R ecent studies here shown that the addition of solid particles can have a profound effect on the characteristics of liquid/liquid dispersions (Nienow et.al., 2000; Nienow et.al., 2003). A particularly important parameter is the wettability of these particles which has been shown to strongly affect the rate of drop coalescence. On the other hand, though for gas-liquid systems the impact of the addition of particulate solids has been quite extensively studied (Nienow and Bujalski, 2002), there has been little work done related to solids wettability. This paper addresses this question for the first time in relation to stirred vessels. The new issues particularly addressed are rates of gas-liquid mass transfer in the presence of solids of different wettability and the impact of wettability on the ability to ingest or draw-down floating solids. Literature Mass TransferMany studies have been carried out on gas-liquid mass transfer, especially of oxygen to or from water and have recently been well summarised elsewhere (Nienow, 2003). Van't Riet ( 1979) was the first to show that k L a data could be related to mean specific energy dissipation rate, (e T ) g (W/kg) and superficial gas velocity, v s (m/s), by an empirical equation of the form:where a and b are about 0.5. He also concluded that, within ±25%, the literature data showed that k L a was independent of the type of impeller used. In general, moving from water to solutions of electrolytes or alcohols significantly enhances k L a whilst the addition of antifoam greatly reduces it (Martin et.al., 1994). In other words, A in Equation 1 is very sensitive to the chemical composition of the solution. Such changes in composition also affect a and b but to a much lesser extent but A, a and b are still independent of impeller type (see for example, Two-and three-phase systems have been studied using particles that are water-wetted and denser than water and with similar sized, small floating particles, both wetted and nonwetted. The parameters investigated have been the agitator speed required to disperse the different types of particles under sparged and non-sparged conditions and rates of gas-liquid mass transfer. With the wetted, dense particles, only at concentrations of > ~20% w/w did the k L a fall significantly (by ~20 to 30%) whilst solid suspension followed the trends of earlier work. With the less dense particles, even with < ~1% w/w that were not wetted, it was very difficult to disperse them throughout the liquid phase, though up-pumping impellers were more effective than radial. In addition, bubble size significantly increased and k L a fell by a factor of 2 to 3. With the wetted, less dense ones up to 2% w/w, uniform dispersion was achieved even at much lower speeds and k L a did not change compared to the case without solids.On a étudié des systèmes bi et triphasiques à l'aide de particules mouillées par l'eau et plus denses que l'eau et de petites particules flottantes de calibre semblable, mouillées et non mouillées. Les paramètres étudiés sont la vitesse ...

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Applications of three-phase bubble columns are increasing in chemical and biotechnical processes. Frequently, these reactors are operated at high solid and gas holdups. Electrodiffusion measurement (EDM) results are presented for local liquid flow velocities under such practically relevant conditions and compared to CFD modeling results. A multifluid model using the k-HH turbulence model and special terms for the direct momentum exchange between gas and solid phase has been implemented using the CFX-4.3 solver. Parabolic radial profiles of axial liquid velocity are obtained with measurements as well as with computations; the influence of increasing superficial gas velocity on liquid circulation is qualitatively well captured by the modelling calculations. 508* Gas Hold-up in Three-phase Multiimpeller Stirred Reactor M a c h o n , V . J a h o d a , M . V e v e r k a , P . M a j i r o v a , HThe gas hold-up has been measured in the tank stirred with two pitch blade turbines downward pumping effect. The used gas was air and liquid charge were suspensions of glass beads of mean diameter 0.14 mm and/or polyvinyl particles 0.94 mm. The concentration was up to 30 % w/w (glass beads) and up to 10 % w/w (polyvinyl particles). The liquid was either water (coalescing system), 0.5 M solution sodium sulphate and 0.1 M ethanol solution (non-coalescing systems). In agreement with literature the higher gas hold-up has been found in non-coalescing systems. The presence of solid particles had ambiguous effect on hold-up: in water was not observed the influence of solids, in sulphate and ethanol solutions the gas hold-up decreased with the concentration of solid particles. The data has been correlated according the equation in which stirred speed and gas flow rates occur. 509* Local Effects of Hydrodynamics andMass Transfer in Multiphase Flows M . S c h l u È t e r S . J o h n S . S c h e i d H . P a r c h m a n n O . B o r k P r o f . D r . N . R a È b i g e r