R CH2N(CH3)3OHR CH2OHN(CH3)3heatR CH2N(CH3)3OHR CH2N(CH3)2CH3OHheatDOE-HDBK-1015/2-93DISSOLVED GASES, SUSPENDED SOLIDS, AND pH CONTROLPrinciples of Water TreatmentCH-04Rev. 0Page 24As solution passes through the resin column, the relative amounts of ammonium andhydrogen ions on the resin change. The actual exchange process occurs primarily in arelatively narrow band of the column rather than over the entire length. This band iscalled the exchange zone. Assuming the column is vertical and that solution flows fromtop to bottom, the resin above the exchange zone is depleted; that is, practically all theexchange capacity has been used. Below the exchange zone, essentially none of theresin's exchange capacity has been used. As more and more solution flows through thecolumn, the exchange zone gradually moves downward as more of the resin is depleted.Eventually, as the exchange zone approaches the end of the column, small amounts ofNH begin to appear in the effluent. The point at which this occurs is called4+breakthrough. If more solution passes through the resin, the concentration of NH in4+the effluent increases until it is the same as the concentration in the influent. Thiscondition is called exhaustion and indicates that essentially all the exchange capacity ofthe resin has been used. (Because of the equilibrium nature of the exchange process,a small amount of the resin may remain in the hydrogen form, but not enough to removeany more ionic impurities.) Note that because the exchange zone in this case wasnarrow, a relatively small volume of solution takes the resin from breakthrough toexhaustion.ResinOverheatingThe potential for elevated temperatures exists during most conditions of facilityoperation, we will examine in detail the processes that occur if the resin in an ionexchanger is overheated. Although the inert polystyrene basic structure of resin isstable up to fairly high temperatures (approximately 300 F), the active exchange sitesare not. The anion resin begins to decompose slowly at about 140 F, and thedecomposition becomes rapid above 180 F. The cation resin is stable up to about250 F. Because these temperatures are well below normal reactor coolanttemperatures, the temperature of the coolant must be lowered before it passes throughthe ion exchange resin.The anion resin (hydroxyl form) decomposes by either of two mechanisms withapproximately equal probability.(4-24)or(4-25)
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