Molecular sieve abstract
In supplying a high volume flow of oxygen-enriched air from a molecular
sieve type gas separation system, two sets of molecular sieve beds
are coupled in parallel. The beds and of each bed set are cyclically
subjected to a charge/adsorption on-stream phase followed by a purge/desorption
regeneration phase and are controlled to operate in overlapping
sequence with respect to each other. At the same time the corresponding
beds are controlled to operate in antiphase relationship to each
other.
Molecular sieve claims
What is claimed is:
1. In a method of operating a molecular sieve type gas separation
system to deliver oxygen-enriched air to an outlet, the system including
a pair of sieve bed sets and control means for cyclically subjecting
each sieve bed to a charge/adsorption on-stream phase followed by
a purge/desorption regeneration phase, the method including the
steps of operating the individual sieve beds of each sieve bed set
in overlapping sequence with respect to each other and operating
the individual sieve beds of one set synchronously with respect
to the sieve beds of the other set but in antiphase so that when
one sieve bed in one set is switched to a charge/adsorption on-stream
phase the corresponding sieve bed in the other set is switched to
a purge/desorption regeneration phase whereby enhanced oxygen concentrations
and smaller outlet pressure swings are achieved by the separation
system.
2. A molecular sieve type gas separation system adapted to deliver
oxygen-enriched air for breathing by aircrew of an aircraft by decreasing
the nitrogen content of air fed into the system, comprising
two bed sets each comprising three molecular sieve beds;
a supply line for delivering air from an air supply source to the
sieve beds;
a respective charge valve connecting a charge inlet port of each
sieve bed with the supply line;
a respective vent valve connecting a vent port of each bed with
ambient atmospheric pressure or a pressure related thereto;
a respective solenoid operated bleed switching valve connected
with the charge and vent valves of each bed and adapted for switching
the charge and vent valves between open and closed positions such
that when the charge valve is open the vent valve is closed and
vice versa;
a sequencer unit connected with the solenoid operated bleed switching
valves and adapted to control the solenoid operated bleed switching
valves to sequentially switch the charge and vent valves of the
beds of each bed set between the open and closed positions such
that the open period of a charge valve is equal to the open period
of a vent valve and the open periods of the charge and vent valves
of one of the beds in a bed overlap the closed periods of the charge
and vent valves of the other beds in the bed set, the sequencer
unit being further adapted to control the solenoid operated bleed
switching valves to switch the charge and vent valves of the beds
of one bed set in synchronisation with the charge and vent valves
of the beds of the other bed set but in anti-phase thereto.
3. A molecular sieve type gas separation system according to claim
2 wherein the sequencer unit comprises a solid state three channel
timer unit having three relays connected so as to sequentially switch
the solenoid operated bleed switching valves.
4. A molecular sieve type gas separation system according to claim
2 wherein respective servo chambers of the charge valves and the
vent valves are connected to the supply line and to the solenoid
operated bleed switching valves.
Molecular sieve description
This invention relates to gas separation systems utilising molecular
sieves and is especially but not exclusively concerned with the
employment of such systems in the aviation field for obtaining oxygen-enriched
air as a breathable gas for aircrew.
A common manner of operating a molecular sieve type gas separation
system employing one or more molecular sieve beds involves charging
the or each bed with feed gas mixture--e.g. air--under pressure
and continuing the feed to the bed to maintain the charge pressure
during delivery of the required product gas constituent(s) to use
or storage. When the bed approaches saturation, the feed is discontinued
and the bed vented to release the charge pressure, whereafter the
bed is purged. The pressurising of the sieve bed promotes adsorption
of the constituents to be retained by the bed, while the subsequent
depressurising promotes desorption of the retained constituents
to facilitate flushing of these from the bed.
In aircraft applications it is normally required that the oxygen
concentration (partial pressure) in the breathable gas supplied
to aircrew shall be related to cabin altitude, i.e. to the ambient
pressure obtaining within their enclosure.
Originally in proposed aircraft applications of molecular sieve
type gas separation systems, such systems were treated as a source
of substantially pure oxygen to be utilised in the manner traditional
in aircrew breathable gas supply systems: that is to say, the substantially
pure oxygen product gas delivered by the separation system is proposed
to be diluted with air to provide a breathable gas mixture of the
required oxygen partial pressure, in a manner analogous to the utilisation
of the oxygen obtained from sources such as high pressure storage
bottles and liquid oxygen converters.
The utilisation of a product gas that is substantially pure oxygen
for producing oxygen-enriched air as breathable gas involves the
use of appropriate mixing valves and related control devices to
derive the desired breathable gas mixture. Such valves and control
devices tend to be heavy and to be potentially unreliable through
their inclusion of capsules and the like for sensing ambient pressure
changes.
It has been proposed, therefore, to operate such a sieve system
to produce a breathable gas of the required oxygen partial pressure
directly from the molecular sieve system rather than by downstream
dilution.
One embodiment of this concept disclosed in GB-A-2029257 (Linde)
is a method of producing breathable gas having an oxygen concentration
dependent on altitude, comprising obtaining the total volume of
breathable gas required in normal continuous service solely by the
controlled adsorptive removal of nitrogen from compressed air by
means of reversible adsorbers operated in a cycle comprising adsorption
and reverse desorption phases. The adsorbers are so operated that,
during each adsorption phase, a continuously increasing proportion
of nitrogen passes through to the outlet end of the adsorber on-stream
and the oxygen concentration of the breathable gas formed is adjusted
within permissible range in accordance with the cabin pressure by
controlling the amount of gas flowing through the adsorbers and/or
the ratio of adsorption to desorption pressure.
In another embodiment of this concept, disclosed in EP-A-0046369
(Normalair-Garrett), control means responsive to the composition
of the product breathable gas are adapted to control regeneration
of the adsorber beds to maintain delivery of a product gas having
the desired oxygen partial pressure. The control means particularly
controls vent valve means for control of the regeneration phase
and may also utilise a microprocessor to provide a variety of other
useful system functions.
The system disclosed in EP-A-0046369 was originally envisaged
primarily for use on advanced high performance combat aircraft,
possibly with development potential still to be realised, so that
the scope accorded by microprocessor-based control was not out of
place and, indeed lends itself to the introduction of more advanced
life support systems. However, with the potential use of molecular
sieve type gas separation systems in more cost-conscious aircraft
such as trainers, attention has turned to the attraction of minimal
ground servicing.
A system meeting this requirement, is disclosed in EP-A-0080300
(Normalair-Garrett) and has control means comprised by a fixed logic
sequencer controlling the sequential operation of charge and vent
valves for cyclically subjecting each sieve bed to a charge/adsorption
on-stream phase followed by a purge/desorption regeneration phase.
The control means provide that during the regeneration phase the
gas pressure in a sieve bed at least substantially equals the ambient
atmospheric pressure or a pressure related thereto and that for
a predetermined range of ambient atmospheric pressure the overall
cycle time and the relative durations of said phases are fixed at
values such that the oxygen content of air delivered to the outlet
remains within physiologically acceptable limits for breathing.
We are now concerned to provide a system which will supply higher
volume flow than the system disclosed in EP-A-0046369 and EP-A-0080300
so as to meet a requirement for supplying oxygen-enriched air from
an on-board molecular sieve type gas separation system to the crew
of large aircraft.
It was first proposed to meet this requirement from two separate
three-bed systems each supplying oxygen-enriched air to a half of
the aircrew. However, a requirement that the oxygen-enriched air
be supplied from a common outlet led us to link the two sets of
beds together and a proposal for sequential and overlapping control
of the beds led to unexpected and advantageous results.
According to the present invention a molecular sieve type gas separation
system adapted to deliver oxygen-enriched air to an outlet by decreasing
the nitrogen content of air fed into the system is characterised
by at least one parallel-coupled pair of sieve bed sets, and control
means for cyclically subjecting each sieve bed to a charge/adsorption
on-stream phase followed by a purge/desorption regeneration phase
in such a manner that the sieve beds of the respective bed sets
of the pair are controlled to operate in overlapping sequence with
respect to each other, and corresponding sieve beds of the bed sets
of the pair to operate substantially in antiphase with respect to
one another.
The control means preferably comprises a multi-channel solid state
sequencer controlling the sequential operation of charge and vent
valves associated with each bed.
The durations of the charge/adsorption phase and the purge/desorption
phase, in the operating cycle of a bed, are preferably in the ratio
of 1:1.
In the regeneration phase it is preferable for obtaining best efficiency
that the bed be opened to the lowest available pressure which will
normally be ambient atmospheric pressure; however, the bed may be
opened to cabin pressure which is directly related to ambient atmospheric
pressure.
A molecular sieve type gas separation system in accordance with
one embodiment of the present invention is characterised by two
three-bed sets coupled in parallel, and control means for controlling
the three beds of each bed set to operate in an overlapping sequence
with respect to each other, the control means further controlling
the first, second and third beds of one of the bed sets to operate
in antiphase to the corresponding first, second and third beds of
the other bed set, the cyclic operation of each bed providing a
charge/adsorption on-stream phase followed by a purge/desorption
regeneration phase, each bed being arranged to be open to ambient
atmospheric pressure or a pressure related thereto during the regeneration
phase.
Such an embodiment has been found on test to give improved oxygen
concentration and lower outlet pressure swings in comparison with
the same parallel-coupled three-bed sets operated with the bed cycles
of one bed set in phase and synchronised with those of the corresponding
beds of the other bed set.
The invention will now be further described by way of example and
with reference to the accompanying drawings in which:
FIG. 1 is a schematic representation of a molecular sieve gas separation
system comprising a parallel-coupled pair of sieve bed sets, suitable
for aircraft;
FIG. 2 is a diagram showing the sequence of the bed sets of FIG.
1 when operated in antiphase;
FIG. 3 is a diagram showing the sequence of the bed sets of FIG.
1 when operated with phase synchronisation;
FIG. 4 graphically illustrates product flow and oxygen concentration
of the bed sets of FIG. 1 both when operated individually and when
operated coupled in antiphase, at ground level and at 7620 meters
(25000 feet); and
FIG. 5 graphically illustrates product pressure fluctuation of
the bed sets of FIG. 1 at 7620 meters (25000 feet), both when
operated individually and when operated coupled in antiphase.
Referring now to FIG. 1 a preferred molecular sieve type gas separation
system for supplying oxygen-enriched air to the several aircrew
members of a large aircraft includes, in accordance with one embodiment
of the invention, two similar bed sets 10 and 20 each comprising
three molecular sieve beds 11 12 13 and 21 22 23 respectively,
suitably contained and arranged to receive atmospheric air from
a source 14 such as a compressor stage of an engine of the aircraft,
so as to deliver product gas in the form of air that is increasingly
enriched with oxygen as the operating altitude increases. The two
bed sets 10 20 are coupled in parallel to the source 14 through
a filter 15 by way of a bifurcated supply line 16 each leg of which
includes a pressure regulating valve 17. The product outlets of
the two sets are conjoined to a single delivery line 18 for delivery
of product gas to suitable demand regulator means (not shown and
forming no part of this invention).
The associated components and method of operation of one bed set
are similar to those of the other and, therefore, the components
and operation of only one bed set, 10 will be described in detail.
The individual sieve beds 11 12 13 of the bed set 10 are arranged
for receiving compressed air from the supply line 16 under the control
of three similar valve arrangements 30 and in this embodiment are
suited to the adsorption of nitrogen by use of a Type 5A molecular
sieve material as available from Bayer AG or Union Carbide Corporation,
while passing oxygen so that the product gas delivered to the delivery
line 18 is oxygen-enriched air having an oxygen concentration, or
partial pressure, determined by the operation of the system as explained
hereinafter.
The sieve beds 11 12 13 are connected to the delivery line 18
by way of check, or non-return valves 31 32 33 respectively,
and are also connected in series, one to another, by a purge loop
34. Between beds 11 and 13 in section 34a of the loop 34 is included
a check valve 35 upstream of a flow regulating orifice 36; in section
34b between beds 13 and 12 a check valve 37 is positioned upstream
of a flow regulating orifice 38 and, likewise, in section 34c between
beds 12 and 11 a check valve 39 is positioned upstream of a flow
regulating orifice 40. The check valves 35 37 39 in the purge
loop are not essential although their inclusion permits a slightly
higher oxygen concentration to obtain in the product gas than obtains
when they are omitted.
Each valve arrangement 30 controls a charge inlet port and a vent
port of a single sieve bed and comprises a charge valve and a vent
valve of similar construction controlled by a servo valve. Thus
the sieve beds 11 12 13 are connected to the supply line 16 via
charge valves 41 42 43 respectively, and to ambient atmospheric
pressure or to a pressure directly related thereto via vent valves
44 45 46 respectively.
The servo chambers of the valves constituting each pair of charge
and vent valves 41/44 42/45 and 43/46 are connected to the supply
line 16 and also to an individual servo valve associated with each
respective pair of charge and vent valves: that is, solenoid operated
bleed switching valves, 47 48 49 are connected to the servo chambers
of the valves of the valve pairs 41/44 42/45 and 43/46 respectively.
Each switching valve 47 48 49 is actuated by a sequencer unit
100 having an operating ratio of 1:1. The switching valves 47 48
49 serve the pairs of charge and vent valves 41/44 42/45 and
43/46 respectively, in such a manner that while a charge valve
is open its companion vent valve is closed and vice versa, and the
sequencer is so connected as to cause the open period of the charge
valves to be equal to that of the vent valves. The cross-sectional
area of the vent flow paths from the sieve beds to ambient is greater
than the corresponding area of the supply air flow path into the
beds.
In this embodiment it is preferred that the charge valves 41 42
43 and the vent valves 44 45 46 are of the resilient diaphragm
type (not illustrated structurally), and are arranged for their
diaphragms to be pressure closed upon their valve seats by supply
pressure fed to their servo chambers via restricted ducts 50 51
respectively, and for relief of these servo chambers via unrestricted
ducts 52 53 respectively. The check valves 31 32 33 and 35
37 39 may be of any suitable form, such as those using a perforated
diaphragm or a flap.
The sieve beds 11 12 13 may be of any profile and form suitable
to the system and available space envelope. However in this embodiment
the base plate of each bed is provided with a charge port and a
vent port positioned to one side of an internal bed-divider wall,
and a purge loop connection and a delivery line connection positioned
at the other side of the wall.
For sequential operation of the individual beds of the two bed
sets 10 and 20 the sequencer unit 100 comprises a solid state three
channel timer unit having three relays connected so as sequentially
to switch the solenoids of the bleed switching valves 47 48 49
of bed set 10 and the corresponding valves of bed set 20 in synchronisation
with the switching of valves 47 48 49 but so as to operate the
corresponding valves in antiphase to the valves 47 48 49.
In this aircraft application, the system delivers oxygen-enriched
air as breathable gas to aircrew usually by way of oronasal breathing
masks. Supply air is obtained from the engine source 14 of the aircraft
and fed into the supply line 16 to the sieve beds by way of filters
15 disposed upstream of suitable pressure control valves 17 which,
for example, may be set to open to a source pressure of 276 kPa
(40 psi) and control at 310 kPa (45 psig). The delivery line 18
feeds to an oronasal breathing mask (not shown) by way of a demand
type breathable gas regulator of suitable form (not shown), upstream
of which is a standby pressurised breathable gas source (not shown),
such as an oxygen cylinder for providing 100% oxygen deliverable
to the regulator in conditions of low supply-air pressure, low cabin
pressure or low concentration (partial pressure) of oxygen in the
product gas delivered from the molecular sieve beds. The standby
gas source may be activated by any suitable monitor/activator system.
In the illustrated embodiment, the beds 11 12 13 are placed on-stream
and regenerated in alternating and overlapping sequence by operation
of the valve arrangements 30 in which the solenoid operated switching
valves 47 48 49 are controlled by the sequencer unit 100 alternately
to open and close the charge and vent ports of their respective
beds by alternate connection of the servo chamber of the charge
valves 41 42 43 and vent valves 44 45 46 to ambient. As previously
stated, the sequencer is arranged to control the charge and vent
valves such that they are open for equal periods of time.
The time of opening of all the valves and the overlapping relationship
of the three bed cycles in each bed set is best understood with
reference to FIG. 2. With the opening of the charge valve 41 of,
nominally, the first bed 11 in sequence, when the switching valve
47 is holding servo pressure in the vent valve 44 and releasing
it from the charge valve 41 supply air is fed into the bed wherein
nitrogen is adsorbed by the molecular sieve material and oxygen-enriched
air is passed into the delivery line 18 on opening of the valve
31. At the end of the period of charging and delivering product
gas, that is half of the way through the bed cycle, the switching
valve 47 is reversed to close the charge valve 41 and open the vent
valve 44 whereupon the bed 11 becomes open to ambient and discharges
thereinto so commencing desorption of nitrogen from the sieve material
while reducing the pressure in the bed 11 and in that part of section
34c of the purge loop 34 which is downstream of the check valve
39. Reduction of pressure in section 34c allows initial stage product
gas from the overlapping, sequentially second, bed 12 to pass the
check valve 39 and flow into bed 11 to purge it clean.
Thus compressed supply air is admitted cyclically to the beds 11
12 13 and 21 22 23 by way of the respective sequencer-controlled
valve arrangements 30 and when a bed is on-stream, oxygen-enriched
air flows from the bed into the delivery line, but shortly after
switching of the charge and vent valves to opposite mode the bed
is purged by new product gas from one of the other beds coming on-stream.
Thus product gas from bed 11 purges bed 13 product gas from bed
12 purges bed 11 and product gas from bed 13 purges bed 12 whereas
product gas from bed 21 purges bed 23 product gas from bed 22 purges
bed 21 and product gas from bed 23 purges bed 22. Evenness of flow
is not lost during the withdrawal of some product gas from one bed
for the purpose of purging another bed because the remaining bed
in each bed set still continues to deliver product gas during this
period.
The partial pressure of oxygen in the oxygen-enriched product gas
changes with change in pressure of the ambient condition to which
the bed is vented during regeneration so that the concentration
of oxygen increases with (although not as fast as) reducing ambient
pressure owing to the efficiency of desorption of nitrogen from
the sieve material increasing. Thus the concentration of oxygen
in the product gas automatically remains satisfactory for most presently
known operational requirements up to a cabin altitude (pressure)
of 4500 meters (15000 feet).
Whilst the embodiment of FIG. 1 shows a system that incorporates
a single pair of bed sets coupled in parallel, in other embodiments
of the invention there may be two or more pairs of bed sets organised
as in FIG. 1 and arranged in parallel between the supply line and
the delivery line. Each pair of bed sets may have an individual
controller or there may be a controller controlling operation of
all of the bed sets. Operation of the bed sets may be arranged such
that each pair of bed sets is run at its own cycle time or the pairs
of bed sets may all be synchronised so that their respective bed
cycle sequences are either in unison or interleaved in time.
A system as shown in FIG. 1 comprising two three-bed sets 10 and
20 respectively, coupled together in parallel with the two product
outlets connected into a common delivery line, was tested in our
high altitude laboratory.
The inlets of the bed sets were supplied with air at separately
controlled pressures so that the performance of the sets could be
separately determined with balanced and unbalanced inlet pressures,
and with different supply pressures.
The solenoids controlling the valves of the beds were wired to
a programmable multi-channel controller to enable the effect of
different bed sequences to be assessed.
With charge and vent times equal at 4.5 seconds each, the three
beds of each bed set were operated with overlapping cycles time-spaced
at equal intervals, (i.e. one bed commenced its cycle every three
seconds in a nine-second total cycle).
The two sets were first operated individually with the timing as
shown for bed set 10 in FIG. 2 at 210 kPa (30 psig) supply and
flows up to 100 liters/min., so that the performance of the bed
sets when coupled could be compared. The oxygen concentrations and
product pressures were recorded.
The two bed sets 10 and 20 were then coupled so that they ran synchronously
but in antiphase, i.e. the beds of bed set 20 were operated in synchronism
but with the opposite phase to the corresponding beds of bed set
10. The resultant complete timing sequence is shown in FIG. 2 showing
that, for instance, bed 21 is switched to vent simultaneously with
the switching of bed 11 to charge.
The performance of the system was recorded at 210 kPa (30 psig)
supply and flow up to 200 liters/min. FIG. 4 shows the oxygen concentrations
obtained for each individual bed set up to a product gas flow of
100 l/min., and for the two bed sets when coupled in parallel and
run synchronously in antiphase for a product gas flow up to 200
liters/min. It can be seen that the oxygen concentration obtained
from the coupled bed sets compares very favourably with that of
the individual bed sets at the appropriate flows, whilst FIG. 5
shows that the outlet pressure swings of the six beds running synchronously
in antiphase are considerably smaller than those of the individual
bed sets.
The tests on the coupled bed sets were repeated with the solenoids
of the bed sets connected in parallel so that they were running
synchronously and with corresponding beds in-phase in a three-bed
sequence as shown in FIG. 3 instead of the six-bed antiphase sequence
shown in FIG. 2. Oxygen concentrations and outlet pressures were
recorded at an inlet pressure of 210 kPa (30 psig) and flows up
to 200 liters/min. and this test also showed that the six-bed antiphase
sequence results in improved oxygen concentration and considerably
smaller outlet pressure swings.
The tests were repeated with different pressures supplied to the
bed sets, firstly with 210 kPa (30 psig) applied to bed set 10 and
195 kPa (28 psig) to bed set 20 and then with the supply pressures
reversed. In each case, the effect of the three-bed in-phase and
six-bed antiphase sequences was determined.
The resultant oxygen concentrations and outlet pressures showed
that when using the three-bed in-phase synchronous sequence, there
is a marked fall in oxygen concentrations especially around 50 liters/min.
N.T.P. demand flow, but that this is not so with the six-bed antiphase
sequence. |