Control of mixing in a continuous-flow microreactor with an integrated bubble

Abstract

Continuous-flow microreactors are increasingly used to replace batch reactors in fine organic synthesis. The interest of pharmaceutical production and similar industries in greater flexibility in reconfiguring synthesis systems motivates the design of increasingly miniature devices, in which mechanical mixing is difficult to apply, while the diffusion mechanism is inefficient. In this paper, using the example of a T-shaped continuous-flow microreactor, we propose to supplement the reaction chamber with a unit that rapidly mixes flowing solutions due to the concentration-capillary mechanism. Two syringe pumps supply a less dense isobutyric acid (\(\mathrm {C_{3}H_{7}COOH}\)) aqueous solution to the upper inlet and denser sodium hydroxide (\(\textrm{NaOH}\)) aqueous solution to the lower inlet, establishing a stably stratified two-layer system at the reaction chamber entrance. The contact between the solutions triggers a neutralization reaction. At a distance from the entrance, an additional inlet helps to form a gas bubble within the chamber. We show experimentally and numerically that the integration of an air bubble in a channel leads to the excitation of a self-oscillatory process near the air-liquid surface. The convective mixing process occurs in a pulsed mode, which includes a phase of rapid homogenization of the medium and subsequent restoration of heterogeneity by the base flow. We have determined the flow stability map on the plane of solution flow rates. We demonstrate that one can control the activation of convection, which enhances the mixing process, by manipulating the flow rates.