GB2257054A

GB2257054A - Oxygen generating system

Info

Publication number: GB2257054A
Application number: GB9213555A
Authority: GB
Inventors: Michael William Harral; Jonathan David Ward
Original assignee: NORMALAIR GARRETT
Current assignee: NORMALAIR GARRETT
Priority date: 1991-07-04
Filing date: 1992-06-25
Publication date: 1993-01-06
Also published as: GB9213555D0

Abstract

Oxygen is separated from air supplied to one side surface of a mixed mode conductor material wall of a ceramic membrane apparatus. A pressure gradient is established across the wall and the wall is heated by heat source 15 to a temperature at which oxygen ions diffuse across the wall to the side surface exposed to low pressure where the oxygen product gas is collected from passages in end 16 leading to delivery duct 17. Examples of suitable mixed mode conductor materials are given and other embodiments are described and illustrated, in which the necessary heat may be supplied by using hot air, eg air from a gas turbine engine in an aircraft, which hot air is used to generate oxygen in the aircraft. <IMAGE>

Description

Description of Invention Title: OXYGEN GENERA#ING SYSTEMS This invention relates to oxygen generating systems and is particularly concerned with generating oxygen by an air separation process.

Most existing small scale oxygen generating systems using air separation processes are based on either pressure swing adsorption (PSA) technology or membrane technology.

Modern day aircraft on-board oxygen generating systems based on PSA technology use zeolite molecular sieve material to separate oxygen from air. This requires at least two zeolite beds which have to be sequentially cycled through on-stream/generating and off-stream/purge cycles. A disadvantage of such systems is the requirement for a valve arrangement to control entry of supply air into the beds and flow of oxygen rich product gas and nitrogen rich purge gas from the beds which results in moving parts that are prone to wear and malfunction. A limitation of such systems is that theoretically the maximum oxygen concentration obtainable in the product gas is 95% unless additional means are provided for the removal of argon and other trace gases in the product air. This gives rise to unacceptable cost, weight and size penalties, particularly in aircraft systems.In practice, even with the most efficient systems, it is difficult to produce better than 93% oxygen concentration product gas.

Current membrane technology is generally based on polymeric membranes with separation on the basis of molecular size and diffusion rate through the membrane material. Such membranes find widespread use for the commercial production of nitrogen from air but their limited selectivity makes them uneconomic for the production of product gas of oxygen concentration above 40 to 50%.

Several ceramic materials are known, e.g. yttria stabilised zirconia, which are so-called ionic conductors of oxygen. Such materials become electrically conductive at elevated temperatures due to the mobility of oxygen ions within the crystal lattice and can be used to make oxygen sensors or pump type oxygen generators; however, since the material is only conductive to oxygen ions, an external electric circuit providing electronic conduction is needed. This requires a large electric current and, hence, high electric power which is a disadvantage in aircraft and certain other systems, e.g. systems for producing oxygen for field medical purposes.

Other ceramic materials are known which exhibit mixed electronic and oxygen ion conductivity. These materials known as mixed mode conductors, exhibit both ionic and electronic conductivity. Such materials may hence permit a sustained flux of oxygen ions without the need for an external circuit as depicted in Figure 1 of the accompanying drawings. A similar mechanism based on the superoxide ion 02 is also possible. The oxygen flux is directly dependent upon the logarithm of the oxygen partial pressure ratio, inversely dependent upon the absolute temperature, and may be limited by either the ionic or electronic conductivity of the material or by the surface reaction rate at which molecular oxygen breaks down into its ionic species.

It is an object of the present invention to provide a method and process for the production of substantially 100% oxygen gas of breathable quality by an air separation process using a mixed mode conductor.

Another object of the invention is the provision of a system which will generate substantially 100% oxygen product gas of breathable quality for use in aerospace and other breathing applications.

Accordingly, the broadest aspect of the present invention consists in a method of generating oxygen by separation from air comprising the steps of: supplying air to one side surface of a mixed mode conductor material wall of a ceramic membrane apparatus; providing a pressure gradient across the wall by exposing said one side surface to an oxygen partial pressure that is high compared to an opposite side surface; heating the wall to a temperature at which oxygen ions diffuse across the wall from the side surface exposed to high pressure to the side surface exposed to low pressure and electronic conduction takes place in the opposite direction; and collecting oxygen product gas at the low pressure side surface of the wall.

The oxygen diffusion process is dependent upon the oxygen partial pressure (PPO2) ratio where PPO2 = %o2 x Pressure.

To produce 100% pure oxygen from air a pressure ratio in the order of 5:1 is required to start the process and a practical pressure ratio of 10:1 only gives a PPO2 ratio of 2:1.

It may be desired, therefore, in increasing efficiency of oxygen production to supply high pressure air, such as from a compressor, to one side of the wall and/or to subject the opposite side surface to a negative pressure, such as by a vacuum pump.

A particular advantage of a mixed mode conductor device is that it is able to use high temperature supply air and, conveniently, the wall may be heated to a temperature appropriate for oxygen diffusion by supplying high temperature air to the high pressure side surface of the wall.

Accordingly, another aspect of the present invention provides a process for producing oxygen by separation from air comprising supplying air at pressure and temperature to one side surface of a wall of a mixed mode conductor device so that a pressure gradient exists between opposite side surfaces of the wall and the wall is heated by the air to a temperature at which oxygen ions diffuse across the wall from said one side surface exposed to high oxygen partial pressure to said opposite side surface and electronic conduction takes place in the opposite direction, and oxygen product gas is made available for collection at said opposite side surface of the wall.

A method and process in accordance with the present invention may be reduced to practice in apparatus comprising ceramic wall means manufactured from mixed mode conductor material, inlet means for supplying air to passage means at one side of the wall means, outlet means for delivering oxygen from passage means at an opposite side of the wall means, means for generating a pressure gradient across the wall means, means for heating the wall means to a temperature at which oxygen ions diffuse thereacross from the air inlet side to the oxygen outlet side, and vent means for venting nitrogen and other trace gases from the passage means at the inlet side of the wall means.

If desired the inlet means may embody compressor means for pressurising the supply air in provision of a pressure gradient across the wall means.

Alternatively or additionally, a negative pressure may be applied at the outlet means such as by a vacuum pump which may be incorporated in the outlet means.

Whilst the wall means may be heated by electrical means which requires a source of electrical power be available, it is preferred that another source of heat energy be used for this purpose.

One such source of heat energy may be provided by supplying air to the inlet means at a temperature suitable for heating the wall means. A convenient source of high temperature air is provided by bleeding high pressure air from a compressor stage of a gas turbine engine.

Alternatively or additionally heat energy may be provided by attaching the apparatus to a hot body such as, for example, the exhaust of a gas turbine engine.

Accordingly, apparatus in accordance with the invention finds particular use in combination with a gas turbine engine and, as such, is suited for generating oxygen on-board of an aircraft which is powered by one or more gas turbine engines. This is particularly advantageous in aircraft systems because substantially 100% oxygen product gas of breathable quality is generated and may be stored in a plenum for subsequent use in an emergency such as cabin decompression.

The ability of the apparatus to use high temperature supply air is a further advantage in aircraft systems when compared with PSA systems which require that engine bleed air be cooled by passing through a heat exchanger before being supplied to the zeolite molecular sieve bed otherwise the performance of the PSA system is unacceptably reduced.

Accordingly, a further aspect of the invention provides an aircraft on-board oxygen generating system characterised by a ceramic membrane apparatus having an inlet end adapted for connection to an outlet delivering air at high pressure and high temperature from a compressor stage of a gas turbine engine installed in the aircraft, means for supplying the air to one side surface of mixed mode conductor ceramic wall means of the apparatus whereby, in operation, a pressure gradient exists across the wall means and the temperature of the wall means is raised to a temperature at which oxygen ions diffuse across the wall means to an opposite side surface thereof, and outlet means for collecting oxygen at the opposite side surface of the wall means.

Additionally, apparatus in accordance with the invention may advantageously be used in combination with a small gas turbine engine to generate oxygen for breathing by patients in a field hospital.

The invention will now be further described by way of example and with reference to the accompanying drawings of which: Figure 1 depicts the ionic and electronic conductivity of a mixed mode conductor membrane when exposed to a high oxygen partial pressure at one side surface and a lower oxygen partial pressure at an opposite side surface; Figure 2 is a diagrammatic illustration of apparatus in accordance with one embodiment of the invention; Figure 3 is a diagrammatic illustration of apparatus in accordance with another embodiment of the invention; Figure 4 is a diagrammatic illustration of apparatus in accordance with a further embodiment of the invention in which supply air is obtained by bleeding air from a compressor stage of a gas turbine engine; and Figure 5 is a diagrammatic illustration of yet another embodiment of the invention.

If a mixed mode (electronic/ionic) conductor material such as, for example, Perovskite type oxides (lea1-x Srx Co1-y Fey O3-"), Bismuth Erbia or Bismuth Terbia, fabricated as a membrane or thin wall is heated to a temperature of 5000C or greater and a first surface A of the wall is exposed to air containing oxygen at a partial pressure that is high compared with the oxygen partial pressure at an opposite surface B of the wall (reference Figure 1), a reaction takes place at the surface A in which oxygen molecules 0 2 are reduced to two oxygen ions:: O2 + 4e o 202 A pressure gradient across the wall due to the high oxygen partial pressure at surface A and the lower oxygen partial pressure at surface B provides for a flux of oxygen ions o2 across the wall from surface A to surface B (ionic conductivity). At the same time there is a reverse flow of electrons & from the surface B to the surface A (electronic conduction). At the surface B there is a reverse reaction in which oxygen ions revert to oxygen molecules: 202- 4 02 + 4e- Effective ceramic membranes for the separation of oxygen from air may be fabricated from: i. perovskite type oxides of the form La1 x Srx Co y Fe 03 O3-# where S represents the deviation from stoichiometry.

One particular perovskite type oxide of this form has the composition LaO - 2SrO 8 Co 5 #Fe, . 5 O3-#.

ii. nickel/cobalt perovskite type oxide having the composition LaO 5 Sr, 5 Co, . 8 Nio .2 iii. bismuth erbia of the form (Bi203)1 x (Er203)x iv. bismuth terbia of the form (Bi2O3 x (Tb 0,)x In minimising the heat energy required for heating the membrane wall the temperature should be kept as low as possible; however, oxygen flux is dependent also upon the diffusion properties of the material, the pressure gradient across the wall, the wall thickness and surface interfacial reactions.

To generate an equivalent oxygen flux across a membrane wall manufactured from the same material, temperature increases with wall thickness so that the wall should be as thin as possible, ideally 150 to 200pm, whilst at the same time having adequate strength to withstand the environment in which it is required to operate.

Referring to Figure 2, apparatus 10 for generating substantially 100% oxygen product gas of breathable quality by separation from air comprises a ceramic membrane assembly 11 having walls (not shown in Figure 2) fabricated from a mixed mode conductor material.

The membrane assembly 11 may take the form of a honeycomb extrudate similar to that used for catalytic converters, with alternate passages being closed at opposite ends. Oxygen flux through the material is driven by a pressure gradient across the walls separating the passages, this pressure gradient being provided by supplying air to those passages which present open ends to one end 12 of the membrane assembly from an inlet 13 which incorporates a compressor 14. The membrane assembly is connected to a source 15 of heat energy, which may be an electrically heated element or some other hot body, and is raised to a temperature of at least 5000C or more. At this temperature diffusion of oxygen ions takes place across separating walls of the passages so that oxygen product gas is made available in those passages which present open ends to an opposite end 16 of the assembly 11.The open ends of these passages are communicated with a delivery duct 17 by which the oxygen product gas is passed to an end user or a storage plenum (not shown). Those passages which present closed ends to the end 16 are crossdrilled so as to connect with each other and with a vent outlet duct 18 by which nitrogen and other residual gases are removed.

The apparatus illustrated in Figure 3 is similar to the apparatus described with reference to Figure 2 but the pressure gradient across the walls of the passages is achieved by providing a negative pressure in those passages having open ends communicating with the oxygen delivery duct 17. This negative pressure is provided by a vacuum pump 19 which is incorporated in the delivery duct 17 and, in this embodiment, the compressor is omitted from the inlet duct 13.

In a non-illustrated embodiment, where weight is not a prime consideration, a compressor is included in the inlet duct and a vacuum pump is provided in the oxygen delivery duct.

Referring to Figure 4, ceramic membrane apparatus 30 in accordance with another embodiment of the invention for generating oxygen by separation from air, is adapted for connection to an outlet delivering air at high pressure and high temperature from a compressor stage 31 of a gas turbine engine 32. The supply air enters an inlet plenum 33 at one end of the apparatus from which it flows into passages 34 each of which has an open end communicating with the inlet plenum. The opposite ends of the passages 34 have restricted openings which communicate with a vent gas plenum 35 and a vent duct 36. The passages 34 are separated by membrane walls 37 from passages 38 that present closed ends to the inlet plenum 33 and which are connected at their opposite open ends with an oxygen gas delivery plenum 39.The membrane walls 37 are manufactured from a ceramic mixed mode conductor material that when heated to a temperature of in excess of approximately 5000C by the high temperature air allows oxygen ions to diffuse across the walls, the flux of oxygen ions being driven by the pressure gradient which exists thereacross because of the higher pressure in the passages 38, with electronic conduction taking place in the opposite direction. The oxygen in the passages 38 passes into the delivery plenum 39 from which it flows by way of a delivery duct 40 to a storage plenum or end user (not shown).

The apparatus 30 may be conveniently formed as an extruded ceramic monolith or fabricated from ceramic parts.

If, as previously discussed, the membrane walls can be made sufficiently thin to require heating to a temperature that is below the temperature of the air bled from the engine compressor, whilst being of sufficient strength to withstand the environment in which the apparatus is required to operate, then no external source of heat energy may be required to heat the wall. Where this is not possible external heat energy, such as from electrical heating means, may be provided to raise the apparatus to its working temperature and thereafter it may be maintained at working temperature by the high temperature supply air.

Apparatus in accordance with this embodiment of the invention finds particular application in the generation of oxygen on-board of an aircraft. The apparatus may be provided as a bolt-on unit to one or more gas turbine engines of the aircraft and the oxygen may be delivered to a storage plenum tank in the aircraft. This is particularly advantageous in that the apparatus can make use of high temperature air available from a compressor stage of the engine and, if required, heat from the engine exhaust to heat it to its working temperature.

Such apparatus may be used to generate oxygen on-board of a passenger carrying aircraft where regulations require that one member of the aircrew continuously breathe oxygen during flight above 12,000 metres to provide protection against the effect of altitude in the event of instantaneous cabin decompression, and that a passenger oxygen breathing system be provided to deliver oxygen for breathing by passengers and cabin crew until the aircraft has descended to a safe altitude of 3,000 metres.

However, the apparatus is not restricted to use in aircraft on-board oxygen generating systems, it being contemplated that within the scope of the present invention the apparatus could be provided as a bolt-on unit of a small gas turbine engine in provision of a system for generation of oxygen on the ground for use in field hospitals and other purposes. A particular advantage of the apparatus is its ability to deliver substantially 100% oxygen product gas free from contaminants, whilst requiring few, if any, moving parts and being insensitive to ambient temperature. Also, in aircraft systems the apparatus has the potential of being light in weight and requiring a small space envelope for its packaging.

Apparatus 50 in accordance with another embodiment of the invention, as shown in Figure 5, provides for reduction in electrical power consumption when a ceramic membrane assembly 51 for the separation of oxygen from air has to be heated to temperature by an electrical heating element 52. The membrane assembly and the heating element are located internally of an insulated enclosure 53 with electrical power being provided to the heating element from a source (not shown) external of the enclosure. Pressurised air from a source (not shown) is supplied to an inlet end 54 of the membrane assembly 51 by way of a duct 55 which connects at its upstream end with an outlet end 56 of one set of passages (not shown) of a heat exchanger 57.In this embodiment the heat exchanger is of cross-flow type and receives supply air at an inlet end 58 by way of a duct 59 that connects with the source of pressurised air. One set of cross-flow passages 60 of the heat exchanger 57 are connected by a duct 61 for receiving oxygen product gas delivered from an outlet end 62 of the membrane assembly 51 opposite the inlet end 54. After passage through the heat exchanger oxygen product gas at reduced temperature is delivered to a storage plenum (not shown) or to an end user by a delivery duct 63. In similar manner nitrogen and other residual gases vented from the outlet end 62 are supplied by way of a duct 64 to another set of cross-flow passages 65 of the heat exchanger 57 and, after passage through the heat exchanger, are vented to ambient by way of a vent duct 66.

Because the membrane assembly 51 has to be heated to a temperature in the order of 5000C, the oxygen product gas and the nitrogen/residual gas mixture are delivered at increased temperature over the temperature of the supply air in the duct 59. By placing these gases in heat exchange relationship with the supply air during its passage through the heat exchanger, the supply air is preheated so that after start-up of the apparatus the supply air assists in maintaining the membrane assembly at temperature and the electrical power provided to the heating element 52 can be reduced accordingly.

Claims

1. A method of generating oxygen by separation from air comprising the steps of: supplying air to one side surface of a mixed mode conductor material wall of a ceramic membrane apparatus; providing a pressure gradient across the wall by exposing said one side surface to an oxygen partial pressure that is high compared to an opposite side surface; heating the wall to a temperature at which oxygen ions diffuse across the wall from the side surface exposed to high pressure to the side surface exposed to low pressure and electronic conduction takes place in the opposite direction; and collecting oxygen product gas at the low pressure side surface of the wall.

2. A method according to Claim 1, wherein the air supplied to said one side surface of the wall is high pressure air.

3. A method according to Claim 1 or Claim 2, wherein the air supplied to said one side surface of the wall is high temperature air.

4. A method according to any one of the preceding claims, wherein a negative pressure is provid#ed at said opposite side surface of the wall.

5. A process for producing oxygen by separation from air comprising supplying air at pressure and temperature to one side surface of a wall of a mixed mode conductor device so that a pressure gradient exists between opposite side surfaces of the wall and the wall is heated by the air to a temperature at which oxygen ions diffuse across the wall from said one side surface exposed to high oxygen partial pressure to said opposite side surface and electronic conduction takes place in the opposite direction, and oxygen product gas is made available for collection at said opposite side surface of the wall.

6. Ceramic membrane apparatus for producing oxygen by separation from air comprising ceramic wall means manufactured from mixed mode conductor material, inlet means for supplying air to passage means at one side of the wall means, outlet means for delivering oxygen from passage means at an opposite side of the wall means, means for generating a pressure gradient across the wall means, means for heating the wall means to a temperature at which oxygen ions diffuse thereacross from the air inlet side to the oxygen outlet side, and vent means for venting nitrogen and other trace gases from the passage means at the air inlet side of the wall means.

7. Apparatus according to Claim 6, wherein the inlet means embody compressor means for pressurising air supplied to said one side of the wall means.

8. Apparatus according to Claim 6 or Claim 7, wherein the outlet means embody means for producing a negative pressure at said opposite side of the wall means.

9. An aircraft on-board oxygen generating system characterised by a ceramic membrane apparatus having an inlet end adapted for connection to an outlet delivering air at high pressure and high temperature from a compressor stage of a gas turbine engine installed in the aircraft, means for supplying the air to one side surface of mixed mode conductor ceramic wall means of the apparatus whereby, in operation, a pressure gradient exists across the wall means, and the temperature of the wall means is raised to a temperature at which oxygen ions diffuse across the wall means to an opposite side surface thereof, and outlet means for collecting oxygen at the opposite side surface of the wall means.

10. Apparatus according to any one of Claims 6, to 9, including electrical means for providing heat energy to heat the wall means.

11. Apparatus according to any one of Claims 6 to 10, wherein the mixed mode conductor material comprises a perovskite type oxide of the form La1~x Srx Coley Fey Fe,

12. Apparatus according to Claim 11, wherein the perovskite type oxide is of the composition ##O. 2 SrO 8 COo 5 Fe, 5

13.Apparatus according to any one of Claims 6 to 10, wherein the mixed mode conductor material comprises a perovskite type oxide of the composition Lao,5 Sir, , Co0 8 Nix.2 3-6-

14 Apparatus according to any one of Claims 6 to 10, wherein the mixed mode conductor material is bismuth erbia or bismuth terbia.

15. Apparatus according to any one of Claims 6 to 14, wherein means are provided for attachment of the apparatus to a hot body.

16. Apparatus according to Claim 6, including heat exchange means for placing the supply air in heat exchange relationship with oxygen delivered by the apparatus and/or nitrogen vented from the apparatus, so that the supply air is pre-heated.

17. Apparatus substantially as hereinbefore described with reference to and as shown in Figure 2 of the accompanying drawings.

18. Apparatus substantially as hereinbefore described with reference to and as shown in Figure 3 of the accompanying drawings.

19. Apparatus substantially as hereinbefore described with reference to and as shown in Figure 4 of the accompanying drawings.

20. Apparatus substantially as hereinbefore described with reference to and as shown in Figure 5 of the accompanying drawings.

21. Any new or improved features, combinations and arrangements described, shown and mentioned, any of them together or separately.