Scale influence on the hydrodynamics of an internal loop airlift reactor

The overall circulation velocity, the overall riser and downcomer gas hold-ups and the effect of reactor scale on a two-phase circulation regimes were studied in this work in three airlift reactors of different scale. The measurements were carried out in airlift reactor with internal loops (IALRs) with a working volume of 10.5, 32 and 200 l at the range of temperatures 18–21 °C, under atmospheric pressure. Air and water were used as gas and liquid media. The three reactors were of similar geometry, the ratio between riser and downcomer cross-sectional areas, the aspect ratio of the column and the shape of the column bottom were taken as similarity criteria. In order to determine the linear circulation velocities, the magnetic tracer method was used. The riser and the downcomer were studied separately. Based on gas hold-up in both the riser and the downcomer, two regimes (homogeneous bubble (HMG) and heterogeneous churn-turbulent (HTG)) of the two-phase flow were observed. These were defined by Daniels [Chem. Eng. 70 (1995)] and described using the correlation proposed by Chisti [Airlift Bioreactors, Elsevier, London, 1989]. The average of the liquid circulation velocities increased with increasing reactor scale for the same superficial gas velocity. The overall circulation velocity was modelled on the basis of the momentum balance proposed in paper [Chem. Eng. Sci. 52 (1997) 25]. The parameters of both the correlation and the model tend to be constant for larger reactor scales. The value of the driving force (_R−_D) was found to be important only for lower values of gas flow rate, because at higher values, the circulation velocity seemed to be governed only by friction in the reactor vessel.