Aqueous separation of syngas components

Details Aqueous separation of syngas components Aqueous separation of syngas components

2002-04-29

2004-04-20

Parsa, J. (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Liquid phase fischer-tropsch reaction

C518S702000, C518S705000, C095S150000

Reexamination Certificate

active

06723756

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a treatment of unreacted synthesis gas (syngas) produced in a gas-to-liquid synthesis and more particularly to a process of contacting unreacted syngas with water to remove CO
2
.
2. Description of Related Art
There is a considerable economic incentive to exploit the production of natural gas which is an abundant resource normally available only at remote sites. Frequently, it is not economically viable to transport natural gas from such remote sites to commercial markets or off-site processing facilities. One approach has been to convert the natural gas into liquified natural gas (LNG) for transport to markets or processing facilities. Another approach has involved converting natural gas into methanol at the remote site without further processing of methanol into gasoline.
Natural gas is a primary source of methane which is used to manufacture synthesis gas. Synthesis gas (syngas) is primarily a mixture composed of CO and H
2
. Techniques are known to convert syngas into useful products such as methanol or into synthetic fuels, lubricants and other hydrocarbonaceous products via Fischer-Tropsch synthesis. One method for the preparation of syngas involves catalytically reacting methane and carbon dioxide. While natural gas is a primary source of methane, coal and petroleum have also been utilized to provide hydrocarbon feeds to generate syngas.
Carbon dioxide is considered by some to be a major factor in global warming. Accordingly, there is an incentive to find means for reducing the production of carbon dioxide and limiting its release into the environment. One advantage of the aforementioned process for preparing syngas is that it utilizes carbon dioxide as a reactant.
At present, there are two gas-to-liquid (GTL) technologies which convert remote natural gas assets or coal into transportation fuels and lubricants. Both use syngas as an intermediate. The first involves the conversion of natural gas or coal into syngas by partial oxidation followed by reaction in a Fischer-Tropsch synthesis with further refining of the Fischer-Tropsch products. The second technology involves conversion of natural gas or coal into syngas by partial oxidation followed by methanol synthesis, the methanol being subsequently converted into highly aromatic gasoline by the Methanol-To-Gasoline (MTG) process.
The Fischer-Tropsch and MTG processes both have relative merits and disadvantages. One advantage of the Fischer-Tropsch process is that the products formed are highly paraffinic. These products have excellent combustion and lubricating properties. A disadvantage of the Fischer-Tropsch process is the relatively large amounts of carbon dioxide that are emitted in the facility during the conversion of natural gas into Fischer-Tropsch products. The MTG process produces a highly aromatic gasoline and LPG fraction. While the gasoline generally is suitable for use in gasoline engines, durene and other polymethyl aromatics may be present. These materials have high crystallization temperatures and can solidify upon standing. The MTG process also suffers from higher capital costs in comparison to the Fischer-Tropsch process and the product cannot be used for lubricants, diesel fuel or jet turbine fuel.
A typical Fischer-Tropsch process is illustrated in
FIG. 1. A
feed of CH
4
, O
2
and H
2
O is forwarded via conduit (
10
) to a syngas generator (
15
). Effluent from the generator containing CO, H
2
and CO
2
is forwarded via conduit (
20
) to a Fischer-Tropsch reactor (
25
). The products of the reaction are forwarded via conduit (
35
) to a separation zone (
40
). Hydrocarbonaceous products including C
5
+ liquids are recovered and forwarded via conduit (
45
) to other areas of the facility for further processing into fuels, lubes, etc. Gaseous products recovered from the separation zone (e.g. tail gas) include CO, H
2
and CO
2
. A portion of the tail gas is forwarded via conduit (
60
) for use as a fuel in the facility. Another portion of the tail gas is recycled via conduit (
50
) to be mixed with the feed to the syngas generator (
15
).
The origin of the CO
2
emissions from the Fischer-Tropsch synthesis can best be understood by examining the stoichiometry of the reaction. The major products of a Fischer-Tropsch reaction are paraffins and olefins, and these can be represented by the formula nCH
2
which represents a paraffinic polymer of n CH2 units This formula is exact for mono-olefins and a close approximation for C
5
+ paraffins. The value of n (the average carbon number of the product) is determined by the reaction conditions, e.g., temperature, pressure, space rate, catalyst type, and syngas composition. The desired net syngas stoichiometry for a Fischer-Tropsch reaction is independent of n, and is approximately 2.0 as determined by the following equation:
n
CO+2
n
H
2
→n
H
2
O+
n
CH
2
where nCH
2
represent the major products of a Fischer-Tropsch reaction (olefins and paraffins).
There are three general reactions that produce syngas from CH
4
. These are:
Steam reforming of CH
4
:
CH
4
+H
2
O→CO+3H
2
However, the ratio of H
2
to CO is 3:1 which is higher than the 2:1 ratio desired for the Fischer-Tropsch conversion.
Dry reforming, or reaction between CO
2
and CH
4
:
CH
4
+CO
2
→2CO+2H
2
However, the ratio of H
2
to CO is 1:1, which is lower than that desired for the Fischer-Tropsch conversion. Also, dry reforming may result in rapid carbon deposition.
Partial oxidation using O
2
:
CH
4
+½O
2
→CO+2H
2
.
This provides the desired 2:1 ratio of CO and H
2
and is the reaction that is to be emphasized.
In commercial practice, an amount of steam is added to a partial oxidation reformer in order to control carbon formation. Likewise, some CO
2
can be tolerated in the feed. So while partial oxidation is the emphasized reaction, all reactions occur to some extent in the reformer.
CO
2
is formed in partial oxidation because the reaction is not perfectly selective. Some CH
4
reacts with O
2
to form CO
2
by complete combustion according to the following:
CH
4
+O
2
→CO
2
+2H
2
and
CH
4
+2O
2
→CO
2
+2 H
2
O
Furthermore, steam added to the reformer to control coking, or produced in the Fischer-Tropsch reaction, can react with CO to form CO
2
by the water gas shift reaction as follows:
CO+H
2
O→CO
2
+H
2
This reaction reaches equilibrium, and the reverse of it is known as the reverse water gas shift reaction:
CO
2
+H
2
→CO+H
2
O
Furthermore, light by-product gases, which include C
1
-C
4
hydrocarbons, are frequently used as fuel in furnaces. This fuel often includes the CO
2
from the GTL facility along with some unreacted CO. The furnaces provide the heat to the process, and contribute significant amounts of CO
2
. With Fischer-Tropsch catalysts that do not promote the water gas shift reaction (Co-based catalyst rather than Fe-based catalysts), and with proper operation of the reformer and other units, the major source of CO
2
is combustion of hydrocarbons in the furnaces.
Thus, a significant amount of CO
2
is formed during the conversion of CH
4
into transportation fuels and lubricants by the Fischer-Tropsch process. This CO
2
exits the GTL-Fischer-Tropsch process in the tail gas from the Fischer-Tropsch unit, i.e., in the gases that are not consumed in the Fischer-Tropsch reactor.
The overall proportion of carbon in the CH
4
that is converted to heavier hydrocarbon products is estimated to be about 68%. The remainder, about 32%, forms significant amounts of CO
2
. These estimates of carbon efficiency were provided by Bechtel Corporation for a GTL complex that uses cryogenic air separation, an autothermal reformer, a slurry bed Fischer-Tropsch unit and a hydrocracker for conversion of the heavy wax into products. Details are described in “CO
2
Abatement in GTL Plant: Fischer-Tropsch Synthesis,” Report # PH3/15, November 2000, published by the IEA Greenhouse Gas R&D P

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