AU2527999A

AU2527999A - Improvements in and relating to gas generators

Info

Publication number: AU2527999A
Application number: AU25279/99A
Authority: AU
Inventors: Stephen Vernon Barnett; Robert Glyn Lewin
Original assignee: British Nuclear Fuels PLC
Current assignee: Sellafield Ltd
Priority date: 1998-02-25
Filing date: 1999-02-19
Publication date: 1999-09-15
Also published as: NO20004231D0; NO20004231L; EP1060009A1; CA2321813A1; ZA991490B; WO1999043424A1; JP2002504424A; GB9803893D0

Description

WO99/43424 PCT/GB99/00345 -1 IMPROVEMENTS IN AND RELATING TO GAS GENERATORS This invention is concerned with improvements in and relating to gas generators, particularly, but not exclusively, for the generation of oxygen. Gas generators are used in a wide variety of applications to produce or separate gases for breathing, to provide gases for use in chemical reactions, to separate chemical compounds into their component parts and for a variety of other purposes. Gas generators should ideally provide a high level of purity, at a high gas flow rate, with maximum efficiency and at as low an operating temperature as possible. Control over the amount of gas generated is also desirable. Existing technology faces problems in one or more of these areas and the present invention aims to provide an improved gas generator. According to a first aspect of the invention we provide a gas generator comprising an assembly comprising a layer of a first material, the first material separating a first space from a second space, and means for provided a differential across the layer for at least one species, the layer being capable of transporting electrons and transporting ions of the at least one species and resisting the flow of gas from one space to the other. The flow of gas ions from one side to the other results in gas being generated or separated to one space of the generator. Preferably the first material is or includes a mixed conductor, most preferably for both electrons and ions of the gas to be transported. Preferably the gas to be transported is oxygen. Preferably the first material is stable in an oxidising and reducing environment. A first material comprising a ceramic oxide is preferred. Preferably the first material comprises urania. Doped urania, for instance by one or more rare earth metals, is particularly preferred. Urania doped with yttria may be provided. The urania may be provided as a solid solution with a second WO99/43424 PCT/GB99/00345 -2 material. The second material may comprise one or more rare earth metals, such as yttria. The urania may be depleted urania i.e. the U 23s content may be less than for naturally occuring urania. The first material may be provided in a layer less than 200 micrometers and more preferably less than 150 micrometers thick. One or more further layers may be provided on the first layer and/or on each other. One or more of the further layers may comprise the first material. A further layer or layers of material may be provided which have a greater resistance to the flow of gas therethrough than the first material layer with which it is provided. Preferably the further layer(s) acts as a barrier to other materials, gases, ions in the feed. The further layer(s) may be substantially impermeable to the gas or gases and/or other components present in the first and/or second space. A layer impermeable to the passage of oxygen and/or nitrogen and/or carbon dioxide may be provided. Preferably the layer is impermeable to all three. The layer may be or may further be impermeable to methane and/or other hydrocarbon type gases. The further layer may be provided on the first space side of the first layer and/or on the second space side of the first layer. The further layer may be exposed to the first space and/or the second space. The further layer may be provided separated from the first and/or second space by another layer. The another layer may comprise a layer of the first material. A further material may be provided on and/or in association with and/or in the first material layer to control the passage of electrons through the first layer and/or to control the passage of the gas ions through the first layer. The further material may be provided as a layer on the first material and/or as a layer separated from the first layer be another material or layer, such as the further layer. The further material may be provided in a continuous or discontinuous manner. The further material may be absent from some areas or locations relative to the first layer. The level WO99/43424 PCT/GB99/00345 -3 of further material present, for instance the proportion of area for which it is present or absent, may vary with its location relative to the first layer. The thickness of the further material may vary with position relative to the first layer. The nature and/or make up and/or chemical composition and/or structure of the further material may vary with position relative to the first layer. The differential providing means may comprise means for providing a chemical differential in relation to the species to be transported. For instance the level of the species may be increased on the first side of the apparatus relative to the second side and / or the level may be reduced on the second side of the apparatus relative to that on the first side of the apparatus. The species may be substantially absent from the second side of the apparatus, particularly in the gas feed to the second side of the apparatus. The differential may take the form of a concentration gradient across the generator. The concentration gradient may apply to one or more of the species. The differential providing means may comprise means for elevating the pressure of the species to be transported, for instance above atmospheric, on the first side of the apparatus and/or means for reducing the pressure of the species to be transported, for instance below atmospheric, on the second side of the apparatus. The differential providing means may alternatively or additionally comprise means for introducing a gas stream containing the species to be transported to the first side of the apparatus and/or means for introducing a gas stream substantially free of the species to be transported on the second side of the apparatus. The gas generator may be formed of a plurality of assemblies of the type described. The generator assembly or assemblies may be provided in substantially planar form, for instance as a planar layer or a series of planar layers of the various materials. Square, rectangular or elongate assemblies may be provided.

WO99/43424 PCT/GB99/00345 -4 In an alternative form the generator assembly or assemblies may be formed in a tubular and/or cylindrical and/or hollow elongate form. The assembly or assemblies may be provided with a through passage, such as a central passage, enclosed by the first material. The central passage may form the first side and the space outside the layer the second side or vice versa. Where more than one layer is provided the layers may be provided in a concentric manner, for instance to form layers on a right cylinder. Non- circular or non-regular cross-sections may be provided. The through passage may be provided with an inlet end and an outlet end. The inlet and outlet ends may oppose one another. According to a second aspect of the invention we provide a method for generating gas comprising providing a differential for at least one species across an assembly comprising a layer of a first material, the first material separating a first space from a second space, the layer being capable of transporting electrons and ions of the at least one species and resisting the flow of gas from one space to the other. One or both sides of the generator may be maintained in contact with a given batch of gas. One side may be depleted and the other enhanced in one or more gas levels in such a batch process. Alternatively one or both sides of the generator may be contacted with a changing volume of gas. The gas on one or both sides may periodically or constantly be replaced. In this continuous system the gas level on one side may be improved, but the level of gas on one or both sides may remain fairly constant due to replacement. Preferably the differential results in a flow of electrons through the first layer. Preferably the differential results in a flow of gas ions through the first layer from one side to another preferentially. The gas ions may flow from the first side to the second side. Preferably the gas ions reform gas molecules on reaching the second side. Preferably the gas WO99/43424 PCT/GB99/00345 -5 molecules are exhausted to the space surrounding the second side. The gas ions may be formed by the disassociation of a gas molecule in contact with the assembly and/or first layer. The molecule decomposed may be 02. The gas generated and/or purified and/or increased in concentration is preferably oxygen. The gas may be extracted and/or purified and/or increased in concentration from air. The method may include the addition of the gas produced, preferably oxygen, to a gas stream. The addition may occur remote from or at a surface of the generator. The gas stream may include or consist of methane. Other gaseous hydrocarbons may be present. The methane may be generated by the treatment of coal. The methane may be extracted from an oilwell or other oil production or processing facility. The gas stream may flow past the surface of the generator, for instance from an entrance to an exit, most preferably in a continuous manner. Preferably the method includes the reaction of the oxygen produced with methane to give CO and H 2 . The reaction products may be further processed and/or catalysed to give higher weight hydrocarbons, such as diesel or petrol. Other features presented elsewhere in the application, including the first aspect of the invention, may be provided. Various embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which : Figure 1 shows a cross-sectioned view of a first embodiment of the invention; and Figure 2 shows a partially cross-sectioned perspective view of a second embodiment of the invention. In the first embodiment of the invention illustrated in Figure 1 the gas generator is formed of an assembly 1 with a first side 3 and second side 5. The space around the first and second sides are isolated from one another by the support structure 7 so as to maintain differences in the make up of the WO99/43424 PCT/GB99/00345 -6 volume 9 contacting the first side 3 relative to the volume 11 contacting the second side 5. The assembly 1 consists of a layer 13 of mixed conductor material. The layer 13 is formed of a mixture of particles, the first particles comprising a solid solution of yttria and uranium dioxide and the second type comprising zirconia. In use an oxygen containing gas, such as air, is introduced to the cathode side 3 of the assembly 1. The other side 5 contains a far lower level of oxygen so giving rise to a pressure differential, at least with regard to the relative oxygen levels. The pressure differential driving the system makes use of the potential arising according to the Nernst equation nF Po 2 E = ---- ln -------- 4RT Po2" such that an initial differential in the chemical species balance gives rise to a chemical potential. The difference in chemical potential gives rise to an electron flow and this in turn leads to a flow of oxygen ions from side 3 to anode side 5. The process results in the depletion of the oxygen level on the cathode side 3 of the assembly and its enhancement on the anode side 5. As well as arising from a difference in pressure, differences in the level of one or more chemical species across the apparatus and / or concentration gradients can be used to give the variation and give rise to a chemical potential as a result. The net result is the production of oxygen at the anode side 5 which can then be used for the desired purpose at that side or by its transfer to the location of use. The oxygen feeding side can be constantly or periodically replaced. The gas produced, usually oxygen, can be used for a variety of purposes. The purity of the oxygen produced and the careful control of the level of oxygen produced make the WO99/43424 PCT/GB99/00345 -7 technique particularly suitable for sensitive operations, such as those involved in semi-conductor manufacture, chemical vapour deposition and the like. The production of a pure oxygen output also renders the system useful for injecting oxygen into a carbon based gas to achieve oxidation. This technique is applicable to generating CO and H 2 from a methane gas stream, for instance. This reaction is important in forming intermediaries in the production of petrochemicals from methane produced from coal and is also believed to offer a particularly suitable technique for generating useful and more readily handleable compounds from the methane off gas of oil extraction facilities. The generation of oxygen in this way is preferable to the provision of a cryogenic separator as the size and capital cost is reduced and the transportability of the system is greater. The process may be conducted as a batch process or one or both sides of the assembly may be continually replaced, for instance to maintain a suitable level of oxygen on the cathode side from which to extract. The mixed oxide layer can be produced from an ink style suspension produced by mixing particles formed of a yttria/urania solid solution, together with zirconia, cod liver oil, polyvinyl butyral, polyethylene glycol, dibutyl phthalate and ethanol. The constituents can be mixed by ball milling together for several weeks. A typical mix may comprise 17.19g 50mol% yttria/UO 2 solid solution; 13.65g zirconia; 0.81g cod liver oil; 4.5g polyvinyl butyral; 1.33g polyethylene glycol; 1.2g dibutyl phthalate; 36g ethanol; with 20g terpineol added after ethanol evaporation.

WO99/43424 PCT/GB99/00345 -8 The resulting suspension can be screen printed or otherwise used to form the layer 3, for instance by spraying. The layer may then be sintered, at a temperature below 1550 0 C. Depending on the layer 3 structure provided it may be desirable to provide a sealing layer 20 on a side of the layer 3 so as to prevent gas flow, rather than gas ion flow, through the assembly 1. The layer 3 itself may have too high a porosity, in certain configurations, to serve this function alone. As an alternative to the plate style assembly 1 illustrated in Figure 1, a tubular style generator system 48 may be employed. Such a system, illustrated in Figure 2, consists of a series tubes 50 formed from a mixed oxide layer 52. The space 54 outside the tube 50 is subjected to a flow of heated air, as an oxygen source. Each of the tubes 50 is provided with a flow of methane from inlet end 56 to outlet end 58. The relative abundance of oxygen outside the tubes 50 and the relative absence of oxygen inside the tubes 50 gives rise to a pressure differential and gives rise to the effects described above as a result. Oxygen ions are generated, flow through the layer 52 into the tube, the oxygen is the exhausted into the methane stream inside the tubes 50. The subsequent reaction generates CO and H 2 which can then be used in subsequent reactions. As discussed above, the system 48 may be provided with a gas impermeable layer or membrane 60 to maintain the pressure differential. As the level of oxygen introduced into the tube 50 may vary between its inlet end 56 and outlet end 58 a further controlling layer 62 may be provided. In this embodiment the further layer 62 is provided on the inside surface of the tube. The control layer 62 is used to influence the amount of oxygen introduced into the tube 50 and hence gas stream along it length. This may be to balance the level along the length or otherwise influence it.

WO99/43424 PCT/GB99/00345 -9 The layer 62 may control the active area of the tubes inside surface by forming an ion and gas barrier, which is perforated to an increasing extent towards the exit 58 to give an increasing proportion of active area. The control may alternatively or additionally be effected by varying the thickness of the layer 62 along the length of the tube 50, reducing towards the exit 58, as shown. The materials of the present invention offer a significant number of advantages over existing gas generators. In particular the assembly structure used offers a far higher active area due to the urania layer used. The greater area leads to higher product flow rates. Additionally the urania layer has a high catalytic activity which once again increases the performance of the generator due to improved kinetics. The materials employed also allow the separator to be operated at lower temperatures, approximately 8000C, with benefits in terms of the life of the product and the reduced cost of the surrounding structure. Cost savings are also achieved in avoiding the use of platinum group catalysts within the assembly. The urania also offers significantly improved resistance to poisoning than many other materials. The layer is also far more stable in both reducing and oxidising environments than prior art materials. The system is particularly useful for the proposed uses as the pressure differential enables close control of, and hence an accurate delivery of, the desired amount of gas through the generator/separator. The gas exhausted is also particularly pure and suitable for the desired use due to the highly selective nature of the gas transfer through the layer.

Claims

1. A gas generator comprising an assembly having a layer of a first material, the first material separating a first space from a second space, and means for providing a differential across the layer for at least one species, the layer being capable of transporting electrons and transporting ions of the at least one species and resisting the flow of gas from one space to the other.

2. A gas generator according to claim 1, in which the first material is or includes a ceramic oxide.

3. A gas generator according to claim 1 or claim 2, in which the first material comprises urania.

4. A gas generator according to any one of the preceding claims, in which the first material comprises urania doped with one or more rare earth metals.

5. A gas generator according to claim 4, in which the urania is provided as a solid solution with a second material, the second material comprising one or more rare earth metals.

6. A gas generator according to any one of the preceding claims, the gas generator being provided with a further layer or layers of material which have a greater resistance to the flow of gas therethrough than the first material layer with which it is provided.

7. A gas generator according to any one of the preceding claims, in which a further material is provided on and/or in association with and/or in the first material layer, to control the passage of electrons through the first layer and/or to control the passage of the gas ions through the first layer. WO99/43424 PCT/GB99/00345 -11

8. A gas generator according to claim 7, in which the further material is provided in a discontinuous manner.

9. A gas generator according to any one of the preceding claims, in which the differential providing means comprises means for providing a chemical differential in relation to the species to be transported.

10. A gas generator according to any one of the preceding claims in which the differential providing means comprise means for elevating the pressure of the species to be transported, for instance above atmospheric, on the first side of the apparatus and/or means for reducing the pressure of the species to be transported, for instance below atmospheric, on the second side of the apparatus.

11. A gas generator according to any one of the preceding claims, in which the differential providing means comprise means for introducing a gas stream containing the species to be transported to the first side of the apparatus and/or means for introducing a gas stream substantially free of the species to be transported on the second side of the apparatus.

12. A method for generating gas comprising providing a differential for at least one species across an assembly comprising a layer of a first material, the first material separating a first space from a second space, the layer being capable of transporting electrons and ions of the at least one species and resisting the flow of gas from one space to the other.

13. A method according to claim 12, in which one or both sides of the generator are maintained in contact with a given batch of gas, one side being depleted and the other enhanced in one or more gas levels in such a batch process. WO 99/43424 PCT/GB99/00345 -12

14. A method according to claim 12, in which one or both sides of the generator are contacted with a changing volume of gas.