FR2771022A1 - PROCESS FOR DEACIDIFYING A GAS WITH A HIGH ACID GAS CONTENT - Google Patents

Abstract

- Process for deacidifying a gas containing at least one acid gas and at least one hydrocarbon, where the acid gas is brought into contact with a solvent phase which is selective with respect to the acid gases, and a gradient of temperature to prevent the condensation of a liquid hydrocarbon phase and obtain a treated gas depleted in acid gases and a solvent phase enriched in acid gases.

Description

1 2771022

  The present invention aims to treat a gas containing at least one hydrocarbon and at least one acid gas in high concentration for the purpose of

  at least partially rid of the acid gases it contains.

  The process according to the invention applies particularly well to deacidification

of natural gas.

  French patents FR-2,605,241 and FR-2,636,857 describe treatment methods using a refrigerated physical solvent making it possible to carry out the various

  gas treatment steps, including deacidification and water removal.

  These methods have as advantages over the prior techniques the possibility of using the same polar solvent for the dehydration and deacidification of the gas, as well as the possibility of regenerating the solvent by direct contact in an absorption column or contactor of the solvent with the gas to be treated (patent FR-2,605,241), leading to significant savings in consumption

  of energy, investment and size of the installation.

  However, for gases containing a very high concentration of acid gas, the implementation of these methods has drawbacks. The gas to be treated must be cooled before it is introduced into the acid gas absorption column. This cooling step results in the condensation of a very large quantity of hydrocarbons and also that of acid gases. This liquid phase must be stabilized, which results in the production of stabilization gas in large quantities and which must then be recompressed and then recycled. The presence of this recompression stage penalizes this

  type of process because it involves high investment and operating costs.

  The present invention overcomes the drawbacks of the prior art by proposing a different approach using an absorption column operating with a thermal gradient. The gas to be treated is introduced, for example without prior refrigeration step, into the column in which a counter-current of physical solvent circulates.

cold fleece.

  Advantageously, the absorption of the acid gases in the column makes it possible to avoid the condensation of these acid gases in a liquid hydrocarbon phase. In fact, the hydrocarbon dew point of the gas at the absorption pressure decreases sharply when

the acid gas content decreases.

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  FIG. 1 represents in a temperature (T), pressure (P) diagram, two envelope curves A and B, corresponding to the gas to be treated and to the treated gas, and the points

  corresponding dew referenced a, b.

  Line C represents the thermal profile AT existing in the absorption column, point a 'corresponds to the operating conditions of the first stage, and point b'

at the last floor.

  Figure 1 shows the importance of the thermal profile of the absorption column for a given calculation example. The evolution of the gas envelope curve in this column makes it possible to avoid the formation of a liquid hydrocarbon phase: whatever the stage considered, the temperature is such that

  the gas is very far from its hydrocarbon dew point.

  In the rest of the description, the expression "zone" is used interchangeably

  contact "or the expression" contactor "to designate the part of the device where we put

  in contact with a fluid to be treated with a solvent selective for acid gases.

  Likewise, the expression "rich solvent phase" corresponds to a solvent phase enriched in acid gases and "poor solvent phase" to a solvent phase depleted in gas.

acids during the process.

  The present invention relates to a process for treating a fluid such as a gas containing at least one acid gas and at least one hydrocarbon, for the purpose of

  at least partially rid said fluid of acid gases.

  It is characterized in that it comprises at least the following steps: * said gas to be treated is brought into contact, for example inside a contactor with a solvent phase selective with respect to said acid gases, such as a mixture of polar solvent and water, and * a temperature gradient is produced inside the contactor to avoid condensation of a liquid hydrocarbon phase and to obtain at the outlet of said contactor

  a treated gas depleted in acid gases and a solvent phase enriched in acid gases.

  The solvent is for example a physical polar solvent.

  For example, the mixture of solvent and water is sent against the current of the gas to be treated, the gas to be treated being introduced at an initial temperature Tg and the solvent mixture at an initial temperature Tm, the temperature values Tg and Tm being chosen for

  generate the necessary gradient inside the contactor, and Tg being greater than Tm.

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  It is possible to take at least a fraction of said gas to be treated at a temperature level Ti, to cool said fraction of gas withdrawn by carrying out a heat exchange with at least a fraction of the treated gas and to reinject it at a level of

temperature below Ti.

  At least a fraction of the treated gas, freed mainly of acid gases, is used, for example, to refrigerate the gas to be treated by circulating said fraction withdrawn at

  against the current of said gas to be treated.

  The temperature difference between the head and the bottom in the absorption column can

  vary, in absolute value, between 5 and 150 C and preferably between 30 and 100 C.

  The solvent phase enriched in acid gases can be regenerated by performing a simple expansion. According to another mode of implementation, the solvent phase enriched in acid gases is regenerated by a distillation process at a pressure lower than the absorption pressure. The solvent phase enriched in acid gases can be regenerated at least in part by expanding it and bringing it into contact with at least one fraction of the solvent phase.

  regenerated and producing a gas with a controlled acid gas content.

  According to another embodiment, the solvent phase enriched in acid gases is regenerated at least in part by a distillation process with simultaneous exchange of heat between said solvent phase which is gradually heated by the solvent phase which results from said regeneration which is cooled by circulating against the current of the

  solvent phase which is regenerated.

  It is possible to purge the water accumulated during the process in the

solvent phase.

  The invention also relates to a device for treating a fluid such as a gas.

  comprising at least one acid gas and at least one hydrocarbon.

  It is characterized in that it comprises in combination * at least one conduit for introducing said gas to be treated, * at least one conduit for introducing a fluid capable of capturing acid gases, * said introduction conduits being connected to an enclosure or contactor, said contactor being adapted to operate with a temperature gradient over at least part of the length of the enclosure to obtain at the output a gas depleted in acid gases evacuated by a conduit and a solvent phase enriched in acid gases

withdrawn by a conduit.

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  The contactor may include means for removing and re-injecting the

  fluid and / or gas circulating inside said contactor.

  The device comprises, for example, a circuit making it possible to purge at least one

  part of the water accumulated in the fluid capable of capturing acid gases.

  The device and method described above are used for the treatment of a

  natural gas containing acid gases such as CO2 and / or H2S.

  The invention and its characteristics will be better understood on reading the

  following description, given solely by way of illustration and without limitation attached

  figures representing respectively: * figure 1 represents the evolution of the curves enveloped by a gas, in an absorption column with thermal gradient, * figure 2 schematizes the principle of the process according to the invention, figures 3 to 5 describe embodiments presenting different stages of regeneration of the solvent, * your figures - 6 and 7 schematize examples of energy integration around the contactor,: FIG. 8 presents an example of diagram making it possible to eliminate the water accumulated in the solvent,

  * Figure 9 shows schematically an example of implementation of the method.

  The principle of the method according to the invention is described in relation to the diagram in FIG. 2 which presents an example of an arrangement of an applied device,

  for example, treating natural gas to deacidify it.

  The natural gas to be treated, containing acid gases and at least one hydrocarbon is sent via line 1 into the absorption column or contact zone C1 where it is brought into contact, for example, against the current with a mixture of solvent. and cold water which arrives via line 2, the solvent phase comprising, for example, at least one polar solvent. A temperature gradient is determined inside the contact zone C1 determined according to the phase diagram of the gas to be treated (FIG. 1) so as to avoid the condensation of a hydrocarbon phase containing acid gases inside. of this area. Inside the contact zone, the acid gases are absorbed

  - selectively in the solvent phase.

  A phase is removed in the upper part of the contactor C1, via the conduit 3

gas depleted in acid gases.

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  In the lower part of the contact zone or contactor C1, it is withdrawn by a

  line 4, a solvent phase charged with acid gases.

  The temperature gradient is generated for example by introducing the gas to be treated at the bottom of the contactor C1 at a temperature between 10 and 50 C, and the mixture of solvent and water at the head of the contactor at a temperature between 0 and - Advantageously, the flow rate and the temperature of the solvent sent to the head of the absorption column are controlled so as to obtain a desired concentration of acid gas.

  in the treated gas and to avoid the condensation of hydrocarbons in the contactor.

  The solvent phase coming from the contactor C1 is expanded through the expansion valve V1, sent to a separator flask B1 which makes it possible to separate it into a partially regenerated solvent phase drawn off at the bottom by a pipe 5 and a gaseous phase drawn off by a pipe 6 at the head of the ball. This gas phase contains acid gases and coabsorbed hydrocarbons. It can optionally be used as

  fuel gas or be recompressed and then recycled to contactor C1.

  The water content of the solvent phase may preferably be at least 10% by

mass fraction.

  The partially regenerated solvent phase from balloon B1 through line 5 is expanded through a valve V2, then sent to a balloon B2 which separates the solvent phase drawn off at the bottom through line 7 and the gas phase drawn off at the head by a pipe 8. The solvent phase is expanded through a valve V3, then sent to a separator flask B3 which performs the separation between a solvent phase drawn off at the bottom of the flask by a pipe 9 and a gas phase

  withdrawn at the head by a conduit 10.

  The gaseous phases discharged through lines 6, 8 and 10 are rich in acid gases. The solvent phase from the flask B3 is sufficiently depleted in acid gases to be recycled to the contactor C1. Beforehand, it may be necessary to withdraw at least part of the solvent through a pipe 11 to avoid the accumulation of water supplied by the gas to be treated introduced through the pipe 1. The rest of the solvent is then taken up by a pump P1 recycling, mixed with an additional solvent arriving via a line 13 in order to compensate for the losses of solvent, in the deacidified gas, in the acid gases produced and possibly in the purge line 11 if the latter is not recycled after elimination of the water it contains. The resulting mixture circulates in a conduit 14, is cooled in an exchanger E1 using an external auxiliary cooling fluid, for example, and recycled to the contact zone C1 by the

conduit 2.

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  The treatment method according to the invention mainly comprises at least the following steps: ò the gas to be treated is brought into contact inside a absorption column with a fluid selective for acid gases, by example a mixture of polar solvent and water, and * a temperature gradient is generated inside this absorption column to achieve the absorption of acid gases without condensation of hydrocarbons or acid gases, or at least if such condensation occurs, the condensed liquid phase is present in minute quantities, * one obtains a treated gas depleted in acid gases and a solvent phase enriched in acid gases. The absorption and the temperature gradient can be obtained by circulating against the current the gas to be treated introduced at the bottom of the contactor at a temperature Tg and the solvent-water mixture introduced at the head of the column at a temperature Tm, with

Tm <Tg.

  The temperature difference | Tm - Tg | can be between 5 and 1 00 C.

  The pressure value prevailing in the absorption column can be understood

between 1 and 20 MPa.

  FIG. 3 illustrates an alternative embodiment of the regeneration step where a distillation column C2 is used, placed after the flask B1 of FIG. 2, making it possible to carry out a more thorough regeneration of the solvent phase, than that obtained in

  using the diagram in Figure 2.

  The partially regenerated solvent from the flask B1 is expanded through the valve V2 and then reheated in an exchanger E2 before being introduced via a line 20 by

  example in the middle of the distillation column C2.

  The acid gases produced by distillation are evacuated at the head of column C2 through a line 21, cooled in an exchanger E3 and sent to a settling or separation tank D1. At the end of this settling flask, a fraction of the acid gases is evacuated via a line 23, and the liquid phase comprising the condensed solvent phase is returned via a line 22 to the distillation column C2 to be used

as reflux.

  The regenerated solvent phase is drawn off at the bottom of the distillation column C2 by a conduit 24, cooled in an exchanger E2 before being sent by a conduit 25 to the purge conduit 11 and the recycling pump P1 according to a diagram identical to

  that described in connection with FIG. 2.

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  FIG. 4 illustrates an alternative embodiment of the device of FIG. 3 o the separating flask B1 is equipped with a contact zone having the function of ensuring a

  specification of acid gas on the gas flow drawn off through line 6.

  For this, a fraction of the regenerated and recycled solvent phase according to the diagram in FIG. 3 is removed and sent via a conduit 26 located at the head of the zone of

contact 27 of ball B1.

  Inside this contact zone 27, the absorption of part of the acid gases is carried out using a determined quantity of solvent in order to obtain, via line 6, a gaseous effluent having a controlled acid gas content. and chosen according to a

io desired specification.

  FIG. 5 illustrates another mode of regeneration of the solvent. The exchanger E2 of FIG. 3 is replaced by a stripper exchanger ES1 which carries out a distillation with simultaneous exchange of heat between the rich solvent phase introduced by the conduit 7 at the level of the upper part of the stripper exchanger ES1 which is gradually heated and the solvent phase resulting from the regeneration step, sent to the lower part by the conduit 24 coming from the distillation column C2 and which is

  cooled by circulating against the solvent phase which is regenerated.

  At the outlet of the stripper exchanger ES1, a rich solvent phase is sent at the bottom sent through line 20 to the distillation column C2, at the head a rich gas discharged through a line 28, and in the upper part of the column the phase regenerated solvent

  drawn off through a conduit 29 connected to the recycling pump P1.

  The internal heat exchange described above between the solvent phase rich in acid gases and the solvent phase poor in acid gases can be carried out by means of

different devices.

  The integrated heat exchanger ES1 can be of the tube and shell type. In this case, the solvent phase regenerated and drawn off at the base of column C2 circulates inside the

  distillation takes place outside the tubes and inside the calender.

  The stripper exchanger ES1 can then be in trays, the tubes being placed above each tray, so as to be submerged by the solvent phase rich in

  recover circulating on each tray.

  The stripper exchanger ES1 can be in continuous contact, the solvent phase rich

then dripping on the tubes.

  The regenerated solvent phase drawn off at the base of column C2 can also circulate outside the tubes and inside the calender. Distillation is then

8 2771022

  performed inside the tubes, which are preferably vertical and provided with a

internal lining.

  For the absorption columns and / or the stripper exchanger described in the preceding figures, it is also possible to use an exchanger with vertical plates. The spaces between the various plates are occupied alternately by the regenerated solvent phase drawn off at the base of the column C2 and by the solvent phase as well as the vapor phase generated flowing against the current in the column during the distillation process. It is, for example, of the brazed aluminum exchanger type or of the stainless steel exchanger type, the plates being welded either edge to edge or by

  metallic diffusion over the entire contact surface between plates.

  FIG. 6 presents an exemplary embodiment of the absorption step in which the cold treated gas is used to cool at least partially the gas to be treated circulating in the absorption column. The gas is withdrawn, during the treatment, at the level of a plate of the column C1 or between two packing zones of the column C1 by a conduit 30 cooled in an exchanger E4 by the treated gas evacuated by the conduit 3, then reintroduced into column C2 via a pipe 31. The withdrawal and injection levels, as well as their number are determined along the column to optimize the recovery of

  frigories on the treated gas recovered in line 3.

  FIG. 7 shows another alternative embodiment of the absorption step where a simultaneous heat exchange is carried out between the treated gas which is gradually heated and the gas to be treated brought into contact with the solvent, which is cooled in flowing against the current. This type of heat exchange can be achieved by

  various devices which have been explained previously in relation to FIG. 5.

  The treated gas coming from the absorption column C1 via the pipe 3 is returned by this same pipe at the top of the column in which it flows against the current of the gas introduced at the bottom of the column through the pipe 1. After having exchanged its frigories with the

  gas to be treated, it comes out heated by a pipe 33, for example located at the bottom of the column.

  The gas to be treated can contain water in different forms, in some cases it may be necessary to eliminate this water to avoid its accumulation in the

solvent phase.

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  FIG. 8 represents a purge diagram making it possible to eliminate the water which has accumulated in the solvent phase during the treatment and which comprises a contactor

  C3 arranged downstream of the contactor C1 with thermal gradient.

  The raw gas to be treated is divided into two fractions passing respectively through conduits 35 and 36. A first fraction passes through conduit 36 which joins conduit 1 (FIG. 1)

  for introducing gas into contactor C1.

  The second fraction of gas to be treated is sent via line 35 to the part

contactor C3.

  The solvent phase loaded with water from the purge pipe 11 (FIG. 1) is

  introduced at the head of contactor C3 using a pump P2 and a conduit 34.

  At the contactor C3 outlet, a gas loaded with solvent discharged at the head is recovered via a conduit 38 which communicates with the introduction conduit 1 and an aqueous phase practically free of solvent which is evacuated at the bottom of the contactor

conduit 37.

  The present invention will be better understood using a nonlimiting example of

  gas treatment described below.

  This example, described in relation to FIG. 9, makes it possible to deacidify a gas

  natural with a very high CO2 content.

  The natural gas to be treated has, for example, the following composition given in mol%:

NITROGEN 1.20

CARBON DIOXIDE 70.50

0.21

HYDROGEN SULPHIDE 0.21

METHANE 27.64

ETHANE 0.24

PROPANE 0.10

ISOBUTANE 0.02

BUTANE 0.02

ISOPENTANE 0.01

PENTANE 0.01

HEXANE 0.03

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  This gas is saturated with water at a temperature of 30 C and has a pressure of 7 MPa. it has a molar flow rate of 22,400 kmol / h. This resulting gas is divided into two fractions in the conduits 35 and 36. The gas from the conduit 35 is sent to the contactor C3 and at a molar flow rate of 17,000 kmol / h. Against the current, a fraction of solvent containing approximately 6 kmol / h of water and 54 kmol / h of methanol is injected through line 34. At the bottom of contactor C3, a flow of 17 kmol / h of water containing

  about 200 ppm molar of methanol.

  The gas coming from the head of the contactor C3, loaded with methanol, is mixed with the gas of the pipe 36, then with the recycled gas coming from the regeneration flask B1 by a pipe 40. It is then sent to the contactor C1. At the head of contactor C1, a solvent containing approximately 5500 kmol / h of water and 49500 kmol / h of methanol is injected via line 2 at a temperature of -30 C. Under these conditions, contactor C1 is operated with a gradient thermal from 23 C at the bottom to -23 C at the top. The treated gas is drawn off at the contactor C1 head via line 3. It has a flow rate of 6603 kmol / h and contains approximately 87% molar of methane and 8% molar of CO2. This gas is heated in the exchanger E at

  a temperature of 50 C, before being evacuated through a pipe 41.

  The rich solvent is withdrawn from the contactor C1 through the conduit 4. It is expanded to a pressure of 3.5 MPa through the valve V1. At the outlet of the separator flask B1, a solvent phase and a gaseous phase evacuated through the conduit 6 are withdrawn through the conduit. The gaseous phase containing approximately 6800 kmol / h of CO2 and 1300 kmol / h of methane is compressed to 7 MPa by a compressor K then cooled to 30 C by an exchanger E

  to be recycled to the base of the contactor C1 through the conduit 40.

  The solvent in line 5 is expanded to a pressure of 1 MPa through the valve V2, is separated in the flask B2 into a gas phase drawn off through line 8 and a solvent phase drawn off through line 7. The gas in line 8 has a flow of 10,153 kmol / h

  and a concentration of 95 mol% of CO2.

  The solvent is again expanded at a pressure of 0.2 MPa through the valve V3, separated in the flask B3 into a gas phase drawn off through line 10 and a solvent phase drawn off through line 9. The gas from line 10a a flow rate of 5666 kmol / h

  and a concentration of 98.5 mol% of CO2.

  A fraction of the solvent is drawn off through the purge pipe 11, sent to the pump P2 ,. to be injected into contactor C3 through line 34. The other fraction of the solvent is sent to pump P1, cooled in exchanger E1 to -30 C before

  to be introduced into contactor C1 through line 2.

  The process makes it possible to eliminate the acid gases contained in a natural gas but also those included in a refinery gas or in any gaseous effluent containing i l 2771022 simultaneously hydrocarbons and acid gases. It eliminates acid gases such as H2S and CO2 and also mercaptans with the chemical formula R-SH, COS and

CS2.

  The various stages of the process according to the invention can be carried out in columns provided with contact zone making it possible to carry out a transfer of material between the gas phases and the liquid phases. To do this, these contact zones can be equipped with perforated plates, with bells, with flaps or in continuous contact zones containing a lining. This packing can be made up of loose elements such as for example Raschig rings, Pall rings, Berl saddles or other packing known to the skilled person. It can also consist of structured packing formed by fabrics, knits, or sheets which can be perforated or provided with corrugations and / or forming channels which

  increase efficiency.

  These trays or these linings can be made of different materials

  such as ceramic, aluminum, stainless steel, plastic.

  The fluid selective for acid gases can comprise a polar solvent chosen from methanol, an alcohol, an ether, a polyethylene glycol ether, propylene carbonate. It is also possible, without departing from the scope of the invention, to use a solvent formed

  with two polar solvents, or a polar solvent and an amine.

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Claims (12)

  1 - Process for treating a fluid such as a gas containing at least one acid gas and at least one hydrocarbon, with the aim of at least partially ridding said fluid of acid gases, characterized in that it comprises at least the following steps: * the said gas to be treated is brought into contact with a selective phase with respect to the said acid gases, such as a mixture of polar solvent and water, and * a temperature gradient is produced to avoid condensation of a phase i0 liquid hydrocarbon and obtain a treated gas depleted in acid gases and a phase solvent enriched in acid gases.   2 - Process according to claim 1, characterized in that the mixture of solvent and water is sent against the current of the gas to be treated, the gas to be treated being introduced at an initial temperature Tg and the solvent mixture at a initial temperature Tm, the temperature values Tg and Tm being chosen to generate the necessary gradient inside   of the contactor, and Tg being greater than Tm.   3 - Method according to one of claims 1 or 2, characterized in that one takes the   at least a fraction of said gas to be treated at a temperature level Ti, said fraction of gas withdrawn is cooled by carrying out heat exchange with at least one   fraction of the treated gas and it is reinjected at a temperature level below Ti.   4 - Method according to one of claims 1 or 2 characterized in that one uses at   at least a fraction of the treated gas freed mainly of acid gases to cool the gas to be treated by circulating said sampled fraction against the flow of said gas to be treated.   - Method according to one of claims 1 to 4 characterized in that one generates at   l Inside the contactor a temperature difference varying, in absolute value, between 5 and    C, and preferably between 30 and 100 C.   6 - Method according to one of claims 1 to 5, characterized in that one regenerates the   solvent phase enriched in acid gases by simple expansion. 13 2771022   7 - Method according to one of claims 1 to 5, characterized in that one regenerates the   solvent phase enriched in acid gases by a pressure distillation process   lower than the absorption pressure.   8 - Method according to one of claims 1 to 5, characterized in that one regenerates at   at least in part the solvent phase enriched in acid gases by expanding it and bringing it into contact with at least one fraction of regenerated solvent phase and producing a   gases with a controlled acid gas content.   9 - Method according to one of claims 1 to 5, characterized in that one regenerates at   at least in part the solvent phase enriched in acid gases by a distillation process with simultaneous exchange of heat between said solvent phase which is gradually heated by the solvent phase which results from said regeneration which is   cooled by circulating against the current of the solvent phase which is regenerated.   - Method according to one of claims, characterized in that one performs a   purging of the water accumulated during the process in the solvent phase.   1 - Device for treating a fluid such as a gas comprising at least one acid gas and at least one hydrocarbon comprising at least one introduction conduit (1) of said gas to be treated, and at least one introduction conduit ( 2) a fluid capable of capturing acid gases, characterized in that said introduction conduits (1) and (2) are connected to an enclosure (C1) or contactor, said contactor being adapted to operate with a gradient of temperature over at least part of the length of the enclosure (C1) to obtain at the output a gas depleted in acid gases evacuated by a conduit (3) and a phase   solvent enriched in acid gases drawn off through a pipe (4).   12 - Device according to claim 1 1, characterized in that said contactor comprises means for withdrawing and reinjecting the fluid and / or the gas circulating inside of said contactor. 14 2771022   13 - Device according to claim 11, characterized in that it comprises a circuit (11, C3) for purging at least part of the water accumulated in the fluid capable to capture acid gases.   14 - Use of the method according to one of claims 1 to 10 and of the device according to   one of claims 11 to 13 for the treatment of a natural gas comprising gases acids such as CO2 and / or H2S.