Packed-bed Microreactors

Section IV.A, packed bed microreactor light-off data are the main source of information for generating kinetics. All these measurements are done with C02 and water in the feed, as this is always present in vehicle exhaust. These components have an inhibiting effect, but once the concentration is above a certain amount, a further increase in concentration has no significant further inhibiting effect, so the concentration dependence of C02 and water can be neglected, provided they are present at a representative level in all experiments. 

Scheme 35 An example of enantioselective synthesis performed in a packed-bed microreactor.

Mini-Packed-Bed Reactors Packed-bed microreactors have a larger flow-through channel that contains particles brought into contact [274,277,278]. The flow of the gas-liquid mixture goes through the interstices, and in this way a dispersive action is given, that is, continuously renewing the interfaces. 

Figure 4.46 Image of a numbered-up 10-channel mini-packed-bed microreactor (by courtesy of IEEE) [278].

The hydrogenation of a-methylstyrene was investigated to demonstrate the performance of a packed-bed microreactor with a palladium catalyst supported on activated carbon [324]. The microreactor was operated at 50 °C, and conversions from 20 to 100% were measured. It was determined that the reaction is first order for hydrogen and zero order for a-methylstyrene. Initial reaction rates were close to 0.01 mol/min per reaction channel and were achieved without additional activation of the catalyst. This is in agreement with the literature data on intrinsic kinetics. 

Srinivas, S., Dhingra, A., Im, H., and Gulari, E. A scalable silicon microreactor for preferential CO oxidation Performance comparison with a tubular packed-bed microreactor. Applied Catalysis. 

Fig. 14 A packed-bed microreactor used for phosgene production by Jensen et al. (A) top view (B) interleaved inlets (C) catalyst filter. (From Ref.t f)...

Microreactors containing a solid-supported organic catalyst have also been developed. For example, solid-supported l,5,7-triazabicyclo-[4.4.0] undec-3-ene is introduced to a microtube reactor (Figure 7.26).This packed-bed microreactor has been used for Knoevenagel condensation... 

Tonkovich and coworkers [42—47] used packed bed microreactors for the production of hydrogen. The authors constructed a reactor consisting of stacked stainless steel plates for the partial oxidation of methane [42]. The microchannels (which were 254 pm wide, 1500 pm deep, and 35 mm long) were filled with mesoporous silica that was impregnated with rhodium. The reactor plates were sandwiched between two integrated heat exchanger plates. 

Infrared imaging was utilized in several studies of spatial effects in exothermic catalytic reactions over model catalysts, such as isolated particles, wafers, plates, discs [2]. Our approach has been to characterize the catalysts directly in a packed-bed microreactor, under realistic reaction conditions. In-situ measurements by infrared thermography of the adsorption properties of catalytic materials have been previously reported [6]. In the present study, the catalytic oxidation of compounds having different chemical properties was investigated by the same technique, with the aim of obtaining comparative data useful to better understand the factors governing the complex phenomena associated with catalytic combustion. 

Bogdan, A.R., Mason, B.P., Sylvester, K.T. and McQuade, D.T. (2007) Improving solid-supported catalyst productivity by using simplified packed-bed microreactors. Angewandte Chemie - International Edition, 46, 1698-1701. 

Example 6.6 Performance comparison of packed bed microreactor and microchannel reactor... 

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. 

A packed-bed microreactor system built from borosilicate glass consisting of an evaporator, aheater, reactor bed and a gas collector was reported by Giorecki et al. [35]. Methanol fuel was used and reformed with a commercial Cu/ZnO catalyst provided by Siid-Chemie. 

Packed-bed microreactors are prepared by filling catalyst powder into the microchannels of the reactor. Since this is the easiest and fastest way for the incorporation of the catalyst, this type of microreactor is frequently used for catalyst screening [82]. Another advantage over other types of catalytic beds is the possibility of using... 

FFMR, falling film microreactor PBMR, packed-bed microreactor MM, micromixer MBC, microbubble column MMR, microstructured mesh reactor SCR, single-channel reactor... 

The highest pressure drops usually occur in packed-bed microreactors with axial flow design. A reduced pressure drop while maintaining catalytic area can be achieved by a cross-flow design packed-bed microreactor [82]. To keep pressure drops down, the use of catalytic wall and catalytic bed microreactors is recommended. 

This criterion is important to consider with respect to the kinetics of the desired reaction. Slow reactions require longer residence times in the reactor and not every reactor provides the same contact time of the gaseous educts with the catalyst Packed-bed microreactors offer the longest residence times, since the gaseous molecules have to pass through a region of closely packed catalyst particles. Unfortunately, this also negatively effects the residence time distribution. 

Several methods for the incorporation of catalysts into microreactors exist, which differ in the phase-contacting principle. The easiest way is to fill in the catalyst and create a packed-bed microreactor. If catalytic bed or catalytic wall microreactors are used, several techniques for catalyst deposition are possible. These techniques are divided into the following parts. For catalysts based on oxide supports, pretreatment of the substrate by anodic or thermal oxidation [93, 94] and chemical treatment is necessary. Subsequently, coating methods based on a Uquid phase such as a suspension, sol-gel [95], hybrid techniques between suspension and sol-gel [96], impregnation and electrochemical deposition methods can be used for catalyst deposition [97], in addition to chemical or physical vapor deposition [98] and flame spray deposition techniques [99]. A further method is the synthesis of zeoUtes on microstructures [100, 101]. Catalysts based on a carbon support can be deposited either on ceramic or on metallic surfaces, whereas carbon supports on metals have been little investigated so far [102]. 

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). 

For a possible heat transfer limitation partide-to-fluid the equation from Mears (1971) can be applied to the packed bed microreactor (Eq. 1). The same is true for a possible heat transport limitation within the catalyst particle (Anderson (1963), Eq. 2). 

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. 

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