Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2001-05-02
2002-09-03
Coerrler, William C. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S620000, C062S929000
Reexamination Certificate
active
06442969
ABSTRACT:
The invention relates to a process and a device for separation of an acid gas fraction comprising carbon dioxide and/or hydrogen sulfide contained in a gas mixture, e.g. a sour gas, that comprises at least one lighter component, such as methane.
The technological background is illustrated by U.S. Pat. Nos. 4,152,129, 5,983,663 and 5,927,103.
The separation of carbon dioxide that is contained in a gas mixture is an operation that is intended either to obtain a gas that contains a reduced content of carbon dioxide, for example in the case of a natural gas, with a view to obtaining an adequate gross calorific value or to obtaining a carbon dioxide-concentrated gas. Such a gas can be used, for example, to carry out an assisted recovery operation of petroleum. In addition, the wish to limit carbon dioxide emissions into the atmosphere leads to a growing advantage for operations for reinjecting carbon dioxide underground. In this case, it is generally preferable to inject underground a carbon dioxide-concentrated gas either to avoid losing enrichable fractions or to reduce the cost of the compression and injection operation.
There exist different processes for treatment of a gas such as a natural gas or a refinery gas to separate the carbon dioxide. These processes are generally based on an operation of absorption by a chemical or physical solvent. These processes are limited, however, to the treatment of a gas containing relatively small contents of carbon dioxide, for example not exceeding 20 mol %.
In the case of a gas mixture that contains high carbon dioxide contents, there exist very few suitable solutions.
The Ryan-Holmes process, which is most used in this case and which is based on a series of stages of fractionation by distillation, is a costly process. This process that operates under cold conditions typically uses four distillation columns, operating in the presence of an additive, and consists of a hydrocarbon fraction that is recycled so as to avoid any risk of crystallization of the carbon dioxide.
In the same way, it may be necessary to separate from a gas mixture, such as a natural gas, another acid gas such as hydrogen sulfide (H
2
S). There exist processes that make it possible to separate such a contaminant that is toxic and corrosive, for example by washing the gas mixture with an amine. These processes become very costly, however, as soon as the gas mixture comprises more than 10 to 15% of H
2
S.
The process according to the invention aims at separating an acid gas such as CO
2
and/or H
2
S in a more economical manner by reducing the amount of equipment needed as well as the energy consumption.
It was discovered, and this is a first object of this invention, that it is possible to carry out such an operation in a simple and economical manner by operating the refrigeration and the rectification of the gas mixture simultaneously in an indirect heat exchanger.
More specifically, the invention relates to a process for separation of an acid gas fraction that comprises carbon dioxide and/or hydrogen sulfide that is contained in a mixture that comprises at least one lighter gas, characterized in that it comprises the following stages:
(a) gas mixture (
1
) is precooled at least once during a heat exchange operation (E
2
),
(b) the gas mixture that is obtained from stage (a) is cooled and rectified simultaneously in an approximately vertical heat exchange zone (ER) by generating a downward liquid reflux,
(c) a gaseous fraction (
8
) that is low in acid gases and high in light gas is collected at the top of the vertical exchange zone, and
(d) an acid gas-enriched liquid fraction (
4
) is collected at the bottom of the exchange zone.
Such a cooling and rectification exchanger is, for example, a plate exchanger. The gas mixture that is to be separated circulates vertically in the upward direction in the exchanger while being cooled. The circulation speed of the gas mixture is kept at a low enough value for the liquid fraction that is generated by cooling to be able to descend again in countercurrent to the gas mixture. In going down, this liquid fraction is reheated and enriched with carbon dioxide while the gas mixture exits at the top with a substantially reduced carbon dioxide content.
By extracting heat by indirect exchange, more reflux is created and thus a better acid gas (CO
2
for example) /light gas (hydrocarbon for example) fractionation is obtained.
The gaseous fraction that is low in acid gases, collected during stage (c), can comprise methane and/or nitrogen and/or hydrogen.
According to a first variant of the invention, it was also discovered that the refrigeration that is necessary to the gas mixture can be ensured at least in part by being expanded at least once and by evaporating in the exchanger at least in part the acid gas-rich liquid fraction collected at the base of the exchanger in which the fractionation of the mixture is carried out.
According to a characteristic of this invention, the expanded liquid fraction is separated in a separation chamber B
2
, and an acid gas-rich gaseous top fraction that is recycled in stage (a) is recovered. It is preferable to carry out at least in part a first expansion of the liquid fraction and to separate a first expansion effluent in a first separation chamber in such a way as to recover at the top a first gaseous fraction that is enriched with lighter components than the acid gases recycled in stage (a) and a first acid gas-enriched liquid fraction. This first liquid fraction can be expanded at least in part, and the second expansion effluent can be separated into a second gaseous top fraction that is enriched with lighter components than the acid gases, recycled in precooling stage (a), and into a second acid gas-enriched liquid fraction that is used as a coolant in the heat exchanger. These first and second gaseous fractions that are enriched with lighter components than the acid gases and at two different pressure levels can be recompressed. The fact of operating in at least two stages makes it possible to reduce the mechanical energy of compression that is necessary for recycling the separated gaseous fractions in stage (a).
According to another embodiment, the acid gas-enriched liquid fraction that is collected at the end of stage (d) can be expanded successively to at least two pressure levels, whereby liquid fraction (
4
a
) that is obtained at the end of a first expansion stage is brought into contact in a zone (C
1
) for countercurrent contact with a vapor reflux (
16
) that is obtained from the evaporation of a portion of liquid fraction (
5
a
) that is collected at the bottom of said contact zone (C
1
).
According to a second variant of the invention, the refrigeration that is required during stage (b) can be ensured at least in part by expansion (TD) of gaseous fraction (
8
) that is low in acid gas and high in light gas that is collected during stage (c).
According to a third variant of the invention, the refrigeration that is required during stage (b) can be ensured at least in part by external refrigeration means, for example an external refrigeration cycle that operates with propane.
According to another characteristic of the process, it is also possible to operate the contact in countercurrent that is carried out in exchanger ER between the acid gas-rich gaseous fraction and the liquid reflux in the presence of solvent.
This offers various advantages. First of all, it thus is possible to inhibit the formation of solid crystals of carbon dioxide and thus to be able to drop to a lower temperature, for example −50° C., at the top of exchanger ER. By operating at a lower temperature, the amount of acid gas(es) entrained with the lighter gas, for example methane and/or nitrogen with which this or these acid gas(es) are mixed, is reduced.
The addition of a solvent may also make it possible to improve the quality of the separation, primarily if this solvent has a selective nature compared to the acid gas(es) to be separated.
This solvent is preferably a polar solvent with a low enough viscosity
Fischer Beatrice
Minkkinen Ari
Rojey Alexandre
Coerrler William C.
Institut Francais du Pe'trole
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