Process for sub-critically drying aerogels

Details Process for sub-critically drying aerogels Process for sub-critically drying aerogels

1999-07-02

2000-10-17

Wilson, Pamela A.

Drying and gas or vapor contact with solids

Process

With contacting of material treated with solid or liquid agent

34337, F26B 300

Patent

active

06131305&

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

Process for the subcritical drying of aerogels
Aerogels are highly porous low-density materials, prepared by forming a gel and subsequently eliminating the liquid with extensive preservation of the gel structure.
According to a narrow definition (see e.g. Gesser and Goswanni, Chem. Rev. 1989, 89, 767) the term aerogel is understood to refer to a material in which the liquid has been removed from the gel under supercritical conditions, whereas, when the gel is dried under subcritical conditions, the resulting product is called a xerogel, and when the liquid is eliminated from the frozen state by sublimation, the product is called a cryogel.
Aerogels within the meaning of the present invention comprise all these materials, and may also contain any other gas besides air. The gases may also be removed from the aerogel under vacuum.
Common aerogels generally contain silicon or metal oxide, and are characterized by especially low densities of 20 to 300 kg/m.sup.3 with very high internal surfaces of over 500 m.sup.2 /g. Because of these properties, they are exceptionally suitable as heat-insulating and sound-absorbing materials, as catalyst carriers and as adsorbents.
According to the prior art, aerogels may be prepared in various ways by a sol-gel process with subsequent supercritical or subcritical drying, where the gel, before drying, may be present as a hydrogel (pore liquid is water) or as a lyogel (pore liquid is an organic solvent).
According to the prior art, all subcritical processes for the drying of lyogels to form aerogels provide the heat required for evaporation of the solvent by contact with a heated surface (contact drying), by electromagnetic waves (e.g. microwave drying) or by flow of a gas (convection drying) (see e.g. DE-A- 43 16 540). Since low densities are necessary for the application of aerogels for heat insulation, lyogels, before drying, have only very low solids levels (e.g. 6 to 8% SiO.sub.2). The remaining 92 to 94% of the gel are solvent that must be evaporated. In the present case the transfer of heat for drying is difficult, since aerogels are excellent thermal insulators. Except in great energy-consuming and costly dielectric drying processes, the already dry aerogel particles inhibit the input of the heat required for drying of the wet particles so much that with conventional processes, despite high temperature gradients, only very lengthy drying courses are possible. On a commercial scale this necessarily leads to large cost-intensive dryers and poor utilization of heat.
The object of the invention was therefore to provide a process for the drying of lyogels to form aerogels that does not have the disadvantages of the known drying processes and can readily be accomplished commercially.
Surprisingly, it has now been found that heat from a liquid is transferred so well to the solvent-wetted gel particles distributed therein that the particles are protected against penetration of the surrounding heat-transfer liquid into the porous solids structure by the escaping vapor, and that, despite low temperature gradients, drying takes place in a fraction of the drying time otherwise required for aerogels.
The subject matter of the invention therefore is a process for the subcritical drying of a lyogel to form an aerogel, characterized in that the lyogel is treated with a heat-transfer liquid which has a temperature above the boiling temperature of the pore liquid of the lyogel under system pressure, and the dried aerogel is then separated from the heat-transfer liquid.
The process according to the invention allows lyogels to be dried to form aerogels in inexpensive fashion and with high utilization of heat.
In principle, all organic and inorganic lyogels which may be used as the preliminary step for an aerogel are suitable as the starting product in the known pathways of synthesis for the process according to the invention (see e.g. Jeffrey Brinker, George W. Scherer, Sol/Gel Science: The Physics and Chemistry of Sol/Gel Processing, Academic Press Ltd., London 1990; U.S. Pat

REFERENCES:
patent: 2503913 (1950-04-01), Kimberlin et al.
patent: 4667417 (1987-05-01), Graser et al.
patent: 5243769 (1993-09-01), Wang et al.
patent: 5473826 (1995-12-01), Kirkbir et al.
patent: 5680713 (1997-10-01), Forbert et al.
patent: 5811031 (1998-09-01), Jansen et al.
PCT Application, WO 94/25149, World Intellectual Property Organization, Nov. 1994.

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