Heat transfer in falling film evaporation of black liquor - Experiments and theory

Sammanfattning: Pulp and paper industry uses a large part of the total amount of energy in industry in Sweden. The evaporation plant has the major energy demand in the kraft pulp mill, which is the dominant type of chemical pulp mill. 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 thermal integration of the evaporation plant with other parts of the process. A question is whether the costs for measures needed in the evaporation plant will increase more than the gains from the live steam savings. This study is intended to provide a scientific basis for such cost estimations. Its aim is to develop basic experimental and theoretical knowledge about heat transfer in evaporation of black liquor, especially for new operating conditions resulting from measures needed to accomplish live steam savings. An important part of the study has been measurement of black liquor evaporation behaviour under controlled conditions in a research evaporator built in cooperation with Kvaerner Power AB. The heart of this evaporator is a 4.5-m steam-heated tube with black liquor flowing on the outside. In the literature there are only a few scientific studies of black liquor evaporation heat transfer, and none have been found for the falling film type. There are, however, numerous studies of falling film evaporation in general, and a compilation of correlations suggested in the literature has been carried out. It shows that for high viscosities, i.e. for Prandtl numbers above 52, no valid general correlation has been presented. 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 through 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. This is important, firstly, since the black liquor viscosity is decreased during the experiments because of its exposure to thermal treatment. A second reason is that measurements with a laboratory rheometer indicated that the used 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 the difference in shear rates. Taking 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.

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