WO2006079782A2 - Nitrogen inerting system for aircraft - Google Patents

Nitrogen inerting system for aircraft Download PDF

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Publication number
WO2006079782A2
WO2006079782A2 PCT/GB2006/000111 GB2006000111W WO2006079782A2 WO 2006079782 A2 WO2006079782 A2 WO 2006079782A2 GB 2006000111 W GB2006000111 W GB 2006000111W WO 2006079782 A2 WO2006079782 A2 WO 2006079782A2
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Prior art keywords
air
method
aircraft
hollow fibre
strands
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PCT/GB2006/000111
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French (fr)
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WO2006079782A3 (en )
Inventor
John Mcneil
Den Gross Andrew Van
Arthur Lyons
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Smartmembrane Corp
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    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D63/00Apparatus in general for separation processes using semi-permeable membranes
    • B01D63/02Hollow fibre modules
    • B01D63/026Wafer type modules or flat-surface type modules
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D53/00Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/22Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D63/00Apparatus in general for separation processes using semi-permeable membranes
    • B01D63/02Hollow fibre modules
    • B01D63/021Manufacturing thereof
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D63/00Apparatus in general for separation processes using semi-permeable membranes
    • B01D63/02Hollow fibre modules
    • B01D63/024Hollow fibre modules with a single potted end or U-shaped
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D63/00Apparatus in general for separation processes using semi-permeable membranes
    • B01D63/10Spiral-wound membrane modules
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B64AIRCRAFT; AVIATION; COSMONAUTICS
    • B64DEQUIPMENT FOR FITTING IN OR TO AIRCRAFT; FLYING SUITS; PARACHUTES; ARRANGEMENTS OR MOUNTING OF POWER PLANTS OR PROPULSION TRANSMISSIONS IN AIRCRAFT
    • B64D37/00Arrangements in connection with fuel supply for power plant
    • B64D37/32Safety measures not otherwise provided for, e.g. preventing explosive conditions
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D2313/00Details relating to membrane modules or apparatus
    • B01D2313/10Specific supply elements
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D2313/00Details relating to membrane modules or apparatus
    • B01D2313/12Specific discharge elements

Abstract

A method and system for inerting an aircraft fuel tank is disclosed. A gas separation module (18) is provided comprising a plurality of strands of hollow fibre separation membrane wound around a core. Cabin air (17) is drawn through the membrane bundle and is separated into nitrogen and oxygen enriched fractions by the pressure difference existing between the cabin air and the atmosphere outside of the aircraft. The nitrogen enriched air is then supplied to the fuel tanks to provide an inert atmosphere.

Description

Nitrogen Inerting System for Aircraft

The present invention relates to an air separation system that is able to utilise the pressure differential between the inside and the outside of a passenger aircraft to separate cabin air into a first fraction slightly enriched with oxygen and a second fraction highly enriched with nitrogen. The nitrogen rich air is then used to provide an inert atmosphere inside the fuel tanks of the aircraft.

An earlier patent application GB 0330257.7 by the applicant describes a nitrogen inerting system that could be used for various transport applications, including road transport vehicles as well as passenger aircraft. Road transport vehicles have relatively low fuel consumption and only a small flow of nitrogen rich air is required to provide an inert atmosphere in the fuel tank of the vehicle. The gas separation module for use inside road vehicles only needs to incorporate short lengths of separation membrane and the module can therefore be relatively small, for example less than 3 litres in volume. A light vacuum applied to the hollow cores of the composite, yet permeable, hollow fibre membranes used in the module is sufficient to separate normal atmospheric air into oxygen rich and nitrogen rich fractions, and provide an adequate enough flow of nitrogen rich air to inert the fuel tank of the vehicle. In contrast, because of the relatively high fuel consumption of passenger aircraft and the atmospheric pressure changes that occur during the flight of an aircraft, a large flow of nitrogen rich air is needed to effectively inert the fuel tanks of an aircraft, especially during the descent phase of the flight. In GB 0330257.7, compressed bleed air from the aircraft engines is used to supplement the pressure differential available between the inside and the outside of the aircraft to ensure that the flow of nitrogen rich air is sufficient to provide fully inert fuel tanks during landing. Alternatively extra pumps are used in the inerting system to change the flow rate of the nitrogen rich air. The additional positive pressure of the compressed air provided by the engines or pumps allows the gas separation system to continue to provide nitrogen rich air to the aircraft fuel tanks during descent, even at low altitude when there is very little pressure differential available between the inside and the outside of the aircraft. However, there are disadvantages associated with using bleed air from the aircraft engines to operate the gas separation module.

For example, using engine bleed air increases fuel consumption and hot bleed air would also be a potential safety hazard if it accidentally escaped directly from the air separation module into an aircraft fuel tank. Feeding hot air to hollow fibre membranes is advantageous in so far as it can improve the permeability of the membranes; however, prolonged exposure to hot bleed air can also accelerate the degradation of organic polymer based membranes. The gas separation modules described in GB 0330257.7 for road transport and passenger aircraft applications contain the same specification of hollow fibre membrane, although the modules would be of a different size.

The present invention describes a nitrogen inerting system that has been specifically designed for use on-board commercial passenger aircraft. The proposed air separation unit comprises one or more gas separation modules able to separate pressurised cabin air into nitrogen rich and oxygen rich fractions by means of the pressure differential available between the inside and the outside of the aircraft only, and the system is not dependent on using bleed air from the aircraft engines to supplement this pressure differential. Because the nitrogen inerting system works under low pressures, the system is safe, lightweight and compact, attributes that are particularly important for aircraft applications.

The one or more gas separation modules are assembled together to form an air separation unit that is able to operate as an integral part of the normal aircraft air conditioning system. For example, during the flight of an aircraft, cabin air is continually replenished with fresh air and stale cabin air, which would normally be dumped overboard, can be used as the air supply for the air separation system. Stale cabin air is normally at a temperature of about 2O0C, and during a flight the pressure differential available between the pressurised cabin air and the outside atmosphere can be up to 7.8 psi when the aircraft is flying at its cruise altitude. The nitrogen inerting system is therefore a low-temperature, low-pressure system that is safe to use in an aircraft environment.

A composite hollow fibre membrane is preferably used as the gas separating medium inside the gas separation module. The materials used in the construction of the membrane are less important than the actual performance properties of the membrane; however, the composite membranes are preferably the same or have similar properties to those described in GB 2397303, where the membranes comprise a polyethersulfσne fibre tube coated on the outside with a thin layer of either polydimethylsiloxane polymer or more preferably a mixture of cross-linked and non cross-linked polydimethylsiloxane polymer.

The gas separation membrane used in the present invention preferably has permeability and selectivity properties similar to those given in Tab IeI . Table 1 Hollow Fibre Membrane Performance Properties

Figure imgf000004_0001

GB 2397303 also describes a modification technique that is preferably used in the present invention to increase the gas permeability properties of the hollow fibre tube before it is coated with the selective outer layer. The modification technique involves the application of liquids to the outer surface of the fibre tube that changes the structure of the open pores located near the surface of the tube. After modification the fibre tube becomes more porous and the outer surface of the tube is able to support a very thin layer of selective polymer. The modification process involves soaking the outer surface of the fibre tube with a solvent solution, such as a solution of acetone, displacing the solvent solution from the pores of the membrane with a non-solvent, such as distilled water, and then quickly drying the tube. Long lengths of modified fibre tube can produced by passing unmodified fibre through a series of baths containing solvent solution and distilled water until the surface of the fibre tube has been sufficiently modified to give the required gas permeability property. The permeability and selectivity properties of composite hollow fibre membranes can also be varied by other means. For example, the hollow fibre tube and the selective coating on the fibre tube can be manufactured from different polymer materials; the process conditions used to produce the fibre tube can be altered; and the internal and external diameters of the fibre tube and the thickness of the selective coating can be varied.

In conventional air separation modules, strands of hollow fibre membrane are usually densely packed in a substantially parallel manner inside the module, and to work effectively most air separation modules constructed in this way need the air to be supplied to the module under high pressure. The high pressure feed air is usually supplied to the hollow bores of the membranes so that permeate oxygen rich air accumulates on the outside of the strands of membrane, whilst retentate nitrogen rich air remains inside the bores of the membranes. In the prefered embodiments of the present invention, instead of packing individual strands of membrane in a parallel manner inside the air separation module, the fibre membrane is wound in a closely intertwined spiral fashion inside the module.

Packing the hollow fibre membrane in a closely entwined spiral manner inside the module improves the packing density and allows a greater surface area of membrane to be contained in a given size of air separation module. Depending on specific circumstances, the air to be separated can be supplied to either the inside bore of the hollow fibre membrane, i.e. the tube side of the membrane, or to the outside of the hollow fibre membrane, i.e. the shell side of the membrane.

To provide an effective nitrogen enrichment gas separation module, it is preferable that the fibre is wound in a spiral manner around a hollow tube located at the centre of the module, and to facilitate the winding operation it is also preferable that the fibre is produced in very long lengths. When the required amount of fibre has been wound onto the central tube, the bundle of hollow fibre membrane is potted at each end with potting compound, and the potted fibre is then cut according to whether the feed air is to be supplied to the tube side or the shell side of the membrane.

It is also preferable that the fibre tube is coated in situ with a thin layer of selective polymer after the fibre tube has been spirally wound into the gas separation module. This is achieved by immersing the wound bundle of fibre tubes into a solution of selective polymer until all of the individual strands of fibre have been, covered with a fine coating of polymer solution. The module is removed from the polymer solution and excess solution is allowed to drain off the fibre tubes. The module is then located inside a drying unit, where a combination of vacuum and gentle heat removes the polymer solvent from the outer surface of the fibre tubes to leave a thin deposit of selective polymer on the outside of each fibre tube.

Hollow fibre tube can also be manufactured in different internal and external diameters to suit specific end-use inerting requirements. For example, for road transport applications the fibre tube will probably have an external diameter of about 800 microns and an internal diameter of approximately 600 microns, whereas for aircraft applications, where a much greater volume of nitrogen rich air is required, it is preferable that the fibre tube has an external diameter of no more than 400 microns and an internal diameter of say 250 microns so that a larger area of membrane can be packed into the air separation module.

Passenger aircraft fly at high attitudes to conserve fuel; for example, many passenger aircraft typically cruise at a height of about 37000 feet, even during short haul flights. Although the proportion of oxygen in the atmosphere remains constant irrespective of altitude, i.e. the composition of the air remains 21% oxygen, 79% nitrogen, as the altitude increases, the atmospheric pressure decreases, as illustrated in Table 2. Table 2 Ambient Pressure at Different Altitudes

Figure imgf000006_0001

To protect the health of passengers when the aircraft is flying at high altitudes, the passenger cabin is usually pressurised to about 10.9 psi, which is equivalent to the atmospheric pressure at a height of around 8000 feet. At a cruising altitude of 37000 feet, the atmospheric pressure on the outside of the aircraft is only about 3.1 psi and there is therefore a pressure difference of 7.8 psi available between the inside and the outside of the aircraft. If the cabin pressure inside the aircraft is higher than 10.9 psi or the aircraft cruises above 37000 feet, then the pressure difference between the inside and the outside of the aircraft will be more than 7.8 psi, although it is unlikely that the pressure differential will ever exceed 10 psi. To preserve passenger comfort, the cabin pressure is gradually reduced as the aircraft climbs after take-off from an atmospheric pressure of about 14.7 psi at ground level to a pressure of about 10.9 psi at 37000 feet. The cabin pressure is maintained at 10.9 psi throughout cruise at 37000 feet, and the pressure change is then gradually reversed during the descent of the aircraft back down to ground level. The pressure differential between the inside and the outside of the aircraft therefore varies from zero at ground level to a maximum of about 7.8 psi at a cruise altitude of 37000 feet. Consequently there is a working pressure differential available throughout the whole flight, although the pressure difference may only be small during parts of the ascent and descent phases of the flight. Because highly permeable hollow fibre membrane is used in the air separation module, application of a low differential pressure of 7.8 psi or less across the wall of the membrane is sufficient to separate normal air into nitrogen rich and oxygen rich fractions. Such differential pressure conditions can be arranged to occur naturally during the flight of a passenger aircraft, and even small pressure differences that exist between the inside and the outside of the aircraft during ascent and descent can be utilised by the air separation process. From a first broad aspect, therefore, the present invention provides a low-pressure air separation system that uses the differential pressure which exists at altitude between the inside and the outside of an aircraft to effect the separation of cabin air into nitrogen rich and oxygen rich fractions. The low pressure on the outside of the aircraft is used to draw permeate oxygen rich air from the air separation system to the outside atmosphere, whilst retentate nitrogen rich air from the system is used to provide an inert atmosphere in the headspace of the aircraft fuel tanks. Fresh cabin air from the aircraft air conditioning system could be used as the air supply for the air separation system; preferably, however, the air supply is either stale cabin air that would otherwise be discharged overboard or a mixture of fresh and stale cabin air. Although in theory any or all of the fuel tanks of an aircraft could be provided with an inert atmosphere from the air separation system, at the present time the aircraft industry considers that inerting the centre tank of the aircraft, i.e. the fuselage tank, is of most importance because the centre tank is probably at most risk of accidental combustion or ignition. The embodiments described therefore concentrate on providing an inert atmosphere to the centre tank of the aircraft only, although the inerting principles could of course be applied to any of the aircraft fuel tanks, such as the wing tanks. By way of example, in a single aisle short haul passenger aircraft the centre tank would normally have a capacity of about 9000 litres. The air separation unit is preferably a sealed unit which can be located in either a pressurised or an unpressurised section of the aircraft. The system has a first pipe connection to a supply of air, preferably stale cabin air, at cabin pressure; a second pipe connection to take permeate oxygen rich air to the outside of the aircraft; and a third pipe connection to take retentate nitrogen rich air to the centre fuel tank of the aircraft.

Whenever there is a pressure difference between the pressurised cabin air and the outside atmosphere, a differential pressure will be exerted between the inside and the outside of each hollow fibre membrane. Because of the permeable nature of the hollow fibre membrane, even a small pressure differential would be enough to draw the cabin air through the walls of the membranes, and the air would become selectively enriched with oxygen as it passed through the membrane walls.

The permeate oxygen rich air is then discharged to the outside atmosphere, and because the centre fuel tank of the aircraft is unpressurised, the retentate nitrogen rich air that remains on the opposite side of the membrane to the permeate oxygen rich air is drawn towards the centre tank by the low pressure prevailing inside the fuel tank.

The permeate oxygen rich air preferably leaves the air separation system at a higher flow rate than the retentate nitrogen rich air. The permeate air is also only slightly enriched with oxygen, whereas the retentate air becomes highly enriched with nitrogen; for example, the permeate air will probably comprise 22% oxygen, 78% nitrogen, whereas the retentate air could contain as little as 5% oxygen and up to 95% nitrogen.

The performance of the nitrogen inerting system is dependent on the flight pattern of the aircraft. For example, the nitrogen inerting system produces the optimum purity of nitrogen rich air when the aircraft is cruising at high altitude, because this is the part of the flight when the pressure differential between the inside and the outside of the aircraft is at its peak. The worst case flight scenario is therefore a short haul flight where the aircraft may only cruise at its maximum altitude for as little as 15 minutes. By way of illustration, a profile of a typical short haul flight is given in Table 3.

Table 3 Description of a Typical Short Haul Flight

Figure imgf000008_0001

The short haul flight described in Table 3 lasts for 65 minutes, after excluding the time spent taxiing on the ground, which obviously varies depending on local circumstances. During the flight only 15 minutes is spent at the cruise altitude of 37000 feet, when the difference in pressure between the inside and the outside of the aircraft is at its maximum of 7.8 psi. The nitrogen inerting system is able to supply nitrogen rich air to the centre tank of the aircraft whenever there is pressure differential between the inside and the outside of the aircraft. The system is obviously most efficient when the pressure differential is at its peak during cruise; however, the system is still able to operate even when the available pressure difference is relatively small as during parts of the ascent and descent of the aircraft. As the aircraft descends, residual gases inside the centre tank reduce in volume due to the increase in atmospheric pressure, and the inerting system should preferably be able to supply enough nitrogen rich air to the tank during descent to compensate for this volume reduction without having to vent outside atmospheric air into the tank.

To satisfy the inerting requirements of a short haul flight, a nitrogen inerting system is preferred that can produce high purity nitrogen rich air during part of the ascent and the entire cruise of the flight, and then continue to produce nitrogen rich air, albeit of a higher volume but lower purity, during the descent of the aircraft. This would ensure that the air in the centre tank is sufficiently enriched with nitrogen to allow the air to remain inert, i.e. contain less than 12% oxygen, at all stages of the descent. It is therefore important that the gas separation system is able to produce a high mass flux and a high purity of nitrogen rich air during the cruise phase of the flight to ensure that the centre tank is adequately inert before the aircraft starts its descent. As the length of the flight at cruise altitude increases, as in a long haul flight, the atmosphere inside the fuel tank will contain a higher concentration of nitrogen and dilution of the inert air by atmospheric air during descent becomes less of a problem. An oxygen content of 13% in the headspace of a fuel tank, i.e. an air composition of 13% oxygen, 87% nitrogen, is usually considered low enough to provide an inert atmosphere. However, to provide an improved safety margin it is generally assumed that the oxygen content in the centre tank of an aircraft should not exceed 12%, especially on landing. To provide a sufficiently high flow rate of high purity nitrogen rich air under relatively low pressure differential conditions, the external diameter of the hollow fibre membrane used in the gas separation module is preferably 400 micron or less, so that the module can contain a large enough area of membrane to produce the requisite amount of nitrogen rich air. For example, a membrane area of between 200m2 and 750m2, or even more, might be required to produce sufficient nitrogen rich air to fully inert the centre tank of an aircraft, depending on the size of the aircraft and the flight conditions. Although the hollow fibre membranes could be contained in a single large air separation module, it is much more likely that the membranes will be contained in a number of small modules, which would then be combined together to form an air separation unit. By spirally winding the hollow fibre membrane inside each module, even an air separation unit that contains a large area of membrane will still be relatively lightweight and compact, important attributes for aircraft applications. It has also been established that the flow rate and the purity of the nitrogen rich air from a module can be dependent to some extent on the relative length and width of the module. For example, when cabin air is fed to the outside, i.e. the shell side, of the fibre membrane, the efficiency of the air separation process might be improved by spirally winding the membrane inside the gas separation module so that the ratio of the active length of the module to the active diameter of the module is between 8:1 and 12:1, and most preferably about 10:1. However, when cabin air is fed to the tube side, i.e. the hollow bore, of the fibre membrane, the air separation process will probably be more efficient if the ratio of the active length to the active diameter of the air separation module is nearer 5:1.

The active length of the module is the length of spirally wound membrane in the direction along the axis of the module and is the length of membrane available for air separation, whilst the active diameter of the module is the maximum diameter of the spirally wound bundle of membranes.

The control systems for the air separation unit are relatively simple in so far as the flow of cabin air into the unit and the flow of permeate oxygen rich air from the unit are initiated and then dependent on the pressure difference that naturally exists between the inside and the outside of the aircraft when it is flying at altitude. Simple on-off regulating valves located in the cabin air supply pipe and the permeate air outlet pipe would be enough to control the operation of the air separation unit. The flow of retentate nitrogen rich air from the air separation unit to the centre tank of the aircraft can also be regulated by a relatively simple on-off, one-way valve, which may include an adjustable outlet orifice to control the flow of the nitrogen rich air into the fuel tank.

The method used to feed the cabin air to the hollow fibre membranes can significantly affect the efficiency of the air separation process. For example, feeding stale cabin air to the outside of the hollow fibre membranes, i.e. the shell side of the membranes, and then drawing the permeate oxygen rich air from the hollow bores of the membranes can be beneficial, in so far as the polymer coating on the outside of the membrane acts as a barrier to pollutants in the stale air, which could help to prolong the life of the membranes. However, with this method of air feed, the flow resistance of the permeate oxygen rich air as it is drawn along the bore of each strand of membrane reduces the pressure differential available for air separation, which can seriously reduce the efficiency of the air separation process. The actual length and internal diameter of the strand of membrane has a significant influence on this effect; for example the longer and narrower the membrane strand, the greater is the loss of differential pressure and the less effective is the air separation process. This detrimental effect of shell side feed can be partially overcome by using strands of membrane that are shorter and wider, and then packing the strands of membrane into a smaller capacity air separation module. A plurality of the smaller sized modules may be combined together to form an air separation unit; however, it is inevitable that the resulting air separation unit will become much larger and heavier and some pressure losses will probably still occur.

An alternative solution is to supply the cabin air to the tube side, i.e. the hollow bore, of each strand of membrane. The pressurised cabin air is introduced into one end of the hollow bores of the membranes, and the outside of the membranes is subjected to the low pressure available on the outside of the aircraft. Permeate oxygen rich air would then be drawn away from the outer surface of the membranes and be discharged to the outside atmosphere, whilst retentate air remaining on the inside of the membranes would become increasingly enriched with nitrogen as the air moves along the bores of the membranes. The retentate nitrogen rich air would eventually be withdrawn from the opposite ends of the bores of the membranes by the low pressure prevailing in the unpressurised centre tank of the aircraft. Because the hollow bore of each strand of membrane is subjected to cabin pressure at one end and low pressure at the other end, the cabin air tends to flow freely along the bore of the membrane. To provide efficient air separation when feeding cabin air to the tube side of the membranes, the fibre membranes are preferably spirally wound into small air separation modules and a multiplicity of modules are preferably combined together to form an air separation unit. The efficiency of the nitrogen inerting system can be influenced by other factors including the performance specification of the hollow fibre membrane; the internal and external diameters of the hollow fibre tube; the area of membrane inside the air separation unit; and the temperature, pressure and flux of the cabin air supplied to the inerting system. Conventional air separation systems usually operate under high-pressure, high-temperature and high-flux conditions, as these conditions encourage high permeability and produce an effective flow rate of nitrogen and oxygen rich air. In contrast, the air separation system described in the present invention is able to operate under low-pressure and low-temperature conditions. However, the flux of the air supply to the air separation unit is also an important consideration, because without an adequate supply of air it would not be possible to produce sufficient nitrogen rich air to inert the centre tank of the aircraft.

To protect the health of passengers during a flight, the air inside the passenger cabin is continually replenished with fresh air introduced from the outside the aircraft, and stale cabin air that exceeds the specified air quality standard is regularly discharged from the aircraft. In theory, up to 20 lb/minute of stale cabin air could be available for the air separation system, however, to allow for potential cabin air losses and an operating safety margin, it is preferable that the inerting system is able to operate efficiently with a lower air consumption. By altering the surface area of the membrane packed inside the air separation unit, the inerting system described in the invention could theoretically operate with an air supply flux ranging from say 6.5 lb/mimite to 20 lb/minute. However, an air flux of about 10 lb/minute is preferred because the air separation unit would only need to contain a relatively small surface area of membrane and yet still provide an efficient nitrogen inerting system. The air separation unit would also still be lightweight and compact.

With an available cabin air flux of 10 lb/minute, a single large air separation module, of either shell side or tube side feed, could theoretically contain sufficient hollow fibre membrane to be able to supply enough nitrogen rich air to inert the centre tank of an aircraft during a short haul flight. However, to ensure an adequate supply of nitrogen rich air at all stages of the flight, particularly during descent, it is much more likely that a plurality of smaller gas separation modules would be combined together to form an air separation unit. The separation unit would be sealed and airtight, and would be equipped with an inlet pipe for the supply of cabin air; on outlet pipe for permeate oxygen rich air; an outlet pipe for retentate nitrogen rich air; and relatively straightforward valves and controls to operate the unit. To inert larger fuel tanks, such as those used on board long haul aircraft, extra gas separation modules would be included in the air separation unit to produce the requisite amount of nitrogen rich air. The nitrogen inerting system is inherently safe because cabin air, which is cooled down to about 200C by the aircraft air conditioning system, is used as the air supply and the system is not directly reliant on the use of hot bleed air from the aircraft engines. The nitrogen rich air from the inerting system will therefore also be at a low temperature and there is no risk of hot air being accidentally discharged from the air separation unit into either the aircraft or the centre fuel tank. The air supply for the inerting system could be fresh cabin air supplied direct from the aircraft .air conditioning system. Preferably, however, the air supply would be stale cabin air, because stale air is normally wasted by being discharged straight out of the aircraft; or alternatively the air supply could be a combination of fresh and stale cabin air. Because the hollow fibre membrane used in the air separation unit is highly permeable, the membrane is capable of efficiently separating air at relatively low operating temperatures of between say O0C and 400C.

The operation of the air separation unit in a low temperature environment would primarily be limited by the dew point and/or the freezing point of the water vapour present in stale cabin air rather than by the performance properties of the fibre membrane. As mentioned earlier, because the air separation unit is a sealed system it could operate in either a pressurised or an unpressurised section of the aircraft. Although the cabin air supplied to the air separation unit would be at a temperature of about 2O0C, the temperature of the ambient air surrounding the air separation unit will depend on whether the unit is stored in a warm, pressurised section of the aircraft or in a cold, unpressurised part of the aircraft.

The nitrogen inerting system has a number of other safety features. For example, because it is a low-temperature, low-pressure system, the risk of membrane degradation is significantly reduced and pressure blow-outs are very unlikely. The permeate oxygen rich air from the gas separation unit is also only slightly enriched with oxygen, for example the permeate air would have a typical composition of 22% oxygen, 78% nitrogen, and accidental leakage of permeate air into the aircraft environment would not pose a combustion hazard.

Also the amount of ozone allowed in cabin air is subject to strict regulation, and the aircraft air conditioning system controls the amount of ozone in the cabin air; consequently nitrogen rich air from the inerting system will also be within specified ozone limits.

A further feature of the inerting system is that the hollow fibre membrane is very effective at separating water vapour from air. Water vapour will be selectively separated from stale cabin air into the permeate oxygen rich air stream, which is then released overboard, and the retentate nitrogen rich air supplied to the centre tank will therefore be in a relatively dry state.

Various embodiments of a nitrogen inerting system that uses cabin air supplied to the outside of the hollow fibre membranes, i.e. the shell side of the membranes, will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure IA is a schematic cross-sectional illustration showing some of the main components used in the construction of the gas separation module, Figure IB is a schematic illustration that shows the hollow fibre membrane wound around a hollow tube at the centre of the gas separation module, and Figure 1C shows the membranes after they have been cut, potted and fixed into place inside the end caps of the gas separation module;

Figure 2A is a side view showing the potted membranes partly covered by protective film, and Figure 2B is a side view showing the gas separation module fitted with a protective cover having an inlet for the supply of cabin air and outlets for nitrogen rich and oxygen rich air;

Figure 3 is a schematic illustration of an aircraft nitrogen inerting system; and

Figures 4 to 6 show idealised graphical models of the operating conditions and performance characteristics of single air separation modules during a short haul flight, based on the shell side feed of cabin air at different fluxes and without making any allowance for the pressure losses that can occur in a shell feed system.

With reference to Figure IA, the main components of the gas separation module are a hollow tube 1, preferably made from a rigid lightweight metal, such as aluminium, aluminium alloy or plastic, and two end caps 2 and 3, which are also preferably made from either lightweight metal or some other rigid lightweight material.

Tube 1 has a series of perforations 4 through the wall of the tube that will eventually allow retentate nitrogen rich air to pass into the hollow core of the tube. The perforations 4 are preferably situated near the end of tube 1 that will eventually be fixed to end cap 3 by means of locking nut 5, and this end of tube 1 is left open. The opposite end of tube 1 is preferably fixed to a hollow fitment 7, which blocks off this end of tube 1.

Fitment 7 has openings 8 that allow permeate oxygen rich air to pass through the fitment. The end of fitment 7 which is not attached to tube 1 is left open and will eventually be fixed to end cap 2 by means of locking nut 6.

Figure IB schematically illustrates of a very long length of hollow fibre wound around tube 1 in a substantially spiral, criss-cross fashion, where alternate layers of fibre have been angled first one way and then another, until the entwined bundle of fibre 9 has reached the required diameter. The fibre could, for example, be manufactured from poly ethersulf one material and the fibre could have been subjected to a modification technique to increase its permeability.

The left hand end of the wound bundle of fibre 9 is then cut along a line marked from A to A' on Figure IB. This exposes the bores of the strands of fibre that have been formed from the large number of separate layers of fibre wound onto tube 1.

As shown in Figure 1C, the ends of the fibre formed by cutting along line A to A' are potted into end cap 3 by potting compound 10 so that the bores of the fibre are completely sealed and airtight. One end of tube 1 is then fixed to end cap 3 by locking nut 5 and the potting compound helps to seal tube 1 to cap 3.

The right hand end of the wound bundle of fibre 9 is then cut along a line marked from B to

B' on Figure IB. This exposes the bores at the opposite ends of the individual strands of fibre that have been wound onto tube 1. As shown in Figure 1C, the exposed bores are then potted into end cap 2 with potting compound 11 in a manner whereby the bores of the fibre strands remain open and lead to an evacuation chamber 12 formed inside cap 2.

The end of fitment 7 is fixed to end cap 2 by locking nut 6 and the potting compound helps to seal fitment 7 in place in cap 2. To facilitate manufacture, the bundle of fibres may be potted into the potting compound 11 first and the potted end is then cut through to expose the open bores of the fibres.

The method of manufacture described in Figures IB and 1C results in a structure with a very large number of closely packed and intertwined membranes 9 that can effect gas separation, and the resulting gas separation module is therefore compact and lightweight. The hollow fibre membrane is wound onto tube 1 so that the ratio of the active length L to the active diameter D, see Figure 1C, is about 10:1.

The intertwined individual hollow fibres 9 are then coated in situ with a thin layer of selective polymer, which could, for example, be a mixture of cross-linked and non cross-linked polydimethylsiloxane polymer, by firstly immersing the module as illustrated in Figure 1C into a tank containing a solution of the selective polymer. The module is immersed in the solution until all of the fibres have been covered with a fine coating of polymer solution. The module is then removed from the polymer solution and excess solution is allowed to drain from the fibres. The module is then positioned inside an evacuation and drying unit, where the polymer solvent is removed from the outer surface of the fibres by a combination of vacuum and gentle heat to leave a thin deposit of selective polymer on the outside of the fibre tubes. With reference to Figure 2A, the entwined coated membranes (illustrated as 9 in Figures IB and 1C) may be wrapped in a thin plastic film 13, except for a small portion of the membranes adjacent to end cap 2, which is left exposed to the air entering the module. The plastic film is preferably of a polymer type that is permeable to air at high pressures but impermeable to air at the low pressure of <10 psi prevailing inside the gas separation module. Alternatively, the plastic film could cover the whole active length L of the entwined membranes, and the film 13 would be suitably perforated so as to allow air to pass through the film 13 at a flow rate appropriate to the relative position in the bundle of membranes. For example, there would be very few perforations in the film near end cap 3, so that the flow of air into the bundle of membranes close to end cap 3 was restricted, and the number of perforations would then gradually increase across the length of the film 13 in a direction towards end cap 2 so that the highest flow of air into the bundle of membranes was adjacent to end cap 2. Figure 2B shows a rigid cylindrical protective cover 14 which is preferably fitted around the outside of end caps 2 and 3.

The cover 14 may be manufactured from lightweight metal or some other rigid lightweight material, such as a composite or a plastic material, and the cover is designed to make the module airtight under the conditions prevailing on board an aircraft. Cover 14 has an inlet or inlets 21 near end cap 2 to allow cabin air 17 to enter the bundle of fibre membranes at the appropriate position in the module and be fed to the shell side, i.e. the outside of the membranes.

A first exit tube 15 is fitted to locking nut 5 to allow the removal of retentate nitrogen rich air from tube 1 at the centre of the module, and a second exit tube 16 is fitted to locking nut 6 to allow removal of permeate oxygen rich air from the bores of the Fibres within the module.

The operation of the air separation module will now be described with reference to Figure 3 and further reference to Figures IA, IB, 1C, 2A, and 2B.

The gas separation module operates by using the differential pressure that exists between the inside and the outside of a commercial passenger aircraft to selectively separate cabin air into oxygen rich and nitrogen rich fractions. A differential pressure can exist at virtually any time during a flight, although the maximum pressure difference occurs when the aircraft is flying at high altitudes.

Figure 3 illustrates a nitrogen inerting system that supplies cabin air to the outside, i.e. the shell side, of the hollow fibre membranes. Exit tube 16 from the gas separation module 18 is connected to an on/off valve 19 that leads directly to the low-pressure atmosphere on the outside of the aircraft. The module is operated by opening valve 19 which causes a low differential pressure to be exerted in the evacuation chamber 12 inside module 18. The system includes a further regulating valve 22 connected to pipe 21, and opening valve 22 allows pressurised cabin air, preferably at a flux of about 10 lb/minute, to enter module 18. Air separation by the membranes inside module 18 will take place whenever there is a pressure difference between the pressurised cabin air in contact with the outside of the membranes and the low pressure in the bores of the membranes.

Permeate air, which is selectively enriched with oxygen, is drawn from the bores of the membranes into pipe 16 by the low pressure on the outside of the aircraft, and the permeate air is released into the outside atmosphere.

The differential pressure between the inside and the outside of the membranes increases with altitude, which correspondingly increases the flow of permeate oxygen rich air out of the module and the flow of cabin air into the module. The air separation process is most efficient when the aircraft is cruising at 37000 feet when the pressure differential is at its peak.

As the permeate air is released out of module 18 into the outside atmosphere, the retentate air left on the outside of the membranes gradually becomes increasingly enriched with nitrogen as it passes over the large number of membranes packed inside module 18. At the opposite end of the gas separation module 18, exit tube 15 supplies nitrogen rich retentate air from the gas separation module to the centre tank of the aircraft via a one-way control valve 20. The centre tank of the aircraft is unpressurised, and on opening valve 20 the retentate nitrogen rich air tends to flow naturally from module 18 through outlet tube 15 towards the centre tank. The one-way valve 20 prevents vapours in the headspace of the centre tank being accidentally drawn back into the module in the event of an unexpected pressure change inside the air separation system. Valve 20 also has a restricted orifice that can be regulated in size to allow the flow of nitrogen rich air into the centre tank to be controlled at different stages of the flight. For example, during ascent and cruise a low flow of high purity nitrogen rich air is required, whereas during descent a high flow of low purity nitrogen rich air is more suitable, and the different flow rates of the retentate nitrogen rich air can be achieved by regulating the discharge orifice in valve 20. This avoids having to use additional pumps in the inerting system to control the flow and purity of nitrogen rich air from the air separation module to the centre tank at different stages of the flight.

The centre fuel tank is not pressurised whereas cabin air supplied to the air separation module is pressurised relative to the atmosphere outside the aircraft. The drop in pressure within the fuel tank exerts a light negative pressure onto tube 15, which in turn applies a negative pressure to the core of the metal tube inside module 18, so that retentate nitrogen rich air is gradually drawn into the metal tube and then out of the module into the fuel tank. To be able to efficiently separate cabin air into oxygen rich and nitrogen rich fractions and produce enough nitrogen rich air to effectively inert the aircraft fuel tank, it is important that the air separation module is supplied with an adequate amount of cabin air. Examples 1, 2 and 3 below illustrate the theoretical performance characteristics of single air separation modules that have been designed to operate with different fluxes of cabin air supplied to the outside, i.e. the shell side, of the hollow fibre membranes. The flight path of the aircraft, the pressure differential between the inside and the outside of the aircraft and the temperature of the cabin air, 2O0C, are assumed to be the same in each example. It was also assumed that at the start of the flight the headspace of the aircraft centre tank would be inert from the previous flight, i.e. the air in the tank would have an oxygen content of 12%. It was also assumed that the cabin air would be supplied at fluxes of 6.5 lb/minute, 10 lb/minute and 20 lb/minute to the modules in examples 1, 2 and 3 respectively. Each air separation module contains the same specification of membrane, i.e. hollow fibre membrane with a bore of 150 micron, a permeability of 2950 nl/m2/hour/atmosphere and an O2/N2 selectivity of 2.2, and the performance of each module is varied by changing the area of membrane packed inside the module to cope with the different air flux conditions.

The examples are supported by the theoretical graphical models given in Figures 4, 5 and 6, which illustrate altitude and cabin pressure during a short haul flight; cabin air flux; air consumption of the module; oxygen content of the nitrogen rich air produced by the module; flow of nitrogen rich air from the module; and oxygen content in the headspace of the centre tank during the flight.

The theoretical graphical models given in Figures 4, 5 and 6 provide idealised performance characteristics of the air separation modules used in examples 1, 2 and 3 respectively. No allowance was made in the models for the pressure losses associated with shell side air feed due to the flow resistance of the permeate air inside the bores of the membranes. The idealised models therefore only provide an indication of how different areas of membrane, and hence different sizes of air separation module, could potentially cope with different cabin air fluxes.

Example 1

In example 1 the air consumption limit, i.e. the cabin air flux, is restricted to 6.5 lb/minute, and it is estimated that the air separation module will have to contain about 750 m2 of hollow fibre membrane if the module is to be able to produce enough nitrogen rich air to adequately inert the centre tank of the aircraft with this particular air supply.

The idealised graphical model in Figure 4 indicates that the proposed air separation module could theoretically provide enough nitrogen rich air to be able to maintain the oxygen content in the centre tank below 12% throughout a short haul flight. However, to provide sufficient nitrogen rich air, the air consumption of the air separation module is always very close to the available cabin air flux of 6.5 lb/minute.

Example 2

In example 2 it is assumed that the cabin air flux is 10 lb/minute, and under these air flow conditions the air separation module will only need to contain about 200 m2 of hollow fibre membrane to produce the requisite amount of nitrogen rich air.

The idealised model in Figure 5 suggests that this air separation module could supply enough nitrogen rich air to adequately inert the centre tank, and that the air flow requirements of the module would be below the available cabin air flux. During the flight, the oxygen content in the headspace of the centre tank will reach a minimum of 5%, and on landing the tank will contain 10.4% oxygen. The theoretical performance model for example 2 suggests that an air flux of about 10 lb/minute could well be enough to provide a reliable and effective nitrogen inerting system, providing there are no significant pressure losses in the system. Details of the nitrogen inerting system used in example 2 are given in Table 4, based on a cabin air flux of 10 lb/minute fed to the shell side of the fibre membranes. The performance characteristics described in Table 4 are idealised in so far as no allowance was made in the model for pressure losses that can occur in the system.

Table 4

Nitrogen Inerting System as described in Example 2 Based on Shell Side Feed of the Cabin Air

Figure imgf000019_0001

Example 3

In example 3 it was assumed that the cabin air flux would be 20 lb/minute, which in theory is the maximum amount of stale cabin air that could be available on some types of aircraft, although in practice less cabin air is likely to be available.

It was also assumed in example 3 that the module would contain about 400 m2 of membrane to illustrate how a combination of high flux and relatively large surface area of membrane would affect the air separation properties of the module.

The performance model in Figure 6 shows that when a large area of membrane is combined with a high flux of cabin air, the oxygen content in the headspace of the centre tank could reach a minimum of 3% and on landing the centre tank could contain as little as 8.4% oxygen. The graphical model in Figure 5 indicated that the single air separation module described in

Table 4 could theoretically produce enough nitrogen rich air to fully inert the centre tank of an aircraft when supplied with cabin air at a flux of 10 lb/minute. However, because the cabin air is supplied to the shell side of the membranes, under practical operating conditions there will undoubtedly be pressure losses in the system and these will affect the efficiency of the air separation process, and for a low pressure system, feeding cabin air to the shell side of the membranes is probably not the most appropriate method of air supply.

A preferred embodiment of the invention is therefore described in Example 4, in which cabin air is supplied to the tube side, i.e. the hollow bores of the membranes, instead of the shell side of the membranes. In tube side feed, the cabin air is fed to one end of the bores of the membranes; permeate oxygen rich air passes through the walls of the membranes to the outside of the membranes; and the retentate nitrogen rich air is left on the inside of the bores of the membranes. The nitrogen rich air is then withdrawn from the opposite end of the bores of the membranes and supplied to the aircraft fuel tank.

The preferred embodiment is explained with the aid of the accompanying illustrations in which:

Figures 7A and 7B are schematic illustrations of air separation modules in which the cabin air is supplied to the tube side, i.e. the bores of the membranes;

Figure 8 shows a graphical model of the performance characteristics of an air separation unit that contains nine gas separation modules in which cabin air is supplied to the tube side, i.e. the bores of the membranes, at a flux of 11 lb/minute.

In figure 7 A, hollow fibre membrane 23 is spirally wound around a hollow tube 24 which is sealed at both ends; for ease of illustration only a single strand of the wound membrane is shown in Figure 7A.

The wound bundle of membranes is then potted at each end into polymer potting compound

25 and 26 so that the hollow core, i.e. the bore, of each strand of membrane is open at both ends 27 and 28.

The potted membranes are then contained in a gas separation module in a manner whereby a pressure chamber 29 is formed at one end of the module and an evacuation chamber 30 is formed at the other end of the module. The bundle of potted membranes is then wrapped in plastic film 31 so that a small area of membranes is left exposed near pressure chamber 29.

Pressurised cabin air is fed to the pressure chamber 29, whilst the low pressure prevailing in the unpressurised centre fuel tank evacuates chamber 30. The cabin air in chamber 29 enters the bores of the membranes, i.e. the air is fed to the tube side of the membranes, and the drop in pressure between chambers 29 and 30 draws the cabin air along the bores of the membranes.

The outside surface of the membranes, i.e. the shell side of the membranes, is subjected to the low atmospheric pressure that exists on the outside of the aircraft so that permeate oxygen rich air is drawn through the walls of the membranes. The permeate air is withdrawn from the module through the exposed portion of membranes located near pressure chamber 29, and the permeate air is then discharged to the outside atmosphere. The retentate nitrogen rich air left inside the bores of the membranes is supplied to the aircraft fuel tank.

A similar air separation module is illustrated in Figure 7B except that the hollow tube 32 at the centre of the module is sealed at the evacuation chamber end and left open at the opposite end. Tube 32 also has apertures 33 that allow permeate oxygen rich air to be drawn into the hollow core of tube 32, so that permeate oxygen rich ah" can be withdrawn from the core of tube 32 as well as from the outside of the bundle of membranes. Under certain circumstances the module construction illustrated in Figure 7B can encourage a greater flow of permeate oxygen rich air from the module, which might be beneficial in some inerting applications. Example 4 uses an air separation unit comprising nine air separation modules constructed as in Figure 7 A, in which the stale cabin air is fed to the tube side of the strands of membranes, i.e. the bores of the membranes. Each air separation module would have a length of 760 mm and a diameter of 157mm, giving an active length to width ratio of 5: 1 for the module. Figure 8 illustrates a graphical performance model for the air separation unit that contains nine of the above air separation modules. Example 4

In example 4 it is assumed that the available cabin air flux would be ll lb/minute. The air separation unit would have an inlet pipe for cabin air; an outlet pipe for retentate nitrogen rich air; an outlet pipe for permeate oxygen rich air; and appropriate regulating valves to control the operation of the unit.

Each module inside the air separation unit would be connected to the inlet pipe supplying cabin air to the separation unit and to the outlet pipes that discharge the retentate nitrogen rich and the permeate oxygen rich air from the unit.

In example 4, the cabin air is supplied to the tube side, i.e. the bores of the fibre membranes, and therefore the potential pressure losses in the air separation unit will be much reduced. In fact the pressure losses should be virtually negligible when compared to inerting systems that incorporate shell side feed of the cabin air.

The graphical model given in Figure 8 of the tube side feed air separation unit is a therefore a reasonably accurate reflection of the actual performance characteristics of the preferred inerting system under practical operating conditions. During a short haul flight, the model suggests that the oxygen content in the headspace of the centre tank will reach a minimum of about 5% and on landing the tank will contain approximately 11% oxygen.

Details of the air separation unit used in example 4 and its performance characteristics are given in Table 5.

Table 5

Nitrogen Inerting System as described in Example 4 Based on Tube Side Feed of the Cabin Air

Figure imgf000022_0001

It was assumed in example 4 that the air separation unit would contain nine individual gas separation modules, each packed with an appropriate area of hollow fibre membrane, and providing the cabin air is supplied at a minimum flux of 11 lb/minute, this particular air separation unit should be able to fully inert the centre tank of a short haul passenger aircraft. The preferred embodiment incorporating tube side feed of cabin air at a flux of 11 Ib /minute to the hollow fibre membranes provides a relatively compact and lightweight air separation unit that is capable of fully inerting the centre fuel tank of a short haul aircraft under normal operating conditions.

If the cabin air flux is less than 11 lb/minute, more air separation modules would be included in the air separation unit to provide a larger surface area of membrane inside the air separation unit, so that the unit would still be able to separate the lower flux air into nitrogen and oxygen rich fractions. If a greater flow of nitrogen rich air were required, either to provide an extra safety margin on board short haul aircraft or to inert large fuel tanks used in long haul aircraft, this can also be achieved by increasing the number of modules in the air separation unit. The present invention therefore describes an air separation unit for use on board aircraft that can supply sufficient nitrogen rich air to fully inert the centre tank of either a short haul or long haul passenger aircraft. The air separation unit uses the pressure differential that exists at altitude between the inside and the outside of the aircraft to separate cabin air, which could be fresh cabin air, stale cabin or a mixture of fresh and stale cabin air, into nitrogen rich and oxygen rich fractions. Preferably the air separation unit comprises a plurality of air separation modules in which the cabin air would be supplied to the tube side, i.e. the bores, of the hollow fibre membranes inside the modules. The preferred air separation unit is able to efficiently separate air into nitrogen and oxygen rich fractions even when the cabin air is supplied at a relatively low flux; for example, an appropriately sized inerting system should be able to work effectively with a cabin air flux of 11 lb/minute or less.

Although the present invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims.

Claims

8820801.212Claims :
1. A method of inerting an aircraft fuel tank comprising separating aircraft cabin air into a nitrogen enriched fraction and an oxygen enriched fraction using an air separation unit comprising one or more gas separation modules and then supplying the nitrogen enriched fraction to the aircraft fuel tank, wherein the rate at which the cabin air is supplied to said air separation unit is 20 lb/minute or less, and wherein each of said one or more gas separation modules comprises a plurality of hollow fibre membranes configured to separate air into nitrogen and oxygen enriched fractions solely by means of the pressure difference that exists between the cabin air inside the aircraft and atmospheric pressure outside the aircraft during a flight .
2. A method as claimed in claim 1 , wherein the air is supplied to the air separation unit at a rate selected from the group consisting of : (i ) < 15 lb/minute; ( ii ) < 10 lb/minute; (iii ) 6.5-20 lb/minute; (iv) 10-20 lb/minute; (v) 10-15 lb/minute ; and (vi ) 15-20 lb/minute .
3. A method of inerting an aircraft fuel tank comprising separating air into a nitrogen enriched fraction and an oxygen enriched fraction using an air separation unit comprising one or more gas separation modules and then supplying the nitrogen enriched fraction to the aircraft fuel tank, wherein each of said one or more gas separation modules comprises a plurality of strands of hollow fibre separation membrane wound around a hollow core, and wherein the air supplied to the air separation unit is either fresh cabin air, waste stale cabin air, or a mixture of fresh cabin air and waste stale cabin air .
4. A method of inerting an aircraft fuel tank comprising separating air into a nitrogen enriched fraction and an oxygen enriched fraction using an air separation unit comprising one or more gas separation modules and then supplying the nitrogen enriched fraction to the aircraft fuel tank, each of said one or more gas separation modules comprising a plurality of strands of hollow fibre separation membrane having an outer diameter of ≤ 400 microns , and wherein the air supplied to the air separation unit is at aircraft cabin pressure .
5. A method of inerting an aircraft fuel tank comprising separating air into a nitrogen enriched fraction and an oxygen enriched fraction using an air separation unit comprising one or more gas separation modules and then supplying the nitrogen enriched fraction to the aircraft fuel tank, each of said one or more gas separation modules comprising a plurality of strands of hollow fibre separation membrane which provide each of said one or more gas separation modules with a ratio of active length to active diameter of about 5 : 1.
6. A method of inerting an aircraft fuel tank comprising separating air into a nitrogen enriched fraction and an oxygen enriched fraction using an air separation unit comprising one or more gas separation modules and then supplying the nitrogen enriched fraction to the aircraft fuel tank, each of said one or more gas separation modules comprising a plurality of strands of hollow fibre separation membrane which provide each of said one or more gas separation modules with a ratio of active length to active diameter in the range of 8 : 1 to 12 : 1.
7. A method as claimed in claim 6, wherein the ratio of active length to active diameter is about 10 : 1.
8. A method of inerting an aircraft fuel tank comprising separating air into a nitrogen enriched fraction and an oxygen enriched fraction using an air separation unit comprising one or more gas separation modules and then supplying the nitrogen enriched fraction to the aircraft fuel tank, each of said one or more gas separation modules comprising a plurality of strands of hollow fibre separation membrane, wherein the air separation unit supplies nitrogen enriched air to the fuel tank through a one way valve .
9. A method as claimed in claim 8 , wherein the one-way valve is a bleed valve having an orifice which is adjustable to control the flow rate of nitrogen enriched air to the fuel tank .
10. A method as claimed in any preceding claim, wherein said method comprises applying a pressure difference across said hollow fibre separation membrane, and wherein said pressure difference is < 10 psi .
11. A method as claimed in claim 10, wherein said pressure difference is selected from the group consisting of : (i) < 9 psi; ( ii) ≤ 8 psi ; ( iii) ≤ 7.8 psi ; and (iv) < 5 psi .
12. A method as claimed in claim 10 or 11 , wherein said pressure difference is solely the difference between cabin air pressure and atmospheric pressure outside the aircraft , and wherein said pressure difference is varied by the change in altitude of the aircraft and/or the regulation of the cabin air pressure .
13. A method as claimed in any preceding claim, wherein the air to be separated is supplied to the hollow bores of the strands of hollow fibre gas separation membrane .
14. A method as claimed in claim 13, wherein the ends of the bores at which the air enters the strands of gas separation membrane are substantially at cabin air pressure and wherein the other ends of the bores are at the atmospheric pressure on the outside of the aircraft .
15. A method as claimed in claim 13 or 14 , wherein the pressure on the outside of the strands of hollow fibre membrane is substantially at the atmospheric pressure on the outside of the aircraft and wherein the difference in pressure between the inside and the outside of each strand of hollow fibre membrane draws permeate oxygen rich air from the bores to the outside of the strands of membrane .
16. A method as claimed in any one of claims 1 to 12, wherein the air is supplied to the outer surfaces of said strands of hollow fibre gas separation membrane .
17. A method as claimed in claim 16, wherein the bores of the hollow fibre membranes are maintained substantially at the atmospheric pressure on the outside of the aircraft .
18. A method as claimed in claim 17, wherein oxygen enriched air is drawn out of the bores of said strands of hollow fibre separation membrane by the pressure difference between the inside and outside of the aircraft .
19. A method as claimed in any preceding claim, wherein the air supplied to the air separation unit is supplied at a temperature selected from the group of : (i ) ≤ 30 °C; ( ii) ≤ 25°C; ( iii) ≤ 200C; and (iv) cabin air temperature .
20. A method as claimed in any preceding claim, comprising withdrawing permeate oxygen enriched air from the air separation unit at a first rate; and withdrawing retentate nitrogen enriched air from the air separation unit at a second, lower, rate .
21. A method as claimed in any preceding claim, wherein the total surface area of the external surfaces of the strands of hollow fibre separation membrane in each gas separation module is selected from the group consisting of: (i ) ≤ 1000 m2; (ii ) ≤ 800 m2; ( iii ) < 600 m2; ( iv) ≤ 400 m2; (v) < 200 m2; (vi ) < 100 m2; (vii ) 50-100 m2/ (viii ) 100-900 m2; (ix) 150-800 m2; and
(x) 200-750 m2.
22. A method as claimed in any preceding claim, wherein the total surface area of the external surfaces of the strands of hollow fibre separation membrane in said air separation unit is selected from the group consisting of : ( i ) < 1000 m2; (ii ) ≤ 800 m2; ( iii) < 600 m2; (iv) < 400 m2; (v) < 200 m2; (vi ) 100- 900 m2; (vii) 150-800 m2/ and (viii) 200-750 m2.
23. A method as claimed in any preceding claim, wherein said strands of hollow fibre separation membrane have an outer diameter selected from the group consisting of : (i ) ≤ 350 microns ; ( ii) ≤ 300 microns ; ( iii) ≤ 250 microns ; ( iv) ≤ 200 microns; (v) < 150 microns ; (vi) 100-400 microns ; (vii) 150-400 microns ; (viii) 200-400 microns ; ( ix) 250-400 microns ; (x) 300-400 microns ; (xi ) 350-400 microns ; (xii ) about 220 microns ; and (xiii) about 150 microns .
24. A method as claimed in any preceding claim, wherein said strands of hollow fibre separation membrane have an inner diameter selected from the groups consisting of : (i ) ≤ 300 microns ; (ii ) ≤ 250 microns ; ( iii ) ≤ 200 microns ; ( iv) ≤ 150 microns ; (v) ≤ 100 microns ; (vi) 100-300 microns ; and (vii) 150-250 microns .
25. A method as claimed in any preceding claim, wherein the strands of hollow fibre separation membrane have a permeability selected from the group consisting of : (i) 2000- 3500 ; (ii) 2100-3500; (iii) 2200-3500; (iv) 2300-3500; (v) 2400-3500 ; (vi ) 2500-3400 ; (vii ) 2600-3300 ; (viii ) 2600-3200 ; (ix) 2650-3250; and (x) about 2950 nl/m2/hour/atmosphere pressure .
26. A method as claimed in any preceding claim, wherein the strands of hollow fibre separation membrane have an oxygen to nitrogen gas selectivity selected from the group consisting of : ( i ) 1.2-4 ; (ii ) 1.3-3.5; ( iii ) 1.4-3.0 ; (iv) 1.5-2.9; (v) 1.6-2.8 ; (vi ) 1.7-2.7 ; (vii) 1.8-2.6; and (viii) about 2.2.
27. A method as claimed in any preceding claim, wherein the flow rate of air into the air separation unit is controlled by a valve .
28. A method as claimed in any preceding claim, wherein the flow rate of the nitrogen enriched air into the fuel tank is varied by controlling a valve .
29. A method as claimed in any preceding claim, wherein the flow rate of oxygen enriched air leaving said air separation unit is varied by controlling an on/off valve .
30. A method as claimed in any preceding claim, wherein the nitrogen enriched air from the air separation unit is supplied to the fuel tank with an oxygen content selected from the group consisting of : (i ) < 13% ; (ii) < 12%; (iii ) ≤ 10%; < (iv) 8% ; (v) < 6% ; (vi ) ≤ 4% ; (vii) < 2% ; (viii) ≤ 1% ; and (ix) 5-12% .
31. A method as claimed in any preceding claim, wherein the atmosphere inside the fuel tank contains 12% or less oxygen .
32. A method as claimed in any preceding claim, wherein the oxygen enriched air from the air separation unit comprises about 22% oxygen, 78% nitrogen .
33. A method as claimed in any preceding claim, wherein the volume of each gas separation module is selected from the group comprising: (i) 0.01-0.04 m3; (ii) 0.013-0.035 m3; (iii) 0.015-0.03 m3; ( iv) 0.017-0.025 m3; and (v) about 0.017 m3.
34. A method as claimed in any preceding claim, wherein the air separation unit comprises a plurality of air separation modules and wherein the total volume of the air separation unit is selected from the group consisting: (i) 0.05-0.5 m3;
(ii) 0.1-0.45 m3; (iii) 0.1-0.4 m3; ( iv) 0.1-0.35 m3; (v) 0.1- 0.25 m3; and (vi ) about 0.15 m3.
35. A method as claimed in any preceding claim, wherein the strands of hollow fibre separation membrane provide each of said one or more gas separation modules with a ratio of active length to active diameter selected from the groups comprising : ( i ) 8 : 1 to 12 : 1 ; ( ii ) 10 : 1 ; and ( iii ) 5 : 1.
36. A method as claimed in any preceding claim, wherein said air separation unit is located in an environment maintained at cabin pressure .
37. A method as claimed in any of claims 1-35, wherein said air separation unit is located in an environment maintained at the atmospheric pressure on the outside of the aircraft .
38. A method as claimed in any preceding claim, wherein each of said one or more gas separation modules comprises a plurality of strands of hollow fibre separation membrane wound around a core .
39. A method as claimed in any of claims 1-2 and 4-37 , wherein said one or more gas separation modules comprises a plurality of strands of hollow fibre separation membrane running substantially parallel to the axis of the module .
40. A method as claimed in any preceding claim, wherein the plurality of strands of hollow fibre separation membrane have an outer coating which is resistant to degradation by ozone .
41. A method as claimed in any preceding claim, wherein said hollow fibre membrane is a composite material comprising a porous hollow fibre tube coated with a selective polymer .
42. A method as claimed in claim 41 , wherein the porous hollow fibre tube is manufactured from polyethersulfone polymer material .
43. A method as claimed in claim 41 or 42 , wherein the selective polymer comprises polydimethylsiloxane .
44. A method as claimed in claim 43, wherein the selective polymer comprises a mixture of cross-linked and non cross- linked polydimethylsiloxane .
45. A method as claimed in any of claims 41 to 44, wherein the external surface of the porous hollow fibre tube has been subj ected to a modification technique so as to increase the porosity of the fibre tube before it is coated with said selective polymer, the modification technique comprising soaking said fibre tube in a solvent solution, displacing the solvent solution from the pores of the fibre tube with a non- solvent solution and then drying the fibre tube .
46. A method as claimed in claim 45, wherein said solvent solution comprises acetone, and wherein said non-solvent solution is preferably distilled water .
47. An aircraft fuel tank inerting system for use in a method as claimed in any preceding claim.
48. An aircraft fuel tank inerting system comprising an air separation unit having one or more gas separation modules for separating air into a nitrogen enriched fraction and an oxygen enriched fraction and means to supply the nitrogen enriched fraction to the fuel tank, wherein each of said one or more gas separation modules comprises a plurality of strands of hollow fibre separation membrane configured to separate air into nitrogen and oxygen enriched fractions solely by means of the pressure difference between cabin air inside the aircraft and the atmospheric pressure outside the aircraft during a flight when air is supplied to the air separation unit at a rate of 20 lb/minute or less .
49. An aircraft fuel tank inerting system as claimed in claim 48 , said system further comprising an on-off valve for controlling the flow of cabin air to the at least one gas separation module .
50. A system as claimed in claim 48 or 49, further comprising a one-way bleed valve having an orifice which is adjustable to control the flow of nitrogen enriched air to the fuel tank .
51. A system as claimed in any of claims 48 , 49 or 50, further comprising an on/off valve for controlling the flow of oxygen enriched air leaving the at least one gas separation module .
52. A method of manufacturing a gas separation module comprising a plurality of strands of hollow fibre separation membrane wound around a core, wherein the strands of hollow fibre separation membrane have a coating of a selective polymer formed by immersing the module into a solution of the selective polymer, removing the module from the polymer solution, draining off excess solution, and drying the module in an evacuation unit .
PCT/GB2006/000111 2005-01-27 2006-01-13 Nitrogen inerting system for aircraft WO2006079782A3 (en)

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CN106326673A (en) * 2016-08-31 2017-01-11 安徽美空气电科技有限公司 Method for separating nitrogen and oxygen from air based on hollow fiber film

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