Centrifugation force

Centrifugal separators make use of the common principle that an object whirled about an axis at a constant radial distance from the point is acted on by a force. Use of centrifugal forces increases the force acting on the particles. Particles that do not settle readily in gravity settlers often can be separated from fluids by centrifugal force. 

The simplest type of centrifugal device is the cyclone separator (Fig. 3.4), which consists of a vertical cylinder with a conical bottom. The centrifugal force is generated by the fluid motion. The mixture enters in a tangential inlet near the top, and the rotating motion so created develops centrifugal force which throws the particles radially toward the wall. 

Figure 3.4 A cyclone generates centrifugal force by the fluid motion.

Figure 3.5 shows centrifuges in which a cylindrical bowl is rotated to produce the centrifugal force. In Fig. 3.5a, the cylindrical bowl is shown rotating with a feed consisting of a solid-liquid mixture admitted at the center. The feed is immediately thrown outward toward the walls of the container. The particles settle horizontally... 

Figure 3 A centrifuge uses a rotating bowl to produce centrifugal force.

Cyclones. Cyclones are also primarily used as prefilters. These also were discussed in Chap. 3 and illustrated in Fig. 3.4. The particle-laden gas enters tangentially and spins downward and inward, ultimately leaving the top of the unit. Particles are thrown radially outward to the wall by the centrifugal force and leave at the bottom. 

Hydrocylones have become common on offshore facilities and rely on centrifugal force to separate light oil particles from the heavier water phase. As the inlet stream is centrifuged oil particles move to the centre of the cyclone, coalesce and are drawn off upwards, while the heavier water is taken out at the bottom. 

The general analysis, while not difficult, is complicated however, the limiting case of the very elongated, essentially cylindrical drop is not hard to treat. Consider a section of the elongated cylinder of volume V (Fig. II-18h). The centrifugal force on a volume element is u rAp, where w is the speed of revolution and Ap the difference in density. The potential energy at distance r from the axis of revolution is then w r Apfl, and the total potential energy for the... 

Illustration showing separation by equilibrium-density-gradient centrifugation. The homogeneous mixture in (a) separates into three bands (b) after applying centrifugal force. 

Secondly, the centrifugal force on the ion as its path is deflected by a magnetic field is equal to the force exerted by the field on a moving charge (Equation 24.2). 

Other Centrifugal Collectors. Cyclones and modified centrifugal collectors are often used to remove entrained Hquids from a gas stream. Cyclones for this purpose have been described (167—169). The rotary stream dust separator (170,171), a newer dry centrifugal collector with improved collection efficiency on particles down to 1—2 pm, is considered more expensive and hence has been found less attractive than cyclones unless improved collection in the 2—10-pm particle range is a necessity. A number of inertial centrifugal force devices as well as some others termed dynamic collectors have been described in the Hterature (170). 

The first differential centrifugal extractor to be used in industry was the PodbieHiiak extractor which was introduced in the 1950s (209,210) and can be regarded as a perforated-plate column wrapped around a rotor shaft. Rotation creates a centrifugal force which results in a great reduction in the equivalent height and contact time that would be needed in a conventional perforated-plate column. 

As observed from Figure 27, the cake removal by fluid shear is also aided by centrifugal force. Other arrangements include stationary filtration media and rotating disks to create the shear effects, and rotating cylindrical elements it has also been shown how such filters can be used for cake washing. 

The scale-up of filtration centrifuges is usually done on an area basis, based on small-scale tests. Buchner funnel-type tests are not of much value here because the driving force for filtration is not only due to the static head but also due to the centrifugal forces on the Hquid in the cake. A test procedure has been described with a specially designed filter beaker to measure the intrinsic permeabiHty of the cake (7). The best test is, of course, with a small-scale model, using the actual suspension. Many manufacturers offer small laboratory models for such tests. The scale-up is most reHable if the basket diameter does not increase by a factor of more than 2.5 from the small scale. 

Fig. 5. Separation in Latham bowl (a) whole blood is pumped down the feed tube and enters bowl at bottom (b) centrifugal force spins denser cellular components outside, leaving plasma or platelet-rich plasma (PRP) in inner band (c) when bowl is full, plasma flows out effluent tube, followed by platelets and then leukocytes, until bowl is almost completely full of ted cells (d) after draw is completed, bowl stops spinning and uncoUected components are...

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