Centrifugal effects solids

In filtering centrifuges, the solids settle out as in the decanter. However, the liquid flows in the same direction and out through the perforated bowl. As the liquid flows in the same direction as the solids, it increases the rate of deposition. In addition, as the liquid flows through the cake, the frictional effect results in sharply increased stresses on the cake. At low rotation speeds, the stress due to the frictional flow may be larger than the centrifugal body forces (Tiller and Homg 1983). 

When cakes of low moisture content are required, use solid-bowl centrifugation if solids are permitted in the mother liquor centrifugal filtration if effective washing is required. 

Figure 10.15 Simulations and experiments in a Couette cell by Conway et al. (2006). (a) Sketch of simulations in a Couette or a shear cell and (b) sketches of experiments in the Couette cell. The tilted base allows gravity to compensate for centrifugal effects in the rotated system, (c and d) Snapshots from simulations of lower and higher solids fractions, respectively, (the larger particles are twice the diameter of the smaller particles and of equal density). (Continued)...

In general, when sepiarating two liquids, they must be immiscible and have different spiecific gravities before a separation technique-such as oil/water separation-would be effective. In the case of finely dispersed liquids or finely dispersed solids, if the dispersed material is below one micron in particle size, centrifuging should be considered. The use of centrifugal force on the differing densities of the material can facilitate the separation technique. 

The principle of the solid bowl centrifuge is illustrated in Figure 4.19. It is essentially a device for enhancing the rate of sedimentation. The application of high G, however, creates stress in the machine and is equivalent to the application of pressure (cf. pressure filtration). There is therefore a balance to be struck between separating effect and stress. 

Separatory operations may isolate the more hazardous components from tlie diluent or inert. Screening of solids, in effect, separates tlie more hazardous dusts (small particles and fines) from the less liazardous larger pieces of material. Centrifuges, like all liigh-speed equipment, must be securely positioned to provide protection to all persomiel working in the vicinity. 

It is obvious that the nitrated product must be separated from the acid in equil with it (spent acid). If the product and the spent acid form two immiscible liq phases, eg, NG, EGDN, or molten TNT, separation is effected by gravity or centrifuging. If the product and spent acid form a solid and a liq phase, eg PA, NC or PETN, separation is effected by centrifuging (PA NC) or filtration (PETN). If the nitration is carried in the vapor phase (NM), separation is effected by distillation... 

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