Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention relates to a process for
separating air by adiabatic pressure swing adsorption.
In the prior art adiabatic pressure swing
processes for air separation, ~he oycle sequence usually
includes a selective adsorption step during which com-
pressed air is introduced at the adsorbent bed inlet
end thereby forming a nitrogen adsorption front, nitrogen
being selectively adsorbed by most adsorbents as for
example, zeolitic molecular sieves. Oxygen is also
coadsorbed but substantially displaced by the more
strongly held nitrogen adsorbate. Oxygen effluent gas
is discharged from the opposite or discharge end of the
bed at about the feed air pressure and the nitrogen
adsorption front moves progressively toward the discharge
end. The adsorption step is terminated when the front
is intermediate the inlet and discharge ends, and the
bed is cocurrently depressurized with oxygen effluent
being released from the discharge end and the nitrogen
adsorption front moving into the previously unloaded
section closer to the discharge end. The cocurrent
depressurization gas may in part be discharged as oxygen
product and in part returned to other adsorbent beds
for a variety of purposes, e.g. purging and pressure
e~ualization with a purged bed for partial repressuriza-
r
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tion thereof, Cocurrent depressurization is terminatedbefore the front reaches the discharge end so that the
oxygen purity of ~he effluent is nearly that of the gas
discharged during the preceeding adsorption step as for
example described more completely in Kiyonaga U,S. Patent
No. 3,176,444.
The cocurrently depressurized bed is usually
further depressurized by releasing waste gas through
the inlet end, i.e. cou~tercurrently depressurized,
until the bed pressure diminishes to a desired low
level for purging. Then oxygen purge gas is flowed
i through the bed to desorb the nitrogen adsorbate and
carry same out of the system. The purged and at least
~;~ partly cleaned bed is then repressurized at least partly
with oxygen and/or feed air and returned to the adsorp-
tion step. One such process delivering product oxygen
; at nearly the feed air pressure is described in Batta
U.S. Patent No. 3,564,816, and requires at leas~ four
adsorbent beds arranged in parallel flow relation.
Another process delivering product oxygen at lower,
slightly above atmospheric pressure is described in
Batta U.S. Patent No. 3,636,679, and re~uires at least
three beds arranged in parallel flow relation. Still
another process requiring any two adsorbent beds arranged
in parallel flow relation is described in McCombs U.S.
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9643
:LC)69(~62
Patent No. 3,738,087. The latter process includes an
increasing pressure adsorption step of introducing feed
air to the inlet end of the partially repressurized ad-
sorption bed at pressure higher than the aforementioned
intermedlate pressure, selectively adsorbing nitrogen
and simultaneously discharging oxygen gas, all at rela
tive rates such that the pressure of the adsorption bed
rises from the intermediate pressure during this step
to higher pressure at the end of such step.
In pilot plant tests relatively high oxygen
recoveries were obtained with both three bed and four
bed systems, For example, in a four bed cal.cium zeolite
A system in which the bed diameter was four inches and
the feed air was supplied at 70F and cycled according
to the teachings of the aforementioned Batta U.S~ Patent
3,564,816, at 90% 2 product purity the oxygen recovery
was 45.5%. However, in commercial-scale equipment
composed of calcium zeolite A beds 26 inch~ in diameter,
the 2 recoveries were substantially less ~han expec~ed,
iOe, 39.4% and 42.3% at air feed temperature of 50F and
78F, respectively~ Also, in a commercial size three bed
calcium zeolite A system (26 inch bed diameter) in which
the feed air was supplied at temperature of 40F, the 2
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.. recovery was less than expected. The system stabilized
. at a product purity of only 66% and with an oxygen recovery
-.~ o only 26.7%.
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An object of this invention is to provide an
; improved adiabatic pressure swing process for air separa- -
tion which permits oxygen recoveries in commercial size
equipment which are equivalent to those obtained in small
pilot plant equipment. ''
Other objects will be apparent 'from the ensuing
disclosure and appended claims,
SU~ARY ,
_ _
This invention relates to an adiabatic pressure
swing process for selectively adsorbing nitrogen from
feed air to provide oxygen effluent product.
One of the more important characteristics of
an adsorbent is ~he selectivity it exhibits for the
components of a multi-componen~ system. Crystalline
zeolitic molecular sieves of at least four Angstroms
pore size co-adsorb oxygen and nitrogen from air, but
selectively adsorb nitrogen relative to oxygen. It is
known that this selectivity is temperature sensitive
and certain prior art suggests that in the crystalline
zeolitic molecular sieve-nitrogen-oxygen system, the
selectivlty for nitrogen improves somewhat with increas-
ing temperature, at least up to room temperature. How-
ever', Heinze U.S. Patent 3,719,205 teaches that tempera-
ture exerts an opposite effect by stating,that with
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calcium zeolite A (Molecular Sieve 5A), the separation
efficiency of an adsorption oxygen enrichment decreases
with increasing temperature.
Another important characteristic of adsorption
processes is adsorbent utilization or capacity for the
adsorbate; it is known that utilization normally decreases
with an increase of adsorption temperature. Karwat U.S.
Patent 3,355,854 teaches that in a pressure swing adsorp-
tion air separation process employing calcium zeolite A~
it ls necessary to take into considera~ion that the
selectivity of the adsorption materlal for nitrogen at
lower temperature is lower than at room temperature~
while the amount of gas adsorbed thereby is much greater
than at room temperature. The patentee also states that
a satisfactory oxygen enrichment is achieved if in this
case an adsorption temperature of -100C and -S0C and
preferably about -70C. However, Skarstrom U.S. Patent
3,237,377 states that room temperature is preferred for
air separation by pressure swing adsorption using zeolitic
molecular sieve adsorbent.
... .
To resolve the conflicting prior art teachings
; regarding the effects of temperature for adiabatic pressure
~:; swing adsorption, air separa~on studies were conducted on
the nitrogen-oxygen-calcium zeolite A system, and Fig. 1
is a graph showing ~he percent oxygen recovery versus gas
temperature relationship for calcium zeolite A (Molecular
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Sieve 5A), sodium zeolite A (Molecular Sieve 4A) and
sodium zeolite X (Molecular Sieve 13X). Zeolite A is ;
described in U.S. Patent 2,882,243; the as~synthesized
sodium form has an apparent pore size of about four
Angstrom units and the calcium exchanged form has an
apparent pore size of about five Angstrom units. Zeolite
X, another synthetic crystalline zeolitic molecular sieve
is described in V.S Patent 2,882,244; the as-synthesized
sodium zeolite X has an apparent pore size of about ten
Angstrom units. In Fig. 1, the sodium zeolite A curve
is shown by a dashed line, the calcium zeolite A curve
is shown by a solid line and ~he sodium zeolite X curve
is shown by a dash-dot-dash line. In general, the curves
show that percent oxygen recovery increases with increas-
ing temperature from O~F up to a maximum of about 90CF
and thereafter diminishes with further increasing tempera-
ture.
Significantly, ~he aforementioned four inch
diameter, four bed system testet at 70~F feed air tempera-
ture yielding 45% oxygen recovery is on the calcium zeolite
A curve, but the commercial size 26 inch system is sub-
stantially below the oxygen recovery predicted from the
curve and based on ~he feed air temperatures.
The prior art has taught that in adiabatic
pressure swing processes (which by definition occur with-
out loss or gain of heat), the end-to-end bed temperature
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should be uniform. Recognizing that the heat effects
of adsorption and desorption produce cyclic tempera~ure
swings in the bed, each active particle of adsorbent
absorbs heat and rises in temperature during adsorption.
Duri~g desorption, ~he particle releases heat and is
recooled. At steady state, the quantity of ~luid adsorbed
by a particle equals the quantity desorbed; also the
quantity of heat absorbed equals the heat released and
the temperature rise equals the temperature fall. There-
fore, over each full cycle the net change in temperature
is zero and the adiabatic concept should be applicable
to every local zone of the active adsorbent bedO Dis-
regarding these cyclic temperature swings, the prior art
has assumed that each adsorbent particle throughout the
bed undergoing pressure swing adsorption experiences a
uniform average temperature su~stantially equal to the
temperature of~the entering feed air.
.
Contrary to the prior art teachings of uniform
adsorbent bed temperature during pressure swing air separa-
tion, it has been unexpectedly discovered that these beds
experience a sharply depressed temperature ~one in the
adsorption bed inlet end. ~s used herein, the "inlet end'
of the zeolitic molecular sie~e adsorbe~t bed is that
portion to which the feed air is in~roduced and which
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adsorbs substantially all of any contaminants contained
in the air feed i.e. C02 and water. The inle~ end of the
bed includes 30% of the full bed length, and is measured
from the point of air feed introduction and extending
in the direction of air flow towards the discharge
end at which ~he oxygen product emerges. In most
instances, the inlet and discharge ends of the adsor-
bent bed are integral; however, the eed end may be
physically separated from the remainder of the bed as
long as both portions are directly joined from the
fluid standpoint. This means that each part experiences
the same process step at the same time.
In some instances, the aforementioned depressed
temperature zone in the inlet end has been observed to
experience temperature drops on the order of 100F below
the feed air tempera~ure. Fig. 2 is a graph showing the
adsorbent bed temperature versus bed depth for prior art
practice and also a three adsorbent bed embodiment of the
instant process. By way of example, the lower curves
in Fig. 2 show that with a feed air temperature of 38F,
a temperature as low as -64F was measured a distance of
three feet from the inlet end support screen. It is
believed that the inlet end temperature depression is
most severe in those systems which experience an inad-
vertent heat regenerative step at such endO Such heat-
regenerative step serves to cyclically receive and store
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9643
the chilling effect of desorption during counter-
flow periods of the process and to cyclically re~urn
the chilling effect to the bed during forward flow
periods of the air separation process. When raw air
which has not been pr~eated is employed as feed,
a water-loaded zone develops in this region and
essentially no oxygen-nitrogen separation occurs therein.
The inlet end temperature depression may be characteri.zed
as producing a temperature difference within the inlet
end (between the point at which feed air is introduced
and the coldestpoint) of at least 15F and with the coldest
temperature within the inlet end being no warmer than 35F.
The temperature depression as hereinbefore described does
not occur in adsorbent beds of less than 12 inches effec-
tive diameter~ As used herein, effective diameter refers
to the minimum cross-sectional dimension of an adsorbent
bed. In smaller beds, there i~s sufficient hea~ inleak
to the adsorbent such that the atmospheric heat moderates
the depression and the process is not truly adiabatic,
Also, the inlet end temperature depression does not
develop unless the feed air is separated to produce
at least 60% oxygen. With lesser oxygen-nitrogen
separations, the chilling effect of desorption is not
sufficient to develop the aforementioned depression,
Although there will always be a degree of depression
irrespective of bed effective diameter or degree of oxygen-
nitrogen separation. In such instances, the depression
is not sufficient to substantially reduce the oxygen re-
.
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covery and require the practice of this invention.
In this invention, only the adsorbent bed
inlet end is heated sufficiently by an external source
to maintain ~he gas flowing therethrough at maximum tem~
pera~ure of at least 20F warmer than such gaswithout
such heating but less than 175F~ preferably between
100F and 150F. The effect of this inlet end heating
is to move the adsorbent temperature to a higher level
along the curves of Fig. 1 and thereby increase the
percent oxygen recovery towards the maximum possible
value. The aforementloned temperature comparisons
should be based on measurements a~ the same point of
time in the cycle and at the same location in the
adsorbent bed. If there is a significant variatio~ in
the temperature difference through the inlet end, the
.
measurements should be made in the region of lowest
absolute temperature and greatest difEerence, as for
example in the on e foot bed depth region of the
Fig. 2 system. In preferred practice, the heat to be
added is in~oduced as sensible heat in warm process
streams entering the feed air inlet end of the adsorbent
bed. In most pressure swing adsorption air separation
processes, the feed air is compressed to superatmospheric
pressure and the heat of compression is more than
adequate to supply the aforementioned inlet end heating.
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~eat addition is for example readily controlled by
removing only a limited amount of ~he heat of compression
from the feed air This may be readily accomplished by
controlling the afteroooler water temperature, or by
employing a controlled feed air bypass around the after-
cooler. Adsorbent bed inlet end heating may also be
achieved by introducing externally generated hea~ to
the a~r feed, as for example with a shell-tube heat
exchanger employing steam as the heating medium. This
latter method is usually unnecessarily expensive for
processes operating atsuperatmospheric pressure but
may be essential for vacuum-purged systems. Similarly,
the heat may be introduced to a recycled process stream
from the discharge end of the adsorbent beds, 8S for
example heating oxygen by an external source prior to
introduction at the feed air inlet end for partial re
pressurization of a purged bed at low pressure.
More specifically, this invention relates to
an adiabatic pressure swing process for air separation
by selectively adsorbing at least nitrogen alternately
in at least two crystallille zeolitic molecular sieve adsorp-
tion beds of at least four Angstroms apparent pore size
at ~mbient temperature wherein the feed air is introduced
at temperature less than 90F to the inlet end of a
first adsorption bed at high pressure and at least
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60% oxygen discharges from the discharge end of the bed.
In this type of process, the first bed is cocurrently
depressurized and the cocurrent depressurization is
terminated when the first bed is at lower pressure.
Part of such oxygen from the cocurrent depressurization
is returned or recycled for repressurization of
another adsorption bed. Waste gas is released
from the first bed inlet end thereby counter-
currently depressurization same to a lowest pressure and
then oxygen gas is introduced from another adsorption
bed discharge end to the first bed discharge end
as purge gas for desorption of the nitrogen adsor-
bate, the adsorbate-containing purged gas being
discharged from the first bed inlet end as waste gas.
Oxygen gas from the discharge end of an other-than-
first adsorption bed in introduced at above said lowest
pressure to the purged first bed for at least partial
repressurization thereof, In this prior art air separa-
tion process the aforedescribed gas flows are such
that the coldest temperature within the first bed inlet ;
end is no warmer than 35F and the temperature difference
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within the first bed inlet end is at least 15F.
Under these conditions the aforedescribed sharply
depressed temperature zone substantially reduces the
oxygen recovery from the process.
In this invention, only the inlet end (and
not the intermediate section nor the discharge end
except to the extent heat may flow to these sec~ions
from the heated inlet end) of the beds is heated by
an external source sufficiently to maintain the gas
flowing therethrough at maximum temperature of at
least 20F warmer than such gas without such heating
but less than 175F, and preferably between 100~ and
150F. As will be hereandafter demonstrated, this ~ :
invention significantly improves the oxygen recovery
from adiabati pressure swing air separation pro-
cesses.
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~ BRIEF DESCRIPTION OF THE DRAWINGS
.'~ .
Fig. 1 is a graph showing the percent
oxygen versus gas temperature relationship for various
forms of zeolite A.
Fig. 2 is a graph showing the adsorbent bed
temperature versus bed depth for prior art practice and
also a three adsorbent bed embodiment of the instant
process.
Fig. 3 is a series of schematic flowsheets
showing various means for heating the adsorbent bed
inlet end according to the invention.
Fig. 4 is a schematic flowsheet of apparatus
` suitable for air separation in four adsorbent beds piped
in parallel flow sequence to produce oxygen at substan-
' tially the feed pressure.
Fig. 5 is a preferred cycle and time program
for various steps of a high pressure oxygen product.
system which can be practiced with the Fig. 4 apparatus.
Fig. 6 is a schematic flowsheet of apparatus `~
suitable for separating air in each of three adsorbent `
beds to produce oxygen at substantially lower pressure
than the feed air.
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Fig. 7 is a sche~atic flowsheet of apparatus
suitable for separating air in each of two adsorbent
beds in parallel flow sequence to produce oxygen.
Fig. 8 is a preferred cycle and time program
for practice with the two bed Fig. 7 apparatus.
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DESCRIPTION OF THE
PREFERRED EMBODIMENTS
The invention may be mathematically described
~ by the introduction of an amount of heat, Q, to the feed
- or inlet end o the zeolitic molecular sieve adsorbent
: beds to satisfy the equation:
Q - F cp (TX ~ TA~ (13
where in Equation 1 and in the following equations:
Q = heat added to the air inlet end of the beds~
F - quantity of air feed,
TA = ambient temperature of the air feed, ~F,
cp = specific heat of the air feed F,
TX = temperature of the air feed F, when the
air feed is the sole source of heat Q,
such that 175F`~TX~90F all in consistent
units.
The invention in its broadest aspect is schemat-
ically illustrated in these mathematical terms by Fig. 3.
Several specific embodiments are illustrated in schematic
Figs. 3A9 3B, 3C and 3D, all as hereinafter described.
Stated otherwise, a quantity of heat is added
to the air inlet end of the beds equivalent to the quantity
of heat, referenced to TA and contained in air feed stream
F, such that the temperature of the air feed stxeam in the
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inlet end is 175F;~TXs 90F.
~he addition of heat Q may or example be
accomplished by:
(A) Controlling the net heat of air compression andheating the inlet end. More than suffici~nt heat
is introduced as heat of compression; therefore,
controlled heat addition can be accomplished con-
viently by selectively bypassing a portion of the
compressed air (ABP~ around the compressor after-
cooler as illustrated in Fig. 3A.
Q = Q _ ~ = F cp (TX - TA) (2)
where; ~ = the heat extracted by the cooling
water ~W)
Qc ~ the heat introduced by compression
(B) The net heat of compression may also be controlled
by selectively cooling the total compressor dis-
charge air. This cooling can be accomplished by
regulating the cooling water temperature or cooling
water flow rate (W) as illustrated in Fig. 3B.
Cooling water systems which include a cooling tower
T are convenient for the practice of cooling water
temperature control since a portion of the return
cooling water can be caused to selectively bypass
the tower ~W~p).
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Q = Qc ~ ~ = F cp (TX ~ T~) (3)
where: Qc = heat introduced by compression
QT = QW = heat extracted by cool;ng tower T
(C) Externally generated heat may be added to the air
feed or directly ~o the adsorbent bed as illustrated
in Fig. 3C.
QF + QB = F CP (TX - TA) (4)
where: QF = heat introduced to the air feed
QB = heat in~roduced directly to the
adsorbent b~d
The heat, QB~ may for example be added by an electric
heater or fluid-conducting tube coil C located in
the dished head of the adsorbent bed-containing
vessels or the device may be embedded in the air
inlet end of the packed bed section. The preferred
loca~ion for embedded heat exchange mechanisms iswith-
in the initial 15 % of bed length. The externally
supplied heat, QF, can be added by any appropriate
heat exchange mechanism as for example a shell-tube
unit employing steam as the heating medium.
~D) Introducing a recycled process stream R to the feed
end of the bed, as for example illustrated in Fig.
3D. Stream R may for example be oxygen gas dis-
charged from another adsorbent bed during its co-
current depressurization step, heated by an external
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source and returned to the inlet end for both
heating and partial repressurization. In this
embodiment:
Q QR QR F CP (TX ~ TA) (5)
where: QR = heat introduced to stream R from an
external hea~ source
~'= sensLble heat~ re~erenced to T~, which
is contained in recycle stream, R,
prior to the introduction of ~R.
(E? Any suitable combination of the above described
methods.
Q Qc+QF+QB-Qw+ ~+QR = F cp (T~-TA) (6)
where Q, QC~ QF' QB' QW~ F, QR' ~R', CP, TX and TA
in the general Equation 6 are as previously described.
Any of aforedescribed systems for in~roducing
heat to the feed air inlet end of an adiabatic pressure
swing adsorption system (as previously characterized)
will substantially improve the oxygen recovery in such
system. Figs. 4 - 9 illustrate such adiabatic pressure
swing adsorption systems ~or air separation, to which
this invention may be successfully applied.
When the product oxygen is needed at substan-
tially the same pressure as the feed air, a four bed
system as for example d~scribed in Batta U.S. Patent
3,564,816 is particularly suitable as hereinafter described
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and illustrated in Figs. 4 and 5. Although the selective
adsorption will be only described in terms of removing
nitrogen from the feed air stream to produce oxygen, it
will be understood that atmospheric impurities, primarily
water and C02 but also trace amounts of light hydro-
carbons, are also selectively adsorbed in preference to
oxygen by crystalline zeolitic molecular sieves of at
least four Angstroms pore size. These impurities are
desorbed from the adsorbent bed during the low pressure
purging, along with the nitrogen.
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Fig.4 shows four adsorbent beds, A, B, C andD connected in parallel flow relation between feed air
manifold 10 and unadsorbed product oxygen manifoldO
11. Autcmatic valves LA, lB, lC and lD direc~ feed
air ~low respectively to first bed A, second bed B,
third bed C and fourth bed D. Automatic valves 2A,
2B, 2C and 2D, respectively, direct product oxygen
from the same beds into product manifold llo
The adsorbed nitrogen rejected by counter-
current depressurization and purge through waste man-
ifold 12 at the inlet end of the beds. Adsorbers A
and B are joined at their inlet ends by conduit 13
having au~omatic valves 3C and 3D thereinO Adsorbers
C and D are joined at their inlet ends by conduit 14
having automatic valves 3C and 3D therein.
~ ~~irst stage ëqualization conduit 15 is pro-
vided joining the discharge ends of adsorbers A and B;
similarly first stage equalization conduit 16 is pro-
vided joining the discharge ends of adsorbers C and D.
To provide first stage pressure equalization, automatic
valves 4AB and 4CD are located in conduits 15 and 16,
respectivelyO Valves 17 and 18 in series with equaliza-
tion valves 4AB and 4CD, respectively, are manual preset
throttling devices which prevent excessively high flow
rates from occurring and which allow adjustment and
balancing of equalization rates between the adsorption
bed pairs AB and DC~
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Au~omatic valves 5A, 5B, 5C and 5D are provided
at the discharge ends of the beds, ~wo of which open tO-
g ~her to pass cocurrent depressurization gas from one
adsorbent bed for use as purge gas in another bed. Manual
valves 19 and 20 in the purge cross-over conduits 21 and
22 respectively serve the same purpose as explained pre-
viously for valves 17 and 18 in the first stage pressure
equalization circuit. The purge cross-over conduits 21
and 22 (piped in parallel flow relation) also contain
back pressure regulators 23 and 24 oriented in opposite
flow directions so as to control flow in either direction
between either bed A or B and bed C or D. The back
pressure regulators 23 and 24 are set to maintain a
minlmum pressure, e.g., 50 p.s.i., in the bed undergo~
ing cocurrent depressurization. When this pressure is
reached the cocurrent depressurization and purge steps
terminate. This arrangement prevents extension of cocur-
rent depressurization to excessively low pressure with
resultant breakthrough of the one component's adsorption
front.
As previously indicated, valves 17, 18, 19 and
20 are flow rate limiting devices which prevent bed
damage due to excessive ~P and fluid velocity. A sim-
ilar precaution may be followed during countercurrent
depressurization, by means of preset throttle valve 25
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9643
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which acts as a bypass around main waste valve 26 in
waste conduit 12. During countercurrent depressuriza-
tion the automatic main waste valve 26 is closed which
forces the gas to follow the bypass route through valve
25. During the following lowest pressure purge step,
valve 26 opens to minimize flow resistance in the waste
conduit 12.
Repressurization conduit 27 having constant
flow ~ontrol valve 28 therein joins product manifold 11
for introduction of unadsorbed product effluent from the
adsorber (on the adsorption step) to a different adsorber
having been partially repressurized to lower intermediate
pressure. Conduit 27 in turn joins product return con-
duit 29 communicating with repressurization valves 6A-6D
joining the product conduits to adsorbers A-D respectively.
Second stage pressure equalization conduit 40
communicates at opposite ends with the bed A discharge
end through valve 5A, bed B discharge end through valv~
5B, bed C discharge end through valve 5C and bed D dis-
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charge end through valve 5C and bed-D discharge end
through 5D. Flow conduit 41 is controlled by valves
41 and 42.
The adsorption step is ~ermina~ed when ~he
nitrogen adsorption front is entirely within the bed.
This point may be determined in a manner well known to
those skilled in the art~ using the feed conditions,
and the adsorbent's capacity and dynamic characteristics.
Also the first pressure equalization step and the co-
current depressurization step are stopped when the adsorp-
tion front is still entirely within the bed and before
breakthrough. This permits removal of the nitrogen
adsorbate from the void space gas by the bed discharge
end, so that the emerging equalization gas and the purge
gas have virtually the same purity as the product gas.
If the cocurrent depressurization step is conducted
before the second equalization step then all void gas
recovery steps must be completed while the adsorption
front isstill entirely within the source bed. If the
second equalization step is carried out after the cocur-
rent depressurization step, the former may continue past
the breakthrough point as the emerging gas is used for
feed end repressurization. Breakthrough may for example
be identified by monitoring the nitrogen concentration
in the discharge gas, and detec~ing the moment at which
this concentration appreciably increases. The purge
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step is most efficiently performed by removing only the
adsorbables deposited in the preceding step. That is,
the bed is not completely cleaned of all nitrogen by
the purge fluid, but the lat~er's coun~er-curren~ flow
insures that the adsorption front is pushed back towards
the inlet end. This insures a clean product during even
the initial portion of the succeeding adsorption step,
The use of the Fig.4 system to practice the
four bed embodiment will be more easily understood by
reference to the Fig.5 cycle and time program. There
are six distinct steps each involving commencement and/or
termination of flows, Streams flowing into and out of
the four-bed system are indicated by vertical lines
connecting the feed manifold 10, the unadsorbed product
oxygen effluent maniold 11 and the desorbate waste nitro-
gen manifold 12. The feed manifold 10 connects vertically
with each of the four adsorption steps and the latter in
turn joîn vertically with the product manifold 11. The
countercurrent depressurization and purge steps, during
which the adsorbed nitrogen is discharged from the beds,
are connected vertically with the desorbate waste manifold
12. The repressurization steps which use a portion of the
unadsorbed product oxygen effluent are connected vertically
with the product manifold 11. All gas flows associated
with the four beds are identified on the figure.
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At least four adsorben~ beds are needed to
match, timewise, those steps in which cocurrent depres-
surization streams become available with those steps
which can utilize these streams. Otherwise large holdup
tanks would be required~ It will be apparent from Fig. 5
that at any moment of time, one of the adsorbent beds is
on its adsorption step delivering product at substantially
constant pressure to the product manifold 11. At the same
moment the other three beds are being cocurrently depres-
surized, or first or second stage pressure equalized,
cleaned of the adsorbed component and/or repressurized
respectively for the succeeding adsorption step. One of
the beds is always receiving product gas for repressuriza-
tion so that the consumption of product for this purpose
is continuous rather than intermittent.
In Fig. 5 the utilization within the system
of the pressure equalization and cocurrent depressuriza-
tion gas is indicated by horizonJcal flow lines. Each
first (I) pressure equalization step is connected horizon-
tally with a repressurization step in another bed having
already been partially repressurized, and each second (II)
pressure equalization step is connected horizontally with
a repressurization step of a different bed having just
been purged. Each cocurrent depressurization step is
connected horizontally with a purge step in a different
bed.
9643
1~69~
Each step in the cycle of bed A will ~ow be
outlined and related to those components of Fig. 4 which
are involved in the cycle changes. Pressures illustrative
of such operation are included.
,- Time 0-60 seconds: Bed A is on adsorption at
40 psig. Valves lA and 2A are open, and valves 3A, 4AB,
5A and 6A are closed.
Time 60-78 seconds: At the end o~ the adsorp-
tion step, valves LA and 2A close, and valve 4AB opens
to commence first-stage pressure equalization between
beds A and second bed B. At this moment, all other valves
associated with bed B are closed except valve 6B (valves
lB, 2B, 3B, 7B and 5). Valve 17 limits the flow rate of
equalization gas to avoid bed fluidization, the direc-
tion being countercurrent to feed gas flow in bed B.
Time 78-102 seconds: When pressures in beds A
and B have equalized at a higher intermediate level of
about 26 psig, valve 4AB closes and valves 5A, 19 and SC
open allowing purge gas to flow from bed A into third
bed C countercurrent to feed gas flow. At this moment,
all other valves associated with bed C except valve 3C
are closed (valves 2C, lC, 4CD and 6C). Valve 23
throttles and limits the flow of purge gas so that bed
C remains at substantially one atmosphere pressure.
Time 102-120 seconds: At the end of the purge
step for third bed C, first bed A will have depressurized
- 28 -
9643
~OG9(~Z
to about 16 psig. At this point, ~alve 3C closes so
that the continued flow of gas from bed ~ in~o bed C
is bottled up. The continued flow of gas cannot be
carried by the purge crossover (conduit 21, valves 23
and l9) because the regulator valve 23 is set to terminate
the purge flow when the pressure in bed A has dropped to
the predetermined lower limit for the withdrawal of
purge gas (e.g., 16 psig). There~ore, the continued
gas flow for countercurrent pressurization of bed C is
shunted through conduit 43 by opening valve 7C and
closing valve 23. Beds A and C equalize at a lower
intermediate pressure of about 8 psig.
Time 120-138 seconds: First bed A is now
countercurrently depressurized to essentially one atmo-
sphere pressure as the lowest pressure of the process
by closing valve 5A and opening valve 3A. Valve 26 in
the waste conduit 12 also closes forcing the blowdown
gas through flow-restrictive device 25.
Time 138-162 seconds: Purge gas for first bed
A is obtained from concurrent depressurization of fourth
bed D which is between its two pressure-equalization
steps. Valves 5A, 20 and 5D open to permit this flow
countercurrent to the previously flowing feed gas. At
this time, all valves associated with bed D other than
valve 5D are closed. Valve 24 throttles and limits the
flow of purge gas so that bed A remains at substantially
one atmosphere. Valve 26 in the waste conduit 12 is also
reopened so as to minimize flow resistance to the low-
- 29 -
9643
~0~
pressure purge gas.
Time 162-180 seconds: Bed A is now cleaned and
ready to be repressurized cocurrently. The initial phase
of repressurization is accomplished by continued intro-
duction of void space gas from fourth bed D. Valves 3A
and 20 close and valve 7A opens to permit flow of gas
from bed D to bed A. This par~ial cocurrent repressuri-
zation of first bed A continues untîl it is pressure
equalized with fourth bed D at lower intermediate pressure,
e.g., about 8 psig. This is also the second or lower
pressure equalization stage of bed D.
Time 180-198 seconds: The next phase of bed A
repressurizatlon is accomplished by higher pressure equali-
zation with second bed B which has just completed its
adsorption step and is initially at full feed pressure.
Valves 5A and 7A close, and valve 4AB opens to admit void
space gas discharged cocurrently from bed B. Valve 17
limits the flow to prevent bed fluidization. This further
countercurrent repressurization of first bed A continues
until it is pressure equalized with second bed B at higher
intermediate pressure, e.g., about 26 psig. This is the
first or higher pressure equalization stage of bed B.
Time 198-240 seconds: The final phase of bed A
repressurization to substantially feed pressure is accom-
plished with product gas discharged from third bed C
through manifold 11, which gas is preliminarily flow-
- 30 -
.
9643
690~
regulated by device 28 into repressurization mani~old 29.
Valve 4AB is closed and 6A opened to admi~ the regula~ed
product gas into bed A. In preferred practice, this final
phase of repressurization using product gas commences at
minute 18 and proceeds simultaneously with the first
higher pressure equalization step of bed B. Such over-
lapping of the two sources of repressurization gas is
advantageous because it smooths the internal utilization
of product and avoids fluctuations of product flow and
pressure. When bed A reaches the pressure level of mani-
fold 29, valve 6A is closed and the bed is again ready to
receive feed air for separation repeating the aforedescribed
cycle step sequence.
The above described cycle for bed A is typical
for beds B, C ~nd D. As represented in Fig. 4, the time
sequence for placing beds on-stream for adsorption is
A, D9 B and C, i.e. the first, fourth, second and third
beds. The cycling of the system was accomplished by
advancing a stepping mechanism which was activated by
the closing of appropriate time delay and pressure switch
relays. The stepping switch mechanism controlled the
opening and closing of the automatic valves according
to the described sequence.
EXAMPLE 1
In experiments performed using the above-
described four bed system of Figs. 4 and 5, the beds
- 3~ -
9643
were 96 incheslong and contained in 26 inch inside
diameter vessels. The adsorbent was 1/16 inch pellets
of calcium zeolite A. The feed air was not pretreated
to remove C02 and was water saturated. Each of the
vessels contai~ed~l200 lb. of adsorbent and the system
was fed air at the rate of 9246 scfh. The temperature
of the feed air was 64F (due to ~y partial after-
cooling to remove heat of compression) although the
ambien~ temperature (TA in equation 1) was only 45F.
Part of the separated oxygen gas was returned to t~e
feed end of the purged bed for partial repressurization
of same, as for example illustrated as stream R in
Fig. 3d. This recycled stream contains sensible heat
Q'R' but no additional heat from an external source
was introduced to the gas prior to in~roduc~ion at the
feed end. Using equation 1, it may be calculated that
the additional heat introduced to the feed air above
the 45F ambient ~y virtue of the 64F actual tempera-
ture plus tha recycle oxygen heat) is equivalent to 3130
Btu/hr., and would provide a TX value of 67.4F.
In operation, the coldest and warmest gas tem-
peratures in the bed inlet end were measured at 18~F and
58F respectively, so that the temperature difference
within the inlet end was 40F. Continued cycling of
the system at the aforedescribed conditions resulted in
- 32 -
9643
~ ~6~
a decrease of the oxygen content of the product, so
that the product extraction rate was adjusted to main-
tain a product purity of 90% oxygen. The system stabilized
at a product rate of 855 scfh~ representing a recovery
of only 39.8%
EXAMPI.E II
The same four bed system used in Example I
was used for this experiment. The feed air (not pretreated
for removal of atmospheric impurities) to the compressor
was at 75F, corresponding to TA in equation 1. The
compressed and warmed air was only partially aftercooled
in the manner of Fig. 3b, i.e~ by controlling the cooling
water temperature, and introduced adsorbent bed inlet end
at temperature of 110F and flow rate of 8960 scfh. The
oxygen product was used for partial inlet end repressuriza-
tion of the purged bed in the manner of Fig. 3d but no
external heat was added to this return stream, i.e. the
value ~ QR was zero. The total Q (Qc-Qw+QR ) added to
the feed air inlet end of the adsorption beds was 6470 Btu
per hour, corresponding to a TX (as defined in equation 1)
of 115.2F. In operation, the coldestand warmest tempera-
tures of the feed air inlet end were measured at 53F and
90F respectively, so that the temperature difference with-
in the inlet end was still very substantial - 37F. How-
ever, with a product oxygen extraction rate of 972 scfh,
the system stabilized at a product purity of 89.5%, repre-
senting an oxygen recovery of 46.4%. It will be apparent
- 33 -
D-9643-C
~)6~0~;~
from a comparison with the Example I prior art oxygen
recovery of 39.8% that this invention represents a sub-
~stantial improvement.
The Figs. 4-5 embodiment is particularly attrac-
tive where the oxygen product is needed at substantial
pressure, i.e., relatively high pressure approximating
that of the compressed feed air, but Fig. 6 illustrates
a three adsorbent bed system which may be preferred
when the oxygen product is to be consumed at only
slightly above atmospheric pressure, e.g., as the
aeration gas for an activated sludge waste treatment
system. In the latter emb~diment, at least the major
part of the feed air is introduced attendant a rise in
adsorbent bed pressure. The bed pressure rises because
the net instantaneous rate of gas introduction ~inflow
minus outflow) exceeds the adsorption capability of the
bed. These embodiments are distinct from those wherein
at least the major part of the feed air is introduced
during a steady pressure adsorption step, i.e., wherein
the net rate of feed air introduction equals the adsorp-
tion capability of the bed.
Referrin~ now to Fig. 6, it shows three adsor-
bent beds A, B and C connected in parallel flow relation
between feed air manifold 11, oxygen effluent gas manifold
12, oxygen purge manifold 13 and waste manifold 1~. Auto-
matic valves 15A, 15B and 15C direct feed air flow respec-
tively to first bed A, second bed B, and third bed C.
- 3~ -
. ~
. .
9643
.
~ ~69 0 ~ 2
Automatic valves 16A, 16B, and 16C respectively dire~t
e~fluent oxygen gas from the same beds into manifold 12.
Purge manifold 13 joins one oxygen effluent gas manifold
12 at the discharge end of the three beds~ and oxygen
purge gas is introduced ~hrough automatic valves 17A,
17B, and 17C to beds A, s, and C countercurrent to ~he
direction of feed air flow. Automatic valves 18A, 18B,
and 18C join waste manifold 14 at the inlet end of the
corresponding beds for discharge of countercurrent depres-
surization gas and purge gas. Valves 19A, l9B, and l9C
at the discharge end upstream of oxygen effluent valves
16A, 16B, and 16C respectively are the manual trim type
for limiting.the flow of pressure equalization gas.
One timing sequence suitable for use with the Fig.6 sys~em
comprises Fig.2 of the aforementioned Bata patent U.S.3,636,679 employing six
distinct steps each involving commencement and/or termination of
flows. Streams flowing into and out of the three bed
system are indicated by vertical line flows in the feed
manifold 11 and in the oxygen effluent gas manifold 12.
The feed air manifold 11 connects horizontally with each
of the three adsorbent beds and the latter in turn join
horizontally with the oxygen effluent manifold 12. The
repressuriza~ion and purge steps which use a portion of
the oxygen effluent are connected horizontally wi~h the
steps, e.g., cocurrent depressurization and pressure
- 35 -
~4:~
~IL0690~2
equalization which supply the returned oxygen gas. All
inter-bed flows are identified on the figure.
It will ~e apparent ~rom the aforementioned timing figure
that at any moment of time one of the adsorbent beds is delivering
product oxygen at progressively diminishing pressure to
the produc~ manifold 12 as follows: bed C during 0-40
seconds, bed A during 40-80 seconds and bed B during 80-
1~0 seconds. Acc~rdingly, product oxygen flow to ~he
consuming ~.eans is continuous.
The utilization within the system of the pressure
equalization and cocurrent depressurization gas is indicated
by horizontal flow lines. Each pressure equalization step
is connected horizontally with a repressurization step in
another bed having already been purged, and each cocurrent
depressurization step is connected horizontally with a
purge step of a different bed having just been counter-
currently depressurized.
Each step in the cycle of bed A will now be
outlined and related to those components of Fig. 6 which
are involved in the cycle changes Pressures illustrative
of such operation for air separation using calcium zeolite
A adsorbent are included.
Time 0-15 seconds: Bed A is being repressurized,
bed B countercurrently depressurized, and bed C pressure
equalized. Valves 15A and 16~ are open, and valves 17A
and 18A are closed. Feed air is introduced to bed A at
- 36 -
.
.~
9643
~0~
its inlet end from manifold 11, and one component depleted
gas from manifold 12 is simultaneously introduced at the
bed A discharge end. The latter is derived from bed C
through trim valve l9C and valve 16C, and flows consec-
utively through valves 16A and trim valve l9A into bed A.
Bed C is cocurrently depressurized during this period and
the flow continues until pressure between beds A and C
is substantially equalized at about 15 psig. During this
period, the flow of equalization gas is rapid while the
flow of feed air from compressor 20 is limited so that
the major portion of the gas for repressurizing bed A
from O to 15 psig is oxygen gas. Duri~g this period,
another part of the gas released from bed C is discharged
as product oxygen in manifold 12.
Time 15-40 seconds: Valve 16A is now closed
and only the flow of feed air continues to bed A until
the terminal pressure of 30 psig is reachedO This com-
pletes the repressurization period for bed ~.
Time 40-55 seconds: The pressure equalization
step for bed A co~nences by closing valve 15A and opening
valves 16A and 16B cocurrently depressurizing the bed by
releasing gas from the discharge end. Oxygen product
gas flows through control valve 21 in manifold 12 to the
product oonduit downstream valve 21 at a rate serving
to hold the product conduit at a suitable low pressure
such as 3 psig. The remainder and major part of the
- 37 -
.' .. . :
9643
lO~ Z
oxygen gas flows through valves 16B and 19B to the discharge
end of bed B for partial repressurization thereof. Bed B
has previously been purged o~ nitrogen adsorbate and is
initially at about 0 psig. This flow of product gas from
bed A to bed B continues for about 15 seconds until the
two beds are at substantially the same pressure, eOg.
15 psig.
Time 55-80 seconds: Additional nitrogen-depleted
gas is released from the bed A discharge end for further
cocurrent depressurization ~hereo~, with one part intro-
duced to the bed C discharge end by closing valve 16B and
opening automatic valve 17C in the purge manifold for
purging nitrogen at slightly above 0 psig. Valves 23 and
24 reduce the purge gas pressure to essentially one atmo-
sphere and also hold the flow rate of purge gas constant.
This, in turn, holds the total quantity of purge gas
constant since the purge step is preferably a fixed length
of time. The flow rate is controlled at a steady value
by regulating valve 23 which holds the pressure constant
between the two valves 23 and 24. The waste gas emerging
from bed C inlet end flows through automatic valve 18C
in waste manifold 14 and is released through automatic
waste discharge valve 25. The last-mentioned valve is a
flow-limiting device rather than the shut-off typeO When
"closed," it introduces a flow restriction into the waste
manifold 14 which reduces the depressurization rate to a
- 38 -
,
value below that causing attrition of the adsorbent
particles. However, for the discharge o~ purge gas,
valve 25 is open to remove the restriction inas~uch as
flow is already limited by valve systems 23 and 24.
Another part of the additiorlal nitrogen-depleted gas from
bed A is discharged as oxygen product~ During this step,
the pressure of bed A and manifold 12 continues to decrease
until it reaches about 11 psig~ which occurs after an addi-
tional 25 seconds (80 seconds into cycle). The lowest
pressure limit for cocurrent depressurization, e.g , 4
psig, should be maintained because the pressure corre-
sponds to imminent breakthrough of the adsorption front
at the discharge end of the bed. This completes the pro-
duction phase for bed A.
Time 80-95 seconds: Bed A now begins its
nitrogen adsorbate rejection (desorption) phase by clos-
ing valves 16A and 17C, and opening valve 18A. Additional
gas at 4 psig is released from the bed A inlet end from
countercurrent depressurization thereof through waste
manifold 14 and discharge valve 25. The latter valve is
"closed" for this step in order to introduce the afore-
said restriction and avoid excessive flow rates from the
bed. This step continues to essentially o~e atmosphere
in about 15 seconds.
Time 95-120 seconds: Bed A is purged of remain-
ing nitrogen adsorbate by opening valves 17A and 25.
_ 39 _
D-9643-C
Additional nitrogen-depleted gas from the discharge end
of bed B flows through manifold 12 through valves 23 and
24 and purge manifold 13, then through valve 17A to the
bed A discharge end. The nitrogen-containing purge gas
emerging through the bed A inlet end flows through valve
18A and is discharged through waste valve 25. Purging
continues for 25 seconds. This completes the cycle and
bed A is in a condition to commence repressurization with
feed air.
Beds B and C are consecutively cycled through
the aforediscussed steps with bed B entering the simul-
taneous feed air-product oxygen repressurization with
the bed A pressure equalization step (time 40-55 seconds).
Bed C enters the simultaneous feed air-product oxygen
repressurization with the bed A countercurrent depres-
surization step (time 80-95 seconds). The necessary valve
changing for these steps will be recognized from Fig. 6
and Batta U.S. Patent No. 3,636,679, Fig. 2 and the
foregoing description. A cycle control system is necessary
to initiate and coorainate these valve changes. The cycle
controller may for example receive a signal from pressure
sensing means in ~eed air conduit 11 downstream compressor 20.
Summarizing the aforedescribed three adsorption
bed system which is preferred when oxygen gas product is
to be discharged at low pressure, the first bed is initially
at the lowest pressure and purged of nitrogen adsorbate.
- 40 -
.~
`~ 9643
~La~9~Z
Feed air and oxygen gas are simultaneously introduced
respectively to the first bed inlet end and discharge
end. Oxygen gas is simul~aneously released from the
discharge end of a third bed initially at the highest
superatmospheric pressure with one part discharged as
product and the balance returned ~o the first bed
discharge end for such simul~aneous introduction, the
gas flows continuing until the first and third beds
are first higher pressure equalized. After terminating
the oxygen gas introduction to ~he discharge end, the
feed air introduction to the first bed lnlet end is
continued until the bed is repressurized to the highest
superatmospheric pressure. Oxygen is thereafter released
from the repressurized first bed discharge end with one
part thereof discharged as product and the balance
returned to the discharge end of a partially repressurized
second bed for simultaneous introduction during feed air
introduction to the second bed inlet end until the first
and second beds are first higher pressuxe equalized.
The first bed is then cocurrently depressurized to about
21 psia. The cocurrent depressurization of the first
bed is continued with one part of the oxygen discharged
as product and the balance returned to the third bed
discharge end for purging of nitrogen adsorbate there-
from. The first bed is thereafter countercurrently
- 4~ -
9643
~1690~
depressurized and oxygen from a cocurrent depressurizing
second bed is then returned to the first bed discharge
end or purging thereof. The aforedescribed steps are
consecutively followed wi~h the second and third beds
in accordance with the flow cycle sequence of Fig.2 in Bata
U.S. 3,6a~,379.
EXAMPLE III
In experiments performed using the above-
described thrae bed system of Fig.6 herein and Flg.2 of Bata
U.S. 3,636,379, ~he beds were 96 inches long and contained in 26 inch -
inside diame~er vessels~ The adsorben~ was l/16 inch pellets of
calcium zeolite A. The feed was not pretreated to remove
C2 and was water saturated. Each of the vessels con~
.
tained 1200 lbs. o~ adsorbent and the system was fed
air at the rate of 9030 scfh. and at temperature of 38
wh~ch was ambient (TA in accordance with equation 1).
The aforementioned highest superatmospheric pressure was
45 psia. The value o TX (as defined) was 38F and the
corresponding value of Q (as defined) was zero. Bed A
was equipped with thermocouples located at the axis
of the vessels and at spaced distances from the
air inlet end to the discharge end. Beds B and
C were equipped with an axially positioned thermocouple
located two feet into the air inlet end of the bed.
Product was extracted at the rate of 770 scfh.
and analyzed for oxygen content. Continued cycling of
- 42 -
.
9643
~69(~~2
the system resulted in a depressed tempera~uxe zone
at the inlet end as depicted in the two lower curves
of Fig. 2. In the latter 5 the curves are used to show
the range of temperatures between the coldest and warmest
section of the beds at the same point in~time. Two
curves are plotted for Example III to depict the temper
ature variation which is experienced at a single point
in the bed. This temperature variation is a measure of
t~e cyclic temperature effect which is common to adia-
batic pressure swing adsorption processes, and is very
small as compared to the magnitude of the stable end-
to-end bed temperature gradient which developed. It is
significant to note that relatively little temperature
drop occurs in the first f~w inches adsorbent bed length
because this section is loaded with preferentially ad-
sorbed air impurities (primarily water and CO2) and
virtually no nitrogen is adsorbed ~herein. The ~empera-
ture drops sharply farther into the first foot o~ bed
... .
lengt~ to a low point about -65F at a distance one foot
from the support screen, so that the temperature difference
within the inlet end is 103F. The system stabillzed at
a product purity of only 66% oxygen with 26.7% oxygen
recovery.
..:
- 43 -
9643
.
9~
EXAMPLE IV
The same three bed system used in Example III
was used for this experiment. The feed air at 9000 scfh.
and 38F was preheated as illustrated in Fig. 3C by in
direct hea~ exchange with steam to 110F, so that TX
assumes this value. The Q (as defined) added was 11,300
Btu per hour, and with a product ex~raction rate of
774 scfh., the operation stabliized at a product purity
of 82% oxygen representing a recovery of 33. 6~/o~ The
temperatures within the beds are depicted by the two
upper curves of Fig. 2. It will be observed that ~he
practice of this invention did not eliminate the adsoxbent
bed end-to-end temperature gradient but instead moved the
level of the inlet end temperature depression substantiaIly
upwardly so that the sys~em operated in a higher range of
the Fig. 1 oxygen recovery vs. g a s temperature
curve.
Another significant observation is that the
temperature depression in the three bed system of
Examples III and IV was greater than the temperature
depression experienced with the four bed systems of
Examples I and II.
EXAMPLE V
Additional tests were performed using the
three bed equipment and cycle described in connection
- 44 -
9643
~Q~9~;Z
with ~xamples III and IV, and with feed air temperatures
(Tx) of 100F to 175F within the adsorbent bed inlet
end, the oxygen recovery and purity were comparable tO
that achieved in Example IV. For example, with an
inlet end air temperature of 175F, the 2 recovery was
32.7% at 87% oxygen purity. When the feed air was
heated sufficiently for an inlet end temperature of 220F,
the 2 recovery was only 29.5%. Accordingly, more energy
has been consumed with lower oxygen recovery and for
thisreason, there is no significant advantage in operating
the present process at inlet end temperatures above 175F.
The preferred range of maximum inlet end temperatures,
between 100F and 150F, represents a balance between
increasing energy requirement and oxygen recovery.
EX~MP~E VI
In still another test using the three bed
equipment and cycle described in connection with Examples
III and IV, the Eeed air was pretreated to remove atmo-
spheric impurities, so that the entering air had a dew
point o -40F and only 1 ppm. CO2. The beds each com-
prised 1200 lbs. of calcium zeolite A. The system was
fed at the rate of 8,670 scfh of 38F pretreated air,
and product was extracted at the rate of 840 scfh. The
system stabilized at a product purity of 75% oxygen,
representing a recovery of 34.7% of contained oxygen.
- 45 -
9643
~L()69Vti;~
Although a depressed temperature zone was developed
at the inlet end, it was not nearly as severe as in
the Example III - V experiments wherein the feed air
was not prepurified. In this experimen~, the tempera-
ture dropped only to 23F in the feed air inlet end
(second foot of bed length), so tha~ the temperature
difference within the inlet end was only 15F. This
resulted in an oxygen recovery approaching that obtained
under otherwise comparable conditions without pretreated
air wherein the adsorbent bed is heated according to
this invention.
Notwithstanding the foregoing, another test
showed significant advantages in practicing this inven-
tion with pretreated air. The same system was fed with
8060 scfh. of pretreated air heated by indirect heat
exchange with steam to 100F. Since the ambient was
54F, the added heat was 5900 BTU/hr. The product was
withdrawn at a rate of 714 scfh. and comprised 90%
oxygen. This resulted in an 2 recovery of 38.1%.
It follows ~rom Example VI that use of pre-
treated air improved performance of the adiabatic~pressure
swing adsorption system for air separation by reducing
the severity of the inle~ end temperature depression.
However, practice of this invention additlonally moved
performance of the system to a higher range of oxygen
- ~6 -
. .
9643
t;Z
r~covery as generally depicted in Fig. 2.
It should also be nclted that when the adsor-
bent beds used for air prepurification and air separa-
tion are placed in separate vessels but are joined by
appropriate piping so that the two sections are integral
from the process standpoint, ~his invention may be
advantageously employed.
The present invention may also be practiced
with two adsorbent beds, as for example illustrated in Figs.
8-9. In this type of process, the purged adsorption zone at
the lowest pressure level is partially repressurized to
an intermediate pressure by introducing oxygen gas. The
process is characterized by an increasing pressure adsorp-
tion step of introducing feed air to the inlet end of the
partially repressurized adsorption zone at pressure
higher than said intermediate pressure, ~electively
adsorbing ni~rogen and simultaneously discharging oxygen
from the zone discharge end, with the feed gas intro-
duction, the nitrogen adsorption and the oxygen discharge
at relative rates such that the pressure of the adsorp-
tion zone rises from the intermediate pressure during
this step to higher pressure at the end of such step.
Stated otherwise, during the increasing
pressure adsorption step the net molal rate of gas
- 47 -
'~ .
. .
9643
~069 l)~;2
introduction to the adsorption zone is greater than
the net molal rate of gas adsorption on the bed. In
this relationship, "net molal rate of gas introduction"
is the rate at which feed air is introduced minus the
above-zero rate at which gas is discharged from the bed,
and the "net molal rate of adsorption" is the rate at
gas phase into the adsorbed phase minus the rate at
which components of the feed are displaced or other-
wise released from the adsorbed phase. When the net
molal rate of gas introduction exceeds the net molal
rate o~ gas adsorption, the adsorption pressure will
rise. This may be accomplished by restricting the
discharge of oxyge~ gA9 rela,tive to the inflow of
feed, The increasing pressure adsorption step
preferably continues until the highest Pressure
level of the process has been attained and the nitrogen
adsorption front has moved from the adsorp~ion zone
inlet end to a position intermediate the inlet and
discharge ends, The location of the one component
adsorption front is such that a substantial portion of
the zone length downstream of the front is unusad, i.e.
not yet significantly loadçd with the one component.
The adsorption zone is thereafter cocurrently depres-
surized for sufficient duration to move the nitrogen
adsorption front to the zone discharge end. During
- 48 -
. 9643
~69~Z
this period, oxygen is released from the zone and may
be used to repressurize or purge another adsorption
zone, andtor be discharged as product. In this manner,
~he adsorbent is fully utilized and maximum recovery
of the less strongly adsorbed components is achieved
at high purity.
F.ach step in theicycle of bed A will now be
outlined and related to those components of Fig. 7 ~:
which are involved in the cycle changes. Pressures
illustrative of such operation for air separation
using calcium zeolite A adsorbent are included and are
related to the following terms used herein to identify
the terminal pressure in a relative sense:
TermIllustrative psi~. .
.: :
lowest pressure ~1
lower intermediate pressure 10
equalization pressure 20
higher intermediate pressure32
highest intermediate pressure35
highest pressure 40
Time 0-10: Bed A is being repressurize~
from the lowest process pressure (less than 1 psig.~
to the equalization pressure (20 psig.), and bed B is being
pressure equalized. Valves 15A and 16A are open and
valves 17A and 18A are closed. Feed air is intro-
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:. : .. : . . . .
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duced to bed A at its inlet end from manifold 11
through valve 15A and one component-depleted gas from
manifold 12 is simultaneously introduced at the bed
A discharge end through valve 16A. The latter is
d~rived from bed B undergoing pressure equalization
through trim valve l9B, valve 16B, and flows con-
secutively through valves 16A and trim valve l9A
into bed A. Bed B is cocurrently depressurized during
this period and the flow continues for about 10 seconds
until pressures between beds A and B are substantially
equalized at about 20 psig. During this period,
the flow of equalization gas is rapid while the flow
of feed air from the compressor is limited, so th~t
the ma;or portion of the gas for repressurizing bed
A from 0 to 20 psig. is one component-depleted gas,
e.g. 85 7O for air separation. During this period,
another part-of the gas released from bed B is dis-
charged as product in manifold 12.
Time 10-30: Valve 16A is now closed and
flow of feed air only continues to bed A for
an additional twen~y seconds to a higher intermediate
pressure of about 32 psig. Simultaneously the bed B
cocurrent depressurization continues and all of the
nitrogen-~depleted gas released therefrom is discharged
as product in manifold 12. During this period the
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bed B pressure diminishes from ZO psig. (equalization~
to 10 psig. ~lower intermediate~. During the bed B
pressure equalization and cocurrent depressurization
steps, the nitrogen adsorption front has moved pro-
gressively toward the bed discharge end~ ancl at this
point has reached the discharge end so that break
through is imminent. Therefore it can no longer
deliver produc~ purity gas to manifold 12 and v~lve
16B closes. In order for product flow to be un-
interrupted, the product gas must be derived from
bed A, and in this process the latter dellvers product
during the remainder of its repressurization.
Time 30-35: Valve 16A again opens and flow
of product proceeds from bed A to manifold 12. This
is the ~irst part o~ the bed A increasing pressure
adsorption step and the bed pressure rises from 32
psig. (higher intermediate) to 35 psig. (higbest
intermediate). Simultaneously valve 18B opens, waste
discharge valve 25 closes and bed B i5 count~r-
currently depressurized through its inlet end to less
than 1 psig., the lowest pressure o~ the process.
Time 35-60: During this remaining part of
the bed A increasing pressure adsorption step wherein
the bed pressure rises from 35 psig. (highest
intermediate) to 40 psig. (highest), valves 17B and
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~069(~2
25 are open and part of the nitrogen-depleted gas
discharged from bed A flows through valves 23 and 24
and 17B to purge bed B.
At the beginning of the bed A 0-10 second
repressurization through both the inlet and discharge
ends, a nitrogen adsorption front is established near
the inlet end. This front moves progressively toward
the discharge end during the remainder of the 10 second
period and during the succeeding repressurization s~ps ~r
the first 60 seconds of the cycle. At the end of this
period, a pred~termined length of unloaded bed re-
mains between the nitrogen adsorption front and the
discharge cnd.
Ti.me 60-70: Valve 15A closes and valve 16B
is opened and bed A now commences pressure equalization
with bed B while continuing to deliver product. Bed A
is cocurrently depressuriæed by releasing gas from the
discharge end. The gas flows through the unloaded bed
length wherein the nitrogen component is adsorbed and the
emerging nitrogen-depleted gas is employed in two
parts. Oxygen product gas flows ~hrough control valve
21 in manifold 12 to the consumer conduit downstream
valve 21 at a rate serving ~o hold the consumer con-
duit at a suitable low pressure such as 3 psig. Theremainder and major part of the nitrogen-depleted gas
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~ 0~ 2
flows through valves 16B and l9B to the discharge end
of bed B for partial repressuriza~ion thereof. Bed B
has previously been purged of nitrogen adsorbate and
is initially at the lowest pressure level of ~he
process. This flow of one c~mponent-depleted gas
from bed A to bed B continues for about 10 seconds
until the two beds are at substantially ~he same
equalization pressure of 20 psig. During this step
valve 15B is open and bed B is also being repres-
surized thro~gh its inlet end with feed air from
manifold 11.
Time 70-90: Valve 16B closes and additional
nitrogen-depleted gas is released from the bed A discharge
end for cocurrent depressurization to about 10 psig.
(lower intermediate), the entire quantity of this gas
from bed A being discharged as product. Sim~ltaneously only
the feed~air flow is continued to the bed B inlet end for
further repressurization thereof from 20 psig. to 32 psig.
Time 90-95: Bed A is now countercurrently
depressurized to the lowest process pressure by closing
valves 15A, 16A, opening valve 18A and
closing valve 25 so the nitrogen desorbate is released
through waste manifold 14. Simultaneously valve 16B
opens and nitrogen-depleted gas emerges from the bed B
discharge end for flow through manifold 12 and valve
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9643
106~636Z
21 as product. This is the first part of the bed s
increasing pressure adsorption step wherein the bed
pressure rises from 32 to 35 psig. during nitrogen
adsorption from feed air flowing through the bed.
Time 95-120: Valves 17A and 25 open and
part of the nitrogen-depleted gas emerging from bed B
is returned from manifold 12 through valves 23 and 24
to the bed A discharge end as purge gas. The latter
~lows through bed A countercurrently to the feed gas
flow direction and desorbs the remaining nitrogen
adsorbate. The resulting waste gas is discarded
through valve 18A and manifold 14. Simultaneously
with the bed A purging, the bed B increasing pressure
adsorption step is continued until the bed pressure
reaches 40 psig., the highest pressure of the process.
At this point valves 17A and 18A are closed and purged
bed A is again ready for repressurization in accordance
with the foregoing sequence.
As previously stated, the crystalline zeolitic
molecular sieves useful in the pr~ctice of this invention
have an apparent pore size of at least about four ~ngstrom
units. Crystalline zeoliteshaving apparent pore sizes
of at least 4.6 Angstroms are preferred because they
permit more rapid adsorption and desorption of the nitro-
gen molecules particularly in the lower temperature
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region, leading to faster operating cycles than atta~n-
able~with small pore size zeolites.
The term apparent pore size as used herein may
be defined as ~he maximum critical dimension of the
molecular species whieh is adsorbed by the zeolitic
molecular sie~e in question under normal conditions.
The apparent pore size will always be larger than the
effective pore diameter, which may be defined as the
free diameter of the appropriate silicate ring in the
zeoli~e structure.
The term "zeolite," in general, refers to a
group of naturally occurring and synthetic hydrated
metal aluminosilicates, many of which are crystalline
in structure. There are, however, signiflcant differences
between the various synthetic and natural materials in
chemical composition crystal structure and physical
properties such as X-ray powder diffractLon patterns.
The structure of crystalline zeolitic molecular
sieves may he described as an open three-dimensional
framework of SiO4 and A104 tetrahedra. The tetrahedra
are cross-linked by the sharing of oxygen atoms, so that
the ratio of oxygen atoms to the total of the aluminum
and silicon atoms is equal ~o two, or Ot(Al~Si)=2~ The
negative electro-valence of tetrahedra containing alumi-
num is balanced by the inclusion within the crystal of
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, ~ .
~ ~ 9 ~ ~ 2
cations, for example, alkali metal and alkaline earth
metal ions such as sodium, potassium, calcium and mag-
nesium ions. One cation may be exchanged for another
by ion-exchange techniques.
The zeolites may ba activated by driving off
substantially all of the water of hydration, The space
remaining in the crystals after activation is available
for adsorption of ads~rbate molecules. Any of this space
not occupied by reduced element metal atoms will be avail-
able for adsorption of molecules having a size, shape and
energy which permits entry of the adsor~ate molecules
into the pores of the molecular sieves.
The zeolites occur as agglomera~es of fine
crystals or are synthesized as fine powders and are
preferably tableted or pelletized for large scale adsorp-
tion uses. Pelletizing methods are known which are very
satisfactory because the sorptive character of the zsolite,
both with regard to selectivlty and capacity, remains
essentially unchanged.
Among the naturally occurring zeolitic molecular
sieves suitable for use in the present invention include
erionite, calcium-rich chabazi~e and faujasite. The
natural materials are adequately described in the chemical
art. Suitable synthetic crystalline zeolitic molecular
sieves include types A, R, X, Y9 L and T. Zeolites such
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9~Z
as types X, Y, L and chabazite are particularly useful
because of their relatively large pore sizesO
Zeolite A is a crystalline zeolitic molecular
sieve which may be represented by the ~ormula:
1.0+0.2M 2 O:Al2o3:l~85~o-5sio2:yH2o
n
where M represents a metal, n is the valence of M, and y
may have any value up to about 6. The as-synthesized
zeolite A contains primarily sodium ions and is desig-
nated sodium zeolite Ao All of the monovalent cation
forms of zeolite A have an apparent pore size of about
4 Angstroms, excepting the potassium form which has a
pore size of about 3~Angstroms and consequently is un-
suitable for use in Ithe present invention. When at least
about 40 percent f Lhe monovalent cations sites are
satisfied with di- or trivalent metal cations, zeolite
A has an app~rent pore size of about 5 Angstroms.
Zeolite R ls described in U.S. Patent No.
3,030,181.
Zeolite T has an apparent pore size of about
. 5 Angstroms 7 and is described in U S. Patent No. 2,950,952.
Zeolite X has an apparent pore size of about
10 Angstroms, and is described in U.S. Patent No.2,882,244.
Zeolite Y has an apparent pore size of about
10 Angstroms, and is described in U.S. Patent No. 3,130,007.
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9643
~ ~69 ~ ~ ~
Although preferred embodiments of the inven-
tion have been described in detail, it is contemplated
that modifications of the process may be made and that
some features may be employed without others, all with-
in the spirit and scope of the invention.
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