TABLE OF CONTENTS The heart of the demineralization process lies in regeneration and how efficiently and well the resin can be restored to optimum capacity after it becomes exhausted.
Figure 5-6 outlines a cation exchange vessel. Feedwater is introduced through distributors to the top of the vessel above the resin. Due to the resin's varying affinity for cations, calcium is preferentially removed, followed by magnesium and then sodium. Relatively distinct bands of these cations form in the resin, with the more tightly held ions forming more narrow bands. Eventually the resin becomes exhausted, upon which sodium begins to break through. At this point the resin must be regenerated to prevent contamination of the boiler. (In actuality, the resin never becomes totally exhausted because pockets of unreacted resin still remain due to flow variations through the bed. One such pocket, known as the heel, often develops at the bottom of the vessel along the outside walls.)
Cation Exchanger Showing Layers of Ionic Separation.
When the resin exhausts, it must be regenerated with an acid solution. In the United States, sulfuric acid is the typical regenerant, while in Europe many systems are designed to use hydrochloric acid. The chemical process is similar in both. Equation 5.1 shows that the equilibrium of ion exchange during process operation is greatly shifted to the right. This is what makes the process so effective. During regeneration, a concentrated (relative to the ionic concentration of the process water) hydrogen ion (acid) solution is introduced...
