Air purification process

An air prepurification process carried out in a battery of three adsorption vessels arranged in parallel. The process includes three steps: a first step in which non-steady state PSA is carried out in the first and second vessels operated in alternating adsorption and bed regeneration mode while the adsorbent in the third vessel undergoes thermal regeneration; a second step in which non-steady state PSA is carried out in the second and third vessels operated in alternating adsorption and bed regeneration modes while the adsorbent in the first vessel undergoes thermal regeneration; and a third step in which non-steady state PSA is carried out in the first and third vessels operated in alternating adsorption and bed regeneration modes while the adsorbent in the second vessel undergoes thermal regeneration. Each vessel contains at least two adsorbent layers, including a first layer of activated alumina, which adsorbs substantially all moisture and some carbon dioxide from the feed air, and a second layer of zeolite, which adsorbs substantially all of the remaining carbon dioxide in the feed air. The feed air may be passed through beds of hydrogen oxidation and carbon monoxide oxidation catalysts positioned between the first and second layers, to convert any hydrogen and carbon monoxide in the feed air to water vapor and carbon dioxide, respectively, these components being removed from the feed air as it passes through the layer of zeolite.

FIELD OF THE INVENTION 
This invention relates to the purification of gas streams and more 
particularly to the removal of carbon dioxide and water vapor from gas 
streams by adsorption. Specifically, the invention relates to the 
purification of air by the removal of water vapor and carbon dioxide 
therefrom by a combination of pressure swing adsorption (PSA) and 
temperature swing adsorption (TSA). 
BACKGROUND OF THE INVENTION 
In many industrial processes using a gaseous feed stream it is desirable or 
necessary to remove carbon dioxide from the gaseous feed stream prior to 
certain steps of the process. For example, in the separation of 
atmospheric air into its component parts by cryogenic distillation; it is 
necessary to prepurify the air by removing carbon dioxide and water vapor 
from the air feed prior to refrigerating the air; otherwise, these gases 
would condense and freeze in the refrigeration heat exchange equipment and 
eventually clog the equipment, thereby necessitating removal of the 
equipment from service for removal of the frozen carbon dioxide and ice. 
The carbon dioxide and water vapor can be removed from the air by a number 
of techniques. 
One well known method of removing carbon dioxide and water vapor from gas 
streams is by the use of pairs of reversing heat exchangers that are 
operated alternately, such that one heat exchanger is in purification 
service while the other is undergoing frozen carbon dioxide and ice 
removal. Specifically, in this method the gas feed is passed through one 
heat exchanger in exchange with a refrigerant, which causes the carbon 
dioxide and water vapor to freeze onto the surfaces of the heat exchanger. 
When the buildup of frozen carbon dioxide and ice in the heat exchanger 
reaches a certain level, the heat exchanger is taken out of service to 
remove, by sublimation and melting, the frozen carbon dioxide and ice. The 
other heat exchanger of the pair, from which frozen carbon dioxide and ice 
have been removed, is then placed into purification service. This method 
has the disadvantage that a considerable amount of purge gas is required 
to remove the frozen carbon dioxide and ice. 
A popular method of removing carbon dioxide and water vapor from gas 
streams is adsorption. One common adsorption method used for air 
prepurification is PSA using two serially-connected adsorption layers, the 
first layer containing a desiccant, such as silica gel or activated 
alumina for water vapor removal, and the second layer containing a carbon 
dioxide-selective adsorbent, such as sodium-exchanged type X zeolite (13X 
zeolite). Typical two-layer air prepurification PSA processes are 
described in U.S. Pat. Nos. 5,110,569 and 5,156,657, the disclosures of 
which are incorporated herein by reference. This method has a number of 
disadvantages. It is difficult to desorb carbon dioxide from the 13X 
zeolite. The zeolite develops "cold spots" in the upstream region of the 
layer of zeolite adsorbent and the adsorbent loses some of its adsorption 
capacity with time. TSA has also been practiced using this combination of 
layers. U.S. Pat. No. 5,110,569, mentioned above, shows such a process. A 
major disadvantage of the described TSA process is that a great quantity 
of heat energy is required in the adsorbent regeneration step, since both 
layers must be heated sufficiently to drive off the adsorbed moisture and 
carbon dioxide. 
Air prepurification by PSA has also been practiced using a single bed of 
adsorbent which removes both water vapor and carbon dioxide. Such a 
process is disclosed in U.S. Pat. No. 5,232,474, the disclosure of which 
is incorporated herein by reference. The principal disadvantages of this 
method of air prepurification are that it is difficult to efficiently 
produce high purity air (air containing less than 1 ppm carbon dioxide) by 
this method, and a high volume of purge gas is required to effect adequate 
adsorbent regeneration 
Japanese Patent Publication No. Sho 55-27034 discloses an air purification 
process in which moisture and carbon dioxide are removed from air by a 
combination of PSA and TSA in an adsorption system comprising three 
adsorption vessels arranged in parallel. Each of the vessels contains a 
synthetic zeolite adsorbent. At any given time during the process two of 
the adsorption vessels are operated in an alternating PSA cycle while the 
adsorbent in the third vessel is thermally regenerated. Following 
regeneration of the adsorbent in the third vessel one of the other two is 
taken out of PSA service and thermally regenerated and the freshly 
regenerated vessel is put into PSA service with the other vessel that was 
in PSA service. This procedure is repeated continuously throughout the air 
purification process. 
Methods of producing air containing very low levels of water vapor and 
carbon dioxide are continuously sought. The present invention provides a 
method which accomplishes this, and does so with low energy and capital 
expenditures 
SUMMARY OF THE INVENTION 
In its broadest aspect the invention comprises a method of removing water 
vapor and carbon dioxide from a gas in a system comprised of first, second 
and third adsorption sections arranged in parallel. At any given time 
during the process a non-steady state PSA cycle is practiced in two of the 
vessels, while the adsorbent in the third vessel is thermally regenerated 
to drive off carbon dioxide and water vapor (if any) remaining in the 
adsorbent from the previous PSA cycle. Periodically, one of the vessels in 
the PSA cycle is taken out of service and replaced by the thermally 
regenerated vessel. Throughout the process, the three vessels are rotated 
through the above cycle on a regular basis. 
Each section of the system has a first adsorption zone containing activated 
alumina absorbent and a second adsorption zone positioned downstream of 
the first adsorption section and containing a carbon dioxide-selective 
adsorbent other than activated alumina. Specifically the process of the 
invention comprises repeatedly performing the steps: 
(a) subjecting the gas being treated to a cyclic non-steady state PSA 
process comprising alternating adsorption steps and adsorbent regeneration 
steps in the first and second adsorption sections, thereby removing 
substantially all of the water vapor and carbon dioxide from the gas, 
while desorbing water vapor and carbon dioxide from the adsorbent in the 
first and second zones of the third adsorption section by heating the 
adsorbent. In the PSA process the two sections are operated 180.degree. 
out of phase, so that one section is in the adsorption mode while the 
other is in the adsorbent regeneration mode. 
(b) subjecting the gas to the above-described non-steady state PSA process 
in the first and third adsorption sections, thereby removing substantially 
all of the water vapor and carbon dioxide from the gas, while desorbing 
water vapor and carbon dioxide from the adsorbent in the first and second 
zones of the second adsorption section by heating the adsorbent. 
(c) subjecting the gas to the above-described non-steady state PSA process 
in the second and third adsorption sections, thereby removing 
substantially all of the water vapor and carbon dioxide from the gas, 
while desorbing water vapor and carbon dioxide from the adsorbent in the 
first and second zones of the first adsorption section by heating the 
adsorbent. 
The adsorption steps of the PSA cycle are generally carried out at a 
pressure in the range of about 1.5 to about 30 bara, and are preferably 
carried out at a pressure in the range of about 3 to about 30 bara. The 
adsorbent regeneration steps of the PSA cycle are generally carried out at 
a pressure in the range of about 0.15 to about 2 bara, and are preferably 
carried out at a pressure in the range of about 0.3 to about 2 bara. The 
adsorption and adsorbent regeneration steps are generally carried out at a 
temperature in the range of about 0 to about 60.degree. C. The thermal 
regeneration step is generally carried out at a temperature in the range 
of about 75 to about 300.degree. C., and is preferably carried out at a 
temperature in the range of about 100 to about 250.degree. C. 
The duration of each of steps (a), (b) and (c) is generally in the range of 
about 4 hrs to about 16 hrs. 
The process of the invention is particularly useful for removing carbon 
dioxide and, if present, water vapor from air, for example atmospheric 
air. 
Carbon dioxide-selective adsorbents suitable for use in the invention 
include sodium type X zeolite, calcium type X zeolite, alkali-washed 
activated alumina and mixtures of these. A preferred carbon 
dioxide-selective adsorbent is sodium type low silicon X zeolite (NaLSX) 
having a silicon to aluminum ratio in the range of about 0.95 to 1.05 and 
particularly about 1.0. 
When air is purified by the process of the invention, the substantially 
water vapor-free and carbon dioxide-free air can be separated by cryogenic 
distillation process, to produce one or both of a nitrogen-enriched 
product and an oxygen-enriched product. In this case, it is advantageous 
to use a waste gas stream from the cryogenic distillation process to purge 
the adsorbent in the first and second adsorption zones during at least 
part of the adsorbent regeneration steps. The waste gas stream from the 
distillation process can also be used to purge the adsorbent in the first 
and second adsorption zones during at least part of the thermal 
regeneration step. 
The gas being treated may contain hydrogen as an impurity. The hydrogen can 
be removed by passing the gas through a zone containing a hydrogen 
oxidation catalyst positioned between the first and second zones, which 
converts the hydrogen to water vapor. The gas may alternatively, or 
additionally contain carbon monoxide as an impurity. The carbon monoxide 
can be removed by passing the gas through a zone containing a carbon 
monoxide oxidation catalyst positioned between the first and second zones, 
thereby converting the carbon monoxide to carbon dioxide. The water vapor 
and/or carbon dioxide produced in the reaction are removed by the second 
carbon dioxide-selective adsorbent.

DETAILED DESCRIPTION OF THE INVENTION 
PSA is a well known process for separating the components of a mixture of 
gases by virtue of the difference in the degree of adsorption among them 
on a particulate adsorbent. Typically, this process is carried out in one 
or more beds which are operated in a cycle which includes an adsorption 
step and a bed regeneration step. The process is often carried out in 
pairs of adsorbent beds arranged in parallel and cycled 180.degree. out of 
phase, such that one bed is in the adsorption mode while the other bed is 
undergoing bed regeneration. This provides a pseudo-continuous flow of the 
desired product or products, which may be obtained during either or both 
phases of the process. The adsorption step may be carried out at 
atmospheric pressure, but is generally carried out at superatmospheric 
pressure, and the desorption or bed regeneration step is carried out at a 
relatively low pressure or under a vacuum. The PSA cycle may contain other 
steps in addition to the fundamental steps of adsorption and regeneration, 
such as pressure equalization between a bed which has just completed its 
adsorption step and another bed which has just completed its bed 
regeneration step, and partial repressurization with product gas following 
bed regeneration or bed equalization. 
As used herein to describe the condition of a gas stream, the terms 
"substantially water vapor-free" or "substantially free of water vapor" 
mean that the gas stream contains not more than about 1 ppm water vapor. 
Similarly, the terms "substantially carbon dioxide-free" or "substantially 
free of carbon dioxide" mean that the gas stream being described contains 
not more than about 1 ppm carbon dioxide. When "substantially all" of the 
water vapor and/or "substantially all" of the carbon dioxide are removed 
from a gas, the gas is substantially free of water vapor and/or 
substantially free of carbon dioxide. 
The invention will be described as it applies to the prepurificaticn of air 
by the removal of water vapor and carbon dioxide and, optionally, 
hydrocarbons, hydrogen and carbon monoxide therefrom, however it is to be 
understood that the process can be used for the purification of any gas by 
the removal of carbon dioxide therefrom, where the carbon dioxide is more 
strongly adsorbed by the adsorbent that is used than is the gas to be 
purified. The invention can be best understood upon consideration of the 
appended drawing, which shows a system comprising a trio of adsorption 
vessels, A, B and C, arranged in parallel. 
In the drawing, line 2 is connected to a source of air. At a point upstream 
of the system illustrated in the drawing, line 2 may be provided with a 
compressor and a cooler/moisture condenser (not shown). On its downstream 
end line 2 is connected to lines 4, 6 and 8, which are provided with 
valves 10, 12 and 14, respectively. These valve s control flow of feed gas 
into vessels A, B and C. Adsorbed gas discharge lines 16, 18 and 20, 
provided with valves 22, 24 and 26, respectively, are also connected to 
the inlet end of vessels A, B and C. Lines 16, 18 and 20 are connected to 
gas discharge line 28, which may vent directly to the atmosphere or may be 
connected to the inlet end of a vacuum pump (not shown), which, in turn, 
is in communication with a downstream application or an atmospheric vent. 
Vessels A, B and C each have first adsorbent layers 30a, 30b and 30c, which 
comprise activated alumina. The activated alumina layer is used for 
removing most of the water and some or most of the carbon dioxide. 
Positioned in vessels A, B and C above layers 30a, 30b and 30c are 
optional layers 32a, 32b and 32c, respectively, which comprise an 
adsorbent selective for hydrocarbons such as propane, propylene and 
ethylene, for example 4A or 5A zeolite or activated carbon. 
Vessels A, B and C may also contain carbon monoxide oxidation catalyst 
layers 34a, 34b and 34c, which may be upstream or downstream of hydrogen 
oxidation layers 36a, 36b and 36c, when these layers are present. The 
hydrogen oxidation catalyst and the carbon monoxide oxidation layer can be 
combined as a single mixed layer, if desired. The carbon monoxide 
oxidizing agent may be, for example, a metal oxide such as nickel oxide, 
copper oxide, manganese dioxide or a mixtures of two or more of these. 
Furthermore, the metal oxide may be supported on a porous substrate. The 
preferred metal oxide catalyst is a mixture of copper oxide and manganese 
dioxide. 
Vessels A, B and C also optionally contain hydrogen oxidation catalyst 
layers 36a, 36b and 36c, which are shown positioned above optional layers 
34a, 34b and 34c, respectively. These catalysts may be noble metal-based 
materials, such as palladium- or platinum-based compositions, and, if 
desired, they may be mounted on an inert support, such as alumina. 
Also contained in vessels A, B and C, downstream of the hydrogen and carbon 
monoxide oxidation catalyst layers 36a, 36b and 36c (when these are 
present), are carbon dioxide-selective adsorbent layers 38a, 38b and 38c, 
respectively. These adsorbent layers are intended to adsorb any carbon 
dioxide that passes through the activated alumina layer in vessels A, B 
and C and any carbon dioxide and water vapor produced by the oxidation of 
carbon monoxide and hydrogen in optional layers 34a, 34b, 34c, 36a, 36b 
and 36c. Suitable carbon dioxide-selective adsorbents include the 
alkali-washed activated aluminas such as selexsorb.RTM. COS from Alcoa 
Chemical, and sodium-exchanged type X zeolite. The preferred carbon 
dioxide adsorbent is sodium-exchanged LSX, i.e. sodium-exchanged type X 
zeolite having a silicon-to-aluminum atomic ratio in the range of about 
0.95 to about 1.1. 
The height of the CO.sub.2 adsorbent layer can be reduced significantly 
(reducing the cost and regeneration requirements for this layer) by using 
an adsorbent with high CO.sub.2 capacity at a very low CO.sub.2 partial 
pressure. Experiments were run with 10 ppm CO.sub.2 in nitrogen at a 
pressure of 35 psig, a temperature of 35.degree. C. and a flow rate of 100 
SCFH. The bed diameter was 1.625" and the bed length was 18". CO.sub.2 
capacities for various adsorbents for these conditions are listed below. 
______________________________________ 
CO.sub.2 
Adsorbent Capacity, wt % 
______________________________________ 
UOP 4A zeolite (8 .times. 12 mesh) 
0.14 
UOP 13X zeolite (8 .times. 12 mesh) 
0.20 
13X zeolite with Si/Al = 1.25 (8 .times. 12 mesh), NaMSX 
0.65 
13X zeolite with Si/Al = 1.00 (8 .times. 12 mesh), NaLSX 
1.00 
Alkali-washed activated alumina (7 .times. 14 mesh), 
1.20 
Alcoa Selexsorb .RTM. COS 
______________________________________ 
The last three adsorbents are the preferred adsorbents for the carbon 
dioxide layer of this invention. 
The outlet ends of vessels A, B and C are connected to product gas 
discharge lines 40, 42 and 44, which are provided with valves 46, 48 and 
50, respectively. Lines 40, 42 and 44 are connected to product line 52. 
Purge gas line 54, which is connected to a source of purge gas, such as a 
waste gas stream from a downstream cryogenic distillation unit, is also 
connected to PSA regeneration purge gas line 56 and thermal regeneration 
purge gas line 58. Line 56, which is provided with valve 60, communicates 
with the outlet ends of vessels A, B and C through lines 62, 64 and 66, 
respectively. Lines 62, 64 and 66 are provided with valves 68, 70 and 72. 
Line 58 passes through heater 86 and is connected to the outlet ends of 
vessels A, B and C, through lines 74, 76 and 78, respectively. Lines 74, 
76 and 78 are provided with valves 80, 82 and 84, respectively. 
Vessels A, B and C may be provided with support screens (not shown) 
positioned beneath layers 30a, 30b and 30c and top screens (not shown) 
positioned above layers 38a, 38b and 38c. The support screens are 
typically displaced from the bottom of vessels A, B and C to provide gas 
distribution chambers for the feed gas entering these vessels. Gas 
collection spaces are typically provided above the top screens. 
The various layers are preferably contained in single vessels, as shown in 
the drawing, although each layer or selected groups of layers may be 
contained in separate vessels, if desired. 
The activated alumina in layers 30a, 30b and 30c serves to remove 
substantially all of the moisture and a considerable portion of the carbon 
dioxide that is contained in the gas being treated, and, as noted above, 
the adsorbent in layers 38a, 38b and 38c serves to remove all remaining 
carbon dioxide, including carbon dioxide produced by the catalytic 
oxidation of carbon monoxide in layers 36a, 36b and 36c and any moisture 
produced by the oxidation of hydrogen in layers 34a, 34b and 34c. It is 
desirable to remove all free moisture contained in the incoming feed gas 
in layers 30a, 30b and 30c to permit the use of a small bed of highly 
efficient carbon dioxide-selective adsorbent in layers 38a, 38b and 38c. 
It is even more important to remove substantially all moisture from the 
incoming gas when the adsorption vessels contain hydrogen and/or carbon 
monoxide oxidation layers, because moisture causes degradation of the 
catalysts used in these layers. 
In the system used for the process described below, each of vessels A, B 
and C contains a first layer of activated alumina, a second layer of 
activated carbon, a third layer comprising a carbon monoxide oxidation 
catalyst such as Carulite-300 from Carus Chemical, a fourth layer of 
palladium-coated alumina and a fifth layer of alkali-washed alumina or 
sodium LSX zeolite. The duration of each cycle of the PSA stage is, at 
most, several minutes, while the duration of the thermal regeneration is 
generally several hours; accordingly, during any single phase of the 
process the two vessels in the PSA mode will undergo many PSA alternations 
while the third vessel undergoes a single thermal regeneration step. For 
purposes of description, it will be assumed that the PSA process is 
carried out with pressurization to superatmospheric pressure during the 
adsorption step and reduction of pressure to atmospheric pressure during 
the bed regeneration step. The pressure in the vessel undergoing thermal 
regeneration is assumed to be at or near atmospheric pressure. 
The process described below comprises three phases; a first phase, in which 
vessels A and B are initially operating in an alternating non-steady state 
PSA cycle and the adsorbent in vessel C is undergoing thermal 
regeneration; a second phase, in which vessels A and C are operating in an 
alternating non-steady state PSA cycle while the adsorbent in vessel B 
undergoes thermal regeneration; and a third phase, in which vessels B and 
C are operating in an alternating non-steady state PSA cycle while the 
adsorbent in vessel A undergoes thermal regeneration. 
At the start of stage 1 of the first phase of the process, one of vessels A 
or B, for example vessel A, is in the adsorption mode and the other vessel 
(vessel B) is in the adsorbent regeneration mode. With vessel A starting 
in the adsorption mode, valves 10, 24, 26, 60, 82 and 84 are initially 
open and all other valves are closed. Atmospheric air is compressed and 
cooled and introduced into vessel A through lines 2 and 4. When the 
pressure in vessel A reaches the desired adsorption pressure, valve 46 is 
opened and gas flows through vessel A at the selected adsorption pressure. 
As the feed air passes cocurrently (in the direction from the feed end 
towards the nonadsorbed gas outlet end of the vessels) through layer 30a, 
substantially all water vapor (if the air contains any) and part of the 
carbon dioxide contained in the air are adsorbed by the activated alumina. 
The air passes out of layer 30a substantially free of water vapor and 
containing a small amount of carbon dioxide and it next passes through 
layer 32a which removes any hydrocarbons such as ethylene, propylene or 
propane from the gas. It then passes through layers 34a and 36a, and as it 
does so, any hydrogen and carbon monoxide that is present in the air is 
oxidized to water vapor and carbon dioxide. The air then passes through 
carbon dioxide adsorbent layer 38a containing, for example, sodium LSX, 
which removes all water vapor generated in layer 34a and all carbon 
dioxide remaining in the gas. The purified air, now substantially free of 
hydrogen, water vapor, carbon monoxide and carbon dioxide, passes out of 
vessel A through line 40 and leaves the system through line 52. 
Meanwhile, regeneration purge gas that is free of moisture carbon dioxide 
and hydrocarbons is introduced into the system through line 54. As noted 
above, the regeneration gas may be a waste stream from a downstream 
cryogenic distillation unit or from other air separation equipment. Part 
of the regeneration gas flows through line 56, and then flows 
countercurrently (in the direction opposite to the flow of feed gas 
through the vessels) through the layers of adsorbent and catalyst in 
vessel B. As it passes through layer 38b it desorbs at least part of the 
water vapor and carbon dioxide contained in this layer. The gas then flows 
through layers 36b and 34b and then through layer 32b, where it desorbs 
hydrocarbons accumulated in the activated carbon, then through layer 30b, 
where it desorbs water vapor and carbon dioxide from the alumina 
adsorbent. The purge gas, together with the gas components desorbed from 
the adsorbents in vessel B, pass out of vessel B and leave the system 
through lines 18 and 28. Although not necessary, the purge gas may be 
heated prior to its use in the PSA process. 
The remainder of the purge gas entering the system through line 54 flows 
through line 58, is heated in heater 86 and then flows countercurrently 
through the layers in vessel C. As the heated purge gas passes through the 
layers in vessel B, it desorbs from the various layers the residual carbon 
dioxide, water vapor and/or hydrocarbons from different layers that have 
gradually built up in this vessel over the previous two PSA stages carried 
out in this vessel. The regeneration gas, together with the desorbed 
carbon dioxide, water vapor and hydrocarbons, leaves vessel C through line 
20 and exits the system through line 28. 
As the PSA adsorption step proceeds, the adsorption fronts in layers 30a, 
32a and 38a advance toward the outlet end of these layers. Prior to 
breakthrough of the sorbed gases from these layers, stage 1 of the first 
phase of the PSA cycle is terminated and stage 2 of the first phase 
begins. 
During stage 2, vessel B, which has completed its PSA adsorbent 
regeneration phase, is put into adsorption service and the adsorbents in 
vessel A are regenerated. The changeover is accomplished by having open 
valves 12, 22, 26, 60, 80 and 84 and having all other valves closed. 
Atmospheric air is compressed and cooled and introduced into vessel B 
through lines 2 and 6. When the pressure in vessel B reaches the desired 
adsorption pressure, valve 48 is opened and gas flows through vessel A. 
Feed air now passes cocurrently through layers 30b, 32b, 34b, 36b and 38b 
which remove substantially all water vapor, hydrocarbons, carbon monoxide, 
hydrogen, and carbon dioxide from the air. The dry purified air next 
passes out of vessel B through line 42 and leaves the system through line 
52, as in stage 1 of this first phase of the process. Also during this 
stage 2 of phase 1, regeneration gas flows through lines 54, 56, 62 and 
countercurrently through the layers of adsorbent and catalyst in vessel A, 
whereupon it desorbs hydrocarbons from layer 32a and water vapor and 
carbon dioxide from layers 38a and 30a. The purge gas, together with the 
gas components desorbed from vessel A, passes out of vessel A through line 
16 and leaves the system through line 28. 
Upon completion of stage 2 of the first phase, stage 1 of this phase begins 
again. This procedure is continuously repeated during the first phase. 
Thermal regeneration of vessel C continues for the duration of this phase. 
As the PSA cycle proceeds in vessels A and B, water vapor and/or carbon 
dioxide gradually build up in 30a and 30b and/or in 38a and 38b, and the 
level of hydrocarbon may build up in layers 32a and 32b. The buildup will 
be more advanced in the layers of vessel A, which has been in continuous 
PSA service longer than vessel B. When the buildup of these components in 
one or more of the vessel A layers reaches the point where it threatens to 
adversely affect the efficiency of the gas purification process, the first 
phase of the process is terminated and the second phase is started. 
The gradual build-up of residual carbon dioxide results from the non-steady 
state PSA cycle that is being carried out. By "non-steady state PSA 
process" is meant that the PSA process is carried out in an adsorption 
vessel under conditions such that the amount of carbon dioxide remaining 
on the adsorbent in the vessel upon completion of the adsorbent 
regeneration step of a given PSA cycle ("residual carbon dioxide") is 
greater than the amount of residual carbon dioxide remaining on the 
adsorbent in that vessel upon completion of the adsorbent regeneration 
step of the PSA cycle immediately preceding the given PSA cycle. While 
carbon dioxide removal during PSA regeneration cycle is non-steady state, 
water removal is expected to be under steady state (no water build-up 
between cycles) since the amount of purge gas needed for water removal is 
much smaller than that needed for steady state removal of carbon dioxide. 
Non-steady state operation results, for example, by using less purge gas 
than is necessary to regenerate the adsorbent to the extent necessary to 
maintain steady state operation, i.e. to reduce the residual carbon 
dioxide to approximately the same extent in each cycle of the PSA process, 
or by not evacuating the adsorption vessel sufficiently during adsorbent 
regeneration to maintain steady state operation. The advantages of 
operating the PSA cycle on a non-steady state basis are that smaller 
adsorption beds can be used in the process, regeneration energy savings 
are realized and less regeneration gas is required because the beds are 
not regenerated to the full extent during the PSA cycles of the process. 
During the second phase of the process vessels B and C are in alternating 
PSA service and the adsorbents in vessel A undergo thermal regeneration. 
At the start of stage 1 of the second phase of the process, one of vessels 
B or C, for example vessel B, is in the adsorption mode and the other 
vessel (vessel C) is in the adsorbent regeneration mode. With vessel B in 
the adsorption mode, valves 12, 22, 26, 60, 80 and 84 are initially open 
and all other valves are closed. Atmospheric air is compressed and cooled 
and introduced into vessel B through lines 2 and 6. When the pressure in 
vessel B reaches the desired adsorption pressure, valve 48 is opened and 
gas flows through vessel B at the adsorption pressure. The feed air now 
passes cocurrently through vessel B, and as it does so water vapor, 
hydrocarbons, hydrogen, carbon monoxide and carbon dioxide are removed 
therefrom. The purified air passes out of vessel B through line 42 and 
leaves the system through line 52. 
Meanwhile, regeneration gas that is free of moisture, carbon dioxide and 
hydrocarbons continues to flow into the system through line 54. Part of 
the regeneration gas flows through line 56, and then flows 
countercurrently through the layers of adsorbent and catalyst in vessel C. 
As it passes through vessel C it sweeps water vapor and carbon dioxide 
from the adsorbent in layers 38b and 30b and hydrocarbons from the 
adsorbent in layer 32b The purge gas, together with the gas components 
desorbed from the adsorbents in vessel C, passes out of vessel C and 
leaves the system through lines 20 and 28. 
The remainder of the purge gas entering the system through line 54 flows 
through line 58, is heated in heater 86 and then flows countercurrently 
through the layers in vessel A. As the heated purge gas passes through 
vessel A, it desorbs residual carbon dioxide and water vapor from layers 
38a and 30a and residual hydrocarbons from layer 32a. The regeneration 
gas, together with any desorbed carbon dioxide, water vapor and 
hydrocarbons, leaves vessel A through line 16 and exits the system through 
line 28. 
When the adsorption fronts in one or more of layers 30b, 32b and 38b reach 
the desired end point, prior to breakthrough, stage 1 of the second phase 
of the PSA cycle is terminated and the second phase stage 2 begins. 
During stage 2, vessel C, which has completed its PSA adsorbent 
regeneration phase, is put into adsorption service and the adsorbents in 
vessel B are regenerated. The changeover is accomplished by initially 
having open valves 14, 22, 24, 60, 80 and 82 and all other valves are 
closed. Atmospheric air is compressed and cooled and introduced into 
vessel B through lines 2 and 8. When the pressure in vessel B reaches the 
desired adsorption pressure, valve 50 is opened and gas flows through 
vessel C at the adsorption pressure. Feed air now passes cocurrently 
through layers 30c, 32c, 34c, 36c and 38c, which remove substantially all 
water vapor, hydrocarbons, carbon monoxide, hydrogen, and carbon dioxide 
from the air. The dry purified air passes out of vessel C through line 44 
and leaves the system through line 52. Also during this stage 2 of phase 
2, regeneration gas flows through lines 54, 56, 64 and countercurrently 
through the layers of adsorbent in vessel B, whereupon it desorbs 
hydrocarbons from layer 32b and water vapor and carbon dioxide from layers 
38b and 30b. The purge gas, together with the gas components desorbed from 
vessel B, passes out of vessel B through line 18 and leaves the system 
through line 28. 
Upon completion of stage 2 of the second phase, stage 1 begins again. This 
procedure is continuously repeated during the second phase. Thermal 
regeneration of vessel A continues for the duration of the second phase. 
As the PSA cycle proceeds in vessels B and C, water vapor and/or carbon 
dioxide gradually build up in 30b and 30c and/or in 38b and 38c, and the 
level of hydrocarbon may build up in layers 32b and 32c. The buildup will 
be more advanced in vessel B, which has been in continuous PSA service 
longer than vessel C. When the buildup of these components in one or more 
of the vessel B layers reaches the point where it threatens to adversely 
affect the efficiency of the gas purification process, the second phase of 
the process is terminated and the third phase is started. 
During the third phase of the process vessels A and C are in PSA service 
and the adsorbent in vessel B undergoes thermal regeneration. 
In stage 1 of the third phase of the process, one of vessels A or C, for 
example vessel C, is in the adsorption mode and the other vessel (vessel 
A) is in the adsorbent regeneration mode. With vessel C starting in the 
adsorption mode, valves 14, 22, 24, 60, 80 and 82 are initially open and 
all other valves are closed. Atmospheric air is compressed and cooled and 
introduced into vessel C through lines 2 and 8. When the pressure in 
vessel C reaches the desired adsorption pressure, valve 50 is opened and 
gas flows through vessel C at the adsorption pressure. The feed air now 
passes cocurrently through vessel C, and as it does so, water vapor, 
hydrocarbons, hydrogen, carbon monoxide and carbon dioxide are removed 
therefrom. The purified air passes out of vessel B through line 44 and 
leaves the system through line 52. 
Meanwhile, regeneration gas that is free of moisture, carbon dioxide and 
hydrocarbons continues to flow into the system through line 54. Part of 
the regeneration gas flows through lines 56 and 74, and then flows 
countercurrently through the layers of adsorbent and catalyst in vessel A. 
As it passes through vessel A it sweeps water vapor, carbon dioxide from 
the adsorbent in layers 38a and 30a and hydrocarbons from the adsorbent in 
layer 32a. The purge gas, together with the gas components desorbed from 
the adsorbents in vessel A, passes out of vessel A and leaves the system 
through lines 16 and 28. 
The remainder of the purge gas entering the system through line 54 flows 
through line 58, is heated in heater 86 and then flows countercurrently 
through the layers in vessel B. As the heated purge gas passes through 
vessel B, it desorbs residual carbon dioxide and water vapor from layers 
38b and 30b and residual hydrocarbons from layer 32b. The regeneration 
gas, together with the desorbed carbon dioxide and water vapor, leaves 
vessel B through line 18 and exits the system through line 28. 
When the adsorption fronts in one or more of layers 30c, 32c and 38c reach 
the desired end point, prior to breakthrough, stage 1 of the third phase 
of the PSA cycle is terminated and the third phase stage 2 begins. 
During stage 2, vessel A, which has completed its PSA adsorbent 
regeneration phase, is put into adsorption service and the adsorbents in 
vessel C are regenerated. The changeover is accomplished by initially 
having open valves 10, 24, 26, 60, 82 and 84 and all other valves are 
closed. Atmospheric air is compressed and cooled and introduced into 
vessel A through lines 2 and 4. When the pressure in vessel A reaches the 
desired adsorption pressure, valve 46 is opened and gas flows through 
vessel A at the adsorption pressure. Feed air now passes cocurrently 
through layers 30a, 32a, 34a, 36a and 38a, which remove substantially all 
water vapor, hydrocarbons, carbon monoxide, hydrogen, and carbon dioxide 
from the air. The dry purified air next passes out of vessel A through 
line 40 and leaves the system through line 52. Also during this stage 2 of 
phase 3, regeneration gas flows through lines 54, 56, 66 and 
countercurrently through the layers of adsorbent in vessel C, whereupon it 
desorbs hydrocarbons from layer 32c and water vapor and carbon dioxide 
from layers 38c and 30c. The purge gas, together with the gas components 
desorbed from vessel C, passes out of vessel C through line 20 and leaves 
the system through line 28. 
Upon completion of stage 2 of the third phase, stage 1 begins again. This 
procedure is continuously repeated during the third phase. Thermal 
regeneration of vessel B continues for the duration of the third phase. 
As the PSA cycle proceeds in vessels A and C, water vapor arid/or carbon 
dioxide gradually build up in 30a and 30c and/or in 38a and 38c, and the 
level of hydrocarbons may build up in layers 32a and 32c. The buildup will 
be more advanced in vessel C, which has been in continuous PSA service 
longer than vessel A. When the buildup of these components in one or more 
of the vessel C layers reaches the point where it threatens to adversely 
affect the efficiency of the gas purification process, the third phase of 
the process is terminated and the first phase is started over again. The 
above-described cycle continues throughout the run. 
Thermal regeneration of the adsorbent beds of this invention removes 
impurities which accumulate due to non-steady state operation of the PSA 
beds and the impurities that are not removed in the PSA regeneration. 
These include carbon dioxide in activated alumina layer which accumulates 
during the PSA operation, hydrocarbons in the hydrocarbon layer, 
chemisorbed impurities in the catalyst layer and CO.sub.2 /H.sub.2 O 
impurities in the top carbon dioxide layer. There may be a small heel of 
water left in the activated alumina layer during PSA operation but there 
should not be any build-up of water during PSA operation. The thermal 
regeneration of the beds may be stopped before the water remaining in the 
activated alumina layer starts desorbing. 
Compared to a standard TSA process this process does not need thermal 
energy for the removal of water adsorbed on activated alumina layer which 
constitutes a significant portion of the overall energy requirement. Also, 
energy required for CO.sub.2 removal is substantially reduced. Compared to 
a standard PSA process, this process has a significantly higher sieve 
specific product which reduces both the vessel size and vent loss leading 
to substantial savings. Compared to the three bed process described in 
Japanese Patent Sho 55-27034, the sieve specific product of this process 
is higher, leading to capital and power savings. Also, this process 
provides for the removal of carbon monoxide, hydrogen and hydrocarbons 
impurities not provided for in Sho 55-27034. 
It will be appreciated that it is within the scope of the present invention 
to utilize conventional equipment to monitor and automatically regulate 
the flow of gases within the system so that it can be fully automated to 
run continuously in an efficient manner. 
The invention is further illustrated by the following example in which, 
unless otherwise indicated, parts, percentages and ratios are on a volume 
basis. 
EXAMPLE 1 (COMATIVE) 
Experiments were carried out using activated alumina H-156 from Alcoa 
Chemical Co. in 7.times.14 mesh size. A bed with a cross-sectional area of 
0.1 ft.sup.2 was filled with 65 inches of this adsorbent. Steady state PSA 
experiments were carried out with feed air at 85 psia and 40.degree. C. 
containing 350 ppm carbon dioxide and saturated with water vapor. The PSA 
cycle used was as follows: 
______________________________________ 
Step Time 
______________________________________ 
Bed pressurization with impurity-free gas 
240 sec 
Air purification 900 sec 
Depressurization 48 sec 
Purge with impurity free gas 
612 sec 
Cycle Time 30 minutes 
______________________________________ 
Nitrogen free of CO.sub.2 and H.sub.2 O impurities was used for both the 
bed pressurization step and the purge step. The ratio of purge flow (in 
std cubic ft per min) to feed flow (in std cubic ft per min) was 0.5. The 
standard conditions refer to a temperature of 70.degree. F. and a pressure 
of one atm. For about 5 ppb CO.sub.2 in the product at steady state, the 
sieve specific product defined as air flow/amount of adsorbent in all the 
beds was 12 std. cfm/ft.sup.3. 
EXAMPLE 2 
The experiment described in Example 1 was run using the same adsorbent and 
the same cycle time. Prior to the start of the experiment the bed was 
thermally regenerated (at 200.degree. C. with 7.5 std cfm nitrogen) to 
remove any impurities from the bed. The purge flow to the feed flow ratio 
was the same as in Example 1. For feed air at 85 psia and 40.degree. C. 
containing 350 ppm carbon dioxide and saturated with water vapor and 
starting with a thermally regenerated bed, the feed flow rate was varied 
until the product CO.sub.2 concentration at the end of eight hours (16 
cycles) was 5 ppb. The sieve specific product in this case (defined as air 
flow rate/amount of sieve in all three beds) was 18 std cfm/ft.sup.3 or 
about 50% higher than the steady state PSA operation. The vent loss was 
reduced by more than 65% in this case compared to the steady state PSA 
case. Product CO.sub.2 concentration for the steady state case (Example 1) 
and the non-steady case of this invention are compared in Table 1. 
TABLE I 
______________________________________ 
Comparison of Product CO.sub.2 Profiles for Steady and 
Non-Steady Operation 
Prod. CO.sub.2 for 
Prod. CO.sub.2 for 
Time, Example 1, Example 2, 
hrs End of Cycle # 
ppb ppb 
______________________________________ 
0.50 1 5.0 &lt;1 
1.00 2 5.0 &lt;1 
1.50 3 5.0 &lt;1 
2.00 4 5.0 &lt;1 
2.50 5 5.0 &lt;1 
3.00 6 5.0 &lt;1 
3.50 7 5.0 &lt;1 
4.00 8 5.0 &lt;1 
4.50 9 5.0 &lt;1 
5.00 10 5.0 &lt;1 
5.50 11 5.0 &lt;1 
6.00 12 5.0 &lt;1 
6.50 13 5.0 &lt;1 
7.00 14 5.0 2.0 
7.50 15 5.0 3.0 
8.00 16 5.0 5.0 
______________________________________ 
EXAMPLE 3 
The bed used in Example 2 was filled with 59" of Alcoa H-156 activated 
alumina and 6" of 13X zeolite with silicon to aluminum ratio of 1.25 
(NaMSX). Prior to the start of the experiment the bed was thermally 
regenerated (at 200.degree. C. with 7.5 std cfm nitrogen) to remove any 
impurities from the bed. The bed was operated in the PSA mode with the 
same cycle as in Example 1. The purge flow to the feed flow ratio (at 
standard conditions) was 0.34. For feed air at 85 psia and 40.degree. C. 
containing 350 ppm carbon dioxide and saturated with water vapor and 
starting with a thermally regenerated bed, the feed flow rate was varied 
until the product CO.sub.2 concentration at the end of eight hours (16 
cycles) was 5 ppb. The sieve specific product in this case (defined as air 
flow rate/amount of sieve in all three beds) was 20 std cfm/ft.sup.3. By 
using a layer of an adsorbent with high CO.sub.2 capacity near the bed 
outlet, an improvement in sieve specific product of over 11% was obtained 
over Example 2 and an improvement of about 67% was obtained over Example 
1. 
EXAMPLE 4 
The bed in Example 2 was filled with 51" of Alcoa H-156 activated alumina, 
4" of Carulite-300 from Carus Chemical, 4" of 0.5%-Pd-on-activated alumina 
catalyst from Engelhard Corp. and 6" of 13X zeolite with silicon to 
aluminum ratio of 1.25. Prior to the start of the experiment the bed was 
thermally regenerated (at 200.degree. C. with 7.5 std cfm nitrogen) to 
remove any impurities from the bed. The bed was operated in the PSA mode 
with the same cycle as in Example 1. The purge flow to the feed flow ratio 
(at standard conditions) was 0.34. For feed air at 85 psia ard 40.degree. 
C. containing 350 ppm carbon dioxide and saturated with water vapor and 
starting with a thermally regenerated bed, the feed flow rate was varied 
until the product CO.sub.2 concentration at the end of eight hours (16 
cycles) was 5 ppb. The sieve specific product in this case (defined as air 
flow rate/amount of sieve in all three beds) was 18.5 std cfm/ft.sup.3. 
The product contained less than 5 ppb each of impurities consisting of CO, 
CO.sub.2, H.sub.2 and H.sub.2 O. 
This example illustrates the removal of impurities other than CO.sub.2 and 
H.sub.2 O by non-steady operation of the 3-bed cycle of this invention. 
Although the invention has been described with particular reference to 
specific equipment arrangements and to specific experiments, these 
features are merely exemplary of the invention and variations are 
contemplated. For example, The PSA cycles of the process may include other 
steps, such as bed equalization and nonadsorbed product gas backfill. 
Also, the vessels may contain fewer layers, or the order of the layers may 
be changed, subject to the recommendation that the alumina layer be 
adjacent the vessel inlets and the carbon dioxide-selective layer be 
adjacent the vessel outlets. Furthermore, the PSA cycle can be operated in 
a vacuum swing adsorption (VSA) cycle with adsorption carried out at or 
above atmospheric pressure and bed regeneration conducted under a vacuum, 
with or without a purge. The scope of the invention is limited only by the 
breadth of the appended claims.