Process for recycling an effluent gas from a chemical vapor deposition or infiltration

Abstract

Process for recycling an effluent gas from a chemical vapor-phase deposition or infiltration. The invention relates to a process for recycling an effluent gas (50) from a chemical vapor-phase deposition or infiltration and comprising dihydrogen, light unsaturated hydrocarbons in the C1 to C4 range and heavy hydrocarbons in the C5 and higher ranges, comprising the following steps: a) removal (60) of the heavy hydrocarbons (501) from the effluent gas, b) catalytic hydrogenation (80) of the light unsaturated hydrocarbons so as to remove them by transforming them into light saturated hydrocarbons in the C1 to C4 range, this catalytic hydrogenation being carried out on the effluent gas obtained after implementation of step a) and consuming a portion of the dihydrogen present in said effluent gas,and c) the introduction of the effluent gas (402) obtained after implementation of step b) into a reaction vessel (30) and the formation of pyrocarbon by deposition or chemical vapor-phase infiltration from the light saturated hydrocarbons in C1 to C4 present in the effluent gas thus introduced. Figure for the abstract: Fig. 1.,

Description

Title of the invention: Process for recycling an effluent gas from a chemical vapor deposition or infiltration. Technical field

[0001] The invention relates to chemical vapor deposition (CVD) and chemical vapor infiltration (CVI) techniques for the formation of pyrocarbon, or pyrolytic carbon. More specifically, the invention relates to the recycling of the effluent gas from this technique in order to obtain high-quality pyrocarbon from this recycled gas. The invention finds particular application in the manufacture of carbon / carbon composite parts, especially friction parts such as brake discs for aircraft or automobiles. Previous technique

[0002] The technique for manufacturing carbon / carbon by pyrocarbon densification uses as raw materials reagents from the family of light hydrocarbons such as natural gas, propane, butane, or more generally alkanes, chosen to optimize the chemical vapor-phase infiltration reaction, in particular its material yield, and the reaction kinetics. The reagents are introduced into a reaction vessel where densification takes place. After their passage and the deposition reaction in the reaction vessel, the effluent gas is extracted from the installation.

[0003] The chemical vapor-phase pyrocarbon densification process uses conditions that allow the reactive gas molecules in the reaction chamber to be cracked, thereby releasing carbon atoms and promoting their deposition onto the fibers. The material yield of the process is low: approximately 8% of the carbon atoms in the incoming gases are deposited. The remaining carbon atoms recombine into numerous hydrocarbon compounds and exit the reaction chamber as an effluent gas. The infiltration time required to obtain a carbon / carbon material can be several hundred hours.

[0004] The effluent gas contains a high proportion of methane and dihydrogen, but also a large number of hydrocarbons, including unsaturated hydrocarbons (alkenes, alkynes), aromatics, and heavy hydrocarbons (C5 and above), in low concentrations. The complexity of this mixture, some of whose compounds are harmful, and others which tend to condense upon cooling in installations as oils or solids, leads in most cases to their destruction by combustion in atmospheric oxygen in order to recover energy in the form of heat.

[0005] The principle of recycling the effluent gas for reinjection as a reactant in The reaction vessel has been proposed in the literature. In this regard, US patent 8,084,079 can be cited, which proposes the reintroduction into the gas phase, admitted at the inlet of the reaction vessel, of at least a fraction of the gas stream extracted from the effluent gas. Before its reintroduction, heavy hydrocarbons are removed from the effluent gas by oil scrubbing or condensation. Nevertheless, it is possible to improve the quality of the material produced by this recycling process, as well as the densification kinetics. Description of the invention

[0006] The present invention relates to a process for recycling an effluent gas from a chemical vapor phase deposit or infiltration and comprising dihydrogen, light unsaturated hydrocarbons in the C1 to C4 range and heavy hydrocarbons in the C5 and higher ranges, comprising the following steps: a) removal of heavy hydrocarbons from the effluent gas, b) a catalytic hydrogenation of light unsaturated hydrocarbons so as to eliminate them by transforming them into light saturated hydrocarbons in C4 to Ci, this catalytic hydrogenation being carried out on the effluent gas obtained after implementation of step a) and consuming a part of the dihydrogen present in said effluent gas, and c) the introduction of the effluent gas obtained after implementation of step b) into a reaction vessel and the formation of pyrocarbon by deposition or chemical infiltration in vapor phase from the light saturated hydrocarbons in C4 to Ci present in the effluent gas thus introduced.

[0007] The invention proposes introducing into the reaction vessel a recycled effluent gas having a composition closer to that of the initial gas than in the solution described in US patent 8,084,079. The invention is particularly noteworthy in that, after the removal of heavy hydrocarbons, it proposes catalytic hydrogenation to eliminate light unsaturated hydrocarbons and transform them into light saturated hydrocarbons useful for deposition. The consumption of unsaturated hydrocarbons during catalytic hydrogenation improves the quality of the material produced from the recycled gas, and the consumption of dihydrogen, which may be supplemented by subsequent removal of all or part of the unconsumed dihydrogen, increases the deposition kinetics.This step takes advantage of the presence of dihydrogen in the effluent gas to chemically transform unsaturated hydrocarbons into hydrocarbons beneficial to the deposit, without the need for external material or energy input. The recycling process according to the invention significantly reduces the consumption, and therefore the cost, of reactive gases without degrading the quality of the material produced or the kinetics of the reaction. The invention also contributes to a reduction in the production of hydrocarbon residues intended for combustion, thereby reducing the amount of greenhouse gases emitted into the atmosphere.

[0008] In one embodiment, the effluent gas obtained after implementation of step a) is compressed before implementation of step b) to bring it to a pressure of at least 4 bar, for example at least 6 bar or even between 4 bar and 12 bar or between 6 bar and 12 bar.

[0009] Such a characteristic advantageously promotes catalytic hydrogenation and brings the composition of the treated effluent gas even closer to that of the initial gas.

[0010] In one embodiment, the removal of residual dihydrogen from the effluent gas is carried out after implementation of step b) and before step c).

[0011] Such a characteristic makes it possible to further increase the deposition kinetics of the pyrocarbon formed from the recycled gas.

[0012] In one embodiment, a densification of a fibrous preform made of carbon fibers by pyrocarbon is carried out during step c).

[0013] The present invention also relates to a chemical vapor phase deposition or infiltration installation, comprising: - a reaction vessel in which pyrocarbon is intended to be formed by chemical vapor deposition or infiltration, having an inlet through which a reaction gas phase is intended to be admitted and an outlet through which an effluent gas is intended to be discharged, and - an effluent gas treatment and recycling circuit defining a loop between the outlet and the inlet of the reaction vessel, comprising (i) a device for removing heavy hydrocarbons in C5 and above from the effluent gas having an inlet in communication with the outlet of the reaction vessel and an outlet, and (ii) a catalytic hydrogenation device having an inlet in communication with the outlet of the heavy hydrocarbon removal device and an outlet in communication with the inlet of the reaction vessel.

[0014] This installation can be intended for the implementation of a process as described above.

[0015] In one embodiment, the effluent gas treatment and recycling circuit further includes a compressor located between the device for removing heavy hydrocarbons in C5 and above and the catalytic hydrogenation device.

[0016] In one embodiment, the effluent gas treatment and recycling circuit further includes a residual dihydrogen removal device located between the catalytic hydrogenation device and the inlet of the reaction vessel. Brief description of the drawings

[0017] [Fig-1] Fig. 1 illustrates, schematically and partially, some steps of a example of a method according to the invention. Description of the implementation methods

[0018] An example of a process and installation according to the invention will be described in connection with [Fig. 1]. This example relates to the densification of porous carbon fiber substrates by pyrocarbon in order to obtain parts made of carbon / carbon composite material, but those skilled in the art will recognize that the invention is not limited to this case, also covering the chemical vapor deposition of pyrocarbon on the surface of substrates to be coated.

[0019] Carbon fibers 10 or a carbon precursor undergo one or more textile operations, for example in a loom 20, so as to obtain a fibrous preform having the shape of the part to be obtained and intended to form the fibrous reinforcement thereof. The preform is produced by textile operations known per se depending on the intended application. For example, a plurality of annular fibrous layers can be superimposed or a helical texture can be wound flat into spirals to form superimposed annular layers and then bonded by needle punching. Other techniques can be implemented, such as the formation of the fibrous preform by three-dimensional weaving. By "three-dimensional weaving" or "3D weaving," we mean a weaving method in which at least some of the warp yarns bind weft yarns over several weft layers.A reversal of roles between warp and weft is possible in the present text and should be considered as covered within the scope of the invention.

[0020] When the textile operation(s) are carried out on carbon precursor fibers, these are transformed into carbon fibers by carbonization heat treatment at a temperature typically between 750°C and 1100°C.

[0021] The fibrous preform(s) to be densified are positioned in a reaction chamber 30 of a chemical vapor infiltration furnace for densification. Densification is carried out by introducing a reaction gaseous phase 400 through an inlet 301 of the reaction chamber 30. The gaseous phase 400 is obtained from the outlet of a gas mixer 40 which receives, at a first inlet, a gas 401 of nominal composition and, at a second inlet, a recycled effluent gas 402. At the beginning of densification, the gaseous phase 400 consists solely of the gas 401, and as densification progresses, effluent gas is recycled, and this recycled gas 402 forms, in whole or in part, the gaseous phase 400 introduced into the chamber 30.A process is thus proposed in which the effluent gas is recycled continuously from a vapor-phase pyrocarbon formation cycle, so that the recycled effluent gas is used during this cycle. The gas 401 can typically be a mixture of methane and propane. A temperature of approximately 850°C to 1050°C and a pressure of [specific pressure] can be imposed in chamber 30. reduced pressure ranging approximately from 0.5 kPa to 3.3 kPa. The formation of pyrocarbon in enclosure 30 is accompanied by the generation of by-products, such as dihydrogen from the decomposition of the pyrocarbon precursor gas, light unsaturated hydrocarbons in the C1 to C4 range, and heavy hydrocarbons with at least 5 carbon atoms, such as polycyclic aromatic hydrocarbons (PAHs), benzene hydrocarbons, or tars. An effluent gas 50 containing these by-products, as well as unreacted light saturated hydrocarbons in the C1 to C4 range, is discharged through an outlet 302 of enclosure 30. By way of illustration, the composition of the effluent gas 50 at outlet 302 of enclosure 30, expressed as volume percentages, may be as follows: - light saturated hydrocarbons at a rate of 50% to 70%, - light unsaturated hydrocarbons at a rate of 5% to 10%, - heavy hydrocarbons at a rate of less than 2%, and - dihydrogen at a rate of 20% to 40%.

[0022] The installation according to the invention comprises a gas treatment and recycling circuit for the effluent gas 50, defining a loop between the outlet 302 and the inlet 301 of the chamber 30. This loop produces recycled effluent gas 402, which is then reintroduced into the chamber 30 (via the gas mixer 40) to be used to produce additional pyrocarbon. The following section provides details of the recycling implemented in the illustrated example installation.

[0023] First, the heavy hydrocarbons 501 are removed from the effluent gas 50 (step a)). This removal corresponds to a technique known per se. The effluent gas 50 passes through a suitable device 60 which has an inlet 601 in communication with the outlet 302 of the reaction vessel 30. The removal can be carried out by at least one of the following methods: washing the gas 50 with a capture liquid, such as an oil, or condensation. As an example, reference may be made to US patent 8,084,079, which describes a possible solution for removing heavy hydrocarbons from the effluent gas 50.

[0024] In the illustrated example, the installation includes a compressor 70 connected to an outlet 602 of the heavy hydrocarbon removal device 501. The effluent gas 50 obtained after step a) passes through the compressor 70 to be brought to a pressure of at least 4 bar, for example, at least 6 bar or at least 10 bar. This pressure can be between 4 bar and 12 bar, for example, between 6 bar and 12 bar or between 10 bar and 12 bar. As mentioned above, this compression promotes catalytic hydrogenation but is not mandatory.

[0025] The installation includes a catalytic hydrogenation device 80 comprising an inlet 801 in communication with the outlet 602 (and with the compressor 70 in (the illustrated example). The effluent gas 50, obtained after implementation of step a) and after possible passage through the compressor 70, undergoes catalytic hydrogenation (step b)). In the illustrated example, the gas 50 is in a compressed state at the pressure values ​​indicated above during catalytic hydrogenation.

[0026] Catalytic hydrogenation takes advantage of the presence of hydrogen in the gas 50 to eliminate light unsaturated hydrocarbons (alkenes and alkynes) by transforming them into light alkanes, such as ethane, propane, or butane. This reaction requires neither an input of external matter, since hydrogen is present in sufficient quantity in the mixture, nor an input of energy, since the reaction is exothermic.

[0027] Partial hydrogenation of unsaturated hydrocarbons is a known reaction in petroleum refining and the production of major intermediates for the petrochemical industry. Indeed, it is desirable to remove the most unsaturated hydrocarbons (alkynes) from light olefinic petroleum fractions to enable their use in petrochemicals or the polymer industry, where very high olefin purities are required. In these techniques, the main objective is to eliminate triple bonds and to avoid excessive hydrogenation, in order to conserve the ethylene, which is then used for polymer synthesis.

[0028] The catalytic hydrogenation process proposed here seeks a more complete hydrogenation of unsaturated hydrocarbons compared to known processes for transforming them into saturated hydrocarbons. Such more complete hydrogenation can be achieved: - by increasing the residence time in the reactor, - by increasing the total specific surface area of ​​the catalyst, - by increasing the temperature up to a certain limit.

[0029] By way of example, palladium, or a palladium-based catalyst, can be used as a catalyst, it being understood that other catalysts are conceivable.

[0030] In general, the effluent gas obtained after implementation of step b) may advantageously have a volume content of light unsaturated hydrocarbons less than or equal to 10%, for example less than or equal to 8%.

[0031] In the illustrated example, the residual dihydrogen 502 is removed from the effluent gas obtained after step b) using a suitable device 90. Those skilled in the art will recognize that several methods are possible for carrying out this removal. For example, the dihydrogen can be separated from the rest of the effluent gas by membrane separation, which provides a very high-quality separation. Other techniques, such as cryogenic distillation or cryogenic condensation, can be used to achieve this separation. It should be noted that the removal of the residual dihydrogen is not mandatory insofar as where dihydrogen slows the pyrocarbon deposition kinetics but does not affect the quality and properties of the deposit. The presence of residual dihydrogen in the effluent gas introduced into the furnace can therefore be accepted if a longer densification time can be implemented. Furthermore, it should be noted that gas 401 can advantageously be devoid of dihydrogen or incorporate a small amount, which reduces the proportion of dihydrogen in the gas phase 400 resulting from the mixture of gas 401 and recycled gas 402. Generally, supplementing recycled gas 402 with gas 401 compensates for the gas flow lost during processing and maintains the total flow rate entering the reaction vessel. It should be noted that gas 401 can be a minor component, having a lower mass concentration than gas 402 in the reaction mixture entering the vessel.The mass content of gas 401 in this reaction mixture can be less than or equal to 30%, for example less than or equal to 20% or 10%. The use of gas 401 is even optional and the system can operate in a closed loop; in this latter case, it may be desirable to remove the residual dihydrogen in the effluent gas after implementation of step b).

[0032] If the effluent gas has been compressed beforehand, the treated effluent gas can be depressurized, for example after the possible removal of residual dihydrogen, before implementation of step c) of introduction into the chamber 30. Following this depressurization, the pressure of the effluent gas can be increased to a maximum of 4 bar, for example to a maximum of 3.5 bar, for example to a pressure between 3 and 4 bar or between 3 and 3.5 bar. The recycled gas 402 is introduced through the inlet 301 of the chamber 30, which is connected to the outlet 802 of the catalytic hydrogenation device 80 (and to the residual dihydrogen separation device 90 in the example considered).

[0033] By way of illustration, the composition of the recycled effluent gas 402, which is intended to be reintroduced into the enclosure 30, expressed in volume percentages, can be as follows: - light saturated hydrocarbons at a rate of 72% to 83% in the case where there is no removal of residual dihydrogen after catalytic hydrogenation, or at a rate of 95% to 99% when such removal is carried out, - residual light unsaturated hydrocarbons, possibly still present, at a rate of no more than 1%, - residual heavy hydrocarbons, possibly still present, at a rate of no more than 1%,

[0034] - dihydrogen at a rate of no more than 28% in the case where there is no elimination of di residual hydrogen after catalytic hydrogenation, or at most 2% when such removal is carried out.

[0035] In general, the recycling of the invention produces a strong reduction in the quantity of light unsaturated hydrocarbons and heavy hydrocarbons, between the outlet 302 and the inlet 301 of the enclosure 30, by providing a recycled effluent gas 402 having a volumetric content of at most 1% in light unsaturated hydrocarbons and at most 1% in heavy hydrocarbons.

[0036] The invention is remarkable in that it proposes a single loop intended to treat the effluent gas from the enclosure 30 to transform it into a gas of composition close to the nominal composition of gas 401 without external input of matter, and minimal energy input limited to the possible compression of the treated gas upstream of the catalytic hydrogenation.

[0037] The formed part can be made of carbon-carbon composite material. For example, it could be a friction part such as an aircraft or automobile brake disc, a nozzle neck, or a heat shield.

[0038] The expression "between ... and ..." should be understood as including the limits.

Claims

Claims

1. A method for recycling an effluent gas (50) from a chemical vapor deposition or infiltration and comprising dihydrogen, light unsaturated hydrocarbons of C1 to C4 and heavy hydrocarbons of C5 and above, comprising the following steps: a) elimination (60) of the heavy hydrocarbons (501) from the effluent gas, b) catalytic hydrogenation (80) of the light unsaturated hydrocarbons so as to eliminate them by transforming them into light saturated hydrocarbons of C1 to C4, this catalytic hydrogenation being carried out on the effluent gas obtained after implementation of step a) and consuming a portion of the dihydrogen present in said effluent gas, and c) introduction of the effluent gas (402) obtained after implementation of step b) into a reaction chamber (30) and formation of pyrocarbon by chemical vapor deposition or infiltration from the hydrocarbons light saturates in C1 to C4 present in the effluent gas thus introduced.

2. Method according to claim 1, in which a compression of the effluent gas (50) obtained after implementation of step a) is carried out before implementation of step b) to bring it to a pressure of at least 4 bar.

3. Method according to claim 1 or 2, in which a removal (90) of the residual dihydrogen (502) from the effluent gas is carried out after implementation of step b) and before step c).

4. A method according to any one of claims 1 to 3, wherein densification of a carbon fiber fibrous preform by pyrocarbon is carried out during step c).

5. Chemical vapor deposition or infiltration installation, comprising: - a reaction chamber (30) in which pyrocarbon is intended to be formed by chemical vapor deposition or infiltration having an inlet (301) through which a reaction gas phase (400) is intended to be admitted and an outlet (302) through which an effluent gas (50) is intended to be evacuated, and - a circuit for treating and recycling the effluent gas defining a loop between the outlet and the inlet of the reaction chamber, comprising (i) a device (60) for eliminating heavy hydrocarbons of C5 and above from the effluent gas having an inlet (601) in communication with the outlet of the reaction chamber and an outlet (602), and (ii) a catalytic hydrogenation device (80) having an inlet (801) in communication with the outlet of the heavy hydrocarbon removal device and an outlet (802) in communication with the inlet of the reaction chamber.

6. Installation according to claim 5, in which the effluent gas treatment and recycling circuit further comprises a compressor (70) located between the device (60) for eliminating heavy hydrocarbons of C5 and above and the catalytic hydrogenation device (80).

7. Installation according to claim 5 or 6, in which the circuit for treating and recycling the effluent gas further comprises a device (90) for eliminating residual dihydrogen located between the catalytic hydrogenation device (80) and the inlet (301) of the reaction chamber (30).

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