Chemistry: fischer-tropsch processes; or purification or recover – Liquid phase fischer-tropsch reaction
Reexamination Certificate
2000-02-11
2001-05-01
Richter, Johann (Department: 1621)
Chemistry: fischer-tropsch processes; or purification or recover
Liquid phase fischer-tropsch reaction
C518S702000, C518S703000, C518S704000, C518S708000, C518S722000
Reexamination Certificate
active
06225358
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a system and method for converting light hydrocarbons into heavier hydrocarbons and more particularly to a system and method for converting light hydrocarbons into heavier hydrocarbons with improved water disposal.
BACKGROUND OF THE INVENTION
As concerns over depletion of traditional sources of energy and over pollution rise, modern society continues to seek new sources of clean energy. One helpful approach is to convert natural gas to a synthesis gas and then synthesize longer-chain hydrocarbons with a Fischer-Tropsch reaction. To make this effective, however, it needs to be sufficiently economical.
A. Introduction To The Fischer Tropsh Process
The synthetic production of hydrocarbons by the catalytic reaction of carbon monoxide and hydrogen is well known and is generally referred to as the Fischer-Tropsch reaction. The Fischer-Tropsch reaction for converting synthesis gas (primarily CO and H
2
) has been characterized by the following general reaction:
2H
2
+CO
{right arrow over (catalyst)}
—CH
2
—+H
2
O
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 have been used in carrying out the Fischer-Tropsch 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.
The Fischer-Tropsch process was developed in early part of the 20
th
century in Germany. It has been practiced commercially in Germany during World War II and later in South Africa. An ongoing quest has existed, however, to improve the economics of the process.
B. Synthesis Gas Production
Synthesis gas may be made from natural gas, gasified coal, and other sources. Three 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 two traditional methods are steam reforming, wherein one or more light hydrocarbons such as methane are reacted with steam over a catalyst to form carbon monoxide and hydrogen, and partial oxidation, wherein one or more light hydrocarbons are combusted sub-stoichiometrically to produce synthesis gas. The steam reforming reaction is endothermic and a catalyst containing nickel is often utilized. Partial oxidation is the non-catalytic, sub-stoichiometric combustion of light hydrocarbons such as methane to produce the synthesis gas. The partial oxidation reaction is typically carried out using high purity oxygen. High purity oxygen, however, can be quite expensive.
In some situations these synthesis gas production methods may be combined to form the third method. A combination of partial oxidation and steam reforming, known as autothermal reforming, has also been used for producing synthesis gas heretofore. For example, U.S. Pat. Nos. 2,552,308 and 2,686,195 disclose low-pressure hydrocarbon synthesis processes wherein autothermal reforming with air is utilized to produce synthesis gas for the Fischer-Tropsch reaction. Autothermal reforming is a combination of partial oxidation and steam reforming where the exothermic heat of the partial oxidation supplies 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 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 hydrogen to CO ratio of about 3:1 or higher while the partial oxidation of methane results in a ratio of less than about 2:1. A good ratio for the hydrocarbon synthesis reaction carried out at low or medium pressure 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 required to maintain the ratio of hydrogen to carbon monoxide in the synthesis gas at the optimum ratio of about 2:1. For this reason steam and/or CO
2
may be added to the synthesis gas reactor. See for example U.S. Pat. Nos. 4,883,170 and 4,973,453, which are assigned to the owner of this application and which are incorporated by reference herein for all purposes.
C. Introduction to Conversion Techniques and by Product Water
Numerous types of systems and reactors have been used for carrying out the Fischer-Tropsch reaction. The commercial development of the Fischer-Tropsch reactor system has included conventional fixed bed and three-phase slurry bubble column designs. Due to the complicated interplay between heat and mass transfer and the relatively high cost of Fischer-Tropsch catalysts, however, no single reactor design has dominated the commercial developments.
Many synthesis plants produce large quantities of waste water such as process condensate containing small amounts of contaminates. For example, a Fischer-Tropsch synthesis plant may produce waste water containing small amounts of alcohol and other oxygenates. A water treatment facility typically has been necessary. Such a facility might use biological treatments, which are fairly capital intensive. An approach using a stripper has been suggested in U.S. Pat. No. 5,053,581, entitled “Process For Recylcing And Purifying Condensate From A Hydrocarbon or Alcohol Synthesis Process,” but has not recognized nor addressed the relative vapor to liquid (V/L) ratio required to efficiently strip the stream in order to produce high purity water.
SUMMARY OF THE INVENTION
In accordance with teachings of the present invention, a conversion system and method are provided that address shortcomings and problems with previous systems and methods. According to one aspect of the present invention, a system for converting lighter hydrocarbons to longer-chain hydrocarbons includes a synthesis gas subsystem for receiving an oxygen-containing gas, such as air, and light hydrocarbons and producing a synthesis gas; a synthesis subsystem for receiving synthesis gas from the synthesis gas subsystem and producing longer-chain hydrocarbons therefrom and an aqueous byproduct stream having contaminates; and a stripper subsystem for receiving the aqueous byproduct stream and removing contaminates therefrom, wherein the stripper subsystem includes a concentrator column for concentrating contaminates in an aqueous by product stream, and a stripper column for receiving light hydrocarbons to strip contaminates from a concentrated aqueous byproduct stream.
According to another aspect of the present invention, a method for producing heavier hydrocarbons from lighter hydrocarbons includes the steps of: reacting air (or other oxygen-containing gas) and a light hydrocarbon feedstock to produce a synthesis gas; delivering the synthesis gas to a Fischer-Tropsch reactor; using a Fischer-Tropsch reaction in the Fischer-Tropsch reactor to convert the synthesis gas into heavier hydrocarbons; removing contaminates from an aqueous byproduct stream; and wherein the step of removing contaminates from the aqueous byproduct stream comprises the steps of: concentrating the contaminates in a concentrator column, and using the light hydrocarbons in a stripper column to remove the contaminates from the byproduct stream.
Technical advantages of the present invention include that it provides a system and method for cleaning up water for disposal at relatively reduced cost from previously known systems and techniques. Another technical advantage of the invention is that it allows for an increase in the carbon content of the light hydrocarbon feedstock.
REFERENCES:
patent: 2552308 (1951-05-01), Buchmann et al.
patent: 2686195 (1954-08-01), McAdams et al.
patent: 4973453 (1990-11-01),
Johnston Robert H.
Parson J.
Richter Johann
Syntroleum Corporation
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