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By Ken-Ichiro Sotowa, and Katsuki Kusakabe, Department of Applied
Chemistry, Kyushu University, Japan; and David Street, Fluent Asia Pacific
A microreactor fabricated on a silicon substrate (sealed with a glass
plate) at Kyushu University
In recent years there has been an
increased interest throughout
Japan in the research and development
of microreactors. During this
time, the Japanese government,
through three MITI (Ministry of
International Trade and Industry)
national projects, has committed nearly
10 million US dollars to the widespread
investigation of these unique
devices. Leading the research effort
are several prominent Japanese universities,
including Tokyo University,
Kyoto University, The Tokyo Institute
of Technology, and Kyushu University.
Many leading Japanese chemical companies
are also participating in this
important long-term national project.
Microreactors, as the name implies,
are very small chemical reactors. They
are typically only a few centimeters
long and the channels through
which the fluids flow are on the order
of 10 to 100 microns in diameter. The
reactors themselves are made using
materials such as silicon, quartz, polymers,
and metals that have well-defined
physical and chemical properties. They
are manufactured using micro-fabrication
techniques developed in the
fields of microelectronics and MEMS
(micro-electro-mechanical systems)
engineering. The reactor manufacturing
processes may therefore involve
photolithography, etching, and thin
film deposition to build the flow channels,
micro-heaters, and various
micro-sensors. Micromilling has been
used for the fabrication of certain
microscale structures. Microscale
pumps, driven by gears or piezoelectric
devices, have also been developed.
Some of these microfluidic devices are
not much bigger than the head of
a ball-point pen, but they can be effectively
used to drive the flow through
the tiny microreactor channels.
There are numerous applications for microreactors, ranging from biomedical
diagnostic devices to catalytic gas phase reactors operating at elevated
temperatures. There is considerable interest in the use of microreactors
for the production of on-demand hydrogen for fuel cells, and on-demand
drug production and delivery. In fact, some of the research into the more
far-reaching applications of microreactors is going on behind closed doors,
in top-secret programs at some of Japan’s largest and most wellrespected
companies.
Flow patterns within an oil bubble injected into an aqueous
stream
There are several reasons why so
much effort is being devoted to develop
such tiny reactors with such limited
production capacities. Because
of their size, it is possible to construct
a chemical plant consisting of
microreactors that is small enough
to be moved from place to place.
In the future, portable plants will be
used as the primary workhorses in
the distributed production of chemicals,
in which chemicals are produced
at the point of consumption. In addition
to providing on-demand production
of hydrogen for fuel cells,
these plants will be used for on-site
production of hazardous chemicals,
which currently incur considerable
risk to humans, animals, and the environment
when transported on
roads or rails. Another advantage of
microreactors is the high surface area
to volume ratio that can be achieved
with tiny channel sizes. This makes
it easier to control the fluid temperature, which is an important
parameter influencing reaction rate
and selectivity. In addition, appropriate
arrangement of the microchannels
makes it possible to attain
microscale mixing of two fluids almost
instantaneously.
CFD has been widely used to better
understand microreactor flows
and help design ways to improve
their efficiency. For example, velocities
through a single microreactor
chip are typically in the range of a
few milliliters per second. To
increase the throughput and make
the devices commercially viable, many
channels can be used together in
parallel. In an effort to design headers
for dividing the flow uniformly
among the channels, some research
groups have developed bifurcating
channels, similar in principle to the
human lung, whereas others have
developed simpler, open headers with
porous regions or baffles to create
more uniform flow. In both cases,
CFD is being used to aid in the design.
It is also being used to determine
the residence time distribution
(RTD) through microreactor channels.
Whereas large scale reactors
typically operate in the turbulent
regime, the flow inside a microreactor
is usually laminar. Without turbulent
eddies, very tight control over
the residence time distribution can
be achieved, so that the reactor conditions
can be well understood. CFD
offers one of the quickest and easiest
ways to determine RTD for simple
or complex channel designs.
At Kyushu University, FLUENT has
recently been used to investigate
microreactors that work with immiscible
fluids. Using the volume of fluid
(VOF) model, a small bubble of hexane
(oil) is injected into a flowing aqueous
stream. The oil bubble grows
in size and eventually breaks away
from the oil inlet stream. Conventional
chemical engineering modeling
approaches assume that mass transfer
from the bubble of hexane to
the bulk fluid is purely by diffusion.
By contrast, the FLUENT results
suggest that there is considerable
convective mass transport occurring
as well. This mechanism dramatically
enhances mass transfer by
more than a factor of 100.
In another project, two streams are brought into direct contact as they
flow side-by-side through a microreactor. When these streams – solutions
of sodium hydroxide (NaOH) and BTB, a ph-indicator – are brought
into close contact, a BTB-alkali reaction takes place at the interface,
even though the fluids are miscible with each other. This is because of
the laminar nature of the flow in microchannels. FLUENT predictions of
the distribution of NaOH concentration have been found to agree well with
experimental results. In some applications, it is necessary to enhance
the mixing rate of two fluids by disturbing the interface. For these applications,
CFD can be used to study the channel structure, which effectively disturbs
the interface and improves the mixing.
Schematic representation of a microreactor contacting device
Experimental observation of mixing at a Y-junction (channel width=400
micrometers)
Simulated concentration profiles in the two-fluid stream
Simulations like these are just
two examples of the considerable
research effort currently being
directed at microreactor applications.
Many other areas in this growing
field are being investigated using
CFD, since it can provide engineers
and scientists with a cost effective
technological advantage in their
attempt to understand these important
devices.
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