Sulfur Oxides Transfer Additives

Sulfur emissions from FCC units cause to atmospheric pollution problems and refineries have to control the sulfur oxide (SOX) content of regenerator flue gas to comply with local or national restrictions. 

Wt% sulfur Feed HjS Gasoline Light-cycle oil Heavy-cycle oil Coke 

In 1978 the South Coast Air Quality Management District (SCAQMD) of California announced that the hmit on FCC flue gas SOX emissions would be 130 kg SOX per 1000 barrels of feed by 1981. This was then reduced to 60 kg SOX per 1000 barrels of feed by 1987. A further proposal, in 1990, suggested that emissions should eventually be lowered to 6-kg SOX per 1000 barrels of feed. The Federal Environmental Protection Agency, while establishing no limits, has specified the options available to reduce SOX emissions. These are the use of flue gas scrubbing, feed hydrodesulfurization, SOX additives, or low sulfur feeds. In Europe SOX emissions were included in an overall refinery sulfur emission limit and not treated separately. The sulfur content of reformulated gasoline has also been restricted by the EPA. 

Additives must rapidly absorb sulfur dioxide and trioxide as it is produced during catalyst regeneratiom The resulting sulfate is then reduced in the reactor and stripper to regenerate the additive and continue the cycle. The reactions taking place are shown schematically in Table 5.14. 

It was observed during the 1970s that high-alumina catalysts partly fulfilled these requirements. However, sulfur oxides in the flue gas were only decreased by about 20% because aluminum sulfate decomposes at a relatively low temperature of 580 C. Better absorbeuts were then investigated. Cerium oxide supported on alumina improved the absorption of sulfur trioxide, but performance 

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Sulfur oxide transfer additives work more effectively if a combustion promoter such as platinum is used to oxidize sulfur dioxide to sulfur trioxide more efficiently. More additive is required when a unit is operating under less oxidizing conditions and the coke is only partially converted to carbon dioxide. 

III. Electron transfer begins with the oxidation of UQH2 by the iron-sulfur protein in complex III, which generates ubisemiquinone (UQH ). Then the reduced iron-sulfur protein transfers an electron to cyt ch which transfers it to cyt c. See Figure 10.7 for additional details. 

A relationship between the redox state of an iron—sulfur center and the conformation of the host protein was furthermore established in an X-ray crystal study on center P in Azotobacter vinelandii nitroge-nase (270). In this enzyme, the two-electron oxidation of center P was found to be accompanied by a significant displacement of about 1 A of two iron atoms. In both cases, this displacement was associated with an additional ligation provided by a serine residue and the amide nitrogen of a cysteine residue, respectively. Since these two residues are protonable, it has been suggested that this structural change might help to synchronize the transfer of electrons and protons to the Fe-Mo cofactor of the enzyme (270). 

Proteins containing iron-sulfur clusters are ubiquitous in nature, due primarily to their involvement in biological electron transfer reactions. In addition to functioning as simple reagents for electron transfer, protein-bound iron-sulfur clusters also function in catalysis of numerous redox reactions (e.g., H2 oxidation, N2 reduction) and, in some cases, of reactions that involve the addition or elimination of water to or from specific substrates (e.g., aconitase in the tricarboxylic acid cycle) (1). 

Similarly, the reaction between sulfur tetrafluoride and tris(dimethylamino)phosphane proceeds with oxidation of the phosphane in addition to fluorine transfer to give tris(dimethylamino)-difluoro-A5-phosphane (18) as the main product. In contrast, the reaction with tris(methyl-sulfanyl)phosphane does not give a straightforward fluorination extensive Arbuzov-type rearrangements and fluorination reactions occur.220... 

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