Spray flash desalination is one of the most promised distillation methods, which induces flash evaporation on over-saturated droplets to generate the vapor. One common method to realize spray flash desalination is spraying the superheated droplets into a vacuum-assisted depressurized chamber, the droplet will evaporate tempestuously due to the sudden pressure drop, and the generated vapor is continuously extracted out of the chamber by vacuuming to maintain a constant evaporative pressure. This method significantly increased the evaporation rate and evaporation efficiency due to increased effective evaporating area by spray atomization. In this process, the freshly generated vapor will be hotter than the liquid residue since the latent heat is self-supplied by the evaporating droplets, which will be continuously cooling down with evaporation going on until it becomes saturated or discharged out of the evaporator. Hence, in the evaporator with an active vacuuming, non-equilibrium exists not only in the flash evaporation driven by the pressure difference between droplet and ambient but also in the temperatures of yields, namely, generated vapor and discharged liquid with or without precipitates. And this temperature gap between two exited phases can be beneficial to many industrial processes such as heat recovery in a distillation process. Most studies of spray flash desalination are focused on heat and mass transfer process on droplets, and little investigation has been done yet to evaluate this thermal non-equilibrium between two phases with the effect of multi-component of droplets.

In this study, to deeper understand the spray flash desalination process and such non-equilibrium phenomena under the operation with active vapor extraction(vacuuming), a mechanistic model of droplets sizes sub-grouped and pressure-controlled flash evaporation is developed to reveal the momentum-heat-mass transfer coupling in the dynamic transport of spray within a bounded depressurized environment. Instead of using assumptions of uniform concentration of salinity in droplets as most previous works did, the evaporating process in this model is dominated by coupled effective heat conduction and mass diffusion of non-volatile substances(NaCl) within in the droplet, as well as a corrected molecular kinetic expression on evaporating interface. A Lab-scaled semi-loop spray flash desalination system is designed and built up for evaporative coefficient determination and model validation. A laser-scanning aided optical measurement system is set up for the spray calibration (droplet size distribution and jetting velocity). A series of experiments are performed and compared with theoretical results under different operating conditions (superheat level and feedwater salinity), to investigate variations of major characteristics, such as evaporation rate, terminal temperatures of two phases (liquid and vapor), and evaporation completeness.


Non-Volatile Fraction Effects in Dispersed Vacuum Spray Flash Evaporation

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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