Device for chemical fluidized bed vapor deposition

Description

Technical Field

[0001] The present invention relates to a device for chemical vapor deposition in a fluidized bed making it possible to stabilize the temperature of the fluidized bed during deposition as well as the use of this device. Prior art

[0002] Fluidized bed chemical vapor deposition is a known technique and allows particles to be coated with various coatings adapted to the desired application. US 2010 / 047136 discloses a fluidized bed reactor. However, it is desirable to improve the efficiency of fluidized bed chemical vapor deposition techniques. Statement of the invention

[0003] The present invention relates to a device for chemical vapor deposition in a fluidized bed, comprising at least: a reactor comprising a treatment zone in which fluidized bed chemical vapor deposition is intended to be carried out from a gaseous phase, an inlet zone through which the gaseous phase is intended to be introduced into the treatment zone and an outlet zone through which the gaseous phase is intended to be evacuated from the treatment zone, a heating system configured to heat the treatment zone, and a cooling system configured to cool the inlet zone, the device being characterized in that it further comprises a porous thermal insulator present in the inlet zone and configured to be crossed by the gas phase, said porous thermal insulator having an effective thermal conductivity at 20°C less than or equal to 3.5 Wm -1 .K -1 .

[0004] In the following, the expression "porous thermal insulation" will be referred to as "insulator" for the sake of brevity.

[0005] Effective thermal conductivity of the insulator is understood to mean the thermal conductivity of the volume occupied by the insulator, including the interstitial gas in its porosity, and not the thermal conductivity of the material constituting the insulator. The interstitial gas present in the porosity of the insulator reduces its effective thermal conductivity and thus limits heat dissipation during deposition between the treatment zone, which is heated, and the inlet zone, which is cooled. This stabilizes the temperature of the fluidized bed in the treatment zone and reduces the phenomenon of reduction of the useful area in which the deposition is actually carried out, thus improving the deposition efficiency.

[0006] The effective thermal conductivity of porous thermal insulation λ eff filled with a gas of thermal conductivity λ g can be determined by verifying the relationship below: [Math. 1] λ eff λ g = 1 + 2 βϕ + 2 β 3 − 0 , 1 β ϕ 2 + ϕ 3 × 0 , 05 exp 4 , 5 β 1 − β ϕ avec β = λ s − λ g λ s − 2 λ g

[0007] In the above relationship, λ S denotes the thermal conductivity of the material forming the porous thermal insulator (i.e., the grain material in the case of an insulator in the form of a granular bed), Φ denotes the solid volume fraction of the porous insulator, or (1- Φ) denotes the volume porosity rate of this porous insulator. Reference may be made to the publication GONZO (EE). - Chem. Eng. J., 90, p. 299 (2002).

[0008] In an exemplary embodiment, the porous thermal insulator has an effective thermal conductivity at 20°C less than or equal to 0.42 Wm -1< .K -1< .

[0009] Such a feature helps to further improve the efficiency of the coating deposition operation.

[0010] In particular, the porous thermal insulator may have an effective thermal conductivity at 20°C less than or equal to 0.11 Wm -1< .K -1< , for example substantially equal to 0.11 Wm -1< .K -1< .

[0011] In an exemplary embodiment, the material forming the porous thermal insulator has a thermal conductivity at 20°C less than or equal to 40 Wm -1< .K -1< . Such a characteristic helps to further improve the efficiency of the coating deposition operation.

[0012] In this case, the insulator is formed by a material that is itself a poor heat conductor, which further stabilizes the temperature of the fluidized bed during the deposition of the coating. This case, however, remains optional since a material with a higher thermal conductivity can be used to form the insulator because the porosity of the insulator - filled with a poorly heat-conducting interstitial gas - reduces the effective thermal conductivity of the insulator to an acceptable value even if the latter is formed by a material that is intrinsically relatively heat-conductive.

[0013] In particular, the material forming the porous thermal insulator may have a thermal conductivity at 20°C less than or equal to 4 Wm -1< .K -1< , for example less than or equal to 3 Wm -1< .K -1< .

[0014] In an exemplary embodiment, the volume porosity rate of the porous thermal insulator is greater than 26%, for example greater than or equal to 32%.

[0015] Such a characteristic advantageously contributes to further reducing the effective thermal conductivity of the insulation, further stabilizing the temperature of the fluidized bed during the deposition of the coating. In the particular case of a granular bed, such porosity values ​​correspond to a “loose” or non-compact filling state.

[0016] In an exemplary embodiment, the porous thermal insulator is a granular bed. The invention also relates to a method for coating particles using a device as described above, comprising at least: introducing the gas phase into the treatment zone through the inlet zone and the porous thermal insulator, depositing a coating on the particles present in the treatment zone by fluidized bed chemical vapor deposition from the introduced gas phase, and recovering, after deposition of the coating, the coated particles.

[0017] In an exemplary embodiment, the gas phase has a thermal conductivity at 20°C less than or equal to 0.04 Wm -1< .K -1< .

[0018] In an exemplary embodiment, after the coating has been deposited, a mixture of the coated particles and the grains of the granular bed is recovered, and the coated particles are separated from said grains. Brief description of the drawings

[0019] [ Fig. 1 ] There figure 1 illustrates, schematically and partially, a device for implementing chemical vapor deposition in a fluidized bed outside the invention. Fig. 2 ] There figure 2 represents the evolution of the average temperature of the fluidized bed obtained during a deposit made with the device of the figure 1 . [ Fig. 3 ] There figure 3 represents the normalized pressure drop of the fluidization gas as a function of its speed when implementing a deposit with the device of the figure 1 . [ Fig. 4 ] There figure 4 compares the particle bed heights obtained before and after deposition for different fluidization gas velocities when the device figure 1 is implemented. [ Fig. 5 ] There figure 5 illustrates, schematically and partially, a device for implementing chemical vapor deposition in a fluidized bed according to the invention. Fig. 6 ] There figure 6 represents the evolution of the average temperature of the fluidized bed obtained during a deposit made with the device of the figure 5 . Description of the embodiments

[0020] We will describe in connection with the figure 1 the structure of a device 1 outside the invention capable of producing a coating of particles by chemical vapor deposition in a fluidized bed. Chemical vapor deposition in a fluidized bed is a technique known per se.

[0021] The device 1 comprises a reactor 3 defined by a wall which may be made of quartz or Inconel ®<. The reactor 3 comprises a treatment zone 31 in which the chemical vapor deposition in a fluidized bed is intended to be carried out from a gaseous phase. The reactor 3 further comprises an inlet zone 35 in communication with a source 6 of gaseous phase through which the gaseous phase comprising the precursor of the coating to be formed is intended to be introduced into the treatment zone 31. The inlet zone 35 comprises a porous diffuser 36 ensuring the diffusion of the gaseous phase in the reactor 3. The gaseous phase coming from the source 6 of gaseous phase is introduced into the reactor 3 through the diffuser 36. The reactor 3 further comprises an outlet zone 37 through which the gaseous phase is intended to be evacuated from the treatment zone 31 to the outside of the reactor 3.The outlet zone 37 is in communication with a pumping system (not shown) in order to ensure the creation of a vacuum in the reactor 3. The gaseous phase coming from the gaseous phase source 6 first passes through the inlet zone 35, then through the treatment zone 31 and is then discharged outside the reactor 3 by passing through the outlet zone 37. The reactor 3 can extend along a vertical axis X. The inlet 35, treatment 31 and outlet 37 zones follow one another along the X axis. The treatment zone 31 is present between the inlet zone 35 and the outlet zone 37.

[0022] The particle bed to be treated initially rests on the diffuser 36 and the gas phase is injected under the diffuser 36 and passes through the particle bed from bottom to top. This upward movement of gas makes it possible to support the weight of the particle bed and guarantees the latter's state of fluidization.

[0023] The device 1 comprises a heating system 9 configured to heat the treatment zone 31 to a temperature allowing the formation of the coating on the particles from the precursor contained in the gas phase. The heating system 9 can be configured to perform resistive heating or alternatively inductive heating. The heating system 9 can be arranged around the entire circumference of the treatment zone 31 or around only a part of the latter. Thermocouples 17 located inside a sheath 15 centered at the reactor 3 are arranged at different positions in the reactor 3 and make it possible to monitor the evolution of the temperature at different heights in the fluidized bed. The device 1 further comprises a cooling system 8 configured to cool the inlet zone 35.The cooling system 8 may comprise a circuit in which a cooling fluid circulates, regulated at a temperature less than or equal to 50°C, for example 20°C. The cooling system 8 may be in the form of cooling flanges. The cooling fluid may be water. The cooling system 8 makes it possible to prevent the temperature of the diffuser 36 from being too high in order to avoid clogging of the latter by a deposit produced from the gaseous phase. The heating system 9 is offset from the cooling system 8 along the X axis of the reactor 3. The heating system 9 is offset from the diffuser 36 along the X axis of the reactor 3. The gaseous phase introduced into the reactor 3 for producing the coating depends on the nature of the coating to be obtained and it is within the general knowledge of a person skilled in the art to choose it and to choose the associated deposition conditions, in terms of temperature, pressure and flow rate.The gas phase typically comprises an inert carrier gas, such as nitrogen. Gaseous precursors can be used under standard conditions, in which case they can be mixed with the carrier gas before being introduced into reactor 3. Liquid precursors can also be used under standard conditions, in which case the carrier gas can bubble into the liquid precursor to enrich it and then be introduced into reactor 3.

[0024] In the non-limiting example of a deposition of a pyrocarbon coating, nitrogen can be used as a carrier gas. The flow rate of introduction of the carrier gas into a reactor 3 as shown diagrammatically in figure 1 having a diameter of 5 centimeters and a height of 1 meter can be between 500 standard cubic centimeters per minute and 3000 standard cubic centimeters per minute, for example between 750 standard cubic centimeters per minute and 1250 standard cubic centimeters per minute. The temperature imposed in the treatment zone 31 can be between 850°C and 1100°C, for example between 925°C and 1000°C. The heating system 9 can be positioned 15 centimeters above the diffuser 36. In this non-limiting case, propane can be used as a precursor of the pyrocarbon to be deposited. Its flow rate can be between 5% and 30% of the flow rate of the carrier gas, for example between 10% and 20% of the flow rate of the carrier gas. By way of example, other precursors for pyrocarbon can be mentioned, such as gaseous precursors such as methane / propane mixtures or natural gas. Liquid precursors such as toluene, cyclohexane or ethanol can also be mentioned.

[0025] The deposit made using device 1 of the figure 1 gives satisfactory results. However, the inventors have found that the average temperature of the fluidized bed decreases during deposition. Indeed, maintaining a moderate temperature at the diffuser 36 in order to avoid clogging by a deposit from the gas phase is such that the cooled inlet zone 35 constitutes a heat dissipation zone. The heat exchanges between the wall of the reactor 3 and the fluidized bed are essentially by particulate convection and radiation. Thus, the fluidized bed easily gives off heat to the wall of the reactor 3 in the inlet zone 35. This heat dissipation leads to a drop in the average temperature within the fluidized bed during deposition and to a reduction in the useful area where the deposition is actually carried out, which results in a drop in efficiency.

[0026] The experimental results obtained by the inventors which made it possible to implement this phenomenon are detailed below.

[0027] The inventors carried out a carbon deposition operation on a porous alumina powder having the following characteristics: the particles of the powder belong to Geldart categories A or B, the diameter d50 of the particles is between 4 µm and 20 µm, the specific surface area of ​​the latter is 200 m 2 < / g and the initial void rate of the fixed bed constituted by these raw particles (before deposition) was equal to 80%.

[0028] The carbon was deposited from propane with a set temperature in treatment zone 31 of 1000°C and a pressure in reactor 3 of 400 mbar. The initial mass of the particles used during deposition was 290 grams, corresponding to an initial height of the fluidized bed of 23.3 cm. The propane flow rate was maintained at 200 standard cubic centimeters per minute and the nitrogen flow rate at 600 standard cubic centimeters per minute.

[0029] The evolution of the average temperature of the fluidized bed during deposition is provided at figure 2 . In this figure, the injection of propane is shown by the dotted line "A" and the stopping of the injection of propane by the dotted line "B". It can be seen that the average temperature within the fluidized bed tends to decrease during deposition. The active volume of the particle bed tends to reduce, which leads to a decrease in the deposition efficiency. The inventors have noted that these thermal instabilities are accompanied by a decrease in the external temperature of the tube wall and an increase in the temperature of the cooling fluid circulating in the cooling system 8. Thermal monitoring also shows that after the propane is stopped, all of the thermal disturbances are reduced and the temperature of the fluidized bed tends to increase again without, however, returning to its value before deposition.

[0030] There figure 3 shows the evolution of the normalized pressure drop (ΔP*) experienced by the gas as a function of its speed.

[0031] ΔP* is equal to the ratio between the measured pressure drop experienced by the gas when crossing the fluidized bed and the theoretical value of this pressure drop. The theoretical value is calculated from the weight of the particle bed relative to its surface area. according to the relationship: ΔP ∗ = ΔP experimental ΔP thearique et ΔP theorique = m lit g S

[0032] The value of ΔP* therefore constitutes a way of verifying the quality of the fluidization regime. A value of ΔP* equal to 1 indicates that all the particles are in a state of fluidization that can be described as homogeneous.

[0033] It is noted that a value of ΔP* close to 1 is obtained, which indicates that the powders, after deposition, retain a capacity to fluidize homogeneously. On the other hand, a reduction in the expanded height of the powder bed was observed after deposition (see figure 4 ). The height of the powder bed at the minimum fluidization is reduced by approximately 13%. The figure 4 shows with crosses the heights of the particle bed after deposition and with circles the heights of the particle bed before deposition.

[0034] All of these results confirm the phenomenon of heat dissipation by particle convection at the level of the cooled inlet zone 35.

[0035] Following this observation, the inventors proposed an evolution of device 1 of the figure 1 allowing to reduce the thermal exchanges in the inlet zone 35. A diagram of the device 10 according to the invention which responds to this problem is provided at figure 5 .

[0036] The device 10 comprises the same elements as the device 1 which are referenced in the same way. In order to reduce heat dissipation at the cooled inlet zone 35, the invention proposes to provide the reactor 3 with a porous thermal insulator 40 which has, as indicated above, a relatively low effective thermal conductivity. The porosity of the insulator 40 allows it to be crossed by the gas phase and contributes to the reduction of its effective thermal conductivity. The volume porosity rate of the insulator 40 may be between 26% and 48%, for example between 34% and 40%, for example substantially equal to 37%. The porosity of the insulator 40 may have a tortuous shape. The insulation 40 is present between the diffuser 36 and the treatment zone 31. The insulation 40 may be present over more than 50%, or even at least 75%, or even over substantially the entire height of the inlet zone 35.The insulation 40 may be present over more than 50%, or even at least 75%, or even substantially the entire height over which the cooling system 8 extends. The insulation 40 may extend from the diffuser 36 at least to the level of the lower part 91 of the heating system 9 which is present at the height H 1 . The lower part 91 of the heating system 9 corresponds to the end of the heating system 9 located on the side of the inlet zone 35. However, those skilled in the art will ensure that the insulation 40 does not extend too high beyond the lower part 91 of the heating system 9 in order to prevent the insulation 40 from being subjected to too high a temperature which would lead to a deposit originating from the gaseous phase clogging its porosity and hindering the circulation of the gaseous phase. The person skilled in the art knows, thanks to his general knowledge, how to determine the height over which the insulation 40 must extend in order to obtain the desired advantage.A person skilled in the art may, for example, choose the height of the insulator 40 so as to obtain a maximum drop in the average temperature of the fluidized bed of 20°C after 2 hours of deposition. As indicated above, the material forming the insulator 40 may or may not be thermally conductive. It is advantageous to use a material that is a poor conductor of heat to form the insulator 40, such as a ceramic material such as zirconia. The thermal conductivity at 20°C of the material forming the insulator 40 may be between 0.15 Wm -1< .K -1< and 418 Wm -1< .K -1< , for example between 0.15 Wm -1< .K -1< and 40 Wm -1< .K -1< , for example between 0.15 Wm -1< .K -1< and 4 Wm -1< .K -1< , for example between 0.15 Wm -1< .K -1< and 3 Wm -1< .K -1< . The insulator 40 may be in the form of a granular bed. In this case, the insulator 40 is formed by a stack of grains. The grains forming the insulator 40 may be stacked in bulk (non-regular stacking).The insulator 40 may be formed from a material distinct from the particles to be coated. Alternatively, an insulator 40 formed from a partially sintered block could be used, retaining sufficient open porosity to allow the gas phase to pass through while exhibiting the desired effective thermal conductivity. Of course, those skilled in the art will take care to choose the insulator 40 so that it remains fixed during the passage of the gas phase, so that the particles to be coated do not become lodged in its porosity and so that it does not cause a significant pressure drop of the fluidizing gas. It is within the general knowledge of those skilled in the art to choose the density of the material forming the insulator 40 and its porosity so as to meet these conditions.For example, it is possible to use a granular bed formed by loose grains having an average diameter d50 less than or equal to 10 mm, for example between 0.5 µm and 10 mm, and a density greater than or equal to 3 g / cm 3< , for example between 3.2 g / cm 3< and 9 g / cm 3< . In the case of the example reactor 3 described above having a diameter of 5 centimeters and a height of 1 meter, it is possible, for example, to use a loose stack of zirconia beads having an average diameter d50 equal to 1 mm to constitute the insulator 40.

[0037] As is usually done in fluidized bed chemical vapor deposition processes, care will be taken to obtain a fluidization phenomenon during the deposition. To allow monitoring of the fluidization, the device 10 comprises a differential pressure sensor 4, allowing the evaluation of the pressure drop of the gas at the crossing of the bed. The person skilled in the art can advantageously maintain this pressure drop at a value close (+ / - 20%) to the ratio between the weight of the bed and the section of the reactor 3 in order to guarantee the fluidization state. The person skilled in the art can also ensure a relative homogeneity of temperature inside the bed. This thermal property, characteristic of fluidized beds operating optimally, is obtained by the presence of bubbles which ensure excellent agitation of the particles to be coated. The thermocouples 17 make it possible to control the isothermicity of the bed.The quality of the fluidization can therefore be assessed from the maximum temperature difference displayed by the thermocouples. For example, a person skilled in the art can carry out the deposition with a maximum temperature difference less than or equal to 20°C, for high temperature depositions (above 800°C).

[0038] The coating may be obtained by chemical vapor deposition from a gas phase. The gas phase may have a thermal conductivity at 20°C of less than or equal to 0.04 Wm -1< .K -1< , for example less than or equal to 0.035 Wm -1< .K -1< . The thermal conductivity of the gas phase may be between 0.01 Wm -1< .K -1< and 0.04 Wm -1< .K -1< , for example between 0.01 Wm -1< .K -1< and 0.035 Wm -1< .K -1< . After deposition, particles comprising a core and a surface coating coating the core are obtained, this coating being obtained by fluidized bed vapor deposition. The particles are called "core-shell" particles. The particles to be coated can be of any kind, for example ceramic, carbon or metallic material. The coating deposited can be of any kind, for example ceramic or carbon or metallic, depending on the desired application.The particle size and shape can vary depending on the desired application. The particles to be coated can belong to Geldart categories A or B. The deposited coating can be single-layer or multi-layer by changing the nature of the precursor between the deposition of the different layers in order to deposit layers of different chemical nature. The particles coated using the process according to the invention can be used in different applications, constituting for example catalysts, or sintering aids.

[0039] A test was carried out to coat the porous alumina powder detailed previously, this time using the example device 10 according to the invention illustrated in figure 5 , the other operating conditions being preserved.

[0040] Monitoring of the average temperature of the fluidized bed during deposition is provided at the figure 6. In this figure, the injection of propane is shown by the dotted line “A” and the stopping of the injection of propane by the dotted line “B”. It can be seen that the implementation of the insulator 40 leads to high thermal stability of the fluidized bed during the deposition operation and to an improved deposition yield. After deposition of the coating, the coated particles are extracted from the reactor 3.

[0041] When the insulator 40 is in the form of a granular bed, it may be necessary to carry out a step of separating the coated particles from the grains forming the insulator 40. Different methods are possible for this. In particular, sieving may be carried out so as to separate the coated particles from the grains due to their difference in size or shape. In the case where the coated particles are separated from the grains by sieving, it may advantageously be possible to choose grains with an average diameter D50 greater than or equal to 0.5 µm. Alternatively, the coated particles or the grains may be magnetically attracted in order to separate them.

[0042] The expression "between ... and ..." must be understood as including the limits.

Claims

1. An apparatus (10) for fluidised-bed chemical vapour deposition, comprising at least: - a reactor (3) comprising a treatment zone (31) in which the fluidised-bed chemical vapour deposition is intended to be carried out from a gaseous phase, an inlet zone (35) through which the gaseous phase is intended to be introduced into the treatment zone and an outlet zone (37) through which the gaseous phase is intended to be removed from the treatment zone, - a heating system (9) configured to heat the treatment zone, and - a cooling system (8) configured to cool the inlet zone, the apparatus being characterised in that it further comprises a porous thermal insulator (40) present in the inlet zone and configured to be passed through by the gaseous phase, said porous thermal insulator having an effective thermal conductivity at 20°C less than or equal to 3.5 W.m-1.K-1, and in that the porosity of the insulator has a tortuous shape.

2. The apparatus (10) according to claim 1, in which the porous thermal insulator (40) has an effective thermal conductivity at 20°C less than or equal to 0.42 W.m-1.K-1.

3. The apparatus (10) according to any one of claims 1 or 2, in which the material forming the porous thermal insulator (40) has a thermal conductivity at 20°C less than or equal to 40 W.m-1.K-1.

4. The apparatus (10) according to any one of claims 1 to 3, in which the porous thermal insulator (40) is a granular bed.

5. The apparatus (10) according to any one of claims 1 to 4, in which the volume pore ratio of the porous thermal insulator is greater than 26%.

6. A method for coating particles using an apparatus (10) for fluidised-bed chemical vapour deposition, the apparatus comprising at least: - a reactor (3) comprising a treatment zone (31) in which the fluidised-bed chemical vapour deposition is intended to be carried out from a gaseous phase, an inlet zone (35) through which the gaseous phase is intended to be introduced into the treatment zone and an outlet zone (37) through which the gaseous phase is intended to be removed from the treatment zone, - a heating system (9) configured to heat the treatment zone, and - a cooling system (8) configured to cool the inlet zone, the apparatus further comprising a porous thermal insulator (40) present in the inlet zone and configured to be passed through by the gaseous phase, said porous thermal insulator having an effective thermal conductivity at 20°C less than or equal to 3.5 W.m-1.K-1, the method comprising at least: - introducing the gaseous phase comprising a precursor of the coating to be formed into the treatment zone (31) through the inlet zone (35) and the porous thermal insulator (40), - depositing a coating on the particles present in the fluidised-bed chemical vapour deposition treatment zone from the gaseous phase introduced, and - recovering, after depositing of the coating, the coated particles.

7. The method according to claim 6, in which the gaseous phase has a thermal conductivity at 20°C less than or equal to 0.04 W.m-1.K-1.

8. The method according to claim 6 or 7, related to claim 4, in which after the deposition of the coating, a mixture of coated particles and grains from the granular bed are recovered, and in which the coated particles are separated from said grains.