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
