FR2917982A1 - Pretreating natural gas having e.g. hydrocarbon, comprises separating liquid aqueous phase from cooled gas, contacting gas with liquid rich in hydrogen sulfide, separating into permeate and retentate, using membrane, and condensing - Google Patents

Abstract

Pretreating natural gas (I) having hydrocarbon, hydrogen sulfide (H 2S) and water, comprises (a) cooling (I) and introducing the cooled natural gas to a separation device (B1) to separate a liquid aqueous phase from (I); (b) contacting (I) with a liquid (11) rich in H 2S to obtain a gas (6) that is free of water and an effluent liquid (4); (c) separating the gas, obtained in step (b), to obtain a permeate (8) rich in H 2S and a retentate (7) free of H 2S; and (d) partially condensing the permeate by cooling to obtain a liquid (10) rich in H 2S, and a gas (12) that is free of H 2S. Pretreating natural gas (I) having hydrocarbon, hydrogen sulfide (H 2S) and water, comprises (a) cooling (I) and introducing the cooled natural gas to a separation device (B1) to separate a liquid aqueous phase from (I); (b) contacting (I) with a liquid (11) rich in H 2S, obtained in step (d), to obtain a gas (6) that is free of water and an effluent liquid (4) rich in water and H 2S; (c) separating the gas, obtained in step (b), through a membrane (M) to obtain a permeate (8) rich in H 2S and a retentate (7) free of H 2S; and (d) partially condensing the permeate by cooling to obtain a liquid (10) rich in H 2S, which is recycled to step (b), and a gas (12) that is free of H 2S.

Description

The present invention relates to the field of gas deacidification

  natural. Document FR 2 814 378 describes a pretreatment process for a natural gas, saturated with water, containing a substantial amount of hydrogen sulphide (H2S) and, optionally, carbon dioxide (CO2) and other sulfur compounds. The pretreatment process makes it possible to obtain, at lower cost, a gas rich in methane, depleted of H2S and water. At the same time, an aqueous hydrocarbon-depleted liquid is obtained, containing a large part of the H 2 S which is generally injected into an underground reservoir, for example a petroleum production well. The present invention provides an improvement of the process described in FR 2,814,378 by using membrane separation to improve the separation performance of H 2 S.

  The present invention describes a process for pretreating a natural gas comprising hydrocarbons, hydrogen sulfide (H2S) and water, wherein the following steps are performed: a) the natural gas is cooled, then introducing the cooled natural gas into a separation device so as to separate a liquid aqueous phase from the natural gas, then b) contacting a natural gas in a distillation column with a liquid rich in H2S obtained in step d ) so as to obtain a water-depleted gas and a liquid effluent rich in water and H 2 S, c) the gas obtained in step b) is separated through a membrane so as to obtain a permeate enriched in H 2 S and a retentate depleted in H2S, d) partially condensing said permeate by cooling so as to obtain said H2S-rich liquid used in step b) and a gas depleted of hydrogen sulfide.

  According to the invention, the membrane may be chosen so as to be more permeable to H2S than to methane. The membrane may have a permeability ratio of the H2S with respect to the methane greater than 15. The membrane may have a permeability relative to the H2S between 10 and 10 Barrer Barrer. The membrane may comprise a polymeric material selected from the group consisting of polyether-block-amides, silicones impregnated with polar solvents, non-impregnated silicones, vitreous polymers. It is possible to impose a pressure difference between the retentate located upstream of the membrane and the permeate situated downstream of the membrane, the pressure difference being greater than 5 bars. The liquid obtained in step d) can be compressed before being used in step b). The natural gas may comprise at least 10 mol% of hydrogen sulfide.

  Other features and advantages of the invention will be better understood and will become clear from reading the description given below with reference to FIG. 1 which schematically represents the method according to the invention.

  In FIG. 1, the natural gas to be treated from a production well arrives via line 1. At the outlet of the well, the natural gas is mainly composed of methane, ethane, propane, and butane, and also contains lower amounts of heavier hydrocarbons. Natural gas also contains various acidic compounds, typically carbon dioxide (CO2), hydrogen sulphide (H2S), but also carbon oxysulphide (COS) and mercaptans (RSH). Mercaptans consist essentially of methyl mercaptan (CH3SH) and ethyl mercaptan (C2H5SH), and possibly mercaptans having longer hydrocarbon chains, up to six carbon atoms. The natural gas leaving the well is, moreover, saturated with water.

  Preferably, the process according to the invention is applied to strongly acidic gases, that is to say with more than 10 mol% of H 2 S, and which may comprise CO 2, for example between 0 and 10 mol% of CO 2. . The pressure of the raw natural gas may be adapted, by compression or expansion in the member P, to reach a value between 40 bar and 100 bar, preferably between 60 and 80 bar. Then the natural gas is cooled by the heat exchanger E1 at a temperature of between 15 ° C. and 40 ° C. The cooling temperature is chosen in particular so as to avoid the problems related to the formation of gas hydrates in the zone B1. and Cl. Cooling makes it possible to condense a fraction of the water contained in the natural gas. Then the natural gas is introduced into the gas / liquid separation device B1, for example a separator flask, in which the condensed water is separated from the gas. The condensed water is discharged at the bottom of the flask B1 via the duct 3. The natural gas saturated with water is discharged at the top of the flask B1 via the duct 2.

  The gas flowing through the duct 2 is introduced into the contacting zone C1 so as to be brought into contact with the liquid rich in H2S arriving via the duct 11. The gas flowing in the duct 2 can be heated by the exchanger El '. in order to avoid or limit the formation of hydrates in the zone C1. In the contacting zone C1, the liquid rich in H2S arriving via the conduit 11 allows, by absorption, to capture and entrain the water residual water contained in the natural gas arriving via the conduit 11. Thus, a gas depleted of water and H2S is removed at the head of the zone C1 and a H2S-rich aqueous liquid is withdrawn at the bottom of the zone C1.

  The contacting zone C1 can operate at a pressure of between 45 bars and 75 bars, preferably between 60 bars and 75 bars. The pressure levels of the contact zone C1 and the temperature of the liquid flowing in the conduit 11 are chosen so as to dehydrate and deacidify the natural gas outside the thermodynamic range of hydrate formation. The liquid fraction rich in water and H2S obtained at the bottom of Cl is discharged via the conduit 4, the pump P1 and the conduit 5. The zone C1 can be a simple contacting flask without a control system. temperature. Preferably, the zone C1 may also be a distillation column provided with internals, for example trays, loose or structured packings, and equipped at the bottom of a reboiler R, as shown schematically in FIG. 1. The reboiler allows to maintain the temperature at the bottom of the column C1 at a sufficient level so as to limit the losses of hydrocarbons in the liquid fraction discharged through the conduit 4. According to the invention, the dehydrated gas obtained at the head of the zone C1 is introduced through line 6 in the membrane separation device S to separate the methane from the H2S. The device S consists of a closed chamber whose inner space is separated into two compartments by the membrane M. According to the invention, a membrane is chosen which is more permeable to H2S than to methane, that is to say that is, the ability of the membrane to pass H2S is greater than its ability to pass methane. In general, the permeability of a membrane corresponds to the flow of material passing through a membrane of given section and thickness, the membrane being subjected to a given pressure difference. For example, a membrane is chosen whose permeability ratio of H2S to methane is greater than 15. An excellent value of this ratio of permeability of H2S with respect to methane, of the membrane, is greater than 30. or 40. In addition, one can choose a membrane whose permeability with respect to HZS is between 10 and 105 Barrer (1 Barrer = 10-10Ncm3.cm/cm2.s.cmHg). The pressure difference between the two compartments tends to migrate through the membrane the HZS contained in the gas arriving via the conduit 6. The methane is retained upstream of the membrane. Thus, the device S is evacuated, on the one hand, a gaseous retentate depleted in H2S via line 7 and, on the other hand, a gas permeate rich in H2S via line 8. The surface of the membrane is chosen in particular according to the flow of natural gas to be treated. The pressure difference between the two compartments may be greater than 5 bar, an excellent pressure difference value being between 30 and 150 bar. The membrane M may be made of polymeric materials offering a selectivity of HZS with respect to methane. It is possible to use elastomers, "block" polymers, or vitreous polymers.

  Elastomers are materials having flexible polymer chains at room temperature, that is to say that the glass transition temperature of these materials is below room temperature, generally below 20 C. For example, it is possible to use silicone "hydrophobic" type elastomers, such as PDMS (polydimethylsiloxane) or PTMSP (polytrimethylsilylpropyne). It is also possible to use silicone-based apolar elastomers impregnated with polar solvents. The membrane can thus be made, for example, based on silicones impregnated with polyethylene glycol and optionally with glycerol. The addition of these polar solvents, which have a strong affinity for hydrogen sulfide, in the silicone matrix contributes to the high selectivity of these membranes with respect to HZS. For example, use is made of polar-type elastomers such as poly (ether-urethane) or polyphosphazenes.

  The polymers of "block" type developed in particular by MTR are polyether-block-amide type polymers (sold under the trade name PEBAX by Arkema). The chains of these polymers consist of a succession of rigid segments comprising amide functions, and of flexible segments comprising ether functions, which have a high affinity for H2S. The vitreous polymers are mainly tight rigid polymers characterized by a glass transition temperature greater than ambient temperature and more particularly greater than 100 ° C. These rigid polymers, most often of a polar nature, exhibit molecular sieving properties, for example the polyimides, polysulfones, and cellulose acetate. Various membrane geometries can be used. For example, the membranes are in the form of hollow fibers, the gas from the distillation column C1 flowing inside the fibers. The membrane separation device S may also have a spiral-type geometry: the thin-film membrane is disposed between two thin compartments in which the retentate and the permeate circulate, this assembly being wound in a spiral so as to obtain a device more compact and less bulky. The permeate rich in H2S is sent through line 8 into the heat exchanger E2 to be partially condensed by cooling. The gas and liquid mixture is sent through line 9 into the gas / liquid separation device B2, for example a balloon. B2 makes it possible to separate the condensate rich in H2S from the gas rich in methane. The condensate is discharged at the bottom of B2 via line 10, then pressurized by pump P2 and sent via line 11 into column Cl. The gas obtained at the top of B2 can be compressed in organ K and then collected. with the methane-rich gas circulating in the pipe 7. The methane rich gas fractions circulating in 7 and 13 can be combined to circulate in the conduit 14. This gas assembly can then be deacidified by a process using an absorbent solution. , for example one of the processes described by the documents FR 2 605 241 and FR 2 636 857.

  The combination of the separation by distillation and the membrane separation, as proposed by the present invention, makes it possible simultaneously to optimize the operation of the membrane and the operation of the distillation column. 1 o Indeed, the distillation column Cl allows in particular to deplete the natural gas in water and heavy hydrocarbons. However, the presence of water significantly accelerates the hydrolysis of the membrane material M and heavy hydrocarbons, for example C5 +, can lead to swelling of the membrane material leading to a loss of selectivity of the membrane M. As a result, the membrane that is implemented to treat the flow from the top of the distillation column Cl is not or not affected by these disadvantages. The operation and the lifetime of the membrane M are improved by the distillation operation carried out in Cl. The membrane separation step makes it possible to concentrate the H 2 S in the reflux which is introduced at the top of the distillation column. Cl. In Cl, it is the hygroscopic nature of the H2S contained in the reflux which conditions the progressive exhaustion in water contained in the natural gas: the more the reflux is rich in H2S, the more the water content of the gas at the top of Cl is weak, and therefore the hydrolysis of the membrane is limited. Therefore, the membrane separation device S is closely related to the distillation column C1 and they work synergistically.

  The numerical example presented in Table 1 below illustrates the operation of the method according to the invention. A membrane based on PEBA 4011, sold by the company Arkema, offering a permeability ratio of H2S with respect to the CH4 of 30 and a methane permeability of 5 × 10 -6 Ncm 3 / cm 2 .s.cmHg is used. has an exchange surface of 200 m2 and operates with a pressure difference between the two sides of the membrane of 70 bar. Load Gas outlet Bottom Liquid _ 1 14 5 N of duct P (bar) 80 80 80 T (C) 30 17 72 _ Composition 64 88.6 9.1 (% mol.) Methane H2S 30 8.6 78.0 CO2 - - - Ethane 2 2.1 1.8 Butane 2 0.9 4.4 Propane 2 0.03. 6.4 H70 1000 ppm mol <1 ppm mol 0.3 Flow rate (kmol / h) 100 69.4 30.6 Table 1

  Under the operating conditions identical to those presented in relation with Table 1, Table 2 compares the process according to the invention with the method according to the prior art (see document FR 2 814 378) operating without membrane separator .

  Process according to the process according to the invention prior art Elimination H9S (%) 80.0 64.0 Methane losses (%) 3.9 3.9 Tplateau (C) 20.0 20.1 Q condenser (kW) 144 152 Q pump (kW) 1.8 - Q compression (kW) 15 - Acidic gases contained 6.0 10.6 in the pretreated gas (kmollh) Table 2

  The membrane separation device S according to the invention makes it possible to increase the amount of H2S eliminated at the bottom of the zone C1 and thus to minimize the amount of H2S contained in the treated gas. The increase in process performance according to the invention is obtained with a low additional energy intake and without prejudice to possible additional loss of methane.

Claims (9)

  1) Process for the pretreatment of a natural gas comprising hydrocarbons, hydrogen sulphide (H2S) and water, in which the following steps are carried out: a) the natural gas is cooled and then the cooled natural gas is introduced; in a separation device (B1) so as to separate a liquid aqueous phase from the natural gas, then b) the natural gas is contacted with a liquid rich in H2S (11) obtained in step d) so as to obtain a gas depleted in water (6) and a liquid effluent rich in water and H2S (4), c) the gas obtained in step b) is separated through a membrane (M) so as to obtain a permeate enriched with H2S (8) and an H2S-depleted retentate (7), d) partially condensing said permeate (8) by cooling to obtain said H2S-rich liquid (10) implemented in step b) and a hydrogen sulphide depleted gas (12).   2) The method of claim 1, wherein the membrane is selected to be more permeable to H2S than methane.   3. The process according to claim 2, wherein the membrane has a permeability ratio of H2S to methane greater than 15.   4) Method according to one of claims 2 and 3, wherein the membrane has a permeability relative to the H2S between 10 Barrer and 105 Barrer.   5) Method according to one of claims 2 to 4, wherein the membrane comprises a polymeric material selected from the group consisting of polyether-block-amides, silicones impregnated with polar solvents, non-impregnated silicones, glassy polymers .   6) Method according to one of the preceding claims, wherein imposes a pressure difference between the retentate located upstream of the membrane and the permeate downstream of the membrane, the pressure difference being greater than 5 bar.   7) Method according to one of the preceding claims, wherein in step a), a flask or a distillation column is used to contact the natural gas with the H 2 S rich liquid.   8) Method according to the preceding claim, wherein the liquid obtained in step d) is compressed before being implemented in step b).   9) Method according to one of the preceding claims, wherein the natural gas comprises at least 10 mol% of hydrogen sulfide.