Modelling and Parametric Analysis of a Brine Treatment Unit Using a High-Temperature Heat Pump and a Vacuum Evaporator

The brine produced from desalination systems is a highly concentrated mixture, including cleansing chemicals from the water treatment processes that can possibly degrade ecosystems in discharge areas. Evaporation is a widely used method for the treatment of high salinity mixtures; however, it requires careful monitoring of the temperature and pressure in order to protect the equipment from the highly corrosive environment of the brine discharge. The proposed brine treatment system is based on the principle of vacuum evaporation with the use of a high-temperature heat pump, which is classified as “green technology”. In this study, a thermodynamic analysis of a vacuum evaporation system with a nominal freshwater production capacity of 160 L/h has been carried out, employing a numerical tool to model the flash evaporator and the heat pump. The analysis focuses on the parameters that present the most significant impact on the system’s efficiency and water production, such as the recirculation ratio, the set-point temperature of the heat pump and the pressure difference provided by the vacuum pump. The results show that, for the constant vacuum pressure difference, the water production increases with the increase in the set-point temperature and the recirculation ratio, but leads to the reduced COP of the heat pump and to an elevated specific energy consumption. Moreover, it is shown that an increased vacuum pressure difference leads to increased water production, but reduces the COP. Finally, the minimum specific energy consumption of 150 kWh/m3 of produced freshwater can be achieved for a set-point at 75 °C and vacuum of 0.21 bar, leading to a levelized cost of water about 11 €/m3.