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 into another liquid phase.
History
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. In the past years the centrifuge design has been further improved and scaled up to flow rates of several hundred liters per minute. 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 caesium from tank wastes stored there.
Monostage centrifugal extractor
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. Annular centrifugal contactors are relatively low revolutions-per-minute, moderate gravity enhancing machines, and can therefore be powered by a direct drive, variable speed motor. Typical RPM for small units is approximately 3600RPM while larger units would operate at lower RPM depending on their size. The effectiveness of a centrifugal separation can be easily described as proportional to the product of the force exerted in multiples of gravity and the residence time in seconds or g-seconds. Achieving a particular g-seconds value in a liquid–liquid centrifuge can be obtained in two ways: increasing the multiples of gravity or increasing the residence time. Creating higher g-force values for a specific rotor diameter is a function of rpm only.
Multistage centrifugal extractor
The feed solution initially containing one or more solutes, and an immiscible solvent having a different density flow counter-currently through the extractor’s rotor, designed with a stack of mechanical subassemblies representing the required number of separate stages. The successive mixing and separation operations performed in each mechanical stage permit the mass transfer of the solutes from the feed solution to the solvent.
Each stage consists of
Mixing chamber where the two phases are mixed and where the transfer of solutes to be extracted is achieved. A fixed disk allows the two phases to be mixed and to create an emulsion. It operates as a pump to draw the two phases from the preceding stage.
Decantation chamber where the two previously mixed liquids are thoroughly separated by centrifugal force. Overflow weirs stabilize the separation area independently of flow rates. The interphase position depends on the diameter of the heavy phase overflow weir, which is interchangeable and to be selected according to the phase density ratio.
Configurations
Mix and separation
As described above, the mix & separation configuration is the standard operation for centrifugal contactors used for liquid-liquid extraction processes. The two liquids and an organic phase ) enter the annular mixing zone where a liquid-liquid dispersion is formed and extraction occurs as solutes are transferred from one phase into the other. Inside the rotor, the liquids will be separated into a heavy and a light phase by their respective densities. This proportion of each phase, total flow rate, rotor speed, and weir sizes are varied to optimize separation efficiency. The separated liquids are discharged without pressure and flow by gravity to exit the stage.
Separation by direct feed
For applications requiring only separation of a pre-mixed dispersion, the direct feed offers the option to feed the mixed liquid stream at a low sheer force directly into the rotor. Inside the rotor, the liquids will be separated into a heavy and a light phase. This principle is used to optimize the separation efficiency. The separated liquids will be discharged without pressure.
Multi-stage processing
Typically for solvent extraction processes in stage-wise equipment such as the centrifugal contactor, you would have multiple contactors in series for extraction, scrubbing, and stripping. The number of stages needed in each section of the process would depend on process design requirements. In the case in Fig. 6, four interconnected stages provide a continuous process in which the first stage is a decanting stage. The next two stages show a counter current extraction. The last stage is a neutralization as a cross stream interconnection.
