Sodium Salt Scaling in Black Liquor Evaporators
Sodium salt scaling has long been recognized as a problem in the recovery cycle of kraft pulp mills, especially in black liquor evaporators. In the black liquor evaporation, as the black liquor becomes more concentrated, the sodium salts, particularly sodium carbonate and sodium sulfate, will saturate and precipitate from the solution. The precipitated crystals may form on the heat transfer surface or can adhere to it, or they can remain in the solution. If crystals are accumulated on the heat transfer surface an insulating layer of scales can build up, reducing the amount of transferred heat and, eventually, forcing the evaporator to be shut down for cleaning.
For the purpose of studying black liquor evaporation, a 4.5 m falling film tubular evaporator was built in cooperation with Metso Power. With the evaporator, a number of aspects of sodium salt scaling were analyzed. To investigate the scaling behavior, two evaluation methods were developed: one based on heat transfer measurements to measure the fouling rate and one to measure different crystallization rates, in the bulk and on surfaces.
Among the aims of this project was to find operating conditions that decrease scaling. The examined parameters included heat flux, circulation flow rate, and internal residence time. The main conditions influencing the scaling were the circulation flow rate and heat flux, whereas the fouling rate was seen to be independent of the internal residence time. To minimize scaling, evaporators should have a high circulation flow rate and a low heat flux. Local fouling variations along the heat transfer surface were also observed. The scales were found to be formed at the bottom first and expand upwards with decreasing initiation times in the direction of the flow.
From crystallization measurements in the evaporator it was shown that the sodium salts exhibit high supersaturation at the metastable limit, which can give very high fouling rates during a nucleation event. It is therefore important to prevent primary nucleation from occurring on the heat transfer surface of black liquor evaporators. The fouling behavior is also strongly influenced by the composition of the liquor. Liquors with high shares of carbonate, crystallizing either sodium sulfate dicarbonate or sodium carbonate, were seen to cause more serious fouling than liquors with lower carbonate shares, crystallizing burkeite.
falling film evaporation