JPH0559770B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a method and apparatus for the purification of noble gases other than nitrogen or helium, and in particular to the purification of noble gases other than nitrogen or helium.

Certain methods of producing nitrogen on an industrial scale produce products containing at least trace amounts of oxygen gas. One of the most widely used methods for producing nitrogen for use in industry is to cool compressed air to a temperature below the liquefaction point and then pass the resulting liquid through a rectification column in which the liquid is A mass change is made between the descending liquid, which becomes richer in oxygen as it moves down the column, and the vapor, which becomes richer in nitrogen as it moves up the column. Single or dual columns are used to produce a substantially pure nitrogen product that is then liquefied for ease of storage and transportation.
Such nitrogen typically contains a few ppm of oxygen and is suitable for use in most industries. The requirement for nitrogen for industrial use is the use of adsorption beds to selectively adsorb oxygen from compressed air streams at high pressures, thereby collecting the non-adsorbed and nitrogen-rich gases. The adsorbent is then brought to a very low pressure to accept the incoming air for separation. Typically two adsorption beds are used,
One is then regenerated, while the other is used to adsorb oxygen from the compressed air stream. This adsorption method, often known as pressure swing adsorption (PSA), can produce nitrogen enrichments with oxygen concentrations typically as low as 0.1%, and the production is directly proportional to its oxygen concentration. Although nitrogen of such purity is generally acceptable in many industries, it is not acceptable in the electronic industry.

It is particularly unsuitable in the manufacture of semiconductors and integrated circuits.

There is a need for a plant to purify such nitrogen by excluding oxygen from the gas. This requirement is normally met by a catalytic reactor that causes the impurity oxygen to react with hydrogen to form water vapor.

Depending on the choice of catalyst (typically platinum), the plant can be operated at room temperature or substantially below room temperature. When used in the electronics industry, hydrogen, for example, is an unacceptable impurity, as is oxygen. There is a requirement to ensure that the purified nitrogen is free of hydrogen. In the past, by providing plants with relatively complex controls, the amount of oxygen in the incoming nitrogen was accurately measured and slightly more than the stoichiometric amount of hydrogen was passed through the plant, eliminating all the oxygen. Attempts were made to satisfy this requirement by ensuring that the gas reacted without leaving sufficient unreacted hydrogen in the gas stream. The resulting gas stream, consisting of water and water vapor, is then passed through a conventional dryer (using a typical zeolite molecular sieve) to remove the water vapor.

The control systems required to ensure the exact amount of hydrogen needed to react with virtually all the oxygen add considerable cost to the plant. Therefore, there is a need for alternative methods and apparatus for removing oxygen-containing impurities from noble gases other than nitrogen or helium containing such impurities.

In accordance with the present invention, a method is provided for removing oxygen-containing impurities from a compressed stream of nitrogen gas or a noble gas other than helium. The method involves passing the compressed gas stream through a catalytic reactor and reacting oxygen impurities with a stoichiometric excess of hydrogen in the reactor to react hydrogen, water vapor and a noble gas other than nitrogen or helium. and passing it through a membrane separator comprising at least one semi-permeable membrane whose permeability to hydrogen and water vapor is substantially greater than its permeability to nitrogen; The separator contains a portion of the noble gas other than nitrogen or helium,
Substantially all of the water vapor and substantially all of the hydrogen are also selectively passed through, leaving substantially pure nitrogen or noble gas products other than helium.

The present invention provides an apparatus for removing impurities from a compressed gas stream consisting of a noble gas other than nitrogen or helium contaminated with oxygen-containing impurities, comprising an inlet for the compressed gas stream, an inlet for hydrogen and an inlet for nitrogen or helium. a catalytic reactor comprising an outlet for a reacted gas stream consisting of a noble gas other than helium, hydrogen and water vapor, said outlet being connected to an inlet of a membrane separation means, said membrane separation means being permeable to water vapor and hydrogen; comprises at least one semi-permeable membrane having a relatively greater permeability to a noble gas other than nitrogen or helium, whereby a portion of the noble gas other than nitrogen or helium, substantially all of the water vapor and the hydrogen substantially all of the reacted gases can be separated from the reacted gases, leaving a substantially pure nitrogen product.

The method and apparatus according to the invention makes it possible to avoid using complex controls in connection with the catalytic reactor to ensure that the correct amount of hydrogen is added. The catalytic reactor may be of any conventional type unless otherwise specified.

The nitrogen source may be a plant for separating nitrogen from air by means of PSA.
If a PSA plant is used, substantially all of the water vapor and substantially all of the hydrogen is preferably mixed with the separation air. This gas stream is permeate (i.e. the gas passes through the membrane);
It is then produced at relatively low pressure and is preferably combined with air upstream of or in the compressor running the PSA plant. If desired, water vapor can be removed from the stream during passage from the membrane separation plant to the PSA plant. alternatively
The PSA plant may include molecular sieves or other means of selectively adsorbing water vapor.

The method and apparatus of the invention will be described with reference to the drawing, which is a schematic diagram illustrating a plant for purifying nitrogen according to the invention.

The disclosed plant comprises an apparatus 2 for separating N ₂ from air by PSA, reacting O ₂ impurities in the N ₂ produced by the apparatus 2 with a stoichiometric excess of H ₂ ; It comprises a catalytic reactor 4 for producing hydrogen and a membrane separator 6 for separating hydrogen and water vapor from the gas stream produced by the catalytic reactor 4. The device 2 is a compressor 10,
It consists of adsorbent beds 12 and 14 and _N2 product storage 18. The adsorbent bed consists of carbon molecular sieves 16 for selectively adsorbing O ₂ from incoming air. A pressure regulator 20 is provided downstream of the reservoir to prevent gas from flowing therefrom below a predetermined pressure. Valve 2 placed in it
Inlet manifold 22 with 4 and 26 is connected to floor 1
2 and 14 and the outlet of the compressor 10.

A similar manifold 28 with valves 30 and 32 disposed therein communicates tank or reservoir 18 with beds 12 and 14. Beds 12 and 14 are interconnected at either end (actually at the top and bottom) by pipelines 34 and 36 having respective valves 38 and 40 respectively disposed therein. floor 12
and 14 are provided with vent lines 42 and 44 having valves 46 and 48 respectively disposed therein.

In operation of the PSA device 2 to separate _N2 from air, all valves except 24, 30 and 48 are closed. The compressor 1
0 sends compressed air to the floor 12. Molecular sieves 16 in the bed selectively adsorb _O2 from the air and _N2- enriched gas is transferred from bed 12 to storage 18.
to go into. While bed 12 produces N2 _- enriched gas containing 99.5% by volume of _N2 , bed 14 containing adsorbed _O2 from the previous operating cycle is regenerated by exposing molecular seeds to the atmosphere. . Such exposure causes O ₂ to adsorb and is vented to the atmosphere through line 44. After the scheduled time, close valves 24, 30 and 48 and open valves 38 and 40;
Gas flows from beds 12 to 14 until the pressures in beds 12 and 14 are approximately equal. This step is only a short period of time during which the bed 12 is in communication with the compressor. Such pressure equalization steps introduce N ₂ into the bed 14 in preparation for separating _it from the incoming air. At the end of the pressure equalization step, bed 12 is switched to regeneration and bed 14 is used to separate _N2 from the air. Thus, at the end of the pressure equalization step, valves 38 and 40 are closed and valves 26, 32 and 46 are opened. The air passes through the floor 14 and
O ₂ is adsorbed from the air by molecular sieves 16 within bed 14 . The N ₂ enriched gas enters the reservoir 18 from 99.5% by volume of N ₂ . At that time, floor 12
It is regenerated by desorbing _O2 and releasing it to the atmosphere. After the scheduled time, floors 12 and 14
The pressure of is again equalized and then bed 12 communicates with compressor 10 and stores product in reservoir 18 while bed 14 is regenerated again. The reservoir 18 can supply N ₂ to the catalytic reactor for a relatively short period of time while the beds 12 and 14 are homogenized. Although there is no continuous supply of N ₂ to reservoir 18, there is a continuous supply of nitrogen from the reservoir.

The catalytic reactor 4 is a conventional reactor consisting of a catalyst bed through which nitrogen from the PSA unit passes.
The catalytic reactor 4 has such an inlet 50 for N ₂ and an inlet 52 for H ₂ . The catalyst is preferably platinum. A practically stoichiometric excess of H ₂ is supplied. The hydrogen reacts with oxygen impurities to form water vapor. A catalytically reacted gas stream consisting of N ₂ , a small amount of H ₂ and water vapor leaves the reactor through outlet 54 and enters membrane separator 6 . The membrane separator consists of a generally cylindrical closed housing 60 in which a group of tubes 62 are arranged. The lower end of the tube 62 communicates with an outlet 72 for gas permeation. The tube is connected in a fluid tight manner to a header 66 at the bottom of the housing 60 in which it resides. tube 62
Each consists of a capillary tube with an internal diameter of less than 1 mm, the wall of which tube acts as a semipermeable membrane. Typically, but not necessarily, the membrane is formed from polymethylpentene. The upper end of the tube is sealed. Header 70 receives purifying N ₂ during operation of the apparatus shown in FIG. in fact,
The catalyzed gas flow passes upwardly through the space in housing 60 not occupied by tube 72. The _H2 and water vapor diffuse through the walls of capillary tube 62 at a faster rate than the rate at which _N2 passes through the membrane. Non-permeate gas exits as product through pipeline 76. It becomes possible to remove substantially all H ₂ and water vapor from the catalyzed gas stream. More than one such separation unit can be used if necessary.

Typically the gas produced by the PSA separator 2 contains 1% by volume of _O2 . Typically, a 10% excess of the required stoichiometric amount of hydrogen to react with this O ₂ is fed to the catalytic reactor 4 . That is, the gas passing through the main separator 6 contains 2% by volume of water vapor and % by volume of _H2 , but no _O2 . The remainder of the gas stream is _N2 . Some of the N ₂ in the flow passes through the walls of tube 62 along with H ₂ and water vapor. A portion of the permeate gas is compressor 10
and the PSA device 2, with the remainder preferably being discharged to the atmosphere via line 74. This helps maximize N ₂ yield since H ₂ and water vapor fed to adsorbent beds 12 and 14 with such N ₂ will tend to pass therethrough with the N ₂ . That is, the H ₂ passing through the walls of the tube 72 is partially reused, reducing the operating costs of the catalytic reactor 4. It is also preferred that the membrane separator does not require a separate compressor and can rely on the compressor of the PSA device to provide high pressure on the tube side of the separator 6. Membrane separator 6
The permeate gas from the membrane separator 6 is partially recycled to the compressor 10 of the PSA unit 2.
It may be desirable to install a conventional dryer to remove water vapor from the circulating gas so as to reduce the amount of water vapor in the gas entering the system. Alternatively, the gas product outlet of the membrane separator may be provided with a dryer to remove traces of water vapor.

Various modifications of the embodiments shown in the accompanying drawings are possible. For example, instead of circulating the permeate gas to the compressor, it may be circulated directly to beds 12 and 14. The gas is passed through the bed for a period of time to undergo regeneration. In this method, the gas acts as a purge gas and helps displace _O2 from the bed during the regeneration process.

Although the invention was described for the purification of _N2 ,
It is not limited to that. For example, it is possible to produce argon by the PSA method. The method and apparatus of the present invention can be used to remove oxygen impurities from such argon.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the apparatus of the present invention.

Claims (1)

[Scope of Claims] 1. A method for removing impurities from a compressed stream of gaseous nitrogen or a noble gas other than helium containing impurities, including oxygen, by passing the compressed gas stream through a catalytic reactor; The oxygen impurity is reacted with a stoichiometric excess of hydrogen,
forming a reacted gas stream consisting of hydrogen, water vapor and a noble gas other than nitrogen or helium; passing the reacted gas stream through at least one semipermeable membrane having a permeability to hydrogen and water vapor greater than that to nitrogen; passing the reacted gas stream through a membrane separator comprising:
removing by permselectivity from the reacted gas stream a portion of noble gases other than nitrogen or helium, substantially all of the water vapor, and substantially all of the hydrogen;
A gas purification process that involves leaving a substantially pure nitrogen stream or a noble gas other than helium. 2. The method of claim 1, wherein the nitrogen source is an air separation plant by means of pressure swing adsorption. 3. The method of claim 2, wherein at least a portion of said gas mixture consisting of a portion of nitrogen, a substantially majority of its water vapor and a substantially majority of its hydrogen is mixed with separation air. 4. A process according to claim 3, wherein the mixture is carried out upstream of a compressor carrying out a pressure swing adsorption factory plant. 5. A method according to any of claims 2 to 4, wherein water vapor is removed from the gas mixture upstream of being mixed with separation air. 6. The method according to any one of claims 2 to 4, wherein the air separation plant by means of pressure swing adsorption comprises a molecular sieve that selectively adsorbs water vapor. 7. Apparatus for removing a compressed gas stream consisting of a noble gas other than nitrogen or helium contaminated with oxygen-containing impurities shall include an inlet for the compressed gas stream, an inlet for hydrogen and an inlet for a noble gas other than nitrogen or helium. a catalytic reactor comprising an outlet for a reacted gas stream consisting of a noble gas, hydrogen and water vapor, said outlet being connected to an inlet of a membrane separation means, said membrane separation means having a permeability to water vapor and hydrogen of or at least one semi-permeable membrane having a relatively greater permeability to a noble gas other than helium, whereby a portion of the nitrogen or noble gas other than helium, substantially all of the water vapor and the hydrogen An apparatus characterized in that substantially all of the reacted gases can be separated, leaving a substantially pure nitrogen product. 8. Apparatus according to claim 7, further comprising a plant for the production of nitrogen by air separation using pressure swing means, the production outlet of said plant being connected to the production inlet of said catalytic reactor. 9. Apparatus according to claim 8, wherein the membrane separation means has an outlet for permeate gas connected to the inlet of the plant. 10. The apparatus according to claim 8 or 9, wherein the plant includes a molecular sieve that selectively adsorbs water vapor. 11. Apparatus according to claim 8 or 9, comprising means for removing water vapor from the permeate gas.