Note: Descriptions are shown in the official language in which they were submitted.
~3~ 39
Back round of the Invention
Field of the Invention - The invention relates
the purification of gases in a pressure swing adsorption
system. More particularly, it relates to improve-
ments in the processing cycle and syst:em enabtingimproved performance to be achieved;
Description of the Prior Art - The pressure
swing adsorption (PSA) process provides a commercially
desirable technique for separa~ing and purifyi.ng
at least one component of a feed gas mixture contain-
ing said 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 reducing the adsorbend bed pressure to a
lower desorption pressure. The carrying out of the
PSA process in multi-bed systems is illustrated by
the Wagner patent~ U.S. 3,430,418, relating to a
system having at Least four beds. As is generally
known and described in this patent, the PSA process
is commonly carried out, on a cyclic, in a processing
sequence that includes, in each bed, (1) higher
pressure adsorption with release of product effluent
from the product end of the bed, (2) cocurrent
depressurization to intermediate pressure with release
of void space gas from the product end thereo~,
(3) countercurrent depressurization to a lower de-
sorption pressure, (4) purge and (5) repressurization.
The void space gas released during the cocurrent de-
pressurization step is commonly employed for pressureequalization purposes and to pro~ide purge gas to a
bed a~ its lower desorption pressure.
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3~ ~ ~3
--3--
In a variation of said PSA processing described
above with reference to systems having four or more
absorbent beds, a conventional three ~ed system was
devised for use in the separation and recovery of
air and other such separations. This system was based
on the increasing pressure adsorption step described
in the McCombs patent, U.S. 3,738,087. In one embodi-
ment thereof, air is added to an adsorbent bed for
the repressurization thereof, with nitrogen being
selectively adsorbed and with oxygen being discharged
from the product end of the bed at rates such that the
bed pressure increases to upper adsorption pressure.
A PSA cycle incorporating said increasing pressure
adsorption step includes (1) said increasing pressure
adsorption step, (2) cocurrent depressurization to
intermediate pressure with release of void space gas
from the product end thereof, (3) countercurrent
depressurization to a lower desorption pressure,
(4) purge and ~5) depressurization. The void space
gas released during the cocurrent depressurization
step is employed, in this embodiment, for passage to
other beds in the system in a pressure equalization-
provide purge pressure equalization sequence. This
latter cycle makes unnecessary a constant pressure
adsorption step as employed in the Wagner cycle. This
enables more time for bed regeneration, i.e. counter-
current depressurization and purge, within a given
cycle time so as to enable greater productiv~ty and
recovery and/or purity to be obtained from a given
system, particularly in systems designed for relative-
ly short overall cycle time operation.
Using such a three bed system with each bed
. containing commercial 13X, 8 x 12 head form, molecular
s~eve in air separation operations, an oxygen
. . .
,
D- 14,716
~ 39
recovery of 43% and a productivity (BSF) of 4,000 lb.
13 X molecular sieve per one ton per day (TPD) of oxygen
have been obtained. Said xecovery is defined as the
percent or volume fraction of the feed air oxygen
removed from the feed stream and delivered as oxygen
product. Productivity is defined as the pounds of
molecular sieve required to generate 1 TPD of contained
oxygenO The recovery and productivity values referred
to above were obtained on the basis of a 180 second
total cycle time for the 3-bed PSA system, with feed
air being introduced at a maximum pressure of 40 psig,
with product being discharged at 20 psig.
While such standard 3-bed system is desirable
for various commercial applications, there is,
nevertheless, a desire in the art to improve product
recovery and productivity. Difficulties have been
encountered, however, in achieving such objectives.
Thus, the total cycle time had to be reduced to less
than said 180 seconds to yield a significant BSF re
duction ~productivity increase~ compared to said
standard 3-bed operation. Howe~Jer, reductions in
individual step times, i.e. the purge and pressure
equali~ation steps, are limited by gas velocity and
bed fluidization limits, or by applicable cycle
performance standards. Such limitations prevent
the achieving of substantial cycle ~ime reductions
only by means of reductions in the duration of the
`individual cycle steps. With respect to the standard
4-bed system, on the other hand, the addition of a
fifth adsorbent bed to increase single bed capacity
limits by means of standard cycling techniques
applicable to said systems would necessarily result
in an increase in total cycle times and in the BSF
values for any given application. Such an increase
D- 14,716
~3(~039
in BSF value would compromise any potential increase
in productive capacîty derived from an increase in the
number of vessels employed in the PSA system. In
addition, size limitations on PSA-oxygen adsorbent
beds limit the maximum capacity of a single PSA
train, so that the development of means to reduce
the BSF would be required to increase the maximum
capacity limits of such a single PSA train. There
remains in the art, therefore, a need to develop
improvements in the PSA art enabling reductions in
BSF and increased single train capacity to be achieved.
Such improvements advantageously would enable the
overall cycle time to be reduced, while enabling
sufficient time to complete each individual cyc}e
time without degradation of product purity ar recovery.
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 for the enhanced
separation and recovery of oxygen from air.
It is another obJect of the invention to pro-
vide a PSA process and system enabling overall cycle
times to be minimized while enabling sufficient time
to complete each individual cycle step without
degradation in product purity or recovery.
With these and other objects in mind, the
in~ention is hereinafter described in detail, the
novel features thereof being particularly pointed
out in the appended claims.
D- 14,716
~ 30 ~ ~3~
Summary of the Invention
The PSA process and system of the invention
advantageously employs novel simultaneous cycle
steps that enable the total cycle time to be reduced,
product recovery to be enhanced, and BSF reductions
to be achieved. The time required for the separate
provide purge step is desirably shorter than the
purge step, with the overall countercurren~ depressuriza-
tion and purge time enabling adequate bed regenera-
tion to be accomplished with the overall shorter total
cycle times permissible in the practice of the inven-
tion.
Detailed Descri tion of the Invention
_ _ P
The objects of the invention are accomplished
by the incorporation into the PSA process and system
as described above with reference to the Wagner patent
of simultaneous cycle steps relating to the cocurrent
depressurization and the countercurrent depressuriza-
tion portions of the overall cycle. Such
simultaneous steps enable the total cycle time to be
reduced and BSF reductions, i.e. increased adsorbent
produc~ivity, to be obtained. Enhanced bed capacity
can be achieved in the practice of the invention at
minimized overall cycle times without degradation of
product effluent purity or recovery. The invention
has been found to actually result in significant
improvements in product recovery as compared with
the standard commercial 3-bed PSA process and system
referred to above.
It should be noted that the invention, in a
desirable embodiment, is based upon the use of a
constant adso-ption pressure step, as in the Wagner
cycle referred to above, wherein feed gas is passed
D-14,716
~31)~39
- 7 -
to an adsorption bed maintained at an upper
adsorption pressure level, with the more readily
adsorbable component being selectively adsorbed and
with the less readily adsorbable component being dis-
charged from the product end of the bed as product
effluent. Such constant adsorption pressure cycles,
modified in accordance with the invention, enable
lower BSF requirements to be achie~ed, which result
in higher single train capacities and lower adsorbent
inventory requirements for a given product capacity
and purity. The following description of the
invention will be understood, therefore, to represent
a desirable modification of the overall processing
cycle of Wagner as recited above.
The invention can advantageously be employed
in multi-bed PSA systems having at least four
adsorbent beds therein, with systems containing four
beds being highl~ desirable for some applications.
Five, six or seven adsorbent bed systems are also
desirable in some instances. While the invention can
be practiced in systems having 8 or more beds~ it
is commonly expedient to employe two 4-bed systems,
or the like, as an alterna~ive to such higher number
of beds in a single system. 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 with
respect to conventional practice, the PSA process
desirably employs two, three or more pressure equali-
zation 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 and to provide purge to a bed at lower
~_ 14,716
~3~)~1)39
--8--
desorption pressure. Thus, the inven~ion can be
used in a variety of processing cycles such as,
for example, those involving five or more beds
with two beds on adsorption at any g:iven time,.in
overlapping sequence, during the overall PSA p~o-
cessing cycle. Those skilled in the art will
appreciate that various other PSA processes and systems
can be adapted so as to take advantage of the desir-
able benefits of the invention in desirable PSA
cycles.
The practice of the invention can be
illustrated by Table I below with respect to a
four bed embodiment of the invention:
TABLE I
15 Bed No. Cycle (412) El/pp, E2/BO
~ _ ~ ~ ~/1 ~
2 ~ .
3 .
4 ~ ~ ~ ~Z ~ 2
In th~s Table with respect to each bed, A
represents an adsorption step at an upper adsorption
pressure with discharge of the less readily adsorbable
component as product effluent from the product end
of the bed; PP represents a cocurrent depressurization
step i~ which ~oid space gas is released from
the product end of the bed ~or use in providing purge
gas to anot~er bed typically at its lower desorption
pressure, said bed being depressurized from an upper
intermediate pressure to an intermediate pressure
level; P represents a purge step typically at lower
desorption pressure in which void space gas released
from another bed is passed directly to said bed under-
going its purge step, with said purge step being seen
D~ ,716
139~)039
to be of longer duration than the provide purge step PP;
R represents repressurization to upper adsorption
pressure; El/PP represents one of the novel steps
of the invention, namely a cocurrent depressuriza-
tion step in which void space gas released duringcocurrent depressurization from said upper
adsorption pressure to an upper intenmediate pressure
is passed simultaneously to one othe.r bed in the
system being partially repressurized to said upper
intermediate pressure and to a second o.ther bed as,
purge gas for said bed at its lower desorption
pressure level; and E2/BD represents the other novel
step wherein further cocurrent depressurization
from said intermediate pressure level to a lower
intermediate pressure is carried out with release
of additional void space gas from the product end
of the bed, said void space gas being passed to
another bed in the system for pressure equaliza-
tion therebetween at said lower intermediate pres-
sure, while'the bed is being simultaneously de-
pressurized countercurrently by the discharge of
gas from the feed end of the bed. The counter-
current, or BD, portion of said E2-BD step is con-
tinued after completion of said pressure qualiza-
tion down to the lower desorption pressure of thebed. ,In the process of the embodiment illustrated
in said Table I, it will be seen that only one of
the four beds in the system is on the adsorption
step, on a cyclic basis, at any given time in the
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cycle. As two pressure equalization steps are
employed, i.e. said El/PP and E2/BD and their counter-
- parts E2 and El, the overall cycle is referred to
in the heading of the Table as a (412) El/PP - E2/BD
cycle, the 4 representing the number of beds, the 1
representing the number of beds on adsorption at any
given time, and the 2 representing the number of
direct pressure equalization steps, and El/PP -
E2/BD denoting the point of novelty of the invention
wherein the two simultaneous processing features
described above are employed in the PSA processing
cycle to obtain the benefits referred to herein.
In the processing cycle illustrated in
Table I, the El/PP step is carried out, e.g. in bed l,
by the cocurrent depressurization of said bed with
the void space gas released from the product end of
the bed being simultaneously passed to bed 3 for
pressure equalization at upper intermediate pressure
and to bed 4 for the initial portion of the purge
step in said bed. Following the continuing cocurrent
depressurization wherein void space gas from bed 1
is passed to said bed 4 as purge gas, with bed 1 being
depressurized further to an intermediate pressure
level, the E2/BD s~ep is carried out with additional
void space gas being released from the produc~ end of
bed 1, which is cocurrently depressurized to a lower
intermediate pressure, said gas being.passed to
bed 4 for pressure equalization at said lower inter-
mediate pressure. Bed 1 is simultaneously depressur-
ized countercurrently by the discharge of gas fromthe feed end thereof. T~e BD portion of the step is
continued after completion o the E2 portion upon
pressuré equalization of depressurizing bed 1 and
repressurizing bed 4 at said lower intermediate
pressure. It will be seen from this example that the
D- 14,716
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E2 step represents partial repressurization of a
bed from its lower desorption pressure to lower
intermediate pressure by the passage of void space
gas thereto, typically directly, from a bed being co-
currently depressurized in its E2/BD step fromintermediate pressure to said lower intermediate
pressure by pressure equalization with said bed
being partially repressurized from its lower
desorption pressure to said lower intermediate
pressure. Similarly, El represents Eurther partial
repressurization to upper intermediate pressure by
the passage of void space gas thereto, typically
directly, from a bed being cocurrently depressurized
in its El/PP step from upper adsorption pressure to
upper intermediate pressure by pressure qualization
with said bed being partially repressurized from
its lower to its upper intermediate pressure.
When the invention as represented by the
processing cycle of Table I is employed in a
practical commercial air separation operation,
each bed being operated at an upper adsorption
pressure of 40 psig, a 160 second total or overall
cycle time can be effectively utilized. The
BSF of commercial 13X, 8 x 12 beaded, molecular
sieve adsorbent, has 3,000 lb. of said 13X/TPD
of oxygen product at 90% product purity. Recovery
of oxygen product was 53%. By contrast, at the same
40 psig adsorption pressure, using com-
mercial 5A, 8 x 12 beads and producing product
oxygen at said 90% purity, a 4-bed Wagner cycle
system required a 240 second total cycle time,
with a higher BSF, i.e. lower productivity, of
6,000 lb. of said 5A /TPD of oxygen product.
D- 14,716
~3~039
In another desirable embodiment of ~he
system, five adsorbent beds are employed with two
beds on adsorption at all times. As in the prevîous
illustrated embodiment, two pressure equalization
steps are employed, together with the El/PP and
E2tBD steps of the inven~ion. ~ence, ~he cycle is
designated as a (522) El/PP - E2/BD cycle in the
heading of Table II below.
TABLE II
10 Bed No Cycle (522~ El/PP -E2/BD
_ --
1 ~ ~ ~d ~¦ f~
2 ~/ ~? ~ o~ ~ ~-
3 f~ ~ /J~i
4 ~ ~ ~ f~ 9
, ~
5 ~ ~ A~
In the cycle of Table I~, A, El/PP, PP,
E2/BD, P, E2, El and R all have the same meanings
a were indicated above with respect to the Table I
embodiment. In the practice of said (522) El/PP-
E2/BD Cycle for oxygen recovery at said 90%purity by air separation, employing an upper
adsorption pressure of 40 psig and using the same
commercial 13 X molecular sieve adsorbent, in 8 x 12
beaded form, a total cycle time of 200 seconds is
employed, with an BSF of about 3,800 lb. of said
BX/TPD of oxygen product. Recovery of product
oxygen is_54%. By contrast at the same 40 psig
adsorption pressure, using the same commercial 13X,
8 x 12 beads and producing product oxygen at said
90% purity, a standard 3-bed cycle as described above
involves the use of a 180 second total cycle time,
~- 14,716
~ 3~0 O~ ~
with a BSF of 4,005 lb. of said 13X/TPD of oxygen
product. The recovery of oxygen product was only
49%.
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 described herein without departing from the
scope of the invention as recited in the appended
claims. It will also 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 can readily be employed using conventional
conduits and control features well known in the art.
For purposes of the invention, the PSA system will
comprise conduit means for passing void space gas
released from the product end of a bed during
cocurrent depressurization from said upper inter-
mediate pressure simultaneously to other beds inthe system, said gas being passed to one bed for
pressure equalization at said upper intermediate
pressure, and to another bed for providing purge
gas to said bed. Commercially available control
means can readily be employed for enabling the
passage of void space gas from the bed being co-
currently depressurized to continue until an
intermediate pressure level is reached, with the
released gas being passed to the bed being purged,
following termination of the passage of gas to the
bed ~eing pressure equalized at said upper inter-
mediate pressure. Conduit means are also pra-
vided for passing additional void space gas
released from the product end of the bed, upon
further cocurrent depressurization thereof from
said intermediate pressure to a lower intermediate
pressure, to another bed in the system for pressure
D- 14,716
130~6~39
-14-
equalization therebet~een at lower intermediate
pressure and for simultaneously dischargin~ gas from
the feed end of the bed. Control means can likewise
be provided for precluding the passage o~ gas
from the bed to which gas had been passed during
pressure equalization at lower intermediate
pressure upon continuance of the discharge of gas
from the feed end of the bed, i.e. the continued
BD portion of the E2/BD step, down to said lower
desorption pressure upon completion of the pressure
equalization at lower intermediate pressure. It is
convenient to employe an in-line check valve as
said latter control means, with said check valve
being adapted to prevent back-flow of gas at lower
intermediate pressure into said bed being further
depressurized from -aid lower intermediate pressure
to lower desorption pressure. Those skilled in
the art will appreciate that such variations or
modifications of the PSA process and system of the
2n invention can include in appropriate circumstances,
the inclusion of additional pressure equalization
steps or the providing of additional adsorbent
beds on the adsorption step at any given time in
overlapping processing sequence.
While the invention has been described above
with reference to a constant pressure adsorption step
in contrast to the standard 3-bed system employing
an increasing pressure adsorption step without sub-
sequent adosrption at a constant upper adsorption
pressure level, it should be noted that the invention
can be practiced by incorporating`an inc~easing
pressure adsorption step ad the repressurization step,
i.e. step R of Tables I and II, following partial
D-14,716
~L30~:)039
repressurization of a bed by pressure equalization,
i.e. steps E2 and El of said Tables. In such an
embodiment, therefore, the repressurization to upper
adsorption pressure is carried out with product
effluent being simultaneously discharged from the
product end of the bed. In such circumstance, it
will be appreciated that increased amounts of product
gas can be recovered in any given cycle time without
sacrifice of the time available for regenration of
the bed, or alternatively, the constant pressure
adsorption step can thus be made shorter to allow
for more time for bed regeneration purposes, there-
by enabling product purity andtor recovery to be
enhanced. It should also be noted that said increas-
ing pressure adsorption step can advantageously beemployed, together with the E2/BD step of the invention
in 3-bed PSA systems without a constant pressure
adsorption step or the El/PP step of the invention.
This processing variation can also be employed in
PSA systems having more than three beds. Thus, a
processing sequence of El (depressurization from
upper adsorption pressure), PP, E2/BD, P, E2 (partial
repressurization ) and increasing pressure adsorption
to upper adsorption pressure could be employed in the
practice of this variation.
The pressure swing adsorption process and
system herein disclosed and claimed can be advantageous-
ly employed to selectively adsorb at least one
component of a feed gas mixture, thereby separating
and purifying a desired product effluent gas, While
the invention is particularly advantageous for
separating and recovering oxygen as the less readily
adsorbable component of air from nitrogen as the
more readily adsorbable component thereof, it will be
appreciated by those skilled in the art that various
other separations, including the recovery of hydrogen
D- 14,716
~300039
-16-
from feed gas mixtures or even the separation and
recovery of nitrogen as the product effluent from
feed air is feasible depending upon the performance
characteristics of the particular adsorbent employed
in the PSA system and its ability to selectively
adsorb one component from a feed gas mixture in
preference to another, less readily adsorbable
component Suitable adsorbent materials may
include zeolitic molecular sieves, activated carbon,
silica get, adtivated alumina and the like. Zeolitic
molecular sieve adsorbents are generally desirable
for said oxygen separation and recovery ~rom air, with
said 13 X adsorbent or 5A molecular sieve being
standard materials that can readily be employed in
the commercial practice of the prior art approaches
improved as herein disclosed and claimed.
It will be understood that various operating
conditions can be employed in the practice of the
invention, depending upon t'ne particular separation
being carried out, the purity level desired, the
adsorbent material employed, and the ike. It has
been found, however, particularly with respect to
the separation and recovery of oxygen from air, that
an upper adsorption pressure of from about 40 to
about 60 PSIG, preferably about 45 to about 55 psig,
is desirable. Desorption LS conventionaLly at about
atmospheric pressure, but other, higher or lower
desorption pressures can also be employed in
particular applications. The invention enables
the overall cycle time to be desirably minimi2ed,
with cycle times of from about 140 to about 180
seconds being ~easible in various embodiments, parti-
cularly in 4-bed systems, while somewhat longer -
times may.be requred in embodiments such as illus-
trated in Table II wherein a 5-bed system was
D- 14~7l6
1300039
employed with two beds in adsorption at any given
time and in which an increase in re overy was
obtainable as compared with the 4-bed system
illustrated in Table I. In general, oxygen
product recovery in air separation applications of
the invention are readily obtainable wi~hin the range
of from about 50% go about 60%, typically from
about 53% ~o about 55~. -
The invention will thus be seen to satisfy
the desire in the are for improvements in the PSA
technology as applied to various gas separation
operations, such as the separation and recovery of
oxygen from air. The simultaneous cycle steps o
the invention thus enable increased absorbent produc-
tivity to be achieved, providing increased bedcapacity, while product recovery improvements
averaging about 5 to 6% can be obtained as compared
with the commercial 3-bed PSA process. The process-
ing cycles of the invention advantageously employ
provide purge steps shorter in time than the time
provided for the actual purge of the bed, with
the overall cycle times being minimized without
degradation of product effluent purity. The
invention thus enhances the feasibility of applying
~5 PSA technology in practicaL, commercial gas
separation operations in a more efficient, effective
manner than was heretofore possible utilizing the
PSA technology as developed heretofore in the art.
D-14,716
