Principles of Water TreatmentDOE-HDBK-1015/2-93WATER TREATMENT PROCESSESRev. 0CH-04Page 7A particular resin may be prepared in different forms according to the identity of theexchangeable ion attached. It is usually named according to the ion present on the active sites.For example, the resin represented by R-Cl is said to be the chloride form of the anion resin, orsimply the chloride form resin. Other common forms are the ammonium form (NH -R), hydroxyl4form (R-OH), lithium form (Li-R), and hydrogen form (H-R).The mechanics of the ion exchange process are somewhat complicated, but the essential featurescan be understood on the basis of equilibrium concepts discussed in Module 1 and recognitionthat the strength of the ionic bond between the resin and an ion varies with the particular ion.That is, for a particular resin, different ions experience different attractions to the resin. Theterm affinityis often used to describe the attraction between a resin and a given ion. This affinitycan be described quantitatively by experimental determination of a parameter called the relativeaffinity coefficient. For a qualitative discussion, it suffices to note the relative affinities betweena resin and different ions.In order of decreasing strength, the relative affinities between a cation resin and various cationsare as follows.Ba > Sr > Ca > Co > Ni> Cu> Mg > Be+2+2+2+2+2 +2 +2+2Ag > Cs > Rb > K NH > Na > H > Li++++++++Similarly, the relative affinities between an anion resin and various anions are as follows.SO> I > NO > Br > HSO > Cl > OH > HCO > F4333-2--------The physical arrangement of one type of ion exchange vessel for purifying water is shown inFigure 3. The ion exchange resin is contained in a vessel with a volume of several cubic feet.Retention elements at the top and bottom consist of screens, slotted cylinders, or other suitabledevices with openings smaller than the resin beads to prevent the resin from escaping from thevessel. The resin bed is a uniform mixture of cation and anion resins in a volume ratio of 2 partscation resin to 3 parts anion resin. This arrangement is called a mixed-bed resin, as opposed toan arrangement of cation and anion resins in discrete layers or separate vessels. The use ofdifferent volumes of the two types of resins is due to the difference in exchange capacity betweencation and anion resins. Exchange capacity is the amount of impurity that a given amount ofresin is capable of removing, and it has units of moles/ml, equivalents/ml, or moles/gm. Theanion resin is less dense than the cation resin; thus, it has a smaller exchange capacity, and alarger volume is needed for anion resins than for the cation resins to obtain equal total exchangecapabilities.
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