METHOD FOR OPTIMISING LIQUEFACTION OF NATURAL GAS

Abstract

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

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.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0039] 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:

[0040] The sole FIGURE illustrates one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0041] 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.

[0042] 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, CO.sub.2, methanol, sulfur-containing compounds), is fed into a heat exchanger 2 in order to be liquefied.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] Temperature approximately equal to another temperature means the same temperature ?5? C.

[0055] 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.

[0056] The method according to the present invention notably offers the following advantages: [0057] 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. [0058] 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.

[0059] 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.