GB2063094A - Water purification by ion exchange - Google Patents

Abstract

A condensate polishing or other water purification process for purifying low solids content water is conducted by passing the water downflow through cation and anion exchange resins in one or more primary beds 4, 5 and finally through a final bed 6 of cation exchange resin and the cation exchange resin of the final bed is regenerated as part of a combined bed 12 containing cation exchange resin from the primary bed(s), and after regeneration the cation exchange resin from the least contaminated part 12a of the combined bed only is used as the resin in the final bed 6. The anion exchange resin of the primary bed(s) may be regenerated as a bed 13 in the same regeneration vessel 7 as the cation exchange resin. If the primary beds are mixed anion-cation exchange resin beds, then the part 12b of the cation bed 12 and the anion bed 13 may be formed in situ in the vessel 7 by separation of the two resins. <IMAGE>

Description

SPECIFICATION Water purification process and apparatus This invention relates to the purification by ion exchange of water that is already of low solids content. For instance it will normally have a total dissolved solids content of below 10 mg/l (10 parts per million).

Typical water of this very low solids content is condensate and methods of purifying such water are known and are often termed condensate polishing processes. Traditional condensate polishing processes have aimed at the production of final dissolved solids contents of 5 to 50 Flg/i (parts per billion) and in practice it has been substantially impossible to obtain values below 5 ll9/l.

Ion exchange processes were initially developed for the purification of water containing relatively high dissolved solids contents, e.g., above 20 mg/l, and it is known that these conventional demineralisation ion exchange processes, which have been used for very many years, may be conducted using various combinations of anion and cation exchange resins. Mixed bed systems, in which the bed is a mixture of anion exchange resin and cation exchange resin, were long ago accepted as being very advantageous for some purposes. When condensate polishing processes began to be developed many years ago mixed bed systems were selected for them and they have been used almost without exception for all condensate polishing processes ever since.Thus there were some proposals to pass the waterfirst through a bed of cation exchange resin and then through a mixed bed but many condensate polishing processes have merely involved the use of a mixed bed. Traditionally the bed is at least 1 metre deep and if very low impurity levels are required the bed is generally deeper.

Users of extremely pure water continually request even lower impurity levels. For instance in modern power stations such as those involving pressurised water reactors or advanced gas cooled reactors the users may now ask for total dissolved solids contents of less than 5'ig/l, for instance a total dissolved sodium ion of less than 1 Cig/l and similar figures for sulphate and chloride ions. Accordingly there is a continuing need for producing the lowest possible impurity levels in an economic manner.

The attainment of very low impurity levels is particularly difficult when, as is commonly the practice in power station water, the water has a high pH as a result of having been dosed with ammonia or other nitrogenous base since this constitutes a heavy load on an orthodox hydrogen-hydroxide form mixed bed. This causes considerable interest in the operation of mixed beds in the NH40H form with their concomitant addition difficulties in meeting the very low sodium, sulphate, and chloride figures that are desired.

Particularly serious problems are also liable to arise when sea water is used for the indirect cooling of condensate since even minute leakage of ocean sea water into the condensate will give a large increase in the dissolved solids content of the condensate.

Although it has long been recognised that the mixed bed is the best system for condensate polishing it has also long been recognised that the use of the mixed bed creates problems which have to be solved, and a lot of effort has been put into solving these problems.

The principle disadvantage of mixed bed operation for removal down to the very low levels now required is the very fact that it requires intimate mixing of the two ion exchange materials, followed by very complete separation for regeneration purposes. If separation prior to regeneration is not absolutely complete, there will be some cation resin entrained in the main body of the anion resin and vice versa. During regeneration any cation resin in the anion resin component will be contacted with caustic soda and converted to the sodium form, and similarly any anion resin in the main body of the cation resin will be contacted with an acid, typically sulphuric or hydrochloric, and will thus be converted to that ionic form.

Since the final quality of treated water depends on the equilibrium between the resin phase and the aqueous phase, this cross contamination can make it difficult, if not impossible, to reach the very low levels currently required, particularly when operating in the ammonium hydroxide form.

This intrinsic disadvantage of mixed bed operation has been much researched and various schemes have been used to ameliorate the problem. There have been three main directions of research. One has been to improve the separation techniques by the incorporation of a third inert material into the mix. A second has been to achieve chemical separation of the two resins by use of an aqueous solution of relatively high specific gravity typically caustic soda.

The third has accepted a certain degree of "cross contamination" as inevitable, and steps are taken to remove the offending ions by chemical means, typically involving recycling of ammonia or lime solutions through the resins at certain stages of the regeneration procedure.

In our European Patent Publication 002342 we describe how exceedingly low impurity levels, e.g.

below 1 ll9/l can be obtained by passing the water through a bed of anion exchange resin, generally after passage through a bed of cation exchange resin, and then finally through a bed of cation exchange resin at a high flow rate, for instance well in excess of 100 metres per hour. The cation and anion resins are at all times kept separate from one another and preferably they are positioned in a single tower on horizontal supports. This process gives the very low impurity levels now desired but necessitates maintaining the resins separate from one another at all times, and this can necessitate the use of expensive apparatus.

It has been our object to devise a condensate polishing process which requires simple apparatus and is easy to operate to give satisfactorily low impurity levels, especially when operated at high pH.

We have now devised a process in which low solids content water is purified by passage down flowthrough an ion exchange vessel in which it is subjected to a primary treatment with cation and anion ion exchange resins in one or more primary beds followed by final treatment in a final bed of cation exchange resin at the base of the vessel.

When regeneration is necessary this final bed can be regenerated as part of a combined bed containing other cation exchange resin, for instance from the primary beds and after regeneration the part of the combined bed that is least contaminated with anion exchange resin is used as the resin for the final cation exchange bed.

In one process of the invention low solids content water is purified by passage downflow through an ion exchange vessel in which it is subjected to a primary treatment with cation and anion ion exchange resins in one or more primary beds followed by a final treatment in a final bed of cation exchange resin at the base of the vessel, and the cation exchange resin of the final bed and cation exchange resin of the one or more primary beds are regenerated as a combined bed in a vessel by passing acid regenerant upflow through the combined bed, and after regeneration only the cation exchange resin from near the base of the combined bed is used as the resin in the final bed.

The low solids content water to be purified may be any water of low solids content, e.g. 0.1 to 10 mg/l usually 1 to 5 or 1 to 10 mg/l and is preferably a condensate for power station use, as discussed above. It may have a substantially neutral pH or, as is common in condensate purification processes, it may have an elevated pH, e.g. above 8 or 8.5, for instance 9 to 10, as a result of the addition of ammonia or other volatile nitrogenous base, such as morpholine orcyclohexylamine, all in conventional manner.

To purify it, it is passed through one or more primary beds and then through a final bed of cation exchange resin. Its flow rate through this final bed, and preferably also through the other bed or beds, should be high, generally being above 100 metres per hour. Typically it is 120 to 300 metres per hour, generally being in the range 120 to 240 metres per hour with 120 to 150 metres per hour being preferred. The direction of flow is downflow.

Before the treatment in the final bed of cation resin the water is treated in one or more preceding (primary) beds that contain cation and anion resin.

The cation and anion resins may be fixed together as a mixed bed or they may be in separate beds.

Preferably the water is first treated with cation resin in order to extract heavy metals and various other impurities. Thus preferred treatments consist of three beds, with cation beds first and last and generally an anion bed but optionally a mixed bed of cation and anion resins inbetween the cation beds.

All the beds may be in a single vessel and they may be in contact with one another. Thus whereas in European Application 0002342 it was essential to keep the beds separate from one another in the present invention they preferably rest on one another. Some inter-mixing of resins at the interfaces between adjacent beds is thus inevitable.

Regeneration of the cation resin of the final bed, and indeed of all the resins, is best conducted in a separate, regeneration, vessel and this vessel should contain additional cation resin, from the primary bed or beds. Preferably the final bed is reformed in the service flow vessel, after regeneration, using cation exchange resin from near the base of the combined bed. Preferably the cation exchange resin from the final bed is transferred for regeneration to the regeneration vessel to form the lower part of the combined bed substantially without being mixed with other cation exchange resin.

The cation resin in the regeneration vessel will in practice always be contaminated by anion resin but by conducting regeneration upflow, usually after backwashing, the anion resin accumulates at the top of the cation resin. Provided the amount of additional cation resin is sufficient, the volume of cation resin near the base of the tower that is required for reforming the final cation bed is wholly free of anion resin. It is therefore desirable for as much as possible of the cation resin from the primary beds to be included in the combined bed and generally all of the cation resin from the primary beds is in the combined bed.

Generally all the anion resin from the primary beds is also introduced into the regeneration vessel, the combined bed of cation resin from the primary and final beds being at the base of the vessel and the anion resin from the primary bed resting on the top of the cation resin. This arrangement can be achieved by introducing all the resins into the bed and then backwashing, the anion resin having a natural tendency to rise to the top of the resin system. Generally the cation resin of the final bed present at the foot of resin system is introduced into the tower first and the other resins are introduced on top of it. If the primary cation resin is separate from the primary anion resin then the primary cation resin can be introduced second with the anion resin being introduced to the top of the tower last.Alternatively the primary cation and anion resins can be introduced into the tower together, and this will be inevitable if all the primary cation resin is being used as a mixed bed with an anion resin.

During transferance of resin from one vessel to the other, for instance to the regeneration vessel, the receiving vessel may be substantially full of water in order to assist stratification of the resin. Thus the heavier resin will fall quicker than the lighter resin and so some separation of resin particle size or types can be achieved. For instance it is desirable, during the formation of the primary cation bed in the service flow vessel, for the larger particles to be at the bottom ofthe bed.

Preferably if there is a primary cation bed before the primary anion or primary mixed bed this primary cation resin is cleaned in known manner, for instance by backwashing, air mixing or a process described in British Patent Specification No.

1,318,102, before being introduced into the regeneration tower. This will tend to remove insoluble substances such as iron and copper oxides and hydroxides which will be attached to the primary cation resin.

After introducing the resins into the regeneration tower backwashing may be conducted to promote stratification of the resins into a deep bed of cation resin (containing all the primary and final cation resin) with the anion resin above it. The anion resin may be contaminated with a small amount of cation resin and there will additionally be mixing of the anion and cation resins at the interface between the two layers.

If desired an inert particulate material may be introduced to facilitate separation of the anion and cation layers. This material generally consists of resin beads having specific gravity approximately 1.2 and a particle size similar to the ion exchange beads. During backwashing this will tend to form a layer of inert beads between the cation and anion layers, thus minimising cross-contamination at the interface.

Regeneration can be conducted in the regeneration vessel in a variety of ways but regenerant for the cation resin should pass upflow and generally the anion resin is regenerated in the vessel at the same time or subsequently. Regeneration of the two resin types can be conducted simultaneously with acid regenerant entering the bottom of the vessel at the same time as alkali regenerant enters the top and with the two used regenerants meeting and being collected and removed from the vessel at a position between the anion and cation beds, generally at the interface between the beds when the anion resin rests on the cation resin.Instead of regenerating simultaneously one can be regenerated before the other, for instance the cation bed can first be regenerated and then the anion bed may be regenerated by downflowing alkali while rinse water flows up through the cation bed, so as to prevent alkali contacting the cation bed.

Conventional alkali regenerants may be used for the anion bed and conventional cation regenerants may be used for the cation bed. Thus sodium hydroxide may be used as the anion regenerant and acids such as hydrochloric or sulphuric acid may be used for the cation regenerant. However it is often preferred that any anions in the treated water should be sulphate ratherthan chloride and so regeneration with sulphuric acid is generally preferred. Some and more usually most (e.g. 80 to 90%) of the sulphuric acid may be passed through the anion bed after passing through the cation bed and before regeneration of the anion bed. This ensures that the cation resin particles at the interface are regenerated and also tends to displace monovalent ions, such as chloride, on the anion resin with sulphate.

Since the cation resin is regenerated upflow and since the regeneration tower is usually incompletely filled with resins it is desirable to provide means for holding the resins down in the tower. Thus inert plastics material may be provided above the uppermost resin and air or other fluid may be pumped onto the top of the uppermost resin in order to hold it down. Suitable methods are described in British Patent Specification No. 1,228,366.

After regeneration of the cation resin, the anion resin is regenerated. It may be transferred to a separate regeneration vessel after the cation regeneration for regeneration in conventional manner.

Preferably however it is regenerated in the same vessel as the cation regeneration, generally by passing sodium hydroxide or other suitable regenerant downflow through the anion resin and removing it substantially at the interface between the anion and cation resins. To prevent sodium hydroxide contacting the main bed of cation resin there is preferably upflowing liquid in the cation resin bed while there is downflowing sodium hydroxide in the anion bed, both liquids being removed at the interface. The upflowing liquid through the cation bed is normally provided by introducing rinse water at the bottom of the vessel upflow, which will have the result that for most of the anion regeneration the water will displace sulphuric acid and it will be sulphuric acid of approximately the same specific gravity as the downflowing caustic soda that is removed at the interface.The sodium hydroxide regenerant will displace the monovalent ions, such as chloride, on the anion resin in preference to the divalent sulphate ions and so even when regeneration of the anion resin is imperfect that resin may be substantially free of chloride.

After regeneration of the anion resin is complete it is rinsed and both resins are then ready for reuse.

Perfect regeneration of the two types of resins will not have occurred, especially at the interface, but the invention can result in very low impurity levels in the treated water despite this imperfect regeneration.

Optimum positioning of the separator in the regeneration vessel between the anion and cation resins can easily be determined by routine experiment.

The final treatment bed of cation resin is formed for reuse using the cation resin from at or near the base of the regeneration tower. Generally the final bed is formed using cation resin solely from the lowest 80% and preferably from the lowest 60% or less of the total depth of the cation resin in the regeneration tower. This bottom part of the cation resin was at all times distant from the anion resin and was regenerated first by the upflowing regenerant and thus irrespective of any sodium contamination near the interface with the anion resin this cation resin will be very pure and free from contamination with alkali metal or anion resin.

The extreme bottom of the cation resin in the regeneration tower may be contaminated with heavy impurities such as iron and it is desirable not to transfer these into the final cation bed. Thus for instance the lowermost 2% and preferably the lowermost 10% of the depth of the bed, e.g. up to 10 cm, is preferably left behind while drawing off the cation resin that is to form the final bed.

Once the optimum quality cation resin has been withdrawn, for forming the final bed, the remaining resins, which are to form the primary beds may then be returned to the primary bed or beds. This return may be conducted in conventional manner either keeping the resins substantially separate from one another or mixed together to form a mixed bed and before the return the resins may, if desired, be air mixed in the regenerator and, if appropriate, reseparated in the regenerator by backwashing.

The primary and final beds are now ready for treating water again. Leakage of chloride from the anion resin will be minimal provided the separator was positioned appropriately. A small amount of sodium may leak from the cation resin in the primary bed or beds onto the final cation bed but the amount entering the final bed will be small and because of the high flow rate and the thorough regeneration of the final cation bed that bed will be capable of taking up substantially all the sodium.

Thus the invention permits the production of satisfactorily low impurity levels, e.g. below 5 and often below 2 ug/l in apparatus that is of simple construction and design even though there will inevitably be cross-contamination between the resins.

The invention includes both the process and the apparatus for carrying out the process. Such apparatus comprises one or more vessels for containing primary treatment anion and cation resins and for containing a final bed of cation resin, a regeneration tower, means for transferring the final bed of cation resin to the bottom of the regeneration tower and for transferring the other resins to the tower above the final bed of cation resin, means for backwashing the resins in the tower, means for regenerating the cation resin upflow and for regenerating the anion resin and means for transferring cation resin from at or near the base of the tower to reform the final bed and for transferring the remaining resins to reform the primary treatment bed or beds.Generally each regeneration tower serves several, e.g. 3 to 10, treatment vessels and generally each treatment vessel is intended to contain the anion or mixed primary bed, and generally all primary beds, on top of the final cation bed.

The invention is illustrated in the accompanying drawing which illustrates diagrammatically suitable apparatus.

The treatment of water is conducted in tower 1 by downflow of water entering through pipe 2 and leaving through pipe 3 having passed through primary beds 4 and 5 and a final bed 6. Beds 4 and 6 are of cation resin while bed 5 is of anion resin although it can be a mixed bed. Conveniently each of the beds is from 25 to 75 cm deep and each may be, for instance, cm deep. Slight mixing of the resins at the interfaces between the beds may have occur red, and is tolerable.

The apparatus also includes a regeneration tower 7, which will generally be empty of resin during service flow through vessel 1, although often there will be several vessels 1 associated with each tower 7 so that resin from of the vessels 1 may be in the tower 7 while service flow continues through the other vessels 1.

When it is desired to regenerate, resin from bed 6 is transferred by line 8, optionally via a backwashing andior air mixing device, from the bottom of the bed 6 to the top of the regeneration tower 7, and falls to the bottom of that tower. The removal of bed 6 means that beds 4 and 5 drop into the position of, respectively, original beds 5 and 6. Cation resin from bed 4 is then transferred by line 9 from the bottom of the bed to a vessel 10 in which it may be cleaned before being passed by line 11 to the top of the vessel 7. It will fall onto the cation resin already in the bed to form a combined cation layer 12, the lower part 12a being formed from bed 6 and the upper part 12b being formed from bed 5.

Anion resin from bed 5 is then transferred, for instance through line 8, to the top of the regeneration tower 7 to form layer 13. Inert polystyrene or plastics beads may then be introduced on top of the layer 13.

The resins in the tower 7 are thoroughly backwashed so as to complete stratification into layers 12 and 13, generally before introducing the layer 14 of inert beads. Preferably the backwashing is so conducted that it does not intermix layers 1 2a and 12b.

Sulphuric acid regenerant is then introduced at 15 and withdrawn at 15 until regeneration of the layer 12 is complete. Rinse water is then introduced at 15 and withdrawn from a collector 17 at the interface between the layers 12 and 13 and simultaneously with the introduction of rinse water into the base of the tower sodium hydroxide regenerant is introduced at inlet 18 and passed downflowthrough the layer 13 and also is withdrawn at the interface collector 17. The layer 13 is then rinsed with rinse water, either upflow or downflow.

Cation resin from a point about 5 cm above the base of the tower is then transferred from the bottom of the tower 7 through line 19 to reform the final bed 6. The remainder of the cation resin in layer 12 and the anion resin in layer 13 are then returned to the tower 1 to form the beds 4 and 5 by usual resin transfer methods, for instance through line 21.

In one modification beds 4 and 5 can be replaced by a single primary bed that is a mixed bed of anion and cation resins. In this process the combined bed may be transferred by line 8, for instance through a device such as 10, after transfer of the cation bed 6.

Thorough backwashing of the resins in the regeneration tower before regeneration will result in stratification into the layers 12 and 13. After regeneration layer 1 2a is returned to form bed 6 through line 19 as before but then the remaining cation resin, layer 12b, and the anion resin, 13, are air mixed and returned to the top of the vessel 1 by line 21 to form the mixed bed.

In another modification a shallow bed 4 is provided consisting of cation resin and a deeper bed 5 is provided consisting of a mixed bed of anion and cation resins. These may be transferred to the regeneration tower 7 and stratified as described above to form the combined bed 1 2a and 1 2b and the anion bed 13 and may then be regenerated.

Layer 1 2a may then be returned to the vessel 1 by line 19 and then the resin above the point marked 20 may be air mixed and then returned to the top of the vessel by line 22 to form the mixed bed. The cation resin between point 20 and the base of the tower is then returned by line 21 to form the top shallow cation bed 4.

Claims (13)

1. A process in which low solids content water is purified by passage downflow through an ion ex change treatment vessel in which it is subjected to a primary treatment with cation and anion ion exchange resins in one or more primary beds followed by a final treatment in a final bed of cation exchange resin at the base of the vessel, and the cation exchange resin of the final bed and cation exchange resin of the one or more primary beds are regenerated as a combined bed and, after regeneration, only the cation exchange resin least contaminated with anion exchange resin is used as the cation exchange resin in the final bed.

2. A process according to claim 1 in which the primary beds comprise a cation exchange resin bed followed by an anion exchange resin bed or by a mixed bed.

3. A process according to claim 1 in which the primary beds comprise a single mixed bed.

4. A process according to any preceding claim in which the beds are arranged one above the other in the treatment vessel with each higher bed resting on the bed beneath it.

5. A process according to any preceding claim in which the water being treated passes downflow at a speed of at least 120 m/h.

6. A process according to any preceding claim in which regeneration of the combined bed is conducted by passing acid regenerant upflow and, after regeneration, only the cation exchange resin from near the base of the combined bed is used in the final bed.

7. A process according to claim 6 in which regeneration is conducted in a separate regeneration vessel and after regeneration the final bed is reformed using cation exchange resin from near the base of the combined bed.

8. A process according to claim 7 in which the cation exchange resin from the final bed is transferred for regeneration to the regeneration vessel to form the lower part of the combined beds substantially without being mixed with other cation exchange resin.

9. A process according to claim 7 or claim 8 in which the anion exchange resin is regenerated in the same vessel as the cation exchange resin while positioned above the combined bed by passing alkali downflow through the anion bed while passing rinse liquid or acid regenerant upflow through the combined bed and thereby substantially preventing alkali contamination of the combined bed, the downflowing and upflowing liquids being removed at a position substantially between the anion and combined beds.

10. A process according to claim 9 in which the cation resin is first regenerated by passing sulphuric acid regenerant upflowthrough the combined and anion beds and thereafter the anion resin is regenerated by passing sodium hydroxide downflow while passing sulphuric acid and/or rinse water upflow through the combined bed.

11. A process according to any of claims 7 to 10 in which the final cation bed is formed from the lowest 60% of the combined bed.

12. A process according to any of claims 7 to 10 in which the lowermost 2 to 10% of the combined bed is left in a regeneration vessel.

13. A process according to claim 1 substantially as herein described with reference to the accompanying drawing.