Method for capturing co2 from a mixture containing at least carbon dioxide, at least 1 mol% oxygen, nitrogen and at least one hydrocarbon

Abstract

The invention relates to a method for capturing CO2 from a mixture (1, 3) containing at least carbon dioxide, at least 1 mol%, or even at least 3 mol%, oxygen, and at least one hydrocarbon, in which method hydrocarbons are removed from the mixture by catalytic oxidation (CAT), upstream of the low-temperature separation, using the oxygen already present in the mixture.

Description

The present invention relates to a process and to an apparatus for the capture of CO2 from a mixture containing at least carbon dioxide, at least 1 mol% of oxygen, nitrogen and at least one hydrocarbon. The gas mixture can contain at least carbon dioxide, at least 1 mol%, indeed even at least 3 mol%, of oxygen, nitrogen and at least one CnHm hydrocarbon of greater than or equal to C2 and can 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%, indeed even less than 5 mol%, of methane. The carbon dioxide (CO2) produced by the capture units can either be sequestered in geological structures, such as saline aquifers or exhausted gas fields; or be used for various possible applications, such as the synthesis of methanol or else the food industry. For this latter application, the specifications with regard to the purity of the CO2 produced are more stringent than those of CCS (Carbon Capture and Storage). In addition, the sizes of the CO2 capture units for the food industry are significantly smaller because the demand is limited. The scheme of the capture process must be adapted to these specific constraints. State of the art Many CO2 capture process schemes are known to a person skilled in the art. There exist many schemes: capture by chemical or physical absorption, by adsorption or by cryogenic separation. It is often necessary to add a catalytic oxidation (CATOX) stage to remove traces of hydrocarbons in order to achieve the purity of the food-grade CO2 produced. In this reactor, the hydrocarbons react with oxygen (O2) to produce CO2 and water. When CO2 is captured with regard to the tail gas of an H2 PSA, the mixture contains a great deal of hydrogen, carbon monoxide and hydrocarbons (mainly methane). If these molecules are not removed before the CATOX, their economic values / gross calorific values will be lost in the CATOX. It is thus preferable to extract these molecules before the CATOX. In this case, the state of the art consists in installing this catalytic reactor downstream of the capture process (figure 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 remove the excess O2). Food-grade CO2 can be produced from virtually pure CO2 by a liquefier. The CATOX then serves solely to remove the traces of hydrocarbons. In this case, the CATOX is installed upstream of the dryer and also of the cryogenic separation of the impurities lighter than CO2 (figure 2), in order to install only a single dryer and also to separate the O2 from the CO2 only once. Problem solved by the invention Food-grade CO2 can also be produced from flue gases. However, these flue gases contain compounds which are not present in the CO2 sources of the state of the art. The flue gases can contain nitrogen oxides (NOx) or sulfur-based impurities or heavy metals. It is still possible to use the scheme of figure 1; however, the invention will present a more competitive configuration. Some catalysts for the CATOX (containing palladium) are poisoned by the presence of sulfur compounds. Two options are conceivable: to choose a catalyst tolerant to sulfur compounds or to remove the sulfur compounds before the CATOX. Catalysts tolerant to sulfur compounds can be more expensive and often have to be employed at a higher temperature to achieve the same performance qualities. These tolerant catalysts make it possible to use the scheme of the state of the art of figure 2; however, their decrease in performance quality does not make this configuration competitive. Activated carbon is a good adsorbent for stopping sulfur-based impurities; however, the other impurities present in flue gases, such as NOx or heavy hydrocarbons, will also be adsorbed thereon. It is thus necessary to remove a majority of the impurities before separation by adsorption with activated carbon to avoid rapidly saturating it. Washing with liquid CO2 is effective in stopping the molecules which are less volatile (such as NO2 or heavy hydrocarbons) than CO2. The invention provides a novel scheme where the catalytic oxidation reactor is integrated within the CO2 capture unit. The impurities incompatible with the CATOX are removed before the reactor. This scheme makes it possible to prepare food-grade CO2 with a limited number of items of equipment, thus reducing the capital cost of the CO2 capture unit. According to a subject matter of the invention, there is provided a process for the capture of CO2 from a mixture containing at least carbon dioxide, at least one component which is less volatile than CO2, at least 1 mol%, indeed even at least 3 mol%, of oxygen, nitrogen and at least one CnHm hydrocarbon of greater than or equal to C2 and which can 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 stages: i)     compression of the mixture, ii)     drying of the compressed mixture, iii)    cooling of the compressed and dried mixture in a heat exchanger, iv)    purification of the compressed and dried mixture to produce a fluid enriched in the at least one component which is less volatile than CO2 with respect to the mixture and a fluid depleted in the at least one component which is less volatile than CO2 and enriched in at least one component which is more volatile than CO2 with respect to the mixture, v) heating of the fluid depleted in the at least one component which is less volatile than CO2 in the heat exchanger, vi) catalytic oxidation at a temperature of less than or equal to 520°C of the fluid depleted in the at least one component which is less volatile than CO2, producing a fluid depleted in the at least one CnHm hydrocarbon of greater than or equal to C2 containing water resulting from the oxidation, vii) drying of the fluid depleted in the at least one hydrocarbon and viii) cooling of the dried fluid from stage vii) in the heat exchanger and separation by partial condensation and / or by distillation and / or by desublimation in order to obtain a liquid enriched in CO2 with respect to the mixture and a gas depleted in CO2 with respect to the mixture. According to other optional subject matters of the invention: • stage iv) consists of a stage of scrubbing with liquid CO2 at a temperature below -50°C, • stage iv) produces a fluid depleted in the at least one component which is less volatile than CO2 and enriched in oxygen with respect to the mixture, • if the mixture to be treated contains at least one sulfur compound, a stage of separation by adsorption is added between stage iv) and the catalytic oxidation, • if the mixture to be treated contains mercury, a second stage of separation by adsorption is added between stage iv) and the catalytic oxidation, •     the CO2-enriched liquid contains a total of hydrocarbons as methane equivalent of less than 50 ppmv and preferentially of less than 10 ppmv and a total of hydrocarbons excluding methane as methane equivalent of less than 20 ppmv and preferentially of less than 5 ppmv. According to the methane equivalent principle, a molecule which contains two carbon atoms counts as two methane equivalents, a molecule which contains three carbon atoms counts as three methane equivalents, and so on. • the fluid dried in stage vii) comprises oxygen and the separation of stage viii) comprises at least one partial condensation stage and optionally a distillation stage, the dried fluid cooled in the heat exchanger being partially condensed in the heat exchanger and separated in a phase separator into a gas enriched in oxygen and depleted in CO2 and a liquid depleted in oxygen and in CO2, the liquid depleted in oxygen being separated by being sent to the top of a distillation column or constituting the CO2-rich product, • the gas enriched in oxygen is heated in the heat exchanger and separated by permeation downstream of the exchanger to remove remaining oxygen, forming a recycle gas containing CO2 which is mixed with the mixture during either of stages i) and ii), • no oxygen flow is added to the fluid depleted in the at least one component which is less volatile than CO2 upstream of the catalytic oxidation, • the mixture contains less than 10 mol%, indeed even less than 5 mol%, of methane. According to another subject matter of the invention, there is provided an apparatus for the capture of CO2 from a mixture containing at least carbon dioxide, at least one component which is less volatile than CO2, at least 1 mol%, indeed even at least 3 mol%, of oxygen, nitrogen and at least one CnHm hydrocarbon of greater than or equal to C2 and which can 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 and dried mixture cooled in the heat exchanger to produce a fluid enriched in the at least one component which is less volatile than CO2 with respect to the mixture and a fluid depleted in at least one component which is less volatile than CO2 and enriched in at least one component which is more volatile than CO2 with respect to the mixture, a pipe connected to the means for purifying the mixture and to the exchanger for sending thereto fluid depleted in the at least one component which is less volatile than CO2 to be heated up, a catalytic oxidation unit capable of operating at a temperature of less than or equal to 520°C, means for sending fluid depleted in the at least one component which is less volatile than CO2 heated in the heat exchanger to the catalytic oxidation unit, a second dryer, a pipe for sending a water-containing fluid depleted in the at least one CnHm hydrocarbon of greater than or equal to C2 from the oxidation unit to the second dryer, producing dried fluid depleted in the at least one hydrocarbon, a pipe for sending the dried fluid depleted in the at least one hydrocarbon to the heat exchanger, means for separation by partial condensation and / or by distillation and / or by desublimation, a pipe for sending the dried fluid depleted in the at least one hydrocarbon cooled in the heat exchanger to the separation means, a pipe for discharging, from the separation means, a liquid enriched in CO2 with respect to the mixture and a pipe for discharging, from the separation means, a gas depleted in CO2 with respect to the mixture. Description of the invention The invention will be described in greater detail while referring to the figures, where: [FIG. 1] represents an alternative process which is not that of the invention. [FIG. 2] represents a process according to the invention. [FIG. 1] represents a process in which a gas 1 is compressed in a compressor having four stages 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 which is less volatile than CO2, for example NO2, at least 1 mol%, indeed even at least 3 mol%, of oxygen, nitrogen and at least one CnHm hydrocarbon of greater than or equal to C2 and which can 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. Cooling produces condensed water W1, W2, W3, W4 which is collected, forming a flow W. The compressed mixture dried by temperature swing adsorption in an adsorption unit TSA1. The dried mixture is cooled in a heat exchanger E1 and leaves the exchanger at an intermediate temperature thereof to be separated in a scrubbing column K1 fed at the top with liquid CO2 33. The column produces a liquid 5 enriched in the at least one component which is less volatile than CO2 with respect to the mixture 1 and a gas 3 depleted in the at least one component which is less volatile than CO2 and enriched in at least one component, in particular oxygen, which is more volatile than CO2 with respect to the mixture. The liquid 5 is vaporized by the heater H1 and mixed with the gas 1. The gas 3 is cooled in the heat exchanger E1 up to the cold end, where it is partially condensed. The two-phase flow is sent to a phase separator S1, producing a gas 7 and a liquid 9. The gas 7 is heated in the heat exchanger E1 and is separated by permeation M, forming a permeate gas enriched in CO2 11, which is recycled to the gas 1 upstream of C1, and a retentate 13 which is a waste gas. 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 the at least one component which is more volatile than CO2 and, at the bottom, a liquid 17 depleted in the at least one component which is more volatile than CO2. The gas 15 is heated in the heat exchanger E1 and sent between the cooler R3 and the compressor C4. The liquid 17 is vaporized and heated in the heat exchanger E1 and the gas formed is mixed with gaseous oxygen 19. The gas 21 formed is heated in a heat exchanger E2, heated by a heater H2 and sent to a catalytic oxidation unit CAT where at least one CnHm hydrocarbon of greater than or equal to C2 reacts with oxygen, forming water. The gas 23 leaves the oxidation unit CAT and is cooled in the exchanger E2 and subsequently 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 reboiling gas. Another part 33 is the scrubbing liquid of the column C1. The unit TSA1 produces a condensate W6. The unit TSA2 produces a wet gas W5. Figure 2 differs from figure 1 in that the column K2 which reduces the content of the at least one component which is 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 the gas 1 being sufficient to feed the catalytic oxidation. The dried mixture is cooled in a heat exchanger E1 and leaves the exchanger at an intermediate temperature thereof to be separated in a scrubbing column K1 fed at the top with liquid CO2 33. The column produces a liquid 5 enriched in at least one component which is less volatile than CO2 with respect to the mixture 1 and a gas 3 depleted in at least one component which is less volatile than CO2 and enriched in at least one component, in particular oxygen, which is more volatile than CO2 with respect to the mixture. The liquid 5 is vaporized by the heater H1 and mixed with the gas 1. If the mixture to be treated 1, 3 contains sulfur compounds, a stage of separation by adsorption is added between the scrubbing with CO2 in K1 and the catalytic oxidation CAT. If the mixture to be treated 1, 3 contains mercury, a stage of separation by adsorption is added between the scrubbing with CO2 in K1 and the catalytic oxidation CAT. 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 CnHm hydrocarbon of greater than or equal to C2 reacts with oxygen, forming water. The gas 4 leaves the oxidation unit CAT and is cooled in the exchanger E2 and subsequently 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 is partially condensed therein. The two-phase flow is separated in a phase separator S1. The liquid 9 from the separator S1 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 and is sent to a pump forming a pressurized liquid, 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 reboiling gas. Another part 33 is the scrubbing liquid of the column C1. The column K3 is not necessarily present. The partial condensation can comprise at least two partial condensation stages. The separation can comprise a desublimation stage. The gas 25 from the column K3 is heated in the heat exchanger E1 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 C1. The gas 10 depleted in CO2 and enriched in oxygen serves as waste gas. The unit TSA1 produces a condensate W6. The unit TSA2 produces a wet gas W5.

Claims

1. A process for the capture of CO2 from a mixture containing at least carbon dioxide, at least one component which is less volatile than CO2, at least 1 mol%, indeed even at least 3 mol%, of oxygen, nitrogen and at least one CnHm hydrocarbon of greater than or equal to C2 and which can 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 stages:i) compression (C1, 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 (E1),iv) purification (K1) of the compressed and dried mixture to produce a fluid enriched in the at least one component which is less volatile than CO2 with respect to the mixture and a fluid (3) depleted in the at least one component which is less volatile than CO2 and enriched in at least one component which is more volatile than CO2 with respect to the mixture,v) heating of the fluid depleted in the at least one component which is less volatile than CO2 in the heat exchanger,vi) catalytic oxidation (CAT) at a temperature of less than or equal to 520°C of the fluid depleted in the at least component which is less volatile than CO2, producing a fluid (4) depleted in the at least one CnHm hydrocarbon of greater than or equal to C2 containing water resulting from the oxidation,vii) drying (TSA2) of the fluid depleted in the at least one hydrocarbon andviii) cooling of the dried fluid from stage vii) in the heat exchanger and separation (S1, K3) by partial condensation and / or by distillation and / or by desublimation in order to obtain a liquid (9, 27, 29, 31) enriched in CO2 with respect to the mixture and a gas (7, 25) depleted in CO2 with respect to the mixture.

2. The process as claimed in the preceding claim, in which stage iv) consists of a stage of scrubbing with liquid CO2 (33) at a temperature below -50°C.

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

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

5. The process as claimed in one of the preceding claims, in which the CO2-enriched liquid (27, 29, 31) contains a total of hydrocarbons as methane equivalent of less than 50 ppmv and preferentially of less than 10 ppmv and a total of hydrocarbons excluding methane as methane equivalent of less than 20 ppmv and preferentially of less than 5 ppmv.

6. The process as claimed in one of the preceding claims, in which the fluid (4) dried in stage vii) comprises oxygen and the separation (S1, K3) of stage viii) comprises at least one partial condensation stage and optionally a distillation stage, the dried fluid cooled in the heat exchanger (E1) being partially condensed in the heat exchanger and separated in a phase separator into a gas (7) enriched in oxygen and depleted in CO2 and a liquid (9) depleted in oxygen and in CO2, the liquid depleted in oxygen being separated by being sent to the top of a distillation column (K3) or constituting the CO2-rich product.

7. The process as claimed in claim 6, in which the gas (7) enriched in oxygen is heated in the heat exchanger (E1) and separated by permeation (M) downstream of the exchanger to remove remaining oxygen, forming a recycle gas containing CO2 (8) which is mixed with the mixture during either of stages i) and ii).

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

9. The process as claimed in one of the preceding claims, in which the mixture (1) contains less than 10 mol%, indeed even less than 5 mol%, of methane.

10. An apparatus for the capture of CO2 from a mixture (1) containing at least carbon dioxide, at least one component which is less volatile than CO2, at least 1 mol%, indeed even at least 3 mol%, of oxygen, nitrogen and at least one CnHm hydrocarbon of greater than or equal to C2 and which can 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 (C1, C2, C3, C4) for compressing the mixture, a first dryer (TSA1) for drying the compressed mixture, a heat exchanger (E1) for cooling the compressed and dried mixture, means (K1) for purifying the compressed and dried mixture cooled in the heat exchanger to produce a fluid enriched in the at least one component which is less volatile than CO2 with respect to the mixture and a fluid (3) depleted in the at least one component which is less volatile than CO2 and enriched in at least one component which is more volatile than CO2 with respect to the mixture, a pipe connected to the means for purifying the mixture and to the exchanger for sending thereto fluid (3) depleted in the at least one component which is less volatile than CO2 to be heated up, a catalytic oxidation (CAT) unit capable of operating at a temperature of less than or equal to 520°C, means for sending fluid (3) depleted in component which is less volatile than CO2 heated in the heat exchanger to the catalytic oxidation unit, a second dryer (TSA2), a pipe for sending a water-containing fluid depleted in the at least one CnHm hydrocarbon of greater than or equal to C2 from the oxidation unit to the second dryer, producing dried fluid depleted in the at least one hydrocarbon, a pipe for sending the dried fluid depleted in the at least one hydrocarbon to the heat exchanger, means (S1, K3) for separation by partial condensation and / or by distillation and / or by desublimation, a pipe for sending the dried fluid depleted in the at least one hydrocarbon cooled in the heat exchanger to the separation means, a pipe for discharging, from the separation means, a liquid (9, 27, 29, 31) enriched in CO2 with respect to the mixture and a pipe (7, 25) for discharging, from the separation means, a gas depleted in CO2 with respect to the mixture.

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