Patent Application
20090321271
The present invention relates to a method of, and apparatus for, pumping a gas stream containing a combustible gas, such as hydrogen or a hydrocarbon.
Many processes use or generate potentially flammable mixtures containing fuels such as hydrogen and hydrocarbons. For example, epitaxial deposition processes are increasingly used for high-speed semiconductor devices, both for silicon and compound semiconductor applications. Epitaxial deposition utilizes a silicon source gas, typically silane or one of the chlorosilane compounds, such as trichlorosilane or dichlorosilane, in a hydrogen atmosphere at high temperature, typically around 800-1100° C., and under a vacuum condition. In such deposition processes, the residence time of the deposition gases in the processing chamber is relatively short, and only a small proportion of the gas supplied to the chamber is consumed during the deposition process. Consequently, the majority of the deposition gases supplied to the chamber are exhausted from the chamber together with by-products from the deposition process.
Whilst the exhaust gas itself is not flammable, the pumping of such mixtures requires great care to be placed on the leak integrity of the foreline and exhaust lines from the pump to ensure that there is no ingress of air into the lines. Once the gas mixture is above its lower explosive limit (LEL), any ignition sources within the pump could result in the generation of hazardous flame fronts travelling through the pump into the exhaust lines.
A common technique used to avoid ignition of a flammable gas stream is introduce into the gas stream an excess of an inert purge gas, typically nitrogen, to bring the gas stream below its LEL, and then to subsequently convey the gas stream to an abatement device, typically a burner, to controllably convert the combustible gases into other species prior to the emission of the gas stream into the atmosphere. However, supplying purge gas into the gas stream can reduce the efficiency of the abatement device and is both wasteful and costly. Furthermore, the use of a burner to destroy the combustible gases may lead to the release into the atmosphere of other, undesirable chemicals as a by-product of the destruction of the combustible gas.
In a first aspect, the present invention provides a method of pumping a gas stream containing a combustible gas, comprising the steps of conveying the gas stream at a sub-atmospheric pressure to one side of an ionic conducting membrane, exposing the other side of the membrane to an oxidising gas, applying a potential difference across the membrane so that reactive oxidising species permeate across the membrane to react with the combustible gas to produce at least water vapour, and subsequently receiving the gas at a vacuum pump, wherein the water vapour is condensed from the gas stream upstream of or within the vacuum pump.
As the total pressure of a potentially explosive gas mixture is reduced, the catastrophic nature of a gas phase chemical reaction involving a combustible gas and an oxidant also reduces. By conveying reactive oxidising species such as oxygen anions or radicals to a combustible gas at a sub-atmospheric pressure, preferably less than 50 mbar, more preferably less than 10 mbar, the amount of inert purge gas that need to be added to the gas stream to reduce the gas stream below its lower explosive limit can be reduced, preferably to zero so that complex purge gas systems, and the cost associated therewith, can be avoided.
An advantage associated with the use of an ionic conducting membrane such as an yttrium stabilised zirconium or a gadolinium doped ceria oxygen anion conductor is that a potential difference needs to be applied across the membrane in order to actuate the permeation of oxidising species through the membrane, otherwise the membrane is impervious to the oxidising species. Thus, in the event of a power failure, transference of oxygen anions to the gas stream is inhibited, thereby avoiding the generation of a potentially explosive atmosphere within the membrane. By controlling the magnitude of the potential difference applied across the membrane, the rate of permeation of the oxidising species can be controlled so that the rate at which the oxidising species enter the gas stream is sufficient to react with substantially all of the combustible gas within the gas stream. The absolute magnitude of the current used to drive the oxygen anions across the membrane will generally depend upon the surface area of electrodes located on the sides of the membrane, the partial pressure of the oxidising gas and the amount of combustible gas within the gas stream. The potential difference applied across the membrane can be adjusted in dependence on the output of a sensor used to measure the amount of oxygen present within the gas stream downstream from the membrane. For example, if the sensor detects no oxygen, this can provide an indication that there is insufficient permeation of oxygen anions across the membrane to react with all of the combustible gas, and so the potential difference or current density may be increased gradually until the sensor detects that oxygen is present in the gas stream.
Furthermore, since at least one, or in the case of hydrogen all, of the products of the reaction between the combustible gas and the oxidising species are is condensable, the size of the vacuum pump required to pump the gas stream can be reduced, as for example each slm of hydrogen contained within the gas stream generates an additional slm of pumping speed when the condensable species such as water vapour are condensed from the gas stream.
The water vapour may be condensed from the gas stream within a condenser located between the membrane and the vacuum pump. Alternatively, the vacuum pump may have a pumping mechanism that exposes the gas stream to water and wherein the water vapour is condensed from the gas stream. Examples of a vacuum pump having such a pumping mechanism include a liquid ring pump and a liquid ejector pump.
As mentioned above, the potential difference can be applied across the membrane using a first electrode on said one side of the membrane and a second electrode on said other side of the membrane, wherein at least one of the electrodes comprises catalytic material, such as platinum, to improve the reaction kinetics.
Means may be provided for controlling the temperature of the membrane. Depending on the nature of the membrane, the membrane may need to be heated to an elevated temperature to have the required degree of conductivity for the oxidising species. At temperatures below a critical temperature (Tc) the electrolyte material may be non-conducting, but at temperatures above Tc the electrolyte may become progressively more conductive. A heater may be conveniently provided about the membrane to heat the membrane to the required temperature.
In the preferred embodiment, the membrane is in the form of a cylinder having a bore through which the gas stream is drawn by the vacuum pump. In order to increase the efficiency of the destruction, a plurality of said cylindrical membranes may be provided in parallel. Alternatively, the membrane may have a plate-like structure.
The oxidising gas can be derived from a gaseous source of oxygen. Typically atmospheric air is used as a gaseous source of oxygen, although other gas compositions can be used.
In a second aspect the present invention provides a system for pumping a gas stream containing a combustible gas, the system comprising an ionic conducting membrane, a vacuum pump for drawing the gas stream at a sub-atmospheric pressure to one side of the membrane, the other side of the membrane being exposed to an oxidising gas, means for applying a potential difference across the membrane so that reactive oxidising species permeate across the membrane to react with the combustible gas to produce at least water vapour, the vacuum pump being arranged to receive the gas stream from the membrane and having a pumping mechanism that exposes the gas stream to water and wherein the water vapour is condensed from the gas stream.
In a third aspect the present invention provides a system for pumping a gas stream containing a combustible gas, the system comprising an ionic conducting membrane, a vacuum pump for drawing the gas stream at a sub-atmospheric pressure to one side of the membrane, the other side of the membrane being exposed to an oxidising gas, means for applying a potential difference across the membrane so that reactive oxidising species permeate across the membrane to react with the combustible gas to produce at least water vapour, the vacuum pump being arranged to receive the gas stream from the membrane and having a pumping mechanism that exposes the gas stream to water and wherein the water vapour is condensed from the gas stream.
Features described above in relation to method aspects of the invention are equally applicable to system aspects of the invention, and vice versa.
Preferred features of the present invention will now be described with reference to the accompanying drawings, in which:
With reference to
Examples of the electrochemical cell 12 and the condenser 14 are illustrated in more detail in
A first electrode 24 is formed on the inner surface of the membrane 20 for exposure to the gas stream, and a second electrode 26 is formed on the outer surface of the membrane 20 for exposure to an oxidising gas, for example air. The electrodes 24, 26 may be deposited using a technique such as vacuum sputtering or applying any suitable commercially available “ink” to the surface. In the event that one of the electrodes is formed on the surface of the membrane 20 using ink, the whole assembly must be fired in a suitable atmosphere determined by the nature of the ink. The first electrode 24 is preferably formed from a material that is able to catalyse the oxidation of a hydrocarbon to carbon dioxide and water, and/or the oxidation of hydrogen to water. One suitable example is platinum. The second electrode 26 is preferably formed from a material that is above to catalyse the dissociative adsorption of oxygen or other reactive oxidising species. Again, one suitable example is platinum. Where a gas other than air is to be used as the source of the oxidising gas, the cell 12 may be surrounded by a housing having an inlet for receiving a stream of the oxidising gas and an outlet for expelling this stream from the housing.
Depending on the nature of the material used to form the membrane 20, the membrane may require heating to raise its temperature above a critical temperature Tc at which the membrane 20 is able to conduct the reactive oxidising species, for example oxygen anions. In view of this, the cell 12 may comprise a heater 28 extending about the membrane 20 for heating the membrane 20 to the required temperature, which, depending again on the material used to form the membranes 26 may be at least 300° C. A heater controller 30 may be provided for controlling the heater 28, for example in response to temperature signals received from a thermocouple located proximate the membrane 20.
A low voltage power supply 32 is provided to apply a potential difference between the first and second electrodes 24, 26 and thus across the membrane 20. A voltammeter 34 is also provided to measure the potential difference between the electrodes 24, 26. Where a housing surrounds the membrane 20, a gas tight electrical feed through may permit electrical connections to the constant power supply 32 and the voltammeter 34 to pass to the electrodes 24, 26.
In use, the outer surface of the membrane 20 is exposed to an oxidising gas, conveniently air, and a potential difference is applied across the membrane 20 using the power supply 32. At the second electrode 26, oxygen within the air is reduced to form oxygen anions:
O2+2Vo+4e−⇄2Oo
where Vo is a doubly charged oxygen anion vacancy and Oo is a filled oxygen anion site in the membrane 20. When the temperature of the membrane 20 is above Tc, oxygen anions will permeate across the membrane 20 at a rate dependent, amongst other things, on the surface area of the electrodes 24, 26, the partial pressure of oxygen at the outer surface of the membrane 20 and the potential difference applied across the membrane 20.
At the first electrode 24, the oxygen anions react with a combustible gas contained within the gas stream conveyed through the bore 22 of the membrane and thus to the inner surface of the membrane. For example, for a combustible gas having the general formula CxHy, where x≧0 and y≧2, the reaction proceeds according to the general equation:
CxHy+(2x+y/2)O2⇄xCO2+y/2H2O+(4x+y)e−
to produce water vapour and, when x>1, carbon dioxide.
As illustrated in
For relatively high flow rates of hydrogen (several slm) or hydrocarbon (above 100 slm) within the gas stream, the number of membranes 20 may be increased to increase the surface area of membrane exposed both to the combustible gas and the oxidising gas. For example, as illustrated in
Returning to
A second embodiment of a system for pumping a gas stream containing a combustible gas is illustrated in
The gas stream enters the pump 60 through gas inlet 104, and is pulled into the spaces 106 between adjacent blades 98. The water vapour contained in the gas stream condenses within the annular ring 102 formed within the pump 60. The pump 60 is provided with an exhaust 108 on the outlet side thereof for exhausting from the pump 60 a liquid/gas mixture of a liquid solution comprising the liquid from the annular ring 102, any liquid-soluble components of the gas stream, and any gaseous species remaining from the gas stream. As water is conveyed from the exhaust 108, the annular ring 102 is replenished by supplying fresh liquid to the housing 92 via inlet 100. The liquid/gas mixture stream exhaust from the pump 60 may be subsequently separated in a discharge separator (not shown) located downstream from the exhaust 108 of the pump 60. The gas may be exhaust to the atmosphere, and the liquid collected for safe disposal. Alternatively, the liquid may be treated for return to the source for re-use.
This second embodiment of the pumping system therefore does not require a separate condenser for condensing the water vapour from the gas stream upstream from the vacuum pump, which can reduce system costs.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0520472.2 | Oct 2005 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2006/003076 | 8/17/2006 | WO | 00 | 8/26/2009 |
