In many process boiler systems, zeolite softening of water is not sufficient to meet feed water quality standards. In these cases (typically high-pressure boilers up;1000 PSIG), the water is demineralized. This process removes most of the mineral content of the raw water.
Demineralizers depend upon both cationic and anionic exchange processes. They can be arranged in many different combinations, based upon incoming water contaminants and desired final water quality. By the end of this chapter, you should have a basic understanding of what demineralization does, how these systems work, and the basic steps necessary to troubleshoot a system
Demineralization on a commercial scale became possible after resin manufacturers developed methods of mass-producing a new form of ion exchange resin. When regenerated with caustic (NaOH), this resin removed silica and carbon dioxide from the effluent of a zeolite resin operated in the hydrogen cycle. A standard cation resin of the type commonly found in zeolite softeners, which is regenerated with sulfuric acid as opposed to salt (hence: hydrogen cycle cation), is called a strong acid cation resin.
An anion resin with a strong affinity for silica and carbon dioxide is described as a strong base anion resin. An offshoot of the strong base anion resin development was the production of a chemically different resin with a strong affinity for strong-acid anions: sulfate (SO4), chloride (CI), nitrate (NO3), phosphate (PO4), etc.This was called a weak base resin.
The weak base resin does not remove silica or carbon dioxide, as does the strong base resin. Therefore, it cannot be used alone to produce the water purity demanded by high-pressure steam plant operations. Less caustic is required to regenerate the weak base resin and it is often coupled with a strong base resin in the same vessel to produce the most economical anion unit operation. The weak base resin is always loaded into the vessel last when the unit is readied for downflow service.
Putting the weak base resin on top of the anion resin bed uses its higher capacity for strong acid removal to insure that the strong base resin can concentrate on removing the weak acids associated with the carbon dioxide and silica (carbon and silicic acids).
The cation and anion vessels of a demineralized train are piped in series so that the effluent of the cation unit enters the top of the anion unit and the discharge from the anion unit goes to the treated water header or into a polishing unit.
A discussion of what takes place as the raw water is processed through the various units in a demineralized may help to explain the resins and their function.
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