Scale formation and growth when evaporating black liquor with high carbonate to sulphate ratio

Abstract

Scaling in black liquor evaporators has for long been recognised as a problem. Most problems are connected to the precipitation of sodium salts. In the black liquor evaporation, as the black liquor gets concentrated, sodium carbonate and sodium sulphate will saturate and precipitate from the solution. When precipitation occurs the salts may crystallise on the heat transfer surface or adhere to the surface as well as they can crystallise in the bulk. If crystals remain on the heat transfer surface an insulating layer of scales can quickly build up causing the heat flux to drop and eventually forcing the evaporator to be shut down for cleaning. The research in this paper focused primarily on scaling for black liquors with high ratio of carbonate to sulphate, a condition shown to be important in industrial black liquor evaporators (Frederick et al. 2004). The scaling behaviour was investigated experimentally in a falling film evaporator. The aim was to find favourable operating conditions decreasing or eliminating scaling. The examined parameters were different heat flux, circulation flow rate, internal residence time and black liquor feed concentration. For the investigated operating conditions the main parameters affecting scaling were the circulation flow rate and heat flux whereas the scaling rate was seen independent of the internal residence time.