Influence of gas separator and scale-up on the hydrodynamics of external loop circulating bubble columns

Gas separators play a significant role on the operation of circulating bubble columns (CBCs), and affect the system hydrodynamics of gas hold-up and the liquid circulation. To study these effects experimentally, a 1351 external loop CBC made from Perspex was used in this work. The CBC had riser and downcomer diameters of 0.153 and 0.082 m, respectively. The liquids used were water and glycerol, while the sparging gas was air. The ratio of the liquid volume in the gas separator to the liquid volume in the reactor, volume ratio TVR, was varied up to 57%. Discernible effects of the volume-ratio on riser, downcomer gas hold-ups and liquid circulation velocity were observed at TVR ≤ 18%. Smooth operation of the CBC was observed at an optimum volume ratio TVRO = 30%. Transition of operation between that of bubble column BC and CBC occurred at TVR = 0% and UGR = 0.07 m s-1 for air-water system, and TVR = 0% and UGR = 0.03 m s-1 for air-glycerol system. The drift flux model was found useful, with an average distribution parameter C0 equal to 0.965, 1.91, 0.78 for homogenous, transitional and heterogenous regimes, respectively. Simple correlations for predicting gas hold-up in the riser, gas hold-up in the downcomer, and liquid circulation velocity were developed taking into account the effect of the volume ratio and liquid viscosity. To account for the scale-up factor AD/AR, the 170 1 external loop CBC data of Al-Masry (1999b), with dR=0.19m and dD=0.14 m, were utilized along the data from this work. The two combined effects, i.e. TVR and AD/AR were recorded in new correlations which predict gas holdup and liquid circulation with good accuracy