MX2008006371A - Process for the recovery of carbon dioxide - Google Patents

Abstract

A process for the recovery of carbon dioxide, comprising:(a) an absorption step of bringing a carbon dioxide-containing gaseous feed stream into gas-liquid contact with an absorbing fluid, whereby at least a portion of the carbon dioxide present in the gaseous stream is absorbed into the absorbing fluid to produce (i) a refined gaseous stream having a reduced carbon dioxide content and (ii) an carbon dioxide-rich absorbing fluid, (b) a regeneration step of treating the carbon dioxide-rich absorbing fluid at a pressure of greater than 3 bar (absolute pressure) so as to liberate carbon dioxide and regenerate a carbon dioxide-lean absorbing fluid which is recycled for use in the absorption step, wherein the absorbing fluid is an aqueous amine solution containing a tertiary aliphatic alkanol amine and an effective amount of a carbon dioxide absorption promoter, the tertiary aliphatic alkanol amine showing little decomposition under specified conditions of temperature and pressure under co-existence with carbon dioxide. The tertiary aliphatic alkanol amine does not contain, in its molecular structure, a nitrogen atom that is substitutedby more than one 2-hydroxyethyl moieties, or a nitrogen atom that is substituted by both a 2-hydroxyethyl moiety and a methyl moiety. Preferred tertiary aliphatic alkanol amines are selected from the group consisting of N-ethyl-diethanolamine, 2-(dimethylamino)-ethanol, 2-(diethylamino)-ethanol, 3-(dimethylamino)-1-propanol, 3-(diethylamino)-1-propanol, 1-(dimethylamino)-2-propanol, and 2-(diisopropylamino)-ethanol.

Description

PROCESS FOR THE RECOVERY OF CARBON DIOXIDE The present invention relates to a process for the recovery of carbon dioxide from gaseous streams containing carbon dioxide, in which the carbon dioxide is recovered at a pressure above atmospheric pressure.

Natural gas produced in a gas field usually contains an appreciable amount of carbon dioxide. In order to reduce the cost of transporting said natural gas from its point of production to a site of distant consumption, and to adjust its calorific value to the normal value at the point of consumption, some carbon dioxide is previously removed from it for produce refined natural gas with a reduced content of carbon dioxide.

On an industrial scale, aqueous solutions of organic bases, for example alkanol amines, are frequently used as absorbent fluids to remove carbon dioxide from gas streams. When the carbon dioxide dissolves, ionic products are formed from the base and carbon dioxide. The absorbent fluid can be regenerated by expansion at a lower pressure, or by stripping, the ionic products reacting again to release the carbon dioxide and / or the carbon dioxide being entrained by steam. The absorbent fluid can be reused after the regeneration process. Common alkanolemines used in the removal of acid gas impurities from hydrocarbon gas streams comprise monoethanolamine (MEA), di-ethanolamine (DEA), triethanolamine (TEA), diethylethanolamine (DEEA), diisopropylamine (DIPA), aminoeyethanol ( ESA) and methyldiethanolamine (MDEA).

Conventionally, carbon dioxide separated from natural gas at the point of production is seldom used. That is, said carbon dioxide has been de-charged directly into the atmosphere or has rarely been used as an injection gas for the tertiary recovery of oil in an oil field. Therefore, little consideration has been given to the pressure of the carbon dioxide separated by the aforementioned refining process.

In recent years, global warming due to an increase in atmospheric carbon dioxide has come to be considered a problem. Accordingly, the current situation is such that the carbon dioxide separated in the manner described above has to be pressurized in order to inject it into an underground aquifer for the purpose of permanent storage or to use it positively for the purpose of improved oil recovery.

However, despite the fact that natural gas is treated at high pressure, the carbon dioxide separated from natural gas by a conventionally used process has a low pressure close to atmospheric pressure. This is disadvantageous in that for the above-described purpose of permanent storage or improved oil recovery, carbon dioxide has to be pressurized from a low pressure to a pressure of approximately 150 bar (absolute pressure) which is necessary for the invention.

The prior art describes several processes in which carbon dioxide is recovered at a pressure greater than atmospheric pressure. An advantage of the implementation of the regeneration step at higher than atmospheric pressure is that the compression steps at low pressure can be eliminated.

Thus, EP-A 768 365 discloses a process for the removal of highly concentrated carbon dioxide from natural gas at high pressure comprising an absorption step consisting of putting natural gas having a pressure of 30 kg. / cm2 (30 bar absolute pressure) or greater in gas-liquid contact with an absorbent fluid; and a regeneration step that consists in heating the absorbent fluid rich in carbon dioxide wit depressurizing it, whereby carbon dioxide is released at high pressure. Specific examples of the absorbent fluid mentioned in this reference are an aqueous solution of N-methyldiethanolamine (MDEA), an aqueous solution of triethanolamine and an aqueous solution of potassium carbonate. It is stated that these solutions may have a C02 absorption promoter (e.g., piperazine) added to them.

US 6,497,852 discloses a carbon dioxide recovery process by preferential absorption of carbon dioxide from a feed stream in a liquid absorbent fluid, pressurization of the resulting stream at a pressure sufficient to allow the stream to reach the head of a volatile material separator at a pressure of 35 psia (2.4 bar absolute pressure) or higher, and dragging the carbon dioxide from the stream in the volatile separator at a pressure of 35 psia (2, 4 bar absolute pressure) or higher. The absorbent fluid is preferably an aqueous alkanolamine solution. Specific examples mentioned are monoethanolamine, diethanolamine and N-methyl diethanolamine.

WO 2004/082809 discloses a process for the removal of an acid gas from a feed gas stream comprising a regeneration step consisting of heating the absorbent fluid rich in acid gas to a pressure greater than atmospheric pressure. The absorbent fluid comprises an aqueous solution of tertiary alkylamines selected from diamines, triamines and tetramines such as tetramethylethylenediamine, tetraethylethylenediamine, tetramethyl-1,3-propanediamine, tetraethyl-1,3-propanediamine, tetramethyl-1,3-butanediamine, tetramethyl-1. , 4-butanediamine, tetraethyl-1,3-butanediamine, tetraethyl-1,4-butanediamine, pentamethyldiethylenetriamine, pentaethyldiethylenetriamine, pentamethyl-dipropylenetriamine and pentamethyl- (2-aminoethyl) -1, 3-propanetriamine or hexamethyltriethylenetrotemine and hexaethyltrietylenetetramine. It is claimed that these amines have a high stability at the heating temperature of the regeneration step and have a high charge of acid gas. However, these amines show deficient carbon dioxide transfer rates.

WO 2005/009592 relates to an acid gas regeneration process which is conducted at a pressure exceeding 50 psia (3.5 bar absolute pressure) and does not exceed 300 psia (20 bar absolute pressure). The separate gaseous stream emerging from the regenerator is compressed and injected into an underground reservoir. The absorbent fluid as illustrated in the practical examples is made up of 43% by weight of N-methyldiethanolamine (MDEA) and 57% by weight of water.

WO 03/076049 describes a washing liquid for deacidifying gas streams, which contains 3-dimethylamino-1-propanol and a secondary amine as activator. The reference suggests that the absorbent liquid is regenerated by sudden vaporization at a pressure of 1 to 2 bar (absolute pressure) or by dragging the volatile materials at a pressure of 1 to 3 bar (absolute pressure).

The higher the pressure at which the carbon dioxide is recovered in the regeneration step, the greater the temperature at which the carbon dioxide-rich absorbent fluid has to be heated to liberate the carbon dioxide and regenerate the absorbent fluid. . The elevated temperatures impose thermal stress on the absorbent fluid. It has been found that, with the known processes employing aqueous solutions of alkanol amines, the absorption capacity of the absorbent fluid deteriorates in the long term and does not recover completely in the regeneration. It is likely that the amines present in the absorbent fluid gradually undergo thermal decomposition.

It is an underlying object of the invention to specify an absorbent fluid and a process for deacidifying gaseous streams, maintaining the absorptive capacity of the absorbent fluid in the long term.

In a first aspect, the invention provides a process for the recovery of carbon dioxide, comprising: a) an absorption step consisting of placing a gas stream containing carbon dioxide in gas-liquid contact with an absorbent fluid , whereby at least a portion of the carbon dioxide present in the gas stream is absorbed in the absorbent fluid to produce (i) a stream of refined gas having a reduced content of carbon dioxide and (ii) a rich absorbent fluid. in carbon dioxide, b) a regeneration step consisting of treating the absorbent fluid rich in carbon dioxide at a pressure greater than 3 bar (absolute pressure) in order to liberate carbon dioxide and regenerate an absorbent fluid poor in carbon dioxide that is recycled to Use in the absorption step, wherein the absorbent fluid is an aqueous amine solution containing a tertiary aliphatic alkanol amine and an effective amount of a carbon dioxide absorption promoter, showing the tertiary aliphatic alkanol-amine less than 5% decomposition when an aqueous solution of the tertiary aliphatic alkanol-amine at a concentration of 4 mol / 1 is maintained for 300 hours in a vapor-liquid equilibrium condition at a temperature of 162 ° C and a total pressure of 6.3 bar (absolute pressure) with co-existence of the aqueous solution and carbon dioxide.

The authors of the present invention devised an assay to evaluate the suitability of a tertiary aliphatic alkanol-amine in the process of the invention. This test involves maintaining an aqueous solution of the aliphatic tertiary alkanolamine (without the addition of a carbon dioxide uptake promoter) at a concentration of 4 mol / l of the aliphatic tertiary alkanol-amine for 300 hours in a condition of vapor-liquid equilibrium at a temperature of 162 ° C and a total pressure of 6.3 bar (absolute pressure) with co-existence of the aqueous solution and carbon dioxide. Samples of the aqueous solution are taken at the beginning and after the period of 300 hours and analyzed, e.g., by gas chromatography. The amount of unchanged amine is calculated from the detected signals. Suitable amines show a decomposition less than 5% based on the weight of the original amine, preferably less than 3% by weight. The test is described in greater detail in the practical examples that follow.

The stability test according to the invention is conducted in the presence of high concentration carbon dioxide. Surprisingly, the thermal stability of the alkanol amines as determined under these conditions can be significantly different from the thermal stability in the absence of carbon dioxide. Therefore, the test according to the invention allows a valid evaluation of the stability of the alkanol amines under the conditions found in the regeneration step of the medium to high pressure regeneration of absorbent fluids rich in carbon dioxide.

In a second aspect, the invention provides a process for the recovery of carbon dioxide, comprising: a) an absorption step consisting in placing a gas stream containing carbon dioxide in gas-liquid contact with an absorbent fluid, whereby at least a portion of the carbon dioxide present in the gas stream is absorbed into the gas stream. absorbent fluid to produce (i) a refined gas stream having a reduced content of carbon dioxide and (i) an absorbent fluid rich in carbon dioxide, b) a regeneration step consisting of treating the absorbent fluid rich in carbon dioxide at a pressure greater than 3 bar (absolute pressure) in order to liberate carbon dioxide and regenerate an absorbent fluid poor in carbon dioxide that is recycled to use in the absorption step, wherein the absorbent fluid is an aqueous amine solution containing a tertiary aliphatic alkanol amine other than N-methyldiethanolamine, and an effective amount of a carbon dioxide absorption promoter.

The absorbent fluid contains a tertiary aliphatic alkanol amine other than N-methyldiethanolamine. It has been shown that tertiary alkanol amines are superior to primary and secondary alkanol amines in terms of stability under conditions of high temperature and high carbon dioxide pressure.

The tertiary amine used in the present invention is an alkanolamine, that is to say it comprises in its molecular structure a nitrogen atom which is substituted with at least one hydroxyalkyl radical. In general, the tertiary aliphatic alkanolamine comprises 4 to 12 carbon atoms. Preferably, the tertiary aliphatic alkanol amine comprises a single nitrogen atom in its molecular structure, ie the tertiary aliphatic alkanol amine is preferably a monoamine.

Typically, the hydroxyalkyl moiety has 2 to 4 carbon atoms, preferably 2 or 3 carbon atoms. Preferably, the hydroxyalkyl moiety is selected from the group consisting of 2-hydroxyethyl, 3-hydroxypropyl, 2-hydroxypropyl and 2-hydroxybutyl. The substituent (s) on the nitrogen atom (s) other than the hydroxyalkyl radical are preferably alkyl radicals, preferably 1 to 3 carbon atoms, such as methyl, ethyl, propyl or isopropyl.

It has been found that a tertiary aliphatic alkanol amine which contains in its molecular structure a nitrogen atom which is substituted with more than one 2-hydroxyethyl moiety, or a nitrogen atom which is substituted at the same time with a 2-hydroxyethyl moiety and a methyl residue, such as, eg N-methyldiethanolamine (MDEA), has a limited thermal stability in the process defined above.

In preferred embodiments, the aliphatic tertiary alkanolamine does not therefore contain in its molecular structure a nitrogen atom which is substituted with more than one 2-hydroxyethyl moiety.

In other preferred embodiments, the tertiary aliphatic alkanol amine does not contain in its molecular structure a nitrogen atom which is substituted at the same time with a 2-hydroxyethyl moiety and a methyl moiety.

In preferred embodiments, the aliphatic tertiary alkanolamine contains in its molecular structure a single nitrogen atom, the nitrogen atom being substituted with a hydroxyalkyl moiety and two alkyl moieties.

Specific examples of useful tertiary aliphatic alkanol amines include N-ethyldiethanolamine (2- [ethyl- (2-hydroxyethyl) -amino] -ethanol, EDEA), 2- (dimethylamino) -ethanol (α, β-dimethylaminoethanol, DMEA), 2- (diethylamino) -ethanol (N, N-diethylethanolamine, DEEA), 3- (dimethylamino) -1-propanol (DMAP), 3- (diethylamino) -1-propanol, 1- (di-methylamino) -2- propanol (?,? - dimethyl isopropanolamine), and 2- (diisopropylamino) -ethanol (N, N-diisopropyl-ethanol-amine).

A particularly preferred tertiary aliphatic alkanol amine for use in the present invention is 3- (dimethylamino) -1-propanol (DMAP). Another preferred tertiary aliphatic alkanol amine is 2- (diethylamino) -ethanol (α, α-diethylethanolamine, DEEA).

It is useful to designate the carbon carrying the nitrogen as a and the carbon atom (s) adjacent to it as β. It is assumed that, once the nitrogen atom has been protonated or quaternized, the replacement or replacement of the nitrogen in carbon a by a nucleophilic reagent is involved in degradation reactions. The nature of the substituents, if any, in the β-carbons influences the ease of such nucleophilic substitution. It is considered that a hydroxy group on the β-carbon, such as that which is present in the 2-hydroxyethyl moiety, mediates said nucleophilic attack. The inventors further believe that the number of hydroxyethyl moieties attached to a nitrogen atom is related to the stability of the alkanol amines. Compounds that incorporate a nitrogen atom or atoms that are substituted with more than one 2-hydroxyethyl moiety are more prone to decomposition. Conversely, if the β-carbon carries, in addition to the hydroxy group, an alkyl group, such as, e.g., in the 2-hydroxypropyl moiety, the β-effect of the hydroxy group is reduced by the steric effect of the alkyl group. Accordingly, a 2-hydroxypropyl moiety or a 2-hydroxybutyl moiety is a preferred hydroxyalkyl moiety in the context of this invention.

The mediating effect of the 2-hydroxyethyl moiety is also, at least in part, attributable to the ease with which an intermediate 3-membered cyclic structure is formed. Since 4-membered cyclic structures are formed less easily, a 3-hydroxypropyl moiety lacks significant mediating effect. Thus, a 3-hydroxypropyl moiety is a preferred hydroxyalkyl moiety in the context of this invention. Actually, it has been shown by experiments that when 3- (dimethylamino) -1-propanol is exposed to sufficient thermal fatigue, the 3-hydroxypropyl moiety is released as allyl alcohol instead of as a 4-membered cyclic structure.

Without intending to be bound by theory, it is believed that the initial steps of MDEA degradation can be illustrated by the following reaction scheme: + MDEA H3C CH., H HO 'OH HO ^ OH O + CO, H20 Thus, an MDEA molecule is initially protonated by an acid proton of carbonic acid that is formed from carbon dioxide and water. The protonated MDEA molecule can then transfer one of its methyl residues to another MDEA molecule to produce a di (2-hydroxyethyl) -dimethylammonium cation and a diethanolamine molecule. The cation thus formed can remove an ethylene oxide molecule (which can in turn react with a molecule of water to give an ethylene glycol molecule). Diethanolamine can react with carbon dioxide to form the 5-membered cyclic structure of 3- (2-hydroxyethyl) -1,3-oxazolid-2-one, which undergoes further decomposition.

The absorbent fluid used in the process of the invention also contains an effective amount of a carbon dioxide absorption promoter. The absorption promoter is usually selected from primary or secondary amines, preferably secondary amines, and serves to accelerate the absorption of carbon dioxide by the intermediate formation of a carbamate structure. Specific examples of useful absorption promoters include piperazine, 2-methyl-piperazine, N-methylpiperazine, N-ethylpiperazine, N-hydroxyethyl-piperazine, N- (2-aminoethyl) -piperazine, homopiperazine, piperidine and morpholine.

In general, the absorbent fluid contains 20 to 60% by weight, preferably 25 to 55% by weight, of the tertiary aliphatic alkanolamine.

In general, the solvent fluid contains 0.5 to 20% by weight, preferably 1 to 15% by weight, of the carbon dioxide absorption promoter.

The gaseous feed stream can be a gaseous feed stream at high pressure, for example, a stream of natural gas or gas stream or high pressure synthesis. Alternatively, the feed gas stream may be a gaseous feed stream at low pressure, for example a flue gas stream from a thermal power plant or a refinery. A gaseous stream of high-pressure supply means a gas stream having a pressure of 10 bar (absolute pressure) or higher, for example from 10 bar to 150 bar. A gaseous stream of low pressure feed means a gas stream having a pressure equal to or close to atmospheric pressure.

In addition to carbon dioxide, the gaseous stream may comprise other acid gases, in particular hydrogen sulphide (H2S), and / or S02, CS2, HCN, COS, disulfides or mercaptans. If present, these acid components are removed, at least in part, from the gas stream together with the carbon dioxide when the gas stream is subjected to treatment by the process according to the invention. Preferably, the amount of carbon dioxide in the gaseous feed stream at high pressure is in the range of 1 to 95% by volume. Typically, a stream of unrefined high pressure natural gas comprises 1 to 40 vol.% Carbon dioxide, preferably 5 to 25 vol.%, By volume. 3 0 example, 10 to 15% by volume. Conveniently, the amount of hydrogen sulfide in the unrefined high pressure natural gas stream is at least 0.02% by volume (at least 200 ppmv). In cases where the refined high-pressure natural gas stream has to be transported through a pipeline, for example to a thermal power plant or to a domestic gas distribution system, it is desired to reduce the amount of dioxide carbon in the refined high pressure natural gas stream at a level below 3% by volume, preferably less than 2% by volume. Preferably, the amount of carbon dioxide in the refined high pressure natural gas stream can be further reduced using the process of the present invention. For example, in the case where the stream of refined high-pressure natural gas must be used as a feed stream for a cryogenic processing unit for generation of liquefied natural gas (LNG), it is desired to reduce the amount of carbon dioxide in the stream of refined high-pressure natural gas at a level below 100 ppmv, preferably at a level of 50 ppmv or less. In both cases, it is preferred to reduce the amount of hydrogen sulphide in the refined high pressure natural gas stream to a level of less than 10 ppmv, more preferably 4 ppmv or less.

The amount of carbon dioxide in the gaseous feed stream at low pressure is at least 1.5% by volume. Preferably, the amount of carbon dioxide in the refined low pressure gas stream is reduced to a value less than 100 ppmv, more preferably 50 ppmv or less.

In the absorption step, the gaseous feed stream containing carbon dioxide is brought into gas-liquid contact with the absorbent fluid. To this end, any suitable absorber can be used. The absorber may contain contact means such as trays, infill beds or other contact devices that provide intimate contact between the gas stream and the absorbent liquid. The gaseous stream can be introduced into the lower section of the absorber and rise towards the head of the absorber. The absorbent fluid can be introduced into the upper portion of the absorber and down to the bottom of the absorber in countercurrent with the gas stream.

In the absorption step, the temperature of the absorbent fluid should not exceed 100 ° C, since at higher temperatures the charge of carbon dioxide is lower and, in general, high temperatures cause a corrosion undesirable. The absorption step is carried out, as a general rule, at a temperature at the head of the absorber from 60 ° to 80 ° C, although the temperature can be as high as 95 ° C. The absorption step can also be carried out at lower temperatures, e.g. from 40 ° C up. However, said low temperatures result in increased energy consumption, particularly if the regeneration is carried out by entrainment of volatile materials. The bottom temperature of the absorber should not be higher than 100 ° C.

In the regeneration step, the absorbent fluid rich in carbon dioxide is treated in order to release carbon dioxide (and other acid gases), if present in the feed gas stream) and regenerate an absorbent fluid poor in carbon dioxide. which is recycled for use in the absorption step. The regeneration step requires heating of the absorbent fluid rich in carbon dioxide, typically at a temperature above 130 ° C, preferably above 150 ° C.

When the absorption step is conducted at an elevated pressure (typically in the treatment of a high pressure feed gas stream), the regeneration step typically comprises expansion or flash vaporization of the carbon dioxide-rich absorbent fluid from the elevated pressure that prevails in the absorber at a lower pressure. The expansion of the pressure can be carried out, for example, by using a throttle valve. Additionally or alternatively, the absorbent fluid can be passed through an expansion turbine that can drive a generator and produce electrical energy. In this step of flash vaporization, inert gases are preferably released, such as the absorbed components of the feed gas stream.

When the absorption step is conducted at a low pressure (typically in the treatment of a low pressure feed gas stream), the absorbent fluid rich in carbon dioxide has to be pressurized at least up to the pressure used in the regeneration step, before the absorbent fluid rich in carbon dioxide enters the regeneration step.

Preferably, in the regeneration step it comprises stripping the absorbent liquid of volatile materials with an inert fluid. For this purpose, the absorbent liquid and a volatile entrainment medium (a hot inert gas, preferring nitrogen or water vapor) are passed in countercurrent mode through a desorption column provided with compacted fillers, neat fillers or dishes.

Before being fed to the absorber, the absorbent fluid poor in carbon dioxide is usually passed through a heat exchanger and brought to the temperature required for the absorption step. The heat removed from the regenerated absorbent fluid exiting the volatile entrainment column can be used to preheat the absorbent fluid rich in carbon dioxide leaving the absorption step.

According to the invention, the regeneration step is conducted at a pressure greater than 3 bar, preferably greater than 3 bar at 10 bar, e.g. from 3.5 bar to 10 bar.

Preferably, the stream comprising the carbon dioxide that is released during the regeneration step is discharged to an underground zone for the purpose of storage. For example, the stream comprising the carbon dioxide can be injected into an underground hydrocarbon carrier formation, in particular, an underground petroleum-bearing formation for storage and / or improved oil recovery. The gaseous stream released will require pressurization at a pressure that is high enough to allow injection into the underground zone. An advantage of the implementation of the regeneration step at higher than atmospheric pressure is that the compression steps at low pressure can be eliminated. For example, carrying out the regeneration step at a pressure of 5 bar (absolute pressure) allows at least one compression step to be removed, while carrying out the regeneration step at a pressure of 9 bar has the potential to eliminate up to 2 steps. compression steps when compared to the release of acid gas at atmospheric pressure.

The invention will now be described in greater detail on the basis of the figures or figures attached and the examples given below.

FIG. 1 shows a preferred arrangement for carrying out the inventive process; Y FIG. 2 shows the arrangement used for evaluating the thermal stability of aqueous alkanolamine solutions in the presence of carbon dioxide.

With reference to FIG. 1, the gaseous feed stream, which is, for example, natural gas at a pressure of about 50 bar and comprises gases 2 0 acids such as H2S, CO2 and COS, is passed through a feed line 1 to an absorption column 2. The absorption column 2 ensures the intimate contact of the feed gas stream with an absorbent fluid. The absorbent fluid is introduced through the feed pipe 3 into the head region of the absorption column 2 and is passed in countercurrent with the feed gas stream.

The gaseous stream that is substantially free of gaseous acid constituents leaves the absorption column 2 through an upper outlet 4. 3 0 The absorbent fluid rich in carbon dioxide leaves the absorption column 2 through line 5 and passes to the head region of an expansion column 6.

In the expansion column 6, the pressure of the absorbent fluid is abruptly reduced to approximately 5 to 9 bar so that the lighter components of the gas stream can evaporate from the absorbent fluid. These components can be burned or recirculated to the absorption column 2. The absorbent fluid leaves the first expansion column 6 through line 8 at the bottom of the column, while the vaporized components of the gas stream are withdrawn through the pipeline. 7 at the head of the expansion column 6.

The absorbent fluid then passes to column 10. The carbon dioxide released in column 10 leaves the column at the head thereof. A reflux condenser with collecting container 12 recirculates droplets of absorbent fluid entrained in column 10. Carbon dioxide is removed through line 13 and can be compressed by means of a compression device 19 and introduced through line 20 into a device storage. Part of the regenerated absorbent fluid exiting the bottom of column 10 is heated by means of reboiler 18 and recirculated to column 10.

The regenerated absorbent fluid exiting the bottom of the column 10 is pumped by means of a pump 16 through a heat exchanger 9 in which it serves to preheat the absorbent fluid rich in carbon dioxide passing through the pipe 8. The regenerated absorbent fluid can then pass to the optional poor refrigerant 21, where its temperature is adjusted further. The absorbent fluid enters the absorption column 2 through the pipe 3. A fresh absorbent fluid can be supplied as a supplement through the pipe 17.

With reference to FIG. 2, the arrangement used to evaluate the thermal stability of the aqueous alkanol amine solutions comprises a reaction vessel 1 (having a volume of about 1 liter), equipped with an electrically driven paddle stirrer 2, an inlet pipeline gas 3 with pressure control valve 4, a gas purge pipe 5 with valve 6, and a thermo-pair 7. The reaction vessel 1 can be maintained at a controlled temperature by means of a diaphragm heater (not shown) adapted to the surface of the container. Samples may be removed from the contents of the reaction vessels by means of the sampling line 8. Pressure and temperature signals may be continuously sent to a data logger (not shown) by the electrical data lines 9 and 10.

Example 1 An arrangement as shown in FIG. 2 is used for thermal stability tests. For a typical experiment, 600 ml of the amine solution with the desired amine concentration is placed in the autoclave with stirring. The autoclave and the liquid were purged with gaseous CO 2 by alternative pressurization and depressurization of the autoclave (0 to 5 bar above atmospheric pressure) with C02 10 times. The autoclave was gradually heated and maintained at the desired temperature. The CO2 entrained as volatile matter was purged and the pressure remained at the desired value. The evaporated water vapor was cooled to 5 ° C and the condensed water was returned as reflux to the autoclave. After the temperature and pressure in the autoclave were stabilized at the desired values, the degradation test was started. The preparatory procedure before the test took approximately 30 minutes.

Liquid samples were removed from the autoclave at appropriate intervals and analyzed by gas chromatography.

The degradation test was carried out at a temperature of 162 ° C and a pressure (absolute pressure) of 6.3 bar for 300 hours. No promoter of the carbon dioxide uptake was added.

The conditions used for gas chromatography were as follows: Model: Shimadzu GC-14A equipped with a capillary column TC-5 (30m by 0.32mm ID), GL Sciences Inc., Tokyo, Japan); column temperature: 40 ° C (5 min) - 10 ° C / min - 280 ° C (15 min); Carrier gas: He; Sample injection volume: 1 μ ?; detector: FID.

The results are summarized below in the table. C0 denotes the amine concentration at the beginning of the experiment; c denotes the unchanged amine concentration after 300 hours.

Table 1: Thermal stability of tertiary aliphatic alkane-amines at 162 ° C and 6.3 bar.

MEA denotes 2-hydroxyethylamine (also referred to as mono-ethanolamine) DEA denotes bis (2-hydroxyethyl) amine (also referred to as diethanolamine) DIPA denotes 1- (2-hydroxypropylamino) propan -2-ol (also referred to as diisopropanolamine) DMAP denotes 3-dimethylamino-1-propanol MDEA denotes 2- (2-hydroxyethyl-methylene-amine) ethanol (also referred to as N) -methyldiethanol-amine) DMEA denotes 2- (dimethylamino) ethanol (also referred to as N, N-dimethylethanolamine) DEEA denotes 2- (diethylamino) ethanol (also referred to as?,? - diethylethanol - amine) EDEA denotes (2- [ethyl- (2-hydroxyethyl) -amino] -ethanol (also referred to as N-ethyldiethanolamine) DEAP denotes 3-diethylaminopropanol The results show that the stability of the primary and secondary alkanolamines (such as MEA, DEA or DIPA) is significantly lower than that of the tertiary alkanol amines. The assay also shows that MDEA (which has two hydroxyethyl moieties at its nitrogen atom) is less stable than DMAP and DEAP (which have only one 3-hydroxypropyl moiety) or DMEA and DEEA (which have only one 2-hydroxyethyl moiety). Surprisingly, although both MDEA and EDEA have two hydroxyethyl moieties at their respective nitrogen atoms, EDEA proves to be more stable than MDEA. This can be added to the fact that the abstraction or replacement of an ethyl group (such as the content in the EDEA molecule) takes place less easily than that of a methyl group (such as the content in the MDEA molecule).

Claims (12)

1 . A process for the recovery of carbon dioxide, comprising: a) an absorption step consisting in placing a gaseous feed stream containing carbon dioxide in gas-liquid contact with an absorbent fluid, whereby at least a portion of the carbon dioxide present in the gas stream is absorbed in the absorbent fluid to produce (i) a stream of refined gas having a reduced content of carbon dioxide and (ii) an absorbent fluid rich in carbon dioxide, b) a regeneration step consisting of treating the absorbent fluid rich in carbon dioxide at a pressure greater than 3 bar (absolute pressure) in order to liberate carbon dioxide and regenerate an absorbent fluid poor in carbon dioxide that is recycled to Use in the absorption step, wherein the absorbent fluid is an aqueous amine solution containing a tertiary aliphatic alkanol amine and an effective amount of a carbon dioxide absorption promoter, showing the tertiary aliphatic alkanol-amine less than 5% decomposition when an aqueous solution of the tertiary aliphatic alkanol-amine at a concentration of 4 mol / 1 of the aliphatic tertiary alkanol amine is maintained for 300 hours in a vapor equilibrium condition -liquid at a temperature of 162 ° C and a total pressure of 6.3 bar (absolute pressure) with co-existence of the aqueous solution and carbon dioxide.

2. A process for the recovery of carbon dioxide comprising: a) an absorption step that consists in placing a gas stream containing carbon dioxide in gas-liquid contact with an absorbent fluid, with which at least one portion of the carbon dioxide present in the gas stream is absorbed in the absorbent fluid to produce (i) a refined gaseous stream having a reduced content of carbon dioxide and (ii) an absorbent fluid rich in carbon dioxide, b) a regeneration step that consists in treating the absorbent fluid rich in carbon dioxide at a pressure greater than 3 bar (absolute pressure) in order to liberate carbon dioxide and regenerate an absorbent fluid poor in carbon dioxide that is recycled to Use in the absorption step, wherein the absorbent fluid is an aqueous amine solution containing a tertiary aliphatic alkanol amine other than N-methyldiethanolamine, and an effective amount of a carbon dioxide absorption promoter.

3. A process according to claim 1 or 2, wherein the tertiary aliphatic alkanolamine does not contain in its molecular structure a nitrogen atom that is substituted with more than one 2-hydroxyethyl moiety.

4. A process according to claim 1 or 2, wherein the tertiary aliphatic alkanolamine does not contain in its molecular structure a nitrogen atom which is substituted at the same time with a 2-hydroxyethyl moiety and a methyl moiety.

5. A process according to any one of claims 1 to 4, wherein the tertiary aliphatic alkanolamine contains in its molecular structure a single nitrogen atom, the nitrogen atom being substituted with a hydroxyalkyl residue and two residues I rent.

6. A process according to claim 4, wherein the hydroxyalkyl moiety is selected from the group consisting of 2-hydroxyethyl, 3-hydroxypropyl, 2-hydroxypropyl and 2-hydroxybutyl.

7. A process according to claim 1 or 2, wherein the tertiary aliphatic alkanolamine is selected from the group consisting of N-ethyldiethanolamine, 2- (dimethylamino) -ethanol, 2- (diethylamino) -ethanol, 3- (dimethylamino) -1-propanol, 3- (diethylamino) -1-propanol, 1- (dimethylamino) -2-propanol, and 2- (diisopropylamino) -ethanol. A process according to any one of the preceding claims, wherein the carbon dioxide absorption promoter is selected from the group consisting of piperazine, 2-methylpiperazine, N-methylpiperazine, N-ethylpiperazine, N-hydroxyethyl-piperazine, N- (2-aminoethyl) -piperazine, homopiperazine, piperidine and morpholine. 9. A process according to any one of the preceding claims, wherein the absorbent fluid contains 20 to 60% by weight of the tertiary aliphatic alkanolamine. 10. A process according to any one of the preceding claims, wherein the absorbent fluid contains 0.5 to 20% by weight of the carbon dioxide absorption promoter. 11. A process according to any one of the preceding claims, wherein the regeneration step is carried out at a pressure ranging from a value greater than 3 bar to 10 bar (absolute pressure). 12. A process according to any one of the preceding claims, wherein the carbon dioxide released is introduced into a compression device.