Global modelling of a gas-liquid-solid airlift reactor

This paper presents a global model of three phase flow (gas-liquid-solid) in an internal airlift reactor. The airlift is composed of four zones: a riser (on the aerated side on the internal wall), a downcomer (on the opposite side) and two turning zones above and below the internal wall. Tap water is the liquid continuous phase and the dispersed phases are air bubbles and polyethylene particles. The global modelling of the airlift involves mass and momentum equations for the three phases. The model enables phase velocities and phase volume fractions to be estimated, which can be compared to experimental data. Closure relations for the gas and solid drift velocities are based on the model proposed by Zuber and Findlay. The drift flux coefficients are derived from CFD numerical simulations of the airlift. Gas bubble and solid particle averaged slip velocities are deduced from momentum balances, including drag coefficient correlations. The link between Zuber and Findlay model and the two-fluid model is established. In the experiment as well as in the model, the gas flow rate is fixed. However, the liquid and solid flow rates are unknown. Two closure relations are needed to predict these flow rates: the first closure relation expresses that the volume of solid injected into the airlift remains constant; the second closure relation expresses a global balance between the difference of column height in the riser and the downcomer and the total pressure drop in the airlift. The main parameters of a three phase airlift reactor, like gas and solid volume fractions, are well predicted by the global model. With increasing solid filling rate (40%), the model starts to depart from the experimental values as soon as coalescence of bubbles appears.