Installation and process for the production of gaseous biomethane and liquid CO2

Abstract

Installation and method for producing gaseous biomethane (24) and liquid carbon dioxide (11) from biogas (2) comprising, arranged in series in a biogas circuit (100), a first compressor (3), a first membrane separation unit (5a), a second membrane separation unit (5b) and a conduit (23) for recycling the second permeate, which extends from the second permeate outlet (12) to the biogas circuit (100) upstream of the first compressor (3), the installation (1) further comprising an apparatus (9) for separating and liquefying the first permeate, a conduit (15) for supplying the first permeate to the apparatus (9) for separating and liquefying the first permeate, a device (7) for regulating the pressure of the first permeate (16) arranged in the supply conduit (15), and a recycling conduit (17), which extends from the head of the distillation column to the biogas circuit (100) downstream of the first compressor (3). Abstract figure: Fig. 1

Description

Title of the invention: Installation and method for producing gaseous biomethane and liquid CO2.

[0001] The invention relates to an installation and a method for producing gaseous biomethane and liquid CO2.

[0002] The invention relates more particularly to a plant for producing gaseous biomethane and liquid CO2 from biogas, comprising, arranged in series in a biogas circuit: a first compressor configured to compress the biogas, a first membrane separation unit comprising a first membrane more permeable to carbon dioxide than to methane, a biogas inlet, a first permeate outlet and a first retentate outlet, said first membrane separation unit being configured to receive the biogas from the first compressor and to provide a first permeate and a first retentate, a second membrane separation unit comprising a second membrane more permeable to carbon dioxide than to methane, a first retentate inlet, a second permeate outlet and a second retentate outlet,said second membrane separation unit being configured to receive the first retentate at its first retentate inlet and provide a second permeate at its second permeate outlet and a second retentate at its second retentate outlet, and a second permeate recycling conduit, which extends from the second permeate outlet to the biogas circuit upstream of the first compressor, configured to recycle the second permeate upstream of the first compressor.

[0003] Biogas is the gas produced during the degradation of organic matter in the absence of oxygen (anaerobic fermentation), also called methanization. It can be a natural degradation - it is observed in marshes or household waste dumps - but the production of biogas can also result from the methanization of waste in a dedicated reactor, the conditions of which are controlled, called a methanizer or digester, then in a post-digester, similar to the digester and allowing the methanization reaction to be pushed further.

[0004] Biomass will be any group of organic materials that can be transformed into energy through this methanization process, for example: sludge from sewage treatment plants, manure / slurry, agricultural residues, food waste, etc.

[0005] The digester, that is to say the reactor dedicated to the methanization of biomass, is a closed tank, heated or not (operation at a fixed temperature, between ambient temperature and 55°C) and whose contents consisting of biomass are mixed, continuously or sequentially. The conditions in the digester are anaerobic and the biogas generated is ends up in the digester headspace (gaseous headspace), where it is taken. Post-digesters are similar to digesters.

[0006] Due to its main constituents - methane and carbon dioxide - biogas is a powerful greenhouse gas; it also constitutes, at the same time, a significant source of renewable energy in a context of increasing scarcity of fossil fuels.

[0007] Biogas mainly contains methane (CH4) and carbon dioxide (CO2) in variable proportions depending on the method of production and the substrate but can also contain, in smaller proportions, water, nitrogen, hydrogen sulfide (H2S), oxygen, as well as other organic compounds, in trace amounts, including H2S, between 10 and 50,000 ppmv.

[0008] Depending on the degraded organic matter and the techniques used, the proportions of the components differ, but on average the biogas contains, on dry gas, 30 to 75% methane, 15 to 60% CO2, 0 to 15% nitrogen, 0 to 5% oxygen and trace compounds.

[0009] Biogas is used in different ways. After light treatment, it can be used close to the production site to provide heat, electricity or a mixture of the two (cogeneration); the high carbon dioxide content reduces its calorific value, increases compression and transport costs and limits the economic interest of its use to this local use.

[0010] Further purification of biogas allows its wider use, in particular, further purification of biogas makes it possible to obtain a purified biogas to the specifications of natural gas and which can be substituted for it; the biogas thus purified is "biomethane". Biomethane thus supplements natural gas resources with a renewable part produced in the heart of the territories; it can be used for exactly the same purposes as natural gas of fossil origin. It can supply a natural gas network, a vehicle filling station, it can also be liquefied to be stored and transported in the form of liquid natural gas (bioLNG)...

[0011] On the other hand, biogas is also largely made up of carbon dioxide. This carbon dioxide is usually released into the atmosphere. However, there is also a demand for CO2 for the food industry (carbonation of beverages), the pharmaceutical industry, the chemical industry (manufacture of solvents, paints) or for injection into greenhouses or algae ponds. This demand for CO2 can also be seasonal (injection into greenhouse crops for example). There is currently no CO2 distribution network equivalent to the natural gas distribution network and therefore which facilitates the recovery of methane.

[0012] Currently, CO2 is generally a by-product of mineral fertilizer production. These plants are centralized and can involve significant transportation distances. In addition, mineral fertilizer production is energy intensive. (1 to 3% of the energy consumed globally). So, when energy prices are too high, these plants can close and break the CO2 supply chain.

[0013] In addition, since these sources are centralized, when the CO2 consumer is not in direct proximity to the source, the CO2 is generally liquefied then transported by truck, which results in additional energy expenditure.

[0014] Purifying biogas into biomethane requires several steps. A first step consists of compressing the biogas that has been produced and transported to atmospheric pressure, this compression can be achieved - in a conventional manner - via a lubricated screw compressor. The following steps aim to rid the biogas of corrosive components such as hydrogen sulfide and volatile organic compounds (VOCs), the technologies used are conventionally pressure swing adsorption (PSA) and activated carbon trapping. Next comes the step of separating the carbon dioxide to ultimately obtain methane at the purity required for its subsequent use.Various technologies are used for this depending on the situation; among these, membrane technology is particularly effective when the CO2 content is high; it is therefore used to separate the CO2 present in biogas, coming from landfills or digesters of plant or animal waste.

[0015] While the process of purifying biogas into biomethane allows the biomethane to reach the required quality, the vents contain the different gases entering the purification unit in varying proportions depending on the technology for separating carbon dioxide / nitrogen / oxygen from methane used. To also valorize the carbon dioxide, it is necessary to separate the carbon dioxide from the rest of the gaseous compounds in the vents. A CO2 liquefaction unit allows the separation of carbon dioxide from the rest of the methane / nitrogen / oxygen gaseous compounds. For carbon dioxide to be valorized in very demanding markets such as the food industry, the purity of the CO2 must be ensured and the process for obtaining it must be controlled.

[0016] In the case where the biogas treatment unit must be operated with or without a CO2 liquefier, the existing solution is to install a biogas purification unit into biomethane making it possible to produce biomethane of the required quality and to achieve a biomethane yield greater than 99% of recovered methane and a collection then purification of the vent flow to recover the CO2.

[0017] Document KR20160055653A describes the separation of methane and dioxide carbon from biogas using three separation membranes, with concentrated methane generated by a second separation membrane and concentrated carbon dioxide generated by a third separation membrane. The carbon dioxide is liquefied and purified in the liquefaction purification unit located in downstream of the third separation membrane.

[0018] Document KR101327337B1 describes a multi-stage membrane system for producing biomethane and recovering carbon dioxide. Carbon dioxide exiting the third membrane separation unit is pressurized and cooled to liquefaction, then stored in a separate storage tank.

[0019] To produce methane and carbon dioxide at the required quality, the membrane biogas purification process comprises at least 3 stages and preferably 4 stages. The disadvantage of this solution is that the number of membranes required and the size of the biogas compressor to achieve these performances is significant and the investment is high.

[0020] The process of purifying CO2 from the vent stream collects a stream that is at ambient pressure. Purification can be carried out by simple liquefaction.

[0021] To ensure the liquefaction of CO2, it is advantageous to increase the pressure of the flow in order to have a liquefaction temperature that is as high as possible because increasing the pressure of a fluid is less energy-intensive than lowering its temperature. Thus, collecting a flow at the vent and therefore at ambient pressure is not optimal. Especially since once the CO2 has been purified, it must be injected into the transport chain which is carried out at a pressure usually between 15 and 22 bara.

[0022] When a vent stream is treated by liquefaction alone, particularly without cryogenic distillation, it is difficult to achieve a high purity level (%CO2 > 99.7%) with traces of methane below 50 ppm and this by only increasing the pressure level and lowering the temperature significantly beyond the liquefaction and distillation purification process. However, in order to be able to supply CO2 for demanding applications, such as the food and beverage market, it is necessary to reduce the residual CH4 level to levels below 50 ppm.

[0023] When the vent stream is used to recover CO2, the process does not use the advantage of installing a CO2 liquefaction unit to treat and partially recover the methane contained in the biogas. The investment and operating costs are not optimized.

[0024] Furthermore, while the demand for biomethane is almost constant, the demand for CO2 can be seasonal, particularly in the case of its use for enriching crop greenhouses or for the food industry. Thus, it is advantageous to have a methane recovery unit sufficiently independent of the CO2 recovery unit, in order to maintain limited electricity consumption and a high methane recovery efficiency, when the CO2 recovery unit is stopped.

[0025] The present invention aims to effectively remedy all or part of the drawbacks of the prior art noted above.

[0026] To this end, the installation according to the invention, moreover in accordance with the generic definition given in the preamble above, is essentially characterized in that the installation further comprises an apparatus for separating and liquefying the first permeate comprising at least one distillation column, a conduit for supplying the first permeate to the apparatus for separating and liquefying the first permeate, a device for regulating the pressure of the first permeate arranged in the supply conduit, the device for regulating the pressure of the first permeate comprising a second compressor, for example a variable speed compressor, configured to maintain a determined pressure setpoint at its suction and thus to maintain the composition of the first permeate at less than 20 mol% of components other than carbon dioxide, preferably from 5% to 11 mol% of CH4,the separation and liquefaction apparatus being configured to receive the first permeate supplied by the pressure regulating device and to produce liquid carbon dioxide purified of methane by distillation in the bottom of the distillation column and a gas enriched in methane and depleted in carbon dioxide relative to the first permeate at the top of the distillation column, and a recycling conduit, which extends from the top of the distillation column to the biogas circuit downstream of the first compressor, configured to send the gas enriched in methane and depleted in carbon dioxide downstream of the first compressor.

[0027] Such an installation makes it possible to optimize the operating process of biogas purification and CO2 liquefaction to reduce investment, limit electricity consumption and increase methane production. In particular, the invention as described proposes to recover the feed stream in the separation and liquefaction apparatus on the first permeate. The first permeate is at a higher pressure than the third permeate which is used in the state of the art as feed in the separation and liquefaction apparatus. The size of the compressor required for the liquefaction of CO2 is reduced as well as its electrical consumption. The treatment in the CO2 separation and liquefaction apparatus of the first permeate also makes it possible to eliminate the recycle to the compressor of the membrane separation of the retentate of the third membrane stage.The size of the compressor required for membrane separation is therefore reduced, as is its power consumption. The number of membranes used to achieve biomethane purity is also reduced.

[0028] In addition, the extraction of CO2 from biomass by coupling a CO2 recovery unit to a CH4 recovery unit has several advantages: - biomass methanization units are decentralized (small units distributed across territories in rural areas) they can significantly reduce transport distances. - units with a positive energy balance are not stopped when energy prices increase; they operate as long as biomass is available.

[0029] In addition, a CO2 recovery unit coupled with a methane recovery unit as proposed by the invention makes it possible to produce food-grade CO2 by minimizing energy expenditure, by improving the efficiency of the methane recovery unit while allowing it to operate even when the CO2 recovery unit does not need to operate (i.e. it is stopped by choice). Thus, it is also possible to reduce the operating cost of the two units to treat the same flow of biogas while allowing the biomethane recovery unit to operate when the CO2 recovery unit is stopped.

[0030] Furthermore, embodiments of the invention may include one or more of the following features: - the installation includes a flow control valve connected to the second retentate outlet, - the installation comprises a unit for purifying the first permeate, the unit for purifying the first permeate being a third membrane separation unit configured to receive at least a portion of the first permeate and provide a third permeate and a third retentate, the third membrane separation unit comprising a third membrane more permeable to carbon dioxide than to methane, and in that the installation comprises a conduit for recycling the third retentate configured to recycle the third retentate upstream of the first compressor, - the installation comprises a first first permeate diversion conduit connecting the first permeate feed conduit to the first permeate purification unit of the installation, a second first permeate diversion conduit connecting the conduit to the first diversion conduit, and a set of diversion valve(s) configured to allow a portion of the first permeate to be directed to the first permeate purification unit, - the first permeate pressure regulation device comprises at least one first permeate pressure sensor, a compressor speed variator and a first permeate pressure regulation and control loop, the second compressor being configured to maintain the pressure setpoint determined at its suction level as a function of the measurement from the pressure sensor, - the apparatus for separating and liquefying the first permeate comprises a tank boiler, a condenser, a liquid and gas phase separator pot and an element for bringing the gas and liquid into contact called filling, - the first permeate bypass pipe is connected to the feed pipe upstream of the first permeate pressure regulating device, - the third retentate recycling pipe includes a control valve, - the recycling duct is configured to send the methane-enriched gas and depleted in carbon dioxide: upstream of the first membrane separation unit; and / or upstream of the second membrane separation unit and downstream of the first membrane separation unit; and / or at the bypass conduit, upstream of the first permeate purification unit and downstream of the first membrane separation unit.

[0031] The invention also relates to a method for producing gaseous biomethane and liquid carbon dioxide from biogas comprising the following steps: a step of compressing the biogas in a first compressor to a first pressure, a step of separating the biogas at the first pressure, in a first membrane separation unit, to produce a first permeate enriched in carbon dioxide and depleted in methane compared to the biogas and a first retentate enriched in methane and depleted in carbon dioxide compared to the biogas, a step of separating the first retentate, in a second membrane separation unit, to produce a second permeate enriched in carbon dioxide and depleted in methane compared to the first retentate and a second retentate enriched in methane and depleted in carbon dioxide compared to the first retentate, a step of recycling the second permeate upstream of the first compressor,and a step of recovering the second retentate as gaseous biomethane. The method further comprises a step of regulating the pressure of the first permeate to a set value, a separation and liquefaction step comprising distillation in a distillation column of the first permeate to produce liquid carbon dioxide purified of methane and a gas enriched in methane and depleted in carbon dioxide relative to the first permeate, a step of recycling the gas enriched in methane and depleted in carbon dioxide downstream of the first compressor, and a step of recovering the liquid carbon dioxide.

[0032] According to other possible particularities: - the process comprises a step of pre-treatment of the biogas to remove at least part of the water and / or hydrogen sulfide and / or volatile organic compounds before the compression step, - the method comprises a step of post-treatment of the first permeate, to remove at least part of the residual water and / or oil vapors and other impurities such as hydrogen sulfide and / or or-

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[0039] volatile organic compounds, - the set value is between 0.5 barg and 3 barg, - the step of regulating the pressure of the first permeate comprises a compression of the first permeate in a second compressor to a pressure between 6 barg and 25 barg, - the method comprises a step of separating a portion of the first permeate in a third membrane separation unit to produce a third permeate enriched in carbon dioxide relative to the first permeate and a third retentate enriched in methane relative to the first retentate, and a step of recycling the third retentate upstream of the first compressor, - the gas enriched in methane and depleted in carbon dioxide compared to the first permeate is recycled: upstream of the first membrane separation unit; and / or upstream of the second membrane separation unit and downstream of the first membrane separation unit; and / or upstream of the first permeate purification unit and downstream of the first membrane separation unit. The invention will be better understood by reading the following description and examining the accompanying figures. These figures are given only for illustrative purposes but in no way limit the invention. [Fig.l] represents a schematic and partial view illustrating a first example of the structure and operation of an installation according to the invention, [Fig.2] represents a schematic and partial view illustrating a second example of the structure and operation of an installation according to the invention, and [Fig.3] represents a schematic and partial view illustrating a third example of the structure and operation of an installation according to the invention. Example of a gaseous biomethane and liquid CO2 production facility 11 from biogas 2 illustrated in [Fig.l] comprises, arranged in series in a biogas circuit 100, a first compressor 3, a first membrane separation unit 5a, a second membrane separation unit 5b and a conduit 23 for recycling the second permeate. The biogas circuit 100 comprises an upstream end intended to be connected to a source of biogas, for example the outlet of a biogas production unit, in particular a digester, to receive a flow of biogas 2. The first compressor 3 is configured to compress the biogas 2. The first compressor 3 may be an oil-lubricated or water-lubricated or non-oil-lubricated medium-pressure compressor configured to increase the pressure and enable the efficient separation of carbon dioxide from methane in the purification unit 5. A oil removal system can be placed downstream of compressor 3 to avoid contamination of purification unit 5a, 5b.

[0040] Preferably, the biogas 2 is pretreated before being compressed to a first pressure PI in the first compressor 3. The installation may comprise, upstream of the first compressor 3, a pretreatment unit configured to eliminate at least part of the water and / or hydrogen sulfide and / or VOCs present in the raw biogas 2.

[0041] The first membrane separation unit 5a comprises a first membrane more permeable to carbon dioxide than to methane, a biogas inlet 4, a first permeate outlet 6 and a first retentate outlet 8. This first membrane separation unit 5a is configured to receive the biogas 2 at the first pressure PI from the first compressor 3 at its inlet 4 and to produce a first permeate 16 at its first permeate outlet 6 and a first retentate 18 at its first retentate outlet 8.

[0042] The second membrane separation unit 5b comprises a second membrane more permeable to carbon dioxide than to methane, a first retentate inlet 10, a second permeate outlet 12 and a second retentate outlet 14. This second membrane separation unit 5b is configured to receive the first retentate 18 at its first retentate inlet 10 and to produce a second permeate 22 at its second permeate outlet 12 and a second retentate 24 at its second retentate outlet 14.

[0043] The second permeate recycling conduit 23 is configured to recycle the second permeate 22 upstream of the first compressor 3. The second permeate 22 is recycled to be combined with the biogas stream 2, in particular the pretreated biogas. The second retentate 24 is recovered as gaseous biomethane which is the first product of the process.

[0044] The installation further comprises an apparatus 9 for separating and liquefying the first permeate comprising at least one distillation column 90, a conduit 15 for supplying the first permeate to the apparatus 9 for separating and liquefying the first permeate and a device 7 for regulating the pressure of the first permeate arranged in the supply conduit 15. The conduit 15 connects the first permeate outlet 6 to the pressure regulating device 7.

[0045] The device 7 for regulating the pressure of the first permeate comprises a second compressor 30, for example a variable speed compressor, for example via a speed variator. The second compressor 30 is configured to compress the first permeate 16 and maintain a determined pressure setpoint P2 of this permeate at its suction and thus to maintain the composition of the first permeate 16 at less than 20 mol%, in particular at less than 15 mol%, components other than CO2, preferably from 5% to 11% by mole of CH4. The speed of the second compressor 30 is set to maintain a pressure at its suction, namely the pressure P2 of the first permeate. When the pressure of the first permeate is maintained, the composition of the first permeate is also maintained, because the composition of the first permeate depends partly, but not exhaustively, on the pressure difference between the feed in the first membrane separation unit 5a and the first permeate 16.

[0046] The device 7 for regulating the pressure of the first permeate 16 comprises at least one pressure sensor of the first permeate 16, a speed variator of the compressor 30 and a regulation and control loop 32 of the pressure of the first permeate 16. The regulation loop 32 acts on the speed of the compressor 30 through the variator, in order to maintain the pressure setpoint P2 of the permeate 16 in the conduit 15 according to the measurement of the pressure sensor. If the measurement of the sensor is higher than the setpoint P2, the speed of the compressor 30 is increased. If the measurement of the sensor is lower than the setpoint P2, the speed of the compressor 30 is reduced.

[0047] The device 7 for regulating the pressure of the first permeate may also comprise an analyzer of the composition of the first permeate 16. The speed of the compressor 30 then makes it possible to maintain a composition of this first permeate 16.

[0048] The pressure setpoint P2 can be between 0.1 and 10 barg, in particular between 0.5 and 3 barg. The higher the pressure, the lower the electrical consumption of the compressor 30.

[0049] The compressor 30 also makes it possible to increase the pressure of the first permeate 16 at the outlet of the pressure regulating device 7, at the discharge of the compressor 30. Advantageously, the compression pressure is between 6 and 25 barg. The higher the pressure, the higher the liquid formation temperature.

[0050] The installation may comprise a first permeate post-treatment unit configured to remove at least a portion of the residual water and / or oil vapors and other impurities such as hydrogen sulfide and / or volatile organic compounds. This first permeate post-treatment unit 16 may be located downstream of the pressure regulating device 7. A drying unit may be located downstream of the post-treatment unit to remove the water by a cyclic water adsorption process.

[0051] The apparatus 9 for separating and liquefying the first permeate is configured to receive the first permeate 16 supplied by the pressure regulating device 7. This apparatus 9 is configured to produce liquid CO2 11 purified of methane by distillation in the bottom of the distillation column 90 and a gas 13 enriched in methane and depleted in CO2 relative to the first permeate 16 at the top of the distillation column 90.

[0052] As illustrated in [Fig.4], the separation and liquefaction apparatus 9 may comprise a bottom boiler 91, a condenser 93 separate from the distillation column, a separator pot 97 for the liquid 94 and gas 13 phases following this condenser 93 and an element for bringing the gas and the liquid into contact called packing 92. The bottom boiler 91 is located at the bottom of the distillation column 90 and makes it possible to create a gaseous current allowing a gas and a liquid to be brought into contact in the packing 92. It also makes it possible to pre-cool the gas flow (the first permeate) to be distilled before passing into the condenser 93. The condenser 93 is a heat exchanger located at the top of the distillation column 90. It makes it possible to preferentially liquefy the CO2. The liquid 94 then falls by gravity through a packing 92 which is located between the tank boiler 91 and the condenser 93.The purity of the liquid recovered in the tank boiler 91 is ensured by the good contact of the gas and the liquid in the packing 92 and by the good ratio between the liquid flow which circulates from the condenser 93 to the boiler tank 91 and the gas which circulates from the tank boiler 91 to the condenser 93.

[0053] Advantageously, the first pressurized permeate 16 entering the apparatus 9 is pre-cooled in a heat exchanger, for example a tubular exchanger, included in the tank boiler 91. The tank boiler therefore has two functions: creation by evaporation of a gaseous stream which makes it possible to ensure a transfer of material between gas and liquid in the packing and pre-cooling of the first permeate 16. The first pressurized and pre-cooled permeate 16 96 is then mixed with the gaseous stream 95 which has undergone the transfer of material in the packing 92, then this mixture is cooled in the condenser 93 in order to partially condense this mixture forming a gas 13 enriched in methane and depleted in carbon dioxide compared to the mixture and a liquid 94 enriched in carbon dioxide and depleted in methane compared to the mixture.The liquid 94 enriched with carbon dioxide is introduced at the top of the distillation column 90 on the packing to 92 constitute a liquid flow descending in the packing. The evaporation in the bottom boiler 91 constitutes a gas flow 95 rising in the packing. The packing 92 ensures the transfer of material which allows the depletion of the gas flow 95 in CO2 and the enrichment of the liquid flow 94 in CO2. The liquid in the bottom boiler 91 is at the required purity. The liquid CO2 11 purified into methane by distillation in the bottom of the distillation column 90 is ready to be recovered.

[0054] The installation further comprises a recycling conduit 17 which extends from the head of the distillation column 90 to the biogas circuit 100 downstream of the first compressor 3.

[0055] The installation may comprise a unit 5c for purifying the first permeate, a first diversion conduit 19 connecting the conduit 15 to the unit 5c for purifying the first permeate, a second diversion conduit 33 connecting the conduit 15 to the first diverter conduit 19 and a set of diverter valve(s) 21, 34.

[0056] As illustrated in [Fig.l], the recycling conduit 17 is configured to send the gas 13 at the top of the distillation column enriched in methane and depleted in CO2 to the downstream of the first compressor 3. The recycling conduit 17 can be configured to send the gas 13 enriched in methane and depleted in CO2 relative to the first permeate 16 upstream of the first membrane separation unit 5a. The gas flow 13 is thus mixed with the compressed biogas flow 2, in particular the compressed and pretreated biogas flow 2, and fed into the first membrane separation unit 5a.

[0057] The first permeate 16 of the first stage can be treated either in the CO2 liquefaction apparatus 9 or in the first permeate purification unit 5c using a set of diverting valve(s). The set of diverting valve(s) 21, 34 is configured to allow a portion of the first permeate 16 to be directed to the first permeate purification unit 5c via the first diverting conduit 19 and / or the second diverting conduit 33.

[0058] The diverter valve 21 can be closed so that the first permeate 16 is fully treated by the CO2 liquefaction apparatus 9.

[0059] In a particular embodiment, the second diversion conduit 33 may comprise at least one diversion valve 34. The diversion valve 34 may be electrically connected to the pressure sensor of the control loop 32. The control loop 32 maintains the pressure of the first permeate 16 at the pressure setpoint P2 thanks to the speed variator of the compressor 30. When the compressor 30 reaches saturation, that is to say that the compressor has reached its maximum speed, the diversion valve 34 will open to discharge towards the unit 5c for purifying the first permeate the excess gas flow that cannot be absorbed by the compressor 30. The diversion valve 34 partially sends the first permeate 16 to the unit 5c for purifying the first permeate via the second diversion conduit 33 and then the first diversion conduit 19.

[0060] Advantageously, the installation may comprise a valve 25 for controlling the pressure of the biogas 2 compressed and ready to be treated in the separation units 5a, 5b and 5c and in the CO2 liquefaction apparatus 9. This pressure control valve 25 is located on the second retentate outlet 14. This valve makes it possible to regulate the purity of the biomethane produced.

[0061] Preferably, the first conduit 19 for diverting the first permeate is connected to the supply conduit 15 upstream of the device 7 for regulating the pressure of the first permeate.

[0062] Preferably, the second conduit 33 for diverting the first permeate is connected to the first conduit 19 for diverting the first permeate upstream of the re- regulation of the pressure of the first permeate.

[0063] The unit 5c for purifying the first permeate may comprise or be a PSA type unit and / or a washing type unit, in particular an absorption treatment unit using a washing column, and / or a membrane permeation treatment unit comprising, for example, at least one membrane separation unit.

[0064] As illustrated in [Fig.l], the unit 5c for purifying the first permeate may be a third membrane separation unit, called a third membrane separation stage. The third membrane separation unit comprises a third membrane more permeable to carbon dioxide than to methane, a first permeate inlet 16, a third permeate outlet 26 and a third retentate outlet 28. The third membrane separation unit is configured to receive at least a portion of the first permeate 16 and provide a third permeate 26 and a third retentate 28.

[0065] The installation may also comprise a conduit 27 for recycling the third retentate configured to recycle the third retentate 28 upstream of the first compressor 3. The third permeate 26 is the vent. The membrane separation unit 5c makes it possible to reduce the methane lost at the vent and in the case where a liquefaction apparatus 9 is used and to treat a temporary increase in the biogas flow rate.

[0066] The conduit 27 for recycling the third retentate may comprise a control valve 29. This valve 29 makes it possible to adjust the pressure of the permeate 16 of the first stage in the case where the liquefaction apparatus 9 is not operated. Adjusting the pressure of the permeate 16 makes it possible to limit the loss of methane to the third permeate 26, i.e. to the vent.

[0067] The installation shown in [Fig. 2] is another example of a device for producing gaseous biomethane 24 and liquid CO2 11 from biogas. The embodiment of [Fig. 2] differs from that of [Fig. 1] essentially in that the recycling conduit 17 is configured to send the gas 13 to the top of the distillation column of the apparatus 9 enriched in methane and depleted in CO2 relative to the first permeate 16 upstream of the second membrane separation unit 5b and downstream of the first membrane separation unit 5a. The gas flow 13 is thus mixed with the first retentate 18 and fed into the second membrane separation unit 5b.

[0068] The installation shown in [Fig. 3] is another example of a device for producing gaseous biomethane 24 and liquid CO2 11 from biogas. The embodiment of [Fig. 3] differs from that of [Fig. 1] essentially in that the recycling conduit 17 is configured to send the gas 13 to the top of the distillation column enriched in methane and depleted in CO2 relative to the first permeate 16 upstream of the first-stage permeate purification unit 5c and downstream of the first membrane separation unit 5a. That is, the recycling conduit 17 extends from the top of the distillation column to the first bypass conduit 19. The top of the distillation column is fluidically connected to the first bypass conduit 19 by the recycling conduit 17. The gas flow 13 is thus mixed with the first permeate 16 which is directed to the purification unit 5c and fed into the purification unit 5c. This operating case is used during transient phases such as start-up, so that the gas flow 13 does not disrupt the achievement of the required biomethane quality.

[0069] The present invention can also combine the three embodiments described in Figures 1, 2 and 3 within the same installation. The conduit 17 can comprise at least one upstream end connected to the apparatus 9 and at least one, two or three downstream ends. The conduit (17) can then connect the apparatus 9 to at least one or two or all three purification units 5a, 5b and 5c.

[0070] The present invention also relates to a method for producing gaseous biomethane 24 and liquid CO2 11 from biogas which can be implemented by the installation for producing gaseous biomethane 24 and liquid CO2 11 from biogas described previously.

Claims

1. Claims Installation (1) for producing gaseous biomethane (24) and liquid carbon dioxide (11) from biogas (2) comprising, arranged in series in a biogas circuit (100): - a first compressor (3) configured to compress the biogas (2), - a first membrane separation unit (5a) comprising a first membrane more permeable to carbon dioxide than to methane, a biogas inlet (4), a first permeate outlet (6) and a first retentate outlet (8), said first membrane separation unit (5a) being configured to receive the biogas (2) from the first compressor (3) and to provide a first permeate (16) and a first retentate (18), - a second membrane separation unit (5b) comprising a second membrane more permeable to carbon dioxide than to methane, a first retentate inlet (10), a second permeate outlet (12) and a second retentate outlet (14), said second membrane separation unit (5b) being configured to receive the first retentate (18) at its first retentate inlet (10) and provide a second permeate (22) at its second permeate outlet (12) and a second retentate (24) at its second retentate outlet (14), and - a conduit (23) for recycling the second permeate, which extends from the second permeate outlet (12) to the biogas circuit (100) upstream of the first compressor (3), configured to recycle the second permeate (22) upstream of the first compressor (3), said installation being characterized in that it further comprises: - an apparatus (9) for separating and liquefying the first permeate comprising at least one distillation column (90), - a conduit (15) for supplying the first permeate to the apparatus (9) for separating and liquefying the first permeate, - a device (7) for regulating the pressure of the first permeate (16) arranged in the supply conduit (15), the a device (7) for regulating the pressure of the first permeate comprising a second compressor (30), for example a variable speed compressor, configured to maintain a determined pressure setpoint P2 at its suction and thus to maintain the composition of the first permeate (16) at less than 20 mol% of the components other than carbon dioxide, preferably from 5% to 11 mol% of CH4, - the separation and liquefaction apparatus (9) being configured to receive the first permeate (16) supplied by the pressure regulating device (7) and to produce liquid carbon dioxide (11) purified of methane by distillation in the bottom of the distillation column (90) and a gas (13) enriched in methane and depleted in carbon dioxide relative to the first permeate (16) at the top of the distillation column (90), and - a recycling conduit (17),which extends from the head of the distillation column to the biogas circuit (100) downstream of the first compressor (3), configured to send the gas (13) enriched in methane and depleted in carbon dioxide downstream of the first compressor (3).,

2. Installation according to claim 1, comprising a flow control valve (25) connected to the second retentate outlet (14).

3. Installation according to claim 1 or 2, characterized in that it comprises a purification unit (5c) of the first permeate, the purification unit (5c) of the first permeate being a third membrane separation unit configured to receive at least a portion of the first permeate (16) and provide a third permeate (26) and a third retentate (28), the third membrane separation unit comprising a third membrane more permeable to carbon dioxide than to methane, and in that the installation comprises a conduit (27) for recycling the third retentate configured to recycle the third retentate (28) upstream of the first compressor (3).

4. Installation according to claim 3, further comprising: - a first conduit (19) for diverting the first permeate (16) connecting the conduit (15) for supplying the first permeate (16) to the unit (5c) for purifying the first permeate of the installation, - a second conduit (33) for diverting the first permeate (16) connecting the conduit (15) to the first divert conduit (19), and - a set of divert valve(s) (21, 34) configured to allow a portion of the first permeate (16) to be directed towards the unit for purifying the first permeate (5c).

5. Installation according to any one of claims 1 to 4, characterized in that the device (7) for regulating the pressure of the first permeate comprises at least one sensor for the pressure of the first permeate (16), a speed variator of the compressor (30) and a regulation and control loop (32) for the pressure of the first permeate (16), the second compressor (30) being configured to maintain the pressure setpoint determined at its suction level as a function of the measurement of the pressure sensor.

6. Installation according to any one of claims 1 to 5, characterized in that the apparatus (9) for separating and liquefying the first permeate comprises a tank boiler (91), a condenser (93), a separator pot (97) for the liquid and gas phases and an element for bringing the gas and liquid into contact called packing (92).

7. Installation according to any one of claims 1 to 6, characterized in that the first conduit (19) for diverting the first permeate is connected to the supply conduit (15) upstream of the device (7) for regulating the pressure of the first permeate.

8. Installation according to any one of claims 3 to 7, characterized in that the conduit (27) for recycling the third retentate comprises a control valve (29).

9. Installation according to any one of claims 1 to 8, characterized in that the recycling conduit (17) is configured to send the gas (13) enriched in methane and depleted in carbon dioxide: - upstream of the first membrane separation unit (5a); and / or - upstream of the second membrane separation unit (5b) and downstream of the first membrane separation unit (5a).

10. ; and / or - to the diversion conduit (19), upstream of the purification unit (5c) of the first permeate and downstream of the first membrane separation unit (5a). A method for producing gaseous biomethane (24) and liquid carbon dioxide (11) from biogas (2) comprising the following steps: - a step of compressing the biogas (2) in a first compressor (3) to a first pressure PI, - a step of separating the biogas at the first pressure PI, in a first membrane separation unit (5a), to produce a first permeate (16) enriched in carbon dioxide and depleted in methane compared to the biogas (2) and a first retentate (18) enriched in methane and depleted in carbon dioxide compared to the biogas (2), - a step of separating the first retentate (18), in a second membrane separation unit (5b), to produce a second permeate (22) enriched in carbon dioxide and depleted in methane compared to the first retentate (18) and a second retentate (24) enriched in methane and depleted in carbon dioxide compared to the first retentate (18), - a step of recycling the second permeate (22) upstream of the first compressor (3), and - a step of recovering the second retentate (24) as gaseous biomethane, the method being characterized in that it further comprises: - a step of regulating the pressure of the first permeate (16) to a set value P2, - a separation and liquefaction step comprising distillation in a distillation column (90) of the first permeate (16) to produce liquid carbon dioxide (11) purified of methane and a gas (13) enriched in methane and depleted in carbon dioxide compared to the first permeate (16), - a step of recycling the gas (13) enriched in methane and depleted in carbon dioxide downstream of the first compressor (3), and - a step of recovering the liquid carbon dioxide (11).

11. A method according to claim 10, comprising a step of pretreating the biogas (2) to remove at least part of the water and / or hydrogen sulfide and / or volatile organic compounds before the compression step.

12. A method according to claim 10 or 11, comprising a step of post-treatment of the first permeate (16), to remove at least a portion of the residual water and / or oil vapors and other impurities such as hydrogen sulfide and / or volatile organic compounds.

13. Method according to any one of claims 10 to 12, characterized in that the set value P2 is between 0.5 barg and 3 barg.

14. A method according to any one of claims 10 to 13, characterized in that the step of regulating the pressure of the first permeate (16) comprises compressing the first permeate in a second compressor (30) to a pressure between 6 barg and 25 barg.

15. A method according to any one of claims 10 to 14, comprising a step of separating a portion of the first permeate in a third membrane separation unit (5c) to produce a third permeate (26) enriched in carbon dioxide relative to the first permeate (16) and a third retentate (28) enriched in methane relative to the first retentate (16), and a step of recycling the third retentate (28) upstream of the first compressor (3).

16. Method according to any one of claims 10 to 15, characterized in that the gas (13) enriched in methane and depleted in carbon dioxide relative to the first permeate (16) is recycled: - upstream of the first membrane separation unit (5a); and / or - upstream of the second membrane separation unit (5b) and downstream of the first membrane separation unit (5a); and / or - upstream of the purification unit (5c) of the first permeate and downstream of the first membrane separation unit (5a).

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