PSA process with dynamic purge control

Oxygen of uniform purity is produced in a two-bed air-fed oxygen pressure swing adsorption process in which the beds are operated out of phase. The steps of the adsorption cycle include a pressurization/production step and a bed regeneration step, with the bed undergoing regeneration being purged with a low pressure stream of the oxygen-enriched gas produced as the nonadsorbed product of the process. The oxygen concentration in the purged gas effluent is continuously periodically monitored, and the maximum oxygen concentration in the effluent during selected purge steps is compared with the maximum oxygen concentration in the effluent during a previous purge step, and the difference is used to adjust the timing and duration of a purge step following the selected purge step in a manner that reduces the difference between the oxygen concentration in the sequential purge steps.

FIELD OF THE INVENTION 
This invention relates to a multiple bed pressure swing adsorption (PSA) 
process which includes a purge period as part of its cycle, and more 
particularly to a method of controlling the cycle carried out in each bed 
of a multiple bed PSA system to minimize variation of the nonadsorbed gas 
composition produced by the process. 
BACKGROUND OF THE INVENTION 
PSA processes have been employed for many years to separate the components 
of a gas mixture. PSA processes are carried out in an elongate vessel 
which has a feed gas inlet end and a nonadsorbed gas outlet end and which 
is packed with an adsorbent which preferentially adsorbs one or more of 
the components of the gas mixture. The gas mixture is passed cocurrently 
(from feed gas inlet to nonadsorbed gas outlet) through the vessel, 
thereby removing the preferentially adsorbed component from the gas 
stream. A product gas enriched in the component or components that are not 
preferentially adsorbed passes through the adsorbent bed and exits the bed 
through the nonadsorbed gas outlet. The adsorbed component initially 
accumulates at the inlet end of the bed, and as the adsorption step 
proceeds, the adsorbed component forms a front which gradually moves 
toward the nonadsorbed outlet end of the bed. When the adsorbed gas front 
reaches a certain point in the bed the adsorption step is terminated and 
the adsorbent is regenerated by desorbing the adsorbed component from the 
bed. This is generally accomplished by countercurrently depressurizing the 
bed, and/or by countercurrently purging the bed with nonadsorbed component 
gas. When the bed is regenerated to the desired extent, the cycle is 
repeated. 
A typical PSA cycle includes a pressurization step, in which the pressure 
in the adsorption vessel is raised to the pressure at which it is desired 
to conduct the adsorption step of the process, by introducing a gas 
(usually the gas mixture being separated) into the vessel; an adsorption 
or production step; and a bed regeneration step. The cycle may include 
other steps, such as multiple pressurization and depressurization steps. 
In conventional PSA processes, the adsorption step is generally conducted 
at moderate to high pressures, e.g. pressures in the range of about 5 to 
about 20 bar, absolute(bara), and the bed regeneration step is frequently 
carried out at or below atmospheric pressure. These processes are 
generally efficient and result in the production of consistently high 
purity nonadsorbed gas product. Such processes are, however, energy 
intensive, since considerable energy must be expended to compress the feed 
gas to the adsorption step operating pressure. 
Low pressure PSA processes with low adsorption pressure to regeneration 
pressure ratios have recently been developed. These processes are 
generally operated with adsorption pressures up to one to three bara and 
bed regeneration pressures of about atmospheric pressure. The feed gas can 
be easily pressurized to these pressures by means of low energy equipment, 
such as blowers, and since the bed is regenerated at atmospheric pressure 
there is no need to use high energy vacuum generating equipment. In such 
low pressure processes, it is common to use a portion of the nonadsorbed 
gas product produced in each cycle to purge the bed of adsorbed gas 
component in order to enhance process performance. 
It is highly desirable that the variation of quality of product gas 
produced in the various beds of a multiple bed adsorption system processes 
be very low. However, when multiple bed adsorption systems are used for 
adsorption processes that are carried out at low adsorption pressure to 
vent pressure ratio operating conditions, e.g. 3 bara/atmospheric 
pressure, significant variations in product quality are experienced. This 
does not present a problem when the equipment is used for moderate or high 
pressure adsorption processes, since the variation of product quality 
diminishes with increasing adsorption pressures. 
U.S. Pat. No. 4,472,177 discloses a vacuum swing adsorption process for 
producing oxygen and nitrogen from an air stream. According to the 
disclosure of this patent, nitrogen is adsorbed from air which is at near 
ambient pressure to produce oxygen as nonadsorbed product. After 
completion of the adsorption step of the process nitrogen is passed 
through the beds to rinse oxygen from the void spaces in the beds. The 
rinse step is terminated when low oxygen is detected in the purge gas 
effluent. 
Copending U.S. patent application Ser. No. 08/189,008, filed Jan. 28, 1994, 
now U.S. Pat. No. 5,490,871, discloses a process for preventing excessive 
loss of purge gas in a PSA process by analyzing the purged waste gas 
stream from the process for purge gas, and terminating the purge step when 
the concentration of purge gas in the waste stream reaches a preselected 
volume of the waste gas stream. 
Because of the attractiveness of low pressure adsorption processes, 
improvements that reduce product quality variation in multiple vessel 
systems are constantly sought. This invention presents an efficient and 
cost effective method of accomplishing this goal. 
SUMMARY OF THE INVENTION 
The invention comprises a cyclic PSA process for recovering a first gas 
component from a gas mixture containing the first gas component and a 
second gas component in a PSA system comprised of two or more beds of 
adsorbent which more strongly adsorbs the second gas component than the 
first gas component. The beds of the system are arranged in parallel and 
operated out of phase such that at least one bed is in adsorption service 
while at least one other bed is being regenerated. A partial cycle of the 
process of the invention comprises at least the following steps: 
(a) flowing the gas mixture into one or more beds that have just completed 
bed regeneration, thereby pressurizing the bed(s) to a selected adsorption 
pressure, usually in the range of about atmospheric pressure to about 20 
bara, and producing a nonadsorbed product enriched in the first gas 
component. At the same time, one or more of the other beds of the system 
undergo regeneration by causing a gas enriched in the second gas component 
to be desorbed from these beds at a selected vent pressure, which is lower 
than the adsorption pressure. During at least part of the bed regeneration 
period the beds being regenerated are purged with the nonadsorbed product. 
Step (a) is repeated with the roles of the beds in adsorption service and 
the beds undergoing regeneration being changed until all the beds of the 
system have undergone the partial cycle of steps (a) to (c), thus 
completing a full cycle. The full cycle described above is repeatedly 
carried out so that the process is a substantially continuous cycle. 
(b) periodically determining the absolute difference between the 
concentration of first gas component in the waste gas stream from the 
bed(s) being regenerated at the time of occurrence of a specific event 
during bed regeneration of one or more beds and the concentration of first 
gas component in the gas stream exiting these beds at the time of 
occurrence of the specific event during an earlier regeneration of one or 
more other beds, and 
(c) periodically adjusting the duration of the purge period of one or both 
of the one or more beds and the one or more other beds in a manner that 
will reduce the absolute difference between the concentration of first gas 
component in the beds. 
In one preferred embodiment the first component is oxygen and the second 
component is nitrogen, and in another preferred embodiment the first 
component is preferably nitrogen and the second component is preferably 
oxygen. In the most preferred embodiment the gas mixture being processed 
is air. 
The specific event of the process may be the occurrence of an extreme 
concentration of first component in the gas stream exiting the bed(s) 
being regenerated or the passage of a specific period of time after 
initiation of the purge period. In a preferred embodiment, the specific 
event is the occurrence of an extreme concentration of first component in 
the gas stream exiting the beds being regenerated. In the most preferred 
embodiment, the specific event is the occurrence of a maximum 
concentration of first component in the gas stream exiting the bed(s) 
being regenerated. 
The determination of absolute difference between the concentration of 
nonadsorbed gas component in the vent stream may be made during each bed 
regeneration step or during selected bed regeneration steps. Similarly, 
the adjustment of the duration of the purge period may be made in every 
bed regeneration period or in selected bed regeneration periods. 
Furthermore, the absolute difference determination may be made more 
frequently than the adjustment of the purge period. The periods of 
regeneration in which adjustment step (c) is put into effect can be 
separated by a fixed or variable number of periods of regeneration in 
which no adjustment of purge period is made. In a preferred embodiment, 
the purge period is adjusted only when the absolute difference determined 
in step (b) exceeds a selected value. 
The determination of absolute difference in nonadsorbed gas in the vent 
stream is based on a comparison of measurements made when one bed is being 
regenerated and at an earlier time when another bed is being regenerated. 
The comparison may be based on measurements made during consecutive bed 
regeneration periods or on measurements made during nonconsecutive bed 
regeneration periods. Similarly, the purge period adjustment may be put 
into effect in the bed regeneration period immediately following the 
regeneration period in which absolute difference determination is made, or 
at some subsequent bed regeneration period. 
The adjustment of the purge period may be made to one bed of the system, or 
it may be made to two or more beds. In a preferred embodiment, the 
adjustment is made to two beds, and in cases where more than two beds are 
sequentially operated, it is preferred to adjust the two most divergent 
beds, i.e. those in which the difference in concentration of nonadsorbed 
gas in the vent stream is greatest. 
The process may have as an additional step prior to step (a), the step of 
partially depressurizing the bed(s) entering the regeneration period and 
partially pressurizing the bed(s) entering the adsorption step by flowing 
gas from the former beds to the latter beds (bed equalization). The 
process may also have as an additional step prior to step (a), the step of 
partially pressurizing the bed(s) entering the adsorption step by flowing 
first component-enriched gas into the bed(s) (product backfill), while at 
the same time, partially depressurizing the bed(s) entering the 
regenerating period by venting gas from these beds. Further, the process 
may have both of these steps, with the bed equalization step preceding the 
product backfill step, and the product backfill step preceding step (a). 
The process of the invention is preferably carried out in a pair of twin 
beds, or in multiple pairs of twin beds. 
When the gas being treated is air, the adsorbent is preferably a synthetic 
or natural zeolite. Preferred adsorbents for the separation of air are 
synthetic zeolites selected from type X and type A zeolites. The zeolite 
may have as exchangeable cations, ions selected from Group 1A, Group 2A, 
Group 3A, Group 3B, and the lanthanide series of the Periodic Table. 
Preferred zeolites are type X zeolites selected from lithium-exchanged 
type X zeolite, calcium-exchanged type X zeolite, lithium- and calcium- 
exchanged type X zeolite and combinations of these. 
In the most preferred embodiment of the invention, the process is a PSA 
process for recovering oxygen-enriched gas from air in a system comprised 
of two beds of adsorbent which more strongly adsorb nitrogen than oxygen, 
the beds being arranged in parallel and operated out of phase such that 
one bed is in adsorption service while the other bed is being regenerated. 
The steps of this most preferred process include: 
(a) passing air through the first bed at an adsorption pressure in the 
range of about atmospheric pressure to about 3 bar, absolute, thereby 
producing oxygen-enriched gas as a nonadsorbed product, while regenerating 
the second bed by desorbing nitrogen therefrom at a pressure lower than 
said adsorption pressure, the second bed being purged with oxygen-enriched 
gas during at least part of the regenerating period, and the concentration 
of oxygen in the stream exiting the second bed being monitored during at 
least part of the regenerating period; 
(b) repeating steps (a) to (c) with the roles of the first and second beds 
being reversed. 
(b) periodically determining the absolute difference between the maximum 
concentration of oxygen in the gas stream exiting the second bed during a 
given regenerating period with the maximum concentration of oxygen in the 
gas stream exiting the first bed during a preceding regenerating period, 
and 
(c) adjusting the duration of the purge in one or both beds during a 
subsequent bed regeneration period in a manner that will reduce the 
absolute difference between the maximum concentration of oxygen in the gas 
stream exiting the second bed during the current regenerating period and 
the maximum concentration of oxygen in the gas stream exiting the first 
bed in the next succeeding regenerating period.

Only equipment, valves and lines necessary for an understanding of the 
invention have been included in the drawing figures. 
DETAILED DESCRIPTION OF THE INVENTION 
The invention may be practiced using any pressure swing adsorption system 
comprising two or more adsorbent-containing vessels arranged in parallel 
and operated out of phase. For example, the system may consist of a single 
pair of adsorption beds, multiple pairs of beds that alternate through 
various phases of an adsorption cycle, or three or more beds that are 
operated in sequence as a set. The system is operated in repeating cycles, 
a cycle being completed when each bed of the series passes once through 
each step of the adsorption cycle sequence. The term "partial cycle" is 
used herein to describe the part of a cycle in which one adsorption bed 
has sequenced through all of the steps of the process. When the process is 
carried out in two alternating beds, a partial cycle is a half-cycle, and 
when three beds are used in series in the process, a partial cycle is a 
one third-cycle. There is one bed regeneration period in a partial cycle. 
The steps of the adsorption cycle include, as a minimum, a bed 
pressurization step, an adsorption or production step and a bed 
regeneration step. The pressurization step may be carried out in one or 
more stages, including one or more of a bed equalization step, in which 
cocurrent expansion gas from a bed that has just completed its adsorption 
step is countercurrently introduced into a bed that has just completed its 
bed regeneration step; as a second stage, a product backfill step, in 
which nonadsorbed product gas is countercurrently introduced into the bed 
being pressurized and a feed pressurization step, in which the gas mixture 
being processed is cocurrently introduced into the bed. Preferred cycles 
include, as a first pressurization stage, a bed equalization step, and as 
a final pressurization stage, a feed pressurization step. A product 
backfill pressurization step may be substituted for either the 
equalization step or the feed pressurization step, or it may be sandwiched 
between the bed equalization step and the feed pressurization step. In a 
similar manner, bed regeneration may be carried out using multiple 
depressurization steps, including the counterpart of the above-described 
bed equalization pressurization step, and a countercurrent 
depressurization or vent step. 
A key step of the adsorption cycle, insofar as this invention is concerned, 
is the purge or rinse step. During this step nonadsorbed product gas 
obtained from the nonadsorbed product gas storage vessel, or from another 
bed that is in its pressurization or production step, is passed through 
the bed being regenerated at low pressure. This step may begin upon or 
after initiation of the feed pressurization step and may continue for part 
or all of the remainder of the bed regeneration step. 
The process of the invention makes it possible to consistently produce a 
nonadsorbed gas product stream that is consistent in composition, while 
maintaining the amount of nonadsorbed gas that is lost during the purge 
period to a minimum. It accomplishes by sensing the concentration of 
nonadsorbed gas in the purge stream exiting the adsorption vessel at the 
time of occurrence of a particular event during a bed regeneration period. 
As mentioned above, the event may be the occurrence of an extreme 
concentration of nonadsorbed gas in the purge effluent during a bed 
regeneration period, i.e. the occurrence of a maximum concentration of 
nonadsorbed gas in the purge effluent or the occurrence of a minimum 
concentration of nonadsorbed gas, i.e. the occurrence of a maximum 
concentration of desorbed gas, in the purge effluent. The event may also 
be the passage of a specific period of time after the purge step occurs, 
or even the occurrence of a point of inflection of the curve representing 
the concentration of nonadsorbed gas in the purge effluent from a bed, 
plotted against time. 
The preferred event is the occurrence of a maximum concentration of 
nonadsorbed gas in the purge effluent. Using this event produces the most 
rapid correction of disparity of nonadsorbed product purity occurring 
between or among the partial cycles of a PSA process. 
The process of the invention can be used to separate any gas that is weakly 
adsorbed by an adsorbent from any other gas that is more strongly adsorbed 
by the adsorbent. Typical of the separations that can be effected by the 
process of the invention include oxygen-nitrogen separations, carbon 
dioxide-methane or carbon dioxide-nitrogen separations, hydrogen-argon 
separations and olefin-paraffin separations. The process is particularly 
suitable for the separation of nitrogen or oxygen from a nitrogen- and 
oxygen-containing gas, such as air, especially separations in which the 
adsorbent adsorbs nitrogen more strongly than oxygen. 
Although the invention can be used in PSA processes in which the adsorption 
step is carried out at higher pressures, for example separations conducted 
at pressures up to about 20 bara or higher, it is most advantageously used 
in processes in which the pressure during the adsorption step does not 
exceed about 5 bara. In general, the invention is applied with beneficial 
results to PSA processes in which the adsorption is carried out at 
pressures in the range of just above atmospheric pressure to about 5 bara, 
and is most suitable for use in processes in which the adsorption pressure 
is in the range of about 1.25 to about 3 bara. Low pressure processes are 
preferred because the pressure of the feed gas to the system can be easily 
increased to the range at which it is desired to conduct the adsorption 
step by means of a low energy-consuming device, such as a blower. The bed 
regeneration step is carried out at pressure lower than the adsorption 
pressure, and although it can be carried out at pressures as low as 200 
millibar, absolute or less, it is preferable to avoid vacuum pressures, 
and to conduct this step at about atmospheric pressure or above, to avoid 
the use of high energy-consuming vacuum generating equipment. The 
adsorption to regeneration pressure ratio of the process of the invention 
is generally in the range of about 1.1 to about 3 and is preferably in the 
range of about 1.2 to about 2.5, on an absolute pressure basis. 
The temperature at which the adsorption step is carried out is not 
critical, and, in general, can vary from a temperature of about 
-50.degree. C. to a temperature of about 100.degree. C., or higher. The 
optimum adsorption temperature of the process will depend, inter alia, 
upon the particular adsorbent being used, the pressure at which the 
process is carried out and the specific gases being separated. 
For convenience, the invention will be described in detail as it is applied 
to the separation of oxygen-enriched product gas using an adsorbent that 
preferentially adsorbs nitrogen relative to oxygen, in a two bed system, 
with the beds being operated 180.degree. out of phase in alternating 
half-cycles, such that one bed is in adsorption service while the other 
bed is undergoing bed regeneration. It is to be understood, however, that 
such a system is only exemplary of systems suitable for practicing the 
process of the invention. A system suitable for practice of the invention 
is illustrated in FIG. 1. Referring now to FIG. 1, there is shown therein 
an adsorption system comprising parallel adsorption units A and B, each of 
which contains an adsorbent which selectively adsorbs nitrogen from air, 
oxygen-enriched product gas storage reservoir C, oxygen sensor D and 
programmable logic controller (PLC) E. Feed air is provided to adsorbers A 
and B through feed line 2. Line 2 joins feed lines 4 and 6, which, in turn 
are connected to the feed inlet of adsorption units A and B, respectively. 
Lines 4 and 6 are respectively fitted with valves 8 and 10, so that feed 
air can be alternately directed into adsorption units A and B. On their 
nonadsorbed product outlet ends, adsorption units A and B are joined to 
oxygen-enriched product gas discharge lines 12 and 14, respectively. Lines 
12 and 14 are fitted with valves 16 and 18, respectively, which provide 
for the selective removal of oxygen-enriched product gas from either one 
of adsorption units A and B. Lines 12 and 14 connect to line 20 at a point 
between valves 16 and 18. Line 20, in turn, is connected to 
oxygen-enriched product gas reservoir C. Flow of oxygen-enriched gas 
through line 20 is controlled by valve 21. Oxygen-enriched product gas can 
be discharged from reservoir C to product storage or to an end use 
application, as desired, through line 22. 
Oxygen-enriched product purge gas can be provided to adsorbers A and B via 
line 24, which can be placed in fluid communication with lines 12 and 14 
through valves 28 and 30, respectively. Line 24 is provided with pressure 
reducing means 26 to reduce the pressure of the nonadsorbed product gas to 
the pressure at which the purge step is to be carried out. 
The nonadsorbed product end of adsorbers A and B can be placed in fluid 
communication through adsorber pressure equalization line 32. Flow of gas 
through line 32 can be effected by opening valve 34. 
The illustrated system is provided with adsorption unit vent line 36 so 
that desorbed nitrogen-enriched waste gas can be removed from adsorption 
units A and B. Vent line 36 communicates with lines 4 and 6, respectively, 
through valves 38 and 40. 
Oxygen sensor D measures the oxygen concentration in waste gas line 36 via 
test line 42 either continuously or at selected times at a frequency of 
e.g. about 0.01 to about 5 seconds, and it transmits the collected analog 
information to PLC E via line 44. The concentration may be an 
instantaneous value or an averaged value. The sampling frequency and the 
number of samplings used in the averaging (if an average value is used) 
are so chosen to optimize both sensitivity and accuracy of the collected 
concentration information. PLC E analyzes the information received from 
oxygen sensor D and sends signals to the various valves of the system 
through lines connecting PLC E with the valves, thereby controlling the 
opening and closing of the valves. Signals sent through line 48 control 
the operation of valves 8 and 10; signals sent through line 50 control the 
operation of valves 38 and 40; a signal sent through line 52 controls the 
operation of valve 34; signals sent through line 54 control the operation 
of valves 28 and 30; signals sent through line 56 control the operation of 
valves 16 and 18; and a signal sent through line 58 controls the operation 
of valve 21. 
In the process of the invention, PLC E makes a determination of the 
absolute difference in the concentration of nonadsorbed gas in the purge 
effluent by comparing the nonadsorbed gas concentration in the purge 
effluent from one bed of the system in a selected partial cycle with the 
nonadsorbed gas concentration in the purge effluent from another bed of 
the system in a partial cycle earlier than the selected partial cycle. The 
earlier partial cycle may be the partial cycle immediately preceding the 
selected partial cycle, or it may be a partial cycle prior to the partial 
cycle immediately preceding the selected partial cycle. In preferred 
embodiments of the invention, the determination is based on a comparison 
of the nonadsorbed gas concentration in the purge effluent in consecutive 
partial cycles. This method provides the most up-to-date information and 
is thus most reliable. 
PLC E may be set to make adjustments to selected valves in any of several 
patterns. For example, it may be instructed to make adjustments to only 
purge valves 28 and 30, so that only the duration of the purge step is 
altered, or it may make adjustments of valves 8 , 10, 16, 18, 28 and 30 to 
alter both the duration of the production step and that of the purge step 
in a given partial cycle. When simultaneous adjustments are to be made to 
more than one bed of a system comprising three or more beds sequenced 
through partial cycles in serial order, it is preferred to adjust those 
beds whose concentrations of nonadsorbed gas are most divergent from the 
average concentration of nonadsorbed gas in the purge effluent from the 
beds during the regeneration steps of the cycle. Other adjustment 
combinations may be employed if desired. 
The adjustments may be made in a particular sequence of partial cycles or 
at random. For example, valve adjustments may be made in each partial 
cycle that follows a concentration difference determination, or in every 
second partial cycle, or in every third partial cycle, etc after a 
concentration difference determination is made. The adjustments may be 
made in evenly spaced partial cycles, i.e. adjustments may be made in 
partial cycles that are separated by a fixed number of partial cycles in 
which no adjustment is made; or in variably spaced partial cycles, i.e., 
adjustments are made in partial cycles that are separated by a variable 
number of cycles in which no adjustment is made. In either case there are 
preferably 0 to about 5 partial cycles in which no adjustment is made 
separating those partial cycles in which an adjustment is made. 
The PLC E may be set to make an adjustment of the purge duration of a bed 
or adjustments of various steps of the PSA process only when the disparity 
between the nonadsorbed gas concentration in the purge gas effluent from 
different beds of the system exceeds a selected value. This method of 
correcting nonadsorbed gas purity inconsistency is often preferable to 
other methods since it may result in fewer corrections and less chance of 
overcorrection. 
The following table shows one of the many adsorption cycles in which the 
invention can be beneficially used. Each step of the cycle is described in 
detail as it applies to the production of oxygen-enriched gas from air in 
the apparatus illustrated in FIG. 1. 
TABLE I 
______________________________________ 
Step Adsorber A Adsorber B 
______________________________________ 
1 Equalization Equalization 
2 Feed Pressurization 
Vent Depressurization 
3 Feed Pressurization 
Purge/Vent 
4 Production Purge/Vent 
5 Production Vent 
6 Equalization Equalization 
7 Vent Depressurization 
Feed Pressurization 
8 Purge/Vent Feed Pressurization 
9 Purge/Vent Production 
10 Vent Production 
______________________________________ 
At the beginning of step 1, adsorber A in FIG. 1 has just completed the 
regeneration phase and adsorber B has just completed the production phase 
of the cycle; thus, the pressure in adsorber A is at the lowest point and 
the pressure in adsorber B is at the highest point of the pressure cycle. 
During step 1, valve 34 is open and all other valves of the system are 
closed. Depressurization gas flows cocurrently out of adsorber B, through 
line 32 and countercurrently into adsorber A, thereby partially 
pressurizing adsorber A. The purpose of this step is to conserve some of 
the pressure energy that is stored in the adsorber that has just completed 
production and to recover some of the relatively oxygen-rich void space 
gas contained in adsorber B prior to regenerating this adsorber. Although 
this step is designated as an equalization step, it is not necessary that 
the step be continued until complete equalization between adsorbers A and 
B is effected. 
Upon completion of step 1, valve 34 is closed and valves 8 and 40 are 
opened. Step 2, the feed pressurization/vent pressurization step, begins 
and adsorber A now undergoes further partial pressurization by the 
cocurrent flow of fresh feed through line 2, valve 8 and line 4 and into 
adsorber A. Simultaneously, nitrogen-rich gas is countercurrently desorbed 
from the adsorbent in vessel B by allowing the gas to vent through line 6, 
valve 40 and line 36. 
Upon completion of step 2, valves 8 and 40 remain open, valves 16 and 30 
are opened and pressure controller 26 adjusts the pressure of 
oxygen-enriched gas flowing through line 24 to the desired purge pressure, 
and step 3 begins. All other valves remain closed during step 3. During 
this step, pressurization of adsorber A continues with fresh feed until 
the pressure in adsorber A reaches the desired adsorption pressure. 
Meanwhile, countercurrent purging of adsorber B begins with the flow of 
oxygen-enriched gas through line 12, valve 16, lines 20 and 24, valve 30, 
line 14 and into adsorber B at purge pressure. The purge gas passes 
through vessel B and exits the system through line 6, valve 40 and line 
36, carrying with it nitrogen-enriched gas that has been desorbed from the 
adsorbent contained in adsorber B. 
Upon pressurization of adsorber A to the desired adsorption pressure, step 
3 ends and step 4 begins. During step 4, valves 8, 16, 30 and 40 remain 
open and valve 21 is opened. All other valves remain closed. 
Oxygen-enriched product gas is now produced in adsorber A and transferred 
to vessel C through line 12, valve 16, line 20 and valve 21. Meanwhile, 
vessel B continues to be purged by the passage of low pressure 
oxygen-enriched product gas into vessel B via line 24, valve 30 and line 
14, and out of vessel B and into the atmosphere through line 6, valve 40 
and line 36. 
Upon purging of vessel B to the desired extent, step 4 is terminated. At 
this point valve 30 is closed and valves 8, 16, 21 and 40 remain open. 
Flow of purge gas through adsorber B is stopped, but adsorber B continues 
to vent through open valve 40 and line 36. Meanwhile, feed air continues 
to enter adsorber A and oxygen-enriched product gas continues to flow out 
of adsorber A to vessel C. As the feed air passes through adsorber A, the 
adsorbed nitrogen gas front moves forward in vessel A and approaches the 
nonadsorbed product outlet of this vessel. When the front reaches a 
certain point in adsorber A, step 5 is terminated by the closing of valves 
8, 16, 21 and 40. The termination point is optimally near the nonadsorbed 
product gas outlet of the adsorber, so that the adsorption system can be 
operated with maximum efficiency. Substantial breakthrough of nitrogen 
from adsorber A is avoided to prevent reduction of the purity of the 
oxygen-enriched product gas to below the acceptable minimum level. At the 
conclusion of step 5, the first half of the cycle of the process of the 
invention is completed. 
The second half of the cycle is carried out by reversing the functions 
conducted in adsorbers A and B during steps 1-5, by manipulation of valves 
corresponding to the valves operated during the respective preceding 
steps. Thus, during step 6, only valve 32 is open and equalization gas 
flows from adsorber A to adsorber B; during step 7, only valves 10 and 38 
are open, and fresh feed flows into adsorber B through lines 2 and 6 while 
adsorber A undergoes countercurrent vent depressurization; during step 8, 
only valves 10, 18, 28 and 38 are open, and adsorber B is pressurized to 
operating pressure with fresh feed gas while adsorber A undergoes purge 
and venting; during step 9, only valves 10, 18, 21, 28, and 38 are open 
and adsorber B continues to produce oxygen-enriched gas and adsorber A 
continues to undergo purging and venting; and during step 10 only valves 
10, 18, 21 and 38 are open, and adsorber B continues to produce 
oxygen-enriched gas while under adsorber A undergoes final venting. At the 
conclusion of step 10 the current cycle is completed and the next cycle 
begins with adsorber A in adsorption service and adsorber B undergoing 
regeneration. 
The above cycle is a typical operating cycle which makes possible the 
production of a relatively high purity, e.g. as high as 93% by volume pure 
or higher, oxygen-enriched product stream. As a variation of this process, 
the cycle can be modified by eliminating the equalization steps (steps 1 
and 6) and/or by adding product backfill steps. In these variations the 
pressurization formerly provided by the equalization steps can be provided 
by the product backfill steps and/or the feed pressurization steps. 
Furthermore, the purge gas for the purge/vent operations of all of steps 3 
and 8 can be provided from vessel C, if desired. 
The key feature of the invention is the measurement of the oxygen 
concentration in the waste gas passing through line 36 during the 
purge/vent and final vent steps of each partial cycle. This measurement is 
preferably made continuously during these steps, although this is not 
strictly necessary, the important thing being the measurement of the 
oxygen concentration in the waste gas during the happening of a specific 
event. This event may be the occurrence of an extreme oxygen concentration 
in the gas stream, which, for purposes of this invention, is defined as 
the maximum oxygen concentration or a minimum oxygen concentration 
occurring during the period that includes both the purge/vent and the 
final vent steps of a partial cycle. The event may also be the passage of 
a specific period of time after initiation of the purge step of the 
partial cycle. For purposes of this invention the term "event" is used to 
denote the occurrence of a maximum oxygen concentration in the waste gas 
stream during a partial cycle, the occurrence of a minimum oxygen 
concentration in the waste gas stream during a partial cycle or the 
passage of a specific period of time after initiation of the purge 
sequence of a partial cycle. 
The oxygen concentration measurement made at the exact time of the 
occurrence of the event during a partial cycle is transmitted to PLC E. 
PLC E compares the value of the measurement made during the current 
partial cycle with the value of the corresponding measurement made during 
the partial cycle immediately preceding the current cycle. PLC E then 
adjusts the total duration of the purge step of the partial cycle 
immediately succeeding the current partial cycle in the direction that 
will tend to reduce the difference between the critical measurement of the 
current partial cycle and that of the immediately preceding partial cycle. 
The adjustment may entail advancing or retarding the starting time and/or 
the ending time of the purge sequence of the partial cycle immediately 
following the current partial cycle. This process is repeated for each 
partial cycle of the process, thus resulting in a dynamically controlled 
adsorption process. 
The preferred event is the occurrence of an extreme value of oxygen 
concentration, and the most preferred event is the occurrence of the 
maximum oxygen concentration during the purge and vent steps of a partial 
cycle. Use of the maximum value provides the most rapid reduction of 
oxygen concentration differences between successive partial cycles. 
It can be appreciated that modification of the duration of the purge 
sequence of a partial cycle entails modification of one or more of the 
other steps of a partial cycle and/or the duration of the partial cycle. 
For example, increasing the duration of the purge sequence of a partial 
cycle will require shortening the extent of the vent step preceding the 
purge sequence and/or the vent step following the purge sequence and/or 
increasing the duration of the entire partial cycle. In preferred 
embodiments it is preferred to adjust the duration of the vent steps and 
keep the total partial cycle time constant. 
The adsorption cycle may contain steps other than the fundamental steps of 
adsorption and regeneration. For example, it may be advantageous to 
depressurize the adsorption bed in multiple steps, with the first 
depressurization product being used to partially pressurize one bed and 
the second depressurization product being used to partially pressurize 
another bed in the adsorption system. 
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 
This experiment was carried out in a two-bed laboratory-scale PSA system 
similar to the system illustrated in FIG. 1, using atmospheric air as 
feed. The adsorbent used in the beds was UOP PSA O.sub.2 HP 13X zeolite, 
sold by UOP. The entire PSA system was enclosed in a controlled 
environment chamber maintained at a constant temperature of 20.degree. C. 
(the feed gas temperature). The operating cycle was the similar to the 
cycle set forth in Table I. The duration of the cycle was 120 seconds, 
distributed as follows: steps 1 and 6--5 secs; steps 2 and 7--6 secs; 
steps 3 and 8--24 secs; steps 4 and 9, 25 secs, and steps 5 and 10--0 secs 
(i.e. steps 5 and 10 were eliminated). The production steps were carried 
out at adsorption pressure of 0.5 bar, gauge (barg) and the vent pressure 
was atmospheric pressure (0 barg). 
During the course of the experiment, the duration of steps 2, 3, 7 and 8 
were adjusted automatically in response to readings made by a PLC. An 
oxygen sensor measured the oxygen concentration in the vent gas during the 
purge/vent steps. Comparisons of the maximum oxygen concentrations in the 
vent stream in consecutive half-cycles were made, and the durations of the 
above-mentioned steps were adjusted in the direction that would reduce the 
difference oxygen concentration maximum in the purge/vent steps of the 
next half-cycle. Steady state conditions were attained. The steady state 
results are tabulated in Table II. A profile of the oxygen concentration 
in the vent stream during a single cycle of the test run is illustrated in 
FIG. 2 as curve A. 
TABLE II 
______________________________________ 
Oxygen Purity, % 92.1 
Specific Product, Nm.sup.3 /m.sup.3 
4.9 
Oxygen Yield, % 22.6 
______________________________________ 
EXAMPLE II 
The Procedure of Example I was repeated except that no attempt was made to 
adjust differences in the maximum oxygen concentration occurring during 
purge/vent steps. Steady state conditions could not be established, and 
the purity of the oxygen product gas fluctuated between 60% and 90% during 
the test period. A profile of the oxygen concentration in the vent stream 
during a single cycle is illustrated in FIG. 2 as curve B. 
FIG. 2 illustrates the benefit attained by use of the invention in a low 
pressure PSA process. As shown in FIG. 2, the variation of oxygen 
concentration in the vent stream from the system was much less when the 
procedure of the invention was used (curve A) compared with the oxygen 
concentration variation in the vent stream when the procedure of the 
invention was not used. 
Although the invention has been described with particular reference to 
specific adsorption cycles, and to specific experiments, these features 
are merely exemplary of the invention and variations are contemplated. For 
example, the adsorption cycle may include more than two bed equalization 
steps, and the invention can be used in PSA processes used to separate 
gases other than oxygen and nitrogen. Furthermore, the duration of the 
individual steps and the operating conditions may be varied. The scope of 
the invention is limited only by the breadth of the appended claims.