Filtration is a process used to separate solids from liquids or gases by passing them through a porous medium. The main objectives of filtration are to produce clean drinking water, effluent, air, and prevent furnace fouling. Key types of filters are cake filters, clarifying filters, and cross-flow filters. Cake filters separate large amounts of solids, clarifying filters remove small amounts of solids, and cross-flow filters use high liquid velocity to prevent solids buildup. Selection factors for filters include slurry properties, throughput needs, and whether the solid or liquid product is more valuable. Common filtration equipment includes rotary drum filters, plate and frame presses, and bag filters.
2. Definition:
“Filtration is the solid liquid separation
where the liquid passes through the
porous medium to remove the fine
suspended solids.”
3. Objectives
• The main objective of filtration is to produce
high quality drinking water (surface water) or
high-quality effluent (wastewater)
• Filtration also cleans up air streams or other
gas streams.
• Furnaces use filtration to prevent the furnace
elements from fouling with particulates.
• Pneumatic conveying systems often employ
filtration to stop or slow the flow of material
that is transported, through the use of a bag
house
5. Uses
• Filtration is used for the purification of fluids: for
instance separating dust from the atmosphere to
clean ambient air.
• Filtration, as a physical operation is very important in
chemistry for the separation of materials of different
chemical composition in solution (or solids which can
be dissolved) by first using a reagent to precipitate
one of the materials and then use a filter to separate
the solid from the other materials.
• Filtration is also important and widely used as one of
the unit operations of chemical engineering.
6. Filter Media
The following points should be considered
while selecting the filter media:
– Ability to build the solid.
– Minimum resistance to flow the filtrate.
– Resistance to chemical attack.
– Minimum cost.
– Long life.
– It must permit the cake formed to be discharged
cleanly and completely.
7.
8. Factors to be considered when selecting
filtration equipment
1. The nature of the slurry and the cake formed.
2. The solids concentration in the feed.
3. The throughput required.
4. The nature and physical properties of the
liquid: viscosity, flammability, toxicity,
corrosiveness.
5. Whether cake washing is required.
6. The cake dryness required.
7. Whether the valuable product is the solid or
the liquid, or both.
9. Types Of Filter
Filters are divided into three main groups:
– Cake filters,
– Clarifying filters
– Cross flow filters.
10.
11. Cake Filters
• It is separate relatively large amount of solids
as a cake of crystals or sludge.
• Often they include provisions for washing the
cake and for removing some of the liquid from
the solids before discharge.
• At the start of filtration in a cake filter, some
solid particles enter the pores of the medium
and are immobilized, but soon others begin to
collect on the septum surface.
12. Cake Filtration
• In cake filtration the liquid passes through two
resistances.
1. Cake resistance
2. Filter resistance
• The cake resistance is zero at the start and
increase with time as filtration proceeds.
• If the cake is washed after it is filtered both
resistances are constant during the washing
period.
13. Clarifying Filters
• Clarifying filters remove small amount
of solids to produce a clean gas or a
sparkling clear liquid such as beverage.
• The solid particles are trapped inside
the filter medium or on its external
surfaces.
• Clarifying filters differ from screens in
that the pores of the filter medium are
much larger in diameter than the
particles to be removed.
14. Cross Flow Filter
• In a cross flow the feed suspension
flows under pressure at a fairly high
velocity across the filter medium.
• A thin layer of solids may form on the
surface of the medium but the high
liquid velocity keeps the layer from
building up.
15. The Rotary Drum Filter
• Rotary vacuum filter drum consists of a drum rotating in a
tub of liquid to be filtered.
• The technique is well suited to high solids liquids that
would blind or block other forms of filter.
• The drum rotates through the liquid and the vacuum
sucks liquid and solids onto the drum pre-coat surface,
the liquid portion is "sucked" by the vacuum through the
filter media to the internal portion of the drum, and the
filtrate pumped away.
• The solids adhere to the outside of the drum, which then
passes a knife, cutting off the solids and a small portion of
the filter media to reveal a fresh media surface that will
enter the liquid as the drum rotates.
• The knife advances automatically as the surface is
removed.
18. Selection Criteria
In broad terms drum filters are suitable to the following
process requirements:
• Slurries with solids that do not tend to settle rapidly
and will remain in a uniform suspension under gentle
agitation.
• Cakes which do not require long drying times.
• Cakes when a single washing stage is sufficient to
remove residual contaminants from the cake or yield
maximum recovery of filtrate.
• Filtrates that generally do not require a sharp
separation between the mother and wash filtrates.
21. Plate and Frame Filter Press
• Plate and frame filter presses are
dewatering machines which utilize
pressure (60-80 psi, typically) to remove
the liquid from a liquid-solid slurry.
• They are particularly suited for low
solids (<2% solids), or solids composed
of fines (- 200 mesh), however they will
essentially dewater many combinations
of particle size distribution and percent
solid slurries.
22.
23. WORKING
• The feed enters the press at the bottom of the plate,
using a pump suitable for pumping up to 80-90 psi.
• Then, the feed travels the path of least resistance (up
between the filter plates), which has filter media
inserted between the plates, and the void between
the plates is filled with the slurry, as the liquid passes
through the filter media, and travels up to the outlet
port at the top of the plate.
• This liquid is referred to as the "filtrate", and is
discharged from the press. The solids remain in the
void between the plates, until the plates discharge
the filtered solids
24.
25. Bag Filters
• Bag filters have now been almost entirely superseded for
liquid filtration by other types of filter, although one of the
few remaining types is the Taylor bag filter which has been
widely used in the sugar industry.
• A number of long thin bags are attached to a horizontal
feed tray and the liquid flows under the action of gravity so
that the rate of filtration per unit area is very low.
• It is possible, however, to arrange a large filtering area in
the plant of up to about 700 m2.
• Bag filters are still extensively used for the removal of dust
particles from gases and can be operated either as
pressure filters or as suction filters.
