Note: Descriptions are shown in the official language in which they were submitted.
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CHEMICAL REACTOR WITH PRESSURE SWING ADSORPTION
FIELD OF THE INVENTION
The invention relates to chemical reactions conducted in the gas or vapour
phase. Reactions may be exothermic or endothermic, and may be conducted over a
catalyst or under an appropriate form of external excitation to stimulate the
reaction.
Some particular applications include ammonia synthesis, methanol synthesis,
conversion of natural gas or syngas to liquid fuels, hydrogenation and
dehydrogenation reactions, and controlled oxidation reactions.
BACKGROUND OF THE INVENTION
Fundamental problems in the chemical process industry include management
of reaction equilibrium and kinetics to achieve high conversion with desired
selectivity
under moderate reaction conditions, and management of the heat of reaction to
control
reaction temperature and to achieve high energy efficiency.
Typically, single pass conversion of the feed reactants) is incomplete because
of equilibrium limitations. It is then often necessary to provide a separation
system
to extract useful products from the reactor effluent, and then to recycle
unconsumed
reactants to the reactor inlet. The prior art provides known separation
processes
based on condensation, distillation, membrane permeation, absorption, and
adsorption. In most cases, these prior art separation processes are
incompatible with
the operating temperature of the reaction itself. Most conventional separation
processes operate at ambient or sub-ambient temperature, while the reaction
operates
at elevated temperature so that costly heat exchangers are required for the
recycle
loop.
High temperatures generally promote good reaction rates, but shift the
equilibrium of exothermic reactions toward lower conversion. The high cost of
heat
exchangers, recycle compressors, and other auxiliary equipment then
incentivizes
operation at relatively severe pressure or temperature reaction conditions to
minimize
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the need for recycle. In the typical exothermic reaction example of ammonia
synthesis, satisfactory conversion is achieved by forcing the equilibrium with
high
pressure operation, while product separation from the recycle loop is achieved
by
condensation usually after refrigeration.
Important applications exist where the separation of carbon dioxide is desired
at elevated temperature from a reactive gas mixture containing steam, or where
such
separation could greatly enhance process efficiency, simplicity and economics.
An
important example is hydrogen production by steam reforming of natural gas.
Several prior art processes have proposed sorption to remove carbon dioxide
from
reacting mixtures of steam and methane in order to drive the steam reforming
and
water gas shift reaction equilibria in order to produce moderately pure
hydrogen at
high conversion. Use of lime as a thermally regenerated sorbent in a fluidized
bed
reactor was proposed by Brun-Tsekhovoi et al, "The Process of Catalytic Steam-
Reforming of Hydrocarbons in the Presence of Carbon Dioxide Acceptor",
Hydrogen
Energy Progress VII, Proceedings of the World Hydrogen Energy Conference,
Pergamon Press, p. 885 (1988). More recently, fixed bed pressure swing
adsorption
reactor processes for steam methane reforming have been developed by Gaffney
et
al (U.S. Patent No. 5,917,136) using modified alumina adsorbents, and by J.R.
Hufton, S.G. Mayorga and S. Sircar ("Sorption Enhanced Reaction Process for
Hydrogen Production", AIChEJ 45, 248 (1999)) using mixed metal oxides derived
from hydrotalcite and promoted with potasssium carbonate.
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.
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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. 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.
SUMMARY OF THE INVENTION
The present invention integrates the chemical reactor with pressure swing
adsorption as the product/reactant separation process.
The present invention conducts a chemical reaction in cooperation with a
rotary module for pressure swing adsorption separation of the reaction
products)
from the reactant(s), with high energy efficiency and with compact machinery
of low
capital cost.
The inventive apparatus includes a rotary module 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
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adsorbed component, and a light product gas enriched in the less readily
adsorbed
component and depleted in the more readily adsorbed component. The apparatus
typically includes axial or 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 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 reflux" 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 include multistage axial or
centrifugal compressors and expanders.
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
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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,
5 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 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.
A preferred way to provide a large number of adsorbers in a mechanically
simple PSA module is to instal 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
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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 principle of using compression and expansion machinery (with
compression performed predominantly in communication with the first ends of
the
adsorbers, and expansion energy recovery performed on cocurrent blowdown from
the second ends of the adsorbers) to generate a high performance PSA cycle is
referred to as "Thermally Coupled Pressure Swing Adsorption" or TCPSA, because
of the inherent heat pumping aspect resulting from a close mechanical analogy
to
Stirling or Ericsson cycle thermodynamic engines.
Energy recovery is performed by expansion of countercurrent blowdown gas
(when those steps are performed at superatmospheric pressure), and by
expansion
over the pressure letdown expanders between the light reflux exit and return
compartments. The present invention provides multistage or split stream
compression/expansion machinery for the multiple gas flows at multiple closely
spaced intermediate pressure levels, enabled by the present PSA process. The
multistage machinery may be provided as separate machines operating in series
or
parallel, or preferably as multiple impellers in tandem on a single shaft
within a
single casing. Heat exchangers may be optionally provided as compression
intercoolers to reject heat of compression, and as heaters to heat either (or
both) the
countercurrent blowdown gas or the light reflux gas about to be expanded. The
heater may be provided with heat from an external source, or may use heat of
compression from the compression intercoolers as another mode of energy
recovery
within the apparatus and process of the invention. Heat may also be provided
to or
removed the adsorbers by providing heat exchange surfaces within the rotor.
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.
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Fig. 3 shows a sectional view of a rotary module configured for radial flow.
Fig. 4 shows a sectional view of a rotary module configured for axial flow.
Fig. 5 shows sectional views of the first valve face, the adsorbers, and the
second valve face of the rotary module of Fig. 4.
Fig. 6 is a simplified schematic of an apparatus for conducting an exothermic
reaction, with the chemical reaction performed within a zone of the adsorbers
in the
rotary module.
Fig. 7 is a simplified schematic of an apparatus for conducting an exothermic
reaction, with the chemical reaction performed in reactors external to the
rotary
module.
Fig. 8 is a simplified schematic of an apparatus for conducting an endothermic
reaction, with the chemical reaction performed in reactors external to the
rotary
module.
Fig. 9 is simplified schematic of an apparatus for conducting an endothermic
reaction in the example of steam methane reforming, with the reaction
performed
within the adsorbers of the rotary module and with heat exchange to the
adsorbers.
Fig. 10 is a sectioned view of the rotary module of the apparatus of Figure 9.
Fig. 11 shows a cross-section of a single absorber of the rotary module of
Fig.
10.
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DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Figures 1 and 2
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 3
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 3 are first
valve
means 7 and second valve means 8. Arrow 9 indicates the direction of
progression
of the adsorbers 3 in being connected to ports of the first and second valve
means 7,
8 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 3600
section about its rotary axis so that rotation causes the adsorbers 3 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 3600 /M. This has the disadvantage of
greater complexity of fluid connections to the first and second valve means 7,
8, 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 180 ° 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 3 is displaced in
phase
by a time interval of T/N. In Fig. 2 the vertical axis 10 indicates the
working
pressure in an adsorber element 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 the first adsorber 3 on the left in Figure 1 is pressure 18.
Starting from
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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 adsorber 3 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 11 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
8 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 24, the second end 6 is next closed to light product delivery conduit
25, and
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 30 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.
The first end 5 of adsorber 3 is then closed by the first valve means 7, while
the
second end 6 is opened successively by the second valve means 8 to (a) drop
the
adsorber pressure to the first cocurrent blowdown pressure 32 while delivering
light
reflux gas by conduit 33 to second light reflux pressure letdown means 34, (b)
drop
the adsorber pressure to the second cocurrent blowdown pressure 36 while
delivering
light reflux gas by conduit 37 to third light reflux pressure letdown means
38, and (c)
drop the adsorber pressure to the third cocurrent blowdown pressure 40 while
delivering light reflux gas by conduit 41 to fourth light reflux pressure
letdown means
42. Second end 6 is then closed for an interval, until the light reflux return
steps
following the countercurrent blowdown steps.
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The light reflux pressure let-down means 30, 34, 38, 42 may be mechanical
expansion stages for expansion energy recovery, or may be restrictor orifices
or
throttle valves for irreversible pressure let-down.
5 Either when the second end 6 is closed after the final light reflux exit
step (as shown
in Fig. 2), or earlier while light reflux 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
46.
10 Next, first end 5 is opened to second exhaust means 50, dropping the
adsorber
pressure to the second countercurrent blowdown intermediate pressure 52 while
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 54. The heavy gas from the first, second and third
exhaust
46, 50, 54 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 3 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,
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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 pressurization is still underway, the process begins feed
pressurization
for the next cycle after time 17 as soon as the third light reflux
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 3 simultaneously, and may have a restricted entrance 74
so as
to provide a gradual throttling equalization of pressure as each adsorber 3 is
opened
to feed compartment 72. First feed pressurization compartment 72 is fed by
conduit
70, second feed pressurization compartment 75 is fed by conduit 76, and feed
production supply compartment 77 is fed by conduit 78.
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 3 simultaneously, and may have a restricted
entrance 84
so as to provide a gradual throttling equalization of pressure as each
adsorber 3 is
opened to exhaust compartment 82. To label the exhaust compartments, first
countercurrent blowdown exhaust compartment 82 exhausts to conduit 80, a
second
countercurrent blowdown exhaust compartment 61 exhausts to conduit 63, and a
purge exhaust compartment 65 exhausts to conduit 67.
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 reflux pressure letdown means (e.g. 34), the light reflux flow
is delivered by a conduit 90 through an optional surge absorber chamber 91
from a
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light reflux exit compartment 92 opening to a light reflux exit port 93 in
second valve
means 8. Light reflux 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 3 is opened to light
reflux exit
compartment 92.
From each light reflux pressure letdown means (e.g. 34), the light reflux flow
is delivered by a conduit 95 through an optional surge absorber chamber 96 to
a light
reflux entrance compartment 97 opening to a light reflux entrance port 98 in
second
valve means 8. Light reflux exit compartment 97 may be open to several
adsorbers
3 simultaneously, and may have a restricted entrance 99 so as to provide a
gradual
throttling equalization of pressure as each adsorber 3 is opened to light
reflux
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 7, 8,
or by throttling in the ports at first and second ends 5, 6, of the adsorbers
3, resulting
in the typical pressure waveform depicted in Fig. 2. Excessively rapid rates
of
pressure change would subject the adsorber 3 to mechanical stress, while also
causing
flow transients which would tend to increase axial dispersion of the
concentration
wavefront in the adsorber 3. 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 7, 8.
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 light reflux. In particular, adsorber
pressurization could be achieved entirely by feed pressurization (or by
pressurization
with a component of the feed), or by light reflux represssuization.
Furthermore, the
length of the steps may be changed readily by changing the angular width of
the
ports. Thus, it may be desirable to extend the duration of the production and
purge
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steps at respectively the higher and lower pressures of the process in order
to reduce
pressure drop in the adsorbers 3 during those steps. Conversely, relatively
gradual
pressurization and blowdown steps may be desirable to overcome kinetic
constraints
or mechanical stress limitations.
The first end 5 and second end 6 of the adsorbers 3 will be maintained at
temperatures Tl and T2 respectively.
Fi ug re 3
Fig. 3 shows a sectional view of a rotary module 100 configured for radial
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 3. 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 72, a second first
feed
pressurization supply compartment 75, a feed production supply compartment 77
at
substantially the higher pressure, a first countercurrent blowdown exhaust
compartment 82, a second countercurrent blowdown exhaust compartment 61, and
a purge exhaust compartment 65 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 87 at
substantially the higher pressure, a first light reflux exit compartment which
is here
simply the downstream end of compartment 87 delivering gas to conduit 25, a
second
light reflux exit compartment 92 delivering gas to conduit 33, third and
fourth light
reflux exit compartments delivering gas to conduits 37 and 41 respectively, a
fourth
light reflux return compartment 97 receiving purge gas from conduit 55 at
substantially the lower pressure, a third light reflux return compartment
receiving gas
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from conduit 59, a second light reflux return compartment receiving gas from
conduit
61, and a first light reflux return compartment receiving gas from conduit 63.
The
angular spacing of ports communicating to the compartments in the first and
second
valve stators 108, 105 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 the in sealing faces 117 and 119 is achieved by clearance seals.
The
clearance seals are provided as slippers 130 attached to the first and second
valve
stators 108, 105 by partitions 131. Partitions 131 provide static sealing
between
adjacent compartments. Slippers 130 engage the sealing faces 107, 119 with
narrow
fluid sealing clearances, which also provide throttling of gas flows between
the
adsorbers 3 and functional compartments in each pressure-changing step, so
that each
adsorber may smoothly equalize in pressure to the pressure of the next
functional
compartment about to be opened to that adsorber 3. In addition to the
functional
compartments, static pressure balancing compartments (e.g. 132 and 133) are
provided behind some clearance seal slippers 130. The static pressure
balancing
compartments are disposed in angular sectors of the first and second valve
stators
108, 105 not used as functional compartments, in order to establish a
controlled
pressure distribution behind the clearance slippers 130 so as to maintain
their positive
sealing engagement without excessive contact pressure and consequent friction.
Figures 4 and 5
Fig. 4 shows a sectional view of a rotary module 200 configured for axial
flow, while Fig. 5 shows sectional views of the first valve face, the
adsorbers 3, and
the second valve face of the rotary module of Fig. 4. 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 3. Sections 200A, 200B and 200C are cross sections of module 200
in
the planes respectively defined by arrows 202 - 203, 204 - 205, and 206 - 207.
Fig.
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4 is an axial section of module 200 through compartments 77 and 87 at the
higher
pressure, and compartments 65 and 97 at the lower pressure. The adsorber rotor
2
contains the "N" adsorbers 3 in adsorber wheel 208, and revolves within stator
103.
5 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
10 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.
Section 200A shows the first valve face of embodiment 200 of Fig. 4, at
section 202 -
203, with fluid connections to feed and to countercurrent blowdown. Arrow 270
15 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 77, 82, 61, 65, 72, 75, and 77 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 between two adjacent
compartments. 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 3
when
pressurization or blowdown is being performed from the other end.
Section 200 C is the second valve face of embodiment 200 of Fig. 4, at section
206
207. Similar principles and alternatives apply to radial flow and axial flow
geometries, respectively sealing on cylindrical or disc faces.
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Section 200 B is an adsorber wheel configuration for the embodiment of Fig. 4,
at
section 204 - 205. The adsorber configuration 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 which is laminated
of
adsorbent sheets 282, with spacers 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 3.
The adsorbent sheets 282 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. Typical adsorbents include zeolites, many
of which
are also active as catalysts for reactions of interest. 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 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 microns, and adsorber flow channel length
approximately
20 cm.
Fi ug re6
An apparatus 300 for conducting an exothermic reaction, in the example of
ammonia synthesis, conducts the reaction within a zone 301 adjacent the second
ends
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6 of the adsorbers 3 in the rotary module. Zone 301 has catalyst active for
stimulating the reaction in an extended portion of the adsorbers 3, with the
catalyst
preferably supported on sheets in a laminated parallel passage structure as
described
above for adsorbent sheets. The flow paths through the rotor include flow
channels
contacting adsorbent material, and the catalyst in zone 301 thereof.
The lower part of Fig. 6 shows the PSA cycle pressure pattern, coordinated
on horizontal time axis 305 with the angular sequence of the functional
compartments
in the first and second valve faces 7 and 8 as shown in the upper part of Fig.
6. The
vertical pressure axis 306 spans the PSA cycle lower pressure 307, upper
pressure
308, and an intermediate pressure 309.
The first end 5 of the adsorbers 3 are maintained at approximately a first
temperature T1 by heat exchanger means 310 cooperating with the conduits
communicating with the second function compartments. The second end 6 of the
adsorbers 3 are maintained at approximately a second temperature T2 by heat
exchanger means 311 cooperating with the conduits communicating with the
second
function compartments. In this embodiment, the reaction is conducted at
elevated
temperature, approximately temperature T2. A product of the reaction is
condensed
in a product separator 320 at a lower temperature, which may be approximately
temperature TI.
With a large temperature difference between Tl and T2, the adsorbers 3 must
support a corresponding temperature gradient along the flow path, and
consequently
cyclic regenerative heat exchange will take place between the cyclically
reversing gas
flow in the flow path and the heat capacity of solid material in the flow path
including
the adsorbent material, any adsorbent support material and reinforcement
thereof, and
any spacers associated with flow channels. It is therefore preferred to
incorporate
ample solid material heat capacity in the flow path, and with intimate thermal
contact
to the flow channels. In this embodiment the thermal conduction path in the
solid
material along the flow path would preferably be interrupted at frequent
intervals, so
as avoid a large thermal conductance of the solid material along the flow
path. It
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is also desirable to layer the adsorbent material with optimally selected
adsorbents in
different zones (e.g. adsorbent zones 328 and 329 divided by zone boundary
330) of
differing temperature.
Apparatus 300 includes a multistage compressor 340 with multiple inlet and
delivery ports for receiving gas enriched in the more adsorbed component from
conduits 80, 63 and 67, and compressing that gas back to conduits 80, 63 and
67.
The process thus includes withdrawing gas enriched in the more readily
adsorbed
component from the first valve face, compressing that gas to an increased
pressure,
and refluxing that gas to the first valve face and thence the flow paths at
the increased
pressure, so as to increase the concentration of the more readily adsorbed
component
adjacent the first valve face. Following a stage 341 of compressor 340, a
stream gas
enriched in the more readily adsorbed component is passed though condenser 320
from which a liquid product of reaction is delivered in conduit 342, a purge
stream
is optionally delivered by conduit 343 to remove any accumulating inert
components,
and an overhead stream is returned by conduit 344 to the inlet of the next
stage of
compressor 340 and thence to the first valve face, or else directly to the
first valve
face.
The apparatus 300 also includes a multistage expander for receiving gas
relatively enriched in the less readily adsorbed component from conduits 25,
33, 37
and 41 communicating to the second valve face, and expanding that gas in
parallel
streams for return to the second valve face by conduits 61, 59, 55 and 63
respectively .
The apparatus 300 includes a feed supply conduit 359 and an optional feed
compressor 360 for supplying a feed gas of the reactants(s) to one or more gas
streams entering the first valve face.
Compressor 340 is a heavy reflux compressor and expander 350 is a light
reflux expander for performing the PSA cycle. Heavy reflux compressor 340 and
feed compressor 360 may be coupled on a single shaft 361 to light reflux
expander
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350 and a prime mover 362. If the power output of expander 350 exceeds power
consumption of compressors 341 and 360, prime mover 362 may be replaced with a
generator or other mechanical load to absorb power usefully.
In the example of ammonia synthesis, the reactants are hydrogen and nitrogen
which react to produce ammonia, typically over a promoted iron catalyst at a
temperature T2 in the approximate range of 4000 C to SOOo C. A bench scale
apparatus, using a compression piston to provide the function of the heavy
reflux
compressor and using an expansion piston to provide the function of the light
reflux
compressor, was operated with a single granular adsorber in the mechanical
embodiment of U.S. Patent No. 4,702,903. The adsorber was loaded with reduced
iron catalyst 301, 13-X zeolite as the adsorbent in zone 309, and silica gel
as the
adsorbent in zone 308 according to the reference numerals of the present
invention.
The upper pressure of the PSA cycle was approximately 800 kPa, and the lower
pressure was approximately 400 kPa. The feed was a mixture of hydrogen and
nitrogen. Gas composition adjacent the second end 6 was approximately 70%
hydrogen, 28 % nitrogen, and 2 % ammonia. Composition of product (delivered as
vapour) from adj acent the second end was approximately 0 % hydrogen, 40 %o
nitrogen
and 60% ammonia. Hence the directly integrated reactor and PSA device was able
to produce and concentrate ammonia product, achieving 100 % conversion of feed
hydrogen, while shifting the reaction equilbrium so as present a low
concentration of
the product component over the catalyst so as to enhance the reaction rate of
this
exothermic reaction at remarkably low pressure.
The apparatus of the present invention enables practicable scale-up and
economic realization of a process similar to that of U.S. Patent No.
4,702,903, using
rotational rather than reciprocating machinery.
Figure 7
Embodiment 400 is another apparatus for conducting an exothermic reaction,
similar to embodiment 300 but with the chemical reaction performed in reactors
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external to the rotary module. This apparatus may also be applied to ammonia
synthesis.
Reactors 401, 402, 403 and 404 contain an appropriate catalyst, and are
5 interposed respectively in conduits 25, 33, 37 and 41 to receive gas
enriched in the
less readily adsorbed reactant components. It will be noted that the reactors
operate
at different pressures which are steady for each reactor. Optionally, reactors
402,
403 and 404 could be deleted so that the entire reaction is conducted in
reactor 401
at the higher pressure.
The exothermic heat of reaction is taken up as heat of expansion in the light
reflux expander 350, so as to maintain the desired reaction temperature T2,
while also
recovering that heat as work of mechanical expansion.
The lower part of Fig. 7 again shows the PSA cycle pressure pattern, as in
Fig. 6.
Figure 8
Embodiment 500 is an apparatus for conducting an endothermic reaction, with
the chemical reaction performed in reactors external to the rotary module. The
PSA
cycle pattern is shown in the lower part of Fig. 8. An example application is
ammonia dissociation to generate reducing gas as hydrogen.
Reactors 501, 502 and 503 are interposed in conduits 78, 76 and 70 supplying
gas to the first valve face 7. If desired, only one reactor (e.g. 501) could
be
provided. A feed gas containing a relatively more readily adsorbed reactant is
supplied to a stage of heavy reflux compressor 340 by infeed conduit 504,
which may
as needed include a vaporizer 505 to ensure that the reactants) are in the
vapour
phase. Gas enriched in a more readily adsorbed reactant component is withdrawn
as
heavy reflux gas by conduits 80, 63 and 67 from countercurrent blowdown and
exhaust compartments in the first valve face, and is thence delivered to inlet
ports of
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heavy reflux compressor stages. The heavy reflux gas is compressed and is
thereby
heated by the heat of compression. Optionally, the compressed heavy reflux gas
may
be further heated from an external source of higher temperature heat in heat
exchanger 510, before admission to reactors 501, 502 and 503 . In this
embodiment,
T2 is typically greater than T1, while the temperature of the compressed heavy
reflux
gas stream entering the reactors (after heavy reflux compression and heating,
and any
further heating by a heat exchanger 510) will be much greater than T2 (the
exit
temperature of the exothermic reactors) so that the endothermic heat of
reaction may
be carried into the reactors as sensible heat of the reactor feed.
Alternatively or
additionally, the endothermic reactors may be heated externally by a furnace
or by
other means known in the art.
A purified stream of gas enriched in the less readily adsorbed product
components is delivered at approximately the upper pressure by product
delivery
conduit 520. This stream would be hydrogen and nitrogen, completely purified
of
ammonia, in the example of ammonia dissociation. In that example, the present
invention provides the important advances of 100 % conversion of the feed,
delivery
of a purified product, and practicable operation of the catalytic reactor at
much
reduced temperature compared to the prior art, because the reactant is
concentrated
over the catalyst.
Figures 9, 10 and 11
Embodiment 600 is an apparatus for conducting an endothermic reaction, with
the chemical reaction performed within the rotary module which itself is a
heat
exchange reactor. An example application is steam methane reforming to produce
hydrogen from natural gas. A steam reforming catalyst (e.g. nickel or a
platinum
group metal supported on alumina) and a high temperature carbon dioxide
sorbent are
supported in the adsorbers 3. The carbon dioxide sorbent may be based on
potasssium carbonate promoted hydrotalcite as developed by J.R. Hufton, S.G.
Mayorga and S. Sircar ("Sorption Enhanced Reaction Process for Hydrogen
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Production", AIChEJ 45, 248 (1999)), or another high temperature carbon
dioxide
sorbent likewise effective in the presence of high steam partial pressure.
This
sorbent has good working capacity for carbon dioxide in the temperature range
of
400o C to SOOo C.
J
In the steam methane reforming application of embodiment 600, the working
temperature of the adsorbers may desirably be near the upper end of the
temperature
range of 400o C to SOOo C, and T1 and T2 may be substantially the same
temperature or moderately different with T2 > T1. The feed mixture of
desulphurized natural gas and steam is introduced at inlet 601 of multistage
feed
compressor 603. Preferably, the compressed feed mixture is delivered at
successively increasing pressures by conduits 70, 76 and 78 to the first valve
face 7
at Tl after heating by heater 605 and recuperator 606.
On entering the adsorbers 3, the feed gas mixture contacts the catalyst which
stimulates reaction to hydrogen and carbon oxides. The reacting gas mixture
simultaneously contacts the sorbent which withdraws carbon dioxide, thus
driving the
further reaction of methane with steam to produce hydrogen and the
simultaneous
reaction of carbon monoxide with steam to produce more hydrogen and carbon
dioxide abstracted by the sorbent. Heat is provided to the strongly
endothermic
reaction by heat exchange within the adsorber, and in part from the exothermic
heat
of adsortion on the sorbent. Thus, product gas delivered through second valve
face
8 to conduit 25 will be hydrogen containing steam, a minor concentration of
unconverted methane, and only trace amounts of carbon dioxide and carbon
monoxide. The majority portion (other than a fraction for light reflux) of the
product in conduit 25 is withdrawn through steam condenser 610 and then
delivered
from conduit 611 as product for subsequent further purification e.g. by
pressure
swing adsorption at substantially ambient temperature if required.
Hydrogen-rich gas from conduits 25, 33, 37 and 41 is expanded as light reflux
gas through multistage expander 350 cooperating with heat exchanger 311. Low
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pressure steam may be added to this gas particularly at the lower pressure by
purge
stream infeed conduit 615 to assist the purge step.
Exhaust gas rich in carbon dioxide is withdrawn from valve face 7 at
successively lower pressures during countercurrent blowdown and purge steps
through
conduits 80, 63 and 67. These streams are cooled in recuperator 606 to assist
preheating the feed, and then further cooled by heat exchanger 620. The carbon
dioxide exhaust streams are then expanded or compressed as desired for
delivery
(after catalytic combustion to recover residual energy value from any slip of
methane,
carbon monoxide and hydrogen) as a second product stream of pure carbon
dioxide
or for disposal to atmosphere. In depicted embodiment 600, the exhaust gas
from
conduits 80, 63 and 67 is compressed by multistage compressor 622 and
delivered as
the second product by conduit 623. This embodiment allows the lower pressure
of the
process to be subatmospheric if the first stage 624 of compressor 622 is
operating as
a vacuum pump.
Heat must be supplied to the reactive adsorbers 3 in order to provide the
endothermic heat of reaction. This heat of reaction is less than the
endothermic
requirement for conventional steam reforming processes, because the more
endothermic reaction branch producing carbon monoxide is suppressed by carbon
dioxide sorption in the present process. Furthermore, a substantial fraction
(estimated
to be about 25 %) of the endothermic requirement during the higher pressure
production step is provided by the exothermic heat of carbon dioxide sorption,
while
this heat of sorption must of course be provided to the adsorbers 3 during
carbon
dioxide desorption at lower pressures.
Some or all of the endothermic heat requirement may be provided as sensible
heat provided to the incoming feed gas by heat exchangers 605 and 606, and to
the
incoming light reflux and purge gases by heat exchanger 311. Unlike the case
of
embodiment 300 of Fig. 6 where axial thermal conductivity in the adsorbers was
desirably low to reduce heat leakage, in this embodiment a high axial thermal
conductivity of the adsorbers is most desirable to improve heat transfer
between the
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reaction zone in the adsorbers and external heat exchangers 605, 606 and 311.
Furthermore, a high solid phase heat capacity is desirable within the reactive
adsorbers 3 to provide some heat storage and reduce reaction temperature
swings as
the endothermic reation rate will be highest during the higher pressure
production step
of the cycle. Use of a metallic foil or mesh support for the catalyst and
sorbent in
adsorbers 3 will provide the desired enhancements of axial thermal
conductivity and
solid phase heat capacity.
The heat demand of heat exchangers 605 and 311 may be made up by
combustion of residual fuel components in the second product delivered by
conduit
623. This fuel may be supplemented by methane either from the feedstock or
else
recovered as low pressure tail gas from downstream final purification of the
product
hydrogen delivered from conduit 611.
Some or all of the endothermic heat requirement may alternatively be provided
within adsorbers 3 by admitting a fraction of oxygen or air to purge stream
infeed
conduit 615 so that partial catalytic combustion of fuel components
(particularly
unreacted methane) takes place during the purge step. This is an "autothermal
reforming" process option. Direct combustion heat release during the purge
step at
the lower pressure will of course assist carbon dioxide desorption, while
storing the
remaining heat in the solid matrix of the adsorbers to provide endothermic
heat of
reaction during the next production step at the higher pressure.
Alternatively, heat may be provided by transverse heat transfer through
extended heat exchange surfaces constituting the walls of adsorbers 3. Rotary
module
2 is shown in Fig. 10, in the view corresponding to section 204 - 205 of Fig.
4, and
in a simplified configuration suitable for heat exchange to the adsorbers
within the
rotor 2. As in view 200B of Fig. 5, the adsorbers 3 are depicted as
rectangular flat
packs of laminated flat sheets. In order to provide heat exchange surfaces,
each
adsorber is contained in a jacket 630 whose external surfaces contact heat
exchange
channels 631, in turn bounded by walls 632 and 633. Heat exchange in channels
631
may in general be achieved by any means, including radiant heating, sensible
heat
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transfer from hot flue gas, latent heat transfer from condensing vapors,
sensible heat
transfer from liquid metals, etc. For heat transfer from a fluid, the
direction of
fluid flow may be either substantially parallel or transverse to the process
flow
direction within the adsorbers 3, or a combination thereof. Baffles may be
used to
5 reverse the direction of heat exchange fluid flow along the length or width
of the
adsorbers.
While Fig. 10 shows a single annular ring of adsorbers in rotor 2, more
complicated arrays (e.g. including two or more annular rings of adsorbers) may
be
10 considered within the invention.
Fig. 11 shows a detailed cross section of a single adsorber 3 within its
jacket
630. Adsorber 3 is a parallel pack of flat sheets 640, here supporting both
the
catalyst and the adsorbent in contact with the flow channels 641 defined by
spacers
15 642 between each adjacent pair of sheets 640. The jacket 630 includes side
walls
651 and 652 parallel to the sheets 640, and edge walls 653 and 654 terminating
the
edges of sheets 640. For good heat transfer with minimal transverse thermal
gradients across the adsorber pack, it is desirable that there be intimate
thermal
contact and ample thermal conductance between the sheets 640 and the edge
walls
20 653,654; and between the side walls 651,652, spacers and sheets.
The sheets 640 may in principle be comprised of any reinforcement material
compatible with reacting gas species and with the operating temperatures, e.g.
metal
foil, metal mesh, woven or nonwoven fabrics of glass or mineral fibers, or
mineral
25 or glass fiber papers. For desired thermal properties of high thermal
conductivity and
high heat capacity, metal foils or wire cloth are highly preferred. A mixture
of
catalyst (e.g. nickel on alumina support) and the carbon dioxide sorbent may
readily
be coated on a metallic support to form sheet 640, either with inorganic
binders or
self-bound by the sorbent. Spacers 642 may be provided as narrow metal strips
parallel to the flow direction in order to define channels 641, or
alternatively may be
formed by etching the channels through metal foil by photolithographic
techniques.
Jacket 630 may then be fabricated by diffusion bonding a stack of metal foils,
which
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are alternatingly those coated with sorbent and catalyst, and those etched to
form the
longitudinal flow channels 641. The edges of metal foils from both the
sorbent/catalyst coated sheets and the spacers are extended through the edge
walls
which will be formed by bonding the foils together with plates forming the
side walls
in fluid sealing contact.
It will be appreciated that the heat exchange reactor configuration of Figs.
10
and 11 could be applied to exothermic as well as endothermic reactions.
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.
20
