Study of Hydrodynamic and Mixing Behaviour of Continuous Stirred Tank Reactor Using CFD Tools

Abstract

The effect of fluid flow and impeller characteristics on the mixing and hydrodynamic behaviour of a continuous stirred tank reactor (CSTR) has been studied using computational fluid dynamics software, Ansys Fluent 12.0. Various mathematical models like equation of continuity,momentum, volume of fluid method,laminar and the turbulent equations have been used to describe the flow behaviour in the reactor.The hydrodynamic behaviour was understood by velocity and pressure profiles and also by the vorticity plots.There are different ways to understand mixing phenomena in the reactor.In the present work, residence time distribution (RTD) method was used to observe the fluid flow in the reactor with and without stirrer and baffles.RTDs are evaluated by the swept volume of the impeller and also by tracer injection method where concentration of tracer (KCl) was determined at the exit of the reactor.The computed values of KCl concentration at the outlet were used to find out the age distribution function I   .The laminar flow and MRF model were used to solve the moving stirrer case.The present RTD data were compared with the available experimental values in the open literature.The simulated results were found in good agreement with the experimental data.Along with RTD, the mixing characteristics were also studied in terms of number of ideal CSTR in series,Ncstr, hold back, segregation, mean residence time, m  and variance, 2  . The effect of RPM of the impeller, tank Reynolds number, viscosity and density of the liquid on the mixing efficiency was found.The mixing behaviour was changed from dispersion to ideal mixing state with the increase of impeller rpm and tank Reynolds Number.The hydrodynamic behaviour of CSTR with water-air interface were also studied in presence of different kinds of impellers, baffles etc.The volume of fluid method was used to capture air-water interface.The realizable k  turbulence model has been adopted to describe the flow behaviour of each phase, where water and air are treated as different continua, interpenetrating and interacting with each other everywhere in the computational domain.The results show that the air-water interface was disturbed by the inflow to the tank, and level of water was also increased with time.