Method and facility for continuously manufacturing granular aerogel and supercritical CO2 drying method

Abstract

The invention relates to a method and a facility for continuously manufacturing granular aerogel and a supercritical CO2 drying method. The present invention provides a process for the continuous production of a particulate aerogel from a precursor, comprising the steps of:-mixing the precursor with a synthesis solvent and a hydrolysis reagent and, if appropriate, a catalyst to obtain a gel,-granulating the resulting product to produce particulates,-maintaining the particulates in contact with the synthesis solvent and the hydrolysis reagent, -washing the granulate by adding a washing solvent in order to extract, in particular, the hydrolytic agent, if appropriate, the catalyst,-drying the granulate by sending an excess of supercritical CO2 to the granulate in order to extract the synthesis solvent and / or the washing solvent, said steps of granulating, holding, washing and drying being carried out at a pressure higher than the critical point of CO2, and maintaining these conditions between these steps. The invention also provides a facility specially designed to implement the method of the invention, as well as a supercritical CO2 drying method.

Description

[0001] This application is a divisional application of the invention patent application filed on July 3, 2017, entitled "Continuous Method for Manufacturing Aerogel" with application number 2017800405885. Technical Field

[0002] This invention relates to the field of materials. Specifically, it relates to a continuous method for manufacturing aerogels. Background Technology

[0003] Aerogels are solid materials whose structure consists of a porous matrix, with most pores having a diameter between 2 and 50 nm (mesopores). This allows for the acquisition of excellent properties such as high porosity (~95%) and high specific surface area (up to 1000 m²). 2 It possesses high light transmittance (90%), low thermal conductivity (~0.01 W / mK), low refractive index (~1.05), and high light transmittance (90%). For example, it is very effective as a thermal insulator. Aerogel production is based on the conversion of colloidal molecular solutions or similar substances into cross-linked gels. Depending on the nature of the precursors, aerogels can be inorganic (e.g., silica, zirconium, or titanium-based precursors), organic (e.g., resorcinol-formaldehyde, polyurethane, or cellulose polymers), or a mixture (obtained from organic-mineral precursors or through combination).

[0004] Aerogels are obtained by replacing the liquid portion of the gel with gas during a drying step. This conversion requires very cumbersome methods and facilities, resulting in high production costs. In practice, to produce high-quality aerogels, the drying step must be carried out without forming a biphase system with associated capillary forces, which would lead to partial or complete destruction of the gel's nanostructure. Therefore, it must be carried out in a supercritical drying (SCD) environment at high temperature and pressure. This is unsuitable for efficient industrial production, especially since the product is introduced in batches into the supercritical drying module. To produce industrial quantities of aerogels at a reasonable cost, the drying step can be carried out at ambient pressure (APD, ambient pressure drying), but the quality of the product is compromised due to solvent evaporation, leading to additional difficulties related to the necessary chemical modification of the product. Furthermore, the production of aerogels from precursors requires different steps, often performed separately and sometimes by different participants at different locations, which further increases production time and costs, as well as the risks associated with variations in the final quality state of the product.

[0005] US6670402B1 discloses an accelerated method for producing aerogels by injecting supercritical CO2 during a drying step, sending a pressure wave, and using a non-reactive and non-condensable gas (NRNC) during decompression. However, the described method uses an extractor that operates discontinuously in batch mode. Summary of the Invention

[0006] The object of this invention is to at least partially overcome these drawbacks. To this end, a method for producing granular aerogels from precursors is proposed, comprising the following steps: - The precursor is mixed with a synthetic solvent and a hydrolysis agent such as water, and, if appropriate, a catalyst, to obtain a gel. - The resulting product is granulated, particularly by cutting a jet of the gel to produce granules. - Keep the granules in contact with the synthesis solvent and hydrolysis reagent. - The granules are washed with a washing solvent to extract, in particular, hydrolytic reagents, and, if appropriate, a catalyst. - Drying particulate matter by sending an excess of supercritical CO2 to extract synthetic and / or wash solvents.

[0007] What makes this method special is that the granulation, holding, washing and drying steps are carried out at pressures higher than the CO2 critical point, and these conditions are maintained between these steps.

[0008] Because of these settings, the aerogel manufacturing process can be carried out continuously, with pressure increases occurring while the product is still in a fluid state. In fact, once the product is solid (once it has been granulated), pressure increases can cease to be continuous. Due to this invention, once the product is solid, no pressure buildup (pressurization) or depressurization is required, except for a final depressurization. This significantly reduces manufacturing time and costs, and improves product quality by minimizing risky state changes and depressurization steps.

[0009] Based on other characteristics: The mixing step can also be carried out at pressures higher than the CO2 critical point, which allows the step to be slightly accelerated. - In the drying step, the solvent-loaded particles can be subjected to a supercritical CO2 jet to place them in a fluidized bed under temperature and pressure conditions where the CO2 is supercritical and the solvent-loaded particles are heavier than the CO2-loaded particles. This enables continuous drying and accelerates the drying process. - The synthesis and / or washing solvents can be organic solvents, and the drying step can be carried out at a pressure of 100 to 200 bar and a temperature of 35 to 50°C. Ethanol is an inexpensive and suitable product for this method, and the conditions between 100 and 200 bar and 35 and 50°C allow CO2 to be injected into the fluidized bed at a certain rate without ethanol-containing particles escaping, while those containing only supercritical CO2 escape from the top of the column and can be recovered for further processing. - Following the drying step, the aerogel manufacturing method may include a step of replacing supercritical CO2 with an inert gas, preferably nitrogen, followed by a decompression step, preferably gradual, which allows for rapid decompression without damaging the aerogel particles. - In the step of replacing supercritical CO2 with an inert gas, the supercritical CO2-loaded particles can be subjected to a jet of the inert gas to place them in a fluidized bed condition at a temperature and pressure such that the CO2 is supercritical and the supercritical CO2-loaded particles are heavier than the particles loaded with the inert gas, thereby enabling the replacement of supercritical CO2 with an inert gas to be performed continuously and accelerating the process.

[0010] The present invention also relates to an apparatus for manufacturing granular aerogels from precursors, comprising: - Mixing reactor, - Granulation equipment capable of forming granules from a jet of gel-like liquid from a mixing reactor, and, if appropriate, located inside an aging reactor. - Aging reactor, - Washing reactor, - Drying equipment, - Pressure relief equipment.

[0011] What makes this facility special is that the aging reactor, washing reactor, and drying reactor, as well as the means for transferring products between these reactors, are configured to operate such that the products can be held from one reactor to another at pressures higher than the critical point of CO2.

[0012] Because of these features, the facility allows for the continuous production of aerogel granules, with pressure increased while the product is still in a fluid state.

[0013] Based on other characteristics: - The mixing reactor can also be configured to operate such that the product can be carried from one reactor to another at pressures higher than the critical point of CO2; in particular, this also reduces reaction time. The facility may also include a first fluidized bed column configured to replace the solvent contained in the granules with supercritical CO2, thereby allowing for continuous drying of the granules and accelerating the drying process. - The facility may also include a second fluidized bed tower configured to replace the supercritical CO2 contained in the granules with a pressurized inert gas, preferably nitrogen, for rapid decompression without damaging the aerogel granules. Attached Figure Description

[0014] The invention will be better understood by reading the following detailed description with reference to the accompanying drawings, in which: - Figure 1 This is a diagram illustrating a facility capable of performing the method according to the invention. Detailed Implementation

[0015] The method according to the invention includes the continuous production of aerogel 1. To do this, all steps in the production of aerogel 1, from the granulation step of turning the aerogel into a solid to the drying step, are carried out at pressures higher than the CO2 critical point required for the drying step. If the production of precursors is included in the method, the production of the precursors can also be carried out at pressures higher than the CO2 critical point. The mixing step may or may not be carried out at pressures higher than the CO2 critical point. The CO2 critical point is located at a temperature of about 31°C and a pressure of about 73 bar. Since the raw materials for producing the aerogel are liquids, they can be pressurized continuously, for example, by a pump. This avoids the need to pressurize the gel (solid) during the drying stage, which can only be carried out in batches (batch process).

[0016] The raw material used to produce aerogel 1 is a precursor. For the production of silica aerogel 1, the precursor can be made from a silica-rich source such as sand. It can be an alkoxysilane, more specifically TMOS (tetramethyl orthosilicate) or TEOS (tetraethyl orthosilicate), which are preferred because methanol and ethanol are byproducts of their respective reactions; or it can be derived from hydrated silicic acid and its oligomers (polysilicic acid), also known as sodium silicate. Other types of precursors can be used without departing from the scope of the invention, for example, to produce carbon, alumina, metal oxide aerogel 1, or organic precursors such as cellulose, polyurethane, or products derived from them.

[0017] The following description relates to the production of silica aerogel 1, but those skilled in the art can readily convert these methods to produce other types of aerogel 1.

[0018] The method according to the invention may include production precursor 2, in which case a pump is used to introduce the product required for production precursor 2.

[0019] Precursor 2 can also be produced outside of this method, in which case precursor 2 is introduced before being mixed.

[0020] Precursor 2 is then mixed with a hydrolysis agent such as water; a synthesis solvent 3 such as ethanol, for example 95% ethanol, methanol, or acetone; and, if appropriate, a catalyst 4. This mixing takes place in a mixing reactor 5. Two reactions occur: hydrolysis and condensation. Hydrolysis is caused by the presence of the hydrolysis agent and can form, for example, silica from precursor 2. Silica forms a colloidal solution with the synthesis solvent 3. Condensation involves the aggregation of colloidal particles into a continuous three-dimensional network called a gel; hence, we refer to it as gelation. The relative rates of the condensation and hydrolysis reactions can be controlled by introducing catalyst 4. The type of catalyst 4 chosen, more precisely, its pH, affects the type of network produced by condensation, and thus the type of aerogel 1 as the final product. For example, ammonia can be chosen as a basic catalyst 4. The hydrolysis and condensation reactions can be carried out simultaneously by mixing all products, or sequentially by preparing intermediate solutions and then mixing these intermediate solutions together; these are referred to as one-step or two-step synthesis.

[0021] After mixing, a granulation stage is performed. When the viscosity of the gel increases sufficiently, the gel is cut to obtain granules. This is achieved by a granulation device 6, such as a jet cutter or by other methods (solutions) known to those skilled in the art, such as dripping or spraying. The type of granulation device 6 can affect the particle size determination of the obtained granules. Jet cutters particularly produce larger granules, typically greater than 50 micrometers and up to millimeters or even larger, while spraying produces finer granules, up to about 5 micrometers or even smaller; such fine granules are sometimes referred to as powders, but within the scope of this invention, they are referred to as granules.

[0022] In this invention, the mixing step may or may not be carried out at a pressure higher than the CO2 critical point, but the granulation step must be carried out at a pressure higher than the CO2 critical point. This does not prevent them from operating smoothly, but rather tends to slightly accelerate the operation. High-pressure granulation is known to those skilled in the art and does not cause any particular problems.

[0023] The next step after granulation is an aging step. After gel formation, a large number of particles remain that have not yet completed their reaction. The aging step can, for example, involve prolonged immersion of the gel in a solution in an aging reactor 7, which may contain the same solvent, hydrolyzing agent, and catalyst as during synthesis. After the aging step, essentially all the particles have reacted, all intermolecular connections are complete, and the gel becomes more solid. The resulting structure type varies depending on various parameters such as time, solution pH, type of synthesis solvent, and temperature.

[0024] In a preferred embodiment of the invention, the granulation and aging steps are carried out in the same aging reactor 7. The gel is then introduced from the top of the aging reactor 7, or the gel is cut into granules before falling into the aging reactor 7, wherein hydrolysis and condensation reactions can continue.

[0025] Following the aging step, a washing step is performed to remove impurities and residues of unreacted compounds from the particulate matter. Most of the impurities consist of catalyst and water. The washing step is important because the presence of water or other components during the drying step can lead to degradation of the aerogel 1 network, as the mixture may not be completely soluble in supercritical CO2, resulting in poor final product quality. Washing can be carried out by immersion in a washing solvent solution in washing reactor 8. The solvent used for the washing step can be ethanol, or other solvents soluble in supercritical CO2, such as acetone, isopropanol, or methanol.

[0026] The washing step can provide an opportunity to introduce hydrophobic agents such as hexamethyldisiloxane (HMDSO) or hexamethyldisilazane (HMDZ). These agents interact with the gel surface to make it hydrophobic. This is necessary for some applications so that the final aerogel 1 does not degrade upon contact with ambient humidity. The hydrophobic agents can also be introduced during or even after the drying stage.

[0027] Finally, the solvent, such as ethanol, present in the particulate matter must be removed from the gel to obtain aerogel 1. Drying by simple evaporation does not produce high-quality aerogel 1 because evaporation may break the bonds between molecules within the gel network due to the capillary pressure (stress) generated by the two-phase state. To produce high-quality aerogel 1, the method according to the invention includes a supercritical drying step. This type of drying allows damage to the aerogel during drying to be avoided by avoiding the two-phase system and the associated capillary forces, which would otherwise lead to partial or complete destruction of the gel nanostructure.

[0028] Several techniques for drying with supercritical CO2 are known to those skilled in the art. According to a preferred embodiment of the method of the present invention, the following method is proposed: In the first stage of supercritical drying, the granular material is introduced from below into a first fluidized bed column 9, in which supercritical CO2 is also injected from below. CO2 is chosen because of its relatively low supercritical pressure and temperature, and because solvents such as ethanol dissolve in CO2. The use of a fluidized bed enables rapid drying, where each particle directly faces a CO2 jet that dissolves the solvent to be discharged.

[0029] In a fluidized bed, the gas injection rate is adjusted. As long as this rate is below the fluidization rate, the particles remain clumped together. From this rate, up to the evaporation rate, the particles rise and move through the gas jet, but do not fly away. It is this state that allows CO2 to contact each particle very quickly. Above the evaporation rate, the particles are carried away by the gas jet and discharged through the top of the column.

[0030] If the particles contain ethanol or supercritical CO2, the evaporation rates differ at the temperature and pressure selected for the fluidized bed. For the fabrication of silica aerogel 1 and using ethanol as the solvent, a pressure of 130 bar and a temperature of 45°C are suitable. This difference in evaporation rates is used to regulate the fluidized bed rate so that particles containing ethanol do not escape, while particles containing only CO2 escape from the top of the column and can be recovered for further processing. The supercritical CO2 is discharged to a decanter and then to a solvent separator. The ethanol-containing particles remain in the first fluidized bed 9 and continue to face the CO2 jet until the ethanol is replaced by CO2, at which point they escape.

[0031] To produce silica aerogel 1 and ethanol as a solvent, the fluidized bed can be adjusted to, for example, 130 bar and 45°. Such conditions enable favorable rate ranges for all particle sizes relevant to production.

[0032] At the outlet of the first fluidized bed 9, supercritical CO2 mixed with ethanol enters the CO2 / ethanol separator 10, which allows pure supercritical CO2 to be reinjected into the fluidized bed 9, thereby gradually replacing all the solvent with supercritical CO2. Ethanol can be reinjected into the mixing or washing steps.

[0033] In the second stage of supercritical drying, the granular material is injected into a second fluidized bed column 11. Nitrogen is injected into the second fluidized bed column under supercritical conditions, or other inert gases, such as dry air, argon, or krypton, are injected under fixed conditions. The second fluidized bed 11 is configured such that, just as CO2 replaces ethanol in the first fluidized bed 9, nitrogen replaces CO2 in the second fluidized bed 11.

[0034] Nitrogen can be produced from air in a nitrogen production unit, as part of the method according to the invention; or it can be produced outside the facility. Within the scope of the invention, it is described as incompressible because it is far less compressible than CO2.

[0035] A decanter 12 can be used at the outlet of the first fluidized bed 9 to separate the supercritical CO2 to be returned to the first fluidized bed 9 from the CO2-laden particulate matter to be returned to the second fluidized bed 11. In a preferred embodiment of the invention, the same decanter 12 is also used at the outlet of the second fluidized bed 11 to separate the supercritical CO2 and supercritical nitrogen to be returned to the first fluidized bed 9 and the second fluidized bed 11, respectively. Under supercritical CO2 conditions, the density difference between the two gases is significant, and the decanting is instantaneous.

[0036] At the outlet of the second fluidized bed 11, the aerogel must be depressurized. Since nitrogen is far less compressible than CO2, depressurization does not damage the aerogel and can be performed much faster than if some CO2 is still present in the particles. Therefore, the depressurization stage takes only a few minutes, whereas depressurization of an aerogel still containing CO2 requires several hours because the depressurization rate must be less than 0.3 bar / minute. Depressurization is performed in a depressurization device 13. Depressurization can be performed directly or through a series of depressurization reactors 13.

[0037] At each level, nitrogen can be recovered and sent to the second fluidized bed 11. At the outlet of the first level, residual CO2 can be returned to the decanter 12.

[0038] Upon returning to atmospheric conditions, nitrogen is separated using known methods, and aerogel 1 is recovered. Dust 14 or particles 15 that are too fine can be separated and used for specific applications. This allows the granular aerogel 1 to have a particle size corresponding to the desired particle size.

Claims

1. A method for continuously manufacturing granular aerogel (1) from a precursor (2), comprising the following steps: - In a mixing reactor (5), the precursor (2) is mixed with a synthesis solvent (3) and a hydrolysis agent such as water, and, if appropriate, a catalyst (4) to obtain a gel. - The resulting product is granulated, particularly by cutting a jet of the gel to produce granules. - In the aging reactor (7), the granules are kept in contact with the synthetic solvent (3) and the hydrolysis reagent. - In the washing reactor (8), the granules are washed by adding a washing solvent to extract, in particular, the hydrolysis reagent, and if appropriate, the catalyst (4). - In a drying apparatus, the granules are dried by sending an excess of supercritical CO2 to extract the synthetic solvent (3) and / or washing solvent. The granulation, holding, washing and drying steps are carried out at pressures higher than the CO2 critical point, and these conditions are maintained between these steps.

2. The method according to the preceding claim, wherein, The mixing step is also carried out at a pressure higher than the CO2 critical point.

3. The method according to any one of the preceding claims, wherein, In the drying step, the solvent-loaded particles are subjected to a supercritical CO2 jet to place them in a fluidized bed under temperature and pressure conditions such that the CO2 is supercritical and the solvent-loaded particles are heavier than the CO2-loaded particles.

4. The method according to the preceding claim, wherein, The rate of the fluidized bed is adjusted so that the particles loaded with solvent do not fly off and remain facing the CO2 jet until the ethanol is replaced by CO2, while the particles containing only CO2 fly off from the top of the column and can be recovered for further use in the method.

5. The method according to the preceding claim, wherein, The solvent is ethanol.

6. The method according to the preceding claim, wherein, The synthesis solvent (3) and / or washing solvent are organic solvents, and the drying step is carried out at a pressure between 100 and 200 bar and a temperature between 35 and 50°C.

7. The method according to any one of the preceding claims, comprising, after the drying step, replacing supercritical CO2 with an inert gas, preferably nitrogen, followed by a decompression step, preferably in stages.

8. The method according to the preceding claim, wherein, In the step of replacing supercritical CO2 with an inert gas, the particles carrying supercritical CO2 are subjected to a jet of the inert gas to place them in a fluidized bed condition, under temperature and pressure conditions such that the CO2 is supercritical and the particles carrying supercritical CO2 are heavier than the particles carrying the inert gas.

9. An apparatus for the continuous production of granular aerogel (1) from precursor (2), comprising: - Mixing reactor (5) - Granulation equipment (6), capable of forming granules from a jet of gel-like liquid from the mixing reactor (5), and, if appropriate, located inside the aging reactor (7), - Aging reactor (7). - Washing reactor (8) - Drying equipment, - Pressure reducing equipment (13). The aging reactor (7), the washing reactor (8), and the drying equipment, as well as the means for transferring products between these reactors, are configured to operate and hold the product from one reactor to another at a pressure higher than the critical point of CO2.

10. The facility according to the preceding claim, wherein, The mixing reactor (5) is also configured to operate and enable the product to be carried from one reactor to another at pressures higher than the CO2 critical point.

11. The facility according to the preceding claim further includes a first fluidized bed tower (9) configured to allow supercritical CO2 to replace the solvent contained in the particulate matter.

12. The facility according to the preceding claim, wherein, The tower includes means for injecting supercritical CO2 from below, means for allowing solvent-loaded particles to enter from below, and means for allowing CO2-loaded particles to exit from below.

13. The facility according to the preceding claim further includes a second fluidized bed tower (11) configured to replace the supercritical CO2 contained in the granules with a pressurized inert gas, preferably nitrogen.

14. The facility according to the preceding claim, wherein, The tower includes a device for injecting supercritical N2 from below, a device for allowing particulate matter loaded with CO2 to enter from below, and a device for allowing particulate matter with pressurized inert gas to exit from above.

15. A supercritical CO2 drying method comprising introducing particulate matter from below into a first fluidized bed column, wherein supercritical CO2 is also injected from below into the first fluidized bed column such that supercritical CO2 replaces the solvent contained in the particulate matter; then, injecting the particulate matter into a second fluidized bed column, wherein nitrogen is injected in the second fluidized bed column in a supercritical state such that nitrogen replaces the supercritical CO2 contained in the particulate matter.

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