Abstract:
The electrodes of the described CDI cell are porous and permeable. The liquid to be deionized (e.g. salt water to be desalinated) flows through the electrodes. The electrodes are arranged in a stack, alternating anode/cathode, and water being treated passes through every electrode in the whole stack. For regeneration, the cells are connected (short-circuited) together, and the ions are dislodged mainly by flushing action. The through-flow arrangement can be realized in a number of different configurations.

Description:
[0001]    This technology relates to the removal of dissolved contaminants from a liquid, and will be described as it particularly relates to the desalination of salt water. 
       BACKGROUND TO THE INVENTION 
       [0002]    It is known to desalinate salt water by Capacitive Deionization (CDI) (also sometimes known as Electrostatic Deionization). The process basically consists in passing the saltwater between a pair of electrodes, each of large surface area, between which a DC voltage is applied. Positive ions (e.g. Na+ ions) migrate to the cathode, and negative ions (e.g. Cl− ions) migrate to the anode. The adsorbed ions are then bound to the respective electrodes. From time to time, the stored ions are removed from the electrodes by an appropriate regeneration process. 
         [0003]    Typically, in the conventional CDI cells, the electrodes are in the form of flat plates or sheets of e.g. activated carbon. Salt water flows along the space between the plates, the ions being attracted to the appropriate electrode by electrostatic forces. Thus, the ions are adsorbed onto the respective electrodes from the passing water. 
         [0004]    A conventional CDI-based treatment apparatus generally includes several of the cells, arranged in a stack of cells, and includes suitable structure for mounting the electrodes of the individual CDI cells, and for conveying the water into and through the spaces between the electrodes. 
         [0005]    Ions are adsorbed into the porous material of the electrodes, and are retained and stored therein, whereby the effluent water from the CDI cell is less salty than the influent water. 
         [0006]    For regeneration, usually the flow of salt water undergoing treatment is switched off, or re-routed, and a flow of regeneration water is now passed through the CDI cell. (In some cases, the regeneration water can be the same salt water.) Traditionally, the polarity of the cells is reversed, whereby the adsorbed ions are repelled from the electrodes, and enter the regeneration water. Typically, regeneration is carried out a few times per hour, and the regeneration process is typically completed in a few minutes. The treatment/regeneration cycle preferably should be automated. 
         [0007]    The salt content of the effluent regeneration water is usually considerably higher than (e.g. ten times) that of the salt water being desalinated. Where the salt water is drawn from the sea, the high-salt regen-water is simply discharged into the sea. If disposal in the sea is not available, further treatment of the concentrate stream might be required; however, the volume of the concentrate is typically only about five percent of the treated water stream. 
         [0008]    Conventional CDI cells may or may not be provided with charge-barriers, which are ion-permeable membranes that are impervious to water, and placed over one or both of the electrodes. Charge barriers are aimed at preventing contamination of the electrode pore volume with the source water and to prevent re-adsorption of the ions during regeneration. 
       THE INVENTION IN RELATION TO THE PRIOR ART 
       [0009]    In the traditional CDI cells, the liquid to be deionized flows through the cell, through the space between the anode and the cathode, in a direction parallel to the plane of the electrodes. This arrangement may be described as the traditional flow-by configuration. 
         [0010]    In the new CDI treatment systems as described herein, the water passes through the electrodes themselves. The water passes through the space between the electrodes in the direction predominantly at right angles to the plane of the electrodes. That is to say, the velocity vector of the water has a predominant component that lies at right angles to the plane of the electrodes. This arrangement may be described as the through-flow configuration. 
         [0011]    The electrode being in the form of a thin sheet of porous material, the sheet having opposed sides, the liquid (e.g. salt water) to be deionized flows right though the pores of the electrode, from the upstream side to the downstream side. Therefore, in the present technology, the electrode must have a sufficient degree of permeability to permit the desired through-flow of water. 
         [0012]    One benefit of the through-flow configuration is that the anodes and cathodes can be comparatively much closer together. In the traditional flow-by configuration for CDI cells, the space between the electrodes has to be large enough for the water to flow parallel to the plane of the electrodes. The closer spacing of the electrodes permitted in the new through-flow configuration means a stronger electrostatic field for a given voltage. 
         [0013]    Since they are generally impermeable, charge-barriers are contra-indicated for use with through-flow electrodes. However, the problem that charge barriers are aimed at curing, i.e. to prevent re-adsorption of the ions during regeneration is less significant when the water passes through anode, then cathode, then anode, then cathode, many times. The omission of charge barriers is advantageous from the cost and complexity standpoint. 
     
    
     
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
         [0014]    The technology will now be further described with reference to the accompanying drawings, in which: 
           [0015]      FIG. 1  is diagram of a two-electrode CDI cell, in which the flow of to-be-treated salt water through the cell is arranged in the through-flow configuration. 
           [0016]      FIG. 2  is a similar diagram of a stack of electrodes, arranged in the through-flow configuration. 
           [0017]      FIG. 3  is a diagram showing the arrangement of some of the components of the apparatus associated with the stack of  FIG. 2 . 
           [0018]      FIG. 4  is a diagram, similar to  FIG. 1 , showing another arrangement of CDI cells having the through-flow configuration. 
       
    
    
       [0019]    The scope of the patent protection sought herein is defined by the accompanying claims. The apparatuses and procedures shown in the accompanying drawings and described herein are examples. 
         [0020]      FIG. 1  shows a single CDI cell  20 . A DC voltage of (typically) 1.3 volts is supplied to the electrodes  23 A, 23 C, whereby  23 A is an anode and  23 C is a cathode. Water to be desalinated is passed through the cell  20  from left to right, as indicated by the arrows  27 . 
         [0021]    The electrodes  23  are made of a high-surface-area porous material, such as activated carbon. The electrodes  23  are prepared from carbon in the form of flat sheet of a constant thickness; in the example, the thickness is 0.5 millimetres. Also, in the example, the electrode is 5,000 square centimetres (0.5 sq.metres) in area. 
         [0022]    The electrode  23  contains a mesh structure  29 , or grid of wires, which is attached to (or embedded in) the carbon material. The wires are of titanium, or other material that is substantially inert in saltwater. The grid serves the dual purposes of providing mechanical support for the carbon material and for even distribution of current, and of smoothing out any voltage differences and gradients that might otherwise be present in the electrode  23 —activated carbon being not so conductive, electrically, as titanium. 
         [0023]    The electrodes  23 A, 23 C are identical as to structure. The electrodes are held apart by an electrode spacer  30 , sufficiently that the anode and cathode cannot touch each other and thereby make an electrical short circuit. The spacer  30  is made of a suitably-inert plastic, which is structured to hold the electrodes apart, substantially without inhibiting the through-flow of water through the cell. In the example, the spacer  30  is of an open-weave structure. 
         [0024]    As shown in  FIG. 2 , a number of the cells  20  may be arranged as a stack  32  of cells; or rather, the electrodes  23  may be arranged as a stack of electrodes. In  FIG. 2 , the electrodes  23  and the spacers  30  are so arranged as to form an intercalated, anode-spacer-cathode-spacer-anode-spacer-cathode-spacer-etc, configuration. In the example, all the odd-numbered electrodes in the stack are connected together electrically and are so charged as to become anodes, while all the even-numbered electrodes are connected together and so charged as to become cathodes. Alternatively, pairs of electrodes can be connected electrically in series. In the example, the stack includes a hundred anodes  23 A, a hundred cathodes  23 C, and a hundred-ninety-nine spacers  30 . 
         [0025]    In  FIG. 2 , salt water to be treated is fed into the stack at a water-inlet-port  34 , located to the left. Treated water, having passed through the stack  32  of electrodes, is discharged through a water-outlet-port  36 , located to the right. 
         [0026]    The electrodes  23 , though very porous, nevertheless have a relatively low permeability to the through-flow of water (i.e. a low ability to conduct through-flow)—to the extent that a considerable hydraulic pressure is required (from e.g. a pump  38 —see  FIG. 3 ) in order to force the water through the stack  32  of electrodes  23  and spacers  30 . 
         [0027]    The designers would typically aim for the stack  32  as a whole to be of such resistance to the desired magnitude of flowrate that the pressure head between the inlet-port  34  and the outlet-port  36  is between about five pounds/sq.inch (thirty-five kN/m2) per hundred electrodes in the stack and about thirty psi. Below about five psi per hundred electrodes, the water will pass through the electrodes too quickly, whereby the residence time per electrode would be too short for adequate and efficient removal of the ions. Above about thirty psi, the energy needed to pump the water through the stack makes the process start to become uneconomic. 
         [0028]    Thus, in  FIG. 2 , where there are two hundred electrodes  23 , the designer should so arrange the permeabilities of the electrodes  23  that the overall pressure drop through the stack, at the desired flowrate, is between about ten psi and sixty psi. 
         [0029]    It is assumed, in the above, that the electrode-spacers  30 , by contrast, offer only a negligible resistance to the flow of water through the stack. If the spacers  30  do have significant resistance, the pump pressure would be increased accordingly. 
         [0030]    The considerations re hydraulic pressure above apply during treatment. Other factors apply during regeneration. Especially during regeneration, it can be advantageous to use suction to aid the flow of regeneration water through the electrode stack. 
         [0031]    The cells as described are effective to lower the salination percentage of water passing through the cell over the whole range of salination, from seawater having about four percent (40,000 ppm) salt, through brackish water at about one percent salt, to almost-pure water. Thus, the treatment system can be tuned to a particular salt-removal requirement simply by adding or removing electrodes to or from the stack. The water should be passed through the electrodes in the stack one after the other; that is to say, the water being treated is routed through the CDI cells on an in-series-flow basis. 
         [0032]    The ability of the described system simply to use more or fewer of the same components to cater for a variety of treatment conditions can also be understood in relation to changing the magnitude of the liquid flow. Of course, if more water needs to be treated, extra facilities are required. However, this need not be a matter of adding further whole, separate, systems. Rather, the designers can often effect economic savings, when the use of several stack units is contemplated, by arranging the stack units in parallel from the standpoint of dividing and treating the liquid flow, and in series from the standpoint of electrical energization. 
         [0033]    It will be understood that, in  FIG. 2 , every pair of adjacent electrodes in the stack can be regarded as an individual CDI cell, irrespective of whether the salt water engages the pair anode-first or cathode-first. Preferably, but not essentially, the number of anodes should exactly equal the number of cathodes, or rather, preferably the effective aggregate area of all the cathodes should equal the effective aggregate area of all the anodes. 
         [0034]      FIG. 3  shows the control system for operating the apparatus, diagrammatically. The apparatus is capable of being operated in the treatment condition, or in the regeneration condition. The controller  40  is set up so as to cycle between the two conditions. In the treatment condition, salt water requiring desalination is routed (via pipe  43 ) to the inlet-port  34 , and the treated water from the outlet-port  36  is conveyed away (via pipe  45 ) to a storage tank  47 . 
         [0035]    In the regeneration condition, the controller connects (shorts) all the electrodes  23  together, so that all are at the same voltage. Regeneration water is now passed through the stack. The regeneration water is routed (via pipe  49 ) into the inlet-port  34 . The ions, now released from the electrodes, are picked up by and in the regeneration water, and conveyed out of the outlet-port  36 . The regeneration water is then routed for disposal (via pipe  50 ). 
         [0036]    The controller is arranged to operate cyclically between the treatment and regeneration conditions. The period of time for treatment, per cycle, is TT. The period for regeneration is TR. In the example, TT is five minutes, and TR is two minutes. The designers wish to keep TR as short as possible, and they wish to use as little regeneration water as possible, since both the time and the water represent inefficiencies in the overall operation of the apparatus. 
         [0037]    Generally, the designers will wish to optimize the design of the components of the stack from the standpoint of operating efficiency during the treatment part of the cycle, and will usually arrange for the water to be fully treated in just one pass through the stack. That being so, during regeneration, it might be necessary for the regeneration water to be circulated and recirculated through the stack, for the most cost-effective compromise between effective regeneration of the electrodes versus the amount of regeneration water required and the time TR. Also, in some cases, the designers might wish to employ recirculation of the salt water during the treatment period. 
         [0038]    As mentioned, for regeneration of the through-flow CDI cells and electrodes as described herein, the electrodes are all connected, i.e. shorted, together. This may be contrasted with regeneration in a traditional CDI cell with charge-barriers, where the flow of water is parallel to the electrode. In that traditional case, the designers arrange for the polarity of the electrodes to be reversed, during regeneration, so that the ions that have been adsorbed into the electrodes are positively repelled, electrostatically, out into the stream of regeneration water. In the traditional cell, if the electrodes were simply shorted, with no repulsive component, the ions would only enter the regeneration water stream by diffusion, which would be very inefficient. 
         [0039]    However, in the case of a traditional CDI cell without charge barriers, by contrast, the practice has been to short the electrodes together during regeneration, and that practice is followed in the systems described herein. 
         [0040]    In the present case, the adsorbed ions are positively flushed out of the pores of their home electrode by the physical velocity of the regeneration water passing through those same pores. In fact, with the through-flow configuration, it would be disadvantageous to reverse the polarity of the electrodes—in that, although the ions might be repelled, electrostatically, from their home electrode, they would be quickly re-adsorbed into the adjacent electrode. In the through-flow configuration, the ions have to travel right through the stack, or rather, they have to travel through all the porous electrodes between their home electrode and the outlet. Thus, through-flow regeneration can be expected to be more efficient than traditional parallel-flow regeneration, just as through-flow treatment can be expected to be more efficient than traditional parallel-flow treatment. 
         [0041]    Other arrangements of the electrodes are possible, using the through-flow configuration.  FIG. 4  is a version in which the velocity vector of the incoming salt water at first is parallel to the upstream electrode  54 , but then the vector assumes a component at right angles to the electrode, and the flow passes through the electrode-spacer  30  in that direction. As the cleaned water emerges from the downstream electrode  56 , its vector once again becomes parallel to the electrodes. The cleaned water passes out between the two electrodes. 
         [0042]    In this specification, some of the components and features in the drawings are given numerals with letter suffixes, to indicate anode, cathode, etc, versions thereof. The numeral without the suffix is used herein to indicate the component generically. 
         [0043]    The numerals that appear in the accompanying drawings can be summarized as:—
         20  CDI cell     23  electrode     23 A anode     23 C cathode     27  flow path arrow     29  wire mesh current collector     30  electrode spacer     32  stack of electrodes     34  water inlet port     36  water outlet port     38  water pump     40  controller     43  pipe—salt water in     45  pipe—treated water out     47  storage tank     49  pipe—regen water in     50  pipe—regen water out     54  upstream electrode     56  downstream electrode