Note: Descriptions are shown in the official language in which they were submitted.
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ROTARY PRESSDRS SWING ADSORPTION APPARATUS
FIELD OF THE INVENTION
The invention relates to gas separations
conducted by pressure swing adsorption, and in
particular applications to oxygen or nitrogen separation
from air and to hydrogen purification. A particular
application is for oxygen enrichment to mobile fuel cell
power plants, for which efficient and compact machinery
will be required.
BACKGROUND OF THE INVENTION
Gas separation by pressure swing adsorption
(PSA) is achieved by coordinated 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 is elevated during
intervals of flow in a first direction through the
adsorbent bed from a first end to a second end of the
bed, and is reduced during intervals of flow in the
reverse direction. As the cycle is repeated, the less
readily adsorbed component is concentrated in the first
direction, while the more readily adsorbed component is
concentrated in the reverse direction.
A "light" product, depleted in the more
readily adsorbed component and enriched in the less
readily adsorbed component, is then delivered from the
second end of the bed. A "heavy" product enriched in
the more strongly adsorbed component is exhausted from
the first end of the bed. The light product is usually
the desired product to be purified by PSA, and the heavy
product often a waste product, as in the important
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examples of oxygen separation over nitrogen-selective
zeolite adsorbents and hydrogen purification. The heavy
product is a desired product in the example of nitrogen
separation over nitrogen-selective zeolite adsorbents.
Typically, the feed is admitted to the first end of a
bed and the second product delivered from the second end
of the bed when the pressure in that bed is elevated to
a higher working pressure, while the second product is
exhausted from the first end of the bed at a lower
working pressure which is the low pressure of the cycle.
The conventional process for gas separation by
pressure swing adsorption uses two or more 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, thus
establishing the changes of working pressure and flow
direction. This conventional pressure swing adsorption
process makes inefficient use of applied energy, because
of irreversible expansion over the valves over large
pressure differences while switching the adsorbent beds
between higher and lower pressures.
Keefer (U.S. Pat. No. 5,256,172) discloses the
use of expansion turbines to recover power by the
principle of thermally coupled pressure swing
adsorption, in which expansion energy of the PSA cycle
is recovered and heat may be applied directly through an
integrated regenerative thermodynamic cycle
(regenerative Brayton cycle, or a modified Ericsson
cycle) as a supplemental energy source to perform
pressure swing adsorption gas separations.
Schartz (PCT publication WO 94/04249), Firey
(U. S. Pat. No. 4,530,705), and Watson et al (U. S. Pat.
No. 5,656,067) disclose the use of expanders for
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partial recovery of energy from countercurrent blowdown
gas. In these prior art devices, the expanders,
compressors and vacuum pumps do not operate under steady
pressure conditions, since they withdraw gas from (or
supply gas to) one adsorbent bed at a time, while the
pressure in that adsorbent bed is changing as the gas
flow is withdrawn or supplied.
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 (U.S. Patent No.
5,133,784), and Petit et al (U. S. Patent No. 5,441,559)
disclose PSA devices using rotary adsorbent bed
configurations. Ports for multiple angularly separated
adsorbent beds mounted on a rotor assembly sweep past
fixed ports for feed admission, product delivery and
pressure equalization; with the relative rotation of the
ports providing the function of a rotary distributor
valve. Related devices are disclosed by Kagimoto et al
(U. S. Patent No. 5,248,325) and LaCava et al (U. S.
Patent No. 5,487,775). All of these prior art devices
use multiple adsorbent beds operating sequentially on
the same cycle, with multiport distributor rotary valves
for controlling gas flows to, from and between the
adsorbent beds.
Parallel passage adsorbers are disclosed by
Keefer (U.S. Patent No. 4,702,903 ) and by Davidson and
Lywood (U. S. Patent No. 4,758,253). High surface area
laminated adsorbers, with the adsorbent supported in
thin sheets separated by spacers to define flow channels
between adjacent sheets, formed either as stacked
assemblies or as spiral rolls, have been disclosed by
Keefer (U.S. Patent No. 4,968,329 and U.S. Patent No.
5,082,473).
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The present invention is related to U.S.
Patent Application No. 08/995,906, which uses rotary
modules to provide an inherently simple valuing function
for connecting a large number of cooperating adsorber
elements sequentially to pressure sources and exhaust
sinks at multiple narrowly spaced pressure intervals.
Pressure and flow pulsations are preferably minimised by
using a large number of equally angularly spaced
adsorber elements in each module, while scale-up to the
largest capacity ratings may be achieved by combining
multiple rotary modules in parallel with a large
compressor with a single prime mover.
DISCLOSURE OF INVENTION
The present invention is intended to enable
high frequency operation of pressure swing and vacuum
swing adsorption processes, with high energy efficiency
and with compact machinery of low capital cost. The
invention applies in particular to air separation.
The invention provides an apparatus for PSA
separation of a gas mixture containing a more readily
adsorbed component and a less readily adsorbed
component, with the more readily adsorbed component
being preferentially adsorbed from the feed gas mixture
by an adsorbent material under increase of pressure, so
as to separate from the gas mixture a heavy product gas
enriched in the more readily adsorbed component, and a
light product gas enriched in the less readily adsorbed
component and depleted in the more readily adsorbed
component. The apparatus includes centrifugal
compression machinery cooperating with one or multiple
PSA modules in parallel. Each PSA module comprises a
plurality of adsorbers, with each adsorber having a flow
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path contacting adsorbent material between first and
second ends of the flow path.
Each PSA module further has a first valve
means cooperating with the adsorbers to admit feed gas
to the first ends of the adsorbers, and to exhaust heavy
product gas from the first ends of the adsorbers. Each
PSA module also has a second valve means cooperating
with the adsorbers to deliver light product gas from the
second ends of the adsorbers, to withdraw light reflux
gas from the second ends of the adsorbers, and to return
light reflux gas to the second ends of the adsorbers.
The term "light reflex" refers to withdrawal of light
gas (enriched in the less readily adsorbed component)
from the second ends of adsorbers via the second valve
means, followed by pressure let-down and return of that
light gas to other adsorbers at a lower pressure via the
second valve means. The first and second valve means
are operated so as to define the steps of a PSA cycle
performed sequentially in each of the adsorbers, while
controlling the timings of flow at specified total
pressure levels between the adsorbers and the
compression machinery.
The PSA process of the invention establishes
the PSA cycle in each adsorber, within which the total
working pressure in each adsorber is cycled between a
higher pressure and a lower pressure of the PSA cycle.
The PSA process also provides a plurality of
intermediate pressures between the higher and lower
pressure. The compression machinery of the apparatus in
general includes a feed gas centrifugal compressor and a
second product gas exhauster. The exhauster would be an
expander (e. g. radial inflow turbine) when the lower
pressure is at least atmospheric pressure. The exhauster
would be a vacuum pump when the lower pressure is
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subatmospheric. A light reflux gas expander may also be
provided for energy recovery from light reflux pressure
let-down, and may for example be used to drive a light
product compressor.
In the present invention, the feed compressor
will typically supply feed gas, in several stages at
discrete intermediate pressures for feed pressurization
of the adsorbers as well as the higher pressure for
light product production, to the first valve means. The
exhauster will typically receive second product gas, in
several stages at discrete intermediate pressures for
countercurrent blowdown of the adsorbers as well as the
lower pressure, from the first valve means. The light
reflux expander may also perform pressure let-down on
several separate light reflux stages, sequentially drawn
from the second valve means at a set of discrete
intermediate pressure levels, and after expansion
returned to the second valve means at a lower set of
discrete intermediate pressure levels. Heat exchangers
may be provided to heat gas streams about to be
expanded, for thermally boosted energy recovery.
In order for the flowing gas streams entering
or exiting the compression machinery at each pressure
level to be substantially uniform in pressure and
velocity, each PSA module will preferably have a
sufficiently large number of adsorbers for several
adsorbers to be undergoing each step of the PSA cycle at
any moment. During pressurization and blowdown steps,
the several adsorbers passing through the step would be
in sequentially phased converging approach 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
being provided to the adsorbers in a pressurization
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step or withdrawn in a blowdown step by the 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.
As set forth in copending U.S. Patent
Application No. 08/995,906, a preferred way to provide a
large number of adsorbers in a mechanically simple PSA
module is to install those adsorbers as angularly spaced
elements in a rotor, whose opposed faces engaging across
sealing faces with a ported stator sealing faces will
provide the first and second valve means. By providing
a sufficient number of ports with suitable angular
spacing to accommodate each of the desired pressure
levels (higher, lower and intermediate) in each of the
first and second valve faces, a desired PSA cycle can be
achieved.
If a smaller number of adsorbers is used in
each PSA module, surge absorber chambers will be needed
to isolate each stage of the compression machinery from
excessive pulsations of flow and pressure. With
sufficiently large surge absorber chambers, flow and
pressure pulsations seen by the compression machinery
are again minimized.
The architecture of adsorbers has three main
hierarchial levels to be addressed:
1) the micropores where selective adsorption
takes place within the adsorbent media
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2) the macropores providing access into the
adsorbent media at approximately micron scale
from the flow channels, and desirably with
minimal mass transfer resistance so that
departures from equilbrium between the
micropores and the adjacent flow channels are
always minimized,
3) the flow channels between bodies of adsorbent
media, and along which a concentration
gradient is established by the process.
In PSA gas separation using zeolite molecular
sieve adsorbents, the conventional art has established a
remarkable, precisely organized architecture at the
atomic scale by which the micropores are defined by the
zeolite crystal framework. The micropores are at
approximately manometer scale, and are organized up to
the typical scale of zeolite crystallites of one or a
few microns.
In conventional PSA technology, the zeolite
crystallites are agglomerated into an amorphous
macroporous structure to form adsorbent pellets or
beads. The macropores are provided by the more or less
random network of interconnecting cavities between the
crystallites, allowing for space taken up by the binder.
The resulting macropores will have a rather high
tortuosity factor, multiplying the effective length of
the macropores by a factor of typically three to
increase mass transfer diffusional resistance
correspondingly.
The adsorbent beads are typically formed at
the scale of one or a few millimeters, and are loaded
into the adsorber containment vessel to form a packed
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bed. The flow channels are provided by the voidage
fraction between the beads, and typically have a length
of the order of one meter. The random assembly of the
packed bed, along with mixing events as the flow splits
and recombines around the beads, results in axial
dispersion which degrades the sharpness of the
concentration wavefront established by the separation
process. The packed bed also has inherently high
pressure drop in the flow channels.
While prior art adsorbent beds based on
zeolite molecular sieves are ideally organized at the
micropore scale of the zeolite crystal lattice, their
architecture is far from satisfactory at the scale of
the macropores (bead architecture) and the flow channels
(adsorber architecture). Packed beds of granular beads
are subject to pressure drop and fluidization
constraints which make it impracticable to operate with
small diameter beads, much smaller than 1 millimeter
diameter. The mass transfer macropore diffusional
resistance of relatively large beads, further
exacerbated by the macropore tortuosity factor, preclude
efficient sustained operation at PSA cycle frequencies
greater than approximately 10 cycles per minute.
Previous investigations by Reefer (U. S. Patent
No. 4,968,329 and U.S. Patent No. 5,082,473) have
established a much improved architecture of the
adsorbent media bodies and the flow channels, in which
the adsorbent is supported in the form of "adsorbent
sheets". The adsorbent sheets are thin sheets (either
as the adsorbent with a composite reinforcement, or as
an inert sheet or foil coated with the adsorbent), with
the flow channels established by spacers as parallel
channels between adjacent pairs of sheets. This
"adsorbent laminate" configuration has much lower
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pressure drop than packed beds, and avoids the
fluidization problem of packed beds. In experimental
adsorbers tested to date, the adsorbent sheets are in
the range of 100 to 175 microns thick. The channel
width between adjacent adsorbent sheets of the
experimental adsorbers has been in the range of 50~ to
100 of the adsorbent sheet thickness.
Intermediate between the microscale of the
zeolite crystallites and the macroscale of the laminate,
the mesoscale architecture of the macropore network
remains a challenge to be organized. The challenge is
to improve on the highly tortuous macropore network
provided by the amorphous structure of zeolite
crystallites cemented together by conventional binders.
Typical tortuosity factors in zeolite adsorbent pellets
are in the order of 3 to 4. Straightening the
macropores into a parallel bundle of straight pores
orthogonal to the external surface of the adsorbent
sheet would ideally result in a tortuosity factor of 1,
greatly reducing macropore diffusional resistance which
usually controls mass transfer. As the macropore
diffusional time constant is proportional to the
tortuosity factor and inversely proportional to the
adsorbent characteristic dimension (pellet diameter or
laminate adsorbent sheet thickness), a factor of 4
reduction in tortuosity is equivalent to a factor of 2
reduction in the characteristic dimension. Hence, for
equal macropore mass transfer resistance at the same
cycle frequency, the adsorbent characteristic dimension
may be increased to reduce adsorbent flow channel
surface area and consequently adsorbent manufacturing
cost, and also to reduce pressure drop in the flow
channels. Alternatively, the characteristic dimension
may be held the same, and the reduced tortuosity and
reduced macropore resistance may then be exploited to
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increase cycle frequency. This reduces the volume of the
adsorbent, and again reduces the installed cost of the
adsorbent.
Accordingly, an important aspect of the
invention is alignment of macropore channels for
improved high frequency PSA adsorbers.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a simplified schematic of a PSA
apparatus, and Fig. 2 shows a typical PSA cycle, in the
format to which the invention shall be applied.
Fig. 3 shows a radial flow rotary module for
VPSA oxygen production with a compressor and a vacuum
pump exhauster.
Figs. 4, 5, 6 and 7 show an axial flow rotary
module for PSA oxygen production.
Fig. 8 shows an alternative rotor with curved
adsorbers.
Fig. 9 shows the radial flow rotor of Fig. 3,
with the adsorbers as angular sectors.
Figs. 10 and 11 show a laminated sector
adsorber for the rotor of Fig. 9, with the adsorbent
sheets normal to the axis of the rotor.
Fig. 12 is a section of the rotor of Fig. 9,
using the adsorbers of Fig. 10.
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Fig. 13 shows an alternative laminated sector
adsorber for the rotor of Fig. 9, with the adsorbent
sheets parallel to the axis of the rotor.
Figs. 14 and 15 show an alternative radial
flow adsorber wheel configuration, with the adsorbent
sheets provided as annular discs.
Figs. 16 and 17 show details of adsorbent
laminate spacer configurations.
Fig. 18 shows layering of the adsorbents
selected between the first and second ends of the
adsorbers.
Fig. 19 and 20 show alternative spiral wrapped
axial flow laminate adsorbers.
Figs. 21, 22 and 23 show alternative spacers.
Fig. 24 shows an alternative laminated sector
adsorber for the rotor of Fig. 9, with the adsorbent
sheets parallel to the axis of the rotor, and with
radial tapering provided by interspersing adsorbent
sheets of differing width in the radial direction.
Fig. 25 shows a detail of adsorbent sheets
assembled from zeolite-coated strips of aluminum foil,
stacked so as to define macropore channels between
adjacent strips, the said macropore channels being
substantially straight and orthogonal to the surface of
the adsorbent sheet contacting the main flow channels.
MODES FOR CARRYING OUT THE INVENTION
Figs. 1 and 2
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Fig. 1 shows an elementary PSA apparatus 1
with an adsorber assembly 2 having a plurality of "N"
cooperating adsorbers 3 in parallel. Each adsorber has
a flow path 4 between first end 5 and second end 6 of
the adsorber 3, with adsorbent material contacting the
flow path. Cooperating with the adsorbers are first
valve means 7 and second valve means 8. Arrow 9
indicates the direction of progression of the adsorbers
in being connected to ports of the first and second
valve means as shown in Fig. 1. In the case of a
rotary adsorber, as in the preferred embodiments of the
invention, adsorber rotor 2 is shown in Fig. 1 unrolled
in a 360° section about its rotary axis so that rotation
causes the adsorbers to advance in the direction of
arrow 9 to undergo the cycle of Fig. 2.
The left hand edge 13 of the unrolled view of
rotor 2 returns to right hand edge 14 after rotation of
360°. It is also possible within the invention to have
an integral multiple of "M" groups of "N" adsorbers in a
single rotor 2, so that the angular extent for edge 13
to edge 14 is 360°/M. This has the disadvantage of
greater complexity of fluid connections to the first and
second valve means, but the advantages of slower
rotational speed (by a factor of "M" for the same PSA
cycle frequency) and a symmetric pressure and stress
distribution. With "M" - 2, Fig. 1 represents each 360°
side of a rotor according to the invention.
Fig. 2 shows the PSA cycle undergone
sequentially by each of the "N" adsorbers 3 over a cycle
period "T". The cycle in consecutive adsorbers is
displaced in phase by a time interval of T/N. In Fig. 2
the vertical axis 10 indicates the working pressure in
the adsorbers and the pressures in the first and second
compartments. Pressure drops due to flow within the
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adsorber elements are here neglected. The higher and
lower working pressures of the PSA process are
respectively indicated by dotted lines 11 and 12.
The horizontal axis 15 indicates time, with
the PSA cycle period defined by the time interval
between points 16 and 17. At times 16 and 17, the
working pressure in adsorber 3 is pressure 18. Starting
from time 16, the cycle begins as the first end 5 of
adsorber 3 is opened by the first valve means 7 to first
feed supply means 20 at the first intermediate feed
pressure 21. The pressure in that adsorber rises from
pressure 18 at time 17 to the first intermediate feed
pressure 21. Proceeding ahead, the first end 5 is
opened next to second feed supply means 22 at the second
intermediate feed pressure 23. The adsorber pressure
rises to the second intermediate feed pressure.
Then the first end 5 is opened to a third feed
supply means 24 at the higher pressure of the PSA
process. Once the adsorber pressure has risen to
substantially the higher working pressure, its second
end 6 is opened by the second valve means to light
product delivery conduit 25 to deliver purified light
product. While feed gas is still being supplied to the
first end of adsorber 3 from the third feed supply
means, the second end 6 is next closed to light product
delivery conduit 25, arid is opened to deliver "light
reflux" gas (enriched in the less readily adsorbed
component, similar to second product gas) by conduit 29
to first light reflux pressure let-down means 30. The
light reflux pressure let-down means may be an expander
with optional heat exchangers such as an inlet heater,
or a throttle orifice. All or some of the feed supply
means may be feed compression stages. One of the feed
supply means may be an external source, such as the
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ambient atmosphere for air purification or air
separation applications.
The first end 5 of adsorber 3 is then closed
by the first valve means, while the second end 6 is
opened successively by the second valve means to (a)
drop the adsorber pressure to the first cocurrent
blowdown pressure 32 while delivering light reflex gas
by conduit 33 to second light reflex pressure letdown
means 34, (b) drop the adsorber pressure to the second
cocurrent blowdown pressure 36 while delivering light
reflex gas by conduit 37 to third light reflex pressure
letdown means 38, and (c) drop the adsorber pressure to
the third cocurrent blowdown pressure 40 while
delivering light reflex gas by conduit 41 to fourth
light reflex pressure letdown means 42. Second end 6 is
then closed for an interval, until the light reflex
return steps following the countercurrent blowdown
steps.
The light reflex pressure let-down means may
be mechanical expansion stages for expansion energy
recovery, or may be restrictor orifices or throttle
valves for irreversible pressure let-down.
Either when the second end 6 is closed after
the final light reflex exit step (as shown in Fig. 2),
or earlier while light reflex exit steps are still
underway, first end 5 is opened to first exhaust means
46, dropping the adsorber pressure to the first
countercurrent blowdown intermediate pressure 48 while
releasing "heavy" gas (enriched in the more strongly
adsorbed component) to the first exhaust means. Next,
first end 5 is opened to second exhaust means 50,
dropping the adsorber pressure to the second
countercurrent blowdown intermediate pressure 52 while
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releasing "heavy" gas. Then first end 5 is opened to
third exhaust means 54, dropping the adsorber pressure
to the lower pressure 12 of the PSA process while
releasing "heavy" gas. Once the adsorber pressure has
substantially reached the lower pressure while first end
5 is open to the third exhaust means 54, the second end
6 is opened to receive fourth light reflux gas (as purge
gas) from fourth light reflux pressure let-down means 42
by conduit 55 in order to displace more heavy gas into
the third exhaust means. The heavy gas from the first,
second and third exhaust means may be delivered together
as the heavy product 56. All or some of the exhaust
means may be mechanical exhauster stages, alternatively
either expansion stages if the pressure is to be
reduced, or vacuum pumping stages if the pressure is to
be increased to ambient pressure, or exhaust compression
stages if the exhaust of second product is to be
delivered at an elevated pressure. An exhaust means may
also be provided by venting to an external sink, e.g.
the ambient atmosphere.
The adsorber is then repressurized by light
reflux gas after the first end 5 is closed. In
succession, the second end 6 is opened (a) to receive
light reflux gas by conduit 59 from the third light
reflux pressure letdown means 38 to raise the adsorber
pressure to the first light reflux pressurization
pressure 60, (b) to receive light reflux gas by conduit
61 from the second light reflux pressure letdown means
34 to raise the adsorber pressure to the second light
reflux pressurization pressure 62, and (c) to receive
light reflux gas by conduit 63 from the first light
reflux pressure letdown means 30 to raise the adsorber
pressure to the third light reflux pressurization
pressure. Unless feed pressurization has already been
started while light reflux return for light reflux
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pressurization is still underway, the process begins
feed pressurization for the next cycle after time 17 as
soon as the third light reflex pressurization step has
been concluded.
From each feed supply means (e.g. 20), the
feed flow is delivered by a conduit 70 through an
optional surge absorber chamber 71 to a feed compartment
72 opening to a feed port 73 in first valve means 7.
Feed compartment 72 may be open to several adsorbers
simultaneously, and may have a restricted entrance 74 so
as to provide a gradual throttling equalization of
pressure as each adsorber is opened to feed compartment
72.
To each exhaust means (e. g. 46), the exhaust
flow is delivered by a conduit 80 through an optional
surge absorber chamber 81 from an exhaust compartment 82
opening to an exhaust port 83 in first valve means 7.
Exhaust compartment 82 may be open to several adsorbers
simultaneously, and may have a restricted entrance 84 so
as to provide a gradual throttling equalization of
pressure as each adsorber is opened to exhaust
compartment 82.
To light product delivery conduit 25, the
light product is delivered through an optional surge
absorber chamber 86 from light product exit compartment
87 opening to a light product port 88 in second valve
means 8.
To each light reflex pressure letdown means
(e.g. 34), the light reflex flow is delivered by a
conduit 90 through an optional surge absorber chamber 91
from a light reflex exit compartment 92 opening to a
light reflex exit port 93 in second valve means 8.
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Light reflex exit compartment 92 may be open to several
adsorbers simultaneously, and may have a restricted
entrance 94 so as to provide a gradual throttling
equalization of pressure as each adsorber is opened to
light reflex exit compartment 92.
From each light reflex pressure letdown means
(e.g. 34), the light reflex flow is delivered by a
conduit 95 through an optional surge absorber chamber 96
to a light reflex entrance compartment 97 opening to a
light reflex entrance port 98 in second valve means 8.
Light reflex exit compartment 97 may be open to several
adsorbers simultaneously, and may have a restricted
entrance 99 so as to provide a gradual throttling
equalization of pressure as each adsorber is opened to
light reflex entrance compartment 97.
The rate of pressure change in each
pressurization or blowdown step may thus be restricted
by throttling in compartments of the first and second
valve means, or by throttling in the ports at first and
second ends of the adsorbers, resulting in the typical
pressure waveform depicted in Fig. 2. Excessively rapid
rates of pressure change would subject the adsorber to
mechanical stress, while also causing flow transients
which would tend to increase axial dispersion of the
concentration wavefront in the adsorber. Pulsations of
flow and pressure are minimized by having a plurality of
adsorbers simultaneously transiting each step of the
cycle, and/or by providing surge absorbers in the
conduits connecting to the first and second valve means.
It will be evident that the cycle shown in
Fig. 2 could be generalized by having more or fewer
intermediate stages in each major step of feed
pressurization, countercurrent blowdown exhaust, or
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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 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.
Figs-3
Fig 3 shows a simplified schematic of a VPSA
(vacuum pressure swing adsorption) air separation system
100, with a multistage or split stream centrifugal
compressor 101 and a multistage or split stream exhaust
vacuum pump 102. The rotary adsorber module 103
includes adsorber rotor 2, and a stator assembly
comprising a first valve stator 104 and a second valve
stator 105. Rotor 2 may be configured for radial flow
as suggested in Fig. 3, or for axial flow.
Rotor 2 contains the "N" adsorbers 3, with the
flow path here oriented radially between first end 5 and
second end 6 of the adsorbers. The adsorber first ends
5 open by apertures 106 to a sealing face 107 with the
first valve stator 108, which has ports 109 to define
the first valve means 7. First valve stator 108 has a
plurality of functional compartments in fluid
communication to sealing face 107 by ports 109,
including a first feed pressurization supply compartment
111, a second first feed pressurization supply
compartment 112, a feed production supply compartment
113 at substantially the higher pressure, a first
countercurrent blowdown exhaust compartment 114, a
second countercurrent blowdown exhaust compartment 115,
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and a purge exhaust compartment 116 at substantially the
lower pressure.
The adsorber second ends 6 open by apertures
118 to a sealing face 119 with the second valve stator
105 which has ports 120 to define the second valve means
8. Second valve stator 105 includes, with each
compartment in fluid communication to sealing face 119
by ports 120, a light product delivery compartment 121
at substantially the higher pressure, a first light
reflex exit compartment 122 which is here simply the
downstream end of compartment 121, a second light reflex
exit compartment 123, a third light reflex exit
compartment 124, a fourth light reflex exit compartment
125, a fourth light reflex return compartment 126 for
purge at substantially the lower pressure, a third light
reflex return compartment 127, a second light reflex
return compartment 128, and a first light reflex return
compartment 129. The angular spacing of ports
communicating to the compartments in the first and
second valve stators defines the timing of the PSA cycle
steps similar to the cycle of Fig. 2.
In this example, sealing faces 107 and 119 are
respectively defined by the outer and inner radii of the
annular rotor 2. Fluid sealing between the functional
compartments in sealing face is achieved by clearance
seals. The clearance seals are provided as slippers 130
attached to the first and second valve stators by
partitions 131. Partitions 131 provide static sealing
between adjacent compartments. Slippers 130 engage the
sealing faces with narrow fluid sealing clearances,
which also provide throttling of gas flows between the
adsorbers and functional compartments in each pressure-
changing step, so that each adsorber may smoothly
equalize in pressure to the pressure of the next
CA 02274286 1999-06-09
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functional compartment about to be opened to that
adsorber. In addition to the functional compartments,
static pressure balancing compartments (e.g. 132 and
133) are provided behind some clearance seal slippers.
The static pressure balancing compartments are disposed
in angular sectors of the first and second valve stators
not used as functional compartments, in order to
establish a controlled pressure distribution behind the
clearance slippers so as to maintain their positive
sealing engagement without excessive contact pressure
and consequent friction.
Apparatus 100 has a feed air inlet filter 140,
from which feed air is conveyed through optional
dehumidifier 141 and conduit 142 to feed compressor
inlet 143. In this example, the first intermediate
feed pressurization pressure is selected to be
substantially atmospheric pressure, so conduit 142 also
communicates to first feed pressurization compartment
111. The feed compressor 101 has a first discharge port
144 at the second intermediate feed pressurization
pressure communicating by conduit 145 and optional
dehumidifier 146 to compartment 112, and a second
discharge port 147 at substantially the higher pressure
of the cycle pressure communicating by conduit 148 and
optional dehumidifier 149 to compartment 213.
Exhaust vacuum pump 102 has a first inlet port
150 at substantially the lower pressure of the cycle in
fluid communication with exhaust compartment 116, a
second inlet port 152 at the second countercurrent
blowdown pressure in fluid communication with
compartment 115, and a third inlet port 154 at the first
countercurrent blowdown pressure in fluid communication
with compartment 114. Vacuum pump 102 compresses the
combined exhaust and countercurrent blowdown gas as the
CA 02274286 1999-06-09
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second product gas enriched in the more readily adsorbed
component to substantially atmospheric pressure, and
discharges the second product gas from discharge port
156.
In the option of light reflux pressure let-
down without energy recovery, throttle valves 160
provide pressure let-down for each of four light reflux
stages, respectively between light reflux exit and
return compartments 122 and 129, 123 and 128, 124 and
127, and 125 and 126 as illustrated. Actuator means 165
is provided to adjust the orifices of the throttle
valves.
Fig~s.4, 5, 6 and 7
Fig. 4 shows an axial flow rotary PSA module
200, particularly suitable for smaller scale oxygen
generation. The flow path in adsorbers 3 is now
parallel to axis 201. The steps of the process and
functional compartments are still in the same angular
relationship regardless of a radial or axial flow
direction in the adsorbers. Figs. 5, 6 and 7 are cross
sections of module 200 in the planes respectively
defined by arrows 202 - 203, 204 - 205, and 206 - 207.
Fig. 4 is an axial section of module 200 through
compartments 113 and 121 at the higher pressure, and
compartments 126 and 117 at the lower pressure. The
adsorber rotor 2 contains the "N" adsorbers 3 in
adsorber wheel 208, and revolves between the first valve
stator 103 and the second valve stator 105. Compressed
feed air is supplied to compartment 113 as indicated by
arrow 211, while nitrogen enriched exhaust gas is
exhausted from compartment 117 as indicated by arrow
212.
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At the ends of rotor 2, circumferential seals
215 and 216 bound first sealing face 107, and
circumferential seals 217 and 218 bound second sealing
face 119. The sealing faces are flat discs. The
circumferential seals also define the ends of clearance
slippers 130 in the sealing faces between the functional
compartments. Rotor 2 is supported by bearing 220 in
housing 225, which is integrally assembled with the
first and second valve stators. Rotor 2 is driven by rim
drive motor 230, which may have a friction, geared or
belt engagement with the outer rim of rotor 2.
Illustrating the option of light reflex
pressure letdown with energy recovery, a split stream
light reflex expander 240 is provided to provide
pressure let-down of four light reflex stages with
energy recovery. The light reflex expander provides
pressure let-down for each of four light reflex stages,
respectively between light reflex exit and return
compartments 122 and 129, 123 and 128, 124 and 127, and
125 and 126 as illustrated.
Light reflex expander 240 is coupled to a
light product pressure booster compressor 245 by drive
shaft 246. Compressor 245 receives the light product
from conduit 25, and delivers light product (compressed
to a delivery pressure above the higher pressure of the
PSA cycle) to delivery conduit 250. Since the light
reflex and light product are both enriched oxygen
streams of approximately the same purity, expander 240
and light product compressor 245 may be hermetically
enclosed in a single housing. This configuration of a
"turbocompressor" oxygen booster Without a separate
drive motor is advantageous, as a useful pressure boost
of the product oxygen can be achieved without an
external motor and corresponding shaft seals, and can
CA 02274286 1999-06-09
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also be very compact when designed to operate at very
high shaft speeds.
Fig. 5 shows the first valve face of
embodiment 200 of Fig. 3, at section 202 a 203, with
fluid connections to a multistage or split stream feed
compressor 101 and to a multistage or split stream
countercurrent blowdown expander 260 as in Fig. 4.
Arrow 270 indicates the direction of rotation
by adsorber rotor 2. The open area of valve face 107
ported to the feed and exhaust compartments is indicated
by clear angular segments 111 a 217 corresponding to
those functional compartments, between circumferential
seals 215 and 216. The substantially closed area of
valve face 107 between functional compartments is
indicated by cross-hatched sectors 275 and 276 which are
clearance slippers 130. Typical closed sector 275
provides a transition for an adsorber, between being
open to compartment 114 and open to compartment 115.
Gradual opening is provided by a tapering clearance
channel between the slipper and the sealing face, so as
to achieve gentle pressure equalization of an adsorber
being opened to a new compartment. Much wider closed
sectors (e. g. 276) 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.
Fig. 6 shows the second valve face of
embodiment 200 of Fig. 3, at section 204 a 205, with
fluid connections to a split stream light reflux
expander 240 and light product booster compressor 245 as
in Fig. 5. Fluid sealing principles and alternatives
are similar to those of Fig. 5. Similar principles and
alternatives apply to radial flow and axial flow
CA 02274286 1999-06-09
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geometries, respectively sealing on cylindrical or disc
faces .
Fig. 7 shows an adsorber wheel configuration
for the embodiment of Fig. 3, at section 206 - 207. The
adsorber configuration of Fig. 7 is similar to a radial
flow geometry shown in copending U.S. Patent Application
No. 08/995,906. Here, "N" - 72. The adsorbers 3 are
mounted between outer wall 280 and inner wall 281 of
adsorber wheel 208. Each adsorber comprises a
rectangular flat pack of adsorbent sheets 282, with
spacers 283 between the sheets to define flow channels
here in the axial direction. Separators 284 are
provided between the adsorbers to fill void space and
prevent leakage between the adsorbers.
The adsorbent sheets comprise a reinforcement
material, in preferred embodiments glass fibre, metal
foil or wire mesh, to which the adsorbent material is
attached with a suitable binder. For air separation to
produce enriched oxygen, typical adsorbents are X, A or
chabazite type zeolites, typically exchanged with
lithium, calcium, strontium, magnesium and/or other
cations, and with optimized silicon/aluminum ratios as
well known in the art. The zeolite crystals are bound
with silica, clay and other binders, or self-bound,
within the adsorbent sheet matrix.
Satisfactory adsorbent sheets have been made
by coating a slurry of zeolite crystals with binder
constituents onto the reinforcement material, with
successful examples including nonwoven fiber glass
scrims, woven metal fabrics, and expanded aluminum
foils. Spacers are provided by printing or embossing
the adsorbent sheet with a raised pattern, or by placing
a fabricated spacer between adjacent pairs of adsorbent
CA 02274286 1999-06-09
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sheets. Alternative satisfactory spacers have been
provided as woven metal screens, non-woven fiber glass
scrims, and metal foils with etched flow channels in a
photolithographic pattern.
Typical experimental sheet thicknesses have
been 150 microns, with spacer heights in the range of
100 to 150 microns, and adsorber flow channel length
approximately 20 cm. Using X type zeolites, excellent
performance has been achieved in oxygen separation from
air at PSA cycle frequencies in the range of 30 to 150
cycles per minute.
FiQ.8
Fig.8 shows an alternative configuration of
rotor 208, in which the adsorbers 3 are again formed of
a pack of rectangular adsorbent sheets with spacers, but
with the sheets here curved to circular arcs rather than
flat. Voids between the circularly curved adsorber
packs are filled by separators 484. Such circularly
curved adsorber packs may be made by forming the
adsorbent sheets with spacers in a spiral roll on a
circular cylindrical mandrel, and then slitting the
spiral roll longitudinally to obtain the desired packs.
Packing density could be further improved by forming the
adsorber packs to a spiral rather than circular curve,
for example by a pleating technique, or by
longitudinally cutting a spiral roll formed on a
noncircular mandrel whose shape in two to four identical
angular sectors defines the desired spiral.
Fig. 9
Fig. 9 shows an enlarged view of the radial
flow rotor 2 of Fig. 3. The adsorbers 3 are contained
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in trapezoidal angular sectors between radial partitions
301. Partitions 301 are attached to outer rotor shell
303 and inner rotor shell 304. Outer shell 303 engages
sealing face 107 and is perforated with apertures 106
communicating with the first ends 5 of adsorbers 3.
Inner shell 304 engages sealing face 119 and is
perforated with apertures 118 communicating with the
second ends 6 of adsorbers 3. An important advantage
of this geometry is the tapering of the adsorbers from
the first to second ends of the flow path, thus reducing
feed flow velocity and pressure drop adjacent the first
end 5.
Figs . 10 - 12
Figs. 10 and 11 show a laminated sector
adsorber for the rotor of Fig. 9, with the adsorbent
sheets normal to the axis of the rotor. The flat
adsorbent sheets 310 are separated by spacers 311 to
define flow channels 312 between first end 5 and second
end 6. Sheets 310 are on edges 320 to fit the
trapezoidal shape of adsorbers 3 between partitions 301.
In Fig. 11, radial spacers 325 define radially tapered
channels 312. Spacers 325 may be printed onto the
adsorbent sheets, and may have the structure shown in
Fig. 16. Alternative spacer geometries based on Figs.
17 or 21 a 23 may also be adapted by tapering to the
radial flow pattern required by the trapezoidal shape.
Fig. 12 shows an axial section of the radial flow rotor
of Fig. 9, with the sector adsorber of Fig. 10.
Fig. 13
Fig. 13 shows an alternative laminated sector
adsorber for the rotor of Fig. 9, with the adsorbent
sheets in radial planes parallel to the axis of the
CA 02274286 1999-06-09
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rotor. Rectangular adsorbent sheets 341 and/or the
spacers defining channels 342 are tapered so that the
combination of an adsorbent sheet and the adjacent
channel has a constant angular width.
Figs. 14 and 15
Figs. 14 and 15 show an alternative radial
flow adsorber wheel configuration, with the adsorbent
sheets provided as complete annular discs. As in Fig.
11, radial spacers 325 define flow channels 312. The
spacers of Fig. 15 substantially prevent transverse flow
between adjacent channels, which thus each define very
narrow adsorbers.
Figs. 16 and 17
Figs. 16 and 17 show details of adsorbent
laminate spacer configurations. In Fig. 16, spacer
ridges 331 are formed by calendaring or by printing one
of both sides of adsorbent sheets 310. Preferably,
spacers 331 are aligned between adjacent sheets so as to
provide compressive strength and stability.
In Fig. 17, the adsorbent is applied as
coating layers 360 to both sides of a support aluminum
foil 361 whose surface has been anodized for good
adhesion. Spacers 362 are printed or embossed on the
coated foil.
Fic~. 18
Fig. 18 shows layering of the adsorbents
selected between the first and second ends of the
adsorbers in a radial flow configuration. Similar
axial layering may be applied to axial flow embodiments
CA 02274286 1999-06-09
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of the invention. From first end 5 to second end 6 of
the adsorbers, the flow path passes through first zone
401, second zone 402 and third zone 403 of the
adsorbent. The first zone adsorbent may be alumina gel,
silica gel or 13-X zeolite for water vapour removal.
For air separation, the second zone adsorbent
may be highly lithium exchanged low silica X zeolite for
most efficient bulk nitrogen removal. The third zone
adsorbent may advantageously be magnesium, calcium or
strontium exchanged chabazite or low silica zeolite X or
zeolite A, for most efficient removal of nitrogen from
lower concentrations.
For hydrogen separation from syngas, the
second zone adsorbent may be 13-X zeolite for efficient
bulk carbon dioxide removal. The third zone adsorbent
may advantageously be calcium or strontium exchanged
chabazite or low silica X zeolite, for efficient removal
of carbon monoxide and any nitrogen.
The zones of different adsorbent composition
between the first and second ends of the adsorbent
elements may be provided by coating the adsorbent sheets
in bands of different composition prior to assembly of
the elements, or by assembling the elements from
separate sheets of the respective compositions so that
the gas flowing along the flow path encounters different
sheets coated with the respective composition between
the first and second ends.
Figs. 19 and 20
Figs. 19 and 20 show alternative spiral
wrapped axial flow laminate adsorbers. The Fig. 19
configuration of a spiral wound adsorber wheel with one
CA 02274286 1999-06-09
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or a few laminate sheets rolled around hub 411 is
suitable for axial flow only. The Fig. 20 configuration
of a steep spiral roll of a very large number of leaves
may in principle be used for either axial or radial
embodiments. Each of these geometries may use spacers
such as those of Fig. 16 to isolate the flow channels
against transverse flow, so that each flow channel is an
independent adsorber.
Figs. 21, 22 and 23
Figs. 21, 22 and 23 show alternative spacers
for use in adsorbers or adsorber sections not requiring
compartmentalization against transverse flow, for
example the embodiments of Figs. 7, 8 and 13. The
direction of flow is defined by arrow 501. In Fig. 21,
raised spacers 505 are applied by printing. In Figs. 22
and 23, woven spacers of wire screen are used. The mesh
of Fig. 23 is a Dutch weave, with fine stabilizing wires
531 and 532 criss-crossing heavier longitudinal spacer
wires 533 in the flow direction. Some or all of the
stabilizing wires may be replaced by polymer fibres
which are burnt out during adsorbent activation.
Fig. 24
Fig. 24 shows a angularly narrow portion of an
alternative laminated sector adsorber for the rotor of
Fig. 9, with a group of adsorbent sheets parallel to the
axis of the rotor, and with radial tapering provided by
interspersing adsorbent sheets of differing width in the
radial direction. Adsorbent sheets 601, 602, 603, 604,
605 and 606 have printed spacers 610 between adjacent
pairs of the sheets, to establish flow channels in the
radial direction between first end 5 and second end 6.
Spacers 610 are printed in a pattern such as that shown
CA 02274286 1999-06-09
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in Fig. 21. Vrhile some of the adsorbent sheets (601,
602, 603 and 604) extend the entire radial distance
between the first and second ends, others (605 and 606)
extend from the first end 5 only varying fractions of
the radial distance to the second end 6. Thus, sheet
606 has only approximately a third of the radially
extending width, and sheet 605 two thirds of the
radially extending width, as the other sheets 601 a 604.
Since the stack of adsorbent sheets is thicker at the
first end than at the second, it can be tapered by
appropriately selecting the fraction of sheets to have
reduced radial widths, so as to have an approximately
constant angular width between the first and second
ends.
In this embodiment, the sheets are flexible so
as to flex around the terminations 611 and 612 of sheets
606 and 605, so as to minimize disturbances of flow
distribution and channel pressure resistance adjacent
the sheet terminations.
Fig.25
Fig. 25 shows a detail portion of two adjacent
adsorbent sheets 701 and 702 assembled from zeolite-
coated strips of aluminum foil, stacked so as to define
macropore channels between adjacent strips, the said
macropore channels being substantially straight and
orthogonal to the surface of the adsorbent sheet
contacting the main flow channels.
An adsorbent sheet (e. g. 100 to 250 microns
thick) 701 may be assembled from strips 703 of aluminum
foil of approximately 12 microns thickness (equivalent
to cigarette wrapper foil). The foil is slit into
strips whose width is nominally equal to the final
CA 02274286 1999-06-09
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thickness of the sheet. The strips are preferably
anodized for good adhesion of zeolite, and are coated on
each side with a coating layer 704 or film of zeolite
crystals approximately 4 to 6 microns thick. The
zeolite crystals may be grown in situ from a zeolite
synthesis solution, as with the known art of depositing
crystalline zeolite films for membranes, although here
there is no concern about avoiding minor crevices or
pinholes through the zeolite film. Alternatively, the
zeolite coatings may be formed by conversion of a
metakaolin coating applied to the foil. Alternatively,
the zeolite crystals may be formed separately, and then
attached by binders (e.g. clay or silica) to the
anodized sheet. The strips are then stacked on each
other, with their edges in contact with parallel
longitudinal support members 706 for sheet 701 and
members 707 for sheet 702, orthogonal to the strips so
as to form the adsorbent sheets, constituted by the
stacked strips being retained by the parallel
longitudinal support members to retain the strips and
provide structural integrity. The adsorbent sheets
are then stacked to form a parallel passage adsorber,
with the parallel longitudinal support members serving
as spacers members defining flow channels with flow
direction indicated by arrows 708 and 709, orthogonal to
the strips and between adjacent pairs of adsorbent
sheets. Between each pair of adsorbent sheets 701 and
702, the parallel longitudinal support members 706 are
separated tranversely so as provide open flow channels,
while providing support to the zeolite coated strips 704
constituting each sheet.
The zeolite crystal coating on the aluminum
strips will have a granular surface texture, so that
sufficient voidage will exist between the contacting
zeolite surfaces of adjacent coated strips to provide
CA 02274286 1999-06-09
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direct fluid access from the flow channels. This
voidage between zeolite films of adjacent strips serves
as primary macropore channels, which are substantially
straight with minimal tortuosity, and are orthogonal to
the surface of the adsorbent sheets contacting the flow
channels.
while only preferred embodiments of the
invention have been described herein in detail, the
invention is not limited thereby and modifications can
be made within the scope of the attached claims.
