Investigations of the Gas-Liquid Multiphase System Involving Macro-Instability in a Baffled Stirred Tank Reactor

Shuo Zhang,Jan Talaga,Michal Dylag,David Müller,Günter Wozny

doi:10.1155/2016/3075321

Abstract

Bubble Sauter Mean Diameter (SMD) in gas-liquid multiphase system is of particular interest and the quantification of gas characteristics is still a challenge today. In this contribution, multiphase Computational Fluid Dynamic (CFD) simulations are combined with Population Balance Model (PBM) to investigate the bubble SMD in baffled stirred tank reactor (STR). Hereby, special attention is given to the phenomenon known as the fluid macro-instability (MI), which is a large-scale low-frequency fluid velocity variation in baffled STRs, since the fluid MIs have a dominating influence on the bubble breakage and coalescence processes. The simulations, regarding the fluid velocity, are validated with Laser Doppler Anemometry (LDA) experiments, in which the instant radial velocity is analyzed through Fast Fourier Transform (FFT) spectrum. The frequency peaks of the fluid MIs are found both in the simulation and in the experiment with a high degree of accuracy. After the validation, quantitative predictions of overall bubble SMD with and without MIs are carried out. Due to the accurate prediction of the fluid field, the influence of the fluid MI to bubble SMD is presented. This result provides more adequate information for engineers working in the field of estimating bubble SMDs in baffled STRs.

Highlights

In multiphase mixing processes of the chemical and biotechnological industry, massive means of agitation are required
The injected gas flow rate is zero where no bubble coalescence and breakage behaviors are involved and the Computational Fluid Dynamic (CFD) simulation results are validated by the Laser Doppler Anemometry (LDA) experiment
On the basis of verified simulation models, the influence of the fluid MI on the bubble Sauter Mean Diameter (SMD) is estimated among the transient steady Population Balance Model (PBM)-CFD simulation and multiblock model [19] with the same geometry and boundary conditions

Summary

Introduction

In multiphase mixing processes of the chemical and biotechnological industry, massive means of agitation are required. Details on conditions of the occurrence of two fundamental types of the MI are given in the comments [8] from Kresta and Roussinova, as well as the comments reply [9] from Yianneskis to the editors In such gas-liquid multiphase STRs, less attention has been paid to the influence of fluid MIs on the bubble size distribution, the bubble Sauter Mean Diameter (SMD) in gasliquid STRs. The fluid MIs resulting in the fluid turbulence condition variation are believed to have the dominating impact on the bubble breakage and coalescence during the bubble SMD prediction. The simulation results with the PBM-CFD modeling could reveal the dynamics behavior of bubbles and might contribute to achieve the optimal design of reactors

Theoretical

Experimental

Results and Discussion

Conclusions