Doctoral Procedure

Procedure scheme of the doctoral procedure in the variant according to Kalinowski (1954)

The doctor method , also ( Engl. ) Doctor Treatment or Doctor sweetening process is a chemical-physical process of the petrochemical industry for desulfurization. Within the refinery process, the process is used to desulphurise motor gasoline , aviation gasoline and solvents with the aid of lead oxide in an alkaline solution. During this process , the mercaptans responsible for odor formation are converted to disulfides through oxidation. One speaks in this context of sweetening oils ( Engl. Sweetening ).

history

The process for treating petroleum distillates with alkaline sodium plumbite solution has been known since 1866 and goes back to the German chemist Rudolf Wagner. After 1869, it became the preferred method of desulfurization for Canadian refined products, which commonly suffered from the problem of high sulfur content. Hermann Frasch developed the process further by performing the distillation in the presence of solid metal oxides. The chemistry of the process was extensively described in 1924 by G. Wendt and S. Diggs.

However, since the process had some disadvantages, including a loss of hydrocarbons, alternative processes were developed and patented as early as the late 1930s. A gasoline doctor treater was still in operation at the former Shell Haven refinery in 1950 , but in the second half of the 20th century the doctor process was increasingly replaced by catalytic processes.

Chemistry of the process

Wendt and Diggs demonstrated in 1924 that the lead oxide solution was able to oxidize the odor-forming mercaptans into comparatively odorless organic disulfides . Lead oxide ( litharg ) dissolves in somewhat highly concentrated sodium or potassium hydroxide solutions to form a soluble compound called sodium plumbite :

${\displaystyle \mathrm {PbO+2\ NaOH\longrightarrow Na_{2}PbO_{2}+H_{2}O} }$

If this alkaline solution is shaken with petroleum, the two liquids do not dissolve with one another, but the mercaptan in the oil phase combines with an equivalent amount of lead (which is then transferred into the oil phase) and forms a lead captide that is soluble in the oil phase :

${\displaystyle \mathrm {2\ RSH+Na_{2}PbO_{2}\longrightarrow (RS)_{2}Pb+2\ NaOH} }$

If this mixture is then treated with sulfur powder - sulfur has a very high affinity for lead - a black suspension of lead sulfides is formed, which is followed by the conversion of the mercaptide into an organic disulfide:

${\displaystyle \mathrm {-(RS)_{2}Pb+S\longrightarrow RS-SR+PbS} }$

The organic disulfide remains dissolved in the oil phase.

This reaction takes place in the presence of atmospheric oxygen even without the presence of sulfur powder; in this case, however, the conversion is much slower and probably incomplete:

${\displaystyle \mathrm {2\ (RS)_{2}Pb+4\ NaOH+O_{2}\longrightarrow 2\ RS-2\ SR+2\ Na_{2}PbO_{2}+2\ H_{2}O} }$

From this description it becomes clear that the overall process does not remove the sulfur from the oil phase, but that the sulfur content can even increase if too much sulfur powder is added.

The chemical reactions described are also the basis for the so-called doctor test . In analytics, gasoline is described as "doctor sweet" if, after shaking out with sodium plumbite, the addition of sulfur powder is unable to precipitate a dark precipitate of lead sulfite.

process technology

Various process variants for the industrial implementation of the doctoral process are described in the literature, which represent the respective technical status of their time and were usually protected by patents. The following describes the Kalinowski doctoral procedure , published on January 27, 1959 as a patent in the name of Mathew L. Kalinowski employed by Standard Oil .

The sulfur-containing product is first preheated in the heat exchanger 1 and flows to the mixer 1. In doing so, doctor solution ( material flow 8 ) at a temperature of about 120 ° C. is added from the regenerator. The regenerated doctor solution also contains free oxygen, which was previously supplied in excess to the regeneration process. A partial flow of the starting product is passed through a receiver filled with crushed sulfur lumps and is enriched there with elemental sulfur.

The decisive procedural step in this phase is that the main stream enriched with doctor's solution only comes into contact with the sulfur-enriched partial stream after at least 45 seconds. During this time the actual reaction - the precipitation of the mercaptans by sodium plumbite - can take place. Technically, this time specification is achieved by selecting the appropriate pipe lengths and separately feeding the partial flows into the mixer.

In mixer 1, the substreams are now mixed at 63 ° C. for about three minutes. Excess gas is separated off via a venting device in the head of the mixer. The mixture of desulphurized oil and doctor solution is drawn off and separated in the separator 1 due to the differences in density. The doctor's solution is fed to the regenerator ( material flow 1 ), while the desulphurized oil phase flows into mixer 2 ( material flow 2 ).

Since the desulfurized oil now contains excess doctor solution and residues of lead sulfide, the quality of the product is further improved in mixer 2. For this purpose, the product is stirred for a further five minutes at 63 ° C. and fed to the separator 2 ( material flow 3 ). After cooling to room temperature, the light oil product is freed from the last traces of doctor's solution and lead sulfide in a receiver filled with rock salt and leaves the process as a desulphurised end product. The lead sulphide-containing doctor solution ( material flow 5 ) deposited in the salt reservoir can either be disposed of or sent for regeneration.

The doctor solution enriched with lead sulfide from the separator 2 can, depending on the concentration, either be fed into the circuit enriched with fresh doctor solution ( material flow 6 ) or fed to the regenerator ( material flow 7 ).

In the regenerator, the lead sulfide is oxidized back into plumbite by free oxygen.

literature

• McBryde, WAE: Petroleum deodorized: Early Canadian history of the doctor sweetening process , Annals of Science, Volume 48, Issue 2, Taylor & Francis, 1991; doi : 10.1080 / 00033799100200161
• LM Henderson, WB Ross, CM Ridgway: Tetraethyllead Susceptibilities of Gasoline Doctor Treatment vs. Caustic Washing ], Ind. Eng. Chem., 1939, 31 (1), pp. 27-30; doi : 10.1021 / ie50349a005
• Naphtali, Max: Advances in the field of mineral oils. The technical development of the petroleum industry after the war , Angewandte Chemie, Volume 42, Issue 20, pp. 508-518, WILEY-VCH Verlag GmbH, May 18, 1929
• Otto Rotton, William Archer: Deodorizing Petroleum , American Artisan and Patent Record (New York), new series 5, p. 310, 1867
• GL Wendt, SH Diggs: The Chemistry of "Sweetening" in the Petroleum Industry , Industrial and Engineering Chemistry, Issue 16, pages 1113-1115, 1924
• ML Kalinowski: Doctor sweetening process using sulfur , U.S. Patent 2871187, January 27, 1957

Individual evidence

1. ^ McGraw-Hill Dictionary of Scientific and Technical Terms, 6th edition, published by The McGraw-Hill Companies, Inc., 2003.
2. ↑ McBryde, petroleum deodorized, Summary.
3. ↑ WAE McBryde: PETROLEUM DEODORIZED: Early Canadian History of the 'Doctor Sweetening' Process , Department of Chemistry, University of Waterloo.
4. ↑ Process for Treating Mineral Oils , United States Patent Office, Patent 2,085,527, June 29, 1937.
5. ↑ Mobil Oil Corp .: GASOLINE IMPROVEMENT METHOD , Patent DE69324260T2, July 8, 1999.
6. ↑ ML Kalinowski: Doctor sweetening process using sulfur , US Patent No. 2871187 of 27 January 1957th