Ion exchange uses are not limited to process and
boiler water makeup. Ion exchange can be used to purify, or polish, returned
condensate, removing corrosion products that could cause harmful deposits in
Typically, the contaminants in the condensate system
are particulate iron and copper. Low levels of other contaminants may enter the
system through condenser and pump seal leaks or carry-over of boiler water into
the steam. Condensate polishers filter out the particulates and remove soluble
contaminants by ion exchange.
Most paper mill condensate polishers operate at
temperatures approaching 200 o F, precluding the use of anion resin. Cation
resin, which is stable up to temperatures of over 270 o F, is used for deep bed
condensate polishing in these applications. The resin is regenerated with
sodium chloride brine, as in a zeolite softener. In situations where sodium
leakage from the polisher adversely affects the boiler water internal chemical
program or steam attemperating water purity, the resin can be regenerated with
an ionized amine solution to prevent these problems.
The service flow rate for a deep bed polisher (20-50
gpm per square foot of resin surface area) is very high compared to that of a
conventional softener. High flow rates are permissible because the level of
soluble ions in the condensate can be usually very low. Particulate iron and
copper are removed by filtration, while dissolved contaminants are reduced by
exchange for the sodium or amine in the resin.
The deep bed cation resin condensate polisher is
regenerated with 15 lb of sodium chloride per cubic foot of resin, in a manner
similar to that used for conventional sodium zeolite regeneration. A
solubilizing or reducing agent is often used to assist in the removal of iron.
Sometimes, a supplemental backwash header is located just below the surface of
the resin bed. This subsurface distributor, used prior to backwashing,
introduces water to break up the crust that forms on the resin surface between
An important consideration is the selection of a
resin for condensate polishing. Because high pressure drops are generated by
the high service flow rates and particulate loadings, and because many systems
operate at high temperatures, considerable stress is imposed on the structure
of the resin. A premium-grade gelular or macroreticular resin should be used in
deep bed condensate polishing applications.
In systems requiring total dissolved solids and
particulate removal, a mixed bed condensate polisher may be used. The
temperature of the condensate should be below 140 o F, which is the maximum
continuous operating temperature for the anion resin. Additionally, the flow
through the unit is generally reduced to approximately 20 gpm/ft≤.
Ion exchange resins are
also used as part of a precoat filtration system, as shown in Figure 8-14, for
polishing condensate. The resin is crushed and mixed into a slurry, which is
used to coat individual septums in a filter vessel. The powdered resin is a
very fine filtering medium that traps particulate matter and removes some
soluble contaminants by ion exchange. When the filter media becomes clogged,
the precoat material is disposed of, and the septums are coated with a fresh
slurry of powdered resin.