Fabric filters can be employed for the separation of dust particles with difficult properties.
Usually, separation of the particles is not a major point of concern. Only if extremely free flowing dust particles e. With extremely fine particles, two main topics have. Firstly, the gas flow in the raw gas section of the bag-house should assist sedimentation of the fine particles to the hopper, i. Secondly, the adhesion forces of the dust cake must be overcome by the separating forces during regeneration see Fig. A proper design of the regeneration system pulse-jet regeneration for instance enables high separation forces due to the pressure wave moving along the filter bag.
Systems working according to these principles are already in operation for several years. The raw gas inlet is from top through the clean gas chamber to bottom so that an almost ideal downflow configuration is realised. The bag-house is heated and insulated so that condensation of water vapour is avoided. Regeneration of the filter medium is realised online during operation by means of a pulse-jet regeneration system.
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The pulse gas has to be heated in order to avoid condensation in the pulse-jet . TiO2 in a flame hydrolysis process can also be collected directly after the flame reactor. Surface filters are also employed to separate fumed silica, carbon black and a variety of other nanoparticles produced in the gas phase.
Physical models for the determination of filtration velocity, and thus for the size of a bag-house have for the time being no relevance in industrial practice. The design depends on the experience of suppliers and users. In those cases where physical design models are not available, simple experimental techniques for determination of relevant parameters are required.
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Based on the results of Sievert , Gang and Loffler , a new VDI guideline  was developed by Gang for the characterisation of regenerable filter media. In this test, particles are fed, well-dispersed, into the vertical raw gas duct and transported to a filter element, which is mounted at the end of the horizontal clean gas duct. The measured parameters are pressure drop, residual pressure drop directly after regeneration, regeneration efficiency in dependence of over-pressure during pulsing, and clean gas concentration. It becomes thus possible to select a filter medium for the collection of a specific dust.
Sievert showed that results, with regard to regeneration efficiency in dependence of over-pressure obtained. In order to compare the results obtained from the VDI-unit with the filtration behaviour of a larger industrial pilot filter, the clean gas part of the VDI-system was installed in a bag-house with 40 bags of 2. One filter bag was removed and the clean gas duct of the VDI-tester was mounted in the bag-house according to Fig. The small filter sample was operated with the filtration velocity corresponding to the mean filter rate of the bag house.
Regeneration was triggered simultaneously to row 4 of the bag house. The maximum over-pressure at the filter sample was similar to the value in the filter bag at the position of the filter sample. Both curves are similar, indicating that dust cakes structures on the filter sample and on the filter bags are comparable.
Particle size distribution of the dust limestone on the filter sample and on various locations on the bag surface were found to the similar, i.
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However, maximum filtration velocity for the bag-house cannot yet be determined by the VDI-tester. At filter rates above 2. One reason for this difference is supposed to be in fluid flow distribution in the bag-house. The coupon tester gives the maximum value of the filter rate for which the medium can still be regenerated without taking fluid flow distribution into consideration. These results show, however, that the small filter tester can conveniently be used in applications with difficult dust properties.
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The tests give reliable data on the pressure drop to be expected and if the dust cake can be removed. The design of the bag-house regeneration system can be based on a preliminary test with the VDI-system . Dry scrubbing systems consist of an entrained flow reactor and a subsequent collection device, e. The adsorbent particles are fed into the reactor, whereby dispersion is essential in order to achieve maximum mass transfer of the gaseous pollutants to the surface of solid adsorbent particles.
Adsorption takes place during transport to the bag filter and on the surface of the bags where the solid particles are.
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The adsorbent particles are usually smaller than mm, so that a dust cake is formed during separation with pores in the mm-size range. The dust cake is then used as a highly efficient fixed bed reactor with very short diffusion distances. In the following, an example for separating extremely sticky solid and liquid particles is given. Carbon in form of coke, pitch and tar is an important raw material in the fabrication of electrodes. These electrodes are used, for instance, in the aluminium industry as anodes for electrolysis.
Fumes from anode baking furnaces contain, besides SO2 and HF, substantial concentrations of solid, liquid and gaseous polyaromatic hydrocarbons PAH , which are extremely sticky. Traditionally, fume treatment systems consisted of evaporative cooling towers with subsequent electrostatic precipitators.
The evaporative coolers lower the gas temperature in order to condense the hydrocarbons, which are collected as droplets in the electrostatic precipitators. Fume treatment plants are not only evaluated by its emissions but also by the residues which are produced by the cleaning process itself. A decisive progress in fume treatment technology for closed-ring type baking furnaces is the concept which is schematically represented in Fig.
The flue gas is cooled with an indirect cooler down to 70oC. A considerable amount of hydrocarbons is condensed on the outside of heat-exchanger pipes. The droplets are collected together with solid dust and tar particles in the electrostatic precipitator.
After the electrostatic precipitator, a dry adsorption unit is used for collection of the remaining particles and the adsorption of gaseous components. Petrol coke, ball mill dust or alumina may be used for adsorption. These materials originate from the process either from electrode production or aluminum electrolysis , and can be recycled back into the process so that no additional materials have to be used.
Essential for a long-term stable operational behaviour is a sufficiently thick dust layer of the precoating material that protects the filter medium from clogging due to the extremely sticky tar particles. The industrial experience shows that even under these extreme conditions, a reliable operation is possible over several years without changing the filter bags. Results of measurements in a fume treatment plant are presented in Fig. Concentrations of PAH components have been measured in the raw gas, after the electrostatic precipitator and after the dry scrubber in the clean gas.
Analytical determination was done by GC-MS analysis. The vapour pressure of the components decreases from left to right, the lowest boiling component being phenanthrene PHE and the highest boiling one being dibenzopyrene. Generally, it can be said that collection becomes easier with decreasing vapour pressure of the components. The dry scrubber, which was operated with petrol coke, reduces components up to pyrene PYR below the detection limit. The two components with the highest vapour pressure, i. This example shows that even extremely sticking tar particles can be reliably removed by fabric filters.
Concentrations of gaseous PAH components measured in a fume treatment with indirect cooler after a closed type ring furnace.
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Due to space limitations, high-temperature filtration will be only briefly touched in this paper. High-temperature gas cleaning technology is reviewed in several conference proceedings . Effect of dust properties on gas cleaning using rigid ceramic filters is described in Ref. Generally, application of high-temperature filtration technology is limited due to two important factors:.
At high temperatures, dust particles may sinter and therefore irreversibly clog the filter medium. They found that small amounts of chlorides in the ashes e. NaCl, CaCl2 affected the regeneration behaviour considerably. Therefore, the sintering behaviour of dust particles should be carefully considered. Dilatome-ter tests, as shown in Table 2, can indicate the danger of sintering. When mixtures occur, low melting eutectica have also to be taken into consideration. Problems due to sintering particles can be overcome by means of precoating the filter medium with an inert dust layer.
Integration of precoating into a high pressure and high-temperature filter system is, however, nontrivial and expensive. High-temperature filter elements can be distinguished into glass, ceramic and metal filter media made of fibres or granules. For ceramic and metal materials, different structures in the form of fibre fleeces, woven tissues and sintered granules, are available. Most of the commercially available filter media show excellent separation characteristics, and to some extent, also a reliable long-term filtration behaviour.
Excellent separation efficiencies were also found for nanoparticles like fumed silica. Specific cake resistance of fumed silica were about three orders of magnitude higher than those obtained for quartz . The economical disadvantages of high-temperature filtration technology due to high prices, not only for filter elements but of course also for the materials of construc-. Therefore, efforts were undertaken to develop an efficient but economically viable filter medium.
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This goal was achieved by using glass fiber woven media, coated with a thin ceramic layer and supported by a specially designed retainer. This new filter medium shows excellent chemical resistance against most atmospheres, with exception of fluorine and strong alkaline. There is a variety of solutions to separate solid and liquid particles with difficult dust properties. By looking at the physical fundamentals of particle separation, new and innovative solutions can be found.
Careful examination of the dust properties in the process is a precondition for the design of a gas cleaning process. The principal ways to reach this goal are:. ESP ,. Seville Ed. Iinoya et al.