The purpose of hydrocarbon liquid treating is for removal or chemical conversion of objectionable compounds or elemental sulfur in order that the liquid products meet corrosion, doctor, and total sulfur content specifications. Failure of a hydrocarbon product to meet corrosion specifications can be due to the presence of hydrogen sulfide and/or free sulfur. Failure to pass a doctor test is caused by the presence of mercaptans. Off-specification products are commonly referred to as "corrosive" if a positive corrosion test is obtained or "sour" if a positive doctor test results; sometimes the liquid product is both "corrosive" and "sour".
It is noted that the use of the term "sour" differs as applied to liquids and gases. "Sour" or "doctor sour" as applied to liquid hydrocarbon products indicates the presence of mercaptans, whereas "sour" used in connection with gaseous hydrocarbon products indicates the presence of hydrogen sulfide. If a liquid hydrocarbon product is negative to the doctor test, it is called "sweet" or "doctor sweet". If a negative corrosion test results, the liquid product is called "noncorrosive". It should be noted that a sweet or doctor sweet liquid hydrocarbon product can result from mere conversion of mercaptans to sulfides or disulfides even though the total sulfur content remains the same or is even higher in some methods.
Removal of hydrogen sulfide and/or free sulfur is desired to prevent corrosion or plugging of users equipment, such as carburetor parts, needle valves, etc. Removal or chemical conversion of mercaptans is desired to eliminate the offensive odor of mercaptans. However, processes that actually remove the mercaptans or their conversion products yield a superior product because of improved tetraethyl lead susceptibility and elimination of sulfur oxides from the products of combustion.
For many years liquid hydrocarbon streams of the type which can be treated in accordance with the present invention have been sweetened by subjecting them to oxidizing conditions in a sodium hydroxide or potassium hydroxide solution, generally in the presence of agitation and a metal phthalocyanine catalyst or equivalent. The mercaptans are converted to the corresponding disulfides at the interface of the aqueous caustic solution and the liquid hydrocarbons with the resulting disulfides dissolving in the liquid hydrocarbon.
The sweetening process has also been carried out in fixed bed systems in the presence of a catalyst and an oxidizing agent. The hydrocarbon stream to be treated can be passed in contact with an aqueous caustic solution over a solid, usually supported, catalyst in a suitable treating vessel. The caustic solution can be regenerated or supplemented as it becomes spent as the result of accumulation of acidic and other nonhydrocarbon impurities. The catalyst, when necessary, can be reactivated by means of well-known in-place regeneration procedures.
One widely known industrial method for treating mercaptan-containing hydrocarbon streams is the Merox Process. See, for example, the Oil and Gas Journal - 57 (44), 73-78 (1959) which has a discussion of the Merox Process and other prior art procedures. Like other known sweetening processes it uses a catalyst to oxidize the mercaptans to disulfides in the presence of oxygen and caustic.