|Title of Invention
A METHOD FOR IMPROVING THE EFFICIENCY OF EXCHANGING A FLUID
|A method for improving the efficiency of exchanging a first fluid such as herein described within a get by a second fluid such as herein described comprising applying pulses of pressure having at least one freouency to the gel, the first fluid and the second fluid during the exchange.
|RAPID AEROGEL PRODUCTION PROCESS
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 CO2 extraction of whatever solvent(s) is utilized to form the wet
gels followed by a supercritical CO2 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 CO2. 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 CO2 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" x 9"
x 1", (12.7 cm x 22.9 cm x 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 CO2, it takes about 30 hours to replace the solvent in
the gels using liquid CO2. The solvent exchange step takes so long because it must
rely on the diffusivity of the solvent and the liquid CO2 and the solute solubility of
liquid CO2. It is performed by adding CO2 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 CO2 (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" x 9" (12.7 x 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 pre-
pared 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 aerogel
products to be cost competitive with alternative products, e.g. other types of insula-
tion. Moreover, the time is highly dependent upon the physical size of the 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 CO2 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 CO2 that in turn has to transmit the heat throughout the entire
aerogel volume and the extractor to minimize the possibility of thermal and fluid
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 CO2 to pre-
pressurize an extractor that is loaded with wet gels for flash-free fast injection of
supercritical CO2 without causing any flow-induced damage to the gel structures.
More particularly, this invention is directed to the use of CO2 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 CO2 injected
into an extractor as a means of displacing the bulk of the free solvent before
supercritical CO2 is injected into the extractor.
More particularly, this invention is directed to the use of supercritical CO2
injected into an extractor to displace the bulk of the solvent before supercritical CO2
is injected into the extractor.
More particularly, this invention is directed to the use of supercritical CO2
injection as a means of direct heat exchange into the supercritical CO2 in the
extractor during depressurization to prevent condensation of supercritical CO2. This
eliminates most of the solvent remaining in the gels as the supercritical CO2 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 CO2 and gas
exchange with gaseous CO2 inside the gels during depressurization to prevent
condensation of CO2. This significantly shortens the duration of depressurization
compared to the conventional slow depressurization in that heat is supplied indirectly
through the extractor wall.
More particularly, this invention is directed to the use of a liquid pump
followed by a heat exchanger to increase the pressure and temperature of CO2 in a
pipe to supercritical conditions to feed into an extractor.
More particularly, this invention is directed to the use of a liquid pump
followed by a heat exchanger to increase the pressure and temperature of CO2 in an
extractor to supercritical conditions.
More particularly, this invention is directed to the use of continuous flow
supercritical CO2 extraction right from the outset to perform solvent exchange and
More particularly, this invention is directed to the use of a non-reacting, non-
condensing gas to remove most of the gaseous CO2 during depressurization to sig-
nificantly reduce the total amount of heating required during depressurization to
prevent condensation of the remaining CO2. The use of the non-reacting, non-
condensing gas also enables direct heating of aerogels by a non-reacting non-
condensing gas during the depressurization.
More particularly, this invention is directed to the use of a non-reacting non-
condensing gas as a direct heating medium during depressurization. The gas takes
the heat energy to the gel pieces where the heat energy is needed to significantly
reduce depressurization time.
More particularly, this invention is directed to an aerogel drying process the
duration of which is substantially equipment scale/size insensitive.
More particularly, this invention is directed to the use of pressure fluctuation
to enhance the solvent exchange procedure for the wet gels such as water/ethanol
exchange for wet gels made from water glass and acid catalysts.
More particularly, this invention is directed to the use of pressure fluctuations
to enhance the supercritical fluid/solvent exchange process, in that high frequency
fluctuations increase the effective mass and heat diffusivity at the interface between
the supercritical fluid phase and the solvent, and low frequency fluctuations increase
the effective mass transport and heat transfer rates through the gel structure.
More particularly, this invention is directed to the use of pressure fluctuation
to reduce the time required for depressurization and still avoid condensation of the
super-critical fluid into a liquid by low frequency pressure fluctuations that increase
the effective mass transport and heat transfer rates through the gel structure.
DETAILED DESCRIPTION OF THE INVENTION
The process of the present invention is directed to an improved process for the
manufacture of any aerogel product, including beads, monoliths and composites.
As used herein, an "aerogel" includes (unless context requires a narrower
mean-ing) not only a conventional aerogel, but also similar structures that have a
micro-porous or nanoporous lattice structure from which a solvent has been removed,
such as a xerogel, silica gel, zeolite, or water glass. The term "beads" refers to
aerogel bodies of generally spherical shape having a diameter, that is typically in the
range of tenths of millimeters to about a centimeter. The term "monolith" refers to
a single aerogel body having a minimum dimension, i.e. thickness, with the other two
dimensions being larger than the thickness, or to a cylindrical object having a
diameter. The thickness or diameter is typically in the range of millimeters to tens
of centimeters. The term "composite" refers to an aerogel that has been formed with
another substance, e.g. glass fibers, in the gels.
The term "solvent" refers to the liquid dispersion medium used to form the
gels and that is removed to form aerogels in accordance with this invention. It is a
non-supercritical fluid at the pressure and temperature of interest.
The term "gas" denotes a fluid where the pressure is below the supercritical
pressure for that fluid and the temperature is higher than the vapor pressure at the
The term "fluid" refers to any of a gas, a vapor, and a liquid.
The term "supercritical fluid," refers to a fluid having a pressure above the
critical pressure and a temperature above the critical temperature required to make
a particular fluid supercritical.
The term "pulse" refers to a brief disturbance of pressure in a fluid by the
application of vibrational energy, generally in the form of separate and discrete
pulses, for example a shock wave or a cycle of a continuous wave, or a discrete
period of application of a continuous wave. The pulse (or wave) preferably has a
sinusoidal wave form, but other wave forms, e.g. saw-tooth, square, Gaussian, and
harmonics of any of these, may be used. The frequency or the amplitude of a series
of pulses can be ramped.
While the process of the present invention is generally described hereinafter
referring to supercritical carbon dioxide as the supercritical extraction fluid, all such
references are intended to include alternative supercritical extraction fluids unless
otherwise specified as specific to carbon dioxide. All references to "critical tempera-
ture," "critical pressure," and "critical conditions," refer to the temperature and
pressure conditions that apply for the specific supercritical fluid being discussed.
Aerogels are open pore materials with about 80 or more vol. % porosity and
pore sizes ranging from about 0.5 to 500 nanometers. Aerogels may be prepared
from any gel-forming materials from which the solvent used for gelation can be
removed by drying without destroying or substantially shrinking the pore structures
during the drying. The drying can be accomplished through supercritical extraction,
atmospheric drying, freeze drying, vacuum evacuation, or the like. Preferably, aero-
gels are produced by supercritical extraction of the solvent (or any liquid replace-
ment for the solvent) that was used to prepare the starting gels. Preferably aerogels
possess a porosity of at least 85 vol. %, more preferably about 90 vol. % and higher.
For purposes of the present invention, aerogels include xerogels which are
prepared by air evaporation, i.e. by slow direct drying, without a supercritical extrac-
tion step. This is typically accomplished by including a surfactant or pore surface
modifier in the gel-forming mixture. Either additive sharply reduces the surface
tension and thus the force exerted on the gel by the evaporation fluid, and/or imbues
a springback force to reverse pore shrinkage during drying. The drying time is also
very long for xerogels. U.S. Pat. No. 5,877,100 to Smith et al. describes a composite
form of a xerogel-like material. For a detailed discussion regarding the production
of both aerogels and xerogels, see Aerogels: Proceedings of the First International
Symposium, Wurzburg, Federal Republic of Germany, Sep. 23-25, 1985, J. Fricke,
ed., Springer-Verlag, Berlin-Heidelberg (1986).
The aerogels of the present invention may be organic, inorganic, or a mixture
thereof. The wet gels used to prepare the aerogels may be prepared by any of the gel-
forming techniques that are well-known to the art including alcogel, hydrogel,
templating, and the like. These techniques are all merely different forms of polym-
erization depending on the solvent or method used for attaining particular micro-
structures. Suitable materials for forming inorganic aerogels are oxides of most of
the metals that can form oxides such as silicon, aluminum, iron, copper, zirconium,
hafnium, magnesium, yttrium, etc. Suitable materials for forming organic aerogels
are polyacrylates, polystyrenes, polyacrylonitriles, polyurethanes, polyimides,
polyfurfural alcohol, phenol furfuryl alcohol, polyfurfuryl alcohol, melamine for-
maldehydes, resorcinol formaldehydes, cresol formaldehyde, phenol formaldehyde,
polyvinyl alcohol dialdehyde, polycyanurates, polyacrylamides, various epoxies,
agar, and the like.
Without being bound to a specific type of aerogel or its method of preparation,
for the sake of convenience the alcogel route of forming inorganic aerogels is used
below to illustrate the invention. The invention is applicable to other aerogels and
Generally the principal synthetic route for the formation of an inorganic
aerogel is the hydrolysis and condensation of an alkoxide. The most suitable metal
alkoxides are those having about 1 to 6 carbon atoms, preferably about 2 to 4 carbon
atoms, in each alkyl group. Specific examples of such compounds include tetra-
ethoxysilane (TEOS), tetramethoxysilane (TMOS), tetra-n-propoxysilane, aluminum
isopropoxide, aluminum sec-butoxide, cerium isopropoxide, hafnium tert-butoxide,
magnesium aluminum isopropoxide, yttrium isopropoxide, zirconium isopropoxide,
and the like.
Suitable materials for use in forming the aerogels to be used at low
temperatures are the non-refractory metal alkoxides based on oxide-forming metals.
Preferred such metals are silicon and magnesium as well as a mixture thereof. For
higher temperature applications, suitable alkoxides are generally refractory metal
alkoxides that will form oxides, e.g. such as zirconia, yttria, hafnia, alumina, titania,
ceria, and the like, as well as mixtures thereof such as zirconia and yttria. Mixtures
of non-refractory metals with refractory metals, such as silicon and/or magnesium
with aluminum, may also be used.
Major variables in the inorganic aerogel formation process include the type of
alkoxide, solution pH, and alkoxide/alcohol/watcr ratio. Control of these varia-bles
can permit control of the growth and aggregation of the aerogel species through-out
the transition from the "sol" state to the "gel" state during drying at supercritical
conditions. While properties of the resulting aerogels are strongly affected by the
pH of the precursor solution and the molar ratio of the reactants, any pH and any
molar ratio that permits the formation of gels may be used in the present invention.
Generally, the solvent will be a lower alcohol, i.e. an alcohol having 1 to 6
carbon atoms, preferably 2 to 4, although other liquids can be used as is known in the
art. Examples of other useful liquids include but not limited to: ethyl acetate,
acetone, dichloromethane, and the like.
For silica aerogel-containing low temperature insulation, the currently
preferred ingredients are tetraethoxysilane (TEOS), water and ethanol (EtOH) and
the preferred ratio of TEOS to water is about 0.2-0.5:1, the preferred ratio of TEOS
to EtOH is about 0.02-0.5:1, and the preferred pH is about 2 to 9. The natural pH of
a solution of the ingredients is about 5. While any acid may be used to obtain a
lower pH solution, HC1, H2SO4 or HF are the currently preferred acids. To generate
a higher pH, NH4OH is the preferred base.
After identification of the aerogel to be prepared, a suitable metal alkoxide-
alcohol solution is prepared. While techniques for preparing specific solutions are
described below, the preparation of aerogel-forming solutions in general, and having
specific compositions, is well known in the art. See, for example, S.J. Teichner et al,
"Inorganic Oxide Aerogel," Advances in Colloid and Interface Science, Vol. 5, 1976,
pp 245-273, and L.D. LeMay, et al., "Low-Density Microcellular Materials," MRS
Bulletin, Vol. 15, 1990, p 19.
While a single alkoxide-alcohol solution is generally used, a combination of
two or more alkoxide-alcohol solutions may be used to fabricate mixed oxide
aerogels. After formation of the alkoxide-alcohol solution, water is added to cause
hydrolysis so that a metal hydroxide in a "sol" state is present. The hydrolysis
reaction, using tetraethoxysilane as an example, is:
Si(OC2H5)4 + 4 H2O - Si(OH)4 + 4 (C2H5OH) (1)
To form an aerogel monolith, this sol state alkoxide solution is then aged for a
sufficiently long period (commonly overnight) that a condensation reaction, as shown
in Eq. 2:
Si(OH)4 ? SiO2 + 2 H2O (2)
occurs and forms precursors which after supercritical drying in accordance with this
invention become aerogels.
EXTRACTION OF SOLVENT-WETTED GELS
The wet gels that form an aerogel product of this invention may be prepared
by any gel-forming procedure as previously described. The present invention is
independent of the gel-forming technique and specific gel-forming materials, and is
believed to be widely applicable to the solvent extraction and drying of such gels.
Drying of the cast wet gels is slow, and requires prolonged operations under
high pressure and controlled temperature. As noted above, this significantly raises
the cost of aerogels, and prevents their use in numerous otherwise desirable
situations. A good description of the most advanced techniques of the current art
may be found in M. J. van Bommell and A. B de Haan, J. Materials Sci. 29 943-948,
The wet gels are formed in or placed into a suitable mold and then aged, com-
monly overnight. In prior art methods, the gels at room temperature are placed into
an extractor at room temperature. In one embodiment of the invention, the wet gels,
either before or after molding, are brought to a temperature above the critical
temperature of the extraction fluid, most commonly CO2 (though other extraction
fluids may be used), by any suitable means that does not cause thermal damage to the
gels. For example, if the wet gels have been prepared at room temperature, they may
be placed in a solvent bath and gradually heated to the appropriate temperature. The
maximum heating rate is determined by the thermal conductivity of the particular gel
formulation, and should be determined experimentally so that cracking is avoided.
Alternatively, the gels may be prepared in a suitable solvent at a temperature about
or above the critical temperature. Still further alternatively, the gels may be placed
in a solvent bath that has been heated to somewhere below the critical temperature,
and then the gels and the solvent bath are heated to above the critical temperature.
In a second embodiment of the method of the invention, the wet gels are
inserted into a heated unpressurized extractor, the walls of which are maintained at
a temperature above the critical temperature of CO2. So long as no problem with
unwanted drying of the gels occurs during loading, then only wet gels need be
loaded, i.e. without additional solvent. If the wet gels would suffer damage due to
drying during loading, then they can be loaded with solvent, preferably inside a
container that is inserted into the extractor. Alternatively, the wet gels can be loaded
into an extractor that has previously been filled with saturated vapor of the solvent
to prevent drying of the wet gel surfaces. The loading process for gel monoliths is
generally performed either by lowering the gel monoliths into a vertical chamber or
by horizontally inserting them into a horizontally-positioned extractor, i.e. by open-
ing the lid or one side of the extractor. The loading process for beads is generally
per-formed by introducing the beads in solvent or a carrier gas, pouring by gravity
or pumping into the extractor through a valve or opening. If the wet gels are not
loaded with additional solvent, the extractor is then filled with extra solvent in liquid
or gaseous form and at a temperature about or above the critical temperature to be
used in the eventual supercritical extraction as described further below.
The next step is the removal of the solvent, both any free solvent surrounding
the wet gels and that solvent contained within the gels. In the prior art, this has been
done either by extraction of the solvent with liquid CO2 (or other suitable fluid.) or
as described in M. J. van Bommell et al, supra, with supercritical CO2.
In the method of the invention, the removal begins in any of several ways in
which the wet gels are al a temperature that is about or above the process temperature
but at a pressure below the eventual process pressure. The solvent removal from the
gels is then completed by addition of a supercritical fluid, e.g. CO2.
For example, if the gels in the extractor are surrounded by solvent outside the
wet gels, that solvent (referred to as "free solvent") can be drained by opening a
valve at the bottom of the extractor. If the gels and solvent are in a container within
the extractor, it is preferred that the container be opened both on top and bottom to
enhance fluid flow. Then the extractor is pressurized by injecting into the extractor
gaseous CO2 at a temperature above the critical temperature and at a pressure that is
below the critical pressure. The gaseous CO2 is above the critical temperature and
it has a much lower density than the liquid solvent. The gaseous CO2 is fed prefer-
ably from the top and will form a bubble space gradually expanding from the top to
squeeze the free solvent toward the bottom of the extractor where it is discharged.
The free solvent is preferably continually separated from the gas and recovered.
Once the bulk of the solvent has been discharged, the gaseous CO2 injection stops.
At this point in the process, the temperature inside the extractor is substantially the
same as that which will be used in the eventual supercritical solvent extraction
process and the pressure is slightly below the critical pressure. Now the supercritical
extraction of the solvent from within the gels is performed by feeding supercritical
CO2 (or other supercritical extraction fluid) into the extractor, either from the top or
the bottom, to remove by diffusion the solvent inside the gels. Supercritical CO2 has
a solute diffusivity that is about 10 times higher than that of liquid CO2 while having
a viscosity that is only about one tenth that of liquid CO2. Thus the diffusion/infil-
tration/extraction is more rapid with supercritical CO2 than with liquid CO2. As the
supercritical CO2 feeding continues, the extractor pressure increases beyond the
critical pressure and all of the CO2 inside the extractor turns supercritical.
Once the extractor has become filled with supercritical CO2, supercritical CO2
containing extracted solvent is continually drained from the bottom of the extractor,
since it is of higher density than pure supercritical CO2. The supercritical CO2 injec-
tion docs not create any violent fluid motion since the extractor is already near, i.e.
within about 1380 kPa, preferably about 345 kPa, and most preferably about 69 kPa
of the supercritical process pressure. For supercritical CO2 the pressure must be
above 7,378 kPa. Preferably it is about 7,585 to 12,411 kPa. The injection process
does not create any thermal shock because the temperatures of the solvent, gels, and
the supercritical CO2 are all practically identical.
An alternative when the gels are in free solvent in the extractor avoids the
initial draining of the solvent. Rather, the clearance volume of the extractor is di-
rectly pressurized by injecting gaseous CO2 at about or above the critical temperature
but below the critical pressure, into the clearance space below the top of the extractor
and above the top surface of the solvent covering the wet gels. The CO2 injection
continues until the pressure builds beyond the critical pressure and all of the CO2
turns supercritical. At this point injection of supercritical CO2 begins and a valve
at or near the bottom of the extractor is opened to allow discharge of the solvent. In
this case the supercritical CO2 that forms an expanding "bubble" from the top, simul-
taneously removing mostly free solvent along with a small amount of the solvent
within the wet gels, by forcing the solvent out the bottom of the extractor. After the
supercritical pressure is reached, then the solvent is drained by the further infusion
of supercritical CO2 into the top of the extractor at substantially the same tempera-
ture as the gels/sol-vent. This process does not create any violent fluid motion since
the clearance volume above the solvent is already pressurized to just below the
critical pressure and the wet gels are immersed in the solvent when supercritical CO2
is first present within the extractor. The injection process does not create any
thermal shock because the temperatures of the solvent, gels and the supercritical CO2
are all substantially the same.
An alternative to the addition of gaseous CO2 into the clearance volume is the
direct injection of supercritical CO2 into that space while the space is at about the
desired supercritical temperature. In this case, the supercritical CO2 will initially
expand into the clearance space and cool down until the pressure builds up to and
beyond the critical pressure Since the wet gels arc immersed in and protected by the
solvent when the supercritical CO2 is added and it expands into the clearance
volume, there will be no thermal or fluid dynamic shock.
A further alternative when the wet gels (either by themselves or on a carrying
tray) are loaded into an extractor without excess solvent is the use of gaseous CO2
to pre-pressurize the extractor to just below the critical pressure, followed by
injection of supercritical CO2 into the extractor. The gaseous CO2 pre-pressurization
does not create any violent fluid motion since it occurs gradually. The supercritical
CO2 injection does not create any violent fluid motion since the extractor is already
substantially at the process pressure. The supercritical CO2 injection does not create
any thermal shock because the temperatures of the solvent, gels and the supercritical
CO2 are all practically identical.
Several steps are proposed here to accelerate the process of preparing an
aerogel. First, during or after the gel being prepared, the temperature of the gel is
raised to near, preferably above, the critical temperature of the extracting super-
critical fluid by the point in time when the gel is to be placed into the extractor.
Likewise, the extractor, and the extracting fluid, and any excess solvent required to
prevent drying, should also be near or preferably at least just above the critical
temperature. Then the wet gel and solvent can be pressurized as rapidly as the gel
can withstand the pressure gradient. A temperature is sufficiently near the critical
temperature for purposes of this invention if it is not more than about 10°C,
preferably not more than about 5°, and more prefer-ably not more than about 2°C
below the critical temperature.
Second, the extracting fluid can be introduced directly in the supercritical
state, or rapidly compressed (with due control of heating) to create extracting liquid
in the supercritical state in the extractor. Then the extraction of the solvent is
performed with supercritical rather than liquid CO2. Because of its increased
diffusivity, extraction with supercritical CO2 is faster than with liquid CO2.
In large scale production, these steps can save significant time compared to
the prior art, by reducing the time required to prepare the system to begin the process
of extracting the alcohol from the wet aerogel with supercritical carbon dioxide.
EXTRACTION OF SOLVENT / INCREASE IN EFFICIENCY BY PULSATIONS
As described above, the current state-of-the art extraction process is
performed by circulating an extraction fluid, either liquid or supercritical, past the
solvent-wet gels, and (in an industrial-scale operation) separating the supercritical
gas from the extracted solvent followed by re-introducing it into the process.
Because the pores in the gels are small, liquid flows through the gels very slowly if
at all. Instead, the primary method of solvent removal from the gels is by diffusion
of solvent out of the gels and into the stream of circulating extraction fluid and
diffusion of the extraction fluid into the gel. The inter-diffusion is inherently and
relatively slow even using supercritical CO2 instead of liquid CO2, and the time
required tends to increase with the square of the thickness of the aerogel body being
produced. In laboratory situations, a few hours may be needed for the exchange. In
production, however, ten to thirty hours may be required to reliably produce multiple
crack-free aerogels of reasonable thickness such as 1 inch. This makes the process
expensive to be commercially viable for wide scale applications.
The efficiency of the solvent exchange procedures with an extraction fluid may
be enhanced by increasing the fluid's effective mass diffusivity. More particu-larly,
improved solvent exchange efficiency may be obtained by cycling or pulsing the
extractor pressure. For example, high frequency low amplitude pressure fluctua-
tions can be used to promote mixing and mass diffusion. Alternatively, low frequen-
cy high amplitude pulsations can be used to effectively pump out higher solvent
concentration solution (e.g. solution of ethanol in supercritical CO2) from inside the
gels and pump in lower solvent concentrated solution into the gel if the extraction
fluid is compressible which is the case with supercritical fluids such as CO2. Prefer-
ably two different pulsations arc used simultaneously for compressible fluids. The
pressure cycling/pulsations result in an active pumping and/or enhanced diffusion
and mixing process that is more effective than passively relying on simple diffusion
of solvent from the gels into the supercritical fluid at slowly changing or constant
The fluid exchange process is considered to be satisfactorily performed by the
method of extraction when the solvent content in the extraction fluid at the discharge
of the extractor is less than a predetermined level, the exact value of which will
depend upon the specific process being performed, the properties of the fluids
involved (the diffusivities ands viscosities), pore size distribution, physical size and
shapes of the gels being processed, as well as the frequencies and amplitudes of the
pulsations used. Generally however, satisfactory levels will be less than about SO
ppm, preferably less than about 20 ppm, and most preferably less than 1 ppm,
provided that the discharge solvent content is representative of the solvent content'
within the gels.
Without wishing to be bound by a theory, it is applicants' belief that high
frequency pulsing accelerates fluid exchange within an aerogel because pulsing
rapidly dilutes the solvent that is near the boundary between the extraction fluid
phase and the liquid solvent phase, inside the gels. This applies to both liquid/liquid
and liquid/-supercritical fluid exchanges.
For mixtures of supercritical fluid and solvent, there is a single phase region,
called a "mixed fluid supercritical region," wherein the mixture with the dissolved
solvent is supercritical. It has unexpectedly been discovered that the single phase
region of supercritical conditions for many solvent/supercritical fluid mixtures
occurs sufficiently near the critical point of the supercritical fluid to use that super-
criticality of the mixed fluid to enhance the rate of extraction of the solvent from
within the gels, thereby reducing the overall process time.
Using the example of ethanol as the solvent and CO2 as the supercritical fluid,
a mixture containing about 50% of each is a single phase and supercritical at
pressures above about 7,585 kPa) and at temperatures above about 35°C. This is near
the usual operating pressures and temperatures for maintaining CO2 in the
supercritical state. Therefore, at the initial moment of contact between the wet gels
and the super-critical fluid, inter-diffusion begins at the interface between the
solvent ethanol inside the wet gels and the supercritical CO2 outside the gels will
begin. The inter-diffusion is enhanced by transmitting a high frequency fluctuation
through the super-critical CO2- As the mixing at the interface continues, the thick-
ness of the mixing region increases. Soon the external portion of the mixing layer
will reach a threshold of "turning supercritical." Since a supercritical fluid is readily
compressible like a gas, as opposed to poorly compressible like a liquid, when the
mixed fluid turns supercritical it is in corn-pressed form both within the extractor
generally and within the gel specifically, more of the molecules will on average move
into the gel. When the gas re-expands, molecules - not necessarily the same
molecules - move out of the "mixed fluid super-critical region" of the gel. Then,
when the next pulse compresses the supercritical fluid, a fresh load of supercritical
CO2 is pushed into the mixed fluid supercritical layer that has by now increased in
thickness. Therefore, to the extent that there is mixing or mutual inter-diffusion
between the solvent liquid and the supercritical mixed fluid, then molecules of the
solvent liquid are mixed into the supercritical mixed fluid and the new mixed fluid
remains supercritical and is removed from the aerogel. This solvent removal is
supplemental to the solvent removal due to pure diffusion, and is much faster.
Without wishing to be bound by a particular theory, the mechanism of
diffusion enhancement by high frequency pressure pulses at the interface region of
the solvent (liquid ethanol) and the supercritical mixed fluid phase (containing CO2
and ethanol) is believed to be due to differences in wave propagation speed and
acoustic impedance within the solvent vs. within the supercritical mixed fluid phase.
The pressure wave will first travel through the supercritical phase outside the gel,
then through the mixed fluid supercritical phase near and in the gel, and then arrive
at the interface with the solvent liquid in the gel. Due to the impedance discontinu-
ity, the pressure wave will be split into two waves at the interface: a transmitted
wave and a reflected wave. The fluid particles at two sides of the interface region
will tend to move at different speeds due to different wave propagation speeds. To
accommodate the impedance discontinuity-induced wave phenomena and the particle
velocity discrepancies between the two sides, the interface region between the
portion of the gel still containing solvent liquid and the rest of the gel containing
supercritical mixed fluid will be perturbed and well mixed, thereby promoting
enhanced diffusion across the interface region. This pulse-enhanced diffusion is
much faster than natural diffusion.
As the enhanced diffusion process proceeds, the interface region moves in the
direction of the remaining solvent liquid region of the gel until that region complete-
ly disappears and the entire gel structure contains only supercritical phase fluid.
Once this happens, the whole gel structure participates in the mass transport
enhanced mostly by slower pulses that generate longer distance pumping effect. The
pumping action of the slower pressure pulses rapidly lowers the solvent concentra-
tion inside the gel at a rate much faster than simple diffusion process relying on
concentration gradient. When the concentration of the solvent in the supercritical
phase at the innermost portion of the gel, or the highest local concentration of the
solvent inside the gel, reaches a low level, e.g. less than about 50 ppm, preferably
less than about 20 ppm, and most preferably less than 1 ppm, the solvent extraction
process is considered finished and depressurization can begin.
For low frequency high amplitude pulses are used, they serve to enhance
solvent removal in the following manner. During the expansion period with low
frequency (long wavelength) fluctuations, supercritical mixed fluid having a higher
concentration of dissolved solvent will flow out of the aerogels enhancing the rate
of solvent removal from the gel while during the compression period the supercritical
fluid having a lower concentration of solvent will be forced back into the gel replen-
ishing the gel with fresh supercritical fluid charge at a lower solvent concentration
and at the process temperature of the extractor. For example, the density of super-
critical CO2 will nearly double when the pressure increases from 7585 to 10,340 kPa
at 40°C. In other words, after a compression swing of this amount, fully 50% of the
molecules inside the "mixed-fluid supercritical" layer inside the gel will come from
the fresh supercritical CO2 stream outside the gel, thereby lowering the concentration
of the solvent in the "mixed fluid supercritical layer" and supplying heat to the
"mixed-fluid supercritical" layer that had undergone an expansion-related
temperature drop. During the subsequent expansion stroke, the heat provided during
compression will prevent condensation of the expanding fluid and fully 50% of the
molecules inside the "supercritical** layer will come out of the gel into the super-
critical fluid to be swept away. The low frequency high amplitude compression and
expansion cycles can be repeated until the entire aerogel body is engulfed by "mixed-
fluid supercritical" region and until the innermost part of the aerogel contains mostly
supercritical fluid with only trace amount of solvent.
Also as the pressure increases during the low frequency compression period
and as the fluid is pushed into the gel, the solubility of the supercritical fluid
increases almost as a linear function of the supercritical fluid density, thereby
promoting the diffusion/solvation process inside the gel. Once the solvent has
dissolved into the supercritical fluid and the pressure has decreased, the density of
the supercritical fluid lowers causing the fluid to expand out of the gel. The increase
in solute solubility of a supercritical fluid such as supercritical CO2 with compres-
sion is an additional factor enhancing diffusion of solute from within the gels.
Moreover, it has been found useful to gradually increase the wavelength (decrease
the frequency) of the pressure pulses as the front between the supercritical fluid and
the normal solvent moves into the aerogel and therefore the distance to travel from
the mixing layer to the surface of the gel increases.
Therefore, there is independent utility in each of high-frequency pulses, low-
frequency pulses, and ramping upward (gradual increase) of the wavelength of at
least the low frequency pulses. The combination of two different frequency of
pulses, with optional ramping, is expected to be especially effective.
The amplitude of the high frequency pulses at a given frequency is less
critical. Higher amplitudes will tend to accelerate the exchange process - ideally
linearly, but in practice at less than linearly due to dissipation. The amplitude at a
given frequency also has an upper limit, above which the gradient of pressure during
a pulse is large enough to damage the structure of the gel. Because the pores of
aerogels are so small, the frictional force exerted on the gel structure by passage of
fluid is surprisingly large. For many aerogels, the upper limit will be in the range of
34.5 kPa or so. A pressure amplitude range of about 0.7 to 27.6 kPa will be typical
for most aerogel materials. The maximum permitted pressure amplitude is dependent
on the frequency or wavelength of the sound waves. This is because as the frequency
increases, the rate of gas movement increases, and this can place a higher pressure
gradient across local regions of the aerogel than is found at lower frequencies of the
For a supercritical CO2 extraction (generally performed at pressures of about
7585 to 12,411 kPa), suitable high frequency pulses will have a frequency in the
range of about 1 to about 100,000 Hz, more typically 2,000 to 50,000 Hz, and in
many cases in the range of about 5000 to 30,000 Hz. Corresponding maximum
allowable pressure amplitudes, which will decrease as the frequency increases and
which will depend on the pore structure of the gel, will typically be in the range of
about 0.07 to about 137.9 kPa, more typically about 2.1 to 34.5 kPa, and often 3.4 to
For the slower pulses, the frequency can be in the range of about 0.0001 to
about 10 Hz, more typically in a range of about 0.001 to about 1 Hz. Pressure
amplitudes generally range from about 69 kPa to up to 6,895 kPa, more preferably
689.5 kPa to 4,137 kPa, provided that the material can tolerate the pressure gradient,
and allowing for the pressure amplitude of the high frequency pulses when used
Specific pressure amplitude/frequency combinations have to be determined for
particular compositions of aerogels by routine experimentation taking into considera-
tion the specific porosity, pore size distribution, compressive and tensile strengths
of the aerogel lattice structure and physical size and shape. The aerogels are not
damaged during the active extraction process either by fluid dynamic erosion,
pressure difference induced stress, or otherwise. Also, the resulting temperature
swing is not so large as to cause stress failures or loss of supercriticality of the fluid
inside the gel.
Other frequencies or wavelengths be used. It is specifically contemplated that
higher frequencies, for example in the range of 100,000 to 10 million Hz (used in
ultrasound and lithotripsy), may prove to be as useful or more useful than the
presently explored range of about 1 to about 100,000 Hz. Such faster cycles require
lower amplitudes to avoid creating excessive pressure gradients.
In selecting a pulse amplitude, it should also be recalled that an excessive
pressure drop, starting from a particular pressure and temperature, can cause a phase
change of a supercritical fluid into a conventional liquid or gas. If the amplitude is
sufficiently large, it can also cause re-condensation of the solvent into a separate
liquid phase due to a reduction in solubility when the density is reduced by pressure
reduction even though the extraction fluid remains supercritical. If the phase of the
extraction fluid changes from supercritical to a gas, most of the solvent will
recondense due to a drastic reduction in solubility.
If appropriately limited, however, a moderate degree of lowering of the
pressure or density will not cause re-condensation of the solvent, since most super-
critical fluids have very high solute solubility for the usual solvents used to form
aerogels. It should be noted that during pressure fluctuations, the shape and size of
the gel lattices and their pores do not undergo any appreciable dimensional changes
because the pressure will remain sufficiently balanced isometrically provided that (i)
the speed of the change is slow enough to be quasi-steady for the slower pressure
fluctuations, and (ii) the amplitudes of the faster fluctuations are much smaller than
the mean pressure and lower than the threshold pressure to cause structural changes.
There is hydrostatic quasi-equilibrium inside the entire gel volume and during the
cycling that status does not change.
The pressure fluctuation process relies, in one mode, on the fact that super-
critical CO2 behaves like a gas in terms of compressibility. So when compressed
more supercritical CO2 can be packed into the same pore volume as before and when
expanded, the solvent laden supercritical CO2 tends to come out of the gels.
Pulses suitable for the practice of the invention may be generated by any
means or method that gives the required frequency and amplitude of pulsations in
pressure inside the extractor. The source of the pulses can be inside the extractor;
outside the reactor (and typically in intimated contact with it); or forming a part of
the reactor. The pulses may be generated by one or more of a piezoelectric device,
an electromechanical device, a piston, a mechanical device, a diaphragm, a bellows,
an inflatable device, or by variation of the input pressure or the backpressure of a
fluid or a gas flowing through the extractor. For example, a piezo-electric device can
be the driver for a hydrophone, and an electromechanical device can be a solenoid,
as is used in a loudspeaker. A mechanical device could include a striking hammer,
as is used to strike a bell. An inflatable device could be an expandable balloon or
bellows, either within the extractor or exterior to it and connected by a port. An
inflatable device could be inflated by a gas or liquid. Likewise, a piston could be
internal, or external via a port, and could be moved by pressure or by mechanical
force. Each of these ways of generating a series of pressure waves is well known.
For example, back pressure can be varied under electronic control by opening and
partially closing the exit port or the entrance port of an extractor (or other closed
vessel) while applying a constant pressure to a fluid entering or exiting through
another port. Coupling of a source of pulsation to the extractor may be by any
DEPRESSURIZATION PROCESS / INCREASE EFFICIENCY BY PULSATIONS
Once the solvent has been exchanged for supercritical fluid throughout the
entire volume of the aerogel, the next step to complete the preparation of the aerogel
is to release the pressure in the extractor so that the aerogel can be returned to atmos-
pheric pressure. This is a slow process in current conventional practice because if
the pressure is simply released in an uncontrolled manner, the supercritical fluid will
return to a liquid state damaging the aerogels. The supercritical fluid remaining
inside the gels as a result of the solvent exchange procedure will approximately fol-
low isentropic expansion during an uncontrolled depressurization. In other words,
supercritical CO2 will tend to cool as it expands unless sufficient heat is supplied to
the interior of the gels to prevent the supercritical CO2 from turning into liquid CO2
that will damage the aerogels that have been prepared.
Depressurization is typically performed in multiple stages. In a first stage, the
extractor is depressurized to just below the critical pressure while maintaining the
temperature above the critical temperature. This may be done in any conventional
manner, but requires supplying heat to the aerogel so that the CO2 remains a gas as
the pressure crosses through the critical pressure. After the pressure is below the
critical pressure, it is then reduced very gradually until reaching atmospheric
pressure. Since aerogels are efficient insulators, this second stage of the depressur-
ization process is necessarily a slow process. Generally this is done at a rate of about
103 kPa/minute or below, depending on the aerogel sample size, pore distri-bution,
initial pressure, etc.
The difficulty with heat transfer into the gel during depressurization is com-
pounded by the fact that the CO2 must move out of the gel while the heat has to flow
up-steam of the CO2 flow! Moreover, the low solid content of an aerogel structure
through which that heat must flow into the interior of the gels means that solid
conductivity of the heat is also extremely low. The problem is compounded by the
fine lattice structure and torturous heat conduction pathways. However, for a more
rapid depressurization, the heat must be delivered into the interior of the gel faster
than is currently possible.
Two techniques have now been discovered to speed up the depressurization
process. The first technique employs low frequency pressure fluctuations to help de-
liver the necessary heat into the interior of the gel. As a first stage, the process
entails reducing the pressure from above the critical value for the extraction fluid to
just below the critical pressure without condensing the extraction fluid into a liquid.
As the mean pressure is reduced by opening a discharge valve, supercritical CO2, is
pumped into the extractor at a gradually decreasing pressure. While the pressure is
being steadily reduced, a superimposed low frequency pulsation of the pressure is
performed. During the compression portion of the pulse, the relatively warm
supercritical fluid from outside the gel is packed into the interior of the gel, thereby
delivering necessary heat to allow speeding up this first stage of the depressurization
process. Then during the expansion portion of the pulse, the supercritical fluid is
removed from the gels. Thus there is improved heat transfer into the gels.
As a second stage the pressure is reduced from just below the critical pressure
to atmospheric pressure. As the mean pressure is reduced by opening a discharge
valve, heated gas, e.g. CO2 gas, not supercritical CO2, is pumped into the extractor
at a gradually decreasing pressure which is just above the extractor pressure.
Simultaneously, a low frequency pulsation is superimposed to pack in (due to the
compression) warmer CO2 into the interior of the gels, thereby delivering necessary
heat during the superimposed compression cycle and removing the CO2 during the
superimposed expansion cycle to speed up the second stage of the depressurization
An alternative technique for more rapidly performing the depressurization
begins with exchanging the supercritical fluid, e.g. CO2, with an inert gas that will
not turn liquid at the temperatures and pressure ranges encountered during the
depressurization of the reactor to atmospheric temperature. To achieve this, an
injection of a non-reacting non-condensing (NRNC) gas is used. The timing and
method of the NRNC gas injection may be performed in a variety of ways.
Method 1 - Complete exchange of the supercritical CO2 with a non-reacting
non-condensing gas while maintaining the temperature and pressure within the
supercritical region of CO2 until all of it is replaced. This is a good method when the
solvent concentration in the supercritical CO2 inside the gels is so low that
recondensation will not cause harm to the gels as the NRNC gas is injected.
The gas exchange is diffusive, and like solvent extraction, will suffer from the
slowness of a diffusion-limited process. While diffusion of a gas is faster than of a
liquid, it can still require too long to exchange the CO2 for a NRNC gas. Thus, pre-
ferably the inert gas is exchanged for the supercritical gas with the use of pressure
pulses, as described above for the solvent extraction. High frequency pulses accel-
erate gas exchange in the pores. Low frequency, long wavelength pulses pump the
CO2 out from within the gels as well as pump fresh NRNC gas into the gels. The
wavelength can be matched to a characteristic dimension of the aerogels in the
extractor. The inert gas can also be heated, thereby allowing depressurization to
proceed more rapidly even while the gases are being exchanged.
Method 2 - Gas exchange after the supercritical CO2 has been depressurized
to a constant mean pressure just below the critical pressure while pressure pulsations
occur. This method is preferred if the solvent concentration at the end of the solvent
exchange process is still not as low as desired and a final draw down of the solvent
is wanted. In this method, right after the supercritical CO2 is depressurized to just
below the critical pressure in a conventional manner, the CO2 gas is replaced by a
non-reacting, non-condensing gas (NRNC). This process requires that once the
pressure is reduced to below the supercritical range, any residual solvent is at a
sufficiently low concentration as to not cause damage to the aerogels due to either
condensation or sudden reduced solvent solubility of the extraction fluid. Thus at
the beginning of the depressurization process, the fluid in the extractor begins as
supercritical CO2, then it becomes gaseous CO2, then a mixture of gaseous CO2 and
a NRNC gas, and finally NRNC gas after the exchange is completed.
The NRNC gas is preferably heated to about the critical temperature of the
supercritical C02 to maintain a uniform temperature inside and outside of the gels.
This initiates a gas exchange procedure that removes first the gaseous CO2 from
within the aerogel structures and from within the extractor. The NRNC gas will form
a bubble pushing the remaining CO2 out of the extractor. The exchange can be
performed entirely by diffusion into a flowing stream of the NRNC gas.
Preferably, the gas exchange is conducted at a substantially constant mean
pressure just below the critical pressure. This may be performed by continually add-
ing a heated non-reacting non-condensing gas at the same or higher pressure than is
in the extractor until the CO2 is sufficiently removed that there is no longer any risk
of damage from CO2 condensation within the aerogels. To speed up this gas-to-gas
exchange step of the depressurization process, low frequency pressure pulses around
the mean pressure are applied. The low frequency pulses pump the CO2 out from
within the gels as well as pump fresh NRNC gas into the gels. The two mechanisms
will work in tandem with each other. First, by enhancing inter-diffusion between
CO2 gas and NRNC gas, and second, by the pumping effect. After the highest con-
centration of CO2 within the gels drop below a level (e.g., SO ppm, preferably 20
ppm, most preferably 10 ppm) that could pose a threat of condensation related dam-
ages, then the extractor pressure may be reduced to atmospheric pressure without
needing to provide heat transfer to the inside of the aerogels without further pressure
There are several advantages to performing the gas exchange before depressur-
ization. By replacing the CO2 with a heated non-reacting, non-condensing gas, the
de-pressurization can proceed rapidly since the NRNC gas cannot liquefy during
depressurization, no matter how fast it occurs. Of course, even in this case the
depressurization must not be so rapid that the physical strength of the aerogel to
tolerate the pressure differential between inside and outside the gels is exceeded.
Since the risk of phase change during rapid depressurization is precluded, thermal
stress is not a problem since the temperatures both inside and outside the gels will
fall simultaneously, i.e. the temperature will remain uniform spatially but not
temporally. The time needed for the gas exchange is limited only by the gas-to-gas
diffusion inside the gels.
Method 3 - Gas exchange from the start of the depressurization process until
the end as the pressure floats down, preferably with pressure pulses being used
throughout the entire process. While this method does not require aay waiting time
at a fixed pressure to exchange gases, the depressurization in later stages cannot be
as fast as the other two methods because the possibility of damage caused by
recondensation of the extraction fluid, e.g. CO2, remains. This third method entails
simultaneous gas exchange and gradual pressure reduction. In this case, a heated
non-reacting, non-condensing (NRNC) gas is injected into the extractor and exchang-
ed with the super-critical CO2 inside the aerogels to avoid condensation of the CO2.
The NRNC gas serves as the primary means of supplying heat to the aerogels evenly
and quickly to prevent condensation of the remaining CO2 within the aerogel pores.
The pressure of the gas is floated down along with the pressure of the extractor. This
gradual reduction of pressure can be accomplished by means of a compressor with
appropriate intake and discharge valves to divert excess non-reacting, non-condens-
ing gas flow into a suitable gas storage tank. Interdiffusion of the NRNC gas and the
CO2 through the aerogel thickness determines how fast the heat and gases can be
transmitted or interdiffused to the interior of the aerogels. The speed of the simul-
taneous gas exchange and depressurization is mainly limited by gas-to-gas diffusion
inside the gels and the rate of heat transfer from the gases outside the gels to the CO2
inside the gels. Since there is a close coupling of gas diffusion/exchange and heat
diffusion, the process of heating the remaining CO2 in the interior of gels is more
efficient than simple heat diffusion through the gels.
Suitable non-reacting, non-condensing gases useful herein for any of the above
mentioned three depressurization methods include, but are not limited to, nitrogen,
helium, argon, and dry air. The gas must not unintentionally react with the gels or
the solvent and it must not condense into a liquid at the temperatures and pressures
of use. Preferably the gas is nitrogen, dry air, or helium. Nitrogen will generally be
used because it is relatively inexpensive. When dry air is available, it will be even
less expensive than nitrogen. Helium, with its much higher thermal and mass
diffusivity than either of the other two gases is the preferred gas when the rate of
heat transfer and gas inter-diffusion through a gel block becomes crucial, such as
when large wet gel monoliths are to be dried quickly. Suitable but less preferred
gases for CO2 exchange include hydrogen, oxygen, methane, ethane, neon and argon.
The frequency ranges of suitable pulses described above for the exchange of
a supercritical fluid for a solvent liquid also apply to gas/supercritical fluid ex-
change. The pressure amplitudes acceptable for a particular frequency and aerogel
will tend to increase, since the fluids are on average less dense and have much lower
viscosity which decreases the stress on the aerogel at a particular pressure amplitude
for a given frequency.
The pressure cycling/pulsations produce an active gas exchange process that
is more effective than relying solely on the passive method of simple inter-diffusion
of gases inside aerogels at slowly changing or constant pressure conditions. The
process does not involve any changes in the pore matrix shape or size -- there is
quasi-hydro-static equilibrium inside the entire gel volume and during cycling that
status will not change.
In short, use of the preferred embodiment of Method 2 of the aerogel drying
process of this invention has reduced the time of extraction and drying from the
nearly 40 hours required for a conventional liquid CO2 aerogel process to produce
high quality aerogels of 1 inch (2.54 cm) thickness to less than 8 hours and with
optimization is expected to decrease the time to 4 hours or less.
ALTERNATIVE AEROGEL PREPARATIONS
The invention is applicable to aerogels that are prepared by a process that does
not include supercritical fluid extraction and depressurization as described above.
More particularly, it is applicable to aerogels prepared by a liquid-liquid
extraction process at atmospheric conditions as is used to wash salt-laden water from
hydro-gels made from water glass or to exchange water for an alcohol for hydrogels
made from water glass. Water glass (sodium silicate) contains the essential compon-
ent to form silica aerogels and is one of the least expensive precursors capable of
making silica aerogels, generally costing less than 10% of that of tetra-ethoxysilane.
Adding a catalyst such as sulfuric acid to water glass produces a wet gel that also
contains salt and water which must be removed to make pure aerogels. A long and
laborious salt washing step using fresh water is required to remove the salt from the
water glass derived hydrogel. And even after the salt is removed, since water is
effectively immiscible with common solvent exchange fluids such as CO2 or super-
critical CO2, the water in the wet gel must then be replaced with a solvent like
ethanol that is miscible with or highly soluble in CO2 or other fluids for supercritical
extraction. Direct drying of a water glass hydrogel gives a dense, collapsed struc-
tures unless a xerogel drying process is utilized.
Use of high frequency pulses to promote the diffusion enables washing of the
salt from inside the hydrogel into the water to the outside in a much more expe-
ditious manner than simple diffusion-limited water soaking. High frequency pulses
can also be used to expedite the exchange of water with ethanol. Then the ethanol
can be re-moved from the wet gels to form aerogels by conventional processing.
More preferably, however, it is removed by the more rapid supercritical processing
and rapid gas exchange processes described above for supercritical extraction pro-
cessing. The operating parameters for pulse frequency, amplitude, method of genera-
tion and the like are substantially the same as described. Specific preferred condi-
tions will depend upon the specific system and equipment available for use. Since
such conditions can be determined by routine trial and error based upon the princi-
ples of pulsation described, further details are not necessary. This process of liquid-
liquid extraction via pulsation opens up new possibilities of speeding up the process
and render it inexpensive for making aerogels with significantly lower raw materials
Xerogels are formed by slow direct drying of wet gels containing one or more
special additives that enable the drying to occur without substantial reduction in
porosity and without the supercritical extraction step described above. This type of
drying of the wet gels to form xerogels can also be enhanced by use of the pulsation
technique described herein. More specifically, when a wet gel containing a solvent
liquid (e.g. water) is exposed to a drying gas (e.g. air), the vapor pressure in the dry
air is lower than the vapor pressure of the liquid solvent contained in the pores of the
gel. Therefore, the solvent liquid will begin to evaporate. As the evaporation front
moves inward, a layer of pores will form that are filled with vapor of the solvent
liquid starting from the saturation concentration at the liquid interface and gradually
decreasing toward the surface of the gel. The vapor pressure of the solvent is much
higher within the pores than outside the gel. The normal drying process relies on
simple diffusion of high concentration vapor from inside the gel into the outside.
Since the liquid is surrounded or bathed in the high vapor concentration gas inside
the gel, the rate of drying is relatively slow. When pressure pulses are used as
described herein, however, during compression low frequency pulses will effectively
pump in largely fresh air from outside into the pores of the gel where there are high
concentration vapor. During expansion the concentrated vapor inside the pores will
be pumped out. So, instead of having to wait for the concentration gradient to work
its way through passive diffusion, the slow frequency pulsation will much more
quickly remove the high concentration vapor from within the gels. As a result, the
remaining liquid is exposed to a low concentration vapor pressure at the interface
with the drying gas within the gel structure. High frequency small amplitude pulsa-
tions of the drying gas to promote the vaporization may also be used, either alone or
in combination with the low frequency pulses. Use of the high frequency pulses will
be akin to "ocean spray" effect by creating disturbances of particles at the liquid-gas
interface thereby increasing the surface area of the liquid vapor interaction and
enhancing the vaporization.
Frequencies and amplitudes for the pulsations will depend on properties such
as density, viscosity, diffusivity, saturation vapor pressure at the process tempera-
ture of the liquid, gas; gel pore dimensions, distribution, tortuosity of the open pores,
size and shape of the gels, etc.
The use of the pressure pulsation techniques described herein make previous
diffusion-limited exchange processes much more rapid which should lead to new
synthetic pathways for the preparation of aerogels and similar materials. The pulsa-
tion methods are useful in any of the nine possible exchange types, i.e. each of
liquid, gas, and supercritical fluid with each of liquid, gas and supercritical fluid.
TEST CELL FOR OBSERVING EXTRACTIONS
A convenient way to optimize the sequence or sequences of pulses is to set up
a pressurizable cell with windows, as an analog of the extractor. Standard pieces of
solvent-filled aerogels can be made, placed in the cell, surrounded with supercritical
fluid, and observed under the influence of pulses. Any evaluation method that does
not require opening the test cell to measure the rate of efflux of the solvent from the
gel may be used. For example, a change in the refractive index is potentially measur-
able for any solvent or supercritical fluid or combination. Alternatively, the concen-
tration of solvent in the exiting supercritical fluid can be measured. This is an
advantageous method, especially if correlated with other methods, because it requires
no special apertures or sensors in the extractor.
A more direct method is to place a dye in the solvent used to prepare the wet
gels or in the supercritical fluid and to observe the rate of removal of the dye from
the gels, or the penetration of the dye into the gels, under various pulsing conditions.
Optimizing the rate of change of the boundary location by varying the frequency or
amplitude of the pulses becomes a rapid and straightforward procedure, readily
giving a significantly improved exchange procedure by minimal experimentation.
Two wet gel samples were prepared from tctra-ethoxysilane (TEOS) esscn-
tially as described in the art. A red dye soluble in ethanol but not in CO2 was dis-
solved into ethanol. During the gel preparations, the ethanol-dye solution was used
in place of conventional pure ethanol. This resulted in red-colored wet gel samples.
To evaluate the effect of sonic pulses on the enhancement of diffusion process, two
wet gel samples were processed as follow after overnight curing. The first was
simply immersed in a jar containing pure ethanol and the diffusion of the red dye was
monitored. The second was also immersed in a jar of pure ethanol, but then the jar
was placed in a small sonic cleaning bath with a 1.25 cm thick sponge pad placed
between the bath and the jar to attenuate the sonic amplitude and to prevent pre-
mature breakage of the gels by the sonicator. The sonic cleaning bath generated
fixed-wavelength pulses at 20 kHz. The diffusion of the red dye out of the gels into
the ethanol was observed and periodically photographed. A UV spectrometer
measured the frequency of the UV light transmitted through the ethanol.
The results show that the dye was extracted more rapidly in the sample that
was sonicated. Extraction to approximate equilibrium was obtained in about 45
minutes in the pulse-treated gel, but has not reached completion after 16 hours in the
absence of pulsation. UV spectrometer plots confirmed that the sonically enhanced
diffusion was approximately 20 times faster than the natural diffusion.
The diffusion enhancement by pressure pulsation technique is not limited to
supercritical fluids or gas-to-gas exchanges. Liquid-liquid extractions can also be
accelerated by pulsation. Although liquids (here, ethanol) are relatively incom-
pressible in comparison with supercritical CO2 or gases), it was found that use of a
fixed-wavelength source of pulsation at 20 kHz decreased the extraction time by a
factor of about three in early stages, and probably in excess of twenty or more at later
stages to accomplish an extraction in about 45 minutes that required about 16 hours
1. A method for improving the efficiency of exchanging a
first fluid such as herein described within a get by a second
fluid such as herein described comprising applying pulses of
pressure having at least one frequency to the gel, the first
fluid and the second fluid during the exchange.
2. The method as claimed in Claim 1, wherein the first
fluid is selected from the group consisting of liquid, gas, and
supercritical fluid and the second fluid is selected from the
group consisting of liquid, gas, and supercritical fluid.
3. The method as claimed in Claim 1, wherein the first
fluid is a solvent liquid used to prepare the gel and the second
fluid is a supercritical fluid.
4. The method as claimed in Claim 1, wherein the gel is an
inorganic gel which is an oxide of a metal selected from the
group consisting of silicon, aluminum, iron, copper, zirconium,
hafnium, magnesium, yttrium, and mixtures thereof.
5. The method as claimed in Claim 1, wherein the first
fluid is a supercritical fluid and the second fluid is a non-
reacting, non-condensing gas.
6. A method as claimed in Claim 1 comprising: providing an
extractor containing the wet gel having a porous structure, said
gel containing within its pores a solvent liquid) providing a
supercritical fluid in the extractor in contact with and in
approximate equilibrium of pressure and temperature with the
solvent liquid-containing wet gels! and applying pulses of
pressure to said supercritical fluid, thereby accelerating the
mixing of the supercritical carbon dioxide and the solvent liquid
to exchange a solvent liquid in a wet gel with a supercritical
fluid to form an aerogel.
7. The method of Claims 1-6, comprising applying pressure
pulses of two different frequencies.
3. The method as claimed in claim 1 comprising providing
the solvent liquid within the gel at a temperature no more than
10 C below the critical temperature of the supercritical fluid
before the supercritical fluid contacts the solvent liquid to
reduce the time required to exchange a solvent liquid located
within a gel with a supercritical extracting fluid in a means for
performing the exchange during preparation of an aerogel.
9. The method as claimed in claim 1, comprising the steps
of: (i) placing in an extractor at atmospheric pressure wet gels
having pores and containing a solvent liquid in the pores end
around the wet gels, (ii) raising the temperature of the
extractor, (iii) adding carbon dioxide at substantially the same
temperature as that of the extractor, (iv) gradually increasing
the pressure in the extractor so as to form supercritical carbon
dioxide in said extractor, wherein the rate of increase of
pressure or of temperature is sufficiently low that it does not
adversely affect the properties or integrity of the gel to
prepare an aerogel.
10. The method as claimed in claim 1 providing an extractor
containing wet gets, said wet gels containing in their pores a
solvent liquid, and further containing a supercritical fluid in
contact with, and in approximate equilibrium of pressure and
temperature with, said solvent-containing wet gels}
wherein each of the solvent liquid, the wet gels, the
extractor, and the gas, liquid, and supercritical phases of the
extracting supercritical fluid are maintained above the critical
temperature of the supercritical fluid frost the beginning of the
introduction of the extracting fluid into the extractor, until
the solvent is extracted from the gel.
11. The method as claimed in claim 1 comprising supplying
heat into the device through injection of a heated supercritical
fluid down to just below its critical pressure and thereafter
injection of a heated gas down to about atmosphereic pressure for
rapid depressurization of a supercritical fluid within a porous
medium in a device.
12. The method as claimed in claim 1 wherein the aerogel is
filled with a supercritical fluid, the method comprising
exchanging the supercritical fluid with a non-reacting non-
supercritical gas, followed by depressurization.
13. The method as claimed in claim 1 comprising exchanging
the supercritical fluid with a non-reacting, non-condensing gas
before or during the depressurization for rapid depressurization
of a supercritical fluid within and around a porous medium.
14. The method as claimed in claims 8-13 comprising applying
pressure pulses of at least one frequency to a fluid during the
15. The method as claimed in claim 14 comprising applying
pressure pulses of two different frequencies.
16. The method as claimed in claims 1-7 and 14-15, wherein
the pulses are generated by one or more of a piezoelectric
device, an electromechanical device, a mechanical device, liquid
piston, a piston, a diaphragm, an inflatable device, audio
frequency speakers, mechanical tapping, vibrating table, and a
variation in the pressure or the back pressure of a fluid or a
17. The method as claimed in Claims 7 and 15, wherein two sets
of pulses are applied to said supercritical fluid, wherein a
first set of pulses has frequencies of about 1Hz to 100,000 Hz,
and the second set of pulses has frequencies in the range of
about 0.001 to 10 Hz, and the second frequency is lower than the
A method for improving the efficiency of exchanging a
first fluid such as herein described within a get by a second
fluid such as herein described comprising applying pulses of
pressure having at least one freouency to the gel, the first
fluid and the second fluid during the exchange.
|Indian Patent Application Number
|PG Journal Number
|Date of Filing
|Name of Patentee
|ASPEN AEROGELS, INC.
|30 FORBES ROAD, NORTHBOROUGH, MA 01606
|PCT International Classification Number
|PCT International Application Number
|PCT International Filing date