Enhancement of Mass Transfer in a Spray Tower Using Swirling Gas Flow

Abstract

Spray towers are commonly used in the chemical and process industries for a number of applications including absorption, desorption and humidification. However, the main disadvantage of a spray tower compared with that of a packed tower is its lower contact efficiency. The present study is concerned with the enhancement of mass transfer between a continuous gas phase and liquid droplets in a spray tower by imparting swirl to the axial gas flow through the tower. It is well known that swirling flow has the ability to augment the rates of heat and mass transfer. Experimental investigations into the hydrodynamics and mass transfer in a laboratory-scale spray tower for air-NH3/H2O system using axial and swirling gas flows have been carried out. The hydrodynamic studies included measurements of the gas velocity distributions and overall pressure drop in the tower, and characterization of water sprays generated by a pressure-swirl nozzle where radial liquid distributions, droplet size and its distribution and mean droplet size in terms of SMD were measured. As for the mass transfer performance of the spray tower, the effect of the gas and liquid flow rates on the overall gas phase mass transfer coefficient, Kga, was investigated for both swirling and non-swirling axial gas flows in order to quantify the effect of swirl. It has been found that Kga increases with increasing gas/liquid flow rates and imparting swirl in the gas flow enhances Kga up to 20% compared with that in axial flows. Correlations of Kga as a function of the gas/liquid flow rates, and also as a function of the gas flow rate and initial droplets SMD are developed. A design methodology to determine the height of a spray tower required to achieve a specified amount of removal of a solute from a gas mixture is proposed.