(74d) Re-Dispersion Microreactor to Achieve Staged Liquid-Liquid Dispersion for Use in Polycondensation

To ensure mass transfer in biphasic reactions a sufficient interface area is required. Within the scope of micro process engineering two concepts are available here. The first one is the continuous-phase contacting and the second one is the dispersing- mixing technique. The latter can be realised in a micromixer-capillary reactor combination, which can successfully be used, if the emulsions are long-term stable [1, 2]. An example for such application is the transient catalyst screening for the isomerisation of allylic alcohols [3].

In some cases, an emulsifier is added to stabilise the interface. In this case often a single-point provision of energy is sufficient to form and preserve an emulsion. In many other cases, however, addition of emulsifiers is prohibited, since that may negatively affect product purification or quality. The then correspondingly faster coalescence of the droplets poses a challenge to the micromixer technology. Flow in microstructured column arrays, highly regular analogues to ceramic foams and metallic fleeces, leads to steady energy provision via constant creation of inlet flows and consequently to repeated breakage of droplets.

Because dispersions of various immiscible solvents typically used for organic synthesis are stable at least for some seconds, a reactor concept has been developed avoiding such energy-intensive solutions. This so-called re-dispersion microreactor contains exchangeable serial re-dispersion units with nozzles and an observation window for flow inspection. The re-dispersion units are separated by multichannels with stage-wise increasing channel length.

For one first model system investigated, the heptane / water (SDS) system, hydrodynamic characterisation was carried out using a high-speed camera; droplet size spectra were gained by a droplet analyser. A decrease in average droplet size with increasing number of re-dispersion units is found. In the process flow, the solvents are initially roughly pre-mixed by a standard interdigital micromixer, resulting in large droplets when entering the inlet chamber of the re-dispersion reactor. The crude dispersion is then reduced to a much smaller size already by passing through the first re-dispersion unit via the respective nozzles. A finer-dispersed multi-droplet flow with a few large droplets with diameters in the order of the microchannel is found. The following re-dispersion steps lead to further decrease in droplet size, finally forming in the outlet segment a fine emulsion. For lower flow rates the effect of the stepwise reduction of the droplet size is stronger than for higher flow rates. The same tendency can be observed for symmetric (1:1) flow ratios as opposed to asymmetric (4:1) ones. Further parametric dependencies will be presented.

To judge the performance of this special microreactor a comparative study using standard interdigital micromixers with heptane / water (SDS) was accomplished. These devices show a decrease of the average droplet size with energy, respectively flow rate, and flow compression through a nozzle-like geometric confinement (?slit').

A subsequent real-case solvent system without emulsifier is ought to be investigated in the new developed re-dispersion reactor. This system constitutes the solvent mixture for a polycondensation reaction to synthesize OLED materials. Here, high demands are made on conversion and product purity.

[1] Löb, P., Hessel, V., Men, Y., Pennemann, H. Chem. Eng. Sci. 61 (2004) 2959-2967. [2] Pennemann, H., Hardt, S., Hessel, V., Löb, P., Weise, F. Chem. Eng. Tech. 28, 4 (2005) 501-508. [3] de Bellefon, Tanchoux, N., Caravieilhes, S., Grenouillet, P., Hessel, V. Angew. Chem. 112, 19 (2000) 3584-3587.