U.S. patent application number 10/620933 was filed with the patent office on 2004-05-06 for rapid aerogel production process.
Invention is credited to Altiparmakov, Zlatko, Begag, Redouane, Lee, Kang P..
Application Number | 20040087670 10/620933 |
Document ID | / |
Family ID | 22576981 |
Filed Date | 2004-05-06 |
United States Patent
Application |
20040087670 |
Kind Code |
A1 |
Lee, Kang P. ; et
al. |
May 6, 2004 |
Rapid aerogel production process
Abstract
Methods of more rapidly producing aerogel products by means of a
rapid solvent exchange of solvent inside wet gels with
supercritical CO.sub.2 by injecting supercritical, rather than
liquid, CO.sub.2 into an extractor that has been pre-heated and
pre-pressurized to substantially supercritical conditions or above.
Preferably, pressure waves are applied to the supercritical
CO.sub.2 to enhance the solvent exchange. The rapid solvent
exchange process is followed by depressurization, optionally with a
gas exchange. Preferably, pressure waves are used to speed up the
depressurization. The process greatly reduces the time for forming
aerogel products.
Inventors: |
Lee, Kang P.; (Sudbury,
MA) ; Begag, Redouane; (Hudson, MA) ;
Altiparmakov, Zlatko; (Shrewsbury, MA) |
Correspondence
Address: |
Bruce Jacobs
P.O. Box 390438
Cambridge
MA
02139
US
|
Family ID: |
22576981 |
Appl. No.: |
10/620933 |
Filed: |
July 16, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10620933 |
Jul 16, 2003 |
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09693390 |
Oct 20, 2000 |
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6670402 |
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60160464 |
Oct 21, 1999 |
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Current U.S.
Class: |
516/99 |
Current CPC
Class: |
Y02P 20/54 20151101;
C01B 33/1585 20130101; B01J 13/0091 20130101 |
Class at
Publication: |
516/099 |
International
Class: |
C08J 003/02 |
Claims
What is claimed is:
1. A method for improving the efficiency of exchanging a first
fluid within a gel by a second fluid 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 of claim 1, wherein the first fluid is a solvent
liquid used to prepare the gel.
3. The method of claim 1, wherein the second fluid is a
supercritical fluid.
4. The method of claim 1, wherein the pulses have a frequency of
about 1 to 100,000 Hz and an amplitude of about 0.1 to 20 psi.
5. The method of claim 1, wherein the pulses have a frequency of
about 2,000 to 50,000 Hz and an amplitude about 0.3 to 5 psi.
6. The method of claim 1, wherein the pulses have a frequency of
about 0.0001 to about 10 Hz and an amplitude of about 10 to 1,000
psi.
7. The method of claim 1, wherein the pulses have a frequency in
the range of about 0.001 to about 1 Hz and an amplitude about 100
to 600 psi;
8. The method of 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.
9. The process of claim 1, wherein the gel is an inorganic gel.
10. The process of claim 9, wherein the inorganic gel is an oxides
of a metal selected from the group consisting of silicon, aluminum,
iron, copper, zirconium, hafnium, magnesium, yttrium, and mixtures
thereof.
11. The process of claim 1, wherein the gel is an organic gel.
12. The process of claim 11, wherein the organic gel is selected
from the group consisting of polyacrylate, polystyrene,
polyacrylonitrile, polyurethane, polyimide, polyfurfural alcohol,
phenol furfuryl alcohol, polyfurfuryl alcohol,
melamine-formaldehyde resin, resorcinol-formaldehyde resin, cresol
formaldehyde resin, phenol-formaldehyde resin, polyvinyl alcohol
dialdehyde, polycyanurate, polyacrylamide, epoxy resin, agar, and
mixtures thereof.
13. The method of claim 1, wherein the first fluid is a solvent
liquid used to prepare the gel and the second fluid is a
supercritical fluid.
14. The method of claim 1, wherein the first fluid is a
supercritical fluid and the second fluid is a non-reacting,
non-condensible gas.
15. The method of claim 14, wherein the inert non-reacting
non-condensible gas is selected from the group consisting of air,
nitrogen, oxygen, helium, neon, argon, hydrogen, and mixtures
thereof.
16. The method of claim 1, wherein the first fluid is a
supercritical fluid and the second fluid is a gas.
17. The method of claim 1, wherein the first fluid is a liquid and
the second fluid is a liquid.
18. The method of claim 1, wherein the gel is an inorganic gel
prepared by the hydrolysis and condensation of a metal
alkoxide.
19. The method of claim 18, wherein the metal alkoxide has about 1
to 6 carbon atoms in each alkyl group.
20. The method of claim 18, wherein the metal alkoxide is selected
from the group consisting of 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 mixtures thereof.
21. The method of claim 1 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 flowing gas.
22. A method for reducing 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 the preparation of an
aerogel, comprising providing the solvent liquid within the gel at
a temperature no more than 10.degree. C. below the critical
temperature of the supercritical fluid before the supercritical
fluid contacts the solvent liquid.
23. The method of claim 22, wherein the means for performing the
exchange, the supercritical extracting fluid, and any excess
solvent liquid required to prevent drying, are at a temperature no
more than 10.degree. C. below the critical temperature.
24. The method of claim 22, further comprising applying pressure
pulses of at least one frequency to the gel, the solvent liquid and
the supercritical extracting fluid during the exchange.
25. The method of claim 24, comprising applying pressure pulses of
two different frequencies.
26. The method of claim 25, wherein the two different frequencies
are a first frequency of about 1 to about 100,000 Hz and a second
frequency of about 0.0001 to about 10 Hz. and the second frequency
is lower that the first frequency.
27. The method of claim 26, wherein the pulses of the first
frequency have an amplitude of about 0.1 to 20 psi and the pulses
of the second frequency have an amplitude of about 10 to 1,000 psi
and the amplitude at the second frequency is higher than the
amplitude of the first frequency.
28. The method of claim 24, 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 flowing gas.
29. A method for rapid depressurization of a supercritical fluid
within and around a porous medium, the method comprising exchanging
the supercritical fluid with a non-reacting, non-condensing gas
before or during the depressurization.
30. The method of claim 29, wherein the porous medium is an
aerogel.
31. The method of claim 29, further comprising applying pulses of
pressure during said exchange.
32. The method of claim 31, comprising applying pulses of two
different frequencies.
33. The method of claim 32, wherein the two different frequencies
are a first frequency of about 1 to about 100,000 Hz and a second
frequency of about 0.0001 to about 10 Hz. and the second frequency
is lower that the first frequency.
34. The method of claim 33, wherein the pulses of the first
frequency have an amplitude of about 0.1 to 20 psi and the pulses
of the second frequency have an amplitude of about 10 to 1,000 psi,
and the amplitude at the second frequency is higher than the
amplitude of the first frequency.
35. The method of claim 32, wherein at least one of the two
different frequencies is systematically varied during the
depressurization.
36. The method of claim 31, 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 flowing gas.
37. A method for rapid depressurization of a supercritical fluid
within a porous medium in a device, the method 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 atmospheric pressure.
38. The method of claim 37, wherein the porous medium is an
aerogel.
39. The method of claim 37, further comprising applying pulses of
pressure during said exchange.
40. The method of claim 39, comprising applying pressure pulses of
two different frequencies.
41. A method of preparing an aerogel comprising the steps of: (i)
placing in an extractor at atmospheric pressure wet gels having
pores and containing a solvent liquid in the pores and 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.
42. A method of exchanging the solvent liquid in a wet gel with a
supercritical fluid to form an aerogel, the method 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.
43. A method for decreasing the time required for preparing an
aerogel in an extractor 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.
44. The method of claim 43, wherein the inert non-reacting
non-supercritical gas is selected from air, nitrogen, oxygen,
helium, neon, argon, hydrogen, and mixtures thereof.
45. The method of claim 43, wherein the gas exchange is accelerated
by applying pulses of pressure to the non-reacting
non-supercritical gas phase.
46. The method of claim 44, wherein two sets of pulses are applied
to said supercritical fluid, wherein a first set of pulses has
frequencies of about 1 Hz 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 that the first frequency.
47. An improved method of exchanging the solvent in wet gels for a
supercritical fluid, the method comprising: providing an extractor
containing wet gels, 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 from the beginning of the introduction of the
extracting fluid into the extractor, until the solvent is extracted
from the gel.
Description
BACKGROUND OF THE INVENTION
[0001] 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.
[0002] Aerogel products, after wet gel formation, are
conventionally prepared by a process of liquid CO.sub.2 extraction
of whatever solvent(s) is utilized to form the wet gels followed by
a supercritical CO.sub.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.sub.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.sub.2 as the pressure is decreased to 1
atmosphere.
[0003] 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".times.9".times.1", (12.7 cm.times.22.9 cm.times.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.sub.2, it
takes about 30 hours to replace the solvent in the gels using
liquid CO.sub.2. The solvent exchange step takes so long because it
must rely on the diffusivity of the solvent and the liquid CO.sub.2
and the solute solubility of liquid CO.sub.2. It is performed by
adding CO.sub.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.sub.2 (1070
psi (7378 kPa) and 31.06.degree. 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 1/2 hour. Finally, the
depressurization commonly takes about 6 hours.
[0004] In total, it currently takes about 40 hours of extractor
time to produce a single batch of five 5".times.9" (12.7.times.22.9
cm) flawless 1" (2.5 cm) thick aerogel panels in a 40 liter
extractor.
[0005] 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.
[0006] 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 insulation. 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 CO.sub.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.sub.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.
[0007] 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.
[0008] 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.
[0009] It is a further object to rapidly produce aerogel products
while avoiding creating surface tension induced failures within the
aerogels.
[0010] 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.
[0011] 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.
[0012] These and still further objects will be apparent from the
following detailed description of this invention.
SUMMARY OF THE INVENTION
[0013] This invention is directed to methods of preparing aerogel
products by an improved supercritical drying process.
[0014] 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.
[0015] 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.
[0016] More particularly, this invention is directed to the use of
gaseous CO.sub.2 to pre-pressurize an extractor that is loaded with
wet gels for flash-free fast injection of supercritical CO.sub.2
without causing any flow-induced damage to the gel structures.
[0017] More particularly, this invention is directed to the use of
CO.sub.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.
[0018] More particularly, this invention is directed to the use of
gaseous CO.sub.2 injected into an extractor as a means of
displacing the bulk of the free solvent before supercritical
CO.sub.2 is injected into the extractor.
[0019] More particularly, this invention is directed to the use of
supercritical CO.sub.2 injected into an extractor to displace the
bulk of the solvent before supercritical CO.sub.2 is injected into
the extractor.
[0020] More particularly, this invention is directed to the use of
supercritical CO.sub.2 injection as a means of direct heat exchange
into the supercritical CO.sub.2 in the extractor during
depressurization to prevent condensation of supercritical CO.sub.2.
This eliminates most of the solvent remaining in the gels as the
supercritical CO.sub.2 is removed from the gel by depressurization
to just below critical pressure.
[0021] 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.sub.2 and gas exchange with gaseous
CO.sub.2 inside the gels during depressurization to prevent
condensation of CO.sub.2. This significantly shortens the duration
of depressurization compared to the conventional slow
depressurization in that heat is supplied indirectly through the
extractor wall.
[0022] 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 CO.sub.2 in a pipe to supercritical conditions
to feed into an extractor.
[0023] 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 CO.sub.2 in an extractor to supercritical
conditions.
[0024] More particularly, this invention is directed to the use of
continuous flow supercritical CO.sub.2 extraction right from the
outset to perform solvent exchange and extraction.
[0025] More particularly, this invention is directed to the use of
a non-reacting, non-condensing gas to remove most of the gaseous
CO.sub.2 during depressurization to significantly reduce the total
amount of heating required during depressurization to prevent
condensation of the remaining CO.sub.2. 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.
[0026] 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.
[0027] More particularly, this invention is directed to an aerogel
drying process the duration of which is substantially equipment
scale/size insensitive.
[0028] 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.
[0029] 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.
[0030] 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 supercritical
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
[0031] The process of the present invention is directed to an
improved process for the manufacture of any type of aerogel
product, including beads, monoliths and composites.
[0032] As used herein, an "aerogel" includes (unless context
requires a narrower meaning) not only a conventional aerogel, but
also similar structures that have a microporous 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.
[0033] 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.
[0034] 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 temperature.
[0035] The term "fluid" refers to any of a gas, a vapor, and a
liquid.
[0036] 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.
[0037] 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.
[0038] 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 temperature," "critical pressure," and
"critical conditions," refer to the temperature and pressure
conditions that apply for the specific supercritical fluid being
discussed.
[0039] Aerogel Materials
[0040] 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, aerogels are produced by supercritical
extraction of the solvent (or any liquid replacement 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.
[0041] For purposes of the present invention, aerogels include
xerogels which are prepared by air evaporation, i.e. by slow direct
drying, without a supercritical extraction 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).
[0042] 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
polymerization depending on the solvent or method used for
attaining particular microstructures. 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
formaldehydes, resorcinol formaldehydes, cresol formaldehyde,
phenol formaldehyde, polyvinyl alcohol dialdehyde, polycyanurates,
polyacrylamides, various epoxies, agar, and the like.
[0043] 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
preparation methods.
[0044] 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.
[0045] 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.
[0046] Major variables in the inorganic aerogel formation process
include the type of alkoxide, solution pH, and
alkoxide/alcohol/water ratio. Control of these variables can permit
control of the growth and aggregation of the aerogel species
throughout 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.
[0047] 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.
[0048] 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, HCl, H2SO4 or HF are the
currently preferred acids. To generate a higher pH, NH.sub.4OH is
the preferred base.
[0049] 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.
[0050] 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(OC.sub.2H.sub.5).sub.4+4H.sub.2O.fwdarw.Si(OH).sub.4+4(C.sub.2H.sub.5OH-
) (1)
[0051] 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).sub.4.fwdarw.SiO.sub.2+2H.sub.2O (2)
[0052] occurs and forms precursors which after supercritical drying
in accordance with this invention become aerogels.
[0053] Extraction of Solvent-Wetted Gels
[0054] 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.
[0055] 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, (1994).
[0056] The wet gels are formed in or placed into a suitable mold
and then aged, commonly 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
CO.sub.2 (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.
[0057] 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 CO.sub.2. 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 per-formed either by
lowering the gel monoliths into a vertical chamber or by
horizontally inserting them into a horizontally-positioned
extractor, i.e. by opening 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.
[0058] 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 CO.sub.2 (or other suitable fluid.) or
as described in M. J. van Bommell et al, supra, with supercritical
CO.sub.2.
[0059] In the method of the invention, the removal begins in any of
several ways in which the wet gels are at 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.
CO.sub.2.
[0060] 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 CO.sub.2 at a temperature
above the critical temperature and at a pressure that is below the
critical pressure. The gaseous CO.sub.2 is above the critical
temperature and it has a much lower density than the liquid
solvent. The gaseous CO.sub.2 is fed preferably 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 CO.sub.2 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 CO.sub.2 (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
CO.sub.2 has a solute diffusivity that is about 10 times higher
than that of liquid CO.sub.2 while having a viscosity that is only
about one tenth that of liquid CO.sub.2. Thus the
diffusion/infiltration/extraction is more rapid with supercritical
CO.sub.2 than with liquid CO.sub.2. As the supercritical CO.sub.2
feeding continues, the extractor pressure increases beyond the
critical pressure and all of the CO.sub.2 inside the-extractor
turns supercritical.
[0061] Once the extractor has become filled with supercritical
CO.sub.2, supercritical CO.sub.2 containing extracted solvent is
continually drained from the bottom of the extractor, since it is
of higher density than pure supercritical CO.sub.2. The
supercritical CO.sub.2 injection does not create any violent fluid
motion since the extractor is already near, i.e. within about 200
psi, preferably within about 50 psi, and most preferably within
about 10 psi of the supercritical process pressure. For
supercritical CO.sub.2 the pressure must be above 1,070 psi (7,378
kPa). Preferably it is about 1,100 to 1,800 psi (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 CO.sub.2 are all practically identical.
[0062] 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 directly pressurized by
injecting gaseous CO.sub.2 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 CO.sub.2 injection continues
until the pressure builds beyond the critical pressure and all of
the CO.sub.2 turns supercritical. At this point injection of
supercritical CO.sub.2 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 CO.sub.2 that forms an expanding "bubble"
from the top, simultaneously 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 CO.sub.2 into the top of the
extractor at substantially the same temperature as the
gels/solvent. 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 CO.sub.2 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 CO.sub.2 are all substantially the same.
[0063] An alternative to the addition of gaseous CO.sub.2 into the
clearance volume is the direct injection of supercritical CO.sub.2
into that space while the space is at about the desired
supercritical temperature. In this case, the supercritical CO.sub.2
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 are immersed in and protected by the solvent when the
supercritical CO.sub.2 is added and it expands into the clearance
volume, there will be no thermal or fluid dynamic shock.
[0064] 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 CO.sub.2 to
pre-pressurize the extractor to just below the critical pressure,
followed by injection of supercritical CO.sub.2 into the extractor.
The gaseous CO.sub.2 pre-pressurization does not create any violent
fluid motion since it occurs gradually. The supercritical CO.sub.2
injection does not create any violent fluid motion since the
extractor is already substantially at the process pressure. The
supercritical CO.sub.2 injection does not create any thermal shock
because the temperatures of the solvent, gels and the supercritical
CO.sub.2 are all practically identical.
[0065] 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 supercritical
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.degree. C., preferably not more than
about 5.degree., and more preferably not more than about 2.degree.
C. below the critical temperature.
[0066] 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 CO.sub.2. Because of its increased
diffusivity, extraction with supercritical CO.sub.2 is faster than
with liquid CO.sub.2.
[0067] 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.
[0068] Extraction of Solvent/Increase in Efficiency by
Pulsations
[0069] 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 CO.sub.2
instead of liquid CO.sub.2, 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.
[0070] The efficiency of the solvent exchange procedures with an
extraction fluid may be enhanced by increasing the fluid's
effective mass diffusivity. More particularly, improved solvent
exchange efficiency may be obtained by cycling or pulsing the
extractor pressure. For example, high frequency low amplitude
pressure fluctuations can be used to promote mixing and mass
diffusion. Alternatively, low frequency high amplitude pulsations
can be used to effectively pump out higher solvent concentration
solution (e.g. solution of ethanol in supercritical CO.sub.2) 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 CO.sub.2. Preferably two
different pulsations are 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
pressure conditions.
[0071] 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 50 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.
[0072] 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.
[0073] 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 supercriticality of the mixed fluid
to enhance the rate of extraction of the solvent from within the
gels, thereby reducing the overall process time.
[0074] Using the example of ethanol as the solvent and CO.sub.2 as
the supercritical fluid, a mixture containing about 50% of each is
a single phase and supercritical at pressures above about 1100 psi
(ca. 80 MPa) and at temperatures above about 35.degree. C. This is
near the usual operating pressures and temperatures for maintaining
CO.sub.2 in the supercritical state. Therefore, at the initial
moment of contact between the wet gels and the supercritical fluid,
inter-diffusion begins at the interface between the solvent ethanol
inside the wet gels and the supercritical CO.sub.2 outside the gels
will begin. The inter-diffusion is enhanced by transmitting a high
frequency fluctuation through the super-critical CO.sub.2. As the
mixing at the interface continues, the thickness 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 com-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 supercritical region" of the gel. Then, when the next
pulse compresses the supercritical fluid, a fresh load of
supercritical CO.sub.2 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.
[0075] 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 CO.sub.2 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 discontinuity, 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.
[0076] 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 completely 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 concentration 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.
[0077] 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 replenishing 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 supercritical CO.sub.2 will nearly double when the pressure
increases from 1100 psi to 1500 psi at 40.degree. 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 CO.sub.2 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
supercritical 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.
[0078] 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 CO.sub.2
with compression 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.
[0079] 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.
[0080] 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 5 PSI or so. A pressure amplitude range of about 0.1
to 4 PSI 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 same amplitude.
[0081] For a supercritical CO.sub.2 extraction (generally performed
at pressures of about 1,100 to 1,800 psi (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
0.01 to about 20 psi, more typically about 0.3 to 5 psi, and often
0.5 to 3 psi.
[0082] 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
10 psi to up to 1000 psi, more preferably 100 psi to 600 psi,
provided that the material can tolerate the pressure gradient, and
allowing for the pressure amplitude of the high frequency pulses
when used simultaneously.
[0083] Specific pressure amplitude/frequency combinations have to
be determined for particular compositions of aerogels by routine
experimentation taking into consideration 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.
[0084] 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.
[0085] 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.
[0086] If appropriately limited, however, a moderate degree of
lowering of the pressure or density will not cause re-condensation
of the solvent, since most supercritical 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.
[0087] The pressure fluctuation process relies, in one mode, on the
fact that supercritical CO.sub.2 behaves like a gas in terms of
compressibility. So when compressed more supercritical CO.sub.2 can
be packed into the same pore volume as before and when expanded,
the solvent laden supercritical CO.sub.2 tends to come out of the
gels.
[0088] 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 conventional method.
[0089] Depressurization Process/Increase Efficiency by
Pulsations
[0090] 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 atmospheric
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 follow isentropic expansion during an uncontrolled
depressurization. In other words, supercritical CO.sub.2 will tend
to cool as it expands unless sufficient heat is supplied to the
interior of the gels to prevent the supercritical CO.sub.2 from
turning into liquid CO.sub.2 that will damage the aerogels that
have been prepared.
[0091] 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 CO.sub.2
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
depressurization process is necessarily a slow process. Generally
this is done at a rate of about 15 psi/minute or below, depending
on the aerogel sample size, pore distribution, initial pressure,
etc.
[0092] The difficulty with heat transfer into the gel during
depressurization is compounded by the fact that the CO.sub.2 must
move out of the gel while the heat has to flow up-steam of the
CO.sub.2 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.
[0093] Two techniques have now been discovered to speed up the
depressurization process. The first technique employs low frequency
pressure fluctuations to help deliver 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 CO.sub.2, 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.
[0094] 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. CO.sub.2
gas, not supercritical CO.sub.2, 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 CO.sub.2 into the
interior of the gels, thereby delivering necessary heat during the
superimposed compression cycle and removing the CO.sub.2 during the
superimposed expansion cycle to speed up the second stage of the
depressurization process.
[0095] An alternative technique for more rapidly performing the
depressurization begins with exchanging the supercritical fluid,
e.g. CO.sub.2, 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 will be used. The timing and method of the NRNC gas injection
may be performed in a variety of ways.
[0096] Method 1--Complete exchange of the supercritical CO.sub.2
with a non-reacting non-condensing gas while maintaining the
temperature and pressure within the supercritical region of
CO.sub.2 until all of it is replaced. This is a good method when
the solvent concentration in the supercritical CO.sub.2 inside the
gels is so low that recondensation will not cause harm to the gels
as the NRNC gas is injected.
[0097] 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 CO.sub.2 for a NRNC gas. Thus, preferably
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 accelerate gas exchange in the pores. Low
frequency, long wavelength pulses pump the CO.sub.2 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.
[0098] Method 2--Gas exchange after the supercritical CO.sub.2 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 CO.sub.2 is depressurized to just below the critical
pressure in a conventional manner, the CO.sub.2 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 CO.sub.2, then it becomes gaseous
CO.sub.2, then a mixture of gaseous CO.sub.2 and a NRNC gas, and
finally NRNC gas after the exchange is completed.
[0099] The NRNC gas is preferably heated to about the critical
temperature of the supercritical CO.sub.2 to maintain a uniform
temperature inside and outside of the gels. This initiates a gas
exchange procedure that removes first the gaseous CO.sub.2 from
within the aerogel structures and from within the extractor. The
NRNC gas will form a bubble pushing the remaining CO.sub.2 out of
the extractor. The exchange can be performed entirely by diffusion
into a flowing stream of the NRNC gas.
[0100] Preferably, the gas exchange is conducted at a substantially
constant mean pressure just below the critical pressure. This may
be performed by continually adding a heated non-reacting
non-condensing gas at the same or higher pressure than is in the
extractor until the CO.sub.2 is sufficiently removed that there is
no longer any risk of damage from CO.sub.2 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
CO.sub.2 out from within the gels as well as pump fresh NRNC gas
into the gels. The two mechanism will work in tandem with each
other. First, by enhancing inter-diffusion between CO.sub.2 gas and
NRNC gas, and second, by the pumping effect. After the highest
concentration of CO.sub.2 within the gels drop below a level (e.g.,
50 ppm, preferably 20 ppm, most preferably 10 ppm) that could pose
a threat of condensation related damages, 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 pulsations.
[0101] There are several advantages to performing the gas exchange
before depressurization. By replacing the CO.sub.2 with a heated
non-reacting, non-condensing gas, the depressurization 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.
[0102] 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 any 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. CO.sub.2, 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 exchanged with the supercritical CO.sub.2
inside the aerogels to avoid condensation of the CO.sub.2. The NRNC
gas serves as the primary means of supplying heat to the aerogels
evenly and quickly to prevent condensation of the remaining
CO.sub.2 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-condensing gas flow into a suitable gas storage
tank. Interdiffusion of the NRNC gas and the CO.sub.2 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 simultaneous 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 CO.sub.2
inside the gels. Since there is a close coupling of gas
diffusion/exchange and heat diffusion, the process of heating the
remaining CO.sub.2 in the interior of gels is more efficient than
simple heat diffusion through the gels.
[0103] 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 CO.sub.2 exchange) include hydrogen, oxygen,
methane, ethane, neon and argon.
[0104] 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 exchange. 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.
[0105] 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-hydrostatic equilibrium inside the entire gel
volume and during cycling that status will not change.
[0106] 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 CO.sub.2 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.
[0107] Alternative Aerogel Preparations
[0108] The present invention is applicable to aerogels that are
prepared by a process that does not include supercritical fluid
extraction and depressurization as described above.
[0109] 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 hydrogels made from water glass
or to exchange water for an alcohol for hydrogels made from water
glass. Water glass (sodium silicate) contains the essential
component 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
CO.sub.2 or supercritical CO.sub.2, the water in the wet gel must
then be replaced with a solvent like ethanol that is miscible with
or highly soluble in CO.sub.2 or other fluids for supercritical
extraction. Direct drying of a water glass hydrogel gives a dense,
collapsed structures unless a xerogel drying process is
utilized.
[0110] 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 expeditious 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 removed 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 processing.
The operating parameters for pulse frequency, amplitude, method of
generation and the like are substantially the same as described.
Specific preferred conditions 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 principles 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
costs.
[0111] 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 pulsations 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.
[0112] Frequencies and amplitudes for the pulsations will depend on
the properties such as density, viscosity, diffusivity, saturation
vapor pressure at the process temperature, of the liquid, gas; gel
pore dimensions, distribution, tortuosity of the open pores, size
and shape of the gels, etc. Suitable such conditions can be
determined by routine experimentation.
[0113] 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 pulsation
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.
[0114] Test Cell for Observing Extractions
[0115] 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 measurable for any
solvent or supercritical fluid or combination. Alternatively, the
concentration 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.
[0116] 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.
EXAMPLE 1
[0117] Two wet gel samples were prepared from tetra-ethoxysilane
(TEOS) essentially as described in the art. A red dye soluble in
ethanol but not in CO.sub.2 was dissolved 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.
[0118] 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 premature 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.
[0119] 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.
[0120] The diffusion enhancement by pressure pulsation technique is
not limited to supercritical fluids or gas-to-gas exchanges. As
shown in Example 1 above, liquid-liquid extractions can also be
accelerated by pulsation. Although liquids (here, ethanol) are
relatively incompressible in comparison with supercritical CO.sub.2
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 without pulsation.
* * * * *