TT

Figure 4. Disc stack centrifuge. The solid bowl disc centrifuge is a solid bowl version liquid-liquid separator operating at 5,00010,000 g with some models as high as 15,000 g. They can handle a wide range of rates up to 2,400 L/min.
Figure 5. Disc stack centrifuge: nozzle version. Same as Figure 4 except that the solids are continuously discharged as a pumpable slurry. Rates as high as 5,000 L/min are possible.
Figure 6. Disc stack centrifuge: solids ejecting version. Used where the solids volume is low, the bowl opens up intermittently to dump the solids as pumpable slurry. Maximum rates 40 L/min solids, 2,500 L/min liquid; g force 5,000-10,000 g.

Figure 7. Decanter centrifuges. Solid bowl decanter centrifuges develop 1,000-3,000 g. They are used where large volumes of solids are present and the solid phase must be as dry as possible. Rates vary from 8 to 4,000 L/min. Very high speed decanters from 5-10,000 g are available but rates are lower, 10-200 L/min.

discharge discharge

Figure 7. Decanter centrifuges. Solid bowl decanter centrifuges develop 1,000-3,000 g. They are used where large volumes of solids are present and the solid phase must be as dry as possible. Rates vary from 8 to 4,000 L/min. Very high speed decanters from 5-10,000 g are available but rates are lower, 10-200 L/min.

Light phase Heavy phase

Feed

Figure 8. Tubular centrifuges. Manual disassembly to remove solids limits these to applications with little or no solids in the feed. As liquid-liquid separators, they develop much higher g force than most other centrifuges; maximum feed rates are 40-80 L/ min.

fluid in the centrifuge directly affects its capacity. Physically, the separation follows Stokes' law for the settling velocity of a particle in a fluid, except that the acceleration of the rotational field is substituted for the earth's gravitational field.

STOKES' LAW

Vg lfyT

where Vg = settling velocity, w = rotational speed, r = radial position of the particle, d = diameter of particle, A<p = difference in the specific gravity of the liquid and solid, and n = viscosity of fluid. Stokes' law is used to design separation systems because the faster the settling velocity of a particle, the greater the chance of capture will be. Generally the temperature and the particle size are, to some extent, variables in a system.

For example, in rendering the fat from animal tissues, the amount of fat separated from the tissue increases with increasing temperature. Above 45°C, the residual tissue changes character, and its value as a meat additive drops. For edible rendering, the separation is made at 45°C, for the maximum fat yield with high-value solids. Inedible rendering is done at 90-95°C where the fat yield is at a maximum, because the value of the inedible solids is quite low.

Stokes' law can be generalized as follows. Separation is improved if

1. The difference in specific gravity between the phases is large.

2. The particle size is large.

3. The viscosity is low.

4. The centrifuge speed is high.

5. The feed rate to the centrifuge is low.

Scale-up to estimate between different types of centrifuges is difficult, but within a class (eg, all disk centrifuges) a reasonably accurate scale-up can be developed. Centrifuges are usually scaled up based on g times volume or g times area. More detailed scale-up is usually left to centrifuge specialists.

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