Introduction to Air Filtration

The hazard to human health from exposure to air-borne dust is primarily due to the smaller particles with aerodynamic diameters of less than a few 

Science and Technology of Polymer Nanofibers. By Anthony L. Andrady Copyright 2008 John Wiley Sons, Inc. 

The removal of particles by a fibrous filter relies on five main mechanisms, of which four are discussed here  

Interception. A particle approaching the fibers surface to a distance equal to or less than its radius (rparticie) or within the contact range of the fiber, without crossing a flow streamline tends to adhere to and deposit on the surface. It is an important mechanism for particles larger than about 100 nm, and its effectiveness depends on tbe ratio of particle diameter to the fiber diameter. If the interfiber distance or the porosity of the mat is smaller than the particle diameter dp, then a special case of interception or particle sieving oeeurs, but this is usually not an important mechanism in air or gas filtration. 

Impaction. The path of airflow results in a curvature of streamlines in the vicinity of a fiber. Because of their inertia (at high enough particle velocities), particles cross the streamlines, impact on the nanofibers, and are deposited on them. This mechanism too is normally important for particles larger than a few hundred nanometers. 

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All drum filters (except the single-compartment filter) utilize a rotary-valve arrangement in the drum-axis support trunnion to facih-tate removal of filtrate and wash hquid and to allow introduction of air or gas for cake blowback if needed. The valve controls the relative duration of each cycle as well as providing dead portions of the cycle through the use of bridge blocks. A typical valve design is shown in Fig. 18-121. Internal piping manifolds connect the valve with various sections of the drum. 

A common problem of this synthesis is the decomposition of the product to a black insoluble and pyrophoric powder (presumably metallic Mo), which is possibly caused by the use of an impure starting material, to the adventitious introduction of air, to excessive exposure to light, or to a combination of those factors. If a black powder is formed, this can be removed at the end of the reaction by filtration through a glass frit the subsequent workup is as described above. Such decomposition reduces the yield but does not lead to a less pure product. If larger amounts of Mo(CO)6 are used, the sublimation of larger quantities of Mo(CO)6 to the condenser occurs, which may require interruption of the procedure and manual scraping of the sublimed solid back into the flask with a spatula under a flow of N2. 

All drum filters (except the single-compartment filter) utilize a rotary-valve arrangement in the drum-axis support trunnion to facilitate removal of filtrate and wash liquid and to aUow introduction of air... 

Introduce 197 g. of anhydrous brucine or 215 g. of the air-dried dihydrate (4) into a warm solution of 139 g. of dZ-acc.-octyl hj drogen phthalate in 300 ml. of acetone and warm the mixture vmder reflux on a water bath until the solution is clear. Upon cooling, the brucine salt (dA, IB) separates as a crystalline solid. Filter this off on a sintered glass funnel, press it well to remove mother liquor, and wash it in the funnel with 125 ml. of acetone. Set the combined filtrate and washings (W) aside. Cover the crystals with acetone and add, slowly and with stirriug, a slight excess (to Congo red) of dilute hydrochloric acid (1 1 by volume about 60 ml.) if the solution becomes turbid before the introduction of... 

To a solution of 40 g (1.0 moles) of sodium hydroxide in 500 ml of methanol was added 242 g (l.Omoles) of 2.6-dichloro-4-bromophenol. The pH was adjusted between 9.0 and 10.0 (preferably 9.5) by means of one or another of the reactants. The pH was determined by diluting a 2.5 g aliquot with 100 ml of 50% aqueous methanol. The alcohol and water were removed by distillation, fn a one liter round bottom flask there was introduced 100 g of the sodium salt of 2.6-dichloro-4-bromophenol, 350 ml of chlorobenzene and 40 ml of N,N-dimethylformamide. The mixture was agitated until the salt was in solution then immediately there was added 26 ml of dimethylsulfoxide. A suspension forms. The air was removed by alternate evacuation and introduction of nitrogen then there was added 1.0 g of benzoyl peroxide dissolved in 10 ml of toluene. The mixture was stirred for 80 min at 29—33° C then for 5 hours at 54—59° C. The formation of polymer was indicated by the disappearance of the particles of the suspension and an increase in the viscosity of the solution. The polymer was isolated by precipitation into acetone. After filtration the polymer was washed thoroughly with water, then with acetone and then dried at 100° C. There was obtained 60 g (theoretical) of poly-(2.6-dichloro-1.4-phenylene ether). 

Mechanical filtration systems are intended to limit the introduction of pollutants from outdoors to indoors. The efficiency of such systems generally depends on the filter properties and the aerodynamic properties of filtered particles [26]. The efficiency of filters varies from 5% to 40% for low-efficiency filters, such as dry media filters, panel and bag filters, from 60% to 90% for electrostatic precipitators to over 99% for high-efficiency particulate air filters. Not only the filters, but the whole heating, ventilation and air-conditioning system contributes to particle reduction, owing to particle losses on the cooling/heating coil and other parts of the system. The selection of a system depends on the type of indoor environment, outdoor and indoor sources, the demand on the level of reduction of pollutant concentrations and the cost associated with purchase, operation and maintenance of the system. 

All hydrogenation experiments were carried out at atmospheric pressure in a reactor, consisting of a flask attached to a burette filled with mercury and equipped with stopcocks that permit removal of air before the introduction of hydrogen. The mixed catalyst/solid substrate was placed at the bottom of the flask reactor to form a thin layer (2-3 mm) and then evacuated to approximately 10-3 Torr for 10 min. After introducing hydrogen (760 Torr), the reaction was carried out until complete transformation of the solid had occurred usually the reaction was completed within 18 h. This transformation was accompanied by the formation of thin needles on the surface of the solid catalytic bed, which correspond to the resulting products cA-4-tert-butylcyclohexanol, /ra s-4-/err-butylcyclohexanol and 4-tert-butylcyclohexanone. The reaction mixture was extracted with ethyl ether, and the catalyst separated by simple filtration. Analysis of the products was carried out by gas chromatography and mass spectroscopy. 

An enclosed space constructed and operated in such a manner and equipped with appropriate air handling and filtration systems to reduce to a pre-defined level the introduction, generation and retention of contaminants within it... 

The resultant brown solution is heated in a boiling-water bath for 30 min (see Properties) with introduction of an air stream to give dark green solution. Then, the solution is cooled to room temperature. After the precipitates of boric acid are filtered off by suction, the filtrate is subjected to Sephadex G-15 (or G-10) column chromatography (eluent, IMHCl diameter 4 cm, length 80 cm Fig. 2). 

Bottom-feed units are characterized by the introduction of dnst-laden gas through the baghouse hopper and then to the interior of the filter tube. In top-feed units, dustladen gas enters the top of the filters to the interior or clean-air side. When the gas flow is fiom inside the bag to the outside, by virtue of the pressure differential, the internal area of the filter element will be open and self-supporting irrtsupported filter elements are tubular. When the filtration process is reversed, with the gas flow fiom outside the bag to inside, it is necessary to support the media agairrst the developed pressures so that the degree of collapse is controlled. Supported filter elemerrts are either of the tubirlar or envelope shape. 

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