Study On Flow And Mass Transfer In Airlift Loop Reactors

Airlift-loop reactor is an easy performance and regular flow field reactor with simple structure. It is used widely in biology, chemistry, wastewater treatment for its predominant fluid mixing performance and high liquid volume mass transfer coefficient without mechanical stirrer. However, the two-phase flow field in airlift-loop reactors is too complex to be characterized with analytical method. As a result, its structure choosing, designing and scale-up are still depended only on experimental validation and expert experience, lacking in support of theories and consuming much money and time. It is very urgently to study about flow field and mass transfer in airlift-loop reactors.To study on fluid flow and mass transfer in detail and grope for a theory of reactor designing and scale-up, the gas-liquid-two-phase flow and mass transfer in the airlift-loop reactors has been studied in terms of the CFD (computational fluid dynamics) simulation and experimental research. Major research work and conclusions are summarized as follows:(1) Based on the analysis of fluid flow in airlift-loop reactors, a mathematical model of the hydrodynamics of two configurations of internal airlift loop reactors had been developed with the two-fluid model, K - s equation of two-dimension pseudo-homogeneous phase and the kinetic theory of two-phase flow. The model for predicting the local liquid volumetric mass transfer coefficient was set up based on Higbie's penetration theory and Kolmogoroff's theory of homogenous and isotropic turbulence. Interactions between the bubbles and the liquid were taken into account by means of a momentum exchange, or drag and added mass force. Besides, the numerical model was combined with two-film model to describe the oxygen mass transfer process.(2) The established equations were solved by means of FLUENT. The profiles of flow field, local gas holdup, bubble diameter and dissolved oxygen concentration, etc. were obtained, and the flow and mass transfer in reactor were showed roundly. The mixing time was got by the two-step method. The simulation results reveal that the airlift-loop reactor has characteristics such as well macro-mixing, low dead-zone volume, and regular flow.(3) The effect of operational and structural parameters of airlift-loop reactors ongas-liquid two-phase flow and mass transfer was studied. The results reveal that superficial gas velocity, static liquid height, H/D , D1/D , D2/D and a are important to the gas-liquid-two-phase flow and mass transfer in the airlift-loop reactors. The optimalstructural parameters of airlift-loop reactors are obtained.(4) The equipment used to measure flow field of airlift-loop reactors was constructed,with the experimental data, such as the gas holdup and liquid velocity and mixing time etc., being acquired by a computer. The comparison between the experimental measurement and the numerical results reveals that they have same development trends though there is a little deviation.