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The results of experimental studies on the parallelization of both single-phase and biphasic liquid−liquid slug flow in capillary microreactors are presented. Various flow distributor designs were investigated with respect to the uniformity of liquid−liquid flow in eight parallel capillaries and the two most promising concepts were identified. A novel multichannel phase splitting unit was installed at the downstream end of the capillaries to ensure well-defined phase separation. Further investigations on more complex reaction systems, in which the hydrodynamics depend on conversion and/or selectivity (e.g., polymerization), suggest that an active flow control for individual capillaries will be necessary in such cases. The uniformity of flow rate and flow structure could be achieved using a noninvasive capacitative measurement to characterize the flow in conjunction with specially developed microvalves and actuators for the regulation of flow rates and the manipulation of slug flow structure in each capillary.

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The liquid-liquid slug flow capillary microreactor offers an excellent mass transfer performance for extraction and biphasic reactions. In combination with a simple phase separator based on wettability discrimination between the two liquids, it provides a powerful tool for process intensification and microscale processing. By new visualization techniques, the interfacial surface and slug vortex structures dictating inter-and intraphase mass transfer have been revealed to be more complex than previously assumed. Suspending fine catalyst particles in one phase of a two-phase slug flow is an effective technique for using heterogeneous catalysts in microreactors, owing to the very good mass transfer characteristics and because catalyst recovery becomes simply a matter of separating the catalyst carrier phase from the reaction medium. To exploit the performance attributes of capillary microreactors at higher throughputs, distributor and control strategies for parallelisation were developed to provide a flow distribution uniform to within 1 % or less.

Reactions of n-decene with ozone and subsequent quenching of the formed ozonides were carried out under flow conditions using the standard Vapourtec flow system equipped with a cooled flow cell. The reactions were performed continuously and in the annular flow regime within the circular cross-section channels. Typical flow rates were 0.25À1 mL min À1 for liquid and 25À100 mL min À1 for gas, reactor volumes were 0.07À10 mL formed of 1 mm ID PFA tubing. The reaction temperature was À10°C. The flow was not always smooth, while waves in the liquid film and droplets in the gas core were observed. Liquid residence times were found to be independent of gas flow rates and increasing with decreasing liquid flow rates. Substrate residence times in the ozonolysis reactor ranged between 1 and 80 s, and complete conversion could be achieved at ∼1 s residence time. Two common reductants, triethylphosphite and triphenylphosphine, were examined as to their suitability under flow conditions. Triphenylphosphine achieved faster reduction of the intermediate ozonides, resulting in a greater than 10:1 selectivity for the aldehyde over the corresponding acid. The cooling system provided a safe and efficient control of the highly exothermic reaction system. The configuration of the system allowed the production of chemically significant amounts (1.8 g h À1 at 1.3 ozone equivalents), with minimal amounts of ozonides present at any time. ' INTRODUCTIONOzone is an oxidant that has been used for decades in organic synthesis 1 and in the purification/deodorisation of water. 2 Ozonolysis is the addition of ozone to a substance, resulting initially in a primary ozonide which then usually rearranges to a more stable secondary ozonide in the case of alkenes. 3 The most common substrates for ozonolysis are unsaturated species containing either double or triple carbonÀcarbon bonds, which result usually in aldehydes/ketones and acids. This transformation has great utility in synthesis both in a research and an industrial setting. Ozone can also be used to oxidise phosphorus 1,4 and sulphur 1,5 compounds as well as organometallics 6 and other inorganic species. 6 The advantages of using ozone over other oxidants is the low cost and toxicity of the byproduct, i.e. DMSO from DMS, Ph 3 PO from PPh 3 as compared with high oxidation level metallic oxidants, e.g. chromium. Some of the drawbacks of using ozone at scale are the high exotherms associated with the initial reaction of substrates with ozone. 7 Also the intermediate ozonides are unstable (although some can be isolated at room temperature) and are an explosion risk due to the instability of the OÀO single bond; the build-up of these materials must be avoided. The use of continuous flow can alleviate both the exotherm problem and the build-up of potentially explosive materials (if suitable reagents are used to quench them in flow).Microreactors have shown several benefits in the past years. Microstructured devices provide short diffusion pathways, improved heat and mass transfer rates, and increa...