Dust particle capture

Filtration of particles through nonwoven filters is depth filtration rather than surface filtration. The particle capture capacities of a nonwoven filter are based on the interactions between targeted particles, individual fibres of the filter, and the fluid molecules, and the dust particles captured by nonwoven filters are described by using single fibre particle collection theory. The filter efficiency of nonwoven filter, which is composed of many individual fibres, depends on the particle collection efficiency of a single fibre. The equation defining overall filter efficiency of a nonwoven fabric in dry air filtrations, Y(d(), for any particle size, (L, and the set of conditions is as follows ... 

A simple model has been developed by Deutsch (1922) to predict the extent of dust particle capture and consequent gas cleanup in turbulent flow in a duct using a number of simplifying assumptions. 

With the advancement of technologies (e.g., GC/MS, ICP-MS, ICP-AES) available for analysis of the biogeochemistry of dust particles captured in cyclones and on impactor plates, one might hope that modeling of the air-borne flux of identified contaminants is a relatively straightforward operation, provided the windspeed field is known or can be estimated. Unfortunately, this is not the case. Empirical relationships between the vertical dust flux (E) and windspeed (m ) are notorious for order of magnitude variation in E for a given airflow rate. 

Panels of siUca aerogels have already been flown on several Space Shuttle missions (74). Currently a STARDUST mission has been planned by NASA to use aerogels to capture cometary samples (>1000 particles of >15 micron diameter) and interstellar dust particles... 

Both fossil fuels and hazardous waste fuels used in Southdown cement kilns contain metals. The raw materials (limestone, clay, sand) used to make cement clinker also contain metals. In fact, certain metals, such as iron and aluminum, are essential components of the final product. While metals cannot be destroyed, the Southdown cement kiln process effectively manages them in the following ways (a) cement kiln operators limit emissions by carefully restricting the metals content in wastes accepted for recycling (b) dust particles containing metals are returned to the kUn through closed-loop mechanisms, where metals are chemically bonded into the cement clinker (c) particles not returned to the kiln are captured in state-of-the-art pollution control devices and (d) small amounts are emitted from the stack in quantities strictly hmited by USEPA s BIF mle. 

To avoid the problems with vacuum cleaner surface dust sampling a special surface dust sampler, HVS-3, was developed (Roberts et al, 1991). The sampler consists of a nozzle that can be adjusted to a well defined distance to the surface, a cyclone that collects the dust particles, an air pump, and an exhaust filter to capture particles that are not retained in the cyclone. SVOCs may break through the cyclone as vapors and a PUF plug can be inserted after the cyclone. HVS-3 has been modified to ensure a more constant suction pressure and volume, and a known sampled area (Gyntelberg et al, 1994). The design and use of the HSV3 has now been standardized (ASTM, 1997, D 5438-94). 

A process emits 50,000 acfin of gas containing a dust (it may be considered ash and/or metal) at a loading of 2.0gr/ft. A particulate control device is employed for particle capture and the dust captured Ifom the unit is worth 0.03/lb of dust. Experimental data have shown that the collection efficiency, E, is related to the... 

The degree of enclosure provided by the hood and the air capture velocity should be such that dust particles are prevented from entering the workplace. 

Like many original methods of mechanical process technology, an old technique for the successful removal of airborne dust was first observed in nature. The capacity of rain to clear the air has been used since ancient times to remove suspended dust particles by passing the contaminated gas through a water spray (wet scrubbers). The liquid droplets capture the solid (and some of the gaseous) pollutants and collect them in a sump. While this reduces air pollution it transfers part of the separation problem to a secondary cleaning process the removal of fine particulate solids from a liquid. 

A further technology for the production of instant (agglomerated) materials in a spray dryer uses the recirculation of dry, already pre-agglomerated particles into the spraying zone of the dryer. Recirculating flnes are particles captured in cyclones of the dust collection system and product screen undersize. Figs. 6.4-10 and 6.4-11... 

Wet scrubbers are in use in melting shops to clean the cupola off-gas. Wet serubbers use water to remove dust particles (average loading 10-15 g/Nm ) and gases sueh as sulphur dioxide from cupola off-gas. In the settling tank, a large part of the solid partieles are captured and thereby almost completely removed from the scrubbing water. 

For dust and particle removal, both wet and dry systems are used. The main advantage with using dry systems is that the dust is captured dry, which allows its subsequent re-use. Furthermore, no pollution is transferred to another medium, as is the case for wet systems. For gaseous compounds, such as SO2 and chlorides, bag filters are inefficient, since they are not adsorbed onto the filter surface. Experiments with lime injection have not provided good results. The following describes the system used for dust and particle removal ... 

It has been argued that the theories behind these mechanisms may be valid for certain air filtration applications, where total-particle capture is vital however, for the purposes of industrial dust collection, they are of limited value. A sieving mechanism is probably more appropriate wherein the size of the apertures in the medium assumes a more dominant role, at least until the fibres have accumulated a layer of dust which then takes over the sieving action. 

The failure to collect dust particles efficiently will inevitably lead to atmospheric pollution, which will at the very least be undesirable, if not positively harmful. It is therefore important that, in the first instance, the fabric is designed to capture the maximum... 

The design requirements include resistance to stretching from the mass of the dust cake, resistance to flex fatigue from the shake cleaning mechanism, a surface that will facilitate efficient dust release, and a construction that will affect maximum particle capture whilst at the same time providing minimum resistance to gas flow. 

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