Evaporation of Na2CO3-Na2SO4 solutions: A method to evaluate the distribution between bulk and surface crystallization
Journal article, 2011
Precipitation of sodium salts in black liquor evaporators causes problems by forming scales on the evaporator surface, reducing heat transfer and cleaning intervals. Most problems are connected with the crystallization of sodium carbonate and sodium sulfate. As the solubility of these salts is exceeded, a crystal mass must form somewhere. Crystallization can occur either in the bulk solution, on the heat transfer surface, or on other surfaces. It is always desirable to create bulk crystals. If crystals form and remain on the surfaces, a layer of scales will build up with time. A method for estimating the distribution of crystal masses between the bulk and on surfaces has been developed in this work. The method is primarily based on inline density measurements combined with inline measurements of the system's water mass. It has been applied to aqueous solution of sodium carbonate and sodium sulfate in a research black liquor falling film evaporator. Experiments have proven that the method gives valuable information on the crystallization process. It shows where crystals are formed during primary nucleation, as well as during the subsequent continuous crystallization. In an industrial black liquor evaporator, the metastable limit can be passed if it is operated under non-steady-state conditions. During evaporation, upon passing the metastable limit, the experiments showed that the surface crystallization is as high as or higher than the bulk crystallization. During the subsequent crystallization process, when concentration is further increased, the crystallization rate is higher in the bulk solution than on the surfaces.