ELECTRODIALYSIS AND ELECTRODEIONIZATION SPACERS

Abstract

An improved spacer for use in electrodialysis and electrodeionization stacks can provide close contact between the spacer mesh and its adjacent ion exchange membranes, reducing the water flow cross-section through the cell. This in turn can lead to higher flow velocities and increased flow turbulence between ion exchange membranes, thereby reducing membrane polarization effects and increasing the limiting current density. The improved spacer can be combined with a voluminous spacer gasket for receiving a volume of electroactive media, the voluminous spacer gasket comprising an outer gasket edge having an open central area for receiving the electroactive media, and holes on the top and bottom of the outer gasket edge whose dimensions match the holes on the spacer.

Claims

1. A spacer for use in electrodialysis and electrodeionization systems, the spacer comprising: a) a mesh component, the mesh component comprising a central mesh sheet shaped to define a plurality of protrusions, each of the plurality of mesh component protrusions including a hole; and b) a gasket component, the gasket component comprising a gasket edge, the gasket edge defining an open central area for receiving the central mesh sheet and including a plurality of protrusions, each of the plurality of gasket edge protrusions including a hole, wherein the plurality of gasket edge protrusions defines a plurality of recesses within the gasket edge for receiving the plurality of mesh component protrusions, and wherein the gasket edge has substantially the same thickness as the central mesh sheet when the spacer is compressed within an electrodialysis/electrodeionization stack, thereby allowing close contact between the mesh component of the spacer and adjacent ion exchange membranes within the stack.

2. The spacer of claim 1, wherein the central mesh sheet has a thickness of between 0.1 mm and 2.0 mm, and wherein the thickness of the gasket edge is substantially the same as the mesh after compression within the stack.

3. The spacer of claim 1 in combination with a voluminous spacer gasket for receiving a volume of electroactive media, the voluminous spacer gasket comprising an outer gasket edge, an open central area for receiving the electroactive media, and holes on the top and bottom of the outer gasket edge, wherein the dimensions and the position of the holes of the voluminous spacer gasket match the holes on the spacer.

4. The spacer of claim 3, wherein the thickness of the outer gasket edge of the voluminous spacer gasket is greater than 2.0 mm.

5. The spacer of claim 1, wherein the gasket component comprises materials such as silicon rubber, nitrile rubber, plastic polymers, or other similar material.

6. A spacer for reducing ion exchange membrane polarization effects and increasing the limiting current density in an electrodialysis system, the electrodialysis system comprising: a stack of alternating pairs of ion exchange membranes, each ion exchange membrane creating a concentrate compartment on one side and a dilute compartment on the other side when the system is filled with a feed solution and acted upon by a direct current; a first electrode housed in a first endplate positioned on one side of the stack; a second electrode housed in a second endplate positioned on the other side of the stack; a plurality of input and output passages leading into and out of the endplates and the stack; and a direct current electric power supply for establishing a potential difference between the first electrode and the second electrode to cause the passage of electric current through the feed solution, wherein the spacer comprises: a) a mesh component, the mesh component comprising a central mesh sheet shaped to define a plurality of protrusions, each of the plurality of mesh component protrusions including a hole; and b) a gasket component comprising a gasket edge, the gasket edge defining an open central area for receiving the central mesh sheet and including a plurality of protrusions, each of the plurality of gasket edge protrusions including a hole, wherein the plurality of gasket edge protrusions defines a plurality of recesses within the gasket edge for receiving the plurality of mesh component protrusions, and wherein the gasket edge has substantially the same thickness as the central mesh sheet when the spacer is compressed within the stack, thereby allowing close contact between the mesh component of the spacer and adjacent ion exchange membranes within the stack.

7. The spacer of claim 6, wherein the central mesh sheet has a thickness of between 0.1 mm and 2.0 mm, and wherein the thickness of the gasket edge is substantially the same as the mesh after compression within the stack.

8. The spacer of claim 6 in combination with a voluminous spacer gasket for receiving a volume of electroactive media, the voluminous spacer gasket comprising an outer gasket edge, an open central area for receiving the electroactive media, and holes on the top and bottom of the outer gasket edge, wherein the dimensions and the position of the holes of the voluminous spacer gasket match the holes on the spacer.

9. The spacer of claim 8, wherein the thickness of the outer gasket edge of the voluminous spacer gasket is greater than 2.0 mm.

10. The spacer of claim 6, wherein the gasket component comprises materials such as silicon rubber, nitrile rubber, plastic polymers, or other similar material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings illustrate the prior art and preferred embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, explain the principles of the invention.

[0032] FIG. 1 illustrates an ion exchange membrane with water passage holes on it;

[0033] FIG. 2 illustrates a prior art regular spacer in two orientations;

[0034] FIG. 3 illustrates an end spacer;

[0035] FIG. 4 illustrates a schematic presentation of an electrodialysis cell;

[0036] FIG. 5 illustrates a pattern of water conveyance passages on endplates;

[0037] FIG. 6 illustrates areas of low contact stress areas on prior art spacers;

[0038] FIG. 7 illustrates the gasket and separate central mesh portion of one embodiment of the inventive spacer;

[0039] FIG. 8 illustrates two views of one embodiment of an assembled spacer according to the present invention; and

[0040] FIG. 9 illustrates a component of another embodiment of the inventive spacer.

DETAILED DESCRIPTION OF THE INVENTION

[0041] The present invention improves on the prior art spacers used in electrodialysis and electrodeionization systems which provide close contact between the spacer mesh and its adjacent ion exchange membranes. Their design can reduce the water flow cross-section through the cell, in turn leading to higher flow velocities and increased flow turbulence between ion exchange membranes, thereby reducing membrane polarization effects and increasing the limiting current density.

[0042] Definitions—As defined herein, the terms “ion” or “ions” refer to an atom or molecule with a net electric charge due to the loss or gain of one or more electrons. In electrolytes, ions are hydrated ions which means that they are covered by a shell of water molecules. The amount of charge of an ion depends on the number of electrons lost or gained. For any ion missing or gaining one electron, the net charge is equals to that of an electron, equal to 1.60217662×10.sup.−19 Coulombs. This results in the fact that one mole of electrons is equivalent to Avogadro's number (6.02214×10.sup.23) of electrons or 96,485.3 Coulombs.

[0043] As used herein, the terms “electrolyte” and “electrolyte solution” are interchangeable. The principals disclosed herein are therefore applicable to any solute or chemically defined salt or salt mixture dissolved in any polar liquid, wherein the result is the formation of an electrolyte solution. Therefore, when referring to ion-containing or salty waters, irrespective of the variety and concentration of the salts present in unit volume of the liquid, it is to be interpreted as to mean and include an electrolyte solution. As such the term water can mean any polar solvent and the term salt can mean any solute which together with a polar solvent forms an electrolyte solution.

[0044] As used herein the terms “ion exchange membrane” and “ion selective membrane” refer to semi-permeable membranes which can function as either cation-selective or anion-selective membranes; such terms are interchangeable when used in this document.

[0045] As used herein the terms “electroactive media” and “ion exchange resin beads” are interchangeable and may include any shapes or forms as long of they can perform the intended function of conducting ions in a sparingly conductive solution under the influence of an electric field, while maintaining sufficient mechanical integrity. Many types of electroactive media can be used to define a lower resistance path for ion flow in electrolytes acted upon by an electric field. The most common type is in the form of ion exchange resin beads, but electroactive media can also be in the form of beads bonded to one another by a bonding agent, or in the form of fabrics, and depending on the specifics of a design can be mixed anion and cation exchange beads or singular polarity bead layers filling one compartment or distinct sections of both types of resin beads in a single compartment.

[0046] As used herein, “gasketing material” used in manufacture of these invented spacers are meant to define any elastomer such as silicon rubber, neoprene, nitrile, PTFE, rubber, and various polymers such as polychlorotrifluoroethylene or other similar material that can form a sealing gasket sheet that could be used to manufacture the gasket part of the invented spacer. Any reference to elastomer material or silicone rubber in this document means “gasketing material” as defined here.

[0047] The terms “electrodeionization” and “electrodialysis” as they apply to specific processes used are technically different. As noted above, electrodeionization devices are typically used for production of higher purity products from higher purity feeds while electrodialysis systems are used to produce water for such uses as for human consumption or for agriculture from brackish waters and seawater, Further, electrodeionization systems may be distinguished from electrodialysis systems by incorporation of specific voluminous spacers (or separators) placed between the ion exchange membranes while electrodialysis devices typically use rather thin spacers made up a plastic mesh. Such spacers, as they apply to the present invention and electrodeionization devices, are typically filled with electroactive media such as ion exchange resin beads, which facilitate ion flow in the low conductivity input and sparingly conductive high purity output product which is generated in the dilute compartments. Further, while electrodialysis systems are typically used for input solutions having 1000 mg/liter and higher salt content, such as brackish water and seawater, electrodeionization systems typically are used for input solutions already having a low salt content, such as aqueous salt solutions that are the product of passing through one or more reverse osmosis systems. Typically, these feeds have conductivities of less than 50 μS/cm corresponding to about 18 to 20 ppm equivalent NaCl.

[0048] Improvements—The present invention improves on conventional prior art spacers used in electrodialysis systems by reducing/preventing gaps between the spacer mesh and the ion exchange membranes, which serves to prevent leakage from the dilute (i.e. low concentration) compartments to the concentrate (i.e. high concentration) compartments. The spacer embodiments disclosed herein can also prevent leakage from the concentrate compartments to the dilute compartments, when operational and design requirements require high pressure in the concentrating compartments.

[0049] The spacer embodiments disclosed herein allow the electrodialysis stack compartments to be filled, allowing for complete contact between the spacer's central mesh portion and its adjacent ion exchange membranes. This results in a reduced water flow cross-section through the stack, in turn leading to higher flow velocities and more turbulence in the water flow between ion exchange membranes, compared to spacers that do not make contact with their adjacent ion exchange membranes. This in turn can reduce or eliminate membrane polarization effects, and increase the limiting current density. When compressed, the spacer's gasketed edges have the same thickness as it's central mesh, such that current density and energy consumption are decreased.

[0050] FIGS. 7a and 7b illustrate components which, when combined, constitute a preferred embodiment of the inventive spacer. The gasket component 200 of FIG. 7a includes holes 204 and is typically constructed by punching or cutting its shape from sheets of silicon rubber or similar elastomer material. Each gasket component 200 also typically includes what can be described as a plurality of extensions, flanges or protrusions 202, as well as a plurality of cut-outs or recesses 203 alternating with the protrusions 202, and a cut corner 205. FIG. 7a and its related cross-sections C-C and D-D show that the gasket component 200 includes a gasket edge 201 bounding, surrounding the perimeter of, or otherwise defining an open central area 220. The gasket component 200 is constructed to receive a central mesh sheet 206 of a mesh component 210, described below. FIG. 7a also illustrates that the gasket edge 201 also defines the gasket component's protrusions 202, recesses 203, holes 204, and cut corner 205. Typically the thickness of the gasket edge is between about 0.1 to 2.0 mm and more preferably between 0.2 mm and 1.0 mm.

[0051] The mesh component 210 of the spacer is presented in FIG. 7b, which includes a cut-out of a central mesh sheet 206 made of a woven plastic or equivalent thereto as is known in the art, the sheet 206 being shaped to define a plurality of mesh protrusions 207 and cut-outs/recesses 209, each mesh protrusion including a hole 208. The central plastic mesh sheet 206 typically has a thickness of between 0.1 mm and 2.0 mm, and more preferably between 0.2 mm and 1.0 mm. The protrusions 207, and the mesh sheet 206 in general, are intended to fit into the open central area 220, including the recesses 203 of the surrounding edge 201 of the gasket 200 of FIG. 7a. It is noted that the dimensions of all of the recesses 203 and protrusions 207, including their holes 204, 208, are intended to be the same for every spacer throughout the stack, and arranged such that their holes 204, 208 match the holes 11 and 12 of their adjacent ion exchange membranes 10 (see FIG. 1).

[0052] FIG. 8 illustrates two views 22 and 23 of the assembled spacer for use in electrodialysis cells, along with three sectional views E-E, F-F, and G-G. As noted above, each of the spacers 22, 23 are a combination of a gasket 200 and a central plastic mesh 206, as shown in FIGS. 7a and 7b. The spacers 22 and 23 are substantially the same if not identical mirror images of one another, with spacer 23 simply being spacer 22 flipped over or turned over along its longer side, as can be identified in FIG. 8 by the position of the triangular cuts 205. For use of the inventive spacers in any electrodialysis or electrodeionization cell, the dimensions of the spacers and the location of the holes 204, 208 should match the holes 11, 12 of the ion exchange membranes 10 (see FIG. 1), and the dimensions of the central plastic mesh 206 should match the cavities 42, 43 of the endplates 40, 41 (see FIG. 5), such that the holes 204 and protrusions 202 of the gasket 200, and the holes 208 and protrusions 207 of the central mesh component 210 can be supported by the endplates 40 and 41 which house the electrodes (not shown). Thus, in a completed stack, it is intended that the holes 11 and 12 of the ion exchange membranes 10 shown in FIG. 1 match up with the holes 204 and 208 of the spacers 22, 23 shown in FIG. 8, which match up with the water conveyance holes 54, 55, 56 and 57 at the top and bottom of the endplates 40 and 41 of FIG. 5, which in turn connect to water conveyance passages 50, 51, 52 and 53 entering from the sides of the endplates 40, 41.

[0053] With the dimensions of the ion exchange membranes, the spacers, and the endplates matching as described above, the assembly of electrodialysis stacks using the inventive spacer can begin with an end spacer (30, see FIG. 3), followed by a first ion exchange membrane (10, see FIG. 1). A first gasket 200 is then placed on the first ion exchange membrane with holes and dimensional boundaries matching the ion exchange membrane and the endplate 40 (see FIG. 5). This is then followed by placement of the central plastic mesh 206 of the first spacer placed within the open central area 220 of the first gasket 200. Experience has shown that when the underlying membrane is wet, as is the norm, the surface tension of the water on the membrane helps the gasket and the mesh to easily adhere to the membrane for the duration of the time it takes to place the next membrane on them. This operation is then followed by placement of an oppositely charged membrane on the previous membrane-spacer arrangement followed by placement of the next gasket 200 with the cut corner 205 placed on the opposite side, followed by completion of the spacer by placement of its central plastic mesh 206 within the open central area 220 of the newly placed gasket 200, as before. This pattern is then repeated until the stack is completed with an end spacer at the other side.

[0054] As noted above, if the gaps between the spacer mesh and the ion exchange membranes can be reduced/prevented, this would result in a reduced water flow cross-section, leading to higher flow velocities and more turbulence in the water flow between ion exchange membranes. This in turn would reduce or eliminate any membrane polarization effects, and increase the limiting current density. Thus, in order to achieve perfect sealing between the dilute compartments and the concentrate compartments, and also to the outside of the stack, the thickness of the central plastic mesh 206 and the gasket edge 201 after compression within electrodialysis stack should be substantially the same. This is done by careful selection of the materials, their thicknesses and compressibility. In practice, the central mesh is slightly thinner than the gasket edge prior to compression. The resulting stack will have sealing edges between adjacent membranes at the locations of the gasket protrusions 202, and easy flow of water into and out of the compartments through the mesh sheet protrusions 207. Further, with the thickness of the gasket edges 201 and the central plastic mesh 206 being the same within the assembled and compressed stack, the ion exchange membranes 10 will be better supported, as there will be a minimal or no gap between them and the central plastic mesh 206 of the spacer. Furthermore, an electrodialysis stack assembled using the inventive spacer can have better seal between the compartments while reducing the flow cross-sections in both sets of compartments, thus reducing polarization effects and improving limited current density.

[0055] For electrodeionization cells where the spacers need to also house electroactive media to facilitate the flow of ions, the thickness of the inventive spacer can be increased. This can be done by placement of a voluminous spacer gasket 70, as shown inn FIG. 9. The gasket 70 can have a thickness compatible with the intended volume of the resin beads, ranging in thickness from a minimum of 2.0 to 3.0 mm to more than 10.0 mm. The gasket can accommodate resin beads, or any other equivalent shape or form of electroactive media placed on top of the spacer shown in FIG. 8. Like the gasket 200 in FIG. 8, the gasket 70 of FIG. 9 includes an outer gasket edge 324, an open central area 323, and holes 311, 312 on the top and bottom of the edge 324. The gasket 70 is typically constructed by punching or cutting its shape from sheets of silicon rubber or similar elastomer material. The dimensions and the position of the holes 311, 312 on this voluminous gasket 70 are intended to match the holes on the spacer of FIG. 8, as well as those of the ion exchange membranes (e.g., see FIG. 1) and endplates, as described in detail above. For assembly of an electrodeionization stack and after an end spacer and the first membrane, the spacer embodiment 22 of FIG. 8 is placed and is then followed by a voluminous gasket embodiment of FIG. 9 on top of the spacer, creating the required volume for placement of electroactive material. Once the electroactive material is placed in, the next membrane with opposite polarity with respect to the previous membrane would be placed on top of this new spacer assembly, followed by the spacer embodiment 23 of FIG. 8, with reverse orientation compared to the spacer embodiment 22 of FIG. 8, followed by another voluminous gasket embodiment of FIG. 9 and placement of electroactive media. This sequence is then repeated until the stack is completed with another end spacer.

[0056] Test Results—A plurality of the spacers illustrated in FIG. 8 were manufactured, each having a gasket component 200 including an edge 201 around the perimeter of an open central area 220, protrusions 202, recesses 203, holes 204, and a cut corner 205. The spacers 22, 23 had a height of about 15.0 cm and width of about 12.5 cm from the top gasket edge to the bottom edge, with three (3) protrusions 202 on both the top and the bottom of each gasket component, each protrusion being about 1.5 cm long and about 1.2 cm wide and having hole at their center about 5.0 mm in diameter. The open central area 220 of each gasket component, for receipt of the central mesh sheet 206 (see FIGS. 7a, 7b and 8), was about 10 cm wide and 9 cm in height. The silicone rubber used was about 0.4 mm thick, had a shore hardness of about 50, and was supplied by the American Rubber products of Santa Ana California. The central mesh sheets were each made of a punched woven plastic and included a plurality of protrusions 207 that matched the number and dimensions of the recesses 203 and the open central area of the gasket 200, and holes 208 which were the same diameter as the holes 204 of the gasket component 200. See, e.g., FIG. 8. The mesh used was a woven mesh with a thickness of about 0.39 mm, made in China using polyethylene yarns and purchased from Skycan Manufacturing Ltd. of Saskatchewan, Canada.

[0057] These spacers were used in an electrodialysis cell equipped with fifty (50) anion exchange membranes (AEM, Type 12) and fifty-one (51) cation exchange membranes (CEM, Type 12), purchased from Fujifilm of the Netherlands. High density polyethylene endplates, which housed the capacitive electrodes, measured about 5.0 cm in thickness and had a height of 18.0 cm and a width of 15.5 cm. The endplates were tightened by eight (8) one-quarter inch (¼ inch) bolts torqued to 25 in-lbs (2.825 newton meters). This cell was tested with a pressure of 1.5 Bars on one set of compartments and free flow on the other side. There were no external leaks observed, and the internal leaks were less than a maximum one ml per minute, which was far less than for a similar cell using prior art spacers. This cell functioned as expected with feeds TDS values ranging 1200 ppm to 15000 ppm with total feed flows ranging from 25 to 10 liters per hour respectively.

[0058] While the present invention has been illustrated by the description of embodiments and examples thereof, it is not intended to restrict or in any way limit the scope of the appended claims to such details. Additional advantages and modifications will be readily apparent to those skilled in the art. Accordingly, departures may be made from such details without departing from the scope of the invention.