GB2160439A

GB2160439A - Purification of gases

Info

Publication number: GB2160439A
Application number: GB08512561A
Authority: GB
Inventors: Michael Ernest Garrett
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1984-05-21
Filing date: 1985-05-17
Publication date: 1985-12-24
Also published as: JPS6157220A; GB8412932D0; GB2160439B; GB8512561D0; JPH0559770B2; ZA853718B

Abstract

Apparatus for the production of substantially pure nitrogen (or noble gas other than helium) comprises a plant for the separation of nitrogen from air by pressure swing adsorption, a catalytic reactor for removing oxygen impurity by reaction with hydrogen, and a membrane separator for separating hydrogen and water vapour from the nitrogen to form a substantially pure product nitrogen stream. A part of the permeate gas from the membrane separator is recycled to the compressor of the plant whereby it is mixed with incoming air for separation. The need for precise control of the amount of hydrogen added to the impure nitrogen entering the catalytic reactor is obviated as excess hydrogen is removed from the nitrogen in the membrane separator.

Description

SPECIFICATION Purification of Gases This invention relates to a method and apparatus for the purification of gases, and in particular for the purification of nitrogen or a noble gas other than helium.

Current methods for producing nitrogen on a commercial scale result in a product containing at least a trace of gaseous oxygen. The most widely used method of preparing nitrogen for use in industry is to cool compressed air to a temperature below its liquefaction point and then to pass the resulting fluid into the rectification column in which mass exchange takes place between a descending liquid which becomes progressively richer in oxygen as it descends the column and a vapour which becomes progressively richer in nitrogen as it ascends the column. Single or double columns may be used to produce a substantially pure nitrogen product which if desired can then be liquified so as to facilitate its storage and distribution.Such nitrogen typically includes only a few volumes per million of oxygen and is suitable for use in most industries having a requirement for nitrogen for industrial use is by using a bed of absorbent that at elevated pressure preferentially adsorbs oxygen from an incoming compressed air stream thereby allowing a nitrogen-rich gas to be collected as the unadsorbed gas. The adsorbent is then regenerated by subjecting it to a much lower pressure than that at which it receives the incoming air for separation.

Typically, two beds of adsorbent are employed and one is regenerated while the other is used to adsorb oxygen from an incoming compressed air stream.

This adsorptive method, often known as pressure swing adsorption, is generally able to provide a nitrogen-rich gas containing as little as 0.1% by volume oxygen, the volume produced being directly proportional to the oxygen concentration. Although, nitrogen of such purity is generally acceptable for many industrial uses it is not in general acceptable for use in the electronics industry, particularly in manufacturing semi-conductors and integrated circuits.

There is thus a need for a plant to purify such nitrogen by removing the oxygen therefrom. This need is conventionally met by catalytic plant for reacting the oxygen impurity with hydrogen to form water vapour. By appropriate selection of a catalyst (typically platinum) the plant can operate at ambient temperature or at least one not substantially above ambient. When used in the electronics industry, for example, hydrogen is as unacceptable an impurity as oxygen. Thus, there is a requirement to ensure that the purified nitrogen contains no hydrogen.

Attempts have been made in the past to meet this requirement by providing a plant with relatively sophisticated control means whereby the proportion of oxygen in the incoming nitrogen is accurately measured and only a trace more than the stoichiometric quantity of hydrogen is passed into the plant to ensure that all the oxygen is reacted without there being any significant unreacted hydrogen left over in the gas stream. The resulting gas stream comprising hydrogen and water vapour is# then passed through a conventional drier (typically employing a zeolite molecular sieve) in order to remove the water vapour.

In practice, the control system necessary for ensuring that only the precise quantity of hydrogen required to react with all the oxygen is provided adds considerably to the expense of the plant.

Accordingly, there is a need for alternative methods and apparatus for removing impurity comprising gaseous oxygen from a pressurised gas stream comprising nitrogen (or noble gas other than helium) including such impurity.

According to the present invention there is provided a method of removing an impurity comprising oxygen from a pressurised stream of gaseous nitrogen or noble gas other than helium containing such impurity, comprising the steps of passing the pressurised gas stream through a catalytic reactor and therein reacting its oxygen impurity with a stoichiometric exces of hydrogen so as to form a reacted gas stream comprising hydrogen, water vapour and the nitrogen or noble gas other than helium, and passing the reacted gas stream through a membrane separator including at least one semipermeable membrane whose permeability to hydrogen and water vapour is substantially greater than its permeability to nitrogen wherebythe membrane separator is able to separate from the reacted gas stream by selective permeation of a gas stream comprising some of the nitrogen or noble gas other than helium, substantially all the water vapour and substantially all the hydrogen so as to leave a substantially pure product nitrogen stream.

The invention also provides apparatus for removing an impurity comprising oxygen from a pressurised gas stream comprising nitrogen or a noble gas other than helium contaminated by the impurity, comprising a catalytic reactor having an inlet for the pressurised gas stream, an inlet for hydrogen and an outlet for a reacted as stream comprising nitrogen or noble gas other than helium, hydrogen and water vapour, said outlet communicating with the inlet of a membrane separation means including at least one semipermeable membrane whose permeability to water vapour and hydrogen is relatively greater than its permeability to nitrogen or the noble gas other than helium, whereby a gas mixture comprising some of the nitrogen or noble gas other than helium, substantially all the water vapour and substantially all the hydrogen can be separated from the reacted gas stream so as to leave a substantially pure nitrogen product.

The method and apparatus according to the present invention makes it possible to avoid using sophisticated controls in association with the catalytic reactor to ensure that a precise amount of hydrogen is added. The catalytic reactor may otherwise be of a conventional kind.

The source of nitrogen may be a plant for separating nitrogen from air by means of pressure swing adsorption. Where a pressure swing adsorption plant is employed, at least some of the said gas mixture comprising some of the nitrogen, substantially all the water vapour and substantially all the hydrogen is preferably mixed with the air for separation. Since this gas stream is produced at relatively low pressure being the permeate (i.e. the gas permeates through the membrane) it is preferably combined with the incoming air upstream of a compressor or the compressor that serves the pressure swing adsorption plant. If desired, water vapour may be removed from the stream during its passage from a membrane separation plant to the pressure swing adsorption plant.Alternatively, the pressure swing adsorption plant may incorporate molecular sieve or other means that selectively adsorbs water vapour.

A method and apparatus according to the present invention will now be described by way of example with reference to the accompanying drawing which is a schematic diagram illustrating a plant for purifying nitrogen in accordance with the invention.

Referring to the drawing, the illustrated plant comprises apparatus 2 for separating nitrogen from air by pressure swing adsorption (PSA), a catalytic reactor 4 for reacting a stoichiometric excess of hydrogen with the oxygen impurity in the nitrogen stream produced by apparatus 2, and a membrane separator 6 for separating hydrogen and water vapour from the gas stream produced by the catalytic reactor 4. The apparatus 2 comprises a compressor 10, adsorbent beds 12 and 14 both comprising carbon molecular sieve 16 that selectively adsorbs oxygen from the incoming air, and a product nitrogen reservoir 18. A pressure regulator 20 is provided downstream of the reservoir 18 to ensure that gas is not withdrawn therefrom below a pre-determined pressure.An inlet manifold 22 with valves 24 and 26 disposed therein connects the outlet of the compressor 10 to the beds 12 and 14.

A similar manifold 28 with valves 30 and 32 disposed therein connects the outlets beds 12 and 14to the tank or reservoir 18. Beds 12 and 14 are interconnected at either end (in practice top and bottom) by pipelines 34 and 36 having valves 38 and 40 respectively disposed therein. The beds 12 and 14 are provided with vent lines 42 and 44 having valves 46 and 48 respectively disposed therein. In operation of the pressure swing adsorption apparatus 2 to separate nitrogen from air, with all valves closed except 24,30 and 48, the compressor 10 delivers compressed air to the bed 12. The molecular sieve 16 in the bed selectively adsorbs oxygen therefrom and a nitrogen enriched gas passes out of the bed 12 into the reservoir 1 8.While the bed 12 is thus used to produced a nitrogenenriched gas typically including 99.5% by volume of nitrogen, the bed 14 which contains adsorbed oxygen from a previous operating cycle is being regenerated by virtue of the exposure of its molecular sieve 16 to ambient pressure, such exposure causing the oxygen to be desorbed and to be vented through the line 44 to the atmosphere.

After a chosen period of time, valves 24,30 and 48 are closed and the valves 38 and 40 open to allow gas to flow from the bed 12 into the bed 14 until the pressures therein have become substantially equalised. This process typically takes only a small proportion of the time during which the bed 12 is in communication with the compressor 10. Such a pressure equalisation step enables the bed 14 to be charged with nitrogen ready for its use to separate nitrogen from the incoming air. At the end of the pressure equalisation step, the bed 12 is switched to a regeneration mode and the bed 14 used to separate nitrogen from incoming air.

Accordingly, at the end of equalisation period, the valves 38 and 40 are closed and the valves 26,32 and 46 are open. Air thus passes into the bed 14 and oxygen is adsorbed therefrom by the molecular sieve 16 in the bed 14. Nitrogen-enriched gas typically comprising 99.5% by volume of nitrogen thus passes into the reservoir 18. Meantime, the bed 12 is regenerated by allowing oxygen to be desorbed therefrom and vented to the atmosphere.

After a chosen time, the pressure in the beds 12 and 14 is equalised again and then the bed 12 is placed in communication with the compressor 10 and the product reservoir 18, while the bed 14 is regenerated again. The reservoir 18 is able to supply nitrogen to the catalytic reactor 4 during those relatively short periods in which the pressure in the beds 12 and 14 is being equalised. Thus, although there is not a continuous supply of nitrogen to the reservoir 18, a continuous supply of nitrogen may be taken from it.

The catalytic reactor 4 is of a conventional kind comprising a bed of catalyst through which the nitrogen from the pressure swing adsorption apparatus is passed. The catalytic reactor 4 has an inlet 50 for such nitrogen and an inlet 52 for hydrogen. The catalyst is preferably platinum. In operation a stoichiometric excess of hydrogen is supplied. It reacts with the oxygen impurity in the hydrogen to form water vapour. Thus, the catalytically reacted gas stream comprising nitrogen and small quantities of hydrogen and water vapour passes out of the reactor 4 through outlet 54 and flows into the membrane separator 6. The membrane separator 6 comprises a generally cylindrical closed housing 60 in which is disposed vertically a bundle of tubes 62.The lower ends of the tube 62 are connected in a fluid tight manner to a header 66 at the bottom of the housing 60 which places the tubes in communication with an outlet 72 for permeate gas. The tubes 62 each comprise a capilliary having an internal diameter of less than 1 mm whose wall functions as a semi-permeable membrane. Typically, but not necessarily, the membrane is formed of polymethylpentene. The upper ends of the tubes 62 are sealed. A header 70 is provided through which nitrogen for purification flows in operation of the apparatus shown in Figure 1. In operation, as the catalytically reacted gas stream passes upwardly through the spaces in the housing 70 not occupied by the tubes 72. Its hydrogen and water vapour content diffuses through the walls of the capilliarytubes 62 at a rate substantially faster than that at which the nitrogen diffuses through the membrane. The nonpermeating gas exits as product through the pipeline 76. It it thus possible to remove substantially all the hydrogen and water vapour from the catalytically reacted gas stream. If necessary, more than one such membrane separation unit 6 may be employed.

Typically, the gas produced by the PSA separation apparatus 2 may include 1% by volume of oxygen.

Typically, in the order of 10% in excess of the stoichiometric quantity of hydrogen required to react with this oxygen is supplied to the catalytic reactor 4. Thus, the gas passing into the main separator 6 will include 2% by volume of water vapour and 0.2% by volume of hydrogen but no oxygen, the remainder of the gas stream being nitrogen. Some of the nitrogen in the stream will inevitably pass through or permeate the walls of the tubes 62 with the hydrogen and water vapour. It is preferred that a part of the permeate gas be recycled to the compressor 10 the PSA apparatus 2, the remainder being vented via line 74 to the atmosphere. This helps to keep to a maximum the yield of nitrogen from the apparatus while the hydrogen and water vapour supplied with such nitrogen to the adsorbent beds 12 and 14 will tend to pass therethrough with the nitrogen.Thus, the nitrogen permeating the walls of the tubes 72 is able to part to be re-used thus keeping down the running costs of the catalytic reactor 4. It is also to be appreciated that the membrane separator 6 does not need an independent compressor but can rely on the compressor 10 of the PSA apparatus 2 in order to supply the necessary high pressure on the tube side of the separator 6. In the event that the permeate gas from the membrane separator 6 is in part re-cycled to the compressor 10 of the pressure swing adsorption apparatus 2, it may be desirable to include a conventional dryer to remove water vapour from the recycled gas so as to keep down the amount of water vapour in the gas entering the membrane separator 6. Alternatively, the product gas outlet of the membrane separator 6 may be provided with a dryer so as to remove traces of water vapour.

Various modifications and alterations to the plant shown in the accompanying drawing are possible.

For example, instead of recycling permeate gas to the compressor, it may be recycled to the beds 12 and 14 directly, the gas being passed to the bed for the time being undergoing regeneration. In this way the gas functions as a purge gas which helps to displace oxygen from the bed during the regeneration step.

The invention is not restricted to the purification of nitrogen. It is for example possible to produce argon by a pressure swing adsorption route, and the method and apparatus according to the invention may be used to remove oxygen impurity from such argon.

Claims

1. A method of removing an impurity comprising oxygen from a pressurised stream of gaseous nitrogen or noble gas other than helium containing such impurity, comprising the steps of passing the pressurised gas stream through a catalytic reactor and therein reacting its oxygen impurity with a stoichiometric excess of hydrogen so as to form a reacted gas stream comprising hydrogen, water vapour and the nitrogen or noble gas other than helium, and passing the reacted gas stream through a membrane separator including at least one semipermeable membrane whose permeability to hydrogen and water vapour is substantially greater than its permeability to nitrogen whereby the membrane separator is able to separate from the reacted gas stream by selective permeation a gas stream comprising some of the nitrogen or noble gas other than helium, substantially all the water vapour and substantially all the hydrogen so as to leave a substantially pure product nitrogen stream.

2. A method as claimed in claim 1, in which the source of nitrogen is a plant for the separation of air by means of pressure swing adsorption.

3. A method as claimed in claim 2, in which at least some of the said gas mixture comprising some of the nitrogen, substantially all the water vapour and substantially all the hydrogen is mixed with the air for separation.

4. A method as claimed in claim 3, in which the mixing takes place upstream of a compressor (in the compressor) that serves the pressure swing adsorption plant.

5. A method as claimed in any one of claims 2 to 4, in which water vapour is removed from the said gas mixture upstream of where it is mixed with air for separation.

6. A method as claimed in any of claims 2 to 4, in which the plant for the separation of air by means of pressure swing adsorption includes molecular sieve that selectively adsorbs water vapour.

7. A method of removing an impurity comprising oxygen from a pressurised stream of gaseous nitrogen or noble gas other than helium, substantially as herein described with reference to the accompanying drawing.

8. Nitrogen or noble gas (other than helium), when produced by a method as claimed in any one of the preceding claims.

9. Apparatus for removing an impurity comprising oxygen from a pressurised gas stream comprising nitrogen or a noble gas other than helium contaminated by the impurity, comprising a catalytic reactor having an inlet for the pressurised gas stream, an inlet for hydrogen and an outlet for a reacted gas stream comprising nitrogen or noble gas other than helium, hydrogen and water vapour, said outlet communicating with the inlet of a membrane separation means including at least one semi-permeable membrane whose permeability to water vapour and hydrogen is relatively greater than its permeability to nitrogen or the noble gas other than helium, whereby a gas mixture comprising some of the nitrogen or noble gas other than helium, substantially all the water vapour and substantially all the hydrogen can be separated from the reacted gas stream so as to leave a substantially pure nitrogen product.

10. Apparatus as claimed in claim 9, additionally including a plant for the production of nitrogen by the separation of air by means of pressure swing adsorption, the product outlet of said plant communicating with the inlet of the catalytic reactor.

11. Apparatus as claimed in claim 10, wherein the membrane separation means has an outlet for permeate gas communicating with the inlet of the said plant.

12. Apparatus as claimed in claim 10 or claim 11, in which said plant includes molecular sieve that selectively adsorbs water vapour.

13. Apparatus as claimed in claim 10 or claim 11, additionally including means for removing water vapour from the permeate gas.

14. Apparatus for removing an impurity comprising oxygen from a pressurised gas stream of gas nitrogen or noble gas other than helium substantially as herein described with reference to the accompanying drawing.