CFD simulations of gas–liquid–solid stirred reactor: Prediction of critical impeller speed for solid suspension

Abstract

In this work, simulations have been performed for three phase stirred dispersions using computational fluid dynamics model (CFD). The effects of tank diameter, impeller diameter, impeller design, impeller location, impeller speed, particle size, solid loading and superficial gas velocity have been investigated over a wide range. The Eulerian multi-fluid model has been employed along with the standard k– ε turbulence model to simulate the gas–liquid, solid–liquid and gas–liquid–solid flows in a stirred tank. A multiple reference frame (MRF) approach was used to model the impeller rotation and for this purpose a commercial CFD code, FLUENT 6.2. Prior to the simulation of three phase dispersions, simulations were performed for the two extreme cases of gas–liquid and solid–liquid dispersions and the predictions have been compared with the experimental velocity and hold-up profiles. The three phase CFD predictions have been compared with the experimental data of Chapman et al. [1983. Particle–gas–liquid mixing in stirred vessels, part III: three phase mixing. Chemical Engineering Research and Design 60, 167–181], Rewatkar et al. [1991. Critical impeller speed for solid suspension in mechanical agitated three-phase reactors. 1. Experimental part. Industrial and Engineering Chemistry Research 30, 1770–1784] and Zhu and Wu [2002. Critical impeller speed for suspending solids in aerated agitation tanks. The Canadian Journal of Chemical Engineering 80, 1–6] to understand the distribution of solids over a wide range of solid loading (0.34–15 wt%), for different impeller designs (Rushton turbine (RT), pitched blade down and upflow turbines (PBT45)), solid particle sizes (120– 1000 μ m ) and for various superficial gas velocities (0–10 mm/s). It has been observed that the CFD model could well predict the critical impeller speed over these design and operating conditions.