Note: Descriptions are shown in the official language in which they were submitted.
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SURGE ABSORBER FLOW REGULATION FOR
MODULAR PRESSURE SWING ADSORPTION
FIELD OF THE INVENTION
The present invention relates to an apparatus for separating gas fractions
from a gas
mixture having multiple gas fractions. In particular, the present invention
relates to a rotary
valve gas separation system having a plurality of rotating adsorbent beds
disposed therein for
implementing a pressure swing adsorption process for separating out the gas
fractions.
BACKGROUND OF THE INVENTION
Pressure swing adsorption (PSA) and vacuum pressure swing adsorption (VPSA)
separate gas fractions from a gas mixture by coordinating pressure cycling and
flow reversals
over an adsorbent bed which preferentially adsorbs a more readily adsorbed
component
relative to a less readily adsorbed component of the mixture. The total
pressure of the gas
mixture in the adsorbent bed is elevated while the gas mixture is flowing
through the
adsorbent bed from a first end to a second end thereof, and is reduced while
the gas mixture is
flowing through the adsorbent from the second end back to the first end. As
the PSA cycle is
repeated, the less readily adsorbed component is concentrated adjacent the
second end of the
adsorbent bed, while the more readily adsorbed component is concentrated
adjacent the first
end of the adsorbent bed. As a result, a "light" product (a gas fraction
depleted in the more
readily adsorbed component and enriched in the less readily adsorbed
component) is
delivered from the second end of the bed, and a "heavy" product (a gas
fraction enriched in
the more strongly adsorbed component) is exhausted from the first end of the
bed.
The conventional system for implementing pressure swing adsorption or vacuum
pressure swing adsorption uses two or more stationary adsorbent beds in
parallel, with
directional valuing at each end of each adsorbent bed to connect the beds in
alternating
sequence to pressure sources and sinks. However, this system is often
difficult and expensive
to implement due to the complexity of the valuing required. Further, the
adsorbent beds are
often exposed to variations in pressure and gas flow, thereby reducing the
efficiency and
yield of the gas separation process.
Numerous attempts have been made at overcoming the deficiencies associated
with
the conventional PSA system. For example, Siggelin (U.S. Patent No.
3,176,446), Mattia
(U.S. Patent No. 4,452,612), Davidson and Lywood (U.S. Patent No. 4,758,253),
Boudet et al
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(U.S. Patent No. 5,133,784) and Petit et al (U.S. Patent No. 5,441,559)
disclose PSA devices
using rotary distributor valves whose rotors are fitted with multiple
angularly separated
adsorbent beds. Ports communicating with the rotor-mounted adsorbent beds
sweep past
fixed ports for feed admission, product delivery and pressure equalization.
However, these
prior art rotary valves have considerable dead volume for flow distribution
and collection,
and expose the adsorbent beds to pulsations in pressure and gas flow. As a
result, the rotary
valves have poor flow distribution, particularly at high cycle frequencies.
Hay (U.S. Pat. No. 5,246,676) and Engler (U.S. Pat. No. 5,393,326) provide
examples
of vacuum pressure swing adsorption systems which reduce throttling losses in
an attempt to
improve the efficiency of the gas separation process system. The systems
taught by Hay and
Engler use a plurality of vacuum pumps to pump down the pressure of each
adsorbent bed
sequentially in turn, with the pumps operating at successively lower
pressures, so that each
vacuum pump reduces the pressure in each bed a predetermined amount. However,
with
these systems, the vacuum pumps are subjected to large pressure pulsations,
stressing the
compression machinery and causing large fluctuations in overall power demand.
Because
centrifugal or axial compression machinery cannot operate under such unsteady
conditions,
rotary lobe machines are typically used in such systems. However, such
machines have
lower efficiencies than modern centrifugal compressors/vacuum pumps working
under steady
state conditions.
Accordingly, there remains a need for a PSA system which is suitable for high
volume and high frequency production, and which also reduces pulsations in
pressure and gas
flow.
SUMMARY OF THE INVENTION
According to the invention, there is provided a gas separation system which
addresses
deficiencies of the prior art gas separation systems.
The gas separation system, according to the invention, includes a stator, and
a rotor
rotatably coupled to the stator, and at least one surge absorber in
communication with the
stator. The stator includes a stator valve surface and a plurality of function
compartments
opening into the stator valve surface. The rotor includes a rotor valve
surface in
communication with the stator valve surface, and a plurality of flow paths for
receiving
adsorbent material therein. The rotor also includes a plurality of apertures
provided in the
rotor valve surface and in communication with the flow paths for cyclically
exposing the flow
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paths to the function compartments.
The surge absorbers are configured to reduce pressure variations in the
function
compartments and to maintain each function compartment at one of a plurality
of discrete
pressure levels. Preferably, the surge absorbers comprise a primary surge
chamber coupled to
one of the function compartments, and at least one secondary surge chamber
coupled to the
primary surge chamber and a respective one of the function compartments.
Typically, each
secondary surge chamber communicates either with an adjacent secondary surge
chamber or
the primary surge chamber through a flow restrictor so as to subdivide the
pressure interval
between adjacent primary function compartments into a larger number of more
closely
spaced pressure levels. Alternately, in one variation, each surge absorber
comprises a
plurality of parallel plates inclined relative to the function compartments
and defining parallel
flow restriction channels for maintaining each function compartment at one of
the plurality of
discrete pressure levels. In this manner, substantially uniform gas flow can
be maintained
through the function compartments and the flow paths without recourse to
multistage
compression machinery.
Preferably, the increments between adjacent pressure levels are sized so that
the gas
flows entering or exiting the gas separation system are substantially steady
in both flow
velocity and pressure. As a result, the gas separation system can be operated
with centrifugal
or axial flow compressor and expander compression machinery, each having a
small number
of compression stages separated by relatively wide pressure intervals.
In a prefer ed embodiment of the invention, during pressurization and blowdown
steps, the several adsorbers passing through the step will converge to the
nominal pressure
level of each step by a throttling pressure equalization from the pressure
level of the previous
step experienced by the adsorbers. Flow is provided to the adsorbers in a
pressurization step
or withdrawn in a blowdown step by compression machinery at the nominal
pressure level of
that step. Hence flow and pressure pulsations seen by the compression
machinery at each
intermediate pressure level are minimal by averaging from the several
adsorbers passing
through the step, although each adsorber undergoes large cyclic changes of
pressure and flow.
During the pressurization steps for each adsorber, either (or both) of the
apertures of
an adsorber already at a pressure is (are) opened respectively to a first or
second
pressurization compartment at a stepwise higher pressure. Similarly, during
the
pressurization steps for each adsorber, either (or both) of the apertures of
an adsorber already
at a pressure is (are) opened respectively to a first or second pressurization
compartment at a
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stepwise lower pressure. Equalization then takes place by flow through the
open apertures)
from the pressurization/blowdown compartment into the adsorber, which by the
end of the
pressurization/blowdown step has attained approximately the same pressure as
the
pressurization/blowdown compartment(s). Each pressurization/blowdown
compartment is in
communication with typically several adsorbers being pressurized (in differing
angular and
time phase) at any given time, so the pressure in that compartment and the
pressurization
flow to that compartment are substantially steady.
Since the orifices providing the valuing function are immediately adjacent to
the ends
of the flow paths, the dead volume associated with prior art distribution
manifolds is
substantially reduced. Also, since the compartments communicating with the
first and
second valve surfaces are external to the valuing function, the compartments
do not
contribute to dead volume of the adsorbers. As a result, high frequency
pressure/vacuum
swing adsorption is possible.
BRIEF DESCRIPTION OF THE DRAWINGS
The preferred embodiments of the present invention will now be described, by
way of
example only, with reference to the drawings in which:
Fig. 1 is a sectional view of a rotary PSA module according to the present
invention,
showing the stator, the rotor and the adsorber situated in the rotor;
Fig. 2 is a sectional view of the module of Fig. l, with the stator deleted
for clarity;
Fig. 3 is a sectional view of the stator shown in Fig. 1, with the adsorbers
deleted for
clarity;
Fig. 4 is an axial section of the module of Fig. 1;
Fig. 5 shows a typical PSA cycle attainable with the present invention;
Fig. 6 shows one variation of the PSA cycle with heavy reflux, attainable with
the
present invention;
Fig. 7 shows a pressure swing adsorption apparatus according to the present
invention, depicting the rotary module shown in Fig. 1 and a compression
machine coupled
to the rotary module;
Fig. 8 shows a radial-flow-configured rotary PSA module, with the compression
machine deleted for clarity;
Fig. 9 shows an axial-flow-configured rotary PSA module, with the compression
machine deleted for clarity;
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Fig. 10 shows the first valve face of the rotary PSA module shown in Fig. 9;
Fig. 11 shows the second valve face of the rotary PSA modules shown in Fig. 9;
Fig. 12 shows a PSA system using flow restrictors for pressure let-down; and
Figs. 13 and 14 show a vacuum-PSA system using alternative flow restrictors
for
pressure let-down.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig-s.l, 2, 3 and 4
A rotary module 10 according to the present invention is shown in Figs. 1, 2,
3 and 4.
The module includes a rotor 11 revolving about axis 12 in the direction shown
by arrow 13
within stator 14. In general, the apparatus of the invention may be configured
for flow
through the adsorber elements in the radial, axial or oblique conical
directions relative to the
rotor axis. However, for operation at high cycle frequency, radial flow has
the advantage that
the centripetal acceleration will lie parallel to the flow path for most
favourable stabilization
of buoyancy-driven free convection, as well as centrifugal clamping of
granular adsorbent
with uniform flow distribution.
As shown in Fig. 2, the rotor 11 is of annular section, having concentrically
to axis 12
an outer cylindrical wall 20 whose external surface is first valve surface 21,
and an inner
cylindrical wall 22 whose internal surface is second valve surface 23. The
rotor has (in the
plane of the section defined by arrows 15 and 16 in Fig. 4) a total of "N"
radial flow absorber
elements 24. An adjacent pair of absorber elements 25 and 26 are separated by
partition 27
which is structurally and sealingly joined to outer wall 20 and inner wall 22.
Adjacent
absorber elements 25 and 26 are angularly spaced relative to axis 12 by an
angle of [360° /
N].
Absorber element 24 has a first end 30 defined by support screen 31 and a
second end
32 defined by support screen 33. The absorber may be provided as granular
adsorbent, whose
packing voidage defines a flow path contacting the adsorbent between the first
and second
ends of the absorber.
First aperture or orifice 34 provides flow communication from first valve
surface 21
through wall 20 to the first end 30 of absorber 24. Second aperture or orifice
35 provides
flow communication from second valve surface 23 through wall 22 to the second
end 31 of
absorber 24. Support screens 31 and 33 respectively provide flow distribution
32 between
first aperture 34 and first end 30, and between second aperture 35 and second
end 32, of
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absorber element 24. Support screen 31 also supports the centrifugal force
loading of the
adsorbent.
As shown in Fig. 3, stator 14 is a pressure housing including an outer
cylindrical shell
or first valve stator 40 outside the annular rotor 1 l, and an inner
cylindrical shell or second
valve stator 41 inside the annular rotor 11. Outer shell 40 carnes axially
extending strip seals
(e.g. 42 and 43) sealingly engaged with first valve surface 21, while inner
shell 41 carries
axially extending strip seals (e.g. 44 and 45) sealingly engaged with second
valve surface 23.
The azimuthal sealing width of the strip seals is greater than the diameters
or azimuthal
widths of the first and second apertures 34 and 35 opening through the first
and second valve
surfaces.
A set of first compartments in the outer shell each open in an angular sector
to the
first valve surface, and each provide fluid communication between its angular
sector of the
first valve surface and a manifold external to the module. The angular sectors
of the
compartments are much wider than the angular separation of the absorber
elements. The first
compartments are separated on the first sealing surface by the strip seals
(e.g. 42).
Proceeding clockwise in Fig. 3, in the direction of rotor rotation, a first
feed pressurization
compartment 46 communicates by conduit 47 to first feed pressurization
manifold 48, which
is maintained at a first intermediate feed pressure. Similarly, a second feed
pressurization
compartment 50 communicates to second feed pressurization manifold 51, which
is
maintained at a second intermediate feed pressure higher than the first
intermediate feed
pressure but less than the higher working pressure.
For greater generality, module 10 is shown with provision for sequential
admission of
two feed mixtures, the first feed gas having a lower concentration of the more
readily
adsorbed component relative to the second feed gas. First feed compartment 52
communicates to first feed manifold 53, which is maintained at substantially
the higher
working pressure. Likewise, second feed compartment 54 communicates to second
feed
manifold 55, which is maintained at substantially the higher working pressure.
A first
countercurrent blowdown compartment 56 communicates to first countercurrent
blowdown
manifold 57, which is maintained at a first countercurrent blowdown
intermediate pressure.
A second countercurrent blowdown compartment 58 communicates to second
countercurrent
blowdown manifold 59, which is maintained at a second countercurrent blowdown
intermediate pressure above the lower working pressure. A heavy product
compartment 60
communicates to heavy product exhaust manifold 61 which is maintained at
substantially the
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lower working pressure. It will be noted that compartment 58 is bounded by
strip seals 42
and 43, and similarly all the compartments are bounded and mutually isolated
by strip seals.
A set of second compartments in the inner shell each open in an angular sector
to the
second valve surface, and each provide fluid communication between its angular
sector of the
second valve surface and a manifold external to the module. The second
compartments are
separated on the second sealing surface by the strip seals (e.g. 44).
Proceeding clockwise in
Fig. 3, again in the direction of rotor rotation, light product compartment 70
communicates to
light product manifold 71, and receives light product gas at substantially the
higher working
pressure, less frictional pressure drops through the adsorbers and the first
and second orifices.
According to the angular extension of compartment 70 relative to compartments
52 and 54,
the light product may be obtained only from adsorbers simultaneously receiving
the first feed
gas from compartment 52, or from adsorbers receiving both the first and second
feed gases.
A first light reflux exit compartment 72 communicates to first light reflux
exit
manifold 73, which is maintained at a first light reflux exit pressure, here
substantially the
higher working pressure less frictional pressure drops. A first cocurrent
blowdown
compartment 74 (which is actually the second light reflux exit compartment),
communicates
to second light reflux exit manifold 75, which is maintained at a first
cocurrent blowdown
pressure less than the higher working pressure. A second cocurrent blowdown
compartment
or third light reflux exit compartment 76 communicates to third light reflux
exit manifold 77,
which is maintained at a second cocurrent blowdown pressure less than the
first cocurrent
blowdown pressure. A third cocurrent blowdown compartment or fourth light
reflux exit
compartment 78 communicates to fourth light reflux exit manifold 79, which is
maintained at
a third cocurrent blowdown pressure less than the second cocurrent blowdown
pressure.
A purge compartment 80 communicates to a fourth light reflux return manifold
81,
which supplies the fourth light reflux gas which has been expanded from the
third cocurrent
blowdown pressure to substantially the lower working pressure with an
allowance for
frictional pressure drops. The ordering of light reflux pressurization steps
is inverted from
the ordering or light reflux exit or cocurrent blowdown steps, so as to
maintain a desirable
"last out - first in" stratification of light reflux gas packets. Hence a
first light reflux
pressurization compartment 82 communicates to a third light reflux return
manifold 83,
which supplies the third light reflux gas which has been expanded from the
second cocurrent
blowdown pressure to a first light reflux pressurization pressure greater than
the lower
working pressure. A second light reflux pressurization compartment 84
communicates to a
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second light reflux return manifold 85, which supplies the second light reflux
gas which has
been expanded from the first cocurrent blowdown pressure to a second light
reflux
pressurization pressure greater than the first light reflux pressurization
pressure. Finally, a
third light reflux pressurization compartment 86 communicates to a first light
reflux return
manifold 87, which supplies the first light reflux gas which has been expanded
from
approximately the higher pressure to a third light reflux pressurization
pressure greater than
the second light reflux pressurization pressure, and in this example less than
the first feed
pressurization pressure.
Additional details are shown in Fig. 4. Conduits 88 connect first compartment
60 to
manifold 61, with multiple conduits providing for good axial flow distribution
in
compartment 60. Similarly, conduits 89 connect second compartment 80 to
manifold 81.
Stator 14 has base 90 with bearings 91 and 92. The annular rotor 11 is
supported on end disc
93, whose shaft 94 is supported by bearings 91 and 92. Motor 95 is coupled to
shaft 94 to
drive rotor 11. The rotor could alternatively rotate as an annular drum,
supported by rollers at
several angular positions about its rim and also driven at its rim so that no
shaft would be
required. A rim drive could be provided by a ring gear attached to the rotor,
or by a linear
electromagnetic motor whose stator would engage an arc of the rim. Outer
circumferential
seals 96 seal the ends of outer strip seals 42 and the edges of first valve
surface 21, while
inner circumferential seals 97 seal the ends of inner strip seals 44 and the
edges of second
valve surface 23. Rotor 11 has access plug 98 between outer wall 20 and inner
wall 22,
which provides access for installation and removal of the adsorbent in
adsorbers 24.
Figs. 5 and 6
Fig. 5 shows a typical PSA cycle which would be obtained using the gas
separation
system according to the invention, while Fig. 6 shows a similar PSA cycle with
heavy reflux
recompression of a portion of the first product gas to provide a second feed
gas to the
process.
In Figs. 5 and 6, the vertical axis 150 indicates the working pressure in the
adsorbers
and the pressures in the first and second compartments. Pressure drops due to
flow within the
absorber elements are neglected. The higher and lower working pressures are
respectively
indicated by dotted lines 151 and 152.
The horizontal axis 155 of Figs. 5 and 6 indicates time, with the PSA cycle
period
defined by the time interval between points 156 and 157. At times 156 and 157,
the working
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pressure in a particular absorber is pressure 158. Starting from time 156, the
cycle for a
particular absorber (e.g. 24) begins as the first aperture 34 of that absorber
is opened to the
first feed pressurization compartment 46, which is fed by first feed supply
means 160 at the
first intermediate feed pressure 161. The pressure in that absorber rises from
pressure 158 at
time 157 to the first intermediate feed pressure 161. Proceeding ahead, first
aperture passes
over a seal strip, first closing absorber 24 to compartment 46 and then
opening it to second
feed pressurization compartment 50 which is feed by second feed supply means
162 at the
second intermediate feed pressure 163. The absorber pressure rises to the
second
intermediate feed pressure.
First aperture 34 of absorber 24 is opened next to first feed compartment 52,
which is
maintained at substantially the higher pressure by a third feed supply means
165. Once the
absorber pressure has risen to substantially the higher working pressure, its
second aperture
35 (which has been closed to all second compartments since time 156) opens to
light product
compartment 70 and delivers light product 166.
In the cycle of Fig. 6, first aperture 34 of absorber 24 is opened next to
second feed
compartment 54, also maintained at substantially the higher pressure by a
fourth feed supply
means 167. In general, the fourth feed supply means supplies a second feed
gas, typically
richer in the more readily adsorbed component than the first feed gas provided
by the first,
second and third feed supply means. In the specific cycle illustrated in Fig.
6, the fourth feed
supply means 167 is a "heavy reflux" compressor, recompressing a portion of
the heavy
product back into the apparatus. In the cycle illustrated in Fig. 5, there is
no fourth feed
supply means, and compartment 54 could be eliminated or consolidated with
compartment 52
extended over a wider angular arc of the stator.
While feed gas is still being supplied to the first end of absorber 24 from
either
compartment 52 or 54, the second end of absorber 24 is closed to light product
compartment
70 and opens to first light reflux exit compartment 72 while delivering "light
reflux" gas
(enriched in the less readily adsorbed component, similar to second product
gas) to first light
reflux pressure let-down means (or expander) 170. The first aperture 34 of
absorber 24 is
then closed to all first compartments, while the second aperture 35 is opened
successively to
(a) second light reflux exit compartment 74, dropping the absorber pressure to
the first
cocurrent blowdown pressure 171 while delivering light reflux gas to second
light reflux
pressure letdown means 172, (b) third light reflux exit compartment 76,
dropping the
absorber pressure to the second cocurrent blowdown pressure 173 while
delivering light
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reflux gas to third light reflux pressure letdown means 174, and (c) fourth
light reflux exit
compartment 78, dropping the absorber pressure to the third cocurrent blowdown
pressure
175 while delivering light reflux gas to fourth light reflux pressure letdown
means 176.
Second aperture 35 is then closed for an interval, until the light reflux
return steps following
the countercurrent blowdown steps.
The light reflux pressure let-down means may be mechanical expanders or
expansion
stages for expansion energy recovery, or may be restrictor orifices or
throttle valves for
irreversible pressure let-down.
Either when the second aperture is closed after the final light reflux exit
step (as
shown in Figs. 5 and 6), or earlier while light reflux exit steps are still
underway, first
aperture 34 is opened to first countercurrent blowdown compartment 56,
dropping the
absorber pressure to the first countercurrent blowdown intermediate pressure
180 while
releasing "heavy" gas (enriched in the more strongly adsorbed component) to
first exhaust
means 181. Then, first aperture 34 is opened to second countercurrent blowdown
compartment 58, dropping the absorber pressure to the first countercurrent
blowdown
intermediate pressure 182 while releasing heavy gas to second exhaust means
183. Finally
reaching the lower working pressure, first aperture 34 is opened to heavy
product
compartment 60, dropping the absorber pressure to the lower pressure 152 while
releasing
heavy gas to third exhaust means 184. Once the absorber pressure has
substantially reached
the lower pressure while first aperture 34 is open to compartment 60, the
second aperture 35
opens to purge compartment 80, which receives fourth light reflux gas from
fourth light
reflux pressure let-down means 176 in order to displace more heavy gas into
first product
compartment 60.
In Fig. S, the heavy gas from the first, second and third exhaust means is
delivered as
the heavy product 185. In Fig. 6, this gas is partly released as the heavy
product 185, while
the balance is redirected as "heavy reflux" 187 to the heavy reflux compressor
as fourth feed
supply means 167. Just as light reflux enables an approach to high purity of
the less readily
adsorbed ("light") component in the light product, heavy reflux enables an
approach to high
purity of the more readily adsorbed ("heavy") component in the heavy product.
The absorber is then repressurized by light reflux gas after the first and
second
apertures close to compartments 60 and 80. In succession, while the first
aperture 34 remains
closed at least initially, (a) the second aperture 35 is opened to first light
reflux pressurization
compartment 82 to raise the absorber pressure to the first light reflux
pressurization pressure
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190 while receiving third light reflux gas from the third light reflux
pressure letdown means
174, (b) the second aperture 35 is opened to second light reflux
pressurization compartment
84 to raise the absorber pressure to the second light reflux pressurization
pressure 191 while
receiving second light reflux gas from the second light reflux pressure
letdown means 172,
and (c) the second aperture 35 is opened to third light reflux pressurization
compartment 86
to raise the absorber pressure to the third light reflux pressurization
pressure 192 while
receiving first light reflux gas from the first light reflux pressure letdown
means 170. Unless
feed pressurization has already been started while light reflux return for
light reflux
pressurization is still underway, the process (as based on Figs. 5 and 6)
begins feed
pressurization for the next cycle after time 157 as soon as the third light
reflux pressurization
step has been concluded.
The pressure variation waveform in each absorber would be a rectangular
staircase if
there were no throttling in the first and second valves. In order to provide
balanced
performance of the adsorbers, preferably all of the apertures are closely
identical to each
other.
The rate of pressure change in each pressurization or blowdown step will be
restricted
by throttling in ports (or in clearance or labyrinth sealing gaps) of the
first and second valve
means, or by throttling in the apertures at first and second ends of the
adsorbers, resulting in
the typical pressure waveform depicted in Figs. 5 and 6. Alternatively, the
apertures may be
opened slowly by the seal strips, to provide flow restriction throttling
between the apertures
and the seal strips, which may have a serrated edge (e.g. with notches or
tapered slits in the
edge of the seal strip) so that the apertures are only opened to full flow
gradually.
Excessively rapid rates of pressure change would subject the absorber to
mechanical stress,
while also causing flow transients which would tend to increase axial
dispersion of the
concentration wavefront in the absorber. Pulsations of flow and pressure are
minimized by
having a plurality of adsorbers simultaneously transiting each step of the
cycle, and by
providing enough volume in the function compartments and associated manifolds
so that they
act effectively as surge absorbers between the compression machinery and the
first and
second valve means.
It will be evident that the cycle could be generalized by having more or fewer
intermediate stages in each major step of feed pressurization, countercurrent
blowdown
exhaust, or light reflux. Furthermore, in air separation or air purification
applications, a stage
of feed pressurization (typically the first stage) could be performed by
equalization with
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atmosphere as an intermediate pressure of the cycle. Similarly, a stage of
countercurrent
blowdown could be performed by equalization with atmosphere as an intermediate
pressure
of the cycle.
F
Fig. 7 is a simplified schematic of a PSA system, in accordance with the
present
invention, for separating oxygen from air, using nitrogen-selective zeolite
adsorbents. The
light product is concentrated oxygen, while the heavy product is nitrogen-
enriched air usually
vented as waste. The cycle lower pressure 152 is nominally atmospheric
pressure. Feed air
is introduced through filter intake 200 to a feed compressor 201. The feed
compressor
includes compressor first stage 202, intercooler 203, compressor second stage
204, second
intercooler 205, compressor third stage 206, third intercooler 207, and
compressor fourth
stage 208. The feed compressor 201 as described may be a four stage axial
compressor or
centrifugal compressor with motor 209 as prime mover coupled by shaft 210, and
the
intercoolers are optional. With reference to Fig. 5, the feed compressor first
and second
stages are the first feed supply means 160, delivering feed gas at the first
intermediate feed
pressure 161 via conduit 212 and water condensate separator 213 to first feed
pressurization
manifold 48. Feed compressor third stage 206 is the second feed supply means
162,
delivering feed gas at the second intermediate feed pressure 163 via conduit
214 and water
condensate separator 215 to second feed pressurization manifold 51. Feed
compressor fourth
stage 208 is the third feed supply means 165, delivering feed gas at the
higher pressure 151
via conduit 216 and water condensate separator 217 to feed manifold 53. Light
product
oxygen flow is delivered from light product manifold 71 by conduit 218,
maintained at
substantially the higher pressure less frictional pressure drops.
The apparatus of Fig. 7 includes energy recovery expanders, including light
reflux
expander 220 (here including four stages) and countercurrent blowdown expander
221 (here
including two stages), coupled to feed compressor 201 by shaft 222. The
expander stages
may be provided for example as iadial inflow turbine stages, as full admission
axial turbine
stages with separate wheels, or as partial admission impulse turbine stages
combined in a
single wheel.
Light reflux gas from first light reflux exit manifold 73 flows at the higher
pressure
via conduit 224 and heater 225 to first light pressure letdown means 170 which
here is first
light reflux expander stage 226, and then flows at the third light reflux
pressurization pressure
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192 by conduit 227 to the first light reflux return manifold 87. Light reflux
gas from second
light reflux exit manifold 75 flows at the first cocurrent blowdown pressure
171 via conduit
228 and heater 225 to second light reflux pressure letdown means 172, here the
second
expander stage 230, and then flows at the second light reflux pressurization
pressure 191 by
S conduit 231 to the second light reflux return manifold 85. Light reflux gas
from third light
reflux exit manifold 77 flows at the second cocurrent blowdown pressure 173
via conduit 232
and heater 225 to third light reflux pressure letdown means 174, here the
third expander stage
234, and then flows at the first light reflux pressurization pressure 190 by
conduit 235 to the
third light reflux return manifold 83. Finally, light reflux gas from fourth
light reflux exit
manifold 79 flows at the third cocurrent blowdown pressure 175 via conduit 236
and heater
225 to fourth light reflux pressure letdown means 176, here the fourth light
reflux expander
stage 238, and then flows at substantially the lower pressure 152 by conduit
239 to the fourth
light reflux return manifold 81.
Heavy countercurrent blowdown gas from first countercurrent blowdown manifold
57
flows at first countercurrent blowdown intermediate pressure 180 by conduit
240 to heater
241 and thence to first stage 242 of the countercurrent blowdown expander 221
as first
exhaust means 181, and is discharged from the expander to exhaust manifold 243
at
substantially the lower pressure 152. Countercurrent blowdown gas from second
countercurrent blowdown manifold 59 flows at second countercurrent blowdown
intermediate pressure 182 by conduit 244 to heater 241 and thence to second
stage 245 of the
countercurrent blowdown expander 221 as second exhaust means 183, and is
discharged from
the expander to exhaust manifold 243 at substantially the lower pressure 152.
Finally, heavy
gas from heavy product exhaust manifold 61 flows by conduit 246 as third
exhaust means
184 to exhaust manifold 243 delivering the heavy product gas 185 to be vented
at
substantially the lower pressure 152.
Heaters 225 and 241 raise the temperatures of gases entering expanders 220 and
221,
thus augmenting the recovery of expansion energy and increasing the power
transmitted by
shaft 222 from expanders 220 and 221 to feed compressor 201, and reducing the
power
required from prime mover 209. While heaters 225 and 241 are means to provide
heat to the
expanders, intercoolers 203, 205 and 207 are means to remove heat from the
feed compressor
and serve to reduce the required power of the higher compressor stages. The
intercoolers
203, 205, 207 are optional features of the invention.
If light reflux heater 249 operates at a sufficiently high temperature so that
the exit
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temperature of the light reflux expansion stages is higher than the
temperature at which feed
gas is delivered to the feed manifolds by conduits 212, 214 and 216, the
temperature of the
second ends 35 of the adsorbers 24 may be higher than the temperature of their
first ends 34.
Hence, the adsorbers have a thermal gradient along the flow path, with higher
temperature at
their second end relative to the first end. This is an extension of the
principle of "thermally
coupled pressure swing adsorption" (TCPSA), introduced by Keefer in U.S.
Patent No.
4,702,903. Absorber rotor 11 then acts as a thermal rotary regenerator, as in
regenerative gas
turbine engines having a compressor 201 and an expander 220. Heat provided to
the PSA
process by heater 225 assists powering the process according to a regenerative
thermodynamic power cycle, similar to advanced regenerative gas turbine
engines
approximately realizing the Ericsson thermodynamic cycle with intercooling on
the
compression side and interstage heating on the expansion side. In the instance
of PSA
applied to oxygen separation from air, the total light reflux flow is much
less than the feed
flow because of the strong bulk adsorption of nitrogen. Accordingly the power
recoverable
from the expanders is much less than the power required by the compressor, but
will still
contribute significantly to enhanced efficiency of oxygen production.
If high energy efficiency is not of highest importance, the light reflux
expander stages
and the countercurrent blowdown expander stages may be replaced by restrictor
orifices or
throttle valves for pressure letdown. The schematic of Fig. 7 shows a single
shaft supporting
the compressor stages, the countercurrent blowdown or exhaust expander stages,
and the light
reflux stages, as well as coupling the compressor to the prime mover. However,
it should be
understood that separate shafts and even separate prime movers may be used for
the distinct
compression and expansion stages within the scope of the present invention.
Fig.8
Fig. 8 shows a radial flow rotary PSA module 300 in which the first and second
valve
surface 21, 23 are respectively provided as hard-faced ported surfaces on the
first and second
valve stators 40 and 41. Sliding seals 380 are provided on rotor 11 between
each adsorber
24 and its neighbours, to engage both valve surfaces 21, 23 in fluid sealing
contact. Seals
380 may have a wear surface of a suitable composite material based on PTFE or
carbon, and
should be compliantly mounted on rotor 11 so as to compensate for wear,
deflections and
misalignment. Ports 381 may be sized, particularly at the leading edge of each
compartment,
to provide controlled throttling for smooth pressure equalization between
adsorbers and that
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compartment, as each adsorber in turn is opened to that compartment.
Split stream vacuum pump 260 receives the countercurrent blowdown and exhaust
flow in three streams receiving exhaust gas at incrementally reduced pressures
from
countercurrent blowdown compartment 56, compartment 58 and compartment 60. The
combined exhaust gas is discharged as heavy product gas. In this example,
initial feed
pressurization is performed from atmosphere, so a first feed pressurization
conduit 382
admits feed air directly from inlet filter 200 to first feed pressurization
compartment 46 at
substantially atmospheric pressure. The first discharge port of feed
compressor 201 now
communicates to second feed pressurization compartment 50. The compressor is
shown as a
split stage machine with inlet 391, and three discharges 392, 393 and 394 at
incrementally
higher pressures.
To achieve light reflux pressure letdown with energy recovery, a split stream
light
reflux expander 220 is provided to provide pressure let-down of four light
reflux stages with
energy recovery. The light reflux expander 220 provides pressure let-down for
each of four
light reflux stages. The stages may optionally be compartmentalized within the
light reflux
expander 220 to minimize mixing of gas concentration between the stages. The
light product
purity will tend to decline from the light reflux stages of higher pressure to
those of lower
pressure, so that a desirable stratification of the light reflux can be
maintained if mixing is
avoided.
Light reflux expander 220 is coupled to drive light product pressure booster
compressor 396. Compressor 396 receives the light product from compartment 70,
and
delivers light product (compressed to a delivery pressure above the higher
pressure of the
PSA cycle) from delivery conduit 218. Since the light reflux and light product
are both
enriched oxygen streams of approximately the same purity, expander 220 and
light product
compressor 396 may be hermetically enclosed in a single housing similar to a
turbocharger.
Fi~9
Fig. 9 is an axial sectional view of an axial flow rotary PSA module 600 for
small
scale oxygen production. The view is taken through compartments 54 and 70 at
the higher
pressure, and compartments 60 and 80 at the lower pressure. The flow path in
adsorbers 24 is
now parallel to axis 601. A better understanding will be obtained from Figs.
10 and 1 l,
which are cross sections of module 600 in the planes respectively defined by
arrows 602 -
603 and 604 - 605.
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The adsorber rotor 11 contains the "N" adsorbers 24 in adsorber wheel 608, and
revolves between the first valve stator 40 and the second valve stator 41.
Compressed feed
air is supplied to compartment 54 as indicated by arrow SO1, while nitrogen
enriched exhaust
gas is exhausted from compartment 60 as indicated by arrow 502.
At the ends of rotor 11, circumferential seals 608 and 609 bound first sealing
face 21,
and circumferential seals 610 and 611 bound second sealing face 23. The
sealing faces are
flat discs. The circumferential seals also define the ends of seals between
the adsorbers, or
alternatively of dynamic seals in the sealing faces between the stator
compartments. Rotor 11
has a stub shaft 511 supported by bearing 512 in first bearing housing 513,
which is integral
with first valve stator 40. Second valve stator 41 has a stub shaft engaging
the rotor 11 with
guide bushing 612.
A flanged cover plate 615 is provided for structural connection and fluid
sealing
enclosure between the first valve stator 40 and the second valve stator 41.
Rotor 11 includes
seal Garner 618 attached at joint 619 to adsorber wheel 608, and extending
between the back
of second valve stator 41 and cover plate 615 to sealing face 621 which is
contacted by
dynamic seal 625. Seal 625 prevents contamination of the light product gas by
leakage from
chamber 626 adjacent the first valve sealing face 21 to chamber 627 adjacent
the second
valve sealing face 23.
Seal 625 needs to be tight against leakage that could compromise product
purity. By
manufacturing this seal to a smaller diameter than the valve faces outer
diameter, frictional
torque from this seal is greatly reduced than if this seal were at the full
rotor diameter. The
circumferential perimeter exposed to leakage is also reduced. As in Fig. 8, a
split stream
light reflux expander 220 with close-coupled light product compressor 396, may
be installed
inside the light valve stator.
Figs. 10 and 11
Fig. 10 shows the first valve face 21 of the axial flow rotary PSA module 600
shown
in Fig. 9, at section 602 - 603, with fluid connections to a split stream feed
compressor 201
and a split stream countercurrent blowdown expander 221. Fig. 11 shows the
second valve
face 23 of the axial flow rotary PSA module 600 shown in Fig. 9, at section
604 - 605, with
fluid connections to a split stream light reflux expander 220 and light
product booster
compressor 396.
Arrow 670 indicates the direction of rotation by absorber rotor 11. The open
area of
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valve face 21 ported to the feed and exhaust compartments is indicated by
clear angular
segments 46, 50, 52, 56, 58, 60 corresponding to those compartments, between
circumferential seals 608 and 609. The closed area of valve face 21 between
compartments is
indicated by cross-hatched sectors 675 and 676. Similarly, the open area of
valve face 23
ported to the light reflux exit and return compartments is indicated by clear
angular segments
70, 72, 74, 76, 78, 80, 82, 84, 86 corresponding to those compartments, while
the closed are
of valve face 23 between the light reflux and return compartments is indicated
by the cross-
hatched sectors.
Typical closed sector 675, shown in Fig. 10, provides a transition for an
absorber,
between being open to compartment 56 and open to compartment 58. Gradual
opening is
provided at the leading edges 677 and 678 of compartments, so as to achieve
gentle pressure
equalization of an absorber being opened to a new compartment. Much wider
closed sectors
(e.g. 676) are provided to substantially close flow to or from one end of the
adsorbers when
pressurization or blowdown is being performed from the other end.
Sealing between compartments at typical closed sectors (e.g. 675) may be
provided by
rubbing seals on either stator or rotor against a ported hard-faced sealing
counter face on the
opposing rotor or stator, or by narrow gap clearance seals on the stator with
the area of the
narrow sealing gap defined by the cross hatched area of the nominally closed
surface.
Rubbing seals may be provided as radial strip seals, with a self lubricating
solid material such
as suitable PTFE compounds or graphite, or as brush seals in which a tightly
packed brush of
compliant fibers rubs against the counter face.
If the rubbing seals are on the rotor (between adjacent adsorbers), cross-
hatched
sectors 675 and 676 would be non-ported portions of the hard-faced sealing
counter face on
the stator. If the rubbing seals are on the stator, the ported hard-faced
counter face is on the
rotor valve face. Those rubbing seals could be provided as full sector strips
for narrow closed
sectors (e.g. 675). For the wider closed sectors (e.g. 676), narrow radial
rubbing seals may be
used as the edges 678 and 679, and at intervals between those edges, to reduce
friction in
comparison with rubbing engagement across the full area of such wide sectors.
Clearance seals are attractive, especially for larger scale modules with a
very large
number "N" of adsorbers in parallel. The leakage discharge coefficient to or
from the
clearance gap varies according to the angular position of the absorber, thus
providing gentle
pressure equalization as desired. The clearance gap geometry is optimized in
typical
nominally closed sectors (e.g. 675) so that the leakage in the clearance gap
is mostly used for
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absorber pressure equalization, thus minimizing through leakage between
compartments.
Preferably, the clearance gap is tapered in such sectors 675 to widen the gap
toward
compartments being opened, so that the rate of pressure change in pressure
equalization is
close to linear and rubbing friction is reduced. For wide closed sectors (e.g.
676) the
clearance gap would be relatively narrow to minimize flows at that end of
adsorbers passing
through those sectors.
For all types of valve face seals described above, it is preferable that
consistent
performance be achieved over time, and that all "N" adsorbers experience the
same flow
pattern after all perturbations from seal imperfections. This consideration
favours placing
rubbing seals on the stator so that any imperfections are experienced
similarly by all
adsorbers. If the seals are mounted on the rotor between adsorbers, it is
preferable that they
are closely identical and highly reliable to avoid upsetting leakages between
adjacent
adsorbers.
To compensate for misalignment, thermal distortion, structural deflections and
wear
of seals and bearings, the sealing system should have a suitable self aligning
suspension.
Thus, rubbing seal or clearance seal elements may be supported on elastomeric
supports,
bellows or diaphragms to provide the self aligning suspension with static
sealing behind the
dynamic seal elements. Rubbing seals may be energized into sealing contact by
a
combination of elastic preload and gas pressure loading.
Clearance seals require extremely accurate gap control, which may be
established by
rubbing guides. However, gap control for blowdown compartments may also be
achieved
through a self regulating seal in which the correct gap is maintained by a
balance between gas
pressure in the gap of a clearance seal segment, and the pressures of adjacent
blowdown
compartments loading the seal behind that segment. For pressurization
compartments, gap
control may be achieved through a self regulating seal in which the correct
gap is maintained
by a balance between gas pressure in the gap of a clearance seal segment, and
an intermediate
pressure loading the seal behind that segment, with the intermediate pressure
being the
average of the pressure of the flow paths approaching the clearance seal
segment and the
pressure of flow paths leaving the clearance seal segment.
Figs. 12. 13 and 14
In order to use centrifugal or axial compression machinery with rotary PSA
devices
having a relatively smaller number of angularly discrete adsorbers, it is
necessary to provide
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means to stabilize the flow and pressure conditions at the first function
compartments to
which feed compression stages, countercurrent blowdown exhausters (expanders
or vacuum
pumps) are connected. It is also desirable to provide means to regulate flows
entering or
exiting the adsorbers through the valve faces during pressurization or
blowdown steps, so as
to prevent abrupt transient pressure changes and flow surges detrimental to
the process and
potentially damaging to the adsorbers.
Stabilization is provided within the invention in part by providing surge
absorbers or
equivalent volume in or communicating with each function compartment to which
compression stages, expander stages, or vacuum pump stages are coupled. The
surge
absorber volume includes the internal volume of the function compartment, any
additional
surge chamber communicating thereto, and the internal volume of conduits
communicating
between the compression machinery stages and the function compartment. The
said surge
absorber volume is preferably at least equal in volume to the total volume of
the adsorbers
communicating to that function compartment at any time. More preferably, the
surge
absorber volume is preferably at least twice as large in volume as the total
volume of the
adsorbers communicating to that function compartment at any time.
Further benefits of stabilization and transient flow regulation may be
achieved by
providing some first function compartments as primary function compartments
communicating to compression machinery stages and with surge absorber volume
cooperating therewith, and then also providing secondary function compartments
communicating to the primary function compartments through flow restrictors
cooperating
with the function compartments in the stator valve faces. The flow restrictors
may be fixed
orifices, adjustable throttle valves, pressure regulators, or differential
pressure regulators.
The function compartments cooperate in groups, so that one primary function
compartment
cooperates with one or a plurality of secondary compartments.
In Fig. 12, feed pressurization compartment 902 is a primary function
compartment
supplied by feed compressor stage 903 through surge absorber 904. Feed
pressurization
compartment 905 is a secondary function compartment supplied through flow
restrictor 906
from compartment 902.
Similarly, countercurrent blowdown compartment 912 is a primary function
compartment exhausting to expander stage 913 through surge absorber 914.
Countercurrent
blowdown pressurization compartment 915 is a secondary function compartment
supplied
through flow restrictor 916 from compartment 912. As will be appreciated, the
present
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invention may include a greater number of secondary function compartments than
that shown
in Fig. 12.
In Fig. 13, a vacuum-PSA system is shown, in which feed pressurization
compartment
922 is a primary function compartment supplied from atmosphere. Feed
pressurization
compartment 925 is a secondary function compartment supplied through flow
restrictor 926
from compartment 922. Feed pressurization compartment 935 is a secondary
function
compartment supplied through flow restrictors 926 and 936 from compartment
922. The
flow restrictors cooperate with the adsorbers to establish incremental
intermediate pressure
levels in the function compartments so that the pressure changes in the
adsorbers are small
and the corresponding pressurization flows are smooth.
Similarly, exhaust compartment 942 is a primary function compartment
exhausting to
vacuum pump stage 943 through surge absorber 944. Countercurrent blowdown
pressurization compartment 945 is a secondary function compartment supplied
through flow
restrictor 946 from compartment 942. As above, the vacuum-PSA system may
include a
greater number of secondary function compartments than that shown in Fig. 13.
Thus the
compression machinery (compressors and vacuum pumps) coupled to the primary
function
compartments can have a small number of stages separated by relatively wide
pressure
intervals, while pressurization and blowdown steps of the adsorbers are
conducted over a
much larger number of relatively narrow pressure intervals.
Fig. 14 shows a variation of the vacuum-PSA system shown in Fig. 13, in which
the
secondary function compartments and flow restrictors associated with each
primary function
compartment are finely divided to become almost a continuum. For each primary
function
compartment, a flow distributor is provided. The flow distributor comprises a
stack of
substantially parallel plates separated by narrow spacers (e.g. wire mesh) to
define flow
restriction channels between each adjacent pair of plates, with each flow
restriction channel
communicating with the primary function compartment and the first valve face.
At its
opening to the valve face, each flow restriction channel defines a secondary
function
compartment. The flow restriction channel are inclined relative to the valve
face so that the
length of the flow restriction channels from the primary function compartment
to the valve
face progressively increases in proportion to the angular separation of the
secondary function
compartment from the corresponding primary function compartment.
In Fig. 14, feed pressurization compartment 922 is a primary function
compartment
supplied from atmosphere and cooperating with flow distributor 950. The flow
distributor
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includes a plurality of parallel plates 951 with spacers which define narrow
flow restrictor
channels 952. Each flow restrictor channel 952 defines a secondary feed
pressurization
compartment 925 supplied by feed pressurization compartment 922. Flow
distributor 960
cooperates with feed supply compartment 961 fed from feed compressor 903
through surge
absorber and optional aftercooler 904.
Similarly, flow distributors 970 and 971 cooperate respectively with exhaust
compartment 942 and countercurent blowdown compartment 972, with the
compartments
942, 972 being exhausted respectively by vacuum pump stages 943 and 973. In
flow
distributor 970, flow restrictor channels 980 and secondary function
compartments 945 are
defined between parallel plates 981. The flow restrictors cooperating with the
adsorbers
establish the incremental intermediate pressure levels in the compartments so
that the
pressure change in the adsorbers and the corresponding pressurization flows
are smooth.
Thus the compression machinery (compressors and vacuum pumps) coupled to the
primary
function compartments can have a small number of stages separated by
relatively wide
pressure intervals, while pressurization and blowdown steps are conducted over
a smooth
pressure gradient whose profile is dictated by the flow distributors and the
gas capacity of the
adsorbers.
The present invention is defined by the claims appended hereto, with the
foregoing
description being illustrative of the preferred embodiments of the present
invention. Those of
ordinary skill may envisage certain additions, deletions or modifications to
the described
embodiments which, although not explicitly disclosed herein, do not depart
from the spirit or
scope of the invention as defined by the appended claims.
