Method for liquefying a methane-rich feed gas, and corresponding facility

Abstract

The invention relates to a method for liquefying a methane-rich feed gas (12), the method comprising: - a step of mixing the feed gas with recycled gas (18), a step of compressing the gas to a pressure of between 19 and 70 bars absolute, and a step of purifying the gas; - a first step of pre-cooling the gas to a temperature of between -40°C and -15°C, and a second step of pre-cooling the gas by heat exchange with the recycled gas; - a step of liquefying the gas by heat exchange only with a liquefaction refrigeration cycle (46); - a step of sub-cooling the gas by heat exchange with at least the recycled gas; and - expanding the sub-cooled liquid (52) to obtain a liquefied gas (14) and wherein the recycled gas represents a mole fraction of less than 35%.

Description

[0001] Process for liquefying a methane-rich feed gas, and corresponding installation

[0002] The present invention relates to a method for liquefying a methane-rich feed gas, comprising cleaning the feed gas to obtain a cleaned gas, pre-cooling the cleaned gas to obtain a pre-cooled gas, liquefying the pre-cooled gas to obtain a liquid stream, sub-cooling the liquid stream to obtain a sub-cooled liquid stream, and expanding the sub-cooled liquid stream to obtain a liquefied gas.

[0003] The invention also relates to an installation suitable for implementing such a method.

[0004] The gas to be treated is, for example, biogas (from the fermentation of organic matter). The market concerned is, for example, that of retail LNG (Liquefied Natural Gas), with final storage of the LNG produced at a pressure of less than 3 bars (10 5 Pa) absolute. This market requires relatively low liquefied gas production capacities, typically less than 20 tonnes of gas to be liquefied per day, or a mechanical power consumption of less than 1 MW.

[0005] Liquefaction of the feed gas consists of cooling it to a temperature cold enough for it to remain in liquid form at thermodynamic equilibrium for pressures making it transportable by state-of-the-art storage techniques, i.e. at pressures below 20 bars absolute and more often below 3 bars absolute. At these pressures, thermodynamic equilibrium of a methane-rich gas is reached at temperatures below -100°C and more often below -140°C.

[0006] To cool the feed gas to these temperatures, it is usually pretreated to remove any compounds that might crystallize at these cryogenic temperatures. Then, the feed gas can also be compressed to a pressure higher than its initial pressure. Then, cooling is usually carried out in three main steps summarized below.

[0007] The gas is first pre-cooled, but not sufficiently to condense to the pressure in question. This pressure is, apart from pressure losses, the pressure at the outlet of the pre-treatment and is generally higher than the final storage pressure of the liquefied gas.

[0008] The gas then undergoes a condensation or liquefaction stage, during which it effectively liquefies and remains, apart from pressure drops, at the cooling inlet pressure. The gas finally undergoes a subcooling stage which continues to cool the liquid and, after a final expansion to reach the storage pressure, the remaining liquid is collected and stored.

[0009] The last two steps are carried out at cryogenic temperatures, typically below -80°C, and are very expensive. On the one hand, the heat extracted, in other words the cooling provided, at these low temperatures requires a lot of energy and, on the other hand, the equipment adapted to these cryogenic temperatures is much more specific and expensive than that designed for lower temperatures, such as pre-cooling.

[0010] It would be possible to increase the temperature of the liquid leaving the subcooling exchanger, just before final expansion. Unfortunately, when the liquid is less subcooled, expansion produces a large quantity of end-flash gas. All or part of this final vapor is most often reinjected at the inlet of the liquefaction process, but this requires recompressing the vapor. Thus, raising the temperature of the subcooled liquid significantly increases the flow rate of gas to be liquefied and the compression requirements of the process.

[0011] For large capacity plants, greater than 20 tonnes per day, the feed gas is usually already at high pressure, greater than 40 bar absolute, because the gas comes either directly from a geological reservoir or from a network. Generally, subcooling is continued down to very low temperatures, around -150°C, in order to generate less than 10% by volume of steam during the final expansion.

[0012] For smaller capacity installations, less than 20 tonnes per day, for example biogas-biomethane applications and reliquefaction of stored LNG vapors or "BOG" (from the English Boil-Off Gas), it is known to carry out compression of the feed gas at higher pressure, beyond 80, or even 120, bars absolute, combined with more moderate cooling which stops at -80°C, or even -50°C. The advantage of raising the final subcooling temperature is to limit the cost of liquefaction and subcooling equipment, but at the cost, on the one hand, of the production of a large quantity of steam during the final expansion (more than 45% by volume) and, on the other hand, of an initial compression of the feed gas which requires expensive compressors and imposes equipment specifically adapted to very high pressures.

[0013] Furthermore, in current processes, the cold from the steam that may be produced is generally recovered and used to cool the feed gas over the entire cooling range, i.e. during pre-cooling, liquefaction and sub-cooling, in order to reduce the overall energy consumption of the cooling refrigeration cycle(s). However, this does not reduce the cost of the most expensive equipment, the contribution of which is significant in small capacity plants.

[0014] Thus, the cost of liquefying methane-rich gas remains high, especially on a small scale.

[0015] One aim of the invention is therefore to propose a liquefaction process making it possible to reduce the overall production cost, in particular for capacities of less than 20 tonnes per day.

[0016] To this end, the invention relates to a process for liquefying a feed gas comprising at least 40% by volume of methane, the process comprising the following steps:

[0017] - mixing the feed gas with a first stream of recycled gas to obtain a gas to be treated, and compression of the gas to be treated to a treatment pressure of between 19 and 70 bars absolute,

[0018] - purification of the gas to be treated to obtain a purified gas,

[0019] - first pre-cooling of the purified gas to obtain a first pre-cooled gas having a temperature less than or equal to -15°C and greater than or equal to -40°C,

[0020] - second pre-cooling of the first pre-cooled gas by heat exchange with at least a second recycled gas flow to obtain a second pre-cooled gas and the first recycled gas flow,

[0021] - liquefaction of the second pre-cooled gas to obtain a liquid stream, by heat exchange only with a liquefaction refrigeration cycle,

[0022] - subcooling the liquid stream by heat exchange with at least a third recycled gas stream to obtain a subcooled liquid stream and the second recycled gas stream, the subcooled liquid stream being at a subcooling temperature, and

[0023] - expansion of the subcooled liquid stream to obtain a liquefied gas and the third recycled gas stream, said expansion and the subcooling temperature being adapted so that the third recycled gas stream represents a fraction, relative to the subcooled liquid stream, of less than 35 mol%.

[0024] According to particular embodiments, the method comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations:

[0025] - the subcooling of the liquid stream comprises: a first subcooling of the liquid stream by heat exchange with the third recycled gas stream, to obtain an intermediate subcooled liquid stream and the second recycled gas stream; and a second subcooling of the intermediate subcooled liquid stream to obtain the subcooled liquid stream;

[0026] - the second sub-cooling of the intermediate sub-cooled liquid stream is carried out by exchange with a liquid nitrogen stream, the second sub-cooling producing the sub-cooled liquid stream and a vaporized nitrogen stream, the second pre-cooling of the first pre-cooled gas being further carried out by heat exchange with the vaporized nitrogen stream;

[0027] - the purified gas undergoes the first pre-cooling in a first pre-cooling unit including a pre-cooling refrigeration cycle, by heat exchange with a refrigerant fluid to form the first pre-cooled gas without heat exchange with the first recycled gas flow, the refrigerant fluid being produced by the pre-cooling refrigeration cycle;

[0028] - the pre-cooling refrigeration cycle and the liquefaction refrigeration cycle are separate;

[0029] - the treatment pressure is less than 45 bars absolute;

[0030] - the liquid stream leaving the liquefaction unit has a temperature between -113°C and -90°C;

[0031] - the second pre-cooled gas is liquefied by the liquefaction unit with sub-cooling less than or equal to 5°C;

[0032] - the liquefaction refrigeration cycle is a Stirling cycle or an inverse Brayton cycle; and

[0033] - the expansion of the subcooled liquid stream is carried out in at least one Joule-Thomson valve or by expansion turbine.

[0034] The invention also relates to an installation suitable for implementing a method as described above, comprising:

[0035] - a mixer for mixing the feed gas with the first recycled gas stream and obtaining the gas to be treated, and at least one compressor suitable for compressing the gas to be treated to the treatment pressure,

[0036] - a purification unit adapted to purify the gas to be treated and obtain the purified gas,

[0037] - a first pre-cooling unit adapted to pre-cool the purified gas and obtain the first pre-cooled gas,

[0038] - a second pre-cooling unit adapted to pre-cool the first pre-cooled gas by heat exchange with at least the second recycled gas stream and to obtain the second pre-cooled gas and the first recycled gas stream,

[0039] - a liquefaction unit for liquefying the second pre-cooled gas and obtaining the liquid stream, the liquefaction unit including the liquefaction refrigeration cycle, - a subcooling unit adapted to subcool the liquid stream to the subcooling temperature by heat exchange with at least the third recycled gas stream and to obtain the subcooled liquid stream and the second recycled gas stream, and

[0040] - an expansion unit for expanding the subcooled liquid stream and obtaining the liquefied gas and the third recycled gas stream, the subcooling unit and the expansion unit being configured so that the third recycled gas stream represents a fraction, relative to the subcooled liquid stream, of less than 35 mol%.

[0041] The invention will be better understood by reading the following description, given solely by way of example and with reference to the attached drawing, in which:

[0042] - figure 1 is a schematic view of an installation according to the invention adapted to implement a method according to the invention; and

[0043] - figure 2 is a schematic view of an installation according to the invention constituting a variant of the installation shown in figure 1.

[0044] In all that follows, the same references will be used to designate a current flowing in a pipe and the pipe that transports it. The terms "upstream" and "downstream" generally extend in relation to the normal direction of circulation of a fluid.

[0045] 1 Nm 3 / h means in this document one cubic meter per hour at a pressure of 101325 Pa and a temperature of 0°C.

[0046] With reference to Figure 1, an installation 10 according to the invention is described. The installation is adapted to liquefy a feed gas 12 comprising at least 40% by volume of methane and to obtain a liquefied gas 14 (i.e. a liquid), for example with a view to its marketing on the retail LNG (Liquefied Natural Gas) market.

[0047] The feed gas 12 is for example at low pressure, close to atmospheric pressure. The feed gas 12 is at a temperature close to ambient temperature, i.e. much hotter than its bubble temperature at atmospheric pressure (101325 Pa).

[0048] Feed gas 12 is, for example, biogas.

[0049] Liquefied gas 14 is advantageously stored at a pressure of less than 3 bars absolute (300 kPa).

[0050] In the example, the installation 10 comprises a mixer 16 for mixing the feed gas 12 with a first recycled gas flow 18 and obtaining a gas to be treated 20. The installation 10 comprises at least one compressor 22 for compressing the gas to be treated 20, for example followed by a cooler 24 and a purification unit 26 adapted to purify the gas to be treated 20 and obtain a purified gas 28. The installation 10 comprises a first pre-cooling unit 30 adapted to pre-cool the purified gas 28 and obtain a first pre-cooled gas 32, the first pre-cooling unit including in the example a pre-cooling refrigeration cycle 34.

[0051] By "refrigeration cycle" is meant a set of pipes and elements (not shown), such as compressors or turbines, adapted to subject a fluid to a series of transformations with the aim of generating cold at a point in the cycle, in a manner known per se.

[0052] The installation 10 comprises a second pre-cooling unit 36 ​​adapted to pre-cool the first pre-cooled gas 32 by heat exchange with a second recycled gas flow 38 and to obtain a second pre-cooled gas 40 and the first recycled gas flow 18.

[0053] The installation 10 comprises a liquefaction unit 42 for liquefying the second pre-cooled gas 40 and obtaining a liquid stream 44, the liquefaction unit including a liquefaction refrigeration cycle 46.

[0054] The installation 10 comprises a subcooling unit 48 adapted to subcool the liquid stream 44 to a subcooling temperature by heat exchange with at least a third recycled gas stream 50 and to obtain a subcooled liquid stream 52 and the second recycled gas stream 38.

[0055] The installation 10 comprises an expansion unit 54 for expanding the subcooled liquid stream 52 and obtaining the liquefied gas 14, for example received in a storage facility 56, and the third recycled gas stream 50.

[0056] The compressor 22 is adapted to compress the gas to be treated 20 to a treatment pressure of between 19 and 70 bars absolute, which makes the gas to be treated 20, after purification, liquefiable at cryogenic temperatures, nevertheless remaining above - 113°C.

[0057] The treatment pressure is advantageously less than 45 bars absolute.

[0058] The purification unit 26 is adapted to remove from the gas to be treated 20 the compounds likely to crystallize downstream. The purification unit 26 is conventionally adapted to remove volatile compounds and heavy hydrocarbons (called "C6+"), for example using activated carbons (not shown and known per se). To lower the water level to a few thousand ppmv (parts per million, by volume), the purification unit 26 comprises for example a condensation system (not shown). To lower the CO2 level to less than 2.5 mol%, a membrane system (not shown) is for example used. To lower the CO2 level below 50 ppmv and the water level below 2 ppmv, molecular sieves can be used (not shown). The purified gas 28 comprises at least 90%, or even 99%, of methane by volume.

[0059] The first pre-cooling unit 30 comprises, for example, a heat exchanger 58 adapted to carry out a heat exchange between the purified gas 28 and a refrigerant fluid 60 produced by the pre-cooling refrigeration cycle 34, without heat exchange with the first recycled gas flow 18.

[0060] In this example, the pre-cooling refrigeration cycle 34 is separate from the liquefaction refrigeration cycle 46. By “separate” is meant that the two refrigeration cycles do not share a refrigerant fluid that would be common to them.

[0061] The pre-cooling refrigeration cycle 34 used is for example a glycol water cycle, a CO2 cycle, an ammonia cycle, a freon cycle, or a propane cycle, known in themselves and which will not be described in detail.

[0062] The temperature of the first pre-cooled gas 32 is between -40°C and -15°C.

[0063] The second pre-cooling unit 36 ​​comprises, for example, a heat exchanger 62 for carrying out the heat exchange with the second recycled gas flow 38.

[0064] The liquefaction unit 42 comprises, for example, a heat exchanger 64 adapted to carry out a heat exchange between the second pre-cooled gas 40 and a refrigerant fluid 66 produced by the liquefaction refrigeration cycle 46, without heat exchange with the second recycled gas flow 38.

[0065] In the example, the liquefaction refrigeration cycle 46 is adapted to provide all the necessary cold to the liquefaction unit 42.

[0066] The liquefaction refrigeration cycle 46 is for example a Stirling cycle.

[0067] By "Stirling cycle" is meant here a refrigeration cycle implemented by a Stirling engine known per se to those skilled in the art.

[0068] Alternatively, the liquefaction refrigeration cycle 46 is for example a reverse Brayton cycle, also known in itself to those skilled in the art.

[0069] The subcooling unit 48 and the expansion unit 54 are configured so that the third recycled gas stream 50 represents a fraction, relative to the subcooled liquid stream 52, of less than 35 mol%, and preferably between 10% and 30 mol%. This is in particular possible by sufficiently lowering the temperature of the subcooled liquid stream 52.

[0070] Still in the example shown in FIG. 1, the subcooling unit 48 comprises a first heat exchanger 68, a second heat exchanger 70 and a subcooling refrigeration cycle 72.

[0071] The first heat exchanger 68 is adapted to carry out a first sub-cooling of the liquid stream 44 by heat exchange with the third recycled gas stream 50, and to obtain an intermediate sub-cooled liquid stream 74 and the second recycled gas stream 38.

[0072] The second heat exchanger 70 is adapted to carry out a second sub-cooling of the intermediate sub-cooled liquid stream 74 to obtain the sub-cooled liquid stream 52, by heat exchange with a refrigerant fluid 76 produced by the sub-cooling refrigeration cycle 72, without heat exchange with the third recycled gas stream 50.

[0073] The expansion unit 54 advantageously comprises an expansion member 78 for expanding the subcooled liquid stream 52 and obtaining an expanded subcooled stream 80, for example at a pressure lower than 3 bars absolute. The expansion unit 54 comprises for example a flash drum 82 for separating the expanded subcooled stream 80 into the liquefied gas 14 and a vapor forming the third recycled gas stream 50.

[0074] The expansion member 78 is for example a Joule-Thomson valve or an expansion turbine.

[0075] A method according to the invention, implemented by the installation 10, will now be briefly described.

[0076] The feed gas 12 and the first recycled gas flow 18 (i.e. the steam from the flash drum 82, after successive reheating in the sub-cooling unit 48 then in the second pre-cooling unit 36) are mixed by the mixer 16 to form the gas to be treated 20.

[0077] The gas to be treated 20 is compressed in the compressor 22, then cooled to approximately room temperature, for example 20°C, in the cooler 24. Then the gas to be treated 20 is purified in the purification unit 26 to form the purified gas 28.

[0078] The purified gas 28 undergoes a first pre-cooling in the first pre-cooling unit 30, by heat exchange with the refrigerant fluid 60, to form the first pre-cooled gas 32.

[0079] The first pre-cooled gas 32 undergoes a second pre-cooling in the second pre-cooling unit 36, by heat exchange with the second recycled gas stream 38, to form the second pre-cooled gas 40. The second recycled gas stream 38 heats up and becomes the first recycled gas stream 18.

[0080] The second pre-cooled gas 40 is liquefied in the liquefaction unit 42 and forms the liquid stream 44.

[0081] In other words, the second pre-cooling unit 36 ​​does not carry out liquefaction. Liquefaction is carried out entirely by the liquefaction unit 42.

[0082] The second pre-cooled gas 40 is liquefied by the liquefaction unit 42, with sub-cooling advantageously less than or equal to 5°C, for example approximately 3°C. In other words, the temperature of the liquid stream 44 at the outlet of the liquefaction unit 42 is for example 3°C below the bubble temperature of the second pre-cooled gas 40. The temperature of the liquid stream 44 at the outlet of the liquefaction unit 42 is preferably between -90°C and -113°C.

[0083] The liquefaction refrigeration cycle 46 advantageously provides all the cold allowing the liquefaction of the second pre-cooled gas 40.

[0084] The liquid stream 44 is then subcooled in the subcooling unit 48 to form the subcooled liquid stream 52, by heat exchange with at least the third recycled gas stream 50, i.e., the vapor from the flash drum 82. The third recycled gas stream 50 heats up and becomes the second recycled gas stream 38.

[0085] In the example, the liquid stream 44 undergoes a first subcooling in the first heat exchanger 68 by heat exchange with the third recycled gas stream 50, then a second subcooling in the second heat exchanger 70 by heat exchange with the refrigerant fluid 76 to form the subcooled liquid stream 52.

[0086] The applied subcooling makes it possible to reduce the evaporation rate at the outlet of the flash tank 82 to a value lower than 35 mol%. Advantageously, the evaporation rate remains greater than or equal to 20 mol%.

[0087] The subcooled liquid stream 52 is expanded in the expansion member 78 to form the expanded subcooled stream 80, which is received in the flash tank 82. The liquefied gas 14 is for example recovered at the bottom of the flash tank 82 and sent to the storage 56.

[0088] The steam from the flash drum 82 is recycled into the gas to be treated 20. This steam forms the third recycled gas stream 50, which first becomes the second recycled gas stream 38 after passing through the subcooling unit 48, then becomes the first recycled gas stream 18 after passing through the second precooling unit 36.

[0089] This steam does not pass into the liquefaction unit 42, or in any case this steam does not give off cold to the liquefaction unit 42.

[0090] In the example shown in Figure 1, this steam also does not pass into the first pre-cooling unit 30, or in any case does not give off cold to the first pre-cooling unit 30.

[0091] According to a variant not shown, this steam can release part of its cold in the pre-cooling unit 30, in particular depending on the size of the installation 10. For example, if the production of liquefied gas 14 is less than 20 tonnes per day, the recovery of cold in the first pre-cooling unit 30 from the first recycled gas flow 18 will be avoided, as shown in FIG. 1.

[0092] On the other hand, if the production of liquefied gas 14 is greater than or equal to 20 tonnes per day for example, this recovery will be preferred. To achieve this, for example, the first stream of recycled gas 18 is passed through the first pre-cooling unit 30.

[0093] With reference to Figure 2, an installation 100 according to the invention is described, constituting a variant of the installation 10. The installation 100 is similar to the installation 10 shown in Figure 1. Similar elements bear the same numerical references and will not be described again. Only the differences will be described in detail below.

[0094] In the installation 100, the first pre-cooling of the purified gas 28 is carried out by heat exchange with a liquefaction refrigeration cycle 146. In other words, the liquefaction unit 42 and the first pre-cooling unit 30 share the same refrigeration cycle 146, which provides the cold used for the first pre-cooling and for the liquefaction.

[0095] The refrigerant 66 gives off cold to the second pre-cooled gas 40 in the heat exchanger 64 of the liquefaction unit 42, and becomes the refrigerant 60. The refrigerant 60 gives off cold to the purified gas 28 in the heat exchanger 58 of the first pre-cooling unit 30.

[0096] As in installation 10, the second pre-cooling unit 36 ​​of installation 100 does not receive cold from the refrigeration cycle 146.

[0097] Alternatively or additionally, in the subcooling unit 48, the second heat exchanger 70 does not receive cold from a refrigeration cycle dedicated to subcooling, but from an open loop 172 with liquid nitrogen.

[0098] In the open loop 172, a liquid nitrogen stream 174 (from a source not shown, such as a liquid nitrogen storage) transfers cold to the intermediate subcooled liquid stream 74 in the second heat exchanger 70 of the subcooling unit 48, and vaporizes to become a gaseous nitrogen stream 176.

[0099] Advantageously, the flow of gaseous nitrogen 176 then gives off cold to the first pre-cooled gas 32 in the heat exchanger 62, and becomes a flow of nitrogen 178.

[0100] According to a particular embodiment, a nitrogen top-up 180 is carried out in the gaseous nitrogen flow 176 before the latter enters the second pre-cooling unit 36. This top-up 180 has a temperature lower than that of the gaseous nitrogen flow 176 before the top-up. The top-up 180 is advantageously carried out in liquid form. Thus, the second pre-cooling is carried out by heat exchange, on the one hand, with the second recycled gas flow 38 and, on the other hand, advantageously with the gaseous nitrogen flow 176, possibly increased by the top-up 180.

[0101] The operation of installation 100 is similar to that of installation 10.

[0102] According to yet another variant (not shown), the second heat exchanger 70 of the subcooling unit 48 is absent, as well as the subcooling refrigeration cycle 72 (figure 1) or the open loop 172 (figure 2). The subcooling carried out by the subcooling unit 48 then takes place only in the first exchanger 68, by heat exchange with the third recycled gas flow 50.

[0103] Examples

[0104] These examples include one or more of the following characteristics, in all possible combinations.

[0105] The treatment pressure is 40 bars absolute (pressure of the gas to be treated after compression by compressor 22).

[0106] The second pre-cooled gas 40 has a temperature of -53.5°C.

[0107] The liquid stream 44, leaving the liquefaction unit 42, has a temperature of -90°C.

[0108] The subcooling unit 48 and the expansion unit 54 are configured to obtain an evaporation rate of between 20% and 30% molar.

[0109] The following two tables define four cases and allow them to be compared with each other:

[0110] - case 1 (counter-example): no recovery of the cold contained in the flash gas (the third gas flow 50 does not pass through the exchangers 68 and 62), nor open loop 172 with liquid nitrogen;

[0111] - case 2 (example according to the invention): with recovery of the cold contained in the flash gas (the third gas flow 50 passes through the exchangers 68 and 62), but no open loop 172 with liquid nitrogen. Case 2 corresponds substantially to figure 1;

[0112] - case 3 (counter-example): no recovery of the cold contained in the flash gas (the third gas flow 50 does not pass through the exchangers 68 and 62), but presence of the open loop 172 with liquid nitrogen; and

[0113] - case 4 (example according to the invention): with recovery of the cold contained in the flash gas (the third gas flow 50 passes through the exchangers 68 and 62), and presence of the open loop 172 with liquid nitrogen. Case 4 corresponds substantially to figure 2.

[0114] Case 1 represents a simple process, namely just liquefaction without subcooling and without cold recovery from the flash. We then seek to evaluate the energy gains resulting from the progressive addition of cold recovery systems from the flash gas and liquid nitrogen subcooling systems.

[0115] To evaluate this gain, as a first approach, we consider only the thermal load of the liquefaction unit 42, because this part is the most expensive of the process. In addition, we use the thermal load rather than the mechanical power consumed by the liquefaction cycle, because the mechanical power depends on the type of cycle used (inverse Brayton, MR, Stirling, etc.), but we are interested in the energy gain independently of the type of liquefaction cycle.

[0116] In conclusion, case 4 allows to reduce the size of the liquefaction unit by 44%, resulting in a significant reduction in the overall production cost of liquefied gas.

[0117] Thanks to the characteristics described above, the process makes it possible to reduce the overall production cost of liquefied gas 14, particularly for production capacities of less than 20 tonnes per day. Indeed, the treatment pressure, between 19 and 70 bars absolute, is high enough so that the liquefaction temperature is not too low, i.e. preferably above -90°C. Thus, the equipment used is less specific and less expensive. Since the investment weight is significant for small capacities, this has a favorable impact on the unit production cost. In addition, the energy used to provide cold is also lower when the temperature of the fluid to be cooled is lower.

[0118] However, the treatment pressure remains relatively low and allows a sufficiently low subcooling temperature, which keeps the volume fraction of recycled flash gas upstream of the compressor in a reasonable proportion, which reduces the energy expended for the compression of the gas to be treated 20. In addition, the lower pressure also allows savings on the equipment, which does not have to withstand very high pressures.

[0119] In addition, the cold present in the recycled flash gas (third recycled gas stream 50) is used specifically to amplify the pre-cooling of the gas to be treated and its sub-cooling. This cold is not used in the liquefaction unit 42. This reduces the cooling range of the liquefaction unit 42, and reduces the size of the liquefaction refrigeration cycles 46, 146. In addition, this avoids having to modify the liquefaction refrigeration cycles themselves to integrate a recycled gas stream.

[0120] Subcooling is advantageously carried out using liquid nitrogen, the cold of which is also used, for example, to amplify precooling.

[0121] The use of flash gas cold, and possibly liquid nitrogen, is not "spread out" throughout the cooling range as in some prior art solutions, but rather concentrated on pre-cooling and sub-cooling, thereby specifically reducing the most expensive parts of the cooling process, including the liquefaction unit 42.

Claims

DEMANDS 1. A process for liquefying a feed gas (12) comprising at least 40% by volume of methane, the process comprising the following steps: - mixing of the feed gas (12) with a first recycled gas stream (18) to obtain a gas to be treated (20), and compression of the gas to be treated (20) to a treatment pressure between 19 and 70 bar absolute, - purification of the gas to be treated (20) to obtain a purified gas (28), - first pre-cooling of the purified gas (28) to obtain a first pre-cooled gas (32) having a temperature less than or equal to -15°C and greater than or equal to -40°C, - second pre-cooling of the first pre-cooled gas (32) by heat exchange with at least one second recycled gas stream (38) to obtain a second pre-cooled gas (40) and the first recycled gas stream (18), - liquefaction of the second pre-cooled gas (40) to obtain a liquid stream, (44) by heat exchange only with a liquefaction refrigeration cycle (46; 146), - subcooling of the liquid stream (44) by heat exchange with at least a third recycled gas stream (50) to obtain a subcooled liquid stream (52) and the second recycled gas stream (38), the subcooled liquid stream (52) being at a subcooling temperature, and - expansion of the subcooled liquid stream (52) to obtain a liquefied gas (14) and the third recycled gas stream (50), said expansion and the subcooling temperature being adapted so that the third recycled gas stream (50) represents a fraction, relative to the subcooled liquid stream (52), of less than 35% molar.

2. A method according to claim 1, wherein the subcooling of the liquid stream (44) comprises: - a first subcooling of the liquid stream (44) by heat exchange with the third recycled gas stream (50), to obtain an intermediate subcooled liquid stream (74) and the second recycled gas stream (38), and - a second subcooling of the intermediate subcooled liquid stream (74) to obtain the subcooled liquid stream (52).

3. A method according to claim 2, wherein the second subcooling of the intermediate subcooled liquid stream (74) is achieved by exchange with a flow of liquid nitrogen (174), the second subcooling producing the subcooled liquid stream (52) and a vaporized nitrogen stream (176), the second precooling of the first precooled gas (32) being further achieved by heat exchange with the vaporized nitrogen stream. (176) 4. A method according to any one of claims 1 to 3, wherein the purified gas (28) undergoes the first pre-cooling in a first pre-cooling unit (30) including a pre-cooling refrigeration cycle (34), by heat exchange with a refrigerant (60) to form the first pre-cooled gas (32) without heat exchange with the first recycled gas stream (18), the refrigerant (60) being produced by the pre-cooling refrigeration cycle (34), the pre-cooling refrigeration cycle (34) and the liquefaction refrigeration cycle (46) being separate.

5. A method according to any one of claims 1 to 4, wherein the processing pressure is less than 45 absolute bars.

6. A method according to any one of claims 1 to 5, wherein the liquid stream (44) exiting the liquefaction unit (42) has a temperature between -113°C and -90°C.

7. A method according to any one of claims 1 to 6, wherein the second pre-cooled gas (40) is liquefied by the liquefaction unit (42) with a subcooling of less than or equal to 5°C.

8. A method according to any one of claims 1 to 7, wherein the liquefaction refrigeration cycle (46; 146) is a Stirling cycle or a reverse Brayton cycle.

9. A method according to any one of claims 1 to 8, wherein the expansion of the subcooled liquid stream (52) is carried out in at least one Joule-Thomson valve or by expansion turbine.

10. Installation (10; 100) adapted to implement a process according to any one of claims 1 to 9, comprising: - a mixer (16) for mixing the feed gas (12) with the first recycled gas stream (18) and obtaining the gas to be treated (20), and at least one compressor (22) suitable for compressing the gas to be treated (20) to the treatment pressure, - a purification unit (26) adapted to purify the gas to be treated (20) and obtain the purified gas (28), - a first pre-cooling unit (30) adapted to pre-cool the purified gas (28) and obtain the first pre-cooled gas (32), - a second pre-cooling unit (36) adapted to pre-cool the first pre-cooled gas (32) by heat exchange with at least the second recycled gas stream (38) and to obtain the second pre-cooled gas (40) and the first recycled gas stream (18), - a liquefaction unit (42) for liquefying the second pre-cooled gas (40) and obtaining the liquid stream (44), the liquefaction unit (42) including the liquefaction refrigeration cycle (46; 146), - a subcooling unit (48) adapted to subcool the liquid stream (44) to the subcooling temperature by heat exchange with at least the third recycled gas stream (50) and to obtain the subcooled liquid stream (52) and the second recycled gas stream (38), and - a decompression unit (54) to decompression the subcooled liquid stream (52) and obtain the liquefied gas (14) and the third recycled gas stream (50), the subcooling unit and the decompression unit being configured so that the third recycled gas stream (50) represents a fraction, relative to the subcooled liquid stream (52), of less than 35% molar.

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