The Rate Problem of a Thermally Regenerable Ion-Exchange System

Earlier equilibrium studies have established the thermal dependence of the equilibrium between salt solution and a mixed bed of weakly basic and weakly acidic ion-exchange resins. High resin utilization can be achieved if the resin properties and equilibrium conditions are optimized; the equilibrium characteristics of polyacrylic acid and polyvinylbenzyldiethylamine resins are quite suited for the practical desalination of brackish waters.

However, the adsorption rates exhibited by normal-sized resin beads of this type are much too slow for satisfactory operation of the process because of the low concentration of protons available for transfer between the resins. It is shown that increasing the porosity of the resins improves amine resin kinetics 10-fold and carboxylic acid resin kinetics 6-fold. Nevertheless such improvements are still inadequate for practical purposes, and it is concluded that for satisfactory rates to be achieved systems having much shorter diffusion paths are necessary.

Two further approaches to the rate problem are discussed, both involving the synthesis of novel resin systems. A mixed bed of microbeads (10-20 p) reacts at acceptable rates but presents mechanical problems; the magnetic flocculation of finely divided magnetic resins is reported as one possible solution to this problem. Another avenue is the synthesis of normal-sized beads of the amphoteric and snake-cage variety. Resins of this type that exchange at suitable rates are described.