United States Patent Office P_d
3 788 978
PROCESS FOR THE DESULFURIZATION OF PETROLEUM OIL STOCKS Roby Bearden, Jr., Baton Rouge, La., Robert B. MacMuIIin, Lewiston, N.Y., WilKam E. Lewis, Baton Rouge, La., and Albert B. Welty, Jr., Westfield, N.J., assignors to Esso Research and Engineering Company Filed May 24,1972, Ser. No. 256,547 Int. CI. ClOg 77/00, 79/00 U.S. CI. 208—208 M 23 Claims
BACKGROUND OF THE INVENTION
Field of the invention
The present invention relates to a process for the desulfurization of sulfur-containing petroleum oil stock. More particularly, the process comprises contacting a sulfur-containing oil stock with an alkali metal or alkali metal alloy.
Description of the prior art
In the last several years there has been an ever-increasing concern about air pollution. Some of the objects of this concern have been the discharge of sulfur oxides to the atmosphere upon burning sulfur-containing fuels. Over a period of many years several studies have been conducted with the object of developing efficient and economcal means for reducing the sulfur content of crude petroleum oils and other virgin hydrocarbon fractions.
To the present, the most practical desulfurization process has been hydrogenation of sulfur-containing oils at elevated pressures and temperatures in the presence of an appropriate catalyst. The process requires the use of hydrogen pressures ranging from about 200 to about 2500 p.s.i.g. and temperatures ranging from about 650 to about 800° F., depending on the nature of the oil to be desulfurized and the amount of sulfur required to be removed.
The process is efficient in the case of distillate oil feedstocks and less efficient when used with those containing undistilled oil such as whole crudes or residua. This is due to several factors. First, most of the sulfur in the oils is contained in high molecular weight molecules, and it is difficult for them to diffuse through the catalyst pores to the catalyst surface. Furthermore, once at the surface, it is difficult for the sulfur atoms contained in the molecules to "see" the catalyst surface. Additionally, the feedstocks may contain large amounts of asphaltenes which tend to form coke deposits under the process conditions on the catalyst surface thereby deactivating the catalyst. Moreover, high boiling organometallic compounds pres
ent in such stocks decompose and deposit metals on the catalyst surface thereby diminishing the catalyst life time. The severe operating conditions employed in the process cause appreciable cracking of high boiling oils thereby
5 producing olefinic fragments which, themselves, consume hydrogen, thereby lowering the process efficiency and increasing costs.
Alternate desulfurization processes that have been employed in the past used alkali metal dispersions, such as
10 sodium, as desulfurization agents. Bascially, the process involved contacting a hydrocarbon fraction with the sodium dispersion, the sodium reacting with the sulfur to form dispersed sodium sulfide (Na2S). However, such a process was not proven to be economically advantageous,
15 particularly for treatment of high boiling, high sulfur content feedstocks due to (a) the high cost of sodium, (b) problems related to removal of sodium sulfide formed in the process from the oil and (c) the impracticability heretofore of regenerating sodium from the sodium sul
In theory, it has been determined that the best approach to this problem of sodium regeneration would be to electrolyze an alkali metal salt that melts at about the same temperature as used for the desulfurization process and
25 which can be electrolyzed with minimum consumption of electrical energy. The alkali polysulfides, preferably the sodium polysulfides meet this requirement. There are three sodium polysulfides with melting points as follows: Na2S2 (885° F.), Na2S4 (545° F.) and Na2S5 (485° F.).
30 These polysulfides are mutually soluble and intermediate compositions, having intermediate properties, can form. The eutectic is at about Na2S3.i with a melting point of about 450° F. Moreover, the electrolysis of molten sodium polysulfide consumes less electrical energy than
35 electrolyzing molten sodium chloride, the traditional electrolysis salt.
The cost of sodium as a reagent used on a oncethrough basis is prohibitively high, and it is therefore clear that one must be able to recover the sodium from
40 the sodium sulfide in order to provide an economically viable process. Much thought has been given to this problem in the past, but until now no economical process has been developed. One of the major difficulties has been the separation of the sodium sulfide from the oil.
SUMMARY OF THE INVENTION
In accordance with this invention, it has now been discovered that an economically feasible desulfurization
50 process vis-a-vis hydrodesulfurization of whole crude or residual oils can be achieved and that outstanding sulfur removal can be realized. Specifically, the process involves contacting a sulfur-containing petroleum oil stock with a desulfurization agent comprising an alkali metal, such
55 as lithium, sodium, potassium, and the like, preferably sodium, or an alkali metal alloy, preferably sodium/lead, at desulfurization conditions, thereby forming a mixture comprising an oil of diminished sulfur content containing alkali metal salts and contacting at least a portion of said
60 mixture with H2S.
The alkali metal salts comprise in addition to alkali metal sulfide, by-product alkali metal salts such as organo metal salts, metal oxides, mercaptides, amides and the like. Hereinafter the invention will be described with
65 respect to sodium although it is understood that other alkali metals as hereinbefore disclosed may be used.
In a preconditioning step for salt recovery at least a portion of the oil-salt mixture (generally in the form of a dispersion of submicron sodium salts in oil) is con
70 tacted with H2S in amounts ranging from about 10 to about 100 mole percent, based on the total number of moles of salt present in the mixture, preferably 30 to 60