Note: Descriptions are shown in the official language in which they were submitted.
~ 8383~
ENHANC~I ) HET TUM RECOVERY
FTFT n OF TH~ INVENTION
Thi~ ap~l~c~tion i~ a cont~uation in part of U.S. A~lic~Lol~ Se.;al No.
0~J326,917 filed Octobcr 21, 1~9~.
( R Ch~ ~a~ C~ a~J5 a", ~4-.~ 7as~ c~.
This invention relates to processes for the recovery of~ ~)fromra natural gas)
containing stream. More particularly, this invention relates to a combined membrane
separation and pressure swing adsorption system and processes for the recovery of a ~ $7Rs
(a~L~as ~e.~. ~
( a.~. heliu~from~ natural gas)containing stream using this membrane separation and
10 pressure swing adsorption system.
BACKGROUND OF TH~ T~VENTION
The principal source of helium is its separation from natural gas streams prior
to the natural gas streams being used as a fuel or as a chemical feedstock. Natural gas
streams can contain up to about 10 percent helium. It is economically feasible to
15 recover helium from a natural gas stream down to a con~ent of as low as about 0.1
~ercent.
The economic feasibility depends on both the capital cost for a system and the
ongoing operational cost. It is an objective of the present invention to set out a system
where the economic efficiency of the system is maximized through the use of a
20 combined membrane and pressure swing adsorption system.
There is disclosed in U.S. application serial no. 08/326,917 an efficient pressure
swing adsorption process for the recovery of helium from natural gas streams. This
pressure swing adsorption system will take a typical natural gas which contains from
about 1 to 6 percent helium and produce a helium stream which contains more than2S 99.9 ~elcent helium. This is a very high purity helium. This is done by solely using a
pressure swing adsorption system.
One problem in the use of such pressure swing adsorption systems in the
recovel ~ of helium from natural gas streams which contain low levels of helium is the
voltllnes of gas that must pass through the adsorbent beds of the pressure swing
2 2 1 83~32
adsorption system to recover a given volume of helium. In order to accommodate
large volumes of helium, the adsorbent containing columns and other equipment
must be sized for this gas flow. However, as the volume of gas flow increases, the
diameter and/or length of the adsorption columns is increased and the amount of
5 adsorbent needed is likewise increased. It is this capital equipment cost plus the
operating cost that will determine if a natural gas stream with a particular helium
conlent can be used as a source of helium.
The efficiency of recovering helium from lower helium content natural gas
streams can be increased if there is a membrane partial separation of the helium from
lO the natural gas with a permeate natural gas with an increased helium contellt being
fed to a pressure swing adsorption system. The processes of this application aredirected to increasing the heliurn conlent of a natural gas stream from about two to
ten times the initial concentration up to about 8 to 20 percent helium by volume in a
membrane separation stage. When the helium content of the natural gas is doubled,
15 the amount of gas to be flowed through the pressure swing adsorption system to
recover the same volume of helium is halved. If tripled, the volume is reduced to one
third. And when the helium content is quadrupled the amount of natural gas to beflowed through the pressure swing adsorption system to recover the same volume of
helium is reduced to a quarter. This has a significant effect on the capital cost of the
20 pressure swing adsorption system and its operating cost. There is a greater efficiency
even though there is a capital cost and an operating cost for the membrane separation
system. There is a net cost savings when the helium content in the natural gas stream
is inaeased a given amount.
Using the present pressure swing adsorption system, There is no need to
25 increase the helium content to above about 20 percent by volume. This pressure
swing adsorption system can very effectively increase the helium conlent in this lower
range to more than 99.99 percent by volume. This is not feasible with the prior art
helium recovery systems.
3 2 1 8383~
The efficiency of the pressure swing adsorption system also can be improved.
By the use of some of the high pressure residue gas from the membrane unit as the gas
to aid in removing primarily nonadsorbed gas FROM the adsorbent bed that has
completed an adsorption phase, the need for an ~itional compressor in the process
5 is obviated. This results in a savings in the capital cost of the pressure swing
adsorption unit.
It is known to separate helium from natural gas by means of pressure swing
adsorption. Such a process is disclosed in U.S. Patent 5,089,048. This patent ~ rloses
a pressure swing adsorption system for helium enrichment. The process in this patent
10 can be used with helium streams which contain less than 10 percent helium. The
~rocess consists of a three step pressure build-up phase, an adsorption phase, a three
step pressure relief phase, and an evacuation phase. In the pressure build-up phase, a
cocurrent first depressurization gas is flowed cocurrently into an adsorbent bed which
has been evacuated to increase the gas pressure in this bed. This is followed by a
1 5 countercurrent flow of a second cocurrent depressurization gas from another
adsorbent bed which has completed an adsorption phase. This is then followed by a
countercurrent flow of product gas to bring the bed up to the operating pressure. This
process will produce a purified helium stream but at a lower efficiency. One problem
is that there is a loss of product helium in the gases that are discharged as waste gases.
20 Since the amounts of helium in the waste gas are relatively high, their loss creates an
the inPffi~i~ncy of the process. In the processes of the present invention, helium is
maintained in the pressure swing adsorption system as a gas inventory and not
removed as part of a waste gas or off-gas. In ~ lition the multi-step pressuri7~tion
and depressuri_ation techniques are not used in the pressure swing adsorption
25 system.
European Patent 092,695 and U.S. Patent 3,636,679 also ~ rlose pressure swing
adsorption systems for helium purification. However, in European Patent 092,695, the
feed gas should contain about 50 to 95 percent by volume heli-lm It is not suitable for
- - 2~3a32
gas streams containing less than about 50 percent helium, and is clearly not useful
where the helium content of the gas stream is less than about 25 percent helium.It likewise is known to separate helium from other gases using a combination
of membrane separation and pressure swing adsorption separation. However, it is
5 not known to take a helium source having a helium content of less than about 10
percent by volume helium and providing a product that is 99.99 percent by volumehelium. In U.S. Patent 4,701,187 the feed to the membrane has a helium concentration
of 58.2 mole ~ercellt. This is a high feed concentration. In U.S. Patent 4,717,407, there
is described a process for the recovery of helium where the feed stream is first treated
10 in a non-membrane unit and then in a membrane unit. The non-membrane unit canbe an adsorption unit. This is the reverse of the present process and would require a
greater capital cost for the adsorption unit since làrge gas volumes have to be
proresse~ in the adsorption unit. U.S. Patent 5,006,132 discloses a process for
upgrading the main product in a pipeline by means of a membrane process. The main
1 5 product permeates through the membrane with contaminants rejected. The rejected
gas is put back into the pipeline and the product gas is used. Helium can be one of the
rejected gases. U.S. Patent 5,224,350 discloses a process for recovering helium from a
hydrocarbon/nitrogen stream by first removing the hydrocarbons in a liquid
extraction followed by a nitrogen/helium separation in a membrane unit. The helium
20 from the membrane unit that is in a concentration of more than 50 mole percent can be
flowed to a pressure swing adsorption unit to increase the helium conle,lt to more
than 99 mole percent. None of these processes is a highly efficient process. In the
~resellt system and processes, there is no need for a membrane unit to increase the
helium content of a feed gas to 50 mole percent or more. The present membrane and
2 S pressure swing adsorption system and processes have a high efficiency at a feed to the
pressure swing adsorption unit from the membrane unit of about 10 to 20 volume
~crcent helium.
2~ 3 2~ ~
Bl~F SUMMARY OF TH~ INVENTION
~ ,a b ~
It is an object of this invention to provide a process for h~liun~ which
tili7~s at least one stage of membrane enrichment followed by at least two stages of
c~ g~5
pressure swing adsorption. The membrane enrichment will increase the h~ u
r~. ~ a ~ -~
S concentration of from about two to ten times the initiah~h~entration with the
pressure swing adsorption stages increasing theCb~li~tration to 98 percent by
( 7~ ~ ~RS~ "a ~4~ Z~j----OO~
volurne or mo~he source of the helium containing stream can be a gas stream
exiting a natural gas well, a natural gas pipeline or any other gaseous source
containing helium.
When the gas source is a pipeline, or a wellhead gas that is to be flowed into apipeline, a non-permeate residue gas from the membrane stage is compressed to
pipeline pressure and put into the pipeline. In this way there is a minimal loss of
natural gas in the pipeline or of the natural gas being flowed to the pipeline. The off-
gas from the first pressure swing stage can be used to power compressors or is flared.
As a pre~erLed embodiment, some residue gas from the membrane unit is fed to
the stage I pressure swing adsorption process. This gas is used in phase II to flow a
gas having an essentially feed gas composition from the adsorbent bed that is
undergoing phase II processing in place of pressurizing a depressurization gas from
phase m and using this pressurized gas for this purpose. There then is produced a
20 recycle feed gas from phase II that is used in phase VI to repressurize an adsorbent
bed that is to go back onto a phase I adsorption step. By the use of residue gas to
produce the recycle feed gas a compressor is not needed to pressurize part of the
depressurization gas from phase m for use in phase II to produce the recycle feed gas.
Essentially any membrane that is permeation selective for helium versus
2 5 nitrogen, methane, carbon dioxide and hydrocarbons can be used. The feed pressure
to the membrane module will be in the range of about 400 to 1200 psig. The helium
enriched gas that permeates through the membrane will be at about 65 to 100 psiapressure, or at a lower pressure if a vacuum is drawn on the permeate side of the
~ 6 2 1 838~
membrane. If more than one stage of membrane separation is used this permeate gas
will be pressurized to about 400 to 1200 psig and fed to the second stage. After the
membrane enrichment of the gas stream the permeate gas as needed is pressurized to
about 25 to 100 psia and fed to one or more stages of pressure swing adsorption to
5 increase the concentration of helium. In order to increase the helium concentration to
98 percent by volume or more, it is preferred to use two stages of pressure swing
adsorption.
Although each stage of pressure swing adsorption also can be used alone
without the other stage, it is the preferred embodiment to use two stages of pressure
10 swing adsorption. Only the first stage of pressure swing adsorption can be used
where a helium gas in a purity of 75 to 95 percent by volume will be sufficient. Such a
purity is sufficient for party balloons and advertising balloons. The second stage of
pressure swing adsorption can be used alone where the helium gas has been
concentrated to about 50 to 90 percent by volume using a membrane or cryogenic
15 technique but then must be increased to a purity of 98 percent by volume or more.
This second stage of pressure swing adsorption is very efficient in producing high
purity helium gas streams.
The first pressure swing adsorption stage is comprised of a plurality of
adsorbent beds with each adsorbent bed sequentially undergoing six pressure swing
20 adsorptionphases. Theseare:
- Adsorption
II - Recycle
m - Depressurization
IV - Evacuation
V - Helium Pressurization
VI - Feed Recycle Pressurization
The second pressure swing adsorption stage is comprised of a plurality of
adsolbel~t beds with each adsorbent bed undergoing five phases. These are:
- 218383Z
- Adsorption
II - Depressurization
m - Evacuation
IV - Purge
V - Helium Pressurization
In the first stage pressure swing adsorption system the adsorbent bed enters a
phase I adsorption phase and produces a crude helium product. Following the
adsorption phase the adsorbent bed is regenerated. In regeneration the adsorbent bed
first enters a phase II recycle phase where a recycle feed gas is produced. This is
10 produced by feeding a part of the residue gas from the membrane unit to this
adsorbent bed. The recycle feed gas that is produced as it exits the adsorbent bed
which has just completed an adsorption phase is flowed to an adsorbent bed about to
go onto a phase I adsorption phase. In the phase II recycle phase the residue gas flows
through the adsorbent bed pushing the gas in the void space (which has
15 approximately feed composition) to the exit of the bed. The phase m depressurization
comprises countercurrently reducing the pressure in the adsorbent bed and
recovering a depressurization gas that is combined with the Phase IV evacuation
phase gas, compressed for use as energy to power plant equipment, or to the flare
unit. At this point, the adsorbent bed undergoing phase m depressurization is at20 about ambient pressure and undergoes a phase IV evacuation phase to remove
essentially all of the adsorbed components. The adsorbent bed on phase IV
evacuation is lowered in pressure to less than ambient pressure to countercurrently
remove the adsorbed substances from the adsorbent bed. This gas is recovered andcompressed with the gas from phase m as described above. This gas primarily will be
2 S hydrocarbons and nitrogen. The adsorbent bed then undergoes a phase V heliumpressurization where an enriched helium gas from phase I adsorption is flowed
counterc~urenlly into the adsorbent bed. In a final phase the adsorbent undergoes a
phase VI recycle feed pressurization where recycle feed gas from phase II recycle is
8 2183832
fed cocurrently into the adsorbent bed. The adsorbent bed then is at about input gas
pressure and is in a condition for a phase I adsorption.
The crude helium from the first pressure swing adsorption stage is fed to the
second pressure swing adsorption stage. In the second pressure swing adsorption
5 stage the adsorbent bed on phase I adsorption receives the enriched helium product
from the first pressure swing adsorption phase. Upon the completion of the phase I
adsorption phase, the adsorbent bed undergoes a phase II depressurization phase.This consists of countercurrently reducing the pressure in the adsorbent bed to about
ambient pressure. All of the depressurization gas produced in the depressurization
10 phase is compressed and flowed to the adsorbent bed on an adsorption phase. Upon
the completion of the phase II depressurization phase, the adsorbent bed then
undergoes a phase m evacuation phase. This consists of reducing the pressure to less
than ambient pressure. The off-gas from this phase can be collected and in whole or in
part flowed to the feed of the first pressure swing adsorption stage or fed to the input
15 to the membrane separation unit. It will be primarily nitrogen and some helium.
Prior to completion of the evacuation phase, the adsorbent bed is purged with anamount of helium product from this second pressure swing adsorption stage. This
consists of flowing some of the product helium gas countercurrently into the
adsorbent bed. This removes traces of non-helium gases from the adsorbent bed and
20 void space. The adsorbent bed then undergoes helium pressurization phase V which
consists of flowing product helium gas countercurrently into the adsorbent bed. At
this point the adsorbent bed has been regenerated and is ready for another adsorption
phase.
Each pressure swing adsorption system is comprised of a plurality of adsorbent
25 beds. Usually there are about three to five adsorbent beds in each pressure swing
adsorption stage and ~referably about four. Each adsorbent bed in each stage
sequentially will undergo the noted phases. The number of adsorbent beds used will
be an economic balance between the capital cost of the installation and operating
-- - 2183~
costs. The timing of the phase in each stage will to a degree be dependent on the
composition of the feed streams., the feed stream flow rates and the size of theadsorbent beds.
Brief Description of the Drawing~
Figure 1 is a schematic diagram of a hybrid membrane and pressure swing
adsorption system for helium recovery.
Figure 2 is a schematic diagram of the pressure swing adsorption phases of the
first stage of the pressure adsorption system section of the helium recovery yrocess.
Pigure 3 is a schematic diagram of the pressure swing adsorption phases of the
second stage of the pressure swing adsorption section of the helium recovery process.
Figure 4 is a detailed schematic diagram of the first stage pressure swing
adsorption system.
Figure S is a detailed schematic diagram of the second stage pressure swing
adsorption system.
Figure 6 is a table which sets out the phase sequences by time for the first stage
pressure swing adsorption system.
Figure 7 is a table which sets out the phase sequences by time for the second
stage pressure swing adsorption system.
1 )etailed l:)escription of the Invention
The present processes will be described in more detail with reference to the
figures. The processes yrererably consist of at least one membrane separation stage
and at least two stages of pressure swing adsorption. A feed gas is derived from a
wellhead gas or from a natural gas pipeline. This gas will be at a pressure of about 25
psia to 1200 psia. This feed gas is fed to the membrane stage of the process at a
pressure of about 400 to 1200 psia. The membrane stage produces a permeate stream
and a non-permeate residue stream. The residue stream which is almost 100 percent
non-helium containing gas is pressurized and fed back to the gas source while the
2 1 B38~
permeate stream, depending on its pressure, will be used at permeate pressure orpressurized to about 25 to 100 psia and fed to the first stage of pressure swingadsorption. In the first stage, a gas stream which contains up to about 20 percent by
volume helium is enriched in helium to more than about 50 percent by volume, and5 ~rerelably to more than about 90 percent by volume helium. This is accomplished by
~rererenlially adsorbing the other gases that are present along with the helium and
removing the other gases. Then in a second stage of pressure swing adsorption,
depending on the helium content of the feed gas, the helium content of the gas stream
is increased to more than 95 percent by volume, and preferably to more than about 99
10 ~ercent. At this concentration the helium is commercially usable.
A primary source of helium is from natural gas. This can be from the natural
gas at the wellhead or from a natural gas in a pipeline. The present combined
membrane and pressure swing adsorption system is economic to operate to recover
helium present at low concentrations. The membrane separation will increase the
15 concentration of the helium and decrease the volume of the other gases that must be
removed to recover the helium. This makes it economically viable to recover helium
from gas streams having a low helium content.
The membrane section of the system can be any membrane device with some
selectivity for helium over nitrogen, carbon dioxide, methane and higher
20 hydrocarbons. Each membrane section may consist of a single membrane device, or in
the alternative, several membrane modules interconnected and operated in order to
most efficiently produce a final permeate stream with a maximum helium content.
Each of the modules would contain a semipermeable membrane selective for the
particular separation. Suitable semipermeable membranes include polysulfane,
25 cp~ lose acetate, polyimide, polyamide, silicone rubber and polyphenylene oxide
membranes. These membranes will increase the content of the helium in the gas
stream from two to ten times. These membranes ~refelably will be in the form of
tubes having a very small diameter. This provides for a high gas/membrane contact
21 83~2
11
and a selective permeation of the helium through the membrane. The initial helium
content of the feed gas will be about 0.5 to 5 percent by volume helium, visually about
0.5 to 2 percent by volume helium.
Each of the pressure swing adsorption systems uses an adsorbent which has no
5 affinity for helium. Essentially any adsorbent that has an affinity for nitrogen and
hydrocarbons can be used. The ~refer~ed adsorbents are activated carbons.
Aluminosilicate and silica gel adsorbents also can also be used.
In Figure 1, there is shown a combined membrane and pressure swing
adsorption system that gets a feed gas from conduit 13. This conduit 13 can be a gas
10 pipeline, such as an interstate gas pipeline, or a feed from a wellhead. Compressor 17
will boost the pressure to about 400 to 1200 psia with this gas being fed into
membrane separation section 19. This compressor is not needed where the gas is at a
pressure of greater than about 400 psia. In this membrane separation unit is a
semipermeable membrane highly selective for helium with a low permeability for the
15 other components of the gas stream. The residue gas exits at conduit 23 and is
boosted in pressure by compressor 27 prior to being fed back into pipeline 13.
Preferably, some of this residue gas is flowed to the pressure swing adsorption units
through conduit 61. The permeate gas which is at a pressure of about 65 to 100 psia or
lower flows to compressor 33 where it is increased in pressure to about 25 to 100 psia
20 as is needed and fed into the first stage of pressure swing adsorption 37. The gas is
further enriched in helium and flows by conduit 39 to a second stage of pressureswing adsorption 41. There is produced a product gas which is more than 98 ~ercent
by volume helium and a by-product gas that is rich in the other components,
primArily nitrogen is fed, in whole or in part, back to any of the feed to the first stage
25 of pressure swing adsorption or to the feed to the membrane separation unit. This gas
flows through conduit 43 to compressor 45 where it is boosted in pressure. Valve 49
which is in line 47 is a three way valve. This valve will direct the flow of gas to
conduit 55 and the first stage of pressure swing adsorption, or to compressor 57 and
12 2 1 838~2
through line 59 to the input to the membrane separation stage. The non-helium
containing gas from the first stage of pressure swing adsorption will be flowed
through conduit 63 for use in providing power to the plant or flared. The membrane
unit will be comprised of a section containing from 10 to 30 tubes of a polymer
5 selective for helium. These tubes will be about 8 to 16 in diameter and 3 to 6 ft. in
length The pressure drop across the membrane unit is about 5 psid. The membrane
unit processes a gas flow of about 350 to 3500 scfm. The output gas will have a helium
conlent of about 4 to 20 percent by volume helium.
The first pressure swing adsorption stage consists of six phases which are set
10 out diagramatically in Fig. 2. In phase I, adsorption, an input gas stream is fed to the
adsorbent bed. An enriched helium gas flows from this adsorbent bed with
substantial quantities of the other gases being adsorbed in the adsorbent bed. Aportion of the enriched helium is used in phase V helium pressurization. After the
adsorbent bed that is undergoing adsorption has become saturated with adsorbed
15 other gases, it enters recycle phase. This consists of flowing a residue gas from the
membrane unit to this adsorbent bed on phase II recycle. This residue gas removes
helium and feed gas that is in the void space in the adsorbent bed to produce a recycle
feed gas. The recycle feed gas with the helium from the void space flows from this
adsorbent bed and is fed to the adsorbent bed that is on a phase VI recycle feed20 press-~ri7~tion phase. This recycle feed gas may be supplemented with feed gas, i.e.
permP~te gas from the membrane unit.
Concurrently an adsorbent bed is undergoing a phase m depressurization
phase. The phase m depressurization phase consists of decreasing the pressure in an
adsorbent bed to about ambient pressure. The effluent gases which are hydrocarbons
2 S and nitrogen contain little or no helium and may have a high fuel value. They are
used to power plant equipment or are flared. At the same time another adsorbent bed
is undergoing a phase IV evacuation phase. The phase IV evacuation consists of
drawing a vacuum on the adsorbent bed and countercurrently removing substantially
13 2 1 83832
all of the adsorbed gases from the adsorbent bed. These gases, like the phase m
depressurization gas, usually are recompressed and fed back to the pipeline or other
source. If not fed back to the pipeline or other source, they are used as a fuel in the
plant or flared.
Simultaneously another adsorbent bed is undergoing a helium pressurization
phase. In this phase enriched helium product from phase I adsorption is flowed into
the adsorbent bed, preferably countercurrently. Immediately upon the completion of
helium pressurization the adsorbent bed is further pressurized with recycle feed gas
from phase II recycle and feed gas. Preferably this gas is flowed cocurrently into the
adsorbent bed. This adsorbent bed then will be at about the pressure of the input gas.
The enriched helium gas from phase I that is not used in the stage I pressure
swing adsorption process is flowed to the second stage pressure swing adsorptionprocess for additional enriching. The second stage pressure swing adsorption process
is ~iPS. rihed in Fig. 3 This is a five phase process and is different from the first stage
pressure swing adsorption process. The first phase of the second stage process is an
adsorption phase and consists of passing the enriched helium gas from stage I into the
adsorbent bed undergoing a second stage adsorption phase. A further enriched
helium gas flows from the adsorbent bed on phase I adsorption with the non-helium
other gases being adsorbed. A portion of this enriched helium is used in the phase IV
purge phase to countercurrently flow and purge the other gases from the void space
and the adsorbent in the bed undergoing this phase. Another portion of the enriched
helium is flowed to phase V helium pressurization. The adsorbent bed undergoing
phase V pressurization is pressurized prior to undergoing a phase I adsorption phase.
Concurrently, there is an adsorbent bed undergoing a phase II
2 5 depressurization. This consists of countercurrently depressurizing the adsorbent bed
which has completed the phase I adsorption phase to produce a recycle gas. The
recycle gas from this phase II depressurization is pressurized to about the feed gas
2 i 83~3~
14
pressure or higher and flowed along with feed gas into the adsorbent bed on phase I
adsorption.
At the same time, an adsorbent bed is undergoing a phase m evacuation. This
consists of reducing the pressure from ambient to more than 20 inches of Hg v~clll.m,
5 and ~referably to more than about 28 inches of Hg vacuum. This removes
substantially all of the non-helium other gases from the adsorbent bed. This gas from
the evacuation phase is usually recycled to stage I input gas since it can contain up to
50 percent or more by volume helium.
These are the pressure swing adsorption phases that are used in stage I and in
10 stage II. Each adsorbent bed undergoing the pressure swing adsorption process in
each stage sequentially goes through the respective phases for stage I and for stage II.
The timing for the phases in each stage varies with the composition of the input gas
stream, gas flow rates and the size of the adsorbent beds. The timing will be governed
also by the time that it takes an adsorbent bed on an adsorption phase to reach
1 5 breakthrough. The input to the adsorbent bed on an adsorption phase will cease just
prior to the adsorbed gases exiting (breaking through) the end of the adsorbent bed.
This then will govern the timing of the other phases.
The stage I process will be described with particular referellce to Fig. 4. The
input gas stream is fed through conduit 10 and valve 12 into conduit 14 which delivers
20 the input gas stream to the adsorbent beds. Since adsorbent bed A is on an adsorption
phase inlet valve 32 is open as is exit valve 38. Valves 30, 34 and 36 are closed as are
inlet valves 44, 56 and 68 for the other adsorbent beds. Valve 24 also is closed. An
enriched helium product exits adsorbent bed A and flows through conduits 21 and 23
to stage II. Throttle valve 28 controls the pressure in conduits 21 and 23 and enriched
25 helium storage tank 26 which stores some of the enriched helium product from this
stage I. The stored enriched helium gas is used in stage I.
Upon the completion of the adsorption phase adsorption bed A enters a recycle
phase. In the recycle phase, part of the residue gas from the membrane unit and
2 1 838;32
residue gas from storage tank 25 is fed to adsorbent bed A from conduit 22. During
this time valve 32 is closed and valve 30 is opened. On the exit end of adsorbent bed
A valve 38 is closed and valve 40 is open. A recycle feed gas flows from adsorbent
bed A through conduit 20 to storage tank 16 during this phase. This recyde feed gas
S will be used to pressurize adsorbent bed B which concurrently is undergoing a phase
VI recycle feed pressurization. In order to flow this gas to adsorbent bed B valve 12 is
dosed and valve 24 opened. Input valves 32, 68 and 56 on the other adsorbent beds
are closed and valve 44 opened. The residue gas during the recycle phase removes a
helium gas having a content of about the input gas from the void space in adsorbent
10 bedA.
While adsorbent bed A is on an adsorption phase and a recycle phase,
adsorbent bed D has been on a phase m depressurization phase. In this phase, outlet
valves 74 and 76 are closed as are inlet valves 66, 68 and 72. Depressurization gas
flows from adsorbent bed D through valve 70 and into conduit 18. This
1 5 depressurization can be used as a fuel in the plant or flared.
While adsorbent bed A is on an adsorption phase and a recycle phase, and
adsorbent bed D on a depressurization phase, adsorbent bed C is on a phase IV
evacuation phase. In this phase outlet valves 62 and 64 and inlet valves 54, 56 and 58
of adsorbent bed C are closed. Valve 60 is opened. Evacuation gas, which is
20 substantially the more highly adsorbed non-helium gases, flows through vacuumpump 35 and is flowed for further use. Upon the completion of the evacuation phase,
adsorbent bed C is substantially clean of the non-helium more highly adsorbed gases.
Concurrently with these operations, adsorbent bed B has been on
re~ressllri7~tion. The first part of re~ress lrization consists of the phase V helium
25 pressurization phase. In this phase, all of the inlet valves to adsorbent bed B, valves
42, 44, 46 and 48, are closed. Outlet valve 52 also is closed. However, outlet valve 50
is opened so that enriched helium gas will flow countercurrently into adsorbent bed B.
Upon the completion of helium pressurization, valve 50 is closed and inlet valve 44 is
~ ~~ 16 2 1 83832
opened. At this time inlet valve 12 is closed and recycle valve 24 is opened. This
allows a recycle feed gas to flow into adsorbent bed B through conduit 14 from storage
tank 16. This will repressurize adsorbent bed B to about the input feed gas pressure.
A ~refe~red option is to incorporate a short input gas repressurization just prior
5 to the adsorption phase. In this mode up to about half of the recycle feed
repressurization time is transferred to an input gas repressurization time. In this
~refelled option it only is required that valve 24 be closed and valve 12 opened. Since
adsorbent bed B will be entering an adsorption phase in the next sequence, Ws will
remain the position of these valves. During an input gas pressurization of adsorbent
10 bed B outlet valves 50 and 52 remain closed. When adsorbent bed B enters the
adsorption phase it then will be necessary only to open valve 50.
This describes a full sequence of the operation of the stage I pressure swing
adsorption system. This produces an enriched helium which contains more than
about 7S volume percent helium, and preferably more than about 90 volume percent15 helium. The adsorbent beds then sequentially go through the phase sequences as set
out in Fig. 6. A useful timing for a full cycle is 480 seconds. However, timing is
dependent on input gas stream composition, pressure and flow rates as well as
adsorbent bed size. If a feed pressurization step is used in the repressurization of the
adsorbent beds this usually will be for a period of about 40 seconds. This timing is for
20 adsorbent beds which contain about 18,000 pounds of adsorbent, a feed gas pressure
of about 50 psia and a flow rate of about 1000 to 3000 cubic feet per minute, and
~,eferably about 2000 cubic feet per minute. Table 1 gives the valve position versus
time during a cycle of the stage I pressure swing adsorption system. The full
operation of the stage I process is fully described with reference to Fig. 2, Fig. 4, Pig. 6
25 and Table 1.
TABLE 1
Valve ~ 0-120 sec. 120-240 sec. 240-360 sec. 360-480 sec.
12 1 O/C(l) I O/C(l) I O/C(l) I O/C(l)
17 2 1 83832
24 C/O(2) C/O(2) C/O(2) C/O(2)
C/0(4) C C C
32 0/C(3) C C C/0(5)
34 C O C C
36 C C O C
38 0/C(3) C C O/C(6)
C/o(4) C C C
42 C C/0(4)
44 C/O(s) o/C(3) C C
46 C C O C
48 C C C O
O/C(6) 0/C(3) C C
52 C C/0(4) C C
54 C C C/0(4) C
56 C c/o(5) 0/C(3) C
58 C C C O
O C C C
62 C O/C(6) 0/C(3) C
64 C C C/O(4) C
66 C C C C/O(4)
68 C C C/0(5) 0/C(3)
O C C C
n c o c c
74 C C O/C(6) 0/C(3)
76 C C C C/O(4)
78 C/O(~ C/O(~ C/O(~ C/O(~
- 2 1 83~32
18
(1) Open during adsorption and closed during recycle feed pressurization
(2) Closed during adsorption and open during recycle feed pressurization
(3) Open during adsorption and closed during recycle
(4) Closed during adsorption and open during recycle
5 (5) aosed during helium pressurization and open during recycle feed
pressurlzahon
(6) Open during helium pressurization and closed during recycle feed
pressurization
(7) Closed during the first part of depressurization.
The helium enriched gas from stage I is fed as the feed gas to the stage II
pressure swing adsorption system as shown in Fig. 5. This feed gas flows from
conduit 23 of stage I through valve 112 and into conduit 114. With adsorbent bed E on
a phase I adsorption phase, outlet valve 136 is open as is inlet valve 130. Valve 102 is
1 S dosed. Valves 132 and 134 of adsorbent bed E are closed, as are inlet valves 138, 146
arld 154 to adsorbent beds F, G and H respectively. The feed gas flows into adsorbent
bed E as also does a recycle gas from adsorbent bed H which is on a phase II
depressurization phase. Optionally some of the recycle gas can be stored in tank 121.
This depressurization phase recycle gas flows through conduit 118 to compressor 122
20 where its pressure is increased up to about that of the feed gas or higher. A purified
heliurn product flows through valve 136 and through conduit 124 to a product outlet
with some purified helium held in storage tank 126. Throttle valve 128 regulates the
pressure in conduit 124 and tank 126. Part of this purified helium will be used in the
purge phase and in the phase V helium pressurization phase. The remainder is
25 productgas.
While adsorbent bed E has been on a phase I adsorption phase, adsorbent bed
H has been on a phase II depressurization phase. In this phase, outlet valve 160 and
110 are closed, as are inlet valves 154 and 156. The depressurization recycle gas flows
21 83832
19
through valve 158 and conduit 118 to compressor 122. Pressurized to about feed gas
pressure or higher, the recycle gas flows through conduit 120 to conduit 114. In this
part of the sequence, this recycle gas will be fed to adsorbent bed E.
Concurrently adsorbent bed G is on a phase III evacuation phase and a phase
IV purge phase. In the evacuation phase outlet valve 152 and 108 are closed as are
inlet valves 146 and 150. A vacuum is drawn on conduit 116 by vacuum pump 135.
T~is decreases the pressure in adsorbent bed G to more than about 20 inches of Hg
vacuum and ~refelably to more than about 28 inches of Hg vacuum, which
substantially removes all of the more highly adsorbed gases from this adsorbent bed
G. For the phase IV helium purge phase which takes place at the end of the
evacuation phase, valve 108is opened to allow purified helium to enter adsorbent bed
G and to flow countercurrently down into adsorbent bed G. Valve 148 remains openand adsorbent bed G is under a vacuum. This serves to remove the more highly
adsorbed gases from the void space and the adsorbent in adsorbent bed G. The gases
1 S flowing from adsorbent bed G are compressed in compressor 45 and flowed to the
input to stage I through conduits 47 and 55 as a part of the stage I input gas or to the
feed to the membrane unit through conduits 47 and 59. If flowed to the membrane
unit it will be compressed to the membrane unit pressure by compressor 57.
Additionally concurrently adsorbent bed F has been in repressurization. This
first consists of a phase V helium pressurization phase. In this phase inlet valves 138,
140 and 142 are closed. Outlet valve 144 is open so that purified heliurn gas which is
at a pressure about that of the feed gas flows countercurrently into adsorbent bed F to
increase the pressure of adsorbent bed F to about that of the feed gas. Valve 106 is
dosed.
A preferred option is to also incorporate a feed gas pressurization into the
sequence. This entails the closing of valve 106 about half to three fourths of the time
through the helium pressurization phase, and preferably about two thirds of the time,
and opening valve 138. This permits a repressurization to full feed gas pressure by
2 183832
the use of feed gas prior to valve 144 being opened and an adsorption phase initiated.
When this adsorbent bed enters an adsorption phase it only will be rle-~essAry to open
valve 144.
Valves 100, 104, 107 and 111 are throttle valves which are open through all
5 phase sequences. These valves are interconnected to line 124 and the adsorbent bed
exits via conduits 101, 103, 105 and 109 respectively. The flow of helium gas as a
purge gas is controlled by the respective companion valve to each of these throttle
valves.
This completes an operating sequence for the stage II pressure swing system.
10 Each of the adsorbent beds sequentially goes through each of the phases. Thissequence versus time is set out in Fig. 7. This is shown for a 360 second cycle which is
a ~ref~l~ed timing. However, as with stage I, the timing is dependent on feed gas
composition, feed gas pressure and flow rate and on the adsorbent bed size. In this
stage each adsorbent bed contains about 1600 pounds of adsorbent.
Table 2 describes the position of each valve during a cycle of Fig. 7 using the
pressure swing adsorption system of Fig. 5 and the phase sequence of Fig. 3.
21 8383~
21
TABLE 2
Valve # 0-90 sec. 90-180 sec. 180-270 sec. 270-360 sec.
102 C C C/O(l) C
106 C C C C/O(l)
108 C/O(l) C C C
110 C C/O(l) C C
130 O C C C/O(2)
132 C C O C
134 C O C C
136 C C O/C(3)
138 C/O(2) O C C
140 C C C O
142 C C O C
144 O/C(3) O C C
146 C C/O(2) O C
148 O C C C
150 C C C O
152 C O/C(3) O C
154 C C C/O(2) O
156 C O C C
158 O C C C
160 C C O/C(3) O
(1) Closed during evacuation and open during purge
S (2) Closed during helium pressurization and open during an optional feed gas
pressurization of 15 seconds
(3) Open during helium pressurization and closed during an optional feed gas
pressurization
22 2~838~
This valve position sequence is for a preferred operation of the present processas is the phase and cycle timing. The valve position sequence and the phase and cycle
tin~ing can be modified and yet remain within the scope of the present processes.
The two position valves that are used are valves which are either open or
S closed usually are butterfly valves. Valves 28 and 128 are throttle valves that remain
open in a constricted condition.
The combined stage I and stage II pressure swing systems will produce a
h~ gas product of more than about 90 percent by volume helium, and ~rererably
more than about 98 percent by volume helium. The pressure swing system can be
10 fully automated with a central processor controlling all of the flows and valve
sequencing. The valves are rated for the pressures of the systems. The tanks andconduits likewise must be rated for the operating pressures.
This description sets out the preferied operation of the stage I and stage II
pressure swing systems to produce a highly énriched product. The full scope of the
15 invention is more particularly set out and described in the appended claims.
As noted above the first stage of pressure swing adsorption or the second stage
of pressure swing adsorption can be used alone; however, it is ~refelred that they be
used together in tandem as ~esrriked. The first stage will produce a helium product
stream of about 75 to 90 percent by volume helium from a gas stream of less than20 about 20 percent by volume. Such a helium product can be used in balloons anddirigibles. The second stage of pressure swing adsorption will bring this heliumstream up to a helium content of 98 percent by volume or more.
A factor in the increased efficiency of the present pressure swing adsorption
~rocPssPs is the inventory of gas that is maintained within each of pressure swing
25 adsorption stage. The only gases that are discharged from stage I is the product
enriched helium gas and the adsorbed gases which primarily are hydrocarbons and
nitrogen. These will contain trace amounts at most of helium. In stage I the only non-
product gas that leaves the system is the gas from the evacuation phase which is only
23 2 ~ 8~8~
adsorbed gases and has essentially no helium content and the depressurization gas
that has essentially no helium content. The effluent gas from the recycle phase flows
to a recycle feed pressurization phase. The gas in the first stage of pressure swing
adsorption functions as an inventory gas until it is essentially devoid of helium and
5 then is vented or flared.
The second stage of pressure swing adsorption likewise maintains a high
il~V~ntCl,y of gas. When used in combination with the first stage of pressure swing
adsorption the gas from the evacuation phase and from the helium purge phase is
flowed to the input gas to stage I pressure swing adsorption. In this way no helium
10 leaves the system. A high inventory of gas is maintained in stage II through the flow
of all of the depressurization gas into the adsorbent bed in an adsorption phase.
The ~rere~led embodiments of the present helium enrichment proress~s have
been rlicclosed in this specification. However various modiffcations can be made to
the processes and yet comprise the present concepts. Such modifications are
1 5 co~ci~lered to be within the present discoveries.
EX~MPT ~
An input gas containing 1.6 percent helium by volume, 26 percent
hydrocarbons by volume and 72.4 percent nitrogen by volume is flowed to membraneunit 19 at a pressure of 925 psig and 5250 scfm. The permeate stream which contains 4
20 ~ercent by volume helium exits at 65 psia and flows through conduit 9 to the first
stage pressure swing adsorption 37. Since the permeate stream is already at a
pressure of 65 psia, compressor 33 is not needed. The residue stream exits the
membrane unit at 920 psia through conduit 23, is compressed by compressor 27 to the
source pressure and flowed back to the source. This residue stream will contain less
25 than 0.05 percent helium.
The membrane unit consists of 16 modules containing a polyimide. The
polyimide membrane is in the form of hollow fibers. Each module is 10 inches in
diameter and 4 feet in length.
- _ 24 218~32
The permeate stream is flowed to a two stage pressure swing adsorption plant
which consists of four adsorbent beds as shown in Figures 4 and 5. Each adsorbent
bed in stage I contains 11 cubic meters of an activated carbon adsorbent and each
adsorbent bed in stage II contains 1 cubic meter of activated carbon adsorbent. The
S input gas is fed at a pressure of 65 psia and a flow rate of 2100 scfm to the stage I
adsorbent beds. The phases of stage I are as set out in Figure 6 and the phases of stage
II are set out in Figure 7. The valves are on a time cycle as described in Table 1 for
stage I and Table 2 for stage II. An enriched helium gas stream having a helium gas
colltel~t of 90 percent helium flows through conduit 23 at 127 scfm to stage II
10 adsorbent beds. This enriched helium gas is further purified in stage II to a product
helium gas having a helium content of 99.999 percent helium. This is produced at a
flow rate of 80 scfm.
The gas from the adsorbent bed in the stage II evacuation phase and the gas
from the stag II purge phase is recycled to the input gas to stage I. This gas has a
15 helium content of 73 percent and flows at a rate of 47 scfm. The depressurization gas
and the evacuation gas from stage I pressure swing adsorption is compressed and fed
to the residue gas stream.
The process is operated continuously until a general maintenance is required.
