Thermodynamic assessment of MVR implementation in multistage evaporation plants

A study on process alternatives for reducing steam consumption was carried out in a multistage evaporation system of a chlorine-alkali industrial complex. A phenomenological model was developed, based on the mass and energy balances and on gas-liquid equilibrium correlations for the ternary mixture of caustic soda, sodium chloride and water. The design specifications of the plant were adopted as a reference to validate the mathematical model, which comprises 153 nonlinear equations. Different process conditions were simulated by taking into account the most important process variables and their effect on steam consumption. Differently from previously published studies, in which the conservation laws are mainly focused on the heat exchangers and their optimization, the present study considers the whole multistage evaporation unit. The model also enabled to evaluate the effect of changes in the plant configuration, either by considering a side product stream at a specified concentration, designated to another production unit in the industrial complex, or by including mechanical vapor recompression, which has proven to be a better solution to reduce steam consumption from the energy and exergy points of view. Results indicate an expressive decrease in steam consumption to around 1% of the original configuration. Moreover, from the exergy perspective, if the power generation is considered, the MVR is a better choice from the energy and exergy point of view.