Optimum solids concentration for solids suspension and solid-liquid mass transfer in agitated vessels

The effect of solids concentration on specific impeller power consumption and solid-liquid mass transfer coefficient was investigated in a 0.2 m diameter baffled agitated vessel with standard six-bladed Rushton turbine for solids concentration up to 0.40 (v/v). It was found that the increase of solids concentration significantly increases the mass transfer coefficient up to an optimum solids concentration and decreases thereafter when the system is operated at a just-suspended condition. The increase in mass transfer coefficient with an increase of solid concentrations is mainly due to the increase in N-js (critical impeller speed) with increasing solids concentration, thereby leading to an upsurge of turbulence around the particles. The solids concentration at which the highest mass transfer coefficient is obtained is designated as the effective solids concentration. In a geometrically similar 0.3 m diameter tank, the trends in impeller energy efficiency and solid-liquid mass transfer coefficient values with increasing solids concentration are similar. The experimental data are satisfactorily correlated using the concept of the Kolmogoroffs theory of isotropic turbulence to develop an equation to estimate the solid-liquid mass transfer coefficient in agitated vessels. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.