U.S. patent application number 11/126012 was filed with the patent office on 2006-11-09 for regenerable adsorption system.
Invention is credited to Jay Perry, Subir Roychoudhury, Dennis Walsh.
Application Number | 20060249019 11/126012 |
Document ID | / |
Family ID | 36916871 |
Filed Date | 2006-11-09 |
United States Patent
Application |
20060249019 |
Kind Code |
A1 |
Roychoudhury; Subir ; et
al. |
November 9, 2006 |
REGENERABLE ADSORPTION SYSTEM
Abstract
A method for regenerable adsorption includes providing a
substrate that defines at least one layer of ultra short channel
length mesh capable of conducting an electrical current
therethrough, coating at least a portion of the substrate with a
desired sorbent for trace contaminant control or CO.sub.2 sorption,
resistively heating the substrate, and passing a flowstream through
the substrate and in contact with the sorbent.
Inventors: |
Roychoudhury; Subir;
(Madison, CT) ; Perry; Jay; (Madison, AL) ;
Walsh; Dennis; (Richboro, PA) |
Correspondence
Address: |
Robert L. Rispoli;Precision Combustion, Inc.
410 Sackett Point Road
North Haven
CT
06473
US
|
Family ID: |
36916871 |
Appl. No.: |
11/126012 |
Filed: |
May 10, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60569714 |
May 10, 2004 |
|
|
|
Current U.S.
Class: |
95/90 |
Current CPC
Class: |
B01J 20/3441 20130101;
Y02C 10/08 20130101; B01D 2259/4575 20130101; B01D 53/0431
20130101; B01D 2257/406 20130101; B01D 2257/504 20130101; B01D
53/0454 20130101; B01D 2259/40096 20130101; B01D 2253/102 20130101;
B01D 2253/302 20130101; Y02C 20/40 20200801; B01J 20/183 20130101;
B01D 2253/108 20130101; B01D 2257/70 20130101; B01D 2259/41
20130101; B01J 20/28033 20130101; B01D 53/32 20130101 |
Class at
Publication: |
095/090 |
International
Class: |
B01D 53/02 20060101
B01D053/02 |
Goverment Interests
GOVERNMENT RIGHTS
[0002] This invention was made with government support under NASA
Contract No. NAS8-02108. The U.S. government holds certain rights
in this invention.
Claims
1. A method for regenerable adsorption comprising: a) providing a
substrate defining at least one layer of ultra short channel length
mesh capable of conducting an electrical current therethrough; b)
coating at least a portion of the substrate with sorbent; c)
resistively heating the substrate; and d) passing a flowstream
through the substrate and in contact with the sorbent.
2. The method of claim 1 wherein the substrate further comprises a
second insulating layer of ultra short channel length mesh.
3. The method of claim 1 wherein the substrate defines a coil such
that the flowstream passes radially through the coil.
4. The method of claim 2 wherein the dual-layer substrate defines a
coil such that the flowstream passes radially through the coil.
5. The method of claim 1 wherein the coating comprises zeolite.
6. The method of claim 2 wherein the coating comprises zeolite.
7. The method of claim 3 wherein the coating comprises zeolite.
8. The method of claim 4 wherein the coating comprises zeolite.
9. A method for regenerable adsorption comprising: a) providing a
substrate defining at least one layer of ultra short channel length
mesh; b) coating at least a portion of the substrate with sorbent;
c) means for resistively heating the substrate; and d) means for
passing a flowstream through the substrate and in contact with the
sorbent.
10. The method of claim 9 wherein the substrate further comprises a
second insulating layer of ultra short channel length mesh.
11. The method of claim 9 wherein the substrate defines a coil such
that the flowstream passes radially through the coil.
12. The method of claim 10 wherein the dual-layer substrate defines
a coil such that the flowstream passes radially through the
coil.
13. The method of claim 9 wherein the coating comprises
zeolite.
14. The method of claim 10 wherein the coating comprises
zeolite.
15. The method of claim 11 wherein the coating comprises
zeolite.
16. The method of claim 12 wherein the coating comprises
zeolite.
17. The method of claim 1 wherein the substrate defines a linear
stack of a plurality of layers of ultra short channel length mesh
capable of conducting an electrical current therethrough.
18. The method of claim 17 wherein the linear stack further
comprises a second insulating layer of ultra short channel length
mesh.
19. The method of claim 18 wherein the coating comprises zeolite.
Description
CROSS-REFERENCE
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/569,714 filed May 10, 2004.
FIELD OF THE INVENTION
[0003] The present invention is generally directed to a method and
apparatus for removing trace contaminants from a flowstream. In
particular, this invention provides an improved adsorption process
for spacecraft cabin air quality control. More particularly, this
invention provides for an energy efficient, lightweight sorption
system for the removal of environmental contaminants in space
flight applications.
BACKGROUND OF THE INVENTION
[0004] Adsorption methods for removing trace contaminants from a
flowstream typically comprise passing the flowstream over or
through a sorbent structure. The sorbent structure may be defined
by a plurality of pellets or an array of tubes or plates or the
like, and such structure typically is positioned within the
flowpath of the flowstream to be treated. The sorbent structure may
comprise, or be coated with, sorbent particles that adsorb targeted
impurities from the flowstream. Although such systems are well
known in the art, several problems or shortcomings associated with
conventional adsorption methods correspondingly also are well known
in the art.
[0005] For example, when the sorbent becomes saturated, the sorbent
must be regenerated or removed and replaced. Typically, the entire
sorbent structure simply is replaced. Preferably, the sorbent
structure is regenerable. In some systems, the sorbent structure is
removed from the adsorption stream, subjected to a desorption
process, and then re-exposed to the adsorption stream. One
alternative method is described in U.S. Pat. No. 6,712,878 to
Chang, et al., wherein sorbent particles are injected into the
flowstream and then the flowstream is passed into contact with the
sorbent structures. The saturated sorbent periodically is removed
and fresh sorbent again is injected into the flowstream.
[0006] Another problem associated with conventional adsorption
methods is the efficiency of the adsorption technique employed.
Often, the unique characteristics of the targeted impurities and
the sorbent itself dictate that the adsorption process operate
within a desired temperature range. Several methods are known for
raising the temperature of the process including heating the
flowstream or the sorbent structure by employing an auxiliary heat
source. However, non-uniform heat distribution within a fixed-bed
substrate or other sorbent structure negatively impacts the
efficiency of the process. In addition, the time it takes for an
auxiliary heat source to raise the temperature of the sorbent
structure, and thereby raise the temperature of the sorbent and the
working fluid, further negatively impacts the efficiency of the
process. Moreover, less-complex auxiliary heat sources may not
provide the capability to reach and hold a narrow operating
temperature range as may be required for the subject adsorption
goal. Although more complex auxiliary heating systems may be
capable of achieving and holding a narrow operating temperature
range in a comparatively short time interval, such devices add
considerable weight and cost to the adsorption process.
[0007] In addition to conventional applications for adsorption
processes, such processes occupy an important niche in spacecraft
environmental control and life support systems. Primary
applications for adsorption processes exist in the area of cabin
air quality control. Since the beginning of crewed space
exploration, adsorption processes have been at the forefront for
ensuring that cabin air is suitable for the crew to breathe by
removing trace chemical contaminants and CO.sub.2. The ability to
remove trace contaminants (e.g. alcohols, ketones, aromatics,
halocarbons, and ammonia) from cabin air is a necessary aspect of
spacecraft life support systems such as that employed on the
International Space Station ("ISS"). Currently, this trace
contaminant control system ("TCCS") requirement is met on the ISS
by employing a 50 lb. bed of acid-treated activated carbon, which
is not regenerated. Due to its long life (>2 yrs.), the carbon
bed is simply replaced periodically. The current CO.sub.2 removal
system on the ISS employs two pellet bed canisters of 5A molecular
sieve that alternate between regeneration and sorption via heating
and exposure to space vacuum.
[0008] It is anticipated that adsorption processes will continue to
remain at the forefront of spacecraft cabin air quality control
technologies. As mission durations increase and exploration goals
reach beyond Earth orbit, the need for regenerable adsorption
processes becomes paramount. Thus, there is a need in the art for
an adsorption process that is capable of regenerably removing trace
contaminants from a flowstream in an efficient, cost-effective, and
robust manner suitable for conventional applications as well as for
aerospace applications. There also is a need in the art for an
adsorption process that is capable of regenerably removing CO.sub.2
from a flowstream in an efficient, cost-effective, and robust
manner suitable for conventional applications as well as for
aerospace applications.
SUMMARY OF THE INVENTION
[0009] The present invention provides an energy efficient,
lightweight sorption system for removal of environmental
contaminants in space flight applications. More particularly, the
present invention provides an alternative technology for removing
both CO.sub.2 and trace contaminants within a single unit employing
a sorption bed comprising ultra-short-channel-length metal meshes
coated with zeolite sorbents. The metal meshes further define a
means for direct, resistive electrical-heating thereby providing
the potential for short regeneration times, reduced power
requirement, and net energy savings in comparison to the
conventional system. The present invention eliminates the need for
a separate trace contaminant control unit resulting in an
opportunity for significant weight and volume savings.
[0010] It has now been found that zeolites deposited on
ultra-short-channel-length metal mesh elements, known as
Microlith.RTM. and commercially available from Precision
Combustion, Inc., located in North Haven, Conn., effectively adsorb
a number of the contaminants of interest. The inert Microlith.RTM.
ultra-short-channel-length metal mesh substrate and the use of a
binder during deposition of the zeolites on the mesh substrate
results in volumetric sorbent loadings considerably lower than if
the adsorbents were used as a packed bed (e.g., the carbon bed
currently employed). However, the ability to directly resistively
heat the metal mesh support provides for relatively rapid periodic
regenerations. Therefore, the weight and volume of the current TCCS
may be substantially reduced (by as much as 75 wt %) by use of
zeolites supported on Microlith.RTM. ultra-short-channel-length
metal mesh elements in conjunction with periodic sorbent
regeneration.
[0011] Since cabin air is fed directly to the TCCS, humidity may
have a significant negative impact on the performance of the
zeolite sorbents. Drying agents are used to mitigate the effect of
humidity on sorbent effectiveness and precede the current carbon
dioxide removal assembly ("CDRA"). In one embodiment of the present
invention, a Microlith.RTM. ultra-short-channel-length metal mesh
based TCCS is combined with the CDRA function. The system
incorporates the existing desiccant beds located upstream of the
current pellet-based CDRA bed. These driers mitigate the negative
impact of humidity on the effectiveness of 5A for CO.sub.2
sorption. Locating the Microlith.RTM. ultra-short-channel-length
metal mesh based TCCS system behind the CO.sub.2 removal bed
provides a dry stream to the zeolites used for trace contaminant
removal.
[0012] In another embodiment of the present invention, the packed
bed for CO.sub.2 removal was replaced with a Microlith.RTM.
ultra-short-channel-length metal mesh supported 5A zeolite sorbent.
As with the TCCS, the Microlith.RTM. ultra-short-channel-length
metal mesh provided considerably lower volumetric sorbent loadings
than the 5A pellet bed (approximately 30%). However, resistive
heating of the Microlith.RTM. ultra-short-channel-length metal mesh
support permits faster periodic regenerations. In yet another
embodiment of the present invention, successful integration of both
the CDRA and TCCS systems based entirely on Microlith.RTM.
ultra-short-channel-length metal mesh supported sorbents--and
similar in size to the current CDRA unit alone--eliminates the
current TCCS unit entirely with corresponding weight and volume
savings. Depending upon the design and regeneration requirements,
the integrated system offers power savings, as well as additional
weight savings, versus the current CDRA pellet bed. The current
TCCS and CDRA system is illustrated in FIG. 1A and the integrated
Microlith.RTM. ultra-short-channel-length metal mesh based
CDRA/TCCS system is illustrated in FIG. 1B.
[0013] Microlith.RTM. ultra-short-channel-length metal mesh
technology is a novel reactor engineering design concept comprising
of a series of ultra-short-channel-length, low thermal mass metal
monoliths that replaces the long channels of a conventional
monolith. Microlith.RTM. ultra-short-channel-length metal mesh
design promotes the packing of more active area into a small
volume, providing increased adsorption area for a given pressure
drop. The advantages of employing Microlith.RTM.
ultra-short-channel-length metal mesh as a substrate include the
feature of electrically heating the substrate to promote a reaction
on a fluid flowing therethrough as described in U.S. Pat. No.
6,328,936 to Roychoudhury, et al., incorporated in its entirety
herein. Whereas in a conventional honeycomb monolith, a fully
developed boundary layer is present over a considerable length of
the device, the ultra short channel length characteristic of the
Microlith.RTM. substrate avoids boundary layer buildup. Since heat
and mass transfer coefficients depend on the boundary layer
thickness, avoiding boundary layer buildup enhances transport
properties. The advantages of employing Microlith.RTM.
ultra-short-channel-length metal mesh as a substrate to control and
limit the development of a boundary layer of a fluid passing
therethrough is described in U.S. patent application Ser. No.
10/832,055 which is a Continuation-In-Part of U.S. Pat. No.
6,746,657 to Castaldi, both incorporated in their entirety
herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1A provides a schematic representation of the current
TCCS and CDRA system employed on the ISS.
[0015] FIG. 1B provides a schematic representation of the
integrated Microlith.RTM. ultra-short-channel-length metal mesh
based CDRA/TCCS system of the present invention.
[0016] FIG. 2 provides an isometric view of a simplified
Microlith.RTM. ultra-short-channel-length metal mesh based radial
flow sorber configuration of the present invention.
[0017] FIG. 3A provides a schematic representation of an external
view of a Microlith.RTM. ultra-short-channel-length metal mesh
based radial flow sorber configuration of the present
invention.
[0018] FIG. 3B provides a schematic representation of an internal
view of a Microlith.RTM. ultra-short-channel-length metal mesh
based radial flow sorber configuration of the present
invention.
[0019] FIG. 4 provides an isometric view of a radial flow reactor
configuration of the present invention.
[0020] FIG. 5 provides a graphical depiction of durability data of
an adsorption system according to the present invention.
[0021] FIG. 6 provides a graphical depiction of test data
comprising cycle-to-cycle variations in CO.sub.2 sorption of an
adsorption system according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] FIG. 1A provides a diagram of the system currently employed
on the ISS. The dehumidification and CO.sub.2 adsorption beds
operate in swing mode. A separate charcoal bed for adsorption of
TCCS also is shown. FIG. 1B provides a diagram of an embodiment of
the present invention. The charcoal bed is eliminated and replaced
with a regenerable CO.sub.2 and TCCS adsorption bed which also may
operate in swing mode.
[0023] One preferred embodiment of the present invention, as shown
in FIG. 2, comprises an axial flow coiled sorber substrate
configuration 10, commonly referred to as a "jelly-roll"
configuration, defined by a coiled Microlith.RTM.
ultra-short-channel-length metal mesh 12. Unlike its axial flow
counterpart, bypassing of the flowstream around the sorber
substrate is not a concern in the radial flow configuration. In
addition, the radial flow arrangement provides volumetric sorbent
loadings at least comparable to a linear bank of screen elements.
Furthermore, from the electrical and hardware assembly vantage
points, a continuous length of coated screen largely mitigates the
complicating issues encountered with a stack of screens. However,
employing a stack of screens as an adsorption bed is considered
within the scope of this invention.
[0024] In a preferred embodiment of the radial flow configuration
of the present invention, as shown in FIG. 4, the sorbent structure
14 comprises a two layer "sandwich" system consisting of a
sorbent-coated glass fiber insulating layer 16 and a sorbent-coated
Microlith.RTM.9 ultra-short-channel-length metal mesh layer 18
(total coated thickness of the combined layers is approximately 0.1
cm). For dual function CO.sub.2 removal/TCCS, the appropriate
lengths of the glass fiber insulating layer and Microlith.RTM.
ultra-short-channel-length metal mesh layer are coated with the
desired amounts of the preferred sorbents 20. The resultant dual
layer system is coiled around a centerline feed supply tube 22 as
shown in FIG. 2. Direct electrical heating of the Microlith.RTM.
ultra-short-channel-length metal mesh layer in order to regenerate
the sorbent is implemented more readily in this arrangement through
electrical leads 24 and 26. However, a radial flow configuration
comprising only one layer of a sorbent-coated Microlith.RTM.
ultra-short-channel-length metal mesh also is considered within the
scope of the invention, wherein the coating positioned on the metal
mesh serves as the insulating layer between metal mesh layers.
Multi-layered radial flow configurations comprising any number of
uncoated or sorbent-coated Microlith.RTM.
ultra-short-channel-length metal mesh layers, with or without any
number of uncoated or sorbent-coated glass fiber insulating layers,
also are considered within the scope of the invention. The length
of the coil or substrate configuration may be selected in order to
advantageously employ an available source of electrical
current.
[0025] As shown in FIG. 3A and FIG. 3B, one embodiment of an
adsorption unit 30 according to the present invention comprises a
dual-layer sorber coil 32 that defines a diameter of approximately
7.4 inches and a length of approximately 7.2 inches, the centerline
of which is occupied by feed tube 34 defining a diameter of
approximately 3 inches. Perforated metal tube 36 is positioned
between feed tube 34 and sorber coil 32 for support and flow
distribution purposes. Sorber coil 32 is fixed within housing 38 by
any conventional means such as threaded fasteners 40, tie rods or
the like. Sorber coil 32 and housing 38 are positioned within the
adsorption unit casing 42, which casing 42 defines a plurality of
apertures 44 in order to provide access for electrical connections
46 or for a variety of testing and measurement components, such as,
for example, a port 48 for air quality testing. Adsorption unit 30
further defines exit tube 50, inlet plate 52, and flanges 54 and
56. Adsorption unit casing 42 is joined to inlet plate 52 by any
conventional means such as welding, brazing, threaded fasteners,
tie rods, or the like. Alternatively, the adsorption unit casing
may be fabricated to comprise a front face such that an inlet plate
is not required. Flanges 54 and 56 are application dependent and
enable installation of adsorption unit 30 into a system
assembly.
[0026] This embodiment of the present invention is targeted to
operate at approximately the same nominal contact time as the
current CDRA sorber (approximately 1.8 seconds). This corresponds
to a volumetric flow rate of approximately 5 cfm through the
annular sorber coil volume of 258.5 in.sup.3 (4235 cc). Average
sorbent loading (based on 5A, Y, and ZSM-5) on the coated coil was
70 mg/in.sup.2 of washcoat (70% zeolite and 30% binder) on each
layer (i.e., Microlith.RTM. ultra-short-channel-length metal mesh
layers and woven glass fiber mesh layers).
[0027] The sorber coil 32 plus the 3-inch centerline feed tube 34
occupied approximately 5075 cc, or approximately 30% of the sorbent
volume in one of the canisters employed in the current CDRA. The
coated sorber coil 32 itself occupied approximately 26% of the
current CDRA sorbent volume. Deducting the feed tube volume of
approximately 840 cc resulted in a net volume for the total sorber
coil (CO.sub.2 & trace contaminant removal) of approximately
4235 cc. Devoting 82.5% of this net sorber volume to CO.sub.2
sorption corresponded to 213.2 in.sup.3 (3494 cc) of sorber volume
containing 5284 in.sup.2 at 140 mg 5A sieve/in (70 mg 5A
sieve/in.sup.2/double layer) or 740 gm of 5A washcoat.
[0028] The CO.sub.2 sorber regeneration requirement sets the cycle
time for the TCCS sorption section of the coil, which occupied
17.5% of the total annular sorption volume, or 45.1 in.sup.3 (740
cc). The TCCS coating followed the 5A coating and therefore was
located at the end of the screen, i.e. at the outer portion of the
wound coil. The initial 57.5% of this TCCS segment was washcoated
with zeolite Y and the balance with ZSM-5. A length/loading
calculation similar to that described for 5A sieve above indicated
that the TCCS segment contained approximately 90 grams of Y and
approximately 67 grams of ZSM-5. Assuming no contribution to trace
contaminant removal from the 5A molecular sieve, this allocation of
15-20% of the sorber volume to a post-5A TCCS function (Y/ZSM-5
approximately 1.35) is sufficient to maintain the exit
concentration of the trace contaminants below their inlet levels
during the sorption cycle.
[0029] Power requirements were examined for direct electrical
heating of the coil based on estimates of the total energy required
to heat the mass of sorber components and to supply the heat of
desorption (dominantly that of CO.sub.2, but also that of the trace
components). Radiation losses are ignored since these are likely to
be small at the target regeneration temperatures for a
well-insulated system having a low ratio of external surface area
to total sorber volume. Assuming no other losses, the total
required energy during heat up to the regeneration temperature is
the sum of mC.sub.p.DELTA.T for the washcoat, substrate, and
insulating layer plus the heats of desorption (where m=mass;
C.sub.p=specific heat capacity; and .DELTA.T =temperature
difference). The Microlith.RTM. ultra-short-channel-length metal
mesh sorber coil has a lower bulk density in comparison to a pellet
bed of 5A (approximately 65% of the weight of an equivalent pellet
volume), and a lower overall estimated specific heat. Assuming a 45
minute sorption/regeneration cycle, approximately 110-150 watts is
required per module depending upon the final target regeneration
temperature (230-300.degree. C.).
[0030] The use of the inert Microlith.RTM.
ultra-short-channel-length metal mesh substrate to support the
zeolite sorbents resulted in a reduced volumetric sorption capacity
in comparison to a conventional pellet bed. A preferred embodiment
of the present invention employed sorbent comprising 5A zeolite
(the calcium form of A) for CO.sub.2 removal, and high Al content
zeolite Y (CBV-400), and moderate Al content ZSM-5 (CBV-5524) for
trace contaminant removal. The zeolite loadings on the
Microlith.RTM. ultra-short-channel-length metal mesh substrate
should be greater than approximately 20 mg/in.sup.2, preferably
greater than approximately 70 mg/in.sup.2, and more preferably in
the range of approximately 70-100 mg/in.sup.2.
[0031] Bench-scale testing of a stack arrangement for all five of
the representative trace components (acetone, dichloromethane,
toluene, ethanol, and ammonia) was conducted in order to evaluate
the performance of candidate sorbents. The testing ensured that all
of the delivered trace contaminant reached the sorbent inlet and
could be properly accounted for, despite the particularly
challenging ammonia component. An NH.sub.3/N.sub.2 stream was
introduced and mixed with the stream containing the other trace
contaminants and/or CO.sub.2. Ammonia injection and mixing was
located just upstream of the sorber section. A mixer was employed
to permit uniform flow and mixing across the cross sectional area
of the flow path. Analysis of NH.sub.3 in the effluent was
conducted via chemiluminescence after catalytically converting it
to NO.sub.x. The adequacy of system performance was determined in
repeated tests employing sampling before and after the empty sorber
section, and bypassing the entire system.
[0032] Multi-cycle durability testing with both CO.sub.2 and the
five trace components also were conducted in order to examine the
robustness of the Microlith.RTM. ultra-short-channel-length metal
mesh supported sorbents. A thermocouple was located in the sorber
housing in order to ascertain the mid-bed centerline temperature.
To reduce heat losses during regeneration, the sorber housing was
removed and subjected to vacuum and external heating within a
programmable oven. Bed regeneration temperatures of approximately
230.degree. C. were maintained for approximately 90 minutes after
the thermocouple in the center of the bed registered the target
value. Thereafter, the oven cooled to room temperature over several
hours. Vacuum was maintained on the sorbent housing during the oven
cool-down period. Prior to the first sorption cycle, the freshly
charged unit was subjected to the modified vacuum regeneration
procedure. Regeneration was also conducted after each subsequent
sorption cycle.
[0033] The reactor design comprising alternate layers of coated
Microlith.RTM.9 ultra-short-channel-length metal mesh layers and
woven glass fiber mesh, with approximately 80% of the available
volume devoted to supporting 5A sorbent for CO.sub.2 removal and
approximately 20% devoted to supporting Y and ZSM-5 for trace
contaminant removal, was tested for 30 sorption/regeneration cycles
over 520 hours to demonstrate durability. The end of a sorption
cycle was defined as the point at which any of the delivered feed
components closely approached (approximately 98%) its feed
concentration. The feed concentrations were approximately 1/2 SMAC
(Spacecraft Maximum Allowable Concentration) for all components
with the exception of NH.sub.3 which was delivered at 75%-100% SMAC
to facilitate analysis. Results of the 30 cycle durability testing
are summarized in Table 1 and graphically represented in FIG. 5.
TABLE-US-00001 TABLE 1 ##STR1##
[0034] The results demonstrate that CO.sub.2 breakthrough defined
the end of cycle, i.e. trace contaminants were below their inlet
concentration at the point where CO.sub.2 exit concentration
approached the inlet concentration. The data in Table 1 illustrates
that vacuum regeneration at 230.degree. C. over the first 15 cycles
resulted in decreasing CO.sub.2 sorption. Regeneration at mid-bed
temperatures above 230.degree. C., however, was able to restore the
lost CO.sub.2 sorption capacity, as illustrated by the performance
in cycle 16 after a 275.degree. C. regeneration. Back-to-back
cycles regenerated at 275.degree. C. exceeded the sorption observed
in cycle 1 (1.3.times. greater CO.sub.2 sorption). This may result
from the creation of a cleaner, drier sorbent surface in comparison
to that of the fresh sorbent that was subjected to a single milder
pretreatment at the 230.degree. C. temperature.
[0035] Returning to the 230.degree. C. regeneration temperature for
the next 5 cycles (cycles 18 through 22), again resulted in
declining cycle-to-cycle performance. Again, back-to-back
regeneration at higher temperatures in cycles 23 and 24
(275.degree. C. and 300.degree. C., respectively) resulted in
CO.sub.2 sorption 1.25.times. greater than cycle 1. Finally,
performance decline over the next 4 cycles (with regeneration at
230.degree. C.) were once again overcome by back-to-back
regenerations at the higher temperatures in cycles 29 and 30
(1.2.times. and 1.5.times. greater CO.sub.2 sorption,
respectively). These results indicate that the system is robust and
suggest that an acceptable operating regimen may employ lower
temperature regeneration (i.e. lower power requirement) over
several sorption cycles in conjunction with periodic higher
temperature regenerations to fully restore bed sorption capacity.
Over the 30 cycles, the trace contaminants remained at <25% of
their inlet concentrations at end-of-cycle. The exit concentrations
increased as the number of cycles increased and showed some
variation with regeneration temperature. Overall, however, the data
suggested the attainment of an "equilibration" value with
increasing number of cycles, which--over the regeneration
temperature range employed--was relatively invariant (Runs 20, 23,
27, 29,and30).
[0036] FIG. 6 provides a graphical representation of CO.sub.2
sorption data obtained from a prototype system that was tested with
CO.sub.2 and trace contaminants. The data indicates 3 wt % CO.sub.2
co-adsorption capacity and a 45-50 minute cycle during which
approximately 55% of the delivered CO.sub.2 was adsorbed. During
this test, all trace contaminant levels remained less than or equal
to their respective inlet concentrations.
[0037] While the present invention has been described in
considerable detail, other configurations exhibiting the
characteristics taught herein for an improved method for adsorption
employing electrically heated ultra-short-channel-length metal mesh
elements are contemplated. Therefore, the spirit and scope of the
invention should not be limited to the description of the preferred
embodiments described herein.
* * * * *