Chemical processing and micromixing in confined impinging jets

Abstract

Abstract Rapid processes such as certain organic reactions or precipitations at high supersaturation require the rapid mixing provided by jet mixers. Micromixing in a confined impinging jets (CIJ) mixer was characterized employing the Damköhler number to correlate processing. A scaling theory for the characteristic micromixing time, proportional to momentum diffusion starting at the Kolmogorov microscale, is shown as sufficient to express the micromixing performance of the CIJ mixer. A recently characterized second‐order competitive reaction set is used as a “chemical ruler” to assign an absolute value to the mixing time in the CIJ mixer. A wide range of characteristic time (320 to 5 ms) is evaluated with hydrochloric acid competing for sodium hydroxide neutralization or 2,2‐dimethoxypropane acid catalyzed hydrolysis. This reaction set was sensitive enough to detect the onset of a turbulent‐like flow at a Re of 90 and was able to demonstrate a decrease in undesired products up to the highest Re tested, 3,800 or a jet velocity of 19 m/s. It represents a significant advancement to the reaction sets and techniques used for previous mixing studies, which are reviewed. Experiments verify the characteristic mixing time in a CIJ mixer scales as the inverse of the jet velocity to the three halves power, and the “mesomixing volume” (the volume over which the majority of flow energy was dissipated) is best approximated as proportional to the internozzle separation cubed. For each of the different jet diameters, chamber diameters and outlet configurations tested, the selectivity of the reaction scaled linearly with the Damköhler number, as determined from the known reaction kinetics and the calculated Kolmogorov diffusion time. The first full characterization is provided of micromixing in impinging jets that allows the prediction of mixing performance, reaction selectivity, and scale‐up criteria.