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12.34 CHAPTER TWELVE

1.5
CG8
e~0

ResinTech
~9
eto
cD
0.5
Operating data courtesy
ResinTech Technical Dept.
Countercurrent
-7- -- = - _n
0-r
4 5 6 7 8 9 1-0
lb HC1/ft 3
FIGURE 12.6 Regeneration method versus leakage.

claim the last portion of the regenerant and reuse it as the first portion of regenerant dur-
ing the next cycle. This is surprisingly difficult to do effectively, especially at low dose
levels. If too much is reclaimed, then the ratio of the unwanted ion will be high in the re-
claimed portion of the regenerant and will compete with the regenerant ions. When this
happens, it can virtually eliminate the effectiveness of the reclaim. When done correctly,
reclaiming regenerant can save up to one-third of the total regenerant dose.
Various methods of countercurrent regeneration have been devised to overcome the
limitations of coflow regeneration. Countercurrent regeneration is primarily used in de-
mineralization, and except for being occasionally used in nitrate removal, it is almost
never used in salt cycle exchange. In all countercurrent regenerations, the dilute regener-
ant solution is introduced in a flow direction that is opposite to the service flow such that
the least used portion of the resin (polishing zone) bed is contacted by the fresh regener-
ant first. This results in very low leakages even at the lowest regenerant dose levels, which
provides higher regeneration efficiency. Countercurrent systems save on chemical oper-
ating costs, and waste discharge is reduced. These advantages are offset by higher capi-
tal costs and a more complex operating system. Maintenance costs are usually higher than
for cocurrent regenerated systems. A comparison of leakages at equal regenerant levels
is shown in Figure 12.6.

Cocurrent versus Countercurrent
The design of a demineralizer, at least in the United States, is often based on customer
preference. It may also be based on the skill of the company building the equipment and
its experience with various types of demineralizer designs. The method of regeneration
chosen will most likely be made based on effluent water quality requirements and on the
inlet and outlet water quality. The cleaner the raw water and the higher the effluent qual-
ity required, the more likely it is that countercurrent regeneration will be employed. The
dirtier the raw water and the less stringent the effluent quality requirement, the more likely
a coflow unit will be employed.