Spatially dependent salinity effect in actively vacuumed spray flash desalination

Document Type

Article

Publication Date

9-1-2022

Abstract

Spray flash desalination is one of the most promised distillation methods, which is realized by spraying the superheated droplets into a depressurized chamber to generate flash evaporation. This intensive evaporation leads to a strongly temperature-salinity coupled polarization near the surface region within droplets, which in turn reduces the evaporation rate. In a spray flash desalination chamber with active vacuuming of vapor extraction, such internal polarizations also weaken the thermal non-equilibrium between the extracted hot vapor and the discharged cooler feed residue during the process. This paper aims to establish an integrated modeling-experiment-CFD methodology for quantifying the spatially dependent salinity effect within droplets to the overall processing characteristics in an actively vacuumed spray flash desalination, such as the evaporation rate and efficiency, the coupled internal heat conduction and salt diffusion within droplets, the thermal non-equilibrium between vapor and spray, as well as the non-uniformed vapor flow from vacuum extraction. Specifically: the heat conduction and salt diffusion coupled droplet flash model is proposed; a further simplified point-based droplet flash model is established for CFD simulations; an experimental system of spray flash desalination with active vacuuming is developed for model validations. The evaporation rate and the thermal non-equilibrium are found to be positively related to the initial superheat levels of the spray but negatively impacted by the initial salinity. Modeling predictions indicate that ignoring the spatial dependence of salinity can over-estimate the flash rate by up to 30% in high salinity cases of salty-water distillation. The thermal non-equilibrium between vapor and feed can be effectively generated under active vacuum, even with a low initial superheat level and high salinity. Increasing feed salinity from 0% to 10% in different cases will narrow the temperature gap between outlet vapor and feed residue by about 10% on average. For typical industrial flash desalination (with salinity up to 10%), with or without considering the spatial dependence of salinity will cause relatively minor differences (less than 5%) on the prediction of evaporation rate.

Identifier

85130938793 (Scopus)

Publication Title

Desalination

External Full Text Location

https://doi.org/10.1016/j.desal.2022.115868

ISSN

00119164

Volume

537

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