(734f) Separation Processes in Micro Reaction Technology

Micro reaction technology has been investigated for nearly 20 years. Several micro mixers, micro heat exchangers, micro reactors for different purposes and applications have been developed. For a variety of reactions it could be shown that there are certain advantages of this technology, namely better control of the reaction, leading to a more efficient, economical and ecological way of producing chemical products. Looking into more detail, it turns out that most of the published examples are single step reactions. In some cases multiple reaction steps could combined, but only when the first steps showed total conversion and by-products could be avoided. Even though micro reaction technology allows a much better conversion and yield, still by-products and unused starting compounds are present in the product stream. Therefore, it is necessary to provide a continuous separation unit which allows to collect the unwanted parts of the mixture before the product stream enters the next process step. Although first developments have been published already [1] most of the necessary downstream processes are not available, yet. This presentation will show new separation modules and their first test results for the separation of gas-liquid and liquid-liquid mixtures.

SEPARATION MODULES

All developed and tested modules of this work are made out of FOTURAN glass, a special photosensitive material which allows to manufacture micro channels in glass with high precision [2]. The material was chosen because of its chemical stability and its optical transparency. With these modules it was possible to observe their behavior during the separation process.

It will be shown in the presentation that the separation quality depends on different physical parameters. Gas-liquid and liquid-liquid separations in a triangular shaped separation chamber are strongly flow rate dependent. At low flows the mixture has time to settle: the heavy phase can be extracted from the bottom, the light phase from the top. Above a certain velocity of the solution (in the range of a few ml/min, depending on the liquids) the residence time is not sufficient for a good separation. The efficiency can be increased by a surface modification according to the properties of the used liquid: e.g. hydrophilic for water or hydrophobic for organic solvents. This kind of surface modification also helps to separate liquids in a "Y"-separator. In this case, the reaction channel leads into a Y-shaped structure with two exits. The surface of one branch of the "Y" has been modified by a plasma process to make it hydrophobic, the other one is hydrophilic. In the reaction channel before the "Y" separator, the mixture forms plugs, which then leave the separator through the hydrophilic or the hydrophobic branch according to their respective properties. Another very effective separation module for gas-liquid mixtures is the "capillary separator". At the end of a micro channel reactor, a system of capillaries take out the liquid phase by capillary forces. Depending on the geometry and the number of capillaries as well as the surface properties and the flow of the components, it is possible to get a stream of gas-free liquid out of the separator. All different separation modules showed good behavior in multi-step reactions. The separation efficiency was in an acceptable range and could be increased by using more than one module in a row. The separation module could also be used for cleaning and re-use of process solvents

ACKNOWLEDGEMENTS

The authors would like to thank the German Federal Ministry of Education and Research and the Deutsche Bundesstiftung Umwelt for funding parts of this work within the projects MIRFA, HotChip, and TRANSKAT.

REFERENCES

[1] T.R.Dietrich (Ed.), Microchemical Engineering in Practice: A. Gavriilidis, J.E.A.Shaw, Separation Units, John Wiley & Sons, Hoboken NJ, 2009, pp. 131-163 [2] T.R.Dietrich, W.Ehrfeld, M.Lacher, M.Krämer, .Speit, Fabrication technologies for Microsystems utilizing photoetchable glass, Microelectr. Eng. 30 (1996) 497-504.