Rotor-rotor spinning disc for intensified high shear gravity extraction

This paper presents a novel centrifugal extractor based on rotor-rotor spinning disc technology. The device consists of an inner and an outer rotor that rotate with individual speeds with a gap distance between the rotors between 1-3 mm. The diameter of the rotors is 17 and 18 cm respectively, the volume of each contacting stage is therefore only 100 mL. The inner rotor has a special design that allows countercurrent contacting of a light and a heavy phase over several contacting stages. The investigated device has three physical contacting stages. Experiments are performed with n-heptane as light phase liquid and water as heavy phase liquid. For inner and outer rotation speeds up to 2500 rpm, a >99.9% physical separation efficiency of light and heavy phase is observed in two distinct operating regions. One region in the area where the outer rotor has a higher speed than the inner rotor and one region where the inner rotor has a higher speed than the outer rotor. Hold-up measurements show that the physical separation efficiency drops when the extractor is almost completely filled with (1) the heavy phase for loss of purity of the light phase and (2) with light phase for loss of purity of the heavy phase. Mass transfer experiments with benzoic acid extracted from the n-heptane phase to the water phase show that three contacting stages are obtained, in accordance with the three physical stages. Liquid-liquid mass transfer coefficients up to 1 m/s were observed at the highest shear conditions. Consequently, extraction can be performed effectively with very short contact times and low solvent volumes. The rotor-rotor spinning disc extractor is especially attractive when quick extraction of unstable (intermediate) reaction products is desired, for pressurized systems, and for corrosive systems. Furthermore, it provides a continuous countercurrent extraction step for flow chemistry, which facilitates continuous production of fine chemicals and pharmaceuticals.