CA1053810A - Method of desalting treatment of condensate water - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
In the ion-exchange demineralization of power plant con-densate using a mixed-bed exchanger, wherein the anion and cation exchange resins are separated from each other for the purpose of regeneration, sodium leakage is minimized by the use of hydrazine to displace sodium from the anion bed. The hydrazine is recircu-lated through the anion bed and a hydrazine regeneration vessel containing a cation exchange resin preferably of the divinylben-zene/styrene sulfonate type or of the carboxylic weak acid type.

Description

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This invention is related to a method of desalting treat-ment of condensate water. More specifically, the invention is re-lated to a method of using hydrazine in the regeneration of the charged ion exchange resins9 after the desalting treatment of con-densate water by use of a mixed becl of ion exchange resins. The invention is especially suitable for application to steam turbine power systems and to atomic energy power generation systems The most widely used condensate water treatment method is that of de-salting by a mixed bed of H type cation exchange resin and OHtype anion exchange resin. In the process of regenerating the cation exchange resin and anion exchange resin used in this method, the two resins are separated into layers. However, it happens that an amount of the cation exchange resin is contained in the layer of anion exchange resin. In the process of regeneration and se-paration of the mixed bed type ion exchange resin tower, the se-paration of the strongly acidic cation exchange resin and the an-ion exchange resin is ordinarily done by reversing water washing, and because of the difference of specific gravity between the two ion exchange resins, the anion exchange resin separates into the upper layer and the cation exchange resin separates into the lower layer, thus the resins separating into two layers However, the boundary between the two layers which are separated by this difference of specific gravity is frequently not straight. Also, depending on the operating conditions of each -cycle, the boundary moves from the set position. This is caused by the imperfectness in grade separation, and wear and tear of the cation exchange resin. Thus, it is almost impossible to complete-ly prevent the anion exchange resin from being contaminated by the cation exchange resin.
Regenera-tion of the resins is carried out either in the same tower9 with the resins separated into two layers, or with one of the two resins transferred to a separate regenerationzone;

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o at this time, the anion exchange resin is regenerated by passing sodium hydroxide through the resin layer, and the cation exchange resin which contaminated said layer would be present in the form of an Na type. When the anion exchange resin which was regenera-ted in this state is again mixed with the regenerated cation ex-change resin layer and is used for treatment of passing water, impurities in the condensate wat0r spill the Na component from the Na type cation exchange resin into the condensate water, which could possibly cause damagss to a steam turbine power system~
10Furthermore, the impurities of condensate water which were adsorbed in the anion exchange resin layer, especially the -iron components, cannot be desorbed by -the regeneration with caus-tic soda alone. And as -the cycle of passing water is repeated, the impurities accumulate in the said resin, gradually reducing its ion exchange capacity.
Very recently, in order to improve the first point of difficulty mentioned above, a method has been proposed in which, following layers separation and the regeneration of the anion ex-change resin layer with caustic soda and the cation exchange re-sin layer with acid, the anion exchange resin layer is treatedwith ammonia water and the cation exchange resin layer is ammo-niated. This will have the effect of converting the Na type ca-tion exchange resin contaminating the anion exchange resin into an ammonia type. However, this method requires a long time, usually more than several hours, for the ammonia treatment of the cation exchange resin contaminating the anion exchange resin layer. Furthermore~ the method has no improving effect on the above mentioned second point of difficulty The present inventors have worked to perfect a method of purifying condensate water, overcoming all of the above men-tioned points of difficulty related to the anion exchange resin layer which is used in the mixed bed. As a result, we have dis-: . -

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covered that all of the above mentioned points of difficul-ty can be solved by treating the anion exchange resin layer with hydra-zine after it is regenerated with caustic soda and that, when this hydrazine treatment is carried out by a specific method, consumption of hydrazine can be limited to a minimum, thus pro-viding an industrially advantageous regeneration system. These findings brought us to this inventionO
The objective of this invention is to provide a method of desalting treatment of condensate water which is safe, effi-cient and, moreover, industrially advantageous. In the method ofdesalting treatment of condensate water by use of a mixed bed of ion exchange resins, this objective can be achieved easily by the following procedure: in regenerating the charged resin after passing condensate water through the mixed bed of ion exchange resins, the above said mixed bed is separated into a layer of anion exchange resin and a layer of cation exchange resin; after -the said anion exchange resin layer is regenerated with caustic soda, it is treated with aqueous solution of hydrazine; the aqueous solution of hydra7ine, after the said treatment, is purified by passing the liquid through a bed of hydrazine type cation exchange resin; the said purified aqueous solution of hydrazine is reused in the treatment of the above said anion exchange resin layerO
The attached figure aids in explaining the invention.
The figure illustrates an example of the device which is used for the application of this inventionO 1 is the mixed bed type ion exchanger; many of the mixed bed type exchangers can be used in parallel. When the exchange capability decreases, the exchangers are taken from the condensate water treatment system in sequence and are subjected to the regeneration process~ When this inven- -tion is applied to a typical steam turbine power system, the con-densate water which was desalted by this exchanger is fed into the boiler through a heat exchanger; the water becomes steam and :-;~,1 ,", .`.;
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is sent to the turbine and then is`exhausted to the condenserThen9 together with the new supply of water~ which is provided as needed, it is fed into the exchanger.
In the figure, the mixed bed type exchanger 1 is shown in a single unit. It can be used this way or~ as mentioned above, it could be one of the many exchangers which work in parallelO ~-Inlet and outlet of the condensate water, which are needed in the operation of the exchanger 1 for the condensate water purifica-tion, are indicated by pipes 101 and 102. When the desalting ca-pacity of the resin decreases to a point where it requires regen-eration, the flow of condens~te water is stopped and the resin is sent to the separator-regenerator 2 through pipes 103 or 104.
Granules of the resin thus transferred are separated, and then the resin is backwashed in order to remove the solid which came into being in the resin as the result of the filtration effect of the resinO
The back-washing is carried out by flowing water in the upward direction under a suitable rate, and also flowing gas~ such as air9 interchangeably or simultaneously with the water. The in-let for the water and gas is indicated by pipe 105 or 106, and their outflow by pipe 1070 ~-When the solid components, such as metallic oxides, are removed substantially, the flow of water into the exchanger is stopped or adjusted. Then, following the ordinary method, the two resins are separated into 2 layers as illustrated in figure lo As the spe~ific gravity of the anion exchange resin is smaller, the layer of anion exchange resin 3 forms at the upper part and the layer of cation exchange resin 4 forms at the lower portionO
Next, each of the resins is regenerated; the process can be preferably carried out separately by transferring the anion ex-change resin into a separate regenerator. In this case, an open-ing of the resin transfer tube 7 is installed at a suitable dis-- 4 -~
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8~) tance above the boundary surface 5 o~ $he two resins, and by the transfer water which is fed from pipe 108, the anion exchange re-sin is transferred into the anion exchange resin regenerator ~
through the resin transfer tube 7O In the said regenerator 8, it is fed from pipe 110 and is regeneralted by the aqueous solution of caustic soda which is taken in from pipe lll. Also, it is ~ed :~
from pipe 112 and is washed with the washing water which is taken out from pipe 111.
~ven if the two resins are separated into two layers very carefully, and a suitable distance is provided between the opening of the resin transfer tube 7 and the boundary surface 5 between the two resin layers, to provide a barrier of the anion exchange resin, it is still practically impossible to avoid, com-pletely, the presence of cation exchange resin in the layer of an-ion exchange resinO Many factors can be raised to explain this:
even with resin granules of standard size, it is difficult to ac-complish a perfect layer separation; to make things worse, the ca- -tion exchange resin granules break into finer particles during the :~
continuous operation, and these small particles at approximately the same speed as that of standard sized anion exchange resin granulesO Consequently, contamination occurs and, usually, a per-centage of the Na type cation exchange resin is mixed in the anion -~exchange resin layer which was regenerated by caustic sodaO Then, `-when the regenerated resins are mixed and provided for use in the purification treatment of condensate water~ there arises the pro-blem of Nat ion leakage in the initial periodO
In this invention, after regeneration and washing, the anion exchange resin which contains a small amount of ~a type ca-tion exchange resin is treated with hydrazine~ and when the fol- -lowing speci~ic method is chosen for the treatment9 consumption of ~;

hydrazine can be kept at a minimum amount while achieving effective treatment.

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From the storage tank 9 for the aqueous solution of hy-drazine, the hydrazine solution is contacted~ via pipe 113, with the anion exchange resin layer which contains a small amount of Na type cation exchange resin. By this contact~ the aqueous so-lution of hydrazine comes to contain a small amount of Na+ ion;
this is taken out by pipe 114 and is sent to the hydrazine puri-fication tank 10 which is filled with a hydrazine type cation ex- ~
change resin. In the said tank, the small amount of Na+ ionwhich ~; -accompanied the hydrazine is trapped by the cation exchange resin, and $he corresponding amount of hydrazine separates out and flows into the aqueous solutionO Thus, the aqueous solution o~ hydra-zine which passes through the hydrazine purification tank and is taken out by pipe 115 is of the same quality level as that of the solution which is fed from pipe 113. And this can be used~ with-out any further purification, again for the hydrazine treatment of ~`
the anion exchange resin layer. The purified aqueous solution is preferably returned to the storage tank 9 by pipe 115, and hydra-zine is also fed into the storage tank 9 by pipe 116 to make up ~ -the natural loss. Continuous operation of the hydrazine purifi-cation tank 10 is regulated by the set point value of Na+ ion in the purified aqueous solution of hydrazine which is taken out by pipe 115. When the amount of Na+ exceeds the set point value or is immediately be~ore that value, the operation is stopped, and ~ -the cation exchange resin in the said purification tank is regen- -erated by the acid which is fed from pipe 116 and is taken out by pipe 1170 Then, i-t is washed, and is converted back to the hydra-zine type by use of the aqueous solution of hydrazine which is fed from pipe 118. After this, it is again provided for use in the purification treatment. When the period in which the purification process in the purification tank 10 is stopped, overlaps with the period in which the hydrazine treatment operation in the regener-ator 8 is occurring, for example, a buffer storage tank can be installed in the middle of pipe 11~ to temporarily store the aqueous solution oE hydrazine after the treatment, and the Eresh hydrazine solution can be continuously Eed ~rom the hydrazine storage tank 9; in this way, even during the period in which the operation oE the puriEication tank 10 is stopped, hydrazine treat- -: ment in the regenerator 8 can be favourably continued without in-terruption.
By this hydrazine treatment, the Na type cation exchange resin which was contaminated in the anion exchange resin layer is converted to the hydrazine type, and the above mentioned problem of Na+ ion leakage during the initial stage in the puriEication oE condensate water is eliminated. The previously proposed method oE using ammonia in place of this hydrazine treatment eliminates ~:-the above mentioned problem in a similar manner, but it takes a :
relatively long time and also requires a large amount of ammonia.
In this invention, the time needed Eor removing ~a+ ion to a satis-factory level is shortened to less than halE a minute, and conse-quently, the amount of treating agent used can also be reduced~
Furthermore, as will be described later, in the ammonia treatment, the anion exchange resin gradually loses the ion exchange capacity because of contamination by iron components during the repeated cycles oE condensate water puri~ication and regeneration In this invention, however, the hydrazine treatment removes these iron components simultaneously, and thus there is the advantage of achieving more per:Eect recovery of ion exchange capacity.
Furthermore, consumption oE hydrazine can be limited to a minimum level by installing the hydrazine puriEication tank as mentioned aboveO
Also, it is possible to eliminate the hydrazine storage tank 9~ iE the puriEied hydrazine being returned by pipe 115 can be fed immediately into the same or other regenerator 80 On the other handg the layer of cation exchange resin - 7 - :~
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which was left ill the separator-regenerator is regenerated by the acidic regeneration agent (usually sulfuric acid is suitable for use) which is fed from pipe 109 and taken out by pipe ll9o After this, it is washed by the ordinary method.
The figure illustrates the method of regenerating the layer of anion exchange resin and the layer of cation exchange resin in separate zones. But, the invention is not limited to this method. For example, at a suitable distance above the boun-dary surface between the two resin layers, the collecting anddis-tributing pipe 6 for the regeneration agent and hydrazine can beinstalled and the two layers can be regenerated in the same zone with the two layers as they are. And it is also possible to have the layer of anion exchange resin subjected to the hydrazinetreat-ment; in this case, the resin transfer tube 7 and the regenerator 8 are eliminated and the separator-regenerator requires more pip-ing; as for the combinations, various methods can be adopted as is obvio~ls to those engaged in this field. Also, the figure does not indicate the accessory parts such as the piping for the wash-ing water and valves; ~or this also, the well known methods may be used.
In this way, the anion exchange resin is regenerated in ~ -to the OH type, the contaminated cation exchange resin is regen-erated into the hydrazine type, and the main cation exchange resin is regenerated into the H type. Each of these regenerated resins is washed thoroughly, and the resins are mixed again (a tank for mixing can be installed if necessary) and returned to the mixed bed type exchanger l; then it can be used for purification of con-densate water. Thus, in the method of this invention, a major part of the cation exchange resin contributes to the purification of condensate water in the form of H type. But, as mentionedear-lier, the condensate water contains a large amount of ammonium ion used for the pH adjustment, and as the flow of liquid continues, the resin converts gradually to the ammonium type, except for the portion which trapped the metallic ions of the impurities. And even after the layer of cation exchange ion is staurated with am-monium, the said resin layer still has the ability of trapping various metallic ions, insluding Na+ ion. There~ore, when the ammonium ion starts leaking from the mixed bed type exchanger 1, the operation may be stopped right then or immediately before that, but the operation can also be continued. In this case, it is desirable to detect the leakage of Na+ ion and use the result as the criterion for stopping the operation. Thus, in this in-vention, the cation exchange resin which is the main agent, is used in the form of H type without modification for the condensate water purification cycles, and when it is done so, effective trap-ping of ~a+ ion is possible even when an unexpectedly large amount of Na+ ion is introduced, as in the case of contamination by sea water. This makes the invention especially suitable for such cases (in the cases requiring strict management as in atomic en-ergy power generation, -this method is especially desirable). In principle, this invention is not limited to this; as in the well known methods, the regenerated cation exchange resin layer can be ammoniated beforehand, and, if needed, it can be hydrazinated be-fore use in the condensate water purification cycles.
The hydrazine solution treatment of the anion exchange resin layer contaminated with the cation exchange resin previous-ly described, can be done in various modes. A preferred method is to pass the aqueous solution of a relatively low concentration in a relatively high flow rate; ordinarily an aqueous solution of hydrazine of a concentration of 0O05 - 5 weight %9 preferably of 0.1 - 1 weight % is passed at a flow rate of 1 - 50 m/hr, prefer-ably of 5 - 20 m/hr.
As described above in detail 7 in the regeneration pro-cess of this invention, the danger of leakage of Na+ ions during , ~i3~
the initial period, due to the cation exchange resin contamina-ted in the anion exchange resin layer, is completely eliminated, and at the same time, the following advantages are obtained. ~on-ventionally, in the feed water managemen-t for the boiler water,in-cluding the condensate water, a method called volatile treatment has been used. This is one of the methods of feed water manage-ment for boiler water, and it has been used in the water quality control of the feed wa-ter, especially in the pH control with am-monia and in the deoxygenation with hydrazine. In this invention, the hydrazine in the hydrazine type cation exchange resin seldom affects the pH control of the feed waterO In other words, the hy-drazine type cation exchange resin is ion-exchanged by other metal-lic ions in the water tower and, even if it should mix with the feed water, most of it will decompose into nitrogen gas and H~0 (water) by reacting with the dissolved oxygen contained in the feed water, when the temperature is above 200 C, giving no effect to the p~ value of feed water; at the same time, this helps more complete elimination of the dissolved oxygen contained in the con-densate waterO Among the impurities contained in the condensate ~ -water, it is believed that the amount of iron component is especi-ally large, and the property and state of the irons contained in the condensate water have been discussed extensively. In the mixed bed of ion exchange resins, a part of the iron components is -~adsorbed to the ca-tion exchange resin and a part of it is adsorbed to the anion exchange resinO In this case, the iron components which are adsorbed to the anion exchange resin can accumulate in the resin when the cycles of regenerating by use of caustic soda alone are repeated~ This invention is characterized by use of hy-drazine whose activity as a reducing agent is effectively utili-zed to easily remove the iron components adsorbed to the anion ex-change resin. By this method, the ion exchange capacity of the anion exchange resin decreases less, and the treatment capacityof ,, - 1 0 -- ' ' ~ 7 i3~
the device ~or desalting condensate water can be advantageously kept at a high e~iciency at all times.
This invention can be applied to the ion exchange re-sin used in the devices for desalting condensate water which have - been used in the past~ And as for the resins used, any known kinds of resins can be usedO

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of desalting treatment of condensate water by a mixed bed of ion exchange resins, the method being charac-terized by the following procedure: in regenerating the charged resins, after passing the condensate water through the mixed bed of ion exchange resins, the above said mixed bed is separated in-to a layer of anion exchange resin and a layer cation exchange re-sin; the said anion exchange resin layer is regenerated by caus-tic soda and is then treated with an aqueous solution of hydra-zine; after the said treatment, the aqueous solution of hydrazine is purified by passing it through a bed of hydrazine type cation exchange resin; then, the said purified aqueous solution of hy-drazine is reused in the treatment of the above said layer of an-ion exchange resin.