Gas purification process by membrane separation

Abstract

Process for purifying a gas by membrane separation The present invention relates to a process for purifying a raw gas (1) mixture of at least A+B, A more permeable than B, with 3 stages of membrane separation (E1, E2, E3), where after compression in (C1), the following are separated: -the compressed raw gas (2) in a 1st stage (E1) to obtain a 1st retentate (3) enriched in B and a 1st permeate (4) enriched in A; -the retentate (3) in a 2nd stage (E2) to obtain a 2nd retentate (5) enriched in B, and a 2nd permeate (6) enriched in A returned to (C1) and mixed with the raw gas (1); -the 1st permeate (4) in a 3rd stage (E3) to obtain a 3rd retentate (7) returned to (C1) and mixed with the raw gas (1), and a 3rd permeate (8) enriched in A; the mole (or mass) fraction of constituent A at the inlet of (E1) being controlled to be less than or equal to its mole (or mass) fraction in the incoming raw gas (1) before recycling by pressure adjustment of the 3rd permeate (8) or the 2nd permeate (6).Figure to be published: Figure 1.

Description

Title of the invention: Method for purifying a gas by membrane separation

[0001] The present invention relates to a gas purification process using three membrane separation stages. An example of a gas to be purified is biogas, from which a stream rich in carbon dioxide (CO2) and a stream rich in methane (CH4) are to be obtained.

[0002] European patent application EP 0 799 634 A1 discloses a process for purifying a raw gas by means of three membrane separation stages, the raw gas being a mixture of at least two components A and B, A being the component having the highest permeance. This process, which aims to separate the raw gas stream into a stream enriched in A and a stream enriched in B, is illustrated by the diagram in [Fig. 4].

[0003] As can be seen in this diagram: - The raw gas stream 1 is compressed in a compressor Cl to obtain a compressed gas stream 2 which is then passed through a first membrane separation stage El to obtain: • a retentate stream 3; and • a permeate current 4; - The retentate 3 current is passed through a second membrane separation stage E2 to obtain: - a retentate current 5, which constitutes the desired B-enriched current; and

[0004] a permeate current 6; - The permeate stream 4 is passed through a third membrane separation stage E3 to obtain: • a retentate current 7; and • a permeate current 8, which constitutes the A-enriched current research.

[0005] A gas recompression is carried out between the permeate stream 4 of stage El and the supply of stage E3. This recompression is carried out using the compressor C2 located on the path of the permeate 4.

[0006] Figure 4 also shows pressure regulating valves VI and V2 located in the path of streams 5 and 7, respectively. The permeate stream 6 and the retentate stream 7 are recycled into stream 1 before being introduced into compressor CL

[0007] In the current context of rising energy costs and in particular of With regard to electrical energy, it is always desirable to reduce the energy consumption of a process while maintaining the expected performance, such as, for example, the final purities sought for the gas whose constituents are to be separated or the recovery efficiency of the gas that is to be concentrated.

[0008] It is this problem that the Applicant Company has tackled. On this occasion, it discovered that by controlling, for example by applying a vacuum in a chosen location of the process, the mole fraction (or mass fraction) of a constituent (constituent A) recycled relative to the incoming gas, it was possible to appreciably reduce the overall energy consumption of the process.

[0009] The present invention therefore relates to a method for purifying a gas using three stages of membrane separation, characterized by the fact that: - in a compressor, a raw gas consisting of a mixture of at least two components A and B is compressed, the permeability of component A being greater than that of component B; then - The compressed raw gas is passed through a first stage of membrane separation to obtain: - a first retentate enriched in constituent B; and - a first permeate enriched in constituent A; - The retentate is passed through a second membrane separation stage to obtain: • a second retentate which is a gas enriched in constituent B; • a second permeate enriched in constituent A which is returned to upstream of said compressor to be mixed with the incoming raw gas; - The first permeate is passed through a third membrane separation stage to obtain: - a third retentate which is returned upstream of said compressor to be mixed with the incoming raw gas; and - a third permeate which is a gas enriched in constituent A,

[0010] the mole fraction or mass fraction of constituent A at the inlet of the first stage of membrane separation being controlled so that it is at every instant less than or equal respectively to the mole fraction or mass fraction of constituent A in the raw gas entering before the recycling input, by reducing the pressure at the outlet of the third stage of membrane separation in the third permeate and by adjusting the pressure of at least one of the third permeate and the second permeate.

[0011] By permeability of a gaseous constituent, also called permeance, we mean the intrinsic property of the material constituting the membrane wall to be left to pass through said gaseous constituent through said membrane wall. Permeability is expressed as a molar or mass flow rate of the constituent per unit area and per unit pressure such as for example in words '.mAPa*.

[0012] In accordance with a first embodiment of the process according to the present invention: • A pressure-reducing device is planned, positioned along the path of the third permeate and designed to decrease the pressure on the third permeate side. • The pressure of the third permeate is controlled by acting on said pressure reduction device in such a way that the mole fraction or mass fraction of constituent A at the inlet of the first stage of membrane separation is at each instant less than or equal respectively to the mole fraction or mass fraction of constituent A in the raw gas entering before the recycling input.

[0013] Controlling the pressure of the third permeate allows control of the mole fraction or mass fraction of component A in the third permeate and consequently the mole fraction or mass fraction of component A in the third retentate. Since the third retentate is returned upstream of the compressor, it is then possible to control the mole fraction or mass fraction of component A at the inlet of the first membrane stage.

[0014] In what follows, the following notations may be used, for example: - %A_IN is the mole fraction (expressed as a percentage) of component A in the incoming raw gas; and - %A_F1 is the mole fraction (expressed as a percentage) of component A in the gas at the inlet of the first stage of membrane separation (El), i.e. after recycling and compression.

[0015] For example, the mole fraction of component A is measured in the raw gas, i.e., before the recirculation of the second-stage permeate E2 and the third-stage retentate E3 (mole fraction %A_IN); the mole fraction of component A is also measured before entering the first stage E1, i.e., after the recirculation of the second-stage permeate E2 and the third-stage retentate E3 (mole fraction %A_F1); the mole fraction %A_IN is the input setpoint to a control loop, and the mole fraction %A_F1 is the input measurement to this control loop. The output signal of the control loop acts on the pressure-variation device. If the latter is a vacuum pump, it acts, for example, on the speed of the vacuum pump motor by means of a variable-speed drive or by acting on a vacuum relief valve located at the vacuum pump suction.

[0016] It is also possible to perform this regulation of the mole fraction of the component A at the entrance of the first stage of membrane separation El by means of 2 cascaded regulation loops (master / slave).

[0017] According to a second embodiment of the process according to the present invention: • pressure is reduced to a fixed value in the third permeate side; • A pressure regulating valve is planned to be positioned in the path of the second permeate; and • The pressure of the second permeate is controlled by acting on said pressure regulating valve in such a way that the mole fraction or mass fraction of constituent A at the inlet of the first stage of membrane separation is at each instant less than or equal respectively to the mole fraction or mass fraction of constituent A in the raw gas inlet before the recycling input.

[0018] Controlling the pressure of the second permeate allows control of the mole fraction or mass fraction of component A in the second permeate. Since the second permeate is returned upstream of the compressor, it is then possible to control the mole fraction or mass fraction of component A at the inlet of the first membrane stage, respectively.

[0019] In particular, the pressure reduction in the third permeate side can be achieved using a pressure reducing device disposed on the path of the third permeate.

[0020] For example, the pressure of the third-stage permeate is maintained at a constant value (e.g., -0.5 barg (-0.5 x 10⁵ Pa on the gauge)), for example, using a pressure-variating device, such as a vacuum pump. A pressure regulating valve (V3) is added to the second-stage permeate to control its pressure. This regulating valve (V3) is controlled by a pressure regulator that measures the pressure of the second-stage permeate (E2) and sets the output of constituent A (carbon dioxide) at the first-stage inlet.

[0021] A vacuum pump can be used as a pressure reduction device.

[0022] According to the invention, the recycled flow rate ratio can advantageously be adjusted, to knowledge of the second permeate and the third retentate, on the flow rate of raw gas entering at a value less than 0.2.

[0023] In particular, constituent A of the raw gas is carbon dioxide, and constituent B is methane.

[0024] Membranes having a selectivity of component A with respect to component B greater than 30, preferably greater than 50 and even more preferably greater than 70, can be advantageously used.

[0025] Selectivity is defined as the ratio of the permeability or permeance of a first pure constituent to the permeability or permeance of a second pure constituent. (Description of Figures)

[0026] - Fig. 1 is a schematic representation of the general process according to the present invention; - Fig. 2 is a schematic representation of the process according to a first embodiment with 3 cascaded control loops; - Figure 3 is a schematic representation of the process according to a second embodiment with 3 cascaded control loops; and - Fig. 4 is a schematic representation of a process using an interstage compression step.

[0027] In what follows, mole fractions are used, but the present invention applies equally to mass fractions.

[0028] In [Fig. 1], a diagram of the process according to the invention can be seen. This diagram corresponds to that of [Fig. 4] with the installation of a PV vacuum pump on the permeate path 8 and without an interstage compressor, the same reference numbers and letters as in [Fig. 4] having been retained to designate similar pipes and devices.

[0029] Fig.2 is a reproduction of Fig.1 on which the regulating elements of the mole fraction of A in the gas mixture entering the El stage have been shown thanks to the pressure of the permeate 8 exiting the E3 stage, a controllable vacuum pressure being applied to the permeate 8 of the E3 stage.

[0030] The following regulatory organs appear in [Fig.2]: • AT_A_IN, measuring device, placed on stream 1, upstream of the recycling input of streams 6 and 7, whose role is to measure the mole fraction of constituent A in the incoming raw gas; • AT_A_F1, measuring device, placed on stream 2, between compressor Cl and stage El, whose role is to measure the mole fraction of constituent A in stream 2; • AT_B, measuring device, placed on the retentate stream 5 of E2, upstream of VI in the example shown, whose role is to measure the mole fraction of constituent B in the retentate at the outlet of the second membrane separation stage E2; • The following pressure sensors • PT1, placed on the retentate current 5 of E2, upstream of VI; • PT2, placed on the retentate current 7 of E3, upstream of V2; • PT3, placed on the permeate stream 8 of E3, upstream of PV, • Regulatory bodies, denoted by blocks: • SP_INT: Internal setting point (setpoint) of the master controller • SP_EXT: External setting point (setpoint) of the slave controller corresponding to the output of the master controller • SP_MAX: Maximum adjustment point • MEASUREMENT: Measurement entering the regulator • OUT: Regulator output • OUT_MAX: Maximum output of the regulator • OUT_MIN: Minimum output of the regulator

[0031] and bearing the reference letters OR1 to OR6.

[0032] The OR1 and OR2 components constitute a cascade regulation of 2 regulators whose function is to control the mole fraction of component B in the retentate 5 by acting on the working pressure PT1 of the membrane separation stages El and E2 by means of the pressure regulating valve VL

[0033] The desired setpoint value of the mole fraction of component B in the retentate 5 is entered as the SP_INT value of the OR1 element and the OR1 element receives as the MEASURE value the value of the mole fraction of component B in the retentate 5 measured by the AT_B sensor.

[0034] The output of the OR1 organ is a working pressure command sent in SP_EXT to the OR2 organ.

[0035] The OR2 component compares the SP_EXT setpoint from OR1 to the MEASURED pressure value of the retentate 5 measured by the PT1 pressure sensor and acts on the VL pressure regulating valve

[0036] The OR3 and OR4 components (2 cascaded regulators) control the purification efficiency of the process. The purification efficiency is defined as the flow rate of component B recovered from the retentate 5 divided by the flow rate of component B entering the raw gas 1.

[0037] The desired setpoint value for the purification efficiency is entered as the SP_INT value of the OR3 component. It also receives the measured value of the flow rate of component B in the retentate 5 divided by the flow rate of component B entering the raw gas 1 as the MEASURE value.

[0038] The OR4 organ receives as a MEASUREMENT value the pressure of the retentate 7 measured by the pressure sensor PT2.

[0039] By comparing the desired setpoint value and the measured value of the purification efficiency, the OR3 organ will send the pressure value to be reached for the retentate 7 as an output value OUT to the OR4 organ, the OR2 organ receiving it as the SP_EXT value.

[0040] The OR4 component will then control the retentate pressure 7 by sending the output value OUT to the regulating valve V2 in order to maintain the yield value purification.

[0041] The OR5 and OR6 organs (2 regulators in cascade) ensure that the mole fraction of constituent A at the inlet of the first stage of membrane separation (El) is at every instant less than or equal to the mole fraction of constituent A in the incoming raw gas (1) before the recycling input.

[0042] The OR5 organ receives as value SP_INT, the mole fraction of constituent A in stream 1 measured by sensor AT_A_IN and receives as value MESURE, the mole fraction of constituent A in stream 2 after compressor Cl and measured by sensor AT_A_F1.

[0043] The OR6 organ receives as a MEASUREMENT value the pressure of the permeate 8 measured by the pressure sensor PT3.

[0044] The OR5 organ sends the pressure of the permeate 8 for which the mole fraction of constituent A in the stream 1 is equal to the mole fraction of constituent A in the stream 2 as output signal OUT and which is received by the OR6 organ as signal SP_EXT.

[0045] By comparison between the SP_EXT signal and the MEASURE value, the OR6 component will control the PV vacuum pump by sending the output value OUT to it so that the mole fraction of component A in stream 2 remains less than or equal to the mole fraction of component A in stream 1.

[0046] In [Fig.3], the diagram shown corresponds to that of [Fig.1] with the installation of a valve V3 on the permeate stream 6 of stage E2, whose role is to allow the regulation of the mole fraction of A in stream 2 with no longer a controllable vacuum pressure as in [Fig.2], but a fixed constant vacuum pressure in the permeate 8 and a variable pressure in the permeate stream 6 of stage E2 as will be developed below.

[0047] Figure 3 shows: • the same quantity measuring devices as those in [Fig.2], labeled identically; • the same pressure sensors PT1 and PT2 as those in [Fig.2], labeled identically, as well as the PT4 sensor placed on the permeate stream 6 of stage E2, upstream of valve V3; • the same OR1 to OR4 regulating organs as those in [Fig.2], noted identically, as well as the OR7 and OR8 regulating organs, the block notations being identical to those above.

[0048] The OR7 and OR8 components ensure that the mole fraction of component A at the inlet of the first membrane separation stage (El) is at every instant less than or equal to the mole fraction of component A in the incoming raw gas (1) before the recycling input.

[0049] The OR7 organ receives as value SP_INT, the mole fraction of constituent A in stream 1 measured by the sensor AT_A_IN and receives as value MESURE, the mole fraction of constituent A in stream 2 after compressor Cl and measured by the sensor AT_A_F1.

[0050] The OR8 organ receives as a MEASUREMENT value the pressure of the permeate 6 measured by the pressure sensor PT4.

[0051] The OR7 organ sends the pressure of the permeate 6 for which the mole fraction of constituent A in the current 1 is equal to the mole fraction of constituent A in the current 2 as output signal OUT and which is received by the OR8 organ as signal SP_EXT.

[0052] By comparison between the SP_EXT signal and the MEASURE value, the OR8 component will control the V3 regulating valve by sending the output value OUT to it so that the mole fraction of component A in stream 2 remains less than or equal to the mole fraction of component A in stream 1.

[0053] The following examples illustrate the present invention without however limiting its scope.

[0054] Examples 1 and 2 are examples of the invention. Example 1 illustrates the embodiment in which the pressure of the permeate 8 is controlled, as shown in [Fig.2], and Example 2 illustrates the embodiment in which the pressure of the permeate 6 is controlled, as shown in [Fig.3].

[0055] Example 3 is a comparative example with the process according to EP 0 799 634 AL

[0056] In these Examples:

[0057] All percentages indicated are molar percentages.

[0058] Specific consumption is defined as the quantity of electrical energy required by the process, expressed per unit of raw gas processed. It is therefore expressed in kWh / Nm3 of raw gas.

[0059] In Examples 1 and 2, it is calculated using the following formula (1) and in Example 3, using the following formula (2)

[0060] [Math.l]

[0061] [Math.2] Cùr = ------------------------------------ ls

[0062] Formulas in which: • Q is the raw gas flow rate in Nm3 / h; • Pucomp is the power required (in kW) by the compressor Cl when making

[0063]

[0064]

[0065]

[0066]

[0067]

[0068]

[0069] the assumption of an isentropic efficiency of 0.7; • PuPAV is the power required by the PV vacuum pump assuming an isentropic efficiency of 0.75; • PuCF is the power required by the cooling unit located at the outlet of compressor Cl in order to cool the gas, assuming a coefficient of performance of 2.5; • PuINTercomp is the power required (in kW) by the interstage compressor C2, assuming an isentropic efficiency of 0.7; and • PuINTergf is the power required by the cooling unit located at the outlet of the interstage compressor C2 in order to cool the gas, assuming a coefficient of performance of 2.5. In all these examples: • A mixture composed of 55% methane (constituent B) and 45% carbon dioxide (constituent A) was separated. The raw gas flow rate is set at 200 Nm3 / h; • The temperature of all currents passing through the membranes is 25°C. Pressure losses in the pipes and in the membranes are neglected; • the purification efficiency is defined as the molar flow rate of methane recovered on the retentate 5 divided by the molar flow rate of methane entering the raw gas 1; • The results were obtained for a mole fraction of 97% methane in retentate 5 and for a purification efficiency of 99.2%; and • The supply pressure of the El stage is controlled by a V1 regulating valve located on the retentate 5. This regulation aims to guarantee a molar fraction of methane in the retentate 5 equal to 97%. Example 1 (according to the invention) as shown in [Fig.2]: UBE-branded CO-810-FSC type hollow fiber membranes with a selectivity of 72.4 are used on the three separation stages, respectively 2, 1 and 2 membranes on stages E1, E2 and E3. The pressure obtained at the inlet of the El stage is 11.03 barg (1.103 MPa relative to the manometer). The measured flow rate of retentate 5 is 112.65 Nm3 / h and the flow rate of permeate 6 is 16.05 Nm3 / h with a methane mole fraction of 46.26%. The treatment efficiency is controlled by the pressure of the permeate 4, which is 1.08 barg (0.108 MPa relative to the pressure gauge). This pressure is controlled by a regulating valve V2 located on the retentate 7. The flow rate of permeate 4 is 105.75 Nm3 / h with a methane mole fraction of 11.75%.

[0070] The pressure applied to the permeate 8 by means of the vacuum pump PV is -0.42 barg (-0.42* 105 Pa relative to the manometer), which makes it possible to verify the equality of the mole fractions of carbon dioxide between streams 1 and 2.

[0071] The discharge pressure of the PV vacuum pump is 0.051 barg (0.051 Pa relative to the pressure gauge). The flow rate of the permeate 8 is 87.26 Nm3 / h with a carbon dioxide mole fraction of 98.99%.

[0072] The recycling rate of the process is 0.1727. (Definition of the recycling rate = sum of the molar flow rates 6 and 7 divided by molar flow rate 1)

[0073] The calculated theoretical electrical consumption is 0.179 kWhe / Nm3 of raw gas.

[0074] Example 2 (according to the invention) according to the diagram in [Fig.3]:

[0075] Hollow fiber membranes marketed by UBE of type CO-810-FSC with selectivity 72.4 are used on the three separation stages, respectively 1, 1 and 4 membranes on stages E1, E2 and E3.

[0076] The pressure obtained at the inlet of the El stage is 12.39 barg (1.239 MPa relative to the manometer).

[0077] The measured flow rate of the retentate 5 is 112.65 Nm3 / h and the flow rate of the permeate 6 is 15.96 Nm3 / h with a methane mole fraction of 51.23%.

[0078] The pressure applied to the permeate 6 by means of the regulating valve V3 is 0.40 barg (0.40* 105 Pa relative to the manometer), which makes it possible to verify the equality of the mole fractions of carbon dioxide between the streams 1 and 2.

[0079] The treatment efficiency is controlled by the pressure of the permeate 4, which is 0.25 barg (0.25 * 10⁵ Pa relative to the pressure gauge). This pressure is controlled by means of a regulating valve V2 located on the retentate 7.

[0080] The flow rate of permeate 4 is 97.50 Nm3 / h with a methane mole fraction of 7.30%.

[0081] The pressure applied to the permeate 8 by the vacuum pump is -0.5 barg (-0.5 * 10⁵ Pa relative to the manometer). The discharge pressure of the vacuum pump is 0.051 barg (0.051 * 10⁵ Pa relative to the manometer).

[0082] The flow rate of the permeate 8 is 87.27 Nm3 / h with a molar fraction of carbon dioxide of 98.99%.

[0083] The recycling rate of the process is 0.1309.

[0084] The calculated theoretical electrical consumption is 0.184 kWhe / Nm3 of raw gas.

[0085] Example 3 (comparative) according to the diagram in [Fig.4]:

[0086] Hollow fibre membranes marketed by UBE of type CO-810-FSC with selectivity 72.4 are used on the three separation stages, respectively 1, 1 and 1 membranes on stages E1, E2 and E3.

[0087] The pressure obtained at the inlet of the El stage is 11.10 barg (1.110 MPa relative to the manometer).

[0088] The measured flow rate of the retentate 5 is 112.65 Nm3 / h and the flow rate of the permeate 6 is 16.37 Nm3 / h with a methane mole fraction of 45.64%.

[0089] The purification efficiency is controlled by the supply pressure of stage E3 after compressor C2, which is also the discharge pressure of compressor C2. This pressure is 3.06 barg (3.06*105 Pa relative to the manometer).

[0090] The pressure of the permeate 4, which is also the suction pressure of the compressor C2, is 0.1 barg (0.1*105 relative Pa on the manometer).

[0091] The flow rate of permeate 4 is 94.27 NmVh with a methane mole fraction of 6.75%.

[0092] The pressure of the permeate 8 is 0.051 barg (0.051 * 105 Pa relative to the manometer) because there is no vacuum pump.

[0093] The flow rate of permeate 8 is 87.26 NmVh with a carbon dioxide mole fraction of 98.99%.

[0094] The recycling rate of the process is 0.1168.

[0095] The calculated theoretical electrical consumption is 0.194 kWhe / Nm3 of raw gas. Summary of results

[0096] Table 1 summarizes the specific consumptions obtained in Examples 1 to 3.

[0097] [Tables 1] Example Specific electrical consumption (kWh / Nm3 of raw gas) 1 (invention) 0.179 2 (invention) 0.184 3 (comparative) 0.194

[0098] As can be seen in the Table above, the specific electrical consumption is lower for Examples 1 and 2 of the invention compared to the comparative example.

Claims

Claims

1. - Method for purifying a gas using three separation stages membrane, characterized by the fact that: - in a compressor (Cl), a raw gas (1) consisting of a mixture of at least two constituents A and B is compressed, the permeability of constituent A being greater than that of constituent B; then - the compressed raw gas (2) is passed through a first membrane separation stage (El) to obtain: • a first retentate (3) enriched in constituent B; and • a first permeate (4) enriched in constituent A; - the retentate (3) is passed through a second membrane separation stage (E2) to obtain: • a second retentate (5) which is a gas enriched in constituent B; • a second permeate (6) enriched with constituent A which is returned upstream of said compressor (Cl) to be mixed with the incoming raw gas (1); - the first permeate (4) is passed through a third membrane separation stage (E3) to obtain: • a third retentate (7) which is returned upstream of said compressor (Cl) to be mixed with the incoming raw gas (1); and • a third permeate (8) which is a gas enriched in constituent A, the molar fraction or the mass fraction of the constituent A at the inlet of the first membrane separation stage (El) being controlled so that it is at each instant less than or equal respectively to the molar fraction or the mass fraction of the constituent A in the incoming raw gas (1) before the addition of the recycling, by reducing the pressure at the outlet of the third membrane separation stage (E3) in the third permeate (8) and by adjusting the pressure of at least one of the third permeate (8) and the second permeate (6).

2. - Method according to claim 1, characterized in that: • a pressure reduction device (PV) is provided on the path of the third permeate (8) and intended to reduce the pressure in the third permeate (8) side; and • the pressure of the third permeate (8) is controlled by acting on said pressure reduction device (PV) so that the molar fraction or the mass fraction of the constituent A at the inlet of the first membrane separation stage (El) is at each instant less than or equal respectively to the molar fraction or the mass fraction of the constituent A in the incoming raw gas (1) before the recycling is added.

3. - Method according to claim 1, characterized in that: • a pressure reduction is carried out to a fixed value in the third permeate side (8); • a pressure regulating valve (V3) is provided arranged on the path of the second permeate (6); and • the pressure of the second permeate (6) is controlled by acting on said pressure regulating valve (V3) in such a way that the molar fraction or the mass fraction of the component A at the inlet of the first membrane separation stage (El) is at all times less than or equal to the molar fraction or the mass fraction of the component A in the incoming raw gas (1) before the recycling is added.

4. - Method according to claim 3, characterized in that the pressure reduction in the third permeate side (8) is carried out using a pressure reduction device (PV) arranged in the path of the third permeate (8).

5. - Method according to one of claims 2 and 4, characterized in that the pressure reduction device (PV) is a vacuum pump.

6. - Method according to one of claims 1 to 5, characterized in that the ratio of the recycled flow rate, namely of the second permeate (6) and of the third retentate (7), to the flow rate of incoming raw gas (1) is adjusted to a value less than 0.

2.

7. - Process according to any one of claims 1 to 6, characterized in that the constituent A of the raw gas is carbon dioxide, and the constituent B is methane.

8. - Process according to any one of claims 1 to 7, characterized in that membranes are used having a selectivity of constituent A with respect to constituent B greater than 30, preferably greater than 50 and even more preferably greater than 70.

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