Experimental study of evaporation and crystallization of brine droplets under different temperatures and humidity levels

Abstract

The evaporation of brine droplets has critical impacts on the zero-liquid-discharge thermal desalination process. A good understanding of the evaporation of brine droplets provides guidelines for the design of thermal desalination systems, and offers insights for applications when solvent droplet evaporation processes are involved. In this study, brine droplets with different salt mass concentrations were placed in a chamber with controlled temperature and relative humidity. The evaporation and crystallization processes were then visualized through a high speed camera by employing the pendant droplet method. The results showed that the evaporation rate increases with the decrease of relative humidity, the increase of temperature, and the decrease of salt mass concentration. After the evaporation was finished, we can observe that the crystal grew along the filament during the evaporation and remained on it. A salt shell was formed at the outside, while the droplet still contained some amount of brine inside, when the evaporation rate was low. Consequently, the evaporation mechanism was changed once the salt shell was formed, the water molecules needed to overcome the pressure difference inside and outside the salt shell, and diffused through the shell for further evaporation. A multi-variable fitted quadratic regression model was developed with R2 = 0.974 to describe the relationship among the evaporation rate, mass concentration, relative humidity and temperature. Because the regression model was based on experimental data with temperature varying from 30 °C to 60 °C, a predicted result of brine droplet's evaporation rate with various mass concentrations under 75 °C and 0% RH showed a good agreement with the experiment data. Therefore the developed regression model can be extended for high temperature (<100 °C, no boiling) applications.