Crystallization in a Pilot Evaporator: Aqueous Solutions of Na2CO3 and Na2SO4

Sodium carbonate and sodium sulfate cause problems by forming scales in black liquor evaporators, reducing heat transfer and cleaning intervals. An experimental investigation of the crystallization behavior during evaporation of different aqueous solutions of sodium carbonate and sodium sulfate has been carried out. The analysis is based oil changes in heat transfer coefficients and crystal masses. The results from this work show large variation in the distribution of the crystal mass, depending on the composition of the solution. Solutions with the crystalline form of either dicarbonate or sodium carbonate exhibit high propensity to form scales on the heat transfer surface. Solutions with the crystalline form of burkeite, however, do not show the same attraction to the heat transfer surface; they are more likely to crystallize in the circulating solution. Thus, the heat transfer is not as affected as for dicarbonate or sodium carbonate.