Falling film micro reactor

For toluene fluorination, the impact of micro-reactor processing on the ratio of ortho-, meta- and para-isomers for monofluorinated toluene could be deduced and explained by a change in the type of reaction mechanism. The ortho-, meta- and para-isomer ratio was 5 1 3 for fluorination in a falling film micro reactor and a micro bubble column at a temperature of-16 °C [164,167]. This ratio is in accordance with an electrophilic substitution pathway. In contrast, radical mechanisms are strongly favored for conventional laboratory-scale processing, resulting in much more meta-substitution accompanied by imcontroUed multi-fluorination, addition and polymerization reactions. 

Figure 5.1 Photograph of a falling film micro reactor [1],...

The falling film micro reactor (Figure 5.1) transfers this well-known macro-scale concept to yield films of a few tens of micrometers thickness [1-3]. For this reason, the streams are guided through micro channels. To obtain a reasonable throughput, many micro channels are operated in parallel. 

Internal heat exchange is realized by heat conduction from the microstructured reaction zone to a mini channel heat exchanger, positioned in the rear of the reaction zone [1,3,4], The falling film micro reactor can be equipped, additionally, with an inspection window. This allows a visually check of the quality of film formation and identification of flow misdistribution. Furthermore, photochemical gas/liquid contacting can be carried out, given transparency of the window material for the band range of interest [6], In some cases an inspection window made of silicon was used to allow observation of temperature changes caused by chemical reactions or physical interactions by an IR camera [4, 5]. 

Reactor type Falling film micro reactor Mini heat exchange channel width depth 1500 pm 500 pm... 

Figure 5.17 Comparison of performance of a typical laboratory column (LBC) with those of micro-reactor devices falling film micro reactor (FFMR) micro bubble column (MBC I and MBC II) [38].

The laboratory and the micro bubble column show decreasing selectivity with increasing conversion. The falling film micro reactor shows a near-constant selectivity-conversion relationship [3, 38]. 

GL 1] [R 1] [R 3] [P le] The falling film micro reactor has a better selectivity-conversion performance than the two micro bubble columns tested (Figure 5.18) (3, 38]. The micro bubble column with narrow channels has a better behavior at large conversion than the version with wide channels. The behavior of the falling film micro reactor and the micro bubble column with narrow channels is characterized by a nearly constant selectivity with increasing conversion, while the bubble column with wide channels shows notably decreasing selectivity with conversion (similar to the laboratory bubble column). 

GL 1] [R 1] [R 3] [P la-d] Space-time yields higher by order of magnitude were found for the falling film micro reactor and the micro bubble column as compared with the laboratory bubble column [38], The space-time yields for the micro reactors ranged from about 20 000 to 110 000 mol monofluorinated product m h. The ratio with regard to this quantity between the falling film micro reactor and the micro bubble column was about 2. The performance of the laboratory bubble column was of the order of 40-60 mol monofluorinated product m" h. ... 

Figure 5.19 Conversion of the direct fluorination of toluene in different reactor types as a function of the molar ratio of fluorine to toluene (a) and efficiency of these reactors, defined as conversion normalized by the molar ratio of fluorine to toluene, as a function of the molar ratio of fluorine to toluene (b). Falling film micro reactor (FFMR) micro bubble column (MBC) laboratory bubble column (LBC) [38].

GL 1] [R 1] [R 3] [P la-d] For micro-channel processing, an analysis of the content of fluorine actually consumed as a function of the fluorine-to-toluene ratio was made [38]. The curves for two micro reactors and one laboratory bubble column do not show the same trend a decrease of converted fluorine with increasing ratio results for the falling-film micro reactor, whereas the micro and laboratory bubble columns show increasing performance. The two micro reactors use about 50-75% of all fluorine offered, whereas the laboratory tool has an efficiency of only 15%. 

GL 1[ [R 1[ [R 3[ [P la-dj On increasing the temperature for micro-channel processing, conversion for the direct fluorination rises, as expected [38]. For the falling film micro reactor, conversion is increased from 15 to 30% on going from -40 to -15 °C. The selectivity varies widely between 30 and 50% without a clear tendency for this temperature range. The origin of this fluctuation is not understood. 

GL 1[ [R 1[ [P la[ The residence time distribution between the individual flows in the various micro channels on one reaction plate of a falling film micro reactor was estimated by analysing the starting wetting behavior of an acetonitrile falling film [3]. For a flow of 20 ml h it was found that 90% of all streams were within a 0.5 s interval for an average residence time of 17.5 s. 

GL 1[ [R 1[[P la[ By autofocus laser imaging, the average position of the liquid surface in all micro channels of a reaction plate of a falling film micro reactor was determined [3]. It was found that very thin films of the order of 20-25 pm were formed for total volume flows of 20-80 ml h The thickness of the films in the various channels differed, but by no more than 30% on average. At high flows, e.g. > 180 ml hr, flooding of the channels occurs. 

GL 1[ [R 1[ [R 3[[P la-d[ The fluorine content in the gas phase of a falling film micro reactor was varied at 10, 25 and 50% [38]. A nearly linear increase in conversion results at constant selectivity. The substitution pattern, i.e. the ratio of ortho-to para-isomers, is strongly affected by this. 

P 12] A falling film micro reactor was applied for generating thin liquid films [6]. A reaction plate with 32 micro channels of channel width, depth and length of 600 pm, 300 pm and 66 mm, respectively, was used. Reaction plates made of pure nickel and iron were employed. The micro device was equipped with a quartz window transparent for the wavelength desired. A 1000 W xenon lamp was located in front of the window. The spectrum provided ranges from 190 to 2500 nm the maximum intensity of the lamp is given at about 800 nm. 

GL 13] [R 1] [P 12] By using a nickel plate, space-time yields up to 401 mol 1 h were achieved in the falling film micro reactor [6]. Control experiments in a batch reactor at a 30 min reaction time resulted in a space-time yield of only 1.3 mol 1 h , hence orders of magnitude smaller. By using an iron plate, space-time yields up to 346 mol h were achieved in the falling film micro reactor. 

Acetic acid and 10, 15, or 20% acetyl chloride were fed as a mixture into a modified falling film micro reactor (also termed micro capillary reactor in [57]) at a massflow rate of 45 g min and a temperature of 180 or 190 °C [57]. Chlorine gas was fed at 5 or 6 bar in co-flow mode so that a residual content of only 0.1% resulted after reaction. The liquid product was separated from gaseous contents in a settler and collected. By exposure to water, acetyl chloride and acetic anhydride were converted to the acid. The hydrogen chloride released was removed. 

Mass transfer efficiency by conversion anaiysis for the falling film micro reactor... 

GL 22] [R 1] [P 23] The mass transfer efficiency of the falling film micro reactor as a function of the carbon dioxide volume content was compared quantitatively (Figure 5.30) [5]. The molar ratio of carbon dioxide to sodium hydroxide was constant at 0.4 for all experiments, i.e. the liquid reactant was in slight excess. 

It is remarkable that the falling film micro reactor achieved complete conversion for all process variations applied [5]. This is unlike conventional reactor operation reported for this reaction, displaying pronounced mass transfer resistance. 

C. De Bellefon, T. Lamouille, N. Pestre, F. Bornette, H. Pennemann, F. Neumann, V. Hessel, Asymmetric catalytic hydrogenations at micro-litre scale in a helicoidal single channel falling film micro-reactor, Catal. Today 110 (2005) 179. 

Owing to the complex and often dedicated equipment required to perform gas-liquid phase reactions within research laboratories, this area of synthetic chemistry is somewhat underutilized. Over the past decade, however, numerous research groups have developed an array of continuous flow reactors capable of conducting such reactions in a safe and efficient manner, including microchannel contactors, falling film micro reactors, and packed-bed reactors [68, 69]. 

K. Jahnisch, U. Dingerdissen, Photochemical generation and [4 + 2)-cycloaddition of singlet oxygen in a falling-film micro reactor, Chem. Eng. Tech. 2005, 28, 426-427. 

---