidic media under anodic conditions, for example titanium, tantalum, tungsten, niobium, zirconium and the like. The opposite surface of the backplate 31, has a catholyte resistant surface 61 thereon. By a "catholyte resistant surface" is meant a surface of iron, or an iron alloy e.g. steel, stainless steel or low carbon mild steel.

Additionally, but not shown, there may be a third layer between the anolyte resistant surface 41 and the catholyte resistant surface 61 of the backplate. The third layer has means for preventing hydrogen embrittlement of the anolyte resistant surface 41 of the backplate 31. This may take the form for example of copper cladding, plate, or sheet, or a sheet of a platinum group metal interposed between the anolyte resistant surface 41 and the catholyte resistant surface 61.

Alternatively, there may be a sheet of material substantially impermeable to the flow of nascent hydrogen interposed between the catholyte liquor and the catholyte resistant surface 61 of the backplate 31.

The individual anodes 43 are fabricated of a valve metal, as described above, having an electrocatalytic surface thereon. The individual anodes 43 may be welded to the anolyte resistant surface 41 of the backplate 31. Alternatively, the individual anodes 43 may be welded to bars, not shown, or to extensions of the cathodic conductors.

The individual anode blades a3 may be for example perforated, foraminous, metal mesh, sheets or plates. They are parallel to each other and extend perpendicularly from the anolyte resistant surface 41 of the backplate 31, whereby the electrodes are fingered and interleaved between the fingered electrodes of opposite polarity.

The cathode structure includes individual hollow cathode fingers 63 having sidewalls, a top edge, a bottom edge, and a leading edge or tip.

The cathode fingers are formed of a suitable material, i.e., one that is electrically conductive, alkali resistant, and in an electro!yte permeable form. That is, they allow electrolyte to flow between the permionic membrane 81 and the catholyte compenment. The electrolyte permeable form may be provided by perforated plate, perforated sheet, metal mesh, or expanded metal mesh so as to provide an open area of from about 30 percent to about 70 percent.

The materials of construction of the individual cathode fingers 63 may be Iron, or an iron alloy e.g. steel, mild low carbon steel, or stainless steel.

Additionally, the cathode 63 may have hydrogen overvoltage reducing catalysts or depolarizing catalysts thereon.

The cathode finger 63 has openings 64 in the base thereof where the cathode fingers 63 are interposed against the backscreen 67, which has similar openings 68. By "opening" is meant that there is a substantial absence of metal mesh, perforated plate, or the like, so as to allow the unimpeded flow of catholyte liquor and hydrogen gas between the compartments.

The cathode backscreen 67 is substantially parallel to and spaced from the cathodic surface 61 of the backplate 31. The cathodic backscreen 67 is substantially coextensive with the cathodic surface 61 of the backplate 31. It may be fabricated of the same material as the cathode fingers 63 and be in the same form, That is, it may be formed of an electroconductive, electrolyte impermeable metal in an electrolyte permeable form, e.g. perforated plate, perForated sheet, metal mesh or expanded metal mesh having from 30 to 70 percent open area and being iron or an iron alloy e.g. steel, low carbon mild steel, or stainless steel.Alternatively, the cathodic backscreen 67 may be fabricated of a nonconductive material, for example a heavy polymeric material or polymer coated material, and may be substantially electrolyte impermeable.

The hollow cathode fingers 63 are mounted on, in fluid communication with, and extend perpendicularly from the cathode backscreen 67.

The volume within the cathode fingers 63 and the volume between the cathode backscreen 67 and the cathodic surface 61 of the backplate 31 are one catholyte volume. The feed of electrolyte, for example water, to the catholyte volume is through water header 103 and water feed line 105, while the recovery of gas therefrom is through hydrogen line 109 to hydrogen header 107.

The synthetic separator 81 separates the anolyte compartment from the catholyte compartment, and bears upon the hollow electrodes 63. While in the exemplification herein contemplated, the hollow electrodes are cathodes, it is to be understood the hollow electrodes may also be anodes and that by merely reversing the choice of materials of construction a cell may be prepared having a catholyte compartment surrounding hollow anodes, which hollow anodes bear the permionic membrane or microporous diaphragm thereon.

The synthetic separator may be a permionic membrane. That is, it may be substantially impermeable to the flow of anions and substantially permeable to the flow of cations whereby to allow the flow of alkali metal ions therethrnugh while preventing the flow of chloride ions. Alternatively, the synthetic separator 81 may be a microporous diaphragm, that is a synthetic separator that is substantially permeable to the bulk flow of electrolyte therethrough including both anions and cations.

Typically, the synthetic separators are fluorocarbon. Where the synthetic separator 81 is a permionic membrane, it is typically a fluorocarbon having pendant acid groups thereon, for example, sulfonic groups, carboxylic groups, phosphonic groups, phosphoric groups, precursors thereof or reaction products thereof.

Fluorocarbon synthetic separator material is characterized by difficulties in joining sheets of the fluorocarbon material to itself as well as in fitting fluorocarbon sheets to complex shapes. This generally has required ultrasonic, thermal, pressure, or chemical procedures to join the sheets of the synthetic separator 81 to each other.

It has been found that particularly desirable results are obtained where the sheets are joined directly to each other with the joining procedure, for example ultrasonic, chemical, pressure, or thermal, with necessary apparatus on each side of the joint, e.g., presses and heating elements.

As herein contemplated, the combination of independently removable individual cathode fingers 63 with each cathode finger bearing its own synthetic separator sheet 81, which sheet is perforate 82 between the cathode finger 63 and the backscreen 67 and sealed remotely from the base of the cathode finger 63, is particularly desirable.

Independently removable cathodes are shown, for example in FIG. 5. As there shown, bolt means 71 extend outwardly from the base of the cathode fingers 63. The bolt means may be threaded bolt means, of a suitable electro-conductive material e.g. copper, iron or the like, and have a diameter of from about 3/1 6 inch (0.48 cms) to about 5/1 6 inch (0.80 cms), for example, as described in U.K. Patent No. 1 545226.

As there described, the bolt means 71 are electrically and mechanically joined to the cathode finger 63. For example, the bolt may be welded to the cathode walls by tap welding, spot welding or the like. According to a further exemplification there described, the bolt means 71 may be welded to a stud which is in turn welded to the walls of the cathode fingers 63.

The cathodic backscreen 67 has apertures therein. The apertures correspond to the bolt means 71 and are of a diameter greater than the diameter of the bolt means 71 to allow, for example, the movement such as the slideable movement of the cathode fingers 63, while being close enough in size to the diameter of the bolt means 71 to allow the bolt means 71 to be fastened thereto. As described in the aforementioned patent of Cunningham et al, the diameter of the apertures adapted to receive the bolt means 71 is from about 1/4 inch (0.64 cms) and to about 1/2 inch (1.3 cms) greater than the diameter of the bolt means 71.

Additionally, the cathodic backscreen 67 includes apertures 68 of sufficient size to allow the unimpeded passage of cell liquor and hydrogen gas between the hollow interiors of the cathode fingers 63 and the volume between the cathodic backscreen 67 and the cathodic surface 61 of the bipolar backplate 31.

The electrical contact between the bipolar backplate 61 and the individual hollow cathode fingers 63 is provided by a system of flexible elastic conductor means substantially as described in the aforementioned patent of Cunningham et al. The flexible, elastic conductor means includes conductors 69 mounted on the individual cathode fingers 63 by bolt 71 and nuts 72, as shown in FIG. 5, electrically and mechanically connected thereto, and extending outwardly from the cathode fingers 63 toward the cathodic surface 61 of the bipolar backplate 31.

By flexible and elastic is meant that the conductor means are yieldable to allow movement and positioning of the cathode fingers 63, while being elastic to allow a tight connection between the two pairs of electrical conductor means.

One pair of conductor means 69 are joined to the individual cathode fingers 63, for example, by welding. A second pair of conductor means 73 are joined to the catholyte resistant surface 61 of the bipolar backplate 31 or directly to the bipolar backplate 31 by bolting by bolt 75 and nut 76, welding or the like.

Each of the cathode fingers 63 bears an independent ion permeable synthetic separator sheet 81. As shown in FIG. 4, the membrane 81 envelopes an individual cathode finger 63 and is compressed between the base of the cathode finger 63 and the cathodic backscreen 67 whereby to provide an electrolyte seal therebetween.

As shown in FIG. 5, there are perforations or openings 64 in the base of the cathode finger 63 corresponding to openings 82 in the base of the permionic membrane 81, which further correspond to openings 68 in the backscreen 67 and, where present, openings 66 in gasket 65.

These openings or perforations allow for the passage of the conductor element 69, as well as for the passage of the electrolyte and hydrogen.

The synthetic separator sheet 81 surrounds the cathode finger 63 as described above, and is jointed upon itself, through overlaps 83. The joining may be accomplished by ultrasonic means, chemical means or thermal means, after envelopment of the hollow cathode fingers 63.

According to a further exemplification of the structure herein contemplated, a resilient layer 65 may be provided between the backscreen 67 and the permionic membrane or microporous diaphragm 81 which further has perforations 66 corresponding to the perforations or openings 82 and perforations or openings 64 in the synthetic separator 81 and cathode 63 respectively. The resilient layer 65 provides additional electrolyte sealing as well as means of increasing the tightness of fit of the permionic membrane-backscreen seal.

While the invention has been described with certain specific and illustrated embodiments, it is not intended to be so limited, except insofar as it appears in the accompanying claims. For example, according to an alternative exemplification, there may be provided a bipolar electrolyzer having a plurality of bipolar elements electrically and mechanically in series where each of the bipolar elements comprises a backplate having an anolyte resistant surface on one side and a catholyte resistant surface on the opposite side where the backplate separates the anolyte compartment of one cell from the catholyte compartment of the prior adjacent cell in the electrolyzer and has cathodes extending from the catholyte resistant surface thereof and anodes extending from the anolyte resistant surface thereof.The anodes may include an anodic backscreen parallel to and spaced from the anolyte resistant surface of the backplate, and hollow anode fingers mounted on, in fluid communication with, and perpendicularly extending outwardly from the anodic backscreen whereby the volume within the hollow anode fingers and between the hollow backscreen and anolyte resistant surface of the backplate defines the anolyte volume, with the ion permeable synthetic separator being mounted upon the hollow anodes. Where a structure as thus described is utilized, the anode fingers are independently removable from the anodic backscreen, and each of the hollow anode fingers bears an ion permeable synthetic separator sheet.

As so contemplated, the ion permeable synthetic separator sheet surrounds an individual hollow anode finger, being perforate, that is having openings, between the hollow anode finger and the anodic backscreen whereby to allow the flow of anolyte liquor therebetween and being jointed upon itself remote from the anodic backscreen.

Additionally, the anodic backscreen may have a layer thereon adjacent the synthetic separator sheet whereby to provide an electrolyte tight seal therebetween. In this way with brine being fed into the anolyte compartment between the anodic backscreen and the anolyte resistant surface of the backplate, the individual hollow anode fingers are electrically in communication with each other through the common anolyte compartment, with means being provided for recovering chlorine and depleted brine from the anolyte compartment between the anodic backscreen and the anolyte resistant surface of the backplate. In the alternative exemplification herein described, the synthetic separator may be a microporous diaphragm or permionic membrane.

In a further exemplification, non-conductive non-catalytic spacer means may be inserted on the membrane bearing electrode between the membrane and the electrode, to space the membrane from the electrode. Alternatively, or additionally, the spacer means may be placed on the opposite electrodes.

Claims (9)

1. A bipolar electrolyzer having a plurality of bipolar elements electrically and mechanically in series, each of said bipolar elements comprising: a. a backplate having an anolyte resistant surface on one side and a catholyte resistant surface on the opposite side, said backplate separating the anolyte compartment of one cell from the catholyte compartment of the prior adjacent cell in the electrolyzer; b. anodic electrodes extending from the anolyte resistant surface of the backplate; c. cathodic electrodes extending from the catholyte resistant surface of the backplate;; d. at least one of said sets of electrodes including a backscreen parallel to and spaced from the surface of the backplate, and hollow electrode fingers mounted on, in fluid communication with, and perpendicularly extending from the backscreen, whereby the volume within said hollow electrode fingers and between said backscreen and the backplate defines an electrolyte volume; and e. an ion permeable synthetic separator on said hollow electrode fingers; wherein (i) said hollow electrode fingers are independently removable from said backscreen; (ii) each of said hollow electrode fingers bears an ion permeable synthetic separator sheet; and (iii) said ion permeable separator sheet surrounds said individual hollow electrode finger being perforate between the electrode finger and the backscreen whereby to allow the flow of electrolyte therebetween, and being jointed upon itself remote from the backscreen.

2. A bipolar electrolyzer according to claim 1 wherein said backscreen has a resilient layer thereon, adjacent said synthetic separator sheet whereby to provide an electrolyte tight seal therebetween.

3. A bipolar electrolyzer according to claim 1 or 2 comprising means for feeding electrolyte into the electrolyte compartment between the backscreen and the backplate.

4. A bipolar electrolyzer according to claim 1, 2 or 3 comprising means for recovering gas and electrolyte frown the electrolyte compartment between the backscreen and the backplate.

5. A bipolar electrolyzer according to any of claims 1 to 4 wherein said synthetic separator is a microporous diaphragm.

6. A bipolar electrolyzer according to any of claims 1 to 4 wherein said synthetic separator is a permionic membrane.

7. A bipolar electrolyzer according to any of claims 1 to 6 wherein the electrodes including a backscreen are cathodes.

8. A bipolar electrolyzer according to any of claims 1 to 5 wherein the electrodes including a backscreen are anodes.

9. A bipolar electrnlyzer substantially as herein described with reference to and as illustrated in the accompanying drawings.