An ion-exchange process with thermal regeneration. IV. Equilibria in a mixed bed of weak-electrolyte resins

Abstract

The titration curve data at c. 20º and 80º for weakly basic amine and carboxylic acid resins suggest that if a mixed bed of such resins is in equilibrium with a salt solution, then when the temperature of the system is raised, protons will be transferred from the base to the acid resin to an extent determined by the overlap of the titration curves of the resins. Maximum overlap would occur with two resins of appropriate acid and basic strength and which possess plateau-type titration curves, but such acid resins are not available. The theory has been confirmed by experiment. The change in salt adsorption on heating (effective capacity) depends critically on pH and may be enhanced for amine resins of homo-functional structure by increasing the ratio of acid to base resin. It is shown that to achieve the maximum effective capacity for each resin combination there is an optimum pH value and resin ratio. A screening procedure, which uses titration curve data at c. 20º and 80º and allows for the effect of pH and resin ratio, has been devised for selecting resin combinations. In a 0.030M salt solution the effective capacity of the commercial resins studied is greatest, for a poly(vinylbenzyldiethy1amine) resin in combination with a poly(acry1ic acid) resin. A-rigorous procedure has been devised for obtaining equilibrium data, which are presented for this resin combination at c. 20º and 80º in sodium chloride solutions with concentrations ranging from 0.0017 to 0.13M.