Three-phase Eulerian simulations of bubble column reactors operating in the churn-turbulent regime: a scale up strategy

Abstract

This paper develops a strategy for scaling up bubble column reactors operating in the churn-turbulent flow regime using computational fluid dynamics (CFD). The bubble column is considered to be made up of three phases: (1) liquid, (2) "small" bubbles and (3) "large" bubbles and the Eulerian description is used for each of these phases. Interactions between both bubble populations and the liquid are taken into account in terms of momentum exchange, or drag, coefficients, which differ for the "small" and "large" bubbles. The interactions between the large and small bubble phases are ignored. The turbulence in the liquid phase is described using the k-ε model. The three-phase description of bubble columns was implemented within the Eulerian framework of a commercial code CFX 4.2 of AEA Technology, Harwell, UK. Two types of approaches were first compared: (a) a simulation model assuming axi-symmetry and (b) a complete three-dimensional model for the cylindrical columns. The three-dimensional simulation showed chaotic behaviour. After averaging with respect to time and in the azimuthal direction, the radial distribution of liquid velocities corresponded closely with the two-dimensional axi-symmetric model. The total system gas hold-up predicted by these two simulation variants were also comparable though there was a significant difference in the radial distribution of the hold-up profiles of the large and small bubbles. For purposes of validation of the three-phase Eulerian simulation model, experiments were carried out in columns of 0.1, 0.174, 0.19, 0.38 and 0.63 m diameter. Three types of experiments were carried out: (1) dynamic gas disengagement experiments to determine the hold-ups of small and large bubble populations, (2) radial distribution of the axial component of the liquid velocity, and (3) centre-line liquid velocity. Demineralized water and Tellus oil, with a viscosity 75 times that of water, were used as liquid phase and air as gaseous phase. Comparison of the experimental measurements with the Eulerian simulations was used to conclude that the two-dimensional axi-symmetric model is adequate for scale up purposes. Simulations for columns with diameters ranging from 1 to 6 m were carried out to emphasise the strong influence of scale on the hydrodynamics.