DOWEX Ion Exchange Resins WATER CONDITIONING MANUAL

Demineralization (Deionization) Process
7.3.1 Equipment Sizing
See Section 11.7.
Note: CADIX can also be used to perform these calculations.
7.4 Product Water Quality
If it is a process water, it is likely that complete removal of carbon dioxide and silica is unnecessary. If so,
a weak base anion resin is used. If removal of carbon dioxide and silica is required, a strong base anion
resin is chosen. Demineralizers can produce waters with varied quality, depending on the type of system
and the water supply. They usually produce water free of suspended solids. Determination of dissolved
solids by evaporation will include any organic matter that may be present in the deionized water. Water
quality is often measured in terms of the amount of suspended solids, dissolved solids concentration, and
conductivity (µS/cm) or resistivity (MΩ.cm). Since conductivity is the reciprocal of resistivity, 1 µS/cm is
equivalent to 1 MΩ.cm and represents approximately 0.5 ppm. A mixed bed unit is required if water purity
in excess of 5 MΩ.cm is needed. The mixed bed may be the primary unit or a polishing unit following a
multiple bed system.
7.5 Product Water Quantity
Usually, if the water demand is less than approximately 50 gpm (11.4 m 3 /h), the plant will benefit from the
simplest piece of equipment at the expense of a higher chemical operating cost. For that reason, it is
common to find mixed bed demineralizers widely used for small plant requirements because both the
anion and cation resin can be contained in one unit and no degasifier is used. On the other hand, when
plant demands exceed 200 gpm, it is almost certain that several units will be built into the demineralizer
and one will probably be a degasifier.
7.6 Other Demineralization Techniques
The various ion exchange resin combinations indicated in the flow diagrams in Table 11 represent the
majority of system designs employed today. Variations from these standard designs, however, are being
increasingly utilized, especially those techniques that demonstrate significant chemical regenerant
utilization improvements. Two of these techniques are outlined here.
7.6.1 Counter-current Operation
This term refers to the relative flow directions of the service and regeneration cycles. As opposed to the
typical downflow service and regeneration of many systems, counter-current operation is typically
downflow service and upflow regeneration. The primary advantage of counter-current operation is that
very low leakage can be obtained at moderate regeneration levels, thus minimizing operating and waste
disposal costs. Counter-current operation is also advantageous in softening.
7.6.2 Layered Beds
A layered bed of ion exchange resin involves the use of two cation resins or two anion resins in a single
unit. A cation layered bed is composed of a weak acid resin upper layer and a strong acid resin lower
layer, while an anion layered bed uses either a weak base resin upper layer with a strong base resin
lower layer or a strong base resin upper layer and weak base resin lower layer depending on the density
of the resins. In general, the use of layered beds allows some of the advantages of weak acid and weak
base resins to be realized in the operation of a single cation or anion bed. Improved regenerant
efficiencies and, in some cases, improved operating capacities over the corresponding strong acid or
strong base units alone are attained. The layered bed concept is made possible by the density and
particle size differences between the resins used. Upon exhaustion, the backwashing operation separates
the resin layers that may have become partially mixed during service. In order for the full advantages of
the weak acid and weak base resins to be realized, good resin separation is important.
DOWEX Ion Exchange Resins 39 Water Conditioning Manual