Dissolution rate of sodium salt scales in falling film evaporators

In pulp mills, fouling can be a significant problem in the black liquor evaporation plant. Part of the black liquor evaporation plant works as a crystallizer where large amounts of sodium carbonate and sodium sulphate will saturate and precipitate. In this work, the dissolution rate of sodium scales was experimentally investigated and modelled. Experiments were conducted in a pilot falling film evaporator where a sodium carbonate and sodium sulphate solution was evaporated on the outside of a vertical stainless steel tube to form the scales. The aim was to relate the dissolution rate to wetting degree (i.e. the mass flow rate per unit circumference) and temperature of the wash liquid. An effective experimental method to measure the removal of sodium scales in falling film evaporators was developed and a 1st order model with respect to the difference in mass fraction between the scaling layer surface and the bulk of the wash liquid was shown to fit the experimental data well.