Flow regimes, mixing degree and reaction yield in microreactors

Experimental flow visualizations, PIV, and simulations are carried out jointly to investigate how the flow regimes affect the reaction yield in T- and X-microreactors. The microreactors are fed with an aqueous solution of ascorbic acid into one of the inlet channels and an aqueous solution of methylene blue and hydrochloric acid into the other which, upon reacting, produce a colorless reaction product that can be directly measured in experiments. Different kinetic constants are considered. The interaction between chemical reactions, flow topology, and mixing is not trivial. At low Reynolds number, Re, the two reactant streams remain stratified in the mixing channels due to their different densities, and mixing is promoted only by diffusion. By increasing the Reynolds number, the engulfment regime occurs, which is characterized by the presence of vortical structures (different in T- and X-microreactors) in the confluence region and in the outlet channels, enhancing mixing thanks to convection. In the stratified regime, the reaction yield decreases with the Reynolds number due to a decrease in residence time. Instead, with the onset of the engulfment regimes, the reaction yield starts to increase with Re because the enhancement in the mixing degree between the streams overcomes the reduction of the residence time. The reaction yield also depends on the Damköhler number (Da), which represents the ratio between the characteristic time of chemical reaction and that of convection, and on the reaction kinetic constant. In the stratified regime, the reaction yield follows a Da^0.3 dependence, independently of the kinetic constant. In the engulfment regime, on the other hand, there is a strong dependence also on kinetic constant for both T- and Xmicroreactors.