Inert gas purifier for bulk nitrogen without the use of hydrogen or other reducing gases

The present invention relates to a three stage process using copper, copper oxide and molecular sieve adsorbent beds for the sequential removal of oxygen, hydrogen, carbon monoxide, carbon dioxide and water from an inert feed gas. The process is especially suited to the purification of nitrogen gas from an air separation plant, which can be purified from a contaminant level of 30 vppm oxygen+carbon monoxide+hydrogen to less than 10 vppb each of oxygen, carbon monoxide, hydrogen, carbon dioxide and water, without the addition of hydrogen or another reducing gas to the process.

TECHNICAL FIELD 
The present invention relates to a process for the purification of nitrogen 
produced from an air separation unit to remove hydrogen, carbon monoxide, 
oxygen, carbon dioxide and water. 
BACKGROUND OF THE INVENTION 
A number of techniques are known for the removal of impurities (either 
singular impurities or combined impurities) from inert gas, among these 
are the following. 
A first method utilizes metal "getters", typically composed of mixtures of 
zirconium, aluminum, iron and vanadium, to remove impurities from an inert 
gas by reaction or chemisorption. A major disadvantage of this method is 
the necessity for operation at high temperature (400.degree. C.) with even 
higher temperatures for initial activation (500.degree.-700.degree. C.). 
Additionally, these materials have a limited capacity and can only be 
regenerated and reused a small number of times before their effectiveness 
is lost. 
A second method utilizes a platinum group catalyst (e.g., platinum and 
palladium) to remove oxygen from an inert gas by reaction with added 
hydrogen at temperatures from ambient to 300.degree. C.; this second 
method is described in U.S. Pat. No. 3,535,074. In this particular 
description, a second absorber bed utilizing copper or nickel is used to 
remove any transient high concentrations of oxygen. The added hydrogen is 
removed by distillation of the product. 
A third method utilizes reduced copper or nickel containing beds at 
temperatures from ambient to 250.degree. C. for removal of oxygen. These 
beds are regenerated from the oxidized state by reduction with a stream 
containing hydrogen. 
Two methods have been described in the art for the removal of combined 
impurities of oxygen, hydrogen, carbon monoxide, carbon dioxide and 
hydrogen. 
The first of these two methods is described in U.S. Pat. No. 4,579,723, 
wherein a commercial catalyst material (e.g., Engelhard Deoxo A containing 
Cr and Pt) is used to react carbon monoxide and hydrogen with oxygen at 
ambient temperature forming carbon dioxide and water. Residual oxygen and 
carbon dioxide are removed in a second bed containing a gettering material 
(e.g., Dow Q1) which is effective to remove oxygen and carbon dioxide. 
Water is removed by adsorption in one or both beds. It is necessary to 
regenerate the beds with a hydrogen containing stream at about 200.degree. 
C. to maintain their effectiveness. 
In the second of these methods a nickel containing bed is used to 
simultaneously remove oxygen, carbon monoxide, hydrogen, water and carbon 
dioxide from an inert gas at ambient temperature; this method is disclosed 
in U.S. Pat. No. 4,713,224. The nickel containing bed is subsequently 
regenerated with a hydrogen containing stream. 
In all of these above processes, it is necessary that the hydrogen be added 
to the process either for the primary removal of the impurity or for 
regeneration. This addition of hydrogen adds the cost of the hydrogen 
supply and the provision of equipment to ensure the safe handling of 
hydrogen. 
Other processes known in the art are disclosed in U.S. Pat. Nos. 3,061,403; 
3,682,585 and 4,459,270 and Australian Pat. No. 16826/53. 
SUMMARY OF THE INVENTION 
The present invention is a process for the purification of a bulk inert gas 
stream, wherein the bulk inert gas stream contains oxygen, carbon monoxide 
and hydrogen impurities and wherein the molar concentration of the carbon 
monoxide plus hydrogen impurities on a time-averaged basis exceeds two 
times the molar concentration of the oxygen impurity. The process 
comprises three sequential steps: (a) the oxygen present in the bulk inert 
gas stream is reacted with the carbon monoxide and hydrogen present in the 
bulk inert gas stream in the presence of a reduced copper containing 
catalyst at a temperature from 150.degree. to 250.degree. C. to form 
carbon dioxide and water; (b) unreacted carbon monoxide and hydrogen from 
step (a) are reacted with the oxygen component of a copper oxide 
containing catalyst at a temperature from 150.degree. to 250.degree. C. to 
from carbon dioxide, water and reduced copper; and (c) water and carbon 
dioxide are removed by adsorption on an adsorption, preferably a molecular 
sieve adsorbent. 
The reduced copper formed during step (b) is intermittently reoxidized to 
copper oxide with an oxygen containing stream at a temperature in the 
range between about 50.degree. and about 150.degree. C. The adsorbent of 
step (c) is regenerated at intervals with a portion of the purified bulk 
inert gas stream at temperatures of about 150.degree. to about 250.degree. 
C.

DETAILED DESCRIPTION OF THE INVENTION 
The manufacture of semiconductors in the electronics industry comprises 
many process steps in which the materials are exposed to inert process 
gases (especially nitrogen). Impurities contained in the inert gases react 
with the semiconductor surface to generate undesirable properties. This is 
a particular problem in the production of advanced semiconductor devices 
with very small feature sizes, higher device density and larger chip 
sizes. It is therefore necessary to produce inert gases with minimal 
impurity levels (preferably below 10 vppb). 
For the case of nitrogen, large quantities of gas are required; many 
installations use up to 100,000 SCFH. Because of these large quantities, 
the preferred manner of production of such bulk gas is by cryogenic air 
separation, which results in a product which contains oxygen, hydrogen and 
carbon monoxide impurities at parts per million concentrations where the 
sum of the hydrogen and carbon monoxide impurities significantly exceeds 
the oxygen; thus creating the problem to efficiently and safely remove 
these impurities. 
The present invention is an improved process for the purification of bulk 
quality inert gas (nitrogen) from impurity levels of up to 30 vppm of 
carbon monoxide+hydrogen+oxygen in which the carbon monoxide+hydrogen 
content exceeds two times the oxygen content. The resultant product of the 
process of the present invention contains less than 0.1 vppm of each of 
the impurities oxygen, carbon monoxide, hydrogen, carbon dioxide and 
water. 
The process comprises three sequential purification stages. The first stage 
utilizes a reduced copper containing catalyst at a temperature from 
150.degree. to 250.degree. C. to react oxygen with carbon monoxide and/or 
hydrogen to carbon dioxide and/or water. The copper catalyst is maintained 
in a continuously reduced state by the excess of reducing gas impurities 
(i.e., carbon monoxide and hydrogen) in the inert gas. The second stage is 
a copper oxide containing bed also operated at 150.degree. to 250.degree. 
C. in which the residual hydrogen and carbon monoxide are converted to 
water and carbon dioxide by reaction with the oxygen component of the 
copper oxide catalyst thereby producing reduced copper. This bed is 
reoxidized at intervals using an oxygen containing stream at 50.degree. to 
150.degree. C. The third stage is an adsorbent bed preferably containing a 
molecular sieve for the adsorption of water and carbon dioxide. The 
adsorber bed is operated at ambient conditions. This molecular sieve bed 
is regenerated at intervals with a portion of the product nitrogen stream 
at temperatures of 150.degree. to 250.degree. C. 
The operation of the process is best described with reference to the single 
figure of the drawing. With reference to the single figure, inert gas 
(nitrogen) in bulk gas quality is produced from an air separation unit 1 
and removed via line 2 at a pressure from 15 to 250 psia, typically 130 
psia. The bulk inert gas in line 2 typically contains from about 0.2 to 2 
vppm, oxygen; about 0.2 to 2 vppm, carbon monoxide and about 0.2 to 2 
vppm, hydrogen, in addition to other trace inert gases such as argon, 
helium and neon. The bulk nitrogen is fed at near to ambient temperature 
via line 2 to heat exchanger 3, wherein it is warmed to approximately 
150.degree. C. and then via line 4 to heater 5 wherein it is heated to 
approximately 175.degree. C. The warmed inert gas then is fed via line 6 
to deoxidizer vessel 10. Deoxidizer vessel 10 contains a reduced copper 
catalyst, which causes the oxygen to react with the hydrogen and/or carbon 
monoxide to form water and carbon dioxide. During temporary periods of 
operation when insufficient hydrogen or carbon monoxide are present to 
fully react with the oxygen, the residual oxygen will be removed by 
reaction with the reduced copper catalyst to form copper oxide. The molar 
recess of hydrogen and carbon monoxide to oxygen on average in the bulk 
inert gas ensures that reduction of any copper oxide formed to copper 
occurs on a continual basis. Thus, there is no requirement for 
regeneration of this bed. 
The gas, from deoxidizer 10, now containing hydrogen, carbon monoxide, 
carbon dioxide and water impurities is fed via line 11 through isolation 
valve 17 and line 19 to oxidizer vessel 20. Oxidizer vessel 20 is one of a 
pair of oxidizer vessels (with vessel 21) which are operated sequentially 
on a cycle of several days. Oxidizer vessel 20 (or alternatively 21) 
contains a copper oxide catalyst (e.g., BASF catalyst R3-11) which totally 
oxidizes any residual hydrogen and carbon monoxide impurities to water and 
carbon dioxide. 
The hot gas from oxidizer 20, which is lean in oxygen, carbon monoxide and 
hydrogen, is removed via line 22 through isolation valve 24 and then line 
25 to heat exchanger 3 where it is cooled to close to ambient temperature 
against the bulk nitrogen feed gas to the purifier process. The cooled gas 
containing water and carbon dioxide impurities is then fed via line 37 
through isolation valve 38 and then line 39 to adsorber bed 41 where the 
water and carbon dioxide are removed by adsorption on, for example, a 
molecular sieve type 13x. Adsorber bed 41 is one of a pair of vessels 
(with vessel 42) operated sequentially with a cycle time of 12 to 48 
hours. 
Purified inert gas product is obtained from the exit of vessel 41 
sequentially through line 43, isolation valve 45, line 46 and product line 
60. 
A portion, from about 5 to 10%, of the product gas is withdrawn from line 
46 as regeneration gas via line 47, fed to electric heater 57 wherein it 
is heated to approximately 230.degree. C., and reduced in pressure across 
valve 48 to a pressure just sufficient to drive the gas flow through 
adsorber vessel 42. This low pressure heated gas is fed via line 49, 
through isolation valve 50 and line 52 to adsorber vessel 42 where the 
combined effect of heat and reduced pressure is used to desorb carbon 
dioxide and water from the adsorbent. The regeneration gas is vented via 
line 53, isolation valve 54 and line 56 to the atmosphere. After a 
sufficient time has elapsed for the adsorbent to be heated and carbon 
dioxide and water to be released, the electric heater 57 is switched off 
and the gas flow continued to cool vessel 42 and the adsorbent in 
preparation for the subsequent adsorption cycle. The sequence of operation 
of vessels 41 and 42 is controlled by activation of the isolation valves 
38, 40, 44, 45, 50, 51, 54 and 55 and electric heater 57 by an automatic 
timer. 
The regeneration of the oxidizer in vessels 20 and 21 is effected in a 
similar manner to the absorber but with the addition of oxygen. A portion, 
approximately 5%, of the hot gas leaving vessel 20 via line 25 is removed 
via line 26 and reduced in pressure across valve 27 to a sufficient 
pressure to drive regeneration gas through vessel 21 to vent to 
atmosphere. A flow of clean dry air is added to the nitrogen flow in a 
proportion to produce a mixture concentration of approximately 1% oxygen 
in nitrogen. The mixture, at a temperature of approximately 120.degree. 
C., is fed via line 29, isolation valve 30 and line 32 to vessel 21 where 
it is used to reoxidize the partially reduced copper oxide catalyst. After 
the catalyst has been reoxidized, the air flow is shut off and the vessel 
is purged with purified nitrogen. The regeneration as is vented from 
vessel 21 via line 33, isolation valve 34 and line 36 to the atmosphere. 
The sequence of operation of vessels 20 and 21 for oxidation and 
regeneration is controlled by activation of the isolation switching valve 
positions. 
In order to demonstrate the efficacy of the present inventions, the process 
of the present invention was operated using a nitrogen feed with varying 
impurity contents. These tests were designed to measure the performance of 
the various stages of the system. All measurements were made at a design 
flow rate of 20,000 SCFH. 
EXAMPLE 1 
Performance of Deoxidizer and Oxidizer With Typical Purity Bulk Gas 
______________________________________ 
Impurity Feed Composition 
Product Composition 
______________________________________ 
Oxygen: vppm 0.23 0.03* 
Hydrogen: vppm 
0.49 &lt;0.05** 
Carbon Monoxide: vppm 
0.43 &lt;0.05** 
Inerts Balance Balance 
______________________________________ 
*Estimated sensitivity of analyzer 0.02 vppm. 
**Limiting sensitivity of analytical instrument. 
EXAMPLE 2 
Performance of Deoxidizer and Oxidizer With a Low Hydrogen/High Carbon 
Monoxide Impurity Inert Feed 
______________________________________ 
Impurity Feed Composition 
Product Composition 
______________________________________ 
Oxygen: vppm 0.19 0.04 
Hydrogen: vppm 
&lt;0.05** &lt;0.05** 
Carbon Monoxide: vppm 
4.28 &lt;0.05** 
Inerts Balance Balance 
______________________________________ 
**Limiting sensitivity of analytical instrument. 
EXAMPLE 3 
Performance of Deoxidizer and Oxidizer with a Temporary High Oxygen/Low 
Hydrogen Impurity Inert Feed 
______________________________________ 
Impurity Feed Composition 
Product Composition 
______________________________________ 
Oxygen: vppm 4.85 0.04 
Hydrogen: vppm 
&lt;0.05** &lt;0.05** 
Carbon Monoxide: vppm 
0.35 &lt;0.05** 
Inerts Balance Balance 
______________________________________ 
**Limiting sensitivity of analytical instrument. 
These examples demonstrate the effectiveness of the process to operate 
under the normally expected condition with hydrogen in the feed gas; also 
at low hydrogen concentrations with carbon monoxide present as the excess 
reducing gas, and under a temporary condition with an excess of oxygen. 
EXAMPLE 4 
In a subsequent test after several months operation with bulk feed gas of a 
normal operating composition (i.e., Example 1) and more extensive and 
sensitive analytical equipment, a pure product inert gas of the following 
composition was measured downstream of the adsorber system. 
__________________________________________________________________________ 
Impurity Product Content 
Type of Analyzer 
__________________________________________________________________________ 
Oxygen: vppb 
&lt;10** Teledyne 356 O.sub.2 Analyzer 
Hydrogen: vppb 
&lt;7** Trace Analytical Reduction Gas 
Detector 
Carbon Monoxide: vppb 
&lt;2** Trace Analytical Reduction Gas 
Detector 
Carbon Dioxide: vppb 
&lt;5** Hewlett Packard Gas Chromatograph 
with Flame Ionization Detector 
Water: vppb 7 Endress + Hauser Ondyne 
Capacitance Cell 
Inerts Balance 
__________________________________________________________________________ 
**Limiting sensitivity of analytical instrument. 
These results demonstrate the ability of the purifier to achieve ultrapure 
nitrogen quality to meet the most stringent requirements of the 
semiconductor industry. 
As can be seen, the process solves the problem of economically and safely 
providing purification of bulk inert gas by using a three stage system of 
impurity removal in which no hydrogen or other reducing gas is added. In 
the first stage, the natural excess of hydrogen and carbon monoxide found 
in bulk nitrogen gas produced by cryogenic air separation is used to react 
oxygen to water and carbon dioxide and to maintain a reducing atmosphere 
over a reduced copper catalyst. In the second stage a copper oxide 
catalyst oxidizer bed is used to convert residual hydrogen and carbon 
monoxide to water and carbon dioxide. This bed is regenerated using a 
dilute oxygen stream. The third stage uses an adsorbent (e.g., 13x 
molecular sieve) to remove the residual water and carbon dioxide at 
ambient temperature. Regeneration of the absorbent is carried out with a 
portion of the purified product stream. 
The process of the present invention has several features which 
distinguishes it from prior art, among these are the following: 
(1) Hydrogen is not required to be added to the feed or for regeneration of 
the reactor beds. This eliminates the cost and hazard resulting from the 
incorporation of hydrogen handling equipment. 
(2) The sequence of operation with a first stage deoxidizer and a second 
stage oxidizer is novel. 
(3) The process allows that the feed may contain either hydrogen or carbon 
monoxide as the reducing impurity with a time average excess over the 
oxygen impurity. The use of a first stage copper deoxidizer permits a 
temporary excess of oxygen impurity over reducing gas which is held on the 
copper catalyst surface as copper oxide. 
(4) The use of a second stage oxidizer provides conversion of residual 
hydrogen and carbon monoxide to readily removable adsorbable impurities. 
The process of the present invention has been described above in reference 
to a specific embodiment thereof. This embodiment should not be viewed as 
a limitation on the scope of the present invention, however, the scope of 
the present invention should be ascertained by the following claims.