Optimal pressure swing adsorption refluxing

Segregated external gas storage tanks are used to store gases of varying purity for use in the purge and pressure equalization and product repressurization steps of pressure swing adsorption operations, thereby enabling the bed size factor and the power requirements of pressure swing adsorption-gas separation operations to be significantly reduced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to pressure swing adsorption processing for the 
separation of gas mixtures. More particularly, it relates to enhanced 
efficiency in the use of pressure swing adsorption processing for the 
large scale production of oxygen from air. 
2. Description of the Prior Art 
Pressure swing adsorption (PSA) processes are well known for use in air or 
other gas separation operations. Such PSA processing generally includes a 
processing sequence comprising: (1) adsorption, with feed gas being passed 
to the feed end of an adsorbent bed at an upper adsorption pressure for 
the selective adsorption of a more readily adsorbable component, and with 
discharge of a less readily adsorbable component from the product end of 
the bed; (2) desorption, with depressurization of the adsorbent bed from 
the upper adsorption pressure to a lower desorption pressure, and with 
discharge of the more readily adsorbable component from the bed; (3) 
purging, by the passing of a purge gas to the adsorbent bed to facilitate 
the removal of said more readily adsorbable component from the adsorbent 
bed; (4) repressurization, with the pressure of the bed being increased 
from its lower desorption pressure to the upper adsorption pressure, and 
(5) passage of additional quantities of feed gas to the adsorbent at the 
upper adsorption pressure in step (1) as the processing sequence is 
continued on a cyclic basis. Such PSA processing is disclosed in the 
Skarstrom patent, U.S. Pat. No. 2,944,627, and a wide variety of 
processing variations are known in the art for the modification of the 
basic adsorption/depressurization/purge/repressurization sequence for 
various purposes. 
Wagner, U.S. Pat. No. 3,430,418, discloses an adsorption system having at 
least four adsorbent beds wherein, as part of the desorption step in each 
bed, void gas, generally comprising the less readily adsorbable component, 
is released from the product-end of the bed and passed to the product end 
of another bed in the system initially at a lower pressure to equalize the 
pressure between the beds at an intermediate pressure level. Following 
such cocurrent depressurization-pressure equalization step, the bed is 
countercurrently depressurized from the intermediate pressure to a lower 
pressure with release of more readily adsorbable component from the feed 
end of the bed. The Doshi patent, U.S. Pat. No. 4,340,398, discloses a PSA 
process using three or more adsorbent beds, wherein void gas is passed 
from the product end of a bed, not directly to another bed, but to a 
storage tank from which gas is passed to a bed for repressurization 
purposes. Likewise, Krishnamurthy et al., U.S. Pat. No. 4,816,039, 
discloses the use of one or more storage tanks in a two-bed PSA system. 
Following direct pressure equalization between two beds, the patent 
discloses the passage of additional void gas from the product end of the 
bed being depressurized to at least one storage tank. Following 
regeneration of the bed at the lower desorption pressure, the void gas is 
returned from the tank to the bed for pressure equalization purposes. 
Recovery of the less readily adsorbable component product gas is enhanced 
due to a decrease in the loss of void space gas during subsequent 
countercurrent depressurization and purge steps. 
In the Yamaguchi et al. patent, U.S. Pat. No. 5,258,059, a PSA process and 
system are described in which at least three adsorbent beds are employed, 
with direct bed-to-bed pressure equalization being carried out during the 
depressurization/repressurization portion of the processing cycle. A 
holding column, i.e., a segregated storage tank, of a feed-in/feed-out 
sequence returning type, is used for storing void space gas recovered 
during a cocurrent depressurization step of the cycle, with release of gas 
from the product end of the bed. This void space gas is then used for 
purging the adsorbent bed during the bed regeneration portion of the 
cycle. The holding column is specifically designed to prevent gas from 
mixing therein, i.e., an impurity concentration gradient is maintained in 
the holding column. 
In currently used PSA cycles, the adsorbent bed undergoing a pressure 
equalization-pressure rising step receives product gas with decreasing 
purity levels from another bed currently on the make product step, i.e., 
the cocurrent depressurization portion of the overall make product step 
that includes the feed-upper adsorption pressure step and the cocurrent 
depressurization step. Consequently, at the end of this pressure 
equalization-pressure rising step, the lowest purity gas is at the product 
end of the bed. In addition, the gas used for purging the adsorbent bed is 
of decreasing purity when it is obtained from another bed currently on the 
make product step. If the purge gas were obtained from a product storage 
tank, a constant purity purge gas would be available. 
It should also be noted that, in order to maintain desired product purity 
in prior art PSA cycles, the production and pressure equalization-falling 
steps must be terminated much earlier than the time required before the 
impurity front of more readily adsorbable component breaks through from 
the product end of the bed. As a result, the adsorptive capacity of the 
adsorbent bed is not fully utilized. Furthermore, using less readily 
adsorbable gas of decreasing purity during the purging, pressure 
equalization-rising, and repressurization steps, results in additional 
contamination of the product end of the bed, due to the use of the lowest 
purity product gas at the end of these bed refluxing steps. This added 
contamination of the product end of the bed results in a significant 
reduction in product purity in the early stage of the make product step, 
and causes a decrease in the average purity of the less readily adsorbable 
product gas. In addition, by using product gas of decreasing purity, the 
spreading of the mass transfer zone within the bed is undesirably 
enhanced. Furthermore, in order to contain the mass transfer zone and 
maintain product purity, more adsorbent material is required, resulting in 
a higher bed size factor, and a more costly overall PSA process. 
In a typical prior art pressure equalization cycle, the PSA process 
comprises the following sequence: 
(I) Feed (air) pressurization (FP) to an upper adsorption pressure level. 
(II) Adsorption and gross product production (AD). 
(III) Depressurization-Equalization falling (EQ) (cocurrent), wherein the 
gas is transferred to another bed that is undergoing the equalization 
rising step (EQ). 
(IV) Depressurization/Evacuation (EV) to waste (countercurrent) at a lower 
desorption pressure. 
(V) Depressurization/Evacuation to waste while purging (PG) 
(countercurrently). 
(VI) Equalization rising step (EQ), wherein the gas is supplied by another 
bed undergoing the equalization falling step (step III). 
In another prior art product pressurization cycle, the gas required for 
purging and repressurization, i.e., refluxing, comes from another bed 
undergoing the adsorption/production step. In this mode of operation, the 
purge gas is obtained from another bed at an early stage of the adsorption 
step, with product gas being obtained from the bed during a later stage of 
said adsorption step. Since the effluent purity decreases with time as the 
impurity front of more readily adsorbable component approaches a 
breakthrough condition, a higher purity gas is used for purging than for 
product repressurization. Ideally, however, it would be desirable to use 
the lowest purity gas at the start of the purging step, followed by the 
use of product gas of increasing purity in the latter stages of such 
purging step. However, due to the mode of operation in such prior art PSA 
cycles, it is very difficult to arrange for the use of the highest purity 
gas last. Consequently, in order to maintain a given product purity, the 
percentage of the total cycle time allocated to the production of the less 
readily adsorbable component product gas is reduced, with a concomitant 
and undesired increase in bed size factor and power consumption. 
In order to use the lowest purity gas at the start of the purge step, 
followed by product gas of increasing purity during the rest of the 
refluxing steps, it is necessary to produce multiple purity products, so 
that the highest purity gas can be used last. However, during the 
production step at the upper adsorption pressure, the purity of the gas 
removed from the product end of the bed decreases with time. Thus, the 
purity of the gas recovered is initially high and gradually decreases to a 
lower level. Thus, there is a need in the art for a means to reverse this 
purity order, and for the production of multiple purity products. 
Since multiple purity products are required for refluxing and bed 
repressurization, the PSA cycle becomes inherently more complicated. In 
one approach to this problem, the use of two storage tanks has been 
considered, so that, at different times in the production step (b), the 
effluent gas can be directed to different storage tanks. In such a mode of 
operation, the time allocated for each storage tank to receive effluent 
gas controls the quantity of each purity gas collected. However, the use 
of more than one storage tank adds to the complexity and the capital cost 
of the PSA process, particularly since additional valves and associated 
piping are required thereby. 
Alternatively, a single segregated storage tank can be used to store 
multiple purity products. In such a tank, no mixing of the product gas is 
allowed, and one end contains the lowest purity gas and the other end 
contains the highest purity gas. Such segregated storage tanks can be of 
the type described in the Yamaguchi et al. patent referred to above or can 
be a tank packed with layers of adsorbent(s) or inert materials, or simply 
an empty tank containing baffles to suppress mixing. 
It will be appreciated from the above that there is a need in the art for 
the development of PSA processing improvements to enable gases of 
increasing purity to be used in purging at lower desorption pressure, 
pressure equalization-rising, and bed repressurization to the upper 
adsorption pressure. Such improvements would serve to lower the bed size 
factor and the power consumption required as compared to the requirements 
of prior art PSA processing cycles. 
It is an object of the invention, therefore, to provide a process for using 
gas of increasing purity in various steps of bed regeneration to lower the 
bed size factor and power consumption requirements of a PSA operation. 
It is another object of the invention to provide a process in which gas of 
increasing purity can be used throughout the purging, pressure 
equalization-rising and pressurization steps of a PSA cycle instead of the 
decreasing purity of direct bed-to-bed gas passage. 
With these and other objects in mind, the invention is hereinafter 
described in detail, the novel features thereof being particularly pointed 
out in the appended claims. 
SUMMARY OF THE INVENTION 
Gases are stored in segregated storage tanks so that such gas of increasing 
purities can be employed during refluxing, i.e. the purging, pressure 
equalization-rising, and product repressurization portions of a PSA 
processing sequence prior to final feed gas repressurization.

DETAILED DESCRIPTION OF THE INVENTION 
The objects of the invention are accomplished by the use of one or more 
segregated external gas storage tanks in the practice of PSA processes and 
systems. Gases withdrawn from an adsorbent bed are stored in a well 
defined order for use in purge and pressure equalization-rising and 
product gas repressurization steps. For example, in the purge step, the 
invention enables product gas of lowest purity to be used initially, 
followed by the use of product gas of increasing purity for the latter 
stages of the purging operation. Similarly, at the beginning of the 
pressure equalization-rising step, the lowest purity product gas from the 
segregated storage tank is used initially, with gas of increasing purity 
being used at latter portions of the pressure equalization-rising step. In 
this regard, it should be noted that, in the corresponding pressure 
equalization-falling, make product step, the gas withdrawn from the 
product end of a bed during cocurrent depressurization thereof and passed 
to the segregated gas storage tank will have decreasing purity levels. 
Such stored product gas will be withdrawn from the storage tank in reverse 
order, with gas purity increasing during withdrawal for passage to a bed 
undergoing refluxing, i.e. purge and/or the pressure equalization-rising 
step and/or product repressurization. It will be appreciated that, since 
multiple purity products are employed in the practice of the invention, 
the withdrawal of gases of lower purities from the product end of a bed 
during the depressurization thereof can be tolerated, and the adsorptive 
capacity of the bed can be more fully utilized, enabling the adsorption 
step to be terminated just prior to breakthrough of the more readily 
adsorbable component at the product end of the bed. 
The incorporation of the segregated storage tank in the PSA processing 
sequence of the invention enables greater processing flexibility and 
processing efficiency to be achieved as compared to prior art PSA cycles. 
In particular, the inclusion of segregated gas storage tank in the 
processing sequence of the invention, for any given PSA cycle, results in 
a lower or comparable Bed Size Factor (BSF), and a 5-15% power reduction 
compared with the same PSA cycle without the use of the segregated 
external storage tank as described and claimed herein. By contrast, prior 
art PSA cycles employ, for example, gas of constant purity, or gas of 
decreasing purity, as obtained directly from another bed in the PSA 
system, for refluxing, i.e., for purge, and pressure equalization-rising. 
For inclusion of the segregated storage tank, as employed in the practice 
of the invention, enables the production of multiple purity gas in various 
quantities for a refluxing, as well as for supplying, if desired, 
quantities of each purity gas to meet variable product demands of the 
consumers of gas from the PSA system. 
It will be understood that, since various amounts of multiple purity 
products are employed, the time allocated, and the process control of the 
gas production step, is important in the operation of the PSA cycle of the 
invention. In addition, those skilled in the art will appreciate that 
various modifications of the PSA processing steps will desirably be 
employed, in the practice of the invention, as a result of the use of a 
segregated external storage tank. Such modifications may include the 
overlapping of various process steps to reduce total cycle time, the 
choice of operating conditions employed, e.g., the upper adsorption 
pressure, the lower desorption pressure, the pressure at the end of the 
pressure equalization step, and the amounts of multiple purity products 
used for refluxing, as well as the time period allocated for each step, 
and the order in which the steps of the overall PSA cycle are carried out. 
In the embodiment of the invention illustrated in FIG. 1 of the drawings, 
the segregated tank is used in a product pressurization cycle as shown. 
Multiple purity products are produced and can be used in a well defined 
order for refluxing during the regeneration and repressurization of the 
adsorbent bed. 
In the practice of the invention in the embodiment illustrated in said FIG. 
1, the following steps are carried out separately or in any desired 
combination: 
(a) The feed (e.g., air) is introduced at one end of the adsorbent bed for 
pressurization (FP) from an intermediate pressure level desirably of 
0.60-1.0 atm. (1.0 atm.=14,696 psi), and preferably between 0.7-0.9 atm., 
to an upper pressure level, selected between 1.30-1.50 atm., and 
preferably between 1.37-1.52 atm. A lower desorption pressure level for 
the process of between 0.30-0.39 atm., and preferably between 0.34-0.37 
atm is desirably employed in mid embodiment. 
(b) The pressure during the production step (AD) could be at rising 
pressure from the intermediate pressure level of 0.60-1.0 atm. (the 
pressure at the end of the product pressurization step) to the adsorption 
pressure of 1.30-1.50 atm. Alternatively, feed pressurization without 
bleed off occurs during feed pressurization (FP) to reach the adsorption 
pressure, after which a control valve opens to produce product. In this 
latter case, the pressure during the production step is at constant 
pressure. The effluent gas stream is directed into a segregated storage 
tank, wherein multiple purity products are stored without significant 
mixing, or the effluent stream is directed to the respective product 
storage tanks at different times in the production step. 
(c) The feed input is terminated, and the adsorbent bed is depressurized 
cocurrently (henceforth referred to as the equalization falling step, 
which is not shown in FIG. 1), to recover the void gas and light component 
that co-adsorbed on the adsorbent, or the adsorption step is continued as 
shown in FIG. 1. The pressure in the former case, decreased from the 
adsorption pressure (1.30-1.50 atm.) to about 1.0 atm. This gas could be 
stored in another segregated storage tank, or could be fed directly to the 
same segregated storage tank used in the previous step. 
(d) Countercurrent depressurization/evacuation (EV) down to the low 
pressure level of about 0.35 atm. 
(e) Purging (PG) the bed countercurrently, wherein, the purge gas is 
returned to the bed in the order of increasing purity, starting with the 
lowest (L) purity product at the beginning of the purge step. 
(f) Product Pressurization, countercurrently with product gas of increasing 
purity, from the low pressure of 0.35 atm., to an intermediate pressure of 
0.60-1.0 atm. At the end of this step, the highest (H) purity gas from the 
segregated storage tank was used. 
The basic features of the invention are illustrated by describing the 
operation of a two-bed PSA process. However, it is anticipated that 
systems having only one bed or having more than two beds can also be 
employed in the practice of this invention. FIG. 1 is a schematic diagram 
of a two-bed PSA process consisting of two adsorption beds, feed 
compressor(s) or interconnected lines and valves. 
The practice of the invention in the embodiment of FIG. 1 is further 
illustrated in FIG. 2 of the drawings. As shown therein, the PSA system 
consists of two adsorbent beds, i.e., A and B filled with adsorbents, each 
having inlets 33 and 35, and outlets 5 and 6. The feed inlets 33 & 35 are 
connected to an air conduit 10 by a blower machine or compressor 11; 
whereas, the exhaust valves 34 and 36 are connected to conduit 12 
incorporating a vacuum pump 13. The outlets 14 and 15 communicate with 
valves 5 and 6 to a production conduit 16 through a control valve 17 that 
connects a segregated product storage tank 18. Valves 10A and 12A allow 
the two beds to communicate, when a conventional purge step is used. For 
example, valve 12A when opened, allows a portion of the product gas from 
bed A to supply a purge stream to bed B. Similarly, valve 10A when opened, 
allows a portion of the product gas from bed B to supply the purge gas to 
bed A. However, in the practice of this invention, all of the purge gas 
comes from the segregated storage tank 18, in the order of increasing 
purity. Thus, at the start of the purge step, the lowest (L) purity gas is 
used, followed by increasing purity during the step. 
While the outlet conduits 14 and 15 are connected to each other by valves 2 
and 4 to allow for direct bed-bed pressure equalization, it will be 
understood however, in the practice of this invention, no direct bed-bed 
pressure equalization is used. Thus, all of the pressure equalization 
falling gas goes to the segregated storage tank 18 in the order of 
decreasing purity, and is then returned to an adsorbent bed, in the order 
of increasing purity, for purging and pressurization of the bed at the 
product end. 
All the valves in the diagram are operated electronically via a computer 
system and program logic. Conduit 19 is connected to the segregated 
product storage tank, and supplies all of the refluxing gas, in the order 
of increasing, purity, for the purge and product pressurization steps. For 
example, when reflux gas is required for bed A, valve 9 is opened to allow 
product gas from the segregated storage tank 18 to enter said bed, in the 
order of increasing purity. Similarly, valve 8 is opened when bed B needs 
refluxing gas. 
Referring to said FIGS. 1 and 2, the two-bed process is described below to 
illustrate the opening and closing of the valves for each step of the 
cycle. 
Step 1 (FP): Feed (air) is introduced at one end of the bed. In the case of 
bed A, valve 33 is opened to allow feed gas to enter the bed. During this 
time, valve 36 is opened and the other bed B is undergoing evacuation. 
Step 2 (AD): Gross product make step. Valves 33 and 5 are opened. Control 
valve 17 program logic dictates when this valve will open to allow product 
gas to enter the segregated product storage tank 18. For instance, if 
constant pressure is required during the make product step, then control 
valve 17 only opened when the bed reached a predetermined pressure level 
to allow product gas to enter the segregated product storage tank 18. 
During the make product step (step 2), valves 8 and 36 are opened. Thus, 
bed B is undergoing the purge step and evacuation simultaneously. The gas 
required for the purge step was received from the segregated storage tank, 
in the order of increasing purity, starting with the lowest purity gas at 
the beginning of the step. 
Step 3 (AD or EQ): Continuation of the adsorption step (AD), wherein, 
valves 33 and 5 remained opened, or closed valve 33 and allow bed A to 
undergo a cocurrent depressurization step (EQ). For either of the two 
cases, additional product gas is directed to the segregated product 
storage tank 18. During this time valve 36 is closed, and valve 8 remained 
opened, so that product gas is obtained from the segregated product 
storage tank, in the order of increasing purity, for product 
pressurization of bed B. 
Step 4 (EV): Valve 34 is now opened to evacuate bed A countercurrently, and 
valve 35 is opened so that bed B undergoes feed pressurization at one end 
of the bed. 
Step 5 (PG): Valve 9 is now opened, so that bed A receives product gas for 
purging, in the order of increasing purity, from the segregated product 
storage tank. During this time, valve 34 remained in the opened position 
for continued evacuation. During this time, valves 35 and 6 are opened so 
that bed B is in the production step (AD). Control valve 17 logic 
determines when product gas from B enters the segregated product storage 
tank (18). 
Step 6 (PP): During this time valve 34 is closed, and valve 9 remained 
opened, so that product gas is obtained from the segregated storage tank, 
in the order of increasing purity, for product pressurization of bed A. In 
the case of bed B, either the adsorption step is continued, wherein valves 
6 and 35 remained opened, or valve 35 is closed to allow bed B to undergo 
a cocurrent depressurization step. For either of the two cases, additional 
product gas is sent to the segregated product storage tank 18. 
Based on the cycle described above in relation to FIGS. 1 and 2, several 
modifications can be made to alter one or more of the steps without 
deviating from the scope of the invention. For example, the feed and 
product pressurization steps can occur simultaneously, rather than 
sequentially as described above. Also, the countercurrent depressurization 
step can be preceded by opening to air until the pressure in the bed 
dropped to 1.0 atm., before evacuation begins. 
FIG. 3 of the drawings illustrates another embodiment in which a segregated 
product storage tank 18 and a segregated equalization tank 20 are employed 
in a system otherwise as shown in the FIG. 2 embodiment in which 
segregated product storage tank 18 is employed, but without use of a 
second external gas storage tank. It will be understood that the carrying 
out of the various processing steps, and the related opening and closing 
of valves, is generally similar to that described above with respect to 
the FIG. 2 embodiment. The use of the two segregated storage tanks 18 and 
20 allows for greater flexibility in the carrying out of the PSA-gas 
separation process. For example, the individual steps in the PSA cycle do 
not have to be carried out for fixed periods of time in the FIG. 3 
embodiment. Thus, physical variables, such as pressure and composition, 
can be readily used to determine the desired time allocated to each step, 
thereby adjusting the process for changes in temperature, pressure and 
variable product demand. In this embodiment, all of the pressure 
equalization-falling gas can conveniently be directed to the segregated 
pressure equalization tank 20. It is particularly pointed out that in the 
practice of various embodiments of the invention, no direct bed-to-bed 
flow of gas is employed, and all of the reflux gas is passed to a bed 
undergoing regeneration in an order of increasing purity, starting with 
the lowest purity gas at the beginning of the step. In addition, since no 
direct bed-to-bed flow is employed, it is possible to operate each 
adsorbent bed independently, with the overall PSA process being treated as 
a collection of single bed units. It will be appreciated that for proper 
sizing and sharing of compressor(s) and vacuum pump(s), however, some 
synchronization of the overall cycle in each bed with the corresponding 
cycles in other beds in the system is necessary or desirable. 
In another embodiment, segregated equalization tank 20 of the FIG. 3 
embodiment can be eliminated from the system, and all of the pressure 
equalization-falling gas can be passed directly from one bed to another. 
However, during such direct bed-to-bed pressure equalization step, the bed 
undergoing the pressure equalization-rising step receives product gas of 
decreasing purity, although, in preferred embodiments, it is desirable 
that the bed receive product gas of increasing purity. Upon completion of 
the pressure equalization-rising step, the bed is further pressurized with 
gas from the segregated product storage tank 18, or it undergoes feed 
pressurization, or product gas and feed gas repressurization 
simultaneously. 
Although the invention has been described above particularly with respect 
to the use of a single segregated product storage tank 18, it is within 
the scope of the invention to employ multiple segregated product storage 
tanks, wherein the effluent gas from a bed is directed to respective tanks 
at different times in the make product step. Likewise, the invention is 
not restricted to the use of cylindrical adsorbent beds with shallow 
dished heads at the top and bottom of the adsorbent vessel, with gas flow 
in the axial direction, and other desired bed configurations can also be 
used. For example, radial beds may be used to achieve a reduction in 
pressure losses, with concomitant reduction in power consumption. In 
addition, layered beds can be used with different adsorbents packed at 
various positions within the bed. For example, activated alumina can be 
placed at the feed end of the bed to remove water and carbon dioxide from 
the feed stream, with LiX zeolite adsorbent being placed on top of the 
activated alumina to perform the separation, for example, of feed air into 
an oxygen-enriched product gas comprising the less readily adsorbable 
component of said feed air. 
FIG. 4 of the drawings illustrate a single adsorbent bed C embodiment of 
the invention in which separate segregated storage are employed, i.e., 
product tank 18 and equalization tank 20. In order to achieve high machine 
utilization in this embodiment, a single compressor/blower is used to 
perform the pressurization and evacuation steps of the process. The 
operating steps of the PSA process as carried out in the FIG. 4 embodiment 
are as set forth below, 
The cycle is considered as beginning after product pressurization. In this 
step (FP), valves 10 and 33 are opened, and the other valves are closed. 
Valve 17 is a differential pressure check valve that opens only when the 
pressure in the adsorbent vessel becomes greater than the pressure in the 
segregated product storage tank 18. After feed pressurization, step 2 (AD) 
begins. During step 2 the make product step, valves 10 and 33 remained 
opened, and the differential check valve 17 opens when the pressure in the 
adsorbent vessel exceeds the pressure in the segregated product storage 
tank 18. Upon the opening of valve 17, product gas enters the segregated 
product storage tank 18. At the end of step 2, valve 33 is closed, and 
valve 36 opened to unload the compressor. During this time, the bed 
undergoes cocurrent depressurization with valve 4 in the opened position 
to collect the void gas into the segregated equalization tank 20. Note 
that the check valve 17 will be in the closed position during the 
cocurrent depressurization step (step 3), since the pressure of the 
adsorbent bed C will fall below that of the segregated product tank 18. 
During step 3 execution, valves 9, 10, and 33 are in the closed positions. 
Upon the termination of step 3, valves 12 and 34 are closed. During this 
step (step 4 or EV), gas in the adsorbent vessel leaves via valve 34 and 
enters through the inlet of the compressor. The next step (step 5 or PG), 
depicted in FIG. 1 is the purge step. During this step, valves 4, 34 and 
12 are opened, and the gas from the segregated equalization tank (20), 
supplies product gas, in the order of increasing purity, to purge the 
adsorbent bed C. The final step (step 6), product pressurization, is then 
executed with valves 12 and 34 closed while valve 4 remains in the opened 
position. If additional product gas is required for product 
pressurization, then valve 4 is closed, and valve 9 is opened to complete 
the product pressurization step. 
It will be understood the various modifications of the single bed process 
can be readily made without departing from the scope of the invention as 
recited in the appended claims. 
EXAMPLE 1 
In an illustrative example of the practice of the invention the processing 
steps of FIG. 1 are employed using the two-bed PSA system of FIG. 2. In 
the example, reflux gas purity is initially relatively low, e.g., 85%, and 
increases to about 93% over a period of about 16 seconds. The symbols used 
below have the following meaning: 
TPD=metric ton (1 ton=2,000 lb) per day of oxygen; KPa=1,000 Pa=S.I. unit 
for pressure (1.0 atm.=101.325 kPa); s=time unit in seconds; and 
kW=kilowatt 
The PSA process conditions and theoretical results obtained by computer 
simulation, are as follows: 
______________________________________ 
Adsorbent Li--X zeolite 
Cycle time 72 sec 
Upper adsorption pressure 
151.99 kPa 
Lower adsorption pressure 
40.53 kPa 
Pressure at the end of step 6 
64.85 kPa 
Feed rate 233.19 NCFH 
Product rate 32.76 NCFH 
Oxygen purity 92.22% 
Oxygen recovery 66.89% 
Bed size factor (BSF) 
663 lb.sub.m /TPD O.sub.2 
Power 5.61 kw/TPD O.sub.2 
______________________________________ 
From this example, it will be seen that the two-bed PSA process 
advantageously produces high purity oxygen product, with high product 
recovery; low Bed Size Factor, i.e., the amount of adsorbent required to 
produce a given amount of product gas; and low power consumption. 
EXAMPLE 2 
Illustrative example 1 was repeated, for comparative purposes, with the 
segregated storage tank being replaced by a prior art storage tank, 
wherein mixing of the gas therein is unavoidable. In this comparative 
embodiment, a portion of the product gas removed from the product end of 
the bed during the adsorption-product recovery step at the upper 
adsorption/pressure is diverted to the storage tank to provide pressure 
equalization gas for the process, and an additional portion thereof is 
likewise diverted to said storage tank to provide purge gas for bed 
regeneration purposes. In this mode of operation, the purity vs. time 
profile has the opposite characteristic of that applicable in Example 1, 
i.e., the oxygen purity varies from an initial higher purity to a 
subsequent lower purity. 
Computer simulated results for this comparative example are as follows: 
______________________________________ 
Adsorbent Li--X zeolite 
Cycle time 70 sec 
Upper adsorption pressure 
149.96 kPa 
Lower adsorption pressure 
40.53 kPa 
Pressure at the end of step 6 
64.85 kPa 
Feed rate 233.19 NCFH 
Product rate 27.89 NCFH 
Oxygen purity 92.59% 
Oxygen recovery 56.96% 
Bed size factor (BSF) 
736.36 lb.sub.m /TPD O.sub.2 
Power 6.48 kW/TPD O.sub.2 
______________________________________ 
Upon comparing the results obtained in Example 1 with those of Example 2, 
it will be seen that, for comparable product purity, the incorporation of 
a segregated storage tank in the PSA system results in a lower bed size 
factor, i.e., about 8%, and a lower power consumption, i.e., about 12%, 
when compared to the same PSA processing cycle with the use of a 
conventional external storage tank. In addition, the incorporation of the 
segregated storage tank serves to reverse the product purity order of the 
refluxing (purge) gas. For example, in the practice of the prior art, the 
highest purity product is used at the beginning of the refluxing step, 
followed by the use of lower purity purge gas. However, in the practice of 
the invention, the segregated storage tank enables the product gas to be 
stored unmixed. When reflux gas is needed, it is removed from the 
segregated storage tank in an order of increasing purity, with the lowest 
purity gas being used first, followed by higher purity gas. At the end of 
the refluxing step, the highest purity gas was used, and the product end 
of the bed has the least contamination with the more readily adsorbable 
component gas. Consequently, during the subsequent production step, higher 
purity product can be produced, or longer processing time can be allocated 
for the product production step to achieve a desired purity level. 
Those skilled in the art will appreciate that various changes and 
modifications can be made in the details of the invention as herein 
described without departing from the scope of the invention as hereinafter 
claimed. For example, it will be understood that product gas of varying 
purity can be used not only for refluxing, but portions thereof can also 
be withdrawn from the process and system as lower purity product gas 
passed to one or more downstream applications. As will also be understood, 
the portion of the product gas of varying purity used as reflux gas can be 
used solely for the pressure equalization-rising step in a bed, or can be 
used for both purge and said pressure equalization rising, or can be used 
for said pressure equalization-rising step together with product 
repressurization, with or without use of a portion of said product gas 
also being used for purge purposes. In any event, as noted above, the 
product gas is passed to and from the segregated gas storage tank(s), with 
no passage of said product gas directly from one bed to another as in 
conventional operations. 
It is also within the scope of the invention to practice the subject 
invention in a variety of adsorption systems, using a variety of PSA 
processing sequences, including two or more stage systems in which 
separate adsorbent beds are used to selectively adsorb different 
components of a feed gas mixture. In feed air separation systems, for 
example, two or more adsorbent stages may be employed, with the adsorbent 
bed or beds in one stage being adapted to selectively adsorb nitrogen as 
the more selectively adsorbed component, and with the adsorbent bed or 
beds in another stage being adapted to selectively adsorb oxygen as the 
more selectively adsorbed component of the feed air. As will be 
appreciated by those skilled in the art, molecular sieves such as 5A and 
13X material are commonly employed adsorbents for the selective adsorption 
of nitrogen from feed air, while activated carbon adsorbents are commonly 
used to selectively adsorb oxygen from feed air. 
Those skilled in the art will appreciate that, in embodiments of the 
invention in which gas of high purity is passed from the external, 
segregated gas storage tank of the invention to the product end of an 
adsorbent bed as part of the bed refluxing operation following the 
pressure equalization step, the adsorbent bed pressure obtained thereby 
will be somewhat less than the desired upper adsorption pressure. Final 
repressurization to the upper adsorption pressure will be achieved upon 
the addition of feed gas to the feed end of the bed at the desired 
adsorption pressure. 
The invention thus represents a significant advance in the PSA field. The 
reduction in Bed Size Factor and the reduction in the power requirements 
of a desired PSA gas separation operation obtainable in the practice of 
the invention enhances the ability of the desirable PSA technology to 
satisfy the ever growing needs for the separation operations in a wide 
variety of commercial applications.