Chemistry: fischer-tropsch processes; or purification or recover – Liquid phase fischer-tropsch reaction
Reexamination Certificate
2002-04-09
2004-02-17
Parsa, J. (Department: 1621)
Chemistry: fischer-tropsch processes; or purification or recover
Liquid phase fischer-tropsch reaction
C208S133000, C208S141000
Reexamination Certificate
active
06693138
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to the reduction of carbon dioxide emissions from Fischer-Tropsch GTL facilities.
2. Description of the Related Art
The conversion of remote natural gas assets into transportation fuels has become more desirable because of the need to exploit existing natural gas assets as a way to satisfy the increasing need for transportation fuels. Generally, the term “remote natural gas” refers to a natural gas asset that cannot be economically shipped to a commercial market by pipeline.
Conventionally, two approaches exist for converting remote natural gases into conventional transportation fuels and lubricants, including but not limited to, gasoline, diesel fuel, jet fuel, lube base stocks, and the like. The first approach comprises converting natural gas into synthesis gas by partial oxidation, followed by a Fischer-Tropsch process, and further refining resulting Fischer-Tropsch products. The second approach comprises converting natural gas into synthesis gas by partial oxidation, followed by methanol synthesis wherein the synthesized methanol is subsequently converted into highly aromatic gasoline by a Methanol To Gasoline (MTG) process. Both of these approaches use synthesis gas as an intermediate. Also, while other approaches exist for using natural gas in remote locations, such approaches do not produce conventional transportation fuels and lubricants, but instead produce other petroleum products including, but not limited to, liquified natural gas (LNG) and converted methanol.
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
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 high 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.
Accordingly, in view of the above disadvantages of the Fischer-Tropsch and MTG processes, there is a need for a process that is capable of producing desirable Fischer-Tropsch petroleum products while significantly minimizing CO
2
emissions commonly generated during the production of such products.
Catalysts and conditions for performing Fischer-Tropsch reactions are well known to those of ordinary skill in the art, and are described, for example, in EP 0 921 184A1, the contents of which are hereby incorporated by reference in their entirety. A schematic of a conventional Fischer-Tropsch process is shown in
FIG. 1. A
feed stream
11
comprising CH
4
, O
2
and H
2
O is introduced into a synthesis gas formation reactor
13
. Although feed stream
11
is depicted as a single stream, it may be desirable to introduce the feed as multiple separate streams. In fact, because it is undesirable to mix O
2
and CH
4
before introduction to the synthesis gas formation reactor
13
, it may be especially beneficial to introduce at least the O
2
and CH
4
in separate streams. A synthesis gas stream
14
comprising CO, H
2
and CO
2
is produced from the synthesis gas formation reactor
13
and introduced into a Fischer-Tropsch reactor
15
. A Fischer-Tropsch process is conducted to produce a Fischer-Tropsch product stream
16
that is fed into a first separator
17
. The first separator
17
separates the Fischer-Tropsch product stream into an unreacted gas stream
18
, comprising CO, H
2
and CO
2
, and a hydrocarbon products stream
22
comprising principally C
5
+
liquids with small amounts of dissolved C
1
-C
5
gaseous products. The unreacted gas stream
18
can be recirculated in a stream
21
to be mixed with the synthesis gas
14
before entering the Fischer-Tropsch reactor
15
. In addition, a portion of the unreacted gas stream
18
can be removed in an exit stream
19
where excess CO, H
2
and CO
2
are ignited by a flare or used as low-BTU fuel.
The generation of CO
2
emissions from Fischer-Tropsch processes 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 nCH
2
. While this formula accurately defines mono-olefin products, it only approximately defines C
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
2
nH
2
O+nCH
2
where nCH
2
represents typical Fischer-Tropsch reaction products such as, for example, olefins and paraffins.
The three general reactions that produce synthesis gas from methane are as follows:
steam reforming of methane: CH
4
+H
2
O CO+3H
2
; dry reforming, or reaction
between CO
2
and methane: CH
4
+CO
2
2CO+2 H
2
; and partial oxidation using oxygen:
CH
4
+½O
2
CO+2H
2
.
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. (In the instant application, all ratios are molar ratios, unless otherwise noted.) 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 for a Fischer-Tropsch conversion. 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.
Generally, the proportion of carbon in methane that is converted to heavier hydrocarbon products in Fischer-Tropsch/GTL processes is estimated to be about 68%. Thus, the remaining 32% of the carbon is left to form significant amounts of CO
2
. Although these estimated values were provided for a GTL facility using cryogenic air separation, an autothermal reformer, a slurry bed Fischer-Tropsch unit and a hydrocracker for converting heavy wax into saleable products, GTL facilities using alternative technologies would exhibit similar carbon conversion efficiencies and CO
2
emissions. A detailed description of the above estimates is described in “CO
2
Abatement in GTL Plant: Fischer-Tropsch Synthesis,” Report #PH3/15, November 2000, published by IEA Greenhouse Gas R&
Burns Doane Swecker & Mathis L.L.P.
Chevron U.S.A. Inc.
Parsa J.
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