Process and apparatus for distilling a mixture of carbon dioxide, carbon monoxide and nitrogen

Abstract

Title of the invention: Process and apparatus for distilling a mixture of carbon dioxide, carbon monoxide, and nitrogen. In a process for separating a gas containing carbon dioxide, carbon monoxide, water, and nitrogen, the gas (1) is separated (5) by adsorption to produce a gas (7) enriched in carbon monoxide and a gas (9) enriched in carbon dioxide. The carbon dioxide-enriched gas is cooled by indirect heat exchange with at least one fluid (45, 47) from a distillation column (43), then by indirect heat exchange with the tank liquid (67) of the distillation column, and then by heat exchange with at least one closed-circuit refrigeration system (53, 55). Fig. 1

Description

Title of the invention: Method and apparatus for distilling a mixture of carbon dioxide, carbon monoxide and nitrogen

[0001] The present invention relates to a method and apparatus for distilling a mixture of carbon dioxide, carbon monoxide and nitrogen, for example a top gas from a blast furnace or another waste gas from a ferrous metal production unit.

[0002] A blast furnace gas contains between 15 and 35 mol% of carbon monoxide.

[0003] The typical molar composition of a blast furnace top gas is as follows: • H2: 4-5% • CO: 24-25% • CO2: 23-25% • N2: 40-45% • H2O: 3%

[0004] The extraction of CO2 and N2 present in the top gases of blast furnaces aims to recover them: • as fuel to gas turbines (e.g. for electricity generation for an integrated steel mill) or to other users of the steel mill (such as heat recovery units for blast furnaces mixed with other fuel gases (coke oven gas or natural gas) to preheat enriched air) • as a reducing gas for the coal in the blast furnace itself • as raw material for the production of biofuels

[0005] Several CO2 extraction / separation technologies can be considered: adsorption with pressure swing or in English "Pressure Swing Adsorption" (PSA), amine washing. The separation of nitrogen from CO is more difficult.

[0006] The blast furnace produces a top gas which is at low pressure, then compressed by a compressor and sent to separate by adsorption to produce a flow enriched in CO and depleted in CO2 also containing hydrogen and nitrogen and a flow enriched in CO2 and depleted in CO also containing hydrogen.

[0007] In the case of a PSA treating a top gas compressed to approximately 8-10 bars, the CO recovery efficiency at high pressure is typically greater than 80% (or even 85%) for corresponding CO2 extraction efficiencies at low pressure. pressure around 88% and N2 of 15%. We seek to maximize both the CO recovery efficiency and the CO2 / N2 extraction efficiencies.

[0008] Pre-treatment of the top gas (capture of particles and certain undesirable compounds) may be necessary upstream of the PSA.

[0009] The CO-enriched flow generated by the PSA therefore also contains a lot of nitrogen (approximately 85 mol% of the treated top gas flow) because the CO / N2 selectivity is very low on conventional adsorbents. It also contains hydrogen and CO2 (approximately 12% of the treated top gas flow)

[0010] The CO2-enriched flow produced at low pressure by the PSA also contains unadsorbed CO (approximately 15% of the treated top gas flow), H2, nitrogen and water present in the top gas.

[0011] WO2012 / 076786 describes a method for treating top gas in which the Compressed top gas feeds a PSA, the PSA product is returned to the blast furnace and the PSA waste is separated at a temperature below 0°C.

[0012] Example of material balance obtained for a PSA operating in “High CO efficiency” mode on top gas at approximately 8 bars (numerical references corresponding to [Fig.l]):

[0013] [Tab 1] PSA Inlet 5 CO Enriched Flow 7 CO2 Enriched Flow 9 Molar Flow 100 67 33 H2 5 8 1 CO 25 32 11 CO2 26 5 68 N2 43 55 18 H2O 1 0 2

[0014] CO recovery efficiency = 85.8%

[0015] N2 / CO ratio approx. 1.72 PSA inlet and in CO-rich gas (PSA product)

[0016] It is known to separate the CO2-enriched PSA H2 waste gas produced at low pressure to extract the CO2 in liquid or gaseous form by partial condensation and / or by distillation. The aim of this separation is then to separate the CO2 from the H2, the latter being recovered by re-treatment in the PSA. This idea can be applied to the waste gas from a PSA positioned on blast furnace top gas to separate the CO2 from the CO. The CO / H2-rich gas thus produced can thus be recovered either as fuel or as a product with or without reprocessing.

[0017] According to an object of the invention, there is provided a method for separating a gas containing carbon dioxide, carbon monoxide, water and nitrogen in which: i. The gas is separated by adsorption or permeation to produce a gas enriched in carbon monoxide and depleted in carbon dioxide and a gas depleted in carbon monoxide and enriched in carbon dioxide ii. The carbon dioxide-enriched gas is dried, compressed and cooled, being first cooled by indirect heat exchange with at least one fluid from a distillation column, then by indirect heat exchange with the bottom liquid of the distillation column, then by heat exchange with at least one closed-circuit refrigeration cycle, being partially condensed by heat exchange with the refrigeration cycle or with the last of the refrigeration cycles iii. The partially condensed gas is sent to a phase separator, the liquid from the phase separator is expanded and sent to the top of the distillation column for separation, without having been reheated upstream of the expansion, and the gas from the phase separator is reheated by indirect heat exchange with the gas enriched in dried and compressed carbon dioxide iv. The tank liquid is vaporized by heat exchange with the gas and the vaporized liquid is sent to the column tank and v. A liquid rich in CO2 is extracted from the column bottom.

[0018] According to other optional aspects: • another portion of the CO2-rich liquid vaporizes by indirect heat exchange with the compressed and cooled dried carbon dioxide-enriched gas to form a CO2-rich gaseous product • a gas at the top of the column is heated by indirect heat exchange with the gas enriched in dried and compressed carbon dioxide • a plate and fin heat exchanger is used to cool the dried and compressed carbon dioxide enriched gas against the other part of the CO2 rich liquid and / or • the column overhead gas and / or the phase separator gas • in the at least one refrigeration cycle, a refrigerant is compressed by a screw or piston compressor, cooled, expanded, condensed, reheated and vaporized by heat exchange with the gas enriched in dried and compressed carbon dioxide having already been used to vaporize the tank liquid and returned to the compressor. • the gas enriched with dried and compressed carbon dioxide having already been used to vaporize the tank liquid is cooled by a first refrigeration cycle, for example up to a temperature between -28°C and -30°C, in which a first refrigerant is compressed by a first screw or piston compressor, cooled, expanded, heated by heat exchange with the dried and compressed carbon dioxide-enriched gas having already served to vaporize the tank liquid and returned to the first compressor. • the dried and compressed carbon dioxide-enriched gas having already been cooled by the first refrigeration cycle is cooled by a second refrigeration cycle, for example to a temperature between -52°C and -53°C, in which a second refrigerant is compressed by a second screw or piston compressor, cooled, expanded, reheated by heat exchange with the dried and compressed carbon dioxide-enriched gas having already been cooled by the first refrigeration cycle and returned to the second compressor. • the first refrigerant is ammonia. • the second refrigerant is carbon dioxide or propane. • a portion of the first refrigerant expanded and condensed is used to cool the second refrigerant after compression in the second compressor and before its expansion, the portion of the first refrigerant vaporizing by heat exchange with the second refrigerant in a heat exchanger. • the part of the first refrigerant used to cool the second refrigerant is at the same temperature as the part of the first refrigerant sent to cool the gas enriched in carbon dioxide. • the column operates at a pressure less than or equal to 21 bars abs. • the column operates at a pressure between 10 and 15 bars abs, or even between 12 and 14 bars and carbon dioxide is produced at a pressure below 10 bars abs. • the gas to be separated contains between 15 and 35% mol of CO. • the CO2-rich liquid contains at least 80 mol% CO2, or even at least 90 mol% CO2 or even 95 mol% CO2. • the CO2-rich liquid extracted from the column bottom is supplied to a customer at 7 bara after subcooling and expansion and the pressure of the carbon dioxide-enriched gas which exchanges indirect heat with the distillation column bottom liquid is between 30 and 40 bara.

[0019] According to another object of the invention, there is provided an apparatus for separating a gas containing carbon dioxide, carbon monoxide, water and nitrogen comprising a distillation column, an adsorption or permeation separation unit for separating the feed gas containing carbon dioxide, carbon monoxide, water and nitrogen to produce a gas enriched in carbon monoxide and depleted in carbon dioxide relative to the feed gas and a gas depleted in carbon monoxide and enriched in carbon dioxide relative to the feed gas, a dryer for drying the carbon dioxide-enriched gas is dried, a compressor for compressing the dried gas, means for cooling the dried and compressed gas by indirect heat exchange with at least one fluid from a distillation column, by indirect heat exchange with the bottom liquid of the distillation column and by heat exchange with at least one closed-circuit refrigeration cycle, in order to partially condense the dried and compressed gas by heat exchange with the refrigeration cycle or with the last of the refrigeration cycles, a phase separator supplied with the partially condensed gas, a pipe for removing a liquid from the phase separator, means for expanding the liquid and means for sending the expanded liquid to the top of the distillation column for separation there,without having been heated upstream of the expansion, a pipe to send gas from the phase separator to be heated by indirect heat exchange with the gas enriched in dried and compressed carbon dioxide, a pipe to send liquid from the tank is vaporized by heat exchange with the gas in the tank of the column and a pipe to extract a liquid rich in CO2 is extracted in the tank of the column.

[0020] The invention will be illustrated in more detail with reference to the figures where:

[0021] [Fig-1] illustrates a separation unit according to the invention

[0022] [Fig.2] illustrates an optional part of a separation unit according to the invention

[0023] [Fig.3] illustrates an optional part of a separation unit according to the invention

[0024] [Fig.4] is a graph illustrating the relationship between the specific energy of a separation unit according to the invention and the pressure of the column.

[0025] [Fig. 1] illustrates a separation unit whose distillation part is carried out at less than 0°C allowing CO2 45 to be extracted from a residual gas of a pressure swing adsorption unit of the PSA 5 type, the unit 5 treating the top gas 1 of a blast furnace. The unit 5 produces a gas flow 7 enriched in CO, nitrogen and hydrogen and depleted in CO2 and water and a gas flow 9 enriched in CO2 and water and depleted in CO, nitrogen and hydrogen, constituting the residual gas of the low pressure PSA.

[0026] Unit 5 can alternatively separate the top gas by permeation or by another type of adsorption.

[0027] The gas flow 7 can be returned to the blast furnace or used to produce bioethanol.

[0028] The separation unit includes • dryers 23 to extract the water contained in the residual gas from the PSA • possibly at least one membrane separation unit to extract the CO and H2 from CO2 • a compressor 17, 27 of residual gas 15, 19 • at least one refrigeration cycle (C3 and / or NH3 and / or CO2 and / or other...) • condensation of CO2 upstream of a phase separator 37 • a stripping column 43 producing purified CO2 at least in liquid form 45 • an exchange line El for cooling the compressed residual gas 31 from the adsorption unit 5 and vaporizing the purified CO2 45 if required.

[0029] A size of at most 1000 t / d of CO2 is typical.

[0030] In this example, the water vapor present in the top gas 1 of the blast furnace is adsorbed by the adsorption unit 5. The wet residual gas 9 from the adsorption unit 5 is dried by passing into the capacity 11 where water and / or other condensates 13 are removed. The dried flow 15 is compressed in a compressor 17 forming the gas 19 and then dried by TSA type adsorption in a dryer 23 at the inlet of the separation unit operating at low temperature. The dryers are generally located at one of the inter-stages of the compressor 17, 27 of residual gas 19 (but not necessarily).

[0031] The regeneration of the dryer 23 can be carried out by an external gas 73 not generated by the low-temperature separation unit (a nitrogen-rich gas for example) or by using a dry flow generated by the low-temperature separation unit, for example gas 39 and / or gas 47. The wet gas 25, which is the gas 73 loaded with water, can be: • either recycled at a point upstream of the dryers 23, or even the PSA adsorption unit 5 (itself to increase the recovery yields of H2 / CO molecules (possibly present in the regeneration gas 73 and the PSA products) and CO2 45 • be exported as fuel.

[0032] The regeneration of the dryer 23 can be carried out at low or high pressure.

[0033] The water content in the residual gas 19 prior to the dryers 23 may be reduced by a preliminary treatment of the water washing type in a tower 21. This tower 21 may not only allow solid particles and soluble impurities to be removed but also allow the gas 19 to be cooled upstream of the dryer 23.

[0034] The dried and compressed residual gas 31 at a pressure between 20-60 bars, typically 40-45 bars) in the compressor 27 is then cooled to around -50°C to partially condense a phase rich in CO2 and is sent to a phase separator 37. The liquid 41 from the phase separator 37 feeds the column 43. The gaseous phase 39 coming from the separator pot 37 contains mainly compounds lighter than CO2 but also non-condensed CO2. The condensation temperature of the residual gas 31 will be limited (generally around -52°C) so as to avoid the risk of formation of CO2 crystals at any point in the process. The waste gas 31 can be cooled by a number of fluids: • Water in the cooler 29 • The gas fraction 39 of pot 37 in a plate and fin exchanger El for example • The head gas 47 from the CO2 exhaust column 43 in a plate and fin exchanger El for example • One or more (depending on the energy optimum that one wishes to obtain) external refrigerants (NH3, C3H8, CO2, others...) generated by classic cold cycles (compression / expansion of the refrigerant), integrated together or not • The reboiler R of the CO2 stripping column 43

[0035] The CO2-rich liquid 41 from the pot 37 is then expanded (typically to a pressure of around 20 bars) and sent to the stripping column 43. The formation of solid is avoided by regulating the column to a sufficient pressure such that T > - 54.5 °C after the expansion valve without preheating the CO2-rich stream 41 before the expansion valve.

[0036] In the example of the figure, the residual gas was cooled in the heat exchanger E1 and then in the reboiler R by heat exchange with the bottom liquid 67 of the column 43.

[0037] Then the residual gas cooled to approximately -12°C is cooled in a heat exchanger 33 by indirect heat exchange with a first closed circuit refrigeration cycle, for example, with ammonia.

[0038] In this first cycle 53, a first refrigerant is compressed in a screw or piston compressor 51 to a pressure greater than 14 bar abs, cooled to 35°C by a water cooler 52, expanded in a valve 53 to atmospheric pressure and a temperature of -30°C to form a liquid and sent to the exchanger 33 to cool the waste gas to between -28°C and -30°C. The first refrigerant heated and vaporized in the heat exchanger 33 is sent to the compressor 51.

[0039] All of the first refrigerant can be sent to the exchanger 33 and then to the compressor 51. According to a variant, the first refrigerant expanded in the valve 53 is divided into two liquid parts 63, 65, only part 65 being sent to the exchanger 33 to cool the residual gas to between -28°C and -30°C, the other part 63, at the same temperature as part 65, being reheated and vaporized in a heat exchanger E2 by indirect heat exchange with the second refrigerant cycle. The two reheated and vaporized parts are then mixed and sent to the compressor 51.

[0040] The cooled waste gas in the exchanger 33 to between -28°C and -30°C continues its cooling in the exchanger 35 by indirect heat exchange with the second closed-circuit refrigerant cycle where a second refrigerant circulates, which may be CO2 or ethane.

[0041] The second refrigerant is compressed in a screw or piston compressor 57 from a pressure of 5.5 bar to a pressure greater than 16 bar, the second compressed refrigerant being cooled in the exchanger E2 against the first liquefied refrigerant 63, expanded to 5.5 bara and a temperature of -54°C in a valve, sent to the heat exchanger 35 at -54°C AND 5.6 bara in liquid form to cool the waste gas to between -52°C and -53°C. The second refrigerant, heated in the heat exchanger 35, is sent as flow 61 to the inlet of the compressor 57.

[0042] The vaporization of the first refrigerant 63 in the heat exchanger E2 at -30°C makes it possible to cool the second refrigerant down to -28°C, thus gaining on the expansion rate of the second refrigeration cycle 55.

[0043] The presence of the exchanger E2 connecting the two cycles 53, 55 is optional.

[0044] The residual gas leaves the exchanger 35 at -52°C in two-phase form.

[0045] For each given compression pressure, there exists a column pressure of minimal depletion 43 avoiding the formation of solid after the expansion valve of the liquid 4L The formation of solid CO2 appears at around -56.4°C. It is desirable to maintain an approach of 2°C with respect to this temperature, that is to say to obtain a temperature at the lowest of -54.4°C downstream of the expansion valve on the liquid 4L The lower the compression pressure chosen, the more the pressure of the depletion column 43° can be reduced; but the specific energy associated with the capture of CO2 increases sharply when the compression pressure is lower than 30 bars, due to a large portion of non-condensed gas recycled to the adsorption unit 5, and therefore compressed in a loop. However, at a compression pressure lower than 20 bars, the scheme requires one less compression stage.

[0046] With the claimed scheme, it is therefore possible to produce liquid CO2 45 directly at pressures lower than 20 bars without preheating the CO2-rich flow 41 upstream of the expansion valve even if this is not the energy optimum.

[0047] The stripping column 43 makes it possible to extract gases lighter than CO2, for example H2, CO, N2 and other impurities present in the CO2. The specification required for the CO2 at the bottom of column 43 will be obtained by varying the stripping parameters, for example the reboiling rate at the bottom. The condensation rate at the top of the stripping column is provided by the expansion level of the CO2-rich liquid 4L. The heat required for reboiling R can be taken from the compressed residual gas 31.

[0048] The gas phase 39 coming from the pot 37 is heated by the residual gas compressed 31 in the exchanger E1 generating a first fluid. The overhead gas 47 of the column 43 is heated by the compressed residual gas 31 generating a second fluid. The first and second fluids are combined to form a residual gas 49.

[0049] If the purified CO2 45 is to be produced in liquid form, it will be directly withdrawn from the bottom of the stripping column 43.

[0050] If the purified CO2 must be produced in gaseous form (not shown), it may be partially vaporized by the residual gas from the compressed PSA 31 before passing into the heat exchanger E1 and being exported (not shown). The recovery of the cold from the purified liquid CO2 45 also makes it possible to reduce the additional cold requirements for cooling the compressed residual gas 31 and therefore also to reduce the energy consumption of the external cold cycles 53, 55.

[0051] A typical material balance of the process is shown below:

[0052] [TAB 2] CO2-enriched gas 9 CO2-rich liquid 41 Gas 39 Gas 47 Liquid CO2 45 Pressure bara 1.05 45 45 21 21 T °C 40 -52 -52 -53 -20 Flow rate mole % 100 65 35 6 59 h2 1 0 2.9 0.2 0 CO 11 2.2 29.9 24.6 0 co2 68 94.1 21.2 33.9 100 N2 18 3.7 46 41.3 0 h2o 2 0 0 0 0 N2 / CO ratio 1.53 1.7

[0053] The optimization of the waste gas compressor 17, 27 and the external cold cycles make it possible to obtain specific energy consumptions of the order of 200 kW / ton liquid CO2 produced.

[0054] Each given compression pressure can be associated with an operating pressure domain for the column, allowing both • to avoid the formation of solids (Minimum pressure) • to optimize thermal integration with reboiling (maximum pressure).

[0055] The specific energy decreases with increasing stripping column pressure, resulting in a higher pressure liquid CO2 product. Below a compression pressure of 30 bar, the specific energy increases sharply.

[0056] With the claimed scheme, it is advantageous to produce liquid CO2 at a pressure greater than 8 bars, preferably greater than 12 bars (pressure of the stripping column) if one seeks to minimize the specific energy.

[0057] If liquid CO2 was to be produced at lower pressure (i.e. at a pressure lower than the minimum pressure of the stripping column), it would be necessary to subcool the liquid CO2 45 produced by the stripping column 43 (down to -52°C for example) before expanding it, as illustrated in Figures 2 and 3.

[0058] In [Fig.2], the liquid 45 is cooled in a heat exchanger E3 by heat exchange with an external fluid 48 forming cooled liquid 46 which is then expanded in a valve VI. This liquid 46 constitutes the product at lower pressure.

[0059] In [Fig.3], the liquid 45 is cooled in a heat exchanger E3 by taking a portion 50 of the liquid, expanding it in a valve V2 and sending it to the heat exchanger E3, leaving another portion of the liquid 46 which is then expanded in the valve VI. This liquid 46 constitutes the lower pressure product. This method corresponds to that of FR3122918B.

[0060] The methods of Figures 2 and 3 make it possible to expand the liquid 45 of [Fig. 1]. In this case, it is possible, for example, to produce liquid CO2 at 7 bars with 100 ppm of CO, and a CO2 purity > 99.9% from the top gas composition 1 mentioned on the first page of this document.

[0061] [Fig.4] shows the evolution of the minimum pressure of the stripping column 43 and the specific energy to produce liquid CO2 45 at 7 bars after subcooling and expansion (as illustrated in Figures 2 and 3) as a function of the compression pressure of the residual gas 31.

[0062] The temperature of the liquid CO2 downstream of the exchanger E3 and upstream of the valve V1 is equal to or less than -50°C. The pressure to which the CO2 is expanded by the valve VI is 7 bars and the temperature of the expanded CO2 is equal to or less than the liquid equilibrium temperature at 7 bars.

[0063] To produce liquid CO2 at 7 bars, the production of liquid CO2 45 is subcooled to T < -50°C (downstream of exchanger E3 and upstream of valve VI) before expanding the liquid in valve VL. This subcooling can be done: • via the existing external CO2 refrigeration unit (in cascade with the NH3 cycle) - [Fig.2] • or by expansion in valve V2 of a fraction 50 of the production allowing an approach of at least 2°C in the exchanger E3- [Fig.3]; this fraction 50 can be recycled in the inter-stage of the compression train 17, 27 of the residual gas if we want to improve the CO2 recovery efficiency.

[0064] The specific energy has a minimum for a compression pressure of the residual gas 31 between 30 and 40 bars. The specific energy is reduced with the installation of an existing external cycle.

[0065] The specific energy for the production of liquid CO2 at 7 bars varies by less than 1% between the minimum and maximum pressure stripping column schemes: in fact, separation is facilitated by a higher pressure column but subcooling requires more energy.

[0066] It is possible to directly produce liquid CO2 at 7 bars in the stripping column; the compression of the residual gas 31 can then be limited to 10 bars. The specific energy calculated in the case of our example is 260 kWh / tCO2. This scheme has the advantage of having a small residual gas compressor.

[0067] It is possible to produce liquid CO2 at 7 bars without risk of solid formation for less energy (~230kWh / t CO2) by expansion of the liquid production of the stripping column 43 operated at a pressure higher than 7 bars after subcooling. There is an optimum between 30 and 40 bars of compression of the residual gas 9, 31 with a column 43 working between 10 and 25 bars. The specific energy is lower the lower the pressure of the column 43 is, and the subcooling provided by an external cycle. On the other hand, the compressor 17, 27 of residual gas will be larger.

[0068] With the scheme according to the invention, if we want to produce liquid CO2 at a pressure PI lower than 12 bars, we will prefer • Compress the waste gas to a pressure of 30 to 40 bar • Operate the stripping column 43 at a pressure of 10 to 25 bars • Subcool the liquid production before expansion with a closed external CO2 cycle

[0069] rather than directly producing CO2 at the required pressure PL

[0070] Example: production of gaseous CO2 at 7 bars with the scheme according to the invention: —> For a compression pressure of the residual gas 31 of 45 bars, the minimum operating pressure of the stripping column 43 is 15 bars. The fluid at 45 bars is cooled to -52°C, the liquid fraction 41 expanded to 15 bars (and to -54.5°C) is introduced into the stripping column 43. 38% (flow rate 50) of the liquid production 45 (15 bars, -28°C) is expanded to 5.7 bars (-54°C) (#41) in order to be able to cool the fluid 46 to -52°C (and assuming 2°C approach in the exchanger). The MP 46 and BP 50 fluids are heated in the EL exchange line. The MP 46 fluid is then expanded to 7 bars, the BP 50 fluid is compressed to 7 bars and are mixed to form the gaseous CO2 product at 7 bars. The specific energy is 160 kWh / tC02- If the CO2 gas must be produced at a pressure higher than the pressure of the stripping column, the MP 46 fluid is not expanded and the compression of the BP 50 and MP 46 fluids is shared.

[0071] To avoid adding a machine to compress the product, the LP fluid 50 can be expanded to (7 bars + pressure drop) before the exchange line El, the MP fluid 46 remaining expanded to 7 bars after the exchange line El, then the MP fluid 46 and the LP fluid 50 are mixed. In this case, the fluid 31 can no longer be cooled to -52°C but to a higher temperature (in the example of a compression pressure of the residual gas of 45 bars, to - 46.5°C). The recycled gas fraction 39 is therefore greater and the specific energy increases (+7 kWh / tC02 in the example). On the other hand, the minimum pressure of the column is lower than in the previous case (10.8 bars in our example).

Claims

Claims

1. A process for separating a gas containing carbon dioxide, carbon monoxide, water and nitrogen wherein: i. The gas (1) is separated (5) by adsorption or permeation to produce a gas (7) enriched in carbon monoxide and depleted in carbon dioxide and a gas (9) depleted in carbon monoxide and enriched in carbon dioxide ii. The carbon dioxide-enriched gas is dried (11, 23), compressed (17, 27) and cooled, being first cooled by indirect heat exchange with at least one fluid (45, 47) from a distillation column (43), then by indirect heat exchange with the bottom liquid (67) of the distillation column, then by heat exchange with at least one closed-circuit refrigeration cycle (53, 55), being partially condensed by heat exchange with the refrigeration cycle or with the last of the refrigeration cycles iii.The partially condensed gas is sent to a phase separator (37), the liquid (41) from the phase separator is expanded and sent to the top of the distillation column to separate there, without having been heated upstream of the expansion, and the gas from the phase separator is heated by indirect heat exchange with the dried and compressed carbon dioxide-enriched gas iv. The bottom liquid (67) is vaporized by heat exchange with the gas and the vaporized liquid is sent to the bottom of the column and v. A CO2-rich liquid (45) is extracted at the bottom of the column.

2. A method according to claim 1 wherein in the at least one refrigeration cycle (53, 55), a refrigerant is compressed by a screw or piston compressor (51, 57), cooled, expanded, condensed, reheated and vaporized by heat exchange (33, 35) with the dried and compressed carbon dioxide-enriched gas having already served to vaporize the tank liquid and returned to the compressor.

3. The method of claim 2 wherein the dried and compressed carbon dioxide-enriched gas having already been used to vaporize the liquid of tank is cooled by a first refrigeration cycle (53), for example to a temperature between -28°C and -30°C, in which a first refrigerant is compressed by a first screw or piston compressor (51), cooled, expanded, reheated by heat exchange (33) with the dried and compressed carbon dioxide-enriched gas having already served to vaporize the tank liquid and returned to the first compressor.

4. A method according to claim 3 wherein the dried and compressed carbon dioxide enriched gas having already been cooled by the first refrigeration cycle (53) is cooled by a second refrigeration cycle (55), for example to a temperature between -52°C and -53°C, wherein a second refrigerant is compressed by a second screw or piston compressor (57), cooled, expanded, reheated by heat exchange with the dried and compressed carbon dioxide enriched gas having already been cooled by the first refrigeration cycle and returned to the second compressor.

5. A method according to claim 3 or 4 wherein the first refrigerant is ammonia.

6. A method according to claim 4 or 5 wherein the second refrigerant is carbon dioxide or propane.

7. A method according to claim 4, 5 or 6 wherein a portion (63) of the expanded and condensed first refrigerant serves to cool the second refrigerant after compression in the second compressor (57) and before its expansion, the portion of the first refrigerant vaporizing by heat exchange with the second refrigerant in a heat exchanger (E2).

8. A method according to claim 7 wherein the portion (63) of the first refrigerant used to cool the second refrigerant is at the same temperature as the portion (65) of the first refrigerant sent to cool the carbon dioxide enriched gas.

9. Method according to one of the preceding claims in which the column (43) operates at a pressure less than or equal to 21 bars abs.

10. Method according to one of the preceding claims in which the column (43) operates at a pressure between 10 and 15 bars abs, or even between 12 and 14 bars and the carbon dioxide (49) is produced at a pressure lower than 10 bars abs.

11. Method according to one of the preceding claims in which the gas to be separated (1) contains between 15 and 35 mol% of CO.

12. Method according to one of the preceding claims in which the liquid rich in CO2 (45) contains at least 80 mol% CO2, or even at least 90 mol% CO2 or even 95 mol% CO2.

13. Method according to one of the preceding claims in which the CO2-rich liquid (45) extracted from the bottom of the column is supplied to a customer at 7 bara after subcooling and expansion and the pressure of the carbon dioxide-enriched gas which exchanges indirect heat (R) with the bottom liquid (67) of the distillation column is between 30 and 40 bara.

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