US20100293997A1

US20100293997A1 - Method and apparatus for cooling and/or liquefying a hydrocarbon stream

Info

Publication number: US20100293997A1
Application number: US12/745,787
Authority: US
Inventors: Francois Chantant; Chun Kit Poh
Original assignee: Individual
Current assignee: Shell USA Inc
Priority date: 2007-12-04
Filing date: 2008-12-02
Publication date: 2010-11-25
Also published as: AU2008333301B2; RU2010127331A; JP2011506893A; RU2499962C2; WO2009071538A3; JP5259727B2; EP2215414A2; WO2009071538A2; AU2008333301A1

Abstract

A refrigerant stream is provided at a refrigerant pressure, and passed through at least three heat exchange steps operating at different pressures. A hydrocarbon stream is passed through at least two of these heat exchange steps to provide a cooled stream. A fraction of the refrigerant stream is expanded and evaporated at each heat exchange step to a different pressure, to provide a first evaporated refrigerant stream at a first evaporation pressure, and at least two other evaporated refrigerant streams at evaporation pressures lower than the first. The first evaporated refrigerant stream is compressed through a highest-pressure compressor to provide at least a fraction of the refrigerant stream at the refrigerant pressure, and the other evaporated refrigerant streams are compressed through at least two parallel lower pressure compressors to provide two or more part-compressed refrigerant streams, all of which part-compressed refrigerant streams are passed through the highest pressure compressor.

Description

The present invention relates to a method and apparatus for cooling a hydrocarbon stream. In another aspect, the present invention relates to a method of liquefying a hydrocarbon stream.
A common example of a hydrocarbon stream to be cooled and/or liquefied is natural gas.
Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at a high pressure.
Natural gas can be liquefied using a compressed refrigerant. U.S. Pat. No. 6,962,060 describes a compressor system for a multistage refrigeration of LNG comprising first and second refrigerant compressors, each with first and second stages, and piping means to combine the discharge from the second stages of the first and second compressors to provide a compressed gas. However, the need to combine the highest pressure discharges from the two second stages of the first and second compressors requires the two discharges to be at the exact same pressure, flow etc., to prevent any back pressure and possible surge. This has the disadvantage of limiting the variability allowed between the two discharges to avoid any combining problems. Refrigerant compressors commonly run at different discharge pressures, for example so as to temporarily avoid surge in one compressor, as is known in the art, but the high pressure of the two discharges makes it extremely difficult to accommodate different discharge pressures and maintain downstream flow, without any disturbance also leading to further complications upstream and/or downstream of the compressors.
Moreover, U.S. Pat. No. 6,962,060 only shows one arrangement of first and second compressors having first and second stages. It does not have any flexibility to provide a compression system for other refrigeration arrangements.
The present invention provides a method of cooling a hydrocarbon stream such as natural gas comprising at least the steps of:
(a) providing a refrigerant stream at a refrigerant pressure;
(b) passing the refrigerant stream through at least three heat exchange steps operating at different pressure levels;
(c) passing a hydrocarbon stream through at least two of the heat exchange steps of step (b) thereby progressively lowering the temperature of the hydrocarbon stream to provide a cooled hydrocarbon stream;
(d) expanding and evaporating a fraction of the refrigerant stream at each heat exchange step to a different pressure, to provide a first evaporated refrigerant stream at a first evaporation pressure, and at least two other evaporated refrigerant streams at evaporation pressures lower than the first evaporation pressure;
(e) compressing the first evaporated refrigerant stream through a highest-pressure compressor stage of a single compressor casing to the refrigerant pressure to provide at least a fraction of the refrigerant stream at the refrigerant pressure of step (a);
(f) compressing the other evaporated refrigerant streams through at least two parallel lower pressure compressor stages to provide two or more part-compressed refrigerant streams; and
(g) passing all of the part-compressed refrigerant streams through said highest pressure compressor stage of step (e).
The present invention further provides a method of liquefying a hydrocarbon stream, to provide a liquefied hydrocarbon stream, comprising cooling of the hydrocarbon stream in accordance with the method as defined above or using the apparatus as defined below.
The present invention further provides an apparatus for cooling a hydrocarbon stream such as natural gas at least comprising:
a refrigerant stream at a refrigerant pressure;
at least three heat exchangers comprising pressure reduction means for operating the heat exchange steps at different pressure levels
refrigerant passage means for passing the refrigerant stream passes through the at least three heat exchangers;
hydrocarbon passage means for passing a hydrocarbon stream through at least two of the heat exchangers steps to provide a cooled hydrocarbon stream;
a first evaporated refrigerant stream at a first evaporation pressure,
at least two other evaporated streams at evaporation pressures lower than the first evaporation pressure;
a highest-pressure compressor stage in a single compressor casing for compressing the first evaporated refrigerant stream to provide at least a fraction of the refrigerant stream at the refrigerant pressure;
at least two parallel lower pressure compressor stages for compressing the other evaporated refrigerant streams to provide one or more part-compressed refrigerant streams; and
a pathway to pass all of the part-compressed refrigerant streams through the highest pressure compressor stage in said single compressor casing.

Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying non-limiting drawings in which:

FIG. 1 is a first scheme of a hydrocarbon cooling process according to one embodiment of the present invention; and

FIG. 2 is a second scheme of a hydrocarbon cooling process according to another embodiment of the present invention.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components.
Embodiments of the present invention provide an improved method of cooling a hydrocarbon stream such as natural gas, which has greater flexibility in its refrigerant compressor arrangements.
It is presently proposed to provide a method of cooling a hydrocarbon stream using a single compressor discharge of refrigerant at the refrigerant pressure from the highest-pressure compressor stage of the refrigerant compressor(s). In other words, the highest-pressure compressor stage is located in a single compressor casing (possibly together with one or more lower compressor stages of the same compressor train).
One advantage of this is that there is no requirement for any balancing of discharge pressure streams from different compressors at the refrigerant pressure. This has the further advantage of increasing the variability allowed in discharges from lower pressure compression stages or steps. There can therefore be wider compressor variability, such as may be required to avoid surge, at the lower pressures of such discharges.
Another advantage of the present invention is that there is little or no additional CAPEX or OPEX required to create any new stream flow arrangement required by the present invention, such as the embodiment shown in the accompanying FIG. 1 hereinafter described, from prior art stream flow arrangements such as that shown in U.S. Pat. No. 6,962,060.
With simple re-arrangements of the interconnections between the evaporated and part-compressed streams and the compressor stages, much greater flexibility can be achieved in matching the overall cooling duty curve provided by the refrigerant stream through the heat exchange steps against the compressor duty requirement.
FIG. 1 shows a first general scheme 1 for a hydrocarbon cooling process, generally involving cooling a hydrocarbon stream 20 such as natural gas.
The hydrocarbon stream may be any suitable gas stream to be cooled, but is usually a natural gas stream obtained from natural gas or petroleum reservoirs. As an alternative the natural gas stream may also be obtained from another source, also including a synthetic source such as a Fischer-Tropsch process.
Usually the natural gas stream is comprised substantially of methane. Preferably the hydrocarbon feed stream comprises at least 50 mol % methane, more preferably at least 80 mol % methane.
Depending on the source, the natural gas may contain varying amounts of hydrocarbons heavier than methane such as ethane, propane, butanes and pentanes, as well as some aromatic hydrocarbons. The composition varies depending upon the type and location of the gas. Hydrocarbons heavier than methane generally need to be removed from natural gas for several reasons, such as having different freezing or liquefaction temperatures that may cause them to block parts of a methane liquefaction plant. C_2-4hydrocarbons can be used as a source of natural gas liquids.
The natural gas stream may also contain non-hydrocarbons such as H₂O, N₂, CO₂, Hg, H₂S and other sulphur compounds.
If desired, the hydrocarbon stream containing the natural gas may be pre-treated before use either as part of a hydrocarbon cooling process, or separately. This pre-treatment may comprise reduction and/or removal of non-hydrocarbons such as CO₂and H₂S or other steps such as early cooling, pre-pressurizing. As these steps are well known to the person skilled in the art, their mechanisms are not further discussed here.
Preferably, a hydrocarbon stream to be used in the present invention undergoes at least the minimum pre-treatment required to subsequently liquefy the hydrocarbon stream. Such a requirement for liquefying natural gas is known in the art.
A refrigerant stream used in the present invention may be formed from a single component such as propane or nitrogen, or may be a mixed refrigerant formed from a mixture of two or more components selected from the group comprising: nitrogen, methane, ethane, ethylene, propane, propylene, butanes, pentanes, etc.
The present invention may comprise, or may be part of, a multistage cooling process involving two or more cooling stages, each stage having one or more steps, parts etc. For example, each cooling stage may comprise one to five heat exchange steps. A first cooling stage could reduce the temperature of a hydrocarbon stream to below 0° C., usually in the range −20° C. to −160° C., more usually −20° C. and −70° C. Such a first cooling stage is sometimes also termed a ‘pre-cooling’ stage.
Any second cooling stage is typically separate from a first cooling stage. That is, the second cooling stage comprises one or more separate heat exchange steps using a second refrigerant circulating in a second refrigerant circuit, although the refrigerant of the second refrigerant stream may also pass through one or more heat exchange steps of the first cooling stage. Such a second cooling stage is sometimes also termed a ‘main cooling’ stage.
Referring to the drawings, FIG. 1 shows a hydrocarbon stream 20 passing through four heat exchange steps in series, comprising a first heat exchange step 12, a second heat exchange step 14, a third heat exchange step 16 and a fourth heat exchange step 18. Each heat exchange step 12, 14, 16, 18 may comprise one or more heat exchangers, with multiple heat exchangers operating in series, parallel or a combination of same.
Typically, each heat exchange step 12, 14, 16, 18 comprises a single heat exchanger such as a kettle or plate-and-fin heat exchanger known in the art. Arrangements of series of kettles or plate-and-fin heat exchangers are known in the art.
Typically, each heat exchange step 12, 14, 16, 18 progressively lowers the temperature of the hydrocarbon stream 20, to provide therefrom a cooled hydrocarbon stream 30. For example, the heat exchange steps, 12, 14, 16, 18 shown in FIG. 1 may reduce the temperature of a hydrocarbon stream such as natural gas to below −0° C., usually in the range −20° C. to −70° C., in a manner known in the art.
One example of the above arrangement is a first or pre-cooling stage in a hydrocarbon cooling process, which may be one stage in a hydrocarbon liquefaction plant such as an LNG plant.
FIG. 1 also shows a first refrigerant circuit 3 which provides a refrigerant stream 10 such as propane at a refrigerant pressure ready to provide cooling to the heat exchange steps 12, 14, 16, 18 by passage therethrough, along with the hydrocarbon stream 20.
The four heat exchange steps 12, 14, 16, 18 operate at different pressure levels, and in each of the heat exchange steps 12, 14, 16, 18, a fraction of the refrigerant stream 10 is expanded and evaporated. The expanding usually is accomplished by passing the refrigerant stream though pressure reduction means (not shown) in the form of an expander, a valve, or the like. Usually, the remaining fraction of the refrigerant stream passes to the next heat exchange step, until the final remaining fraction passing into the fourth heat exchange step 18 is fully expanded and evaporated. As each heat exchange step 12, 14, 16, 18 is operating at a different pressure, each heat exchange step 12, 14, 16, 18 will provide an evaporated refrigerant stream therefrom at a different evaporation pressure.
Typically, the first heat exchange step 12 provides a first evaporated refrigerant stream 40 at a first evaporation pressure, typically at the ‘highest’ pressure closest to the refrigerant pressure of the refrigerant stream 10. The highest evaporation pressure may also be termed ‘a high-high-pressure’, and the first evaporated refrigerant stream 40 may also be termed a ‘high-high-pressure evaporated refrigerant stream’ 40.
The second heat exchange step 14 receives a first remaining portion 10 a of the original refrigerant stream 10, and expands and evaporates a fraction of the first remaining portion 10 a to provide a second or other refrigerant stream 50 at a pressure lower than the first evaporation pressure of the first evaporated refrigerant stream 40. The second evaporated stream 50 may also be termed a ‘high-pressure evaporated refrigerant stream’.
A second remaining portion 10 b of the first remaining portion 10 a not expanded or evaporated in the second heat exchange step 14 passes to the third heat exchange step 16, wherein another fraction of the refrigerant stream 10 is expanded and evaporated to provide a third or other evaporated refrigerant stream 60 at an evaporation pressure lower than the first evaporation pressure. The third or other evaporated refrigerant stream 60 may also be termed an ‘intermediate-pressure evaporated refrigerant stream’.
A third remaining portion 10 c of the refrigerant stream 10 passes to the fourth heat exchange step 18, where it is expanded and evaporated to provide a fourth or other evaporated refrigerant stream 70 at an evaporation pressure lower than the first evaporation pressure. The fourth evaporated stream 70 may also be termed a ‘low-pressure evaporated refrigerant stream’ 70.
The process, set-up and arrangement of heat exchange steps 12, 14, 16 and 18, and the multi-stage expansion and evaporation of the refrigerant stream 10, are known in the art, examples of which are shown in U.S. Pat. Nos. 6,962,060 and 6,637,238.
The expansions and evaporations of part or all of the remaining fractions 10 a, 10 b and 10 c of the refrigerant stream 10 in each of the second, third and fourth heat exchange steps 14, 16, 18 allow further or lower-temperature cooling of the hydrocarbon stream 20 as it passes therethrough. However, each of the evaporated refrigerant streams 40, 50, 60 and 70 must then be re-compressed back to the refrigerant pressure of the original refrigerant stream 10.
As shown in FIG. 1, the first evaporated refrigerant stream 40 is compressed through a highest-pressure compressor stage 22 to provide at least a fraction of the refrigerant stream 10 at the refrigerant pressure. The term “highest-pressure compressor stage” refers to the compressor stage having the highest inlet pressure. Where the first evaporated refrigerant stream is a high-high-pressure evaporated refrigerant stream 40, then the highest-pressure compressor stage could also be termed a high-high-pressure compressor stage.
As shown in FIG. 1, the highest-pressure compressor stage 22 is part of a compressor train A, which also comprises a third compressor stage 26 therewith. A “compressor train” as used herein may comprise a single compressor stage, or two or more conjoined compressor stages. Typically in a compressor train, all of the compressed refrigerant from a lower-pressure stage passes straight into the next higher compression stage, and there is a single discharge from the highest-level compression stage in the compressor train.
The arrangement of two or more compressors, compression stages or a combination of same, to form a train is well known in the art. Typically, a compression train comprises one compression stage in a single casing, or two, three or four conjoined compression stages in a single casing. In the latter cases, the compressed refrigerant from the lowest compressor stage passes through each subsequently higher compression stage, usually in combination with one or more higher pressure feeds from one or more intermediate inlets.
FIG. 1 shows the compressor train A comprising a third compressor stage 26 for compressing the third evaporated refrigerant stream 60. The third compressor stage 26 does not fully compress the third evaporated refrigerant stream 60 back to the refrigerant pressure of the original refrigerant stream 10, but provides a ‘part-compressed refrigerant stream’ 60 a.
The part-compressed refrigerant stream 60 a from the third compressor stage 26 then passes directly into the highest-pressure compressor stage 22. The highest-pressure compressor stage 22 is comprised in a single casing, either alone or, for instance, together with the other, lower pressure, stage of train A.
The second or high evaporated refrigerant stream 50 passes into a second or high pressure compressor stage 24. The fourth or low evaporated refrigerant stream 70 passes into a fourth or low-pressure compression stage 28, the part-compressed refrigerant stream 70 a from which passes directly into the second or high-pressure compressor stage 24 along with the second evaporated refrigerant stream 60.
The second and fourth compressor stages 24, 28 comprise a second compression train B, separate from the first compression train A. Thus, at least two of the lower pressure compressor stages 24, 26, 28 are in at least two compressor trains A and B.
The second and fourth compressor stages 24, 28 are also parallel with the third compressor stage 26, such that there are at least two parallel lower pressure compressor stages shown in the general scheme 1 of FIG. 1. The lower pressure compressor stages are parallel in the sense that the lower compressor stages 24, 26, 28 are not in series, or aligned within one casing, and/or that the other evaporated refrigerant streams, being in FIG. 1 the second, third and fourth evaporated refrigerant streams 50, 60, 70, are not compressed in series, and/or that the evaporated stream at the lowest pressure, being in FIG. 1 the fourth evaporated refrigerant stream 70, is not passed through all the compressor stages.
The discharge from the second compressor stage 24 is still a part-compressed refrigerant stream 50 a, which passes through an outlet 24 a of the second compressor train B. As shown in FIG. 1, the part-compressed refrigerant stream 50 a is combined by a combiner 34 with the first evaporated refrigerant stream 40 to pass through an inlet 32 of the first or high-high-pressure compressor stage 22 in the first compressor train A.
In this way, all of the part-compressed refrigerant streams 50 a, 60 a and 70 a commonly pass through the highest pressure compressor stage 22 of a single casing.
FIG. 1 also shows an example of how the discharge (being line 50 a in FIG. 1) of the or each lower pressure compressor stage (being the second and fourth compressor stages 24, 28 in FIG. 1) parallel with the highest pressure compressor stage (being the first compressor stage 22 in FIG. 1) passes through the highest pressure compressor stage.
FIG. 2 shows a second general scheme 2 for a hydrocarbon cooling process, generally involving cooling a hydrocarbon stream 20 such as natural gas. The hydrocarbon stream 20 may pass through four heat exchange steps 12, 14, 16, 18 in the same manner as that described hereinabove in relation to the first general scheme 1 shown FIG. 1.
As a first alternative, the hydrocarbon stream may not pass through all the same heat exchange steps 12, 14, 16, 18 as the refrigerant stream 10. Stream 20 a in FIG. 2 represents a hydrocarbon stream which is only cooled in the second, third and fourth heat exchange steps 14, 16, 18. The first heat exchange step 12 could be used to cool one or more other streams, such as another refrigerant stream or another refrigerant circuit.
FIG. 2 shows a second refrigerant circuit 4 similar to the first refrigerant circuit 3 shown in FIG. 1, where a refrigerant stream 10 passes into a first heat exchange step 12, and successive fractions 10 a, 10 b, 10 c of the refrigerant stream 10 pass into successive second, third and fourth heat exchange steps 14, 16, 18, whilst a fraction of the refrigerant stream 10 at each heat exchange step 12, 14, 16, 18 is expanded and evaporated to provide a first evaporated refrigerant stream 40, and similarly to provide second, third and fourth lower pressure evaporated streams 50, 60 and 70 as described hereinabove.
The first evaporated stream 40 passes into a highest-pressure compressor stage 22 to be compressed and provide a fraction of the original refrigerant stream 10 at the refrigerant pressure.
In the second general scheme 2 shown in FIG. 2, the highest-pressure compressor stage 22 is part of a third compressor train C, which also comprises second and fourth lower-pressure compressor stages 24, 28 therewith. The second and fourth compressor stages 24 and 28 are parallel with a third compressor stage 26. Thus, there are at least two of the lower pressure compressor stages 24, 26, 28 being in at least two compressor trains C and D.
The fourth evaporated stream 70 passes into the fourth compressor stage 28, and the part-compressed refrigerant stream 70 a therefrom passes directly into the second compressor stage 24 along with the second evaporated stream 50. The part-compressed refrigerant stream 50 a from the second compressor stage 24 then passes directly into the highest-pressure stage 22.
Meanwhile, the third evaporated refrigerant stream 60 passes into a parallel third compressor stage 26, which comprises a fourth compressor train D. The fourth compressor train D thus comprises only one compressor stage 26, which is separate from the compressor stages 22, 24, 28 in the third compressor train C.
The discharge of a part-compressed refrigerant stream 60 a from the third compressor stage 26 passes through an outlet 26 a of the second compressor train D to combine with the first evaporation stream 40 by a combiner 36, and so also pass through the highest-pressure stage 22.
FIGS. 1 and 2 show two examples of the flexibility of the present invention to be arranged to receive at least three evaporated refrigerant streams at different evaporation pressures, and for such streams to be recompressed via a variety of arrangement or systems by three lower-pressure compressor stages 24, 26 and 28 in at least two separate compressor trains.
Table 1 lists a number of examples of arrangements for four compressor stages to recompress evaporated refrigerant streams from four different heat exchange steps according to the present invention, using the nomenclature of the compressor stages used in FIGS. 1 and 2 for ease of reference only.

TABLE 1

	Compressor	Compressor	Discharge to
Example	Trains	Stages	different Train

1	A	22, 26	24 → 22
(FIG. 1)	B	24, 28
2	C	22, 24, 28	26 → 22
(FIG. 2)	D	26
3	E	22, 26, 28	24 → 22
	F	24
4	G	22, 24, 28	26 → 24
	H	26
5	I	22, 24, 26	28 → 24
	J	28
6	K	22, 24, 26	28 → 22
	L	28
7	M	22, 24	26 → 22
	N	26, 28
8	O	22, 28	24 → 22
	P	24	26 → 22
	Q	26

Table 1 confirms how the discharge for each train not compressing a refrigerant stream up to the highest compressor, is transferred either straight to the highest pressure stage 22, or into an earlier compressor stage in the same train as the highest pressure stage 22, through which all such refrigerant will then pass.
Examples 1 and 2 in Table 1 are shown in accompanying FIGS. 1 and 2.
Examples 4 and 5 in Table 1 illustrate that a part-compressed refrigerant stream discharged from either compressor stage 26 as compressor train H, or compressor stage 28 as compressor stage J, can be first passed into the second compressor stage 24 rather than directly into the highest pressure compressor stage 22. As the second compressor stage 24 is part of the same compressor train G, I as the highest-pressure compressor stage 22, the part-compressed refrigerant stream will still commonly pass through the highest-pressure compressor stage 22.
Table 1 also provides Example 8 comprising three compressor trains O, P and Q, of which compressor trains P and Q comprise only one compressor stage 24, 26 respectively. However, the part-compressed discharge from each of the lower pressure compressor stages 24 and 26 can be commonly passed through the highest-compressor stage 22.
Table 1 relates to examples involving four compressor stages. The present invention is not limited thereto, and arrangements involving 3 or 5+ compressor stages are also within the scope of the present invention. The skilled man can see different combinations of compressor stages in different compressor trains, wherein the discharge from one or more lower pressure compressor stages can pass into the inlet or inlets of one or more higher-pressure compressor stages, as long as all the part-compressed streams commonly pass through the highest pressure compressor stage.
In summary, disclosed are methods and compressor line-ups for compressing three or more evaporated refrigerant streams through three or more refrigerant compressor stages, wherein:
(i) a first evaporated refrigerant stream (40) is compressed through a common highest-pressure compressor stage (22) to provide at least a fraction of a fully-compressed refrigerant stream (10) at a refrigerant cooling pressure;
(ii) the other evaporated refrigerant streams (50, 60, 70) are compressed through at least two parallel lower pressure compressor stages (24, 26, 28) to provide one or more part-compressed refrigerant streams (50 a, 60 a, 70 a); and
(iii) all of the part-compressed refrigerant streams (50 a, 60 a, 70 a) are commonly passed through the common highest pressure compressor stage (22).
Different arrangements or line-ups of the compressor stages provide different profiles for the re-compression of the refrigerant stream. In this way, the present invention provides flexibility for better matching the compression required of the evaporated refrigerant streams either with the attendant compressor power requirement, or with the cooling duty requirement, or a combination of same, so as to make the arrangement or set-up more efficient.
Persons skilled in the art will readily understand that the present invention may be modified in many ways without departing from the scope of the appended claims.

Claims

1. A method of cooling a hydrocarbon stream, comprising at least the steps of:

(a) providing a refrigerant stream at a refrigerant pressure;

(b) passing the refrigerant stream through at least three heat exchange steps operating at different pressure levels;

(c) passing a hydrocarbon stream through at least two of the heat exchange steps of step (b) thereby progressively lowering the temperature of the hydrocarbon stream to provide a cooled hydrocarbon stream;

(d) expanding and evaporating a fraction of the refrigerant stream at each heat exchange step to a different pressure, to provide a first evaporated refrigerant stream at a first evaporation pressure, and at least two other evaporated refrigerant streams at evaporation pressures lower than the first evaporation pressure;

(e) compressing the first evaporated refrigerant stream through a highest-pressure compressor stage of a single compressor casing to the refrigerant pressure to provide at least a fraction of the refrigerant stream at the refrigerant pressure of step (a);

(f) compressing the other evaporated refrigerant streams through at least two parallel lower pressure compressor stages to provide two or more part-compressed refrigerant streams; and

(g) passing all of the part-compressed refrigerant streams through said highest pressure compressor stage of step (e).

2. The method as claimed in claim 1, wherein said passing of the hydrocarbon stream through at least two of the heat exchange steps of step (b) comprises passing the hydrocarbon stream through at least three of the heat exchange steps of step (b).

3. The method as claimed in claim 1, comprising four or five heat exchange steps, and wherein expanding and evaporating the refrigerant stream provides four or five evaporated refrigerant streams at four or five different pressures respectively.

4. The method as claimed in claim 3, wherein the refrigerant stream and the hydrocarbon stream pass through the same heat exchange steps.

5. The method as claimed claim 1 wherein evaporating a fraction of the refrigerant stream in at least two of the heat exchange steps of step (d) comprises exchanging heat with the hydrocarbon stream passing through said heat exchange steps.

6. The method as claimed in claim 1 wherein the refrigerant stream is propane.

7. The method as claimed in claim 1, further comprising:

expanding and evaporating a first fraction of the refrigerant stream at a first heat exchange step to provide a high-high-pressure evaporated refrigerant stream being the first evaporated refrigerant stream, and having a high-high pressure being the first evaporation pressure,

expanding and evaporating a second fraction of the refrigerant stream at a second heat exchange step to provide a high-pressure evaporated refrigerant stream having a high pressure lower than the high-high pressure,

expanding and evaporating a third fraction of the refrigerant stream at a third heat exchange step to provide an intermediate-pressure evaporated refrigerant stream having an intermediate pressure lower than the high pressure,

expanding and evaporating a fourth fraction of the refrigerant stream at a fourth heat exchange step to provide a low-pressure evaporated refrigerant stream having a low pressure lower than the intermediate pressure,

compressing the high-high-pressure evaporated refrigerant stream through a high-high-pressure compressor stage being said highest pressure compressor stage of step (e), to provide a refrigerant stream at the refrigerant pressure of step (a),

compressing the low pressure-evaporated refrigerant stream through a low-pressure compression stage,

compressing the intermediate-pressure evaporated refrigerant stream through an intermediate-pressure compressor stage, and

compressing the high-pressure evaporated refrigerant stream through a high-pressure compression stage,

wherein the intermediate-pressure compressor stage and the high-high-pressure compressor stage are comprised in a first compressor train, and

the low-pressure compression stage and the high-pressure compression stage are comprised in a second compressor train at least partly separate from the first compressor train.

8. The Method as claimed in claim 7, wherein the second compressor train has an outlet to provide a part-compressed refrigerant stream, and wherein said part-compressed refrigerant stream is passed to the inlet of the high-high-pressure compressor stage in the first compressor train.

9. The method as claimed in one or more of the preceding claims, wherein at least two of the lower pressure compressor stages are in at least two compressor trains.

10. The method as claimed in claim 1 wherein the hydrocarbon stream comprises, preferably essentially consists of, natural gas.

11. A method of liquefying a hydrocarbon stream to provide a liquefied hydrocarbon stream, comprising cooling of the hydrocarbon stream in accordance with a method as claimed in one or more of claims 1 to 10.

12. An apparatus for cooling a hydrocarbon stream, comprising:

a refrigerant stream at a refrigerant pressure;

at least three heat exchangers comprising pressure reduction means for operating the heat exchange steps at different pressure levels

refrigerant passage means for passing the refrigerant stream passes through the at least three heat exchangers;

hydrocarbon passage means for passing a hydrocarbon stream through at least two of the heat exchangers steps for progressively lowering the temperature of the hydrocarbon stream to provide a cooled hydrocarbon stream;

a first evaporated refrigerant stream at a first evaporation pressure,

at least two other evaporated streams at evaporation pressures lower than the first evaporation pressure;

a highest-pressure compressor stage in a single compressor casing for compressing the first evaporated refrigerant stream to provide at least a fraction of the refrigerant stream at the refrigerant pressure;

at least two parallel lower pressure compressor stages for compressing the other evaporated refrigerant streams to provide one or more part-compressed refrigerant streams; and

a pathway to pass all of the part-compressed refrigerant streams through the highest pressure compressor stage in said single compressor casing.

13. The apparatus as claimed in claim 12, wherein at least two of the lower pressure compressor stages are in at least two at least partly mutually separate compressor trains.