Exit discontinuity in slurry bubble columns

Abstract

The exit discontinuity in slurry bubble columns, i.e., the difference in the apparent solids concentration at the very top of the column (Ct) and the concentration in the effluent (C e ), was studied in a 0.3 m ID bubble column, using air as the gas phase, water, a light hydrocarbon oil (Varsol) and trichloroethylene as liquids and glass beads of different sizes and density as solids. The results showed that the drop in solids concentration occurs in a very small layer at the gas/liquid interface. By changing the column exit configuration and removing the gas/liquid interface the exit discontinuity disappeared. The extent of the exit discontinuity depended on the liquid properties and appeared to be related to the foaming tendency of the liquid. In addition, in those systems where the exit discontinuity was significant, it depended on the solids properties and the gas and liquid superficial velocities. The magnitude of the exit discontinuity, expressed as Ct/C e , decreased with increasing superficial gas velocity in the water and Varsol systems. In trichloroethylene no exit discontinuity was observed. The correlation Ct/Ce = 1 + 0.5(U g /Vt)4. 4 derived by others from measurements in an air/water system,accurately predicted the exit discontinuity in water. Predictions for the other systems were not in agreement with our measurements. The exit discontinuity in Varsol was very much larger, particularly when foaming was not suppressed.