Evaporation and Evolution of Suspended Solution Droplets at Atmospheric and Reduced Pressures

Abstract

Thermal history and solute precipitation behavior of suspended solution droplets of sodium chloride (NaCl), magnesium sulphate (MgSO₄), and zirconium hydroxychloride (ZrO(OH)Cl) evaporating at atmospheric and reduced pressures are studied. Experimental measurements on the variation of droplet diameter, solution concentration, and temperature during the evaporation period are presented and discussed. The results of solute precipitation behavior in solution droplets observed under an optical microscope are displayed and discussed. Results indicate that reducing the pressure (∼ 33 kPa) results in a change in the solution droplet evaporation rate, but the thermal histories of a particular solution droplet are similar at the atmospheric and reduced pressures. At atmospheric and reduced pressures used in this study, the d ² law for solution droplets is valid at early stages of the evaporation and before the solute precipitation initiates. Drying of MgSO₄ and ZrO(OH)Cl solution droplets results in the formation of spherical particles, whereas drying of spherical NaCl solution droplets results in the formation of cubic particles.

Keywords:

• Crystal formation
• Powder morphology
• Reduced pressure effect
• Solute precipitation
• Spray drying and pyrolysis
• Suspended droplet evaporation

ACKNOWLEDGEMENTS

Experiments were carried out in the Thermodynamics and Kinetics Laboratory at the University of Toronto. The cooperation and fruitful discussion with the director of the lab, Professor C. A. Ward, is greatly appreciated. The authors are also grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC) for partial support of this research. The first author acknowledges the financial support from the ministry of Science, Research and Technology of Iran.