Filtration

FiltrationFiltration

• Filter media:

– Structural considerations: rigid, semi-rigid

– Size and shape of pores and path through medium

– Number of pores per unit area and uniformity of pores

• Criteria for evaluation of filter media • Criteria for evaluation of filter media

– Measured data of how small particle the media can stop

– Permeability (the ability of medium to allow flow)

– Relationship between buildup of cake in the medium and the rate of

increase of resistance to the flow

Filter mediaFilter media

• Cartridge media

– Have integral cylindrical configuration with

disposable or cleanable filter medium with

structural hardware

– Most widely used

– High specific area, low cost

– Can be made of wound media, bonded fibers

made of glass, wool, cotton, etc

Cartridge filter

made of glass, wool, cotton, etc

• Rigid porous media

– Porcelain, ceramics, sintered metals, etc

– Fragile

• Woven and non woven media

• Materials for filter media:

– Cellulose acetate, acrylic, fluorocarbons, glass,

nylon, polyethylene, polypropylene, PVDF, etc

Woven media

Mode of FiltrationMode of Filtration

• Crossflow/Tangential filtration

• Dead End filtration

Feed Retentate

Permeate

• Dead End filtration

Feed

Permeate

Types of FiltrationTypes of Filtration

• Deep-bed filtration

– Particles penetrate in to pores of filter medium

– Surface of filter medium responsible for

filtration

– Used for very dilute suspensions

– Recovery of particles is not desired

– Filter-bed get clogged with particles,

Suspension

Mechanism of Deep-bed filtration– Filter-bed get clogged with particles,

resistance increases to an unacceptable high

level – leading to replacement of bed

• Cake Filtration

– Particles from suspension deposited on porous

filter

– With solid buildup on the filter, initial layers

effectively act as a filter

Mechanism of Deep-bed filtration

Mechanism of Cake filtration

Suspension

Deep-bed filtrationDeep-bed filtration

• Low depth of filter media

– Early breakthrough, i.e. quicker appearance of turbidity

– Low pressure drop

• Large depth of filter media

– High pressure drop, but more time before appearance of turbidity

Deep-bed filtrationDeep-bed filtration

• Low depth of filter media

– Early breakthrough, i.e. quicker appearance of turbidity

– Low pressure drop

• Large depth of filter media

– High pressure drop, but more time before appearance of turbidity

Depth of filter media

Time to turbidity

breakthrough

tbTime to

renew bed

Deep-bed filtrationDeep-bed filtration

• Low depth of filter media

– Early breakthrough, i.e. quicker appearance of turbidity

– Low pressure drop

• Large depth of filter media

– High pressure drop, but more time before appearance of turbidity

Depth of filter media

Time to turbidity

breakthrough

tb

Time to design

pressure drop

th Manifests as

pump duty

Time to

renew bed

Deep-bed filtrationDeep-bed filtration

• Low depth of filter media

– Early breakthrough, i.e. quicker appearance of turbidity

– Low pressure drop

• Large depth of filter media

– High pressure drop, but more time before appearance of turbidity

Optimum

depth

Optimum

depth

Depth of filter media

Time to turbidity

breakthrough

tb

Time to design

pressure drop

th

Region of possible

operation

depthdepth

Manifests as

pump duty

Time to

renew bed

Deep-bed filtrationDeep-bed filtration

• Particle diameter

– Small diameter: larger area (also, high pressure drop)

– Larger diameter: lower area (low pressure drop)

• Rate of filtration

– Higher rate: desirable, but may lead to early breakthrough

Increasing rate and/or particle diameter may lead to degradation in

performance

• Compensate using greater depth?

Deep-bed filtrationDeep-bed filtration

• Assuming, particulate screening to be first order phenomena,

we get:

Filtration coefficient

Iwasaki Equation

Deep-bed filtrationDeep-bed filtration

• Assuming, particulate screening to be first order phenomena,

we get:

Filtration coefficient

Iwasaki Equation

Depth of filter media (L)

Deep-bed filtrationDeep-bed filtration

• Assuming, particulate screening to be first order phenomena,

we get:

Filtration coefficient

Iwasaki Equation

Depth of filter media (L)

Improvement achieved with depth

has diminishing returns, but rate of

filtration can be increased

Improvement achieved with depth

has diminishing returns, but rate of

filtration can be increased