Design and evaluation of a parallel-connected double-effect mechanical vapor recompression evaporation crystallization system

Abstract

This paper proposes a parallel-connected double-effect mechanical vapor recompression (MVR) evaporation crystallization system, which combines the falling film evaporator with the forced circulation evaporator. With the purpose of treating high salinity wastewater, the circulation process of the system was designed and mathematical models were established according to the mass and energy balance as well as thermodynamic properties in the heat transfer process. Model calculation was conducted using 5% sodium sulfate solution, based on which the effects of evaporation temperature, feed concentration, and temperature difference between saturated boiling solution and condensate on the system performance were analyzed by a parametric study. The results showed that the optimal values of evaporation temperature and saturation temperature difference could be determined based on a trade-off between power consumption and heat transfer area. After that, the performance of the proposed system was compared with that of a traditional multi-effect evaporation crystallization system, and it was revealed that the coefficient of performance (COP) and exergy efficiency of the proposed system were 21.4 and 49.1%, respectively, 82.2% and 51.5% higher than those of the traditional system under the same working condition. Meanwhile, the exergy destruction of the proposed system was 24.7% lower than that of the traditional system, indicating that the new system performs better regarding energy saving and thermodynamic perfection.