WO2022106768A1

WO2022106768A1 - Method for cryogenic separation of a biomethane-based feed stream, method for producing biomethane that includes said cryogenic separation and associated facility

Info

Publication number: WO2022106768A1
Application number: PCT/FR2021/051967
Authority: WO
Inventors: Guénaël PRINCE; Antonio TRUEBA
Original assignee: Waga Energy
Priority date: 2020-11-23
Filing date: 2021-11-08
Publication date: 2022-05-27
Also published as: FR3116445A1; FR3116445B1

Abstract

Method for cryogenic separation of a feed stream containing methane, nitrogen and/or oxygen, characterised in that the heated stream of vent gas (11) is compressed, the stream of compressed vent gas is cooled, the stream of cooled compressed vent gas (29) is subjected to at least one membrane separation (15) to partially separate the methane from the oxygen and the nitrogen.

Description

Title of the invention: Process for cryogenic separation of a feed stream based on biomethane, process for producing biomethane integrating said cryogenic separation and associated installation.

Technical area

The present invention relates to the field of biomethane production.

[0002] In particular, the invention relates to a process for the cryogenic separation of a feed stream containing methane, dinitrogen and/or dioxygen. The invention also relates to a method for producing a flow enriched in methane as well as the associated installation.

The method of the invention finds a particularly advantageous application in the context of the production of biomethane by purification of biogas from non-hazardous waste storage facilities (ISDND).

Prior technique

[0004] Biogas is recovered in the same way as natural gas; it can, for example, undergo combustion in a boiler to produce heat or be used as fuel in vehicles. Biogas is particularly interesting because it belongs to the category of renewable energies.

[0005] Biomethane is a biogas produced by a biological process of degradation of organic matter in an anaerobic environment. During its production, biomethane is mixed with carbon dioxide, water vapor and other impurities in varying amounts depending on the organic matter of origin. The main impurities found are hydrogen sulphide, and when the organic matter comes from household or industrial waste, volatile organic compounds (VOCs).

[0006] Biomethane can be produced in dedicated reactors (otherwise called "digesters") where the biological reaction takes place, in a perfectly anaerobic medium and with a controlled temperature. It can also be produced naturally and in large quantities in non-hazardous waste storage facilities (ISDND), in which household waste is stored in cells, covered with a membrane when they are full. When the cell is closed, the organic matter methanation process can begin. The biogas thus produced is then withdrawn by suction into a booster via collection pipes introduced into the alveoli, thus creating a slight depression in said alveoli. As these are not perfectly sealed, air is sucked in and ends up in the biogas in varying proportions. The gases in the air are therefore added to the impurities mentioned above and must be removed to purify the biomethane.

[0007] There are, however, other sources of gas, of non-renewable origin, containing methane, such as mine gas. The latter is produced by outgassing coal seams in abandoned mines, and also mixing with air as it accumulates in mine voids. In order to recover this gas, the impurities it contains must also be eliminated.

[0008] In all cases, in order to obtain a flow enriched in methane, it is necessary to remove the impurities which are carbon dioxide, nitrogen and oxygen, to a level such that the flow enriched in methane as well product can be recovered in the form of natural gas, liquefied natural gas, or vehicle fuel. Depending on the uses mentioned above, the level of impurity required may vary. Nevertheless, a typical target for these impurities in the methane-enriched stream is: less than 2% molar carbon dioxide, less than 1% oxygen, and less than 1% nitrogen. When the methane-enriched flow no longer contains carbon dioxide, the level of impurity required is: less than 2.5% molar nitrogen, and less than 1% oxygen.

[0009] A process making it possible to obtain a good separating power between methane on the one hand and oxygen and nitrogen on the other hand is cryogenic distillation. A distillation column makes it possible to separate the methane at the bottom of the column from the dioxygen and the dinitrogen at the top of the column, thanks to the differences in volatility between these molecules.

[0010] Document FR 3,051,892 describes a cryogenic separation process by distillation of a mixture of CH, N2 and O2 at low pressure. The yield obtained by this process is limited. Indeed, part of the methane is lost at the level of the vent gas of the column.

[0011] A solution to improve the yield could be that of recondensing the gas phase obtained after expansion of the flow cooled at the top of the column by means of a condenser. However, this solution has the disadvantage of requiring a large amount of energy to produce the cold necessary for condensation, which makes it an economically unviable solution.

[0012] SKIBOROWSKI MIRKO ET AL (XP55822279A) presents different methodologies for calculating a hybrid process combining distillation and pervaporation membranes in order to optimize its design. Examples of these methodologies relate to the azeotropic distillation for the dehydration of ethanol, the production of MTBE (Methyl tert-butyl ether) and the separation of MTBE from butene and MeOH (methanol), or even the separation of a ternary mixture of acetone, water and IPA (isopropyl booze). None of the cited examples relates specifically to the cryogenic distillation of a ternary oxygen, nitrogen and methane, and its specific difficulties.

[0013] Document US Pat. No. 6,035,641 suggests replacing cryogenic distillation with separation membranes in a process for separating methane and nitrogen, without the presence of oxygen and therefore without taking into consideration the specific difficulties that this brings to separation.

The technical problem that the invention proposes to solve is to obtain a higher methane recovery rate than that obtained with existing cryogenic separation techniques, in particular those described in the aforementioned documents, and this, by means of an economically more attractive process.

Technical solution

To address this problem, the Applicant has developed a process for the cryogenic separation of a feed stream containing methane, dinitrogen and dioxygen in which:

- the feed flow is cooled in a recuperator exchanger,

- the cooled feed flow is at least partially condensed,

- the at least partially condensed cooled feed stream is expanded, which then contains a liquid fraction and a vapor fraction,

- the liquid fraction is separated from the vapor fraction of the expanded flow,

- the liquid fraction of the cooled at least partially condensed feed stream is sent to an upper level of a distillation column,

- a tank flow is withdrawn from the distillation column, the tank flow being enriched in methane with respect to the feed flow,

- a first part of the tank flow is vaporized,

- the first vaporized part of the tank flow is introduced at a lower level of the distillation column then it is brought into contact with the liquid fraction of the cooled feed flow at least partially condensed, introduced at an upper level of the distillation column distillation.

- a flow of vent gas enriched in oxygen and nitrogen compared to the feed flow is withdrawn from the distillation column and heated in the recuperator exchanger.

The process is characterized in that: - the heated flow of vent gas is compressed,

- the flow of compressed vent gas is cooled to ambient temperature, in practice a temperature between -40°C and 45°C,

- the flow of cooled compressed vent gas is subjected to at least one membrane separation to partially separate the methane from the oxygen and the nitrogen.

[0016] In other words, it has been found that the compression of the vent gas at the column outlet, its cooling to ambient temperature, its filtration by a membrane system and finally the reinjection of the methane-enriched flow from the membrane separation in the feed flow or the flow of methane-enriched gas from the distillation makes it possible to significantly increase the methane recovery rate of the overall process, that is to say to reduce the methane losses of the process . In addition and above all, the increase in energy consumption of the process linked to the membrane separation gas compressor is lower than if a condenser had been added at the top of the column to recondense the methane, the condenser operating with a system for producing energy-intensive cryogenic cold. Finally, the membrane separation of the vent gas can be carried out under conditions of temperature but also of pressure similar to those of the feed flow before cooling in the recuperator exchanger, which allows, if necessary, the reinjection of the flow enriched in methane directly into this feed gas, without any additional form of treatment.

In the rest of the description, by ambient temperature, when we speak of an installation of the type of that of the invention, we designate a temperature between - 40 ° C and + 45 ° C, i.e. between 233K and 318K.

In a specific embodiment, at least one non-combustible dilution flow and more volatile than oxygen is introduced into the distillation column at at least a level lower than that at which the liquid fraction of the expanded flow is introduced, cooled at least partially condensed, after separation, and a level higher than that at which the first vaporized part of the tank flow is introduced, the dilution flow being formed by the vapor fraction of the expanded flow after separation.

[0019] This dilution flow makes it possible to avoid the concentration of O2 in the packing, because the latter can lead to the formation of gas mixtures with explosive concentrations in the distillation column.

According to a preferred embodiment, the feed flow cooled by heat exchange is at least partially condensed with the first part of the flow of tank, said first part of the tank flow being vaporized in contact with the cooled feed flow.

This embodiment makes it possible to transfer the calories from the cooled supply flow to the tank flow. Thus, it is not necessary to provide an external cold source to condense the cooled feed flow or an external heat source to vaporize the tank flow. This embodiment therefore allows a non-negligible energy saving of the process.

According to the invention, a membrane system is composed of two compartments separated by at least one membrane. The two compartments are under different pressures to allow molecules to cross the membrane.

In practice, the flow of compressed vent gas is cooled to a temperature between -40 and 45°C.

[0024] To be able to at least partially separate the methane from the oxygen and nitrogen from the vent gas flow at room temperature, a membrane suitable for separating at room temperature is selected. Indeed, the temperature of the vent gas flow is adapted to the type of membrane used for the separation. An optimal temperature maximizes the rate of methane recovery by the membrane. Some membranes have maximum efficiency for negative fluid temperatures.

According to a preferred embodiment, the a(N2-CH4) selectivity of the membrane is greater than 1.

Thus, the affinity of dinitrogen with the membrane is greater than the affinity of the membrane with methane. The dinitrogen molecules will then find it easier to cross the membrane. The flow having crossed the membrane, also called permeate, will therefore be rich in nitrogen. The flow that has not crossed the membrane, also called retentate, will be rich in methane.

This type of membrane is called positive selectivity membrane, as opposed to another type of membrane, called reverse selectivity, whose selectivity a(N2-CH4) is less than 1. In this case, the flow having crossed the membrane, also called permeate, will be rich in methane and the flow that has not crossed the membrane, also called retentate, will be rich in nitrogen.

[0028] As a variant, the membrane can be used to separate several chemical species. In a preferred embodiment, the membrane has both an a(N2-CH4) selectivity greater than 1 and an a(C>2-CH4) selectivity greater than 1. The invention also relates to a process for producing biomethane by purification of a feed stream containing methane, dinitrogen and dioxygen implementing the cryogenic separation process described above, in which:

- subjecting said feed stream containing methane, nitrogen and oxygen to be purified to said cryogenic separation process, and

- A second part of the tank flow is recovered as well as the flow enriched in methane resulting from the membrane separation.

[0030] This process makes it possible to increase the methane recovery rate. Thus, the recovered methane can be used in several ways.

In a first embodiment, liquid methane is produced.

To do this,

- the second part of the tank flow is recovered in liquid form,

- the feed flow in the recuperator exchanger is cooled with a refrigerant coming from an external source, and

- the flow enriched in methane resulting from the membrane separation is introduced into the feed flow.

The refrigerant is typically liquid nitrogen. It can be in the form of a sacrificial fluid or circulate within a thermodynamic cycle comprising a stage of compression, cooling and expansion of the refrigerant.

In a second embodiment, methane gas is produced.

According to a first option, the second part of the tank flow is vaporized in the recovery exchanger so as to recover a flow of gas enriched in methane. At the same time, the methane-enriched flow from the membrane separation is introduced into the feed flow.

In this first option, the second part of the tank flow circulating in the recovery exchanger absorbs the calories from the feed flow and thus allows it to be cooled. Furthermore, the introduction of the flow enriched in methane resulting from the membrane separation into the flow of the feed gas modifies the composition and the flow of the feed gas of the distillation column, which has the consequence of increasing the methane yield of the process.

[0035] Nevertheless, to avoid the risk of explosion in the distillation column which could occur following an enrichment in O2, the membrane is adapted to allow a sufficient quantity of oxygen to pass, preventing the reinjection of the stream enriched in methane in the column feed gas, and containing oxygen, leads to the accumulation of this oxygen in the distillation column.

[0036] According to a second option,

- the second part of the tank flow is vaporized in the recovery exchanger so as to recover a flow of gas enriched in methane,

- the flow of gas enriched in methane is compressed,

- the flow of compressed gas enriched in methane is cooled,

- the flow enriched in methane resulting from the membrane separation is introduced into the flow of cooled compressed gas, to form a final flow of methane, and

- the final methane flow is recovered.

In other words, the flow from the methane-rich tank obtained at the end of the distillation is conditioned to be injected into the town gas network. Then, the methane recovered during the membrane separation is injected into the conditioned tank flow. This second option also makes it possible to increase the overall yield of the distillation.

According to another aspect, the invention relates to an installation for the production of biomethane by purification of a feed stream containing methane, dinitrogen and dioxygen implementing the method according to the first embodiment of the invention and/or the first option of the second embodiment and comprising:

- a heat exchanger capable of cooling a supply flow; heating the vent gas flow and vaporizing the second part of the tank flow,

- means of at least partial condensation of the cooled feed flow,

- means for vaporizing the first part of the tank flow,

- a pipe capable of transporting the first part of the vaporized tank flow to a lower level of the distillation column (26),

- a flow expansion means at least partially condensed,

- a separation tank for the liquid and vapor fractions of the flow at least partially condensed after expansion,

- a distillation column,

- a pipe capable of transporting the liquid fraction from the separator flask to a higher level of a distillation column,

- means for compressing the flow of vent gas withdrawn from the column,

- a heat exchanger capable of cooling the flow of vent gas once compressed,

- at least one separating membrane capable of partially separating methane from 1'02 and from N2, - a means of recovering the flow enriched in methane resulting from the membrane separation,

- a means of evacuating the flow enriched in oxygen and nitrogen resulting from the membrane separation, and

- a means of introducing the flow enriched in methane resulting from the membrane separation into the feed flow.

In the second option of the second embodiment of the installation described above, the heat exchanger capable of cooling the feed flow also makes it possible to vaporize the second part of the tank flow and said installation further comprises :

- vaporized flow compression means,

- a heat exchanger capable of cooling the compressed flow, and

-a means of introducing the methane-enriched flow into the cooled compressed gas flow.

[0040] Optionally, for all the embodiments, the installation may also comprise a reboiler, said reboiler being capable of at least partially condensing the cooled flow and of vaporizing the first part of the vessel flow.

The reboiler is a heat exchanger which makes it possible to achieve thermal integration by the transfer of calories from the cooled feed flow to the tank flow. Thus, it is no longer necessary to provide an external cold source to condense the cooled feed flow or an external heat source to vaporize the tank flow. The reboiler can take the form of an exchanger independent of the distillation column. In this case, the cooled flow and the first part of the bottom flow circulate countercurrently in pipes within the reboiler. Alternatively, the reboiler is integrated into the bottom of the distillation column. In this case, the cooled flow and the liquid methane coming from the bath located in the column vessel circulate in an exchanger integrated in the column vessel.

According to one characteristic of the invention, the installation comprises a pipe capable of transporting the vapor fraction from the separator drum into the column at at least one level lower than that at which the liquid fraction is introduced.

This line avoids the concentration of O2 in the packing, because the latter can lead to the formation of gas mixtures at explosive concentrations in the distillation column. The separating membranes are made of a material included in the group including polyimides, cellulose acetate, polycarbonates, polysulfone, poly(dimethylsiloxane-dimethylstyrene), poly(dimethylsiloxane), poly(siloctylene-siloxane), poly(p- silphenylene-siloxane), polyamide-polyester copolymer.

According to a first embodiment, the methane recovery means include a valve capable of varying the pressure in the membrane system.

[0046] Furthermore, the means for evacuating the nitrogen and the oxygen include a valve capable of adjusting the back pressure of the membrane system.

To act on the speed at which the molecules cross the membrane, it is possible either to modify the membrane, or to place several membranes in parallel, or to vary the pressure difference between the compartments. The two valves mentioned above make it possible to modulate the pressure in the two compartments of the membrane system. Thus, for example, if it is estimated that the membrane lets through too many methane molecules, it is possible to act on the back pressure of the membrane system to reduce this flow.

Brief description of the drawings

The various aspects of the present invention are described with reference to the indexed figures, illustrating the various embodiments of the invention. The figures are provided by way of illustration and not for the purpose of limitation. The drawings are a schematic representation and are therefore not to scale.

Fig.1

[0049] [fig.1] Figure 1 is a diagram of a cryogenic separation unit integrated into a plant for the production of bio methane by biogas purification comprising a membrane system according to a first embodiment.

Fig.2

[0050] [fig.2] Figure 2 is a diagram of a cryogenic separation unit integrated into an installation for the production of bio methane by biogas purification comprising a membrane system according to a second embodiment.

Description of embodiments

As mentioned, Figures 1 and 2 schematically illustrate a cryogenic separation unit integrated into a bio methane production facility by purification of a feed stream containing methane, dinitrogen and/or dioxygen, comprising a membrane system according to one of two embodiments.

In both cases, the installation essentially comprises an exchanger (2), a distillation column (26), a compressor and its cooler (13 and 14), and a membrane system (15).

The exchanger (2) is fed by a supply gas flow (1). This has a pressure between 5 and 25 bar absolute, preferably a pressure between 8 and 15 bar absolute, a temperature between 273 and 313K, typically 288K, and comprises between 50 and 100% methane, up to 50 % nitrogen and up to 4% oxygen. The feed gas flow (1) is for example obtained at the outlet of a purification unit by adsorption and/or membrane separation, not shown in FIGS. 1 and 2, making it possible to lower the content of organic compounds volatiles (VOC) and CO2 at a value less than or equal to 50 ppmv.

The feed gas flow (1) is cooled and partially liquefied to a temperature of between 100 and 200 K, in a heat exchanger (2) by exchange with the tank flow (9), liquid and enriched in methane, and the vent flow (11), enriched in O2 and N2 and depleted in methane, resulting from the distillation. This embodiment is a thermal integration which makes it possible to transfer the calories from the supply flow (1) to the tank flow (9) and the vent flow (11). In an embodiment not shown in the figures, the supply gas flow (1) may also or alternatively be cooled by heat exchange with a refrigerant.

The cooled flow (8) is then partially condensed. In FIGS. 1 and 2, the cooled flow (8) is sent to a reboiler (3) where it is cooled to a lower temperature and partially or totally liquefied by heat exchange with part (34) of the vessel flow ( 24), up to a temperature slightly higher than that of the tank flow (24). In practice, the temperature of the cooled flow is between 90 and 130K. This embodiment is also a thermal integration which at the same time allows the vessel flow (24) to heat up and boil in order to generate methane-rich vapor which will be used for distillation. Alternatively, it is possible to vaporize the tank flow (24) using an electric pin or any other device providing heat.

Subsequently, the vaporized part (34) of the tank flow (24) is introduced at a lower level of the distillation column (26). According to the embodiments, the reboiler (3) can take the form of an exchanger independent of the distillation column (26), as illustrated in FIGS. 1 and 2. In this case, the cooled flow (8) and the part (34) of the tank flow (24) circulate in countercurrent in pipes within the reboiler (3). In an embodiment not shown in the figures, the reboiler (3) is integrated into the tank (24) of the distillation column (26).

The partially or totally liquid flow (25) is then expanded in an expansion device (4) to the operating pressure of the distillation column (26), between 1 and 5 bars absolute. The expansion device (4) is generally a valve producing a pronounced cooling of the expanded fluid (Joule-Thomson effect). In practice, the temperature of the expanded fluid is between 90K and 130K.

At the outlet of the expansion device (4), the expanded flow (27) contains 2 fractions, respectively a predominantly liquid fraction, and a vapor fraction. The liquid phase is then separated from the vapor fraction. In practice, the expanded flow (27) is injected into a vessel for separating liquid and vapor fractions (5) located at the top of the distillation column (26).

In one embodiment not shown, a flow of liquid nitrogen from a liquid nitrogen storage not shown in Figures 1 and 2 is mixed with the expanded flow (27), before introduction into the separation drum (5). In a second embodiment, not shown, the liquid nitrogen is injected directly into the separation drum (5). The liquid nitrogen makes it possible to dilute the expanded flow (27) and also acts as a cold source making it possible to condense the vapor at the top of the column (26).

For all the embodiments mentioned above, the liquid fraction (7) coming from the separation drum (5) is then introduced into at one level of the distillation column (26), in practice in the upper part so as to run off the packing of the column (26). The vapor fraction (6) is introduced into the lower part of the packing of the distillation column (26), more precisely at a level below which the liquid fraction (7) is introduced to constitute the sweep gas avoiding the concentration of C>2, problematic in the packing because it can lead to the formation of mixtures of methane and oxygen at explosive concentrations in the distillation column.

Distillation thus produces two flows: a flow in liquid form enriched in methane (24), located in the column tank (26) and a flow depleted in methane (11), but rich in O2 and N2, at the top of the column. distillation column (26). A fraction of the liquid flow enriched in methane (24) whose temperature is between 90K and 130K is sent to the exchanger (2) to be vaporized and form a gas flow (10). This gas flow (10) comprises between 95 and 100% methane and is at a pressure of between 1 and 5 bars absolute, and at ambient temperature, typically between 273K and 313K, advantageously 288K. This gas flow (10) is recovered to be stored, as shown in Figure 1 or it can be injected directly into the natural gas network, as shown in Figure 2, via compression by the compressor (27 ) and cooling by the exchanger (28).

The flow depleted in methane (11), also called vent gas from the column, contains a variable fraction of methane, typically between 2 and 50%, in practice around 25%. This gas at the column outlet has a temperature of between 90K and 130K and a pressure of between 1 and 5 bar absolute. This vent gas (11) circulates against the current in the exchanger (2) and heats up. At the outlet of the exchanger (2), the gas has a temperature between 273K and 313K then it is compressed in a compressor (13) to a pressure between 5 and 20 bars absolute and advantageously between 8 and 15 bars absolute, i.e. the same pressure as the supply flow (1). This stream is cooled in an exchanger (14) to a temperature between -40 and 45° C. to then be sent to a membrane system (15). The cooling temperature in the exchanger (14) is chosen according to the nature of the membrane so as to optimize its separating properties.

The membrane system (15) is composed of two compartments (30, 31) subjected to different pressures, separated by at least one gas permeation membrane (33). The gas molecules dissolve on the membranes (33) then cross them by diffusion. The membranes (33) making up the membrane system (15) are chosen according to their affinity with the gaseous species considered.

In practice, the passage rate of a species (i) through a membrane (33) is calculated as follows:

[0067] [Math 1] Qi = Permi * A * APi

With (Qi) the passage rate of the species (i) through the membrane (33), (Permi) the permeance coefficient of the species (i) through the membrane (33), and ( APi) the partial pressure difference of species (i) between the two compartments (30, 31). The flow having crossed the membrane (33) constitutes the permeate (30), and the flow which has not crossed the membrane (33) constitutes the retentate (31). [0069] Two parameters make it possible to play on the rate at which a species (I) passes through the membrane. The first parameter is the pressure difference applied between the two compartments (30, 31) of the membrane system (15). Indeed, the higher the pressure difference (APi) between the two compartments (30, 31), the higher the flow rate (Qi). The second parameter is the permeability coefficient (Permi), ie the ability of a species (i) to easily cross the membrane (33).

To meet the needs of the invention, the choice of membranes meets a criterion of good separation between the CP and the O2 and the N2 present in the vent flow (29). To do this, the membranes (33) used must have high selectivity, that is to say that the ratio between the permeance coefficient of the species (i) by that of the species (j): a(ij) = Permi / Permj has a value greater than 1 .

There are two types of membranes (33) which correspond to these criteria: membranes (33) with positive selectivity and membranes (33) with negative or inverse selectivity.

The membranes (33) with positive selectivity have a selectivity coefficient a(O ₂ -CH ₄ ) and a(N2-CH4) greater than unity. Thus, the affinity of the gaseous species of dioxygen and dinitrogen with the membrane (33) is much better than that of methane. Dinitrogen and oxygen will therefore find it easier to cross the membrane (33). In this case, the permeate (30) will be enriched in O2 and N2 while the retentate (31) will be enriched in CP.

The membranes (33) with negative selectivity have a selectivity coefficient a(C>2-CP14) and a(N2-CP) of less than unity. Thus, the affinity of methane with the membrane (33) is much better than that of oxygen and nitrogen. Methane will therefore find it easier to cross the membrane (33). In this case, the permeate (30) will be enriched in methane while the retentate (31) will be enriched in O2 and N2.

For example, the membranes (33) made of polyimide (6FDA-BAPIF) have an a(N2-CH4) selectivity of 2.3. As a variant, the membranes (33) can be made of a material included in the group including polyimides, cellulose acetate, polycarbonates, polysulfone, poly(dimethylsiloxane-dimethylstyrene), poly(dimethylsiloxane), poly(siloctylene-siloxane), poly( p-silphenylene-siloxane), polyamide-polyester copolymer...

At the end of this membrane separation, the flow enriched in CPU (16) is recovered.

As illustrated in Figures 1 and 2, if the membrane (33) used is of positive selectivity, it is the retentate (31) which is enriched in CP and which is recovered. On the other hand, in an embodiment not shown in which the membrane used has reverse selectivity, it is the permeate (30) which is enriched in CP and which is recovered.

Once the flow enriched in CPU (16) has been recovered, it is sent to a pressure control valve (17) making it possible to vary the pressure in the membrane system (15), and therefore to control the quality of the separation.

[0077] In a first embodiment illustrated in Figure 1, the stream enriched in CH4 (16) is reinjected into the feed flow (1), thus forming a recycling loop making it possible to recover part of the methane flow. contained in the vent gas (12) and to reinject it into the supply flow (1) of methane.

In a second embodiment illustrated in Figure 2, the stream enriched in CH4 (16) is injected into the flow of purified methane (10) from the distillation.

The flow enriched in O2 and N2 and also comprising the loss of CH4 from the process is sent to a second control valve (20) making it possible to adjust the back pressure of the membrane system (15). The expanded stream (21) is then vented (23) to the atmosphere or to an oxidizer to oxidize the lost methane before venting the stream to the atmosphere. A bypass line comprising a valve (22) makes it possible to evacuate an excess of vent gas which could not be treated by the membrane system (15).

In an embodiment not shown, the liquid methane (24) is recovered directly from the tank outlet. To do this, the installation further comprises means for cooling the feed flow (1), in the exchanger (2), which are for example in the form of a refrigerant flowing counter-current to the feed stream (1) or a refrigerant bath in which the feed stream (1) circulates. In all cases, the refrigerant is preferably colder than the supply flow (1) to recover the calories. For example, the refrigerant can be in the form of a sacrificial fluid or be cooled in a closed thermodynamic cycle comprising a stage of compression, cooling and expansion of the refrigerant.

The following material balance is given as an indication for the overall system coupling a distillation and a membrane system (15) as illustrated in FIG. 1. In this system, the waste gas is the evacuated stream (23), which contains 7.2% CH4. The methane recovery rate is then 96.75%. The formula for calculating the methane yield is given below, for a system comprising an inlet F (1), and two outputs: a vent V (19) and a product P (10). x is the molar fraction of compound i whose recovery rate is to be calculated: i = [x_ * (x_F - x_V)] / [x_F * (x_P - x_V)]

Thus, the methane yield is calculated as follows: imethane=0.99*(0.7-0.072)/(0.7*(0.99-0.072)=96.75%.

[Table 1]

By way of comparison, a system with distillation alone would have the following material balance. In this system, the waste gas is the vent gas (11), which contains 12% CPU, therefore a higher concentration of CP than the system coupling the distillation and the membranes. The methane recovery rate is therefore 92%. The formula for calculating the methane yield is identical, for a system comprising an inlet F (1), and two outlets: a vent V (12) and a product P (10).

Thus, the methane yield is calculated as follows: imethane=0.99*(0.7-0.16)/(0.7*(0.99-0.16)=92%.

[Table 2]

By comparing the methane recovery rate with the membranes, to the methane recovery rate without membrane, the gain in yield is therefore 4.75% under the conditions studied. To conclude, the invention makes it possible to obtain a higher methane recovery rate than existing cryogenic separation techniques, while avoiding the generation of an explosive mixture in the distillation.

Claims

Claims [Claims 1] Process for the cryogenic separation of a feed stream

(1) containing methane, nitrogen and oxygen in which:

- the feed flow (1) is cooled in a recuperator exchanger (2),

- the cooled feed flow (8) is at least partially condensed (3),

- the at least partially condensed cooled feed stream (25) is expanded (4), which then contains a liquid fraction and a vapor fraction,

- the liquid fraction is separated from the vapor fraction of the expanded flow (27),

- the liquid fraction is sent to an upper level of a distillation column (26),

- a tank flow (24) is withdrawn from the distillation column (26), the tank flow (24) being enriched in methane with respect to the feed flow (1),

- a first part of the tank flow (24) is vaporized (3),

- the first vaporized part of the tank flow (34) is introduced at a lower level of the distillation column (26) then it is brought into contact with the liquid filling introduced at a higher level of the distillation column (26), and

- a flow of vent gas (11) enriched in oxygen and nitrogen relative to the feed flow (1) is withdrawn from the distillation column (26) and heated in the recuperator exchanger (2), characterized in that :

- the heated flow of vent gas (11) is compressed,

- the flow of compressed vent gas is cooled to ambient temperature,

- the flow of cooled compressed vent gas (29) is subjected to at least one membrane separation (15) to partially separate the methane from the oxygen and the nitrogen.

[Claims 2] Process according to claim 1, characterized in that at least one dilution flow (6) which is non-combustible and more volatile than oxygen is introduced into the distillation column (26) at at least a level lower than that into which the liquid fraction of the expanded flow (27) is introduced after separation, the dilution flow being formed by the vapor fraction of the expanded flow after separation.

[Claims 3] Method according to claim 1 or 2, characterized in that the cooled feed flow (8) is at least partially condensed (3) by heat exchange with the first part of the tank flow (24), said first part of the tank flow (24) being vaporized in contact with the cooled feed flow (8).

[Claims 4] Method according to one of Claims 1 to 3, characterized in that the flow of compressed vent gas is cooled to a temperature of between -40 and 45°C.

[Claims 5] Process according to one of Claims 1 to 4, characterized in that the selectivity a(N2-CH4) of the membrane is greater than 1.

[Claims 6] Process for the production of biomethane by purification of a feed stream containing methane, dinitrogen and dioxygen implementing the cryogenic separation process according to one of claims 1 to 5, in which:

- Recovering a second part (9) of the tank flow (24) and the flow enriched in methane (16) from the membrane separation.

[Claims 7] Process for the production of biomethane according to claim

6, characterized in that:

- the second part (9) of the tank flow (24) is recovered in liquid form,

- the feed flow (1) in the recuperator exchanger (2) is cooled with a refrigerant coming from an external source, and

[Claim 8] Process for the production of biomethane according to claim 6, characterized in that:

- the second part (9) of the tank flow (24) is vaporized in the recovery exchanger (2) so as to recover the gas flow (10) enriched in methane, and

- the methane-enriched flow (16) resulting from the membrane separation is introduced into the feed flow (1).

[Claims 9] Process for the production of biomethane according to claim 6, characterized in that:

- the second part (9) of the flow from the tank (24) is vaporized in the recuperator exchanger (2) so as to recover the flow of gas enriched in methane (10),

- the flow of gas enriched in methane (10) is compressed,

- the flow of compressed gas enriched in methane is cooled,

- the flow enriched in methane (16) resulting from the membrane separation is introduced into the flow of cooled compressed gas (32), to form a final flow of methane (36), and

- Recovering the final methane flow (36).

[Claims 10] Installation for the production of biomethane by purification of a feed stream containing methane, dinitrogen and dioxygen implementing the method according to one of Claims 5 to 7 and comprising: 19

- a heat exchanger (2) able to cool a feed flow (1), to heat the vent gas flow (11) and if necessary to vaporize the second part of the tank flow (9); at least partial condensation means (3) of the cooled feed flow (8);

- vaporization means (3) of the first part of the tank flow;

- a pipe capable of transporting the first part of the vaporized tank flow (3) to a lower level of the distillation column (26),

- an expansion means (4) of the at least partially condensed flow (25);

- a separating drum (5) for the liquid (7) and vapor (6) fractions of the flow at least partially condensed (25) after expansion;

- a distillation column (26);

- a pipe capable of transporting the liquid fraction (7) from the separator flask (5) to an upper level of a distillation column (26);

- compression means (13) of the vent gas flow (11), withdrawn from the column (26);

- a heat exchanger (14) capable of cooling the vent gas flow (11) once compressed;

- at least one separator membrane able to partially separate the methane from the O ₂ and from the N ₂ ;

- a recovery means (16-18) of the flow enriched in methane resulting from the membrane separation;

- a means of evacuation (19-23) of the flow enriched in dioxygen and dinitrogen resulting from the membrane separation, and

- a means of introducing the flow enriched in methane resulting from the membrane separation into the feed flow (1).

[Claims 11] Installation for the production of biomethane by purification of biogas implementing the process according to one of claims 5 or 8 and comprising:

- a heat exchanger (2) able to cool a feed flow (1), to heat the vent gas flow (11) and to vaporize the second part of the tank flow (9) into a vaporized flow ( 10); at least partial condensation means (3) of the cooled feed flow (8);

- vaporization means (3) of the first part of the tank flow (24);

- a pipe capable of transporting the first part of the vaporized tank flow (3) to 20 a lower level of the distillation column (26),

- an expansion means (4) of the at least partially condensed flow (25);

- a distillation column (26);

- at least one separating membrane (33) able to partially separate the methane from the O ₂ and from the N ₂ ;

- a means for recovering the flow enriched in methane (16-18) resulting from the membrane separation;

- a means of evacuating the flow enriched in oxygen and nitrogen (19-23) resulting from the membrane separation,

- compression means (27) of the vaporized flow (10),

- a heat exchanger (28) capable of cooling the compressed flow, and

- a means of introducing the flow enriched in methane (18) into the flow of cooled compressed gas (32).

[Claims 12] Installation according to claim 9 or 10, further comprising a reboiler (3), said reboiler being adapted to at least partially condense the cooled flow (8) and to vaporize the first part of the bottom flow.

[Claims 13] Installation according to one of Claims 10 to 12, comprising a pipe capable of transporting the vapor fraction (6) from the separator drum (5) into the column (26) at least at a level lower than that at which it is introduced. the liquid fraction (7).

[Claims 14] Installation according to one of Claims 10 to 13, characterized in that the separating membranes (33) are made of a material included in the group including polyimides, cellulose acetate, polycarbonates, polysulfone, poly(dimethylsiloxane- dimethylstyrene), poly(dimethylsiloxane), poly(siloctylene-siloxane), poly(p-silphenylene-siloxane), polyamide-polyester copolymer. 21

[Claims 15] Installation according to one of claims 10 to 14, characterized in that the methane recovery means include a valve (17) able to vary the pressure in the membrane system (15).

[Claims 16] Installation according to one of Claims 10 to 15, characterized in that the means for evacuating the dinitrogen and the dioxygen include a valve (20) able to adjust the back pressure of the membrane system (15).