EP0535752B1 - Method for liquefying natural gas - Google Patents

Description

The invention relates to a natural gas liquefaction process comprising the separation of hydrocarbons heavier than methane.

Natural gas and other methane-rich gas streams are commonly available at sites away from places of use, and it is therefore common to liquefy natural gas for transportation by land or sea. Liquefaction is widely practiced in the present day and the literature and patents describe numerous processes and apparatuses for liquefaction. US-A-3945 214, 4251 247, 4274 849, 4339 253 and 4539 028 are examples of such methods.

It is also known to fractionate the streams of light hydrocarbons, containing for example methane and at least one higher hydrocarbon such as ethane to hexane or higher, by cryogenics.

Thus, US Pat. No. 4,690,702 describes a process in which the charge of hydrocarbons under high pressure (P₁) is cooled so as to cause the liquefaction of part of the hydrocarbons, a gas phase (G₁) is separated from a liquid phase (L₁), the gas phase is relaxed (G₁) to lower its pressure to a value (P₂) lower than (P₁), the liquid phase (L₁) and the gas phase (G₁) are sent under pressure (P₂) in a first fractionation zone, for example a purification column - contact refrigeration, a residual gas (G₂) rich in methane is drawn off at the head, the pressure of which is then raised to a value (P₃), it is withdrawn in melts a liquid phase (L₂), the phase (L₂) is sent to a second fractionation zone, for example a fractionation column, a liquid phase is drawn off at the bottom (L₃), enriched in higher hydrocarbons, for example C₃ +, a gas phase (G₃) is drawn off at the head, at least part of the gas phase (G₃) is condensed and at least part of the resulting condensed liquid phase is sent (L₄) as additional feed at the head of the first fractionation zone. In this method, the second fractionation zone operates at a pressure (P₄) greater than the pressure of the first fractionation zone, for example 0.5 MPa for the first zone and 0.66 MPa for the second zone.

Advantageously, in the aforementioned method, the expansion of G₁ takes place in a turboexpander which transmits at least part of the energy collected to a turbocharger which raises the pressure of G₂ to the value P₃.

The advantage of such a process is to collect, with a high yield, condensates such as C₃, C₄, petrol, etc. which are valuable products.

It has already been proposed to associate a natural gas fractionation unit with a liquefaction unit, so as to be able to collect both liquid methane and condensates such as C₃, C₄ and / or higher. Such proposals are made for example in US-A-3,763,658 and US-A-4,065,278, the liquefaction unit can be of a conventional type.

The difficulty to be overcome in this type of installation is to obtain a reduced operating cost. In particular, it is inevitable to collect the recompressed gas under a pressure (P₃) lower than that (P₁) under which it was initially, unless consuming additional energy. Now the subsequent liquefaction of methane is all the easier as its pressure is higher.

There is therefore room in the art for an economical process for fractionating hydrocarbons from natural gas and subsequent liquefaction of methane.

The process of the invention is distinguished, for its fractionation part, from the process of US-A-4 690 702, in that the pressures used in the fractionation zones are higher than those previously used and in that the second fractionation zone operates at a lower pressure than the first fractionation zone.

Furthermore, US Pat. No. 4,657,571 discloses a process corresponding to the preamble of claim 1.

According to the invention, the gaseous charge of hydrocarbons containing methane and at least one hydrocarbon heavier than methane, under pressure P₁, is cooled in one or more stages so as to form at least one gaseous phase G₁, the gas phase G₁ to lower its pressure from the value P₁ to a value P₂ lower than P₁, the pressure reduction product P la is sent to a first contact fractionation zone, a residual gas G₂ enriched in methane is drawn off at the head , a liquid phase L₂ is drawn off at the bottom, the liquid phase L₂ is sent to a second fractionation zone by distillation operating under a pressure P₄ lower than the pressure P₂ of the first fractionation zone, it is drawn off at the bottom thereof at least one liquid phase L₃, enriched in hydrocarbons heavier than methane, a gas phase G₃ is drawn off at the head and it is returned to the first fractionation zone as reflux.

According to the characteristic of the invention, at least part of the gas phase G₃ is condensed, to produce a condensed phase L₄ and the pressure of at least part of the condensed phase L₄ is raised, which is sent to the first zone fractionation as reflux and the residual gas G₂ is then further cooled under a pressure at least equal to P₂; in a methane liquefaction zone, so as to obtain a methane-rich liquid.

By way of example, the gas is initially available under a pressure P₁ of at least 5 MPa, preferably at least 6 MPa.

During expansion, its pressure is usefully brought to a value P₂ such that P₂ = 0.3 to 0.8 P₁, P₂ being chosen for example between 3.5 and 7 MPa, preferably between 4.5 and 6 MPa. The pressure P₄ of the second fractionation zone is advantageously such that P₄ = 0.3 to 0.9 P₂, P₄ having a value for example between 0.5 and 4.5 MPa, preferably between 2.5 and 3, 5 MPa.

Several embodiments can be implemented.

According to a preferred embodiment, the expansion of G₁ is done in one or more turboexpanders coupled (s) to one or more turbocharger (s) which recompresses (s) the waste gas G₂ from the pressure P₂ to a pressure P₃.

According to another preferred embodiment, during the initial cooling of the gas, at least one liquid phase L₁ is formed in addition to the gas phase G₁, and the liquid phase L₁ is sent, after expansion, to said first fractionation zone by contact.

According to another variant, the gas phase G₃ is completely condensed and a portion is sent to the second fractionation zone as internal reflux and the complement to the first fractionation zone as reflux. To achieve this result, one can act on the reboiler of the first fractionation zone, so as to control the ratio C rapport / C₂ of the liquid phase L₃.

If the cooling of the G₃ phase is not sufficient to fully condense this phase, which is preferred, the condensation can be completed by further compressing, with subsequent cooling of the said G₃ phase.

The invention is illustrated by the attached figure. The natural gas from line 1 passes through one or more exchangers 2, for example of the propane or C mélange / C₃ liquid mixture type, and advantageously one or more exchangers using cold process fluids. Preferably, the cold fluid comes from line 5 of the first contact column 7. The gas, which here is partially liquefied, is fractionated in the flask 4 into liquid conveyed to column 7 via line 6 equipped with a valve. V₁ and in gas supplied by line 8 to the turboexpander 9. The expansion causes partial liquefaction of the gas and the product of the expansion is sent by line 10 to column 7. This column is of a conventional type, for example at trays or filled. It includes a reboiling circuit 11. The liquid effluent from the bottom of the column is expanded by the valve 12 and sent by line 13 to column 14. This column, which operates at a lower pressure than column 7, has a reboiler 15. The liquid effluent, enriched in hydrocarbons higher than methane, for example C₃ +, leaves via line 16. At the top, the vapors are partly or completely condensed in the condenser 17. The phase resulting liquid is returned at least in part to column 14 as reflux by line 18. The gas phase (line 19 and valve V₂) is then condensed, preferably entirely, by cooling preferably in the exchanger 20 supplied by minus part of the residual gas at the top of column 7 (lines 21 and 22).

As a variant, the valve V₂ is closed if all of the vapor phase has been condensed in 17. The valve V₃ is open and it is then the liquid phase which is sent to column 7 via line 19a. You can also open the 2 valves V₂ and V₃ and thus send a mixed phase.

The liquid phase resulting from the cooling in the exchanger 20 passes into the balloon 23, the recompression pump 24 and returns to column 7 via line 25 as reflux. If the condensation in the exchanger 20 is not total, which is less preferred, the residual gas can be evacuated by line 26. The waste gas coming from the head of column 7 by line 21, in the form of the aforementioned embodiment, passes through the exchanger 20 before being sent to the turbocharger 27 via the lines 28 and 29. The turbocharger is driven by the turboexpander 9.

Alternatively, at least part of the waste gas from line 21 is sent via line 30 to the exchanger 3 to cool the natural gas. It then joins the turbocharger 27 via lines 5 and 29.

In another variant not shown, the waste gas (line 21) passes successively through the exchangers 20 and 3, or vice versa, before joining the turbocharger 27.

Other arrangements can be provided, as will be understood by specialists, making it possible to ensure the necessary cooling of the gas from lines 1 and 19. It is possible, for example, to directly send the gas from line 21 to the compressor 27 via line 31 and provide different cooling for exchangers 3 and 20.

After recompression in the turbocharger 27, the gas is sent via line 32, which may include one or more exchangers, not shown, to a conventional methane liquefaction unit, shown here in a simplified manner. It passes through a first cooling exchanger 33, then the expansion valve V₄ and a second cooling exchanger 34 where the liquefaction and subcooling are completed. The refrigerant circuit, of conventional or improved type (one can for example use the circuit of US-A-4 274 849) is shown here by the use of a multi-component fluid for example a mixture of nitrogen, methane , ethane and propane, initially in the gaseous state (line 35), which is compressed by one or more compressors such as 36, cooled by the external medium, air or water, in one or more exchangers such as 37, further cooled in exchanger 38, for example with propane or a C₂ / C₃ liquid mixture. The partially condensed mixture reaches the flask 40 through line 39. The liquid phase passes through line 41 in the exchanger 33, is expanded by the valve 42 and returns to line 35 through the exchanger 33 where it heats up in cooling the streams 32 and 41. The vapor phase of the tank 40 (line 43) passes through the exchangers 33 and 34, where it is condensed, then is expanded in the valve 44 and passes through the exchangers 34 and 33 via the lines 45 and 35.

In summary form, the liquefaction of methane is carried out by indirect contact with one or more fractions of a multicomponent fluid during vaporization and circulating in a closed circuit comprising compression, cooling with liquefaction giving one or more condensates and vaporization. said condensates constituting said multicomponent fluid.

By way of nonlimiting example, a natural gas having the following composition is treated, in mol%: Methane 90.03 Ethane 5.50 Propane 2.10 C₄ - C₆ 2.34 Mercaptans 0.03 100.00
under a pressure of 8 MPa.

After cooling with liquid propane and with the top effluent from column 7, the gas reaches the flask 4 at the temperature of -42 ° C. The liquid phase is sent via line 6 to column 7, and the gaseous phase expanded by the turboexpander up to 5 MPa. The liquid phase collected (line 13) at the temperature of + 25 ° C. is expanded to 3.4 MPa in the valve 12 and then fractionated in column 14 which receives the reflux from line 18. This column 14 has a temperature bottom temperature of 130 ° C and a head temperature of -13 ° C.

Reductory gas leaves column 7 at -63 ° C and it is directed partly towards the exchanger 3 and partly towards the exchanger 20. After recompression in 27 using only the energy of the turboexpander 9, the gas pressure is 5.93 MPa. This gas, the temperature of which is -28 ° C., has the following% molar composition: Methane 93.90 Ethane 5.51 Propane 0.53 C₄ - C₆ Mercaptans less than 10 ppm 0.06 100.00

This current represents 95.88 mol% of the load current of the installation.

It can be seen that the installation made it possible to eliminate almost all of the mercaptans from the gas to be liquefied.

Liquefaction takes place as follows:
The gas is cooled and condensed to -126 ° C in a first bundle of the heat exchanger 33 then expanded to 1.4 MPa and sub-cooled in a second bundle of the heat exchanger 34 to at -160 ° C. From there it is sent to storage.

The refrigerant has the following molar composition: N2 7% Methane 38% Ethane 41% Propane 14%

This fluid is compressed to 4.97 MPa, cooled to 40 ° C in a water exchanger 37, then cooled to -25 ° C in the exchangers represented diagrammatically by 38 in indirect contact with a liquid mixture C₂ / C₃, then fractionated in the separator 40 giving the liquid 41 and gas 43 phases. The gas phase is condensed and cooled to -126 ° C in a second bundle of the exchanger 33 then sub-cooled to -160 ° C in a bundle of the exchanger 34. After expansion to 0.34 MPa, it is used to cool the natural gas and returns to the compressor 36 after having passed through the shell of each of the exchangers 34 and 33 and having received the liquid current from the line 41 which passed through the valve 42 after being sub-cooled to -126 ° C in 33.

At the compressor inlet (line 35), the pressure is 0.3 MPa and the temperature is -28 ° C.

By way of comparison, all other things being substantially equal, when column 7 is operated at 3.3 MPa with a temperature of + 1 ° C at the bottom and -64 ° C at the head and column 14 at 3.5 MPa , with a temperature of 131 ° C. at the bottom and -11.7 ° C. at the head, that is to say under conditions which are deduced from the teaching of the patent US Pat. No. 4,690,702, already named, the gas pressure at the outlet of the turbocharger 27 reaches only 5.33 MPa, and the temperature -24 ° C, which is much less advantageous for the subsequent liquefaction and will require a much higher energy expenditure.

Claims (9)

1. Method for liquefying natural gas, according to which said gas including methane and a hydrocarbon heavier than the methane under a pressure P₁, is cooled in view to form at least a gaseous phase G₁, one decreases the pressure of the gaseous phase G₁ to lower its pressure and to bring it to a value P₂ which is smaller than P₁, one sends the product of the pressure decrease at a pressure P₂ in a first zone of fractionation by contact, one draws off at the head a rest gas G₂ enriched with methane, one draws off at the bottom a liquid phase L₂, one sends the liquid phase L₂ in a second zone for fractionation by distillation working at a pressure P₄ lower than the pressure P₂ of the first fractionation zone, one draws off at the bottom of the second fractionation zone at least a liquid phase L₃ enriched with hydrocarbons which are heavier than the methane, one draws off at the head of the second fractionation zone a gaseous phase G₃ and send it back to the first fractionation zone as flow back, characterized in that one performs said flow-back by condensating at least a part of the gaseous phase G₃ in view to produce a condensated phase L₄ and by increasing the pressure of at least a part of the condensated phase L₄ one sends as flow-back to the first fractionation zone, and in that one cools then further the residual gas G₂ at a pressure at least equal to P₂, in a methane liquefaction zone, in view to get a liquid enriched with methane.
2. Method according to claim 1, wherein one performs the pressure decrease of the gaseous phase G₁ in a turbo pressure reducing means and performs the pressure increase of the residual gas from the value P₂ to a value P₃ in a turbocompressor and uses the energy furnished by the pressure decrease for actioning the turbocompressor.
3. Method according to claim 1 or 2, wherein the pressure P₁ is at least equal to 5 MPa, the pressure P₂ is such that P₂ = 0.3 to 0.8 P₁ with P₂ between 3.5 and 7 MPa, and the pressure P₄ is such that P₄ = 0.3 to 0.9 P₂, with P₄ between 0.5 and 4.5 MPa.
4. Method according to claim 3, wherein P₁ is at least equal to 6 MPa, P₂ being between 4.5 and 6 MPa and P₄ being between 2.5 and 3.5 MPa.
5. Method according to any of claims 1 to 4, wherein at least a part of the residual gas G₂ exchanges heat with the natural gas in view to contribute to the cooling thereof, before the increase of the pressure of the gas G₂ from P₂ to P₃.
6. Method according to any of claims 1 to 5, wherein at least a part of the residual gas G₂ exchanges heat with at least a part of the gaseous phase G₃ for cooling the same and produce the condensed phase L₄.
7. Method according to any of claims 1 to 6, wherein the liquefying of the methane is performed by putting into indirect contact with one or a multitude of fractions of a multicomponent fluid during the vaporization and circulation in a closed circuit comprising a compression zone, a cooling zone with liquefaction producing one or a multitude of condensates and a vaporization zone of said condensates for reconstituting said multicomponent fluid.
8. Method according to any of claims 1 to 7, wherein during the initial cooling of the gas, one forms at least one liquid phase L₁ supplementary to the gaseous phase G₁ and sends the liquid phase L₁ after pressure decrease in said first fractionation zone.
9. Method according to any of claims 1 to 8, wherein one condenses completely the gaseous phase G₃ and sends a part thereof to the second fractionation zone as internal flow-back and the complement to the first fractionation zone as flow-back.

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