Transcript Filtration By Sidra Jabeen Department of Chemical Engineering, University of Engineering & Technology Lahore.

Filtration
By
Sidra Jabeen
Department of Chemical Engineering,
University of Engineering & Technology Lahore
Lecture includes:
 What is filtration?
 Understanding of the basic terms
 How filtration is done?
 Types of filtration
 Rate of filtration
 Filter Media
 Filter Aid
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Filtration
Removal of solid particles from a fluid by passing it through
a septum on which the solids are deposited
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1. Solid Particles
Solubility
Dissolved
Concentration
Size
Traces to High %
Valuability
Desired
Waste
Undissolved
Micro filtration
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Nano Filtration
Ultra filtration
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2. Fluids
A continuous substance which can Flow
Gas
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Liquid
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3. Septum or Filtering Media
Filter Paper
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Membrane
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4. Solid Deposition
In the form of cake on the septum
Or
Trapped inside the pores of the septum
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Example
 The solid remaining in the
filter paper is called the
residue.
 The residue can be dried by
spreading it out on the filter
paper and allowing the liquid
to evaporate.
 The liquid which has passed
through the filter paper is
called the filtrate.
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How filtration is done?
Fluid flows through the filter medium by virtue of pressure
difference across the medium.
Through Feed Side
 Pump
 Blower
 Liquid height of column etc
Through Product Side
By generating Vacuum
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Types of filtration
Two basic types of filtration are
1. Cake filtration  Solid particles form a layer of solids
on filtering media (called as cake).
2. Deep or Depth bed filtration  Solid particles are
caught inside the pores of filtering media.
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1. Cake Filtration
Solid particles are retained on septum as a layer of solids
called as “cake”
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2. Deep Bed Filtration
Further of two types
a) Clarifying filtration
b) Cross flow filtration
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a. Clarifying Filtration
Solid particles are trapped inside the septum
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b. Cross flow Filtration
Some of the liquid passes through the septum leaving
behind conc. solution
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Factors affecting Rate of Filtration
 Drop in pressure from the feed to the far side of septum
 Area of the filtering surface
 Viscosity of the solution
 Resistance of the filter cake
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Filter Media
Common names
Filter Paper
Membrane
Screen
Porous Solids
Desired Properties
Good Filtration
Should not Plug
Chemically Resistant
Physically Strong
Cheap
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Material of construction of filter media
 Woven Materials (Cotton, Wool, Silk etc)
 Perforated Metal Sheeting
 Granular Materials (Sand, Asbestos etc)
 Porous Solid
 Felted-Fiber materials (Porous Paper)
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Filter Media Structure
•Cloth
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•Metal
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Filter Aids
o Filter aids are granular solids which are used in
combination with filtration to enhance rate of filtration.
For example, Diatomaceous silica, perlite, purified wood
cellulose etc.
o Filter aids are used in two different ways:
 Added before filtration
 As a pre-coat
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Filter Aids
 Added before filtration
This increases the porosity of the cake and reduces resistance of
the cake during filtration.
 As a pre-coat
They can be used as a precoat before the slurry is filtered. This
will prevent solids from plugging the filter medium and also give
a clearer filtrate.
Use of filter aids is usually limited to cases where the cake is
discarded.
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Types of Filters
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Classification of Filters
Filters are classified on following three different basis:
I. Mechanism
II. The driving force
III. Operation
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On the basis of mechanism
1. Cake filters
2. Clarifying filters
3. Cross flow filters
On the basis of driving force
1. Pressure Filters
2. Vacuum filters
3. Centrifugal filters
On the basis of operation
1. Continuous filters
2. Batch or discontinuous filters
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Classification of filters
 Pressure filters  Pressure applied at feed side
 Vacuum filters Vacuum applied at product side
 Centrifugal separators  Spinning the slurry to force
separation
 Continuous filters  Flow of fluid and discharge of
solids (cake) is continuous or uninterrupted.
 Batch filters Flow of fluid is continuous but it has to
be interrupted to permit discharging of the accumulated
solids.
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Cake filters
 They can be pressure filters (filter press, shell & leaf
filter)
 Or they can operate under vacuum (rotary drum filter)
 Or cake filtration can be carried in centrifugal filters
(suspended batch centrifuges)
Either type can be batch or continuous.
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1.Filter Press
 Pressure filter operate in batch mode
 Sets of plates (circular or square)
 Plate faces covered by septum
 Slurry enters at one end
 Solids (cake) collect in chamber and liquid pass out
 ‘Jammed’ filter
 Wash liquid, air or steam used to remove residual liquid
 Press opened and cake removed
 Washing is time consuming and labor intensive
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Filter Press
Pink : Plates
Yellow : filter Cloth
Blue : Filtrate
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Filter Press
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2. Shell and leaf filter
 Pressure filter operating in batch mode
 Stack of leaves inside the closed tank
 Feed enters in the side of the tank
 Pass through leaves covered with filter
 Liquid leaves into discharge manifold
 Cake is removed by gravity force and vibration
 Cake is effectively removed and economize labor
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3. Shell and leaf filter
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Shell and leaf filter cake disposal
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3. Rotary Drum Filter
 Vacuum filter with continuous operation
 Drum turns at 0.1 – 2 rpm in slurry trough
 Filter covers the face of the drum
 Rotary drum filter has four zones:
1.
2.
3.
4.
Filtering zone
Washing and drying zone
Cake removal zone
Dead zone
 Vacuum is applied in first two zones
 Permeate and washing liquid discharge into separate
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compartments
 Doctor blade is used in third zone
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Rotary Drum Filter
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Rotary Drum Filter
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Rotary Drum Filter
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Rotary Drum Filter
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4. Suspended Batch Centrifuge
 Centrifugal filter operating in batch mode
 Perforated baskets 750 – 1200 mm in dia. and 18 – 30 inch deep
 Basket rotates at 750 – 1200 rpm
 Feed taken from inlet is centrifuged
 Clear liquid pass out leaving behind solid cake in basket
 Wash liquid sprayed to remove soluble solids and cake dried
 For cake removal, basket is turned at 30 – 50 rpm and cake is cut
off with unloader knife
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Suspended Batch Centrifuge
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Clarifying Filtration
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Clarifying Filtration
Removal of the Small amounts of solids or liquids droplets from
the fluids.
Principle of Clarification
•
•
•
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Particles are trapped inside the filter media or on its
surfaces
Caught by the surface forces
Reduce the active dia. of the channel
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Liquid Clarification: Cartridge Filter
 Series of thin disks with close clearances in a vertical stack
 Disks carried on a vertical hollow shaft
 Liquid admitted under pressure
 Trapping of the solids between disks
 Liquid flows out through the casing
 Comb cleaner pass between the disks and drop solids to the
bottom of casing
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Cartridge Filter
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Gas Clarification
 For atmospheric dust – Pad filters
 For process dusts – Bag Filters
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Pad Filter
 Air passed through pads of filter media
 Pad may be dry or coated with viscous oil
 For light duty – pads are disposable
 For heavy duty – reused by rinsing and recoating
Before
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Bag Filter or Bag House
 Contains one or more large bags of fabric material inside
metal casing
 Dust laden gas enters the filter at the bottom
 Solid particles trap inside the bag and clean gas leave at the
top
 Solids are discharged by shaking mechanism
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Cross flow Filtration
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Definition
Filtration in which flow of the suspension is parallel to the filtering
media used to get concentrated slurry
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Cross flow filtration Operation
The feed is passed across the filter membrane at positive
pressure relative to the permeate side. A proportion of
the material which is smaller than the membrane pore
size passes through the membrane as permeate or
filtrate; everything else is retained on the feed side of the
membrane as retentate.
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Difference b/w cross flow & Conventional filtration
1.
2.
3.
4.
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Cross Flow Filtration
Flow of fluid is parallel
to the surface
Majority of the feed
travels across the
surface of the filter
Process feed remains in
the form of a mobile
slurry, suitable for
further processing
It is possible to
fractionate particles by
size
1.
2.
3.
4.
Conventional Filtration
Flow of fluid is
perpendicular to the
surface
Solids in the feed trap
inside the filter or retain
on the filter
Process feed is
transformed into solid
cake & filtrate
It is simply the
separation of solids
from liquids, no
fractionation
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Performance of cross flow filtration





Fouling of the membrane
Permeate flux
Percent rejection
Back flushing
Factors for optimizing permeate flux
1.
2.
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Trans-membrane pressure
Shear rate
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Types of cross flow filtration
 Reverse Osmosis
 Microfiltration
 Ultrafiltration
 Nanofiltration
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Reverse Osmosis
The pressure driven transport of solvent from a solution
through a membrane is known as reverse osmosis
Applications
 Separating low-molecular-weight substances in solution
 Concentrating
 Cleaning wastewater
 Demineralization
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Points of discussion
 Osmosis
 Osmotic pressure
 Reveres Osmosis
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Micro filtration
 Used for the particles in the size range of 0.1- 5 μm.
Applications
• Fermentation
• Biomass clarification
and recovery
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Ultra filtration
 Used for the particles in the size range from 1μm down to
10-3 μm.
Applications
 used in the fractionation of milk as well as protein
fractionation
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Nano filtration
 Used for the particles up to 1nm
 Nanofiltration is the process with the characteristics b/w
those of Ultrafiltration and reverse osmosis.
Applications
 Pharmaceutical processing
 Water treatment
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