Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
2002-07-16
2004-08-10
Silverman, Stanley (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C423S418200, C252S373000, C048S198100, C048S198700
Reexamination Certificate
active
06773691
ABSTRACT:
RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a process for the preparation of synthesis gas (“syngas”), i.e., a mixture of carbon monoxide and hydrogen, from natural gas. More particularly, the present invention relates to controlling the exit stream composition of a syngas reactor by controlling the feed hydrocarbon composition.
BACKGROUND
Large quantities of methane, the main component of natural gas, are available in many areas of the world. However, a significant portion of that natural gas is situated in areas that are geographically remote from population and industrial centers (“stranded gas”). The costs of compression, transportation, and storage often makes the use of stranded gas economically unattractive. Consequently, the stranded natural gas is often flared. Flaring not only wastes the energy content and any possible economic value the natural gas may have but may also create environmental concerns.
To improve the economics of natural gas transportation and utilization, much research has focused on using the methane component of natural gas as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to higher hydrocarbons is typically carried out in two steps. In the first step, methane is reacted to produce carbon monoxide and hydrogen (i.e., synthesis gas or “syngas”). In a second step, the syngas is converted to higher hydrocarbon products by processes such as Fischer-Tropsch synthesis. For example, fuels with boiling points in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes may be produced from the syngas. In addition, syngas may be used for the manufacture of ammonia, hydrogen, methanol, and other chemicals. Less traditional uses of syngas continue to be developed and have increased in importance in recent years, such as in the production of acetic acid and acetic anhydride manufacture. Among the promising new developments in syngas chemistry are routes to ethylene.
There are currently three primary methods for converting methane to syngas. Those methods include: steam reforming (the most widespread), dry reforming (also called CO
2
reforming), and partial oxidation. Steam reforming, dry reforming, and partial oxidation ideally proceed according to the following reactions respectively:
CH
4
+H
2
O+heat→CO+3H
2
(1)
CH
4
+CO
2
+heat→2CO+2H
2
(2)
CH
4
+½O
2
→CO+2H
2
+heat (3)
For a general discussion of steam reforming, dry (or CO
2
) reforming, and partial oxidation, please refer to HAROLD GUNARDSON,
Industrial Gases in Petrochemical Processing
41-80 (1998), the contents of which are incorporated herein by reference.
Although a theoretical H
2
:CO ratio can be calculated for any given reaction, relative amounts of hydrogen and carbon in a syngas product stream depend on many factors including the type of reaction, the process technology, the feedstock composition, and the reactor operating conditions. The theoretical ratio of hydrogen to carbon monoxide in the reactant stream of reactions 1, 2, and 3 can easily be calculated as 3:1, 2:2 (i.e., 1:1), and 2:1. The actual ratio of hydrogen to carbon monoxide in syngas product streams can range as low as 0.6 with CO
2
reforming of natural gas or partial oxidation of petroleum coke to as high as 6.5 with steam methane reforming. In addition, it has been noticed in GUNARDSON on pages 68-71 the actual molar ratio of H
2
:CO in the product stream can vary depending upon the feedstock used.
There are many processes, such as the production of methanol, in which an H
2
:CO molar ratio of about 2:1 is desired. There are also processes in which a molar ratio of hydrogen and carbon monoxide of less than 2:1 is preferable. One such process is hydroformylation, which is the addition of one molecule of carbon monoxide and one molecule of hydrogen to an olefin to make an aldehyde. The following reaction illustrates one of the simplest examples of hydroformylation:
C
2
H
4
+CO+H
2
→CH
3
CH
2
CHO (4)
Hydroformylation is, inter alia, an intermediate step in both methyl methacrylate synthesis and the oxo process to produce alcohols. Additionally, there may be other processes in which an H
2
:CO ratio of between 2:1 and about 1:1 is desirable.
As noted above, one method of producing syngas with a molar ratio of hydrogen to carbon monoxide of between about 2:1 and about 1:1 is by the partial oxidation of methane followed by the CO
2
reforming of methane. Unfortunately, CO
2
reforming is endothermic and requires external heating to drive the reaction, which increases the capital cost of CO
2
reforming. In addition, this scheme of partial oxidation followed by CO
2
reforming requires two reactors thereby also increasing the capital cost. Thus, in many situations, partial oxidation followed by CO
2
reforming may be economically or physically (or both) unfeasible or undesirable.
There is, therefore, a need for a less capital intensive process in which the H
2
:CO molar ratio in the product stream can be varied and controlled between about 2:1 and about 1:1
SUMMARY OF THE PREFERRED EMBODIMENTS
The present invention provides a method for controlling the H
2
:CO molar ratio between about 2:1 and about 1:1 in a syngas product stream by controlling the feed hydrocarbon composition.
An embodiment of the present method generally includes predetermining a desired syngas product stream H
2
:CO molar ratio, selecting a hydrocarbon with an actual natural H
2
:CO molar ratio greater than the desired molar ratio, selecting a hydrocarbon with an actual natural H
2
:CO molar ratio less than the desired molar ratio, mixing the two hydrocarbons on-line such that the actual natural H
2
:CO molar ratio of the mixture is equal to the desired molar ratio, and net catalytically partially oxidizing the mixture to produce syngas with the desired H
2
:CO molar ratio.
It is also possible to control and vary the product stream composition by controlling and varying the feed stream composition.
REFERENCES:
patent: 6488907 (2002-12-01), Barnes et al.
Gunardson, Harold,Industrial Gases in Petrochemical Processing, pp. 41-88, Ch. 2 “Synthesis Gas Manufacture,” Marcel Dekker, Inc. Pub. (1998).
Texaco—Brochure entitled“Texaco Gasification Process for Gaseous or Liquid Feedstocks” (Before Sep. 3, 1997).
Andrzej Cybulski and Jacob A. Moulijn eds.,Structured Catalysts and Reactors, pp. 179-208, 599-615 (1998).
Allison Joe D.
Keller Alfred E.
Ramani Sriram
Conley & Rose, P.C.
ConocoPhillips Company
Medina Maribel
Silverman Stanley
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