Method for separating a pollutant from an effluent gas

Abstract

The invention relates to a chemical vapour infiltration method, comprising at least: - a pyrolysis step; - a step of recovering the polluted effluent gases; - a first step of separating the polluted effluent gases by exposing said gases to an absorption liquid (17); - a second step of separating the purified effluent gases (7) in order to obtain a gas stream (10) predominantly comprising species constituting the reactive gas phase and a gas stream (13) predominantly comprising species that are not present in the reactive gas phase; - reintroducing the gas stream (10) into the chamber of the furnace; - a step of purifying the absorption liquid; the method being characterised in that the extraction gas phase used in the purification step comprises at least a portion of the gas stream (13) predominantly comprising species that are not present in the reactive gas phase.

Description

Description Title of the invention: Method for separating a pollutant from an effluent gas Technical Field

[0001] The invention falls within the field of purification of an effluent gas containing pollutants. Previous technique

[0002] Carbon / pyrolytic carbon material parts are attracting increasing technological interest due to the good compromise they offer in terms of strength and weight.

[0003] It is known to produce carbon / carbon pyrolytic parts by chemical infiltration or chemical vapor deposition. Such processes use a carbon-rich reactive gas phase brought into contact with porous substrates under conditions such that the gas phase can react with the substrate to form a matrix within the substrate's pores. The choice of gas constituting the gas phase and the temperature and pressure conditions prevailing in the furnace determine the nature of the matrix formed within the composite parts.

[0004] However, such processes generate gaseous effluents to produce a final part. These effluents are generally not recovered in current processes. Recovering these carbonaceous effluents would reduce costs and the environmental footprint of the preparation processes, and therefore of the parts obtained.

[0005] However, recycling the valuable vapor phase, for example including reagents that have not been used, requires separation steps which themselves require special facilities and specific solvents.

[0006] Methods for separating valuable species from pyrolytic process effluent gases have been proposed, for example in document WO 03 / 047725. US 11 753 594, which describes a process for treating oil contaminated with polycyclic aromatic hydrocarbons, is also known. It remains Nevertheless, it is in the interest of the person skilled in the art to propose improvements to such processes. Description of the invention

[0007] The invention specifically aims to improve existing effluent gas treatment processes and proposes for this purpose a chemical vapor-phase infiltration process comprising at least: - a pyrolysis step of a reactive gaseous phase within the enclosure of a chemical vapor deposition furnace; - a step of recovering polluted effluent gases exiting the enclosure of the chemical vapor deposition furnace; - a first step of separating polluted effluent gases by exposing them to an absorption liquid, allowing the pollutants to be separated from the polluted effluent gases by loading the absorption liquid with pollutants and obtaining after the first step on the one hand purified effluent gases and on the other hand an absorption liquid loaded with pollutants; - a second stage of separation of the purified effluent gases to separate from the purified effluent gases common to the reactive gas phase and other species remaining in the purified effluent gases and to obtain on the one hand a gas stream comprising mainly species constituting the reactive gas phase and on the other hand a gas stream comprising mainly species not present in the reactive gas phase; - the reintroduction of the gas stream, consisting mainly of species constituting the reactive gas phase, into the enclosure of the chemical vapor deposition furnace; - a step of purifying the pollutant-laden absorption liquid by exposing the latter to an extraction gaseous phase; the process being characterized in that the extraction gaseous phase used in the purification step comprises at least a part of the gaseous stream comprising mainly species not present in the reactive gaseous phase.

[0008] This process enables a chemical vapor deposition process ensuring optimal recycling of polluted effluent gases.

[0009] In particular, the process allows continuous recycling of the absorption liquid, optimal recovery of the constituent species of the reactive gas phase and finally a reduction in the need for extraction gas phase thanks to the reuse of the gas stream made up of non-constituent species of the reactive gas phase.

[0010] In one embodiment, the first separation step can be carried out in a counter-current separator, for example an absorption column, comprising: - an absorption zone designed to bring the polluted effluent gases into contact with the absorption liquid; - an absorption liquid inlet intended to be supplied by the absorption liquid; - an absorption liquid outlet intended to be traversed by the absorption liquid loaded with pollutant; the absorption liquid inlet and outlet being arranged so that the absorption liquid and the polluted effluent gases circulate in an absorption zone of the separator in counter-current to each other.

[0011] This method of implementation allows for a particularly efficient separation.

[0012] In one embodiment, the first separation step is carried out in a counter-current separation device which further includes an introduction device, configured to cool and pressurize the absorption liquid before its entry into the separator.

[0013] In this embodiment, separation is further improved because the polluted effluent gas is warmer than the cooled absorption liquid. Thus, the partition coefficient of the pollutant between the polluted effluent gas and the absorption liquid shifts in favor of pollutant migration towards the absorption liquid. This results in a greater affinity of the pollutant for the absorption liquid and therefore improved separation.

[0014] In such an embodiment, the introduction device can be broken down into two elements, one for pressurizing the absorption liquid, and the other for cooling it.

[0015] For example, the introduction device may include a pump to pressurize the absorption liquid and a heat exchanger configured so that the absorption liquid transfers heat to a colder flow, thereby cooling itself.

[0016] In one embodiment, the absorption liquid is chosen so that the pollutant's affinity for the absorption liquid is as high as possible. It is particularly advantageous to choose an absorption liquid that is a solvent for the pollutant. Indeed, the better the pollutant's affinity for the absorption liquid, the better the separation.

[0017] In one embodiment, the pollutant is a benzene hydrocarbon (BTX) for example chosen from benzene, toluene, ethylbenzene, a xylene, a polycyclic aromatic hydrocarbon (PAH) or a mixture of two or more of these species.

[0018] In this embodiment, the absorption liquid can be chosen from the following product families: hydrotreated or non-hydrotreated mineral oils, synthetic oils, esters (vegetable oils) and mixtures of compounds from these families.

[0019] This embodiment is particularly preferred when the effluent gas is derived from a pyrolytic gas used for pyrocarbon deposition, especially in the context of the densification of composite materials.

[0020] The invention is described using the plural expression "polluted / purified effluent gas..." but this should not be understood in a limiting way and the described process also works in the case where the effluent gas is a single compound.

[0021] In one embodiment, the purification step can be carried out in a purifier comprising: - an inlet configured to be supplied by the absorption liquid loaded with the pollutant; - a purification zone intended to bring the charged absorption liquid into contact with the extraction gaseous phase; - an outlet intended to be traversed by the purified absorption liquid.

[0022] The purifier purifies the absorption liquid, thus making it more effective for subsequent use.

[0023] Indeed, the affinity of pollutants for the absorption liquid is greater when the latter is less loaded with pollutants.

[0024] Thanks to the purification step, the pollutants initially present in the effluent gas and which were recovered by the absorption liquid during the separation step can be separated from the absorption liquid to allow the latter to be reused.

[0025] In one embodiment, the purifier is a desorption column, i.e. the purification step is carried out in a desorption column.

[0026] In one embodiment, the purifier is a desorption column further comprising: - a gas inlet intended to be supplied by the gaseous extraction phase; - a gas outlet intended to be traversed by the pollutant-laden extraction gas phase; the gas inlet and outlet being arranged so that the polluted absorption liquid and the recovery gas flow in counter-current to each other in the purification device.

[0027] In the process described, at least part of the extraction gas phase is composed of the gas stream consisting mainly of species not present in the reactive gas phase described above.

[0028] In one embodiment of the process, the extraction gas phase does not include any species other than the gas stream consisting mainly of species not present in the reactive gas phase obtained at the end of the second separation step.

[0029] In one embodiment the process further includes a conditioning step to heat and / or expand the pollutant-laden absorption liquid before it enters the purifier.

[0030] The conditioning step is for example carried out by a conditioning device comprising heating means arranged to heat the absorption liquid loaded with pollutant and / or a pressure regulator arranged to expand the absorption liquid loaded with pollutant. [003 l]This embodiment allows for even better purification of the pollutant-laden absorption liquid because by increasing its temperature and decreasing its pressure, the distribution equilibrium of the pollutant between the absorption liquid and the recovery gas is shifted in favor of greater migration towards the recovery gas.

[0032] In one embodiment, the inlet of the purifier is in communication with an absorption liquid outlet of the separator, and an absorption liquid inlet of the separator is in communication with the outlet of the purifier.

[0033] This embodiment allows continuous recycling of the absorption liquid, as the absorption liquid becomes loaded with pollutants in the separator and is then sent to the purifier where it is purified and then sent back to the separator.

[0034] Continuous recycling in this embodiment allows the use of a reduced quantity of absorption liquid while ensuring excellent separation, because the absorption liquid used in the separator has always just been purified, and the separation rate is all the better when the absorption liquid used in the separator is lightly loaded with pollutants.

[0035] Of course, it is possible that the introduction and / or conditioning devices described above may be arranged between the purifier outlet and the separator inlet on the one hand, and between the separator outlet and the purifier inlet on the other. This particular embodiment allows for the achievement of all the effects described above.

[0036] In one embodiment, the purification step can be carried out under reduced pressure, for example at a pressure between 5.0 times and 15.0 times the saturated vapor pressure of the solvent at the temperature chosen for the treatment.

[0037] Indeed, the inventors determined that carrying out this step under reduced pressure promotes the transfer of pollutants to the gaseous extraction phase.

[0038] In one embodiment, the second separation step can be carried out by membrane separation, molecular sieve separation, cryogenic condensation separation, or adsorption. Brief description of the drawings

[0039] [Fig. 1] Figure 1 is a schematic representation of an installation enabling the implementation of a process according to an embodiment of the invention. Description of the implementation methods

[0040] The invention is now described using figures, which should not be considered as limiting the invention. These figures only depict certain specific embodiments to aid understanding, and these embodiments should not be interpreted as limiting the invention. Finally, the figures illustrate several embodiments in combination, without these embodiments necessarily needing to be combined to achieve the desired technical effects.

[0041] Figure 1 schematically represents an installation according to one embodiment of the invention.

[0042] In the embodiment shown, the polluted effluent gases 1 from a chemical vapor deposition furnace enter a compressor 101.

[0043] This compressor is optional and in one embodiment allows the first stage of separation under pressure to be carried out, which increases the transfer rate between the polluted effluent gases and the absorption liquid.

[0044] In one embodiment, the chemical vapor deposition process can be a fibrous preform densification process.

[0045] In particular, the process described in this application can be a process for manufacturing aeronautical parts in carbon / carbon composite material and in particular for the manufacture of aeronautical brakes.

[0046] Whether the compressor 101 is present or not, the polluted effluent gases 1, or where applicable the compressed polluted effluent gases 2, enter a first separation device 103.

[0047] In Figure 1, the first separation unit 103 is an absorption column.

[0048] The absorption column 103 enables the first separation step of the process to be carried out in one embodiment.

[0049] Column 103 is supplied by compressed polluted effluent gases 2 on the one hand, and by an absorption liquid 17 on the other hand.

[0050] For example, in the embodiment shown, the absorption liquid 17 is introduced through an absorption liquid inlet in the upper part of the absorption column 103, and the polluted effluent gases enter through a gas inlet 201 of the column 103 located in a lower part thereof.

[0051] In the embodiment shown, the compressed polluted effluent gases 2 pass through the absorption column 103 against the current of the absorption liquid 17 which, by gravity, flows into the column 103.

[0052] The purified effluent gases 7 can exit the first separation unit 103 through a gas outlet 202, for example, located at the top of the column 103, after the pollutant has migrated to the absorption liquid.

[0053] In one embodiment, the absorption liquid 17 is chosen so that the pollutant's affinity for the absorption liquid 17 is as high as possible. It is particularly advantageous to choose an absorption liquid that is a solvent for the pollutant. Indeed, the better the pollutant's affinity for the absorption liquid, the better the separation.

[0054] In one embodiment the pollutant is a benzene hydrocarbon (BTX) such as benzene, toluene, ethylbenzene, xylene or a polycyclic aromatic hydrocarbon (PAH).

[0055] In this embodiment, the absorption liquid can be chosen from the following product families: hydrotreated or non-hydrotreated mineral oils, synthetic oils, esters (vegetable oils) and mixtures of compounds from these families.

[0056] This embodiment is particularly preferred when the effluent gases come from pyrolytic gases used for pyrocarbon deposition, especially in the context of the densification of composite materials.

[0057] Alternatively, if the pyrolytic gases originate from biomass processing, the pollutant is generally a compound selected from among a benzene hydrocarbon (BTX) such as benzene, toluene, ethylbenzene, xylene, or a polycyclic aromatic hydrocarbon (PAH), or an oxygenated compound such as phenol, cresol, or a compound containing heteroatoms. The pollutant may also be a mixture of one or more elements from the preceding list.

[0058] In this embodiment, the absorption liquid 17 can be chosen from among polar solvents having a good affinity for pollutants.

[0059] In one embodiment, the absorption liquid 17 can be a mixture of solvents and preferably of a polar solvent and a nonpolar solvent.

[0060] In the described process, the purified effluent gases 7 exit at the top of the column, which then enter a second separation unit 112.

[0061] Such a second organ 112 may be another adsorption column, a membrane, a cryogenic separation organ.

[0062] In the process described, the second separation step, carried out here in the second organ 112, allows the purified effluent gases to be separated into two streams, one consisting mainly of species constituting the reactive gas phase and the other consisting mainly of species not present in the reactive gas phase.

[0063] Mostly means in an embodiment with more than 80% by volume, or even more than 90% by volume.

[0064] Indeed, traces of the other portion may remain in one of the gas streams due to the separation processes used.

[0065] In the described process, the stream 13, which mainly comprises species not present in the reactive gas phase, is used as the extraction gas phase for a purification step.

[0066] The expression "species not present in the reactive gas phase" is intended to characterize species that are not intended to be introduced into the deposition furnace and which are not present in the reactive gas phases introduced intentionally.

[0067] For example, the species not present are those that would impair the quality, yield or kinetics of the desired reaction in the chemical vapor deposition furnace.

[0068] The "species not present in the reactive gas phase" can, for example, be dihydrogen, water, carbon dioxide, carbon monoxide, dinitrogen.

[0069] As shown, the stream consisting mainly of species not present in the reactive gas phase is routed to purifier 107 as extraction gas phase for a purification step.

[0070] This reduces the need for gaseous phase extraction and thus offers a process that consumes fewer external resources than prior art processes.

[0071] The other portion, consisting mainly of species constituting the reactive gas phase, can be returned directly to the chamber of a chemical vapor deposition furnace.

[0072] For example, such species can be hydrocarbons, particularly methane and / or propane.

[0073] In the embodiment shown in Figure 1, the pollutant-laden absorption liquid can be conveyed from the outlet of column 103 to the inlet configured to be supplied by the pollutant-laden absorption liquid from purifier 107.

[0074] In one embodiment, the purifier 107 is a desorption column which corresponds to a device known per se.

[0075] In one embodiment, the absorption liquid 17 introduced into the separator 103 can be, beforehand, cooled for example by a heat exchanger 104 and pressurized for example by a pump 108, while the pollutant-laden absorption liquid 11 introduced into the purifier 107 can be heated for example by the heat exchanger 104 and expanded for example by the valve 106, for example to atmospheric pressure (1 bar) before its introduction.

[0076] For example, the temperature of the polluted effluent gas at the outlet of compressor 101 or at the inlet of separator 103 is advantageously between 5 and 30°C to ensure good industrial compatibility of the process.

[0077] The specific process parameters will be chosen based on the physical properties of the selected absorption liquid, particularly its vapor pressure, dynamic viscosity, and cold density. Specifically, the temperature in separator 103 will be selected to ensure the absorption liquid's viscosity is sufficiently low to avoid significant pressure drops.

[0078] In one embodiment, the pollutant-laden absorption liquid 8 can pass through the heat exchanger 104 before being introduced into the purifier 107 to be heated.

[0079] The heat exchanger 104 through which the pollutant-laden absorption liquid 8 passes allows a heated absorption liquid 9 to be obtained at the outlet before its introduction into the purifier 107. Heating the absorption liquid 9 before introducing it into the purifier 107 is advantageous because a higher temperature of the absorption liquid in the purifier 107 improves the separation of the pollutant.

[0080] Similarly, in one embodiment, the absorption liquid loaded with pollutant 9 can pass through the valve 106 before entering the purifier, allowing its expansion.

[0081] The purpose of valve 106 is to relax the heated pollutant-laden absorption liquid 9 before its introduction into purifier 107. Indeed, a lower pressure of the pollutant-laden absorption liquid in purifier 107 further improves the separation of the pollutant in purifier 107.

[0082] It is understood that the heat exchanger 104, and the valve 106, may or may not be present independently of each other and may be arranged in any order.

[0083] In addition, alternatively, if a higher temperature than that which the heat exchanger 104 can achieve is desired for the pollutant-laden absorption liquid 9, additional heating means can be provided before the purifier 107.

[0084] For example, an additional heating means can be arranged between the heat exchanger 104 and the valve 106.

[0085] However, in embodiments where the pollutant-laden absorption liquid 8 is under pressure, for example because it has been introduced under pressure into the separator 103, it is preferable that the valve 106 be located after the heat exchanger 104, and where applicable after any additional heating means, because the pressure of the pollutant-laden absorption liquid then overcomes the pressure losses of these components without requiring additional elements. Furthermore, the pressurization of the pollutant-laden absorption liquid 8 prevents the formation of bubbles up to valve 106, thus preventing any vaporization of volatile compounds.

[0086] The heat exchanger 104 and the valve 106 can form a conditioning device to heat and / or expand the pollutant-laden absorption liquid 8 before it enters the purifier 107.

[0087] In the embodiment shown, the purifier is a desorption column 107 which is further supplied by a gaseous phase 13.

[0088] In the process of the invention, at least part of the extraction gaseous phase comes from the second separation step described above.

[0089] Indeed, it is to the credit of the inventors that they determined that the gas stream, consisting mainly of species not present in the reactive gas phase obtained after the separation step and usually considered as waste, had a composition perfectly suited to participate in the purification step.

[0090] In one embodiment, the gaseous phase 13 may include dinitrogen or natural gas, in addition to the gaseous stream comprising mainly species not present in the reactive gaseous phase.

[0091] Indeed, these species can be mixed with the gas phase 13 when it is necessary to increase the amount of extraction gas phase for the purification step.

[0092] In one embodiment, the gaseous phase 13 used in the purification step does not include any other species than the gaseous stream comprising mainly species not present in the reactive gaseous phase obtained at the end of the second separation step carried out in the separator 112.

[0093] This embodiment makes it possible to propose a chemical vapor phase infiltration process ensuring effluent recycling using only gases already present in the process.

[0094] In the embodiment shown, the desorption column 107 is provided with an outlet shown at the bottom of the column 107 in Figure 1, through which the purified absorption liquid 12 passes.

[0095] The gaseous phase 13 becomes charged with pollutant in column 107, and the gaseous phase charged with pollutant 14 passes through an outlet intended to be crossed by a gaseous phase represented in figure 1 at the top of column 107.

[0096] In one embodiment, the pollutant-laden gaseous phase 14 may be intended to be destroyed, stored or reused in another process.

[0097] In the embodiment shown, the purified absorption liquid 12 passes through a compressor 108 configured to pressurize the purified absorption liquid 15.

[0098] In the embodiment shown, the compressed purified absorption liquid 15 can be cooled by means of the heat exchanger 104 before its introduction into the separator 103.

[0099] In the embodiment shown, the heat exchanger 104 is traversed on one side by the cold pollutant-laden absorption liquid 8 which needs to be heated and on the other side by the hot purified absorption liquid 15 which needs to be cooled, so that at the outlet of the heat exchanger 104, the pollutant-laden absorption liquid 9 comes out hotter and the purified absorption liquid 17 comes out colder.

[0100] In this embodiment, the process is even more economical because part of the thermal energy is supplied by the absorption liquid itself, which reduces the amount of heat to be supplied and therefore the overall cost of the process.

[0101] However, it is not necessary for the process of the invention that the same heat exchanger 104 be traversed by the absorbing liquid loaded with pollutant 8 and by the purified absorbing liquid 15. In one embodiment, the absorbing liquid loaded with pollutant 8 or the purified absorbing liquid 15 can be heated or cooled by heating or cooling elements distinct from each other.

[0102] The purified absorption liquid 17, having passed through the heat exchanger, is cooled. It can then be directly introduced into the inlet of the separator 103, which is intended to be supplied with the absorption liquid.

[0103] Alternatively, and not shown in Figure 1, if a temperature even lower than that which can be achieved through the heat exchanger 104 is desired for the purified absorption liquid 17, it can be arranged with other cooling elements before the inlet of the separator 103 configured to be supplied by the absorption liquid.

[0104] For example, one (or more) additional cooling element may be arranged between the heat exchanger 104 and the separator 103. The heat exchanger 104, the compressor 108 and, where applicable, any additional cooling element may be present independently of each other, and are not necessarily arranged in the order described in Figure 1.

[0105] However, in an embodiment where a heat exchanger 104 and a compressor 108 are present, it is advantageous to arrange the compressor 108 before the heat exchanger 104 so that it is not necessary to add an element to overcome the pressure losses in the purified absorption liquid circuit.

Claims

Demands

1. A chemical vapor-phase infiltration process comprising at least: - a pyrolysis step of a reactive gaseous phase within the enclosure of a chemical vapor deposition furnace; - a stage of recovering polluted effluent gases (1) exiting the enclosure of the chemical vapor deposition furnace; - a first step of separating the polluted effluent gases by exposing them to an absorption liquid (17), allowing the separation of pollutants from the polluted effluent gases by loading the absorption liquid with pollutant and obtaining after the first step on the one hand purified effluent gases (7) and on the other hand an absorption liquid loaded with pollutant (8); - a second stage of separation of the purified effluent gases (7) to separate from the purified effluent gases species common with the reactive gas phase and other species remaining in the purified effluent gases and to obtain on the one hand a gas stream (10) comprising mainly species constituting the reactive gas phase and on the other hand a gas stream (13) comprising mainly species not present in the reactive gas phase; - the reintroduction of the gas stream (10) consisting mainly of species constituting the reactive gas phase into the enclosure of the chemical vapor deposition furnace; - a step of purifying the pollutant-laden absorption liquid (11) by exposing the latter to an extraction gaseous phase; the process being characterized in that the extraction gaseous phase used in the purification step comprises at least a part of the gaseous stream (13) comprising mainly species not present in the reactive gaseous phase.

2. A chemical vapor-phase infiltration process according to claim 1, wherein the first separation step is carried out in a counter-current separator comprising (103): - an absorption zone intended to bring the polluted effluent gas flow (2) into contact with the absorption liquid (7); - an absorption liquid inlet intended to be supplied by the absorption liquid (17); - an absorption liquid outlet intended to be traversed by the absorption liquid loaded with pollutant (8); the absorption liquid inlet and outlet being arranged so that the absorption liquid and the polluted effluent gas flow in an absorption zone of the separator in counter-current to each other.

3. A chemical vapor phase infiltration process according to claim 1 or 2, wherein the pollutant is selected from benzene, toluene, ethylbenzene, a xylene, a polycyclic aromatic hydrocarbon (PAH) or a mixture of two or more of these species.

4. Chemical vapor phase infiltration process according to any one of claims 1 to 3, the absorption liquid being selected from the following product families: hydrotreated or non-hydrotreated mineral oils, synthetic oils, esters (vegetable oils) and mixtures of compounds from these families.

5. A chemical vapor phase infiltration process according to any one of claims 1 to 4, wherein the purification step is carried out in a desorption column (107).

6. A chemical vapor phase infiltration process according to any one of claims 1 to 5, wherein the extraction gas phase does not comprise any species other than the gas stream (13) comprising predominantly species not present in the reactive gas phase obtained at the end of the second separation step.

7. A chemical vapor phase infiltration process according to any one of claims 1 to 6, wherein the purification step is carried out at a pressure between 5.0 times and 15.0 times the saturated vapor pressure of the solvent at the temperature chosen for the treatment.

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