Method and apparatus for producing a gas mixture

Abstract

The invention relates to a method for producing a gas mixture, wherein a feed gas (1) containing more than 10 mol% CO2 and at least one compound lighter than CO2 is cooled in a heat exchanger (E) and separated by partial condensation and / or distillation to generate a CO2-rich liquid (17), a secondary gas (19) is cooled in the heat exchanger and is mixed with the CO2-rich liquid to form a two-phase fluid (23), and the two-phase fluid is heated in the heat exchanger to form a gas mixture (25) of hydrogen and carbon dioxide which constitutes a synthesis gas.

Description

[0001] Description

[0002] Title of the invention: Method and apparatus for producing a gaseous mixture

[0003] The present invention relates to a method and apparatus for producing a gaseous mixture, for example a synthesis gas.

[0004] It concerns the production of a gaseous mixture comprising at least two components, for example, a synthesis gas which is a mixture of CO2 and hydrogen. The ratio between the two components varies depending on the final product to be formed by the reaction of the synthesis gas.

[0005] It concerns CO2 capture units by partial condensation and distillation producing CO2 intended for the synthesis of a product, for example methanol, methane or carbon monoxide.

[0006] One proposed way to utilize captured CO2, for example from large industrial emitters, is to use it as a feedstock for methanol synthesis, ideally using decarbonized hydrogen. This methanol can then be used as a raw material for industry or as an alternative fuel.

[0007] The synthesis reaction is based on the following reaction:

[0008] [CHEM1]

[0009] CO2 + 3H2 -^CH3OH +H2O

[0010] It is also possible to utilize the captured CO2 as a raw material for the synthesis of carbon monoxide with the following reaction:

[0011] [CHEM2]

[0012] C₂O + H₂ → CO₂ + H₂O

[0013] It is also possible to valorize the captured CO2 as a raw material for the synthesis of methane with a methanation reaction (Sabatier reaction): [CHEM3]

[0014] CO2+4H2^CH4+2H2O

[0015] The methanol production reaction takes place at relatively high pressure, typically between 80 and 90 bara. [FIG. 1] shows a methanol synthesis gas production process in which hydrogen is produced by water electrolysis (E) and CO2 is produced by partial condensation and distillation (CC). The CO2 is taken as tank liquid from the distillation column and vaporized at at least two different pressures, lower than the pressure required for the reaction. Therefore, it is necessary to compress the CO2 and hydrogen separately in a compressor (VCO2) before entering the reaction loop (S) that produces methanol (MEOH). This is because cryogenic CO2 capture units produce CO2 at relatively low pressures, typically 5 and 10 bara. The hydrogen is compressed in a hydrogen compressor (VH2) upstream of the loop (S) before being mixed with the CO2.

[0016] The described invention aims to reduce the compression work for this case by integrating the reactive hydrogen into the CO2 capture unit.

[0017] A CO2 capture unit by partial condensation and distillation processes a gas rich in CO2 (>50mol%, ideally >65mol% or even >80mol%), also containing light gases (typically H2, CO, CH4, N2, Ar, O2).

[0018] The gas undergoes partial condensation at high pressure (25-60 bar) in a heat exchanger, generating a liquid rich in CO2 (>95 mol%). This liquid is then purified of its light impurities in a distillation column. Typically, the liquid obtained at the bottom of the column is then vaporized at various pressures to generate the cold necessary for further partial condensation.

[0019] The invention aims to modify the process by incorporating the following steps:

[0020] 1. The secondary gas (for example the hydrogen required for the reaction described above), for example available at high pressure (80-90 bara) and at a temperature above -40°C, or even at 0°C, is cooled in the heat exchanger.

[0021] 2. The purified CO2 available at the bottom of the distillation column is pumped up to the pressure of hydrogen, then mixed with the latter, generating a two-phase mixture.

[0022] 3. The liquid in the two-phase mixture is vaporized in the heat exchanger, providing the cooling necessary for partial condensation. The advantage of this invention lies in the fact that instead of being vaporized at 5 or 10 bar, the CO2 is vaporized at a higher pressure, even at the pressure required for the reaction to synthesize methanol. This allows for a reduction in the size of the CO2 compressor, or even eliminates the need for one altogether. This also results in reduced energy consumption, since pressurizing a liquid consumes less energy than compressing an equivalent gas.

[0023] It is known from EP2692411 and EP2656898 to pressurize a CO2-rich liquid flow from a partial condensation or distillation separation process and vaporize the pressurized flow in a heat exchanger against the flow to be separated, which cools there. No flow containing at least 90 mol% hydrogen is present.

[0024] The presence of hydrogen lowers the partial pressure of the mixture and allows for non-isothermal vaporization, with a large temperature glide that follows the condensation curve of the feed gas relatively well.

[0025] [Fig. 3] shows the heat exchange diagram of the apparatus according to the invention (solid lines), compared to the conventional diagram (dashed lines), with the heat exchanged H on the ordinate and the temperature T on the abscissa. The vaporization of the H2 / CO2 mixture allows for very satisfactory thermal integration in a single vaporization step.

[0026] The two-phase mixture can be introduced into the exchanger via a dedicated device, or directly without any special equipment.

[0027] Depending on the CO2 content of the gas to be purified, the process may or may not be autothermal. If it is not, all or part of the high-pressure non-condensables can be expanded in a turbine to generate the necessary additional cooling. Additional cooling can also be provided by an external mechanical refrigeration unit or by the cooling from the vaporization of a liquid external to the process. The invention has been described in the example where the mixture formed is a mixture of hydrogen and CO2. However, the invention applies to other cases where a secondary gas, other than hydrogen, must be mixed with CO2 to form a gaseous mixture, preferably under pressure. For example, the secondary gas may comprise at least one gas selected from the group: oxygen, nitrogen, hydrogen, helium, argon, carbon monoxide, carbon dioxide, methane, ethane. Thus, the gaseous mixture formed would be a mixture of CO2 and at least one of these gases.

[0028] One application would be to form a gaseous mixture for enhanced oil recovery or inerting from a flow of nitrogen gas, which would be cooled in the heat exchanger, mixed with liquid CO2 to form a larger flow of inert gas.

[0029] Another application would be to form synthetic air consisting of a mixture of oxygen and CO2.

[0030] According to one aspect of the invention, a process for producing a synthesis gas is provided in which: a) a feed gas containing more than 10% mol CO2 typically more than 50 mol% CO2, preferably more than 65% or even more than 80 mol% CO2 as well as at least one compound lighter than CO2 chosen from the group H2, CO, CH4, N2, Ar and O2 is cooled in a heat exchanger and separated by partial condensation and / or distillation generating a liquid rich in CO2, typically greater than 95 mol% CO2, and a gas poor in CO2 b) a secondary gas is cooled in the heat exchanger and is mixed with the liquid rich in CO2 forming a two-phase fluid c) the two-phase fluid is heated in the heat exchanger forming a gaseous mixture of secondary gas, for example hydrogen, and carbon dioxide and d) the gaseous mixture optionally constitutes a synthesis gas.

[0031] According to other optional aspects:

[0032] • The CO2-rich liquid is pressurized to the pressure of the gas containing at least 90 mol% hydrogen, cooled in the heat exchanger, before being mixed with the secondary gas, for example, a gas containing at least 90 mol% hydrogen, also cooled in the heat exchanger. • The secondary gas, for example, a gas containing at least 90 mol% hydrogen, enters the heat exchanger at a pressure equal to or greater than 20 bar, preferably greater than or equal to 40 bar.

[0033] • the secondary gas, for example containing at least 90% mol of hydrogen, enters the heat exchanger at a pressure equal to or greater than 80 bars, preferably less than or equal to 90 bars.

[0034] • the feed gas is separated by distillation in a column system comprising at least one distillation column generating a CO2-rich liquid in the distillation column tank.

[0035] • Part of the liquid produced in the column tank is vaporized in the heat exchanger, without having been pressurized, and returned to the column in gaseous form.

[0036] • Part of the liquid produced in the column tank is vaporized, without having been pressurized, by heat exchange with the secondary gas, for example containing at least 90% mol of hydrogen, cooled in the heat exchanger and returned to the column in gaseous form either in the heat exchanger or in another dedicated heat exchanger.

[0037] • the synthesis gas is at least a methanol synthesis gas and is sent to a methanol synthesis unit, preferably without having been compressed downstream of the heat exchanger.

[0038] • Synthesis gas is at least a synthesis gas from a methanation process and is sent to a methane synthesis unit, preferably without having been compressed downstream of the heat exchanger.

[0039] • Synthesis gas is at least a synthesis gas from a reverse water gas reaction and is sent to a carbon monoxide synthesis unit, preferably without having been compressed downstream of the heat exchanger.

[0040] • all or part of a CO2-depleted gas generated by partial condensation and / or distillation is heated in the heat exchanger, expanded in a turbine and heated again in the heat exchanger.

[0041] • All or part of the gas containing at least 90 mol% hydrogen is cooled by a mechanical refrigeration unit external to the process. • Hydrogen and / or carbon dioxide is added to the gas mixture downstream of the heat exchanger.

[0042] • The gas mixture contains hydrogen and CO2 in a 1:1 ratio at the outlet of the heat exchanger

[0043] • The gas mixture contains hydrogen and CO2 in a 3:1 ratio at the outlet of the heat exchanger

[0044] • The gas mixture contains hydrogen and CO2 in a 4:1 ratio at the outlet of the heat exchanger

[0045] • The gas mixture contains hydrogen and CO2 in a 1:1 ratio once mixed with hydrogen and / or CO2 downstream of the heat exchanger

[0046] • The gas mixture contains hydrogen and CO2 in a 3:1 ratio once mixed with hydrogen and / or CO2 downstream of the heat exchanger

[0047] • The gas mixture contains hydrogen and CO2 in a 4:1 ratio once mixed with hydrogen and / or CO2 downstream of the heat exchanger

[0048] • the gas containing at least 90 mol% hydrogen arrives in the heat exchanger at a temperature above 0°C

[0049] • The gas containing at least 90 mol% hydrogen enters the heat exchanger at a temperature equal to or lower than 0°C

[0050] • The gas, containing at least 90 mol% hydrogen, enters the heat exchanger at a temperature of 0°C or lower, coming from a cryogenic distillation unit for the separation of carbon monoxide and hydrogen, from which it was extracted at a temperature of 0°C or lower.

[0051] • the gas containing at least 90% mol of hydrogen arrives in the heat exchanger at a temperature equal to or less than 0°C from a hydrogen liquefier from which it was extracted at a temperature equal to or less than 0°C.

[0052] • the secondary gas contains at least 90 mol% hydrogen.

[0053] • the secondary gas contains less than 5% mol, or even less than 1% mol of CO2.

[0054] • the secondary gas cools in the heat exchanger without condensing and mixes with the CO2-rich liquid while in gaseous form.

[0055] • The secondary gas that cools in the heat exchanger has a dew point below -50°C. • The secondary gas comprises at least one gas chosen from the group: oxygen, nitrogen, hydrogen, helium, argon, carbon monoxide, carbon dioxide, methane, ethane.

[0056] • the secondary gas includes at least one gas chosen from the group: oxygen, nitrogen, hydrogen, helium, argon, carbon monoxide, carbon dioxide, methane, ethane.

[0057] • The secondary gas consists mainly of nitrogen, and the gas mixture is enhanced oil recovery gas.

[0058] • the secondary gas includes only one gas chosen from the group: oxygen, nitrogen, hydrogen, helium, argon, carbon monoxide, carbon dioxide, methane, ethane.

[0059] • The secondary gas includes several gases, all without exception chosen from the group: oxygen, nitrogen, hydrogen, helium, argon, carbon monoxide, carbon dioxide, methane, ethane.

[0060] According to another object of the invention, an apparatus for producing a gaseous mixture, for example a synthesis gas, is provided, comprising a heat exchanger, a separation apparatus by partial condensation and / or distillation, means for sending a feed gas containing more than 10% mol CO2, typically more than 50 mol% CO2, preferably more than 65% or even more than 80 mol% CO2, as well as at least one compound lighter than CO2 selected from the group H2, CO, CH4, N2, Ar and O2, to be cooled in the heat exchanger and then separated by partial condensation and / or distillation in the separation apparatus generating a liquid rich in CO2, typically greater than 95 mol% CO2, and a gas poor in CO2, means for sending a secondary gas, for example containing at least 90% mol hydrogen, to be cooled in the heat exchanger and then mixed with the liquid rich in CO2 forming a two-phase fluid,means for sending the two-phase fluid to be heated in the heat exchanger, forming the gas mixture, the gas mixture possibly constituting a synthesis gas. Preferably, the apparatus includes a pump to compress the CO2-rich liquid upstream of the heat exchanger.

[0061] Preferably, the apparatus includes means for mixing the secondary gas, for example containing at least 90 mol% hydrogen, and the CO2-rich liquid downstream of the pump.

[0062] The invention will be described in more detail with reference to the figures where: [FIG.2] represents at least some features of a process according to the invention [FIG.4] represents at least some features of another process according to the invention.

[0063] In [FIG.2] in a process according to the invention, a feed flow 1 containing more than 10% mol CO2 typically more than 50 mol% CO2, preferably more than 65% or even more than 80 mol% CO2 as well as at least one compound lighter than CO2 chosen from the group H2, CO, CH4, N2, Ar and O2 is cooled in a heat exchanger E to an intermediate exchanger temperature, for example -35°C, where it partially condenses; the two-phase flow is separated in a phase separator S1, forming a CO2-depleted gas 3 and a CO2-enriched liquid 5. Gas 3 is cooled in the exchanger E to the cold end by partially condensing and then is separated in a second phase separator S2 at a cold temperature, for example -52.5°C forming a gas 9 depleted in CO2 containing between 15 and 25% mol CO2 and a liquid 10 enriched in CO2 compared to gas 3. Liquid 10 is mixed with liquid 5 forming a liquid 11 containing about 95% mol CO2.

[0064] Liquid 11 is expanded and sent to the top of a distillation column C as the sole feed stream, where it separates, forming a head gas containing approximately 65 mol% CO2 and a tank liquid containing more than 95 mol% CO2. The liquid is divided into two, forming liquid 15 and liquid 17. Liquid 15 is not pressurized, vaporizes in the heat exchanger E, which it enters at an intermediate temperature, and is returned as a gas to the tank of column C as a reboiling gas.

[0065] Liquid 17 is pressurized to a pressure greater than or equal to 20 bar, preferably greater than or equal to 40 bar, or even greater than or equal to 80 bar, possibly less than 90 bar by a pump P. The exchanger E is designed to withstand such a pressure.

[0066] A flow 19 of a secondary gas, for example a gas containing at least 90 mol% hydrogen from, for instance, a water electrolysis unit, enters the heat exchanger at its hot end and is cooled to an intermediate temperature while remaining gaseous (flow 21). Flow 21 mixes with the pressurized flow 17, forming a two-phase flow 23 which is heated from the cold end of the exchanger E, where the liquid portion vaporizes. The proportions of the flow rates 17 and 19 can be chosen so that the mixture 25 exiting the exchanger E contains 25% CO2 and 75% hydrogen, so that the mixture can serve as the methanol synthesis gas. Alternatively, it is also possible to add gaseous CO2 and / or gaseous hydrogen downstream of the exchanger E to obtain the desired percentages.

[0067] Preferably the pressures of the flow rates 17, 19 are chosen so that the mixture 25 exiting the exchanger E can be sent directly to a synthesis unit S, for example a methanol synthesis unit without having been pressurized downstream of the exchanger E. However, the presence of a compressor is not excluded.

[0068] Preferably the pump outlet pressure P is equal to the hydrogen pressure 21.

[0069] The secondary gas preferably contains less than 5% mol, or even less than 1% mol of CO2.

[0070] The secondary gas cools in the heat exchanger, preferably without condensing, and mixes with the CO2-rich liquid in its gaseous form. The secondary gas that cools in the heat exchanger preferably has a dew point below -50°C.

[0071] The secondary gas may include at least one gas chosen from the group: oxygen, nitrogen, hydrogen, helium, argon, carbon monoxide, carbon dioxide, methane, ethane.

[0072] Preferably, the secondary gas comprises only one gas chosen from the group: oxygen, nitrogen, hydrogen, helium, argon, carbon monoxide, carbon dioxide, methane, ethane. The secondary gas may comprise several gases, all chosen from the group: oxygen, nitrogen, hydrogen, helium, argon, carbon monoxide, carbon dioxide, methane, ethane.

[0073] The secondary gas may consist primarily of nitrogen. In this case, the gas mixture is an enhanced oil recovery gas.

[0074] The column reboiling is shown in Figure 2 as being integrated into the main exchange line.

[0075] However, this reboiling could be done in a separate heat exchanger R, as shown in Figure 4, for example using hydrogen-19 as the hot fluid. Thus, the hydrogen cooled in heat exchanger E and then in heat exchanger R is mixed with the liquid CO2 from pump P. The remainder of Figure 4 corresponds to Figure 2.

[0076] The process according to the invention may be advantageous given the high operating pressure of the heat exchanger E: limiting the number of fluids and heads, in particular the number of vaporizations, can simplify the thermomechanical design of the exchanger.

[0077] Depending on the CO2 content of the gas to be purified, the process may or may not require additional cooling. If cooling is required, all or part of the high-pressure non-condensables 9, 13 can be heated in the heat exchanger, expanded in a turbine, and reheated in the heat exchanger to generate the necessary additional cooling. Additional cooling can also be provided by an external mechanical refrigeration unit or by the cooling from the vaporization of a liquid external to the process.

[0078] The invention has been described in the context of the production of a methanol synthesis gas.

[0079] It will be understood that by varying the ratios of hydrogen to CO2, it is possible to form a methanation synthesis gas with a 4:1 ratio or a reverse water-gas shift reaction (RWSD) synthesis gas. The methanation reaction takes place at pressures between 5 and 30 bar, or even between 15 and 30 bar, or up to 50 bar. Therefore, it will be necessary to mix pressurized CO2 and hydrogen at a pressure equal to or lower than the pressure of the methanation synthesis gas.

[0080] The reverse reaction of gas to water takes place at a pressure between 1 and 50 bars abs, preferably between 12 and 50 bars abs.

[0081] It is possible to produce a mixture of hydrogen and CO2 which does not exactly match the desired proportions, not containing enough hydrogen, and to add hydrogen to the mixture downstream of the heat exchanger to obtain the desired proportions.

[0082] It is also possible to produce a mixture with the desired proportions for one type of synthesis and to add hydrogen downstream of the exchanger to obtain a synthesis gas for another type of synthesis, in addition to or instead of the other gas.

[0083] In all cases, heat generated by the reaction of the synthesis gas to form methanol, methane or carbon monoxide can be used to preheat water for electrolysis.

[0084] For example, one could produce methanol synthesis gas according to the invention and add hydrogen downstream of the heat exchanger to obtain in addition (or instead) a methanation synthesis gas.

[0085] It is possible to take hydrogen for Figures 2 and 4 at a temperature below ambient, for example by taking hydrogen at a temperature below 0°C taken from a unit separating a mixture of carbon monoxide and hydrogen by cryogenic distillation or from a hydrogen liquefier.

Claims

Demands 1. A process for producing a gaseous mixture, for example a synthesis gas, in which: a) a feed gas (1) containing more than 10 mol% CO2, typically more than 50 mol% CO2, preferably more than 65 mol% or even more than 80 mol% CO2, and at least one component lighter than CO2 selected from the group H2, CO, CH4, N2, Ar, and O2, is cooled in a heat exchanger (E) and separated by partial condensation and / or distillation, generating a liquid (17) rich in CO2, typically greater than 95 mol% CO2, and a gas (13) poor in CO2; b) a secondary gas (19) is cooled in the heat exchanger and mixed with the CO2-rich liquid, forming a two-phase fluid (23); c) the two-phase fluid is heated in the heat exchanger, forming a gaseous mixture (25) of the secondary gas and carbon dioxide; and d) the mixture gaseous constitutes a synthesis gas.

2. A method according to claim 1 in which the CO2-rich liquid is pressurized to the pressure of the secondary gas cooled in the heat exchanger (E) before being mixed with the secondary gas (21) cooled in the heat exchanger.

3. Method according to claim 1 or 2 wherein the secondary gas (19) enters the heat exchanger at a pressure equal to or greater than 20 bar, preferably greater than or equal to 40 bar.

4. Method according to claim 3 wherein the secondary gas (19) enters the heat exchanger (E) at a pressure equal to or greater than 80 bar, preferably less than or equal to 90 bar.

5. A method according to any one of the preceding claims wherein the feed gas (1) is separated by distillation in a column system comprising at least one distillation column (C) generating a CO2-rich liquid in the distillation column tank.

6. A method according to claim 5 in which part of the liquid produced in the column tank (15) is vaporized in the heat exchanger (E), without having been pressurized, and returned to the column in gaseous form.

7. A process according to claim 5 or 6 in which part of the liquid produced in the column tank is vaporized, without having been pressurized, by heat exchange with the secondary gas cooled in the heat exchanger and returned to the column (C) in gaseous form either in the heat exchanger (E) or in another dedicated heat exchanger (R).

8. A method according to any one of the preceding claims wherein the synthesis gas (25) is at least a methanol synthesis gas and is sent to a methanol synthesis unit (S), preferably without having been compressed downstream of the heat exchanger.

9. A process according to any one of the preceding claims wherein the synthesis gas is at least one synthesis gas (25) from a methanation process and is sent to a methane synthesis unit, preferably without having been compressed downstream of the heat exchanger.

10. A method according to any one of the preceding claims wherein the synthesis gas (25) is at least a synthesis gas from a reverse water gas reaction and is sent to a carbon monoxide synthesis unit, preferably without having been compressed downstream of the heat exchanger.

11. A process according to any one of the preceding claims in which the secondary gas contains at least 90 mol% hydrogen.

12. A process according to any one of the preceding claims wherein the secondary gas contains less than 5 mol%, or even less than 1 mol%, of CO2.

13. A method according to any one of the preceding claims in which the secondary gas cools in the heat exchanger without condensing and mixes with the CO2-rich liquid while in gaseous form.

14. A method according to any one of the preceding claims wherein the secondary gas which cools in the heat exchanger has a dew point below -50°C.

15. Apparatus for producing a gaseous mixture comprising a heat exchanger (E), a separation apparatus by partial condensation and / or distillation, means for supplying a feed gas containing more than 10% mol CO2, typically more than 50 mol% CO2, preferably more than 65% or even more than 80 mol% of CO2 and at least one compound lighter than CO2 chosen from the group H2, CO, CH4, N2, Ar and O2 to be cooled in the heat exchanger and then separated by partial condensation and / or distillation in the separation apparatus generating a CO2-rich liquid, typically greater than 95 mol% CO2, and a CO2-poor gas, means for sending a secondary gas (18) to be cooled in the heat exchanger and then mixed with the CO2-rich liquid forming a two-phase fluid, means for sending the two-phase fluid to be heated in the heat exchanger forming the gas mixture, the gas mixture possibly constituting a synthesis gas.

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