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FALLING FILM TECHNOLOGY FOR LIQUOR CONCENTRATION, EXPERIENCES FROM A RECENT START-UP

Ainscow, S.

Two principal evaporative techniques are used to concentrate aluminate liquor. In flash evaporation the liquor is preheated before it is flashed through a series of evaporators. In multiple effect evaporation the liquor receives heat in each evaporator separately. Multiple effect evaporation is carried out using a variety of evaporators such as, rising film, falling film or forced circulation.

The recent commissioning of an evaporation plant in India offers a good opportunity to look at the advantages that falling film evaporator technology offers the alumina refinery operator.

The plant consists of two evaporation trains each of which contains six effects. The plant is designed to accommodate various load requirements such as, economy (6 effects / 6 bodies), high capacity (5 effects / 6 bodies) and washing (5 effects / 5 bodies). To minimise downtime during configuration changes, motorised isolation valves are provided in the liquor lines and vapour ducts.

Due to the high liquor velocity that prevails in the evaporator tubes, the residence time is very short and the super-saturated liquor does not have sufficient time to reach equilibrium. Precipitation of solids is hindered and scaling minimised. As a result the frequency of the washing cycles is low and the plant has a high utilisation factor.

By installing high efficiency demisters in the vapour ducts, condensate with a low conductivity is recovered and can be used as a source of make-up water for the boilers.

The extensive use of frequency controlled pumps in place of control valves not only reduces the electrical power requirements but also leads to very stable control of the level in the evaporators.