Small scale expansion cycle methane liquefaction process

Abstract

The present invention relates to expansion cycle methane liquefaction processes for feed gases containing methane and nitrogen, which processes are especially suitable for small-scale plants, such as biogas liquefaction plants. The processes use a portion of the pressurized fluid to cool the fluid to be expanded prior to expansion and recycles the entirety of the spent cooling portion and vapour generated in the expansion.

Description

[0001] Field of the invention The present invention relates to a method for producing liquid me- thane in small scale by an expansion process, in particular to such methods producing liquid biogas (LBG). Background Methane is an important energy source and is sometimes needed in liquid form, where it more easily and safely can be handled and transported. When liquid methane is produced from natural gas, i.e. fossil gas, it is re- ferred to as liquid natural gas (LNG) and when it is produced from biogas it is referred to as Liquid Biogas (LBG). Liquid methane is produced in liquefaction processes where methane is cooled to condense methane to a liquid at or close to atmospheric pressure. A number of different liquefaction processes exist, and they can in general be classified as expansion cycle, mixed- refrigeration cycle and cascade cycle. Cascade processes involve multiple stages of refrigerant cycles to sequentially cool and condense the feed gas. Typical refrigerant cycles use propane, ethylene and methane. Mixed- refrigerant cycles use specific refrigerants designed to achieve cooling tem- peratures allowing condensing of methane in a cryogenic heat exchanger. Cascade processes and mixed-refrigerant processes are energy efficient but are also complex and require high capital expense. Consequently, these are typically used for large-scale plants. Expansion cycles are less energy effi- cient, but simpler and more compact and are thus an attractive option for small-scale plants, such as producing LBG at a biogas production plant. In typical expansion processes the feed gas is compressed and pre- cooled before it is expanded to cool and condense methane. The expansion process yields liquid methane and a vapour fraction. WO15110779 discloses an expansion process where the feed gas is split in a first and a second stream, the second stream is expanded and used to cool the first stream. The first stream is then expanded to provide LNG and a vapour fraction can be recycled into the feed gas, discharged or used as fuel gas. When the feed gas contains impurities an external refrigeration sys- tem may be combined with removal of carbon dioxide and / or other impurities. When the feed gas for such an expansion process contains impuri- ties such as nitrogen and oxygen, recycling the vapour fractions may lead to accumulation of impurities in the process due to the impurities disproportion- ately separating into the vapour fraction in the expansion step. This is not ad- dressed in WO15110779 which only mentions the possibility of removing car- bon dioxide and / or other impurities from the feed or recycle streams. Another and similar expansion process is disclosed in WO2012172281. In this disclosure the problem of nitrogen accumulation in the recycle gas is mentioned and the proposed solution is to route a vapour fraction generated from expansion with the boil-off product LNG to be dis- charged as fuel gas. Another expansion process is disclosed in WO2017 / 162566 where a fuel bleed is efficiently placed to prevent contamination of the recycled vapour fractions with nitrogen. Hence it is known that the issue of accumulation of contaminants can be solved by bleeding (purging) a part of the vapour fraction recycle, but this will entail a loss of methane from the process. Part of the value of the me- thane lost in such a bleed can be recovered if the bleed can be used as fuel in another process, but this may not always be possible. Also, using the bleed as a fuel may not recuperate the full value of the methane compared to if it was recovered as LBG. Hence, there is a need for further and / or improved expansion cycle processes where the methane loss is reduced, especially in LBG liquefaction processes where the gas feed still contains some contaminants, such as ni- trogen and possibly oxygen, even after purification. In addition, there is a need for liquefaction processes for small-scale use, with mild operational conditions and thus inexpensive and commonly available equipment. Summary of the invention These and further objects are achieved by a process according to the invention, where there is provided a method for producing liquid methane from a feed gas comprising methane and contaminants, which contaminants comprise nitrogen, said method comprising the steps of a) compressing the feed gas in a compressor section to provide a pressurized fluid having a pressure in the range of 30 to 70 bar(a), b) cooling the pressurized fluid in an intermediate cooling section to provide a cooled, pressurized fluid having a temperature in the range of -70 to -100 °C, c) in an expansion section, - further cooling the cooled, pressurized fluid, or a remaining portion thereof in a first cooling unit, using a first cooling portion to pro- vide a further cooled, pressurized fluid having a temperature in the range of -110 to -160 °C, or suitably -120 to -160 °C, and a spent first cooling portion, and - expanding the further cooled, pressurized fluid, or portion thereof, and separating the resulting expanded mixed fluid in a first sep- aration unit into a vapour portion and a liquid methane portion, d) recycling the entirety of the nitrogen and / or the entirety of the me- thane in the spent first cooling portion and the entirety of the nitrogen and / or the entirety of the methane in the vapour portion to the compressor section, optionally mixed with the feed gas, wherein the first cooling portion is obtained by expanding a portion of the cooled, pressurized fluid, said first cooling portion is used to cool the re- maining portion, or wherein the first cooling portion is obtained by expanding a portion of the further cooled, pressurized fluid, said further cooled, pressurized fluid is used to cool the cooled, pressurized fluid. The inventors have found that by using a substantial portion of the feed gas itself as the first cooling portion, obtained either from cooled, pres- surized fluid or from the further cooled, pressurized fluid, and cooling the fluid to -120 to -160 °C prior to the expansion, it has been possible to recycle the entirety of the nitrogen and methane in the vapour portion resulting from the expansion without accumulation of nitrogen in the process, even though the vapour portion from the expansion is enriched in said contaminants. Hence, according to the invention there is no purge of methane and nitrogen from the recycled portions and therefore the methane recovery is essentially 100 % disregarding losses which are unavoidable in practice, such as by leaks. Fur- ther, by cooling to -70 to -100 °C in the intermediate section, commonly avail- able refrigerants can be used in external refrigeration cycles, providing part of the cooling required and thus reducing the cooling to be supplied in the ex- pansion section. Similarly, compressing to 30 to 70 bar(a) allows for the use of inexpensive compressors, typically with one or two compression stages, thereby avoiding the need for high pressure compressors which are more costly. The process according to the invention finds application in small-scale methane liquefaction plants, such as with biogas as the feed gas being lique- fied to produce LBG. The first cooling portion can be obtained from either the cooled, pressurized fluid or from the further cooled, pressurized fluid. Obtaining the first cooling portion from the further cooled, pressurized fluid may provide a colder first cooling portion and thus allow for cooling to lower temperatures in the first cooling unit. As the first cooling portion is obtained from the cooled pressurized fluid or further cooled pressurized fluid it will have the same composition as the fluid from which they are obtained. The spent first cooling portion will thus also have the same composition which allows it to be recycled without nitro- gen accumulating the process. The first cooling portion may be said to be ob- tained directly from the cooled pressurized fluid or the further cooled, pressur- ized fluid, by splitting off a portion thereof and expanding said portion to pro- vide the first cooling portion. By “obtained directly” is understood that the split- ting is done without phase separation or separation of components, so there is no change in composition in the split. Conversely, the vapour portion pro- vided by the expansion of the further cooled, pressurized fluid will be enriched with respect to nitrogen and possibly other contaminants. By letting the first cooling portion provide a substantial cooling of the cooled, pressurized fluid prior to expansion, the flow rate of the vapour portion is reduced, which allows the process to reach a steady state in which nitrogen does not accumulate even with a full recycle of the nitrogen and methane in the vapour portion and spent first cooling portion and without purging of nitrogen and methane from the process. In some variations of the method, such as with biogas, the feed gas may contain trace amounts of hydrogen which is more volatile than nitrogen and is thus more difficult to handle in the condensed phase in the process. In these variations the recycles would typically include a step of oxidation of the hydrogen with oxygen, which also found in for example biogas as a contami- nant, and subsequent removal of the resulting water. Such an oxidation may be catalytic, e.g. using platinum, whereby hydrogen can be “burned” off at sufficiently low temperatures so as not to burn methane, e.g. about 150 °C. The resulting water can be removed without loss of methane using for exam- ple an adsorption dryer. The presence of such a hydrogen removal step in the recycle(s) would thus still allow for recycling the entirety of at least the nitro- gen and methane in the spent first cooling portion and vapour portion. Hence, if the feed gas contains hydrogen, the method may comprise a hydrogen removal step from the spent first cooling portion and / or from the vapour portion as detailed above. In some embodiments, if the feed gas does not contain hydrogen, then the spent first cooling portion and vapour portion may be recycled in their entirety. It will be appreciated that the pressure of the further cooled, pressur- ized fluid will correspond substantially to the pressure of the pressurized fluid, barring any inevitable pressure losses in the passage from the compressor section through the first cooling unit. In some embodiments, the temperature of the further cooled, pres- surized fluid is in the range of -145 to -159°C, preferably -150 to -158 °C. In this way, the flow rate of the vapour portion is further reduced. This can be achieved by setting the pressure of the first cooling portion to provide an ap- propriate temperature difference. In some embodiments, where the temperature of the further cooled, pressurized fluid is in the range of -110 to -150 °C, preferably -120 to 140 °C, the expansion section in step c) further comprises the steps of - cooling the liquid methane portion, or a portion thereof, using a second cooling portion in a second cooling unit to provide a cooled liquid methane portion having a temperature in the range of -140 to -159 °C, preferably, -145 to -159 °C, more preferably -150 to -158 °C, and a spent second cooling portion, and - expanding the cooled liquid methane portion and separating the resulting expanded mixed fluid in a second separation unit into a fur- ther vapour portion and a further liquid methane portion, and the method further comprising e) recycling the spent second cooling portion and further vapour por- tion to compressor section, optionally mixed with the feed gas, and further wherein the second cooling portion is obtained by ex- panding a portion of the liquid methane portion, said second cooling portion is used to cool said remaining portion, or wherein the second cooling portion is obtained by expanding a por- tion of the further cooled liquid methane portion, said second cooling portion is used to cool the liquid methane portion. This embodiment is referred to as a two-stage process wherein the expansion section comprises two cooling units and two expansions of the gas to be liquefied. Such two-stage processes may be advantageous for applica- tions wherein the feed gas is provided at a pressure which is higher than the pressure of the expansion yielding the further liquid methane portion, i.e. the final liquid methane product of the process. In such applications, recycling the further vapour portion and spent second cooling portion to the feed gas / compressor section requires recompression using recycle compressor(s) due to the pressure difference. By providing two stages in the expansion sec- tion, the first stage comprising the first cooling unit and second cooling unit, can be operated at a pressure greater than or substantially equal to the feed gas pressure, allowing the vapour portion and spent first cooling portion to be recycled with recompression, providing a more efficient recycle of gas in the process. It will be appreciated that the pressure of the cooled liquid will sub- stantially correspond to the pressure of the liquid methane portion, barring any inevitable pressure losses in the passage from the first separation unit through the second cooling unit. In such two stage embodiments, the temperature of the further cooled, pressurized fluid (30) is preferably in the range of -125 to -135 °C. In the same manner as the first cooling portion described above, the second cooling portion will have the same composition as the liquid methane portion as it obtained as a portion thereof (or as a portion of the further cooled liquid methane portion), without any separation of phases or components, in the split. In some embodiments, the entirety of the nitrogen and the entirety of the methane in the spent second cooling portion and the entirety of the nitro- gen and the entirety of the methane in the further vapour portion are recycled to the feed gas in step e). As described above, if hydrogen is present in the feed gas, there may be a hydrogen removal step. Hence, if the feed gas contains hydrogen, the method may, comprise a step of removing hydrogen from the spent sec- ond cooling portion and / or from the further vapour portion. If the feed gas does not contain hydrogen, then the spent second cooling portion and the further vapour portion may be recycled in their entirety. By the entirety of the portions are recycled in steps d) and e) is meant that all of the methane and nitrogen is recycled and added to the feed gas before or in the compressor section. In some embodiments, a fluid path of the feed gas through to the liq- uid methane portion and if present, through to the further liquid methane por- tion defines a liquefaction train, further characterized in that no purge of nitro- gen and methane from the liquefaction train or from any of the recycle streams in the liquefaction train takes place. It will be understood that the liq- uefaction train is the fluid path from the feed gas through to the liquid me- thane portion which has the lowest pressure in the process. The lowest pres- sure liquid methane will be the product liquid methane obtained by a final ex- pansion step. Hence, the liquefaction train is defined as the fluid path from the feed gas through to the final expansion of the process. In a single stage pro- cess, where the expansion section comprises a first cooling unit, a single ex- pansion, and a separation unit, this expansion produces the lowest pressure liquid methane in the process. In a two-stage process, the lowest pressure liquid methane is produced in the expansion associated with the second sep- aration unit. Hence, the liquefaction train encompasses the compressor sec- tion and all expansions of the feed gas through to the product liquid methane. Methods according to the invention, allow for the liquefaction train and all re- cycle streams in the liquefaction train not to have any purge of nitrogen or methane from. Recycle streams are understood to be portions of fluid ob- tained from said fluid path which are recycled back into said fluid path. As previously described, if the feed gas contains hydrogen, a step of removing hydrogen from one or more of the recycle streams may be included, which allows for a liquefaction train where no purge of methane and nitrogen from the liquefaction train or from any of the recycles stream in the liquefac- tion train takes place. If the feed gas does not contain hydrogen, the liquefac- tion train can be characterized in that no purges of contaminants from the liq- uefaction train or from any of the recycle streams in the liquefaction train take place. Generally, the entirety of the first cooling portion is used to cool the cooled, pressurized fluid, or the remaining portion thereof, and if present, the entirety of the second cooling portion is used to cool the liquid methane por- tion, or the remaining portion thereof. In some embodiments, the contaminants of the feed gas constitute 5 mol% or less, preferably 4 mol% or less, preferably 3 mol% or less, preferably 2 mol% or less or preferably 1 mol% or less. In some embodiments, nitrogen constitutes 3 mol% or less of the feed gas, preferably 2.5 mol% or less, more preferably 2 mol% or less, more preferably 1.5 mol% or less, more preferably 1 mol% or less. In some embodiments, the contaminants further include oxygen, and oxygen constitutes 3 mol% or less of the feed gas, preferably 2.5 mol% or less, more preferably 2 mol% or less, more preferably 1.5 mol% or less, more preferably 1 mol% or less. In some embodiments, nitrogen and oxygen collectively constitute 3 mol% or less of the feed gas, preferably, 2.5 mol% or less, more preferably 2 mol% or less, more preferably 1.5 mol% or less, more preferably 1 mol% or less. In some embodiments, the intermediate cooling section comprises a first cooling cycle using propane or propylene as refrigerant, and a second cooling cycle using ethane or ethylene as refrigerant. The refrigerants are typically coupled with the first cooling cycle using propane and the second using ethane or using propylene and ethylene respectively. When using ethane, the pressurized fluid can be cooled to about -88 °C while using eth- ylene allows for cooling to about -100 °C. Hence, by using ethylene a greater part of the cooling duty required for liquefaction can be achieved in the inter- mediate cooling section, compared to using ethane. This lowers the cooling duty to be supplied by the expansion section. Lower temperatures in the in- termediate cooling section thus allows for reducing the pressure in the com- pressor section, lowering the energy use therein and reducing the pressure rating requirements of the components in the liquefaction train. Hence, when using ethylene and propylene as the refrigerant, the pressure of the pressur- ized fluid may be in the range of 30 to 50 bar(a), preferably 31 to 40 bar(a). Using ethane and propylene in the intermediate cooling section, may lead to a pressure in the range of 40 to 70 bar(a), preferably 50 to 65 bar(a). In some embodiments, one or more of the spent first cooling portion, spent second cooling portion, the vapour portion and the further vapour por- tion is / are used to indirectly cool the cooled, pressurized fluid upstream of the expansion section. Indirect cooling refers to cooling by way of a heat- exchanger. Cooling the cooled, pressurized fluid upstream of the expansion section here refers to cooling at a position in the fluid path from the feed gas through to the first cooling unit. This may contemplate cooling within the com- pressor section, in the intermediate cooling section or after the intermediate cooling section. In some embodiments, the pressure of the first cooling portion is greater than a pressure of the feed gas, preferably the pressure of the first cooling portion is within 1 bar of the pressure of the feed gas, preferably with- in 0.5 bar. In this way the spent first cooling portion can be recycled without recompression. This is especially contemplated for two-stage processes. Within 1 bar or 0.5 bar of the pressure of the feed gas means that the pres- sure is at most 1 bar or 0.5 bar greater than the pressure of the feed gas. In some embodiments, the further cooled, pressurized fluid is ex- panded in step c) to a pressure which is greater than a pressure of the feed gas, preferably to a pressure within 1 bar of the pressure of the feed gas. In some embodiments, the pressure of the first cooling portion is the same as the pressure to which the further cooled, pressurized portion is ex- panded in step c). In some embodiments, the liquid methane portion or the further liquid methane portion, if present, has a pressure in the range of 1 to 4 bar(a), pref- erably 1 to 3 bar(a), even more preferably 1 to 2 bar(a). Subsequently, the (further) methane portion may be pumped to a higher pressure so as to provide a sub-cooled liquid methane product. In some embodiments, the pressure of the second cooling portion is in the range 1 to 4 bar(a), preferably 1 to 3 bar(a), more preferably 1 to 2 bar(a). In some embodiments, the pressure of the feed gas is in the range of atmospheric to 20 bar(a), preferably 2 to 10 bar(a). In a preferred embodiment, - the feed gas has a pressure in the range of 1 to 10 bar(a), pre- ferably in the range 3 to 8 bar(a) and more preferably 4 to 6 bar(a), - the cooled, pressurized fluid has a temperature in the range of -70 to -100 °C, preferably -76 to -88 °C, and a pressure in the range 40 to 70 bar(a), - the further cooled, pressurized fluid has a temperature in the range of -125 to -135 °C, - the cooled liquid methane portion has a temperature in the range of -145 to -158 °C. Reaching a temperature of the cooled, pressurized fluid in the range of -76 to -88 °C can be achieved by using ethane as a refrigerant in the intermediate cooling section. In another preferred embodiment, - the feed gas has a pressure in the range of 1 to 10 bar(a), pre- ferably in the range 3 to 8 bar(a) and more preferably 4 to 6 bar(a), - the cooled, pressurized fluid has a temperature in the range of -90 to -100 °C, and a pressure in the range 30 to 50 bar(a), - the further cooled, pressurized fluid has a temperature in the range of -125 to -135 °C, - the cooled liquid methane portion has a temperature in the range of -145 to -158 °C. Reaching a temperature of the cooled, pressurized fluid in the range of -90 to -100 °C can be achieved by using ethane as a refrigerant in the intermediate cooling section. Using ethylene and lowering the tempera- ture reached in the intermediate cooling section allows for the pressure reached in the compressor section to be lower. In a particular embodiment, a fluid path from the feed gas through to the further liquid portion consists of the compressor section, the intermediate cooling section and the expansion section and where the spent first cooling portion, spent second cooling portion, vapour portion and further vapour por- tion constitute all the recycled portions from said fluid path. In this embodiment, the portion of said fluid path in the compressor section consists of one or more compressors with intercooling. The portion of the fluid path in the intermediate cooling section consists of one or two cool- ing stages, preferably the first and second cooling cycles previously de- scribed. The portion of the fluid path in the expansion section consists of the first cooling unit, a means for expanding the further cooled pressurized fluid, the first separation unit, the second cooling unit, a means for expanding the further cooled liquid methane, and the second separation unit. This preferred embodiment may be combined with the preferred embodiment in the preced- ing paragraph. The invention will now be illustrated in more details below. Brief description of the drawings In the following the invention will be described with reference to the exemplary drawings, where Fig. 1 shows a schematic view of a process for producing liquid me- thane according to an embodiment of invention wherein the expansion sec- tion comprises one expansion, Fig. 2 shows a schematic view of a process for producing liquid me- thane according to another embodiment of invention wherein the expansion section comprises two expansions, Fig. 3 shows a schematic view of an embodiment similar to the em- bodiment of Fig.2 with additional details, and Fig.4 shows a schematic view of a comparative process not accord- ing to the invention, wherein a purge of the recycled vapour portions is re- quired. Detailed description Unless otherwise noted all pressures described herein refer to abso- lute pressure. Fig.1 is a schematic diagram of a process according to the invention wherein the expansion section C has one heat-exchanging step at cooling unit C1 and one separation step at separation unit D1. Such a single stage process may be advantageous when the feed gas 1 is provided at low pres- sure such as about atmospheric pressure and / or when the pressure of the liquid methane portion 40 is at higher pressure than the feed gas 1. In these situations, spent first cooling portion, 31, and vapour portion, 41, can be recy- cled without recompression, in which case a single stage process is advanta- geous at least because of reduced equipment count. If the pressure of the feed gas 1 is higher than the pressure of the liquid methane produced, an embodiment such as than the one in Fig. 2 may be advantageous as will be described in greater detail below. The feed gas 1 may be a biogas which comprises mainly methane but also some contaminations including nitrogen. Within the context of me- thane liquefaction plants, typical biogas plants would be considered small- scale plants, where the expansion cycle type processes find application. The biogas is obtained by ways known to the skilled practitioner such as anaero- bic digestion or as landfill gas and is upgraded to remove carbon dioxide, wa- ter and hydrogen sulphide present in the biogas. Biogas upgrade processes are also well known to the skilled practitioner and includes absorption pro- cesses using physical or chemical solvents or membrane based processes. Upgraded biogas is sometimes referred to as biomethane, denoting that gas has been purified to satisfy certain criteria, such grid quality. For producing liquefied biogas, LBG, a high level of purity with respect carbon dioxide and water is required as these compounds may solidify in the liquefaction train. Techniques for achieving these purities are also well known to the skilled practitioner and a suitable example is the applicant’s own WO2022 / 101509 using chemical absorption. The type of upgrade process used will typically determine the pressure at which the feed gas 1 is provided, where chemical absorption is typically operated at lower pressure than physical absorption processes, which in turn is typically at lower pressures than membrane based upgrade processes. Throughout this disclosure the term “feed gas” refers to a gas comprising methane and nitrogen contaminants, suitable for liquefaction, meaning that contaminants posing an issue of liquefaction have been re- moved to acceptable levels. Contaminants posing an issue for liquefaction are generally those which are less volatile than methane and for which there is a risk of solidification in the liquefaction train, e.g. carbon dioxide. With reference to Fig. 1, in the first step of the liquefaction process feed gas 1 is compressed to a pressurized fluid 10 in a compressor section A. The pressure of the pressurized fluid is 40 to 70 bar(a) which can typically be achieved using one or two compression stages in the compressor section A. The compressor section A will in the manner known in the art include a num- ber of intercooling stages to remove at least part of the heat generated by compression. Hence the pressurized fluid 10 may have a temperature of about ambient temperature, but it depends on the type of cooling used. In the compressor section A the feed gas is mixed with a recycle of a spent cooling portion 31 and a vapour portion 41 (herein after denoted recycled portions 31, 41) which will be described below, such that the pressurized fluid 10 includes the feed gas 1 and recycled portions 31, 41. The mixing of these portions is shown schematically in Fig.1, but it will be appreciated that this mixing is per- formed as is appropriate accounting for the pressures of the streams. Hence, if the recycled portions 31, 41 correspond in pressure to feed gas 1, they may be mixed before any compression thereof, or if the recycled portions 31, 41 have a higher pressure than the feed gas 1, they may be mixed in at an ap- propriate compression stage. The recycled portions 31, 41 may also be used to provide intercooling in the compressor section A. In the next step of the liquefaction process, the pressurized fluid 10 is cooled in an intermediate cooling section B to provide a cooled, pressurized fluid 20. The cooled, pressurized fluid 20 has a temperature in the range of - 70 to -100 °C which can be achieved using readily available refrigerants such as ethane, propane, ethylene and propylene in external refrigeration cycles which have low capital and operational cost. The term external refrigeration cycles is used to denote cooling achieved by refrigerants which are not ob- tained from the feed gas 1 and are thus external to the liquefaction process. At this point the cooled, pressurized fluid 20 remains the same as the pres- surized fluid 10, barring inevitable pressure drops in the components of the intermediate cooling section B. In the next step of the liquefaction process, the cooled, pressurized fluid 20 is sent to the expansion section C where it is further cooled in the cooling unit C1, expanded (shown by a valve) and separated into a liquid me- thane portion 40 and vapour portion 41 in separation unit D1. Cooling unit C1 uses a first cooling portion 21 to further cool the cooled, pressurized fluid 20 to provide a further cooled, pressurized fluid 30. Fig. 1 shows two embodi- ments of the first cooling portion 21. In the embodiment shown in fully drawn lines, the first cooling portion 21 is obtained by expanding (shown by a valve) a portion 20b of the cooled, pressurized fluid 20 leaving remaining portion 20a to be cooled in the first cooling unit C1 to yield the further cooled, pressurized fluid 30. In the other embodiment shown by dashed lines, the first cooling por- tion 21 is obtained by expanding (shown by a valve) a portion 30b of the fur- ther, cooled pressurized portion 30, leaving a remaining portion of the further, cooled pressurized fluid for further processing in separation unit D1. In both embodiments of the first cooling portion 21, the portions 20b and 30b have the same composition as the streams from which they originate, meaning that there is no separation of phases or components in the branching off of the portions 20b or 30b. Hence, the spent first cooling portion 31 will have the same composition as the cooled, pressurized fluid 20 which is why it can be recycled without contributing to accumulation of contaminants. The further cooled, pressurized portion 30 has a temperature in the range of -120 to -160 °C °C, preferably in the range of -145 to -159 °C and more preferably -150 to - 158 °C. These temperatures are achievable with the first cooling portion 21 which comprises mainly methane, maintaining a practical temperature differ- ences across the cooling unit C1, which is a heat-exchanger in the embodi- ment shown. In general, the cooling units in the processes according to the inven- tion may be operated such that the temperature of the cooling portions used in the cooling unit is at least 5 °C lower than the fluid cooled in and leaving the cooling unit. Said temperature can be set by the pressure of the cooling portions. In addition, the volume / flow of the cooling portions used in a cooling unit is selected so as to provide the desired temperature of the fluid leaving the cooling unit having been cooled, and such that the spent cooling portion has a temperature which is at least 3 °C lower than the temperature of the fluid which is to be cooled in the cooling unit. For example, the cooling unit C1 in Fig.1 may be operated such that the temperature of the first cooling portion 21 is at least 5 °C lower than the further cooled, pressurized portion 30 which leaves the first cooling unit C1 and the volume / flow of the first cooling portion 21 is selected such that the temperature of the spent first cooling portion 31 is at least 3 °C lower than the temperature of the remaining portion 20a of the cooled, pressurized fluid to be cooled in the first cooling unit C1. Returning to Fig. 1, at this stage the pressure of the further cooled, pressurized fluid 30 remains the same as the cooled, pressurized fluid 20, barring inevitable pressure drops in the cooling unit C1 and is sub-cooled to a great degree. The further cooled, pressurized fluid 30 is then expanded (shown by a valve) and the resulting expanded mixed fluid is separated in separator unit D1 providing the vapour portion 41 and liquid methane portion 40. The pressure at this stage is generally from atmospheric to 3 bar(a), typi- cally about 1 to 2 bar(a). Due to the expansion the liquid methane portion 40 has a lower temperature than the further, cooled pressurized fluid 30 and is then pumped to a storage vessel E providing subcooled liquid methane 100. In the expansion, nitrogen, and if present oxygen, will separate disproportion- ately into the vapour portion 41, meaning that the vapour portion 41 is en- riched in nitrogen compared to the further cooled, pressurized fluid 30. How- ever, due to the further cooling unit C1, the entirety of the vapour portion 41 is recycled to the compressor section A without requiring purging. The dispro- portionate split of methane and nitrogen is avoided in the spent first cooling portion 21 by branching the portions 20b or 30b off without separation of phases and / or components. In the embodiment of Fig. 1 the fluid path from feed gas 1 through to compressor section A, intermediate cooling section B, expansion section C to liquid methane portion 40 is the liquefaction train of the process. The recycle streams in this liquefaction train is the portion 20b or portion 30b as well as the spent cooling portion 31, and the vapour portion 41. The process in Fig. 1 has no purges from the liquefaction train or any of the recycle streams thereof. In the embodiment of Fig.1, the pressures of the first cooling portion 21 and in separator unit D1 are each greater than the pres- sure of the feed gas 1 allowing recycling of streams without recompression. Fig.2 shows a further embodiment, wherein the expansion section C comprises two cooling units C1, C2, two expansions and two separation units D1, D2. Such an embodiment may be advantageous for applications where the pressure of the feed gas 1 is lower than the pressure of the liquid me- thane obtained in the last expansion, in which case the vapour portion of the last expansion requires recompression for recycling. Including a second cool- ing unit C2 and a second separation unit D2 allows for the first cooling unit C1 and first separation unit D1 to be operated at an intermediate pressure at or greater than the feed gas pressure, where the resulting vapour portions can be recycled without recompression. This is advantageous as the recycled streams for the first cooling units C1 and first separation unit D1 are compa- rably larger than those of the recycle of second cooling unit C2 and second separation unit D2. Up until the further cooled, pressurized fluid 30 the pro- cess is the same as in Fig.1, but the temperature of the further cooled, pres- surized fluid 30 is in the range of -120 to -140 °C. The expansion at separator unit D1 is at an intermediate pressure as described above allowing direct re- cycle with recompression but does not provide the liquid methane portion 40 at the desired temperature of the product liquid methane, so the liquid me- thane portion 40 is further processed in the second cooling unit C2 and sec- ond separation unit D2. This second stage operates in the same manner as in the first stage in units C1 and D1, hence the liquid methane portion 40 or a remaining portion 40a thereof is cooled using a second cooling portion 42, to provide the further cooled liquid methane portion 50 having a temperature in the range -145 to -159 °C, preferably -150 to -158 °C. As described in relation to Fig. 1 the second cooling portion 42 is obtained by expansion of a portion 40b of the liquid methane portion 40 as shown in fully drawn lines or by ex- pansion of a portion 50b of the further cooled liquid methane portion 50 shown in dashed lines. As above, these portions are branched off without any separation of phases or components, resulting in the spent second cooling portion 51 having the same composition as the liquid methane portion 40, preventing accumulation of nitrogen from the spent second cooling portion 51. The further cooled liquid methane portion 50 is then expanded and the result- ing fluid mixture is separated into a further vapour portion 61 and further liquid methane portion 60. The pressure at this stage is generally from atmospheric to 3 bar(a), typically about 1 to 2 bar(a). The further liquid methane portion 60 is then pumped to storage vessel E providing subcooled liquid methane as the product liquid methane. The further vapour portion 61 and spent second cooling portion 51 are both recycled to the compressor section A and mixed with the feed gas 1 and form part of the pressurized fluid 10. Recycle com- pressors are shown in the recycle lines indicating that the feed gas 1 is at a higher pressure than the feed gas 1. As before, the recycled portions 51, 61 are mixed with the feed gas 1 or in the compressor section as is appropriate in view of the prevailing pressures. The two recycle compressors shown in the recycle lines 51, 61 could be a single recycle compressor. As previously described, by having two stages in the expansion section, the recycled por- tions 31, 41 can be recycled without recompression. In the embodiment of Fig. 2 the fluid path from feed gas 1 through to compressor section A, inter- mediate cooling section B, expansion section C to further liquid methane por- tion 60 is the liquefaction train of the process. The recycles in this liquefaction train is the portion 20b or portion 30b as the spent cooling portion 31, the va- pour portion 41, the portion 40b or portion 50b as the spent cooling portion 51, and the further vapour portion 51. The process in Fig. 2 has no purges from the liquefaction train or any of the recycle streams. Turning now to Fig.3 which shows an embodiment of a process simi- lar to that of embodiment in Fig.2 but showing further details. The features in Fig. 3 are indicated by the same reference numerals used in Figs. 1 and 2 and reference is made to the description of Figs.1 and 2 for those features. In Fig. 4, the compressor section A comprises two compressor stages A1 and A2 with a plurality of intercooling steps shown by the typical symbol for a heat-exchanger. The intercooling in the compressor stage is provided by cool- ing water in circuit W. The intermediate cooling section B comprises two cool- ing cycles, a first cooling cycle B1 using propane and a second cooling cycle B2 using ethane. These two cooling cycles cool the pressurized fluid 10 at heat-exchanger B1a and B2a respectively to provide the cooled, pressurized fluid 20. As can be seen, the first cooling cycle B1 is also used to provide cooling in the second cooling cycle B2 and in the compressor section A and the water in circuit W is used to provide cooling in the first cooling cycle B1. Upstream of the expansion section C, starting from the first cooling unit, a number of heat-exchangers are provided wherein the recycled portions 31, 41, 51, 61 are used to indirectly cool the fluid to be liquefied. As can be seen the spent first cooling portion 31 and vapour portions 41 are recycled as a single recycle portion and mixed directly into the feed gas 1. Hence, the pressure of the first cooling portion 21 and the pressure in the separation unit D1 are about the same and correspond to the pressure of the feed gas 1. The pressure of the second cooling portion 42 and in separation unit D2 is lower than the pressure of the feed gas and thus recompression is needed as indi- cated by the presence of a recycle compressor and subsequent cooling using the water circuit W. By having the two stages in the expansion section C, the recycle becomes more efficient by lowering recompression duty. The fluid path from the feed gas 1 through to the further liquid methane 60 is the lique- faction train of the process and no purges are present in the liquefaction train or in any of the recycles therein. The embodiments in Figs.1 to 3 do not comprise a hydrogen remov- al step and therefore the entirety of the spent first cooling portion 31, vapour portion 41, spent second cooling portion 51, and further vapour portion 61 are recycled to the compressor section A as part of the feed gas 1. The entirety of the nitrogen and methane and thus recycled by recycling the entirety of the recycle streams 31, 41, 51, 61. If hydrogen is present in the feed 1, then a hydrogen removal step, such as a catalytic oxidation step, can be implement- ed in one or more of the recycle streams. The hydrogen removal step could be implemented after the recycle streams 31, 41, 51, 61 have been mixed as is illustrated in Fig.3. A hydrogen removal step can be implemented by pass- ing part of a recycle stream into a catalytic oxidation unit, possibly through a pre-heater, in the oxidation unit hydrogen reacts with oxygen under the for- mation of water. The water is then removed in a dryer, such as an adsorption dryer, and the dried stream is returned to the recycle stream. In this way the entirety of the methane and nitrogen is recycled to the compressor section. Referring now to Fig. 4 which shows a comparative process not ac- cording to the invention. The process is similar to the one in Fig. 3 but does not have the first cooling unit C1 and second cooling unit C2 in the expansion section C. The features in Fig. 4 corresponding to those in Figs. 1 to 3 are indicated by the same reference numerals but using prime ('). The cooled pressurized fluid 20' is cooled by vapour portion 41' to provide stream 30' which is the expanded at separation unit D1'. The stream 30' will not have the temperature of the further cooled, pressurized fluid 30 in Figs. 1 to 3, as the cooling capacity in vapour portion 41' in sufficient. The liquid methane portion 40' is cooled in a similar manner using further vapour portion 61'. As the pro- cess in Fig.4 does not include the first and second cooling units C1 and C2 a purge 101 of the recycled portions is necessary. A comparison of the processes of Figs. 3 and 4 is described in Ex- ample I. Example I The processes shown in Figs. 3 and 4 were simulated using com- mercially available software. Each process was simulated in four cases hav- ing different levels of contaminants in the feed gas, Case 1 to 4. Table I summarizes the results of the process according to Fig.4, i.e. the comparative example. Table II summarizes the results of the process ac- cording to Fig. 3, i.e. an embodiment of the invention. The results are the steady state of the respective process, hence there is no accumulation of contaminants in the processes. In the process of Fig. 4 the purge 100 is re- quired to balance the system, whereas the process of Fig. 3 is in balance without a purge. In Case 1 the feed is pure methane and both processes yield 100 % methane recovery, as there is no need to purge the recycled portions. As the level of contaminants, nitrogen and oxygen, increases in Case 2 to 4, the me- thane recovery rate in the comparative process (Table I) declines to 91.91 % in Case 4 due to the required purge from the recycled portions. By compari- son, the results in Table II shows 100 % methane recovery in all 4 Cases for methods according to the invention. The process according to the invention has a greater total power us- age than the comparative process, 700.21 kW vs 680.38 kW in Case 4, but due to the better methane recovery, the specific power per ton methane in the product increases with the process according to the invention, which is thus a more efficient process. Power usage is evaluated as the compressor duty of the compres- sors in the compressor section, recycle compressors and the compressors in the external refrigerant cycles for the intermediate cooling section, which will the main power consumption in the liquefaction processes. Table I Case 1 Case 2 Case 3 Case 4 Feed [kmol / h] 911.95 911.95 911.95 911.95 yCH4[%] 100.00 99.60 99.20 98.80 yN2[%] 0.00 0.20 0.40 0.60 yO2[%] 0.00 0.20 0.40 0.60 CH4in product [kg] 911.95 905.15 898.40 891.69 kg CH4in product [kg] 911.96 863.50 838.11 819.52 CH4recovery [%] 100.00 95.40 93.29 91.91 Total power [kW] 655.87 658.04 666.51 680.38 Specific power [kW / ton CH4feed] 719.20 726.99 741.89 763.02 Specific power [kW / ton CH4product] 719.19 762.06 795.25 830.21 Table II Case 1 Case 2 Case 3 Case 4 Feed [kmol / h] 911.95 911.95 911.95 911.95 yCH4[%] 100.00 99.60 99.20 98.80 yN2[%] 0.00 0.20 0.40 0.60 yO2[%] 0.00 0.20 0.40 0.60 CH4in product [kg] 911.95 905.15 898.40 891.69 kg CH4in product [kg] 911.96 905.15 898.40 891.70 CH4recovery [%] 100.00 100.00 100.00 100.00 Total power [kW] 656.43 667.74 682.35 700.21 Specific power [kW / ton CH4feed] 719.81 737.72 759.51 785.26 Specific power [kW / ton CH4product] 719.80 737.72 759.51 785.25 The details of the results are shown in Tables III and IV for the pro- cess of Fig.3 and 4 respectively. Referring first to Table III (Fig.3): In all four cases the feed gas has a pressure of 5 bar(a) and a temperature of 20 °C and is compressed, together recycled vapour portions and spent cooling portions to a pressure of about 60 bar(a). It is then cooled in the intermediate section and by indirect heat ex- change with the recycled vapour portions and spent cooling portions to a temperature of about -79 °C as can be seen from the data of the remaining portion 20a of the further cooled pressurized portion. In the first cooling unit C1 it is then cooled to about -132 to -133 °C and then expanded to 5 bar(a) and separated in separation unit D1. The pressure of the first cooling portion is 5.20 bar(a). The resulting liquid methane portion 40 from separation unit D1 is then cooled by the second cooling portion 42, having a pressure of 1.3 bar(a), to temperature of -153.3 to -154.7 °C. It is then expanded to 1.3 bar(a) reaching temperature of -158.4 to -161 °C as the further liquid methane por- tion 60. This further liquid methane portion 60 is the product LBG and can be pumped to higher pressure, e.g. less than 3 bar(a), to obtain a sub-cooled liquid methane 100. Referring now to Table IV showing the process of Fig. 4: Here the feed gas has the same pressure of 5 bar(a) and temperature of 20 °C. It is pressurized to 60 bar(a) and then cooled in the intermediate section and us- ing the recycled vapour portions. Hence, the temperature of the gas prior to expansion 30' is -85-86 °C. As in Table III the two expansions steps are to 5 bar(a) and 1.3 bar(a) respectively. As can be seen the two vapour portions 41 and 61 are enriched in nitrogen and oxygen. In order to prevent accumulation of these gases, a purge stream 101 has to be implemented, resulting in a re- duced methane recovery rate.

[0002] 0 d5,0970e+s-y d -rsn-6 , 3, 00,00,030, 5, 9010, 4,6, 99,460, 5, 9990, 4,0, 91016,09978 ,4,89552 4080 0062 4488900624987 ,91,16 ,2455 ,e ac eop t099 2,1F gerlcoi1rerhtun83,036rooi 1 8 ,10,00070292930601 9708091,0,02, 341, 611, 369, 919, 381,8, 0upt1 200,0,1,2,4,2,1,2,8,4,1,2,Farvo659p1539066 0 1 26-0118 6 8 1-1-1- reh n8 t3,0340rdie o, 8, 00,00072,03560 ,6,020231,03540 ,2,040479,03620 ,8,0606u- n itr068160,0,09 ,169 ,0,00 ,169 ,0,00 ,168 ,u0,0qe ao5 505 5 9 6 5 9 6 59Filmht p1- 11-1-1-deln52odoie o ,590000000096596043838261 5911568775665931490268u- n it qe a r0o53 ,54 ,9 , , ,,,, ,, ,,,, ,, ,,,, ,, ,150000315 491599 00454980 044971 015 9 5159Cilmht p - - - -tdgn762910000004623848382062845877764406nneo niloi 9t1,1 ,pc o r 54 1,7,0,0,00,2 ,1,63,,0,01,2 ,1,66,,50,00,22,11,69, 2,81,0Sesocop1-01 4919 489 479-1-1- dnogcnin51loi 23910000009133848861 858761 4406t2,8 ,eo 1,7,0,0,06,8 ,1,63, 3,20,01,93,41,66, 7,50,06,93,11, 9, 2,8,oro451 01 5919 589 5 6791 0S c p - -1-1- run3007700000030004046607073907379op oi 8tr1,47 ,35,3,0,0,09,8 ,35,41,41,79 4,10,00,05,53,82,48,31,10,75,57,22,29,a o 04 8 4 8 1 4V p1- 11-1-1- -n f30a069000000300921267360056123879004100po m 8x naoi er-,7 ,03 5 ,9 , , , 9,,,0,, , 0,,,9,, , 1,0,7, 1 26000083 507990004507791 01 ,45176 ,92,1E sts 31- 11-1-1-t gn 01515200000088515712671 59128250100net nilooitr13,2,5 ,7 ,60,0,0,1,5 ,00,9 ,30,80,11,52,95, 3,7811,0,1,50,7, 1,22,1c p0 87 9877 186ps8- - 989Srifo o- 1 -re ,d d n20600008091267605128504100htr el-eoseziori 1tr0,632,939, 0,05960,00,03,62,2,0,,339507990,06,62,6,9, 3,7390771,09,62,0,7, 1,239176u2,1Focrpusop1- 115 9 5 9-1-1-gtn202520000008302571267366029212387590201010nsi orl i 1,,,, , ,,,,0,, ,,,,9,, ,,, 0,7,,2,iotor1o273576000075090075571 0751 62 11 3179 3 7 9 3 89F c p1- -1-1- -n56 591 55 -n o48, 6,9,000,00,07,7, 6,2,0,267,30,7, 6,61 ,9,238,72,65410, 6,0,7, 10,2,eiaita r0287959600008795079900879507791 08795176921R mgniop - 1 - - -+d -979 1 0 ,092ey,20,00,00,005, 8, 9,02, 0, 67,304, 0, 9,912, 9, 38,7404, 1, 9,017,0102ec- ee3l2 4730 0034190044571 044,26 ,2,1101 249 249 259FRc 1 1 10 4 0 50 0 50 0 60 d0,00e,es10258,0600,00050,0,00,00,56,6,020,20, 0,00, 4,2,040,4, 0,000, 2,8, 60,6,Fag0 0126599 002565990002565890 0)3.egc]in)]ear ] r ](r h / %[] ])]]a(rr ] h / %] ])]]a(rr ])h / %] ] ]a(rr ] h / %] ]F(IeICal %[%[Cal[%[%[Cal[%[%[Cal[%[%[Ife °[b[ o4 H 2 2 °[bo4 H 2 2 °[bo4 H 2 2 °[bo4 H 2 2 RT pmkyCyNyO[T pmkC N O[m C N O[m C N Oey y yT p ky y yT p ky y ylb a T 1 '5,51680,00000008 24,0,0,0,05,5682,77 24,23,5588,74 92,16,5685,00 24,48,7224,99 4,26,5686,93 4,46,4043,95,3egr09 2 9u1Psad1 e5l 581000000858275887455074956343s,56 ,4 ,,0, , 5,, 1,3,, , 6, 8, 5,8, 2,9, 6, 8,7, 6, 4,9,gc 290 064752 164324264005 3dy n702 89 2 9 9 209e ceeori- t1 1rF +op830322000000603168756380303673627037095060r,, , , , 7,, , 0,, 9,, , 3,, 1,, , 8er' un 81 900,081 8 1,181 8 2,291 ,, ,o151 01 579 559 5 8393 3htropit6 -1-1-1- urFavop- 4 2 0 re-r8 o3,0emp'83,18,0600,0006 0,0,07,083,191 ,873,902,18 0,09,083,149 ,602, 10,27 0,01,0939 ,129 ,531, 18,20,0hte05r di 1501 5159 5 599 5 599uun6an- -1-1-Fqilhtoit-e-r5 o0,5997 ,40,0600,0008 0,0,08,5994 ,454 ,472, 29,25 0,03,5090 ,470 ,1 10, 49,45596135660,07,,43,8,,0,dlmp'3 69 9 0 980e0 03 69 4 6 9 4 5 9odie 5un 1n- 11-1-1-oCqialhtoi t38r,0u709 ,52,0000,0002 0,0,07,0801 ,559 ,1 98,593,82005 2,12,2559 ,4004009 5,5 6555,2 ,6004,36,,5 ,3,,5,4on' poi 13tr 417-01 3179 3 8 9932909-1-1- aVopn3 oi8s m,005300000027,005070,875122,005972,775946,006056,5696n a7 ,a er -35,6,0,0,08 , , , , , , , , , , , ,1 3101 35171 195552195233 2141 9 31 41 9 31 51 9p txs- - - -f ' Eo0391065300000905078758059777570605569,,, , ,056,0, 1 16,2, 986,6, 6re ,-r-rh d uo'695600,0,59 ,517 ,91,1,59 ,55 ,2,1,49 ,23 ,3,2tr elss p 08d3 -31018-418- 541 98- 551 9uo erp en Foczioit77,58530000000958507087512058597775971 586055696+el3 ,4,6,0,0,0 ,3 ,4,1 ,7 ,1,1 ,4 ,4,52,5 ,2,1 ,,4,26,,3,2dc-23ey 101 241 9 241 9425319ecFeR s 0040000050 0 50 0 60 a0, 0, 8, 0,0,0, 0, 06,6,02020,004,2,04040,002,8,0606g'10256500000 ,256 ,5990,00 ,256 ,599 0,00 ,256 ,5890,0de1e)F4giec] ] ] ]F n)])])])] (ear(rVe]rah / ] ]al %[%(r [%[]rah / ] ]al %[%(r [%[]rah / ] ]al %[%(r ] ] [%[]rah / l %[%[%[IfeC°[b[ o4 RT pmH k yC2 yN2CyO°[b[ o4 TpmH 2 2C°[b[ o4 H 2 2C°[b[ o4 H 2 2 k yCyN O m C N O m C N OelyT p ky y yT p ky y ybaT List of references 1 Feed gas 10 Pressurized fluid 20 Cooled, pressurized fluid 20a Remaining portion of cooled, pressurized fluid 20b Portion of cooled, pressurized fluid 21 First cooling portion 30 Further cooled, pressurized fluid 30b Portion of further cooled, pressurized fluid 31 Spent first cooling portion 40 Liquid methane portion 40a Portion of liquid methane portion 41 Vapour portion 42 Second cooling portion 50 Cooled liquid methane portion 50b Portion of cooled liquid methane portion 51 Spent second cooling portion 60 Further liquid methane portion 61 Further vapour portion 100 Liquid methane product / LBG 101' Purge stream A Compressor section A1 First compressor stage A2 Second compressor stage B Intermediate cooling section B1, B1a First cooling cycle, heat exchanger B2, B2a Second cooling cycle, heat exchanger C Expansion section C1 First cooling unit C2 Second cooling unit D1 First separation unit D2 First separation unit W Cooling water circuit ' Corresponding feature in comparative process in Fig.4

Claims

C L A I M S 1. A method for producing liquid methane from a feed gas (1) com- prising methane and contaminants, which contaminants comprise nitrogen, said method comprising the steps a) compressing the feed gas (1) in a compressor section (A) to pro- vide a pressurized fluid (10) having a pressure in the range of 30 to 70 bar(a), b) cooling the pressurized fluid (10) in an intermediate cooling sec- tion (B) to provide a cooled, pressurized fluid (20) having a temperature in the range of -70 to -100 °C, c) in an expansion section (C), - further cooling the cooled, pressurized fluid (20), or a remain- ing portion (20a) thereof in a first cooling unit (C1), using a first cooling portion (21) to provide a further cooled, pressurized fluid (30) having a temperature in the range of -110 to -160 °C, and a spent first cooling portion (31), and - expanding the further cooled, pressurized fluid (30), or portion thereof, and separating the resulting expanded mixed fluid in a first sep- aration unit (D1) into a vapour portion (41) and a liquid methane portion (40), d) recycling the entirety of the nitrogen and / or the entirety of the me- thane in the spent first cooling portion (31) and the entirety of the nitrogen and / or the entirety of the methane in the vapour portion (41) to the compres- sor section (A), optionally mixed with the feed gas (1), wherein the first cooling portion (21) is obtained by expanding a por- tion (20b) of the cooled, pressurized fluid (20), said first cooling portion (21) is used to cool the remaining portion (20a), or wherein the first cooling portion (21) is obtained by expanding a por- tion (30b) of the further cooled, pressurized fluid (30), said further cooled, pressurized fluid (30) is used to cool the cooled, pressurized fluid (20).

2. The method according to claim 1, wherein the temperature of the further cooled, pressurized fluid (30) is in the range of -145 to -159°C, preferably -150 to -158 °C.

3. The method according to claim 1, wherein the temperature of the further cooled, pressurized fluid (30) is in the range of -110 to -150 °C, prefer- ably -120 to -140 °C, and the expansion section (C) in step c) further com- prises the steps of - cooling the liquid methane portion (40), or a portion (40a) thereof, using a second cooling portion (42) in a second cooling unit (C2) to provide a cooled liquid methane portion (50) having a tempera- ture in the range of -140 to -159 °C, preferably -145 to -159 °C, more preferably -150 to -158 °C, and a spent second cooling portion (51), and - expanding the cooled liquid methane portion (50) and separat- ing the resulting expanded mixed fluid in a second separation unit (D2) into a further vapour portion (61) and a further liquid methane portion (60), and the method further comprising e) recycling the spent second cooling portion (51) and further vapour portion (61) to compressor section (A), optionally mixed with the feed gas (1), and further wherein the second cooling portion (42) is obtained by expanding a portion (40b) of the liquid methane portion (40), said second cooling portion (42) is used to cool said remaining portion (40a), or wherein the second cooling portion (42) is obtained by expanding a portion (50b) of the further cooled liquid methane portion (50), said second cooling portion (42) is used to cool the liquid methane portion (40).

4. The method according to claim 1 or 3, wherein the entirety of the nitrogen and the entirety of the methane in the spent second cooling portion (51) and the entirety of the nitrogen and the entirety of the methane in the vapour portion (61) are recycled to the feed gas (1) in step e).

5. The method according to any one of the preceding claims, wherein a fluid path of the feed gas (1) through to the liquid methane portion (40) and if present, through to the further liquid methane portion (60) defines a lique- faction train, further characterized in that no purge of nitrogen and methane from the liquefaction train or from any of the recycle streams in the liquefac- tion train take place.

6. The method according to any one of the preceding claims, wherein a composition of the spent first cooling portion (31) is the same as a composi- tion of the cooled, pressurized fluid (20), and a composition of the spent sec- ond first cooling portion (51), if present, is the same as a composition of the liquid methane (40) portion.

7. The method according to any one of the preceding claims, wherein the contaminants of the feed gas (1) constitute 5 mol% or less, such as 4 mol% or less, 3 mol% or less, 2 mol% or less or 1 mol% or less.

8. The method according to any one of the preceding claims, wherein the intermediate cooling section (B) comprises a first cooling cycle (B1) using propane or propylene as refrigerant, and a second cooling cycle (B2) using ethane or ethylene as refrigerant.

9. The method according to any one of the preceding claims, wherein one or more of the spent first cooling portion (31), spent second cooling por- tion (51), the vapour portion (41) and the further vapour portion (61) is / are used to indirectly cool the cooled, pressurized fluid (20) upstream of the ex- pansion section (C).

10. The method according to any of the preceding claims, wherein the pressure of the first cooling portion (21) is greater than a pressure of the feed gas (1), preferably the pressure of the first cooling portion (21) is within 1 bar of the pressure of the feed gas (1), preferably within 0.5 bar.

11. The method according to any of the preceding claims, wherein the further cooled, pressurized fluid (30) is expanded in step c) to a pressure which is greater than a pressure of the feed gas (1), preferably to a pressure within 1 bar of the pressure of the feed gas (1).

12. The method according to any of the preceding claims, wherein the liquid methane portion (40) or the further liquid methane portion (60), if present, has a pressure in the range of 1 to 4 bar(a), preferably 1 to 3 bar(a), more preferably 1 to 2 bar(a).

13. The method according to claim 3 or claims 4 to 12 when depend- ing on claim 3, wherein the pressure of the second cooling portion is in the range 1 to 4 bar(a), preferably 1 to 3 bar(a), more preferably 1 to 2 bar(a).

14. The method according to claim 3 or claims 4 to 13 when depend-ing on claim 3, wherein - the feed gas (1) has a pressure in the range of 1 to 10 bar(a), pre- ferably in the range 3 to 8 bar(a), and more preferably 4 to 6 bar(a), - the cooled, pressurized fluid (20) has a temperature in the range of -70 to -100 °C, preferably -76 to -88 °C, and a pressure in the range 40 to 70 bar(a), - the further cooled, pressurized fluid (30) has a temperature in the range of -125 to -135 °C, - the cooled liquid methane portion (50) has a temperature in the range of -145 to -158 °C.

15. The method according to claim 4 and 9, wherein a fluid path from the feed gas (1) through to the further liquid portion (60) consists of the com- pressor section (A), the intermediate cooling section (B) and the expansion section (C), and the spent first cooling portion (31), spent second cooling por- tion (51), vapour portion (41) and further vapour portion (61) constitute all the recycled portions from said fluid path.

Images