4 Filter media selection

Dr. Geeta

Introduction

Filtration is a physical separation of one or more components from solution in a fluids (liquids /gases), by passing of that medium through or across a barrier which is permeable to only few components. Filtration is oldest technologies for the separation of particulate matter forms the industrial gases. It is very simple process where dust laden gas is pass through fabrics or other filter media. The dust particles are separated from the gas by inertial impaction or direct interception followed by diffusion in the filter media.

There are basically four possible mechanisms to separate dust particle from gas i.e. inertial impaction, direct interception, diffusion and electrostatic capture as shown in Fig1.

1. Inertial impaction: It occurs when inflowing gas flow around the fibers whereas particles travel straight due to inertia and hit the fibers and fall down in the filter media. This is due to larger particles size which is unable to adjust with streamline direction.

2. Direct interception: The smaller particles due to low inertia followed the gas streamline path and come in contact with fiber and being coverage the fiber. In this way they are eliminated from the gas flow.

3. Diffusion: The very small size particle ranging from 0.001 to 0.05 µm diameter in gas stream colloid with gas molecules and create random path known as Brownian diffusion. This random motion causes the particles to impact and adhere on fiber surface.

4. Electrostatic attraction: The electrostatic force generated due to friction between gas and fibers play an important role. Other electrostatic force is developing between fibers and particles. It plays a minor role in filtration process.

Figure 1. Showing mechanism of Filtration

(Source: http://totobobo.com/blog/2008/08/does-smaller-particle-always-penetrate-filters-easier)

Filter: It is a device in which separation is achieved by mechanical means. Filtration is depends upon size of the particles, droplets or molecules being separated. Some filtration process involves electrical as well as mechanical forces and some require chemical forces as well as physical ones. All filters have simple purpose of holding a filter medium firmly parallel or across to flow of fluid. Filter differs from each other widely in complexity (from simple tubular to rotary vacuum drum filter). When surface filter are used then particles are trapped and cover the surface of the media whereas in case of depth filter larger particles are held on the surface and smaller particles are trapped within the filter media. Filter are classified into four categories according to European classification as course, fine, HEPA (high efficiency particulate air filter) and ULPA (ultra low particulate air filter).

Course filter: These filter are made up of glass or synthetic fibers and able to separate particles of 5mm or larger than 5mm.

Fine filter: They are made up of glass fibers and plastic fibers

HEPA and ULPA filter: They are being used by hospitals, military and nuclear power plant to clean the air. These are classify according to their level for separation. HEPA filter removed 99.9% of particles from ambient air.

Chemical filter: Theses are based on adsorption process where chemical substance likes activated charcoal.

11.2 Filter medium: The barrier is known as filter medium, and a filter is a mechanical structure that holds the medium in most efficient way. Filter medium is defined as any permeable material used in filtration and upon which or within which, particles are deposited by the process of filtration. A filter medium is any material that, under the operating conditions of the filter, is permeable to one or more components of a mixture, solution or suspension and is impermeable to the remaining component. Filter media is consist of variety of material from metal plates with hole, microporous membranes, sheets of cloths to bed of sand. There is material like inorganic minerals, metals, glass, synthetic sheet material, synthetic organic fibers, glass, metal oxides, ceramic, carbon and charcoal are made into permeable form to being used as filter media. These materials are mold into different shape like rod, sheet, granules, wire, and bar, bonded fibers, loosely packed as per the requirement.

11.3 Types of filter media

Cartridge filters: It consist of rolls of filter media wound so that several layers cover each other. They are used as surface filters or depth filters.
Bag filters: It consist of porous membrane or layer of mesh made up of natural or synthetic fibers. The bag filter used in food, pharmaceutical or beverage are polypropylene based. The large filter bags are used in cement and metal industries for collecting dust and other conatminants.The main advantage of these filters are that they are simple and inexpensive.
Vacuum filters: They are two main types: horizontal belt and rotary drum filters and operate continuously. They are used in production of food ingredient, waste water treatment and infiltration of chemical wash. The main disadvantage is that they are very expensive and require large floor space.
Fabric filters: Also known as bag houses method and it is used for separating dry particles from gas stream. It is categories into three types as reverse air, shaker and pulse-jet baghouse.It is used in cleaning and collection of dust from the bags.
Depth bed filters: In this method the slurry passes through the bed, the solid particles filtered out due to gravity settling, diffusion molecular or electrostatics forces and being separated. These filters are used in water treatment, in removing solid and oil residue from the industrial fluids.
Magnetic filters: Use of magnetic field for separation of particles. Theses filters are used for coolant filtration in machine.
Ceramic filters: They are made up of alumina, silicon carbide and refractory fibers. The main advantage of these filters is that they work at 900oC and at high pressure. They are used to clean up high temperature off-gas streams.
Nutsche filters: Both filtration and reaction occur in nutsche filters. They are mostly shed for hazardous materials. They are used in dye, pesticide and pharmaceutical industries.
Panel filters: It come in different shapes and sizes depending upon the application. They are used in ventilation and air condition units.

11.4 Selection of media

Selection of media is a tough scientific work which gives excellent result with longer life of bag, less maintenance and no environmental pollution. So proper selection require good understanding of the whole process (i.e. composition of gas stream), how various types of filter media are design according to the requirement of industries. There are three major categories on the basis of which selection of filter media become easier (Table 1).These are discuss below:

11.4.1 Selection based upon Machine

Strength: It is defined as strength of a filter system to withstand the physical structure of filter system without rupturing and tearing of the bed. Tensile strength of porous ceramics is measured by standard test and uses the term cross breaking strength. The paper and textiles industries are also measuring the strength in term of resistance to tearing and bursting.

Resistance to abrasion: Abrasion resistance is a basic requirement of industry which measure the hardness of the material from which the medium is formed. In case of textiles, martindale wear tester technique is used to check the abrasive surface.

Table 1: Showing the various selection criteria for selecting suitable filter media for specific purpose

Rigidity: Elasticity of the metal play an important role in choosing which kind of media is used. In case of paper and textile the term stiffness is used and measured as flexural rigidity.

Gasketing and sealing function: Sealing is done by natural fibers and with synthetic fibres.Natural fibers give good result in sealing having high absorbing capacity to liquid as compared to synthetic fibers.

Dimensions: Dimension of filter media are controlled by various machine and technique so can be easily fit.

Ability to be fabricated: Various fabricating techniques like welding, bending and cutting are used depending upon the medium used.

11.4.2 Selection based upon Filtration properties

Particle shape and size: The shape of particles varies from spherical to any shape and it’s also affect in selecting filter media of particular shape depending upon their efficiency. Size should be taken between 0.425-0.85 mm for better result. Narrow size distribution are used to increase the good porosity ,long time runs of filter and less loss of filter media.

Permeability: It is the rate of flow of fluid under defined pressure. Water and air are mostly used in the assessment of the permeability. Different media have different permeability.

Mechanism of filtration: There are various filtration mechanism used in filtration.

Retention efficiency: Retention efficiency of filter medium is reduced by decreasing in the particles size.

Dirt holding capacity: High dirt holding capacity indicates it’s cleaning and replacing of filter medium. Fiberglass filter are thicker than synthetic fiber media. The structural stability of glass is more so it can retain under pressure drop.

Structure of filter media: Filtration process is depending upon the intrinsic properties of the material .Metal sheet is designed by engineer by cutting or drilling, the holes form are uniform in nature. In case of woven fabrics they are not in uniformity. It affects the flow rate.

11.4.3 Selection based upon application properties

Biological, thermal and chemical stability: It is the ability for filter medium to withstand biological degradation, temperature and chemical stability. Natural fibers are more susceptible to biological attack as compared to synthetic fibers.

Cost: It is concern with the price of unit, its maintenance and disposal. Cost of filter medium is varies with their types. Reusability of media make its high cost.

Disposability: Proper disposal of waste is done so that it does not cause any harmful effect to the environment. Recycling the waste material is the need of hours.

Suitable for reuse: Some type of filter media are used only one time and then discarded but other has long life. Filter media chosen with long time working capacity lower down the cost.

Absorptive and adsorptive characteristics: Absorption may be defined as the ability of in which one substance as associated with other substance and form a homogenous mixture. When a fibers made up of cellulose absorb moisture and swell up then the space between fibers is reduced and filtration rate is change. Adsorption means which occurs on the surface of solid or liquid. If some molecules are absorbed on the surface of filter media then it’s directly affect the performance.

Health and environmental safety aspects: Any chemical which is inhaled caused health effects. Proper care should be taken during their disposal because hazardous waste caused environmental problem.

Summary

Selection of good filter media helps in removal of specific chemical compounds which may be gas or liquid.
Natural and synthetic fibers are used in the construction of filter media.
Filter media can be selected on the basis of various application, machine and filtration properties.
The best filter media should be inert in nature, having sufficient mechanical strength, reusability, withstand the filtration pressure, easy disposal and low cost.

Glossary

Abrasion Resistance– The level of resistance to withstand repeated rubbing and scuffing

Absorption-It is a physical or chemical process in which atoms or ions enter into liquid or solid material.

Diffusion- It is net movement of molecule or atoms from a region of high concentration to low concentration.

Efficiency-It is degree to which filter will perform in removing solid or liquid.

Electrostatic Forces -The forces between particles that are caused by their electric charges–

Retention– It is the ability of filter medium to retain particles of given size.

References

Chrys Philip Z (1991). Improving quality and productivity through filter media selection.
Purchas Derek B and Sutherland K. Introduction of filter media (Handbook of filter media) Elsevier Science, 2002.
Thomas J. Scanlan(2014) Filter media selection in amine gas. Sweetening systems:1-8.

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