PURIFICATION OF A LIQUID STREAM

Abstract: Method and apparatus for purifying a liquid stream comprising at least two ion exchange material beds (17, 18), one of anion exchange material and the second of cation exchange material. The beds (17, 18) are connected at at least one point such that a junction (4) which is permeable to ions is formed therebetween. Liquid to be purified is passed serially through one bed than the other. A pair of electrodes (2, 3) is provided one each being associated with a respective material bed such that at least some ion exchange material is sandwiched between each electrode (2, 3) and the junction (4), and an electrical potential is applied across the pair of electrodes (2, 3) so as to produce an electric field therebetween and regenerate the ion exchange material (10, 11). In another form the beds may contain a mixture of anion and cation exchange material. A method of forming a junction between the beds is also disclosed.

Description: Title: Purification of a Liquid Stream Technical Field The present invention relates to a method and apparatus for the purification of water containing streams using ion exchange materials and the regeneration of said material electrochemically. Background of the Invention and Prior Art There is an enormous market for high purity water. It is used extensively in industrial applications, with notable major users including the pharmaceutical, electronics and power generation industries. In the final polishing step for most applications, dissolved salts are commonly removed from the water by passage through ion exchange resins. A combination of a cation exchange resin (in the H form) and an anion exchange resin (in the OH " form) are used to remove the cations and anions respectively. Two such beds may be used in tandem for the removal of salts. If the aqueous solution is first passed through a bed of cation exchange resin (in the H + form), the cations in the solution are taken up by the cation exchanger, and an equivalent amount of H ions are released into solutions (thus preserving electroneutrality). This (now acidic) solution is then passed through a bed of anion exchange resin (in the OH " form), and the anions in the solution are taken up by the anion exchanger. The OH " that is released, neutralises the H in the solution, forming water. The dissolved salts have effectively been removed from the solution. (If the sequence of the desalting beds is reversed, a corresponding scenario, with an intermediate alkaline solution, still applies). However, for a number of applications, a mixed bed of ion exchange resin is preferable, either in the place of the tandem bed configuration, or in addition to it, as a final polishing step. In this case, the cation and anion resins are intimately mixed, and the cations and anions are removed from the solution at the same time, with immediate neutralisation of the H and OH " that are released by the resin. It is possible to achieve a greater level of salt removal from aqueous solution with this configuration, since the chemical equilibrium process is "driven" by the loss of H + and OH " by mutual neutralisation. In either configuration, once the resins are exhausted (i.e. once there is no more resin in the H + and OH " form), the cation and anion beds must be regenerated by treatment with acid (typically H 2 SO 4 ) and alkali (typically NaOH) respectively. This regeneration involves the storage and handling of concentrated acid and caustic solutions, as well as the disposal of the effluent regeneration and wash solutions after they have been used to treat the resins. This is undesirable from the point of view of cost, safety and environmental considerations. In the present application, we describe a method of regenerating the resin using mild, inexpensive and ecologically acceptable electrochemical techniques. This novel process does away with the regenerating chemicals, and has the added advantage that the water purification process and ion exchanger regeneration can be carried out in the same vessel, with no need to disturb the resin bed. In the case of the mixed bed systems, this process also has the advantage that the regeneration can be carried out on the resin beds without needing to separate the cation exchange resin from the anion exchange resin. For the conventional chemical regeneration processes, the cation and anion resins in the mixed resin bed must be physically separated before chemical regeneration (using acid and alkali respectively) can be carried out. Prior Art The use of conventional ion exchange technology, with chemical regeneration of the exhausted resin, remains the principal means of removing dissolved salts from water to very low concentration levels. The following patents and patent applications illustrate the prior art relating to the present invention. Electrodeionization apparatus (TJS Patent 4.632.745. Intl. Patent WO 92/1 1089 The Elix™ Electrodeionization Process, (marked by Millipore TM), is used to deionise solutions almost to the level of purity attained by conventional mixed bed ion exchange systems. In its simplest form, the system comprises four alternating cation (c) and anion (A) permeable membranes sandwiched between a pair of electrodes ( + and -), in the configuration + | C | A | C | A | -. The space between the central pair of membranes is filled with mixed bed ion exchange resin (principally in order to improve electrical conductivity). The feed water flows down between the sheets of ion exchange membrane (which allow the flow of relevant ions but not water), perpendicular to the applied electric field. The dissolved cations in the feed solution (in the central cavity) pass through the cation permeable membrane towards the cathode, while OH " ions generated at the cathode pass through the anion permeable membrane. A concentrated stream of hydroxide salt of the cations passes out through this (concentrating) chamber. Likewise, the dissolved anions in the feed solution (in the central cavity) pass through the anion permeable membrane towards the anode, while H ions generated at the anode pass through the cation permeable membrane. A concentrated stream of the acid form of the anions passes out through this (concentrating) chamber. The water flowing out of the central chamber is thus depleted of dissolved salts, while the (waste) water flowing out of the two outer chambers contains the cations and anions from the feed solution. A bank of such cells can be used to generate water that has a low concentration of dissolved ions. In this case, the intermediate concentrating chambers have anions and cations entering through the membranes on opposite sides of the chamber, and concentrated salt solutions (rather than concentrated acid and alkali solutions) flow out through these chambers. It is only from the outermost chambers (that house the electrodes) that acid and alkali solutions are released in this configuration. Although this system is also used for polishing water, the technology is totally different from that of the present invention. Furthermore, the modules involve a stack of individual compartments, each of which has a complex design. The large number of membranes increases the potential for leakage (even the smallest rupture in one membrane renders the system ineffective) from the concentrating compartment into the pure water compartment, as well as providing a large membrane surface area with a potential for fouling and being relatively expensive. The current invention has significant advantages both in simplicity of operation, reliability and lower cost. Electrochemical deionisation (US Patent 5.584.981) This is very similar to the above invention, except that the ion exchange material is structured as a porous layer, so that water from the feed solution passes through the sheets of ion exchanger along with the ions. Purified ion exchange resi and process (US Patent 5.259.936) The resin to be purified is placed in the ion-depletion chambers of the "Elix" type configuration above (US Patent 4,632,745). Once again, this process operates using extensive banks of ion exchange membranes. Process for purifying resins using a bipolar interface (US Patent 5.211.823) This patent discloses a method for the regeneration of ion exchange resins within the compartments of an electrodeionization apparatus of the kind described in US Patent 4,632,745. However, each of the dilution compartments (comprising a compartment defined by an anion and a cation exchange membrane), is further divided by a bipolar membrane, with an anion permeable face directed towards the anion permeable membrane, and the cation permeable face directed towards the cation permeable membrane. The two subcompartments thus formed are filled with anion exchange resin and cation exchange resin respectively. Under the influence of an electric field (+ve potential applied on an anion exchange side, -ve potential applied on the cation exchange side), water is split into OH " and H + at the bipolar membrane, and these ions migrate through the anion and cation resin beds respectively, replacing the associated anions and cations. These flow out of the compartment through the anion and cation permeable membranes, and into concentrating chambers that are formed by sandwiching a series of such cells in parallel. Concentrated solutions of salt are thus removed, leaving the anion and cation resins in the OH " and H + forms respectively. In the terminal concentrating chambers (in which the electrodes are housed, and where OH " and H are generated), the cations and anions are removed as the OH " and H + forms respectively. Once again, this system is exceedingly complex, comprising multiple layers of anion, bipolar and cation membranes, with their attendant potential for leakage and fouling. Furthermore, this system, in reality, is an electrodialysis system rather than an electrochemical regeneration system, as it relies principally on the electric field and the bipolar membrane to generate the H + and OH " that are used for regeneration, rather than producing them as a consequence of the electrolysis of water. Process and device for demineralising aqueous solutions using ion exchangers (US Patent 5.423.965. This patent discloses a process for the electrochemical regeneration of a cation exchange resin bed placed between a pair of electrodes, with the H ions displacing the cations. These move toward the cathode, and combine with the OH " produced there. This alkaline solution is then used to chemically regenerate the anion exchanger in a traditional manner. However, here the regeneration of the two beds of resin does not take place together as a result of the simultaneous generation of the H and OH " coupled to their electrically assisted migration through the resin bed. It more closely resembles the electrochemical regeneration of a single resin bed, with the effluent then being used as a conventional regenerant of the other bed. Electrochemical ion exchange (UK Patent Appl. GB 2.178.759A: Appl. No. 8614401 ;. This patent application discloses the electrochemical regeneration of a single bed of either cation or anion exchange resin. It involves establishing an electrochemical cell with the working electrode surrounded by ion exchange resin, which is held within a compartment permeable to the ions generated at the electrode (and therefore, also the ions exchanged from the resin). The whole is immersed in an electrolyte in which is placed the counter electrode to complete the electrochemical cell. In the case of the working electrode being the anode, H + is generated, and displaces ions associated with the cation exchange resin surrounding the electrode, and these migrate through the ion permeable compartment and combine with the ions generated at the cathode (presumably OH " ). The regenerated resin is then removed and is ready for use. A similar system of opposite polarity can be used to regenerate anion exchange resin. This patent application also involves electrochemical regeneration, but the novelty of the system of the present invention resides in the simultaneous regeneration of a pair of anion and cation exchange resin beds due to the generation of OH " and H at the cathode and anode respectively, and the consequent efficient use of all the generated ions, and simultaneous regeneration of a complete deionising system. Even in the mixed bed form of the present invention, in any one cycle, the anion resin is one mixed bed and the cation resin is the other mixed bed, are being simultaneously regenerated. Process and Device for Regenerating Ion-Exchange Material (Intl. Patent Application No. WO 89/00453) This patent application discloses a process for the regeneration of an ion exchange resin by the application of an alternating voltage or pulsating direct voltage to an electrochemical cell containing either anion exchange or cation exchange resin. A two bed system is also described, wherein one bed contains anion exchange resin, the other contains cation exchange resin, and the beds are separated by an electrode and a liquid layer. A high frequency (I KHz to 10 MHz) alternating potential (of tens of hundreds of volts) is applied. This system is quite different in conception from that of the present invention, and other than claiming to use electricity to regenerate an ion exchange resin, the two have little if anything in common. A process for regeneration of ion exchange resins .Indian Patent 15 08 70) This patent discloses a method for partial regeneration of ion exchange resin in an electrochemical cell. In this patent application, the electrodes are buried in the centre of the compartments of ion exchange resin. By contrast, the sandwich design of the system of the present invention (where the resin is contained between the two electrodes) is fundamental to its successful operation. Furthermore, there is no release of liquid during the regeneration process in the process described in this patent, which means that there is a build up of salt, leading to equilibration between the salt in the water and on the resin, preventing further regeneration. In the examples given, the amount of salt solution passed through the resin bed amounts to -10% of the total capacity of the resin bed each time. Only four aliquots of this solution were passed, so it is not clear whether the regeneration occurred to any significant extent, or whether the desalting was simply due to the presence of more unused resin than required by the solution to be desalted. The difference in the conductivity of the effluent solution after passage of the electric current may simply be due to the difference in mobility of the H /Na and OH7CT ions. The highest purity level achieved corresponded to water of 277 micromho conductivity (3600 ohms water). Disclosure of the Invention According to the present invention a method and apparatus is disclosed which allows for the continuous production of purified water and regeneration of the ion exchange material beds. According to one aspect of the present invention there is provided apparatus for purifying a liquid stream comprising at least two ion exchange material beds, one of anion exchange material and the second of cation exchange material, the beds being connected at at least one point such that a junction which is permeable to ions is formed therebetween, means for passing liquid to be purified serially through one bed then the other, at least one pair of electrodes one each being associated with a respective material bed such that at least some ion exchange material is sandwiched between each said electrode and said junction, means for applying an electrical potential across said pair of electrodes so as to produce an electric field therebetween and regenerate said ion exchange material. According a further aspect, the present invention provides apparatus for purifying a liquid stream comprising at least two ion exchange material beds, each bed containing a mixture of anion and cation exchange material, the beds being connected at at least one point such that a junction which is permeable to ions is formed therebetween, means for passing liquid to be purified through at least one of said beds, at least one pair of electrodes one each being associated with a respective material bed such that at least some ion exchange material is sandwiched between each said electrode and said junction, means for applying an electrical potential across said pair of electrodes so as to produce an electric field therebetween and regenerate said ion exchange material. The invention also provides a method of purifying a liquid stream comprising the steps of passing liquid to be purified serially through at least a pair of ion exchange material beds, one of anion exchange material and the second of cation exchange material, the beds being connected at at least one point such that a junction which is permeable to ions is formed therebetween, concurrently regenerating said ion exchange material beds by applying an electrical potential across a pair of electrodes, one each being associated with a respective material bed, so as to produce an electric field between the electrodes and said electrodes being arranged such that at least some ion exchange material is sandwiched between each said electrode and said junction. When using separate anion and cation exchange material beds, the cathode is placed in the anion exchange material bed and the anode is placed in the cation exchange material bed. A liquid stream to be purified is fed into one of the beds, preferably to the base of one of the beds. It is then allowed to flow through and exit the first bed and then be fed into a second bed, preferably to the base of the second bed. The liquid stream then flows through the second bed whereupon it is purified of ions. The liquid then exits the second bed as a purified stream. While these processes are occurring an electrical potential is applied between the electrodes in contact with the ion exchange material beds such that a current flows between the electrodes. This causes an electric field to exist across the ion exchange material packed between the electrodes, and the reduction and oxidation of water to produce hydroxide and hydrogen ions at the cathode are anode respectively. The electric field causes the contaminant ions that had been absorbed by the ion exchange material to collect at the bed junction where they form salts. The contaminant ions are replaced in the ion exchange material by the hydroxide and hydrogen ions that have been generated at the electrodes. A small flow of liquid is caused in the vicinity of the bed junction, from one or both sides of the junction, which carries away the salts formed, for external collection. In this way, according to the present invention, the ion exchange material in the beds is continuously regenerated while purified water is being produced. This allows the continuous production of purified water for extended periods without the need to interrupt the process to regenerate the ion exchange beds. When using beds filled with mixed anion and cation exchange materials, the liquid to be purified can be passed through a single bed or through multiple beds of ion exchange material. In this case the hydrogen and hydroxide ions generated by the electrochemical reactions do not necessarily come into contact with the bed of ion exchange material. These products may be flushed away by a liquid stream passing over the electrodes. Optionally, the cathode may be separated from the ion exchange beds by a cation exchange membrane and the anode by a anion exchange membrane. The separation of salt in this case is achieved by the salt being absorbed from the liquid stream by the mixed ion exchange material. The presence of the applied electric field, by causing an electric current to flow between the electrodes, causes the ions in the ion exchange material beds to migrate towards and accumulate as salts at the bed junctions. As before, a small flow of fluid in the vicinity of the bed junction serves to remove the accumulated salt. As this salt is continuously removed from the bed it is continually replaced with salt from the stream to be purified, resulting in a continuous purification of the liquid stream entering the apparatus. Preferably, the direction of flow of liquid to be purified through the ion exchange material beds should be such that, at least in part, it is different to the direction of flow of ions under the influence of the electric field, more preferably in a counter-current or orthogonal fashion. In a second aspect of the present invention, the junction of the ion exchange material beds is formed such that it is permeable to anions or cations or both anions and cations but substantially restricts or prevents the flow of water through it. In this way an ion connection path can exist between the ion exchange material beds without allowing the substantial transport of water. This prevents hydraulic short-circuiting of the ion exchange material beds by the liquid to be purified. In a preferred embodiment of this aspect of the invention such a junction may be formed by using a sheet of anion or cation exchange membrane to divide the two ion exchange material beds. More preferably, a cation exchange membrane is used. Examples of such membranes are Nafion™ (DuPont), Tosflex™ (TOSH) or Ionac™ (Sybron Chemicals Inc.). Another method for forming the junction is to pack ion exchange resin beads into a grid such that there is a large resistance to water flow through the packed grid. The resin particles could be compressed and held within the grids of a single mesh or sandwiched between two or more grids. For example, a grid with a larger mesh size could be used as the primary support for the resin beads, which are then further sandwiched between two other grids with finer mesh size. Preferably, all these grids would be made of relatively inert, electrically non-conductive material. If desired, the flow in the vicinity of the bed junction used to flush the salt away can be distributed via a flow distributor. It is desirable in operating the present invention to reduce the electrical resistance to ion flow through the ion exchange material beds. In a third aspect of the present invention it has been found that by applying pressure to compress the bed of ion exchange material it is possible to lower the resistance of the bed to ion flow by a factor of two or more. A preferred embodiment of how to achieve this compression is to use the electrode structure placed in or on the bed to apply pressure to the ion exchange material sandwiched between the electrode and the bed junction. The electrode structure could be used as it is or reinforced with suitable structural members to prevent undue flexing when compressive force is applied. In another embodiment of this aspect of the invention, a separate compressive structure could be placed above the electrode structure in contact with the resin bed. This could, for example be the lid of the vessel which contains the ion exchange material, where by over-filling the vessel with ion exchange material during assembly the material would be compressed as the lid was assembled onto the vessel. Apart from compressing the ion exchange material there are other methods for lowering its resistance to ion flow. The traditional form of ion exchange material for water purification is in the form of beads. In a fourth aspect to the current invention it has been discovered that ion exchange materials made into other structures can have lower resistance to ion flow than resin particles when used in beds. Examples of other structures are filaments of ion exchange material or a macroporous monolith of the material. These structures can be made by any suitable method. For example, a macroporous monolith could be fabricated by adding foaming agents to a suitable polymer dope. Filaments of ion exchange material can be formed, for example, by melt blowing filaments of polymer which could subsequently be spun bonded or similarly processed to form a sheet of conjoined non- woven filaments. These sheets could then optionally be compressed and either stacked or wound to form a porous mass of ion exchange material. The polymer which forms the filaments could contain ion exchange groups prior to being formed into filaments or the ion exchange groups could be formed on the polymer after formation of the sheets by chemical treatments. Examples of polymers that would be suitable for use in this aspect of the current invention are polystyrene or polyacrylates. Examples of suitable ion exchange groups are sulfonate, carboxylic acid, amine and quaternary amines. A method of lowering resistance to ion flow of mixed ion exchange materials is to use a filamental material, such as disclosed above, where some of the filaments contain cation exchange groups and some contain anion exchange groups. Where these two types of fibres are uniformly interspersed within the assembled mass of ion exchange material. Another method is to use filaments, beads or a monolith of suitable polymer material and either before or after fabrication of the structure chemically react the materials such that the final ion exchange materials contain mixed cation and anion exchange groups. For example, polystyrene beads could be chemically treated such that each bead contained both cation exchange groups and anion exchange groups. According to a fifth aspect of the present invention there is provided an apparatus for purifying a liquid stream comprising at least two ion exchange material containing beds, one of anion exchange material and the second of cation exchange material, the beds being connected at one point at least such that a junction is formed therebetween, means for passing liquid to be purified through each bed, either through a single bed or through at least one anion exchange material containing bed and at least one cation exchange material containing bed, said ion exchange material containing beds being arranged in a bank and having at least one pair of electrodes associated with said bank and means for applying an electrical potential across said electrodes and said bed junction or junctions. In one embodiment of this aspect of the invention using a single pair of ion exchange beds, two types of junction between the ion exchange material beds can be formed. In the first type of junction the electrical potential is applied across the beds junction such that the mobile cations and anions residing on the ion exchange material are attracted away from the junction and into the cation and anion exchange material beds respectively. This will tend to cause water to dissociate into protons and hydroxide ions in the junction. The protons and hydroxide ions thus formed will then travel through the beds of ion exchange material under the influence of the applied electric field to continuously regenerate the material. In the second type of junction the electrical potential is applied across the junction such that cations and anions present in the bed are attracted towards the junction where they form salts. These salts can then be carried out of the beds by a small flow of water caused to flow in the vicinity of the junction. A gutter may be provided at the base of the junction to assist in carrying away the salt solution. In an embodiment of this aspect of the invention using more than one pair of ion exchange material beds, arranged in a bank, each bed of ion exchange material has at least two junctions either with an adjacent bed or an electrode containing compartment. When an electrical potential is applied in this case, one junction functions as a source of protons or hydroxide ions and the second junction as a place for removing contaminant ions from the beds. According to one preferred form of this aspect of the invention, the junctions can be formed, for example, by simply abutting the two exchange material beds, using a woven mesh or other macroporous separator between the two exchange material beds, forming a mesh or other porous material into which ion exchange material is compressed, or using a semipermeable membrane to separate the beds. In the case of the junction where water is dissociated, the semipermeable membrane may be either a bipolar membrane or a cation or anion or cation and anion permselective membrane. In the case of the salt collecting junction the membrane may be either a cation or anion or a cation and anion permselective membrane. Examples of such membranes are Nafion™ (DuPont), Tosflex™ (TOSH) or Ionac™ (Sybron Chemicals Inc.). The electrodes used in the embodiments of the present invention can be fabricated from any material that is inert under the conditions used. Preferably, the anode is constructed from gold, platinum, palladium, iridium, carbon or more preferably, titanium coated with gold, platinum, palladium or iridium. Preferably the cathode is fabricated from gold, platinum, palladium, iridium, carbon, stainless steel, silver, copper, titanium or titanium coated with gold, platinum, palladium or iridium. In a preferred form of the present invention the pressure of the liquid flowing through each of the beds of ion exchange material can be substantially equal. When this is the case, there is no need to have the junctions between the beds which substantially restrict the flow of liquid through the junction and so a simple abutting of the exchange materials is all that is necessary. The equalisation of hydraulic pressure between the beds can be assisted by having connection pathways for the feed liquid between the feed inlets to the separate beds. For example, a shared manifold for the feed liquid to all the beds may be used. When it is desired that the liquid to be purified is passed serially through more than one bed in the bank of ion exchange material beds it is necessary that at least some of the bed junction means are such that they substantially restrict the flow of liquid through the junction at the hydraulic pressure differences that exist across the junction during operation. For example, if the flow of liquid to be purified was made such that for each pair of cation/anion exchange material beds the liquid first flowed through an ion exchange bed of one type then through an adjacent ion exchange bed of the opposite type, the salt collecting junctions between these beds may restrict the flow of liquid but the water dissociation junctions not. In this case it is desirable to manifold together the feeds into the first ion exchange material beds to assist in pressure equalisation between the pairs of beds. In a further aspect, the present invention provides a method for forming a junction between at least two regions of ion exchange material comprising the step of providing a densely packed structure of ion exchange material between said regions to form said junction, said structure having a packing density sufficient to substantially restrict the flow of liquid across said junction. Brief Description of the Drawings Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which; Figure 1 shows a schematic of a one preferred embodiment of a first aspect of the present invention; Figure 2 shows a schematic of a second prefeπed embodiment of the first aspect of the present invention; Figure 3 shows a schematic of a third preferred embodiment of the first aspect of the present invention. Figure 4 shows a schematic of a one preferred embodiment of the fifth aspect of the present invention; Figure 5 shows a schematic of a second preferred embodiment of the fifth aspect of the present invention; and Figure 6 shows a schematic of a third preferred embodiment of the fifth aspect of the present invention. Modes of Performance of the Invention Referring to the Figure 1, two vessels 17 and 18 contain anion exchange material 10 and cation exchange material 11. They are connected by a piece of cation exchange membrane 4 which forms an ion conducting junction between the two beds but substantially prevents the passage of water therebetween.. The liquid to be purified is fed to the base of the anion bed 10 at 5 via a suitable distributor which ensures the desired liquid flow patterns in the bed, it rises through the bed and exits at port 6. Optionally, a collector could be positioned near the top of the bed 10 prior to port 6, again to ensure the desired flow patterns within the bed. The liquid is then transferred via a pipe 7 to be fed to the base of the cation bed 11 at 8 through a second distributor. Purified water exits the bed 11 at 9, again optionally through a suitable collector. As an example, let us assume that the liquid stream that we wish to purify is a solution of water containing sodium chloride. As the solution rises through the bed 10 the chloride ions are absorbed by the ion exchange material and replaced with hydroxide ions. The absorption occurring mostly near the base of the bed 10. The liquid therefore exits at 6 as a solution of sodium hydroxide. This solution is then fed to the cation exchange material bed 11. As the solution rises through this bed 11 the sodium ions are absorbed by the cation exchange material, again mostly near the bed base, and replaced with hydrogen ions, which react with the hydroxide ions present to form water. The purified liquid then exits the apparatus at 9 for use. While liquid is being fed to the beds to be purified an electrical potential is applied between electrodes 2 and 3 by a power supply 19. The potential is of sufficient size to cause water to be reduced at the cathode 2 and oxidised at the anode 3, these electrodes being fabricated from titanium mesh coated with platinum. The reduction of water produces molecular hydrogen and hydroxide ions 14 and the oxidation of water produces molecular oxygen and hydrogen ions 13. The small amount of gases produced could dissolve in the liquid stream and exit with the purified water at 9. Optionally, suitable vents could be incorporated into the tops of vessels 17 and 18 through which any build up of gas could be removed. Due to the electrical potential that is applied between the electrodes, an electric field exists which attracts anions 14 and 16 towards the anode 3 and cations 13 and 15 towards the cathode 2. This draws the anions 14 down through the anion exchange material bed and the cations 13 down through the cation exchange material bed. Since sodium ions 15 and chloride ions 16 are being continuously deposited, mostly near the base of the ion exchange material beds, it is these ions that will tend to arrive at the bed junction rather than hydroxide or hydrogen ions. This is desirable as pairs of hydroxide 14 and hydrogen ions 13 that arrive at the junction will react to form water, leading to coulombic inefficiency in the system. The hydroxide and hydrogen ions 13, 14 formed at the electrodes will tend to migrate down through the beds and replace the chloride or sodium ions 15, 16 as the latter migrate towards the bed junction 4. This continual replenishment of the hydroxide and hydrogen ions in the ion exchange material ensures that the liquid purification process can continue without interruption. Anions will tend to migrate to the interface between the cation exchange membrane, that forms the junction between the beds, and the anion exchange material, as the former is substantially impermeable to anions. Cations from the bed 11 pass through the cation exchange membrane and combine with the anions at the cation exchange membrane/anion exchange material interface 4 to form salts. A small portion of the liquid feed stream is allowed to exit the anion exchange bed at 12. This liquid serves to carry away the salts formed at the cation exchange membrane/anion exchange material interface 4. The removal of salts from this interface is aided by the fact that the concentrated salt solution formed at the interface will be denser than the liquid in the rest of the bed and so will tend to fall towards the exit point 12. This process can be further aided by sloping the base of the vessel 18 in the vicinity of the bed junction such that the exit 12 is located at the lowest point. In a second preferred embodiment of the present invention as illustrated in Figure 2, multiple beds 21 of ion exchange material 30 are placed between the electrodes 22 and 23. According to this embodiment the ion exchange beds may contain mixed anion and cation exchange material 30. The beds may be such that the liquid flow through each beds follows a relatively straight path. For example, the liquid may enter the base of the bed at 25 and exit at the top of the bed at 26 such as is illustrated in Figure 2. In another preferred embodiment the liquid to be purified may travel through several beds in the sandwich sequentially either in the same direction or alternating directions in adjacent beds. In this case the liquid would be successively purified as it travelled through the beds until it exits as a purified stream. As the liquid to be purified travels through the stack of such beds, salt is removed at each junction as in the previous embodiment such that by the time that the liquid has exited the stack of bed it is purified to the required extent. By a suitable system of liquid distributors and collectors placed at the ends of the beds the flow of the liquid to be purified is directed such that it does not disturb the liquid near the membrane/exchange material interface where the salt accumulates. This ensures that the accumulated salt is washed out the exit points 24 and is substantially prevented from mixing in the stream to be purified. According to this embodiment the anode 23 is placed at one end of the stack of ion exchange material beds 21 and optionally may be separated from the adjacent bed of ion exchange material by a anion exchange membrane 28. Similarly, the cathode 22, placed at the other end of the stack of ion exchange material beds may be separated from the adjacent bed of ion exchange material by an cation exchange membrane 27. A flow of liquid would be caused to flow past the electrodes to assist in carrying away the products of the electrochemical reactions proceeding at the electrodes when a potential is applied between them. This liquid may go directly to waste or be recirculated around past the electrodes with a small bleed in of fresh liquid and a small bleed out of liquid containing the products of the electrochemical reactions. A third embodiment of the present invention is depicted in Figure 3. This figure depicts three units sandwiched together for illustrative purposes however one unit or many more units can be assembled to form the embodiment, if desired. In this embodiment the vessels 21 are filled with mixed anion and cation exchange material 30. Each vessel is separated by a central baffle 29 which extends part way up the height of the bed and the length of the bed. The liquid to be purified enters each unit at 25 travels up through the bed, over the baffle 29 and down the other side to exit as a purified stream at 26. While this liquid is being purified a potential is applied between the cathode 22 and the anode 23. This causes the reduction and oxidation of water at the electrodes and causes cations to be attracted towards the cathode 22 and anions to be attracted towards the anode 23. The beds of ion exchange material 30 in the vessels 21 are connected to one another for ion transport by pieces of cation exchange membrane 27, for example. Salt tends to accumulate at the interface between the cation exchange membrane 27 and the ion exchange material on the side closest to the cathode 22 as in the previous embodiment. Again these salts are removed by a small flow of liquid exiting the vessel at 24. Optionally, rather than having mixed ion exchange material adjacent to the cation exchange membrane 27, some anion exchange material can be placed adjacent to the cation exchange membrane 27 on its side closest to the cathode 22 and/or cation exchange material adjacent to the side of the membrane closest to the anode 23. This assists in ensuring the ions to be removed at the interface do not re-enter the stream of liquid to be purified. Also, optionally a piece of anion exchange membrane 28 can be used to prevent the hydroxide ions formed at the cathode 22 from entering the exchange material beds. While the potential is being applied between the electrodes 22 and 23 a small flow of liquid is directed over them to provide water for the electrode reactions and to assist in carrying away the electrode reaction products. This liquid could go directly to waste or be recirculated around past the electrodes with a small bleed in of fresh liquid and a small bleed out of liquid containing the products of the electrochemical reactions. Referring to a first embodiment of a fifth aspect of the invention shown in Figure 4, the liquid to be purified 49 enters the bank of ion exchange material beds 41 via a common manifold 54. The liquid travels up through either a cation, 44 or an anion, 45 exchange material bed. Upon which either cations or anions in the feed stream are replaced with protons or hydroxide ions. The product liquid 50 then exits the beds via the common manifold 53. While this is occurring an electrical potential is applied between the cathode 42 and the anode 43 such that an electrical current flows between the electrodes. The electrodes being separated from the beds of ion exchange material by a cation exchange membrane 51 or an anion exchange membrane 52. This causes contaminant ions, shown here as Na + and Cl " , to migrate towards the bed junctions 47. At the bed junctions 47 the contaminant ions combine to form salts. The gutters 48 have a drain point open to the outside of the apparatus which causes a small flow of liquid towards the junction 47 and into the gutter 48. This flow of liquid causes the salts formed at the junction 47 to be flushed into the gutters 48 and so removed from the beds 44, 45. The gutters 48 shown at the base of the junctions 46 are optional and may be required when treating feed streams with relatively high concentrations of dissolved salts. At the junctions 46 there is only a relatively low concentration of ions in the liquid compared to the concentration of mobile ions on the ion exchange material phase. These mobile ions are attracted away from the bed junctions 46 by the electric field and so in order to carry current across the junction water will dissociate into protons and hydroxide ions. These ions will then migrate through the cation and anion exchange material beds respectively, due to the presence of the electric field, continuously regenerating the beds. According to this embodiment, approximately half the contaminant salts will be removed from the feed stream 49 and the junctions 46 and 47 need only be an interface between the two material beds or a piece of macroporous material such as a mesh or sintered sheet. Alternatively, the manifolding of the liquid flow through the beds may be changed such that the liquid to be purified first travels through one ion exchange material bed and then a second bed of the opposite type of ion exchange material. This may substantially remove all the dissolved salts from the liquid to be purified. This requires that either the junctions 46 or 47 or both substantially restrict the flow of liquid through them to restrict undesirable mixing of the liquids between adjacent beds of ion exchange material. In a further form of this embodiment, two banks of ion exchange material beds, of the type depicted in Figure 4, may be used such that after the liquid to be purified passes through the first bank of beds it enters a second bank of beds where it is further purified. Further, according to this form of the embodiment, liquid that has passed through a cation exchange material bed in the first bank of beds is transferred to the second bank of beds so as to pass through an anion exchange material bed. Similarly, liquid that has passed through an anion exchange material bed in the first bank of beds is transferred to the second bank of beds so as to pass through a cation exchange material bed. In this way both anions and cations are removed from the liquid to be purified. According to this form of the embodiment, a separate pair of electrodes may be associated with each bank of beds or they may have one common pair of electrodes. In a second prefeπed embodiment of the fifth aspect of the invention, shown in Figure 5, the two banks of beds are placed one on top of the other such that each bed of ion exchange material in the first bank has a bed of the opposite type of ion exchange material directly above it. A cathode 42 is placed at the one end of this double bank of ion exchange material beds and an anode 43 at the other end. In this embodiment, a single anode and cathode supply current to both banks of beds simultaneously. If desired, the electrodes can be separated from the end beds of ion exchange material either by a cation exchange membrane 51 or an anion exchange membrane 52. The flow of liquid to be purified 49 is straight through the two banks of beds. The other numbers in Figure 5 designate the similar structure as defined in Figure 4. In a second form of this embodiment, the two banks of beds can be arranged in any suitable fashion such that the liquid exiting from the first bank of beds can be fed into the appropriate bed in the second bank via suitable connectors. In this form of the embodiment, a separate pair of electrodes is used for each bank of beds. Preferably these pairs of electrodes are connected together in series. In this form of the embodiment it is desirable that the means for transferring the liquid from the first bank of beds to the second bank of beds is long enough and of small enough cross-section such that the resistance to ion flow through said means, due to the applied electric field,, is large compared to the resistance to ion flow through the bank of ion exchange material beds. This prevents significant electrical short-circuiting of the ion exchange material beds. In a third preferred embodiment of the fifth aspect of the invention, an end bed of one of the banks of ion exchange material beds has a junction with an end bed of the second bank of ion exchange material, where this junction is such that it allows electrical connection between the beds but restricts or substantially prevents the flow of liquid through it. Such a junction means could be an ion exchange membrane. Means for transferring the liquid exiting the first bank of beds to the second bank of beds is provided as in the second preferred embodiment. According to this third embodiment, one pair of electrodes is used, where the cathode is placed at one end of the joined banks of beds and the anode is placed at the other end. In this embodiment the banks of beds could be joined so as to form a rod shape or joined so as to form a U-shape, or other suitable shape. An example of this embodiment in a rod shape is given is shown in Figure 6. In a further aspect of the present invention, a method for forming a bed junction is disclosed. This method is applicable to junctions of the type referred to as 46 or 47 in the Figures 4, 5 and 6. According to this further aspect, a structure of densely packed ion exchange material is formed to make the junction. This ion exchange material may be in the form of a fine powder, a compressed filament material, spun bonded filament material or other material form that allows dense packing. The density of packing of the material is such that in operation there is a sufficient resistance to liquid flow through the densely packed material that only a small fraction of the flow of feed water can pass through. This resistance may be such that, by itself it restricts the liquid flow to the desired level or, such that the junction resistance is used in combination with a valve or capillary to give the desired flow through the junction or out of the junction and into a gutter when one is used. Preferably, the densely packed material has two regions abutting one another, one containing anion exchange material and the second containing cation material. The region containing the anion exchange material faces the anion exchange bed and that containing cation exchange material faces the cation exchange bed. Optionally, one or more pieces of macroporous material or mesh can be placed between the regions to give a relatively low liquid flow resistance path compared to flow through the densely packed material. When using spun bonded material for this aspect, the junction structure may be self supporting. When using finely powdered material a bag formed out of mesh may be used to contain the powder. The mesh is preferably fine enough to substantially retain the powdered material but optionally also allows some of the material to extrude through the mesh. This material which extrudes through the mesh allows more intimate contact between the ion exchange material in the junction and that of the bed, lowering electrical resistance. Optionally, this extruded material can be sprayed with a binder to increase the robustness of the structure. Fine powdered material may also be mixed with a binder and formed into a self-supporting junction structure. In a preferred embodiment of this further aspect of the present invention, the ion exchange material is formed into a wedge shape with the thickest end of the wedge at the base of the junction. According to this embodiment, the resistance to liquid flow through the thinner portion of the wedge is less than through the thicker portion of the wedge so that liquid entering the junction through the densely packed material tends to preferentially enter towards the top of the junction. The liquid then tends to flow down the junction assisting in the removal of salts if required. It will be appreciated that further embodiments and exemplifications of the invention are possible without departing from the spirit or scope of the invention described.

Claims: CLAIMS: 1. A method of purifying a liquid stream comprising the steps of passing liquid to be purified serially through at least a pair of ion exchange material beds, one of anion exchange material and the second of cation exchange material, the beds being connected at at least one point such that a junction which is permeable to ions is formed therebetween, concurrently regenerating said ion exchange material beds by applying an electrical potential across a pair of electrodes, one each being associated with a respective material bed, so as to produce an electric field between the electrodes and said electrodes being arranged such that at least some ion exchange material is sandwiched between each said electrode and said junction. 2. A method of purifying a liquid stream according to claim 1 wherein the cathode is placed in the anion exchange material bed and the anode is placed in the cation exchange material bed. 3. A method of purifying a liquid stream according to claim 1 or claim 2 including the step of feeding the liquid stream to be purified into one of the beds, allowing the stream to flow through and exit said one of the beds and then be fed into a second bed, and allowing the liquid stream to flow through the second bed whereupon it is purified of ions and exit the second bed as a purified liquid stream. 4. A method of purifying a liquid stream according to any one of the preceding claims including a step of providing a small flow of fluid in the vicinity of the bed junction which serves to remove accumulated impurities. 5. A method of purifying a liquid stream according to any one of the preceding claims wherein the direction of flow of liquid to be purified passing through the ion exchange material beds is such that, at least in part, it is different to the direction of flow of ions under the influence of the electric field. 6. A method of purifying a liquid stream according to claim 5 wherein the direction of flow of liquid is in a counter-current or orthogonal fashion to the direction of flow of ions under the influence of the electric field. 7. A method of purifying a liquid stream according to any one of the preceding claims including forming the junction of the ion exchange material beds such that it is permeable to anions or cations or both anions and cations but substantially restricts or prevents the flow of liquid therethrough. 8. A method of purifying a liquid stream according to claim 7 wherein the junction is formed by using a sheet of anion or cation exchange membrane to divide the two ion exchange material beds. 9. A method of purifying a liquid stream according to claim 7 wherein the junction is formed by packing ion exchange resin beads into a grid. 10. A method of purifying a liquid stream according to claim 9 wherein the resin particles forming said junction are compressed and held within the grids of a single mesh or sandwiched between two or more grids. 11. A method of purifying a liquid stream according to claim 9 or claim 10 wherein the grid is formed by of relatively inert, electrically non-conductive material. 12. A method of purifying a liquid stream according to any one of the preceding claims including the step of applying pressure to compress the bed or beds of ion exchange material so as to lower the resistance of the bed to ion flow. 13. A method of purifying a liquid stream according to claim 12 wherein the pressure is applied by means an electrode structure placed in or on the bed to apply pressure to the ion exchange material sandwiched between the electrode and the bed junction. 14. A method of purifying a liquid stream according to claim 13 wherein the electrode structure is reinforced with suitable structural members to prevent undue flexing when compressive force is applied. 15 A method of purifying a liquid stream according to claim 12 wherein the pressure is applied by means of providing a lid on the bed which contains the ion exchange material, and includes the step of over-filling the bed with ion exchange material during assembly such that the material is compressed as the lid was assembled onto the bed. 16. A method of purifying a liquid stream according to any one of the preceding claims wherein the ion exchange material is filamental. 17. A method of purifying a liquid stream according to any one of claims 1 to 15 wherein the ion exchange material is a macroporous monolith of the material. 18. A method of purifying a liquid stream as claimed in any one of the preceding claims wherein the ion exchange materials are formed from a chemically treated polymeric material. 19. Apparatus for purifying a liquid stream comprising at least two ion exchange material beds, one of anion exchange material and the second of cation exchange material, the beds being connected at at least one point such that a junction which is permeable to ions is formed therebetween, means for passing liquid to be purified serially through one bed then the other, at least one pair of electrodes one each being associated with a respective material bed such that at least some ion exchange material is sandwiched between each said electrode and said junction, means for applying an electrical potential across said pair of electrodes so as to produce an electric field therebetween and regenerate said ion exchange material. 20. Apparatus for purifying a liquid stream comprising at least two ion exchange material beds, each bed containing a mixture of anion and cation exchange material, the beds being connected at at least one point such that a junction which is permeable to ions is formed therebetween, means for passing liquid to be purified through at least one of said beds, at least one pair of electrodes one each being associated with a respective material bed such that at least some ion exchange material is sandwiched between each said electrode and said junction, means for applying an electrical potential across said pair of electrodes so as to produce an electric field therebetween and regenerate said ion exchange material. 21. Apparatus for purifying a liquid stream comprising at least two ion exchange material containing beds, one of anion exchange material and the second of cation exchange material, the beds being connected at one point at least such that a junction is formed therebetween, means for passing liquid to be purified through each bed, either through a single bed or through at least one anion exchange material containing bed and at least one cation exchange material containing bed, said ion exchange material containing beds being arranged in a bank and having at least one pair of electrodes associated with said bank and means for applying an electrical potential across said electrodes and said bed j unction or j unctions . 22. Apparatus for purifying a liquid stream according to any one of claims 19 to 21 including means for applying pressure to compress the bed or beds of ion exchange material so as to lower the resistance of the bed to ion flow. 23. Apparatus for purifying a liquid stream according to claim 22 wherein the means for applying pressure includes an electrode structure placed in or on the bed to apply pressure to the ion exchange material between the electrode and the bed junction. 24. Apparatus for purifying a liquid stream according to claim 23 wherein the electrode structure is reinforced with suitable structural members to prevent undue flexing when compressive force is applied. 25. Apparatus for purifying a liquid stream according to claim 22 wherein the means for applying pressure includes a lid on the bed containing the ion exchange material, and wherein the bed is overfilled with ion exchange material during assembly such that the ion exchange material is compressed as the lid is assembled onto the bed. 26. Apparatus for purifying a liquid stream according to any one of claims 19 to 25 wherein the ion exchange material is filamental. 27. Apparatus for purifying a liquid stream according to any one of claims 19 to 25 wherein the ion exchange material is a macroporous monolith of the material. 28. Apparatus for purifying a liquid stream according to claim 21 having a single pair of ion exchange beds and two types of junction formed between the ion exchange material beds, the first type of junction having an electrical potential applied across the junction such that mobile cations and anions residing on the ion exchange material are attracted away from the junction and into the cation and anion exchange material beds respectively, and a second type of junction having an electrical potential applied across the junction such that cations and anions present in respective beds are attracted towards the junction where they form salts. 29. Apparatus for purifying a liquid stream according to claim 21 wherein said salts are carried out of the beds in solution by a small flow of liquid caused to flow in the vicinity of the junction and a gutter is provided at the base of the junction to assist in carrying away the salt solution. 30. Apparatus for purifying a liquid stream according to claim 21 having more than one pair of ion exchange material beds, arranged in a bank, each bed of ion exchange material has at least two junctions either with an adjacent bed or an electrode containing compartment, one of said junctions having an electrical potential applied across the junction such that mobile cations and anions residing on the ion exchange material are attracted away from the junction and into the cation and anion exchange material beds respectively and the other of said junctions having an electrical potential applied across the junction such that cations and anions present in respective beds are attracted towards the junction where they form salts. 31. Apparatus for purifying a liquid stream according to any one of claims 21 to 30 wherein the junction is formed by abutting the ion exchange material beds, using a woven mesh or other macroporous separator between the ion exchange material beds, forming a mesh or other porous material into which ion exchange material is compressed, or using a semipermeable membrane to separate the beds. 32. Apparatus for purifying a liquid stream according to claim 31 wherein at a junction where water is dissociated, the semipermeable membrane may be either a bipolar membrane or a cation or anion or cation and anion permselective membrane. 33. Apparatus for purifying a liquid stream according to claim 31 wherein at a junction where salt is collected the semipermeable membrane may be either a cation or anion or a cation and anion permselective membrane. 34. Apparatus for purifying a liquid stream according to any one of claims 21 to 33 wherein the pressure of the liquid flowing through each of the beds of ion exchange material is substantially equal. 35. Apparatus for purifying a liquid stream according to claim 34 wherein the equalisation of pressure between the beds is assisted by having connection pathways for the feed liquid between feed inlets to the separate beds. 36. A method for forming a junction between at least two regions of ion exchange material comprising the step of providing a densely packed structure of ion exchange material between said regions to form said junction, said structure having a packing density sufficient to restrict the flow of liquid across said junction. 37. A method for forming a junction between at least two regions of ion exchange material according to claim 36 wherein the ion exchange material forming the densely packed structure is in the form of a fine powder or a compressed, filament material. 38. A method for forming a junction between at least two regions of ion exchange material according to claim 36 or 37 wherein the junction restriction is used in combination with a valve or capillary to give the desired flow through the junction. 39. A method for forming a junction between at least two regions of ion exchange material according to claim 36, 37 or 38 wherein the densely packed material is formed from two regions abutting one another, one containing anion exchange material and the second containing cation exchange material. 40. A method for forming a junction between at least two regions of ion exchange material according to any one of claims 36 to 39 wherein each region containing one type of ion exchange material is arranged to face an exchange bed of the same material type. 41. A method for forming a junction between at least two regions of ion exchange material according to claim 40 wherein at least one piece of macroporous material or mesh is placed between the regions to give a relatively low liquid flow resistance path compared to flow through the densely packed material. 42. A method for forming a junction between at least two regions of ion exchange material according to any one of claim 36 to 41 including the step of forming the ion exchange material forming the junction into a wedge shape with the thickest end of the wedge at the base of the junction, such that the resistance to liquid flow through the thinner portion of the wedge is less than through the thicker portion of the wedge so that liquid entering the junction through the densely packed material tends to preferentially enter towards the top of the junction.

Inventors: HODGES ALASTAIR MCINDOE, HODGES, ALASTAIR, MCINDOE
Applicants: HODGES ALASTAIR MCINDOE, USF FILTRATION & SEPARATIONS, HODGES, ALASTAIR, MCINDOE, USF FILTRATION AND SEPARATIONS GROUP INC.
Application year: 1998
Main classification: C02F1
IPC codes: B01D61, B01J47, C02F1