Miniature bubble column

Dream reactions can be performed using chemical micro process engineering, e.g., via direct routes from hazardous elements [18]. The direct fluorination starting from elemental fluorine was performed both on aromatics and aliphatics, avoiding the circuitous Anthraquinone process. While the direct fluorination needs hours in a laboratory bubble column, it is completed within seconds or even milliseconds when using a miniature bubble column. Conversions with the volatile and explosive diazomethane, commonly used for methylation, have been conducted safely as well with micro-reactors in a continuous mode. 

Mixing in microreactors is almost exclusively assumed to be laminar dne to the small flow channel width. Laminar flow through microchannels reqnires the phase flow to be alternated in some fashion in order to create the mixing environment. This operation is important because mass transfer, in this case, is driven only by molecular diffusion. Hence, the creation of larger gas-liquid interfaces is the only practical course of action for gas-limited operations. Miniaturized bubble columns are able to accomplish this task well. For example, a standard reactor can create... 

Direct fluorination of toluene using elemental fluorine is not feasible since the heat release cannot be controlled with conventional reactors. So the process is deliberately slowed down. Hence, the direct fluorination needs hours in a laboratory bubble column. It can be completed within seconds or even milliseconds when using a miniature bubble column, operating close to the kinetic limit. The Bayer-Villiger oxidation of cyclohexanol to cyclohexanone with fluorine and aqueous formic acid (5% water) is done in miniature bubble column reactors at 60% conversion at 88% selectivity. 

Similar performance enhancement could be realized by the miniaturization for mass transfer leading to efficient mixing. For multiphase systems within microdevices, the interfacial surface to volume ratio between the two fluids is notably Increased. Indeed, the miniaturized systems possess high interfacial area up to 30000m m . The traditional bubble columns do not exceed a few 100 m m- [7]. 

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. 

Similar to the numbered up Taylor-flow reactor, the micro bubble column achieved dispersion by an interdigital mixing element with many miniaturized mixing tees (Figures 9.17 and 9.18) [4,26,66,67,71,74,75]. This was one of the very... 

---