Chemical Engineering Science 60 (2005) 5945–5954 www.elsevier.com/locate/ces Experimentalandtheoreticalanalysisofaxialdispersionintheliquid phaseinexternal-loopairliftreactors Ch.Vial a, b , S. Poncin a , ∗ , G. Wild a , N. Midoux a a LSGC - CNRS-ENSIC - 1 rue Grandville, BP 451, F-54001 Nancy Cedex, France b LGCB - UBP - 24 Av. des Landais, BP 206, F-64174 Aubiere Cedex, France Received 15 November 2004; received in revised form 22 March 2005; accepted 13 April 2005 Available online 6 June 2005 Abstract A model has been developed for the prediction of the axial dispersion coefﬁcient in the riser of an external-loop airlift reactor. It takes into account both radial velocity and gas hold-up proﬁles as a function of ﬂow regime, but also the two-phase turbulence. It is based on the mixing length model developed by [Vial et al., 2002. Chem. Eng. Sci. 57, 4745–4762] for the ﬂow ﬁeld prediction and uses a turbulent diffusion coefﬁcient to estimate the inﬂuence of two-phase turbulence on mixing. The relations established using experimental data from an airlift reactor have been validated experimentally using a second reactor.A comparison with theoretical dispersion coefﬁcients deduced from CFD calculations and correlations from the literature is also provided. The results show that in all the hydrodynamic regimes, dispersion stems from bubble-induced turbulence, despite the presence of a nearly parabolic liquid velocity proﬁle in the homogeneous regime and marked liquid hold-up proﬁles in churn-turbulent ﬂow.  2005 Elsevier Ltd. All rights reserved. Keywords: Aeration; Axial dispersion; Bubble columns; Hydrodynamics; Mixing; Two-phase turbulence 1. Introduction Bubble columns are reactors in which a continuous liquid phase and a gas ﬂow in the form of bubbles are brought into contact. Their main advantages are their easy construction and their low energy consumption, as mix- ing is induced only by gas aeration (Deckwer, 1992). Their main drawback is a severe degree of backmixing in the liquid phase due to the low liquid ﬂow rate. Back- mixing is known to increase drastically when local liq- uid circulation develops. This occurs particularly when transition from homogeneous to heterogeneous regime begins. Backmixing results both from the presence of marked radial velocity proﬁles in the liquid phase, but also from the “two-phase turbulence”. Turbulence in bubbly ∗ Corresponding author. Tel.: +330383175223; fax: +330383322975. E-mail addresses: poncin@ensic.inpl-nancy.fr, souhila.poncin@ensic.inpl-nancy.fr (S. Poncin). 0009-2509/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2005.04.028 ﬂows includes ﬁrst the classical single-phase turbulence, but also the pseudo-turbulence due to bubble passage and slip velocity (Joshi et al., 1990) and an additional con- tribution due to bubble wakes (Schmidt et al., 1992). To reduce backmixing, modiﬁed bubble columns have been designed, such as packed bubble columns. Airlift loop re- actors (ALRs) constitute another important class of such modiﬁed bubble columns in which an overall circulation of the continuous phase is induced by a differential aera- tion between the riser and the downcomer. Higher liquid velocity reduces local liquid circulation and therefore back- mixing. This gives ﬂatter liquid velocity proﬁles, reducing shear, which improves life preservation of microorgan- isms in biological applications (Chisti and Moo-Young, 1987). However, ALRs have been far less studied than conventional bubble columns in the literature and their geometry is more complex, which makes their design trick- ier: speciﬁc phenomena may take place in each section and thus, the sections do not scale necessarily in propor- tion! Indeed, the present design practice of these reactors