Note: Descriptions are shown in the official language in which they were submitted.
CA 02452536 2004-O1-14
TITLE OF THE INVENTION
PRESSURE SWING ADSORPTION PROCESS WITH MULTIPLE BEDS ON
PURGE AND/OR WITH TEN BEDS AND FOUR PRESSURE
EQUALIZATION STEPS
This application is a divisional application of copending
Canadian Application Serial No.: 2,331,034, filed January 15,
2001.
BACKGROUND OF THE INVENTION
This invention relates to pressure swing adsorption (PSA)
processes, and more particularly to such processes employing
multiple adsorbent beds and multiple pressure equalization
steps.
PSA processes are well-known for the separation of gas
mixtures that contain components with different adsorbing
characteristics. For example, hydrogen production via pressure
swing adsorption (H2 PSA) is a multi-million dollar industry
supplying high purity hydrogen for chemical producing
industries, metals refining and other related industries.
In a typical PSA system, a multicomponent gas is passed
to at least one of multiple adsorption beds at an elevated
pressure to adsorb at least one strongly sorbed component while
at least one component passes through. In the case of H2 PSA,
HZ is the most weakly adsorbed component which passes through
the bed. At a defined time, the feed step is discontinued and
the adsorption bed is depressurized with flow co-
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CA 02452536 2004-O1-14
current to the direction of the feed in one or more steps which permits
essentially pure
HZ product to exit the bed with a high recovery of the most weakly adsorbed
component,
H2. Then a countercurrent desorption step is carried out, followed by
countercurrent
purge and repressurization.
U.S. Patent No. 3,986,849 to Fuderer et al. discloses PSA processes employing
at least seven adsorbent beds, at least three steps of pressure equalization
per bed.
This patent teaches that an undesirable reversion of the desorbate profile
from the inlet
to the discharge end of the bed is substantially reduced when at least three
pressure
equalization stages are employed. Fuderer et al. does not disclose any
embodiments
comprising performing four pressure equalization steps in a ten-bed apparatus,
or
process cycles with on average two or more beds being purged at the same time.
The prior art in general teaches that increasing the number of beds typically
facilitates increasing the number of equalizations, which minimizes the
production costs
of a PSA system. Unfortunately, increasing the number of beds typically
increases the
cost of a PSA system as well.
Accordingly, it would be very desirable to provide an improved PSA process
which increases production andlor recovery per bed in a multiple bed system.
BRIEF SUMMARY OF THE INVENTION
The invention provides a pressure swing adsorption process comprising
providing a pressure swing adsorption apparatus having ten beds, and
equalizing a
pressure of each of said ten beds in four steps. At all times during the
process, an
average of at least two of said ten beds are being simultaneously regenerated
by
simultaneously providing off-gas from a product end of each of said two beds
to an off
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gas line. Each of the four pressure equalization steps comprises a
depressurization
phase spanning about 1/20 of a total cycle time of said process and a
repressurization
phase spanning about 1/20 of said total cycle time. The preferred product of
the
process is hydrogen.
In accordance with one embodiment of the present invention there is
provided a pressure swing adsorption process comprising providing a pressure
swing
adsorption apparatus having ten beds, and equalizing a pressure of each of the
ten
beds in four steps represented by the following cycle chart:
A A A A A A 1 2 3 P' P B B G G 4' 3' 2' 1' R
1' R A A A A A A 1 2 3 P' P B B G G 4' 3' 2'
3' 2' 1' R A A A A A A 1 2 3 P' P B B G G 4'
G 4' 3' 2' 1' R A A A A A A 1 2 3 P' P B B G
B G G 4' 3' 2' 1' R A A A A A A 1 2 3 P' P B
P B B G G 4' 3' 2' 1' R A A A A A A 1 2 3 P'
.3 P' P B B G G 4' 3' 2' 1' R A A A A A A 1 2
1 2 3 P' P B B G G 4' 3' 2' 1'R A A A A A A
A A 1 2 3 P' P B B G G 4' 3'2' 1' R A A A A
A A A A 1 2 3 P' P B B G G 4' 3' 2' 1 R A A
'
wherein A is adsorption, 1 is providing gas for a first step of pressure
equalization, 2 is
providing gas for a second step of pressure equalization, 3 is providing gas
for a third
step of pressure equalization, P' is providing purge gas and simultaneously
providing
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gas for a fourth step of pressure equalization, P is providing purge gas, B is
counter-
current blowdown, G is counter-current purge, 4' is receiving gas for the
fourth step of
pressure equalization, 3' is receiving gas for the third step of pressure
equalization, 2' is
receiving gas for the second step of pressure equalization, and 1' is
receiving gas for the
first step of pressure equalization and simultaneously receiving product gas
for
repressurization, and R is repressurizing with product gas.
In accordance with another embodiment of the present invention there is
provided a pressure swing adsorption process comprising providing a pressure
swing
adsorption apparatus having ten beds, and equalizing a pressure of each of the
ten
beds in four steps represented by the following cycle chart:
A A A A 1 2 3 4 P P P B G G G 4' 3' 2' 1' R
1'R A A A A 1 2 3 4 P P P B G G G 4' 3' 2'
3'2' 1' R A A A A 1 2 3 4 P P P B G G G 4'
G 4' 3' 2' 1' R A A A A 1 2 3 4 P P P B G G
G G G 4' 3' 2' 1' R A A A A 1 2 3 4 P P P B
P B G G G 4' 3' 2' 1' R A A A A 1 2 3 4 P P
.PP P B G G G 4' 3' 2' 1' R A A A A 1 2 3 4
3 4 P P P B G G G 4' 3' 2' 1' R A A A A 1 2
1 2 3 4 P P P B G G G 4' 3' 2' 1' R A A A A
A A 1 2 3 4 P P P B G G G 4' 3' 2' 1' R A A
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wherein A is adsorption, 1 is a first step of pressure equalization, 2 is a
second step of
pressure equalization, 3 is a third step of pressure equalization, 4 is a
fourth step of
pressure equalization, P is purge, B is counter-current blowdown, G is counter-
current
purge, 4' is receive gas for the fourth step of pressure equalization, 3' is
receive gas for
the third step of pressure equalization, 2' is receive gas for the second step
of pressure
equalization, and 1' is receive gas for the first step of pressure
equalization and
simultaneously receive product gas for re-pressurization, and R is bed
repressurizing
with product gas.
In accordance with a further embodiment of the present invention there is
provided a pressure swing adsorption process comprising providing a pressure
swing
adsorption apparatus having ten beds, and equalizing a pressure of each of the
ten
beds in four steps wherein at all times during the process, an average of at
least two of
the ten beds are being counter-currently purged represented by the following
cycle
chart:
A A A A 1 2 3 P' P B B G G G G 4' 3' 2' 1' R
1 R A A A A 1 2 3 P' P B B G G G G 4' 3' 2'
'
3' 2' 1' R A A A A 1 2 3 P' P B B G G G G 4'
G 4' 3' 2' 1' R A A A A 1 2 3 P' P B B G G G
G G G 4' 3' 2' 1' R A A A A 1 2 3 P' P B B G
B G G G G 4' 3' 2' 1' R A A A A 1 2 3 P' P B
.P B B G G G G 4' 3' 2' 1' R A A A A 1 2 3 P'
3 P' P B B G G G G 4' 3' 2' 1' R A A A A 1 2
1 2 3 P' P B B G G G G 4' 3' 2' 1' R A A A A
A A 1 2 3 P' P B B G G G G 4' 3' 2' 1' R A A
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wherein A is adsorption, 1 is providing gas for a first step of pressure
equalization, 2 is
providing gas for a second step of pressure equalization, 3 is providing gas
for a third
step of pressure equalization, P' is providing purge gas and simultaneously
providing
gas for a fourth step of pressure equalization, P is providing purge gas, B is
counter-
current blowdown, G is counter-current purge, 4' is receiving gas for the
fourth step of
pressure equalization, 3' is receiving gas for the third step of pressure
equalization, 2' is
receiving gas for the second step of pressure equalization, and 1' is
receiving gas for the
first step of pressure equalization and simultaneously receiving product gas
for
repressurization, and R is repressurizing with product gas.
In accordance with a still further embodiment of the present invention there
is
provided a pressure swing adsorption process comprising: (a) providing an
adsorption
apparatus having a plurality of beds;(b) at feast one adsorption step
comprising feeding
a feed mixture to a feed end of a first bed, adsorbing impurities onto an
adsorbent in the
first bed and permitting a product gas to exit a product end of the first bed;
(c) at least
one depressurizing equalization step comprising reducing a pressure in said
first bed by
closing a feed valve and sequentially releasing gas from a product end of the
first bed to
other beds or to at least one other bed and a tank; (d) at least one pressure
reduction
step comprising further reducing said pressure of the first bed co-currently
and/or
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counter-currently; (e) at least one counter-current purging step, comprising
counter-currently purging the first bed with gas from another bed for a
duration such that
at least two of the beds of the apparatus are being purged simultaneously
throughout
the process; (f) at least one pressure augmenting equalization step comprising
increasing the pressure of the first bed with gas released from at least one
other bed
and/or tank undergoing the pressure reducing step; and (g) at least one
repressurization
step comprising further increasing the pressure of the first bed by feeding to
the first bed
at least one of a counter-current stream of product gas and a co-current
stream of feed
gas, wherein the apparatus has ten beds and four pressure equalizations per
bed are
performed throughout the process represented by the following cycle chart:
A A A A A A A A 1 2 3 P'P B B G G G G 4' 3'2' 1' R
1' R A A A A A A A A 1 2 3 P,F B B G G G G 4' 3' 2'
3' 2' 1'R A A A A A A A A 1 2 3 P'P B B G G G G 4'
G 4' 3'2' 1'R A A A A A A A A 1 2 3 P' P B B G G G
G G G 4' 3'2' 1'R A A A A A A A A 1 2 3 P' P B B G
D G G G G 4' 3'2' 1' R A A A A A A A A 1 2 3 P' P B
P B B G G G G 4' 3' 2'1' R A A A A A A A A 1 2 3 P'
3 P' P B B G G G G 4'3' 2'1' R A A A A A A A A 1 2
~1 2 3 P' P B B G G G G 4'3' 2'1' R A A A A A A A A
A A 1 2 3 P' P B B G G G G 4'3' 2'1' R A A A A A A
A A A A 1 2 3 P' P B B G G G G 4'3' 2' 1'R A A A A
A A A A A A 1 2 3 P'P B B G G G G 4' 3'2' 1'R A A
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wherein A is adsorption, 1 is providing gas for a first step of pressure
equalization, 2 is
providing gas for a second step of pressure equalization, 3 is providing gas
for a third
step of pressure equalization, 4 is providing gas for a fourth step of
pressure
equalization, P is providing purge gas, B is counter-current blowdown, G is
counter-current purge, 4' is receiving gas for the fourth step of pressure
equalization, 3'
is receiving gas for the third step of pressure equalization, 2' is receiving
gas for the
second step of pressure equalization, and 1' is receiving gas for the first
step of
pressure equalization and simultaneously receiving product gas for
repressurization, and
R is repressurizing with product gas.
In accordance with a yet further embodiment of the present invention there is
provided pressure swing adsorption process comprising: (a) providing an
adsorption
apparatus having a plurality of beds; (b) at least one adsorption step
comprising feeding
a feed mixture to a feed end of a first bed, adsorbing impurities onto an
adsorbent in the
first bed and permitting a product gas to exit a product end of the first bed;
(c) at least
one depressurizing equalization step comprising reducing a pressure in the
first bed by
closing a feed valve and sequentially releasing gas from a product end of the
first bed to
other beds or to at least one other bed and a tank; (d) at least one pressure
reduction
step comprising further reducing the pressure of the first bed co-currently
and/or
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counter-currently; (e) at least one counter-current purging step, comprising
counter-currently purging the first bed with gas from another bed for a
duration such that
at least two of the beds of the apparatus are being purged simultaneously
throughout
the process; (f) at least one pressure augmenting equalization step comprising
increasing the pressure of the first bed with gas released from at least one
other bed
andlor tank undergoing the pressure reducing step; and (g) at least one
repressurization
step comprising further increasing the pressure of the first bed by feeding to
the first bed
at least one of a counter-current stream of product gas and a co-current
stream of feed
gas, wherein the apparatus has ten beds and four pressure equalizations per
bed are
performed throughout the process represented by the following cycle chart:
A A A A A A 1 2 3 P'P B B G G G G G G 4' 3'2' 1'R
1' R A A A A A A 1 2 3 P'P B B G G G G G G 4' 3'2'
3' 2' 1'R A A A A A A 1 2 3 P'P B B G G G G G G 4'
G 4' 3'2' 1'R A A A A A A 1 2 3 P' P B B G G G G G
G G G 4' 3'2' 1'R A A A A A A 1 2 3 P' P B B G G G
G G G G G 4' 3'2' 1' R A A A A A A 1 2 3 P' P B B G
B G G G G G G A' 3' 2'1' R A A A A A A 1 2 3 P' P 8
P B B G G G G G G 4'3' 2'1' R A A A A A A 1 2 3 P'
.3 P' P B B G G G G G G 4'3' 2'1' R A A A A A A 1 2
1 2 3 P' P B. B G G G G G G A'3' 2' 1'R A A A A A A
A A 1 2 3 P' P B B G G G G G G 4' 3'2' 1'R A A A A
A A A A 1 2 3 P' P B 8 G G G G G G 4' 3'2' 1'R A A
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wherein A is adsorption, 1 is providing gas for a first step of pressure
equalization, 2 is
providing gas for a second step of pressure equalization, 3 is providing gas
for a third
step of pressure equalization, P' is providing purge gas and simultaneously
providing
gas for a fourth step of pressure equalization, P is providing purge gas, B is
counter-
current blowdown, G is counter-current purge, 4' is receiving gas for the
fourth step of
pressure equalization, 3' is receiving gas for the third step of pressure
equalization, 2' is
receiving gas for the second step of pressure equalization, and 1' is
receiving gas for the
first step of pressure equalization and simultaneously receiving product gas
for
repressurization, and R is repressurizing with product gas.
In accordance with one embodiment of the present invention there is provided a
pressure swing adsorption process comprising providing a pressure swing
adsorption
apparatus having ten beds, and equalizing a pressure of each of the ten beds
in four
steps, represented by the following cycle chart:
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A A A A 1 2 3 4 P P B B G G I 4' 3' 2' 1'R
1' R A A A A 1 2 3 4 P P B B G G I 4' 3'2'
3' 2' 1' R A A A A 1 2 3 4 P P B B G G I 4'
I 4' 3' 2' 1' R A A A A 1 2 3 4 P P B B GG
G G I 4' 3' 2' 1' R A A A A 1 2 3 4 P P B B
B B G G I 4' 3' 2' 1' R A A A A 1 2 3 4 P P
P P B B G G I 4' 3' 2' 1' R A A A A 1 2 3 4
3 4 P P B B G G I 4' 3' 2' 1' R A A A A 1 2
1 2 3 4 P P B B G G I 4' 3' 2' 1' R A A AA
A A 1 2 3 4 P P B B G G I 4' 3' 2' 1' R AA
wherein A is adsorption, 1 is providing gas for a first step of pressure
equalization, 2 is
providing gas for a second step of pressure equalization, 3 is providing gas
for a third
step of pressure equalization, 4 is providing gas for a fourth step of
pressure
equalization, P is providing purge gas, B is counter-current blowdown, G is
counter-current purge, I is idle, 4' is receiving gas for the fourth step of
pressure
equalization, 3' is receiving gas for the third step of pressure equalization,
2' is receiving
gas for the second step of pressure equalization, and 1' is receiving gas for
the first step
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of pressure equalization and simultaneously receiving product gas for
repressurization,
and R is repressurizing with product gas.
In accordance with another embodiment of the present invention there is
provided a pressure swing adsorption process comprising: (a) providing an
adsorption
apparatus having a plurality of beds; (b) at least one adsorption step
comprising feeding
a feed mixture to a feed end of a first bed, adsorbing impurities onto an
adsorbent in the
first bed and permitting a product gas to exit a product end of the first bed;
(c) at least
one depressurizing equalization step comprising reducing a pressure in the
first bed by
closing a feed valve and sequentially releasing gas from a product end of the
first bed to
other beds or to at least one other bed and a tank; (d) at least one pressure
reduction
step comprising further reducing the pressure of the first bed co-currently
and/or
counter-currently; (e) at least one counter-current purging step, comprising
counter-currently purging the first bed with gas from another bed for a
duration such that
at least two of the beds of the apparatus are being purged simultaneously
throughout
the process; (f) at least one pressure augmenting equalization step comprising
increasing the pressure of the first bed with gas released from at least one
other bed
and/or tank undergoing the pressure reducing step; and (g) at least one
repressurization
step comprising further increasing the pressure of the first bed by feeding to
the first bed
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at least one of a counter-current stream of product gas and a co-current
stream of feed
gas, represented by the following cycle chart:
A A A A A A 1 2 3 P' P B B G G G G 4' 3' 2' 1'I R R
R R A A A A A A 1 2 3 P'P B B G G G G 4' 3'2' 1' I
1' I R R A A A A A A 1 2 3 P'P B B G G G G 4' 3' 2'
3' 2'1' I R R A A A A A A 1 2 3 P' P B B G G G G 4'
G 4'3' 2' 1'I R R A A A A A A 1 2 3 P' P B B G G G
G G G 4' 3'2' 1'I R R A A A A A A 1 2 3 P' P B B G
B G G G G 4' 3'2' 1'I R R A A A A A A 1 2 3 P' P B
P B B G G G G 4' 3'2' 1' I R R A A A A A A 1 2 3 P'
'3 P'P B B G G G G 4' 3' 2'1' I R R A A A A A A 1 2
1 2 3 p' p B B G G G G 4'3' 2'1' I R R A A A A A A
- -
A A 1 2 3 P' P f3 B G G G G 4'3' 2' 1'I R R A ~ A
A A A A 1 2 3 P' P B B G G G G 4' 3'2' 1' I R R A A
wherein A is adsorption, 1 is providing gas for a first step of pressure
equalization, 2 is
providing gas for a second step of pressure equalization, 3 is providing gas
for a third
step of pressure equalization, P' is providing purge gas and simultaneously
providing
gas for a fourth step of pressure equalization, P is providing purge gas, B is
counter-
current blowdown, G is counter-current purge, I is idle, 4' is receiving gas
for the fourth
step of pressure equalization, 3' is receiving gas for the third step of
pressure
equalization, 2' is receiving gas for the second step of pressure
equalization, and 1' is
receiving gas for the first step of pressure equalization and simultaneously
receiving
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product gas for repressurization, and R is repressurizing with product gas.
DETAILED DESCRIPTION OF THE INVENTION
In a first embodiment of the invention, the improved PSA system employs ten
adsorbent beds and four steps of equalization. Preferably, an average of at
least two of
said ten beds are simultaneously regenerated by simultaneously providing off-
gas from
a feed end of each of said two beds to an off-gas line throughout said
process.
Preferably, each of said four steps comprises a depressurizing provide
equalization
phase spanning about 1/20 of a total cycle time of said process and a
repressurization
phase "R" spanning about 1/20 of said total cycle time. The preferred product
of the
invention is hydrogen.
A second embodiment of the invention is based on the inventor's discovery that
the average bed pressure during the purge step is very important in
determining the
recovery and productivity of the adsorption system. It is desirable to use a
longer purge
time to reduce the pressure drop in the bed, and therefore reduce the average
bed
pressure during the purge step. The slower the purge rates, the more effective
a system
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CA 02452536 2004-O1-14
is in removing adsorbed gases. Thus, the second embodiment of the invention
requires
that, at all times throughout the process, at least two beds are being counter-
currently
purged at the same time.
In a preferred embodiment, the process comprises:
(a) at least one adsorption step comprising feeding a feed mixture to a feed
end of a first bed, adsorbing impurities onto an adsorbent in said first bed
and permitting a product gas to exit a product end of said first bed;
(b) at least one depressurizing equalization step comprising reducing a
pressure in said first bed by closing a feed valve and sequentially
releasing gas from a product end of said first bed to other beds or to at
least one other bed and a tank;
(c) at least one pressure reduction step comprising further reducing said
pressure of said first bed co-currently and/or counter-currently;
(d) at least one counter-current purging step, comprising counter-currently
purging said first bed with gas from another bed for a duration such that
at least two of said beds of said apparatus are being purged
simultaneously throughout said process;
(e) at least one repressurizing equalization step comprising increasing the
pressure of said first bed with gas released from at least one other bed
andlor tank undergoing said pressure reducing step; and
(f) at least one pressure augmentation step comprising further increasing
said pressure of said first bed by feeding to said first bed at least one of a
counter-current stream of product gas and a co-current stream of feed
gas.
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In certain embodiments, steps (a) to (f) are sequential.
The preferred product gas is hydrogen, but the invention is not limited
thereto.
Preferably, the feed mixture comprises hydrogen and at least one member
selected from the group consisting of methane, carbon dioxide, carbon
monoxide,
nitrogen and water vapor.
In embodiments, the feed gas is obtained by steam reforming of hydrocarbons.
In other embodiments, the feed gas is obtained by partial oxidation of
hydrocarbons. In a number of these embodiments, the partial oxidation can
occur in the
presence of at least one catalyst.
In embodiments wherein the oxygen used for oxidation is provided by an ion
transport membrane, it is preferred that the ion transport membrane be
integrated with a
reactor in which the partial oxidation occurs.
In embodiments, at least a part of said at feast one repressurizing
equalization
step overlaps in time with said at least one pressure augmentation step.
In embodiments, at least a part of said at least one repressurizing
equalization
step overlaps in time with at least one of counter-current repressurization by
the product
gas and co-current repressurization by the feed gas.
In embodiments, gas from the product end of the bed in said at least one
depressurizing equalization step is transferred directly to a bed in said at
least one
repressurizing equalization step.
In embodiments, gas from the product end of the bed in said at least one
depressurizing equalization step is transferred to a tank before being
transferred to a
bed in said at least one repressurizing equalization step.
In embodiments, a bed being co-currently depressurized according to said at
least one pressure reduction step provides purge gas to more than one other
bed.
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In embodiments, the depressurizing equalization step precedes said provide
purge step, in other embodiments, the depressurizing equalization step
overlaps in time
with at least some portion of said provide purge step, and in still other
embodiments, the
depressurizing equalization step follows said provide purge step.
In embodiments, a bed being co-currently depressurized according to said at
least one pressure reduction step provides gas for pressure equalization of at
least one
other bed, as well as purge gas to more than one other bed.
In embodiments, the bed at step (c) has at feast one counter-current
depressurization step.
In embodiments, the counter-current depressurization step immediately precedes
the counter-current purge step (d).
The invention is most efficiently described through the use of cycle charts,
which
are readily understood by those of ordinary skill in the art. In the cycle
charts shown
herein, each column represents a time period of the cycle and each row
represents a
bed. The following table shows the cycle chart of a process that is presented
for
purposes of comparison only.
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TABLE 1
A A A A A A A A 1 2 3 4 P P B B G G I 4' 3'2' 1'
R
1'R A A A A A A A A 1 2 3 4 P P B B G G I 4' 3'2'
3'2' 1' R A A A A A A A A 1 2 3 4 P P B B G G I
4'
I 4' 3' 2'1' R A A A A A A A A 1 2 3 4 P P B B GG
G G I 4'3' 2'1' R A A A A A A A A 1 2 3 4 P p B
B
B B G G I 4'3' 2'1' R A A A A A A A A 1 2 3 4 P
P
P P B B G G I 4'3' 2'1' R A A A A A A A A 1 2 3
4
3 4 P P B B G G I 4'3' 2'1' R A A A A A A A A 1
2
1 2 3 4 P P B B G G I 4'3' 2' 1'R A A A A A A AA
A A 1 2 3 4 P P B B G G 1 4' 3'2' 1'R A A A A AA
A A A A 1 2 3 4 P P B B G G I 4' 3'2' 1'R A A AA
A A A A A A 1 2 3 I P P B B G G I 4' 3'2' 1'R A
A
' In the first step of this cycle for the first bed, the feed goes to a bed
from the feed
end for 1/3 of the cycle time to produce hydrogen from the product end in the
first step.
During this period, the pressurized feed flows through the bed. The impurities
in the
feed, such as COZ, methane, CO and nitrogen, are adsorbed by the adsorbents.
Hydrogen, on the other hand, is less strongly adsorbed and is driven out of
the bed as
hydrogen product. This step is called the "adsorption" step and is indicated
by "A's" in
Table 1.
In the second step, feed to the first bed is stopped. The hydrogen from the
product end of this bed (the first bed) is now fed to the product end of
another bed (the
second bed) whose pressure is being increased. That process goes on for
approximately 1124 of the cycle time, until the pressures in these two beds
are almost
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equal. This step is indicated by "1" in the cycle chart, standing for the
depressurization
phase of pressure equalization step one.
In the third step, the product end of the first bed is disconnected from the
second
bed and connected to the product end of the third bed whose pressure is lower
than the
first and second beds. This goes on for approximately 1124 of the cycle time,
until the
pressures in these two beds are close to each other. This step is indicated by
"2" in the
cycle chart, meaning that this is the depressurization phase of pressure
equalization
step two.
!n the fourth step, the connection between these two beds is then cut off, and
hydrogen from the product end of the first bed is fed to the product end of
the fourth bed,
whose pressure is yet lower than the third bed at the beginning of the
previous step.
This process again goes on for approximately 1/24 of the cycle time until the
pressures
of the two beds are close to each other. This step is indicated by "3" in the
cycle chart,
standing for the depressurization phase of pressure equalization step three.
In the fifth step, the connection between the first bed and the fourth bed is
closed, and the hydrogen from the product end of the first bed is introduced
into the fifth
bed whose pressure is lower than that of the fourth bed at the beginning of
the previous
step, and which has just finished the "purge" step (step 8) and idle step
(step 9). This
process again lasts for 1/24 of the cycle time, and at the end of this step,
the pressures
in these two beds are again close to each other. This step is indicated by "4"
in the
cycle chart, representing the depressurization phase of pressure equalization
step four.
In the sixth step, the connection between the first bed and the fifth bed is
closed,
and the gas from the product end of the first bed is introduced to the product
end of the
sixth bed. The feed end of the bed being purged (the sixth bed) is connected
to the
offgas line and the gas purged from this bed enters the offgas line. This step
lasts for
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CA 02452536 2004-O1-14
approximately 1112 of the cycle time, and is called the "provide purge" step,
indicated by
"P's" in the cycle chart.
In the seventh step, the product end valve is closed and the feed end is
connected to the offgas line. The gas in the bed goes to the offgas line. This
step lasts
for 1/12 of the cycle time and is called the "countercurrent blowdown" step
and is
indicated by "B's" in the chart.
In the eighth step, the product end of the bed is connected to the product end
of
a bed in its sixth step (the "provide purge" step). The gas from the "provide
purge" bed
purges the bed and the impurities and some hydrogen gas leave this first bed
(the bed
being purged) from the feed end and enters the offgas line. This step lasts
about for
1/12 of the cycle time and is called the "countercurrent purge" step, It is
indicated by
"G's" in the cycle chart.
In the ninth step, the bed is closed on both ends for 1/24 of the cycle time
and
this step is called the "idle" step, indicated by "I" in the cycle chart.
In the tenth step, the product end of the bed is connected to the product end
of a
bed at its fifth step. Gas flows from the product end of the bed at its fifth
step and enters
this bed from its product end. This takes about 1124 of the cycle time until
the pressures
in the two beds are close to each other. This step is indicated by "4'",
indicating that it is
at the receiving gas end of the fourth pressure equalization step following
the sequence
order of the pressure equalization steps. That is, "4"' indicates the
repressurization
phase of the fourth pressure equalization step.
In the eleventh step, the product end of the bed is connected to the bed at
the
fourth step. Gas flows from the product end of the bed at its fourth step and
enters this
bed from its product end. This takes about 1/24 of the cycle time until the
pressures in
the two beds are close to each other. This step is indicated by "3"',
indicating that it is at
_g_
CA 02452536 2004-O1-14
the receiving gas end of third pressure equalization step. That is, "3"'
indicates the
repressurization phase of the third pressure equalization step.
In the twelfth step, the product end of the bed is connected to the bed at the
third
step. Gas flows from the product end of the bed at its third step and enters
this bed from
its product end. This takes about 1124 of the cycle time until the pressures
in the two
beds are close to each other. This step is indicated by "2"', indicating that
it is at the
receiving gas end of the second pressure equalization step. That is, "2"'
indicates the
repressurization phase of the second pressure equalization step.
- In the thirteenth step, the product end of the bed is connected to the bed
at the
second step. Gas flows from the product end of the bed at its second step and
enters
this bed from its product end. This takes about 1/24 of the cycle time until
the pressures
in the two beds are close to each other. Some product gas also enters the
product end
of this bed. This step is indicated by "1 "', indicating that it is at the
receiving gas end of
the first pressure equalization step. That is, "1"' indicates the
repressurization phase of
the first pressure equalization step.
Then in the fourteenth step, the connection between these two beds is closed
whiVe the gas from the product line continues to re-pressurize the bed until
the pressure
in the bed is close to that of the product hydrogen. This step takes
approximately 1/24
of the cycle time and is indicated by the letter "R" in the chart for
"repressurization."
After this step is finished, the bed returns to step 1.
The cycles of the twelve beds of Table 1 are staggered such that one bed
starts
adsorption 1112 of the cycle time later than the previous one. Such a cycle
has four
beds generating products all the time. The time for pressure equalization does
not have
to be exactly 11(2 x n), in which "n" is the number of beds in the system.
When the time
for the first step of pressure equalization, "1", is "t" less than 1/(2 x n)
of the cycle time,
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CA 02452536 2004-O1-14
the time for the receiving pressure equalization step, 1', is decreased by the
same
amount, while the time for re-pressurization step "R" is increased by the same
amount.
The time for the second pressure equalization step on both providing gas end
and
receiving gas end, 2 and 2', can be increased by the same amount. If the first
and
second equalization share a common valve/line arrangement and the third and
fourth
equalization share a common valve/line arrangement, the time for the third
pressure
equalization step on both providing gas end and receiving gas end, 3 and 3',
must be
reduced by at feast t. The time for the fourth pressure equalization step on
both
providing gas end and receiving gas end, 4 and 4', can be increased by the
same
amount. The time for the idle can be decreased by the same amount. The same
holds
true for all the cycles discussed below (the idle step and the step solely
consisting of
providing gas for the lowest pressure equalization step, 4, do not exist for
all the cycles).
The following table shows the cycle chart of a ten-bed cycle of the first
embodiment of the invention with only two beds on adsorption:
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., CA 02452536 2004-O1-14 .. _............. _,. ... . ._.
TABLE 2
A A A A 1 2 3 4 P P B B G G I 4' 3' 2' 1'
R
1' R A A A A 1 2 3 4 P P B B G G I 4' 3'2'
3' 2' 1' R A A A A 1 2 3 4 P P B B G G 1 4'
I 4' 3' 2' 1' R A A A A 1 2 3 4 P P B B GG
G G I 4' 3' 2' 1' R A A A A 1 2 3 4 P P B B
B B G G I 4' 3' 2' 1' R A A A A 1 2 3 4 P P
-P P B B G G I 4' 3' 2' 1' R A A A A 1 2 3 4
3 4 P P B B G G I 4' 3' 2' 1' R A A A A 1 2
1 2 3 4 P P B B G G I 4' 3' 2' 1' R A A AA
A A 1 2 3 4 P P B B G G I 4' 3' 2' 1' R AA
In this cycle, the bed is on adsorption for 1/5 of the cycle time. It then
provides
gas from the product end of the bed in the first, second, third, and fourth
steps of
pressure equalization sequentially, with each step lasting approximately 1/20
of the cycle
time. The bed then provides gas for purge, again from the product end of the
bed, for
approximately 1110 of the cycle time. Then the product end of the bed is
closed and the
feed end valve to the offgas line is opened. The gas in the bed is sent to the
offgas line
for approximately 1/10 of the cycle time, until the pressure in the bed is not
much greater
than that in the offgas line. Then the product end of the bed is connected to
the product
end of a bed that is at its "provide purge" step. This mostly hydrogen gas
flows from the
bed at its "provide purge" step to the bed under description, driving out some
of the
impurities in the bed to the offgas line. This lasts for approximately 1/10 of
the cycle
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CA 02452536 2004-O1-14
time. Then the bed sits idle for approximately 1/20 of the cycle time. It then
receives
gas from the product end of the bed in the steps of receiving pressure
equalization,
indicated by 4', 3', 2', and 1' in the cycle chart (i.e., Table 1 ), with each
step lasting for
approximately 1120 of the cycle time, until the pressure of the bed is close
to that in the
bed providing gas for pressure equalization in each step. In the last of these
receiving
pressure equalization gas steps, some product gas is also fed to the product
end of the
bed. Then, in the last step, the connection of this bed with the provide
equalization gas
bed is cut and the product gas continues to enter the bed from the product end
for
approximately 1/20 of the cycle time until the pressure of the bed is close to
that of the
product. This bed thereby completes one cycle and returns to the adsorption
step. The
ten beds are staggered by starting the cycle of the subsequent bed 1/10 of the
cycle
time later than the previous step so that there are two beds on adsorption all
the time. In
addition, two beds are under regeneration at any given time.
Given the fact that the mass transfer layer has certain length, it is still
desirable
to have more beds on feed if doing so does not cause a large negative impact
on
performance by reducing the time for other steps. Notice that in the process
whose
cycle chart is shown in Table 2, 1120 of the cycle time is idle, meaning that
the bed is not
used in that 5% of the cycle time. That idle time can be used for adsorption
by shifting it
to the adsorption step. In order to maintain synchronization of the beds
providing gas for
pressure equalization and those receiving gas for pressure equalization, and
to have
three beds on adsorption ail the time, one more of these 1120 cycle slots has
to be
allocated to adsorption. This can be accomplished by eliminating the fourth
step of
providing gas for pressure equalization, and providing the gas for the fourth
step of
pressure equalization from the bed that is providing gas for purge at that
time, as shown
in the following table:
-13-
CA 02452536 2004-O1-14
TABLE 3
A A A A A A 1 2 3 P' P B B G G 4' 3' 2' 1' R
1' R A A A A A A 1 2 3 P' P B B G G 4' 3' 2'
3' 2' 1' R A A A A A A 1 2 3 P' P B B G G 4'
G 4' 3' 2' 1' R A A A A A A 1 2 3 P' P B B G
B G G 4' 3' 2' 1' R A A A A A A 1 2 3 P' P B
P B B G G 4' 3' 2' 1' R A A A A A A 1 2 3 P'
.3 P' P B B G G 4' 3' 2' 1' R A A A A A A 1 2
1 2 3 P' P B B G G 4' 3' 2' 1' R A A A A A A
A A 1 2 3 P' P B B G G 4' 3' 2' 1' R A A A A
A A A A 1 2 3 P' P B B G G 4' 3' 2' 1' R A A
In this cycle, in the first 1/20 of the cycle time of the provide purge step
(indicated
by "P' "s), the provide purge bed simultaneously provides gas to the bed in
its purge step
(indicated by "G"s) and to the bed on the receiving end of the fourth step of
pressure
equalization (indicated by "4"'). The rest is kept as in the cycle shown in
Table 2.
In yet another embodiment, two beds are always on adsorption. After the
completion of the adsorption step, the bed goes through four steps of pressure
equalization on the provide gas side. Then the bed provides gas for purge for
3/20 of
the cycle time, followed by 1/20 of the cycle time for blowdown, 3/20 of the
cycle time for
purge, 4 steps of pressure equalization on the receiving gas end, and 1/20 of
the cycle
time for re-pressurization, as shown in the following table:
-14-
CA 02452536 2004-O1-14
TABLE 4
A A A A 1 2 3 4 P P P B G G G 4' 3' 2' 1' R
1' R A A A A 1 2 3 4 P P P B G G G 4' 3' 2'
3' 2' 1' R A A A A 1 2 3 4 P P P B G G G 4'
G 4' 3' 2' 1' R A A A A 1 2 3 4 P P P B G G
G G G 4' 3' 2' 1' R A A A A 1 2 3 4 P P P B
P B G G G 4' 3' 2' 1' R A A A A 1 2 3 4 P P
.P P P B G G G 4' 3' 2' 1' R A A A A 1 2 3 4
3 4 P P P B G G G 4' 3' 2' 1' R A A A A 1 2
1 2 3 4 P P P B G G G 4' 3' 2' 1' R A A A A
A A 1 2 3 4 P P P B G G G 4' 3' 2' 1' R A A
Tables 2-4 show the first embodiment of the invention, wherein the PSA system
employs ten adsorbent beds and four steps of equalization. Table 5, below,
shows a
cycle chart for a process in accordance with the first and second embodiments
of the
invention. That is, in addition to having ten adsorbent beds and four steps of
equalization, the process of Table 5 has at feast two beds being counter-
currently
purged at any given time.
Increasing the purge time can improve the adsorption capacity and increase
recovery simultaneously. Increasing the purge time also increases the provide
purge
time following the conventional ways of constructing cycles. In the following
embodiment, one provide purge bed provides the purge gas for two beds:
-15-
CA 02452536 2004-O1-14
TABLE 5
A A A A 1 2 3 P' P B B G G G G 4' 3' 2' 1' R
1' R A A A A 1 2 3 P' P B B G G G G 4' 3' 2'
3' 2' 1' R A A A A 1 2 3 P' P B B G G G G 4'
G 4' 3' 2' 1' R A A A A 1 2 3 P' P B B G G G
G G G 4' 3' 2' 1' R A A A A 1 2 3 P' P B B G
B G G G G 4' 3' 2' 1' R A A A A 1 2 3 P' P B
.P B B G G G G 4' 3' 2' 1' R A A A A 1 2 3 P'
3 P' P B B G G G G 4' 3' 2' 1' R A A A A 1 2
1 2 3 P' P B B G G G G 4' 3' 2' 1' R A A A A
A A 1 2 3 P' P B B G G G G 4' 3' 2' 1' R A A
In addition, this bed is also used to provide the gas for the fourth step of
pressure
equalization during the first half of the provide purge step (P'). This
enables doubling the
purge time, and at the same time employing four steps of pressure equalization
for a
ten-bed system with two beds on adsorption.
The second embodiment of the invention is not limited to ten-bed devices or
four
equalization steps, as in the process depicted in Tabfe 5.
For example, a 12 bed cycle with double purge time, 4 steps of pressure
equalization and 3 beds on adsorption is shown in the following table:
-16-
CA 02452536 2004-O1-14
TABLE 6
A A A A A A 1 2 3 P'P B B G G G G 4' 3'2' 1' I R R
R R A A A A A A 1 2 3 P'P B B G G G G 4' 3' 2'1'
I
1' I R R A A A A A A 1 2 3 P'P B B G G G G 4'3'2'
3' 2' 1'I R R A A A A A A 1 2 3 P'P B B G G G G 4'
G 4' 3'2' 1'I R R A A A A A A 1 2 3 P' P B B G G G
G G G 4' 3'2' 1'I R R A A A A A A 1 2 3 P' P B B G
B G G G G 4' 3'2' 1' I R R A A A A A A 1 2 3 P'P B
P B B G G G G 4' 3' 2'1' I R R A A A A A A 1 2 3 P'
3 P' P B B G G G G 4'3' 2'1' I R R A A A A A A 1 2
1 2 3 P' P B B G G G G 4'3' 2'1' I R R A A A A A A
A A i 2 3 P' P B B G G G G 4'3' 2'1' I R R A A A A
A A A A 1 2 3 P' P B B G G G G 4'3' 2' S'I R R A A
In the process shown in Table 6, the highest pressure equalization step is
separate from the product re-pressurization step. The bed providing purge gas
provides
purge gas to two beds. In the first half of the providing purge gas step (P'),
this bed also
provides gas to the lowest pressure step of pressure equalization.
A 12 bed cycle with double purge time, 4 steps of pressure equalization and 4
beds on adsorption is shown in the following table:
-17-
CA 02452536 2004-O1-14
TABLE 7
IAA A A A A A A 1 2 3 P'P B B G G G G 4'3' 2' 1'R
1'R A A A A A A A A 1 2 3 P'P B B G G .GG 4' 3'2'
3'2' 1'R A A A A A A A A 1 2 3 P'P B B G G G G4'
G 4' 3'2' 1'R A A A A A A A A 1 2 3 P'P B B G G
G
G G G 4' 3'2' 1'R A A A A A A A A 1 2 3 P'P B B
G
B G G G G 4' 3'2' 1'R A A A A A A A A 1 2 3 P' P
B
P B B G G G G 4' 3'2' 1' R A A A A A A A A 1 2 3
P'
3 P' P B B G G G G 4' 3' 2'1' R A A A A A A A A 1
2
.12 3 P' P B B G G G G 4'3' 2'1' R A A A A A A AA
A A 1 2 3 P' P B B G G G G 4'3' 2'1' R A A A A A
A
A A A A 1 2 3 P' P B B G G G G 4'3' 2'1' R A A AA
A A A A A A 1 2 3 P' P B B G G G G 4'3' 2'1' R AA
In the process shown in Table 7, the bed providing purge gas provides purge
gas
to two beds. In the first half of the providing purge gas step, this bed also
provides gas
to the lowest pressure step of pressure equalization.
A 12 bed cycle with triple purge time, 4 steps of pressure equalization and 3
beds
on adsorption is shown in the following table:
-18-
CA 02452536 2004-O1-14
TABLE 8
A A A A A A 1 2 3 P'P B B G G G G G G 4' 3'2' 1' R
1'R A A A A A A 1 2 3 P' P B B G G G G G G 4' 3' 2'
3'2' 1'R A A A A A A 1 2 3 P' P B B G G G G G G 4'
G 4' 3'2' 1' R A A A A A A 1 2 3 P' P B B G G G G G
G G G 4' 3' 2'1' R A A A A A A 1 2 3 P' P B B G G G
G G G G G 4'3' 2'1' R A A A A A A 1 2 3 P' P B B G
B G G G G G G 4'3' 2'1' R A A A A A A 1 2 3 P' P B
P B B G G G G G G 4'3' 2' 1'R A A A A A A 1 2 3 P'
.3P' P B B G G G G G G 4' 3'2' 1'R A A A A A A 1 2
1 2 3 P' P B B G G G G G G 4' 3'2' 1'R A A A A A A
A A 1 2 3 P'P B B G G G G G G 4' 3'2' 1'R A A A A
A A A A 1 2 3 P'P B B G G G G G G 4' 3'2' 1'R A A
In the process shown in Table 8, the bed providing purge gas provides purge
gas
to three beds. In the first half of the providing purge gas step (P'), this
bed also provides
gas to the lowest pressure step of pressure equalization.
Tables 5-8 are illustrative and not limiting. For example, the increased purge
time embodiment of the invention encompasses the use of any number of beds,
although 10-12 beds are preferred, and encompasses cycles with less or more
than four
steps of pressure equalization.
The invention will be illustrated in more detail with reference to the
following
Examples, but it should be understood that the present invention is not deemed
to be
limited thereto.
_19_
CA 02452536 2004-O1-14
Example 1
The unexpected advantages of the invention are demonstrated by the following
simulation comparing a ten-bed embodiment of the invention (see Table 2,
above) with a
twelve-bed cycle in accordance with the prior art (see Table 1, above).
The pressure and temperature specified in the simulations were 30 atmospheres
and 100°F, respectively. The composition specified in the simulations
contained 0.5%
nitrogen, 6% methane, 16% carbon dioxide, 3.5°!° carbon monoxide
and 74% hydrogen.
The key operating parameters and results are summarized in Table 9:
TABLE 9
Cycle Table 1 (Comparative)Table 2 (Invention)
Bed diameter (ft) 11.0 11.5
Bed length (ft) 22.4 22.4
Feed time (s) 240 128
Production (million 84.2 83.5
scf/d)
Recovery 88.2% 88.0l0
CO in product (ppm) 1.0 1.0
N2 in product (ppm) 620 332
It can be seen from Table 9 that the recoveries of the two cycles are
virtually the
same. The N2 content is much lower with the ten-bed cycle according to the
invention.
The amount of adsorbent needed for a certain production rate is reduced by 9%,
and the
number of beds is reduced from 12 to 10. That not only means that the costs of
vessels
-20-
CA 02452536 2004-O1-14
and skids are reduced, it also means that the costs of valves and connecting
pipes
connecting these beds are reduced as the consequence of bed reduction.
Example 2
The efficacy of increasing the purge time in accordance with the invention can
be
demonstrated by comparing the simulation results of the process of Table 1
(i.e., a 12
bed cycle with 4 beds on adsorption and 4 steps of pressure equalization) with
those of
Table 5 (i.e., a 10 bed cycle with 4 steps of pressure equalization, double
purge time,
and 2 beds on adsorption). The pressure and temperature specified in the
simulations
were 30 atmospheres and 100°F, respectively. The composition specified
in the
simulations contained 0.5% nitrogen, 6% methane, 16% carbon dioxide, 3.5%
carbon
monoxide and 74% hydrogen. Each bed has two sections: (1) a section closer to
the
feed nozzle, with 50% of the bed length, containing carbon adsorbent; and (2)
a section
further from the feed nozzle containing a zeolite adsorbent. The key operating
parameters and results are summarized in Table 10:
TABLE 10
Cycle Table 1 (Comparative)Table 5 (Invention)
Bed Diameter (ft) 11.0 11.0
Bed length (ft) 22.4 22.4
Hydrogen Production 84.2 79.9
(million scf/d)
Hydrogen Recovery 88.2 89.21
CO in product (ppm) 1.0 0.2
Nitrogen in product 620 1005
(ppm)
It can be seen from Table 10 that using the cycle in Table 5, it is possible
to
eliminate two beds and the associated valves, piping and adsorbents, and at
the same
time increase hydrogen recovery by 1 %. Moreover, the diameters of the beds in
this
-21
CA 02452536 2004-O1-14
cycle are smaller. The total amount of adsorbent is reduced by 36%. If the bed
diameters were equal, the hydrogen production would be greater for the cycle
in Table 5
with a similar recovery benefit. The modifications that may be needed to
achieve these
are (1) arranging piping and valves such that one provide purge bed can
provide gas to
three beds with the desired flows, and (2) arranging pipes and valves such
that off-gas
can be collected from one blowdown bed and two purge beds. One way of doing so
is to
use a valve with a positioner on the off-gas line of each bed so that the off-
gas flow from
each individual bed can be independently controlled.
While the invention has been described in detail and with reference to
specific
examples thereof, it will be apparent to one skilled in the art that various
changes and
modifications can be made therein without departing from the spirit and scope
thereof.
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