Page 316 - Biosystems Engineering

Bioseparation Pr ocesses 293

Slide
head
Fixed Frame
head Filter
media Plate
Filtrate

Feed
slurry
Side bar

Fixed Slide
head plate head plate

FIGURE 9.5 Plate and frame fi lter.

Industrial filtrations range from simple straining to highly complex
separation. The solid particles may be coarse or fine, rigid or plastic,
round or elongated, separate individuals or aggregates. The feed sus-
pension may carry a heavy load of solids or almost none. The fluid
may be very hot or very cold and can operate in a vacuum or under
high pressure. Sometimes, the fluid is the valuable phase, the solid, or
sometimes both. As a result, many filters have been developed to
meet specific problems. Some important pressure filters are the plate-
and frame-filter presses (Fig. 9.5), the Kelly filter, the Sweetland filter,
and the Vallez filter. There are mainly two types of suction—the disc
filter and the drum filter. Pressure filters have flexibility in operation
and can give a very clear filtrate. The suction filter, on the other hand,
gives a muddy filtrate and is also limited by the high vacuum that
can be created. The major advantage of a suction filter is that it is
continuous in operation.

Theory of Filtration
Filtration is primarily a problem of fluid flow. The generally accepted
view is that under conditions of filtration, the flow of filtrate through
the porous filter cake and filter medium is laminar. Therefore, Poiseuille’s
law holds well, the filtration rate is directly proportional to the applied
pressure. Assuming laminar flow through filter channels, the filtration
rate equation can be written as

U = dV = Pg c (9.8)
Adθ μ( R + R )
c f