Electrostatic Filtration

As indicated above, there is a relationship between particle concentration, equilibrium factor and the amount of highly mobile radioactive particles. Removal of the accumulation mode particles may decrease the decay product exposure, but increase the dose because of the high effectiveness of the "unattached activity in dose deposition. Thus, air cleaning may not succeed in lower risk unless both factors are taken into account. Jonassen explores electrostatic filtration in this context. Finally, design considerations are presented for a possible alternative control system using activated carbon in an alternating bed system. 

Wood pulp fibres are frequently less than 1 pm in fibre size and dominate wet-laid nonwoven filter media they are able to provide higher filtrafion efficiency and are environmentally fiiendly. Synthetic fibres (eg, polyolefin, polyester, and polycarbonate (PC) fibres) could work as electret fibres to provide additional electrostatic filtration mechanism, and also could be made binder-fitee, thermobond nonwoven fabrics if they are made Ifom thermoplastic polymers. 

Deep Bed Filters. Deep bed filtration is fundamentally different from cake filtration both in principle and appHcation. The filter medium (Fig. 4) is a deep bed with pore size much greater than the particles it is meant to remove. No cake should form on the face of the medium. Particles penetrate into the medium where they separate due to gravity settling, diffusion, and inertial forces attachment to the medium is due to molecular and electrostatic forces. Sand is the most common medium and multimedia filters also use garnet and anthracite. The filtration process is cycHc, ie, when the bed is full of sohds and the pressure drop across the bed is excessive, the flow is intermpted and solids are backwashed from the bed, sometimes aided by air scouring or wash jets. 

The carbon black (soot) produced in the partial combustion and electrical discharge processes is of rather small particle si2e and contains substantial amounts of higher (mostly aromatic) hydrocarbons which may render it hydrophobic, sticky, and difficult to remove by filtration. Electrostatic units, combined with water scmbbers, moving coke beds, and bag filters, are used for the removal of soot. The recovery is illustrated by the BASF separation and purification system (23). The bulk of the carbon in the reactor effluent is removed by a water scmbber (quencher). Residual carbon clean-up is by electrostatic filtering in the case of methane feedstock, and by coke particles if the feed is naphtha. Carbon in the quench water is concentrated by flotation, then burned. 

Fabric filters are limited by physical size and bag-life considerations. Some sacrifices in efficiency might be tolerated if higher air-cloth ratios could be achieved without reducing bag life (improved pulse-jet systems). Improvements in fabric filtration may also be possible by enhancing electrostatic effects that may contribute to rapid formation of a filter cake after cleaning. 

Gas cleaning by filtration, demisting, electrostatic precipitation, wet collection of particulates, cyclonic separation. 

Direct recycle Dispersion Electrostatic precipitation Filtration Needle bonded fab Reverse jet Reverse pressure Shaker type ric... 

Destruction Direct recycling Electrostatic precipitation Filtration Gravity settlement Total enclosure Wet scrubbing... 

Fabric filters can be more costly to operate and maintain than electrostatic precipitators, cyclones, and scrubbers however, fabric filters are more practicable for filtration of specific dusts. For example fabric systems are the typical control method for toxic dusts from insecticide manufacturing processes, salt fumes from heat treating, metallic fumes from metallurgical processes, and other applications. Any other control method may not be as efficient, nor economically feasible for such applications. 

Methods of dust removal depend mainly on the particle size of the dust and the temperature and moisture content of the gas. The methods used are broadly divided into dry methods and wet methods. The dry methods involve the use of gravity and baffle chambers, cyclones, filters, and electrostatic precipitators, while the wet methods involve the use of spray towers and venturi scrubbers. In principle, wet cleaning is preferred to dry cleaning because of the excessive wear associated with and the difficulty in handling the fine dusty material removed in the dry methods. The wet methods, however, must be followed by such operations as filtration, drying of filter cakes, and recycling of water. 

A series of other removal processes, such as filtration, diffu-sional plateout and electrostatic deposition, may, however, be effective in the case of the airborne radon progeny, and since the major part of the radiological exposure in a given environment connected with radon is due to the radon progeny rather than to the radon gas itself, it may be of interest to look into these processes... 

Electrostatic fluidized-bed coating, 7 55-56 Electrostatic forces, 9 569, 570 11 800 and adsorbent selectivity, 1 584 in adsorption, 1 583 in solvent-solute interactions, 23 91-92 Electrostatic particle forces, in depth filtration theory, 11 339 Electrostatic precipitators (ESP), 11 714 13 180 23 552 26 699-706 advantages of, 26 700 applications of, 26 701-703, 705t design considerations related to,... 

Airborne particles, bacteria, fungi spores and allergens will be removed by air filtration, which is often cheaper and more effective than electrostatic precipitators and ionizers. A disposable, high-efficiency filter capable of removing at least 95 percent of particles with size down to 0.3 micron will be installed. A washable coarse filter that removes fifty percent of particles larger than 10 microns will be used as a pre-filter to extend the life of the high efficiency filter by at least a third. An inexpensive sensor will be included to indicate when filter replacement is necessary. 

Adsorption affects the electrostatic properties of suspended particles and colloids, which, in turn, influences their tendency to aggregate and attach (coagulation, settling, filtration). 

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