Simulation of Droplet Entrainment in Multistage Flash Evaporating Units

Abstract

Evaporators are widely used in industrial processes to produce thermally treated distillate by evaporation with concentration of impurities in the bulk of water. As to their thermal efficiency, multistage flash evaporators (MFEs) belong to one of the most promising types of evaporating equipment, in which the consumption of thermal energy for evaporating water in each stage is compensated for by the recovery of this energy in heating the treated water during the condensation of the produced steam. Improvement of the methods for design and operation of these units on the basis of mathematical models of heat- and mass-transfer processes is an urgent problem facing developers and designers of evaporative equipment. Within the scope of the performed study, a combinatorial model is proposed for calculation of the size distribution of superheated water droplets during evaporation in the low-pressure zone of the dirty compartment of an MFE. A mathematical model was developed of a droplet separator designed to prevent water droplet entrainment and ingress of impurities into the distillate. The flowrates of distillate and treated water and the content of impurities in these flows were calculated for each stage of the evaporation unit, taking into consideration the water droplet entrainment from the dirty compartment. The effect of droplet entrainment in multistage evaporators on the quality of distillate was assessed. An approach has been proposed to the design of multistage evaporators providing the specified process indicator under different operating conditions. This approach can also be used in the construction of regime maps of equipment, implementation of measures to improve the efficiency of equipment, and development of systems for diagnostics of heating surface in the evaporators for proper condition.