SCALING UP BUBBLE COLUMN REACTORS WITH THE AID OF CFD R. KRISHNA and J. M. VAN BATEN Department of Chemical Engineering, University of Amsterdam, Amsterdam, The Netherlands B ubble column reactors, used widely in industry, often have large column diameters (up to 6 m) and are operated at high super®cial gas velocities (in the range of 0.1 to 0.4 m s ±1 ) in the churn-turbulen t ¯ow regime. Experimental work on bubble column hydrodynamic s is usually carried out on a scale smaller than 0.3 m, at super®cial gas velocities lower than 0.25 m s ±1 . The extrapolation of data obtained in such laboratory scale units to the commercial scale reactors requires a systematic approach based on the understanding of the scaling principles of bubble dynamics and of the behaviour of two-phase dispersions in large scale columns. We discuss a multi-tiered approach to bubble column reactor scale up, relying on a combination of experiments, backed by Computational Fluid Dynamics (CFD) simulations for physical understanding. This approach consists of the following steps: (a) description of single bubble morphology and rise dynamics (in this case both experiments and Volume-of-Fluid (VOF) simulations are used); (b) modelling of bubble-bubbl e interactions, with experiments and VOF simulations as aids; (c) description of behaviour of bubble swarms and the development of the proper interfacial momentum exchange relations between the bubbles and the liquid; and (d) CFD simulations in the Eulerian framework for extrapolation of laboratory scale information to large-scale commercial reactors. Keywords: bubble columns; ¯ow regimes; scale up strategy; hold-up; bubble rise; bubble interactions; CFD. INTRODUCTION When a column ®lled with a liquid is sparged with gas, the bed of liquid begins to expand as soon as the gas is introduced. As the gas velocity is increased the bed height increases almost linearly with the super®cial gas velocity, U, provide d the value of U stays below a certain value U trans . This regime of operation of a bubble column is called the homogeneous bubbbly ¯ow regime. The bubble size distributio n is narrow and a roughly uniform bubble size, generally in the range 1±7mm, is found. When the super®cial gas velocity U reaches the value U trans coales- cence of the bubbles takes place to produce the ®rst fast- rising `large’ bubble. The appearance of the ®rst large bubble changes the hydrodynami c picture dramatically. The hydrodynamic picture in a gas-liquid system for velocities exceeding U trans is commonly referred to as the hetero- geneous or churn-turbulen t ¯ow regime 1 . In the hetero- geneous regime, small bubbles combine in clusters to form large bubbles in the size range 20±70 mm 2 . These large bubbles travel up through the column at high velocities (between 1±2 m s ±1 ), in a more or less plug ¯ow manner 3 . These large bubbles churn up the liquid phase and because of their high rise velocities they account for a major fraction of the gas throughput 4 . Small bubbles, which co-exist with large bubbles in the churn-turbulen t regime, are `entrained’ in the liquid phase and, as a good approximation, have the same back-mixing characteristics of the liquid phase. The two regimes are portrayed in Figure 1, which also shows qualitatively the variation of the gas hold-up e as a function of the super®cial gas velocity U. When the gas distributio n is very good, the regime transition region is often characterised by a maximum in the gas hold-up 5 . The transition between homogeneous and churn-turbulent regime is often dif®cult to characterize and it is not prudent to design a reactor to operate in the transition zone. The estimation of the gas hold-up in the bubble column reactor of industrial scale is an important but extremely dif®cult task. The gas hold-up varies signi®cantly with liquid properties. See data in Figure 2 for air±paraf®n oil (r L = 795 kg m ±3 ; m L = 0.0029 Pa s; j = 0.029 N m ±1 ), air±water (r L = 1000; m L = 0.001; j = 0.072) and air±Tellus oil (r L = 862; m L = 0.075; j = 0.028) measured in a column of 0.38 m diameter with a sintered plate distributor . The gas hold-up appears to also depend on the column diameter. See data in Figure 3 for air±paraf®n oil, air±water and air±Tellus oil measured in a columns of 0.1 and 0.38 m diameter, both with a sintered plate distributor . Literature correlations for the gas hold-up bubble columns show a wide spread in their capabilities to predict the variation with respect to U and with respect to the column diameter D T . Figure 4(a) compares air±water experimental data in a 0.38 m diameter column as a function of U with several literature correlations 6±12 . Only the Krishna-Ellenberger 3 correlation matches the data successfully but this is to be expected because their correlation was developed including the data set shown. Figure 4(b) compares air±water gas 283 0263 ±8762/01/$10.00+0.00 q Institution of Chemical Engineers Trans IChemE, Vol. 79, Part A, April 2001