The influence on the mass transfer in liquid-liquid extraction was investigated during droplet formation in a quiescent aqueous continuous phase for the two transition components, acetone and acetonitrile, in toluene. Both transition components have similar characteristics. However, an approximately eight times slower mass transfer of a droplet hanging on a capillary in relation to a rising droplet could be observed. The droplet formation time and the initial solute concentration are decisive for the mass transfer behaviour. A lower volumetric flow leads to slower droplet formation and a higher specific mass transfer area enhancing mass transfer, which is visualized via laser induced fluorescence (LIF). Additionally, as expected, higher initial solute concentrations promote Marangoni turbulences and thus mass transfer, which is measured via confocal Raman spectroscopy inside a fixed hanging droplet. K E Y W O R D S droplet formation, interfacial phenomena, Marangoni convection, mass transfer

The impact of mass transfer in liquid-liquid extraction during droplet formation in a quiescent continuous phase was investigated. The impact of droplet size, concentration, and formation rate on a hanging droplet was analyzed via laser induced fluorescence (LIF) using rhodamine 6G as tracer in the system of toluene and acetone in water. The droplet formation rate has a major impact on mass transfer and the mixing pattern inside the droplet. Very fast mass transfer induced by Marangoni convection was visualized at a high local resolution independently of concentration and formation rate.

In liquid-liquid extraction processes, the mass transfer coefficient is one of the key design parameters. In the present study, the impact on mass transfer during droplet formation in a quiescent continuous phase is investigated analytically. Therefore, confocal Raman spectroscopy in a novel measurement cell combined with interfacial tension measurements are applied. It could be shown, that Marangoni effects in the system toluene-acetonitrile-water (mass transfer direction from the dispersed to the continuous phase) lead to 50 % of mass transferred in the first 5 s when 90 % is transferred in 1 min.

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