Sulfur dioxide removal, absorption

As worldwide attention has been focused on the dangers of acid rain, the demand to reduce sulfur dioxide [7446-09-5] emissions has risen. Several processes have been developed to remove and recover sulfur dioxide. Sulfur can be recovered from sulfur dioxide as Hquid sulfur dioxide, sulfuric acid, or elemental sulfur. As for the case of hydrogen sulfide, sulfur dioxide removal processes are categorized as adsorption, absorption, or conversion processes. 

Regenerable absorption processes have also been developed. In these processes, the solvent releases the sulfur dioxide in a regenerator and then is reused in the absorber. The WelLman-Lord process is typical of a regenerable process. Figure 11 illustrates the process flow scheme. Sulfur dioxide removal efficiency is from 95—98%. The gas is prescmbbed with water, then contacts a sodium sulfite solution in an absorber. The sulfur dioxide is absorbed into solution by the following reaction ... 

Throwaway processes generally remove sulfur dioxide by absorption into a lime or limestone slurry or a clear solution. Figures 3-5 show general diagrams for these processes (22,23,24, 25). 

As a direct result of this pilot effort, a wetted film packing was found which exhibits excellent sulfur dioxide absorption with an exceptionally low pressure drop. This packing has outstanding mass transfer characteristics and high specific surface area. The packing developed by Munters Corp. is a key element in the limestone-based sulfur dioxide removal system. 

Research-Cottrell is also supplying a gas cleaning system using an electrostatic precipitator for dry particulate collection followed by a multi-stage gas liquid contactor for sulfur dioxide removal. The first stage is a cyclonic quencher for gas saturation and moderate sulfur dioxide absorption followed by a spray and packed-tower section where the major sulfur dioxide absorption takes place. Again the reagent is applied countercurrent to the gas flow. 

The reagent stream must be controlled to permit calcium salt desupersaturation external to the scrubber and absorber while maintaining adequate concentration levels for good absorption efficiency. In order to do this a reagent stream containing 8-15% solids is circulated. The solid portion is composed of some fly ash components but mainly calcium carbonate, sulfite, and sulfate. Sulfur dioxide removal efficiency dictates the carbonate level. Sulfite crystals enhance and control desupersaturation of calcium sulfate while providing nucleation sites for crystal growth... 

Feeding solutions from the absorber system and the regeneration system through surge tanks enables the entire recovery process to operate smoothly and reliably despite frequent gas flow and concentration fluctuations. In addition, the surge tanks allow the regeneration section to be shut down for up to 3 days without interfering with the sulfur dioxide removal in the absorption section. This is possible because the absorber is the only part of the system that contacts the flue gas and removes the sulfur dioxide. 

Gas cooling, cleaning, and sulfur dioxide removal is accomplished by adiabatically cooling flue gas with quench water, passing into a venturi-type water scrubber to remove fly ash, followed by absorption of the sulfur dioxide in an aqueous solution of sodium citrate and citric acid. The pilot plant has demonstrated the feasibility of a commercial plant consistently to remove more than 95% of the sulfur dioxide in the inlet gas. The pilot unit has operated for prolonged periods with exit gas of 25-50 ppm sulfur dioxide. 

Tezcan Un U, Koparal AS, Bakir Ogiitveren U (2007) Sulfur dioxide removal from file gases by electrochemical absorption. Sep Purif Technol 53(l) 57-63. doi 10.1016/j.seppur.2006.06.016... 

Operating data for four absorption systems of plants utilizing the Cominco process are presented in TaUe 7-25. Obsraved sulfor dioxide removal efficiencies vary fonn 8S to 97%. The degree of sulfur dioxide removal attainable in a system of this type is obviously d en-doit upon a large nuniba of variables. Chief among these are... 

The absorption of sulfur dioxide in alkaline (even weakly alkaline) aqueous solutions affords sulfites, bisulfites, and metabisulfites. The chemistry of the interaction of sulfur dioxide with alkaline substances, either in solution, slurry, or soHd form, is also of great technological importance in connection with air pollution control and sulfur recovery (25,227,235—241). Even weak bases such as 2inc oxide absorb sulfur dioxide. A slurry of 2inc oxide in a smelter can be used to remove sulfur dioxide and the resultant product can be recycled to the roaster (242). 

This reaction can be forced to effective complete conversion by first carrying out the reaction to approach equilibrium. The sulfur trioxide is then separated (by absorption). Removal of sulfur trioxide shifts the equilibrium, and further reaction of the remaining sulfur dioxide and oxygen allows effective complete conversion of the sulfur dioxide, Figure 6.6. 

Hydrogen sulfide in the air is oxidized at a relatively slow rate by molecular oxygen (02) but at a much faster rate by hydroxide (OH) radicals, forming the sulfhydryl radical and ultimately sulfur dioxide or sulfate compounds (Hill 1973 NSF 1976). Sulfur dioxide and sulfates are eventually removed from the atmosphere through absorption by plants and soils or through precipitation (Hill 1973). 

One way to control gaseous pollutants like SO2 and SO3 is to remove the gases from fuel exhaust systems by absorption into a liquid solution or by adsorption onto a solid material. Absorption involves dissolving the gas in a liquid while adsorption is a surface phenomenon. In each case, a subsequent chemical reaction can occur to further trap the pollutant. Lime and limestone are two solid materials that effectively attract sulfur dioxide gas to their surfaces. The ensuing chemical reaction converts the gaseous pollutant to a solid nontoxic substance that can be collected and disposed or used in another industry. 

ATS [Ammonium thiosulfate] A process for removing residual sulfur dioxide from Claus tail gas by absorption in aqueous ammonia to produce ammonium sulfite and bisulfite. Addition of hydrogen sulfide from the Claus unit produces saleable ammonium thiosulfate. Developed by the Pritchard Corporation and first operated by the Colorado Interstate Gas Company at Table Rock, WY. 

Citrate A process for flue-gas desulfurization by absorption of the sulfur dioxide in aqueous sodium citrate, reacting with hydrogen sulfide to produce elemental sulfur, and recycling the citrate solution. Subsequent modifications involved removing the sulfur dioxide from the citrate solution by steam stripping, or by vacuuming the gas used to make sulfuric acid. 

Hydromag A process for removing sulfur dioxide from industrial gas streams by absorption in magnesia. Developed in Japan by Nissan Chemical Industries. 

Peracidox A process for removing sulfur dioxide from the tail gases from sulfuric acid plants by absorption in peroxomonosulfuric acid (Caro s acid). The peroxomonosulfuric acid is generated on-site by the electrolytic oxidation of sulfuric acid. Developed by Lurgi and Sud-Chemie and first operated in 1972. 

Townsend A process for removing hydrogen sulfide from natural gas by absorption in triethylene glycol containing sulfur dioxide. Part of the sulfur produced is burnt to sulfur dioxide in order to provide this solution. The hydrogen sulfide and sulfur dioxide react in the presence of water to generate elemental sulfur. Invented in 1959 by F. M. Townsend. 

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