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

Artikel i vetenskaplig tidskrift, 2011

In industrial black liquor evaporation plants, large amounts of sodium crystals are produced every day which all have 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 scale 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 (between 0.2 and 3.7 kg/m.s), the residence time for the black liquor in the evaporator (between 76 and 226 s), and the temperature difference between the liquor and the heating steam (between 11 and 26 degrees C). The sodium scale formed during evaporation is 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.