Mini-packed bed reactor

Figure 3.20 Photograph of the mini packed-bed reactor (a) mini packed-bed reactor packed with 50 pm glass beads (b) detailed SEM image (c) [80],...

Figure 3.21 Schematic of the fabrication sequence for the mini packed-bed reactor [80],...

Reactor type Mini packed bed-reactor Catalyst-loading channels width 400 pm... 

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. 

In one mini-packed-bed reactor, standard porous catalyst particles were inserted in a mini-flow-through chamber (see Figure 4.45) [274,277,278]. Filling of the catalyst... 

Figure 4.45 A mini-packed-bed reactor (by courtesy of the American Chemical Society) [277],...

Nunes MAP, Rosa ME, Fernandes PCB, Ribeiro MHL. Operational stability of naringinase PVA lens-shaped microparticles in batch stirred reactors and mini packed bed reactors-one step closer to industry. Bioresource Technology 2014 164 362-370. 

Figure9.22 A mini-packed bed reactor. Source By courtesy ofthe American Chemical Society[68].

Figure 9.23 Image of a numbered up 10-channel mini-packed bed reactor microreactor. Source By courtesy of IEEE) [69].

Speed-up of mixing is known not only for mixing of miscible liquids, but also for multi-phase systems the mass-transfer efficiency can be improved. As an example, for a gas/liquid micro reactor, a mini packed-bed, values of the mass-transfer coefficient K a were determined to be 5-15 s [2]. This is two orders of magnitude larger than for typical conventional reactors having K a of 0.01-0.08 s . Using the same reactor filled with 50 pm catalyst particles for gas/Hquid/solid reactions, a 100-fold increase in the surface-to-volume ratio compared with the dimensions of laboratory trickle-bed catalyst particles (4-8 mm) is foimd. 

The central element of this reactor is an elongated channel in which small catalyst particles can be filled to give a mini-packed bed (Figure 3.20) [79, 80]. Gas streams enter this reaction zone as a mixture via an interleaved channel section, which also prevents the small particles penetrating the gas-feed channels. A similar type of microstmctured frit is placed at the end of the packed bed for the same function. 

Mini-columns for analyte separation/concentration can also behave as reactors, resembling the packed bed reactor. In this context, organic polymer monoliths, largely used in the medical and biological fields [73], should be highlighted. Monolithic mini-columns consist of continuous beds with macropores and mesopores which are characterised by low back-pressure effects. These columns offer several other advantages [74], as emphasised in Chapter 8. In the context of flow analysis, monolithic mini-columns were implemented in a sequential injection analyser in 2003 [75] and the potential and limitations of the approach, called Sequential Injection Chromatography, were recently reviewed [76]. 

Especially the favorable mass transfer of micro reactors is seen to be advantageous for the oxidation of benzyl alcohol [58]. As one key to this property, the setting and knowledge on flow patterns are mentioned. Owing to the special type of microreactor used, mixing in a mini trickle bed (gas/liquid flows over a packed particle bed) and creation of large specific interfaces are special aspects of the reactor concept. In addition, temperature can be controlled easily and heat transfer is large, as the whole micro-reactor construction acts as a heat sink. 

Column packing is very important during construction of the reactor. When the mini-column is not properly packed and/or the particles used are too heterogeneous, preferential pathways are established, through which most of the sample flows without exploiting the benefits of the packed bed. 

Tubular microreactors , which are in reality mini- or millimeter-scale reactors, have been used to study these reactions. Tubular microreactors take the form of a fixed-bed reactor with various materials of construction. Chao et al. used a quartz tubular microreactor (Sqm i.d.) to compare oxidative dehydrogenation of ethane over vanadium- and magnesium-based catalysts of different preparations [15]. The catalysts were introduced in the middle of the reactor tube with quartz granules used to fill the space above the catalysts and quartz wool used to retain the packing. This quartz microreactor was placed in a tubular furnace with the catalyst bed held in a constant-temperature zone. After performing experiments to study selectivity and conversion on the catalysts, surface analysis of the catalyst materials was used to identify the best catalyst preparation method. 

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