DNS-based prediction of the selectivity of fast multiphase reactions: Hydrogenation of nitroarenes

Abstract

The catalytic hydrogenation of nitroarenes is a reaction of significant importance for the pharmaceutical, agro-chemical and dye-manufacturing industries. It proceeds through a hydroxylamine intermediate, which is toxic, carcinogenic and thermally unstable. While many studies have been conducted, attempting to alter the catalyst to minimize the accumulation of hydroxylamine, the influence of the hydrodynamics on the product and intermediate formation has so far not been investigated. Due to the fast rate of reaction most of the hydrogen dissolved from the bubbles will be consumed in close proximity of the bubble surface. In such cases local mixing in the bubble wake impacts the reaction network far surpassing that of the reactor-scale transport. For the purposes of studying the local hydrodynamics in bubbly flows, direct numerical simulations (DNS) were performed of freely deformable gas bubbles rising in a liquid phase. Different cases were considered, including both single bubbles and clusters of multiple bubbles. The hydrodynamic model has been coupled with the equations describing species transport and chemical reaction of all reactants, which allows the direct investigation of the effects of the local hydrodynamics on the product distribution. The effect of different Hatta and Schmidt numbers has been investigated. The simulation results have been compared with a solution obtained from film theory and suggestions have been made how to incorporate the local mixing effect into a modified film theory. We found that the different bubble wake types observed at different conditions will result in different rates of hydroxylamine formation and selectivity. These differences will persist even in the case of clusters consisting of multiple bubbles. The results of this study can be useful for the future design and operation of bubble reactors, as it provided a straightforward method for decreasing the accumulation of harmful intermediates in the catalytic hydrogenation of nitroarenes.