Method for capturing CO2 from a mixture containing at least carbon dioxide, at least 1 mol% oxygen, nitrogen and at least one hydrocarbon

Abstract

Title: Process for capturing CO2 from a mixture containing at least carbon dioxide, at least 1 mol% of oxygen, nitrogen and at least one hydrocarbon In a process for capturing CO2 from a mixture (1, 3) containing at least carbon dioxide, at least 1 mol%, or even at least 3 mol% of oxygen, and at least one hydrocarbon, the mixture is purified of hydrocarbons by catalytic oxidation (CAT), upstream of the low-temperature separation, using the oxygen already present in the mixture. FIG. 2

Description

Title of the invention: Method for capturing CO2 from a mixture containing at least carbon dioxide, at least 1 mol% oxygen, nitrogen and at least one hydrocarbon

[0001] The present invention relates to a method and apparatus for capturing CO2 from a mixture containing at least carbon dioxide, at least 1 mol% of oxygen, nitrogen and at least one hydrocarbon.

[0002] The gas mixture may contain at least carbon dioxide, at least 1 mol%, or even at least 3 mol% of oxygen, nitrogen and at least one hydrocarbon CnHm greater than or equal to C2 and may also contain one of the following compounds: water, argon, at least one nitrogen oxide, at least one sulfur compound and at least one heavy metal. Preferably it contains less than 10 mol%, or even less than 5 mol%, of methane.

[0003] The carbon dioxide (CO2) produced by capture units can either be sequestered in geological structures, such as saline aquifers or depleted gas fields; or it can be used for various possible applications, such as methanol synthesis or the food industry. For the latter application, the specifications on the purity of the CO2 produced are more stringent than those for CCS (carbon capture and storage). In addition, the sizes of CO2 capture units for food are significantly smaller because the demand is limited. The capture process flowsheet must be adapted to these specific constraints.

[0004] State of the art

[0005] Many CO2 capture process schemes are known to those skilled in the art. There are many schemes: capture by chemical or physical absorption, by adsorption, or by cryogenic separation.

[0006] It is often necessary to add a catalytic oxidation (CATOX) step to remove traces of hydrocarbons in order to achieve food-grade purity of the CO2 produced. In this reactor, hydrocarbons react with oxygen (O2) to produce CO2 and water.

[0007] When CO2 is captured from the tail gas of an H2 PSA, the mixture contains a lot of hydrogen, carbon monoxide and hydrocarbons (mainly methane). If these molecules are not removed before CATOX, their economic values ​​ / calorific values ​​will be lost in CATOX. It is therefore preferable to extract these molecules before CATOX. In this case, the state of the art is to install this catalytic reactor downstream of the capture process ([Fig. 1]). It is then necessary to install a second dryer (to remove the water produced by the reaction) and to carry out a second separation (to eliminate excess O2).

[0008] Food grade CO2 can be produced from almost pure CO2 by a liquefier. The CATOX is then used only to remove traces of hydrocarbons. In this case, the CATOX is installed upstream of the dryer as well as the cryogenic separation of impurities lighter than CO2 ([Fig.2]), in order to install only one dryer as well as to separate the O2 from the CO2 only once.

[0009] Problem solved by the invention

[0010] Food grade CO2 can also be produced from flue gases. However, these flue gases contain compounds that are not present in state-of-the-art CO2 sources. The flue gases may contain nitrogen oxides (NOX) or sulfur impurities or heavy metals. It is still possible to use the scheme of [Fig.l], however the invention will present a more competitive configuration.

[0011] Some catalysts for CATOX (containing palladium) are poisoned by the presence of sulfur compounds. Two options are possible: choose a sulfur-tolerant catalyst or eliminate sulfur compounds before CATOX.

[0012] Sulfur-tolerant catalysts can be more expensive and often require operation at higher temperatures to achieve the same performance. These tolerant catalysts allow the use of the state-of-the-art scheme of [Fig.2], however their reduced performance does not make this configuration competitive.

[0013] Activated carbon is a good adsorbent for stopping sulfur impurities, however other impurities present in the fumes, such as NOX or heavy hydrocarbons, will also be adsorbed. It is therefore necessary to eliminate a majority of the impurities before separation by adsorption with activated carbon to avoid saturating it quickly. Washing with liquid CO2 is effective for stopping less volatile molecules (such as NO2 or heavy hydrocarbons) than CO2.

[0014] The invention proposes a new scheme where the catalytic oxidation reactor is integrated within the CO2 capture unit. Impurities incompatible with CATOX are removed before the reactor. This scheme makes it possible to produce food-grade CO2 with a limited number of pieces of equipment, thus reducing the investment cost of the CO2 capture unit.

[0015] According to an object of the invention, there is provided a method for capturing CO2 from a mixture containing at least carbon dioxide, at least one component less volatile than CO2, at least 1 mol%, or even at least 3 mol% of oxygen, nitrogen and at least one hydrocarbon CnHm greater than or equal to C2 and which may also contain one of the following compounds: water, argon, at least one nitrogen oxide, at least one sulfur compound and at least one heavy metal comprising the following steps: i. Compression of the mixture. ii. Drying of the compressed mixture. iii. Cooling of the compressed and dried mixture in a heat exchanger. iv. Purifying the compressed and dried mixture to produce a fluid enriched in at least one component less volatile than CO2 relative to the mixture and a fluid depleted in at least one component less volatile than CO2 and enriched in at least one component more volatile than CO2 relative to the mixture. v. Heating the fluid depleted in at least one component less volatile than CO2 in the heat exchanger. vi. Catalytic oxidation at a temperature less than or equal to 520°C of the fluid depleted in at least one component less volatile than CO2 producing a fluid depleted in at least one hydrocarbon CnHm greater than or equal to C2 containing water resulting from the oxidation. vii. Drying of the fluid depleted in at least one hydrocarbon. viii. Cooling the dried fluid from step vii) in the heat exchanger and separation by partial condensation and / or distillation and / or desublimation in order to obtain a liquid enriched in CO2 compared to the mixture and a gas depleted in CO2 compared to the mixture.

[0016] According to other optional objects of the invention: • step iv) consists of a washing step with liquid CO2 at a temperature below -50°C. • step iv) produces a fluid depleted in at least one component less volatile than CO2 and enriched in oxygen compared to the mixture • if the mixture to be treated contains at least one sulfur compound, an adsorption separation step is added between step iv) and the catalytic oxidation. • if the mixture to be treated contains mercury, a second adsorption separation step is added between step iv) and catalytic oxidation. • the CO2-enriched liquid contains a total of hydrocarbons in methane equivalent of less than 50 ppmv and preferably less than 10 ppmv and a total of hydrocarbons excluding methane in methane equivalent of less than 20 ppmv and preferably less than 5 ppmv.

[0017] According to the methane equivalent principle, a molecule containing two carbon atoms counts as two methane equivalents, a molecule containing three carbon atoms counts as three methane equivalents, and so on. • the fluid dried in step vii) comprises oxygen and the separation of step viii) comprises at least one partial condensation step and optionally one distillation step, the fluid dried and cooled in the heat exchanger being partially condensed in the heat exchanger heat and separated into an oxygen-enriched, CO2-depleted gas and an oxygen- and CO2-depleted liquid in a phase separator, the oxygen-depleted liquid is separated by being sent to the top of a distillation column or constituting the CO2-rich product. • the oxygen-enriched gas is reheated in the heat exchanger, separated by permeation downstream of the exchanger to remove remaining oxygen forming a recycle gas containing CO2 which is mixed with the mixture during one of steps i) and ii). • no oxygen flow is added to the fluid depleted in at least one component less volatile than CO2 upstream of the catalytic oxidation. • the mixture contains less than 10 mol%, or even less than 5 mol%, of methane.

[0018] According to another object of the invention, there is provided an apparatus for capturing CO2 from a mixture containing at least carbon dioxide, at least one component less volatile than CO2, at least 1 mol%, or even at least 3 mol% of oxygen, nitrogen and at least one hydrocarbon CnHm greater than or equal to C2 and which may also contain one of the following compounds: water, argon, at least one nitrogen oxide, at least one sulfur compound and at least one heavy metal comprising a compressor for compressing the mixture, a first dryer for drying the compressed mixture, a heat exchanger for cooling the compressed and dried mixture, means for purifying the compressed, dried and cooled mixture in the heat exchanger to produce a fluid enriched in the at least one component less volatile than CO2 relative to the mixture and a fluid depleted in at least one component less volatile than CO2 and enriched in at least one more volatile component than CO2 compared to the mixture,a pipe connected to the means for purifying the mixture and to the exchanger to send there fluid depleted in at least one component less volatile than CO2 to heat up, a catalytic oxidation unit capable of operating at a temperature less than or equal to 520°C, means for sending fluid depleted in at least one component less volatile than CO2 heated in the heat exchanger to the catalytic oxidation unit, a second dryer, a pipe for sending a fluid depleted in at least one hydrocarbon CnHm greater than or equal to C containing water from the oxidation unit to the second dryer producing fluid depleted in at least one dried hydrocarbon, a pipe for sending the fluid depleted in at least one dried hydrocarbon to the heat exchanger, means for separation by partial condensation and / or distillation and / or desublimation,a pipe for sending the fluid depleted in at least one dried hydrocarbon cooled in the heat exchanger to the separation means, a pipe for outputting a liquid enriched in CO2 relative to the mixture from the separation means and, a pipe to release a gas depleted in CO2 compared to the mixture of separation means.

[0019] Description of the invention

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

[0021] [Fig-1] represents an alternative method which is not that of the invention.

[0022] [Fig.2] represents a method according to the invention.

[0023] [Fig.l] represents a process in which a gas 1 is compressed in a four-stage compressor C1, C2, C3, C4, each stage being followed by a cooler R1, R2, R3, R4. The gas 1 is a mixture containing at least carbon dioxide, at least one component less volatile than CO2, for example NO2, at least 1 mol%, or even at least 3 mol% of oxygen, nitrogen and at least one hydrocarbon CnHm greater than or equal to C2 and which may also contain one of the following compounds: water, argon, at least one nitrogen oxide, at least one sulfur compound and at least one heavy metal. The cooling produces condensed water W1, W2, W3, W4 which is collected forming a flow W. The compressed mixture is dried by temperature-swing adsorption in an adsorption unit TSA1.The dried mixture cools in a heat exchanger E1 and leaves the exchanger at an intermediate temperature thereof to be separated in a washing column K1 supplied at the top with liquid CO2 33. The column produces a liquid 5 enriched in at least one component less volatile than CO2 compared to the mixture 1 and a gas 3 depleted in at least one component less volatile than CO2 and enriched in at least one component more volatile, in particular oxygen, than CO2 compared to the mixture. The liquid 5 is vaporized by the heater H1 and mixed with gas 1.

[0024] The gas 3 cools in the heat exchanger El to the cold end where it partially condenses. The two-phase flow is sent to a phase separator SI producing a gas 7 and a liquid 9. The gas 7 heats up in the heat exchanger El and is separated by permeation M forming a permeate gas enriched in CO2 11 which is recycled to the gas 1 upstream of Cl and a retentate 13 which is a residual gas.

[0025] The liquid 9 is expanded and sent to the top of a column K2. The column K2 produces at the top a gas 15 enriched in at least one component more volatile than CO2 and at the bottom a liquid 17 depleted in at least one component more volatile than CO2. The gas 15 is heated in the heat exchanger El and sent between the cooler R3 and the compressor C4. The liquid 17 is vaporized and heated in the heat exchanger El and the gas formed is mixed with gaseous oxygen 19. The gas formed 21 is heated in a heat exchanger E2, heated by a heater H2 and sent to a catalytic oxidation unit CAT where at least one hydrocarbon CnHm greater than or equal to C2 reacts with the oxygen forming water. Gas 23 leaves the CAT oxidation unit and cools in the E2 exchanger and then in the cooler R5. The cooled gas 23 is dried in the second temperature swing adsorption unit TSA2 producing a dried gas 24 which is cooled in the heat exchanger E2. The gas 24 is expanded to form a liquid at the top of a column K3. A CO2-rich product 27 is produced at the bottom of the column K3, a part 29 of which constitutes the product. Another part 31 is vaporized in the heat exchanger and sent to the bottom of the column K3 as a reboiling gas. Another part 33 is the washing liquid of the column CL

[0026] The TSA 1 unit produces a condensate W6. The TSA2 unit produces a wet gas W5.

[0027] [Fig.2] differs from [Fig. 1] in that the column K2 which reduces the content of at least a component more volatile than CO2 is no longer present and in that no oxygen flow is added upstream of the catalytic oxidation, the oxygen present in gas 1 being sufficient to feed the catalytic oxidation.

[0028] The dried mixture cools in a heat exchanger E1 and leaves the exchanger at an intermediate temperature thereof to be separated in a washing column K1 supplied at the top with liquid CO2 33. The column produces a liquid 5 enriched in at least one component less volatile than CO2 compared to the mixture 1 and a gas 3 depleted in at least one component less volatile than CO2 and enriched in at least one component more volatile, in particular oxygen, than CO2 compared to the mixture. The liquid 5 is vaporized by the heater H1 and mixed with gas 1.

[0029] If the mixture to be treated 1, 3 contains sulfur compounds, an adsorption separation step is added between the CO2 washing in Kl and the catalytic oxidation CAT.

[0030] If the mixture to be treated 1, 3 contains mercury, an adsorption separation step is added between the CO2 washing in Kl and the catalytic oxidation CAT.

[0031] The gas 3 is heated in the heat exchangers E1, E2 and by the heater H2. The heated gas 3 is sent to a catalytic oxidation unit CAT where at least one hydrocarbon CnHm greater than or equal to C2 reacts with oxygen forming water. The gas 4 leaves the oxidation unit CAT and cools in the exchanger E2 and then in the cooler R5. The cooled gas 4 is dried in the second temperature swing adsorption unit TSA2 producing a dried gas 24 which is cooled in the heat exchanger and partially condenses there. The two-phase flow is separated in a phase separator SL. The liquid 9 from the separator SI is expanded to form a liquid at the top of a column K3. A CO2-rich product 27 is produced in the bottom of the column K3 and sent to a pump forming a pressurized liquid, a portion 29 of which constitutes the product.Another part 31 is vaporized in the heat exchanger and sent to the bottom of column K3 as reboiling gas. Another part 33 is the washing liquid of column CL.

[0032] Column K3 is not necessarily present. Partial condensation may comprise at least two partial condensation stages. Separation may understand a desublimation step.

[0033] The gas 25 from column K3 is reheated in the heat exchanger El still contains oxygen and is separated by permeation M to form a permeate 8 enriched in CO2 and depleted in oxygen which is sent to be mixed with the gas 1 upstream of the compressor Cl. The gas 10 depleted in CO2 and enriched in oxygen serves as waste gas.

[0034] Unit TSA 1 produces a condensate W6. Unit TSA2 produces a wet gas W5.

Claims

Claims

1. Method for capturing CO2 from a mixture containing at least carbon dioxide, at least one component less volatile than CO2, at least 1 mol%, or even at least 3 mol% of oxygen, nitrogen and at least one hydrocarbon CnHm greater than or equal to C2 and which may also contain one of the following compounds: water, argon, at least one nitrogen oxide, at least one sulfur compound and at least one heavy metal, comprising the following steps: i. Compression (Cl, C2, C3, C4) of the mixture (1) ii. Drying (TSA1) of the compressed mixture. iii. Cooling of the compressed and dried mixture in a heat exchanger (El). iv. Purification (Kl) of the compressed and dried mixture to produce a fluid enriched in at least one component less volatile than CO2 compared to the mixture and a fluid (3) depleted in at least one component less volatile than CO2 and enriched in at least one component more volatile than CO2 compared to the mixture. v. Heating the fluid depleted in at least one component less volatile than CO2 in the heat exchanger. vi. Catalytic oxidation (CAT) at a temperature less than or equal to 520°C of the fluid depleted in at least one component less volatile than CO2 producing a fluid (4) depleted in at least one hydrocarbon CnHm greater than or equal to C2 containing water resulting from the oxidation. vii. Drying (TSA2) of the fluid depleted in at least one hydrocarbon viii. Cooling of the dried fluid from step vii) in the heat exchanger and separation (SI, K3) by partial condensation and / or distillation and / or desublimation in order to obtain a liquid (9, 27, 29, 31) enriched in CO2 compared to the mixture and a gas (7, 25) depleted in CO2 compared to the mixture.

2. Method according to the preceding claim, in which step iv) consists of a step of washing with liquid CO2 (33) at a temperature below -50°C.

3. Method according to claim 1 or 2, in which if the mixture to be treated contains at least one sulfur compound, a step of separation by adsorption is added between step iv) and the catalytic oxidation (CAT).

4. Method according to one of the preceding claims, in which, if the mixture to be treated contains mercury, a second step of separation by adsorption is added between step iv) and the catalytic oxidation (CAT).

5. Method according to one of the preceding claims in which the CO2-enriched liquid (27, 29, 31) contains a total of hydrocarbons in methane equivalent of less than 50 ppmv and preferably less than 10 ppmv and a total of hydrocarbons excluding methane in methane equivalent of less than 20 ppmv and preferably less than 5 ppmv.

6. Method according to one of the preceding claims in which the fluid (4) dried in step vii) comprises oxygen and the separation (SI, K3) of step viii) comprises at least one partial condensation step and optionally a distillation step, the fluid dried and cooled in the heat exchanger (El) being partially condensed in the heat exchanger and separated into a gas (7) enriched in oxygen and depleted in CO2 and a liquid (9) depleted in oxygen and CO2 in a phase separator, the oxygen-depleted liquid being separated by being sent to the top of a distillation column (K3) or constituting the CO2-rich product.

7. A method according to claim 6 wherein the oxygen-enriched gas (7) is reheated in the heat exchanger (El), separated by permeation (M) downstream of the exchanger to remove remaining oxygen forming a CO2-containing recycle gas (8) which is mixed with the mixture during one of steps i) and ii).

8. Method according to one of the preceding claims in which no flow of oxygen is added to the fluid (3) depleted in at least one component less volatile than CO2 upstream of the catalytic oxidation (CAT).

9. Method according to one of the preceding claims in which the mixture (1) contains less than 10 mol%, or even less than 5 mol%, of methane.

10. Apparatus for capturing CO2 from a mixture (1) containing at least carbon dioxide, at least one component less volatile than CO2, at least 1 mol%, or even at least 3 mol% of oxygen, nitrogen and at least one hydrocarbon CnHm greater than or equal to C2 and capable of also contain one of the following compounds: water, argon, at least one nitrogen oxide, at least one sulfur compound and at least one heavy metal comprising a compressor (Cl, C2, C3, C4) for compressing the mixture, a first dryer (TSA1) for drying the compressed mixture, a heat exchanger (El) for cooling the compressed and dried mixture, means (Kl) for purifying the compressed, dried and cooled mixture in the heat exchanger to produce a fluid enriched in the at least one component less volatile than CO2 compared to the mixture and a fluid (3) depleted in the at least one component less volatile than CO2 and enriched in at least one component more volatile than CO2 compared to the mixture, a pipe connected to the means for purifying the mixture and to the exchanger to send therein fluid (3) depleted in the at least one component less volatile than CO2 to heat up, an oxidation unit catalytic converter (CAT) capable of operating at a temperature less than or equal to 520°C,means for sending fluid (3) depleted in less volatile components than the CO2 heated in the heat exchanger to the catalytic oxidation unit, a second dryer (TSA2), a pipe for sending a fluid depleted in at least one hydrocarbon CnHm greater than or equal to C2 containing water from the oxidation unit to the second dryer producing fluid depleted in at least one dried hydrocarbon, a pipe for sending the fluid depleted in at least one dried hydrocarbon to the heat exchanger, means (SI, K3) for separation by partial condensation and / or distillation and / or desublimation, a pipe for sending the fluid depleted in at least one dried hydrocarbon cooled in the heat exchanger to the separation means, a pipe for outputting a liquid (9, 27, 29 31) enriched in CO2 relative to the mixture from the separation means and a pipe (7, 25) to release a gas depleted in CO2 compared to the mixture of the separation means.,

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