WO1999032218A1 - A method for the sub-critical drying of lyogels to produce aerogels - Google Patents

A method for the sub-critical drying of lyogels to produce aerogels Download PDF

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Publication number
WO1999032218A1
WO1999032218A1 PCT/EP1998/008244 EP9808244W WO9932218A1 WO 1999032218 A1 WO1999032218 A1 WO 1999032218A1 EP 9808244 W EP9808244 W EP 9808244W WO 9932218 A1 WO9932218 A1 WO 9932218A1
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WO
WIPO (PCT)
Prior art keywords
drying
fixed bed
gel
drying gas
sub
Prior art date
Application number
PCT/EP1998/008244
Other languages
French (fr)
Inventor
Johannes Hartel
Rainald Forbert
Original Assignee
Cabot Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Cabot Corporation filed Critical Cabot Corporation
Priority to BR9813736-0A priority Critical patent/BR9813736A/en
Priority to CA002310317A priority patent/CA2310317A1/en
Priority to JP2000525200A priority patent/JP4450989B2/en
Priority to DE69811948T priority patent/DE69811948T2/en
Priority to EP98966343A priority patent/EP1042059B1/en
Priority to KR10-2000-7005717A priority patent/KR100523465B1/en
Priority to AU22737/99A priority patent/AU2273799A/en
Publication of WO1999032218A1 publication Critical patent/WO1999032218A1/en
Priority to US09/594,000 priority patent/US6378229B1/en

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F26—DRYING
    • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
    • F26B3/00—Drying solid materials or objects by processes involving the application of heat
    • F26B3/02—Drying solid materials or objects by processes involving the application of heat by convection, i.e. heat being conveyed from a heat source to the materials or objects to be dried by a gas or vapour, e.g. air
    • F26B3/06—Drying solid materials or objects by processes involving the application of heat by convection, i.e. heat being conveyed from a heat source to the materials or objects to be dried by a gas or vapour, e.g. air the gas or vapour flowing through the materials or objects to be dried
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/14—Colloidal silica, e.g. dispersions, gels, sols
    • C01B33/157—After-treatment of gels
    • C01B33/158—Purification; Drying; Dehydrating
    • C01B33/1585—Dehydration into aerogels

Definitions

  • the invention relates to a method for the sub-critical drying of lyogels to produce aerogels.
  • Lyogels are gels which contain a liquid, the dispersing agent. In the particular instance where the gel fluid is water, they are also referred to as hydrogels. In the present Application, the term lyogel' also includes hydrogels. Aerogels in the wider sense, i.e. in the sense of "gel with air as the dispersant", a suitable gel is produced by drying. The term “aerogels” in this sense includes aerogels in the narrower sense, xerogels and cryogels. In this respect, a dried gel is referred to as an aerogel in the narrower sense if the fluid of the gel is eliminated at temperatures above the critical temperature and starting from pressures which are above the critical pressure. On the other hand, if the fluid of the gel is removed sub-critically, for example with the formation of a fluid-vapour interface, then the resulting gel is also frequently referred to as a xerogel.
  • Aerogels have a very low density and high porosity for solid substances. Therefore, and on account of the minimal pore size, aerogels, particularly those with porosities of over 60% and densities below 0.6 g/cu.cm, display an extremely low thermal conductivity and are therefore used as heat insulating materials such as are described for instance in EP-A 0 171 722.
  • aerogels are predominantly used as granulate.
  • aerogels also exhibit low mechanical stability, particularly in respect of sheer loadings and against abrasion.
  • aerogels are dried by dielectric processes. Due to the necessary electrical energy and the relatively high investment for an appropriate drying apparatus, however, these methods are not sufficiently economical.
  • the object of the present invention was to provide a method for the sub-critical drying of lyogels which are suitable for sub-critical drying to produce aerogels and which minimises the destruction of aerogel particles and the abrasion of aerogel particles during drying and which can be used on a large industrial scale.
  • the problem is resolved by a method for the sub-critical drying of lyogels to produce aerogels and which is characterised in that the lyogel particles are disposed as a static bed and have a dry gas flowing through them. Carried out in this way, the drying does not lead to grain fracture or abrasion since the particles are stationary in their bulk. Surprisingly, the drying can be carried out in relatively short times and in layer heights which are considerable for fixed bed drying.
  • any lyogels which can be dried sub-critically are suitable for the method, i.e. their gel structure must be sufficiently stable to withstand collapse of the framework by the action of capillary forces and are if necessary suitably surface modified in order for example to prevent a reaction between pore walls in the shrunken condition.
  • these may according to the type of gel structure be organic or inorganic lyogels.
  • they can be produced on a basis of metal oxides which are suitable for the Sol-gel technical (G. J. Blinker, G. W. Scherer, Sol-Gel Science 1990, chapters 2 and 3), such as for example Si- or Al-compounds or on the basis of organic substances which are suitable for the sol-gel technique such as for example melamine formaldehyde condensates (US-A-5 086 085) or resorcin formaldehyde condensates (US-A-4 873 218) or even on the basis of mixtures of the above-mentioned materials.
  • they are SiO 2 gels and particularly preferably organically surface modified SiO 2 gels such as are described for example in the unpublished German Patent Application No. 19648798.
  • the gel fluid may in principle be pure substances or mixtures, the gel fluid preferably containing more than 50% by weight of organic solvents, preferably acetone or hexamethyl disiloxane. Naturally, the gel fluid may also contain small quantities of other substances such as for example hydrochloric acid or residues of water.
  • the lyogel particles can in principle be of any desired form and size but preferably substantially spherical particles with diameters between 100 ⁇ m and 5 cm and particularly preferred are those with diameters between 0.5 mm and 5 mm. It is also possible to dry mixtures of particles of different shape and/or different size. In order to avoid already dried and therefore lightweight aerogel particles being blown away, it would among other things be necessary for the velocity of the dry gas flow to be low so that no dried aerogel particles are carried away. However, this results in low gas flows and therefore a very limited energy input. For drying aerogels, a rapid energy input is advantageous. For using relatively high drying gas flow velocities, the bulk must therefore, at those places where the gas flow emerges from the fixed bed, be limited by at least one arrangement which is permeable to the drying gas but not to the particles.
  • the fixed bed is preferably and in per se known manner, traversed by the drying gas flow passing through it downwards, so that the arrangement which is permeable to the drying gas but not to the particles is constituted by the carrier of the fixed bed.
  • the drying apparatus which is then to be used is consequently not only simpler and less expensive but grain fracture is also reduced, since the upper layers which press down onto those underneath are the first to be dried.
  • the dried and mechanically sensitive aerogels then lie on the heavier and less dried and also mechanically loadable gel particles and are not heavily loaded mechanically.
  • the fixed bed may also be moved with the carrier, e.g. on a suitable belt.
  • the gas flow then passes through the fixed bed crosswise to the direction of movement.
  • the thickness of the fixed bed in the direction of flow of the drying gas stream can be surprisingly great. Preferably, it is between 20 cm and 100 cm and particularly preferably between 20 and 60 cm.
  • any gas suitable for drying can be used as the drying gas; if the gel fluid contains large fractions of organic solvents, then possibly an inert gas such as nitrogen should be used.
  • the input gas flow can already contain solvent gas which the process allows, in other words a circulatory pattern of gas flow, whereby, as usual, solvent gas is constantly eliminated from the circuit, for example by condensation. To accelerate the drying as a whole, it may be advantageous first to dry the gel in a circulatory fashion but to use fresh drying gas for drying the last residues of gel fluid.
  • the method according to the invention can be carried out by apparatus known to the man skilled in the art, e.g. fixed bed driers, suitable containers with screen bottoms or suitable drying belts, etc., which may possibly be modified in a manner evident to a man of the art.
  • the method for drying surface modified SiO 2 aerogels such as disclosed for example in DE-A-43 42 548 or in the unpublished German Patent Application 196487 98 will be used.
  • the temperature of the drying gas is preferably between 100°C and 200°C and with particular preference between 140°C and 180°C.
  • the incident flow velocities are preferably in the range between 1 and 40 cm/s and particularly preferably between 5 and 30 cm/s.
  • the method is particularly suitable for producing aerogels with densities below 300 g/cu.cm, it is also possible to use it for drying xerogels of greater density.
  • a surface modified SiO 2 lyogel which has been produced in accordance with examples 1 to 4 in the unpublished German Patent Application 1964877798 and which has a sharp grain size distribution around 1 mm diameter is dried in a fixed bed with a height of 50 cm with a stream of nitrogen flowing through the fixed bed from the top downwards at an incident flow velocity of 20 cm/s and with a gas inlet temperature of 160°C.
  • the gel is dried for 2 l A hours in a circulatory manner, the drying gas leaving the fixed bed saturated with moisture and after thorough condensation with a charge of lOg/kg inert gas, it is again introduced into the fixed bed. Afterwards, drying is continued for half-an- hour with fresh drying gas.
  • the dried aerogel displays virtually no grain fracture and hardly any abrasion.

Abstract

The present invention refers to a method for the sub-critical drying of lyogels to produce aerogels. The method for the sub-critical drying of lyogels to produce aerogels provides for the lyogel particles to be disposed as a fixed bed and for a drying gas to be passed through them.

Description

Description
A method for the sub-critical drying of lyogels to produce aerogels
The invention relates to a method for the sub-critical drying of lyogels to produce aerogels.
Lyogels are gels which contain a liquid, the dispersing agent. In the particular instance where the gel fluid is water, they are also referred to as hydrogels. In the present Application, the term lyogel' also includes hydrogels. Aerogels in the wider sense, i.e. in the sense of "gel with air as the dispersant", a suitable gel is produced by drying. The term "aerogels" in this sense includes aerogels in the narrower sense, xerogels and cryogels. In this respect, a dried gel is referred to as an aerogel in the narrower sense if the fluid of the gel is eliminated at temperatures above the critical temperature and starting from pressures which are above the critical pressure. On the other hand, if the fluid of the gel is removed sub-critically, for example with the formation of a fluid-vapour interface, then the resulting gel is also frequently referred to as a xerogel.
Where the use of the term aerogels in the present Application is concerned, there are gels which are sub-critically dried.
Aerogels have a very low density and high porosity for solid substances. Therefore, and on account of the minimal pore size, aerogels, particularly those with porosities of over 60% and densities below 0.6 g/cu.cm, display an extremely low thermal conductivity and are therefore used as heat insulating materials such as are described for instance in EP-A 0 171 722.
For industrial applications, aerogels are predominantly used as granulate. For the application, it is in this respect essential for the aerogel granulate to be used to consist of particles of a suitable form, preferably spherical form, and size distribution. By virtue of their low density, however, aerogels also exhibit low mechanical stability, particularly in respect of sheer loadings and against abrasion.
By reason of the capillary forces which occur in sub-critical drying and the shrinkage involved, not all lyogels are suitable for sub-critical drying to produce an aerogel. During drying, the gel shrinks considerably if the meniscus of the fluid migrates into the interior of the gel, in order, with effect from a certain point in the drying process, to spring back more or less completely into its starting form again. Consequently, depending upon the qualities of the inner gel surface, a certain minimum stability of the gel network is essential, modification of the inner gel surface often being required in order to prevent a reaction by adjacent pore walls in the shrunk condition and a collapse of the gel which this would entail.
Corresponding processes in which the inner surface of an SiO2 lyogel is organically modified and the resulting gel is sub-critically dried to produce an aerogel, are disclosed for example in US- A-5 565 142, DE-A-43 42 548 and in the unpublished German Patent Application 19648798.
Gels which are unsuitable for sub-critical drying collapse under sub-critical drying, with a loss of the porous structure and therefore they no longer display the favourable properties of aerogels.
According to the gel, surface modification, granular form and size and drying conditions, so gel particles can, during drying, be destroyed on a macroscopic scale, i.e. while retaining the nanoporous structure. It is true that the aerogel retains its properties but on account of the now smaller granular size an irregular or undefined grain shape, it can no longer be used so satisfactorily. Drying methods which at first sight appear obvious for such drying problems are not readily suited to major industrial production of aerogel granulate of a definite form. By reason of the low density of the aerogels, a fluid bed drying system has been found to be unsuited to major industrial production. In order not to carry the aerogel particles out of the layer, it would be necessary to work below the fluidising point; the necessary gas flow velocities would then be so low that there would be no guarantee of heat being supplied and vapour dispersed in an acceptable time for drying. Were greater gas flow velocities to be used, then the aerogel would not be completely dried since it will be conveyed out of the dryer. Furthermore, in the fluidised phase, gel particles collide with one another so that there is considerable abrasion and grain fracture.
Contact drying proves to be not sufficiently effective since due to the high heat insulating capacity of the aerogels, a transfer of heat into layers more remote from the contact surface does not take place quickly enough so that only minimal layer thicknesses and therefore, in view of the quantities needed, excessively great surface areas would be needed.
In accordance with DE-A-43 16 540, aerogels are dried by dielectric processes. Due to the necessary electrical energy and the relatively high investment for an appropriate drying apparatus, however, these methods are not sufficiently economical.
Therefore, the object of the present invention was to provide a method for the sub-critical drying of lyogels which are suitable for sub-critical drying to produce aerogels and which minimises the destruction of aerogel particles and the abrasion of aerogel particles during drying and which can be used on a large industrial scale.
Surprisingly, the problem is resolved by a method for the sub-critical drying of lyogels to produce aerogels and which is characterised in that the lyogel particles are disposed as a static bed and have a dry gas flowing through them. Carried out in this way, the drying does not lead to grain fracture or abrasion since the particles are stationary in their bulk. Surprisingly, the drying can be carried out in relatively short times and in layer heights which are considerable for fixed bed drying.
In principle, any lyogels which can be dried sub-critically are suitable for the method, i.e. their gel structure must be sufficiently stable to withstand collapse of the framework by the action of capillary forces and are if necessary suitably surface modified in order for example to prevent a reaction between pore walls in the shrunken condition.
Where the lyogels are concerned, these may according to the type of gel structure be organic or inorganic lyogels. For example, they can be produced on a basis of metal oxides which are suitable for the Sol-gel technical (G. J. Blinker, G. W. Scherer, Sol-Gel Science 1990, chapters 2 and 3), such as for example Si- or Al-compounds or on the basis of organic substances which are suitable for the sol-gel technique such as for example melamine formaldehyde condensates (US-A-5 086 085) or resorcin formaldehyde condensates (US-A-4 873 218) or even on the basis of mixtures of the above-mentioned materials. Preferably, they are SiO2 gels and particularly preferably organically surface modified SiO2 gels such as are described for example in the unpublished German Patent Application No. 19648798.
Where the gel fluid is concerned, this may in principle be pure substances or mixtures, the gel fluid preferably containing more than 50% by weight of organic solvents, preferably acetone or hexamethyl disiloxane. Naturally, the gel fluid may also contain small quantities of other substances such as for example hydrochloric acid or residues of water.
The lyogel particles can in principle be of any desired form and size but preferably substantially spherical particles with diameters between 100 μm and 5 cm and particularly preferred are those with diameters between 0.5 mm and 5 mm. It is also possible to dry mixtures of particles of different shape and/or different size. In order to avoid already dried and therefore lightweight aerogel particles being blown away, it would among other things be necessary for the velocity of the dry gas flow to be low so that no dried aerogel particles are carried away. However, this results in low gas flows and therefore a very limited energy input. For drying aerogels, a rapid energy input is advantageous. For using relatively high drying gas flow velocities, the bulk must therefore, at those places where the gas flow emerges from the fixed bed, be limited by at least one arrangement which is permeable to the drying gas but not to the particles.
Surprisingly, the fixed bed is preferably and in per se known manner, traversed by the drying gas flow passing through it downwards, so that the arrangement which is permeable to the drying gas but not to the particles is constituted by the carrier of the fixed bed. The drying apparatus which is then to be used is consequently not only simpler and less expensive but grain fracture is also reduced, since the upper layers which press down onto those underneath are the first to be dried. The dried and mechanically sensitive aerogels then lie on the heavier and less dried and also mechanically loadable gel particles and are not heavily loaded mechanically.
For a continuous operation, the fixed bed may also be moved with the carrier, e.g. on a suitable belt. Preferably, the gas flow then passes through the fixed bed crosswise to the direction of movement.
The thickness of the fixed bed in the direction of flow of the drying gas stream can be surprisingly great. Preferably, it is between 20 cm and 100 cm and particularly preferably between 20 and 60 cm.
Any gas suitable for drying can be used as the drying gas; if the gel fluid contains large fractions of organic solvents, then possibly an inert gas such as nitrogen should be used. The input gas flow can already contain solvent gas which the process allows, in other words a circulatory pattern of gas flow, whereby, as usual, solvent gas is constantly eliminated from the circuit, for example by condensation. To accelerate the drying as a whole, it may be advantageous first to dry the gel in a circulatory fashion but to use fresh drying gas for drying the last residues of gel fluid.
The method according to the invention can be carried out by apparatus known to the man skilled in the art, e.g. fixed bed driers, suitable containers with screen bottoms or suitable drying belts, etc., which may possibly be modified in a manner evident to a man of the art.
Preferably, the method for drying surface modified SiO2 aerogels such as disclosed for example in DE-A-43 42 548 or in the unpublished German Patent Application 196487 98 will be used.
In this case, if the gel fluid contains more than 50% by weight hexamethyl disiloxane or acetone, then the temperature of the drying gas is preferably between 100°C and 200°C and with particular preference between 140°C and 180°C.
In the case of the said surface modified SiO2 aerogels, the incident flow velocities are preferably in the range between 1 and 40 cm/s and particularly preferably between 5 and 30 cm/s.
Although the method is particularly suitable for producing aerogels with densities below 300 g/cu.cm, it is also possible to use it for drying xerogels of greater density.
The method according to the invention will be described hereinafter with reference to an example of embodiment but without however being limited thereby.
Example:
A surface modified SiO2 lyogel which has been produced in accordance with examples 1 to 4 in the unpublished German Patent Application 1964877798 and which has a sharp grain size distribution around 1 mm diameter is dried in a fixed bed with a height of 50 cm with a stream of nitrogen flowing through the fixed bed from the top downwards at an incident flow velocity of 20 cm/s and with a gas inlet temperature of 160°C. Initially, the gel is dried for 2lA hours in a circulatory manner, the drying gas leaving the fixed bed saturated with moisture and after thorough condensation with a charge of lOg/kg inert gas, it is again introduced into the fixed bed. Afterwards, drying is continued for half-an- hour with fresh drying gas. The dried aerogel displays virtually no grain fracture and hardly any abrasion.

Claims

Patent claims
1. A method for the sub-critical drying of lyogels to produce aerogels, characterised in that the lyogel particles are disposed as a fixed bed and have drying as passed through them.
2. A method according to claim 1, characterised in that, at those locations at which the gas flow emerges from the fixed bed, the fixed bed is bounded by at least one arrangement which is permeable to the drying gas but not to the particles.
3. A method according to claim 2, characterised in that the fixed bed lies on the said means and has the flow passing through it from the top downwards.
4. A method according to at least one of claims 1 to 3, characterised in that the lyogel is an organically modified gel the framework of which contains SiO2.
5. A method according to claim 4, characterised in that the gel fluid predominantly contains hexamethyl disiloxane.
6. A method according to claim 4, characterised in that the gel fluid predominantly contains acetone.
7. A method according to at least one of claims 4 to 6, characterised in that the temperature of the drying gas at the inlet is between 100┬░C and 200┬░C.
8. A method according to 6 or 7, characterised in that the incident flow velocity is between 1 cm/s and 40 cm/s.
9. A method according to at least one of the preceding claims, characterised in that the thickness of the fixed bed is between 20 cm and 100 cm in the direction of flow of the drying gas stream.
10. A method according to at least one of the preceding claims, characterised in that the drying gas follows a circulatory system.
11. A method according to at least one of the preceding claims, characterised in that in a first phase of drying the drying gas is circulated while in a second phase fresh drying gas is used.
12. A method according to at least one of the preceding claims, characterised in that the fixed bed is moved together with the means bounding it, the stream of drying gas being passed through the fixed bed crosswise to the direction of movement.
PCT/EP1998/008244 1997-12-19 1998-12-16 A method for the sub-critical drying of lyogels to produce aerogels WO1999032218A1 (en)

Priority Applications (8)

Application Number Priority Date Filing Date Title
BR9813736-0A BR9813736A (en) 1997-12-19 1998-12-16 Method for subcritically drying lyogels to produce aerogels
CA002310317A CA2310317A1 (en) 1997-12-19 1998-12-16 A method for the sub-critical drying of lyogels to produce aerogels
JP2000525200A JP4450989B2 (en) 1997-12-19 1998-12-16 Airgel production method by subcritical drying of liogel
DE69811948T DE69811948T2 (en) 1997-12-19 1998-12-16 METHOD FOR THE SUBCRITICAL DRYING OFLYO-GELS FOR THE PRODUCTION OF AEROGELS
EP98966343A EP1042059B1 (en) 1997-12-19 1998-12-16 A method for the sub-critical drying of lyogels to produce aerogels
KR10-2000-7005717A KR100523465B1 (en) 1997-12-19 1998-12-16 A method for the sub-critical drying of lyogels to produce aerogels
AU22737/99A AU2273799A (en) 1997-12-19 1998-12-16 A method for the sub-critical drying of lyogels to produce aerogels
US09/594,000 US6378229B1 (en) 1997-12-19 2000-06-14 Method for the sub-critical drying of lyogels to produce aerogels

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE19756633.2 1997-12-19
DE19756633A DE19756633A1 (en) 1997-12-19 1997-12-19 Lyogels and aerogels subcritically dried in a packed bed with minimal particle breakdown

Related Child Applications (1)

Application Number Title Priority Date Filing Date
US09/594,000 Continuation US6378229B1 (en) 1997-12-19 2000-06-14 Method for the sub-critical drying of lyogels to produce aerogels

Publications (1)

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WO1999032218A1 true WO1999032218A1 (en) 1999-07-01

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PCT/EP1998/008244 WO1999032218A1 (en) 1997-12-19 1998-12-16 A method for the sub-critical drying of lyogels to produce aerogels

Country Status (11)

Country Link
US (1) US6378229B1 (en)
EP (1) EP1042059B1 (en)
JP (1) JP4450989B2 (en)
KR (1) KR100523465B1 (en)
CN (1) CN1111445C (en)
AU (1) AU2273799A (en)
BR (1) BR9813736A (en)
CA (1) CA2310317A1 (en)
DE (2) DE19756633A1 (en)
ES (1) ES2191989T3 (en)
WO (1) WO1999032218A1 (en)

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US9242900B2 (en) 2009-12-01 2016-01-26 Arizona Board Of Regents, A Body Corporate Of The State Of Arizona Acting For And On Behalf Of Arizona State University Porous geopolymer materials
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US9605427B2 (en) 2011-10-14 2017-03-28 Enersens Process for manufacturing xerogels
US10170759B2 (en) 2013-06-21 2019-01-01 Arizona Board Of Regents On Behalf Of Arizona State University Metal oxides from acidic solutions
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