NL1041358A

NL1041358A - Rapid conversion of biomass into char, low water content oil, aqueous acids and fuel gas.

Info

Publication number: NL1041358A
Application number: NL1041358A
Authority: NL
Inventors: Rob Vasbinder Ir
Original assignee: Nettenergy B V
Priority date: 2015-06-12
Filing date: 2015-06-12
Publication date: 2016-12-14
Also published as: NL1041358B1

Abstract

A system for converting biomass into char, fuel gas and multiple liquids comprises a reactor from which char is continuously removed, followed by a first separation unit (11), for separating solids and tar, a second separation unit (15), for separating an oil having a relatively low water content (<5% w/w), and a condensing unit (18), for condensing and separating an aqueous solution having a relative high acidity (pH <4). The produced fuel gas may be used for generating power for the system. Also a method is claimed for converting biomass into char, fuel gas and multiple liquids, which comprises the system previously described.

Description

Rapid conversion of biomass into char, low water content oil, aqueous acids and fuel gas

The present invention relates to a system and method for the rapid conversion of biomass, e.g. wood into valuable liquid products, e.g. bio-oil.

Numerous processes for the thermal conversion of biomass are known as reviewed e.g. by M. Garcia-Perez et al. in Methods for producing biofuels and biochar in Washington State. Part 1: Literature review of pyrolysis reactors. Washington State University, 2011.

Depending on the conditions used, pyrolysis converts biomass into a combustible gas and/or liquid and/or char by the action of heat, normally in the absence of oxygen. The production of charcoal is a typical example of a conventional pyrolysis process, which takes several hours at temperatures not exceeding 400°C to reach completion.

Following the oil crisis in 1973 -1974, the pyrolysis of wood at elevated temperatures and very short reaction times was found to produce organic vapours in high yields and char. In addition the organic vapours were found to be relatively reactive, and in order to prevent degradation and/or polymerisation thereof, these vapours have to be cooled or quenched relatively quickly after being released from the pyrolysis reactor. Upon cooling or quenching, the organic vapours partly condense into liquid oily products and pyrolysis gas.

In this description, the term "biomass" refers to biodegradable substances irrespective of their origin, including but not limited to wood chips, wood pellets, straw, seed husks and water treatment sludge; the rapid conversion of biomass is referred to as "flash pyrolysis"; the vapours and gases resulting therefrom are referred to as "fluids"; the essentially organic liquid fraction separated and collected from these fluids is referred to as "bio-oil"; the essentially aqueous liquid separated and collected from these fluids is referred to as "pyroligneous acid" or "wood vinegar"; and the carbonized solid residue is referred to as "biochar". US2011/0123407 discloses a system for thermally converting biomass into a vapour comprising a fluidized bed reactor using a solid heat carrier e.g. sand, and a condensing chamber where the vapour is rapidly quenched. In order to sustain the fluidized bed, this >13 5 8 system requires a considerable airstream, which also lifts the fine char produced in the reactor and therefore relatively large cyclones are needed for separating the char from the vapour. Such a system therefore has a relatively large footprint and requires a considerable capital investment.

Conventional flash pyrolysis processes yield bio-oils containing relatively high amounts of water and organic acids, resulting in a reduced energy density. Furthermore the high water content of the bio-oil from conventional pyrolysis may cause phase separation. The following table gives the typical composition of a bio-oil from conventional flash pyrolysis.

US4,344,770 discloses a method for thermally converting biomass to fuel oil comprising pyrolytic conversion in an agitated reactor, condensation, gravity separation into water and oil, and distillation of a water soluble oil. The water-soluble oil is combined with the gravity separated oil to lower the viscosity and increase the energy content thereof. The gravity separation requires relatively large amounts of water to be effective and the distillation step consumes a considerable amount of energy and may induce polymerisation of the bio-oil.

Bio-oil may be used as fuel for industrial boilers and static diesel engines. It may also be used as a valuable source for the production of organic intermediates, bio-aromatics, coatings, bitumen and flavours and fragrances. Pyro-ligneous acid may be used as biodegradable deicing salt, as a valuable source for the production of organic intermediates or the acidification of e.g. manure, as insecticide, as plant growth stimulant or as food preservative.

It is an object of the present invention therefore to provide a system and method for converting biomass to bio-oil having low water content (<5% w/w) and a substantially increased storage stability. It is also an object of the present invention to provide a system and method for converting biomass to bio-oil having a lower energy consumption and a smaller footprint than conventional pyrolysis processes. It is a further object of the present invention to provide a system and method for converting biomass to bio-oil having a lower capital investment than conventional pyrolysis processes.

To this end, the invention provides a system apparatus for converting biomass into a fluid by means of a gas stream, as defined in Claim 1.

In one embodiment, the invention provides a system for separating a fluid from the conversion of biomass, as defined in Claim 5.

In another embodiment, the invention provides a system for enhancing the conversion of biomass into fluids by means of an oxygen enriched airstream, as defined in Claim 13.

In one embodiment, the invention provides a method for converting biomass into fluids by means of a gas stream, as defined in Claim 17.

In one further embodiment, the invention provides a method for separating a fluid from the conversion of biomass, as defined in Claim 20.

In yet another embodiment, the invention provides a method for enhancing the conversion of biomass into fluids by means of an oxygen enriched airstream, as defined in Claim 23.

This system and this method result in a compact unit since the heat for converting the biomass is transferred directly without substantial system losses. Furthermore the system may be operated as an autonomous unit since the fuel gas produced from the biomass may be used in a generator in order to provide electrical power to the system of the invention. As a result the system can be provided as a mobile conversion unit, which is transported to a location where biomass is available in abundance.

The invention will be described in more detail below with reference to the figures that follow as examples. The figures are in no way intended to limit the scope of the invention, and only serve as an illustration thereof.

Fig. 1 diagrammatically shows an overview of a system for converting biomass into fluids by means of an airstream and separating the fluids according to an embodiment of the invention.

Fig. 2 diagrammatically shows an overview of a system for enhancing the conversion of biomass into fluids by means of an oxygen enriched airstream according to an embodiment of the invention.

The description of the invention below is made by way of example with reference to the conversion of wood chips into fluids. It is to be understood however, that the embodiments described, both with regard to the system and method, can likewise be used for the conversion of other types of biomass.

Fig. 1 diagrammatically shows an overview of a system for converting biomass into fluids by means of an airstream and separating said fluids according to an embodiment of the invention.

The system comprises a conversion unit 1 comprising a first heat transfer zone 1A, a second heat transfer zone IB, a drying zone 1C and a discharge zone 10. The conversion unit 1 is further provided with an inlet 5 for introducing biomass, an inlet 6 for introducing an airstream, an auger 8 for removing carbonized material, having an outlet 9 to a storage container (not shown) and an outlet 10 for releasing a fluid. The system further comprises a hopper 2, provided with an inlet 3 and an auger 4 which is connected to and in open or closed communication with inlet 5 of conversion unit 1.

The outlet 10 is connected to and in open communication with a first separation unit 11, which is provided with an inlet 12, a first outlet 13, and a second outlet 14, which is connected to and in fluid communication with a second separation unit 15.

The second separation unit 15 is further provided with means for exerting centrifugal forces on the introduced fluids (not shown), a first outlet 16 and a second outlet 17, which is connected to and in fluid communication with a condensing unit 18.

The condensing unit 18 is further provided with an inlet 19, means for introducing and releasing cooling media (not shown), a first outlet 20 and a second outlet 21, which is connected to and in fluid communication with a ventilator 22, having a ventilator outlet 23, which is connected to and in fluid communication with a third separation unit 24, which is provided with a first outlet 25, and a second outlet 26, which is connected to and in fluid communication with a gas burner or engine (not shown).

The rate determining parameters for the conversion of biomass in a conversion unit include without limitation temperature, pressure, concentration, residence time and exposed contact area. Biomass generally has a relatively low heat capacity; therefore to quickly heat same and accomplish a rapid conversion thereof, the size of the biomass particles preferably is between 1 and 50 mm, more preferably between 5 and 25 mm and most preferably between 8 and 10 mm.

The inlet 5 for introducing biomass is preferably arranged in a central location in the upper part of conversion unit and comprises a horizontal section, provided with transport means such as an auger, and a vertical section having an open end or chute which is exactly centered relative to the walls of conversion unit 1. Most preferably, said vertical section has an adjustable height relative to the bed of biomass deposited therefrom., in order to accomplish a homogeneous and perfect conical deposition of fresh biomass. The conical shape of the biomass bed also results in a lower flow resistance for any fluid introduced thereto than would be accomplished by a horizontal or sloped flat bed.

Furthermore, inlet 5 preferably is provided with locking means in order to prevent any inflow of ambient air into the upper part of conversion unit 1 where such inflow may result in the formation of explosive mixtures of fluids and ambient air.

Rapid conversion also requires an intimate contact between biomass particles and the medium transferring heat thereto. Ambient air is used as the primary source for the heat transfer medium and the homogeneous distribution thereof is accomplished by preferably arranging inlet 6 in a central location in the lower part of conversion unit 1. More preferably, inlet 6 comprises at least an air distributor having a horizontal section, and a vertical section provided with holes for distributing air to the biomass bed. The homogeneous distribution of air may require inlet 6 to comprise a number of such air distributors, which are preferably arranged and evenly distributed in a horizontal plane, thus creating an initially horizontal and ultimately upward airstream. Most preferably, the at least one air distributor guides at least part of the airstream directly adjacent the wall of conversion unit 1 in order to create a very effective conversion zone preventing any biomass from passing said zone prior to being completely converted. The conversion unit 1 may be provided with a heat exchanger (not shown) for preheating ambient air with heat from the pyrolysis process prior to introducing the air to the system. The use of preheated air will increase the biomass conversion capacity of conversion unit 1.

After complete conversion, the carbonized residue preferably is removed from discharge zone ID in such a way that the perfect conical shape accomplished by the homogeneous deposition of fresh biomass from inlet 5 is maintained. To that effect auger 8 and/or the geometry of discharge zone ID are appropriately adapted to effect an even removal of carbonized residue from the unit. The continuous removal of carbonized biomass from discharge zone ID maximizes the capacity of the unit 1 for the conversion of biomass.

In order to start conversion, the hopper 2 is filled with biomass e.g. wood chips. The auger 4 is started in order to load wood chips to the conversion unit 1 through inlet 5, until the wood chips have reached a desired level or height therein. The ventilator 22 is started and the wood chips in the first heat transfer zone 1A are ignited with external ignition means e.g. a gas torch (not shown).

The airstream, distributed from inlet 6, supports the at least partial combustion of carbonized biomass in the first heat transfer zone 1A by providing oxygen thereto and is thereby essentially depleted of oxygen and directly heated to a first temperature level, resulting in a mixed and heated first fluid, which subsequently serves as heat transfer medium for transferring heat to dry wood chips in the second heat transfer zone IB, located directly above the first heat transfer zone 1A.

The first fluid released from the first heat transfer zone 1A enters the second heat transfer zone IB and rapidly heats the dry wood chips, resulting in a rapid conversion of wood and the formation of a mixed second fluid having a second temperature level. The porous bed of wood chips allows the mixed second fluid to enter the drying zone 1C, located directly above the second heat transfer zone IB, and subsequently transfer heat to freshly deposited wood chips. The second fluid rapidly heats the fresh wood chips resulting in a rapid drying thereof and the formation of a mixed third fluid having a third temperature level, which is subsequently released from outlet 10.

The carbonization and at least partial combustion in the first heat transfer zone most preferably takes place at a temperature exceeding 600°C. The carbonized wood chips are discharged continuously and evenly from the discharge zone (ID) and collected as bio-char.

The third fluid released from outlet 10 of conversion unit 1 is introduced to the first separation unit 11. A first liquid, such as a water spray is simultaneously introduced to inlet 12, which liquid cools said fluids and gases to a first temperature level in order to remove solid and/or tar particles which may have formed during the preceding thermal conversion and which may have been entrained in the third fluid. The temperature of outlet 14 preferably is between 160 and 200°C in order to separate a first fraction, which comprises essentially tar and solids and collect same from outlet 13 and release a quenched fourth fluid from outlet 14 and introduce same at the bottom of the second separation unit 15, where the fourth fluid is subjected to centrifugal forces in order to essentially coalesce and separate at least part of the fourth fluid. The temperature of outlet 17 preferably is between 80 and 200°C. From outlet 16, a second fraction is separated and collected, which essentially comprises bio-oil having a relatively high energy content and a relatively low acidity and low water content (<5% w/w).

The second separation unit 15 may be provided as a single unit having a certain outlet temperature, thus providing a single fraction of bio-oil. Alternatively, separation unit 15 may comprise various different stages, each operating at a different temperature range or may be provided as multiple sequential units, which are connected to and in fluid communication with each other in series, each unit having a different outlet temperature, e.g. a first outlet temperature of 160°C, a second outlet temperature of 140°C, and a third outlet temperature of 120°C, in order to collect individual sub-fractions of bio-oil.

Preferably, the separation unit 15 is provided with at least one horizontal rotatable disk comprising at least one blade extending at least partly from the periphery towards the centre of the disk, whereby the intersection of blade and disk is at an angle with the disk radius of between 0 and 180 degrees, and extending at least partly from the disk in vertical direction, whereby the perpendicular line extending from the peripheral intersection of blade and disk is at an angle with the horizontal plane of between 0 and 90 degrees. The disk is rotated at such a rate that even very small oil droplets are accelerated in a horizontal direction and thus are forced to impact the wall of the separation unit 15. As a result, the oil droplets coalesce on said wall and by the action of gravity flow as a film in a downward direction where they are collected. The oil film continuously cleans the wall of separation unit 15 and prevents fouling. Furthermore, the second separation unit 15 is very compact and the second fraction, separated therein and collected from outlet 16, has a relatively stable water content irrespective of the water content in the wood chips introduced to the conversion unit 1.

After separation of the second fraction, the remaining fifth fluid is released from outlet 17 and introduced to the upper part of indirect condensing unit 18. At the same time a cooling medium is provided to indirect condensing unit 18 at such a rate that the fifth fluid is cooled to a second temperature level in order to condense a third fraction, which essentially comprises an aqueous solution having a relatively high acidity (pH <4) and a relatively low energy content, and collect same from outlet 20. After condensing said third fraction, the residual sixth fluid is released from outlet 21. The temperature of outlet 21 preferably is between 10 and 50°C.

The ventilator 22 provides the driving force for the fluid and gas stream through the system. Upon actuation, ventilator 22 reduces the pressure in outlet 21 and since this outlet is connected to and in fluid communication with condensing unit 18, second separation unit 15 and first separation unit 11, all the way through to conversion unit 1, said reduced pressure is ultimately communicated to inlet 6 of conversion unit 1. Thus, ventilator 22 provides the driving force for the introduction of ambient air to inlet 6, the upward airstream in conversion unit 1 and the resulting stream fluids through the system. The reduced system pressure also facilitates the evaporation of fluids and stabilizes the temperature ranges of the various system components.

The sixth fluid passes through ventilator 22 and is released from outlet 23 and introduced to a third separation unit 24, which has a higher pressure than system upstream ventilator 22. As a result, a fourth liquid fraction is separated and collected from outlet 25, which essentially comprises a bio-oil having a relatively high water content (z 20%). The residual fuel gas is released from outlet 26 to a gas burner or engine (not shown).

Since the residual gas, released from outlet 26 of separation unit 24, may have a useful energy content, it is very advantageous to use said gas, as fuel gas for e.g. an engine connected to a generator unit. It is furthermore very advantageous to directly connect such a generator, powered by an engine operated with residual gas from the conversion of biomass, to a compressor for compressing ambient air in order to drive same without transmission losses.

Fig. 2 shows an engine 30, which is connected to and in fluid communication with outlet 26 of separation unit 24. The engine is provided with outlet 31 and is connected to generator 33 through drive shaft 32, which is also in direct connection with compressor 35 through drive shaft 34. Compressor 35 has an inlet 36 and an outlet 37, which is connected to and in fluid communication with a membrane separation unit 38. The membrane separation unit 38 is provided with outlet 39 and outlet 40.

Residual fuel gas, released from outlet 26 of separation unit 24, is introduced to engine 30 where it is mixed with ambient air and combusted in order to provide the energy for driving generator 33, through drive shaft 32. The formed combustion gases are released from outlet 31. The generator 33 provides electrical power to the system of the invention. In this way the system of the invention may be operated as an autonomous unit, which solely relies on the supply of biomass.

At the same time, drive shaft 34, which is in direct connection with drive shaft 32, drives compressor 35. Ambient air is introduced to inlet 36 and compressed to a first pressure level e.g.10 bars before being released from outlet 37 and introduced into membrane separation unit 38, which separates the compressed air into a first gas fraction released from outlet 39, comprising essentially pure nitrogen at a second pressure level e.g. 9,7 bars, and a second gas fraction released from outlet 40, which essentially comprises ambient air enriched with oxygen at a third pressure level e.g. approximately ambient pressure. The oxygen content of the second gas fraction is between 28% and 40%.

It is particularly advantageous to introduce said second gas fraction into inlet 6 of conversion unit 1 since the capacity for converting biomass thereof will increase proportional to the oxygen content of said second fraction.

Obviously, the system according to the invention will preferably be provided with various sensors and controls e.g. pressure and temperature sensors and control valves in order to control and regulate the method according to the invention. Furthermore, the system according to the invention is preferably provided with a processor for automatic operation thereof. Such sensors, controls and processor are considered to be state of the art which is well-known to the skilled person and therefore require no further explanation. 413 5 8

Claims

CLAIMS:

1. System for converting biomass into fluids by means of a gas stream, the system comprising: - a first heat transfer zone (1A) for heating the gas stream by direct contact between the gas stream and carbonized biomass having a first temperature level, resulting in the at least partial combustion thereof and a first fluid having a first temperature level; - a second heat transfer zone (IB) for heating dried biomass by direct contact between the first fluid having a first temperature level and the dried biomass, resulting in the at least partial conversion thereof and the release of a second fluid having a second temperature level; - a drying zone (1C) for drying a fresh biomass by direct contact between the second fluid having a second temperature level and the fresh biomass, resulting in a dried biomass and the release of a third fluid having a third temperature level - a discharge zone (ID) for continuously removing carbonized biomass.
2. System according to Claim 1, characterized in that the first heat transfer zone (1A), the second heat transfer zone (IB), the drying zone (1C) and the discharge zone (ID) are all incorporated in a conversion unit (1), in which the first heat transfer zone (1A) comprises a layer of carbonized biomass (1A), the second heat transfer zone (IB) comprises a layer of dried biomass (IB), which layer during operation is arranged above the layer of carbonized biomass (1A), and the drying zone (1C) comprises a layer of fresh biomass (1C), which layer during operation is arranged above the layer of dried biomass (IB), the conversion unit (1) is further provided with an inlet (5) for introducing biomass, an inlet (6) for introducing a gas stream, an outlet (8) for removing carbonized material, and an outlet (10) for releasing a second fluid.
3. System according to Claim 1 or 2, characterized in that the system is provided with a heat exchanger for preheating the gas stream prior to introducing the gas stream to the first heat transfer zone (1A). 1 j r s
4. System according to one of the preceding claims, characterized in that the gas stream is ambient air.
5. System according to one of the preceding claims, the system also comprising: - a first separation unit (11), for separating solids and tar from the third fluid by direct contact between the fluid and a first liquid, resulting in a in fourth fluid having a fourth temperature level and a first fraction, comprising essentially tar and solids; - a second separation unit (15), for separating an oil from the fourth fluid by subjecting same to centrifugal forces, resulting in a fifth fluid having a fifth temperature level and a second fraction essentially comprising an oil; - a condensing unit (18), for condensing and separating residual liquids from the fifth fluid by direct contact between the fifth fluid and a second liquid and indirect contact with a cooling medium, resulting in a sixth fluid having a sixth temperature level and a third fraction, essentially comprising a mixture of water and acids.
6. System according to Claim 5, characterized in that the second separation unit (15) comprises at least one horizontal rotatable disk comprising at least one blade extending at least partly from the periphery towards the centre of the disk, whereby the intersection of blade and disk is at an angle with the disk radius of between 0 and 180 degrees, and extending at least partly from the disk in vertical direction, whereby the line extending from the peripheral intersection of blade and disk in a direction perpendicular to the blade is at an angle with the horizontal plane of between 0 and 90 degrees.
7. System according to Claim 5 or 6, characterized in that the second separation unit (15) comprises one or multiple sequential units wherein the outlet temperature of the last unit equals the fifth temperature level and the inlet temperature of the first unit equals the fourth temperature level in order to separate the second fraction in one or multiple sub-fractions.
8. System according to one of the claims 5 to 7, characterized in that the system comprises a ventilator (22) for creating a gas stream, the ventilator being located downstream relative to the condensing unit (18).
9. System according to one of the claims 5 to 8, characterized in that the system comprises a third separation unit (24) for separating residual liquids from the sixth fluid, resulting in a gas stream and a fourth fraction, essentially comprising a mixture of oil and water.
10. System according to one of the claims 5 to 9, characterized in that the first liquid is water.
11. System according to one of the claims 5 to 10, characterized in that the second liquid at least partly comprises the first fraction collected from the first separation unit (11).
12. System according to one of the claims 5 to 11, characterized in that the cooling medium is water or ambient air.
13. System according to one of the claims 9 to 12, the system also comprising: - an engine (30), for converting a fuel gas to mechanical power; • a generator (33), for converting the mechanical power provided by the engine (30) through a first drive shaft (32) into electrical power; - a compressor (35), for compressing ambient air to a first pressure level, using mechanical power provided by the engine (30) through a second drive shaft (34), which is directly connected to the first drive shaft (32); - a membrane separation unit (38), for separating the compressed air having a first pressure level into a first gas fraction, comprising essentially pure nitrogen at a second pressure level, and a second gas fraction, comprising ambient air having an increased oxygen level and a third pressure level.
14. System according to Claim 13, characterized in that the engine (30) is directly or indirectly connected to and in fluid communication with the third separation unit (24).
15. System according to Claim 13 or 14, characterized in that the membrane separation unit (32) is connected to and in fluid communication with the first heat transfer zone (1A).
16. System according to one of the claims 13 to 15, characterized in that the electrical power required to operate the system is provided by generator (27).
17. Method for converting biomass into fluids by means of a gas stream, the method comprising: - heating the gas stream by direct contact in a first heat transfer zone (1A) between the gas stream and carbonized biomass having a first temperature level, resulting in the at least partial combustion thereof and a first fluid having a first temperature level; - heating dried biomass by direct contact in a second heat transfer zone (IB) between the first fluid having a first temperature level and the dried biomass, resulting in the at least partial conversion thereof and the release of a second fluid having a second temperature level; - drying fresh biomass by direct contact in a drying zone (1C) between the second fluid having a second temperature level and the fresh biomass, resulting in a dried biomass and the release of a third fluid having a third temperature level - continuously removing carbonized biomass from a discharge zone (ID).
18. Method according to Claim 17, characterized in the fact that the gas stream is preheated in a heat exchanger prior to introducing the gas stream to the first heat transfer zone (1A).
19. Method according to Claim 17 or 18, characterized in that the particle size of the biomass is between 1 and 50 mm.
20. Method according to one of the claims 17 to 19, the method also comprising: - separating solids and tar from the third fluid by direct contact between the third fluid and a first liquid in a first separation unit (11), resulting in a in fourth fluid having a fourth temperature level and a first fraction comprising essentially tar and solids; - separating an oil from the fourth fluid by subjecting same to centrifugal forces in a second separation unit (15), resulting in a fifth fluid having a fifth temperature level and a second fraction essentially comprising an oil; - condensing and separating residual liquids from the fifth fluid by direct contact between the fifth fluid and a second liquid and indirect contact with a cooling medium in a condensing unit (18), resulting in a sixth fluid having a sixth temperature level and a third fraction, essentially comprising a mixture of water and acids.
21. Method according to Claim 20, the method also comprising separating residual liquids from the sixth fluid in a third separation unit (24), resulting in a gas stream and a fourth fraction, essentially comprising a mixture of oil and water.
22. Method according to Claim 20 or 21, characterized in that the outlet temperature of the second separation unit (15) is not less than 80eC.
23. Method according to any of the claims 20 to 22, the method also comprising: - converting a fuel gas to mechanical power by an engine (30); - converting the mechanical power provided by the engine (30) into electrical power by a generator (33) through a first drive shaft (32); - compressing ambient air to a first pressure level by a compressor (35), using mechanical power provided by the engine (30) through a second drive shaft (34), which is directly connected to the first drive shaft (32); • separating the compressed air having a first pressure level by a membrane separation unit (38) into a first gas fraction, comprising essentially pure nitrogen at a second pressure level, and a second gas fraction, comprising ambient air having an increased oxygen level and a third pressure level.
24. Method according to Claim 23, characterized in that the gas stream released from the third separation (24) is directly or indirectly introduced to the engine (30).
25. System according to Claim 24 or 25, characterized in that the second gas fraction released from the membrane separation unit (32) is introduced to the first heat transfer zone (1A).
26. Method according to one of the claims 23 to 25, wherein the oxygen content of the second gas fraction is between 28% and 40%. 4 13 5 8