Chemistry: fischer-tropsch processes; or purification or recover – Liquid phase fischer-tropsch reaction
Reexamination Certificate
1999-08-17
2001-10-23
Kumar, Shailendra (Department: 1621)
Chemistry: fischer-tropsch processes; or purification or recover
Liquid phase fischer-tropsch reaction
C518S702000, C518S703000, C518S715000, C518S721000
Reexamination Certificate
active
06306917
ABSTRACT:
FIELD OF INVENTION
This invention relates to improved processes for the conversion of carbon-containing liquid and solid feedstocks into valuable liquid hydrocarbon products by subjecting the feedstock to partial oxidation to produce synthesis gas and converting the synthesis gas into valuable products using a Fischer-Tropsch reactor and an iron-based catalyst. In these processes, high carbon feedstocks are converted to relatively low carbon fuels and the excess carbon dioxide is separated. The production of power, carbon dioxide or hydrocarbons can be selectively maximized to provide greater operational flexibility and economic return.
BACKGROUND OF THE INVENTION
Carbon dioxide emissions to the atmosphere have risen steadily since the beginning of the industrial revolution. At present, worldwide combustion of fossil fuels emits about 22 Gt of carbon dioxide to the atmosphere annually. The measured annual increase in atmospheric carbon dioxide is approximately 13 Gt. The difference between total output, which includes some additional emissions from deforestation and other anthropogenic sources, and the observed increase in atmospheric carbon dioxide is absorbed into natural sinks like the ocean and the biosphere. The substantial absorption indicates that the current state of the atmosphere is far from a steady-state equilibrium. The level of atmospheric carbon dioxide has risen by 30 percent from its pre-industrial value of 280 ppm to about 365 ppm today. Most of this rise (about 60 ppm) has occurred during the past 50 years.
The size of readily accessible fossil fuel deposits is extremely large. Easily accessible oil and gas may be limited, but oil shales, tar sands and coal deposits are virtually inexhaustable. Coal deposits alone are estimated at 10,000 Gt, which may be compared to a worldwide annual consumption of about 6 Gt of carbon. Methane hydrate deposits have become of interest recently, and may dwarf all other carbon resources. It can thus be concluded that fossil fuel resources are not ultimately limited by availability, or even for that matter by the cost of extraction. Past history suggests that technological advances can be expected to keep up with a gradual degradation of the quality of the energy resources. Furthermore, various hydrocarbon sources can be regarded as virtually interchangeable at some incremental cost over current energy costs.
Today, fossil energy contributes about 85 percent of the world energy supply. It is the cheapest, most readily available energy source. Thus, fossil energy is likely to remain the dominant energy resource for satisfying the growing world energy demand. World energy demand is growing rapidly as the developing countries are becoming industrialized. The potential for further growth is extremely large. A world population of 10 billion with a per capita energy consumption equal to that of the U.S. today would consume ten times more energy than the world consumes today. Even though most energy forecasts assume far less growth over the next fifty years, higher growth resulting in additional improvements in living standards and a consequent increase in political stability would be highly desirable. These lower estimates actually assume that economic growth in the first half of the 21st century will be smaller than that in the second half of the 20th century. Even so, growth in energy demand will still be very large. Even with the extensive use of alternative forms of energy, the demand for fossil fuels will increase significantly.
Unless environmental considerations artificially limit the use of fossil energy, there is no end in sight for the demand for fossil fuels. Combustion of such quantities of fossil fuels could drive atmospheric carbon dioxide levels very much higher. The available 10,000 Gt of carbon correspond to 4700 ppm of atmospheric carbon dioxide. While the detailed effects of carbon dioxide on climate and environment are still debated, it is known that carbon dioxide is a greenhouse gas which can cause climate change. Carbon dioxide affects the acidity of the ocean, it is of physiological importance and thus can directly affect the ecological balance of species. To continue current energy consumption patterns could eventually lead to a doubling of natural carbon dioxide levels. To stabilize carbon dioxide at 600 ppm would require a drastic reduction in carbon dioxide emissions, ultimately to about 30 percent of those of 1990. For 10 billion people sharing in such a carbon dioxide budget, the per capita allowance would come to about 3 percent of that of the average U.S. citizen today.
In summary, it appears to be extremely difficult to stop the growth of fossil energy demand, yet to stabilize atmospheric carbon dioxide levels would require a drastic reduction in carbon dioxide emissions. The logical solution appears to be methods of collecting and subsequently disposing of the gas after it has been generated. While it is acknowledged that it is easier to collect carbon dioxide from a concentrated stream than from a dilute stream, it has actually been suggested that carbon dioxide could be collected from the atmosphere to accomplish these objectives. See Lackner et al., “Carbon Dioxide Extraction From Air: Is it an Option?”, Proceedings of 24th International Technical Conference on Coal Utilization and Fuel Systems, March 1999, Clearwater, Fla.
Given the expected increases of carbon dioxide in the atmosphere, it is clearly desirable to separate this gas from emissions by power plants or other sources, or even from the atmosphere itself, in order to dispose of or sequester carbon dioxide. Sequestration of carbon dioxide means its removal or segration from the atmosphere for a significant period of time, if not permanently. There are various approaches, including disposal in the deep ocean, injection into underground reservoirs and chemical stabilization as carbonate minerals. It is becoming increasingly important to prevent emissions from systems involving the combustion of fossil fuels from increasing the proportion of carbon dioxide in the air. Such removal and disposal, whether viewed as permanent sequestration or longterm term segregation, has economic value which can be awarded by national authorities as tax or pollution credits. For example, Norway presently levies a tax of over $50 U.S. per ton on carbon dioxide emissions. (See “Technology to Cool Down Global Warming,” infra.) Equivalent amounts may be awarded to organizations sequestering carbon dioxide from combustion processes.
A significant fraction of the crude oil fed to a refinery consists of heavy material generally having a high content of sulfur. This material is oftentimes an environmental liability to the refinery with high disposal costs. Recently it has been considered that a more economical solution to the problem is to convert the heavy crude oil to synthesis gas using partial oxidation (POX).
The partial oxidation (POX) reaction can be expressed as:
CH
z
+0.5O
2
→z/
2H
2
+CO
where z is the H:C ratio of the hydrocarbon feedstock. The water gas shift (WGS) reaction also takes place:
H
2
O+CO←→H
2
+CO
2
The synthesis gas can then be used as fuel in a gas turbine to generate electrical power. An example of this approach is the api Energia S.p.A integrated combined cycle plant (IGCC) described in the Dec. 9, 1996 issue of the Oil & Gas Journal. In many instances, it is not desirable or practical to use all of the synthesis gas produced in the POX reactor for production of electricity. In these instances it may be desirable to convert some or all of the synthesis gas to liquid hydrocarbons which are free of aromatics and sulfur using Fischer-Tropsch (FT) chemistry.
The Fischer-Tropsch (FT) synthesis reaction is expressed by the following stoichiometric relation:
2
n
H
2
+n
CO→C
n
H
2n
+n
H
2
O
The aliphatic hydrocarbons produced by the Fischer-Tropsch reaction have an H:C atom ratio of 2.0 or greater.
Fischer-Tropsch catalysts such as iron-based composites also catalyze the water gas shift (WG
Benham Charles S.
Bohn Mark S.
Kumar Shailendra
Martin Rick
Parsa J.
Patent Law Offices of Rick Martin P.C.
Rentech Inc.
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