Power Plant Water Demineralization and How Does It Work?

DM PLANT PROCESS 

De-mineralization is the process of removing all dissolved salts as far as possible. The product of a D.M. (De-mineralisation) plant is almost of the quality of distilled water. The de-mineralisation system consists of one or more ION exchange resin columns which includes a strong cation unit and a strong anion unit. The cation resins exchanges hydrogen for raw water cation as shown by the reactions 

The above reactions indicate complete removal of cations and anions from the water, but in reality, these are equilibrium reaction and even with very efficient operation leakage will occur. The leakage will vary according to the demineralizer system used and for any given system, according to the raw water system composition and dimineralizer regenerate level (the amount of acid and caustic used for regenerated). To minimize cation leakage, the exchanger may be regenerated upflow and operated downflow in the service cycle. This insures that the water will pass through the most highly regenerated resin just before leaving the unit. Demineralizers which removes silica, use strong base anion resins, and both the silica content and specific conductance are important water quality criteria. Both values are high after regeneration and low after rinsing to produce satisfactory water quality. The silica level, nearly constant during the entire service run, increases sharply at the end; conductivity, also nearly constant during the service run, drop briefly at the end then rises.

The temporary drop in conductivity when the resin capacity is first exceeded is easily explorised. During the normal service cycle a small amount of sodium leaks from the cation resin. As the anion resins is converting the anions to hydroxyl (OH) ions, sodium ions combine with these hydroxyl groups to produce small amounts of sodium hydroxide (NaOH) which is highly conductive.

When the capacity of the anion resin is exhausted, silica leakage converts the sodium leakage to sodium silicate, a material less conductive than NaOH. When a silica increase is detected, the demineralizer must be removed promptly from service. If the demineralizer is allowed to remain in service until the conductivity increases not only will raw water silica leak through but raw water sulphate and chloride ions will displace additional silica from the resin. Under these circumstances the treated water silica can substantially exceed the raw water silica level.

DM systems for producing water of extremely high purity use mixed bed demineralizers (an intimate mixture of strongly acidic action and strongly basic anion resins in the same column). A typical mixed bed demineralizer will rinse down to low conductivity and silica values after regeneration. These values will remain low and nearly constant during the service cycle but then the resin is exhausted both values will increase concurrently.

When the cation and anion resins are exhausted the column are taken out of service and regenerated to get back the original power.

Cation resins are regenerated with either hydrochloric or sulfuric acid. The reactions are as follows

The relatively high concentration of caustic soda provides the driving force necessary to reverse the service cycle reactions and service cycle resin.

   

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