Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
2002-07-23
2004-06-15
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C252S373000, C423S580100
Reexamination Certificate
active
06749829
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a process for cost-effectively producing commercial products from natural gas. More particularly, this invention relates to an improved process for manufacturing synthesis gas, an intermediate product useful for subsequent conversion to more easily stored and transported hydrocarbon products.
Natural gas generally refers to rarefied or gaseous hydrocarbons found in the earth. Non-combustible natural gases occurring in the earth, such as carbon dioxide,-helium and nitrogen are generally referred to by their proper chemical names. Often, however, non-combustible gases are found in combination with combustible gases and the mixture is referred to generally as “natural gas” without any attempt to distinguish between combustible and non-combustible gases. See Pruitt, “Mineral Terms-Some Problems in Their Use and Definition,” Rocky Mt. Minn. L. Rev. 1, 16 (1966).
Natural gas is often plentiful in regions where it is uneconomical to develop those reserves due to lack of a local market for the gas or the high cost of processing and transporting the gas to distant markets.
The technologies currently employed to move natural gas to remote markets generally include, but are not limited to, pipelines, the cryogenic manufacture of liquefied natural gas (LNG), and the production of Gas to Liquids (GTL) products. Each of these technologies feature benefits and incur penalties which can make one technology preferred over another for any given commercial environment.
Traditional GTL products include, but are not limited to, methanol, acetic acid, olefins, dimethyl ether, dimethoxy methane, polydimethoxy methane, urea, ammonia, fertilizer and Fischer Tropsch (FT) reaction products. The FT reaction produces mostly hydrocarbon products of varying carbon chain length, useful for producing lower boiling alkanes, alkenes, naphtha, distillates useful as jet and diesel fuel and furnace oil, and lubricating oil and wax base stocks.
GTL products, such as those produced from FT synthesis, including FT diesel fuels, have chemical and physical properties and environmental qualities that permit these products to most easily benefit from traditional energy infrastructure, including fuel storage and dispensing equipment. Additionally, fuel consuming equipment such as automotive/airplane engines as well as other utility, transportation and domestic power systems are currently more receptive or adaptable to the use of GTL products.
The broader implementation of GTL technology commercially, however, has been limited by the high capital cost and operational efficiency of the GTL manufacturing plants. Moreover, the largest single component of capital cost and the single largest contributor to operational efficiency (or inefficiency) has generally been the synthesis gas manufacturing units at the GTL manufacturing plant.
Therefore, there is a great need in industry for improved synthesis gas technology that is cost effective and operationally efficient.
For purposes of the present invention, methods for producing GTL products are categorized as either indirect synthesis gas routes or as direct routes. The indirect synthesis gas routes involve the production of synthesis gas comprising hydrogen and carbon monoxide as an intermediate product whereas the direct routes shall be construed as covering all others.
The most common commercial methods for producing synthesis gas are steam/methane reforming, auto-thermal reforming, gas heated reforming, partial oxidation, and combinations thereof.
Steam/methane reforming reacts steam and natural gas at high temperatures and moderate pressures over a reduced nickel-containing catalyst to produce synthesis gas where the reaction heat is applied externally to the process.
Autothermal reforming processes steam, natural gas and oxygen through a specialized burner where only a portion of the methane from the natural gas is combusted. Partial combustion of the natural gas provides the heat necessary to conduct the reforming reactions that will occur over a catalyst bed located in proximity to the burner.
Gas heated reforming consists of two reactors, a gas heated reformer reactor and an autothermal reformer reactor. Steam and natural gas are fed to the gas-heated reformer where a portion of the natural gas reacts, over catalyst, to form synthesis gas. This mixture of unreacted natural gas and synthesis gas is then fed to the autothermal reformer, along with oxygen, where the remaining natural gas is converted to synthesis gas. The hot synthesis gas stream exiting the autothermal reformer is then routed back to the gas reformer to provide the heat of reaction necessary for the gas-heated reformer.
Partial oxidation generally processes steam, natural gas and oxygen through a specialized burner where a substantial portion of the methane is combusted at high temperatures to produce synthesis gas. Contrary to autothermal reforming, no catalyst is present in the partial oxidation reactor.
Autothermal reforming, gas heated reforming, and partial oxidation all require an internal natural gas oxidation step where hydrocarbon is partially oxidized, generally in the presence of less than 60 mole % of the stoichiometric amount of oxygen necessary to fully oxidize natural gas to carbon dioxide and water. In each case, the hydrocarbon oxidation step results in a measurable portion of the hydrocarbon being converted into carbon dioxide. Carbon dioxide formation reduces the overall thermal efficiency of the GTL process resulting in lower product yields. The natural gas oxidation step, at high temperatures, can also lead to the formation of coke, soot, and coke and soot precursors that deactivate the synthesis gas catalyst, increase reactor pressure drop, and/or cause the synthesis gas reactors to plug.
Steam/methane reforming converts natural gas to synthesis gas in the presence of steam at elevated temperatures without the use of oxygen but with other means of external heat input. Since the steam/methane reforming reaction itself does not include internal oxidation or partial oxidation, substantially all of the energy required to facilitate the reaction must be supplied from an external source. These external sources generally include high reactor feed preheat temperatures supplemented by heat input supplied externally by the complete combustion of natural gas. In each case, the combustion products from the external heat supplied (water and carbon dioxide) are released to the atmosphere and are not converted to synthesis gas.
The steam/methane reforming reaction can also result in the formation of a synthesis gas having a higher hydrogen to carbon monoxide ratio than is desirable for certain GTL commercial uses. Such steam reforming processes often necessitate installation of equipment to remove excessive hydrogen prior to any downstream conversion steps or, after downstream conversion steps, result in unconverted hydrogen, carbon dioxide, light hydrocarbon and unrecovered synthesis gas being recycled within the process or consumed as fuel.
It has now been found that hydro-steam reforming utilizing steam recovered from the oxidation of hydrogen to water increases synthesis gas reforming process efficiency compared to steam/methane reforming processes requiring external energy sources that ultimately exhaust higher levels of carbon in the form of carbon dioxide to the atmosphere.
It has also been found that hydro-steam reforming utilizing the energy recovered from the oxidation of hydrogen to water substantially reduces or eliminates any external energy inputs that would otherwise be required to reach steam/methane reforming reaction temperatures or to sustain an endothermic steam/methane reforming process.
It has also been found that utilizing energy recovered from the oxidation of hydrogen substantially reduces coke and particulate accumulation in the reforming reaction zone compared to processes that oxidize substantial portions of natural gas hydrocarbon for reaction energy.
It has also been found that where hydro-steam reforming utilizing
BP Corporation North America Inc.
Medina Maribel
Silverman Stanley S.
Wood John L.
Yassen Thomas A.
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