Note: Descriptions are shown in the official language in which they were submitted.
113Z918
BACKGROUND OF THE INVENTION
This inventïon relates to the art of separation of gas
mixtures, and more particularly to a new and improved process and
system for separating gas mixtures by pressure swing adsorption.
One area of use of the present invention is in separating
air to provide a product stream of high purity oxygen, although
the principles of the present invention can be variously applied.
In basic pressure swing adsorption processes and systems for
separating air, adsorption is carried out at a high pressure and
desorption is carried out at a low pressure. Compressed air is
introduced into a fixed bed of adsorbent material and nitrogen is
then preferentially adsorbed to produce oxy~en rich gas product.
When the adsorption bed is about saturated, the bed pressure is
reduced to desorb nitrogen from the adsorbent material and re-
generate the adsorption capacity. To increase the efficiency of
regeneration, a purge by some of the product or an intermediate
process stream often is used. To facilitate continuous operation,
two or more adsorption beds are employed so that while one bed
performs adsorption the other bed undergoes regeneration.
In the design and operation of pressure swing adsorption
processes and systems, it would be highly desirable to provide
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~1~291~
maximum utilization of adsorbent material in the adsorption beds,
reduction in energy requirements for operation of the system, a
substantially constant degree of product purity, and reduction in
adsorbent material requirements while maintaining a high degree
of product purity, along with improved efficiency and reliability.
Sl~:RY OF THE INVENTION
It is, therefore, a primary object of this invention to
provide a new and improved process and system for separation and
fractionation of gas mixtures by pressure swing adsorption.
It is a further object of this invention to provide such
a process and system characterized by maximum utilization of
adsorbent material in the adsorption beds.
It is a further object of this invention to provide such
a process and system having reduced energy requirements for
operation.
It is a further object of this invention to provide such
a process and system which is balanced and yields a substantially
constant degree of product purity.
It is a further object of this invention to provide such
a process and system which has reduced adsorbant material
requirements along with a high degree of product purity.
~132918
It is a further object of this inventiol) to prov~.de
such a process and system which maintains a reserve supply
of product for use during a system malfunction or in augment-
ing system functions.
It is a further object of this invention to provide
such a process and system which is reliable, efficient and
economical.
In general terms, the invention provides, in one
aspect thereof, a pressure swing adsorption process using
at least one adsorption bed having a gas inlet and a gas
outlet, conduit means for flowing product gas from said
bed outlet to product delivery means and into a product
gas reservoir, comprising the steps of normally preventing
flow of product gas out of said reservoir, and flowing
product gas from said reservoir to said delivery means
upon sensing failure of normal gas flow, whereby product
gas continues to be delivered even after failure of normal
product gas flow until said reservoir is exhausted.
In another aspect, the present invention can be
broadly defined as providing, in a system for fractionating
at least one component from a qaseous mixture by pressure
swing adsorption including an adsorption bed having a gas
inlet and a gas outlet, a product outlet and output conduit
means for coupling said gas outlet of said adsorption bed to
said product outlet, a reservoir, reservoir conduit means
connected to said product outlet and output conduit means
and to said reservoir, said reservoir conduit means including
flow control means controlling gas flow from said output
conduit means to said reservoir and valve means for control-
ling flow of product gas from said reservoir to said output
conduit.
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113291B
The foregoing and additional advantages
and characterizing features of the present invention
will become clearly apparent from the ensuing detailed
description wherein:
BRIEF DESCRIPTION OF THE DRAWING FIGURES
Figure 1 is a schematic diagram of a pressure
swing adsorption system according to the present in-
vention;
Figure 2 is a cycle sequence chart illustrating
the pressure swing adsorption process of the present
invention;
Figure 3 is a schematic diagram of a pressure
swing adsorption system with parts removed according
to another embodiment of the present invention; and ::
Figure 4 is a schematic block diagram of a
control arrangement according to the present invention
for a pressure swing adsorption system.
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~1~2918
DET~ILED DE5CRIPTION OF THE ILLUSTRAT~D EMBODIMENTS
Referring now to Fig. 1, there is shown a system ac-
cording to the present ;nvention for fractionating at least one
component from a gaseous mixture by pressure swing adsorption.
The gaseous mixture is supplied to the system by a feed gas
stream which flows along an input conduit 10 and is moved there-
along by means of a pump or compressor 12. Although the present
system and process is specifically described and illustrated in
relation to the application of pressure swing adsorption to the
fractionation of air to produce an oxygen rich stream, the
present invention is broadly applicable to the separation of
organic and/or inorganic gas mixtures.
The system includes a first adsorption bed 16, also
designated A, having a gas inlet 18 and a gas outlet 20. The
system further includes at least one additional adsorption bed
24, also designated B, having a gas inlet 26 and a gas outlet 28.
Adsorption beds A and B are the type comprising a vessel con-
taining adsorbent material and are well known to those skilled
in the art. A preferred vessel construction includes an outer
pressure cell with an inner annulus, and one skilled in the art
can provide suitable pressure vessels, piping or tubing, con- -
nectors, valves and auxiliary devices and elements. Likewise,
adsorbent materials are well-known in the art, and one skilled
in the art may select an adsorbent material~s~ which is com-
mercially recommended for the separation or fractionation of the
particular gas to be purified. Examples of typical adsorbent
materials for use in adsorption beds include natural or synthetic
zeolites, silica gel, alumina and the like. Generally, the ad-
sorbent beds in a system contain the same adsorbent material,
however, each bed may contain a different type of adsorbent
material or different mixtures of adsorbent material as desired.
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The particular adsorbent material or mixtures used are notcritical in the practice of this invention as long as the
material separates or fractionates the desired gas components.
The system of the present invention further comprises a
segregated storage adsorption bed 32, also designated C, and in
the system shown in Fig. 1 gas is introduced to and withdrawn
from the segregated storage adsorption bed C at the same end
which is provided with a conduit 34. The segregated storage ad- :
sorption bed C likewise is a vessel containing adsorbent material,
but bed C does not communicate with the feed gas stream from con-
duit 10. In the system shown, adsorpticn bed C is approximately
the same size as the adsorption beds A and B and may contain the
same type of adsorbent material, but the segregated storage ad-
sorption bed C can be smaller in size, include different adsorb-
ent material, and be operated at a different capacity as com- -
lS pared to t~.e adsorption ~eds A and B.
The gas inlet 18 of adsorption bed A is connected to
conduit lQ containing the feed gas stream by suitable conduit
means including an automatic valve 40A and, similarly, the gas
inlet 26 of adsorption bed B is connected to the feed gas stream
in conduit 10 by suitable conduit means including an automatic
valve 4QB. The system further.includes a waste gas outlet 44
which can be open to the atmosphere or which can be in fluid
communication with a ~aste gas stream. The gas inlets 18 and 26
of adsorption beds A and B, respectively, also are connected to
25 the waste gas outlet 44 by suitable corresponding conduit means -:
including automatic valves 46A and 46B, respectively. The auto- .:
matic valves 4Q and 46 and those additional automatic valves to
be described can be of the solenoid-operated type, ~ut in any
event are of the type which are operated to be either fully open
3Q or fully closed.
The system of the present invention furth.er comprises
means such as suitable conduits or piping defining a ~as flow
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ll~Z91~
path connected at one end to gas outlet 20 of adsorption bed Aand connected at the opposite end to gas outlet 28 of adsorption
bed B. A first flow control valve 50A is in the gas flow path
between gas outlet 20 of adsorption bed A and adsorption bed B.
Valve 50A allows unrestricted gas flow in a direction from the
outlet 20 of bed A through the valve toward adsorption bed B, and
the valve provides controlled flow therethrough in a direction
to gas outlet 20 of adsorption bed A. The controlled flow pre-
ferably is provided by manual adjustment. A second flow control
valve SOB is in the gas flow path between gas outlet 28 of ad-
sorption bed B and the adsorption bed A. Valve 50B allows un-
restricted gas flow therethrough in a direction from gas outlet
28 of adsorption bed B toward adsorption bed A, and it provides
controlled flow therethrough in a direction to gas outlet 28 of
adsorption bed B. The controlled flow preferably is provided by
manual adjustment. Valves 50A, 50B preferably are identical and
can be of the type known commercially as Parker-Hannifin flow
control valves. An isolation valve in the fonm of an automatic
valve 54 is provided in the gas flow path between gas outlets 20
and 28 of the adsorption beds, and in the system shown valve 54
is connected between gas outlet 20 of adsorption bed A and the
fIow control val~e 50A.
The system of the present invention includes a second
gas flow path provided by suitable conduits or piping which joins
the gas outlets 20 and 28 of the adsorption beds A and B, re-
spectively. A first automatic valve 60A is connected in the pathadjacent outlet 20 of bed A, and a second automatic valve 60B is
connected in the path adjacent outlet 28 of adsorption bed B.
The segregated storage adsorption bed C is connected through an
automat~c valve 62 to a point in the gas flow path between the
automatic valves 60A and 60B.
The system of the present invention further comprises a
product outlet designated 66 and output conduit means for
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` ~32918
coupling the gas outlets of the adsorption beds to the product
outlet 66. In the system shown the output conduit means is con-
nected to the first flow path at a point between the flow control
valves 50A and 50B and includes a first section 70 including an
automatic valve 72 and a second section 74 including the series
S combînation of a pressure regulator 76, a throttle valve 78 and a
flow meter 80. The flow rate of product to the outlet 66 is con-
trolled by valve 78 which preferably is a manually adjustable
needle-type valve, and the flow rate is indicated visually by the
meter 80.
The system of the present invention further comprises a
reservoir 84 which functions primarily to store product gas re-
ceived through a conduit 86 and serve as a reserve supply of pro-
duct for use in the event of a system malfunction. A first reser-
voir conduit means is connected at one end of the system output
conduit means and at the other end to the reservoir 8~ through
conduit 86 and includes flow control means in the form of check
valve 90 which allows gas flow only in one direction from the
system output conduit means to the reservoir 84. Another valve
92 in the form of a throttle valve which preferably is manually
adjustable is connected in the conduit and preferably ~etween
check valve 9Q and reservoir 84. Valve 92 can be used to control
the rate of flow of gas product into reservoir 84. A second
reservoir conduit means i5 connected at one end to reservoir 84
through conduit 86 and at the other end to the system output con-
duit means and includes valve means 96 for controlling the flowof product gas from reservoir 84 to the output conduit means. A
control lQ0 is connected by lines 102 and 103 to valves 72 and
96, respectively, and functions to open the normally closed valve
96 in response to closing of valve 72. A pressure indicator
meter 104 can be connected to the output of reservoir 84 for the
purpose of indicating the pressure of gas product remaining
therein.
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~ Z918
In general, the present invention is illustrated in
terms of a process and system utilizing a first adsorption bed, a
second adsorption bed and a segregated storage adsorption bed.
However, the process and system can employ more than one first
adsorption bed, more than one second adsorption bed and more than
one segregated storage adsorption bed. The adsorption beds com-
municate with the feed gas stxeam which supplies the gaseous mix-
ture, and the segregated storage adsorption bed never directly
communicates with or is directly exposed to the feed gas stream.
Although the process and system of the present invention
are described with particular reference to separation or fraction-
ation of air to provide a high purity product oxygen by removal of
nitrogen, essentially any gas mixture may be separated by the pro-
cess and system of the present invention by the proper selection
of time for each cycle and step and by the selection of a proper
adsorbent material, adsorbent materials or mixtures of adsorbent
materials.
As used herein, depressurizing or depressurization
refers to the reduction of pressure in the vessel and associated
piping o~ an adsorption bed and thR level to which pressure is
reduced can be selected by those skilled in the art depending
upon operating conditions and the nature of the gas mixture being
fractionated. Desorption and purging pressures are selected in a
similar manner. Pressurizing or pressurization refers to the
increa8e of pressure in the vessel and associated piping of an
adsorption bed. The process and system of the present invention
have the capability of product gas delivery in a low pressure
range down to about 2 p.s.i.g. and in a high pressure range up
to about 40 p.s.i.g. The present invention is not limited to
particular pressures of the product gas or any other pressures,
3Q and one skilled in the art can manipulate and adjust pressures
throughout the system to provide the desired delivery or product
gas pressure. For example, when air is fractionated to deliver
1 O-
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11329i8
high purity oxygen gas product, a delivery pressure of around 3
p.s.i.g. is employed for medical uses and breathing devices
whereas ahigher delivery pressure of up to about 40 p.s.i.g. is
ideally suited for commercial uses such as in metal cutting or
welding equipment.
Fig. 2 illustrates a process timing sequence according
to the present invention for use with the system of Fig. 1. In
Fig. 2 preferred times in seconds are indicated for each step,
and preferred pressures in each adsorption bed for each step are
shown parenthetically and given in pounds per square inch gage.
The particular operation carried out in each adsorption bed
during each step is shown în F;g. 2, most of which are ab-
breviated for convenîence in illustration. Thus "FEE" refers to
feed end equalization and will be explained in further detail
presently, "ISOL" refers to isolation of a particular adsorption,
"EQ" refers to pressure equalîzation of two adsorption beds and
will be explained in further detail presently, "REP" refers to
repressurization or repressurizing to increase the pressure in
an adsorption bed, and "P~RGE" refers to introduction of purge
gas or purging.
2q Reerring now in detail to Fig. 2, prior to step No. 1
the gaseous mixture i.e. ordinary air, has been flowing from the
feed gas stream in conduit 10 and through valve 40A which is open
into and through adsorption bed A wherein nitrogen is adsorbed.
High purity oxygen gas leaves bed A through outlet 20 and flows
through the opened valve 54 and flow control valve 50A and then
flows along conduit section 70 through the opened valve 72, along
conduit section 74 and through the series combination of pressure
regulator 76, needle valve 78 and flow meter 80 to the product
outlet 66 for use. Just prior to the beginning of step No. 1,
adsorption bed A is about saturated and nearing the end of the
adsorption operation therein. Also just prior to the beginning
of step No. 1, adsorption bed A is at a higher pressure than the
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~3Z918
adsorption beds B and C.
At the beginning of step No. 1, valve 40B is opened,
and valve 40A is kept open as well as valve 72. As indicated in
Fig. 2, at the beginning of step No. 1, typical pressures in beds
A, B and C are 30, 7 and 7, respectively.
Durinq this step, gas flows from the bottom or feed end
of adsorber A in a reverse direction through valve 40A whereupon
it mixes with the incoming feed air stream from conduit 10 and
flows through valve 40B into the bottom or feed end of adsorber
B. Adsorption bed A is very near the end of the adsorption step
therein. As a result, durîng this step, adsorption bed A is de-
pressurized countercurrently to feed flow, and adsorption bed A
is pressure equalized with adsorption bed B causing the pressure
in bed B to r~se. Also in this step, adsorption bed A continues
to supply oxygen gas product, but this is terminated by the end
of the step. Step No. 1 prefera~ly has a duration of about 7
seconds. Th~oughout this step and all other steps there is con-
tinuous air flow into the system and continuous product flow out.
Cocurrent to feed flow is in a direction from the inlet to the
outlet of the adsorption bed and countercurrent to feed flow is
in a direction from the outlet to the inlet of the adsorption
bed.
T~e process of step No. 1 may be described as continuing
to discharge product gas from the outlet of the first bed while
simultaneously equalizing the pressures of the first and second
adsorption beds from the feed ends thereof b~ withdrawing gas
from the feed inlet of the first adsorption bed at the end of
the adsorption operation therein in a direction countercurrent
to feed flow and introducing the withdrawn gas along with the
gaseous mixture from feed gas stream to the feed inlet of the
second adsorption bed in a direction cocurxent with feed flow
and after pressurization thereof.
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113Z918
As shown in Fig. 2, at the transition bet~een the end o~
step ~o. 1 and beginning of step No. 2, t~e pressures in beds A
and B are equalized at 20 p.s.i.g. and the pressure in the seg-
regated storage adsorption bed C has remained at 7 p.s.i.g. At
the beginning of step No. 2, valve 40B remains open, valve 4QA
S closes, and valve 6QA opens. No product gas is obtained from ad-
sorption bed A. During this step, feed air continues to flow
into the feed inlet 26 of adsorber B, and oxygen rich gas is
taken as product ~rom the outlet 28 of adsorber B and flows
through flow control valve 50B into conduit section 70 and
through the remaining s~stem components as previously described
to product outlet 66. At the same time, low purity gas flows
from the outlet 20 of adsorber A through valve 6QA and valve 62
into the segregated storage adsorption bed C. AS a result,
during this step adsorption bed A is pressùre equalized with the
-15 segregated storage adsorption ~ed C. The automatic valve 62 can
remain open during all steps or it can be opened and closed when
necessary. Step No. 2 preferably has a duration of about 7
seconds.
; The process of step No. 2 may be described as sLmul-
taneousl~ terminating the pressure equalization of step 1, ad-
~orbing the gaseous mixture from the feed gas stream in the
second adsorptîon bed~ releasing product gas from the outlet of
the second adsorption bed, and eqalizing the pressures of the
first adsorption bed and the segregated storage adsorption bed
by withdrawing low purity gas from the outlet of the first ad-
sorption bed in a direction cocurrent with feed flow and intro-
ducing the low purit~ gas into the segregated storage adsorption
bed.
As is evident from steps 1 and 2 on Fig. 2 and from the
foregoing descriptions thereof, it can be seen that bed A has
undergone a decreasing pressure adsorption process~ i.e., it has
been producing product gas while s;multaneously experiencing a
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113Z918
reduction in pressure. While this is shown in Fig. 2 as oc-
curring at the same time as beds A and B are undergoing FEE, it
will be clear to those skilled in the art that concurrence with
a FEE step is not essential, e.g., a bed could be made to per-
form a decreasing pressure adsorption step while connected to an
SST, or to atmosphere, or otherwise.
As shown in Fig. 2, àt the transition between the end
of step No. 2 and the beginning of step No. 3, the pressures in
adsorption beds A and C are equalized at 14 p.s.i.g. and the
pressure in adsorption bed B has risen to 28 p.s.i.g. At the
~eginning of step No. 3, valve 40B remains open, valve 6QA closes
and valve 46A opens. During this step feed air continues to
enter bed B, and product quality oxygen rich gas continues to be
taken as product from the outlet of bed B and is available at
product outlet 66. Also during this step, adsorption bed A is
lS depressurized to the atmosphere through va~ve 46A and waste out-
let 44 in a direction countercurrent to feed flow. As a result,
nitrogen rich waste gas is rejected to the atmosphere, and the
pressure in adsor~er A drops from 14 p.s.i.g. to 0 p.s.i.g. Con-
currently with the foregoing depressurization, a portion of the
oxygen gas product flowing from adsorber B through flow control
valve 50B flows through valve 50A and valve 54 into adsorber A.
The product quality oxygen gas flows through bed A and out
thxough Yalve 46A and waste ou~let 44 in a direction opposite to
that of air separation. This oxygen purge flowing countercurrent
to feed flow displaces nitrogen from the adsorbent material in
bed A, and nitrogen rich stream leaves the system through valve
46A and outlet 44 to the atmosphere. As a result, product
~uality oxygen gas is taken from the adsorbing bed B to purge
the nitrogen loaded bed A in a reverse direction to reject un-
wanted impurity to the atmosphere. Step No. 3 preferably has aduration of about 39 seconds.
The process of step No. 3 may be described as simul-
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.. . . .
. :, " ~i - , . - ~ -
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11~2918
taneously terminating the pressure equalization of step 2, con-
tinuing adsorption of the gaseous mixture from feed gas stream in
the second adsorption bed, releasing product gas from the outlet
of the second adsorption bed, and depressurising the first ad-
sorption bed in a direction countercurrent to feed flow and
purging the first adsorption bed by diverting some product gas
from the outlet of the second adsorption bed into the first ad-
sorption bed in a direction countercurrent to feed flow.
As shown in Fig. 2, at the transition between the end of
step No. 3 and the beginning of step No. 4, the pressure in bed A ~
10 is at Q p.s.i.g., the pressure in segregated storage adsorption ~-
bed C has remained at 14 p.s.i.g., and the pressure in bed B has
risen to 30 p.s.i.g. At the beginning of step No. 4, valve 40B
remains open, valve 46A closes, and valve 60A opens. Valve 62 if
not already open is opened at the beginning of this step. During
this step feed air continues to enter bed B, and product quality
oxygen gas continues to be taken as product from the outlet of
bed B and is available at product outlet 66. At the same time,
gas flo~s from the segregated storage tank C through valves 62
and 60A into adsorber A through the outlet 20 thereof. This gas
2Q withdrawn from adsorpt;on bed C during step 4 is a version of the ~-
gas supplied to bed C during step 2 which gas has been influenced
b~ travel in bed C.
As a result, during this step the segregated storage
adsorption bed C is pressure equalized with the adsorption bed
A. At least during the initial portion of step 4, there is some
additional flow of gas from bed B through valves 50B, SQA and 54.
Step No. 4 preferably has a duration of about 7 seconds.
The process of step No. 4 may be described as simul-
taneously terminating the depressurizing and purging of the first
3a adsorption bed, continuing adsorption of the gaseous mixture from
the feed gas stream in the second adsorption bed, releasing pro-
duct gas from the outlet of the second adsorption bed and
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1~329i~
equalizing the pressures of the segregated storage adsorption
bed and the first adsorption bed by withdrawing gas from the
segregated storage adsorption bed and introducing the withdrawn
gas into the first adsorption bed in a direction countercurrent
to feed flow.
The foregoing process steps are repeated consecutively
beginning with pressure equalization of the adsorption beds from
the feed ends thereof reversing the functions of the adsorption
beds A and B. In particular, as shown in Fig. 2, at the transi-
tion between the end of step No. 4 and the beginning of step No.
5, the pressures in beds A and C are equalized at 7 p.s.i.g. and
the pressure in bed B has remained at 30 p.s.i.g. At the begin-
ning of step No. S, valve 40B remains open, valve 60A closes and
valve 40A opens. During this step, gas flows from the bottom or
feed end of adsorber B, which is near the end of its adsorption
operatîon, in a reverse direction through valve 40B whereupon it
mixes with the incoming feed air stream from conduit 10 and the
resulting mixture flows through valve 40A into the bottom or feed
end of adsorber A. As a result, adsorption bed B is pressure
equalized with adsorption bed A, and bed A begins to adsorb the
feed gas mixture. This feed end equalization is similar to that
which occurred during step No. 1 but in this step the roles of
the beds A and B are interchanged. Also during this step, pro-
duct quality oxygen rich gas continues to be taken as product
from bed B and is available at product outlet 66. This step be-
gins the second half of the process cycle wherein steps 5 - 8
are similar to 1 - 4 with the roles of beds A and B interchanged
and with the valve sequence being the same with the A and B
designations interchanged. For example the process of step No.
6 tthe same as step 2 with beds reversed~ may be described as
simultaneously terminating the pressure equalization of step No.
5, repressurizing the first adsorption bed while withdrawing
product gas therefrom, and equalizing pressures in the second
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~1~2918
adsorption bed and the segregated storage adsorption zone.
Equalizing the pressures of the adsorption beds A and Bat the feed ends thereof according to the present invention, as
illustrated in step No. 1, advantageously reduces energy require-
ments and increases oxygen recovery. When an adsorption bed at
the end of the adsorption step therein is depressurized counter-
currently to feed flow, i.e. as bed A from 30 p.s.i.g. to 20
p.s.i.g. in step No. 1, this gas can be introduced into the feed
end of a repressurizing adsorber, i.e. adsorption bed B in step
No. 1, without any appreciable loss in system performance com-
pared to repressurizing with air from the system compressor 12.Feed and equalization according to the present invention thus
greatly reduces the feed air requirement and increases oxygen
recovery, i.e. decreases the size of compressor 12 required to
produce a given amount of oxygen. Feed end equalization recovers
energy, increases system efficiency and can be used for both low
and high product deli~ery pressures. The foregoing ad~antages
of course apply to both of the feed end equalizations which occur
during a single cycle as illustrated in step Nos. 1 and 5.
The feed end equalization according to the pxesent in- `
vention requires less adsorbent material in a given bed as com-
pared to product end equalization fox the following reasons. In
product or outlet end equalization, the bed at the higher pres-
sure depressurizes in a direction cocurrent to feed flow during
the pressure equalization step. This causes the mass transfer
zone to advance toward the product end of the bed as the pressure
decreases. In order to contain the mass transfer zone during
this step to maintain product purît~, a larger ~ed, i.e. more
adsorbent material, is required. In feed end equalization ac-
cording to the present invention, on the other hand, the bed at
the higher pressure depressuxizes in a direction countercurrent
to feed flow during the equalization step. In this step the
mass transfer zone does not advance due to the direction of the
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~i3291B
flow. The countercurrent depressurization also is beneficial
for the subsequent purge step because nitrogen starts to flow
toward the feed end of the bed during this step. The combination
of no advancing of the mass transfer zone and countercurrent de-
pressurization reduces the amount of adsorbent material required.
Bed size factor ;s a quantity used to compare the amount
of adsorbent material required from one system or cycle to an-
other. At a given bed size factor, it has been determined that
using feed and equalization according to the present invention
produces oxygen at a higher purity as compared to using product
end equalization.
The combination of equaliz;ng pressures of an adsorption
bed and the segregated storage adsorption bed when the adsorption
bed is at the end of the adsorption operation therein and prior
to purging of the bed as illustrated in step No. 2 and thereafter
equalizîng pressures between these same two components after
purging of the adsorption bed when it is at a relatively low
pressure as illustrated in step No. 4 maximizes the utilization
of the adsorption bed while at the same t;me maximizing purity
of the product. In particular, during step No. 2 as tbe de-
2Q pressurizing adsorber A equalizes cocurrently to feed flow intosegregated storage adsorption bed C, part of the nitrogen con-
tained in the mass transfer zone of bed A will be transferred
into the bed C. This allows for maximum and continual utiliza-
tion of adsorption bed A, i.e. the mas-s transfer zone can be
moved along bed A from inlet to outlet as far as possible. At
the beginn~ng of the flow from bed A to bed C the gas is rich
in oxygen but as flow continues the gas becomes more like air.
In addition, the segregated storage adsorption bed recovers some
potential energy from the depressurizing adsorber and this, in
turn, reduces system blowdown pressure and increases recovery
and efficiency. Providing the segregated storage adsorption
bed C in effect provides a mixing volume to smooth out any
~ -18-
~13Z918
fluctuations in product purity which otherwise might occur when
the front of the mass transfer zone breaks out of the output end
of an adsorption bed. The foregoing advantages result when the
system is operating at equilibrium conditions and at flow con-
ditions for which the system is optimally designed. For example,
when the system is used to supply oxygen for medical use, design
conditions occur at a flow rate of about 3.0 liters per minute.
During step No. 4 as the segregated storage adsorption
bed C pressuxe equalizes countercurrently to feed flow into ad-
sorber A, the gas returned to adsorber A is distributed or dis-
lQ persed therethrough in a manner which does not adversely affectproduct purity. The gas is not returned to adsorber A in a lump
quantity concentrated in the output region of bed A but instead
is spaced, equalized or dispersed through and along the bed A.
The foregoing is believed to result from the fact that gas
return to adsorber A occurs when the latter is at a relatively
low pressure, i.e. O p.s.i.g. after purging of adsorber A, which
low pressure allows the gas to disperse through the bed. It is
believed that low or zero pressure in bed A allows the incoming
gas to move along the bed in a manner such that a large amount
2Q of nitrogen is not taken up by the adsorbent material adjacent
the outlet end of the bed. At the beginning of gas flow from
bed C to bed A, the gas is rich in nitrogen but as the flow con-
tinues it becomes more rich in oxy~en. The foregoing advantages
are of course equally associated with the relationship between
adsorption bed B and segregated storage adsorption bed C during
step Nos. 6 and 8.
Providing the flow control valves 5Q~ and 5Q~ allo~
the s~stem to be balanced by providing individual control or
adjustment of the purge gas flow to each of the adsorption beds
3a A and B. Providing an adjustable flow control va7ve associated
with each bed permits compensating for differences in the beds
and piping by simple manual adjustment of valves 5QA, 5QB.
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An unbalanced system is characterized by the front of the masstransfer zone breaking through the output end of one bed sooner
than in the other bed. In order to maintain purity, this would
limit system operation to that of the bed which is first to ex-
perience nitrogen breakthrough thereby causing the other ad-
sorber to be underutilized with the result that the entire systemproduces less oxygen at a given purity. System balance and
optimization are achieved by the independently adjustable flow
control valves 50A, 50B. Advantageously, product gas also
travels through these same valves toward the system product out-
let 66. Alternatively, flow control valves 50A and 50B could bereplaced by two needle valves for independently controlling purge
flow and then the combination of two check valves would be con- `
verted in parallel with the needle valves and poled to transmit
product gas from the bed outlets to the system product outlet 66.
lS The automatic valve 54 in the path containing valves
50A, 50B is a shut down isolation valve which serves to isolate
beds A and B when the system is shut down to maintain the re-
; spective pressures in the beds and prevent pressure equalization.
When the system is shut down, all the other automatic valves
close also. Then when the system is placed in operation, less
time is required to reach desired operating conditions by virtue
of the beds A and B having been maintained at the respective
pressures prior to shut down.
Table I presents data illustrating the effect of the
segregated storage tank or segregated storage adsorption bed C
on system performance. The data presented in Table I is for
oxygen product at a purity of 90% and the oxygen recovery in
percent is presented for both low pressure and high pressure
delivery conditions. The abbreviations S.S.T. for segregated
3a storage tank and F.E.E. for feed end equalization are used.
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TABLE I
Low High
Pressure Pressure
Deliverv Delivery
S.S.T. Absent 21% 21
S.S.T. Present
But Empty 25% 23%
S.S.T. Half Full of
Adsorbent Material 35% 31%
S.S.T. Full And
With F.E.E. 49~ 48%
Fig. 3 shows a system acoording to another embodiment of the
present invention wherein gas product can be withdrawn from the other end of
t~e segregated storage adsorption bed. In the system shown in Fig. 3, ocmr
ponents identical to those of Fig. 1 are provided with the same reference
numerals but with a prime designation. In addition, the system of Fig. 3
would also include adsorpticn beds identical to those designated A and B in
the system of Fig. 1, along with similar connections of the feed gas stream
to the gas inlets of the beds, connections of the gas inlets to the waste
outlet, and connections of the gas outlets of the beds to the gas flow path
containing the flow control valves 5QA' and 50B'. Thus, the arrcwheads at
opposite ends of the path shown in Fig. 3 containing automatic valves 60A',
60B' and the path containing flow control valves 5QA' and 50B' indicate con-
nection to the gas outlets of the corresponding adsorption beds A and B.
Similarly, the output of regulator 76' is connected through a throttle valve
and flow indicator to a product outlet as indicated by the arrowhead in the
portion 74' of the gas flow path.
The opposite end of the segxegated storage adsorption
bed C' is connected by a conduit 108 which contains an automatic
valve 110 to the output conduit means, in particular to portion
74' thereof and upstream from regulator 76'. Upon opening of
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valve 110, product quality gas can be withdrawn from the segre-
gated storage adsorption bed C' and introduced to the output con-
duit means. Withdrawing product gas from the segregated storage
adsorption bed can be advantageous in situations where low pres-
sure rather than high pressure product delivery is needed. In
addition, when product is delivered from the segregated storage
adsorption bed C', the bed serves also as a product surge tank
enabling product to be withdrawn from the system at a high flow
rate for a short period of time before the mass transfer zone
breaks through that end of the bed. On the other hand, recovery
from a breakthrough condition can be relatively slow. Another
advantage of withdrawing product gas from the segregated storage
adsorption bed C' is that it provides a relatively higher rate of
recovery of product. This is because withdrawal of product from
bed C' reduces the pressure therein so that when the pressure
equalizes with.either of the adsorption beds that adsorption bed,
in time, will be at a lower pressure. The lower pressure, in
turn, imposes a lower blowdown requirement for that bed with a
result that less gas is released to the atmosphere. This re-
duction in the ~aste losses, in time~ results in a higher per-
2Q centage of pxoduct recovery. Another advantage associated withth.e segregated storage adsorption bed involves feed end equaliz-
tion ~hich.lowers the front of the mass transfer zone in each of
the other two beds so that when the beds are equalized from the
tops wi.th.the segregated storage adsorption bed there is less
nitrogen to be taken up by the segregated storage adsorption bed.
As shown in ~ig. 3, the system can also include a third
reservoir conduit designated 114 connected at one end to the
reservoir 84' and coupled at the other end to the adsorption
beds. In the present illustration, th.e other end of conduit 114
is connected to the flow path containing the automatic yalves
6OA' and 6Q8' and is connected between these valves. Conduit
114 contains an automatic valve 116. Upon opening of valve 116,
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product gas from reservoir 84', flows to the adsorption beds and
it can be used for operations such as purging and repressuriza-
tion.
The primary role of the reservoir in the system of the
present invention is a reserve supply of product gas in the event
S of equipment malfunction or power failure. This is of particular
importance when the system of the present invention supplies
oxygen for medical use. Under normal operating conditions the
reservoir is at a pressure of 28-29 p.s.i.g., and product oxygen
flows through valve 72 and regulator 76 to product outlet 66.
If electrical power is interrupted, valve 72 closes and this is
sensed by control 100 which opens valve 96. Oxygen flow con-
tinues from the reservoir through valve 96 to the output conduit
to outlet 66 until the supply in the reservoir is depleted. An
alarm can be sounded to indicate the power interruption.
The reservoir also can be used to supply part or all of
the purge oxygen required for an adsorber during its purge step.
This ;s accomplished by opening valve 116 at the appropriate
time. The reservoir also can be used as another surge tank.
Pressure equalizations to and from the adsorbers can be accom-
2Q plished through the correct sequencing of valves 116 and 62.
The primary purpose of the reservoir is a reserve
oxygen supply in the event of a malfunction. The length of time
the reserve oxygen lasts depends on the pressure in the reser-
voir at the time of the malfunction. If the reservoir is used
only as a back-up oxygen supply, the reservoir pressure will be
at its maximum at all times. If the reservoir is used to supply
supplemental purge and or repressurization gas, the pressure in
the reservoir will vary as will the reserve supply of oxygen.
The reservoir can comprise an adsorption bed but it also can
comprise an ordinary tank of larger size.
Fig. 4 shows an arrangement for controlling the system
and process of the present invention. The output conduit means
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can be connected to a tank or similar storage receptacle orvessel 120 and gas product can be withdrawn therefrom through
a conduit or path 122 for use. The sequencing and timing of the
system including the control of the automatic valves is performed
by a system control designated 124, and control signals or com-
S mands generated by the control 124 are transmitted by linescollectively designated 126 to the valves and other appropriate
components of the system. Persons skilled in the art are readily
familiar with such controls so that a detailed description there-
of is believed to be unnecessary. Generally, the control 124 is
responsive to the pressure of product gas within the storage
element 12Q, and to this end a pressure sensor 130 is operatively
connected to the storage element 120 by the connection designated
132. In accordance with the present invention, the output from
sensor 130 is connected ~y a line 134 to an additional control
means 136 which, in turn, is connected in controlling relation
to the system control 124 by the connection designated 138. In
accordance with the present invention~ it has been determined
that once operation of the process and system has begun there is
an optimum time at which to terminate operation, both in terms
2Q of a minimum number of cycles to be completed and a point within
a cycle to terminate operation. The additional control functions
to cause the system control 124 to maintain operation of the
system, once begun, for a predetermined number of cycles. It
has been determined that in a system of the present invention
for producing oxygen from feed air that a total of t~o complete
cycles pro~ides desirable results. One complete cycle includes
step Nos. 1-8 described in Fig. 2. Furthermore, it has been
determined that there is an optimum point within a cycle at
which operation of the system and process should be terminated.
3Q This is when the pressures are equal in the two adsorption beds
A and B which is at the beginning of step Nos. 2 and 6 described
in Fig. 2. Thus, the additional control 136 also functions to
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113Z918
stop the system only after two complete cycles have been com-
pleted and only at an optimum point within the next cycle when
the pressures are equal in the two adsorption beds A and B. The
additional control can be of. the cam type or step switch type,
for example, and persons skilled in the art are readily familiar
with the construction and operation of these and other types
which can be used for additional control 124 so that a detailed
description thereof is believed to be unnecessary. Thus, the
system control means 124 is responsive to gas pressure in
storage means 120 signalled by sensing means 130 for stopping
operation of the process and system normally when gas pressure
in storage means 120 reaches a predetermined magnitude. The
additional control means 136 overrides the system control means
to terminate operation of the process and system only at a
predetermined time.
It is therefore apparent that the present invention ac-
complishes its intended objects. While several embodiments of :
the present invention have been described in detail, this is for
:~: the purpose of illustration, not lîmitation.
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