Abstract: A device for separating a flowing medium mixture into at least two different fractions with differing average mass density, comprising an elongate separating space which is circle-symmetrical in axial direction and enclosed by a stationary casing (2), wherein the casing is provided with feeds for a mixture for separating and at least two discharges (9, 10) for discharging at least two fractions with differing mass density, and rotation means located in the separating space for causing the mixture to rotate as a vortex in the separating space. Also disclosed is a method for separating a flowing medium mixture.