Title:
Process to manufacture low sulfur fuels
Kind Code:
A1


Abstract:
The instant invention relates to a process for producing high octane, low sulfur naphtha products through skeletal isomerization of feed olefins and hydrotreating.



Inventors:
Welch, Robert C. (Baton Rouge, LA, US)
Stuntz, Gordon F. (Baton Rouge, LA, US)
Shih, Stuart S. (Baton Rouge, LA, US)
Application Number:
10/887680
Publication Date:
02/03/2005
Filing Date:
07/09/2004
Assignee:
WELCH ROBERT C.
STUNTZ GORDON F.
SHIH STUART S.
Primary Class:
Other Classes:
208/213, 208/216R, 208/217, 208/134
International Classes:
B01J23/85; B01J23/882; B01J29/06; B01J29/70; C10G65/04; (IPC1-7): C10G69/08; C10G59/02
View Patent Images:
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Primary Examiner:
SINGH, PREM C
Attorney, Agent or Firm:
ExxonMobil Research & Engineering Company (P.O. Box 900 1545 Route 22 East, Annandale, NJ, 08801-0900, US)
Claims:
1. A process for producing low sulfur naphtha products from an olefin and sulfur containing naphtha boiling range feedstream comprising: a) contacting a naphtha boiling range feedstream containing organically bound sulfur and olefins in a first reaction zone operated under effective isomerization conditions and in the presence of hydrogen-containing treat gas with a first catalyst selected from medium pore zeolites to produce a first reaction zone effluent; and b) hydrotreating at least a portion of the first reaction zone effluent of step a) above in a second reaction zone operated under effective hydrotreating conditions and in the presence of hydrogen-containing treat gas and a second catalyst selected from hydrotreating catalysts comprising about 2 to 20 wt. % of at least one Group VIII metal oxide and about 1 to 50 wt. % of at least one Group VI metal oxide to produce a desulfurized product.

2. The process according to claim 1 wherein said first and second reaction zones comprise one or more catalyst beds selected from the group consisting of fluidized beds, ebullating beds, slurry beds, fixed beds, and moving beds wherein each of said one or more catalyst beds contains a catalyst suitable for the reaction zone in which the catalyst bed is located.

3. The process according to claim 2 wherein said first and second reaction zones are located in the same reaction vessel.

4. The process according to claim 3 wherein said first and second reaction zones comprise one or more fixed catalyst beds.

5. The process according to claim 2 wherein said process further comprises interstage cooling between said first and second reaction zone, or between catalyst beds in said first and second reaction zone.

6. The process according to claim 2 wherein said first catalyst is selected from ZSM-23 and ZSM-48.

7. The process according to claim 2 wherein said first catalyst is ZSM-48.

8. The process according to claim 6 wherein said second catalyst is a hydrotreating catalyst comprising about 4 to about 12% of a Group VIII metal oxide and about 10 to about 40 wt. % of a Group VI metal oxide.

9. The process according to claim 6 wherein said second catalyst is a hydrotreating catalyst comprising about 1 to 25 wt. % MoO3, about 0.1 to 6 wt. % CoO wherein said CoO and MoO3 are present in an atomic ratio of about 0.1 to about 1.0 Co/Mo, and said catalyst has a median pore diameter of about 75 Å to about 175 Å, wherein said second catalyst has a MoO3 surface concentration of about 0.5×10−4 to about 3×10−4 g and an average particle size diameter of less than 2.0 mm.

10. The process according to claim 1 wherein said effective isomerization conditions are selected to cause skeletal isomerization of at least about 20 wt. % of the n-olefins present in said naphtha boiling range feedstream.

11. The process according to claim 10 wherein said first reaction zone effluent has a higher ratio of iso-olefins to n-olefins than the naphtha boiling range feedstream.

12. The process according to claim 9 wherein said second catalyst further comprises a suitable binder or matrix material selected from zeolites, alumina, silica, titania, calcium oxide, strontium oxide, barium oxide, carbons, zirconia, diatomaceous earth, lanthanide oxides including cerium oxide, lanthanum oxide, neodymium oxide, yttrium oxide, and praseodymium oxide; chromia, thorium oxide, urania, niobia, tantala, tin oxide, zinc oxide, and aluminum phosphate.

13. The process according to claim 12 wherein said suitable binder or matrix support of said second catalyst also contains less than about 1 wt. % of contaminants, such as Fe, sulfates, silica, and various metal oxides that can be introduced during the preparation of the support.

14. The process according to claim 13 wherein said suitable binder or matrix support of said second catalyst also contains about 0 to 5 wt. % of an additive selected from the group consisting of phosphorus and metals or metal oxides from Group IA (alkali metals) of the Periodic Table of the Elements.

15. The process according to claim 12 wherein said suitable binder or matrix material is selected from alumina, silica, and silica-alumina.

16. The process according to claim 12 wherein said suitable binder or matrix material is alumina.

17. The process according to claim 10 wherein said first catalyst further comprises a suitable porous binder or matrix material selected from clays, silica, and/or metal oxides such as alumina.

18. The process according to claim 17 wherein said suitable porous binder or matrix material is selected from silica, alumina, or a kaolin clay.

19. The process according to claim 16 wherein said suitable porous binder or matrix material is alumina present in a ratio of less than about 15 parts zeolite to one part binder.

20. The process according to claim 19 wherein said effective hydrotreating conditions are selected in such a manner that said desulfurized naphtha product has a sulfur level less than 100 wppm sulfur.

21. The process according to claim 20 wherein said effective hydrotreating conditions are selective hydrotreating conditions.

22. The process according to claim 20 wherein said desulfurized naphtha product has a higher concentration of iso-paraffins than n-paraffins.

23. The process according to claim 22 wherein the naphtha boiling range feedstream containing organically bound sulfur and olefins is preheated prior to entering said first reaction zone.

24. A process for producing low sulfur naphtha products from an olefin and sulfur containing naphtha boiling range feedstream comprising: a) contacting a naphtha boiling range feedstream containing organically bound sulfur and olefins in a first reaction zone operated under effective isomerization conditions and in the presence of hydrogen-containing treat gas with a first catalyst selected from ZSM-23 and ZSM-48 to produce a first reaction zone effluent having a higher ratio of iso-olefins to n-olefins than the naphtha boiling range feedstream; and b) hydrotreating at least a portion of the first reaction zone effluent of step a) above in a second reaction zone operated under effective hydrotreating conditions and in the presence of hydrogen-containing treat gas and a second catalyst selected from hydrotreating catalysts comprising about 1 to 25 wt. % MoO3 about 0.1 to 6 wt. % CoO wherein said CoO and MoO3 are present in an atomic ratio of about 0.1 to about 1.0 Co/Mo, and said catalyst has a median pore diameter of about 75 Å to about 175 Å, wherein said second catalyst has a MoO3 surface concentration of about 0.75×10−4 to about 2.5×10−4 g and an average particle size diameter of less than 2.0 mm to produce a desulfurized product having a sulfur level less than 100 wppm sulfur and a higher concentration of iso-paraffins than n-paraffins.

25. The process according to claim 24 wherein said first catalyst is ZSM-48.

26. The process according to claim 25 wherein said second catalyst is a hydrotreating catalyst comprising about 4 to 19 wt. % MoO3, about 0.5 to 5.5 wt. % CoO wherein said CoO and MoO3 are present in an atomic ratio of about 0.20 to about 0.80 Co/Mo, and said catalyst has a median pore diameter of about 75 Å to about 175 Å, wherein said second catalyst has a MoO3 surface concentration of about 0.5×104 to about 3×104 g and an average particle size diameter of less than 1.6 mm.

27. The process according to claim 26 wherein said effective isomerization conditions are selected to cause skeletal isomerization of at least about 20 wt. % of the n-olefins present in said naphtha boiling range feedstream.

28. The process according to claim 26 wherein said second catalyst further comprises a suitable binder or matrix material selected from zeolites, alumina, silica, titania, calcium oxide, strontium oxide, barium oxide, carbons, zirconia, diatomaceous earth, lanthanide oxides including cerium oxide, lanthanum oxide, neodymium oxide, yttrium oxide, and praseodymium oxide; chromia, thorium oxide, urania, niobia, tantala, tin oxide, zinc oxide, and aluminum phosphate.

29. The process according to claim 28 wherein said suitable binder or matrix material is selected from alumina, silica, and silica-alumina.

30. The process according to claim 24 wherein said first catalyst further comprises a suitable porous binder or matrix material selected from clays, silica, and/or metal oxides such as alumina.

31. The process according to claim 30 wherein said first reaction zone catalyst further comprises suitable porous binder or matrix material selected from silica, alumina, or a kaolin clay.

32. The process according to claim 31 wherein said effective hydrotreating conditions are selected in such a manner that said desulfurized naphtha product has a sulfur level less than 100 wppm sulfur.

33. The process according to claim 32 wherein said second effective hydrotreating conditions are selective hydrotreating conditions.

34. A process for producing low sulfur naphtha products from an olefin and sulfur containing naphtha boiling range feedstream comprising: a) contacting a naphtha boiling range feedstream containing organically bound sulfur and olefins in a first reaction zone operated under effective isomerization conditions selected to cause skeletal isomerization of at least about 20 wt. % of the n-olefins contained in said feedstream, and in the presence of hydrogen-containing treat gas, with a first catalyst comprising ZSM-48 and an alumina binder, wherein said binder and ZSM-48 are present in a ratio of less than about 15 parts zeolite to one part binder, to produce a first reaction zone effluent having a higher ratio of iso-olefins to n-olefins than the naphtha boiling range feedstream; and b) hydrotreating the first reaction zone effluent of step a) above in a second reaction zone operated under selective hydrotreating conditions and in the presence of hydrogen-containing treat gas and a second catalyst selected from hydrotreating catalysts comprising about 5 to 16 wt. % MoO3, about 1 to 5 wt. % CoO wherein said CoO and MoO3 are present in an atomic ratio of about 0.25 to about 0.72 Co/Mo, and said catalyst has a median pore diameter of about 80 Å to about 150 Å, wherein said second catalyst has a MoO3 surface concentration of about 1×10−4 to 2×10−4 g and an average particle size diameter of less than 1.4 mm to produce a desulfurized product having a sulfur level less than 50 wppm sulfur and a higher concentration of iso-paraffins than n-paraffins.

Description:

CROSS REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Patent Application Ser. No. 60/492,081 filed Aug. 1, 2003.

FIELD OF THE INVENTION

The instant invention relates to a process for upgrading hydrocarbon mixtures boiling within the naphtha range. More particularly, the instant invention relates to a process to produce high octane, low sulfur naphtha products through the skeletal isomerization of feed olefins and hydrotreating.

BACKGROUND OF THE INVENTION

Liquid hydrocarbon streams that boil within the naphtha range, i.e. below about 232° C., and produced from the Fluidized Catalytic Cracking Unit (“FCC”) are typically used as blending components for motor gasolines. Environmentally driven regulatory pressure concerning motor gasoline sulfur levels is expected to result in the widespread production of less than 50 wppm sulfur mogas by the year 2004. Levels below 10 wppm are being considered for later years in some regions of the world, and this will require deep desulfurization of naphthas in order to conform to emission restrictions that are becoming more stringent. The majority, i.e., 90% or more, of sulfur contaminants present in motor gasolines are typically present in naphtha boiling range hydrocarbon streams. However, the naphtha boiling range streams are also rich in olefins, which boost octane, a desirable quality in motor gasolines.

Thus, many processes have been developed to produce low sulfur products from naphtha boiling range streams while attempting to minimize olefin loss, such as, for example, hydrodesulfurization processes. However, these processes also typically hydrogenate feed olefins to some degree, thus reducing the octane number of the product. Therefore, processes have been developed that recover octane lost during desulfurization. Non-limiting examples of these processes can be found in U.S. Pat. Nos. 5,298,150; 5,320,742; 5,326,462; 5,318,690; 5,360,532; 5,500,108; 5,510,016; and 5,554,274, which are all incorporated herein by reference. In these processes, in order to obtain desirable hydrodesulfurization with a reduced octane loss, it is necessary to operate in two steps. The first step is a hydrodesulfurization step, and a second step recovers octane lost during hydrodesulfurization.

Other processes have also been developed that seek to minimize octane lost during hydrodesulfurization. For example, selective hydrodesulfurization is used to remove organically bound sulfur while minimizing hydrogenation of olefins and octane reduction by various techniques, such as the use of selective catalysts and/or process conditions. For example, one selective hydrodesulfurization process, referred to as SCANfining, has been developed by ExxonMobil Research & Engineering Company in which olefinic naphthas are selectively desulfurized with little loss in octane. U.S. Pat. Nos. 5,985,136; 6,013,598; and 6,126,814, all of which are incorporated by reference herein, disclose various aspects of SCANfining. Although selective hydrodesulfurization processes have been developed to avoid significant olefin saturation and loss of octane, such processes have a tendency to liberate H2S a portion of which may react with retained olefins to form mercaptan sulfur by reversion.

Thus, there still exists a need in the art for a process to reduce the sulfur content in naphtha boiling range hydrocarbon streams while minimizing octane loss and mercaptan reversion.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows research octane versus desulfurization results from the example.

FIG. 2 shows iso-olefin to n-olefin ratio results from the example.

FIG. 3 shows iso-paraffin to n-paraffin ratio results from the example.

SUMMARY OF THE INVENTION

The instant invention is directed at a process for producing low sulfur naphtha products. The process comprises:

    • a) contacting a naphtha boiling range feedstream containing organically bound sulfur and olefins in a first reaction zone operated under effective isomerization conditions and in the presence of hydrogen-containing treat gas with a first catalyst selected from medium pore zeolites to produce a first reaction zone effluent; and
    • b) hydrotreating at least a portion of the first reaction zone effluent of step a) above in a second reaction zone operated under effective hydrotreating conditions and in the presence of hydrogen-containing treat gas and a second catalyst selected from hydrotreating catalysts comprising about 0.1 to 27 wt. % of at least one Group VIII metal oxide and about 1 to 45 wt. % of at least one Group VI metal oxide to produce a desulfurized product.

DETAILED DESCRIPTION OF THE INSTANT INVENTION

It should be noted that the terms “hydrotreating” and “hydrodesulfurization” are sometimes used interchangeably herein, and the prefixes “i-” and “n” are meant to refer to “iso-” and “normal”, respectively.

In the hydrotreating of naphtha boiling range feedstreams, olefins are typically saturated in the hydrotreating zone resulting in a decrease in octane number of the desulfurized product. However, the present invention reduces the decrease in octane of the desulfurized product through the use of a novel process involving contacting a naphtha boiling range feedstream in a first reaction zone operated under effective isomerization conditions. This first reaction zone contains a first catalyst selected from medium pore zeolites, and the naphtha boiling range feedstream is contacted with the first catalyst in the presence of a hydrogen-containing treat gas. The contacting of the naphtha boiling range feedstream with the first catalyst produces a first reaction zone effluent. The first reaction zone effluent is then contacted in a second reaction zone operated under effective hydrotreating conditions, and in the presence of hydrogen-containing treat gas, with a second catalyst comprising at least one Group VIII metal oxide and at least one Group VI metal oxide supported on a suitable substrate.

The desulfurized product thus obtained has a higher iso-paraffin to n-paraffin ratio, and thus a higher octane than a desulfurized naphtha treated by a selective or non-selective hydrotreating process only, i.e., without an octane recovery step. The higher octane of the desulfurized product results from the unexpected finding by the inventors hereof that by operating the first reaction zone under conditions effective for encouraging the skeletal isomerization of n-olefins to iso-olefins results in a desulfurized naphtha product having a higher octane number than a desulfurized product resulting from a selective hydrodesulfurization process only. The inventors hereof have found that the degree of skeletal isomerization of n-olefins to iso-olefins benefits the final product because the saturation of iso-olefins to iso-paraffins that occurs in the second reaction zone herein provides for less octane loss in the final product when compared to the saturation of n-olefins to n-paraffins. It should be noted that iso-paraffins typically have a much higher octane than their corresponding n-paraffin. Further, the rate of saturation of iso-olefins is typically slower than that of n-olefins. Therefore, by increasing the ratio of iso-olefins to n-olefins present in the first reaction zone effluent, the resulting desulfurized naphtha product exiting the second reaction zone also has a higher iso-olefin to n-olefin ratio as well as a higher olefin content, and thus a higher octane than a desulfurized naphtha treated by a selective or non-selective hydrotreating process only.

In the hydroprocessing of naphtha boiling range hydrocarbon feedstreams, it is typically highly desirable to remove sulfur-containing compounds from the naphtha boiling range feedstreams with as little olefin saturation as possible. It is also highly desirable to convert as much of the organic sulfur species of the naphtha to hydrogen sulfide with as little mercaptan reversion as possible. By mercaptan reversion we mean the reaction of hydrogen sulfide with olefins during the hydrotreating to form undesirable alkylmercaptans. The inventors hereof have unexpectedly found that through the use of the presently claimed invention, high levels of sulfur can be removed from an olefinic naphtha stream without excessive olefin saturation or mercaptan reversion taking place.

Feedstreams suitable for use in the present invention include naphtha boiling range refinery streams that typically boil in the range of about 50° F. (10° C.) to about 450° F. (232° C.) containing olefins as well as sulfur-containing compounds. Thus, the term “naphtha boiling range feedstream” as used herein includes those streams having an olefin content of at least about 5 wt. %. Non-limiting examples of naphtha boiling range feedstreams that can be treated by the present invention include fluid catalytic cracking unit naphtha (FCC catalytic naphtha or cat naphtha), steam cracked naphtha, and coker naphtha. Also included are blends of olefinic naphthas with non-olefinic naphthas as long as the blend has an olefin content of at least about 5 wt. %, based on the total weight of the naphtha feedstream.

Cracked naphtha refinery streams generally contain not only paraffins, naphthenes, and aromatics, but also unsaturates, such as open-chain and cyclic olefins, dienes, and cyclic hydrocarbons with olefinic side chains. The olefin-containing naphtha feedstream can contain an overall olefins concentration ranging as high as about 70 wt. %, more typically as high as about 60 wt. %, and most typically from about 5 wt. % to about 40 wt. %. The olefin-containing naphtha feedstream can also have a diene concentration up to about 15 wt. %, but more typically less than about 5 wt. % based on the total weight of the feedstock. The sulfur content of the naphtha feedstream will generally range from about 50 wppm to about 7000 wppm, more typically from about 100 wppm to about 5000 wppm, and most typically from about 100 to about 3000 wppm. The sulfur will usually be present as organically bound sulfur. That is, as sulfur compounds such as simple aliphatic, naphthenic, and aromatic mercaptans, sulfides, di- and polysulfides and the like. Other organically bound sulfur compounds include the class of heterocyclic sulfur compounds such as thiophene, tetrahydrothiophene, benzothiophene and their higher homologs and analogs. Nitrogen can also be present in a range from about 5 wppm to about 500 wppm.

The feedstreams used herein are typically preheated prior to entering the first reaction zone herein and final heating is typically targeted to the effective hydrotreating temperatures in the second reaction zone. If the naphtha boiling range feedstream is preheated, it can be reacted with the hydrogen-containing treat gas stream prior to, during, and/or after preheating. At least a portion of the hydrogen-containing treat gas can also be added at an intermediate location in the first reaction zone. Hydrogen-containing treat gasses suitable for use in the presently disclosed process can be comprised of substantially pure hydrogen or can be mixtures of other components typically found in refinery hydrogen streams. It is preferred that the hydrogen-containing treat gas stream contains little, more preferably no, hydrogen sulfide. The hydrogen-containing treat gas purity should be at least about 50% by volume hydrogen, preferably at least about 75% by volume hydrogen, and more preferably at least about 90% by volume hydrogen for best results. It is most preferred that the hydrogen-containing stream be substantially pure hydrogen.

In the first reaction zone, the above-described naphtha boiling range feedstream is contacted with a first catalyst comprising a medium pore zeolite. Zeolites are porous crystalline materials, and medium pore zeolites as used herein can be any zeolite described as a medium pore zeolite in Atlas of Zeolite Structure Types, W. M. Maier and D. H. Olson, Butterworths. Typically, medium pore zeolites are defined as those having a pore size of about 5 to about 7 Angstroms, such that the zeolite freely sorbs molecules such as n-hexane, 3-methylpentane, benzene and p-xylene. Another common classification used for medium pore zeolites involves the Constraint Index test which is described in U.S. Pat. No. 4,016,218, which is hereby incorporated by reference. Medium pore zeolites typically have a Constraint Index of about 1 to about 12, based on the zeolite alone without modifiers and prior to treatment to adjust the diffusivity of the catalyst. Preferred medium pore zeolites for use herein are selected from the group consisting of ZSM-23 and ZSM-48 with ZSM-48 being the most preferred.

The medium pore zeolite used as the first catalyst may be combined with a suitable porous binder or matrix material. Non-limiting examples of such materials include active and inactive materials such as clays, silica, and/or metal oxides such as alumina. Non-limiting examples of naturally occurring clays that can be composited include clays from the montmorillonite and kaolin families including the subbentonites, and the kaolins commonly known as Dixie, McNamee, Georgia, and Florida clays. Others in which the main mineral constituent is halloysite, kaolinite, dickite, nacrite, or anauxite may also be used. The clays can be used in the raw state as originally mixed or subjected to calcination, acid treatment, or chemical modification prior to being combined with the medium pore zeolite.

It is preferred that the porous matrix or binder material comprises silica, alumina, or a kaolin clay. It is more preferred that the binder material comprise alumina. In this embodiment the alumina is present in a ratio of less than about 15 parts zeolite to one part binder, preferably less than about 10, more preferably less than about 5, and most preferably about 2.

The first reaction zone can be comprised of one or more fixed bed reactors or reaction zones each of which can comprise one or more catalyst beds of the same first catalyst. Although other types of catalyst beds can be used, fixed beds are preferred. Such other types of catalyst beds include fluidized beds, ebullating beds, slurry beds, and moving beds. Interstage cooling or heating between reactors, or between catalyst beds in the same reactor, can be employed since some olefin saturation can take place, and olefin saturation and the desulfurization reaction are generally exothermic. A portion of the heat generated during hydrotreating can be recovered. Where this heat recovery option is not available, conventional cooling may be performed through cooling utilities such as cooling water or air, or through use of a hydrogen quench stream. In this manner, optimum reaction temperatures can be more easily maintained.

As stated above, the above-defined first catalyst is placed in a first reaction zone that is operated under effective isomerization conditions. By effective isomerization conditions, it is meant those conditions that provide for the skeletal isomerization of at least about 20 wt. % of the n-olefins present in the feedstream to iso-olefins, preferably at least about 40 wt. %, more preferably at least about 50 wt. %. By skeletal isomerization, it is meant the reorientation of the molecular structure of the normal olefins (n-olefins) with a preference for branched chain iso-olefins over straight. Thus, skeletal isomerization, as used herein, refers to the conversion of a normal olefin to a branched olefin or to the rearranging or moving of branch carbon groups, which are attached to the straight chain olefin molecule, to a different carbon atom, and non-skeletal isomerization can be described as the rearranging of the position of the double bond within the straight chain or branched olefin molecule. These conditions typically include temperatures ranging from about 150° C. to about 425° C., preferably about 200° C. to about 370° C., more preferably about 230° C. to about 350° C. Typical weight hourly space velocities (“WHSV”) range from about 0.1 to about 20 hr−1, preferably from about 0.5 to about 5 hr−1. Any effective pressure can be utilized, and pressures typically range from about 4 to about 70 atmospheres, preferably from about 10 to about 40 atmospheres.

The contacting of the naphtha boiling-range feedstream with the first catalyst under effective isomerization conditions produces a first reaction zone effluent. At least a portion of the first reaction zone effluent, preferably substantially all of the first reaction zone effluent, is then passed to a second reaction zone wherein the first reaction zone effluent is contacted in a second reaction zone with a hydrotreating catalyst in the presence of a hydrogen-containing treat gas under effective hydrotreating conditions. The second reaction zone can also be comprised of one or more fixed bed reactors or reaction zones each of which can comprise one or more catalyst beds of the same catalyst. Although other types of catalyst beds can be used, non-limiting examples of suitable bed types include fluidized beds, ebullating beds, slurry beds, and moving beds. Preferred are fixed catalyst beds and it is more preferred that the first reaction zone and the second reaction zone be in the same reaction vessel. Interstage cooling or heating between reactors or reaction zones, or between catalyst beds in the same reactor, can be employed since some olefin saturation can take place, and olefin saturation and the desulfurization reaction are generally exothermic. A portion of the heat generated during hydrodesulfurization can be recovered. Where this heat recovery option is not available, conventional cooling may be performed through cooling utilities such as cooling water or air, or through use of a hydrogen quench stream. In this manner, optimum reaction temperatures can be more easily maintained.

Suitable second reaction zone catalysts are those that are comprised of at least one Group VIII metal oxide, preferably an oxide of a metal selected from Fe, Co and Ni, more preferably Co and/or Ni, and most preferably Co; and at least one Group VI metal oxide, preferably an oxide of a metal selected from Mo and W, more preferably Mo, on a high surface area support material, preferably alumina. Other suitable second reaction zone catalysts include zeolitic catalysts, as well as noble metal catalysts where the noble metal is selected from Pd and Pt. It is within the scope of the present invention that more than one type of catalyst be used in the same reaction vessel. The at least one Group VIII metal oxide of the second reaction zone catalysts is typically present in an amount ranging from about 2 to about 20 wt. %, preferably from about 4 to about 12%. The at least one Group VI metal oxide will typically be present in an amount ranging from about 1 to about 50 wt. %, preferably from about 1 to about 10 wt. %, and more preferably from about 1 to about 5 wt. %. All metal oxide weight percents are on support. By “on support” we mean that the percents are based on the weight of the support. For example, if the support were to weigh 100 g. then 20 wt. % Group VIII metal oxide would mean that 20 g. of Group VIII metal oxide was on the support.

Preferred catalysts of the second reaction zone will also have a high degree of metal sulfide edge plane area as measured by the Oxygen Chemisorption Test described in “Structure and Properties of Molybdenum Sulfide: Correlation of O2 Chemisorption with Hydrodesulfurization Activity,” S. J. Tauster et al., Journal of Catalysis 63, pp 515-519 (1980), which is incorporated herein by reference. The Oxygen Chemisorption Test involves edge-plane area measurements made wherein pulses of oxygen are added to a carrier gas stream and thus rapidly traverse the catalyst bed. For example, the oxygen chemisorption will be from about 800 to 2,800, preferably from about 1,000 to 2,200, and more preferably from about 1,200 to 2,000 μmol oxygen/gram MoO3.

The most preferred catalysts for the second reaction zone can be characterized by the properties: (a) a MoO3 concentration of about 1 to 25 wt. %, preferably about 2 to 10 wt. %, and more preferably about 3 to 6 wt. %, based on the total weight of the catalyst; (b) a CoO concentration of about 0.1 to 6 wt. %, preferably about 0.5 to 5 wt. %, and more preferably about 1 to 3 wt. %, also based on the total weight of the catalyst; (c) a Co/Mo atomic ratio of about 0.1 to about 1.0, preferably from about 0.20 to about 0.80, more preferably from about 0.25 to about 0.72; (d) a median pore diameter of about 60 Å to about 200 Å, preferably from about 75 Å to about 175 Å, and more preferably from about 80 Å to about 150 Å; (e) a MoO3 surface concentration of about 0.5×10−4 to about 3×10−4 g. MoO3/m2, preferably about 0.75×10−4 to about 2.5×10−4, more preferably from about 1×10−4 to 2×10−4; and (f) an average particle size diameter of less than 2.0 mm, preferably less than about 1.6 mm, more preferably less than about 1.4 mm, and most preferably as small as practical for a commercial hydrodesulfurization process unit.

The catalysts used in the second reaction zone of the present invention are preferably supported catalysts. Any suitable refractory catalyst support material, preferably inorganic oxide support materials may be used as supports for the catalyst of the present invention. Non-limiting examples of suitable support materials include: zeolites, alumina, silica, titania, calcium oxide, strontium oxide, barium oxide, carbons, zirconia, diatomaceous earth, lanthanide oxides including cerium oxide, lanthanum oxide, neodymium oxide, yttrium oxide, and praseodymium oxide; chromia, thorium oxide, urania, niobia, tantala, tin oxide, zinc oxide, and aluminum phosphate. Preferred are alumina, silica, and silica-alumina. More preferred is alumina. Magnesia can also be used for the second reaction zone catalysts. It is to be understood that the support material can also contain small amounts of contaminants, such as Fe, sulfates, silica, and various metal oxides that can be introduced during the preparation of the support material. These contaminants are present in the raw materials used to prepare the support and will preferably be present in amounts less than about 1 wt. %, based on the total weight of the support. It is more preferred that the support material be substantially free of such contaminants. It is an embodiment of the present invention that about 0 to 5 wt. %, preferably from about 0.5 to 4 wt. %, and more preferably from about 1 to 3 wt. %, of an additive be present in the support, which additive is selected from the group consisting of phosphorus and metals or metal oxides from Group IA (alkali metals) of the Periodic Table of the Elements.

As previously stated, the first reaction zone effluent is contacted with the above-defined second catalyst in a second reaction zone under effective hydrotreating conditions to produce a desulfurized product. By effective hydrotreating conditions, it is meant those conditions chosen that will achieve a resulting desulfurized naphtha product having less than 100 wppm sulfur, preferably less than 50 wppm sulfur, more preferably less than 30 wppm sulfur. These conditions typically include temperatures ranging from about 150° C. to about 425° C., preferably about 200° C. to about 370° C., more preferably about 230° C. to about 350° C. Typical weight hourly space velocities (“WHSV”) range from about 0.1 to about 20 hr−1, preferably from about 0.5 to about 5 hr−1. Any effective pressure can be utilized, and pressures typically range from about 4 to about 70 atmospheres, preferably 10 to 40 atmospheres. It should be noted that although the range of operating conditions for the second reaction zone is similar to that for the first reaction zone, both reaction zones could operate under different conditions. In a most preferred embodiment, the effective hydrotreating conditions are selective hydrotreating conditions configured to achieve a sulfur level within the above-defined sulfur ranges, most preferably the desulfurized naphtha product has a sulfur level sufficiently low to meet current regulatory standards in place at that time. By selective hydrotreating conditions, it is meant conditions such as those contained in U.S. Pat. Nos. 5,985,136; 6,013,598; and 6,126,814, all of which have already been incorporated by reference herein, which disclose various aspects of SCANfining, a process developed by the ExxonMobil Research and Engineering Company in which olefinic naphthas are selectively desulfurized with little loss in octane.

As previously stated, the desulfurized product thus obtained will have a higher iso-paraffin to n-paraffin ratio, and thus a higher octane than a desulfurized naphtha treated by a selective or non-selective hydrotreating process. Typical iso-paraffin to n-paraffin ratios in the desulfurized product resulting from the present process are typically greater than about 1, preferably about 2, more preferably about 3. Thus, compared to selective hydrodesulfurizafion processes, the presently claimed process produces desulfurized naphtha products with a higher octane at constant olefin saturation even when both processes maintain similar desulfurization/olefin saturation selectivity.

The above description is directed to several embodiments of the present invention. Those skilled in the art will recognize that other embodiments that are equally effective could be devised for carrying out the spirit of this invention.

The following example will illustrate the improved effectiveness of the present invention, but is not meant to limit the present invention in any fashion.

EXAMPLE

An FCC naphtha was treated with acidic materials (Amberlyst-15 and alumina) to remove nitrogen-containing compounds. The naphtha feed having a reduced amount of nitrogen compounds was used in the present example, and its properties are outlined in Table 1 below.

TABLE 1
API Gravity56
Sulfur606 wppm
Nitrogen 1 wppm
Bromine Number72
Research Octane Number92.1
N-Paraffins 3.22 wt. %
I-Paraffins23.22 wt. %
Naphthenes 8.38 wt. %
Aromatics29.69 wt %
N-Olefins11.95 wt. %
I-Olefins17.35 wt. %
Other Olefins 6.20 wt. %
DistillationASTM D2287
10% 42° C.
30% 79° C.
50%109° C.
70%138° C.
90%174° C.

The feed described in Table 1 above was then subjected to two side-by-side experiments to demonstrate the concept of olefin isomerization/desulfurization to preserve octane of the desulfurized naphtha product. These experiments were conducted in identical down-flow, fixed-bed pilot units that share a common sand bath for control of reactor temperature.

In these experiments, two units, Unit A and Unit B were used to evaluate the effectiveness of the present invention. Unit A utilized a stacked bed configuration and Unit B used a single bed. The catalyst loadings of Unit A were 2.5 cc of ZSM-48 as the first catalyst in the first reaction zone and 2.5 cc of a catalyst comprising 4.3 wt. % MoO3, 1.2 wt. % CoO, on alumina with a median pore diameter of 95 Å was used as the second catalyst in the second reaction zone. Unit B utilized 2.5 cc of a catalyst comprising 4.3 wt. % MoO3, 1.2 wt. % CoO, on alumina with a median pore diameter of 95 Å only.

The feed was contacted with the catalyst(s) system contained in both Unit A and Unit B under the same conditions. These conditions included a flow rate of 10 cc/hr, a hydrogen treat gas rate of 59.4 cc/min of substantially pure hydrogen, and a total system pressure of 1.84 MPa. The reactor temperature (sand bath) was varied from 250° C. to 290° C. The results of the two experiments were then evaluated and are contained in FIGS. 1, 2, and 3 below. Based on the results contained in FIGS. 1, 2 and 3, the catalyst system of the instant invention saves octane because the products resulting from treating a naphtha boiling range feed stream with the present process unexpectedly have more branched olefins and paraffins.

FIG. 1 shows that at constant desulfurization, the stacked bed system of Unit A produced a product having a higher research octane number than the catalyst system of Unit B.

FIG. 2 shows that at constant olefin saturation, the stacked bed catalyst system of Unit A gave a higher iso-olefin to n-olefin ratio in the first reaction zone effluent than the catalyst system of Unit B. The olefin saturation is expressed as a reduction of bromine number (HDBr), which is directly related to the olefin content. The reduction in bromine number was measured according to ASTM 1159

FIG. 3 shows that at constant olefin saturation, the stacked bed catalyst system of Unit A produced a product having a higher iso-paraffin to n-paraffin ratio that the catalyst system of Unit B.