Mineral oils: processes and products – Refining – Sulfur removal
Reexamination Certificate
2000-10-12
2002-04-09
Preisch, Nadine (Department: 1764)
Mineral oils: processes and products
Refining
Sulfur removal
C208S243000, C208S244000, C208S249000, C208S088000
Reexamination Certificate
active
06368495
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a novel process for removing organic sulfur compounds (e.g. thiophenes) from liquid hydrocarbon streams. The process comprises subjecting the liquid hydrocarbon stream to oxidation conditions, thereby oxidizing at least a portion of the organic sulfur compounds to oxidized organic sulfur compounds (e.g. sulfones), followed by catalytically decomposing the oxidized organic sulfur compounds to provide a treated hydrocarbon product of reduced sulfur content.
BACKGROUND OF THE INVENTION
Sulfur is present in a wide range of mostly organic forms in both straight run and refined hydrocarbon streams, including, for example, gasoline, diesel fuel, and kerosene. Sulfur contaminants, while ubiquitous in hydrocarbon products, are suspected of causing adverse environmental effects when converted to sulfur oxides (SO
x
) upon combustion. SO
x
emissions are believed to contribute to not only acid rain, but also to reduced efficiency of catalytic converters designed to improve motor vehicle exhaust quality. Furthermore, sulfur compounds are thought to ultimately increase the particulate content of combustion products. Because of these issues, the reduction of the sulfur content in hydrocarbon streams has become a major objective of recent environmental legislation worldwide. For instance, Canada, Japan, and the European Commission have all recently adopted a 0.05 wt-% limit on diesel fuel sulfur.
For the oil refiner, complying with such increasingly stringent specifications has primarily meant using more severe hydrotreating conditions. Hydrotreating refers to a well-known process whereby hydrogen is contacted with a hydrocarbon stream and catalyst to effect a number of desirable reactions, including the conversion of sulfur compounds to hydrogen sulfide. This reaction product is then separated into a gaseous hydrotreater effluent stream and thus effectively removed from the hydrocarbon product. Hydrotreating can readily reduce the level of several common classes of sulfur compounds such as sulfides, disulfides, and thiols (mercaptans), present in refinery products. Unfortunately, however, hydrotreating (or hydrodesulfurization) often fails to provide a treated product in compliance with the strict sulfur level targets demanded currently. This is due to the presence of sterically hindered sulfur compounds such as unsubstituted and substituted thiophenes that are essentially refractory in hydrotreating environments. Attempts to completely convert these species, which are more prevalent in heavier stocks such as diesel fuel and fuel oil, have resulted in increased equipment costs, more frequent catalyst replacements, degradation of product quality due to side reactions, and continued inability to comply with sulfur specifications.
Several prior art disclosures address sulfur contamination in refinery products. U.S. Pat. No. 2,769,760, for example, describes a hydrodesulfurization process with an additional conversion step that does not further reduce the sulfur level but converts some sulfur species to less-corrosive forms, allowing the product to meet acidity requirements. Other disclosures are more specifically directed toward essentially complete sulfur removal from hydrocarbons. Particularly, the ability to oxidize sulfur compounds that are resistant to the aforementioned hydrogenation method is recognized in a number of cases. Oxidation has been found to be beneficial because oxidized sulfur compounds have an increased propensity for removal by a number of separation processes that rely on the altered chemical properties such as the solubility, volatility, and reactivity of such compounds. Techniques for the removal of oxidized organic sulfur compounds therefore include extraction, distillation, and adsorption.
In U.S. Pat. No. 3,163,593, organic sulfur compounds contained in petroleum fractions are oxidized by contact with a mixture of H
2
O
2
and a carboxylic acid to produce sulfones, which are then degraded by thermal treatment to volatile sulfur compounds. In U.S. Pat. No. 3,413,307, thiophene and thiophene derivatives are oxidized to sulfones in the presence of a dilute acid. The sulfones are then extracted using a caustic solution. In U.S. Pat. No. 3,341,448, the oxidation and thermal treatment steps are combined with hydrodesulfurization to greatly reduce the hydrocarbon sulfur content. As noted previously, the oxidation and hydrogenation techniques are effective for converting different types of organic sulfur-containing species, thereby leading to a synergistic effect when these methods are combined. In U.S. Pat. No. 3,505,210, sulfur contaminants in a hydrocarbon fraction are oxidized using hydrogen peroxide or other suitable oxidizing agent to convert bivalent sulfur to sulfones. The hydrocarbon, after having been subjected to oxidation conditions, is then contacted in this case with molten sodium hydroxide to produce a treated product of reduced sulfur content. Another example of a two-step oxidation and extraction method is provided in U.S. Pat. No. 3,551,328, where the extractant is a paraffinic hydrocarbon comprising a 3-6 carbon number alkane. Also, EP-0565324 A1 teaches the effectiveness of oxidizing sulfur-containing compounds followed by removal according to a number of possible separations known in the art.
In contrast to the prior art, applicant has determined that organic sulfur contaminants in petroleum fractions can be first oxidized and then catalytically decomposed to hydrocarbons and volatile sulfur compounds. The hydrocarbons formed by this conversion remain in the treated liquid petroleum fraction as valuable components while the volatile sulfur is easily separable and can therefore be ultimately sent for typical caustic scrubbing and/or sulfur recovery procedures currently practiced commercially. The conversion of oxidized organic sulfur compounds such as sulfones according to the present invention has been determined to occur in the presence of a number of solid catalysts under a wide range of reaction conditions.
Compared to other techniques for the removal of oxidized sulfur compounds from hydrocarbons, heterogeneous catalytic decomposition offers distinct advantages. For instance, in prior art methods for extracting sulfones, liquid extractants are continually consumed due to solution losses and invariable contamination of the treated hydrocarbon product. Also, the high energy costs and incomplete component separations associated with distillative separations, as taught in other disclosures, are avoided using the process of the present invention. Lastly, the frequent replacement of adsorbent beds when hydrocarbons with high sulfur levels are treated is also overcome.
Regarding the oxidative/adsorptive processes of the prior art in particular, U.S. Pat. No. 3,945,914 teaches, as a first step, the oxidation of sulfur compounds in hydrocarbons using any conventional oxidant to form an oxidized sulfur compound. In a second step, the oxidized sulfur-containing hydrocarbon is contacted with a metal to form a metal-sulfur-containing compound. This process therefore relies on the adsorption of oxidized sulfur compounds from the hydrocarbon using a metal capable of forming a metal sulfide. The metal is selected from the group consisting of Ni, Mo, Co, W, Fe, Zn, V, Cu, Mn, Hg, and mixtures thereof. This process is distinguished from conventional hydrodesulfurization in that the sulfur is immobilized in the form of a metallic sulfur compound (e.g. a metal sulfide) rather than converted to hydrogen sulfide. For this reason, the addition of free molecular hydrogen, as is required in hydrodesulfurization, is overcome. A hydrogen atmosphere, however, is apparently needed to effect the reduction of oxidized sulfur to the metal sulfur compound, based on the Examples I-III of this reference.
Adsorptive processes for the removal of oxidized sulfur compounds may provide a higher degree of overall sulfur reduction than traditional hydrodesulfurization processes. However, several disadvantages are associated with this
Brandvold Timothy A.
Kocal Joseph A.
Molinaro Frank S.
Preisch Nadine
Tolomei John G.
UOP LLC
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