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

Scaling in the recovery cycle of kraft pulp mills has long been recognised as a problem, especially in black liquor evaporators. Most problems are connected with the precipitation of sodium salts. In the black liquor evaporation, as the black liquor becomes 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.In this research the scaling rate, or the reduction in heat transfer rate over time, has been analysed. For the research a pilot evaporator has been used, built in cooperation with Metso Power (former Kvaerner Power). The falling film type evaporator is a 4.5 m tube with the black liquor on the outside.One of the objectives of the thesis was to investigate whether there are local scaling variations at different positions on the heat transfer area by monitoring the fouling growth rate at different local positions in the pilot evaporator. The scaling was found to depend on the vertical position along the heat transfer surface. The scales start to form on the bottom of the evaporator tube where the concentration is highest, and then expand upwards. A clean surface starts to foul only when the area beneath it has fouled.Furthermore the research in this thesis 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 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 to be independent of the internal residence time.