Three-phase Eulerian simulations of bubble column reactors operating in the churn-turbulent regime: a scale up strategy
Introduction
Bubble column reactors operated in industry have several distinguishing features: (1) large column diameters are involved, ranging to 6 m, (2) high superficial gas velocities, in the 0.1–0.4 m/s range, are usually used, (3) the system pressure can range to 6 MPa and (4) the liquid phase often consists of a non-aqueous hydrocarbon mixture (Krishna, Ellenberger & Sie, 1996). Laboratory studies on bubble column hydrodynamics are usually carried out with the air-water system, at ambient pressure conditions, in columns that are smaller than say 0.5 m in diameter (Deckwer, 1992). Even for the air–water system, available literature correlations give significantly different results. This is demonstrated by the predictions of the total gas hold-up and the centre-line liquid velocity as a function of the superficial gas velocity and column diameter; see Fig. 1, Fig. 2. Only two correlations plotted in Fig. 1 anticipate that the gas hold-up decreases with increasing column diameter. We see from Fig. 2(b) that the predictions of the centre-line velocity for a bubble column of diameter 6 m diameter operating at U=0.3 m/s varies between 0.9 and 4.5 m/s. This represents a variation of a factor of five and so there is a clear need for a reliable scale up strategy.
The major objective of the present paper is to develop a model for predicting the scale dependence of the hydrodynamics of bubble column reactors operating in the churn-turbulent regime. The model is based on computational fluid dynamics (CFD) and uses an Eulerian description for the fluid phases. We attempt to validate, at least partially, the scale dependence predicted by the CFD model by comparison with experimental data generated in our laboratory in columns ranging in diameter from 0.1 to 0.63 m. Both experimental data from our data bank, partly published previously (Krishna & Ellenberger, 1996; Krishna, Urseanu, van Baten & Ellenberger, 1999b), and new experimental data generated in this work have been used for validation purposes. Furthermore, for purposes of validation of the CFD simulations we also use the experimental data on the radial distribution of gas hold-up and liquid velocity obtained by Hills (1974) in a 0.14 m diameter column with the air–water system.
Section snippets
Experimental
Two types of experiments were performed: (1) Dynamic gas disengagement experiments to determine the hold-ups of the “small” and “large” bubble populations and (2) Measurement of the radial distribution of the axial component of the liquid velocity.
The description of the dynamic gas disengagement experiments data analysis procedure has been discussed earlier (Ellenberger & Krishna, 1994; Krishna & Ellenberger, 1996; Krishna, Van Baten & Ellenberger, 1998). For this study additional measurements
Development of CFD model
For the homogeneous regime of operation of bubble columns a more or less uniform bubble size is obtained (Clift, Grace & Weber, 1978). Many CFD approaches have been successfully developed to cater for this homogeneous regime of operation using the Eulerian description for the gas and liquid phases (Boisson & Malin, 1996; Grevskott, Sannæs, Dudukovic, Hjarbo & Svendsen, 1996; Grienberger & Hofmann, 1992; Jakobsen, Sannæs, Grevskott & Svendsen, 1997; Kumar, Vanderheyden, Devanathan, Padial,
Simulations vs. experiments
We first compare the results of two-dimensional axi-symmetric simulation with a complete three-dimensional simulation of a 0.38 m diameter column at U=0.23 m/s with the air–water system. Fig. 4(a) shows the transient approach to steady state in the 2D simulation. The parameter values at the end of the simulation were taken to be the steady-state values. The corresponding 3D simulation shows chaotic behaviour (cf. Fig. 4(b) and (c)), which can best be appreciated by viewing the animations on our
Conclusions
The following major conclusions can be drawn from the present work.
1. For reasonable predictions of radial distribution of liquid velocity and gas hold-up we must resort to complete three-dimensional Eulerian simulations.
2. For estimation of average gas hold-ups in the dispersion and circulating liquid velocities, typified by the centre-line velocity VL(0), two-dimensional simulations assuming cylindrical axi-symmetry are of adequate accuracy.
3. On the basis of the comparison of Eulerian
Acknowledgements
The Netherlands Organisation for Scientific Research (NWO) is gratefully acknowledged for providing financial assistance to J.M. van Baten.
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