EFFECT OF GAS AND LIQUID PROPERTIES ON GAS-PHASE DISPERSION IN BUBBLE-COLUMNS

The effect of gas and liquid properties on the gas phase dispersion has been investigated in bubble column reactors. Data were obtained in two 3.0 m tall bubble columns (of diameters 0.15 m and 0.25 m) and by varying superficial phase velocities. A novel experimental technique, a quadrupole mass spectrometer, was used to measure the tracer gas concentration. Data analysis was accomplished via a simple axial dispersion model with the inclusion of a mass transfer term. Results indicate that an increase in liquid viscosity and decrease in the liquid surface tension leads to a decrease in the gas phase dispersion. Further, the gas properties have no influence on the gas phase dispersion so long as the mass transfer effects are properly accounted for in the model. A hydrodynamic model has been proposed to predict the gas phase dispersion in bubble column reactors. The model distinguishes various bubble fractions present in the column based on the differences in their rise velocities. The model assumes a bimodal distribution of the gas phase, i.e. fast-rising bubbles following a plug-flow behavior and slow-rising bubbles which are being entrained and partially backmixed in the liquid phase. The model has been validated by predicting experimental as well as literature data on gas phase dispersion under various operating conditions. The proposed model is easy to use since it requires few easily obtainable parameters for the prediction of gas phase dispersion, which is an essential parameter in the design and upscaling of bubble column reactors.