Phase transfer catalysis in a microchannel: Paradoxical effect of transition from kinetic control to mass transfer control

Abstract

The high surface area per unit volume available in microfluidic devices has led to their use in process intensification of mass transfer limited reactions. The commonly observed flow regimes parallel and slug flows, play a major role in determining the performance of these systems. The strong internal circulations and high interfacial area per unit volume makes the slug flow regime a preferred regime for mass transfer limited reactions. The reaction between benzyl alcohol and sodium hypochlorite to yield benzaldehyde via phase transfer catalysis (PTC) is chosen as a test reaction system. Batch experiments were carried out at three different pH values viz. 10.4, 8.4 and 7.4. The rate of reaction decreased as the medium became alkaline. A mathematical model where the species in the aqueous phase is considered to be in equilibrium was developed. In the slug flow regime of the microchannel the yield of the reaction is found to increase with an increase in organic phase residence time at high pH. The yield however decreased with residence time at low pH. This paradoxical behavior is attributed to a shift in the system from the kinetic control regime to a mass transfer control regime. The effect of residence time, mass transfer time and reaction time on the test reaction is analyzed in this work. Criteria in terms of dimensionless numbers have been proposed to determine if a system is kinetically controlled or mass transfer controlled.