Pressure swing adsorption process

Pressure swing adsorption operations for the selective adsorption of a component of a gas mixture is carried out, in multi-bed systems, so that the gas released from a bed during the cocurrent depressurization, provide-purge step is passed partly to another bed and partly to an external vessel. The gas in said vessel is thereafter used to partially repressurize said other bed after it has completed its purge step.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the purification of gases in a pressure swing 
adsorption system. More particularly, it relates to the improvement of 
product recovery and/or purity in such a system. 
2. Description of the Prior Art 
The pressure swing adsorption (PSA) process provides a commercially 
desirable technique for separating and purifying at least one gas 
component from a feed gas mixture of said gas component and at least one 
selectively adsorbable component. Adsorption occurs in an adsorbent bed at 
a higher adsorption pressure, with the selectively adsorbable component 
thereafter being desorbed by pressure reduction to a lower desorption 
pressure. The PSA process is commonly employed in multi-bed systems as is 
indicated by the Wagner U.S. Pat. No. 3,430,418, relating to a system 
having at least four beds, and by the Fuderer, et al U.S. Pat. No. 
3,986,849, which discloses the use of at least seven adsorbent beds. As is 
generally known and described in these patents, the PSA process is 
commonly carried out, on a cyclic basis, in a processing sequence that 
includes, in each bed, higher pressure adsorption with the release of 
product effluent from the product end of each bed, cocurrent 
depressurization to intermediate pressure with release of void space gas 
from the product end of the bed, countercurrent depressurization to a 
lower desorption pressure, purge and repressurization. The void space gas 
released during the cocurrent depressurization step is commonly employed 
for pressure equalization purposes and to provide purge gas to a bed at 
its lower desorption pressure. 
Multi-bed systems have the inherent advantage of greater productivity 
coupled with a substantially uniform flow of product effluent therefrom. 
The necessary cycling of the processing sequence from one bed to another 
is recognized, however, as creating conditions that somewhat limit the 
recovery of product from such systems. Such a loss of product gas, while 
tolerable in light of the overall objects of particular commercial 
applications of the PSA process and system is nevertheless undesired. 
It is an object of the invention, therefore, to provide an improved PSA 
process and system. 
It is another object of the invention to provide a PSA process and system 
having improved product recovery. 
SUMMARY OF THE INVENTION 
The PSA process and system of the invention are employed so that the void 
space gas released from each bed during the cocurrent depressurization, 
provide-purge gas step is used partly for passage directly to another bed 
for such purge purposes and partly for passage to an external vessel. The 
gas thus passed to the external vessel is thereafter used to provide 
repressurization gas to said other bed after it completes its purge step.

DETAILED DESCRIPTION OF THE INVENTION 
The objects of the invention are accomplished by the addition of a pressure 
equalization step for partial repressurization of a purged bed using gas 
recovered in an external vessel simultaneously with the providing of purge 
gas to said bed. 
The PSA process and system of the invention relates to such conventional 
PSA technology in which each adsorbent bed of the system undergoes on a 
cyclic basis, higher pressure adsorption, cocurrent depressurization to 
intermediate pressure levels with release of void space gas from the 
product end of the bed, countercurrent depressurization to a lower 
desorption pressure with the release of desorbed gas from the feed end of 
the bed, purge and repressurization to said higher adsorption pressure. As 
is disclosed in the patents referred to above, a portion of the void space 
gas released from one bed during its cocurrent depressurization is 
commonly passed, directly or through external storage tanks, to a bed or 
beds initially at lower pressure to equalize the pressure between said 
beds, i.e., in one or more pressure equalization steps. Another portion of 
said void space gas is used to provide purge to a bed undergoing the purge 
step. For this purpose, the released void space gas can advantageously be 
passed directly from the bed undergoing cocurrent depressurization to the 
bed being purged. Alternatively, the prior art has employed systems in 
which said released void space gas is passed, not directly to another bed, 
but to an external storage tank for passage therefrom to the bed to be 
purged, typically at an economic penalty vis-a-vis direct pressure 
equalization and provide-purge systems. 
In the practice of the invention, a portion of the released void space gas 
that is to be used for purge purposes is introduced, as in the patents 
referred to above, directly into an adsorbent bed that is to be purged at 
that point in the processing cycle of the overall PSA system. The 
remaining portion of released void space gas, however, is simultaneously 
introduced into an external vessel. Such gas is thereafter passed from the 
external vessel, as purge gas, to the same adsorbent bed that received a 
portion of said void space gas directly as purge gas. 
The invention can advantageously be practiced in multi-bed PSA systems 
having at least four adsorbent beds therein, preferably in systems having 
from five to eight adsorbent beds, although the invention can also be used 
in systems having a larger number of beds. It will be understood that, in 
such multi-bed systems, the feed gas may be passed to more than one bed at 
any particular stage of the processing cycle. Thus, the feed gas is often 
passed to at least two beds at any given time in the operation of such 
multi-bed systems. As indicated above with respect to conventional 
practice and the practice of the invention, the PSA process desirably 
employs, in multi-bed operations, one, two, three or more pressure 
equalization steps in which cocurrent depressurization gas released from 
one bed at an elevated pressure is used to partially repressurize another 
bed initially at lower pressure. Thus, the invention can be used in a 
variety of processing cycles such as, for example, those involving five 
adsorbent beds, with two on adsorption at any time, and one pressure 
equalization step, those involving six adsorbent beds with two on 
adsorption at any time, and two pressure equalization steps, and those 
involving eight adsorbent beds, with two on adsorption at any time and 
three pressure equalization steps. Those skilled in the art will 
appreciate that various other PSA processes and systems can be adapted so 
as to take advantage of the desirable benefits of the invention. 
The practice of the invention can be illustrated by the Table below with 
respect to a six bed embodiment of the invention: 
TABLE 
__________________________________________________________________________ 
Bed No. 
Cycle (622) E/PP 
1 
##STR1## 
2 
##STR2## 
3 
##STR3## 
4 
##STR4## 
5 
##STR5## 
6 
##STR6## 
External 
Vessel 
##STR7## 
__________________________________________________________________________ 
In this Table with respect to each bed, A represents an adsorption step at 
a high adsorption pressure; the numeral 1 represents a cocurrent 
depressurization-pressure equalization step between a bed that has just 
completed its adsorption step and is being depressurized and a partially 
repressurized bed that has completed its repressurization step 2 and is 
initially at a lower pressure than said bed being depressurized; 2 
represents a second cocurrent depressurization-pressure equalization step 
between a bed that has completed its step 1 depressurization and is being 
further depressurized and a partially repressurized bed that has completed 
its repressurization step 3 in which a portion of the void space gas 
released from the product end of a bed is passed directly to another bed 
undergoing its purge step and the remaining portion of said gas is 
simultaneously introduced into an empty external vessel for the 
repressurization or filing thereof; D represents a countercurrent 
depressurization step in which gas is released from the feed end of the 
bed; P represents a purge step at lower desorption pressure in which void 
space gas released from another bed during its E/PP step is passed 
directly to the bed undergoing said purge step; 3 represents an indirect 
pressure equalization step in which void space gas previously introduced 
into the external vessel during the E/PP step in one bed is passed 
therefrom during an emptying step therein and is passed to the bed into 
which void space gas was passed directly during said E/PP step for the 
repressurization thereof, with said step 3 occurring immediately after 
said bed has completed its purge step P; and R represents repressurization 
to higher adsorption pressure. In the Table with respect to the external 
vessel, E represents external vessel emptying and F represents external 
vessel filling. In the process of the embodiment illustrated in the Table, 
it will be seen that two of the six beds are in their adsorption step, in 
overlapping sequence, at any given time in the cycle. As two direct 
pressure equalization steps are employed, i.e. steps 1 and 2, the overall 
cycle is referred to in the heading of the Table as a (622) E/PP cycle, 
the 6 representing the number of beds, the first 2 representing the number 
of beds on adsorption, the second 2 representing the number of direct 
pressure equalization steps, and E/PP denoting the point of novelty of the 
invention wherein a portion of the cocurrent depressurization, 
provide-purge gas passes directly from one bed to another for such purge 
purposes, while another portion passes simultaneously to an empty external 
vessel for subsequent use as repressurization gas as described above. An 
eight bed system having two beds on adsorption and two direct pressure 
equalizations would thus similarly be referred to as having an (822) E/PP 
cycle. 
In the processing cycle illustrated in the Table, the cocurrent 
depressurization, provide-purge step of bed 1 involves passing released 
void space gas directly from the product end of bed 1 to the product end 
of bed 6, i.e. to the fifth higher numbered bed, to provide purge gas for 
said bed 6, which is on its purge step following step D. At the same time, 
released void space gas from bed 1 is introduced into an empty external 
vessel for the repressurization thereof. During the initial portion of 
countercurrent depressurization step D in bed 1, void space gas is passed 
from the thus repressurized external vessel drum to said bed 6 for partial 
repressurization purposes. After countercurrent depressurization step D in 
bed 1, void space gas is passed from the external vessel to bed 1 for 
initial repressurization purposes. In the illustrated embodiment, it will 
be seen that the void space gas used to fill and repressurize the empty 
external vessel, for subsequent use in partially repressurizing bed 1, 
comes from bed 2. Thus, a part of the void space gas from bed 2 is used 
indirectly to partially repressurize the fifth higher numbered bed as the 
numbering reverts to bed 1 after bed 6 in cyclic operations. Similarly, 
each bed passes void space gas to the third higher numbered bed during 
direct pressure equalization step 1, and to the fourth higher numbered bed 
during said direct equalization step 2. Similarly, it will be seen that 
each bed passes void space gas directly to the fifth higher numbered bed 
during the direct provide purge portion of the E/PP step of the invention. 
It will also be seen that, in the illustrated cycle, each bed undergoes an 
idle or delay period between repressurization steps 2 and 1. The (622) 
E/PP embodiment of the invention has costs and performance characteristics 
essentially equivalent to those obtainable in a ten bed Fuderer et al 
system. 
Those skilled in the art will appreciate that various changes and 
modifications can be made in the details of the PSA process and system as 
herein described without departing from the scope of the invention as 
recited in the appended claims. Thus, while the invention has been 
described in particular with reference to a desirable six bed system, it 
will be appreciated that other systems having a different number of beds 
in the system can be employed, and that various PSA processing features 
can be incorporated with any particular cycle or system incorporating the 
particular E/PP step as herein disclosed and claimed. In applying the 
invention generally, it will readily be appreciated that PSA systems 
necessarily incorporate various conduits, valves and other control 
features to accomplish the necessary switching of the adsorbent beds from 
one processing step to the next in appropriate sequence. The invention 
employs conventional conduits and control features well known in the art, 
as indicated by reference to the patents referred to above. For purposes 
of the invention, it will be understood that the external vessel is 
employed together with means, i.e. conduits and suitable conventional 
controls, for passing a portion of the void space gas released during the 
cocurrent depressurization, provide-purge step to said external vessel for 
repressurization of the empty vessel simultaneously with the introduction 
of the remaining portion of said released void space gas directly into a 
bed to be purged through conventional conduit means. Means are similarly 
provided for passing void space gas from said external vessel to the same 
bed purged by the remaining portion of said void space gas, and for 
passing feed gas to two or more adsorbent beds at all stages of the 
processing cycle, and to accomplish other desired processing steps in 
particular embodiments of the invention. 
The pressure swing adsorption process and system herein disclosed and 
claimed can be advantageously employed to selectively adsorb at least one 
component of a feed gas mixture, thereby separating and purifying a 
desired product effluent gas. For example, the invention can be used to 
advantage in separating and purifying hydrogen present as a major 
component of a feed gas mixture also containing carbon dioxide as a 
selectively adsorbable component, commonly together with one or more 
additional minor components to be removed as undesired impurities, such as 
nitrogen, argon, carbon monoxide, light saturated and unsaturated 
hydrocarbons, aromatics, light sulfur compounds and the like. Those 
skilled in the art will appreciate that the invention can also be 
advantageously employed for other desirable separations in which at least 
one component of a feed gas mixture is selectively adsorbed in an 
adsorption system of the type herein described. The separation and 
purification of oxygen from air, and methane purification from mixtures 
thereof with carbon dioxide, ammonia, hydrogen sulfide and the like, or 
from other heavier hydrocarbon gases, are examples of other applications 
of the invention. It should be noted that the PSA process in general, and 
the invention in particular, can be carried out using any suitable 
adsorbent material having a selectivity for one component of a feed gas 
mixture over another, as for the impurity over the desired product gas. 
Suitable adsorbents include zeolitic molecular sieves, activated carbon, 
silica gel, activated alumina and the like. Zeolitic molecular sieve 
adsorbents are generally desirable in the separation and purification of 
hydrogen contained in mixtures thereof with carbon dioxide, nitrogen and 
the like. Further information concerning suitable adsorbents, including 
such zeolitic molecular sieves is contained in the Kiyonaga U.S. Pat. No. 
3,176,444, and various other patents such as those referred to above. 
As was indicated above, various changes and modifications can be made in 
the PSA process and system to which the invention is directed without 
departing from the scope of the invention as herein disclosed and claimed. 
Thus, the manner in which the pressure equalization steps are carried out, 
i.e., either directly or indirectly through external equalization vessels, 
the number of such equalizations, the manner in which repressurization to 
higher adsorption pressure, i.e., by feed gas or by a portion of the 
product effluent from the system, is not critical to the invention or to 
the obtaining of the benefits therefrom. In this regard, it should also be 
noted that, while the purge step has been described herein as occurring at 
the lower desorption pressure, those skilled in the art will appreciate 
that the purge step can be carried out at a pressure above said lower 
desorption pressure, although it is more commonly carried out after 
countercurrent depressurization to a lower desorption pressure. 
In an illustrative example of the invention, it has been found that a 
particular (622) E/PP system can be used to achieve a performance 
approaching that of a conventional (10,3,3) Fuderer et al system at a 
relatively small incremental cost over that of a conventional six bed 
system employing direct equalizations without the use of an external 
vessel. In the operation of such a system with the higher adsorption 
pressure being about 290 psia, cocurrent depressurizations down to about 
35 psia and a countercurrent depressurization or blowdown, i.e., waste, 
pressure of about 21 psia, recovery of product hydrogen of 99.99+% purity 
is on the order of 87.5%. By contrast, the use of only two direct pressure 
equalization steps, without the indirect repressurization provided by the 
E/PP and indirect pressure equalization 3 steps of the invention, there is 
too much gas storage in each bed, so that either the countercurrent 
depressurization, i.e. dump, step or the purge step must be increased. In 
either case, product recovery is necessarily reduced. Conversely, if a 
system employing three direct pressure equalizations were to be employed, 
there would be too little gas storage, and product gas would have to be 
used for providing purge purposes, again reducing product recovery. It 
would also appear that, for a three direct pressure equalization system of 
comparable performance, a minimum of eight adsorbent beds would be 
required, rendering the system economically unattractive as compared with 
the six bed (622) E/PP system of the invention. 
In an alternative approach, it is possible to employ the void space gas 
released from each bed during its cocurrent depressurization steps for (1) 
a first direct pressure equalization step, (2) a second direct pressure 
equalization step, (3) a provide-purge gas step, and (4) a third direct 
pressure equalization step. Such an approach, however, has attendant 
disadvantages. For example, in the third equalization step following the 
purge step, the least pure gas is being used for repressurization rather 
than for purge and bed regeneration. In addition, no waste gas flows into 
the waste surge drum during part of the processing cycle. This 
circumstance necessitates the use of higher purge pressures into the waste 
surge drum with an accompanying decrease in product recovery. 
By comparison, the practice of the invention enables the void space gas 
used for a third pressure equalization to be taken into an external vessel 
during the early part of the cocurrent depressurization, provide-purge gas 
step, with the gas then being passed from said external vessel to the same 
bed as is purged with said provide-purge gas for partial repressurization 
thereof. The invention has very significant advantages compared to the use 
of a third direct pressure equalization after the cocurrent 
depressurization, provide-purge step. Thus, the gas employed for the third 
pressure equalization is purer than the purge gas. In addition, those 
skilled in the art will appreciate from a review of the Table that a 
continuous waste gas flow to a waste surge drum occurs in the system, as 
all points in the processing cycle, have either a countercurrent 
depressurization step D or a purge step P in which a waste gas is produced 
and is passed to such waste surge drum. Because the surge drum does not 
have to accumulate gas for periods in which there is no purge or waste 
effluent, the average purge pressure is lower than for the indicated 
alternative approach in which a third pressure equalization is employed 
after the cocurrent depressurization, provide-purge gas step. In a further 
advantage, the practice of the invention enables the purge step to be 
longer, and the pressure drop for purge purposes to be lower than in said 
alternative approach, further enhancing product recovery and/or purity. 
Additionally, it is found that the external vessel employed in the 
practice of the invention is relatively small, i.e. from less than 50% of 
an adsorber bed volume up to one bed volume. As such, the external vessel 
is smaller than the difference in waste surge drum size between the lower 
requirements of the invention and the higher requirements of the 
alternative approach. This factor further enhances the overall 
technical-economic benefits obtainable by the use of the process and 
system of the invention. 
The invention enables pressure swing adsorption performance characteristics 
to be enhanced in systems having a lesser number of adsorbent beds, and 
favorable overall costs, compared with the practices of the highly 
desirable process of the Fuderer et al patent referred to above. A highly 
desirable processing flexibility is thereby provided, so as to enable the 
advantageous pressure swing adsorption technology to be even more 
precisely adapted to the requirements of a given application than has 
heretofore been possible. The invention thus contributes in a very 
significant manner to the desired development of PSA processes and systems 
in meeting the ever more restrictive overall requirements of industrial 
gas separation and purification operations.