Heat transfer in evaporating black liquor falling film

Black liquor is a fluid with high viscosity, implying high Pr numbers. Most of the previously developed correlations for falling film evaporation heat transfer were developed for fluids with relatively low Pr numbers, and therefore are not valid for most black liquor evaporation conditions. Experimental heat transfer data from black liquor evaporation are presented here. They show that the Nu number, as expected, increases in the turbulent region. However, at a specific Re number for each Pr number level, the Nu number ceases to increase with increasing Re number. A new correlation, taking this observation into account, has been developed on the basis of experimental data in the region of 4.7 < Pr < 170 and 47 < Re < 6740, and is presented in this paper. A dry solid content dependence is included in the new correlation and improves its predictions for black liquor.