EP0007219B1

EP0007219B1 - A process for catalytically reforming a naphtha in the presence of hydrogen

Info

Publication number: EP0007219B1
Application number: EP19790301330
Authority: EP
Inventors: Louis Dauber; George Alexander Swan
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1978-07-10
Filing date: 1979-07-09
Publication date: 1985-10-02
Also published as: EP0007219A1; MX6147E

Description

The present invention relates to a process for catalytically reforming a naphtha in the presence of hydrogen.
Reforming with hydrogen, or hydroforming, is a well established industrial process employed by the petroleum industry for upgrading virgin or cracked naphthas for the production of high octane products. Noble metal, notably platinum type catalyst are currently employed. It has become the practice to employ a plurality of adiabatic fixed-bed reactors in series with provision for interstage heating of the feed to each of the several reactors. In all processes the catalyst must be periodically regenerated by burning off the coke in the initial part of the catalyst reactivation sequence, since coke deposition gradually deactivates the catalyst. In a cyclic type of process, the reactors are individually isolated, or in effect swung out of line by various piping arrangements, the catalyst is regenerated to remove the coke deposits, and then reactivated while the other reactors of the series remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, and is then put back in series. In such processes hydrogen is produced in net yield, the product being separated into a C_S ⁺ liquid product, e.g., a C_s/221°C (430°F) product, and a hydrogen-rich gas a portion of which is recycled to the several reactors of the process unit.
In a cyclic reforming unit, individual reactors of the multi-reactor unit can be isolated, the catalyst regenerated, and reactivated, and the reactor placed back on stream without significantly affecting unit feed rate or octane quality. By adjusting the regeneration frequency, the unit can be economically designed for the minimum loading of hydrogenation-dehydrogenation metal, or metals components on the catalyst while maintaining an optimum yield of C₅ ⁺ reformate at given conditions.
Essentially all petroleum naphtha feeds contain sulfur, a well known catalyst poison which can gradually accumulate upon and poison the catalyst. Most of the sulfur, because of this adverse effect, is generally removed from feed naphthas, e.g., by hydrofining or by contact with nickel or cobalt oxide guard chambers, or both. In use of the more recently developed multi-metallic platinum catalysts wherein an additional metal, or metals hydrogenation-dehydrogenation component is added as a promoter to the platinum, it has become essential to reduce the feed sulfur to only a few parts per million parts by weight of feed (ppm). For example, in the use of platinum-rhenium catalysts it is generally necessary to reduce the sulfur concentration of the feed well below about 10 ppm, and preferably well below about 2 ppm, to avoid excessive loss of catalyst activity and C₅ ⁺ liquid yield. The role of sulfur on the catalyst presents something of an anomaly because the presence of sulfur in the feed can adversely affect the activity of the catalyst and reduce liquid yield: and yet, sulfiding of the multi-metallic catalyst species, which is a part of the catalyst reactivation procedure, has been found essential to suppress excessive hydrogenolysis which is particularly manifest when a reactor is first put on stream after regeneration and reactivation of the catalyst. Excessive hydrogenolysis caused by use of these highly active catalysts cannot only produce acute losses in C_S ⁺ liquid yield through increased gas production, but the severe exotherms which accompany operation in hydrogenolysis mode can seriously damage the catalyst, reactor, and auxiliary equipment.
US-A-3578582 describes a cyclic regenerative process for reforming a naphtha with hydrogen in a reforming unit which contains a plurality of catalyst-containing on-stream reactors connected in series, the hydrogen and naphtha feed flowing from one reactor of the series to another to contact the catalyst contained therein at reforming conditions. The catalyst used comprises 0.01 to 3 weight percent of a platinum group component (e.g. Pt) and 0.01 to 5 weight percent of a rhenium component and is regenerated by contact at an elevated temperature with an oxygen-containing gas after less than 100 hours of on-stream time to remove carbonaceous deposit. At start up of the cyclic regenerative reforming process, either initially or after regeneration with the catalyst in the unsulfided state, the catalyst is contacted with hydrogen for at least 0.1 hour, then contacted with a source of sulfur (preferably in the presence of hydrogen) so as to add to the catalyst from 0.05 to 2 mols sulfur per mol of platinum group component and rhenium component, and then the catalyst is contacted with hydrogen for at least 0.1 hour prior to contacting of the naphtha feed with the catalyst.
In cyclic reforming, it has been found that when a reactor containing highly active rhenium-promoted platinum catalysts is reinserted in the multiple reactor series of the unit, albeit it contains regenerated, reactivated, sulfided catalyst, there occurs an initial upset period when the catalyst activity and C_S ⁺ liquid yield of the unit is reduced. It has been observed that this effect is first noted in the reactor immediately downstream of the swing reactor which when first put on-stream contains a freshly sulfided catalyst. A quantity of sulfur is released when the freshly sulfided catalyst is contacted with the feed, the resulting sulfur wave travelling downstream from one reactor to the next of the sequence. Concurrent with the sulfur wave there results a loss in C_S ⁺ liquid yield which, like a wave, also progresses in seriatim from one reactor of the series to the next until finally the C₅ ⁺ liquid yield loss is observed throughout the unit. Over a sufficiently long period after the initial decline in C₅ ⁺ liquid yield, the C₅ ⁺ liquid yield in the several reactors of the unit, and consequently the overall performance of the unit, gradually improves. However, although the performance gradually improves, often the improvement is not sufficient to return each of the reactors of the unit, or unit as a whole, to its original higher performance level.
The effect of this phenomenon is that, in the overall operation, the catalyst contained in the several reactors briefly becomes less active, and a transient, but profound C,¹ liquid yield loss is observed.
It is, accordingly, the primary object of this invention to provide a new and improved process which will obviate these and other disadvantages of the present start-up procedures for reforming units, particularly those employing highly active promoted noble metal containing catalysts.
The present invention provides a process for reforming a naphtha with hydrogen in a reforming unit which comprises a plurality of on-stream reactors containing reforming catalyst to which sulfur has been added, said reactors being connected in series, the hydrogen and naphtha feed flowing from one reactor of the series to another to contact the catalyst therein at reforming conditions and wherein the on-stream flow of naphtha and hydrogen to each reactor is intermittently interrupted and the catalyst therein issubjectto a regeneration and reactivation sequence during said interruption, and sulfur is added to the catalyst of at least one reactor from a source of sulfur outside the reforming unit to maintain a sulfur content on the said catalyst of more than 0.01 wt.% sulfur based on the total weight of the said catalyst in said one reactor before the catalyst thereof has been subjected to more than five regeneration and reactivation sequences from its freshly-prepared state, characterized in that after any regeneration/reactivation sequence subsequent to the fifth sequence, no more than 0.01 wt.% sulfur (based on the total weight of the said catalyst in the said reactor) is added to the said catalyst from a source of sulfur outside the reforming unit.
Thus, the present invention comprises a new and improved mode of operating a cyclic or semi-regenerative reforming unit wherein in the sequence of regeneration and reactivation of the catalyst of any given reactor, the ability of a catalyst to operate in a hydrogenolysis mode can be effectively suppressed after the freshly prepared catalyst has been regenerated and reactivated more than five times by the addition to the catalyst of at least one reactor of the unit of sufficient sulfur to maintain an equilibrium amount of sulfur on the catalyst, as hereinafter defined, preferably by the addition thereto of no more than 0.01 percent sulfur, more preferably from 0.001 percent to 0.005 percent sulfur, based on the total weight of the catalyst (dry basis).
In a further embodiment of the present invention, the desired suppression of sulfur release by the catalyst after it has been regenerated and reactivated more than five times is effected by the addition or injection of small and infinitesimal amounts of water or hydrogen halide, or both, or a substance which can produce in situ water or hydrogen halide, or both, during the reforming operation to displace previously adsorbed sulfur, or to suppress the adsorption of sulfur by a catalyst to control the amount of sulfur added to the catalyst to an equilibrium level. It has been found in the sequence of regeneration and reactivation of the catalyst, that the ability of a catalyst to operate in a hydrogenolysis mode and effect sulfur release can be effectively suppressed after the freshly prepared catalyst has been regenerated and reactivated more than five times, by presulfiding the catalyst with sulfur to deposit, as hereinafter defined, no more than 0.01 percent sulfur, more preferably from 0.001 percent to 0.005 percent sulfur, based on the total weight of the catalyst (dry basis) and that these proper amounts of sulfur can be maintained on the catalyst by the continuous addition or injection of from 0.5 to 50 wppm, preferably from 1 to 20 wppm water or hydrogen halide, notably hydrogen chloride, or both, into the system. Alternatively the equivalent of from 0.05 to 0.2 wt.% catalyst of water or halide may be injected. Water and halide should preferably be added simultaneously to prevent unduly disturbing the catalyst halide content.
This mode of operation differs profoundly from a prior art operation wherein from about 0.05 percent to about 0.10 percent sulfur, based on the weight of the catalyst, is added to a catalyst to suppress hydrogenolysis, and wherein, when a reactor containing such catalyst is initially put on stream a release of sulfur as hydrogen sulfide in concentration ranging from about 10 to about 20 parts per million parts based on volume, (vppm) occurs, which is released in the recycle gas to poison catalyst dehydrogenation sites, thereby temporarily causing excessive cracking and lowered C₅ ⁺ liquid yield.
This invention is based on the recognition that, in a cyclic reforming unit, an in situ water wave immediately follows a sulfur wave when a reactor containing a freshly sulfided catalyst is put on-stream, and that a water wave, on contacting a freshly sulfided catalyst, causes release of sulfur from the catalyst. Sulfur release has also been observed in the operation of a semi-regenerative reforming unit by injecting halide and/or water into the system during on-oil operation. It is believed that, initially, the sulfur associates itself with the active sites of a catalyst, but thereafter when the catalyst is contacted by water, the water and sulfur moieties compete with each other for association with the active sites of the catalyst. Concurrent with such consideration, it has also been found, quite surprisingly, that residual sulfur remains on the catalyst even after catalyst regeneration, and reactivation, despite the high temperature burn to which the catalyst is subjected to remove coke deposits.
This phenomenon suggests an unusually high affinity of sulfur for catalyst sites: albeit sulfur is so readily displaced from a freshly regenerated, reactivated catalyst by water. As a consequence, it has been found that far smaller amounts of sulfur than are conventional can be beneficially employed in overall catalyst presulfiding operations, particularly in sulfiding catalysts which have previously been regenerated, and reactivated a number of times. Pre-sulfided catalysts which have been previously regenerated, and reactivated require far less sulfur to maintain an effective sulfide level, and apparently after several regeneration/reactivation sequences of treatment the sulfide level reaches an equilibrium level of from 0.03 to 0.04 wt.% sulfur on the catalyst. Thereafter, only minimal sulfur need be added to the system, if any, to maintain the effective sulfide level on the catalysts of the several reactors. Added sulfur can be effectively distributed from the catalyst of any given reactor to the catalysts of other reactors for maintenance purposes by adding sulfur, e.g., as by pre-sulfiding only the catalyst of a selected reactor, or reactors, because sulfur will be carried throughout the reactor system by recycle hydrogen, and sulfur will be adsorbed by the catalysts if they are undersulfided, and in situ water waves or injected water will remove sulfur from oversulfided catalyst and redistribute sulfur to undersulfided catalysts or release it from the catalyst system.
A feature of the invention then also resides in the discovery that even when a reactor containing a freshly prepared catalyst is put on stream, benefits can also be derived by use of a modified catalyst presulfiding regimen wherein the amount of sulfur added to the catalyst is progressively, and preferably proportionately reduced from one regeneration, reactivation sequence to the next until such time that an equilibrium amount of residual sulfur has been retained by the catalyst. After the fifth regeneration and reactivation sequence, no more than 0.01 percent sulfur, based on the total weight of the catalyst (dry basis), is added to the catalyst. In a preferred sequence of operation, a maximum of from 0.05 percent to 0.10 percent sulfur, based on the total weight of the catalyst (dry basis), is added ab initio to the fresh catalyst, the maximum amount of sulfur added to the catalyst being reduced about twenty percentto about forty percent with each regeneration, and reactivation of the catalyst. Thus, e.g., if 0.05 weight percent sulfur is put on the fresh catalyst, about 0.04 weight percent is put on the catalyst after the first regeneration, and reactivation of the catalyst: about 0.03 weight percent is put on the catalyst after the second regeneration and reactivation of the catalyst; about 0.025 weight percent is put on the catalyst after the third regeneration and reactivation of the catalyst; about 0.015 weight percent is put on the catalyst after the fourth regeneration and reactivation of the catalyst: and about 0.01 weight percent is put on the catalyst after the fifth regeneration, and reactivation of the catalyst. Similarly, if 0.10 percent weight percent sulfur is put on the fresh catalyst; about 0.08 weight percent sulfur is put on the catalyst after the first regeneration, and reactivation of the catalyst; about 0.06 weight percent sulfur is put on the catalyst after the second regeneration, and reactivation of the catalyst; about 0.04 weight percent sulfur is put on the catalyst after the third regeneration, and reactivation of the catalyst; about 0.02 weight put on the catalyst after the fourth regeneration, and reactivation of the catalyst; and about 0.01 weight put on the catalyst after the fifth regeneration, and reactivation of the catalyst.
In a preferred operation, a minimum amount of sulfur is released into the recycle gas of the cyclic system, and consequently less sulfur is available for poisoning of the dehydrogenation sites of the catalyst, such that substantially optimal Cs liquid yield is achieved with smoother operation, and better catalyst utilization following reactor swings. In this embodiment, after the fifth sequence of regeneration and reactivation of a catalyst, only the catalyst in the tail reactor, or reactors (i.e., the reactor, or reactors, in the rearward part of the series) are sulfided while the catalyst in the lead reactor or reactors (i.e., the reactor, or reactors in the forward part of the series), are left unsulfided. For example, in a reactor series which includes Reactors A, B, C and D, and a swing Reactor S, after the fifth sequence of regeneration and reactivation of the catalysts, only the catalyst of Reactors B, C and D, or preferably only the catalysts of Reactors C and D are sulfided, or more preferably only the catalyst of Reactor D is sulfided, while the catalyst of Reactor A, or preferably the catalysts of Reactors A and B, or more preferably the catalysts of Reactors A, B and C, are left unsulfided. If the catalyst of swing Reactor S is regenerated, reactivated and returned to service and moved into the first position (position A), or preferably the first or second position of the series (position A or B), or more preferably the first, second, or third position of the series (position A, B or C) the catalyst, after five regeneration/reactivation sequences of treatment, is not presulfided. On return of a reactor to the A, the A or B, or the A, B or C position of the series, however, there is no sulfur release when the reactor is initially put on stream since the catalyst is undersulfided. However, a water wave from the catalyst of each reactor successively returned to service passes through the downstream reactors and displaces sulfur from the catalysts of these reactors, the emitted sulfur emerging as hydrogen sulfide in the recycle gas which is recycled to the lead reactor, or reactors, to sulfide the unsulfided, or undersulfided catalyst. As the unsulfided, or undersulfided catalyst is sulfided by adsorption onto the catalyst, the hydrogen sulfide concentration in the recycle gas is decreased. The net effect is that marginally excess sulfur on the catalyst in the tail reactor, or reactors, is redistributed to the lead reactor, or reactors, and the hydrogen sulfide in the recycle gas rapidly lines out, e.g., within two to three hours from the time the reactor containing the unsulfided, or undersulfided catalyst is put on stream, to a base level of about 1 vppm sulfur in the recycle gas. This is sharply contrasted with conventional presulfiding wherein the catalysts of all the reactors are sulfided to levels ranging from 0.05 weight percent to 0.10 weight percent, and wherein a reactor swing of approximately 24 hours duration is produced before line out occurs, this resulting in a significantly greater C₅ ⁺ liquid yield loss, principally due to C₃/C₄ cracking.
In one mode of operation, following an upset which results in introducing a large quantity of sulfur into the system, water is injected with the feed, or gas, or separately injected, for contact with the catalyst to suppress adsorption of sulfur or effect release of sulfur in the form of hydrogen sulfide from the catalyst. Organic halides can also be added, to effect sulfur release, and also to increase correspondingly the level of halogen, and maintain the proper catalyst halide level since water will cause a loss of halogen and reduction in the halide level of the catalyst. The released hydrogen sulfide is purged from the system over a period of time via the unit make gas or removed from the recycle gas, or both, by means of desiccants or specific sulfide removal agents, e.g., zinc oxide. Or, if the sulfur content of the naphtha feed is higher than desired in normal operation for any reason, water and halogen can be injected to reduce the sulfur-on-catalyst level and to maintain the desired halide level on the catalyst, thereby obtaining improved catalyst activity and yields. The amount of water and halide injected is increased in proportion to the feed sulfur level. In a cyclic operation, more rapid displacement of presulfiding sulfur can be obtained by controlling the regeneration conditions to produce a higher water and halide level on the catalyst before swinging the reactor back on-stream, or by direct injection of water into the system.
In all embodiments a minimum amount of sulfur, particularly after the sulfide level of the catalysts has equilibrated (which occurs after the fifth sequence of regeneration and reactivation of a catalyst), is released into the recycle gas of the cyclic system, and consequently less sulfur is available for poisoning the dehydrogenation sites of the catalyst, such that substantially optimal C₅ ⁺ liquid yield is achieved with smoother operation, and better catalyst utilization.
Water injected into the system, e.g., the lead reactor of the series, passes successively through downstream reactors and displaces sulfur from the catalysts of these reactors, the emitted sulfur emerging as hydrogen sulfide in the recycle gas of which a portion can, if desired, be recycled to the lead reactor, or reactors, of the series to sulfide the unsulfided, or undersulfided catalyst. As the unsulfided or undersulfided catalyst is sulfided by adsorption onto the catalyst, the hydrogen sulfide concentration in the recycle gas passed downstream is decreased. The net effect is that excess, or marginally excess, sulfur on the catalyst of a lead reactor, or reactors, is redistributed to a downstream reactor, or reactors, and the hydrogen sulfide in the recycle gas rapidly lines out, e.g., within only about one hour or less from the time of the upset, or time that a reactor, or reactors containing an unsulfided, or undersulfided catalyst is put on stream, to a base level of less than 1 vppm sulfur in the recycle gas. This is sharply contrasted with conventional presulfiding wherein the catalysts of all of the reactors are sulfided to levels ranging from 0.05 weight percent to 0.10 weight percent, and wherein in a cyclic operation an upset operation of more extended period is produced before line-out occurs. This extension of the upset periods, of course, results in a significantly greater C₅ ⁺ liquid yield loss, principally due to C₃/C₄ cracking and the elevated system sulfur level.
In many reforming units it is customary to employ recycle gas drying in order to control the moisture level in the gas recirculating to the catalyst. In all embodiments the off gas from the last reactor of the series, predominantly an admixture of hydrogen and hydrocarbon containing moisture and hydrogen sulfide, is passed through a drier wherein essentially all or a major portion of the moisture is removed, suitably by contact with an adsorbent, after which time the gas is recycled to the process. Preferably, the moisture level of the recycle gas exiting the reactors is maintained below about 50 parts, more preferably below about 20 parts, per million parts of hydrogen. Suitably also, some of the hydrogen sulfide can be removed from the recycle gas should its concentration become excessive. Generally, the hydrogen sulfide level in the recycle gas is maintained below about 10 parts, or more preferably below about 5 parts, per million parts of hydrogen.
Sulfur can also be introduced into the system through the hydrocarbon feed, and consequently the feed sulfur level is normally maintained at very low level. On the other hand, where the sulfur level of the catalyst of the several reactors of a unit have already substantially equilibrated, or reached an equilibrium sulfur level, a major portion of the sulfur required to maintain an equilibrium amount thereof on the catalyst of the several reactors can be added to the feed, i.e., to make up for small loss of sulfur during regeneration.
These features and others will be better understood by reference to the following more detailed description of the invention, and to the drawing to which reference is made.
In the drawing the Figure depicts, by means of a simplified flow diagram, a preferred cyclic reforming unit inclusive of multiple on-stream reactors, and an alternative or swing reactor inclusive of manifolds for use with catalyst regeneration and reactivation equipment (not shown).
Referring to the Figure, generally, there is described a cyclic 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 being manifolded to Reactors A, B, C, 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, D, are arranged so that while one reactor is off-stream for regeneration and reactivation of the catalyst, the swing Reactor S can replace it and provision is also made for regeneration and reactivation of the catalyst of the swing reactor.
In particular, the on-stream Reactors A, B, C, D, each of which is provided with a separate furnace or heater F_A, or reheater F_B, F_c, F_o, respectively, are connected in series via an arrangement of connecting process piping and valves so that feed can be passed in seriatim through F_AA, F_BB, F_cC, F_DD, respectively; or generally similar grouping wherein any of Reactors A, B, C, D are replaced by Reactor S. This arrangement of piping and valves is designated by the numeral 10. Any one of the on-stream Reactors A, B, C, 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 in "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 side of said reactor so that fluid enters and exits from said swing Reactor S. Regeneration facilities, not shown, are manifolded to each of the several Reactors A, B, C, D, 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.
In conventional practice the reactor regeneration sequence is practiced in the order which will optimize the efficiency of the catalyst based on a consideration of the amount of coke deposited on the catalyst of the different reactors during the operation. Coke deposits much more rapidly on the catalyst of Reactors C, D and S than on the catalyst of Reactors A and B and, accordingly, the catalysts of the former are regenerated and reactivated at greater frequency than the latter. The reactor regeneration sequence is characteristically in the order ACDS/BCDS, i.e., Reactors A, C, D, B, etc., respectively, are substituted in order by another reactor, typically swing Reactor S, and the catalyst thereof regenerated and reactivated while the other four reactors are left on-stream. In the practice of the present invention, virtually any reactor regeneration sequence can be followed.
With reference to the Figure, for purposes of illustrating a regeneration, reactivation sequence, it is assumed that all of Reactors A, B, C, D and S were charged ab initio with fresh catalyst presulfided to deposit 0.05 weight percent sulfur on the catalyst, and Reactors A, B, C, D then put on-stream. The catalyst of each of the several Reactors A, B, C, D are then each removed from the unit as the catalyst is deactivated, the catalyst of each subsequently regenerated, and reactivated in conventional sequence, supra. With each progressive presulfiding the level of sulfur deposited on the catalyst of each of Reactors A, B, C, D and S is progressively, and proportionately reduced until at the end of the fifth catalyst regeneration, and reactivation, the catalyst is found to equilibrate at a level of from 0.03 to 0.04 weight percent sulfur. Thereafter, only the catalysts of Reactor D and S are presulfided, and the catalyst of Reactor S is only presulfided when employed as a substitute for Reactor D, these reactors acting, on their successive return to service, to restore the level of sulfur on the undersulfided catalyst of all of the reactors to about 0.03 to 0.04 weight percent.
In conducting the reforming operations, substantially all or a major portion of the moisture is scrubbed, or adsorbed from the hydrogen recycle gas which is returned to the unit to maintain a dry system. The recycle gas of the system should be dried sufficiently such that it contains a maximum of about 50 parts, preferably 20 parts, per million parts of water.
The invention, and its principle of operation, will be more fully understood by reference to the following examples, and comparative data which characterized a preferred mode of operation.

Example 1

In an operating run, Reactors A, B, C, D and S were each charged with a commercially supplied catalyst which contained platinum and rhenium well dispersed upon the surface of a gamma alumina support. The catalyst was dried, calcined, and then sulfided by contact with an admixture of n-butyl mercaptan in hydrogen, the gas having been injected into the reactor to provide a catalyst (dry basis) of the following weight composition, to wit:
A reforming run was then initiated, Reactors A, B, C and D having been placed on-stream with Reactor S in stand-by position, by adjusting the hydrogen and feed rates to the reactors, the feed being characterized as Light Arabian/West Texas Virgin naphtha blend which had, as shown in Table I, the following inspections:
The temperature and pressure of the reactors were then adjusted to the operating conditions required to produce a 102 RONC octane C_S ⁺ liquid product, and the run was continued at generally optimum reforming conditions by adjustment of these and other major process variables to those given below:
The run was continued until such time that sufficient coke had deposited on the catalyst of a reactor that regeneration, and reactivation was required. Each reactor of the series was periodically replaced and the catalyst thereof regenerated, and reactivated a multiple number of times as required; Reactors C and D, and Reactor S when placed in the position of Reactors C and D, requiring regeneration and reactivation about twice as often as Reactors A and B. The regeneration in each instance was accomplished by burning the coke from the coked catalyst, initially by burning at 510°C (950°F) by the addition of a gas which contained 0.6 mole percent oxygen: and thereafter the temperature was raised to 527°C (980°F) while the oxygen concentration in the gas was increased to 6 mole percent. Reactivation in each instance was conducted by the steps of: (a) redispersing the agglomerated metals by contact of the catalyst with a gaseous admixture containing sufficient carbon tetrachloride to decompose in situ and deposit 0.1 wt.% chloride on the catalyst: (b) continuing to add a gaseous mixture containing 6% oxygen for a period of 2 to 4 hours while maintaining a temperature of 950°F (510°C): (c) purging with nitrogen to remove essentially all traces of oxygen from the reactor: and (d) reducing the metals of the catalyst by contact with a hydrogen-containing gas at 850°F (454.4°C).
The amount of sulfur directly added to each reactor subsequent to each regeneration/reactivation sequence is given in Table II. Initially, in each instance after a regeneration/reactivation sequence, the activation of the catalyst was completed by sulfiding the catalyst of all of Reactors A, B, C, D and S by direct contact with a gaseous admixture of n-butyl mercaptan in hydrogen, sufficient to deposit a target amount of sulfur on the catalyst. After initial sulfiding of the catalyst, the amount of sulfur added to the catalyst of each reactor was progressively reduced, and after the fifth regeneration/reactivation sequence, no additional sulfur was directly added to the catalyst of Reactors A and B. The catalysts of Reactors A and B were thereafter sulfided in situ by contact with the hydrogen sulfide containing recycle hydrogen, previously passed through a recycle drier to remove essentially all of the water: and, no further regeneration and reactivation of the catalyst was necessary. The amount of sulfur added to the catalysts of Reactors C, D and S was progressively reduced through the ninth regeneration/reactivation sequence and thereafter no sulfur was directly added to Reactor C. The catalyst of Reactor C, like that of Reactors A and B was thereafter sulfided only by contact with the hydrogen sulfide containing recycle gas stream, and no further regeneration and reactivation of the catalyst was necessary.
It was found that when a reactor is returned to service after regeneration and reactivation of the catalyst, some sulfur remains on the catalyst, even though the catalyst is not directly sulfided. An in situ water wave from the freshly regenerated/reactivated catalyst of an upstream reactor containing a platinum-rhenium catalyst which contains more than 0.03 to 0.04 wt.% sulfur will redistribute the excess sulfur to an undersulfided catalyst of a down-stream reactor, i.e., one which contains platinum-rhenium catalyst having less than 0.03 to 0.04 wt.% sulfur. In such a system therefore, excess sulfur from an oversulfided catalyst will be distributed to the undersulfided catalyst of a downstream reactor, or recycled with dry hydrogen and redistributed to the undersulfided catalyst of an upstream reactor. This means, in effect, that the addition of sulfur to the system in any amount more than that required to provide an equilibrium level on the catalyst results in decreased catalyst activity and loss of C_S ⁺ liquid yield.
The advantages of operating the process at conditions required to maintain as close as possible to an equilibrium level of sulfur on the catalyst, and low concentration of hydrogen sulfide in the recycle gas, can be conveniently shown by comparison of the effect of the concentration of hydrogen sulfide contained in the recycle gas on the activity of the catalyst and the Cs liquid yield obtained under two different conditions of operation which are both in accordance with the process of the invention. Consequently, to show such comparison, reference is again made to the tabulation given in Table II, and to the data given in Table III which presents a comparison, during reforming operations in accordance with the invention, of the average amount of hydrogen sulfide contained in the recycle gas (as measured on the exit side of the last reactor of the series, and upstream of the recycle gas drier), the average catalytic activity and the total C₅ ⁺ liquid yield produced after a final direct sulfiding of the catalysts in Reactors A and B subsequent to the fifth regeneration/reactivation sequence (which added 0.011 wt.% and 0.013 wt.% sulfur, respectively, on the catalyst) and the ninth-sulfiding of the catalysts of Reactors C, D and S following the ninth regeneration/ reactivation sequence (which added 0.007 wt.%, 0.009 wt.% and 0.011 wt.% sulfur, respectively, on the catalyst) (Reforming Operation I) vis-a-vis the subsequent reforming operation (Reforming Operation II) made after directly sulfiding only the catalysts of Reactors D and S at their tenth regeneration/reactivation sequences, without direct sulfiding of the catalysts of any of Reactors A, B and C.
In swinging reactors containing undersulfided catalyst into their operating positions in the reforming unit, it was found that the sulfur concentration in the recycle gas actually dropped, and after several hours began to line out as the sulfur level on the catalyst equilibrated. When, e.g., Reactor C was returned to service without direct sulfiding of the catalyst of this reactor subsequent to regeneration and reactivation of the catalysts, a water wave from this reactor caused displacement of some sulfur from the D reactor as hydrogen sulfide, which was recycled with hydrogen to the upstream reactors of the unit, and redistributed throughout the system.
As clearly shown, this procedure in accordance with the process of the invention, provides a means for eliminating excess sulfur by using in situ water to redistribute minimal sulfur on catalyst, instead of directly presulfiding the catalyst of each reactor following a regeneration/reactivation sequence.
It will be apparent that as it becomes possible to add less and less sulfur to the catalyst of the reforming unit to maintain a required threshold level of sulfur on the catalyst to suppress hydrogenolysis, and the level of sulfur on the catalyst reaches an equilibrium level, catalyst activity and C₅ ⁺ liquid yield are both improved. The data in Tables II and III show at what part of the reforming operations this can occur, and the data in Table III show the correlation which appears to exist between the average concentration of hydrogen sulfide in the recycle gas and the catalyst activity and C₅ ⁺ liquid yield. It appears from the data in Table III that an average H_ZS concentration in the recycle gas of 0.5 vppm (Run II) gives better results in terms of catalyst-activity and C_S ⁺ liquid yield than an average H₂S concentration of 3 vppm (Run I). Operation at a 3 vppm H₂S concentration results in a 1.4 LV% C₅ ⁺ liquid yield loss and about 20% catalyst activity loss relative to operation at 0.5 vppm H₂S concentration.

Example 2

This example is not an example of the process of the invention as claimed, but illustrates competitive adsorption between sulfur and water on the catalyst in the reactors of a reforming unit comprising a plurality of on-stream series-connected reactors. It is believed that the competitive adsorption between sulfur and water illustrated in this example provides the mechanism by which sulfur is distributed and/or redistributed throughout the on-stream reactors of the reforming unit in accordance with the process of the present invention.
In an operating unit utilizing a platinum-rhenium (Pt/Re) catalyst, feed sulfur had been elevated above the maximum desirable level due to operating problems in the feed preparation section. Hence, recycle gas H₂S concentration was too high and ranged from 2.7 to 3.4 vppm. After the feed problem was alleviated, it was desired to return the catalyst to its normal state by removal of excess sulfur as rapidly as possible. Accordingly, a "pulse" of water was introduced into the system via the feed, the feed water level being increased to approximately 50 wppm for about a 2 hour period. The hydrogen sulfide concentration in the recycle gas was immediately noticed to increase to 5.3 wppm from a base level of 2.5 wppm in the preceding hours, demonstrating desorption of excess sulfur from the catalyst. Over the next 12 hours, as the catalyst dried down, the hydrogen sulfide concentration declined and levelled out at 1.5 wppm. It was then found possible to resume normal operations.

Example 3

The following test operation was conducted in a cyclic operating unit with a Pt/Re catalyst; the lead reactor catalyst was regenerated, rejuvenated, and the catalyst thereof reduced by treatment with hydrogen in preparation for reinsertion into the reaction circuit. The catalyst was not presulfided, however, the catalyst was equilibrated with a moist gas containing about 8,000 vppm water at 850°F (454.4°C) and 125 psig (861.9 kPa gauge) during the regeneration procedure. The hydrogen sulfide level of the recycle gas had been lined out at 1 vppm. The recycle gas drier was temporarily bypassed.
As soon as the reactor containing the freshly regenerated, reactivated catalyst was introduced into the reaction circuit, a wave of water travelled through the downstream reactors. This resulted in desorption of sulfur; the hydrogen sulfide concentration immediately rising above 10 vppm (the upper limit of the on stream analyzer). The recycling hydrogen sulfide sulfided the fresh catalyst of the lead reactor, and no exotherms were observed. The excess hydrogen sulfide which was released was gradually purged from the system via the make gas. The hydrogen sulfide analyser returned to an on-scale reading of less than 10 vppm after 2 hours; the prereactor swing hydrogen sulfide level of 1 vppm was obtained after another 5 hours. Although the recycle gas drier was not inserted following the swing in order to evaluate hydrogen sulfide release dynamics, it could normally be put on stream to help absorb the hydrogen sulfide from the recycle gas, thus considerably accelerating the lineout period.

Example 4

In a semi-regenerative reformer loaded with a bi-metallic platinum-iridium (Pt/ir) catalyst the catalyst had been too heavily sulfided prior to oil-in and acid during early operations. Catalyst activity was below par and little improvement was apparent. It was believed that the catalyst was also somewhat deficient in chloride. In view of this deficiency, it was decided to "pulse" the equivalent of 0.3 wt.% chloride to the last reactor; this simultaneously improved catalyst chloride level and reduced its sulfur level. At the same time, t-butyl alcohol was added to the feed in order to redistribute the chloride over the catalyst of all the catalyst beds and further assist in sulfur desorption.
The chloride was added in the form of trichlorethylene in seven separate pulses over a two-day period. The following table shows the results of this procedure:
During the addition of the chloride, the hydrogen sulfide concentration in the stabilizer off gas rose from 2 to 18 vppm and then declined again to 2 vppm.
It was noted that all the common criteria used for evaluating reforming catalyst activity and selectivity showed improvement, i.e., the recycle gas purity, reactor T's, and the overall activity index (based on octane requirement, feed characteristics, operating conditions). Although higher chloride on catalyst can partly explain the higher activity, only a reduction in catalyst sulfur can explain the overall performance improvement of the catalyst.
The present invention finds its greatest utility in cyclic reforming processes wherein the new "bimetallic" or multi-metallic catalysts are employed, notably Group VIII platinum group, or noble metals (ruthenium, rhodium, palladium, osmium, iridium and platinum), e.g., platinum-rhenium, platinum-rhenium-iridium, palladium-rhenium, platinum-palladium-rhenium, etc. Fresh, or reactivated catalysts of this type are particularly hypersensitive. Exotherms or heat fronts can be produced which pass through a catalyst bed at startup, i.e., when new or freshly regenerated catalyst is initially contacted with hydrocarbons at reforming temperatures. The temperature excursions or heat fronts are attributed to the hyperactivity of the catalyst which causes excessive hydrocracking of the hydrocarbons or hydrogenolysis, sometimes referred to as "runaway hydrocracking". These temperature excursions or heat fronts are undesirable because the resultant temperature often results in damage to the catalyst, or causes excessive coke laydown on the catalyst with consequent catalyst deactivation and, if uncontrolled, may even lead to damage to the reactor and reactor internals. The present invention serves to suppress, or even eliminate this severe hydrocracking problem.
Other catalysts suitable for the practice of this invention contain a hydrogenation-dehydrogenation component constituted of a platinum group metal, or admixtures of these and/or one or more additional non-platinum group metallic components such as germanium, gallium, tin, rhenium, tungsten, and the like. A preferred type of catalyst contains the hydrogenation-dehydrogenation component in concentration ranging from 0.01 to 5 wt.%, and preferably from 0.2 to 1.0 wt.%, based on the total catalyst composition. In addition, such catalysts also usually contain an acid component, preferably halogen, particularly chlorine or fluorine, in concentration ranging from 0.1 to 5 wt.%, and preferably from 0.3 to 1.0 wt.%. The hydrogenation-dehydrogenation components are composited with an inorganic oxide support, such as silica, silica-alumina, magnesia, thoria, zirconia, or the like, and preferably alumina.
Methods of regeneration, and reactivation of these catalysts are known and per se form no part of the present invention. Conventionally, an isolated reactor which contains a bed of catalyst, the latter having reached an objectionable degree of deactivation due to coke deposition thereon, is first purged of hydrocarbon vapors with a nonreactive or inert gas, e.g., helium, nitrogen, or flue gas. The coke or carbonaceous deposits are then burned from the catalyst by contact with an oxygen-containing gas at controlled temperature below the sintering point of the catalyst, generally below about 1300°F (704.4°C), and preferably below about 1200°F (648.9°C). The temperature of the burn is controlled by controlling the oxygen concentration and inlet gas temperature, this taking into consideration, of course, the amount of coke to be burned and the time desired in order to complete the burn. Typically, the catalyst is treated with a gas having an oxygen partial pressure of at least about 0.1 psi (pounds per square inch) (0.6895 kPa), and preferably in the range of 0.3 psi (2.0685 kPa) to 2.0 psi (13.79 kPa) to provide a temperature ranging from 575°F (301.7°C) to about 1000°F (537.8°C), at static or dynamic conditions, preferably the latter, for a time sufficient to remove the coke deposits. Coke burn-off can be accomplished by first introducing only enough oxygen to initiate the burn while maintaining a temperature on the low side of this range, and gradually increasing the temperature as the flame front is advanced by additional oxygen injection until the temperature has reached optimum. Most of the coke can be readily removed in this way.
Typically in reactivating multimetallic catalysts, sequential halogenation and hydrogen reduction treatments are required to reactivate the reforming catalysts to their original state of activity, or activity approaching that of fresh catalyst after coke or carbonaceous deposits have been removed from the catalyst. Suitably, the coke is burned from the catalyst, initially by contact thereof with an admixture of air and flue gas or nitrogen to give about 0.75 wt. percent oxygen at temperatures ranging to about 750°F (398.9°C), and thereafter the oxygen is increased within the mixture to about 6 wt. percent and the temperature gradually elevated to about 950°F (510°C).
The agglomerated metals of the catalyst are redispersed and the catalyst reactivated by contact of the catalyst with halogen, suitably a halogen gas or a substance which will decompose in situ to generate halogen. Various procedures are available dependent to a large extent on the nature of the catalyst employed. Typically, e.g., in the reactivation of a platinum-rhenium catalyst, the halogenation step is carried out by injecting halogen, e.g., chlorine, bromine, fluorine or iodine, or a halogen component which will decompose in situ and liberate halogen, e.g., carbon tetrachloride, in the desired quantities into the reaction zone. A halogen gas or halogen-containing gaseous mixture is introduced into the reforming zone and into contact with the catalyst at temperature ranging from 550°F (287.8°C) to 1150°F (621.1°C), and preferably from 700°F (371.1°C) to 1000°F (537.8°C). The introduction may be continued up to the point of halogen breakthrough, or point in time when halogen is emitted from the bed downstream of the location of entry where the halogen gas is introduced. The concentration of halogen is not critical, and can range, e.g., from a few parts per million (ppm) to essentially pure halogen gas. Suitably, the halogen, e.g., chlorine, is introduced in a gaseous mixture wherein the halogen is contained in concentration ranging from 0.01 mole percent to 10 mole percent, and preferably from 0.1 mole percent to 3 mole percent.
After redispersing the metals with the halogen treatment, the catalyst can then be rejuvenated by soaking in an admixture of air which contains about 6 wt. percent oxygen, at temperatures ranging from 850°F (454.4°C) to 950°F (510°C).
Oxygen is then purged from the reaction zone by introduction of a nonreactive or inert gas, e.g., nitrogen, helium or flue gas, to eliminate the hazard of a chance explosive combination of hydrogen and oxygen. A reducing gas, preferably hydrogen or a hydrogen-containing gas generated in situ or ex situ, is then introduced into the reaction zone and contacted with the catalyst at temperatures ranging from 400°F (204.4°C) to 1100°F (593.3°C), and preferably from 650°F (343.3°C) to 950°F (510°C), to effect reduction of the metal hydrogenation-dehydrogenation components, contained on the catalysts. Pressures are not critical, but typically gauge pressures range between 5 psig (34.48 kPa) to 300 psig (2068.5 kPa). Suitably, the gas employed comprises from 0.5 to 50 percent hydrogen, with the balance of the gas being substantially nonreactive or inert. Pure or essentially pure, hydrogen is, of course, suitable but is quite expensive and therefore need not be used. The concentration of the hydrogen in the treating gas and the necessary duration of such treatment, and temperature of treatment, are interrelated, but generally the time of treating the catalyst with a gaseous mixture such as described ranges from 0.1 hour to 48 hours, and preferably from 0.5 hour to 24 hours, at the more preferred temperatures.
The catalyst of a reactor may be presulfided, prior to return of the reactor to service. Suitably a carrier gas, e.g., nitrogen, hydrogen, or admixture thereof, containing from 500 to 2000 ppm of hydrogen sulfide, or compound, e.g., a mercaptan, which will decompose in situ to form hydrogen sulfide, at from 700°F (371.1°C) to 950°F (510°C), is contacted with the catalyst for a time sufficient to incorporate the desired amount of sulfur upon the catalyst.

Claims

1. A process for reforming a naphtha with hydrogen in a reforming unit which comprises a plurality of on-stream reactors containing reforming catalyst to which sulfur has been added, said reactors being connected in series, the hydrogen and naphtha feed flowing from one reactor of the series to another to contact the catalyst therein at reforming conditions and wherein the on-stream flow of naphtha and hydrogen to each reactor is intermittently interrupted and the catalyst therein is subject to a regeneration and reactivation sequence during said interruption, and sulfur is added to the catalyst of at least one reactor from a source of sulfur outside the reforming unit to maintain a sulfur content on the said catalyst of more than 0.01 wt.% sulfur based on the total weight of the said catalyst in said one reactor before the catalyst thereof has been subjected to more than five regeneration and reactivation sequences from its freshly-prepared state, characterized in that after any regeneration/reactivation sequence subsequent to the fifth sequence, no more than 0.01 wt.% sulfur (based on the total weight of the said catlyst in the said reactor) is added to the said catalyst from a source of sulfur outside the reforming unit.

2. A process according to claim 1 further characterized in that the said catalysts are platinum catalysts promoted with a hydrogenation-dehydrogenation component, or components, which increase the rate of hydrogenolysis as contrasted with an unpromoted platinum catalyst.

3. A process according to claim 2 further characterized in that the platinum catalyst is promoted with rhenium.

4. A process according to any one of claims 1 to 3 further characterized by injecting water into an on-line reactor of the series in a concentration ranging from about 0.5 to about 50 wppm based on feed to maintain and control the sulfur level of the catalyst.

5. A process according to any one of claims 1 to 4 further characterized in that sulfur is added to the catalyst in at least said one reactor during the first five catalyst regeneration/reactivation sequences, the sulfur added to the catalyst being progressively reduced between the first catalyst regeneration/ reactivation sequence and the fifth sequence at which time the maximum amount of sulfur added to the catalyst in said one reactor is about 0.01 wt. percent sulfur based on the total weight of catalyst in said one reactor.

6. A process according to any one of claims 1 to 5 further characterized in that the equivalent of 0.05 to 0.2 wt.% on said catalyst of halide and proportionate amounts of water are injected over short term periods to rapidly remove excess sulfur from the said catalyst.

7. A process according to any one of claims 1 to 6 further characterized in that after the fifth regeneration/reactivation sequence, the catalysts of the lead reactors of the series are no longer directly sulfided, but only the catalysts of the tail reactors are directly sulfided.

8. A process according to any one of claims 1 to 7 further characterized in that the unit contains at least three on-stream reactors in series, and after the fifth regeneration/reactivation sequence, the last reactor of the series is substituted by a swing reactor, and only the catalyst of the swing reactor is directly sulfided prior to the substitution.

9. A process according to any one of claims 1 to 8 further characterized in that the hydrogen gas from the last reactor of the series is dried to remove moisture, and recycle hydrogen containing less than about 50 parts of water, per million parts of hydrogen, is circulated within the unit.

10. A process according to any one of claims 1-9 further characterized in that a maximum of from 0.001 percent to 0.005 percent sulfur is added to the catalyst of any reactor of the series after the fifth sequence of regeneration and reactivation.