A new test facility for black liquor evaporation

In the pulp and paper industry, the black liquor evaporation process is a large energy consumer. Black liquor is generated during the production of chemical pulp from wood or annual plants and its main constituents are dissolved wood (lignin), cooking chemicals and a large amount of process water. Research on the energy efficiency of the pulp and paper process has shown that heat integration of the evaporation with other parts of the process might considerably increase energy recovery and thus save significant amounts of live steam. The aim of our research is thus to develop basic experimental and theoretical knowledge about heat transfer during evaporation of black liquor under new conditions. To be able to obtain this knowledge a research evaporator has been built at Chalmers in cooperation with Kvaerner Power. The evaporator consists of one 4.5 m long vertical tube with a falling film of black liquor flowing on the outside. Heating is accomplished by condensing steam on the inside of the tube. To be able to measure heat flux locally, the evaporator tube is equipped with 18 pairs of thermocouples mounted at different positions both vertically and circumferentially. The paper will focus on the design of the temperature and heat flux measurement system, but will also include examples of measurement data obtained. Heat transfer coefficients from the experimental measurement are in the same range as the heat transfer coefficient calculated from the theoretical correlations from literature, however the measurement conditions in the initial tests have not been stable enough to reach acceptable accuracy levels