Falling film evaporation of black liquor - comparison with general heat transfer correlations
Journal article, 2006

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 Schlünder (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 Schlünder (1980) leads to values that are closest to our sparse experimental results under these conditions.

non-Newtonian viscosity

evaporation

heat treatment

black liquor

heat transfer

falling film

high Prandtl number

thermal treatment

Author

Miriam Johansson

Industrial Energy Systems and Technologies

Lennart Vamling

Industrial Energy Systems and Technologies

Lars Olausson

Industrial Energy Systems and Technologies

Nordic Pulp and Paper Research Journal

0283-2631 (ISSN)

Vol. 21 4/2006 496-506

Subject Categories

Energy Engineering

Paper, Pulp and Fiber Technology

Other Engineering and Technologies not elsewhere specified

More information

Created

10/6/2017