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ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12.43

in that they are inherently noncorrosive. However, they are not available in very large
sizes and are generally limited to tank diameters of 8 ft and less. They are not generally
used for pressures above 150 psig or temperatures above 100 ° F (under pressure). Steel
tanks can be made in almost any configuration. However, steel tanks are heavier than
fiberglass tanks. They are almost always lined and, therefore, more expensive. Where high
temperatures are employed, stainless steel is sometimes preferred over carbon steel. The
cost of building a stainless steel vessel makes it prohibitively expensive for all but a few
specialty applications.

Distributor Design
As a bare minimum, the tank should be equipped with an inlet distributor, to get the wa-
ter into the tank, and an underdrain collector, to get the water out of the tank. It is ad-
visable to have if there are separate connections for the introduction of the resin into the
tank and the removal of resin from the tank. For tanks that are larger than 3 ft in diame-
ter, it is necessary to have an access manway, so that a worker can climb inside the tank
to install or repair the internal distributors and collectors. Manways are best located at the
top of the tank. Larger manways are preferable to small manways. If the manway is lo-
cated on the side of the tank, it can create distribution problems. View ports are a feature
used in more sophisticated ion exchange systems. They allow the operator to visually in-
spect resin to make sure that resin has not been lost due to excessive backwash or prob-
lems with the underdrain collector, and to watch the resin during the regeneration and
backwash cycles to ensure that the resin bed remains packed and does not fluidize when
it is not supposed to.
To ensure the proper flow of liquid through the resin bed, the water must enter the
tank in such a fashion as to provide minimum turbulence and laminar flow into the resin
bed. Any protuberance will create a disturbance and an area of maldistribution. The resin
bed should present a flat surface that the water can flow straight down through and into
the underdrain collector. The path length should be equal for all portions of the bed. Most
pressure vessels have dished heads. The dish creates a disturbance that distorts the flow
and reduces the efficiency of the ion exchange resin. Large tanks benefit from the addi-
tion of either flat, false bottoms or subfills such as concrete or sometimes gravel that fill
the bottom head and provide a flat surface for the resin bed to lay upon. By far the most
important factor in the efficiency and quality produced by an ion exchange system is the
design of the internal distributors and collectors. The inlet distribution system must allow
the water and chemical to enter the vessel in such a way as to spread out equally through-
out the entire tank diameter with equal velocity. The velocity must be sufficiently low as
to not disturb the resin bed below. Distributor design is much more crucial in tanks with
limited freeboard. Underdrain collectors must be located as close to the bottom of the ves-
sel as possible with the least amount of resin underneath the collector as possible, and
should have collection spacing of about 6 in. Hub and lateral collectors have a problem
in the larger tank sizes for two reasons: (1) The radials are far apart at the edges of the
tank. (2) The relative surface area close to the edge of the tank is large; however, the num-
ber of collection points in a radial is small. Therefore, it is very difficult in large tanks to
get even distribution with the hub and lateral design. Another problem is that the hub it-
self causes an area of poor distribution right in the center of the tank.
If the underdrain is not located within 1 to 2 in. of the bottom of the tank, the rinse
requirement will increase significantly due to the difficulty of removing the regenerant
chemical that lays out and hides below the collector. The underdrain must also have suf-
ficient open area to collect the water without becoming clogged with resin. The slotted
pipe type of collectors sometimes used in the less expensive and smaller systems have