Oxygen recovery pressure swing adsorption process

A pressure swing process for air separation to produce concentrated gaseous oxygen at an elevated pressure without the requirement of further compression of the gaseous oxygen product. The process comprises the steps of compressing the feed air to a pressure in the range of approximately 45 psig to 105 psig, preheating the feed air to each of the adsorption beds to a temperature in the range of approximately 100.degree. F. to 200.degree. F., then directing flow of the feed air cyclically into and through at least two crystalline zeolite molecular sieve adsorption beds for selectively adsorbing at least nitrogen therein. In this manner, oxygen having a purity of approximately 88% to 93% at a recovery of approximately 30% to 45% and a bed size factor in the range of 2,500 pounds to 4,000 pounds of adsorbent per ton per day of oxygen can be delivered to a receptor tank at an elevated pressure of approximately 40 psig to 100 psig.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to a process for separating air by 
pressure swing adsorption and, more specifically, for recovering oxygen at 
an elevated pressure without requiring product compression. 
2. Description of the Prior Art 
In the prior art pressure swing processes for air separation, the cycle 
sequence usually includes a selective adsorption step during which 
compressed air is introduced at the adsorbent bed inlet, or feed end, 
thereby forming a nitrogen adsorption front, nitrogen being selectively 
adsorbed by adsorbents as, for example, zeolitic molecular sieves. Oxygen 
is also coadsorbed but substantially displaced by the more strongly held 
nitrogen adsorbate. Oxygen effluent gas is discharged from the opposite 
discharge, or product end of the bed, at about the feed air pressure and 
the nitrogen adsorption front moves progressively toward the product end. 
The adsorption step is terminated when the front is intermediate the feed 
and product ends, and the bed is co-currently de-pressurized with oxygen 
effluent being released from the product end and the nitrogen adsorption 
front moving into the previously unloaded section closer to the product 
end. The co-current de-pressurization gas may in part be discharged as 
oxygen product and in part returned to other adsorbent beds for a variety 
of purposes, for example, purging and pressure equalization with a purged 
bed for partial re-pressurization thereof. Co-current de-pressurization is 
terminated before the front reaches the product end so that the oxygen 
purity of the effluent is nearly that of the gas discharged during the 
preceding adsorption step. 
The term "co-current flow" through an adsorbent bed represents the flow of 
gas in the direction of the air feed flow through the same bed. In 
contrast, counter-current flow through an adsorbent bed represents the 
flow of gas in a direction opposite that of the air feed flow. 
The co-currently de-pressurized bed is usually further de-pressurized by 
releasing waste gas through the feed end, i.e. countercurrently 
de-pressurized, until the bed pressure diminishes to a desired low level 
for purging. Then oxygen purge gas is caused to flow through the bed to 
desorb the nitrogen adsorbate and carry it out of the system. The purged 
and at least partly cleaned bed is then re-pressurized at least partly 
with oxygen and/or feed air and returned to the adsorption step. 
SUMMARY OF THE INVENTION 
The present invention relates to such a pressure swing process for air 
separation and serves to produce concentrated gaseous oxygen at an 
elevated pressure without the requirement of further compression of the 
gaseous oxygen product. The process comprises the steps of compressing the 
feed air to a pressure in the range of approximately 45 psig to 105 psig, 
preheating the feed air to each of the adsorption beds to a temperature in 
the range of approximately 100.degree. F. to 200.degree. F., then 
directing flow of the feed air cyclically into and through at least two 
crystalline zeolite molecular sieve adsorption beds for selectively 
adsorbing at least nitrogen therein. In this manner, oxygen having a 
purity of approximately 88% to 95% at a recovery of approximately 30% to 
45% and a bed size factor in the range of 2,500 pounds to 4,000 pounds of 
adsorbent per ton per day of oxygen can be delivered to a receptor tank at 
an elevated pressure of approximately 40 psig to 100 psig. 
The objective of the invention is to produce about 93% purity oxygen 
through the pressure swing adsorption process with product produced at 
pressures exceeding 70 psig with recoveries of at least 35% and small bed 
size factors of less than 3,500. As noted, no product compressor is to be 
used to achieve this final product pressure. 
The above objectives could be met with known prior art provided the product 
pressure was kept under 45 psig. However, heretofore, when product 
pressure in the range of over 70 psig was desired without using product 
compression, the known system required high feed air pressure of about 100 
psig with air compression ratio of over 7. This, in turn, exacerbated the 
adverse thermal gradients in the adsorbent beds making it difficult to 
achieve the desired combination of high product pressure and purity, good 
product recovery, and small bed size factors. 
A significant advance in the state of the art occurred with the advent of 
U.S. Pat. No. 3,673,931 to J. J. Collins entitled "Air Separation by 
Adsorption". Collins recognized that the adsorbent beds experience a 
sharply depressed temperature zone at the inlet or feed end of the 
adsorption bed due mainly to the desorption of air contaminants such as 
CO.sub.2 and moisture, especially when high O.sub.2 recovery is desired. 
His proposed solution was to heat the inlet end of the bed to achieve high 
air feed temperatures (below 175.degree. F.), preferably between 
100.degree. F. and 150.degree. F. to mitigate the average temperature 
depression in the bed. He achieved product O.sub.2 purity of about 90% 
with recovery of 55% at a product pressure of 40 psig using a bed size 
factor of about 4,800 pounds of sieve per tons per day (TPD) of oxygen 
product. 
The Collins teachings considered four, three, and two bed systems and 
required about 1,200 pounds of sieve per bed for a 4-bed system. Using a 
3-bed cycle, Collins achieved bed size factors of over 5,500 pounds per 
ton of oxygen produced. Although no figures are provided for a two-bed 
process, the bed size factors achieved by the present invention of 3,000 
to 3,500 pounds per ton of oxygen produced are a significant improvement 
over the Collins system Collins' figures were achieved with a feed 
dewpoint of -40.degree. F. and with the CO.sub.2 removed, while a much 
higher product pressure of about 70 psig has been achieved according to 
the present invention using a lower bed-size-factor (BSF) and with a much 
higher dewpoint of +40.degree. F. and without removing CO.sub.2. 
Collins' two-bed cycle used a 120 second cycle to achieve a product 
pressure of 40 psig. In contrast, by extending the cycle to 240 seconds 
and with the addition of a product surge tank, according to the present 
invention, the product pressure has been increased to a magnitude in 
excess of 70 psig. Furthermore, by accepting the lower product purity 
(90%) mentioned in the Collins patent, the recovery attainable by the 
process of the present invention can exceed 35% and still achieve higher 
product pressures. 
During the pressure equalization step, there is an opportunity to transfer 
some of the gas from the top, or product end, of one vessel or bed to the 
bottom, or feed end, of the other vessel or bed. Unlike Collins who 
suggests introducing product oxygen from the product end of one bed to the 
feed end of the other bed, one method according to the present invention 
would be to transfer initially from the product end of one bed to the 
product end of the other bed, thereby introducing the higher oxygen 
content gas to the product end of the bed being pressurized. Near the end 
of the step is the best time to transfer the oxygen from the product end 
of bed B to the feed end of bed A because that is the time during the step 
when the pressure in bed A is closer to the designated maximum pressure 
during the step. The relatively high pressure in bed A reduces the 
migration of nitrogen and other impurities from the feed end of the bed 
toward the product end. Another reason for doing this top-to-bottom 
transfer late in the step is that the concentration of impurities in the 
gas being transferred is highest near the end of the step. Concentrating 
the top-to-bottom equalization near the end of the step reduces migration 
of nitrogen toward the product end of the bed being pressurized while 
still using the top-to-bottom equalization to reduce the temperature 
gradient in bed A. 
In a four-minute equalization (Step A as shown in FIG. 2) one might use 
top-to-top equalization for the first three minutes and follow that with a 
one-minute top-to-bottom equalization. Since the pressure in bed A rises 
during the step, its pressure would be higher during the top-to-bottom 
equalization. 
In comparing the present invention with that disclosed in the Collins 
patent, significant differences include the number of beds utilized, the 
maximum product pressure, and the feed air temperature. These can be 
clearly seen in Table A provided below. 
TABLE A 
__________________________________________________________________________ 
Product 
Product 
Air Feed 
Cycle 
# of 
O.sub.2 Purity, 
0.sub.2 Pressure, 
Temp, 
Time, 
Oxygen 
BSF, 
Item 
Data Source 
Beds 
% psig .degree.F. 
Sec 
Recovery 
#/TPD 
__________________________________________________________________________ 
1 Present Invention 
2 93.3 77.5 170 240 
36.9 3,056 
2 Prior Art, 
3 87 1-3 175 120 
32.7 4,500 
3 Collin U.S. 
3 90 1-3 100 120 
38.1 5,000 
4 Pat. No. 
4 90 40 64 240 
39.8 5,600 
5 3,973,931 
4 89.3 40 100 240 
46.4 4,950 
__________________________________________________________________________ 
With the four bed cycle, Collins is said to have achieved a product purity 
of 89.3% with a product pressure of 40 psig and a bed size factor of 4950 
pounds per TPD while achieving a recovery of 46.4%. The present invention 
utilizes a two-bed cycle with a product surge tank which would typically 
suggest a higher bed size factor. However, a lower bed size factor was 
achieved of about 3,000 pounds per TPD at a higher purity, specifically 
93.3%, and much higher product pressures, specifically 77.5 psig. In 
addition, much higher feed air temperatures were used, 170.degree. F. 
versus 100.degree. F., while relatively good recovery of 36.9% was 
achieved. The three-bed process of Collins ran lower feed air temperatures 
(100.degree. F. versus 175.degree. F.) and at much lower pressures (1 to 3 
psig) while achieving lower product purity and similar recoveries. One 
would have expected much higher recoveries at the lower pressure and 
better bed size factors with more beds that Collins used than for the 
two-bed system of the present invention. 
A small (1-2 tons per day) and relatively simple pressure adsorption system 
for oxygen recovery utilizing the principles of the invention is thus seen 
to produce 93% pure oxygen at pressures above 70 psig with over 35% 
recovery. Further, such a low cost system avoids the use of a product 
compressor and associated gas coolers as previously required. 
The key achievement of the invention is production of high pressure (&gt;70 
psig) oxygen with higher purity and good recovery at much lower bed-size 
factor and without product compression and without full prepurification of 
air contaminants as in the Collin's patent. Referring to Table A, the 
primary difference of the invention as compared with the prior art as 
disclosed in the Collins patent that result in the above improved 
performance is the unique combination of conditions, comprising: 
1. long cycle time of approximately 240 seconds at which point Collins paid 
significant bed-size-factor (BSF) penalty (items 4 and 5). 
2. high feed air temperature of 170.degree. F., which in combination with 
lower product pressure and faster cycle resulted in lower purity and 
higher BSF for Collins (item 2). 
3. use of air as well as product surge tanks; 
4. optionally, use of a compound bed using alumina at the feed end followed 
by a 13.times. molecular sieve for the bulk air separation could further 
enhance the system performance of the invention. 
It should be noted that product recovery is the only parameter, for which 
Collins has an edge over the present invention but not without penalties 
in all other desired parameters, namely high product pressure, high 
product purity and low BSF (items 3, 4 and 5). 
The above indicated improved performance at much higher product pressure is 
surprisingly achieved in spite of the higher feed air pressure which tends 
to cause greater adverse thermal gradients in a bed and higher feed air 
dewpoint (40.degree. F. versus -40.degree. F. in the prior art), which 
also contributes to greater temperature depression at the feed end of the 
bed, using only 2 beds as compared to 3 and 4 bed systems as in the 
Collins patent, using long cycle time (240 second) generally associated 
with larger BSF and using high feed air temperature (170.degree. F.) which 
tends to reduce adsorption capacity and thus increase BSF. 
Other and further features, advantages, and benefits of the invention will 
become apparent in the following description taken in conjunction with the 
following drawings. It is to be understood that the foregoing general 
description and the following detailed description are exemplary and 
explanatory but are not to be restrictive of the invention. The 
accompanying drawings which are incorporated in and constitute a part of 
this invention, illustrate one of the embodiments of the invention, and, 
together with the description, serve to explain the principles of the 
invention in general terms. Like numerals refer to like parts throughout 
the disclosure.

DETAILED DESCRIPTION OF THE INVENTION 
Turn now to the drawings and initially to FIG. 1A which illustrates, 
schematically, a pressure swing adsorption system 20 provided in 
accordance with the present invention for producing high pressure oxygen. 
The system 20 includes an air compressor unit 22 which encompasses filter 
24, compressor 26, after cooler 28, and moisture separator 30, a 
refrigerated air dryer unit 32 which encompasses a heat exchanger 34, a 
further moisture separator 36, an air surge tank 38, a heater 72, two 
adsorbent beds A and B, and a product surge tank 62. The air surge tank is 
particularly desirable for increasing the production rate if the size of 
the compressor 26 is limited. However, the primary function of the air 
surge tank is to even out flow into the beds A and B, to assure a more 
uniform flow and pressure distribution through and within the beds. 
To achieve the favorable results stated above, a feed air stream is 
directed through the air compressor unit 22, cleansed by the filter 24, 
then compressed by the compressor 26 to a pressure 8-10 psig higher than 
the desired product pressure. 
If a final product pressure of 77 psig is attained, the maximum pressure in 
the pair of sieve beds A and B may reach 87 psig. The sieve beds A and B 
are substantially identical and may be, for example, crystalline zeolite 
molecular sieve adsorption beds of at least 4 angstroms apparent pore size 
(13.times. molecular sieve) capable of adsorbing at least nitrogen 
therein. In this instance, certain allowances would be made for piping 
losses from the air compressor to the sieve beds such that the feed air 
pressure from the compressor would be about 95 psig. Unlike Collins' use 
of prepurifiers, the moisture in the feed air stream is removed by cooling 
the feed air stream in a refrigerated dryer so that the dewpoint of the 
feed air stream does not exceed 40.degree. F. versus -40.degree. F., in 
the case of Collins. The feed air stream is then reheated in heater 72 
prior to injection into the sieve beds. While Collins indicated that the 
optimum feed air temperature to maximize recovery was between 100.degree. 
F. and 150.degree. F. the experience of the inventors showed that by using 
13.times.sieve material, preheating of the air should be at temperatures 
in excess of 140.degree. F. up to approximately 170.degree. F. to achieve 
good recoveries. In any event, it is desirable for the bottoms of the beds 
to maintain a temperature around 40.degree. F. 
Viewing FIG. 2A, the cycle herein proposed is initiated with bed B at 
maximum pressure, typically 88 psig, and bed A which is at or close to 
ambient pressure. With all other valves depicted in FIG. 1 being 
preferably closed, a pressure equalization valve 40 connecting the two 
beds is opened and pressurized gas is fed to bed A from bed B under a 
controlled flow rate. The beds should come as close to each other in 
pressure as possible within a reasonable time, with a likely optimum 
pressure differential of 5-7 psig resulting after a duration for this step 
of 30 to 35 seconds. 
Preferably, this step should end with the configuration shown in the 
optional step, as seen in FIG. 2AA. In this instance, the last part of the 
equalization thus ends with gas flowing from the top of bed B into the 
bottom of bed A. It will be understood that the configurations of the 
systems illustrated in FIG. 1A and FIG. 1B would be modified accordingly 
to accomodate such an outcome. 
Next, viewing FIG. 2B, a feed air valve 42 on a feed air line 44 is opened 
and the pressure in bed A is raised to a pressure, generally in the range 
of 74 psig, at the rate of approximately 1 psi per second. In this 
instance, preferably, the only other valve of the system 20 which is 
opened is a blowdown valve 46 in discharge line 48, as will be discussed 
below. Raising the pressure in bed A is accomplished by the introduction 
of preheated feed air from the compressor unit 22 through the feed end of 
the bed A, that is, the opposite end from which the equalization flow 
occurred. It is important that the feed air be heated to a level 
sufficient to raise the minimum temperature that the sieve experiences to 
a level above 40.degree. F. at a level one foot above the bottom of the 
sieve bed. The preheating of the feed air can be accomplished in a variety 
of ways but the most cost effective manner is to recuperate the heat of 
compression from the air feed compressor unit 22 and by means of the heat 
exchanger 72 elevate the feed air temperature. The feed air must have also 
been pretreated by having the dewpoint of the feed air reduced to 
40.degree. F. This can be achieved by the use of the commercially 
available refrigerant dryer unit 32. Faster rates may be used if care is 
taken to prevent lifting or fluidization of the sieve at the top of bed A. 
If care is taken not to fluidize the sieve in the bed, this step could be 
reduced time-wise, improving the output of the system and reducing the bed 
size factor. 
While bed A is being pressurized to 85 to 90 psig (FIG. 2C), bed B has the 
remaining pressure released from the bottom or feed air end by opening the 
blowdown valve 46 to atmosphere. This blowdown step typically lasts 55 
seconds, or about 1 psig per second, and brings the bed B to atmospheric 
pressure, discharging the adsorbed nitrogen from bed B via discharge line 
48, suitable sound muffler 50, and into a waste nitrogen receiver 52. Once 
bed B is substantially de-pressurized, a purge valve 54 is opened to 
connect the product ends of both of the beds, and a controlled amount of a 
high pressure, high purity, stream of oxygen from the pressurized bed A is 
fed to the de-pressurized bed B. This purge step should last about 33 
seconds. The amount of purge gas should be controlled such that the purity 
of the gas exiting the bottom, or feed end, of the de-pressurized bed B 
remains at 10 to 13% oxygen until near the end of the step and then the 
oxygen content of the gas flow should reach about 16 to 19% oxygen. This 
is to ensure that an excessive amount of oxygen gas is not used to purge 
the bed as this would detract from the overall system performance. While 
the purge step is being performed, the bed at pressure (bed A) is not only 
providing a small amount of purge gas, it is also filling the product 
surge tank to a pressure close to that of the maximum pressure seen at the 
product end of the bed at pressure. This is accomplished through a check 
valve 60 with a low cracking pressure. The product surge tank 62 is sized 
so that during the equalization step (FIG. 2A), the pressure in the tank 
does not fall below a minimum pressure requirement. The time of the step 
depicted in FIG. 2C is dictated by the length of time it takes to reach 
the maximum pressure of the bed. 
Oxygen product, pressurized, is caused to flow from the product end of bed 
A through a check valve 60 in product line 58, then through a product make 
valve 56 in exit line 61 into an oxygen product storage tank 62. By reason 
of the process just described, the oxygen product in the storage tank 62 
is already at an elevated pressure, ready for use. The oxygen product can 
then be withdrawn from the storage tank 62, as desired, by operation of an 
outlet valve 64, for delivery to a diagrammatically illustrated pipeline 
66 or other suitable receptacle. 
FIGS. 2D, 2E, and 2F illustrate steps in the process which are identical to 
the steps depicted in FIGS. 2A, 2B, and 2C except that the roles of beds A 
and B and their associated lines and valves are reversed. 
Preferably, the step illustrated in FIG. 2D should end with the 
configuration shown in the optional step, as seen in FIG. 2DD. In this 
instance, the last part of the equalization thus ends with gas flowing 
from the top of bed A into the bottom of bed B. It will be understood that 
the configurations of the systems illustrated in FIG. 1A and FIG. 1B would 
be modified accordingly to accomodate such an outcome. 
In another embodiment, as shown in FIG. 1B, a pair of supplemental product 
lines 68 may interconnect the product ends of the beds A and B with the 
exit line 61 and the flow through the lines 68 may be controlled by check 
valves 70 to allow flow of oxygen product only in the direction of the 
beds. With this construction, in the step depicted in FIG. 2C, for 
example, with the pressure equalization valve 40 and the purge valve 54 
being closed, oxygen product produced from vessel A is directed into the 
product end of bed B to achieve the purging of bed B which was previously 
achieved with operation of the purge valve 54. 
To produce one ton per day of contained oxygen with a purity of 93% 
(minimum) one would have to use 3,000 pounds of 13.times. sieve. This 
could be tried with other sieves such as 5A but with sieves that are 
designed to operate under vacuum during the de-pressurization step, one 
would expect a poorer performance. 
For achieving the desired objective, tests were conducted involving 
variations in feed temperature, cycle time and valve size and its 
associated pressure drop. As indicated by Table A, above, various changes 
to the above parameters were made with varying results in product purity 
and pressure. For example, the 32 second time duration for the pressure 
equalization step of FIG. 2A could be further reduced with increased valve 
sizing, which should result in even higher product recoveries. If the 55 
second time duration for the pressurization step of FIG. 2B is also 
reduced with improved valving and the bed maintained so as to prevent 
lifting, the recoveries should approach 40%. 
There are other variations that could be tried to achieve similar 
improvements in performance. The predrying of the feed stream could be 
accomplished by a desiccant or pressure or thermal swing dryer or membrane 
dryer which would lower the amount of moisture or dewpoint of the feed air 
stream and possibly improve the performance. The feed air could also be 
preheated by using an auxiliary heating source in the air feed line, such 
as an immersion heater. Another way to take the water vapor and CO.sub.2 
out of the air feed stream prior to the air coming in contact with the 
sieve in one or the other of the beds A and B would be to put a layer of 
activated alumina in the bottom of the sieve bed. This would act as a weak 
adsorbent and reduce the refrigeration effect during the de-pressurization 
step. An optimum method of preheating the air feed stream would be to use 
a heat exchanger to capture the heat of compression from the air 
compressor and transfer this to the feed air stream. Other sources of 
energy are available including excess stream, warm process, and 
electricity. If sufficiently inexpensive, any of these can be used to 
pretreat the air stream. 
While a preferred embodiment and an alternate embodiment of the invention 
have been disclosed in detail, it should be understood by those skilled in 
the art that various other modifications may be made to the illustrated 
embodiments without departing from the scope of the invention as described 
in the specification and defined in the appended claims.