Microstructured packed beds

Liquid was delivered into the packed bed with a needle. The gas feed line was connected to the feed section with a T-junction therefore, the gas flowed through the annular area between the liquid feed line and the outer pipe. When the gas and liquid flows were stopped, the liquid entirely remained in the micro-structured bed. Thus, the bed has zero dynamic holdup and only static holdup. The static holdup (gj.) is expressed as the fraction of the space between the particles that is, on average, filled with liquid  

Different flow regimes in microstructured packed bed reactor were investigated in details by van Herk et al. [19]. These authors observed pulsations and the formation of segmented flow regime at high gas flow velocities. The segmented gas-liquid flow was also reported by Tadepalli et al. [25] in a MPBR with a Pd/zeolite catalyst with particles of 45-150 pm and a length of catalyst bed of 6-8 cm. 

The interaction between gas and liquid is limited at low Re numbers. Neither holdup nor dispersion is strongly affected by the gas flow rate, and molecular diffusion coefficient has a rather small effect on the particle Peclet number [20]. 

As per the guidelines available in the literature to overcome the wall effects in microstructured packed bed reactors, the maximum catalyst particle diameter that can be used in a reactor with an internal diameter of 10 mm is 0.4 mm [26, 27]. These reactors can be considered as microstructured packed bed reactors, as the flow of gas and liquid is not affected by gravity [28]. 

In the analysis of trickle beds, draining experiments are used to distinguish dynamic holdup from static holdup, where the static 

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High gas flow velocities in microreactors lead to pulsations and the formation of segmented flow. The various flow patterns observable in microstructured packed beds were investigated in detail by van Herk et al. [123]. The results confirm that segregated flow patterns form at high gas fractions. 

Recently, Tadepalli et al. [124] investigated the catalytic hydrogenation of o-nitroanisole in a microstructured packed-bed reactor. The reactor had an inner diameter of 0.775 mm and was filled with a Pd/zeolite catalyst with a particle diameter in the range of 45-75 and 75-150 pm. The length of the catalyst bed could be varied between 60 and 80 mm. The authors observed segmented gas-liquid flow. Further details of the hydrodynamics were not provided. 

Example 6.2 Comparison of pressure drop in microstructured packed bed reactor, microchannel reactor and foam reactor... 

Microstructured packed beds have been used for gas-hquid-sohd reactions. An advantage of microstructured packed beds for heterogenous catalytic processes stems from the fact that active and selective catalysts are commercially... 

The mixing results of the bimodal intersecting mixer show that microstructured analogs of packed-bed structures can mix efficiently. Small micro channels resemble... 

A major problem in using microstructured reactors for heterogeneously catalyzed gas-phase reactions is how to introduce the catalytic active phase. The possibilities are to (i) introduce the solid catalyst in the form of a micro-sized packed bed, (ii) use a catalytic wall reactor or (iii) to use novel designs. Kiwi-Minsker and Renken [160] have discussed in detail these alternatives. 

Although MSRs have been shown to be suitable for the optimization of many synthetic procedures, they have not yet received enough attention for catalytic chemistry. The main reason for the reluctance to apply them is the difficulty of introducing a solid catalyst into the microchannels of the reactor. Micropacked-bed reactors are easy to fabricate, but they usually produce a high pressure drop during the flow of gases. To overcome this problem, microstructured packings such as foams or fibrous supports may be used instead. 

As demonstrated in Figure 11.12, the trade-off index of microstructured channels is roughly five times higher compared to that of packed beds. This means that the energy dissipation for the same mass transfer performance in a packed bed is five times higher than in a microchannel. 

Example 6.1 Pressure drop in packed bed microstructured reactors... 

Figure 6.8 Influence of the bed porosity on pressure drop in microstructured packed...

The comparion of pressure drop in three different types of microstructured reactors, foam reactor, square channels and packed bed, is shown in Example 6.2. 

For three types of microstructured devices - the multichannel catalytic wall microreactor, the micro packed bed, and the catalytic metallic foam - the mass transfer effectiveness was calculated with the relations for mass transfer and pressure loss given in the previous sections. For the metallic foam, characteristic data were taken from [46] and [48]. The effectiveness is not dependent on the size or length of the device. 

The new reactor types that have been proposed are (i) structured-type monoliths and packed beds with structured packings (de Deugd et al. 2003 Pangarkar et al. 2008) and (ii) microstructured devices with catalyst coating (Deshmnkh et al. 2010 Glasser et al. 2012). 

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. 

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. 

Comparison of a single-tube packed-bed reactor with a traditional batch reactor was also published in the case of o-nitroanisole hydrogenation, not for productivity purposes but rather as laboratory tools for kinetic studies (Scheme 9.11) [46]. It was shown that the better efficiency of mass transfer enables the microreactor to obtain intrinsic kinetic data for fast reactions with characteristic times in the range 1-100 s, under isothermal conditions, which is difficult to achieve with a stirred tank reactor. However, the batch reactor used in this study was not very well designed since a maximum mass transfer coefficient (kia) of only 0.06 s was measured at 800 rpm, whereas kia values of up to 2 s are easily achieved in small stirred tank reactors equipped with baffles and mechanically driven impellers [25]. This questions the reference used when comparing microstructured components with traditional equipment, with the conclusion that comparison holds only when the hest traditional technology is used. 

Find et al. [25] developed a nickel-based catalyst for methane steam reforming. As material for the microstructured plates, AluchromY steel, which is an FeCrAl alloy, was applied. This alloy forms a thin layer of alumina on its surface, which is less than 1 tm thick. This layer was used as an adhesion interface for the catalyst, a method which is also used in automotive exhaust systems based on metallic monoliths. Its formation was achieved by thermal treatment of microstructured plates for 4h at 1000 °C. The catalyst itself was based on a nickel spinel (NiAl204), which stabUizes the catalyst structure. The sol-gel technique was then used to coat the plates with the catalyst slurry. Good catalyst adhesion was proven by mechanical stress and thermal shock tests. Catalyst testing was performed in packed beds at a S/C ratio of 3 and reaction temperatures between 527 and 750 °C. The feed was composed of 12.5 vol.% methane and 37.5 vol.% steam balance argon. At a reaction temperature of 700°C and 32 h space velocity, conversion dose to the thermodynamic equilibrium could be achieved. During 96 h of operation the catalyst showed no detectable deactivation, which was not the case for a commercial nickel catalyst serving as a base for comparison. 

Wall-coated microchannel reactor [48, 51] A reactor which incorporates either a conventional packed bed or a stack of microstructured wafers with Pd catalysts is used... 

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