Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
2003-02-21
2004-10-12
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C252S373000
Reexamination Certificate
active
06803029
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for reducing coke formation in organic compound conversion processes.
BACKGROUND OF THE INVENTION
Fouling of catalysts and/or process equipment by coke is a major problem in high temperature organic compound conversion processes. Coke can cover active catalyst sites and plug catalyst pores, thereby reducing activity. In process equipment, it can build up on furnace tubes and reactors leading to heat transfer and pressure drop problems. Coking in some cases can be so severe as to completely plug the process with coke. While there are many methods of controlling coking such as careful selection of catalysts and plant metallurgy, application of low coking coatings, and/or the addition of steam or sulfur, coking still remains a major problem. Application of low coking coatings, often referred to as Metal Protection Technology (MPT) is taught in U.S. Pat. Nos. 5,866,743; 5,849,969; 6,019,943; 5,676,821; 5,593,571; 5,863,418; and 5,413,700 all of which are incorporated herein by reference. In some processes such as delayed coking or flexi-coking, coke is a by-product of the process that has a very low value. A particular type of coking that is a problem in hydrocarbon processing is Metal Catalyzed Coking. Metal catalyzed coking occurs when hydrocarbons and/or carbon monoxide present in certain processes react with the plant or process metallurgy at temperatures typically above 800° F. to produce carbon-containing deposits. These carbon containing deposits can build up to a level where they are detrimental to the process by creating, for example, pressure drop problems, blocking off catalytic sites, and/or impeding the transfer of heat, in for example, a furnace tube. Metal catalyzed coking is also an indication that the plant or process metallurgy is undergoing metal dusting and possibly carburization. Both of these later processes can affect the structural integrity of the metallurgy. Typically, iron, cobalt and/or nickel containing alloys show metal catalyzed coking at temperatures of 800° F. or higher. Of particular interest to this invention are high temperature processes such as steam reforming, partial oxidative reforming, and/or autothermal reforming associated with the conversion of hydrocarbons to carbon monoxide and hydrogen for use in Fischer-Tropisch plants, syngas to methanol plants, fuel cells or any other process that require or consume hydrogen and/or carbon monoxide. There are many methods taught in the art to control metal catalyzed coking. Some techniques such as addition of sulfur to the process stream cannot be used in certain processes such as steam or autothermal reforming processes due to sulfur poisoning of the reformer catalyst. Thus the addition of steam is normally used to control coking. Typically, the steam to carbon mole ratio used in reforming processes range from about 0.5 to 6 and more commonly from about 2 to 4. However addition of steam is frequently not sufficient to control metal catalyzed coking. In one form of the art, iron, cobalt and/or nickel containing alloys are treated with aluminum at high temperature to form aluminum diffusion coating. Methods for preparing such coatings are taught in the “Metals Handbook”, 9
th
Ed, Vol 5 page 611-613 and for example in U.S. Pat. Nos. 1,988,217, 3,486,927 and 3,254,969 all of which are incorporated herein by reference. U.S. Pat. No. 1,988,217, to Sayles, which is incorporated herein by reference, teaches that aluminum diffusion coatings with a surface concentration of about 5 to 35% Al can be used to protect oil furnace tubes and other high temperature processes from the corrosive action of oil. U.S. Pat. No. 3,827,967 to Nap et. al. which is incorporated herein by reference, teaches that an aluminum coated alloy will resist coking in the thermal cracking of hydrocarbon feedstocks. The beneficial effects of aluminum diffusion coatings on specifically suppressing metal catalyzed coking under low sulfur conditions is claimed in U.S. Pat. No. 5,849,969 to Heyse et. al. Alternatively, aluminum-containing alloys have been shown to be resistant to coking. U.S. Pat. No. 4,532,109 to Maeda teaches that alloys containing 1-10% Al that have been oxidized prior to high temperature contact with hydrocarbons, or carbon monoxide show reduced metal catalyzed coking rates. U.S. Pat. Nos. 4,532,109, and 5,849,969 are incorporated herein by reference in their entirety. Unfortunately, metal catalyzed coking can still occur on Al containing alloys or Al rich surfaces under conditions commonly encountered in high temperature processes. Clearly any method that can reduce the amount of metal catalyzed coke formed would be very beneficial. The present invention provides such a method that can be used to minimize coking in a wide variety of processes and applications.
SUMMARY OF THE INVENTION
The present invention provides a process for reducing metal catalyzed coke formation in organic compound conversion processes. The present invention dramatically reduces or eliminates metal catalyzed coking on aluminum coated or aluminum containing nickel and/or cobalt containing alloys by the presence of carbon dioxide and steam in the process stream.
One embodiment of the present invention describes a process for steam reforming of hydrocarbons to produce hydrogen and carbon monoxide, comprising: passing a feed comprising hydrocarbons, steam, and CO
2
over a steam reforming catalyst, at steam reforming conditions comprising a temperature of at least 800 degrees F., in a steam reformer to form an effluent comprising CO and hydrogen, wherein said steam reformer is constructed at least in part of a material comprising of a cobalt or nickel containing alloy further comprising aluminum or an aluminum coating, cladding, or paint.
Another embodiment of the present invention involves a method for prevention of metal catalyzed coking in a reforming process producing carbon monoxide and hydrogen, comprising constructing the reformer using alloys comprising a metal selected from the group consisting of nickel and cobalt, said alloy also comprising aluminum or being coated, cladded, plated or painted with a material comprising aluminum, and; feeding a hydrocarbon to said reforming process comprising at least 0.5% CO
2
by volume and a steam to carbon mole ratio of at least 0.3.
The present invention also describes a process for forming hydrogen for use in a fuel cell comprising: passing a feed comprising hydrocarbons, and CO
2
, in combination with steam in a reforming process in the presence of a nickel or cobalt containing alloy, at reforming conditions sufficient to form carbon monoxide and hydrogen, wherein said alloy further comprises aluminum or an aluminum diffusion layer and said CO
2
content in the feed is sufficient to suppress metal catalyzed coking.
A particularly preferred embodiment of the present invention describes a process for forming hydrogen for use in a fuel cell, comprising: passing a feed comprising hydrocarbons, and CO
2
, in combination with steam in a reforming process in the presence of a nickel and/or cobalt containing alloy, at reforming conditions sufficient to form carbon monoxide and hydrogen, wherein said alloy further comprises aluminum or an aluminum diffusion layer and said CO
2
content in the feed is sufficient to suppress metal catalyzed coking.
In an alternative embodiment of the present invention describes a process for forming hydrogen and carbon monoxide for use in a Fischer-Tropsch plant, comprising: passing a feed comprising hydrocarbons, and CO
2
, in combination with steam in a reforming process in the presence of a nickel or cobalt containing alloy, at reforming conditions sufficient to form carbon monoxide and hydrogen, wherein said alloy further comprises aluminum or an aluminum diffusion layer and said CO
2
content in the feed is sufficient to suppress metal catalyzed coking.
Among other factors I have surprisingly discovered that metal catalyzed coking can be suppressed if not completely eliminated by the use of a nickel and/or
Chevron U.S.A. Inc.
Medina Maribel
Silverman Stanley S.
Tuck D. M.
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