Mixed-bed (MB) exchangers contain both SAC and SBA resins intimately intermixed. The primary purpose of a MB exchanger is to polish the already purified water. MB quality water is needed for high-pressure boiler makeup and at some manufacturing facilities, particularly in the electronics and semiconductor industries.

Because a MB exchanger has such a light ionic load, flow rates can be increased. A typical cross-sectional flow rate is 20 GPM/ft ². The mixed bed is sized to provide very long term operation with infrequent regenerations. This can be done without excessive quantities of resin because the ion loading is so low. Consider a SAC/SBA demineralizer with an effluent that contains 1 ppm of cations as CaCO ³. If the cation exchange resin from the earlier example is used in the MB exchanger, 50 cubic feet could theoretically process 13,500,000 gallons of water. At 100 GPM, the MB cation resin would not exhaust for three months. (In actuality, the higher flow rate can reduce the operating capacity somewhat. Even so, very long run times are still normal.)

Although regeneration of mixed-beds is performed infrequently, it is a very important process and must be handled with care, since two resins are involved. Regeneration is usually performed in the exchange vessel. Anion resin is lighter than cation resin, and when the MB resin is backwashed, the process causes the two resins to settle into distinct layers. By design, a collector system is installed at the cation/anion resin interface. During regeneration,...