Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02501094 2002-07-16
TITLE OF THE INVENTION:
PRESSURE SWING ADSORPTION PROCESS
WITH REDUCED PRESSURE EQUALIZATION TINE
This is a division of copending Canadian Application
Serial No.: 2,393,683, filed July 16, 2002.
BACKGROUND OF THE INVENTION
[0001] Pressure swing adsorption is an important gas separation
process which is widely used in the chemical process industries.
The process has been highly developed in particular for use in the
recovery of high purity hydrogen from synthesis gas, refinery
offgases, and other hydrogen-containing gas mixtures. The process
as known in the art uses multiple adsorbent beds operating in
parallel with overlapping cycles to provide selective gas flow
between beds for pressure equalization and purge steps. Highly
developed cycles are known which use up to 16 beds with multiple
beds on feed and multiple beds on purge at any given time. Each
bed can undergo multiple pressure equalization steps with other
individual beds in order to increase product gas recovery.
[0002] Each adsorbent bed in a pressure swing adsorption (PSA)
cycle typically utilizes a sequence of steps which begins with a
feed or adsorption step in which a pressurized feed gas mixture is
passed through a bed of adsorbent which selectively adsorbs one or
more of the components in the mixed feed gas. A product gas
containing the desired component at acceptable purity is withdrawn
from the bed until the adsorption step is terminated at a
predetermined time.
[0003] After termination of the adsorption step, the pressure in
the bed is reduced in a series of pressure equalization steps in
which gas is transferred at decreasing pressure to a succession of
other beds to provide pressurization to those beds. Final
depressurization typically is completed by withdrawing a waste as
in a final blowdown step. The depressurized bed then is purged
with purge gas provided from other beds, thereby removing
additional adsorbed components from the bed.
[0004] Upon completion of the purge step, the bed is repressurized
to an intermediate pressure by a succession of pressure
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equalization steps in which gas is transferred from other beds,
and the bed is pressurized further to the feed pressure with feed
and/or product gas. The steps are repeated in a cyclic manner.
[0005] An objective in the development of pressure swing
adsorption cycles is to reduce the cycle time in order to reduce
the amount of adsorbent required in the beds for a given feed rate
while continuing to provide product at an acceptable product
purity. This has the desirable effect of reducing the capital
cost of the process equipment required for a given volumetric
production rate. The present invention as described below and
defined by the claims which follow addresses this need by reducing
the required pressure equalization time in a cycle, thereby
reducing the overall cycle time and increasing product recovery
per unit of adsorbent used.
BRIEFSUMMARY OF THE INVENTION
[0006] A pressure swing adsorption process is disclosed for
recovering a less strongly adsorbable component from a feed gas
mixture comprising at least one less strongly adsorbable component
and at least one more strongly adsorbable component, which process
comprises performing sequential process steps in an adsorbent bed
which include an adsorption step, two or more pressure
equalization steps at decreasing pressure, a provide purge step, a
blowdown step, a purge step, two or more pressure equalization
steps at increasing pressure, and a final repressurization step,
wherein the duration of each pressure equalization step is less
than about 25 seconds and the adsorbent bed is one of at least
four parallel adsorbent beds undergoing the sequential process
steps in a cyclic manner.
[0007] In the process, four or more pressure equalization steps
can be carried out at decreasing pressure and four or more
pressure equalization steps can be carried out at increasing
pressure in the adsorbent bed. Optionally, the provide purge
step and the blowdown step can overlap such that gas is withdrawn
from the product end of the bed for providing purge to another bed
while additional blowdown gas is withdrawn from the other end of
the bed. Optionally, the final pressure equalization step at
decreasing pressure and the provide purge step can overlap such
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that gas is withdrawn from one end of the bed for providing purge
to another bed while additional gas is withdrawn from the other
end of the bed to pressurize yet another bed. Three beds can be
on the purge step at any given time.
[0008] The adsorbent bed can be one of 12 or more parallel
adsorbent beds. The feed gas mixture can contain hydrogen,
carbon monoxide, carbon dioxide, and one or more
hydrocarbons containing one or more carbon atoms, wherein
hydrogen is the less strongly adsorbable component. In this
process, a series of two or more pressure equalization steps
can be effected between an adsorbent bed and other adsorbent
beds, and the final differential pressure in one of the
pressure equalization steps can be greater than the final
differential pressure in any earlier step in the series of
two or more pressure equalization steps.
[0009] In one embodiment, the process uses 16 parallel
adsorbent beds which undergo the sequential process steps in
a cyclic manner. Four beds can be on the adsorption step at
any given time. In additional, four pressure equalization
steps can be carried out at decreasing pressure and four
pressure equalization steps can be carried out at increasing
pressure in each of the 16 adsorbent beds. Also, three beds
can be on the purge step at any given time. Preferably,
each bed on the purge step is purged exclusively by gas
provided exclusively from another bed on the provide purge
step. Optionally, an idle period can follow the purge step.
[0010] In another embodiment, 14 parallel adsorbent beds are
utilized which undergo the sequential steps in a cyclic
manner. Three or four beds can be on the adsorption step at
any given time and two or three beds can be on the purge
step at any given time. Preferably, four or five pressure
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equalization steps can be carried out at decreasing pressure
and four or five pressure equalization steps can be carried
out at increasing pressure in each of the 14 adsorbent beds.
Optionally, the provide purge step and the blowdown step can
overlap such that gas is withdrawn from one end of the bed
for providing purge to another bed while additional blowdown
gas is withdrawn from the other end of the bed.
[0011] In yet another embodiment, 12 parallel adsorbent beds
are utilized which undergo the sequential steps in a cyclic
manner. Two or three beds can be on the adsorption step at
any given time. At least one of the pressure equalization
steps at decreasing pressure and the provide purge step can
overlap such that gas is withdrawn from the product end of
the bed for providing purge to another bed while additional
gas is withdrawn from the other end of the bed to pressurize
yet another bed. Two beds can be on the purge step at a
given time and three beds can be on the purge step at
another given time. Optionally, the provide purge step and
the blowdown step can overlap such that gas is withdrawn
from the product end of the bed for providing purge to
another bed while additional blowdown gas is withdrawn from
the other end of the bed.
[0012] The invention includes a pressure swing adsorption
system for operation in a pressure swing adsorption process,
wherein the system comprises 16 parallel adsorbent beds
which are manifolded such that three beds can be on a purge
step at any given time and each bed on the purge step can be
purged exclusively by gas provided exclusively from another
bed on a provide purge step. The 16 parallel adsorbent beds
can be manifolded into four groups of four beds each such
that each bed in any given group can provide purge gas
exclusively to another bed in the group and can receive
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purge gas exclusively from yet another bed in the group. The 16 parallel
adsorbent beds
can be manifolded such that a group of four beds can be isolated from the
other 12 beds
and the other 12 beds can be operated in a cycle with two beds on feed at any
given
time, 2 beds on purge at any given time, and two pressure equalizations.
[0013] Another embodiment of the invention includes a pressure swing
adsorption
system for operation in a pressure swing adsorption system, wherein the system
comprises 12 parallel adsorbent beds which are manifolded such that two beds
can be
on a purge step at any given time and each bed on the purge step can be purged
exclusively by gas provided exclusively from another bed on a provide purge
step. The
12 parallel adsorbent beds can be manifolded into three groups of four beds
each such
that each bed in a group can provide purge gas exclusively to another bed in
the group
and can receive purge gas exclusively from yet another bed in the group. The
pressure
12 parallel adsorbent beds can be manifolded such that the given group of 4
beds can
be isolated from the other 8 beds, wherein the other 8 beds can be operated in
a cycle
with one or two beds on feed at any given time, one or two beds on purge at
any given
time, and three pressure equalizations.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
[0014] Fig. 1 is a cycle chart for an embodiment of the invention which
utilizes 16
adsorbent beds, four beds on feed, three beds on purge, and four pressure
equalization
steps.
[0015] Fig. 2 is a cycle chart for an embodiment of the invention which
utilizes 16
adsorbent beds, four beds on feed, three beds on purge, and five pressure
equalization
steps.
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[0016] Fig. 3 is a cycle chart for an embodiment of the invention which
utilizes 16
adsorbent beds, four beds on feed, three beds on purge, and five pressure
equalization
steps, with a simultaneous provide purge and blowdown step.
[0017] Fig. 4 is a cycle chart for an embodiment of the invention which
utilizes 14
adsorbent beds, three beds on feed, two beds on purge, and five pressure
equalization
steps.
[0018] Fig. 5 is a cycle chart for an embodiment of the invention which
utilizes 14
adsorbent beds, three beds on feed, two or three beds on purge, and five
pressure
equalization steps, with a simultaneous provide purge and blowdown step.
[0019] Fig. 6 is a cycle chart for an embodiment of the invention which
utilizes 14
adsorbent beds, three beds on feed, three beds on purge, and four pressure
equalization
steps, with a simultaneous provide purge and blowdown step.
[0020] Fig. 7 is a cycle chart for an embodiment of the invention which
utilizes 14
adsorbent beds, four beds on feed, two or three beds on purge, and four
pressure
equalization steps.
[0021] Fig. 8 is a cycle chart for an embodiment of the invention which
utilizes 12
adsorbent beds, two beds on feed, two or three beds on purge, and four
pressure
equalization steps, with a simultaneous provide purge and blowdown step.
[0022] Fig. 9 is a cycle chart for an embodiment of the invention which
utilizes 12
adsorbent beds, three beds on feed, two beds on purge, and four pressure
equalization
steps, with a simultaneous provide purge and blowdown step.
[0023] Fig. 10 is a cycle chart for an embodiment of the invention which
utilizes 12
adsorbent beds, three beds on feed, two beds on pufge, and four pressure
equalization
steps, with a simultaneous pressure equalization and provide purge step.
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[0024] Fig. 11 is a cycle chart for an embodiment of the invention which
utilizes 12
adsorbent beds, three beds on feed, two beds on purge, and five pressure
equalization
steps, with three simultaneous pressure equalization and provide purge steps.
DETAILED DESCRIPTION OF THE INVENTION .
[0025] The present invention is a pressure swing adsorption (PSA) process for
recovering a less strongly adsorbable component from a feed gas mixture
comprising at
least one less strongly adsorbable component and at least one more strongly
adsorbable
component, which utilizes sequential steps in an adsorbent bed which include
an
adsorption step, two or more pressure equalization steps at decreasing
pressure, a
provide purge step, a blowdown step, a purge step, two or more pressure
equalization
steps at increasing pressure, and a final repressurization step. The invention
has a
number of embodiments which can utilize combinations of 12, 14, or 16 beds,
three or
four beds on feed simultaneously, two or three beds on purge simultaneously,
and two or
more, preferably four or five, pressure equalization steps. The invention can
be carried
out in PSA systems with as few as four beds, which is the minimum number of
beds
required for two pressure equalizations.
[0026] The common feature of all embodiments of the invention is that the
duration of
each pressure equalization (PE) step is less than about 25 seconds, which is a
shorter
pressure equalization time than previously used in PSA cycles. It was
discovered in the
development of the invention that shorter PE times are possible when two or
more beds
are on purge at any given time.
[0027] When the overall cycle time is shortened such that the PE step is
reduced to
less than about 30 seconds, two things happen. First, the purge time is
reduced in
proportion to the shortened cycle time, and therefore the purge rate increases
and the
pressure drop through the purged bed also increases. The higher pressure drop
during
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the purge step results in higher bed pressures during the purge step, since
the pressure
at the outlet of the bed during the purge step is essentially a constant. From
the
adsorption isotherm it is known that more adsorbed components are retained in
the bed
at a higher purge pressure for a given volume of purge gas, and therefore
purge at
higher pressure is less effective at desorbing the more strongly adsorbable
components
from the bed compared with purge at lower pressure. As a result, while the
amounts of
waste gas from the blowdown and purge steps are essentially constant, the
product
recovery is reduced because less feed gas can be processed in a given cycle.
[0028] Second, as PE time is reduced, the purge step requires a higher purge
gas flow
rate, and the lifting force in the bed providing purge gas will increase. This
can cause
undesirable bed fluidization, which occurs because the bed providing purge is
depressurized in an upward direction. In order to avoid this problem,
conventional PSA
cycles have used PE times of greater than about 25 seconds.
[0029] When two or more beds are on purge, the gas flow rate in each bed on
purge is
reduced compared to the gas flow rate with only one bed on purge. This reduces
the
pressure drop through the bed being purged, and results in improved product
recovery
and adsorbent working capacity. As noted above, inrhen the cycle time is
reduced, the
purge time also will be reduced, and this results in a higher pressure drop in
the purged
bed. However, since the purge flow per bed is reduced by a factor of two or
more when
two or more beds are on purge, a doubling of the total purge flow results in a
pressure
drop no higher than when only one bed is on purge.
[0030] Reducing the cycle time by 50%, for example, can reduce the amount of
adsorbent by almost 50% if mass transfer resistance does not affect adsorption
rate
significantly, and this greatly reduces the cost of adsorbent and bed vessels.
If the
preferred cycle time requires a PE time of 40 seconds when one bed is on
purge, the PE
time can be reduced to 20 seconds or less when two or more beds are
simultaneously
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CA 02501094 2002-07-16
on purge. This is the theoretical basis for using shorter cycle times with two
or more
beds on purge according to the present invention.
[0031] As mentioned above, as the cycle time decreases, the gas flow rate
between
the beds during each PE step increases. When a depressurizing bed is long
enough or
the gas flow velocity is high enough, the potential for adsorbent fluidization
in the bed
becomes a concern. This concern about fluidization is a factor which prevented
the use
of shorter PSA cycle times in the past.
[0032] In the present invention, partial pressure equalization can be used
during the
pressure equalization steps at lower pressures to minimize fluidization in the
depressurizing bed. The term "partial pressure equalization" defines a
pressure
equalization step in which the final differential pressure between the two
beds is greater
than zero. For example, when four steps of pressure equalization are used in
a.cycle,
the final differential pressure between two beds undergoing pressure
equalization is set
typically at 0.2 atm. The final differential pressure is defined as the
absolute differential
pressure between the two beds at the end of a pressure equalization step. The
highest
lifting force in a depressurizing bed occurs in the last step of pressure
equalization
because much greater gas volumes are transferred at lower pressures. This
occurs in
part because of the lower absolute pressure and in part because of the greater
amount
of gas desorbed from the adsorbent at lower pressures. In addition, in the
first PE step
the pressurizing bed can be receiving repressurization gas from another
source, which
reduces the gas flow from the depressurizing bed. As mentioned above, when a
depressurizing bed is long enough or the gas flow velocity is high enough, the
potential
for adsorbent fluidization in the bed becomes a concern. This is addressed in
the
present invention by using a final differential pressure of 0.2 atm or less
for the initial PE
steps, and then using a final differential pressure of at least 0.4 atm and up
to 2.5 atm for
the last PE step.
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[0033] When the final differential pressure increases, the amount of gas
transferred
during that equalization step is reduced. If this results in insufficient
pressurization gas
for the receiving bed, one or more additional steps of pressure equalization
can be
added. Typically, the final differential pressure during the last equalization
step should
be greater than that the final differential pressure which occurs.in earlier
PE steps at a
higher pressure levels. Thus when the PE steps are reduced to less than 25
seconds
each, the overall PSA cycle time is reduced in proportion, and this results in
greater
single train feed capacity.
[0034] The cycles of the present invention can be used to recover hydrogen
from a
steam methane reformate in the 15-40 atma pressure range. Typical components
and
composition ranges for a steam methane reformate feed to the PSA process are
(in
volume %) 70-79°J° hydrogen, 14-23% carbon dioxide, 1-7%
methane, 0.1-5% carbon
monoxide, 0-3% nitrogen, and 0.1-1% water. The cycles disclosed here also may
be
used for the separation of other gas mixture such as hydrogen-containing
offgases from
ethylene or methanol production plants. The adsorbent or adsorbents used in
the PSA
process can be selected from numerous commercially-available adsorbents known
in the
art. In this application, the adsorbent bed can contain a layer of activated
carbon near
the feed end and a layer of zeolite near the product end of the bed, such as
for example
a type 4A, 5A, or X type zeolite. A typical carbon to zeolite ratio is about
1:1. When the
feed is in the pressure range of 15-40 atria, the optimum number of pressure
equalization steps is typically three to five. When partial pressure
equalization is used,
additional pressure equalizations may be required.
[0035] The PSA process of the present invention can operate in a number of
embodiments. In each of these embodiments described below, a plurality of
adsorbent
beds is operated in parallel and each bed proceeds through identical cyclic
steps. The
cycles of the beds are staggered to allow the transfer of purge and pressure
equalization
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gas between beds, and also to allow the simultaneous operation of multiple
beds on the
feed and purge steps at any given time in the overall cycle.
[0036] In each of the embodiments of the invention, a single adsorbent bed
proceeds
in sequence through the steps described and defined below.
[0037] 1 ) The feed step begins at an initial feed pressure following
repressurization. A
mixed feed gas containing at least one more strongly adsorbable component and
at least
one less strongly adsorbable component is passed into the feed end of the
adsorbent
bed which contains one or more adsorbents which retain the more strongly
adsorbed
components. The average bed pressure at the beginning of this step is the
initial feed
pressure. A product gas enriched in the less strongly adsorbed components is
withdrawn from the product end of the bed. The feed step continues for a
predetermined
time and is terminated prior to breakthrough of the more strongly adsorbed
components.
[0038] 2) Pressure eaualization at decreasing bed pressure is initiated in
which gas
is withdrawn at decreasing pressure from the adsorbent bed and transferred to
other
beds in turn which are undergoing pressure equalization at increasing pressure
as
described later. Pressure equalization gas is withdrawn cocurrently, that is,
in the same
direction as the gas flow in the feed step. Pressure equalization at
decreasing bed
pressure is effected in two or more steps, preferably four or five steps, in
which the gas
withdrawn from the adsorbent bed is transferred in turn to four or five other
beds which
receive the gas at increasing bed pressures. Preferably the elapsed time for
each
pressure equalization step is less than about 25 seconds as discussed earlier.
[0039] In any given pressure equalization step, the pressure in the bed from
which gas
is withdrawn (i.e. the bed which is decreasing in pressure) approaches the
pressure in
the bed receiving the gas (i.e. the bed which is increasing in pressure). In
the limit, if the
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CA 02501094 2002-07-16
pressure equalization step duration is sufficiently long, these two pressures
will become
equal, and the final differential pressure will be zero. In actual practice,
however, the
pressures may not become equal and the final differential pressure may be
greater than
zero.
[0040] The term "final differential pressure" is defined as the absolute
differential
pressure between the two beds at the end of a pressure equalization step. The
term
"partial pressure equalization" defines a pressure equalization step in which
the final
differential pressure is greater than zero. The term "pressure equalization"
is used
herein to describe the transfer of gas from one bed at decreasing pressure to
another
bed at increasing pressure and applies to any pressure equalization step which
has any
final differential pressure, including zero.
[0041] In one embodiment of the invention, a series of pressure equalization
steps is
used in which the final differential pressure increases in each successive PE
step. In the
most general embodiment of a series of pressure equalization steps, the final
differential
pressure in a pressure equalization step is greater than the final
differential pressure in
any earlier step. The final differential pressure in the final pressure
equalization step can
be up to about 2.5 atm. In one example of this embodiment, the final
differential
pressures in a series of five pressure equalization steps is 0.2, 0.7, 1.2,
1.8, and 2.5 atm
respectively. Other combinations of final differential pressures can be used,
but at least
one final differential pressure is greater than the final differential
pressure in at least one
earlier PE step.
[0042) 3) The provide pur~lc a step begins after the last pressure
equalization step is
completed. Additional gas is withdrawn cocurrently from the adsorbent bed at
further
decreasing pressures and this gas is used to purge other beds to desorb the
more
strongly adsorbed components in these beds. In an optional mode of operation,
an initial
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portion of the provide purge step can overlap with one or more of the pressure
equalization steps such that gas withdrawn from a bed can be used in part for
pressure
equalization and in part for providing purge.
[0043] 4) In the blowdown step, a final volume of gas is withdrawn
countercurrently
from the adsorbent bed and discharged from the system for fuel or other uses.
The
pressure in the bed at the end of the blowdown step is the minimum bed
pressure in the
cycle. In an optional mode of operation, the last portion of the provide purge
step and
the initial portion of the blowdown step can be carried out simultaneously.
[0044] 5) In the purge step, gas from another bed in the provide purge step is
passed
through the bed at minimum bed pressure to sweep void space gas and desorb
remaining adsorbed components, thereby regenerating the adsorbent.
[0045] 6) Pressure egualization at increasinct bed pressure is initiated in
which gas
is introduced stepwise at increasing pressure into the adsorbent bed in turn
from other
beds which are undergoing pressure equalization at decreasing pressure as
described
above. Pressure equalization gas typically is introduced into the bed
countercurrently,
that is, in the opposite direction to the gas flow in the feed step. The
overall pressure
equalization at increasing bed pressure is effected in two or more steps,
preferably four
or five steps, in which the gas introduced into the adsorbent bed is
transferred in turn
from four or Ove other beds which provide the gas in turn at increasing bed
pressures.
Preferably the elapsed time for each pressure equalization step is less than
about 25
seconds as discussed earlier.
[0046] In any given pressure equalization step, the pressure in the bed from
which gas
is withdrawn (i.e. the bed which is decreasing in pressure) approaches the
pressure in
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the bed receiving the gas (i.e. the bed which is increasing in pressure). In
the limit, if the
pressure equalization step duration were sufficiently long, these two
pressures would
become equal, and the final differential pressure would be zero. In actual
practice,
however, the pressures may not become equal and the final differential
pressure may be
greater than zero.
[0047) The term "final differential pressure" is defined as the absolute
differential
pressure between the two beds at the end of a pressure equalization step. The
term
"partial pressure equalization" defines a pressure equalization step in which
the final
differential pressure is greater than zero. The term "pressure equalization"
is used
herein to describe the transfer of gas from one bed at decreasing pressure to
another
bed at increasing pressure and applies to any pressure equalization step which
has any
final differential pressure, including zero.
[0048] In one embodiment of the invention as described above, a series of
pressure
equalization steps is used in which the final differential pressure increases
in each
successive step. In the most general embodiment of a series of pressure
equalization
steps, the final differential pressure in a pressure equalization step is
greater than the
final differential pressure in any earlier step. The 1~inal differential
pressure in the final
pressure equalization step can be up to about 2.5 atm. In one example of this
embodiment, the final differential pressures in a series of five pressure
equalization are
0.2, 0.7, 1.2, 1.8, and 2.5 atm respectively.
[0049] In the above discussion of pressure equalization steps, several terms
are used
which are related as follows. The term "pressure equalization at decreasing
pressure"
refers to an individual bed which is decreasing in pressure by the transfer of
gas in turn
to one or more other beds which are increasing in pressure. The term "pressure
equalization at increasing pressure" refers to an individual bed which is
increasing in
pressure by the receipt of gas in turn from one or more other beds which are
decreasing
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in pressure. For a given pair of beds, a pressure equalization step at
decreasing
pressure and a pressure equalization step at increasing pressure occur
simultaneously.
The generic term "pressure equalization" refers to a pair of beds between
which gas is
transferred wherein one bed is decreasing in pressure and the other bed is
increasing in
pressure. A pressure equalization step between two beds by definition
therefore
includes pressure equalization at decreasing pressure in one bed and pressure
equalization at increasing pressure in the other bed.
[0050] 7) The repressurization step proceeds by introducing pressurized feed
gas
into the feed end of the bed, introducing product gas into the product end of
the bed, or
by simultaneously introducing pressurized feed gas into the feed end of the
bed and
introducing product gas into the product end of the bed. The repressurization
step is
considered complete when the average bed pressure reaches the initial feed
pressure,
at which time the cycle proceeds again to the feed step described above. In an
optional
mode of operation, the last step of pressure equalization at increasing
pressure is carried
out simultaneously with an initial portion of the repressurization step. .
[0051] Typical ranges of the elapsed times for the cycle steps described above
are as
follows: feed, 40-120 seconds; each pressure equalization step at decreasing
pressure,
10-25 seconds; provide purge, 40-200 seconds; blowdown, 10-100 seconds; purge,
40-200 seconds; each pressure equalization step at increasing pressure, 10-25
seconds;
and repressurization, 20-50 seconds. Total cycle time can range from 200 to
1000
seconds. If necessary, an idle step can be inserted between steps so that the
necessary
overlap of steps among beds can be effected. For example, an idle step could
be
inserted after the purge step and prior to pressure equalization at increasing
pressure.
During an idle step, the bed is inactive and typically isolated from the other
beds.
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[0052] Typical pressure ranges for the above steps are as follows: feed step,
20-40
atma; pressure equalization steps at decreasing pressures, 5-40 atma; provide
purge
step, 3-8 atma; blowdown step, 3-5 atma; purge step, 1.1-1.6 atma; pressure
equalization steps at increasing pressures, 1.1-36 atma; and repressurization
step,
16-40 atma.
[0053 A first embodiment of the invention is illustrated by the cycle chart of
Fig. 1,
which is a process cycle utilizing 16 parallel beds identified by the letters
A through P. In
this chart and following charts, the sequential steps are shown as a function
of time for
each bed, wherein the cycles of the beds are staggered to allow the required
transfer of
gas between beds in pressure equalization and purge steps. The cycles also are
arranged such that at any given time in the cycle, four beds are on the feed
step, three
beds are on the purge step, and three beds are on the provide purge step. Four
steps of
pressure equalization are used. In this and the following embodiments, the
final
pressure equalization step at increasing pressure overlaps with the initial
repressurization step. This overlapping step is optional, however, and cycles
without this
overlapping step are within the scope of the invention.
[0054] The 16 parallel beds in this embodiment are manifolded such that three
beds
are on a purge step at any given time and each bed on the purge step is purged
exclusively by gas provided exclusively from another bed on a provide purge
step. The
beds preferably are manifolded into four groups of four beds each such that
each bed in
a given group provides purge gas exclusively to another bed in the group and
receives
purge gas exclusively from yet another bed in the group. In the cycle chart of
Fig. 1, the
four groups of beds are AEIM, BFJN, CGKO, and DHLP respectively. In group
AEIM, for
example, bed A provides purge to bed M and receives purge from bed E, bed E
provides
purge to bed A and receives purge from bed I, bed I provides purge to bed E
and
receives purge from bed M, and bed M provides purge to bed f and receives
purge from
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CA 02501094 2002-07-16
bed A. No purge manifold is needed to connect beds in a given group with beds
in
another group, thereby simplifying piping in the system and simplifying
control of the
purge gas flow.
[0055) An additional feature of the embodiment of Fig. 1 is that the 16 beds
are
manifolded such that any given group of four beds can be isolated from the
other 12
beds and the other 12 beds can be operated in a different cycle with four beds
on feed at
any given time, 2 beds on purge at any given time, and two pressure
equalizations. For
example, the group of beds AEIM can be isolated from the other groups of beds
BFJN,
CGKO, and DHLP, which can be operated as described.
[0056] In Fig. 1 and in Figs. 2-11 below for other embodiments, the following
identifiers
are used: Feed is the feed step; 1, 2, 3, and 4 are pressure equalization
steps at
decreasing pressure; PP is the provide purge step; BD is the blowdown step; Pu
is the
purge step; I is the idle step; 4', 3', and 2' are pressure equalization steps
at increasing
pressure; 1'/R is the optional combined step of pressure equalization at
increasing
pressure and repressurization; and R is the repressurization step. If optional
step 1'/R is
not used, a final pressure equalization step at increasing pressure, 1' (not
shown), is
used. In a typical application of the embodiment of Fig. 1, the durations of
the cycle
steps are as follows: feed, 40-120 seconds; each pressure equalization step at
decreasing pressure, 10-25 seconds; provide purge, 60-150 seconds; blowdown,
20-50
seconds; purge, 60-150 seconds; idle, 10-25 seconds; each pressure
equalization step
at increasing pressure, 10-25 seconds; and repressurization, 20-50 seconds.
[005Tj A second embodiment of the invention is illustrated by the cycle chart
of Fig. 2,
which is a process cycle utilizing 16 parallel beds identified by the letters
A through P. In
this chart, the steps are identified using the same terminology as above. The
cycles are
arranged such that at any given time in the cycle, four beds are on the feed
step, three
beds are on the purge step, and three beds are on the provide purge step. Five
steps of
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CA 02501094 2002-07-16
pressure equalization are used, and the fifth pressure equalization step at
decreasing
pressure overlaps with the provide purge step wherein a portion of the
withdrawn gas
provides equalization to another bed and the remaining withdrawn gas provides
purge to
yet another bed. This step is identified as 5/PP.
[0058] A third embodiment of the invention is illustrated by the cycle chart
of Fig. 3,
which is a process cycle utilizing 16 parallel beds identified by the letters
A through P. In
this chart, the steps are identified using the same terminology as above. The
cycles are
arranged such that at any given time in the cycle, four beds are on the feed
step, three
beds are on the purge step, and three beds are on the provide purge step. Five
steps of
pressure equalization are used. The final portion of the provide purge step
and the initial
portion of the blowdown step overlap in a combined step identified as PP/BD.
[0059] A fourth embodiment of the invention is illustrated by the cycle chart
of Fig. 4,
which is a process cycle utilizing 14 parallel beds identified by the letters
A through N. In
this chart, the steps are identified using the same terminology as above. The
cycles are
arranged such that at any given time in the cycle, three beds are on the feed
step, two
beds are on the purge step, and two beds are on the provide purge step. Five
steps of
pressure equalization are used.
[0060] A fifth embodiment of the invention is illustrated by the cycle chart
of Fig. 5,
which is a process cycle utilizing 14 parallel beds identified by the letters
A through N. In
this chart, the steps are identified using the same terminology as above. The
cycles are
arranged such that at any given time in the cycle, three beds are on the feed
step and
two beds are on the provide purge step. During some portions of the cycle, two
beds are
on purge with purge gas provided by beds on the provide purge step. During
other
portions of the cycle, three beds are on purge with two beds receiving purge
gas from
beds on the provide purge step and one bed receiving purge gas from a bed on a
combined provide purge/blowdown step. The final portion of the provide purge
step and
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CA 02501094 2002-07-16
the initial portion of the blowdown step overlap in this combined provide
purgelbfowdown
step which is identified as PP/BD. Five steps of pressure equalization are
used, and the
final portion of the provide purge step and the initial portion of the
blowdown step overlap
in a combined step identified as PP/BD.
[0061] A sixth embodiment of the invention is illustrated by the cycle chart
of Fig. 6,
which is a process cycle utilizing 14 parallel beds identified by the letters
A through N. 1n
this chart, the steps are identified using the same terminology as above. The
cycles are
arranged such that at any given time in the cycle, three beds are on the feed
step, three
beds are on the purge step, and two or three beds provide purge gas to the
beds being
purged. Four steps of pressure equalization are used, and the final portion of
the
provide purge step and the initial portion of the blowdown step overlap in a
combined
step identified as PP/BD.
[0062 A seventh embodiment of the invention is illustrated by the cycle chart
of Fig. 7,
which is a process cycle utilizing 14 parallel beds identified by the fetters
A through N. In
this chart, the steps are identified using the same terminology as above. The
cycles are
arranged such that at any given time in the cycle, four beds are on the feed
step, two
beds are on the purge step, and two beds are on the provide purge step. Four
steps of
pressure equalization are used.
[0063] An eighth embodiment of the invention is illustrated by the cycle chart
of Fig. 8,
which is a process cycle utilizing 12 parallel beds identified by the letters
A through L. In
this chart, the steps are identified using the same terminology as above. The
cycles are
arranged such that at any given time in the cycle, two beds are on the feed
step. During
portions of the cycle, three beds are on the purge step; during other portions
of the cycle,
two beds are on the purge step. Three beds provide purge gas to the beds being
purged, two of them on the provide purge step and one on a provide
purge/blowdown
step. Four steps of pressure equalization are used, and the final portion of
the provide
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CA 02501094 2002-07-16
purge step and the initial portion of the blowdown step overlap in the
combined provide
purge/blowdown step which is identified as PP/BD.
(0064] The 12 parallel beds in this embodiment are manifolded such that two or
three
beds are on a purge step at any given time and each bed on the purge step is
purged
exclusively by gas provided exclusively from another bed on a provide purge
step and a
provide purge/blowdown step. The beds preferably are manifolded into three
groups of
four beds each such that each bed in a given group provides purge gas
exclusively to
another bed in the group and receives purge gas exclusively from yet another
bed in the
group. In the cycle chart of Fig. 8, the three groups of beds are ADGJ, BEHK,
and CFIL
respectively. In group ADGJ, for example, bed A provides purge to bed J and
receives
purge from bed D, bed D provides purge to bed A and receives purge from bed G,
bed G
provides purge to bed D and receives purge from bed J, and bed J provides
purge to bed
G and receives purge from bed A. No purge manifold is needed to connect beds
in a
given group with beds in another group, thereby simplifying piping in the
system and
simplifying control of the purge gas flow.
(0065] An additional feature of the embodiment of Fig. 8 is that the 12 beds
can be
manifolded such that any given group of four beds can be isolated from the
other 8 beds
such that the other 8 beds can be operated in a different cycle with one or
two beds on
feed at any given time, one or two beds on purge at any given time, and three
pressure
equalizations. For example, the group of beds ADGJ can be isolated from the
other
groups of beds BEHK and CFIL, which can be operated as described.
(0066] A ninth embodiment of the invention is illustrated by the cycle chart
of Fig. 9,
which is a process cycle utilizing 12 parallel beds identified by the letters
A through L. In
this chart, the steps are identified using the same terminology as above. The
cycles are
arranged such that at any given time in the cycle, three beds are on the feed
step, two
beds are on the purge step, and one or two beds are on the provide purge step.
The
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CA 02501094 2002-07-16
final portion of the provide purge step and the initial portion of the
blowdown step overlap
in a combined provide purge/blowdown step identified as PP/BD, which provides
purge
when only one other bed is on a provide purge step. Four steps of pressure
equalization
are used.
[0067] A tenth embodiment of the invention is illustrated by the cycle chart
of Fig. 10,
which is a process cycle utilizing 12 parallel beds identified by the letters
A through L. In
this chart, the steps are identified using the same terminology as above. The
cycles are
arranged such that at any given time in the cycle, three beds are on the feed
step, two
beds are on the purge step, and two beds are on the provide purge step. Four
steps of
pressure equalization are used, and the fourth pressure equalization step at
decreasing
pressure overlaps with the provide purge step wherein a portion of the
withdrawn gas
provides equalization to another bed and the remaining withdrawn gas provides
purge to
yet another bed. This step is identified as 4/PP.
[0068] An eleventh embodiment of the invention is illustrated by the cycle
chart of Fig.
11, which is a process cycle utilizing 12 parallel beds identified by the
letters A through L.
In this chart, the steps are identified using the same terminology as above.
The cycles
are arranged such that at any given time in the cycle, three beds are on the
feed step,
and two beds are on the purge step. Five steps of pressure equalization are
used, and
the third, fourth, and fifth pressure equalization steps at decreasing
pressure overlap with
the provide purge step wherein portions of the withdrawn gas provide
equalization to
other beds and the remaining withdrawn gas provides purge to yet another bed.
These
overlapping steps are identified as 3/PP, 4/PP, and 5/PP. Purge gas to a given
bed on
purge is supplied by another bed which proceeds through steps 3/PP, 4/PP,
5/PP, and
PP (provide purge).
[0069] Thus the present invention offers a method to reduce the cycle time in
a
pressure swing adsorption process by reducing the required pressure
equalization time
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CA 02501094 2002-07-16
in a cycle, thereby reducing the overall cycle time and increasing product
recovery per
unit of adsorbent used. This reduces the amount of adsorbent required in the
beds for a
given feed rate while continuing to provide product at an acceptable product
purity, and
has the desirable effect of reducing the capital cost of the process equipment
required
for a given volumetric production rate.
[0070] The characteristics of the present invention are described completely
in the
foregoing disclosure. One skilled in the art can understand the invention from
this
disclosure and can make various modifications to the invention which are
within the
scope of the claims which follow.
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