Crystallization in a pilot evaporator – Comparison between black liquor and black liquor model substances

Crystallization measurements during semi-batch operation of a pilot evaporator have been performed. The experimental analysis is based on changes in heat transfer coefficients and crystal masses. With this method of operation, the total crystal mass as well as the distribution of the crystal mass between the circulating solution and on surfaces can be measured continuously during the evaporation. Experiments with black liquor and different black liquor model substances, all with equal carbonate-to-sulfate ratio, crystallizing burkeite have been evaluated. The results are compared to each other and to two aqueous reference solutions crystallizing dicarbonate and sodium carbonate. The results presented include solubilities and metastable limits as well as different crystallization rates and fouling rates. Generally, the supersaturation at the metastable limit observed for the different liquors was high (14-25%). All the different liquors crystallizing burkeite were seen to have significantly lower fouling rates than the reference solutions crystallizing dicarbonate and sodium carbonate. The burkeite liquors showed no specific attraction to crystallize on the heat transfer surface as the reference solutions did.