Effects of gas absorption with chemical dissociation reaction on single slurry droplet drying

Abstract

A theoretical model of the absorption of soluble gases during the first stage of slurry droplet drying, at or below room temperature and at atmospheric pressure, is suggested as a basis for the development of alternative drying methods. The suggested approach is based on the theory of slurry droplet evaporation, in conjunction with a model of chemical absorption by evaporating droplets. Slurry droplet evaporation is described by a system of transient conjugate nonlinear energy and mass conservation equations. Numerical calculations are performed for a slurry droplet evaporating in various gaseous mixtures, including a gaseous mixture containing soluble gases. It is shown that the effect of gas absorption is a significant enhancement in the rate of evaporation of a slurry droplet, with higher concentrations of soluble gas in the gas phase leading to a shorter first stage of drying. In particular, in a ternary gaseous mixture (N₂/NH₃/H₂O) with an ammonia mass fraction of 0.5 and at 288 K, the evaporation time of a coal-water slurry droplet with an initial radius of 100$\mathrm{\mu }\mathrm{m}$ was about 20% shorter than that in a mixture free of soluble gases. The existence of a maximum in the temporal dependence of the interfacial temperature of the droplet during evaporation is also revealed. This maximum becomes more pronounced with increasing ambient concentration of the absorbate. Calculations show that relative humidity exerts a minor influence on the dynamics of gas absorption by liquid droplets. By contrast, relative humidity strongly affects the rate of evaporation of slurry droplets. The model for slurry droplet evaporation in the presence of soluble gas presented herein allows for calculation of the appropriate concentration of soluble gas in the gaseous phase to achieve an optimal regime for slurry droplet drying.

Keywords:

• Slurry droplet drying
• gas absorption
• low temperature drying

Additional information

Notes on contributors

Yehonatan David Pour

Mr. Yehonatan David Pour is an M.Sc. student at the Department of Mechanical Engineering at Ben-Gurion University of the Negev, Israel.

Dr. Andrew Fominykh works at the Department of Mechanical Engineering at Ben-Gurion University of the Negev, Israel as Research Associate Professor (Researcher Grade A). He joined Ben-Gurion University in 1994. He got his Ph.D. in 1987 from Novosibirsk State University, Russia. His major field of interest is heat and mass transfer in two-phase flows.

Dr. Boris Krasovitov, Ph.D., currently works at the Mechanical Engineering Department of Ben-Gurion University of the Negev in Israel. He graduated from the Physics Department of Moscow Regional State University, Russia. He got his Ph.D. from the same University. His research focuses on heat and mass transfer in multiphase flows, air pollution modeling and scavenging of polluted aerosols and gases from the atmosphere. He is a member of American Association for Aerosol Research (AAAR) and European Institute of Liquid Atomization and Spray Systems (ILASS). Since October 2000, he is a senior research scientist in the Laboratory of Turbulent Multiphase Flows at the Pearlstone Center for Aeronautical Engineering Studies of Ben-Gurion University of the Negev.

Prof. Avi Levy serves as the Dean of the Engineering Sciences Faculty. He studied Mechanical Engineering at Ben-Gurion University of the Negev, Israel. He got his Ph.D. in 1995 from the same university. He worked as a research scholar in the above university until 1995 and as a research fellow at Glasgow Caledonian University in the Center for Industrial Bulk Solids Handling until 1997. Then he joined the Department of Mechanical Engineering at Ben-Gurion University of the Negev, Israel, where he served as the Head of Department. His main interest centers on ‘transport phenomena in multiphase media’, ‘pneumatic conveying’, ‘drying’, ‘mass momentum and heat transfer’, ‘heat pumps’, ‘bubble pumps’, ‘diffusion absorption refrigeration systems’ and ‘shock wave interaction with porous and granular materials’.