Page 189 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

WATER TREATMENT AND PURIFICATION 4.25

There are a large number of ion exchange resins available. Each of them has been
formulated to obtain optimum performance for different impurities. The affinity for spe-
cific ions in solution is termed the selectivity coefficient. The number of charges (valence)
available on a particular ionic medium is a major factor in the selection of specific resins
to remove the desired impurities, and is based on an analysis of raw water. These resins
are contained in a vessel often referred to as a column. The actual resin bed could be sup-
ported by a mat of graded gravel, screen-wrapped pipe, or perforated plates, which also
act to evenly distribute feedwater over the entire resin bed. The size of the resin beads in
the vessel also creates an effective depth filter. This filtering action leads to fouling and
unpredictable operating runs because of an accumulation of particulates.
Anion resins could be either strong or weak bases. A common anion resin is divinyl
benzene, a gelatinous bead. Anion resin type 1 premium has a very close tolerance of bead
size. Anion resin type 1 regular is generally used for maximum silica reduction. Resin type 2
is used most often unless type 1 is specifically requested. There is a difference in cost and
capacity between the two resins. In general, the higher cost of the type 1 resin is considered
acceptable in order to obtain a more efficient and longer-lasting resin. Weak base exchang-
ers are not effective in the removal of carbon dioxide or silica. They remove strong acids
more by adsorption than by ion exchange. The end result is the same, and the efficiency
of weak base regeneration for acid salt removal is far superior to that of the strong base
material doing the same job. Thus weak base units are superior when the feedwater is high
in sulfates and chlorides.
The two cation exchange resins used most often are strong or weak acids. Strong cation
resins remove all cations regardless of the anion with which they are associated. These
resins have a moderate exchange capacity and require a strong acid regenerant such as
hydrochloric or sulfuric acid.
The deionization process can be arranged as either a two-step (dual-bed) or single-step
(mixed-bed) process. In the dual-bed process, one vessel contains the anion exchange res-
ins and the second vessel, the cation exchange resin. In the mixed-bed unit, a single vessel
contains a mixture of both resins. The dualbed arrangement produces water that is less
pure than that produced by a mixed bed, but the dual bed has a greater removal capacity. A
typical mixed bed contains 40 percent cation resins and 60 percent anion resins. Dual beds
are easier to regenerate. It is not uncommon to have a dual-bed exchanger, often referred to
as a working exchanger, installed in front of a mixed bed to remove the bulk of the impuri-
ties, and then have the mixed bed, often called a polishing exchanger, further purify the
water to the desired high level. A typical single-bed ion exchange unit is illustrated in
Fig. 4.6. A typical dual-bed ion exchange unit is illustrated in Fig. 4.7. A typical mixed-bed
ion exchange unit is illustrated in Fig. 4.8. The piping and valve arrangement for different
manufacturers may be different.

Regeneration Cycle
The ion exchange process is reversible. As the water continues to pass through the ion
exchange resin beds, the number of ions on the resin beads available for exchange declines
with time and gradually becomes exhausted. This starts first at the water entry to the vessel
and progresses down the bed. When the resin has reached the limit of exchange, the bed is
said to have reached its exchange capacity. It is then necessary to take the column out of
service to be regenerated.
Regeneration, which is the reverse of deionization, is the term used for the replacement
of the ions removed by the feedwater. Regeneration generally consists of three steps:
(1) backwashing, (2) application of regenerating solution, and (3) rinsing. Regeneration
can be performed either cocurrently (in the same direction as the flow of feedwater) or
countercurrently (in the opposite direction of the flow of feedwater). All of the water used
for regeneration must be routed to a drain of adequate size. In addition, the acid and caustic

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