Principles of Water TreatmentDOE-HDBK-1015/2-93WATER TREATMENT PROCESSESRev. 0CH-04Page 9Because 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 riseto the top by a process called classification, resulting in a layer of anion resin on top of thecation resin, as shown in Figure 3. In the example shown, the layering results from regenerationand/or backwash. In systems not using a backwash, the anion and cation resin beads areuniformly mixed. Many systems use a backwash procedure, if the resins are regenerated, toremove solids collected by filtration and to separate the resins for regeneration. They areremixed after regeneration.For fixed amounts of anion and cation resins, the efficiency for removal of impurities is greaterin a mixed-bed resin than a layered arrangement. The main reason is that for layered resinsthere may be large pH gradients within the column of resin. If, for example, the hydroxyl formresin is on top, as solution passes through it anionic impurities are removed and replaced byOH 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 toprecipitate because solubility changes with pH. The resin column will filter some undissolvedmaterial, but the efficiency for filtration is usually significantly less than that for removal by ionexchange. 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 otherinformation on resins. For instance, each cubic foot of a mixed-bed resin is capable ofexchanging with 19.8 moles each of monovalent cations and anions. Mixed-bed resins areavailable commercially and in practical applications several cubic feet are used in a purificationsystem.TABLE 2Properties of Ion Exchange ResinsPropertiesCation ResinAnion ResinMixed-Bed ResinIon exchange capacity,moles of single ion/ml1.75 x 101.20 x 100.7 x 10 anion & cation-3-3-3Ion exchange capacity,moles of single ion/ft349.534.019.8 anion & cationDensity of wet resinparticles, grams/ml1.271.10--Bulk density of loaded bed(including voids), grams/ml0.800.620.70Volume fraction----60% anion & 40% cation
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business