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The study relates to the mass transfer and the bubble size in a non standard vessel equipped with various dual-impeller combinations. The effects of the rotational speed, gas flow rate, impeller type and diameter are investigated. The volumetric mass transfer coefficient kLa and the bubble size dbs were studied. The liquid side mass transfer coefficient kL and the volumetric interfacial area a were estimated separately. A comparison has been made with some existing correlations. ass transfer from the gas phase to the liquid phase is M the most important goal in gas-liquid mixing. The volumetric mass transfer coefficient is a key parameter in the characterization and design of industrial agitated gas-liquid reactors which are often equipped with two or more impellers. These reactors, currently used in many chemical and biological processes, are traditionally equipped with radial turbines. However, minimal data has been obtained with multi-impeller axial configurations [Schliiter and Deckwer (1993), Chiampo and Conti (1994), Pinelli et al. (1994) and Bouaifi et al. ( 1995)]. Some of axial impellers are able to disperse gas in liquids and yield to good mass transfer performances and high hydraulic efficiency while reducing energy consumption when they are compared with radial turbines [Charles (19781, Oldshue et al. (1988), Bakker (1991) and McFarlane (1991), Roustan et al. (1994) and Bouaifi (1997)l. Myers et al. (1994 studied various mixed and radial four impeller configurations. They found that gas hold-up obtained with mixed configurations is lower than that of radial ones when compared at the same power input and gas flow rate. However, the mixed configurations exhibit similar or improved gas-liquid mass transfer at high gas flow rates and power Input.We notice also that most of the investigations done in single impeller systems are limited to measuring the volumetric mass transfer coefficient kLa. Unfortunately, this parameter is global and not sufficient to understand the mass transfer mechanisms. However, very little data are found concerning mass transfer in multi-impeller gas-liquid reactors (Roustan, 1978;Myers et al., 1994 and. The separation of the parameters kL and a should be considered for a better comprehension of the gas-liquid mass transfer mechanisms. It allows also to identify the parameter (kL or a ) which limits the mass transfer. Few results concerning the separation of k, and a have been obtained (Deckwer et al., 1978 andKawase andMoo-Young, 1990)."Author to whom correspondence should be addressed. E-mail address: roustan@insatlse.insa-tlse.fr)In a previous work, Bouaifi et al. (1997) have studied the hydrodynamic regimes, the power consumption, the gas hold-up and the mixing time in various axial and mixed dual-impeller systems. The effects of the rotational speed, gas flow rate, impeller type and diameter were investigated. The objectives of this work are to complete the previous study. The bubble size and distribution, the volumetric mass transfer coefficient and the separation of k, a...

Dissipative particle dynamics (DPD) is a widely used coarse-grained technique for the simulation of complex fluids. Although the method is capable of describing the hydrodynamics of any fluid, the common choice of DPD parameters, such as friction coefficient γ, dissipative cutoff radius r D c , coarse-graining factor N m and weighting function exponent s, unrealistically leads to the simulation of liquid water with a low Schmidt number Sc at standard pressure and temperature. In this work we explored the influence of these parameters, finding the set of parameters needed to properly simulate liquid water. Particular attention was devoted to the numerical techniques to calculate the transport properties from equilibrium simulations, especially in the calculation of the viscosity, comparing the most commonly adopted techniques and formulating a recipe that can be used for further investigations.

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