(625a) A Novel Euler-Lagrange Simulation Method for Gas-Liquid Stirred Reactors

         Various
processes in chemical and biochemical industries involve dispersion of gas into
liquid in stirred tank reactors. The complexity within the reactors are
extremely high due to turbulent flow induced by impeller, interactions between
the dispersed-gas and liquid phase, and interactions within the dispersed-gas
phase. A simulation method that provides realistic description of the
multiphase flow within the reactors is a valuable tool for design,
optimization, and scale-up of such processes.

         We
present a novel simulation method for gas-liquid stirred reactors including
bubble breakage and coalescence [1]. The filtered conservation equations for
the liquid phase are discretized using a lattice-Boltzmann scheme. A Lagrangian approach with a bubble parcel concept is used to
represent the dispersed-gas phase. Bubble breakage and coalescence are modeled
as stochastic events. The momentum coupling between phases is realized through the  source term
in the conservation equations resulting in the so-called four-way coupling. The
action of the reactor's components on the dispersed-gas and liquid phases is
described using an immersed boundary condition.

         The
present method was used to simulate gas-liquid flow in a stirred reactor
following the experiments of Montante et al. [2]. The
predicted number-based mean diameter and long-term averaged liquid velocity
components agreed qualitatively well with the experimental data (Fig. 1).
Effects of the presence of the dispersed-gas phase and the gas flow rate on the
hydrodynamics were numerically studied.

         Furthermore, the present method was extended by including a power
law model to represent shear-thinning liquids and empirical correlations for
bubbles in shear-thinning liquids [3]. Simulations of aerated stirred
reactors with shear-thinning liquids following the experiments of Venneker et al. [4] were carried out (Fig. 2). The
predicted flow field of a single-phase stirred reactor with shear-thinning
liquid showed reasonable agreement with the experimental data. For gas-liquid
reactors, the predicted gas holdup distribution agreed qualitatively with the
experimental data. Using the present method, it was found that a change in
rheology significantly alters the number mean diameter, Sauter
diameter, and the shape of bubble size distribution.

Figure
1. Comparison between experimental and predicted long-term
average axial and radial liquid velocity.

Figure
2. Snapshot of predicted bubble dispersion pattern in an aerated stirred
reactor with shear-thinning power law liquid.

References

[1]
Sungkorn R., Derksen J.J., Khinast
J.G. Euler-Lagrange modeling of a gas-liquid stirred reactor with consideration
of bubble breakage and coalescence. AIChE J. 2012;58:1356-1370.

[2] Montante G., Paglianti A., Magelli F. Experimental analysis and computational modelling of gas-liquid stirred vessels. Trans IChemE. 2007;85:647-653.

[3]
Sungkorn R., Derksen J.J., Khinast
J.G. Modeling of aerated stirred tanks with shear-thinning power law liquids.
Heat and Fluid Flows. 2012 (accepted).

[4] Venneker V.C.H., Derksen J.J., Van den Akker
H.E.A. Population balance modeling of aerated stirred vessels based on CFD. AIChE J. 2002;48:673-685.