Hydrodynamic and mass transfer characteristics of bubble and packed bubble columns with downcomer

The hydrodynamic and mass transfer characteristics of bubble and packed bubble columns with downcomer were investigated. The contactor consisted of two concentric columns of 0.11 and 0.2 m i.d., with the annulus acting as the downcomer. The packing used in this investigation was standard 16 mm stainless steel Pall rings. The superficial gas and liquid velocities, V G and V L , were varied from 0.01 to 0.09 and 1 × 10 −3 to 8.8 × 10 −3 m s −1 respectively. Two flow patterns, namely the bubble and pulse flows were observed in the packed bubble column with downcomer, as shown by a flow map. The liquid circulation velocity in both the contactors was observed to be constant throughout the ranges of V G and V L covered in this work. The effect of liquid viscosity (0.8 to 9.5 mPa ṁ s) and surface tension (45 to 72 mN m −1 ) on the flow pattern, liquid circulation, gas hold‐up and pressure drop was investigated. The pressure drop characteristics across the two contactors have been compared with those across a bubble column. Values of the effective interfacial area, a, and the volumetric mass transfer coefficient, k L a, were measured by using chemical methods. Values of a as high as 180 and 700 m −1 and k L a as high as 0.075 and 0.22 s −1 , in the bubble and packed bubble columns with downcomer, respectively, were obtained. The values of true liquid‐side mass transfer coefficient, k L , were found to be independent of V G and were of the order of 5.5 × 10 −4 and 3.5 × 10 −4 m s −1 , respectively, in the two contactors.