WO2008020044A2

WO2008020044A2 - Method and apparatus for liquefying a hydrocarbon-containing feed stream

Info

Publication number: WO2008020044A2
Application number: PCT/EP2007/058461
Authority: WO
Inventors: Marco Dick Jager; Suyog Kalyanji Kotecha; Irina Tanaeva
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2006-08-17
Filing date: 2007-08-15
Publication date: 2008-02-21
Also published as: AU2007285734A1; WO2008020044A3; RU2447382C2; RU2009109420A; EP2052197B1; US20110185767A1; AU2007285734B2; EP2052197A2

Abstract

Method and apparatus for liquefying a hydrocarbon stream such as natural gas from a feed stream. The method comprises at least the steps of: first cooling the feed stream (10) against a first cooling refrigerant being cycled in a first cooling refrigerant circuit (100), wherein the first cooling refrigerant comprises > 90 mol% propane, second cooling the cooled gas stream (20) to obtain a liquefied stream (60) against a first mixed refrigerant being cycled in a first mixed refrigerant circuit (200), wherein said second cooling is in two or more heat exchangers (42, 44), at least two of which are operating at different pressures and sub-cooling the liquefied stream (60) against a second mixed refrigerant or against a nitrogen refrigerant being cycled in a sub-cooling refrigerant circuit (300), thereby obtaining a sub-cooled hydrocarbon stream (70).

Description

METHOD AND APPARATUS FOR LIQUEFYING A HYDROCARBON- CONTAINING FEED STREAM

The present invention relates to a method and apparatus for liquefying a hydrocarbon-containing feed stream, particularly but not exclusively a natural gas feed stream. 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 .

EP 1 340 951 A2 describes a process for liquefying a natural gas stream using three refrigerant cycles. The first and second cooling stages are against a mixed refrigerant while the third cooling stage can be against nitrogen. The second cooling stage is carried out in a single heat exchanger at a single pressure of mixed refrigerant . US 2005/056051 describes a process for liquefying a natural gas stream using three refrigerant cycles. The first cooling stage is against a propane refrigerant, the second cooling stage is against a mixed refrigerant and the third cooling stage can be against nitrogen. The second cooling stage is carried out in a single heat exchanger at a single pressure of mixed refrigerant.

DE 3521060 describes a process for liquefying a natural gas stream using three refrigerant cycles. The first and third cooling stages are against a mixed refrigerant or propane refrigerant, while the second cooling stage is against a mixed refrigerant. There is no disclosure of the use of at least two heat exchangers operating at different pressures of mixed refrigerant in the second cooling stage.

US 6,253,574 Bl describes a process for liquefying a natural gas stream using a mixed-refrigerant cascade cycle of three-mixed refrigerant cycles having different refrigerant compositions. The refrigerant for the first cycle is a mixture of ethylene or ethane, propane and butane. The refrigerant for the second cycle is a mixture of methane, ethylene or ethane and propane, and the third refrigerant is a mixture of nitrogen, methane and ethylene or ethane . The use of mixed refrigerant can have some advantages in certain situations, for example in a large spool-wound cryogenic heat exchanger, which is efficient when providing cooling to go down to or below -100⁰C. However, spool-wound heat exchangers are expensive for pre- cooling.

It is an object of the present invention to improve the efficiency of a three refrigerant cycle liquefying process .

In a first aspect, the present invention provides a method of liquefying a hydrocarbon stream such as natural gas from a feed stream, the method at least comprising the steps of:

(a) providing the feed stream;

(b) first cooling the feed stream against a first cooling refrigerant being cycled in a first cooling refrigerant circuit, thereby obtaining a cooled gas stream, wherein the first cooling refrigerant comprises > 90 mol% propane; (c) second cooling the cooled gas stream obtained in step (b) into a liquid against a first mixed refrigerant being cycled in a first mixed refrigerant circuit, wherein said second cooling is in two or more heat exchangers, at least two of which are operating at different pressures, thereby obtaining a liquefied stream; and

(d) sub-cooling the liquefied stream obtained in step (c) against a second mixed refrigerant or against a nitrogen refrigerant being cycled in a sub-cooling refrigerant circuit, thereby obtaining a sub-cooled hydrocarbon stream.

In a further aspect, the present invention provides apparatus for liquefying a hydrocarbon stream such as natural gas stream from a feed stream, the apparatus at least comprising:

- a first cooling stage comprising one or more heat exchangers arranged to receive the feed stream and to provide a cooled gas stream, the first cooling stage including a first cooling refrigerant circuit using a first refrigerant for removing heat from the feed stream, wherein the first cooling refrigerant comprises > 90 mol% propane;

- a second cooling stage comprising a plurality of heat exchangers arranged to receive the cooled gas stream from the first cooling stage and to provide a liquefied stream, the second cooling stage being a cryogenic system, and including a second refrigerant circuit using a mixed refrigerant for removing heat from the cooled gas stream, wherein at least two of the heat exchangers of the second cooling stage are adapted to operate at different pressures; and - a sub-cooling stage comprising one or more sub- cooling heat exchangers arranged to receive the liquefied stream from the second cooling stage and to provide a sub-cooled liquefied hydrocarbon product stream, the sub- cooling stage including a sub-cooling refrigerant circuit using a mixed refrigerant or a nitrogen refrigerant for removing heat from the liquefied stream.

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;

Figure 1, is a first general scheme of an LNG plant according to one embodiment of the present invention; and

Figure 2 is a second general scheme of an LNG plant 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 may include cooling of a feed stream into a liquefied stream in at least first and second cooling stages. First cooling in the first stage may hereinafter be referred to as "step (b) " while second cooling in the second stage may hereinafter be referred to as "step (c)". The second cooling stage is advantageously operated using a first mixed refrigerant in the second cooling stage in two or more heat exchangers, at least two of which heat exchangers are operating at different pressures. Expanding the first mixed refrigerant at two different pressure levels achieves a reduction of the low-pressure compressor suction flow. This provides a reduction in the required compressor power, and improvement in the process efficiency. In addition, reduced compressor suction flow allows a reduction in the size of the compressor.

Preferably, at least one of the two or more heat exchangers is operating at a pressure of from 4 to 15 bar and at least one other of the two or more heat exchangers is operating at a pressure of from 1 to 8 bar. Whenever in this specification reference is made to bar, this is a reference to bar absolute. More preferably, the pressure difference between at least two heat exchangers is 3 bar or more. In an other more preferred embodiment, at least one heat exchanger is operating at a pressure that is 1.5 times higher than the pressure at which at least one other heat exchanger is operating.

Thus, for instance, if one of the two or more heat exchangers is operating at a pressure of 6 bar, the other of the two or more heat exchangers may for example operate at 11 bar. In another example, if one of the two or more heat exchangers is operating at a pressure of 1.6 bar, the other of the two or more heat exchangers may for example operate at 5.7 bar.

In the first cooling stage, the feed stream may be cooled against a first cooling refrigerant comprising more than 90 mol% propane. Propane can be more conveniently used at different pressure levels than a mixed-refrigerant, such that the first cooling of the feed stream can be more efficiently arranged. Recompression of the first cooling refrigerant is also more efficient because the fraction of the refrigerant that is compressed over the full pressure ratio of the refrigerant compressor is reduced.

Moreover, a propane refrigeration circuit is less expensive than a mixed refrigerant refrigeration circuit, more particularly in the use of multiple heat exchangers and/or multiple pressure levels to effect the cooling. This is because with a single component refrigerant shell and tube heat exchangers can be used, while this is not possible if a mixed refrigerant is used. Apparatus, installations and equipment that can be used as shell and tube heat exchangers are well known the art, and include for example kettles which are relatively inexpensive compared with spool-wound heat exchangers. A line of kettles can be quickly and easily located to allow a stream of single component refrigerant to pass there along, each kettle using a different pressure. Different evaporation rates and vapour pressures from such kettles are also not significant as the vapours will all pass back into one or more compressors, and the use of a single component prevents any imbalance of a mixed refrigerant where one of the components in the mix is evaporating faster than other component ( s ). Thus, the use of single component refrigerant heat exchangers in pre- cooling is generally less expensive than other arrangements. One example is a line of kettles as discussed below.

Preferably, the first cooling refrigerant used in step (b) comprises > 95 mol% propane, preferably > 98 mol% propane, more preferably > 99 mol% propane. The first cooling of step (b) may be provided by the passage of the feed stream through a first cooling stage having one or more heat exchangers. The one or each heat exchanger is preferably wholly or substantially supplied with cooling by the first cooling refrigerant. Preferably, the first cooling comprises at least two, optionally three, four or five, heat exchangers.

In another embodiment of the present invention, each heat exchanger of a multi-exchanger first cooling stage involves a different first cooling refrigerant pressure. For an arrangement in which four heat exchangers are used in a first cooling stage, such pressures are often referred to as: low pressure, medium pressure, high pressure and high high pressure. For example, the low pressure may be 1 bar, the medium pressure 2 bar, the high pressure 4 bar and the high high pressure 8 bar. The expanded refrigerant from each pressure stage could be compressed in one or more compressors known in the art, for example to a pressure in the range of from 16 to 20 bar .

An advantage of the use of different first cooling refrigerant pressures is the better efficiency of providing cooling and/or the recompression of propane over a fraction of the pressure range compared with other refrigerants hitherto used for pre-cooling natural gas, most especially mixed refrigerants.

The second cooling of step (c) is provided by passing the cooled gas stream through a second cooling stage having at least two heat exchangers. At least two heat exchangers of the second cooling stage are supplied with cooling by the first mixed refrigerant in the first mixed refrigerant circuit at different pressures. The heat exchangers may preferably be arranged in series such that the cooled gas stream passes through each exchanger.

Additional cooling of the gas stream and/or the first mixed refrigerant could be provided by one or more other refrigerants or refrigerant circuits, optionally being connected with another part of the method and/or apparatus for liquefying a hydrocarbon stream as described herein.

The heat exchangers in the second cooling of step (c) are preferably spool-wound heat exchangers. Spool-wound heat exchangers provide improved efficiency for the second cooling step.

In another preferred embodiment of the present invention, the circulation of the first mixed refrigerant in step (c) comprises compressing, cooling, and separating the refrigerant into a first high pressure fraction that is evaporated at a high pressure in one heat exchanger, and a second low pressure fraction that is evaporated at a low pressure in another heat exchanger, and recollection of the first and second evaporated fractions, wherein the high pressure fraction is evaporated at a higher temperature than the low pressure fraction.

In another preferred embodiment of the invention, a fraction of the first mixed refrigerant in step (c) does not pass through every heat exchanger of the second cooling step. By passing a portion of the first refrigerant stream through less than the total number of heat exchangers in second cooling, more cooling is made available to the cooled gas stream.

In another preferred embodiment of the invention, the first mixed refrigerant is separated into two or more fractions after passing through at least the first heat exchanger of step (c), and at least one of said fractions is expanded and returned to the first heat exchanger. Separating the first mixed refrigerant after passing through at least the first heat exchanger splits the refrigerant at a point other that the coldest point of the full stream, providing more cooling to the cooled gas stream in second cooling.

The liquefied stream obtained from step (c) may then be sub-cooled in a step (d) . The sub-cooling of step (d) may be provided by passing the liquefied stream through a third cooling stage having one or more sub-cooling heat exchangers. The or each heat exchanger of the sub-cooling is preferably supplied with cooling by the second mixed refrigerant or a nitrogen refrigerant in the sub-cooling refrigerant circuit. Additional cooling of the liquefied stream and/or the second mixed refrigerant could be provided by one or more other refrigerants or refrigerant circuits, optionally being connected with another part of the method and/or apparatus for liquefying a hydrocarbon stream as described herein. An example of this is passing the sub-cooling refrigerant circuit through the second cooling step.

The feed stream may be any suitable gas stream to be liquefied. It may comprise a hydrocarbon stream, 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 feed stream comprises at least 60 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 natural gas stream may also contain undesired non-hydrocarbons such as Hg, H2O, N2,

CO2, H2S and other sulphur compounds. Typically, the feed stream containing the natural gas may be pre-treated to remove any undesired components present such as CO2 and H2S, or there may be other steps such as pre-cooling or pre-pressurizing. As these steps are well known to the person skilled in the art, they are not further discussed here.

The term "feed stream" as used herein relates to any hydrocarbon-containing composition usually containing a large amount of methane. In addition to methane, natural gas contained various amounts of ethane, propane and heavier hydrocarbons. The composition varies depending upon the type and location of the gas. Hydrocarbons heavier than ethane 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. C2-C4 hydrocarbons can be used as a source of natural gas liquids.

The term "feed stream" also includes a composition prior to any treatment, such treatment including cleaning, dehydration and/or scrubbing, as well as any composition having been partly, substantially or wholly treated for the reduction and/or removal of one or more compounds or substances, including but not limited to sulphur, sulphur compounds, carbon dioxide, water, and C2+ hydrocarbons.

Referring to the drawings, Figure 1 shows a general scheme for a liquid natural gas (LNG plant) . It shows an initial feed stream containing natural gas 10, which feed stream may be pre-treated to separate out any of at least some heavier hydrocarbons and impurities such as carbon dioxide, nitrogen, mercury, helium, water, sulphur and sulphur compounds, including but not limited to acid gases, which may be present.

The feed stream 10 undergoes first cooling in a first cooling stage 110 against a first cooling refrigerant being circulated in a first cooling refrigerant circuit 100, thereby obtaining a cooled gas stream 20.

In Figure 1 the first cooling stage 110 is shown in simplified form, and generally involves a first refrigerant circuit 110, four first heat exchangers 112, a first compressor 114, driven by a driver 116, and a water and/or air cooler 118.

The refrigerant of the first cooling refrigerant circuit 100 comprises > 90 mol% propane. The first cooling stage 110 may comprise any suitable number of heat exchangers, e.g. two, three or four, through which the feed stream 10 passes, and each heat exchanger may also have a different pressure level. Using different pressure levels, such as low pressure, medium pressure, high pressure and high high pressure, in each of the four heat exchangers 112 shown in Figure 1, makes a more efficient arrangement where the refrigerant is propane. The use and arrangement of four different pressure levels in a refrigeration circuit allows the use of inexpensive kettle heat exchangers.

Generally, the vapour released from each heat exchanger 112 passes to and along the first compressor 114 in an arrangement known in the art, and the compressed refrigerant is then cooled by the cooler 118 before recirculation through the heat exchangers 112. See in this respect WO01/44734A2 and WO2005/057110A1.

Optionally, the first cooled feed stream 20 is then passed into a separation column (not shown), which column can separate the cooled gas stream 20 into a more liquid or heavier stream, generally being a heavier hydrocarbon rich stream, and a more gaseous or lighter stream, generally being a methane enriched stream, for subsequent cooling and liquefaction. The heavier stream can be recycled or used for other product production.

Preferably, the first cooling cools down the feed stream 10 to approximately -20 to -50⁰C, such as about -25°C.

The cooled gas stream 20 then undergoes second cooling into a liquid in a second cooling stage 210 against a first mixed refrigerant being circulated in a first mixed refrigerant circuit 200. In simplified form, the first mixed refrigerant circuit 200 includes a second compressor 202 driven by a driver 204, a water and/or air cooler 206 and one or more dedicated heat exchangers (e.g. kettle 208) that could be cooled by a refrigerant circuit, preferably provided by or connected with the first refrigerant circuit 100.

The first mixed refrigerant may be any suitable mixture of components including two or more of nitrogen, methane, ethane, ethylene, propane, propylene, butane, pentane, etc. In this specification, a refrigerant is referred to as "mixed" if each component is present in the mixture in an amount of less than 90 mol%, preferably less than 80 mol%.

In one embodiment of the present invention, the first mixed refrigerant used in step (c) comprises: > 50 mol% of a compound selected from the group consisting of ethane and ethylene or a mixture thereof; and > 10 mol% of a compound selected from the group consisting of propane and propylene or a mixture thereof. Preferably, in this embodiment the amount of a compound selected from the group consisting of propane and propylene or a mixture thereof is not more than 30 mol% and the amount of methane is less than 20 mol%. In another embodiment of the present invention, the first mixed refrigerant used in step (c) comprises: > 30 mol% of a compound selected from the group consisting of ethane and ethylene or a mixture thereof; and > 30 mol% of a compound selected from the group consisting of propane and propylene or a mixture thereof. There can be various arrangements for the gas stream and refrigerant stream in the second cooling stage 210. These all involve two or more heat exchangers at different pressure levels.

In the arrangement shown in Figure 1, the first mixed refrigerant circuit 200 comprises a second heat exchanger 42 and a third heat exchanger 44, both being provided with first mixed refrigerant being circulated in the first mixed refrigerant circuit 200. The second and third heat exchangers 42, 44 provide a condensed gas stream 60.

The second and third heat exchangers 42, 44 operate at different pressures, to increase the efficiency of cooling provided to the stream being liquefied. One manner of achieving this is the first mixed refrigerant in the first mixed refrigerant circuit 200 being split prior to the third heat exchanger 44, to provide a separate (high pressure) refrigerant stream that passes via valve 214 into the second heat exchanger 42. Valve

214 reduces the pressure of the high pressure refrigerant stream to medium pressure, preferably in the range of from 4 to 15 bar. Valve 212 reduces the pressure of the remaining first mixed refrigerant prior to entry into the third heat exchanger 44 to a low pressure, preferably in the range of from 1 to 8 bar, to provide a low pressure fraction into heat exchanger 44. More preferably, the pressure of the low pressure fraction is at least 3 bar lower than the pressure of the refrigerant stream that is passed into heat exchanger 42. According to another more preferred embodiment, the pressure of the refrigerant stream that is passed into heat exchanger 42 is at least 1.5 times the pressure of the low pressure fraction that is passed into heat exchanger 44.

In this way, the circulation of the first mixed refrigerant comprises compressing, cooling, and separating the refrigerant into a first high pressure fraction and a second low pressure fraction, evaporation of the first and second fractions in different heat exchangers 42,44, so that the high pressure fraction is evaporated at a higher temperature than the low pressure fraction . Not only does using high and low pressure fractions assist cooling of the gas stream 20, the cooled gas stream 30 may optionally also provide an intermediate temperature stream that can be used to provide reflux for a scrub column when provided as optional gas/liquid separator 52.

Typically, cooling in the second heat exchanger 42 may reduce the temperature of the gas stream 20 to provide a gas stream 30 at a temperature in the range of from -30 to -70 ⁰C, such as about -50⁰C. Typically, cooling in the third heat exchanger 44 may reduce the temperature of the gas stream 30 to provide a liquefied hydrocarbon stream 60 at a temperature in the range of from -70 to -120 ⁰C, such as about -80⁰C.

Optionally, the exit stream 30 from the second heat exchanger 42 passes through a separation vessel 52, so as to provide a lighter gas stream 50, being methane- enriched, and a heavier liquid stream 40 which can be recycled in the liquefaction plant, or used for production of other hydrocarbon streams.

The liquefied stream 60 then undergoes a third cooling, preferably sub-cooling, in a third cooling stage 310 using a fourth heat exchanger 46 and against a second mixed refrigerant or a nitrogen refrigerant being circulated in a sub-cooling refrigerant circuit 300, thereby obtaining a sub-cooled liquefied natural gas stream 70. In simplified form, the sub-cooling refrigerant circuit 300 involves a third compressor 302 driven by a driver 304, an air and/or water cooler 306, and one or more dedicated heat exchanger (s) such as a sub-cooler, e.g. a kettle 308.

Where the second refrigerant is a mixed refrigerant, it may be any suitable mixture of components including two or more of nitrogen, methane, ethane, ethylene, propane, propylene, butane, etc. Preferably, any mixed refrigerant used in step (d) comprises: > 30 mol% of a compound selected from the group consisting of ethane and ethylene or a mixture thereof; and > 30 mol% methane.

The sub-cooling refrigerant circuit 300 may include a heat exchanger 312, which could comprise more than one heat exchanger, to provide additional cooling to the refrigerant of the sub-cooling refrigerant circuit 300. For example, where the refrigerant is a nitrogen refrigerant, the nitrogen refrigerant could be cooled in the heat exchanger 312 against a mixed refrigerant.

In a further embodiment (not shown), the first mixed refrigerant circuit 200 could separately or additionally cool or provide direct or indirect cooling to, the refrigerant of the sub-cooling refrigerant circuit 300, optionally to the same temperature as the liquefied stream 60, and further optionally to the heat exchanger 312.

The scheme shown in Figure 2 is similar to that shown in Figure 1. It has four refrigerant pressure levels in the first cooling stage 110, using kettles 112a, 112b, 112c and 112d, having refrigerant outflow streams 101, 102, 103 and 104 respectively to the first compressor 114.

Figure 2 also shows two alternative embodiments according to the present invention.

Optionally, the sub-cooling refrigerant circuit 300 using a second mixed refrigerant can separate the refrigerant into light and heavy fractions, similar to that described above for the first mixed refrigerant circuit 200. Both mixed refrigerants also can be let down to the same pressure level in one cryogenic heat exchanger, where the light fraction cools the coldest end. The recombined refrigerant can then be sent from the bottom of the cryogenic heat exchanger to the accompanying refrigerant compressor.

In Figure 2, this is shown by the introduction of a separator 54 in the sub-cooling refrigerant circuit 300. The separator can separate the mixed refrigerant into a liquid heavy fraction 303 and a vapour light fraction 305, which can both pass into the fourth heat exchanger 46. Where the fourth heat exchanger 46 is a spool-wound heat exchanger, the light fraction 305 and the heavy fraction 303 can both be passed into the same shell side of the heat exchanger 46, and the light fraction 305 can be used to cool the liquefied hydrocarbon stream 60 entering the fourth heat exchanger 46 at the low end of the temperature interval of the third cooling, and the heavy fraction 303 can be used to cool the liquefied hydrocarbon stream 60 at the high end of the temperature interval of the third cooling. The passage, operation and expansion of the refrigerant lines in the fourth heat exchanger 46 of Figure 2 are known to those skilled in the art. In this way, the heavy fraction 303 and light fraction 305 of the second mixed refrigerant can evaporate in the fourth heat exchanger 46 at the same or substantially the same pressure as is known in the art . In the second alternative embodiment in Figure 2, the first mixed refrigerant circuit 200 still comprises a second heat exchanger 42 and a third heat exchanger 44, but the first mixed refrigerant, after condensing and cooling through a water and/or air cooler 206 and one or more dedicated heat exchangers (e.g. kettle 208 or provided by the first cooling stage 100), all passes through the second heat exchanger 42 to be cooled. The cooled refrigerant stream 203 is then divided between a first fraction which is expanded by a valve 214 to provide a separate low pressure refrigerant stream 205 which is used to provide cooling in the heat exchanger 42, and whose exit stream 201 is passed into the second compressor 202, and a second fraction 207 which passes through the third heat exchanger 44 for cooling, prior to expansion to provide a refrigerant stream 209 which provides cooling in the third heat exchanger 44. The refrigerant exit stream 211 passes to the compressor 202. The division of the condensed first mixed refrigerant stream 203 may take place at a temperature between -30⁰C and -70⁰C. By expanding the condensed first mixed refrigerant at two different pressure levels in the second cooling stage 210, which is the main liquefaction cycle, there is a reduction of the low-pressure compressor suction flow, which can provide a reduction in the required compressor power, and improvement in the process efficiency. In addition, reduced compressor suction flow provides a reduction in the size of the compressor. Further, any air cooler at the exit of the compressor for the first mixed refrigerant may not be necessary, as the temperature of the compressor suction flow can be close to, such as only a few degrees different, that of the first mixed refrigerant temperature when divided, which results in the compressor outlet temperature being below the ambient temperature. This is especially where cooling of the first mixed refrigerant from the compressor is provided, either directly or indirectly, by the first cooling stage 110. As with the scheme or arrangement shown in Figure 1, additional cooling of the refrigerant in the sub-cooling refrigerant circuit 300 can be provided by the second cooling, generally by passing the sub-cooling refrigerant circuit 300 through part or all of the second cooling step, or having an intermediate circuit (s) thereinbetween .

Further the person skilled in the art will readily understand that after liquefaction, the liquefied natural gas may be further processed, if desired. As an example, the obtained LNG may be depressurized by means of a Joule-Thomson valve or cryogenic turbo-expander .

Table I gives an overview of the separate and overall power requirements for one example of the process shown in Fig. 1. Table I

The power requirements for the example of Figure 1 are compared with a comparative scheme using a mixed refrigerant in its first cooling cycle, as shown for instance in US 6,253,574 Bl. As can be seen, although the first cooling power is increased, the power for the second and third cooling cycles is reduced, so that for a similar production of LNG, there is an overall reduction of 17 MW (7%) achieved by the present invention. This is significant in relation to the size and energy requirements of an LNG plant.

The results also show that the first or pre-cooling cycle is loaded to a higher extent than the other cooling cycles. One consequence is that the internal flows of the first or pre-cooling compressor are higher, even though a split propane line-up is used: that is, the propane refrigerant flow rate is higher than in the comparative scheme. However, the third or sub-cooling cycle has a reasonable suction volume for its compressor and a main cryogenic exchanger area that is well in line with the current main cryogenic exchangers . Table II gives an overview of the overall power requirements of an example of the process represented in Figure 2 and a further example of the process represented in Figure 1.

Table II

The power requirements for this example process of the invention represented in Figures 1 and 2 are compared with a comparative scheme using a mixed refrigerant in its first and second cooling cycles in which the second cooling cycle is carried out in a single heat exchanger operated at a single pressure of mixed refrigerant. In this example process of the invention, nitrogen was used as the second single refrigerant in the process of Figure 1 and a mixed refrigerant was used as the second mixed refrigerant in the process of Figure 2.

As can be seen, for a similar production of LNG, there is an overall reduction of 9 MW achieved by the process of the present invention represented in Figure 1 compared to the comparative process. Similarly, the process represented in Figure 2 provides an overall reduction of 17 MW. This is significant in relation to the size and energy requirements of an LNG plant.

Table III gives a representative working example of temperatures, pressures and flows of streams at various parts in an example process referring to Figure 2. Streams 100a, 200a and 300a refer to the respective refrigerant streams of the first, second and third refrigerant circuits 100, 200 and 300, after their compression and cooling.

Table III

The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims .

Claims

C L A I M S

1. Method of liquefying a feed stream comprising a gaseous hydrocarbon stream such as natural gas, the method at least comprising the steps of:

(a) providing the feed stream; (b) first cooling the feed stream against a first cooling refrigerant being cycled in a first cooling refrigerant circuit, thereby obtaining a cooled gas stream, wherein the first cooling refrigerant comprises > 90 mol% propane; (c) second cooling the cooled gas stream obtained in step (b) into a liquid against a first mixed refrigerant being cycled in a first mixed refrigerant circuit, wherein said second cooling is in two or more heat exchangers, at least two of which are operating at different pressures, thereby obtaining a liquefied stream; and

2. Method according to claim 1 wherein at least two, preferably all, of the heat exchangers in the second cooling of step (c) are spool-wound heat exchangers.

3. Method according to any of the preceding claims wherein a fraction of the first mixed refrigerant in the second cooling of step (c) does not pass through every heat exchanger of the second cooling.

4. Method according to any of the preceding claims wherein the first mixed refrigerant is separated into two or more fractions after passing through at least the first heat exchanger in the second cooling of step (c), and at least one of said fractions is expanded and returned to the first heat exchanger.

5. Method according to one or more of the preceding claims, wherein the first cooling refrigerant used in step (b) comprises > 95 mol% propane, preferably > 98 mol% propane, more preferably > 99 mol% propane.

6. Method according to one or more of the preceding claims, wherein the first cooling in step (b) comprises at least two stages, optionally four stages.

7. Method according to claim 6 wherein each stage of the first cooling involves a different first cooling refrigerant pressure.

8. Method according to one or more of the preceding claims, wherein the first mixed refrigerant used in step (c) comprises: > 50 mol% of a compound selected from the group consisting of ethane and ethylene or a mixture thereof; and > 10 mol% of a compound selected from the group consisting of propane and propylene or a mixture thereof .

9. Method according to one or more of the preceding claims, wherein step (d) uses a second mixed refrigerant, and the second mixed refrigerant comprises: > 30 mol% of a compound selected from the group consisting of ethane and ethylene or a mixture thereof; and > 30 mol% methane.

10. Method according to one or more of the preceding claims, wherein in step (c) the circulation of the first mixed refrigerant comprises compressing, cooling, and separating the refrigerant into a subsequent first high pressure fraction and a subsequent second low pressure fraction, evaporation of the first and second fractions in different heat exchangers, and recollecting of the first and second evaporated fractions, wherein the high pressure fraction is evaporated at a higher temperature than the low pressure fraction.

11. Method according to one or more of the preceding claims wherein step (d) uses a second mixed refrigerant, and in step (d) the circulation of the second mixed refrigerant comprises compressing, cooling, and separating the second mixed refrigerant into a first heavy fraction and a second light fraction, evaporating the first and second fractions in different zones at wholly or substantially the same pressure, and collecting the first and second fractions.

12. Method according to one or more of the preceding claims wherein the sub-cooling refrigerant circuit passes through the second cooling of step (c) to cool the second mixed refrigerant or nitrogen refrigerant prior to step (d) .

13. Method according to one or more of claims 1 to 8, 10 and 12 wherein step (d) uses a nitrogen refrigerant, and the sub-cooling refrigerant circuit includes a single- component refrigerant heat exchanger to cool the nitrogen refrigerant prior to its use to sub-cool the liquefied stream.

14. Apparatus for liquefying a hydrocarbon stream such as natural gas stream from a feed stream, the apparatus at least comprising: - a first cooling stage comprising one or more heat exchangers arranged to receive the feed stream and to provide a cooled gas stream, the first cooling stage including a first cooling refrigerant circuit using a first refrigerant for removing heat from the feed stream, wherein the first cooling refrigerant comprises > 90 mol% propane; a second cooling stage comprising a plurality of heat exchangers arranged to receive the cooled gas stream from the first cooling stage and to provide a liquefied stream, the second cooling stage including a second refrigerant circuit using a mixed refrigerant for removing heat from the cooled gas stream, wherein at least two of the heat exchangers of the second cooling stage are adapted to operate at different pressures; and a sub-cooling stage comprising one or more sub- cooling heat exchangers arranged to receive the liquefied stream from the second cooling stage and to provide a sub-cooled liquefied hydrocarbon product stream, the sub- cooling stage including a sub-cooling refrigerant circuit using a mixed refrigerant or a nitrogen refrigerant for removing heat from the liquefied stream.

15. Apparatus according to claim 14 wherein the sub- cooling refrigerant circuit uses a mixed refrigerant, and includes a separator adapted to provide a first light fraction and a second heavier fraction for use in the one or more sub-cooling heat exchangers.