BACKGROUND OF THE INVENTION [0002] Motor gasoline ("mogas") sulfur specifications are being regulated to increasingly lower levels (e. g. , less than 30 wppm. The primary source of naphtha used for mogas blending comes from cracked naphtha, particularly fluid catalytically cracked ("FCC") naphtha that can have a sulfur content in the range of from about 100 to about 7000 wppm. Conventional fixed bed hydrodesulfurization can reduce the sulfur content of an FCC naphtha to very low levels, but this requires substantial hydrogen consumption and may be accompanied by significant octane number loss due to olefin saturation. More recently a highly selective hydrodesulfurization catalyst and processes for its use have been developed for removing sulfur from naphtha with reduced olefin loss and concomitant higher octane number of the desulfurized product. The catalyst and processes are disclosed, for example, in U. S. Patent Nos. 6,013, 598; 6,126, 814; and 6,231, 753.

[0003] Some processes have been developed to remove thiophenic sulfur from naphtha, by contacting the naphtha with a solid acid catalyst to alkylate the thiophenic species to higher boiling sulfur compounds and then hydrodesulfurizing the heavy naphtha fraction that contains the higher boiling sulfur compounds. These processes are disclosed for example, in U. S. Patent No. 5,599, 441 and patent publication WO 01/53432 Al. It would be an improvement if a way could be found to remove sulfur from a naphtha that contains olefins and organic sulfur compounds, including thiophenic sulfur, with little or no reduction in octane number. It would be an even greater improvement if the sulfur removal could be integrated with a naphtha-producing cracking process : [0004] The invention relates to a process for selectively removing sulfur from a naphtha feed with reduced or no octane number loss by (a) adding olefins having from four to six carbon atoms (C4-C6 olefins) to a sulfur-containing naphtha feed to form an olefin-enriched feed, (b) contacting the olefin-enriched feed with a catalytically effective amount of a conversion catalyst at conversion reaction conditions effective to convert C4-C6 olefins to iso-olefins having from eight to twelve carbon atoms (C8-C12 iso-olefins) and form an iso-olefinic naphtha, and (c) selectively hydrodesulfurizing the iso-olefinic naphtha with a sulfur selective hydrodesulfurizing catalyst, at reaction conditions selective for sulfur removal, to remove most of the sulfur and form a desulfurized naphtha useful for mogas, with reduced octane number loss, preferably no octane number loss and more preferably with an octane number increase. By reduced octane number loss, no octane number loss and octane number increase are respectively meant that the octane number of the selectively hydrodesulfurized iso-olefinic naphtha is within about 95%, no less than, and greater than its octane number value prior to the desulfurization. By octane number is meant the average of the Research octane number (R) and Motor octane number (M), or (R + M) /2. By iso-olefinic naphtha is meant naphtha comprising at least about 25 vol. % and preferably at least about 40 vol. % olefins, of which at least about 20 vol. %, preferably at least about 30 vol. % and more preferably at least about 35 vol. % are iso-olefins having from eight to twelve carbon atoms. By sulfur is meant sulfur-containing organic compounds. In a related embodiment, the invention relates to an integrated process in which naphtha is formed by cracking a hydrocarbon feed in a cat cracker, to form a sulfur and olefin-containing naphtha feed (raw naphtha) and isobutene, adding at least a portion of the isobutene to the raw naphtha to form the olefin-enriched naphtha feed, which is then contacted with the catalytically effective amount of the conversion catalyst to form the iso-olefinic naphtha, followed by selectively hydrodesulfurizing the iso-olefinic naphtha to form a desulfurized naphtha whose octane number is within 95% of that of the raw naphtha, preferably with no loss in octane number and more preferably having a higher octane number than that of the raw naphtha.

[0005] If about 10 wt. % or more of the sulfur in the sulfur- containing naphtha comprises thiophenic sulfur compounds, it is preferred that the conversion step also convert them to higher boiling sulfur compounds ("converted thiophenic sulfur compounds") boiling above about 182°F and preferably above about 200°F, so that the iso-olefinic naphtha contains converted thiophenic sulfur compounds. Although the total iso-olefinic naphtha formed by the conversion step may be selectively hydrodesulfurized, it is preferred to separate it into heavy and light fractions, wherein the heavy fraction contains the C8-Cl2 iso-olefins and the light fraction contains C6 olefins, and then selectively hydrodesulfurize only the heavy fraction. The octane number of the selectively hydrodesulfurized heavy fraction will be no less, and preferably greater than it was prior to the desulfurization. The light fraction may then be recombined with the hydrotreated heavy fraction to form a mogas naphtha stock, which will typically have an octane number higher than both the raw naphtha feed and the iso-olefinic naphtha. This minimizes the loss of C6 olefins by saturation, which would otherwise occur if the light fraction remains and is selectively hydrodesulfurized with the heavy fraction. The heavy fraction would also contain the higher boiling sulfur compounds formed by converting thiophenic sulfur compounds. By light and heavy fractions is meant fractions respectively boiling below and above a temperature the range of from about 180°F to about 300°F and preferably about 200°F to about 250°F. The iso-olefinic naphtha feed will have a Bromine Number greater than about 50. In the embodiment in which the iso-olefinic naphtha is formed by converting a naphtha in which most of the sulfur is in the form of thiophenic sulfur compounds, more than about 50 wt. % and preferably more than about 75 wt. % of the sulfur in iso-olefinic naphtha will be in the form of the higher boiling, converted thiophenic sulfur compounds.

[0006] The sulfur-selective hydrodesulfurizing catalyst and conditions are chosen so that most of the sulfur is removed from the naphtha as H2S, while most of the olefins are retained. The sulfur-selective hydrodesulfurizing catalyst can be, for example, is a low metal-loaded catalyst of the type disclosed and described in U. S. Patent No. 6,013, 598, the disclosure of which is incorporated herein by reference. This catalyst comprises an inorganic refractory support component, from about 2 to about 8 wt. % MoO3, about 0.1 to about 5 wt. % CoO, has a Co/Mo atomic ratio of about 0.1 to about 1.0, a median pore diameter of from about 60A to about 200A, a MoO surface concentration in g Mo03/m2 of about 0. 5x10-4 to about 3x10-4, an average particle size diameter of less than about 2.0 mm, and a metal sulfide edge plane area of from about 800 to about 2,800 pmol oxygen/g Mo03 as measured by oxygen chemisorption.

Suitable conversion catalysts comprise solid acid catalysts having alkylating activity, such as those disclosed in the patents and patent publications referred to in paragraph [0003], and discussed in more detail below.

[0007] The particular choice of selective hydrodesulfurizing conditions depend on whether single or multiple-stage selective hydrodesulfurization is used and whether or not the iso-olefinic naphtha feed is in the vapor phase.

Single-stage selective hydrodesulfurization reaction conditions will include a temperature in the range of about 450°F to about 675°F and preferably about 500°F to about 625°F, a pressure in the range of from about 200 to about 800 psig and preferably about 200 to about 500 psig, a liquid hourly space velocity in the range of from about 1.2 to about 15 V/V/Hr and preferably about 1.5 to about 10 VN/Hr, a hydrogen treat gas feed rate of about 200 to about 5000 SCF/B and preferably about 200 to about 2500 SCF/B, with the hydrogen content of the (hydrogen) treat gas ranging from about 50 to about 100 and preferably about 65 to about 100 volume %. Selective hydrodesulfurizing with the naphtha in the vapor state and/or using two or more stages with interstage removal of H2S, will permit the use of slightly higher temperatures and slightly lower space velocities.

DETAILED DESCRIPTION [0008] The invention relates to a process for removing sulfur from a naphtha boiling-range hydrocarbon with little or no loss in the hydrocarbon's octane number. The hydrocarbon can be a naphtha, naphtha feed or feedstock including petroleum naphthas, steam cracked naphthas, coker naphthas, thermally cracked naphthas, FCC naphthas and blends and fractions thereof, with boiling points typically in the range of from about C5 Up to about 450°F for full-range naphthas.

Light fraction naphthas typically have a boiling range of from about C4, C5, or C6 up to about 330°F and heavy naphthas boil in the range of from about 250°F to about 450°F. All cracked naphthas inherently contain olefins, preferably C5+ olefins, and organic sulfur compounds, and have most of the sulfur in the form of thiophenic sulfur compounds. Such naphthas have an initial boiling point typically starting at about C5. By thiophenes and thiophenic is meant thiophene and derivatives thereof such as methylthiophene, dimethylthiophene, tetrahydrothiophene and methyl-tetrahydrothiophene. The sulfur and olefin- containing naphtha feed will typically be a highly olefinic thermally or catalytically (cat) cracked naphtha, preferably a cat cracked naphtha and more preferably a fluid cat cracked naphtha (an FCC naphtha), which generally contains more than about 25 vol. % C5+ olefins and in which most of the sulfur is in the form of thiophenic sulfur compounds. For the integrated process embodiment, the naphtha feed will comprise an FCC naphtha. An FCC naphtha, typically contains up to about 60 vol. % olefins and from about 1,000 to about 7,000 wppm (about 0.1 to about 0.7 wt. %) sulfur, more typically up to about 3,000 wppm, most of which is in the form of thiophenic sulfur compounds. The actual olefin content of a sulfur-containing cracked naphtha typically ranges from about 5 to about 60 vol. %, with about 10 to about 40 vol. % being more typical.

[0009] Thermal and catalytic cracking processes are conventional, i. e. , well known, to those skilled in the art. In a catalytic (or"cat") cracking process, a cracker feed is brought into contact with a hot cracking catalyst. This cracks the heavy feed to lower boiling components, which includes naphtha and cycle oils, light olefins comprising isobutene, butene and propylene and coke. Cat cracker feeds typically include gas oils, including a vacuum gas oil, a straight run gas oil, a deasphalted oil and coker gas oils. These oils typically have an initial boiling point above about 450°F (232°C). In addition, one or more heavy feeds having an end boiling point above about 1050°F may be blended in with the cat cracker feed. These heavy feeds include whole and reduced crudes, resids, asphalts and oils derived from tar sand, coal, syncrudes and the like, as is known.

In an FCC process, the cracking catalyst comprises a fluidized, fine powder in a riser reaction zone. The cracker feed contacts the hot catalyst particles in the riser, in which it is cracked into lower boiling components. In the integrated process of the invention, a cat cracker feed is cracked in a fluidized cat cracker to produce lower boiling components, which include raw naphtha and light olefins. The light olefins comprise C4-C6 olefins, including isobutene. At least a portion of this isobutene is separated from the cracked product and added to the raw cat cracker naphtha to produce the olefin-enriched feed, which is then contacted with the conversion catalyst to convert isobutene and other C4-C6 olefins to iso-olefins and form the iso-olefinic naphtha, which is then selectively hydrodesulfurized. The production of both naphtha and isobutene in a cat cracker is well known and is disclosed, for example, in U. S. Patent No.

4,828, 679, the disclosure of which is incorporated herein by reference.

[0010] The iso-olefins formed by the conversion reaction are made from olefins having from four to six carbon atoms (C4-C6). The total olefin content should be sufficient to form C8-C12 iso-olefins and to convert thiophenes to the higher boiling sulfur compounds. In olefin enrichment, C4-C6 olefins are added to the raw naphtha feed, even if the raw naphtha feed inherently contains some C4-C6 olefins. By C4-C6 olefins in the context of the invention is meant one or more of four, five or six carbon atom olefins, typically five and six carbon atoms, and preferably olefins having four, five and six carbon atoms. By C6- olefins and a naphtha containing C6 olefins is not meant to exclude the presence of C7+ olefins (e. g., C7-Cl2) in the naphtha. In at least some cases C7+ olefins will also be present in a naphtha containing C6 olefins. The added C4-C6 olefins- can comprise linear olefins and/or iso-olefins, and preferably isobutene in the integrated process embodiment. It is preferred that at least a portion of the olefin content of the raw naphtha be inherent and present in an amount of at least about 25, preferably about 30 vol. %, up to at least about 60 and preferably between about 50 and about 60 vol. %. This corresponds to a Bromine number ranging from about 15 to about 30+. Nitrogen compounds, particularly basic nitrogen compounds, are removed from the naphtha feed prior to the conversion reaction, if needed to avoid deactivation of the acid catalyst. This is done by any suitable means, including conventional means such as washing with an aqueous acidic solution and/or passing the naphtha feed through one or more guard beds to adsorb or absorb these compounds, prior to the feed contacting the acid catalyst.

Such guard beds can be conventional, and may contain one or more solid materials such as fresh cracking catalyst, fluorided alumina, any of a number of zeolite materials, alumina, silica, silica-alumina, activated carbon, various clays, etc.

[0011] The iso-olefinic feed is formed using an acidic material having alkylating activity and which is capable of (i) converting C4-C6 olefins to Ce-Cl2 iso-olefins and, for a naphtha feed having 10 wt. % or more thiophenic sulfur, (ii) catalyzing the reaction of an olefin or alcohol with thiophenic sulfur compounds to form higher boiling sulfur compounds. Solid acidic catalysts are particularly desirable and include liquid acids supported on a solid substrate, as well as acidic inorganic oxides and acidic polymeric resins. See, e. g. , U. S.

Patent Nos. 5,863, 419 and 5,599, 411, which are incorporated herein by reference. A catalyst comprising phosphoric acid supported on kieselguhr has been used with particular effectiveness. The preparation and use of this particular type of catalyst is known and disclosed in for example, the'419 patent and in published patent application 20020148757. Catalysts comprising acidic inorganic oxides useful for treating the naphtha include, by way of example, aluminas, silica-aluminas, natural and synthetic pillared clays, and natural and synthetic zeolites. Illustrative, but non-limiting examples include faujasites, mordenites, L, omega, X, Y, beta, and ZSM zeolites. Highly suitable zeolites include zeolite beta, Y, ZSM-3, ZSM-4, ZSM-5 types, ZSM-12, ZSM-18, ZSM- 20, ZSM-35 and ZSM-48, TEA, mordenite, faujasites, USY and members of the MCM group, such as MCM-2.

[0012] Forming iso-olefins and higher boiling sulfur compounds in the naphtha feed reduces the Bromine Number because at least some C4-C6 olefins are converted to C8-C12 iso-olefins useful for octane number, and these iso-olefins participate in the conversion reaction as alkylating agents for converting thiophenic sulfur compounds to the higher boiling sulfur compounds.

Alcohols are also known to be useful alkylating agents and, while it is permissible to add one or more alcohols to the naphtha feed as thiophenic sulfur alkylating agents, it is preferred to use only olefins. Conversion reaction conditions useful for forming C8-C12 iso-olefins and higher boiling sulfur compounds from the naphtha feed to produce an iso-olefinic naphtha, include a temperature in the range of from about 100°F to about 700°F (about 38°C to about 371°C), preferably about 200°F to about 600°F (about 93°C to about 316°C) and more preferably about 300°F to about 400°F (about 149°C to about 204°C). The pressure may range from about 50 to about 10,000 kPa, preferably from about 100 to about 2,000 kPa. Preferred olefins in the naphtha feed for converting thiophenes and forming the iso-olefins will have from about 4 to about 12 carbon atoms and preferably from about 4 to about 8 carbon atoms.

While the formation of the higher boiling sulfur compounds may cause some reduction in the sulfur content of the naphtha, it is usually minor and the amount of reduction will depend on many factors, including the sulfur-containing naphtha feed, type and amount of olefins present, catalyst and reaction conditions. If most of the feed sulfur is in the form of thiophenic sulfur compounds, then most of the sulfur remaining in the iso-olefinic naphtha will be higher boiling, converted thiophenic sulfur and will be concentrated in the heavy fraction boiling in the range of about 180°F to about 300°F+, with only relatively minor amounts of lower boiling and non-thiophenic sulfur compounds left in the light fraction boiling in the range of about 180°F to a about 300°F-. In some many cases the amount of sulfur remaining in the light fraction will be low enough for it not to require hydrodsulfurization. The C8-C12 iso-olefins present in the iso-olefinic naphtha feed formed by the conversion reaction will also be in the heavy fraction. It is preferred that the iso-olefinic naphtha feed or fraction thereof to be selectively hydrodesulfurized have a high olefin concentration, as reflected in a Bromine number greater than about 50, preferably greater than about 60 and more preferably at least about 75.

[0013] Selective hydrodesulfurization uses a sulfur-selective hydrodesulfurization catalyst, sulfur-selective process conditions, or both, to remove sulfur with little or no attendant octane number reduction. Conventional selective hydrodesulfurization can be used. One conventional sulfur-selective hydrodesulfurization catalyst comprises an inorganic refractory support component, from about 1 to about 10 wt. % Mo03, preferably about 2 to about 8 wt. % and more preferably about 4 to about 6 wt. %. The amount of CoO will range from about 0.1 to about 5 wt. %, preferably about 0.5 to about 4 wt. % and more preferably about 1 to about 3 wt. %. The weight percents expressed herein are all based on the total weight of the catalyst. The Co/Mo atomic ratio will range from about 0.1 to about 1, preferably about 0.20 to about 0.80 and more preferably about 0.25 to about 0.72. The catalyst will have a median pore diameter of from about 60A to about 200A, preferably from about 75A to about 175A and more preferably about 80A to about 150A. The MoO surface concentration in g Mo03/m2 will range between about 0. 5x10-4 to about 3x104, preferably about 0. 75x104 to about 2. 5x10-4 and more preferably about lux10-4 to about 2. x104. The average particle size diameter is less than about 2 mm, preferably less than about 1.6 and more preferably less than about 1.4. It will have a metal sulfide edge plane area of from about 800 to about 2, 800 gmol oxygen/g Mo03 as measured by oxygen chemisorption, preferably about 1,000 to about 2,200 and more preferably about 1,200 to about 2,000. The amounts and types of contaminants permitted as well as the amount of phosphorus and alkali metal additives desired to be present and suitable catalyst preparation techniques, may be found in U. S. Patent No. 6,013, 589. Selective hydrodesulfurizing conditions, including conventional selective hydrodesulfurization conditions can be used. Such conditions can include a temperature in the range of about 450°F to about 675°F and preferably about 500°F to about 625°F, a pressure in the range of from about 200 to about 800 psig and preferably about 200 to about 500 psig, a liquid hourly space velocity in the range of from about 1.2 to about 15 V/V/Hr and preferably about 1.5 to about 10 V/V/Hr, a hydrogen treat gas feed rate of about 200 to about 5000 SCF/B and preferably about 200 to about 2500 SCF/B, with the hydrogen content of the (hydrogen) treat gas ranging from about 50 to about 100 and preferably about 65 to about 100 volume %. The selective hydrodesulfurization removes most of the sulfur from the converted naphtha, while retaining most of the olefins in it, to produce a desulfurized naphtha having an octane number within about 95%, preferably no less than, and more preferably greater than its octane number value prior to the desulfurization. As set forth above, selectively hydrodesulfurizing with the naphtha in the vapor state and/or using two or more stages with interstage removal of H2S, will permit the use of slightly higher temperatures and slightly lower space velocities. A multiple-stage process with interstage removal of H2S is disclosed, for example, in U. S. Patent No. No.

6,231, 753 referred to above, the disclosure of which is incorporated herein by reference and which relates to selective naphtha hydrodesulfurization.