Particle electrostatic precipitation

Typical mechanisms for aerosol removal from gas streams by filters are diffusion to surfaces, interception and impaction. Very large particles can be removed by gravitational settling. These mechanisms are quite dependent on the particle size and it is usually found that conventional filters have a minimum in filter efficiency for particles in a narrow size range less than 1 im. When the gas is hot relative to the filter, thermophoresis can enhance particle removal. When the aerosol laden gas stream contains elevated concentrations of steam that condenses within the filter, difflisiophoresis will enhance particle removal. These phoretic enhancements of filtration are attractive because filtration efficiencies by these mechanisms are not especially dependent on the aerosol particle size. Washed Venturi scmbbers involve the injection of water droplets into the aerosol laden gas and these water droplets act much like spray water droplets to remove aerosol particles. Electrostatic precipitation is, in principle, a very attractive decontamination process, but it is difficult to assure that the necessary power will be available to operate the precipitators under accident conditions. 

During the oxidisation process some of the volatile metals are vaporised and contained in the off-gas along with SO2 and other particles. Electrostatic precipitators or filters pick up most of the dispersed particles. Gases low in SO2 are circulated repeatedly through the charge. This produces an enriched gas containing 4-6 per cent SO2, suitable for the subsequent manufacture of sulphuric acid. 

Figure 11.3 Electrostatic precipitation can be used to remove fine particles. (Reproduced with permission from Stenhouse, Pollution Control, in Teja, Chemical Engineering and the Enuironment, Blackwell Scientific Publications, Oxford, U.K., 1981.)...

The gas, along with entrained ash and char particles, which are subjected to further gasification in the large space above the fluid bed, exit the gasifier at 954—1010°C. The hot gas is passed through a waste-heat boiler to recover the sensible heat, and then through a dry cyclone. SoHd particles are removed in both units. The gas is further cooled and cleaned by wet scmbbing, and if required, an electrostatic precipitator is included in the gas-treatment stream. 

N2, and traces of PH, CO2, E, and S large furnaces generate off-gas at a rate of about 120—180 m /min. In most installations the off-gas is passed through a series of Cottrell electrostatic precipitators which remove 80—95% of the dust particles. The precipitators ate operated at temperatures above the 180°C dew point of the phosphoms. The collected dust is either handled as a water slurry or treated dry. Einal disposal is to a landfill or the dust is partially recycled back to the process. The phosphoms is typically condensed in closed spray towers that maintain spray water temperatures between 20 and 60°C. The condensed product along with the accompanying spray water is processed in sumps where the water is separated and recycled to the spray condenser, and the phosphoms and impurities ate settled for subsequent purification. 

To reduce catalyst losses even further, additional separation equipment external to the regenerator can be installed. Such equipment includes third-stage cyclones, electrostatic precipitators, and more recentiy the Shell multitube separator, which is Hcensed by the Shell Oil Co., UOP, and the M. W. Kellogg Co. The Shell separator removes an additional 70—80% of the catalyst fines leaving the first two cyclones. Such a third-stage separator essentially removes from the due gas stream all particles greater than 10 p.m (36). 

Recovered catalyst and blowdown gas (- 3% of the flue gas) exit from the bottom of the separator to an electrostatic precipitator or to a small, fourth-stage cyclone for further concentration of catalyst fines. The flue gas, with 70—90% of the catalyst particles removed, passes from the separator into the power expander. 

Small solid particles, present in dust and grit emissions, have very low settling velocities (Table 4.4) The collection efficiencies of simple cyclones are tlierefore, as shown in Figure 17.3, relatively low. Fabric filters, electrostatic precipitators or wet scrubbers may be required to remove particles <5 pm in size with an acceptable efficiency. Therefore the cost of pollution control inevitably increases when dealing with particle size distributions skewed towards the lower end. 

Electrostatic precipitators (ESPs) remove particles by using an electrostatic field to attract the particles onto the electrodes. Collection efficiencies for well-designed. 

Fabric filters are useful for collecting particles with resistivities either too low or too high for collection with electrostatic precipitators. Fabric filters therefore may be good candidates for collecting fly ash from low-sulfur coals or fly ash containing high unburned carbon levels, which respectively have high and low resistivities, and thus are relatively difficult to collect with electrostatic precipitators. 

The most common equipment for cleaning recirculated air from particles is fabric filters, mechanical collectors, electrostatic precipitators, and cleaners and wet collectors.For cleaning of recirculated air from gases, absorbers and adsorbers such as activated carbon, sometimes with impregnation for specific gases, and impregnated alumina are most common. The performance of different air cleaning equipment is described in many textbooks and handbooks. 

Electrostatic precipitation is one of the fundamental means of separating solid or liquid particles from gas streams. This technique has been utilized in numerous applications, including industrial gas-cleaning systems, air cleaning in general ventilation systems, and household room air cleaners. 

The development of electrostatic precipitators was based mostly on empirical work, and it has produced more than 1000 patents covering all aspects of electrostatic air cleaning. From the theoretical point of view, important milestones were papers published by Deutsch as well as White.These papers deal with the collection efficiency of the electrostatic precipitator. The most important early papers dealing with the electrical charging of particles are the ones published by Arendt and Kallmann, Pautheniet and Moreau-Hanot, and White. ... 

A very important parr of the gas-deatimg process is the removal of the collected particles from the cleaning system. This should be as controlled as possible in order to avoid particle reenrrainmenr to the gas flow. This can be accomplished in the case of liquid particles such as acid fume or tar or oil smoke. olid particles are normally removed by periodic rapping of discharge and collection electrodes. Solid particles can also be removed with the aid of water, as is done in wet electrostatic precipitators. 

The properties of gas ions are of great importance for the electrical performance of an electrostatic precipitator. They also are very important for particle-charging processes. The size of gas ions is normally such that they can be regarded as gas molecules carrying a single elementary charge. It can even be assumed that ions form a gas component with a very low- partial pressure. Thus, the thermal motion of gas ions is assumed to be similar to that of gas molecules. The most important parameters describing the properties of gas ions are... 

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