Method and apparatus for separating a mixture of co2 and NOX by partial condensation

Abstract

In a method for separating a mixture of CO2, NOx and at least one component that is lighter than CO2 by partial condensation, the mixture is cooled and sent into the bottom of a first washing column (K1) fed at the top with a liquid (43, 45) containing at least 90 mol% of CO2, a gas (5) depleted in NOx and enriched in CO2 exits the top of the first washing column and a liquid (3) enriched in NO2 and depleted in CO2 exits at the bottom of the first washing column, the NOx-depleted gas is separated by partial condensation and optionally by distillation forming at least one CO2-enriched liquid (33, 38) and at least one CO2-depleted gas (37), at least one portion (39) of one of the at least one CO2-enriched liquid is vaporized, compressed, cooled and sent in the form of gas into the bottom of a second washing column (K3) fed at the top with a liquid (51) containing at least 90 mol% of CO2, a gas (41) depleted in NOx and enriched in CO2 exits the top of the second washing column and a liquid (43) enriched in NO2 and depleted in CO2 exits at the bottom of the second washing column.

Description

[0001] Method and apparatus for separating a mixture of CO2 and NOx by partial condensation

[0002] The present invention relates to a method and apparatus for separating a mixture of CO2 and NOx by partial condensation. The mixture also contains at least one component lighter than CO2.

[0003] In CO2 capture processes at temperatures below 0°C, the inlet gas is compressed, dried, cooled and separated by partial condensation. In the case of gases containing NOx, gas washing is carried out upstream of the partial condensation as illustrated in WO13 / 135993 and WO15 / 148927. As specifications for the purity of the CO2 produced become increasingly stringent, it is therefore necessary to consider new washing methods to achieve the new required levels.

[0004] The present invention proposes a scheme with two columns dedicated to the washing of heavy molecules.

[0005] For these diagrams, the present invention also proposes possibilities of interconnecting the refluxes allowing savings in investment and energy consumption.

[0006] State of the art

[0007] In current low-temperature CO2 capture schemes, the separation of nitrogen oxides from the flue gases occurs throughout the process. A large part of the NO is oxidized to NO2 during compression and within the adsorbents of the dryers (and the PSA for schemes that include them), part of the NO2 is eliminated in the condensates after hydrolysis to HNO3.

[0008] At the outlet of the adsorption stage(s), the NOx are present mainly in the form of NO2.

[0009] These remaining NOx are usually washed in the cryogenic part by the combination of an adsorbent bed accelerating the conversion of NO to NO2 and a washing column.

[0010] In the latter, the NO2 is mainly removed by direct contact with liquid CO2 from the downstream CO2 separation process. This method makes it possible to achieve a NOx content of less than 10 ppm mol in the final product. With more restricted NOx contents in the CO2 produced (around 1 ppm mol or even lower), the conversion to NO2 may prove insufficient before the scrubbing column. One solution would be to modify the location of this column further downstream in the process where temperatures are favorable for the conversion of NO to NO2 as illustrated in W009 / 007937. But in this case, the other heavy molecules (if present) that were separated in the scrubbing column with the NO2 will be able to migrate downstream of the process where they risk freezing. Additionally, in some applications where the treated flue gases are highly loaded with NO and NO2, NO2 or its dimer N2O4 may freeze during vaporization of liquid CO2 at low pressure (approximately 5.5 bara).

[0011] In this case, it is preferable to keep the first washing column and install a second one after the light distillation column, as in a reversed column scheme. Indeed, low temperatures and high pressures accelerate the oxidation of NO to NO2. Thus, even if the NO2 is largely reduced in the first washing column, NO2 can form again between the first washing column and the light distillation column from the NO remaining at the top of the washing column. This will therefore end up in the final product and will not allow the desired content to be reached. A washing column, installed after the distillation column, can thus remedy this problem.

[0012] The use of two NOx scrubbing columns thus provides greater confidence regarding the NOx content in the final product. In addition, this allows for greater flexibility in the NOx content of the gas leaving the first column, as the latter is no longer the sole source of the NOx purity of the product. It is therefore possible to degrade the quality of the liquid used for refluxing this column or to carry out less effective scrubbing without impacting the NOx content of the final product.

[0013] Problems solved by the invention

[0014] A scheme using two NOx scrubbing columns results in treating a higher flow rate of NOx-rich liquid in their tanks compared to a scheme using only one scrubbing column. Usually, the purge from a NOx scrubbing column is recycled upstream of the filtration as described in WO09 / 007937. The scheme proposed by this invention has the advantage of reducing the purge flow rate recycled upstream of the process. A reduced flow rate results in lower energy consumption thanks to the saved compression energy but also a lower investment thanks to machines sized for a lower volume flow rate. This impact mainly affects the first equipment in the process, in particular the compressor and the wet flue gas dryer.

[0015] The invention also has the advantage of overcoming a lack of frigories in the process. Indeed, using less liquid from partial condensation allows it to be recovered in the form of cold in the main exchanger (in particular at the outlet of the light distillation column). This aspect also has the effect of reducing the volume of the main exchanger.

[0016] Finally, the invention makes it possible to increase the pressure at the outlet of the turbo-supercharger and therefore partial condensation and thus to have a better efficiency of the latter. This aspect is made possible by sub-cooling the reflux of the first column, reducing the evaporation of the latter in the top of the column and therefore minimizing the flow rate at the inlet of the supercharger.

[0017] According to an object of the invention, there is provided a method for separating a mixture of CO2, NOx and at least one component lighter than CO2 by partial condensation in which: i. the mixture is cooled and sent to the bottom of a first washing column fed at the top with a liquid containing at least 90 mol% of CO2, a gas depleted in NOx and enriched in CO2 relative to the mixture leaves the top of the first washing column and a liquid enriched in NOx, for example in NO2 and depleted in CO2 relative to the mixture leaves the bottom of the first washing column ii. the gas depleted in NOx is separated by partial condensation and optionally by distillation forming at least one liquid enriched in CO2 relative to the gas depleted in NOx and at least one gas depleted in CO2 relative to the gas depleted in NOx iii.at least a portion of one of the at least one liquid enriched in CO2 is vaporized, compressed, cooled and sent in the form of gas to the bottom of a second washing column supplied at the top with a liquid containing at least 90 mol% of CO2, a gas depleted in NOx and enriched in CO2 compared to the gas sent to the bottom of the second washing column leaves the top of the second washing column and a liquid enriched in NO2 and depleted in CO2 compared to the gas sent to the bottom of the second washing column leaves the bottom of the second washing column. According to other optional characteristics:.

[0018] • at least part of the liquid sent to the top of the first washing column is a liquid produced by partial condensation and / or distillation of the NOx-depleted gas coming from the top of the first washing column.

[0019] • at least part of the liquid sent to the top of the first washing column is bottom liquid from the second washing column.

[0020] • the tank liquid from the second washing column constitutes the only liquid sent to the top of the first washing column.

[0021] • at least part of the liquid sent to the top of the first washing column is pressurized by a pump.

[0022] • the second washing column is arranged vertically above the first washing column.

[0023] • the bottom liquid from the second washing column is cooled before being sent to the top of the first washing column.

[0024] • the bottom liquid of the second washing column is cooled by indirect heat exchange with the overhead gas of the second washing column and / or the overhead gas of the first washing column.

[0025] • the flow rate of the bottom liquid of the second washing column is regulated before being sent to the top of the first washing column, in order to adjust the reflux rate of the first washing column.

[0026] • the gas mixture contains between 35 and 95% mol of CO2,

[0027] • the gas mixture contains at least one component lighter than CO2, for example nitrogen, oxygen, carbon monoxide

[0028] • the gas mixture contains at least one component heavier than CO2 including NOx and possibly at least one other heavier component such as mercury.

[0029] • the first washing column operates at between 15 and 40 bara, for example between 20 and 25 bara

[0030] According to another object of the invention, there is provided an apparatus for separating a mixture of CO2, NOx and at least one component lighter than CO2 by partial condensation comprising a first washing column, a second washing column, means for cooling the mixture, means for sending the cooled mixture to the bottom of the first washing column, means for feeding the first mixing column at the top with a liquid containing at least 90 mol% of CO2, means for removing a gas depleted in NOx and enriched in CO2 relative to the mixture from the top of the first washing column, means for removing a liquid enriched in NO2 and depleted in CO2 relative to the mixture in the bottom of the first washing column, at least one phase separator, means for sending the gas depleted in NOx to be separated by partial condensation in the at least one phase separator and optionally by distillation in a distillation column,forming at least one liquid enriched in CO2 relative to the gas depleted in NOx and at least one gas depleted in CO2 relative to the gas depleted in NOx, means for vaporizing at least a portion of one of the at least one liquid enriched in CO2, a compressor for compressing the vaporized liquid, means for cooling the vaporized liquid compressed in the compressor and means for sending the cooled vaporized liquid in the form of gas to the bottom of the second washing column, means for feeding the second washing column at the top with a liquid containing at least 90 mol% of CO2, means for removing a gas depleted in NOx and enriched in CO2 relative to the gas sent to the bottom of the second washing column from the top of the second washing column and means for removing a liquid enriched in NO2 and depleted in CO2 relative to the gas sent to the bottom of the second washing column from the bottom of the second washing column.

[0031] Description of the invention

[0032] The invention will be described in more detail with reference to the figures where: [FIG.1] schematically represents a method according to the invention.

[0033] [FIG.2] schematically represents a method according to the invention which is a variant of that of [FIG.1].

[0034] [FIG.3] schematically represents a method according to the invention which is a variant of that of [FIG.2]

[0035] [FIG.4] schematically represents a method according to a simplified variant according to the invention.

[0036] [FIG.5] schematically represents a method according to a simplified variant according to the invention. [FIG.1] shows an apparatus for separating a gas mixture 1 containing between 35 and 95 mol% of CO2, at least one component lighter than CO2, for example nitrogen, oxygen, carbon monoxide and at least one component heavier than CO2 including NOx and possibly at least one other heavier component such as mercury.

[0037] The gas mixture 1 is cooled in a first heat exchanger HX1 and is sent to the bottom of a first washing column K1 operating at between 15 and 40 bara, for example between 20-25 bara. The column is fed at the top with a washing liquid 45 rich in CO2, containing for example at least 90 mol% of CO2, and produces in the bottom a liquid enriched in NOX but also containing CO2. The top gas 5 is enriched in CO2 and in the at least one lighter component compared to the mixture 1.

[0038] A liquid 3 is drawn off from the bottom of column K1, enriched in NOx compared to gas 1. This liquid can be sent upstream of the heat exchanger HX1 in a part of the process where the NOx are removed.

[0039] The overhead gas 5 is heated in the heat exchanger HX1 and then compressed in a booster B to a pressure higher than that of the column K1.

[0040] The supercharger B is preferably coupled to a turbine in which a gas 47 expands.

[0041] The pressurized gas 5 cools in a second heat exchanger HX2 to an intermediate temperature thereof to partially condense. The two-phase flow formed is separated in a phase separator S1 forming a gas 9 depleted in CO2 and enriched in the at least one lighter component compared to the gas 5 and a liquid 11 enriched in CO2 and depleted in the at least one lighter component compared to the gas 5. The gas 9 is partially condensed in the heat exchanger HX2, then sent to the phase separator S2 forming a gas 15 depleted in CO2 and enriched in the at least one lighter component compared to the gas 9 and a liquid 17 enriched in CO2 and depleted in the at least one lighter component compared to the gas 9.

[0042] At least a portion of the liquid 11 and / or the liquid 17 constitutes at least a portion of the liquid 45 sent to the top of the column K1.

[0043] At least a portion of the liquid 11 and / or the liquid 17 constitutes at least a portion of the liquid 21 sent to the top of the column K2 after expansion through the valve V2. The column K2 operates as a stripping column at a pressure between 7 and 20 bara to reduce the content of the at least one lighter component of the liquid 21 producing a gas 37 at the top of the column. The CO2 concentrates in the bottom of the column K2 with the remainder of the NOx in the form of NO2 and is withdrawn as liquid 23.

[0044] The liquid 23 is divided into two flows 25, 27, the flow 27 being vaporized in the exchanger HX2 forming a gas containing CO2 and NOx MP CO2. The flow 25 is expanded in a valve V3 to partially vaporize it and separated in a phase separator S3 forming a gas 31 and a liquid 29. The gas 31 heats up in the heat exchanger HX2 where the liquid 29 vaporizes and the heated gas and the vaporized liquid mix forming a gas 33 which is CO2 containing NOx at a lower pressure than that of the gas 38.

[0045] Gas 33 and / or gas 38 may constitute at least a portion of gas 39 containing more NOx than desired in the final product. Gas 39 is preferably at a pressure between 10 and 50 bars, following a compression step (not shown). Gas 39 cools in heat exchanger HX1 and is sent to the bottom of a second scrubbing column K3 supplied at the top with liquid CO2 51 containing less NOx than gas 39 and containing for example at least 90 mol% CO2. A gas 41 purified of NOx is withdrawn at the top of column K3. A liquid 43 is withdrawn at the bottom of column K3, enriched in NOx compared to gas 39. This liquid 43 may be sent upstream of heat exchanger HX1 in a part of the process where the NOx are removed.

[0046] Otherwise according to a variant which is that illustrated, the liquid 43 is sent as reflux for the first column K1.

[0047] To achieve this scheme, two configurations can be considered. The first, illustrated in [FIG.1] consists of using a pump P to convey the liquid from column K3 to column K1 and thus avoid possible problems linked to a pressure difference between the two columns linked to their location in the process.

[0048] A second version illustrated in [FIG.2] consists of superimposing column K3 vertically above column K1 in order to take advantage of the hydrostatic height and therefore to be able to convey the liquid from the second washing column to the first washing column. This scheme is possible when the second washing column K3 has an operating pressure sufficiently higher than the first K1.

[0049] According to a variant, the pressure of column K3 can be higher than the pressure of column K1 and the liquid 45 can be expanded, without column K3 necessarily being above column K1.

[0050] Depending on the liquid flow rate 43, 45 available at the outlet of the second washing column K3, the available reflux may not be sufficient to guarantee a liquid load allowing efficient distillation in the first column K1. This flow rate can thus be supplemented by adding part of the liquid from the separation process 11 and / or 17 (such as that from partial condensation) in order to achieve a sufficient liquid load.

[0051] Another solution is to ensure excess reflux in the second washing column K3 in order to obtain a sufficient liquid load in the first washing column K1. It is obviously possible to consider intermediate solutions where the reflux of the second column K3 exceeds the minimum value necessary for its proper operation and is still supplemented by liquid from the separation process.

[0052] In order to limit the evaporation of the reflux from the second washing column K3 entering the first column K1, it is possible to subcool this liquid 43, as illustrated in [FIG.3]. In particular, if the first column K1 is entirely fed at the top by the purge 43 of the second washing column K3, this operation makes it possible to significantly reduce the reflux in the second column K3 to guarantee a sufficient liquid load in the first column K1 and therefore to reduce the specific energy because the withdrawal from the production (origin of the reflux from the second washing column K3) is lower.

[0053] The previous point also shows the possibility, in the case of a single feed of the first column K1 by the second washing column K3, of direct control of the liquid load of the first column K1 by this reflux 43. A variation of the reflux 43 coming from the second column K3 will make it possible to have the desired liquid / vapor ratio in the first column.

[0054] In this case, liquids 11, 17 do not feed column K1.

[0055] As noted above, the NOx content of the gas 5 leaving the first column K1 may be higher than according to the prior art, the latter no longer being the sole source of the NOx purity of the product 41. It is therefore possible to degrade the quality of the liquid 45 used for the reflux of this first column. Thus the liquid 45 may contain less CO2 and / or more NOx than the liquid 51, containing for example at least 90 mol% of CO2, used for the reflux of the second washing column K3.

[0056] Similarly, it is possible to carry out a less efficient washing without impacting the NOx content of the final product. Thus the flow rate of liquid 45 can be reduced or the K1 column can include fewer theoretical plates.

[0057] In some cases it is possible to eliminate column K2, all separation of the at least one component lighter than CO2 being achieved by at least one partial condensation step, the produced liquid being vaporized, compressed and sent to the second washing column K3.

[0058] Figure 4 is a simplified variant of Figure 2 where column K2 is not present. At least a portion of the liquid 21 is vaporized and heated and potentially compressed to form the gas 39.

[0059] Figure 5 is a simplified variant of Figure 3 where column K2 is not present. At least part of the liquid 21 is vaporized and reheated to form the gas 39. It differs from Figure 4 by the subcooling of the liquid 43.

Claims

Claims 1. Method for separating a mixture of CO2, NOx and at least one component lighter than CO2 by partial condensation in which: i. the mixture is cooled and sent to the bottom of a first washing column (K1) fed at the top with a liquid (13, 43, 45) containing at least 90 mol% of CO2, a gas (5) depleted in NOx and enriched in CO2 relative to the mixture leaves the top of the first washing column and a liquid (3) enriched in NOx, for example in NO2 and depleted in CO2 relative to the mixture leaves the bottom of the first washing column ii. the gas depleted in NOx is separated by partial condensation and optionally by distillation forming at least one liquid (21, 33, 38) enriched in CO2 relative to the gas depleted in NOx and at least one gas (15, 37) depleted in CO2 relative to the gas depleted in NOx iii.at least a portion (39) of one of the at least one liquid enriched in CO2 is vaporized, compressed, cooled and sent in the form of gas to the bottom of a second washing column (K3) supplied at the top with a liquid (51) containing at least 90 mol% of CO2, a gas (41) depleted in NOx and enriched in CO2 compared to the gas sent to the bottom of the second washing column leaves the top of the second washing column and a liquid (43) enriched in NO2 and depleted in CO2 compared to the gas sent to the bottom of the second washing column leaves the bottom of the second washing column.

2. Method according to claim 1 in which at least part of the liquid (13) sent to the top of the first washing column (K1) is a liquid produced by partial condensation and / or distillation of the gas (5) depleted in NOx coming from the top of the first washing column.

3. Method according to claim 1 or 2 in which at least a portion (43) of the liquid sent to the top of the first washing column (K1) is liquid from the bottom of the second washing column (K3).

4. Method according to claim 3 in which the tank liquid (43) of the second washing column (K3) constitutes the only liquid sent to the top of the first washing column (K1).

5. Method according to one of claims 2, 3 or 4 in which at least part of the liquid sent to the top of the first washing column (K1) is pressurized by a pump (P).

6. Method according to one of claims 1 to 5 in which the second washing column (K3) is arranged vertically above the first washing column (K1).

7. Method according to one of claims 3 to 6 in which the tank liquid of the second washing column (K3) is cooled before being sent to the top of the first washing column (K1).

8. Method according to claim 7 wherein the tank liquid (43) of the second washing column (K3) is cooled by indirect heat exchange with the overhead gas (41) of the second washing column and / or the overhead gas (5) of the first washing column (K1).

9. Method according to one of claims 3 to 8 in which the flow rate of the tank liquid (43) of the second washing column (K3) is regulated before being sent to the top of the first washing column (K1), in order to adjust the reflux rate of the first washing column.

10. Apparatus for separating a mixture of CO2, NOx and at least one component lighter than CO2 by partial condensation comprising a first washing column (K1), a second washing column (K3), means for cooling the mixture (HX1), means for sending the cooled mixture (1) to the bottom of the first washing column, means for feeding the first mixing column at the top with a liquid (45) containing at least 90 mol% of CO2, means for removing a gas (5) depleted in NOx and enriched in CO2 relative to the mixture from the top of the first washing column, means for removing a liquid (3) enriched in NO2 and depleted in CO2 relative to the mixture in the bottom of the first washing column, at least one phase separator (S1, S2), means for sending the gas depleted in NOx to be separated by partial condensation in the at least one phase separator and possibly by distillation in a distillation column (K2),forming at least one CO2-enriched liquid (33, 38) relative to the NOx-depleted gas and at least one CO2-depleted gas (37) relative to the NOx-depleted gas, means (HX2) for vaporizing at least a portion of one of the at least one CO2-enriched liquid, a compressor for compressing the vaporized liquid, means for cooling the, vaporized liquid (HX1) compressed in the compressor and means for sending the vaporized liquid cooled in the form of gas (39) to the bottom of the second washing column, means for feeding the second washing column at the top with a liquid (51) containing at least 90 mol% of CO2, means for removing a gas (41) depleted in NOx and enriched in CO2 compared to the gas sent to the bottom of the second washing column from the top of the second washing column and means for removing a liquid (43) enriched in NO2 and depleted in CO2 compared to the gas sent to the bottom of the second washing column from the bottom of the second washing column.

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