SAE TECHNICAL
PAPER SERIES
972430
Microwave-Powered Thermal Regeneration of
Sorbents for CO2,Water Vapor and
Trace Organic Contaminants
James E. Atwater, John T. Holtsnider, and Richard R. Wheeler, Jr.
UMPQUA Research Co.
Bernadette Luna
NASA Ames Research Center
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II!!!The Engineering
Society
For Advancing Mobility
Land Sea Air and Space ~
INTERNATIONA
L
27th International Conference
on Environmental Systems
Lake Tahoe, Nevada
July 14-17, 1997
400 Commonwealth Drive,Warrendale, PA15096-0001U.S.A. Tel:(412)776-4841 Fax:(412)776-5760
972430
Microwave-Powered Thermal Regeneration of
Sorbents for CO2, Water Vapor and
Trace Organic Contaminants
James E. Atwater,John T. Holtsnider, and Richard R. Wheeler, Jr.
UMPQUA Research Co.
Bernadette Luna
NASA Ames Research Center
ABSTRACT
Feasibility of the use of microwave heating to
achieve fast and efficient thermal regeneration of sorbents for
the removal of carbon dioxide, water vapor, and trace organics
from contaminated air streams has been conclusively
demonstrated. The use of microwave power offers several
advantages, including: improved heat transfer, lower thermal
losses, improved power utilization, and enhanced operational
capabilities. During the initial research, the sorption and
microwave-powered thermal desorption of acetone,
trichloroethylene (TCE), carbon dioxide, and water vapor was
studied at 2.45 GHz using a rectangular waveguide based test
apparatus. Both activated carbon and Carbosieve S-III were
identified as excellent microwave regenerable sorbents for use
in the removal of airborne organics. Water loaded silica gel,
Molecular Sieve I3X, and Molecular Sieve 5A were also
effectively regenerated under microwave irradiation at this
frequency. Molecular Sieve 5A and a carbogenic molecular
sieve prepared at NASA's Jet Propulsion Laboratory were
identified as viable microwave regenerable carbon dioxide
(CO2) sorbents. A sorbent bed containing multiple media was
challenged with air containing 0.5% CO2' 300 parts per
million (ppm) acetone, 50 ppm TCE, and saturated with water
vapor. The layered media bed was shown to effectively purify
the contaminated air stream and to be completely regenerated
by microwave induced heating. Spectral studies of the
reflection, transmission, and phase shifts of microwaves for a
variety of sorbents over the frequency range between 1.3- 2.7
GHz wereperformed.
INTRODUCTION
An investigation has been conducted to determine the
feasibility of using microwave power to promote thermal
regeneration of sorbents applicable to the life support
requirements of astronauts during Extravehicular Activity
(EVA). The primary advantages inherent to the use of
microwaves are the increases in energy efficiency which result
from the heating of materials by direct absorption of energy
(in contrast to the indirect supply of heat by thermal
conduction from heating elements), and the extremely rapid
rates of heating which are possible. This maximizes the
efficiency of energy transfer and minimizes conductive,
convective, and radiative losses as well as the need to heat
additional thermal massl.
These features suggest the
possibility of a more compact and thermally efficient
technology for the removal of CO2' H2O, and trace organic
contaminantsfrom air during EVA. A second generation
thermally regenerable air purification system is envisioned in
which microwave power is applied directly to the Portable
Life Support System (PLSS) via coaxial cable, thus providing
a means for regeneration which does not require removal of
sorbent cartridges from the Extravehicular Mobility Unit
(EMU). A third generation system could provide capability
for sorbent regeneration during EVA. While the current
investigation has been directed specifically toward EVA, the
operational efficiencies which can be gained by the use of
microwave power may apply equally to other life support
requirements such as Air Revitalization within the cabin of
spacecraft or within other enclosed habitats.
Susceptibility to microwave heating is a function of
the dielectric properties of the material.
Microwaves
encompass the upper end of the radio frequency (RF)
electromagnetic spectrum.
Due to their relatively long
wavelengths, the behavior of microwaves is different in many
respects from that of more energetic (and for most researchers
more familiar) regions of the spectrum sucha~IR, visible,and
UV light. If a high frequency RF signal is applied to a
conductor, a current will flow. If the same signal is applied to
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a non-conductor (i.e., a dielectric material) then
electromagnetic waves are propagated. If the electromagnetic
energy is absorbed by the dielectric, the temperature of the
material rises in proponion to the energy absorbed. The most
common embodiment of microwave heating is the microwave
oven in which 2.45 GHz microwaves couple primarily with
the rotational transitions of dipolar water molecules. Different
dipole-dipole loss phenomena occur in solids such as silicon
carbide (SiC) and barium titanate (BaTiO)) which couple very
effectively with microwaves. In these materials the frictional
losses which result in the generation of heat are manifested
through oscillatorily induced polarizations (Maxwell-Wagner
effect, etc.) rather than by rotationally induced internal
friction2,3.
In concept, microwave-powered thermal regeneration
of a loaded adsorbent can occur via three possible
mechanisms: I) the adsorbent only couples with the
microwaves; 2) both adsorbent and sorbate couple; and 3) the
sorbate only couples. From an energy efficiency perspective,
the latter mechanism offers the greatest potential savings in
comparison to conventional thermal regeneration methods
since the energy required for regeneration is transferred
directly to the sorption site. Water bound on silica gel is an
example of this phenomena. A founh mechanism for thermal
regeneration is made possible by mixing a microwave
susceptible material such as silicon carbide with a sorbent
material. This founh mechanism may apply to sorbents such
as lithium hydroxide (LiOH) and silver oxide (Ag20) in
which the contaminant, CO2' is chemically bound, i.e., as
lithium carbonate (Li2CO3) and silver carbonate (Ag2CO3)'
respectively. Regeneration of chemisorbed materials will, in
general, require higher temperatures, longer regeneration
times, or both. The ability of microwave heating to achieve
extremely rapid heat-up of microwave susceptible materials to
very high temperatures may be a panicular advantage when
applied to composite mixtures of these sorbents and
microwave susceptible materials, and may make possible the
regeneration of sorbents which otherwise would not be
practical.
For the purposes of the initial investigation, a
representative variety of candidate sorbent materials which are
potentially useful in the removal of airborne water vapor,
CO2' and trace organics were screened for susceptibility to
thermal regeneration using microwave power.
These
experiments were conducted at 2.45 GHz. In addition,
because the susceptibilities of materials to microwave heating
vary with the frequency of incident radiation, a variety of
sorbent materials were also screened for their dielectric loss
characteristics over a range of frequencies between 1.3 - 2.7
GHz. In these experiments, bulk propenies were measured,
with contributions arising from both the sorbent and the gas
phase filling the pores and intergranular spaces4-9.
regeneration using 2:45 GHz microwaves; and 2) investigation
of the spectral responses of sorbents over a range of incident
microwave frequencies to determine if particular regions of
the microwave spectrum exist which may be especially well
suited for use with a particular sorbent.
Sorption-Desporption Studies. A variety of sorbents
were selected for study, including: activated carbon, carbon
based molecular sieves, lithium hydroxide, silica gel, and
silver oxide. Acetone, carbon dioxide, trichloroethylene
(TCE), and water vapor were selected as challenge
contaminants. Small packed sorbent beds were exposed to
humidified air streams containing the appropriate
contaminants and the relationships between cumulative flow
and breakthrough of the sorbates were monitored. Once
breakthrough occurred, the sorbents were exposed to
microwave energy and the bed temperatures and effluent gas
concentrations were monitored. For the cases in which
successful thermal regeneration was indicated, this was
confirmed by a subsequent re-Ioading of the sorbent with the
contaminant to ensure that substantial sorption capacity had
been regained.
Microwave Spectral Studies. Low power studies
were conducted to examine the dielectric loss characteristics
of a selection of typical sorbent materials over a range of
frequencies. The dielectric loss propenies determine a
material's susceptibility to microwave heating. These
propenies are functions of both frequency of the incident
microwave radiation and of the temperature of the medium.
For this preliminary investigation, room temperature
experiments were conducted over the relatively narrow range
of frequencies between 1.3 - 2.7 GHz. A vector network
analyzer based system was assembled to monitor three
propenies of each test specimen: relative reflected power,
relative transmitted power, and the phase shift of the
transmitted power.
MATERIALS AND APPARATUS
Microwave Powered Thermal Desorption Apparatus
-
The microwave
transmission
and irradiation
testbed
is
illustrated in Figure I. The apparatus is composed of a series
of WR 430 rectangular waveguide elements, and includes a
magnetron, shorting plate, waveguides, directional coupler,
solid state microwave detector, RF power meter, and a water
load. An 800 W magnetron, emitting at 2.45 GHz, is located
in the launcher waveguide section. The launcher section is
terminated at one end by a shorting plate located at a distance
of 'I.-wavelength from the magnetron antenna.
At the
opposite end of the launcher section microwaves are
transmitted in the transverse electric (TE) mode into the test
chamber waveguide section where packed sorbent beds are
placed in the microwave field. The test chamber is connected
to a 60 decibel (dB) directional coupler instrumented with a
Hewlett-Packard (HP) 478A solid state microwave transducer
and HP 432A power meter. Any microwave energy which
has not been absorbed in the test chamber passes into the
directional coupler where the power level is monitored, and
then into a water load which serves as a sink for excess energy
to prevent the reflection of microwaves backward through the
waveguide components toward the magnetron.
Water is
circulated through the load at a flow rate of"" 1 L/min.
EXPERIMENT AL SECTION.
EXPERIMENTAL APPROACH -To explore the
usefulness of microwave power for the thermal regeneration
of sorbents, two primary areas of experimentation were
focused on: I) characterizationof the susceptibilityof a
typical range of sorbents loaded with common airborne
contaminants toward
microwave
powered
thermal
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Figure I.
Microwave Powered Thermal Regeneration
Experimental Apparatus. I-Compressed Gas and Pressure
Regulators, 2-Flow meter,
3-Sparger, 4-Sorbent Bed,
5-Magnetron, 6-Launcher, 7-Dewpoint/Hydrocarbon/CO2
Analyzer, 8-Directional Coupler, 9-Microwave Detector,
10- RF Power Meter, II-Water-Ioad.
Microwave
Choke Tube
Glass Column
Microwave power output is controlled by a variable
transformer which regulates the voltage to the magnetron
power supply. The packed sorbent beds are held in place
within the test chamber using the mechanism illustrated
schematically in Figure 2. The device is mounted vertically
through the center of the test chamber waveguide section.
The packed sorbent bed is confIDedwithin a length of quartz
tubing with an internal diameter of 1.07 em, and held in place
using glass wool end plugs. The sorbent bed is positioned
so that the mid-point of the packed bed is centered in the
waveguide. Gas enters from the bottom and flows upward
through the sorbent bed. A J-type thermocouple is mounted
inside.
Challenge gases were fed into the apparatus Hom
compressed gas cylinders, with initial pressures ranging from
15.5 - 20.7 MPa (2250 - 3000 psi). The gas stream passed
through primary and second stage pressure regulators, to an
Aalburg Instruments #052-04gG variable area flow meter
(Cole-Parmer, Chicago, IL), and then through a sparging
chamber, in which the stream was saturated with water vapor.
In the case of CO2 sorptions, the liquid within the sparging
vessel was maintained at an acidic pH and was pre-saturated
with the contaminant before flow into the packed sorbent bed
was initiated. In the case of the organic contaminants, the
aqueous phase consisted of a solution with an amount of
dissolved organic compound sufficient for equilibrium to exist
between the gas phase and the liquid phase, according to
Henry's Law. After passing through the sorbent bed, the
decontaminated air goes to the appropriate gas analyzer(s).
During a typical sorption/desorption experiment,
contaminated air passes through the sorbent bed at IUmin
until contaminant breakthrough is indicated by an effluent
level approaching the influent level.
For microwave
desorption, the gas feed stream is then switched to a nitrogen
tank and in most cases set to a reduced flow of 100 cm3/min.
Microwave powered desorption then commences. Microwave
power is manually adjusted based on the effluent gas
temperature to maximize the rate of desorption without
compromising the sorbent material with excessive
temperatures.
Sorbents - Type 830lMC activated carbon was
acquired from CECA, Inc. (Tulsa, OK). Coconut Shell
Gas Inlet
t
Figure 2. Device for Mounting Sorbent Beds in the
Rectangular Waveguide.
Charcoal, #5769, was purchased from Alltech (Deerfield, IL).
Carbosieve S-III carbon based molecular sieve was acquired
from Supelco (Belefonte, PA). A second carbon based
molecular sieve was prepared by Dr. Pramad Sharma at the Jet
Propulsion Laboratory using the methods of FoleylO. Lithium
hydroxide was purchased nom Cyprus Foote Mineral
Company (Kings Mountain, NC). Molecular sieves 5A and
13X were donated by UOP (Des Plaines, IL). Silica gel was
acquired nom EM Science (Gibbstown, NJ). Silver carbonate
was purchased nom Aldrich (Milwaukee, WI).
Contaminants - Contaminant levels of carbon dioxide
and water were meant to represent that of expired air. For the
trace organic contaminants SMAC levels of 300 ppm acetone
and 50 ppm trichloroethylene (TCE) were chosen.
Compressed gas cylinders containing 0.5% carbon dioxide in
air were obtained from Pacific Airgas (Portland, OR).
Acetone and trichloroethylene were obtained from Aldrich
(Milwaukee, WI). Separate contaminated air feed streams
containing 300 ppm acetone and 50 ppm trichloroethylene
(TCE) were prepared by addition of the appropriate volume of
contaminant into evacuated high pressure gas cylinders,
followed by pressurization with breathing quality air to 15.5
MPa (2250 psi) for steel cylinders and to 20.7 MPa (3000 psi)
for aluminum cylinders.
Analytical
Instruments
- Carbon dioxide
concentrations were determined using an ASTRO Model 2001
TIC/TOC analyzer (League City, TX) and an ASTRa
5600AT continuous on-line non-dispersive infrared (NDIR)
CO2 monitor. Trichloroethylene and acetone were determined
using an HP-57IOA gas chromatograph with a packed SP1000 column (Supelco, Belefonte, FA) and flame ionization
detection. Water vapor was monitored using an EG&G Model
880 Dew Point Hygrometer. Continuous hydrocarbon
3
monitoring was performed using a Beckman Model 400
Hydrocarbon Analyzer.
Multifrequency Microwave Transmission and
Reflection Apparatus A network analyzer based system was
assembled to measure the dielectric loss characteristics of
candidate sorbents over the frequency range between 1.3 - 2.7
GHz. The apparatus is illustrated schematically in Figure 3.
Variable frequency microwaves are output via coaxial cable
from the Hewlett-Packard (HP) Model 8754A Vector Network
Analyzer sequentially to: 1) a frequency doubler (MA~COM
D-6-4), 2) an HP Model 778D dual directional coupler, and 3)
a coaxial cable to WR 430 rectangular waveguide adapter.
Attenuated signals representing both incident and reflected
power levels are routed back to the network analyzer from the
directional coupler. Microwaves are transmitted through the
WR 430 rectangular waveguide in the transverse electric (TE)
mode. The microwaves pass through a WR 430 to WR 650
waveguide transition and into the Specimen Confmement
Chamber. The Specimen Chamber consists of a hollow
rectangular box (18 x 12.9 x 3 cm) with an internal volume of
414 cm3, constructed from 0.48 cm thickness polycarbonate
sheets. The path length for microwave travel through the
chamber is 2.05 em. Microwaves reflected by the specimen
travel in the reverse direction via the directional coupler to the
network analyzer.
Microwaves transmitted through the
Specimen Confmement Chamber pass through a WR 650 to
WR 430 transition into a WR 430 to coaxial cable adapter,
and then by coaxial cable to the network analyzer. The
network analyzer sweeps through the entire frequency range at
a pre-determined sweep rate. Output power, reflected power,
and transmitted power levels are monitored by the network
analyzer and analog signals are output, via analog to digital
converter (DATAQ 01180), to an IBM Compatible 80486
personal computer for data storage. In essence the waveguide
sections form two symmetrical 'horn antennas' which surround
the specimen and transmit microwaves through the specimen.
Dielectric loss characteristics are determined from a
comparison of transmitted and reflected power.
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Figure 3. Multifrequency Microwave Transmission Test
Apparatus: I) Network Analyzer, 2) Frequency Doubler, 3)
Dual Coaxial Directional Coupler, 4) Incident Power Signal,
5) Reflected Power Signal, 6) Coaxial Cable to WR 430
Waveguide Adapter, 7) WR 430
WR 650 Waveguide
Transition, 8) Specimen Housing, 9) Transmitted Power
Signal, 10) X-Y Plotter, II) Analog to Digital Converter, 12)
Personal Computer.
-
-
Sorption and Thermal DesOl:ption of Acetone
Activated carbon and Carbosieve S-IIl were investigated as
candidate thermally regenerable acetone sorbents for use with
microwave powered heating systems. Based upon previous
work, activated carbon and Carbosieve were known to sorb
acetone strongly11,12. Further, Carbosieve S-IIl had been
identified as a selective sorbent for low molecular weight
species such as acetone and methyl-ethyl-ketone.
A 4.95 cm3 packed bed of activated carbon (2.13 g)
was prepared using #5769 coconut shell charcoal. In the
initial sorption experiment, 300 ppm of acetone in air
saturated with water vapor was fed to the sorbent bed at a flow
rate of I L/min. Effluent acetone levels were monitored by an
on-line hydrocarbon analyzer and confirmed by gas
chromatography. Initial breakthrough was noted after 99
liters of flow. Fifty percent breakthrough occurred at 120 L.
Complete breakthrough was observed after 150 L of
cumulative flow. The loaded activated carbon bed was
thermally regenerated using an initial power level of 23 W
under a dry nitrogen flow at 0.1 L/min. At this relatively low
level of incident radiation, bed temperatures of 180°C were
attained. Upon increasing the power to 35 W, maximum bed
temperatures of 380°C were observed. The adequacy of
thermal regeneration was confirmed by a repeat sorption.
Equivalent performance was attained prior to and after
thermal regeneration. The sorption and regeneration experiments are presented in Figures 4 and 5, respectively.
A 1.9 cm3 bed containing 1.36 g of Carbosieve S-IH
was challenged with humidified air containing acetone.
RESULTS AND DISCUSSION.
Microwave Powered Thermal Regeneration of
Sorbents - The sorption and microwave powered thermal
desorption characteristics of carbon dioxide, water vapor,
acetone and trichloroethylene (TCE) were studied using a
range of typical sorbent materials. Because of the diversity of
the chemical properties of the contaminants, no individual
sorbent was appropriate for the removal of all airborne
species.
Single-sorbent/single-contaminant experimental
results are summarized in Table I. Maximum temperatures
achieved in the exit gas stream during desorption and nominal
microwave power levels used are presented. For brevity, only
representative experiments will be discussed in detail. In
general, with the exceptions of lithium hydroxide (LiGH) and
silver oxide (Ag20), the sorbentmaterials
investigated,
appeared to be adequately regenerated by microwave
desorption as indicated by roughly equivalent contamin~nt
loading between initial and follow-up sorptions.
4
-
Table I Temperature and Power Levels for Individual Desorption Experiments.
300ppm
0.5%
50ppm
saturated
Acetone
CO,
TCE
Water
Activated Carbon
380°C/35W
-
530°C/40W
-
CarbosieveS-III
90°C/86W
-
610C/390W
-
LithiumHydroxide
-
51°C/780W.
-
-
Molecular Sieve SA
-
55°C/780W
-
112°C/780W.
255°C/780W.
-
-
-
SilicaGel
-
Silver Oxide
-
43°C/780W
-
-
Carbogenic Molecular
-
noC/780W
-
-
Molecular Sieve13X
90°c/780W.
Sieve
* Nitrogen flow rate at ILlmin for desorption.
Initial breakthrough occurred after 90 liters of cumulative
flow. Fifty percent breakthrough and total breakthrough were
observed at 150 liters and 240 liters, respectively. The
acetone loaded bed was thermally regenerated using stepwise
increasing applied microwave power levels between 86 600
W, corresponding to exit gas temperatures between 90 230°
e. The bulk of the acetone was desorbed at the 86 W power
level. The subsequent sorption experiment confirmed the
adequacy of the microwave powered thermal regeneration.
Sorption and desorption experimental results are shown in
Figures 6 and 7, respectively.
Sor:ption and Thermal Desor:ptionof Carbon Dioxide
- CO2 sorption experiments were conducted using lithium
hydroxide, Molecular Sieve 5A, the carbogenic molecular
sieve prepared at JPL, silver carbonate, and silver oxide.
During regeneration total effluent was collected in a gas-tight
bag for gross CO2 determination. A lithium hydroxide bed
containing 1.62 g was challenged with 0.5% CO2. During
regeneration the LiOH bed coupled with the microwave field
to a minor extent, but temperatures achieved were not
sufficient to reverse the chemisorption of CO2. In order to
achieve the higher temperatures required for reversal of the
reactions responsible for chemisorption of CO2 by UGH, a
composite bed was prepared consisting of 90% LiGH and
10%activated carbon by weight. This bed was exposed to the
full microwave power of the waveguide irradiation system
(780 W). Under these conditions, the quartz tube melted after
an exposure of less than one minute, indicating that bed
temperatures in excess of 1470°C had been attained. Clearly,
very high temperatures can be achieved using a composite
bed. However, owing to the constraints of time, this line of
investigation was discontinued.
A packed bed containing 2.81 g of Molecular Sieve
5A was challenged with dry air containing 0.5% CO2. The
initial breakthrough occurred almost immediately. Total
breakthrough was observed after 50 liters. Regeneration was
conducted using full microwave power and dry nitrogen at 0.1
Llmin.
During thermal desorption the effluent gas
temperature rose to a maximum of 55°C. The results of
sorption and regeneration experiments are shown in Figure 8.
Integration of the breakthrough curve yielded a total mass
loading of 56 mg CO2' corresponding to 2% of the initial
sorbent bed weight.
The microwave powered thermal
desorption yielded 75 mg CO2. The discrepancy is believed
due to the sorption of CO2 by the sorbent bed while exposed
to the atmosphere during bed assembly. The carbon dioxide
levels recovered upon heating indicated a complete
regeneration of the Molecular Sieve 5A.
To investigate the compatibility of a molecular sieve
CO2 sorbent and a strong microwave susceptor, a composite
bed containing 1.05 g Molecular Sieve 5A and 0.87 g
activated carbon was challenged with 0.5% carbon dioxide in
dry air at a flow rate of 320 mLimin. Effluent CO2 levels
were tracked continuously using an on-line NDIR analyzer.
Following sorption, the bed was thermally regenerated using
-
microwave
power levels ranging from 39
-
210 W.
The
adequacy of thermal regeneration was confirmed by a second
sorption, yielding virtually identical results.
The carbogenic molecular sieve material (0.52 g)
received from NASA's JPL was also evaluated as a CO2
sorbent. Initial tests with this material indicated an extremely
high heat-up rate, actually melting the glass wool end plug. It
is noteworthy that the softening point for borosilicate glass is
:::::
700°e. This temperature was apparently achieved inside the
packed bed, while the indicated exit gas temperature never
exceeded n°e.
A subsequent sorption/desorption cycle
indicated a I% bed weight CO2 loading. It is not known
whether the performance of this material was adversely
affected by temperatures ~ 700°C achieved during the first
microwave desorption. Insufficient material was available to
continue this investigation.
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;
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;
11
:
0
;
--
i503'
c
~
0
n
I
~ 0.2~I,
.
.
water vapor indicated complete regeneration of the molecular
sieve 13X. The initial sorption and second sorption are shown
in Figure II. The temperatures achieved during regeneration
indicate fairly strong coupling of the molecular sieve with the
microwave field. A 5-cm3 packed bed of Molecular Sieve 5A
was prepared.
During sorption maximum exit gas
temperatures of 52°C were observed. Under full microwave
power during desorption, exit gas temperatures rose to 112°C
until irradiation was tenninated 14 minutes into the
regeneration. Similar experiments indicated that silica gel
couples only very weakly with the incident microwave
energy, and that regeneration is in large part attributable to the
direct uptake of energy by adsorbed water molecules. On the
other hand, Molecular Sieve 13X appears to be quite
susceptible to microwave heating. An intennediate case is
presented by Molecular Sieve 5A, for which significant
microwave absorption by the solid medium and by adsorbed
water appears to occur. Though the mechanisms differed, all
water sorbents were adequately regenerated.
To improve the microwave coupling of a given CO2
sorbent material, composite mixtures of these sorbents and
known microwave susceptors, such as activated carbon and
silicon carbide were made. Beds assembled with these
mixtures were placed in the microwave desorption test stand.
The maximum temperatures achieved in the exit gas stream
during microwave heating and nominal microwave power
levels used are presented in Table II, along with the
corresponding single sorbent desorption data for comparison.
Insufficient time was available to pursue this
investigation thoroughly. Methods of thennal control were
not well established for these desorption experiments. The
arbitrary mixtures investigated either resulted in runaway
temperatures destroying the bed, or insufficient regeneration
temperatures. Follow-up experiments were not perfonned due
to time constraints. However, this comparison suggests that
by varying the mixture of a microwave transparent sorbent
material with a microwave susceptor material, a desired
regeneration temperature can be achieved at reduced power
levels. These results suggest the possibility of achieving the
high regeneration temperatures required for chemisorbed CO2
sorbents such as Lithium Hydroxide and Silver Oxide, and
other non-microwave susceptible sorbents, using this method.
Sorption and Regeneration of Layered Media for
.c£h.WaterVaporand TraceOrganics- A layered multimedia
sorGentbed consisting of 0.56 g Molecular Sieve 13X, 0.17 g
Carbosieve s-m, and 2.27 g Molecular Sieve 5A, in sequence
from inflow face to outlet, was prepared for a challenge with
an air stream saturated with water vapor and containing 0.5%
CO2' 628 ppm acetone, and 105 ppm TCE. Sorption was
conducted at a flow rate of 100 mL/min. Effluent levels of
CO2' total hydrocarbon, and dew point were monitored
continuously. Carbon dioxide began initial breakthrough at
5.2 liters of cumulative throughput.
Fifty percent
breakthrough occurred at approximately 9.5 liters, and total
breakthrough was observed at 14 liters.
Hydrocarbon
breakthrough began after 9.2 liters of cumulative flow and
never reached 50%. Water adsorption as indicated by dew
point was anomalous. The dew point fell steadily until the 9
liter mark had been reached, after which it began to rise. Be-
.
1
i
0.111
1
I
0.0 ~
1
j
0
10
20
30
50
40
60
CumulativeFlow (L)
--,60
3.0.,
-
;
~
t
jJ -CO2
.
tt 50
P
G>
:g
x 2.0 .)
0
l40
2
~
2.5
i
Temperature
.
t
~
-;
is
§ 1.5
.J:J
t ~
(ij
(.)
C 1.0
G>
:J
~
w 0.5
~
[30
i 20
~
;
~
~
0.0
0
2
3
4
5
~
~
~
'"
'"
~
'x
10 W
jo
6
Cumulative Flow (L)
Figure 8. Sorption of CO2 by MolecularSieve 5A (top), and
Regeneration (bottom).
Sorption
and
Thennal
Desorption
of
Trichloroethylene (TCE)
The sorption and microwave
powered
thennal
desorption
characteristics
of
trichloroethylene (TCE) were studied using activated carbon
and Carbosieve S-III. A bed of Carbosieve S-III containing
only 0.33 g was prepared. Breakthrough for this bed began at
approximately 30 L of cumulative flow. Thennal desorption
was conducted at a constant 390 W power level, during which
a maximum effluent gas temperature of 63°C was observed.
The repeat sorption experiment (40 ppm TCE) confinned that
the bed had been adequately regenerated. The results of
sorption and thennal regeneration experiments are presented
in Figures 9 and 10. Activated carbon showed a similar
sorption capacity to Carbosieve S-III, and desorption was
verified by a follow-up sorption.
Sorption and Thennal Desorption of Water Vapor Water loaded silica get, Molecular Sieve 13X, and Molecular
Sieve 5A were evaluated for compatibility with microwave
powered thennal regeneration. Water sorption experiments
were perfonned using a packed bed containing 3.47 cm3
(2.12 g) of Molecular Sieve 13X. During sorption, exit gas
temperatures of 62°C were attained, indicative of the strongly
exothermic sorption. A four minute thermal desorption was
perfonned
under full microwave power. Exit gas
temperature rose steadily throughout the regeneration,
reaching 255°C after four minutes. The second sorption of
-
7
501
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45
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40
S
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u
35
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=c
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.= 20
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5
0
1
j:
.10
-15
-20
0
25
0
50
75
100
125
150
175
200
225
~
;;..
25
30
-
25
30
10
.10
-15
J
15
15
C
5
"0
n. 0
~
co
0 .5
~
201
0
1
i!.=
20
20
35
25
15
25
40
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10
250
Cumulative Flow (l)
30
5
Cumulative Flow (L)
10
S
I
10
~
0 ..s
15
'[
~1
:!
-
-20
0
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15
Cumulative
10
20
Flow (L)
Figure 11. First and Second Sorptions of Water Vapor on
5
25
0
50
75
100
125
150
175
200
225
Molecular Sieve 13X.
250
Cumulative Flow (l)
cause the dew point never exceeded -10°C, adequate removal
of water vapor by the layered multimedia bed was indicated.
Based upon 100% breakthrough of CO2' regeneration
was initiated after 14 liters of flow. Maximum effluent gas
temperatures of approximately 45°C were observed during
regeneration. Following regeneration, a second sorption was
conducted, yielding results which were essentially identical
with those obtained initially. This conflllIled the adequacy of
the microwave powered thermal regeneration. The results of
the initial and second sorption experiments are shown in
Figures 12, and 13 respectively.
Microwave Reflection and Transmission Spectra All of the microwave-powered thermal regenerations
attempted in the current study utilized 2.45 GHz as the
frequency of irradiation. To evaluate the potential for more
favorable microwave heating of sorbent materials using other
frequencies, a spectral study of microwave transmission,
reflection, and phase shift over the frequency range betWeen
1.3 - 2.7 GHz was conducted using the vector netWork
analyzer based multifrequency microwave transmission and
reflection apparatus. Transmitted and reflected power levels
were measured in terms of decibels (dB), defined as,
Figure 9. Sorption ofTCE on Carbosieve S-III: Top Initial,
Bottom After Regeneration.
,
6000
.
i
-
Ii
50001
[4000~
~
1
]
i3000]
"
1
~
:/
5.\ 2000
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0
2
3
4
Cumulative
5
6
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70
-
0600
:\
60
j~ \~
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50
40
Q.
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co
C)
.": 20
.
!
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1
:
i 600 ~
i
1500
.,
0
0..
; 400 .,
i
.--
10
o~--0
300
~
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200
:;
,
"
w
Temperature
Power!
i
; 100
i
'---~-'
1
2
3
'
4
5
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j
.-1
t;
P
dB = 10 log~
Pr
0
6
wherePm is the measuredpower level and Pr is a reference
Cumulative Flow (L)
Figure 10. Regeneration
power level, in this case a 20 dB (ill ooth) sample of the
ofTCE Loaded Carbosieve S-III.
netWork analyzer RF output. The phase shift 6.8 measured
8
-
Table II Composite Mixture vs. Individual Sorbent Heatings.
Mass Fractions of Composite Beds
90% Lithium Hydroxide mixed with 10%
Composite Heating
Individual Sorbent
Results
Desorption Results
>1470°C 1780W
51oC/780W
Activated Carbon
. -
55% Molecular Sieve 5A mixed with 45%
.
165°C1210W
55°C/780W
Activated Carbon
60% Silver carbonate mixed with 40%
I83°C1780W
20°C/780W
72°CI245W
43°C/780W
activated carbon
47% Silver oxide mixed with
53% a-silicon carbide
was the difference in phase between the transmitted signal and
the reference signal.
:1
~e = LPI - LPr
~... 0.4
:!2
~
Transmission, reflection, and phase shift spectra were
collected for GAC 830IMC activated carbon, #5769 activated
carbon, lithium hydroxide, Molecular Sieve 5A, Molecular
Sieve 13X, dry silica gel, partially wetted silica gel, and silver
carbonate. The spectra were compared to that obtained from
an empty specimen chamber. Due to the large sample volume
required to fill the specimen chamber, Carbosieve S-III and
the carbogenic material from JPL were not analyzed. A
comparison of the spectra for GAC 8301MC and #5769
activated carbons is shown in Figures 14 and 15. The spectra
for partially wetted and dry silica gel are shown in Figures 16
and 17. Not surprisingly, both activated carbons show strong
absorptions compared to the other materials studied. The
differences in transmission spectra between 3.1% water loaded
silica gel and dry silica gel, while not as well pronounced, do
show a limited frequency range between 1750 and 1950 MHz
where a distinct difference can be seen. However, for the
balance of materials studied, which have low dielectric loss, a
comparison between different frequencies is not realistic due
to low power resolution limitations of our test stand.
The phase shift between GAC 830lMC activated
carbon and the empty specimen chamber is presented in
Figure 18. For this test apparatus, power absorbed by a
material is represented by:
PAb""".d= ~~nm*'"
I00
~.
PT"""mW'd+
{( I00
P",jurnc<
)
PIb:.fkCl<d +
P"""""""
.
(~.
I00
J
.
j
~
0.31
.8
'
~
~
J
. 1
02
j
0.1 1
0.0 .L-..,--.
0
1
2
3
4
5
6
7
Cumulative
170
-,---,
165
.
E,600S:
c:
0
~
8
9
10
11
~
14
15
-20
Dew Point
~
-19
;
Hydrocarbon
,~ -18
/\
.
~
-g,'50,
:z:
.
145
13
~--~
155-
~
12
Flow (L)
i
/
/
140
."
-16
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~
T -15
~ -14 ~
,,
0...
;. -12
j
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c:
i -13
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t -17
,
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i
i. -11
"""
'-10
0123456789101112131415
Cumulative
Flow (L)
Figure 12. Multimedia Bed: Initial Sorption of CO2' Water
Vapor, Acetone and TCE.
Pm"nldUS'
P "'{".n«)}
.
9
0.6
m
0.5
30
~20
...
G)
#~0.4
~
&. 10
co
:3!
><
"C
.&
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--=
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-u
a
a:: -10
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>
+I
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.!!-20
~
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:;'
.
.
G)
0
1
2
3
4
5
6
7
8
9
-30
1200
10 11 12 13 14 15
1400
1600
-20
-
Dew Point
Hydrocarbon
-
150
2200
2400
2600
Figure IS. Relative Reflected Power for Activated Carbons.
-18
30
-17
6'
-18 't,..
1:
-15 l
!140
c:
~ 135
----
~ 130
I
125
I
;/
-14
-13
/.-
120
../
,./
110
n.
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1101,...".,
0 1
2
3
4
5
6
7
8
9
. .
fii'
~20
-12
115 ~.,./
f
I
e -10
rV .
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,
16-20
:i
V
1 ~.
0
F
,1-10
10 11 12 13 14 15
Cumulative Flow (1.)
.
~ -30
1200 1400 1600 1800 2000 2200 2400 2600 2800
Figure 13. Multimedia Bed: Repeat Sorption after
Microwave Powered Regeneration.
Frequency (M-Iz)
-
m 30
::- 20
~
~
0
[l.
~
10
"C
~
~E
0
\
UJ
c
.-E -10
- Cort>on
GAC 83OIMC
~
a::
10
-u
G)
~
G)
0
G)
~
1ii -20
Qj
-20
-30
1200
&.
"C
a:: -10
~
>
Q;
Figure 16. Relative Transmitted Power for 3.1% Water
Loaded Silica Gel and Dry Silica Gel.
30
m
~2O
"C
~r\
'i:
'
a::
1400
1600
1800
2000
2200
2400
2600
2800
-30
1200 1400
;
:::
1600
~.
'
~i
:
1800
~ir~
\.,i-{/
'
2000
~,
/- \.
-Slllc8Ge1
2200
't;f
2400
:
2600
Frequency (MHz)
Frequency (MHz)
Figure 14. Relative Transmitted Power for Activated Carbons.
Figure 17. Relative Reflected Power for 3.1% Water Loaded
Silica Gel and Dry Silica Gel.
;
j
j
i
t
f
2800
-19
'E145
~
2000
Frequency (MHz)
160
155
~
-c:..-v
85789
1800
Cumulative Flow (1.)
'.
/
.
a::
0.0
.~~I~
\.
~
10
2800
After applying this relationship to the transmitted and
reflected spectral data for GAC 830IMC activated carbon and
the empty specimen chamber, the absorbed power spectrum
for this activated carbon is shown in Figure 19. In general, a
calculation of the complex permittivity as a function of
frequency for a material can be made from the phase shift data
and the absorbance data, with corrections made for the effects
of atmosphere filling the pores and intergranular spaces.
Phase shift data are .closely related to the real part of the
complex permittivity, and absorbance data allows an estimate
of the loss factor or imaginary part of the complex
permittivity. For this preliminary investigation however,
further data analysis was not feasible.
--,--,
180
en
,-_.'._,.,.
test apparatus emitting at <1 frequencyof 2.45 GHz. Both
activated carbon and Carbosieve S-III were identified as
excellent microwave regenerable sorbents for use in the
removal of airborne organics. Water loaded silica gel,
Molecular Sieve I3X, and Molecular Sieve 5A were also
effectively regenerated under microwave irradiation at this
frequency. Molecular Sieve 5A and a carbogenic molecular
sieve prepared at NASA's Jet Propulsion Laboratory were
. identifiedas
.
90 '
Q)
Q)
c,
Q)
e.
0-
~
:c
en
Q)
'"
~
Q.
r'v
-90-
-180
1200
\ \ --. '--. J--'
\
V
\J
~---~
1400 1600
1800
,
2000
Frequency
~..~
2200 2400
ACKNOWLEDGMENT.
.
2600
;
2800
Development and testing of the Microwave
Regenerable Air Purification Device was funded by the Ames
Research Center of the National Aeronautics and Space
Administration under contract NAS2-14374. Follow-on work
is currently in progress.
(MHz)
Figure 18. Phase Shift Relative to Empty Specimen Chamber
for GAC 830IMC Act. Carbon.
10
CD
~
--- --
-- - ..,
---..-..--.-.---
REFERENCES.
0
CD
3:
0
Q. -10
"0
Q)
.D
,"\
.,
\;-""""'./
/ .~""
v"v,",
.. :'\.:
I.
Carslaw, H.S., and Jaeger, J.e., Conduction of Heat in
Solids, 2nd Ed., Clarendon Press, Oxford, 1959.
2. von Hippel, A., Dielectrics and Waves, Wiley, New York,
1954.
3. von Hippel, A., Ed., Dielectric Materials and
Applications, Technology Press of MIT, Cambridge,
.
0
~
«
viablemicrowaveregenerableCO2sorbents. A
sorbent bed containiI:tgmultiple media was challenged with air
containing 0.5% CO2' 300 ppm acetone, 50 ppm TCE, and
saturated with water vapor. The multimedia bed was shown to'
effectively purify the contaminated air stream and to be
completely regenerated by microwave induced heating. A
means of potentially regenerating sorbents that do not
otherwise couple with microwaves has been identified. By
mixing a microwave susceptible material with a sorbent
material, otherwise unreachable regeneration temperatures can
be achieved. Spectral studies of the reflection, transmission
and phase shifts of microwaves irradiating a variety of
sorbents over the frequency range between 1.37 - 2.6 GHz
have indicated that interesting differences exist betWeenthe
materials. These results warrant further investigation of
dielectric loss over a larger range of microwave frequencies.
-20
-30 1200
1954.
1400
1600
1800
2000
2200
2400
2600
~
4. Tsang, L, and Kong, lA., Scattering of Electromagnetic
2800
Waves from Random Media with Strong Permittivity
Fluctuations,RadioSci., 16(3), 303-320, 1981.
5. Bergman, DJ., The Dielectric Constant of a Composite
Frequency (MHz)
Figure 19. Absorbed Power for GAC 830IMC Act. Carbon.
Material
-A
Problem
in Classic
Physics,
Phys. Lell. C,43
(9),377-407,1978.
6. Polder, D., and Van Santen, J.H., The Effective
Permeability of Mixtures of Solids, Physica. 12 (5), 257271,1946.
7. Taylor, LS., Dielectric Properties of Mixtures, IEEE
Trans. Antennas Propagat., AP-13 (6), 943-947, 1965.
8. Stogryn, A., The Bilocal Approximation for the Effective
Dielectric Constant of an Isotropic Random Medium,
IEEE Trans. Antennas Propagat., AP-32 (5), 517-520,
1984.
CONCLUSIONS.
Feasibility of the use of microwave heating for the
fast and efficient thermal regeneration of sorbents for the
removal of carbon dioxide, water vapor, and trace organics
from contaminated air streams has been conclusively
demonstrated. Microwave powered thermal regeneration of
single sorbents and multimedia sorbent beds loaded with
acetone, trichloroethylene (TCE), carbon dioxide, and water
vapor has been achieved using a rectangular waveguide based
11
90 Nost, B., Hansen, B.Do, and Haslund, Eo, Dielectric
Dispersion of Composite Material, Physico Scripta, T44,
67-70, 1992.
10. Foley, H.c., Carbogenic Molecular Sieves: Synthesis,
Properties, and Applications, Microporous Mater., 4, 407433, 1995.
II. Atwater, J.E., and Holtsnider, J.T., Airborne Trace
Organic Contaminant Removal Using Thennally
Regenerable Multi-Media Layered Sorbents, SAE Trans.
J Aerosp., 100, 1726, 1991.
12. Atwater, 1.E., and Holtsnider, 1.T., Simple Models for the
Breakthrough of Humidified Acetone and Ethyl Acetate
on a Carbon Based Molecular Sieve, Carbon, 34 (6), 824825, 1996.