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Multi-effect mechanical vapor recompression (MVR) system is an effective technology for concentrating industrial high-salt wastewater that has high boiling point elevation (BPE). To minimize the technical requirements of steam compressor, a superheat eliminator is incorporated in the multi-effect MVR system. By employing the Aspen Plus software, simulations are conducted to evaluate the performance of the modified single-effect to five-effect MVR systems in concentrating calcium chloride solute from 5% to 30%. With the simulation results, impact of the number of evaporators and heat transfer temperature difference, along with the benefit of superheat eliminator are analyzed. This analysis aims to guide the optimal design of a multi-effect MVR device. It is concluded that the heat transfer temperature difference should not be higher than 9°C for systems with more than three evaporators. The addition of evaporators leads to higher pressure ratio of the compressor and increased consumption of fresh steam. The operation costs of two-effect system and three-effect system are similar. Furthermore, the benefit derived from the superheat eliminator, in terms of power and suction volume reduction, become more pronounced as the BPE increases. Incorporating energy-efficient devices for fresh steam generation can further facilitate the utilization of multi-effect MVR systems for high-salt wastewater concentration.

Heat pumps offer an effective heat recovery way for small-capacity thermal desalination plants. A modified single-effect heat pump desalination system is proposed in this paper. The auxiliary condenser for the condensing of high temperature refrigerant in a conventional system is adjusted to condense the low temperature steam. Aspen Plus software is employed to develop simulation models, which are subsequently validated. With the simulation results, performances of the modified system and the conventional system are compared. It is concluded that modified system produces more freshwater for a constant heat pump operation condition and seawater feed condition. Furthermore, the circulating refrigerant mass together with the compression power can be reduced by the modification when the freshwater production is fixed, resulting in a growth rate of 7% in both freshwater production and performance ratio (PR). However, the modification sacrifices the heat transfer area of auxiliary condenser. For both systems, an increase in heat pump evaporating temperature leads to a decrease in freshwater production but an increase in the PR. Conversely, elevating the heat pump condensing temperature results in a reduction of both freshwater production and PR for both systems. Findings offer reference for the further optimal design of small-scale heat pump desalination systems.