Gas separation: apparatus – Apparatus for selective diffusion of gases
Reexamination Certificate
1999-10-25
2001-11-20
Spitzer, Robert H. (Department: 1724)
Gas separation: apparatus
Apparatus for selective diffusion of gases
C096S009000, C096S134000, C096S142000
Reexamination Certificate
active
06319305
ABSTRACT:
BACKGROUND TO THE INVENTION
This invention relates to a gas generating system, and more particularly to such a system which generates two different gases by separating the gases from a supply gas, which may be air.
DESCRIPTION OF THE PRIOR ART
Oxygen generating systems are known. These may typically comprise a molecular sieve oxygen generating system (MSOGS) which utilises pressure swing technology and a molecular sieve bed e.g. a zeolite bed, to adsorb nitrogen from air, thus separating oxygen from the nitrogen. Such MSOGS usually have two or three sieve beds which are cycled through on-stream/generating and off-stream/purge cycles to permit sequential purging of the sieve beds when contaminated with nitrogen. Such MSOGS are capable of producing low pressure oxygen, to a concentration of up to 95% in the product gas. The nitrogen which is purged from the beds typically is a residual or waste gas which is exhausted.
Molecular sieve inert gas generating systems (MSIGGS) have also been proposed which operate on a similar principle to MSOGS, but the molecular sieve bed adsorbs oxygen from the supply gas, so that the product gas is nitrogen enriched and the residual gas (although this may be put to an auxiliary use) is oxygen.
Other kinds of oxygen
itrogen generating systems are known, for example permeable membrane devices which permit a gas component in the supply gas, such as nitrogen, to permeate through the, typically polymeric, membrane, the oxygen or the nitrogen enriched gas being the product gas, and the nitrogen enriched or the oxygen enriched gas comprising residual gas respectively.
More recently it has been proposed to generate oxygen on-board an aircraft using a ceramic membrane oxygen generating device (COG). Such devices operate on the principle that certain ceramic materials, which are ionic conductors of oxygen, become electrically conductive at elevated temperatures due to the mobility of oxygen ions within the crystal lattice. Thus by passing an electrical current through a membrane of such ceramic materials, whilst a supply gas containing oxygen is supplied to one face of the membrane, oxygen in the supply gas diffuses through the membrane by ionic transport when the membrane is at a required elevated temperature, and may be recovered for use from the other face of the membrane.
A COG has advantages in that the product gas may comprise 100% oxygen, and the oxygen may be generated at pressure so that there is a lesser requirement to pressurise the product gas for use, as can be the case with a MSOGS for example.
It has been found that with known COG technologies, a COG operates more efficiently when the supply gas is richer in product gas. Thus for example, a COG will operate relatively inefficiently when used to separate oxygen at a concentration of about 21%, from supply gas comprising air, than where the supply gas has a greater concentration of oxygen than this.
MSOGS, permeable membrane oxygen generating devices and COGS have been put to use to generate oxygen on-board an aircraft and devices which operate according to such technologies will generically be referred to hereinafter as OBOG (on-board oxygen generating) devices. In order for the oxygen generated by such OBOG devices to be usable e.g. for breathing by an aircrew, the oxygen needs to be in a pressurised state. In OBOG devices in which oxygen gas cannot be produced at sufficient pressure, it is a requirement to provide some gas compression means.
It is also a requirement in an aircraft for an inert gas, such as nitrogen to be provided to the aircraft fuel tanks to fill voids in the fuel tanks both to maintain a desired pressure on the fuel and to replace fuel as the fuel is used, as well as to minimise the risk of fire/explosion in the fuel tanks. Conventionally such inert gas has comprised predominantly nitrogen with a concentration of oxygen of 9% or less. Such gas has been provided from storage tanks of compressed nitrogen in the aircraft although it is known to provide an on-board inert gas generator (OBIGG) device of the molecular sieve bed or permeable membrane type to generate such nitrogen from air.
In a high performance aircraft particularly, but not exclusively, great efforts are made to reduce weight to a minimum as well as of course to save space and ensure reliability whilst presenting a minimum maintenance burden. It will be appreciated that the provision of compression equipment and gas storage tanks is therefore undesirable.
In U.S. Pat. No. 4,681,602 there is proposed a system which utilises molecular sieve bed and/or permeable membrane technology, to produce first, oxygen for use for breathing by an aircrew, and second, nitrogen for use as an inert environment in the fuel tanks of an aircraft. Thus the requirement to provide storage tanks for compressed oxygen and/or nitrogen is avoided. However such system still requires the provision of compressors, and for both the oxygen, in order that the oxygen can be delivered at an appropriate pressure for breathing, and for the nitrogen. Also, the concentration of oxygen which can be produced is restricted by virtue of the nature of the conventional OBOG device technology which is used.
SUMMARY OF THE INVENTION
According to a first aspect of the invention we provide a gas generating system for generating a supply of oxygen or oxygen rich gas, and a residual gas, the system including a first gas separation device for separating from a supply gas, first gas being oxygen enriched gas, to leave residual gas, means to provide the first oxygen enriched gas from the first gas separation device to a second gas separation device for further separating from the first oxygen enriched gas, oxygen gas, the second gas separation device generating product gas which is at least highly oxygen enriched and further residual gas, at least one of the first and second gas separating devices including a ceramic membrane through which in use gas, ions diffuse.
Where the ceramic membrane device is an oxygen producing device, the present invention provides the advantage that at least highly oxygen enriched product gas, which may be 100% or substantially 100% oxygen, is produced, but whether the ceramic membrane device is an oxygen producing or inert gas producing device, less or no gas compression before use is required compared with for example, oxygen enriched product gas from more conventional e.g. MSOG device or permeable membrane technologies, because by the nature of a COG device, the product gas is pressurised by the electrical energy which causes the gaseous ions to diffuse through the ceramic membrane.
Thus improved quality product gas is provided, and the use of compressors to compress the product gas may be lessened or avoided altogether.
Typically the residual gas generated by the first and second gas separation devices is generally inert i.e. where the supply gas is air, the residual gas will comprise predominantly nitrogen. Means may be provided to feed residual gas from at least one of the first and second gas separation devices for use as an inert environment.
Preferably residual gas from the gas separation device having the ceramic membrane is fed for use as an inert atmosphere. Thus in the event that the other gas separation device is a MSOG device for example, residual gas from that gas separation device may simply be exhausted. Thus the efficiency of operation of the MSOG device is not compromised as can occur where the there is any resistance to the outflow of residual gas from the MSOG. Of course where both the gas separation devices are COG devices, residual gas from both gas separation devices may be put to use as an inert atmosphere.
Where the invention is applied to aircraft use the residual gas may be fed to provide an inert atmosphere in a fuel tank of the aircraft.
Where the first and second gas separation devices are of different kinds, preferably the second gas separation device is of the kind having a ceramic membrane. The first gas separation device may thus be a pressure swing molecular sieve bed type device and/or a permeable
Kilner John Anthony
Lane Jonathan Andrew
Phillips Robert John
Simons Adrian
Marshall Gerstein & Borun
Normalair-Garret (Holdings) Limited
Spitzer Robert H.
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