Patent Application
20120308462
The present invention relates to the treatment of gases, and in particular to a method of, and apparatus for, recovering an inert gas from a gas stream. In one embodiment, the invention relates to the recovery of an inert (noble) gas such as argon from a gas mixture.
There is a growing demand for renewable energy sources and in recent years silicon wafers have increasingly been manufactured for photovoltaic cells. Modules of photovoltaic cells can be electrically connected together to form photovoltaic arrays, so called solar panels, and are capable of generating electric power by converting energy from the sun into electricity. Solar panels arranged in multiples can provide sufficient power for a domestic house or office building. The demand for photovoltaic devices has advanced dramatically in recent years and simplifying the manufacturing process and reducing the cost of fabrication and processing are current key challenges.
One fabrication technique available uses a vacuum furnace along with an inert argon atmosphere enabling crystallisation and recrystallisation of silicon ingots and wafers to form the starting material for desired photovoltaic structures. In this type of vacuum furnace process over 100,000 litres of argon are typically required for a process cycle lasting 40 hours or more and producing a silicon ingot up to 1.8m long ready for onward processing. The estimated cost of annual usage of argon (at current prices) in a typical facility (for example a facility having 40 vacuum furnaces) can be more than £1 million
Other industries with heavy usage of noble gases such as helium, neon, argon, krypton and xenon include the following; vacuum based metallurgy (argon), lamp filling (argon, neon, xenon), semiconductor fabrication and the manufacture of plasma displays (neon, xenon). Xenon also has a number of medical uses including acting as a neural protector, as a clinical anaesthetic and as a contrast agent in MRI scanning.
In at least a vacuum furnace application the purity of the noble gas is important, the argon generated inert atmosphere and argon purge of the furnace should be oxygen free and of a very high purity around 6N i.e. around 1 ppm total contaminants and 99.9999% pure, to avoid reaction (oxidation) and damage to the silicon wafers and ingots being processed in the furnace.
Noble gases such as argon are present in atmospheric air in low concentrations and require considerable energy and financial cost to extract and purify. Therefore, it is very desirable to recover and re-use the noble gases from the effluent stream exhaust from systems and processes using noble gases such as those mentioned above. Re-using the noble gases from waste products also provides some independence and security of supply for the owner of the plant or facility using the noble gas.
The conventional systems used for the recovery of noble gases utilise a catalytic combustion reactor or cryogenic trapping to effectively remove contaminant species from the exhaust gas. In practice the costs associated with cryogenic separation and trapping preclude their use for all but the largest system wide installations and hence this means such systems are limited in their appeal. For local, point of use, recovery and recycle the current state of the art requires the addition of an excess of molecular oxygen prior to a catalytic combustion reactor operating at ˜600° C. to convert combustible impurities to carbon dioxide and water. The excess oxygen is then removed using a finely dispersed metal bed e.g. nickel. The metal bed used for oxygen removal has a limited capacity and will require frequent regeneration using pure hydrogen gas offline substantially adding to the complexity of the system. Such systems cannot efficiently remove the varying levels of combustible species, due to changes in process conditions and frequent ingress of oil as used in vacuum pumps, seen in the exhaust streams from vacuum furnaces.
It is an aim of at least the preferred embodiment of the present invention to provide a relatively simple and cost effective technique for recovering an inert gas, for example a noble gas such as argon, from a gas mixture which contains the inert gas and oxidisable impurities. It is desirable to provide an efficient and reliable inert gas recovery process.
According to a first aspect, the present invention provides a method of recovering an inert gas from a first gas stream comprising the inert gas and oxidisable impurities (especially organic impurities, and more especially hydrocarbon impurities) the method comprising the steps of supplying the first gas stream to an oxidation reactor containing a solid state oxygen carrier and oxidising the impurities in the first gas stream in the presence of the solid state oxygen carrier, to form a second gas stream containing carbon dioxide and water, supplying the second gas stream to a carbon dioxide removal column, and removing carbon dioxide from the second gas stream in the column to form a third gas stream, removing water from the third gas stream in an absorption column to produce purified inert gas, and collecting the purified inert gas from the absorption column for conveying to a process utilising the inert gas.
The oxidisable impurities in the first gas stream typically include hydrocarbons, such as traces derived from lubricant oils present in valve apparatus or the like. It is advantageous according to the invention that volatile hydrocarbons, and even methane, in the first stream can be oxidised to carbon dioxide and water, permitting removal of methane without resorting to cryogenic separation or the like.
Purification in this way is such that the components are separated and oxidation occurs in the presence of the solid state oxygen carrier, thus eliminating the need to inject oxygen into the gas stream for purification purposes.
As such the proposed method is advantageous over conventional systems which are uneconomic for recovery of all but the most expensive noble gases i.e. helium, krypton, xenon. In addition the relatively high levels of oxygen injection required in current systems, around 1000s ppm, for complete combustion of the contaminant species does not sit well with the requirement of an oxygen free recovered gas which is addressed with the method and apparatus of the present invention.
Further advantages include the possibility for the regeneration of the solid state oxygen carrier, preferably a metal oxide, during the processing.
The step in the oxidation column in the method according to the invention typically employs a Chemical Looping Combustion (CLC) process, in which the metal oxide is provided as a bed material for oxidation of the first stream, and then reoxidised before a further gas stream is supplied. In the CLC process in the method according to the invention an oxygen carrier, typically in the form of a metal oxide, is used instead of air to provide oxygen for combustion. The basic chemical looping combustion steps, for a divalent metal oxide,(MO) are as follows:—
(2x+0.5y)MO(s)+CxHy(g)→(2x+0.5y)M(s)+xCO2(g)+0.5yH2O(g) 1
(2x+0.5y)M(s)+(x+0.25y)O2(g)→(2x+0.5)MO(s) 2
Which lead to an equivalent overall combustion reaction of:—
CxHy+(x+0.25y/)O2→xCO2+0.5yH2O 3
The oxygen carrier MO typically comprises an oxide of a transition metal of Group VIIIb of the periodic table, such as Cu, Ni, Co or the like (or another transition metal such as Mn or Fe). Where the metal is other than divalent, M, the above equations are adjusted accordingly; as will be apparent to the person skilled in the art. The oxide (which is in the solid state) is typically of a transition metal, and typically on a support comprising, for example, silica and/or alumina or a silicate and/or aluminate.
The labels s and g in parentheses denote whether the component is in the gas (g) or solid (s) state. It is known to operate a CLC process with various compositions of metal oxide particles and with various arrangements of powders and composites etc. So far however high temperatures, up to and above 600° C. have been utilised.
In preferred embodiments the method according to the invention provides a chemical looping combustion process to convert combustible species e.g. CO, H2, hydrocarbons and vacuum pump oil etc., in the exhaust gas stream from the process to CO2 and water, followed by efficient removal of CO2 and H2O in regenerable reactor beds. The method is tolerant to the wide fluctuations in contaminant levels observed in vacuum furnace applications.
Oxidative conversion to CO2 and H2O followed by regeneration in this way is such that the components are separated and oxidation occurs by chemical looping combustion in the presence of the solid state oxygen carrier, thus eliminating the need to inject oxygen into the gas stream for purification purposes. The provision of the solid state oxygen carrier also removes the need to use gaseous hydrogen during regeneration of a subsequent oxygen removal reactor.
In one embodiment, the solid state oxygen carrier comprises at least one transition metal oxide. Preferably from periodic table classification groups VIIA, VIIIA, IB or IIB, more preferably an oxide of copper such as copper oxide.
Preferably, the transition metal oxide is combined with an inert support material comprising an oxide of an element chosen from periodic table classification Group IIIA, Group IVA, Group IIIB, Group IVB and the Lanthanide series.
In one embodiment the method includes the step of regenerating the solid state oxygen carrier following the step of oxidation wherein the step of regeneration is undertaken in the presence of a gas phase oxygen carrier mixture, preferably air. The step of regeneration is preferably undertaken in situ and comprises a CLC process.
The method according to the invention can enable recovery of an inert gas stream of about 6N purity (that is 99.9999% purity or 1 ppm total contaminants).
The method preferably further comprises an initial step of filtering the first gas stream and supplying the gas stream to the oxidation reactor. In an embodiment, the method includes a step of cleaning the exhausted inert gas stream using a hot metal getter. Preferably, the hot metal getter comprises a metal selected from titanium, zirconium and alloys thereof. Excess residual air gases can enter the system through air ingress into the furnace or vacuum lines. Residual air gases contain a significant portion of nitrogen which can build up detrimentally within the system. The hot metal getter can be arranged to remove the residual air gases, preferably nitrogen.
In an embodiment, the method may include conveying the purified gas to a collection device for recovered, purified gas, preferably with top up facility to account for and address gas losses. In an embodiment there may be a delivery line back to the process. A preferred method may comprise a monitoring step for monitoring the quality of a gas.
The quantity of solid state oxygen carrier is generally matched to the combustion output (the quantity and level of contamination in the first gas stream), such that the combustible (oxidisable) species is converted into carbon dioxide and water and the inert gas is recovered during a time period of around one process cycle of the process utilising the inert gas. Thus, regeneration of the CLC reactor occurs during the process dead time associated with unload and load of a vacuum furnace and results in an efficient process.
According to a second aspect, the present invention provides apparatus for recovering an inert gas from a first gas stream comprising the inert gas and oxidisable impurities, the apparatus comprising
In an embodiment the oxidisation reactor comprises a CLC reactor. In preferred embodiments the temperature of operation of the CLC reactor is in the range from 250° C. to 450° C., preferably about 400° C., when it is not needed to oxidise volatile hydrocarbons such as methane. The use of lower temperatures provides an efficient low cost system of conversion and purification. When however it is desired to oxidise volatile hydrocarbons such as methane, it is preferred to use higher temperatures, such as up to about 650° C.
In this arrangement the solid state oxygen carrier may comprise a transition metal oxide preferably chosen from classification Group VIIA, Group VIII, Group IB and Group IIB more preferably an oxide of copper.
In an embodiment, the apparatus comprises a filter for filtering the first gas stream. The carbon dioxide removal column and water absorption column may comprise a molecular sieve capable of removing one or more components such as carbon dioxide, water and nitrogen. More preferably, the carbon dioxide removal and water absorption column comprises a first molecular sieve and a second molecular sieve, the first molecular sieve having a higher capacity for removal of carbon dioxide than for nitrogen and the second molecular sieve having a higher capacity for removal of nitrogen.
The material of the molecular sieve may comprise zeolite or may comprise activated carbon or other chemicals designed to trap CO2 and water e.g. metal organic frameworks having a carbon or metal framework with a structure of a porous nature allowing decontamination to occur.
In an embodiment, the apparatus comprises a hot metal getter for cleaning the inert gas mixture, preferably the hot metal getter comprises titanium, zirconium and/or an alloy thereof. In addition there may be a monitoring device for monitoring the first gas stream and exhausted, purified inert gas. A monitoring stage can provide feedback on the recovery of the inert gas and other parameters of the process, adjustment and alteration of the process operation can then be made accordingly.
A single unit (CLC and recovery unit) may be used to recover an additional inert gas, for example, both argon and helium may be recovered from the process.
Further preferred features of the invention are defined in the accompanying claims.
The present invention will now be described in greater detail, by way of example only, with reference to the accompanying drawings, in which:
Referring to
The preferred embodiment of the present invention provides that the removal column 26 may be sized to match the CLC reactor 22. The removal column 26 contains a regenerable CO2 and H2O removal material. The material may be a looping chemical reaction material based on a metal oxide/metal carbonate couple typically from the metals Cu, Ni, Co, Mn or Fe. The material may be an absorber material such as a molecular sieve or zeolite. Preferably the removal column 26 comprises a molecular sieve. The preferred embodiment of the recycle system 1 comprises a water removal column 30. The removal column 30 comprises a material capable of preferentially absorbing moisture and ideally nitrogen from the system, as well as less preferably CO2, the material may be a molecular sieve or zeolite bed for example. It is an advantage of the current embodiment that trace amounts of nitrogen can be removed and absorbed from the system as nitrogen can enter the vacuum furnace and the recovery system along with the gas mixture. Entry of nitrogen can be through small air leaks in a valve or a vacuum pump or seal of the system for example. The removal column 30 includes gas exit point 33 and may include monitoring and sensing apparatus such as an FTIR (not shown) for detecting the purity and quality of the recycled and purified gas mixture. The recycle system 1 further comprises a back pressure regulator 77, and a pressure control valve 35. Pressure control valve 35 is provided in a linked arrangement to a back up gas supply, in this example the back up supply is argon supply 36. The back up argon supply 36 may be controlled in such a way such that gas is preferentially fed from the recycle system 1 unless a pressure drop is detected. If a pressure drop is detected it is possible to supply the gas, from the back up supply 36 instead. A gas delivery line 44 is located in communication with recycle system 1 and back up supply 36 for transferring gas to the vacuum furnace 100.
The recycle system 1 further comprises the following elements utilised in a regeneration mode (operation of which will be described in further detail below). Continuing to refer to
The recycle apparatus further comprises supply lines, compressors, valves etc associated with carbon dioxide removal column 26 and water removal column 30 as will now be explained in more detail
In operation, the gas to be recovered enters the recycle system 1, from the vacuum pump 10 of the exhaust 12 of a vacuum furnace at approximately atmospheric pressure through valve 80, and passes through a filter 14 to remove particles and gross levels of oil vapour. The filtered gas is compressed by compressor 18 to around a 1 to 5 bar gauge pressure, preferably around 2 bar. The gas, which is predominantly argon in the preferred embodiment, passes through line 20 to enter a CLC reactor 22. The CLC reactor 22 preferably operates at between 250° C. and 450° C., preferably about 400° C. The CLC reactor 22 is sized such it has capacity in excess of the equivalent of a process cycle and preferably a capacity of approximately 1.5 process cycles. The CLC reactor converts substantially all combustible material in the gas to CO2 and H2O and the gas then passes via line 24 into removal column 26 sized to match the CLC reactor 22 and equipped with a regeneratable carbon dioxide removal material. The gas is then passed via line 28 to water removal column 30 to absorb residual moisture and CO2, and nitrogen. The gas exiting the removal column 30 via line 32 is generally of the required purity i.e. ˜6N, namely 99.9999% purity and is maintained at approximately 2 bar through the action of the backpressure regulator 77. The recycled gas exits the system via valve 34 and pressure control valve 35 to a backup argon supply 36. The latter is preferably controlled in such a way as gas is preferentially fed from the recycle system 1 (that is from column 30) unless the pressure drops for some reason, then gas would be supplied from the back up supply 36. Gas is then transferred to a tool via lines 38, 44 to the vacuum furnace 100 for use as appropriate.
At the vacuum furnace 100 the process cycle ends with a cool down period of approximately 2 to 4 hours; after the cool down period and once the furnace is at a low enough temperature the vacuum pump 10 will be turned off along with the purge gas from line 44. At this point, and until the furnace is back up to working temperature, the recovery system enters a regeneration mode and finally a standby mode as described further below.
The compressor 18 is turned off and process exhaust gas line 12 is isolated at valve 80. Clean dry air, CDA 101, is admitted to the system 1 through valve 61 to regenerate the CLC reactor 22. The latter self heats because the regeneration reaction is highly exothermic—the temperature in the reactor 22 is in the range from 500° C. to 800° C., preferably around 600° C.; the actual temperature being set by the flow of gas into the reactor via flow restriction 63. The hot gases exiting from the CLC reactor 22 during the regeneration are vented to atmosphere via three way valve 65 which also isolates the CO2 and H2O absorber columns 26, 30. This will continue until sufficient air along air flow path 63, 61, 22 has been reacted to completely regenerate the material of the CLC reactor 22. At that time the CDA supply valve 61 will be switched off and argon from an argon purge valve 50 and flow restriction 53 will be switched on to purge the CLC reactor 22 of residual nitrogen.
The columns 26, 30 are regenerated through a combination of being heated electrically, purged with clean argon via flow restriction 66 and valve 67, and evacuated through vacuum pump 71 via valve 70. The operation of 3-way valves 28 and 33 isolate the columns 26, 30 from the rest of the system and as the removal columns 26 and 30 are heated, purged and evacuated the absorbed CO2 and H2O and any N2 is desorbed and vented and removed via the vacuum pump 71. The removal columns 26 and 30 will continue to be heated, purged and evacuated until essentially all the absorbed CO2, H2O and N2 is vented. At that stage in the process the heating supply is turned off and the columns 26, 30 allowed to cool to their operating temperatures. The cooling stage takes place under argon purge and vacuum pumping at which point valves 50, 67 and 70 are closed, three way valves 65, 28 and 33 switch and the compressor 18 is turned on. The system is in standby mode. On receiving the appropriate signal from the vacuum furnace valves 81 and 34 are opened and the recycle system 1 is ready to recover fresh gas.
Referring now to
In operation, the flow of a first gas, here argon, or a gas mixture, through column 26 and to the process tool along gas line connection 44 is achieved with the combination of opening valves 28, 72 and 37 such that the gas flow is diverted into the fluid flow path for the removal column 26. The switch over valve 35 is selected to allow the argon gas to flow to the process tool and vacuum furnace 100 along connection 44.
In this configuration argon from the purge supply 103 is routed to flow through by-pass valve 38 into column 30 and to exhaust through by-pass valve 73 and vacuum pump 71. Throughout this operation the absorber column 30 is electrically heated to accelerate the processing of the absorbed CO2, H2O, N2 etc to fully desorb and exit the system. At this point the electrical heater is turned off and column 30 cools down to its operating temperature after which point the column 30 regenerates and becomes available to remove CO2, H2O, N2 etc again. The inert gas can thus be routed through column 30 via by-pass valves 38 and 73 and via valve 37 and the appropriate switch over valve 35, 39 to delivery at the process tool via line connection 44. Column 26 is then regenerated as described above for column 30.
As discussed above, such a flow regime is advantageous in situations where the levels of contamination in a gas mixture are high and it is not therefore practical to size removal columns 26 and 30 to the CLC reactor and a single process cycle.
In an alternative configuration the recycling systems 1, described above with reference to
In operation, the by-pass valve 37 is switched on receipt of instructions or a cue such that fluid flow is diverted to flow through gas switching valve 39 and via connection 44 to the process tool and vacuum furnace 100. In a preferred embodiment the time delay between the process system selecting and switching to the second inert purge gas and the by-pass valve 37 switching is commensurate with the transit time of that gas through the recovery system 2.
After recovery and recycling of the second inert gas the second inert purge gas is switched off, valve 37 switches back to argon delivery and the recovery system enters regeneration mode as described above for recycling system 1.
In a third embodiment, not shown, “on the fly” regeneration of the CLC reactor 22 can be effected through the addition of by-pass valves and a second CLC reactor column.
Additionally, at the exit point 34 where the purified, inert gas can be returned to the vacuum furnace, an optional gas quality sensor (not shown) can be fitted. This could be, for example, a process Gas Chromatograph, or a process infrared spectrometer or other sensor known in the art.
In the embodiment described above vacuum regeneration of the or each molecular sieve is achieved by external electrical heating, for example with a direct heating element or using an indirect heat source and a fan. In an alternative embodiment the or each molecular sieve is heated by recovering the usually vented off-gas from the CLC reactor and redirecting it to the one or more molecular sieves. In the embodiment using a vacuum for the regeneration one or more additional valves may be provided to allow the sieve columns to be connected in parallel and to be pumped by a vacuum pump. For example, a separate vacuum pump could be used, or a combined compressor and vacuum pump. Alternatively the vacuum may utilise the process tool apparatus master vacuum pump during the period of apparatus tool load and unload, also known as the “process deadtime” period. In combination with the vacuum pump arrangement selected, a purge flow of clean argon may be used to aid regeneration of the molecular sieve and/or to control the pressure in the system to around 10 to 100 mbar. The exact pressure of a small purge flow is selected depending on the type of vacuum pump used.
In the alternative embodiment for purifying high levels of contamination the molecular sieve columns may need to undergo a regeneration step around every 10 hours. In this embodiment regeneration may be switched between columns part way through the regeneration cycle. Purifying exceptionally high levels of contamination can be achieved by utilising an additional non-regenerative CO2 removing column to “mop up” CO2 that gets through the molecular sieve(s). In this alternative scheme the non-regenerative column should be replaced every 6-12 months, being replaced, for example during a standard system service.
Alterations in the system parameters for example in an operating temperature and pressure range, may be required for different gas mixtures and flows that are processed and recovered. The system parameters may be altered for different ranges and types of contaminants and also if higher purification levels are required.
The following example results further illustrate the present invention but should not be construed as limiting its scope.
The mixture gas of the following examples contained both carbon monoxide and hydrogen in a mix to be processed by the argon recycling system.
Specifically, 5,082 ppm CO and 507 ppm H2 in an argon matrix was supplied to a CLC reactor and molecular sieve 13× absorber column in series with the ability to extract gas samples to a high sensitivity infrared spectrometer as shown on
During the experiment the gas was sampled from the process points A to C on
| Number | Date | Country | Kind |
|---|---|---|---|
| 1001508.9 | Feb 2010 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2011/050142 | 1/28/2011 | WO | 00 | 7/31/2012 |
