WO2005019391A1

WO2005019391A1 - Naphtha desulfurization with no octane loss and increased olefin retention

Info

Publication number: WO2005019391A1
Application number: PCT/US2004/024861
Authority: WO
Inventors: Gordon F. Stuntz; Randolph J. Smiley; Thomas R. Halbert
Original assignee: Exxonmobil Research And Engineering Company
Priority date: 2003-08-19
Filing date: 2004-08-03
Publication date: 2005-03-03
Also published as: WO2005019390A1; WO2005019387A1

Abstract

Naphtha is desulfurized with no octane loss by forming an iso-olefinic naphtha feed and then selectively hydrodesulfurizing it to produce a mogas naphtha. The iso-olefinic naphtha feed may be formed from cracked naphtha, by contacting it with a solid acid catalyst to convert C6- olefins to C8-C12 iso-olefins and convert thiophenes to higher boiling sulftir compounds. Preferably only a heavy fraction containing the isoolefins and higher boiling sulfur is selectively hydrodesulfurized.

Description

NAPHTHADESULFURIZATIONWITHNO OCTANELOSS ANDINCREASED OLEFINRETENTION

FIELD OF THE INVENTION

[0001] The invention relates to a process for removing sulfur from naphtha, with olefin retention and no octane loss. In an embodiment, the process comprises (a) forming, from a sulfur-containing naphtha, an iso-olefinic naphtha feed containing sulfur and iso-olefins having from eight to twelve carbon atoms, and (b) selectively hydrodesulfurizing the iso-olefinic naphtha feed with no octane number loss and increased olefin retention.

BACKGROUND OF THE INVENTION

[0002] Motor gasoline ("mogas") sulfur specifications are being regulated to increasingly lower levels (e.g., less than 30 wppm), due to environmental concerns. 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 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 of the desulfurized product. The catalyst and processes are disclosed for example, in U.S. Patents 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 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 compounds, with reduced olefin loss and little or no reduction in octane number. It would be an even greater improvement if these goals could be achieved with an increase in octane number.

SUMMARY OF THE INVENTION

[0004] The invention relates to a process for selectively removing sulfur from a sulfur and olefin-containing naphtha feed with little or no octane number loss and with retention of most of the feed olefins. The process comprises (a) forming, from a sulfur-containing naphtha, an iso-olefinic naphtha feed containing sulfur and iso-olefins having from eight to twelve carbon atoms (C₈ - Cι₂), and (b) selectively hydrodesulfurizing the iso-olefinic naphtha feed with a sulfur selective hydrodesulfurizing catalyst, at reaction conditions selective for sulfur removal, to remove most of the sulfur while retaining most of the olefins and form a desulfurized naphtha useful for mogas, with little or no octane number loss. In a preferred embodiment the selective hydrodesulfurizing will result in an octane number increase. By no octane number loss and octane number increase is respectively meant the octane number of the desulfurized naphtha is no less than, and preferably greater than, that of the iso-olefinic naphtha feed prior to being selectively hydrodesulfurized. By iso-olefinic naphtha feed is meant naphtha comprising at least about 25 vol.% and preferably at least about 40 vpl.% 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. By octane number is meant the average of the Research octane number (R) and Motor octane number (M), or (R + M)/2.

[0005] The iso-olefinic naphtha feed can be formed by (i) adding Cs-Ci₂ iso-olefins to a sulfur-containing naphtha, by (ii) converting an olefinic naphtha that contains sulfur and C_6- olefins to an iso-olefinic naphtha feed, by contacting the feed with a catalytically effective amount of a conversion catalyst at conversion reaction conditions effective to convert at least some of the C_6- olefms to iso-olefins having from eight to twelve carbon atoms, or (iii) a combination thereof. More typically the sulfur-containing naphtha will be a highly olefinic cracked naphtha, such as an FCC naphtha, which typically contains more than about 25 vol.% olefins and in which most of the sulfur is in the form of thiophenic sulfur compounds. If the sulfur-containing naphtha contains thiophenic sulfur compounds, it is preferred that the conversion step also convert the thiophenic sulfur compounds in the naphtha to higher boiling sulfur compounds ("converted thiophenic sulfur compounds") boiling above about 182°F and preferably above about 200°F, to produce an iso-olefinic naphtha feed containing converted thiophenic sulfur compounds. Although the iso-olefinic naphtha formed by the conversion step (ii) may be selectively hydrodesulfurized, it is preferred to separate it into heavy and light fractions, with the heavy fraction containing the C₈-Ci₂ iso-olefins and the light fraction containing C_6- olefins, and then selectively hydrodesulfurize only the heavy fraction under conditions where the octane number of the selectively hydrodesulfurized heavy fraction will be no less 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 the iso-olefinic naphtha formed in step (ii). This minimizes the loss of C_6- 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, for the preferred embodiment in which the sulfur in the sulfur- containing naphtha feed is mostly thiophenic sulfur. 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 feed 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 feed 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, a low metal-loaded catalyst of the type disclosed and described in U.S. Patent 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.% M0O₃, 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 6θA to about 20θA, a MoO surface concentration in g Mo0₃/m² of about 0.5xl0^"4 to about 3x10^"4, an average particle size diameter of less than 2.0 mm, and a metal sulfide edge plane area of from about 800 to about 2,800 μmol oxygen/g M0O₃ 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 are 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 V/V/Hr, a hydrogen treat gas feed rate of about 200 to about 5000 SCF/B and preferably about 200 to bout 2500 SCF/B, with the hydrogen content of the (hydrogen) treat gas ranging from 50-100 and preferably 65-100 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 and preferably an increase. The hydrocarbon can be a 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 Ce up to about 330°F and heavy naphthas boil in the range of about 250°F to about 450°F. All cracked naphthas inherently contain olefins and organic sulfur compounds, with 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. In an embodiment, the feed is a thermally or cat cracked naphtha, e.g., an FCC naphtha, which 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 bout 60 vol.%, with about 10 to about 40 vol.%) being more typical.

[0009] The iso-olefins formed by the conversion are made from olefins having from four to six carbon atoms (C_6-). The total olefin content should be sufficient to form C₈-C₁₂ iso-olefins and to convert thiophenes to the higher boiling sulfur compounds. If the sulfur-containing naphtha does not contain sufficient C_6- olefins, then C_6- olefins may be added. In an embodiment, the olefin content of the sulfur-containing naphtha is naturally 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+. By C_6- 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 C_6- olefins and a naphtha containing C_6- olefins is not meant to exclude the presence of C ₊ olefins (e.g., C -C₁₂) in the naphtha. In at least some cases C₇₊ olefins will also be present in a naphtha containing C₆_ olefins. 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. Conventional nitrogen removal means can be used, including 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.

[0010] The iso-olefinic feed is formed using an acidic material having alkylating activity and which is capable of (i) converting C -C₆ olefins to Cg-Cι₂ 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. Patents 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 lαiown 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.

[0011] The naphtha feed conversion treatment reduces the Bromine Number because at least some of the feed olefins participate in the conversion reaction as alkylating agents for converting the thiophenic sulfur compounds, while others are converted to branched isoparaffins useful for octane number. 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 alkylating agents prior to the conversion treatment, it is preferred use only olefins as alkylating agents. Reaction conditions useful for treating the naphtha feed to produce a converted 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 conversion 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, catalyst, type and amount of olefins present, catalyst and reaction conditions. Most of the sulfur remaining in the converted 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 nonthiophenic sulfur left in the light fraction (boiling in the range of about 180°F to about 300°F-). In some cases the amount of sulfur remaining in the light fraction will be low enough for it not to require hydrodsulfurization. The Cs-Cι₂ iso-olefins present in the iso-olefinic naphtha feed formed by the conversion reaction will also be in the heavy fraction. In an embodiment, the iso-olefinic naphtha feed or fraction thereof to be selectively hydrodesulfurized has 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.

[0012] 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 6θA to about 200 A, preferably from about 75 A to about 175 A and more preferably about 8θA to about 15θA. The MoO surface concentration in g Mo0₃/m² will range between about 0.5xl0^"4 to about 3xl0^"4, preferably about 0.75xl0^"4 to about 2.5xl0^"4 and more preferably about lxlO^"4 to about 2.x 10^"4. 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 μmol 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 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. 6,231,753 referred to above, the disclosure of which is incorporated herein by reference and which relates to selective naphtha hydrodesulfurization.

Claims

CLAIMS;

1. A process for selectively removing sulfur from a sulfur and olefin-containing naphtha comprising (a) forming, from a sulfur-containing naphtha, an iso-olefinic naphtha feed containing sulfur and iso-olefins having from eight to twelve carbon atoms, and (b) selectively hydrodesulfurizing said iso-olefinic naphtha feed with a sulfur selective hydrodesulfurizing catalyst at reaction conditions selective for sulfur removal, to remove most of said feed sulfur while retaining most of said feed olefins, and forming a desulfurized naphtha useful for mogas having at least the same octane as said iso-olefinic naphtha feed.

2. A process according to claim 1 wherein less than about 50 wt.%) of said feed olefins are saturated by said hydrodesulfurization reaction.

3. A process according to claim 2 wherein said iso-olefinic feed comprises at least about 25 vol.% olefins, of which at least about 20 vol.%> are iso- olefins having from eight to twelve carbon atoms.

4. A process according to claim 3 wherein said sulfur and olefin-containing naphtha contains C_6- olefins.

5. A process according to claim 4 wherein said iso-olefinic naphtha feed is formed from said sulfur and olefin-containing naphtha by contacting said naphtha with a conversion catalyst comprising a solid acid catalyst having alkylating activity, at conversion reaction conditions effective to form said iso-olefins from said C - olefins.

6. A process according to claim 5 wherein said desulfurized naphtha has higher octane than said iso-olefinic naphtha feed.

7. A process according to claim 6 wherein most of said sulfur in said sulfur and olefin-containing naphtha is in the form of thiophenic sulfur compounds.

8. A process according to claim 7 wherein most of said sulfur in said iso-olefinic naphtha feed is in the foπn of thiophenic sulfur compounds that have been converted to higher boiling sulfur compounds by said conversion reaction.

9. A process according to claim 8 wherein said iso-olefinic naphtha feed contains at least about 50 vol.% olefins and wherein said hydrodesulfurizing removes at least about 90 wt.% of said sulfur as H₂S.

10. A process for selectively removing sulfur from naphtha containing C_6- olefins and sulfur in the form of organic sulfur compounds comprises (a) contacting said naphtha with a catalyst comprising a solid acid catalyst having alkylating activity, at reaction conditions effective to convert at least a portion of said C_6- olefins to iso-olefins having from eight to twelve carbon atoms and form an iso-olefinic naphtha feed, and (b) selectively hydrodesulfurizing said iso-olefinic naphtha feed with a sulfur selective hydrodesulfurizing catalyst at reaction conditions selective for sulfur removal, to remove most of said feed sulfur while retaining most of said feed olefins and form a desulfurized naphtha useful for mogas having at least the same octane as said iso-olefinic naphtha feed, wherein said sulfur selective catalyst comprises an inorganic refractory support component, from about 2 to about 8 wt.% Mo0₃, 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 6θA to about 20θA, a MoO surface concentration in g Mo0₃/m² of about 0.5xl0^"4 to about 3xl0^"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 μmol oxygen/g Mo0₃ as measured by oxygen chemisorption.

11. A process according to claim 10 wherein more than about 75 wt.% of said organic sulfur compounds in said C_6- olefin-containing naphtha comprise thiophenic sulfur compounds, which are converted to higher boiling sulfur compounds boiling above about 200°F during said conversion reaction.

12. A process according to claim 11 wherein said iso-olefinic feed comprises at least about 25 vol.% olefins, of which at least about 20 vol.% are iso- olefins having from eight to twelve carbon atoms.

13. A process according to claim 12 wherein said desulfurized naphtha has a higher octane than said iso-olefm naphtha feed.

14. A process according to claim 13 wherein said iso-olefinic naphtha feed is separated into a light fraction and a heavy fraction which respectively boil below and above a temperature in the range of from about 180 to about 300°F and wherein only said heavy fraction is selectively hydrodesulfurized.

15. A process according to claim 14 wherein said heavy fraction contains said iso-olefins having from eight to twelve carbon atoms and said converted, higher boiling sulfur compounds.

16. A process according to claim 15 wherein said sulfur and C_6- olefin- containing naphtha comprises a cracked naphtha containing at least about 40 vol.%) olefins.

17. A process according to claim 16 wherein most of said organic sulfur compounds in said C_6- olefin-containing naphtha comprise thiophenic sulfur compounds.

18. A process according to claim 17 wherein said iso-olefinic naphtha feed comprises at least about 35 vol.% iso-olefins having from eight to twelve carbon atoms.

19. A process according to claim 18 wherein said said iso-olefinic naphtha is vapor during said selective hydrodesulfurization.

20. A process according to claim 10 wherein said selective hydrodesulfurization is conducted in two or more reaction stages with interstage separation and removal of H₂S.