Argon purification system

An argon purification system comprising an ambient temperature molecular sieve adsorption step, an ambient temperature chemisorption step, and a cryogenic temperature adsorption step, particularly useful with liquefaction of the purified argon.

TECHNICAL FIELD 
This invention relates generally to argon purification and more 
specifically to argon purification on-site for recycle and reuse. 
BACKGROUND ART 
Argon is employed in various processes wherein its chemically inert nature, 
specific physical properties, and a cost which is low relative to those of 
other noble gases make its use particularly advantageous. For example, 
argon is used as a blanketing or purge gas, as a heat transfer medium, for 
the degassing of reactive impurities in various metal processing 
operations, and for the atomization of molten metals into fine powder. 
While argon is present in air at a much higher concentration than those of 
the other noble gases, and considerable volumes of argon are available as 
a byproduct of oxygen and nitrogen production by air separation, the cost 
of argon still provides significant incentive toward maximizing recycle 
usage. Therefore, systems have been commercially implemented to conserve 
argon by means of pressure equalization between vessels, recompression and 
recycle, generally with particulate separation. 
However, the operations in which the argon is utilized often involve 
periodic exposure of various parts of the system to the surrounding 
atmosphere. Steps which are conducted at low pressure or vacuum are 
subjected to potential air infiltration. In addition, materials being 
processed may degas various impurities. Thus there is a need to purify the 
spent argon prior to recycle and reuse. 
The operation of systems in which argon is employed is frequently batch in 
nature, resulting in periodic requirements for very high flowrates over 
relatively short time intervals, and other times when throughput is very 
low or absent. High pressure receivers, or reliquefaction for compact 
storage, may be utilized to accommodate these requirements. These 
conditions make it difficult to match the desired gas contaminant removal 
to reasonably-sized separation equipment. 
Cryogenic distillation and catalytic combustion have both been proposed for 
the purification of argon in order to promote additional argon 
conservation. However, both of these methods are costly to implement and 
to operate. Moreover, the design of a cryogenic distillation system is 
generally controlled by the maximum instantaneous demand with respect to 
impurity levels and flowrate. When used in applications such as argon 
recycle purification, where impurity levels and flows may vary greatly 
with time, the equipment may then be considerably oversized with respect 
to the time-averaged requirement. The sizing of catalytic combustion 
equipment is similarly controlled by the maximum instantaneous 
requirement. 
Accordingly it is an object of this invention to provide an improved method 
and apparatus for purifying argon. 
It is a further object of this invention to provide an improved method and 
apparatus for purifying argon which can be effectively employed under 
conditions of wide variations in flows and in impurity concentration 
levels. 
It is yet another object of this invention to provide an improved method 
and apparatus for purifying argon which is less costly than heretofore 
available systems. 
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, one aspect of which is: 
Method for purifying argon comprising: 
(A) providing a gaseous argon stream comprising one or more of oxygen, 
nitrogen, water vapor, hydrogen, carbon monoxide, carbon dioxide and 
hydrocarbon impurities; 
(B) passing the gaseous argon stream through a bed comprising molecular 
sieve adsorbent at ambient temperature and adsorbing thereon water vapor 
and/or carbon dioxide; 
(C) passing the gaseous argon stream through a bed comprising catalytic 
material at ambient temperature and chemisorbing thereon oxygen, hydrogen 
and/or carbon monoxide; 
(D) passing the gaseous argon stream through a bed comprising adsorbent at 
a cryogenic temperature and adsorbing thereon nitrogen and/or hydrocarbon; 
and 
(E) recovering a purified argon stream. 
Another aspect of the present invention comprises: 
(A) a bed comprising molecular sieve and means for providing impurity 
containing gaseous argon through the molecular sieve bed; 
(B) a bed comprising catalytic material and means for providing impurity 
containing gaseous argon through the catalytic material bed; 
(C) a bed comprising adsorbent and means for providing impurity containing 
gaseous argon through the adsorbent bed; 
(D) means for reducing the temperature of the gaseous argon to a cryogenic 
temperature prior to its passage through the adsorbent bed; and 
(E) means for recovering purified argon from the adsorbent bed. 
As used herein the term "ambient temperature" means a temperature within 
the range of from -30.degree. C. to +50.degree. C. 
As used herein the term "cryogenic temperature" means a temperature below 
-120.degree. C. 
As used herein the term "bed" means a permeable aggregate of pelletized 
solid particles held within a vessel. 
As used herein the term "catalytic material" means a solid material which 
under certain conditions of temperature and pressure increases the rate of 
specific chemical reactions while itself remaining unchanged at the 
completion of the reaction. 
As used herein the term "adsorption" means the reversible process whereby 
some components of a gas mixture adhere to the surface of solid bodies 
with which they are in contact. 
As used herein the term "chemisorption" means an adsorption process in 
which certain components of a gas mixture selectively adhere to the 
surface of the solid as a result of chemical forces.

DETAILED DESCRIPTION 
The invention comprises up to three argon purification steps, depending 
upon which species of impurities are present in the argon, the steps being 
an ambient temperature molecular sieve adsorption step, an ambient 
temperature chemisorption step and a cryogenic temperature adsorption step 
which is particularly advantageous if the purified argon is to be 
liquified such as for storage purposes. 
The invention will be described in detail with reference to the FIGURE 
which illustrates a preferred embodiment of the invention wherein the 
subject beds or vessels are arranged in pairs which are installed in 
parallel to allow continuous operation. That is, while the first of each 
pair of vessels is purifying the argon stream the second of the pair is 
undergoing regeneration and at the appropriate time the flows are switched 
so that the first is being regenerated while the second carries out the 
purification. In an alternative arrangement the beds could be dual bed 
single vessel adsorbers. 
Referring now to the FIGURE, there is provided a gaseous argon stream 1 
comprising impurities generally at a concentration within the range of 
from 1 part per million to 1 percent. Typically the impurity-containing 
gaseous argon stream is taken from an industrial process involving the use 
of argon as a blanketing or purge gas, as a heat transfer medium or as an 
atomization carrier gas. The impurities may include one or more of oxygen, 
nitrogen, water vapor, hydrogen, carbon monoxide, carbon dioxide and one 
or more hydrocarbons such as methane, ethane or propane. 
The gaseous impurity-containing argon stream is passed through a bed 
comprising molecular sieves contained in vessel 101. The preferred type of 
molecular sieve is NaX zeolite. Other types of molecular sieves which may 
be employed include NaA, CaA and CaX. Those skilled in the art are 
familiar with molecular sieves and their designations as set forth herein. 
As the gaseous argon stream passes through the molecular sieve bed, water 
vapor and/or carbon dioxide, if present, are adsorbed at ambient 
temperature from the gaseous argon stream onto the molecular sieve bed. 
That is, at least one of water vapor and carbon dioxide are adsorbed onto 
the molecular sieve bed. 
The resulting gaseous argon stream 2 is then passed through a bed 
comprising catalytic material contained in vessel 103. Among the different 
types of catalytic material which may be employed in the bed contained in 
vessel 103 one can name various reduced forms of nickel or cobalt. The 
preferred material comprises extruded pellets containing a high percentage 
of nickel on an alumina-silica support. As the gaseous argon stream passes 
through the bed of catalytic material, oxygen, hydrogen and/or carbon 
monoxide, if present, are chemisorbed at ambient temperature from the 
gaseous argon stream onto the bed of catalytic material. That is, at least 
one of oxygen, hydrogen and carbon monoxide are chemisorbed onto the bed. 
The chemisorption step itself is not catalytic. The chemisorbent is a 
material which under certain conditions, specifically at the regeneration 
temperature, does act as a catalyst for a reaction between at least one of 
the contaminants and another gas which may either be present as an 
adsorbed contaminant or added to the regeneration gas. 
The resulting gaseous argon stream 3 is cooled by passage through heat 
exchanger 108 generally to a cryogenic temperature which is close to its 
dewpoint, generally within the range of from -150.degree. C. to 
-180.degree. C. The resulting gaseous argon stream 4 is then passed 
through a bed comprising adsorbent contained in vessel 105. The preferred 
type of adsorbent is NaX zeolite. Other types of adsorbent which may be 
employed include CaA and CaX. Those skilled in the art are familiar with 
adsorbents and their designations as set forth herein. As the gaseous 
argon stream passes through the adsorbent bed, nitrogen and/or 
hydrocarbons, if present, are adsorbed at a cryogenic temperature from the 
gaseous argon stream onto the adsorbent bed. That is, at least one of 
nitrogen and hydrocarbon are adsorbed onto the bed. 
The resulting stream 5 is purified argon having an argon concentration 
generally of 99.999 percent or more. This purified argon may be recovered 
and may be recycled to the industrial process for reuse. The embodiment 
illustrated in the FIGURE is a preferred embodiment wherein the purified 
argon is condensed for storage and/or for more efficient pressurization if 
higher pressures are desired. In this embodiment purified argon stream 5 
is condensed by passage through heat exchanger 109 by, indirect heat 
exchange with liquid nitrogen 20 supplied to heat exchanger or condenser 
109 from liquid nitrogen storage tank 111. Liquefied argon 6 may then be 
passed to liquid argon storage tank 110. The liquid argon may be withdrawn 
from storage tank 110 as stream 7 and may be pumped to a higher pressure 
by pump 112. In this way, if a higher pressure is desired, the liquid 
pumping raises the pressure of the argon much more efficiently than if 
pressurization of gaseous argon were carried out. Pressurized liquid argon 
8 is then vaporized such as by passage through atmospheric vaporizer 113 
and the resulting argon 9 may be recovered and recycled to the industrial 
process for reuse. 
Gaseous nitrogen 21 resulting from the heat exchange in heat exchanger or 
condenser 109 is warmed to ambient temperature by passage through heat 
exchanger 108 by indirect heat exchange with cooling gaseous argon as was 
previously described thereby recovering additional refrigeration from the 
vaporized nitrogen. The resulting gaseous nitrogen 22 is employed to 
regenerate the molecular sieve bed and the bed of catalytic material. 
As mentioned previously, the embodiment illustrated in the FIGURE employs a 
pair of vessels for each cleaning step, one vessel employed in cleaning 
the gaseous argon while the other undergoes regeneration. Vessel 102 
contains a bed similar so that contained in vessel 101 and vessel 104 
contains a bed similar to that contained in vessel 103. While the beds in 
vessels 101 and 103 are carrying out the aforedescribed cleaning, the beds 
in vessels 102 and 104 are being regenerated by use of regeneration gas 
comprising gaseous nitrogen 22. The regeneration gas is warmed in 
electrical heater 107 during the first part of the regeneration. This gas 
flows countercurrently to the flow during adsorption, entering the bottom 
of vessel 104 and exiting the top of vessel 102, prior to being vented to 
the atmosphere. When the outlet of bed 104 has reached the desired 
desorption temperature, a small amount of externally-supplied hydrogen is 
added to the regeneration gas for a short interval to assist regeneration 
of the chemisorbent. The hydrogen content of the mixed regeneration gas 
during this step is typically about 1 percent. The heating step continues 
after the flow of hydrogen is terminated, until the outlet of adsorber 102 
reaches its desired regeneration temperature. Heater 107 is then 
de-energized and regeneration gas flow continued to cool the two adsorbers 
to near-ambient temperature. The beds in vessels 102 and 104 are then 
ready to be switched into adsorption service. 
In a similar fashion, vessel 106 contains a bed similar to that contained 
in vessel 105. While the bed in vessel 105 is carrying out the 
aforedescribed cleaning, the bed in vessel 106 is being regenerated. The 
initial part of the regeneration is accomplished by closed-loop 
recirculation of argon gas through atmospheric heater 114, blower 115 and 
vessel 106 in a direction countercurrent to the flow during adsorption. 
The gas then returns to the atmospheric heater. When the gas leaving 
vessel 106 approaches ambient temperature, a fraction of the purified 
argon leaving adsorber 105 is diverted through valve 117 to vessel 106, 
and then vented from the cryogenic adsorption system through valve 118. 
This purges the regeneration recirculation loop of the desorbed 
contaminants. 
Cooldown is accomplished by closing valve 118. The flow through valve 117 
is then directed from the top of adsorber 106 through atmospheric heater 
114, blower 115, aftercooler 116 and valve 119 into stream 3 which is the 
feed stream to the cryogenic system. This gas is cooled in heat exchanger 
108 and flows through adsorber 105 with the feed, following which the 
recirculating fraction is split off from the product through valve 117. 
The argon vented through valve 118, to purge the regeneration loop of the 
cryogenic adsorption system, is the only significant argon loss from that 
system. This step is timed to coincide with the last regeneration step of 
the ambient temperature adsorbers. The argon purge replaces the nitrogen 
regeneration gas flow to the latter adsorbers during this interval, 
purging nitrogen from the vessels and thereby preparing for their switch 
to adsorption service. 
The design of the cleaning steps which are employed with the argon 
purification system of this invention are primarily controlled by the 
time-averaged requirement. The amount of each adsorbent which must be 
supplied is a function of the total amount of the associated impurities 
which must be removed during a complete adsorption half-cycle. The bed 
shape can be arranged to constrain pressure drop to an acceptable level at 
the maximum adsorption flowrate. The regeneration system can be designed 
based on the time-averaged adsorbent requirement. This invention thereby 
facilitates economical sizing of equipment. 
Although the invention has been described in detail with reference to a 
certain preferred embodiment, those skilled in the art will recognize that 
there are other embodiments of the invention within the spirit and the 
scope of the claims.