Removal of Sulfur Dioxide

Very large quantities of poisonous sulfur dioxide are formed by the combustion of fossil-derived fuels in factories, power plants, houses 

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For dual alkali scrubbing, the major dissolved species are sodium chloride, sodium bisulfite, sodium sulfite, and sodium sulfate. If a separate prescrubber is provided for the removal of fly ash and hydrogen chloride before the removal of sulfur dioxide, then sodium chloride will not be present in the sulfur dioxide scrubber. 

The removal of sulfur dioxide from air has been studied by Komiyama and Smith (1975) in an agitated slurry reactor using activated carbon particles as catalyst at 25 °C and 1 atm. The gas stream consisted of 2.3% S02and 21% oxygen, and pure water was used as liquid phase. Activated carbon catalyses the oxidation of S02 to S03, which dissolves in the water to produce H2S04 ... 

The production process evolves close to 1 t of gaseous sulfur dioxide and 0.3 t of water-soluble sodium sulfoxides for every tonne of pigment produced. These must be disposed of in an environmentally acceptable manner. If the soluble salts are fully oxidized, they can be discharged safely into tidal waters. Future legislation in all producing countries may require removal of sulfur dioxide from the effluent gases before discharge to the atmosphere. 

R.J. Walker, C.J. Drummond and J.M. Ekmann, Evaluation of Advanced Separation Techniques for Application to Flue Gas Cleanup Processes for the Simultaneous Removal of Sulfur Dioxide and Nitrogen Oxides, Department of Energy Report, DE85102227 (May, 1985). 

An example of industrial relevance is the removal of sulfur dioxide (S02) from vent gases by absorption into water or a lime slurry (48). In the water absorption process, both gas-film and liquid-film resistance to mass transfer occurs. As a result the overall mass transfer rate is proportional to gas-flow rate and acceleration but inversely proportional to liquid-flow rate. Due to the fast reaction of S02 with lime, this system is only gas-film diffusion limited. The overall mass transfer rate is largely unaffected by gas- or liquid-flow rate and is proportional to acceleration, but to a lesser extent than the water absorption process. In both cases the overall mass transfer rate is reportedly much higher than the corresponding conventional packed towers. 

In an effort to avoid discharge of scrubber effluent into the Thames River, Imperial Chemical Industries, Ltd., British Power Authority consultants, and the Howden Construction Company together developed the ICI Howden process, a closed-loop, lime-based system for the removal of sulfur dioxide. This process was installed at the Swansea Power Plant in 1935 and at the Fulham Power Plant in 1937. Both systems operated successfully until their shutdown during World War II. 

In some cases, however, sorbent costs are aggravated by other problems. While nahcolite (a mineral form of sodium bicarbonate) is effective for the removal of sulfur dioxide when injected directly into the duct, the mineral is only beginning to be mined commercially, which increases the costs associated with the supply of this mineral. 

A mixture of humid air and sulfur dioxide forms the model gas for these experiments, as in today s world the removal of sulfur dioxide from exhaust gases is an important gas cleaning process. Calcium hydroxide suspension was injected as a reactant, and the experimental investigations have been subdivided into two parts. 

Removal of sulfur dioxide from a gas stream. If a fuel that contains sulfur is burned, the product gas contains sulfur dioxide. If the gas is released directly into the atmosphere, the SO2 combines with atmospheric oxygen to form sulfur trioxide. The SO3 in turn combines with water vapor in the atmosphere to form sulfuric acid (H7SO4), which eventually precipitates as acid rain. To prevent this occurrence, the combustion product gas is contacted with a liquid solution in an absorption or scrubbing process. The SO2 dissolves in the solvent and the clean gas that remains is released to the atmosphere. 

H. W. Kouwenhoven, F.W. Pijpers, and N. van Lookeren Campagne (Shell Internationale Research Mij. N.V), Removal of sulfur dioxide from oxygen containing gases, Brit. Pat. 

In addition to this wet scrubbing process, dry purification processes are also utilized for the removal of sulfur dioxide from furnace gases, such as the absorption of sulfur dioxide on activated charcoal (BF-process from Bergbau Forschung). 

Use Decolorizing of sugar, water and air purification, solvent recovery, waste treatment, removal of sulfur dioxide from stack gases and clean rooms, deodorant, removal of jet fumes from airports, catalyst for natural-gas purification, brewing, chromium electroplating, air- conditioning. 

Use Refractory for furnaces manufacture of magnesium compounds and magnesium metals as building material in fertilizers stock feeds papermaking ceramics mineral wool removal of sulfur dioxide from stack gases. 

Use Building stone, metallurgy (flux), manufacture of lime, source of carbon dioxide, agriculture, road ballast, cement (Portland and natural), alkali manufacture, removal of sulfur dioxide from stack gases and sulfur from coal. 

Use Refractories, especially for steel furnace linings, poly crystalline ceramic for aircraft windshields, electrical insulation, pharmaceuticals and cosmetics, inorganic rubber accelerator, oxychloride and oxysulfate cements, paper manufacture, fertilizers, removal of sulfur dioxide from stack gases, adsorption and catalysis, semiconductors, pharmaceuticals, food and feed additive. 

Use Soft drinks, photography, frozen desserts, meat products, detergents, special cheeses, electroplating, sequestrant and buffer, nutrient for cultured buttermilk, removal of sulfur dioxide from smelter waste gases, medicine (diuretic, expectorant), anticoagulant for blood withdrawn from body. 

The solid product from each of these sets of reactions is primarily calcium sulfite hemihydrate (CaSOs-5 H2O), which has been confirmed by x-ray diffraction analysis of scrubber sludgesJ l A similar set of reactions collects sulfur trioxide (SO3) from the flue gases, forming gypsum (CaS04 2H2O) as the solid product, but under normal boiler conditions sulfur trioxide makes up only about 0.5% of the total sulfur oxides, and so its removal is less important than the removal of sulfur dioxide.l " ... 

Bjornbom, E.N. Druesne, S. Zwinkels, M.F.M. Jaras, S.G. Study on the regeneration of copper-manganese sorbent for removal of sulfur dioxide from flue gases. Ind. Eng. Chem. Res. 1995, 34 (5), 1853-1858. 

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