METHOD FOR OPTIMISING LIQUEFACTION OF NATURAL GAS

Abstract

A method for liquefying a hydrocarbon stream such as natural gas starting from a feed stream.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International Application PCT/FR2016/052024 filed Aug. 3, 2016, which claims priority to French Patent Application 1560731 filed Nov. 10, 2015, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a method for liquefying a hydrocarbon stream such as natural gas in particular in a method for producing liquefied natural gas. At typical plants for liquefaction of natural gas using a mixed refrigerant cascade, refrigerant streams are used for producing cold at different levels of a main heat exchanger by evaporating against the hydrocarbon stream to be liquefied (typically natural gas).

Liquefaction of natural gas is desirable for a number of reasons. For example, natural gas can be stored and transported over great distances more easily in the liquid state than in gaseous form, as it occupies a smaller volume for a given mass and does not need to be stored at high pressure.

Several methods of liquefaction of a natural gas stream for obtaining liquefied natural gas (LNG) are known. Typically the mixed refrigerant is compressed by means of a compressor and separated into a gas stream and at least one liquid stream, then the two streams are combined to form a two-phase stream. This two-phase stream is fed into the main heat exchanger, where it is liquefied completely and subcooled to the coldest temperature of the process, typically that of the stream of liquefied natural gas. At the coldest outlet of the main heat exchanger, the refrigerant is expanded and fed back into the main exchanger, to be evaporated against the hydrocarbon-rich fraction that is being liquefied.

This solution is not optimized, owing to the two-phase composition of the refrigerant stream once the two phases are recombined and introduced in this state into the exchanger. In fact the liquid refrigerant stream contains the heaviest compounds. The latter will therefore evaporate at a higher temperature than lighter compounds such as nitrogen or methane, for example. It is therefore used for producing cold at an intermediate temperature (typically of the order of −30° C. to −50° C., for precooling and partial liquefaction of the hydrocarbon mixture to be liquefied).

As the gaseous refrigerant stream contains the lightest compounds, it is used for producing cold at a colder temperature (typically below −100° C.), for liquefaction and total subcooling of the hydrocarbon mixture to be liquefied.

Therefore it is not necessary for the liquid refrigerant to be subcooled as much as the gaseous refrigerant before being expanded and evaporated against the hydrocarbon stream to be liquefied. Now, this is what the typical method of the prior art proposes, as described in the preceding paragraph.

Moreover, patent application US2009/0260392 A1 describes the liquefaction of a hydrocarbon-rich fraction against a mixed refrigerant, this refrigerant stream being separated in a phase separator into a gas phase and a liquid phase following a step of compression and cooling of said mixed refrigerant. Next, the two phases of the refrigerant are cooled separately and then recombined only after the two phases have been expanded. Once recombined, these two phases are fed into the exchanger again in the form of a two-phase stream and heated against the natural gas that is being liquefied. This “heating” occurs, both for the liquid phase of the refrigerant and for the gas phase, once these streams of the refrigerant have been expanded.

The inventors of the present invention then developed a solution for solving the problem described above while optimizing the energy expenditure.

SUMMARY

The solution proposed is to present the liquid refrigerant stream and the gaseous refrigerant stream separately in the main heat exchanger. The liquid is then cooled to an intermediate temperature level, whereas the gas is liquefied and cooled as far as the coldest outlet of the main heat exchanger. The liquefied gaseous refrigerant is then expanded and fed back into the main heat exchanger. It is mixed with the cooled liquid refrigerant and also expanded beforehand, once it has reached the correct temperature level.

The present invention relates to a method for liquefying a hydrocarbon stream such as natural gas starting from a feed stream, comprising at least the following steps:

Step a): passing the feed gas against a mixed refrigerant stream through a heat exchanger to supply an at least partially liquefied hydrocarbon stream having a temperature below −140° C.;

Step b): withdrawing a mixed refrigerant stream from the heat exchanger from an outlet where the temperature in the heat exchanger is highest;

Step c): feeding the mixed refrigerant from step b) into a phase separating means in order to produce a gaseous refrigerant stream and a first liquid refrigerant stream;

Step d): passing the first liquid refrigerant stream resulting from step c) into the heat exchanger starting from a first inlet and up to a so-called intermediate outlet, beyond which the refrigerant stream thus obtained is expanded, the temperature T1 at said outlet being such that said expansion produces a gas fraction below 20%, preferably below 10%;

Step e): in parallel with step d), compressing the gaseous refrigerant stream resulting from step c) and then cooling before feeding the refrigerant stream thus obtained into a phase separating means in order to produce a gaseous refrigerant stream and a second liquid refrigerant stream;

Step f): passing the second liquid refrigerant stream resulting from step e) through the heat exchanger starting from a second inlet and up to an outlet, beyond which the refrigerant stream thus obtained is expanded, the temperature T2 at said outlet being above T1 and such that said expansion produces a gas fraction below 20%, preferably below 10%;

Step g): passing the gaseous refrigerant stream resulting from step e) through the heat exchanger starting from a third inlet and up to an outlet at a temperature T3, the level of which is the lowest of the temperature levels of said heat exchanger in order to produce a liquefied stream, and then expansion of the stream thus obtained;

Step h): passing the stream resulting from step g) through the heat exchanger from an inlet at a temperature T3 to an outlet at a temperature approximately equal to the temperature T2;

Step i): mixing the refrigerant stream resulting from step h) with the refrigerant stream resulting from step f), and then passing the mixture thus obtained through the heat exchanger from an inlet having a temperature approximately equal to T2 to an outlet having a temperature approximately equal to T1;

Step j): mixing the refrigerant stream resulting from step i) with the refrigerant stream resulting from step d) and then passing the mixture thus obtained through the heat exchanger up to the outlet.

More particularly, the present invention relates to:

• • A method as defined above, characterized in that the mixed refrigerant stream circulates in a closed-cycle refrigeration circuit.
  • A method as defined above, characterized in that it comprises a step before step c) of compressing the mixed refrigerant resulting from step b) followed by cooling.
  • A method as defined above, characterized in that T1 is between −30° C. and −50° C.
  • A method as defined above, characterized in that T2 is between −80° C. and −110° C.
  • A method as defined above, characterized in that T3 is between −140° C. and −170° C.
  • A method as defined above, characterized in that the mixed refrigerant stream contains constituents among nitrogen, methane, ethylene, ethane, butane and pentane.
  • A method as defined above, characterized in that the gaseous refrigerant stream resulting from step e) contains nitrogen and methane.
  • A method as defined above, characterized in that a pump is not used.

The method according to the present invention makes it possible to optimize the use of the liquid and gaseous refrigerant streams in the liquefaction cycle, since the liquid, which contains the heaviest components, must not be subcooled as much as the gaseous refrigerant.

Moreover, a pump is not used in the method according to the invention, as the intermediate liquid (in the foregoing called the first liquid refrigerant stream resulting from step c)) is not pumped in order to be mixed with the liquid at high pressure (in the foregoing called the second liquid refrigerant stream resulting from step e).

This is notably advantageous in terms of capital expenditure.

Although the method according to the present invention is applicable to various hydrocarbon feed streams, it is particularly suitable for streams of natural gas to be liquefied. Furthermore, a person skilled in the art will easily understand that, after liquefaction, the liquefied natural gas may be further processed, if desired. As an example, the liquefied natural gas obtained may be depressurized by means of a Joule-Thomson valve or by means of a turbine. Furthermore, other intermediate processing steps between gas/liquid separation and cooling may be carried out. The hydrocarbon stream to be liquefied is generally a stream of natural gas obtained from natural gas or oil reservoirs. Alternatively, the stream of natural gas may also be obtained from another source, including a synthetic source such as a Fischer-Tropsch process. Usually the stream of natural gas consists essentially of methane. Preferably, the feed stream comprises at least 60 mol % of methane, preferably at least 80 mol % of methane. Depending on the source, the natural gas may contain amounts of hydrocarbons heavier than methane, such as ethane, propane, butane and pentane as well as certain aromatic hydrocarbons. The stream of natural gas may also contain non-hydrocarbon products such as H₂O, N₂, CO₂, H₂S and other sulfur-containing compounds, and others.

The feed stream containing natural gas may be pretreated before it is fed into the heat exchanger. This pretreatment may comprise reduction and/or removal of the undesirable components such as CO₂ and H₂S, or other steps such as precooling and/or pressurizing. Since these measures are well known by a person skilled in the art, they are not described in more detail here.

The expression “natural gas” as used in the present application refers to any composition containing hydrocarbons, including at least methane. This comprises a “crude” composition (before any treatment such as cleaning or washing), as well as any composition that has been treated partially, substantially or entirely for reduction and/or removal of one or more compounds, including, but not limited to, sulfur, carbon dioxide, water, and hydrocarbons having two or more carbon atoms.

The separator may be any unit, column or arrangement suitable for separating the mixed refrigerant in a stream of refrigerant in vapor form and a stream of liquid refrigerant. Such separators are known in the prior art and are not described in detail here.

The heat exchanger may be any column, a unit or other arrangement suitable for allowing the passage of a certain number of streams, thus allowing direct or indirect heat exchange between one or more lines of refrigerant, and one or more feed streams.

BRIEF DESCRIPTION OF THE DRAWING

For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawing, in which like elements are given the same or analogous reference numbers and wherein:

The sole FIGURE illustrates one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will be described in more detail with reference to the FIGURE, which illustrates the scheme of a particular embodiment of an implementation of a method according to the invention.

In the FIGURE, a stream 1 of natural gas, optionally pretreated beforehand (typically having undergone separation of part of at least one of the following constituents: water, CO₂, methanol, sulfur-containing compounds), is fed into a heat exchanger 2 in order to be liquefied.

The FIGURE therefore shows a method for liquefying a feed stream 1. The feed stream 1 may be a pretreated stream of natural gas, in which one or more substances, such as sulfur, carbon dioxide, and water, are reduced, so as to be compatible with cryogenic temperatures, as is known in the prior art.

Optionally, the feed stream 1 may have undergone one or more steps of precooling, as is known in the prior art. One or more of the precooling steps may comprise one or more refrigeration circuits. As an example, a feed stream of natural gas is generally treated starting from an initial temperature of 30-50° C. Following one or more steps of precooling, the temperature of the feed stream of natural gas may be reduced to −30 to −70° C.

In the FIGURE, the heat exchanger 2 is preferably a coil-wound cryogenic heat exchanger. Cryogenic heat exchangers are known in the prior art, and may have various arrangements of the feed stream(s) and refrigerant streams. Furthermore, heat exchangers of this kind may also have one or more lines to allow the passage of other streams, such as refrigerant streams for other steps of a method of cooling, for example in methods of liquefaction. These other lines or streams are not shown in the FIGURE, for simplicity.

The feed stream 1 enters the heat exchanger 2 via a feed inlet 3 and passes through the heat exchanger via line 4, and then is withdrawn from the exchanger at outlet 5 to supply an at least partially liquefied hydrocarbon stream 6. This liquefied stream 6 is preferably liquefied completely and even subcooled, and may moreover be treated as discussed below. When the liquefied stream 6 is liquefied natural gas, the temperature may be from about −150° C. to −160° C. Liquefaction of the feed stream 1 is accomplished by means of a refrigerant circuit 7. A mixed refrigerant, preferably selected from the group comprising nitrogen, methane, ethane, ethylene, propane, propylene, butane, pentane, etc., circulates in the refrigerant circuit 7. The composition of the mixed refrigerant may vary according to the conditions and the parameters desired for the heat exchanger 2, as is known in the prior art.

In the arrangement of the operation of the heat exchanger 2 shown in the FIGURE, a gaseous refrigerant stream 8 is fed into the exchanger 2 at an inlet 9, then it passes through this inlet and is liquefied and subcooled along line 10 through the heat exchanger 2, up to the outlet 11. The temperature T3 of the outlet 11 is the lowest of the temperatures of the heat exchanger 2. T3 is typically between −140° C. and −170° C., for example −160° C. During its passage through line 10, the stream of gaseous refrigerant 8 is liquefied, so that the refrigerant stream downstream of the outlet 11 is a liquid stream 12. The refrigerant stream 12 is then expanded for example by means of a valve 13, so as to supply a first refrigerant stream at reduced pressure 14. This stream 14 is then fed into the heat exchanger 2 via the inlet 15.

A liquid stream 16 of the refrigerant is fed into the heat exchanger 2 via inlet 17, and then passes through the exchanger 2 along line 18. The liquid stream of refrigerant 16 is withdrawn from the exchanger at outlet 19, at an intermediate level between the top and the bottom of said exchanger, having a temperature T2 above T3. For example, T2 is between −90° C. and −110° C. The refrigerant stream 20 downstream of the outlet 19 is expanded in a pressure reducing valve 21, to form a second stream of refrigerant at reduced pressure 22. The stream 22 then goes, via inlet 23, into the heat exchanger 2 again, and travels as far as the outlet 24 of the heat exchanger.

Another liquid stream 25 of the refrigerant is fed into the heat exchanger 2 via inlet 26, and then passes through the exchanger 2 along line 27. The liquid stream of refrigerant 25 is withdrawn from the exchanger at outlet 28, at an intermediate level between the top and the bottom of said exchanger, having a temperature T1 above T2. For example, T1 is between −30° C. and −50° C. The refrigerant stream 29 downstream of the outlet 28 is expanded in a pressure reducing valve 30, to form a third stream of refrigerant at reduced pressure 31. Preferably, the pressures of the first, of the second and of the third refrigerant at reduced pressure 14, 22 and 31 are approximately the same; for example about 3 bara.

Once it has entered the heat exchanger 2, the stream 14 of refrigerant evaporates, at least partially, up to the outlet 34, then downstream of this outlet 34 it will rejoin stream 22 resulting from expansion of the cooled liquid stream 16 of the refrigerant, and the two streams are then mixed in stream 22. Similarly, this refrigerant stream 22 is mixed with refrigerant stream 31 downstream of outlet 24.

Stream 31 then passes, via inlet 32, into the heat exchanger 2 again and evaporates completely up to the outlet 33 of the heat exchanger. A gaseous refrigerant stream 35 circulates in the refrigeration circuit 7 downstream of the outlet 33 of the heat exchanger at ambient temperature (i.e. the temperature measured in the space where the device for implementing the method according to the present invention is placed. This temperature is for example between −20° C. and 45° C.). The refrigerant stream is compressed by a compressor 36. The method of compression is known from the prior art and the compressor 36 is for example a compressor with at least two adiabatic sections A and B, therefore comprising at least two coolers 37 and 38. Once compressed in the first section A of the compressor 36, the refrigerant stream 35 is cooled by means of a cooler 37 and is then partially condensed and forms a two-phase refrigerant stream 39. For example, the pressure at the outlet of section A of the compressor 36 is of the order of 18 bara and the temperature is of the order of 130° C. Typically the temperature at the outlet of the cooler 37 is of the order of 25° C.

The refrigerant stream 39 is sent to a phase separator 40, which separates said two-phase refrigerant stream into a gas stream 41 and a first liquid stream 25. Said first liquid refrigerant stream 25 consists of the heaviest elements of the refrigerant stream of the refrigeration circuit 7, i.e. in particular the components having more than four carbon atoms. The liquid refrigerant stream 25 then follows the path described above starting from the inlet 26 of heat exchanger 2.

The gaseous refrigerant stream 41 is compressed in section B of the compressor. Typically, the pressure at the outlet of this section B is of the order of 50 bara. After this compression, the refrigerant stream is partially condensed by means of the cooler 38 and forms a two-phase refrigerant stream 42. Typically the temperature is at the level of the ambient temperature. The refrigerant stream 42 is sent to a phase separator 43, which separates said refrigerant stream into a gas stream 8 and a second liquid stream 16. Said second liquid refrigerant stream 16 consists of the elements that are lighter than those contained in the liquid 25 but heavier than those contained in the gas stream 8. This liquid refrigerant stream 16 then follows the path described above starting from the inlet 17 of heat exchanger 2. The gaseous refrigerant stream 8 then follows the path described above starting from the inlet 9 of heat exchanger 2. This gaseous refrigerant stream 8 contains the lightest elements of the refrigerant stream of the refrigeration circuit 7, i.e. typically nitrogen and methane.

“Temperature approximately equal to” another temperature means the same temperature ±5° C.

The liquefied natural gas 6 resulting from the method according to the present invention may then, for example, be transferred to a storage or transport device.

The method according to the present invention notably offers the following advantages:

• • Energy optimization of the refrigeration cycle. In fact, the liquid refrigerant streams are not subcooled more than is necessary (typically characterized by correspondence between the temperature of withdrawal from the exchanger at points 20 and 28), and the composition of the evaporated refrigerant stream (having the lightest components) at the coldest outlet of the main heat exchanger is improved.
  • Optimization of capital expenditure in particular by reducing the size of the exchanger performing liquefaction of the hydrocarbon-rich fraction, as a pump is not used in the refrigeration circuit.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.

Claims

1.-8. (canceled)

9. A method for liquefying a hydrocarbon stream such as natural gas starting from a feed stream comprising at least the following steps:

Step a): passing the feed gas against a mixed refrigerant stream through a heat exchanger to supply an at least partially liquefied hydrocarbon stream having a temperature below −140° C.;

Step b): withdrawing a mixed refrigerant stream from the heat exchanger from an outlet where the temperature in the heat exchanger is highest;

Step c): introducing the mixed refrigerant resulting from step b) into a phase separating means in order to produce a gaseous refrigerant stream and a first liquid refrigerant stream;

Step d): passing the first liquid refrigerant stream resulting from step c) in the heat exchanger starting from a first inlet and up to a so-called intermediate outlet, beyond which the refrigerant stream thus obtained is expanded, the temperature T1 at said outlet being such that said expansion produces a gas fraction below 20%;

Step e): in parallel with step d), compressing the gaseous refrigerant stream resulting from step c) and then cooling before introducing the refrigerant stream thus obtained into a phase separating means in order to produce a gaseous refrigerant stream and a second liquid refrigerant stream;

Step f): passing the second liquid refrigerant stream resulting from step e) in the heat exchanger starting from a second inlet and up to an outlet, beyond which the refrigerant stream thus obtained is expanded, the temperature T2 at said outlet being above T1 and such that said expansion produces a gas fraction below 20%;

Step g): passing the gaseous refrigerant stream resulting from step e) in the heat exchanger starting from a third inlet and up to an outlet at a temperature T3, the level of which is the lowest of the temperature levels of said heat exchanger in order to produce a liquefied stream, and then expanding the stream thus obtained;

Step h): passing the stream resulting from step g) in the heat exchanger from an inlet at a temperature T3 up to an outlet at a temperature approximately equal to the temperature T2;

Step i): mixing the refrigerant stream resulting from step h) with the refrigerant stream resulting from step f), then passing the mixture thus obtained in the heat exchanger from an inlet having a temperature approximately equal to T2 up to an outlet having a temperature approximately equal to T1;

Step j): mixing the refrigerant stream resulting from step i) with the refrigerant stream resulting from step d) and then passing the mixture thus obtained in the heat exchanger up to the outlet.

10. The method as claimed in claim 9, wherein the mixed refrigerant stream circulates in the closed-cycle refrigeration circuit.

11. The method as claimed in claim 9, further comprising a step before step c) of compressing the mixed refrigerant resulting from step b) followed by cooling.

12. The method as claimed in claim 9, wherein T1 is between −30° C. and −50° C.

13. The method as claimed in claim 9, wherein T2 is between −80° C. and −110° C.

14. The method as claimed in claim 9, wherein T3 is between −140° C. and −170° C.

15. The method as claimed in claim 9, wherein the mixed refrigerant stream contains constituents selected from the group consisting of nitrogen, methane, ethylene, ethane, butane and pentane.

16. The method as claimed in claim 9, wherein a pump is not used.