Optimizing Cation to Anion Resin Ratio in Mixed-Bed Ion Exchange

A mixed bed ion exchange (MBIE) column producing ultrapure water will be removed from service well before its capacity is exhausted. There is usually an operational criterion for replacing or regenerating the bed; for example, defining 50 parts per trillion (ppt) Na as an endpoint in Pressurized Water Reactors (PWRs). These plant-specific criteria depend on the water treatment objectives and include low day-today concentrations or the ability to handle an upset event such as a concentration spike. In mixing the bed initially the cation to anion resin ratio is frequently set to where the equivalence of cation capacity equals the equivalence of anion capacity. An alternative approach is to define the resin ratio based on the feedwater pH so that the initial breakthrough time of cations and anions are the same. This ratio calculation procedure using feedwater analysis and a rate limited ion exchange model is described and the impact of this approach is discussed.