A three-year consortium project for the mathematical description of black liquor flow in falling-film evaporators has just been completed with impressive simulation results. The work by Dr. Cyrus Aidun with support from paper science and engineering students such as Emmanuel Doro were presented to their member sponsors in May. The investigation describes phenomena wherein, at high dissolved-solids mass fractions, black liquor evaporation is characterized by erratic soluble scaling with rapid fouling of heat-transfer surface, leading eventually to evaporator shutdown. Falling-film evaporation is defined by high rates of heat transfer at relatively low evaporator surface temperature and short residence time of the process stream, thus making it suitable for black liquor evaporation and concentration. Nevertheless, with liquors at high dissolved-solids mass fraction, its performance still suffers from rapid heat transfer surface fouling, albeit to lesser degrees. Studies have shown that soluble scale deposition in high solids black liquor evaporators is controlled by crystallization and the influence of fluid flow and heat transfer associated with the crystallization and falling film evaporation processes. Thus, a thorough understanding of the soluble scaling process is incomplete without accurate prediction of falling film flow field, heat transfer and evaporation with interfacial evolution. Therefore, research objectives for this project were to develop a computational framework for detailed investigation of the flow physics and transport phenomena associated with free surface evaporation of falling film, and that goal was met. The application to black liquor evaporation focuses on the fundamental structure of free surface waves of falling film and how these impact species and energy transport in the evaporating liquid film.

There are opportunities for further exploration of the phenomenon and application of the model, and additional sponsors are sought. Those interested should contact Professor Cyrus Aidun at cyrus.aidun@me.gatech.edu.