Critical Gas Velocity for Complete Suspension of Solid Particles in Bubble Column

Abstract

The critical superficial gas velocity for suspending solid particles, UG.C is one of the key design parameters to contact the gas, liquid and solid phases in a bubble column. Values of UG.C were measured over a wide range of variables, such as dimension of bubble column, geometry of gas sparger, physical properties of gas and liquid and solid particles. The values of UG.C obtained were correlated in terms of a factor of K defined as the ratio of the values of UG.C in a bubble column to that in an airlift bubble column with external liquid circulation having both the same riser (also downcomer) diameter DT and the same static slurry height Ls above gas distributor as in the bubble column concerned. According to dimensional analysis, K was correlated by Ls/DT, the Reynolds number based on the terminal velocity of a single particle Ut, Re (=DT UtρL/μL) and the ratio of solid particle to liquid densities, ρp/ρL, indicating degree of developing liquid circulation flow over column height, of accompanying solid particles with liquid flow and of suspending solid particles due to buoyancy, respectively. A critical Reynolds number at which the dependence of K on Re changed was found to be 800 for all four spargers used. The two bubble columns with parforated plates as gas spargers showed an identical dependence of K on Ls/DT, Re and ρp/ρL, while the other two columns with single nozzles showed a similar identical dependence. The calculated values of UG.C from the correlation agreed with the observed ones including the literature data within an accurcy of approximately ± 30%. © 1995, The Society of Chemical Engineers, Japan. All rights reserved.