Method for recycling a captured aromatic oil loaded with polycyclic aromatic hydrocarbons

Abstract

The invention relates to a method for reprocessing a loaded captured aromatic oil (11) comprising a captured aromatic oil and one or more polycyclic aromatic hydrocarbons, the method being characterised in that it comprises a step of separating the polycyclic aromatic hydrocarbon from the captured aromatic oil, the separation step being carried out by supplying a continuous counter-current contactor (100) with the loaded captured aromatic oil and with carbon dioxide in the supercritical state (23).

Description

Process for recycling an aromatic capture oil loaded with polycyclic aromatic hydrocarbons Technical Field

[0001] This presentation concerns a recycling process for an aromatic capture oil, notably used to trap effluents from densification processes by chemical vapor phase infiltration processes. 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 obtain parts made of carbon / pyrolytic carbon material by chemical infiltration or by chemical vapor deposition.

[0004] Such processes utilize a carbon-rich reactive gas phase brought into contact with porous substrates under conditions such that the gas phase can react on contact with the substrate to form a matrix directly within the substrate's porosities.

[0005] The choice of gas constituting the gaseous phase and the temperature and pressure conditions prevailing in the furnace determine the nature of the matrix formed within the composite material parts.

[0006] However, under the temperature and pressure conditions involved in the furnace, the gaseous phase can generate polycyclic aromatic hydrocarbons as self-reaction products.

[0007] These by-products no longer react in the oven, and simply pass through it to exit the oven.

[0008] Trapping of such polycyclic aromatic hydrocarbons is generally carried out as soon as they leave the furnace, because the cooling of polycyclic aromatic hydrocarbons generates soot which settles in the pipes downstream of the reaction chamber and which impairs the entire process.

[0009] Solutions have already been proposed to trap polycyclic aromatic hydrocarbons at the outlet of a chemical vapor infiltration furnace, notably by passing the effluents through a trap containing an aromatic oil, in which the polycyclic aromatic hydrocarbons are soluble.

[0010] However, while this trapping does indeed protect the downstream area of ​​the installation by trapping polycyclic aromatic hydrocarbons, the aromatic capture oil containing polycyclic aromatic hydrocarbons becomes a new waste product of the process that must then be managed independently.

[0011] Furthermore, the capture rate of polycyclic aromatic hydrocarbons by the aromatic capture oil decreases as the aromatic capture oil becomes loaded with polycyclic aromatic hydrocarbons, so it is necessary to change the aromatic capture oil repeatedly to continue to ensure effective trapping.

[0012] It has been proposed to recycle aromatic capture oil contaminated with polycyclic aromatic hydrocarbons. For example, processes based on sulfuric acid or vacuum distillation have been suggested.

[0013] However, the first of these processes generate acidic sludge that is difficult to remove and is therefore undesirable for the overall environmental footprint of the process.

[0014] The latter of these processes are extremely energy-intensive and therefore not very viable industrially.

[0015] More recently, solvent extraction solutions have been proposed, but the solvents used are affected by the evolution of REACH regulations and therefore replacements will need to be found in the medium term.

[0016] Therefore, there remains a need for a solution that allows for the treatment of effluents from a chemical vapor infiltration process without the aforementioned disadvantages. Description of the invention

[0017] The present invention is specifically designed to meet this need.

[0018] To this end, it proposes a reprocessing method for a loaded aromatic capture oil comprising an aromatic capture oil and one or more polycyclic aromatic hydrocarbons, the method being characterized in that it includes a step of separating said polycyclic aromatic hydrocarbon from the aromatic capture oil, the separation step being carried out by supplying a counter-current continuous contactor with the loaded aromatic capture oil and with carbon dioxide under supercritical conditions.

[0019] The charged aromatic capture oil can be obtained by trapping, in an aromatic oil, gaseous effluents from a chemical vapor-phase infiltration process for the densification of a carbon fiber fibrous preform by pyrocarbon,

[0020] In one embodiment, the countercurrent continuous contactor is not supplied with water or a solvent in which the polycyclic aromatic hydrocarbon(s) are soluble. Carbon dioxide under supercritical conditions can be the sole solvent for extracting the polycyclic aromatic hydrocarbons contained in the aromatic capture oil. The countercurrent continuous contactor can therefore be supplied solely with the charged aromatic capture oil and carbon dioxide.

[0021] Indeed, it is to the credit of the inventors that they determined that the particular choice of such a process presents advantages compared to the solutions proposed in the prior art.

[0022] Firstly, the particular choice of supercritical carbon dioxide exhibits good selectivity towards polycyclic aromatic hydrocarbons, which allows for excellent extraction of the latter.

[0023] In addition, the particular choice of a continuous counter-current contactor allows continuous recycling of the aromatic capture oil, which ensures that an aromatic capture oil with low polycyclic aromatic hydrocarbon content is always available, thus ensuring excellent capture of polycyclic aromatic hydrocarbons at the outlet of a chemical vapor phase infiltration chamber.

[0024] The use of such a process makes it possible to have, at one of the outputs of the counter-current continuous contactor, an aromatic capture oil depleted in polycyclic aromatic hydrocarbons and therefore reusable as such in a polycyclic aromatic hydrocarbon trap, and at the other, supercritical carbon dioxide loaded with polycyclic aromatic hydrocarbons.

[0025] Here and throughout the application, it shall be understood that the expression "supercritical carbon dioxide" or simply "supercritical carbon dioxide" has the usual meaning it takes in the field of thermodynamics, i.e. describes the fluid state in which carbon dioxide is found when its pressure and / or temperature are above the critical pressure of 73 bars and / or the critical temperature of 31°C.

[0026] In one embodiment, the pressure applied in the counter-current continuous contactor during the process can be between 150 bar and 250 bar, or even between 150 bar and 200 bar.

[0027] The inventors determined that at such pressure, the difference between the solubility of polycyclic aromatic hydrocarbons in supercritical carbon dioxide and the solubility of aromatic capture oil in supercritical carbon dioxide was maximal.

[0028] In other words, it is at these pressures that polycyclic aromatic hydrocarbons are best extracted without co-extracting too much aromatic capture oil.

[0029] Furthermore, at such pressures, supercritical carbon dioxide exhibits excellent selectivity towards polycyclic aromatic hydrocarbons compared to aromatic capture oil.

[0030] In other words, the polycyclic aromatic hydrocarbons present in the aromatic capture oil will preferentially migrate towards supercritical carbon dioxide in the countercurrent continuous contactor.

[0031] In one embodiment, the temperature applied in the counter-current continuous contactor during the process can be between 40°C and 80°C, or even between 50°C and 80°C, or even between 60°C and 75°C.

[0032] Similarly, these temperatures represent optimums with regard to the solubility of polycyclic aromatic hydrocarbons in supercritical carbon dioxide, which is much higher than that of aromatic capture oil.

[0033] In one embodiment, the aromatic capture oil may include 2,3-dibenzyltoluene (also known as 2,3-DBT), or even be composed of 2,3-dibenzyltoluene. A synthetic aromatic capture oil with a consistent chemical composition, regardless of the source, is preferred in order to control its physical properties during the trapping of polycyclic aromatic hydrocarbons. A well-defined chemical composition is obtained, in particular, through a reproducible synthesis process. "Synthetic" aromatic oils are contrasted with "natural" oils, such as those extracted from petroleum fractionation, which are generally mixtures of several compounds and whose composition varies from one raw material to another; some of these oils are aromatic and contain only a single benzene ring.

[0034] Indeed, polycyclic aromatic hydrocarbons have an excellent affinity for such an aromatic capture oil.

[0035] In one embodiment, the polycyclic aromatic hydrocarbons included in the loaded aromatic capture oil may be selected from acenaphthene, fluorene, or a mixture of polycyclic aromatic hydrocarbons comprising at least these two compounds.

[0036] In this application, the term "polycyclic aromatic hydrocarbon" defines a chemical compound comprising carbon and oxygen whose form includes at least two aromatic rings, in particular at least two condensed aromatic rings. An aromatic ring is understood here and in the application in the usual sense of Hückel's rule. Examples of polycyclic aromatic hydrocarbons are acenaphthene, anthracene, benzo[g,h,i]perylene, fluoranthene, fluorene, naphthalene, phenanthrene, and pyrene.

[0037] Aromatic capture oil loaded with PAH(s) comprises one or more polycyclic aromatic hydrocarbons. For example, the compound 2,3-dibenzyltoluene.

[0038] Similarly, an oil will be said to be "aromatic" if it contains an aromatic compound in the usual sense of Hückel's rule, or if it is composed of such a compound or compounds. Aromatic capture oil is, of course, a chemical compound distinct from a polycyclic aromatic hydrocarbon. Aromatic oil preferably comprises a single chemical compound and possibly impurities in negligible quantities; this chemical compound preferably comprises two, three, or more benzene rings. In aromatic oil, the benzene rings are preferably non-condensed.

[0039] In one embodiment, the loaded aromatic capture oil may comprise up to 10% by mass of polycyclic aromatic hydrocarbons, or even up to 5% by mass of polycyclic aromatic hydrocarbons.

[0040] Indeed, since the process allows continuous recycling of the aromatic capture oil, it is preferable to recycle it even if it contains only a low load of polycyclic aromatic hydrocarbons, because this ensures that we always have an aromatic capture oil that is very low in polycyclic aromatic hydrocarbons and therefore ensures excellent capture of these compounds.

[0041] In one embodiment, the process further includes, at the output of the counter-current continuous contactor, a supercritical carbon dioxide expansion step.

[0042] This step then makes it possible to modify the properties of the supercritical carbon dioxide which until then ensured the solvation of polycyclic aromatic hydrocarbons, which causes the formation of a liquid phase rich in polycyclic aromatic hydrocarbons.

[0043] In one embodiment, the reprocessing process includes reintroducing into the countercurrent continuous contactor a portion of the liquid phase rich in polycyclic aromatic hydrocarbons obtained by expansion of the supercritical carbon dioxide exiting the countercurrent continuous contactor (so-called "reflux" current).

[0044] If necessary, this reintroduction can be preceded by bringing this phase rich in polycyclic aromatic hydrocarbons back to the temperature and pressure conditions chosen for the continuous counter-current contactor.

[0045] For example, the portion of liquid phase loaded with polycyclic aromatic hydrocarbons obtained by expansion of supercritical carbon dioxide reintroduced into the countercurrent continuous contactor is between 90% and 97% of the liquid phase obtained by expansion of supercritical carbon dioxide.

[0046] This embodiment, in which a portion of the phase extracted by supercritical carbon dioxide in the countercurrent continuous contactor is reintroduced into the countercurrent continuous contactor, ensures, on the scale of the whole process, better separation of polycyclic aromatic hydrocarbons from the charged aromatic capture oil.

[0047] In one embodiment, the mass ratio of the feed of the countercurrent continuous contactor by carbon dioxide and by the charged aromatic capture oil can be between 50 and 65, for example between 55 and 65.

[0048] In other words, the continuous counter-current contactor can be powered by a mass flow rate between 50 and 65 times greater, or even between 55 and 65 times greater, of supercritical carbon dioxide than of loaded aromatic capture oil.

[0049] The inventors determined that such a ratio represented a desirable optimum between the carbon dioxide consumption required for the process and the selectivity of the extraction obtained.

[0050] In one embodiment, the invention relates in another aspect to a process for densifying a carbon fiber fibrous preform with pyrocarbon, the densification being carried out by a chemical vapor infiltration process, in which the gaseous effluents of the chemical vapor infiltration process are trapped in an aromatic capture oil and the aromatic capture oil is continuously recycled by means of a process as described above. Brief description of the drawings

[0051] [Fig. 1] Figure 1 is a schematic representation of a process in one embodiment of the invention. Description of the implementation methods

[0052] The invention is now described by means of figures, which are provided for descriptive purposes to illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter.

[0053] Figure 1 illustrates a process according to one embodiment.

[0054] The process includes supplying a counter-current continuous contactor 100 with a first supply 13 and a second supply 23.

[0055] As illustrated, the first feed 13 can be obtained after a pressurizing and heating device 12 of a flow 11 of aromatic capture oil loaded with one or more polycyclic aromatic hydrocarbons.

[0056] In one embodiment, the stream 11 may consist of an aromatic capture oil used in a trap disposed downstream of a furnace used for a chemical vapor phase infiltration process, for example used for the manufacture of a part in carbon / carbon composite material.

[0057] Component 12 is known as such and can conventionally include a pump for pressure increase and a heat exchanger for temperature increase.

[0058] As illustrated, the second supply 23 can be obtained after a pressurization and temperature control device 22 for a flow 21 of carbon dioxide. In particular, the pressurization and temperature control device 22 must ensure that the carbon dioxide is in supercritical conditions when it supplies the counter-current DC contactor 100.

[0059] In one embodiment, the second supply 23 consists of carbon dioxide under supercritical conditions, and the supply of the counter-current DC contactor 100 does not include one or more supplies other than the supply 13 and the supply 23, which would provide water or a solvent in which the polycyclic aromatic hydrocarbon(s) are soluble. Carbon dioxide under supercritical conditions can be the sole solvent for extracting the polycyclic aromatic hydrocarbons contained in the aromatic capture oil. The component 22 can conventionally include a pump, a compressor, and a heat exchanger.

[0060] In the embodiment shown, the charged aromatic capture oil stream 13 feeds the top of the countercurrent DC contactor, and the carbon dioxide stream feeds the bottom. It should be kept in mind that the feeds 13 and 23 can be reversed without loss of process effects.

[0061] Indeed, the density of supercritical carbon dioxide varies with pressure and temperature. Depending on the pressure and temperature chosen for the process, the feed to the countercurrent continuous contactor is selected so that the densest fluid of the supercritical carbon dioxide and the charged aromatic capture oil is introduced at the top of the countercurrent continuous contactor 100 and the least dense fluid is introduced at the bottom.

[0062] In this way, it is ensured that gravity allows the fluids from the two supplies 13, 23 to be in contact with each other in the counter-current continuous contactor 100.

[0063] After the continuous counter-current contactor 100, two fluxes are obtained 200, 300.

[0064] The flux 200, obtained at the bottom of the counter-current continuous contactor in the embodiment shown, corresponds to the flux 13 which exchanged with the flux 23 during its passage through the counter-current continuous contactor 100.

[0065] In one embodiment, the countercurrent continuous contactor can comprise between 15 and 30 theoretical stages.

[0066] In Figure 1, the 200 stream is then composed of the initial charged aromatic capture oil 13 from which the polycyclic aromatic hydrocarbons have been extracted by contact with the 23 stream.

[0067] Similarly, the flux 300 corresponds to the supercritical carbon dioxide flux 23, which has become loaded with polycyclic aromatic hydrocarbons during its passage through the counter-current continuous contactor 100.

[0068] In the embodiment shown, the stream 200 corresponding here to the purified aromatic capture oil can pass through a secondary separator 600 allowing the recovery of carbon dioxide 601 on the one hand, and purified aromatic capture oil 602 on the other.

[0069] Indeed, the affinity of polycyclic aromatic hydrocarbons initially contained in the charged aromatic capture oil stream 13 is greater for supercritical carbon dioxide than for aromatic capture oil.

[0070] Therefore, throughout the contactor 100, polycyclic aromatic hydrocarbons will gradually migrate into the supercritical carbon dioxide stream 23, which helps to purify the aromatic capture oil.

[0071] This gives us at the bottom of the separator 600 a stream 602 composed very mainly of aromatic capture oil.

[0072] In an embodiment not shown, the flow 602 can directly supply a trap comprising an aromatic capture oil disposed downstream of a chemical vapor-phase infiltration furnace.

[0073] Still on figure 1, the flux 300 exiting the counter-current continuous contactor 100 corresponds to the flux of supercritical carbon dioxide which has become charged with polycyclic aromatic hydrocarbons.

[0074] In the embodiment shown, the flow 300 passes through a conditioning element 301. This element allows the temperature and / or pressure of the flow 300 to be modified.

[0075] In particular, it is desirable that the supercritical carbon dioxide 300 loaded with polycyclic aromatic hydrocarbons exiting the countercurrent continuous contactor 100 be placed in conditions at the exit of the conditioning unit 301 where the carbon dioxide is no longer in supercritical conditions but in a gaseous state.

[0076] This can be achieved, for example, by expanding and heating supercritical carbon dioxide.

[0077] In this way, after a separator 400, we obtain a phase 401, composed of gaseous carbon dioxide, and on the other hand of a liquid highly concentrated in polycyclic aromatic hydrocarbons 402.

[0078] In one embodiment, however optional and consequently represented in dotted line on figure 1, a portion of this liquid 500 highly concentrated in polycyclic aromatic hydrocarbons can pass through a conditioning element 501 and be reintroduced 502 at the head of the counter-current continuous contactor 100.

[0079] Such a reintroduction 502 of a phase rich in polycyclic aromatic hydrocarbons makes it possible to recover a further portion of the aromatic capture oil still contained in phase 402, and thus further increases the separation rate of the process at full scale. Example

[0080] The invention is now described by means of examples numerically illustrating the benefits of a process such as that described above.

[0081] The data in this example were obtained using a simulation model whose parameters were fitted based on experimental results concerning the specific solubility of two polycyclic aromatic hydrocarbons in supercritical carbon dioxide and its selectivity towards these two compounds. The two polycyclic aromatic hydrocarbons chosen are compounds commonly found in the output of a carbon / carbon composite material densification process using chemical vapor infiltration: acenaphthene and fluorene.

[0082] After determining the selectivity of the separation for these two polycyclic aromatic hydrocarbons, the model is applied for a 100 countercurrent continuous contactor comprising 20 theoretical stages.

[0083] The parameters chosen for the process are a pressure of 200 bars and a temperature of 70°C.

[0084] Starting from a loaded aromatic capture oil 11 comprising 95% by mass of 2,3-dibenzyltoluene and 5% by mass of polycyclic aromatic hydrocarbons, distributed equally between acenaphthene and fluorene.

[0085] The mass ratio between the carbon dioxide flux 21 and the charged aromatic capture oil flux 11 is chosen to be 62.2.

[0086] With these conditions and for a process shown in Figure 1, the recovery rate determined for acenaphthene, expressed as the mass flow rate of acenaphthene exiting through the stream 402 divided by the mass flow rate of acenaphthene composing the incoming stream 11, is 98%.

[0087] The fluorene recovery rate, expressed in the same way, is equal to 68%.

[0088] The recycling rate of the aromatic capture oil, expressed as the mass flow of oil exiting through the 602 stream divided by the mass flow of aromatic capture oil composing the incoming stream 11, is 98.6%.

[0089] Furthermore, at the output of process 602, a 99.1% pure aromatic capture oil stream is obtained from a loaded aromatic capture oil 11 comprising 5% by mass of polycyclic aromatic hydrocarbons.

[0090] These values ​​illustrate that the described process allows for continuous purification of aromatic capture oils, enabling the trapping of polycyclic aromatic hydrocarbons.

Claims

Demands

1. A process for reprocessing a loaded aromatic capture oil (11) comprising an aromatic capture oil and one or more polycyclic aromatic hydrocarbons, wherein the loaded aromatic capture oil is obtained by trapping, in an aromatic oil, gaseous effluents from a chemical vapor-phase infiltration process for the densification of a carbon fiber fibrous preform by pyrocarbon, the process being characterized in that it comprises a step of separating said polycyclic aromatic hydrocarbon from the aromatic capture oil, the separation step being carried out by feeding a countercurrent continuous contactor (100) with the loaded aromatic capture oil and with carbon dioxide under supercritical conditions (23).

2. Reprocessing method according to claim 1, wherein the pressure applied in the countercurrent continuous contactor (100) during the process can be between 150 bar and 250 bar.

3. Reprocessing method according to claim 1 or 2, wherein the temperature applied in the countercurrent continuous contactor (100) during the process can be between 40°C and 80°C.

4. A reprocessing method according to any one of claims 1 to 3, wherein the aromatic capture oil comprises 2,3-dibenzyltoluene.

5. A reprocessing method according to any one of claims 1 to 4, wherein the polycyclic aromatic hydrocarbons included in the loaded aromatic capture oil are selected from acenaphthene, fluorene, or a mixture of polycyclic aromatic hydrocarbons comprising at least these two compounds.

6. A reprocessing method according to any one of claims 1 to 5, wherein the loaded aromatic capture oil (11) comprises up to 10% by mass of polycyclic aromatic hydrocarbons.

7. A reprocessing method according to any one of claims 1 to 6, the reprocessing method further comprising reintroducing into the countercurrent continuous contactor a portion (502) of the polycyclic aromatic hydrocarbon rich liquid phase obtained by expansion of supercritical carbon dioxide (300) exiting the countercurrent continuous contactor (100).

8. A reprocessing method according to claim 7, wherein the portion of liquid phase loaded with polycyclic aromatic hydrocarbons obtained by expansion of reintroduced supercritical carbon dioxide (502) is between 90% and 97% of the liquid phase obtained by expansion of supercritical carbon dioxide.

9. A method for densifying a carbon fiber fibrous preform with pyrocarbon, the densification being carried out by a chemical vapor infiltration process, wherein the gaseous effluents of the chemical vapor infiltration process are trapped in an aromatic capture oil and the aromatic capture oil is continuously recycled by means of a reprocessing process according to any one of claims 1 to 8.

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