Mineral oils: processes and products – Refining – Sulfur removal
Reexamination Certificate
2000-09-01
2002-06-11
Preisch, Nadine (Department: 1764)
Mineral oils: processes and products
Refining
Sulfur removal
C208S237000, C208S238000, C208S219000, C208S222000, C208S223000, C208S196000
Reexamination Certificate
active
06402940
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a process for the removal of organic sulfur compounds by oxidation from hydrocarbon fuels which have relatively low amounts of sulfur present, such as in fuels which have been through a hydrogenation step to remove organic sulfur compounds.
The presence of sulfur in hydrocarbons has long been a significant problem from the exploration, production, transportation, and refining all the way to the consumption of hydrocarbons as a fuel, especially to power automobiles and trucks. Now, it has become an environmental objective to rid fuels such as diesel fuel, gasoline, fuel oils, jet fuel, kerosine, and the like of the troublesome residual organic sulfur present in such hydrocarbons, even though on a relative basis the amount present to begin with is small such as, for example, in diesel fuel the sulfur content may be about 500 parts per million by weight, or less. However, under the present regime even this amount has become too much with extant and prospective regulation of sulfur emissions from many sources becoming increasingly strict.
The prior art is replete with attempts to reduce the sulfur content of hydrocarbon by both reduction and oxidation of organic sulfur present. Much of this prior art relating to oxidation has taught the use of various peroxides in conjunction with a carboxylic acid and, specifically, the preferred species involved in the practice of this invention; i.e., hydrogen peroxide and formic acid. For example, U.S. Pat. No. 5,310,479 teaches the use of formic acid and hydrogen peroxide to oxidize sulfur compounds in crude oil, limiting the application of the technology only to aliphatic sulfur compounds. There was no hint of the removal of aromatic sulfur compounds. This patent discussion is directed to the removal of sulfur from crude oil rich (about 1-4%) in sulfur compounds. The acid to peroxide ratio was indiscriminately broad and failed to recognize the economic disadvantages to using hydrogen peroxide in attempts to remove large amounts of sulfur, while at the same time failing to recognize the importance of controlling the presence of water to the successful operation. Water was used to extract the sulfones from the treated hydrocarbon in a separate wash step. Further, the prior art also fails to recognize the beneficial effect of limiting the peroxide concentration to low values without compromising either the rate or extent of oxidation of the sulfur compounds.
A recent study entitled “Oxidated Desulfurization of Oils by Hydrogen Peroxide and Heteropolyanion Catalyst,” Collins, et al., published
Journal of Molecular Catalysis A: Chemical
, 117 (1997) 397-403, discusses other studies to oxidatively remove sulfur from fuel oil, but large quantities of hydrogen peroxide were required. However, the experimental work did show that unacceptable amounts of hydrogen peroxide were consumed thus suggesting the cost of oxidative reduction of sulfur in feedstocks for diesel fuel to be impermissibly high.
In European Patent Application Publication No. 0565324A1, a method for recovering organic sulfur compounds from liquid oil is described. While the stated objective of the patent publication is to recover the organic sulfur compound, the treatment involves using a mixture of a number of oxidants, one of which is disclosed as a mixture of formic acid and peroxide. The distillation products, the organic sulfones, are removed by a number of methods including absorption on alumina or silica adsorbent materials. The treatments described are characterized by use of a low ratio of formic acid to the hydrogen peroxide.
While this and other prior art recognize the reaction kinetics and mechanism of hydrogen peroxide and other peroxides with organic sulfur compounds present in various fuels, none recognize the combination of factors necessary to successfully and economically remove relatively small amounts of sulfur present in fuels such as diesel oil, kerosine, gasoline, and light oils down to residual levels approaching zero. While low amounts of sulfur will be construed to mean in the context of this invention, those amounts which are less than about 1500 parts per million, an eaxample demonstrates effective removal of 7000 ppm of sulfur such that the present invention is applicable to higher levels of sulfur. Of course in some instances, the practice of this invention may be economically and technically applicable to the treatment of fuels having a sulfur content at these elevated levels. It has been found in the practice of this invention that the sulfur content of the fuel which is left unoxidized is less than about 10 ppm of sulfur, often as low as between 2 ppm and 8 ppm. Oxidation alone does not necessarily ensure total removal of the sulfur to the same low residual sulfur values since some of the oxidized sulfur species do have a non-zero solubility in the fuel, and a partition coefficient that defines their distribution in the oil phase in contact with a substantially immiscible solvent phase, whether it is an organic solvent as in prior art, or the high acid aqueous phase of this invention. In addition to the substantially complete and rapid oxidation of the relatively low amounts of sulfur in the fuel feed, the present invention also teaches the substantially complete removal of the oxidized sulfur to residual levels approaching zero, and the recovery of the oxidized sulfur compounds in a form suitable for their practical further disposition in an environmentally benign way.
The sulfur compounds which are most difficult to remove by hydrogenation appear to be the thiophene compounds, especially benzothiopene, dibenzothiopene, and other homologs. In an article, Desulfurization by Selective Oxidation and Extraction of Sulfur-Containing Compounds to Economically Achieve Ultra-Low Proposed Diesel Fuel Sulfur Requirements (Chapados, et al., NPRA Presentation, Mar. 26-28, 2000) the oxidation step involved the reaction of the sulfur in a model compound using dibenzothiophene with a peroxyacetic acid catalyst made from acetic acid and hydrogen peroxide. The reaction with the peroxyacid was conducted at less than 100° C. at atmospheric pressure and in less than 25 minutes. After extraction, the process resulted in a reduction of the sulfur content in the diesel fuel. Still, the cost was indicated to be high with the hydrogen peroxide being the biggest cost item and consumed in the process due in large part to the lack of recognition of the part excessive water plays in the efficient utilization of low amounts of hydrogen peroxide.
SUMMARY OF THE INVENTION
It has been discovered that fuel oils such as diesel fuel, kerosene, and jet fuel, though meeting the present requirements of about 500 ppm maximum sulfur content, can be economically treated to reduce the sulfur content to an amount of from about 5 to about 15 ppm, in some instances even less. In practicing the process of the present invention the hydrocarbon fuel containing low amounts of organic sulfur compounds, i.e., up to about 1500 ppm, is treated by contacting the sulfur-containing fuel with an oxidizing solution containing hydrogen peroxide, formic acid, and a limit of a maximum of about 25 percent water. The amount of the hydrogen peroxide in the oxidizing solution is greater than about two times the stochiometric amount of peroxide necessary to react with the sulfur in the fuel. The oxidizing solution used contains hydrogen peroxide at low concentration, the concentration, in its broadest sense, being from about 0.5 wt % to about 4 wt %. The reaction is carried out at a temperature of from about 50° C. to about 130° C. for less than about 15 minutes contact time at close to, or slightly higher than atmospheric pressure at optimum conditions. The oxidizing solution of the invention has, not only a low amount of water, but small amounts of hydrogen peroxide with the acid, with the formic acid being the largest constituent. The oxidation products, usually the corresponding organic sulfones, become soluble in the oxidizing solution and, therefore, may b
Jenkens & Gilchrist
Preisch Nadine
Unipure Corporation
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