Chemistry: fischer-tropsch processes; or purification or recover – Treatment of feed or recycle stream
Reexamination Certificate
1997-10-14
2001-01-09
Kumar, Shailendra (Department: 1621)
Chemistry: fischer-tropsch processes; or purification or recover
Treatment of feed or recycle stream
C518S702000, C518S703000, C518S728000
Reexamination Certificate
active
06172124
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for converting gases to liquids and in one of its aspects relates to a highly-efficient process for converting hydrocarbon gas (e.g. methane) to a hydrocarbon liquid (e.g. gasoline, distillates, etc.) which includes the improved operation for providing the required process-air and wherein waste heat and tail gas is efficiently recovered to be used within the process and/or to generate auxiliary power.
BACKGROUND ART
The desirability to convert light, hydrocarbon gases (e.g. natural gas) to liquids (e.g. methanol, gasolines, distillates, etc.) has long been recognized. Probably the most commonly-proposed process for carrying out this type of conversion is one wherein natural gas is first flowed through an Autothermal Reformer, e.g. a partial oxidation reformer, to convert the natural gas to a synthesis gas (“syngas”, i.e. a gas comprised of carbon monoxide (CO) and hydrogen (H
2
)). The syngas is then fed to a Fischer-Tropsch type of reactor which is loaded with an appropriate catalyst which, in turn, converts the syngas to a desired product (e.g. methanol, gasolines, distillates, etc.) depending on the catalyst and the operating conditions within the reactor. Such processes are well-known in the industry; for examples of Fischer-Tropsch (“F-T”) processes of this type, see U.S. Pat. Nos. 1,798,288; 2,500,533; 2,552,308; 4,579,985; and 4,973,453.
While the type of basic process has been known for some time, efforts are continously being made to improve its efficiency in order to make it more commercially attractable. For example, where possible, air instead of oxygen is used as a reactant in the ATR stage since air is obviously cheaper and more readily available than pure oxygen; e.g. see U.S. Pat. Nos. 2,500,533; 2,552,308, et sec. Further, a continuing search is on-going to find the ultimate catalyst for use in the F-T reactor; e.g. see U.S. Pat. Nos. 5 4,522,939; 4,755,536; et sec. Also, improvements in the various elements (e.g. partial oxidation reformer) used in the process are important considerations in attempting to optimize the process (e.g. see U.S. Pat. Nos. 3,549,335; 4,778,826) for commerical use.
Another very important consideration in the commercialization of such a process is maximizing the recovery of otherwise wasted heat and gases from the process for use in the process, itself, or for generating excess energy (i.e. heat and/or mechanical power) which, in turn, can be sold or used in other applications. For example, (a) energy may be generated by reacting off-gas from the process in a fuel cell, see U.S. Pat. No. 4,048,250; (b) dry or tail gas may be used for, generating heat used in the process, see U.S. Pat. No. 4,048,250; (c) heat recovered from a gas turbine, which is used in the process to both compress the process-air and drive an electrical generator, may be used in the ATR, see U.S. Pat. No. 4,315,893; and (d) heat, recovered from the product after it passes through the reformer, may be used to generate a separate stream of superheated steam while the syngas may be expanded through a turbine to recover mechanical energy, see U.S. Pat. No. 4,074,981. While each of these approaches add to the operating efficiency of the overall conversion process, there is still much which can be done in the optimizing the process to make it more commercially acceptable.
SUMMARY OF THE INVENTION
The present invention provides a process for converting a hydrocarbon gas (e.g. natural gas) to syngas which, in turn, is converted into a liquid hydrocarbon product wherein a substantial amount of the heat generated in the process is recovered for use in the process or to be converted to mechanical energy. Further, the tail gas generated in the process is used to fuel the gas turbine which is used to power the compressors which, in turn, are used to compress the “process-air”. By using tail gas to fuel the gas turbine, less of the compressed combustion-cooling air has to be used to cool the combustion gases (i.e. exhaust gases) from the combustor of the turbine and, instead, can be used to provide a portion of the process-air required in the process. This can save up to 20 to 30 percent of the horsepower which otherwise would be needed to compress the volume of process-air needed for the process.
More specifically, the present invention provides a process for converting a hydrocarbon feed gas to a hydrocarbon liquid wherein the process-air needed for carrying out the process is compressed by a compression unit which is powered by a gas turbine wherein the gas turbine has a compressor section, a combustor, and a turbine section. The compressor section compresses combustion-cooling air, a first portion of which (i.e. “combustion-air”) is supplied to the combustor where it is mixed with tail gas which, in turn, is recovered from the process, itself.
A typical tail gas recovered from the present process is comprised of methane, carbon monoxide, carbon dioxide, hydrogen, nitrogen, and other light hydrocarbons (e.g. C
2
-C
4
) which burns substantially cooler than do higher-BTU fuels such as natural gas thereby producing combustion gases at lower temperatures. This allows a substantially smaller second portion of the compressed combustion-cooling air to be used to cool the the same volume of combustion gases to the temperature required for safe operation of the turbine section of the gas turbine. By using less of the compressed combustion-cooling air air for combustion and cooling, a substantial remaining portion (e.g. about 30 to 40% of the original volume) of the compressed combustion-cooling air from said compressor section can be supplied directly to said process to form a portion of the process-air required to carry out the present process.
Once the process air has been compressed, it is mixed with steam and is heated in a heater before the mixture is passed to an Autothermal Reforming Unit (ATR). A hydrocarbon feed gas (e.g. methane) is also mixed with steam and is heated in the heater (the heater possibly being fueled with tail gas from the process) before this feed gas/steam mixture is also passed into the ATR where it is mixed with the process air/steam mixture in the presence of a catalyst to form a syngas which, in turn, is comprised of nitrogen, carbon monoxide and hydrogen. Heat is recovered from the syngas and is used to generate steam, some of which is mixed with both the process-air and the feed gas.
The syngas is then passed over a catalyst in a Fischer-Tropsch reactor to thereby convert at least a portion of said syngas to liquid hydrocarbons. Heat is also recovered from the reactor as said syngas being converted to a liquid hydrocarbon and can be used in generating the steam needed in the present process. The products from the reactor are passed to a separation section where the unconverted syngas is separated from the liquid hydrocarbon. It is this unconverted syngas and by-products (methane, C
2
-C
4
, carbon dioxide, and nitrogen) which forms the “tail gas” which is used for fuel in the process. Also, at least a portion of the tail gas may be expanded through a turbine to recover mechanical energy therefrom.
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paten
Beer Gary L.
Payne Richard L.
Wolflick John R.
Baker & Botts L.L.P.
Kumar Shailendra
Sybtroleum Corporation
Vollano Jean F
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