Micromixing in oscillatory baffled flows

Abstract

The mesoscale oscillatory baffled reactor (meso-OBR) is a novel screening platform that does not require significant optimisation of the operating conditions, making it ubiquitous. Although it is known that micromixing performance influences the observed reaction kinetics, the micromixing of meso-OBRs has not previously been characterised. Therefore, in this study we have measured the micromixing times in three meso-OBR configurations across a broad range of oscillatory (Reo = 50–1000) and net flow Reynolds numbers (Ren = 5–40) for the first time using the Villermaux-Dushman competing reaction scheme. Helical baffles exhibited the lowest and most consistent micromixing times, followed by the central axial baffle design and the integral baffle design. Using the micromixing times, design equations were developed that showed the ratio of the micromixing time to mean residence time was dependent only on the velocity ratio (ratio of oscillatory to net flow Reynolds numbers). In all baffle designs, micromixing times decreased with increasing velocity ratios (ψ > 25). Therefore, for the central baffle and integral baffle designs, the lowest micromixing times would not be accessible for flow chemistry applications, because the optimal velocity ratios for plug flow in these designs are only around 4–10. Whereas, the helical baffle can achieve plug flow across a much broader range of velocity ratios (up to ψ = 250), because of the additional swirling component to the flow, enabling the lowest micromixing times of the helical baffle to be exploitable for reaction engineering applications. The lowest micromixing time produced was 0.03 s, using the helical baffle with Ren = 10 and Reo = 750.