Region Of Light Phase Liquid

Figure 15. Separation is achieved by use of stack discs.

The position of the separating zone is controlled by adjusting the back pressure of the discharged liquids or by means of exchangeable ring dams. Figure 16 illustrates the main features of a disk-bowl centrifuge, which includes a seal ring (1), a bowl (2) with a bottom (13); a central tube (18), the lower part of which has a fixture (16) for disks; a stack of truncated cone disks (17), frequently flanged at the inside and outer diameters to add strength and rigidity; collectors (3 and 4) for the products of separation; and a feed tank (5) with a tube (6). The bowl is mounted to the tube (14) with a guide in the form of a horizontal pin. This arrangement allows the bowl to rotate along with the shaft. The suspension is supplied from the feed tank (5) through the fixed tube (6), to the central tube (18), which rotates together with the bowl and allows the liquid to descend to the bottom. In the lower part of the bowl, the suspension is subjected to centrifugal force and, thus, directed toward the periphery of the bowl. The distance between adjacent disks is controlled by spacers that usually are radial bars welded to the upper surface of each disk. The suspension may enter the stack at its outside diameter or through a series of vertical channels cut through the disks, as described earlier. The suspension is lifted up through vertical channels formed by the holes in the disks and distributed simultaneously under the action of centrifugal force into the spacings between the discs. These spacings are of tight tolerances and can range from 0.3 to 3 mm.

Figure 16. Details of the disc-bowl centrifuge.

Due to a larger diameter, the disk bowl operates at a lower rotational speed than its tubular counterpart. Its effectiveness depends on the shorter path of particle settling. The maximum distance a particle must travel is the thickness of the spacer divided by the cosine of the angle between the disk wall and the axis of rotation. Spacing between disks must be wide enough to accommodate the liquid flow without promoting turbulence and large enough to allow sedimented solids to slide outward to the grit-holding space without interfering with the flow of liquid in the opposite direction.

The disk angle of inclination (usually in the range of 35 to 50°) generally is small to permit the solid particles to slide along the disks and be directed to the solids-holding volume located outside of the stack. Dispersed particles transfer from one layer to the other; therefore, the concentration in the layers and their thickness are variables. The light component from the spacing near central tube (18) falls under the disk; then it flows through the annular gap between tube (18) and the cylindrical end of the dividing disk, where it is ejected through the port (7) into the circular collector (4) and farther via the funnel (9) on being discharged to the receiver. The heavier product is ejected to the bowl wall and raised upward. It enters the space between the outside surface of the dividing disk and the cone cover (2); then passes through the port (8) and is discharged into the collector (3). From there, the product is transferred to the funnel (10).

One variation of the disk-type bowl centrifuge is the nozzle centrifuge, so named because nozzles are arranged on the periphery or on the bottom of the bowl in a circle that is smaller in diameter than the bowl peripheral diameter.

The Figure 17 (A) shows the conceptual operation of such a unit. The design is advantageous because it provides a high solids concentration in the discharge with nozzles of relatively large diameters. As centrifugal force is less in that area than near the periphery of the bowl, the concen- trated solids are ejected through the nozzles under a comparatively low pressure. Figure 17 (B) shows an actual unit being employed as a yeast concentrator. Substances such as yeast and bacteria are very slippery and slide easily; hence, they will not stick or plug up the channels leading to the nozzles.

The fluid vortices and flow patterns characteristic of gas-cyclone operations are equally descriptive of liquid-hydroclones. However, the density differences between particles and liquids are significantly smaller than for gas-solid systems. For example, the density of water is approximately 800 times greater than that of air. This means that high fluid-spinning velocities cannot be employed in hydroclones as excessive pressure drop becomes a limitation. Obviously, the efficiency of hydroclones is low in comparison to gas cyclones. The design features of a hydroclone are illustrated in Figure 18. It consists of an upper short cylindrical section (1) and an elongated conical bottom (2). The suspension is introduced into the cylindrical section (1) through the nozzle (3) tangentially, whence the fluid acquires an intensive rotary motion. The larger particles, under the action of centrifugal force, move toward the walls of the apparatus and concentrate on the outer layers of the rotating flow. Then they move spirally downward along the walls to the nozzle (4), through which the thickened slurry is evacuated. The largest portion of liquid containing small particles (clear liquid) moves in the internal spiral flow upward along the axis of the hydroclone. The cleared liquid is discharged through the nozzle (5) and fixed at the partition (6) and the nozzle (7). The actual flow pattern is more complicated than described because of radial and closed circulating flows. Because of peripheral flow velocities, the liquid column formed at the hydroclone axis has a pressure that is below atmospheric.

Figure 18. Features of a hydroclone

The liquid bulk flow limits the upward flow of small particles from the internal side and has a significant influence on the separating effect. Hydroclones are applied successfully for classification, clarification and thickening of suspensions containing particles from 5 to 150 ^m in size.

The smaller the hydroclone diameter, the greater the centrifugal forces developed and, consequently, the smaller the size particles that can be separated. The following are typical hydroclone diameters used for various general applications: for classification and degritting process streams, D = 300-350 mm; for thickening of suspensions, D = 100 mm; for clarification (where it is necessary to apply powerful centrifugal fields), D = 10-15 mm. In the last case, multiclones are employed. Figures 19 and 20 provide examples of hydroclones used in a degritting operations. Sand is accumulated in a grit chamber for intermittent blowdown. Such an operation could be used off of a cooling tower installation. Good separation of suspensions is achieved, especially in thickening and clarification, when hydroclones have an elongated shape with the slope of the cone equal to approximately 10-15°. At such a cone shape, the path of solid particles is increased as well as the residence time, which thus increases the separating efficiency.

Figure 19. Hydroclone used in degritting water.

Design methodology consists of determining capacity, approximate sizes of particles settled and horsepower requirements for pumping. The flowrate of a

By combining the above expressions, the following formula is obtained for

Using SI units, and from some literature reported values for hydroclones with D between 125 and 600 mm, and a cone angle of 38°, a value for coefficient K is 2.8 X 10"*. The maximum size of particles in the cleared liquid can be estimated from:

where h is the height of the central flow (assumed equal to 1/3 of the cone height), and (j)x is a parameter that accounts for the change in fluid velocity. For

The power required to operate a hydroclone is the horsepower needed for a pump that supplies the capacity V^ with an acceptable head of pressure.

The separating effect is influenced mostly by the ratio dN/dfiN of nozzles, which may be assumed to be in the range of 0.37 to 0.40. The diameter of the inlet nozzle, dN, is assumed to be 0.14 to 0.3 times D, and the diameter of discharge nozzle dd is in the range of 0.2D to 0.167D. The cone angle for hydroclones used as classifiers is typically 20°; for thickeners it is 10- 15°. The operating arrangements used with hydroclones can be parallel and series operations. The example illustrated in Figure

We have already discussed this technology back in Chapter 8 and will onlyadd a few more general comments. Thickening is practiced in order to remove as much water as possible before final dewatering of the sludge. It is usually accomplished by floating the solids to the top of the liquid (floatation) or by allowing the solids to settle to the bottom (gravity thickening). Other method of thickening are by centrifuge, gravity belt, and rotary drum thickening, as already described. These processes offer a low-cost means of reducing the volumetric loading of sludge to subsequent steps. In the floatation thickening process air is injected into the sludge under pressure. The resulting air bubbles attach themselves to sludge solids particles and float them to the surface of an open tank. The sludge forms a layer at the top of the tank which is removed by a skimming mechanism. This process increases the solids concentration of activated sludge from 0.5-1 % to 3-6 %.

Gravity thickening has been widely used on primary sludge for many years because of its simplicity and inexpensiveness. In gravity thickening, sludge is concentrated by the gravity- induced settling and compaction of sludge solids. It is essentially a sedimentation process. Sludge flows into a tank that is similar to the circular clarifiers used in primary and secondary sedimentation.

The solids in the sludge settle to the bottom where a scraping mechanism removes them to a hopper. The type of sludge being thickened has a major effect on performance. The best results can be achieved with primary sludge. Purely primary sludge can be thickened from 1-3% to 10% solids. As the proportion of activated (secondary) sludge increases, the thickness of settled solids decreases. There are various designs for sludge thickeners. Figure 21 illustrates a tray thickener.

Figure 21. Illustrates a tray thickener
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