Falling film evaporation of black liquor - comparison with general heat transfer correlations

A research evaporator has been built to increase knowledge about heat transfer in evaporation of black liquor. Existing models for heat transfer in falling film evaporation have been compared with the experimental heat transfer coefficients obtained from this research. Experiments with water as the evaporated fluid have been performed to validate the equipment and the method used. The coefficients obtained are mostly within +/- 10 % of those calculated from the correlation for heat transfer in falling film evaporation from Schnabel and Schlunder (1980). Experiments with black liquor as the evaporated fluid have been performed. To be able to compare the experimental results with correlations from other experiments, a crucial factor is the black liquor's viscosity, which decreased during the experiments because of its exposure to thermal treatment. Measurements with a laboratory rheometer indicated that the considered black liquor is non-Newtonian. The viscosity measured with an inline viscometer could for some conditions be less than one third of the viscosity of the black liquor falling film, due to different shear rates. Taken this into account, the correlation by Numrich (1995) agrees best with the experimental values for evaporation of black liquor with higher circulation flows and moderate viscosities. For high viscosities, at Prandtl numbers above 52, there are no valid correlations to compare the experimental results with. However, extrapolation of the correlations by Chun and Seban (1971) and Schnabel and Schlunder (1980) leads to values that are closest to our sparse experimental results under these conditions.