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12.32 CHAPTER TWELVE

bed system, to more complex systems consisting of several different vessels with various
combinations of weak acid, strong acid, weakly basic, and strongly basic resins.
When the bicarbonate alkalinity is more than 50 ppm, it is sometimes, depending on
the flow rate, more practical to employ a forced-draft decarbonator to remove carbon diox-
ide in the effluent of the cation exchange vessel by blowing air through the water. This
reduces the load on the anion exchange vessel, which follows the decarbonator.
Mixed beds of cation and anion resins give the effect of multiple stages of two-bed
systems. Most mixed resin systems consist of strongly acidic and strongly basic cation
and anion resins. Mixed beds are most widely used for polishing water that has previ-
ously been deionized either by a membrane process or by a two-bed demineralizer. The
main advantage of mixed bed demineralization is the improved water quality.

BASICS OF EQUIPMENT DESIGN

This section covers the basics of how ion exchange systems are sized, the requirements
for ancillary systems, and the selection of materials of construction. These vary depend-
ing on the type of equipment and the equipment suppliers' preferences. Therefore, the
subject is presented as a brief introduction rather than as a design manual.

Flow Rates
Most ion exchange systems are limited by flow rate for physical reasons such as pressure
loss or the physical characteristics of the resin rather than by any particular chemical re-
quirements. With respect to linear flow rates, if the linear flow rate exceeds about 20
gpm/ft 2, the pressure exerted on the resin bed, together with other stresses such as os-
motic and thermal, is sufficiently large to begin to break some of the resin beads. The
strength of the individual beads is actually much higher, but when pressure loss across
the bed routinely reaches about 50 psi, excessive bead breakage can be expected. In gen-
eral, the maximum allowable pressure loss across the cation beds should always be less
than 20 psi and across the anion beds less than 20 psi. It is common practice to allow for
a 5- to 10-psi increase in pressure drop across the resin bed over time. Therefore, the de-
sign value for the maximum flow rate across a resin bed is usually in the neighborhood
of 15 gpm/ft 2.
On the low end, the minimum flow rate is that which the liquid distributors can ac-
commodate. Typically this is the flow rate during the regeneration cycle. The maximum
practical turndown is about 5: 1. A liquid distributor designed for a maximum flow rate
of 5 gpm may not give good distribution at flow rates below 1 gpm. Some distributors
can do somewhat better than this with 10:1 being about the maximum, while other dis-
tributors, such as those used in some of the less expensive commercial type tanks, do not
do this well. Rinse water flow rates of less than JA gpm/ft 3 should generally be avoided
due to the potential difficulty in rinsing the regenerant out of the resin bed and the con-
sequent prolonged leakage of regenerant into the product water during service.

Regeneration Frequency and Media Depth

Resin volume requirements are generally selected based on the user preference for cycle
times between regenerations within the limits defined by flow rate and bed depth re-
quirements. Regeneration frequencies are usually kept at less than 2 or 3 times per 24 h.