Analysis of the energy consumption of supercritical water desalination (SCWD)

An experimental and modelling study was done to investigate supercritical water desalination (SCWD) with respect to energy consumption as a function of the NaCl concentration (0 to 20 wt%). Pilot plant experiments were performed for different flow rates and feed concentrations, and used for the validation of the thermodynamic models (eNRTL and Anderko & Pitzer) employed for phase equilibria and enthalpy calculations. Experimental and modelling results showed that the lowered heat capacity of the feed streams, while increasing the concentration from 0 to 7 wt%, leads to improved performance of the feed - supercritical water (SCW) heat exchanger (HEX), evident from a higher feed outlet temperature. For concentrations of ≥14 wt%, pre-heating of the feed, prior to the HEX, is recommended due to the decrease in the SCW recovery in the SCW-brine separator. The calculated duty, of the heater bringing the heat-exchanged feed to the separation temperature, decreases with NaCl concentration due to the decrease in the feed heat capacity. The calculated overall energy consumption of SCWD was between 0.71 and 0.90 MJ_th/kg_feed. For higher concentration feeds, the energy input is divided between low – and high quality (temperature) heat.