Molecular sieve abstract
A molecular sieve type gas separation system capable of delivering
oxygen-enriched air by increasing the oxygen content of air fed
to the system has control means including a valve arrangement associated
with each sieve bed for cyclically subjecting each bed to a charge/adsorption
on-stream phase followed by a purge/desorption regeneration phase.
During the regeneration phase of each sieve bed the gas pressure
in the bed at least substantially equals the ambient atmospheric
pressure or a pressure related thereto such as the ambient pressure
of an outlet to which the oxygen-enriched air is being delivered
and, for a predetermined range of outlet ambient pressure, the overall
cycle time and the relative durations of the phases are fixed at
values such that the oxygen content of air delivered by the system
remains within physiologically acceptable limits for breathing.
In application of the system to an aircraft for supplying oxygen-enriched
air to aircrew, the outlet will generally comprise one or more oral-nasal
breathing masks and during the regeneration phase the beds are preferably
opened to the ambient atmospheric pressure of the aircraft.
Molecular sieve claims
What is claimed is:
1. A method for delivering oxygen-enriched air to a breathing mask
of an occupant of an aircraft cabin comprising the steps of:
supplying air to a molecular sieve bed system, the concentration
of the oxygen-enriched gas exiting from the sieve bed system being
a function of the partial saturation of the sieve bed;
controlling the charge/adsorption on-stream phase and the purge/desorption
regeneration phase so that a desired saturation of the sieve bed
is maintained so as to produce an oxygen-enriched gas having a concentration
appropriate for the actual cabin pressure; said controlling step
including the steps of determining the cabin pressure, and then
selecting the appropriate overall cycle time and relative duration
of each phase from predetermined values appropriate for the determined
cabin pressure such that the oxygen-enriched air remains within
predetermined physiologically acceptable limits for breathing at
the cabin pressure;
venting to aircraft ambient atmosphere the sieve bed during the
purge/desorption regeneration phase; and
delivering the oxygen-enriched air directly to the breathing mask
during the charge/adsorption phase.
2. A molecular sieve type gas separation system adapted to deliver
oxygen-enriched air to an oral-nasal breathing mask for breathing
by an occupant of an aircraft cabin, the system comprising:
a supply line including supply valve means for supplying air to
the system;
at least one molecular sieve bed connected with said supply line
and adapted for decreasing the non-oxygen content of the supply
air;
a delivery line including delivery valve means connected with said
bed for delivering oxygen-enriched air from said bed to said breathing
mask;
a vent line including vent valve means connected with said bed
for venting gas in said bed to aircraft ambient atmosphere or to
aircraft cabin atmosphere;
control means connected with said supply valve means and said vent
valve means for controlling opening and closing thereof;
said control means being adapted to cyclically subject said bed
to a charge/adsorption on stream phase followed by a purge/desorption
regeneration phase, and
pressure transducer means sensing an altitude related pressure
and operatively connected with said control means to vary the total
cycle time of the on stream and regeneration phases of said bed
in accordance with the sensed altitude related pressure such that
for predetermined ranges of altitude related pressure the overall
cycle time and the relative durations of each of the said phases
are at values such that the oxygen content of the oxygen-enriched
air delivered to said breathing mask from said sieve bed remains
within predetermined physiologically acceptable limits for breathing
at the actual cabin pressure.
3. A system according to claim 2 wherein the control means and
pressure transducer means reduce the cycle time in steps as the
sensed altitude related pressure reduces.
4. A system according to claim 2 wherein the control means provide,
at least within one predetermined range of ambient atmospheric pressure
or a pressure related thereto, phase relative durations in the ratio
of 2:1 in favour of the charge/adsorption phase.
Molecular sieve description
DESCRIPTION OF INVENTION
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.
In a gas separation system utilising zeolithic molecular sieves,
a feed gas mixture is caused to flow through a bed of zeolite molecular
sieve material having the capability of retaining unwanted constituent(s)
of the gas mixture while passing the wanted constituent(s); for
instance, in the separation of oxygen from air a molecular sieve
material Type 5A as available, for example, from Bayer AG and Union
Carbide Corporation may be used to hold back nitrogen and to pass
oxygen to provide a delivery of a product gas that is approximately
95% oxygen. In the operation of such a system, the sieve material
becomes progressively saturated with the retained gas constituent(s),
e.g. nitrogen, and eventually becomes incapable of retaining further
amounts of such constituent(s). The sieve material of the bed therefore
has to be regenerated from time to time to purge it of the retained
constituent(s). Typically regeneration is effected by flushing the
bed, usually in the reverse direction, with the product gas while
the outflow from the bed is discharged, e.g. to atmosphere.
To provide for continuous delivery of the desired product gas,
the system may include a reservoir from which product gas is drawn
for flushing the bed for regeneration purposes. Additionally or
alternatively, the system may include a plurality of beds arranged
to be utilised and regenerated in an overlapping sequence to provide
for continuous delivery of a required product gas.
A common manner of operating a 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.
Hitherto in early proposed aircraft applications of molecular sieve
type gas separation systems, such systems have been 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 respiration 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 utilises
a microprocessor for control of the regeneration phase and also
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, the emphasis is now on the attraction of minimal
ground servicing.
According to the present invention a molecular sieve type gas separation
system adapted to deliver oxygen-enriched air to an outlet by decreasing
the non-oxygen content of air fed to the system, comprising at least
one sieve bed and control means for cyclically subjecting said bed
to a charge/adsorption on-stream phase followed by a purge/desorption
regeneration phase is characterised in that said control means provide
that during said regeneration phase the gas pressure in said 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 the oxygen-enriched air delivered to said outlet remains
within physiologically acceptable limits for breathing.
In application of a system in accordance with the present invention
to an aircraft for supplying oxygen-enriched air to aircrew, the
outlet will generally comprise one or more oral-nasal breathing
masks.
In the regeneration phase it is preferable for obtaining best efficiency
that the bed be opened to the lowest available pressure which in
the aircraft application will be ambient atmospheric pressure; however,
the bed may be opened to cabin pressure which is directly related
to ambient atmospheric pressure.
Preferably, the control means set the cycle time and/or the phase
relative durations at different values for respectively different
ranges of outlet ambient pressure.
The control means may comprise a fixed logic sequencer controlling
the sequential operation of charge and vent valves associated with
the bed, and may further include transducer means responsive to
outlet ambient pressure for selectively associating said fixed logic
elements with said charge and vent valves in accordance with the
ambient pressure of the outlet.
Within at least one predetermined range of outlet ambient pressure,
the control means may provide phase relative durations in the ratio
of 2:1 in favour of the charge/adsorption phase. The control means
may reduce the cycle time in steps as the outlet ambient pressure
reduces.
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, including three beds, suitable for aircraft applications;
FIG. 2 is a diagram showing the operating sequence of the principal
valves of the system shown in FIG. 1;
FIG. 3 is a graph showing the band of oxygen concentration available
from two particular grades of molecular sieve material and the band
for the physiological requirement for oxygen, from ground level
to 10000 meters altitude;
FIG. 4 is a graph showing an acceptable band of oxygen concentration
against the physiological requirement for oxygen from ground level
to 8500 meters altitude;
FIG. 4A is a histogram showing the ratio of charge period to vent
period in the total cycle time for obtaining the acceptable band
of oxygen concentration shown in FIG. 4;
FIG. 5 is a graph similar to FIG. 4 showing closer to optimum
bands of oxygen concentration which are obtainable within bands
of altitude between ground level and 10000 meters; and
FIG. 5A is a histogram similar to FIG. 4A, showing the ratio of
the charge to vent periods for the modified cycle times required
to obtain the bands of oxygen concentration shown in FIG. 5.
Referring to FIG. 1 a molecular sieve gas separation system 10
in accordance with one embodiment of the invention, comprises three
molecular sieve beds 11 12 13 suitably contained and arranged
to deliver product gas to a delivery line 14. The sieve beds 11
12 13 are arranged for receiving compressed air from a supply line
15 under the control of three similar valve arrangements 16. In
this embodiment the sieve beds are suited to the adsorption of nitrogen
by use of Type 5A molecular sieve material while passing oxygen
so that the product gas delivered to line 14 is oxygen-enriched
air having an oxygen partial pressure determined by the operation
of the system as explained hereinafter.
The sieve beds 11 12 13 are connected to the delivery line 14
by way of pressure opening valves 17 18 19 respectively, and
are also connected in series, one to another, by a purge loop 20.
Between beds 11 and 13 in section 20a of the loop 20 is included
a pressure opening valve 21 upstream of a flow regulating orifice
22; in section 20b between beds 13 and 12 a pressure opening valve
23 is positioned upstream of a flow regulating orifice 24 and, likewise,
in section 20c between beds 12 and 11 a pressure opening valve 25
is positioned upstream of a flow regulating orifice 26.
Each valve arrangement 16 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
each sieve bed 11 12 13 is connected to the supply line 15 via
a charge valve 27 28 29 respectively, and to ambient atmospheric
pressure or to an ambient pressure directly related thereto via
a vent valve 30 31 32 respectively.
The servo chambers of each pair of charge and vent valves 27/30
28/31 and 29/32 are connected to the supply line 15 and also to
a servo valve comprising for each respective pair of charge and
vent valves, a solenoid operated bleed switching valve, 33 34
35 respectively. Each switching valve 33 34 35 is actuated by
a fixed logic sequencer 50 having an operating ratio of 2:1. The
switching valves 33 34 35 serve the pairs of charge and vent valves
27/30 28/31 and 29/32 respectively, in a manner in which 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 twice 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 all the pressure opening
valves 17 18 19 21 23 25; the charge valves 27 28 29; and
the vent valves 30 31 32 are of resilient diaphragm type (not
illustrated structurally).
The charge and the vent valves are arranged for their diaphragms
to be pressure closed upon their valve seats by supply pressure
fed to their servo chambers via restricted ducts 36 37 respectively,
and for relief of these chambers via unrestricted ducts 38 39
respectively. The pressure opening valves 17 18 19 and 21 23
25 utilise perforated diaphragms urged to close upon their seats
against upstream pressure with a predetermined force applied by
a compression spring, which in this embodiment is equivalent to,
say, 69 kPa (10 psi) in respect of the purge loop valves 21 23
25 and 14 kPa (2 psi) in respect of the product delivery line valves
17 18 19.
Whilst the sieve beds 11 12 13 may be of any profile and form
which is suitable to the system and an available space envelope,
in this embodiment the sieve bed container comprises a tube 43 having
one end closed by a semi-spherical wall 44 and the opposite end
secured to a base plate 46. The container is divided internally
by a diametrical wall 45 extending from the base plate 46 to terminate
short of the semi-spherical wall 44. The base plate 46 is provided
with a charge inlet port and a vent port positioned at one side
of the dividing wall 45 and a purge loop connection and a delivery
line connection positioned at the other side of the dividing wall.
In the aircraft application, the system delivers oxygen-enriched
air as breathable gas to aircrew usually by way of oral-nasal breathing
masks. Supply air is obtained from an engine source 40 of the aircraft
and fed into the supply line 15 to the sieve beds by way of a filter
41 disposed upstream of a suitable pressure control valve 42 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 14
feeds to an oral-nasal 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 operation, a desired concentration (partial pressure) of oxygen
in the delivered product gas, such as to provide, say, the equivalent
of air at an altitude of 1500 meters (5000 feet) is maintained
by control of the regeneration of the molecular sieve beds. In this
connection it should be noted that in operation of a sieve bed,
such as one of the beds 11 12 13 optimum gas separation efficiency
is obtained when the bed is in a fully purged nitrogen-free condition.
As the bed becomes progressively saturated with retained nitrogen,
its separating efficiency declines.
In prior art gas separation systems a molecular sieve bed is placed
on-stream cyclically and into regeneration in accordance with a
schedule that provides for full purging of the bed material at each
regeneration, and subsequent retention of the bed on-stream only
for so long as it is delivering oxygen of the maximum practical
purity. Thus, in such a system full regeneration is routinely effected
before the separating efficiency of the bed material has significantly
declined as a result of nitrogen saturation.
However, in gas separation systems embodying the present invention,
and as in the systems disclosed in our aforementioned EP-A-0046369
the bed is continually operated in a partially saturated condition
so as to deliver air enriched with oxygen to a desired partial pressure.
This is accomplished by restricting purging of the bed so as not
to fully cleanse it at each regeneration. In the system disclosed
in our aforementioned EP-A-0046369 restriction of bed purging
is controlled according to various parameters pertaining to the
physiological well-being of the aircrew, which parameters are continuously
monitored and acted upon in appropriate manner such as through a
microprocessor.
In contradistinction, whilst the sieve beds in the system in accordance
with the present invention are continually operated in a partially
saturated condition so as to deliver air enriched with oxygen to
a desired partial pressure, this is achieved by control means without
use of a feed-back control loop. The control means cyclically subjects
each bed to a charge/adsorption on-stream phase followed by a purge/desorption
regeneration phase and provides that during the regeneration phase
the gas pressure in the bed at least substantially equals the ambient
atmospheric pressure of the aircraft or, alternatively, the cabin
ambient pressure, and that for a predetermined range of cabin ambient
pressures the overall cycle time and the relative durations of the
phases are fixed at values such that the oxygen content of air delivered
to the cabin remains within physiologically acceptable limits for
breathing by an occupant of the cabin.
To obtain most efficient working of the beds their pressure should
be reduced during the regeneration phase to the lowest available
pressure. Thus it will be preferable to vent the beds overboard
to the ambient atmosphere of the aircraft. However, they may be
vented to the aircraft cabin, the cabin ambient pressure being directly
related to ambient atmospheric pressure.
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 16 in which the solenoid operated switching
valves 33 34 35 are controlled by the fixed logic sequencer 50
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 27 28 29 and vent valves 31 32 33 to ambient.
As previously stated, the sequencer is arranged to control the charge
and vent valves such that the former are open for twice the period
of time of the latter. Regulation of the purging phase of each bed
and the delivery of product gas therefrom is automatically achieved
by means of the pressure opening valves 21 23 25 and 17 18 19
respectively.
The time of opening of all the valves and the overlapping relationship
of the three beds is best understood with reference to FIG. 2. With
the opening of the charge valve 27 of, nominally, the first bed
11 in sequence, when the switching valve 33 is holding servo pressure
in the vent valve 30 and releasing it from the charge valve 27
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 14 on opening of the valve 17. At the end of the
period of charging and delivering product gas, that is two thirds
of the way through the sequence, the switching valve 33 is reversed
to close the charge valve 27 and open the vent valve 30 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 20c of the purge
loop 20 which is downstream of the valve 25. Upon the pressure difference
across the valve 25 rising above the closing load, initial stage
product gas from the overlapping, sequentially second, bed 12 flows
into bed 11 to purge it clean.
Thus compressed supply air is admitted cyclically to the beds 11
12 13 by way of the respective sequencer controlled valve arrangements
16 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 bed 11 purges
bed 13 bed 12 purges bed 11 and bed 13 purges bed 12. 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 third
bed still continues to deliver product gas during this period.
Variation in the partial pressure of oxygen in the oxygen-enriched
air occurs with change in pressure of the ambient condition to which
the bed is vented so that the concentration of oxygen increases
with reducing ambient pressure owing to the rate of desorption of
nitrogen from the sieve material increasing. Thus variation in the
concentration of oxygen in the product gas automatically remains
satisfactory for most presently known operational requirements up
to a cabin altitude (pressure) of 6000 meters (20000 feet).
In investigating various types of molecular sieve material we have
also found that use can be made of the fact that the ability of
a sieve bed to adsorb nitrogen varies according to the molecular
sieve material. For example, a medical grade Type 5A sieve material
is appreciably more efficient than a commercial grade Type 5A sieve
material so that at lower altitudes greater degradation of a bed
containing medical grade material is required in order to obtain
an oxygen concentration in the product gas appropriate to the pertaining
altitude than is required of a bed containing commercial grade material.
FIG. 3 is a graph which plots percentage concentration of oxygen
against altitude and shows a band, reference MG, of concentration
of oxygen available from a medical grade material, a band, reference
CG, of concentration of oxygen available from a commercial grade
material, and a band, reference PR, of the physiological requirement
for oxygen from ground level to 10000 meters (33000 feet) cabin
altitude.
Thus, we have found that by selection of a suitable molecular sieve
material and a suitable fixed cycle time it is possible to provide
a simple system which will produce an acceptable, although less
than optimum, oxygen concentration over a range of altitude. This
can be seen in the graph of FIG. 4 which shows a representative
acceptable band of concentration of oxygen, reference AC, and the
band for the physiological requirement for oxygen, reference PR,
from ground level to approximately 8500 meters (28000 feet) cabin
altitude. FIG. 4A is a histogram showing that for a fixed cycle
time T the charge period is twice that of the vent period.
We have found that an improvement over this acceptable, although
less than optimum, band of concentration of oxygen can be made if
the selected sieve material is of medical grade and if the cycle
time of the bed is sequentially reduced with increasing altitude.
Two sequential reductions in bed cycle time are shown in the histogram
of FIG. 5A and reference to the graph shown in FIG. 5 illustrates
that these reductions introduced at cabin altitude thresholds of
3000 meters (10000 feet) and 6700 meters (22000 feet) give three
different bands of oxygen concentration in the product gas for three
bands of altitude. In the two lower altitude bands the oxygen concentration
bands, references A and B, are closer to the corresponding mean
of the band for physiological requirement, reference PR, whilst
at altitudes above 6700 meters (22000 feet) the band of oxygen
concentration, reference C, is maintained substantially at the highest
enrichment level obtainable from a molecular sieve oxygen generator
system, i.e. in the order of 94 to 95 percent. In the lower region
of the altitude band 3000 to 6700 meters, the oxygen concentration
is greater than is physiologically necessary, but this is not medically
detrimental. However, if desired one or more additional thresholds
may be introduced and so provide bands that obtain more closely
to the physiological requirement. The different thresholds can be
effected by pressure transducer means, for example an altitude switch,
arranged to modify the output signal of the sequencer to reduce
the cycle time in each ascending altitude band. |