Continuous Centrifuges

Conventional batch separations are generally unsuitable for many industrial and certain laboratory-scale separations. Continuous-flow centrifugation offers certain advantages when large quantities of sample must be processed, the stream to be recovered is at low concentration, or long acceleration/deceleration times are required. Such units may be used for rate, pelleting, filtration, or isopycnic banding separations. In continuous-flow centrifugation, the sample mixture is introduced continuously to a spinning rotor as the supernatant stream continuously exits. The denser product may either accumulate on the rotor wall, from where it is recovered by stopping the run when the rotor capacity is reached (semi-batch mode), or continuously discharged during the run (continuous mode).

In + I Out
Zonal Rotor

1. Gradient loaded, light end first, with rotor at rest

2. Sampfe solution layered on top of gradient

3. Rotor accelerated. Layers reoriented under centrifugal force

Supernatant Layer

4. Layers vertical. Particles seperated with rotor at speed

5. Rotor decelerated. Layers reoriented

6. Static unloading. Contents disptaced with air pressure, heavy end first

Figure 8 Static loading and unloading of a zonal rotor with a reorienting gradient core. (Courtesy of Beckman Instruments, Inc.)

Zonal Rotor
Figure 9 Dynamic loading and unloading of a zonal rotor. (Courtesy of Beckman Instruments, Inc.)

The rotors previously described can be, and often are, adapted for continuous-flow separations. However, the following discussion focuses on rotors that are designed specifically for continuous operation, particularly for industrial applications such as those depicted in Figure 10.

Disc centrifuges Disc centrifuges operate on the principle of differential sedimentation and are used for two-phase (liquid-solid or liquid-liquid) and three-phase (liquid-liquid-solid) separations. These are highly efficient units with some industrial-scale units generating forces of 10 000gand pelleting of particles as small as 0.1 |im. Disc centrifuges are essentially a rotating bowl equipped with an internal set of conical settling plates or discs mounted at an angle to the axis of rotation (typically 30-40°). The discs serve to decrease the sedimentation pathlength and increase the sedimentation surface area, i.e. capacity factor. Denser materials sediment onto and slide across the plate surfaces before accumulating on the bowl wall (Figure 11) as the clarified supernatant continuously exits. In addition to the parameters of centrifugal force and flow rate, the capacity and performance of disc centrifuges are also dependent on the number, spacing and diameter of the plates. Sample mixtures may be introduced to either the interior or outside of the disc stack, depending on the nature and concentration of solids, with most units configured for liquid-liquid or liquid-liquid-solid mixtures being centre fed.

Three variations of disc centrifuges, as distinguished by their solids-handling capability, are commonly used: solids-retaining, intermittent solids-ejecting and continuous solids-ejecting (Figure 11). Solids-retaining designs (Figure 11A) are appropriate for liquid-solid or liquid-liquid separations where the solids content is less than about 1% by volume. For liquid-solid separations, the solids that accumulate on the bowl wall are recovered when the rotor capacity is reached and the centrifuge is stopped. Removable baskets are incorporated into some designs to facilitate solids removal. Recovery of two liquid streams can be achieved by positioning exit

Liquid Liquid Chromatography Partition
Figure 10 Major industrial applications for continuous centrifuges. (Courtesy of Alfa Laval Separations.)

ports at different radial distances as dictated by the relative concentration of the liquids. Commercial units are available with liquid throughput capacities of 60 m3 h_1 and holding capacities of 30 L. A variation on the solids-retaining disc centrifuge is the cylindrical-bowl design shown in Figure 12, which incorporates a series of concentric cylindrical retainers for processing liquid-solid mixtures. Unlike the disc centrifuge, in which the feed stream is split and makes a single pass through the disc stack, in the cylindrical-bowl design the liquid stream is routed through each chamber in succession, resulting in a longer residence time, more efficient recovery, and generally greater capacity (to 70 L). Applications of solids-retaining centrifuges of the stacked-disc or cylindrical-bowl design include separation of cream from milk, organic waste from water, purification of lubricating oils, or removal of water and solids from jet fuel.

Solids-ejecting stacked-disc centrifuges (Figure 11B) are more suitable for processing samples with solids contents to about 15% by volume. These units operate similarly to the solids-retaining design, only solids or sludge that accumulate on the bowl wall are intermittently discharged through a hydraulically activated, peripheral opening. Laboratory models to 18 cm diameter and industrial units to 60 cm are available, with the latter capable of throughputs in excess of 100m3h~\ Applications for these units include catalyst recovery, clarification of paints and varnishes, treatment of radioactive waste water, and copper extraction.

Continuous solids-discharge disc centrifuges, also called nozzle bowl separators (Figure 11C), are used to process samples with solids contents ranging from 5 to 30% by volume. In this design, solids are continuously discharged via backward-facing orifices, i.e. nozzles, closely spaced around the outer periphery of

Imperforated Centrifuge

Figure 11 Disc centrifuge configurations: (A) solids-retaining; (B) intermittent solids-ejecting; and (C) continuous solids-ejecting. (Courtesy of Alfa Laval Separations.)

tions for continuous-discharge disc centrifuges include production of baker's yeast, dewatering of kaolin clay, titanium dioxide classification, and coal-tar and tar-sand clarification.

Continuous conveyor discharge These centrifuge types integrate an active mechanical solids discharge mechanism in an imperforate bowl for the continuous processing of larger sample volumes. The bowl shape is tubular, having a length-to-diameter ratio of 1.5-5.2, and may operate in either a horizontal or vertical configuration. The vertical configuration is generally preferred for reduced or elevated temperature and/or pressure applications owing to fewer mechanical problems with seals and heat expansion. The solids-discharge mechanism is most commonly, a helical screw turning at a slightly slower rate than the rotor, though pistons or conveyer belts are also used. Figure 13 illustrates a helical-screw configuration used for three-phase separations (liquidliquid-solid). Solid-liquid and liquid-liquid configurations with either concurrent or countercurrent flow regimes are commercially available. Such mechanical discharge units typically operate at lower centrifugal forces (to 5000g) than disc centrifuges. However, they are capable of very high throughput, up to 300 000 Lh"1, and can be used to process feed streams containing up to 50% solids by volume. While a limited number of industrial units operate on materials smaller than 1 |im, particles smaller than about 2 |im are usually not collected in such units, a characteristic that is used to advantage for particle classification. Continuous conveyer centrifuges are widely used in the chemical, mining, pharmaceutical,

Figure 11 Disc centrifuge configurations: (A) solids-retaining; (B) intermittent solids-ejecting; and (C) continuous solids-ejecting. (Courtesy of Alfa Laval Separations.)

the bowl. Due to the high discharge velocities resulting from the centrifugal pressures, nozzle erosion can occur. Thus, the materials used for nozzle construction and the ease of replacement of eroded components should be considered. Newer designs discharge to an internal chamber where the discharge is pumped out as a product stream. Industrial units are available to 200m3h_1 throughput capacity, elevated temperature (<200°C) or pressure (7bar) capability, and particle removal to 0.1 |im. Applica

Imperforated Centrifuge
Figure 12 Schematic of a cylindrical-bowl centrifuge. (Courtesy of Alfa Laval Separations.)
Solid Bowl Centrifuge Diagram

Liquids discharge Solids discharge

Figure 13 Schematic of a horizontal continuous-conveyer centrifuge. (Courtesy of Alfa Laval Separations.)

Liquids discharge Solids discharge

Figure 13 Schematic of a horizontal continuous-conveyer centrifuge. (Courtesy of Alfa Laval Separations.)

biotechnology and food sectors for clarifying, classifying, dewatering and thickening applications.

Tubular centrifuges These centrifuges utilize a vertically mounted, imperforate cylindrical-bowl design to process feed streams with a low solids content. Liquid(s) is discharged continuously and solids are manually recovered after the rotor capacity is reach ed. One configuration, designed for recovery of two immiscible liquids and a solid product, is shown in Figure 14. Other configurations for processing solid-liquid or liquid-liquid mixtures are also widely used. Industrial models are available with diameters up to 1.8 m, holding capacities up to 12 kg, throughput rates of 250m3h_1, and forces ranging up to 20 000g. Laboratory models are available with

Continuous Tubular Centrifuges

diameters of 4.5 cm, throughput rates of 150 L h"1, and centrifugal forces ranging up to 62 000g. Because of their high speed and short settling path, tubular centrifuges are well suited for the pelleting of ultrafine particles, liquid clarification, and separation of difficult-to-separate immiscible liquids. In addition to the standard electric motor used for most laboratory centrifuges, laboratory-scale tubular centrifuges are available with turbine drives. Tubular centrifuges were refined for the separation of penicillin during World War II but since then have largely been replaced by disc centrifuges because of their limited holding capacity. However, they are still widely used for applications that involve the efficient recovery of high value products at high purity, especially in the pharmaceutical and chemical industries. Typical applications include recovery of Escherichia coli cells and flu viruses, removal of colloidal carbon and moisture from transformer oils, removal of small particles from lubricating oils, blood fractionation, and de-inking.

Continuous zonal rotors Zonal rotors are often used for smaller scale, semi-batch separations. Operation is similar to that previously described for batch separation only a larger diameter core with a different flow pattern is inserted as illustrated in Figure 15. Continuous-feed separations in zonal centrifuges are best suited for low concentration, high volume samples. Such separations may be conducted with a homogeneous medium for sample pelleting, or with a density gradient for materials that may be adversely affected by pelleting (e.g. viruses that may lose their activity) or if simultaneous isolation of two or more materials is desired. Applications include purification of viruses from tissue-culture media, harvesting bacteria, or separating fine clay particles in water pollution studies.

Elutriation rotors Another type of laboratory-scale continuous-flow centrifugation is elutriation or counterstreaming, used to separate particles with differing sedimentation rates (rate separation). A schematic of the elutriation process is shown in Figure 16. Conical or funnel-shaped rotors are used with the small end positioned farthest from the axis of rotation. The rotor is initially filled with a buffer solution followed by the sample mixture, introduced at a constant rate to the small end of the spinning rotor, where particles experience the opposing forces of the centrifugal field and the flowing medium. Initially, the frictional force of the carrier medium is greater than the centrifugal force and all particles are swept inward by the flowing carrier. However, as the entrained particles migrate toward the large end of

Zonal Continuous Rotor Diagram
1. Fiow during rotor loading at 2000 rpm

t 11

t 11

2. Sample fiow at operating speed

3. Unloading after a banding or tsopycnic experiment

Figure 15 Flow regimes in a continuous-flow zonal rotor. (Courtesy of Beckman Instruments, Inc.)

3. Unloading after a banding or tsopycnic experiment

Figure 15 Flow regimes in a continuous-flow zonal rotor. (Courtesy of Beckman Instruments, Inc.)

the chamber, the linear velocity of the carrier decreases as the cross-sectional area of the rotor increases. Due to the greater sedimentation rates for larger particles in a centrifugal force field, smaller particles continue to migrate toward the centre of the rotor while larger particles remain suspended or move more slowly, resulting in particle classification. Such separations are semi-batch since, as the concentration of larger particles in the rotor increases to capacity, sample feed must be stopped so that these particles may be eluted with a higher velocity rinse solution. Elutriation rotors typically operate at lower centrifugal forces (10 000g) with throughputs to 400mLmin"1. A common application is the isolation of specific cell types.

Elutriation
Figure 16 The elutriation process. (Courtesy of Beckman Instruments, Inc.)
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