Supercritical water desalination (SCWD)- process development, design and pilot plant validation
Samuel Odu is a PhD-Student in the Research Group Sustainable Process Technology. His supervisor is professor Sascha Kersten from the Faculty Science and Technology.
Conventional desalination technologies such as reverse osmosis (RO), multi-stage flash distillation (MSF) and electro dialysis (ED) have a major drawback; the production of a liquid waste stream with an increased salinity (compared to the feed) that has to be disposed of. The treatment of this waste stream has always presented technical, economic and environmental challenges. With stricter environmental regulations regarding brine disposal into water bodies, the treatment or disposal of this waste stream pose a huge challenge for the sustainability of desalination methods. Currently, research studies are being conducted to develop zero liquid discharge (ZLD) technologies for desalination.
Supercritical water desalination (SCWD) is a new desalination method that allows for the treatment of salt water streams with ZLD. A detailed literature overview of state-of-the-art desalination technologies, their advantages and major drawbacks, as well as motivations for a desalination process that eliminates the production of a waste brine stream are provided in Chapter 1.
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.
In order to design a SCWD process, it is essential to know the phases present under supercritical water conditions and understand how the process conditions (Pressure and Temperature) influence the separation efficiency as well as the overall energy demands of the process. In Chapter 2, visualization of the phase transition of model NaCl-H2O under supercritical water conditions as a function of temperature and pressure was carried out in quartz capillaries. Under supercritical conditions, two distinct regions, V−L and V−S as well as a transition V−L−S were observed. The transition temperature from V−L to V−S was found to be about 450 °C at 250 bar, and 475 °C at 300 bar respectively. In addition, the phase equilibrium solubility of NaCl−H2O 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-H2O 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 an increase in mass flux is observed due to the dominance of natural convection as heat transport mechanism at the low mass fluxes investigated. An increase in pressure leads to a decrease in the magnitude of the measured HTC near the pseudo-critical temperature. Experimental results from the local measurement method are used to further validate the numerical results obtained in Chapter 3.
In Chapter 5, the detailed design (and challenges), selection of materials of construction, operating procedure and control, and experimental results of a first generation modular pilot plant for SCWD with a capacity of 5 kg/hr drinking water, the first of its kind, are presented. Experiments with NaCl feed (3.5 wt.%) have been carried out successfully with the plant running for several hours without operational problems and with good mass balance closure. The pilot plant produces drinking quality water (< 700 ppm salts) and solid salt crystals (2-15 µm).
The findings of the research work is summarized in Chapter 6. In addition, the current bottlenecks of the process are highlighted, and potential scope for further development of the SCWD process is given. For example, the flash operation needs to be optimized, and further tests with other salts and mixture of salts are proposed. SCWD is still more energy intensive compared to the conventional MSF distillation, and as such preliminary evaluation shows it is too expensive as a stand-alone water producing technology. The added value of combining SCWD with other conventional desalination techniques as end of pipe solution for ZLD applications should be explored.
