Microbubble columns

Another modification is the use of microbubble column flotation (13). In this process, smaller bubbles are generated to enhance the recovery of micrometer-sized particles. A countercurrent flow of feed slurry is also used to further enhance the bubble—particle attachment. The process is capable of produciug ultraclean coals containing less than 0.8% ash. 

To achieve an even higher degree ofnumberingup, a modular microbubble column reactor was developed that contains a stack of microstructured plates. The construction encompasses five different assembly groups, a cylindrical inner housing, which... 

Figure 4.40 Schematic of the microbubble column , a numbered-up Taylor-flow microreactor with a mixing for each reaction channel (left) scanning electron micrograph of the mixing element, top view (right). L liquid C gas. The small channels with the semicircular openings are the gas feed and the larger rectangular ones are for liquid feed (by courtesy of VDI Verlag) [275],...

Figure 4.41 Microbubble column (redesigned version). Besides the two inlets for gas and liquid flows and the outlet for the dispersion, two further fluid connectors are there for the incoming and outgoing heat exchange medium (by courtesy ofWiley-VCH Verlag GmbH) [279],...

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). 

Figure 4.52 Special-type multipurpose (1100pm x 170 pm) microbubble column (A)...

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). 

FIGURE 15 Microbubble column with integrated cooling channels [119]. (Adapted with permission from Springer.)... 

Unfortunately, the extreme numbers are somewhat misleading. In order to properly understand their context for biological application, an understanding of microbubble column flow regimes is necessary. Bubble velocity in microreactors is defined by a gas space velocity. This velocity is similar to the superficial gas velocity, but it tends to be slower due to very significant wall effects. So, the gas space velocity is approximated using a sample bubble velocity. 

Bubbles in microbubble colunuis are observed to be separated by a liquid film from the wall at almost all gas space velocities. At low gas space velocities, the microbubble column experiences bubbly flow. The bubbly flow is defined by microbubbles, which are spherical and as large as the channel diameter. Since bubbles of this size are highly unstable without surfactants, the gas space velocity has to be low enough to allow enough space between bubbles in order to prevent coalescing. 

Figure 15.12 Microbubble column with integrated cooling channels [2].

The fluorination of toluene with 10% F2 in N2 by using the falling-film microreactor and the microbubble column has also been studied (Scheme 8.7) [11, 12]. Both the falling-film microreactor and the microbubble column offer advantages over conventional reactors in fluorination reactions. The selectivity of the formation of monofluorinated toluene in a falling-film microreactor is significantly higher than... 

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

Haverkamp V, Hessel V, Lowe H, Menges G, Warnier MJF, Rebrov EV, de Croon MHJM, Schouten JC, Liauw M. Hydrodynamics and mixer-induced bubble formation in microbubble columns with single and multiple channels. Chem. Eng. Technol. 2006 29(9) 1015-1026. 

The mass transfer efficiency of different gas-liquid contactors as a function of residence time was compared, including an interdigital micromixer, a caterpillar minimixer, a mixing tee, and three microbubble column with microchannels of varying diameter (Figure 9.35) [141]. The two microbubble columns comprising the... 

Dispersed-phase contacting, obtained when one of the fluid phases is dispersed into the other phase. Examples are microbubble columns and micro-pacJced-bed reactors. 

Demming, S., Peterat, G., Llobera, A., Schmolke, H., Bruns, A., Kohlstedt, M., Al-Halhouli, A., Klages, C.-P., Krull, R., and Buttgenbach, S. (2012) Vertical microbubble column - a photonic lab-on-chip for cultivation and online analysis of yeast cell cultures. Biomicrofluidics, 6, 034106/1-034106/14. 

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