METHOD AND DEVICE FOR SEPARATING A MIXTURE OF HYDROGEN AND CARBON DIOXIDE

Description

[0001] The present invention relates to a method and apparatus for separating a mixture of hydrogen and carbon dioxide, in particular from a mixture also comprising at least one other component, such as carbon monoxide, methane or nitrogen. The mixture may optionally comprise water.

[0002] It has a particular advantage for such processes in which the mixture to be separated has been previously separated by adsorption, for example in an H2 PSA, to produce a gas enriched in hydrogen and depleted in carbon dioxide and this mixture in gaseous form which is depleted in hydrogen and enriched in carbon dioxide compared to the gas treated in the adsorption.

[0003] For such processes equipped with turbines, the invention makes it possible to reduce the load of the separation process in a stable and efficient manner.

[0004] The adsorption process that produces the mixture to be separated often processes a gas from a hydrogen production process, for example a reforming process such as a steam methane reformer known by the acronym SMR, an autothermal reformer known by the acronym ATR or a partial oxidation known by the acronym POX.

[0005] FR3052684 A1 describes a process for separating a mixture containing carbon dioxide, water and at least one compound chosen from the list: hydrogen, carbon monoxide, nitrogen.

[0006] FR2877939 describes the separation of a residual gas from a PSA by partial condensation, the incondensables from the partial condensation being separated by permeation, the permeate being recycled upstream of the PSA and the residue being sent to reforming as feedstock and fuel.

[0007] WO2012 / 064938, WO2012 / 064941 and WO2012 / 158673 describe the separation by permeation of a flow of incondensables coming from a low temperature separation of CO2, involving the expansion in a turbine of a residue and the compression of a permeate.

[0008] US2011 / 0138852 describes processes in which a mixture is separated by permeation, the CO2-enriched permeate is separated by partial condensation and the residue is expanded in a turbine.

[0009] According to the invention, the method treats a gas, which may be the hydrogen-depleted waste gas from an adsorption process, for example of the pressure swing type, known by the acronym PSA. This gas can be compressed, dried, separated by partial condensation and / or distillation to produce a CO2-enriched fluid, a waste gas from the partial condensation and / or distillation being separated by permeation in two membrane units.

[0010] The heart of the process according to the invention is to associate: A first separation at low temperature (below -40°C) by partial condensation and possibly by distillation, making it possible to recover and purify a percentage of the CO 2 contained in the treated feedstock, for example having a yield of at least 60%, A second membrane treatment step applied to at least a non-condensable part of the low-temperature part, the membrane treatment producing for example: ∘ A hydrogen-rich fluid recycled to the production unit, for example the reforming unit, upstream in order to increase its production or reduce its load in order to produce the same quantity of hydrogen, and / or ∘ A fluid of intermediate composition, rich in H 2 and CO 2 , recycled to the section operating at low temperature to recover the CO 2 and / or ∘ A fluid poor in H 2 and CO 2 , returned to the hydrogen production unit, typically to be used as fuel.

[0011] It is possible to increase the performance of membrane separation by incorporating at least one turbine to recover the pressure energy from the membrane residue.

[0012] According to one object of the invention, there is provided a method for separating a mixture containing hydrogen and carbon dioxide, as well as at least one compound lighter than carbon dioxide chosen from the list: carbon monoxide, methane, nitrogen, comprising the following steps: a) Cooling the mixture from a first temperature in a heat exchanger by sending the mixture to the heat exchanger, resulting in the partial condensation of the mixture into a liquid phase enriched in carbon dioxide and a gaseous phase depleted in carbon dioxide, the mixture leaving the heat exchanger at a second temperature lower than the first temperature, said heat exchanger being at least partially cooled by a gaseous fluid which heats up in the heat exchanger by indirect heat exchange b) Separation of the liquid phase from the gaseous phase in one or more separator pots,possibly in several stages interspersed with successive cooling phases c) Reheating of at least a part of the gas phase coming from at least one of the separator pots in the heat exchanger by indirect heat exchange d) Sending the at least a reheated part from stage c) to a membrane separation unit as the only feed flow of the membrane separation unit, , generating one or more permeates enriched in hydrogen and / or carbon dioxide and depleted in at least one compound lighter than carbon dioxide compared to the reheated part, and at least one residue depleted in hydrogen and carbon dioxide and enriched in at least one compound lighter than carbon dioxide compared to the reheated part and characterized in that it comprises: e) expanding the at least one residue from a first pressure in one or more turbines producing an expanded fluid to a second pressure lower than the first pressure and to a third temperature lower than the first temperature and preferably at the second temperature or lower than the second temperature, f) the fluid at the second pressure produced during step e) constituting the gaseous fluid of step a) which heats up in the heat exchanger by indirect heat exchange with the mixture and g) the at least one residue is sent from the membrane separation unit to the turbine or turbines without having passed through the heat exchanger.

[0013] According to other optional aspects: the first temperature is above 0°C, or even above 10°C. the second temperature is below 0°C, preferably below -30°C, or even below -40°C. the membrane separation unit operates at a temperature above 50°C and the at least one heated portion of step c) is heated downstream of the heat exchanger and upstream of the membrane separation unit in an auxiliary heat exchanger. the fluid expanded to the second pressure is sent to the heat exchanger without passing through the auxiliary heat exchanger. a portion of the gaseous phase produced in the at least one separator pot is heated in the heat exchanger and another portion of the gaseous phase produced in the at least one separator pot is not heated in the heat exchanger, the two portions being mixed downstream of the heat exchanger,the ratio between the flow rates of the two parts being regulated in order to reach a target temperature after mixing the two parts. the mixture of the two parts is sent to the membrane separation unit. the mixture containing hydrogen and carbon dioxide is a waste gas from an adsorption separation process, for example pressure swing. step b) optionally followed by a distillation step, produces a fluid containing at least 60 mol% of carbon dioxide, preferably at least 80 mol% of carbon dioxide, or even at least 90 mol% of carbon dioxide. the membrane separation unit (M) produces a hydrogen-enriched fluid which is recycled to the upstream adsorption process in order to increase its production or reduce its load in order to produce the same quantity of hydrogen and / or a fluid of intermediate composition, enriched in H 2 and CO 2 ,separated in at least one of the separator pots and / or by distillation to recover the CO 2 and / or a fluid depleted in H 2 and CO 2 , returned to a hydrogen production unit, feeding the adsorption process, typically to be recovered as fuel. no part of the permeate(s) is sent to the separation of step b). the residue is sent to the turbine without having passed through a combustion chamber. at least part of the gaseous phase does not cool in the heat exchanger upstream of the membrane separation unit According to another object of the invention, there is provided an apparatus for separating a mixture containing hydrogen and carbon dioxide, as well as at least one compound lighter than carbon dioxide chosen from the list: carbon monoxide, methane, nitrogen, comprising a heat exchanger having a cold end and a hot end,the cold end being adapted to operate at a colder temperature than the hot end, means for sending a mixture to cool from a first temperature in the heat exchanger by sending the mixture to the heat exchanger, resulting in the partial condensation of the mixture into a liquid phase enriched in carbon dioxide and a gaseous phase depleted in carbon dioxide, means for leaving the mixture from the heat exchanger at a second temperature lower than the first temperature, means for sending a gaseous fluid which heats up in the heat exchanger by indirect heat exchange with the mixture, a partial condensation unit comprising one or more separator pots connected in series or in parallel to separate a liquid phase from a gaseous phase of the mixture leaving the exchanger,means for sending at least a portion of the gas phase from at least one of the separator pots into the heat exchanger to be heated by indirect heat exchange, a membrane separation unit, means for sending the at least one heated portion of the gas phase to the membrane separation unit, means for removing one or more permeates enriched in hydrogen and / or carbon dioxide and depleted in at least one compound lighter than carbon dioxide from the membrane separation unit, means for removing at least one residue depleted in hydrogen and carbon dioxide and enriched in at least one compound lighter than carbon dioxide from the membrane separation unit, at least one turbine, means for sending at least a portion of the at least one residue to expand from a first pressure in the turbine(s), without having passed through the heat exchanger,producing a fluid expanded to a second pressure lower than the first pressure at a third temperature lower than the first temperature and preferably at the second temperature and means for sending the fluid at the second pressure to the heat exchanger to heat up like the gaseous fluid which heats up in the heat exchanger by indirect heat exchange characterized in that the separator pot(s) connected in series or in parallel are connected directly to the heat exchanger without passing through the membrane separation unit and the means for sending the fluid at the second pressure to the heat exchanger are connected to introduce the fluid at the second pressure into the heat exchanger at its cold end.

[0014] Preferably, the means for sending a mixture to cool from a first temperature in the heat exchanger are connected to the hot end of the heat exchanger,

[0015] The apparatus may include means for sending a portion of the gas phase to the membrane separation unit without passing through the heat exchanger.

[0016] According to one object of the invention, there is provided a method for separating a mixture containing hydrogen and carbon dioxide, as well as at least one compound lighter than carbon dioxide chosen from the list: carbon monoxide, methane, nitrogen, comprising the following steps: a) Cooling the mixture from a first temperature in a heat exchanger by sending the mixture to the heat exchanger, resulting in the partial condensation of the mixture into a liquid phase enriched in carbon dioxide and a gaseous phase depleted in carbon dioxide, the mixture leaving the heat exchanger at a second temperature lower than the first temperature, said heat exchanger being at least partially cooled by a gaseous fluid which heats up in the heat exchanger by indirect heat exchange. b) Separation of the liquid phase from the gaseous phase in one or more separator pots, possibly in several stages interspersed with successive cooling phases. c) Reheating of at least a portion of the gaseous phase coming from at least one of the separator pots in the heat exchanger by indirect heat exchange.d) Sending the at least one heated portion from step c) to a membrane separation unit, generating one or more permeates enriched in hydrogen and / or carbon dioxide and depleted in the at least one compound lighter than carbon dioxide, and at least one residue depleted in hydrogen and carbon dioxide and enriched in the at least one compound lighter than carbon dioxide, and e) Expansion of the at least one residue from a first pressure in one or more turbines producing an expanded fluid at a second pressure lower than the first pressure at a third temperature lower than the first temperature and preferably at the second temperature. f) The fluid at the second pressure produced during step e) constituting the gaseous fluid of step a) which is heated in the heat exchanger by indirect heat exchange.(g) the at least one residue is sent from the membrane separation unit to the turbine or turbines without passing through the heat exchanger.

[0017] According to other optional aspects: the first temperature is above 0°C, or even above 10°C. the second temperature is below 0°C, preferably below -30°C, or even below -40°C. the membrane separation unit operates at a temperature above 50°C and the at least one heated portion of step c) is heated downstream of the heat exchanger and upstream of the membrane separation unit in an auxiliary heat exchanger. the fluid expanded to the second pressure is sent to the heat exchanger without passing through the auxiliary heat exchanger.a portion of the gaseous phase produced in the at least one separator pot heats up in the heat exchanger and another portion of the gaseous phase produced in the at least one separator pot does not heat up in the heat exchanger, the two portions being mixed downstream of the heat exchanger, the ratio between the flow rates of the two portions being regulated in order to reach a target temperature after mixing the two portions. the mixture of the two portions is sent to the membrane separation unit. the mixture containing hydrogen and carbon dioxide is a waste gas from an adsorption separation process, for example pressure swing. step b) optionally followed by a distillation step produces a fluid containing at least 60 mol% of carbon dioxide, preferably at least 80 mol% of carbon dioxide, or even at least 90 mol% of carbon dioxide.the membrane separation unit produces: a hydrogen-enriched fluid which is recycled to the upstream adsorption process in order to increase its production or reduce its load in order to produce the same quantity of hydrogen and / or a fluid of intermediate composition, enriched in H 2 and CO 2 , separated in at least one of the separator pots and / or by distillation to recover the CO 2 and / or a fluid depleted in H 2 and CO 2 , returned to a hydrogen production unit, feeding the adsorption process, typically to be used as fuel.

[0018] According to another aspect of the invention, there is provided an apparatus for separating a mixture containing hydrogen and carbon dioxide, as well as at least one compound lighter than carbon dioxide chosen from the list: carbon monoxide, methane, nitrogen, comprising a heat exchanger, means for sending a mixture to cool from a first temperature in the heat exchanger by sending the mixture to the heat exchanger, resulting in the partial condensation of the mixture into a liquid phase enriched in carbon dioxide and a gaseous phase depleted in carbon dioxide, means for leaving the mixture from the heat exchanger at a second temperature lower than the first temperature, means for sending a gaseous fluid which heats up in the heat exchanger by indirect heat exchange,a partial condensation unit comprising one or more separator pots connected in series or in parallel to separate a liquid phase from a gaseous phase of the mixture leaving the exchanger, means for sending, at least a portion of the gas phase from at least one of the separator pots in the heat exchanger to be heated by indirect heat exchange, a membrane separation unit, means for sending the at least one heated portion of the gas phase to the membrane separation unit, means for removing one or more permeates enriched in hydrogen and / or carbon dioxide and depleted in at least one compound lighter than carbon dioxide from the membrane separation unit, means for removing at least one residue depleted in hydrogen and carbon dioxide and enriched in at least one compound lighter than carbon dioxide from the membrane separation unit, at least one turbine,means for sending at least a portion of the at least one residue to expand from a first pressure in the turbine(s) producing an expanded fluid at a second pressure lower than the first pressure at a third temperature lower than the first temperature and preferably at the second temperature and means for sending the fluid at the second pressure to the heat exchanger to be heated like the gaseous fluid which heats up in the heat exchanger by indirect heat exchange.

[0019] According to the invention, the cooled gas produced by the expansion by the at least one turbine of a residue from the permeation can be used to cool the mixture to be separated upstream of the low-temperature separation.

[0020] According to one version of the invention, this at least one turbine drives at least one brake compressor, supplied by a permeate from a membrane separation stage, which makes it possible to reduce the permeation pressure.

[0021] An advantage of the scheme according to this version is that it eliminates the need for an external heat source for heating the non-condensables upstream of the membranes, if the latter operate at a temperature requiring this heating. Indeed, the compression of the permeates causes a rise in temperature, which can be used in the upstream multi-fluid exchanger.

[0022] By providing more cold to the cryogenic part, the use of a turbine to expand the residue allows the surface area of the cryogenic exchanger to be reduced, as well as energy consumption. This recovery of cold is even mandatory, because the gas is used at least at room temperature for the regeneration of dryers before being returned for combustion.

[0023] However, this innovation is not without its drawbacks. Indeed, the recovery of the compression heat of the boosters is achieved at the cost of numerous control loops, with multiple bypasses redirecting the heat and cold around the exchanger and the machines to stabilize the system. This complex system is further destabilized by the temperature variations of the non-condensable gases coming from the cryogenic part, ultimately attributable to the temperature variations of the cooling water.

[0024] In addition, the use of rotating machines raises a reliability issue. In the event of failure or degraded operation of one of the machines, it is no longer possible to cool the combustible gas sufficiently to recover it in the cryogenic section. The reduced surface area of the exchanger quickly proves to be a hindrance, greatly limiting the capacity and performance of the unit.

[0025] According to a variant of the invention, it is possible to solve the two aforementioned problems, and is based on the following concept: not to use the frigories brought by the turbine in the cryogenic part as a reduction in exchange surface.

[0026] More specifically, this variant of the proposed invention consists of not heating all of the incondensables from the partial condensation in the cryogenic exchanger, and therefore of extracting at cryogenic temperature a quantity of incondensables corresponding at least to the cold provided by the turbined fluid.

[0027] The invention will be described in more detail with reference to the figure.

[0028] [ FIG.1 ] describes a separation method according to the invention.

[0029] A mixture 1 of the [ FIG.1] containing hydrogen and carbon dioxide, as well as at least one compound lighter than carbon dioxide chosen from the list: carbon monoxide, methane, nitrogen, is at a pressure of at least 5 bars and a first temperature above 0°C, or even above 10°C. Mixture 1 may be a hydrogen-depleted residual gas from an adsorption process, for example by PSA-type pressure swing.

[0030] The mixture 1 is cooled to a second temperature below 0°C, preferably below -30°C, or even below -40°C and partially condensed in a heat exchanger 7 by passing through the exchanger from one end to the other and the two-phase fluid formed is separated in a phase separator S forming a gas 5 and a liquid 3. The liquid 3 enriched in carbon dioxide is separated in a separation unit 15 at a temperature below -40°C by partial condensation and / or by distillation to form a carbon dioxide-rich product. Otherwise the liquid 3 may be the carbon dioxide-rich product of the separation. Here the partial condensation unit comprises only one phase separator but it will be understood that the unit may comprise several phase separators S in series or in parallel.

[0031] The gas 5 is heated in the heat exchanger 7, by indirect heat exchange with the mixture 1, up to a temperature above 0°C but preferably below 80°C, or even 50°C. The gas 9, possibly after a reheating step, is separated by permeation in a permeation unit M where at least one permeation step occurs producing at least one permeate 10 enriched in carbon dioxide and hydrogen compared to the gas 9 and depleted in at least one compound lighter than carbon dioxide compared to the gas 9 and at least one residue 11 depleted in carbon dioxide and hydrogen compared to the gas 9 and enriched in at least one compound lighter than carbon dioxide compared to the gas 9.

[0032] The membrane separation unit M can operate at a temperature above 50°C and the at least one heated part of step c) is heated downstream of the heat exchanger and upstream of the membrane separation unit in an auxiliary heat exchanger (not shown).

[0033] The residue is expanded in a turbine T or several turbines in series to reduce its pressure and lower its temperature to a temperature of at most 0°C. Without having been reheated, the expanded residue 13 is sent to the heat exchanger 7, or even the cold end of the heat exchanger 7 where it heats up from a third temperature, lower than the first temperature, or even the second temperature, by passing through the exchanger, for example from one end to the other, countercurrently with the mixture 1 by indirect heat exchange with it. The expanded and reheated flow 13 is low in H 2 and CO 2 , but can be returned to a hydrogen production unit, feeding the PSA, typically to be used as fuel.

[0034] If the auxiliary heat exchanger is present, the fluid 13 expanded to the second pressure is sent to the heat exchanger 7 without passing through the auxiliary heat exchanger.

[0035] According to this version of the invention, all of the gas 5 is sent to heat up in the heat exchanger 7 by passing through the exchanger from one end to the other, counter-currently with the mixture 1.

[0036] According to another variant, a portion 5A of the gas from the separator S is sent through a valve V and does not heat up in the heat exchanger 7, or only partially heats up in the heat exchanger 7, then being extracted at an intermediate level of the heat exchanger 7. The two gases, including the heated (or partially heated) gas 5 and the unheated gas 5A, are mixed downstream of the heat exchanger to form a gas 9 that is colder than the gas 5. The temperature of the gas 9 is varied by adjusting the relative flow rates of the gases 5, 5A with the valve V.

[0037] By deliberately rejecting the cold from part 5A, we partially deprive ourselves of the benefits of expansion in turbine T in terms of thermal integration. But this has several drawbacks: By not valuing the additional frigories as a reduction in the exchange surface, we keep an exchanger 7 which can possibly do without the cold coming from the turbine T or the turbines T in the event of failure. The unit is therefore more robust. Thanks to this rejection of cold towards the hot end of the section operating at low temperature, it is possible to regulate the temperature of the gas 9 which is returned to the membrane part M. In doing so, we can smooth the temperature variations induced by the variations of the fluid 1 which most often comes from a heat exchanger with an ambient fluid such as cooling water or air. As a result, the control loops around the turbine T operate at a fixed temperature, and therefore with much less variation. The system is therefore stabilized and reliable.This advantage is even more obvious if the membrane separation unit M comprises at least one permeate booster coupled to at least one residue turbine T.

Claims

1. Process for separating a mixture containing hydrogen and carbon dioxide, as well as at least one compound lighter than carbon dioxide selected from the group consisting of carbon monoxide, methane and nitrogen, comprising the following steps a) Cooling of the mixture (1) from a first temperature in a heat exchanger (7) by feeding the mixture to the heat exchanger, causing partial condensation of the mixture into a carbon dioxide-enriched liquid phase (3) and a carbon dioxide-depleted gaseous phase (5), the mixture leaving the heat exchanger at a second temperature lower than the first temperature, said heat exchanger being at least partially cooled by a gaseous fluid (13) which heats up in the heat exchanger by indirect heat exchange b) Separation of the liquid phase (3) from the gaseous phase (5) in one or more separator pots (S), possibly in several stages interspersed with successive cooling phases. c) Heating at least part of the gas phase (5) coming from at least one of the separator pots in the heat exchanger by indirect heat exchange. d) The at least one heated part (9) from step c) is sent to a membrane separation unit (M) as the sole feed flow to the membrane separation unit (M),generating one or more permeates (10) enriched in hydrogen and / or carbon dioxide and depleted in at least one compound lighter than carbon dioxide with respect to the heated part, and at least one residue (11) depleted in hydrogen and carbon dioxide and enriched in at least one compound lighter than carbon dioxide with respect to the heated part and characterised in that it comprises e) Expansion of at least one residue from a first pressure in one or more turbines (T) producing an expanded fluid (13) to a second pressure lower than the first pressure and to a third temperature lower than the first temperature and preferably at or below the second temperature, f) The fluid at the second pressure produced in step e) constitutes the gaseous fluid from step a), which heats up in the heat exchanger by indirect heat exchange. with the mixture and g) the at least one residue is sent from the membrane separation unit to the turbine or turbines without passing through the heat exchanger.

2. Process according to claim 1 in which the first temperature is greater than 0°C, or even greater than 10°C.

3. Process according to claim 1 or 2 in which the second temperature is below 0°C, preferably below -30°C or even below -40°C.

4. Process according to one of the preceding claims in which the membrane separation unit (M) operates at a temperature above 50°C and the at least one reheated part (5) of step c) is reheated downstream of the heat exchanger. and upstream of the membrane separation unit in an auxiliary heat exchanger.

5. Method according to claim 4 in which the fluid (13) expanded to the second pressure is sent to the heat exchanger (7) without passing through the auxiliary heat exchanger.

6. Process according to one of the preceding claims in which a part of the (5) of the gas phase produced in the at least one separator pot heats up in the heat exchanger and another part (5A) of the gas phase produced in the at least one separator pot does not heat up in the heat exchanger, the two parts being mixed downstream of the heat exchanger, the ratio between the flow rates of the two parts being regulated in order to reach a target temperature after mixing the two parts.

7. Process according to claim 6 in which the mixture of the two parts (5, 5A) is sent to the membrane separation unit (M).

8. Process according to one of the preceding claims in which the mixture (1) containing hydrogen and carbon dioxide is a waste gas from an adsorption separation process, for example with pressure swing adsorption.

9. Process according to claim 8 in which the membrane separation unit (M) produces a hydrogen-enriched fluid (10) which is recycled to the upstream adsorption process in order to increase its production or reduce its charge in order to produce the same quantity of hydrogen and / or a fluid of intermediate composition, enriched in H2 and CO2, separated in at least one of the separator pots and / or by distillation in order to recover the CO2 and / or a fluid (11) depleted in H2 and CO2, returned to a hydrogen production unit, supplying the adsorption process, typically for use as a fuel.

10. Process according to one of the preceding claims in which no part of the permeate or permeates is sent for separation in step b).

11. Process according to one of the preceding claims in which the residue is sent to the turbine without having passed through a combustion chamber.

12. Process according to one of the preceding claims in which the at least one part (5) of the gaseous phase does not cool in the heat exchanger (7) upstream of the membrane separation unit (M).

13. Apparatus for separating a mixture (1) containing hydrogen and carbon dioxide and at least one compound which is lighter than carbon dioxide. selected from the group consisting of carbon monoxide, methane and nitrogen, comprising a heat exchanger (7) having a cold end and a hot end, the cold end being adapted to operate at a colder temperature than the hot end, means for supplying a mixture to be cooled, characterised in that the heat exchanger (7) has a cold end and a hot end, the cold end being adapted to operate at a colder temperature than the hot end. from an initial temperature in the heat exchanger by sending the mixture to the heat exchanger, causing partial condensation of the mixture into a liquid phase enriched in carbon dioxide and a gaseous phase depleted in carbon dioxide, means for discharging the mixture from the heat exchanger at a second temperature lower than the first temperature, means for sending a gaseous fluid which heats up in the liquid phase to the heat exchanger, and means for discharging the gaseous fluid which heats up in the gaseous phase to the heat exchanger. the heat exchanger by indirect heat exchange with the mixture, a partial condensation unit comprising one or more separator pots (S) connected in series or in parallel to separate a liquid phase (3) from a gaseous phase (5) of the mixture leaving the exchanger, means for sending at least part of the gaseous phase coming from at least one of the separator pots into the exchanger to be heated by indirect heat exchange, a membrane separation unit (M), means for sending the at least one heated part (9) of the gaseous phase to the membrane separation unit, means for removing one or more permeates (10) enriched in hydrogen and / or carbon dioxide and depleted in the at least one compound lighter than carbon dioxide, means for sending the at least one heated part (9) of the gaseous phase to the membrane separation unit, means for removing one or more permeates (10) enriched in hydrogen and / or carbon dioxide and depleted in the at least one compound lighter than carbon dioxide from the membrane separation unit, means for removing at least one residue (11) depleted in hydrogen and carbon dioxide and enriched in at least one compound lighter than carbon dioxide from the membrane separation unit, at least one turbine (T), means for sending at least one part of the at least one residue expand from a first pressure in the turbine or turbines, without having passed through the heat exchanger, producing a fluid (13) expanded to a second pressure lower than the first pressure at a third temperature lower than the first temperature and preferably at the second temperature and means for sending the fluid to the second pressure to the heat exchanger in order to heat up in the same way as the gaseous fluid which heats up in the heat exchanger by indirect heat exchange, characterised in that the separator pot or pots (S) connected in series or in parallel are connected directly to the heat exchanger without passing through the membrane separation unit and the means for sending the fluid at the second pressure to the heat exchanger are connected directly to the heat exchanger without passing through the membrane separation unit to the heat exchanger are connected to introduce the fluid at the second pressure into the heat exchanger at its cold end.

14. Apparatus as claimed in claim 13 comprising means for sending part (5A) of the gas phase to the membrane separation unit (M) without passing through the heat exchanger (7).