Biogas treatment plant and process

Abstract

Installation (1) for the treatment of a gas stream (10), in particular biogas, comprising methane and carbon dioxide, the installation (1) comprising a first cold source (6) and a gas circuit for the gas stream (10), the gas circuit comprising, arranged in series and fluidly connected by a set of pipes, a first compressor (2), a first scrubbing unit (3) configured to produce a first methane-enriched gas stream (11) with less than 5% carbon dioxide, and a carbon dioxide-enriched gas stream (12), a first heat exchange system (4, 5), and a second scrubbing unit (7) configured to produce a second methane-enriched gas stream (13) with less than 400 ppm carbon dioxide, and a methane-depleted gas stream (14). (Shorthand figure: Fig. 1)

Description

Title of the invention: Biogas treatment plant and process

[0001] The present invention relates to a plant and process for producing purified methane from biogas. Biogas is the gas produced during the decomposition of organic matter in the absence of oxygen (anaerobic fermentation), also known as methanation. This can be a natural decomposition observed in marshes or municipal waste landfills. Biogas production can also result from the methanation of waste in a dedicated reactor, the conditions of which are controlled. This reactor is called a methanizer or digester, and the biogas then passes through a post-digester, similar to the digester, which allows the methanation reaction to continue and potentially complete.Biomass refers to any group of organic matter that can be transformed into energy through the methanation process, for example sewage sludge, manure / slurry, agricultural residues, and food waste.

[0002] Biogas consists mainly of methane (CH4) and carbon dioxide (CO2) in varying proportions depending on the production method and substrate, but may also contain, in smaller proportions, water, nitrogen, hydrogen sulfide (H2S), oxygen, and other organic compounds in trace amounts, including H2S, at concentrations between 10 and 50,000 ppmv. Depending on the organic matter degraded and the techniques used, the proportions of the components differ, but on average biogas comprises (by mole or volume) on a dry gas basis, 30 to 75% methane, 15 to 60% carbon dioxide, 0 to 15% nitrogen, 0 to 5% oxygen, and trace amounts of other compounds.

[0003] More refined purification of biogas allows for its wider use; in particular, refined purification of biogas makes it possible to obtain biogas purified to the specifications of natural gas, which can then be substituted for it. Biogas thus purified is called "biomethane." Biomethane thus complements natural gas resources with a renewable component produced within local areas; it can be used for exactly the same purposes as natural gas of fossil origin. It can supply a natural gas network or a vehicle refueling station. Under certain conditions, it can also be liquefied for storage as liquefied natural gas (bio-LNG).

[0004] Liquefaction plants for producing bio-LNG require, as an input, methane that is sufficiently pure to avoid, in particular, the solidification of impurities and clogging of the pipes or heat exchangers used by the process. The main impurity is CO2, the content of which must be less than 400 ppm to ensure the proper operation of the liquefaction process. To produce purified methane of the necessary quality required for its liquefaction, that is to say, methane having between 0 and 400 ppm of CO2, especially between 50 and 400 ppm of CO2, from biogas sources, the traditional solution is to link two purification steps.

[0005] During a first stage known as "upgrading", the raw biogas is generally purified into biomethane down to a CO2 level of 2 to 4 mol%. This stage is carried out by membrane separation units or amine scrubbing.

[0006] During a second stage known as "fine polishing", amine washing or adsorption units of type PTSA, PSA or TSA are commonly used to obtain a CO2 content of between 50 and 400 ppm, from the methane stream comprising 2 to 4 mol% CO2 which is supplied to them as input.

[0007] Application WO2016 / 034788 Al describes a possible alternative for the implementation of the purification or upgrading step.

[0008] The invention relates in particular to an installation for the treatment of a gas stream, in particular biogas, comprising methane and carbon dioxide, the installation comprising a first cold source and a gas circuit for the gas stream, the gas circuit comprising, arranged in series and fluidically connected by a set of pipes, a first compressor, a first purification unit configured to produce a first gas stream enriched in methane and a gas stream enriched in carbon dioxide, a first heat exchange system, and a second purification unit configured to produce a second gas stream enriched in methane and a gas stream depleted in methane, the set of pipes being configured to transfer the gas stream compressed by the first compressor to an inlet of the first purification unit,to transfer the methane-enriched gas stream from a first outlet of the first purification unit to a first inlet of the second purification unit, and to transfer the methane-depleted gas stream from a first outlet of the second purification unit to a compressor inlet, the first heat exchange system being configured to exchange heat between the cold source and the first methane-enriched gas stream in order to cool the first methane-enriched gas stream.

[0009] These solutions are not energy-optimized, do not take into account the management or recovery of CO2 produced, and are therefore not adapted to the new challenges. These solutions present, in particular, the following drawbacks.

[0010] Amine washes used for the fine grinding or polishing stage are energy-inefficient systems that contain harmful chemicals and require a high temperature.

[0011] Adsorption polishing is a cyclic sequential system that requires regeneration phases and the use of a regeneration gas to desorb CO2. captured; the operation is therefore discontinuous and often consumes a significant amount of purified biomethane for the regeneration phases.

[0012] Adsorption systems have many moving parts and require fine regulation of the cycles.

[0013] These installations represent a high operating cost.

[0014] The integration of the purification process with the liquefaction processes is not taken into account.

[0015] One object of the present invention is to propose an innovative installation and process for the production of purified methane, the liquefaction of purified methane and the valorization of carbon dioxide, and thus to overcome all or part of the disadvantages noted above.

[0016] To this end, the invention, in accordance with the generic definition given above, is characterized in that the first purification unit is configured to produce the first methane-enriched gas stream with less than 5% carbon dioxide, and in that the second purification unit comprises a membrane separation unit configured to produce the second methane-enriched gas stream with less than 400 ppm carbon dioxide.

[0017] The invention thus allows a simple and efficient implementation of the polishing step.

[0018] The invention can be advantageously applied to the treatment of gas streams of biological origin (biogas), fossil origin (for example natural gas), or produced from waste or garbage, particularly within or near landfill sites (landfill gas).

[0019] According to other aspects, embodiments of the installation according to the invention may include one or more of the following characteristics.

[0020] According to one embodiment, the second purification unit consists of a membrane separation stage, in particular configured to produce the second gas stream enriched in methane with less than 50 ppm of carbon dioxide.

[0021] According to one embodiment, the first heat exchange system is further configured to exchange the methane-depleted gas stream with the first methane-enriched gas stream.

[0022] According to one embodiment, the first heat exchange system comprises, in series in the gas circuit, a first heat exchanger and a second heat exchanger, the first heat exchanger being configured to exchange heat between the first methane-enriched gas stream and the methane-depleted gas stream, and the second heat exchanger being configured to exchange heat between the first methane-enriched gas stream and the cold source.

[0023] According to one embodiment, the piping assembly includes a bypass line configured to take a fraction of the second methane-enriched gas stream and transfer it to a second inlet of the second purification unit, and in that the bypass line may include a pressure-reducing device.

[0024] According to one embodiment, the first purification unit comprises a first and a second membrane separation stage in series, and in that the piping assembly comprises a pipe configured to connect an outlet of the first membrane separation stage to an inlet of the second membrane separation stage, the installation comprising a second heat exchange system configured to put at least a portion of the pipe into heat exchange with the cold source.

[0025] According to one embodiment, the installation includes a distillation column and in that the piping assembly is configured to transfer the carbon dioxide-enriched gas stream from a second outlet of the first purification unit to an inlet of the distillation column in order to produce liquefied carbon dioxide.

[0026] According to one embodiment, the piping assembly includes a pipe connecting a first upper outlet of the distillation column to the inlet of the first purification unit to transfer at least a fraction of a top gas stream from the distillation column to the first purification unit.

[0027] According to one embodiment, the installation includes a third heat exchange system configured to cool the overhead gas stream of the distillation column.

[0028] According to one embodiment, the installation includes a liquefaction device and in that the piping assembly is configured to transfer the second methane-enriched gas stream from the second purification unit to an inlet of the liquefaction device in order to liquefy the second methane-enriched gas stream.

[0029] According to one embodiment, the installation includes a fourth heat exchange system configured to cool the second methane-enriched gas stream before it enters the liquefaction device.

[0030] According to one embodiment, at least two of the first, second, third and fourth heat exchange systems are in heat exchange with the same cold source.

[0031] The invention further relates to a process for treating a gas stream, in particular biogas, comprising methane and carbon dioxide, the process comprising the following steps.

[0032] Compress the gas stream to a pressure between 1 and 22 barg, preferably between 7 and 20 barg, preferably between 12 and 16 barg.

[0033] Separate the compressed gas stream into a first gas stream enriched in methane and a gas stream enriched in carbon dioxide.

[0034] Cool the first methane-enriched gas stream to a temperature between -10 and -50 °C, preferably between -20 and -30 °C.

[0035] Separate the first methane-enriched gas stream into a second methane-enriched gas stream and a methane-depleted gas stream.

[0036] The first methane-enriched gas stream contains less than 3% carbon dioxide, and the second methane-enriched gas stream contains less than 400 ppm carbon dioxide, preferably less than 50 ppm carbon dioxide.

[0037] According to other aspects, embodiments of the process according to the invention may include one or more of the following characteristics.

[0038] According to one embodiment, the process includes an additional step of putting the methane-depleted gas stream into heat exchange with the first methane-enriched gas stream.

[0039] According to one embodiment, the step of separating the first methane-enriched gas stream is carried out by a membrane separation unit, and the process includes the following additional steps.

[0040] To take a fraction of the second methane-enriched gas stream,

[0041] Expanding the fraction of the second gas stream enriched in methane,

[0042] Inject the fraction of the second gas stream enriched in methane into the membrane separation unit.

[0043] According to one embodiment, the process includes at least one of the following additional steps: liquefying the carbon dioxide-enriched gas stream, and / or liquefying the second methane-enriched gas stream.

[0044] The invention may also relate to any alternative device or method comprising any combination of the above or below features, particularly within the scope of the claims.

[0045] The invention may also relate to any alternative device or method comprising any combination of the above or below features, particularly within the scope of the claims.

[0046] Other features and advantages will become apparent from the following description, made with reference to the following figures.

[0047] [Fig.1] Fig.1 represents a schematic and partial view, illustrating a first example of a possible embodiment of the invention.

[0048] [Fig.2] Fig.2 represents a schematic and partial view, illustrating a second example of a possible embodiment of the invention.

[0049] [Fig.3] Fig.3 represents a schematic and partial view, illustrating a detail of a third example of a possible embodiment of the invention.

[0050] The installation 1 illustrated by way of example in [Fig. 1] allows the treatment of a gas stream 10 comprising methane and carbon dioxide. This gas stream 10 is in particular of biological origin, for example biogas. Preferably This gas stream 10 has previously been dried and purified of H2S and volatile organic compounds (VOCs).

[0051] The installation 1 includes a first cold source 6 and a gas circuit for the gas stream 10. The gas circuit includes, arranged in series and fluidly connected by a set of pipes, a first compressor 2, a first purification unit 3, a first heat exchange system 4, 5, and a second purification unit 7.

[0052] The first purification unit 3 is configured to produce a first methane-enriched gas stream 11 and a carbon dioxide-enriched gas stream 12. The first purification unit 3 is configured to produce the first methane-enriched gas stream 11 with less than 5% carbon dioxide, for example, less than 3% carbon dioxide. The first methane-enriched gas stream 11 comprises, for example, more than 96% methane. The carbon dioxide-enriched gas stream 12 comprises, for example, more than 80% carbon dioxide. The first purification unit 3 thus enables the implementation of the upgrading step.

[0053] For example, the configuration of the purification unit 3 to produce the first methane-enriched gas stream 11 with a maximum carbon dioxide content is obtained by using a scrubber sized to achieve this level of purification.

[0054] The purification unit 3 can be a membrane permeation treatment unit comprising one or more membrane separation units. These membranes are more permeable to carbon dioxide than to methane. Tubular and / or polyimide membranes can be used, for example. Each membrane separation unit (also called a "stage") can comprise one or more membranes connected in parallel. Several membrane stages (typically between 2 and 4) are required to achieve a CO2 content in the first methane-enriched gas stream 11 of less than 5% or less than 3%. Varying, respectively reducing or increasing, the number of stages or the quantity of membranes actually used allows the CO2 content to be varied, respectively increased or decreased.

[0055] The second purification unit 7 is configured to produce a second methane-enriched gas stream 13 and a methane-depleted gas stream 14. The second purification unit 7 includes a membrane separation unit 7. The membranes used are more permeable to carbon dioxide than to methane. For example, tubular and / or polyimide membranes can be used.

[0056] The second membrane separation unit is configured to produce the second gas stream enriched in methane-13 with less than 400 ppm of carbon dioxide, preferably less than 50 ppm of carbon dioxide. The second stream The methane-enriched gas stream 13, for example, contains more than 99.9% methane. The methane-depleted gas stream 14, for example, contains less than 95% methane, for example, between 80% and 95% methane. The second purification unit 7 thus enables the polishing stage to be carried out.

[0057] The second purification unit 7 may include one or more configurable membrane separation stages, allowing the use of between 50% and 100% of the membrane capacity of the stages, depending on the flow rate of the gas to be purified and the required purity. That is to say, each stage has a predetermined maximum membrane capacity, and one or more of the membrane separation stages can be configured in real time to use a variable portion of their membrane capacity. Membrane capacity refers to the membrane surface area in contact with the stream of gas to be purified.

[0058] For example, the configuration of the second purification unit 7 to produce the second methane-enriched gas stream 13 is obtained by using a scrubber sized to achieve this level of purification.

[0059] In the case where the first methane-enriched gas stream 11, entering the second purification unit 7, has a flow rate of approximately 1000 Nm3 / h and a CO2 content of less than 3%, the second purification unit 7 shall comprise fifty membranes in order to obtain a second methane-enriched gas stream 13 with a CO2 concentration between 0 and 400 ppm, in particular 50 ppm. The second purification unit 7 can be operated between 8 and 12 barg and between -40 and -20°C, preferably -20°C; that is to say, the second methane-enriched gas stream 13 enters the second purification unit 7 at a pressure between 8 and 12 barg and at a temperature between -40°C and -20°C, preferably -20°C.

[0060] The piping system is configured to transfer the gas stream 10 compressed by the first compressor 2 to an inlet 31 of the first purification unit 3, transfer the gas stream enriched in methane 11 from a first outlet 32 ​​of the first purification unit 3 to a first inlet 71 of the second purification unit 7, and transfer the gas stream depleted in methane 14 from a first outlet 72 of the second purification unit 7 to an inlet 21 of the compressor 2. The methane that was not retained by the second purification unit 7 is thus recovered. The production of purified methane is optimized.

[0061] In operation, the gas stream 10 is compressed by the compressor 2 to a pressure between 1 and 22 barg, preferably between 7 and 20 barg, and even more preferably between 12 and 16 barg. The first purification unit 3 carries out the upgrading step and separates the gas stream 10 into the first methane-enriched gas stream 11 and the carbon dioxide-enriched gas stream 12. The first methane-enriched gas stream 11 contains less than 3% carbon dioxide and is then cooled to a temperature between -10 and -50 °C, preferably between -20 and -30 °C, even more preferably -20 °C.

[0062] The carbon dioxide-enriched gas stream 12 is recovered at a second outlet 33 of the first purification unit 3 and can be vented or used in a known manner, or liquefied as illustrated later.

[0063] The second purification unit 7 implements the polishing step and separates the first methane-enriched gas stream 11 into the second methane-enriched gas stream 13 and the methane-depleted gas stream 14. The second methane-enriched gas stream 13 then contains less than 400 ppm of carbon dioxide, and preferably less than 50 ppm of carbon dioxide.

[0064] The second methane-enriched gas stream 13 is recovered at a second outlet 73 of the second purification unit 7 and can be used or sold directly in a known manner, or liquefied as illustrated later.

[0065] The first heat exchange system 4, 5 is configured to put the cold source 6 and the first methane-enriched gas stream 11 in heat exchange in order to cool the first methane-enriched gas stream 11.

[0066] The cold source 6 is specifically configured to provide cooling power at a temperature between -50 °C and -10 °C, preferably between -45 °C and -20 °C, preferably between -30 °C and -20 °C, typically at -20 °C. The first methane-enriched gas stream 11 therefore enters the second purification unit 7 at a temperature determined by the cold source 6.

[0067] The cold source 6 may include a mechanical and / or refrigeration cycle refrigerator, for example a cold fluid reservoir. Heat exchange between the cold source and the gas stream may take place directly in the exchange system and / or via an intermediate heat transfer fluid or material.

[0068] Surprisingly, the inventors observed that cooling a methane gas stream already purified to a quality sufficient for, for example, injection into a natural gas network, and subsequent treatment by a membrane separation unit makes it possible to obtain a much purer methane gas stream than that which known membrane systems make it possible to obtain.

[0069] In one embodiment, the second purification unit 7 may be formed from or consist of a membrane separation stage configured to produce the second methane-enriched gas stream 13 with less than 400 ppm of carbon dioxide, preferably with less than 50 ppm of carbon dioxide. A single membrane separation stage may thus be sufficient for polishing the biomethane. The installation is simplified and operating costs can be reduced.

[0070] In one embodiment, the second purification unit 7 may comprise two membrane separation stages or a single divided membrane separation stage in two substages. Obtaining a second stream enriched in methane 13 with less than 50 ppm of CO2 is facilitated.

[0071] Dividing a membrane stage into two substages, also called "split staging," involves dividing the membranes of the membrane stage into two groups or substages connected in series. For example, a membrane stage comprising four membranes in parallel can be replaced by a first substage comprising two membranes in parallel and connected in series to a second substage comprising two membranes in parallel. The methane-rich gas streams produced by the membranes of the first substage are recombined to enter the second substage. Thus, each substage can operate at a lower selectivity, while achieving better productivity and a lower CO2 concentration in the methane-enriched gas stream exiting the membrane stage.

[0072] In one embodiment, the first heat exchange system 4, 5 can be configured to exchange the methane-depleted gas stream 14 with the first methane-enriched gas stream 11. The methane-depleted gas stream 14 is heated and thus cools the first methane-enriched gas stream 11 to a temperature between -40 °C and -10 °C, preferably -20 °C. The cooling capacity of the methane-depleted gas stream 14 is recovered. The energy efficiency of the installation 1 is improved.

[0073] In one embodiment, the first heat exchange system may include, in series in the gas circuit, a first heat exchanger 4 and a second heat exchanger 5. The first heat exchanger 4 is configured to exchange heat between the first methane-enriched gas stream 11 and the methane-depleted gas stream 14. The second heat exchanger 5 is configured to exchange heat between the first methane-enriched gas stream 11 and the cold source 6.

[0074] The first heat exchanger 4 and the second heat exchanger 5 are preferably separate. This allows for a reduction in the size of this equipment. Heat exchanger 4 thus performs a pre-cooling of the methane-enriched gas stream 11 while recovering the cooling capacity of the methane-depleted gas stream 14, before recycling the latter at the inlet 21 of the compressor 2. The control of the operation of the second heat exchanger can therefore be optimized.

[0075] The first heat exchanger 4 and the second heat exchanger 5 can, alternatively, form part of a single heat exchange block. This heat exchange block would then comprise several dedicated passages allowing first the heat exchange between the first methane-enriched gas stream 11 and the methane-depleted gas stream 14, and then the heat exchange of this same first methane-enriched gas stream. methane 11 with the cold source 6. The temperature of the first gas stream enriched in methane 11 can thus be regulated via a single component.

[0076] The first heat exchange system 4, 5 may for example include a single heat exchanger comprising three passages: a fluid passage for the first methane-enriched gas stream 11, a fluid passage for the methane-depleted gas stream 14 and a fluid passage for a heat transfer fluid from the cold source 6. This single heat exchanger is then configured to exchange heat, in order, the first methane-enriched gas stream 11 with the methane-depleted gas stream 14, then the first methane-enriched gas stream 11 with the heat transfer fluid.

[0077] In one embodiment, the piping assembly includes a bypass line configured to draw a portion of the second methane-enriched gas stream 13 and transfer it to a second inlet 74 of the second purification unit 7. For example, the second outlet 73 of the second purification unit 7 is connected to a main line transferring the second methane-enriched gas stream 13 to a user. The bypass line then comprises a first end connected to this main line and a second end connected to the second inlet 74 of the second purification unit 7.

[0078] The bypass line may include a pressure-reducing device 8. The pressure-reducing device may, for example, be a control valve associated with a gas pressure measuring instrument in the bypass line downstream of the pressure-reducing device 8. The pressure-reducing device is preferably configured to lower the pressure of the fraction taken by the bypass line to the pressure of the methane-depleted gas stream 14.

[0079] During operation, a fraction, for example between 0% and 5%, of the second methane-enriched gas stream 13 is taken, expanded using the expansion device 8, and injected into the membrane separation unit 7. The second methane-enriched gas stream 13 is, for example, at a pressure between 8 and 14 barg, preferably 10 barg. The methane-depleted gas stream 14 is, for example, at a pressure between 0.5 and 4 barg, preferably 1 barg. This operation, also called flash cooling, allows the gas streams to be cooled subsequently in the second purification unit 7.

[0080] In one embodiment, the first purification unit 3 comprises a first 310 and a second 320 membrane separation stage in series. The piping assembly includes a pipe configured to connect a methane-enriched gas outlet from the first membrane separation stage 310 to an inlet of the second membrane separation stage 320. The installation includes a second heat exchange system 9 configured to exchange the enriched gas for heat. Methane exits the first stage 310, along with the cold source 6. The first membrane separation stage 310 thus produces a methane-rich retentate and a carbon dioxide-rich permeate. The heat exchange system 9 cools the retentate using the cold source 6. The cooled retentate is then introduced into the second membrane separation stage 320.

[0081] The first purification unit 3 may include one or more membrane separation stages 310, 320, 330, which are configurable to use, depending on the gas flow rate to be purified and the required purity, between 50% and 100% of the membrane capacity of the stages. That is to say, each stage has a predetermined maximum membrane capacity, and one or more of the membrane separation stages can be configured in real time to use a variable portion of their membrane capacity. Thus, the percentage of available membrane surface area that is actually used can vary to adapt to production constraints.

[0082] In one embodiment, the first purification unit 3 comprises a membrane separation unit formed of or consisting of two or three membrane separation stages 310, 320, 330.

[0083] For example, a first gas stream enriched in methane 11 with less than 3%, typically 2%, of CO2 can be obtained by configuring the first purification unit 3 to include three membrane separation stages. The number of membranes per stage will depend, among other things, on the flow rate and the composition of the gas stream 10. For a biogas flow rate of 1000 Nm3 / h with a methane content of approximately 50%, the first purification unit 3 can be configured with four membranes in the first stage 310, five membranes in the second stage 320, and four membranes in the third stage 330.

[0084] In one embodiment, the first purification unit 3 comprises at least one of: a carbon dioxide adsorption unit, for example a PSA / VSA / TSA, or a carbon dioxide absorption unit, for example an amine scrubber.

[0085] The purification unit 3 can indeed operate according to the principle of separation and / or adsorption and / or absorption. This can be, for example, a pressure alternating adsorption (PSA) system, amine washing, water washing, organic physical washing, cryogenic distillation or membrane separation.

[0086] In one embodiment, the installation comprises a distillation column 100 and the piping system is configured to transfer the carbon dioxide-enriched gas stream 12 from a second outlet 33 of the first purification unit 3 to an inlet of the distillation column 100 in order to produce liquefied carbon dioxide. The carbon dioxide-enriched gas stream 12 is preferably compressed to a pressure between 1 barg and 3 barg, preferably 1.5 barg. The mixture is compressed into a 120-liter compressor before entering the distillation column. Distillation columns suitable for this purpose are well-known and will not be described in further detail here.

[0087] In operation, between 80% and 95%, typically 90%, of the CO2 contained in the carbon dioxide-enriched gas stream is liquefied.

[0088] In one embodiment, the piping system includes a pipe connecting a first upper outlet of the distillation column 100 to the inlet 31 of the first purification unit 3 to transfer at least a fraction of a headstream gas from the distillation column 100 to the first purification unit 3. The headstream gas typically contains between 20% and 40% methane, which is thus recycled into the purification system and recovered. The methane production of the installation is optimized.

[0089] In one embodiment, the installation includes a third heat exchange system 110 configured to cool the overhead gas stream from the distillation column 100 before recycling the overhead gas stream into the first purification unit 3.

[0090] In one embodiment, the installation includes a liquefaction device 200 and the piping assembly is configured to transfer the second methane-enriched gas stream 13 from the second purification unit 7 to an inlet of the liquefaction device 200 in order to liquefy the second methane-enriched gas stream 13. Devices suitable for the liquefaction of methane are known and will not be described here in further detail.

[0091] In operation, more than 95%, preferably all of the CH4 contained in the second methane-enriched gas stream 13 is liquefied.

[0092] In one embodiment, the installation includes a fourth heat exchange system 210 configured to cool the second methane-enriched gas stream 13 before it enters the liquefaction device 200. In this way the cooling power supplied by the liquefaction device is optimized.

[0093] In one embodiment, at least two of the first 4, 5, second 9, third 110 and fourth 210 heat exchange systems are in heat exchange with the same cold source 6. The installation can thus share a cold source for several distinct cooling needs. The installation's construction and operating costs can be optimized.

[0094] Figure 2 shows an example where the installation includes membrane separation units 3, 7, a distillation column 100, and a methane liquefaction device 200. In this example, a single cold source 6 provides the cooling power necessary for the first 5, second 110 and third 210 heat exchange systems.

[0095] In one embodiment, at least a portion of the installation's piping includes thermal insulation. The thermal insulation may, for example, comprise or be formed of perlite, foam, or other suitable materials. The thermal insulation may be vacuum-sealed. For example, the thermal insulation may be provided by a vacuum space around the main piping that prevents or reduces heat exchange between the outside and the gas flow inside the piping.

[0096] In one embodiment, the installation includes a source of pure methane, for example a cylinder of gaseous or liquefied methane, and the process includes a start-up step of the installation during which methane from the source of pure methane is put into circulation in the second purification unit 7.

[0097] Alternatively, at the start-up of the process, the second methane-enriched gas stream 13, which is not yet sufficiently pure to be liquefied, is recycled upstream until this stream 13 reaches a CO2 content low enough to be liquefied, in particular a CO2 content between 0 and 400 ppm, preferably less than 50 ppm. Preferably, if the installation includes a digester, the second methane-enriched stream 13 is injected into the digester. This start-up phase can vary in duration and may, in particular, last approximately 15 minutes.

Claims

Demands

1. Installation (1) for the treatment of a gas stream (10), in particular biogas, comprising methane and carbon dioxide, the installation (1) comprising a first cold source (6) and a gas circuit for the gas stream (10), the gas circuit comprising, arranged in series and fluidly connected by a set of pipes, a first compressor (2), a first scrubbing unit (3) configured to produce a first gas stream enriched in methane (11) and a gas stream enriched in carbon dioxide (12), a first heat exchange system (4, 5), and a second scrubbing unit (7) configured to produce a second gas stream enriched in methane (13) and a gas stream depleted in methane (14), the set of pipes being configured to - transfer the gas stream (10) compressed by the first compressor (2) to an inlet (31) of the first scrubbing unit (3),- transferring the methane-enriched gas stream (11) from a first outlet (32) of the first purification unit (3) to a first inlet (71) of the second purification unit (7), and - transferring the methane-depleted gas stream (14) from a first outlet (72) of the second purification unit (7) to an inlet (21) of the compressor (2), the first heat exchange system (4, 5) being configured to exchange heat between the cold source (6) and the first methane-enriched gas stream (11) in order to cool the first methane-enriched gas stream, characterized in that the first purification unit (3) is configured to produce the first methane-enriched gas stream (11) with less than 5% carbon dioxide, and in that the second purification unit (7) comprises a membrane separation unit (7) configured to produce the second methane-enriched gas stream (13) with less than 400 ppm of carbon dioxide.

2. Installation (1) according to claim 1, characterized in that the second purification unit (7) consists of a membrane separation stage, in particular configured to produce the second gas stream enriched in methane (13) with less than 50 ppm of carbon dioxide.

3. Installation (1) according to any one of claims 1 or 2, characterized in that the first heat exchange system (4, 5) is further configured to heat exchange the methane-depleted gas stream (14) with the first methane-enriched gas stream (11).

4. Installation (1) according to any one of claims 1 to 3, characterized in that the first heat exchange system (4, 5) comprises, in series in the gas circuit, a first heat exchanger (4) and a second heat exchanger (5), - the first heat exchanger (4) being configured to exchange heat between the first methane-enriched gas stream (11) and the methane-depleted gas stream (14), and - the second heat exchanger (5) being configured to exchange heat between the first methane-enriched gas stream (11) and the cold source (6).

5. Installation (1) according to any one of claims 1 to 4, characterized in that the piping assembly includes a bypass line configured to take a fraction of the second methane-enriched gas stream (13) and transfer it to a second inlet (74) of the second scrubbing unit (7), and in that the bypass line may include a pressure-reducing device (8).

6. Installation (1) according to any one of claims 1 to 5, characterized in that the first purification unit (3) comprises a first (310) and a second (320) membrane separation stage in series, and in that the piping assembly comprises a pipe configured to connect an outlet of the first membrane separation stage (310) to an inlet of the second membrane separation stage (320), the installation comprising a second heat exchange system (9) configured to exchange heat between at least a portion of the pipe and the cold source (6).

7. Installation (1) according to any one of claims 1 to 6, characterized in that it comprises a distillation column (100) and in that the piping assembly is configured to transfer the carbon dioxide-enriched gas stream (12) from a second outlet (33) of the first purification unit (3) to an inlet of the distillation column (100) to produce liquefied carbon dioxide.

8. Installation according to claim 7, characterized in that the piping assembly includes a pipe connecting a first upper outlet of the distillation column (100) to the inlet (31) of the first purification unit (3) to transfer at least a fraction of a top gas stream from the distillation column (100) to the first purification unit (3).

9. Installation according to claim 8, characterized in that it comprises a third heat exchange system (110) configured to cool the overhead gas stream of the distillation column (100).

10. Installation (1) according to any one of claims 1 to 9, characterized in that it comprises a liquefaction device (200) and in that the piping assembly is configured to transfer the second methane-enriched gas stream (13) from the second purification unit (7) to an inlet of the liquefaction device (200) in order to liquefy the second methane-enriched gas stream (13).

11. Installation according to claim 10, characterized in that it comprises a fourth heat exchange system (210) configured to cool the second methane-enriched gas stream (13) before its entry into the liquefaction device (200).

12. Installation according to any one of claims 9 or 11, characterized in that at least two of the first (4, 5), second (9), third (110) and fourth (210) heat exchange systems are in heat exchange with the same cold source (6).

13. A process for treating a gas stream (10), in particular biogas, comprising methane and carbon dioxide, the process comprising the following steps: - compressing the gas stream (10) to a pressure of between 1 and 22 barg, preferably between 7 and 20 barg, preferably between 12 and 16 barg, - separating the compressed gas stream (10) into a first gas stream enriched in methane (11) and a gas stream enriched in carbon dioxide (12), - cool the first methane-enriched gas stream (11) to a temperature between -10 and -50 °C, preferably between -20 and -30 °C, - separate the first methane-enriched gas stream (11) into a second methane-enriched gas stream (13) and a methane-depleted gas stream (14), characterized in that the first methane-enriched gas stream (11) contains less than 3% carbon dioxide, and the second methane-enriched gas stream (13) contains less than 400 ppm carbon dioxide, preferably less than 50 ppm carbon dioxide.

14. A method according to claim 13, characterized in that it includes an additional step of heat-exchanging the methane-depleted gas stream (14) with the first methane-enriched gas stream (11).

15. A method according to any one of claims 13 or 14, characterized in that the step of separating the first methane-enriched gas stream (13) is carried out by a membrane separation unit (7), and in that the method comprises the following additional steps: - taking a fraction of the second methane-enriched gas stream (13), - expanding the fraction of the second methane-enriched gas stream (13), - injecting the fraction of the second methane-enriched gas stream (13) into the membrane separation unit (7).

16. A process according to any one of claims 13 to 15, characterized in that it comprises at least one of the following additional steps: - liquefying the carbon dioxide-enriched gas stream (12), and / or - liquefying the second methane-enriched gas stream (13).

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