Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
2002-05-30
2004-08-24
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C252S373000
Reexamination Certificate
active
06780395
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for forming a tailored synthesis gas that has a controlled hydrogen/carbon monoxide (H2/CO) ratio. More particularly this invention, by way of gas separation by a membrane, forms a synthesis gas whose H2/CO ratio is deliberately controlled to meet the H2/CO ratio best suited or otherwise desired in a feed stock for one or more specific, predetermined downstream processes. For example, by this invention a synthesis gas can be formed which consistently has its H2/CO ratio controlled to be about 1.6/1 which is ideal for a synthesis gas feed stock to a Fischer Tropsch process.
2. Description of the Prior Art
Heretofore steam reforming of a hydrocarbon feed containing methane, e.g., natural gas, has been performed in order to produce a synthesis gas rich in hydrogen by converting the carbon in the methane to carbon monoxide and freeing hydrogen from the steam.
Depending upon the composition of the hydrocarbon feed, the reforming conditions, the catalyst used, and many other variables, the synthesis gas product from such steam/methane reforming can vary widely as to its composition in general, and its H2/CO ratio in particular.
The synthesis gas product aforesaid is then used in one or more different downstream processes to make one or more chemical products of commercial value. For example, a conventional synthesis gas product can be used as a source of hydrogen for hydrotreating other hydrocarbon streams in a crude oil refinery, or, after suitable compression, can be used as a feed stock for making ammonia or methanol.
However, downstream processes which use synthesis gas as a feed operate more efficiently when the H2/CO ratio in their feed is more carefully controlled than can currently be done in a conventional steam/methane reforming system.
Accordingly, it is desirable to be able to control the H2/CO ratio of a steam/methane reformer product so as to tailor the synthesis gas product from that reformer to better meet or otherwise suit the requirements of the specific downstream processing unit or units for which that particular synthesis gas product will be used as a feed material.
SUMMARY OF THE INVENTION
In accordance with this invention, there is provided a method for producing a tailored synthesis gas that has a controlled H2/CO ratio, which ratio is tailored to meet the desired or optimum operating requirements of at least one specific, predetermined downstream process that uses synthesis gas as a feed stock.
Synthesis gas generators (reformers) are well known in the art, having been used in various forms in industry since the classical Haber Bosch Process was developed in 1917. All these processes operate on the basic reforming reaction which reacts carbon in some form, e.g., coke, natural gas, naphtha, and the like with steam, usually, but not always, in the presence of a catalyst, to produce hydrogen and carbon monoxide. The H2/CO ratio of the synthesis gas varies widely depending on the form of the carbon source feed, but if one mol of methane was reformed with steam it would produce a synthesis gas rich in hydrogen, viz., 3 mols of hydrogen and 1 mol of carbon monoxide or a H2/CO ratio of 3/1. A competing reaction known as the water gas shift reaction also takes place in a reformer to some extent wherein carbon monoxide reacts with steam to form carbon dioxide and additional hydrogen. The shift reaction is a secondary reaction in a reformer because the high temperature and lower pressure of the conventional reformer favor the strongly endothermic reforming reaction, whereas the exothermic shift reaction is favored by low temperatures and is largely unaffected by pressure change. Normal operating conditions for a steam/methane reformer are from about 750 to about 900° C. at from about 100 to about 500 psig over a catalyst such as nickel, cobalt, and the like.
This invention is useful with any device that performs a reforming reaction on a methane containing feed to produce a synthesis gas that contains at least in part a mixture of hydrogen and carbon monoxide, preferably a substantial amount of said mixture, still more preferably a major amount on a volume basis of said mixture.
The aforesaid synthesis gas will vary in its composition depending upon the feed stock to the reformer, e.g., ethane propane, natural gas or the like, and mixtures of two or more thereof. Natural gas can vary in its composition depending upon where geologically and geographically it was produced but it generally contains a preponderance of methane with minor amounts of hydrocarbons having from two to five carbon atoms per molecule and one or more of carbon dioxide, nitrogen, sulfur, and the like. A synthesis gas from a natural gas feed can, for example, contain about 70 volume percent (hereinafter “vol %”) hydrogen, about 17 vol % carbon monoxide, about 8 vol % carbon dioxide with the remainder essentially methane plus trace amounts of inerts such as nitrogen, all vol % being based on the total volume of the synthesis gas. Hereafter all percentage figures will be volume percent figures, and all volume percent figures herein are on a dry basis. Considering the conventional range of commercial reformer feed stocks synthesis gas compositions can generally be from about 65 to about 75 vol % hydrogen, from about 13 to about 20 vol % carbon monoxide, and from about 6 to about 10 vol % carbon dioxide with the remainder various combinations of one or more of methane, nitrogen, and the like. The H2/CO ratios will also vary with the reformer feed stock composition. For example, for a natural gas feed the H2/CO ratio can be about 4/1, with propane 3.5/1, and with naphtha 3/1. Generally, the synthesis gas reformer product useful in this invention will have an H2/CO ratio of from about 3/1 to about 4/1, depending on the reformer feed composition.
The synthesis gas issuing from the reformer is, as stated above, at a relatively low pressure of from about 100 to about 500 psig. Normally the synthesis gas is next compressed in a conventional manner to a pressure of at least about 600 psig, preferably from about 600 to about 2000 psig, at essentially ambient temperature, e.g., from about 80 to about 120° F. The precise degree of compression depends upon the desired downstream use of the synthesis gas. For example, if the gas is going to an ammonia synthesis plant it will be compressed to about 1500 psig, whereas for a methanol plant it will be compressed to about 2000 psig.
All or any part of the compressed synthesis gas is then, in accordance with this invention, passed to a membrane separation unit. To the extent part of the compressed synthesis gas is not subjected to such membrane separation unit, it can be passed to one or more conventional manufacturing plants for the production of one or more of ammonia, methanol, and the like.
The compressed synthesis gas feed for the membrane separation unit will be at essentially the same temperature and pressure conditions stated above for the compression step, e.g., at least about 600 psig and essentially ambient temperature.
Chemical separation with a membrane had its start in the early nineteenth century with the diffusion work of Thomas Graham, the father of Graham's law—the rate of diffusion of a gas is inversely proportional to the square root of its molecular weight. Considerable advancements have been made since the time of Graham in materials and processes for making and using membranes as well as in the mechanistic understanding of membrane transport phenomena. For example, the development of the asymmetric membrane morphology by Loeb and Sourirajan was a major breakthrough. Asymmetric membranes are essentially anisotropic (having a graded distribution of pore sizes) membranes with a well-defined skin on one side of the membrane. Immediately below this skin, the pore size is very small (less than one nanometer). The pore size increases as it moves away from the skin. There may or may not be a skin on the opposite face of the membrane. Membrane separation can be ca
Equistar Chemicals LP
MacDonald Roderick W.
Medina Maribel
Silverman Stanley S.
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