Abstract:
An electrodialysis method and apparatus include a source of concentrate fluid, a source of dilute fluid, a collector of treated concentrate fluid, a collector of dilute fluid, an anode and a cathode. A plurality of generally planar spacers are interleaved with a plurality of membranes to define a plurality of cells providing electrically conductive fluid connection between the anode and the cathode. Each of the spacers comprises a gasket that defines a first aperture and a second aperture. Each of said first and second apertures define an independent cell between interleaved membranes. The symmetrical, multiple split cell spacer configuration channels fluid flow through two or more narrow and elongated paths. The split cell arrangement allows for operation of the stack in parallel or in series. The invention improves the ion removal efficiency of a given membrane area, requires significantly less energy than other electrodialysis systems and substantially reduces stack assembly, materials and fabrication costs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/080,302, filed Feb. 21, 2002 now abandoned, and also claims priority to PCT Application, Ser. No. PCT/US03/05185, filed Feb. 21, 2003. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention pertains to a method and apparatus for the purification and reuse or disposal of polluted liquids. 
     More particularly, this invention pertains to an electrodialysis stack for the removal and concentration of ions from aqueous solutions and certain aqueous/organic solutions. 
     2. Description of the Related Art 
     There are presently a number of systems for treating and recycling aqueous and aqueous/organic waste streams on the market. Present state of the art systems, including de-ionization methods that are available to industrial waste stream generators, are deficient in their ability to consistently and economically produce a cleansed fluid of sufficient quality that can be continuously recycled and reused, especially in the case of small to medium volume liquid waste generation. The initial high cost of purchasing many of these systems is beyond the economic resources of many businesses, thus prohibiting cost-effective recycling for environmental compliance or beneficial reuse. 
     Multi-cell electrodialysis stacks are normally built up of membrane sheets separated from each other by suitable gaskets. For efficient separations, the distance (gap) between the sheets is as small as possible. In most designs, a spacer is introduced between the individual membrane sheets, both to assist in supporting the membrane and to help control the liquid flow distribution. The stacks for most electrodialysis processes are assembled in the same fashion as a plate-and-frame filter press, the gaskets corresponding to the frames and the membrane sheets corresponding to the plates. The manifolds that are needed to distribute the process fluids to the various compartments or channels are formed by ingenious patterns of mating holes and slots punched in the gaskets and sometimes in the membranes themselves, prior to assembly of the stack. Several different gasket and spacer materials and arrangements and channel geometries have been utilized or proposed. 
     In typical electrodialysis systems, the flow pattern within each compartment (i.e., between any two successive membranes) is determined by the configuration of the spacer element used between the membranes. Two distinctively different flow arrangements are typically used. One is known as the tortuous-path design; the other makes use of the sheet-flow principle. The most serious design problem for both flow arrangements for multi-membrane and multi-cell stacks is that of assuring uniform fluid flow to the various compartments and effective transport of the ions to the membrane surfaces. These difficulties are the major obstacles to simple, single stage demineralization of brackish liquids. 
     In particular, reducing concentration polarization is one of the most important design issues for electrodialysis. Concentration polarization is the reduction of ion concentrations near the membrane surface compared to those in the bulk solution flowing through the membrane compartment. With substantial concentration polarization, electrolytic water splitting in order to provide the requisite electric current carriers through the membranes occurs due to the deficiency of solute ions adjacent to the membranes that can carry the current. This water splitting is extremely detrimental to electrodialysis efficiency. The tendency of concentration polarization to take place at the surface of the membranes is due to the hydrodynamic characteristic of channel flow, in which there is a central turbulent core of flow bounded by thin viscous boundary layers adjacent to the confining surfaces. These viscous boundary layers impose a resistance to the passage of ions much greater than that of a layer of like thickness in the turbulent core, and hence increase the likelihood of polarization at the membrane surfaces. Polarization is objectionable not only from the standpoint of the inefficient increase in energy consumption, but also the change of pH of the concentrate stream as a result of water splitting, which tends to cause scale deposition. 
     When dealing with fluids with very low total dissolved solids (TDS), back diffusion can take place. Back diffusion occurs when the ion concentration in the concentrate stream is substantially higher than the ion concentration in the de-mineralized stream. The result is that some of the ions from the concentrate stream diffuse back through the membrane, against the force of the DC potential, into the de-mineralized stream. 
     The number of cells in a stack is limited mainly by the practical considerations of assembly and maintenance requirements. Since the failure of a single membrane can seriously impair stack performance, the necessity to be able to disassemble and reassemble a stack to replace a membrane, and the necessity to be able to perform this quickly and easily, effectively limits the number of membranes that can be practically utilized in a stack. As a result, it is often desirable to use several smaller modular-size stacks rather than one large one. This problem has been attacked by using several small subassemblies or packs containing about 50 to 100 cell pairs (CP), and arranging as many as 10 of these packs in series in a single clamping press. A single set of electrodes may be used for the entire assembly (stack) or several electrodes may be used to provide electric staging. However, use of single electrodes for larger assemblies typically causes end-cell heating that results in rapid membrane deterioration. 
     The present invention serves to expand the possible applications of electrodialysis in that it represents an efficient, small scale electrodialysis system with a configuration allowing cost-effective small-scale applications, while making the large scale applications even more cost-competitive than they currently are. 
     In accordance with the present invention, a unique gasket design reduces hydraulic pressure drop across the cell stack assembly by eliminating narrow inlet/outlet manifold cutouts inherent with conventional designs. The reduction of hydraulic pressure permits the use of higher flow rates that further reduce concentration polarization, as well as thinner membranes, resulting in improved desalting efficiency, especially for sparingly conductive solutions, and also less sensitivity to the presence of suspended matter. 
     The novel multiple split cell design can be operated in parallel as a roughing de-mineralizer (or operated in a batch recirculation mode) or operated in series allowing for single-pass continuous flow. When operated in the series mode, the split cell design permits separate voltage and flow control when a higher purity fluid is desired. The split cell design permits separate cell control of concentrate stream salinity content. The roughing cell may be operated with a higher concentrate stream TDS, with the salinity of the polish cell concentrate stream correspondingly reduced to the salinity content of the de-mineralized stream. This prevents back diffusion and allows for efficient removal of ions in feed water of low TDS. In short, the split cell design incorporates the benefits of hydraulic and electrical staging without the inherent complexity and expense of commercial electrodialysis systems. 
     The split cell design minimizes the voltage potential across the stack, thereby reducing end-cell heating that leads to membrane deterioration. 
     It is an object of the present invention to provide a simpler stack assembly of low production cost. Stack assembly cost is reduced as a result of the novel split cell/gasket geometry. A reduced number of expensive machined components are required. Simpler and lighter components lower material costs for a given membrane area. Inexpensive center bolts provide an alternative to typical hydraulic force application arrangements, which also improves the uniformity of the clamping force distribution on the gasket area. Threaded bolts also reduce assembly labor time, i.e., it is easier to hold the configuration in place and also facilitate change-out of membranes when they are spent, as the cell geometry reduces stress on the end points as is found inherent with some conventional stack assemblies. 
     It is another object of the present invention to provide an apparatus and method that allows for the cost-effective arrangement of two or more split membrane cells that enables the ingenious arrangements of plumbing for optimizing deionization processing. 
     The cell gasket geometry can be more easily and inexpensively fabricated from a larger range of materials in comparison to conventional designs, allowing the process to be used in more harsh environments through the use of a wide range of chemically resistant materials. It is still another object of the present invention to provide an apparatus and method that combines a unique arrangement of small to intermediate scale unit operations for the economical recovery/reclamation of a wide range of fluids and that can also be scaled to a large system size, further improving the economics of large scale electrodialysis systems by reducing both capital and operating costs. 
     BRIEF SUMMARY OF THE INVENTION 
     According to one embodiment of the present invention, a dialysis stack is provided in which each generally planar gasket defines a first cell and a second cell. A membrane is located adjacent to each side of each gasket. A turbulence spacer is located within each cell. Each cell is provided with an inlet and an outlet to provide fluid access into and out of each cell. Fluid flows sequentially through the two cells defined in each gasket. Preferably, the fluid flows through a plurality of first cells defined by a plurality of spacers and then flows through a plurality of second cells defined in the plurality of spacers. Separate anodes and cathodes provide electrical energy to the two parallel sets of first cells and second cells. Separate rectifiers can be used to apply specific electric potential across the first set of cells and second cells when operated in series, or a single rectifier can power both the first and second cell sets when operated in parallel. A system of bolts extending through parallel compression plates are used to secure the plurality of spacers and interleaved membranes in register to define conduits extending between the plurality of cells. 
     The electrodialysis stack is included in an electrodialysis system. The system includes a mixing tank for the solution being processed. Mixed solution is passed through filters for removing particulate matter and potential precipitants. The filtered solution is collected in a dilute tank. Concentrated fluid is collected in a concentrate tank. Electrolyte is provided from an electrolyte tank to an anode chamber and to a cathode chamber. The anode chamber and the cathode chamber have an electrically conductive fluid connection through the cell stack. The membranes alternate between anion exchange membranes and cation exchange membranes. The cells alternate between concentrate stream cells and dilution stream cells. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The above-mentioned features of the invention will become more clearly understood from the following detailed description of the invention read together with the drawings in which: 
         FIG. 1  is a laterally exploded view of a cell stack embodying various of the features of the present invention; 
         FIG. 2  is a plan view of a cell stack embodying various features of the present invention; 
         FIG. 3  is a side elevation view of a cell stack embodying various features of the present invention; 
         FIG. 4  is an end elevation view of a cell stack embodying various features of the present invention; 
         FIG. 5  is an elevation view of an electrodialysis system embodying various features of the present invention; 
         FIG. 6  is a flow diagram of an electrodialysis system embodying various features of the present invention; 
         FIG. 7   a  is a schematic diagram of split cell spacer having two cells arranged in series; 
         FIG. 7   b  is a schematic diagram of split cell spacer having two cells arranged in parallel; 
         FIG. 7   c  is a schematic diagram of split cell spacer having three cells arranged in series; 
         FIG. 7   d  is a schematic diagram of split cell spacer having three cells arranged in parallel; 
         FIG. 7   e  is a schematic diagram of split cell spacer having four cells with two parallel cells arranged in series with two parallel cells; 
         FIG. 7   f  is a schematic diagram of split cell spacer having four cells arranged in series; and 
         FIG. 7   g  is a schematic diagram of split cell spacer having four cells with three cells arranged in parallel arranged in series with a single polish cell. 
         FIG. 8  is a chart of Current Utilization Efficiency. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, wherein similar reference numbers denote similar elements throughout the several drawings, there are disclosed a method and an apparatus for electrodialysis treatment of a fluid in which a salt is dissolved. One example of such a fluid is used antifreeze, which can be cleaned and recycled in accordance with the present invention. 
     In  FIG. 1  there is illustrated one embodiment of an electrodialysis cell stack  10 , exploded laterally. At one end is an electrode stream spacer  12  defining two rectangular apertures  14  and  14 ′ separated by partition  17  having aligned holes  69  therein for insertion of threaded members such as bolts  68 . In the depicted embodiment the electrode stream spacer is approximately 14 inches by 24 inches, though it will be recognized that various sizes may be used. Also defined in the electrode stream spacer  12  are eight conduit apertures  16   a ,  16   b ,  16   c ,  16   d ,  16   e ,  16   f ,  16   g  and  16   h.    
     Adjacent to the electrode stream spacer  12  is an anion exchange membrane  18 , many of which are well known in the art. One commercially available material is Neosepta AFN produced by Tokuyama Corporation. The anion exchange membrane  18  is shaped and sized substantially identically to the electrode stream spacer  12  and includes conduit apertures  19   a–h  in register with the conduit apertures  16   a–h  defined in the electrode stream spacer  12 . The anion exchange membrane  18 , and similar shaped and sized membranes  29 ,  40 ,  52  described herein, have aligned holes  69  along a mid-length in register with holes  69  defined in electrode stream spacer  12 . 
     Adjacent to the anion exchange membrane  18  is a concentrate split cell spacer  20  defining two apertures  22  and  22 ′ separated by partition  63  having aligned holes  69  therein. Each of the apertures  22  and  22 ′ has the shape of an abbreviated rectangle in which two squares have been removed from diagonally opposed corners and all corners have been rounded. Conduit apertures  24   a ,  24   c ,  24   e  and  24   h  are defined in the concentrate split cell spacer. The concentrate split cell spacer  20  is shaped and sized substantially identical to the electrode stream spacer  12 . The aperture  22  is in register with the rectangular aperture  14  and the aperture  22 ′ is in register with the rectangular aperture  14 ′. 
     A concentrate turbulence spacer  28  is located within the aperture  22  and a concentrate turbulence spacer  28 ′ is located within the aperture  22 ′. Each of the concentrate turbulence spacers  28  and  28 ′ are formed from a mesh to maintain turbulence within the apertures  22  and  22 ′ as concentrate fluid passes through the apertures  22  and  22 ′. 
     Adjacent to the concentrate turbulence spacer  28  is a cation exchange membrane  29 , many of which are well known in the art. One commercially available material is Neosepta CMX produced by Tokuyama Corporation. The cation exchange membrane  29  is shaped and sized substantially identical to the electrode stream spacer  12  and includes conduit apertures  30   a–h  in register with the conduit apertures  16   a–h  defined in the electrode stream spacer  12 . 
     Adjacent to the cation exchange membrane  29  is a dilution stream split cell spacer  32  defining two apertures  34  and  34 ′ separated by partition  65  having aligned holes  69  therein. Each of the apertures  34  and  34 ′ has the shape of an abbreviated rectangle in which two squares have been removed from diagonally opposed corners and all corners have been rounded. The apertures  22  and  22 ′ are mirror images of the apertures  34  and  34 ′. Conduit apertures  36   b ,  36   d ,  36   e  and  36   g  are defined in the dilution stream split cell spacer  32 . The dilution stream split cell spacer  32  is shaped and sized substantially identically to the electrode stream spacer  12 . The aperture  34  is in register with the rectangular aperture  14  and the aperture  34 ′ is in register with the rectangular aperture  14 ′ to provide electrically conductive fluid connection to the apertures  14  and  14 ′, respectively. 
     A dilution stream turbulence spacer  38  is located within the aperture  34  and a concentrate turbulence spacer  38 ′ is located within the aperture  34 ′. Each of the concentrate turbulence spacers  38  and  38 ′ is formed from a mesh to maintain constant turbulence within the apertures  34  and  34 ′ as dilution fluid passes through the apertures  34  and  34 ′. 
     Adjacent to the dilution stream turbulence spacer  38  is an anion exchange membrane  40 , which is identical to anion exchange membrane  18 . The anion exchange membrane  40  defines conduit apertures  42   a–h  in register with the conduit apertures  16   a–h  defined in the electrode stream spacer  12 . 
     Adjacent to the anion exchange membrane  40  is a concentrate split cell spacer  44  defining two apertures  46  and  46 ′ separated by partition  67  having aligned holes  69  therein. The concentrate split cell spacer is identical to the concentrate split cell spacer  20  and defines conduit apertures  48   a ,  48   c ,  48   f  and  48   h . The aperture  46  is in register with the rectangular aperture  14  and the aperture  46 ′ is in register with the rectangular aperture  14 ′ to provide electrically conductive fluid connection to the apertures  14  and  14 ′, respectively. 
     A concentrate turbulence spacer  50  is located within the aperture  46  and a concentrate turbulence spacer  50 ′ is located within the aperture  46 ′. Each of the concentrate turbulence spacers  50  and  50 ′ is formed from a mesh to maintain constant turbulence within the apertures  46  and  46 ′ as concentrate fluid passes through the apertures  46  and  46 ′. 
     Adjacent to the concentrate turbulence spacer  50  is a cation exchange membrane  52 , many of which are well known in the art. The cation exchange membrane  52  is shaped and sized substantially identically to the electrode stream spacer  12  and includes conduit apertures  54   a–h  in register with the conduit apertures  16   a–h  defined in the electrode stream spacer  12 . 
     Adjacent to the cation exchange membrane  52  is an electrode stream spacer  56  defining two rectangular apertures  58  and  58 ′ separated by partition  61  having aligned holes  69  therein. The electrode stream spacer  56  is substantially identical to the electrode stream spacer  12 . Also defined in the electrode stream spacer  56  are eight conduit apertures  60   a–h , which are in register with the conduit apertures  16   a–h  respectively. 
     A first end section  62   b  of the aperture  22  overlays the conduit apertures  19   b  and  30   b  to cooperatively define a concentrate outlet port for the aperture  22 . A diagonally opposed second end section  62   e  overlays conduit apertures  19   e  and  30   e  to cooperatively define a concentrate inlet for the aperture  22 . A first end section  62   d  of the aperture  22 ′ overlays the conduit apertures  19   d  and  30   d  to cooperatively define an outlet port for the aperture  22 ′. A diagonally opposed second end section  62   g  of the aperture  22 ′ overlays the conduit apertures  19   g  and  30   g  to cooperatively define an inlet port for the aperture  22 ′. 
     A first end section  64   a  of the aperture  34  overlays the conduit apertures  30   a  and  42   a  to cooperatively define a dilution outlet port for the aperture  34 . A diagonally opposed second end section  64   f  overlays conduit apertures  30   f  and  42   f  to cooperatively define a dilution inlet for the aperture  34 . A first end section  64   c  of the aperture  34 ′ overlays the conduit apertures  30   c  and  42   c  to cooperatively define an outlet port for the aperture  34 ′. A diagonally opposed second end section  64   h  of the aperture  34 ′ overlays the conduit apertures  30   h  and  42   h  to cooperatively define an inlet port for the aperture  34 ′. 
     A first end section  66   b  of the aperture  46  overlays the conduit apertures  42   b  and  54   b  to cooperatively define a concentrate outlet port for the aperture  46 . A diagonally opposed second end section  66   e  overlays conduit apertures  54   e  and  42   e  to cooperatively define a concentrate inlet for the aperture  46 . A first end section  66   d  of the aperture  46 ′ overlays the conduit apertures  42   d  and  54   d  to cooperatively define an outlet port for the aperture  46 ′. A diagonally opposed second end section  66   g  of the aperture  46 ′ overlays the conduit apertures  42   g  and  54   g  to cooperatively define an inlet port for the aperture  46 ′. 
     In  FIGS. 2 and 3  the cell stack  10  is depicted as it is mounted with threaded bolts  68  between an opposed pair of electrolyte flow distribution endplates  70   a  and  70   b . Preferably, the bolts  68  are coated with a plastic or other high electrically resistant material. The threaded bolts  68  are arranged around the periphery of the end plates  70   a  and  70   b  and also, or alternatively, extend through the holes  69  aligned in each partition providing separation space between the split cells as shown in  FIG. 4 . As depicted in  FIG. 5 , a cathode  72  extends through the endplate  70   a  and an anode  74  extends through the endplate  70   b . A rectifier  75  applies a potential between the cathode  72  and the anode  74 . An electrolyte solution supplied to the endplates  70   a  and  70   b , a concentrate stream sequentially supplied to the apertures  22 ,  22 ′,  46 ′ and  46  and a dilution stream sequentially supplied to the apertures  34  and  34 ′ provide electrically conductive fluid connection between the cathode  72  and the anode  74 . 
     The split-cell spacers comprise EPDM (ethylene propylene diene terpolymer) sold under the name Nordel by E. I. Du Pont de Nemours and Company. When assemble and secured with threaded bolts  68  no glue or other adhesive is required between the membranes and the spacers. 
     Referring now to  FIG. 6 , there is depicted a flow diagram of an electrodialysis system adapted for using the cell stack described hereinabove. The system is portable and may be easily moved to locations where fluids require cleaning. For example, used antifreeze is stored in a mixing tank  76 , where it is mixed with a metal reducing agent to precipitate metals in the fluid. The mixing tank  76  is in flow communication with a desalinated tank  78  through a filter pump  80 , a 1 micron filter  82 , a carbon adsorber  84  and a second 1 micron filter  86 . 
     The desalinated tank  78  is in flow communication by conduits, through a pump  88  to the conduit apertures  60   f ,  54   f ,  48   f  and  42   f  (in series) to enter the inlet of aperture  34 . The outlet of the aperture  34  is connected in flow communication with the inlet of the aperture  34 ′ by a conduit  91 . The outlet of the aperture  34 ′ is in flow communication with the desalinated tank by conduit apertures  30   c ,  24   c ,  19   c  and  16   c.    
     An electrolyte is stored in an electrolyte rinse tank  90 , which is connected through conduits to the inlets  92   a  and  92   b  of the end plates  70   a  and  70   b , respectively. The outlets  94   a  and  94   b  from the endplates  70   a  and  70   b , respectively, are connected back to the electrolyte rinse tank  90 . A pump  96  circulates the electrolyte. 
     A pump  100  sends concentrated brine from a concentrate brine tank  98  through the conduit apertures  60   e  and  54   e  to enter the inlet of aperture  46 . From the outlet of the aperture  46  the brine is directed through the conduit apertures  42   b ,  36   b  and  30   b  to the inlet of the aperture  22 . From the outlet of the aperture  22  the brine is directed through the conduit apertures  19   e  and  16   e , a conduit  102 , and conduit apertures  60   g  and  54   g  to the inlet of aperture  46 ′. From the outlet of the aperture  46 ′ the brine is directed through the conduit apertures  42   d ,  36   d  and  30   d  to the outlet of the aperture  22 ′. From the outlet of the aperture  22 ′ the brine is directed back to the concentrated brine tank  98  via the conduit apertures  19   g  and  16   g . The concentrated brine tank  98  is in flow communication with a concentrate neutralization tank  104 . 
     In operation, electrolyte is circulated between the electrolyte rinse tank  90  and the end plates  70   a  and  70   b . The pH of the electrolyte is monitored for maintenance in a generally constant range. As required, neutralization acid may be added from the tank  104 . 
     Concentrated brine is circulated from the tank  98 , sequentially through the apertures  46 ,  22 ,  46 ′ and  22 ′ and then back to the tank  98 . The concentration of the brine is monitored for maintenance in a generally constant range. As required, water may be added to the tank  98 . A “feed and bleed” mode of operation is provided for make-up water. The pH is also monitored and controlled. 
     The fluid to be cleaned, such as used antifreeze, is entered into the mixing tank  76  where a stirrer  106  maintains agitation of the fluid with a metal reducing agent. The fluid is then pumped through the filter  82 , the carbon adsorber  84  and the filter  86  to the desalinated tank  78 . The fluid is circulated from the desalinated tank  78 , sequentially through the apertures  34  and  34 ′, and then back to the desalinated tank. As is well recognized in the field of electrodialysis, the potential applied between the cathode  72  and anode  74  induce the ions of salts in the fluid to pass through the membranes into the brine solution passing through the adjacent aperture, thus increasing the concentration of salts in the brine solution and reducing the concentration of salts in the treated fluid. By cycling the fluid repeatedly through the apparatus, the concentration of salts can be reduced to the desired minimal level. A conductivity sensor  108  monitors the fluid as it leaves the pump  88  to determine when a satisfactory level has been reached. A control panel  110  provides visual readouts and controls for operating the system. 
     EXAMPLE 1 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Conventional 
                 Multi-path 
               
               
                   
                   
                 Multi- 
                 Split Cell 
               
               
                   
                   
                 Compartment 
                 (Operated 
               
               
                   
                 Parameter 
                 Stack 
                 in parallel) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Glycol content % w. 
                 40.0 
                 40.0 
               
               
                   
                 Glycol Retention % 
                 91.8 
                 99.9 
               
               
                   
                 Starting conductivity (μMho/cm) 
                 3,800 
                 3,800 
               
               
                   
                 Finish Conductivity (μMho/cm) 
                 1,000 
                 1,000 
               
               
                   
                 Cell pair Voltage (V) 
                 1.0 
                 1.0 
               
               
                   
                 Membrane type 
                 Conventional 
                 Conventional 
               
               
                   
                 Solution temperature (° F.) 
                 76 
                 76 
               
               
                   
                 Production Rate 
                 0.44 
                 2.0 
               
               
                   
                 (m 3 /day/m 2  of membrane) 
               
               
                   
                 Gasket Material 
                 EPDM 
                 EPDM 
               
               
                   
                   
               
               
                   
                 (The anion exchange membrane used was Neosepta AFN produced by Tokuyama Corporation. The cation exchange membrane used was Neosepta CMX produced by Tokuyama Corporation.) 
               
             
          
         
       
     
     The multi-path split cell system was substantially less costly to produce than the conventional multi-compartment stack, yet operated at a production rate over four times greater. 
     Studies indicate that the configuration of the invention is a substantial improvement over traditional designs. Example 2 shows the production rate and typical % removal of NaCl for the current invention; those skilled in the art will recognize these values allow the invention to be economically competitive for a variety of feeds. Example 3 shows typical membrane area and energy requirements for desalination using traditional ED stack designs contrasted with the performance of the current invention. Those skilled in the art will recognize that the improved design of the current invention results in a stack requiring significantly less membrane area and that is significantly more energy efficient. 
     EXAMPLE 2 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 1. NaCl Feed 
                 Production Rate 
                 % NaCl 
               
               
                   
                 Concentration 
                 (m 3 /m 2  day) 
                 Removal 
               
               
                   
                   
               
             
             
               
                   
                 1.65 g/L 
                 5.74 
                 91 
               
               
                   
                 16.5 g/L 
                 1.50 
                 99 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 3 
     
       
         
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 NaCl Feed 
                 Traditional 
                   
                 % 
               
               
                   
                 Concentration 
                 Designs** 
                 Split cell 
                 Reduction 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 A. Membrane Area (m 2 ) for 1 m 3 /day Capacity* 
               
             
          
           
               
                   
                  1 g/L 
                 0.3 
                 0.17 
                 42% 
               
               
                   
                 10 g/L 
                 1.2 
                 0.67 
                 44% 
               
             
          
           
               
                 B. Energy Requirements (kw-hr/m 3  product)* 
               
             
          
           
               
                   
                  1 g/L 
                 1.2 
                 0.26 
                 78% 
               
               
                   
                 10 g/L 
                 3.4 
                 2.67 
                 21% 
               
               
                   
                   
               
               
                   
                 *For a product concentration of 500 ppm TDS. 
               
               
                   
                 **Source: Strathmann, H., “Design and Cost Estimates”, in Membrane Handbook, pp. 246–254, W. S. W. Ho and K. K. Sirkar, eds., Van Nostrand Reinhold, New York (1992). 
               
             
          
         
       
     
     An important variable describing an ED system is the current utilization efficiency. The current utilization efficiency is primarily influenced by the ED stack design and flow velocities but also to a lesser extent by the concentration and composition of the feed stream. For a given ED stack (gasket design, spacer design, etc.) and feed stream, the current efficiency is [1,2,3,4,5]: 
                   ξ   =           zFQ   f     ⁡     (       C   inlet   d     -     C   outlet   d       )       NI     ×   100   ⁢   %             (   1   )               
where
     ξ=current utilization efficiency, %   z=charge of ion   F=Faraday&#39;s constant, 96,485 Amp-s/mol   Q f =diluate flow rate, L/s   C d   inlet =diluate ED cell inlet ion concentration, mol/L   C d   outlet =diluate ED cell outlet ion concentration, mol/L   N=number of cell pairs   I=applied current, Amps.   
     Those skilled in the art will recognize that current utilization efficiencies should be &gt;70% for efficient use of ED for desalting typical brackish water feeds, and that current utilization decreases as the product water concentration decreases. Chart 1 shows that the invention provides excellent current utilization efficiencies (&gt;90%) over a wide range of product water concentrations. The figure also shows that good current utilizations are achieved even when producing high quality product (&lt;5 mg/L Cl − ). Also, studies indicate that the invention is capable of producing a product with extremely low conductivity levels (down to as low as 2.6 μMho/cm). Those skilled in the art will recognize that this represents a substantial improvement compared to traditional ED designs, which are typically limited to product with conductivities &gt;30 μMho/cm. As a result, the invention would represent a new pretreatment option for production of ultrapure water. 
     While the depicted embodiment has been described in terms of three split cell spacers and four membranes, it will be recognized that additional split cell spacers and membranes are desirable to speed the process. Such additional apparatus would function in substantially the same manner. 
     As depicted schematically in  FIGS. 7   a  to  7   g , the split cells may be arranged with more than two cells and the cells may be arranged in a variety of parallel, serial and parallel/serial arrangements.  FIG. 7   a  depicts the arrangement described herein above.  FIG. 7   b  depicts an arrangement wherein the two split cells are arranged in parallel.  FIG. 7   c  depicts a split cell having three apertures that are arranged serially.  FIG. 7   d  depicts a split cell having three apertures that are arranged in parallel.  FIG. 7   e  depicts a split cell having four apertures that are arranged with two parallel cells arranged serially with another set of parallel cells.  FIG. 7   f  depicts a split cell having four apertures that are arranged serially.  FIG. 7   g  depicts a split cell having four apertures with three cells arranged in parallel and all three serially feeding the fourth cell. It will be recognized by those skilled in the art that the multiple cells may be arranged in a variety of ways to accommodate many different electrodialysis situations. 
     Benefits of the process include a recovery rate in excess of 95%, high throughput and low capital and operating cost. The system does not generate hazardous by-products. It is easy to operate, control and automate, and easy to maintain. Also, studies indicate that the invention is capable of producing a product with extremely low conductivity levels (down to as low as 2.6 μMho/cm). Those skilled in the art will recognize that this represents a substantial improvement compared to traditional ED designs, which are typically limited to product with conductivities &gt;30 μMho/cm. As a result, the invention would represent a new pretreatment option for production of ultrapure water. 
     The multi-path split-cell spacer design permits use of a single or multiple central bolts, eliminating the need for an expensive hydraulic clamping assembly for applying central pressure on the stack and providing a uniform force distribution over the gasket area, improving the seals between membranes and improving ion removal efficiency, while also reducing assembly labor time. Expensive machined components are replaced with simpler, lighter components having lower material costs, for a given membrane area assemblies. 
     In addition to the described use of the method and apparatus to clean used antifreeze, the system may be used to clean and/or recycle: wash water (vehicular, laundry, mop water, trailer/tank washout, textile rinses, metal, aqueous parts cleaners), oil and gas field fluids (glycol base natural gas dehydration fluids, glycol/water heat transfer fluids, amines from treatment of natural gas, produced water), other thermal transfer fluids (secondary coolants from HVAC systems and coolants from ice-skating rinks), cooling water reuse, nuclear wastewater, mixed (nuclear and hazardous) wastewater, hazardous wastewater, desalination of sea or brackish water, drinking water production and pretreatment for ultra-pure water production. 
     While the present invention has been illustrated by description and while the illustrative embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept.