Crystallization fouling of sodium salts in black liquor falling film evaporators – development of experimental method and first results

Artikel i vetenskaplig tidskrift, 2013

The black liquor evaporation plant is the largest steam consumer at the pulp mill. At the same time, its performance is heavily influenced by fouling, where crystallization of sodium salts (double salts of Na2SO4 and Na2CO3) is a major contributor. In this work, an experimental method was developed for a pilot evaporator that greatly improved the possibility to measure crystalizing fouling. The main improvements were that the heat transfer surface could be cleaned online to reset the fouling, and that the scales’ chemical composition could be determined from the wash liquid, i.e. no need to open the evaporator and take scale samples. The method was evaluated at different operational modes and a wide range of operational conditions where crystallization of sodium salts can occur, i.e. 50-85% dry solids content. The method enabled fast cleaning of the heat transfer surface up to about 72% dry solids content. In addition, the measured rate of fouling showed no increase at higher dry solids contents. The observed fouling was mainly formed due to primary nucleation at operation close to metastable limits, where the first metastable limit showed the highest fouling rate. It was also found that the fouling rate measurement results could not be repeated, despite operation under the apparent same conditions with the same liquor. This means that the fouling mechanism is more complex and the rate cannot be predicted using only liquor composition and operational conditions.