Chemical Engineering and Processing 48 (2009) 1012–1019 Contents lists available at ScienceDirect Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep Mixing in milli torus reactor under aerated conditions R. Rihani a,∗ , J. Legrand b , A. Bensmaili c a Centre de Développement des Energies Renouvelables, C.D.E.R., BP 62 Route de l’observatoire. Bouzareah, Algeria b Université de Nantes, CNRS, GEPEA, UMR6144, CRTT, BP 406, 44602 Saint-Nazaire Cedex, France c Université des Sciences et de la Technologie Houari Boumediene, U.S.T.H.B., BP 32 El-Alia, Bab-Ezzouar, Algeria article info Article history: Received 17 May 2008 Received in revised form 9 November 2008 Accepted 19 January 2009 Available online 29 January 2009 Keywords: Milli torus reactor Aerated conditions Mixing Gas hold up Axial dispersion coefficient abstract The present paper contributes to improve the understanding of two-phase flow hydrodynamics charac- teristics in milli torus reactor. To reach this goal, experiments were performed in a vertical milli torus reactor, having a capacity of 0.140l. The effect of the aeration numbers and the impeller rotation speeds has been studied. It was found that the homogenization of the tracer inside the gas–liquid milli torus reactor was nearly achieved after more recirculations than those obtained in one-phase flow. Under aer- ated conditions, the flow pattern depended on both impeller rotation speeds and aeration numbers. Two flow regimes have been distinguished: not-dispersed and dispersed flow regimes. The flow induced by a rotating marine impeller reduced bubble size and prevented bubbles coalescence for aeration numbers Fl>0.0021, at which the dispersed flow regime can be maintained in the milli torus reactor. The flow behaviour inside the milli torus reactor was modelled by the dispersed plug flow model. Correlations have been proposed to predict gas hold up and axial dispersion. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. 1. Introduction Mixing plays a very important role in different industries such as chemistry, biochemistry, food,.... According to [1,2], it was esti- mated that the annual cost of an inefficient industrial mixing or inadequate understanding of mixing was of the order of 10 billions of dollars in the USA. To obtain high-added value products, mix- ing must satisfy not only the need of heat and mass transfer but also the required homogeneity in the reactor in the shortest pos- sible time. Also it promotes a better contact between the different phases. Gas–liquid reactors are widely employed in many fields of indus- try in biotechnology in general and in fermentation processes in particular. The contact between gas and liquid phases is carried out in several types of reactors such as: stirred reactors, bubble column reactors, airlifts, static mixers, packed columns, tubes in U, hydroe- jectors, etc. But several reactors are frequently used: the bubble columns, the cylindrical internal or external-loop airlift reactors, in which the required stirring is provided by the gas via sparger. Their disadvantage consist of an intense liquid backmixing, to reduce the latter one, Wei et al. [3] proposed a multi-stage internal-loop airlift reactor, by analogy with the tanks-in-series concept. Stirred tank reactors constitute another important type of gas–liquid reactors, in which the overall circulation of the continuous phase is induced by the impeller. Different impellers design or impellers combina- ∗ Corresponding author. Tel.: +213 21 90 15 03; fax: +213 21 90 15 60. E-mail address: rachida riha@yahoo.fr (R. Rihani). tions will be often used to improve the dispersion and the mixing. Martín et al. [4] studied the effect of different impeller types on bubbles characteristics. These authors found that small bubbles are difficult to be broken and move with the flow. Whereas big bubbles can be easily deformed and broken. In the torus reactor the flow pat- tern is practically the same in all parts of the reactor, it offers certain flexibility in operation, it can be operated in batch as well as in con- tinuous mode. It can be easily set vertically or horizontally. Such geometry seems an attractive way to carry out gas–liquid system in order to reach good mixing conditions. Different types of flow regimes can be observed. For exam- ple, bubble column reactors may be operated into two main flow regimes: the homogeneous regime with uniform bubbles or the heterogeneous regime with wide bubbles. The pseudo- homogeneous regime was encountered at relatively low gas flow rates in bubble column equipped with porous sparger, at this regime discrete bubbles of non-uniform size were formed while coalescence between them was negligible [5]. In this reactor the transitions between flow regimes have been investigated by Gourich et al. [6] using signal processing techniques. The stirred reactors may be also operated in different flow regimes depending on the type and location of the impeller, impeller rotation speed and superficial gas velocity. The effect of these parameters and the geometry factor on mixing time was investigated by [7] in a Bioflow batch dual turbine-stirred reactor. They found that some configu- rations considerably reduce the mixing time, in top injection case, while for others the mixing time is longer than that in a single liq- uid phase. Therefore, both mixing and aeration had a very important influence on flow pattern and mixing time. 0255-2701/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2009.01.009