TABLE OF CONTENTS Demineralization, which is also called deionization, is the name given to the removal of ionic substances. It is the result of exchanging all the positively charged ions for hydrogen ions and all the negatively charged ions for hydroxide ions. The hydrogen and hydroxides then react to form water molecules: Cation exchange for hydrogen ions:
Anion exchange for hydroxide ions:
Hydroxides and hydrogen ions react to form water:
In acid exchanges, cation resins are used in the hydrogen form to exchange hydrogen ions for all other ions. Strongly acidic cation resins are normally used, although weakly acid cation resins are sometimes used in conjunction with the strong acid resins to increase regeneration efficiency in larger systems where the increased capital cost can be justified. The cation bed is normally used as the first step in the deionization process. Otherwise, the divalent cations would precipitate as hydroxides in the anion vessel. Some systems used in pharmaceutical applications do practice what is called reverse deionization, usually including a softener that precedes a hydroxide form anion exchanger followed by a hydrogen form cation exchanger.
Generally speaking, demineralization always requires cation and anion exchange. Several varieties of each kind of resin can be selected depending on the specific water analysis, quality requirements, and operating conditions. The cation and anion resins can be in separate vessels (separate beds or two beds) or in a single vessel (mixed beds). In the latter, the reaction products disappear by forming water molecules, which drives...
