Computational flow modelling and design of bubble column reactors

Abstract

The present design practice of the bubble column reactors is still closer to an art than science because of the complexity of the fluid mechanics. In view of this, there have been continuous attempts to understand the complex three-dimensional turbulent two-phase flow. The present paper reviews the modelling efforts on the flow patterns published in the last 30 years with relatively more focus on the last 10 years. Over this period, there have been sustained efforts to improve our understanding of the governing equations of the change (equations of continuity and motion) for two-phase flows. Both Eulerian and Lagrangian approaches have been extensively used. The development has been mainly on three fronts: (i) formulation of interfacial forces (ii) closure problem for the eddy viscosity and (iii) modelling of the correlations arising out of Reynolds averaging procedure. As regards to interface force terms, the published literature has been critically analysed. The present status of our understanding of the drag force, virtual mass force and lift force has been presented. The physical significance of the various formulations has been brought out. The mechanism of the energy transfer from gas to liquid phase has been explained. The developments in closure problem have been most dramatic. The progress of the past 30 years has been reviewed with a focus on the past 10 years. The published literature has been critically analysed and chronology of development has been presented. The effort has been concentrated on cylindrical bubble columns where results on flow pattern could be extended to the design. The studies on transient flow pattern in two-dimensional columns have not been covered because the subject is still under development and the results cannot be extended to the design objective. The closure problem is intimately linked with the physics of turbulence. An attempt has been made to develop a complete correspondence between an operation of real column and the model simulation. Attention has been focused on the cylindrical bubble columns because of their widespread applications in the industry. The effects of the superficial gas velocity, column diameter and bubble slip velocity on the flow pattern have been examined. Extensive comparison has been presented between the predicted and the experimental velocity profiles. For the design of the bubble columns, the knowledge of various design parameters (such as pressure drop, rate of mixing, residence time distribution of both the phases, heat and mass transfer coefficients) is needed. For the estimation of these parameters, the prevailing procedures are largely empirical. The fundamental basis for the estimations is possible through the understanding of the detailed macro- and micro-flow patterns. This basic direction has been the subject of several publications, particularly during the last 5 years. All these studies have been critically analysed in the present review paper. A coherent and holistic approach has been presented on the modelling of fluid mechanics and design of bubble column reactors.