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Equipment: Dealkalisation

A deakalisation plant is used to remove the temporary hardness from raw water in order to reduce condensate line corrosion and boiler blowdown rates in steam boilers.

The bicarbonate (temporary hardness) present in raw waters when used for steam boilers causes operational problems with condensate line corrosion due to the breakdown of the bicarbonate in the boilers to carbon dioxide gas. This gas will condense in the boiler condensate forming carbonic acid which is corrosive to pipework if it is made from mild steel.

A complete dealkalisation plant will typically consist of:

  • Raw water break tank
  • A pressure vessel containing weak hydrogen form cation resin
  • A degasser tower
  • A water softening plant
  • Pumps for water transfer
  • pH correction pumps
  • Control panel
  • Effluent neutralisation

The raw water containing bicarbonate ions (also known as temporary hardness) is passed through the weak hydrogen cation exchange resin in the dealkalisation column. This resin selectively removes the calcium and magnesium salts associated with the alkaline hardness and then in the outlet water the bicarbonate ion exists as a solution of carbon dioxide and water. This carbon dioxide is then removed by passing the water through a degasser column. Remaining in the water is the permanent hardness which then passes through the simple softening process where the hardness is converted to sodium salts.

The raw water containing bicarbonate ions (also known as temporary hardness) is passed through the weak hydrogen cation exchange resin in the dealkalisation column.

This resin selectively removes the calcium and magnesium salts associated with the alkaline hardness and then in the outlet water the bicarbonate ion exists as a solution of carbon dioxide and water. This carbon dioxide is then removed by passing the water through a degasser column. Remaining in the water is the permanent hardness which then passes through the simple softening process where the hardness is converted to sodium salts.

For most waters, the process will produce the following water:

  • Bicarbonate alkalinity - less than 5% of the input total alkalinity
  • Total dissolved solids reduction - approximately equal to the total hardness
  • Total hardness to service - zero hardness

When the resin in the dealkalisation column becomes fully used up, i.e. exhausted, then the unit regenerates with hydrochloric acid to replace the hydrogen sites on the resin, ready for the next service run. Similarly, when the softening plant resin becomes exhausted, it is regenerated by brine (sodium chloride) which replaces the sodium ions back onto the resin, ready for the next service run.

The range of dealkalisation plants regenerate automatically according to pH. In some instances, a water meter can be used but pH is normally the preferred option.When the pH of the outlet water rises to 5.6, the unit automatically educts acid from the acid measure tank into the dealkalisation column. After regeneration of the resin, the outlet pH is 3.8. The service cycle then resumes and carries on until the pH reaches 5.6 again and the regeneration process is repeated.

The complete plant can be skid mounted for ease of installation

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