WO2012029021A1

WO2012029021A1 - Method for treating a natural gas containing carbon dioxide

Info

Publication number: WO2012029021A1
Application number: PCT/IB2011/053789
Authority: WO
Inventors: Xavier Renaud
Original assignee: Total S.A.
Priority date: 2010-09-03
Filing date: 2011-08-30
Publication date: 2012-03-08
Also published as: FR2964390B1; MY160227A; FR2964390A1

Abstract

The invention relates to a method for treating a natural gas containing carbon dioxide, involving the following steps: - at least some of the carbon dioxide contained in the natural gas is transformed into the solid state by cooling the natural gas in a heat-exchange module, the pressure of the natural gas during cooling being between 5 and 30 bar; - the solid carbon dioxide is deposited in the heat-exchange module; - a carbon-dioxide-lean stream of natural gas is retrieved at the outlet of the heat-exchange module; the solid carbon dioxide deposited in the heat-exchange module is melted by heating; and - a stream of liquid carbon dioxide is recovered at the outlet of the heat-exchange module. The invention also relates to a facility for implementing this method.

Description

PROCESS FOR TREATING A NATURAL GAS CONTAINING

CARBON DIOXIDE

FIELD OF THE INVENTION

The present invention relates to a method of treating natural gas to at least partially remove the carbon dioxide it contains. The invention also relates to an installation adapted to the implementation of this method.

TECHNICAL BACKGROUND

In the context of the production of natural gas or liquefied natural gas, it is necessary to purify said natural gas from a pool of a number of contaminants, foremost among which are acid gases such as hydrogen sulphide. (H ₂ S) and carbon dioxide (CO2).

In particular, carbon dioxide may represent an important part of the gaseous mixture from a natural gas field, typically from 3 to 70% (in molar concentration).

Several methods are known in the art to reduce the carbon dioxide content of natural gas.

The most common treatment is based on the use of amino solvents. This method allows a very selective separation of CO2 from hydrocarbons; it enables the CO2 concentration to be lowered below the threshold of 50 ppm. But this method requires significant energy for the regeneration of the solvent. It is therefore unsuitable in case of high concentration of CO2 in the original gas. Regeneration is almost atmospheric, and requires compression consuming a lot of energy if one considers a reinjection of the separated CO2 (which is to be envisaged in a more and more systematic way taking into account the environmental stakes).

Another type of treatment relies on the use of semi-permeable membranes. The applications of these membranes for gases with a content average in CO2 have grown significantly in recent years. Membrane treatment is advantageous for significant CO2 concentrations and for a certain range of "inlet / retentate" partial pressure ratios. However, when CO2 specifications are relatively low, the associated methane losses can become significant. It is then possible to provide several stages of membranes to concentrate the CO2 in the permeate and reduce methane losses, which requires the provision of intermediate compression of the permeate and the associated mechanical energy. The reinjection of CO2, when it is sought, requires additional compression, starting from the low pressure of the final permeate, which further increases the energy bill for this type of process.

Cryogenic processes are another type of treatment. Their energy value is even greater than the concentration of CO2 in the original gas is high. An example of a cryogenic process by distillation is found in US 4,152,129. However, due to limitations related to the crystallization of CO2 or to the proximity of critical conditions at the column head, such a method does not make it possible to achieve binding specifications for CO2, but frequently limited specifications around 20% of CO2. . A finishing treatment, for example of the amine type, is therefore essential if a severe CO2 specification is required.

Some cryogenic treatment variants have been presented more recently, in particular the CFZ (Controlled Freeze Zone) process, whose particularity is to manage the crystallization of CO2 in the problem zone of the distillation column, which makes it possible to envisage specifications. outbreaks with very low treatment temperatures (around -90 ° or -120 ° C). In this regard, reference may be made for example to US 4,533,372.

Another variant of cryogenic treatment has been developed by

Cool Energy Limited. This process, with the trade name "Cryocell", makes it possible to obtain specifications around 2 to 3% of CO2, from a gas pretreated by cryogenic distillation, or directly for raw gases with a medium CO2 concentration (typically 25 to 35%). This process uses a liquefaction of the gas under pressure, then a relaxation of the fluid which creates an intense cold and a partial crystallization of the CO2. The liquid and solid fractions are recovered in a flask designed to maintain the bottom temperature in the liquid domain. WO 2007/030888 and WO 2008/095258 illustrate this technique. Another variant, illustrated by the document WO 03/062725, aims a selective recovery of CO2 crystallized by a mechanical system, but its practical realization and its effectiveness do not seem assured.

Another variant of cryogenic treatment consists of the family of processes known as "Ryan Holmes". These processes allow a good separation of components, but use 3 or 4 distillation columns, and are therefore relatively complex and heavy investment and consumption.

A disadvantage of the simplest cryogenic methods is that they separate the components according to their volatility and trap with liquid CO2, almost all C3 + hydrocarbons contained in natural gas. This is a handicap sometimes very important depending on the composition of the gas. It is estimated that from 8 to 15% by weight of the hydrocarbons are generally lost when CO2 separation is carried out by distillation; moreover, the lost hydrocarbons are mainly hydrocarbons of intermediate molar mass, therefore the most valued ones.

There is therefore a real need to develop a natural gas treatment to effectively reduce its carbon dioxide content without inducing significant loss of hydrocarbons.

SUMMARY OF THE INVENTION

The invention firstly relates to a method of treating a natural gas containing carbon dioxide, comprising the following steps:

- The solid state of at least a portion of the carbon dioxide contained in the natural gas by cooling the natural gas in a heat exchange module, the pressure of the natural gas during cooling being 5 to 30 bar;

deposition of the solid carbon dioxide in the heat exchange module;

recovery of a stream of natural gas depleted in carbon dioxide at the outlet of the heat exchange module;

melting of the solid carbon dioxide deposited in the heat exchange module by heating; and recovering a stream of liquid carbon dioxide at the outlet of the heat exchange module.

According to one embodiment, the method comprises a step of:

separation of the stream of carbon dioxide depleted natural gas into a C2 + rich liquid phase and a C2 + poor gas phase.

According to one embodiment, the method comprises a step of:

fractionation of the C2 + rich liquid phase into a stream rich in methane and in a stream that is poor in methane;

the methane-rich stream being preferably mixed with the C2 + -low gas phase.

According to one embodiment, the method comprises a step of:

fractionation of the poor methane stream into a C3 + rich stream and a C3 + poor stream.

According to one embodiment, the pressure during cooling is 10 to 20 bar.

According to one embodiment, at least two heat exchange modules are used alternately, the solid state transition of the carbon dioxide and the deposition of the solid carbon dioxide in a first heat exchange module being carried out simultaneously with the melting of the carbon dioxide in a second heat exchange module.

According to one embodiment, the natural gas containing carbon dioxide undergoes a deacidification and / or dehydration and / or fractionation prior to the solid state transition of the carbon dioxide by cooling.

According to one embodiment, the methane-rich stream is further treated to remove carbon dioxide by washing with a methanol-based solvent.

The invention also relates to a plant for treating a natural gas containing carbon dioxide, comprising:

at least one heat exchange module adapted to cool and heat a gas flow;

a natural gas supply line supplying the heat exchange module;

a collection line of natural gas depleted in carbon dioxide at the outlet of the heat exchange module; a collection line for liquid carbon dioxide at the outlet of the heat exchange module;

liquid / gas separation means supplied by the carbon dioxide depleted natural gas collection line; a liquid phase collection line and a gas phase collection line at the outlet of the liquid / gas separation means. According to one embodiment, the liquid / gas separation means consist of a liquid / gas separator, the liquid phase collection line is a C2 + rich liquid phase collection line, the gas phase collection line is a line C2 + low gaseous phase collection, and the installation preferably further comprises:

- a C2 + rich liquid phase fractionation unit, fed by the C2 + rich liquid phase collection line;

a methane rich flow collection line at the outlet of the C2 + rich liquid flow fractionation unit;

a methane-poor stream collection line at the outlet of the fractionation unit of the C2 + rich liquid phase;

the methane-rich stream collecting line preferably being connected to the C2 + lean gas phase collecting line to form a purified natural gas collection line.

According to another embodiment, the liquid / gas separation means consist of a fractionation unit, the liquid phase collection line is a methane-poor stream collection line and the gas phase collection line is a line of collection of purified natural gas.

According to one embodiment, the installation furthermore comprises:

a contactor supplied on the one hand by the purified natural gas collection line and on the other hand by a solvent supply line;

a highly purified gas collection line and a solvent collection line loaded at the contactor output; and

solvent regeneration means connected at the inlet to the loaded solvent collection line and at the outlet of the solvent supply line.

According to another embodiment, the installation comprises a column, the liquid / gas separation means being arranged in the column, the liquid phase collection line being a methane-poor stream collection line, the collection line of gas phase being a highly purified gas collection line, and the installation further comprises: a solvent supply line feeding the column;

a solvent collection line loaded at the outlet of the column; and

According to one embodiment, the installation comprises:

a methane-poor fraction fractionation unit fed by the methane-poor flux collection line;

a flow collection line rich in C3 + at the outlet of the methane-poor flux fractionation unit;

a low C3 + stream collection line at the outlet of the methane-poor flux fractionation unit.

According to one embodiment, the installation comprises at least two heat exchange modules, the natural gas supply line being connected to an inlet of each heat exchange module, the depletion of natural gas collection line in dioxide of carbon being connected to an output of each heat exchange module and the liquid carbon dioxide collection line being connected to an output of each heat exchange module, the installation also comprising a plurality of valves adapted to that the heat exchange modules, alternatively:

- are fed by the natural gas supply line and feed the carbon dioxide-depleted natural gas collection line; and

- feed the liquid carbon dioxide collection line.

According to one embodiment, the plant comprises a deacidification unit and / or a dehydration unit and / or a fractionation unit upstream of the heat exchange module.

According to one embodiment, the above method is implemented in an installation as described above.

The present invention overcomes the disadvantages of the state of the art. In particular, it provides a natural gas treatment by which the carbon dioxide content can be significantly reduced, to a high specification, while recovering most of the CO ₂ -dependent hydrocarbons. Thus, said treatment is implemented with limited hydrocarbon losses. This is accomplished through the use of heat exchange modules, in which the pressurized natural gas is cooled to produce solid CO2. The solid CO2 is deposited on the cold surface of the heat exchange module, and the natural gas is purified of a part of the CO2. In a second phase, the heat exchange module operates a heating to recover a flow of CO2 in liquid form. A portion of the natural gas hydrocarbons are eventually liquefied during cooling, but the liquefied hydrocarbons as well as the initially liquid hydrocarbons remain predominantly in the natural gas stream and are trapped only marginally in the crystallized CO2.

According to some particular embodiments, the invention also has one or preferably more of the advantageous features listed below.

The invention can be implemented with a wide range of cooling temperatures, which determines the CO2 specification obtained. There is no strict limitation to the process as is the case in conventional cryogenic methods, where the occurrence of solid CO2 must be avoided. Thus, it is possible to achieve, with the method according to the invention, the desired CO2 specification in the treated gas, and in particular to achieve the specification specific to the pipelines (of about 2.5% of CO2) for a temperature close to -90 ° C.

To achieve a much lower specification, such as that required in natural gas liquefaction processes (50 ppm molar CO2), it is however preferable to provide a finishing treatment, for example an amine solvent treatment or a treatment with use of refrigerated methanol. This latter alternative offers the advantage of good thermal integration with the cryogenic process which is the subject of the invention. It also avoids the humidification of the gas and its final dehydration, necessary in the finishing treatment with amines.

According to the invention, the crystallized CO2 in the heat exchange module is recovered in liquid form, and it can be pressurized by simple pumping for injection into geological structures (unlike processes based on an amine solvent or on a semi-permeable membrane). The process of the invention is particularly useful and suitable for a natural gas with a significant content of CO ₂ (from 20 to 40%), a low content of hydrogen sulphide, and comprising a large fraction of C2 + hydrocarbons. .

The process of the invention is also possible, as a complementary treatment downstream of a conventional cryogenic distillation producing a pretreated gas with about 20% CO ₂ , almost free of C3 +. In this case, the pressure of the crystallization module can be chosen at the maximum pressure which prevents the condensation of liquid hydrocarbons, which simplifies the flow diagram and limits the recompression power of the treated gas downstream of the unit.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 schematically shows an embodiment of an installation according to the invention.

FIG. 2 schematically represents another embodiment of the invention (variant in which the gas / condensate separation and the stabilization of the condensate are carried out in a single column).

FIG. 3 schematically represents another embodiment of the invention (variant with a complementary treatment by absorption with refrigerated methanol, to obtain a high CO ₂ specification on the treated gas).

FIG. 4 schematically represents another embodiment of the invention (variant grouping the gas / condensate separation, the first condensate stabilization column and the refrigerated methanol contactor in a single column). DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention is now described in more detail and without limitation in the description which follows.

All pressures are given in absolute values. All percentages are given in molar values unless otherwise stated. The terms "upstream" and "downstream" refer to the direction of flow of natural gas. Installation

With reference to FIG. 1, the plant according to the invention comprises a natural gas supply line (for example from a dewatering and pre-cooling unit), which supplies at least one module with heat exchange and preferably at least two heat exchange modules 1, 2. An inlet valve 21, 22 is provided before each heat exchange module 1, 2.

Each heat exchange module 1, 2 is a heat exchanger which comprises on the one hand an enclosure or a space in which the natural gas can circulate, and on the other hand a refrigerant circuit (in particular a compression circuit of vapor) suitable for both cooling and heating the inner surface of the enclosure or space in which the gas flows.

Each heat exchange module 1, 2 is preferably designed so as to limit the liquid or gaseous accumulations that may affect the selectivity of the process. Thus, the heat exchange modules 1, 2 are preferably compact, with a downward slope on the side of the gas to be treated, and with low dead volumes upstream and downstream of their active surface.

Two separate pipes are connected at the outlet of each heat exchange module 1, 2, namely a carbon dioxide depleted natural gas collection line 1 1 and a liquid carbon dioxide collection line 14. An outlet valve natural gas 24, 26 is provided at the outlet of each heat exchange module 1, 2 upstream of the natural gas collection line depleted in carbon dioxide 1 1. A carbon dioxide outlet valve 23, 25 is also provided at the outlet of each heat exchange module 1, 2 upstream of the liquid carbon dioxide collection line 14.

The natural gas collection line depleted in carbon dioxide

1 1, advantageously provided with a heating means 31, opens into a liquid / gas separator 3 of the balloon type. At the bottom thereof is connected a C2 + 13 rich liquid phase collection line, and at the head is connected a gas phase collection line low in C2 + 12.

The C2 + 13 rich liquid phase collection line is advantageously provided with pumping means as well as with heating means 28. It feeds a liquid phase fractionation unit. rich C2 + 4, provided with a reboiler 29. At the bottom thereof is connected a low methane flow collection line 16 and at the head of it is connected a methane-rich flow collection line 15.

The methane-rich stream collection line 15 joins the C2 + 12 lean gas phase collection line (advantageously provided with heating means 27) to form a purified natural gas collection line 17.

The methane-poor stream collection line 16 is advantageously provided with pumping means. It feeds a methane-poor stream fractionation unit 5 equipped with a reboiler 30. At the bottom of the latter is connected a flow collection line rich in C3 + 19 and at its head is connected a line of collection of C3 + poor flux 18.

The installation can be modified in many variants. For example, it is possible to provide three or four heat exchange modules, or more. It is also possible to use heat tracing on cold lines and equipment.

Upstream of the natural gas supply line 10, preliminary treatment units may be provided, in particular one or more units selected from a gas / liquid separation unit, a fractionation unit, a dewatering unit, a unit of deacidification, comprising for example an amine solvent washing unit.

The deacidification unit may also comprise a unit dedicated to a preliminary treatment of carbon dioxide according to any of the techniques known in the state of the art (washing with an amine solvent, cryogenic distillation, separation by membrane, etc.) . This can be useful in the case of a gas with a very high CO _{2 content} . It is preferred, however, that the plant be devoid of a preliminary treatment unit for carbon dioxide.

The purified natural gas collection line 17 may be provided with recompression means and / or feed a complementary treatment unit, for example an absorption unit with amine or refrigerated methanol, if a finishing purification of the gas is necessary. Downstream, this purified natural gas collection line 17 can be connected to recompression means and / or to a transport and / or gas distribution network. If the gas is intended to feed a liquefaction unit natural gas, it must undergo a complementary treatment, for example absorption type amines or refrigerated methanol.

Process

The natural gas that is treated by the process according to the invention is a gaseous mixture (which may contain a minor liquid fraction) comprising at least methane and CO2. Preferably, this gaseous mixture comprises at least 20% methane, and generally at least 30% or at least 40% or at least 50% methane. Preferably, this gaseous mixture comprises at least 10% of CO2, and generally at least 20% of CO2 or at least 25% of CO2 or at least 30% of CO2 or at least 35% of CO2. Preferably, this gaseous mixture contains less than 10% nitrogen, and generally less than 5% nitrogen or less than 2% nitrogen.

Natural gas may undergo one or more preliminary treatments to remove it from its solid contaminants or liquid fraction, dehydrate it and / or reduce its hydrogen sulphide content. It can also undergo a preliminary treatment to reduce its CO2 content according to a technique known from the state of the art (for example by cryogenic distillation process); however, according to a preferred embodiment, it undergoes no treatment specifically aimed at reducing its CO2 content prior to its cooling in the heat exchange modules.

Then, the natural gas is fed into at least one heat exchange module, via the natural gas supply line 10. Preferably, at least two heat exchange modules 1, 2 operate alternately, respectively in heating mode. production of purified natural gas and in liquid CO2 production mode, in order to ensure continuous production of each of these streams.

In the example shown in FIG. 1, the inlet valve 21 before the first heat exchange module 1 is closed and the inlet valve 22 before the second heat exchange module 2 is open; the natural gas outlet valve 24 at the outlet of the first heat exchange module 1 is closed and the natural gas outlet valve 26 at the outlet of the second heat exchange module 2 is open; finally, the carbon dioxide outlet valve 23 at the outlet of the first heat exchange module 1 is open, and the carbon dioxide outlet valve 25 at the outlet of the second heat exchange module 2 is closed. In this way, the second heat exchange module 2 operates in purified natural gas production mode, and the first heat exchange module 1 operates in liquid CO2 production mode.

The second heat exchange module 2, which is in the purified natural gas production mode, functions as a cooling means for the natural gas. This cooling is carried out at a pressure of 5 to 30 bar, and preferably of 10 to 20 bar. The temperature of the natural gas during cooling can reach from -100 to -70 ° C, preferably from -95 to -80 ° C, and more preferably about -90 ° C. The cooling is adjusted so that a portion of the CO2 contained in the natural gas passes to the solid state, until the desired specification of CO2 in the treated gas is reached.

The solid CO2 is deposited on the internal walls of the heat exchange module, which makes it possible to separate it from the natural gas stream, which is recovered in the natural gas collection line depleted in carbon dioxide 1 1.

The first heat exchange module 1, which is in liquid CO2 production mode, functions as a solid CO2 heating means. The CO2 is heated to a typical temperature of between -55 and -40 ° C, and more particularly about -50 ° C, at a pressure close to the gas / liquid equilibrium of pure CO2. At this stage, the solid CO2 deposited melts, and a stream of liquid CO2 is recovered in the liquid carbon dioxide collection line 14.

When the surface (inner wall) of the second heat exchange module 2 is saturated with solid CO2, the situation is reversed, that is to say that the first heat exchange module 1 goes into natural gas production mode purified and the second heat exchange module 2 goes into liquid CO2 production mode, and this by actuating the inlet and outlet valves 21, 22, 23, 24, 25, 26.

The flow of liquid CO2 produced can be pumped and injected into an underground formation, or be used or upgraded in another way.

The stream of purified natural gas, transported in the collection line of natural gas depleted in carbon dioxide 1 1, can optionally be reheated to guarantee a margin for solid CO2 deposits downstream of the heat exchange modules 1, 2. , this stream of purified gas comprises a gaseous phase and a liquid phase, in particular because of the condensation of a portion of the hydrocarbons during the cooling step.

If it is desired to valorize heavy hydrocarbons of natural gas separately from methane, it is then possible to take advantage of this phenomenon by separating the gaseous phase and the liquid phase by means of the liquid / gas separator 3, as shown in FIG. 1. The gas phase is richer in methane than the liquid phase, while the liquid phase is richer in CO2 and C2 + type hydrocarbons (with at least 2 carbon atoms) than the gas phase.

The gas phase is recovered in the C2 + 12 low gas phase collection line. It can be reheated to avoid solid CO2 deposition and can be the final purified natural gas.

The liquid phase is recovered in the C2 + 13 rich liquid phase collection line. This liquid phase may be recovered as such or fractionated before recovery, as shown in FIG.

Thus, in the illustrated embodiment, this liquid phase is pumped, optionally reheated in order to avoid any solid CO2 deposition, then sent to the C2 + 4 rich liquid phase fractionation unit. This fractionation unit operates a separation. between a head flow and a foot flow, the head flow being richer in methane than the foot flow, and the foot flow being richer in CO2 and C2 + hydrocarbons than the flow of the head.

The head stream is recovered in the methane-rich stream collection line 15. This stream can be mixed with the gas phase flowing in the C2 + 12 low gas phase collection line to provide the purified natural gas recovered from the purified natural gas collection line 17. Preferably, the purified natural gas comprises less than 10%, or less than 8%, or less than 6%, or less than 5%, or less than 4%, or less than 3%, or less than 2.5% CO2.

This purified natural gas can optionally be recompressed and / or undergo a finishing treatment of CO2 removal (especially if it is intended to be liquefied). It can be later liquefied or transported and distributed as such to end-users.

The foot flow is recovered in the methane-poor stream collection line 16. It is advantageously pumped and then sent to the methane-poor flux fractionator unit 5. This fractionation unit separates a head flow and a flow of foot, the flow of head being richer in CO2 and ethane than footflows, and the footflowing being richer in C3 + hydrocarbons (having at least 3 carbon atoms) than the overhead flow. Both streams are virtually free of methane.

These two streams are respectively recovered in the C3 + lean stream collection line (head stream) and in the C3 + rich stream collection line (foot stream). The overhead stream, although rich in CO2 can be used as a fuel gas or fuel gas supplement, for example locally for an energy requirement of the treatment site. The foot stream, substantially free of CO2, can be fractionated later, for example to obtain more individualized hydrocarbon fractions of C2, C3 / C4, C5 + type. These splits are not detailed here.

The system of heat exchange modules operating alternately used in the present invention can be adapted from the system described in FR 2820052. This document relates to the treatment of combustion fumes containing a relatively low CO2 content (lower at 20% or even 15%) and a high nitrogen content (more than 70%). The pressure at which the CO2 goes to the solid state is necessarily the atmospheric pressure according to this document. Correlatively, the melting of solid CO2 requires increasing the pressure. A pressure change of this order is avoided within the scope of the invention.

variants

A first possible variant consists in grouping the gas / liquid separation function (liquid / gas separator 3) and the first stabilization of the condensate (C2 + 4 rich liquid phase fractionation unit).

To do this, and with reference to FIG. 2, the carbon dioxide depleted natural gas collection line 11 feeds directly a fractionation unit 40 equipped with a reboiler 41. At the bottom thereof is connected a methane-poor stream collection line 42 analogous to the methane-poor stream collection line 16 described above (it optionally feeds the methane-lean stream fractionator 5 as described above), and at the head of the latter is directly connected a purified natural gas collection line 43 similar to the collection line purified natural gas 17 described above, advantageously provided with heating means 44.

Another possible variant consists in providing a complementary treatment with refrigerated methanol for the purified natural gas, capable of providing a thorough specification, for example that required for the liquefaction of the gas (50 ppm of CO ₂ ). The treatment makes it possible to obtain a highly purified gas.

More specifically, with reference to FIG. 3, the purified natural gas collection line 17 (or 43) supplies a contactor 50 equipped with a mass transfer device, such as trays or packing. A methanol-based solvent feeds the upper part of the contactor 50 via a solvent supply line 53. The solvent is mainly charged with CO ₂ in the contactor 50. In this way, a highly purified gas is recovered at the top of the contactor 50. in a highly purified gas collection line 57, and at the bottom of the contactor 50 a methanol solvent charged with CO ₂ in a charged solvent collection line 54.

The regeneration of the solvent is carried out by means of a separation device 51 fed by the loaded solvent collection line 54. The separation device 51 uses a pressure drop and a temperature increase (or a combination of both). At the outlet of this device, the regenerated solvent is recovered on the one hand in a regenerated solvent collection line 56, and on the other hand a rich flow of CO ₂ in a CO _2- rich stream withdrawal line 55.

The heating means and / or pressure drop for the regeneration of the solvent are not shown. The regenerated solvent is cooled in an exchanger 52, for example by heat exchange with the highly purified gas: it thus feeds the solvent supply line 53.

The highly purified gas, after heat exchange with exchanger 52, is recovered in a highly purified gas collection line at increased temperature 58.

Another possible variant consists in grouping the gas / liquid separation functions (liquid / gas separator 3), condensate stabilization (liquid phase fractionation unit rich in C2 + 4) and refrigerated methanol solvent wash (contactor 50).

Referring to Figure 4, a single column 60 groups these three functions. To do this, the column 60 comprises a lower section 61 and an upper section 62, separated by an intermediate plate for recovering the liquid from the upper section, provided with chimneys passing gas from the lower section 61 to the upper section 62.

The natural gas is fed through the natural gas collection line depleted in carbon dioxide 11 and enters the lower section 61 of the column 60, under the intermediate plateau. In this zone, the gas / liquid gravity separation takes place. The lower section 61 of the column 60 stabilizes the separated liquid, by means of a mass transfer device and a reboiler 63, in a manner similar to the stabilization performed in the liquid phase fractionating unit 63. C2 + 4 of Figure 1. At the outlet of the lower section 61 is thus connected a methane-poor stream collection line 64 similar to the methane-poor stream collection line 16 described above.

The upper section 62 of the column 60 treats the purified gas from the lower section 61 by a complementary treatment by absorption in contact with a solvent based on methanol, similar to that described with reference to FIG. The methanol base injected to the top of the top section 62 is primarily CO 2 fired, and produces a CO2-loaded methanol solvent which is recovered on the middle plateau of the column 60 in the charged solvent collection line 54.

A highly purified gas exits the upper portion of the upper section 62 and is recovered in a highly purified gas collection line 65, analogous to the highly purified gas collection line 57 described above.

The regeneration of the CO2-based methanol solvent is unchanged from FIG. 3.

EXAMPLE

The following example illustrates the invention without limiting it.

A numerical simulation has been carried out to characterize the operation of an installation corresponding to FIG. 1. Tables 1a, 1b, 2a and 2b below give the composition of the starting natural gas as well as the flow rates obtained and the composition the flow obtained in each line of the installation. Line of installation 10 11 12 13 14

Liquid (L) or gaseous state G + L G + L G L L (G)

Temperature (° C) -55.0 -92.6 -92.6 -92.6 -50.0

Pressure (bar) 15.0 15.0 15.0 15.0 15.0

Molecular Weight 25.98 18.70 17.12 34.44 44.01

Flow (tonnes / h) 457.86 234.73 195.26 39.47 223.13

Composition (% in

mass)

N ₂ 1, 08 2.10 2.51 0.08 0.00

CO ₂ 52.51 7.36 6.16 13.31 100.00

Methane 39.39 76.84 88.71 18.14 0.00

Ethane 2.08 4.06 2.18 13.40 0.00

Propane 2.38 4.63 0.41 25.53 0.00 l-butane 0.67 1, 31 0.02 7.69 0.00

Butane 0.67 1, 31 0.01 7.73 0.00

C5 + 1, 22 2.38 0.00 14.13 0.00

Table 1a - general data and mass data

Table 1b - General data and mass data (continued) Line of 10 11 12 13 14 the installation

Flow (x 10 ³ mol / h) 17622 12552 11406 1146 5070

Composition (% in

moles)

N ₂ 1.00 1.40 1.54 0.09 0.00

CO ₂ 31.00 3.13 2.40 10.41 100.00

Methane 63.80 89.57 94.66 38.93 0.00

Ethane 1.80 2.53 1.24 15.34 0.00

Propane 1.40 1.97 0.16 19.94 0.00 l-butane 0.30 0.42 0.01 4.56 0.00

Butane 0.30 0.42 0.00 4.58 0.00

C5 + 0.40 0.56 0.00 6.15 0.00

Table 2a - molar data

Table 2b - molar data (continued)

Claims

A method of treating a natural gas containing carbon dioxide, comprising the steps of:

deposition of the solid carbon dioxide in the heat exchange module;

melting of the solid carbon dioxide deposited in the heat exchange module by heating; and

recovering a stream of liquid carbon dioxide at the outlet of the heat exchange module.

The method of claim 1, comprising a step of:

The method of claim 2 comprising a step of:

the methane-rich stream being preferably mixed with the C2 + -low gas phase.

The process of claim 3 comprising a step of:

5. Method according to one of claims 1 to 4, wherein the pressure during cooling is 10 to 20 bar. Process according to one of Claims 1 to 5, in which at least two heat exchange modules are used alternately, the solid state transition of the carbon dioxide and the deposition of the solid carbon dioxide in a first module. heat exchange being carried out simultaneously with the melting of the carbon dioxide in a second heat exchange module.

Process according to one of Claims 1 to 6, in which the carbon dioxide-containing natural gas undergoes deacidification and / or dehydration and / or fractionation prior to the solid state transition of the carbon dioxide by cooling.

Process according to one of Claims 1 to 7, in which the methane-rich stream is further treated for the removal of carbon dioxide by washing with a methanol-based solvent.

Plant for treating a natural gas containing carbon dioxide, comprising:

- At least one heat exchange module (1, 2) adapted to cool and heat a gas stream;

a natural gas supply line (10) supplying the heat exchange module (1, 2);

a depleted carbon dioxide (1 1) natural gas collection line at the outlet of the heat exchange module (1, 2);

a liquid carbon dioxide collection line (14) at the outlet of the heat exchange module (1, 2);

liquid / gas separation means (3, 40, 60) supplied by the carbon dioxide depleted natural gas collection line (1 1);

- a liquid phase collection line (13, 42, 64) and a gas phase collection line (12, 43, 65) at the outlet of the liquid / gas separation means (3, 40, 60). Plant according to claim 9, wherein the liquid / gas separation means consist of a liquid / gas separator (3), the liquid phase collection line is a C2 + rich liquid phase collection line (13), the line The gas phase collection system is a C2 + lean gas phase collection line (12), the plant preferably further comprising:

- a C2 + rich liquid phase fractionation unit (4) fed by the C2 + rich liquid phase collection line (13);

a methane rich stream collection line (15) at the outlet of the C2 + rich liquid flow fractionating unit (4);

a methane-poor stream collection line (16) at the outlet of the fractionation unit of the C2 + rich liquid phase (4);

the methane-rich stream collecting line (15) being preferably connected to the C2 + lean gas phase collection line (12) to form a purified natural gas collection line (17).

Plant according to claim 9, wherein the liquid / gas separation means consist of a fractionation unit (40), the liquid phase collection line is a methane-poor stream collection line (42) and the collection line gas phase is a purified natural gas collection line (43).

An installation according to claim 10 or 11, further comprising:

- a contactor (50) supplied on the one hand by the purified natural gas collection line (17, 43) and on the other hand by a solvent supply line (53);

- a highly purified gas collection line (57) and a charged solvent collection line (54) at the output of the contactor (50); and - solvent regeneration means (51, 52) connected at the inlet to the charged solvent collection line (54) and at the outlet to the solvent supply line (53).

Plant according to claim 9, comprising a column (60), the liquid / gas separation means being arranged in the column (60), the liquid phase collection line being a methane-poor stream collection line (64), the gas phase collection line being a highly purified gas collection line (65), the installation further comprising:

a solvent supply line (53) supplying the column (60);

- a loaded solvent collection line (54) at the outlet of the column (60); and

- solvent regeneration means (51, 52) connected at the inlet to the charged solvent collection line (54) and at the outlet to the solvent supply line (53).

Installation according to one of claims 10 to 13, comprising:

a methane-poor stream fractionation unit (5) fed by the methane-poor stream collection line (16, 42, 64);

a C3 + rich stream collection line (19) at the outlet of the methane-poor flux fractionation unit (5);

a C3 + poor flux collection line (18) at the outlet of the methane-poor flux fractionation unit (5).

Installation according to one of claims 9 to 14, comprising at least two heat exchange modules (1, 2), the natural gas supply line (10) being connected to an inlet of each heat exchange module ( 1, 2), the depleted carbon dioxide gas collection line (1 1) being connected to an outlet of each heat exchange module (1, 2) and the liquid carbon dioxide collection line (14). ) being connected to an output of each heat exchange module (1, 2), the installation also comprising a a plurality of valves (21, 22, 23, 24, 25, 26) adapted so that the heat exchange modules (1, 2), alternatively:

- are fed by the natural gas supply line (10) and supply the depleted carbon dioxide (1 1) natural gas collection line; and

- feed the liquid carbon dioxide collection line (14). 16. Installation according to one of claims 9 to 15, comprising a deacidification unit and / or a dewatering unit and / or a fractionation unit upstream of the heat exchange module (1, 2).

17. Method according to one of claims 1 to 8, implemented in an installation according to one of claims 9 to 16.