Continuous flow mode catalysis

As indicated in the introduction of Section 2.2.2, continuous flow reactors (generally fixed bed reactors) are not frequently used in the investigation of the liquid phase zeolite catalysed synthesis of Fine Chemicals. However, fixed bed reactors present some significant advantages on batch reactors  

The size of the catalyst particles will be chosen so as to respect these conditions, but also to avoid a too high pressure drop. For this latter reason, zeolite powder cannot be used. Generally, the powder is pelletized, the resulting pellets being crushed and sieved to yield the desired particle size. 

Additional information on the plug flow fixed bed reactors and on the heat and mass balance equations can be found in the Handbook of Heterogeneous Catalysis[15] and in the classical books devoted to chemical engineering kinetics.113,141 

External mass transfer limitations, which cause a decrease in both the reaction rate and selectivity, have to be avoided. As in the batch reactor, there is a simple experimental test in order to verify the absence of these transport limitations in isothermal operations. The mass transfer coefficient increases with the fluid velocity in the catalyst bed. Therefore, when the flow rate and amount of catalyst are simultaneously changed while keeping their ratio constant (which is proportional to the contact time), identical conversion values should be found for flow rate high enough to avoid external mass transfer limitations.[15] 

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For examples of organocatalyzed reactions carried out under the continuous-flow mode see (a) Ishiara, K., Hasegawa, A. and Yamamoto, H. (2002) Synlett, 1296-8. (b) Briinjes, M., Sourkouni-Argirusi, G. and Kirschning, A. (2003) Advanced Synthesis Catalysis, 345, 635—42. 

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