FR2824492A1 - Pre-treating natural gas containing acid compounds, involves cooling natural gas to condense water, contacting partially dehydrated natural gas with liquid stream and cooling dehydrated natural gas - Google Patents

Abstract

Natural gas containing acid compounds is pre-treated by performing processes (I-IV). In process (I), the natural gas is cooled. In process (II), the gas phase obtained in process (I) is contacted with liquid phase obtained in process (III), and in process (III), the gas phase obtained in process (II) is contacted with liquid phase obtained in process (IV). The gas phase obtained in process (III) is cooled. Pre-treatment of natural gas containing hydrocarbons, water and acid compounds chosen from hydrogen sulfide and carbon dioxide, involves cooling natural gas to condense portion of water, contacting partially dehydrated natural gas with a liquid stream containing hydrogen in contact zones and cooling dehydrated natural gas to condense and separate acid compounds, in sequential processes (I-IV). The process (I) involves cooling the natural gas to produce a liquid phase and a gas phase. The process (II) involves contacting the gas phase obtained in process (I), in a contact zone (I) (202), with a liquid phase obtained in process (III) to produce a gas phase and a liquid phase. The process (III) involves contacting, in a contact zone (II) (203), the gas phase obtained in process (II) with a liquid phase obtained in process (IV) to produce a gas phase and a liquid phase. The gas phase obtained in process (III) is cooled to produce a liquid phase and a gas phase.

Description

16 through a siphon (8).

  The invention relates to a process for the pretreatment of a highly acidic natural gas containing a substantial amount of acid compounds such as hydrogen

  sulfide (H2S) and carbon dioxide (CO2).

  The work carried out by the applicant had made it possible to propose in patent EP 0 665 046 a process making it possible to eliminate a substantial quantity of the acid compounds present in the natural gas leaving the well, a process whose simplicity allowed an easy realization and an investment minimal. According to the process, the initial natural gas is brought into contact in a cyclone-type enclosure with a liquid rich in acid compounds in order to obtain on the one hand, at the top of the cyclonic enclosure, a gaseous fraction depleted in acid compounds and on the other hand, at the bottom of the cyclonic enclosure, a liquid phase containing the majority of the acid compounds and of water. The liquid phase recovered at the bottom of the cyclone enclosure is reinJected into a well in the process of exhaustion. The gaseous fraction obtained at the head of the cyclonic enclosure is cooled at low temperature (down to -30 ° C.) then sent to a separation flask to obtain on the one hand a gas purified of acid compounds and on the other hand a condensate rich in acidic compounds which is

  recycled to the cyclonic enclosure.

  However, the process described in patent EP 0 665 046 has drawbacks: 1) The presence of water in the gaseous fraction cooled at low temperature is likely to cause throughout the circuit the formation of solid hydrates which may eventually obstruct the pipes, or even damage the components of the device used. This is why the process described by patent EP 0 665 046 recommended the use of an anti-hydrate, preferably methanol, to prevent the formation of hydrates during

  cooling of the gaseous fraction from the cyclonic enclosure.

  2) A non-negligible quantity of hydrocarbons is entrained with the liquid phase recovered at the bottom of the cyclonic enclosure. The loss of hydrocarbons entrained with the liquid at the bottom of the cyclonic enclosure can amount to 10% of the quantity of gas treated. The current; nvention aims in particular to remedy, O.

  disadvantages mentioned above.

  l O It has been discovered by the applicant that it is possible, under appropriate thermodynamic conditions, to concentrate the initial natural gas in methane while removing the majority of the acid gases and substantially all of the water it contains. By substantially all of the water, it is understood that the amount of water present in the final gas is less than 50 ppm molar, preferably less than 10 ppm molar, and even more preferably less than 5 ppm molar. The present invention proposes to replace the cyclonic contact point with several contacting zones, each contacting zone operating under pressure conditions and

of determined temperatures.

  The present invention also proposes to implement different means of cooling the gaseous fraction obtained at the head of the cyclonic enclosure. The present invention provides a process for the pretreatment of a natural gas under pressure containing hydrocartures, at least one of the acid compounds H2S and CO2 and water, the process comprises the steps: a) the natural gas is cooled to produce a liquid phase and a gas phase, b) the gas phase obtained in step a) is brought into contact in a first contact zone with a liquid phase obtained in step c) to produce a gas phase and a liquid phase, c) the gas phase obtained in step b) is brought into contact in a second contact zone with a liquid phase obtained in step d) to produce a gas phase and a liquid phase, d) the gas phase is cooled obtained in step c) to produce a phase

liquid and a gas phase.

  In step d) of the process according to the invention, the gas phase obtained in step c) can be cooled by means of a heat exchanger and / or at

by means of an expansion turbine.

  The process according to the invention may include the step: e) the gas phase obtained in step d) is cooled by means of an expansion turbine to produce a gas phase and a liquid phase which is

recycled in step c).

  If the process according to the invention uses an expansion turbine, it may include the step: f) at least one of the gaseous phases obtained in step d) is compressed and at

  step e) using the energy recovered from the expansion turbine.

  In step d) of the method according to the invention, the gas phase obtained in step c) can be cooled by means of a Venturi neck, said liquid phase being drawn off at the Venturi neck and said gas phase being collected at the outlet of the divergent tube from the Venturi neck. The liquid phase drawn off at the Venturi neck can be cooled to produce the

  liquid recycled in step c) and a gas phase.

  The gas phases obtained in step d) and in step e) can be used to cool the gas phase obtained in step c) and / or for

  re * oidid natural gas in step a).

  In the first contact zone, the liquid obtained can be heated to

step b).

  In step a) of the process according to the invention, the natural gas can be at a

  l O pressure of 8 MPa and at a temperature above 15 C.

  The liquids obtained in step a) and in step b) can be introduced

in a well.

  According to the present invention, after treatment of the natural gas leaving the production well, a final gas is recovered containing the majority of the hydrocarbons contained in the gas before treatment. The majority of hydrocarbons means at least 90% of hydrocarbons, preferably at least 95% of hydrocartures and very preferably at least 97% of hydrocarbons relative to the hydrocarbons contained in the gas before treatment. The present invention advantageously makes it possible to avoid the use of an anti-hydrate, such as methanol, the transport, use and regeneration of which are generally expensive, complex and the handling

dangerous.

  The invention will be better understood on reading the description given below.

  after by way of exemplary embodiments, in the context of non-limiting applications with reference to the accompanying drawings in which: FIG. 1 shows a block diagram of the method according to the invention, FIG. 2 represents a variant of the method according to the invention using an expansion turbine, - Figure 3 shows a variant of the method according to the invention using a Venturi neck type separator. In the embodiment of the process of the invention according to FIG. 1, a very acidic natural gas coming from a production well via a pipe 10 (1) at a pressure of 8 MPa and a temperature of 50 C, saturated in water (3600 ppm molar), containing 32 mol% of H2S, 11 mol% of CO2 and 57 mol% of methane (less than 1 mol% of C2 +) is introduced into a heat exchanger (101) where it is cooled at 30 C. The cooling temperature is chosen so as to be slightly higher than the hydrate formation temperature at the pressure of the natural gas arriving through the pipe 1. The fluid leaving the exchanger (101) is introduced by a line (2) in a separator (201), and a liquid effluent containing essentially water and very few dissolved acid compounds is drawn off via a line (3). This liquid effluent can optionally be reheated in the heat exchanger (102). Is also withdrawn from the separator (201) through a conduit (4) a gas saturated with water containing 1650 ppm molar water. The cooling in the exchanger (101) thus makes it possible to obtain a gas

  with a much lower water content.

  The gas from the separator (201) through the conduit (4) is introduced into a first contacting zone (202). This contact area (202)

  operates at a pressure of 7.97 MPa and a temperature of around 17 C.

  It may consist of a balloon provided with linings known to those skilled in the art. It receives, via a conduit (5), a liquid flow composed of a majority of H2S (approximately 70 mol%) which is at a temperature of approximately 5 C. Thus a mixture is formed by bringing the gas arriving through the pipe (4) into contact with the liquid arriving through the pipe (5) in the zone (202). This contacting simultaneously makes it possible: to raise the molar fraction of H2S in the mixture contained in the zone (202); - dissolving any solid particles (mainly sulfur-based) from the mixture contained in the zone (202). The work of Alberta Sulfur Rescarch Ltd. and presented in the publication "Recent Developments in the Mitigation of Sulfur Deposition in Sour Gas Facilities" (PD Clark, P. Davis, J. Simion, E. Fitzpatrick and CSC Lau Laurance Reid Gas Conditioning Conference 1996 Norman Oklahoma) indeed show that the solubility of sulfur is appreciably increased when the molar percentage of the mixture of H2S exceeds 40%; and obtaining a partially dehydrated gas leaving the zone (202) via the conduit (8). Indeed, the water contained in the gas arriving through the pipe (4) is absorbed by the liquid arriving through the pipe (5) because the water has a more affinity

  stronger for H2S than for hydrocarbons.

  A liquid is formed at the bottom of the zone (202) formed at more than 75 mol% by H 2 S, the rest being water, CO2 and a little entrained methane. The hydrates can possibly be recovered in the form of a deposit at this level. This liquid is discharged through the conduit (6), possibly reheated in the exchanger (103), mixed with the liquid effluent recovered at the bottom of the separator (201) and discharged through the conduit (7) by means of a pump (301) to one

  pressure of 38 MPa to be reinjected into an exhausted oil well.

  The temperature of the zone (202) is adjusted so as to be high enough to protect against the formation of hydrates. However, in the case where hydrates are formed, the contacting zone (202) can be produced as a cyclone-type enclosure so as to avoid any blockage or clogging by solid particles. A heating means (107) can also be used so as to raise the temperature of the mixture in the zone (202) above the hydrate formation temperature. This heating means can consist of a reboiler (107) heating the liquid at the bottom of the zone (202). The addition of heat by the reboiler (107) also makes it possible to vaporize the hydrocarbons present in the liquid at the bottom of the zone (202) and therefore to limit the losses of hydrocarbons entrained with the liquid

  l 0 discharged through the conduit (6) at the bottom of the zone (202).

  At the head of the contact zone (202), a gas formed essentially of H2S (33 mol%), CO2 (12 mol%) and methane (64 mol%) and

  strongly depleted in water (200 ppm molar) is evacuated through the conduit (8).

  The gas from the first contact zone (202) through the conduit (8) is introduced into a second contacting zone (203). This contacting zone (203) operates at a pressure of 7.94 MPa and a temperature of approximately C. It receives a liquid flow composed of approximately 50 mol% of H2S via a conduit (9) which is at a temperature of approximately -30 C. This addition of liquid H2S, containing practically no more water (40 ppm molar) makes it possible to remove the majority of the water present in the gas contained in the zone (203) due to the affinity of liquid H2S dehydrated for water stronger than

  the affinity of hydrocartures for water.

  At the head of the contact zone (203), a gas essentially consisting of H2S, CO2 and methane is removed via the conduit (10) and does not contain

  significantly more water (about 16 ppm molar).

  The gas transported by the conduit (10) passes through various cooling systems to liquefy the acid compounds. First a gas-gas heat exchanger (104). A fluid is evacuated from the exchanger (104) by the conduit (11) at approximately -5 C. This fluid is introduced into a heat exchanger (105), using for example a propane refrigerant, and leaves it through the conduit. (12) at the temperature of -30 C The fluid circulating in the line (12) is inkoduit in the separator flask (204). The balloon (204) is at a temperature of -30 ° C. and at the pressure of 7.88 MPa. A gas partially purified of acid compounds is evacuated from the flask (204) by the conduit (13) and a condensate rich in H2S and CO2 by the conduit (14). The condensate circulating in the conduit (14) is recycled thanks to

  the pump (302) in the contact zone (203) via the conduit (9).

  The methane contained in the condensate circulating in the conduit (14) is largely recovered in the contact zone (203) The gas circulating in the conduit (13) can be used as fluid

  refrigerant in the exchanger (104) then in the exchanger (101).

  Finally, there is a loss of methane of 823 kmol / h, or less than 7 mol% of the amount present in the charge arriving via the pipe (1). The feed gas was purified from 4750 kmollh of H2S, or 72 mol% of the amount present in the feed. The main advantage of the process according to the invention is to always use streams having molar fractions of water and associated temperatures which are such that the formation of hydrates is impossible. This is in particular due to the use of the balloon (201) making it possible to reduce the amount of water present in the gas and to the use of the two contact zones (202) and (203) making it possible to obtain a gas very poor in water can then be cooled without risk of hydrate formation. Thus the method according to the invention does not require mixing the

  natural gas with an anti-hydrate compound.

  Without departing from the scope of the invention, it is possible to implement more than two contact zones. One or more contact zones operating under conditions may be disposed between the contact zone (202) and (203).

  thermodynamics intermediate between those of zones (202) and (203).

  Table 1 shows, in the embodiment described in relation 10 with FIG. 1, the material balance obtained with the process: Line I 1 | 3 | 4 | 6 | 13 | 14 Temperature (C) 50.0 30.0 30.0 1 17.0 -30.0 -30.0 Pressure (MPa) 8.00 7.97 7.97 1 7.97 7.88 7.88 Flow molar (kmol / h)

H2O 76.3 42.9 32.3 32.3 0.0 * 0.4

N2 8.1 0.0 * 8.1 0.2 7.9 1.0

  C02 2219.4 0.1 2219.3 614.9 1604.3 1715.2

  H2S 6,570.7 0.9 6,669.9 4,749.6 1,820.4 4,365.9

  Methane 11,839.8 0.0 * 11,839.8 823.8 11,015.6 3,235.5 Ethane 96.2 0.0 * 96, 2 24.3 71.9 65.4 Propane 37.2 0.0 * 37, 2 22.8 14.4 29.4 Butane 5.0 0.0 * 5.0 3.0 4.6 0.4 1.8 Pentane 2.3 0.0 * 2.3 2.3 0.0 * 0, 2 Total (kmoVh) | 20854, 0 | 44.0 1 20810.1 1 6274.9 1 14534.9 1 9405.0 * less than 0.05

Table 1

  Another possible configuration of the device described in FIG. 1 allowing the application of the present method is presented in FIG. 2. The modification with respect to FIG. 1 relates to the cooling means

  serving to cool the fluid circulating in the conduit (11).

  In FIG. 2, the flow discharged from the exchanger (104) through the conduit (11) at the temperature of -5 ° C. is sent to a separator tank (205). This balloon (205) makes it possible to separate a liquid effluent rich in acid compounds, discharged through the conduit (16) and a gas, discharged through the conduit (17). The conduit (17) brings the gas to an expansion turbine (401) in which it undergoes isentropic expansion. The stream from the expansion turtine (401) is at low temperature (around -30 C) and is sent through the conduit (18) into the separator flask (204). A gas partially purified of acid compounds is removed from the flask (204) via the pipe (19) and a condensate rich in H2S and CO2 through the pipe (14). The separation of the partially purified gas into acidic compounds and the condensate rich in H2S and CO2 is favored by the low pressure value in the balloon (204) obtained following the expansion of the gas in the turbine (401). The condensate circulating in the line (14) is raised in pressure by the pump (302) and is mixed with the liquid stream from the balloon (205) through the conduit (16). This mixture is recycled in the contact zone (203) thanks to the

pump (303).

  The gas from the balloon (204) through the conduit (19) can be used as refrigerant in the exchanger (104) and then in the exchanger (101). This gas leaving the exchanger (101) is directed by the conduit (20) to the compressor (402) to be recompressed before being exported by the conduit (21). The compressor (402) can be coupled to the turbine (401) to use the work of the isentropic expansion as a source of energy. A second compressor supplied with energy by a source external to the process of the invention can also

  compress the gas from the balloon (204) to compensate for the loss of energy.

  FIG. 3 represents a variant of the method according to the invention using a separator of the Venturi neck type. The gas leaves the separator (501) through the conduit (23). The Venturi neck separator is a means of

  gas cooling not requiring energy input.

  Thanks to the fact that the flow circulating in the conduit (10) contains only a low water content (around 16 ppm molar), hydrates are not formed at the separator (501) nor at the effluent liquid recovered through the conduit (22). Thus the method according to the invention does not require the use of an anti-hydrate continuously. The effluent flowing in the conduit (22) is cooled to -30 C through the heat exchanger (106) which can use a propane coolant. The cooled effluent is directed to the flask (204) through the conduit (24). The balloon (204) produces a liquid effluent rich in H2S discharged through the conduit (14) and a gas discharged through the conduit (25). This gas flowing in the conduit (25) is remixed with the gas flowing in

  the conduit (23) to produce a mixture of gases flowing in the conduit (26).

  This gas mixture is used as refrigerant in the exchanger (104) then in the exchanger (101) before being exported. The liquid effluent flowing in the conduit (14) is redirected by the pump (302) and the conduit (9) to the

contact area (203).

  Table 3 shows, in the embodiment described in relation to Figure 3, the material balance obtained: Conduit 1 1 1 11 1 23 1 25 1 26 Temperature (C) 50.0 -6.0 19.0 -30.0 1 15. 4 Pressure (Mpa) 10.0 9.95 7.5 7.5 1 7.5 Molar flow (kmoVh)

H20 75.3 1 0.4 0.0 * 0.0 * 0.0 *

N2 8.1 8.9 7.1 0.7 7.8

  C02 2219.4 3320.0 1584.5 83.9 1668.4

  H2S 6570.7 6176.1 1085.6 96.5 1182.1

  Methane 11839.8 14251.1 10953.5 619.4 11572.9 Ethane 96.2 137.0 79.9 3. 4 83.3 Propane 37.2 44.0 12.4 0.8 13.2 Butane 5.0 2.2 0.2 0.0 * 0.2 Pentane 2.3 0.3 0.0 * 0.0 * 0.0 * Total (kmoVh) 1 20854.0 1 23940.0 1 13723. 2 1 804.7 1 14527.9 * less than 0.05

Table 3

Claims (10)

  1) Process for the pretreatment of a pressurized natural gas containing hydrocarbons, at least one of the hydrogen sulfide acid and carbon dioxide compounds, and water, in which: a) the natural gas is cooled to produce a liquid phase and a gas phase, b) the gas phase obtained in step a) is brought into contact in a first contact zone with a liquid phase obtained in step c) to produce a gas phase and a liquid phase, c) the gas phase obtained in step b) is brought into contact in a second contact zone with a liquid phase obtained in step d) to produce a gas phase and a liquid phase, d) the gas phase obtained is cooled to step c) to produce a phase liquid and a gas phase.   2) Method according to claim 1 wherein in step d), the   gas phase obtained in step c) by means of a heat exchanger.   3) Method according to one of claims 1 and 2 wherein in step d),   the gas phase obtained in step c) is cooled by means of an expansion turbine. 4) Method according to claim 2 wherein: e) the gas phase obtained in step d) is cooled by means of an expansion turbine to produce a gas phase and a liquid phase which is recycled in step c).   6) Method according to one of claims 3 and 4 wherein:   f) at least one of the gaseous phases obtained in step d) is compressed and   step e) using the energy recovered from the expansion turbine.   6) Method according to one of claims 1 and 2 wherein in step d),   the gaseous phase obtained in step c) is cooled by means of a Venturi neck, said liquid phase being drawn off at the Venturi neck and said gaseous phase being recovered at the outlet of the diverging tube from the neck of the l O Venturi.   7) The method of claim 6 wherein in step d), said liquid phase drawn off at the venturi neck is cooled to produce the   liquid recycled in step c) and a gas phase.   8) Method according to one of the preceding claims in which:   g) using at least one of the gas phases obtained in step d) and   step e) to cool the gas phase obtained in step c).   9) Method according to one of the preceding claims, in which:   h) at least one of the gas phases obtained in step d) is used and   step e) to cool the natural gas in step a).   ) Method according to one of the preceding claims wherein, in   said first contact zone, the liquid obtained in step b) is heated.   11) Method according to one of the preceding claims wherein, at   step a) the natural gas is at a pressure of 8 MPa and at a temperature greater than 15 C.   12) Method according to one of the preceding claims wherein: