Experimental investigation and modelling of sodium scale dissolution with black liquor - A pilot plant investigation

Abstract

In this thesis the dissolution of sodium scales with black liquor has been investigated. The experimental part was carried out at a falling film pilot plant evaporator at the division of Heat and Power Technology at Chalmers University of Technology. In order to study the dissolution, a scaling layer was build up by evaporation of a saline solution which was then washed off by black liquor at different temperatures and flow rates. During washing the outgoing dry solid content of the wash liquid was measured along with the scaling layer thickness. The dissolution rate constant was found to be lower than in previous research carried out by dissolution with water at corresponding temperature and flow rate. The dissolution was dependant on the concentration difference between the saturation concentration and the concentration of the bulk liquid, the flow rate and the temperature of the washing liquid. The dissolution seems to be decreasing with increasing viscosity, but since temperature affects the viscosity, the solubility and the dissolution rate constant the effect of only the viscosity cannot be confirmed. In contrast to previous work the results suggests that the amount of wash liquid needed to dissolve a given amount of scales decreased with increasing temperature and flow which suggest that there might be beneficial to wash at a high wetting degree and at a high temperature.