Colloid systems and wetting agents; subcombinations thereof; pro – Continuous or semicontinuous solid phase – The solid phase contains silica
Reexamination Certificate
2000-10-20
2003-12-30
Lovering, Richard D. (Department: 1712)
Colloid systems and wetting agents; subcombinations thereof; pro
Continuous or semicontinuous solid phase
The solid phase contains silica
C034S405000, C034S413000, C423S338000, C501S012000, C516S099000, C516S102000, C516S104000
Reexamination Certificate
active
06670402
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to an improved method for preparing an aerogel product, e.g. bead, composite or monolith, in which the time required to perform solvent exchange and drying, is substantially reduced.
Aerogel products, after wet gel formation, are conventionally prepared by a process of liquid CO
2
extraction of whatever solvent(s) is utilized to form the wet gels followed by a supercritical CO
2
extraction. More particularly, a sol-gel technique is used to prepare wet gels in a solvent such as ethanol or ethyl acetate. The wet gels are placed into a suitable mold and then aged, commonly overnight. As practiced by the assignee of this application in making sample quantities of aerogel products, and as disclosed in, for example, U.S. Pat. No. 5,395,805 to Coronado et al., the solvent must next be removed to form a desired aerogel monolith. To do this, the wet gels are quickly placed into an extractor that is filled with liquid carbon dioxide and a relatively long solvent exchange process begins during which the temperature and pressure are maintained below critical conditions. Once the solvent exchange is complete, the extractor is sealed and the sealed extractor is heated to above the critical point of the CO
2
. After a short thermal stabilization period, the extractor is slowly depressurized while it is heated to maintain the temperature inside the aerogels sufficiently high to avoid condensing the CO
2
as the pressure is decreased to 1 atmosphere.
The time required to perform these steps is highly dependent upon the physical size of the extractor, and the physical size of the extractor determines the maximum physical size of an aerogel monolith piece which can be produced. For example, to prepare five high quality crack-free aerogel monolith panels each of which is 5″×9″×1″, (12.7 cm×22.9 cm×2.5 cm) a 40 liter extractor is used and the extractor time required to produce the monolith panels is about 40 hours. This total time begins with about 5 to 20 minutes to quickly place the wet gels into the extractor. After the extractor is filled with liquid CO
2
, it takes about 30 hours to replace the solvent in the gels using liquid CO
2
. The solvent exchange step takes so long because it must rely on the diffusivity of the solvent and the liquid CO
2
and the solute solubility of liquid CO
2
. It is performed by adding CO
2
into the top of the extractor while draining it out of the bottom until close to 100% of the solvent used to prepare the gels has been extracted. Then it takes about 2 to 2.5 hours to heat the extractor to above the critical point of CO
2
(1070 psi (7378 kPa) and 31.06° C.). It takes this long because the heating must be done at a sufficiently low rate to avoid causing damage to the resulting aerogels. Next there is a thermal stabilization period of about ½ hour. Finally, the depressurization commonly takes about 6 hours.
In total, it currently takes about 40 hours of extractor time to produce a single batch of five 5″×9″ (12.7×22.9 cm) flawless 1″ (2.5 cm) thick aerogel panels in a 40 liter extractor.
The length of time for aerogel drying is also dependent upon the pore size distribution, tortuosity of the pores and thickness of the aerogel products being prepared since it is the thickness, i.e. the smallest dimension, that determines the distance required for heat and mass diffusion during the drying. The times needed for solvent exchange and depressurization steps vary approximately proportionally to the thickness squared.
Quite simply, this time period has been found to be far too long for alerogel products to be cost competitive with alternative products, e.g. other types of insulation. Moreover, the time is highly dependent upon the physical size &f he extractor, and larger extractors would require an even greater operating time for a single batch of the same sized and shaped gels so that the initial capital investment for large scale production of large aerogel monoliths is too high. In addition to the physical -size and shape of wet gels to be dried, the solvent exchange step depends upon the total amount of solvent that must be extracted. The heating step requires heat to be applied on the extractor walls and then travel through liquid CO
2
to reach the gels while avoiding a temperature gradient that is so steep that it causes thermal shock or damage to the still wet gels. And the depressurization is conducted very slowly to supply an adequate amount of heat, again through the extractor walls, to heat the immediate layers of CO
2
that in turn has to transmit the heat throughout the entire aerogel volume and the extractor to minimize the possibility of thermal and fluid dynamic-induced damage.
The present invention is the result of research focused on reducing the processing time for preparing aerogel products once wet gels have been placed inside an extractor for supercritical drying.
It is an object of the present invention to substantially reduce the time needed for supercritical drying of wet gels to form an aerogel product.
It is a further object to rapidly produce aerogel products while avoiding creating surface tension induced failures within the aerogels.
It is a still further object to produce aerogel products while maintaining the temperature within the wet gels sufficiently spatially uniform to avoid thermal-induced stress fractures within the gels.
It is a still further object to produce aerogel products while maintaining the fluid surrounding the wet gels at substantially the same temperature and pressure as the fluid within the wet gels.
These and still further objects will be apparent from the following detailed description of this invention.
SUMMARY OF THE INVENTION
This invention is directed to methods of preparing aerogel products by an improved supercritical drying process.
More particularly, this invention is directed to the preparation and/or loading of gels at process temperature to eliminate extractor time to reach the process temperature after the loading.
More particularly, this invention is directed to maintaining the extractor wall temperature at the process temperature to eliminate the time to heat the solid mass of the extractor that will be well insulated thermally.
More particularly, this invention is directed to the use of gaseous CO
2
to pre-pressurize an extractor that is loaded with wet gels for flash-free fast injection of supercritical CO
2
without causing any flow-induced damage to the gel structures.
More particularly, this invention is directed to the use of CO
2
the temperature of which is about the supercritical extraction process temperature and the gel temperature to pre-pressurize an extractor loaded with wet gels without causing any temperature gradient induced damage to the gel structures during pre-pressurization.
More particularly, this invention is directed to the use of gaseous CO
2
injected into an extractor as a means of displacing the bulk of the free solvent before super-critical CO
2
is injected into the extractor.
More particularly, this invention is directed to the use of supercritical CO
2
injected into an extractor to displace the bulk of the solvent before supercritical CO
2
is injected into the extractor.
More particularly, this invention is directed to the use of supercritical CO
2
injection as a means of direct heat exchange into the supercritical CO
2
in the extractor during depressurization to prevent condensation of supercritical CO
2
. This eliminates most of the solvent remaining in the gels as the supercritical CO
2
is removed from the gel by depressurization to just below critical pressure.
More particularly, this invention is directed to the use of a non-reacting, non-condensing gas as a means of direct heat exchange into gaseous CO
2
and gas exchange with gaseous CO
2
inside the gels during depressurization to prevent condensation of CO
2
. This significantly shortens the duration of depressurization compared to the conventional slow depres
Altiparmakov Zlatko
Begag Redouane
Lee Kang P.
Aspen Aerogels, Inc.
Jacobs Bruce F.
Lovering Richard D.
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