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  Hydrogenation of polynuclear aromatic compoundsUSPTO Application #: 20070289900Title: Hydrogenation of polynuclear aromatic compounds Abstract: A process for reducing the polynuclear aromatics (PNA) content of a sulfur-containing hydrocarbon stream. The process includes contacting the sulfur-containing hydrocarbon stream with a dearomatization composition comprising a promoter metal component and zinc oxide. The dearomatization composition has enhanced resistance to sulfur poisoning and is also effective to remove sulfur from the hydrocarbon stream. (end of abstract)
Agent: Conocophilips Company - I.p. Legal - Bartlesville, OK, US Inventors: Walter E. Alvarez, Glenn W. Dodwell, Tushar V. Choudhary USPTO Applicaton #: 20070289900 - Class: 208255 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20070289900. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND [0001]This invention relates generally to the hydrogenation of polynuclear aromatic compounds (PNA) in low-sulfur hydrocarbon-containing streams, such as diesel fuel. In another aspect, this invention relates to compositions suitable for use in the desulfurization and dearomatization of diesel fuels while increasing the cetane number of the fuel. [0002]Diesel fuel inherently contains certain amounts of aromatic hydrocarbon compounds (i.e., hydrocarbon compounds containing one or more "benzene-like" ring structures). Commercial diesel fuel is typically made by blending "straight run" diesel (produced by simple distillation of crude oil) and a catalytically cracked stock (typically cracked residual oil). Much of the aromatic content of blended diesel fuels originates from the cracked stock because catalytic cracking increases aromatics content. [0003]Aromatic hydrocarbons have poor self-ignition qualities, so that diesel fuels containing a high fraction of aromatics tend to have low cetane numbers. Typical cetane values for straight run diesel are in the range of 50 to 55, while the cetane value for highly aromatic blended diesel fuel is typically in the range of 40 to 45. These low cetane values can produce more difficulty in cold engine starting and can increase combustion noise. The ignition delay experienced with low cetane diesel fuel can also cause increased hydrocarbon emissions and increased NOx emissions. [0004]Polynuclear aromatics (PNA), also called polycyclic aromatic hydrocarbons (PAH), are a type of aromatic compound whose presence in diesel fuel is particularly undesirable. PNA are aromatic compounds having more than one aromatic ring. Some tests have indicated that certain PNA compounds can have adverse health affects on animals. Thus, it is desirable to limit PNA emissions from vehicles, especially in densely populated high traffic urban areas. Scientists have found that a linear relationship exists between fuel PNA input and PNA emissions. Thus, by reducing fuel PNA content in commercially available diesel fuel, PNA emissions to the environment will be reduced. [0005]A variety of commercial processes exist for reducing the total aromatic and PNA content of diesel fuel. The most popular dearomatization processes involve hydrogenation of aromatics in the presence of a catalyst from Group VIII of the Periodic Table of Elements. However, conventional Group VIII catalysts are very susceptible to sulfur poisoning from sulfur-containing compounds in diesel fuel, even when the diesel subjected to hydrodearomatization is low sulfur diesel containing less than 30 parts per million by weight (ppmw) sulfur. OBJECTS AND SUMMARY OF THE INVENTION [0006]It is, therefore, an object of the present invention to provide an improved process for reducing the polynuclear aromatics (PNA) content of hydrocarbon-containing streams that employs a catalyst having a high sulfur tolerance. [0007]Accordingly, one aspect of the present invention concerns a process for removing polynuclear aromatics from a low-sulfur hydrocarbon-containing stream containing less than about 500 parts per million by weight (ppmw) sulfur. This process comprises contacting the low-sulfur stream with a dearomatization composition comprising a promoter metal component and zinc oxide under dearomatization conditions sufficient to reduce the PNA content of the low-sulfur stream by at least about 25 percent by weight thereby providing a PNA-reduced stream, wherein the sulfur content of the PNA-reduced stream is at least about 5 percent by weight less than the sulfur content of the low-sulfur stream. [0008]Another aspect of the present invention concerns a process for treating an initial hydrocarbon-containing stream having a sulfur content of at least about 500 ppmw and a PNA content of at least about 5 percent by weight, the process comprising: (a) contacting the initial stream with a catalyst and/or sorbent composition in a first zone under desulfurization conditions sufficient to reduce the sulfur content of the initial stream by at least about 25 percent by weight, thereby producing a sulfur-reduced stream having a sulfur content of less than about 500 ppmw and a PNA content of at least about 5 percent by weight; and (b) contacting at least a portion of the sulfur-reduced stream with a dearomatization composition comprising nickel and zinc oxide in a second zone to thereby produce a PNA-reduced stream having a PNA content at least about 25 percent by weight less than the PNA content of the sulfur-reduced stream. BRIEF DESCRIPTION OF THE DRAWINGS [0009]FIG. 1 plots the PNA hydrogenation activity of three different compositions as a function of the amount of sulfur across the catalyst bed. [0010]FIG. 2 compares the resistance to deactivation of two different compositions by plotting the amount of sulfur contained in a feed stream as a function of the amount of sulfur across the catalyst bed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0011]One embodiment of the present invention concerns a dearomatization composition with enhanced tolerance to sulfur poisoning. The dearomatization composition generally comprises a promoter metal component and zinc oxide. [0012]The promoter metal component employed in the dearomatization composition comprises, consists of, or consists essentially of a promoter metal. The promoter metal is a metal capable of catalyzing a hydrogenation reaction, such as hydrogenation of polynuclear aromatics (PNA). Preferably, the promoter metal is selected from the group consisting of nickel, cobalt, iron, manganese, copper, zinc, molybdenum, tungsten, silver, tin, antimony, vanadium, gold, platinum, ruthenium, iridium, chromium, palladium, titanium, zirconium, rhodium, rhenium, and combinations thereof. Most preferably, the promoter metal is nickel. [0013]In one embodiment of the present invention, substantially all of the promoter metal component is present in the dearomatization composition in a reduced-valence state. As used herein, "reduced-valence state" denotes a state of a composition/component where the number of oxygen atoms associated therewith has been reduced. Preferably, substantially all of the promoter metal component is present in a zero valence state, having no oxygen atoms associated therewith. Accordingly, as will be discussed in further detail below, it is preferred for the dearomatization composition to be subjected a reduction step during its preparation. In addition, it is preferred for the dearomatization composition to be subjected to an acid treatment step during its preparation, preferably pretreatment with tartaric acid and/or citric acid, most preferably pretreatment with citric acid. [0014]In one embodiment of the present invention, the promoter metal component of the dearomatization composition comprises, consists of, or consists essentially of, a substitutional solid metal solution containing the promoter metal in solid solution with another metal. The substitutional solid metal solution preferably is characterized by the formula: M.sub.AZn.sub.B, wherein M is the promoter metal, Zn is zinc, and A and B are each numerical values in the range of from 0.01 to 0.99. In the above formula for the substitutional solid metal solution, it is preferred for A to be in the range of from about 0.70 to about 0.97, and most preferably in the range of from about 0.85 to about 0.95. It is further preferred for B to be in the range of from about 0.03 to about 0.30, and most preferably in the range of from about 0.05 to 0.15. Preferably, B is equal to (1-A). [0015]Substitutional solid solutions have unique physical and chemical properties that are important to the chemistry of the dearomatization composition described herein. Substitutional solid solutions are a subset of alloys that are formed by the direct substitution of the solute metal for the solvent metal atoms in the crystal structure. For example, it is believed that the substitutional solid metal solution (M.sub.AZn.sub.B) found in the dearomatization composition is formed by the solute zinc metal atoms substituting for the solvent promoter metal atoms. There are three basic criteria that favor the formation of substitutional solid solutions: (1) the atomic radii of the two elements are within 15 percent of each other; (2) the crystal structures of the two pure phases are the same; and (3) the electronegativities of the two components are similar. The promoter metal (as the elemental metal or metal oxide) and zinc oxide employed in the dearomatization composition described herein preferably meets at least two of the three criteria set forth above. For example, when the promoter metal is nickel, the first and third criteria, are met, but the second is not. The nickel and zinc metal atomic radii are within 10 percent of each other and the electronegativities are similar. However, nickel oxide (NiO) preferentially forms a cubic crystal structure, while zinc oxide (ZnO) prefers a hexagonal crystal structure. A nickel-zinc solid solution retains the cubic structure of the nickel oxide. Forcing the zinc oxide to reside in the cubic structure increases the energy of the phase, which limits the amount of zinc that can be dissolved in the nickel oxide structure. This stoichiometry control manifests itself microscopically in a 92:8 nickel zinc solid solution (Ni.sub.0.92Zn.sub.0.08) that is formed when the nickel oxide-zinc oxide solid solution is reduced. [0016]In addition to zinc oxide and the reduced-valence promoter metal component, the dearomatization composition may further comprise a porosity enhancer and a promoter metal-zinc aluminate substitutional solid solution. The promoter metal-zinc aluminate substitutional solid solution can be characterized by the formula: M.sub.ZZn.sub.(1-Z)Al.sub.2O.sub.4), wherein Z is a numerical value in the range of from 0.01 to 0.99. [0017]The porosity enhancer, when employed, can be any compound which ultimately increases the macroporosity of the dearomatization composition. Preferably, the porosity enhancer is perlite. The term "perlite" as used herein is the petrographic term for a siliceous volcanic rock which naturally occurs in certain regions throughout the world. The distinguishing feature, which sets it apart from other volcanic minerals, is its ability to expand 4 to 20 times its original volume when heated to certain temperatures. When heated above 1,600.degree. F., crushed perlite expands due to the presence of combined water with the crude perlite rock. The combined water vaporizes during the heating process and creates countless tiny bubbles in the heat softened glassy particles. It is these diminutive glass sealed bubbles which account for its light weight. Expanded perlite can be manufactured to weigh as little as 2.5 lbs per cubic foot. Typical chemical analysis properties of expanded perlite are: silicon dioxide 73%, aluminum oxide 17%, potassium oxide 5%, sodium oxide 3%, calcium oxide 1%, plus trace elements. Typical physical properties of expanded perlite are: softening point 1,600-2,000.degree. F., fusion point 2,300.degree. F.-2,450.degree. F., pH 6.6-6.8, and specific gravity 2.2-2.4. The term "expanded perlite" as used herein refers to the spherical form of perlite which has been expanded by heating the perlite siliceous volcanic rock to a temperature above 1,600.degree. F. The term "particulate expanded perlite" or "milled perlite" as used herein denotes that form of expanded perlite which has been subjected to crushing so as to form a particulate mass wherein the particle size of such mass is comprised of at least 97% of particles having a size of less than 2 microns. The term "milled expanded perlite" is intended to mean the product resulting from subjecting expanded perlite particles to milling or crushing. [0018]The dearomatization composition preferably comprises zinc oxide, the reduced-valence promoter metal component (M.sub.AZn.sub.B), the porosity enhancer (PE), and the promoter metal-zinc aluminate (M.sub.ZZn.sub.(1-Z)Al.sub.2O.sub.4) in the ranges provided below in Table 1. TABLE-US-00001 TABLE 1 Components of the Dearomatization Composition M.sub.AZn.sub.B PE M.sub.ZZn.sub.(1-Z)Al.sub.2O.sub.4 Range ZnO (wt %) (wt %) (wt %) (wt %) Preferred 5 80 5 80 2 50 1 50 More Preferred 20 60 20 60 5 30 5 30 Most Preferred 30 50 30 40 10 20 10 20 [0019]The dearomatization composition is preferably in the form of solid particles suitable for used in a fixed bed or moving bed reactor. The dearomatization composition can be in one or more of the following forms: a granule, an extrudate, a tablet, a sphere, a pellet, or a microsphere. Preferably, the dearomatization composition is in the form of granules, extrudates, tablets, spheres, or pellets having an average minimum particle diameter of at least about 0.0625 inches, more preferably at least 0.125 inches. Continue reading... Full patent description for Hydrogenation of polynuclear aromatic compounds Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Hydrogenation of polynuclear aromatic compounds patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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