Prediction of flow rates and stability in large scale airlift reactors

Large-scale High Recirculation Airlift Reactors have been used to treat biodegradable waste waters since the mid nineteen seventies. The system is particularly attractive for situations where the land to locate wastewater works is restricted. Little is known, however, about the fluid dynamics of the gas-liquid mixture flowing around the reactor. This makes the determination of air injection rates difficult if effluent quality and dynamic stability are to be maintained. When the air injected is not sufficient to maintain stable operation the reactor contents may reverse violently resulting in down time, failure to achieve target discharge quality and possible damage to the reactor itself. As a result many reactor installations operate at air injection rates above those necessary for the biological processes. The extra air injected results in higher capital and process costs. This paper considers the effect of air injection rates on the hydrodynamic stability of Airlift Reactors and a two-phase model is proposed to predict stable operation at a reduced air injection rate. Results are presented which show the effect of reactor design on stability.