Falling film microreactor

The housing consists of a capped stainless steel tube [266]. For high-pressure experiments, a stainless steel housing can be used up to 50 bar at 20 °C. Additionally, the stainless steel tube can be replaced by a transparent PMM A housing. This allows a visual inspection of the microchannels in the case of low-pressure experiments (up to 5 bar). 

Continuous Contactor with Partly Overlapping Channels Solute transfer can occur between immiscible phases each flowing in separate adjacent but displaced microchannels, having only a small conduit in which the fluid interface is stable (partial overlap) [267,268]. 

Mesh Microcontactor A mesh microcontactor contains a microstructured plate with regular circular openings through which separate gas and liquid streams come into contact [270,271]. Stability of the interface and prevention of breakthrough are achieved by adjusting the pressure. Gas-liquid operation requires a low gas flow 

A coolant channel is guided through the metal block in a serpentine fashion so that reactant and coolant flows are orthogonal [272]. A thermocouple measures the temperature at the product outlet. This microreactor was developed for the (very fast) fluorination reactions with elemental fluorine. Therefore, the surface of the microchannel was inactivated by exposure to the increasing concentration of fluorine in nitrogen. 

After the initial manufacture of a single-microchannel version [272], a numbered-up (scale-out) three-microchannel version followed later [273], 

Initially, taper cone-shaped devices were made from aluminum by computerized numerically controlled (CNC) turning as manufacturing technique using a lathe [47]. 

These devices are easily amenable to inspection of the complete fluidic path. The empty channel surface and the liquid surface of the Ailed channel were accessible by white light interferometry. Using n-butanol as liquid flowing in microchannels of 100 pm X 300 pm cross-section, a complete filling of the microchannels without any flooding or undesired wetting of liquid at the walls of the cylinder next to the channel was achieved. 

Principles of a falling film microreactor have been applied to other falling film microdevices where enhanced mass transfer is needed, such as a falling film microabsorber and a falling film micro evaporator [43,50-52]. 

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Degussa/Evonik (Germany) Falling film microreactors for ozonolysis and other chemical applications with IMM reactors [3] and DEMIS project collaboration [4, 5]... 

Jahnisch, K., Baerns, M., Hessel, V, Haverkamp, V, Lowe, H., Wille, C., Selective reactions in microreactors -fluorination of toluene using elemental fluorine in a falling film microreactor, in Proceedings of tlie 37tli ESF/EUCHEM Conference on Stereocliemistry (13-19 April 2002), BUrgenstoclc, Switzerland. 

Gas-Liquid Processing Scale-out Falling Film Microreactor. . . 225... 

Fig. 12. Cylindrical falling film microreactor for pilot operation at tenfold capacity increase as compared to the laboratory falling film microreactor...

Jahnisch et al. used an IMM falling-film microreactor for photochlorination of toluene-2,4-diisocyanate [38] (see also Chapter 4.4.3.3, page 161). As a result of efficient mass transfer and photon penetration, chlorine radicals were well distributed throughout the entire film volume, improving selectivity (side chain versus aromatic ring chlorination by radical versus electrophilic mechanism) and spacetime-based yields of l-chloromethyl-2,4-diisocyanatobenzene compared to those obtained using a conventional batch reactor. 

Figure 4.28 Principle to generate a falling film on a structured plate with channels (left). Falling-film microreactor, laboratory version up to 1 l/h (by courtesy of IMM).

Figure 4.29 Numbered-up, cylindrical , (left) and scaled-up, large , (right, laboratory version in front) falling-film microreactors (by courtesy of IMM).

Figure 4.30 Helical falling-film microreactor (right). Flow of the liquid in the helical microchannel, visualized by injection of a fluorescent tracer (by courtesy of Elsevier and IMM) [266].

Conversion rises with the increasing temperature, as expected [308]. For the falling-film microreactor, conversion is increased from 15 to 30% when heating up from —40 to —15 °C. The selectivity varies largely and exhibits no clear trend. 

With the use of falling-film microreactor or a microbubble column, yields of up to 28% were obtained with acetonitrile as solvent at conversions ranging from 7 to 76% and selectivities from 31 to 43% with regard to the monofluorinated product [308]. With the use of dual-channel reactor, conversions from 17 to 95% and selectivities from 37 to 10% were achieved using methanol as solvent [274]. The conversion of a laboratory bubble column, taken for comparison, ranged from 6 to 34% with selectivities of 17-50%, which is equivalent to yields of 2-8% [308],... 

Figure 4.47 Reduction of reaction times by several orders of magnitude using a falling-film microreactor (FFMR) or microbubble columns (MBC I and II, denoting different dimensions, as given in [308]) as compared to standard organic laboratory processing with a laboratory bubble column (LBC). x residence time (by courtesy of IMM).

The most striking point about the fluorination results is the high intrinsic speed of the reaction (see Figure 4.47). The falling-film microreactor was operated at seconds scale and the microbubble column even at microseconds scale [308]. This is in contrast to fluorinations in laboratory flasks taking hours. 

Accordingly, the respective space-time yields are higher by orders of magnitude [308]. The space-time yields for these microreactors ranged from about 20000 to 110 000 mol monofluorinated product/(m3 h). The falling-film microreactor had two times higher space-time yields than the microbubble column. The performance of the laboratory bubble column was in the order of 40-60 mol monofluorinated product/(m3h). 

The fluorine content in the gas phase of a falling-film microreactor varied between 10, 25 and 50% [308], A nearly linear increase in conversion results at constant selectivity. The substitution pattern rather than the ratio of ortho to para isomers is strongly affected. 

Figure 4.49 Microplant for toluene sulfonation with falling-film microreactor as central apparatus and unitized backbone as fluidic bus system (by courtesy of Elsevier) [315],...

The photooxygenation of cyclopentadiene with Rose Bengal as photosensitizer was performed using a falling-film microreactor in methanol [317]. The endoperoxide is first generated and then reduced to 2-cyclopenten-l,4-diol, which is used as an intermediate in pharmaceutical drug synthesis. This route is not easily possible by batch processing because the explosive endoperoxide intermediate is formed in substantial amounts. 

The mass transfer efficiency of the falling-film microreactor was determined at various carbon dioxide volume contents (0.1,1.0 and 2.0 M) [318]. The molar ratio of carbon dioxide to sodium hydroxide was constant at 0.4 for all experiments, that is, the liquid reactant was in light excess. The higher the base concentration, the higher was the conversion of carbon dioxide. For all concentrations, complete absorption was, however, achieved at different carbon dioxide contents in the gas mixture. The results show the interdependency of the carbon dioxide content, the gas flow velocity and the sodium hydroxide concentration. 

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). 

A more detailed mass transfer study on the carbon dioxide absorption in sodium hydroxide solution was performed using a falling-film microreactor [319]. Experimental investigations were made at a liquid flow of 50ml/h, with three NaOH concentrations (0.1,1 and 2 M), at a fixed inlet molar ratio C02 NaOH of 0.4, and for a range of C02 concentration of 0.8-100%. A two-dimensional reactor model was developed, and the results are similar to the experimental data at low NaOH concentrations (0.1 and 1 M). The agreement is less pronounced for higher concentrations such as 2 M NaOH, which could be explained by either maldistribution of... 

An even wider variation of preparation procedures for the palladium catalyst was investigated for the hydrogenation of nitrobenzene using a falling-film microreactor [321,323],... 

Discovery Technologies) with a circular arrangement of 12 screw-capped gas-tied glass tubes agitated by magnetic stirring. The second reactor is a commercial pressure batch reactor with a turbine and baffles (Series 4590 from Parr Instruments). The third reactor was the helical falling-film microreactor. 

Figure 4.57 Reaction performance comparison of three reactors with the most active catalysts Rh/Josiphos and Rh/Diop. Caroussel (car), helical falling-film microreactor (p) and Parr (batch) reactor (by courtesy of Elsevier) [266]. 9% conv. and 46% conv. denote a fixed conversion of 9 and 46%, respectively, which have to be achieved.

Enantioselectivity was roughly the same for the three reactors, being 80-90 and 62-65% for the Rh/Josiphos and Rh/Diop catalysts, respectively [266]. Conversion was very different. For fixed reaction time, the batch reactor and the falling-film microreactor had higher conversions than the Caroussel reactor. This was indicative of operation under mass transfer regime in the latter. On the basis of these data, it was concluded that the mass transfer coefficients kya of the helical falling-film microreactor are in between the boundaries given by the known kta values of 1-2 s 1 for small batch reactors and about 0.01 s-1 for the Caroussel reactor. 

A multistep gas-liquid process using a falling-film microreactor was carried out for the ozonolysis of a steroid that is a fast and highly exothermic reaction [50],... 

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