U.S. patent application number 12/911712 was filed with the patent office on 2011-02-17 for fluidization and solids processing in microchannel devices.
This patent application is currently assigned to VELOCYS INC.. Invention is credited to Eric Daymo, David J. Hesse, Kai Jarosch, Steven T. Perry, Laura J. Silva, Anna Lee Tonkovich.
Application Number | 20110039955 12/911712 |
Document ID | / |
Family ID | 34966438 |
Filed Date | 2011-02-17 |
United States Patent
Application |
20110039955 |
Kind Code |
A1 |
Jarosch; Kai ; et
al. |
February 17, 2011 |
Fluidization and Solids Processing In Microchannel Devices
Abstract
This invention describes gas-solid, liquid-solid and
gas-solid-liquid processes in microchannels devices including such
processes as heterogeneous catalysis, particle formation, particle
attrition, particle separation and adsorption or desorption of
selected species. Various processes can be enhanced by the unique
properties of microchannels such as the predominance of laminar
flow, high rates of shear, high rates of heat transfer and high
rates of mass transfer. Also encompassed by this invention are
methods for the introduction to and removal from microchannels of
particle containing fluid streams.
Inventors: |
Jarosch; Kai; (Bexley,
OH) ; Tonkovich; Anna Lee; (Dublin, OH) ;
Hesse; David J.; (Columbus, OH) ; Daymo; Eric;
(Dublin, OH) ; Perry; Steven T.; (Galloway,
OH) ; Silva; Laura J.; (Dublin, OH) |
Correspondence
Address: |
FRANK ROSENBERG
P.O. BOX 29230
SAN FRANCISCO
CA
94129-0230
US
|
Assignee: |
VELOCYS INC.
Plain City
OH
|
Family ID: |
34966438 |
Appl. No.: |
12/911712 |
Filed: |
October 25, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10830710 |
Apr 22, 2004 |
7820445 |
|
|
12911712 |
|
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Current U.S.
Class: |
518/709 |
Current CPC
Class: |
B01J 2219/0081 20130101;
B01J 19/0093 20130101; B01J 2219/00869 20130101; B01J 2219/00873
20130101; B01J 2219/00783 20130101; B01J 2219/00905 20130101; B01J
2219/00835 20130101; B01J 2219/00891 20130101; B01J 2219/00824
20130101; B01J 2219/00889 20130101; B01J 2219/0086 20130101; B01J
2219/00822 20130101 |
Class at
Publication: |
518/709 |
International
Class: |
C07C 29/151 20060101
C07C029/151 |
Claims
1-8. (canceled)
9. A method of conducting a chemical process, comprising: passing a
gaseous stream through a microchannel; controlling the flow of the
gaseous stream to obtain a laminar flow regime; wherein said
gaseous stream comprises solid particulates; wherein, after the
solid particulates flow through the microchannel, further
comprising a step of recycling the solid particulates so that the
solid particulates pass through the microchannel for a second
time.
10. The method of claim 9 wherein the solid particulates are made
in the microchannel.
11. The method of claim 10 wherein the microchannel comprises an
inlet, an outlet and walls defining the microchannel; wherein a
second gas enters the microchannel through an orifice in a wall of
the microchannel; wherein the orifice is located between the inlet
and the outlet; and wherein the second gas is a reactant, diluent
or quenching agent.
12. (canceled)
13. The method of claim 9 wherein the solid particulates are
catalyst particles.
14. The method of claim 13 further comprising a step of segregating
the catalyst particles after they have been passed through the
microchannel.
15. The method of claim 9 comprising passing the gaseous stream
through a flow modifying manifold connection and through a
microchannel array.
16. (canceled)
17. A method of conducting a chemical process, comprising: flowing
a fluid stream into a header and through a flow modifying manifold
connection to form a distributed flow; wherein there is an
interface between the header and a microchannel array; and wherein
(a) the distributed flow carries solid particulates through the
microchannel array, or (b) the distributed flow entrains solid
particulates.
18. The method of claim 17 wherein the distributed flow entrains
particulates and carries the entrained particulates through the
microchannel array.
19. The method of claim 17 wherein the flow modifying manifold
connection tends to distribute flow equally into the microchannel
array.
20. The method of claim 17 wherein the fluid stream is a gaseous
stream carrying solid particulates; wherein the microchannels in
the microchannel array have the same length and the same gap; and
further comprising a step of flowing the gaseous stream into the
microchannel array such that such that the run up length of the
microchannels in the microchannel array, x.sub.H, is less than the
microchannel length; wherein x H = 0.338 ( H 2 ) 2 ( U v )
##EQU00006## wherein H is the smallest dimension (gap) of the
microchannel, U=superficial velocity of the gas in the microchannel
v=dynamic viscosity of the gas in the microchannel.
21. A method of conducting a chemical process, comprising: flowing
a first fluid stream carrying solid particulates through a
microchannel; wherein at least a portion of the solid particulates
are agglomerated; and passing a second fluid stream into the
microchannel at a rate sufficient to cause a decrease in particle
agglomeration.
22. A method of conducting a chemical process, comprising:
providing a first fluid stream carrying solid particulates into a
microchannel in a direction opposing gravity; wherein the
microchannel has an inlet, an outlet and a length between the inlet
and the outlet; wherein the microchannel is further defined by at
least one microchannel wall; wherein said at least one microchannel
wall comprises at least one orifice; wherein said at least one
orifice is located between the inlet and the outlet; wherein the
first fluid stream flows into the microchannel at a rate such that
at least a portion of said particulates are pulled down by gravity
and are not carried along the entire length of the microchannel;
and providing a second fluid that passes through the orifice, into
the microchannel, and out of the outlet.
23. The method of claim 22 wherein at least a portion of the solid
particulates are agglomerated; and wherein the second fluid passes
into the microchannel at a rate sufficient to cause a decrease in
particle agglomeration.
24. A method of conducting a catalyzed chemical reaction in a
microchannel reactor, comprising: providing a first fluid stream
carrying solid catalyst particulates into a microchannel; using the
solid catalyst particulates to catalyze a reaction in the
microchannel; wherein the solid catalyst particulates move out of
the microchannel; and subsequently, recycling the solid catalyst
particulates back into the microchannel in a continuous
fashion.
25-29. (canceled)
Description
INTRODUCTION
[0001] Fluidization is the process by which a fluid is passed
through a bed of solids at a velocity that suspends the solids in
the flowing stream. Types of fluidization have been categorized by
authors such as Grace, J. R., "Contacting Modes and Behaviour
Classification of Gas-Solid and other Two-Phase Suspensions", Can.
J. Chem. Eng., v 64, pp 353-363, 1986. In industry, fluidization
has been applied to a number of chemical engineering unit
operations including chemical reaction (for example, gas oil
cracking, phthalic anhydride production, acrylonitrile production,
and combustion reactions), material separation and classification,
drying, calcining, and ore roasting amongst others.
[0002] An advantage of fluidizing catalyst particles is the very
short mass transfer distances, from both the bulk of the gas to the
catalyst surface and from the surface of the catalyst to internal
sites, involved. An alternative method to achieve short mass
transfer distances is to conduct catalyzed reactions in a
microchannel reactor. However, the conventional wisdom has been
that microchannel reactors should not be used with fluidized
particulates since the particulates would clog the microchannels.
See, for example, Wegeng et al., U.S. Pat. No. 5,811,062.
[0003] On the other hand, microchannel reactors have been
successfully used for liquid phase reactions that produce very
small, highly uniform particles. See Meeh et al., U.S. Pat. No.
5,342,609 and Nickel et al, U.S. Pat. No. 6,566,519. A subsequent
paper by a group of authors including Nickel reported that the
liquid-phase synthesis of pigment particles in a microchannel
reactor resulted in reactor pigments with D.sub.50 values smaller
than 250 nm with a standard deviation for the particle size
distribution of s=1.5. Willie et al., "Progress in Transferring
Microreactors from Lab into Production--an Example in the Field of
Pigments Technology," IMRET Int'l Conf. on Microreaction
Technology," Nov. 30, 2001.
[0004] As described below, the present invention describes chemical
processes utilizing the fluidization of particulates including gas
phase processes and chemical reactions catalyzed by particulate
catalysts moving in fluids through microchannels.
SUMMARY OF THE INVENTION
[0005] In a first aspect, the invention provides a method of
conducting a chemical process, comprising: flowing a gaseous stream
carrying solid particulates through a microchannel; controlling the
flow such that operating conditions and microchannel dimensions are
such that the run up length, x.sub.H, is less than the microchannel
length; wherein
x H = 0.338 ( H 2 ) 2 ( U v ) . ##EQU00001##
These variables are described elsewhere in the specification. In
preferred embodiments, the microchannel has a rectangular
cross-section. In another preferred embodiment, the solids flux
through the microchannel cross section is continuously greater that
0.01 kg/m.sup.2s for at least 8 hours. In more preferred
embodiments, the run up length is up to 75, 50, 20 or 10% of the
microchannel length.
[0006] In another aspect, the invention provides a method of
conducting a chemical process, comprising: passing a gaseous stream
through a microchannel, and controlling the flow of the gaseous
stream to obtain a laminar flow regime. The gaseous stream
comprises solid particulates. In one preferred embodiment, the
particulates are synthesized within the microchannel. In another
preferred embodiment, a second gas enters the microchannel;
preferably the second gas is a second reactant, a diluent, or a
quench gas, thus limiting the reaction to the microchannel.
Optionally, the particulates can be recycled or removed.
[0007] The invention further provides a method of conducting a
chemical process, comprising: flowing a fluid stream into a header
and through a flow modifying manifold connection to form a
distributed flow. This process occurs in an apparatus in which
there is an interface between the header and a microchannel array.
The distributed flow (a) carries solid particulates through the
microchannel array, or (b) entrains solid particulates. In some
preferred embodiments, the distributed flow both entrains
particulates and carries the entrained particulates through the
microchannel array. In some embodiments, the flow modifying
manifold connection tends to distribute flow equally into the
microchannel array. In a preferred embodiment, flow is distributed
with a Q factor of less than 30%.
[0008] In yet another aspect, the invention provides a method of
conducting a chemical process, comprising: flowing a first fluid
stream carrying solid particulates through a microchannel; wherein
at least a portion of the solid particulates are agglomerated; and
passing a second fluid stream into the microchannel at a rate
sufficient to cause a decrease in particle agglomeration.
[0009] In a still further process, the invention provides a method
of conducting a chemical process, comprising:
[0010] providing a first fluid stream carrying solid particulates
into a microchannel in a direction opposing gravity;
[0011] wherein the microchannel has an inlet, an outlet and a
length between the inlet and the outlet;
[0012] wherein the microchannel is further defined by at least one
microchannel wall;
[0013] wherein said at least one microchannel wall comprises at
least one orifice;
[0014] wherein said at least one orifice is located between the
inlet and the outlet;
[0015] wherein the first fluid stream flows into the microchannel
at a rate such that at least a portion of said particulates are
pulled down by gravity and are not carried along the entire length
of the microchannel; and
[0016] providing a second fluid that passes through the orifice,
into the microchannel, and out of the outlet. The invention also
provides a chemical system having these components. In some
embodiments, this method can be used to separate particles. In some
embodiments, the apparatus has an array of microchannels and in
some embodiments, the microchannel or array of microchannels are
substantially parallel to gravity. In some preferred embodiments,
the second fluid comprises a quenching agent.
[0017] In a further aspect, the invention provides a method of
conducting a chemical reaction in an integrated microchannel
device, comprising:
[0018] entraining catalyst particles in a fluid to form a fluidized
stream;
[0019] passing the fluidized stream from an entrainment zone to at
least one process microchannel;
[0020] passing the fluidized stream around a dividing wall;
[0021] wherein, after the fluidized stream passes around the
dividing wall, particles in the fluidized stream pass back down at
least one return microchannel;
[0022] wherein the dividing wall separates the at least one process
microchannel and the at least one return microchannel; and
[0023] wherein, after the fluidized stream passes around the
dividing wall fluid from the fluidized stream exits through an
outlet; and
[0024] wherein the stream exiting through the outlet has a lower
volume fraction of particulates than the fluidized stream. In a
preferred embodiment, the second fluid flows into microchannel to
sweep particles down the return side. Preferably, the at least one
process microchannel and the at least one return microchannel are
disposed on a common plane. This method can be used for chemical
reactions, for example, methanol synthesis.
[0025] In yet another aspect, the invention provides a method of
conducting a catalyzed chemical reaction in a microchannel reactor,
comprising: providing a first fluid stream carrying solid catalyst
particulates into a microchannel; and using the solid catalyst
particulates to catalyze a reaction in the microchannel. In this
method, the solid catalyst particulates move out of the
microchannel, and are subsequently recycled back into the
microchannel in a continuous fashion.
[0026] In a further aspect, the invention provides a method of
entraining particulates in a gas stream, comprising: flowing a
liquid/particle slurry into a wick inside a microchannel to form a
slurry in a wick in a microchannel; exposing the slurry in a wick
in a microchannel to pressure and temperature conditions such that
the liquid evaporates; and forming a solids-in-gas suspension
comprising particles from the slurry. Preferably, this method
further comprises a step of passing a gas into the microchannel to
assist in entraining the particles in a gas.
[0027] The invention also provides a method of removing
particulates from a suspension, comprising: flowing a liquid in a
wick in a microchannel; flowing a solids-in-gas suspension into the
microchannel comprising the liquid in the wick; wherein solid
particulates in the solids-in-gas suspension are transferred into
the liquid in the wick. The liquid suspension thus formed can flow
out of the microchannel and may be easily separated from the
gas.
[0028] While several of the above-described inventive aspects have
been described in terms of methods, it should be appreciated that
the invention includes any of the apparatus or chemical systems
(apparatus in combination with fluids and/or process conditions)
that are described herein. For example, the invention also provides
a chemical system comprising an apparatus comprising a wick in a
microchannel wherein there is a slurry in the wick and a
solids-in-gas suspension in the microchannel.
[0029] In some inventive aspects, solid particulates can be carried
along at substantially the same speed as the fluid--this happens
with suspensions of very small particles that are held in solution
by sol-type intermolecular interactions. In some preferred
embodiments, the particulates move at a rate that is significantly
slower (i.e., at least 10% longer mean residence times) than the
carrier fluid--this happens where the velocity of the particles is
decreased by a force other than the fluid drag on the particles
including, but not limited to, gravity, a magnetic field, or
electrostatic interactions with the walls. For example for
particles which are large enough so that a gravitational body force
acting on the particles has a non-trivial component in a direction
opposite to the direction of fluid flow, the particles are slowed
down relative to the bulk fluid flow. Flow of the carrier fluid can
be laminar, transition, or turbulent. In some preferred
embodiments, the fluid is a gas. In some preferred embodiments, the
contact time of gas through the microchannels is less than 500 ms,
in some embodiments less than 100 ms, in some embodiments less than
50 ms, and in some embodiments, 1 to 500 ms. For a system with a
liquid carrier fluid as opposed to a gaseous carrier fluid, the
expected range of reaction contact times may be up to three orders
of magnitude higher than the preferred ranges for gaseous
reactions. In some embodiments of the invention, the particulates
are acted upon by a force other than fluid drag inside the
microchannel(s) and thus may have an average residence time that is
at least 10% longer, in some embodiments at least 30% longer than
the carrier fluid. The average residence time of solids can be
measured (preferably) by introducing tracer particles and measuring
the time needed for them to emerge or (if tracer particles are
impracticable) by introducing solids and using optical or fiber
optical probes to assess the solids content of the gas exiting the
microchannels.
[0030] In some embodiments, the median particle size is at least
0.1 micron (.mu.m), in some embodiments at least 5 .mu.m, in some
embodiments at least 20 .mu.m, in some embodiments no larger than
50 .mu.m, in some embodiments no larger than 100 .mu.m, in some
embodiments no larger than 500 .mu.m, and in some embodiments
within the ranges set by these preferred sizes. In some
embodiments, smaller mean particle diameters may also be used in
the inventive process, including mean particle diameters in the
range of 5 to 100 nanometers. In some embodiments the volume
fraction solids in a fluid stream is at least 0.01, in some
embodiments at least 0.05, in some embodiments at least 0.10, in
some embodiments no more than 0.20, in some embodiments no more
than 0.40, in some embodiments no more than 0.70, and in some
embodiments within the ranges set by these preferred volume
fractions. In some embodiments the solids flux through a
microchannel, array of microchannels, or device is at least 0.1
kg/m.sup.2s, in some embodiments at least 1.0 kg/m.sup.2s, in some
embodiments at least 10 kg/m.sup.2s, in some embodiments at least
50 kg/m.sup.2s, in some embodiments at least 100 kg/m.sup.2s and in
some embodiments in the range of 10 to 500 kg/m.sup.2s.
[0031] Catalyst particles can include metal particles such as Ni,
Fe, and Co; or metals dispersed on a metal oxide such as transition
metals on a metal oxide, in some preferred embodiments precious
metals such as Pt, Rh, Ir, Pd, Os, Re, and Ru on a metal oxide. In
some embodiments, at least 50 volume %, and in some systems at
least 90 volume % of the solids present in the system are catalyst
particles.
[0032] Back-mixing in transported beds can have a significant
negative impact on product yield in processes such as the
conversion of gas oil to gasoline, including a reduction in
selectivity to the desired product by flowing the desired product
back through reaction conditions and thus allowing secondary
reactions. In some embodiments of this invention, high shear rates
and laminar flow regime present in a microchannel substantially
reduce the presence of back-mixing.
[0033] Various embodiments of the present invention offer further
advantages in process efficiency and intensification. In preferred
embodiments, the process microchannels are straight-through
channels that lack any curves or serpentine configurations. In some
preferred embodiments, pressure drop through the microchannel,
microchannel array, or entire device, is less than 10 bar, in other
embodiments is less than 1 bar, in other embodiments is less than
0.5 bar, more preferably less than 0.25 bar, and in some
embodiments is in the range of 0.035 to 0.34 bar. In some preferred
embodiments, contact time of the fluid and/or the entrained mixture
through the microchannel, microchannel array, or entire device, is
less than 1000 ms, in other embodiments is less than 500 ms and in
other embodiments is less than 250 ms more preferably less than 50
ms, and in some embodiments is in the range of 10 ms to 100 ms. In
some preferred embodiments for solids entrained in a liquid
reactant, the contact time may be less than 1000 seconds, in other
embodiments less than 500 seconds, and in other embodiments less
than 50 seconds. In preferred embodiments, an inventive process can
be operated continuously for at least 8 hours without clogging and
at substantially undiminished productivity.
[0034] One advantage to the use of microchannels for carrying out
particulate fluidization is the ability to tightly control the
manner in which various fluids are allowed to interact with the
particles. Specifically, the local fluid composition and the time
over which fluid(s) and particles are allowed to remain in contact
can be tightly controlled, a distinct advantage over conventional
fluidized beds. Additional fluid(s) can be separately distributed
to each microchannel and mixed with an already fluidized bed
flowing within the microchannel at temperature and pressure
conditions necessary to carry out the desired chemical process.
Because of the small hydraulic diameter of the microchannels,
mixing is quick and temperature and residence time are more easily
controlled. For example, in one embodiment, one or more additional
reactants may be added to a fluidized bed within a microchannel or
set of microchannels, each comprising at least one other reactant
and an entrained, fluidized particulate catalyst. In this way,
fast, uniform mixing of the reactants can be carried out in close
proximity to the fluidized catalyst particles, allowing tight
control over reaction times for heterogeneous catalyzed reactions.
In another embodiment, a second fluid is distributed and introduced
into each microchannel in order to quench a chemical reaction
occurring in the microchannels comprising a fluidized particulate
stream. In yet another embodiment, one reactant in a chemical
reaction is distributed along the length over which the fluid in
the fluidized channel flows in order to control the rate of
reaction. In yet another embodiment, a liquid product is removed
via wick as it is formed in the microchannel containing the
fluidized particles, allowing the local equilibrium of the process
to shift as the chemical process is carried out.
[0035] For any of the apparatus and methods, the heights of
connecting channels and/or manifolds are preferably in the range of
20 .mu.m to 50 mm, more preferably 20 mm or less, and even more
preferably less than 10 mm. The thickness of walls separating
channels or manifold walls are preferably in the range of 20 .mu.m
to 5 mm, more preferably 2 mm or less. Connecting channels
preferably have a length of 1 cm to 10 m In a stacked device, the
web thickness between layers is preferably the thickness of a sheet
(in other words, in some preferred embodiments, devices are made by
cutting features through a sheet rather than etching). Any of the
articles described herein may have multiple layers and repeating
sets of layers (repeating units). For example, 2, 10, 50 or more
repeating units within a laminate. This multiplicity, or "numbering
up" of layers creates added capacity of microchannel laminated
devices. Throughout all aspects, the figures are merely
illustrative and do not limit all aspects of the invention).
GLOSSARY
[0036] As is standard patent terminology, "comprising" means
"including" and neither of these terms exclude the presence of
additional or plural components. For example, where a device
comprises a lamina, a sheet, etc., it should be understood that the
inventive device may include multiple laminae, sheets, etc.
[0037] A "chemical process" is a process in which materials undergo
a chemical or physical change. Examples of chemical processes
include chemical reactions, mixing, separations, crystallization,
emulsion formation, drying, and phase changes.
[0038] "Connecting channels" are channels connected to a manifold.
Typically, unit operations occur in connecting channels. Connecting
channels have an entrance cross-sectional plane and an exit
cross-sectional plane. Although some unit operations or portions of
unit operations may occur in a manifold, in preferred embodiments,
greater than 70% (in some embodiments at least 95%) of a unit
operation occurs in connecting channels. In preferred embodiments,
the connecting channel walls are straight.
[0039] "Downdraft" is the movement of fluid or of solid
particulates or of fluid and solids particles (suspension) in a
direction counter to that of the superficial velocity caused by
separation of a portion of the flow from the bulk of the flow.
Downdraft encompasses the formation of recirculation cells in which
fluid and or fluid and solids flow in a `loop` separate from the
bulk of the fluid and or suspension.
[0040] A "header" is a manifold arranged to deliver fluid to
connecting channels and a "footer" is a manifold arranged to take
away fluid from connecting channels.
[0041] In the present invention, a microchannel is defined as a
channel having at least one dimension of 10 millimeters or less, in
some embodiments 2 millimeters or less, and in some embodiments,
0.1 to 1 millimeter. Microchannels are not merely orifices; the
length of a microchannel (that is, the direction of flow during
normal operation) is not the shortest dimension of a microchannel.
Both height and width of a microchannel are substantially
perpendicular to the direction of flow of reactants through the
reactor. Microchannels are also defined by the presence of at least
one inlet that is distinct from at least one outlet--microchannels
are not merely channels through zeolites or mesoporous materials.
The height and/or width of the reaction microchannel is preferably
about 2 mm or less, and more preferably 1 mm or less. Preferably,
the length of a microchannel is greater than 1 cm, in some
embodiments in the range of about 1 to 50 cm. The sides of the
microchannel are defined by a microchannel wall of walls. The
choice of material for the walls depends on the intended use. These
walls are preferably made of a hard material such as a ceramic, an
iron based alloy such as steel, or monel. In some embodiments, the
microchannel walls are comprised of a stainless steel or
Inconel.RTM. which is durable and has good thermal conductivity.
The microchannel devices can be made by known methods, and in some
preferred embodiments are made by laminating interleaved plates
(also known as "shims"), and in some preferred embodiments, shims
designed for reaction channels are interleaved with shims designed
for heat exchange. In some preferred embodiments, the microchannel
devices are microchannel reactors that include a plurality of
microchannel reaction channels, preferably in thermal contact with
a plurality of adjacent heat exchange microchannels. A plurality of
microchannels may contain, for example, 2, 10, 100, 1000 or more
channels. In preferred embodiments, the microchannels are arranged
in parallel arrays of planar microchannels, for example, at least 3
arrays of planar microchannels. In some preferred embodiments,
multiple microchannel inlets are connected to a common header
and/or multiple microchannel outlets are connected to a common
footer. During operation, the heat exchange microchannels (if
present) contain flowing heating and/or cooling fluids.
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 and 6,219,973 (both of
which are hereby incorporated by reference). Performance advantages
in the use of this type of architecture include their relatively
large heat and mass transfer rates, and the substantial absence of
any explosive limits. Microchannel reactors can combine the
benefits of good heat and mass transfer, excellent control of
temperature, residence time and minimization of by-products.
Pressure drops can be low, allowing high throughput. Furthermore,
use of microchannel reactors can achieve better temperature
control, and maintain a relatively more isothermal profile,
compared to conventional systems. In addition to the process
microchannel(s), additional features such as microchannel or
non-microchannel heat exchangers may be present. Microchannel heat
exchangers are preferred. Heat exchange fluids may flow through
adjacent heat transfer microchannels, and 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 process microchannels.
[0042] In the present invention, unless specified otherwise, the
fluid that fluidizes or carries a particulate may be a gas, vapor,
liquid or mixture of any two or all three.
[0043] Particulate size--In the present invention, "particulates"
(also called "particles") are solids. In the broadest embodiments
of the invention, particulates are just smaller than the
microchannel through which they pass. Generally, particulates can
be of any shape such as granular, dendridic, flake (including
plates), rods (including needles), and irregular shape. In some
preferred embodiments, the particulates are substantially
spherical.
[0044] In the present invention, "particle size" is a distribution
that is characterized per ASTM D 4464-00 entitled `Standard Test
Method for Particle Size Distribution of Catalytic Material by
Laser Light Scattering`. ASTM D 4464-00 has qualified this test for
particles in the range 30 to 300 .mu.m but it is capable of
measuring particles in the range of 0.02 to 2000 .mu.m. Particle
size is quantified by referring to the center value median bin when
a particle size distribution is broken down into at least 10 size
ranges. This test uses a volume weighted particle diameter. In some
preferred embodiments, the particle size distribution has a median
value of 200 to 800 .mu.m, and in some embodiments the particle
size distribution has a median value of 100 to 300 .mu.m and in
some embodiments the particle size distribution has a median value
of 75 to 150 .mu.m, and in other embodiments the particle sized
distribution has a median value of 25 to 75 .mu.m, and in other
embodiments the particle sized distribution has a median value of
10 to 30 .mu.m. In some embodiments there is no particle greater
than 1000 .mu.m, and in some embodiments there is no particle
greater than 100 .mu.m.
[0045] Quality Index factor "Q.sub.1" is a measure of how effective
a manifold is in distributing flow among more than one
microchannel. In the description below, a microchannel is also
referred to as a "connecting channel". It is the ratio of the
difference between the maximum and minimum rate of connecting
channel flow divided by the maximum rate of connecting channel
flow. For systems of connecting channels with constant channel
dimensions it is often desired to achieve equal mass flow rate per
channel. The equation for this case is shown below, and is defined
as Q.sub.1.
Q 1 = m max - m min m max .times. 100 % ##EQU00002##
where m.sub.max[kg/sec]=maximum connecting channel mass flow rate
m.sub.min[kg/sec]=minimum connecting channel mass flow rate For
cases when there are varying connecting channel dimensions it is
often desired that the residence time, contact time, velocity or
mass flux rate have minimal variation from channel to channel such
that the required duty of the unit operation is attained. For those
cases we define a quality index factor Q.sub.2:
Q 2 = G max - G min G max .times. 100 % , ##EQU00003##
where G is the mass flux rate. For cases when all the connecting
channels have the same cross sectional area, the equation for
Q.sub.2 simplifies to Q.sub.1. The quality index factor gives the
range of connecting channel flow rates, with 0% being perfect
distribution, 100% showing stagnation (no flow) in at least one
channel, and values of over 100% indicating backflow (flow in
reverse of the desired flow direction) in at least one channel.
Q.sub.1 and Q.sub.2 are defined based on the channels that comprise
95% of the net flow through the connecting channels, the lowest
flow channels are not counted if the flow through those channels is
not needed to account for 95% of the net flow through the
connecting channels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIG. 1 illustrates the relationship between boundary layer
thickness (B) and displacement thickness (A) showing the location
of potential downdraft (back mixing) cell formation (C).
[0047] FIG. 2 illustrates a system in which a jet of fluid is
injected through an orifice in a microchannel wall to assist in
particle elutriation.
[0048] FIG. 3 is a cross-sectional view of a device for entraining
particulates and distributing the entrained particulates in an
array of microchannels.
[0049] FIG. 4a illustrates a device for a jet-assisted process in a
fluidized bed.
[0050] FIG. 4b is a magnified view of a portion of the jet section
of the device of FIG. 4a.
[0051] FIG. 5a illustrates a cross-sectional view of an apparatus
in which particles are recycled.
[0052] FIG. 5b illustrates an optional, adjacent heat exchange
layer.
[0053] FIG. 6 illustrates a race track structure in a header for
equal distribution of flow in an array of microchannels.
[0054] FIG. 7 shows a flow diagram of a wick-assisted process of
entraining solid particles and illustrates the cross-section of a
microchannel device comprising a wick with liquid in a wick and
solid particulates entrained in a gas.
DETAILED DESCRIPTION OF THE INVENTION
Solids Transport
[0055] Solids bridging is an important cause of channel blockage
when transporting slurried or suspended material. The potential for
bridging is higher when the particles are adhesive and/or cohesive
to each other and to the channel walls.
[0056] Industrially, bridging is avoided by increasing the pipe
diameter, injecting air to break up agglomerates, or intense
agitation in feed tanks (mechanical energy to break up solids).
See, for example,
http://www.magnumsystems.com/caseHist/smootCase004.htm.
Furthermore, industrial slurry transport systems operate in the
turbulent regime so that the velocity is above the minimum required
for complete fluidization. Usually such velocities (combined with
the larger diameter pipes in conventional systems) result in
turbulent flow.
[0057] Another important factor is critical velocity--the velocity
above which the solids are suspended in a slurry system. Slurry
pipelines are normally operated above the critical velocity so that
the slurry is homogeneous. There are many theoretical equations and
correlations for calculating the critical velocity. One popular
theoretical method to calculate the critical velocity is the
Ororskar and Turian equation.
[0058] Despite the high shear forces in laminar flow, conventional
slurry pipelines do not usually operate in laminar flow because the
critical velocity is not exceeded. A microchannel system may be
designed to prevent agglomeration by having one or more of the
following features:
1. Walls with a coating that inhibits the adhesion of particles to
the wall. Surface effects (such as surface charges on the particles
and on the walls and charges created by flowing past a wall) can
play a large role in the agglomeration behavior of particles
flowing in a microchannel. Coatings may include Teflon, polymeric,
metallic, metal oxide, hydrophobic, hydrophilic, and combinations
thereof. 2. Fluid jets that allow fluid flow to break up particle
agglomerates. Micro-jets can be designed to deliver a much higher
shear force than larger air holes in larger systems. The greater
forces imparted by microjets into microchannels can break
agglomerated particles into even smaller particles than would
otherwise be possible.
[0059] Furthermore, it may be desirable to operate the microchannel
solids processing system at low flow rate--perhaps even laminar
flow rates--and reduce pumping requirements. Because of the small
diameters inherent in microchannel systems, it is easier to stay
above the critical velocity and maintain homogeneous flow even
while in the laminar regime. Furthermore, the higher wall shear
forces present with laminar flow may also cause agglomerations to
break up more easily than in turbulent flow. Higher shear forces
are observed in laminar flow (with parabolic velocity profiles)
than in turbulent flow (with nearly flat velocity profiles). This
higher shear rate is due to the higher velocity gradient associated
with the parabolic laminar velocity profile:
Fs=.mu.dV/dr, where Fs is the shear force, .mu. is the viscosity,
and dV/dr is the velocity gradient.
[0060] Shear force on a fluid control element (in discretized form)
in direction of velocity u is calculated by Fx=mu*du/dy, where mu
is viscosity, and du/dy is velocity gradient of the fluid flow
normal to the flow direction. However, as in a location of fluid
(represented by a control element) the velocity generally has 3
components, shear force also has three components. For a channel
flow near and at the surface, a one dimensional assumption can be
generally made and Fx can approximate the net shear at an element
surface of the fluid-wall interface. It is seen that the velocity
field u as function of distance to wall y has to be known in the
equation. Generally one has to solve momentum and mass conservation
equations and eventually also energy equation if temperature field
is non-uniform. The use of computational fluid dynamics, including
commercial software packages such as Fluent.RTM. or FEMLAB.RTM.,
are used to simultaneously solve all transport equations such that
the surface shear force may be calculated. The surface shear force
may be further calculated along the channel length, parallel to the
direction of flow. Shear force may also be calculated between
parallel microchannels, where flow distribution effects are
included to determine the mass flux into each parallel channel as a
function of the detailed channel and manifold geometry. More
stringent expressions and calculation methods can be found in text
books, such as "Fundamentals of Fluid Mechanics, 3.sup.rd Ed." B.
R. Munson, D. F. Young and T. H. Okiishi, John Wiley &Son,
Inc., Weinheim, 1998".
REFERENCES
[0061] A. Shekarriz et. al. 1997. "Cross Site Transfer System at
Hanford: Long-Term strategy for Waste Acceptance." PNNL-11497.
Pacific Northwest National Laboratory, Richland, Wash.
Fluidized Bed Microchannel Advantage--Design and Operation to Avoid
Back-Mixing and Stagnant Zones
[0062] When the design intent of a fluidized bed system is to
actively transport solids, such as is done in fast fluidized
systems or pneumatic transports systems, a major limitation to
operation in the plug flow regime is the development of
recirculation cells otherwise known as downdraft zones. These cells
form locally in the channels used in conventional systems
(regardless of the shape of the cross sectional area) when
separation occurs between the boundary layer and the core of the
developed flow. In many instances, for example fluid catalytic
cracking of hydrocarbons, the formation of recirculation zones is
undesirable because it can lead to increase solids hold up,
resulting in over cracking of the hydrocarbon. Wei et al report in
a paper entitled `A Pilot Plant Study and 2-D Dispersion-Reactor
Model for a High-Density Riser Reactor` in Chemical Engineering
Science v56 pg 613-620 (2001) that for the acrylonitrile process
yield increases from approximately 72% to 84% as back mixing is
eliminated from the fluidized system.
[0063] When properly designed, a fluidized bed application operated
microchannel apparatus can have the advantage of avoiding downdraft
zones and the ensuing recirculation of particulate suspensions
along process channel walls. This advantage relates to the boundary
layer thickness relative to the overall channel dimensions.
[0064] For most macrochannel applications, the boundary layer is
the thin fluid layer adjacent to the surface of a wall or body in
the flow where strong viscous effects exist. These viscous effects
take place due to the large velocity gradients which take place
between the bulk flow in the middle of the channel and the surface
of the wall where a no slip condition holds, i.e., at the wall the
flow is essentially stagnant. Within this viscous sublayer, the
superficial flow velocity may be very low, and the associated
hydrodynamic force may be too low to maintain suspended particles
against the opposing force of gravity, leading to a recirculation
downdraft of the fluidized bed along the channel walls. This
downdraft phenomena can adversely impact the overall process and
can be altogether avoided in some microchannel applications. To
understand how this is true, a careful examination of the boundary
layer is required.
[0065] A commonly used metric of a boundary layer is the boundary
layer thickness, denoted .delta., defined as the distance from the
solid boundary, in this case the wall of the microchannel at which
the local value of the velocity magnitude essentially reaches 0.99
of the free-stream or outer-flow value. Another measure of boundary
layer thickness which is related to .delta. and is more relevant to
the understanding of the downdraft problem is the displacement
thickness, .delta.* (see FIG. 1). This thickness is defined as the
distance which the undisturbed outer flow is displaced from the
boundary (microchannel wall) by a stagnant layer that removes the
same mass flow from the flow fields as the actual boundary layer.
Alternatively, it can be thought of as a zero-velocity layer which
has the same mass-flow defect as the actual boundary layer. The
displacement thickness is therefore the region that defines the
separation zone between the free-stream flow and the wall or
channel boundary where the potential downdraft region exists. If
this separation zone could be removed by merging of the
displacement thicknesses from each of the opposing microchannel
wall the entire flow region would then be encompassed by the
displacement thickness and the downdraft phenomena can be
eliminated above the point at which the displacement thicknesses
meet. The phenomenon can be understood by considering that the
stream lines of the fluid flowing into the microchannel initially
have approximately the same magnitude (the bulk flow value) and are
parallel to one another, this magnitude is greater that that of the
streamlines in the displacement thickness. When the incoming
streamlines contact the flow in displacement thickness they can be
deflected and this shedding can set up the down draft cell as
indicated in FIG. 1. Past the point in the channel at which the
displacement thicknesses from each wall meet (the run up length)
all the streamlines are again parallel and down draft cells cannot
develop.
[0066] The mathematical expressions for .delta. and .delta.* are as
follows:
.delta. = 5.48 x R N ##EQU00004## and ##EQU00004.2## .delta. * =
1.72 x R N ##EQU00004.3##
where x=Distance from inlet (leading edge) of microchannel (see
FIG. 1)
R.sub.N=Ux/v
[0067] U=superficial velocity of the fluid in the microchannel
v=dynamic viscosity of the fluid Thus, both boundary layer metrics
initially grow in thickness as {square root over (x)} and the
displacement thickness is approximately 1/3 of the boundary layer
thickness for the same axial location, x, measured from the inlet
of the channel.
[0068] Typically, macrochannel dimensions are sufficiently large
that the boundary layer does not completely occupy the entire
channel cross-section. However, microchannel dimensions are
sufficiently small that the boundary layer displacement thickness
can exceed 1/2 the minimum distance between the microchannel walls
(H), thereby resulting in complete removal of the associated
stagnation/separation zone between the free-stream and boundary
layer. This minimum distance (H), or critical dimension is also
referred to as the channel `Gap`.
[0069] As design criterion for microchannel fluidized bed
configurations the above can be used to determine the distance at
which the displacement thickness completely occupies the channel
and thus any subsequent axial distance downstream will not exhibit
downdraft. Taking the equation given above for .delta.* and solving
for x=x.sub.H this is to say solving for the distance from the
inlet of the microchannel at which the displacement distances
.delta.* from each opposing channel wall meet (i.e. .delta.*=H/2)
and thus finding the point beyond which the downdraft zones are no
longer significant, we obtain:
x H = 0.338 ( H 2 ) 2 ( U v ) ##EQU00005##
In the above equation variable x.sub.H is also known as the run up
length. The run up length is the distance from the inlet where the
displacement thicknesses meet. The above equation is applicable to
rectangular openings can also be used for other geometries to
estimate the run up length for circular cross sections, in the case
of circular cross-sections the dimension for the gap (H) is
replaced with the value of the circular cross sectional channel
diameter. In some preferred embodiments velocity is controlled such
that the run up length is less than 5% of the total channel
microchannel length, in another embodiment the velocity is
controlled such that the run up length is less than 20% of the
total microchannel length in another embodiment less than 50% and
in another less than 100%. In various embodiments, conditions can
be controlled, for example: per microchannel gas flow less than 100
liters per min (actual volumetric flow rate based on inlet
conditions of pressure and temperature a molar flow not the
volumetric flow rate based on standard conditions of temperature
and pressure) and gap in the range of 0.25 mm to 10 mm with aspect
ratio in the range 1:1 to 50:1; per microchannel gas flow less than
50 liter per min and gap in the range of 0.25 mm to 10 mm with
aspect ratio in the range 1:1 to 50:1; per microchannel gas flow
less than 10 liter per min and gap in the range of 0.25 mm to 10 mm
with aspect ratio in the range 1:1 to 50:1; per microchannel gas
flow less than 1 liter per min and gap in the range of 0.25 mm to
10 mm with aspect ratio in the range 1:1 to 50:1; or a gas flow in
the range of 0.25 liter per minute and a gap in the range of 2 mm
to 10 mm with an aspect ratio in the range of 4:1 to 10:1.
Selective Elutriation
[0070] In some embodiments of this invention microchannel
architecture is used to produce selective separation (elutriation)
of specific sizes shapes or densities of particles from a mixture
of particles by varying the velocity of the fluidizing medium. In
one embodiment it would be desirable to remove light or small
particles and to retain solids of greater than or equal to a
certain density or size, in another it would be desirable to remove
light or small particles less than or equal to a certain density or
size such that they could be collected in a subsequent processing
step. Particle shape may also be used as the determinate of
separation. In yet another embodiment particle capture can be
enhanced by placing a jet or staging point in the channel by which
more fluidizing medium can be injected into the channel such that
the velocity can be increased ensuring any particles that reach the
staging point are carried away (to be rejected, recycled or
collected as desired).
[0071] One embodiment is illustrated in FIG. 2. In this embodiment,
the walls 202, 204 are configured so that the cross-sectional area
of the microchannel increases with distance up the channel. This
increase in area reduces velocity and reduces fluid viscosity so
that particles 206 will fall more easily. Channel walls above and
below the plane of the page (not shown) constrain flow. An orifice
208 in at least one of the microchannel walls allows in-flow 210
from a high pressure gas source. In some preferred embodiments, the
high pressure gas is a gas flow injected from an adjacent
microchannel. The injected flow sweeps up the higher (or otherwise
more readily fluidized) particulates and carries them out of the
microchannel.
Manifolding to Distribute Flow in Particulate-Containing
Microchannels
[0072] Flow from a header into a microchannel array and/or from a
microchannel array into a footer can be controlled through the use
of manifolding structures. One embodiment for manifolding within
microchannel devices including structures and methods of
controlling flow are described in detail in U.S. patent application
Ser. No. 10/695,400 filed Oct. 27, 2003, which is incorporated
herein as if reproduced in full below. This invention includes any
of the structures or methods described in the incorporated
reference with the additional requirement of a
particulate-containing stream as described herein.
[0073] In the present invention, features (such as described in the
incorporated application) that distribute flow from a manifold to
plural microchannels are called "flow modifying manifold
connections" because these features modify flow as compared to a
simple header in which flow is passed through a manifold to the
microchannels in an unobstructed, unmodified fashion. In a typical
case, flow enters a manifold and, without obstruction, turns into
an array of microchannels; a problem is that momentum carries a
disproportionately large part of the flow into the microchannels
that are farthest from the manifold inlet. One type of flow
modifying manifold connection is race tracks (see FIG. 6) that are
walls disposed in a header that segregate flow in the header at a
point sufficiently upstream of a turn to reduce flow
maldistribution caused by momentum effects. Another type of flow
modifying manifold connection is projections into the microchannels
that obstruct flow to control flow in a desired manner.
[0074] In some preferred embodiments, at least two, more preferably
at least 5, in some embodiments at least 10 or at least 100 or, in
some embodiments, 5 to 500 connecting channels are served by a
single manifold. In many preferred embodiments, flow is controlled
to be distributed equally over multiple connecting channels with Q
factors (as described in the Glossary, and in alternate embodiments
include Q.sub.1 or Q.sub.2) of 30% or less, more preferably 20% or
less, and in some embodiments, in the range of 0.1% to 15%.
[0075] In one embodiment, (see FIG. 3) the microchannel portion of
a device can be placed directly on a wind box D and entrainment
section (302). The wind box is an open volume into which a
fluidizing medium is brought prior to distribution into an
entrainment section or process microchannel. It may either be empty
or contain features (such as race tracks, see FIG. 6) to aid in
fluid distribution. A distribution plate 304, optionally, can be
placed between the wind box and the entrainment zone. In a
preferred embodiment, the distribution plate has microchannels for
fluid flow to create jets to assist particulate entrainment. In a
preferred embodiment, the distribution plate is a flow modifying
manifold connection that tends to equalize flow over the length of
the entrainment zone. "Tends to equalize" does not mean that flow
will be perfectly equally distributed to the microchannels, but
that flow is more equalized than if there were no design features
tending to equalize flow. Alternatively, or in addition, a flow
modifying manifold connection can be placed in the entrainment zone
and/or at the border between the entrainment zone and the process
microchannels C.
[0076] The entrainment section 302 is a zone in which particles are
contacted with the fluidizing medium. In some preferred embodiments
the zone will be maintained with a solids fraction of less than
0.05 in some embodiments with a solids fraction of less that 0.25
and in some embodiments with a solid fractions fraction of less
than 0.7. In some embodiments, solids fraction is at least 0.01 and
in some embodiments at least 0.05. A stream of fluidizing medium
(E) can be brought into the wind box (D) and distributed in to the
entrainment section (C). The parts can be modular components that
are attached to each other, or the parts can be regions in an
integrated design. In the illustrated design, the footer is on top.
The distribution may be accomplished using a conventional or
microchannel based distribution plate. The microchannel
distribution plate can be a plurality of parallel microchannels or
a set of microchannels tending to produce a distribution of
fluidizing medium uniformly across the distribution surface and in
other embodiments to produce a non-uniform distribution. In a
preferred embodiment, the interface between wind box (D) and the
entrainment section (C) includes a structure that tends to equalize
flow in the microchannels (see e.g., the incorporated patent
application).
[0077] When assembled as a unit, the wind box D is preferably
disposed at the `bottom` with respect to gravity. Particles can be
returned to the entrainment section via conduit (H) and entrained
in the fluidizing medium and subsequently carried into the
microchannels (G). The particulate-containing fluid stream exits
the microchannels enters the header (A). The header may be designed
such that the velocity of the combined streams carries the
particles out of the header to a subsequent separation step. This
may be done by reduction of the header cross section or a sweeping
stream can be used to assist the removal of the particles. In some
preferred embodiments the entrainment section is maintained with a
solids fraction of less than 0.02, in some embodiments less than
0.05, and in some embodiments 0.05 to 0.70
[0078] The microchannel distribution plate creates a flow pattern
in the entrainment zone that is conducive to suspending and
carrying particles into the process microchannels.
[0079] In some embodiments, the channels in the microchannel
distribution plate create fluid bubbles of a desired size. In a
preferred embodiment, particles are returned from the footer to the
entrainment vessel via an external loop, their movement being
assisted by a flow of fluidizing medium. Once in the entrainment
section the particles are suspended by the fluidizing medium
exiting the microchannel distribution plate and carried into the
process microchannels.
[0080] Once in the process microchannels, a solid suspension can be
temperature controlled by external heat exchanger J or can be
rapidly brought to temperature target reaction temperature by heat
exchange fluid flowing in interleaved heat transfer channels (not
shown). In a preferred embodiment of catalyzed reactions, a desired
reaction takes place and the solids suspension of catalyst
particles exits the process microchannels. If desired, a multiple
temperature zone approach can be used to provide a thermal quench
to stop the reaction or to conduct additional reactions at
different temperatures prior to the suspension leaving the process
microchannels. Alternatively, a temperature gradient might be
maintained along the flow length of the microchannels within a
given temperature zone. Integrated heat addition or removal is
enabled by this invention, allowing fluidized reactions to benefit
from the tight temperature control possible in microchannels with
its associated benefits of control over selectivity and conversion.
Additional reactants can also be added at any point in the
microchannels, perhaps between temperature zones. Products and/or
solids can also be separated within the microchannels at any point
downstream of other unit operations within the same microchannel
hardware. Finally, the solids suspension enters the footer. Once in
the footer, the solids suspension can be carried by the fluidizing
medium to a gas solids separator. If necessary, a supplemental
sweep stream, preferably taken as a slip stream of product from the
solid/gas separator (not shown), can be used to ensure particles
remain entrained.
Jet-Assisted Methods for Microchannel Fluidized Beds
[0081] Another mode for operating a circulating bed in a
microchannel device would be to maintain the entrainment vessel in
one of the following states: Particulate, Bubbling, Slugging or
Turbulently fluidized. For a general description of these states
see Grace, J. R., "Contacting Modes and Behaviour Classification of
Gas-Solid and other Two-Phase Suspensions", Can. J. Chem. Eng., v
64, pp 353-363, 1986. Some preferred embodiments use a bed expanded
to the turbulent state. In some preferred embodiments, the bed can
be expanded in the fluidized state until portions of it extend it
the microchannels. The expanded bed 402 can be formed by fluid jets
formed through a distributor plate 404 (FIG. 4a). More fluidizing
medium 406 would then be introduced into the process microchannels
at the jet (orifice) location from an interleaved channel (not
shown) adjacent to the process channel(s). A jet is an opening in
any channel wall that allows a fluid (which may contain suspended
solids and/or a fluid mixture) to leave one channel and enter
another. This would have the effect of increasing the fluid
velocity in the process microchannels. The lighter particulates
would be carried away from the expanded bed into the upper reaches
of the process microchannels 407 (and optionally to a solids
separator 410). The jet may have any cross sectional aspect that
can be described by simple geometrical form for example a
rectangle, a circle or a triangle or may be irregular for example
like an ink blot. The ratio of the cross sectional area of the
channel to the cross sectional area of the orifice or jet in some
embodiments will be less that 250, and in some embodiments less
than 100:1, and in some preferred embodiments less than 70:1 and
greater than 1:1. These size ratios for the orifices can apply to
any of the various aspects of this invention. In some embodiments
the fluid entering the process microchannel via the jet may be a
second or third species required to conduct a desired chemical
reaction or process, thus reaction in the entrainment section can
be avoided. In some embodiments additional jets can introduce a
second or third species required to conduct a desired chemical
reaction or process above the run up length for the channel thereby
further reducing the effects of back mixing. After passing through
the process microchannels the suspension enters the header 412. If
desired, temperature in the process microchannels can be controlled
by a heat exchange fluid 416 flowing in a channel or channels (not
shown) adjacent the process channels. Gas velocity in the header
can be increased by reducing the cross section (see 414) for flow
thus aiding in the maintenance of the suspension. The suspension
then leaves the device and the solids are separated from the fluid
stream in device such as a cyclone. Particulate matter can then
return to the entrainment zone either by hydrostatic forces
(gravitationally induced flow), a sweep of fluidizing medium, via
application of vibrations or some combination of methods. In some
preferred embodiment solids fraction in the microchannel would be
less than 0.05, in some embodiments less than 0.10 and in some
embodiments 0.005 to 0.25 volume %. Alternatively, if the cross
section of the microchannel flow area is smaller than the bubbling
bed area upstream of the microchannels, a similar effect can be
achieved.
Circulating Microchannel Fluidized Bed with Internal Recycle
[0082] An example of a circulating bed is illustrated in FIG. 5. In
this case, particles are transported into process microchannels 502
but instead of allowing the suspension to flow out of the device
for gas/solids separation, the header is devised such that the
fluid velocity is reduced thus allowing the solids particles to
disengage and return via microchannels 506 to the entrainment zone
508. Some product containing fluid may return to the entrainment
zone via this path. Disengagement may or may not be complete and
some portion of the solids may leave with the product containing
process fluid. Process fluid, typically containing product gas,
exits through fluid outlet 504. A dividing wall 510 in the plane of
the microchannels separates the process microchannels from the
return microchannels. Optionally, a second fluid can be added
through orifices 512 on the return side and/or in the footer
section; this second fluid could be a quench fluid, a sweep fluid,
or additional reactant (and could perform more than two or all of
these functions). The second fluid could be added from an adjacent
channel (not shown). Optionally, there can be an adjacent channel
514 for heat transfer; in some embodiments, there can be a first
circuit 516 for temperature control of the process side and a
second circuit 518 for temperature control on the return side. This
mode of operation allows rapid quenching of the reaction by
removing the catalyst from the reactant mixture rapidly.
Synthesis of Fine Powders in Microchannel Systems
[0083] The formation or synthesis of fine powders, including
nanoparticles to particles that are up to about 40% the hydraulic
diameter of the microchannel, is enabled by formation in
microchannels. Synthesis methods include combustion synthesis,
nucleation and growth, and crystallization.
[0084] Control of the particle size and distribution of particle
sizes is enabled through control of local reaction conditions,
including temperature, pressure, and low axial dispersion or
back-mixing. Flow in microchannels can create high surface shear
with the microchannel walls thus enabling the synthesis of smaller
particles. The residence time within the microchannel is preferably
short (<500 ms, and more preferably less than 100 ms). For a
liquid carrier, the preferred contact times may be up to 1000 times
longer, including up to 500 seconds or less. Rapid thermal quench
at a rate of less than 10.degree. C. per second, or more preferred
less than 100.degree. C. per second will aid in the formation of
small, equally-sized fine particulates. Fast quench may also
inhibit the agglomerization of particles with each other.
[0085] Examples of nanoparticles include titania, silica, alumina,
iron, and carbon among others including alloys. The small particles
may be used as a catalyst support (homogeneous or heterogeneous),
an adsorbent, or other small particle application.
[0086] The use of a high shear environment can control the
morphology of the particles. Particles may be made more perfectly
spherical or elongated based on the flow characteristics and
corresponding shear force environment in the microchannel synthesis
reactor. The high shear environment may prevent particles to
continue to grow in a direction normal to the direction of flow. As
an example, for the case of emulsion formation, the high surface
shear with the wall sweeps the emulsion droplets off an aperatured
surface and prevents their continued growth in the continuous phase
to larger diameters (see U.S. patent application Ser. No.
10/440,056, filed May 16, 2003, incorporated herein by reference).
The high shear force environment may also inhibit agglomerization
of particles to form larger and less desired particles.
Wicks
[0087] Because of the typically large density differences between
solids and gases, the uniform distribution of mixtures of particles
and gases into parallel microchannel paths can be problematic.
Typically less problematic is the uniform distribution of slurries
made up of particles entrained in liquids into parallel
microchannel paths. Using this principle, several novel options for
distribution of solid/gas mixtures into microchannels are included
in the present invention. In one embodiment, a slurry is first made
in any standard mixing apparatus by mixing a liquid with solid
particulates. The slurry is then introduced into the microchannel
device and distributed among the several microchannel paths. Once
inside the individual microchannels, a gas is introduced into and
distributed (for example, through an orifice in a microchannel
wall) among the same microchannels and allowed to mix with the
liquid/solid slurry. Finally, the liquid is removed from the
mixture by passing the mixture over a wicking material and
separating the liquid (and the wick structure) from the gas/solid
mixture as the gas/solid mixture flows downstream. Suitable wicking
structures for use in microchannels have been well-described in the
literature; see, for example, TeGrotenhuis et al., U.S. Pat. No.
6,666,909 and U.S. patent application Ser. Nos. 011,386 filed Dec.
5, 2001 (Published Patent Application 20020144600) and 384,179
filed Mar. 7, 2003 (Published Patent Application 20030221554) which
are all incorporated herein. The openings in the wick material
should be designed such that the particles cannot pass into the
wicking channels but are left in the gas stream.
[0088] Another novel option to accomplish a uniform distribution of
particle/gas mixtures into parallel microchannel paths is to size
the particles and the wick features such that the liquid/particle
slurry can flow in the wick (wherein the wick is an array of
microgrooves, and the particles are no more than one-fourth the
diameter of the microgrooves, preferably no more than one tenth).
The liquid/solid slurry is then distributed evenly into the
microchannels or into wick material on the wall or walls of each
microchannel via wick based pathways. Once the liquid/solid slurry
has been distributed into the wick material in the microchannels,
the liquid is evaporated into the microchannel either by raising
the temperature or lowering the pressure (or both) and the slurry
becomes a vapor/particulate mixture. In some preferred embodiments,
a gas is introduced into the microchannel containing the wick
material before the liquid is evaporated and the particulates are
entrained in this gas flow as the liquid evaporates.
[0089] Particulates can be removed from a microchannel in a related
fashion. A liquid filled wick is formed on the wall or walls of a
microchannel containing a flowing gas/particle mixture. Because of
the small hydraulic diameter of the microchannel, particles can
quickly be captured from the gas phase and into the liquid phase
which fills the wick without the use of electrostatic charging. The
liquid/solid slurry can then be collected from each microchannel
into a common footer or system of footers. The use of such a system
can help improve uniformity of particle and gas distribution from
channel to channel because the solids are removed from the gas
phase before collecting the gas stream in the footer.
Aggregation of Fine Particles
[0090] A high shear environment can affect the size and structure
of flocculations of suspended small particles. This is an important
technology, for example, for pulp and paper, mining, and waste
water treatment. In some embodiments, lower levels of flocculating
agents are required to maintain a stable suspension in an
increasing shear force environment. This is especially true for
higher molecular weight materials, including polymers.
Catalyzed Reactions
[0091] The following discussions of methanol synthesis and
Fischer-Tropsch synthesis are contemplated examples. Neither these
processes nor any of the above-described processes have been
conducted; however, it is believed that all of the processes
described will operate effectively.
Adiabatic Circulating Microchannel Fluidized Bed for Methanol
Production
[0092] The most commonly employed reactor for the commercial
production of methanol is the Adiabatic Quench Reactor. These
reactors generally operate with a catalyst bed temperature in the
range 225-275.degree. C. and produce a reactor effluent with a
methanol content of 4 mole percent. Reactor pressures are in the
range of 50-80 atm. In order to achieve a high overall process
conversion of carbon monoxide (CO), a recycle ratio of 6:1 is
typically used. Productivity is in the range of 1300
kg.sub.CH3OH/kg.sub.cat-d. The reaction is net exothermic and
strongly equilibrium limited. Compression represents a major
portion of the operating cost of a methanol synthesis train.
[0093] Utilizing methods of the present invention, the production
of methanol can be conducted isothermally in a microchannel
reactor. For example, a reactant gas composition can be used that
consists essentially of H.sub.2, CO and CO.sub.2. A preferred
composition may contain 50 to 80 (more preferably 60-70) mole
percent hydrogen, 15-30 (more preferably 20-30) mole percent CO and
10 mole percent CO.sub.2. The reactant gas carries catalyst
particulates into a microchannel or a set of parallel
microchannels. The contact time of the gas stream is preferably
less than 500 ms, more preferably less than 100 ms, and in some
embodiments in the range of 0.1 to 50 ms. In some embodiments the
catalyst particulate will be distributed with particle diameters of
less than 250 micron but preferably greater than 1 micron, or
preferably distributed with particle diameters of less than 120
micron but greater than 20 micron or more preferably distributed
with particle diameters of less than 90 micron but greater than 40
micron (the foregoing ranges refer to 95% of the particles; 5% can
be outside the stated ranges). In a preferred embodiment the
particles will have a Gaussian distribution, in another embodiment
a log normal distribution, and in another embodiment a Weibul
distribution. In a preferred embodiment, the catalyst is of the
Cu/Zn oxide type. In some embodiments, the total solids flux
through a microchannel(s) is at least 300 kg/m.sup.2s, in some
embodiments at least 20 kg/m.sup.2s, in some embodiments at least
0.5 kg/m.sup.2s, and in some embodiments 0.5 to 5 kg/m.sup.2s. The
microchannels can be designed with any desired set of dimensions;
for example, a height of 0.05 to 10 mm (more preferably 1 to 3 mm),
a width of 5 to 1000 mm (more preferably 12 to 100 mm), and a
length of 2 to 500 cm (more preferably 15 to 120 cm). In some
preferred embodiments, the temperature of the process is in the
range of 25 to 1000.degree. C., more preferably 250 to 750.degree.
C. Reaction temperatures can be controlled to a constant
temperature or variable temperature by heat exchange, or,
alternatively operated adiabatically (by eliminating the heat
exchange). In microchannel apparatus, heat can be carefully
controlled by heat exchange channels that are adjacent to the
reaction microchannels; for example, as is known in the prior art,
a set of at least 3 layers of reaction microchannel layers can be
interleaved with a set of at least 3 layers of heat exchanger
(preferably microchannel heat exchanger) layers. In preferred
embodiments, the flow of reactant is oriented such that both the
gas and catalyst particles flow down with respect to gravity.
[0094] Elimination of the fixed bed reduces the total pressure drop
of the cycle and thus the capital and operating costs of the
synthesis train are reduced. The pressure drop from microchannel
reactor to particle separator to condenser to recycle is preferably
less than 7 psig. It is believed that in the inventive method, the
gas leaving the microchannels has a methanol content that is 90% of
the value predicted at equilibrium.
Fischer-Tropsch Reaction to Produce Paraffins from Mixtures of CO
and Hydrogen
[0095] Paraffins can be produced from mixtures of CO and hydrogen
via the Fischer-Tropsch reaction when passed over cobalt or iron
based catalysts. Selectivity to the desired product slate is
dictated by the temperature of the reaction environment as well as
by diffusional constraints inside the catalyst particle.
Operational temperatures of commercial systems are generally in the
range of 210.degree. C. to 250.degree. C. Early commercial
Fischer-Tropsch synthesis systems were operated as tube cooled
fixed-bed reactor containing cobalt catalysts. Current commercial
production is based on the three-phase SASOL slurry bed technology
in which 50 micron iron catalyst is suspended in the paraffinic
product and synthesis gas is percolated through the mixture at
pressure (.about.275 psig). The contact time is approximately 10 s
and the conversion of CO is 87%. Catalyst productivity is 0.2 kg of
hydrocarbons/kg of catalyst/hour.
[0096] The Fischer-Tropsch reaction can be conducted in a
cross-flow microchannel reactor with a cross flow structure; that
is, in a reactor in which two streams flow orthogonally to each
other in alternating planes. Preferably, the process stream flows
upwards against gravity. A coolant stream flows in the other plane.
In some embodiments, the working fluid on the cooling side is water
and cooling is via boiling heat transfer.
[0097] Preferably, the catalyst is a cobalt based catalyst disposed
on a suitable fluidizable support such as spray dried kaolin and
diatomatious earth. The fluidizable support can be tailored to have
a low skeletal density for ease of fluidization and a catalyst
porosity structure suitable for the retention of hydrocarbons (to
promoted chain growth to the desired degree). The process stream is
a mixture of hydrogen and CO in a molar ratio of 1.65:1. At the
inlet to the process microchannels the pressure is 35 atm. The
coolant side is operated to maintain a uniform temperature of
225.degree. C. Under these conditions the conversion of CO will be
at least 70% when the contact time is 25 ms and the solids flux
through the open area of the microchannels is 0.12 kg/m2-s. In some
embodiments the molar ratio of hydrogen to CO in the feed is
greater than 1:1 in another embodiment the molar ratio of hydrogen
to CO in the feed is greater than 1.5:1 and in another preferred
embodiment the molar ratio of hydrogen to CO in the feed is greater
than 2.1 and in some embodiments is in the range of 1.8:1 to 2.2:1.
In some preferred embodiments, contact time of the fluid and/or the
entrained mixture through the microchannel, microchannel array, or
entire device, is less than 1000 ms, in other embodiments is less
than 500 ms and in another embodiment less than 300 ms and in some
embodiments is in the range of 20 ms to 250 ms. In some preferred
embodiments, the median particle size is less than 500 .mu.m, in
other embodiments is less than 250 .mu.m and in another embodiment
less than 100 .mu.m and in some embodiments is in the range of 20
.mu.m to 200 .mu.m. In some preferred embodiments, the inlet
pressure is less than 200 atm, in other embodiments is less than 50
atm and in another embodiment less than 25 atm and in some
embodiments is in the range of 20 atm to 50 atm. In some preferred
embodiments, the productivity is greater than 1000 kg of
hydrocarbon product per kg of catalyst per hour and in another
embodiment is greater than 100 kg of hydrocarbon product per kg of
catalyst per hour and in another embodiment is greater than 10 kg
of hydrocarbon product per kg of catalyst per hour and in some
embodiments is in the range of 2 kg of hydrocarbon product per kg
of catalyst per hour to 20 kg of hydrocarbon product per kg of
catalyst per hour. In some preferred embodiments, the CO
selectivity to hydrocarbons with carbon numbers greater than 4 is
greater than 50% and in other embodiments the CO selectivity to
hydrocarbons with carbon numbers greater than 4 is greater than 75%
and in other embodiments the CO selectivity to hydrocarbons with
carbon numbers greater than 4 is greater than 95% and in some
embodiments is in the range of 88% to 98%. In some preferred
embodiments, the CO selectivity to methane is less than 2% and in
other embodiments the CO selectivity to methane is less than 4% and
in other embodiments the CO selectivity to methane is less than 6%
and in some embodiments is in the range of 5% to 10%.
EXAMPLES
[0098] Testing involving the flow of solid particulates through a
microchannel has been conducted with a microchannel test apparatus.
In this testing, particles carried by an air stream were passed
though a 0.31 mm diameter hole (the hole was 1.7 mm in length). The
orifice opened into a perpendicular microchannel having dimensions
of 1.00 mm height.times.2.4 mm width.times.2.5 mm length. The
orifice was positioned in the center of the microchannel by width
and length; thus particulates carried by the flow traversed the
1.00 mm height and impinged against the opposite wall of the
microchannel before being carried out along the length of the
microchannel. The test devices were constructed of Inconel 625.
[0099] Testing was conducted in three identically-configured test
devices with a stream of particulates entrained in a steam of
compressed air. One portion of the particles were obtained from
Arizona Road Test Dust, which is a standardized dust used by
automotive companies when automotive tests are needed in a dusty
environment. This dust has been sieved into different sizes; the
size purchased and used for this experiment was 0 to 10 micron with
99% of the particles being in this range. Laser light scattering
was used to determine the volume based particle size distribution a
constant density was assumed for the particles and mass weighted
average particle diameter of 3.2 micron was calculated. The density
of these particles was 2650 kg/m.sup.3. The Arizona Road Test Dust
was fed into the system through a fluidized-bed dust feeder. This
feeder consisted of a plastic tube with an outer diameter of 5/8''
and an inner diameter of 3/8'' and air was inserted into the bottom
of this tube. This tube contained larger zirconia particles of
diameter 0.0059'' (0.15 mm) and the test dust; the zirconia
particles were present to help fluidization and were significantly
heavy such that they did not leave the dust feeder (that is, the
zirconia did not pass through the microchannel test device). The
mass ratio of zirconia to test dust was 2:1 (i.e. 2 grams of
zirconia for each gram of test dust). Various testing parameters
and erosion results are shown in the Table below (surface
temperature refers to the surface temperature of the test device.
No plugging or change in pressure drop was observed during the
test.
TABLE-US-00001 Test device # 1 2 3 Outside surface temperature
.degree. C. 850 850 1050 Total amount of dust fed mg 49.4 45.2 60.0
Total duration of test hours 12 8 8 Orifice Particle Mass Flux
kg/m.sup.2hr 84.8 77.6 103 Flowrate of air SLPM 0.360 0.360
0.360
* * * * *
References