US006007606A

United States Patent (19) 11 Patent Number: 6,007,606

Baksh et al. (45) Date of Patent: Dec. 28, 1999
54 PSA PROCESS AND SYSTEM 5,258,059 11/1993 Yamaguchi et al. .................. 95/116 X
5,370,728 12/1994 LaSala et al. ....... ... 95/130 X
(75) Inventors: Mohamed Safdar Allie Baksh; Frank 5,505,765 4/1996 Kaaji et al. .. ... 9.5/103 X
Notaro, both of Amherst, N.Y. 5,518,526 5/1996 Baksh et al. .......................... 95/130 X
5,565,018 10/1996 Baksh et al. .......................... 95/105 X
Assignee: Praxair Technology, Inc., Danbury, 5,620,501 4/1997 Tamhankar et al. .................... 95/96 X
Conn. 5,658,371 8/1997 Smolarek et al. .. ... 9.5/105 X
5,702,504 12/1997 Schaub et al. .. ... 9.5/105 X
5,735,938 4/1998 Baksh et al. ... ... 95/105 X
Appl. No.: 08/987,791
Filed: Dec. 9, 1997 Primary Examiner Robert Spitzer
Attorney, Agent, or Firm-Robert J. Follet
Int. Cl." ............................................ B01D 53/053
U.S. Cl. ................................... 95/98; 9.5/101; 95/105; 57 ABSTRACT
95/117; 95/139 A PSA process involving the Storage of products of various
Field of Search ......................... 95/96–98, 100-105, purities in Segregated Storage tanks for Subsequent usage is
95/116-119, 121, 122, 130, 135-140 disclosed. Products of increasing purities, admitted at the
References Cited
product end of the bed, are used during purging and repres
Surization Steps. In addition, different composition Streams
U.S. PATENT DOCUMENTS collected at the feed end of the bed during the countercurrent
depressurization Step are admitted at the feed end of the bed,
3,788,036 1/1974 Lee et al. .................................. 95/101 in the order of increasing product component content, during
4,263,018 4/1981 McCombs et al. ... 95/98 X the rising pressure step(s). This cycle gives higher recovery
4,340,398 7/1982 Doshi et al. .............................. 95/103 and lower bed size factor than prior art PSA cycles.
4,468,237 8/1984 Fuderer .................... ... 9.5/100
4,816,039 3/1989 Krishnamurthy et al. ... 95/97
5,250,088 10/1993 Yamaguchi et al. ........................ 95/98 4 Claims, 13 Drawing Sheets

TE P2 P2 TE
C - RS D SC e PP
EqD PPG BD PG Equ PP AD
BED A
III IV WI VII VIII
C-BE) C-BE
TE1
lePP TE1 C) Cn-P1 C P1
AD AD Eqd PPG | BD PG
BEDB
V W WI
C-BE1
TE2 TE2 P2
O PP C) Ca-P2 C
Equ PP AD AD EqD PPG BD PG
BED C
VII VIII I I II IV W VI
C-BE2 Cn-BE2
P TE2
C) C PP
TE2 C
o
BD PG | Equ PP AD AD PPG
BED D
V WI VII VII IV
C-BE2 C-BE2
TE2 P2 BE2
O GO GD
SEGREGATED
ST ST2 ST3 TANKS
P1 BE1

TE
CD C C2
U.S. Patent 6,007,606

|SOu0n?9ON 0119u ng
U.S. Patent Dec. 28, 1999 Sheet 2 of 13 6,007,606

B3 - LAYER 3

B2 - LAYER 2

B1 - LAYER 1

FIG. 2
U.S. Patent Dec. 28, 1999 Sheet 3 of 13 6,007,606

HYDROGEN
PRODUCT

TOP EOUALIZATION

ST3

BOTTOM
EQUALIZATION
U.S. Patent Dec. 28, 1999 Sheet 4 of 13 6,007,606

TE P2 P2 TE1
C -os) D SC e PP
PPG BD PG Equ PP AD
BED A
IV W VI VII VIII I
O-BE1 C - BE1
TE1 TE1
ePP C Cn P1 C) P1
AD AD EqD PPG BD PG Equ PP
BEDB
I IV W V VII VIII
CN BE1 BE1

TE2 TE2 P2
C PP C) CN-P2 O
Equ PP AD AD EqD PPG BD PG
BED C
WII VIII I IV W WI
C-BE2 CN-BE2
P1 TE2 TE2 P1
C C PP C C
BD PG Equ PP AD AD PPG
BED D
V VI VII VIII IV
G-BE2 C-BE2
TE2 P2 BE2
O GO CD
SEGREGATED
ST1 ST2 ST3 TANKS
P1 BE1

TE1
CD C C2

FIG. 4
U.S. Patent Dec. 28, 1999 Sheet 5 of 13 6,007,606

P
AD AD PP BD PG Equ
BED A
I IV V VI VII

PP
Equ PP AD AD EqD PP BDI PG
BED B
VII VIII I III IV W WI

BD PG Equ AD AD EqD PP
BED C
V VI VII

PP
PP BD PG E U PP D
BED D q
IV V WI VII VIII

FIG. 5
U.S. Patent Dec. 28, 1999 Sheet 6 of 13 6,007,606

7. O
G660
OW)/OW) ()ITAZO
8
W8 Id
GZ660

9 y
-- -- --- -
660 09.1,7Z8|6 B(SCWN0I3S)
U.S. Patent Dec. 28, 1999 Sheet 7 of 13 6,007,606

09.19
0,9 9

0 79
08ZG B(SCWN0I3S)
ZNOIWAJH30N
0709

G10 AO G90 90 GGO

OW)/ION) ()ITAO
GO GO ty'O GSO 0 87
y O 8 9
W8 I d' I
U.S. Patent Dec. 28, 1999 Sheet 9 of 13 6,007,606

09.19

OZ9

08Z9 (BSOWNI3S)
NZOIWAJHE|0N

gto G90 GGO Gy'O

OWX/ION) ()ITAO
GSO 0 9?
y O
V8 I d'I
U.S. Patent Dec. 28, 1999 Sheet 10 Of 13 6,007,606

HYGRODEN
PRODUCT
U.S. Patent Dec. 28, 1999 Sheet 11 of 13 6,007,606

EË? | ?| EF- -E E |
U.S. Patent Dec. 28, 1999 Sheet 12 of 13 6,007,606

HYGRODEN
PRODUCT
U.S. Patent Dec. 28, 1999 Sheet 13 of 13 6,007,606

ST ST
PG

BEDA

ST2 T2
ST1 ST
PG

BED B

ST2 ST2

ST SOL ? ISO SOL -

ST2 SO

F. G. 3
 6,007,606
1 2
PSA PROCESS AND SYSTEM dation reactor produces high purity hydrogen and carbon
monoxide. Hydrogen and carbon monoxide are separated
FIELD OF THE INVENTION from the gas mixture (containing carbon dioxide, methane
This invention relates to a pressure Swing adsorption and water vapor, etc.) by a combination of PSA and distil
(PSA) process and system. More particularly the invention 5 lation or possibly only PSA. At least one stream rich in high
relates to the production of hydrogen gas via pressure Swing hydrocarbons (and water vapor etc.) is recycled to the
adsorption (PSA). reactor. A preferred embodiment of PSA system comprises
Serially connected adsorption Zones (or beds of Zeolite 5A or
BACKGROUND OF THE INVENTION 13X), in each Zone carbon monoxide is less Strongly
adsorbed than higher hydrocarbons, carbon dioxide and
There are a variety of known processes for producing water vapor, but more Strongly than hydrogen. According to
hydrogen. Some examples include the following: (1) Steam this invention, a basic PSA cycle is used, and the
reforming of natural gas or naphtha, (2) catalytic reforming depressurization/desorption Step is divided into two parts:
of hydrocarbons, e.g. gasoline and fuel oil, and (3) partial the first part, middle depressurization during which the top
oxidation of heavy oils or natural gas. In the aforementioned 15
bed undergoes countercurrent depressurization while the
processes, Steam reforming of natural gas is probably the bottom bed undergoes cocurrent depressurization, and car
most widely used process for hydrogen production. FIG. 1 bon monoxide is withdrawn between two adsorption beds;
shows the key proceSS units in a Steam methane reforming the Second part is a normal countercurrent depressurization
process to produce hydrogen. Referring to FIG. 1, a for both beds. The proceSS can produce, as claimed, at least
feedstock, e.g., natural gas is compressed and fed to a 98% pure hydrogen and carbon monoxide, and the gas
purification unit to remove Sulfur compounds. The desulfu entering the PSA unit has about 55% H and 42% CO.
rized feed is then mixed with Superheated Steam and fed to McCombs et al., in U.S. Pat. No. 4,263,018, disclose a
a reformer to produce primarily H and CO. The effluent preSSure Swing adsorption process utilizing at least two
Stream from the reformer is sent to a heat recovery unit, then adsorption beds, and a storage tank that is either an empty
to a shift converter to obtain additional hydrogen. The tank or a packed bed. The Storage tank is used for Storing
25 equalization falling gas from the top of the bed that is
effluent from the shift converter goes through a proceSS Subsequently used to pressurize another bed during the
cooling and recovery unit prior to Sending to a purification equalization rising Step. This invention practices a Sequen
unit (e.g. PSA) to produce high purity hydrogen. The tial refluxing Strategy wherein the Void gas captured during
following gives a brief introduction of Some of the prior art the equalization falling Step is returned in the order of
processes for producing hydrogen. increasing purity during the equalization rising Step. In
Sircar et al., in U.S. Pat. No. 4,077,779, describe an addition, this invention discloses the use of bottom-to
improved four bed hydrogen PSA process, wherein, a cocur bottom bed equalization Step; wherein, the bed receiving the
rent displacement Step, commonly referred to as high pres bottom equalization rising gas is also receiving feed
Sure rinse Step, is used prior to depressurization of each Simultaneously, i.e., uninterrupted feed during the bottom
adsorbent bed. By using a high pressure rinse Step prior to 35 to-bottom bed equalization Step.
depressurization of the bed, enhanced light component Yamaguchi et al., in U.S. Pat. No. 5,258,059, describe a
recovery is achieved. In addition, according to this PSA process using at least three adsorbent beds and a
invention, an evacuation Step is used between the purging holding column (Segregated Storage tank) of the feed-in/
and repressurization steps, and high purity H (99-99%) can feed-out Sequence retaining type. The holding column is
be produced at high recovery (96.5%) from a feed of 75% 40 used for Storing the Void gas recovered during the cocurrent
H and 25% CO. depressurization Step of the cycle. This gas is then used for
Fuderer et al., in U.S. Pat. No. 5,553,981, disclose a purging of the adsorbent bed during the regeneration Step of
proceSS for enhancing hydrogen recovery by combining shift the cycle. This holding column is specifically designed to
conversion, scrubbing and PSA. Carbon dioxide is mainly prevent gas from mixing, i.e., an impurity concentration
removed by scrubbing. Hydrogen is then purified by PSA. 45 gradient exists in this holding column. This invention is
The recycling of a portion of the waste from PSA system to restricted to the use of at least three adsorbent beds and a
shift conversion unit improves the hydrogen recovery. In an holding column to Store cocurrent depressurization gas that
example case, H can be finally purified by PSA up to is Subsequently used for purging. It does not describe for
99.99% from a feed typically around 97% after methanation. example, how this holding column can Supply both the purge
The recovery can reach 99% if 80% of PSA waste is 50 gas and product pressurization gas in advanced PSA cycles.
recycled to the shift converter, 15% is recycled to the partial Baksh et al. in U.S. Pat. No. 5,565,018, disclose the use
oxidation unit and 5% is discharged for use as fuel gas. of Segregated external gas Storage tanks to Store gases of
In U.S. Pat. No. 5,152,975, Fong et al., disclosed a varying purity for later use, in the order of increasing purity,
proceSS for producing high purity hydrogen. The basic Steps in the purging, equalization rising, and product pressuriza
in the process include the following: (1) partially oxidizing 55 tion steps of the PSA cycle. Significant reduction in both the
a gaseous hydrocarboneous feedstock to produce a Synthesis bed size factor and power consumption are achieved with
gas mixture of H and CO, (2) reacting the Synthesis gas this novel PSA cycle.
mixture with Steam to convert CO into a raw gas mixture Although, many modifications and variations of the basic
that primarily contains CO2 and H2, and (3) passing the raw PSA cycle have been studied and applied to commercial
gas mixture to a PSA process to produce high purity hydro 60 processes, Such as for the production of hydrogen from
gen and a reject gas mixture of impurities. Synthesis gas, yet PSA processes remain inefficient and
Kapoor et al., in U.S. Pat. No. 5,538,706, disclose a uneconomical for high purity production of hydrogen for
proceSS for hydrogen and carbon monoxide production from large plants when compared to the alternative methods. In
hydrocarbons. It consists of partial oxidation and PSA particular, current PSA processes (such as those disclosed
Separation, and cryogenic distillation as one possibility. The 65 above) both produce an enriched product having a purity
System provides a better performance by improving PSA that is averaged over the whole “make product' step and
Separation and reducing Starting materials. The partial oxi possess Several inherent problems.
 6,007,606
3 4
For example, during the top-to-top bed equalization Step, high purity hydrogen from a hydrogen feed mixture (e.g.,
the bed going through the equalization rising Step receives Synthesis gas) with higher H recovery, lower power
product gas with decreasing purity. Consequently, at the end requirement, and bed size factor than prior art PSA cycles.
of this equalization rising Step, the lowest purity gas is at the It is a further object of the invention to produce multiple
product end of the bed. Thus, during the Subsequent make purity products that can be used in different steps of the PSA
product Step, the product purity Spikes downwards, which cycle, or for use at Specified time within a step.
when averaged over the whole production Step, decreases
the purity of the product. In addition, the gas used for SUMMARY OF THE INVENTION
purging is of decreasing purity when obtained from another The invention preferably comprises a PSA process involv
bed currently in the make product step. If this purge gas was ing Storing of products of various purities in Segregated
obtained from a storage tank, then a constant purity gas is Storage tanks for Subsequent usage. Products of increasing
used for purging. Also, a similar problem (using products of purities, admitted at the product end of the bed, may be used
decreasing purity) exists in PSA cycles that utilized a during purging and repressurization Steps. In addition, dif
product pressurization Step in the PSA cycle, wherein, the ferent composition Streams collected at the feed end of the
product gas was obtained from another bed in the production 15 bed during the countercurrent depressurization Step is admit
Step.
Typically, in prior art H. PSA cycles, the bed going ted at the feed end of the bed, in the order of increasing
through the equalization rising Step receives product gas product content, during the rising pressure step(s). This
with decreasing purity. Consequently, at the end of this cycle gives higher product recovery and lower bed size
equalization rising Step, the lowest purity gas is at the factor than prior art PSA cycles having no concentration
product end of the bed. In addition, the gas used for purging reversal.
of the adsorbent bed was obtained during a Second Stage BRIEF DESCRIPTION OF THE DRAWINGS
equalization falling Step, and is used in the order of decreas
ing purity. The Statements made for the equalization and Other objects, features and advantages will occur to those
purging StepS are equally valid for the gas used in PSA skilled in the art from the following description of preferred
cycles that utilized a product pressurization Step. In addition, 25 embodiments and the accompanying drawings in which:
during the bottom-to-bottom bed equalization Step, the bed FIG. 1 is a process flow diagram showing how Steam/
that is being pressurized is receiving gas of decreasing methane reforming integrates with PSA to produce high
hydrogen concentration, Starting with a H. concentration purity hydrogen.
that is equal to the feed concentration, and as the bottom FIG. 2 is a schematic diagram of a PSA adsorption
to-bottom bed equalization Step continues, the hydrogen column containing three layers of adsorbents.
concentration at the feed end of the bed drops due to heavy FIG. 3 is a schematic diagram of a four bed PSA process
component(s) desorption. using reverse concentration profiles at both ends of the bed.
The aforementioned concentration degradation, i.e., gas FIG. 4 is a Schematic diagram of a preferred process of the
of decreasing H2 concentration, is being used for refluxing
and repressurization, giving rise to process irreversibility invention using a four bed PSA column cycle with concen
and mixing losses, viz, a degradation in the PSA proceSS 35 tration reversal at both ends of the bed.
efficiency. For example, during the purge Step and top-to-top FIG. 5 is a schematic diagram of a four bed PSA column
bed equalization Step, gas of decreasing H2 concentration is cycle wherein a Segregated Storage tank or tanks is (are) not
being used for refluxing. During the bottom-to-bottom bed used. This is outside the Scope of the present invention.
equalization Step, at the Start of the equalization, the H2 mole FIG. 6 illustrates PSA simulation results using the PSA
fraction is about 0.7403 (same as the feed concentration in 40
cycle of FIG. 5. Pressure and hydrogen concentration pro
Synthesis gas), and as the bottom-to-bottom equalization files at the product end of the bed during one PSA cycle are
progresses, the hydrogen mole fraction at the feed end of the illustrated.
bed falls from about 0.74 to about 0.65. Thus, in order to FIG. 7 illustrates PSA simulation results using the PSA
maintain desired product purity in prior art PSA cycles, the cycle of FIG. 5. Pressure and hydrogen concentration pro
production and equalization falling Steps must be terminated 45
files at the feed end of the bed during one PSA cycle are
much earlier than the time required for the bed to break illustrated.
through. This results in a failure to fully utilize the adsorbent FIG. 8 illustrates PSA simulation results using a PSA
bed.
Further, by using products of decreasing purities, for cycle (as in FIG. 4) of this invention (see FIG. 4). Pressure
and hydrogen concentration profiles at the product end of the
example during purging and repressurization Steps, addi 50
bed during one PSA cycle are illustrated.
tional contamination of the product end of the bed results, FIG. 9 illustrates PSA simulation results using a PSA
due to the use of the lowest purity product gas at the end of cycle (as in FIG. 4) of this invention (see FIG. 4). Pressure
the refluxing Steps. This added contamination of the product and hydrogen concentration profiles at the feed end of the
end of the bed brings about a Significant reduction in the bed during one PSA cycle are illustrated.
product purity in the early Stage of the make product Step. 55
FIG. 10 illustrates another embodiment of the invention
This causes a decrease of the average purity of the product, using a two bed PSA process using reverse purity profile at
or for a given purity, a significant reduction in product (light the product end of the bed.
component) recovery. In addition, by using product gas of FIG. 11 illustrates an embodiment of the invention involv
decreasing purity, the Spreading of the mass transfer Zone is ing a two bed PSA column cycle using reverse purity profile
60
enhanced. Finally, in order to contain the mass transfer Zone at the product end of the bed.
and maintain product purity, more adsorbent is required, FIG. 12 illustrates an embodiment of the invention
resulting in higher bed size factor and a more costly PSA involving a two bed PSA proceSS using reverse purity profile
proceSS.
at both ends of the bed.
OBJECTS OF THE INVENTION 65 FIG. 13 illustrates an embodiment of the invention
It is therefore an object of this invention to provide a involving a two bed PSA column cycle using reverse purity
highly efficient PSA process for producing large Volumes of profile at both ends of the bed.
 6,007,606
S 6
DETAILED DESCRIPTION OF THE Storage tanks at both ends of the bed in accordance with this
INVENTION invention, results in 10–15% higher H recovery when
This invention discloses high efficiency PSA processes for compared with PSA cycles that do not utilize hydrogen
hydrogen production by reducing the irreversibility and concentration reversal during purging and repressurization
asSociated gas mixing losses that are present in prior art PSA StepS e.g., top-to-top and bottom-to bottom bed equalization
proceSSeS.
StepS.
The invention possesses the following features: A preferred embodiment of the invention consists of the
(1) Novel PSA cycles using sequential refluxing at both utilization of a segregated Storage tank(s) in the PSA cycle
ends of the bed, i.e., product (light component) in the order to Store gases of various purities obtained from one or both
of increasing purity at the product end of the bed, and gas of ends of the bed. These variable purity gases are then used in
increasing hydrogen (the light component) concentration at the order of increasing concentration for purging and repres
Surization.
the feed end during repressurization step(s) of the PSA For example, in the bottom-to-bottom bed equalization
cycle;
(2) Reduction of process irreversibility and gas mixing 15
Step, gas of lowest hydrogen concentration is used in the
losses, via the use of Segregated Storage tank(s); early Stage of repressurization at the feed end, then gas of
increasing hydrogen content, up to the feed concentration, is
(3) Novel PSA cycles using sequential refluxing for two used in the latter Stage of repressurization. Thus, at the end
and four beds PSA processes; of the bottom-to-bottom equalization rising Step, the con
(4) Enhanced H2 recovery and productivity, and centration at the feed end of the bed coincides with the feed
(5) Simultaneous top-to-top and bottom-to-bottom equal gas concentration. During the Subsequent feed Step(s), the
izations with concentration reversal at both ends of the bed, concentration at the feed end of the bed matches exactly the
viz., the use of Segregated Storage tank(s) at both ends of the incoming feed, thereby reducing process irreversibility and
bed. mixing losses in the PSA process.
In order to use the lowest hydrogen purity gas at the Start 25 In addition, at the product end of the bed, gas of increas
of refluxing, followed by gas of increasing hydrogen purity ing H. purity is used for purging and repressurization. Note
during purging and repressurization Steps, it is necessary to that the use of concentration reversals at both ends of the bed
produce multiple hydrogen concentration products. Accord is quite different from prior art PSA cycles. In those cycles,
ing to this invention, the highest purity gas for refluxing product of constant purity, or product of decreasing purity
(purging and top-to-top bed equalization) is used last, and (obtained from another bed in the production Step) are used
gas of increasing H concentration is also used at the feed for refluxing at the top of the bed, and gases of decreasing
end during repressurization Step(s). Thus, we need a reversal He concentration are used during the bottom-to-bottom
of the H concentrations at both ends, viz. the use of equalization Step. Thus, the inclusion of the Segregated
Segregated Storage tanks at both ends of the bed. Storage tank enables the production of multiple purity gases
Since multiple purity products are required for purging 35 at both ends of the bed in various quantities for refluxing and
and repressurization, the PSA cycle becomes inherently repressurization. Also, by using additional Segregated Stor
more complicated, and Several modes of operation can be age tanks to capture gases of various purities during the high
envisioned. For example, if hydrogen concentration reversal pressure adsorption step(s) the process provides the capa
is required at both ends of the bed, then two Segregated bility for Supplying the necessary amounts of each purity gas
Storage tanks can be used, one at each end of the bed. Typical 40 to meet product demand of customers.
Segregated Storage tank designs could be of the type The PSA process of this invention will be described with
described by Yamaguchi et al., U.S. Pat. No. 5,258,059, or reference to the drawings and a particularly preferred
could be a vessel packed with layer(s) of adsorbent(s) or embodiment. The feed to the PSA process is synthesis gas
inert materials, or simply an empty column containing having a composition about 74% H, 22.5% CO2, and trace
baffles to Suppress mixing. 45 levels of CO, CH, N, and H.O.
By incorporating Segregated Storage tanks in the PSA FIG. 2 shows a PSA adsorption column containing three
cycle, we are able to Store multiple hydrogen concentration layers of adsorbents to remove the impurities from the
gases, which can be used in the order of increasing hydrogen Synthesis gas for producing high purity hydrogen. The first
concentration for purging and repressurization. For example, layer (B1) primarily removes H2O from Synthesis gas, and
the gas collected during the equalization falling Step is 50 the adsorbent is alumina, or Zeolite-Y, or alumina mixed
Stored in the Segregated Storage tank. At the beginning of the with zeolite-Y. The second layer (B2), commonly referred to
equalization rising Step, the lowest purity gas from the as the bulk Separation Zone, removes primarily CO2 from the
Segregated Storage tank is consumed first, then product of Synthesis gas, and the adsorbent is activated carbon or
increasing purity is used at a later time. Thus, during the Zeolite. The third layer (B3) is a zeolite based adsorbent that
equalization falling Step product gas of decreasing purities 55 removes the residual impurities to produce high purity
enters the Segregated Storage tank, and leaves the Storage hydrogen as the desired product.
tank in the reverse order (increasing product purities) during The basic features of the invention can be illustrated by
the equalization rising Step. Similarly, by using another describing the operation of a four-bed PSA process for H
Segregated Storage tank at the feed end of the bed, the gas production from Synthesis gas. It should be noted that the
collected at the feed end of the during depressurization is 60 Scope of this invention is not limited to hydrogen
used in the order of increasing hydrogen concentration production, and the key features of this invention could also
during the bottom-to-bottom equalization rising Step. be applied to other PSA Separation processes, e.g. air Sepa
The incorporation of the Segregated Storage tanks in the ration to produce oxygen using Zeolite as the adsorbent, or
PSA cycle to facilitate purity reversal at both ends of the bed, in the production of N from air using O Selective adsor
allows for greater proceSS flexibility and improvements in 65 bents. The process/apparatus of the System may also be used
efficiency, when compared to prior art cycles. In particular, in Various purification processes, e.g., crude argon purifica
for a given PSA cycle, the inclusion of the Segregated tion to produce ultra high purity argon. In addition, although
 6,007,606
7 8
the features of this invention are illustrated using a four bed Step V. (BD): Valves 21, 44, 51, 52, 73 and 74 are closed;
PSA process to produce H from Synthesis gas, the process Valve 22 is opened and Bed B continues to depressurize
may also be employed with fewer than four beds or more Supplying burner gas to the Steam methane reformer until
than four beds. the minimum bed pressure is achieved. During the same
In the four bed hydrogen cycle, preferred total cycle times time, valves 14, 44, 77 and 78 are opened to allow feed
may range from about 2 to about 16 minutes, preferably 2 to gas to flow through Bed D. In addition, valves 31, 33, 61,
8 minutes. The purity of hydrogen recovered is preferably 63, 71, 72, 75 and 76 are opened, and Beds A and C
between about 99 to about 99.9999 vol.% hydrogen, pref undergo Simultaneous top-to-top and bottom-to-bottom
erably 99.99 Vol.%. Preferred product recovery ranges from
about 80 to about 95%, preferably 95%. bed equalization via Segregated tanks ST1 and ST3,
FIG. 3 is a schematic diagram of a four-bed PSA process respectively.
consisting of four adsorption beds, a feed compressor or Step VI: (PG): Valve 22 remains open; valves 52, 53, 73 and
blower, and three segregated Storage tanks (ST1, ST2, and 74 are opened. The bed is purged countercurrently from
ST3), and interconnected lines and valves. the top using gas from the bottom of the purge Storage
In order to more particularly describe the invention, 15 vessel (ST2). During the same time, valves 14, 44, 77 and
preferred features will be illustrated via an example of using 78 remain opened, and valve 41 is opened to allow Bed
a four bed PSA process to produce H2 from Synthesis gas. A to receive product preSSurization gas from Bed D.
All the valves in the diagram are operated electronically via Step VII: (EQU): Valves 14, 41, 44, 52, 53.73 and 74 are
a computer System and program logic. closed; valves 32, 34, 62, 64, 71, 72, and 75 and 76 are
Consider the cycle to start after Bed B has completed the opened. The bed is simultaneously pressurized with
product pressurization (PP) step. Now referring to FIGS. equalization gas from both the top and the bottom equal
3-4, the following describes the sequence of steps that Bed ization vessels (ST1 and ST3). During the same time,
B goes through using the preferred mode of operation. Note valves 11, 41, 77 and 78 are opened to allow feed gas to
that AD=Adsorption; PP=Product Pressurization; EQD= flow through Bed A. In addition, valve 23 is opened, and
Equalization Down; PPG = Provide Purge Gas; 25 Bed C undergoes countercurrent depressurization to Sup
BD=Blowdown; PG=Purge; EQU=Equalization Up. ply burner fuel.
Step I: (AD): Feed gas (e.g., Synthesis gas) is introduced at Step VIII: (PP): Valves 32,34, 62, 64, 71,72, and 75 and 76
the bottom of the bed and product is withdrawn from the are closed; valves 41 and 42 are opened to allow Bed B
top. In our example, valves 12, 42, 77 and 78 (see FIG. 3) to receive product pressurization gas from Bed A. The
are opened to allow the feed gas to flow through the bed. preSSurization continues to the maximum operating pres
During the same time, valve 24 is opened and Bed D Sure using product Supplied by Bed A which is on the
undergoes countercurrent depressurization to Supply adsorption Step. During the same time, Valves 11, 23, 41,
burner fuel. In addition, valves 31, 33, 61, 63, 71, 72, 75 77 and 78 remain opened, and Bed A continues to receive
and 76 are opened, and Beds A and C undergo Simulta feed gas. In addition, valves 53, 54,73 and 74 are opened
neous top-to-top and bottom-to-bottom bed equalization 35 to allow Bed C to receive purge gas from Bed D via
via segregated tanks ST1 and ST3, respectively. segregated Tank No.2 (ST2).
Step II: (AD & PP): Valves 12, 42, 77 and 78 remain open Tables 1 and 2 give examples of the operating conditions
as in Step I. In addition, valve 43 is opened. This provides and the PSA proceSS performance using three layers of
product pressurization gas to Bed C. In addition, Valves adsorbents (e.g. as in FIG. 2: alumina at the feed end (B1),
24, 51, 54, 73 and 74 are opened and Bed A continues to 40 followed by activated carbon (B2), then Na-Y Zeolite at the
depressurize while Supplying purge gas to Bed D Via product end of the bed (B3)). In Table 1, the results are in
segregated tank No. 2 (ST2). accordance with this invention that used Segregated tanks to
Step III: (EQD): Valves 12, 42, 43, 77 and 78 are closed; achieve concentration reversal during purging and repres
valves 32,34, 62, 64, 71,72, 75 and 76 are open. The bed Surization StepS at both ends of the bed. The comparative
is allowed to partially depressurize. Valves 32 and 75 45 data in Table 2 is from exactly the same PSA cycle, but
allow gas from the bottom of Bed B to pressurize the without any concentration reversal. For clarity, the PSA
bottom equalization vessel, Segregated Tank No. 3 (ST3). cycle used to obtain the results of Table 2 is shown in FIG.
Valves 62, 64, 71 and 72 allow gas from the top of Bed 5. Note from FIGS. 4 and 5, the only difference in the PSA
B to pressurize the top of Bed D via, Segregated Tank No. cycles is the use of the Segregated tanks in FIG. 4 to achieve
1 (ST1). In addition, valves 24, 51, 54, 73 and 74 are 50 concentration reversal during the purging and repressuriza
opened and Bed A continues to depressurize while Sup tion steps at both ends of the bed.
plying purge gas to Bed D Via Segregated tank No. 2 In the tables, the Symbols have the following meaning:
(ST2). These storage vessels are flow-through and baffled kPa 1000 Pa=S.I. unit for pressure (1.0 atm.-101.325 kPa,
as described in U.S. Pat. No. 5,258,059 (Yamaguchi et al., S=time unit in seconds, m=meter, and NCFH=Normal cubic
1993). During the same time, valves 13,43, 77 and 78 are 55 foot per hour.
opened to allow feed gas to flow through Bed C. In Table 1: A non-limiting example using this invention and
addition, Valve 21 is opened and Bed A undergoes coun three layers of adsorbents in the four-bed PSA process
tercurrent depressurization to Supply burner fuel. depicted in FIGS. 2-4. The results shown below were
Step IV: (PPG): Valves 32,34, 62, 64, 71,72, 75 and 76 are obtained from PSA simulations using Synthesis gas (dry
closed; valves 51, 52, 73 and 74 are opened. The bed 60 basis:74.03% H, 22.54% CO, 0.36% CO, 2.16% CH,
continues to depressurize while Supplying product purge and 0.91% N) as the feed. The feed typically contains
gas to the bottom of the purge vessel, Segregated Tank about 1500 ppm H.O.
No. 2 (ST2). Again, this storage vessel is flow-through
and baffled as described previously. During the same time, Layer 1 Adsorbent: Alumina
valves 13, 21, 43, 77 and 78 remain opened, and valve 44 65 Layer 1 Bed Height O.1270 m
is opened to allow Bed D to receive product pressuriza Amount of Alumina: 97.663 Kg per bed
tion gas from Bed C.
 6,007,606
9 10
-continued purity of the product. In addition, by using product gas of
decreasing purity, the Spreading of the mass transfer Zone is
Layer 2 Adsorbent: Activated Carbon enhanced. Finally, in order to contain the mass transfer Zone
Layer 2 Bed Height 17907 in and maintain product purity, more adsorbent is required,
Amount of Carbon: 900 Kg per bed
Layer 3 Adsorbent: Na-Y Zeolite resulting in higher bed size factor and a more costly PSA
Layer 3 Bed Height O.1270 m proceSS.
Amount of Zeolite: 514 Kg per bed In order to use the lowest hydrogen purity gas at the Start
Cycle Time (s) 96.O of refluxing, followed by gas of increasing hydrogen purity
High Pressure: 1187.53 kPa during purging and repressurization Steps, it is necessary to
Low Pressure: 144.57 kPa
Feed Rate: 1841 NCFH 1O
produce multiple hydrogen concentration products. Accord
Hydrogen Purity: 99.55% ing to this invention, the desirable purity versus time profile
Hydrogen Recovery: 80.11% at the product end of the bed is shown in FIG. 8; wherein,
Temp (K.): 3OO the highest purity gas for refluxing (purging and top-top
bed equalization) is used last. Therefore, to practice the
Table 2 An example using three layers of adsorbents in the 15
invention described herein, utilization of a product gas of
four-bed PSA process depicted in FIG. 5 without the use increasing purity is required, as shown in FIG. 8. Thus, we
of Segregated tank(s), i.e., no concentration reversal used need a reversal of the purity order, and the production of
at either end of the bed. The results shown below were multiple purity products, viz. the use of Segregated Storage
obtained from PSA simulations using Synthesis gas (dry tank(s).
basis:74.03% H, 22.54% CO, 0.36% CO, 2.16% CH, FIG. 9 shows the desirable hydrogen concentration profile
versus time at the feed end of the bed. Note from FIG. 9 that
and 0.91% N) as the feed. The feed typically contains at the end of the equalization rising Step, the concentration
about 1500 ppm H2O.
of the gas entering the bottom of the bed coincides with the
feed concentration. However, in PSA processes that do not
Layer 1 Adsorbent: Alumina practice concentration reversal at the feed end, at the Start of
Layer 1 Bed Height O.1270 m 25 the bottom-to-bottom bed equalization Step, the concentra
Amount of Alumina: 97.663 Kg per bed tion of H2 coincides with the feed concentration, then drops
Layer 2 Adsorbent: Activated Carbon
Layer 2 Bed Height 17907 in as the equalization continues, as shown in FIG. 7.
Amount of Carbon: 900 Kg per bed Since multiple purity products are required for refluxing
Layer 3 Adsorbent: Na-Y Zeolite and product pressurization, the PSA cycle becomes inher
Layer 3 Bed Height O.1270 m
Amount of Zeolite: 514 Kg per bed ently more complicated, and Several modes of operation can
Cycle Time (s): 96.O be envisioned. For example, if hydrogen concentration
High Pressure:
Low Pressure:
1187.53 kPa
144.57 kPa
reversal is required at both ends of the bed, then two
Feed Rate: 156.70 NCFH
Segregated Storage tanks can be used, one at each end of the
Hydrogen Purity: 99.6% bed. Typical Segregated Storage tank designs could be of the
Hydrogen Recovery: 71% 35 type described by Yamaguchi et al., U.S. Pat. No. 5,258,059,
Temp (K.): 3OO or could be a vessel packed with layer(s) of adsorbent(s) or
inert materials, or simply an empty column containing
Note from Tables 1 and 2 the higher H recovery (80.11 baffles to SuppreSS mixing.
versus 71%) for the four-bed PSA process using this inven By incorporating Segregated Storage tanks in the PSA
tion with Segregated tanks for concentration reversal (Table 40 cycle, we are able to Store multiple hydrogen concentration
1) versus the case, wherein no Segregated Storage tank(s) are gases, which can be used in the order of increasing hydrogen
used to achieve concentration reversal. concentration for purging and repressurization. For example,
In addition, the purity degradation in PSA cycles that do the gas collected during the equalization falling Step is
not practice the use of Segregated tank(s) for concentration Stored in the Segregated Storage tank. At the beginning of the
reversal during purging and repressurization step(s) (i.e., gas 45 equalization rising Step, the lowest purity gas from the
of decreasing product purity used for refluxing) is shown in Segregated Storage tank is consumed first, then product of
FIGS. 6 and 7 for the product end and feed ends of the bed, increasing purity is used at a later time.
respectively. For example, FIG. 6 shows that during the Note that in this example (see table 1 and FIGS. 3, 4 and
purge Step and top-to-top bed equalization Step, gas of FIG. 8 versus FIG. 6), during the equalization falling step,
decreasing H2 concentration is being used for refluxing. 50 product gas of decreasing purities enters the Segregated
FIG. 7 shows during the bottom-to-bottom bed equalization Storage tank, and leaves the Storage tank in the reverse order
Step, at the Start of the equalization, the H2 mole fraction is (increasing product purities) during the equalization rising
0.7403 (same as the feed concentration), and as the equal Step. Similarly, by using another Segregated Storage tank at
ization progresses, the hydrogen mole fraction falls from the feed end of the bed, the gas collected at the feed end of
0.7403 to about 0.64. 55 the bed during depressurization is used in the order of
Thus, in order to maintain desired product purity in prior increasing hydrogen concentration during the equalization
art PSA cycles, the production and equalization falling Steps rising step (see FIGS. 3, 4 and 9 versus 7).
must be terminated much earlier than the time required for The incorporation of the Segregated Storage tanks in the
the bed to breakthrough; thus, the adsorbent bed is not fully PSA cycle to facilitate purity reversal at both ends of the bed,
utilized. Furthermore, by using products of decreasing 60 allows for greater proceSS flexibility and improvement in the
purities, for example during the equalization rising and efficiency, when compared to prior art PSA cycles. In
purging Steps, additional contamination of the product end particular, for a given PSA cycle, the inclusion of the
of the bed results, due to the use of the lowest purity product Segregated Storage tank, in accordance with this invention
gas at the end of the refluxing Steps. This added contami (see Table 1), results in 10-15% higher H recovery when
nation of the product end of the bed, brings about a signifi 65 compared with the same PSA cycle without the use of
cant reduction in the product purity in the early Stage of the concentration reversal at both ends of the bed during purging
make product step, and causes a decrease of the average and repressurization Steps (see Table 2).
 6,007,606
11 12
In an alternative mode of operation, FIGS. 10 and 11 show complete the pressurization Step. At the end of this Step,
the PSA process and the Sequence of Steps, respectively, for Bed A is pressurized to the adsorption pressure.
a two-bed Hydrogen PSA process using Reverse Purity Step II: (AD): Valve 41 is closed. Valve 11 remains open as
Profiles at the top end, i.e. the product end of the bed. in Step I. In addition, valves 32, 51, 62, 71, 72,73 and 74
Referring to FIGS. 10 and 11 the two-bed PSA process is 5 are opened. This allows feed gas to flow through the Bed
now described with respect to Bed A. A while Bed B is depressurizing, Supplying equalization
Step I: (FP & EQU) Feed gas (e.g., H synthesis gas) is gas to the top and bottom of the Segregated Storage vessels
introduced at the bottom of the Bed A and equalization gas (ST1 and ST2).
from Bed B is introduced at the top of Bed A. In our Step III: (AD): Valves 32 71 and 72 are closed. Valves 11,
example, valves 11, 61, and 62 (see FIG. 10) are opened. 51, 62, 73 and 74 remain open with Bed A on adsorption.
Bed B is on the equalization falling step. After Beds A and In addition, Valves 22, 61 and 62 are opened, this Supplies
B are Substantially equalized in preSSure, then Valves 61 purge gas to Bed B. Bed B is on the purge Step.
and 62 are closed, and Valve 41 is open to allow gas from Step IV: (AD): Valves 22 and 61 are closed. Valves 11,51,
the product Source tank to enter the top of Bed A to 62, 73 and 74 remain open with Bed A on adsorption.
complete the preSSurization Step. At the end of this Step, 15 Also, valves 32, 71 and 72 are open, and Bed B is on the
Bed A is pressurized to the adsorption pressure. equalization raising Step, receiving gas from both the top
Step II: (AD): Valve 41 is closed. Valve 11 remains open as and bottom Segregated Storage vessels (ST1 and ST2).
in Step I. In addition, valves 51, 62, 71, 73 and 74 are Step V: (EQD): Valves 11, 32, 51, 71, 72, 73 and 74 are
opened. This allows feed gas to flow through the Bed A closed; valves 61 and 62 are opened. Bed A is partially
and allows Bed B to Supply equalization gas to the equalized with Bed B. In addition, valve 12 is open, and
Segregated Storage vessel (ST1). Bed B is undergoing feed pressurization and equalization
Step III: (AD): Valves 11, 51, 62, 73, and 74 remain open raising Step. After Beds A and B are Substantially equal
with Bed A on adsorption. In addition, valves 22, 61 and ized in pressure, then Valves 61 and 62 are closed, and
62 are opened, this Supplies purge gas to Bed B. Bed B is Valve 42 is open to allow gas from the product Source tank
on the purge Step. 25 to enter the top of Bed B to complete the pressurization
Step IV: (AD): Valves 22 and 61 are closed. Valves 11, 51, step. At the end of this step, Bed B is pressurized to the
71, 73 and 74 are open with Bed A on adsorption. Bed B adsorption pressure.
is on the equalization raising Step, receiving gas from the Step VI: (EQ): Valve 42 is closed; valve 12 remains open. In
Segregated Storage vessels (ST1 and ST2). addition, valves 31, 52, 61, 71, 71, 72, 73 and 74 are
Step V: (EQD): Valves 11, 51, 71, 73 and 74 are closed; opened. This allows feed gas to flow through Bed B while
Valves 61 and 62 are opened. Bed A is partially equalized Bed A is depressurizing, Supplying equalization gas to
with Bed B. In addition, valve 12 is open and Bed B is both the top and bottom segregated vessels (ST1 and
undergoing feed preSSurization and equalization raising ST2). During the same time Bed B is on an adsorption
Step. After Beds A and B are Substantially equalized in Step.
pressure, then valves 61 and 62 are closed, and valve 42 35 Step VII: (PG): Valves 31, 71 and 72 are closed. Valves 12,
is open to allow gas from the product Source tank to enter 52, 61, 71, 73 and 74 remain open, in addition valves 62
the top of Bed B to complete the pressurization Step. At and 21 are opened. Bed A is purged with product from
the end of this step, Bed B is pressurized to the adsorption Bed B and the waste stream is sent back to the steam
preSSure. methane reformer as burner gas. Bed B is on an adsorp
Step VI: (EQ): Valve 42 is closed; valve 12 remains open. In 40 tion Step.
addition, valves 52, 61,71, 73 and 74 are open, while Bed Step VIII: (PP): Valves 21 and 62 are closed. Valves 12, 52,
A continues to depressurize to Supply equalization gas to 61, 73 and 74 remain open. Valves 31, 71 and 72 are
the segregated vessel (ST1). During the same time, Bed B opened. Bed A undergoes partial repressurization by
is on an adsorption Step. Simultaneously receiving equalization gas from both the
Step VII: (PG): Valve 71 is closed. Valve 61 remains open, 45 top and bottom segregated vessels (ST1 and ST2). Bed B
in addition valves 62 and 21 are opened. Bed A is purged is on an adsorption Step.
with product from Bed B and the waste stream is sent Based on the PSA cycles described above, several modi
back to the Steam methane reformer as burner gas. Bed B fications can be made to alter one or more of the Steps
is on an adsorption Step. without deviating from the application or the general fea
Step VIII: (PP): Valve 21 is closed. Valves 61 and 71 remain 50 tures of the invention. For example, the top-to-top and
open. open. Bed A undergoes partial repressurization bottom-to-bottom repressurization Steps can occur as
receiving equalization gas from the Segregated vessel Sequentially rather than Simultaneously as described above.
(ST1). Bed B is on an adsorption step. In addition, Since various amounts of multiple purity
In another alternative mode of operation, FIGS. 12 and 13 products are collected, the time allocated and the proceSS
show the PSA process and the Sequence of Steps, 55 control of each Step becomes crucial in the operation of the
respectively, for a two-bed Hydrogen PSA proceSS using PSA cycle. Also, further improvements in the PSA cycle that
Reverse Purity Profiles at both ends, i.e. the product and feed utilized the Segregated Storage tank, include the overlapping
ends of the bed. Referring to FIGS. 12 and 13 the two-bed of various Steps in the PSA cycle to reduce total cycle time,
PSA process is now described with respect to Bed A. the choice of operating conditions (high pressure, low
Step I: (FP & EQU) Feed gas (e.g., H synthesis gas) is 60 preSSure, pressure at the end of equalization falling Step,
introduced at the bottom of the Bed A and equalization gas etc.), the times allocated for each Step, and the order in
from Bed B is introduced at the top of Bed A. In our which each Step of the cycle is executed.
example, valves 11, 61, and 62 (see FIG. 12) are opened. In another mode of operation, the Segregated equalization
Bed B is on the equalization falling step. After Beds A and tank (e.g. ST1) can be eliminated from the process, and all
B are Substantially equalized in preSSure, then Valves 61 65 of the equalization falling gas goes directly to the other bed,
and 62 are closed, and Valve 41 is open to allow gas from i.e., direct bed-to-bed equalization is allowed. However, in
the product Source tank to enter the top of Bed A to this mode of operation, during the bed-to-bed equalization
 6,007,606
13 14
Step, the bed undergoing the equalization rising Step, Thus, the inventive reversal of the concentrations at both
receives product gas of decreasing purity (in this invention, the top and bottom ends of the bed, via the use of Segregated
we want to use product of increasing purity). Upon the Storage tanks, allows for the reversal of the purity order and
completion of the equalization rising Step, the bed is pres the use of multiple product purities in the proper Sequence
surized with product from another bed that is in the produc to remove the inherent problems in prior art PSA cycles.
tion Step, or undergoes feed pressurization, or product and Further, Since, multiple purity products are required, the
feed pressurization simultaneously. adsorbent bed is fully utilized, i.e., the bed may be kept
Although the invention described here uses a Segregated onstream just prior to the heavy component(s) breakthough
product storage tanks (ST1, ST2 and ST3), multiple storage at the effluent end of the bed.
tanks can be conceived, wherein the effluent gas is directed Specific features of the invention are shown in one or
to the respective tank at different times in the make product more of the drawings for convenience only, as Such feature
Step. Also, this invention is not restricted to the use of may be combined with other features in accordance with the
cylindrical adsorbent beds with shallow dished heads on the invention. Alternative embodiments will be recognized by
top and bottom, and gas flow in the axial direction, i.e., other those skilled in the art and are intended to be included within
bed configurations may be used. For example, radial beds 15 the Scope of the claims.
may be used to achieve a reduction in pressure losses with What is claimed is:
a concomitant reduction in power consumption. In addition, 1. A proceSS for the Separation of one or more Selectively
mixed layered beds can be used with mixed adsorbents adsorbable components from a gas mixture including a leSS
packed in each layer at various positions in the bed. For Selectively adsorbable component, wherein the gas mixture
example, Na-Y Zeolite mixed with alumina and pelletized is contacted in one or more fixed adsorbent beds containing
could be placed at the feed end of the bed to remove water adsorbent material Selective for the adsorption of Said one or
and carbon dioxide from the feed Stream, then Subsequent more Selectively adsorbable components by means of cyclic
layers are placed on top of the mixed alumina Zeolite Steps comprising:
composite to perform the Separation of Synthesis gas to (a) adsorption, during which the gas mixture is passed into
produce hydrogen enriched product. 25 Said one or more fixed adsorbent beds in contact with
In addition, using multiple Storage tanks allow for greater the adsorbent at an upper adsorption pressure and Said
flexibility in the process. For example, the individual steps one or more Selectively adsorbable components of the
in the cycle shown in FIG. 4 do not have to occupy fixed gas mixture are Selectively adsorbed and the leSS Selec
periods of time. Thus, physical variables Such as pressure tively adsorbable component thereof is recovered from
and composition can be used easily to determine the time the product end of Said one or more fixed adsorbent
allocated for each Step; thereby, adjusting the proceSS for beds, and
changes in temperature, pressure and variable product (b) depressurization, during which the passage of the gas
demand. Since no bed-to-bed gas transfer is required when mixture into Said one or more fixed adsorbent beds is
additional Storage tanks are used, then it is possible to run discontinued and the preSSure in the bed is reduced
each bed independently, and regard the proceSS as a collec 35 from the upper adsorption preSSure to a lower desorp
tion of Single bed units. However, for proper Sizing and tion pressure to desorb and recover Said one or more
Sharing of compressor(s) and vacuum pump(s), Some Syn Selectively adsorbable component from Said one or
chronization of the overall cycle of each bed with the cycles more fixed adsorbent beds;
of the other beds is necessary. wherein
Finally, although the PSA cycle has been described in 40 i) a first portion of said less selectively adsorbable
relation to hydrogen production from Synthesis gas, wherein component is passed from the product end of Said
particular embodiments have been shown, other embodi one or more fixed adsorbent beds to one or more
ments are contemplated along with modification of the external, Segregated gas storage tank(s) adapted for
disclosed features as being within the Scope of the invention. precluding mixing of gas passed thereto, Said first
The aforementioned embodiments with the implied modifi 45 portion passed to Said external, Segregated gas Stor
cations are also applicable to other PSA Separation pro age tank(s) being initially of higher purity, followed
cesses. Also, although the PSA cycles described herein by lower purity, without passage of any of Said first
utilized Super-atmospheric cycles, it should be noted that the portion directly from one of Said one or more fixed
PSA cycle is not restricted to Super-atmospheric pressure adsorbent beds to another;
Swing adsorption (PSA) cycles, and trans-atmospheric or 50 ii) said first portion from Said external, Segregated gas
Subatmospheric PSA cycles may also be used. In addition, Storage tank(s) is passed to the product end of Said
the PSA process of this invention could also be used for one or more fixed adsorbent beds initially at said
Separating other mixtures (e.g. air) to produce oxygen, or lower desorption pressure for repressurization to an
nitrogen, or oxygen and nitrogen co-product). intermediate pressure with Said first portion at Said
The invention allows for the use of the highest purity gas 55 lower purity being initially passed from Said external
last, or the use of products of increasing purity in the Segregated gas storage tank(s), followed by Said first
equalization rising, purging and product pressurization portion at Said higher purity, Said one or more fixed
Steps, instead of the reverse order as in the prior art. In adsorbent beds thereby having said first portion of
addition the invention allows for operation of the PSA Said leSS Selectively adsorbable component of
proceSS in Such a manner that at the end of the bottom-to 60 increasingly higher purity therein at the product end
bottom bed equalization Step, the concentration at the feed thereof,
end of the bed coincides with the feed concentration. Note iii) a portion of Said less Selectively adsorbable com
that in prior art PSA processes, during the bottom-to-bottom ponent utilized to purge Said one or more fixed
bed equalization Step, the concentration at the feed end of adsorbent beds during step (b) is recovered from the
the bed deviates from the feed concentration as the bottom 65 product end of Said one or more fixed adsorbent beds
to-bottom bed equalization progresses with rising heavy and is passed from Said product end of Said one or
component(s) concentrations. more fixed adsorbent beds to Said one or more
 6,007,606
15 16
external, Segregated gas storage tank(s) adapted for the feed end of said one or more fixed adsorbent beds
precluding mixing of gas passed thereto, Said portion initially at Said lower adsorption pressure for repres
passed to Said external, Segregated gas Storage tank Surization to Said intermediate pressure with Said
(s) being initially of higher purity, followed by lower feed end gas having a lower purity being initially
purity, without passage of any of Said portion directly passed from Said external Segregated gas Storage
from one of Said one or more fixed adsorbent beds to
another;
tank(s), followed by said feed end gas having a
iv) Simultaneously with Step (ii) recovering feed end higher purity, Said one or more fixed adsorbent beds
thereby having gas of increasingly higher purity
gas from the feed end of Said one or more adsorbent therein at the feed end thereof.
beds and passing Said feed end gas recovered from 10 2. The process of claim 1 in which Said gas mixture
the feed end of said one or more fixed adsorbent beds
to Said one or more external, Segregated gas Storage comprises Synthesis gas, with hydrogen comprising the leSS
tank(s) adapted for precluding mixing of gas passed readily adsorbable component.
thereto, Said feed end gas passed to Said external, 3. The process of claim 1 in which said first portion is
Segregated gas storage tank(s) being initially of 15 passed, in Said step (i), to a single external, Segregated gas
higher purity, followed by lower purity, without Storage tank.
passage of any of Said feed end gas directly from one 4. The process of claim 1 in which said portion is passed,
of Said one or more fixed adsorbent beds to another; in Said step (iii) to a single external, Segregated gas Storage
V) said feed end gas recovered from said feed end of tank.
Said one or more fixed adsorbent beds in Said 20
external, Segregated gas storage tank(s) is passed to k . . . .