CA2105305C - Low-sulfur reforming processes - Google Patents

Abstract

Disclosed is a method for reforming hydrocarbons comprising contacting the hydrocarbons with a catalyst in a reactor system (10, 20, 30) of improved resistance to carburization and metal dusting under conditions of low sulfur.

Description

The present invention relates to improved techniques 6 for catalytic reforming, particularly, catalytic 7 reforming under low-sulfur, and low-sulfur and low-water 8 conditions. More specifically, the invention relates to 9 the discovery and control of problems particularly acute with low-sulfur, and low-sulfur and low-water reforming 11 processes.

13 Catalytic reforming is well known in the petroleum 14 industry and involves the treatment of naphtha fractions to improve octane rating by the production of aromatics.
16 The more important hydrocarbon reactions which occur 17 during the 1 reforming opeiation include the dehydrogenation of 2 cyclohexanes to aromatics, dehydroisomerization of 3 alkylcyclopentanes to aromatics, and 4 dehydrocyclization of acyclic hydrocarbons to aromatics. A number of other reactions also occur, 6 including the dealkylation of alkylbenzenes, 7 isomerization of paraffins, and hydrocracking 8 reactions which produce light gaseous hydrocarbons, 9 e.g., methane, ethane, propane and butane. It is important to minimize hydrocracking reactions.during 11 reforming as they decrease the yield of gasoline 12 boiling products and hydrogen.

14 Because there is a demand for high octane gasoline, extensive research has been devoted to the 16 development of improved reforming catalysts and 17 catalytic reforming processes. Catalysts for 18 successful reforming processes must possess good 19 selectivity. That is, they should be effective for producing high yields of liquid products in the 21 gasoline boiling range containing large 22 concentrations of high octane number aromatic 23 hydrocarbons. Likewise, there should be a low yield 24 of light gaseous hydrocarbons. The catalysts should possess good activity to minimize excessively high 26 temperatures for producing a certain quality of WO 92/15653 2 1 0 5 3 ~ ~ PCT/US92/01856 1 products. It is also necessary for the catalysts to 2 either possess good stability in order that the 3 activity and selectivity characteristics can be 4 retained during prolonged periods of operation; or be sufficiently regenerable to allow frequent 6 regeneration without loss of performance.

8 catalytic reforming is also an important process 9 for the chemical industry. There is an increasingly larger demand for aromatic hydrocarbons for use in il the manufacture of various chemical products such as 12 synthetic fibers, insecticides, adhesives, 13 detergents, plastics, synthetic rubbers, 14 pharmaceutical products, high octane gasoline, perfumes, drying oils, ion-exchange resins, and 16 various other products well known to those skilled in 17 the art.

19 An important technological advance in catalytic reforming has recently emerged which involves the use 21 of large-pore zeolite catalysts. These catalysts are 22 further characterized by the presence of an alkali or 23 alkaline earth metal and are charged with one or more 24 Group VIII metals. This type of catalyst has been found to advantageously provide higher selectivity 26 and longer catalytic life than those previously used.

1 Having discovered selective catalysts with 2 acceptable cycle lives, successful commercialization 3 seemed inevitable. Unfortunately, it was 4 subsequently discovered that the highly selective, large pore zeolite catalysts containing a Group VIII
6 metal were unusually susceptible to sulfur poisoning.
7 See U.S. Patent No. 4,456,527. Ultimately, it was 8 found that to effectively address this problem, 9 sulfur in the hydrocarbon feed should be at ultra-low levels, preferably less than 100 parts per billion 11 (ppb), more preferably less than 50 ppb to achieve an 12 acceptable stability and activity level for the 13 catalysts.

After recognizing the sulfur sensitivity 16 associated with these new catalysts and determining 17 the necessary and acceptable levels of process 18 sulfur, successful commercialization reappeared on 19 the horizon; only to vanish with the emergence of another associated problem. It was-found that 21 certain large pore zeolite catalysts are also 22 adversely-sensitive to the presence of water under 23 typical reaction conditions. Particularly, water was 24 found to greatly accelerate the rate of catalyst deactivation.

210 530a 1 Water sensitivity was found to be a serious 2 drawback which was difficult to effectively address.
3 Water is produced at the beginning of each process 4 cycle when the catalyst is reduced with hydrogen.

And, water can be produced during process upsets when 6 water leaks into the reformer feed, or when the feed 7 becomes contaminated with an oxygen-containing =

8 compound. Eventually, technologies were also 9 developed to protect the catalysts from water.

11 Again commercialization seemed practical with 12 the development of various low-sulfur, low-water 13 systems for catalytic reforming using highly 14 selective large-pore zeolite catalysts with long catalytic lives. While low-sulfur/low-water systems 16 were initially effective, it was discovered that a 17 shut down of the reactor system can be necessary 18 after only a matter of weeks. The reactor system of 19 one test plant had regularly become plugged after = 20 only such brief operating periods. The plugs were 21 found to be those associated with coking. However, 22 although coking within catalyst particles is a common 23 problem in hydrocarbon processing, the extent and 24 rate of coke plug formation exterior to the catalyst particles associated with this particular system far 26 exceeded any expectation.

N1053"5 PCT/US92/01856 _ 6 -2 Accordingly, one object of the invention is to 3 provide a method for reforming hydrocarbons under 4 conditions of low sulfur which avoids the aforementioned problems found to be associated with 6 low-sulfur processes, such as brief operating 7 periods.

9 It is another object of the invention to provide a reactor system for reforming hydrocarbons under 11 conditions of low sulfur which permits longer 12 operating periods.

14 After a detailed analysis and investigation of the coke plugs of low-sulfur reactor systems, it was 16 surprisingly found that they contained particles and 17 droplets of metal; the droplets ranging in size of up 18 to a few microns. This observation led to the 19 startling realization that there are new, profoundly serious, problems which were not of concern with 21 conventional reforming techniques where process 22 sulfur and water levels were significantly higher.
23 More particularly, it was discovered that problems 24 existed which threatened the effective and economic operability of the systems, and the physical 26 integrity of the equipment as well. It was also 1 discovered that these problems emerged-due to the 2 low-sulfur conditions, and to some extent, the low 3 levels of water.

For the last forty years, catalytic reforming 6 reactor systems have been constructed of ordinary 7 mild steel (e.g., 2; Cr 1 Mo). Over time, experience 8 has shown that the systems can operate successfully 9 for about twenty years without significant loss of physical strength. However, the discovery of the 11 metal particles and droplets in the coke plugs 12 eventually lead to an investigation of the phvsical 13 characteristics of the reactor system. Quite 14 surprisingly, conditions were discovered which are symptomatic of a potentially severe physical 16 degradation.of the entire reactor system, including 17 the furnace tubes, piping, reactor walls and other 18 environments such as catalysts that contain iron and 19 metal screens in the reactors. Ultimately, it was discovered that this problem is associated with the 21 excessive carburization of the steel which causes an 22 embrittlement of the steel due to injection of 23 process carbon into the metal. Conceivably, a 24 catastrophic physical failure of the reactor system could result.

1 With conventional reforming techniques 2 carburization simply was not a problem or concern;
3 nor was it expected to be in contemporary low-4 sulfur/low-water systems. And, it was assumed that conventional process equipment could be used.

6 Apparently, however, the sulfur present in 7 conventional systems effectively inhibits 8 carburization. Somehow in conventional processes the 9 process sulfur interferes with the carburization reaction. But with extremely low-sulfur systems, 11 this inherent protection no longer exists.

13 Figure lA is a photomicrograph of a portion of 14 the inside (process side) of a mild steel furnace tube from a commercial reformer. The tube had been 16 exposed to conventional reforming conditions for 17 about 19 years. This photograph shows that the 18 surface of the tube has remained essentially 19 unaltered with the texture of the tube remaining normal after long exposure to hydrocarbons at high 21 temperatures (the black portion of the photograph is 22 background).

24 Figure 1B is a photomicrograph of a portion of a mild steel coupon sample which was placed inside a 26 reactor of a low-sulfur/low-water demonstration plant 1 for only 13 weeks. The photograph shows the eroded 2 surface of the sample (contrasted against a black 3 background) from which metal dusting has occurred.
4 The dark grey-like veins indicate the environmental carburization of the steel, which was carburized and 6 embrittled more than 1 mm in depth.

8 Of course, the problems associated with 9 carburization only begin with carburization of the physical system. The carburization of the steel 11 walls leads to "metal dusting"; a release of 12 catalytically active particles and melt droplets of 13 metal due to erosion of the metal.

The active metal particulates provide additional 16 sites for coke formation in the system. While 17 catalyst deactivation from coking is generally a 18 problem which must be addressed in reforming, this 19 new significant source of coke formation leads to a new problem of coke plugs which excessively 21 aggravates the problem. In fact, it was found that 22 the mobile active metal particulates and coke 23 particles metastasize coking generally throughout the 24 system. The active metal particulates actually induce coke formation on themselves and anywhere that 26 the particles accumulate in the system resulting in 1 coke plugs anu hot regions of exothermic 2 demethanation reactions. As a result, an 3 unmanageable and premature coke-plugging of the 4 reactor system occurs which can lead to a system shut-down within weeks of start-up. Use of the 6 process and reactor system of the present invention, 7 however, overcomes these problems.

9 Therefore, a first aspect of the invention relates to a method for reforming hydrocarbons 11 comprising contacting the hydrocarbons with a 12 reforming catalyst, preferably a large-pore zeolite 13 catalyst including an alkali or alkaline earth metal 14 and charged with one or more Group VIII metals, in a reactor system having a resistance to carburization = 16 and metal dusting which is an improvement over 17 conventional mild steel reactor systems under 18 conditions of low sulfur and often low sulfur and low 19 water, and upon reforming the resistance being such that embrittlement from carburization will be less 21 than about 2.5 mm/year, preferably less than 1.5 22 mm/year, more preferably less than 1 mm/year, and 23 most preferably less than 0.1 mm/year. Preventing 24 embrittlement to such an extent will significantly reduce metal c:usting and coking in the reactor WO 92/15653 PCT/L'S92/01856 1 system, and permits operation for longer periods of 2 time.

4 And, another aspect of the invention relates to a reactor system including means for providing a 6 resistance to carburization and metal dusting which 7 is an improvement over conventional mild steel 8 systems in a method for reforming hydrocarbons using 9 a reforming catalyst such as a large-pore zeolite catalyst including an alkaline earth metal and 11 charged with'one or more Group VIII metals under 12 conditions of low sulfur, the resistance being such 13 that embrittlement will be less than about 2.5 14 mm/year, preferably less than 1.5 mm/ye&r, more preferably less than 1 mm/year, and most preferably 16 less than 0.1 mm/year.

18 Thus, among other factors, the present invention 19 is based on the discovery that in low-sulfur, and low-sulfur and low-water reforming processes there 21 exist significant carburization, metal dusting and 22 coking problems, which problems do not exist to any 23 significant extent in conventional reforming 24 processes where higher levels of sulfur are present.
This discovery has led to intensive work and 26 development of solutions to the problems, which WO 92/15653 { PC.'T/US92/01856 1 solutions are novel to low-sulfur reforming and are 2 directed to the identification and selection of 3 resistant materials for low-sulfur reforming systems, 4 ways to effectively utilize and apply the resistant materials, additives (other than sulfur) for reducing 6 carburization, metal dusting and coking, various 7 process modifications and configurations, and 8 combinations thereof, which effectively address the 9 problems.

11 More particularly, the discovery has led to the 12 search for, identification of, and selection of 13 resistant materials for low-sulfur reforming systems, 14 preferably the reactor walls, furnace tubes and screens thereof, which were previously unnecessary in 16 conventional reforming systems such as certain alloy 17 and stainless steels, aluminized and chromized 18 materials, and certain ceramic materials. Also, it 19 was discovered that other specific materials, applied as a plating, cladding, paint, etc., can be 21 effectively resistant. These materials include 22 copper, tin, arsenic, antimony, brass, lead bismuth 23 chromium, intermetallic compounds thereof, and alloys 24 thereof, as well as silica and silicon based coatings. In one preferred embodiment of the 1 invention there is provided a novel and resistant tin-2 containing paint.

4 Furthermore, the discovery led to the development of certain additives, hereinafter referred to as 6 anticarburizing and anticoking agents, which out of 7 necessity are essentially sulfur free, preferably 8 completely sulfur free, which are novel to reforming.
9 Such additives include organo-tin compounds, organo-antimony compounds, organo-bismuth compounds, organo-11 arsenic compounds and organo-lead compounds.

13 Also, the problems associated with low-sulfur 14 reforming has lead to the development of certain process modifications and configurations previously unnecessary 16 in conventional reforming. These include certain 17 temperature control techniques, the use of superheated' 18 hydrogen between reactors, more frequent catalyst 19 regenerations, the use of staged heaters and tubes, the use of staged temperature zones, the use of superheated 21 raw materials, and the use of larger tube diameters 22 and/or higher tube velocities.

24 According to one aspect of the invention, there is provided an improved method for catalytically reforming a 26 hydrocarbon, comprising the step of contacting, under 27 conditions of low sulfur, a sulfur-sensitive zeolite 28 reforming catalyst with a hydrocarbon in a reactor system 29 having a plurality of furnace tubes, wherein a portion of ~. .~i a 1 said reactor system has F-i resistarn.ue t_o carburization and 2 metal dusting under low sulfua- ref;,Drml.ng conditions at 3 least as great as that of: st.ainles:_a steel.

According to another aspect o,E the invention, there 6 is provided a method for t.i7e <:~ata'.,yFti.c reforming of a 7 hydrocarbon stream to produce an aromatic compound using 8 a reactor system, having a sulfur--.~.~ens i.tive reforming 9 catalyst charged witta at least one Group VIII metal, over a prolonged pex:-ic_>d of: oper-~~iti.on w:it:hout significant coke-11 plugging, comprising the steps of:

12 providing a low sl.zlfur hyydroc.:~3-(rbc~n.--c;ontain.ing stream 13 prepared by reducing tile Sulf:ur i:~c:>nte.nt of said 14 hydrocarbon-containing streani to :1-(.:~ss than 50 ppx) sulfur;
providing a reforming r..E.~actor system of improved 16 resistance to carburization and met:::al dusting upon 17 reforming sai(I hydrocarbon-c:ontairi:.:i.ny sta:-earn, said 18 reactor systern having at ieast one furnace to heat said 19 hydrocarbon-containirrg stream to eatal.ytic reforming temperatures, said furnace having, in contact with said 21 hydrocarbon-containi.ng stx-etIlTi, a pJ.ux.ality of furnace 22 tubes having a resistance t.c:> ca:rbe_ir;i.zati.on and metal 23 dusting at least as great as t:hat of ~,4 7 stainless steel;
24 and passing said hydroca.rbon--c_ontaini-ng stream thr-ough 26 said reactor system to contact said hydrocarbon-27 cont.ainiri.g streatrI wi.th 5a i..d _r-eforrri.,ng catalyst tc) produce 28 an aromatic.

1.:31:) 1 According to a further aspect of' the invention, 2 there is provided a method for imp..ovi.ng the 3 carburization resistance of at lea..at ~j portion of an 4 apparatus for hydrocarbori c.~or7.ver.sion, comprising the steps of:

6 applying a reducible, r_i:r].-cOnt_:air,~ing paint t:o a 7 portion of an apparatus for hydroc.,a.rbc,n conversion; and 8 heating said reducibl.e paint under reducing 9 condit.ions to form a protec.t::. J..-ve layer wiiich provides improved carburization resistance.

12 According to another rIspect ()f the invent:ion., there 13 is provided a catalytic reforming z-eac:tor system for 14 catalytically reformi.ng hydr_c>carbor:s under Low sulfur conditioris, comprising:

16 a furnace having a pl.ura:lity c,f:: turnace t.ubes;

17 a reforming reactor ilavi ng a catalyst bed containing 18 a sulfur-sensitive cata:Lyst, and 19 wherein a portion of sa.id cat~:~lytic reforming reactor system, which cont:.ac-ts a low-sulfur stream 21 containing a hydroca.rbon, has an improved resistance to 22 carburizat.ion.
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~lU53 0 5 1 BRIEF DtSCRIPTION OF THE DRAWING

2 As noted above, Figure 1A is a photomicrograph 3 of a portion of the inside (process side) of a mild 4 steel furnace tube from a commercial reformer which had been in use about 19 years; and as also noted 6 above, 8 Figure 1B is a photomicrograph of a portion of a 9 mild steel coupon sample which was placed inside a reactor of a low-sulfur/low-water demonstration plant 11 for only 13 weeks.

13 Figure 2 is an illustration of a suitable 14 reforming reactor system for use in the present invention.

18 The metallurgical terms used herein are to be 19 given their common metallurgical meanings as set forth in THE METALS HANDBOOK of the American Society 21 of Metals. For example, "carbon steels" are those 22 steels having no specified minimum quantity for any 23 alloying element (other than the commonly accepted 24 amounts of manganese, silicon and copper) and containing only an incidental amount of any element 26 other than carbon, silicon, manganese, copper, sulfur 1 and phosphorus. "Mild steels" are those carbon 2 steels with a maximum of about 0.25% carbon. Alloy 3 steels are those steels containing specified 4 quantities of alloying elements (other than carbon and the commonly accepted amounts of manganese, 6 copper, silicon, sulfur and phosphorus) within the 7 limits recognized for constructional alloy steels, 8 added to effect changes in mechanical or physical 9 properties. Alloy steels will contain less than 10%
chromium. Stainless steels are any of several steels 11 containing at least 10, preferably 12 to 30%, 12 chromium as the principal alloying element.

14 Generally, therefore, one focus of the invention is to provide an improved method for reforming 16 hydrocarbons using a reforming catalyst, particularly 17 a large pore zeolite catalyst including an alkali or 18 alkaline earth metal and charged with one or more 19 Group VIII metals which is sulfur sensitive, under conditions of low sulfur. Such a process, of course, 21 must demonstrate better resistance to carburization 22 than conventional low-sulfur reforming techniques.

24 One solution for the problem addressed by the present invention is to provide a novel reactor 26 system which can include one or more various means WO 92/15653 PCT/L'S92/01856 1 for improving resistance to carburization and metal 2 dusting during reforming using a reforming catalyst 3 such as the aforementioned sulfur sensitive large-4 pore zeolite catalyst under conditions of low sulfur.

6 By "reactor system" as used herein there is 7 intended at least one reforming reactor and its 8 corresponding furnace means and piping. Figure 2 9 illustrates a typical reforming reactor system suitable for practice of the present invention. It 11 can include a plurality of reforming reactors (10), 12 (20) and (30). Each reactor contains a catalyst bed.
13 The system also includes a plurality of furnaces 14 (11), (21) and (31); heat exchanger (12); and separator (13).

17 Through research associated with the present 18 invention, it was discovered that the aforementioned 19 problems with low-sulfur reforming can be effectively addressed by a selection of an appropriate reactor 21 system material for contact with the hydrocarbons 22 during processing. Typically, reforming reactor 23 systems have been constructed of mild steels, or 24 alloy steels such as typical chromium steels, with insignificant carburization and dusting. For 26 example, under conditions of standard reforming, 2;

1 Cr furnace tubes can last twenty years. However, it 2 was found that these steels are unsuitable under low-3 sulfur reforming conditions. They rapidly become 4 embrittled by carburization within about one year. For example, it was found that 2{ Cr 1 Mo steel carburized 6 and embrittled more than 1 mm/year.

8 Furthermore, it was found that materials considered 9 under standard metallurgical practice to be resistant to coking and carburization are not necessarily resistant 11 under low-sulfur reforming conditions. For example, 12 nickel-rich alloys such as Incoloy 800T"" and 825; Inconel 13 600T""; Marcel and Haynes 230T"", are unacceptable as they 14 exhibit excessive coking and dusting.

16 However, 300 series stainless steels, preferably 17 304, 316, 321 and 347, are acceptable as materials for at 18 least portions of the reactor system according to the 19 present invention which contact the hydrocarbons. They have been found to have a resistance to carburization 21 greater than mild steels and nickel-rich alloys.

23 Initially it was believed that aluminized materials 24 such as those sold by Alon Corporation ~~~~3 05 1 ("Alonized Steels") would not provide adequate 2 protection against carburization in the reforming 3 reactor system and process of the invention. It has 4 sincebeen discovered, however, that the application of thin aluminum or alumina films to metal surfaces 6 of the reforming reactor system, or simply the use of 7 Alonized Steels during construction, can provide 8 surfaces which are sufficiently resistant to 9 carburization and metal dusting under the low-sulfur reforming conditions. However, such materials are 11 relatively expensive, and while resistant to 12 carburization and metal dusting, tend to crack, and 13 show substantial reductions in tensile strengths.

14 Cracks expose the underlying base metal rendering it susceptible to carburization and metal dusting under 16 low sulfur reforming conditions.

~=, 18 While aluminized materials have been used to 19 prevent carburization in ethylene steam cracking processes, such processes are operated at 21 significantly higher temperatures than reforming;
22 temperatures where carburization would be expected.
23 Carburization and metal dusting simply have not been 24 problems in prior reforming processes.

WO 92/15653 PC'T/US92/01856 1 Therefore, another solution to the problems of 2 carburization and metal dusting involves the 3 application of thin aluminum or alumina films on, or 4 the use of aluminized materials as, at least a portion of the metal surfaces in the reactor system.
6 In fact, the metal surfaces particularly susceptible 7 to carburization and metal dusting can be provided in 8 that manner. Such metal surfaces include but are not 9 limited to, the reactor walls, furnace tubes, and furnace liners.

12 When applying an aluminum or alumina film, it is 13 preferable that the film have a thermal expansivity 14 that is similar to that of the metal surface to which it is applied (such as a mild steel) in order to 16 withstand thermal shocks and repeated temperature 17 cycling which occur during reforming. This prevents 18 cracking or spalling of the film which could expose 19 the under2ying metal surface to the carburization inducing hydrocarbon environment.

22 Additionally, the film should have a thermal 23 conductivity similar to that of, or exceeding, those 24 of metals conventionally used in the construction of reforming reactor systems. Furthermore, the aluminum 26 or alumina film should not degrade in the reforming 1 environment, or in the oxidizing environment 2 associated with catalyst regeneration, nor should it 3 result in the degradation of the hydrocarbons in the 4 reactor system.

6 Suitable methods for applying aluminum or 7 alumina films to metal surfaces such as mild steels 8 include well known deposition techniques. Preferred 9 processes include powder and vapor diffusion processes such as the "Alonizing" process, which has 11 been commercialized by Alon Processing, Inc., 12 Terrytown, Pa.

14 Essentially, "Alonizing" is a high temperature diffusion process which alloys aluminum into the 16 surface of a treated metal, such as e.g., a 17 commercial grade mild steel. Zn this process, the 18 metal (e.g., a mild steel) is positioned in a retort 19 and surrounded with a mixture of blended aluminum powders. The retort is then hermetically sealed and 21 placed in an atmosphere-controlled furnace. At 22 elevated temperatures, the aluminum deeply diffuses 23 into the treated metal resulting in an alloy. After = 24 furnace cooling, the substrate is taken out of the retort and excess powder is removed. Straightening, 26 trimming, beveling and other secondary operations can 1 then be performed as required. This process can 2 render the treated ("alonized") metal resistant to 3 carburization and metal dusting under low-sulfur 4 reforming conditions according to the invention.

6 Thin chromium or chromium oxide films can also 7 be applied to metal surfaces of the reactor system to 8 render the surfaces resistant to carburization and 9 metal dusting under low-sulfur reforming conditions.
Like the use of alumina and aluminum films, and 11 aluminized materials, chromium or chromium oxide 12 coated metal surfaces have not been used to address 13 carburization problems under low-sulfur reforming 14 conditions.

16 The chromium or chromium oxide can also be 17 applied to carburization and metal dusting 18 susceptible metal surfaces such as the reactor walls, 19 furnace liners, and furnace tubes. However, any surface in the system which would show signs of 21 carburization and metal dusting under low-sulfur 22 reforming conditions would benefit from the 23 application of a thin chromium or chromium oxide 24 film.

WO 92/15653 PCI"/US92/01856 2 105 3{3 1 When appiying the chromium or chromium oxide 2 film, it is preferable that the chromium or chromium 3 oxide film have a thermal expansivity similar to that 4 of the metal to which it is applied. Additionally, the chromium or chromium oxide film should be able to 6 withstand thermal shocks and repeated temperature 7 cycling which are common during reforming. This 8 avoids cracking or spalling of the chromium or 9 chromium oxide film which could potentially expose the underlying metal surfaces to carburization 11 inducing environments. Furthermore, the chromium or 12 chromium oxide film should have a thermal 13 conductivity similar to or exceeding those materials 14 conventionally used in reforming reactor systems (in particular mild steels) in order to maintain 16 efficient heat transfer. The chromium or chromium 17 oxide film also should not degrade in the reforming 18 environment or in the oxidizing environment 19 associated with catalyst regeneration, nor should it induce degradation of the hydrocarbons in the reactor 21 system.

23 Suitable methods for applying chromium or 24 chromium oxide films to surfaces such as e.g., mild steels, include well known deposition techniques.
26 Preferred processes include powder-pack and vapor 2105305) 1 diffusion processes such as the "chromizing" process, 2 which is commercialized by Alloy Surfaces, Inc., of 3 Wilmington, Delaware.

The "chromizing" process is essentially a vapor 6 diffusion process for application of chromium to a 7 metal surface (similar to the above described 8 "Alonizing process"). The process involves 9 contacting the metal to be coated with a powder of chromium, followed by a thermal diffusion step.

11 This, in effect, creates an alloy of the chromium 12 with the treated metal and renders the surface 13 extremely resistant to carburization and metal 14 dusting under low-sulfur reforming conditions.

16 In some areas of the reactor systems, localized 17 temperatures can become excessively high during 18 reforming (e.g., 900-1250 F). This is particularly 19 the case in furnace tubes, and in catalyst beds where exothermic demethanation reactions occur within 21 normally occurring coke balls causing localized hot 22 regions. While still preferred to mild steels and 23 nickel-rich alloys, the 300 series stainless steels 24 do exhibit some coking and dusting at around 1000 F.
Thus, while useful, the 300 series stainless steels Attorney's Docket No. 005950-314 WO 92/15653 PCI'/US92/01856 ~lU~3 0 5 1 are not the most preferred material for use in the 2 present invention.

4 Chromium-rich stainless steels such as 446 and 430 are even more resistant to carburization than 300 6 series stainless steels. However, these steels are 7 not as desirable for heat resisting properties (they 8 tend to become brittle).

Resistant materials which are preferred over the 11 300 series stainless steels for use in the present 12 invention include copper, tin, arsenic, antimony, 13 bismuth, chromium and brass, and intermetallic 14 compounds and alloys thereof (e.g., Cu-Sn alloys, Cu-sb alloys, stannides, antimonides, bismuthides, 16 etc.). Steels and even nickel-rich alloys containing 17 these metals can also show reduced carburization. In' 18 a preferred embodiment, these materials are provided 19 as a plating, cladding, paint (e.g., oxide paints) or other coating to a base constructionmaterial. This' 21 is particularly advantageous since conventional 22 construction materials such as mild steels can still 23 be used with only the surface contacting the 24 hydrocarbons being treated. Of these, tin is especially preferred as it reacts with the surface to 26 provide a coating having excellent carburization WO 92/15653 PCT/L'S92/01856 1 resistance at higher temperatures, and which resists 2 peeling and flaking of the coating. Also, it is 3 believed that a tin containing layer can be as thin 4 as 1/10 micron and still prevent carburization.

6 Where practical, it is preferred that the 7 resistant materials be applied in a paint-like 8 formulation (hereinafter "paint") to a new or 9 existing reactor system. Such a paint can be sprayed, brushed, pigged, etc. on reactor system 11 surfaces such as mild steels or stainless steels. It 12 is most preferred that such a paint be a 13 decomposable, reactive, tin-containing paint which 14 reduces to a reactive tin and forms metallic stannides (e.g., iron stannides and nickel/iron 16 stannides) upon heating in a reducing atmosphere.

18 It is most preferred that the aforementioned 19 paint contain at least four components (or their functional equivalents); (i) a hydrogen decomposable 21 tin compound, (ii) a solvent system, (iii) a finely 22 divided tin metal and (iv) tin oxide as a reducible 23 sponge/dispersing/binding agent. The paint should 24 contain'finely divided solids to minimize settling, and should not contain non-reactive materials which ?10b3U5 1 will prevent reaction of reactive tin with surfaces 2 of the reactor system.

4 As the hydrogen decomposable tin compound, tin octanoate is particularly useful. Commercial 6 formulations of this compound itself are available 7 and will partially dry to an almost chewing-gum-like 8 layer on a steel surface; a layer which will not 9 crack and/or split. This property is necessary for any coating composition used in this context because 11 it is conceivable that the coated material will be 12 stored for months prior to treatment with hydrogen.
13 Also, if parts are coated prior to assembly they must 14 be resistant to chipping during construction. As noted above, tin octanoate is available commercially. 16 It is reasonably priced, and will decompose smoothly 17 to a reactive tin layer which forms iron stannide in 18 hydrogen at temperatures as low as 600 F.

Tin octanoate should not be used alone in a 21 paint, however. It is not sufficiently viscous.
22 Even when the solvent is evaporated therefrom, the 23 remaining liquid will drip and run on the coated 24 surface. In practice, for example, if such were used to coat a horizontal furnace tube, it would pool at 26 the bottom of the tube.

1 Component (iv), the tin oxide 2 sponge/dispersing/binding agent, is a porous tin-3 containing compound which can sponge-up an organo-4 metallic tin compound, yet still be reduced to active tin in the reducing atmosphere. In addition, tin 6 oxide can be processed through a colloid mill to 7 produce very fine particles which resist rapid 8 settling. The addition of tin oxide will provide a 9 paint which becomes dry to the touch, and resists running.

12 Unlike typical paint thickeners, component (iv) 13 is selected such that it becomes a reactive part of 14 the coating when reduced. It is not inert like formed silica; a typical paint thickener which would 16 leave an unreactive surface coating after treatment.

18 Finely divided tin metal, component (iii), is 19 added to insure that metallic tin is available to react with the surface to be coated at as low a 21 temperature as possible, even in a non-reducing 22 atmosphere. The particle size of the tin is 23 preferably one to five microns which allows excellent 24 coverage of the surface to be coated with tin metal.
Non-reducing conditions can occur during drying of 26 the paint and welding of pipe joints. The presence WO 92/15653 PCr/US92/01856 1 of metallic tin ensures that even when part of the 2 coating is not completely reduced, tin metal will be 3 present to react and form the desired stannide layer.

The solvent should be non-toxic, and effective 6 for rendering the paint sprayable and spreadable when 7 desired. It should also evaporate quickly and have 8 compatible solvent properties for the hydrogen 9 decomposable tin compound. Isopropyl alcohol is most preferred, while hexane and pentane can be useful, if 11 necessary. Acetone, however, tends to precipitate 12 organic tin compounds.

14 In one embodiment, there can be used a tin paint of 20 percent Tin Ten-Cem (stannous octanoate in 16 octanoic acid), stannic oxide, tin metal powder and 17 isopropyl alcohol.

19 The tin paint can be applied in many ways. For example, furnace tubes of the reactor system can be 21 painted individually or as modules. A reforming 22 reactor system according to the present invention can 23 contain various numbers of furnace tube modules 24 (e.g., about 24 furnace tube modules) of suitable width, length and height (e.g., about 10 feet long, 26 about 4 feet wide, and about 40 feet in height).

1 Typically, each module will include two headers of 2 suitable diameter, preferably about 2 feet in 3 diameter, which are connected by about four to ten u-4 tubes of suitable length (e.g., about 42 feet long).
Therefore, the total surface area to be painted in 6 the modules can vary widely; for example, in one 7 embodiment it can be about 16,500 ft2.

9 Painting modules rather than the tubes individually can be advantageous in at least four 11 respects; (i) painting modules rather than individual 12 tubes should avoid heat destruction of the tin paint 13 as the components of the modules are usually heat 14 treated at extremely elevated temperatures during production; (ii) painting modules will likely be 16 quicker and less expensive than painting tubes 17 individually; (iii) painting modules should be more 18 efficient during production scheduling; and (iv) 19 painting of the modules should enable painting of welds.

22 However, painting the modules may not enable the 23 tubes to be as completely coated with paint as if the 24 tubes were painted individually. If coating is insufficient, the tubes can be coated individually.

1 It is prFferable that the paint be sprayed into 2 the tubes and headers. Sufficient paint should be 3 applied to fully coat the tubes and headers. After a 4 module is sprayed, it should be left to dry for about 24 hours followed by application of a slow stream of 6 heated nitrogen (e.g., about 150 F for about 24 7 hours). Thereafter, it is preferable that a second 8 coat of paint be applied and also dried by the 9 procedure described above. After the paint has been applied, the modules should preferably be kept under 11 a slight nitrogen pressure and should not be exposed 12 to temperatures exceeding about 200 F prior to 13 installation, nor should they be exposed to water 14 except during hydrotesting.

16 Iron bearing reactive paints are also useful in 17 the present invention. Such an iron bearing reactive 18 paint will preferably contain various tin compounds 19 to which iron has been added in amounts up to one third Fe/Sn by weight.

22 The addition of iron can, for example, be in the 23 form of Fe203. The addition of iron to a tin 24 containing paint should afford noteworthy advantages;
in particular: (i) it should facilitate the 26 reaction of the paint to form iron stannides thereby 1 acting as a flux; (ii) it should dilute the nickel 2 concentration in the stannide layer thereby providing 3 better protection against coking; and (iii) it 4 should result in a paint which affords the anti-coking protection of iron stannides even if the 6 underlying surface does not react well.

8 Yet another means for preventing carburization, 9 coking, and metal dusting in the low-sulfur reactor system comprises the application of a metal coating 11 or cladding to chromium rich steels contained in the 12 reactor system. These metal coatings or claddings 13 may be comprised of tin, antimony, bismuth or 14 arsenic. Tin is especially preferred. These coatings or claddings may be applied by methods 16 including electroplating, vapor depositing, and 17 soaking of the chromium rich steel in a molten metal 18 bath.

It has been found that in reforming reactor 21 systems where carburization, coking, and metal 22 dusting are particularly problematic that the coating 23 of the chromium-rich, nickel-containing steels with a 24 layer of tin in effect creates a double protective layer. There results an inner chromium rich layer 26 which is resistant to carburization, coking, and - 32 - +
1 metal dusting and an outer tin layer which is also 2 resistant to carburization, coking and metal dusting.
3 This occurs because when the tin coated chromium rich 4 steel is exposed to typical reforming temperatures, such as about 1200 F, it reacts with the steel to 6 form iron nickel stannides. Thereby, the nickel is 7 preferentially leached from the surface of the steel 8 leaving behind a layer of chromium rich steel. In 9 some instances, it may be desirable to remove the iron nickel stannide layer from the stainless steel 11 to expose the-chromium rich steel layer.

13 For example, it was found that when a tin 14 cladding was applied to a 304 grade stainless steel and heated at about 1200 F there resulted a chromium 16 rich steel layer containing about 17% chromium and 17 substantially no nickel, comparable to 430 grade 18 stainless steel.

When applying the tin metal coating or cladding 21 to the chromium rich steel, it may be desirable to 22 vary the thickness of the metal coating or cladding 23 to achieve the desired resistance against 24 carburization, coking, and metal dusting. This can be done by, e.g., adjusting the amount of time the 26 chromium rich steel is soaked in a molten tin bath.

1 This will also affect the thickness of the resulting 2 chromium rich steel layer. It may also be desirable 3 to vary the operating temperature, or to vary the 4 composition of the chromium rich steel which is coated which in order to control the chromium 6 concentration in the chromium rich steel layer 7 produced.

9 It has additionally been found that tin-coated steels can be further protected from carburiza=tion, 11 metal dusting, and coking by a post-treatment process 12 which involves application of a thin oxide coating, 13 preferably a chromium oxide, such as CrZO3. This 14 coating will be thin, as thin as a few m.

Application of such a chromium oxide will protect 16 aluminum as well as tin coated steels, such as 17 Alonized steels, under low-sulfur reforming 18 conditions.

The chromium oxide layer can be applied by 21 various methods including: application of a chromate 22 or dichromate paint followed by a reduction process;
23 vapor treatment with an organo-chromium compound; or 24 application of a chromium metal plating followed by oxidation of the resulting chromium plated steel.

- , , WO 92/15653 PC'T/US92/01856 1 Examination of tin-electroplated steels which 2 have been subiected to low-sulfur reforming 3 conditions for a substantial period of time has shown 4 that when a chromium oxide layer is produced on the surface of the stannide layer or under the stannide 6 layer, the chromium oxide layer does not cause 7 deterioration of the stannide layer, but appears to 8 render the steel further resistant to carburization, 9 coking and metal dusting. Accordingly, application of a chromium oxide layer to either tin or aluminum 11 coated steels will result in steels which are further 12 resistant to carburization and coking under the low-13 sulfur reforming conditions. This post-treatment 14 process has particular applications for treating tin or aluminum coated steels which, after prolonged 16 exposure to low-sulfur reforming conditions, are in 17 need of repair.

19 It has further been found that aluminized, e.g., "Alonized" steels which are resistant to 21 carburization under the present reforming conditions 22 of low sulfur can be rendered further resistant by 23 post-treatment of the aluminum coated steel with a 24 coating of tin. This results in a steel which is more carburization resistant since there are 26 cumulative effects of carburization resistance 2,105305 1 obtained from both the aluminum coating and the tin 2 coating. This post-treatment affords an additional 3 benefit in that it will mend any defects or cracks in 4 the aluminum, e.g., Alonized, coating. Also, such a post-treatment should result in a lower cost since a 6 thinner aluminum coating can be applied to the steel 7 surface which is to be post-treated with the tin 8 coating. Additionally, this post-treatment will 9 protect the underlying steel layer exposed by bending of aluminized steels, which can introduce cracks in 11 the aluminum layer, and expose the steel to 12 carburization induced under reforming conditions.
13 Also, this post-treatment process can prevent coke 14 formation on the treated steel surfaces and also prevent coke formation that occurs on the bottom of 16 cracks which appear on steels which have been 17 aluminized, but not additionally coated with tin.

19 Samples of Alonized Steels painted on one side with tin, were found to show a deposit of black coke 21 only on the untreated side under low-sulfur reforming 22 conditions. The coke that forms on an aluminized 23 surface is a benign coke resulting from cracking on 24 acidic alumina sites. It is incapable of inducing additional coke deposition. Accordingly, a post-26 treatment application of a tin coating to aluminized WO 92/15653 PC'T/1JS92/01856 1 steels can provide further minimization of the 2 problems of carburization, coking, and metal dusting, 3 in reactor systems operating under reforming 4 conditions according to the invention.

6 While not wishing to be bound by theory, it is 7 believed that the suitability of various materials 8 for the present invention can be selected and 9 classified according to their responses to carburizing atmospheres. For example, iron, cobalt, 11 and nickel form relatively unstable carbides which 12 will subsequently carburize, coke and dust. Elements 13 such as chromium, niobium, vanadium, tungsten, 14 molybdenum, tantalum, and zirconium, will-form stable carbides which are more resistant to carburization 16 coking and dusting. Elements such as tin, antimony 17 and bismuth do not form carbides or coke. And, these 18 compounds can form stable compounds with many metals 19 such as iron, nickel and copper under reforming conditions. Stannides, antimonides and bismuthides, 21 and compounds of lead, mercury, arsenic, germanium, 22 indium, tellurium, selenium, thallium, sulfur and 23 oxygen are also resistant. A final category of 24 materials include elements such as silver, copper, gold, platinum and refractory oxides such as silica 26 and alumina. These materials are resistant and do 1 not form carbides, or react with other metals in a 2 carburizing environment under reforming conditions.

4 As discussed above, the selection of appropriate metals which are resistant to carburization and metal 6 dusting, and their use as coating materials for metal 7 surfaces in the reactor system is one means for 8 preventing the carburization and metal dusting 9 problem. However, carburization and metal dusting can be prevalent in a wide variety of metals; and 11 carburization resistant metals can be more costly or 12 exotic than conventional materials (e.g., mild 13 steels) used in the construction of reforming reactor 14 systems. Accordingly, it may be desirable in the reactor system of the invention to use ceramic 16 materials which do not form carbides at typical 17 reforming conditions, and thus are not susceptible to' 18 carburization, for at least a portion of the metal 19 surfaces in the reactor system. For example, at least a portion of the furnace tubes, or furnace 21 liners or both may be constructed of ceramic 22 materials.

24 In choosing the ceramic materials for use in the present invention, it ispreferable that the ceramic 26 material have thermal conductivities about that or 1 exceeding those of materials conventionally used in 2 the construction of reforming reactor systems.

3 Additionally, the ceramic materials should have 4 sufficient structural strengths at the temperatures which occur within the reforming reactor system.

6 Further, the ceramic materials should be able to 7 withstand thermal shocks and repeated temperature 8 cycling which occur during operation of the reactor 9 system. When the ceramic materials are used for constructing the furnace liners, the ceramic 11 materials should have thermal expansivities about 12 that of the metal outer surfaces with which the liner 13 is in intimate contact. This avoids undue stress at 14 the juncture during temperature cycling that occurs during start-up and shut-down. Additionally, the 16 ceramic surface should not be susceptible to 17 degradation in the hydrocarbon environment or in the 18 oxidizing environment which occurs during catalyst 19 regeneration. The selected ceramic material also should not promote the degradation of the 21 hydrocarbons in the reactor system.

23 Suitable ceramic materials include, but are not 24 restricted to, materials such as silicon carbides, silicon oxides, silicon nitrides and aluminum 26 nitrides. Of these, silicon carbides and silicon 1 nitrides are particularly preferred as they appear 2 capable of providing complete protection for the 3 reactor system under low-sulfur reforming conditions.

At least a portion of the metal surfaces in the 6 reactor system can also be coated with a silicon or 7 silica film. In particular, the metal surfaces which 8 can be coated include, but are not limited to the 9 reactor walls, furnace tubes, and furnace liners.
However, any metal surface in the reactor system, 11 which shows signs of carburization and metal dusting 12 under low-sulfur reforming conditions would benefit 13 from the application of a thin silicon or silica 14 film. =

16 Conventional methods can be used for applying 17 silicon or silica films to coat metal surfaces.

18 Silica or silicon can be applied by electroplating 19 and chemical vapor deposition of an alkoxysilane in a steam carrier gas. it is preferable that the silicon 21 or silica film have a thermal expansivity about that 22 of the metal surface which it coats. Additionally, 23 the silicon or silica film should be able to 24 withstand thermal shocks and repeated temperature cycling that occur during reforming. This avoids 26 cracking or spalling of the silicon or silica film, "N =, 1 and potential exposure of the underlying metal 2 surface to the carburization inducing hydrocarbon 3 environment. Also, the silica or silicon film should 4 have a thermal conductivity approximate to or exceeding that of metals conventionally used in 6 reforming reactor systems so as to maintain efficient 7 heat transfer. The silicon or silica film also 8 should not degrade in the reforming environment or in 9 the oxidizing environment associated with catalyst regeneration; nor should it cause degradation of the 11 hydrocarbons themselves.

13 Because different areas of the reactor system of 14 the invention (e.g., different areas in a furnace) can be exposed to a wide range of temperatures, the 16 material selection can be staged, such that those 17 materials providing better carburization resistances 18 are used in those areas of the system experiencing 19 the highest temperatures.

21 With regard to materials selection, it was 22 discovered that oxidized Group VIII metal surfaces 23 such as iron, nickel and cobalt are more active in 24 terms of coking and carburization than their unoxidized counterparts. For example, it was found 26 that an air roasted sample of 347 stainless steel was WO 92/15653 PCT/L'S92/01855 2105'N5 1 significantly more active than an unoxidized sample 2 of the same steel. This is believed to be due to a 3 re-reduction of oxidized steels which produces very 4 fine-grained iron and/or nickel metals. Such metals are especially active for carburization and coking.
6 Thus, it is desirable to avoid these materials as 7 much as possible during oxidative regeneration 8 processes, such as those typically used in catalytic 9 reforming. However, it has been found that an air roasted 300 series stainless steel coated with tin 11 can provide similar resistances to coking and 12 carburization as unroasted samples of the same tin 13 coated 300 series stainless steel.

Furthermore, it will be appreciated that 16 oxidation will be a problem in systems where sulfur 17 sensitivity of the catalyst is not-of concern, and 18 sulfur is used to passivate the metal surfaces. If 19 sulfur levels in such systems ever become insufficient, any metal sulfides which have formed on 21 metal surfaces would, after oxidation and reduction, 22 be reduced to fine-grained metal. This metal would 23 be highly reactive for coking and carburization.

24 Potentially, this can cause a catastrophic failure of the metallurgy, or a major coking event.

1 As noted above, excessively high temperatures 2 can occur in the catalyst beds when exothermic 3 demethanation reactions within cokeballs cause 4 localized hot regions. These hot spots also pose a problem in conventional reforming reactor systems (as 6 well as other areas of chemical and petrochemical 7 processing).

9 For example, the center pipe screens of reformers have been observed to locally waste away 11 and develop holes; ultimately resulting in catalyst 12 migration. In conventional reforming processes the 13 temperatures within cokeballs during formation and 14 burning are apparently high enough to overcome the ability of process sulfur to poison coking, 16 carburization, and dusting. The metal screens, 17 therefore, carburize and are more sensitive to 18 wasting by intergranular oxidation (a type of 19 corrosion) during regeneration. The screen openings .20 enlarge and holes develop.

22 Thus, the teachings of the present invention are 23 applicable to conventional reforming, as well as 24 other areas of chemical and petrochemical processing.
For example, the aforementioned platings, claddings 26 and coatings can be used in the preparation of center 1 pipe screens to avoid excessive hole development and 2 catalyst migration. In addition, the teachings can 3 be applied to any furnace tubes which are subjected 4 to carburization, coking and metal dusting, such as furnace tubes in coker furnaces.

7 In addition, since the techniques described 8 herein can be used to control carburization, coking, 9 and metal dusting at excessively high temperatures, they can be used in cracking furnaces operating at 11 from about 1400 to about 1700 F. For example, the 12 deterioration of steel occurring in cracking furnaces 13 operating at those temperatures can be controlled by 14 application of various metal coatings. These metal coatings can be applied by melting, electroplating, 16 and painting. Painting is particularly preferred.

18 For example, a coating of antimony applied to 19 iron bearing steels protects these steels from carburization, coking and metal dusting under the 21 described cracking conditions. In fact, an antimony 22 paint applied to iron bearing steels will provide 23 protection against carburization, coking, and metal 24 dusting at 1600 F.

1 A coating of bismuth applied to nickel rich 2 steel alloys (e.g., INCONEL 600) can protect those 3 steels against carburization, coking, and metal 4 dusting under cracking conditions. This has been demonstrated at temperatures of up to 16000F.

7 Bismuth coatings may also be applied to iron 8 bearing steels and provide protection against 9 carburization, metal dusting, and coking under cracking conditions. Also, a metal coating 11 comprising a combination of bismuth, antimony, and/or 12 tin can be used.

14 Looking again to low-sulfur reforming, other techniques can also be used to address the problem 16 discovered according to the present invention. They 17 can be used in conjunction with an appropriate 18 material selection for the reactor system, or they 19 can be used alone. Preferred from among the additional techniques is the addition of non-sulfur, 21 anti-carburizing and anti-coking agent(s) during the 22 reforming process. These agents can be added 23 continuously during processing and function to 24 interact with those surfaces of the reactor system which contact the hydrocarbons, or they may be 26 applied as a pretreatment to the reactor system.

1 While not wishing to bound by theory it is 2 believed that these agents interact with the surfaces 3 of the reactor system by decomposition and surface 4 attack to form iron and/or nickel intermetallic compounds, such as stannides, antimonides, 6 bismuthides, plumbides, arsenides, etc. Such 7 intermetallic compounds are resistant to 8 carburization, coking and dusting and can protect the 9 underlying metallurgy.

11 The intermetallic compounds are also believed to 12 be more stable than the metal sulfides which were 13 formed in systems where H:S was used to passivate the 14 metal. These compounds are not reduced by hydrogen as are metal sulfides. As a result, they are less 16 likely to leave the system than metal sulfides.

17 Therefore, the continuous addition of a carburization 18 inhibitor with the feed can be minimized.

Preferred non-sulfur anti-carburizing and anti-21 coking agents include organo-metallic compounds such 22 as organo-tin compounds, organo-antimony compounds, 23 organo-bismuth compounds, organo-arsenic compounds, 24 and organo-lead compounds. Suitable organo-lead compounds include tetraethyl and tetramethyl lead.

1 Organo-tin compounds such as tetrabutyl tin and 2 trimethyl tin hydride are especially preferred.

4 Additional specific organo-metallic compounds include bismuth neodecanoate, chromium octoate, 6 copper naphthenate, manganese carboxylate, palladium 7 neodecanoate, silver neodecanoate, 8 tetrabutylgermanium, tributylantimony, 9 triphenylantimony, triphenylarsine, and zirconium octoate.

12 How and where these agents are added to the 13 reactor system is not critical, and will primarily 14 depend on particular process design characteristics.
For example, they can be added continuously or 16 discontinuously with the feed.

18 However, adding the agents to the feed is not 19 preferred as they would tend to accumulate in the initial portions of the reactor system. This may not 21 provide adequate protection in the other areas of the 22 system.

24 It is preferred that the agents be provided as a coating prior to construction, prior to start-up, or 26 in-situ (i.e., in an existing system). If added in-1 situ, it should be done right after catalyst 2 regeneration. Very thin coatings can be applied.

3 For example, it is believed that when using organo-4 tin compounds, iron stannide coatings as thin as 0.1 micron can be effective.

7 A preferred method of coating the agents on an 8 existing or new reactor surface, or a new or existing 9 furnace tube is to decompose an organometallic compound in a hydrogen atmosphere at temperatures of 11 about 900 F. For organo-tin compounds, for example, 12 this produces reactive metallic tin on the tube 13 surface. At these temperatures the tin will further 14 react with the surface metal to passivate it.

16 Optimum coating temperatures will depend on the 17 particular organometallic compound, or the mixtures 18 of compounds if alloys are desired. Typically, an 19 excess of the organometallic coating agent can be pulsed into the tubes at a high hydrogen flow rate so 21 as to carry the coating agent throughout the system 22 in a mist. The flow rate can then be reduced to 23 permit the coating metal mist to coat and react with 24 the furnace tube or reactor surface. Alternatively, the compound can be introduced as a vapor which 1 decomposes and reacts with the hot walls of the tube 2 or reactor in a reducing atmosphere.

4 As discussed above, reforming reactor systems susceptible to carburization, metal dusting and 6 coking can be treated by application of a 7 decomposable coating containing a decomposable 8 organometallic tin compound to those areas of the 9 reactor system most susceptible to carburization.
Such an approach works particularly well in a 11 temperature controlled furnace.

13 However, such control is not always present.
14 There are "hot spots" which develop in the reactor system, particularly in the furnace tubes, where the 16 organometallic compound can decompose and form 17 deposits. Therefore, another aspect of the invention 18 is a process which avoids such deposition in 19 reforming reactor systems where temperatures are not closely controlled and exhibit areas of high 21 temperature hot spots.

23 Such a process involves preheating the entire 24 reactor system to a temperature of from 750 to 1150, preferably 900 to 1100, and most preferably about 26 1050 F, with a hot stream of hydrogen gas. After 1 preheating, a colder gas stream at a temperature of 2 400 to 800, preferably 500 to 700, and most 3 preferably about 550 F, containing a vaporized 4 organometallic tin compound and hydrogen gas is introduced into the preheated reactor system. This 6 gas mixture is introduced upstream and can provide a 7 decomposition "wave" which travels throughout the 8 entire reactor system.

Essentially this process works because the hot 11 hydrogen gas'produces a uniformly heated surface 12 which will decompose the colder organometallic gas as 13 it travels as a wave throughout the reactor system.
14 The colder gas containing the organometallic tin compound will decompose on the hot surface and coat 16 the surface. The organometallic tin vapor will 17 continue to move as a wave to treat the hotter 18 surfaces downstream in the reactor system. Thereby, 19 the entire reactor system can have a uniform coating of the organometallic tin compound. It may also be 21 desirable to conduct several of these hot-cold 22 temperature cycles to ensure that the entire reactor 23 system has been uniformly coated with the 24 organometallic tin compound.

1 In operation of the reforming reactor system 2 according to the present invention, naphtha will be 3 reformed to form aromatics. The naphtha feed is a 4 light hydrocarbon, preferably boiling in the range of about 70 F to 450 F, more preferably about 100 to 6 350 F. The naphtha feed will contain aliphatic or 7 paraffinic hydrocarbons. These aliphatics are 8 converted, at least in part, to aromatics in the 9 reforming reaction zone.

11 In the "low-sulfur" system of the invention, the 12 feed will preferably contain less than 100 ppb 13 sulfur, and more preferably, less than 50 ppb sulfur.
14 If necessary, a sulfur sorber unit can be employed to remove small excesses of sulfur.

17 Preferred reforming process conditions include a 18 temperature between 700 and 1050 F, more preferably 19 between 850 and 1025 F; and a pressure between 0 and 400 psig, more preferably between 15 and 150 psig; a 21 recycle hydrogen rate sufficient to yield a hydrogen 22 to hydrocarbon mole ratio for the feed to the 23 reforming reaction zone between 0.1 and 20, more 24 preferably between 0.5 and 10; and a liquid hourly space velocity for the hydrocarbon feed over the 1 reforming catalyst of between 0.1 and 10, more 2 preferably between 0.5 and 5.

4 To achieve the suitable reformer temperatures, it is often necessary to heat the furnace tubes to 6 high temperatures. These temperatures can often 7 range from 600 to 1800 F, usually from 850 and 8 1250 F, and more often from 900 and 1200 F.

As noted above, the problems of carburization, 11 coking and metal dusting in low-sulfur systems have 12 been found to associated with excessively high, 13 localized process temperatures of the reactor system, 14 and are particularly acute in the furnace tubes of the system where particularly high temperatures are 16 characteristic. In conventional reforming techniques 17 where high levels of sulfur are present, furnace tube 18 skin temperatures of up to 1175 F at end of run are 19 typical. Yet, excessive carburization, coking and metal dusting was not observed. In low-sulfur 21 systems, however, it has been discovered that 22 excessive and rapid carburization, coking and metal 23 dusting occurred with CrMo steels at temperatures 24 above 950 F, and stainless steels at temperatures above 1025 F.

WO 92/15653 PC.'T/L'S92/01856 (rl~e3c)U5 - 52 -1 Accordingly, another aspect of the invention is 2 to lower the temperatures of the metal surfaces 3 inside the furnace tubes, transfer-lines and/or 4 reactors of the reforming system below the aforementioned levels. For example, temperatures can 6 be monitored using thermocouples attached at various 7 locations in the reactor system. In the case of 8 furnace tubes, thermocouples can be attached to the 9 outer walls thereof, preferably at the hottest point of the furnace (usually near the furnace outlet).

11 When necessary, adjustments in process operation can 12 be made to maintain the temperatures at desired 13 levels.

There are other techniques for reducing exposure 16 of system surfaces to undesirably high temperatures 17 as well. For example, heat transfer areas can be 18 used with resistant (and usually more costly) tubing 19 in the final stage where temperatures are usually the highest.

22 In addition, superheated hydrogen can be added 23 between reactors of the reforming system. Also, a 24 larger catalyst charge can be used. And, the catalyst can be regenerated more frequently. In the 26 case of catalyst regeneration, it is best 2~0 5- 30 5 1 accomplished using a moving bed process where the 2 catalyst is withdrawn from the final bed, 3 regenerated, and charged to the first bed.

Carburization and metal dusting can also be 6 minimized in the low-sulfur reforming reactor system 7 of the invention by using certain other novel 8 equipment configurations and process conditions. For 9 example, the reactor system can be constructed with staged heaters and/or tubes. In other words, the 11 heaters or tubes which are subjected to the most 12 extreme temperature conditions in the reactor system 13 can be constructed of materials more resistant to 14 carburization than materials conventionally used in the construction of reforming reactor systems;

16 materials such as those described above. Heaters or 17 tubes which are not subjected to extreme temperatures 18 can continue to be constructed of conventional 19 materials.

21 By using such a staged design in the reactor 22 system, it is possible to reduce the overall cost of 23 the system (since carburization resistant materials 24 are generally more expensive than conventional materials) while still providing a reactor system ' 26 which is sufficiently resistant to carburization and 1 metal dusting under low-sulfur reforming conditions.
2 Additionally, this should facilitate the retrofitting 3 of existing reforming reactor systems to render them 4 carburization and metal dusting resistant under low-sulfur operating conditions; since a smaller portion 6 of the reactor system would need replacement or 7 modification with a staged design.

9 The reactor system can also be operated using at least two temperature zones; at least one of higher 11 and one of lower temperature. This approach is based 12 on the observation that metal dusting has a 13 temperature maximum and minimum, above and below 14 which dusting is minimized. Therefore, by "higher"
temperatures, it is meant that the temperatures are 16 higher than those conventionally used in reforming 17 reactor systems and higher than the temperature 18 maximum for dusting. By "lower" temperatures it is 19 meant that the temperature is at or about the temperatures which reforming processes are 21 conventionally conducted, and falls below that in 22 which dusting becomes a problem.

24 Operation of portions of the reactor system in . 25 different temperature zones should reduce metal 26 dusting as less of the reactor system is at a 1 temperature conducive for metal dusting. Also, other 2 advantages of such a design include improved heat 3 transfer efficiencies and the ability to reduce 4 equipment size because of the operation of portions of the system at higher temperatures. However, 6 operating portions of the reactor system at levels 7 below and above that conducive for metal dusting 8 would only minimize, not completely avoid, the 9 temperature range at which metal dusting occurs.
This is unavoidable because of temperature 11 fluctuations which will occur during day to day 12 operation of the reforming reactor system;

13 particularly fluctuations during shut-down and start-14 up of the system, temperature fluctuations during cycling, and temperature fluctuations which will 16 occur as the process fluids are heated in the reactor 17 system.

19 Another approach to minimizing metal dusting relates to providing heat to the system using 21 superheated raw materials (such as e.g., hydrogen), 22 thereby minimizing the need to heat the hydrocarbons 23 through furnace walls.

Yet another process design approach involves 26 providing a pre-existing reforming reactor system WO 92/15653 PCT/L'S92/01856 1 with larger tube diameters and/or higher tube 2 velocities. Using larger tube diameters and/or 3 higher tube velocities will minimize the exposure of 4 the heating surfaces in the reactor system to the hydrocarbons.

7 As noted above, catalytic reforming is well 8 known in the petroleum industry and involves the 9 treatment of naphtha fractions to improve octane rating by the production of aromatics. The more 11 important hydrocarbon reactions which occur during 12 the reforming operation include the dehydrogenation 13 of cyclohexanes to aromatics, dehydroisomerization of 14 alkycyclopentanes to aromatics, and dehydrocyclization of acyclic hydrocarbons to 16 aromatics. In addition, a number of other reactions 17 also occur, including the dealkylation of 18 alkylbenzenes, isomerization of paraffins, and 19 hydrocracking reactions which produce light gaseous hydrocarbons, e.g., methane, ethane, propane and 21 butane, which hydrocracking reactions should be 22 minimized during reforming as they decrease the yield 23 of gasoline boiling products and hydrogen. Thus, 24 "reforming" as used herein refers to the treatment of a hydrocarbon feed through the use of one or more 26 aromatics producing reactions in order to provide an 210530~

1 aromatics enriched product (i.e., a product whose 2 aromatics content is greater than in the feed).

4 While the present invention is directed primarily to catalytic reforming, it will be useful 6 generally in the production of aromatic hydrocarbons 7 from various hydrocarbon feedstocks under conditions 8 of low sulfur. That is, while catalytic reforming 9 typically refers to the conversion of naphthas, other feedstocks can be treated as well to provide an 11 aromatics enriched product. Therefore, while the 12 conversion of naphthas is a preferred embodiment, the 13 present invention can be useful for the conversion or 14 aromatization of a variety of feedstocks such as paraffin hydrocarbons, olefin hydrocarbons, acetylene 16 hydrocarbons, cyclic paraffin hydrocarbons, cyclic 17 olefin hydrocarbons, and mixtures thereof, and 18 particularly saturated hydrocarbons.

Examples of paraffin hydrocarbons are those 21 having 6 to 10 carbons such as n-hexane, 22 methylpentane, n-haptane, methylhexane, 23 dimethylpentane and n-octane. Examples of acetylene 24 hydrocarbons are those having 6 to 10 carbon atoms such as hexyne, heptyne and octyne. Examples of 26 acyclic paraffin hydrocarbons are those having 6 to WO 92/15653 i~ PCT/US92/01856 ~

1 10 carbon atoms such as methylcyclopentane, 2 cyclohexane, methylcyclohexane and 3 dimethylcyclohexane. Typical examples of cyclic 4 olefin hydrocarbons are those having 6 to 10 carbon atoms such as methylcyclopentene, cyclohexene, 6 methylcyclohexene, and dimethylcyclohexene.

8 The present invention will also be useful for 9 reforming under low-sulfur conditions using a variety of different reforming catalysts. Such catalyst 11 include, but are not limited to Noble Group VIII
12 metals on refractory inorganic oxides such as 13 platinum on alumina, Pt/SN on alumina and Pt/Re on 14 alumina; Noble Group VIII metals on a zeolite such as Pt, Pt/SN and Pt/Re on zeolites such as L-zeolites, 16 ZSM-5, silicalite and beta; and Nobel Group VIII

17 metals on alkali- and alkaline-earth exchanged L-18 zeolites.

A preferred embodiment of the invention involves 21 the use of a large-pore zeolite catalyst including an 22 alkali or alkaline earth metal and charged with one 23 or more Group VIII metals. Most preferred is the 24 embodiment where such a catalyst is used in reforming a naphtha feed.

1 The term "large-pore zeolite" is indicative 2 generally of a zeolite having an effective pore diameter 3 of 6 to 15 AngstroMs. Preferable large pore crystalline 4 zeolites which are useful in the present invention include the type L zeolite, zeolite X, zeolite Y and 6 faujasite. These have apparent pore sizes on the order 7 to 7 to 9 Angstroms. Most preferably the zeolite is a 8 type L zeolite.

The composition of type L zeolite expressed in terms 11 of mole ratios of oxides, may be represented by the 12 following formula:

14 (0.9-1.3)M2/nO:AL203 (5.2-6. 9) SiOZ:yH2O

16 In the above formula M represents a cation, n 17 represents the valence of M, and y may be any value from 18 0 to about 9. Zeolite L, its X-ray diffraction pattern, 19 its properties, and method for its preparation are described in detail in, for example, U.S. Patent No.

21 3,216,789. The actual formula may vary without changing 22 the crystalline structure. For example, the mole ratio 23 of silicon to aluminum (Si/Al) may vary from 1.0 to 3.5.

The chemical formula for zeolite Y.expressed in 26 terms of mole ratios of oxides may be written as:

1 (0. 7-1. 1) Na20 : A1203 : xSiO2 : yH2O

3 In the above formula, x is a value greater than 3 and up 4 to about 6. y may be a value up to about 9. Zeolite Y
has a characteristic X-ray powder diffraction pattern 6 which may be employed with the above formula for 7 identification. Zeolite Y is described in more detail in 8 U.S. Patent No. 3,130,007.

Zeolite X is a synthetic crystalline zeolitic 11 molecular sieve which may be represented by the formula:

13 (0.7-1.1)M2/nO:A12O3: (2.0-3.0)SiO2:yH2O

In the above formula, N represents a metal, particularly 16 alkali and alkaline earth metals, n is the valence of M, 17 and y may have any value up to about 8 depending on the 18 identity of M and the degree of hydration of the 19 crystalline zeolite. Zeolite X, its X-ray diffraction pattern, its properties, and method for its preparation 21 are described in detail in U.S. Patent No. 2,882,244, 23 An alkali or alkaline earth metal is preferably 24 present in the large-pore zeolite. That alkaline 1 earth metal may be either barium, strontium or 2 calcium, preferably barium. The alkaline earth metal 3 can be incorporated into the zeolite by synthesis, 4 impregnation or ion exchange. Barium is preferred to the other alkaline earths because it results in a 6 somewhat less acidic catalyst. Strong acidity is 7 undesirable in the catalyst because it promotes 8 cracking, resulting in lower selectivity.

In another embodiment, a= least part of t-he 11 alkali metal can be exchanged with barium using known 12 techniques for ion exchange of zeolites. This 13 involves contacting the zeolite with a solution 14 containing excess Ba'''ions. In this embodiment the barium should preferably constitute from 0.1% to 35%
16 by weight of the zeolite.

18 The large-pore zeolitic catalysts used in the 19 invention are charged with one or more Group VIII
metals, e.g., nickel, ruthenium, rhodium, palladium, 21 iridium or platinum. The preferred Group VIII metals 22 are iridium and particularly platinum. These are 23 more selective with regard to dehydrocyclization and 24 are also more stable under the dehydrocyclization reaction conditions than other Group VIII metals. If WO 92/15653 PL"T/US92/01856 ~~.,1FlJe1 1 used, the preiarred weight percentage of platinum in 2 the catalyst is between 0.1% and 5%.

4 Group VIII metals are introduced into large-pore zeolites by synthesis, impregnation or exchange in an 6 aqueous solution of appropriate salt. When it is 7 desired to introduce two Group VIII metals into the 8 zeolite, the operation may be carried out 9 simultaneously or sequentially.

11 To obtain a more complete understanding of the 12 present invention, the following examples 13 illustrating certain aspects of the invention are set 14 forth. It should be understood, however, that the invention is not limited in any way to the specific 16 details set forth therein.

19 Tests were run to demonstrate the effect of sulfur and water on carburization in reforming 21 reactors.

23 In these tests, eight inch long, ; inch outside 24 diameter copper tubes were used as a reactor to study the carburization and embrittlement of 347 stainless 26 steel wires. Three of these stainless steel wires 21~5 3 05 1 having a diameter of 0.035 inches were inserted into 2 the tube, while a four inch section of the tube was 3 maintained at a uniform temperature of 1250 F by a 4 furnace. The pressure of the system was maintained at 50 psig. Hexane was introduced into the reactor 6 at a rate of 25 microliters/min. (1.5 ml/hr) with a 7 hydrogen rate of about 25 cc/min. (ratio of H2 to HC
8 being 5:1). Methane in the product effluent was 9 measured to determine the existence of exothermic methane reactions.

12 A control run was made using essentially pure 13 hexane containing less than 0.2 ppm sulfur. The tube 14 was found to be completely filled with carbon after only three hours. This not only stopped the flow of 16 the hydrogen and hexane feeds, the growth of carbon 17 actually split the tube and produced a bulge in the 18 reactor. Methane in the product effluent was 19 approaching 60-80 wt% before plugging.

21 Another run was conducted using essentially the 22 same conditions except that 10 ppm sulfur was added.
23 The run continued for 50 hours before it was shut 24 down to examine the wires. No increase in methane was noted during the run. It remained steady at 26 about 16 wt% due to thermal cracking. No coke plugs . . . . . . , , ~ , . .. . . . ' : ... . . .

1 were found and no carburization of the steel wires 2 was observed.

4 Another identical run was made except that only 1 ppm sulfur was added (10 times lower than the 6 previous run). This run exhibited little methane 7 formation or plugging after 48 hours. An examination 8 of the steel wires showed a small amount of surface 9 carbon, but no ribbons of carbon.

11 Another run was conducted except that 1000 ppm 12 water (0.1%) was added to the hexane as methanol. No 13 sulfur was added. The run lasted for 16 hours and no 14 plugs occurred in the reactor. However,=upon splitting the tube it was discovered that about 50 16 percent of the tube was filled with carbon. But the 17 carbon buildup was not nearly as severe as with the 18 control run.

21 Tests were conducted to determine suitable 22 materials for use in low-sulfur reforming reactor 23 systems; materials which would exhibit better 24 resistance to carburization than the mild steels conventionally used in low-sulfur reforming 26 techniques.

WC192/15653 PCT/L'Syzru ~o~u u u S

1 In these tests there was used an apparatus 2 including a Lindberg alumina tube furnace with 3 temperatures controlled to within one degree with a 4 thermocouple placed on the exterior of the tube in the heated zone. The furnace tube had an internal 6 diameter of 5/8 inches. Several runs were conducted 7 at an applied temperature of 1200 F using a 8 thermocouple suspended within the hot zone (=2 9 inches) of the tube. The internal thermocouple constantly measured temperatures from 0 to 10 F lower 11 than the external thermocouple.

13 Samples of mild steels (C steel and 2; Cr) and 14 samples of 300 series stainless steels were tested at 1100 F, 1150 F and 1200 F for twenty-four hours, and 16 1100 F for ninety hours, under conditions which 17 simulate the exposure of the materials under 18 conditions of low-sulfur reforming. The samples of 19 various materials were placed in an open quartz boat within the hot zone of the furnace tube. The boats 21 were one inch long and h inch wide and fit well 22 within the two-inch hot zone of the tube. The boats 23 were attached to silica glass rods for each placement 24 and removal. No internal thermocouple was used when the boats were placed inside the tube.

PCT/US92/01856=

1 Prior to start up the tube was flushed with 2 nitrogen for a few minutes. A carburizing gas of a 3 commercially bottled mixture of 7% propane in 4 hydrogen was bubbled through a liter flask of toluene at room temperature in order entrain about 1% toluene 6 in the feed gas mix. Gas flows of 25 to 30 cc/min., 7 and atmospheric pressure, were maintained in the 8 apparatus. The samples were brought to operating 9 temperatures at a rate of 144 F/min.

11 After exposing the materials to the carburizing 12 gas for the desired period at the desired 13 temperature, the apparatus was quenched with an air 14 stream applied to the exterior of the tube. When the apparatus was sufficiently cool, the hydrocarbon gas 16 was swept out with nitrogen and the boat was removed 17 for inspection and analysis.

19 Prior to start up the test materials were cut to a size and shape suitable for ready-visual 21 identification. After any pretreatment, such as 22 cleaning or roasting, the samples were weighed. Most 23 samples were less than 300 mg. Typically, each run 24 was conducted with three to five samples in a boat.
A sample of 347 stainless steel was present with each 26 run as an internal standard.

WC992/15653 PCf/US92/01856 1 After completion of each run the condition of 2 the boat and each material was carefully noted.

3 Typically the boat was photographed. Then, each 4 material was weighed to determine changes while taking care to keep any coke deposits with the 6 appropriate substrate material. The samples were 7 then mounted in an epoxy resin, ground and polished 8 in preparation for petrographic and scanning electron 9 microscopy analysis to determine the coking, metal dusting and carburization responses of each material.

12 By necessity, the residence time of the 13 carburizing gas used in these tests were considerably 14 higher than in typical commercial operation. Thus, it is believed that the experimental conditions may 16 have been more severe than commercial conditions.

17 Some of the materials which failed in these tests may 18 actually be commercially reliable. Nevertheless, the 19 test provides a reliable indication of the relative resistances of the materials to coking, carburization 21 and metal dusting.

WO 92/15653 2105305 PCI'/US92/01856 The results are set forth in the Table below.
2 Table*

4 Wt. % C
Gain Dustina Composition 6 1200 F; 24 hours 7 C Steel 86 Severe 3 2; Cr 61 Severe 9 304 little No 18 Cr 10 Ni 347 little No 18 Cr 10 Ni 12 1150 F; 24 hours 13 C Steel 63 Severe 14 2; Cr 80 Severe 304 1 No 16 347 1 No 18 1100 F; 24 hours 19 C Steel Trace Trace, localized 2; Cr 0 No 21 304 0 No 22 347 0 No 24 1100 F; 90 hours C Steel 52 Severe 26 2; Cr 62 Severe 27 304 5 No 28 347 1 No * 15% C7Hs + 50% C3H3 + H2 (by weight) 33 Of course, the above results are qualitative and 34 depend on surface morphology, i.e., microscopic roughness of the metals. The carbon weight gain is 36 indicative of surface coking which is autocatalytic.

39 The same techniques used above were used again to screen a wide assortment of materials at a 41 temperature of 1200 F for 16 hours. The results are Attorney's Docket No. 005950-314 0:5 set forth below. Each group represents a side-by-2 side comparison in a single boat under identical 3 conditions.

4 TABLE (1) Wt. % C
6 Gain Dustinq Composition 8 Group I
9 Inconel 600 57 Severe 15 Cr 75 Ni 347 oxid.(2) 21 Moderate 11 347 Fresh 4 No- 18 Cr 10 Ni 13 Group II
14 Inconel 600 40 Severe 15 Cr=75 Ni 310 8 Mild 25 Cr 20 Ni 16 Incoloy 800 5 Moderate 21 Cr 32 Ni 17 347 1 Trace 19 Group IIi Incoloy 825 <1 Moderate 21 Haynes 230 2 Mild 22 Cr 64 Ni 22 Alonized 347 3 Trace 23 347 <1 Trace Grouo IV
26 Ni (Pure) 656 Severe 100 Ni 27 Cu (Pure) 0 No 100 Cu 28 Sn (Fused) 0 No 29 100 Sn Tin Can 0 No 31 Sn + C Steel (1) 15% C7Hb + 50% C3Ha + H2 (By Wt.) 37 (2) Roasted in air 2 hours at 38 1000 C to produce a thin 39 oxide crust.

WO 92/15653 PCr/US92/01850 2 Additional materials were tested, again using 3 the techniques described in Example 2 (unless stated 4 otherwise).

6 Samples of 446 stainless steel and 347 stainless 7 steel were placed in a sample boat and tested 8 simultaneously in the carburization apparatus at 9 1100 F for a total of two weeks. The 446 stainless steel had a thin coating of coke, but no other 11 alteration was,detected. The 347 stainless steel, on 12 the other hand, had massive localized coke deposits, 13 and pits more than 4 mils deep from which coke and 14 metal dust had erupted.

16 Samples were tested of a carbon steel screen 17 electroplated with tin, silver, copper and chromium.
18 the samples had coatings of approximately 0.5 mil.
19 After 16-hour carburization screening tests at 1200 F, no coke had formed on the tin-plated and 21 chromium-plated screens. Coke formed on the silver-22 plated and copper-plated screens, but only where the 23 platings had peeled. Unplated carbon steel screens 24 run simultaneously with the plated screens, exhibited severe coking carburization, and metal dusting.

WO 92/15653 210 53 il ~~ PCT/US92/01856 1 Samples were tested of a 304 stainless steel 2 screen; each sample being electroplated with one of 3 tin, silver, copper and chromium. The samples had 4 coatings with thicknesses of approximately 0.5 mil.
After 16-hour carburization screening tests at 6 1200 F, no coke had formed on any of the plated 7 screens, except locally on the copper-plated screen 8 where the plating had blistered and peeled. Thin 9 coke coatings were observed on unplated samples of 304 stainless steel run simultaneously with the 11 plated screens.

13 Samples were tested of a 304 stainless steel 14 screen; each sample being electroplated with one of tin and chromium. These samples were tested along 16 with a sample of 446 stainless steel in a 17 carburization test at 1100 F. The samples were 18 exposed or five weeks. Each week the samples were 19 cooled to room temperature for observation and photographic documentation. They were then re-heated 21 to 1100 F. The tin plated screen was free of coke;
22 the chromium-plated screen was also free of coke, 23 except locally where the chrome plate had peeled; and 24 the piece of 446 stainless steel was uniformly coated with coke.

~~~5,)0-1 Samples of uncoated Inconel 600 (75% Ni) and 2 tin-coated (electroplated) Inconel 600 (75% Ni) were 3 tested at 1200 F for 16 hours. The tin-plated sample 4 coked and dusted, but not to the extent of the uncoated sample.

8 The following experiments were conducted to 9 study the exothermic methanization reaction occurring during the formation and burning of cokeballs during 11 reforming under conditions of low-sulfur. In 12 addition tin, as an additive to reduce methane 13 formation was studied.

In low-sulfur reforming reactor systems, coke 16 deposits containing molten particles of iron have 17 been found. This formation of molten iron during 18 reforming at temperatures between 900 and 1200 F is 19 believed to be due to very exothermic reactions which occur during reforming. It is believed that the only 21 way to generate such temperatures is through the 22 formation of methane which is very exothermic. The 23 high temperatures are particularly surprising since 24 reforming is generally endothermic in nature and actually tends to cool the reactor system. The high 26 temperatures may be generated inside the well insulated cokeballs by diffusion of hydrogen into the 2 interior catalytic iron dust sites where they 3 catalyze methane formation from coke and hydrogen.

In this experiment steel wool was used to study 6 methane formation in a micro pilot plant. A; inch 7 stainless steel tube was packed with 0.14 grams of 8 steel wool and placed into a furnace at 1175 F.

9 Hexane and hydrogen were passed over the iron and the exit stream was analyzed for feed and products. The 11 steel wool was.pretreated in hydrogen for twenty 12 hours before introduction of the hexane. Then hexane 13 was introduced into the reactor at a rate of 25 14 microliters/min. with a hydrogen rate of about 25 cc/min.

17 Initially, methane formation was low, but 18 continued to increase as the run progressed; finally 19 reaching 4.5%. Then, 0.1 cc of tetrabutyl tin dissolved in 2 cc of hexane was injected into the 21 purified feed stream ahead of the iron. The methane 22 formation decreased to about 1% and continued to 23 remain at 1% for the next three hours. The data is 24 summarized in the Table below.

Y
19.2 0.0 0.5 0.3 98.6 6 20.7 1.06 2.08 1.74 93.4 7 21.2 2.62 4.55 3.92 85.3 8 21.5 3.43 4.23 3.83 84.6 9 21.9 4.45 4.50 4.32 82.0 11 22 Tetrabutyl Tin Added 13 22.6 1.16 3.81 4.12 86.2 14 23.0 1.16 3.96 4.24 85.9 23.3 1.0 4.56 3.77 87.5 16 24.3 0.97 3.60 3.76 87.6 17 25.3 1.0 4.47 3.57 88.0 From the results above it can be seen that the 21 addition of tin to the steel wool stops the 22 acceleration of methane formation, and lowers it to 23 acceptable levels in the product.

26 Additional tests were conducted using tetrabutyl 27 tin pre-coated steel wool. In particular, as in 28 Example 5, three injections of 0.1 cc of tetrabutyl 29 tin dissolved in 2 cc of hexane were =injected into a=
; inch stainless steel tube containing 0.15 grams of 31 steel wool. The solution was carried over the steel 32 wool in a hydrogen stream of 900 F.

34 The hydrocarbon feed was then introduced at 1175 F at a hydrocarbon rate of 25 microliters/min 1 with a hydrogen rate of about 25 cc/min. The exit 2 gas was analyzed for methane and remained below 1%
3 for 24 hours. The reactor was then shut down, and 4 the reactor tube was split open and examined. Very little carburization had occurred on the steel wool.

7 In contrast, a control was run without 8 tetrabutyl tin pre-treatment. It was run for one day 9 under the same conditions described above. After 24 hours, no hydrogen or feed could be detected at the 11 tube exit. The inlet pressure had risen to 300 lbs.
12 from the original 50 lbs. When the reactor tube was 13 split open and examined, it was found that coke had 14 completely plugged the tube.

16 Thus, it can be seen that organo-tin compounds 17 can prevent carburization of steel wool under 18 reforming conditions.

21 Another run like the control run of Example 1 22 was conducted to investigate the effect of 23 carburization conditions on vapor tin coated 24 stainless steel wires in a gold plated reactor tube.
The only other difference from the control run was 26 that a higher hydrogen rate of 100 ml/min was used.

The run continued for eight hours with no 2 plugging or excessive methane formation. When the 3 tube was split and analyzed, no plugs or carbon 4 ribbons were observed. only one black streak of carbon appeared on one wire. This was probably due 6 to an improper coating.

8 This experiment shows that tin can protect 9 stainless steel from carburization in a manner similar to sulfur. Unlike sulfur, however, it.does 11 not have to be continuously injected into the feed.
12 Sulfur must be continuously injected into the feed to 13 maintain the partial pressure of hydrogen sulfide in 14 the system at a sufficient level to maintain a sulfide surface on the steel. Any removal of sulfur 16 from the feedstock will lead to a start of 17 carburization after sulfur is stripped from the 18 reactor system. This usually occurs within 10 hours 19 after cessation of sulfur.

21 While the invention has been described above in 22 terms of preferred embodiments, it is to be 23 understood that variations and modifications may be 24 used as will be appreciated by those skilled in the art. For example, portions of steel in the reactor 26 system can be coated with niobium, zirconium, silica 1 ceramics, tungsten, or chromium (chromizing), 2 although these techniques could be excessively 3 difficult to do or use, or prohibitively expensive.
4 or, the use of heat exchangers to heat hydrocarbons to reaction temperature could be minimized. The heat 6 could be provided by super-heated hydrogen. or, the 7 exposure of heating surfaces to hydrocarbons can be 8 reduced by using larger tube diameters and higher 9 tube velocities. Essentially, therefore, there are many variations and modifications to the above 11 preferred embodiments which will be readily evident 12 to those skilled in the art, and which are to be 13 considered within the scope of the invention as 14 defined by the following claims.

Claims (65)

WHAT IS CLAIMED IS:

1. An improved method for catalytically reforming a hydrocarbon, comprising the step of contacting, under conditions of low sulfur, a sulfur-sensitive zeolite reforming catalyst with a hydrocarbon in a reactor system a plurality of furnace tubes, wherein a portion of said reactor system has a resistance to carburization and metal dusting under low sulfur reforming conditions at least as great as that of stainless steel.

2. The method of claim 1, wherein said sulfur-sensitive zeolite reforming catalyst is an L-zeolite catalyst.

3. The method of claims 1 or 2, wherein said improved resistance to carburization and metal dusting under low sulfur reforming conditions is at least as great as that of 300 series stainless steel.

4. The method of any one of claims 1-3, wherein said improved resistance to carburization and metal dusting under low sulfur reforming conditions is at least as great as that of 347 stainless steel.

5. The method of any one of claims 1-4, wherein said reactor system comprises a fixed furnace.

6. The method of claim wherein said plurality of furnace tubes are positioned within a single furnace.

7. The method of any one of claims 1-6, wherein said portion of said reactor system at least one of said plurality of furnace tubes.

8. The method of any one of claims wherein said portion of said reactor system is a portion of a wall of said reactor system.

9. The method of any one of 1-3, wherein said conditions of low sulfur comprise less than about 100 ppb sulfur.

10. The method of any one of claims 1- 9, wherein said conditions of low sulfur comprise, less than about 50 ppb sulfur.

11. The method of any one of claims 1-10, wherein said contacting step is conducted under conditions of low water content.

12. The method of any one of claims 1-11, wherein said contacting step is conducted under conditions of 1000 ppm of water, or less.

13. The method of any one of claims 1 or 3-12, wherein said sulfur-sensitive reforming catalyst comprises a large-pore zeolite comprising an alkali or alkaline earth metal and at least one Group VIII metal.

14. The method of claim 13, wherein said Group VIII
metal comprises platinum.

15 . The method of any one of claims 1-14, wherein said improved resistance to carburization and metal dusting is provided by a cladding, coating, or paint on said portion of said reactor system.

16. The method of claim 15, wherein said paint comprises a reducible paint and further comprising the step of heating said reducible paint under reducing conditions.

17. The method of claim wherein said heating step is conducted in the presence of hydrogen.

18. The method of claims 15 or 16, wherein said paint comprises a tin-containing paint.

19. The method of claim 18, wherein said tin-containing paint further comprises a hydrogen decomposable tin compound, a solvent system, -a finely divided tin metal, and a tin oxide.

20. The method of claims 18 or 19, wherein said tin-containing paint further comprises a tin-containing compound and an iron compound, and wherein the iron:tin ratio is less than approximately 1:3 by weight.

21. The method of claims 15 or 16, wherein said paint comprises a metal oxide.

22. The method of claims 15 or 16, wherein said paint comprises a hydrogen decomposable compound.

23. The method of claim 15, wherein said coating is selected from the group consisting of copper, tin, intermetallic tin compounds, tin alloys, arsenic, antimony, brass, lead, bismuth, chromium, intermetallic compounds thereof, alloys thereof, a copper-tin alloy, a copper-antimony alloy, and mixtures thereof.

24. The method of claim 23, wherein said coating retains said resistance to carburization and metal dusting after oxidation.

25. The method of claim 15, wherein said coating is selected from the group consisting of aluminum, alumina, chromium, chromium oxide, an aluminized material, a chromized material and mixtures thereof.

26. The method of claim 15, wherein said coating comprises a ceramic coating.

27. The method of claim 15, wherein said coating comprises a silica coating.

28. The method of any one of claims 1-14, wherein said resistance to carburization and metal dusting is provided by a layer of a metallic stannide on said portion of said reactor system.

29. The method of any one of claims 1-14, wherein said portion is constructed of a material that provides said resistance to carburization and metal dusting.

30. The method of claim 29, wherein said portion is 347 stainless steel or has a resistance to carburization and metal dusting at least as good as 347 stainless steel.

31. The method of any one of claims 1-14, wherein said resistance to carburization and metal dusting is provided by the step of implementing a process modification selected from the group consisting of operating said reactor system at a lower reforming temperature, superheating said low-sulfur stream, operating said reaction system with two temperature zones, operating said reactor system with staged heaters or furnace tubes, operating said reactor system with staged heaters and staged furnace tubes, operating said reactor system by minimizing the exposure of the heating surfaces of said reactor system to said hydrocarbon, and combinations thereof, thereby providing said portion of said reactor system with an improved resistance to carburization and metal dusting.

32. The method of any one of claims 1-14, wherein said resistance to carburization and metal dusting is provided by the step of adding a non-sulfur, anti-carburizing, anti-coking agent to said reactor system.

33. The method of claim 32, wherein said non-sulfur, anti-carburizing agent is selected from the group consisting of organo-metallic compounds, organo-tin compound, organo-antimony compounds, organo-bismuth compounds, organo-arsenic compounds, organo-lead compounds, tetraethyl lead, tetramethyl lead, tetrabutyl tin, trimethyl tin hydride, bismuth neodecanoate, chromium octoate, copper naphthenate, manganese carboxylate, palladium neodecanoate, silver neodecanoate, tetrabutyl germanium, tributyl antimony, triphenyl antimony, triphenyl arsine, zirconium octoate and combinations thereof.

34. The method of any one of claims 1-33, wherein said reactor system is an existing reactor system.

35. The method of any one of claims 1-33, wherein said reactor system is a new reactor system.

36. The method of any one of claims 1-35, wherein said portion of said reactor system, prior to having an improved resistance to carburization and metal dusting, is oxidized steel.

37. A method for the catalytic reforming of a hydrocarbon stream to produce an aromatic compound using a reactor system, having a sulfur-sensitive reforming catalyst charged with at least one Group VIII metal, over a prolonged period of operation without significant coke-plugging, comprising the steps of:

providing a low sulfur hydrocarbon-containing stream prepared by reducing the sulfur content of said hydrocarbon-containing stream to less than 50 ppb sulfur;
providing a reforming reactor system of improved resistance to carburization and metal dusting upon reforming said hydrocarbon-containing stream, said reactor system having at least one furnace to heat said hydrocarbon containing stream to catalytic reforming temperatures, said furnace having, in contact, with said hydrocarbon-containing stream, a plurality of furnace tubes having a resistance to carburization and metal dusting at least as great as that of 347 stainless steel; and passing said hydrocarbon-containing stream through said reactor system contact said hydrocarbon-containing stream with said reforming catalyst to produce an aromatic.

38. The method of claim 37, wherein said sulfur-sensitive reforming catalyst comprises a large-pore zeolite comprising an alkali or alkaline earth metal and at least one Group VIII metal.

39. The method of claims 37 or 38, wherein said sulfur-sensitive zeolite reforming catalyst is an L-zeolite catalyst.

40. A method for improving the carburization resistance of at least a portion of an apparatus for hydrocarbon conversion, comprising the steps of:

applying a reducible, tin-containing paint to a portion of an apparatus for hydrocarbon conversion;
and heating said reducible paint, under reducing conditioris to form a protective layer which provides improved carburization resistance.

41. The method of claim 40, wherein said reducible, tin-containing paint comprises a hydrogen decomposable tin compound, a solvent system, a finely divided tin metal and a tin oxide.

42. The method of claims 40 or 41, wherein said heating step is conducted in the presence of hydrogen.

43. The method of any one of claims 40-42, wherein said protective layer comprises a metallic stannide.

44. A catalytic reforming reactor system for catalytically reforming hydrocarbons under low sulfur conditions , comprising:

a furnace having a plurality of furnace tubes;
a reforming reactor having catalyst bed containing a sulfur-sensitive catalyst; and wherein a portion of said catalytic reforming reactor system, which contacts a low-sulfur stream containing a hydrocarbon, has an improved resistance to carburization.

45. The catalytic reforming reactor system of claim 44, wherein said sulfur-sensitive zeolite reforming catalyst is an L-zeolite catalyst.

46. The catalytic reforming reactor system of claims 44 or 45, wherein said portion of said catalytic reforming reactor system is at least one of said plurality of furnace tubes.

47. The catalytic reforming reactor system of claims 44 or 45, wherein said portion of said catalytic reforming reactor system is a portion of a wall of said catalytic reforming reactor system.

48. The catalytic reforming reactor system of any one of claims 44-47, wherein said low sulfur stream has less than about 100 ppb su1fur.

49. The catalytic reforming reactor system of any one of claims 44-48, wherein said low sulfur stream has less than about 50 ppb sulfur.

50. The catalytic reforming reactor system of any one of claims 44-49, wherein said sulfur-sensitive catalyst is a large-pore zeolite comprising an alkali or alkaline earth metal and at least one Group VIII
metal.

51. The catalytic reforming reactor system of claim 50, wherein said Group VIII metal comprises platinum.

52. The catalytic reforming reactor system of any one of claims 44-51, wherein said improved resistance to carburization is provided by a cladding, a coating or a paint on said portion of said reforming reactor.

53. The catalytic reforming reactor system of claim 52, wherein said paint comprises a reducible paint which has been heated under reducing conditions.

54. The catalytic reforming reactor system of claims 52 or 53, wherein said paint comprises a tin-containing paint.

55. The catalytic reforming reactor system of claim 54, wherein said tin-containing paint further comprises a hydrogen decomposable tin compound, a solvent system, a finely divided tin metal, and a tin oxide.

56. The catalytic reforming reactor system of claims 52, wherein said coating is selected from the group consisting of copper, intermetallic tin compounds, tin alloys, arsenic, antimony, brass, lead, bismuth, chromium, intermetallic compounds thereof, alloys thereof, a copper-tin alloy, a copper-antimony alloy, and mixtures thereof.

57. The catalytic reforming reactor system of claim 52, wherein said coating is selected from the group consisting of aluminum, alumina, chromium, chromium oxide, an aluminized material, a chromized material and mixtures thereof.

58. The catalytic reforming reactor system of claim 52, wherein said coating comprises a ceramic coating.

59. The catalytic reforming reactor system of claim 52, wherein said coating comprises a silica coating.

60. The catalytic reforming reactor system of any one of claims 44-51, wherein said portion of said catalytic reforming reactor system comprises a protective layer comprising a metallic stannide which provides said improved resistance to carburization.

61. The catalytic reforming reactor system of any one of claims 44-51, wherein said portion is constructed of a material that provides said resistance to carburization and metal dusting.

62. The catalytic reforming reactor system of claim 61, wherein said material is 347 stainless steel or has a resistance to carburization and metal dusting at least as good as 347 stainless steel under low sulfur reforming conditions.

63. The catalytic reforming reactor system of any one of claims 44-62, wherein said catalytic reforming reactor system is an existing catalytic reforming reactor system.

64. The catalytic reforming reactor system of any one of claims 44-62, wherein said catalytic reforming reactor system is a new catalytic reforming reactor system.

65. The catalytic reforming reactor system of any one of claims 44-64, wherein said portion of said catalytic reforming reactor system, prior to having an improved resistance to carburization and metal dusting, is oxidized steel.