Principles, theory and
operation
Filtration
Filtration Process
Introduction:
•Cake filtration is widely used in industry to
separate solid particles from suspension in
liquid.
•Cake filtration is a common application for
the flow through packed beds of particles.
•Filtration process involves the build up of a
bed or ‘cake’ of particles on a porous surface
known as the filter medium, which
commonly takes the form of a woven fabric.
The pore size of the medium is less than the
size of the particles to be filtered.
Diagram of simple
filtration: oversize
particles in the feed
cannot pass through the
lattice structure of the
filter, while fluid and small
particles pass through,
becoming filtrate
•In general, filtration process can be analyzed
in terms of the flow of fluid through a packed
bed of particles, the depth of which is
increasing with time. The voidage of cake
may be changed with time “ compressible
cake” but we can assume, in some cases, the
cake voidage is constant “ incompressible
cake”.
Principle of filtration
Summary & Definitions
•Filtration is a separation of solid particles
from liquid with the aid of a semi-permeable
medium, which retains the solids and allows
the liquid to pass through.
•Classification of equipment: gravity, pressure
and vacuum filters.
•Filter cake: a bed of solid particles.
•Filtrate: liquids which pass through the filter
medium.
Cake
Compressible Incompressible
Porosity varied Porosity constant
Filter media Septum
a) it must retain solids
b) it must not plug
c) it must be strong enough chemically and
physically to withstand different operating
conditions.
d) it must allow the cake to be removed cleanly
and completely.
e) it must be cheap.
Typical filter media
(A) plain, square weave,
(B) twill weave,
(C)plain, reverse Dutch,
(D)Double layer weave,
(E) plain weave in metal media,
(F) twilled weave in metal media,
(G)twilled weave in metal media
Filter aids
Substances used to increase the porosity of
solid cake in order to permit the liquid to
pass at a reasonable rate. Filter aids such as
diatomaceous earth, asbestos, purified wood
cellulose or other inert porous solids.
Note: The support of media affects the way
of the cake growth.
factors to be considered when selecting
equipment and operating
•The properties of the fluid, particularly its viscosity,
density, and corrosive properties.
•The nature of the solid—its particle size and shape, size
distribution, and packing characteristics.
•The concentration of solids in suspension.
•The quantity of material to be handled, and its value.
•Whether the valuable product is the solid, the fluid, or
both.
•Whether it is necessary to wash the filtered solids.
•Whether the feed liquor may be heated.
Theory of filtration
•Filtration is similar to the flow of a fluid
through a granular bed. The only difference is
that the thickness of cake is growing with
time.
