Falling-Film Black Liquor Evaporation --- Pilot-Plant Practice and Scale Study, 1995 Pulping Conference Proceedings

Michael X. Meng

Georgia Tech.

Jeffery S. Hsieh

Georgia Tech.

Black liquor evaporation is crucial to the chemical recovery of Kraft pulping in that it greatly determines the overall energy efficiency and the economy of a pulp mill. The advances in pulping processes lead to the development of cold-blow pulping and high solid burning. The falling-film evaporation is usually used in conjunction with such new technologies to produce a slurry with with solid content (such as 75-80%) for effective combustion. However, during the evaporation process, the scaleup of the solid deposition on the surface of heating element will be encoutered as the solid concentration increases. It results in a drastic decrease in the heat transfer efficiency, and may force the evaporator to only operate under the critical solid content. In this study, a falling-film evaporator with Dimpled-plates as heating element has been built. This pilot-scale evaporator is capable of processing one ton black liquor per day. The analysis and technical examination of the system is presented here. An effort is made to prolong the operation by controlling the solid concentration of the-final product. Some crystallization concepts are applied to the case and it shows promising in process improvement.