Air fractionation by pressure swing adsorption

Improved oxygen recovery in the operation of a pressure swing adsorption system for air fractionation is obtained by passng the air, freed of water and CO.sub.2, through an adsorbent bed maintained at elevated temperature throughout the cycle and which is selective for retention of nitrogen, and consequent withdrawal of an oxygen-rich primary effluent product. In a preferred embodiment ambient air is passed through a pretreater section providing an adsorbent bed for removal of water and carbon dioxide. The thus purified air is compressed with consequent rise in temperature and then only partly cooled down by exchange with cooler desorbed and purged nitrogen-rich gas products withdrawn from the main adsorbent beds. These nitrogen-rich products thus heated by the exchange are employed in regenerating a water and carbon dioxide laden bed of the pretreater section. While the adsorption-desorption is operated in a pressure swing cycle, the pretreater section is operated in an independent thermal swing cycle. Alternatively, the feed air may be compressed prior to its introduction into the pretreater and the thus generated heat utilized by heat exchange for heating the purified air effluent to be fractionated.

BACKGROUND OF THE INVENTION 
The present invention relates to the fractionation of air by selective 
adsorption, and is particularly directed to a pressure swing adsorption 
system obtaining enhanced recovery of oxygen from ambient air. 
Fractionation of air by pressure swing adsorption essentially consists of 
two key operations: an adsorption step, where the compressed feed air is 
contacted with a sorbent capable of selectively adsorbing one of the main 
constituents of air, and a regeneration step, where the adsorbed 
components are removed from the sorbent so that it can be reused. 
Commonly, alumino-silicate zeolites, which selectively adsorb N.sub.2 from 
air, are employed as sorbents, thereby producing an O.sub.2 enriched air 
stream during the adsorption step. 
The common methods for sorbent regeneration are (i) desorption by pressure 
reduction and (ii) desorption by elution or purging. Depressurization at 
the end of the sorption stroke reduces the superatmospheric pressure 
inside the sorption columns causing desorption of the sorbed components 
along with removal of the void gases from the column. The purging step 
following depressurization consists of eluting the column with a gas 
stream rich in the non-adsorbed species of the air (usually O.sub.2 
enriched product gas) which reduces the partial pressure of the sorbed 
components in the voids of the column causing further desorption. 
Unfortunately, O.sub.2 is lost by both of these regeneration procedures, 
thereby lowering the recovery of that species in the oxygen-enriched 
product gas. 
Among the numerous systems found in the patent literature for production of 
an oxygen-enriched product gas from air by pressure swing adsorption (PSA) 
are those described in U.S. Pat. Nos. 2,944,627; 3,564,816; 3,636,679; 
3,717,974 which utilize zeolite molecular sieves as selective adsorbents 
for nitrogen-oxygen separation. Also, it is known to employ a 
pre-treatment section comprising one or more adsorbent beds for removal of 
water and CO.sub.2 from the feed prior to contact with the main adsorbent 
bed. Typical patents employing this feature include: U.S. Pat. Nos. 
2,944,627; 3,140,931; 3,533,221; 3,796,022; 3,797,201; 3,957,463; 
4,013,429. However, in these prior art systems, the air introduced into 
the main adsorption beds is at about ambient temperature, while a 
variation is proposed in U.S. Pat. No. 3,973,931. The latter patent 
discloses that, where the water-carbon dioxide impurities of the ambient 
air are removed by adsorbing these impurities in the feed inlet end of the 
same adsorbent column where the nitrogen-oxygen separation is carried out, 
an undesired cooling of that front section takes place. This is described 
as being responsible for the observed lower oxygen recovery of these 
processes in actual large scale application. Consequently, it is proposed 
in this patent to supply heat from an external source to only the inlet 
end section of the column sufficient to maintain the inlet end at least 
20.degree. F. (11.degree. C.) warmer than without heating, but below 
175.degree. F. (79.4.degree. C.). Similarly, a companion patent, U.S. Pat. 
No. 4,026,680, concerned with the same general problem of reduced 
temperature at the inlet end of the adsorption bed, uses metallic elements 
embedded in the column. This procedure is stated to have helped reduce the 
problem of the inlet end temperature depression and to result in improved 
oxygen recovery. 
In the preliminary studies leading to the development of the present 
invention, two important characteristics of pressure swing adsorption 
systems for air separation were investigated. These were: (1) the 
temperature effect on the N.sub.2 capacity of the adsorbent during 
adsorption from air at superatmospheric pressure and (2) the temperature 
effect on the quantity of oxygen purge gas needed for cleaning the 
adsorbent column after depressurizing the column to a near ambient 
pressure level. A synthetic mordenite molecular sieve was used in the 
tests conducted in this investigation. 
It was found from these tests that both the N.sub.2 capacity and the 
O.sub.2 purge requirements decreased as the base column temperature 
increased, as was expected. However, there was a significant difference in 
the temperature coefficients of these two properties, not heretofore 
appreciated. That is, the temperature coefficient for the oxygen purge gas 
was found to be 4 to 10 times that for the nitrogen capacity, depending 
upon the conditions of operation. 
Based on the foregoing exploratory tests, it was discovered that the 
performance of a pressure swing adsorption process (PSA) for air 
separation could be improved by operating the system at an elevated 
temperature throughout the columns. While in such operation a larger 
inventory of adsorbent may be required due to decrease in the adsorption 
capacity, the O.sub.2 loss during the elution of the bed would be 
substantially reduced. In addition, it was found that the oxygen loss 
during the depressurization of the columns could also be reduced when 
operating these at an elevated base temperature throughout the bed and 
during the entire operating cycle. Also, it was discovered that the 
greater oxygen recovery would more than offset any anticipated increase in 
the capital cost, and thereby substantially improve the total economics of 
generating product oxygen by pressure swing adsorption, such for example, 
as an overall cost decrease in the order of 10 to 30%. 
In addition to obtaining higher oxygen recovery, operation of the air 
fractionation process in accordance with the present invention provides 
another important advantage. By operating the pressure swing air 
fractionation system during the entire cycle at substantially the same 
elevated temperature at which the hot feed air is introduced thereto, the 
pretreatment of the feed air for removal of water and carbon dioxide prior 
to oxygen-nitrogen separation can be conveniently carried out by a thermal 
swing adsorption scheme because the hot desorbed nitrogen and the hot 
waste purge gas from the N.sub.2 --O.sub.2 separation can be used for 
thermal regeneration of the pretreatment columns. Ordinarily, the 
adsorbent in the front section of the main N.sub.2 --O.sub.2 separation 
column is utilized as a trap for H.sub.2 O and CO.sub.2, and a cyclic 
regeneration of this section is achieved during the depressurization and 
purging steps for the regeneration of the N.sub.2 --O.sub.2 section. Such 
procedure for regeneration of the H.sub.2 0--CO.sub.2 section is not very 
efficient because most of the desorption of water and carbon dioxide is 
achieved by the " purging effect" of the H.sub.2 O--CO.sub.2 free gases 
from the N.sub.2 --O.sub.2 separator section. A portion of the 
oxygen-enriched purge gas is thus often consumed only to clean the 
pretreatment section, particularly to remove water, and thereby decreases 
the amount of oxygen which can be recoverd as a dry product. 
Thus, by operating at elevated temperature and in accordance with the 
particular steps of the present invention, one can judiciously utilize the 
heat of compression to supply the energy required for operation at the 
desired elevated temperature, as well as the energy needed for thermal 
swing removal of the H.sub.2 O and CO.sub.2 impurities in the feed air, 
such that the requirement for an external heat source is eliminated. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a pressure-swing adsorption 
process for production of product oxygen is provided which obtains 
substantially enhanced recovery of product oxygen. The invention includes 
the steps of 
(1) introducing the feed air previously freed of water and CO.sub.2, at 
superatmospheric pressure and at elevated temperature, into a bed of 
adsorbent which is selective for adsorption of nitrogen, and which bed is 
maintained throughout the cycle and throughout its length at or near the 
elevated temperature of the feed air, and discharging an oxygen-enriched 
effluent gas from the bed; 
(2) then reducing the pressure of the bed with simultaneous withdrawal of 
the desorbed and void gases from the bed in a direction opposite to that 
of feed air flow in step (1) until the pressure of the bed reaches a near 
ambient pressure level; 
(3) then purging the bed with a part of the oxygen-enriched effluent gas in 
a direction opposite to that of the feed air flow; 
(4) then repressuring the bed to an intermediate pressure level by 
introduction thereinto of the desorbed and void gases from one or more 
companion beds in the same direction as that of the feed air flow; and 
(5) finally, further repressuring the bed to about the designed adsorption 
pressure level by introduction therein of part of the high pressure 
oxygen-rich effluent in a direction opposite to that of the initial feed 
air flow. 
In accordance with the preferred embodiment, the water and carbon dioxide 
impurities of the ambient air are removed by adsorbing them in a 
pretreatment bed prior to introduction of the pretreated air into the 
pressure swing adsorption section for separation of the oxygen and the 
nitrogen. The adsorption step in the pretreatment section is carried out 
at near ambient temperature either before the compression of the ambient 
air or after it. The regeneration of the adsorbent in the pretreatment 
beds is effected by simultaneously heating and purging the beds with 
nitrogen-rich desorbed and purge gas effluents from the pressure swing 
adsorption section of the system. Thus, the heat for regeneration of the 
pretreatment beds is obtained by at least partly recovering the heat of 
compression of the feed air. This is accomplished by heat exchanging the 
waste regenerating gas with the hot compressed feed air leaving the 
compressor.