The latest generation of demineralizers include systems based on packed-bed technology. In these demineralizers, the vessel is almost completely filled with resin, with perhaps only 10% freeboard. Two processes are available, both based on countercurrent operation. In one, the process water flows from top to bottom of the vessel and the regenerant from bottom to top. Unlike conventional countercurrent regeneration, however, the regenerant is introduced at enough pressure to lift the entire bed as a plug to the top of the vessel. The other type of packed-bed system essentially reverses these functions. The process water raises the bed, and the resin is regenerated from top to bottom. These processes require different distributor and collection systems than conventional units, whose distributor laterals would be subjected to mechanical stress by the moving plug of resin. Distributor and collector plates with screened openings are located at the bottom and top of the packed-bed vessel, and serve as the restraining barrier for the resin.

Manufacturers of these systems claim several advantages versus regular countercurrent demineralizers. One advantage is that a smaller vessel will hold the same amount of resin as a standard ion exchange unit. Better regeneration efficiency is another reported advantage. This is due to the more intimate contact of regenerant with the tightly compacted bed than occurs in a standard counter-current unit where the bed may be somewhat fluidized.

One area where packed-bed technology has been applied is retrofitting of existing co-current demineralizers. Water quality can be significantly improved by...