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Figure 3   Schematic Diagram of a Typical Ion Exchanger
Ion  Exchange  Process

Chemistry Volume 2 of 2
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Principles of Water Treatment DOE-HDBK-1015/2-93 WATER TREATMENT PROCESSES Rev. 0 CH-04 Page 9 Because of the different densities of anion and cation resins, the flow of solution (impure water) is from top to bottom.  If the flow were reversed, the lighter anion resin would gradually rise to the top by a process called classification, resulting in a layer of anion resin on top of the cation resin, as shown in Figure 3.  In the example shown, the layering results from regeneration and/or  backwash.    In  systems  not  using  a  backwash,  the  anion  and  cation  resin  beads  are uniformly mixed.  Many systems use a backwash procedure, if the resins are regenerated, to remove  solids collected by filtration and to separate the resins for regeneration.  They are remixed after regeneration. For fixed amounts of anion and cation resins, the efficiency for removal of impurities is greater in a mixed-bed resin than a layered arrangement.  The main reason is that for layered resins there may be large pH gradients within the column of resin.  If, for example, the hydroxyl form resin is on top, as solution passes through it anionic impurities are removed and replaced by OH  ions; thus, the pH increases.  This increase in pH may decrease the efficiency in lower - portions  of  the  resin  bed  for  removing  impurities.    It  may  also  cause  some  impurities  to precipitate because solubility changes with pH.  The resin column will filter some undissolved material, but the efficiency for filtration is usually significantly less than that for removal by ion exchange.  Thus, the overall efficiency is less than in a mixed-bed resin. The capacity of ion exchange resins to remove impurity ions is given in Table 2 along with other information  on  resins.    For  instance,  each  cubic  foot  of  a  mixed-bed  resin  is  capable  of exchanging  with 19.8 moles each of monovalent cations and anions.  Mixed-bed resins are available commercially and in practical applications several cubic feet are used in a purification system. TABLE 2 Properties of Ion Exchange Resins Properties Cation Resin Anion Resin    Mixed-Bed Resin Ion exchange capacity, moles of single ion/ml 1.75 x 10 1.20 x 10 0.7 x 10    anion & cation -3 -3 -3 Ion exchange capacity, moles of single ion/ft3 49.5 34.0 19.8 anion & cation Density of wet resin particles, grams/ml 1.27 1.10 -- Bulk density of loaded bed (including voids), grams/ml 0.80 0.62 0.70 Volume fraction -- -- 60% anion & 40% cation







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