Natural gas deacidizing method

The present invention relates to the field of dehydration, deacidizing and stripping of natural gas, using a mixture of water, amine and methanol.

Treating a natural gas requires dehydration, stripping when the gas contains condensable hydrocarbons and deacidizing this gas when the acid gas content is too high.

It is possible to dehydrate and to strip a natural gas by cooling it in the presence of methanol so as to prevent formation of ice or hydrates.

It is well-known from the prior art to use chemical solvents such as amines and physical solvents such as methanol or triethylene glycol dimethylether (TEGDME) to carry out the deacidizing operation. Chemical solvents afford the advantage of absorbing acid gases, even in very low proportions in the gas to be treated. They thus allow to obtain high-level specifications for the gas coming from the deacidizing plant, i.e. to obtain a gas containing less than 1% by mole of acid gases. Physical solvents allow to absorb large amounts of acid gases when the acid gas contents are high. However, it is more difficult to obtain high-level specifications for the gas treated with physical solvents. It is also well-known to use a mixture of physical solvent and chemical solvent to treat highly acid gases by means of high-level specifications for the gas treated. It is notably known to use a mixture of amine such as DIPA (diisopropanolamine) or MDEA (methyldiethanolamine) with a relatively heavy physical solvent such as Sulfolane used by the Shell company.

Document FR-2,743,083 provides a natural gas dehydration, deacidizing and stripping method using a mixture of physical and chemical solvents.

The present invention aims to improve the method provided by document FR-2,743,083.

The present invention thus relates to a method of deacidizing a natural gas containing acid gases wherein the following stages are carried out:

(a) contacting the natural gas with a first solvent containing methanol, water and an amine, obtained in stage e), and with a third solvent containing acid gases obtained in stage b) so as to obtain a methanol-containing gas and an acid gas-containing solvent, (b) contacting the gas obtained in stage a) with a second solvent containing methanol, water and an amine, obtained in stage d), so as to obtain a gas partly freed of acid gases and the third solvent containing acid gases, (c) expanding the acid gas-containing solvent obtained in stage a) so as to release acid gases and to obtain a partly regenerated solvent, (d) distilling in a distillation column a first fraction of the partly regenerated solvent obtained in stage c) so as to obtain a gas phase containing acid gases and the second solvent, (e) mixing a second fraction of the partly regenerated solvent obtained in stage c) with a fourth solvent drawn off at a point of the distillation column so as to obtain the first solvent, the draw-off point being determined for the proportion by mole of methanol of the first solvent to be higher than that of the second solvent.

The second solvent can contain less than 1% by weight of methanol.

In stage d) of the method, the second solvent can be drawn off from the bottom of the distillation column.

In stage e) of the method, the fourth solvent can be drawn off at a point located between the bottom and the top of the distillation column. In stage e), the fourth solvent can also be drawn off in gaseous form at the top of the distillation column and cooled.

In stage c) of the method, the acid gas-containing solvent obtained in stage a) can be expanded successively to two pressure levels.

The method according to the invention can comprise the following stages

(f) cooling the gas obtained in stage b) so as to obtain a methanol-poor gas, a hydrocarbon-containing liquid phase and a methanol-containing aqueous phase, (g) prior to stage a), contacting part of the natural gas with the aqueous phase obtained in stage f).

In stage d) of the method, the first fraction of the partly regenerated solvent obtained in stage c) can be expanded, then heated by heat exchange with the second solvent obtained in stage d).

According to the method of the invention, a third fraction of the partly regenerated solvent obtained in stage c) can be fed into the distillation column, the feeding point in the distillation column for the first fraction of the partly regenerated solvent being different from the feeding point of the third fraction of the partly regenerated solvent.

The amine can be selected from the following compounds: diethanolamine (DEA), methyldiethanolamine (MDEA), methylethanolamine (MEA), triethanolamine (TEA), diglycolamine (DGA) and diisopropanolamine (DIPA), and a mixture of these amines.

The method according to the invention can comprise the following stages:

(h) cooling the acid gas-containing gas phase obtained in stage d) so as to obtain a liquid phase and an acid gas-rich gas phase, the liquid phase being separated from the gas phase in a separating drum, (i) cooling the acid gas-rich gas phase obtained in stage h) and the acid gases released in stage c) so as to obtain a liquid phase and a gas phase, (j) feeding the liquid phase obtained in stage i) into the separating drum. The operating conditions of the method according to the invention can be as follows:

stages a) and b) are carried out at a temperature ranging between 20° C. and 80° C. and at a pressure ranging between 3 MPa abs. and 10 MPa abs., in stage c), the acid gas-containing solvent obtained in stage a) is expanded to a pressure ranging between 0.1 MPa abs. and 1 MPa abs., stage d) is carried out at a temperature ranging between 30° C. and 130° C. and at a pressure ranging between 0.1 MPa abs. and 1 MPa abs.

Other features and advantages of the invention will be clear from reading the description hereafter, given by way of non limitative example, with reference to the accompanying drawings wherein:

FIG. 1 diagrammatically shows the method according to the invention,

FIGS. 2 and 3 diagrammatically show variants of the method according to the invention.

In FIG. 1 , the natural gas to be treated flows in through line 0 . It contains for example methane, ethane, propane, butane, as well as heavier hydrocarbons, water and acid gases such as, for example, H 2 S and CO 2 , and possibly mercaptans.

A fraction of this gas is sent to contact column CS where it is subjected to countercurrent contact with a methanol solution flowing in through line 12 . At the bottom of column CS, an aqueous phase substantially freed from methanol is discharged through line 15 . The methanol-containing gas recovered at the top of column CS is mixed with the natural gas that has not flowed through column CS. This gas mixture is sent through line 1 into contact column CA.

Makeup methanol flowing in through line (a) is possibly added to the gas circulating in line 1 prior to sending it to contact column CA. Column CA comprises two absorption sections CA 1 and CA 2 . In first section CA 1 , the gas flowing in through line 1 is subjected to countercurrent contact with a solvent consisting of a mixture of water, methanol and amine flowing in through line 4 . The gas leaving first section CA 1 is brought into countercurrent contact in second section CA 2 with a solvent consisting of a mixture of water, methanol and amine flowing in through line 9 . The methanol content (in % by mole) of the solvent flowing in through line 9 is lower than the methanol content (in % by mole) of the solvent flowing in through line 4 . An acid gas-containing solvent is discharged at the bottom of column CA through line 3 . A gas substantially freed of the acid gases and containing methanol is recovered at the top of column CA through line 2 . Makeup methanol flowing in through line (b) allows the methanol content of the gas circulating in line 2 to be increased.

The gas flowing from column CA through line 2 is cooled in indirect heat exchangers E 5 and E 6 , then sent into separating drum BR 2 . A gas phase substantially freed of methanol and discharged through line 13 after heat exchange in exchanger E 5 , a hydrocarbon-containing liquid phase discharged through line 14 and a methanol-containing aqueous phase introduced by means of pump P 5 and line 12 into column CS are separated in drum BR 2 .

The solvent discharged through line 3 is expanded to a first pressure level through valve V 1 by releasing a gas phase which contains at least part of the hydrocarbons that may have been co-absorbed in the solvent in column CA. The gas phase and the liquid phase obtained by expansion of the solvent are separated in separating drum B 1 . The solvent-containing liquid phase is discharged from drum B 1 through line 5 . The gas phase forms a combustible gas FG commonly referred to as fuel gas. The gas phase is possibly freed of the acid gases it contains by washing in drum B 1 with a fraction of the solvent circulating in line 9 . Washing can also be carried out in any other suitable system known to the man skilled in the art.

The solvent circulating in line 5 is expanded through expansion valve V 2 . The acid gases released through expansion are separated from the liquid solvent in separating drum B 2 . The liquid solvent recovered at the bottom of drum B 2 is thus partly regenerated. The acid gases are discharged through line 6 . A first part of the liquid solvent is discharged through line 7 , then mixed with the solvent circulating in line 11 . This solvent mixture is fed through line 4 into column CA. A second part of the liquid solvent is expanded through expansion valve V 3 , then it flows into exchanger E 1 where it is heated by indirect heat exchange and sent through line 8 into regeneration column CR. In column CR, the solvent is regenerated by distillation. Reboiler ER heats the liquid solvent available at the bottom of column CR. Regenerating the first part of the solvent circulating in line 7 only by expansion allows to limit the amount of solvent treated by column CR, and consequently to limit the energy required for regeneration of the solvent from column CA.

The regenerated solvent, i.e. substantially free of acid gases (the acid gas content can be less than 1% by mole), is recovered at the bottom of column CR. The regenerated solvent is cooled by indirect heat exchange with the solvent circulating in line 8 in exchanger E 1 , then cooled in exchanger E 2 , and fed into column CA through line 9 .

Line 11 draws off a solvent at an intermediate point of column CR, i.e. at a point located above the point of draw-off of the regenerated solvent through line 9 . The solvent circulating in line 11 is pumped by pump P 2 , cooled in exchanger E 7 , mixed with the solvent circulating in line 7 . This solvent mixture is fed through line 4 into column CA. The position of the draw-off point of line 11 on column CR depends on the composition of the solvent to be fed into column CA through line 4 . The closer the draw-off point to the top of column CR, the richer in methanol the solvent circulating in line 11 . The composition of the solvent fed through line 4 into column CA can thus be adjusted according to the position of the draw-off point of line 11 on column CR. Furthermore, draw-off a methanol-rich solvent from regeneration column CR allows to increase the amine concentration of the solvent drawn off at the bottom of column CR. In fact, the amount of methanol contained in the solvent drawn off at an intermediate point through line 11 will not be contained in the solvent drawn off through line 9 .

It has also been discovered that it is possible, by means of the method according to the invention, to obtain at the bottom of regeneration column CR a solvent having a very low methanol content. Under such conditions, a gas substantially free of methanol is obtained at the top of absorption column CA. This treated gas requires no cooling treatment to recover the methanol it contains. For example, it is possible to obtain at the bottom of column CR a regenerated solvent containing less than 1% by weight of methanol. The methanol thus separated from the solvent is discharged at the top of the regeneration column and sent to the top of first absorption section CA 1 through lines 11 and 4 . Such a separation requires a temperature and a pressure allowing to reboil a liquid consisting of amine and substantially free of methanol. For example, if the pressure of column CR is close to the atmospheric pressure, i.e. between 0.1 MPa and 1 MPa abs., the temperature at the bottom of column CR can range between 110° C and 130° C.

The vapor phase discharged at the top of regeneration column CR is cooled in condenser E 3 . After cooling, the methanol-rich liquid phase is separated from the acid gases in drum BR 1 . The methanol-rich liquid phase is sent to the top of column CR by means of pump P 3 . The acid gases from drum BR 1 are mixed with the acid gases circulating in line 6 . The acid gas mixture is cooled in exchangers E 8 and E 4 , then fed into drum B 3 . The methanol-rich liquid phase obtained at the bottom of drum B 3 is fed into drum BR 1 by means of pump P 4 and line 17 . The acid gases obtained at the top of drum B 3 are heated in exchanger E 8 by heat exchange with the acid gas mixture from drum BR 1 and line 6 , then discharged through line 16 . The acid gases discharged through line 16 can be liquefied and injected into the subsoil, for example in an oil well.

The method according to the invention allows on the one hand to regenerate part of the solvent by simple expansion, which is economical, and on the other to adjust the methanol contents in the two absorption sections CA 1 and CA 2 . The method according to the invention thus allows to treat a highly acid gas because, during stage a), the first methanol-rich solvent allows to absorb most of the acid gases and, during stage b), the amine-rich solvent allows to absorb the remaining acid gases.

An example of implementation of the method according to the invention as defined above is given hereafter in connection with FIG. 1 . The gas is fed through line 0 at a temperature of 45° C. and an absolute pressure of 7.6 MPa with the following composition (in % by mole):

In column CS, the gas is contacted with the aqueous methanol solution flowing in through line 12 and flows out of column CS charged with methanol. It is then fed into absorption column CA.

After washing with a methanol-rich solvent flowing in through line 4 at the top of section CA 1 , this gas is washed with a diethanolamine (DEA) rich solvent flowing in through line 9 at the top of section CA 2 . The gas flows out free of its acid gases and it is cooled to −30° C. in exchangers E 5 and E 6 after addition of methanol through line (b). A deacidified, dehydrated and stripped gas phase discharged through line 13 , a liquid hydrocarbon phase discharged through line 14 and a methanol-containing aqueous phase discharged through line 12 to column CS to recover the methanol by stripping with a fraction of the gas flowing in through line 0 are recovered in three-phase drum BR 2 . A methanol-poor aqueous phase (98.91% by mole of H 2 O) is discharged through line 15 at the bottom of column CS.

The treated gas discharged through line 13 contains 4.4 ppm H 2 S and 2.9 ppm CO 2 .

The acid gas-containing solvent recovered at the bottom of column CA is expanded to 2.6 MPa abs. to release the co-absorbed hydrocarbons. The latter are washed in drum B 1 by a fraction of regenerated solvent from line 9 . The solvent freed from the dissolved hydrocarbons circulating in line 5 is expanded to an absolute pressure of 0.4 MPa through valve V 2 to release part of the dissolved acid gases which are sent to the top of column CR through line 6 . A fraction of the partly regenerated solvent is sent to the top of the first section of CA 1 through line 7 and the other fraction is sent to regeneration column CR after expansion to 0.24 MPa abs. through valve V 3 , then heating in exchanger E 1 . A flow of regenerated solvent with a low methanol concentration (11.40% by mole) is recovered at the bottom of column CR and sent to the top of column CA. A fraction of solvent with a high methanol concentration (19.93% by mole) is drawn off from column CR through line 11 and sent with the solvent circulating in line 7 to the top of the lower section of CA 1 after cooling by E 7 . The draw-off point of line 11 is located between the bottom of column CR and the feeding point of column CR through line 8 .

The solvent sent to the top of absorption column CA through line 9 contains 0.26% H 2 S and 0.08% CO 2 in molar fraction. The solvent discharged from separating drum B 1 and sent to the absorption column through line 7 contains 1.93% CO 2 and 4.44% H 2 S in molar fraction.

Finally, the acid gases at the top of regeneration column CR containing methanol and water are cooled to ambient temperature by exchanger E 3 to generate a reflux sent to column CR by pump P 3 . The acid gases at the outlet of BR 1 are cooled in E 8 and E 4 . The methanol fraction they contain is recycled to BR 1 and the acid gases are discharged through line 16 after heating in exchanger E 8 .

The elements of FIGS. 2 and 3 having the same reference numbers as the elements of FIG. 1 are identical thereto.

In FIG. 2 , in relation to FIG. 1 , the draw-off point of line 11 has been modified. Line 11 draws off a fraction of the methanol-rich liquid phase obtained in drum BR 1 . This fraction is mixed with the solvent circulating in line 7 , the mixture being introduced at the top of absorption section CA 1 .

In FIG. 3 , in relation to FIG. 1 , the liquid solvent from drum B 2 is separated into three parts. A first part of the solvent is mixed with the solvent circulating in line 11 , the mixture being introduced at the top of absorption section CA 1 . A second part of the solvent is expanded through valve V 3 , then exchanges heat in E 1 with the regenerated solvent circulating in line 9 , then it is fed into column CR. A third part of the solvent is expanded through valve V 4 , then fed into column CR at a level located above the level of introduction of the second part of the solvent in column CR.

Absorption column CA and regeneration column CR can be plate or packed columns. Various types of plates and/or packings, random or stacked, can be used.

Heat exchangers E 1 to E 8 can be plate or shell-and-tube exchangers, made of brazed aluminium or stainless steel.

Without departing from the scope of the invention, regeneration can be carried out in two or more distillation columns working under different pressure and temperature conditions. It is thus possible, for example, to obtain acid gas fractions with different compositions, for example a CO 2 -concentrated fraction and an H 2 S-concentrated fraction.

Various amines can be used in the method according to the invention. It is notably possible to use secondary amines such as diethanolamine (DEA) or tertiary amines such as methyldiethanolamine (MDEA). It is possible to use these amines pure or in admixture. It is also possible to add various additives intended to limit degradation of the amine by oxidation or to limit foaming.

It is advantageous to use an H 2 S selective amine. The CO 2 contained in the mixture of solvents is thus separated upon expansion through valves V 1 and V 2 . The H 2 S is separated from the mixture of solvents during a stage of regeneration by distillation.