Mixed-Bed Polishers

In very high-pressure boiler operations, even trace contaminants are extremely detrimental. Demineralized water requirements are more stringent than can be produced by the two-bed unit. To cope with these specifications, many operations follow the two-bed demineralizer with a mixed-bed polisher. The mixed bed polisher consists of a special vessel into which are loaded both strong base anion and strong acid cation resins. The cation resin anion, being denser, is always on the bottom of the vessel after backwashing. The regenerant piping is designed so that caustic enters the top of the vessel, flows down through and regenerates the strong base anion resin, and exits the side of the vessel near the interface between the anion and cation resins. Similarly, the sulfuric acid enters the side of the vessel near the resin interface and flows downward through the strong acid cation resin, regenerates it, and flows out of the bottom of the vessel to the sewer. Prior to the addition of the caustic, the vessel is filled with water and kept under pressure (blocking flow) to insure that no caustic channels down to the cation resin before it leaves the waste line at the resin interface.

After regeneration, air is admitted to the bottom of the vessel by opening the vent. This is called air mix and serves to commingle the resins in a random fashion. As the anion and cation resins mix, the result is a vessel filled with small two-bed demineralizers. Each cation resin bead and its companion anion resin bead function as a two bed unit. The combinations on the top that receive the water first are successful in removing a great percentage of the contaminants entering the unit. Each pair below remove, in succession, a high percentage of the contaminants in the water passing the adjacent pair of resin beads. Extremely high-quality effluent water is possible with this arrangement. The water generally contains silica and sodium in the parts per billion range.