EP0186555B1 - Self-refrigerating process for the extraction of heavy hydrocarbon fractions - Google Patents

Abstract

This new process provides for the treatment of a gas containing a light fraction and a heavy fraction, so as to extract at least a portion of the heavy fraction. The process is characterized in that the gas to be treated (1) is contacted with a solvent (8) under cooling conditions to selectively absorb at least a portion of the heavy fraction of the gas in the solvent; the light unabsorbed fraction is discharged (16) and the solution of the heavy fraction in the solvent (18) is subjected to desorption (B1): the solvent (6) is recycled and the evaporated heavy fraction (9) is condensed (C), expanded (V1) and evaporated in thermal exchange contact (E2) with the mixture of the gas and the solvent to be cooled; the heavy fraction is discharged (14). The process may be used for the treatment of gas on the field (associated gas, natural gas) and/or in refining and petrochemical operations.

Description

The present invention relates to a new self-cooled process, based on the principle of absorption-desorption in a solvent and making it possible to extract hydrocarbons of which the number of carbon atoms is at least equal to two, for example from 2 to 6 , of a gas which contains them. This process can find applications in field gas treatment if the gas to be treated is for example a natural gas or an associated gas, and / or in refining and petrochemicals.

When a gas, for example a petroleum gas, is rich in heavy components, i.e. easily condensable hydrocarbons and if this gas has to be transported, it is necessary to rid it of part of these components to avoid the formation of liquid plugs which would disrupt transport. On the other hand, the products recovered can prove to be financially attractive.

There are three types of processes most commonly used to achieve these separations: expansion, absorption in a solvent, and refrigeration.

Gas expansion is the simplest of the processes; it has the disadvantage of greatly lowering the gas pressure and leads to a low recovery rate.

In absorption processes, the gas is brought into contact with a solvent, in which the heaviest constituents of the gas are absorbed preferentially; the enriched solvent is then subjected to expansion and / or heating to be regenerated and to release the absorbed constituents; certain stages of these processes, in particular the absorption and recovery of heavy fractions, can be refrigerated by an external refrigeration cycle.

The most conventional method consists in cooling the gas using an external refrigerating machine; it allows working in a wide range of temperature and pressure and leads to significant recovery.

The present process applies in particular to the treatment of a gas containing a light fraction and a heavy fraction, the light fraction containing at least one light constituent of the group comprising hydrogen, nitrogen and hydrocarbons having from 1 to 2 carbon atoms and the heavy fraction containing at least one heavy constituent from the group comprising hydrocarbons having from 2 to 6 carbon atoms, provided that, when the light fraction contains a hydrocarbon containing 2 carbon atoms, the heavy fraction contains at least a hydrocarbon having 3 to 6 carbon atoms.

• a) on met en contact le gaz avec la phase liquide de solvant, de manière à absorber majoritairement au moins une partie de la fraction lourde du gaz dans la phase liquide et à abaisser majoritairement la teneur du gaz en cette fraction lourde, et l'on évacue la chaleur résultante,
• b) on fractionne le produit de l'étape (a) pour recueillir séparément la phase liquide résultante et le gaz non-absorbé,
• c) on élève la pression de la phase liquide séparée à l'étape (b) et on élève suffisamment sa température pour qu'elle laisse se désorber au moins une partie du gaz qu'elle avait absorbé à l'étape (a),
• d) on fractionne le produit de l'étape (c) pour recueillir séparément une solution appauvrie et une phase gazeuse désorbée,
• e) on refroidit la solution appauvrie, on abaisse sa pression et on la renvoie à l'étape (a) pour y reconstituer au moins une partie de la phase liquide de solvant,
• f) on refroidit la phase désorbée de manière à en condenser au moins une partie et l'on recueille le condensat résultant,
• g) on détend la pression du condensat et fournit de la chaleur au condensat détendu, de manière à en évaporer au moins une partie, et on recueille le produit résultant de cette évaporation au moins partielle, enrichi en constituant lourd, par rapport au gaz de départ,
• le procédé étant caractérisé en outre en ce que au moins une partie de la chaleur évacuée à l'étape (a) constitue au moins une partie de la chaleur fournie à l'étape (g).

The process according to the invention for treating a gas as defined above, by extraction in a liquid solvent phase, the solubility of the heavy fraction in the solvent being greater than that of the light fraction and the liquid solvent phase being substantially non-distillable under the process conditions, is characterized in that:

• a) the gas is brought into contact with the liquid solvent phase, so as to mainly absorb at least part of the heavy fraction of the gas in the liquid phase and to lower the majority of the content of the gas in this heavy fraction, and we remove the resulting heat,
• b) the product from step (a) is fractionated to collect the resulting liquid phase and the non-absorbed gas separately,
• c) the pressure of the liquid phase separated in step (b) is raised and its temperature is raised enough to allow at least part of the gas it had absorbed to be desorbed in step (a),
• d) the product of step (c) is fractionated to collect separately a depleted solution and a desorbed gas phase,
• e) the depleted solution is cooled, its pressure is lowered and it is returned to step (a) in order to reconstitute therein at least part of the liquid solvent phase,
• f) the desorbed phase is cooled so as to condense at least part of it and the resulting condensate is collected,
• g) the pressure of the condensate is relaxed and heat is supplied to the expanded condensate, so as to evaporate at least part of it, and the product resulting from this at least partial evaporation is collected, enriched with a heavy constituent, relative to the gas of departure,
• the method being further characterized in that at least a portion of the heat removed in step (a) constitutes at least a portion of the heat provided in step (g).

Non-distillable means that the major part, and preferably at least 90%, most often at least 98%, of the liquid phase is not vaporized under the operating conditions of the process. By majority (or selective) absorption of the heavy fraction, it is meant that the relative proportion of heavy fraction absorbed is greater than the relative proportion of light fraction absorbed. The majority lowering of the heavy fraction gas content means that the relative lowering of the heavy fraction gas content is greater than the relative lowering of the light fraction gas content.

In the process, in step (g), the condensate can receive heat not only from absorption (step a), but also from condensation (step f) and circulating fluids: depleted solution from step (e) and gas separated in step (b).

Preferably, the absorption (step a) is carried out in two stages: the mixture of gas and liquid phase yields heat first to a medium external to the process and then to the condensate of step (g).

Similarly, it is preferable to carry out the condensation (step f) in two stages: the gaseous phase releases heat first towards the external medium then towards the condensate expanded in step (g).

Depending on the degree of evaporation in step (g), the heavy fraction desorbed may come out entirely vaporized or only partially vaporized. In the latter case, a liquid / vapor fractionation will make it possible to collect a condensate, for example butane, and the remaining gaseous fraction can either be collected or recycled to be treated to a level mixed with the fresh gas.

In the process according to the invention, at least one of steps (a) and / or (f) can advantageously take place at least partially at a temperature below room temperature thanks to the cold produced in step (g) .

An example of the method according to the invention is illustrated in the figure. The gas to be treated, called rich gas (GR), arrives via line 1; it is mixed with a gas flow coming from line 13; the total gas flow, circulating in line 23, is brought into contact with a solvent phase coming from line 8, and an addition of solvent coming from line 24. The mixture circulating in line 21 enters the exchanger A in which it is cooled by heat exchange with an external fluid entering through line 25 and emerging through line 26; during this heat exchange, part of the heaviest components of the gas is absorbed in the solvent; the mixture leaves the exchanger A via line 2, and enters the heat exchanger E2 in which it cools; another fraction of the heaviest components of the gas is absorbed in the solvent; the mixture leaves the exchanger E2 via line 17 and enters the tank B2 in which the gas and the liquid are separated; the gas, depleted in heavy constituents, called lean gas (GP), leaves the cylinder B2 via line 19, enters the exchanger E2 in which it heats up by heat exchange and leaves the process through line 16. The liquid phase , called rich solution (SR), consisting of solvent and heavy constituents absorbed, leaves the balloon B2 via line 18, passes through pump P which raises the pressure, enters via line 20 in exchanger E2 in which it heats up by heat exchange, exits through line 3, enters heat exchanger E 1 in which it heats up by heat exchange, exits through line 4, enters heat exchanger G in which it heats up by heat exchange with a hot external fluid which enters the exchanger G via the pipe 29 and exits from it via the pipe 30. The mixture leaves the exchanger G via the pipe 5 partially vaporized; the liquid phase is separated from the vapor phase in the flask B1; the liquid phase, consisting mainly of solvent, called lean solution (SP), leaves the balloon B1 via line 6, enters the exchanger E1 in which it cools by heat exchange, leaves via line 7, passes through the valve V2 undergoing a drop in pressure, and arrives via line 8 to be brought back into contact with the gas; the vapor phase (LPG), consisting mainly of the constituents absorbed by the solvent, leaves the flask 81 via line 9, enters the exchanger E1 in which it cools by heat exchange, leaves via line 22, enters the condenser C in which it cools by heat exchange with a cold external fluid entering through line 27 and emerging through line 28; during this cooling at least partial condensation occurs. The at least partially condensed fluid leaves the exchanger C via the line 10, enters the exchanger E2 in which it cools, which causes total condensation and / or sub-cooling, exits through the line 15, is expanded in valve V1, enters via line 11 into exchanger E1 in which it vaporizes by heat exchange producing cold, exits through line 12 partially vaporized, and enters balloon B3 in which the two liquid phases and steam are separated; the liquid phase, which contains heavy constituents extracted from the treated gas, leaves the process via line 14, the vapor phase leaves the flask B3 through line 13 and is mixed with the gas to be treated coming from line 1.

In this example of the process according to the invention, the absorption of the heavy constituents of the treated gas in the solvent is carried out partially at low temperature; on the other hand, the condensation of the desorbed heavy fractions can be completely carried out at a temperature close to ambient, that is to say that the flow leaving the exchanger C via line 10 can be completely in the liquid state if the pressure in this area of the process is high enough.

The rich gas which can be treated by the process according to the invention is a petroleum gas which can come from a production field (natural gas or associated gas) or be a gas from a refinery or from a petrochemical unit. This gas can contain hydrocarbons, saturated or not, with a branched or cyclic straight chain, such as for example methane, etane, propane, butane, pentane, hexane, ethylene, propylene, butene, acetylene. Heavier constituents may be present, for example heptane, octane, nonane and decane. The higher the proportions of hydrocarbons present the lower their number of carbon atoms. The method according to the invention makes it possible to recover at least part of the hydrocarbons other than methane. The gas may also contain some constituents which are not part of the hydrocarbon family and which are not recovered by the process, such as for example hydrogen, nitrogen, carbon dioxide, hydrogen sulfide, l 'water.

The solvent used in the process according to the invention is chosen so as to allow the absorption of the heavy constituents of the gas; it is preferably characterized by a boiling point at least 50 ° C and preferably at least 100 ° C higher than that of the heaviest component of the heavy fraction of the gas; it can be a pure body or a mixture. It can be chosen from hydrocarbons; in this case the number of carbon atoms is at least equal to 6; the hydrocarbons can be paraffinic, aromatic or naphthenic, and can be chosen, for example, from oils. One or more hydrogen atoms may be substituted by other atoms such as F, CI, Br and the solvent may contain alcohol, aldehyde, ketone, ester, ether, ether, carboxylic acid functions having in particular the formulas R-CH ₂ 0H , R-CHOH-R ', RR'R "C-OH, R-CHO, RR'C = O, R-COO-R', RO-R ', R-COOH in which R, R', and R "denote hydrocarbon radicals which may themselves be partially substituted, the said solvent being able to consist of a mixture of several of these products.

In the process according to the invention, the external cooling fluid circulating in the exchangers A and C can be water or ambient air or a fluid coming from a refrigeration machine external to the process.

In the process according to the invention, certain exchangers can be of multiple flow: 3 for E 1, 5 for E2, but one can also carry out exchanges between fluids two by two.

The method according to the invention can treat gases whose pressure is preferably between 0.1 and 20 MPa. The pressure of steps (a, b and g) is preferably within the same range and that of steps (c, d and f) preferably between 0.2 and 20 MPa, with the additional condition that the pressure of steps ( c, d and f) is at least 0.1 MPa, preferably at least 0.5 MPa, higher than that of steps (a, b and g). In step (e) cooling can precede or follow the pressure reduction: we then go from the pressure of step (d) to that of step (a).

The heat supplied to the process in the exchanger G, to carry out the desorption of the absorbed constituents, is at a temperature level preferably between 100 and 300 ° C.

The cold produced by the vaporization of a part of the absorbed constituents and which serves to cool the absorption of a part of the heavy constituents of the rich gas and / or to condense said heavy constituents reseparated from the solvent is at a temperature level preferably between 10 and -50 ° CV.

The process according to the invention is illustrated by the following example:

Example

In this example, we proceed according to the diagram shown in the figure.

The gas to be treated, called rich gas, enters the process through line 1; its composition is given in Table 1; its temperature is 35 ° C, its pressure is 0.15 MPa absolute, its flow rate is 1184 kg / h. It is mixed with the gas coming from line 13 whose flow rate is 1264 kg / h, and with the solvent coming from line 8 whose flow rate is 6650 kg / h; the makeup of solvent provided by line 24 is 27 kg / h. The solvent used is a paraffinic oil whose normal boiling temperature is between 300 and 350 ° C.

The gas and solvent mixture passes through the exchanger A which is cooled by cooling water; at the outlet of the exchanger A, the mixture is at a temperature of 35 ° C. It enters the exchanger E2, in which it is cooled down to a temperature of -10 ° C. The two liquid and vapor phases are separated in the flask B2; the gas which has not been absorbed, called lean gas and the composition of which is given in Table 1, leaves the cylinder B2 via line 19, passes through the exchanger E2, and leaves the process at a temperature of 30 ° C. via the line 16; its flow rate is 872 kg / h. The solvent phase enriched in absorbed constituents called rich solution, leaves the tank B2 via line 18, passes through the pump P which raises its pressure to 0.74 MPa; it enters the exchanger E2 in which it heats up, exits through line 3, enters the exchanger E1 in which it heats up, exits through line 4, enters the exchanger G in which it heats up by exchange thermal with a hot external fluid, comes out through line 5 at a temperature of 200 ° C, partially vaporized. The two liquid and vapor phases are separated in tank B1; the liquid phase, rich in solvent called lean solution, leaves tank B1 through line 6, enters exchanger E1 in which it cools, exits through line 7, crosses valve V2 undergoing a pressure loss which reduces its pressure substantially to the value of that of the rich gas at the inlet of the process and passes through line 8 to be brought back into contact with the gas to be treated. The gaseous phase of tank B1, exits via line 9, enters exchanger E1 in which it cools, exits via line 22, enters condenser C in which it cools to 35 ° C by heat exchange with cooling water; at the outlet of the condenser C, the fluid is fully condensed, it enters the exchanger E2 in which it is sub-cooled to -11 ° C, expanded in the valve V1, up to approximately 0.15 MPa and partially vaporized in the exchanger E1, producing cold, exits through line 12 at a temperature of 30 ° C, and enters the flask B3 where the two liquid and vapor phases are separated; the liquid phase consisting mainly of constituents extracted from the rich gas, leaves the process via line 14, its composition is given in Table 1. The vapor phase of the flask B3 leaves through line 13 and is mixed with the rich gas.

Claims (8)

1. A process for treating a gas containing a light fraction and a heavy fraction, in order to extract at least a portion of the heavy fraction, the light fraction containing at least one light constituent from the group comprising hydrogen, nitrogen, and hydrocarbons having 1 to 2 carbon atoms and the heavy fraction containing at least one heavy constituent from the group comprising hydrocarbons having 2 to 6 carbon atoms, provided that, when the light fraction contains a hydrocarbon of 2 carbon atoms, the heavy fraction contains at least one hydrocarbon having from 3 to 6 carbon atoms, said process involving an extraction in a solvent liquid phase so selected that the solubility of the heavy fraction in the solvent be higher than that of the light fraction, and that the solvent liquid be substantially undistillable under the operating conditions of the process, said process being characterized by the steps of:

a) contacting the gas with the solvent liquid phase so as to absorb to a major extent at least a portion of the heavy fraction of the gas in the liquid phase and to decrease to a major extent the heavy fraction content of the gas, and releasing the resultant heat,

b) fractionating the product from step (a) to separately recover the resultant liquid phase and the unabsorbed gas,

c) increasing the pressure of the liquid phase separated in step (b) and sufficiently increasing its temperature to desorb at least a portion of the gas absorbed therein in step (a),

d) fractionating the product from step (c) to separately recover a lean solution and a desorbed gas phase,

e) cooling said lean solution, decreasing its pressure and feeding it back to step (a) to reconstitute at least a portion of the solvent liquid phase,

f) cooling the desorbed gas phase so as to condense at least a portion thereof and recovering the resultant condensate,

g) expanding the condensate and supplying heat to said expanded condensate so as to epavorate at least a portion thereof, and recovering the resultant product of higher content of heavy constituent, as compared with the initial gas, the process being further characterized in that at least a portion of the heat released in step (a) forms at least a portion of the heat supplied to step (g).

2. A process according to claim 1, characterized in that step (a) is conducted by transferring released heat, first partially to an external cooling fluid and then, for at leat a part of the remaining portion, to step (g).

3. A process according to claim 1 or 2, characterized by a partial evaporation in step (g) and by the recovery of a relatively heavy unevaporated liquid phase and a relatively light evaporated gas phase.

4. A process according to claim 3, characterized in that the relatively light gas phase is fed back to step (a) to be contacted again with the solvent liquid phase.

5. A process according to one of claims 1 to 4, wherein the solvent is a paraffinic, aromatic or naphthenic hydrocarbon of at least 6 carbon atoms, a hydrocarbon oil, a halogenated hydrocarbon, an alcohol of formula R-CH₂-OH, R-CHOH-R', or RR'R"C-OH, an aldehyde of formula R-CHO, a ketone of formula RR' C = 0, an ester of formula R-COO-R', an ether of formula R-O-R', an organic acid of formula R-COOH, or a mixture thereof, R, R' and R" being substituted or unsubstituted hydrocarbon radicals.

6. A process according to one of claims 1 to 5, characterized in that the pressure in steps a, b and g ranges from 0.1 to 20 MPa and the pressure in steps c, d and f ranges from 0.2 to 20 MPa, with the additional condition that the pressure in steps c, d and f is at least 0.1. MPa higher than that of steps a, b and g.

7. A process according to one of claims 1 to 6, characterized in that the temperature in step (c) ranges from 100 to 300°C.

8. A process according to one of claims 1 to 7, characterized in that the vaporisation temperature in step (g) ranges from + 10 to -50°C.

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