Sodium Salt Scaling in Black Liquor Evaporators – Pilot Plant Results and Industrial Significance

Paper in proceeding, 2010

In industrial black liquor evaporation plants large amounts of sodium crystals are produced every day which all has the potential to scale the evaporator surfaces. In this project, research on the crystallization during evaporation of softwood black liquor has been performed. The results are based on measurements in a research black liquor evaporator. The distribution of scales vertically along the heat transfer surface has been investigated as well as the influence of different operating conditions, such as the circulating flow rate of black liquor, the residence time for the black liquor in the evaporator and the temperature difference between the liquor and the heating steam. The sodium scales formed during evaporation are mainly in the form of double salts of sodium carbonate and sodium sulfate. The ratio between the two salts is one important parameter affecting the crystallization and scaling. The effect of the carbonate-to-sulfate ratio has been investigated for aqueous sodium carbonate – sodium sulfate solutions. In this paper, the results of these investigations are presented and a number of industrial significances are illustrated.