Chemical scrubbings

Sodium hydroxide, caustic soda Electrolytic Chlorine Mercury Alkaline scrubbers Chemical scrubbing and adsorbers... 

Mercury is emitted from the mercury cell process from ventilation systems and by-product streams. Control techniques include (1) condensation, (2) mist elimination, (3) chemical scrubbing, (4) activated carbon adsorption, and (5) molecular sieve absorption. Several mercury cell (chloralkali) plants in Japan have been converted to diaphragm cells to eliminate the poisonous levels of methyl mercury found in fish (9). 

Ventilation of sewers may not only reduce the hydrogen sulfide concentration in the sewer atmosphere but also the moisture that is a fundamental requirement for establishment of microbial activity on the sewer walls. It is important that the ventilation be well controlled otherwise, odorous problems in the vicinity of the sewer network may occur. In some cases, operational procedures like treatment of the vented air, e.g., by wet oxidation, by chemical scrubbing or by passing a biofilter, may need to be considered. 

The SW-400 soil washing unit is an ex situ, treatment technology that removes contaminants from soil using biodegradable chemical surfactants and/or coUoidals. The water-based, soil washing process mechanically and chemically scrubs excavated soUs. Contaminants are removed from soils by suspension or dissolution within the wash solution. The SW-400 is mobile and transportable and can operate either as a stand-alone technology or in combination with others. The SW-400 system is currently commercially available. 

The efficiency of reactant utilization in CVD on float glass can be as low as 10%, necessitating the installation of expensive chemical scrubbing units or incinerators and requiring landfill of more than one million kg/year of waste. 

Chemical scrubbing systems for SO/ absorption fall into two broad categories (a) Disposable systems and (b) regenerative systems. Typical of systems in use for a number of years are those that use an aqueous slurry of an insoluble caldum compound, which can be discarded after use. Disposable 02-removal systems use aqueous slurries of finely ground materials, such as lime, limestone or dolomite, to produce a mixture of insoluble sulfites and sulfates. On passing through the scrubber, S02 from the waste gas dissolves to form sulfurous acid S02 ... 

Removal of degradation products from spent solvents. Several methods of regeneration have been used to maintain the PUREX process solvent quality (143) chemical scrubbing treatment, specific management of solvent streams, and regeneration of solvent by distillation. 

Recycle designs of this type are limited to applications in which the components of the gas mixture, if sufficiently concentrated, can be separated from the gas by some other technique. With organic vapors, condensation is often possible adsorption, chemical scrubbing or absorption can also be used. The process shown in Figure 8.20 is used to separate VOCs from nitrogen and air or to separate propane, butane, pentane and higher hydrocarbons from natural gas (methane). 

FIG. 22-20 Relative (and approximate) treatment costs for removing VOCs from air (Kosky and Neff, 1988). Assumption 40,000 m3/h at 1982 price level. (Note Current costs for chemical scrubbing can be 2 or more times greater than for biofiltration.)... 

Combinations of physical and chemical absorption are also used, as in Shell s Sulfinol process in which a mixture of diisopropanolamine and sulfolane in water is utilized. For the hydrogen sulfide-free gases from the steam-reforming process, chemical scrubbing with activated potassium carbonate solutions or alkanolamines is preferred. In the case of hydrogen sulfide-containing gases from the partial oxidation process, physical absorption alone or in combination with chemical absorption is preferred. 

Chemical scrubbing Amines are strong bases, and react with sulphuric or phosphoric acid. The pH of the solution is normally controlled to below 3. As a consequence, the scrubbing solution is replaced from time to time to remove concentrated salts, which subsequently require disposal as a hazardous waste. It is technically possible to process the scrubbing solution to reclaim the amine for re-use. This is discussed in Section 4.6.5 See Section 4.5.S.6. 

Recently, a new exhaust gas treatment based on the chemical scrubbing of the unreacted SO3 (by reaction with freshly added LAB) has been developed and retrofitted in several industrial plants located in areas where the acidic dripping disposal indeed has a high cost and poor feasibility. 

Usually, natural gas treatment on the basis of thermal process engineering takes place in three steps (see Fig. 7.9). The first step that may consist of partial steps just like all other subsequent steps, serves the preparation of the crude gas for its processing. Here, for example, acid-forming gas components, such CO2, H2S and other sulphuric compounds are removed. Usually, chemical scrubbing with amines (MEA, DEA, MDEA) is applied in which the adsorbent is being regenerated. Then the natural gas is dried. In case of moderate water dew point requirements, glycol is used as wash liquor. The lowest water contents (< 1 ppm) are achieved with the application of zeolitic molecular sieves. Finally, mercury is removed in case aluminium will be used as material of construction for equipment. Mercury in contact with aluminium may lead to catastrophic corrosion. 

Dunson, J. B., Jr., 1991. Chemical Scrubbing for Control of Air Emissions, presented at AIChE Summer Meeting, Pittsburgh, PA, Aug. 18-21. 

SO2 is released into the atmosphere by a variety of industrial and natural processes. Typical concentrations of this gas from stack emissions range from 100 to 5200 ppm requiring chemical scrubbing before release into the atmosphere.(i) SO2 emissions pose a serious problem to the environment because they readily form acid rain in the higher atmosphere. Presently, few analytical procedures are available for the quantitation of this gas.(2) Rapid, selective, and economical detection of this gas would be useful for regulating its atmospheric discharge. 

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