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Nonwoven for filtration

Abstract:

Nonwovens are rapidly advancing to help filter media manufacturers offer improved efficiencies; better pressure drops and overall better performance. Manufacturers have more choices than ever before when deciding which filtration media will offer the best performance. Whether the goal is cleaner air to breathe, water to drink or a more efficient fuel system, nonwoven is becoming the substrate of choice.Nonwoven fabrics are broadly defined as sheet or web structures bonded together by entangling fiber or filaments mechanically, thermally, or chemically.In the coming decades, filter fabric filtration will play a very critical role in day to day life, and there is no single type of fabric being used in all the applications. The usage of filter fabrics varies according to their end use.

Keywords: Nonwoven, filtration, filtration efficiency etc.

Introduction:

Filtration can be defined as separation of one material from another. The main purpose of filtration is improving the purity of filtered material. Filtration plays a critical role in our day-to-day life by providing healthier and cleaner products and environment. Textile materials are used in the filtration of air, liquids, in food particles and in industrial production. Filtration fabrics are used widely in vacuum cleaners, power stations, petrochemical plants, sewage disposal, etc. Textile materials, particularly ‘nonwovenare suitable for filtration because of their complicated structure and thickness. Dust particles have to follow a tortuous path around textile fibers. Thus, due to their structure, they have high filtration efficiencies i.e. 25-99.9%.

Although, filtration plays a critical role in our day-to-day life, there is not a single type of fabric used in all the applications. The usage of the filter fabrics varies according to their end-use. This depends on the properties the filters have which ultimately depends on the characteristics of the raw material used for the manufacturing of the filter fabric. A filter fabric intended to use for heavy chemical filtration may or may not be used at high temperatures. Similarly, a filter fabric intended to use at the high temperatures may not be a good chemical resistant. Hence, it is totally depending on the type of filter fabric intended to use and the specific end use. Filter material are generally used in solid-gas separation and solid –liquid separation. [1]

INDA, North America’s Association of the Nonwoven Fabrics Industry, describes nonwoven fabrics as sheet or web structures bonded together by entangling fibers or filaments, by various mechanical, thermal and/or chemical processes. These were made directly from separate fibers or from molten plastic or plastic film.

EDANA, (The European Disposables and Nonwovens Association) defines a nonwoven as ‘a manufactured sheet, web of directionally or randomly orientated fibers, bonded by friction, and/or cohesion and/or adhesion. [2]

The nonwoven manufacturing process basically consists of web laying and web bonding. The web lying mostly consist of air & dry and the web bonding are mechanical, Chemical and thermal bonding. The basic concept employed in making a nonwoven fabric is transform fiber based material into two-dimensional sheet structure with fabric like properties such as porosity, flexibility and mechanical integrity. Nonwovens offer many advantages in all types of filtration. Whether a filter is designed to keep the air in your home clean, an operating room sterile or to remove dirt and grit from the oil in your car before it reaches the engine, nonwoven fabrics get the job done.

Anyone who suffers from allergies already knows that having a good heating and air conditioning filter in the home can help immeasurably in clearing the air. Air filters capture pollen, allergens, mold spores, and other microscopic particles to reduce symptoms and help everyone breathe easier. Medical filters made from nonwovens are key in any operating room to keep out pathogens, bacteria or microbes that might otherwise contaminate a sterile environment.

Oil filters are an important component in your car. The nonwoven fabrics in the filters help remove contaminants from the engine oil, hydraulic oil, transmission oil and lubricating oil. [3]

Figure 1; Non- Woven Filters

Filtration:

Filtration is a mechanical or physical operation, which is used for the separation of solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass. Oversize solids in the fluid are retained, but the separation is not complete; solids will be contaminated with some fluid and filtrate will contain fine particles depending on the pore size and filter thickness. Filtration is used to separate particles and fluid in a suspension, where the fluid can be a liquid, a gas or a super-critical fluid. Depending on the application, either one or both of the components may be isolated.

Filtration, as a physical operation is very important in chemistry for the separation of materials of different chemical composition. A solvent is chosen which dissolves one component, while not dissolving the other. By dissolving the mixture in the chosen solvent, one component will go into the solution and pass through the filter, while the other will be retained. This is one of the most important techniques used by chemists to purify compound.[4]

Principles of filtration

Objective of filter medium is to maximize the possibility of collision and the subsequent retention of the suspended particles with fibrous structures while minimizing the energy loss of the system. The efficiency of filtration in industrial fabrics is affected by their porosity. Knowledge of air permeability is also important for suitability for use.Air permeability of a fabric is measure of ease with which air can flow through the material.

A large portion of total volume occupied by the fabric is in fact air space. The ratio of volume of the air to void contain in the fabric to the total fabric volume is defined as “Porosity”. The amount and distribution of this air space influences a number of important fabric properties including the efficiency of filter fabric.

Permeability is capacity of porous materials to transmit the fluids. Liquid and gas permeability increases with the increase in porosity of the fabric.The type of finish also affects the permeability. When Porosity increases pressure drop tends to decrease. When flow rate increases, pressure drop increases. [1& 5]

1) Interception: When a particle tries to pass the fiber surface from the distance smaller than the radius of particle, it may collide with the fiber and may be stopped or arrested.

2) Inertial disposition: When heavy particles are carried in the flow, they may be thrown out from the streamline flow due to its inertia. This may cause the particle to be trapped in the fibers.

3) Random diffusion: Due to random vibrations and zigzag movement of particles in the flow, particles may follow zigzag route causing chances of trapping.

4) Electrostatic disposition: Micro particles are very difficult to capture with

5) Gravitational forces: Under the influence of the gravity, a particle that is sinking may collide with the fibers and get caught. [5]

Types of filtration:

1. Ultra-Filtration(UF)

Ultra-filtration is a variety of membrane filtration in which forces like pressure or concentration gradients leads to a separation through a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained in the so-called retentate, while water and low molecular weight solutes pass through the membrane in the permeate. This separation process is used in industry and research for purifying and concentrating macromolecular (103 – 106 Da) solutions, especially protein solutions. Ultra-filtration is not fundamentally different from microfiltration. Both of these separate based on size exclusion or particle capture. It is fundamentally different from membrane gas separation, which separate based on different amounts of absorption and different rates of diffusion. Ultra-filtration membranes are defined by the Molecular Weight Cut off (MWCO) of the membrane used. Ultra-filtration is applied in cross-flow or dead-end mode.

UF is used extensively in the dairy industry.UF can also be used for the removal of particulates and macromolecules from raw water to produce potable water.

2. Nano-filtration

Nano-filtration is a membrane filtration based method that uses nanometer sized cylindrical through-pores that pass through the membrane at a 90°. Nano-filtration membranes have pore sizes from 1-10 Angstrom, smaller than that used in microfiltration and ultrafiltration, but just larger than that in reverse osmosis. Membranes used are predominantly created from polymer thin films. Materials that are commonly used include polyethylene terephthalate or metals such as aluminum. Pore dimensions are controlled by pH, temperature and time during development with pore densities ranging from 1 to 106 pores per cm2. Membranes made from polyethylene terephthalate and other similar materials, are referred to as “track-etch” membranes, named after the way the pores on the membranes are made. Membranes created from metal such as alumina membranes, are made by electrochemically growing a thin layer of aluminum oxide from aluminum metal in an acidic medium.

Table No. 1 – Application of Nano-Filtration

Application Field Uses
Fine chemistry andPharmaceuticals Non-thermal solvent recovery and management

Room temperature solvent exchange

Oil and Petroleum chemistry Removal of tar components in feed

Purification of gas condensates

Natural Essential Oils and similar products Fractionation of crude extracts

Enrichment of natural compounds Gentle Separations

Medicine Able to extract amino acids and lipids from blood and other cell culture.

3. Microfiltration

The typical particle size used for microfiltration ranges from about 0.1 to 10 µm.In terms of approximate molecular weight these membranes can separate macromolecules generally less than 100,000 g/mol.The filters used in the microfiltration process are specially designed to prevent particles such as sediment, algae, protozoa or large bacteria from passing through a specially designed filter. More microscopic, atomic or ionic materials such as water (H2O), monovalent species such as Sodium or Chloride ions, dissolved or natural organic matter and small colloids and viruses will still be able to pass through the filter.

 

Micro-filtration applications

A)Water Treatment

Perhaps the most prominent use of microfiltration membranes pertains to the treatment of potable water supplies. The membranes are a key step in the primary disinfection of the uptake water stream. Such a stream might contain pathogens such as the protozoa Cryptosporidium and Giardia lamblia which are responsible for numerous disease outbreaks. Both species show a gradual resistance to traditional dis infectants i.e. chlorine. The use of MF membranes presents a physical means of separation (a barrier) as opposed to a chemical alternative. In this sense, both filtration and disinfection take place in a single step, negating the extra cost of chemical dosage and the corresponding equipment. Similarly, the MF membranes are used in secondary wastewater effluents to remove turbidity but also to provide treatment for disinfection.

B) Sterilization

Another crucial application of MF membranes lies in the cold sterilization of beverages and pharmaceuticals. Historically, heat was used to sterilize refreshments such as juice, wine and beer in particular, however a palatable loss in flavor was clearly evident upon heating. Similarly, pharmaceuticals have been shown to lose their effectiveness upon heat addition. MF membranes are employed in these industries as a method to remove bacteria and other undesired suspensions from liquids, a procedure termed as ‘cold sterilization’, which negate the use of heat.

C) Petroleum Refining

Furthermore, microfiltration membranes are finding increasing use in areas such as petroleum refining, in which the removal of particulates from flue gases is of particular concern. The key challenges/requirements for this technology are the ability of the membrane modules to withstand high temperatures i.e. maintain stability, but also the design must be such to provide a very thin sheeting to facilitate an increase of flux. In addition the modules must have a low fouling profile and most importantly, be available at a low-cost for the system to be financially viable.

D) Dairy Processing

Aside from the above applications, MF membranes have found dynamic use in major areas within the dairy industry, particularly for milk and whey processing. The MF membranes aid in the removal of bacteria and the associated spores from milk, by rejecting the harmful species from passing through. This is also a precursor for pasteurization, allowing for an extended shelf-life of the product. However, the most promising technique for MF membranes in this field pertains to the separation of casein from whey proteins i.e. serum milk proteins.

4. Reverse Osmosis(RO)

RO is a water purification technology that uses a semipermeable membrane. This membrane technology is not properly a filtration method. In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property that is driven by chemical potential, a thermodynamic parameter. Reverse osmosis can remove many types of molecules and ions from solutions, and is used in both industrial processes and the production of potable water. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be “selective”, this membrane should not allow large molecules or ions through the pores, but should allow smaller components of the solution to pass freely. Moreover reverse osmosis involves a diffusive mechanism, so that separation efficiency is dependent on solute concentration, pressure, and water flux rate.

Application of reverse osmosis

Reverse osmosis (RO) is most commonly known for its use in drinking water purification from seawater, removing the salt and other effluent materials from the water molecules. [6]

Why nonwovens for filtration?

Natural randomness of textile structure increases as probability of the particle being caught by fibers. Therefore, nonwoven fabric, in general provide higher filtration efficiency than woven or knit fabric. Woven and knit fabric isconsidered to have a two- dimensional structure while nonwoven fabrics may give a three-dimensional structure with a larger thickness which increases the distance for the particle to travel. Also nonwoven can be constructed in layers. A coarse open fabric removes larger particles. As the fluid progresses through the filter, the fibers and pores become finer to trap finer particles. [1]

Advantages of nonwoven filter over the woven filters are

  • High filtration efficiency.
  • High permeability
  • Less blinding tendency
  • No yarn slippage as in woven media
  • Good cake discharge.
  • There is no limitation for thickness.
  • High production rate.
  • Continuous process line.

As fiber diameter increases, filtration efficiency decreases. Also, void volume, which is required to reduce the pressure drop is more in case of nonwoven (98%) than that of woven or knitted (70%), thus nonwoven are advantageous to use.[7]

Synthetic fibers for filtrations:

Fibers and fabrics play a large role in everyday applications. They are the smallest visible unit of a fabric and are denoted by being extremely long in relation to their width. Fibers can be spun into yarn and made into fabrics.Synthetic fibers are a subset of the larger area of textiles. Textiles can be natural or synthetic. Natural fibers include cotton, fur, wool, etc. Regenerated fibers are natural materials that have been processed into a fiber structure. Regenerated fibers such as cellulose and wood pulp are used to make materials such as rayon and acetate. Synthetic fibers are man-made from chemicals. They are generally based on polymers and are stronger than natural and regenerated fiber.

Synthetic fibre and fabric features

In terms of structural features, some synthetic fibres and synthetic fabrics include:

  • Chemical/fuel resistant– Materials are designed to resist damage caused by acids, alkalis, general chemicals, fuel and oils. These materials are used to seal fuel or oil tanks.
  • Electrically conductive– Textiles or fabrics include fibres with high electrical conductivity or low electrical resistivity. Often, conductive filler is added to increase conductivity. Products are used in electronic, anti-static or electrostatic discharge (ESD) applications.
  • Flame retardant fabrics– Flame retardant products reduce the spread of flames or resist ignition when exposed to high temperature, or insulate the substrate and delay damage. A UL 94 rating indicates that the material is flame retardant in accordance with Underwriters Laboratories, Inc(UL) Flame Class 94V-0 or other equivalent ISO standards.
  • Hydrophilic/absorbent– The surfaces of hydrophilic materials absorb water. They are often used when high absorbency (many times the basis weight of the material) is important.
  • Thermal insulation/fireproofing– Thermal insulation materials provide a barrier between a component and a heat source.
  • Hydrophobic/waterproof– Waterproof materials do not dissolve or degrade when exposed to water. The fabric may still absorb water if the product is hydrophilic and has open porosity.
  • Weather/UV resistant– Plastic or elastomer foams are resistant to ultraviolet (UV) light or sunlight. Some non-UV resistant foam will crack, yellow, or degrade on exposure to UV light. Weather resistant materials can withstand exposure to the elements, such as wind, rain, snow dust, humidity, heat, cold, and other weather conditions. (8)

Table No. 2 – Fibers used for filtration with their properties [8]

Testing of filters

Filtration tests are conducted to measure the filtering capacity of fabric for the intended suspension in liquid filtration. In afiltration test the relationship between filter delivery and filtration time is determined using different pressure and liquid suspension temperature. Other factors that need to be consider for commercial liquid filtration system include the rate of filter choking, service life of the filter cloth, filtrate purity and cake removal.The efficiency of a filter has direct relation with the particle size. The high efficiency of fibrous filters is obtained by the design of the fiber system such as a size of a fiber, orientation and packing.Important test characteristics include, Permeability, Differential pressure, Filtration Efficiency, Strength, Chemical resistance. [4]

Conclusion

Nonwovens can be engineered very precisely to meet exacting specifications and stringent regulatory requirements for the filtration of air, liquid, dust, gas etc.Nonwoven webs have high barrier properties, good stability and strength at extremely high temperatures; they can filter almost anything ranging from macro to nano scale particle sizes.Nonwoven nanofibre filtration media is now filling the micro-filtration performance gap that had existed in the past, offering benefits such as enhanced air quality, reduced energy cost, and longer service life.Nonwovens are ideal in filtration applications due low cost, ease of strikethrough and increased efficiency.

References

  • SabitAdnoor, Handbook of Technical Textile, Woodhead publication.
  • S.J.Russell, Handbook of Nonwoven, Woodhead publication
  • http://www.inda.org/about-nonwovens/nonwoven-markets/filtration.
  • Horrock A R and Anand S C: Handbook of Technical Textiles, Woodhead Publication, 2000
  • Prof. U.J.Patil, Filtration in textile, Indian Textile Journal, March 81-86(2010).
  • www.Wikipedia.com
  • Kothari V K, Das A and Sing S: Filtration Behavior of Woven and Nonwoven Fabrics, IJFTR, June 2007.
  • http://www.globalspec.com/learnmore/materials_chemicals_adhesives/compositestextiles_reinforcements/synthetic_fibers_fabrics_polymer_textiles
  • Heinrich Jacob, Application Fields for Nonwoven, Nonwoven, Indian journal of fibre and textile research, 216-223(1994).
  • Dr. Shrinivasan and S Katharvelu: Nonwoven Fabric Filtration Media, Synthetic Fibres, April and June 2006.

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