Method and apparatus for separating a gas comprising CO2, nitrogen and at least one toxic and / or flammable compound

Abstract

Title: Process and apparatus for separating a gas comprising CO2, nitrogen, and at least one toxic and / or flammable compound. In a process for separating a gas (1) comprising CO2, nitrogen, and at least one toxic and / or flammable compound, the gas is compressed in a compressor (3) whose barrier gas (4) is nitrogen, the leakage rate of the barrier gas into the gas being less than 0.5 mol% of the gas flow rate, producing a compressed gas. The compressed gas is cooled (R) to a temperature below -20°C and then separated (15) by partial condensation and / or distillation and / or washing and / or solidification, producing a fluid enriched in CO2 and depleted in N2 (13), and a fluid depleted in CO2 and enriched in N2 (11). Figure: Fig. 1

Description

Title of the invention: Method and apparatus for separating a gas comprising CO2, nitrogen and at least one toxic and / or flammable compound

[0001] The present invention relates to a method and apparatus for separating a gas comprising CO2, nitrogen and at least one toxic and / or flammable compound, this method comprising a gas compression step upstream of the separation.

[0002] When compressing gas containing at least one highly toxic and / or flammable compound, this gas being called the process gas, it is typical to use seals that minimize leakage from the process gas to the barrier gas. Furthermore, leakage from the barrier gas to the process gas is usually avoided to prevent contamination of the process gas.

[0003] For applications of CO2 capture from a gas containing CO2 and nitrogen, it may be advisable to proceed in another way due to the differences in composition of the compressed gases and the interest in maximizing the amount of CO2 recovered.

[0004] Nitrogen is commonly used as a barrier gas in compressors.

[0005] In order to prevent leaks of process gas into the dam gas, very Sealed chambers are commonly used, and for stages where the pressure exceeds that of nitrogen, the barrier is often created by the process gas itself, which is recycled into a first chamber. A second chamber is then used where nitrogen is injected. In this case, leakage into the nitrogen is limited, and nitrogen does not leak into the process because it is at a lower pressure than the process pressure. For stages where the pressure is lower than that of nitrogen, the nitrogen is typically expanded to limit the pressure difference, and / or pressure drops are generated (using labyrinths, for example) to reduce the nitrogen pressure and thus limit leakage.

[0006] High-sealing systems are considerably more expensive and often more fragile than other systems, and in the configuration used, the process gas containing hazardous impurities and CO2 leaks into the barrier gas. These hazardous impurities have a cost, at a minimum that of their calorific value when burned (e.g., CO or H2). These compounds may even be process products, and therefore have commercial value. Minimizing these leaks thus makes it possible to limit fuel consumption when they are burned or to maximize the unit's output.

[0007] The invention mainly consists of intentionally allowing the barrier nitrogen to contaminate the process gas, and separating downstream this nitrogen which is introduced into the process. In particular, this is possible by maintaining a nitrogen pressure in the compressor dam chambers always higher than that of the process gas in the compression stage (=first stages of a machine) or in an intermediate chamber between the compression stage and the nitrogen (=higher pressure stage of the machine), the leakage gas from this intermediate chamber then being recycled to a lower pressure stage of the compressor.

[0008] One objective is to allow the barrier gas to escape into the gas to be compressed. Another objective is to remove the barrier gas from the CO2 produced without using a dedicated purification system.

[0009] According to one aspect of the invention, a process is provided for separating a gas comprising CO2, nitrogen and at least one toxic and / or flammable compound in which

[0010] i) The gas having a flow rate is compressed in a compressor whose blocking gas is nitrogen, the leakage rate of the blocking gas into the gas being less than 0.5 mol% of the gas flow rate but not zero, producing a compressed gas

[0011] ii) the compressed gas is cooled to a temperature below -20°C, preferably below -45°C, then separated by partial condensation and / or distillation and / or washing and / or solidification producing at least two fluids, one fluid enriched in CO2 and depleted in N2, and one fluid depleted in CO2 and enriched in N2.

[0012] According to other optional aspects of the invention, which can be combined with each other in any way compatible with logic and science:

[0013] -the gas comprising CO2, nitrogen and at least one toxic and / or flammable compound does not leak towards the barrier nitrogen.

[0014] at least one toxic and / or flammable compound is carbon monoxide, hydrogen, methane, mercury or NOx.

[0015] -the gas comprising CO2, nitrogen and at least one toxic and / or flammable compound is a synthesis gas resulting from a reforming reaction of a hydrocarbon, preferably natural gas.

[0016] -the compressor suction pressure is less than 6 bar abs, preferably less than 2 bar abs.

[0017] -the compressor discharge pressure is greater than 20 bar abs, preferably greater than 30 bar abs.

[0018] -the pressure of the barrier gas (therefore nitrogen) is less than 10 bar, preferably less than 8 bar.

[0019] -the nitrogen leakage rate is less than 0.3% of the compressed flow rate, preferably less than 0.1%.

[0020] -the fluid enriched in CO2 and depleted in N2 contains less than 0.5 mol% nitrogen, pre- differentially less than 0.1 mol% nitrogen.

[0021] -the gas contains NOx and the compressed gas is separated in a column to produce a liquid enriched in NOx and depleted in CO2 and nitrogen and a gas enriched in nitrogen and CO2 and depleted in NOx, the gas enriched in nitrogen and CO2 and depleted in NOx is separated to form the fluid enriched in CO2 and depleted in N2 and the fluid depleted in CO2 and enriched in N2.

[0022] -at least 50% of nitrogen leaks end up in the fluid depleted in CO2 and enriched in N2.

[0023] -the fluid depleted in CO2 and enriched in N2 serves as a barrier gas.

[0024] - the nitrogen pressure used as a barrier gas is greater than that of a chamber Intermediate between a first compression stage of the compressor and the nitrogen inlet, nitrogen is injected into the chamber, and a leakage gas from the chamber is then recycled to a second compression stage of the compressor upstream of the first stage.

[0025] - at least one toxic and / or flammable compound is found at least partially in the fluid enriched in CO2 and depleted in N2, or the fluid depleted in CO2 and enriched in N2

[0026] - at least one toxic and / or flammable compound is found at least partially in or constitutes another fluid or solid produced by at least one partial condensation step and / or at least one distillation step and / or at least one washing step and / or at least one solidification step of the compressed gas

[0027] - the gas to be compressed contains, on a dry basis, at least 10% mol of carbon dioxide, at least 20% mol of carbon dioxide, at least 30% mol of carbon dioxide.

[0028] - the gas to be compressed does not contain water and contains at least 10 mol% of dioxide of carbon, at least 20 mol% of carbon dioxide, at least 30 mol% of carbon dioxide.

[0029] - the gas to be compressed contains, on a dry basis, at least 2% nitrogen, at least 5% of nitrogen, at least 10% nitrogen, or even at least 20% nitrogen, these percentages being molar percentages.

[0030] - the gas to be compressed does not contain water and contains at least 2% nitrogen, at least 5% nitrogen, at least 10% nitrogen, or even at least 20% nitrogen, these percentages being molar percentages.

[0031] -the compressed and cooled gas is separated by at least one partial condensation step and / or at least one distillation step and / or at least one washing step and / or at least one solidification step and optionally a permeation and / or adsorption step to produce the CO2-enriched and N2-depleted fluid and the CO2-depleted and N2-enriched fluid.

[0032] According to another object of the invention, a gas separation apparatus is provided including CO2, nitrogen and at least one toxic and / or flammable compound including

[0033] i) a compressor, a line for sending the gas having a flow rate to the compressor suction, means for sending a barrier gas to the compressor, the compressor being adapted to allow leakage of the barrier gas into the gas to be compressed, the leakage rate of the barrier gas into the gas being less than 0.5% mol of the gas flow rate but not zero, means for extracting the compressed gas from the compressor

[0034] ii) a cooler for cooling the compressed gas and means for separating the compressed gas cooled in the cooler by partial condensation and / or distillation and / or washing and / or solidification producing at least two fluids, one of which is a fluid enriched in CO2 and depleted in N2, and the other a fluid depleted in CO2 and enriched in N2.

[0035] Thus for example the compressor is sized to allow the barrier gas to leak towards the gas to be compressed or the means of sealing the compressor is chosen to be less efficient.

[0036] The apparatus may include means for treating the fluid depleted in CO2 and enriched in N2 to serve as the dam gas.

[0037] The invention will be described in more detail with reference to the figure.

[0038] [Fig-1] represents a method according to the invention.

[0039] In [Fig. 1], a wet gas 1 containing carbon dioxide and nitrogen and at least one toxic and / or flammable compound, which may be a compound lighter than nitrogen, such as nitrous oxide, carbon monoxide, methane, hydrogen cyanide or hydrogen or a compound heavier than carbon dioxide, for example NO2, mercury, is compressed in a compressor 3. This compressor 3 comprises four stages 3A, 3B, 3C, 3D, each being followed by a cooling means 5A, 5B, 5C, 5D.

[0040] The gas 1 can, for example, be a synthesis gas resulting from a reforming reaction of a hydrocarbon, preferably natural gas.

[0041] The gas 1 may contain on a dry basis at least 2% nitrogen, at least 5% nitrogen, at least 10% nitrogen, or even at least 20% nitrogen, these percentages being molar percentages.

[0042] Gas 1 may contain on a dry basis at least 10% mol of carbon dioxide, at least 20% mol of carbon dioxide, at least 30% mol of carbon dioxide.

[0043] The suction pressure of the compressor 3 is less than 6 bar abs, preferably less than 2 bar abs.

[0044] The discharge pressure of the compressor 3 is greater than 20 bar abs, preferably greater than 30 bar abs.

[0045] A barrier gas 4, which is nitrogen, is sent to at least one bearing of the compressor 3, and leaks of barrier nitrogen constitute an amount corresponding to Less than 0.5% mol of the gas flow rate 1 is found in the compressed gas 7. The leakage rate can be less than 0.3% mol, or even less than 0.1% mol. However, it is not zero. The pressure of nitrogen 4 is less than 10 bar, preferably less than 8 bar.

[0046] Thus, nitrogen 4 is preferably injected into the compressor where the nitrogen 4 pressure is greater than that of the intermediate chamber between the compression stage 3B, 3C, 3D and the nitrogen inlet, the leakage gas from this intermediate chamber then being recycled to a lower pressure stage of the compressor 3A, 3B, 3C. The details of the intermediate chamber are not illustrated.

[0047] After cooling in the cooling means 5D, the compressed gas 1 is sent to a dryer 9 (if it contains water), cooled by a cooler R to partially condense it and sent to a phase separator 15 to produce a liquid enriched in CO2 and depleted in nitrogen 13 and a gas enriched in nitrogen and depleted in CO2 11.

[0048] Gas 11 can possibly be used as a barrier gas, if it is sufficiently pure.

[0049] Here the separation unit is reduced to its simplest expression but it goes without saying that it can include several stages of partial condensation in parallel or in series and / or at least one distillation stage and / or at least one washing stage and / or at least one solidification stage.

[0050] Depending on its volatility, at least one toxic and / or flammable compound may be found at least partly in the fluid enriched in CO2 and depleted in N2 or in the fluid depleted in CO2 and enriched in N2.

[0051] At least one toxic and / or flammable compound may be found in or constitute another fluid or solid produced by at least one partial condensation step and / or at least one distillation step and / or at least one washing step and / or at least one solidification step of the compressed gas.

[0052] It will be understood that gas 1 does not necessarily contain moisture, in which case no drying is necessary.

Claims

Claims

1. A method for separating a gas (1) comprising CO2, nitrogen and at least one toxic and / or flammable compound in which i) the gas is compressed having a flow rate in a compressor (3) whose barrier gas (4) is nitrogen, the leakage rate of the barrier gas to the gas being less than 0.5 mol% of the gas flow rate but not zero, producing a compressed gas ii) the compressed gas is cooled (R) to a temperature below - 20°C, preferably below -45°C, then separated (15) by partial condensation and / or distillation and / or washing and / or solidification producing at least two fluids including a CO2-enriched and N2-depleted fluid (13), and a CO2-depleted and N2-enriched fluid (H).

2. A method according to claim 1 wherein the gas comprising CO2, nitrogen and at least one toxic and / or flammable compound (1) does not leak towards the nitrogen barrier (4).

3. Method according to one of the preceding claims in which the at least one toxic and / or flammable compound is carbon monoxide, hydrogen, methane, mercury or NOx.

4. Method according to one of the preceding claims in which the gas comprising CO2, nitrogen and at least one toxic and / or flammable compound (1) is a synthesis gas resulting from a reforming reaction of a hydrocarbon, preferably natural gas.

5. Method according to one of the preceding claims in which the suction pressure of the compressor (3) is less than 6 bar abs, preferably less than 2 bar abs.

6. Method according to one of the preceding claims in which the discharge pressure of the compressor (3) is greater than 20 bar abs, preferably greater than 30 bar abs.

7. Method according to one of the preceding claims in which the pressure of the barrier gas (4) is less than 10 bar, preferably less than 8 bar.

8. Method according to one of the preceding claims in which the nitrogen leakage rate is less than 0.3% of the compressed flow rate, preferably less than 0.1%.

9. Method according to one of the preceding claims in which the CO2-enriched and N2-depleted fluid (13) contains less than 0.5 mol% nitrogen, preferably less than 0.1 mol% nitrogen.

10. A method according to one of the preceding claims wherein the gas (1) contains NOx and the compressed gas is separated in a column to produce a liquid enriched in NOx and depleted in CO2 and nitrogen and a gas enriched in nitrogen and CO2 and depleted in NOx, the gas enriched in nitrogen and CO2 and depleted in NOx is separated to form the fluid enriched in CO2 and depleted in N2 and the fluid depleted in CO2 and enriched in N2.

11. Method according to one of the preceding claims in which at least 50% of the nitrogen leaks (4) are found in the CO2-depleted and N2-enriched fluid (11).

12. Method according to one of the preceding claims in which the CO2-depleted and N2-enriched fluid (11) serves as a barrier gas.

13. Method according to one of the preceding claims in which the pressure of nitrogen serving as barrier gas is higher than that of an intermediate chamber between a first compression stage of the compressor (3B, 3C, 3D) and the nitrogen inlet (4), the nitrogen is injected into the chamber and a leakage gas from the chamber is then recycled to a second compression stage of the compressor (3A, 3B, 3C) upstream of the first stage.

14. Apparatus for separating a gas comprising CO2, nitrogen and at least one toxic and / or flammable compound comprising i) a compressor (3), a pipe for sending the gas having a flow rate at the suction of the compressor, means for sending a barrier gas (4) to the compressor, the compressor being adapted to allow leakage of the barrier gas to the gas to be compressed, the leakage rate of the barrier gas to the gas being less than 0.5 mol% of the flow rate of the gas but not zero, means for extracting the compressed gas from the compressor ii) a cooler (R) for cooling the compressed gas and means (15) for separating the compressed gas cooled in the cooler by partial condensation and / or distillation and / or washing and / or solidification producing at least two fluids including a fluid enriched in CO2 and depleted in N2 (13) and a fluid depleted in CO2 and enriched in N2 (11).

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