Compartments for electrodeionization apparatus

Abstract: The present invention provides compartments for electrodeionization apparatus including an arrangement including a plurality of stacked groups of compartments. Each group of compartments is defined in part by exterior membranes at first and second ends of the group, and each group includes a plurality of compartments including at least one ion-concentrating compartment positioned adjacent at least one ion-depleting compartment. The plurality of compartments are defined in part by spaced, alternating membranes permeable to ions of a first polarity and membranes permeable to ions of a second polarity opposite the first polarity. An exterior membrane of at least one group of compartments co-extensively contracts an exterior membrane of an adjacent group in the plurality of stacked groups of compartments. Each ion-concentrating and ion-depleting compartment contains an ion-exchange material. The compartments of the present invention may be used in any of a wide variety of electrodeionisation system to carry out any of a variety of methods for purification of a fluid, including those systems and methods described above.

Description: FIELD OF THE INVENTION The present invention relates to compartments for electrodeionization apparatus. BACKGROUND OF THE INVENTION The purification of liquids by reducing the concentration of ions or molecules in the liquid has been an area of substantial technological interest. Many techniques have been used to purify and isolate liquids or to obtain concentrated pools of specific ions or molecules from a liquid mixture. Well-known processes include distillation, electrodialysis, reverse osmosis, liquid chromatography, membrane filtration, ion exchange, and electrodeionization. Modern electrodeionization units are described, for example, in commonly-owned U.S. patents 5,308,466, issued May 3, 1994, and 5,316,637, issued May 31, 1994, both to Ganzi et al. One problem associated with many liquid purification devices is the formation of scale on various liquid-contacting surfaces. In the case of electrodeionization apparatus, for example, scale is known to form on ion-exchange membranes which define ion-depleting and ion-concentrating compartments as well as on ion-exchange resins contained within at least some of the compartments. Numerous methods to prevent or remove scale formation in electrical water purification apparatus have been attempted. For example, U.S. Patent No. 2,854,394 to Kollsman describes the use of polarity reversal in electrodialysis devices as a way to reduce clogging of pores contained in the membranes used in such devices. According to one aspect of the present invention, there is provided compartments for electrodeionization apparatus, comprising: an ion-depleting compartment containing an ion-exchange material and defined in part by a first membrane permeable to ions of a first polarity and a second membrane spaced apart from the first membrane and permeable to ions of a second polarity opposite the first polarity; an ion-concentrating compartment containing an ion-exchange material and defined in part by a third membrane permeable to ions of the first polarity and a fourth membrane spaced from the third membrane and permeable to ions of the second polarity, wherein the first membrane co-extensively contacts the third membrane. According to a second aspect of the present invention, there is provided compartments for electrodeionization apparatus, comprising: a plurality of stacked groups of compartments, each group defined in part by exterior membranes at first and second ends thereof, and each group comprising a plurality of compartments including at least one ion-concentrating compartment positioned adjacent to at least one ion-depleting compartment, the ion-concentrating and ion-depleting compartments each containing an ion-exchange material and the plurality of compartments being defined in part by spaced, alternating membranes permeable to ions of a first polarity and membranes permeable to ions of a second polarity opposite the first polarity, wherein an exterior membrane of at least one group co-extensively contacts an exterior membrane of an adjacent group. In one embodiment of the present invention the exterior membrane of at least one group and the exterior membrane of an adjacent group are each selectively permeable to ions of the first polarity. In another embodiment of the present invention, the exterior membranes of each group of compartments are each selectively permeable to ions of the first polarity. In a further embodiment of the present invention, the compartments of electrodeionization apparatus further comprise: a first electrode on a first side of the stacked group of compartments and in electrical communication therewith; a second electrode on a second side of the stacked group of compartments and in electrical communication therewith; an electrical source in electrical communication with the first and second electrodes, which, in combination with the electrodes and the plurality of compartments, defines an electrical circuit; and a fluid circuit constructed and arranged to establish fluid streams through said at least one ion-concentrating compartment and said at least one ion-depleting compartment and to recover a depleted fluid product from said at least one ion-depleting compartment. The compartments of the present invention may be used in any of a wide variety of electrodeionization systems to carry out any of a variety of methods for purification of a fluid, including those systems and methods described in our co-pending EP-95 916 389 and EP-A-1 044 717. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded view of an electrodeionization stage; FIG. 2 is a schematic view illustrating the operation of the stage illustrated in FIG. 1; FIG. 3a is a schematic sectional view of a preferred embodiment of the inventive apparatus of two separate electrodeionization subcompartments from adjacent stacked compartment groupings prior to being compressed; and FIG. 3b is a schematic sectional view of a preferred embodiment of the inventive apparatus of two separate electrodeionization subcompartments from adjacent stacked compartment groupings after being compressed. DETAILED DESCRIPTION OF THE INVENTION In accordance with the invention, an electrodeionization apparatus is provided including one or more electrodeionization stages, wherein each electrodeionization stage includes at least one ion-concentrating compartment positioned adjacent at least one ion-depleting compartment. Preferably, the inventive apparatus includes a series of ion-concentrating compartments which alternate with a series of ion-depleting compartments. The compartments are defined, in part, by spaced apart, alternating, anion and cation-permeable membranes. Each of the concentrating compartments and depleting compartments contains an ion exchange material such as a mixture of anion-exchange resin and cation-exchange resin. Representative suitable ion exchange materials include fibers, woven fibers, beads or the like. The inventive apparatus includes at least first and second electrodes provided in electrical communication with the ion-concentrating and ion-depleting compartments, and a source of electrical potential provided in electrical communication with the first and second electrodes which, in combination with the electrodes and the compartments, defines an electrical circuit. In this way, an electrical potential may be provided across the ion-concentrating and ion-depleting compartments, or across one or more electrodeionization stages including a plurality of such compartments, so that an ion-depleted product fluid stream may be established during apparatus operation. In one embodiment of a preferred electrodeionization apparatus, the concentrating and depleting compartments are configured to include a plurality of independent discrete subcompartments, each of which have a width of about 102mm (4 inches) or less, preferably between 13mm (0.5) and about 38mm (1.5 inches). The discrete subcompartments are formed by securing, such as by bonding, an anion-permeable membrane and a cation-permeable membrane to the periphery of compartment spacers each having ribs which extend across the thickness of and along the length of the compartments. Thus, each subcompartment is defined by a pair of ribs, an anion permeable membrane, and a cation permeable membrane. The subcompartments are each filled with ion exchange material. Preferred embodiments of the present invention include electrodeionization apparatus including one or more groups of compartments, or "dual compartments". Such dual compartments are described in U.S. Patent No. 4,956,071 to Giuffrida et al., issued September 11, 1990. The dual compartments are each defined by an odd number of selectively-permeable membranes, at least one depleting compartment, and at least one concentrating compartment, each of which compartments are divided into subcompartments, as described above. The ion-permeable membranes are arranged so that the anion-permeable membranes and the cation-permeable membranes alternate along the thickness of the dual compartments. Thus, the dual compartment can include one more cation-permeable membrane than anion-permeable membrane or can include one more anion-permeable membrane or cation-permeable membrane. The apparatus of the present invention may also comprise one or more electrodeionization stages. An exemplary stage suitable for use in the apparatus of the present invention is described in co-pending, commonly-owned U.S. patent application serial no. 07/938,329, filed August 28, 1992. In each stage, a stack of depleting and concentrating compartments is positioned between first and second electrodes. Each stage of the apparatus of the present invention could include a plurality of stacked groups of compartments, each group defined in part by exterior membranes at either end thereof. An exterior membrane of at least one group co-extensively contacts an exterior member of an adjacent group, and according to a particularly preferred embodiment, all the groups in a stack are arranged adjacent each other with exterior membranes of adjacent groups in co-extensive contact with each other. This arrangement is described more fully below with reference to FIGS. 3a and 3b. Each electrode is provided in electrical communication with an ion-permeable membrane defining a boundary of a stack of compartments. Such electrical communication may be effected by any known method, for example via an electrolyte in an electrode compartment containing the electrode and defined in part by the ion-permeable membrane. According to one preferred embodiment, each electrode is provided with an adjacent electrode spacer and an ion-permeable membrane wherein an electrolyte passes through the electrode spacer. Optionally, the electrode spacer can be filled with ion exchange resin as well. The electrode spacer may comprise an expanded sheet of material, a screen, or the like, and may be electrically conductive or electrically nonconductive. The magnitude and variability of the electrical resistance between the electrodes and the membranes immediately adjacent to them are important factors in the performance of electrodeionisation apparatus and the time to quality upon polarity reversal and upon restart from a standby condition. The electrode compartment electrical resistance is particularly important when electrodeionization apparatus is operated with pretreatment by reverse osmosis or is fed other feed streams of relatively low ionic content and is cases where the electrode compartment construction comprises spacers and screens that are not intrinsically conductive. In the case where feed water has a relatively low ionic content, such as downstream of reverse osmosis apparatus, ionic impurities are often predominantly weakly ionized compounds, such as carbonic acid, mixed with relatively low concentrations of strong ions and intermediate strength ions, such as sodium and bicarbonate ions, respectively. It is preferred to conserve and increase the relative concentration of strong ions within the apparatus and streams by the choice of the liquids fed to the electrode compartments and by the choice of membranes immediately adjacent to the electrodes. It is also preferred to minimize the variability of the electrode stream electrical resistance to avoid the need to control apparatus voltage. For apparatus that reverse polarity, it is preferred that the membrane be of the same type adjacent to each electrode. In such apparatus, the electrode stream conditions are the same for each half cycle of operation. The preferred choice for the feed to the electrode streams is the feed water. Feed water, although lower in ionic strength than the concentrating stream at steady state, is more consistent in concentration. Also, during polarity reversal and immediately upon apparatus startup, the feed stream is often higher in ionic strength than the concentrating stream. The remaining portion of each stage comprises a series of alternating depleting and concentrating compartments constructed as set forth herein. The liquid to be depleted of ions can be passed in parallel through each depleting compartment in each stage and a second liquid can be passed through each concentrating compartments in parallel in each stage in order to effect removal of ions from the first liquid in the depleting compartments and into the second liquid in the concentrating compartment. Referring to FIG. 1, purification center 150 comprises a plurality of stacked compartments bounded by a pair of opposed end plates 154. Adjacent to each end plate is an end block 156 which serves as a mounting for a ?air of opposed electrodes 158. Each electrode has a tab 160 which extends through its adjacent end block and end plate. In the perspective of FIG. 1, a lower electrode 158 is visible and the tab 160 of the upper electrode is visible. Adjacent to each electrode is an electrode spacer 162 having a plurality of ribs 164 which define fluid flow paths through the electrode spacer. An inert screen (not shown) can be positioned adjacent to the ribs to provide turbulent fluid flow therethough. A stack 166 of alternating ion-depleting and ion-concentrating compartments is positioned between the opposing electrode spacers 162. The stack 166 comprises a series of alternating anion exchange membranes 168 and cation exchange membranes 170 is secured to, and separated by, spacers 172. The alternating membranes 168, 170 and spacer 172 define individual ion-depleting and ion-concentrating compartments, 176 and 178, respectively. Each spacer 172 includes a plurality of ribs 174 to define flow paths through the ion-depleting or ion-concentrating compartments. Each of the compartments 176, 178 is filled with an ion exchange resin, preferably a mixture which comprises an anion exchange resin and a cation exchange resin. Holes 179 in end plates 154 and in end blocks 156 are designed to accommodate tie bars (not shown) which hold together and compress the plurality of stacked compartments of purification center 150. The spacers 172 are manifolded such that each of the compartments is in fluid communication with its like compartments. Thus, the ion-depleting compartments 176 are each in fluid communication with one another, and the ion concentrating compartments 178 are each in fluid communication with one another. The manifolding can be achieved by providing each spacer with a chamber inlet 180 and a chamber outlet 182 as well as pair of transfer channels 184 which allow fluid to be passed through the spacer from adjacent compartments without entering the chamber defined, in part, by that spacer. The chamber inlet 180 and chamber outlet 182 can be positioned on diagonally opposing corners of each spacer so that if a adjacent, identical, spacers are inverted, a pair of interleaved, but isolated flow paths through the stack 166 is formed. As such, it is possible to use a single spacer design which provides a stack having a plurality of ion-depleting compartments in fluid communication with each other, a plurality of ion-concentrating compartments in fluid communication with each other, but no fluid communication between the ion-depleting compartments and the ion-concentrating compartments. Of course, the ion-depleting compartments and the ion-concentrating compartments can be said to achieve some mutual communication via ions which transfer across the ion-exchange membranes. Each end plate 154 is provided with a plurality of ports to allow fluid flow through the device. Thus, the end plate can include a concentrate-inlet port 186, a diluting stream inlet port 188, a concentrate outlet port 190, a dilute product outlet port 192, an electrode inlet port 194 and an electrode outlet port 196. Additionally, an electrode contact port 198 is also provided to allow the tab 160 of each electrode 158 to project from the ends of the device. Referring to FIG. 2, the flow paths of the liquids in the various compartments of the purification center illustrated in FIG. 1 are explained. Shown is a two-stage device, although single or multiple-stage devices can be used as well. Liquid to be purified enters inlet 188, passes through depletion compartment 176, is then passed through a second depletion compartment 176 and is recovered from outlet 192. It is to be understood that liquid flow through the depletion compartments can be in one direction in each stage. Also, the liquid can be made to flow through more than one depletion compartment in each stage. In addition, the liquid effluent from a depletion compartment can be split into multiple streams and then passed through a second set of depletion compartments. Concentrating liquid is passed through inlet 186 through concentrating compartments 178 and thence through outlet 190 to drain. Liquid electrolyte is circulated through electrode compartments 210 and 212 from inlets 194 and is discarded to drain through outlets 196. In the apparatus depicted in FIG. 1 above, each ion-depleting and ion-concentrating compartment shared a common membrane wall with its adjacent compartment. Since each of the ion-depleting and ion-concentrating compartments is filled with ion exchange resin, separating the compartments if, for example, maintenance is needed, is difficult. One common approach to addressing this problem is to provide inert screen spacers periodically within the stack of ion-depleting and ion-concentrating compartments. However, that approach adds further complications since fluid and electrical communication must be maintained across the screens. As a result, complicated additional manifolding is required. The use of inert screen spacers can be avoided by providing each of the ion-depleting and ion-concentrating compartments as a separate, independent module maintained in a coextensively contacting relationship with its adjacent modules. As shown in FIG. 3a, each module 250, 250' includes a spacer 252, 252' having an anion exchange membrane 254, 254' mounted on one side and an opposing cation exchange membrane 256, 256' mounted on its opposite side. The membranes and the spacer define a compartment 258, 258' that is filled with an ion exchange medium 260, 260', preferably a mixture of anion and cation resins. Standing alone, the membranes of each module tend to bulge outward as a result of resin swelling. Although the swelling affects the intermembrane spacing, when the individual modules are held together under pressure as shown in FIG. 3b the resins are compressed and the membranes are forced into a parallel relationship with desired intermembrane spacing. As can be seen in FIG. 3b, the modules 250, 250' are forced together under pressure provided by adjacent modules and, ultimately, the end plates positioned at either end of the purification center stack. The pressure compresses the resin 260, 260' contained in each compartment 258, 258' and also forces the membranes into a parallel configuration conforming with the desired intermembrane spacing. Unlike conventional electrodeionization apparatus, in the configuration of FIG. 3b the cation membrane 256 of the first module 250 is pressed into coextensive contact with the cation exchange membrane 256' of the adjacent module 250'. Although not shown in the Figure, if additional modules were provided adjacent to module 250' the anion exchange membrane 254' of module 250' would be in coextensive contact with an anion exchange membrane of the adjacent module. Thus, unlike a configuration in which each compartment is separated by a single ion exchange membrane, in the configuration of FIG. 18b, each compartment is separated from the adjacent compartment by a pair of like, coextensively contacting membranes. Unlike the bipolar membrane configurations of some prior art devices (see, for example, U.S. patent no. 4,871,431 to Parsi), the apparatus of FIG. 3b results in what is, for all practical purposes, a single anion or cation exchange membrane of double thickness. Such a configuration offers performance that is equivalent to devices having the compartments separated by a single ion exchange membrane. Since, however, the apparatus of FIGS. 3a and 3b provides a modular design, the need to provide inner screens between inner portions of the stack is eliminated and, if desired, it becomes possible to replace even a single ion exchange compartment if so desired. Finally, since the resins contained within the module stack will be maintained under pressure on either side of the pairs of coextensively contacting like membranes, the resins act to prevent movement of the membranes and to maintain the desired intermembrane spacing across each ion exchange compartment. The following examples are intended to illustrate the benefits of the present invention. However, they are not intended to exemplify the entire scope of the invention. For example, although a particular polarity reversal protocol is exemplified, it may be advantageous in some circumstances to rearrange the order of protocol steps. Example 1: Exemplary Polarity Reversal Protocol Two identical 4 cell pair single stage electrodeionization apparatus were assembled in a configuration of the type shown in FIG. 1, each comprised of the following components: two 13mm (0.5 inch) thick aluminum end plates, two 13mm (0.5 inch) thick polypropylene electrode blocks, two iridium oxide coated titanium electrodes each separated from their nearest (cation permeable) ion exchange membranes by a 0.625mm (0.025 inch) thick woven polyester electrode compartment screen gasketed around the periphery by a frame of thermoplastic elastomer, an alternating stack of nine heterogeneous polyethylene-based cation and anion ion exchange membranes in a sequence beginning and ending with a cation membrane, each membrane adhered with adhesive to polypropylene intermembrane spacers (intermembrane spacing of about 2.4mm (0.1 inches) and flow path length of 330mm (13 inches), each spacer comprised of 3 subcompartments each of width approximately 30.5mm (1.2 inches), each subcompartment filled with Rohm and Haas Amberjet mixed ion exchange resin in a 60:40 anion to cation ratio by volume. The two apparatus were fed in parallel with a well water pretreated by a UV sanitizing treatment, followed by granular activated carbon, and followed by 0.5 micrometer nominal filtration. Each ion-depleting and each ion concentrating stream was fed in parallel in a downflow direction with no stream recirculation and each stream was operated at a flow rate of about 0.9 lpm. A DC voltage of about 19.5 volts was applied between the electrodes of each apparatus. The feed water to each apparatus contained about 235 ppm total dissolved solids (as CaCO 3 ), including 102 ppm calcium, 57 ppm magnesium, and 117 ppm bicarbonate ions, 9 ppm silica, and a pH of about 8.05. The product and concentrate outlets were substituted for one another and the polarity was reversed at 30 minute intervals on each apparatus. Over a 4 day period, the operating voltage was gradually increased to 41 volts on each apparatus in order to create a readily discernable pH shift within the apparatus. Beginning on the fourth day and every 24 hours thereafter, the flow directions of all streams on the first apparatus were reversed so as to alternate on a daily basis between the upflow and downflow directions. There was no flow reversal on the second apparatus and both apparatus continued identical polarity reversal sequences at 30 minute intervals. On day 4, purification levels in the ion depleting streams of both apparatus as monitored by electrical conductance were about 80%. Beginning with day 5 of operation, the ion depleting streams of both apparatus tended to increase in pH to a maximum of about 9.5 and then upon polarity reversal, the pH of the streams - now ion concentrating streams - tended to decrease to a minimum of about 6.6. Langelier saturation index calculations indicated that the concentrate streams had a consistently low scaling propensity, and that at the high pH extreme, the depleting streams had a high scaling propensity. On day 5 of operation, the purification performance of the two apparatus began to differ - the first maintained approximately 80% purification through day 12 and the second apparatus gradually declined to about 60% purification. During this time, the first apparatus maintained an electrical resistance of about 25 ohms, and the second apparatus gradually increased in electrical resistance to about 31 ohms. After day 12, the test was ended and the apparatus were disassembled. Little or no scale was observed on the membrane or electrode surfaces. Scale was detected in both apparatus within the interstices of the resin bead filler. There was considerably less scale within the filler of the first apparatus. Example 2: Second Exemplary Polarity Reversal Protocol Two identical 4 cell pair single stage electrodeionization apparatus as described in Example 1 were fed in parallel with water pretreated by reverse osmosis. The first apparatus was fed in an upflow direction and the second apparatus was fed in a downflow direction. Each stream operated at a flow rate of about 1.1 lpm. Each concentrating stream was recirculated and makeup water was fed at a rate of about 0.1 lpm. The feed conductivity of the ion depleting stream and concentrate makeup was about 13 microsiemen-cm -1 and contained mixed ionic salts, approximately 4 ppm dissolved carbon dioxide, and approximately 250 parts-per-billion (ppb) dissolved silica. A DC voltage of about 20 volts was applied between the electrodes of each apparatus. Polarity was reversed every 24 hours by performing the steps of: (1) shutting off power (at time = 0); (2) substituting the depleting and concentrating streams for one another (at time = 10 seconds); (3) reversing polarity (at time = 15 seconds); and (4) turning on power (at time = 20 seconds). Product water was continuously obtained throughout the cycle. Immediately prior to initiation of the polarity reversal protocol, the product water electrical resistivity was about 17 megohm-cm and the silica concentration was about 7 ppb for both apparatus. The product water resistivity for both apparatus declined to a minimum of about 6 megohm-cm for both apparatus at time = 1 minute. For the first and second apparatus the maximum silica concentration was about 164 ppb and 260 ppb respectively, also occurring at time = 1 minute. The first apparatus and second apparatus attained a product water resistivity of about 15 megohm-cm at time = 25 and 45 minutes respectively, and they attained a purity of 8 ppb silica at time = 6 minutes and 15 minutes respectively. Example 3: Third exemplary Polarity Reversal Protocol An electrodeionization apparatus was constructed in a manner identical to that described in Example 1 except that the cell compartments were filled with Dow Marathon ion exchange resin filler. The apparatus was fed permeate water from a reverse osmosis apparatus. Each ion-depleting and each ion-concentrating stream was fed in parallel in a downflow direction and each diluting and concentrating stream was operated at a flow rate of about 0.2 lpm per cell pair. A DC voltage of about 20 volts was applied between the electrodes of the apparatus. The feed water to the apparatus contained about 4 ppm total dissolved solids (as CaCO 3 ), including 2.5 ppm carbon dioxide and 1.5 ppm mixed ions. (i) In a first polarity reversal protocol, considered as a control test indicative of background art, the ion-depleting and ion-concentrating streams were operated with no concentrate recirculation and the streams were interchanged at approximately the same time as the electrical polarity was reversed; (ii) In a second polarity reversal protocol, the ion-concentrating stream was recirculated and combined at the concentrate inlet of the apparatus with a feed of RO permeate. Combined concentrate was removed from the recirculating stream at a rate such that the overall product water recovery from the apparatus was approximately 90% of the total RO permeate being fed to the apparatus. The streams were interchanged at approximately the same time as the electrical polarity was reversed; (iii) In a third polarity reversal protocol, the streams were fed and recirculated in a manner identical to example (3,ii). The reversal protocol comprised the following steps: (a) the DC power was switched off; and (b) the streams were interchanged, the electrical polarity was reversed, and the DC power was reapplied at about the time that the product resistivities of the streams were equal; (iv) In a fourth polarity reversal protocol, the streams were fed and recirculated in a manner identical to example (3,ii). The reversal protocol comprised the following steps: (a) the feed and product stream flows were modified to operate in a manner identical to example (3,i); (b) after operating in mode (iv, a) for about two minutes the streams were interchanged and the electrical polarity was reversed; and (c) the stream flows were returned to a manner identical to example (3,ii) at about the time that the product resistivities of the streams were equal; (v) In a fifth polarity reversal protocol, the streams were fed and recirculated in a manner identical to example (3,ii). The reversal protocol comprised the following steps: (a) the feed and product stream flows were modified to operate in a manner identical to example (3,i); (b) after operating in mode (v, a) for about two minutes, the DC power was switched off; (c) the stream flows were interchanged and the DC power reversed and reapplied at about the time that the product resistivities of the streams were equal; and (d) the stream flows were returned to a manner identical to example (3,ii) after operating in mode (v,c) for about 10 minutes; and (vi) In a sixth polarity reversal protocol, the streams were fed and recirculated in a manner identical to example (3,ii). The reversal protocol comprised the following steps: (a) RO permeate feed to the recirculating concentrate stream was shut off; (b) a concentrated aqueous sodium chloride solution was fed to the recirculating concentrate stream in an amount sufficient to create a 2% salt solution within the concentrating stream; (c) after operating in mode (vi,b) for about 5 minutes, the feed and product stream flows were modified to operate in a manner identical to example (3,i); (d) after operating in mode (vi,c) for about 2 minutes, the streams were interchanged and the DC polarity was reversed; and (e) at about the time that the product resistivities of the streams were equal, the stream flows were returned to a manner identical to example (3,ii). For each polarity reversal protocol test, water electrical resistivity was monitored and product water was continuously collected from the stream producing the best water quality. Product water was collected at steady state operation prior to the initiation of the reversal cycle, during the time throughout the reversal cycle, and subsequent to the reversal cycle until steady state was reachieved. Table I below tabulates the length of time that the product water quality was lower than given resistivity values at any time during the continuous operation of the apparatus. Resistivity Time Duration Lower Than Resistivity Value (hours) Value For test protocol: (meqohm-cm ) #1 #2 #3 #4 #5 #6 4 0 0 0 0 0 0 6 0.7 0 0 0 0 0.5 8 2.1 0 0 0 0 0.7 10 3.8 0 0 0 0 1.0 12 4.9 0 0 0 0 1.1 14 6.3 1.6 0.5 0 0 1.6 16 8.7 4.4 1.0 0 0 2.6 Protocol Number Description of time = 0 Time of lowest Product Resistivity (hours) 1 Polarity reversal 5.1 2 Polarity reversal 3.2 3 Power off 24 4 Remove recirculation 21 5 Remove recirculation 7.2 6 Sodium chloride addition 0.1 Example 4: First comparison of electrodeionization apparatus with and without cell pair groupings Two apparatus were assembled. The first comprised components as described in Example 1, except that it comprised two cell groupings of two cell pairs each, and except that the resin filler was Dow Marathon ion exchange resin. Each cell grouping was comprised of five alternating cation and anion membranes beginning and ending with a cation membrane. The second apparatus was identical to the first except that it comprised five cell pairs in one cell grouping comprised of eleven alternating cation and anion membranes beginning and ending with a cation membrane. Each apparatus was fed with water pretreated by reverse osmosis containing 4 ppm calcium, 6 ppm dissolved carbon dioxide and 296 ppb silica at a flow rate to the ion depleting streams of 0.2 lpm per cell. The concentrating streams were recirculated with no make-up feed water at a flow rate of 0.2 lpm per cell. A DC voltage of 3 volts per cell pair was applied to each apparatus. After 24 hours operation the first apparatus produced product water with a resistivity of 11 megohm-cm, apparatus amperage was 0.58 amps, and the concentrate stream contained 140 ppm calcium. The second apparatus produced product water with a resistivity of 7 megohm-cm, apparatus amperage was 0.78 amps, and the concentrate stream contained 120 ppm calcium. After 72 hours operation, both apparatus were disassembled. Scale was found within the concentrating cells of both apparatus. There was no scale in the first apparatus at the interface between the cell pair groupings. Example 5: Second comparison of electrodeionization apparatus with and without cell pair groupings Two apparatus were assembled and fed water as described in Example 4, except that the recirculating concentrating streams were fed makeup water at a rate of about 0.02 lpm per concentrating cell. The first and second apparatus were operated at 15 volts and amperage was about 0.60 and 0.58 amps, respectively. After 8 days operation, product water electrical resistivity was about 16.6 megohm-cm for the first apparatus and about 15 megohm-cm for the second apparatus. On the ninth day of operation, depleting and concentrating streams were substituted for one another, followed by reversing the polarities of each apparatus. On the eleventh day of operation, product water electrical resistivity was about 17 megohm-cm for the first apparatus and about 15 megohm-cm for the second apparatus. Upon termination of the test, the first apparatus was disassembled and there was no scale or any sign of membrane damage at the interface between the cell pair groupings. Equivalents Those skilled in the art will readily appreciate that all parameters listed herein are meant to be exemplary and that actual parameters will depend upon the specific application for which the inventive electrodeionization apparatus is being used. It is, therefore, to be understood that the foregoing embodiments and examples are presented by way of example only, and that, within the scope of the appended claims, the invention may be practised otherwise than as specifically described.

Claims: Compartments for electrodeionization apparatus, comprising: an ion-depleting compartment (176) containing an ion-exchange material and defined in part by a first membrane (256) permeable to ions of a first polarity and a second membrane (254) spaced apart from the first membrane and permeable to ions of a second polarity opposite the first polarity; (178) an ion-concentrating compartment containing an ion-exchange material and defined in part by a third membrane (256') permeable to ions of the first polarity and a fourth membrane (254') spaced from the third membrane and permeable to ions of the second polarity, wherein the first membrane co-extensively contacts the third membrane. Compartments for electrodeionization apparatus, comprising: a plurality of stacked groups of compartments, each group defined in part by exterior membranes at first and second ends thereof, and each group comprising a plurality of compartments including at least one ion-concentrating compartment (178) positioned adjacent to at least one ion-depleting compartment (176), the ion-concentrating and ion-depleting compartments each containing an ion-exchange material and the plurality of compartments being defined in part by spaced, alternating membranes permeable to ions of a first polarity and membranes permeable to ions of a second polarity opposite the first polarity, wherein an exterior membrane of at least one group co-extensively contacts an exterior membrane of an adjacent group. Compartments for electrodeionization apparatus of claim 2, wherein the exterior membrane of at least one group and the exterior membrane of an adjacent group are each selectively permeable to ions of the first polarity. Compartments for electrodeionization apparatus of claim 2 wherein the exterior membranes of each group of compartments are each selectively permeable to ions of the first polarity. Compartments of electrodeionization apparatus of claim 2, which further comprise: a first electrode on a first side of the stacked group of compartments and in electrical communication therewith; a second electrode on a second side of the stacked group of compartments and in electrical communication therewith; an electrical source in electrical communication with the first and second electrodes, which, in combination with the electrodes and the plurality of compartments, defines an electrical circuit; and a fluid circuit constructed and arranged to establish fluid streams through said at least one ion-concentrating compartment (178) and said at least one ion-depleting compartment (176) and to recover a depleted fluid product from said at least one ion-depleting compartment (176).

Inventors: GALLAGHER CHRISTOPHER J, GANZI GARY C, WILKINS FREDERICK, GALLAGHER, CHRISTOPHER J., GANZI, GARY C., WILKINS, FREDERICK
Applicants: UNITED STATES FILTER CORP, UNITED STATES FILTER CORPORATION
Application year: 1995
Main classification: B01D61
IPC codes: B01D61, C02F1, B01J47