Method and apparatus for separating a gas comprising co2, nitrogen and at least one toxic and / or flammable compound

Abstract

In a method for separating a gas (1) comprising CO2, nitrogen and at least one toxic and / or flammable compound, the gas, having a flow rate in a compressor (3) for which the barrier gas (4) is nitrogen, the leakage rate of the barrier gas to the gas being less than 0.5 mol% of the flow rate of the gas, is compressed, producing a compressed gas containing barrier gas, and the compressed gas containing barrier 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 CO2-enriched and N2-depleted fluid (13) and a CO2-depleted and N2-enriched fluid (11).

Description

[0001] Method and apparatus for separating a gas comprising CO2, nitrogen and at least one toxic and / or flammable compound

[0002] The present invention relates to a method and an apparatus for separating a gas comprising CO2, nitrogen and at least one toxic and / or flammable compound, this method comprising a step of compressing the gas upstream of the separation.

[0003] When compressing gas containing at least one highly toxic and / or flammable compound, this gas being called process gas, it is typical to employ seals that minimize leakage of the process gas into the barrier gas. In addition, leakage of the barrier gas into the process gas is usually avoided to prevent pollution of the process gas.

[0004] For CO2 capture applications from a gas containing CO2 and nitrogen, it may be advisable to proceed in another way due to the differences in the composition of the compressed gases and the interest in maximizing the quantity of CO2 recovered.

[0005] Nitrogen is commonly used as a barrier gas in compressors.

[0006] In order to avoid leaks of process gas into the barrier gas, very tight systems are commonly used and for stages whose pressure is higher than that of the nitrogen, the barrier is often made by the process gas itself which is recycled in a first chamber and a second chamber is used where the nitrogen is injected. In this case, the leakage to the nitrogen is limited, and the nitrogen does not leak into the process because it is at a lower pressure than that of the process. For stages whose pressure is lower than that of the nitrogen, the nitrogen will typically be expanded to limit the pressure difference and / or pressure losses will be generated (by means of labyrinths for example) in order to reduce the nitrogen pressure and thus limit leaks.

[0007] High-tightness systems are much more expensive and often more fragile than other systems, and in the configuration used, the process gas containing dangerous impurities and CO2 leaks into the barrier gas. These dangerous impurities have a cost, at least that of their calorific value when they are burned (for example, CO or H2). These compounds can even be products of the process, therefore having a commercial value. Minimizing these leaks therefore makes it possible to limit fuel consumption when they are burned or to maximize the unit's production.

[0008] The invention mainly consists of deliberately allowing the barrier nitrogen to contaminate the process gas, and of separating downstream this nitrogen which is introduced into the process. In particular, this is possible by maintaining a nitrogen pressure in the barrier chambers of the compressor 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 leak gas from this intermediate chamber then being recycled to a lower pressure stage of the compressor.

[0009] One objective is to leak the barrier gas into the gas to be compressed. Another objective is to remove the barrier gas from the produced CO2 without using a dedicated scrubbing system.

[0010] According to an object of the invention, there is provided a method for separating a gas 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 whose barrier gas is nitrogen or a nitrogen-rich gas, the leakage rate of the barrier gas towards the gas being less than 0.5 mol% of the gas flow rate but not zero, producing a compressed gas comprising barrier gas ii) - the gas comprising CO2, nitrogen and at least one toxic and / or flammable compound does not leak towards the barrier gas. iii) the compressed gas comprising barrier 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 including a fluid enriched in CO2 and depleted in N2, and a fluid depleted in CO2 and enriched in N2.

[0011] According to other optional aspects of the invention, which can be combined with each other in any manner compatible with logic and science:

[0012] • at least one toxic and / or flammable compound is carbon monoxide, hydrogen, methane, mercury or NOx.

[0013] • 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. • the suction pressure of the compressor is less than 6 bar abs, preferably less than 2 bar abs.

[0014] • the compressor discharge pressure is greater than 20 bar abs, preferably greater than 30 bar abs.

[0015] • the pressure of the barrier gas (therefore nitrogen) is less than 10 bar, preferably less than 8 bar.

[0016] • the nitrogen leak rate is less than 0.3% of the compressed flow, preferably less than 0.1%.

[0017] • the fluid enriched in CO2 and depleted in N2 contains less than 0.5% mol of nitrogen, preferably less than 0.1% mol of nitrogen.

[0018] • the gas contains NOx and the compressed gas comprising barrier 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.

[0019] • at least 50% of nitrogen leaks are found in the fluid depleted in CO2 and enriched in N2.

[0020] • the fluid depleted in CO2 and enriched in N2 serves as a barrier gas.

[0021] • the pressure of nitrogen or a nitrogen-rich gas serving as a barrier gas is higher than that of an intermediate chamber between a first compression stage of the compressor and the nitrogen inlet, the nitrogen is injected into the chamber and a leak gas from the chamber is then recycled to a second compression stage of the compressor upstream of the first stage.

[0022] • the compressed gas in which the barrier gas has leaked is compressed in at least one last stage (3D) of the compressor.

[0023] • at least one toxic and / or flammable compound is found at least in part in the CO2-enriched and N2-depleted fluid, or the CO2-depleted and N2-enriched fluid.

[0024] • the at least one toxic and / or flammable compound is at least partly present 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. • the gas to be compressed contains on a dry basis at least 10 mol% carbon dioxide, at least 20 mol% carbon dioxide, at least 30 mol% carbon dioxide.

[0025] • the gas to be compressed does not contain water and contains at least 10 mol% carbon dioxide, at least 20 mol% carbon dioxide, at least 30 mol% carbon dioxide.

[0026] • the gas to be compressed contains 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.

[0027] • 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.

[0028] • the compressed and cooled gas containing barrier 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 possibly a permeation and / or adsorption step to produce the CO2-enriched and N2-depleted fluid and the CO2-depleted and N2-enriched fluid.

[0029] According to another object of the invention, there is provided an apparatus for separating a gas comprising CO2, nitrogen and at least one toxic and / or flammable compound comprising: i) a compressor, a pipe for sending the gas having a flow rate to the suction of the compressor, means for sending a barrier gas to the compressor, the compressor being adapted to allow leakage of the barrier gas to the gas to be compressed and to prevent leakage of gas to be compressed to the barrier gas, 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 comprising barrier gas from the compressor,ii) a cooler for cooling the compressed gas comprising barrier gas and means for separating the cooled compressed gas comprising barrier gas in the cooler 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, and a CO2-depleted and N2-enriched fluid. Thus, for example, the compressor is sized to allow the barrier gas to leak towards the gas to be compressed and / or a compressor sealing means is chosen to be less effective in the direction towards the gas to be compressed. Similarly, the compressor is sized to prevent the gas to be compressed from leaking towards the barrier gas or a compressor sealing means is chosen to be fully effective in the direction towards the barrier gas.,

[0030] The apparatus may include means for treating the CO2-depleted and N2-enriched fluid to serve as the barrier gas.

[0031] The apparatus may comprise means, for example a PSA type adsorption unit or a permeation unit for enriching the compressed gas containing CO2 barrier gas upstream of the means for separating the compressed gas. The invention will be described in more detail with reference to the figure.

[0032] [Fig. 1] represents a method according to the invention.

[0033] In Figure 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.

[0034] Gas 1 may, for example, be a synthesis gas resulting from a reforming reaction of a hydrocarbon, preferably natural gas. Such reforming processes include steam reforming, partial oxidation, and autothermal reforming. They are described, for example, in "Industrial Gases Processing" by Haring, Wiley-VCH, 2008.

[0035] 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.

[0036] The 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. The suction pressure of the compressor 3 is less than 6 bar abs, preferably less than 2 bar abs.

[0037] The discharge pressure of the compressor 3 is greater than 20 bar abs, preferably greater than 30 bar abs. A barrier gas 4 which is nitrogen is sent to at least one bearing of the compressor 3 and leaks of barrier nitrogen constituting a quantity corresponding to less than 0.5 mol% of the gas flow 1 are found in the compressed gas 7. The leak rate may be less than 0.3 mol%, or even less than 0.1 mol%. On the other hand, it is not zero. The pressure of the nitrogen 4 is less than 10 bar, preferably less than 8 bar.

[0038] Thus, nitrogen 4 is preferably injected into the compressor where the pressure of nitrogen 4 is higher than that of the intermediate chamber between the compression stages 3B and 3C or 3C and 3D and the nitrogen inlet, the leak 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.

[0039] In the example in the figure, the compressed gas in stage 3D is the mixture of the gas to be compressed and the leakage barrier gas. This mixture can be compressed in stages further upstream, depending on the arrival point of the barrier gas.

[0040] 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.

[0041] Gas 11 may possibly serve as a barrier gas, if it is sufficiently pure. Here the separation unit is reduced to its simplest expression but it goes without saying that it may comprise several partial condensation stages 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. For example, liquid resulting from the partial condensation of the gas contaminated by the barrier gas may be sent to the top of a stripping column and the CO2 may be recovered at the bottom of the column. An example of a separation unit for separating such a mixture is described in “Industrial Gas Handbook” by Kerry, CRC Press, 2007. US8535638, FR3073835, FR2877939, FR3143106 or FR3116526 may also be consulted.

[0042] Depending on its volatility, the at least one toxic and / or flammable compound may be found at least partly in the CO2-enriched and N2-depleted fluid or in the CO2-depleted and N2-enriched fluid. The 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. It will be understood that the gas 1 does not necessarily contain moisture in which case no drying is necessary.

[0043] The apparatus may comprise means for enriching the compressed gas containing CO2 barrier gas upstream of the means for separating the compressed gas, for example a pressure swing adsorption unit or a permeation unit.

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 or a nitrogen-rich gas (11), 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 containing barrier gas, ii) the gas comprising CO2, nitrogen and at least one toxic and / or flammable compound (1) does not leak to the barrier gas (4), iii) the compressed gas containing barrier gas is cooled (R) to a temperature below -20°C, preferably below -45°C, and then separated (15) by partial condensation and / or distillation and / or washing and / or solidification producing at least two fluids, one of which is enriched in CO2 and depleted in N2 (13), and a fluid depleted in CO2 and enriched in N2 (11).

2. Method according to claim 1 in which the at least one toxic and / or flammable compound is carbon monoxide, hydrogen, methane, mercury or NOx.

3. 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.

4. 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.

5. 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.

6. 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.

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

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

9. Method according to one of the preceding claims in which the gas (1) contains a NOx and the compressed gas containing barrier 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.

10. 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).

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

12. 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 leak gas from the chamber is then recycled to a second compression stage of the compressor (3A, 3B, 3C) upstream of the first stage.

13. Method according to one of the preceding claims, in which the compressed gas into which the barrier gas has leaked is compressed in at least one last stage (3D) of the compressor.

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 and to prevent leakage of gas to be compressed to the barrier gas, the leakage rate of the barrier gas to the gas being less than at 0.5 mol% of the gas flow rate but not zero, means for extracting the compressed gas from the compressor, ii) a cooler (R) for cooling the compressed gas containing barrier gas and means (15) for separating the compressed gas containing barrier 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).

15.

15. Apparatus according to claim 14 comprising means for enriching the compressed gas containing CO2 barrier gas upstream of the means (15) for separating the compressed gas.

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