With an ever-increasing demand for freshwater, it becomes essential to find ways of producing freshwater. Due to its abundance, desalination of seawater near coastal regions is a logical choice. It becomes imperative to develop desalination systems that are scalable, energy efficient and cost-effective. Flash evaporation has been used for producing potable water of which multi-stage and multi-effect desalination systems are popular. However, they are energy intensive processes. We have developed a novel dynamic flash evaporation and vapor separation system which encompass evaporation and separation in one step. In the experimental work&nbsp;described in this study water from a primary tank is pumped to pressures above one atmosphere and heated to temperature close to saturation. Thereafter it is injected into a separator tube mounted with tangential injectors near the inlet. The separator tube is made of borosilicate glass which allows for visual observation of the process. The end of the separator tube is mounted to an exit block on which a retrieval tube is mounted. The pressurized water passes through tubes that connect to the injectors. This causes pressure drop due to friction. As the pressure drops, sub-cooled water becomes superheated which triggers flashing forming a two-phase mixture which further increases pressure drop in the flow direction. This two-phase mixture is then injected tangentially into the test section. Tangential injection causes centrifugal force which pushes the denser liquid to the annulus and vapor forms a stable core in the middle of the separator tube. The retrieval tube positioned in the vapor core removes the separated vapor which proceeds to a condenser where it is condensed at atmospheric pressure. The mass flowrate of condensate is measured for calculating its purity. The remaining liquid from the separator exits to a secondary tank. Experiments were performed for different flowrates and superheats to determine the optimal operating conditions. The retrieval tube position was strategically adjusted to obtain maximum phase separation efficiency. The effect of injector, separator tube dimensions and the length of tubes connecting the injectors were studied pertaining to thermal conversion and phase separation efficiencies. Additionally, two and four injector configurations were investigated. Thermal conversion efficiency is defined based on the utilization of maximum superheat available. Phase separation efficiency is defined based on the purity of separated vapor after condensation by mass. As high as 90% and 99% efficiencies were achieved for thermal conversion and phase separation, respectively. The present work opens new opportunities for compact, scalable flash evaporation and vapor separation system for desalination of seawater that can be operated with solar power and/or waste heat.&nbsp;* This work received support from NSF.

A Dynamic Flash Evaporation and Vapor Separation System for Seawater Desalination

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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