Water purification by ion exchange mixed beds
Ion exchange is used extensively for the removal of ionised impurities found in natural waters. The final stage in the production of ultra pure water is normally a bed of mixed anion and cation exchange resins. Three areas within the operating cycle of a regenerable mixed bed - resin separation, resin mixing and anion exchange kinetics - have been investigated. Complete separation of the two resins by backwashing, prior to chemical regeneration, is necessary to prevent the subsequent release of trace impurities into the purified water. Various published models of particle segregation by backwashing were examined but none accurately described the separation of two ion exchange resins with similar bead size distributions and densities. A new model has been proposed based on variations in fluidised bed porosity combined with overlapping bulk circulation cells of particles. A graphical technique has been developed to predict resin separability and the predictions compared with practical data. The effects of variations in bead size, bead density, backwash flow rate and temperature have been calculated. The variations in bead density with ionic form and polymer/matrix type of the exchanger have been measured. Following regeneration the resins are remixed by air agitation of a resin/water slurry. A mechanism to describe the progressive stages of air mixing has been proposed, based on bubble transport and bulk circulation of resin beads. The subsequent sedimentation of the resins was also considered. Laboratory and full scale studies confirmed the predicted effects of mixing fault conditions, particularly re-separation of the mixed resins. A mass transfer equation has been developed to describe the leakage of influent ions through a column of exchange resins. In conjunction with laboratory column tests the equation has been used to investigate the influence on anion exchange of polymer/matrix type, influent anion and the presence of foulants on the resin beads. Sulphate and phosphate ions exchange more slowly than monovalent chloride and nitrate ions. On a fouled exchanger the rate of sulphate exchange deteriorates more rapidly and seriously than for chloride exchange. This has been attributed to steric hindrance of the divalent sulphate ion. A laboratory method has been developed for the routine assessment of mixed bed anion exchangers and the prediction of their performance potential in service, with particular application to condensate purification for boiler feedwater.