CONTROL OF SOLUBLE SCALE IN BLACK LIQUOR EVAPORATORS AND CONCENTRATORS:PART 1. PILOT PLANT STUDIES

摘要

Evaporation studies were conducted with a falling film, pilot evaporator to evaluate the impact of chemicalcomposition and process variables on the rate of fouling of the heat-transfer surface. The experiments wereconducted with both black liquor and aqueous solutions of sodium carbonate and sodium sulfate. Fouling of theheat-transfer surface was inferred from changes in the heat-transfer coefficient during evaporation, after accountingfor changes in black liquor viscosity. The apparent particle size distribution of sodium salt crystals produced duringevaporation was also monitored during the experiments.The experimental results showed that two types of fouling could occur at the conditions where black liquor isconcentrated to high solids. The first is a slow, steady deposition that occurs at relatively low evaporation rates,when supersaturation of the sodium salts is moderate. The second is a rapid deposition that occurs simultaneouslywith a primary nucleation event. The scale is a double salt with the approximate composition 2Na_2CO_3·Na_2SO_4.Burkeite scale does not appear to deposit this way.