(436b) Anomalous Behavior of Liquid-Liquid Two-Phase Reaction in a Slug Flow Microreactor

This work depicts the experimental investigation of a two-phase heterogeneous reaction in the slug flow regime in a micro-reactor. The high surface area to volume ratio and shorter diffusion paths offer by microfluidic devices intensify the performance of these systems. Phase transfer catalyzed benzyl alcohol (BA) oxidation using sodium hypochlorite yielding benzaldehyde is chosen as a test system. BA oxidation is dependent on pH of aqueous phase with a lower pH favoring the reaction. The reaction rate constants at three different pH values 10.4, 8.4 and 7.4 is determined using batch experiments. Slug flow experiments carried out at these pH values are analyzed for benzaldehyde yield. The yield of benzaldehyde increases with an increase in organic phase residence at higher pH i.e. 10.4 and 8.4. However at pH 7.4 the system shows an anomalous behavior i.e. benzaldehyde yield decreases with an increase in organic phase residence time.

The anomalous behavior observed in BA oxidation is explained using hydrodynamics of slug flow and is shown to be due to a change in the rate determining step. The relative importance of three time scales i.e. residence time, mass transfer time and reaction time in two-phase reaction in microfluidic devices is captured using dimensionless Damkohler numbers. Criteria in terms of these dimensionless numbers are obtained to determine where a particular step is rate determining.