EP3849697A1

EP3849697A1 - Method for continuous supercritical drying of aerogel particles

Info

Publication number: EP3849697A1
Application number: EP19766026.9A
Authority: EP
Inventors: Sohajl MOVAHHED; Wibke LOELSBERG; Dirk Weinrich; Marc Fricke; Raman SUBRAHMANYAM; Irina Smirnova; Pavel GURIKOV; Fynn MISSFELDT
Original assignee: BASF SE
Current assignee: Aerogel It GmbH
Priority date: 2018-09-12
Filing date: 2019-09-12
Publication date: 2021-07-21
Also published as: JP2022500536A; WO2020053349A1; KR20210056417A; JP7500080B2; CN112672819A; US20220041817A1

Abstract

The present invention relates to methods for drying gel particles, in particular for producing aerogels, comprising: providing a suspension containing gel particles (P1) and a solvent (LM); filling the suspension into a column through which carbon dioxide is passed in counter-current; and separating the dried aerogel particles from the column, the suspension being filled in the head region of the column and the dried aerogel particles being separated in the lower region of the column, and wherein pressure and temperature are set in the column such that the mixture of carbon dioxide and solvent is practically supercritical or is supercritical. The aerogel particles can be discharged both via discharge containers and via a continuous expansion. The present invention also relates to aerogel particles obtainable or obtained according to such a method, and to the use of the aerogel particles according to the invention for medical and pharmaceutical applications, as additive or carrier material for additives for foods, as catalyst support, for cosmetic, hygiene, washing and cleaning applications, for the production of sensors, for heat insulation or as core material for VIPs.

Description

Process for the continuous supercritical drying of airgel particles

The present invention relates to methods for drying gel particles, in particular for the production of aerogels, comprising the provision of a suspension containing gel particles (P1) and a solvent (LM), the filling of the suspension into a column in the counterflow of carbon dioxide is flowed through and the separation of the dried airgel particles from the column, the suspension being introduced into the top region of the column and the dried airgel particles being separated off in the lower region of the column, and pressure and temperature being set in the column such that the mixture from carbon dioxide and solvent is almost supercritical or supercritical. The airgel particles can be discharged both via discharge containers and via a continuous release. Furthermore, the present invention relates to airgel particles obtainable or obtained according to such a method and to the use of the airgel particles according to the invention for medical and pharmaceutical applications, as an additive or carrier material for additives for food, as a catalyst carrier, for cosmetics, hygiene, washing and Cleaning applications, for the production of sensors, for thermal insulation or as core material for VIPs.

Porous materials such as inorganic or organic aerogels are suitable for various applications. Porous materials with particle diameters in the size range of a few micrometers and a high porosity of at least 70% are particularly good thermal insulators, for example due to theoretical considerations.

Organic and inorganic aerogels and xerogels and processes for their production are known from the prior art. In order to obtain a porous material, for example an airgel, from the gel, the liquid must be removed. This step is referred to as drying for the sake of simplicity.

In the continuous production of particles in pressurized apparatus, the continuous expansion of the generated particles to ambient pressure is particularly problematic. Problems that arise when depressurizing particle-fluid flows by means of valves include abrasion and blockage of the valves. In addition to continuous expansion via valves or semi-continuous expansion via discharge containers, continuous, valveless methods for expanding particle-fluid flows with different devices for regulating the outlet flow are known.

Methods for supercritical drying of gels are also known from the prior art. In supercritical drying, the interfacial tension of the fluid contained in the mesoporous particles is completely or largely removed with the aim of largely avoiding shrinkage of the mesoporous and macroporous particles during drying, since characteristic properties of the porous particles, in particular those of the meso- and macroporous particles, are lost in whole or in part. Such a Product obtained by supercritical drying is called airgel in gels. In contrast to conventional drying without special precautions, in which the gels suffer a large volume contraction and xerogels are formed, only a small (<15%) volume contraction takes place when drying near or above the critical point.

The patents US 2868280 and US 4845056 disclose continuous processes for the production of airgel particles by means of supercritical drying. The gel suspensions are brought to a pressure above the critical pressure of the fluid surrounding the gel particles with the aid of a pump. The suspension flows through a heated tube and is heated to a temperature above the critical temperature of the fluid and then relaxed adiabatically. The resulting airgel particles are separated from the gas stream by means of gas cyclones and / or filters.

US 2868280 describes a continuous process for the production of supercritically dried inorganic airgel particles. The sol is gelled continuously in a tubular reactor at elevated temperatures and pressures and, at the end of the reactor, in which the fluid is present in the supercritical state, expanded via a valve.

US 4845056 discloses a continuous process for the production of airgel ceramic powders with continuous production of the sol by hydrolysis of an alkoxide, above the critical pressure of the liquid phase, in a tubular reactor. After the hydrolysis, the sol is continuously conveyed into a heating zone, and after reaching a temperature above the critical temperature of the liquid phase, the ceramic powder, which is suspended in a supercritical fluid, is relaxed adiabatically.

A distinction must be made between the two types of airgel production in the continuous relaxation of airgels: 1. High-temperature supercritical drying (HTSCD) and 2. Low-temperature supercritical drying (LTSCD)) . The processes listed above are continuous processes based on the HTSCD process, which also include continuous relaxation of the airgel particles.

In the LTSCD process with C0 ₂ as the supercritical fluid, there is no real continuous process with continuous relaxation of the particles. With the current state of the art, the pressure release of the CO2-dried aerogels is carried out in batches with slow relaxation rates of 0.05-5 bar / min. (US 7,781, 492 B2, US 9,073,759 B2, US 2017/0081494 A1, WO 2018/007740 A1, US 5,306,555)

By means of special devices for enclosing the aerogels in solid containers, relaxation rates in US Pat. No. 5,686,031 can be selected 20-200 times higher than usual. The usual relaxation rate is 0.35 bar / min. In WO 2018/007740 A1, C0 ₂ is exchanged for nitrogen before the relaxation, in order to enable faster relaxation. The relaxation rates are not specified exactly, but relaxation within minutes instead of hours is possible.

US Pat. No. 6,670,402 discloses a technique for increasing the pressure release rate by exchanging the supercritical CO2S with a non-reacting, non-condensing fluid before or during the relaxation and by using pressure pulses with different frequencies to accelerate mass transfer.

US Pat. No. 3,195,613 discloses a process for the continuous expansion of solid-liquid mixtures from about 30 bar to ambient pressure using the example of ethylene polymerization via a long tube. Due to the pressure release, a (partial) flash evaporation of the liquid occurs and with the accompanying volume expansion, the pressure loss is further increased. The outlet flow is regulated by superimposing it with an inert gas flow or by changing the cross-sectional area or length of the outlet nozzle.

US Pat. No. 7,731,783 B2 discloses a method for continuously depressurizing a gas-solid system via a long pipe with additional internals for increasing the pressure loss and a device for regulating the emerging particle gas flow.

The disadvantages of the above-mentioned methods for continuous drying according to the HTSCD method are the guidance of drying fluid and gel particles in cocurrent. The entire fluid flow in which the particles are suspended must be in the supercritical state at the outlet. This heating to temperatures above the critical temperature of the fluid requires a high energy requirement. In addition, due to the high temperatures required, the processes are not suitable for the continuous supercritical drying of organic, e.g. suitable for biopolymer-based gels.

A widely used method for the supercritical drying of temperature-sensitive airgel is the extraction of the solvent with CO2 at a pressure above the critical pressure of the mixture and a temperature above the critical temperature of the CO2S. This is usually done in batches. A large amount of CO2 per kg of airgel is used here, since towards the end of the process there must be an exit concentration of solvent in the C0 ₂ stream of <1-2%.

An alternative to the batch process is quasi-continuous countercurrent drying using a cascade operation, as is customary in solid extraction with supercritical fluids. However, this process is associated with a high level of process engineering and high equipment costs due to the large number of containers (number of stages = number of containers). US5962539 describes a semi-continuous process for supercritical drying of aerogels with CO ₂ , which is operated with a simulated continuous mode of operation. Here, n pressure vessels are operated in parallel, each of which is offset by a time period At in the extraction process of the supercritical drying. In this way, constant drain flows and constant required inlet flows occur over time.

KR20100086297 discloses a process for the continuous countercurrent drying of airgel particles using supercritical fluids. Airgel particles with a diameter of 0.1-1 mm are fed with a screw conveyor in counterflow to supercritical CO2 or supercritical methanol.

The US6516537 patent discloses a process for the continuous supercritical or near-critical drying of microporous silica beads (2 to 12 mm) in the countercurrent process, in which the particles are guided in a moving bed against a drying fluid stream from isopropanol. The particles are located in a supercritical fluid or a supercritical mixture of fluids at the bottom of the container and are expanded via a discharge container or two alternating discharge containers.

The methods known from the prior art have the disadvantage that they are associated with a high outlay on equipment or that long dwell times and a large consumption of materials are incurred.

One object on which the present invention is based, based on the prior art, was to provide a method for drying mesoporous and macroporous gel particles which can be operated economically and with little outlay on equipment.

According to the invention, this object is achieved by a method for drying gel particles, in particular for producing aerogels, comprising the steps

(i) providing a suspension containing gel particles (P1) and a solvent (LM),

(ii) Pouring the suspension into a column countercurrent to carbon dioxide

is flowed through,

(iii) separation of the dried airgel particles from the column, the suspension being introduced into the top area of the column and the dried airgel particles being separated off in the lower area of the column, pressure and temperature being set in the column such that the mixture of Koh - Lenikdioxid and solvent is supercritical or almost supercritical. A gel in the sense of the present invention is a crosslinked system based on a polymer which is in contact with a liquid (known as a solvogel or lyogel) or with water as a liquid (aquagel or hydrogel). Here the polymer phase forms a continuous three-dimensional network.

In the context of the present invention, the term “supercritical” is understood to mean that the mixture of CO2 and solvent is in the supercritical state if the operating pressure is above the critical pressure of the mixture at the respective operating temperatures.

In the context of the present invention, the state of a mixture of solvent and CO2 is referred to as “almost supercritical” if the pressure and or the temperature of the mixture is or are below the critical pressure or the critical temperature of the mixture, the interfacial tensions that occur between however, the phases are already reduced in such a way that they do not lead to destruction of the pore structure.

Suitable pressure and temperature ranges can vary widely. If, for example, ethanol is used as the solvent, the process is carried out, for example, at a temperature in the range from 30 to 260 ° C. and a pressure in the range from 70 to 160 bar.

According to the invention, the supply and removal of CO2 can take place, for example, in side draws, for example via a partial crossflow. This mode of operation has proven to be particularly advantageous for longer drying times and / or larger particle diameters.

In the context of the present invention, the particles can be removed semi-continuously via two or more alternating discharge containers. The column can advantageously be installed above the discharge containers, so that the particles fall into the containers by gravity.

In the context of an alternative embodiment, it is also possible for the removal to be carried out continuously, without a valve, via a capillary. The column is preferably installed above an intermediate collection container into which the particles fall due to gravity. A fluidization aid is preferably used to discharge the particles from the intermediate container by means of a CO ₂ stream, in order to prevent the capillary inlet from becoming blocked.

According to a further embodiment, the present invention accordingly relates to a method for drying gel particles as described above, the airgel particles obtained being removed continuously, without a valve. In the method, for example, airgel particles are expanded continuously, without valves, to a ambient pressure via a capillary from a pressurized storage container, for example the countercurrent extraction column. In the context of the present invention, the pressure can be, for example, in the range from 80 to 200 bar, preferably in the range from 100 to 150 bar. The pressure loss due to fluid-wall friction, particle-wall friction and particle-particle friction / collision over the entire pipe length, depending on the gas mass flow, must preferably be just large enough to overcome the pressure difference between the reservoir and the environment. This means that no valve is required to build up an additional pressure loss. The particles released to ambient pressure (or almost ambient pressure) are then separated from the gas flow and collected by cyclone separators or filters.

According to the invention, drying takes place under supercritical conditions. Such drying is known per se to the person skilled in the art. Supercritical conditions characterize a temperature and a pressure at which CO2 or any solvent mixture used to remove the gelling solvent is present in the supercritical state.

In this way, the shrinkage of the gel body when removing the solvent can be reduced. Carbon dioxide is particularly well suited for thermally sensitive substances due to its favorable critical temperature of 31 ° C. In general, the choice of drying fluid depends on various points. If you want to set "close" critical conditions, the thermal stability of the particles to be dried or the end product determines, among other things, the selection of the drying fluid and thus also limits the critical temperature of the drying fluid.

The continuous expansion is preferably carried out through tubes / capillaries with a small diameter in order to keep the pressure loss high. However, a certain diameter must be maintained in order to convey particles without blocking. Suitable inner tube diameters are in the range from 1.5 mm to 50 mm, preferably in the range from 1.6 mm to 20 mm. The inside diameter of the pipe, the volume of solids, the length of the pipe, the type and number of fittings and the gas mass flow result in the pressure loss across the pipe.

Pipe lengths can range from 30 m to 400 m and the gas mass flows can range from 3 kg / h to 20 t / h.

The particles are discharged from the storage container, for example from the bottom of the column for continuous drying, into the capillary preferably using fluidization aids and metering devices, but can also take place without fluidization aids and metering devices, depending on the material properties of the airgel. The relaxation takes place through an externally heated capillary to avoid C0 ₂ condensation or CO2 freezing in the pores of the aerogels. For heating, the pipeline / capillary can be routed through a jacket with a heat transfer fluid, as in a tube bundle heat exchanger with a single tube and a large number of passages. In the capillary Various internals, such as pipe constrictions and pipe extensions, are included to increase the pressure loss.

In a further embodiment of the invention, a less compressible, with supercritical CO2 miscible inert gas, such as nitrogen, can be added before or during the expansion. As a result, the pressure-dependent change in volume of the gas mixture is less and the relaxation rates which occur can be reduced if the gas flow in the cyclone is partly separated off with the same total gas mass flow.

In a further embodiment of the invention, the relaxation can also be carried out in stages: the incremental relaxation consists of 2 or more capillary pieces, such as the heat exchangers described above. The capillary pieces / heat exchangers can be operated at different temperature levels, for example to reduce the energy required to compensate for the Joule-Thomson effect or to use the Joule-Thomson effect to cool brine at low pressures.

For example, in the case of gradual relaxation with different temperature levels, a first stage can be operated at a pressure in the range from 60 to 120 bar at 50 ° C, a further stage at a pressure in the range from 40 to 60 bar at 25 ° C and a third stage at a pressure in the range of 1 to 40 bar at 5 ° C.

Another embodiment of the stepwise expansion is the interconnection of cyclones between the steps. Between the individual relaxation stages, the airgel particles can be separated from part of the gas flow in a cyclone or filter. The particle-free gas stream can then be returned. The remaining gas flow with the particles goes into the next relaxation stage. In this way, the compression work required for the expansion gas flow can be reduced.

The combination of the two embodiments of the gradual relaxation is of course also possible.

According to the invention, for example, the gradual relaxation with different temperature levels and partial return of the CO2S can be combined with the separation in a cyclone. For example, a first stage is possible at a pressure in the range from 80 to 120 bar at 50 ° C., a separation in a cyclone, a further separation stage at a pressure in the range from 55 to 80 bar at 40 ° C., a further stage at Pressure in the range of 40 to 55 bar at 25 ° C; and optionally a further stage at a pressure in the range from 1 to 40 bar at 5 ° C.

A suitable ratio or a suitable setting of the material flows and flow velocities of drying fluid and particles to be dried for the production and maintenance of the counterflow can be determined by a person skilled in the art in the course of tests customary in the art. This setting depends, among other things, on the height of the column, the internal Ren mass transfer and heat transfer in the particles to be dried and the vortex point, ie from the density and particle size or particle size distribution of the porous particles to be dried.

The method is preferably carried out continuously in the context of the invention. According to a further preferred embodiment, the carbon dioxide stream is circulated.

Surprisingly, it was found that gel particles can be dried quickly and gently using the method according to the invention. According to the invention, it is in particular possible to carry out the drying at low temperatures, so that even sensitive materials can be dried well.

According to the invention, the method is carried out such that the gel particles preferably sediment in the carbon dioxide stream.

According to a further embodiment, the present invention accordingly relates to a method for drying gel particles as described above, the gel particles sedimenting in countercurrent.

Suitable ranges for solvent mass fractions at the C0 ₂ outlet can vary widely depending on the CO2 mass flow and the column height. In the context of the present invention, the mass fraction of solvent, for example ethanol in CO2, can be in the range from 30 to 98%.

In the context of the present invention, the mass flow is adjusted depending on the diameter of the column according to principles known to those skilled in the art so that the particles still sediment.

According to a further embodiment, the present invention accordingly relates to a method for drying gel particles as described above, the CO ₂ mass flow being adjusted so that dried airgel particles are obtained

In the context of the present invention, the C0 ₂ mass flow is set, for example, in the range from 0.75 kg / h to 25t / h.

According to a further embodiment, the present invention accordingly relates to a method for drying gel particles as described above, the CO ₂ mass flow being set in the range from 0.75 kg / h to 25 t / h.

In the context of the present invention, gel particles can be dried which, for example, have an average diameter in the range from 20 pm to 1000 pm, preferably in the range from 20 pm to 500 pm, more preferably in the range from 50 pm to 250 pm. According to a further embodiment, the present invention accordingly relates to a method for drying gel particles as described above, the gel particles having an average diameter in the range from 20 pm to 1000 pm.

The free-flowing particles are preferably passed through the column in countercurrent to supercritical CO2. Surprisingly, it could be shown that the significantly shorter residence times of the free fall compared to the moving bed are sufficient to dry the particles and that no conveyance and the associated additional installations are necessary.

The process presented is particularly suitable for the continuous countercurrent drying of smaller particles in the range from 20 pm to 500 pm and has the advantage that no conveyor screw or moving installations in the drying container are required.

The airgel particles are continuously dried in counterflow with CO2 above the critical point of the mixture in a kind of counterflow extraction column. The gel particle suspension, consisting of gel particles suspended in an organic solvent that is readily soluble in CO2, is fed in at the top of the column with a pump. The gel particles sediment in the gravitational field against the upward flowing CO2 to the bottom of the column. The supercritical CO2 flows through the column from the bottom to the top and extracts the solvent from the gel particles. Completely dried (solvent-free), preferably mesoporous airgel particles are thus obtained in the bottom of the column, while a highly laden solvent / CO ₂ stream is obtained in the top of the column.

The particle size is a crucial factor in continuous countercurrent extraction. The drying time and residence time in the column are determined by the particle size at a constant CO ₂ mass flow. The ratio of drying time / dwell time increases with constant particle conditions with increasing particle diameter, because on the one hand the required drying time is increased and at the same time the dwell time decreases.

Likewise, with a constant particle diameter, the ratio of drying time / dwell time can be adjusted in particular by changing the dwell time based on the process parameters. The dwell time is influenced by the C0 ₂ density as well as the upward directed C0 ₂ mass flow. The increase in the CO ₂ mass flow directed upwards against the direction of fall of the particles leads to a decrease in the absolute speed of the particles with a constant relative speed and thus to an increase in the residence time. The ratio of drying time / residence time required should preferably be <1.

Suitable methods for producing gel particles are known per se. In the context of the present invention, the gel can be, for example, an organic or an inorganic gel.

In principle, a gel can be produced by hydrolysis of a suitable precursor and subsequent condensation (gelation). Suitable gel material precursors for inorganic or hybrid materials can be inorganic or a mixture of organic and inorganic components. Brine can be catalyzed to induce gelation by various methods. Examples include adjusting the pH and / or temperature of a dilute metal oxide sol to a point where gelation occurs. Suitable materials for the formation of inorganic aerogels are oxides of metals, transition metals and semimetals which can form oxides, such as silicon, aluminum, titanium, zirconium, hafnium, yttrium, vanadium and the like.

The main synthetic route for the formation of an inorganic airgel can be the hydrolysis and condensation of a suitable metal alkoxide.

Suitable precursors for gelation for organic gels are also known to the person skilled in the art.

Suitable gels are, for example, those based on materials containing polysaccharides, such as alginates. For example, it is known that alkali alginates such as sodium alginate are water soluble, while alkaline earth alginates such as calcium alginates are insoluble in water. In this way, gels can be produced from water-soluble polysaccharides, in particular natural polysaccharides such as alginates. According to the present invention, water-soluble polysaccharides can preferably be used to form gels. Among them, the use of natural polysaccharides and / or their derivatives is particularly attractive because of their stability, availability, renewability and low toxicity.

In the context of the present invention, water-soluble means that the solubility in water is sufficient to form a solution that can be used to prepare a gel. According to the present invention, a gel is formed from the water-soluble polysaccharide and a suitable crosslinking agent. The polysaccharide used for the process of the present invention must be suitable for forming a gel with a crosslinking agent, in particular it must have suitable functional groups. Natural polysaccharides such as agar, alginate, carrageenan, cellulose, hyaluronic acid, pectin, starch and xanthan gum as well as semi-synthetic polysaccharides such as modified cellulose, chitin and chitosan are particularly preferred.

According to the present invention, a hydrogel is formed, which is then subjected to a solvent exchange. According to the present invention, the water-soluble polysaccharide is preferably selected from the group consisting of agar, alginate, carrageenan, cellulose, hyaluronic acid, pectin, starch, xanthan gum, modified cellulose, chitin and chitosan.

Other natural or synthetic hydrocolloid-forming polymers include (partially) water-soluble, natural or synthetic polymers that form gels or viscous solutions in aqueous systems. They are carefully selected from other natural polysaccharides, synthetically modified derivatives thereof or synthetic polymers. Further Polysaccharides include, for example, carrageenan, pectins, tragacanth, guar gum, locust bean gum, agar, gum arabic, xanthan gum, natural and modified starches, dextranes, dextrin, maltodextrins, chitosan, glucans, such as beta-carboxymethyl cellulose. 1, 3-glucan, beta-1, 4-glucan, cellulose, mucopolysaccharides, such as especially hyaluronic acid. Synthetic polymers include cellulose ether, polyvinyl alcohol, polyvinyl pyrrolidone, synthetic cellulose derivatives, such as methyl cellulose, carboxy cellulose, carboxymethyl cellulose, in particular sodium carboxymethyl cellulose, cellulose esters, cellulose ethers, such as hydroxypropyl cellulose, polyacrylic acid, polymethacrylic acid, poly (methyl meth - acrylate) polymethacrylate (PMMA) (PMMA) , Polyethylene glycols, etc. Mixtures of these polymers can also be used.

The reaction temperature can be in the range from 0 to 100 ° C., preferably 5 to 75 ° C., in particular 10 to 50 ° C. The sol concentration, i.e. H. The concentration of the reagents in the solvent can be in the range from 0.25 to 65% by weight, preferably 0.5 to 60% by weight, in particular 1 to 10% by weight.

In principle, any solvent (LM) can be used as long as it is miscible with carbon dioxide or has a sufficient boiling point that enables the solvent to be removed from the resulting gel. Generally, the solvent is a low molecular weight organic compound, i.e. H. an alcohol of 1 to 6 carbon atoms, preferably 2 to 4, although other liquids known in the art can be used. Possible solvents are, for example, ketones, aldehydes, alkyl alkanoates, amides such as formamide, N-methylpyrrolidone, N-ethylpyrollidone, sulfoxides such as dimethyl sulfoxide, aliphatic and cycloaliphatic halogenated hydrocarbons, halogenated aromatic compounds and fluorine-containing ethers. Mixtures of two or more of the above compounds are also possible.

Examples of other useful liquids include, but are not limited to: ethyl acetate, ethyl acetoacetate, acetone, dichloromethane, isopropanol, methyl ethyl ketone, tetrahydrofuran, propylene carbonate, and the like.

Other possibilities of solvents are acetals, especially diethoxymethane, dimethoxymethane and 1,3-dioxolane.

Dialkyl ethers and cyclic ethers are also suitable as solvents. Preferred dialkyl ethers are in particular those having 2 to 6 carbon atoms, in particular methyl ethyl ether, diethyl ether, methyl propyl ether, methyl isopropyl ether, propyl ethyl ether, ethyl isopropyl ether, dipropyl ether, propyl isopropyl ether, diisopropyl ether, methyl butyl ether, methyl isobutyl ether, methyl t-butyl ether, ethyl -n-butyl ether, ethyl isobutyl ether and ethyl t-butyl ether. Preferred cyclic ethers are especially tetrahydrofuran, dioxane and tetrahydropyran. Aldehydes and / or ketones are particularly preferred as solvents. Aldehydes or ketones which are suitable as solvents are in particular those which correspond to the general formula R 2 - (CO) -R 1, where R 1 and R 2 each represent hydrogen or an alkyl group having 1 stand, 2, 3, 4, 5, 6 or 7 carbon atoms. Suitable aldehydes or ketones are, in particular, acetaldehyde, propionaldehyde, n-butyraldehyde, isobutyraldehyde, 2-ethylbutyraldehyde, valeraldehyde, isopentaldehyde, 2-methylpentaldehyde, 2-ethylhexaldehyde, acrolein, methacrolein, crotonaldehyde, furfural methacrole, acrurol, acrurol, acrurol, acrurol, acrurol, acrurol, acrurol, acrurol, acrurol, acrurol, acrurol, acrurol, acrylonitrile , 2,3,6-tetrahydrobenzaldehyde, 6-methyl-3-cyclohexenaldehyde, cyanoacetaldehyde, ethylglyoxylate, benzaldehyde, acetone, diethyl ketone, methyl ethyl ketone, methyl isobutyl ketone, methyl n-butyl ketone, methyl pentyl ketone, dipropyl ketone ethyl isopropyl ketone, ethyl, diisobutyl ketone, ethyl Methyl-2-acetylfuran, 2-acetylfuran, 2-methoxy-4-methylpentan-2-one, 5-methylheptan-3-one, 2-heptanone, octanone, cyclohexanone, cyclopentanone and acetophenone.

The above aldehydes and ketones can also be used in the form of mixtures. In many cases, particularly suitable solvents are obtained by using two or more completely miscible compounds selected from the solvents mentioned above.

According to a further embodiment, the present invention accordingly relates to a process for drying gel particles as described above, the solvent (LM) being selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol and hexanol.

In addition, one or more auxiliary substances could be contained in the suspension according to the present invention. Auxiliary substances include: fillers, pH adjusting agents, such as buffer substances, stabilizers, co-solvents, pharmaceutically and cosmetically customary or other dyes and pigments, preservatives, nutritional supplements, vitamins, active substances, plasticizers, lubricants and lubricants.

The method of the present invention can also include further steps, for example suitable treatment steps.

At the end of the supercritical extraction, the pressure is released at a rate that enables optimal material properties in combination with a suitable time consumption, and the particles are removed from the column.

It was found that the method according to the invention makes it possible to obtain airgel particles with improved particle size and porosity. In particular, stable, dry and highly porous airgel particles with high surfaces and a high pore volume are obtained.

There is currently no continuous relaxation for temperature-sensitive substances after drying with the LTSCD process via valves, since the airgel particle C0 ₂ stream has to be heated strongly before the relaxation in order to compensate for the Joule-Thomson effect. The method presented allows the maximum temperature experienced by the airgel product to be reduced, since heating over a longer distance is at a moderate temperature.

Continuous valve-less relaxation is known, but has not previously been used for porous materials. In particular for aerogels, a continuous relaxation did not appear possible due to the necessary low relaxation rates for maintaining the pore structure given in the prior art. Thanks to the valve-free continuous relaxation of aerogels, a real continuous process for the production of aerogels using the LTSCD process is possible for the first time. This means that no separate large-volume discharge containers are required. The valveless relaxation also avoids the problems of abrasion and blocking associated with the use of valves to relax particles.

The product obtained in the process of the present invention is a micron-sized powder of porous airgel with a porosity of at least 70% by volume. In general, the size of the particles can vary, the particle size is in the range from 20 pm to 1000 pm. The aerogels obtained according to the invention can be inorganic or organic aerogels.

In further embodiments, the airgel comprises average pore diameters from approximately 2 nm to approximately 100 nm, for example in the range from 5 to 55 nm or in the range from 10 to 50 nm. In additional embodiments, the average pore diameter of dried gel materials can be approximately 4 nm, about 6 nm, about 8 nm, about 10 nm, about 12 nm, about 14 nm, about 16 nm, about 18 nm, about 20 nm, about 25 nm, about 30 nm, about 35 nm, about 40 nm or about 45 nm, about 50 nm or about 55 nm. In the context of the present invention, the surface, the pore sizes and the pore volumes were measured by BET according to ISO 9277: 2010, unless stated otherwise. This International Standard specifies the determination of the total specific outer and inner surface of disperse (eg nanopowders) or porous solids by measuring the amount of physically adsorbed gas according to the Brunauer, Emmett and Teller (BET) method. It takes into account the recommendations of the International Union for Pure and Applied Chemistry (IU-PAC) from 1984 and 1994.

According to a further embodiment, the present invention accordingly relates to a method for drying gel particles as described above, the gel particles having an average pore diameter in the range from 2 to 100 nm.

According to a further aspect, the present invention also relates to airgel particles obtainable or obtained according to a method for drying gel particles as described above.

The airgel particles obtained or obtainable by the process of the present invention are suitable for various applications. The invention also relates to building materials and vacuum insulation panels which comprise the powdery nanoporous materials and the use of powdered nanoporous materials for thermal insulation. The materials obtained according to the invention are preferably used for thermal insulation, in particular in buildings or for cold insulation, in particular in mobile transport applications or in stationary applications, for example in refrigerators or for mobile applications. Fibers can be used as additives for mechanical reinforcement.

According to a further aspect, the present invention also relates to the use of airgel particles, obtainable or obtained according to a method for drying gel particles as described above, for medical and pharmaceutical applications, as an additive or carrier material for additives for food, as a catalyst carrier, for cosmetics -, hygiene, washing and cleaning applications, for the production of sensors, for thermal insulation or as core material for VI Ps.

Further embodiments of the present invention can be found in the claims and the examples. It goes without saying that the features mentioned above and those explained below of the object / method according to the invention or of the uses according to the invention can be used not only in the respectively specified combination but also in other combinations, without the scope of the invention to leave. So z. B. also implicitly includes the combination of a preferred feature with a particularly preferred feature, or a feature that is not further characterized with a particularly preferred feature, etc., even if this combination is not expressly mentioned.

Exemplary embodiments of the present invention are listed below, but these do not restrict the present invention. In particular, the present invention also includes those embodiments which result from the back references and thus combinations given below.

1. A method for drying gel particles comprising the steps

(i) providing a suspension containing gel particles (P1) and a solvent (LM),

(ii) filling the suspension into a column through which carbon dioxide flows in countercurrent,

(iii) separating the dried airgel particles from the column, the suspension being introduced into the top area of the column and the dried airgel particles being separated off in the lower area of the column, the pressure and temperature in the column being set such that the mixture of carbon dioxide and solvent is almost supercritical or supercritical.

2. Method according to embodiment 1, wherein the gel particles sediment in countercurrent.

3. Method according to one of the embodiments 1 or 2, wherein the airgel particles obtained are removed continuously, without a valve.

4. The method according to one of the embodiments 1 to 3, wherein the C0 ₂ mass flow is adjusted so that dried airgel particles are obtained

5. The method according to one of the embodiments 1 to 4, the CO ₂ mass flow being set in the range from 0.75 kg / h to 25 t / h.

6. The method according to one of the embodiments 1 to 5, wherein the gel particles have an average diameter in the range from 20 pm to 1000 gm.

7. The method according to one of the embodiments 1 to 6, wherein the gel particles have an average pore diameter in the range from 2 to 100 nm.

8. The method according to one of the embodiments 1 to 7, wherein the solvent (LM) is selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol and hexanol.

9. Airgel particles, obtainable or obtained according to a method according to one of the embodiments 1 to 8.

10. Airgel particles, obtainable or obtained according to a method for drying gel particles comprising the steps

(i) providing a suspension containing gel particles (P1) and a solvent (LM),

11. Airgel particles according to embodiment 10, the gel particles sedimenting in countercurrent.

12. Airgel particles according to one of the embodiments 10 or 11, the airgel particles obtained being removed continuously, without a valve.

13. Airgel particles according to one of the embodiments 10 to 12, wherein the CO2 mass flow is adjusted so that dried airgel particles are obtained

14. Airgel particles according to one of the embodiments 10 to 13, the CO2 mass flow being set in the range from 0.75 kg / h to 25t / h.

15. Airgel particles according to one of the embodiments 10 to 14, wherein the gel particles have an average diameter in the range from 20 pm to 1000 pm.

16. Airgel particles according to one of the embodiments 10 to 15, wherein the gel particles have an average pore diameter in the range from 2 to 100 nm.

17. Airgel particles according to one of the embodiments 10 to 16, wherein the solvent (LM) is selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol and hexanol.

18. Use of airgel particles according to one of the embodiments 10 to 17 or of airgel particles obtained or obtainable according to a method according to one of the embodiments 1 to 8 for medical and pharmaceutical applications, as an additive or carrier material for additives for food, as a catalyst carrier, for cosmetics -, hygiene, washing and cleaning applications, for the production of sensors, for thermal insulation or as core material for VIPs.

The invention is explained in more detail below by examples.

Examples

I. Design examples

Below are some rough calculations and a sample design. In the following interpretation, the operating conditions (120 bar, 50 ° C) and the particle properties (particle porosity e = 0.93 and tortuosity T = 2.5) are constant. In the following In addition, the throughput is initially very low, so that the fluid phase is approximately described with pure C0 ₂ .

1. Calculation of drying times

The drying times for particles of different diameters were simulated assuming a one-dimensional mass transfer. The substance sizes of the fluid (CO2 and ethanol) were modeled using the Peng-Robinson equation of state with the corresponding mixing rules. The particles are described with a particle porosity of e = 0.93 (corresponds to V _pores = 8 cm ³ / g) and a tortuosity of T = 2.5.

1.1 Diffusion in the particle

The following drying times (Table 1) were calculated on the assumption that the rate-determining step was the diffusion within the particle. The mass transfer from the particle to the surrounding fluid phase was assumed to be very large. The drying times for 5 pm particles are only a few milliseconds and in the range up to 100 pm under one second.

Table 1: Calculated drying times of alginate airgel particles at 120 bar and 50 ° C

Particle diameter [pm] _ 5 _ 25 _ 50 _ 100 _ 200 _ 300 _ 500 calculated drying time [s] 0.0019 0.0479 0, 1916 0.7664 3.0655 6.897 19, 159

1.2 Consideration of mass transfer

The mass transfer from the particle to the surrounding fluid phase was taken into account in the drying times summarized in Table 2. The mean mass transfer coefficients were calculated using a Sh correlation for a single sphere. Interestingly, the mass transfer coefficient is only a function of the particle diameter and not the flow velocity, since the relative velocity and therefore Re is constant and only changes as soon as the particles are discharged.

Table 2: Calculated drying times of alginate airgel particles at 120 bar and 50 ° C taking into account the average mass transfer coefficient

Particle diameter [pm] _ 5 _ 25 _ 50 _ 100 200 300 500

ß [m / s] 8, 11E-03 2.05E-03 l, 40E-03 l, HE-03 9.62E-04 8.98E-04 8.28E-04 calculated drying time [s] 0.0028 0 , 0644 0.2382 0.8765 3.2914 7.255 19.609

The relative change in the drying times when considering the mass transfer compared to the assumption of an infinite mass transfer coefficient is greatest for small particle diameters and becomes smaller with increasing particle diameter. ner. However, the absolute drying time changes relatively little and the drying times remain in the same order of magnitude.

2. Discharge of particles: mass flow CO2 and particle diameter

In addition to the drying time, another critical aspect of continuous supercritical drying in a countercurrent column is the fluid dynamics and the associated residence time of the particles. The rate of sinking of particles can be described by a connection between the Archimedes number and Reynolds number. According to MARTIN, the following applies to the transition range between the Stokes and Newton ranges:

The (sink) speed calculated from the Re number represents the relative speed between the particles and the surrounding fluid and depends on the particle diameter. Depending on the size of the upward C0 ₂ stream, the absolute sink rate of the particles is reduced or particles with the C0 ₂ stream are discharged over the top of the column.

For a column with an inner diameter of di = 20.57 mm and thus a free cross section of A = 3.32E-04 m ² and a column height of 500 mm, the calculated residence times of alginate airgel particles of different diameters are for different C0 ₂ -Mass flows summarized in Table 3. Here, an average apparent density of the particles of wet and completely dried particles was assumed.

Table 3: Calculated residence times of alginate airgel particles with e = 0.93

Parti keldameter [mhi]

25 50 100 200 300 500 mass flow C0 ₂ [kg / h]

0.5 carried out carried out 286.6 73.4 29.2 19.0 12.0

1 carried out carried out 756.5 84.6 30.7 19.6 12.2

1.5 carried out carried out carried out 100.5 32.4 20.3 12.5

2 carried out carried out carried out 125.1 34.3 21.0 12.7

3 carried out carried out carried out 274.2 39.0 22.6 13.3

The porosity of the airgel particles has a major influence on the theoretical sinking rate and residence time of the particles in the C0 ₂ stream. At lower porosities, as can be seen in Table 4, smaller particles can also be dried without discharge or higher C0 ₂ flow rates and thus higher particle throughputs with the same column height. Table 4: Calculated residence times of alginate airgel particles with e = 0.85

Parti keldameter [mhi]

25 50 100 200 300 500 mass flow C0 ₂ [kg / h]

0.5 carried out 971.566186 136.7 44.6 19.5 13.2 8.6

1 carried out carried out 176.5 47.9 20.1 13.4 8.7

1.5 carried out carried out 256.5 51.8 20.7 13.7 8.8

2 carried out carried out 526.6 56.5 21.4 14.0 8.9

3 carried out carried out carried out 69.3 22.9 14.6 9.2

3. Influence of the mass flow on the ratio of the required drying time / residence time.

For the design of the column length, the ratio of residence time to drying time should understandably be> 1. Table 5 shows the ratio of residence time / drying time for a column height of 500 mm. The particles with a diameter of 500 gm would not be completely dried for the CC> 2 mass flows shown from 0.5 kg / h to 3 kg / h. This could be counteracted by a further increase in the CC> 2 mass flow, whereby smaller particles would, however, be discharged. Alternatively, an increase in the column height leads to a proportional increase in the residence time and thus to a proportional increase in the residence time / drying time ratio. Doubling the column height to, for example, 1 m would double the residence time / drying time ratio and thus also lead to drying of particles with the diameter d = 500 gm without smaller particles being discharged.

Table 5: Ratio of residence time / drying time for 0.5 m column height at infinite

Mass transfer coefficients

Parti keldameter [mhi]

25 50 100 200 300 500

Mass flow C0 ₂ [kg / h]

0.5 carried out carried out 1495.89 95.81 9.51 2.76 0.63

1 carried out 3948.27 110.38 10.01 2.84 0.64

1.5 carried out carried out carried out 131.10 10.56 2.94 0.65

2 carried out carried out carried out 163.27 11.18 3.04 0.67

3 carried out carried out carried out 357.75 12.72 3.28 0.69 Table 6: Ratio of residence time / drying time for 1 m column height with infinite mass transfer coefficient

Parti keldameter [mhi]

25 50 100 200 300 500 mass flow C0 ₂ [kg / h]

0.5 carried out carried out 2991.77 191.61 19.03 5.51 1.25

1 carried out carried out 7,896.54 220.76 20.01 5.69 1.28

1.5 carried out carried out carried out 262.20 21.12 5.88 1.30

2 carried out carried out carried out 326.54 22.37 6.09 1.33

3 carried out carried out carried out 715.50 25.45 6.55 1.39

4. Influence of particle loading on drying

In previous rollover calculations, the assumption was made that almost pure CO2 is also present at the outlet, which means that only very low particle loads are driven. In technical applications, a CC> 2 stream loaded with ethanol (ETOH) as high as possible should be drawn off in the countercurrent operation at the top of the column in order to reduce the use of CO2 per kg of airgel. To do this, the extended drying time must be compensated for by increasing the column height accordingly. For example, the drying of 1 L / h of 500 pm particles shown in Table 6 in a 1 m high column with CC> 2 mass flows of 1 kg / h and less is not achieved. (See Table 7) If the column is increased to 3 m, drying is achieved even with 1 kg CC> 2 / h and the high exit mass fraction of E-TOH of 64% (w / w) can be maintained.

Table 7: Drying 1 L / h of 500 pm airgel particles with different CO2 mass flows in counterflow in a column with a height of 1 m

C0 ₂ mass flow mass fraction of mass fraction of ETOH

[kg / h] ETOH in C0 ₂ [-] in particles [-]

0.50 0.97 0.24

0.75 0.80 0.06

1.00 0.63 0.02

2.00 0.32 0.00

3.00 0.21 0.00

4.00 0.16 0.00

II. Examples

The basic feasibility of the drying process was demonstrated on a pilot plant.

A suspension of alginate gel particles with diameters of 50-300 μm (14% (v / v)) in ethanol with 23 ml was made from a storage container into the top of a column of length 0.5 m and an inner diameter of di = 20.6 mm / min promoted. At an operating pressure of 120 bar and an operating temperature of 50 ° C, supercritical sches C0 _{2 promoted} in counterflow to the particle flow at 40 g / min. After the experiment, the collecting container was relaxed.

The gel particles sediment against the C0 ₂ flow, while the free ethanol, in spite of the greater density of the ethanol-C0 ₂ mixture than CO2, in the C0 ₂ -ethanol-

Mixture flow is discharged over the head. The comparatively short residence time of the particles compared to a moving bed surprisingly led to a complete drying of the airgel particles. With pore volumes of 9.4 cm ³ / g and BET surface areas of 500 m ² / g, the particles that have fallen into the collecting container have properties similar to those of the airgel particles produced and dried in batches using the same gelation process.

Claims

1. A method for drying gel particles, in particular for the production of aerogels, comprising the steps

(i) providing a suspension containing gel particles (P1) and a solvent (LM),

(iii) separating the dried airgel particles from the column, the suspension being introduced into the top area of the column and the dried airgel particles being separated off in the lower area of the column, pressure and temperature being set in the column such that the mixture of carbon dioxide and solvent is almost supercritical or supercritical.

2. The method according to claim 1, wherein the gel particles sediment in countercurrent.

3. The method according to any one of claims 1 or 2, wherein the removal of the airgel particles obtained is continuous, valve-less.

4. The method according to any one of claims 1 to 3, wherein the C0 ₂ mass flow is adjusted so that dried airgel particles are obtained.

5. The method according to any one of claims 1 to 4, wherein the gel particles have an average diameter in the range of 20pm to 1000pm.

6. The method according to any one of claims 1 to 5, wherein the gel particles have an average pore diameter in the range of 2 to 100 nm.

7. The method according to any one of claims 1 to 6, wherein the solvent (LM) is selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol and hexanol.

8. Airgel particles, obtainable or obtained according to a method according to one of claims 1 to 7.

9. Use of airgel particles according to claim 8 or of airgel particles obtained or obtainable according to a method according to one of claims 1 to 7 for medical and pharmaceutical applications, as an additive or carrier material for additives for food, as a catalyst carrier, for cosmetic, hygiene, washing and cleaning applications, for the production of sensors, for thermal insulation or as core material for VIPs.