Evaporation of lignin lean black liquor

Paper in proceeding, 2014

ABSTRACT Many of the pulp mill biorefinery concepts recently presented includes removal of lignin from black liquor. In this work, the aim was to study how the change in liquor chemistry affected the evaporation of kraft black liquor when lignin was removed using the LignoBoost process. Lignin was removed from a Softwood kraft black liquor and four different black liquors were studied; one reference black liquor (with no lignin extracted), two lignin lean black liquors with a lignin removal rate of 5.5% and 21% respectively and finally one liquor with maximum lignin removal of 60%. Evaporation tests were carried out at the research evaporator in Chalmers University of Technology. Studied parameters were liquor viscosity, boiling point rise, heat transfer coefficient, scaling propensity, changes in liquor chemical composition and tube incrustation. It was found that the solubility limit for incrustation changed towards lower dry solids for the lignin lean black liquors due to an increased salt content. The scaling obtained on the tubes was easily cleaned with thin liquor at 105 °C. It was also shown that the liquor viscosity decreased exponentially with increased lignin outtake and hence, the heat transfer coefficient increased with increased lignin outtake. Long term tests, operated about 6 percentage dry solids units above the solubility limit for incrustation for all liquors, showed that the heat transfer coefficient increased from 650 W/m2K for the reference liquor to 1500 W/m2K for the liquor with highest lignin separation degree, 60%.