Droplet Coalescence Phenomena during Liquid-Liquid Heterogeneous Reactions in Microreactors

The sulfonation of naphthalene with sulfuric acid to produce naphthalene sulfonic acid was carried out in capillary microreactors as a model liquid-liquid heterogeneous reaction to study droplet coalescence phenomena. The effects of various factors associated with droplet coalescence or the reaction kinetics, such as capillary length (residence time), reaction temperature, molar ratio of reactants, and capillary wall wetting properties, on the reaction performance were investigated. In particular, the influence of droplet coalescence on the specific interfacial area, the mass transfer rate, and the reaction performance was evaluated in detail. Surprisingly, the specific interfacial area could decrease up to 93% along the flow direction in the capillary microreactor with the droplet coalescence. Interestingly, the microreactor wall surface properties were found to affect the reaction performance significantly. Moreover, the Hatta number was evaluated, which reflected the competition between the mass transfer and the reaction during the sulfonation in the capillary microreactors with droplet coalescence.