PROCESS FOR THE REMOVAL OF GASEOUS IMPURITIES FROM A STREAM OF HYDROGEN

Abstract: A process for the removal of impurities from a stream of hydrogen, containing more easily removable impurities, like for instance COx, and more hardly removable impurities, mainly consisting of nitrogen and methane, wherein: A) said stream is first brought into contact, at 5-50 DEG C, with a material containing nickel and/or Ni compounds, wherein at least 1 % b.w. of the overall amount of Ni is present in the reduced form; B) the stream from (A), still containing said more hardly removable N2 and CH4, is brought into contact with non-evaporable getter material at a higher temperature.

Description: PROCESS FOR THE REMOVAL OF GASEOUS IMPURITIES FROM A STREAM OF HYDROGEN The invention relates to an improved process for the removal of gaseous impurities from a stream of hydrogen, without residual traces of methane and without causing any formation of new methane, particularly suitable for the long lasting manufacture of purified hydrogen containing less than 50 and preferably 20 ppb of methane, q namely 20 parts per 10 parts (by volume). The semiconductor industry is developing integrated circuits with ever more increasing line densities and it is thus required that the materials used in the manufacturing process be of ever increasing purity. As hydrogen is one of the gases used in these processes it is therefore necessary to ensure that its impurity content be kept as low as possible. The main gaseous impurities in commercial hydrogen are moisture (water vapour), oxygen, carbon monoxide (CO), carbon dioxide (CO * ) and mixtures thereof (CO v ), as well as nitrogen and methane, these last (N, and especially CH^) being very hardly removable. One method for the purification of hydrogen, since a long time, was the selective diffusion of hydrogen through palladium or palladium alloys. The rate of diffusion, however, increased with the pressure drop between the opposite sides of a Pd barrier; moreover the operating temperature, for an economical throughput of purified hydrogen on Pd, was very high. In addition, as the hydrogen impurities are blocked by the Pd barrier, a removal device or expedient has to be provided for; US Patent N° 3,368,329 describes one of such apparatuses and another form of hydrogen purification by means of diffusion membranes is described in US Patent N° 3, ' 534, 531. Such diffusion barriers however, while very efficient, show several drawbacks. Should the barrier be sufficiently thin as to ensure a high throughput of purified hydrogen, it would suffer from mechanical failure, with consequent undesirable leakage of impure hydrogen into the purified gas. The drawback is even more heavy because of the high pressure drop between the two sides of the barrier. If the barrier thickness gets increased, as to avoid mechanical failure, then excessively high temperatures have to be used for ensuring a high throughput of purified gas. The use of high temperatures in the presence of hydrogen is very dangerous, due to the potential existence of explosive hydrogen-oxygen (or air) mixtures, whenever excessively high temperatures are encountered, and an increased barrier thickness would involve the use of greater amounts of expensive palladium. It is therefore one object of the present invention to provide an improved process for the purification of hydrogen free from one or more of the drawbacks of the prior art. It is another object of the present invention to provide an improved process for the purification of hydrogen without requiring diffusion through palladium or Pd alloys. It is still another object of the present invention to provide an improved process for the purification of hydrogen free from high pressure drops. It is a further object of the present invention to provide an improved process for a long lasting manufacture of highly purified hydrogen free from any presence of methane and without any formation of new methane. These and further advantages of the present invention will become clear to one skilled in the art by reference to the following disclosure and drawings. In its broadest aspect the invention relates to an improved process for the removal of gaseous impurities from a stream of hydrogen, containing a first class of more easily removable impurities, like for instance CO , and a second class of more hardly removable impurities, mainly consisting of nitrogen and methane, essentially consisting of the following steps: A. Said stream is first brought into contact, at 5- 50°C, with one or more beds of a particulate material containing nickel and/or nickel compounds and optionally also a carrier, wherein at least 1% b.w. (preferably 5%) of the overall amount of nickel is present in the reduced (elemental) form, until said more easily removable impurities are removed in an essentially complete way; B. The stream coming from step A, essentially free from said more easily removable impurities, but still containing said more hardly removable nitrogen and methane, is brought into contact with one or more beds of a non-evaporable getter material at a higher temperature . Said hydrogen stream has suitably a pressure from 1 to 20 bar and the temperature of step (B) is suitably from 400 to 600°C and preferably from 500°C to 600°C. The (mass) space velocity of the hydrogen stream is generally from 0.5 to 50 normal cm /minute per gram of getter material and the amount of undesired methane is generally up to 5 ppm (5000 ppb) . The elemental nickel and the nickel compounds (e.g. oxide) used in first step (A) of the two-step process according to the invention are suitably supported on a carrier preferably consisting of a silicalite, a titanium- silicalite, a xerogel (see EP-A-537851) or a silica having an effective surface area equal to or higher than 100 m 2 /g (preferably 100-200 m 2 /g), as described in U.S. patent 4,713,224, and the nickel containing bed may be followed or (preferably) preceded by a second sorbing bed, essentially consisting of a natural or synthetic molecular sieve, like for instance natural or synthetic zeolites, silicalites or titanium- silicalites. A getter material for the process according to the invention is suitably a getter alloy selected from: a) the Ti-Ni or Zr-Ni alloys and preferably the alloys containing from 50 to 80% by weight titanium or zirconium (balance = nickel), wherein up to 50% b.w. of the nickel may be replaced by iron and/or manganese and or technetium and/or rhenium. b) the high manganese Ti-V alloys (hereinafter HM alloys), described by U.S. patent 4,457,891; c) the low manganese Ti-V alloys (hereinafter LM alloys ) . The composition of said HM alloys is as follows (% by weight ) : - titanium: 25 - 30.9% - vanadium: 10 - 42%- manganese: 27.1 - 65.1%; wherein from 2 to 2.2 other atoms are present per titanium atom. Preferably, in said HM alloys, up to about 40% of the vanadium atoms may be replaced by iron atoms and up to 10% of the vanadium atoms may be replaced by aluminum atoms, wherein however no more than 40% of the vanadium atoms are replaced by the overall amount of said iron and aluminum atoms. Moreover, in said HM alloys, the following optional modifications of the composition may also occur: i) up to about 20% of the titanium atoms may be replaced by Ca, Y, La, misch metall or mixtures thereof; ii) up to 0.2 atoms of Cr (per titanium atom) may replace a corresponding number of manganese and/or vanadium atoms. iii) up to 0.1 Ni atoms and/or 0.05 Cu atoms (per titanium atom) may be present in the alloy, wherein preferably no more than about 0.1 atom of nickel and copper replace a corresponding number of manganese and/or vanadium atoms. The composition of said LM (low-manganese) alloys is as follows (% by weight): - titanium : 25 - 65% - vanadium: 10 - 52% wherein up to 40% b.w. of vanadium may be replaced by iron and wherein up to 20% b.w. of titanium may be replaced by manganese. The getter material may be used in the form of a loose powder showing an average particle size between 1 and 500 micrometer, preferably between 1 and 250 micrometer and even better between 1 and 128 micrometer, but said powder may optionally be shaped, before use, in the form of formed bodies (pellets, tablets, rings, saddles and so on). The shaping may be carried out by compression or by sintering; in its turn said sintering may be carried out by simple heating or by exploiting both a heating and the presence of a second powder, as described e.g. in GB Patent Publication N° 2,077,487, in order to reach a satisfactory level of porosity. T h e average size of said bodies is generally equal to a few millimeter (0.5-5 mm). The surface of the device containing said impurity adsorbing alloys, in contact with said strea * ι. of hydrogen, has to be very thoroughly polished in a uniform smooth way, in order to minimize contamination. The desired degree of smoothness of said surface may be expressed in terms of roughness of the inner wall surface, to be brought into contact with hydrogen; said roughness in its turn, according to preferred embodiment of the invention, has to be equal to or lower than 0.50 and preferably 0.25 micrometer, in terms of centerline average height ( R ) . Although such values are not critical, they are recommended as a reliable safe condition. Referring now to the drawing of Fig. 1, there is shown a purifier 100 for the removal of impurities from an impurity-containing hydrogen stream, having a gas inlet 102 in fluid communication, through pipings 104, 104' , with preliminar purification chambers 106, 106' . Valves 108, 108' can be alternately opened or closed, as to allow the passage of the impurity-containing gas through the first or the second of the preliminar purification chambers 106, 106' , containing a bed of particulate material 110, based on supported nickel, whereby said bed removes, at a relatively low temperature, the more easily removable impurities (CO, and so on). Chambers 106, 106' may in addition contain a natural or synthetic molecular sieve 111 to better promote the removal of carbon dioxide, or alternatively a separate molecular sieve may be provided for. Said chambers 106, 106' can also remove moisture down to trace levels, but do not remove nitrogen and methane. It is thus possible to obtain a partially purified hydrogen containing only the second class of impurities, essentially consisting of nitrogen and methane. The partially purified gas leaves therefore preliminar purification chambers 106, 106' entering a final purification chamber 112, kept at a much higher temperature, with which chambers 106, 106' are in fluid communication, by means of pipings 114, 114' . Valves 116, 116' control the flow of the partially purified gas from either of the first preliminar purification chambers 106, 106' , which allow the regeneration of nickel in one chamber whilst, the other is working. In said final purification chamber 112 the partially purified hydrogen comes into contact with a bed of non-evaporable getter material 118. The invention may be better understood by reference to the following examples wherein all parts and percentages are by volume unless otherwise indicated; such examples are merely supplied for illustrating purposes and do not limit in any case the spirit and the scope of the invention. EXAMPLE 1 A stream of hydrogen, containing 5 ppm by volume of methane (5000 ppb) as well as traces of nitrogen and CO.,, was allowed to flow, at a flow rate of 100 normal cm'/minute, at a pressure of 4 bar and at a room temperature (lower than 40"C), through a first preliminar chamber (106) containing two beds of sorbing materials; upstream there was a bed (111) consisting of a molecular sieve (synthetic zeolite) and downstream there was another bed (110) containing approximately 20 g of a material containing 58% b.w. of nickel (mainly in the form of nickel oxide) supported on a silica carrier having a surface area slightly higher than 100 m"/g, traded by the Engelhard Company as "Ni 0104T. At least 5%b.w. of said nickel was in the reduced state. At the outlet of such preliminar chamber it was no more possible to retrieve any trace of CO... The gas stream was then allowed to flow, through a second (finishing) chamber, loaded with 40 g of a non-evaporable Ti,Ni getter alloy, in the form of a loose powder, having an average particle size of 1-150 micrometer, consisting of (% b.w. ): 62% Ti; 38% Ni . The temperature of said getter alloy was kept at 550' C throughout the test. The level of the residual CH, concentration was measured, at the outlet of said second chamber, by means of a VALCO gas-chromatograph, working with a metastable helium ionization detector having a sensitivity limit of 5 ppb for methane. At the beginning, the fresh getter alloy did completely adsorb all the methane and no trace of residual methane could be detected at the outlet of said second chamber; then the getter alloy began to be appreciably saturated and the test was arrested when the concentration of residual methane reached the level of 50 ppb. From the elapsed time, it was calculated that an overall amount of methane higher than 1.36 torr x liter/g had been adsorbed; this value is reported as "sorption capacity" in Table I. EXAMPLES 2 AND 3 Example 1 was repeated, replacing the specified getter alloy by other two different kinds of getter alloys according to the invention and precisely two getter alloys showing the following composition { % by weight ) : - for example 2 : 56.7% Ti; 30.2% V; 6.6% Fe ; 6.5% Mn ( LM alloy) ; - for example 3 : 30.1% Ti; 14.4% V; 10.5% Fe ; 44.9% Mn (HM alloy) Data and results are reported in Table I . TABLE I

Claims: WHAT IS CLAIMED IS: 1. An improved process for the removal of gaseous impurities from a stream of hydrogen, containing a first class of more easily removable impurities, like for instance CO , and a second class of more hardly removable impurities, mainly consisting of nitrogen and methane, essentially consisting of the following steps: A. Said stream is first brought into contact, at 5- 50°C, with one or more beds of a particulate material containing nickel and/or nickel compounds, wherein at least 1% b.w. (preferably 5%) of the overall amount of nickel is present in reduced form, until said more easily removable impurities are removed in an essentially complete way; B. The stream coming from step A, essentially free from said more easily removable impurities, but still containing said more hardly removable nitrogen and methane, is brought into contact with one or more beds of a non-evaporable getter material at a higher temperature . 2. The process of claim 1, wherein the pressure of said stream is from 1 to 20 bar and wherein the temperature of step B is from 400 to 600°C and preferably from 500 to 600 = C. 3. The process of claim 1, wherein said nickel and/or nickel compounds are supported on a carrier essentially consisting of a silicalite, a titanium- silicalite, a xerogel or a silica having an effective surface area equal to or higher than 100" m"/g (preferably 100 - 200 m 2 /g) • 4. The process of claim 1, wherein the nickel containing bed is followed or (preferably) preceded by a second sorption bed, consisting of one or more natural or synthetic molecular sieves and in particular of natural or synthetic zeolites, silicalites or titanium- silicalites . 5. The process of claim 1, wherein said getter material is contained in a housing with uniformly and smoothly polished inner walls, the roughness of said inner walls, expressed in terms of centerline average height ( , ) , being equal to or lower than 0.50 and preferably 0.25 micrometer. 6. The process of claim 1, wherein the hydrogen space velocity is from 0.5 to 50 normal cmVminute per gram of getter material and wherein the amount of the hardly removable methane is up to 5 ppm. 7. The process of claim 1, wherein said getter material is in the form of a loose powder having an average particle size from 1 to 500 and preferably from 1 to 250 micrometer. 8. The process according to claim 1, wherein said getter material is in the form of a shaped body, in particular pellets, having preferably an average size from 0.5 to 5 mm. 9. The process of claim 1, wherein said getter material is selected from: a) the Ti-Ni or Zr-Ni alloys and preferably the alloys containing from 50 to 80% by weight titanium or zirconium (balance = nickel), wherein up to 50% b.w. of the nickel may be replaced by iron and/or manganese and/or technetium and/or rhenium. b) the high manganese Ti-V alloys (HM alloys); c) the low manganese Ti-V alloys ( LM alloys). 10. The process of claim 9, wherein the composition of said HM alloys is as follows (% by weight): - titanium: 25 - 30.9% - vanadium: 10 - 42% - manganese: 27.1 - 65.1%; wherein from 2 to 2.2 other atoms are present per titanium atom. 11. The process of claim 10, wherein, in said HM alloys, up to about 40% of the vanadium atoms are replaced by iron atoms and up to 10% of the vanadium atoms are replaced by aluminum atoms, wherein however no more than 40% of the vanadium atoms are replaced by the overall amount of said iron and aluminum atoms. 12. The process of claim 11, wherein, in said HM alloys, the following optional modifications of the composition do also occur: i) up to about 20% of the titanium atoms may be replaced by Ca, Y, La, misch metall or mixtures thereof ; ii) up to 0.2 atoms of Cr (per titanium atom) may- replace a corresponding number of manganese and/or vanadium atoms; iii) up to 0.1 Ni atoms and/or 0.05 Cu atoms (per titanium atom) may be present in the alloy, wherein preferably no more than about 0.1 atom of nickel and copper replace a corresponding number of manganese and/or vanadium atoms. 13. The process of claim 9, wherein the composition of said LM (low-manganese) alloys is as follows (% by- weight) : - titanium : 25 - 65% - vanadium: 10 - 52% wherein up to 40% b.w. of vanadium may be replaced by- iron and wherein up to 20% b.w. of titanium may be replaced by manganese. 14. An apparatus for the removal of gaseous impurities from a stream of hydrogen, containing a first class of more easily removable impurities, like for instance CO and a second class of more hardly removable impurities, mainly consisting of nitrogen and methane, comprising : A) a gas inlet 102 in fluid communication, through pipings 104, 104' , with preliminar purification chambers 106, 106' , as well as valves 108, 108' , which can be alternately- opened or closed, to allow passage of the impurity-containing hydrogen through the first or the second of the preliminar purification chambers 106, 106' , containing a bed of particulate material 110, based on supported nickel, and optionally containing also a second bed of a natural or synthetic molecular sieve material (111); B) a final purification chamber 112, with which preliminar chambers 106, 106' are in fluid communication, by means of piping's 114, 114', and valves 116, 116' , which control the flow of the partially purified hydrogen from either of the preliminar purification chambers 106, 106' , which allows the regeneration of nickel in one chamber whilst the other is working, wherein said final purification chamber 112 contains a bed of non-evaporable getter material 118. 15. The apparatus of claim 14, wherein said getter material is selected from: a) the Ti-Ni or Zr-Ni alloys and preferably the alloys containing from 50 to 80% by weight titanium or zirconium (balance = nickel), wherein up to 50% b.w. of the nickel may be replaced by iron and/or manganese and/or technetium and/or rhenium. b) the high manganese Ti-V alloys (HM alloys); c) the low manganese Ti-V alloys ( LM alloys).

Inventors: D ARCY LORIMER, SOLCIA CAROLINA, SUCCI MARCO, D'ARCY, LORIMER, SOLCIA, CAROLINA, SUCCI, MARCO
Applicants: GETTERS SPA, SAES GETTERS S.P.A.
Application year: 1993
Main classification: C01B3
IPC codes: B01D53, B01J20, C01B3, C22C14, C22C16, C22C22, C22C27