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| [[File:DoctorSweeteningProcess-Kalinowsky.jpg|thumb|Doctor Sweetening Process; version as patented by Kalinowsky (1954)]]
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| The '''Doctor Sweetening Process''' is an industrial [[chemical process]] for converting [[mercaptan]]s in '''sour gasoline''' into disulfides. Sulfur compounds darken gasoline, give it an offensive odor and increase toxic [[sulfur dioxide]] engine emissions.<ref>McGraw-Hill Dictionary of Scientific and Technical Terms, 6th edition, published by The McGraw-Hill Companies, Inc., 2003</ref> However, this process only reduces the odor. | |
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| These sulfur compounds can be removed with the following chemical reactions:<ref>G.L. Wendt, S.H. Diggs: ''The Chemistry of "Sweetening" in the Petroleum Industry'', Industrial and Engineering Chemistry, Ausgabe 16, p. 1113-1115, 1924
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| (sour gasoline) 2RSH + Na<sub>2</sub>PbO<sub>2</sub>+S <sup>in the presence of NaOH</sup>---- R-S-S-R + PbS + 2NaOH (alkyl disulfide)
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| == Chemistry of the process ==
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| The chemistry of ‘doctor sweetening’ was described in detail by G. Wendt and S. Diggs in 1924. They showed that the lead oxide solution brought about oxidation of the mercaptans to the corresponding organic disulfides, which are comparatively odourless. Lead oxide (litharge) will dissolve in reasonably concentrated solutions of sodium or potassium hydroxide owing to formation of a soluble compound, sodium plumbite:
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| :<math>\mathrm{PbO + 2 \ NaOH \longrightarrow Na_2PbO_2 + H_2O}</math>
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| When this alkaline solution is agitated with petroleum, the two liquids do not dissolve in one another, but any mercaptan in the oil will unite with an equivalent amount of the lead (which then passes into the petroleum) to form what is called a lead mercaptide, soluble in the oil:
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| :<math>\mathrm{2 \ RSH + Na_2PbO_2 \longrightarrow (RS)_2Pb + 2 \ NaOH}</math>
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| If the mixture is now treated with powdered sulfur, which has a high affinity for lead, a black suspension of lead sulfide forms, and conversion of the mercaptide into a so-called disulfide (which remains in the oil) is induced:
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| :<math>\mathrm{ - (RS)_2Pb + S \longrightarrow RS - SR + PbS}</math>
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| With no sulfur added, but in the presence of atmospheric oxygen, the same conversion occurs, but only slowly, and probably not completely:
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| :<math>\mathrm{ 2 \ (RS)_2Pb + 4 \ NaOH + O_2 \longrightarrow 2 \ RS - SR + 2 \ Na_2PbO_2 + 2 \ H_2O}</math>
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| It is evident that the process does not remove the sulfur from the oil but even may increase the sulphur content if too much powdered sulfur is added, and some of the lead may remain in the petroleum.
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| The described chemistry is also the basis of the ''doctor test'' for the sweetness or sourness of gasoline (i.e., the extent of sulfur contamination). A gasoline is described as ''doctor sweet'' if, after shaking with sodium plumbite solutions, the addition of powdered sulfur fails to produce a dark precipitate of lead sulfide.
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| == Literature ==
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| * McBryde, W.A.E.: [http://dx.doi.org/10.1080/00033799100200161 ''Petroleum deodorized: Early canadian history of the ‘doctor sweetening’ process''], Annals of Science, Vol. 48, Issue 2, Taylor & Francis, 1991
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| * L. M. Henderson, W. B. Ross, C. M. Ridgway: [http://pubs.acs.org/doi/pdf/10.1021/ie50349a005 ''Tetraethyllead Susceptibilities of Gasoline Doctor Treatment vs. Caustic Washing''], Ind. Eng. Chem., 1939, 31 (1), p. 27–30
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| * Naphtali, Max: ''Fortschritte auf dem Gebiete der Mineralöle. Die technische Entwicklung der Erdölindustrie nach dem Kriege'', Angewandte Chemie, Band 42, Ausgabe 20, p. 508-518, WILEY-VCH Verlag GmbH, May 18, 1929
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| * Otto Rotton, William Archer: ''Deodorizing Petroleum'', American Artisan and Patent Record (New York), new series 5, p. 310, 1867
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| * G.L. Wendt, S.H. Diggs: ''The Chemistry of "Sweetening" in the Petroleum Industry'', Industrial and Engineering Chemistry, Ausgabe 16, pp. 1113-1115, 1924
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| * M.L. Kalinowski: [http://www.google.com/patents/US2871187.pdf ''Doctor sweetening process using sulfur''], US patent 2871187 January 27, 1957
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| ==References==
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| {{reflist}}
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| ==External links==
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| * {{cite patent |country=US |number=2871187 |status=patent | title=Doctor sweetening process using sulfur}}
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| * {{cite book|author1=Saeid Mokhatab|author2=William A. Poe|author3=J. G. Speight|title=Handbook of natural gas transmission and processing|url=http://books.google.com/books?id=eu9FYCoVpAQC&pg=PA397|accessdate=3 August 2010|year=2006|publisher=Gulf Professional Publishing|isbn=978-0-7506-7776-9|pages=397–}}
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| [[Category:Oil refining]]
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