Evaporation
During the washing of the pulped wood chips, the weak black liquor is removed from the digester. Due to the high water content, it is of little value as a fuel as it will not burn. In order to process black liquor, the solids must be concentrated from 14% to 80% before a burnable waste is produced. The concentration of black liquor is accomplished by evaporation. A series of heat exchangers or evaporators operating under vacuum is used to evaporate water from the liquor. Steam is used as the heating medium and introduced into the last stage evaporator and moves towards the first stage unit. Liquor flows in the opposite direction. A typical multiple-effect evaporator system consists of 6 evaporator vessels or effects. Concentrated liquor at 60% solids or higher is typically recovered at the end of the last effect. Further concentration to a burnable solids concentration of 80% takes place in a separate evaporating vessel called a “concentrator”. When concentrated to 80%, the “heavy black liquor” has a fuel value that is similar to a medium grade coal. After the second evaporator, when the black liquor has reached at least 20% solids it is settled in tanks to allow wood oil soaps, resins and alcohols extracted from the wood chips to separate out. The tall oil soaps are subjected to a process to recover sodium and are then sold as an additive for industrial lubricants.
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