Volumetric mass transfer coefficients in slurry bubble columns operating in the churn-turbulent flow regime

Abstract

We report the results of an extensive experimental study of the gas hold-up, ε , and volumetric mass transfer coefficient, k_La, in bubble columns operated at ambient temperature and pressure conditions. The superficial gas velocity U was varied in the range 0-0.4 m/s, spanning both the homogeneous and churn-turbulent flow regimes. Air was used as the gas phase in all cases. Three different measurement campaigns were carried out. In the first campaign the influence of liquid properties (viscosity, surface tension) were investigated in a column of 0.1 m diameter, equipped with a sieve plate distributor with 0.5 mm holes. Four different liquids were investigated: water, tetradecane, paraffin oil and Tellus oil, with viscosities ranging from 1 to 75 mPa s. The gas hold-up ε_G in these systems vary significantly. The volumetric mass transfer coefficient, k_La, closely follows the trend in gas hold-up. For the churn-turbulent regime of operation, i.e. U> 0.08 m/s, the value of k_La/ε_G is found to be practically independent of U; the value of this parameter is found to depend on the liquid phase Schmidt number, showing a Sc^−1/3 dependence. In the second campaign, we investigated the influence of catalyst particles addition (porous silica, mean diameter=38 μm) to the liquid phase (water and tetradecane), with slurry concentrations varying up to 25 vol.%. With increasing slurry concentrations, ε_G is significantly reduced due to enhanced bubble coalescence and for high slurry concentrations, the "small" bubbles are almost completely destroyed. For U>0.08 m/s, the value of k_La/ε_G is again found to be practically independent of U; this parameter is however significantly lowered with increased catalyst concentrations, due to increase in the size of the "large" bubbles. In the third campaign, the influence of increasing column diameter D_T was investigated by experiments with water and Tellus oil. For both systems, k_La/ε_G shows a slight increase with D_T in the churn-turbulent regime. This increase is due to increased liquid circulations with increasing scale, leading enhanced bubble split up. Our studies provide a simple method for estimation of k_La in industrial size slurry bubble columns operating in the churn-turbulent flow regime.