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 boilers.
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 regenerations.
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.