Mathematical model for a multiple effect evaporator system with condensate-, feed- and product- flash and steam splitting

Abstract

In the present investigation a non-linear mathematical model is developed for the analysis of multiple effect evaporators (MEE) system. This model is capable of simulating process of evaporation and takes into account variations in -boiling point rise (BPR) due to variation in concentration and temperature of liquor, -overall heat transfer coefficient (OHTC) of effects and -physico-thermal property of the liquor. Using mass and energy balance around an effect, a cubic polynomial equation is developed to model an effect, which is solved using generalized cascade algorithm. For this purpose, a Septuple effect flat falling film evaporator (SEFFFE) system with backward feed flow sequence, being used for concentrating weak black liquor and operating in a near by paper mill, is selected as a typical MEE system. This system supports different operating strategies such as steam splitting and condensate-, feed- and product- flashing. The empirical correlations for BPR, OHTCs of flat falling film evaporators and heat loss have been developed using the plant data. Using these correlations, average errors of 3.4%, ± 10% and –33 to +29% have been observed for the predictions of BPR, OHTC and heat loss, respectively.