Abstract
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 MJth/kgfeed. For higher concentration feeds, the energy input is divided between low – and high quality (temperature) heat.
Highlights
Brine management has gained increased attention in the past few years due to the growing demand for fresh drinking water and the accompanying increased brine production
The results show that the Anderko & Pitzer (AP) equation of state (EoS) is able to predict both the supercritical water (SCW) concentration and recovery within an accuracy of 10 and 5%, respectively
A decrease in heat capacity of the cold stream will improve heat exchange potential, in that less energy is required for heating the feed, a decrease in the SCW recovery will result in less energy available for heat exchange
Summary
Brine management has gained increased attention in the past few years due to the growing demand for fresh drinking water and the accompanying increased brine production. The demand for zero liquid discharge (ZLD) technology is, increasing as new methods need to be developed for the treatment of brine streams. Supercritical water desalination (SCWD) is a ZLD technology that utilises the non-polar nature of water under supercritical conditions (Temperature > 374 °C, Pressure > 22.1 MPa) to separate salt from water. This approach to desalination has been investigated for the treatment of seawater and more concentrated waste brine streams (≥3.5 wt% NaCl) [6,7,8]. The phase behaviour and removal of different salts from supercritical water (SCW) has been extensively studied using various laboratory scale set-ups [12,13,14,15,16,17,18] and recently pilot plant scale facilities have been built and tested [6,7,8]
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