Filtration processes are usually categorised as dry filtration (air/gas filtration) and wet filtration (liquid filtration), say Dr Senthilkumar P and Punitha V.
Filtration is the process of separating one material which is dispersed in other material using porous structure. The porous structure is obtained using textile fabric of woven, knitted or nonwoven. Among these various structure fabric, nonwoven fabric plays major role in filtration in spite of their wide range of thickness and porosity and pore size distribution. Nonwoven fabric has fine pores along their structure up to nano-level. Pore size is the main parameter which determines the type of usage. For example pore size of filter fabric must be smaller than particles to be filtered out which is helpful for surface filtration where as in case of depth filtration even though particles size is greater than pore size of fabric due to inertia, random motion, electrostatic force of attraction or gravitational motion, particles are captured by fibres inside the fabric.
Textile filtration meets developments regarding pore size variation, stability to thermal and chemical conditions, resistance against clogging which is also called as blinding and filtration efficiency. These various factors decides the filter fabric which type, where (liquid or gas medium of dispersed particles) and how long can be used. Recent days new fibre development and property development encourages the filtration application of textile fabric in this field.
Filtration process simply defined as the separation of one material from another. Filtration processes are usually categorised as dry filtration (air/gas filtration), wet filtration (liquid filtration).
Objectives of filtration are: (i) to obtain the maximum separation of targeted solid/liquid particulates from the fluid flow while (ii) minimising the pressure drop of the fluid flow across the filter thickness.
Classification of filter media
- The nature of the dispersion (dry and wet filtration)
- Surface or depth filters
- Particle size to be filtered
Filter media - Surface or depth filters
Nonwoven filters mostly are operated as depth filters, in which the targeted particles gradually penetrate through the filter in the direction of the filter thickness are captured by individual fibres. Some singed, coated and/or composite nonwoven filters are used for surface filtration in which the targeted particles are blocked on the surface.
The mechanism of filtration varies with the properties of the targeted particles (such as size, shape and physical attraction) and the fluid carriers (viscosity, flow type, flow rate and chemical nature) and physio-chemical interactions of the particle-fluid-filter material system, thus the filtration processes and the performance of a nonwoven filter cannot be treated in a universal model.
If the dimensions of the particle are larger than the pore size in the textile nonwoven material, the particles are stopped easily. For the particles that are smaller than the pore sizes, there are five separate mechanisms of arrest.
- Interception: When a particle tries to pass the fibre surface at a distance smaller than the particle’s radius, it merely collides with the fiber and may be stopped or arrested.
- Inertial deposition: When a heavy particle is carried by the flow, it is thrown out of the flow streamlines due to its inertia (mass x speed).
- Random diffusion (Brownian motion): Due to Brownian-type motion (random vibration and movement of particles in a flow), particles follow a zig-zag route and caught by the fibre.
- Electrostatic deposition: It is well known that strong electrostatic forces of fibres attract the particles.
- Gravitational forces: Due to gravity, a particle that is sinking, may collide with fibre.
The major criteria characterising the performance of a filter include the filter efficiency, pressure drop, filter loading, filter clogging and filter cleaning and filtration cycling time.
Particles are retained by the medium only when aperture of fabric is smaller than the particles themselves. This is advantage when frequent and periodic cleaning is possible during application. Based on cleaning mechanism they again classified such as shake collectors, Reverse air cleaning and pulse jet cleaning. Fibres used are based on particulate size and property, thermal and chemical nature of the medium to be filtered.
The particles are captured through attachment to the fibres within the body of the filter medium, e.g. because of Vander Waals or electrostatic forces, even though they may be smaller than the apertures.
1. High viscosity EP filter: Polyolefin fibres: They have stiff structure. Inside core high melting point polyolefin, out cover is lower melting point Polyolefin, fibre-to-fibre thermal bond forms a stable porosity. Available from 1 micron to 350 micron.
Applications: Automotive, paints, laser cooling, resin & adhesives
Operating conditions: Temperature max: 1760F(800C); pressure max: 80 psi(5.5bar)/700F(210C) and 35psi(2.4bar)/1760F(800C)
2. Nylon melt-blown depth media: This filter is designed for applications where polypropylene filter can’t be used. Excellent chemical compaitibility with solvents hydrocarbos and aromatics Continuously gradient pore structure for maximum dirt holding capacity.
Applications: Pure water prefiltration, Electrophoretic paint Slops reprocessing, Sour water stripper protection, Final product dehazing, Solvent based coating, Liquefied petroleum gas and Water vapor degreasing.
Operating conditions: Temperature max: 1200C.
3. Lenticular filters: It uses composite material high purity lingo-cellulose and inorganic filter aid agent.
Its inner crisscrossing three-dimensional structure let it be a depth filter and provide excellent filtration efficiency, high dirty holding capacity and longer life time.
Operating conditions: Temperature max: 800C ; pressure max: 2.4 bar @ 250C.
Applications: Sterile filtration: Injection, serum, vaccine, biopharmacy products
Fine filtration: Pharmaceutical, health care products, colliod
Clarifiltration: Wine, beer, food & beverage, process fluid
4. High service time guard filter cartridge (GUF): This is made of polypropylene non-woven. It provides excellent life time which is 3 times than general Melt-blown filters. It is an exceptional value for general applications where long life, high dirt-holding and low change-out frequency are required. Suitable for water treatment industry.
Operating conditions: Temperature max: 1580F(700C); pressure max: 43.5psi(3bar)/700F(210C) and 7.6psi(1.2bar)/1580F(700C)
Applications: Guard filter for RO system, potable water filtration, cooling water system and plating baths.
5. Precision string wound filter cartridge (WDC): It has various media available to suit different applications. Removal rating avaliable from 0.5 to 100 microns. The cartridge length is from 4 inch to 40 inch. Filter Medium: Polypropylene/bleached cotton/glass fibre/washed polypropylene
Inner core: Polypropylene/304 Stainless Steel/316 stainless steel
Operating conditions: Temperature maximum
With stainless core: Cotton: 1490C(3000F), Polypropylene: 930C(1990F) and Glass fibre: 3990C(7500F)
With Polypropylene core: Cotton - 600C (1400F), Polypropylene - 600C (1400F)
i. Chemical industry: painting, ink, resin, adhesives
ii. Microelectronics industry: Pre-filter of ultra pure water
iii. Food & beverage: oil, syrup filtration
iv. Water treatment: process water, domestic water, condensate water
SIM polymer hollow-fibre membrane: SRI’s Materials Research Laboratory is developing an advanced sulfonated-imidazole (SIM) polymer hollow-fibre membrane for reverse osmosis. Shipboard desalination of seawater, brackish water & Purify water is extracted during gas and oil production. High thermal and mechanical stability and it is more chlorine-tolerant than currently available reverse osmosis polymer membranes. Withstand high pressures. SIM is inexpensive.
- Gas-borne dust particles arise wherever solid materials are handled.
It is a type of physical filtration process. Membranes with a pore size of 0.1 – 10 µm perform micro filtration. Microfiltration membranes remove all bacteria. Only part of the viral contamination is caught up in the process, even though viruses are smaller than the pores of a micro filtration membrane. This is because viruses can attach themselves to bacterial biofilm.
Examples of micro filtration applications are:
- Clearing of fruit juice, wines and beer
- Effluent treatment
- Separation of oil/ water emulsions
- Pre-treatment of water for nano filtration or reverse osmosis
- Solid-liquid separation for pharmacies or food industries
Ceramic membranes from a low-cost naturally occurring clay material viz. red mud prepared with additives like sodium carbonate, sodium metasilicate and boric acid. Circular disc shaped membranes with 50 mm× 5 mm dimensions were found well suited for oily waste water. A maximum rejection of 53 per cent was achieved with membrane sintered at 8000C.
Since this membranes filters through physical mechanism, physical limitations occurs. e.g water that is filled with particulates or organic materials passed through can clog membranes. Thus, particularly surface water, must be pretreated before passing through a membrane system to reduce clogging. These systems also produce small volumes of highly concentrated solution which requires disposal. Compared to other kinds of membrane technologies, microfiltration is less commonly used today. Based on US technology, melt blown system has the ability to develop and produce micro or nano fiber webs. Their combined effect of low diameter and compact packing also allows efficient and more economical dyeing and finishing.
The process requires very high water pressures to force source fluid through extremely small pores (as small as .001 micrometers or one nanometer, hence the name) in order to remove contaminants. Nanofiltration is used to remove hardness, natural organic matter, and synthetic organic chemicals from water.
Electrospinning is a technique used to spin fibres with diameters less than 100nm up to micrometer level from a wide range of polymers. This electrostatic processing method uses a high-voltage electric field to form solid fibers from a polymeric fluid stream (solution or melt) delivered through a millimeter-scale nozzle. Fibers such as Polyamide-66 nanofibre, polycaprolactone (PCL), Submicron polystyrene (PS) fibres, poly methyl methacrylate (PMMA) nanofibres, polylactic acid fibres, Chitosan fibres, etc., are commercially produced at nano level. Nano size of fiber represents at any direction the dimension of them should be within 10 – 100 nm.
1. Nano-sized activated alumina particles bonded onto a glass fibre matrix to filter out: Sub-micron particulates (0.2-1 micron), Turbidity, Rust particles, Iron bacteria, Colloidal silica and iron and Reduce microorganisms (15)
2. Using sol-gel techniques thin (50–100 nm) amorphous nanoporous layers of silica sol or metal oxide sol, having pore sizes in the micropore (<2 nm) or fine mesoporous (<5 nm) region can be prepared on a porous substrate. These layered porous systems, usually in tubular form, can be used for nanofiltration, pervaporation and gas separation applications (16).
Ultrafiltration membranes, like microfilters, are created in several different designs. For complete removal of viruses, ultra filtration is required. The pores of ultrafiltration membranes can remove particles of 0.001 – 0.1 µm from fluids.
Examples of fields where ultrafiltration is applied are:
- The dairy industry (milk, cheese)
- The food industry (proteins)
- The metal industry (oil/ water emulsions separation, paint treatment)
- The textile industry
Reverse osmosis (RO)
Reverse osmosis filters have a pore size around 0.0001 micron. After water passes through a reverse osmosis filter, it is essentially pure water. Membrane water treatment systems were originally used only in desalination projects. But improvements in membrane technology have made them an increasingly popular choice for treating microorganisms, particulates, and natural organic materials that foul water’s taste and taint its clarity.
These systems consist of thin material sheets that technically do not have pores. Rather, the membrane allows water molecules to pass through it, but catches and retains other dissolved or suspended substances. The system pressurises the solution to such an extent that water flows from a more concentrated solution, through the membrane, and into the more dilute solution—the opposite of natural flow by osmosis.
Wang Ling, et.al studied composite reverse osmosis membranes used in textile wastewater treatment and reutilisation. Ultra filtration combined with coagulation as pretreatment for RO/NF process can remove most turbidity, the rejection for COD and salts in integrated RO membrane process can reach as high as 94 per cent and 97 per cent, respectively. All final RO permeates becomes colourless, and had a fairly good quality for reutilization of water.
Electrodialysis and electrodialysis reversal treatment systems use electricity and a series of membranes to separate salts from source water and to concentrate them into a solution for disposal. When electric current is applied to source water, chloride ions gravitate to the one end and sodium ions are drawn to the other. Moving in either direction, these minerals pass through stacks of membranes, which trap them in channels dedicated to containing the highly concentrated solution. This waste product, which must be disposed of properly, may amount to some 30 per cent of the total source water treated; 15 to 20 per cent is more typical. Source water does not physically pass through membranes in these systems, most organic contaminants are not removed.
Filter efficiency: The filter efficiency, E, is the ability of the filter to retain particles and is defined as the percentage of particles of a given size retained by the filter. It can be calculated from the ratio of the particle concentrations in the upstream (Pin) and downstream (Pout) fluid flows respectively.
Pressure drop: The pressure drop refers to the difference in pressures across the filter thickness. For dry filtration, Dp = pin – pout, where upstream (pin) and downstream pressure (pout). For wet filtration, DP = DPH–P + DPB–P where DPH–P is the pressure drop for a Hagen - Poiseuille fluid, and DPB–P is the pressure drop due to the particle flow resistance.
As the growth and developments in synthetic fibres, filtration media requirement can utilise textile material for their effective performance with minimum fouling and maximum filtration efficiency. These two phenomena have to be equally satisfied for a better filter performance and this can be achieved through proper selection of filter fabric with accurate pore size distribution through over the fabric.
Nano filtration and reverse osmosis are now used worldwide for water purification. Electro-dialysis is newly emerging technique especially for kidney dialysis. Another development of air stop membrane for infusion sets with particle barrier pore size 5-15 µm, provides higher protection against particle related risks. Auto stop function blocks automatically the air passing when the infusion container runs empty. The fluids stay maintained in the infusion line. Further in almost all polymer production, liquid and gas processing systems, textile filtration plays a major role in purifying and separating purposes.
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7. 3D modeling of filtration process via polyurethane nanofibre based nonwoven filters prepared by electrospinning process Wannes Sambaer, Martin Zatloukala & Dusan Kimmer, Chemical Engineering Science, vol. 66, 2011, pp. 613–623.
12. Sandeep Parma, Pradip Chowdhury, Preparation and Characterization of Microfiltration Ceramic Membrane for Oily Waste Water Treatment, IJRET, Vol. 03: 3, May-2014
16. Sol-Gel Approaches in the Synthesis of Membrane Materials for Nanofiltration and Pervaporation, Ben C. Bonekamp, Robert Kreiter & Jaap F. Vente, NATO Science for Peace and Security Series C: Environmental Security pp 47-65, 2008.
19. Performance of Composite Reverse Osmosis Membranes Used in Textile Wastewater Treatment and Reutilization, Wang Ling, Sheng Xinjiang, Zhang Guoliang & Zuo Wenrui, Computer Distributed Control and Intelligent Environmental Monitoring (CDCIEM), 2011 International Conference, pp 1611 – 1614.
• Dr Senthilkumar P is Assistant Professor (Sr.Gr) with the Department of Textile Technology. He can be contacted at: Email: firstname.lastname@example.org.
• Punitha V is M.Tech Scholar with the Department of Textile Technology, PSG College of Technology, Peelamedu, Coimbatore – 641004.