Chemical Engineering July 2016 - 53

Table 1. G-forces developed by various Types of cenTrifuGes
[from ref (1)]
Single-chamber bowl centrifuge
Decanter centrifuge
Multichamber bowl centrifuge
Disk stack centrifuge
Laboratory bottle centrifuge
Tubular centrifuge
Ultracentrifuge
particle to be removed. However,
not only is the particle size important,
but also its shape. For example,
flat, elongated particles tend to settle
more slowly than spherical ones. In
addition, solids tend to settle more
slowly with increasing liquid viscosity,
requiring a larger centrifuge or
higher rotational speed to achieve
the desired capacity.
The undissolved solids content of
the feed must also be considered.
Values can be expressed using either
a weight or volume basis. Percent
by volume is usually used, but not
always, so the basis must be made
clear. In addition to solids content,
the nature of the settled solids must
be understood to allow the best type
of centrifuge to be selected. For example,
solids may pack into a hard,
firm cake, or the packed solids may
be soft and loose.
Additional considerations
Solids discharge may be manual
or automatic, with the latter either
intermittent or continuous, and the
desired method must be specified.
In addition, do the solids need to
be washed, and if so, how much?
If the solids are the product, then
displacement of residual liquid using
a wash solvent (usually water) may
be needed to remove contaminants.
On the other hand, if the liquid is the
valuable phase, washing may be
needed to maximize liquid recovery
from the solid.
Other considerations include the
following: Is the feed flammable,
toxic or corrosive? Are there any
unusual operating conditions, such
as elevated temperature or pressure?
Is aseptic operation needed,
as in some biotechnology applications?
Are the feed solids abrasive
or fibrous? Is there a tendency for
solids to crystallize? Does significant
foaming occur? Suitable centrifuge
designs are available to handle all of
these situations.
600 -1,200
2,000-5,000
5,000-9,000
5,000-15,000
2,000-20,000
12,000-62,000
20,000-1,000,000
Small-scale testing
Much can be learned by using a
simple laboratory centrifuge, such
as the one shown in Figure 1. This
device consists of an even number
(typically four to eight) of tubes that
rotate around the central axis. In
most designs, glass or plastic tubes
are placed inside of stainless steel
holders that are hinged to the rotor.
When at rest, the tubes are vertical;
upon rotation, centrifugal force orients
them horizontally.
The tubes are filled with the starting
material, and the rotational speed is set
so that the applied force is 1,000 times
that of gravity, or 1,000G (see the explanation
below). The time required to
obtain clear liquid or fully settled solids
is then determined; the typical time is
between 30 s and 20 min.
Often the tubes are graduated,
so that the volume percent solids
can be read directly. One can assess
the nature of the settled solids
(for instance, firm and hard-packed,
or soft and loose) simply by poking
them with a glass rod. Information
obtained from this simple test can
provide important insight into the
size and type of centrifuge that may
be suitable for the application.
G-force and sigma factor
The centrifugal force developed by a
centrifuge is expressed as a multiple
of the force of gravity. This force,
known as the G-force, is proportional
to the distance from the axis
of rotation and the square of the rotational
speed:
(1)
Where:
G is the G-force
n is the rotational speed, revolutions
per second (rev/s)
r is the distance from the axis of rotation,
cm
g is the acceleration due to gravity,
which has a conventional standard
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