Applications of
Filtration
Dr. Hala M. Helal
Examples
Filtration, as a
physical
Filtration is also
Filtration is operation is
important and
used to very important
widely used as
separate in chemistry for
one of the unit
particles and the separation
operations of
fluid in a of materials of
chemical
suspension different
engineering.
chemical
composition.
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Filtration
differs
from
Filtration Filtration
removal of
differs differs
magnetic
from from
contamina
sieving adsorption
nts from
fluids with
magnets
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1. Filter
Classification of filtration equipment
(filters)
Based on external force
Centrifugal
Pressure filters Vacuum filters
filters
Plate and
Rotary drum
frame filter
vacuum filter
press
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Positive pressure filters
• Pressure filters/presses most often operate in a batchwise
manner and use positive pressure above the semi-
permeable separating surface(s) to remove liquid and
retain solids in the form of cakes.
• They are used in a wide range of chemical and process
industries for the separation of suspensions which contain
finer particles that settle slowly and exhibit poor
filterability, and/or suspensions that contain higher solids
contents.
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Plate and frame filter press
Structure: Comprises an
alternating sequence of flat
filter solid plates and
hollow frames to facilitate
cake formations, normally
up to 50 mm thick, mounted
on parallel support bars.
Filter medium is supported
between each plate and
frame. Each unit consists of
two plates and a frame. The
surface of the plate is
corrugated
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Each plate has a tape for filtrate
drainage.
Each plate and frame has an
opening, these openings are parallel
to each other for a continuous
channel for entry of slurry or
washing liquid.
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Slurry passage
Wash passage
filtrate
Washing plate
Frame
Non-Washing plate
Operation: Slurry enters the frame from
the feed channel and passes through
the filter medium onto the surface of
the plate. The solids form a filter cake
and remain in the frame. The thickness
of the cake is half of the frame
thickness because on each side of the
frame filtration occurs. Thus, two filter
cakes are formed, which meet
eventually in the center of the frame. In
general, there will be an optimum
thickness of filter cake for any slurry
depending on the solid content in the
slurry and the resistance of the filter
cake.
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Methods of Filtrate discharge
Open discharge system Closed discharge system
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Cake washing
Simple washing
i. The washing liquid takes the same path as the slurry.
ii. Central washing.
iii. Not an efficient as it passes into the cracks of the cake.
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Thorough washing (every other plate)
To wash the filter cake, the outlets of the
washing plates are closed.
Wash water is pumped into the washing
channel. The water enters through the inlets
onto the surface of the washing plates.
Water passes through the filter cloth and
enters the frame, which contains the cake.
Then water washes the cake, passes
through the filter cloth, and enters the plate
down the surface.
Finally washed water escapes through the
outlet of that plate.
It is more effective as the wash liquid is
admitted to the faces of every other plate.
Reverse flow of wash.
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Thorough washing (every other plate)
With the help of special washing plates, it is possible for the wash water to flow over
the entire surface of the washing plate so that the flow resistance of the cake is
equal to all points. Hence, the entire cake is washed with equal efficiency.
It should be noted that water-wash is efficient only if the frames are full with filter
cake.
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Advantages Precautions
i. Simple in construction i. Materials
ii. Low maintenance.
iii. Central washing. ii. Factors
iv. Wide range of Temperature and
pressure. iii. Uses
v. High filtration rate in small
space
Disadvantages
i. Batch process.
ii. High labour cost
iii. Time consuming
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Vacuum Filters
A category of filters that use vacuum induced
driving forces and semi-permeable media to
facilitate a separation. Most units are capable
of processing a wide range of (coarser particle
size) feed materials in a continuous manner.
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Rotary drum vacuum filter
Structure: consists of two
concentric cylinders. The outer
one is perforated and covered
with filter medium. The space
between the two of them is
divided by septa into
compartments connected to
the central valve.
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Operation:
i. The rotary valve applies vacuum to the
section of the drum immersed in the
slurry.
ii. The insoluble solids accumulated on the
drum filter forming the cake.
iii. The filtrate passes through the filter
medium and through the duct to reach the
reservoir.
iv. As rotation proceed, the cake layer
enters the washing zone for being washed.
v. The cake then enters the drying zone for
dewatering.
vi. Finally, at the discharge zone, the
rotary valve applies positive pressure to
loosen the solid cake from the filter
medium for easily scrapping via
advanced scrapping knife leaving a clean
filter medium surface for new cycle.
Operation:
➢ The knife should allow
scrapping without tearing.
➢ Speed of rotation is adjusted
to have reasonable cake
thickness.
➢ The depth of the drum in the
slurry is kept constant.
➢ Continuous stirring of the
slurry or top feed.
➢ A precoat of filter aid is
necessary.
Advantages
➢ The process is rapid, simple Factors affecting cake thickness
and continuous. 1. Speed of rotation
➢ High capacity 2. Depth of drum immersion
➢ The formed cake is washed 3. Slurry stirring
and dried in the same
process.
Disadvantages Uses
➢ vacuum filters are not in the
following cases.
High viscous slurry
Slurry containing small particles
Rapid filtration process
Cake cracking and impermeable.
Centrifugal filters
Filtering centrifuges use centrifugal forces to perform batch and
continuous cake filtration on either cylindrical, or conical, semi-
permeable surfaces. Displacement washing operations can be
accommodated by most centrifuges in addition to efficient cake
de-liquoring.
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Centrifugal filter
Structure: Centrifugal filter is
basically a basket with
perforated wall rotated by a
shaft connected centrally. The
inner wall is wrapped with
filtering medium.
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Operation:
The feed suspension is to be introduced
when the basket is either stationary or
rotating at a moderate speed. The
rotational speed is often varied through a
cycle with cake washing and de-liquoring
being performed at high speed (~1500
rpm) and cake discharge at a lower speed
(~60 rpm).
When basket is rotated, slurry is forced to
move towards the wall. The filtrate thus
comes out from the perforated wall and
collected in an outer basket. The rate of
filtration depends on the centrifugal force
applied to slurry and the suspended solids.
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2. Dust control &
air filtration
Dust control & air filtration
Process of removing suspended
particles from air stream.
Dust can also be classified into:
a) Respirable dust (< 5m).
b) Inhalable dust (10 m).
c) Thoracic dust.
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Reasons of dust
control
Health
threats Workers
Valuable
Types of dust
Inertial collectors Electrostatic
separators precipitators
Wet Fabric
scrubbers filters
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Inertial separators
The main idea is the difference in
momenta between air and dust
particles.
This type of separators includes:
Settling chambers.
Baffle chambers.
Centrifugal cyclones.
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Settling chambers: Centrifugal cyclones
They depend on gravitational Tangential air inlet.
force. Sudden expansion in Baffle chamber: Decreased cross-sectional
chamber size causes settling Simple baffle chamber. area at bottom leading to
of particles. It is efficient only Tray separator. vortex formation.
for coarse particles. They are Air outlet is central causing
used as pre-cleaners. more efficient separation.
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Wet scrubbers Cyclonic spray scrubber
These act by either:
➢ Gas humidification & particle
agglomeration.
➢ Gas-liquid contact leading to
particle adsorption &/or
entrapment within the liquid
droplet.
➢ In both cases the particle size
increases & hence separation
is achieved.
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Fabric Filters
This class removes up to 99% of
dust.
Air enters the bag house and pass
through fabric bags that act as
filters.
Their high efficiency is due to the
dust cake formed on surfaces of
bags.
The filter must be cleaned
periodically.
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Electrostatic precipitators
The basic principle is to make
dust particles acquire
electrostatic charge by passing
air through sparks.
Then the collector area is
equipped with plate electrodes
for particle collection.
This type is rather expensive and
of less common use in pharmacy.
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Air sterilization
HEPA filters:
Glass-asbestos filter sheets folded over corrugated aluminum
support sheets.
Pressure chamber to direct air.
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Thank you
