Studies of Holdup, Mixing, and Heat Transfer in Bubble Column Reactors.

A bubble column is a device in which a gas phase is bubbled through a column of liquid to promote a chemical or biochemical reaction in the presence or absence of a catalyst suspended in the liquid. Bubble columns are now commonly used in chemical, petrochemical and mineral process industries because of their simple construction, ease of operation, and their flexibility with respect to the continuous phase residence time. Prediction of hydrodynamic design parameters, such as bubble size, hold-up, back mixing and heat transfer in these reactors is of considerable importance. In the present study, after an extensive literature review, published theories and experimental results on bubble diameter, hold-up, mixing and heat transfer are collated and systematically analysed. A complete experimental set-up of two and three dimensional columns was constructed. A two dimensional column, and a cylindrical column with 3.5 cm diameter are used for bubble size measurement. Two cylindrical bubble columns with 8 cm and 15.5 cm diameter, and a column with rectangular cross section of 15x10 cm are used for the measurement of other parameters. New experimental results are obtained where published data are insufficient. The effect of system variables where there is not a general agreement is studied. These results and the results of previous investigators are compared with published relations. Bubble size during its formation at single and multiple orifices under constant flow conditions is studied. Using neural network analysis the dependence of system variables on bubble size is detected. A parametric correlation for bubble size prediction based on dimensionless numbers is introduced which is in excellent agreement with the present measurements and previous results of other investigators. Gas holdup is studied for different liquids and at high temperatures. A new criterion is introduced for the prediction of the transition from the homogeneous to the heterogeneous regime in bubble columns, which is based on bubble aggregation. The effect of different operational and geometrical parameters on the transition point is also studied. A new correlation is developed for gas hold-up that is based on flow regimes prevailing in the bubble columns. The presented correlation is in good agreement with the present data and other data found in the literature. Liquid phase dispersion measurements are done utilizing heat as tracer. Using neural network analysis the effects of important variables on dispersion are realized. A new correlation for prediction of dispersion coefficient in the liquid phase is introduced that is in good agreement with the experimental results of present work and that of other investigators. Local heat transfer coefficients are measured in different parts of the column using a rod heater. It is shown that the heat transfer coefficient varies along the column height. A new correlation that is based on theoretical findings of previous investigations is introduced. The present correlation predicts the trend of heat transfer coefficient for different systems with good accuracy