Experimental investigation and modelling of sodium scale dissolution rates in black liquor evaporators

Abstract

Few publications address how to efficiently remove sodium scales in black liquor evaporators. The lack of information makes the cleaning inefficient and subjective. In this work, the sodium scale dissolution rate has been experimentally investigated and a model for the dissolution process has been developed. The aim was to relate the dissolution rate to wetting degree and temperature of the wash liquid. The experiments were conducted in a pilot plant evaporator of the falling film type. A saline solution consisting of sodium carbonate and sodium sulphate was evaporated on the outside of a vertical stainless steel tube. The removal of the resulting scaling layer was then studied at different temperatures and wetting degrees. An effective method for investigating the removal of sodium scales in falling film evaporators was developed and a model was constructed that successfully could predict the obtained experimental data. The washing behaviour proved to be strongly dependent on how the scales were distributed along the tube. It was confirmed that scales grow from the bottom of the tube and upwards until the layer reaches an equilibrium thickness. The dissolution rate was found to follow a 1st order expression with regards to the concentration difference between the scaling layer surface and the bulk of the wash liquid. The model was developed in the temperature interval 45 85 °C and wetting degrees of 0.13 kg/ms up to 0.44 kg/ms. Based upon the model, three correlations applicable to the industry were developed in order to estimate the saturation degree of the outgoing wash liquid.