Hydrodynamic aspects of airlift contactors

This study deals with gas void fraction, liquid circulation velocity and liquid mixing in airlift contactors.;Gas holdup was observed to increase with increasing gas velocity, liquid phase electrolyte concentration and Newtonian and pseudoplastic non-Newtonian liquid viscosity. The effect of the two immiscible liquid phases on gas holdup was relatively complex. Initially, on addition of oil to the continuous phase (water) there was a sharp increase in gas holdup. As the concentration of oil-in-water emulsion increased the gas holdup was observed to decrease and then increase as the pure oil limit was reached.;The liquid velocity and liquid phase mixing in the airlift vessel improved with increasing gas flow rate. But, with an increase in electrolyte concentration, liquid viscosity and oil-in-water emulsion concentration the superficial liquid velocity decreased. Hence, the liquid mixing was found to deteriorate with increasing salt concentration, liquid viscosity and oil-in-water emulsion concentration. For a fixed gas velocity, the local Bodenstein numbers and axial dispersion coefficients indicated that the mixing in the airlift contactor was not uniform in the different regions of the vessel. The downcomer section gave considerably higher axial dispersion coefficient values compared to the riser section for a given gas flow rate. The riser and downcomer Bo numbers indicated that the liquid flow in the riser section approached plug flow, while the flow in the downcomer section was reasonably well mixed. The liquid mixing in the reactor as a whole was somewhere in- between the mixing in the riser and downcomer sections.;A hydrodynamic model was employed to estimate the riser gas holdup and liquid velocity in the airlift vessels. This model was developed using a drift flux model together with an energy balance over an airlift loop. Model predictions were compared with experimental data obtained in this study. In addition, suitable experimental data available in previous literature were used. The applicability of the model to a broad range of airlift reactor types (split-cylinder and draft tube internal loops; external loop) and scales (0.033 m3 ≤ V ≤ 1.058 m3; 1.220 m ≤ Ht ≤ 5.076 m; 0.11 ≤ Ad/Ar ≤ 1.45) and with various Newtonian and non-Newtonian liquids is demonstrated.;Experimental measurements of gas holdup, liquid velocity, mixing and circulation times, Bodenstein number and liquid phase axial dispersion coefficient were obtained with various Newtonian and non-Newtonian fluids using different reactor configurations. The experimental data were obtained in two distinct laboratory scale draft tube internal loop airlift devices (52 L) and an external loop airlift contactor (35 L).The effect of superficial gas velocity (low to intermediate range), liquid properties (salt concentration and liquid viscosity), two immiscible liquid phases (oil/water) and reactor geometry on the gas holdup, liquid velocity and liquid mixing in airlift vessels were examined.;For non-Newtonian media in airlift vessels, a correlation for average shear rate was also developed. This relationship considers the effect of gas holdup, liquid velocity and flow behaviour index on shear rate. A higher apparent liquid viscosity gave a lower average shear rate in the vessel than a lower apparent viscosity, at a given superficial gas velocity.