A computational snapshot of gas-liquid flow in baffled stirred reactors

Abstract

In a stirred reactor, flow around the rotating impeller blades interacts with the stationary baffles and generates a complex, three-dimensional, recirculating turbulent flow. When gas is sparged in such a reactor, gas tends to accumulate in the low pressure region behind the impeller blades forming so-called gas cavities, which significantly alter the flow and turbulence in the reactor. In this paper, a computational technique is developed to predict the turbulent gas-liquid flow in a stirred reactor. The technique is also able to predict the flow around the impeller blades and the accumulation of gas behind these blades. Unlike the past efforts, no empirical (in the form of impeller boundary conditions) is required. A computational snapshot approach has been used to model the gas-liquid flow in a stirred reactor with a disc turbine. This approach essentially boils down to capturing the flow characteristics of a stirred vessel at one time instant from the solution of steady-state equations with boundary conditions corresponding to that particular time instant. A mathematical model is developed for turbulent, dispersed gas-liquid flow. The time-averaged two-phase momentum equations are solved by using a finite volume algorithm. The turbulent stresses are simulated using ak-ε model. The distribution of gas around the impeller blades is predicted for the first time. The model also enables an a priori prediction of the drop in the power dissipated by the impeller in the presence of gas. Predicted flow characteristics of the gas-liquid reactor show good agreement with the experimental data.