Microreactors mass transport

Isobaric applications in the continuum regime, making use of molecular bulk diffusion and/or some viscous flow are found in catalytic membrane reactors. The membrane is used here as an intermediating wall or as a system of microreactors [29,46]. For this reason some attention will be paid to the general description of mass transport, which will also be used in Sections 9.4 and 9.5. 

A decrease in the characteristic dimension of the system (see schematic of parallel plate microreactor in Figure 10.4c) increases the rate of mass transport from the bulk gas to the reactor walls and changes Da. When Da <0.1, surface reaction is limiting and when Da > 10, mass transfer is limiting. The pseudo-first-order reaction rate constant is estimated from k, = a S/C, where o is the rate of fuel consumption (coming from a detailed model), a = 2/dis the catalyst area per unit volume and C is the concentration of the fuel. 

Due to the important ratio between the reactor length and the inter-electrode distance, plate and channel microreactors behave like plug flow reactors. The maximum possible conversion is reached if the reaction is under mass transport control on the entire electrode surface. The combination of the local diffusion-limited... 

When handling strong exothermic processes or hazardous substances, safety issues also became a major driver for the use of microreactors. Finally, several academic studies can be found in the literature focusing on the analysis of mass transport and flow characteristics within microfluidic charmels by using electrophilic aromatic substitutions as model reactions. 

Therefore, one of the major drivers for running Friedel-Crafts alkylations in microstructured reactors is to improve the selectivity of monoalkylation products under reasonable stoichiometric conditions, in particular by achieving significantly accelerated and intensified mixing and mass transport than achievable in macroscopic processes. Moreover, it is also expected that the exothermic alkylation reactions additionally benefit from the improved heat transfer characteristics of microreactors. 

For example, the nitration of benzene and other aromatic compounds is often strongly limited by the mass transfer performance within the reactor that is used. In particular in the case of biphasic nitration reactions, a good mass transfer performance is essential to suppress the formation of unwanted by-products such as higher nitrated compounds (e.g. dinitro and trinitro compounds) or oxidation products. Therefore, the use of microreactors offers a good possibility to overcome common restrictions in mass transport and thus achieve higher yields and selectivities in nitration reactions. 

In the past decade, a wide variety of aromatic substitution reactions has been intensively investigated by applying microreaction technology in conjunction with appropiate process settings to identify routes towards optimized process performance. Enhanced heat and mass transport characteristics achievable in microstruc-tured reactors have been dehberately used to obtain higer product yields, selectivities and purities. Moreover, microreactors have been suceessfuly used to identity synthesis routes towards new products and process conditions which are not attainable in macroscopic bacth processes. 

Due to short diffusion pathways in the microsystem, the overall mass transport in the phases or the transfer via phase boundaries is often magnitudes higher than in conventional reactor systems. However, with regard to the desired high loadings with catalyst and low cost for fluid compression or pumping, the mass transfer to the catalyst and the mass transport within porous catalyst still has to be effective. As for the heat transport the differentiation between packed bed and wall-coated microreactor is necessary for mass transport considerations. The mass transport in packed bed microreactors is not significantly different to normal tubular packed bed reactors, so that equations like the Mears criteria (Eq. 6) can be used. 

The validation of negligible heat transport limitation is only possible via control of observable temperature gradients, e.g. in the cooling fluid or across the reactor walls. A sensor influence is predictable on the temperature measurement due to the small inner structures of the microreactor. In terms of mass transport resistances, some experimental approaches can be used to validate their absence. 

In addition to a better heat exchange, microreactors also intensify mixing and mass transport, which is particularly important in multiphase systems (gas-liquid or liquid-liquid). 

Otherwise so-called microeffects have been intensively discussed, which should cause unexpected potentials of microreactors [22]. Meanwhile, it is known that microeffects are scaling down effects that are relevant or dominant on the microscale (from 100 pm to 1mm). These effects are held responsible for (i) intensified mass transport toward the smaller dimensions, (ii) intensified heat transport toward the smaller dimensions, and (iii) intensified surface phenomena by higher surface area-to-volume ratios as a result of the smaller dimensions. 

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