Fume capture

Enclosure design on retrofit cases becomes very difficult and may require a compromise between process vessel operation and fume capture performance. 

When primary fume capture is performed by the enclosure, furnace off-gas combustion efficiency is lower than experienced by furnace direct evacuation control. The off-gas, rich in carbon monoxide (CO), rises from furnace roof openings and partially burns and cools with enclosure air. Significant levels of CO have resulted in the enclosures and exhaust ducting from this type of combination. These levels are not explosive but present a potential hazard to personnel working in the enclosure or in downstream fume cleaning equipment. 

Fume Capture Fume capture is accomplished by a combination of the following enclosure features ... 

For a high-production furnace the fume volume flow rate after air dilution to 130 °C will be considerably higher than for secondary fume control by enclosure. A separate primary fume capture system would be used for this case. 

Consider a fume control system with an overall hood fume capture efficiency of 60% and a fume cleaning efficiency of 99%. Calculate overall fume control system performance. 

The technology in the fume capture field Is not well developed, and performances of many capture systems are low and typically may be in the 30% to 60% range. There is a paucity of fundamental research and development in the fume capture field. In contrast, hundreds of million of dollars have iteen spent on research and development activities in the gas-cleaning area, which is mature and well developed. It is not uncommon to specify and to measure gas-cleaning equipment performances of over 99.9% colleaion efficiency. As shown in Eq. (13.75), the ovcTall fume control system performance is determined by the product of the capture efficiency and the gas-cleaning efficiency. This equation clearly shows the need to improve the efficiency of capture of the fume at the source in order to obtain significant improvements in the overall fume control system performance. 

Fume capture techniques can be classified into three specific n pes as follows ... 

A pictorial representation of the different fume capture techniques is shown in Fig. 13.33. 

Table 13.17 lists some of the important considerations for the different fume capture techniques. From the point of view of cost effectiveness, the usual preference is source collection or a low-level hood, provided an acceptable scheme can be developed within the process, operating, and layout constraints. The cost of fume control systems is almost a direct function of the gas volume being handled. Flence, the lower volume requirements for the source capture or low-level hood approach often results in significant capital and operating cost savings for the fume control system. 

The use of a source capture or a direct evacuation system is the most positive form of fume capture. A well-designed system can operate at high fume capture efficiencies. For many of these systems, the captured gas temperature for the processing operation is very high (1000-1500 °C), and gas cooling may be required... 

Figure 13.36 is a plot of the preceding equation for the three types of hoods. The plot shows the curve for the actual and the worst hoods requiring a hood flow rate larger than the plume flow rate in order to get 99% fume capture. 

This addition provides about 0.05 % of the magnesium in the melt, most of the remainder oxidises and escapes to atmosphere as MgO, where it will agglomerate fairly rapidly in the air. Where there is no fume capture, the fumes can spread through the foundry and a proportion will drop out in the foundry as dust. There is no accurate information available on this amount but a reasonable figure may be 50 % of the fume released. Therefore, for each tonne of metal treated there would be around 500 g of magnesium released to air at the ladle as MgO (i.e. 833 g of MgO released per tonne of metal treated) and about 400 g of MgO released to the external atmosphere. 

Solid particulates are captured as readily as hquids in fiber beds but can rapidly plug the bed if they are insoluble. Fiber beds have frequently been used for mixtures of liqmds and soluble sohds and with soluble solids in condensing situations. Sufficient solvent (usually water) is atomized into the gas stream entering the collector to irrigate the fiber elements and dissolve the collected particulate. Such nber beds have been used to collect fine fumes such as ammonium nitrate and ammonium chloride smokes, and oil mists from compressed air. 

The actual name dry scrubbing was first publicized by Teller [U.S. Patent no. 3,721,066 (1973)]. He worked both with classical Army-type soda-lime and with his patented water-activated form of the alkaline feldspar nepheline syenite as a flow agent and feedstock sorbent for HF and SO9 in hot, sticky fumes from glass melting furnaces. He claimed capture of more than 99 percent of 180 ppm HF and SO9 for more than 20 hours in a packed bed of 200 X 325 mesh hydrated nephehne syenite at 42,000/hr. 

Basic oxygen furnaces oxygen blowing Fumes, smoke, CO, particulates (dust) Proper hooding (capturing of emissions and dilute CO), scrubbers, or electrostatic precipitator... 

Dry scrubber systems applied to the pot fumes and to the anode baking furnace result in the capture of 97% of all fluorides from the process. 

This latter equation can also be used for systems without a local exhaust hood by setting the capture efficiency to zero. It could also be used to show the result of recirculation from, e.g., a laboratory fume hood with immediate recirculation. In such a hood all contaminants are generated within the hood and usually also all generated contaminants are captured, so the capture efficiency is 1. The equation demonstrates that if the... 

On oxygen steel conversion furnaces, primary fume control is usually achieved by a separate close-capture hood positioned over the vessel mouth. The enclosure is then used for secondary fume control during charging, turndown, tapping, and slagging. 

Step 1 Determine primary emission heat content. This step should be taken early in the design stage to determine if the enclosure will capture both primary and secondary emissions. The heat content of furnace emissions and the temperature limitation on the fume collector are considered for this task. The off-gas heat content is calculated for furnace reactions during melting and refining periods. The maximum heat content should be used for design. Assuming a fabric... 

Gases, vapors, and fumes usually do not exhibit significant inertial effects. In addition, some fine dusts, 5 to 10 micrometers or less in diameter, will not exhibit significant inertial effects. These contaminants will be transported with the surrounding air motion such as thermal air current, motion of machinery, movement of operators, and/or other room air currents. In such cases, the exterior hood needs to generate an airflow pattern and capture velocity sufficient to control the motion of the contaminants. However, as the airflow pattern created around a suction opening is not effective over a large distance, it is very difficult to control contaminants emitted from a source located at a di,stance from the exhaust outlet. In such a case, a low-momentum airflow is supplied across the contaminant source and toward the exhaust hood. The... 

For an existing process plant, the designer has the opportunity to take measurements of the fume or plume flow rates in the field. There are two basic approaches which can be adopted. For the first approach, the fume source can be totally enclosed, and a temporary duct and fan system installed to capture the contaminant. For this approach, standard techniques can be used to measure gas flow rates, gas compositions, gas temperatures, and fume loadings. From the collected fume samples, the physical and chemical characteristics can be established using standard techniques. For most applications, this approach is not practical and not very cost effec tive. For the second approach, one of three field measurement techniques, described next, can be used to evaluate plume flow rates and source heat fl uxes. 

The nature of the preceding analysis does not permit the application of the technique to design of local capture hoods but rather to the design of remote or canopy fume hoods. For this approach to be valid, the hoods must usually be at least two source diameters away from the emission source. 

The use of canopy hoods or remote capture of fume is usually considered only after the rejection of source or local hood capture concepts. The common reasons for rejecting source or local hood capture are usually operating interference problems or layout constraints. In almost all cases, a canopy hood system represents an expensive fume collection approach from both capital and opetating cost considerations. Remote capture depends on buoyant ait curtents to carry the contaminated gas to a canopy hood. The rising fume on its way to the hood is often subjected to cross-drafts within the ptocess buildings or deflected away from the hood by objects such as cranes. For many of these canopy systems, the capture efficiency of fume may be as low as 30-50%. 

Bender, M. Fume Hoods, Open Canopy Types—Their Ability to Capture Pollutants in V ar -ous Environments. Am. Industrial Hygiene Assoc. J. 40 (1979), pp. 118-127. 

Range hood An extraction hood positioned above a cooking range to provide the best possible capture velocity of the fumes. 

Local extract systems are designed specifically to remove fumes, dust, mists, heat, etc. at source from machinery and fume cupboards. The main design considerations are capture of the contaminant, which will normally involve special hoods or cabins, and extract at sufficient velocity... 

Fume scrubber water recycle. The steel finishing industry uses fume scrubbers to capture acid gases from pickling tanks. Scrubber water, which may contain a dilute caustic solution, is neutralized and recirculated continuously to adsorb the acid. Makeup water is added to replace water lost through evaporation and water that is blown down to end-of-pipe metals treatment. 

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