Gas holdup and liquid velocity in three-phase internal-loop airlift reactors

Abstract

The liquid velocities and gas holdups in the riser and downcomer of three-phase internal-loop airlift reactors with Newtonian and non-Newtonian fluids were studied. The reactor was constructed of 19 cm ID Plexiglas column and 250 cm in height. The inside diameters of the draft tube were 9, 12, and 14 cm (wall thickness is 0.5 cm), which made the ratio of the cross-sectional area of the riser to the downcomer equal to 0.69, 1.33 and 3.22, respectively. The draft tube heights used in this study were 70, 110 and 150 cm. Water and various concentrations of carboxymethyl cellulose (CMC) aqueous solutions were employed as the liquid phases. Air and polystyrene particles were used as the gas and solid phases, respectively. The liquid velocities and gas holdups in the riser and downcomer were measured by tracer response and manometric techniques, respectively.It was found that the gas holdups and the liquid velocities generally decreased with an increase in the concentration of CMC or the solids loading. In addition, the gas holdup in the riser decreased but that in the downcomer increased with increasing draft tube length. The liquid velocities increased with increasing draft tube length. Furthermore, the gas holdup and the liquid velocity in the riser increased but those in the downcomer decreased with decreasing draft tube diameter. Based on the drift-flux model and energy balance, a hydrodynamic model was developed. It was shown that using a correlation equation for the gas holdup in the downcomer, the proposed model could predict satisfactorily the gas holdup in the riser and the liquid velocities in the riser and downcomer of the internal-loop airlift reactors with Newtonian and non-Newtonian fluids.