Heat Transfer and Hydrodynamics in Falling Film Evaporation of Black Liquor

Abstract: The evaporation plant is one of the major steam consumers in kraft pulp mills, which are the dominant type of chemical pulp mill in Sweden. Previous studies have shown potential to decrease the live steam demand for the pulp process, e.g. by increasing the number of effects in the evaporation plant, or by heat integration of the evaporation plant with other parts of the process. This study is intended to provide a scientific basis for cost estimations for the design changes needed to accomplish the increased energy efficiency. An important part of the study was measurement of black liquor evaporation behaviour under controlled conditions in a one-effect research evaporation plant built in cooperation with Metso Power AB. In the validation experiments with water as the evaporated fluid, the heat transfer coefficients obtained were mostly within ±10% of those calculated with the correlation for heat transfer in falling film evaporation from Schnabel and Schlünder (1980). From the experiments presented in this study, it is seen that the Nu number, for a specific Re number in the turbulent flow regime at each Pr number level, ceases to increase with increased Re number. Taking this observation into account, a new heat transfer correlation was developed, based on experimental data from the research evaporator. It was found that adding a dry solid content dependence to the new correlation improved the prediction for black liquor. Two comprehensive turbulence models for falling film hydrodynamics were studied, which provided increased qualitative understanding of the velocity, temperature and heat transfer profile in the evaporating black liquor falling film. The heat transfer coefficients calculated with the turbulence models were in agreement with the experimental heat transfer data for water. However, they did not agree with the experimental heat transfer coefficients for black liquor. Several possible reasons for the deviation were discussed. From studying the wave behaviour it was found that there were large local fluctuations in the film thickness. For the water falling film, existing correlations (Brötz’s and Brauer’s) were mostly within the 95% confidence interval of the measured average film thickness. The black liquor film tends to be thicker than the film thickness calculated with the correlations, especially for higher Re numbers. Bubbles were observed in the evaporating falling film of black liquor. A combination of turbulent wave-breaking and effects due to surface-active compounds – surfactants – was found to be the most likely explanation for the bubble creation.