Continuous Centrifugation

These separations are similar to those previously discussed in the sense that separations are based on size or density differences. However, unlike batch-mode separation, in continuous centrifugation the sample mixture is introduced continuously to a spinning rotor as the supernatant stream continuously exits. For pelleting separations, the denser product may either accumulate on the rotor wall from where it is recovered after the rotor capacity is reached (semi-batch) or continuously discharged as the rotor spins (continuous mode). Continuous-feed centrifuges may be used for rate, pelleting, filtration, or isopycnic banding separations. They are best suited for applications in which large volumes of sample must be processed, the stream to be recovered is at low concentration, the particle sedimentation coefficient is high (less than about 50 S), or long acceleration/deceleration times are required.

The parameters of primary concern for continuous separations are centrifugal force and flow rate. These parameters must be carefully controlled to provide sufficient time for solid or denser liquids to sediment before being carried out with the supernatant, but not so long as effectively to under utilize the throughput capacity of the rotor. The parameters controlling particle sedimentation are the same in continuous-flow as in batch-mode separations. Therefore, the maximum flow rate that can be utilized in a specific rotor at a given speed may be estimated by using eqn [15] to determine the time required for a given particle to traverse the radial distance from the rotor exit, re, and to the outer rotor wall, rmax. With information on liquid volume within the rotor and assuming laminar flow of liquid from the entry to the exit port(s), the flow rate can then be adjusted to provide this minimum residence time. The calculation of the minimum residence time is simpler if the rotor k-factor and the particle sedimentation coefficient are known, in which case the minimum residence time required for pelleting can be calculated from eqn [18] (i.e. T = k/S, where T is in hours).

Continuous centrifugation is used extensively in industrial applications, where large sample throughput and recovery is more common. However, laboratory-scale continuous-feed applications are also common, particularly in semi-batch mode where the component to be isolated is present at low concentrations. Owing to the variety of continuous-flow configurations that are available, further discussion of this approach is to be found in the section on centrifugal equipment below.

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