Chemistry: fischer-tropsch processes; or purification or recover – Liquid phase fischer-tropsch reaction
Reexamination Certificate
2000-03-29
2001-08-21
Padmanabhan, Sreeni (Department: 1621)
Chemistry: fischer-tropsch processes; or purification or recover
Liquid phase fischer-tropsch reaction
C518S702000, C518S703000, C518S704000
Reexamination Certificate
active
06277894
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to synthesis systems and more particularly, to a system and method for converting (e.g., through a Fischer-Tropsch reaction) light hydrocarbons, such as natural gas, into heavier hydrocarbons with a plurality of synthesis gas subsystems.
BACKGROUND OF THE INVENTION
As concerns over pollution caused by traditional fossil fuels increases and as sources of crude oil decrease, there has been increased interest in other sources of energy. One promising source of energy is the synthetic production of fuels, lubricants, and other products from natural gas (referred to at times as gas-to-liquids or GTL) preferably through the Fischer-Tropsch process. See for example U.S. Pat. Nos. 4,883,170 and 4,973,453, which are incorporated by reference herein for all purposes.
A. Introduction to the Fischer Tropsch Process
The synthetic production of hydrocarbons by the catalytic reaction of synthesis gas is well known and is generally referred to as the Fischer-Tropsch reaction. The Fischer-Tropsch process was developed in early part of the 20
th
century in Germany. It was practiced commercially in Germany during World War II and later has been practiced in South Africa.
The Fischer-Tropsch reaction for converting synthesis gas (primarily CO and H
2
) has been characterized in some instances by the following general reaction:
The hydrocarbon products derived from the Fischer-Tropsch reaction range from some methane to high molecular weight paraffinic waxes containing more than 50 carbon atoms.
Numerous catalysts incorporating active metals, such as iron, cobalt, ruthenium, rhenium, etc., have been used in carrying out the reaction, and both saturated and unsaturated hydrocarbons can be produced. The synthesis reaction is very exothermic and temperature sensitive whereby temperature control is required to maintain a desired hydrocarbon product selectivity.
B. Synthesis Gas Production
Synthesis gas may be made from natural gas, gasified coal, and other sources. A number of basic methods have been employed for producing the synthesis gas (“syngas”), which is substantially carbon monoxide and molecular hydrogen, utilized as feedstock in the Fischer-Tropsch reaction. The numerous methodologies and systems that have been used to prepare synthesis gas include partial oxidation, steam reforming, auto-reforming or autothermal reforming. Both fixed and fluid bed systems have been employed.
The reforming reactions are endothermic and a catalyst containing nickel is often utilized. Partial oxidation (non-catalytic or catalytic) involves sub-stoichiometric combustion of light hydrocarbons such as methane to produce the synthesis gas. The partial oxidation reaction is typically carried out commercially using high purity oxygen.
In some situations these synthesis gas production methods may be combined to form another method. A combination of partial oxidation and steam reforming, known as autothermal reforming, wherein air may be used as the oxygen-containing gas for the partial oxidation reaction has also been used for producing synthesis gas heretofore. Autothermal reforming, the combination of partial oxidation and steam reforming, allows the exothermic heat of the partial oxidation to supply the necessary heat for the endothermic steam reforming reaction. The autothermal reforming process can be carried out in a relatively inexpensive refractory lined carbon steel vessel whereby a relatively lower cost is typically involved.
The autothermal reforming process results in lower hydrogen to carbon monoxide ratio in the synthesis gas than does steam reforming alone. That is, the steam reforming reaction with methane results in a ratio of about 3:1 or higher while the partial oxidation of methane results in a ratio of less than about 2:1—depending upon the extent of the water gas shift reaction. A good ratio for the hydrocarbon synthesis reaction carried out at low or medium pressure (i.e., in the range of about atmospheric to 500 psig) over a cobalt catalyst is about 2:1. When the feed to the autothermal reforming process is a mixture of light shorter-chain hydrocarbons such as a natural gas stream, some form of additional control is desired to maintain the ratio of hydrogen to carbon monoxide in the synthesis gas at the optimum ratio (for cobalt based F-T catalysts) of about 2:1. For this reason steam and/or CO
2
may be added to the synthesis gas reactor to adjust the H
2
/CO ratio to the desired value with the goal of optimizing the process economics.
C. Improved Economics Desired
It has been a quest for many to improve the economics of processes utilizing the Fischer-Tropsch reaction. Improved economics will allow a wide-scale adoption of the process in numerous sites and for numerous applications. Efforts have been made toward that end, but further improvements are desirable.
SUMMARY OF THE INVENTION
A need has arisen for a system and method for converting light hydrocarbons into heavier hydrocarbons (C
5+
) that addresses disadvantages and problems associated with previously developed systems and methods. According to an aspect of the present invention, a system for converting normally gaseous hydrocarbons into heavier hydrocarbons, which are liquid or solid at standard temperature and pressure, includes: a turbine; a first synthesis gas subsystem; a second synthesis gas subsystem that receives thermal energy from the turbine and which includes a steam reformer; and a synthesis subsystem for receiving synthesis gas from the first synthesis gas subsystem and from the second synthesis gas subsystem and which produce the heavier hydrocarbons.
According to another aspect of the present invention, a method for converting normally gaseous hydrocarbons to heavier hydrocarbons that are normally solid or liquid at standard temperature and pressure is provided that includes the steps of: preparing a synthesis gas in a first synthesis gas unit; providing a steam reformer having a primary reforming zone; providing a turbine having a compressor section, cumbustor, and expansion section; thermally coupling the expansion section of the turbine to the steam reformer to provide at least a portion of the reaction energy required in the steam reformer to produce synthesis gas; preparing a synthesis gas in the steam reformer; delivering the synthesis gas from the first synthesis gas unit and the steam reformer to a synthesis subsystem for conversion to the heavier hydrocarbons. According to another aspect of the present invention, the combustor section of the gas turbine may be combined with the first synthesis gas unit, which may be an autothermal reformer or a steam reformer.
The present invention provides a number of advantages. A few examples follow. A technical advantage of the present invention is that it allows more efficient use of energy in a turbine-powered synthesis system. Another technical advantage of the present invention is that it lowers the nitrogen content in the synthesis gas compared to a straight air-blown autothermal reformer based conversion system.
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“PFBC clean-coal technology. A new generation of combined-cycle plants to meet the growing world need for clean and cost effective power.” ABB Carbon Marketing Department, S-612 82 Finspong, Approximately Feb. 1998.
PCT International Search Report (PCT Rule 44.1), mailed Nov. 2, 2000 re International Application PCT/US00/08371 filed Mar. 29, 2000 (Applicant's reference 062754.0214).
Agee Kenneth L.
Agee Mark A.
Baker & Botts L.L.P.
Padmanabhan Sreeni
Parsa J.
Syntroleum Corporation
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