Method and apparatus for the cryogenic separation of hydrogen and carbon dioxide

Abstract

Method and apparatus for the cryogenic separation of mixtures containing hydrogen and CO2 wherein the CO2 is condensed in the tube bundle of a kettle reboiler type heat exchanger and then separated from the gas phase rich in hydrogen. Condensation is obtained by vaporization through expansion of said CO2 which is recycled to the same heat exchanger outside the tube bundle. The cold vapor phases of CO2 and hydrogen are used to pre-cool the raw material to be treated. Pressure swing adsorption systems are used to treat the heated vapor phase of hydrogen and maximize recovery of CO2 and energy efficiency (Fig. 1).

Description

[0001] METHOD AND APPARATUS FOR THE CRYOGENIC SEPARATION OF HYDROGEN AND CARBON DIOXIDE DESCRIPTION

[0002] The present invention relates to a method and to an apparatus for the cryogenic separation of hydrogen and carbon dioxide from mixtures containing them, in particular deriving from various industrial processes.

[0003] Background of the invention

[0004] Considerable amounts of carbon dioxide and hydrogen are contained in mixtures obtained from various industrial processes, for example in the synthesis gases or in gaseous streams emitted within the context of petrochemical processes, also - but not only - in the oil and natural gas extraction sector.

[0005] On the other hand, there is great interest in the elimination or reduction of CO2 emissions into the atmosphere, and in its capture and reuse in processes that produce it. Recovering and reusing the hydrogen contained in these gaseous streams is also important and interesting.

[0006] The possibility of separating hydrogen from carbon dioxide by cooling and liquefaction of the latter with suitable cryogenic systems is also known. These systems mainly derive from the Linde-Hampson cycle, which is the simplest thermodynamic cycle for the liquefaction of cryogenic gases.

[0007] State of the art

[0008] Many modifications and variations of the Linde-Hampson cycle are described in the scientific and patent literature, also for the cryogenic separation of CO2 and hydrogen.

[0009] WO 2022 / 184646 Al describes a process for the liquefaction of a gas containing at least 95% of CO2, which is liquefied. Only a part of the liquefied CO2 is then expanded and vaporized in a heat exchanger.

[0010] WO 2023 / 117130 Al describes a process for separating carbon dioxide from a mixture also containing hydrogen, for example a mixture consisting of synthesis gas, with pressure swing absorption treatments (PSA) and final cryogenic separation of the CO2 after compression.

[0011] US 2011 / 271713 Al describes a method and an apparatus for the purification of carbon dioxide comprising contaminants such as oxygen and carbon monoxide. Operations of compression of the CO2, condensation and expansion are described, obtaining a vapor phase rich in contaminants separated from liquid CO2. The method does not involve separation and recovery of the CO2 still present in the vapor phase rich in contaminants, in fact limiting the maximum possible recovery of CO2 to what is permitted by the liquid / vapor balance. US 3,001,373 describes a method and an apparatus for producing hydrogen by selective absorption of carbon dioxide from mixtures that contain it, together with gaseous hydrocarbons. The use of systems that maximize recovery of the carbon dioxide and the purity of the hydrogen obtained is not described.

[0012] Consequently, there is the need to provide a method and an apparatus that allow effective separation of carbon dioxide from hydrogen, with carbon dioxide recovery even in excess of 99%, in mixtures containing them even with a CO2 content of less than 95% and also in the presence of other components, such as water and various impurities, for example consisting of sulfur compounds.

[0013] It would also be desirable to provide a method and an apparatus that allow energy consumption to be minimized by making the best use of the heat exchanges possible, in particular making the best use of the cold energy made available by expansion of cryogenic liquids, such as CO2. Summary of the invention

[0014] An aspect of the invention consists of a method for the cryogenic separation of hydrogen and carbon dioxide in a gaseous mixture containing them, characterized by comprising the steps of: a) compressing said mixture to a pressure value between 1 MPa and a value lower than 7.38 MPa, and not higher than the critical pressure of the mixture, obtaining condensation of all or part of any water present; b) purifying said mixture by eliminating or reducing condensable impurities, in particular water, to no more than 50 ppm vol; c) pre-cooling said purified mixture to a temperature T 1 between -30°C and 0°C, obtaining a gaseous mixture or a gas-liquid mixture; d) cooling said pre-cooled mixture to a temperature T2 between -55°C and -10°C, where T2 < Ti, obtaining condensation of at least a part of said carbon dioxide with the formation of a biphasic gas-liquid mixture, in which the liquid phase is rich in carbon dioxide and the gaseous phase is rich in hydrogen; e) separating said gaseous phase from said liquid phase; f) using said gaseous phase rich in hydrogen separated in said step e) as a refrigerant fluid to pre-cool said mixture purified in said step c); g) using all said liquid phase rich in carbon dioxide separated in said step e) as a refrigerant fluid to cool said mixture pre-cooled in said step d), wherein the cooling is obtained by expansion and vaporization of said liquid phase rich in carbon dioxide with formation of a biphasic liquid-vapor mixture; h) using said vapor phase rich in carbon dioxide obtained in said step g) as a refrigerant fluid to pre-cool said mixture purified in said step c); i) recovering said carbon dioxide from said step h) for its storage or use as final product; j) purifying said gaseous phase rich in hydrogen and poor in carbon dioxide obtained in said step f) by: a. removing carbon dioxide by pressure swing absorption treatment, obtaining a gaseous phase further enriched in hydrogen and a gaseous phase poor in hydrogen containing said carbon dioxide, wherein said gaseous phase poor in hydrogen containing said carbon dioxide is combined with said mixture to be treated in said step a); and b. removing further impurities from said gaseous phase further enriched in hydrogen in step j a) by further pressure swing absorption treatment, obtaining hydrogen with a degree of purity equal to or greater than 95 mol% and a gaseous phase poor in hydrogen containing said impurities; and c. recovering said gaseous phase rich in hydrogen from said step f) for the storage or use of said hydrogen as final product; wherein said step g) of cooling by expansion and vaporization of said liquid phase rich in carbon dioxide is carried out in a heat exchanger by making said pre-cooled mixture flow inside a tube bundle arranged inside a chamber into which said biphasic liquid-vapor mixture rich in carbon dioxide is injected, with said tube bundle immersed in the liquid phase, and wherein the condensable impurities not eliminated in said step b) separate from the vapor phase and collect in said liquid phase.

[0015] Another aspect of the invention relates to an apparatus for the cryogenic separation of hydrogen and carbon dioxide in mixtures containing them, comprising: compression means configured to compress gaseous mixtures containing one or more of hydrogen, carbon dioxide and mixtures thereof; purification means of said mixtures; a pre-cooling device consisting of a multi stream heat exchanger, configured to receive a mixture to be cooled and at least two separate streams of refrigerant fluids; a cooling and condensing device consisting of a heat exchanger comprising a tube bundle configured to receive a mixture to be cooled, arranged inside a chamber configured to receive an expanded and vaporized refrigerant fluid (kettle reboiler); a gas-liquid separator a tank configured to contain a pressurized liquid; means for the expansion of pressurized fluids; pressure swing absorption devices for the separation of carbon dioxide (37) and other impurities (39) from gaseous phases rich in hydrogen.

[0016] In the present description the term “carbon dioxide” and the formula “CO2” are used without distinction, as they define the same substance.

[0017] Moreover, in the present description the terms “vapor phase” and “gas phase” are used without distinction.

[0018] In the present description the terms “comprising” and “containing” are used without distinction, and their meaning does not exclude the presence of other elements, in addition to those defined after these terms. The term “consisting of’ is therefore also included within the scope of this meaning. The terms “comprising” and “containing” have a wider meaning than “consisting of’, but do not exclude it.

[0019] Brief description of the figures

[0020] The invention is described in detail below, also with reference to the non-limiting examples of embodiments illustrated in the accompanying figures, wherein:

[0021] - Fig. 1 is a schematic representation of the method and of the apparatus according to an embodiment of the invention; and

[0022] - Fig. 2 is a schematic representation of an embodiment of the cooling and condensing device according to the invention;

[0023] - the Fig. 3 is a schematic representation of the liquid and vapor phases in the cooling and condensing device of Fig. 2.

[0024] Detailed description of the invention

[0025] In light of the significant amounts of CO2 produced in various industrial gas streams, as mentioned above, there is the need to provide processes for the removal of CO2 and / or for its separation and purification, which are not weighed down by excessive costs for energy and equipment.

[0026] Climate change due to global warming is a threat to humanity and the release of a significant amount of carbon dioxide into the atmosphere due to human activities (industry, transport, homes, etc.) is the main cause of this. Therefore, the development of new and more efficient methods for capturing and sequestering or reusing the CO2 emissions from various industrial processes is of fundamental importance.

[0027] Combustible gases are often burned releasing CO2 into the atmosphere and known methods for separating CO2 from the gas streams are considered as having very high costs. For example, these known methods involve the absorption of CO2 by a chemical solvent, such as an amine solution, or a physical solvent, such as cold methanol, or separation by membrane diffusion or by adsorption on a solid adsorbent, such as a zeolite or activated carbon. In any case, these are methods with very high costs.

[0028] Consequently, it is necessary to provide CO2 removal systems at costs that can be easily integrated in existing processes, such as hydrogen generation, capable of recovering almost 100% of C02.

[0029] Therefore, the present invention relates to a method and an apparatus that allows hydrogen and carbon dioxide with a high degree of purity to be obtained from mixtures containing them, advantageously using a self-refrigeration system that uses as refrigerant fluids the carbon dioxide and hydrogen streams separated from their mixture.

[0030] Preferably, the mixtures containing CO2 and hydrogen treated with the method of the invention comprise CO2 in an amount from 40 to 90 mol% and hydrogen in an amount from 5 to 40 mol%. More preferably, these mixtures comprise CO2 in an amount from 60 to 90 mol% and hydrogen in an amount from 5 to 20 mol%.

[0031] Moreover, mixtures containing CO2 and hydrogen treated with the method of the invention can comprise from 0 to 20 mol% of water, preferably from 0 to 15 mol% of water; from 0 to 20 mol% of inert gases such as nitrogen and argon, preferably from 0 to 10 mol% of inert gases, more preferably from 0 to 5 mol% of inert gases; from 0 to 20 mol% of carbon monoxide, preferably from 0 to 5 mol% of carbon monoxide, and traces of sulfur compounds and / or other contaminants, where the term traces is meant as no more than 5 mol% of these compounds. The method of the invention comprises a series of steps consisting of unitary operations aimed at obtaining separation of carbon dioxide from hydrogen, and the use of the carbon dioxide and hydrogen streams thus separated as refrigerant fluids for pre-cooling and cooling the mixture forming the raw material to be treated, before these streams are optionally treated further to obtain substantially pure carbon dioxide and hydrogen, or in any case suitable for storage and final use.

[0032] These steps are described also with reference to Figs. 1 and 2, and are listed according to a logical sequence.

[0033] Step a)

[0034] With reference to Fig. 1, in step a) of the method according to the invention, according to the embodiment illustrated, a gaseous current 10, forming the mixture containing CO2 and hydrogen, is mixed with a recycle current 54, which will be described below, so as to form the raw material to be treated 12, fed to a compressor 14. The raw material 12 is compressed in a compressor 14 to a pressure between 1 MPa and 7.38 MPa, which is the critical pressure of CO2, and in any case not higher than the critical pressure of the mixture. In these conditions, the condensation of all, or almost all, of any water present in the mixture is obtained, which is then discharged through the line 16. The pressure value between 1 and 7.38 MPa is chosen according to the composition of the raw material to be treated and to the pressure to be given to the mixture 18 discharged from the compressor 14 downstream.

[0035] In the embodiment illustrated in Fig. 1, the mixture 18, which is heated to a temperature exceeding 60°C in the compression step, is cooled in a heat exchanger 20 by the circulation of cold water or another refrigerant medium, so as to lower the temperature below 60°C, preferably below 40°C and more preferably below 30 °C. The heat exchanger 20 can in practice consist of a plurality of heat exchangers.

[0036] The cooled mixture 22 is sent to a separator 24, in which the condensed water is separated and discharged to the bottom through the line 26. The vapor phase 28 is instead fed to a purification unit 30.

[0037] Step b)

[0038] Purification comprises drying treatments, hence removal of water residues still present in the gaseous mixture by adsorption with specific adsorbents, for example molecular sieves. Purification can also comprise the removal of contaminants, such as sulfur compounds, by adsorption on activated carbons, or the removal of water-soluble compounds by washing with water.

[0039] Purification - and in particular drying - in the unit 30 is carried out upstream of the steps, and of the apparatus, that comprise the cryogenic treatments for the separation of CO2 by condensation, in order to avoid the formation of ice while carrying out these treatments.

[0040] The apparatus for performing the cryogenic treatments can be contained is the “cold section” (or cold box), which is a thermally isolated space configured for the implementation of cryogenic processes, included in the part indicated by the dashed line with the reference number 11 in Fig. 1.

[0041] Therefore, in the present description, the term “cold section” or “cold box” designates a thermally isolated space designed to minimize heat losses into the environment.

[0042] The cold box acts as housing for various critical apparatus, including heat exchangers, separators, piping, and valves, as known to those skilled in the art.

[0043] According to an embodiment not shown in Fig. 1, the compressed mixture 18 is sent directly to the purification unit 30, in which the treatments described above are carried out.

[0044] Step c) The purified mixture 32 delivered from the purification unit 30 is introduced into the cold box 11 to be pre-cooled to a temperature Ti between -30°C and 0°C, preferably between -30°C and -10°C, more preferably between -30°C and -15°C, obtaining a gaseous mixture or a gas-liquid mixture, depending on the temperature acquired, where, according to the operating conditions, the liquid consists mainly of CO2 that starts to condense. Pre-cooling is carried out in the heat exchanger 13, which is a multistream type heat exchanger, for example made of braze welded aluminum, configured to receive the mixture to be cooled 32 and at least two separate streams of refrigerant fluids generated downstream of the heat exchanger 13, as will be described below. This configuration allows recovery of the cold energy from the cold vapors produced inside the cold box of the system.

[0045] Step d)

[0046] The pre-cooled mixture 34 delivered from the heat exchanger 13 is then cooled to a temperature T2 between -55°C and -10°C, preferably between -55°C -20°C, more preferably between -55°C -30°C, where T2 < Ti, obtaining condensation of at least a part of said carbon dioxide with the formation of a biphasic gas-liquid mixture 36, in which the liquid phase is rich in carbon dioxide and the gaseous phase is rich in hydrogen and poor in carbon dioxide. Cooling to the temperature T2 takes place in the heat exchanger 15, which is thus also a CO2 condenser. The temperature in the condenser is chosen, depending on the operating pressure, according to the degree of separation desired of the condensed CO2.

[0047] Step e)

[0048] The biphasic gas-liquid mixture 36 is sent to a separator 17 to separate the gaseous phase from the liquid phase. The separator 17 is a receptacle in the lower part of which the liquid phase 38 rich in CO2 is collected, while the vapor phase 40 poor in CO2 but rich in hydrogen passes through a filter element 19, which retains the drops of liquid entrained by the gaseous current, and exits from the top of the separator 17. The liquid phase contains at least 90 mol% of CO2, preferably at least the 95 mol% of CO2.

[0049] Step f)

[0050] The vapor phase 40 poor in CO2 but rich in hydrogen released from the separator 17 is used in step c) as refrigerant fluid to pre-cool the purified mixture 32 at the entrance to the cold box 11. Therefore, the vapor phase 40 is sent to the multi stream heat exchanger 13, where - due to heat exchange - it is heated to at least 20°C, preferably to at least 30°C, to maximize pre-cooling of the mixture 32.

[0051] Step g) All the liquid phase 38 rich in carbon dioxide separated in step e) is used as refrigerant fluid to further cool the pre-cooled mixture 34 and cause its partial condensation, as described in step d).

[0052] This cooling and condensation are obtained by expansion and vaporization of the liquid phase 38 rich in carbon dioxide. For this purpose, the liquid phase 38 is discharged from the separator 17 through the line 21 into a storage tank 23, and from here it is sent via the line 40 to an expansion valve VI, upstream of the heat exchanger-condenser 15. The expanded mixture 42 is then sent to the heat exchanger-condenser 15, schematically shown in Fig. 2.

[0053] With reference to Figs. 2 and 3, the heat exchanger-condenser 15 is of the “kettle reboiler” type, comprising a tube bundle 25 configured to receive the pre-cooled mixture 34 to be cooled and condensed, arranged inside a chamber 27. This is configured to receive an expanded refrigerant fluid 42 through the valve VI in the nozzle 29.

[0054] The expanded refrigerant fluid 42 consists of the expanded biphasic vapor-liquid mixture rich in carbon dioxide, with the tube bundle 25 immersed in the liquid phase, as indicated by the level L of the liquid in Figs. 2 and 3. The vapor phase is thus formed both by the vapor produced with expansion through the valve VI in the nozzle 29, designated with the arrow A in Fig. 3, and by a part of the liquid that vaporizes in the chamber 27, designated with the arrow B in Fig. 3. In fact, a part of the liquid phase vaporizes while it absorbs heat from the fluid to be cooled on the tube side and joins the vapor generated in expansion which is used as refrigerant in step c).

[0055] The biphasic gas-liquid mixture 36, where the liquid is rich in CO2, is delivered from the tube bundle 25, and a vapor phase 44, also rich in CO2 as obtained from the expansion and vaporization of the liquid phase rich in CO2, is delivered from the chamber 27.

[0056] The kettle reboiler heat-exchanger-condenser 15 allows precise control of the pressure, and hence also the temperature, of the refrigerant fluid 42 downstream of the expansion valve VI. The pressure in the chamber 27 is selected based on the temperature desired for the biphasic gas-liquid mixture 36 delivered from the tube bundle 25, and is in any case higher than the triple point of CO2, to prevent the formation of solids. This pressure is adjusted via the valve V3 downstream of the heat exchanger 15. The valve V3 acts on the line that transports the vapor phase 44.

[0057] The liquid phase 38 rich in carbon dioxide separated in step e), which is used as refrigerant fluid in the kettle reboiler heat-exchanger-condenser 15, can still contain some less volatile impurities of the carbon dioxide not separated and eliminated in the purification step b), consisting for example of sulfur compounds and nitrogen oxides. It has been found that with the use of the kettle reboiler heat-exchanger-condenser 15 it is possible to collect and concentrate these impurities in the liquid phase of the expanded biphasic vapor-liquid mixture, where the tube bundle 25 is immersed. This makes it possible to obtain a gaseous phase 44 rich in CO2 with a constant composition, not influenced by the systems and by the operations upstream of the heat exchanger-condenser 15.

[0058] By regulating the level of the liquid inside the chamber 27 of the heat exchanger 15, it is thus possible to control the variations in the content of concentrated condensable impurities in the liquid phase by discharging a part of the liquid through the nozzle 31.

[0059] Step h)

[0060] With reference to Fig. 1, also the vapor phase 44 rich in CO2, produced in step g), is used as refrigerant fluid to pre-cool the mixture 32 purified in step c), at the entrance to the cold box 11. Therefore, the vapor phase 44 is sent to the multi stream heat exchanger 13, where it is heated to at least 20°C, preferably to at least 30°C, to maximize pre-cooling of the mixture 32. Step i)

[0061] This step has the main aim of recovering the carbon dioxide vaporized in step h) and optionally compressing it to the pressure desired for its storage or use as final product.

[0062] Therefore, a heated phase rich in carbon dioxide 46 is delivered from the heat exchanger 13 and is sent to a CO2 compressor 35, to obtain the final pressure desired for the CO2, as a rule above 4 MPa. From the compressor 35, which is present in this embodiment but not indispensable, the compressed CO2 can be sent via the line 48 directly to the final user.

[0063] In a different embodiment, part of the compressed CO2 can be sent to the storage tank 23, via the line 50, in which the expansion valve V2 is located. The expanded CO2 52 is then introduced into the tank 23 at a low temperature caused by its expansion, increasing the amount of refrigerant fluid contained therein.

[0064] Recycling of the compressed CO2 to the storage tank 23 can be used both during normal operation of the system and also only in certain steps of management of the system, for example during start up or when an increase in capacity is required.

[0065] Step i)

[0066] The gaseous phase rich in hydrogen is purified by CO2 and H2 separation units, consisting of several stages of pressure swing absorption treatments (PSA) in series, suitable to reach the required purity of the product, and optionally recirculating the gaseous phase poor in hydrogen resulting from purification to step a).

[0067] As shown in Fig. 1, the phase 40 is treated in a unit 37 for removal of carbon dioxide residues. Removal of CO2 is implemented with the pressure swing absorption (PSA) method. This allows the purity of the gaseous phase rich in hydrogen to be increased and simultaneously separation of the CO2 from the initial mixture to be maximized, recovering even over 99%.

[0068] The CO2 obtained recycled with the line 54 is combined with the gaseous flow 10 to form the raw material 12 sent to the compressor 14. Recycling of this current rich in CO2 allows the concentration of CO2 in the raw material 12 to be increased with respect to the gaseous flow 10, thus facilitating separation by condensation of said CO2 in step e).

[0069] The gaseous phase 56 impoverished in CO2 and enriched in hydrogen is sent to a hydrogen separation unit 39, which allows highly pure hydrogen to be obtained, for example over 95 mol%, through the removal of other impurities. Removal of the impurities is carried out with the pressure swing absorption (PSA) method. The separated impurities in gaseous form 58 can be eliminated from the system or combined, via the line 60, with the recycling line 54 to the compressor 14. The highly pure hydrogen is sent via the line 62 for storage or final use.

[0070] The apparatus according to the invention has been described within the description of the method for cryogenic separation of hydrogen and carbon dioxide in mixtures containing them. In brief, the main components are the following: compression means configured to compress gaseous mixtures containing hydrogen, carbon dioxide and mixtures thereof, which in an embodiment consist of the compressors 14 and 35; purification means of said mixtures, which in an embodiment consist of the units 30, 37 and 39; a pre-cooling device, which in an embodiment consists of the multistream heat exchanger 13, configured to receive a mixture to be cooled and at least two separate streams of refrigerant fluids; a cooling and condensation device 15, which in an embodiment consists of a heat exchanger comprising a tube bundle 25 configured to receive a mixture to be cooled, arranged inside a chamber 27 configured to receive an expanded and vaporized refrigerant fluid (kettle reboiler); a gas-liquid separator 17; a tank 23 configured to contain a pressurized liquid; means for the expansion of pressurized fluids, which in an embodiment consist of expansion valves VI, V2 and V3; pressure swing absorption devices (37) for the separation of carbon dioxide and other impurities from gaseous phases rich in hydrogen. In a preferred embodiment, the gas-liquid separator 17 is provided with a filter 19 to retain the drops of liquid entrained by the gaseous stream.

[0071] Moreover, the pre-cooling device (13), the kettle reboiler cooling and condensing device (15), the gas-liquid separator (17), the tank (23) and the means for expansion of pressurized fluids (VI, V2 and V3) can be positioned in a cold section, or cold box 11, which is a thermally isolated space configured for the implementation of cryogenic processes.

[0072] The purification means (30; 37; 39) of the mixtures treated in the method according to the invention comprise adsorption means, such as molecular sieves, activated carbons, solvents of water-soluble compounds, such as water, pressure swing absorbers (PSA), molecular sieves.

[0073] The components of the apparatus having the aforesaid features allow the method to be implemented in an efficient and compact apparatus.

[0074] The method allows effective separation of CO2 and of hydrogen optimizing the use of the refrigerant fluids in order to reduce energy consumptions, while allowing flexible management of the method according to the features of the raw material treated and of the desired features of the final products, for example the purity and the pressure both of CO2 and of hydrogen.

Claims

CLAIMS1. Method for the cryogenic separation of hydrogen and carbon dioxide in a gaseous mixture containing them, characterized by comprising the steps of: a) compressing said mixture to a pressure value between 1 MPa and a value lower than 7.38 MPa, and not higher than the critical pressure of the mixture, obtaining condensation of all or part of any water present; b) purifying said mixture by eliminating or reducing condensable impurities to no more than 50 ppm vol; c) pre-cooling said purified mixture to a temperature Ti between -30°C and 0°C, preferably between -30°C and -10°C, more preferably between -30°C and -15°C, obtaining a gaseous mixture or a gas-liquid mixture; d) cooling said pre-cooled mixture to a temperature T2 between -55°C and -10°C, preferably between -55°C and -20°C, more preferably between -55°C and -30°C, where T2 < Ti, obtaining condensation of at least a part of said carbon dioxide with the formation of a biphasic gas-liquid mixture, in which the liquid phase is rich in carbon dioxide and the gaseous phase is rich in hydrogen and poor in carbon dioxide; e) separating said gaseous phase from said liquid phase; f) using said gaseous phase rich in hydrogen separated in said step e) as a refrigerant fluid to pre-cool said mixture purified in said step c); g) using all said liquid phase rich in carbon dioxide separated in said step e) as a refrigerant fluid to cool said mixture pre-cooled in said step d), wherein cooling is obtained by expansion and vaporization of said liquid phase rich in carbon dioxide, with formation of a biphasic liquid-vapor mixture; h) using said vapor phase rich in carbon dioxide obtained in said step g) as a refrigerant fluid to pre-cool said mixture purified in said step c); i) recovering said vaporized carbon dioxide from said step h) for its storage or use as final product; j) purifying said gaseous phase rich in hydrogen and poor in carbon dioxide obtained in said step f) by: a. removing carbon dioxide by pressure swing absorption treatment, obtaining a gaseous phase further enriched in hydrogen and a gaseous phase poor in hydrogen containing said carbon dioxide, wherein said gaseous phase poor in hydrogen containing said carbon dioxide is combined with said mixture to be treated in said step a); andb. removing further impurities from said gaseous phase further enriched in hydrogen in step ja) by further pressure swing absorption treatment, obtaining hydrogen with a degree of purity equal to or greater than 95 mol% and a gaseous phase poor in hydrogen containing said impurities; and c. recovering said gaseous phase rich in hydrogen from said step jb) for storage or use of said hydrogen as final product; wherein said step g) of cooling by expansion and vaporization of said liquid phase rich in carbon dioxide is carried out in a heat exchanger by making said pre-cooled mixture flow inside a tube bundle arranged inside a chamber into which said biphasic liquid-vapor mixture rich in carbon dioxide is injected, with said tube bundle immersed in the liquid phase, and wherein the condensable impurities not eliminated in said step b) separate from the vapor phase and collect in said liquid phase.

2. Method according to claim 1, characterized in that said mixture containing CO2 and hydrogen treated in said step a) comprises CO2 in an amount from 40 to 90 mol% and hydrogen in an amount from 5 to 40 mol%, preferably comprises CO2 in an amount from 60 to 90 mol% and hydrogen in an amount from 5 to 20 mol%.

3. Method according to claim 1 or 2, characterized in that said mixture containing CO2 and hydrogen treated in said step a) comprises from 0 to 20 mol% of water, preferably from 0 to 15 mol% of water.

4. Method according to one or more of claims 1 to 3, characterized in that said mixture containing CO2 and hydrogen treated in said step a) comprises from 0 to 20 mol% of inert gases, preferably from 0 to 10 mol% of inert gases, more preferably from 0 to 5 mol% of inert gases.

5. Method according to one or more of claims 1 to 4, characterized in that said mixture containing CO2 and hydrogen treated in said step a) comprises from 0 to 20 mol% of carbon monoxide, preferably from 0 to 5 mol% of carbon monoxide.

6. Method according to one or more of claims 1 to 5, characterized in that said cooled mixture is sent to a separator in which the condensed water is separated, and in that the non-condensed vapor phase is instead sent to said purification step b).

7. Method according to one or more of claims 1 to 6, characterized in that in said step e) the separated liquid phase contains at least 90 mol% of CO2, preferably at least 95 mol% of CO2.

8. Method according to one or more of claims 1 to 7, characterized in that in said step f) the gaseous phase rich in hydrogen is sent to a heat exchanger in which said mixture purifiedin said step c) is pre-cooled and said gaseous phase rich in hydrogen is heated to at least 20°C, preferably to at least 30°C.

9. Method according to one or more of claims 1 to 8, characterized in that said liquid phase separated in said step e) containing at least 90 mol% of CO2 is accumulated in a storage tank before being expanded in said step g).

10. Method according to one or more of claims 1 to 9, characterized in that the pressure in said chamber of said heat exchanger where the vapor phase rich in carbon dioxide is injected in said step g) is higher than the triple point of CO2.

11. Method according to one or more of claims 1 to 10, characterized in that said gaseous phase poor in hydrogen containing said impurities obtained in said step jb) is combined with said mixture to be treated in said step a).

12. Apparatus for the cryogenic separation of hydrogen and carbon dioxide in mixtures containing them, comprising:- compression means (14;25) configured to compress gaseous mixtures containing one or more of hydrogen, carbon dioxide and mixtures thereof;- purification means (30; 27; 29) of said mixtures;- a pre-cooling device consisting of a multistream heat exchanger (13), configured to receive a mixture to be cooled and at least two separate streams of refrigerant fluids;- a cooling and condensing device (15) consisting of a heat exchanger comprising a tube bundle configured to receive a mixture to be cooled, arranged inside a chamber configured to receive an expanded and vaporized refrigerant fluid;- a gas-liquid separator (17);- a tank (23) configured to contain a pressurized liquid, connected to said gas-liquid separator (17) and to said cooling and condensing device (15);- means for the expansion (VI; V2; V3) of pressurized fluids;- pressure swing absorption devices for the separation of carbon dioxide (37) and other impurities (39) from gaseous phases rich in hydrogen.

13. Apparatus according to claim 12, characterized in that said means for the expansion of pressurized fluids comprise an expansion valve (VI) arranged between said tank (23) and said cooling and condensing device (15).

14. Apparatus according to claim 12 or 13, characterized in that said means for the expansion of pressurized fluids comprise an expansion valve (V3) located downstream of said cooling and condensing device (15).

15. Apparatus according to one or more of claims 12 - 14, characterized in that said purification means (30; 27; 29) of said mixtures comprise adsorption means, such as molecular sieves, activated carbons, solvents of water-soluble compounds, such as water, pressure swing absorbers (PSA), molecular sieves, membranes.

16. Apparatus according to one or more of claims 12 - 15, characterized in that said precooling device (13), said cooling and condensing device (15), said gas-liquid separator (17), said tank (23) and said means for the expansion of pressurized fluids (VI; V2; V3) are placed in a cold section (11), consisting of a thermally insulated space configured for the implementation of cryogenic processes.

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