The Tubularbowl Centrifuge

This is a centrifuge to consider using for particle size ranges of 0.1 to 200 /am and up to 10% solids in the in-going slurry. Figure 10.16a shows an arrangement used in a Sharpies Super-Centrifuge. The main component of the centrifuge is a cylindrical bowl (or rotor) (A in Fig. 10.16), which may be of a variable design de-

Feed

-Porous lining

- Filter cake

- Perforated wall o

Fig. JO.15. Diagram of basket centrifuge.

pending on application, suspended by a flexible shaft (B). driven by an overhead motor or air turbine (C). The inlet to the bowl is via a nozzle attached to the bottom bearing (D). The feed which may consist of solids and light and heavy liquid phases is introduced by the nozzle (E). During operation solids sediment on the bowl wall while the liquids separate into the heavy phase in zone (G) and the light phase in the central zone (H). The two liquid phases are kept separate in their exit from the bowl by an adjustable ring, with the heavy phase flowing over the lip of the ring. Rings of various sizes may be fitted for the separation of liquids of various relative densities. Thus the centrifuge may be altered to use for:

(a) Light-phase/heavy-phase liquid separation.

(b) Solids/light-liquid phase/heavy-liquid phase separation.

(c) Solids/liquid separation (using a different rotor, Fig. 10.16b).

The Sharpies laboratory centrifuge with a bowl radius of approximately 2.25 cm can be operated with an air turbine at 50,000 rpm to produce a centrifugal force of approximately 62,000 g, but has a bowl capacity of only 200 cm3 with a throughput of 6 to 25 dm3 h The largest size rotor is the Sharpies AS 26, which has a bowl radius of 5.5 cm and a capacity of 9 dm3, a solids capacity of 5 dm3 and a throughput of 390 to 2400 dm3 h"1.

The advantages of this design of centrifuge are the high centrifugal force, good dewatering and ease of cleaning. The disadvantages are limited solids capacity, difficulties in the recovery of collected solids, gradual loss in efficiency as the bowl fills, solids being dislodged from the walls as the bowl is slowing down and foaming. Plastic liners can be used in the bowls to help improve batch cycle time. Alternatively a spare bowl can be changed over in about 5 minutes.

THE SOLID-BOWL SCROLL CENTRIFUGE (DECANTER CENTRIFUGE)

This type of centrifuge is used for continuous handling of fermentation broths, cell lysates and coarse materials such as sewage sludge (Fig. 10.17). The slurry is fed through the spindle of an archimedean screw within the horizontal rotating solids bowl. Typically the speed differential between the bowl and the screw is in the range 0.5 to 100 rpm (Coulson and Richardson, 1991). The solids settling on the walls of the bowl are scraped to the conical end of the bowl. The slope of the cone helps to remove excess liquid from the solids before discharge. The liquid phase is discharged from the opposite end of the bowl. The speed of this type of centrifuge is limited to around 5000 rpm in larger models because of the lack of balance within the bowl, with smaller models having bowl speeds of up to 10000 rpm. Bowl diameters are normally between 0.2 and 1.5 metres, with the length being up to five times the diameter. Feed rates range from around 200 dm3 h 1 to 200 m3 h~1 depending on scale of operation and material being processed. A number of variants on the basic design are available:

(a) Cake washing facilities (screen bowl decanters).

(b) Vertical bowl decanters.

(c) Facility for in-place cleaning.

(d) Bio-hazard containment features; steam sterilization in-situ, two or three stage mechanical seals, control of aerosols, containment casings and the use of high pressure sterile gas in seals to prevent the release of micro-organisms.

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