Indian Journal of Chemical Technology Vol. 5, July 1998, pp. 187-191 Modelling of air-lift reactors based on bubble dynamics Saikat Chakraborty+, Kajari Kargupta & Avijit Bhowal* Department of Chemical Engineering, Jadavpur University, Calcutta 700 032, India Received 13 January 1998; accepted 8 June 1998 Predictions of hydrodynamic performance of an air-lift reactor (ALR) have traditionally considered bulk gas and liquid phase characteristics. This study aims to present an alternative approach of modelling an ALR by estimating the energy input to the reactor and gas hold-up in the riser based on dynamics of individual bubbles. The bubbles have been assumed to rise in discrete layers in the riser having a diameter in equilibrium with the surrounding pressure. The model predictions of gas hold-up and liquid velocity in the downcomer for ALRs have been compared with the reported experimental data. and has calculated the average gas hold-up in the riser by the equation, · .. (1) · .. (2) · .. (3) Besides this, other empirical correlations are also available for estimation of gas hold-up2.4. Thus, previous predictive studies concerning estimation of the hydrodynamic characteristics of ALR have not taken into account the disperse state of the gas in the riser. In order to present a more realistic approach towards modelling of an ALR, this study attempts to correlate the liquid circulation velocity and gas hold-up with the rise velocity, time of formation and volume of individual bubbles. To estimate the liquid velocity in the downcomer from the above equation, Calvo3 has equated the energy input, E to reversible isothermal work of expansion of the bulk gas i.e., Airlift reactors are one of the most promising devices for gas-liquid mass transfer and are being increasingly used in biotechnological processes. In general, these reactors comprise a pool of liquid, divided into two vertical zones--a central zone called riser, where both the gas and the liquid flow upward and the liquid is sparged by a gas at the bottom, and an annular space called downcomer, where the liquid flows down from the top to the bottom of the reactor. Density difference of the fluids between the riser and the downcomer induces a circulation in the reactor-up flow in the riser and downflow in the downcomer. The primary hydrodynamic characteristics influencing the operation of an airlift reactor are gas hold-up and liquid circulation velocity. These affect the heat and mass transfer coefficients by controlling the extent of mixing in the reactor and the mean residence time of gas and hence process yield. Liquid circulation velocity in airlift reactors can be estimated by equating the energy input to the energy dissipated occurring as a result of liquid circulation 1.3. All the mechanical energy consUII1ed in an airlift reactor is introduced by the gas flow. The gas bubbles expanding through the riser provide the mechanical energy. This energy (E) counterbalances the friction losses between the fluids and devices (F) and the friction losses at the gas-liquid interface (S). The overall energy balance as given by Calvo3 is 'Forcorrespondence +Present address: Department of Chemical Engineering, Indian Institute of Science, Bangalore 560 012, India Theory In this context, a literature review has been done to study the dynamics of bubble. Merchuk5