CONDENSATE POLISHING UNIT

The Condensate Polishing Unit removes 'crud' - corrosion products consisting mostly of oxide of iron, copper or nickel, dissolved solids - mostly consisting of sodium, chloride and silica and carbon dioxide. Condensate polishing units are typically installed for super thermal power station with the main objective of improving the boiler water quality. The benefits of condensate polishing is quicker start up and as a result full load conditions are reached early giving economic benefits. Orderly shut down is possible in the case of condenser tube leak conditions.

The condensate polishers are located in the turbine hall and the exhausted resins are hydro pneumatically transferred to the water treatment plant areas where they are regenerated and transferred back to the polisher.

It is normal to operate the polisher initially in the hydrogen cycle in which the cation resin is in hydrogen form and the anion resin is in the hydroxide form. The process typically takes around 7 -8 days after which the cation resin gets converted into ammonium form and the polisher is then operated in the ammonia cycle. Experience has shown that the hydrogen cycle operation is almost always problem free and produces condensate of the required quality. Boiler drum sodium, chloride and silica increases within 2 - 3 days of operation of the polisher in ammonium cycle.

Separation of ion exchange resin in a mixed bed is done by backwashing the unit with water when cation resin settles at the bottom and the light anion resin is at the top. However, the process almost always results in presence of a small percentage of cation resins in the anion portion and vise versa - a phenomenon called cross contamination. On regeneration of the anion resin with alkali, the cation resin presents in the anion portion gets converted into sodium form and simillarly, the anion resin present in the cation portion gets converted into chloride form.

Fig-1 shows the location of the condensate polisher in the boiler turbine circuit.

 

INDION Industrial Mixed Bed Deionisers are designed to produce high purity treated water required by the pharmaceuticals and electronic industries. These deionisers can be used as polishing units after two bed deionisers or directly to obtain high purity water. Mixed Bed Deionisers are single column units, filled with strongly acidic cation and strongly basic anion exchange resins mixed together evenly. Dissolved solids in the water are thus removed, producing water of very high quality - confirming to IP specifications of purified water. The treated water, however is not free from bacteria and pyrogen.

Once the resins are exhausted, the bed is backwashed. Backwashing is initiated by introducing a uniform upward flow of water through the resin bed. The backwash step serves two important functions:

Firstly, it expands the resin bed releasing any entrapped particulate matter and resin fines.

Secondly, the backwash flow separates the denser cation resin from the lighter anion resin, forming two distinct layers in the vessel.

The first stage in the process of regeneration involves passing a dilute solution of acid, usually hydrochloric, through the cation bed. After the cation resin has been regenerated,the anion resin is regenerated by passing a dilute solution of caustic (sodium hydroxide) through the anion resin bed. As a result, the cation resin is reconverted to the hydrogen form and the anion resin to the hydroxyl form.

The final stage of regeneration is to rinse the resins of excess regenerant and then remix with air.