Catalytic reforming process comprising removal of sulfur from recycle gas streams

Abstract: A catalytic reforming process in which sulfur moieties are removed from a gaseous product stream by use of a sulfur trap comprised of about 10 to about 70 wt. % nickel dispersed on a support. The sulfur which is removed is both sulfur which is inherent in the feed as well as sulfur which results from presulfiding the catalyst.

Description: The present invention relates to the removal of sulfur from a process unit for catalytically reforming a naphtha feedstream boiling in the gasoline range. The sulfur is sulfur which is inherent in the feedstock, as well as sulfur resulting from catalyst presulfiding. The removal is accomplished by use of a massive nickel trap in a process gas line. Catalytic reforming is a well established refinery process for improving the octane quality of naphthas or straight run gasolines. Reforming can be defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes, dehydroisomerization of alkylcyclopentanes, and dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcyclopentanes to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst. In catalytic reforming, a multifunctional catalyst is usually employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, usually platinum, substantially atomically dispersed on the surface of a porous, inorganic oxide support, such as alumina. The support, which usually contains a halide, particularly chloride, provides the acid functionality needed for isomerization, cyclization, and dehydrocyclization reactions. Reforming reactions are both endothermic and exothermic, the former being predominant, particularly in the early stages of reforming with the latter being predominant in the latter stages. In view thereof, it has become the practice to employ a reforming unit comprised of a plurality of serially connected reactors with provision for heating of the reaction stream from one reactor to another. There are three major types of reforming: semiregenerative, cyclic, and continuous. Fixed-bed reactors are usually employed in semiregenerative and cyclic reforming, and moving-bed reactors in continuous reforming. In semiregenerative reforming, the entire reforming process unit is operated by gradually and progressively increasing the temperature to compensate for deactivation of the catalyst caused by coke deposition, until finally the entire unit is shut-down for regeneration and reactivation of the catalyst. In cyclic reforming, the reactors are individually isolated, or in effect swung out of line, by various piping arrangements. The catalyst is regenerated by removing coke deposits, and then reactivated while the other reactors of the series remain on stream. The "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, which is then put back in the series. In continuous reforming, the reactors are moving-bed reactors, as opposed to fixed-bed reactors, with continuous addition and withdrawal of catalyst. The catalyst is regenerated in a separate regeneration vessel. In reforming, sulfur compounds, even at a 1-2 ppm level contribute to a loss of catalyst activity and C₅+ liquid yield, particularly with the new sulfur-sensitive multimetallic catalysts. For example, a platinum-rhenium catalyst is so sensitive to sulfur poisoning that it is necessary to reduce sulfur to well below 0.1 wppm to avoid excessive loss of catalyst activity and C₅+ liquid yield. Generally, all petroleum naphtha feeds contain sulfur. Consequently, most of the sulfur is usually removed from the feed by hydrofining with conventional hydrodesulfurization catalysts comprised of molybdenum with nickel or cobalt, or both, on a carrier such as alumina. The severity of the hydrofining can be increased so that essentially all of the sulfur is removed from the naphtha in the form of H₂S. However, small quantities of olefins are also produced. As a consequence, when the exit stream from the hydrofiner is cooled, sulfur can be reincorporated into the naphtha by the combination of H₂S with the olefins to produce mercaptans. Hence, if a refiner is willing to pay the price, a hydrofining process can be employed at high severity to remove substantially all of the sulfur from a feed, but it is rather costly to maintain a product which consistently contains less than 1-2 parts per million by weight of sulfur. Also, during hydrofiner upsets, the sulfur concentration in the hydrofined product can be considerably higher, e.g., as high as 50 ppm, or greater. While hydrofining may remove most of the sulfur from the feedstock sulfur still remains a problem in catalytic reforming because another source of sulfur results from catalyst presulfiding. It is generally necessary to passivate the active metal sites on fresh, or freshly regenerated catalysts prior to contacting with feed. This helps prevent excessive demethylation reactions, low liquid yields, and possible temperature run-aways. Passivation is accomplished by first reducing the catalyst with hydrogen, followed by treating it with about 0.1 wt.% sulfur in the form of H₂S, di-tertiary polysulfide (TNPS), or other suitable sulfur compounds, particularly the organic sulfur compounds. While most of this sulfur is gradually depleted from the catalyst during normal operation of the unit and removed during removal of make fuel gas, the remainder (up to about 30% or original) is recirculated. In cyclic reformers, this remaining recirculating sulfur has the effect of depressing activity in all of the reactors. Various techniques have been used to remove sulfur, primarily from the feed. For example, one method for removing sulfur from feedstreams which has met with a limited amount of success is taught in U.S. Patent No. 4,634,515, which is incorporated herein by reference. This patent teaches removal of sulfur from liquid phase feedstreams by use of a fixed bed of massive nickel catalyst, the nickel being supported on alumina. This method requires use of temperatures in the range of 150°C to 260°C (300°F to 500°F). While such a method does in fact remove the sulfur inherent in the feedstock, it does not teach removal of sulfur resulting from presulfiding the catalyst. U.S. Patent No. 4,519,829 is an improvement on this method, by incorporating, with the massive nickel, from 1 to 15 weight percent iron to suppress the production of PNAs. Various techniques have also been proposed to remove sulfur from gas streams which could be employed on the recycle gas streams. For example, it has been proposed to remove sulfur by use of zinc alumina spinel, see U.S. Patent Nos. 4,263,020 and 4,690,806. The drawback of using spinel compositions is that they have a relatively low capacity for sulfur, e.g. 1-2%, and thus, require their own regeneration facility. It has also been proposed to use zinc traps, such as a zinc oxide trap, see for example U.S. Patent Nos. 4,717,552; 4,371,507; and 4,313,820. Zinc oxide traps tend to deteriorate rapidly in the presence of chloride and thus a chloride trap upstream of the zinc trap is required. Other references teach the use of various high temperature traps, such as U.S. Patent No. 4,187,282 which teaches the use of iron/copper/titanium oxide at a temperature from 249°C to 500°C (480° to 932°F); U.S. Patent No. 4,273,748 which teaches the use of dual iron/nickel oxide beds operating at temperatures of between 450° and 700°C (842° and 1300°F); and U.S. Patent No. 4,140,752 which teaches the use of vanadium, nickel, and/or potassium on activated carbon. While some of the above methods for removing sulfur have met with various degrees of commercial success, there is still a need in the art for the removal of sulfur which is both inherent in the feedstock as well as sulfur resulting from presulfiding the catalyst. In accordance with the present invention, there is provided an improved process for reforming a gasoline boiling range hydrocarbonaceous feedstock in the presence of hydrogen and in a reforming process unit, said process unit comprised of a plurality of serially connected reactors, inclusive of a lead reactor and one or more downstream reactors, the last of which is a tail reactor, and wherein each of the reactors contains a supported noble metal-containing catalyst and wherein a hydrogen-containing gas is recycled from one or more of the downstream reactors to the lead reactor, the improvement which comprises passing the recycle gas through a sulfur trap prior to it entering the lead reactor, said sulfur trap containing a catalyst comprised of about 10 to about 70 wt.% nickel dispersed on a support. In a preferred embodiment of the present invention, the gaseous stream passing through the trap also contains up to about 3.5 wt.% chloride. In another preferred embodiment of the present invention, the process unit is a cyclic unit and at least about 50% of the nickel is in a reduced state and is comprised of metal crystallites having an average size greater than about 7.5 nm (75 angstroms). Figure 1 is a simplified how diagram of a typical cyclic reforming process unit, inclusive of multiple on-stream reactors, an alternate or swing reactor inclusive of manifolds and reactor by-passes for use with catalyst regeneration and reactivation equipment. Figure 2 is a simplified flow diagram of a typical catalyst regeneration and reactivation facility, and the manner in which the coked deactivated catalyst of a given reactor of a cyclic unit can be regenerated and reactivated, as practiced in accordance with the present invention. Feedstocks which are typically used for reforming in accordance with the process of the instant invention are any hydrocarbonaceous feedstock boiling in the gasoline range. Non-limiting examples of such feedstocks include the light hydrocarbon oils boiling from 21°C to 260°C (70°F to 500°F), preferably from 82°C to 204°C (180°F to 400°F). Such feedstocks include straight run naphtha, synthetically produced naphtha such as a coal or oil-shale derived naphtha, thermally or catalytically cracked naphtha, hydrocracked naphtha, or blends or fractions thereof. Catalysts typically suitable for reforming, as practiced by the present invention, include both monofunctional and bifunctional multimetallic Pt-containing reforming catalysts. Preferred are the bifunctional reforming catalysts comprised of a hydrogenation-dehydrogenation function and an acid function. The acid function, which is important for isomerization reactions, is thought to be associated with a material of the porous, adsorptive, refractory oxide, preferably alumina, which serves as the support, or carrier, for the metal component. The metal component is typically a Group VIII noble metal, such as platinum, which is generally attributed the hydrogenation-dehydrogenation function. The support material may also be a crystalline aluminosilicate, such as a zeolite. Non-limiting examples of zeolites which may be used herein include those having an effective pore diameter, particularly L-zeolite, zeolite X, and zeolite Y. Preferably, the Group VIII noble metal is platinum. One or more promoter metals selected from metals of Groups IIIA, IVA, IB, VIB, and VIIB of the Periodic Table of the Elements may also be present. The promoter metal can be present in the form of an oxide, sulfide, or in the elemental state in an amount ranging from 0.01 to 5 wt.%, preferably from 0.1 to 3 wt.%, and more preferably from 0.2 to 3 wt.%, calculated on an elemental basis, and based on the total weight of the catalyst composition. It is also preferred that the catalyst compositions have a relatively high surface area, for example, 100 to 250 m²/g. The Periodic Table of the Elements referred to herein is published by Sergeant-Welch Scientific Company and having a copyright date of 1979 and available from them as Catalog Number S-18806. Reforming catalysts also usually contain a halide component which contributes to the necessary acid functionality of the catalyst. It is preferred that this halide component be chloride in an amount ranging from 0.1 to 3.5 wt.%, preferably from 0.5 to 1.5 wt.%, calculated on an elemental basis on the final catalyst composition. It is generally preferred that the platinum group metal be present on the catalyst in an amount ranging from 0.01 to 5 wt.%, also calculated on an elemental metal basis on the final catalyst composition. More preferably the catalyst comprises from 0.1 to 2 wt.% platinum group metal, especially from 0.1 to 2 wt.% platinum. Other platinum group metals suitable for use herein include palladium, iridium, rhodium, osmium, ruthenium, and mixtures thereof. Referring to Figure 1, there is described a reforming cyclic process unit comprised of a multi-reactor system, inclusive of on-stream reactors A, B, C, D, and a swing reactor S, and a manifold useful with a facility for periodic regeneration and reactivation of the catalyst of any given reactor. Swing reactor S is manifolded to reactors A, B, C, and D so that it can serve as a substitute reactor for purposes of regeneration and reactivation of the catalyst of a reactor taken off-stream. The several reactors of the series A, B, C, and D are arranged so that while one reactor is off-stream for regeneration and reactivation of the catalyst, it can be replaced by the swing reactor S. Provision is also made for regeneration and reactivation of the catalyst of the swing reactor. The on-stream reactors A, B, C, and D are each provided with a separate fumace, or heater, F A , F B , F C , and F D respectively, and all are connected in series via an arrangement of connecting process piping and valves, designated by the numeral 10, so that feed can be passed serially through F A A, F B B, F C C, and F D D, respectively; or generally similar grouping wherein any of Reactors A, B, C, and D respectively, can be substituted by swing Reactor S, as when the catalyst of any one of the former requires regeneration and reactivation. This is accomplished by "paralleling" the swing reactor with the reactor to be removed from the circuit for regeneration by opening the valves on each side of a given reactor which connect to the upper and lower lines of swing header 20, and then closing off the valves in line 10 on both sides of said reactor so that fluid enters and exits from said swing Reactor S. Regeneration facilities, shown in Figure 2 hereof, are manifolded to each of the several Reactors A, B, C, D, and S through a parallel circuit of connecting piping and valves which form the upper and lower lines of regeneration header 30, and any one of the several reactors can be individually isolated from the other reactors of the unit and the catalyst thereof regenerated and reactivated. The product from the fourth, or tail, reactor is flashed off in a gas-liquid separator with primarily hydrogen and methane, and sulfur-containing gases, such as hydrogen sulfide, going overhead. This stream is divided into fuel gas and recycle gas. It is preferred that the recycle gas first be recompressed, then passed through a sulfur trap, and returned to the reactor system where it is combined with fresh feed upstream of the lead reactor F A . The separator bottoms are stabilized of LPG and blended into the gasoline pool. Figure 2 depicts the catalyst regeneration and reactivation circuit, of the illustrated process unit which is used for the regeneration and reactivation of the coked deactivated catalyst of a reactor, e.g., the catalyst of Reactor D, which has been taken off line and replaced by Swing Reactor S. The catalyst regeneration and reactivation circuit generally includes a compressor, regenerator fumace F R , serially connected with the Reactor D which has been taken off line for regeneration and reactivation of the coked deactivated catalyst. The so formed circuit also includes location for injection of water, oxygen, hydrogen sulfide, and hydrochloric acid, as shown. A more detailed discussion of regeneration and reactivation of a reforming catalyst can be found in U.S. Patent No. 4,769,128 which is incorporated herein by reference. During regeneration of a coked deactivated catalyst, oxygen is injected upstream of the recycle gas compressor via regenerator fumace F R into Reactor D. In reactivation of the coke-depleted catalyst, oxygen, hydrogen sulfide, hydrochloric acid, and water if needed, are injected into Reactor D to redisperse the agglomerated catalytic metal, or metals, components of the catalyst. The hydrogen sulfide is added to passivate the catalyst before it is contacted with feed. The hydrogen suede, hydrochloric acid, and water are added downstream of the regenerator fumace F R . The sulfur contained in the separator overhead gas can be removed by use of a massive nickel trap placed in a product gas stream line. It can also be placed in the upper section of the separator. For example, the sulfur trap can be placed: (X) in a section of gaseous product line after the gas-liquid separator but prior to it being divided into a recycle gas stream and a fuel gas stream; (Y) in the recycle gas line, upstream (Y') or downstream of the compresor (Y); or (Z) in the feed line after the recycle gas is mixed with the feedstock, but prior to introduction into the lead furnace. The sulfur trap may also be incorporated into the upper section (X') of the gas/liquid separator. In this way, the sulfur trap would de-entrain the liquid being carried overhead with the gas. The letters X, X', Y, Y', and Z refer to those used in Figure 1 hereof. The sulfur trap is packed with a bed of nickel adsorbent of large crystallite size in highly reduced form, supported on alumina. In general, the nickel concentration ranges from 10 percent to 70 percent, preferably above 45 percent, more preferably from 45 percent to 55 percent, based on the total weight of the catalyst bed (dry basis). At least 50 percent, preferably at least 60 percent of the nickel is present in a reduced state, and the metal crystallites are greater than 7.5 nm (75 Angstrom units), Å, average diameter, and preferably at least about 9.5 (95 Å) average diameter. In particular, the nickel component of the adsorbent ranges from 45 percent to 55 percent, preferably from 48 percent to 52 percent elemental, or metallic nickel, based on the total weight of the supported component (dry basis). The size of the nickel crystallites range above 10 nm to 30 nm (100 Å to 300 Å), average diameter. A nickel adsorbent so characterized is far more effective for sulfur uptake than a supported nickel catalyst, or adsorbent of equivalent nickel content with smaller metal crystallites. The nickel containing absorbent is effective even if the stream contains HCl which is often the case in reforming since chlorides are continuously being depleted from the catalysts and replaced by injection of a small amount of organic chloride with the naphtha feed. The alumina component of the nickel-alumina adsorbent, or catalyst, is preferably gamma alumina, and contains a minimum of contaminants, generally less than about 1 percent, based on the total weight of the catalyst (dry basis). In particular, the alumina has a low silica content. That is, the silica content should not exceed about 0.7 percent, and will preferably range from 0 and 0.5 percent, based on the weight of the alumina (dry basis). Having thus described the present invention and a preferred and most referred embodiment thereof, it is believed that the same will become even more apparent by reference to the following examples. It will be appreciated, however, that the examples are presented for illustrative purposes and should not be construed as limiting the invention. EXAMPLE 1 This example was run to determine if massive nickel will absorb an appreciable amount of H₂S at temperatures as low as about 180°F. A sulfur adsorption test by TGA (Thermo Gravimetric Analysis) was devised to compare the performance of massive nickel in the sulfur trap at a total pressure of 101 KPa (1 atmosphere) and 260°C and 82°C (500°F and 180°F) respectively. Approxiately 100 mg of fresh catalyst were charged and heated to 482°C (900°F) in argon until no further weight loss was observed. Then it was cooled to 260°C (500°F) in flowing argon. After temperature equilibration, a stream consisting of 2 vol.% H₂S/98 vol.% Ar was introduced and weight gain due to sulfur adsorption measured with time until lineout at 260°C (500°F). The same experiment was performed on fresh catalyst for a temperature of 82°C (180°F). The capacity was determined by measuring the weight gain (H₂S uptake), of the massive nickel and is shown in Table 1 below. EXAMPLE 2 This example was run at conditions closer to process conditions, and at a temperature of 82°C (180°F), a temperature representative of the temperature of a recycle gas stream in a cyclic catalytic reforming process unit. A sample of massive nickel was saturated with HCl wherein the resulting massive nickel sample was found to contain about 20 wt.% Cl. The sample was placed in a microbalance and subjected to 0.1 vol.% H₂S in hydrogen for 30 hours at a temperature of 82°C (180°F). H₂S uptake was found to be about 10%. This example also demonstrates that sulfur can removed by use of a massive nickel trap in the presence of chloride. EXAMPLE 3 15 grams of massive nickel were loaded into a packed bed and contacted with a gas stream containing 2 vol.% H₂S in hydrogen at 180°F at a total pressure of 101.3 KPa (1 atmosphere) and a flow rate of 27 liters (STP) per hour. H₂S breakthrough occurred after uptake of 9 wt.% H₂S.

Claims: A process for catalytically reforming a hydrocarbonaceous feedstock boiling in the gasoline range, wherein the reforming is conducted in the presence of hydrogen in a reforming process unit under reforming conditions, said process unit comprised of a plurality of serially connected reactors, each of said reactors containing a reforming catalyst, and which process unit also includes a regeneration circuit for regenerating the catalyst after it becomes coked, said regeneration being effected by a step comprising treatment with a sulfur-containing gas, and which process unit also includes a gas/liquid separator wherefrom a portion of the gas is recycled to one or more of said reactors and the remaining portion is collected or recovered as make-gas, and wherein recycled gas is contacted with a catalyst comprised of from 10 to 70 wt.% nickel dispersed on a support and which catalyst is contained in a sulfur trap, between the gas/liquid separator and the recycle gas path to the first reactor. The process of claim 1 wherein the sulfur trap is comprised of from 45 to 70 wt.% of nickel. The process of claim 1 or claim 2 wherein at least 50% of the nickel is in a reduced state and is comprised of metal crystallites having an average size greater than about 75 Angstroms (7.5 nm). The process of any one of claims 1 to 3 wherein up to about 3.5 wt.% chloride is present in the recycle gas stream. The process of any one of claims 1 to 4 wherein the sulfur trap is in the recycle stream line. The process of any one of claims 1 to 4 wherein the sulfur trap is between the gas/liquid separator and the gas-discharge line therefrom, but before the reycle gas line, or downstream of the said separator and upstream of a make fuel-gas draw-off. The process of any one of claims 1 to 4 wherein the sulfur trap is located in an upper section of the gas/liquid separator. The process of any one of claims 1 to 4 wherein the sulfur trap is located just prior to the first reactor so that a mixture of feedstock and reycle gas is passed through it. The process of any one of claims 1 to 8 wherein the process unit is selected from the group consisting of a semi-regen unit, a semi-cyclic unit, and a cyclic unit.

Inventors: BOYLE JOSEPH PHILIP, BOYLE, JOSEPH PHILIP
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1991
Main classification: C10G35
IPC codes: C10G61, C10G35