Multiphase microstructured reactors

The issues to be solved for direct fluorinations are heat release and mass transfer via the gas-liquid interface. Multiphase microstructured reactors enable process intensification [230,248-250,304—306]. Often geometrically well-defined interfaces are formed with large specific values, for example, up to 20 000 m2/m3 and even more. These areas can be easily accessible, as flow conditions are often highly periodic and transparent microreactors are available. For the nondispersing... 

Kashid, M.N., Renken, A., and Kiwi-Minsker, L. (2010) CFD modelling of liquid-liquid multiphase microstructured reactor slug flow generation. 

Of course, not all multiphase microstructured reactors are presented in Table 9.1. Either because they have attracted (too ) little interest, because they may have been qualified as microreactors in spite of their overall size but caimot be considered as microstmctured , or because they combine several contacting principles. Examples are a reactor developed by Jensen s group featuring a chaimel equipped with posts or pillars, thus resembling more a packed bed but with a wall-coated layer of catalyst [20], and a string catalytic reactor proposed by Kiwi-Minsker and Renken [21], that may applied to multiphase reactions. 

In a multiphase membrane reactor, the conversion of benzylpenicillin to 6-aminopenidllinic acid is performed. The type of microstructured reactor used is a fermentation reactor which contains the enzyme penicillin acylase immobilized on the wall of a hollow-fiber tube. The hollow-fiber tube extracts 6-aminopenicillinic acid at the same time selectively. Benzylpenicillin is converted at the outer wall of the hollow fiber into the desired product, which passes into the sweep stream inside the fiber where it can be purified, e.g. by ion exchange. The non-converted benzylpenicillin is recycled back through the reactor [84],... 

Multiphase microstructured devices can potentially be used to dimmish the limitations of conventional reactors. They generally take advantage of their large interfacial area reducing the mass transfer resistances. 

Various parameters must be considered when selecting a reactor for multiphase reactions, such as the number of phases involved, the differences in the physical properties of the participating phases, the post-reaction separation, the inherent reaction nature (stoichiometry of reactants, intrinsic reaction rate, isothermal/ adiabatic conditions, etc.), the residence time required and the mass and heat transfer characteristics of the reactor For a given reaction system, the first four aspects are usually controlled to only a limited extent, if at aH, while the remainder serve as design variables to optimize reactor performance. High rates of heat and mass transfer improve effective rates and selectivities and the elimination of transport resistances, in particular for the rapid catalytic reactions, enables the reaction to achieve its chemical potential in the optimal temperature and concentration window. Transport processes can be ameliorated by greater heat exchange or interfadal surface areas and short diffusion paths. These are easily attained in microstructured reactors. 

The chapter is organized in two parts. The first section is devoted to a short description of typical microstructured reactors used for multiphase additions. The other sections are then organized according to reaction types, i.e. addition across carbon-carbon and carbon-oxygen double bonds, and other additions. 

Table 9.1 Microstructured reactors mostly used for multiphase addition reactions. 

We first present general criteria for the rational use of MSRs on the basis of fundamentals of chemical reaction engineering [21-24], The main characteristics of MSRs are discussed, and the potential gain in reactor performance relative to that of conventional chemical reactors is quantified (Section 2). Subsequently, the most important designs of fluid-solid and multiphase reaction systems are described and evaluated (Sections 3 and 4). Because microstructured multichannel reactors with catalytically active walls are by far the most extensively investigated MSRs for heterogeneous catalytic reactions, we present their principal design and recent synthetic methods separately in Section 5. 

In recent years, the main focus of the patents has shifted from the development of novel microstructured device to the well-defined application of the device to proper reaction process. Homogeneous liquid reactions are occupying a large part of patents. The main improvement of these patents is to achieve the faster reaction in microreactor than batch reactor or to conduct the reaction with hazardous chemicals without human exposure. Microreactors also can be applied to multiphase and gas-phase reactions. Recently, the carbonylation reaction using carbon monoxide in microreactor has been reported [15]. The polymerization process can be also controlled with microreactors. The microreactor leads to the concise synthesis of polymers with a narrow molecular weight distribution. 

Therefore, high transformation rates can be obtained in fluid-fluid devices with high interfacial area. Microstructured multiphase reactors are characterized by interfacial areas, which are at least 1 order of magnitude higher compared to conventional contactors, and, therefore, suited particularly for very fast reactions. 

Regular flow patterns are provided by the segmented flow in a single capillary or in multi-channel microreactors. Miniaturized packed-bed microreactors follow the paths of classical engineering by enabling tridde-bed or packed bubble column operation. M ost of the microstructured multiphase reactors are at the research stage. Due to the small reaction volumes th will find their appHcation mainly in small-scale production in the fine chemical and pharmaceutical industries. 

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