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12.36 CHAPTER TWELVE

Degasification
Another factor in the selection of a demineralizer design is whether to use a degasifier.
CO2 is present as a gas. It is also created by the reaction of the bicarbonates with the hy-
drogen ions generated in the cation exchange vessel. CO2 can be removed by a forced-
draft degasifier or by vacuum deaeration in between the cation and anion exchangers. This
reduces the ionic load on the anion vessel. A degasifier complicates the design for sev-
eral reasons:
• It may make the anion exchanger much smaller in size than the cation exchanger, and
this creates potential problems in flow rates and regeneration times. However, the
smaller size means lower chemical operating costs.
• In most cases two or more trains of exchangers share a common degasifier. Simulta-
neous regeneration becomes more complicated as the degasifier has to stay in service
while a particular train is regenerating.
• The employment of rinse recycle becomes far more complicated in cases with a de-
gasifier. In fact, it is generally not practical to rinse-recycle the cation exchanger al-
though in many cases, even with a degasifier, the anion regeneration can finish with
rinse recycle.
• Although the reason to use a degasifier is to reduce operating costs, a certain amount
of caustic may be required to neutralize the waste acid from the cation regeneration.
This should be considered.

Cocurrent Exchangers
Cocurrent ion exchange is the oldest of all the designs and is the simplest. Although they
are inherently less efficient and do not provide as good a water quality as other designs,
they are a forgiving design and can be used in dirty water applications with high turbidity.
The cocurrent exchanger consists of a tank that contains a bed of ion exchange resin.
At the top there is an upper distributor (a means of distributing the water over the surface
of the resin bed), and at the bottom there is an underdrain collector (a means of remov-
ing the water from the resin where it exits the vessel). The raw water enters at the upper
distributor and flows down through the resin bed (Figure 12.7). During regeneration the
regenerant chemical also flows downward through the resin bed. In smaller units the re-
generant chemical and feedwater are introduced through the same upper distributor. In
larger units there is generally a separate regenerant distributor located just above the resin
bed. The advantage of a regenerant distributor is that it saves water during regeneration.
Coflow exchangers have freeboard or empty space between the upper distributor and the
resin bed, which allows for expansion of the resin bed during the backwash portion of the
regeneration cycle. During backwash, the inlet distributor becomes the outlet collector.
The regeneration of a coflow ion exchanger consists of four steps:
1. Backwash. During backwash a flow of water is introduced through the underdrain
and flows up through the resin bed at a rate sufficient to expand the resin bed by about
50%. The purpose is to relieve hydraulic compaction, move the finer resin material
such as resin fragments to the top of the bed, and remove any suspended solids from
the bed that have accumulated during the service cycle.
2. Chemical (regenerant) injection. A dilute solution of regenerant chemical flows down
through the resin bed, stripping the ions off the resin that were collected during the
service cycle and restoring the resin into what is called the regenerated form.