Page 40 - Subyek Encyclopedia - Encyclopedia of Separation Science

Sepsci*11*TSK*Venkatachala=BG
I / CENTRIFUGATION 35

tions for continuous-discharge disc centrifuges include
production of baker’s yeast, dewatering of kaolin
clay, titanium dioxide classiRcation, and coal-tar
and tar-sand clariRcation.
Continuous conveyor discharge These centrifuge
types integrate an active mechanical solids discharge
mechanism in an imperforate bowl for the continuous
processing of larger sample volumes. The bowl shape
is tubular, having a length-to-diameter ratio of
1.5}5.2, and may operate in either a horizontal or
vertical conRguration. The vertical conRguration is
generally preferred for reduced or elevated temper-
ature and/or pressure applications owing to fewer
mechanical problems with seals and heat expansion.
The solids-discharge mechanism is most commonly,
a helical screw turning at a slightly slower rate than
the rotor, though pistons or conveyer belts are also
used. Figure 13 illustrates a helical-screw conRg-
uration used for three-phase separations (liquid}
liquid}solid). Solid}liquid and liquid}liquid conRg-
urations with either concurrent or countercurrent
Sow regimes are commercially available. Such mech-
anical discharge units typically operate at lower cen-
trifugal forces (to 5000g) than disc centrifuges. How-
ever, they are capable of very high throughput, up to
1
300 000 L h , and can be used to process feed
streams containing up to 50% solids by volume.
While a limited number of industrial units operate on
materials smaller than 1 m, particles smaller than
about 2 m are usually not collected in such units,
a characteristic that is used to advantage for particle
classiRcation. Continuous conveyer centrifuges are
widely used in the chemical, mining, pharmaceutical,

Figure 11 Disc centrifuge configurations: (A) solids-retaining;
(B) intermittent solids-ejecting; and (C) continuous solids-eject-
ing. (Courtesy of Alfa Laval Separations.)

the bowl. Due to the high discharge velocities result-
ing from the centrifugal pressures, nozzle erosion can
occur. Thus, the materials used for nozzle construc-
tion and the ease of replacement of eroded compo-
nents should be considered. Newer designs discharge
to an internal chamber where the discharge is
pumped out as a product stream. Industrial units
1
3
are available to 200 m h throughput capacity,
elevated temperature (42003C) or pressure (7 bar) Figure 12 Schematic of a cylindrical-bowl centrifuge. (Cour-
capability, and particle removal to 0.1 m. Applica- tesy of Alfa Laval Separations.)