Mixed-Bed DemineralizerA mixed-bed demineralizer is a demineralized in which the cation and anion resin beads are mixed together. In effect, it is equivalent to a number of two-step demineralizers in series. Ina mixed-bed demineralizer, more impurities are replaced by hydrogen and hydroxyl ions, and the water that is produced is extremely pure. The conductivity of this water can often be less than 0.06 micromhos per centimeter.
Mixed-Bed RegenerationThe mixed-bed demineralizer shown in Figure 14 is designed to be regenerated in place, but the process is more complicated than the regeneration of a single-bed ion exchanger. The steps in the regeneration are shown in Figure 14.Figure 14a shows the mixed-bed ion exchanger in the operating, or online mode. Water enters through a distribution header at the top and exits through the line at the bottom of the vessel.Regeneration causes the effluent water to increase in electrical conductivity.The first regeneration step is backwash, as shown in Figure 14b. As in a single-bed unit, backwash water enters the vessel at the bottom and exits through the top to a drain. In addition to washing out entrained particles, the backwash water in a mixed-bed unit must also separate the resins into cation and anion beds. The anion resin has a lower specific gravity than the cation resin; therefore, as the water flows through the bed, the lighter anion resin beads float upward to the top. Thus, the mixed-bed becomes a split bed. The separation line between the anion bed at the top and the cation bed at the bottom is called the resin interface. Some resins can be separated only when they are in the depleted state; other resins separate in either thedepleted form or the regenerated form.The actual regeneration step is shown in Figure 14c. Dilution water is mixed with the caustic solution and introduced at the top of the vessel, just above the onion bed. At the same time, dilution water is mixed with acid and introduced at the bottom of the vessel, below the cautioned. The flow rate of the caustic solution down to the resin interface is the same as the flow rate of the acid solution up to the resin interface. Both solutions are removed at the interface and dumped to a drain.
