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This paper describes a new mathematical model of the fluid dynamic processes in a high recirculation airlift reactor. The model was created to provide information to assist in the design of a reactor, in particular considering the selection of parameters to adjust in order to achieve a steady state solution. The modelling of two phase-flow of air and water in small scale airlift bio-reactors is considered. This modelling was applied to the high recirculation airlift reactor process. A new computer simulation was created and a test program performed to evaluate the models used. The results of this evaluation are presented. The evaluation showed that variation of the superficial gas velocity or the simultaneous variation of the downcomer and riser diameters could be used to produce a steady-state design solution.

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.

SUMMARYAn investigation has been made into the effect of oil concentration on evaporation heat transfer coefficients in refrigerant-oil mixtures flowing in a horizontal tube. A new correlation is presented for heat transfer coefficients in convective evaporation of refrigerant-oil mixtures that predicts the results of the present study within approximately f 20%. The paper reports measurements of evaporation heat transfer coefficients in refrigerants R12 and R22, both oil-free and with two concentrations of Shell Clavus 32 oil. A 1.8 m long 2 in O/D copper tube (8.05 mm I/D) was used, at evaporation temperatures of -5"C, 0°C and + 5°C. Heat flux and mixture mass velocity were kept constant at 2500 W m-2 and 155 kg m-' s -l , respectively, and measured coefficients were in the range of 1400 to 3900 W m-* K-I. The results showed that, for a complete evaporator, 2% oil may be expected to increase the heat transfer coefficient by 12%, but 10% oil returns the coefficient to oil-free values.