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

This paper develops a strategy for scaling up bubble column reactors operat ing in the churn-turbulent Row regime using computational fluid dynamics (C FD). The bubble column is considered to be made up of three phases: (1) liq uid, (2) "small" bubbles and (3) "large" bubbles and the Eulerian descripti on is used for each of these phases. Interactions between both bubble popul ations and the liquid are taken into account in terms of momentum exchange, or drag, coefficients, which differ for the "small" and "large" bubbles. T he interactions between the large and small bubble phases are ignored. The turbulence in the liquid phase is described using the k-epsilon model. The three-phase description of bubble columns was implemented within the Euleri an framework of a commercial code CFX 4.2 of AEA Technology, Harwell, UK. T wo types of approaches were first compared: (a) a simulation model assuming axi-symmetry and (b) a complete three-dimensional model for the cylindrica l 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-dimensi onal axi-symmetric model. The total system gas hold-up predicted by these t wo 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 s imulation model, experiments were carried out in columns of 0.1, 0.174, 0.1 9, 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 compone nt of the liquid velocity, and (3) centre-line liquid velocity. Demineraliz ed 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 m easurements with the Eulerian simulations was used to conclude that the two -dimensional axi-symmetric model is adequate for scale up purposes. Simulat ions for columns with diameters ranging from Ito 6 m were carried out to em phasise the strong influence of scale on the hydrodynamics. (C) 2000 Elsevi er Science Ltd. All rights reserved.