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The capillary microreactor, with four stable operating flow patterns and a throughput range from grams per hour to kilograms per hour, presents an attractive alternative to chip-based and microstructured reactors for laboratory-and pilot-scale applications. In this article, results for the extraction of 2-butanol from toluene under different flow patterns in a water/toluene flow in long capillary microreactors are presented. The effects of the capillary length (0.4À2.2 m), flow rate (0.1À12 mL/min), and aqueous-to-organic volumetric flow ratio (0.25À9) on the slug, bubbly, parallel, and annular flow hydrodynamics were investigated. Weber-number-dependent flow maps were composed for capillary lengths of 0.4 and 2 m that were used to interpret the flow pattern formation in terms of surface tension and inertia forces. When the capillary length was decreased from 2 to 0.4 m, a transition from annular to parallel flow was observed. The capillary length had little influence on slug and bubbly flows. The flow patterns were evaluated in terms of stability, surface-to-volume ratio, throughput, and extraction efficiency. Slug and bubbly flow operations yielded 100% thermodynamic extraction efficiency, and increasing the aqueous-to-organic volumetric ratio to 9 allowed for 99% 2-butanol extraction. The parallel and annular flow operating windows were limited by the capillary length, thus yielding maximum 2-butanol extractions of 30% and 47% for parallel and annular flows, respectively.

Precise control over the interfacial area of aqueous and organic slugs in segmented flow in a microchannel reactor provides an attractive means to optimize the yield and productivity of a phase-transfer-catalyzed reaction. Herein, we report the selective alkylation of phenylacetonitrile to the monoalkylated product in a microchannel of 250-μm internal diameter operated in a continuous and solvent-free manner in the slug-flow regime. The conversion of phenylacetonitrile increased from 40% to 99% as a result of a 97% larger slug surface-to-volume ratio when the volumetric aqueous-to-organic phase flow ratio was raised from 1.0 to 6.1 at the same residence time. The larger surface-to-volume ratio significantly promoted catalyst phase transfer but decreased selectivity because of the simultaneous increase of the rate of the consecutive reaction to the dialkylated product. There exists an optimum flow ratio with a maximum productivity. Conversion and selectivity in the microchannel reactor were both found to be significantly larger than in a stirred reactor.

An interdigital mixer-redispersion capillary assembly was applied to prevent the liquid-liquid bubbly flow coalescence in microreactors. The redispersion capillary consisted of 1-mm-long 0.25 mm inner-diameter constrictions, placed every 0.50 m along the channel length. The system was tested on the phase transfer-catalyzed esterification to produce benzyl benzoate. The application of constrictions to prevent coalescence resulted in a better reproducibility and higher conversion compared to a capillary without constrictions. The bubbly flow generated by the interdigital mixer-redispersion capillary assembly was found to be independent of the redispersion capillary inner diameters (0.50 and 0.75 mm) while being highly dependent on the flow rates. By controlling the total flow rate and the aqueousto-organic ratio, the bubbly flow surface-to-volume ratio could be increased up to 230 700 m 2 m -3. Compared to the conventional phase transfer-catalyzed esterification, the continuous operation in the interdigital mixer-redispersion capillary assembly eliminated the use of solvents and bases, removing an energy-intensive step of distillation while increasing process safety.

Redispersion Microreactor System for Phase Transfer Catalyzed EsterificationAn interdigital mixer -redispersion capillary assembly was applied to prevent the liquid-liquid bubbly flow coalescence in microreactors. The redispersion capillary consisted of 1 mm long and 0.25 mm inner-diameter constrictions placed every 0.50 m along the channel length. The system was tested on the phase transfer catalyzed esterification to produce benzyl benzoate. The application of constrictions to prevent coalescence resulted in a better reproducibility compared to a capillary without the constrictions. By controlling the total flow rate and the aqueous-to-organic ratio the bubbly flow surface-volume ratio could be increased up to 230 700 m 2 m -3. Compared to the conventional phase transfer catalyzed esterification, the continuous operation in the interdigital-redispersion capillary assembly eliminated the use of solvents and bases, removing an energy intensive step of distillation, while increasing process safety.

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