U.S. patent application number 09/963253 was filed with the patent office on 2002-08-29 for processes for the production of hydrocarbons, power and carbon dioxide from carbon-containing materials.
Invention is credited to Benham, Charles B., Bohn, Mark S..
Application Number | 20020120017 09/963253 |
Document ID | / |
Family ID | 26907073 |
Filed Date | 2002-08-29 |
United States Patent
Application |
20020120017 |
Kind Code |
A1 |
Bohn, Mark S. ; et
al. |
August 29, 2002 |
Processes for the production of hydrocarbons, power and carbon
dioxide from carbon-containing materials
Abstract
Apparatus and processes for producing power, liquid hydrocarbons
and carbon dioxide from heavy feedstocks, using a partial oxidation
reactor to produce a synthesis gas, a Fischer-Tropsch reactor to
convert the synthesis gas to hydrocarbon products and tail gases
containing hydrogen and carbon dioxide, and a combined cycle plant
to produce power from steam generated by recovering heat from the
reactors and from combustible tail gases. By varying operating
conditions and utilizing hydrogen for recycle to the
Fischer-Tropsch reactor and/or hydrocracking wax products to
produce lighter hydrocarbons, the process can selectively maximize
the production of power, hydrocarbons or carbon dioxide. In
preferred embodiments, the Fischer-Tropsch reactor can be a slurry
reactor and can employ an iron-based catalyst.
Inventors: |
Bohn, Mark S.; (Golden,
CO) ; Benham, Charles B.; (Littleton, CO) |
Correspondence
Address: |
Patent Law Offices of Rick Martin, P.C.
416 Coffman Street
Longmont
CO
80501
US
|
Family ID: |
26907073 |
Appl. No.: |
09/963253 |
Filed: |
September 25, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09963253 |
Sep 25, 2001 |
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09376709 |
Aug 17, 1999 |
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6306917 |
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09376709 |
Aug 17, 1999 |
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09212374 |
Dec 16, 1998 |
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Current U.S.
Class: |
518/703 |
Current CPC
Class: |
C10J 2300/1659 20130101;
C10J 2300/1612 20130101; F25J 3/04587 20130101; Y02P 20/10
20151101; Y02E 20/18 20130101; C10G 2/332 20130101; F25J 3/04539
20130101; C10J 2300/1884 20130101; C10J 3/00 20130101; F25J 3/04545
20130101; C07C 1/0485 20130101; Y02E 20/16 20130101; C10J 2300/1653
20130101; C10K 1/005 20130101; Y02E 20/185 20130101; C10G 49/007
20130101; C10K 1/003 20130101; Y02P 20/124 20151101; C10G 2/30
20130101 |
Class at
Publication: |
518/703 |
International
Class: |
C07C 027/06 |
Claims
We claim:
1. A process for producing power, carbon dioxide and hydrocarbons
having an average H:C atom ratio of 2 or greater from
carbon-bearing feedstocks having an H:C atom ratio of less than 2,
comprising the steps of: a) reacting a carbon-bearing feedstock
with an oxidizing gas and steam in a partial oxidation reactor to
produce a mixture of gases containing hydrogen, carbon monoxide and
carbon dioxide having a molar ratio of H.sub.2:CO of greater than
0.6; b) reacting the mixture of gases containing hydrogen and
carbon monoxide in a Fischer-Tropsch synthesis reactor containing a
catalyst which catalyzes both hydrocarbon-forming reactions and the
water gas shift reaction; c) condensing the product hydrocarbons
from unreacted hydrogen, carbon monoxide and other gases (tail
gases); d) separating the product hydrocarbons into naphtha, diesel
and wax fractions; e) removing at least a portion of carbon dioxide
from the tail gases; and f) producing steam from heat recovered
from at least said partial oxidation reactor and said
Fischer-Tropsch reactor, directing the steam to the steam turbine
of a combined cycle plant, and directing at least the tail gases to
the gas turbine of said combined cycle plant to produce power,
wherein the process is operated to selectively maximize the
production of at least one of the products power, Fischer-Tropsch
hydrocarbons and carbon dioxide.
2. The process of claim 1 wherein natural gas is introduced to the
partial oxidation reactor to supplement the feedstock.
3. The process of claim 1 wherein natural gas is introduced to the
gas turbine of said combined cycle plant to increase power
production.
3. The process of claim 1 wherein acid gases are removed from the
products of the partial oxidation reactor before they are passed to
said Fischer-Tropsch reactor.
4. The process of claim 1 wherein hydrogen is separated from the
tail gases and directed to at least one of: a) the Fischer-Tropsch
reactor, and b) a hydrocracking reactor where wax products of the
process are hydrocracked to form naphtha and diesel fractions.
5. The process of claim 1 wherein at least a portion of the product
hydrocarbons are directed to the gas turbine of said combined cycle
plant to increase the production of power.
6. The process of claim 1 wherein said Fischer-Tropsch reactor is a
slurry reactor.
7. The process of claim 1 wherein said Fischer-Tropsch catalyst is
an iron catalyst.
8. The process of claim 7 wherein said catalyst is an iron-based
catalyst.
9. The process of claim 8 wherein said catalyst is an unsupported
precipitated iron catalyst promoted with copper and potassium.
10. The process of claim 1, wherein the carbon-bearing feedstock is
selected from the group consisting of heavy residual oil from an
oil refinery, petroleum coke, coal, aqueous emulsions of bitumen,
biomass, rubber tires and mixtures thereof.
11. The process of claim 1 wherein the oxidizing gas is oxygen of a
purity greater than about 90 volume percent.
12. The process of claim 1, wherein the oxidizing gas is a mixture
of gases containing greater than about 20 volume percent
oxygen.
13. The process of claim 1, wherein water is removed from the
mixture of gases from step (a) before they are reacted in step
(b).
14. The process of claim 1 wherein said Fischer-Tropsch reactor is
a slurry reactor.
15. The process of claim 1 wherein the production of
Fischer-Tropsch hydrocarbons is maximized by separating hydrogen
from the tail gases, recycling a portion of the hydrogen to the
Fischer-Tropsch reactor, utilizing the remainder of said hydrogen
to hydrocrack wax products in a hydrocracking reactor, and
directing all of said mixture of hydrogen and carbon monoxide to
said Fischer-Tropsch synthesis reactor.
16. The process of claim 1 wherein power production is maximized
while producing Fischer-Tropsch hydrocarbons by directing all tail
gases to said gas turbine and directing at least the wax and
naphtha fractions of said hydrocarbons to said gas turbine.
17. The process of claim 1 wherein power production is maximized
while producing Fischer-Tropsch hydrocarbons by introducing natural
gas to the gas turbine of said combined cycle plant.
18. The process of claim 1 wherein the production and separation of
carbon dioxide are maximized by increasing the water content of the
synthesis gas introduced to said Fischer-Tropsch reactor.
19. The process of claim 1 wherein the production and separation of
carbon dioxide are maximized by removing and recovering
substantially all carbon dioxide from the tail gases of the
hydrocarbon recovery section prior to directing said tail gases to
said gas turbine.
20. The process of claim 1 wherein the production and separation of
carbon dioxide are maximized by recycling up to about 90 percent of
the Fischer-Tropsch tail gases to the inlet of said Fischer-Tropsch
reactor.
21. The process of claim 1 wherein said Fischer-Tropsch reactor is
a slurry reactor.
22. Apparatus for producing power, carbon dioxide and hydrocarbons
having an average H:C atom ratio of 2 or greater from
carbon-bearing feedstocks having an H:C ratio of less than 2,
comprising: a) a partial oxidation reactor for reacting a
carbon-bearing feedstock with an oxidizing gas and steam to produce
a mixture of gases containing hydrogen, carbon monoxide and carbon
dioxide, having a molar ratio of H.sub.2:CO of greater than 0.6; b)
a Fischer-Tropsch synthesis reactor for reacting the mixture of
gases containing hydrogen and carbon monoxide, said reactor further
containing a catalyst which catalyzes both hydrocarbon-forming
reactions and the water gas shift reaction; c) means for
transporting the gases from the partial oxidation reactor to the
Fischer-Tropsch synthesis reactor; d) means for condensing the
product hydrocarbons from the Fischer-Tropsch synthesis reactor
from unreacted hydrogen, carbon monoxide and other gases (tail
gases); e) means for separating the product hydrocarbons into
suitable fractions; f) means for separating at least one of
hydrogen and carbon dioxide from the tail gases; g) means for
recycling at least a portion of the separated hydrogen to the inlet
of the partial oxidation reactor; h) means for recovering heat from
at least the partial oxidation and Fischer-Tropsch reactors and
generating steam; and i) a combined cycle plant comprising gas and
steam turbines for the production of power from products generated
by the Fischer-Tropsch reactor and the steam generated in (h).
23. The apparatus of claim 22, further comprising a hydrocracking
reactor and means for transporting a wax fraction of the
hydrocarbon products and a portion of the hydrogen separated from
the tail gases to said hydrocracking reactor to produce additional
hydrocarbon fractions.
24. The apparatus of claim 22 wherein said Fischer-Tropsch
synthesis reactor is a slurry reactor.
25. The apparatus of claim 24 wherein said slurry reactor is steam
cooled.
26. The apparatus of claim 22 wherein said catalyst comprises
iron.
27. The apparatus of claim 26 wherein said catalyst is an
iron-based catalyst.
28. The apparatus of claim 27 wherein said catalyst is an
unsupported precipitated iron catalyst promoted with copper and
potassium.
29. The apparatus of claim 22, further comprising a source of
natural gas and means for introducing the natural gas into at least
one of the partial oxidation reactor and the gas turbine of the
combined cycle plant.
30. The apparatus of claim 22, further comprising means for
separating acid gases from the products of said partial oxidation
reactor.
Description
CROSS REFERENCED PATENTS
[0001] This application is a divisional application of U.S.
application Ser. No. 09/376,709 filed Aug. 17, 1999 and issued as
U.S. Pat. No. ------ on ------ which is a continuation-in-part of
U.S. application Ser. No. 09/212,374 filed Dec. 16, 1998.
FIELD OF INVENTION
[0002] 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
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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 long-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.
[0009] 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).
[0010] The partial oxidation (POX) reaction can be expressed
as:
CH.sub.z+0.50.sub.2.fwdarw.z/2H.sub.2+CO
[0011] where z is the H:C ratio of the hydrocarbon feedstock. The
water gas shift (WGS) reaction also takes place:
H.sub.2O+CO.rarw..fwdarw.H.sub.2+CO.sub.2
[0012] 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:
2n H.sub.2+n CO ------.fwdarw.C.sub.nH.sub.2n+n H.sub.2O
[0013] The aliphatic hydrocarbons produced by the Fischer-Tropsch
reaction have an H:C atom ratio of 2.0 or greater.
[0014] Fischer-Tropsch catalysts such as iron-based composites also
catalyze the water gas shift (WGS) reaction:
H.sub.2O+CO.rarw..fwdarw.H.sub.2+CO.sub.2
[0015] If all of the water produced in the FT reaction were reacted
with CO in the WGS reaction, then the overall consumption of
hydrogen would be one-half of the consumption of carbon monoxide.
If none of the water were reacted in the WGS reaction (no WGS
activity) then the consumption of hydrogen would be twice the
consumption of carbon monoxide.
[0016] The oil produced in the FT reaction can be blended and
processed with the lighter refinery crude oil, thereby lowering the
average aromatic and sulfur content of distillate fuels.
[0017] Due to the relatively low hydrogen content of the heavy
crude oil, any FT catalyst useful in converting synthesis gas
produced by partial oxidation of heavy crude oil must possess some
water gas shift activity. Therefore, modern cobalt-based FT
catalysts which have very little WGS activity cannot generally be
used when the POX feedstock is a heavy crude oil, coke or coal.
However, iron-based catalysts as described in U.S. Pat. No.
5,504,118 have high WGS activity and are preferred for use with
low-hydrogen feedstocks.
[0018] For a natural gas feedstock which has a high H:C ratio, U.S.
Pat. Nos. 5,620,670 and 5,621,155 teach that carbon dioxide recycle
back to the synthesis gas producing step (either POX or steam
reforming) decreases the excessively high H.sub.2:CO ratio of the
synthesis gas and increases the yield of Fischer-Tropsch (FT)
hydrocarbons and the attendant carbon conversion efficiency.
[0019] In the case of low H:C ratio feeds, steam reforming is not a
viable means for producing synthesis gas due to the inevitable
formation of carbon when using these high carbon feedstocks. Carbon
deposition on a reforming catalyst cannot be tolerated. Also, solid
fuels are unsuitable for steam reforming. Thus, the only viable
option for gasifying high C:H feeds is POX.
[0020] In the instant case, the aforementioned carbon dioxide
recycle back to a POX reactor is not useful due to the lack of
sufficient hydrogen.
[0021] Another means for increasing the hydrocarbon yield and
carbon conversion efficiency of a system is to recycle part of the
tail gas to the inlet of the POX unit. However, the amount of tail
gas recycle is limited by the resulting low H.sub.2:CO ratio in the
synthesis gas produced in the POX caused by the large amount of
CO.sub.2 in the tail gas.
[0022] The use of combined partial oxidation and Fischer-Tropsch
reactors permits the conversion of a variety of high-carbon solid
and liquid fuels to liquid hydrocarbons and other products which
have lower C:H atom ratios and can thus be combusted or otherwise
used with net lower emissions of carbon dioxide to the atmosphere.
In the present invention, carbon dioxide can be efficiently removed
from tail gases in the process and sequestered to reduce the net
carbon dioxide emissions. Due to environmental and political
considerations, there is increasing interest in reducing carbon
dioxide emissions associated with combustion energy, and in
trapping and sequestering such gases as are emitted. See
"Technology to Cool Down Global Warming," Chemical Engineering,
January 1999 (pp. 37-41). Because of these inherent advantages,
Fischer-Tropsch technology is attracting increasing attention as a
means for utilizing resources such as coal in efficient and
environmentally friendly ways. Countries such as China and India,
having large coal reserves and needs for liquid hydrocarbon fuels,
could benefit immensely from such processes. See, e.g. Arthur W.
Tower III, "Fischer-Tropsch Technology," published by Howard, Weil,
Labouisse, Friedrichs of New Orleans, La., Dec. 18, 1998. See also
"State of the Art in GTL Technology," presented by Dr. Joe Verghese
of ABB Lummus Global at the Gas to Liquids World Forum, London,
November 1998.
[0023] Information about Orimulsion.RTM., an aqueous emulsion of
bitumen produced in Venezuela, can be found in various
publications, including A. R. Jones' "The Commercial Combustion of
Orimulsion," in the book Combustion & Emissions Control III,
ed. M. Adams, Institute of Energy, London 1997 (pp. 318-339). See
also Franzo Marruffo et al., "Orimulsion an alternative source of
energy," presented at the 22nd International Technology Conference
Coal Utility Fuel Systems at Clearwater, Fla., March 1997 (Coal
& Slurry Technology Ass'n), pp. 13-24. Also pertinent is
Rivalta et al., "Orimulsion(.TM.)--A New Fuel for Power Generation
and Future Feedstock Use," Polymer News, Vol. 21, No. 10 (pp.
342-344).
[0024] U.S. Pat. No. 4,549,396 (Mobil Oil) discloses a process of
converting coal to synthesis gas by partial oxidation with air,
then converting the synthesis gas to liquid and gaseous
hydrocarbons. The gas and liquid products are both used in a gas
turbine to generate electrical power.
[0025] U.S. Pat. No. 4,433,065 (Shell Oil) discloses a process for
converting pulverized coal to a synthesis gas, which is
catalytically converted to a gas containing hydrocarbons. Part of
the product gases are recycled to the gasification stage.
[0026] U.S. Pat. No. 4,092,825 (Chevron Research) discloses a
process of gasifying solid carbonaceous feedstocks to form a
synthesis gas, a portion of which is contacted with a
Fischer-Tropsch catalyst to form condensable hydrocarbons. A second
portion of the synthesis gas can be combusted to generate
electrical power, while the condensable hydrocarbons are used as
fuel to generate more power to meet peak loads. This patent is a
C.I.P. of U.S. Pat. No. 3,986,349.
[0027] U.S. Pat. No. 3,972,958 (Mobil Oil) discloses an integrated
process for converting coal to high octane gasoline by gasifying
the coal to form a synthesis gas containing methane, then
contacting the gas with at least one catalyst to form products
including gasoline and light hydrocarbons.
[0028] Gray and Tomlinson of Mitretek Systems disclose in "CO.sub.2
Emissions from Fischer-Tropsch Fuels," presented at Fuels,
Lubricants, Engines and Emissions meeting (sponsored by EFI and
DOE) at Tucson, Ariz. on Jan. 18-20, 1999 a "coproduction cofeed"
concept. Coal-derived synthesis gas is reacted in a liquid
synthesis reactor to form liquid hydrocarbons, and unreacted
synthesis gas is combined with natural gas for combustion in a
downstream combined cycle power generation unit.
[0029] U.S. Pat. No. 5,324,335 (Applicants) discloses the use of
Fischer-Tropsch liquids as a diesel fuel additive.
[0030] U.S. Pat. No. 5,500,449 (Applicants) discloses a method of
recovering a heavy Fischer-Tropsch wax and thermally cracking the
wax to produce diesel and naphtha fractions.
[0031] U.S. Pat. No. 5,504,118 (Applicants) discloses methods for
manufacturing and activating iron-based Fischer-Tropsch
catalysts.
[0032] U.S. Pat. No. 5,506,272 (Applicants) discloses
Fischer-Tropsch diesel fuel products.
[0033] U.S. Pat. No. 5,543,437 (Applicants) discloses methods for
producing Fischer-Tropsch products from coal-derived synthesis gas.
The products are produced at varying rates due to varying amounts
of the synthesis gas being fed to a power generation facility.
[0034] U.S. Pat. No. 5,620,670 (Applicants) discloses a process of
producing synthesis gas in a steam reformer, reacting the synthesis
gas in a Fischer-Tropsch reactor, then separating carbon dioxide
and recycling same to the reformer to enhance carbon conversion
efficiency and product yield.
[0035] U.S. Pat. No. 5,621,155 (Applicants) discloses methods of
producing synthesis gas which is reacted in a Fischer-Tropsch
reactor, then separating and recycling carbon dioxide to the steam
reformer or partial oxidation reactor or recycling light
hydrocarbons from the Fischer-Tropsch reactor to the reactor inlet,
all to increase carbon conversion efficiency.
[0036] U.S. Pat. No. 5,645,613 (Applicants) discloses the use of
Fischer-Tropsch liquids as a blending stock for diesel fuel to
produce oxygenated diesel fuels.
[0037] Even though the technology for conversion of high-carbon
feedstocks to synthesis gas and the subsequent production of
Fischer-Tropsch liquids is well developed, the growing demand for
energy coupled with the need to limit emissions of "greenhouse
gases" and/or to sequester carbon dioxide which is emitted by
combustion processes create a need for more efficient and flexible
processes which can meet the demand for power and hydrocarbon
production while separating carbon dioxide for sequestration or
disposal.
SUMMARY OF THE INVENTION
[0038] It is an aspect of this invention to provide a
Fischer-Tropsch process using an iron-based catalyst where the
hydrocarbon-containing feedstock to the first stage gasifier has a
much lower H:C ratio than natural gas which generally has a H:C
ratio in the range of 3.0 to 4.0.
[0039] It is a further aspect of this invention to increase the
hydrocarbon yields from a POX/FT system. Alternatively, it is an
aspect of this invention to increase the production of power from a
combined cycle system associated with the system of the invention,
and/or to increase the amount of carbon dioxide separated from the
FT reactor tail gases in order to sequester this gas from the
atmosphere.
[0040] It is another aspect of this invention to produce a
synthesis gas from carbon-bearing feedstocks having an H:C atom
ratio of less than about 2.0 which can be used for combustion in
boilers for the production of steam in an electric power generation
system and when the demand for the electric production decreases,
the synthesis gas can be used for a process for producing
hydrocarbons having an average H:C atom ratio of 2.0 or
greater.
[0041] The process produces hydrocarbons having an average H:C atom
ratio of 2.0 or greater from carbon-bearing feedstocks having an
H:C atom ratio of less than 2.0. The carbon-bearing feedstock is
reacted with an oxidizing gas such as oxygen or air containing
oxygen and steam in a partial oxidation reactor to produce a
mixture of gases containing hydrogen and carbon monoxide having a
molar ratio of H.sub.2:CO of greater than 0.6. Then the mixture of
gases containing hydrogen and carbon monoxide is reacted in a
Fischer-Tropsch synthesis reactor containing a catalyst which
catalyzes both hydrocarbon-forming reactions and the water gas
shift reaction. The product hydrocarbons are condensed out from the
unreacted hydrogen, carbon monoxide and other gases referred to as
tail gases.
[0042] The present invention allows the production of FT liquid
hydrocarbons, electrical power and carbon dioxide, all from
high-carbon feedstocks. The claimed apparatus and processes allow
at least one of these outputs to be selectively maximized according
to economic requirements. The process is aimed at future
requirements when carbon dioxide sequestration may be required and
there will be economic benefits to doing so. Under such conditions,
the sacrifice of outputs of high value FT liquids and/or electrical
power to preferentially produce carbon dioxide will be a necessary
capability.
[0043] Enabling features of the invention include the
following:
[0044] Iron catalyst: Iron-based catalysts allow the production of
FT liquids from synthesis gases with low H.sub.2:CO ratios, such as
obtained from high-carbon feedstocks. Also, iron catalysts are
active towards the water-gas shift reaction, which produces carbon
dioxide in the reactor product stream. Reducing the net carbon
dioxide produced from high-carbon feedstocks is an increasingly
important capability.
[0045] High-pressure CO.sub.2: Carbon dioxide is produced in the FT
product stream at high pressure and at high concentration. Under
these conditions, it is much more economical to separate the
CO.sub.2 from the FT process tail gas, compared, e.g. to the
removal of dilute, low pressure CO.sub.2 from a power plant stack
gas. (Typical CO.sub.2 concentration in stack gas from coal-fired
power plants is around 15 percent, while the equivalent
concentration for natural gas-fired plants is less than 10 percent,
according to Plasynski et al., "Carbon Mitigation: A Holistic
Approach to the Issue," Proceedings of 24th International Technical
Conference on Coal Utilization & Fuel Systems.) This results
from the fact that the FT reactor operates more efficiently at high
pressure. The unique combination of a high-pressure FT reactor and
an iron-based catalyst makes it easier to sequester CO.sub.2 from
the high-carbon feedstocks without the need for additional unit
operations such as a separate shift reactor.
[0046] Hydrogen recycle: Hydrogen recycle from the FT product
stream to the FT feed can optionally increase the H.sub.2:CO ratio
of the synthesis gas fed to the FT reactor. This is beneficial,
because although iron catalysts can handle low H.sub.2:CO ratio
synthesis gas, higher H.sub.2:CO ratios produce less CO.sub.2 and
greater yields of FT liquids. In addition, unless there is a local
high-value market for hydrogen, it would be used to produce power.
If power is higher valued then the hydrogen, less hydrogen would be
recycled and more would be used to produce power. If a mixed FT
product stream is acceptable, hydrogen would not be needed for
hydrocracking, and it may be possible to eliminate the hydrogen
removal step.
[0047] Process adjustability: The process can be operated to
optimize the relative production rates of FT liquids, power and
CO.sub.2. The FT reactor can be designed and operated (as to size
and temperature, pressure and space velocity) to produce more or
less FT liquids, and synthesis gas can be bypassed around the FT
reactor directly to the combined cycle plant to produce more power
or for load leveling. The production of carbon dioxide can be
traded off against hydrocarbon production by altering catalyst
operating variables such as temperature and space velocity.
Additionally, the input of water to the FT reactor can be increased
directly or by extracting less heat from the synthesis gas via the
heat exchanger to increase CO.sub.2 production in the FT
reactor.
[0048] Combined cycle plant: A combined cycle plant associated with
the process generates power from both a combustion turbine and a
steam turbine. This is well suited to a process including a FT
reactor, since both combustible tail gases and steam are produced.
Combustible tail gas is produced by the FT reactor as light
overhead gases. Steam is produced by cooling the gasifier, effluent
from the FT reactor and by heat recovery from the gas turbine
exhaust.
[0049] Co-feeding natural gas: Feeding natural gas to the gasifier
in addition to the high-carbon feedstock allows one to supply the
necessary hydrogen to the gasifier with less water. This reduces
the energy penalty of vaporizing water and significantly increases
plant efficiency. Alternatively or in addition to this use of
natural gas, natural gas can optionally be fed directly to the
combustion turbine when increase power production is desired.
[0050] In accordance with the invention, a process for producing
power, carbon dioxide and hydrocarbons having an average H:C atom
ratio of 2 or greater from carbon-bearing feedstocks having an H:C
atom ratio of less than 2 is provided which permits the selective
maximization of at least one of the products power, hydrocarbons
and carbon dioxide, and comprising steps of:
[0051] a) reacting a carbon-bearing feedstock with an oxidizing gas
and steam in a partial oxidation reactor to produce a mixture of
gases containing hydrogen, carbon monoxide and carbon dioxide
having a molar ratio of H.sub.2:CO of greater than about 0.6;
[0052] b) reacting the mixture of gases containing hydrogen and
carbon monoxide in a Fischer-Tropsch synthesis reactor containing a
catalyst which catalyzes both hydrocarbon-forming reactions and the
water gas shift reaction;
[0053] c) condensing the product hydrocarbons from unreacted
hydrogen, carbon monoxide and other gases (tail gases);
[0054] d) separating the product hydrocarbons into naphtha, diesel
and wax fractions;
[0055] e) removing at least a portion of carbon dioxide from the
tail gases; and
[0056] f) producing steam from heat recovered from at least the
partial oxidation reactor and the Fischer-Tropsch reactor,
directing the steam to the steam turbine of a combined cycle plant,
and directing at least the tail gases to the gas turbine of the
combined cycle plant to produce power.
[0057] Optionally, hydrogen can be separated from the tail gases
and utilized for recycle to the partial oxidation reactor and/or
hydrocracking wax F-T products to form more liquid hydrocarbon
products. Natural gas can be introduced into the partial oxidation
reactor to supplement the feedstock. Natural gas can also be fed to
the gas turbine of the combined cycle plant to increase power
production. The production of carbon dioxide in the F-T reactor can
be increased relative to hydrocarbon production by increasing the
water content of the synthesis gas fed to the F-T reactor,
preferably by decreasing the amount of heat extracted from the
partial oxidation reactor effluent.
[0058] Further in accordance with the invention, apparatus for
carrying out the process is provided, comprising the following
operationally-connected components:
[0059] a) a partial oxidation reactor;
[0060] b) means for removing acid gases (such as H.sub.2S and COS)
from the synthesis gas leaving the partial oxidation reactor;
[0061] c) a Fischer-Tropsch reactor connected to receive the
synthesis gas;
[0062] d) heat exchangers connected to remove heat from the partial
oxidation reactor, Fischer-Tropsch reactor and other heat sources
and generate steam;
[0063] e) means for separating carbon dioxide and/or hydrogen from
the F-T tail gases;
[0064] f) means for removing hydrocarbon products from the
Fischer-Tropsch reactor effluent and separating them into
fractions;
[0065] g) optionally, a hydrocracking reactor for utilizing
hydrogen separated by (e) to hydrocrack wax fractions of the
Fischer-Tropsch hydrocarbon products to form more liquid products;
and
[0066] h) a combined cycle plant including gas and steam turbines
for production of power from the steam and tail gases recovered
from the partial oxidation reactor--Fischer-Tropsch reactor
complex.
[0067] Other aspects of this invention will appear from the
following description and appended claims, reference being made to
the accompanying drawings forming a part of this specification
wherein like reference characters designate corresponding parts in
the several views.
BRIEF DESCRIPTION OF THE DRAWINGS
[0068] FIG. 1 is a flow sheet comprehensively illustrating the
processes and apparatus of the invention in their most general
forms.
[0069] FIG. 2 is a flow sheet illustrating the process and
apparatus of the invention configured for maximum power
production.
[0070] FIG. 3 is a flow sheet illustrating the process and
apparatus of the invention configured for maximum carbon dioxide
production.
[0071] FIG. 4 is a flow sheet illustrating the process and
apparatus of the invention configured for maximum hydrocarbon
production.
[0072] FIG. 5 is a graph of daily revenues vs. F-T plant size,
providing an economic analysis of the effects of different process
configurations.
[0073] Before explaining the disclosed embodiment of the present
invention in detail, it is to be understood that the invention is
not limited in its application to the details of the particular
arrangement shown, since the invention is capable of other
embodiments. Also, the terminology used herein is for the purpose
of description and not of limitation.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0074] Candidate feedstocks for the processes of the invention
include those having a low percentage of hydrogen compared to
carbon on an atom basis. Candidate feedstocks include refinery
residual oil (low API gravity fuel oil, VacResid, H-Oil Bottoms, 0
degrees API Asphalt, etc.), petroleum coke, coal, tires, biomass,
and mixtures of these feedstocks. Other suitable feedstocks include
aqueous emulsions of bitumen such as Orimulsion.RTM., available
commercially from Venezuelan sources. (Bitumen is a generic term
describing mixtures of hydrocarbons derived from natural and/or
pyrogenic origins.)
[0075] Refinery oil residues are relatively high in carbon because
they contain a high percentage of ring compounds including
aromatics. The average H:C atomic ratios of various candidate
feedstocks are:
1 Feedstock H:C Atom Ratio Refinery Feedstocks: 9.6 API Fuel Oil
1.35 4.3 API VacResid 1.37 H-Oil Bottoms 1.26 0 deg. API Asphalt
1.26 Delayed Petroleum Coke 0.53 Fluid Petroleum Coke 0.28 Coals:
Pittsburgh #8 0.83 French 0.80 Utah 0 . 84 German 0.75 S. African
0.64 Biomass: Wood Wastes 1. 49 Wheat Straw 1.50 Tires: 1.25
Orimulsion .RTM.: 2.12
[0076] The oxidizing gas which is used can be oxygen in a
relatively pure form such as at a purity of greater than about 90
percent by volume or it can be in the form of air where the air
contains greater than about 20 percent oxygen by volume.
[0077] Any suitable source of natural gas or the equivalent can be
used to provide supplementary feed to the partial oxidation reactor
and/or fuel to the gas turbine of the combined cycle plant. It
would of course be convenient if the system of the invention were
located adjacent to a direct source of natural gas. It is also
advantageous to locate the system in a refinery, where various
feedstocks are available and hydrocarbon products and power can be
readily used.
[0078] Any suitable partial oxidation reactor capable of handling
heavy feedstocks can be employed, e.g. the type(s) disclosed in
Texaco's U.S. Pat. Nos. 4,992,081; 4,902,303 and 5,545,238, all of
which are incorporated herein by reference, and in additional
patents listed in column 1 of U.S. Pat. No. 5,545,238.
[0079] Typical heat exchangers useful in refinery and power plant
applications can be used to recover waste heat from suitable
sources within the system and generate steam for use in the
combined cycle plant.
[0080] Suitable apparatus for removing acid gases such as H.sub.2S
and COS from the synthesis gas produced by the partial oxidation
reactor can be used. Suitable apparatus includes wet or dry
scrubbing systems: an example is disclosed in U.S. Pat. No.
5,403,568, which is incorporated herein by reference.
[0081] Any suitable Fischer-Tropsch reactor capable of converting
synthesis gases produced from heavy feedstocks to typical
Fischer-Tropsch hydrocarbon products can be used. Typical reactors
are disclosed in Rentech's U.S. Pat. Nos. 5,506,272; 5,543,437 and
5,763,716, which are incorporated herein by reference. Preferably,
the Fischer-Tropsch reactor is a slurry reactor, such as disclosed
in the above Rentech patents and described in the article entitled
"Fischer-Tropsch Synthesis in Slurry Phase" by Schlesinger et al.
in Engineering and Process Development, Vol. 43, No. 6 (June,
1951), pp. 1474-1479, and by Kolbel and Ralek in "The
Fischer-Tropsch Synthesis in the Liquid Phase," Catalyst
Review-Science Engineering, Vol. 21(2), pp. 225-274 (1980). Slurry
reactors are vertical vessels in which fine powdered catalyst is
suspended in molten wax. The synthesis gas is introduced into the
bottom of the reactor and F-T products emerge from the top and side
of the reactor. The heavy F-T products are filtered from the
wax/catalyst slurry.
[0082] The Fischer-Tropsch catalyst should be a catalyst which
simultaneously catalyzes the production of hydrocarbon products
from a synthesis gas and activates the water gas shift reaction.
The catalyst is preferably an iron-based catalyst such as disclosed
in U.S. Pat. No. 5,504,118, which is incorporated herein by
reference. As stated in column 4 of the patent, the composition,
preparation and activation of the catalyst are all important to its
operation. The preferred catalyst is an unsupported precipitated
iron catalyst promoted with copper and potassium, which is prepared
using elemental iron and copper as starting materials. The catalyst
can be prepared as disclosed in column 11 of the patent, and
activated as disclosed in columns 12.
[0083] The H.sub.2 separation devices based on membranes are
commercially available from Monsanto, W.R. Grace & Co., and Dow
Corning. Another technology for H.sub.2 separation from a mixed gas
stream uses pressure swing absorption (PSA). Suitable apparatus for
separating carbon dioxide from tail gases include various
absorption systems disclosed in U.S. Pat. Nos. 4,496,371; 4,957,715
and 5,424,051, which are incorporated herein by reference. Removal
of carbon dioxide from the relatively high pressure gases emerging
from the Fischer-Tropsch reactor at pressures in the range of from
about 100 to about 500 psig, and temperatures in the range of from
about 100.degree. to about 300.degree. F. is advantageous when
compared with the alternative of removing the gas from atmospheric
pressure stack gases from furnaces or other combustion systems,
which typically contain less than 15 percent carbon dioxide. In the
processes of the invention, the F-T reactor effluent can contain in
the range of from about 15 to about 60 volume percent carbon
dioxide, preferably from about 20 to about 55 and most preferably
from about 25 to about 50 volume percent.
[0084] The product separation means will generally consist of
partial condensers, distillation columns, and possibly
hydrocracking units, all well known to those possessing ordinary
skill in the art. Tail gas which is not recycled can be used for
fuel in the process, including the gas turbine of the combined
cycle plant.
[0085] Optionally, a typical hydrocracking reactor is provided to
utilize hydrogen separated from tail gases to crack wax products of
the Fischer-Tropsch reactor into liquid hydrocarbons. Suitable
hydrocracking reactors and catalysts are disclosed in U.S. Pat.
Nos. 3,617,498; 4,197,184; 4,404,088; 4,501,655 and 5,026,472,
which are incorporated herein by reference.
[0086] A suitable combined cycle plant is provided which includes
at least one steam turbine to which steam recovered from the
process can be fed, and a gas turbine to burn fuels which can
include natural gas, tail gases from the Fischer-Tropsch reactor,
hydrogen and naphthas separated from the hydrocarbon products. Such
combined cycle plants are well known in the art.
[0087] Turning now to the drawing, FIG. 1 illustrates the system 10
of the invention and the processes carried out therein. Partial
oxidation reactor 12 is provided with feed 14 as described above,
plus oxygen 16 and water 18. Optionally, natural gas 20 can be
introduced to supplement the feed.
[0088] The synthesis gas produced by the partial oxidation reactor
12 is passed to a Fischer-Tropsch reactor 28 via connectors 23 and
26. Preferably apparatus 24 for removing acid gases such as
H.sub.2S and COS is included between the partial oxidation reactor
12 and Fischer-Tropsch reactor 28. Heat exchangers 22, 30 and 34
are provided to remove heat from the partial oxidation reactor,
Fischer-Tropsch reactor and FT effluent connector 32, respectively.
Additional heat exchangers (not shown) can be placed wherever the
system offers a significant source of heat. All the heat exchangers
are also steam generators, producing steam to drive the steam
turbine(s) (not shown) of the combined cycle plant 68 via 70.
[0089] Hydrocarbon recovery unit 36 separates hydrocarbon products
from F-T tail gases delivered through connector 32 and divides the
products into fractions containing water and oxygenates 40, naphtha
42, diesel 44 and waxes 46. Optionally, hydrocracking reactor 78 is
provided to crack wax products 46 into further naphtha and diesel
fractions 80 and 82, respectively. Tail gases 74 generated in the
hydrocracking reactor are typically burned in the gas turbine (not
shown) of the combined cycle plant 68.
[0090] Tail gases 38 discharged from the hydrocarbon recovery unit
36 pass through optional hydrogen separation unit 50 and through
carbon dioxide separation unit 58. Separated hydrogen is used by
recycling to the Fischer-Tropsch reactor via connector 62 to
increase the H.sub.2:CO ratio in the synthesis gas entering the F-T
reactor and/or directed via connector 66 for hydrocracking wax
products in hydrocracking reactor 78. A portion of tail gases 64
can pass directly to the combined cycle plant 68 for combustion in
the gas turbine (not shown).
[0091] (Similarly, tail gases remaining after separation of carbon
dioxide and hydrogen (59 and 52, respectively) also are directed to
the gas turbine. Such tail gases can also be recycled to gasifier
12 via 90.
[0092] Separation of carbon dioxide from the F-T effluent is
advantageous because it can be sequestered for environmental
purposes. For example, the carbon dioxide recovered can be
transported and injected into the depths of the sea, used for
enhanced oil recovery from underground formations or employed in
the synthesis of materials such as urea. A plant for producing such
products (not shown) can be installed adjacent to the system of the
invention.
[0093] Combined cycle plant 68 is configured and connected to
permit the employment of all available steam and combustible gases,
and can be operated to maximize the production of power when and
where this is more valuable than the Fischer-Tropsch products. The
principal feed for the gas turbine of this plant is tail gases 32
from the Fischer-Tropsch reactor (which may have been processed by
further gas separators or the hydrocracking reactor). This fuel can
be augmented by feeding natural gas 72 and/or naphtha fractions 42
of the F-T products to increase power production. The steam and gas
turbines are connected to electrical generators to produce
electrical power. To maximize power without production of F-T
products, all the synthesis gas 73 from the POX can be sent to the
gas turbine of combined cycle plant 68, thus bypassing F-T reactor
28.
EXAMPLES
[0094] Having described the basic aspects of the invention, the
following examples are given to illustrate specific embodiments
thereof.
[0095] Examples were calculated using a proprietary computer
program for thermodynamic calculations based on apparatus for
configurations and process conditions designed to maximize the
production of power, carbon dioxide and hydrocarbons, respectively.
The functions performed by the computer program included modeling
the POX reactor, the FT reactor, all recycle loops, the separation
processes and power generation.
[0096] The examples are based upon the flow sheets of FIGS. 2, 3
and 4. All three flow sheets assume a feedstock feed rate of 2500
tons per day. The feedstock (Orimulsion) contains 60.7 weight
percent carbon, 7.6 weight percent hydrogen, 0.4 weight percent
nitrogen, 2.6 weight percent sulfur, 0.1 weight percent ash and
28.6 weight percent moisture. An efficiency of power generation
from the steam turbine is 33 percent and of the combined cycle
plant is 55 percent, based upon the lower heating value of the
fuel.
Examples 1, 1A
Maximum Power Flow Sheet
[0097] In this flow sheet (FIG. 2) the F-T plant converts syngas
from the gasifier into liquid hydrocarbons. Tail gas from the F-T
plant is burned in the combined cycle plant to produce electricity.
Steam from the three heat exchangers (22, 30, 34) is sent to the
steam turbine part of the combined cycle plant 68 to produce
electricity. Power production is maximized by burning the F-T
naphtha and wax fractions in the combined cycle because these
hydrocarbon products are generally of less value than the diesel
fraction. This gives a power production of 271 MWe and a diesel
production of 869 bbl/day. If the syngas from the gasifier is sent
directly to the combined cycle, i.e., no hydrocarbons are produced
via F-T reactions (Example 1A), the power production is 330 MWe. It
could be argued that the latter configuration produces the maximum
power, but in the absence of F-T hycrocarbon production.
Examples 2 and 2A
Maximum CO.sub.2 Flow Sheet
[0098] By removing all CO.sub.2 from the F-T tail gas prior to the
combined cycle as shown in FIG. 3, one can easily capture a large
flow of CO.sub.2, approximately 46 MMSCFD. Power production will be
approximately the same as in the maximum power flow sheet of
Example 1, 271 MWe, and diesel production is also the same, 869
bbl/day. This example illustrates that if the value of the
CO.sub.2, e.g. through environmental credits, is sufficiently high,
the extra capital cost of the CO.sub.2 removal system can be
justified and there will be no penalty in power production or F-T
liquid production. This example (2) illustrates the ease with which
CO.sub.2 can be removed from the tail gas of an iron catalyst-based
F-T reactor. The amount of CO.sub.2 removed can be varied by
adjusting the size of the CO.sub.2 removal system to maximize plant
revenues.
[0099] The production of CO.sub.2 by the F-T reactor system can be
further increased by taking advantage of the water-gas shift
activity of the iron F-T catalyst preferably used in the processes
of the invention. The equilibrium of the water-gas shift reaction
(shown above on page 7) can be shifted to the right by forcing more
water vapor into the F-T reactor. The results will be increased
production of CO.sub.2, which can then be separated from the F-T
effluent. Water can be introduced directly, but preferably the same
effect is achieved by reducing the cooling effect of heat exchanger
22. Since the effluent from the gasifier is cooled less, the
partial pressure of water vapor entering the F-T reactor will be
greater, with the predictable effects on the water-gas shift
reaction equilibrium. The resultant yield of hydrocarbons will be
less, but under certain economic conditions, the increased
production and separation of CO.sub.2 will make up for this.
[0100] The computer simulation program is used to determine and
compare the yields of CO.sub.2 and hydrocarbons for the cases of
Example 2 and the same simulation (Example 2A) in which more water
is allowed to enter the F-T reactor. The results are shown in Table
1 below.
2TABLE 1 water to CO.sub.2 yield diesel yield power Example case FT
vol % MMSCFD bbl/day MWe 2 Baseline 0 46 869 271 max CO.sub.2 2A
Baseline + 9.2 54 695 282 water to FT
[0101] It can be seen that by allowing a nominal quantity (9.2
volume percent) of water to enter the F-T reactor, the yield of
CO.sub.2 increases by about 17 percent. The yield of diesel
decreases by 20 percent, and power production increases by 4
percent, mainly because of the additional hydrogen produced by
moving the water-gas shift equilibrium to the right.
Example 3
Maximum Hydrocarbons
[0102] In the flow sheet of FIG. 4, the yield of liquid
hydrocarbons is maximized with two modifications from the flow
sheet of FIG. 2. First, waxes 46 produced by the F-T reactor are
hydrocracked to lighter hydrocarbon fractions. Second, a
substantial fraction (95 percent) of the F-T tail gas 90 (mainly
H.sub.2, CO.sub.2 and N.sub.2) is recycled to the gasifier. This
large recycle rate is only feasible if CO.sub.2 is first removed
from the recycle gas; otherwise excessive carbon would be fed to
the gasifier. In this example, power production is 66 Mwe, liquid
hydrocarbon production is 6365 bbl/day and CO.sub.2 production is
47 MMSCFD. Note that the CO.sub.2 production is the same as in that
of Example 2, and that it cannot be adjusted downward without
sacrificing hydrocarbon yield. However, the recycle rate and/or
size of the F-T plant can be reduced to decrease the F-T product
yield and increase power as needed.
[0103] Table 2 below summarizes the power production, CO.sub.2
yield and hydrocarbon yield from Examples 1-3:
3TABLE 2 power CO.sub.2 hydrocarbons Example (MWe) (MMSCFD)
(bbl/day) 1A Max power, no FT 330 0 0 1 Max power with FT 271 0 869
2 Max CO.sub.2 271 46 869 2A Max CO.sub.2 via H.sub.2O 282 54 695
to FT 3 Max hydrocarbons 66 46 6365
Plant Daily Revenues
[0104] The ability of the present invention to maximize actual
plant revenues is demonstrated in the graph of FIG. 5. The plant
configuration of FIG. 4 was used to estimate daily revenues as a
function of the size of the F-T plant. In Example 3, the F-T plant
is considered full size, or 100 percent, to maximize hydrocarbon
yields. Smaller F-T plants produce less hydrocarbons and more
power, but about the same amount of CO.sub.2. It was assumed that
if no F-T plant is used, (0 percent F-T plant size) the system
degenerates to that of FIG. 2 and no CO.sub.2 production. In FIG.
5, the values of the power, CO.sub.2 and hydrocarbons were varied.
A baseline case assumes a power value of 3.5 cents/kilowatt hour, a
CO.sub.2 value of $1 per thousand cubic feet, and a hydrocarbon
value of $30/bbl. In addition to this baseline, six variations were
run for the values of the CO.sub.2, power and hydrocarbons. The low
power value scenario assumes 1.5 cents/kilowatt hour and baseline
values for the CO.sub.2 and hydrocarbons. The high power value
scenario assumes 5.5 cents/kilowatt hour and baseline values for
the CO.sub.2 and hydrocarbons. The low CO.sub.2 value scenario
assumes no value for the CO.sub.2 and baseline values for the power
and hydrocarbons. The high CO.sub.2 value scenario assumes $2 per
thousand cubic feet of CO.sub.2 and baseline values for the power
and hydrocarbons. The low hydrocarbon value scenario assumes
$20/bbl and baseline values for the power and CO.sub.2. The high
hydrocarbon value scenario assumes $40/bbl and baseline values for
the power and CO.sub.2.
[0105] Briefly, the graph of FIG. 5 illustrates that in the case of
high hydrocarbon value or low power value, maximum plant revenues
result from the use of the largest possible F-T plant. In other
scenarios, maximum revenues can be obtained with a relatively small
F-T plant. For example, in the high power value scenario, revenues
are maximized with a 10 percent size F-T plant. A smaller F-T plant
gives up the revenues from the CO.sub.2 and a larger F-T plant
reduces generation of the high value power. In the case of low
(zero) CO.sub.2 value, the revenues trade off 3.5 cent/kwh power
against $30/bbl hydrocarbons. The graph suggests that a smaller F-T
plant is appropriate for those values. A slightly lower power value
or slightly higher hydrocarbon value would suggest a large F-T
plant; i.e., the graph line would slope upwards. It is thus
apparent that the partial oxidation--F-T plant--combined cycle
plant system of the invention can be designed and operated to
selectively maximize the production of at least one of the
principal outputs--power, CO.sub.2 and hydrocarbons--and to
maximize plant revenues as appropriate.
[0106] Although the present invention has been described with
reference to preferred embodiments, numerous modifications and
variations can be made and still the result will come within the
scope of the invention. No limitation with respect to the specific
embodiments disclosed herein is intended or should be inferred.
* * * * *