Environmental Engineering Reference
In-Depth Information
Univalent-divalent exchange ( z A =
2, z B =
1) is an important application of ion-
exchange technology. For this case, the selectivity coefficient is:
C
C
x A ) 2
y A (1
( K c ) B
=
y A ) 2 .
(8.13)
x A (1
The preference for A (divalent ion) is greatly increased as the solution concentration is
decreased. As one increases C , the selectivity will decrease.
8.6.2
Multicomponent ion exchange
The theoretical treatment of multicomponent ion exchange involving species of different
valences is extremely complex. For ternary systems, the theory is reasonable and quite
precise, although the representation relies heavily on the constancy of the relevant sep-
aration factors. Rigorous treatments are available [8-10]. Regrettably, the values of the
selectivity coefficients and separation factors are not constant in practice. More detailed
information on representing and predicting multicomponent ion exchange can be found
in [11-14].
8.7
Equipment and design procedures
8.7.1
Equipment
Because the ion-exchange process is very similar to the adsorption process, the equipment
is also similar. Ion exchange occurs in batch tanks, stirred tanks, fixed-bed columns,
fluidized-bed columns, and moving-bed processes. The most common process is a
fixed-bed column (Figure 8.6), which can be operated in a cocurrent or countercurrent
fashion.
Both cocurrent and countercurrent fixed-bed columns operate in the same way during ion
exchange, but the regeneration steps are different. For a cocurrent column, after the column
reaches a predetermined breakthrough level, it is removed from the process and regenerated
by passing the regenerate solution through the column in the same flow direction as the
solution that is being treated. A countercurrent column will have the regenerate solution
pass through in the opposite flow direction from the solution being treated. Cocurrent
columns are simpler to design and operate, while countercurrent columns have higher
chemical efficiencies [4].
Moving-bed columns are based on a moving packed bed in counterflow to the nor-
mal service flow of solution. It is always necessary to shut off normal flow to achieve
bed movement, and hydraulic displacement is used to move the bed with either a
piston or pump driving the displacement water [1]. The two most successful commer-
cial designs are the Higgins technique and the Asahi moving packed bed (Figures 8.7
and 8.8).
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