Multiphase reactors, flow regimes

Flow Regimes in Multiphase Reactors. Reactant contacting, product separations, rates of mass and heat transport, and ultimately reaction conversion and product yields are strong functions of the gas and Hquid flow patterns within the reactors. The nomenclature of commonly observed flow patterns or flow regimes reflects observed flow characteristics, ie, armular, bubbly, plug, slug, spray, stratified, and wavy. 

This review paper is restricted to stirred vessels operated in the turbulent-flow regime and exploited for various physical operations and chemical processes. The developments in the field of computational simulations of stirred vessels, however, are not separated from similar developments in the fields of, e.g., turbulent combustion, flames, jets and sprays, tubular reactors, and multiphase reactors and separators. Fortunately, there is a strong degree of synergy and mutual cross-fertilization between these various fields. This review paper focuses on aspects specific to stirred vessels (such as the revolving impeller, the resulting strong spatial variations in turbulence properties, and the macroinstabilities) and on the processes carried out in them. 

As is evident from inspection of Table III turbulence modeling of multiphase flow systems requires major attention in the near future. Also the development of closure laws for phenomena taking place in the vicinity of interfaces such as coalescence, breakup, and accumulation of impurities should be considered in more detail. Once these requirements have been met, in principle, it would be possible to predict a.o. flow regime transition and the spatial distribution of the phases with confidence, which is of utmost importance to the chemical engineer dealing with the design of (novel) multiphase reactors. 

Apart from the flow regimes, several other issues control the performance of these multiphase reactors. For example, in a gas-liquid reactor, the rate of mass... 

Based on the considerations presented above, it was found necessary to carry out a critical evaluation of the various model formulations available aiming to achieve a proper description of the multiphase flow regimes occurring in chemical reactors. 

Whereas in a fixed bed reactor with a single fluid phase there exist only two modes of operation, either downflow (which is used in most cases) or upflow, and only two different flow regimes, either laminar or turbulent flow, which can be observed and characterized by a Reynolds number as the single relevant dimensionless group, the fluiddynamics in multiphase catalytic fixed bed reactors are much more complex. 

With respect to the above-given review, it certainly can be imagined that in multiphase catalytic reactions heat and mass transfer as well as phase mixing and stagnant zones have an influence on the chemical conversion in a very different way, depending on the operating variables of the reactor. But more than that, this behavior has consequences on scale-up procedures for these types of reactors, as the most appropriate scale-up procedure depends on mode of operation, flow regimes and the relative importance of mass transfer resistances compared to the chemical... 

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