U.S. patent application number 12/014743 was filed with the patent office on 2009-01-08 for catalysts, in microchannel apparatus, and reactions using same.
This patent application is currently assigned to Velocys Corp.. Invention is credited to Terry J. Mazanec, Laura J. Silva, David P. Vanderwiel, Yong Wang.
Application Number | 20090010823 12/014743 |
Document ID | / |
Family ID | 32093444 |
Filed Date | 2009-01-08 |
United States Patent
Application |
20090010823 |
Kind Code |
A1 |
Mazanec; Terry J. ; et
al. |
January 8, 2009 |
Catalysts, in microchannel apparatus, and reactions using same
Abstract
The present invention provides new microreactor systems,
catalysts, and chemical processes. Methods of making novel
catalysts and reaction apparatus are also described.
Inventors: |
Mazanec; Terry J.; (Solon,
OH) ; Wang; Yong; (Richland, WA) ; Silva;
Laura J.; (Dublin, OH) ; Vanderwiel; David P.;
(Columbus, OH) |
Correspondence
Address: |
FRANK ROSENBERG
P.O. BOX 29230
SAN FRANCISCO
CA
94129-0230
US
|
Assignee: |
Velocys Corp.
Plain City
OH
|
Family ID: |
32093444 |
Appl. No.: |
12/014743 |
Filed: |
January 15, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10279089 |
Oct 22, 2002 |
7404936 |
|
|
12014743 |
|
|
|
|
Current U.S.
Class: |
422/211 ;
264/239; 264/49; 29/890; 427/532; 427/550 |
Current CPC
Class: |
B01J 2208/025 20130101;
B01J 2219/00873 20130101; Y02P 20/52 20151101; Y10T 29/49345
20150115; C01B 3/40 20130101; C01B 2203/0227 20130101; B01J
2219/00783 20130101; C01B 2203/0283 20130101; B01J 2219/00835
20130101; B01J 19/2485 20130101; B01J 19/0093 20130101; C01B 3/386
20130101; B01J 35/0006 20130101; B01J 2219/00117 20130101; B01J
35/04 20130101; C01B 2203/0261 20130101 |
Class at
Publication: |
422/211 ; 29/890;
427/532; 427/550; 264/239; 264/49 |
International
Class: |
B01J 8/00 20060101
B01J008/00; B21D 39/00 20060101 B21D039/00; B05D 3/00 20060101
B05D003/00; B05D 3/14 20060101 B05D003/14; B29C 39/02 20060101
B29C039/02 |
Claims
1. An apparatus comprising: a microchannel; a graded catalyst
disposed in the microchannel; wherein the graded catalyst has a
varying thermal conductivity such that the thermal conductivity in
one part of the graded catalyst is at least 25% higher than in
another part of the catalyst.
2. The apparatus of claim 1 wherein the graded catalyst varies
gradually in thermal conductivity.
3. The apparatus of claim 1 wherein the microchannel comprises an
inlet and an outlet and wherein the graded catalyst has a higher
thermal conductivity nearer the outlet as compared to nearer the
inlet.
4-27. (canceled)
28. A method of forming a catalyst microinsert, comprising: adding
a catalyst precursor into a mold; wherein the mold has at least one
dimension of 5 mm or less; forming a monolithic catalyst
microinsert; and removing the monolithic catalyst microinsert.
29. The method of claim 28 further comprising the step of binder
removal to form pores.
30. The method of claim 28 wherein the step of adding a catalyst
precursor comprises multiple steps and wherein a graded catalyst is
formed.
31. A method of making catalytic apparatus, comprising: applying a
magnetic or electric field to a microchannel; and loading or
orienting particles in the microchannel under the influence of the
magnetic or electric field.
32. The method of claim 31 wherein the step of applying a magnetic
or electric field orients support particles in a microchannel; and
further comprising depositing a catalyst layer on the support
particles.
33. Apparatus made by the process of claim 31.
34. (canceled)
35. Microchannel reactor apparatus comprising: a microchannel; and
hollow or porous catalyst particles disposed in the microchannel,
wherein the porous catalyst particles comprise large pores within
the individual particles; wherein the microchannel has a cross
sectional area; wherein the large pores are defined as a pore size
of at least 1% of the particle size; and wherein the hollow or
porous catalyst particles have a volume average particle
cross-section that is 1 to 40% that of the cross sectional area of
the microchannel.
36. A method of making the microchannel reactor apparatus of claim
35 comprising pouring the hollow or porous catalyst particles into
microchannel.
37. The microchannel reactor apparatus of claim 35 comprising
metallic hollow or porous catalyst particles.
38. The microchannel reactor apparatus of claim 35 wherein the
particles are spherical.
39. Apparatus comprising: a microchannel, high aspect ratio
particles disposed within the microchannel, wherein the high aspect
ratio particles are oriented within the microchannel such that at
least 40% by mass of the high aspect ratio particles are
substantially oriented in one direction that is perpendicular to a
wall of the microchannel.
40. The apparatus of claim 39 wherein the high aspect ratio
particles comprise a thermally conductive material.
41. Apparatus for conducting a chemical reaction, comprising: an
open flow path having a cross sectional area comprising at least
one dimension of 5 mm or less, wherein the cross sectional area is
perpendicular to a direction of flow; a catalyst defining at least
a portion of one wall of the open flow path within the cross
sectional area comprising at least one dimension of 5 mm or less,
such that no point within the cross sectional area in the open flow
path is more than 5 mm away from the catalyst; and a element
disposed in the open flow path, wherein the element is selected
from the group consisting of flow disruptors and micromixers; and
wherein the micromixer, if present, comprises a catalyst.
42. The apparatus of claim 41 comprising a flow disrupter selected
from the group consisting of a bar above the catalyst, corkscrew,
wire, or a strand pulled up from catalyst felt.
43. (canceled)
44. The apparatus of claim 39 wherein the high aspect ratio
particles comprise a magnetic material.
45. The apparatus of claim 1 wherein the microchannel comprises an
inlet and an outlet and wherein the graded catalyst has a lower
thermal conductivity nearer the outlet as compared to nearer the
inlet.
Description
FIELD OF THE INVENTION
[0001] The invention relates to catalysts, microchannel apparatus
and methods of conducting reactions in microchannels.
BACKGROUND OF THE INVENTION
[0002] Microreactors, which can have improved heat and mass
transfer compared to conventional reactors, are a new type of
reactor that offer the potential to significantly improve the
efficiency of chemical processes. This potential has engendered
intense efforts toward developing microreactors, catalysts and
micro-processes. A recent review of this technology, containing 236
citations, has been provided by Gavrilidis et al., "Technology And
Applications Of Microengineered Reactors," Trans. IChemE, Vol. 80,
Part A, pp. 3-30 (January 2002).
[0003] A more conventional approach to reactions in small channels
has involved reactions in honeycombs. In U.S. Pat. No. 5,248,251,
Dalla Betta et al. described a partial combustion process in which
a combustible gas mixture is passed through a monolith that has a
graded catalyst in which the catalyst on a leading portion of the
support has a higher activity than has the catalyst on a trailing
portion of the support.
[0004] A recent patent for producing a hydrogen-rich gas describes
a catalyst on a honeycomb that has a layer of a partial oxidation
catalyst and a layer of a steam reforming catalyst. An embodiment
is described in which the catalyst layers are graded such that the
partial oxidation catalyst has its maximum thickness near the inlet
and diminishes over the length of the catalyst structure to
practically zero near the outlet and the steam reforming catalyst
has a near zero thickness near the inlet and increases over the
length of the catalyst structure to its maximum thickness near the
outlet.
SUMMARY OF THE INVENTION
[0005] The present invention provides new microreactor systems,
catalysts, and chemical processes. Methods of making novel
catalysts and reaction apparatus are also described.
[0006] In one aspect, the invention provides a method of conducting
a reaction, comprising: flowing at least one reactant into a
microchannel, and reacting the at least one reactant in the
presence of the graded catalyst within the microchannel to form at
least one product. In this aspect, the microchannel includes a
graded catalyst that substantially fills a cross section of the
microchannel. The graded catalyst has a distribution of
catalytically active material such that the at least one reactant
is exposed to a higher concentration of catalytically active
material in one area of the catalyst than in another area of the
catalyst. As with all methods mentioned herein, the invention also
includes apparatus for conducting these methods and systems that
include the apparatus and reactants and/or products.
[0007] In another aspect, the invention provides a method of
conducting a reaction, comprising: flowing at least one reactant
into a microchannel through a microchannel inlet and reacting the
at least one reactant in the presence of the graded catalyst within
the microchannel to form at least one product. In this aspect, the
microchannel contains a graded catalyst that has a lower
concentration of catalytically active material near the inlet as
compared with elsewhere in the microchannel. In this aspect, the
microchannel contains only one catalyst--for example, the
microchannel does not contain both a reforming catalyst and a
partial oxidation catalyst. Although the graded catalyst may
consist of more than one catalytic element (such as to include a
promoter metal), the graded catalyst is not in contact with a
different catalyst material. Furthermore, the catalyst is an active
catalyst, not merely a catalyst precursor. For example, in some
preferred embodiments for an exothermic reaction (such as
Fischer-Tropsch), a catalyst is graded such that the least amount
of catalyst is present at (or near) the inlet where the reactant
concentration is greatest and where the least amount of product is
present, and increases along the length of the reaction
microchannel to the most amount of catalyst near the outlet where
only a relatively small amount of reactants remain such that the
heat released or the temperature is relatively uniform along the
flow direction which is important for controlling the product
selectivity.
[0008] In another aspect, the invention provides an apparatus
comprising: a microchannel; and a graded catalyst disposed in the
microchannel; wherein the graded catalyst has a varying thermal
conductivity such that the thermal conductivity in one part of the
graded catalyst is at least 25%, more preferably 50% (and sill more
preferably at least 200%) higher than in another part of the
catalyst. The invention also includes methods of conducting
chemical reactions by flowing at least one reactant into a
microchannel that includes a graded catalyst having a varying
thermal conductivity; and forming at least one product.
[0009] In a further aspect, the invention provides a method of
conducting a reaction, comprising: flowing at least one reactant
into a microchannel through a microchannel inlet and reacting the
at least one reactant in the presence of the graded catalyst within
the microchannel to form at least one product. The microchannel
comprises (includes) a graded catalyst that has a higher
concentration of catalytically active material in the catalyst
nearer a microchannel wall than the concentration of the
catalytically active material in the catalyst nearer the center of
the microchannel. The microchannel wall is adjacent to a heat
exchanger such that, during reaction, heat is transferred between
the microchannel and the heat exchanger. While heat need not be
transferred at all times, it must be transferred during at least
some point of the reaction and preferably during substantially all
of the reaction (not just for startup). Placing more catalyst near
a channel wall that is adjacent to a heat exchanger shortens the
distance for heat transfer.
[0010] In another aspect, the invention provides a method of
conducting a reaction, comprising: flowing at least one reactant
into a first reaction microchannel, and reacting the at least one
reactant in the presence of the graded catalyst within the first
reaction microchannel. A graded catalyst is disposed in the first
reaction microchannel. The graded catalyst has a distribution of
catalytically active material such that the at least one reactant
is exposed to a higher concentration of catalytically active
material in one area of the catalyst than in another area of the
catalyst. During reaction, heat is exchanged between the first
reaction microchannel and an adjacent, second reaction
microchannel. One of the first or second reaction channels
comprises an exothermic reaction and the other of the reaction
channels comprises an endothermic reaction. At least one first
product is formed in the first reaction microchannel and at least
one second product is formed in the second reaction
microchannel.
[0011] In a further aspect, the invention provides a method of
conducting a reaction, comprising: flowing at least one reactant
into a microchannel, and reacting the at least one reactant in the
presence of a graded catalyst within the microchannel to form at
least one product. In this aspect, the catalyst consists of a
graded catalyst having of a distribution of a catalytically active
material such that the at least one reactant is exposed to a higher
concentration of catalytically active material in one area of the
catalyst than in another area of the catalyst. By "consists of," it
is meant that there is only one type of catalyst at any point along
the length of the microchannel (preferably, only one type of
catalyst along the length of the reaction zone in the
microchannel). The reactant, or reactants, and catalyst are
selected such that the step of reacting is selected from the group
consisting of: acetylation, addition reactions, alkylation,
dealkylation, hydrodealkylation, reductive alkylation, amination,
ammoxidation, ammonia synthesis, aromatization, arylation,
autothermal reforming, carbonylation, decarbonylation, reductive
carbonylation, carboxylation, reductive carboxylation, reductive
coupling, condensation, cracking, hydrocracking, cyclization,
cyclooligomerization, dehalogenation, dimerization, epoxidation,
esterification, exchange, Fischer-Tropsch, halogenation,
hydrohalogenation, homologation, hydration, dehydration,
hydrogenation, dehydrogenation, hydrocarboxylation,
hydroformylation, hydrogenolysis, hydrometallation, hydrosilation,
hydrolysis, hydrotreating (HDS/HDN), isomerization, methylation,
demethylation, metathesis, nitration, polymerization, reduction,
reformation, reverse water gas shift, Sabatier, sulfonation,
telomerization, transesterification, trimerization, and water gas
shift.
[0012] In another aspect, the invention provides a method of
conducting a reaction, comprising: flowing at least one reactant
into a microchannel, and reacting the at least one reactant in the
presence of the graded catalyst within the microchannel to form at
least one product. A graded catalyst is disposed in the
microchannel. The graded catalyst comprising a distribution of
catalytically active material such that the at least one reactant
is exposed to a higher concentration of catalytically active
material in one area of the catalyst than in another area of the
catalyst. The reactant, or reactants, and catalyst are selected
such that the step of reacting is selected from the group
consisting of: acetylation, addition reactions, alkylation,
dealkylation, hydrodealkylation, reductive alkylation, amination,
ammonia synthesis, aromatization, arylation, carbonylation,
decarbonylation, reductive carbonylation, carboxylation, reductive
carboxylation, reductive coupling, condensation, cracking,
hydrocracking, cyclization, cyclooligomerization, dehalogenation,
dimerization, epoxidation, esterification, exchange,
Fischer-Tropsch, halogenation, hydrohalogenation, homologation,
hydration, dehydration, hydrogenation, hydrocarboxylation,
hydroformylation, hydrogenolysis, hydrometallation, hydrosilation,
hydrolysis, hydrotreating (HDS/HDN), isomerization, methylation,
demethylation, metathesis, nitration, polymerization, hydrocarbon
reforming, reverse water gas shift, Sabatier, sulfonation,
telomerization, transesterification, trimerization, and water gas
shift.
[0013] In another aspect, the invention provides a method of
forming a catalyst microinsert, comprising: adding a catalyst
precursor into a mold; wherein the mold has at least one dimension
of 5 mm or less; forming a monolithic catalyst microinsert; and
removing the monolithic catalyst microinsert.
[0014] In yet another aspect, the invention provides a method of
making catalytic apparatus, comprising: applying a magnetic or
electric field to a microchannel; and loading or orienting
particles in the microchannel under the influence of the magnetic
or electric field. Here, "applying" (obviously) means something
other than existing in the earth's magnetic field, and does not
mean electroplating or electrochemical deposition.
[0015] In another aspect, the invention provides apparatus
comprising: a microchannel; and hollow or porous catalyst particles
disposed in the microchannel, wherein the porous catalyst particles
comprise large pores within the individual particles; wherein the
microchannel has a cross sectional area; wherein the large pores
are defined as pores having a pore size of at least 1% of the
particle size of the particle in which the pores exist; and wherein
the hollow or porous catalyst particles have a volume average
particle cross-section that is 1 to 40% that of the cross sectional
area of the microchannel. Pore size is measured by microscopy
(usually scanning electron microscopy (SEM)) or mercury
porisimetry. Particle size is mass average and is based on the
conventional understanding of particles fitting through a mesh
screen. "Average" is volume average such that a large particle has
a much greater influence on the average than a small particle. In
some preferred embodiments, the hollow or porous catalyst particles
have a volume average particle cross-section that is 10 to 40% that
of the cross sectional area of the microchannel. The
cross-sectional area of the reaction microchannel is measured at a
point in the reaction microchannel where particles are lodged. In
this aspect of the invention, "porous catalyst particles" includes
both particles and pellets, but does not include foams, felts,
wads, screens, mesh or honeycombs. Hollow particles, however, may
include foams, felts, wads, screens, mesh or honeycombs as well as
particles or pellets (provided the particles meet the definition of
hollow set forth herein). The invention also includes methods of
conducting reactions in this apparatus, and methods of making
apparatus by adding these particles into microchannels.
[0016] The invention also provides a method of conducting a
chemical reaction, comprising: passing at least one reactant fluid
into a reaction microchannel that contains catalyst particles
dispersed in a fluid; wherein at least one reactant within the at
least one reactant fluid reacts to form at least one product;
wherein at least some of the catalyst particles flow out of the
microchannel to form a catalyst particle stream; and passing a
stream of catalyst particles comprising said catalyst particle
stream into the same or a different reaction channel. The invention
also includes systems comprising microchannel apparatus containing
a catalyst particle stream.
[0017] In a further aspect, the invention provides apparatus
comprising: a microchannel, and high aspect ratio particles
disposed within the microchannel. The high aspect ratio particles
are oriented within the microchannel such that at least 40% (more
preferably at least 50% and still more preferably at least 75%) by
mass of the high aspect ratio particles are substantially oriented
in one direction that is perpendicular to a wall of the
microchannel. "Substantially oriented" means within 45.degree. of
one direction as measured along the primary axis (averaged by mass
in each particle) of each particle. The high aspect ratio particles
are preferably a support material, and more preferably a catalyst
material, and in some embodiments a graded catalyst. In some
embodiments, the high aspect ratio particles are mixed with low
aspect ratio particles. The invention also includes methods of
reacting a reactant in this apparatus and methods of making this
apparatus.
[0018] In another aspect, the invention provides apparatus for
conducting a chemical reaction, comprising: an open flow path
having a cross sectional area comprising at least one dimension of
5 mm or less, wherein the cross sectional area is perpendicular to
a direction of flow; a catalyst defining at least a portion of one
wall of the open flow path within the cross sectional area
comprising at least one dimension of 5 mm or less, such that no
point within the cross sectional area in the open flow path is more
than 5 mm away from the catalyst; and a flow disruptor and/or
micromixer; and wherein the micromixer, if present, comprises a
catalyst. The invention also includes methods of conducting
chemical reactions in this apparatus.
Glossary of Terms
[0019] "Channels" refers to the generally accepted meaning and
includes conduits and other means for directing the flow of a
fluid. Channels of the invention include at least one opening,
typically with an inlet and outlet, and may include other openings.
As will be seen in the description below of various embodiments,
numerous functions other than simple transport can occur within
channels. A reaction channel (including a reaction microchannel)
does not include inlet or outlet valves or inlet or outlet orifices
(of course inlet and outlet orifices, valves, etc. may be connected
to a reaction channel but they are not considered part of the
reaction channel itself).
[0020] "Catalyst" is a material that enhances reaction parameters,
for example reaction rate, without itself being consumed.
[0021] A "Cross-sectional area," or "an area of a cross-section,"
of a reaction channel is measured perpendicular to the direction of
net flow and includes all area within a reaction channel including
catalyst particles (or monolith) and/or a catalyst wall coating
(including a thermally grown oxide (if present) but does not
include the reaction channel walls. For reaction channels that
curve along their length, cross-sectional area is measured
perpendicular to the direction of net flow at a selected point
along a line that parallels length and is at the center (by area)
of the reaction channel. Statements such as "a cross sectional area
varies" mean that there is a significant variation in area, not
merely a variation in surface roughness. Dimensions of height and
width are measured from one reaction channel wall to the opposite
wall and are not changed by application of a catalyst coating, and
are average values that account for variations caused by surface
roughness, or variations caused by corrugations, etc.
[0022] "Engineered catalyst" is a single piece or several pieces of
catalysts that can be shaped for a particular reaction channel and
inserted or stacked into a microchannel. Some examples are foams
and felts (that is, a collection of nonwoven fibers or strands).
Pellets, coatings and powders are not engineered catalysts.
[0023] "Catalyst particles dispersed in a liquid" are solid,
typically colloidal, particles that, with the liquid, form a slurry
or colloidal dispersion.
[0024] "Composition" is a gas, a liquid, or a fluid mixture (such
as a colloid which could be a solid/liquid mixture). The
composition may itself be reactive or may be mixed with another
material.
[0025] "Direction of flow" is the direction of net flow through at
least one segment of a reaction channel. For a straight channel,
the direction of flow is from the inlet or inlets of a channel to
the outlet or outlets of the channel.
[0026] "Flow path" is a path in the reactor through which travels a
composition.
[0027] "Fluid Heat exchanger" is a chamber (having an inlet and an
outlet, or multiple inlets and/or outlets) through which a fluid
(i.e., a gas or liquid) flows and, through a wall of the reaction
channel, conducts heat away from or toward a reaction channel. A
fluid heat exchanger is not an electrical heater.
[0028] A "graded catalyst" has a gradient, or gradients, of
catalytic activity (such as by varying concentration or surface
area of a catalytically active metal or the turnover rate of the
active sites or by varying the physical properties and/or form of
the catalyst material) that varies as a function of distance; for
example, a lower active metal concentration at the front of a
reaction channel that increases to a higher concentration near the
back of the reaction channel; or a lower concentration of active
metal nearer the center (i.e., midpoint) of a reaction channel and
a higher concentration nearer a reaction channel wall, etc. One
example of a physical properties is thermal conductivity. Surface
area of catalytically active metal can be varied by varying size of
metal sites on a constant surface area support or by varying the
surface area of the support such as by varying support type or
particle size. A graded catalyst may have a single catalytic
component or multiple components (for example, a bimetallic or
trimetallic catalyst). In some preferred embodiments, the catalyst
gradually changes its properties and/or composition as a function
of distance rather than abrupt or discontinuous changes. A graded
catalyst is not merely a rimmed particle or particles that have an
"eggshell" distribution of catalytically active metal within each
particle.
[0029] "Heat exchanger" is a component that adds or removes heat
from a reaction chamber. It is an active component, not merely
ambient air or a stagnant fluid. Preferably, a heat exchanger is a
fluid heat exchanger.
[0030] "Heat transfer distance" is the distance between the
midpoint of a reaction channel and the wall of a heat exchanger.
The midpoint is the area-weighted center point of a cross-section
of the reaction channel, and distance is typically measured
perpendicular to flow. In other words, the midpoint is the
intersection of lines that bisect (divide in half) the
cross-sectional area of the reaction channel.
[0031] "High aspect ratio particles" are anisotropically shaped
particles that have a length (longest dimension) that is at least 2
times (more preferably, at least 5 times and still more preferably
at least 10 times) greater than either (but not necessarily both) a
width or a height. Examples include wires, whiskers, plates and
flakes.
[0032] "Hollow particles" are particles in which the density of the
outermost 20% (by volume) of the particle has a density that is at
least 20%, more preferably at least 50%, greater than the average
density of the interior 90% by volume.
[0033] "Inlet side" and "outlet side" are relative terms. Every
part of the inlet side of a reaction chamber is closer to an inlet
into a reaction channel, and every part of the outlet side of a
reaction chamber is closer to an outlet from a reaction channel. In
preferred embodiments, there are a single inlet and a single outlet
connected to a reaction channel, however, in other embodiments the
invention includes reaction channels with multiple inlets and/or
outlets.
[0034] A "micromixer" is a component that is disposed within a bulk
flow region of a reaction chamber (preferably a
microchannel)--where the bulk flow region is substantially
unobstructed except for the micromixer. A micromixer has at least
one dimension of 5 mm or less. For purposes of defining a bulk flow
region in this invention, when a micromixer is present, the
presence of a micromixer does not negate the existence of a bulk
flow region.
[0035] A "microchannel," for purposes of the present invention, is
a channel having a height of 5 mm or less, preferably 2 mm or less,
and still more preferably 1 mm or less, and in some preferred
embodiments height is in the range of 0.1 and 2 mm. Length of a
microchannel is typically not crucial but, in some embodiments is
less than 10 cm. Length is defined to be the same direction as net
flow through a microchannel. Channel cross-sections can be, for
example, rectangular, circular, triangular, or irregularly shaped.
Height and width are perpendicular to length and either or both can
vary along the length of a microchannel. Height and width can be
arbitrarily selected; in the present invention, height is defined
as the smallest dimension of a channel that is perpendicular to
flow. In some embodiments, such as steam reforming, width is
preferably 5 cm or less, more preferably 1 cm or less, and in some
embodiments in the range of 0.1 mm and 2 cm.
[0036] "Porous catalyst particles" have the porosity of the "porous
catalyst material" described below.
[0037] "Thermally conductive materials" refers to the generally
accepted understanding. Carbon and metals are thermally conductive
while most of the common metal oxides and plastics are not.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 illustrates a cross-flow microchannel reactor. In
some preferred embodiments, catalysts are placed in the process
channels as either a coating, an insertable felt or foam, or packed
powders. In this illustration, a heat exchange fluid is flowing
downward while a process stream flows in the direction that is into
the page. The heat exchange fluid could alternatively be oriented
as co-flow or counter-flow.
[0039] FIG. 2 is a schematic illustration of a cross-section of a
reaction microchannel containing a flow disrupter 204.
[0040] FIG. 3a schematically illustrates powder loading into a
microchannel reactor under the influence of an electric field.
[0041] FIG. 3b schematically illustrates a channel having
anisotropic particle loading in the presence of a magnetic
field.
[0042] FIG. 4 schematically illustrates the formation of a graded
insert in a mold. The figure shows a microchannel preform filled
with a first catalyst layer (top) and the preform filled with two
more catalyst layers (bottom). The catalyst layers can vary in
composition, particle size, density, etc.
[0043] FIG. 5 schematically illustrates a lengthwise
cross-sectional view of a reaction channel containing a graded
catalyst.
[0044] FIG. 6 schematically illustrates a cross-sectional view of a
reaction channel containing a graded catalyst having a relatively
high concentration of active catalyst near the channel wall.
DESCRIPTION OF PREFERRED EMBODIMENTS
Apparatus
[0045] The invention preferably uses a microchannel reactor.
Microchannel reactors are characterized by the presence of at least
one reaction channel having a (wall-to-wall, not counting catalyst)
dimension of 5 mm (preferably 1 mm) or less, and in some
embodiments 50 to 500 .mu.m. Both height and width are
perpendicular to the direction of flow. The height and/or width of
the reaction microchannel is preferably 5 mm or less, and more
preferably 1 mm or less (in which case the reaction chamber falls
within the classical definition of a microchannel). The length of
the reaction channel is typically longer. Preferably, the length of
the reaction chamber is greater than 1 cm, more preferably in the
range of 1 to 20 inches (2.5 to 50 cm). Typically, the sides of the
reaction channel are defined by reaction channel walls. These walls
are preferably made of a hard material such as a ceramic, an iron
based alloy such as steel, or monel. More preferably, the reaction
chamber walls are comprised of stainless steel or inconel or other
alloy which is durable (capable of withstanding high temperatures
while supporting high pressures inside the reactor for long periods
of time) and has good thermal conductivity.
[0046] Reaction channels that contain post-like supports or baffles
are considered a single channel while channels with a support rib
running the entire channel length are considered two channels.
Reaction channels that are "separated" by a porous material are
also considered two channels.
[0047] The reactors preferably include a plurality of microchannel
reaction channels and/or a plurality of adjacent heat exchange
microchannels. The plurality of microchannel reaction channels may
contain, for example, 2, 10, 100, 1000 or more channels. In some
preferred embodiments, the microchannels are arranged in parallel
arrays of planar microchannels. Layers of reaction channels can be
alternated with layers of heat exchange channels (or layers of
exothermic reaction channels alternated with layers of endothermic
reaction channels) or two layers of reaction channels sandwiched
between heat exchange layers, etc. During operation, the heat
exchange microchannels contain flowing heating and/or cooling
fluids. Flows between layers can be co-flow, counter-flow,
cross-flow or a combination of lows (diagonal flow). Channels
within a single layer can be also co-flow or counter flow.
Non-limiting examples of this type of known reactor usable in the
present invention include those of the microcomponent sheet
architecture variety (for example, a laminate with microchannels)
exemplified in U.S. Pat. Nos. 6,200,536, 6,192,596 and 6,219,973
(both of which are hereby incorporated by reference). Performance
advantages in the use of this type of reactor architecture for the
purposes of the present invention include their relatively large
heat and mass transfer rates, and the substantial absence of any
explosive limits. Use of microchannel reactors can achieve better
temperature control, and maintain a relatively more isothermal
profile (or, in some embodiments, a well-controlled temperature
gradient), compared to conventional reactors.
[0048] In some embodiments, the reaction microchannel (or
microchannels) contains a "bulk flow region" or an open flow path.
The terms "open flow path" or "bulk flow region" refer to an
unobstructed, contiguous bulk flow region within the reaction
chamber. A contiguous bulk flow region allows rapid gas flow
through the reaction chamber without large pressure drops. In
preferred embodiments there is laminar flow in the bulk flow
region. Bulk flow regions within each reaction channel preferably
have a cross-sectional area of 5.times.10.sup.-8 to
1.times.10.sup.-2 m.sup.2, more preferably 5.times.10.sup.-7 to
1.times.10.sup.-4 m.sup.2. The bulk flow regions preferably
comprise at least 5%, more preferably 30-80% of either 1) the
internal volume of the reaction chamber, or 2) the cross-section of
the reaction channel. Flow patterns as well as flowrate can be
tailored to achieve desired temperature gradients within the
reaction channels along the flow direction. Heat transfer fluids
may include any known heat transfer fluids, such as water, aqueous
solutions, silicone oils, molten salts, liquid metals, etc. In some
preferred embodiments, the heat exchange fluid is steam or is a
fluid that undergoes a phase change in the heat exchanger under the
intended process temperatures.
[0049] In addition to the reaction chamber(s), additional features
such as microchannel or non-microchannel heat exchangers may be
present. Microchannel heat exchangers are preferred. Less
preferably, conventional heat exchangers may be attached to the
microchannel reactor by conventional piping and adapters.
Microchannel heat exchangers can be integral to the reactor, i.e.,
they can be formed as one continuous unit with the reactor. In some
preferred embodiments, a reactant stream is preheated by the
reactor, this preheated stream then flows into the reactor and/or
the product stream from the reactor can transfer heat to the
reactant stream--in these embodiments, fluids flow between a heat
exchanger and the reactor. The heat exchanger can be incorporated
into the microchannel reactor in the form of a pre-heat zone. Heat
exchangers can exchange heat between the process stream and a
separate, hot or cold heat exchange fluid stream (typically the
streams are separated by a wall or walls of a microchannel), or
they can exchange heat between the inlet and outlet streams, or
both. In some embodiments of the inventive reactor or method, the
reactor (or method) is configured to send the product stream into a
second reactor or recycle the product stream back into the same
reactor. The heat exchange fluids can be gases or liquids and may
include steam, liquid metals, or any other known heat exchange
fluids--the system can be optimized to have a phase change in the
heat exchanger. In some preferred embodiments, multiple heat
exchange layers are interleaved with multiple reaction
microchannels (for example, at least 10 heat exchangers interleaved
with at least 10 reaction microchannels).
[0050] Many options exist for the design of a microchannel reactor.
In some embodiments, a coolant fluid flows in microchannels
adjacent to the reaction chamber. The flow of coolant may be cross
flow, counter-flow or co-flow. Heat transfer fluids may include any
known heat transfer fluids, such as water, aqueous solutions,
silicone oils, molten salts, liquid metals, gases, etc. In some
preferred embodiments, the heat exchange fluid is steam or is a
fluid that undergoes a phase change in the heat exchanger or
reactor under the intended process temperatures.
[0051] In an alternate microchannel embodiment, a reactant or
reactants could be staged or fed sequentially into the reaction
mixture. The staging could occur in separate devices, through the
use of small orifices or jets within one device, or from a
microporous membrane or alternate sparging sheet. In oxidative
dehydrogenation, for example, staged oxygen addition lowers the
local oxygen partial pressure and thus favors the desired partial
oxidation reaction over the competing and undesired combustion
reaction.
[0052] An alternate microchannel design is the coupling of an
exothermic and an endothermic reaction in adjacent reaction
chambers, preferably adjacent microchannels. The placement of an
endothermic reaction such as a steam reforming reaction next to the
exothermic reaction allows for the highest rate of heat transfer.
In some embodiments, an endothermic reaction such as steam
reforming is conducted in one reaction microchannel while an
exothermic reaction such as combustion is conducted in an adjacent
microchannel or in another microchannel that is in a heat-exchange
relationship in a multi-stream integrated heat exchanger/reactor.
In some preferred embodiments, a reaction channel contains multiple
reaction zones such as a first reaction zone with a first catalyst
followed by a second reaction zone with a second catalyst--two
separate and distinct catalysts are not considered a graded
catalyst, but the first and/or second catalysts could be graded
catalysts. Where two separate and distinct catalysts are employed,
it is often desirable to operate the catalysts at different
temperatures. For combustion reactions, it is sometimes desirable
to conduct a partial oxidation followed by a complete
oxidation--this is especially useful where a partially oxidized
fuel has better reaction properties than the nonoxidized fuel.
[0053] A nonlimiting example of microchannel reactor hardware is
shown in FIG. 1. Coolant microchannels (5 mm, and preferably 2 mm
or less) are adjacent to a microchannel reaction chamber (5 mm, and
preferably 2 mm or less). The wall between the channels is
preferably 2 mm or less. The flow of coolant may be oriented in a
co-current flow, counter-current flow, or cross-current flow. The
length of the process flow channel may be any length, but a typical
range is 1 to 20 inches (2.5 to 50 cm). The height of the process
channel may also be any value, but a typical range is 0.1 inches to
10 inches (0.25 to 25 cm). Each of the process or coolant channel
may be further subdivided with parallel subchannels. The spacing of
subchannels is dependent upon maximizing heat transfer and
minimizing mechanical stresses. Pressure drop considerations are
also important for choosing channel spacing, length and
width/heights.
[0054] At a point where the chamber height or the chamber width is
about 5 mm or less, the chamber height and the chamber width define
a cross-sectional area. In some preferred embodiments, the
cross-sectional area comprises a porous catalyst material and an
open area, where the porous catalyst material occupies up to 95%,
preferably 5% to 95% of the cross-sectional area and where the open
area occupies at least 5%, preferably 5% to 95% of the
cross-sectional area. In some preferred embodiments, the open area
in the cross-sectional area of a single channel occupies a
contiguous area of 5.times.10.sup.-8 to 1.times.10.sup.-2
m.sup.2.
[0055] In some embodiments, the reactor may have a reaction channel
with a cross-sectional area that varies along its length, such as
described in U.S. patent application Ser. No. 10/153,577;
incorporated by reference herein as if reproduced in full below.
The variable cross-section, which permits control of local contact
time, can (in some preferred embodiments) be combined with a graded
catalyst as described herein. For example, the catalyst can be
graded with the highest concentration of catalytically active
material in the outlet side (or, conversely, on the inlet side) of
the reaction channel and a reaction channel of gradually increasing
cross-sectional area with the largest cross-section on the outlet
side (or, conversely, gradually decreasing cross-sectional area
with the largest cross-section on the inlet side) of the reaction
channel--thus enhancing control of reaction conditions.
Methods of Making Apparatus
[0056] The inventive reactors can be fabricated using methods such
as lamination of thin metal sheets (where a reaction channel can be
within one sheet, for example, the channel can be etched in a sheet
or stamped through a sheet with reaction channel walls provided by
adjacent sheets, or a reaction channel can be made up of multiple
sheets), micro-EDM drilling, laser machining, chemical etching,
injection molding, welding. Materials like metal, alloys,
composite, polymers, and ceramics can be utilized. Highly
conductive material will enhance heat transfer efficiency and
mitigate non-uniformity of temperature distribution. Preferably, at
least a portion of a wall or walls of the reaction channel are
composed of a thermally-conductive material such as steel or
aluminum. For devices made from laminated devices, it can be
desirable to stamp reaction channels into a sheet or sheets.
Devices fabricated from such sheets will typically have a reaction
channel in a single sheet or multiple adjacent sheets (preferably
sandwiched between layers of heat exchangers), so that the
assembled device will have reaction channels with constant heights
as defined by top and bottom sheets defining the top and bottom of
a reaction channel.
Catalysts
[0057] For many of the inventive embodiments, a
catalytically-active material is present. In various embodiments,
(a nonlimiting list of) the catalytically-active materials of the
present invention may include: the noble metals including Au and
Ag, base metals including Fe, Co, Cu, and Ni, catalyst materials
comprising at least one metal selected from the group consisting of
Pt, Pd, Os, Rh, Ir, Re and Ru; and/or at least one oxide or salt of
a metal selected from the group consisting of Li, Mo, V, Nb, Sb,
Sn, Zr, Cr, Mg, Mn, Ni, Co, Ce, Al, Fe, rare-earth metals and
mixtures thereof. The catalyst may contain additional components
such as promoters, for example, alkali or alkaline earth promoters.
Preferred support materials include alumina, silica, titania,
zirconia, ceria, other metal oxides or mixtures of these oxides,
mesoporous materials and refractory materials, carbon or silicon
carbide. Catalysts can be, for example, vanadia dispersed on
alumina, or platinum on alumina. Catalysts can also be a noble
metal dispersed on a metal oxide layer that is coated on (such as
by wash coating or chemical vapor deposition) a metal foam or metal
felt (nonwoven metal). In some preferred embodiments, catalyst is
disposed (such as by CVD or wash coating) on a wall or walls of a
microchannel.
[0058] Catalysts employed in the reaction channels are preferably
solids (or contain solids) for heterogeneous reactions that, under
the selected reaction conditions, remain (at least partly) as
heterogeneous material, that is, the catalyst does not completely
dissolve in the process stream. Preferably, the catalyst is
essentially insoluble in the process stream. The catalyst may be
utilized in a flow-by (such as a coating or thin layer) or flow
through (substantially occupying an entire cross-section of the
reaction channel) configuration. Examples of catalyst structures
include: foams, felts (nonwoven fibers), screens, pellets, saddles,
powders, honeycombs, aligned or nonaligned carbon nanotubes and
coatings. In some preferred embodiments, the catalyst is an
engineered catalyst. Any of these structures can have multiple
layers such as a buffer layer, interfacial layer(s), and a layer or
layers containing a catalytically active metal. The catalyst may
contain a single type of material, but more typically will contain
multiple materials such as a support and metal or multiple metals,
promoters, or mixtures of supports, metals, etc. The catalyst may
be a fluidized bed or a fixed bed.
[0059] In some preferred embodiments, the invention utilizes a
porous catalyst, the porous catalyst having a length, a width and a
thickness, the porous catalyst defining at least a portion of at
least one wall of a bulk flow path. In this preferred embodiment,
the surface of the catalyst defines at least one wall of a bulk
flow path through which the reactant and product mixture passes.
During operation, a reaction mixture flows through a reaction
chamber (preferably a microchannel), past and in contact with the
porous catalyst.
[0060] In some preferred embodiments, the porous catalyst is
provided as a porous insert that can be inserted into (or removed
from) each channel in a single piece; preferably the porous insert
is sized to fit within a microchannel with a width of less than 2
mm. The inserts can be made with any desired size, and in some
embodiments have one dimension of 2 mm or less and one dimension of
1 cm or more. In some embodiments, the porous catalyst occupies at
least 60 volume %, in some embodiments at least 90%, of a
cross-sectional area of a microchannel; this volume % includes
pores within a monolith and interstitial spaces between particles.
In another alternative, the catalyst can be provided as a coating
(such as a washcoat) of material within a microchannel reaction
channel or channels. The use of a flow-by catalyst configuration
can create an advantageous capacity/pressure drop relationship. In
a flow-by catalyst configuration, gas preferably flows in a 0.1-1.0
mm gap adjacent to a porous insert or a thin layer of catalyst that
contacts the microchannel wall (preferably the microchannel wall
that contacts the catalyst is in direct thermal contact with a heat
exchanger, preferably a heat exchange stream contacts the opposite
side of the wall that contacts the catalyst). Another inventive
embodiment is combinations of engineered catalysts, wall coatings
and/or powders all in one channel. For example, there may be a
catalyst on a foam at the front of a channel and a second catalyst
(where the catalyst have the same or different elemental
composition) in powder form downstream of it; the same catalyst can
be used in two forms--this would be another way to form a graded
bed.
[0061] Various porous catalysts and apparatus containing catalyst
are described in U.S. Patent Application Serial No. 6,378,278,
which is incorporated herein as if reproduced in full below. A
"porous catalyst material" (or "porous catalyst") refers to a
porous material having a pore volume of 5 to 98%, more preferably
30 to 95% of the total porous material's volume. At least 20% (more
preferably at least 50%) of the material's pore volume is composed
of pores in the size (diameter) range of 0.1 to 300 microns, more
preferably 0.3 to 200 microns, and still more preferably 1 to 100
microns. Pore volume and pore size distribution are measured by
mercury porisimetry (assuming cylindrical geometry of the pores)
and nitrogen adsorption. As is known, mercury porisimetry and
nitrogen adsorption are complementary techniques with mercury
porisimetry being more accurate for measuring large pore sizes
(larger than 30 nm) and nitrogen adsorption more accurate for small
pores (less than 50 nm). Pore sizes in the range of about 0.1 to
300 microns enable molecules to diffuse molecularly through the
materials under most gas phase catalysis conditions. The porous
material can itself be a catalyst, but more preferably the porous
material comprises a metal, ceramic or composite having a layer or
layers of a catalyst material or materials deposited thereon. The
porosity can be geometrically regular as in a honeycomb or parallel
pore structure, or porosity may be geometrically tortuous or
random. Preferably the substrate is a foam metal or foam ceramic.
The catalyst layers, if present, are preferably also porous. The
average pore size (volume average) of the catalyst layer(s) is
preferably smaller than the average pore size of the substrate. The
average pore sizes in the catalyst layer(s) disposed upon the
support preferably ranges from 10.sup.-9 m to 3.times.10.sup.-7 m
as measured by N.sub.2 adsorption with BET method. More preferably,
at least 50 volume % of the total pore volume is composed of pores
in the size range of 10.sup.-9 m to 10.sup.-7 m in diameter.
Diffusion within these small pores in the catalyst layer(s) is
typically Knudsen in nature for gas phase reactions, whereby the
molecules collide with the walls of the pores more frequently than
with other gas phase molecules.
[0062] Catalysts or catalyst supports preferably have good thermal
conductivity. In some preferred embodiments, the catalyst comprises
a support such as a metal, metal alloy, carbon, carbide, sulfide,
nitride, polymer, ceramic or mixture of these. Particularly useful
examples are alumina, zirconia, silicon carbide, aluminum nitride,
carbon, ceria, titania, Ag, Co, Au, Cu, Zn, Ni, or mixture of
these. In some preferred embodiments, Ag, Co, Au, Cu, Zn, or
mixture of these are mixed with less than 50% by volume of a metal
oxide such as alumina, silica, titania, zirconia, ceria, or
mixtures of these. Perovskites are particularly useful as catalyst
supports or coatings on supports. ABO(3-x) perovskites are expected
to be particularly useful in oxidation processes such as combustion
where their oxygen absorption and desorption characteristics can
benefit catalyst activity and stability. In the formula, element A
is selected from a group consisting of a Group II metal, calcium,
strontium, barium, yttrium, lanthanum, a lanthanide series metal,
an actinide series metal, and a mixture thereof. Element B is
selected from a group consisting of iron, manganese, chromium,
vanadium, titanium, copper, nickel, cobalt, and mixtures thereof.
Perovskites are well-known materials and are discussed in texts
such as Wells, "Structural Inorganic Chemistry," Oxford Science
Publications.
[0063] In some preferred embodiments, the invention uses a graded
catalyst that has a varying density of catalytically active sites.
Gradients can be arranged in any desired configuration. For
example, gradients can be arranged with the greatest density of
catalytically active sites in the center of a reaction
microchannel, near a microchannel wall, or in the front (near an
inlet) of a reaction microchannel. In some preferred embodiments,
the concentration of catalytically active sites is least at the
front 502 (that is, the region of a microchannel that contains
catalyst that is nearest an inlet for reactants) of a reaction
microchannel and increases along the length of the microchannel.
One such variation of this "back to front" configuration is
illustrated in FIG. 5. The "back to front" configuration is
especially desirable for reactions in which it is desired to have
an equal amount of reaction over the length of the microchannel and
where the concentration of the reactants can vary significantly
over the length of the microchannel. This may be desirable, for
example, to reduce hot spots in an exothermic reaction.
[0064] In some preferred embodiments, the invention uses a graded
catalyst wherein the physical form varies over the length or width
of the reaction microchannel. For example, the thermal conductivity
of the substrate material may be varied by location to take
advantage of different local rates of reaction, which will respond
different to varying local exotherms or endotherms.
[0065] In some preferred embodiments, the density of catalytically
active sites is greatest near the microchannel walls. See, for
example, FIG. 6 where black dots 602 indicate catalytically active
sites in porous catalyst 604. In this cross-sectional view, there
is an open bulk flow region 608. This type of configuration is
especially advantageous where the microchannel wall 606 contacts a
heat exchanger (not shown). In some preferred embodiments, the
walls are in direct contact with a heat transfer fluid. In addition
to catalyst site density, graded catalysts can also include
catalysts of varying compositions or catalysts with varying
turnover rates.
[0066] Graded catalysts can be utilized either in a flow-by
configuration (FIGS. 5 and 6) or in a flow-through configuration
(not shown) where catalyst substantially fills a reaction
microchannel.
[0067] In some embodiments, catalyst particles are hollow or porous
(as defined above). Hollow particles are substantially hollow in
the center of the particles--these are not merely rim-type catalyst
particles with catalyst metal on the exterior of a catalyst
particle, rather, these particles preferably have substantially no
support material in the particle core; in other words, the particle
core is substantially empty. Preferably, hollow or porous particles
have a mass average particle size (as measured by sieving for
larger particles or electron microscopy for smaller particles) that
is in the range of 1 and 40% of the largest diameter of a
cross-section of a reaction microchannel, more preferably in the
range of 1 and 25%, and in some embodiments 5 to 20%, of the
largest diameter of a cross-section of a reaction microchannel. In
some preferred embodiments, at least 50% (by mass) of the catalyst
particles are in the size range of 1 to 300 .mu.m. Particle sizes
in these ranges are advantageous in ease of loading into
microchannels and in generating turbulent flow around the
particles. Particles of these sizes can be readily made by
processes such as spray drying or spray pyrolysis.
[0068] In another preferred embodiment, the catalyst comprises
tubes (hollow fibers). These tubes preferably have an aspect ratio
of at least 2, more preferably at least 10. Preferably, at least
50% (by mass) of the tubes have an external diameter in the range
of 1 to 20 .mu.m.
Methods of Making Catalysts, Supports and Composites
[0069] In some embodiments of the invention, catalysts can be made
by known processes or purchased from commercial sources. An example
of catalyst preparation is found in U.S. patent application Ser.
No. 09/492,950, now U.S. Pat. No. ______ incorporated herein by
reference. Some known methods of forming a graded catalyst are
described by Dalla Betta et al. in U.S. Pat. No. 5,248,251,
incorporated herein by reference. Among other techniques, graded
catalysts can be formed by spraying an active component on the
surface of a support. Graded catalysts can also be formed by
dipping a microinsert or a microreactor apparatus with open
reaction channels (or partly dipping a microinsert or microreactor
to immerse only a portion of the length of the microinsert or
channel length) into a catalyst coating solution or slurry.
[0070] In a novel method of the present invention, a microinsert
catalyst or microinsert catalyst support is prepared by using a
microchannel as a mold. In this method, a powder, liquid or slurry
precursor composition is added to a microchannel; this precursor
composition is reacted, such as by heating, to form a solid
microinsert catalyst or solid microinsert catalyst support. The
mold can be sized to form inserts of desired dimensions--for
example, one preferred mold has a width of 5 mm or less, more
preferably less than 2 mm. If desired, the resulting microinsert
can be additionally treated, for example coated (e.g., wash-coated
and/or coated by vapor deposition) or sintered, either in the mold
and/or in the microchannel reactor. Optionally, the microchannel
mold can be made of (or lined with) a release material such as
Teflon.RTM.. In another embodiment, the microchannel mold is made
of a material that dissolves or degrades--leaving the microinsert
catalyst or microinsert catalyst support. A graded catalyst could
be made in a mold by layering in layers of varying catalytic
activity (that is varying catalytic activity in the final catalyst
after any sintering and/or activation steps)--see FIG. 4. For
example, a graded catalyst can be made by putting a less active
catalyst in a mold followed by a more active catalyst; more
generally, a first type of catalyst 402 that partially fills a mold
is followed by a second type of catalyst 404 (optionally followed
by a third type 406, etc.). In a variation of this technique, a
preform is made by putting more active material in a binder and
partially filling a mold, then additional layer(s) are added of
less active material. The binder can be a wax or any substance that
can be easily be removed (such as by burning or dissolving) without
damaging the catalyst and which fixes the particles into a spatial
relationship that is largely maintained after the binder is
removed.
[0071] In another technique, at least two different types of
particles are introduced in a microchannel mold or reaction
microchannel. One type contains a catalytically active material and
the other type contains a lower concentration (or none) of the
catalytically active material; the relative quantities of each type
of particle can be varied to produce a graded catalyst. Another
good way to form a graded bed is to load a bed with layers of
progressive larger or smaller particle sizes; even if the particles
all have the same metal loading, the smaller particles will pack
more densely, resulting in a higher metal concentration in the
smaller particle layers. There is also a secondary advantage of
varying the particle diameter down the length of the bed; doing so
effectively changes the hydrodynamics in the bed, resulting in a
change in the bed porosity. For some reactions such as Fischer
Tropsch, there are also mass transfer (interparticle) reasons to do
this.
[0072] The techniques discussed herein can be used to form a
catalyst or matrix in a microchannel either to form a microinsert
or to form a catalyst, matrix or composite in situ. In this
invention, a carbon-based, metal-containing, ceramic, or other
matrix can be formed in a microchannel and used, for example, as a
catalyst support, mixing device, or heat transfer mechanism. A
material formed in situ can have excellent physical contact with a
microchannel wall. In addition, if a flow-through configuration is
desired, the material can completely fill the microchannel (except
for pores in the material) regardless of the uniformity of the
channel dimensions, thus mitigating potential by-pass of flow along
channel walls.
[0073] In one embodiment of the invention, a matrix is formed by
mixing carbon fibers, a carbonizable organic powder and a rigidizer
to form a slurry. The slurry can then be placed into a microchannel
using vacuum techniques to ensure complete filling of the
microchannel. The carbon material can then be dried (for example at
about 50.degree. C.), and then pressure and heat treated (for
example, 200 to 2000 pounds per square inch (psi) of an inert gas
and 130.degree. C. to 400.degree. C.). The resulting matrix can
then be carbonized, for example, by treatment at 650.degree. C. or
more in an inert environment. Some conventional examples of forming
carbon fibers and composite materials that can be adapted for use
in microchannels. Examples of conventional methods are provided in
U.S. Pat. Nos. 5,334,414 and 5,744,075, which are both incorporated
herein as if reproduced in full below.
[0074] In another embodiment of the invention, a matrix precursor
is formed by mixing a catalyst alloy of a Raney metal and a
leachable material (for example, Co and Al), a powder of an active
metal (for example, Co), optionally a pore former (e.g., wax
spheres), and optionally a wetting agent (e.g., water). This
mixture is introduced into a microchannel, and, preferably, fills a
reaction zone within the microchannel. The resulting composition
can be dried and then the pore former removed by calcinations. An
example of a conventional method that can be adapted to a
microchannel environment is described in U.S. Pat. No. 5,536,694
which is incorporated herein as if reproduced in full below.
[0075] The invention also provides methods of orienting particles
(such as support particles or catalyst particles) in a microchannel
using an applied electric or magnetic field. For example, magnetic
particles in a microchannel can be oriented with respect to the
magnetic field and/or drawn to the walls of the microchannel. In a
preferred embodiment, a graded catalyst is prepared in which one
type of particle is magnetic (while the other type is less magnetic
or nonmagnetic) and the particles are introduced into a reaction
microchannel under the influence of a magnetic field (meaning that
the walls of the microchannel are magnetized or that an external
magnetic field is applied). Orientation can be conducted on
catalyst particles or on particles (such as support particles) that
are subsequently treated to form catalyst particles.
[0076] In another method, high aspect ratio catalysts or catalyst
supports are grown on a wall or walls of a reaction microchannel.
This may be done under the influence of a magnetic field in which a
magnetic precursor material is drawn to a microchannel wall; for
example, iron flakes or rods can be aligned in a microchannel,
sintered and then used as a catalyst support (followed by any
desired steps of sintering and/or coating of high surface area
layers and active catalyst material). Similarly electroplating or
electroless plating can be used to grow dendritic whiskers or
fibers in direct contact with a wall of a microchannel. In this
case, the microchannels can be filled with an electrolytic solution
which contains dissolved catalyst constituents; upon application of
an electric potential between the channel walls and the solution,
certain catalytic constituents can be preferentially plated onto
the channel walls. In electroplating, the microchannels are filled
with a solution and subjected to an electrical field wherein ions
from the solution are attracted to the wall and deposited thereon.
In electroless plating a solution of the metal and a reducing agent
is introduced into the microchannel and permitted to react so as to
deposit the reduced metal on the microchannel wall. Often the
electroless plating process is preceded by deposition of a small
amount of a nucleating agent or catalyst that facilitates the
reduction of the metal from ions to metal. These configurations can
be particularly advantageous because the intimate attachment of
catalyst to the microchannel wall enhances heat transfer. In some
preferred embodiments, high aspect ratio, thermally conductive
particles are oriented in a direction within a microchannel that is
substantially perpendicular to flow and in contact with a
microchannel wall--thus enhancing thermal transfer.
[0077] In another embodiment, walls of a reaction microchannel are
etched to form a porous catalyst support. Similar to the techniques
described above for catalyst formation in molds, catalysts
(including graded catalysts) can be formed in situ in microchannels
by techniques such as sintering particles. In some other
embodiments, catalysts can be deposited on the walls of
microchannels using electrochemical deposition.
[0078] For use in catalytic applications, matrices (including
graded matrices) can be treated in one or more steps to generate a
catalytic (or more catalytic) surface. Such steps include: etching,
chemical or plasma vapor deposition, wash coating, impregnation,
precipitation, leaching, etc. For use as a mixing or heat transfer
device, the matrix can be used directly or with further treatment
to enhance desired physical properties such as passivation,
etching, coating, etc.
Methods of Loading Microchannels
[0079] When subsequent manufacturing steps won't destroy a
catalyst, a catalyst can be formed in a microchannel by techniques
such as by wash coating, sol-gel processing, chemical vapor
deposition, etc. Also, relatively large particles such as pellets
or monoliths can be dropped or slid into microchannels. In many
cases where bonding or laminating conditions won't destroy the
catalyst, catalyst components can be placed or formed in a partly
assembled device, or catalyst can be rejuvenated after bonding (or
maybe the catalyst preparation isn't completed until after
bonding--that is, a catalyst precursor is loaded into the partly
assembled device); for example deposited on a sheet before the
sheet is bonded into a laminated device.
[0080] For any of fabrication methods mentioning catalyst particles
it should be observed that such particles may include catalyst
precursor particles that could be activated after loading into a
microchannel.
[0081] Relatively small particles such as powders (for example,
particles having at least one dimension of 150 .mu.m or less, in
some embodiments particles having at least one dimension of 100
.mu.m or less, and in some embodiments particles having no
dimension of 50 .mu.m or more) can be difficult to load into a
microchannel due to static electricity or wind or air currents and
catalyst particles can be lost or drift into undesired locations.
According to some preferred embodiments, a magnetic field is
applied to a microchannel or microchannels and catalyst particles
are loaded under the influence of the magnetic field. Preferably
the catalyst particles are magnetized or are ferromagnetic.
Similarly, in some preferred embodiments, catalyst particles can be
loaded into a microchannel that has an electric potential; in some
preferred embodiments the catalyst particles are charged, however,
in many cases particles are sufficiently polarizable that the
particles need not be charged. Alternatively, small catalyst
particles can be prepared in a sol form and coated or loaded into a
microchannel using sonication.
[0082] In addition, or alternatively, to loading particles,
magnetic or electric fields can be used to orient particles within
a microchannel. For example, particles can be loaded (either with
or without a magnetic or electric field) into a microchannel and
subsequently oriented under a magnetic or electric field. In
preferred embodiments, particles with aspect ratios greater than
1.1 (preferably greater than 3, more preferably greater than 10,
hereinafter known as "high aspect ratio particles") are oriented in
a microchannel such that the longest axes of the particles are
substantially perpendicular (within 10 degrees) to the channel
walls--in preferred embodiments, at least 50% of the high aspect
ratio particles, more preferably at least at least 80% of the high
aspect ratio particles are substantially perpendicular to the
channel walls. In especially preferred embodiments, these particles
contact the channel walls and thus conduct thermal energy between
the microchannel walls and the inside of the reaction channel. A
schematic example of particle loading under the influence of an
applied electric field 301 is illustrated in FIG. 3a. Catalyst
particles 305 are loaded into reaction channels 307 under the
influence of the applied field. In the illustrated embodiment,
reaction channel layers are interleaved between microchannel heat
exchangers 303. In the illustrated embodiment, an insulating layer
309 is present. One schematic example having oriented particles is
illustrated in FIG. 3b which shows a microchannel having walls 302,
304, 306. Rod-like catalyst particles 308 are oriented within the
microchannel in substantial conformity with the applied magnetic
field indicated by the arrow.
[0083] High aspect ratio particles can have a catalyst gradient
along the length of the particle. In some preferred embodiments,
the high aspect ratio (catalyst gradient) particles are polarized
and oriented within a reaction channel such that a majority of
particles (at least 60%) are oriented with the particles' ends
having a greater amount of active catalyst metal near a
microchannel wall and the end with a lesser amount of active
catalyst metal near the center of a reaction channel. In some
embodiments, high aspect ratio polarizable particles (with or
without a graded catalyst) are oriented within a microchannel;
preferably so that at least 50% (by mass), more preferably at least
75%, of the particles are oriented within a microchannel such that
their central axes are parallel within 10 degrees. In some
preferred embodiments, particles are oriented within a microchannel
under the influence of a magnetic field and the particles
(including precursor particles such as metallic catalyst supports)
are sintered to freeze the particles in place so that the magnetic
field can be removed without losing particle orientation. High
aspect ratio particles are particles having a height to width ratio
of at least 3 (more preferably at least 10); where height is the
largest dimension and width the smallest dimension that is
perpendicular to height; "smallest dimension" refers to a dimension
such that the particle can fit through a hole of a screen of a
given dimension--for needles this screening can provide accurate
width measurements, for platelets the width is more typically
measured by microscopy.
[0084] In some preferred embodiments, devices, such as that
illustrated in FIG. 3b, can be made by stacking sheets and bonding,
gluing or pressing to keep the sheets together. In some preferred
embodiments, the particles 308 could be fixed in place such as by
heating to sinter particle tips to a microchannel wall. The
catalyst particles can be coated with a catalyst or interfacial
layer either before or after loading the particles in a reaction
channel. In some preferred embodiments, the particles are coated
with a low melting (relative to the microchannel walls) material
that softens and adheres the particles to the channel walls.
[0085] In another embodiment, after introduction of the catalyst
particles they are fixed or attached to the wall in situ. This
attachment can be accomplished by coating the catalyst particles
with a thin film of a material that forms a strong bond to the
microchannel wall and to the particles. This process optionally can
be performed using the catalyst support and the active catalytic
materials can be added in a further step. The coating step can be
accomplished by electroless plating or electroplating. In one
preferred embodiment the coating material is deposited as a metal
or alloy that has a high conductivity to facilitate heat transfer
between catalyst and microchannel wall. In another embodiment the
coating can be chemically or thermally treated to transform a low
thermal conductivity material to a higher thermal conductivity
material, such as by reduction or partial reduction of the
material.
Methods of Removing Catalyst
[0086] Due to the small dimensions of microreactors, catalyst
replenishment can be a costly and formidable challenge. One method
of removing catalysts from microchannels is to conduct a different
type of reaction such that either the energy released is high
enough to decompose the catalyst or its support, or decompose part
of the catalyst structure such that the overall catalyst structure
collapses and can be readily removed from the channel by a process
such as suction, flushing, rinsing, or further chemical reaction.
Another method of removing a catalyst is to introduce a corrosive
liquid that dissolves the catalyst but does not attack the reactor.
Catalyst removal could require multiple steps such as a combustion
reaction followed by an acid wash.
Flow Disruptors and Micromixers
[0087] A flow disruptor is a component disposed in the open flow
path over a catalyst. The flow disruptor is in the open flow path
at a point wherein the open flow path has at least one dimension of
5 mm or less. The flow disrupter converts laminar or transitional
flow to turbulent flow. The flow disruptor occupies less than about
20%, and preferably less than 10% of the volume of the open flow
path. A simple "vertical" post that provides structural support is
not a flow disrupter (on the other hand, a "vertical" post with
horizontal fins can be a flow disruptor). Also, according to the
present invention, posts or wires that function to retain catalyst
do not qualify as flow disruptors. In one preferred embodiment,
illustrated in FIG. 2, the flow disrupter 204 is a bar. While a
straight bar is depicted, it should be appreciated that the
disrupter may take other shapes such as corkscrew, barbed wire,
etc. The flow disrupter is depicted as a wire (which is preferable
in some embodiments), but can be a strand or other material. The
flow disrupter is disposed over catalyst 202. The catalyst can be
any suitable form including, but not limited to, washcoat, vapor
deposit or monolithic insert. The microchannel 208 has a height h.
The height h' of open flow path 206 is less than 5 mm. There may be
a single flow disrupter in a reaction microchannel, or there could
be multiple disruptors such as at least 2, 5, 10 or more. While the
figure illustrates a flow disrupter that has supports projecting
from the catalyst, it should be appreciated that supports for the
disruptors need not contact the catalyst. Flow disruptors could
also be posts or other projections (that do not provide structural
support) that cause turbulent flow. The flow disruptors could also
be unsupported inserts such as a spiral winding inserted into a
reaction channel. In some preferred embodiments, the disruptors are
noncatalytic (i.e., they do not catalyze the chemical reaction
being run or intended to run in the apparatus), although catalytic
flow disruptors could have catalytic activity. Of course, a "flow
disrupter" should not be interpreted as merely catalyst particles,
pellets, coatings, foams, wads, felts or honeycombs loaded in a
reaction channel. A flow disrupter could be coated or otherwise
treated to enhance its chemical stability, compatibility, or
activity for the desired reaction.
[0088] In another preferred embodiment, the flow disrupter is a
strand that is pulled up from a catalyst felt.
[0089] A micromixer includes static mixers and microturbines.
Micromixers can be, for example, flow disruptors as described
above. In one preferred embodiment, the micromixer is a corkscrew
or spiral winding with the central axis in the direction of flow.
Microtubines can be powered by an external electrical source, but
more preferably, the turbine is not electrically powered, but is
motivated by fluid flow through the turbine. The micromixers are
either made of a nonporous catalyst for the intended reaction or a
porous or nonporous support material that is coated with a
catalytically active material. Preferably, a catalytically active
material is dispersed in isolated islands over the surface of the
micromixer. More preferably the micromixer is formed of a support
material that has a relatively (relative to the support) high
surface area coating (preferably a metal oxide) and a catalytically
active metal dispersed on the high surface area coating. In some
embodiments, a buffer layer can be disposed between the support
(the support may be stainless steel, other metal, or ceramic) and
the high surface area coating. For low temperature applications,
the support can be plastic. A microturbine in a microchannel will
either impart energy to the fluid stream or extract energy from the
fluid stream. In the former case the microturbine needs to be
powered by electrical, mechanical, magnetic, or other power source.
In the latter case, the microturbine can be used to generate power
for use in another part of the process, or simply to modify the
flow characteristics of the fluid stream. In the present invention,
a micromixer is a component that is disposed within a bulk flow
region of a reaction chamber (preferably a microchannel)--where the
bulk flow region is substantially unobstructed except for the
micromixer. A micromixer has at least one dimension of 5 mm or
less.
[0090] According to the present invention, flow disruptors,
micromixers (including microturbines) do not form an impermeable
barrier in the open flow path such as would force substantially all
fluid to flow through the catalyst.
[0091] Flow disruptors are preferably drawn materials such as wires
or strands that can be welded or adhesively bonded in a
microchannel. Micromixers are preferably manufactured separately
and placed in a reaction channel; alternatively, the micromixers
can be co-cast or machined into a reaction channel. Microturbines
can be made using techniques developed in micro-electro-mechanical
system technology. Catalytic coatings can be applied by
wash-coating, chemical vapor deposition, electrochemical
deposition, etc.
Chemical Reactions
[0092] Operating conditions can be adapted in particular to the
particular conformation of the channel, nature and amount of
catalyst, and type of chemical reaction performed. Processes of the
present invention include: acetylation, addition reactions,
alkylation, dealkylation, hydrodealkylation, reductive alkylation,
amination, ammoxidation aromatization, arylation, autothermal
reforming, carbonylation, decarbonylation, reductive carbonylation,
carboxylation, reductive carboxylation, reductive coupling,
condensation, cracking, hydrocracking, cyclization,
cyclooligomerization, dehalogenation, dehydrogenation,
oxydehydrogenation, dimerization, epoxidation, esterification,
exchange, Fischer-Tropsch, halogenation, hydrohalogenation,
homologation, hydration, dehydration, hydrogenation,
dehydrogenation, hydrocarboxylation, hydroformylation,
hydrogenolysis, hydrometallation, hydrosilation, hydrolysis,
hydrotreating (including hydrodesulferization HDS/HDN),
isomerization, methylation, demethylation, metathesis, nitration,
oxidation, partial oxidation, polymerization, reduction,
reformation, reverse water gas shift, Sabatier, sulfonation,
telomerization, transesterification, trimerization, and water gas
shift. For each of the reactions listed above, there are catalysts
and conditions known to those skilled in the art; and the present
invention includes apparatus and methods utilizing these catalysts.
For example, the invention includes methods of amination through an
amination catalyst and apparatus containing an amination catalyst.
The invention can be thusly described for each of the reactions
listed above, either individually (e.g., hydrogenolysis), or in
groups (e.g., hydrohalogenation, hydrometallation and hydrosilation
with hydrohalogenation, hydrometallation and hydrosilation
catalyst, respectively). Suitable process conditions for each
reaction, utilizing catalysts and/or apparatus of the present
invention can be identified through knowledge of the prior art
and/or routine experimentation. To cite one example, the invention
provides a Fischer-Tropsch reaction using any of the catalysts or
apparatus described herein wherein the catalytically active metal
comprises Co, Fe, Ru, Ni, or a combination of these, and optionally
containing a promoter or promoters as known in the art.
[0093] In some embodiments, a process will be conducted at
temperatures of 300.degree. C. or above, or 450.degree. C. or
above, or in some embodiments in the range of 200.degree. C. to
1000.degree. C., or in some embodiments in the range of 300.degree.
C. to 800.degree. C. In its broader aspects, the invention is not
limited to these temperatures. In some preferred embodiments, the
reactants are gaseous and the process is run at greater than 2
atmospheres (atm) absolute, and in some embodiments 20 atm or
greater, in some embodiments 100 atm or greater; however, in some
embodiments of the invention, reactions can be run at low pressures
well below 1 atm.
[0094] In some embodiments of the invention, a dispersion of
catalyst particles in a fluid is passed through a reaction
microchannel. The catalyst particles then pass into a second
reactor or are recirculated into the reaction microchannel. In a
microchannel reactor the slurry particles need to be small enough
to flow smoothly without forming impediments to fluid flow. In a
slurry process at least 80% (by mass), more preferably at least
90%, and still more preferably essentially all of the catalyst
particles are less than 75 micrometers, preferably less than 50
micrometers, and most preferably less than 20 micrometers in their
largest dimension. The exact dimension of the particles is a
function of the viscosity of the slurry fluid, the flow rate, gas
composition and other factors. Slurry reactors are particularly
advantageous for the Fischer-Tropsch reaction where enhanced heat
and mass transport are desired.
[0095] While the invention has generally been referred to under
steady state conditions, it will be appreciated that the heat
exchanger(s) can be used to bring (or maintain) a reaction to a
desired temperature range. In some embodiments, the adjacent
channels could alternatively be used for another reaction--for
example, an exothermic reaction can be conducted in a reaction
channel and an endothermic reaction conducted in an adjacent
reaction channel, in which case the heat exchange channel(s) could
also be conducting a chemical reaction, and could contain an
appropriate catalyst. The invention includes methods of starting up
a reaction using the inventive catalysts and/or apparatus, for
example, operating a combustion reaction in a microchannel
containing a graded catalyst to provide start-up heat to an
adjacent reaction chamber.
Closure
[0096] While preferred embodiments of the present invention have
been shown and described, it will be apparent to those skilled in
the art that many changes and modifications may be made without
departing from the invention in its broader aspects. The appended
claims are therefore intended to include all such changes and
modifications as fall within the true spirit and scope of the
invention.
* * * * *