Filtration

By

Sidra Jabeen

Department of Chemical Engineering,

University of Engineering & Technology Lahore

What is filtration?Understanding of the basic termsHow filtration is done?Types of filtrationRate of filtrationFilter MediaFilter Aid

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Lecture includes:

Removal of solid particles from a fluid by passing it through a septum on which the solids are deposited

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Filtration

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Solubility Valuability

ConcentrationSize

Dissolved

Undissolved

Traces to High %

Micro filtration

Nano Filtration

Ultra filtration

Desired Waste

1. Solid Particles

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A continuous substance which can Flow

Gas Liquid

2. Fluids

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Filter Paper Membrane

3. Septum or Filtering Media

In the form of cake on the septumOr

Trapped inside the pores of the septum

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4. Solid Deposition

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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Example

Fluid flows through the filter medium by virtue of pressure difference across the medium.

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Through Feed Side Pump

Blower

Liquid height of column etcThrough Product Side

By generating Vacuum

How filtration is done?

Two basic types of filtration are1. 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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Types of filtration

Solid particles are retained on septum as a layer of solids called as “cake”

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1. Cake Filtration

Further of two typesa) Clarifying filtrationb) Cross flow filtration

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2. Deep Bed Filtration

Solid particles are trapped inside the septum

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a. Clarifying Filtration

Some of the liquid passes through the septum leaving behind conc. solution

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b. Cross flow Filtration

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Drop in pressure from the feed to the far side of septum

Area of the filtering surfaceViscosity of the solutionResistance of the filter cake

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Factors affecting Rate of Filtration

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Filter Media

Desired Properties Good Filtration

Should not PlugChemically ResistantPhysically StrongCheap

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Woven Materials (Cotton, Wool, Silk etc)

Perforated Metal Sheeting

Granular Materials (Sand, Asbestos etc)

Porous Solid

Felted-Fiber materials (Porous Paper)

Material of construction of filter media

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•Metal•Cloth

Filter Media Structure

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 filtrationAs a pre-coat

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Filter Aids

Added before filtrationThis 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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Filter Aids

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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 forceIII.Operation

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On the basis of mechanism1. Cake filters

2. Clarifying filters3. Cross flow filters

On the basis of driving force1. Pressure Filters

2. Vacuum filters3. Centrifugal filters

1. Continuous filters2. Batch or discontinuous filters

On the basis of operation

Pressure filters Pressure applied at feed side Vacuum filters Vacuum applied at product

sideCentrifugal separators Spinning the slurry to

force separationContinuous 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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Classification of 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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Cake filters

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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1.Filter Press

Pink : PlatesYellow : filter

ClothBlue : Filtrate

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Filter Press

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Filter Press

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2. Shell and leaf filter

Pressure filter operating in batch modeStack of leaves inside the closed tankFeed enters in the side of the tank Pass through leaves covered with filter Liquid leaves into discharge manifoldCake is removed by gravity force and

vibrationCake 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

Vacuum filter with continuous operationDrum turns at 0.1 – 2 rpm in slurry troughFilter covers the face of the drumRotary drum filter has four zones:

1. Filtering zone2. Washing and drying zone3. Cake removal zone4. Dead zone

Vacuum is applied in first two zonesPermeate and washing liquid discharge into

separate compartmentsDoctor blade is used in third zone

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3. 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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Rotary Drum Filter

Centrifugal filter operating in batch modePerforated baskets 750 – 1200 mm in dia. and

18 – 30 inch deepBasket rotates at 750 – 1200 rpmFeed taken from inlet is centrifuged Clear liquid pass out leaving behind solid cake

in basketWash liquid sprayed to remove soluble solids

and cake driedFor cake removal, basket is turned at 30 – 50

rpm and cake is cut off with unloader knife04/21/2338

4. Suspended Batch Centrifuge

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Suspended Batch Centrifuge

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Clarifying Filtration

Removal of the Small amounts of solids or liquids droplets from the fluids.

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Clarifying Filtration

Principle of Clarification

• Particles are trapped inside the filter media or on its surfaces

• Caught by the surface forces• Reduce the active dia. of the channel

Series of thin disks with close clearances in a vertical stack

Disks carried on a vertical hollow shaftLiquid admitted under pressureTrapping of the solids between disksLiquid flows out through the casingComb cleaner pass between the disks and

drop solids to the bottom of casing

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Liquid Clarification: Cartridge Filter

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Cartridge Filter

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For atmospheric dust – Pad filtersFor process dusts – Bag Filters

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Gas Clarification

Before

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After

Pad Filter

Air passed through pads of filter mediaPad may be dry or coated with viscous oilFor light duty – pads are disposableFor heavy duty – reused by rinsing and

recoating

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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

Filtration in which flow of the suspension is parallel to the filtering media used to get concentrated slurry

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Definition

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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Cross flow filtration Operation

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Cross Flow Filtration1. Flow of fluid is parallel

to the surface2. Majority of the feed

travels across the surface of the filter

3. Process feed remains in the form of a mobile slurry, suitable for further processing

4. It is possible to fractionate particles by size

Conventional Filtration1. Flow of fluid is

perpendicular to the surface

2. Solids in the feed trap inside the filter or retain on the filter

3. Process feed is transformed into solid cake & filtrate

4. It is simply the separation of solids from liquids, no fractionation

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Difference b/w cross flow & Conventional filtration

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Fouling of the membrane Permeate flux Percent rejection Back flushing Factors for optimizing permeate flux

1. Trans-membrane pressure2. Shear rate

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Performance of cross flow filtration

Reverse OsmosisMicrofiltrationUltrafiltrationNanofiltration

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Types of cross flow filtration

The pressure driven transport of solvent from a solution through a membrane is known as reverse osmosis

Applications Separating low-molecular-weight

substances in solutionConcentratingCleaning wastewater Demineralization

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Reverse Osmosis

OsmosisOsmotic pressureReveres Osmosis

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Points of discussion

Used for the particles in the size range of 0.1- 5 μm.

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Applications

• Fermentation• Biomass clarification and recovery

Micro 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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Ultra filtration

Used for the particles up to 1nmNanofiltration is the process with the

characteristics b/w those of Ultrafiltration and reverse osmosis.

ApplicationsPharmaceutical processingWater treatment

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Nano filtration

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