TWO-STAGE HYDRODESULFURIZATION OF CRACKED NAPHTHA STREAMS WITH LIGHT NAPHTHA BYPASS OR REMOVAL

FHSLD OF THE INVENTION

[0001] The present invention relates to a multi-stage process for the selective hydrodesulfurization of an olefinic naphtha stream containing a substantial amount of organically-bound sulfur and olefins.

BACKGROUND OF THE INVENTION

[0002] Environmentally-driven, regulatory pressure concerning motor gasoline sulfur levels will result in the widespread production of less than 50 wppm sulfur mogas by the year 2004, and levels below 10 wppm are being considered for later years. In general, this will require deep desulfurization of cat naphthas. That is, naphthas resulting from cracking operations, particularly those from a fluid catalytic cracking unit. Cat naphthas typically contain substantial amounts of both sulfur and olefins. Deep desulfurization of cat naphtha requires improved technology to reduce sulfur levels without the severe loss of octane that accompanies the undesirable hydrogenation of olefins.

[0003] Hydrodesulfurization is one of the fundamental hydrotreating processes of refining and petrochemical industries. The removal of organically-bound sulfur in the feed by conversion to hydrogen sulfide is typically achieved by reaction with hydrogen over non-noble metal sulfided supported and unsupported catalysts, especially those containing Co/Mo or Ni/Mo. This is usually achieved at fairly severe temperatures and pressures in order to meet product quality specifications, or to supply a desulfurized stream to a subsequent sulfur-sensitive process.

[0004] Olefinic naphthas, such as cracked naphthas and coker naphthas, typically contain more than 20 wt.% olefins. Conventional fresh hydrodesulfurization catalysts have both hydrogenation and desulfurization activity. Hydrodesulfurization of cracked naphthas using conventional naphtha desulfurization catalysts under conventional startup procedures and under conventional conditions required for sulfur removal, typically leads to an undesirable loss of olefins through hydrogenation. Since olefins are high octane components, for some motor fuel use, it is desirable to retain the olefins rather than to hydrogenate them to saturated compounds that are typically lower in octane. This results in a lower grade fuel product that needs additional refining, such as isomerization, blending, etc., to produce higher octane fuels. Such additional refining, or course, adds significantly to production costs.

[0005] Selective hydrodesulfurization to remove organically-bound sulfur, while minimizing hydrogenation of olefins and octane reduction by various techniques, such as selective catalysts and/or process conditions, has been described in the art. For example, a process referred to as SCANfining has been developed by Exxon Mobil Corporation in which olefinic naphthas are selectively desulfurized with little loss in octane. U.S. Patent Nos. 5,985,136; 6,013,598; and 6,126,814; all of which are incorporated herein by reference, 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 H ₂ S that reacts with retained olefins to form mercaptan sulfur by reversion.

[0006] Many refiners are considering combinations of available sulfur removal technologies in order to optimize economic objectives. As refiners have sought to

minimize capital investment to meet low sulfur mogas objectives, technology providers have devised various strategies that include distillation of the cracked naphtha into various fractions that are best suited to individual sulfur removal technologies. While economics of such strategies may appear favorable compared to a single processing technology, the complexity of overall refinery operations is increased and successful mogas production is dependent upon numerous critical sulfur removal operations. Economically competitive sulfur removal strategies that minimize olefin saturation and capital investment and operational complexity will be favored by refiners.

[0007] Consequently, there is a need in the art for technology that will reduce the cost of hydrotreating both cracked naphthas, such as cat cracked naphthas and coker naphthas. There is also a need for more economical hydrotreating processes that minimize both olefin saturation and mercaptan reversion.

SUMMARY OF THE INVENTION

[0008] In accordance with the present invention, there is provided a process for hydrodesulfurizing olefinic naphtha feedstreams and retaining a substantial amount of the olefins, which feedstream boils in the range of 5O ⁰ F (10°C) to 45O ⁰ F (232 ⁰ C) and contains organically-bound sulfur and an olefin content of at least 5 wt.%, which process comprises:

a) hydrodesulfurizing the olefinic naphtha feedstream in a first hydrodesulfurization stage in the presence of hydrogen and a hydrodesulfurization catalyst, at hydrodesulfurization reaction conditions including temperatures from 232°C (450 ⁰ F) to 427°C (800 ⁰ F), pressures of 60 to 800 psig (515 to 5,617 kPa), and hydrogen treat gas rates of 1000 to 6000 standard cubic feet per barrel (178 to

1,068 m ³ /m ³ ), to convert at least 50 wt.%, but not all, of the organically-bound sulfur to hydrogen sulfide and to produce a sulfur-containing first product stream;

b) conducting said sulfur-containing first product stream to a first separation zone operated at a temperature from 200 ⁰ F (93 ⁰ C) to 350°F (177°C) where it is contacted with a countercurrent flow of hydrogen treat gas to produce a first lower boiling naphtha product stream and a first higher boiling naphtha product stream, wherein the higher boiling product stream contains greater than 50 wt.% of the sulfur from the first product stream;

c) conducting said first lower boiling naphtha product stream to a second separation zone operated at a temperature at least 10°C (5O ⁰ F) lower than that of said first separation stage wherein a second lower boiling naphtha product stream and a second higher boiling product stream are produced, which second higher boiling product stream contains substantially all of the sulfur from said first lower boiling naphtha product stream;

d) conducting said second lower boiling product stream from said second separation stage to a third separation stage which is maintained at a temperature at least 3O ⁰ F (-1 ⁰ C) lower than that of said second separation stage thereby resulting in a hydrogen containing vapor recycle stream and a desulfurized naphtha product stream;

e) conducting said first higher boiling naphtha product stream from said first separation zone and at least a portion of said second higher boiling naphtha stream from said second separation zone to a second hydrodesulfurization stage in the presence of hydrogen treat gas and a hydrodesulfurization catalyst, at hydrodesulfurization reaction conditions including temperatures from 232 ⁰ C

(450 ⁰ F) to 427°C (800°F), pressures of 60 to 800 psig (515 to 5,617 WPa), and hydrogen treat gas rates of 1000 to 6000 standard cubic feet per barrel (178 to 1,068 m ³ /m ³ ), to convert at least a portion of any remaining organically-bound sulfur to hydrogen sulfide;

f) recycling at least a portion of the hydrogen-containing vapor recycle stream from said third separation zone to said first hydrogenation stage;

g) stripping substantially all remaining hydrogen from said desulfurized naphtha product stream from said third separation zone; and

h) collecting said stripped higher boiling naphtha product stream.

[0009] In a preferred embodiment, at least a portion of said higher boiling naphtha product stream from said second separation zone is conducted to said first separation zone and flows downward countercurrent to an upflowing hydrogen stream.

[0010] In another preferred embodiment, at least a portion of said hydrogen- containing vapor from said third separation zone is conducted to said first separation zone where it flows countercurrent to downflowing naphtha.

[0011] In still another preferred embodiment of the present invention, the hydrodesulfurization catalyst for either the first, second, or both hydrodesulfurization zones is comprised of a Mo catalytic component, a Co catalytic component and a support component, with the Mo component being present in an amount of from 1 to 25 wt.% calculated as MoO ₃ and the Co component being present in an amount of from 0.1 to 5 wt.% calculated as CoO, with a Co/Mo atomic ratio of 0.1 to 1.

BRIEF DESCRIPTION OF THE DRAWING

[0012] The Figure hereof is a representation of one preferred process scheme for practicing the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0013] Feedstocks suitable for use in the present invention are olefϊnic naphtha boiling range refinery streams that typically boil in the range of 1O ⁰ C (5O ⁰ F) to 232°C (45O ⁰ F). The term "olefϊnic naphtha stream" as used herein are those naphtha streams having an olefin content of at least 5 wt.%. Non-limiting examples of olefinic naphtha streams include fluid catalytic cracking unit naphtha (FCC catalytic naphtha or cat naphtha), steam cracked naphtha, and coker naphtha. Also included are blends of olefϊnic naphthas with non-olefinic naphthas as long as the blend has an olefin content of at least 5 wt.%.

[0014] Olefinic 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 olefinic naphtha feedstock can contain an overall olefins concentration ranging as high as 60 wt.%, more typically as high as 50 wt.%, and most typically from 5 wt.% to 40 wt.%. The olefinic naphtha feedstock can also have a diene concentration up to 15 wt.%, but more typically less than 5 wt.% based on the total weight of the feedstock. High diene concentrations are undesirable since they can result in a gasoline product having poor stability and color. The sulfur content of the olefinic naphtha will generally range from 300 wppm to 7000 wppm, more typically from 1000 wppm to 6000 wppm, and most typically from 1500 to 5000 wppm. The sulfur will typically 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 and its higher homologs and analogs. Nitrogen will also be present and will usually range from 5 wppm to 500 wppm.

[0015] As previously mentioned, it is highly desirable to remove sulfur from olefinic naphthas with as little olefin saturation as possible. It is also highly desirable to convert as much as the organic sulfur species of the naphtha to hydrogen sulfide with as little mercaptan reversion as possible. The level of mercaptans in the product stream has been found to be directly proportional to the concentration of both hydrogen sulfide and olefinic species at the reactor outlet, and inversely related to the temperature at the reactor outlet.

[0016] The sole figure hereof is a simple flow scheme of a best mode for practicing the present invention. Various ancillary equipment, such as compressors, pumps, and valves are not shown for simplicity reasons. An olefinic naphtha feed is conducted via line 10 to first hydrodesulfurization zone 1 that is preferably operated in selective hydrodesulfurization conditions that will vary as a function of the concentration and types of organically-bound sulfur species of the feedstream. By "selective hydrodesulfurization" we mean that the hydrodesulfurization zone is operated in a manner to achieve as high a level of sulfur removal as possible with as low a level of olefin saturation as possible. It is also operated to avoid as much mercaptan reversion as possible. Generally, hydrodesulfurization conditions, for both the first and second hydrodesulfurization zones, as well as any subsequent hydrodesulfurization zone include: temperatures from 232 ⁰ C (450°F) to 427°C (800 ⁰ F), preferably from 26O ⁰ C (500 ⁰ F) to 355°C

(671 ⁰ F); pressures from 60 to 800 psig (515 to 5,617 kPa), preferably from 200 to 500 psig (1,480 kPa to 3,549 kPa); hydrogen feed rates of 1000 to 6000 standard cubic feet per barrel (scf/b) (178 to 1,068 m ³ /m ³ ), preferably from 1000 to 3000 scf/b (178 to 534 m ³ /m ³ ); and liquid hourly space velocities of 0.5 hr ^"1 to 15 hr ^"1 , preferably from 0.5 hr ^"1 to 10 hr ^"1 , more preferably from 1 hr ^"1 to 5 hr ^"1 . The terms "hydrotreating" and "hydrodesulfurization" are sometimes used interchangeably herein.

[0017] This first hydrodesulfurization reaction zone can be comprised of one or more fixed bed reactors each of which can comprise one or more catalyst beds of the same, or different, hydrodesulfurization catalyst. Although other types of catalyst beds can be used, fixed beds are preferred. Non-limiting examples of such other types of catalyst beds that may be used in the practice of the present invention include fluidized beds, ebullating beds, slurry beds, and moving beds. Interstage cooling between reactors, or between catalyst beds in the same reactor, can be employed since some olefin saturation can take place, and olefin saturation as well as the desulfurization reaction are generally exothermic. A portion of the heat generated during hydrodesulfurization can be recovered by conventional techniques. Where this heat recovery option is not available, conventional cooling may be performed through cooling utilities such as cooling water or air, or by use of a hydrogen quench stream. In this manner, optimum reaction temperatures can be more easily maintained. It is preferred that the first hydrodesulfurization stage be configured in a manner and operated under hydrodesulfurization conditions such that from 20% to 75%, more preferably from 20% to 60% of the total targeted sulfur removal is reached in the first hydrodesulfurization stage.

[0018] Hydrotreating catalysts suitable for use in both the first and second ^• hydrodesulfurization zones 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 selected from 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 hydrotreating 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 hydrotreating catalyst be used in the same reaction vessel. The Group VIII metal oxide of the first hydrodesulfurization catalyst is typically present in an amount ranging from 2 to 20 wt. %, preferably from 4 to 12 wt.%. The Group VI metal oxide will typically be present in an amount ranging from 5 to 50 wt.%, preferably from 10 to 40 wt.%, and more preferably from 20 to 30 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 grams , then 20 wt.% Group VIII metal oxide would mean that 20 grams of Group VIII metal oxide is on the support.

[0019] Preferred catalysts for both the first and second hydrodesulfurization stage will also have a high degree of metal sulfide edge plane area as measured by the Oxygen Chemisorption Test as described in "Structure and Properties of Molybdenum Sulfide: Correlation of O ₂ 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 800 to 2,800, preferably from 1,000 to 2,200, and more preferably from 1,200 to 2,000 μmol oxygen/gram MoO ₃ .

[0020] The most preferred catalysts for the second hydrodesulfurization zone can be characterized by the properties: (a) a MoO ₃ concentration of 1 to 25 wt.%, preferably 2 to 18 wt.%, and more preferably 4 to 10 wt.%, and most preferably 4 to 8 wt.%, based on the total weight of the catalyst; (b) a CoO concentration of 0.1 to 6 wt.%, preferably 0.5 to 5.5 wt.%, and more preferably 1 to 5 wt.%, also based on the total weight of the catalyst; (c) a Co/Mo atomic ratio of 0.1 to 1.0, preferably from 0.20 to 0.80, more preferably from 0.25 to 0.72; (d) a median pore diameter of 60 A to 200 A, preferably from 75 A to 175 A, and more preferably from 80 A to 150 A; (e) a MoO ₃ surface concentration of 0.5 x 10 ^"4 to 3 x 10 ^'4 grams MoO ₃ /m ² , preferably 0.75 x 10 ^"4 to 2.5 x 10 ^"4 grams MoO ₃ /m ² , more preferably from 1 x 10 ^"4 to 2 x 10 ^"4 grams MoO ₃ /m ² ; and (f) an average particle size diameter of less than 2.0 mm, preferably less than 1.6 mm, more preferably less than 1.4 mm, and most preferably as small as practical for a commercial hydrodesulfurization process unit.

[0021] The catalysts used in the practice of the present invention are preferably supported catalysts. Any suitable refractory catalyst support material, preferably inorganic oxide support materials, can 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, neodynium oxide, yttrium oxide, and praesodymium 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 catalysts with a high degree of metal

sulfϊde edge plane area of the present invention. 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 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 0 to 5 wt.%, preferably from 0.5 to 4 wt.%, and more preferably from 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.

[0022] Returning now to the figure hereof, the total effluent product from first hydrodesulfurization stage 1 is passed via line 12 to first separation zone 2, which is maintained at a temperature from 93 ⁰ C (200 ⁰ F) to 177 ⁰ C (350 ⁰ F), to produce a first lower boiling naphtha product stream and a first higher boiling naphtha product stream. The first lower boiling naphtha product stream exits first separation zone 2 via line 14 and is conducted to second separation zone 3, which is maintained at a temperature at least 15 ⁰ C (59°F), preferably at least 20 ⁰ C (68°F), and more preferably at least 25 ⁰ C (77 ⁰ F) cooler than first separation zone 2.

[0023] Hydrogen treat gas enters first separation zone 2 via line 16 and flows upward and countercurrent to downflowing higher boiling naphtha product stream that exits first separation zone 2 via line 18 and is passed to second hydrodesulfurization zone 4. The upflowing hydrogen treat gas stream strips out dissolved H ₂ S from the hot liquid higher boiling naphtha product stream that is passed to second hydrodesulfurization stage 4. It is preferred that the bottom

section of the first separation zone 2 contain a first gas-liquid contacting zone 8 comprised of suitable trays or other conventional gas-liquid contacting media to aid in the stripping of dissolved H ₂ S from the exiting naphtha.

[0024] A higher boiling naphtha product stream exits second separation zone 3 via line 20 wherein at least of portion thereof is passed to second hydrodesulfurization zone 4. A portion of the higher boiling naphtha product stream from second separation zone 3 can optionally also be passed to first separation zone 2 via line 22 to flow countercurrent to up-flowing hydrogen- containing vapor. Use of this portion of higher boiling naphtha from the second separation zone acts as a reflux and results in the reduction of the amount of high- boiling naphtha in the overhead vapor for a given yield of separated lower boiling naphtha. It is preferred that the first separation zone 2 contain a second gas-liquid contacting zone 9 comprised of suitable trays located vertically above the point of introduction of the effluent from the first hydrodesulfurization stage via line 12, and vertically below the point of introduction of the higher boiling naphtha from the second separation zone via line 22. This also allows for an increase in the yield of separated lower boiling naphtha for a given lower boiling naphtha sulfur content. The more naphtha that bypasses the second hydrodesulfurization zone, the greater the benefit of interstage, or interzone, separation.

[0025] A second lower boiling naphtha product stream exits second separation zone 3 via line 24 and is conducted to third separation zone 5 that is maintained at a temperature of at least 15 ⁰ C (59 ⁰ F), preferably at 2O ⁰ C (68 ⁰ F), and more preferably at least 25°C (77°F) cooler than that of second separation zone 3. A hydrogen containing vapor stream exits third separation zone 5 via line 26 and is passed to scrubbing zone 6 where it is contacted with a basic solution, preferably an amine-

containing solution to remove H ₂ S before recycle via line 28 to first hydrodesulfurization stage 1. A portion of recycle hydrogen can be passed via line 30 to line 16 to flow countercurrent in first separation zone 2. A portion of recycle hydrogen can also be passed, via line 38 to the second hydrodesulfurization zone. The naphtha product effluent stream from second hydrodesulfurization zone 4 is conducted to third separation zone 5 via line 27. A third higher boiling naphtha product stream from third separation zone 5 is passed via line 32 to stripping zone 7 wherein substantially all of any remaining H ₂ S is stripped from the stream and collected via line 34. The stripped naphtha product stream is then collected via line 36.

[0026] In a preferred embodiment, the effluent from second hydrodesulfurization stage is cooled to approximately the temperature of the third separation zone and passed into the third separation zone for concurrent recovery of the desulfurized naphthas from the first and second hydrodesulfurization zones. Hydrogen containing vapor from both hydrodesulfurization stages is likewise concurrently separated from the desulfurized naphthas and passed to amine scrubbing followed by recycle of at least a portion of the gas to either or both hydrodesulfurization stages.