Cryogenic system for producing ultra-high purity nitrogen

A process for further purification of nitrogen gas, thereby producing ultra-high purity nitrogen which is substantially free of hydrogen, oxygen and carbon monoxide, wherein the nitrogen gas is contacted with a metal-containing adsorbent at a temperature of 150 K. or less.

FIELD OF THE INVENTION 
This invention relates generally to the production of ultra-high purity 
nitrogen and, more particularly, to the production of ultra-high purity 
nitrogen using a process operating at a cryogenic temperature. 
BACKGROUND ART 
Consumers of nitrogen in the electronics industry typically require 
ultra-high purity nitrogen which contains less than 1 part-per-billion 
(ppb) of any contaminant such as oxygen, hydrogen and carbon monoxide. 
Concentrations of these substances in nitrogen obtained from a 
conventional cryogenic air separation plant are typically in the range of 
about 0.5-2 parts-per-million (ppm). Oxygen, which has a higher boiling 
point than nitrogen, is almost completely removed by the cryogenic 
distillation. However, since the boiling point of carbon monoxide is very 
close to that of nitrogen, and that of hydrogen is much lower, most of the 
carbon monoxide present in the feed air to the cryogenic air separation 
plant is also present in the nitrogen product stream from the plant, and 
the hydrogen concentration in the nitrogen product stream is about double 
that in the feed air. 
Removal of these contaminants is typically carried out using a conventional 
adsorption process following the cryogenic air separation process. 
However, such a system is disadvantageous because of the large size of the 
adsorption vessels needed to carry out the purification. 
An alternative to the use of conventional ambient temperature adsorption 
processes for producing ultra-high purity nitrogen is the upstream 
oxidation of hydrogen and carbon monoxide to water and carbon dioxide, 
respectively. These oxidation products are then removed in a molecular 
sieve prepurification system prior to the cryogenic air separation. This 
oxidation is typically carried out as a catalytic process. A major 
disadvantage of this oxidation process is that it requires high 
temperatures, increasing the energy requirements, and hence the cost of 
the entire process. Another disadvantage is that the oxygen remaining in 
the product nitrogen stream must be removed by another means, usually a 
separate cryogenic distillation process, which adds further to the cost of 
the overall process. 
Accordingly, it is an object of this invention to provide an improved 
system for producing ultra-high purity nitrogen. 
SUMMARY OF THE INVENTION 
The above and other objects, which will become apparent to one skilled in 
the art upon a reading of this disclosure, are attained by the present 
invention which is: 
A cryogenic adsorption process for producing ultra-high purity nitrogen, 
said process comprising contacting nitrogen gas containing one or more of 
hydrogen, oxygen or carbon monoxide impurities with a metal-containing 
adsorbent at a temperature of 150 K. or less, and producing ultra-high 
purity nitrogen which is substantially free of hydrogen, oxygen and carbon 
monoxide. 
As used herein the terms "cryogenic adsorption" and "cryoadsorption" mean 
an adsorption process carried out at a temperature of 150 K. or less. 
As used herein, the term "column" means a distillation or fractionation 
column or zone, i.e., a contacting column or zone wherein liquid and vapor 
phases are countercurrently contacted to effect separation of a fluid 
mixture, as for example, by contacting or the vapor and liquid phases on a 
series of vertically spaced trays or plates mounted within the column 
and/or on packing elements which may be structured packing and/or random 
packing elements. For a further discussion of distillation columns see the 
Chemical Engineers' Handbook fifth edition, edited by R. J. Perry and C. 
H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous 
Distillation Process. A column may include a top condenser wherein vapor 
is condensed for column reflux. 
Vapor and liquid contacting separation processes depend on the difference 
in vapor pressures for the components. The high vapor pressure (or more 
volatile or low boiling) component will tend to concentrate in the vapor 
phase whereas the low vapor pressure (or less volatile or high boiling) 
component will tend to concentrate in the liquid phase. Partial 
condensation is the separation process whereby cooling of a vapor mixture 
can be used to concentrate the volatile component(s) in the vapor phase 
and thereby the less volatile component(s) in the liquid phase. 
Rectification, or continuous distillation, is the separation process that 
combines successive partial vaporizations and condensations as obtained by 
a countercurrent treatment of the vapor and liquid phases. The 
countercurrent contacting of the vapor and liquid phases is adiabatic and 
can include integral or differential contact between the phases. 
Separation process arrangements that utilize the principles of 
rectification to separate mixtures are often interchangeably termed 
rectification columns, distillation columns, or fractionation columns. 
Cryogenic rectification is a rectification process carried out, at least 
in part, at temperatures at or below 150 degrees Kelvin (K.). 
As used herein, the term "indirect heat exchange" means the bringing of two 
fluids into heat exchange relation without any physical contact or 
intermixing of the fluids with each other. 
As used herein, the terms "upper portion" and "lower portion" of a column 
mean those portions respectively above and below the midpoint of the 
column. 
As used herein, the term "top" of a column means that section of the column 
above the internals, e.g. trays or packing, of the column.

DETAILED DESCRIPTION 
In the method of this invention, nitrogen gas, which contains one or more 
of hydrogen, oxygen or carbon monoxide impurities at the ppm level, is 
contacted with an adsorbent at a temperature of 150 K. or less. 
Preferably, the temperature is no greater than about 120 K., and most 
preferably the temperature is in a range from 80-100 K. The adsorbent 
adsorbs substantially all of hydrogen, oxygen or carbon monoxide which may 
be present in the nitrogen gas, resulting in an ultra-high purity nitrogen 
product which is substantially free of hydrogen, oxygen and carbon 
monoxide. Carrying out the adsorption process at a cryogenic temperature 
allows nitrogen gas produced in a cryogenic air separation system to be 
withdrawn from the separation system and directly purified without 
undergoing a heating step prior to the adsorption step, thus reducing 
operating costs. Nitrogen that is being held in a storage tank may also be 
purified with the method of this invention. In this embodiment, nitrogen 
from a cryogenic storage tank is contacted with the adsorbent at or near 
the storage temperature without preheating. The cryoadsorption system of 
this invention also enables the use of much smaller adsorption vessels 
than those necessary with conventional higher temperature or ambient 
temperature systems thus reducing the capital costs of the purification 
system. 
Suitable adsorbents for the method of this invention which are capable of 
efficiently adsorbing hydrogen, carbon monoxide and oxygen at cryogenic 
temperatures are shown in Table 1 and include adsorbents that contain 
nickel, copper, palladium, or iron. The preferred adsorbents are 
nickel-containing adsorbents, and the most preferred adsorbent is 
nickel(II) oxide on an alumina support. Preferably, the adsorbent is 
contained in the form of an adsorbent bed in a vessel of suitable capacity 
for the required quantity of adsorbent. 
In a preferred embodiment of this invention, adsorbents are regenerated by 
heating in an atmosphere of hydrogen and ultra-high purity nitrogen at a 
temperature greater than 120.degree. C., preferably greater than 
200.degree. C. The most preferred composition of the atmosphere used for 
regeneration of the adsorbent is about 1% hydrogen, by volume of the total 
mixture, in ultra-high purity nitrogen. The hydrogen reacts with the 
carbon monoxide and oxygen on the surface of the adsorbent to form methane 
and water, respectively. The methane and water are more weakly bound to 
the adsorbent and thus can be removed easily from the surface of the 
adsorbent by the nitrogen stream. Regeneration is performed at varying 
intervals depending on the capacity of the adsorbent vessels and the 
concentrations of hydrogen, carbon monoxide and/or oxygen in the nitrogen 
used as a feed for the process. Typically, multiple vessels are employed 
to allow regeneration of one or more vessels while one or more vessels are 
in use for producing ultra-high purity nitrogen. 
FIG. 1 is a process flow diagram showing the cryoadsorption nitrogen 
purification system of this invention integrated with a cryogenic air 
separation plant. Referring to FIG. 1, feed air in piping 1 is compressed 
in compressor 2 and carbon dioxide, water and some hydrocarbons are 
removed by prepurifier 3. Hydrogen and carbon monoxide, which may be in 
the feed air, are not removed by the prepurifier because they are not 
adsorbed by the molecular sieve materials of the prepurifier at the 
compressor discharge conditions. The cleaned air stream in piping 4 is 
then cooled to cryogenic temperatures by indirect heat exchange in a heat 
exchanger 5 against return streams. The cooled air stream in piping 6 is 
fed into a cryogenic rectification column 7, wherein the feed air is 
separated by cryogenic rectification into nitrogen gas and oxygen-enriched 
liquid. The nitrogen gas has a relatively low concentration of oxygen, 
typically less than 1 ppm. The distillation process shown is meant to 
represent a generic distillation process that produces nitrogen having a 
low level of oxygen. The distillation process, per se, does not affect 
purifier design and configuration. Therefore, any process which removes 
oxygen to low levels for the production of nitrogen may be used in 
conjunction with this invention. In a process that does not produce 
nitrogen at cryogenic temperatures, the product stream is cooled before 
carrying out the process of this invention. Ultra-high purity nitrogen can 
be added back into the top of the distillation column to improve 
distillation efficiency, and thereby decrease the oxygen content in the 
nitrogen gas produced by the column. 
Cryogenic nitrogen gas leaves the distillation process through piping 8, 
bypass piping 9 and bypass valve V1 to the cryogenic purifier 12, or to 
oxygen analyzers 10 where its oxygen content is measured prior to entering 
the cryogenic purifier to prevent high-oxygen excursions. Another portion 
30 of the nitrogen gas is passed into top condenser 31 wherein it is 
condensed and returned to column 7 as reflux stream 32. Oxygen-enriched 
liquid is passed from the lower portion of column 7 in stream 33 into top 
condenser 31 wherein it is vaporized by indirect heat exchange with the 
aforesaid condensing nitrogen gas and from when it is removed from the 
system in waste stream 34. 
The cryogenic purifier 12 is shown in FIG. 1. A summary of the valves 
(V1-V18) follows: 
______________________________________ 
V1 Control valve for bypass 9. 
V2 Back-up liquid piping 21. 
V3 Vent valve for piping 11. 
V4 Valve for back up piping 21. 
V5 Valve for purified nitrogen product exiting 
purifier. 
V6 Purge valve for piping 9 or 13. 
V7 Isolation valve to Bed 12A used to isolate from 
product stream for regeneration cycle. 
VB Isolation valve to Bed 12A used to isolate from 
product stream for regeneration cycle. 
V9 Regeneration vent for Bed 12A. 
V10 Product valve from Bed 12A for piping 22. 
V11 Isolation valve to Bed 12B used to isolate from 
product stream for regeneration cycle. 
V12 Isolation valve to Bed 12B used to isolate from 
product stream for regeneration cycle. 
V13 Regeneration vent for Bed 12B. 
V14 Product valve from Bed 12B for piping 22. 
V15 Valve for regeneration cycle to allow purifier 
product flow 14. 
V16 Valve for regeneration cycle to allow pure 
hydrogen to blend with product flow 15. 
V17 Valve for regeneration of Bed 12A. 
V18 Valve for regeneration of Bed 12B. 
______________________________________ 
Two oxygen analyzers are used for redundancy. The nitrogen vapor stream 
passes through piping 11 and enters the cryogenic purifier 12 where 
hydrogen, carbon monoxide and any remaining oxygen are removed to be at a 
concentration of about 1 ppb or less. Valve V3 is a vent valve for piping 
11. The purified product stream leaving the purifier in piping 13 is 
warmed to ambient temperature in heat exchanger 5, then recovered. Valve 
V6 is a purge valve for piping 9 or 13. 
The purifier 12 of this invention comprises two beds, 12A and 12B, that 
contain a metal-containing adsorbent, e.g. at least 5% nickel balanced 
alumina adsorbent material, preferably 10-30% nickel balanced alumina 
adsorbent material, and most preferably, 20% nickel balanced alumina 
adsorbent material. The purified product comes from either Bed 12A or Bed 
12B. There are a pair of isolation valves on each purifier bed to isolate 
it from the streams carried in piping 11 or 21 for regeneration or a 
possible shut-down. Isolation valves V7 and V8 are used on 12A; and valves 
V11 and V12 on 12B. One bed 12A is regenerating while the other bed 12B is 
adsorbing. 
Regeneration is accomplished by mixing about 5% of ultra-high purity 
nitrogen product from piping 14 and valve V15 with hydrogen from tank 27, 
passing through valve V16 and piping 15 to make a mixture containing about 
1 volume percent hydrogen which is heated in heater 16 to a temperature of 
no less than 120.degree. C., and preferably at least 200.degree. C. 
Typically the bed to be regenerated is heated to at least 120.degree. C. 
prior to introducing the 1% hydrogen blend. The hot stream passes through 
piping 17 and valve V17 to purifier bed 12A, or valve V18 for 12B, heating 
the respective adsorbent bed and releasing any adsorbed substances. The 
spent regeneration stream is vented through valve V9 and piping 18 from 
12A, or through valve V13 and piping 18 from 12B. 
The regeneration process requires no less than 120.degree. C., preferably 
at least 200.degree. C. and hydrogen to react with the adsorbed carbon 
monoxide to form methane, and to react with oxygen to form water. The 
methane and water along with the hydrogen are then easily desorbed. The 
regeneration cycle is 24 hours, at which time the beds are switched to 
replace the adsorbing bed with a regenerated bed. Regeneration can be done 
every 24 hours or every few days, depending on the vessel capacity. 
Regeneration flow is countercurrent to the adsorption flow. 
The cryogenic purification system of this invention may also be used to 
remove 0.5-1 ppm oxygen and 1-2 ppm carbon monoxide from nitrogen stored 
and transported in liquid form. Nitrogen is typically stored in liquid 
form to meet usage needs when the cryogenic rectification plant is not in 
service. Stored backup liquid in tank 19 is heated to ambient temperature 
using a vaporizer 20, passing through valve V4, piping 21 and valve V2 to 
the cryogenic purifier. Stored liquid product typically contains little 
hydrogen because hydrogen boils off in transport and in the liquid storage 
tank. Carbon monoxide loading, which is identical at either ambient or 
cryogenic temperatures, dictates vessel size in the cryogenic adsorber. 
Therefore, the cryogenic purifier size may be used for either process or 
liquid nitrogen purification. 
Because the purifier is at a cryogenic temperature when process nitrogen is 
passing through it, backup nitrogen leaving the purifier from bed 12A or 
bed 12B is initially cold. If it is required to warm the product nitrogen, 
it is passed through valve V10, or valve V14, respectively, and piping 22 
to an electric heat glycol/water bath 23. The warmed stream passes through 
piping 24 to the filter skid 25. The streams passing through either piping 
13 or 24 are treated in a filter skid assembly 25 to remove fine particles 
before the product stream exits through piping 26 for recovery. 
In addition, at the same mass flow rate, the volume of vaporized liquid 
nitrogen that would pass through the purifier would be greater than the 
volume of cryogenic nitrogen passing through the regenerator during normal 
operation. Pressure loss through the purifier when vaporized liquid is 
purified is about four times that observed with process vapor. However, 
the pressure available in the liquid storage tank is typically relatively 
high. Therefore, this increased pressure drop will not cause problems. 
FIG. 1 illustrates a particularly preferred embodiment of this invention 
wherein a portion of the ultra-high purity nitrogen produced by the 
cryoadsorption system is returned to upper portion, preferably to the top, 
of the cryogenic rectification column 7 in piping 28. This reflux 
operation improves the efficiency of the air separation system and lowers 
the oxygen content of the nitrogen gas produced by the cryogenic 
rectification column. The ultra-high purity nitrogen passes into the top 
condenser for the production of reflux. If desired, condensed nitrogen gas 
from top condenser 31 may be removed in piping 29 and recovered as 
ultra-high purity liquid nitrogen. 
Table 1 contains data from laboratory characterization work done on 
identified cryoadsorption materials at 87.degree. K. (-183.degree. C.). 
The criterion for selecting a cryoadsorbent material is that the material 
adsorb the maximum number of moles of carbon monoxide, oxygen and hydrogen 
per mole of metal in the cryoadsorbent. The best cryoadsorbing material 
identified from the laboratory work was the Crosfield nickel catalyst 
HTC-500.TM. (available from Crosfield Catalysts, Chicago, Ill.). 
TABLE 1 
__________________________________________________________________________ 
SOURCE/ADSORBENT 
COMPOSITION 
H/M 
CO/M 
O/M 
Comments 
__________________________________________________________________________ 
METADYNE (Elma, NY) 
Tungsten Sponge 
-- &lt;0.01 
-- &gt;700.degree. C. to regenerate 
Union Carbide (Danbury, 
9% CuO-Y Zeolite 
-- 0.23 
-- 
CT) 
Crosfield/HTC-500 .TM. 
20% NiO/Al.sub.2 O.sub.3 
0.01 
0.09 
0.02 
BASF (Mt. Olive, NJ)/ 
CuO/ZnO/Al.sub.2 O.sub.3 
-- &lt;0.01 
-- Loading based on 40% Cu.sup.+ 
R-3-12 .TM. 
Union Carbide/OC-112 .TM. 
50% Cuo; Mno.sub.2 /SiO.sub.2 
-- &lt;0.01 
&lt;0.01 
Carus (Ottawa, IL)/ 
75% MnO.sub.2 ; 15% 
-- &lt;0.01 
&lt;0.01 
Loading based on 75% Mn.sup.+3 
Carulite-300 .TM. 
CuO/Al.sub.2 O.sub.3 
Degussa (Tulsa, 
0.5% Pd/Al.sub.2 O.sub.3 
-- 1.1 -- 
OK)/E-221 .TM. 
United Catalyst 
Fe/Al.sub.2 O.sub.3 
-- &lt;0.01 
0.01 
O.sub.2 loading based on 80% Fe.sup.+2 
(Louisville, 
KY)/C12-4-02 .TM. 
Engelhard (Iselin, 
18% Cr.sub.2 O.sub.3 /Al.sub.2 O.sub.3 
-- &lt;0.01 
&lt;0.01 
NJ)/CR-0211 T 5/32" .TM. 
Engelhard/Silica 2351 T 
90% Silica/Al.sub.2 O.sub.3 
-- -- &lt;0.01 
1/8" .TM. 
Engelhard/Co-0138 E 
30% Co/Silica 
-- 0.02 
&lt;0.01 
1/16" .TM. 
Various 5A Mol. Sieves 
-- 0.04 
-- 
__________________________________________________________________________ 
In Table 1, H/M, CO/M, O/M are mole to mole ratios of atomic hydrogen, 
carbon monoxide and oxygen respectively to the given metal. 
In another preferred embodiment of this invention, a stand-alone 
cryoadsorption system is employed to produce ultra-high purity nitrogen 
from standard grade nitrogen such as from a liquid nitrogen storage tank 
or a liquid trailer. A process flow diagram is shown in FIG. 2. The 
cryoadsorption system of this invention is used to purify nitrogen gas 
containing one or more of oxygen, hydrogen or carbon monoxide at levels of 
0.1-10 ppm to levels of 1 ppb or less by cryogenic gas phase 
chemiadsorption. 
As shown in the process flow diagram in FIG. 2, liquid nitrogen is 
withdrawn from tank 101, flows through piping 102 and passes through a 
vaporizer 103, or through valve 104 bypassing the vaporizer. The vaporized 
nitrogen is maintained in either case at a temperature of 150 K. or less, 
preferably from 80-100 K., and most preferably at about 90 K. The nitrogen 
flow in piping 105 is either at full product flow as a gas or, as a 
liquid, at about 1-10 gallons per hour to the top of the catalyst bed 
where it flash vaporizes. The flash vaporization maintains the cryogenic 
refrigeration of the catalyst bed when not in use. The cryogenic high 
purity nitrogen vapor flows through the catalyst bed 106 where at least 
one of the impurities (oxygen, hydrogen, and carbon monoxide) is 
chemiadsorbed to 1 ppb or less. The vessel 107 that contains the catalyst 
bed has only an inlet and outlet port for flow and/or catalyst handling. 
The vessel employs cryogenic insulation to maintain the cryogenic 
temperature. It is vertically supported. The ultrahigh purity nitrogen 
vapor remains at cryogenic temperature, and exits from the bottom outlet 
of the bed through piping 108. The ultra-high purity nitrogen is warmed to 
ambient temperature in vaporizer 109, its pressure is regulated by 
pressure regulator 110, and it passes through a particulate filter skid 
111 to eliminate any particulate matter before being recovered. 
The preferred nickel catalyst bed has the capacity to purify cryogenic 
nitrogen vapor for 30 days without the need to regenerate. The bed 
preferably is large enough to purify nitrogen from a liquid nitrogen 
storage tank or a liquid trailer for 30 days, and to allow for a less 
frequent regeneration interval, e.g., 6-12 months. For example, for a 
plant that will produce 180,000 cubic feet per hour (CFH) nitrogen, the 
required amount of catalyst is at least 500 pounds, but no greater than 
5,000 pounds. 
The cryopurifier shown in FIG. 2 does not have permanent regeneration 
capability. Regeneration is done using auxiliary equipment on or off-site 
to regenerate the catalyst. The required equipment and setup is 
illustrated in FIG. 3. Regeneration is accomplished by mixing about 5% of 
the ultra-high purity nitrogen product stream, passing through piping 201 
and valve 210 near ambient temperature, with a hydrogen supply stream 
passing through piping 202 and valve 211 to make a regeneration stream in 
piping 203 comprising nitrogen and about 1 volume percent hydrogen. The 
regeneration stream enters a static mixer 204 to ensure good mixing. A 
mixer is not absolutely necessary to carry out the process successfully. 
The regeneration stream then passes through piping 205 and enters a heater 
206, where it is heated to not less than 120.degree. C., preferably at 
least 200.degree. C. As the heated stream exits the heater through piping 
207 it passes through a shut off valve 212 and enters the catalyst 
adsorbent bed 208. The adsorbent bed is heated by the stream and releases 
any adsorbed substances. The spent regeneration stream is vented through 
the bed and piping 209 to exhaust. 
The regeneration process requires a temperature of at least 120.degree. C., 
preferably at least 200.degree. C., and a gas stream containing hydrogen 
to reduce the adsorbed carbon monoxide to methane, and to reduce oxygen to 
form water. The methane and water, along with the hydrogen mixture stream, 
are then easily desorbed. The regeneration cycle, including heating and 
cooling sequences, can be from 24 hours up to 2 weeks, depending on the 
vessel capacity. Regeneration flow is countercurrent to the adsorption 
flow. 
Although the invention has been described in detail with reference to 
certain preferred embodiments, those skilled in the art will recognize 
that there are other embodiments of the invention within the spirit and 
the scope of the claims.