Content

• Introduction

• Principle of filtration

• Filtration mechanism

• Filter types

• Process design parameters

• Equipment design parameters

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Introduction• The separation of solids from a suspension in a

liquid by means of a porous medium or • which retains the solids and allows the liquid to

pass is termed filtration.• Filtrate may contain some fine particles.

(depending on the pore size)

• Filtration of the thick pulp gives rise to a moist filter cake of between 80 to 90% solids

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Introduction

• Filter medium: generally to act as a support for the filter cake.

• The filter medium should be mechanically strong, resistant to the corrosive action of the fluid, offer as little resistance as possible.

• Made up of: Natural (cloth), Synthetic fibers (nylon) or metal wires.

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Introduction

• In filtration, the cake gradually builds up on the medium, form the true filtering medium and the resistance to flow progressively increases.

• Depending on the nature of the slurry, the driving force required for separation may be grouped into four categories:

• gravity• Pressure drop• Centrifugal

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Introduction

• Filtration is adversely affected by slimes, since they tend to choke the filter medium.

• Flocculants are added to aid filtration • Lower molecular weight flocculants are preferred

over those with high molecular weights. • This is because the flocs formed with the latter

are relatively large, and entrain water within their structure. This, in effect, increases the moisture content of the filter cake.

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Principle of filtration

• The volume of filtrate collected per unit time (dV/dt) is termed as the rate of filtration.

• As the filtration proceeds, solid particle accumulate on the filter medium forming a packed bed of solids, called filter cake

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Principle of filtration• As the thickness of the cake increases:

resistance to flow of filtrate increases

rate of filtration gradually decreases.

• Filtration rate constant pressure drop increase

• Pressure drop constant filtration rate decrease

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Principle of filtration

• The factors influencing the rate of filtration: the pressure drop from the feed to the far side of

the filter medium the filtering surface area the filtrate viscosity the resistance offered by the filter cake The resistance of the filter medium.

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Principle of filtration

• DARCY LAW: [2]

fitration rate: dv/dt = KA∆P/ηL

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• V: volume of filtrate• T: time of filtration• K: constant for filter medium & filter cake• A: area of filter medium• ∆P: pressure drop above & below the filter• η: viscosity of filtrate• L: thickness of the filter bed

• V: volume of filtrate• T: time of filtration• K: constant for filter medium & filter cake• A: area of filter medium• ∆P: pressure drop above & below the filter• η: viscosity of filtrate• L: thickness of the filter bed

Filtration mechanism

slurry is pumped into the filter through the manifold located on the stationary head.

the liquid component passes through the cloth medium.

the larger particles in the slurry begin to gradually bridge the openings in the filter media, reducing the size of the openings, allowing the filter cake to form.

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Filtration mechanism

• Filter Cake Formation:

1) suspension flows through the clean filter medium, some solids stay behind

2) suspension continues to flow through the partially coated medium and at either reduced rate or higher pressure differential, more solids stick

3) suspension now flow through the cake solids. Most solids will now be caught, filtrate is cleaner, flow reduces further and cake starts to consolidate

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Filtration mechanism

4) solids have built up on the medium until there is so much resistance that filtration has virtually stopped.

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Filtration mechanism

In case of batch filters this means stopping the filter, discharging the cake and cleaning the medium and restarting, whereas in continuous filters the cake is continuously discharged and the medium cleaned.

finer particles are separated from the incoming slurry, resulting in a filtrate that’s very low in turbidity.

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Filter types• The most common types of filters are ‘cake

filters

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Filter types

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1. Discontinuous Pressure Filters: Apply large P across septum to give economically

rapid filtration with viscous liquids or fine solids.

2. Vacuum Filter:Vacuum filters are simple and reliable machines and

therefore have gained wide acceptance in the chemical, food and pharmaceutical industries.

Filter types• Plate and Frame Filter Press• The plates are clamped together, then a pump starts

feeding the slurry into the filter press to complete a filtering cycle and produce a batch of solid filtered material, called the filter cake. 

• A filter press uses increased pump pressure to maximize the rate of filtration

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Filter types

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Filter types• Drum Filter:• consists of a 1 to 5 m diameter cylindrical drum that

rotates while remaining partially submerged in an open-feed slurry tank.

• As panel leaves slurry zone, a wash liquid is drawn through filter, then cake is sucked dry with air, and finally cake is scraped off.

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Process design parameters

Characterization of the feed• Particle size distribution• Affect cake formation rates and filtration equipment

selection.• the finer the solids(below 44µ), the lower the unit's

capacity, the poorer it's performance and the more restrictive the equipment choices.

• fine particles with a very narrow distribution are much easier to handle than a wide distribution range with the same average particle size.

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Process design parameters

Characterization of the feed• -10 micron solids content• If fine particles are dispersed as true colloids, they

exhibit Brownian movement and do not settle. • Therefore, charge neutralization (coagulation), and

sometimes polymer addition, are required for their removal.

• these finer sizes adversely affect settling rate, filtration cake, and cake moisture content.

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Process design parameters

Characterization of the feed• Particle shape• Particle shape starts to influence performance when it

varies greatly from a general spherical form. • Platelets act as multiple flapper valves within a filter cake

and restrict cake formation rate, particularly at higher vacuums.

• Long needles can cause blinding of filter media by imbedding in the pores of the cloth. In this case, continuous cloth washing on a belt type filter is required to maintain cloth porosity.

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Process design parameters

Characterization of the feed• Suspended solids concentration: • High solids concentrations are concerned with cake

filtration (removing bulk solids from the liquid). • Low solids concentrations (usually less than 5%) involve

clarification (the removal of small amounts of solids from the bulk liquid).

• Dilute slurries must be concentrated, usually in a gravity thickener, before going to the final dewatering device.

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Process design parameters

Characterization of the feed• Dissolved solids concentration:• Impacts solute viscosity and is a function of the degree

of saturation for the component present. • Dissolved solids can cause: o scaling in the internals of the equipment.o Blind the filter cloth due to precipitation (because of

pressure drop or temperature changes).

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Process design parameters

Characterization of the feed• The pH of the feed stream: • the initial pH of the feed stream and the flexibility of

making changes to the pH in the process has impact on filtration performance.

• Changes in pH can provide some flexibility in pretreatment of the slurry as a result of its affect on the behavior of particle surface chemistry.

• Use of coagulants are affected by pH.

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Equipment design parameters

• Process tonnage: (affect the economics of equipment selection)

• For low tonnages and solids concentrations, there is a definite economic advantage in manual or semi-automated batch operations (pressure filtration).

• Some pressure filters are fully automated and specifically designed to handle large tonnages. Pressure filters of this type approach the economics of continuous vacuum filtration.

• The economics of continuous versus batch filtration equipment is governed by the filtration rates achieved, tonnage processed and cake moisture attained.

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Equipment design parameters

• Filtration time• Pressure drop• Filter medium (area and material)

• Whether the valuable product is the solid, the fluid, or both.

• Whether it is necessary to wash the filtered solids.

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Acknowledgement

Thanks to other presentations about filtration existing on www.slideshare.net

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