Catalytic reforming process with sulfur preclusion

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and more specifically for the catalytic reforming of gasoline-range hydrocarbons.
2. General Background
The catalytic reforming of hydrocarbon feedstocks in the gasoline range is an important commercial process, practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petrochemical industry or gasoline components with high resistance to engine knock. Demand for aromatics is growing more rapidly than the supply of feedstocks for aromatics production. Moreover, the widespread removal of lead antiknock additive from gasoline and the rising demands of high-performance internal-combustion engines are increasing the required knock resistance of the gasoline component as measured by gasoline "octane" number. The catalytic reforming unit therefore must operate more efficiently at higher severity in order to meet these increasing aromatics and gasoline-octane needs. This trend creates a need for more effective reforming processes and catalysts.
Catalytic reforming generally is applied to a feedstock rich in paraffinic and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased aromatics and gasoline-octane needs have turned attention to the paraffin-dehydrocyclization reaction, which is less favored thermodynamically and kinetically in conventional reforming than other aromatizations reactions. Considerable leverage exists for increasing desired product yields from catalytic reforming by promoting the dehydrocyclization reaction over the competing hydrocracking reaction while minimizing the formation of coke.
The effectiveness of reforming catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins is well known in the art. The use of these reforming catalysts to produce aromatics from paraffinic raffinates as well as naphthas has been disclosed. The increased sensitivity of these selective catalysts to sulfur in the feed also is known. Nevertheless, this dehydrocyclization technology has not been commercialized during the intense and lengthy development period. The extreme catalyst sulfur intolerance of current reforming catalysts selective for dehydrocyclization, providing surprising results when sulfur is precluded from the feed according to the process of the present invention, is only now being recognized.
3. Related Art
U.S. Pat. No. 2,618,586 (Hendel) discloses a process for removing relatively small amounts of sulfur-containing compounds from a petroleum liquid using an adsorbent which could be manganese oxide. U.S. Pat. No. 3,063,936 (Pearce et al.) discloses a desulfurization process combining sulfuric acid treatment, contact with a material which may be manganese oxide and contact with a hydrodesulfurization catalyst. However, neither Hendel nor Pearce et al. suggest the catalytic reforming process of the present invention.
U.S. Pat. No. 3,898,153 (Louder et al.) teaches a catalytic reforming process including chloride removal, hydrodesulfurization, and zinc oxide adsorbent to reduce the sulfur content of the reformer feed to as low as 0.2 ppm. U.S. Pat. No. 4,634,515 (Bailey et al.) discloses a nickel-catalyst sulfur trap downstream of a hydrofiner to reduce sulfur content to preferably below 0.1 ppm before a reforming unit. However, neither Louder et al. nor Bailey et al. contemplate the first reforming zone and manganese component precluding sulfur from the feed to a second reforming zone of the present invention.
U.S. Pat. Nos. 4,225,417 and 4,329,220 (Nelson) teach a reforming process in which sulfur is removed from a reforming feedstock using a manganese-containing composition, particularly directed to sulfur removal in a halogen atmosphere. Preferably, the feed is hydrotreated and the sulfur content is reduced by the invention to below 0.1 ppm. U.S. Pat. Nos. 4,534,943 and 4,575,415 (Novak et al.) teach an apparatus and method, respectively, for removing residual sulfur from reformer feed using parallel absorbers for continuous operation; ideally, sulfur is removed to a level of below 0.1 ppm. Neither Nelson nor Novak et al., however, suggest the two reforming zones and resulting preclusion of feed sulfur to the second reforming zone of the present invention.
U.S. Pat. No. 4,456,527 (Buss et al.) discloses the reforming of a hydrocarbon feed having a sulfur content of as low as 50 ppb (parts per billion) with a catalyst comprising a large-pore zeolite and Group VIII metal. A broad range of sulfur-removal options are disclosed to reduce the sulfur content of the hydrocarbon feed to below 500 ppb. U.S. Pat. No. 4,741,819 (Robinson et al.) is drawn to a method for removing residual sulfur from a hydrotreated naphtha feedstock comprising contacting the feedstock with a less-sulfur sensitive reforming catalyst, a sulfur sorbent, and a highly selective reforming catalyst. Both Buss et al. and Robinson et al. teach that the sensitive reforming catalyst preferably contains an alkaline earth metal, in contrast to the alkali metal of the present catalyst. Further, the broad list of sulfur-removal options and metals of Buss et al and Robinson et al. do not include a manganese sulfur sorbent and preferably lead one to select a zinc oxide sorbent. In summary, neither Buss et al. nor Robinson et al. teach the reforming process combination of the present invention using a manganese component to preclude sulfur as elucidated hereinafter from the feed to a second reforming zone.
U.S. Pat. No. 4,831,206 (Zarchy) discloses a hydrocarbon conversion process comprising sulfur conversion, liquid-phase H.sub.2 S removal with zeolite, and vaporization of the product to the reaction zone. Zarchy requires condensation and vaporization of the hydrocarbon stream, however, and does not teach the use of a manganese component to achieve the substantially sulfur-free effluent of the present invention. Pending application Ser. No. 354,653 (Zarchy) discloses an isomerization process comprising a first conversion zone, a hydrogen sulfide recovery zone and a second conversion zone, but does not suggest the present reforming process.
SUMMARY OF THE INVENTION
1. Objects
It is an object of the present invention to provide a catalytic reforming process combination, effective for the dehydrocyclization of paraffins, with high catalyst stability. A corollary objective is to preclude sulfur from the feed to a second reforming catalyst having unusual sulfur intolerance.
2. Summary
This invention is based on the discovery that a catalytic reforming process combination comprising two reforming zones and an intermediate sulfur-removal zone using a manganese component demonstrates surprising paraffin-dehydrocyclization catalyst stability relative to the prior art.
3. Embodiments
A broad embodiment of the present invention is a catalytic reforming process combination in which a hydrocarbon feedstock contacts successively a first reforming catalyst in a first reforming zone, a sulfur sorbent comprising MnO in a sulfur-removal zone, and a second reforming catalyst containing L-zeolite and a platinum-group metal in a second reforming zone.
In a preferred embodiment, the feed contacting the second reforming catalyst contains no detectable sulfur. In an especially preferred embodiment, the sulfur content of this feed is less than about 14 mass parts per billion.
Optimally the process is effected in the absence of added halogen, and effluents from the first reforming zone, sulfur-removal zone, and second reforming zone are halogen-free.
An alternative embodiment of the present invention comprises a radial-flow reactor which contains both the first reforming catalyst and the sulfur sorbent.
In another aspect, the process includes a precedent hydrodesulfurization zone to remove most of the sulfur from the feedstock before it contacts the first reforming catalyst.
These as well as other objects and embodiments will become apparent from the detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified block flow diagram showing the arrangement of the major sections of the present invention.
FIG. 2 shows a radial-flow reactor containing perforated cylindrical partitions to define two zones containing respectively the first reforming catalyst and the sulfur sorbent.
FIG. 3 is a graph of the temperature requirement to maintain 55% conversion of the feedstock of Example II in a reforming operation, comparing results based on preclusion of feed sulfur according to the present invention with results corresponding to the prior art.
FIG. 4 is a graph of the temperature requirement to maintain 99 Research octane clear product when reforming the feed of Example III, comparing results based on preclusion of feed sulfur according to the present invention will results corresponding to the prior art.
FIG. 5 is a graph of the temperature requirement to maintain 99 Research octane clear product when reforming the feed of Example IV, comparing results based on preclusion of feed sulfur according to the present invention with results corresponding to the prior art.
FIG. 6 shows the relative compatibility of zinc oxide and manganese oxide to the second reforming catalyst in distinguishing the process of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
To reiterate, a broad embodiment of the present invention is directed to a catalytic reforming process combination in which a hydrocarbon feed contacts successively a first reforming catalyst, a sulfur sorbent comprising MnO, and a second reforming catalyst containing L-zeolite and a platinum-group metal.
FIG. 1 is a simplified block flow diagram representing the present invention. Only the major sections and interconnections of the process are represented. Individual equipment items such as reactors, heaters, heat exchangers, separators, fractionators, pumps, compressors and instruments are well known to those skilled in the art; description of this equipment is not necessary for an understanding of the invention or its underlying concepts.
The hydrocarbon feedstock is introduced to the process by line 11, and joins a hydrogen-containing stream from line 12 as combined feed to the first reforming zone 13. This zone contains the first reforming catalyst, described in more detail hereinafter, which converts substantially all of the sulfur in the feed to H.sub.2 S and produces a first effluent via line 14. The temperature of the first effluent may be adjusted before sulfur removal, using heat exchanger 15, with the need for temperature adjustment depending on feedstock sulfur content and hydrocarbon types as discussed hereinafter. The first effluent, after the heat exchanger, passes via line 16 into sulfur-removal zone 17. Sulfur entering this zone as H.sub.2 S is removed from the process by MnO sulfur sorbent, generally in combination with spent sorbent, via line 18. The second effluent in line 19 is substantially sulfur-free. The temperature of the second effluent may be adjusted, using heat exchanger 20, before passing it via line 21 to the second reforming zone 22 in which paraffins are dehydrocyclized to aromatics. Net hydrogen-rich gas is produced and is removed via line 23 either as recycle to the process via line 12 or to other uses. The aromatics-rich effluent is removed as product in line 24.
The hydrocarbon feedstock will comprise paraffins and naphthenes, and may comprise aromatics and small amounts of olefins, boiling within the gasoline range. Feedstocks which may be utilized include straight-run naphthas, natural gasoline, synthetic naphthas, thermal gasoline, catalytically cracked gasoline, partially reformed naphthas or raffinates from extraction of aromatics. The distillation range may be that of a full-rang naphtha, having an initial boiling point typically from 40.degree.-80.degree. C. and a final boiling point of from about 160.degree.-210.degree. C., or it may represent a narrower range within a lower final boiling point. Light paraffinic feedstocks, such as naphthas from Middle East crudes having a final boiling point of from about 100.degree.-160.degree. C., are preferred due to the specific ability of the process to dehydrocyclize paraffins to aromatics. Raffinates from aromatics extraction, containing principally low-value C.sub.6 -C.sub.8 paraffins which can be converted to valuable B-T-X aromatics, are especially preferred feedstocks.
The hydrocarbon feedstock to the present process contains small amounts of sulfur compounds, amounting to generally less than 10 parts per million (ppm) on an elemental basis. Preferably the hydrocarbon feedstock has been prepared from a contaminated feedstock by a conventional pretreating step such as hydrotreating, hydrorefining or hydrodesulfurization to convert such contaminants as sulfurous, nitrogenous and oxygenated compounds to H.sub.2 S, NH.sub.3 and H.sub.2 O, respectively, which can be separated from the hydrocarbons by fractionation. This conversion preferably will employ a catalyst known to the art comprising an inorganic oxide support and metals selected from Groups VIB(6) and VIII(9-10) of the Periodic Table. [See Cotton and Wilkinson, Advanced Organic Chemistry, John Wiley & Sons (Fifth Edition, 1988)]. Alternatively or in addition to the conventional hydrotreating, the pretreating step may comprise contact with sorbents capable of removing sulfurous and other contaminants. These sorbents may include but are not limited to zinc oxide, iron sponge, high-surface-area sodium, high-surface-area alumina, activated carbons and molecular sieves; excellent results are obtained with a nickel-on-alumina sorbent. Preferably, the pretreating step will provide the first reforming catalyst with a hydrocarbon feedstock having low sulfur levels disclosed in the prior art as desirable reforming feedstocks, e.g., 1 ppm to 0.1 ppm (100 ppb). It is within the ambit of the present invention that the pretreating step be included in the present reforming process.
Operating conditions used in the first reforming zone of the present invention include a pressure of from about atmospheric to 60 atmospheres (absolute), with the preferred range being from atmospheric to 20 atmospheres and a pressure of below 10 atmospheres being especially preferred. Free hydrogen is supplied to the first reforming zone in an amount sufficient to correspond to a ratio of from about 0.1 to 10 moles of hydrogen per mole of hydrocarbon feedstock. By "free hydrogen" is meant molecular H.sub.2, not combined in hydrocarbons or other compounds. Preferably, the reaction is carried out in the absence of added halogen. The volume of the contained first reforming catalyst corresponds to a liquid hourly space velocity of from about 1 to 40 hr.sup.-1. The operating temperature generally is in the range of 260.degree. to 560.degree. C. This temperature is selected to convert sulfur compounds in the feedstock to H.sub.2 S in order to preclude sulfur from the second reforming zone. Operating temperature thus relates to feed sulfur content, difficulty of conversion of sulfur compounds, and other operating conditions in the first reforming zone. Hydrocarbon types in the feed stock also influence temperature selection, as naphthenes generally are dehydrogenated over the first reforming catalyst with a concomitant decline in temperature across the catalyst bed due to the endothermic heat of reaction. The temperature generally is slowly increased during each period of operation to compensate for the inevitable catalyst deactivation.
The first reforming catalyst is a dual-function composite containing a metallic hydrogenation-dehydrogenation component on a refractory support which provides acid sites for cracking and isomerization. This catalyst functions principally to convert small amounts of sulfur in the feedstock, preferably about 0.05 to 2 ppm, to H.sub.2 S in order to preclude sulfur from the feed to the second reforming catalyst. The first reforming catalyst would tolerate episodes of up to about 10 ppm of sulfur in the feedstock with substantial recovery of activity. The first reforming catalyst also preferably effects some dehydrogenation of naphthenes in the feedstock as well as, to a lesser degree, isomerization, cracking and dehydrocyclization.
The refractory support of the first reforming catalyst should be a porous, adsorptive, high-surface-area material which is uniform in composition without composition gradients of the species inherent to its composition. Within the scope of the present invention are refractory support containing one or more of: (1) refractory inorganic oxides such as alumina, silica, titania, magnesia, zirconia, chromia, thoria, boria or mixtures thereof; (2) synthetically prepared or naturally occurring clays and silicates, which may be acid-treated; (3) crystalline zeolitic aluminosilicates, either naturally occurring or synthetically prepared such as FAU, MEL, MFI, MOR, MTW (IUPAC Commission on Zeolite Nomenclature), in hydrogen form or in a form which has been exchanged with metal cations; (4) spinels such as MgAl.sub.2 O.sub.4, FeAl.sub.2 O.sub.4, ZnAl.sub.2 O.sub.4, CaAl.sub.2 O.sub.4 ; and (5) combinations of materials from one or more of these groups. The preferred refractory support for the first reforming catalyst is alumina, with gamma- or eta-alumina being particularly preferred. Best results are obtained with "Ziegler alumina," described in U.S. Pat. No. 2,892,858 and presently available from the Vista Chemical Company under the trademark "Catapal" or from Condea Chemie GmbH under the trademark "Pural." Ziegler alumina is an extremely high-purity pseudoboehmite which, after calcination at a high temperature, has been shown to yield a high-priority gamma-alumina. It is especially preferred that the refractory inorganic oxide comprise substantially pure Ziegler alumina having an apparent bulk density of about 0.6 to 1 g/cc and a surface area of about 150 to 280 m.sup.2 /g (especially 185 to 235 m.sup.2 /g) at a pore volume of 0.3 to 0.8 cc/g.
The alumina powder may be formed into any shape or form of carrier material known to those skilled in the art such as spheres, extrudates, rods, pills, pellets, tablets or granules. Spherical particles may be formed by converting the alumina powder into alumina sol by reaction with suitable peptizing acid and water and dropping a mixture of the resulting sol and gelling agent into an oil bath to form spherical particles of an alumina gel, followed by known aging, drying and calcination steps. The preferred extrudate form is preferably prepared by mixing the alumina powder with water and suitable peptizing agents, such as nitric acid, acetic acid, aluminum nitrate and like materials, to form an extrudable dough having a loss on ignition (LOI) at 500.degree. C. of about 45 to 65 mass %. The resulting dough is extruded through a suitably shaped and sized die to form extrudate particles, which are dried and calcined by known methods. Alternatively, spherical particles can be formed from the extrudates by rolling the extrudate particles on a spinning disk.
An essential component of the first reforming catalyst is one or more platinum-group metals, with a platinum component being preferred. The platinum may exist within the catalyst as a compound such as the oxide, sulfide, halide, or oxyhalide, in chemical combination with one or more other ingredients of the catalytic composite, or as an elemental metal. Best results are obtained when substantially all of the platinum exists in the catalytic composite in a reduced state. The platinum component generally comprises from about 0.01 to 2 mass % of the catalytic composite, preferably 0.05 to 1 mass %, calculated on an elemental basis. It is within the scope of the present invention that the catalyst known to modify the effect of the preferred platinum component. Such metal modifiers may include Group IVA (14) metals, other Group VIII (8-10) metals, rhenium, indium, gallium, zinc, uranium, dysprosium, thallium and mixtures thereof. Excellent results are obtained when the first reforming catalyst contains a tin component. Catalytically effective amounts of such metal modifiers may be incorporated into the catalyst by any means known in the art.
The first reforming catalyst may contain a halogen component. The halogen component may be either fluorine, chlorine, bromine or iodine or mixtures thereof. Chlorine is the preferred halogen component, usually added during incorporation of a metal component. The halogen component is generally present in a combined state with the inorganic-oxide support. The halogen component is preferably well dispersed throughout the catalyst and may comprise from more than 0.2 to about 2 mass % calculated on an elemental basis, of the final catalyst but preferably is no more than about 1 mass % in order to avoid loss of halogen to the reactants.
An optional ingredient of the first reforming catalyst is an L-zeolite. It is within the ambit of the present invention that the same catalyst may be used in the first and second reforming zones. Since the sulfur content of the feedstock to the first reforming zone is at levels taught in the prior art while sulfur is substantially precluded from the feed to the second reforming zone, the optional first reforming catalyst containing L-zeolite is significantly less effective for the dehydrocyclization of paraffins than is the second reforming catalyst even if the catalysts have the same composition. In this option, therefore, the first reforming catalyst containing L-zeolite primarily converts small amounts of sulfur in the feedstock to H.sub.2 S and dehydrogenates naphthenes to aromatics as described hereinabove.
The first reforming catalyst generally will be dried at a temperature of from about 100.degree. to 320.degree. C. for about 0.5 to 24 hours, followed by oxidation at a temperature of about 300.degree. to 550.degree. C. in an air atmosphere for 0.5 to 10 hours. Preferably the oxidized catalyst is subjected to a substantially waterfree reduction step at a temperature of about 300.degree. to 550.degree. C. for 0.5 to 10 hours or more. Further details of the preparation and activation of embodiments of the first reforming catalyst are disclosed in U.S. Pat. No. 4,677,094 (Moser et al.), which is incorporated into this specification by reference thereto.
The feed to each of the first reforming zone, sulfur-removal zone and second reforming zone may contact the respective catalyst or sorbent in each of the respective reactors in either upflow, downflow, or radial-flow mode. Since the present reforming process operates at relatively low pressure, the low pressure drop in a radial-flow reactor favors the radial-flow mode.
The catalyst or sorbent is contained in a fixed-bed reactor or in a moving-bed reactor whereby catalyst may be continuously withdrawn and added. These alternatives are associated with catalyst-regeneration options known to those of ordinary skill in the art, such as: (1) a semiregenerative unit containing fixed-bed reactors maintains operating severity by increasing temperature, eventually shutting the unit down for catalyst regeneration and reactivation; (2) a swing-reactor unit, in which individual fixed-bed reactors are serially isolated by manifolding arrangements as the catalyst become deactivated and the catalyst in the isolated reactor is regenerated and reactivated while the other reactors remain on-stream; (3) continuous regeneration of catalyst withdrawn from a moving-bed reactor, with reactivation and substitution of the reactivated catalyst, permitting higher operating severity by maintaining high catalyst activity through regeneration cycles of a few days; or: (4) a hybrid system with semiregenerative and continuous-regeneration provisions in the same unit. The preferred embodiment of the present invention is fixed-bed reactors in a semiregenerative unit.
Preferably about 75% to 95% of the total catalyst and sorbent volume of the process is represented by the second reforming catalyst. Continuous regeneration shows best results when applied to a large volume of catalyst, justifying the capital cost of the regeneration section. An optional embodiment therefore is a hybrid system with continuous regeneration of the second reforming catalyst.
The first reforming catalyst and sulfur sorbent together thus preferably represent only about 5% to 25% of the total catalyst and sorbent volume of the process. In an alternative embodiment, the first reforming zone containing the first reforming catalyst and the sulfur-removal zone containing the sulfur sorbent are contained within the same reactor vessel. Savings are realized in piping, instrumentation and other appurtenances by employing a single reactor instead of two or more reactors to contain the first reforming and sulfur-removal zones. Preferably, the reactants contact the first reforming catalyst and sulfur sorbent consecutively in a downflow manner. It is within the scope of the invention that a vapor, liquid, or mixed-phase stream is injected between the beds of particles to control the inlet temperature of the reactants to the sulfur sorbent.
In an optional embodiment, the sulfur-removal zone and from about 5% to 30% of the second reforming zone are contained within the same reactor vessel. In yet another optional embodiment, the first reforming zone, sulfur-removal zone, and from about 5% to 30% of the second reforming zone are contained within the same reactor vessel.
In an elective embodiment, the first reforming zone and sulfur-removal zone are contained as annular concentric zones within the same vertically oriented reactor vessel. Each zone is defined by two perforated cylindrical partitions coaxially disposed within the reactor vessel. The first reforming catalyst and sulfur sorbent are retained within the respective zones by top and bottom closures disposed at the two ends of the perforated cylindrical partitions. The cylindrical partitions are perforated in a manner to retain the reforming catalyst and sulfur sorbent while permitting transfer of feed, reactants and associated gaseous materials through the partitions; one or more of the perforated cylindrical partitions may comprise a screen. The perforated cylindrical partitions also define an outer annular manifold and central manifold for distributing feed and reactants to and collecting reactants from the respective zones.
FIG. 2 is an elevational view showing a preferred aspect of this embodiment. A vertically oriented reactor vessel 101 contains a group of perforated cylindrical partitions 106, 108 and 110 coaxial with the reactor vessel, with partitions 106 and 108 defining an outer annular zone 107 and partitions 108 and 110 defining an inner annual zone 109. The combined feed enters the reactor vessel through nozzle 102, and passes through a top opening 103 in a top closure 104 into outer manifold space 105. The top closure 104 prevents the combined feed from communicating with the zones 107 and 109 and a central manifold space 111 before it enters the outer manifold space 105. The combined feed passes from the outer manifold space 105 in an inward radial direction through partition 106 into the outer annular zone 107, which serves as a first reforming zone to produce a first effluent containing hydrogen sulfide. The first effluent passes through partition 108 into inner annular zone 109, which serves as a sulfur-removal zone to remove hydrogen sulfide from the reactants and produce a substantially sulfur-free second effluent. The second effluent passes through partition 110 into central manifold space 111 and through an opening 112 in a bottom closure 113 to exit the reactor vessel 101 through a nozzle 114. The bottom closure 113 prevents communication of the second effluent with outer manifold space 105 and zones 107 and 109.
In an alternative embodiment, the combined feed passes into the central manifold space 111 and flows in an outward radial direction through the zone 109 which now serves as a first reforming zone, and zone 107, which now serves as a sulfur-removal zone, to exit the reactor vessel via the outer manifold space 105. In another aspect, the sulfur-removal zone and a portion of the second reforming zone are defined by perforated cylindrical partitions within a single reactor vessel. In yet another alternative embodiment, the first reforming zone, sulfur-removal zone, and a portion of the second reforming zone are defined by perforated cylindrical partitions within a single reactor vessel.
It is within the scope of the present invention that the reactor vessel described hereinabove contains an annular space between beds of particles defined by perforated cylindrical partitions into which intermediate vapor, liquid, or mixed-phase streams may be injected to control reaction temperature or reactant concentration. In another aspect, nozzles for entry of feed, injection of intermediate streams, and exit of effluent may be located in the cylindrical wall of the reactor vessel. In an optional embodiment, conduits communicate with one or more zones in the reactor vessel through the top and bottom closures respectively for introduction of fresh and removal of deactivated reforming catalyst or sulfur sorbent.
The sulfur sorbent comprises a manganese component, preferably a manganese oxide. Manganese oxide has been found to provide reforming catalyst protection superior to the zinc oxide of the prior art, it is believed, due to possible zinc contamination of downstream reforming catalyst. The manganese oxides include MnO, Mn.sub.3 O.sub.4, Mn.sub.2 O.sub.3, MnO.sub.2, MnO.sub.3, and Mn.sub.2 O.sub.7. The preferred manganese oxide is MnO (manganous oxide). The manganese component may be composited with a suitable binder such as clays, graphite, or inorganic oxides including one or more of alumina, silica, zirconia, magnesia, chromia or boria. Preferably, the manganese component is unbound and consists essentially of manganese oxide. Even more preferably the manganese component consists essentially of MnO, which has demonstrated excellent results for sulfur removal and has shown adequate particle strength without a binder for the present invention.
The efficacy of the manganese component for sulfur preclusion contrasts with the application of a zinc component in similar service, referring to Example V. Zinc oxide is the especially preferred metal oxide taught in the prior art for sulfur removal from a feedstock to an L-zeolite catalyst, as discussed hereinabove in connection with the disclosure of U.S. Pat. No. 4,456,527 (Buss et al.) and U.S. Pat. No. 4,741,819 (Robinson et al.). The catalysts of Buss et al. and Robinson et al. preferably contain an alkaline earth metal, in contrast to the alkali-metal component of the present catalyst. However, it is not known whether zinc oxide is incompatible with the present catalyst through interaction with one of the components of the catalyst or with a combination of the components in the process of the present invention.
The manganese component is provided in an amount effective to preclude sulfur from the second reforming catalyst by providing a substantially sulfur-free second effluent from the sulfur sorbent based upon a feedstock to the first reforming zone as defined hereinabove. Sulfur-free is defined as containing less than 20 parts per billion (ppb), and preferably less than 14 ppb, sulfur. In another aspect, sulfur-free is defined as containing no detectable sulfur. The repeatability of the American National Standard test ASTM D 404587 is 20 ppb at a sulfur level of 0.02 ppm (20 ppb), and "sulfur free" according to this test therefore would be defined as less than 20 ppb sulfur. It is believed, however, that one laboratory testing a series of similar samples can detect differences at lower sulfur levels, e.g., 10 .mu.g/ml or 14 ppb sulfur for the feedstocks described in the examples cited hereinafter. Such differences are reported in the examples.
The feedstock and the effluents from or within the first reforming zone, sulfur-removal zone, and second reforming zone of the process optimally are free of halogen. Absence of halogen, more specifically chloride, is consistent with effective performance of the nonacidic-L-zeolite second reforming catalyst. No halogen is added to the process, and catalyst halogen contents preferably are sufficiently low to avoid halogen loss to effluents. "Free of halogen," or "halogen-free" in this context means that chlorine and hydrogen chloride are not detectable in effluents using standard Drager tubes.
Operating conditions employed in the sulfur-removal zone containing the sulfur sorbent to preclude sulfur from the second reforming zone include a pressure of from about atmospheric to 60 atmospheres (abs), with the preferred range being from atmospheric to 20 atmospheres (abs), and a pressure below 10 atmospheres being especially preferred. The hydrogen to hydrocarbon mole ratio is defined by the operation of the first reforming zone hereinabove, and is from about 0.1 to 10 moles of hydrogen per mole of hydrocarbon in the first effluent. Space velocity with respect to the volume of sulfur sorbent is from about 1 to 40 hr.sup.-1. Operating temperature may be controlled to be independent of the first reforming zone, as shown in FIG. 1. However, it is preferred that this temperature be defined by the temperature of the first effluent, and be within the range of from about 260.degree. to 560.degree. C. As the dehydrogenation of naphthenes in the first reforming zone normally will result in a decline in temperature across this zone due to the endothermic heat of reaction, the operating temperature of the sulfur-removal zone usually is lower than that of the first reforming zone. A temperature of from about 310.degree. to 420.degree. C. is especially preferred for the sulfur-removal zone.
The second reforming zone operates at a pressure, consistent with the first reforming and sulfur-removal zones, of from about atmospheric to 60 atmospheres (abs) and preferably from atmospheric to 20 atmospheres (abs). Excellent results have been obtained at operating pressures of less than 10 atmospheres. The free hydrogen to hydrocarbon mole ratio is from about 0.1 to 10 moles of hydrogen per mole of C.sub.5 + second effluent from the sulfur-removal zone. Preferably, the reaction is carried out in the absence of added halogen. Space velocity with respect to the volume of second reforming catalyst is from about 0.2 to 10 hr.sup.-1. Operating temperature is from about 400.degree. to 560.degree. C., and preferably is controlled independently of temperature in the sulfur-removal zone as indicated hereinabove and in FIG. 1.
Since the predominant reaction occurring in the second reforming zone is the dehydrocyclization of paraffins to aromatics, this zone will comprise two or more reactors with interheating between reactors to compensate for the endothermic heat of reaction and maintain dehydrocyclization conditions. The second reforming zone thus will produce an aromatics-rich effluent stream, with the aromatics content of the C.sub.5 + portion of the effluent typically within the range of about 45 to 85 mass %. The composition of the aromatics will depend principally on the feedstock composition and operating conditions, and generally will consist principally of C.sub.6 -C.sub.12 aromatics. Benzene, toluene and C.sub.8 aromatics will be the primary aromatics produced from the preferred light naphtha and raffinate feedstocks.
The second reforming catalyst contains a non-acidic L-zeolite, an alkali-metal component and a platinum-group metal component. It is essential that the L-zeolite be non-acidic, as acidity in the zeolite lowers the selectivity to aromatics of the finished catalyst. In order to be "non-acidic," the zeolite has substantially all of its cationic exchange sites occupied by nonhydrogen species. Preferably the cations occupying the exchangeable cation sites will comprise one or more of the alkali metals, although other cationic species may be present. An especially preferred nonacidic L-zeolite is potassium-form L-zeolite.
It is necessary to composite the L-zeolite with a binder in order to provide a convenient form for use in the catalyst of the present invention. The art teaches that any refractory inorganic oxide binder is suitable. One or more of silica, alumina or magnesia are preferred binder materials of the present invention. Amorphous silica is especially preferred, and excellent results are obtained when using a synthetic white silica powder precipitated as ultra-fine spherical particles from a water solution. The silica binder preferably is nonacidic, contains less than 0.3 mass % sulfate salts, and has a BET surface area of from about 120 to 160 m.sup.2 /g.
The L-zeolite and binder may be composited to form the desired catalyst shape by any method known in the art. For example, potassium-form L-zeolite and amorphous silica may be commingled as a uniform powder blend prior to introduction of a peptizing agent. An aqueous solution comprising sodium hydroxide is added to form an extrudable dough. The dough preferably will have a moisture content of from 30 to 50 mass % in order to form extrudates having acceptable integrity to withstand direct calcination. The resulting dough is extruded through a suitably shaped and sized die to form extrudate particles, which are dried and calcined by known methods. Alternatively, spherical particles may be formed by methods described hereinabove for the first reforming catalyst.
An alkali metal component is an essential constituent of the second reforming catalyst. One or more of the alkali metals, including lithium, sodium, potassium, rubidium, cesium and mixtures thereof, may be used, with potassium being preferred. The alkali metal optimally will occupy essentially all of the cationic exchangeable sites of the non-acidic L-zeolite. Surface-deposited alkali metal also may be present as described in U.S. Pat. No. 4,619,906, incorporated herein by reference thereto.
The platinum-group component is another essential feature of the second reforming catalyst, with a platinum component being preferred. The platinum may exist within the catalyst as a compound such as the oxide, sulfide, halide, or oxyhalide, in chemical combination with one or more other ingredients of the catalytic composite, or as an elemental metal. Best results are obtained when substantially all of the platinum exists in the catalytic composite in a reduced state. The platinum component generally comprises from about 0.05 to 5 mass % of the catalytic composite, preferably 0.05 to 2 mass %, calculated on an elemental basis. It is within the scope of the present invention that the catalyst may contain other metal components known to modify the effect of the preferred platinum component. Such metal modifiers may include Group IVA(14) metals, other Group VIII(8-10) metals, rhenium, indium, gallium, zinc, uranium, dysprosium, thallium and mixtures thereof. Catalytically effective amounts of such metal modifiers may be incorporated into the catalyst by any means known in the art.
The final second reforming catalyst generally will be dried at a temperature of from about 100.degree. to 320.degree. C. for about 0.5 to 24 hours, followed by oxidation at a temperature of about 300.degree. to 550.degree. C. (preferably about 350.degree. C.) in an air atmosphere for 0.5 to 10 hours. Preferably the oxidized catalyst is subjected to a substantially water-free reduction step at a temperature of about 300.degree. to 550.degree. C. (preferably about 350.degree. C.) for 0.5 to 10 hours or more. The duration of the reduction step should be only as long as necessary to reduce the platinum, in order to avoid pre-deactivation of the catalyst, and may be performed in-situ as part of the plant startup if a dry atmosphere is maintained. Further details of the preparation and activation of embodiments of the second reforming catalyst are disclosed, e.g., in U.S. Pat. No. 4,619,906 (Lambert et al) and U.S. Pat. No. 4,822,762 (Ellig et al.), which are incorporated into this specification by reference thereto.
Using techniques and equipment known in the art, the aromatics-containing effluent from the second reforming zone usually is passed through a cooling zone to a separation zone. In the separation zone, typically maintained at about 0.degree. to 65.degree. C., a hydrogen-rich gas is separated from a liquid phase. The resultant hydrogen-rich stream can then be recycled through suitable compressing means back to the first reforming zone. The liquid phase from the separation zone is normally withdrawn and processed in a fractionating system in order to adjust the concentration of light hydrocarbons and produce an aromatics-containing reformate product.
EXAMPLES
The following examples are presented to demonstrate the present invention and to illustrate certain specific embodiments thereof. These examples should not be construed to limit the scope of the invention as set forth in the claims. There are many possible other variations, as those of ordinary skill in the art will recognize, which are within the spirit of the invention.
Three parameters are especially useful in evaluating reforming process and catalyst performance, particularly in evaluating catalysts for dehydrocyclization of paraffins. "Activity" is a measure of the catalyst's ability to convert reactants at a specified set of reaction conditions. "Selectivity" is an indication of the catalyst's ability to produce a high yield of the desired product. "Stability" is a measure of the catalyst's ability to maintain its activity and selectivity over time.
The examples illustrate the effect especially on reforming catalyst stability of precluding sulfur in the manner disclosed in the present invention.
EXAMPLE I
The capability of combination of a reforming catalyst and an MnO sulfur sorbent in series to achieve a substantially sulfur-free effluent from a naphtha feedstock was determined.
The platinum-tin on alumina reforming catalyst used in this determination had the following composition in mass %:
______________________________________ Pt 0.38 Sn 0.30 Cl 1.06______________________________________
The manganous oxide consisted essentially of MnO in spherical pellets with over 90% in the size range of 4-10 mesh. Equal volumes of reforming catalyst and MnO were loaded in series with the reforming catalyst above the MnO. The sulfur-removal capability of this combination was tested by processing a hydrotreated naphtha spiked with thiophene to obtain a sulfur concentration of about 2 mass parts per million (ppm) in the feed. The naphtha feed had the following additional characteristics:
______________________________________Sp. gr. 0.7447ASTM D-86, .degree.C.:IBP 8050% 134EP 199______________________________________
The naphtha was charged to the reactor in a downflow operation, thus contacting the reforming catalyst and MnO successively. Operating conditions were as follows:
______________________________________Pressure, atmospheres 8Temperature, .degree.C. 371Hydrogen/hydrocarbon, mol 3Liquid hourly space velocity, hr.sup.-1 10*______________________________________ *On total loading of catalyst + MnO
Over the 13-day testing period, there was no detectable sulfur in the liquid or vapor products. Adjusting ASTM D4045 repeatability for laboratory experience, the product sulfur level was reported as less than 14 parts per billion (ppb). The combination of a platinum-tin-alumina catalyst ahead of a bed of manganous oxide thus was able to treat naphtha with a sulfur content higher than would be obtained by standard hydrotreating to yield a product containing no detectable sulfur.
EXAMPLE II
The impact on a second reforming catalyst as described hereinabove of reducing the feed sulfur content to a nondetectable level, similar to that achieved in Example I, was assessed in comparison to a feed sulfur content according to the prior art.
The feed on which the comparison was based was a raffinate from a combination of catalytic reforming followed by aromatics extraction to recover benzene, toluene and C.sub.8 aromatics. The characteristics of the feedstock were as follows:
______________________________________Sp. gr. 0.689ASTM D-86, .degree.C.:IBP 6750% 82EP 118Mass % Paraffins 87.5Olefins 2.0Naphthenes 7.1Aromatics 3.4Sulfur, mass ppb 70______________________________________
Catalytic reforming tests were performed on the above raffinate without and with high-surface sodium treatment for sulfur removal. The catalyst contained 1.07 mass % platinum on a base of 50/50 mass % L-zeolite and alumina. Operating conditions were as follows:
______________________________________Pressure, atmospheres 5Hydrogen/hydrocarbon, mol 5Liquid hourly space velocity, hr.sup.-1 2.5______________________________________
Temperature was adjusted as required to achieve 55 mass % conversion of the charge stock to aromatics plus butane and lighter products, as shown in FIG. 3. No chloride was added during the test. The comparative results may be summarized as follows:
______________________________________Feed sulfur content, ppb 70 <14Deactivation rate, .degree.C./day 2.0 0.7______________________________________
Yields of aromatics and C.sub.5 + product were essentially the same during the two runs, with the sulfur-free feed showing an advantage of about 0.3% in the late stages of the comparison runs. The aromatics content of the respective C.sub.5 + products was approximately as follows:
______________________________________Feed sulfur content, mass ppb 70 <14Aromatics in C.sub.5 +, mass %Benzene 15.0 16.0Toluene 25.2 24.8C.sub.8 aromatics 8.6 8.2C.sub.9 + aromatics 0.1 0.1______________________________________
Thus, the reforming catalyst stability with a sulfur-free feed was about three times better than when processing the same feed containing 70 parts per billion sulfur, and end-of-run yields were slightly improved with a sulfur-free feed.
EXAMPLE III
The impact on second reforming catalyst life of the preclusion of sulfur from a feed with an already low sulfur level of 25 ppb was examined.
The feedstock was a light raffinate, from catalytic reforming followed by extraction of benzene and toluene, with the following characteristics:
______________________________________Sp. gr. 0.682ASTM D-86, .degree.C.:IBP 6950% 78EP 103Mass % Paraffins 90.4Olefins 2.9Naphthenes 5.3Aromatics 1.4Sulfur, mass ppb 25______________________________________
Catalytic reforming tests were performed on the above raffinate without and with high-surface sodium treatment for sulfur removal. The catalyst contained about 0.65 mass % platinum on a base of 85/15% L-zeolite and silica. Operating conditions were as follows:
______________________________________Pressure, atmospheres 5Hydrogen/hydrocarbon, mol 5Liquid hourly space velocity, hr.sup.-1 1.5______________________________________
Temperature was adjusted as required to produce 99 Research-octane-number C.sub.5 + product, as shown in FIG. 4. No chloride was added during the test. The comparative results may be summarized as follows:
______________________________________Feed sulfur content, ppb 25 <14Deactivation rate, .degree.C./day 9 5.4______________________________________
Catalytic reforming of a sulfur-free feed thus demonstrated a significant improvement in deactivation rate, even in comparison to the processing of a feed with a feed sulfur content well below that taught in the prior art.
EXAMPLE IV
The benefit of precluding sulfur from a straight-run naphtha feed to a second reforming catalyst as described hereinabove was studied.
The feed was a desulfurized light naphtha fraction, containing principally C.sub.6 and C.sub.7 hydrocarbons and having the following characteristics:
______________________________________Sp. gr. 0.7152ASTM D-86, .degree.C.:IBP 6950% 79EP 141Mass % Paraffins 54.1Naphthenes 41.2Aromatics 4.7Sulfur, mass ppb 56______________________________________
Catalytic reforming tests were performed on the above naphtha with and without high-surface sodium treatment for sulfur removal. The reforming catalyst contained about 1.07% platinum on a base of 50/50 mass % L-zeolite and silica. Operating conditions were as follows:
______________________________________Pressure, atmospheres 5Hydogen/hydrocarbon, mol 5Liquid hourly space velocity, hr.sup.-1 1.5______________________________________
Temperature was adjusted as required to produce 99 Research-octane-number C.sub.5 + product, as shown in FIG. 5. No chloride was added during the test The comparative results may be summarized as follows:
______________________________________Feed sulfur content, ppb 56 not detectedDeactivation rate, .degree.C./day 3.5 1.0______________________________________
The sulfur-free feedstock thus provided a second-reforming-catalyst deactivation rate about 3.5 times lower in a reforming operation than the desulfurized feedstock containing only 56 ppb sulfur.
Reduction of sulfur content in the feed to a reforming catalyst as described hereinabove to levels well below those described in the prior art thus shows surprising benefits in catalyst stability in the catalytic reforming process of the present invention.
EXAMPLE V
Having demonstrated the sulfur-removal capability of the manganese-oxide sorbent per Example I, the compatibility of the manganese sorbent in the process of the present invention was tested relative to the preferred zinc-oxide sorbent of the prior art. Zinc oxide is known from the prior art to be effective for sulfur removal. Thus, this example demonstrated whether any aspect of either metal oxide would affect the operation of the second reforming catalyst, precluding the known effect of sulfur removal by using a sulfur-free feedstock.
A reactor loading was prepared for the zinc-oxide test which contained a bed of zinc oxide pellets between two beds of reforming catalyst. The cylindrical, down-flow reactor contained the following three layers from top to bottom:
______________________________________Volume Material______________________________________20 cc Reforming Catalyst40 cc Zinc Oxide Pellets80 cc Reforming Catalyst______________________________________
The reforming catalyst contained about 1.1 mass-% platinum on a base of 50/50% L-zeolite and alumina. The zinc oxide was a commercially available desulfurization catalyst obtained from Katalco called "32-4".
For the manganese-oxide test, procedures were similar to those used for zinc oxide with a small variance in reactor loading. In place of the 40 cc of the zinc oxide, we loaded 30 cc of manganous oxide and 10 cc of alpha-alumina pellets. Alumina is known to be inert for sulfur removal or reforming at the conditions employed. The manganous oxide consisted essentially of MnO in spheroidal pellets with over 90% in the size range of 4-10 mesh.
The feedstock to both tests was identical to that employed in Example II, with high-surface-sodium removal of sulfur in order to isolate the impact of incompatibility on the process. Operating conditions in both cases were as follows:
______________________________________Pressure, psig 60Hydrogen/Hydrocarbon, moles 2Liquid Hourly Space Velocity, hr.sup.-1 1.5______________________________________
Temperature was adjusted as required to achieve 70% conversion of the non-aromatics contained in the feed to either aromatics or cracked products (pentane or lighter hydrocarbons). No chloride was added during the test.
FIG. 6 provides test results, showing the rapid loss in activity of the reforming catalyst associated with zinc oxide. Catalyst deactivation was significantly lower with the loading of manganous oxide. Comparison to the deactivation with ZnO is noted below:
______________________________________Material Deactivation (.degree.C./day)______________________________________ZnO >7MnO 0.8______________________________________
EXAMPLE VI
Tests were performed to determine whether chloride present in platinum/L-zeolite catalysts, characterizing the second reforming catalyst, would result in the presence of chloride in reforming reactants. Three different catalysts, two with usual chloride levels and one with a high chloride content, were tested. The feedstock to the tests was a paraffinic raffinate, and operating conditions were consistent with those in previous examples.
Drager tubes were used in the detection of chloride in the reactor off-gas stream. Hydrochloric acid and chlorine tubes both were employed, as indicated below, with respective ranges of 0.0 to 10.0 ppm and 0.0 to 3.0 ppm. Results were as follows:
______________________________________Test Catalyst: Cl, mass % Cl.sub.2 or HCl, ppm______________________________________1 A 0.40 0.0 Cl.sub.22 A 0.40 0.0 Cl.sub.23 B 1.09 0.0 HCl4 C 0.38 0.0 HCl5 C 0.38 0.0 HCl6 C 0.38 0.0 HCl______________________________________
These results indicate that there was no chloride present in the reforming reactants using platinum/L-zeolite catalyst, notwithstanding the chloride content of the catalysts.

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5059304	Field	Oct 1991

	Number	Date	Country
Parent	555962	Jul 1990
Parent	408577	Sep 1989

Catalytic reforming process with sulfur preclusion

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Continuation in Parts (2)