An analysis of black liquor falling film evaporation

Scaling in conventional black liquor evaporators has presented problems for decades, impeding the improvement of productivity in paper mills. Recent investigations suggest that falling film technology may effectively minimize black liquor fouling and improve productivity in a paper mill. This finding motivates the current work to analyze the transport phenomenon, enrichment and scale fouling of black liquor in a falling film evaporator. In the paper, a mathematical model based on a turbulent two-phase flow with multiple components is presented to investigate the transport processes of black liquor in a falling film evaporator. A phenomenological model of crystallization fouling is used to predict the fouling process. The results show the relationship between heat and mass transfer occurring within a very thin viscous sublayer close to the heat transfer surface, and the influence of soluble solids concentration and thermal boundary condition on the enrichment and scale fouling of black liquor.