Note: Descriptions are shown in the official language in which they were submitted.
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PRESSURE AND TEMPERATURE SWING ADSORPTION AND
TEMPERATURE SWING ADSORPTION
The present invention relates to methods of treating gases using pressure and temperature
swing adsorption, or temperature swing adsorption, together with apparatus and materials
for use therein. Particularly the method is directed at purification of gases by removal of
particular components therefrom, and the regeneration of adsorbent materials in the
apparatus used for the method.
The requirements of an adsorbent system for cyclic filtration processes are not comparable
with those of conventional single pass filters. This is because the metallic impregnants on
such filters are an undesirable feature as they promote chemisorption. For maximum
performance and service life cyclic filtration must therefore proceed reversibly by physical
adsorption. Furthermore, the regeneration step must not result in adsorbent degradation;
thermal aging, which can result from overheating of the filter bed, being just one example of
such adsorbent degradation. In addition to these considerations, other factors such as
particle size and hardness must be assessed when selecting adsorbents for PTSA
applications.
Air purification processes, such as those required in collective protection situations wherein
air from an external source is purified before delivery to an enclosed area occupied by
personnel, typically consist of three stages each requiring a different adsorbent material; (i)
removal of high boiling contaminants (e.g. boiling point over 50C), (ii) removal of water
vapour, and (iii) removal of low boiling contaminants (e.g. boiling point below 50C). The
distinction between high and low boiling contaminants in terms of exact temperature range
is not important; the critical requirement is the removal of contaminants having a boiling
point between approximately -90C and 200C).
The identification of an adsorbent for cyclic filtration of high boiling components is especially
demanding as such components are generally strongly physically adsorbed and may be
irreversibly retained. Adsorbent characterisation using nitrogen adsorption will elucidate the
porous characteristics of an adsorbent but are not useful in prediction of adsorption and
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regeneration properties; adsorption and desorption data for dimethylmethylphosphonate,
ethandiol and a range of other high boi!ing simulants provides more useful data in this
regard.
Using such volatile materials the present inventors have studied a variety of adsorbents and
determined that, although many of these have high adsorption capacity, in order for a filter
bed to be adequately regenerable only certain types of adsorbent materials meet
operational requirements.
Thus in a first aspect of the present invention there is provided a method for separating one
or more volatile contaminants from a subject gas using a temperature, or pressure and
temperature, swing adsorbent filtration filter bed apparatus comprising three or more layers
of adsorbent materials characterised in that the layers comprise a first layer of material
capable of adsorbing material of relatively high boiling point, a second layer of desiccant
material and a third layer of material capable of adsorbing material not adsorbed by the first
layer and in that each of the layers may be purged of adsorbed contaminants by a gas
having a higher temperature than that from which the contaminants were adsorbed, or
having a combination of higher temperature and lower pressure than that from which the
contaminants were adsorbed. and the method comprising passing the subject gas through
the filters serially in the order first layer, second layer, then third layer.
Although these three are the only layers illustrated in the following examples, the claims
should be construed as including the cases where other layers are present, adjacent or
between the first, second and third layers described herein.
In a preferred embodiment the layers comprise a first layer of mesoporous adsorbent
material a second layer of desiccant material and a third layer of material capable of
adsorbing contaminants that are not retained by mesoporous material.
Preferably the mesoporous layer is an activated carbon and the third layer is capable of
adsorbing contaminants of relatively low boiling point, e.g. of boiling point less than 50C;
most suitably being a microporous adsorbent. Most preferably the method is arranged to be
particularly suitable for the removal of contaminants from an oxygen and/or nitrogen
containing gas such as air.
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The order of the layers is essential as high boiling point contaminants have been found by
the inventors to be retained on all non-mesoporous adsorbents they have tested at high
levels, even at temperatures of up to 450C. Thus by the inclusion of a mesoporous
adsorbent upstream of the desiccant these downstream layers are protected from the high
boiling point contaminants retained on the mesoporous layer.
The efficiency of filtration of high boiling point compounds is not affected significantly by
water vapour due to the higher enthalpies of condensation compared with water, whereas
the third layer, ideally a molecular sieve such as a zeolite and/or a microporous carbon,
and/or mesoporous carbon, will have its efficiency reduced by water vapour. By the
inclusion of a desiccant layer upstream of the third layer the latter can be protected from
water vapour in the gas, which is known to have a deleterious effect on adsorption of many
agents by adsorbents such as activated carbons (see C R Hall and R J Holmes (1989) Ads
Sci and Technol" 6 83; and ISRP Journal, 6, Summer 1992).
The mesoporous layer may be any of the well known mesoporous materials obtainable from
suppliers such as Westvaco, Norit, Chemviron, Elf Atochem, UOP, Grace, Down and Rohm
& Haas; typical of those suitable are SA1817, Durferrit, CAR and BAX. The most preferred
material of those so far investigated is BAX as it regenerates efficiently and possesses the
most appropriate physical properties; i.e. relatively large particle size and resistance to
oxidation and attrition.
The third layer may employ any of the well known microporous adsorbents such as RB1,
NC35, Zeolites (e.g. 13X) or may be mesoporous carbon. The desiccant layer will
preferably be selected from those known to be efficacious in existing pressure swing
adsorber drier units, e.g. zeolites 13X and 3A.
The arrangement of the layers in the bed should be such that they can each be heated for
the purposes of regeneration in the temperature swing adsorption (TSA) or pressure
temperature swing adsorption (PTSA) fashion. Such arrangement might conveniently be
provided in any fashion that allows controlled heating of the bed layer, and is most
conveniently realized in use of a bed apparatus as described in patent application UK
9422835Ø
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UK patent application 9422835.0 describes in a first aspect a method of heating a pressure
and temperature swing adsorption gas filtration bed unit, or temperature swing adsorption
gas filtration bed unit, the method comprising locating a heating means within the bed
housing and characterised in that the heating means acts to heat gas passing into the bed
or a layer thereof in the purge direction and uses the heated air to heat the adsorbent
material .
In a preferred embodiment of this aspect of UKPA 9422835.0 the method employs a
number of layers of adsorbent material within the filtration bed housing and positions a
heating means upstream. with respect to the purge flow direction, of each of these layers.
Most preferably the heating means are used to separate the multilayer bed units that are the
subject matter of the present invention.
In this manner the temperature selected for regeneration of a particular adsorbent material
may be matched more closely to its particular characteristics, especially with regard to its
thermal degradation characteristics and the amount of heat required to purge a particular
component from a particular layer at a given pressure.
Particularly preferred heaters for the purpose of heating the gas as it enters each layer of a
bed are provided in the form of disc shaped heater units located within divider elements
which may be used for supporting and in turn being supported by the adsorbent of an
adjacent adsorbent material layers. Suitable such heater units are Curie point heaters such
as those provided by Domnick Hunter Filters UK, (these being conveniently located in
batteries of three heater elements each) or any other arrangement including batteries
containing six or more such discs, or elements of different shape.
The heaters are controlled, in the preferred arrangement, using a microprocessor device
(there being no need to monitor bed temperature). The microprocessor device allows rapid
control of the bed heaters including the provision of sequential shutdown in order to
minimise cooling periods required and thus return the bed to operational condition as soon
as possible. The electricai connections to the heater batteries and sensors may
conveniently be provided entering through the ends of the bed, as will be seen in the
Example below.
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The particular use of batteries of air heaters placed within transverse elements to divide a
bed into layers allows placement of heaters at any desired position within the bed. The
preferred Curie point heaters are of honeycomb construction and provide direct gas heating
during passage of gas through the bed with gas temperature controlled by the composition
of the heater element.
The requirements for pure or cleaned air in other applications can also be fulfilled by the
present invention. Other applications may require two adsorbent layers or more than three
adsorbent layers. The present invention will also result in improvements in adsorbent life
and performance in, for example, air drying applications.
In a particular embodiment, the method of the invention is used to provide a collective
protection environment for personnel aboard a military or civilian aircraft. Compressed air,
heating and cooling means are already available in such an environment.
The method, apparatus and adsorbents of the invention will now be described further by
way of illustration only by reference to the following non-limiting examples and figures.
Further embodiments falling within the scope of the claims will occur to those skilled in the
art in the light of these.
FIGURES
Figure 1 is a graph of amount of nitrogen adsorbed mg 9~' v relative pressure (p/p) to
show the mesoporous character for carbons WVA1100 and RB1.
Figure 2 is a graph of thermal desorption of DMMP from BAX and RB1 carbons with
changing temperature.
Figure 3 is a histogram showing the adsorption and desorption of DMMP from
mesoporous carbons.
Figure 4 is a histogram showing adsorption and desorption of DMMP from
microporous adsorbents.
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Figure 5 is a graph of water vapour breakthrough measured at 8 bar, 5 dm3min~' at
25C for 13X and 3A molecular sieve adsorbents.
Figure 6 shows adsorption isotherms of HFC134a. on Norbit RB1 at 25C, 40C, 55C,
70C, 85C, 100C, 125C and 150C (upper to lower traces).
Figure 7 shows a schematic representation of a filter bed arrangement which might
typically be used by the current invention.
Figure 8a: shows a plan view and an elevation of a 3 element Curie point heater
showing its dimensions Figure 8b shows a divider element in which the heater of Figure 8a
is placed during operation.
Figure 9: shows the arrangement of four heater units of Figure 8 within a vertically
oriented bed such as to divide it into four layers.
Figure 10: shows temperature profiles obtained within a bed of zeolite using a 38.5 dm3
fill at airflow rate 3120 dm3 min ' at the thermocouples A1 to A6 of Figure 9.
Figure 11: shows a schematic representation of a system utilising the current invention
to provide a collective protection environment for aircraft personnel.
MATERIALS AND METHODS: Activated carbons, zeolite and polymer based adsorbents
were obtained from Westvaco, Norit, Chemviron, Elf Atochem, UOP, Grace, Dow and Rohn
& Haas. Nitrogen adsorption for characterising the pore structure was carried out by the
method of K S W Slng (1970) 'Surface Area Determination', Edit. Everett and Ottewill,
Butterworths, London pp 25-42. Characterisation of physical properties of the adsorbents
also included particle slze analysis, packing density and voidage measurements. The ability
of the adsorbents to resist attrition during repeated pressure swing cycling was assessed
initially using a small packed bed arrangement employing rapid 7 to 0 bar pressure swings
(approx 5000 cycles).
Assessment of adsorption and desorption properties was carried out after first outgassing
samples at 120C, 3mbar. The adsorption of water vapour and high boiling components,
including dimethylmethylphosphonate (DMMP, simulant for G agents), and 1,2-ethandiol
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and 2-chlorethylether (boiling point and structural simulant for H agents) was carried out by
exposing samples to each vapour in a desiccator at 30C.
Desorption was measured using thermal gravimetric analysis (TG); a 25 cm3 min~~ airflow
was passed through the TG furnace containing the sample during desorption which was
then carried out using a 20C min~~ ramp to 1 80C and the sample maintained at that
temperature for 75 minutes prior to cooling; in some cases desorption being repeated more
than once without re-equilibration, to determine the efficiency of post-challenge regeneration
(cycle experiments). 180C was selected as the regeneration temperature to minimise any
risk of adsorbent decomposition or thermal aging. Adsorption and desorption of low boiling
point components including HFC134a was carried out using a semiautomatic microbalance
IGA system (Hiden Analytical Ltd UK).
Chemical warfare (CW) agent simulants were selected to reflect both structure and boiling
point where possible. Breakthrough data was obtained using a semiautomatic pressure and
temperature swing apparatus (Hiden Analytical Ltd, UK).
ADSORBENT FOR FIRST LAYER: HIGH BOILING POINT MATERIAL ADSORBENT.
Examples of nitrogen isotherms are shown in Figure 1, while Table 1 shows surface area,
pore volume data, particle size and oxidation temperature (C); meso and macro porosity
being preferred pore volume characteristics.
TABLE 1
AdsorbentSurface AreaPore VolMicropore MesoporeParticle(mm) Oxidation
m29-1 cm39~~cm39~~ cm 9 length width Temp
RB1 1084 0.485 0.417 0.068 2.3 1.1 380
-400
NC35 1347 0.584 0.528 0.056 5.7 4.0 240
-255
CAR95 1414 0.795 0.489 0.306 3.7 2.4 270
-400
BAX950 1250 0.810 0.392 0.418 4.0 2.0 330
-410
Zeolite 13X 606 0.302 0.225 0.077 2.4 4.8
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Amberlite 0.875 0.13 0.745 0.4 280
-310
* = temperature stable; RB1 and NC35 are microporous carbons, CAR95 and BAX950 are
mesoporous carbons, Zeolite 13X is microporous, XAD4 is a polymer.
Adsorption data for DMMP and the ethandiol are shown in Table 2 and Table 3 with typical
desorption profiles being shown in Figure 2. All the materials tested proved to have high
adsorption capacity, but predominantly mesoporous materials have significantly greater
capacity.
The microporous adsorbents, including 1 3X zeolite, do not regenerate adequately, retaining
more than 30% of the initial adsorbate load; 13X retaining 10% of the adsorbate load at
temperatures of above 4500C. This contrasts with the mesoporous adsorbents which retain
less than 1 0%. The behaviour of the different adsorbent types is quite general (see
Figures 3 and 4 where loading and residual are the amounts adsorbed and retainedrespectively as weight percent of DMMP, at the beginning and end of a single desorption
experiment) .
The effect of pressure swing alone was shown to have little effect on desorption, illustrating
the need for a thermal swing.
TABLE 2 Adsorption and desorption of DMMP
AdsorbentAmount Adsorbed 180C Isothermal Region Loss Residual % wVwt% 30min 60min 75min
RB1 45.6 50.4 61.3 64.5 34.2
NC35 43.8 41.3 51.4 54.8 43.8
CAR95 85.5 75.3 85.3 87.2 12.5
BAX950 91.3 84.6 90.5 91.4 4.3
Zeolite 13X 46.3 65.3 67.8 68.4 34.3
Amberlite 115.1 97.3 99.4 99.5 0.04
XAD4
Amount adsorbed is w/w%. Residual loss, determined as % of adsorbent remaining after
cooling; original adsorbate loading is 100%.
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A comparison of the data in Table 2 with that of Table 3 below, illustrates a strong time
dependency of the degree of desorption, indicating that the duration of the heat regeneration
phase, including the cooling stage, will be a primary factor in controlling the size and number
of beds used. It is notable that a number of the mesoporous materials generated with
relatively high efficiency.
Since the TG experiments do not enable airflow through the sample, cycle experiments
were carried out to determine the efficiency of post-challenge regeneration. For BAX carbon
the residual DMMP loading was ca. 1% (99% desorption after 5 cycles): for Zeolite 13X the
residual value was 35% (5 cycles, no further loss detected after the third cycle). These
results illustrate that commercially available mesoporous adsorbents exist which possess
appropriate adsorption and regeneration characteristics.
TABLE 3. Adsorption and Desorption of Ethandiol
AdsorbentAmount Adsorbed 180C Isothermal Region Loss Residual %
wVwt% 30min 60min 75min
RB1 42.9 59.4 78.4 82.1 16.6
NC35 41.8 65.6 85.5 90.1 8.5
- CAR95 86.5 79.2 go.o go.g 5 4
BAX950 63.7 85.9 94.9 95.2 2.2
Zeolite 13X 27.0 37.6 40.9 41.7 54.9
Amberlite 45.2 99.4 -0.3 -0.3 -10.8
XAD4
Negative mass change may represent adsorbent decomposition.
A comparison of BAX with microporous carbon data for nitrogen adsorption indicates that
this mesoporous carbon possesses a comparable micropore volume (see Table 1); the
difference in regeneration properties possibly reflecting the well developed mesopore
structure which probably results in more efficient and rapid transport of the organic
chemicals from the micropore structure.
Tables 2 and 3 show that the performance of molecular sieve for water separation purposes
in the desiccant layer(s) will be reduced due to fouling if exposed to high boiling compounds;
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hence illustrating the need for the upstream location of the mesoporous layer. Typical
breakthrough measurements for water vapour through desiccant beds are shown in Figure 5
wherein beds were challenged with a saturated airflow.
As low boiling compounds are only poorly adsorbed under humid conditions, penetration of
such contaminants through the mesoporous layer, and at least part of the desiccant ) bed,
will proceed rapidly
Figure 6 shows characterisation of the microporous adsorbents with adsorption isotherms,
showing the regeneration conditions for removal of HFC134a. Similar such measurements
using zeolites and mesoporous carbons demonstrate the utility of these materials for
separating light gases in the absence of water vapour.
Referring to Figure 7 a typical PTSA adsorbent filter bed of the current invention comprises
a first layer 1 of mesoporous adsorbent, a second layer 2 of molecular sieve and a third
layer 3 of microporous adsorbent. During normal operation, gas to be purified passes
through layer 1 first where high boiling chemicals are removed. The gas then passes
through layer 2 where water vapour and some chemicals of lower boiling point than those
trapped in layer 1 are removed. Finally, the gas passes through layer 3 where chemicals of
a low boiling point relative to those trapped by layer 1 are removed.
Figure 8 illustrates the use of Curie point heaters (three elements per battery) in a PTSA
unit. These and the housings therefor were supplied by Domnick Hunter Filters Ltd, UK.
Referring to Figure 9 four batteries 1 - 4, each independently controlled via a
microprocessor keypad timing device, were located axially within a 38.5 dm3 bed of zeolite,
electrical connection belng made by thin (2mm) insulated wires which pass along the
adsorber bed to connectors enterin9 an upper manifold via gas tight ducts. The upper and
lower manifolds. which form an integral part of the bed housing, also contain the valving
arrangement. Each element is of honeycomb construction and provides direct air heating
during passage of gas along the column of the bed unit. Gas temperature is controlled by
the composition of the heating element at approximately 180-210C. Power consumption is
approximately 2kW (eight three element blocks occupying 10% of the housing volume).
Typical in-bed temperature profiles during heating and cooling measured using
thermocouples shown at positions A1 to A6 are shown in Figure 10 with a gas flow of air at
3120 dm3 min~'. In obtaining these the heater batteries were switched off sequentially.
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During regeneration of the filter beds, gas passes through the filter bed in the opposite
direction to that of the subject gas being filtered and is heated the heaters 4. Each layer has
an adjacent heater downstream thereof with respect to the direction of gas flow in filtration
mode.
Referring to Figure 11, which illustrates use of the invention in an aircraft, inlet air I is fed to
heat exchange 1 which provides primary cooling to reduce the air temperature to not greater
than 50C and preferably below 40C. The inlet air I may be derived from the aircraft
auxiliary power unit, engine bleed or from ram air and should be not less than 4 bar,
preferably about 7 - 8 bar. Total air demand is regulated by venturi 2. Entrained water and
any liquid contamination is removed during passage of the air through coalescer 3. The
coalescer is sited close to the inlet to the adsorber beds 4a, 4b and the liquid condensate is
dumped overboard.
The airflow then passes onto adsorber bed 4a, via valve 5a. The product flow, after passing
through non-return valve 6 passes into a manifold arrangement. The bulk of the air
(perhaps 90%) then passes into the environmental control system supply for the purposes
of systems air, and air for the oxygen concentrator (not shown). The remaining air passes
through heat exchanger 7 which raises its temperature to about 200C prior to passage
through valve 8b (two way, selectable) and onto adsorber bed 4b. The inlet air temperature
to the regenerating bed should be at, or about, 200C.
The regenerating air, and the contaminants desorbed by it, (all of which flows in the opposite
direction to air being filtered) then pass through a further valve 9b as an exhaust flow Z
which would preferably be dumped overboard. The flow rate of regenerative air is regulated
by a fixed constriction in the inlet pipe downstream of valve 8b. This allows the pressure to
fall to about 1 bar during the heating stage.
On completion of the heating stage, bed 4b can be allowed to cool either by natural
convention (termination of the regenerative airflow) or by switching the beds and allowing
the input air I to flow onto the bed. Preferably a cool air purge E is applied via valve 8b prior
to switching the beds over.
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The siting of a flow restrictor downstream of valve 8b also serves to regulate the cooling flow
since the manifold feed pressure will be the same for flow rates D and E (this arrangement
means that flow rates D and E are the same). On completion or regeneration (and cooling)
of bed 4b it becomes the adsorbing bed and bed 4a becomes the regenerating bed.
Although only two adsorbing beds are shown, the use of more than two beds is feasible and
this may allow the design of a relatively compact system.
Heat exchanger 7 is shown as two separate units for clarity. In practice, the regenerative
airflow would be obtained from a single heat exchanger from an appropriate design of the
pipework. The system functions on a fixed cycle time and the valves may be controlled by a
cam arrangement or. more preferably, by software.
The filtration system may be sited anywhere within the confines of the airframe to simplify
integration, as long as the inlet temperature requirements are met. The system lends itself
to incorporation on an aircraft since compressed air and means for heating and cooling are
already available.
Use of a filter bed comprising the layered adsorbents as disclosed in the present application
allows maximum protection to be achieved using a minimally sized apparatus due to
increased capability for efficient regeneration and taking advantage of the temperature and
pressure swing facility This is equally applicable to the temperature swing process, but in
this case the beds will need to be larger, owing to the need to remove larger quantities of
water vapour which would normally be removed as a result of air compression during the
PTSA process.
US Patent 3961919 discloses filtration apparatus incorporating three adsorbent layers, of
which a first is adapted for the removal of oil vapour, a second comprises a desiccant, whilst
the third may comprise activated carbon. The apparatus is neither a TSA nor a PTSA unit.
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