Gas-liquid mass transfer in vessels stirred with multiple impellers—I. Gas-liquid mass transfer characteristics in individual stages

Abstract

Volumetric mass transfer coefficient, holdup and power consumption were measured in individual stages of the vessel stirred with 4 Rushton turbines on a common shaft. Water and a 0.5 M aqueous solution of Na 2SO 4 were used to simulate coalescing and noncoalescing batches. Volumetric mass transfer coefficient, k L a, was measured by the dynamic pressure method (DPM). Distribution of all the measured quantities along the vessel followed a similar pattern: the values of the coefficient in the upper stages were equal and by up to 40% higher than in the bottom stage. The difference of dissipated powers was 50%. The quantities were therefore correlated against the dissipated power separately for the bottom stage and the upper stages. The DPM gives k L a local to individual stages of the vessel even if axial mixing in both phases is significant and the simple model of perfect mixing of phases is used for evaluation. This is demonstrated by comparing the k L a data from the DPM with those measured by the standard dynamic method, with a step change of inlet gas concentration and evaluated using a model of plug flow with axial dispersion for the gas and cascade of four perfectly mixed stages with exchange flows for the liquid. The hydrodynamic parameters of the model were established by tracer experiments.