Supercritical water desalination (SCWD): process development, design and pilot plant validation

Abstract

The research described in this thesis was financially supported by Wetsus, European Centre of Excellence for Sustainable Water Technology. The main objective of the work reported in this dissertation is the development, design and construction of a pilot plant scale SCWD process that produces drinking quality water and solid salt. SUPERCRITICAL WATER DESALINATION (SCWD)PROCESSIn order to design a SCWD process, it is essential to know the phases present under supercritical water conditions and understand how the process conditions °C at 250 bar, and 475 °C at 300 bar respectively. In addition, the phase equilibrium ii solubility of NaCl−H 2 O was studied under isobaric conditions in a lab-scale experimental setup. The results of the visualization experiments and phase equilibrium measurements show that the SCWD process could be operated in two stages: (i) a V−L separator to remove the supercritical product water from the liquid phase at 250/300 bar, and (ii) a V−S separator to obtain the solid salt by flashing the liquid phase (a highly concentrated salt solution, 50 wt.% at 300 bar, 460 °C) to atmospheric pressure. The two-stage operation is necessary to avoid salts precipitation in the early stage of the process which could lead to equipment blockage and downtime.Simulation of the SCWD process using water was carried out in UniSim Design at 250 and 300 bar pressure. Simulation results show that operating the SCWD process at 300 bar offers better heat integration potential as well as a 22% reduction in thermal energy consumption compared to operating at 250 bar. A conceptual design and a lab-scale (12 g/hr) demonstration of the proof-of-concept of the SCWD process using 3.5 wt.% NaCl-H 2 O solution are presented in Chapter 2.The SCWD process is energy intensive, therefore heat integration is essential to regain as much energy as possible from the process in order to make the process a commercial success. Understanding the heat transfer mechanism as well as knowing the heat transfer coefficient of sub-to supercritical water (SCW) flow is essential to design a heat exchanger required for the heat integration. In Chapter 3, 2D numerical simulations were carried out in COMSOL Multiphysics to provide essential insights into the heat transport mechanism of SCW flow at low mass fluxes. The results show that the heat transport mechanism is primarily by buoyancy-induced circulation resulting from gravitational force acting on density gradients (a direct consequence of temperature gradients) across the section of the tube. From the numerical results, a 1D Nusselt correlation for engineering design was developed and validated with experimental temperature measurements.In Chapter 4, two experimental measurement methods -local and spatially averaged -to measure the heat transfer coefficient (HTC) of SCW are presented.Only the local measurement method has enough resolution and accuracy to detect the maximum in HTC near the pseudo-critical temperature. No noticeable effect of iii an increase in mass flux is observed due to th...