Review on Gas-liquid Mixing Analysis in Multiscale Stirred Vessel Using Cfd

The liquid-gas operations in the stirred vessels can be a complex system to analyze because of the presence numerous parameters and operational conditions that effect its performance. Since the past decades, CFD analysis is gaining popularity and even replacing the need for extensive experimentation. The objective of this review is to establish the approaches and equations commonly used in CFD analysis for gas-liquid mixing operation and investigation of their strength and weaknesses. Upon review, it was found that researchers either used Eulerian-Eulerian (E-E) or Eulerian-Lagrangian (E-L) approaches to investigate multi-phase flows. It is concluded that E-L approaches for dispersed multi-phase flows simulate detailed flow structures with a much higher spatial resolution if powerful computing is available.Within these approaches turbulency is another important factor for the study of fluid dynamics.Common turbulence models such as standard, realizable and RNG k-ε RSM, SST k-ε direct numerical simulation (DNS) and large eddy simulation (LES) are described and compared for their respective limitations and advantages. Literature confirmed that k-ε is the most widely used turbulence model. However it suffers from some inherent shortcoming due to the assumption of the isotropy of turbulence and homogenous mixing which is suitable for very high Reynolds number in unbaffled stirred reactors. Therefore LES have been proven to be able to overcome k-ε limitations but it needs a rather intense CPU time.In the applications of CFD simulations for gas–liquid stirred vessels, bubble size distribution is the most important parameter; hence, different techniques for the formulation of the bubble size equations that involves source/sink terms for the coalescence or breakup have been investigated. The techniques include The 4 AUN/SEED Net Regional Conference on Chemical Engineering 83 Quadrature of moments, Direct Quadrature of Moments, Classes Method bubble number density, multiple size group model, parallel parent and daughter classes and the Monte Carlo method for solving population. These techniques provide a framework in which the population balance method together with the coalescence and break up models can be unified into three-dimensional CFD calculations. Sliding mesh (SM), multiple reference frame (MRF) and inner– outer (IO) aspects were also investigated as the three widely used approaches for simulation of flow field in modeling problems. The simulations performed indicate that the type of numerical grids applied did not significantly influence the numerical results. Meanwhile the grid density has a considerable effect on the numerical results especially in Lagrangian simulations where the particles are investigated individually and also for MRF framework where stationary and moving domanis are coupled. In addition, the most common simplification assumptions which have resulted in the major error during simulation are also summarized and factors that contribute to lack of fundamental knowledge on CFD analysis for multiphase flows in stirred reactors are also explored.