U.S. patent application number 10/665181 was filed with the patent office on 2004-04-08 for enhancement of fluid replacement in porous media through pressure modulation.
This patent application is currently assigned to Aspen Aerogels, Inc.. Invention is credited to Lee, Kang P..
Application Number | 20040064964 10/665181 |
Document ID | / |
Family ID | 23095545 |
Filed Date | 2004-04-08 |
United States Patent
Application |
20040064964 |
Kind Code |
A1 |
Lee, Kang P. |
April 8, 2004 |
Enhancement of fluid replacement in porous media through pressure
modulation
Abstract
A method is described for enhancing mass and heat transport of
fluids in a fine pore structure through an appropriate modulation
of the fluid pressure. For example, in an air drying process for a
porous material that contains liquid, the air pressure is modulated
throughout the volume of the drying chamber. Alternatively, the
fluid pressure is modulated in a process stream. As an example,
this method can be used for rapid drying of any open porous
substances ranging from small pored materials such as aerogels and
xerogels, to larger pored substances or articles such as industrial
articles, agricultural articles (e.g., densely stacked vegetables,
coffee beans, hops and other grains), paper-based products, thin
films, pharmaceuticals, cloth, and clothing.
Inventors: |
Lee, Kang P.; (Sudbury,
MA) |
Correspondence
Address: |
EDWARDS & ANGELL, LLP
P.O. BOX 55874
BOSTON
MA
02205
US
|
Assignee: |
Aspen Aerogels, Inc.
Aspen Systems, Inc.
|
Family ID: |
23095545 |
Appl. No.: |
10/665181 |
Filed: |
September 16, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10665181 |
Sep 16, 2003 |
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10124433 |
Apr 16, 2002 |
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10665181 |
Sep 16, 2003 |
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09693390 |
Oct 20, 2000 |
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6670402 |
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60285751 |
Apr 23, 2001 |
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60160464 |
Oct 21, 1999 |
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Current U.S.
Class: |
34/343 ;
34/467 |
Current CPC
Class: |
B01J 20/3408 20130101;
B01J 20/3491 20130101; C01B 33/1585 20130101; B01D 15/40 20130101;
Y10S 977/841 20130101; Y10S 977/961 20130101; Y10S 977/962
20130101; B01J 13/0091 20130101; B01J 20/103 20130101; B01J 20/18
20130101; B01D 12/00 20130101; B01J 20/3433 20130101 |
Class at
Publication: |
034/343 ;
034/467 |
International
Class: |
F26B 003/00 |
Claims
What is claimed is:
1. A method for increasing transport of fluids within a porous
medium, said method comprising applying pressure pulses to the
fluids, said pressure pulses having a predetermined frequency and
amplitude.
2. The method of claim 1, wherein at least one of the fluids is
compressible.
3. The method of claim 2, wherein at least one compressible fluid
is a supercritical fluid.
4. The method of claim 1, wherein the pulses have an amplitude in
the range of about 0.0001% to about 50% of a mean process
pressure.
5. The method of claim 1, wherein the pulses have a frequency in
the range of about 0.001 Hz to about 100 MHz and an amplitude in
the range of about 0.01 psi to about 1000 psi.
6. The method of claim 1, wherein the pulses have a frequency in
the range of about 0.0001 Hz to about 100 kHz and an amplitude in
the range of about 0.01 psi to about 10 psi.
7. The method of claim 1, comprising applying pressure pulses of at
least two different frequencies.
8. The method of claim 7, wherein the at least two different
frequencies include a high frequency pulse, and a low frequency
pulse, and wherein the pressure amplitude of the high frequency
pulse is in the range of about 0.0001% to about 10% of a mean
process pressure and the pressure amplitude of the low frequency
pulse is in the range of about 1% to about 50% of the mean process
pressure.
9. The method of claim 8, wherein the pressure amplitude of the low
frequency pulse is higher than the pressure amplitude of the high
frequency pulse.
10. The method of claim 1, wherein the porous medium is a small
pored structure selected from the group consisting of an aerogel, a
xerogel, a silica gel, and a zeolite.
11. The method of claim 1, wherein the porous medium is a large
pored article selected from the group consisting of an agricultural
article, a paper-based article, an article of clothing, a thin
film, and a pharmaceutical.
12. The method of claim 11, wherein the agricultural article is
selected from the group consisting of a vegetable, a coffee bean,
and a grain.
13. A method for reducing the time required to replace a first
fluid located within a porous medium with a second fluid, which is
miscible with the first fluid, said method comprising applying
pressure pulses to the second fluid at a predetermined frequency
and amplitude.
14. The method of claim 13, wherein at least one of the fluids is a
compressible fluid.
15. The method of claim 14, wherein the compressible fluid is a
supercritical fluid.
16. The method of claim 13, wherein the pulses have an amplitude in
the range of about 0.0001% to about 50% of a mean process
pressure.
17. The method of claim 13, comprising applying pressure pulses of
at least two different frequencies.
18. The method of claim 17, wherein two of the at least two
different frequencies are a first frequency in the range of about 1
Hz to about 100 MHz and a second frequency in the range of about
0.0001 to about 100 kHz.
19. The method of claim 18, wherein the second frequency is lower
than the first frequency.
20. The method of claim 18, wherein the pulses of the first
frequency have an amplitude in the range of about 0.01 to 20 psi
and the pulses of the second frequency have an amplitude in the
range of about 0.1 to 1,000 psi.
21. The method of claim 20, wherein the amplitude of the second
frequency is higher than the amplitude of the first frequency.
22. The method of claim 16, wherein the at least two different
frequencies include a high frequency pulse, and a low frequency
pulse, and wherein the pressure amplitude of the high frequency
pulse is in the range of about 0.0001% to about 10% of a mean
process pressure and the pressure amplitude of the low frequency
pulse is in the range of about 1% to about 50% of the mean process
pressure.
23. The method of claim 22, wherein the pressure amplitude of the
low frequency pulse is higher than the pressure amplitude of the
high frequency pulse.
24. The method of claim 13, wherein the porous medium is a small
pored structure selected from the group consisting of an aerogel, a
xerogel, a silica gel, and a zeolite.
25. The method of claim 13, wherein the porous medium is a large
pored article selected from the group consisting of an agricultural
article, a paper-based article, an article of clothing, a thin
film, and a pharmaceutical.
26. The method of claim 25, wherein the agricultural article is
selected from the group consisting of a vegetable, a coffee bean,
and a grain.
27. A method of drying a porous medium containing a liquid, said
method comprising; providing a fluid to the porous medium under
conditions to vaporize the liquid; and applying pressure pulses to
the fluid at a predetermined frequency and amplitude.
28. The method of claim 27, wherein the fluid is compressible.
29. The method of claim 28, wherein the compressible fluid is a
supercritical fluid.
30. The method of claim 27, wherein the pulses have an amplitude in
the range of about 0.0001% to about 50% of a mean process
pressure.
31. The method of claim 27, comprising applying pressure pulses of
at least two different frequencies.
32. The method of claim 31, wherein the two or more different
frequencies are a first frequency in the range of about 1 Hz to
about 100 MHz and a second frequency in the range of about 0.0001
to about 100 kHz.
33. The method of claim 32, wherein the second frequency is lower
than the first frequency.
34. The method of claim 32, wherein the pulses of the first
frequency have an amplitude in the range of about 0.001 to 20 psi
and the pulses of the second frequency have an amplitude in the
range of about 0.1 to 1000 psi.
35. The method of claim 34, wherein the amplitude of the second
frequency is higher than the amplitude of the first frequency.
36. The method of claim 31, wherein the at least two different
frequencies include a high frequency pulse, and a low frequency
pulse, and wherein the pressure amplitude of a high frequency pulse
is in the range of about 0.0001% to about 10% of a mean process
pressure and the pressure amplitude of a low frequency pulse is in
the range of about 1% to about 50% of the mean process
pressure.
37. The method of claim 36, wherein the pressure amplitude of the
low frequency pulse is higher than the pressure amplitude of the
high frequency pulse.
38. The method of claim 27, wherein the porous medium is a small
pored structure selected from the group consisting of an aerogel, a
xerogel, a silica gel, and a zeolite.
39. The method of claim 27, wherein the porous medium is a large
pored article selected from the group consisting of an agricultural
article, a paper-based article, an article of clothing, a thin
film, and a pharmaceutical.
40. The method of claim 39, wherein the agricultural article is
selected from the group consisting of a vegetable, a coffee bean,
and a grain.
41. A method of extracting a soluble component from a porous
medium, said method comprising; providing a compressible fluid to
the porous medium; and applying pressure pulses to the compressible
fluid at a predetermined frequency and amplitude.
42. The method of claim 41, wherein the fluid is compressible.
43. The method of claim 42, wherein the compressible fluid is a
supercritical fluid.
44. The method of claim 41, wherein the pulses have an amplitude in
the range of about 0.0001% to about 50% of a mean process
pressure.
45. The method of claim 41, comprising applying pressure pulses of
at least two different frequencies.
46. The method of claim 45, wherein two of the at least two
different frequencies are a first frequency in the range of about 1
Hz to about 100 MHz and a second frequency in the range of about
0.0001 to about 100 kHz.
47. The method of claim 46, wherein the second frequency is lower
than the first frequency.
48. The method of claim 46, wherein the pulses of the first
frequency have an amplitude in the range of about 0.001 to 20 psi
and the pulses of the second frequency have an amplitude in the
range of about 0.1 to 1000 psi.
49. The method of claim 48, wherein the amplitude of the second
frequency is higher than the amplitude of the first frequency.
50. The method of claim 45, wherein the at least two different
frequencies include a high frequency pulse, and a low frequency
pulse, and wherein the pressure amplitude of a high frequency pulse
is in the range of about 0.0001% to about 10% of a mean process
pressure and the pressure amplitude of a low frequency pulse is in
the range of about 1% to about 50% of the mean process pressure
51. The method of claim 50, wherein the pressure amplitude of the
low frequency pulse is higher than the pressure amplitude of the
high frequency pulse.
52. The method of claim 41, wherein the porous medium is a small
pored structure selected from the group consisting of an aerogel, a
xerogel, a silica gel, and a zeolite.
53. The method of claim 41, wherein the porous medium is a large
pored article selected from the group consisting of an agricultural
article, a paper-based article, an article of clothing, a thin
film, and a pharmaceutical.
54. The method of claim 53, wherein the agricultural article is
selected from the group consisting of a vegetable, a coffee bean,
and a grain.
Description
CROSS REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation of, claims priority from,
and incorporates by reference the entirety of currently pending
U.S. patent application Ser. No. 10/124,433, which was filed on
Apr. 16, 2002, and which claimed priority from U.S. Provisional
Patent Application Serial No. 60/285,751, which was filed on Apr.
23, 2001, and which is now abandoned, and this application is a
continuation-in-part of, claims priority from and incorporates by
reference the entirety of currently pending U.S. patent application
Ser. No. 09/693,390, which was filed on Oct. 20, 2000, and which
claimed priority from U.S. Provisional Patent Application Serial
No. 60/160,464, which was filed on Oct. 21, 1999, and which is now
abandoned.
FIELD OF THE INVENTION
[0002] The present invention relates to methods for enhancing
replacement of one fluid in a porous medium by a second fluid.
Particularly, the present invention relates to the use of pressure
fluctuation to enhance the mass and heat transport in a porous
medium and, in particular, the replacement of one fluid in a porous
medium by a second fluid. More particularly, the present invention
relates to such methods wherein at least one of the fluids is
compressible. Various applications include drying, solvent
exchange, removal of soluble impurities, and the like.
BACKGROUND OF THE INVENTION
[0003] Many processes involve the replacement of one fluid in a
porous media by a second fluid. For example, the process of drying
involves the replacement of a liquid, frequently water, with a gas,
usually air, through a process of evaporation. In another example,
the extraction of caffeine from coffee beans can be considered as
imbibing a solvent for the caffeine into the bean and replacing the
solvent containing the caffeine with pure solvent, thereby
extracting the caffeine from the bean. Also, aerogel products,
after wet gel formation, are conventionally prepared by a process
of solvent exchange between liquid CO.sub.2 and the solvent that
was utilized to form the wet gels, followed by a supercritical
CO.sub.2 extraction.
[0004] It often is desirable to perform these processes in a
shorter period of time. Frequently, heat is used to hasten or
sustain or support such processes. However, the heat transfer
inside the porous medium can be very slow, and there are times when
the application of high temperature heat can degrade the product.
In drying, often vacuum is applied to hasten the process without
degrading by heat. However, vacuum application requires extra
equipment and expense, and still may require considerable time
periods for completion of the process. Further, repressurization or
depressurization may require care to avoid harm to the product.
[0005] Supercritical fluids can be used as solvents in extraction
instruments, chromatographs and other related instruments. In
supercritical fluid extraction, an extraction vessel is held at a
temperature above the critical point and is supplied with fluid at
a pressure above the critical pressure. Under these conditions, the
fluid within the extraction vessel is a supercritical fluid. In
supercritical fluid chromatography, a similar process is followed
except that the supercritical fluid moves the sample through a
column, separates some of the components of the sample one from the
other, and removes the components from the column.
[0006] The critical temperature is the temperature above which the
distinction between gases and liquids disappears--that is, where
there is one fluid phase for all pressures, and where, no matter
how much pressure is applied, a liquid phase cannot be condensed.
The supercritical region is defined by all temperatures and
pressures above the critical temperature and pressure.
Supercritical fluids are a useful hybrid of gases and liquids as we
commonly perceive them, possessing gas-like viscosities,
liquid-like densities, and diffusivities greater than typical
liquid solvents. The liquid-like density of a supercritical fluid
imparts a variable liquid-like solvent power by an essentially
linear function of density. This allows the solvent power, usually
considered a chemical interaction, to be set ("dialed in") simply
by adjusting a physical parameter, namely density or pressure.
[0007] The supercritical fluid transport properties of relatively
low viscosity and relatively low diffusivity allow enhanced mass
transport within complex matrices, such as coal, plant or animal
tissue, or packed beds. In other words, supercritical fluids
penetrate better and dissolve almost as well as typical liquids.
Therefore, supercritical fluids are more efficient to use for
extractions of complex matrices.
[0008] Carbon dioxide is the principal extracting fluid used in
supercritical fluid extraction systems because it is cheap,
innocuous, readily available at high purities, and has a relatively
low critical temperature of about 31.degree. C. Thus, it is useful
for thermally labile compounds and to avoid the hazards of high
temperature flammable solvents. Furthermore, it is mutually soluble
with many common liquid solvents.
[0009] It has been found that carbon dioxide has a solvent power
similar to that of hexane. Hence, many applications exist that
require great solvent power, the advantageous properties of
supercritical fluids, and mild operating temperatures for thermally
labile compounds. Mixtures of carbon dioxide plus modifiers can
meet these requirements. As is well known to those of ordinary
skill, supercritical fluids can be used as solvents in extractions
and chromatography; in such applications carbon dioxide is the
preferred solvent. Other fluids, e.g., ethane, nitrous oxide,
ethylene, or sulfur hexafluoride, that have critical points near
ambient temperature (25.degree. C.) can also function as the base
solvent. The capability to utilize these alternative solvents is
preferably not exploited because of the potential danger in using
these solvents.
[0010] U.S. Pat. No. 5,133,859 describes a sample preparation
device, which extracts sample components from complex matrices
using supercritical carbon dioxide as the principal extracting
solvent, and which presents the resulting extract in a user-chosen
sample collection vessel. Traditional preparative procedures such
as solvent extraction, Soxhlet extraction, liquid/liquid
extraction, concentration, and evaporation are replaced with the
solvent power stepwise settable by the parameters of density,
modifier concentration, and temperature.
[0011] The supercritical fluid extractor can mimic column
chromatography sample fractionation in some applications.
Accordingly, the fluid flow system apparatus comprises control
apparatus having a variable and controllable flow restriction and a
sample container section. The sample is inserted into the sample
container section, the temperature, pressure, flow rate and
extraction time setpoints are inputted into the control apparatus,
and pressurized fluid is provided.
[0012] By directing the fluid to a pump--which injects the fluid
into the flow system apparatus at the input flow rate setpoint--the
extraction process is initiated. The system pressure is sensed as
fluid is pumped into the system, and the variable flow restriction
is regulated to achieve and to maintain the setpoint pressure.
Extraction is accomplished by directing fluid through the sample at
the setpoint flow rate, and by directing a fluid mixture leaving
the sample container section to an expansion nozzle section.
[0013] Preferably, the methods include maintaining the controlled
setpoints of flow, pressure, and temperature until the input
extraction time is achieved. The methods also contemplate opening
and closing the orifice in order to control the variable flow
restriction, or closing the orifice until setpoint pressure is
achieved and controlling the restriction of the orifice to maintain
the setpoint pressure.
[0014] In one class of supercritical fluid extraction of soluble
components from a sample using a supercritical fluid, the
components dissolved in the extraction fluid are separated from the
fluid by allowing the extraction fluid to vaporize. For extraction,
supercritical fluid flows through material to be extracted.
[0015] As described in U.S. Pat. No. 6,149,814, the fluid flows
through a heat exchanger so that the heat exchanger is at the same
temperature as a pressure vessel and an extraction tube. Before
using the extraction system, the pump is set to the desired
pressure and the heater block is set to the desired temperature.
The internal cavity is then filled or partly filled with sample to
be extracted. Pressurized fluid flows through a valve into the heat
exchanger so that it is at the desired supercritical temperature,
and flows into the cavity. This supercritical fluid flowing through
the interior sample cavity of the extraction cartridge extracts the
soluble components from the sample contained within the cavity.
[0016] In making aerogel products via a conventional process, the
solvent from the gelling step must be removed to form a desired
aerogel monolith. To do this, the wet gels--after a proper aging
process--are quickly placed into an extractor that is filled with
liquid carbon dioxide.
[0017] The relatively long solvent exchange process then 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.
[0018] It is desirable to shorten the time period for completing
all of these and other processes involving replacement in a porous
matrix of one fluid by a second fluid, or involving extraction of a
soluble component using a supercritical fluid.
[0019] In copending application, U.S. Ser. No. 09/693,390 filed
Oct. 20, 2000, the processing time for preparing aerogel products,
once wet gels have been placed inside an extractor for
supercritical drying, is reduced substantially by a method wherein
a first fluid within a gel is replaced by a second fluid while
applying pulses of pressure having at least one frequency to the
gel, the first fluid and the second fluid during the exchange. The
disclosure of U.S. Ser. No. 09/693,390 filed Oct. 20, 2000 is
hereby incorporated by reference.
SUMMARY OF THE INVENTION
[0020] It has been discovered that the time for replacement of one
fluid in a porous medium by a second fluid in various processes can
be shortened substantially by the use of pressure waves applied to
the replacement second fluid. In accord with the present invention,
the pressure waves are applied in addition to any pressurization
and/or depressurization cycle conventionally used for processing.
The application of pressure waves in accord with the present
invention enhances fluid transport in the porous medium. In
preferred aspects of the invention, at least one of the fluids is
compressible. The methods of the present invention can be used, for
example, for rapid drying of any open porous substances ranging
from small pored materials, such as aerogels and xerogels, to
larger pored substances, such as industrial articles, thin films,
agricultural articles (e.g., densely stacked vegetables, coffee
beans, hops and/or other grains), pharmaceuticals, and/or
paper-based products, cloth, clothing, etc.
[0021] Examples of processes contemplated within the scope of the
present invention include, without limitation, the following
exchanges of fluids in porous media:
[0022] liquid to liquid--e. g., water/ethanol exchange for a
hydrogel process using water glass.
[0023] liquid to vapor/gas--e.g., initial process for drying
vegetables, clothes, xerogels, etc. during which water is removed
by evaporation into the air (i.e., air replaces the water in the
porous medium).
[0024] vapor to vapor/gas--e. g., subsequent process for drying
vegetables, clothes, xerogels, etc. once the liquid phase has been
mostly removed from the objects, leaving only vapor phase remaining
inside. Another example is the depressurization process of Alcogel
after solvent exchange.
[0025] liquid to supercritical fluid--e.g., solvent exchange
process for Alcogel in which alcohol from inside the Alcogel is
extracted using supercritical CO.sub.2.
[0026] More particularly, the present invention is directed to the
use of pressure fluctuation, such as a pressure wave or a series of
pressure pulses, to enhance the exchange of fluids in porous media,
or to extract soluble materials from one or more porous media. In
preferred aspects of the invention, high frequency pressure
fluctuations increase the effective mass and/or heat diffusivity at
either the interface between the exchanging fluids in the porous
medium, or the interface between the solvent and the soluble
material contained in the porous medium, and low frequency pressure
fluctuations increase the effective mass transport and/or heat
transfer rates through the porous medium. The pressure pulses
applied in accord with the present invention provide pressure
fluctuations around the pressure set point or pressure profile
conventionally used in a process.
[0027] In certain preferred aspects of the invention, at least one
of the fluids is a compressible fluid. A preferred compressible
fluid for extraction of a soluble component from a porous medium is
a supercritical fluid. Certain alternative aspects of the present
invention also provide a method for increasing the diffusion of a
compressible fluid into a porous structure, e.g., to extract a
soluble component from within the porous medium, by applying to the
compressible fluid pressure pulses at predetermined frequency and
amplitude.
[0028] As used herein, the term "porous medium" includes any
material in which a fluid can diffuse from the exterior into the
interior thereof, or vice versa.
[0029] As used herein, the term "aerogel" includes (unless context
requires a narrower meaning) 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 or zeolite.
[0030] 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.
[0031] 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.
[0032] 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 gels, or to a fluid in which a component contained in
a porous medium is soluble.
[0034] The term "gas" denotes a fluid, wherein the pressure is
below the supercritical pressure for that fluid, and wherein 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 "vapor" refers to the gas in contact with a liquid,
both being composed of molecules of the same substance.
[0037] 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.
[0038] The term "pulse" refers to a fluctuation 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.
[0039] Although certain aspects/embodiments of the present
invention are described generally hereinafter by referring to
supercritical carbon dioxide as the supercritical extraction fluid,
all such references are intended to include alternative
supercritical extraction fluids unless such references are
otherwise specified as being 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.
DETAILED DESCRIPTION OF THE INVENTION
[0040] The present invention provides a method for enhancing fluid
transport in a porous medium. Thus, the present invention provides
a method for enhancing the replacement of a fluid in a porous
medium by a second fluid, or a method for enhancing the extraction
of a soluble component contained in a porous medium. The method
provides one or more pressure pulses with different frequencies
and/or amplitudes to the fluid and porous medium during the
replacement or extraction process. Preferably, a series of pressure
pulses is applied to the fluid and porous medium during the
replacement or extraction process. More preferably, a continuous
pressure pulsation is applied to the fluid and porous medium during
the replacement or extraction process.
[0041] In accord with the present invention, superimposing the
pressure pulsation of the fluid and porous medium upon the
otherwise conventional process for fluid replacement or soluble
component extraction in a porous medium substantially speeds up the
process. Thus, if the conventional process includes a cycle of
pressurization and depressurization, the present invention
superimposes on that conventional process cycle, pressure pulsation
of the fluid and porous medium, thereby enhancing fluid transport
within the medium. The pressure pulsation in accord with the
present invention can be applied for a portion of the time period
during the conventional process cycle, or it can be applied
continuously throughout the conventional process cycle.
[0042] The pressure pulsation can be applied generally with a
frequency in the range of about 0.0001 Hz to about 100 MHz.
Preferred ranges of pressure pulse frequency include low frequency
ranges of about 0.0001 Hz to about 10 Hz or about 0.001 to about 1
Hz and high frequency ranges of about 2,000 Hz to about 50,000 Hz
and about 5000 Hz to 30,000 Hz. However, the optimum frequency can
be higher or lower those measurements encompassed by these ranges,
depending upon the particular fluids, the particular porous medium,
and/or the desired characteristics of the final product being
made.
[0043] Maximum allowable pressure amplitudes for pressure pulses
will depend upon the particular porous medium and upon the
frequency of pulsing. Typically, other things being equal, the
maximum allowable pressure amplitude will decrease as the frequency
increases. The pressure pulse also usually will not cause material
expansion or contraction of the matrix of the porous medium. In
general, the pressure amplitude will be between about 0.0001% and
about 50%. When more than one frequency is used, the pulse
amplitude of the lower frequency will be greater than the pulse
amplitude of the higher frequency.
[0044] In general, the pressure amplitude of the high frequency
pulses will be between about 0.0001% and about 10% of the mean
process pressure. Particularly, the pressure amplitudes of high
pressure pulses will be between about 0.001% and about 5% of the
mean process pressure and, more particularly, between about 0.01%
and about 3% of the mean process pressure. In certain preferred
embodiments, the pressure amplitudes will be between about 0.01%
and about 2% of the mean pressure.
[0045] In general, the pressure amplitude of the low frequency
pulses will be between about 1% and about 50% of the mean process
pressure. Particularly, the pressure amplitudes of low pressure
pulses will be between about 2% and about 40% of the mean process
pressure and, more particularly, between about 3% and about 30% of
the mean process pressure. In certain preferred embodiments of the
invention, the low pressure pulse pressure amplitudes will be
between about 3% and about 20% of the mean pressure.
[0046] For example, if low frequencies in the range of about 0.0001
to about 10 Hz and are encountered during the supercritical
extraction process, pressure amplitudes ranging from about 10 psi
to up to 1000 psi (preferably about 100 psi to 600 psi), are
generally useful, provided that the material can tolerate the
pressure gradient. For high frequencies, useful pressure amplitudes
during supercritical extraction process typically will be in the
range of about 0.01 to about 20 psi, more typically about 0.3 to 5
psi, and often in the range of about 0.5 to 3 psi.
[0047] The efficiency of the solvent exchange procedures that
utilize/incorporate an extraction fluid can 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 at the interface of the two
fluids within the porous media. Alternatively, low frequency/high
amplitude pulsations can be used to effectively pump out high
solvent concentration solution from inside porous media, and to
pump in fresh solvent into the porous media if the extraction fluid
is compressible, which is the case with supercritical fluids such
as CO.sub.2. By supplying fresh fluid, this pumping action also
provides a mechanism for heating or cooling within the porous media
with fluid having a different temperature being supplied externally
of the porous media.
[0048] Preferably, two or more pulsations having different
amplitudes and frequencies are used simultaneously for a
compressible fluid. 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.
[0049] 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 sizes 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
porous medium.
[0050] Without wishing to be bound by a theory, it is believed that
high frequency pulsing accelerates fluid exchange within a porous
medium because pulsing rapidly dilutes the solvent that is near the
boundary between the extraction fluid phase and the liquid solvent
phase, inside the porous medium. This applies also to the solvation
of a soluble component at the boundary between the solid and liquid
in the porous medium.
[0051] For example, for mixtures of supercritical fluid and
solvent, it is postulated that 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 this single phase region of supercritical
conditions for many solvent/supercritical fluid mixtures occurs
sufficiently near the critical point of the supercritical fluid to
use the supercriticality of the mixed fluid to enhance the rate of
extraction of the solvent from within the porous medium, thereby
reducing the overall process time. Similarly, the rate of
extraction of a soluble component by a supercritical fluid is also
enhanced.
[0052] Using an example of ethanol as the solvent and CO.sub.2 as
the supercritical fluid (such as can be encountered in extraction
of solvent from an aerogel), a mixture containing about 50% of each
by volume is a single phase and is 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 super-critical fluid, inter-diffusion begins at the
interface between the solvent ethanol inside the wet gels, and at
the supercritical CO.sub.2 outside the gels as well. The
inter-diffusion is enhanced by transmitting a high frequency
fluctuation through the super-critical CO.sub.2.
[0053] 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 a compressed form both within the
extractor generally and within the gel specifically, such that more
of the molecules will on average move into the gel.
[0054] 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, molecules of the solvent liquid are then 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.
[0055] Without wishing to be bound by a particular theory, the
mechanism of diffusion enhancement by high frequency pressure
pulses, for example, 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.
[0056] 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 will 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.
[0057] 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 fluid inside 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
that relies on a 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.
[0058] For low frequency, high amplitude pulses are used, because
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 (or other soluble component)
will flow out of the porous medium, thus enhancing the rate of
solvent removal from the gel. During the compression period, the
supercritical fluid having a lower concentration of solvent will be
forced back into the gel, thus 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
porous medium will come from the fresh supercritical CO.sub.2
stream outside the porous medium, thereby lowering the
concentration of the solvent (or other soluble component) in the
"supercritical boundary layer" and supplying heat to the
"supercritical" boundary layer that had undergone an
expansion-related temperature drop.
[0059] 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 porous medium into the supercritical
fluid to be swept away. The low frequency high amplitude
compression and expansion cycles are repeated until the soluble
component is substantially removed from the porous medium, i.e.,
until the porous medium contains mostly supercritical fluid with
only a trace amount of soluble component.
[0060] Also, as the pressure increases during the low frequency
compression period and as the fluid is pushed into the porous
medium, 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 porous medium.
Once the solvent (or other soluble component) has dissolved into
the supercritical fluid and the pressure has decreased, the density
of the supercritical fluid lowers, thus causing the fluid to expand
out of the porous medium. 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 porous medium.
[0061] Moreover, in preferred embodiments of the invention, it is
useful to gradually increase the wavelength (decrease the
frequency) of the pressure pulses as the front between the
supercritical fluid and the soluble component moves into the porous
medium and as the distance to travel from the mixing layer to the
surface of the porous medium increases.
[0062] Therefore, there is independent utility in each of
high-frequency pulses, low-frequency pulses, and a 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, can be especially effective.
[0063] The amplitude of the high frequency pulses at a given
frequency generally 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 porous medium.
[0064] When the pores are very small, as in aerogels, the
frictional force exerted on the structure by passage of fluid is
surprisingly large. For example, in many aerogels, the upper limit
of pressure amplitude for high frequency pulses will be about 5
psi. 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
fluid movement increases, and this can place a higher pressure
gradient across local regions of the porous medium than is found at
lower frequencies of the same amplitude.
[0065] For a supercritical CO.sub.2 extraction in an aerogel
(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 Hz to about 100 MHz, 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.
[0066] 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. Corresponding 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.
[0067] Specific pressure amplitude/frequency combinations should be
determined for particular compositions of porous materials by
routine experimentation, when taking into consideration, e.g., the
specific porosity, pore size distribution, compressive and tensile
strengths of the lattice structure, as well as physical size and
shape of the materials. Under suitable selection of frequencies and
pressure amplitudes, the porous materials 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 porous
material.
[0068] Other frequencies or wavelengths can be used, depending upon
the nature of the porous medium and the fluids. It is specifically
contemplated that higher frequencies, for example in the range of
100,000 to 10 million Hz (used, e.g., in ultrasound and
lithotripsy) can be used where suitable. Such faster cycles can
require lower amplitudes to avoid creating excessive pressure
gradients.
[0069] In selecting a pulse amplitude, it should also be noted 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 recondensation of the solvent into a
separate liquid phase due to a reduction in solubility when the
density is reduced by pressure reduction, and despite the
extraction fluid remaining 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.
[0070] If appropriately limited, however, a moderate degree of
lowering of the pressure or density will not cause re-condensation
of the solvent.
[0071] It should be noted that during pressure fluctuations, the
shape and size of the 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.
[0072] 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 soluble component laden supercritical CO.sub.2 tends
to come out of the porous medium.
[0073] Pressure pulses suitable for the practice of the invention
can be generated by any means or method that gives/produces the
required frequency and amplitude of pulsations in pressure inside
the extractor. The source of the pulses can be inside the
extractor, outside the extractor (and typically in intimate contact
with it), or can form a part of the extractor.
[0074] The pulses can 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
piezoelectric 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 can be an
expandable balloon or bellows, either within the extractor or
exterior to it and connected by a port. An inflatable device can be
inflated by a gas or liquid. Likewise, a piston can be internal, or
external via a port, and can be moved by pressure or by mechanical
force.
[0075] Each of these methods 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
can be by any method known to those skilled in the art.
[0076] The methods of the present invention can be utilized in a
variety of cases where pressure fluctuation causes changes
equilibrium vapor pressure, solute solubility, etc. Specific
examples include the following. However, these are only examples
where the methods of the present invention can be used. The
invention is not limited to these specific examples.
EXAMPLE 1
Interdiffusion of Ethanol and Water Inside Wet Gel
[0077] Two wet gel samples were prepared from tetra-ethoxysilane
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. To evaluate the effect of pressure pulses on the
enhancement of diffusion process, two wet gel samples prepared by
the above method were processed as follows.
[0078] The first sample was simply immersed in a jar containing
pure ethanol, and the diffusion of the red dye was monitored. The
second sample also was immersed in a same sized jar of pure
ethanol, but then the jar was placed in a sonic bath. In between
the bottom of a sonic bath and the bottom of the jar containing the
wet gel sample, there was a 1.25 cm thick sponge pad to attenuate
the amplitude of the sonic pulse to the point that the sonic energy
did not breakup the wet gels.
[0079] The sonic cleaning bath generated fixed wavelength pulses at
20 kHz. The diffusion of the red dye out of the wet gels into the
ethanol bath was observed and periodically photographed. A UV
spectrometer measured the frequency of the UV light transmitted
through the ethanol.
[0080] The results showed that the dye was extracted much more
rapidly in the sample under sonication. Extraction to visual color
equilibrium between the wet gel and the ethanol bath was obtained
in about 45 minutes in the pulse-assisted case, as opposed to more
than 16 hours in the case of the wet gel and the ethanol bath
without the pulse-assist.
EXAMPLE 2
Drying of Xerogels
[0081] Unlike aerogels (which, after solvent exchange, can be dried
under supercritical pressure conditions), xerogels almost uniformly
are dried under atmospheric conditions. For example, because of the
nano-sized pores present therein, xerogel beads are generally dried
in ovens, and only after anti-shrinkage materials have been
introduced into the wet xerogels. Because the nanometer-sized pores
are so small, they tend to maintain the liquid and/or vapor trapped
therein under these pressure conditions, thus rendering the drying
process slow (i.e., because of the relatively low rate of diffusion
within the beads, and due to evaporation and heat transfer between
the drying gas and the beads).
[0082] However, in accordance with the methods outlined above, the
process is greatly expedited. In particular, the pressure
pulses/fluctuation facilitates the removal of the trapped liquid
and/or vapor and, in turn, allows for improved diffusion,
evaporation, and heat transfer.
EXAMPLE 3
Evaporation of Water from Porous Articles Such as Clothes,
Vegetables, Paper, etc. into Air
[0083] Water evaporates as long as the vapor pressure in the
receiving gas is lower than the equilibrium vapor pressure for a
given fluid and temperature. When the pressure is lowered by
pressure pulsation suddenly to less than the equilibrium pressure
for the given temperature, the water will evaporate at much higher
rate to reach the equilibrium at that lowered pressure level. Once
the water evaporates into the air--during the expansion cycle in
accord with the present invention--the moistened air is taken away
from the clothes rather than recondensed into liquid in the
materials being dried during the recompression cycle.
[0084] Evaporation of water requires a supply of heat; and
ordinarily, the heat has to be transported mostly by convection
from the drying air onto the surface of the article being dried,
followed by conduction through the porous media, which can be quite
poor conductors. In the meantime, the evaporation process tends to
cool down the porous article, thus further reducing the rate of
heat transport into the porous article to be dried.
[0085] If, for example, one seeks to dry a porous media that is
filled with saturated vapor, except for its interior section, which
is filled with liquid. In accordance with a normal drying process,
the saturated vapor has to diffuse out of the porous media and the
drying air has to diffuse into the porous media based on pure mass
diffusivity. The vapor, which has a relatively poor thermal
conductivity to begin with, is coming out of the porous media, and,
therefore, behaves as an undesirable thermal barrier between the
liquid deep inside the porous media to be evaporated and the heat
contained in the drying air outside the porous media.
[0086] In the case of pulse assisted drying, however, the lingering
vapor phase inside the porous media is rapidly pumped out and fresh
drying air with heat content is pumped back in effectively
increasing the rate of heat and mass transport. In other words, in
accordance with the pulse assisted drying process, the mass
transport between the drying air and the liquid vapor is enhanced,
and, by virtue of the enhanced mass transport, the heat transport
is enhanced as well.
[0087] Thus, another factor favoring pressure pulsation is that due
to increased mass transport, the effective heat transfer rate
between the drying air and the porous article is significantly
enhanced, as is the rate within the porous media. Therefore, the
drying process becomes much more efficient than constant pressure
dryers.
EXAMPLE 4
Interdiffusion of Two Different Vapors/Gases
[0088] Once the liquid phase is evaporated or removed by diffusion
from any part of the porous media, that entire region inside the
porous media becomes "compressible," and the subsequent low
frequency pressure fluctuation will create a so-called "pumping
effect" in the region. In other words, fresh gas from outside will
be compressed into the compressible region inside the porous media,
mixed with high concentration vapor, and the resulting mixture will
be pumped out on the down stroke. It is noted that the gas/vapor
compression/expansion does not physically compress/expand the
porous media.
EXAMPLE 5
"Evaporation" of Solvent from Inside of Wet Gels into Supercritical
Fluid
[0089] Solvent diffusion into supercritical fluid is very similar
to "evaporation" of liquid into gas in that the solute solubility
of the critical fluid also changes with pressure. This is
equivalent to "vapor" pressure equilibrium for water/air system.
When the pressure is increased, e.g., from 1200 pSi.sub.2 to 1400
psi for CO.sub.2 above the critical temperature of 31.1.degree. C.,
the ethanol solubility into supercritical CO.sub.2 significanlty
increases. It therefore will tend to absorb a higher concentration
of ethanol into CO.sub.2, and, as the pressure swings back down to
1200 psi, a large portion of the ethanol rich supercritical
CO.sub.2 from inside of the wet gel is pumped out and cannot go
back in easily. Thus, the slow fluctuation of supercritical
CO.sub.2 increases "effective" diffusion of solvent from wet gel to
the supercritical fluid outside.
EXAMPLE 6
Faster Extraction of Constituent Chemicals using Pulse Assisted
S.C. Spectrochromatography
[0090] U.S. Pat. No. 5,133,859 describes a sample preparation
device, which extracts sample components from complex matrices
using supercritical carbon dioxide as the principal extracting
solvent, and which presents the resulting extract in a user-chosen
sample collection vessel. Traditional preparative procedures such
as solvent extraction, Soxhlet extraction, liquid/liquid
extraction, concentration, and evaporation are replaced with the
solvent power that is stepwise settable by the parameters of
density, modifier concentration, and temperature. This enables the
supercritical fluid extractor to mimic column chromatography sample
fractionation in some applications.
[0091] A judicious use of pressure fluctuations in each column can
speed up the fractionation process reducing the process time
significantly and potentially reducing the size of the equipment
and cost of processing.
EXAMPLE 7
Faster, More Efficient Decaffeination of Coffee Beans, Hops
Extraction, etc.
[0092] Traditional methods of removing caffeine contained in coffee
beans or hops extraction for beer production can also be sped up
using the present invention of pressure pulse assisted extraction
technique. During the conventional CO.sub.2 extraction process, the
pressure of the supercritical extraction fluid, e.g., CO.sub.2, can
be pulsed to enhance removal of dissolved ingredients from inside
the coffee beans or hops.
EXAMPLE 8
Solvent Exchange Process in Wet Gels
[0093] Whenever there is a need to exchange solvents for wet gels,
either to remove contaminants or facilitate a subsequent
supercritical extraction, pulse assisted solvent exchange process
will significantly enhance the speed of the solvent exchange.
[0094] For example, water glass derived wet gels will contain water
and salt within the porous structure. During the salt washing
process by water, high frequency pulses will effectively expedite
the washing process. During the subsequent solvent exchange of
water with ethanol, again high frequency pulses can be used to
speed up the removal of water content from inside the wet gel
structure.
[0095] The amplitude and frequencies one can use for the present
purpose can be determined analytically, empirically or by a
combination of both. General guidelines are as follows.
[0096] The amplitudes should not be high enough to cause physical
damage to the porous media at the frequency used. Also, typically,
the amplitudes should not have any significant effect on the volume
of the matrix. Volume expansion or contraction of the matrix of the
porous medium generally is not desirable, nor practical for the
present invention.
[0097] The total pressure differential between any two points
inside the porous media should be lower than that which will cause
damage to the porous media. For example, some silica aerogels will
have a maximum tensile strength of 5 psi. In such cases, the
pressure fluctuation should not cause more than 5 psi tensile load
between any two points inside the gels. The open porosity and
tortuosity of the lattice structure will determine the transient
pressure drop through which the pressure will change with time. In
the case of drying wet articles in air, the pressure amplitudes
generally will be dictated by other considerations such as
economically allowable pressure containment and pulse generation
methods, in addition to the above considerations when
appropriate.
[0098] The methods of the invention are particularly useful in
porous media that is wet with a liquid for which the removal of the
liquid is through phase change of the liquid, followed by diffusion
or direct diffusion into a "drying" fluid, and in which internal
small flow passages become saturated with the "vapor" or "liquid"
from the liquid and diffusion of mass and heat is a dominant and a
limiting mechanism to remove the "solvent vapor" to the outside of
the porous article.
[0099] As discussed above, the method provides controlled pressure
fluctuations of drying fluid surrounding and inside the porous
material to be dried. In such cases, the phase change also requires
a supply of heat from external sources in order to maintain the
temperature of the liquid inside the porous article being dried,
and that the heat contained in the compressible fluid with enhanced
transportability due to pressure fluctuations will help supply the
heat to the interior of the porous article. In the case of
supercritical fluid used as compressible fluid, this supplied heat
can be used to prevent recondensing of supercritical fluid into
potentially damaging liquid within the porous media.
[0100] In the case of drying of liquid from porous article into the
air or other gases, the heat contained in the air getting pumped
into the porous article will tend to have the effect of increased
heat transfer from the drying gas into the interior, thereby
further increasing the speed of drying. Preferably, pressure waves
are applied to the extracting fluid to pump in fresh extracting
fluid by increasing the pressure, and later to drain out the fluid
by lowering the pressure during each pulse. The application of
pressure waves at a predetermined frequency overcomes the diffusion
limitation, and thereby greatly reduces the time for drying.
[0101] The invention has been described in detail including the
preferred embodiments thereof. However, it will be understood that
those skilled in the art may make modifications and improvements
within the spirit and scope of the invention as set forth in the
claims.
[0102] All documents (including, but not limited to, patents and
patent applications) referred to herein are incorporated by
reference in their entirety.
* * * * *