A combined computational fluid dynamics (CFD) and population balance model (PBM) approach has been applied to the simulation of gas–liquid stirred tanks agitated by (i) a Rushton turbine or (ii) a CD-6 impeller, operating at aeration numbers from 0.017 to 0.038. The multiphase simulations were realised via an Eulerian–Eulerian two-fluid model and the drag coefficient of spherical and distorted bubbles was modelled using the Ishii–Zuber equations. The effect of the void fraction on the drag coefficient was modelled using the correlation by Behzadi et al. [Behzadi, A., Issa, R.I. and Rusche, H., 2004, Modelling of dispersed bubble and droplet flow at high phase fractions, Chem Eng Sci, 59: 759–770]. The local bubble size distribution was obtained by solving the PBM using the quadrature method of moments (QMOM). The local k L a was estimated using both the Higbie penetration theory and the surface renewal model. The predicted gas–liquid hydrodynamics, local bubble sizes and dissolved oxygen concentration were in good agreement with experimental measurements reported in the literature. A slight improvement in the prediction of the aerated power number was obtained using the non-uniform bubble size distribution resulting from the coupled CFD–PBM simulation. Evaluation of the prospective scale-up approaches indicates a higher probability of maintaining a similar level of mass transfer in a larger tank by keeping the P g / V and VVM constant. Considering its predictive capability, the method outlined in this work can provide a useful scale-up evaluation of gas–liquid stirred tanks.

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Modelling of mass transfer in gas–liquid stirred tanks agitated by Rushton turbine and CD-6 impeller: A scale-up study

Multidimensional population balance model for the simulation of turbulent gas–liquid systems in stirred tank reactors

In the first part of the review, published literature regarding the CFD modelling of single-phase turbulent flow in stirred tank reactors with radial flow impellers was critically analysed. A brief overview of different turbulence models (standard k − e model, RNG k − e model, the Reynolds stress model and large eddy simulation) as well as impeller baffle interaction models has been presented in the previous part. This part is concerned with the review of literature regarding CFD simulation of axial flow impellers. Comprehensive simulations have been carried out using various turbulence models and the model predictions (of all the mean velocities, turbulent kinetic energy and its dissipation rate) have been compared with the experimental measurements at various locations in the tank. The strengths and weaknesses of various turbulence models for axial flow impellers is presented. The quantitative comparison of exact and modelled turbulence production, transport and dissipation terms has highlighted the reasons behind the partial success of various modifications of standard k − e model as well as Reynolds stress model. Literature efforts on multiple impeller systems and multiphase systems have been discussed in a separate section. Based on these results, suggestions have been made for the future work in this area.



Dans la premiere partie de l'etude, on a procede a une analyse critique de la litterature concernant la modelisation de la dynamique des fluides numerique de l'ecoulement turbulent a une phase dans les reacteurs a cuve agitee dotes de turbines a ecoulement radial. Une vue d'ensemble rapide des differents modeles de turbulence (modele standard k-e, modele RNG k-e, modele aux tensions de Reynolds et simulation des grandes echelles), ainsi que des modeles d'interaction des deflecteurs de turbine, a ete presentee dans la partie precedente. Cette partie se concentre sur l'analyse de la litterature concernant la simulation de DFN de turbines a ecoulement axial. Des simulations completes ont ete effectuees en utilisant plusieurs modeles de turbulence et les predictions des modeles (de toutes les vitesses moyennes, de l'energie cinetique turbulente et de son taux de dissipation) ont ete comparees aux donnees experimentales relevees a differents endroits de la cuve. On a presente les points forts et les points faibles de plusieurs modeles de turbulence concernant les turbines a ecoulement axial. La comparaison quantitative des donnees exactes et modelisees de la production, du transport et de la dissipation de la turbulence a mis en evidence les raisons qui expliquent la reussite partielle de plusieurs modifications apportees au modele standard k-e ainsi qu'au modele aux tensions de Reynolds. Une partie distincte est consacree a la discussion des resultats indiques dans la litterature concernant les systemes a roues multiples et les systemes multiphases. Sur la base de ces resultats, des etudes a venir dans ce domaine ont ete suggerees. © 2011 Canadian Society for Chemical Engineering

M Skrga

Papers

Two-phase mass transfer coefficient prediction in stirred vessel with a CFD model