Petroleum deodorized: early Canadian history of the ‘Doctor Sweetening’ process

Monday, May 24, 1993

W.A.E. McBryde, Department of Chemistry, University of Waterloo

The Canadian oil industry was grossly disadvantaged throughout the nineteenth century because of several factors: want of capital, long delays in gaining access to appropriate transportation, but above all, crude oil at the well-head containing an unusually high content of sulphur, mainly in the form of foul-smelling mercaptans. Save for a brief interlude during the American Civil War, when the Pennsylvania production was suspended, the Canadians were faced with the predicament of having an abundance of oil for which, because of its legendary bad odour and inferior burning qualities, they could not find a market. It was quite evident that the customary chemical refining procedures, based on treatment with sulphuric acid, were inadequate to yield a satisfactory product for commercial use.

However, toward the end of the 1860’s, local reports conveyed a new mood of optimism among the Canadian producers. We learn that William Spencer, an oil producer in London, Ontario, greatly improved the quality of the oil produced in his refinery by the discovery that litharge would ‘sweeten’ the oil. This process he sold to other refineries that had by this time established in London. ‘Discovery’ is certainly an exaggerated claim on behalf of Spencer, in view of the information recently published in a trade paper that he would almost certainly have seen, not to mention a Canadian patent. It is almost impossible to date precisely when he put this new procedure into operation, but by 1869 a number of press reports were expressing renewed confidence in the ability of Canadian refiners to produce a commodity of distinctly higher quality than before, and Spencer’s firm was one of the two specifically mentioned. Thus, in August 1869, the Canadian News stated: "Since the new mode of treating the oil has been discovered an export trade of important dimensions has sprung up. However, for competitive reasons the ‘new mode’ of treatment was evidently not made public form some time. A complete description of what was undoubtedly the contemporary form of the ‘new mode’ appeared as the preamble to a new patent granted in 1879 to E.J. Hodgens of London, Ontario. This preamble was a detailed description of the procedure carried out previously, and on which the patent claimed an improvement. This existing practice consisted of treatment of the petroleum distillate with sulphuric acid, followed by a wash with water, then agitation with caustic lead, and finally the addition of flowers of sulphur ‘to separate the impurities.’ When, and by whom, this important final step was added is not stated, but from what has already been explained in this paper about ‘doctor sweetening,’ it should be appreciated that this addition marked a significant evolution of the process. As far as I have been able to determine, this is the earliest occasion of its being mentioned. The addition of sulphur serves two purposes: first, to precipitate most of the lead out of the oil and thereby to enable its recovery; and second, to oxidize the lead mercaptide to an odourless disulphide and, at the same time, to discharge the dark colour imparted to the oil by the former compound. Rotton and Archer mentioned this discolouration as a drawback to the use of sodium plumbate. ‘Doctor sweetening’ is the name of a procedure for deodorizing petroleum distillate by means of an alkaline solution of lead oxide (litharge). Much of the unwanted odour of the oils to be so treated is due to mercaptans (sulphur analogues of alcohols) whose foul odour is notorious; removal of the odour was said to render the oil ‘sweet.’ The procedure was extensively used in petroleum refineries during the period between the two world wars in the twentieth century, but it was superseded gradually as the demand for fuels of higher octane number led to changes in refining practice. Even after the original deodorizing practice was no longer in use, its name partially survived in that of the ‘doctor test’ for sulphur compounds in gasoline, which is also based on the use of alkaline solutions of lead oxide.

The chemistry of ‘doctor sweetening’ was worked out and published by G. Wendt and S. Diggs in 1924. These authors showed that the lead oxide solution brought about oxidation of the mercaptans to the corresponding organic disulphides, 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 (sometimes called sodium plumbate):

PbO + 2NaOH = Ha2PbO2 + H2O

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:

2RSH + Na2PbO2 = (RS)2Pb + 2NaOH

If the mixture is now treated with powdered sulphur, which has a high affinity for lead, a black suspension of lead sulphide forms, and conversion of the mercaptide into a so-called disulphide (which remains in the oil) is induced:

- (RS)2Pb + S = RS – SR + PbS

With no sulphur added, but in the presence of atmospheric oxygen, the same conversion occurs, but only slowly, and probably not completely:

(RS)2Pb + 2NaOH + ½O2 = RS – SR + Na2PbO2 + H2O

It is evident from this description that the overall procedure does not remove the sulphur from the oil; indeed, it may even increase the sulphur content if too much powdered sulphur is added. On a small scale, the same chemical reactions form the basis of the ‘doctor test’ for the sweetness or sourness of gasoline (i.e., the extent of sulphur contamination). A gasoline is described as ‘doctor sweet’ if, after shaking with sodium plumbite solutions, the addition of powdered sulphur fails to produce a dark precipitate of lead sulphide.

W.A.E. McBryde, ‘Petroleum Deodorized: Early Canadian History of the ‘Doctor Sweetening’ Process,’ Annals and Science, 48(1991), 103-111.

Otto Rotton and William Archer, ‘Deodorizing Petroleum,’ American Artisan and Patent Record (New York), new series 5, (1867), 310.

G.L. Wendt and S.H. Diggs, ‘The Chemistry of "Sweetening" in the Petroleum Industry,’ Industrial and Engineering Chemistry, 16 (1924), 1113-5.

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