Page 33 - Subyek Encyclopedia - Encyclopedia of Separation Science

28 I / CENTRIFUGATION/ Derivatization

The parameters of primary concern for continuous ual moisture contents lower than generally provided
separations are centrifugal force and Sow rate. These by Rlter presses or vacuum Rlters. Most centrifugal
parameters must be carefully controlled to provide Rltration applications are typically conducted in con-
sufRcient time for solid or denser liquids to sedi- tinuous or semi-batch mode in which the liquids
ment before being carried out with the supernatant, passing the Rlter are continuously discharged and the
but not so long as effectively to under utilize the Rlter cake is continuously discharged or recovered
throughput capacity of the rotor. The parameters post run. Perhaps the most widely used example of
controlling particle sedimentation are the same in centrifugal Rltration is the spin cycle in domestic
continuous-Sow as in batch-mode separations. washing machines.
Therefore, the maximum Sow rate that can be util- Centrifugal Rltration is a complex process that is
ized in a speciRc rotor at a given speed may be dependent on a number of parameters including
estimated by using eqn [15] to determine the time liquid viscosity, cake thickness, centrifugal force,
required for a given particle to traverse the radial screen area and, importantly, the size and packing
distance from the rotor exit, r e , and to the outer rotor characteristics of the particles themselves. Centrifugal
wall, r max . With information on liquid volume within Rltration may be conducted in batch, semi-batch or
the rotor and assuming laminar Sow of liquid from continuous mode. While traditional industrial ap-
the entry to the exit port(s), the Sow rate can then be plications commonly use centrifugal Rltration to re-
adjusted to provide this minimum residence time. The cover solid materials with reduced moisture contents,
calculation of the minimum residence time is simpler many laboratory-scale spin Rlters, particularly in
if the rotor k-factor and the particle sedimentation a test-tube conRguration, are available. This tech-
coefRcient are known, in which case the min- nique is generally not amenable to broad generaliz-
imum residence time required for pelleting can be ations and is, therefore, best approached on a case-
calculated from eqn [18] (i.e. T"k/S, where T is in by-case basis.
hours).
Continuous centrifugation is used extensively in Centrifugal Equipment
industrial applications, where large sample through-
put and recovery is more common. However, laborat- Centrifuges and rotors are commercially available in
ory-scale continuous-feed applications are also com- literally hundreds of shapes, sizes and conRgurations.
mon, particularly in semi-batch mode where the com- They range from small laboratory-scale units equip-
ponent to be isolated is present at low concentrations. ped with capillary tubes, operating at speeds in excess
Owing to the variety of continuous-Sow conRgura- of 100 000 rpm or forces approaching 1 000 000g to
tions that are available, further discussion of this large industrial decanters that may continuously pro-
1
approach is to be found in the section on centrifugal cess up to 300 000 L h . The primary rotor or cen-
equipment below. trifuge selection criteria must centre on the objective
for conducting the separation. Parameters such as
Filtration batch versus continuous; required centrifugal force
Filtration is a mechanical means of separating solids and purity; throughput; the number of components to
from a liquid suspension via a porous medium or be recovered; sample toxicity/corrosiveness; time;
screen that permits the liquid to pass while retaining cost; available space; noise tolerances, and so forth
the solids. Similar to conventional Rltration, achieved must be considered when selecting the appropriate
via a differential pressure across a Rlter, centrifu- centrifuge/rotor for a given application.
gal Rltration is driven by the pressure exerted by Early rotors were often manufactured of steel or
a liquid medium within a centrifugal force Reld. Op- brass, but are now more commonly constructed of
posing the centrifugal pressure is the combined resist- aluminium and titanium. Newer carbon composites
ance of the porous medium and Rlter cake. Centrifu- are also gaining acceptance, with plastics commonly
gal Rlters are commonly used to remove or recover used for small-scale applications and stainless steel for
coarse and crystalline solids from a Suid slurry, often industrial-scale units. Though somewhat more expen-
followed by a rinse cycle to purify the solids and sive, titanium is particularly suitable as it has both
remove the residual mother liquor. In this technique, a higher strength-to-density ratio and a high resistance
a sample slurry is fed to the rotor with the centrifugal to corrosion and erosion. Selected properties for steel,
pressure forcing the carrier liquid through a cylin- aluminium and titanium are shown in Table 2.
drical screen or other permeable medium positioned Centrifuge bottles and tubes are also constructed
around the outer wall to retain the solids or Tlter from a variety of materials. Early tubes were usually
cake. The Rlter cake may be dried by shutting off glass or stainless steel, but these have largely
the slurry feed and spinning the solids to attain resid- been replaced by plastics, e.g. polycarbonate, nylon,