Method and apparatus for producing a liquefied hydrocarbon stream

Abstract

 A hydrocarbon stream of a composition comprising methane, ethane, and one or more hydrocarbon components heavier than ethane is provided (10). A side portion (70) of the hydrocarbon stream having the same composition as the hydrocarbon stream is selectively branched off, whereby a main portion of the hydrocarbon stream passes to a liquefaction system (15). The side portion is passed to an extraction unit (200) where an ethane-enriched stream is extracted from the side portion, which ethane-enriched stream contains a higher relative amount of ethane than the side portion. the ethane-enriched stream is passed to a storage tank where it forms an accumulated amount of the ethane-enriched stream which is stored for subsequent use as a refrigerant fluid (150) in the liquefaction system.

Description

[0001] The present invention relates to a method and apparatus for producing a liquefied hydrocarbon stream.

[0002] A commercially traded type of liquefied hydrocarbon stream is liquefied natural gas (LNG). Several apparatuses and methods of producing liquefied natural gas are known. It is desirable to liquefy natural gas 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.

[0003] In an article entitled "Evaluate separation for LNG plants", published in the September 1978 issue of Hydrocarbon Processing, author Chen-hwa Chiu of Air Products and Chemicals Inc., describes a processes and liquefaction system for producing an LNG stream from natural gas. After acid gas removal and dehydration, the natural gas is pre-cooled against propane and passed to a scrub column. The scrub column is necessary to remove heavy hydrocarbons and aromatic before liquefaction. The scrub column overhead is then passed to a cryogenic heat exchanger that is operated with a multicomponent refrigerant stream. Here the scrub column overhead is condensed to form LNG. The removed heavy hydrocarbons form a fractionation feed to produce ethane and propane for mixed refrigerant make-up, and/or natural gas liquids. Fractionated ethane and propane may be reinjected into the LNG to the limits that the specification will permit.

[0004] A drawback of this process is that it requires a lot of equipment in the vicinity of the liquefaction system, and thus consuming a large available plot space around for instance the cryogenic heat exchanger in 5h3 liquefaction system. For instance, the scrub column needs to be continuously operated in the vicinity of the multicomponent refrigerant system, because the scrub column provides the feed for the mixed refrigerant make-up for the multicomponent refrigerant system.

[0005] In a first aspect, the present invention provides for an apparatus for producing a liquefied hydrocarbon stream, said apparatus comprising:

• a hydrocarbon stream source for providing a hydrocarbon stream of a first composition comprising methane, ethane, and one or more hydrocarbon components heavier than ethane;
• a hydrocarbon feed line for carrying the hydrocarbon stream;
• a feed stream splitter in the hydrocarbon feed line, dividing the hydrocarbon feed line in an upstream feed line section and a downstream feed line section, and arranged to selectively branch off a side portion of the hydrocarbon stream, said side portion having the same composition as the first composition, whereby a main portion of the hydrocarbon stream passes into the downstream feed line section of the hydrocarbon feed line;
• a liquefaction system fluidly connected to the downstream feed line section and arranged to receive and liquefy the main portion of the hydrocarbon stream;
• a side feed stream line fluidly connected to the feed stream splitter for selectively carrying the side portion of the hydrocarbon stream;
• an extraction unit arranged to extract an ethane-enriched stream from the side portion of the hydrocarbon stream from the side feed stream line, said ethane-enriched stream containing a higher relative amount of ethane than the side portion of the hydrocarbon stream from the side feed stream line;
• a storage tank arranged to receive the ethane-enriched stream and store an accumulated amount of the ethane-enriched stream for subsequent use as a refrigerant fluid in the liquefaction system.

[0006] In another aspect, the present invention provides a method of producing a liquefied hydrocarbon stream, the method comprising:

• providing a hydrocarbon stream of a first composition comprising methane, ethane, and one or more hydrocarbon components heavier than ethane;
• selectively branching off a side portion of the hydrocarbon stream, said side portion having the same composition as the first composition, whereby a main portion of the hydrocarbon stream passes to a liquefaction system;
• liquefying the main portion of the hydrocarbon stream in the liquefaction system;
• passing the side portion to an extraction unit and extracting an ethane-enriched stream from the side portion, said ethane-enriched stream containing a higher relative amount of ethane than the side portion;
• passing the ethane-enriched stream to a storage tank, accumulating the ethane-enriched stream in the storage tank to form an accumulated amount of the ethane-enriched stream and storing the accumulated amount of the ethane-enriched stream for subsequent use as a refrigerant fluid in the liquefaction system.

[0007] The present invention will now be further illustrated by way of example, and with reference to the accompanying non-limiting drawings, in which:

Figure 1 schemetically shows a general embodiment of the invention;

Figure 2 schematically shows a first process flow diagram incorporating an embodiment of the invention;

Figure 3 schematically shows a second process flow diagram incorporating another embodiment of the invention; and

Figure 4 schematically shows a third process flow diagram incorporating still another embodiment of the invention.

[0008] 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. The same reference numbers refer to similar components, streams or lines.

[0009] In the method and system disclosed herein, a side portion is selectively branched off from the hydrocarbon stream, while a remaining main portion of the hydrocarbon stream passes to a liquefaction system, to be liquefied. In this context, the term "selectively" is used to indicate that the branching off of the side portion can activated or deactivated (i.e. interrupted). An ethane-enriched stream is extracted from the side portion and passed to a storage tank for subsequent (future) use as a refrigerant fluid in the liquefaction system. The ethane-enriched stream contains a higher relative amount of ethane than the side portion of the hydrocarbon stream.

[0010] Herewith it is achieved that a scrub column or the like does not have to be in continuous operation close to the liquefaction system.

[0011] Moreover, the extraction unit can be relatively small. It does not have to be capable of handling the full the hydrocarbon stream load, but only the load of the side portion that is selectively branched-off from the hydrocarbon stream. The main portion of the hydrocarbon stream passes to a liquefaction system without passing through the extraction unit.

[0012] These advantages render the invention particularly suitable for application in an off-shore location where plot space for equipment is generally sparse. Thus, in preferred embodiments the apparatus for producing the liquefied hydrocarbon stream described herein is located on an off-shore platform, preferably in the form of a floating structure. Likewise, in preferred embodiments the method of producing the liquefied hydrocarbon stream described herein is performed on an off-shore platform, preferably in the form of a floating structure. Alternatively, the off-shore platform may be founded directly on the bed below the water, such as by legs or in the form of a gravity base structure.

[0013] Figure 1 is a conceptual respresentation of a general embodiment of an apparatus and/or method for producing a liquefied hydrocarbon stream. A hydrocarbon stream source 10 provides a hydrocarbon stream of a first composition comprising methane, ethane, and one or more hydrocarbon components heavier than ethane. The hydrocarbon stream is conveyed in a hydrocarbon feed line 30, which extends between the hydrocarbon stream source 10 and a liquefaction system 15. The hydrocarbon feed line 30 comprises a feed stream splitter 40, which divides the hydrocarbon feed line 30 in an upstream feed line section 30a, between the hydrocarbon stream source 10 and the feed stream splitter 40, and a downstream feed line section 30b, between the feed stream splitter 40 and the liquefaction system 15. The feed stream splitter 40 is arranged to selectively branch off a side portion of the hydrocarbon stream, having the same composition as the first composition, while allowing a main portion of the hydrocarbon stream to pass into the downstream feed line section 30b of the hydrocarbon feed line 30 to the liquefaction system 15, wherein it is at least cooled and liquefied to form a liquefied hydrocarbon stream 90.

[0014] A side feed stream line 70 extends between the feed stream splitter 40 and an extraction unit 200. The side feed stream line 70 comprises at least one selection valve to control selective branching off the side protion of the hydrocarbon stream. In the embodiment of Figure 1, the at least one selection valve is represented in the form of an isolation valve 69. Optionally, the at least one selection valve comprises a pressure control valve (not shown in Figure 1). This may be a separate flow control valve in addition to the isolation valve, or alternatively both intermittent isolation and flow control may be done in one valve with combined functionality.

[0015] The extraction unit 200 is capable of extracting an ethane-enriched stream (not shown in Figure 1) from the side portion of the hydrocarbon stream and producing at least one residue stream 84 comprising at least a portion of the side stream from which the ethane-enriched stream has been extracted (not shown in Figure 1). An accumulated amount of the ethane-enriched stream is stored for subsequent use as a refrigerant fluid. Optionally, a refrigerant make-up part 150 of the stored accumulated ethane-enriched stream may selectively be added to a refrigerant circuit in the liquefaction system 15.

[0016] The extraction unit 200 may comprise one or two columns, preferably distillation columns, to distillitively prepare the ethane-enriched stream. The first column, which is arranged to receive the side portion of the hydrocarbon stream, typically produces at least two first effluent streams: a vapourous first overhead stream and a liquid first bottom stream. Depending on the distillation conditions, one of the first effluent streams is or contains the ethane-enriched stream. If this is the vapourous first overhead stream, as would be the case when the first column is for instance a de-ethanizer, a de-propanizer, or a debutanizer, then it has been extracted from the heavy end of the side portion of the hydrocarbon stream. If it is the liquid first bottom stream, as would be the case when the first column is instance a de-methanizer, then it has been extracted from the light end of the side portion of the hydrocarbon stream.

[0017] An optional second column may be used to further extract the ethane-enriched stream from the said one effluent stream. If in the first column it was extracted from the heavy end of the side portion of the hydrocarbon stream, then in the second column the further extraction should preferably extract the ethane-enriched stream from the light ends; and vice versa if in the first column the said one effluent stream was extracted from the light end of the side portion of the hydrocarbon stream, then in the second column the further extraction should preferably extract the ethane-enriched stream from the heavy ends.

[0018] A possible detailed embodiment of the second option will be described below in more detail with reference to Figure 2.

[0019] Referring again to Figure 1, the downstream feed line section 30b bypasses the extraction unit 200 so that the main portion of the hydrocarbon stream passes from the feed stream splitter into the liquefaction system 15 without passing through the extraction unit 200.

[0020] The hydrocarbon stream may be provided from any suitable hydrocarbon stream source 10. It generally has a first composition comprising methane ("C₁") and amounts of hydrocarbons heavier than methane ("C₂+"; "higher hydrocarbons") including ethane ("C₂"), and one or more hydrocarbon components heavier than ethane.

[0021] If desired or necessary, the hydrocarbon stream source 10 may comprise one or more treating units to remove unwanted components from the hydrocarbon stream, or reduce the content of such unwanted components. Natural gas, for instance, and other methane-containing gases, may in addition to methane contain higher hydrocarbons, including ethane, propane ("C₃"), butanes ("C₄"), and alkanes heavier than butanes("C₅+"), such as pentanes ("C₅") and higher. The C₅+ components are generally removed to a very low level, for instance to below 50 ppm, otherwise they will solidify when producing the liquefied hydrocarbon stream 90. The hydrocarbon stream source 10 may also comprise one or more treating units to remove other freezable components, such as CO₂ and H₂O, and/or other unwanted components such as H₂S and mercury.

[0022] In addition thereto or instead thereof, the hydrocarbon stream source 10 may comprise an auxiliary heat exchanger arranged to provide the hydrocarbon stream at a temperature below an initial temperature (usually the initial temperature is the temperature of the ambient). Possible details will be described below with reference to figures 2 to 4.

[0023] Figure 2 shows a specific embodiment in more detail than in Figure 1 in the form of a first process flow diagram for an apparatus and/or method for producing a liquefied hydrocarbon stream.

[0024] A hydrocarbon feed line 30 is provided for carrying a hydrocarbon stream from which the liquefied hydrocarbon stream 80,90 is to be produced.

[0025] In the embodiment of Figure 2, the hydrocarbon stream source comprises an auxiliary heat exchanger 25, arranged to receive an initial hydrocarbon stream 20 and to discharge, into the hydrocarbon feed line 30, the hydrocarbon stream comprising the initial hydrocarbon stream 20.

[0026] The hydrocarbon feed line 30 is provided with a feed stream splitter 40. The feed stream splitter 40 divides the hydrocarbon feed line 30 into an upstream feed line section 30a being upstream of the feed stream splitter 40 as seen from the hydrocarbon stream source, and a downstream feed line section 30b being downstream of the feed stream splitter 40 as seen from the hydrocarbon stream source.

[0027] A liquefaction system is fluidly connected to the downstream feed line section 30b, and arranged to receive and liquefy a main portion of the hydrocarbon stream which is in the downstream feed line section 30b. In the embodiment of Figure 2, the liquefaction system comprises a cryogenic heat exchanger 50 that is fluidly connected to the downstream feed line section 30b. The cryogenic heat exchanger 50 forms part of a refrigerant circuit 100, and is arranged to receive and liquefy the main portion 30b of the hydrocarbon stream 30.

[0028] A side feed stream line 70 is fluidly connected to the feed stream splitter 40. It is also downstream of the feed stream splitter 40 as seen from the hydrocarbon stream source. An extraction unit, generally depicted at 200, is provided in fluid communication with the side feed stream line 70.

[0029] The extraction unit 200 is selectively connected with the feed stream splitter 40 by means of isolation valve 69 in the side feed stream line 70. The isolation valve may be fully closable to isolate the extraction unit 200 from the feed stream splitter 40 and intermittingly openable to partial or full extent.

[0030] The extraction unit 200 of Figure 2 is an example of a semi-autarkic extraction unit, in the sense that the only source of refrigeration in the extraction unit 200 is in the content of the side feed stream line 70 (vested in the thermodynamic quantities, such as temperature and pressure, of the side feed stream in side feed stream line 70). No other source of refrigeration is employed.

[0031] The extraction unit 200 comprises a first column 210, with a first inlet 211 connected to the side feed stream line 70. The first column 210 is arranged to divide the side portion from the hydrocarbon stream in the side feed stream line 70 into a methane-enriched first overhead stream 192 and a methane-depleted first bottom stream 190. If the first column is a de-methanizer, which will be assumed hereinafter, the ethane-enriched stream forms part of the methane-depleted first bottom stream 190. A first export valve 194 is provided in the methane-enriched first overhead stream 192. A first pressure controller 196 is provided to control the first export valve 194.

[0032] The extraction unit 200 may comprise an extraction unit feed condenser 250. Such an extraction unit feed condenser 250 may be arranged in the side feed stream line 70, between the feed stream splitter 40 and the first inlet 211 of the first column 210. Such an extraction unit feed condenser 250 may comprise a first expander, here provided in the form of a first flow control valve 252, arranged to expand the side portion of the hydrocarbon stream and/or a feed stream effluent heat exchanger 254 arranged to remove heat from the side portion of the hydrocarbon stream.

[0033] The feed stream effluent heat exchanger 254 may comprise a warm side and a cold side in heat exchanging contact with each other. The feed stream effluent heat exchanger 254 may be provided in the form of a tube-in-shell type heat exchanger or pipe-in-pipe heat exchanger, but preferred is a plate-type heat exchanger such as a plate-fin heat exchanger and/or a printed circuit heat exchanger.

[0034] The warm side of the feed stream effluent heat exchanger 254 may form part of the side feed stream line 70. The cold side may be in fluid communication with a second outlet 191 in the first column to receive at least part of the a methane-enriched first overhead stream 192. In the embodiment as shown in Figure 2, a feed stream effluent heat exchanger 254 is provided followed by flow control valve 252 in the side feed stream line 70 downstream of the feed stream effluent heat exchanger 254 and upstream of the first inlet 211. The first expander is shown in the form of a Joule-Thomson valve 252, but if a stronger temperature reduction is desired a mechanical expander, such as an expansion turbine, may be used instead of or preferably in addition to the flow control valve 252.

[0035] As assumed above, the methane-depleted first bottom stream 190 is assumed to contain the ethane-enriched stream. A storage tank 240 may thus be arranged to receive and store the the ethane-enriched stream in the form of the full methane-depleted first bottom stream 190. However, in the embodiment of Figure 2, the extraction unit 200 comprises a second column 220 with a second inlet 221 in fluid communication with the first outlet 189 of the first column 210, to further extract the ethane-enriched stream from the methane-depleted first bottom stream 190. A pressure reduction valve 195 may optionally be provided in the methane-depleted first bottom stream 190 between the first outlet 189 in the first column 210 and the first inlet 221 in the second column 220. The second column is arranged to receive the the methane-depleted first bottom stream 190, and to divide the methane-depleted first bottom stream 190 into a ethane-enriched second overhead stream 280 and an ethane-depleted second bottom stream 290. The ethane-enriched second overhead stream 280 comprises the ethane-enriched stream that is to be accumulated and stored in the storage tank 240.

[0036] Preferably, the extraction unit 200 has no more than two distillation columns, in the sense that it does not contain other columns in addition to the first and second columns. It may contain other items in addition to the first and second columns, such as a gas/liquid separator, heat exchangers, pumps, valves, conduits. For example, an optional overhead separator 230 will be discussed below, but this would essentially be a tank or simple vessel and not a column. In this context, a column is understood to comprise one or more of a reflux system, a reboiler, structured packing and/or contacting trays representing at least one theoretical plate.

[0037] The extraction unit 200 may comprise an overhead condenser 285, arranged to receive the ethane-enriched second overhead stream 280 and to at least partly condense said ethane-enriched second overhead stream 280. This allows the ethane-enriched stream that is to be accumulated and stored to be stored in liquid form in the storage tank 240.

[0038] The overhead condenser 285 may comprise a warm side and a cold side in heat-exchanging contact with each other. The overhead condenser 285 may be provided in the form of a tube-in-shell type heat exchanger or pipe-in-pipe heat exchanger, but preferred is a plate-type heat exchanger such as a plate-fin heat exchanger and/or a printed circuit heat exchanger.

[0039] The warm side of the overhead condenser 285 may be in fluid communication with a second outlet 223 in the second column 220. It may be arranged to receive the ethane-enriched second overhead stream 280 from the second outlet 223 in the second column 220, and to at least partly condense said ethane-enriched second overhead stream 280. Preferably, the cold side of the overhead condenser 285 is in fluid communication with the second outlet 191 in the first column 210 to receive the methane-enriched first overhead stream 192 in an indirect heat exchanging relationship with the ethane-enriched second overhead stream 280. In the embodiment shown in Figure 2, the cold side of the overhead condenser 285 is between the second outlet 191 in the first column 210 and the cold side of the feed stream effluent heat exchanger 254.

[0040] Downstream of the overhead condenser 285 an overhead separator 230 may be provided in the ethane-enriched second overhead stream 280. If the ethane-enriched second overhead stream 280 during operation is in a partially condensed state, the separator is arranged to separate the ethane-enriched second overhead stream 280 into an overhead condensate stream 232 and an overhead vapour stream 231. A second export valve 234 may be provided in the overhead vapour stream 231. Preferably, the second export valve 234 is under control by a second pressure controller 236.

[0041] A bottom outlet 233 from the overhead separator 230 is, optionally, in fluid communication with an optional overhead liquid pump 235 and an overhead liquid splitter 238. The downstream side of the overhead liquid splitter 238 is in communication with the storage tank 240 via storage line 237, and in communication with a second inlet 222 in the second column 220 via a reflux control valve 239. Thus, the storage line 237, during operation, may confer the ethane-enriched stream that is to be accumulated and stored in the storage tank 240, from the ethane-enriched second overhead stream 280.

[0042] The storage tank 240 may be provided with a boil off vapour discharge valve 242 in fluid communication with the storage tank 240, to allow controlled discharge of boil off vapour 246 from storage tank 240. Preferably, a pressure controller 224 is provided for control of the boil off vapour discharge valve 242. The boil off vapour line 246 may be connected to a flare system and/or to a fuel system and/or to a residue reinjection line 83 (as will be further explained hereinbelow).

[0043] An optional reprocessing line 158 may be provided between the storage tank 240 and the first inlet 221 of the second column 220. An optional reprocessing pump 157 and optional reprocessing control valve 159 may be provided in the optional reprocessing line 158.

[0044] As briefly mentioned before, the cryogenic heat exchanger 50 forms part of the refrigerant circuit 100. The embodiment of Figure 2 the auxiliary heat exchanger 25 also forms part of the refrigerant circuit 100. The remainder of the refrigerant circuit 100 will now be explained in more detail. A first compressor 120 is provided to compress the refrigerant. The first compressor 120 may be provided in the form of a single compressor or two or more compressors arranged in a series or parallel arrangement. As schematically drawn in Figure 2, it is a single casing compressor with multiple compression stages, mechanically driven via a first rotating shaft 126 coupled to a driver 125. The driver may be provided in any suitable form, such as a gas turbine, a steam turbine, an electric motor, or combinations thereof.

[0045] The first compressor discharge line 130 from the first compressor 120 is fluidly connected to an optional auxiliary refrigerant separator 170 via an optional auxiliary refrigerant separator inlet device 171. A first ambient cooler 135 is provided in the first compressor discharge line 130, to pass heat from the compressed refrigerant in the first compressor discharge line 130 to the ambient. An optional mist mat 172 may be provided internally in the optional auxiliary refrigerant separator 170.

[0046] An auxiliary refrigerant make-up feed line 140 may be provided, to inject a auxiliary refrigereant make-up component into line 130, suitably downstream of the first ambient cooler 135 but possibly upstream thereof. This can be used to add an auxiliary refrigerant component that can be evaporated in the auxiliary heat exchanger 25. The auxiliary refrigerant component preferably has a higher average molecular weight than the accumulated ethane-enriched stream described above that is stored in the storage tank 240. The auxiliary refrigerant component preferably predominantly consists of propane and/or butanes.

[0047] Optionally, at least one auxiliary refrigerant make-up component 140 that can be evaporated in the auxiliary heat exchanger 25 is injected into the refrigerant circuit 100.

[0048] The auxiliary refrigerant make-up component 140 is preferably imported from an external refrigerant component production line-up not comprising the extraction unit 200. This way it is not necessary for the extraction unit 200 to be able to separately produce this auxiliary refrigerant component in addition to the accumulated ethane-enriched stream that is stored in the storage tank 240. Moreover, by importing allows the auxiliary heat exchanger 25 to be operated before the extraction unit 200, which is advantageous in case the extraction unit 200 makes use of the cold vested in the hydrocarbon stream 30a.

[0049] The auxiliary refrigerant make-up component 140 is preferably sourced from an LPG tank (not shown) in which it can be stored until injecting it into line 130. An LPG transfer pump is preferably provided for the injection. A discharge pressure of about 55 bar is preferred for the LPG transfer pump. Optionally a dryer, suitably a sacrificial dryer, is provided in the auxiliary refrigerant make-up component 140 to achieve a pre-determined water specification of, for instance, maximally 1 ppmw.

[0050] In an off-shore location, the storage tank 240 and any optional LPG tank(s) are preferably located on top of the platform.

[0051] The auxiliary heat exchanger 25 has three auxiliary tube bundles (21,22,23). The first of these three auxiliary tube bundles (21) connects the initial hydrocarbon stream 20 to the hydrocarbon feed line 30. The second of these three auxiliary tube bundles (22) is connected to an optional auxiliary refrigerant separator overhead outlet 173 via an auxiliary vapour line 160a. The third of these three auxiliary tube bundles (23) is connected to an optional auxiliary refrigerant separator bottom outlet 174 via an auxiliary liquid line 160b, and to an auxiliary refrigerant return line 165. The auxiliary refrigerant return line 165 is connected to an auxiliary refrigerant inlet divider 26, arranged in a shell side of the auxiliary heat exchanger 25, via an auxiliary Joule-Thomson valve 24. The shell side of the auxiliary heat exchanger 25 is fluidly connected to an intermediate inlet in the first compressor 120 via auxiliary spent refrigerant line 175 and an optional suction drum (not shown).

[0052] The second of the three auxiliary tube bundles (22) is connected to a main refrigerant separator 180 via a main refrigerant line 60 and an optional main refrigerant separator inlet device 181 provided therein. An optional mist mat 182 may be provided internally in the optional auxiliary refrigerant separator 180.

[0053] The cryogenic heat exchanger 50 may suitably be provided in the form of a coil wound heat exchanger known in the art. It may have three bottom part tube bundles (51a,52a,53), and two top part tube bundles (51b,52b). The first of the three bottom part tube bundles (51a) may be in fluid communication with the downstream feed line section 30b on one side and with a first of the two top part tube bundles (51b) on the other side. The second of these three bottom part tube bundles (52a) is connected to a main refrigerant separator overhead outlet 61 in the main refrigerant separator 180 via main refrigerant vapour line 60a, and to the second of the top part tube bundles (52b). The other end of the second of the top part tube bundles (52b) is connected to a top refrigerant return line 66. The top refrigerant return line 66 is connected to a top refrigerant inlet divider 68, arranged in a shell side of the cryogenic heat exchanger 50, via a top Joule-Thomson valve 64. The third of the three bottom part tube bundles (53) is connected to a main refrigerant separator bottom outlet 59 in the main refrigerant separator 180 via main refrigerant liquid line 60b. The other end of the third bottom part tube bundles 53 is connected to a bottom refrigerant return line 65. The bottom refrigerant return line 65 is connected to a bottom refrigerant inlet divider 67, arranged in the shell side of the cryogenic heat exchanger 50, via an bottom Joule-Thomson valve 63. The shell side of the cryogenic heat exchanger 50 is fluidly connected to a suction inlet in the first compressor 120 via main spent refrigerant line 110 and an optional suction drum (not shown).

[0054] Optionally (not shown), a slip stream may be drawn from the main refrigerant liquid line 60b and added to the auxiliary refrigerant return line 165 to add some more relatively light refrigerant components to the shell side of the auxiliary heat exchanger 25.

[0055] The first of the two top part tube bundles (51b) in the cryogenic heat exchanger 50 is connected by a liquefied product line 80 to a liquefied product phase separator 85 via an optional liquefied product phase separator inlet device 87. The liquefied product phase separator 85 may also comprise other internals, such as an optional mist mat 88. A pressure reduction device, such as an expander turbine, a Joule-Thomson valve, or both, may be provided in the liquefied product line 80. In the example of Figure 2, the pressure reduction device is provided in the form of a Joule-Thomson valve 82. A liquefied product discharge line 90 is in fluid communication with the liquefied product phase separator 85 via a bottom outlet 89. A flash vapour discharge line 95 is in fluid communication with the liquefied product phase separator via flash vapour outlet 91.

[0056] The at least one residue stream may be formed from one or more of the group consisting of the overhead vapour stream 231, the methane-enriched first overhead stream 192, and the ethane-depleted second bottom stream 290.

[0057] One or more of the group consisting of the overhead vapour stream 231, the methane-enriched first overhead stream 192, the ethane-depleted second bottom stream 290, and the boil off vapour in line 246 is in fluid communication with the liquefied product phase separator 85 via a residue reinjection line 83. This way, at least some molecules from these stream will end up in the liquefied product in discharge line 90 without having to recompress. The remaining molecules will be processed together with the flash vapour via flash vapour discharge line 95. Particularly, the the ethane-depleted second bottom stream 290 may be cooled by indirect heat exchanging against the methane-enriched first overhead stream 192, for instance against the methane-enriched first overhead stream 192 being discharged from the feed stream effluent heat exchanger 254.

[0058] Alternatively, one or more of these stream may be disposed of, for instance by incineration. In this case, the ethane-depleted second bottom stream 290 may be vaporized in a vaporizer (not shown) before incineration.

[0059] The storage tank 240 is selectively connected to the refrigerant circuit 100 via a make-up line 150. This can be in any suitable location of the refrigerant circuit 100, preferably in the main refrigerant line 60 upstream of the main refrigerant separator 180 and downstream of the auxiliary heat exchanger 25. Optionally, a transfer pump 155 is provided in the make-up line 150.

[0060] The first column 210 and the second column 220 in the extraction unit 200 may be any type of column suitable for cryogenic distillation, optionally comprising internals as known in the art including contacting trays and/or packing, inlet devices, draw-off trays, etc.. Preferably, the first column 210 is a de-methanizer and the second column 220 is a de-ethanizer. In the embodiment of Figure 2, the first column 210 is serviced by an optional first reboiler 219. The optional first reboiler 219 is fluidly connected to the first column 210 via first reboiler line 215 and a third outlet 216 in the first column 210 upstream of the first reboiler 219 and a second inlet 214 into the first column 210 downstream of the first reboiler 219. Likewise, the second column 220 is optionally serviced by an optional second reboiler 229, which is fluidly connected to the second column 220 via second reboiler line 225 and a third outlet 226 in the second column 220 upstream of the second reboiler 229 and a second inlet 224 into the second column 220 downstream of the second reboiler 229.

[0061] The first reboiler 219 is heated by a first heat source 218, the second reboiler by a second heat source 228. These heat sources may be electrical heaters. Alternatively, the heat sources such as electrical heaters may be internal to the respective columns (not shown) to provide reboiler duty. Preferably, the reboilers are of thermosiphon type with electrical heaters.

[0062] During operation, the apparatus described above works as follows.

[0063] A hydrocarbon stream 30a of a first composition comprising methane, ethane, and one or more hydrocarbon components heavier than ethane, is provided in hydrocarbon feed line 30. The providing of this hydrocarbon stream 30a may comprise providing an initial hydrocarbon stream 20 at an intial temperature, and while passing the initial hydrocarbon stream 20 through an auxiliary heat exchanger 25 extracting heat from the initial hydrocarbon stream 20. The hydrocarbon stream 30a comprising the initial hydrocarbon stream may then be discharged from the auxiliary heat exchanger 25 at a lower temperature than the initial temperature. Preferably, the heat is extracted by evaporation of an auxiliary refrigerant component having a higher molecular mass than that of ethane, such as the auxiliary refrigerant liquid in auxiliary liquid line 160b. The auxiliary refrigerant component is preferably imported from an auxiliary source external to the apparatus and method described herein, and not produced in the extraction unit 200.

[0064] Preferably, the first composition meets a specification for a fully liquefied product stream, such as an LNG stream, to the extent that no higher hydrocarbons need to be extracted before liquefaction of the hydrocarbon stream.

[0065] Nevertheless, operation of extraction unit 200 is at times desired to produce and/or replenish a volume of higher hydrocarbons, as refrigerant make-up buffer. The refrigerant make-up buffer produced/replenished this way preferably predominantly consists of ethane, because other components useful as make-up component for the refrigerant including propane and butanes can suitably be imported from an external source. Importing of ethane from an external source is more problematic, because there is currently no existing market of liquid ethane.

[0066] Upon need, a side portion 70 of the hydrocarbon stream 30a may selectively be branched off from feed stream splitter 40 by selectively opening the isolation valve 69 in the side feed stream line 70. The selective branching off may be intermittently activated, when production of refrigerant is needed, whereby no side portion is branched off at times when no refrigerant needs to be produced. The pressure drop in the isolation valve 69 is preferably small compared to the pressure drop in the remainder of the feed stream line 70. Preferably, the isolation valve 69 is controlled by snapaction control (i.e. a two-position on/off control mode) whereby a controller either opens or closes the isolation valve 69.

[0067] The side portion has the same composition as the first composition. The side portion in the side feed stream line 70 is fed to the extraction unit 200 to produce the ethane-enriched stream for storage in storage tank 240 for later use as a refrigerant fluid. The accumulated ethane-enriched stream in the storage tank 240 is a buffer quantity for refrigerant make-up.

[0068] The flow rate of the side portion 70 may be selected in dependence of how much time is available for accumulating a full buffer quantity and in dependence of the available heating capacity for the extraction unit 200. It is typically recommended that the flow rate of the side portion 70 is less than 5 % of the flow rate of the the hydrocarbon stream 30a ("relative flow rate"). In one example the relative flow rate was 1.5 %. By keeping the relative flow rate less than 5 %, the cooling and liquefying of main portion 30b of the hydrocarbon stream 30 in the liquefaction system 15 is not disrupted too much. By allowing a relative flow rate of more than 0.5 % the time required to accumulate enough of the ethane-enriched stream is acceptable, for instance less than 30 days.

[0069] Suitably, the refrigerant 130 in said refrigerant circuit 100 is a mixed refrigerant comprising at least ethane and at least one of the group consisting of nitrogen, methane, propane, butane, isobutane, and pentane. The accumulated ethane-enriched stream in storage tank 240 preferably comprises predominantly ethane. A mixed refrigerant or a mixed refrigerant stream as referred to herein comprises at least 5 mol% of two different components.

[0070] Described in more detail, the side portion is divided in the first column 210 into a methane-enriched first overhead stream 192 and a methane-depleted first bottom stream 190. The side portion is preferably a mixture of vapour and liquid phases. If side portion 70 is in vapour phase, it may be passed through the extraction unit feed condenser 250 upstream of the first column 210, to partially condense the side portion 70 before feeding it to the first column 210.

[0071] In alternative embodiments, if the side portion 70 is in liquid phase, instead of an extraction unit feed condenser 250 it could be passed through an extraction unit feed evaporator (not shown). If the side portion 70 is already of mixed phase, it may be either passed through an extraction unit feed evaporator or an extraction unit feed condenser in order to change the relative amount of vapour in the side portion 70 before feeding it into the first column 210.

[0072] The vapour and liquid phases do not have to be fed to the first column 210 mixed together as shown in Figure 2, but they can be phase separated and then fed to the first column 210 via mutually different inlets.

[0073] In the embodiment of Figure 2, the side portion 70 is heat exchanged against the methane-enriched first overhead stream 192 in the feed stream effluent heat exchanger 254 and subsequently expanded in the Joule Thomson valve 252 to a first separation pressure of for instance less than 35 bar gauge. Preferably, the first separation pressure is less than 20 bar gauge, allowing for equipment of ISO class 300 lbs. In a preferred embodiment, the fist separation pressure is in a range of from 2 bar absolute to 12 bar absolute. The first separation pressure may be controlled by adjusting the first export valve 194.

[0074] The temperature of the side portion 70 as it is being being fed into the first column 210 may be in the range of from -120°C to -80°C, preferably in the range of from -110°C to -90°C, more preferably in the range of from -105°C to -95°C.

[0075] Heat is added to the bottom of the first column 210. The temperature in the bottom of the first column 210 is controlled to maximise C₂+ recovery in the methane-depleted first bottom stream 190. The temperature in the bottom of the first column 210 may for example be in the range of from -15°C to 0 °C. The methane-enriched first overhead stream 192 and the methane-depleted first bottom stream 190 are discharged from the first column 210.

[0076] Optionally, the methane-depleted first bottom stream 190 may further be cryogenically separated in the second column 220 at a second separation pressure, into a ethane-enriched second overhead stream 280 and an ethane-depleted second bottom stream 290. The second separation pressure may essentially be equal to the first separation pressure (for instance within 1 bar absolute), or optionally, the methane-depleted first bottom stream 190 is expanded in the pressure reduction valve 195 to the second separation pressure, before being fed into the second column 220. The second separation pressure may be in the range of from 2 bar absolute to 12 bar absolute.

[0077] The second pressure may be controlled by the pressure in the first column 210, or via separate pressure control which may be preferred if the methane-depleted first bottom stream 190 is optionally expanded. Separate pressure control may for instance be achieved by adjusting the second export valve 234 in response to the second pressure controller 236. The ethane-enriched second overhead stream 280 contains the ethane-enriched stream that is to be accumulated and stored in the storage tank 240. The overhead temperature of the second column 220 is controlled to maximise the amount of ethane in the ethane-enriched second overhead stream 280. The overhead temperature may be in the range of from -30°C to -10 °C. The second reboiler 229 heating duty is controlled based on the feed rate into the second column 220, to ensure that sufficient upward vapour transport takes place in the second column 220 without overloading the overhead condenser 285. The bottom temperature in the second column may be in the range of from 0 °C to 30 °C.

[0078] In the embodiment of Figure 2, the ethane-enriched second overhead stream 280 is passed through the overhead condenser 285, via its warm side, wherein it is at least partially condensed. In embodiments it may fully condensed, preferably fully condensed and subcooled. In a preferred embodiment, the at least partially condensing involves indirectly exchanging heat with the methane-enriched first overhead stream 192, which is passed to the overhead condenser 285 from the second outlet 191 in the first column 210 and through the overhead condenser 285 via its cold side. The temperature of the ethane-enriched second overhead stream 280 after having passed through the overhead condenser 285 may be in the range of from -60 °C to -30 °C.

[0079] The ethane-enriched second overhead stream 280, after having passed through the overhead condenser 285, is optionally phase separated into a second column vapour residual stream 231, which can exisist if the ethane-enriched second overhead stream 280 is not fully condensed, and a ethane-enriched liquid stream 232. Alternatively, the ethane-enriched liquid stream 232 can simply consist of the full effluent from the warm side of the overhead condenser 285.

[0080] The ethane-enriched liquid stream 232 may be passed entirely or in part to the storage tank 240, optionally via overhead liquid pump 235. A reflux portion may optionally be drawn from the ethane-enriched liquid stream 232 using overhead liquid splitter 238 and reflux control valve 239. The reflux portion is fed into the second column 220 via the second inlet 222, after having passed from the overhead liquid splitter 238 through the reflux control valve 239.

[0081] The ethane-enriched liquid stream 232 typically comprises more than 50 mol% ethane, preferably more than 70 mol% ethane. The balance is mostly methane and nitrogen. In preferred embodiments, the ethane-enriched liquid stream 232 is not exported from the method or apparatus or sold as a separate product, but it is only used as refrigerant make-up, and optionally a reflux stream, for in the apparatus and method itself. Therefore the ethane-purity of the ethane-enriched liquid stream 232 typically needs not be very high. An ethane content of less than 95 mol.%, or even less than 90 mol.%, can be afforded. Typically, the ethane-enriched liquid stream 232 is produced at an ethane-purity of about 80 mol.%. Preferably at least 80 mol% of the balance consists of propane and/or preferably at least 90 mol% of the balance consists of propane and butanes.

Example

[0082] A process simulation has been carried out to demonstrate extraction of an ethane-enriched stream with 80 mol.% ethane using the extraction unit 200 as illustrated in Figure 2. For this simulation, a side portion 70 of 1.5 mol.% of the hydrocarbon stream 30a was fed to the extraction unit 200 at a pressure of 68 bar absolute. The composition of the hydrocarbon stream 30a and the side portion 70 was identical and as listed in Table 1. Table 2 shows pressure and temperature in various lines identified with reference numbers corresponding to those used in Fig. 2. The resulting compositions of the streams 232, 290 and 192 are also given in Table 1.

Table 1. Composition (in mol.% unless otherwise specified).

Molecule	Stream 70	Stream 232	Stream 290	Stream 192
N2	2.6	0	0	2.7
C1	91.4	0.8	0	95.5
C2	3.0	80.0	5.0	1.7
C3	2.0	18.0	58.5	0.1
iC4+nC4	1.0	1.2	36.0	0
C5	<0.1	10 ppm	0.5	0

Tabel 2. Temperature (in ºC) and pressure (in bara).

	70	192	190	280	232	290
Temp.	-22	-102	-7	-20	-47	19
Pres.	68	8	8	7	6.5	7

[0083] The temperature and pressure given in Table 2 for line 70 corresponds to the conditions at the inlet of the feed stream effluent heat exchanger 254. Between the feed stream effluent heat exchanger 254 and the Joule Thomson valve 252 the temperature was -50 °C and the pressures 67 bara, and at the first inlet 211 it was respectively -102 °C and 8 bara. The temperature of stream 192 given in Table 2 corresponds to the temperature at the second outlet 191. Between the overhead condenser 285 and the feed stream effluent heat exchanger 254 the temperature was -93 °C, and coming out of the feed stream effluent heat exchanger 254 the temperature was -32 °C at approximately 7 bar.

[0084] The ethane content in the ethane-depleted second bottom stream 290 was set at 5 mol.%, which allowed to achieve an ethane recovery of 42 %. The methane content of the ethane-enriched stream is less than 1.0 mol.%.

[0085] This ends the example.

[0086] The extraction unit 200 may be designed differently than the one shown in Figure 2. For instance, instead of two successive columns, the first and second colums could be combined into a single column, whereby the ethane-enriched stream may be extracted as a side stream drawn off from an intermediate height in the single column.

[0087] Irrespective of the arrangement of the extraction columns in the extraction unit 200, the pressure in the storage tank 240 may be controlled by continuously venting storage boil off vapour 246 over the pressure-controlled boil off vapour discharge valve 242. The pressure may be selected based on finding a compromise by managing the boil-off rate (maintaining the boil-off rate lower than a target maximum boil-off rate) and safety. A typical suitable pressure range to select from is from 5 to 20 bar gauge, depending on the desired bubble point. In one example, the target pressure in the storage tank 240 was 10 bara under which ethane-enriched liquid could have a bubble point of about -30 °C depending on composition of the ethane-enriched stream.

[0088] The composition of the ethane-enriched liquid being stored in the storage tank 240 changes over time as a result of lighter components boiling off. If desired, the content of the storage tank 240 may optionally be reprocessed by pumping liquid from the storage tank, for instance by means of the optional reprocessing pump 157, via the optional reprocessing line 158 back into the second column 220. This can be achieved via a specific inlet means into the second column 220 or suitably by reinjection in into the methane-depleted first bottom stream 190, either downstream or upstream (as shown in Fig. 2) of the pressure reduction valve 195.

[0089] A main portion 30b of the hydrocarbon stream 30a stays in the hydrocarbon feed line 30 and passes to the liquefaction system, where it is liquefied. It does not pass through the extraction unit 200. The liquefying of the main portion 30b of the hydrocarbon stream comprises feeding the main portion 30b into the cryogenic heat exchanger 50, and heat exchanging the main portion 30b against one or more main fractions of the main refrigerant portion in main refrigerant line 60.

[0090] The main portion 30b passes through the first bottom part tube bundle 51a wherein it is essentially fully liquefied, and subsequently through the first top part tube bundle 51b wherein it is subcooled. The subcooled liquefied product is discharged into the liquefied product line 80, let down in pressure 82, preferably to a pressure of between 0 and 1 barg, and fed into the liquefied product phase separator 85. In one example, it is let down to a pressure of 0.2 barg. Any vapour phases entering the liquefied product phase separator 85, such as flash vapour entering the liquefied product phase separator 85 via the liquefied product line 80 and residue vapours optionally entering liquefied product phase separator 85 via optional residue recycle line 83, are separated from any liquid phases and discharged through flash vapour outlet 91 into the flash vapour discharge line 95. These vapours may be compressed to fuel gas pressure and used as fuel gas in the method and apparatuses described herein and/or sent to a flare stack for disposal. The liquid phases are discharged through bottom outlet 89 into the liquefied product discharge line 90, via which they may be passed to product storage or an export vessel.

[0091] The main refrigerant portion is derived from the refrigerant stream 130 being cycled in the refrigerant circuit 100. The refrigerant stream 130 is discharged from the discharge outlet of the first compressor 120 in a pressurized condition. The pressure may be determined based on the refrigeration needs, but a typical pressure range is from 30 bara to 60 bara. In one example, the pressure is between 45 bara and 50 bara. The refrigerant stream 130 is passed through the first ambient cooler 135, to transfer heat from the compressed refrigerant in the first compressor discharge line 130 to the ambient, into the auxiliary refrigerant separator 170. In the embodiment of Figure 2, the passing of the heat from the the compressed refrigerant results in partial condensation of the compressed refrigerant in line 130. The condensed fraction is separated from the vapour fraction and passed to the third auxiliary tube bundle 23 via the auxiliary liquid line 160b. The condensed fraction is subcooled in the auxiliary heat exchanger 25, let down in pressure by means of the auxiliary Joule-Thomson valve 24 and led into the shell side of the auxiliary heat exchanger 25. Here it evaporates using heat from three auxiliary tube bundles 21,22,23. The evaporated refrigerant is then passed to the intermediate inlet in the first compressor 120 via the auxiliary spent refrigerant line 175 and optionally optional suction drum (not shown).

[0092] The not condensed fraction of the refrigerant stream 130 is passed to the second auxiliary tube bundle 22 via the auxiliary vapour line 160a. It is partially condensed in the auxiliary heat exchanger 25, and passed into the main refrigerant separator 180, together with any refrigerant make-up part from the make-up line 150, via the main refrigerant line 60.

[0093] In order to use the refrigerant make-up buffer quantity stored in the storage tank 240, the storage tank 240 is selectively connected to the refrigerant circuit 100, whereby at least a make-up part 150 of the content, preferably the liquid content, of the storage tank 240 is added to the refrigerant in the refrigerant circuit 100. The selective connection to the refrigerant circuit is an intermittent action, needed only when refrigerant make-up is necessary. Described more generally, at least a make-up part 150 of the at least the fraction of the methane-depleted first bottom stream 190 is added from the storage tank 240 to the refrigerant circuit 100.

[0094] One way to achieve this is by operating the optional transfer pump 155. The discharge pressure should exceed the pressure of the refrigerant in the refrigerant circuit 100 at the injection point. In one example, the discharge pressure was 55 bara. The capacity of the transfer pump 155 may be selected in accordance with the target timespan for filling the make-up refrigerant inventory in the refrigerant circuit 100. In one example a target timespan of 6 hours was selected.

[0095] The liquid components entering the main refrigerant separator 180 via the main refrigerant line 60 are separated from vapour components and passed through the third bottom part tube bundle 53 via the main refrigerant liquid line 60b. Here it is subcooled in the cryogenic heat exchanger 50, let down in pressure by means of the bottom Joule-Thomson valve 63 and led into the shell side of the cryogenic heat exchanger 50, preferably at a height above the bottom part tube bundles and below the top part tube bundles. Here it evaporates using heat from three bottom part tube bundles 51a,52a,53.

[0096] The vapour components entering the main refrigerant separator 180 via the main refrigerant line 60 are passed through the second bottom part tube bundle 52a via the main refrigerant vapour line 60a, where it is a least partially condensed, and subsequently through the second top part tube bundle 52b where it is subcooled (after having been fully condensed). Subsequently it is let down in pressure by means of the top Joule-Thomson valve 64 and fed into the shell side of the cryogenic heat exchanger 50, preferably at a height above the top part tube bundles.

[0097] The evaporated main refrigerant is then passed to the suction inlet in the first compressor 120 via the main spent refrigerant line 110 and optionally via optional suction drum (not shown).

[0098] The first compressor 120 is driven by the driver 125, and compresses the evaporated refrigerant from the main spent refrigerant line 110 and the auxiliary spent refrigerant line 175. The refrigerant stream 130 is again discharged from the discharge outlet of the first compressor 120, which completes the cycle.

[0099] In the embodiment described with reference to Figure 2, the refrigerant circuit 100 provides an auxiliary refrigerant fraction (160b) for evaporation in the auxiliary heat exchanger 25 as well as a main refrigerant fractions (60a, 60b) for evaporation in the cryogenic heat exchanger 50 of the liquefaction system. This may be referred to as a single mixed refrigerant process. An example of a single mixed refrigerant process can be found in US Patent 5,832,745. In such a single mixed refrigerant process, the refrigerant being cycled in the refrigerant circuit 100 may be formed of a mixture of two or more components within the following composition:<20 mol% N₂, 20-60 mol% methane, 20-60 mol% ethane, <30 mol% propane, <15% butanes and <5% pentanes; having a total of 100%.

[0100] Figure 3 shows an alternative, wherein a separate auxiliary refrigerant circuit 105 is provided that is decoupled from the refrigerant circuit 100. The refrigerant fluid being cycled in the refrigerant circuit 100 is evaporated in the crycogenic heat exchanger 50 but not evaporated in the auxiliary heat exchanger 25. The auxiliary refrigerant fluid being cycled in the auxiliary refrigerant circuit 105 is evaporated in the auxiliary heat exchanger 25 but not evaporated in the crycogenic heat exchanger 50.

[0101] The auxiliary refrigerant circuit 105 works as follows. An auxiliary refrigerant stream 131 is discharged from the discharge outlet of a second compressor 121, in a pressurized condition. The pressure may be determined based on the refrigeration needs, but a typical pressure range is from 30 bara to 60 bara. In one example, the pressure is between 45 bara and 50 bara. The auxiliary refrigerant stream 131 is passed through a second ambient cooler 136, to transfer heat from the compressed auxiliary refrigerant in the second compressor discharge line 131 to the ambient, and into the third auxiliary tube bundle 23. An optional refrigerant accumulator (not shown) may be provided in the auxiliary refrigerant stream 131 between the second ambient cooler 136 and the start of the third auxiliary tube bundle 23. It is assumed that the auxiliary refrigerant stream is essentially fully condensed in the second ambient cooler 136. The condensed auxiliary refrigerant is subcooled in the auxiliary heat exchanger 25, let down in pressure by means of the auxiliary Joule-Thomson valve 24 and led into the shell side of the auxiliary heat exchanger 25. Here it evaporates using heat from three auxiliary tube bundles 21,22,23. The evaporated refrigerant is then passed to the suction inlet in the second compressor 121 via the auxiliary spent refrigerant line 175 and optionally via an optional suction drum (not shown).

[0102] The second compressor 121 may be driven by a separate second driver 127 via a second rotating shaft 128. Alternatively, the first compressor 120 and the second compressor 121 may be mechanically connected on a single rotating shaft and commonly driven by one or more drivers.

[0103] The refrigerant circuit 100 works almost in the same way as explained above with reference to Figure 2, except that the auxiliary refrigerant separator 170 is not provided in line 130 so that the refrigerant stream 130 having passed through the first ambient cooler 135 is passed straight to the second auxiliary tube bundle 22 of the auxiliary heat exchanger 25.

[0104] The embodiment illustrated in Figure 3 can be a double mixed refrigerant process. An example of a double mixed refrigerant process of the state of the art is published in US Pat. 6,370,910.

[0105] Suitably, the auxiliary refrigerant has a higher average molecular weight than main refrigerant. More specifically the auxiliary refrigerant in the auxiliary refrigerant circuit 105 may be formed of a mixture of two or more components within the following composition: 0-20 mol% methane, 0-40 mol% ethane, 20-80 mol% propane, <20 mol% butanes, <10 mol% pentanes; having a total of 100%. The main refrigerant in the refrigerant circuit 100 may be formed of a mixture of two or more components within the following composition: <10 mol% N₂, 30-60 mol% methane, 30-60 mol% ethane, <20 mol% propane, and <10% butanes; having a total of 100%.

[0106] The auxiliary refrigerant circuit 105 may comprise an auxiliary refrigerant make-up header 132 for allowing injection of auxiliary refrigerant make-up 140 and/or refrigerant makeup from storage tank 240 via line 150'. The refrigerant circuit 100 may, in addition to the make-up line 150, comprise an auxiliary make-up line 140' for injecting refrigerant components having a higher molecular weight than those stored in storage tank 240.

[0107] In still another alternative (not shown), the auxiliary refrigerant circuit is operated using an essentially single component refrigerant stream. Suitably, the single component is propane. In such a case, instead of being provided in the form of a tube-in-shell type heat exchanger, the auxiliary heat exchanger may suitably be provided in the form of one or more kettles. These kettles could be operated under mutually different propane evaporation pressures. Suitably, a string of one or more kettles operated at different pressures is provided instead of the first auxiliary tube bundle 21 of auxiliary heat exchanger 25 and a separate string of one or more kettles operated is provided instead of the second auxiliary tube bundle 22 of auxiliary heat exchanger 25. An example of such parallel strings of heat exchangers is shown for instance in US Pat. 6,962,060.

[0108] Especially, but not exclusively, in cases such with a separate auxiliary refrigerant circuit for cooling the auxiliary heat exchanger the initial hydrocarbon stream 20 may be discharged from the auxiliary heat exchanger in a partially condensed state. In such cases, as illustrated via the embodiments of Figures 3 and 4, the hydrocarbon stream source 10 further may comprise a hydrocarbon stream separator 35 arranged to receive the partially condensed hydrocarbon stream from the auxiliary heat exchanger 25 via a line 31, and to separate the partially condensed hydrocarbon stream in its liquid and vapour parts. Like explained in connection with the optional overhead separator 230, the hydrocarbon stream separator 35 would preferably be essentially a tank or simple vessel rather than a column. An optional mist mat 38 could nevertheless form part of the internals as well as an inlet device 37. The feed stream splitter 40 is in fluid communication with one of the discharge openings of the hydrocarbon stream separator 35 to receive a separated phase from the hydrocarbon stream separator 35.

[0109] In the embodiment of Figure 3 the feed stream splitter 40 is in fluid communication with the hydrocarbon stream separator 35 via a hydrocarbon stream separator liquid discharge opening 41 provided in the bottom area of the hydrocarbon stream separator 35, so that the upstream feed line section 30a extends between the feed stream splitter 40 and the a hydrocarbon stream separator liquid discharge opening 41.

[0110] The main portion of the hydrocarbon stream in the downstream feed line section 30b is passed to cryogenic heat exchanger 50 in the liquefaction system. The vapour part 32 of the partially condensed hydrocarbon stream 31 exits the hydrocarbon stream separator 35 via an overhead vapour discharge opening 42 and may also be passed to the liquefaction system to be liquefied and optionally subcooled. This can be done in a separate tube bundle in the cryogenic heat exchanger 50 (not shown), a separate heat exchanger, or, as depicted in Figure 3, in the same tube bundles (51a,51b) as the main portion of the hydrocarbon stream that is passed to the cryogenic heat exchanger 50 via the downstream feed line section 30b. In the latter case, the vapour part 32 of the partially condensed hydrocarbon stream 31 and the main portion of the hydrocarbon stream from the downstream feed line section 30b are suitably mixed in a mixing header 39 upstream of or integrated with the cryogenic heat exchanger 50.

[0111] The extraction unit 200 may be designed differently than the one shown in Figure 2. For instance, instead of an extraction unit feed condenser 250, an extraction unit feed evaporator (not shown) may be provided since in this embodiment the side portion 70 is in liquid phase. Also, the extraction unit 200 may be based on a single column (preferably a de-ethanizer), because in this embodiment the hydrocarbon stream in the hydrocarbon feed line 30 is already a relatively heavy cut from the initial hydrocarbon stream 20, which has condensed out of most of the methane content of the initial hydrocarbon stream 20.

[0112] Alternatively, as illustrated in Figure 4, the feed stream splitter 40 is in fluid communication with the hydrocarbon stream separator 35 via the hydrocarbon stream separator vapour discharge opening 42, so that the upstream feed line section 30a extends between the feed stream splitter 40 and the a hydrocarbon stream separator vapour discharge opening 42.

[0113] The main portion of the hydrocarbon stream in the downstream feed line section 30b is passed to cryogenic heat exchanger 50 in the liquefaction system. The liquid part 33 of the partially condensed hydrocarbon stream 31 exits the hydrocarbon stream separator 35 via the liquid discharge opening 41 and may also be passed to the liquefaction system, to be subcooled. Suitably, the main portion in the downstream feed line section 30b and the liquid part 33 are mixed in the mixing header 39 upstream of or integrated with the cryogenic heat exchanger 50.

[0114] Also in the case of Figure 4, the extraction unit 200 may be based on a single column (preferably a de-methanizer), because in this embodiment the hydrocarbon stream in the hydrocarbon feed line 30 is already a relatively light cut from the initial hydrocarbon stream 20, from which most of the heavier components have condensed out. The extraction unit 200 could then serve to exctact the ethane-enriched stream from an undesired excess of light components, such as methane.

[0115] In other aspects, the embodiment of Figure 4 works the same as the embodiment of Figure 3.

[0116] The hydrocarbon stream source 10 may take in any suitable gas stream comprising methane, ethane, and one or more hydrocarbon components heavier than ethane. A gas stream can be derived from for instance a natural gas stream obtained from natural gas or petroleum reservoirs a synthetic gas stream such as obtained by a Fischer-Tropsch process.

[0117] 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

1. Apparatus for producing a liquefied hydrocarbon stream, said apparatus comprising:

- a hydrocarbon stream source (10) for providing a hydrocarbon stream of a first composition comprising methane, ethane, and one or more hydrocarbon components heavier than ethane;

- a hydrocarbon feed line (30) for carrying the hydrocarbon stream;

- a feed stream splitter (40) in the hydrocarbon feed line, dividing the hydrocarbon feed line in an upstream feed line section (30a) and a downstream feed line section (30b), and arranged to selectively branch off a side portion of the hydrocarbon stream, said side portion having the same composition as the first composition, whereby a main portion of the hydrocarbon stream passes into the downstream feed line section (30b) of the hydrocarbon feed line (30);

- a liquefaction system (15) fluidly connected to the downstream feed line section (30b) and arranged to receive and liquefy the main portion of the hydrocarbon stream;

- a side feed stream line (70) fluidly connected to the feed stream splitter (40) for selectively carrying the side portion of the hydrocarbon stream;

- an extraction unit (200) arranged to extract an ethane-enriched stream from the side portion of the hydrocarbon stream from the side feed stream line (70), said ethane-enriched stream containing a higher relative amount of ethane than the side portion of the hydrocarbon stream from the side feed stream line (70);

- a storage tank (240) arranged to receive the ethane-enriched stream and store an accumulated amount of the ethane-enriched stream for subsequent use as a refrigerant fluid in the liquefaction system (15).

2. The apparatus of claim 1, wherein the liquefaction system (15) comprises a cryogenic heat exchanger (50) fluidly connected to the downstream feed line section (30b) and arranged to receive and liquefy the main portion of the hydrocarbon stream, by heat exchanging against one or more main fractions (60a,60b) of a refrigerant stream (130) being cycled in a refrigerant circuit (100), and wherein said storage tank (240) is selectively connectable to the refrigerant circuit (100) for selectively adding at least a make-up part (150) of the accumulated amount of the ethane-enriched stream from the storage tank (240) to the refrigerant circuit (100).

3. The apparatus of claim 1 or 2, wherein the extraction unit (200) is a semi-autarkic extraction unit.

4. The apparatus of any one of claims 1 to 3, wherein the extraction unit (200) comprises a first column (210) with a first inlet (211) in fluid communication with the side feed stream line (70), said first column (210) arranged to divide the side portion into a methane-enriched first overhead stream (192) and a methane-depleted first bottom stream (190).

5. The apparatus of claim 4, wherein the extraction unit (200) comprises an extraction unit feed condenser (250) arranged in the side feed stream line (70) between the feed stream splitter (40) and the first inlet (211) of the first column (210).

6. The apparatus of claim 5, wherein the extraction unit feed condenser (250) comprises at least one of the group consisting of a first expander arranged to expand the side portion of the hydrocarbon stream and a feed stream effluent heat exchanger (254).

7. The apparatus of any one of claims 4 to 6, wherein the methane-depleted first bottom stream (190) contains the ethane-enriched stream, and wherein the extraction unit (200) optionally further comprises a second column (220) with a second inlet (221) in fluid communication with the first column (210) to receive the methane-depleted first bottom stream (190) and arranged to divide the methane-depleted first bottom stream (190) into a ethane-enriched second overhead stream (280) and an ethane-depleted second bottom stream (290), wherein the ethane-enriched second overhead stream (280) comprises the ethane-enriched stream that is to be stored in the storage tank (240).

8. The apparatus of any one of claims 4 to 6, wherein the a methane-enriched first overhead stream (192) contains the ethane-enriched stream, and wherein the extraction unit (200) optionally further comprises a second column with a second inlet in fluid communication with the first column to receive the a methane-enriched first overhead stream (192) and arranged to divide the a methane-enriched first overhead stream (192) into a ethane-depleted second overhead stream and an ethane-enriched second bottom stream, wherein the ethane-enriched second bottom stream comprises the ethane-enriched stream that is to be stored in the storage tank (240).

9. The apparatus of any one of the preceding claims, wherein the hydrocarbon stream source comprises an auxiliary heat exchanger (25) arranged to receive an initial hydrocarbon stream (20) at an intial temperature, and to allow the initial hydrocarbon stream (20) to pass through the auxiliary heat exchanger (25) in an indirect heat exchanging arrangement against at least an evaporating auxiliary refrigereant component (140), and to discharge, at a lower temperature than the initial temperature, the hydrocarbon stream comprising the initial hydrocarbon stream, wherein the auxiliary refrigerant component (140) is obtained from a refrigerant component production apparatus not comprising the said extraction unit (200).

10. Method of producing a liquefied hydrocarbon stream, the method comprising:

- providing a hydrocarbon stream (30a) of a first composition comprising methane, ethane, and one or more hydrocarbon components heavier than ethane;

- selectively branching off a side portion (70) of the hydrocarbon stream (30a), said side portion having the same composition as the first composition, whereby a main portion (30b) of the hydrocarbon stream (30a) passes to a liquefaction system;

- liquefying the main portion (30b) of the hydrocarbon stream in the liquefaction system;

- passing the side portion (70) to an extraction unit (200) and extracting an ethane-enriched stream from the side portion, said ethane-enriched stream containing a higher relative amount of ethane than the side portion;

- passing the ethane-enriched stream to a storage tank (240), accumulating the ethane-enriched stream in the storage tank to form an accumulated amount of the ethane-enriched stream and storing the accumulated amount of the ethane-enriched stream for subsequent use as a refrigerant fluid in the liquefaction system (15).

11. The method of claim 10, wherein said liquefaction system comprises a cryogenic heat exchanger (50), and wherein said liquefying of said main portion (30b) of said hydrocarbon stream comprises feeding the main portion (30b) into the cryogenic heat exchanger (50) and heat exchanging the main portion (30b) against one or more main fractions (60a,60b) of a refrigerant stream (130) being cycled in a refrigerant circuit (100), wherein said refrigerant stream (130) comprises a mixed refrigerant comprising at least ethane and at least one of the group consisting of nitrogen, methane, propane, butane, isobutane, and pentane, which refrigerant stream (130) comprises a portion of the accumulated amount of the ethane-enriched stream from the storage tank (240).

12. The method of claim 11, further comprising selectively connecting the storage tank (240) to the refrigerant circuit (100) and selectively adding at least a make-up part (150) of the accumulated amount of the ethane-enriched stream from the storage tank (240) to the refrigerant in the refrigerant circuit (100).

13. The method of any one of claims 10 to 12, wherein the extraction unit (200) comprises a first column (210), said method further comprising dividing, in said first column (210), the side portion into a methane-enriched first overhead stream (192) and a methane-depleted first bottom stream (190), which methane-depleted first bottom stream (190) contains the ethane-enriched stream.

14. The method of claim 13, wherein the extraction unit (200) further comprises a second column (220), said method further comprising dividing the methane-depleted first bottom stream (190), in said second column (220), into a ethane-enriched second overhead stream (280) and an ethane-depleted second bottom stream (290), wherein the ethane-enriched second overhead stream (280) comprises the ethane-enriched stream that is to be stored in the storage tank (240).

15. The method of any one of claims 10 to 14, wherein said providing of said hydrocarbon stream (30a) comprises providing an initial hydrocarbon stream (20) at an intial temperature, passing said initial hydrocarbon stream (20) through an auxiliary heat exchanger (25) and extracting heat from the initial hydrocarbon stream (20) in the auxiliary heat exchanger (25) by using said heat to evaporate an auxiliary refrigerant component (140) in the auxiliary heat exchanger (25), and discharging the hydrocarbon stream (30a) comprising the initial hydrocarbon stream from the auxiliary heat exchanger (25) at a lower temperature than the initial temperature, wherein the auxiliary refrigerant component (140) is obtained from a refrigerant component production apparatus not comprising the said extraction unit (200).

16. The method of any one of claims 10 to 15, wherein the hydrocarbon stream is obtained from natural gas, and wherein the liquefied hydcrocarbon stream comprises liquefied natural gas.

Drawing