ELECTROLYSIS CELL SEAL MEANS

Abstract: A seal means for an electrolysis cell including a substantially solid member adapted to be interposed between at least two electrode frame members of the cell. The substantially solid member has two parallel side walls (21b), a generally planar top wall (21c) adapted for contacting a generally planar surface, and a bottom wall (21d) defined by an uneven surface adapted for contacting at least one electrode frame.

Description: This invention relates to a seal means for electrolysis cells, a method of sealing electrolysis cells and a cell assembly comprising said seal means. Electrolysis cells are used in various applications, for example, electrolyzing an alkali metal salt, such as sodium chloride, to produce a halide and alkali metal hydroxide, such as chlorine and sodium hydroxide. A typical structure for an electrolysis cell is that of the filter press-type, which includes monopolar or bipolar structures. Bipolar structures, for example, are as described in U.S. Patent Nos. 4,108,742 and 4,111,779. Typically, a seal means for an electrolysis cell, in particular a filter press-type cell is well known in the art and includes a gasket placed between two adjacent electrode frame members to form an electrolysis cell assembly. A compressive force is applied to the entire cell assembly to squeeze the elements together to form a fluid-tight, i.e., liquid- and gas-tight, seal. The compressive force typically is applied manually or may be applied utilizing hydraulic rams or other types of pressure-applying apparatus to compress the electrode frame members and gasket together. Upon compression, it is common for conventional gasket material, such as rubber or elastomer to deform and expand outwardly or "ooze-out" of the electrolysis cell assembly as pressure is applied to the frames via the frame members. Gaps between cell frame members, caused by gasket oozing out, may lead to gas or electrolyte leakage especially in pressurized cells, requiring shutdown of the cell's operation. Furthermore, in cells employing a separator, as the gaskets deform outwardly, certain separators which are in contact with the gaskets tend to stretch when they are pulled under the pressure of the outwardly deforming gaskets. The stretching of the separator by gasket deformation can cause the separator, which typically is a flat, thin, sheet-like membrane, to break or tear during compression of the gasket and frame members. Any tears or breaks in the membrane employed in a cell may lead to reduced current efficiency during operation of the cell, greatly increasing electrical current usage while reducing the electrolytic operating efficiency of the cell. Too great a drop in current efficiency and/or electrolytic operating efficiency can require costly shutdown of the entire cell while the damaged membrane or membranes are replaced. In EP-A 51360 a sealing means for filter press cells is described which are disposed between two frame members housing an electrode with a separator positioned between said frames whereby the first sealing means is positioned between the first frame and one side of the separator and the second sealing means is positioned between the second frame member and the opposite side of the separator. Said second sealing means is constucted that way that it has a raised portion which raised portion contacts the other side of the separator and thereby presses the separator against the first sealing means so as to form a substantially fluid-tight seal. JP-A-6156 290 describes a method for attaching ion exchange membranes between electrode frames. According to which the ion exchange membrane must be larger than the size of the electrode frames. The ion exchange membrane is thereby hold by the electrode frames with two gaskets whereby one gasket has a plurality of projecting lines running continuously over the entire periphery. US-A 4 130 468 describes a very special constructed electrolysis cell having a diaphragm separating said cell into an anode compartement and a cathode compartement.According to a special embodiment the cell is sealed with a gasket having a planar and an uneven side. In order to achieve a fluid-tight cell, conventional seal means require a large amount of compressive force for adequate interface tightness between the cell frame members and membrane to prevent electrolyte leakage. This compressive force may become so large as to cause stretching of or damage to the thin and relatively weak membrane. Thus, mechanical failures of the membrane have been attributed to the mechanical inadequacies of the interface seal assemblies employed hitherto, and the service life expectancy of flat, sheet-like membranes for electrolysis cells have been known to be significantly effected by gasket characteristics and by the mechanics of sealing. It is desirous to elongate the service life of the membrane by providing a seal means which minimizes stretching of or damage to the membrane and thus, minimize production stoppages. Furthermore, elimination of gas and electrolyte leakage is desirous as such leakages may cause a safety hazard. Accordingly, one aspect of the present invention resides in a seal means for an electrolysis-cell comprising a substantially solid member. The substantially solid member comprises parallel side walls, a generally planar top wall adapted for contacting a generally planar surface and a bottom wall defined by an uneven surface adapted for contacting at least one electrode frame of the cell, said uneven surface is a plurality of concavities or a generally pyramidal- diamond- or honeycomb-shape. Another aspect of the present invention is an electrolysis cell assembly comprising said above seal means interposed between at least two adjacent electrode frames. Still another aspect of the present invention is the electrolysis cell assembly above including a separator and a second seal means as described above. Yet another aspect of the present invention is a method of sealing an electrolysis cell employing the seal means above. The invention can be better understood by reference to the drawings illustrating the preferred embodiments of the invention. The same numerical designation is used in all the Figures where identical elements are shown. Figure 1 is a perspective view of a frame-shaped seal means of the present invention. Figure 2 is a cross-sectional view taken along line 2-2 of Figure 1. Figure 3, is another embodiment of the seal means of the present invention showing a cross-sectional view of the seal means. Figure 4 is another embodiment of the seal means of the present invention showing a front view of a corner portion of the seal means. Figure 5 is a cross-sectional view taken along line 5-5 of Figure 4. Figure 6 is another embodiment of the seal means of the present invention showing a front view of a corner portion of the seal means. Figure 7 is a cross-sectional view taken along line 7 of Figure 6. Figure 8 is another embodiment of the seal means of the present invention showing a front view of a corner portion of the seal means. Figure 9 is a cross-sectional view taken along line 9-9 of Figure 8. Figure 10 is another embodiment of the seal means of the present invention showing a cross-sectional view of the seal means. Figure 11 is another embodiment of the seal means of the present invention showing a cross-sectional view of the seal means. Figure 12 is another embodiment of the seal means of the present invention showing a cross-sectional view of the seal means with rigidifying material embedded therein. Figure 13 is another embodiment of the seal means of the present invention showing a cross-sectional view of the seal means having a bi-layer construction. Figure 14 is another embodiment of the seal means of the present invention showing a cross-sectional and exploded view of the seal means with a spacer member. Figure 15 is a perspective view of a corner portion of an electrolysis cell assembly showing adjacent filter press cell frames and an embodiment of the seal means of the present invention. Figure 16 is a cross-sectional view of a portion of an electrolysis cell assembly of Figure 15. Figure 17 is a perspective and partially cut-away view of a corner portion of an electrolysis cell assembly showing adjacent filter press cell frames, a membrane, and another embodiment of the seal means of the present invention. Figure 18 is a cross-sectional view of a portion of the electrolysis cell assembly of Figure 17. A frame-shaped seal means, generally identified by reference numeral 20, is shown in Figures 1 and 2. Additional embodiments of seal means 20 are shown in Figures 3-14. The seal means 20, in this instance, is in the shape of a picture frame and rectangular in shape, but is not to be limited thereto as other shapes such as square or circular shapes may be used. The seal means 20 includes a substantially solid body 21a having top and bottom walls 21b which are preferably parallel, a generally planar side wall 21c, and a side wall 21d defined by a nonplanar or an uneven surface. "Substantially solid body" means the body may be porous or contain other materials embedded therein. The uneven surface, in this instance, may be in the form a series of ridges 22 and grooves 23 running parallel to each other. In Figure 1, the ridges 22 and grooves 23 are rounded at the corners of the frame-shaped seal means 20, but this is not necessary for purposes of the invention. In Figure 2, the ridges 22 and groves 23 are generally triangular in shape, although any crosssectional shape may be used. For example, as shown in Figure 10, the ridges 22 and grooves 23 may be of generally rectangular cross-section or, as shown in Figure 11, the ridges 22 and grooves 23 may be of generally semicircular cross-section. Furthermore, although not necessary for the purpose of the present invention, the seal means 20 may contain flat side portions 24, as shown in Figure 2. Figure 3 shows a seal means 20 without such flat side portions. The uneven surface designated generally as the side wall 21d may be in the form of a plurality of concavities with ridge 25 as illustrated in Figures 4-9. Various shapes and sizes of the concavities are possible. However, it is preferred to use a uniform shape and size for the concavities. For example, the concavities may be pyramidal-shaped (Figure 4), diamond-shaped (Figure 6), or honeycomb-shaped (Figure 8). The concavities provide ridges 25 adapted for contacting the gasket surface area of a cell frame member and providing an interface between the seal means and the gasket surface area of the cell frame member for sealing. In addition, the concavities advantageously provide non-interconnecting cup-like traps adapted for catching random gas or electrolyte leaks. In addition, the concavities provide a cup-like suction effect when compressed and are adapted for a gasket-to-frame attachment for vertically walled seal surfaces. Seal means 20 may be constructed of any suitable material. For example, where the seal means is to be used as a gasket for a chlor-alkali cell, e.g., in the electrolysis of an aqueous alkali metal chloride solution, the gasket may be made of rubber or elastomer material. Preferably, the gasket is made of a material which is physically and chemically stable, e.g., when contacted with an electrolyte and products of electrolysis present such as in a chlor-alkali cell. In the production of chlorine and caustic, for example, the gasket must, of course, be substantially inert to acid, brine, chlorine, hydrogen and caustic. In addition, the gasket is preferably electrically nonconductive. Furthermore, the gasket should be of a material having high volume resistivity and good sealability after it has been compressed. Suitable materials which can be employed in accordance with this invention include, but are not limited to, for example, neoprene, butyl rubber, ethylene-propylene diene monomer (EPDM), chlorinated polyethylene (CPE) and a polytetrafluoroethylene (PTFE) material such as GORE-TEX®, a Trademark of W. L. Gore & Associates, Inc., or Teflon®, a Trademark of E. I. duPont de Nemours & Company. The durameter of these materials may range from 50 to 80 Shore A. Figure 12 shows another and preferred embodiment of the present invention wherein seal means 20 includes a reinforcement or rigidifying material 26 embedded in the solid body 21a. Although not shown, a rigidifying material may be used with any of the seal means 20 of the Figures 1-18, inclusive, if desired. The purpose of the rigidifying material is to minimize local compression and stiffen quadralaterally the seal means against stretch. The rigidifying material 26 may be made of any material inert to the gasket materials above, for example, an aromatic polyamide fiber, such as Kevlar®, a Trademark of E. I. duPont de Nemours & Company, polyester, nylon or fabric. The rigidifying material 26 may also be a screen of metal such as, for example, titanium. The rigidifying material 26, viewed in cross-section, may be any shape or form. Preferably, a fabric material is embedded within the solid body 21a. Another embodiment of the present invention is shown in Figure 13 wherein the seal means 20 is of a bi-layered construction comprising a first layer 31 and a second layer 32 attached to each other at interface 33 by well known methods such as adhesives. The materials of the first and second layers may be selected from the materials described above for seal means 20. The first and second layers 31 and 32, respectively, may be made identical or different materials with identical or different hardness. For example, the first layer 31 may be made of CPE with a hardness of 40 shore A and the second layer 32 may be made of EPDM with a hardness of 80 shore A. Yet another embodiment of the present invention is shown in Figure 14. In Figure 14 there is shown a seal means 20 with a back portion 41 and a front portion 42 connected at a bottom portion 43 forming a slot 44 adapted to receive a spacer frame 45. The front portion 42 may contain ridges 22 and grooves 23 of any shape in cross-section as described above. The back portion 41 may be thinner than the front portion 42. The spacer frame 45 is positioned in slot 44. By shimming the spacer frame 45 into the slot 44, the seal means may be provided with further support to minimize excessive compression to the seal means due to nonparallelism or thickness variation of the frame assembly. The spacer frame 45 is preferably wedge-shaped but may be of any desired shaped. The slot 44 may also be any shape to accommodated the spacer frame 45 which may be made of a polyvinylchloride polymer, Kynar®, and other thermoplastic materials that can provide the rigidity necessary to obtain a seal. As one preferred application, the seal means 20 may be used in electrolysis cells not employing a separator such as an electrolysis cell used for the production of a chlorate as illustrated in Figures 15 and 16, wherein the seal means 20 is interposed between two electrode frame members 51 and 52. The seal means 20 is interposed between a pair of adjacent filter press frames 51 and 52 without a separator. For illustrative purposes, frame 51 will be referred hereto as the anode frame 51 and frame 52 will be referred hereto as the cathode frame 52. Typically, the anode and cathode frames are structured such that an anode and a cathode are attached on opposite faces or sides of the structure and electrically connected through the structure. In this instance, the electrolysis cell assembly is typical of a filter presstype electrolytic cell unit which may be monopolar or bipolar. For illustration only, the invention will be described herein with reference to a bipolar, electrolytic filter press-type cell. The seal means of the present invention is useful in filter press-type cells, for example, as described in U. S. Patent Nos. 4,108,742 and 4,111,779. In a more preferred application, the seal means of the present invention may be used in an electrolysis cell with a separator, for example, chlor-alkali cells used for the production of chlorine and alkali metal hydroxide. Referring to Figures 17 and 18, a chlor-alkali cell assembly is shown including a separator 60 and two seal means 20 positioned between frames 51 and 52. The separator 60 is interposed between the two seal means 20 while the two seal means 20 including the separator 60 are interposed between the lateral faces 53 and 54 of frames 51 and 52, respectively. The grooved side wall of each seal means is in contact with the lateral faces 53 and 54 of electrode frames 51 and 52, respectively, and the planar side wall 21c of the seal means is in contact with the separator 60. The top and bottom walls 21b of each of the pair of seal means 20 essentially face opposite each other with the separator 60 interposed between the planar side surfaces of the seal means. Although two seal means 20 are illustrated in Figures 17 and 18, it is within the scope of the invention to use only one seal means between separator 60 and the frame 51 or 52, respectively, of the present invention. Separator 60 used in the present invention may be a diaphragm or a membrane. Particularly useful in the present invention are ion-exchange membrane separators used in filter press-type cells. Preferably, membranes having ion-exchange properties and which are substantially impervious to the hydrodynamic flow of the electrolyte and the passage of gas products produced during electrolysis, are used. The materials of construction for frames 51 and 52 may be any which are resistant to corrosion by the electrolytes and the products of electrolysis. The frames 51 and 52 may be any shape used in typical electrolysis cells, for example, in the shape of rectangular bars, C or U channels, cylindrical tubes, elliptical tubes as well as being I-shaped or H-shaped. Preferably, the cross-section shape of the frame members are I-shaped. In order to effectuate a fluid-tight seal between the adjacent frame members 51 and 52, a pair of the seal means 20 are interposed between the cell frames 51 and 52 so that the ridges 22 bear against the lateral face 35 and 54 of frames 51 and 52, respectively, at the points of contact. The membrane 60 is interposed and in contact with the flat or planar surfaces 21c of the seal means 20 A compressive force, for example, using a hydraulic ram is applied to the cell frames to compress the seal means with the membrane together. Upon compression, the ridges 22 deform slightly which is not shown in Figure 18. The nominal compressive stress normally used may range from 345 to 13,800 kPa. Preferably, from 345 to 4100 kPa and more preferably from 345 to 1400 kPa. However, the lowest stress required for effectuating a desired joint tightness is desirable for material endurance. One advantage of the present seal means over conventional seal means is that an interfacial seal may be obtained at a lower nominal compressive stress with seal means having ridges and grooves than with seal means without ridges and grooves. Consequently, there is less compressive load on the membrane, and the intrinsic oozing of the gasketing material is significantly reduced. Since oozing is reduced, the tearing or breaking of a membrane is also reduced during compression of the seal means and membrane via the cell frame members which in turn enhances the expected service life of the membrane. The following example is illustrative of the invention: A 60 cm by 60 cm test cell consisting of two cell frames provided with piping connections for air and hot water service is used to test the "sealability" of gaskets up to a cell pressure of 345 kPa. After cleaning the flanged surface of the cell frames, a gasket is cemented to the flange using a rubber cement. The membrane is then taped to one of the test cell frames. Care is taken to avoid wrinkles around the periphery of the membrane. The assembly is then placed in a hydraulic press testing machine and squeezed lightly to hold the assembly just enough to inspect for an even seating of the sealing surfaces. The assembly is then squeezed to a gasket loading of approximately 345 kPa and held for about 20 minutes. The loading is then relaxed to approximately 170 kPa and held for about 10 minutes. The loading is then increased to approximately 510 kPa. The cell is then filled with tap water and the water circulated therethrough. If wetting of the membrane or leakage of water is observed, the procedure is started over again. Upon an adequate seal around the entire periphery of the cell, the gasket loading is increased to approximately 7600 kPa in increments of about 103 kPa. Each increment is held for 20 minutes before raising the loading to the next increment. The gasket loading is then decreased to half of its high value. This process is known as "preconditioning" the gasketed assembly. The temperature of the circulating water is incrementally raised to 93.3°C by increments of 11°C per 30 minutes and the internal cell pressure (air-padding) is raised incrementally to 170 kPa at 34 kPa increments per 25 minutes. After the above test conditions are reached, they are maintained for a minimum of 100 continuous hours. Air leakage is calculated by measuring the drop in air pressure during several half-hour intervals; while the temperature remains substantially constant. Thus, a prorated air loss per day was calculated. Upon completion of the test, the cell assembly is disassembled. The gasket is examined for permanent deformation and the membrane is examined for thinning and/or tears. Example 1 The general procedure above was followed to test a pair of gaskets having a plurality of ridges and grooves of triangular shape in cross-section, herein serrated gaskets, shown in Figures 11 and 12. The serrated gaskets were made of EDPM rubber obtained from Sealmaster Corporation, Kent, Ohio. The size of the serrated gaskets were 63 cm by 63 cm by 3.1 mm. The membrane tested was Nafion® 321. The cell frame was maintained at 207 kPa and the water circulated was maintained at 93°C for 140 hours. The gasket loading was about 480 kPa. Visual observation of the serrated gaskets and membrane after the testing period showed some pressure marks with an acceptable amount of stretching, but the membrane showed no sign of tearing or breaking.

Claims: A seal means for an electrolytic cell comprising a substantially solid member, and comprising top and bottom walls, a generally planar side wall adapted for contacting a generally planar surface, and a side wall defined by an uneven surface and adapted for contacting at least one electrode frame, characterized in that the uneven surface is a plurality of concavities of a generally pyramidal-, diamond- or honeycomb-shape. The seal means of Claim 1 wherein the substantially solid member includes rigidifying material embedded therein. The seal means of Claim 2 wherein the rigidifying material is a synthetic resinous material, a metal sheet or screen or a fabric material. The seal means of Claim 3, wherein the synthetic resinous material is selected from ethylene propylene diene and chlorinated polyethylene. A seal means of Claim 1, wherein the substantially solid member comprises a first layer and a second layer, said layers being attached to each other at an interface to form a bi-layered seal. The seal means of Claim 5, wherein the first layer being made from a material having different hardness than the material of the second layer. An clectrolysis cell assembly comprising at least a first seal means according to Claims 1 - 6 interposed between at least two adjacent electrode frames. The cell assembly of Claim 7 characterized in that it comprises a separator sheet interposed between the first seal means and a first electrode frame wherein the planar side wall of the first seal means is in contact with a first side of the separator and the uneven side wall of the first seal means is in contact with a second electrode frame. The cell assembly of Claim 8 characterized in that it further includes a second seal means interposed between the separator and the first electrode frame, whereby the planar side wall is contacting the second side of the separator, and the side wall defined by an uneven surface is contacting the first electrode frame. A method of sealing an electrolysis cell comprising interposing at least a first seal means according to one of Claim 1 - 6 between at least two adjacent electrode frames of the cell and applying a compressive force to the frames. The method of Claim 10 including the step of interposing a separator sheet between the first seal means and a first electrode frame wherein the side wall of the first seal means is in contact with one side of the separator and the side wall of the first seal means is in contact with a second electrode frame. The method of Claim 11 including interposing further a second seal means between the separator an the first electrode frame, whereby the generally planar side wall of the second seal means is contacting the other side of the separator and a side wall defined by an uneven surface is contacting the first electrode frame.

Inventors: GROSSHANDLER, SANDOR
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1987
Main classification: C25B9
IPC codes: C25B9, F16J15