Microreactors conventional reactors, differences

The influence of residence time distribution (RTD) on performance, selectivity and yield is the same in microreactors as in conventional reactors. Therefore, the eflfects are well understood. Nonetheless, the demonstration of this at the microscale has hardly been reported so far. However, some experimental techniques have been developed to measure RTDs in microchannel flows which allow comparison between different types of flows or flows run at different parameters so that at least optimal flow conditions with regard to RTD can be found. 

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. 

What are the reasons that microreactors in many cases produce better results than conventional reactors In order to provide an optimal progress of a chemical reaction, different conditions must be fulfilled in the reactor First, a nearly ideal mixing of the reactants should be ensured, linked with the generation of an extended phase interface in multiphase reactions. Afterward, the required response time must be guaranteed by a residence time with preferentially narrow residence time distribution. Finally, the reactor heat necessary for the reaction must be supplied or carried off. In this connection, control of temperature, pressure, time of reaction, and flow velocity in reactors with small volume is carried out much... 

Table 12.1 Differences between processes in microreactors and conventional reactors.

There are some difFerences between two-phase flows in microreactors and conventional reactors. The importance of surface over volume forces increases. Reynolds number is usually small and laminar flow is established, where viscous forces dominate over inertial ones. The effects of wall roughness, wettability, and flow confinement become important. 

The mass transfer efficiency of the falling-film microreactor and the microbubble column was compared quantitatively according to the literature reports on conventional packed columns (see Table 4.3) [318]. The process conditions were chosen as similar as possible for the different devices. The conversion of the packed columns was 87-93% the microdevices had conversions of 45-100%. Furthermore, the space-time yield was compared. Flere, the microdevices resulted in larger values by orders of magnitude. The best results for falling-film microreactors and the microbubble columns were 84 and 816 mol/(m3 s), respectively, and are higher than conventional packed-bed reactors by about 0.8 mol/(m3 s). 

The hydrogenation of p-nitrotoluene in the presence of a supported Pd catalyst was carried out in a microreactor with stacked plates with complete selectivity [283,320]. The Pd catalyst was prepared in three different ways. Conversions were 58-98% for an impregnated aluminum oxide wash-coat catalyst, depending on the process conditions. The conversions for an electrodeposited catalyst and an impregnated catalyst on electrooxidized nanoporous substrate were 58 and 89%, respectively. The best latter result is similar to that of a conventional fixed-bed reactor (85%), while the maximum yield of 30% in a microreactor was superior because of the high selectivity. 

In discussing microreaction systems, it is helpful to first distinguish the characteristic size of a microreactor in comparison with conventional scale reactors. Two different definitions of the term microreactor are commonly used in the literature. The first defines any reactor that is an order of magnitude or more smaller than its conventional scale coimterpart as a microreactor. For example, an industrial reforming reactor might be 10,000-100,000 L in volume. Using this first definition, a 1 L reforming reactor could be considered a microreactor. This definition is not very useful as it... 

Schwarz et al. [85] studied the efficiency of different microstructured mixers followed by microchannels and their influence on the space time for obtaining high product yields. With increasing mass transfer performance of the micromixer and decreasing channel diameter of the microchannel reactors, shorter reaction times of several minutes at lower reaction temperatures compared to conventional batch reactor were obtained. Similar observations are reported for the synthesis of biodiesel in capillary microreactors [86] and in zigzag microchannels [87]. 

In Pfeifer et al. (2011) this comparison was extended for the packed bed microreactor approach, showing that, with different catalyst systems for the oxidation reaction of SO2 to SC, both approaches can be competitive with conventional tubular reactors (Table 2). 

A common feature of monolithic reactors and microreactors are therefore the gas flow in small channels which creates different reactor properties when compared to conventional fixed-bed... 

It can be foreseen that microstructured FBMRs will have different design paradigms compared to conventional fluidized bed (membrane) reactors. It is for this reason that we also highfight several design aspects of these microreactors in this chapter. 

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