U.S. patent application number 11/261632 was filed with the patent office on 2006-04-06 for control of co2 emissions from a fischer-tropsch facility by use of dual functional syngas conversion.
This patent application is currently assigned to CHEVRON U.S.A. INC.. Invention is credited to Charles L. Kibby, Dennis J. O'Rear.
Application Number | 20060074134 11/261632 |
Document ID | / |
Family ID | 33518248 |
Filed Date | 2006-04-06 |
United States Patent
Application |
20060074134 |
Kind Code |
A1 |
O'Rear; Dennis J. ; et
al. |
April 6, 2006 |
Control of CO2 emissions from a fischer-tropsch facility by use of
dual functional syngas conversion
Abstract
CO.sub.2 emissions from Fischer-Tropsch facilities are
controlled by using multiple reactors. A process for the conversion
of syngas using multiple Fischer-Tropsch reactors comprises
reacting at least a portion of a first syngas in a first
Fischer-Tropsch reactor to form a first hydrocarbonaceous product
and a second syngas. The second syngas is mixed with a
H.sub.2-containing stream to form an adjusted syngas. At least a
portion of the adjusted syngas is reacted in a second
Fischer-Tropsch reactor to form a second hydrocarbonaceous product
and a third syngas. At least a portion of the first and second
hydrocarbonaceous products are blended to obtain a blended
hydrocarbonaceous product.
Inventors: |
O'Rear; Dennis J.;
(Petaluma, CA) ; Kibby; Charles L.; (Benicia,
CA) |
Correspondence
Address: |
BUCHANAN INGERSOLL PC;(INCLUDING BURNS, DOANE, SWECKER & MATHIS)
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
CHEVRON U.S.A. INC.
San Ramon
CA
|
Family ID: |
33518248 |
Appl. No.: |
11/261632 |
Filed: |
October 31, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10720675 |
Nov 25, 2003 |
6992114 |
|
|
11261632 |
Oct 31, 2005 |
|
|
|
Current U.S.
Class: |
518/726 ;
422/600 |
Current CPC
Class: |
C10G 2/32 20130101; Y02P
20/132 20151101; Y02P 20/582 20151101; Y02P 20/129 20151101; C10G
2300/4043 20130101; Y02P 30/446 20151101; Y02P 30/40 20151101 |
Class at
Publication: |
518/726 ;
422/190 |
International
Class: |
B01J 8/04 20060101
B01J008/04; C07C 27/26 20060101 C07C027/26 |
Claims
1.-18. (canceled)
19. A Gas-to-Liquids facility comprising: a) a first
Fischer-Tropsch reactor that reacts a least a portion of a first
syngas, comprising at least about 2 vol % CO.sub.2, to form a first
hydrocarbonaceous product and a second syngas comprising at least
about 2 vol % CO.sub.2; b) a hydrogen source that provides hydrogen
that mixes with the second syngas to form an adjusted syngas; c) a
second Fischer-Tropsch reactor that reacts at least a portion of
the adjusted syngas to form a second hydrocarbonaceous product and
a third syngas comprising a reduced amount of CO.sub.2 than was
present in the adjusted syngas; and d) a blender that blends at
least a portion of the first and second hydrocarbonaceous products
to produce a blended hydrocarbonaceous product.
20. The facility of claim 19, wherein the first Fischer-Tropsch
reactor is a reactor selected from the group consisting of a slurry
bed reactor, a fixed bed reactor, a fluidized bed reactor and
combinations thereof.
21. The facility of claim 20, wherein the reactor is a slurry bed
reactor comprising a Fischer-Tropsch catalyst that comprises
cobalt.
22. The facility of claim 19, wherein the hydrogen source comprises
a hydrogen recovery system that recovers hydrogen from process
streams using a recovery method selected from the group consisting
of adsorption, absorption, cryogenic separation, membrane
separation and combinations thereof.
23. The facility of claim 22, wherein the hydrogen source comprises
a hydrogen recovery system that recovers hydrogen from steam
reforming of methane.
24. The facility of claim 22, further comprising a recycle for at
least a portion of the third syngas so that the portion of the
third syngas mixes with the first syngas to form a blended syngas.
Description
[0001] The present application is a divisional of U.S. Ser. No.
10/720,675, filed on Nov. 25, 2003, hereby incorporated by
reference in its entirety.
FIELD OF THE INVENTION
[0002] The present invention is directed to controlling CO.sub.2
emissions from Fischer-Tropsch facilities. In particular, the
present invention is directed to controlling CO.sub.2 emissions
from a Fischer-Tropsch facility by using multiple reactors.
BACKGROUND OF THE INVENTION
[0003] The conversion of natural gas assets into more valuable
products, including combustible liquid fuels, is desired to more
effectively utilize these natural gas assets. The conversion of
natural gas to more valuable products generally involves syngas
generation. Syngas generation involves converting natural gas,
which is mostly methane, to synthesis or syngas gas, which is a
mixture of carbon monoxide and hydrogen. Syngas may be used as a
feedstock for producing a wide range of products, including
combustible liquid fuels, methanol, acetic acid, dimethyl ether,
oxo alcohols, and isocyanates.
[0004] There are two main approaches to convert remote natural gas
assets into conventional transportation fuels and lubricants using
syngas. Natural gas may be converted into syngas followed by a
Fischer-Tropsch process, or natural gas may be converted into
syngas followed by methanol synthesis, which is followed by a
methanol to gas process (MTG) to convert methanol into highly
aromatic gasoline. The syngas generation is the most costly step of
these processes. A critical feature of these processes is producing
syngas with a desired H.sub.2/CO ratio to optimize formation of the
desired products and to avoid problems in the syngas formation
step. Unless otherwise stated, syngas ratios (and percentage
compositions) as described herein are in terms of molar ratios (and
molar percentages).
[0005] Syngas can be generated from three major chemical reactions.
The first involves steam reforming of methane. The ratio of
hydrogen to carbon monoxide, which is formed from this process, is
typically approximately 3.0. A second process for syngas generation
involves dry reforming of methane or the reaction between carbon
dioxide and methane. An attractive feature of this method is that
carbon dioxide is converted into syngas; however, this method has
problems with rapid carbon deposition. The carbon deposition or
coke forming reaction is a separate reaction from the one that
generates the syngas and occurs subsequent to the syngas formation
reactor. However, the reaction of methane in dry reforming is slow
enough that long residence times are required for high conversion
rates and these long residence times lead to coke formation. The
ratio of hydrogen to carbon monoxide, which is formed from this
process, is typically approximately 1.0. A third process for
synthesis gas generation involves partial oxidation of methane
using oxygen. The ratio of hydrogen to carbon monoxide, which is
formed from this process, is typically approximately 2.0. However,
in commercial practice, some amount of steam is typically added to
a partial oxidation reformer in order to control carbon formation
and the addition of steam tends to increase the H.sub.2/CO ratio
above 2.0.
[0006] It is possible to produce syngas with a H.sub.2/CO ratio
that is above the ratio ideally desired for the process in which
the syngas is to be used, and then to remove excess hydrogen to
adjust the ratio to the desired value. However, the H.sub.2 removal
process employs expensive H.sub.2 separation systems that tend to
foul and decline in performance with use.
[0007] The Fischer-Tropsch and MTG processes both have advantages
and disadvantages. For instance, the Fischer-Tropsch process has
the advantage of forming products that are highly paraffinic.
Highly paraffinic products are desirable because they exhibit
excellent combustion and lubricating properties. Unfortunately, a
disadvantage of the Fischer-Tropsch process is that the
Fischer-Tropsch process emits relatively large amounts of CO.sub.2
during the conversion of natural gas assets into saleable products.
An advantage of the MTG process is that the MTG process produces
highly aromatic gasoline and LPG fractions (e.g., propane and
butane). However, while highly aromatic gasoline produced by the
MTG process is generally suitable for use in conventional gasoline
engines, highly aromatic MTG gasoline may be prone to form durene
and other polymethyl aromatics having low crystallization
temperatures that form solids upon standing. In addition, the MTG
process is more expensive than the Fischer-Tropsch process and the
products produced by the MTG process cannot be used for lubricants,
diesel engine fuels or jet turbine fuels. Furthermore, like the
Fischer-Tropsch process, the MTG process also generates
CO.sub.2.
[0008] It is known to use multiple Fischer-Tropsch reactors for
various purposes. For instance, U.S. Pat. No. 6,169,120, to Beer of
Syntroleum, discloses a two-stage Fischer-Tropsch process employing
intermediate H.sub.2O removal. The process of Beer is intended to
be used to process syngas produced from air that will contain
appreciable amounts of nitrogen.
[0009] U.S. Pat. No. 4,279,830, to Haag et al. of Mobil, discloses
a zeolite-containing catalyst used to maintain a syngas composition
so that a H.sub.2:CO ratio is in a range of about 0.5 to about 1.0.
Haag explains that this range is appropriate for Fe-based FT
catalysts, but is not suitable for Co-based catalysts. A second FT
catalyst contains Fe but does not contain a zeolite. This catalyst
makes a minimal change in the H.sub.2:CO ratio. Haag also explains
that operating within a selective range of process conditions
provides an improved process for upgrading the total effluent from
a Fischer-Tropsch operation.
[0010] U.S. Pat. No. 4,624,968, to Kim et al. of Exxon, discloses a
two-stage Fischer-Tropsch process using specific catalysts intended
for olefin synthesis and conversion.
[0011] WO/0063141, to Clark et al. of Reema International Corp.,
discloses a Fischer-Tropsch process for synthesizing hydrocarbons
that includes multiple Fischer-Tropsch reactor stages arranged in
series. The process of Clark provides very low CO conversion per
Fischer-Tropsch reactor stage and employs intermediate removal of
water between reactor stages. In one embodiment, the process of
Clark uses an iron-based catalyst. In a preferred embodiment,
CO.sub.2 is recycled from the last reactor in a series of
Fischer-Tropsch reactors to the syngas generator.
[0012] EP 0679620 A2, to Long of Exxon, discloses a high conversion
hydrocarbon synthesis achieved by reacting H.sub.2 and CO in a
first stage(s) in the presence of a non-shifting catalyst. The
process further includes separating liquid products and reacting
the remaining gas streams in the presence of shifting
catalysts.
[0013] Additionally, various technologies have been used for the
conversion of CO.sub.2. For instance, "Can Carbon Dioxide be
Reduced to High Molecular Weight in Fischer-Tropsch Products?," In
re Puskas, ACS 213 National Meeting (San Francisco 4/13-17/97) ACS
Division of Fuel Chemistry Preprints, V.42, N.2, 680-86 (1997),
discloses that CO.sub.2 can be converted into hydrocarbonaceous
products of similar structure and composition as obtained with CO.
Also, "Kinetics of CO.sub.2 hydrogenation on a K-promoted Fe
Catalyst," Thomas Riedel and Georg Schaub, Industrial and
Engineering Chemistry Research, 40/5 1355-1363, discloses that
CO.sub.2 can be converted into hydrocarbonaceous products of
similar structure and composition as obtained with CO. Riedel and
Schaub discloses that preferred operating temperatures are below
360.degree. C. to prevent rapid carbon deposition on the
catalyst.
[0014] Similarly, "Iron Catalyzed CO.sub.2 Hydrogenation to Liquid
Hydrocarbons," Fourth International Carbon Dioxide Utilization
Conference (Kyoto, Japan 9/7-11/97) Studies in Surface Science and
Catalysis, V114, 339-44 (1998), discloses that many catalysts
useful in Fischer-Tropsch synthesis can also catalyze CO.sub.2
hydrogenation to hydrocarbons.
[0015] Although multiple Fischer-Tropsch reactors have been used,
an integrated system employing multiple Fischer-Tropsch reactors
wherein hydrocarbonaceous products from different stages are
blended has not been utilized to reduce CO.sub.2 emissions.
[0016] As a result, there is an urgent need for a process that can
reduce CO.sub.2 emissions from syngas conversion processes while
still being able to generate desired hydrocarbonaceous
products.
SUMMARY OF THE INVENTION
[0017] The present invention satisfies the above objectives by
providing a process that uses multiple reactors to control CO.sub.2
emissions generated from a syngas conversion process. By using
multiple reactors, the present invention avoids the need for costly
CO.sub.2 isolation processes.
[0018] The process according to the present invention, for the
conversion of syngas using multiple Fischer-Tropsch reactors,
includes reacting at least a portion of a first syngas, comprising
at least about 2 vol % CO.sub.2, in a first Fischer-Tropsch reactor
to form a first hydrocarbonaceous product and a second syngas
comprising at least about 2 vol % CO.sub.2. The process also
includes mixing the second syngas with a H.sub.2-containing stream
to form an adjusted syngas. The process further includes reacting
at least a portion of the adjusted syngas in a second
Fischer-Tropsch reactor to form a second hydrocarbonaceous product
and a third syngas comprising a reduced amount of CO.sub.2 than was
present in the adjusted syngas. Finally, the process includes
blending at least a portion of the first and second
hydrocarbonaceous products to obtain a blended hydrocarbonaceous
product.
[0019] According to another aspect of the present invention, the
process for the conversion of syngas using multiple Fischer-Tropsch
reactors includes forming a first syngas comprising at least about
2 vol % CO.sub.2. The process also includes reacting at least a
portion of the first syngas in a first Fischer-Tropsch reactor to
form a first hydrocarbonaceous product and a second syngas
comprising at least about 2 vol % CO.sub.2. The process further
includes mixing the second syngas with a H.sub.2-containing stream
to obtain an adjusted syngas having a molar ratio of
H.sub.2:(CO+CO.sub.2) of at least about 1.0, and preferably between
about 1.0 and about 8.0. The process also includes reacting at
least a portion of the adjusted syngas in a second Fischer-Tropsch
reactor to form a second hydrocarbonaceous product and a third
syngas comprising a reduced amount of CO.sub.2 than was present in
the adjusted syngas. Finally, the process includes blending at
least a portion of the first and second hydrocarbonaceous products.
In a preferred embodiment, the process can further include
converting at least a portion of the blended hydrocarbonaceous
product into at least one product selected from the group
consisting of jet fuel, diesel fuel, lubricant base oil, naphtha,
and combinations thereof.
[0020] In yet another embodiment of the present invention, the
process includes reacting at least a portion of a blended syngas
comprising a first syngas and containing at least about 2 vol %
CO.sub.2 in a first Fischer-Tropsch reactor to form a first
hydrocarbonaceous product and a second syngas comprising at least
about 2 vol % CO.sub.2. The process also includes mixing the second
syngas with a H.sub.2-containing stream to form an adjusted syngas.
The process further includes reacting at least a portion of the
adjusted syngas in a second Fischer-Tropsch reactor to form a
second hydrocarbonaceous product and a third syngas comprising a
reduced amount of CO.sub.2 than was present in the adjusted syngas.
The process also includes blending at least a portion of the first
and second hydrocarbonaceous products to obtain a blended
hydrocarbonaceous product. Finally, the process includes recycling
at least a portion of the third syngas to be mixed with the first
syngas to form the blended syngas.
[0021] According to a further aspect of the present invention, a
Gas-to-Liquids facility includes a first Fischer-Tropsch reactor
that reacts at least a portion of a first syngas, comprising at
least about 2 vol % CO.sub.2, to form a first hydrocarbonaceous
product and a second syngas comprising at least about 2 vol %
CO.sub.2. The facility further includes a hydrogen source that
provides hydrogen that mixes with the second syngas to form an
adjusted syngas. The facility also includes a second
Fischer-Tropsch reactor that reacts at least a portion of the
adjusted syngas to form a second hydrocarbonaceous product and a
third syngas comprising a reduced amount of CO.sub.2 than was
present in the adjusted syngas. Finally, the facility includes a
blender that blends at least a portion of the first and second
hydrocarbonaceous products to produce a blended hydrocarbonaceous
product.
[0022] The process of the present invention, through the use of its
multiple reactors, thereby realizes an important advantage in that
it substantially minimizes CO.sub.2 emissions generated from a
syngas conversion process without having to employ costly CO.sub.2
isolation processes such as, for example, CO.sub.2 compression,
liquefaction or solidification to isolate CO.sub.2 from
CH.sub.4-containing gases. In addition, by blending at least a
portion of the first and second hydrocarbonaceous products, the
present invention can produce a blended hydrocarbonaceous product
that substantially minimizes or eliminates the need for additional
distillation and/or upgrading. Further, when at least a portion of
the third syngas is recycled, the present invention provides a
blended syngas that becomes enriched in methane as CO.sub.2 is
removed. The result is a process and facility that can operate more
efficiently and economically in the production of hydrocarbonaceous
products.
BRIEF DESCRIPTION OF DRAWINGS
[0023] FIG. 1 is a schematic view of a conventional Fischer-Tropsch
synthesis.
[0024] FIG. 2 is a schematic view of an exemplary embodiment of a
syngas conversion process of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] In the present invention, CO.sub.2 emissions from a syngas
conversion process are effectively reduced by using multiple
reactors.
[0026] One such syngas conversion process is a Fischer-Tropsch
process. Catalysts and conditions for performing Fischer-Tropsch
reactions are well known to those of skill in the art, and are
described, for example, in EP 0 921 184A1, the disclosure of which
is hereby incorporated by reference in its entirety. A schematic of
a conventional Fischer-Tropsch process is shown in FIG. 1.
[0027] The Fischer-Tropsch process can be understood by examining
the stoichiometry of the reaction that occurs during a
Fischer-Tropsch process. For example, during Fischer-Tropsch
processing, synthesis gas (i.e., a mixture including carbon
monoxide and hydrogen), is generated, typically from at least one
of three basic reactions. Typical Fischer-Tropsch reaction products
include paraffins and olefins, generally represented by the formula
C.sub.nH.sub.2n+2. While this formula accurately defines
mono-olefin products, it only approximately defines C.sub.5+
paraffin products. The value of n (i.e., the average carbon number
of the product) is determined by reaction conditions including, but
not limited to, temperature, pressure, space rate, catalyst type
and synthesis gas composition. The desired net synthesis gas
stoichiometry for a Fischer-Tropsch reaction is independent of the
average carbon number (n) of the product and is about 2.0, as
determined by the following reaction equation:
nCO+2nH.sub.2.fwdarw.nH.sub.2O+C.sub.nH.sub.2n+2 where
C.sub.nH.sub.2n+2 represents typical Fischer-Tropsch reaction
products such as, for example, olefins and paraffins. The amount of
by-product water produced by the reaction is significant. For
instance, when converting the molar ratios to weight ratios, one
can see that the relative weight percentages of water to CH.sub.2
hydrocarbons is 56%/44%.
[0028] The three general reactions that produce synthesis gas from
methane are as follows:
[0029] 1. steam reforming of methane:
CH.sub.4+H.sub.2O.fwdarw.CO+3H.sub.2;
[0030] 2. dry reforming, or reaction between CO.sub.2 and methane:
CH.sub.4+CO.sub.2.fwdarw.2CO+2H.sub.2; and
[0031] 3. partial oxidation using oxygen:
CH.sub.4+1/2O.sub.2.fwdarw.CO+2H.sub.2.
[0032] Although the above general reactions are the basic reactions
used to produce synthesis gas, the ratio of hydrogen to carbon
monoxide produced by the above reactions is not always adequate for
the desired Fischer-Tropsch conversion ratio of 2.0. For example,
in the steam reforming reaction, the resulting ratio of hydrogen to
carbon monoxide is 3.0, which is higher than the desired hydrogen
to carbon monoxide ratio of 2.0 for a Fischer-Tropsch conversion.
Similarly, in the dry reforming reaction, the resulting hydrogen to
carbon monoxide ratio is 1.0, which is lower than the desired
hydrogen to carbon monoxide ratio of 2.0. In addition to exhibiting
a hydrogen to carbon monoxide ratio that is lower than the desired
ratio for a Fischer-Tropsch conversion, the above dry reforming
reaction also suffers from problems associated with rapid carbon
deposition. Finally, because the above partial oxidation reaction
provides a hydrogen to carbon monoxide ratio of 2.0, the partial
oxidation reaction is the preferred reaction for Fischer-Tropsch
conversions.
[0033] In commercial practice, an amount of steam added to a
partial oxidation reformer can control carbon formation. Likewise,
certain amounts of CO.sub.2 can be tolerated in the feed. Thus,
even though partial oxidation is the preferred reaction for
Fischer-Tropsch conversions, all of the above reactions can occur,
to some extent, in an oxidation reformer.
[0034] During partial oxidation, CO.sub.2 forms because the
reaction is not perfectly selective.
[0035] That is, some amount of methane in the reaction will react
with oxygen to form CO.sub.2 by complete combustion. The reaction
of methane with oxygen to form CO.sub.2 is generally represented by
the following reactions: CH.sup.4+O.sub.2.fwdarw.CO.sub.2+2H.sub.2
and CH.sub.4+2O.sub.2.fwdarw.CO.sub.2+2H.sub.2O.
[0036] Furthermore, steam added to the reformer to control coking,
or steam produced during the Fischer-Tropsch reaction can react
with CO to form CO.sub.2 in a water gas shift reaction at various
points in the process including in the syngas generator, in the
syngas shift unit, and in the Fischer-Tropsch reactor. The reaction
is represented by the following general reaction:
CO+H.sub.2O.revreaction.CO.sub.2+H.sub.2.
[0037] In addition, light by-product gases, including
C.sub.1-C.sub.4 hydrocarbons, are frequently used as fuel in
furnaces. These fuels often include CO.sub.2 from a GTL facility
along with some unreacted CO. Moreover, during operation, a furnace
provides heat that can contribute to the generation of substantial
amounts of CO.sub.2.
[0038] Thus, invariably a significant amount of CO.sub.2 is formed
during the conversion of methane into transportation fuels and
lubricants by the Fischer-Tropsch process. The CO.sub.2 produced
during the Fischer-Tropsch process exits the Fischer-Tropsch/GTL
process in a tail gas exiting the Fischer-Tropsch unit. Tail gases
exiting a Fischer-Tropsch/GTL process comprise any gases that
remain unconsumed by the Fischer-Tropsch process.
[0039] The overall proportion of carbon in methane that is
converted to heavier hydrocarbon products has been estimated to be
as high as about 68%. Thus, the remaining 32% can form significant
amounts of CO.sub.2. These estimates of carbon efficiency are
provided, for example, by Bechtel Corporation for a GTL complex
using cryogenic air separation, an autothermal reformer, a slurry
bed Fischer-Tropsch unit and a hydrocracker for conversion of heavy
wax into saleable products. See "CO.sub.2 Abatement in GTL Plant:
Fischer-Tropsch Synthesis," Report # PH3/15, November 2000,
published by IEA Greenhouse Gas R&D Programme, the disclosure
of which is hereby incorporated by reference in its entirety.
Additionally, although the above estimates are provided for a
specific GTL complex, it is believed that similar carbon
efficiencies and CO.sub.2 emissions would be produced by GTL
processes employing alternative technologies.
[0040] The above equations represent general stoichiometric
equations, and do not reflect an optimum synthesis gas composition
for the kinetics or selectivity of a Fischer-Tropsch reaction.
Moreover, depending on the nature of the Fischer-Tropsch catalyst,
synthesis gas ratios other than 2.0, typically less than 2.0, are
used to prepare the feed to a Fischer-Tropsch unit. However,
because Fischer-Tropsch units typically produce products exhibiting
a hydrogen to carbon ratio of about 2.0, the limiting reagent,
typically H.sub.2, is consumed first. The extra reagent, typically
CO, is then recycled back to the Fischer-Tropsch unit for further
conversion. Synthesis gas compositions having hydrogen to carbon
ratios other than 2.0 are typically generated by recycling unused
reagents.
[0041] In order to control the reaction, Fischer-Tropsch processes
often operate at less than 100% conversion of the CO in the syngas.
Typical conversion values are between about 40% and about 70% per
pass. In Fischer-Tropsch processes that use O.sub.2 (typically
99.5% purity), rather than air, unreacted syngas is recycled to the
Fischer-Tropsch process. From the standpoint of economics and
operational efficiency, preferred Fischer-Tropsch processes use a
catalyst in a slurry bed. Also, from a practicality standpoint, the
most common catalyst for use in slurry bed units contains cobalt.
Cobalt does not promote the water-gas shift reaction or the reverse
of the reaction. In these units, CO.sub.2 is, for the most part, an
inert gas. As syngas is recycled to the Fischer-Tropsch unit, its
concentration builds up. As a result, the concentration of the
reactive syngas components (CO+H.sub.2) is effectively lowered,
thereby reducing the rate of reaction. To compensate for the lower
partial pressures of the reactive components, the pressure of the
Fischer-Tropsch reactor is increased. During this recycle
operation, a small amount of CO.sub.2 in the initial syngas
(typically about 5 vol % but always at least about 2 vol %)
increases to much larger values (typically about 40 vol %).
[0042] In commercial practice, a portion of the CO.sub.2-enriched
recycled gas is typically recycled to a syngas generator where it
promotes a dry reforming reaction and reduces the ratio of
H.sub.2:CO in the syngas to a desired level. However, much more
CO.sub.2 is produced than can be consumed in the syngas generator.
Thus, excess CO.sub.2-enriched recycled gas is purged from the
process and used as a low-energy content fuel. This
low-energy-content fuel is a significant source of CO.sub.2
emissions generated from syngas conversion processes.
[0043] In preferred embodiments of the present invention, a first
syngas, preferably containing at least about 2 vol % CO.sub.2, is
reacted in a first Fischer-Tropsch reactor to produce a first
hydrocarbonaceous product and a by-product of unreacted syngas
having an increased CO.sub.2 content. The by-product of unreacted
syngas constitutes a second syngas. The second syngas may be mixed
with a H.sub.2-containing stream to form an adjusted syngas. Next,
the second or adjusted syngas is reacted in a second
Fischer-Tropsch reactor to produce additional hydrocarbonaceous
product and a third syngas comprising a reduced amount of CO.sub.2
than was present in the feed to the second Fischer-Tropsch reactor.
Optionally, a H.sub.2-containing stream may be added to the
Fischer-Tropsch reactor if the second syngas stream is not mixed
with a H.sub.2-containing stream to form an adjusted syngas prior
to entering the reactor. Further, at least a portion of the third
syngas can be recycled to the first Fischer-Tropsch reactor for
additional processing. That is, at least a portion of the third
synthesis gas can be recycled to mix with the first syngas and
optionally a H.sub.2-containing stream to produce a blended syngas
at least one of before, during or after entering the first
Fischer-Tropsch reactor. Because the resulting blended syngas is
rich in methane, the blended syngas is not only easier to burn, but
also provides greater process control. Additionally, the
hydrocarbonaceous products produced by the first two
Fischer-Tropsch reactors can be combined to form a blended
hydrocarbonaceous product. By blending the hydrocarbonaceous
products to obtain a blended hydrocarbonaceous product, the present
invention can substantially minimize or eliminate the need for
further distillation and/or upgrading. As a result, the present
invention is less costly and more efficient than conventional
hydrocarbon product syntheses.
[0044] At least a portion of the blended hydrocarbonaceous product
may be converted into more valuable products including jet fuel,
diesel fuel, lubricant base oil, and naphtha. Furthermore, it may
be desirable to combine one or more of these more valuable
products.
[0045] The adjusted syngas, comprising the second syngas and a
hydrogen containing stream, may be formed in situ in the second
Fischer-Tropsch reactor. In such a case, both the second syngas and
the hydrogen containing stream would be fed directly to the second
Fischer-Tropsch reactor.
[0046] According to the present invention, the third syngas
comprises a reduced amount of CO.sub.2 than was present in the
adjusted syngas, meaning that the adjusted syngas has a greater
total amount or quantity of CO.sub.2 than does the third syngas.
Therefore, although the percent concentration of CO.sub.2 in the
third syngas may be greater than the percent concentration of
CO.sub.2 in the adjusted syngas, the third syngas comprises a
reduced total amount or quantity of CO.sub.2.
[0047] The blender that blends the hydrocarbonaceous products
should be understood to include any suitable means for blending
including, but not limited to, mixers, blenders, agitators and
stirrers, including, for example, those described in Section 18 of
Perry's Chemical Engineer's Handbook, Seventh Edition; (1997), the
disclosure of which is hereby incorporated by reference in its
entirety. Blending can be accomplished in a variety of ways
commonly known to those of still in the art. By way of example,
blending may be accomplished by sending the products to a common
distillation unit or separator, as shown in the preferred
embodiment described below. In such a case, the common distillation
unit or separator would serve as the blender.
[0048] The H.sub.2-containing stream can be mixed with the second
syngas at least one of before, during or after entering the second
Fischer-Tropsch reactor. The H.sub.2-containing stream can be
recovered from any number of sources in the Fischer-Tropsch
process. Suitable sources of a H.sub.2-containing stream include,
but are not limited to, syngas, unreacted gas streams from the
syngas conversion unit, gas streams associated with a
hydrotreator/hydrocracker used for upgrading the product, gas
streams associated with a reformer used to make an aromatic
product, steam reforming of CH.sub.4, combinations thereof and the
like. With regard to steam reforming of CH.sub.4 as a source of a
H.sub.2-containing stream, a steam reforming reactor may be needed
to produce a H.sub.2-containing stream for upgrading of C.sub.10+
product, via hydrotreating and hydrocracking. The
H.sub.2-containing stream can be recovered using any suitable
recovery process including, but not limited to, adsorption,
absorption, cryogenic separation, membrane separation, combinations
thereof and the like. Additionally, while it is preferable to use
recovered hydrogen, it is also acceptable to supplement or replace
recovered hydrogen with hydrogen obtained from alternative sources.
CO.sub.2 is a by-product of steam reforming. Since this
H.sub.2-containing steam will be reacted with CO.sub.2 from other
sources, it is not essential that the CO.sub.2 (or CO) be removed
from it prior to reaction. If CO.sub.2 and CO removal are needed
from the portion of the H.sub.2 intended for upgrading of the
C.sub.10+ product, this recovered CO.sub.2 and CO can be reacted
along with the other sources of CO.sub.2.
[0049] In the processes of the present invention, suitable
Fischer-Tropsch reactors include, but are not limited to, slurry
bed reactors, fixed bed reactors, fluidized bed reactors,
combinations thereof and the like. The first Fischer-Tropsch
reactor is preferably a slurry bed reactor that contains a Co
catalyst. Additionally, process conditions during operation of the
first Fischer-Tropsch reactor and the catalyst used therein are
preferably selected so that the water gas shift reaction occurring
therein is not promoted to an appreciable extent.
[0050] A feature of the second Fischer-Tropsch reactor is that it
is capable of converting CO.sub.2. Catalysts that do so are called
Shifting Fischer-Tropsch catalysts. Preferably, the second
Fischer-Tropsch reactor contains an iron catalyst. In the second
Fischer-Tropsch reactor CO.sub.2 is converted into additional
hydrocarbonaceous product and a second unreacted syngas having a
reduced CO.sub.2 content. The H.sub.2-containing stream added to
both the first and second Fischer-Tropsch reactors should contain
essentially no sulfur, as both cobalt-catalyzed and iron-catalyzed
Fischer-Tropsch catalysts are readily poisoned by sulfur, and
>50% H.sub.2, but may contain impurities such as hydrocarbons,
inert gases, N.sub.2, and light alcohols.
[0051] In addition to minimizing CO.sub.2 emissions, the processes
of the present invention also provide more efficient conversion of
syngas into valuable hydrocarbonaceous products. Sources of syngas
suitable for the present invention, include, but are not limited
to, CH.sub.4, coal, hydrocarbonaceous products, combinations
thereof and the like. Syngas can be derived from a combination of
these sources by using syngas generators.
[0052] Suitable syngas generators, according to the present
invention, can include a light hydrocarbon reformer or a heavy
hydrocarbon reformer. Light hydrocarbon reformers can employ a
variety of technologies including, but not limited to, steam
reforming, partial oxidation, dry reforming, series reforming,
convective reforming, autothermal reforming, combinations thereof
and the like. Generally, light hydrocarbon reformers change the
mixture of hydrocarbon products in a hydrocarbon product mixture.
Regardless of the specific technology used, syngas will be produced
from CH.sub.4 and an oxidant such as, for example, O.sub.2,
CO.sub.2, air, enriched air, combinations thereof and the like. The
resulting gas product typically contains some CO.sub.2 and steam,
in addition to syngas.
[0053] Series reforming, convective reforming and autothermal
reforming incorporate more than one syngas-forming reaction in
order to better utilize the heat of reaction. The above processes
for producing syngas or obtaining syngas from C.sub.1-C.sub.3
alkanes are well known in the art. Steam reformation is typically
performed by contacting C.sub.1-C.sub.3 alkanes with steam,
preferably in the presence of a reforming catalyst, at a
temperature of about 1,300.degree. F. (about 705.degree. C.) to
about 1,675.degree. F. (about 913.degree. C.) and a pressure of
about 10 psia (about 0.7 bars) to about 500 psia (about 34 bars).
Suitable reforming catalysts include, for example, nickel,
palladium, nickel-palladium alloys, combinations thereof and the
like.
[0054] Regardless of the system used to produce syngas, it is
desirable to remove any sulfur compounds such as, for example,
hydrogen sulfide and mercaptans, contained in the C.sub.1-C.sub.3
alkane feed. The removal of sulfur compounds can be achieved, for
example, by passing the C.sub.1-C.sub.3 alkane gas through a packed
bed sulfur scrubber containing a zinc oxide bed or other slightly
basic packing material. If the amount of C.sub.1-C.sub.3 alkanes
exceeds the capacity of the syngas unit, surplus C.sub.1-C.sub.3
alkanes can be used to provide energy throughout the facility. For
example, excess C.sub.1-C.sub.3 alkanes can be burned in an
upstream boiler to provide steam used in a thermal cracking step in
the process of the present invention.
[0055] A heavy hydrocarbon reformer converts coal, heavy petroleum
stocks or combinations thereof into syngas. The temperature in the
reaction zone of the syngas generator is in the range of about
1,800.degree. F. to about 3,000.degree. F. and the pressure is in
the range of about 1 atm to about 250 atm. The atomic ratio of free
oxygen in the oxidant to carbon in the feedstock (i.e., O/C, on an
atom to atom basis) is preferably in the range of about 0.6 to
about 1.5 such as, for example, about 0.80 to about 1.3. The free
oxygen containing gas or oxidant is selected from the group
consisting of air, O.sub.2-enriched air (i.e., about 21 mol % to
about 95 mol % O.sub.2) and substantially pure O.sub.2 (i.e., at
least about 95 mol % O.sub.2). An effluent gas stream leaving the
partial oxidation gas generator has the following composition in
mol %, depending on the amount and composition of the feed streams:
about 8.0 to about 60.0H.sub.2, about 8.0 to about 70.0 CO, about
1.0 to about 50.0 CO.sub.2, about 2.0 to about 75.0H.sub.2O, about
0.0 to about 30.0 CH.sub.4, about 0.1 to about 2.0H.sub.2S, about
0.05 to about 1.0 COS, about 0.0 to about 8.0 N.sub.2 and about 0.0
to about 2.0 Ar. Entrained in the effluent gas stream is
particulate matter comprising about 0.5 wt % to about 30 wt %
particulate carbon, preferably about 1 wt % to about 10 wt %
particulate carbon (based on the weight of carbon in the feed to
the gas generator). Fly ash particulate matter can be present along
with the particulate carbon and molten slag. Conventional gas
cleaning and/or purification steps can also be employed. See, for
example, U.S. Pat. No. 5,423,894 to Texaco, incorporated herein by
reference in its entirety.
[0056] It should be recognized that CO.sub.2 recycled to the first
Fischer-Tropsch reactor serves a useful purpose. That is, CO.sub.2
recycled to the first Fischer-Tropsch reactor serves as a diluent
for reactive syngas components such as, for example, H.sub.2 and
CO. Inert CO.sub.2 acts to dissipate heat generated during
operation and can improve process control. As CO.sub.2 is consumed
by the second Fischer-Tropsch reactor, other inert gas components
build up in the recycled gas. These inert components can include
methane, N.sub.2, Ar and other inert gas elements. Methane
components can originate from unconverted methane in the syngas
generator and methane formation that occurs in the first
Fischer-Tropsch reactor. The N.sub.2, Ar and other inert gas
elements can be generated from an air separation unit or may also
be obtained from an original natural gas feedstock. Normally,
CO.sub.2 is the most abundant inert gas component. However, when
CO.sub.2 is removed, in accordance with processes of the present
invention, the concentration of other inert gas components can
increase. These other inert gas components can act as a substitute
for the removed CO.sub.2 to improve management of heat released in
the first Fischer-Tropsch reactor. As a result, more of the carbon
(as carbon monoxide) in the recycled gas can be processed. The
increased processing of carbon (as carbon monoxide) improves
overall carbon efficiency as does the direct conversion of CO.sub.2
to products in the second Fischer-Tropsch reactor. Carbon
efficiency is defined as the ratio of C.sub.3+ products to feed
methane in the process. However, because of the increased
concentration of other inert gas components, it may eventually be
necessary to purge the other inert gas components from the system
to be used in other applications, thereby producing a
methane-enriched gas. In normal operation, the purged gas is rich
in CO.sub.2 and has a low heating value making its use limited.
However, according to the present invention, the purged gas will be
depleted in CO.sub.2 and enriched in other gases, typically
methane, thus increasing its heating value and uses.
[0057] In the recycling gas operation, the composition of the
blended syngas, introduced to the first Fischer-Tropsch reactor,
will change as the second Fischer-Tropsch reactor converts CO.sub.2
into other products. That is, the concentration of other inert gas
components will increase, and the concentration of reactive syngas
components will remain essentially constant. As previously
explained, the blended syngas is a combination of the first syngas,
from the syngas generator, and the third syngas, that is recycled
to the first Fischer-Tropsch reactor. The following table provides
typical ranges of syngas compositions for a process that generates
syngas from an O.sub.2 source of about 99.5% purity. TABLE-US-00001
Typical FT Unit With low With moderate With high without CO.sub.2
level of CO.sub.2 level of CO.sub.2 level of CO.sub.2 Component,
mol % conversion conversion conversion conversion H.sub.2 30 30 30
30 CO 40 40 40 40 CO.sub.2 20 15 10 5 N.sub.2 + Ar 2 3 4 6
C.sub.1+Hydrocarbons 8 12 16 19
[0058] It should be recognized that not all of the syngas from the
first reactor needs to be processed in the second reactor. Because
CO.sub.2, as an inert, builds up slowly and reasonable
concentrations of CO.sub.2 can be tolerated (e.g., about 60 mol %),
only a portion of the syngas from the first reactor needs to be
processed in the second reactor to achieve a desired CO.sub.2
conversion. Accordingly, the size of the second Fischer-Tropsch
reactor can be relatively small.
[0059] In certain circumstances, it is undesirable to process
syngas from the first Fischer-Tropsch reactor and then recycle the
CO.sub.2-depleted syngas back to the first reactor. It has been
surprisingly discovered that a reduction in CO.sub.2 emissions can
also be achieved by using a portion of the unreacted syngas from a
first Fischer-Tropsch reactor as a feed to a second Fischer-Tropsch
reactor.
[0060] Water is a natural product of the Fischer-Tropsch and
synthesis gas generation reactions. Preferably, water generated
from both the first and second reactors and the syngas generator is
separated from the syngas products and the hydrocarbonaceous
products. If not removed, the water can promote undesirable reverse
water gas shift reactions that can convert CO into additional
CO.sub.2.
[0061] A preferred embodiment of the present invention is depicted
in FIG. 2. In this embodiment, a feedstream 10 comprising CH.sub.4,
O.sub.2, H.sub.2O, and optionally CO.sub.2 enters a syngas
generator 12. The syngas generator 12 generates water 13 and a
first syngas stream 14. The syngas formed in the syngas generator
12 is cooled, and water 13 is removed after being condensed.
Additionally, the first syngas stream 14 exits the syngas generator
12 and enters a first Fischer-Tropsch reactor 16, which generates
stream 17 comprising a second syngas and a first hydrocarbonaceous
product along with water.
[0062] Stream 17, containing the second syngas, exits the first
Fischer-Tropsch reactor 16 and enters a first separator 18, which
separates an unreacted by-product stream 19, a water stream 20 and
a hydrocarbonaceous stream 21, comprising C.sub.5+ liquids. The
water stream 20 exits the first separator 18. A portion of the
unreacted gas stream 19 enters a second Fischer-Tropsch reactor 30.
Separator 18 is a generalized representation of the product
separation, representing one or more separation zones, some of
which may be internal to the Fischer-Tropsch reactor, and some of
which may be external to the Fischer-Tropsch reactor.
[0063] A second Fischer-Tropsch synthesis is conducted in the
second Fischer-Tropsch reactor 30 generating stream 31 comprising a
third syngas and a second hydrocarbonaceous product along with
water. Stream 31, containing the third syngas, exits the second
Fischer-Tropsch reactor 30 and is recycled to the first separator
18. The hydrocarbonaceous product stream 21 exits the first
separator 18 and enters a second separator 22. The second separator
22 separates the hydrocarbonaceous product stream 21 into a product
stream 23 comprising C.sub.1-C.sub.5, having a H:C ratio of about
2.0, a product stream 24 and a C.sub.10+ product stream 25, having
a H:C ratio of about 2.0.
[0064] The product stream 24 exits the second separator 22 and
enters a naphtha reformer 26. The naphtha reformer 26 upgrades the
product stream 24 to produce a C.sub.6-C.sub.10 product stream 27,
having a H:C ratio below about 2.0. The naphtha reformer 26 also
generates a H.sub.2-containing stream that exits the naphtha
reformer 26 in a stream 28 that is recycled to be mixed with the
unreacted gas stream 19, exiting the first separator 18, prior to
entering the second Fischer-Tropsch reactor 30. The stream 28 mixes
with the unreacted gas stream 19 so that a mixture comprising both
unreacted gas, from the first separator 18, and a
H.sub.2-containing stream, from the reformer 26, enters the second
Fischer-Tropsch reactor 30 in a mixed stream 29. Thus, mixed stream
29 is the adjusted syngas mentioned earlier.
[0065] Additionally, a portion of the unreacted gas stream 19,
exiting the first separator 18, can be divided into two additional
streams. The first stream can be comprised of excess unreacted gas
comprising CO, H.sub.2 and CO.sub.2. This stream exits the process
in an exit stream 32, to be used as fuel. The second stream can be
recycled in a stream 33 to be mixed with the feed gas 10, producing
a mixed feed gas 11 that enters the syngas generator 12. At least a
portion of the third syngas in the second product stream 31 is
directed in a stream 35 to be blended with the first syngas stream
14, producing a blended syngas stream 15. Additionally, another
portion of the unreacted gas stream 19 is recycled in a syngas
recycle stream 33. The syngas recycle stream 33, mixes with the
feed gas stream 10 producing a mixed feed gas stream 11 that enters
the syngas generator 12. Also, a portion of the syngas recycle
stream 33 can be directed in a stream 34 that mixes with the
blended syngas stream 15, producing a mixed syngas stream 36 that
is introduced into the Fischer-Tropsch reactor 16. Although the
embodiment depicted in FIG. 2 comprises two Fischer-Tropsch
reactors, additional reactors can be used as desired. For instance,
additional reactors may be used in instances where even greater
CO.sub.2 reduction is desired.
[0066] Generally, the operating conditions for the second
Fischer-Tropsch reactor used to convert CO.sub.2 should be:
TABLE-US-00002 Variable Broad Preferred Pressure, Atmospheres 1-20
10-18 Temperature, .degree. C. 250-425 300-360 CO + CO.sub.2
conversion, % 10-70 30-50 H.sub.2:(CO + CO.sub.2) ratio 1-8
1.25-3
EXAMPLE
[0067] In initial operation of the following example, a first
syngas is fed to a first Fischer-Tropsch reactor. A second syngas
is recovered from the Fischer-Tropsch reactor and fed to a second
Fischer-Tropsch reactor. A third syngas is recovered from the
second Fischer-Tropsch reactor. This third syngas may then be
blended with the first syngas to form a blended syngas, which would
be fed into the first Fischer-Tropsch reactor.
[0068] A first synthesis gas is formed from CH.sub.4 by reactions
that include partial oxidation in a syngas generator. The O.sub.2
used in the first syngas formation has a purity of about 99.5 vol
%. The first syngas from the reactor contains about 5 vol %
CO.sub.2. Water produced in the syngas generator is separated from
other components and is disposed of. The first syngas is blended
with a third syngas, forming a blended syngas. The blended syngas
has a CO.sub.2 content of about 10 vol %, and the blended syngas is
processed in a first Fischer-Tropsch reactor that uses a
Co-containing catalyst in a slurry bed-type reactor. The unreacted
syngas (i.e., the second syngas), contained in the products from
the first Fischer-Tropsch reactor has a higher CO.sub.2 content,
when expressed on the basis of C.sub.3.sup.- and lower molecular
weight species including inerts, than was present in the blended
syngas.
[0069] The gas and liquid products from the first Fischer-Tropsch
reactor are sent to a separation complex comprised of API
separators, distillation columns and other standard equipment
items. With a slurry bed reactor, effluent leaves the reactor in
two or more streams, wherein at least one stream is a vapor stream
and one stream is a liquid stream. The vapor stream leaving the
first Fischer-Tropsch reactor contains the second syngas. The
streams from the separation complex are comprised of water,
hydrocarbonaceous products and unreacted syngas. Since a separation
complex is needed for the products from the first and second syngas
reactors, it is economically desirable to use a single separation
complex for both reactors.
[0070] A portion of the syngas from the separator, that contains
the second syngas along with some of the third syngas, is mixed
with a H.sub.2-containing stream, produced later in the process, to
produce an adjusted syngas with a H.sub.2:(CO+CO.sub.2) ratio of
about 2.0. The portion of the syngas from the separator is
processed in a second Fischer-Tropsch reactor. The third syngas
contained in the products from the second Fischer-Tropsch reactor
comprises a reduced amount of CO.sub.2, when expressed on the basis
of CO, H.sub.2 and CO.sub.2, than was present in the syngas feed to
the second Fischer-Tropsch reactor. In the second Fischer-Tropsch
reactor, the CO.sub.2 conversion is about 40%, the temperature is
about 320.degree. C. and the pressure is about 10 atmospheres.
[0071] The effluent from the second Fischer-Tropsch reactor is sent
to the separation complex. The hydrocarbonaceous product from the
separation complex is then further separated to provide a C.sub.5
product, a C.sub.6-C.sub.10 containing product and a C.sub.10+
containing product. The C.sub.10+ containing product is converted
into products that include, but are not limited to, diesel fuel
having a H:C molar ratio of about 2.0 or greater. The
C.sub.6-C.sub.10 containing product can be reformed to produce an
aromatic product having a H:C molar ratio of below about 2.0 and
H.sub.2. The H.sub.2-containing stream produced during reformation
is recycled to the second Fischer-Tropsch reactor where it is used
to convert the CO.sub.2.
[0072] An additional portion of the syngas from the separation
complex, containing the second syngas and third syngas, is mixed
with the first syngas from the syngas generator, forming a blended
syngas. The blended syngas is then processed in the first
Fischer-Tropsch reactor.
[0073] While the present invention has been described with
reference to specific embodiments, this application is intended to
cover those various changes and substitutions that may be made by
those of ordinary skill in the art without departing from the
spirit and scope of the appended claims.
* * * * *