Evaporation - Process info

The spent liquor from the cooking plant and pulp washing contains almost all the consumed cooking chemicals and the dissolved wood substance. To be able to burn the spent liquor it has to be concentrated. This is done in the evaporation plant where the liquor is concentrated from about 15% dry solids to 65-85%.

To evaporate in an economically efficient way several evaporator bodies are connected in series to give a temperature drop over the evaporation plant. Thus, about 0.2 ton live steam is sufficient to evaporate 1 ton water from the black liquor.

Live steam is used in the first effect. The evaporated liquor from this effect is used as steam in the next etc.

In the last effect the liquor vapour is condensed in the condenser where the vacuum pump is located which acts as the “motor” for the plant. The liquor is processed in different modes through the effects both counter current and current to the steam, depending on the liquor dryness.

The condensates from the effects are collected and reused as warm water. 
The more dirty condensate (foul condensate) is cleaned in a stripper with steam prior to the reuse in the pulp mill.