Chemistry: fischer-tropsch processes; or purification or recover – Plural zones each having a fischer-tropsch reaction
Reexamination Certificate
2000-02-29
2001-12-18
Richter, Johann (Department: 1621)
Chemistry: fischer-tropsch processes; or purification or recover
Plural zones each having a fischer-tropsch reaction
C518S700000, C518S715000, C518S719000, C585S664000, C585S734000
Reexamination Certificate
active
06331573
ABSTRACT:
BACKGROUND OF THE INVENTION
The majority of combustible fuel used in the world today is derived from crude oil. There are several limitations to using crude oil as a fuel source. Crude oil is in limited supply; it includes aromatic compounds believed to cause cancer, and contains sulfur and nitrogen-containing compounds that can adversely affect the environment, for example, by producing acid rain.
Combustible liquid fuels can also be prepared from natural gas. This involves converting the natural gas, which is mostly methane, to synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. An advantage of using fuels prepared from syngas is that they do not contain nitrogen and sulfur and generally do not contain aromatic compounds. Accordingly, they have minimal health and environmental impact.
Fischer-Tropsch chemistry is typically used to convert the syngas to a product stream that includes combustible fuel, among other products. A limitation associated with Fischer-Tropsch chemistry is that it tends to produce a broad spectrum of products, ranging from methane to wax. Product slates for syngas conversion over Fischer-Tropsch catalysts (Fe, Co and Ru) are controlled by polymerization kinetics with fairly constant chain growth probabilities, which fix the possible product distributions. Heavy products with a relatively high wax content are produced when chain growth probabilities are high. Methane is produced with high selectivity when chain growth probabilities are low.
Methane can be recirculated to ultimately yield combustible liquid fuel. Wax can be processed, for example, by hydrocracking and/or hydrotreating followed by oligomerization, to yield combustible liquid fuel. However, it would be advantageous to have new methods for providing a product stream from a Fischer-Tropsch process that has a higher proportion of combustible liquid fuel with less methane to recirculate and less wax to process.
One method used in the past to minimize methane production has been to incorporate olefins in the Fischer-Tropsch reaction. Work in the early 1930's used a roughly 1:1 ratio of hydrogen/carbon monoxide, and added olefins to the reaction mixture (Smith et al.,
J.A.C.S.,
52:3221 (1930). This tended to provide oxygenated material, which is not preferred. U.S. Pat. No. 4,754,092 to Iglesia et al. discloses incorporating olefins into a Fischer-Tropsch reaction, but does not specify the type of chain growth probabilities for the reaction, and discloses using a wide range of hydrogen/carbon monoxide ratios such that it would be difficult to predict whether the product would be oxygenated, olefinic, or saturated.
It would be advantageous to provide methods for improving product yields in Fischer-Tropsch reactions, while minimizing methane and oxygenate production. The present invention provides such methods.
SUMMARY OF THE INVENTION
In its broadest aspect, the present invention is directed to an integrated process for producing liquid fuels from syngas via a two-stage Fischer-Tropsch reaction. The first stage of the Fischer-Tropsch chemistry is performed using conditions in which chain growth probabilities are relatively low to moderate, and the product of the reaction includes a relatively high proportion of low molecular (C
2-8
) weight olefins and a relatively low proportion of high molecular weight (C
30
+) waxes.
The products of the first stage include methane, C
2-4
hydrocarbons, C
5
+ hydrocarbons, water and carbon dioxide, as well as unreacted syngas. Optionally, but preferably, water produced in the first stage is substantially removed before the product stream is sent to the second stage. Optionally, the product is hydrotreated at this stage to remove any oxygenated products. Further, C
5
+ hydrocarbons are preferably isolated. In one embodiment, at least a portion of the olefins is isomerized prior to being fed into the second stage.
The product from the first stage, after any optionally performed processing steps, is then fed into the second stage where the chain growth probabilities are relatively high. The wax and other paraffins produced in the first reaction are largely inert under these conditions. The light olefins compete with heavier olefins for chain initiation, and fewer chains will be initiated at C
20
+. With most chains initiated at C
2-8
, moderate chain growth probability will produce a relatively larger fraction in the C
5-12
range. In this manner, wax yield is minimized.
The syngas used in both stages preferably contains certain ratios of hydrogen to carbon monoxide. In the first stage, hydrogen/carbon monoxide ratios in excess of about 1.0/1.0 tend to provide less olefins and more hydrogenated products, although changing the temperature and/or pressure may mitigate these effects to some degree. In the second stage, using a cobalt-containing catalyst, a ratio of hydrogen to carbon monoxide greater than 1.5/1.0 tends to provide a product that is greater than 80% saturated. At a ratio of about 1.0/1.0, the product tends to include oxygenates and olefins.
In one embodiment, the Fischer-Tropsch synthesis with low to moderate chain growth probability is performed using an iron-containing catalyst in the first reactor, and Fischer-Tropsch synthesis with high chain growth probability is performed using a cobalt-containing catalyst in the second reactor.
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Burns Doane Swecker & Mathis L.L.P.
Chevron U.S.A. Inc.
Parsa J.
Richter Johann
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