Centrifugal extractor

A centrifugal extractor—also known as a centrifugal contactor or annular centrifugal contactor—uses the rotation of the rotor inside a centrifuge to mix two immiscible liquids outside the rotor and to separate the liquids in the field of gravity inside the rotor. This way, a centrifugal extractor generates a continuous extraction from one liquid phase (fermentation broth) into another liquid phase (organic solvent).

Annular centrifugal extractor design and development has been pursued by various Department of Energy laboratories for more than 40 years. Initial design of the annular centrifugal contactor was done at Argonne National Laboratory through modification of a Savannah River Site paddle mixed design. It has been employed in solvent extraction processes for metals valuable to the nuclear industry. A summary of contactor design principles and applications is included in a recent compilation.

Commercialization of this technology began in 1990 when a patent was granted for continuous separation of hydrocarbons from water (Meikrantz, 1990). In the past years the centrifuge design has been further improved and scaled up to flow rates of several hundred liters per minute (Meikrantz et al., 1997). Such contactors are used as part of the Salt Waste Processing Facility at the Savannah River Site for implementation of the CSSX process to extract radioactive cesium from tank wastes stored there.

Two immiscible liquids of different densities are fed to the separate inlets and are rapidly mixed in the annular space between the spinning rotor and stationary housing. The mixed phases are directed toward the center of the rotor by radial vanes in the housing base. As the liquids enter the central opening of the rotor, they are accelerated toward the wall. The mixed phases are rapidly accelerated to rotor speed and separation begins as the liquids are displaced upward. A system of weirs at the top of the rotor allow each phase to exit the rotor where it lands in a collector ring and exits the stage. Flow from between stages is by gravity with no need for inter-stage pumps. The centrifugal contactors thus acts as a mixer, centrifuge and pump. Centrifugal contactors are typical referred to by the diameter of their rotor. Thus, a 5-inch centrifugal contactor is one having a 5-inch diameter rotor.

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