Changes in viscosity and boiling point elevation of black liquor when a fraction of lignin is removed - consequences in the evaporation stage

Artikel i vetenskaplig tidskrift, 2009

Removing lignin from kraft black liquor affects the physico-chemical properties of the liquor. The viscosity and boiling point elevation of black liquor are of crucial importance to the design as well as the operation of the black liquor evaporation plant. Since these properties are strongly influenced by the composition of the black liquor, it is of great interest to understand how they are influenced by the removal of lignin from black liquor. The viscosity and boiling point of black liquor samples taken both before and after lignin removal runs, and a mixture thereof, have therefore been measured in this work. Six different original black liquors (softwood and hardwood) were used as the source for lignin removal runs; a total of 18 liquors of different lignin concentration were studied. It was found that the property of black liquor that was most affected by the removal of lignin was viscosity, at most it was reduced by an order of magnitude (depending on the dry content, mill and wood species). A decrease in boiling point elevation was also found at dry contents above 40-50 % when a fraction of the lignin was removed from the black liquor.