Active scrubbers

Active scrubbers, which include pumps or other active components. These scrubbers generally circulate chemicals (such as water, acid, caustics, or organics) through a packed or trayed tower to absorb and/or condense hazardous materials from a vapor stream. The active components may run continuously, or only when material is released to the scrubber. This approach is most effective when the circulating fluid reacts with the material being adsorbed or condensed. 

Other industries of interest are (1) the manufacturing of spices and flavorings, which may use activated carbon filters to remove odors from their exhaust stream (2) the tanning industry, which uses afterburners or activated carbon for odor removal and wet scrubbers for dust removal and (3) glue and rendering plants, which utilize sodium hypochlorite scrubbers or afterburners to control odorous emissions. 

Large active exposed surface area the specific surface area, e.g., the exposed surface area of the packing per cubic foot of packed volume, is important since larger specific area means a larger area available for contact between the liquid and the gas, resulting in more efficient scrubber operation. 

Other methods have been developed for the removal of oxygen (particularly from flowing streams). These include the use of electrochemical or chemical (zinc) scrubbers, nitrogen-activated nebulizers, and chemical reduction (by addition of sodium sulfite or ascorbic acid). Alternately, it may be useful to employ voltam-... 

Catalytic incineration has been appHed in the abatement of chlorinated VOC emissions in the pharmaceutical industry. The major compounds in the emission mixture are dichloromethane, perchloroethylene, dimethylformamide, oxitol, and toluene. The incinerator operates normally at 400-500 °C, but when emissions contain perchloroethylene the temperature is increased up to 500-600 °C. The emission mixture also contains water, which pushes the selectivity further toward HCl formation instead of formation of CI2. After oxidation, the product gases are washed with NaOH scrubbers. The purification level of over 99% can be achieved with the incinerator, the activity of which has been shown to be very stable after one year of continuous operation [69-71]. 

Oxygen-containing molecules cannot be tolerated in the ammonia synthesis, primarily because they form iron oxide that blocks the active surface. First the CO2 is removed, through a scrubber, by reaction with a strong base. The remaining CO (and CO2) is then removed by the methanation reaction, converting the CO into methane and water. Finally the water is removed by, for example, molecular sieves. Methane does not present problems because it interacts weakly with the catalyst surface. The gas mixture (Tab. 8.6) is compressed to the roughly 200 bar needed for ammonia synthesis and admitted to the reactor. 

Dust created by mining and milling activities blown by the wind from ore stockpiles is a potential source of environmental contamination. The watering of stockpiles or erection of enclosures is resorted to when necessary in order to prevent the spread of dust. Where ventilation systems collect significant amounts of uranium dust in mills, scrubbers are used to remove the dust from the air before it is discharged to the environment. 

Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5). 

For instance, a company might have a 50-day supply of product available, but be incurring a 250-a-day fine from the Environmental Protection Agency for each day it operates without a scrubber. Under these circumstances, it might be expedient not to crash those activities on the critical path whose slopes are greater than 250 per day. 

S02 and NOx in flue gas from coal combustion contribute to smog and acid rain. Methods to remove these pollutants include alkaline wet scrubber systems that fix S02 to solid CaS04, and selective catalytic reduction by metal/metal oxide systems of NO/NOz to N2 and steam in the presence of ammonia. Particulate active carbons have also been used in flue gas decontamination, especially as they avoid costly scrubber processes and can operate at lower temperatures. The potential of active carbon fibers in this application has been explored by a... 

A couple of years ago an investigation was carried out to evaluate the efficiency of odour reducing processes based on different principles, such as chemical scrubbers, soil bed filters, activated carbon filters, iron oxide filters, and combustion. 

Scrubber water is onoculated with activated sludge, i.e. special micro-organisms ... 

Bioscrubbers working on the principle of the activated sludge technique involve high input in design and control. Therefore it is only the more simply designed percolator scrubber that has been used for the practical examples of installation shown in the following. 

There is not much to say about the costs of such implements as they are in each pracitical example individual constructions. In principle, it can be stated that for processes involving waste gases which do not have too strong odour concentration, the percolator scrubbers are the most favourable solution. They are small, compact, efficient and thus more economy-priced than the activated sludge scrubbers. 

Considering that wet scrubbers are in place in many facilities and more are planned for the future, another efficient means for NO removal could be considered. These scrubbing methods are limited by the relatively inert nature of NO. It has been proposed that this difficulty can be overcome by the conversion of NO to the much more active N02 through reaction (8.84), as discussed earlier ... 

TRW Systems, Inc., conducted a laboratory-scale incineration study for the U.S. Army from 1973 to 1975 (9). Eleven individual pesticide formulations and three mixed pesticide formulations containing six different active ingredients (chlordane, 2,4-D, DDT, dieldrin, lindane, and 2,4,5-T) were incinerated in a liquid injection incinerator. The experimental apparatus consisted of a fuel atomizer, combustion chamber, afterburner, quench chamber, and scrubber unit. Destruction efficiencies exceeded 99.99% for a minimum 0.4-s residence time at temperatures above 1000°C with 45 to 60% excess air. 

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