April
9, 2001 Volume
1 University
of Michigan
Ann Arbor, MI
Jennifer Brakel
Microreactors are
miniature housings for carrying out chemical reactions. These reactors have a
channel diameter of 100-500 microns and a channel length of 1-10 mm. The
system of micro fluid channels is etched into one wafer and the second wafer
is glued over the first. Microreactors are typically made out of glass,
quartz, plastic, silicon, and metals such as stainless steel. The branched
microchannels are used for liquid-gas and liquid systems. Due to the small
dimensions of microreactors, and hence small internal volumes, they have very
high surface-to-volume ratios, which increases rate of reactions in comparison
to macroscopic devices. High heat and mass transfer rates in microreactors
allow reactions to be carried out more aggressively which produces high yields
that aren’t achievable with conventional reactors. Another advantage of
microreactors is safety. Even if
a microreactor were to fail or an exothermic runaway reaction occurred, the
small amount of chemicals released could be contained easily. The small
amounts of chemicals used in microreactors are also environmentally friendly
because less waste is produced. Scale-up to production by replication of
microreactors used in lab would eliminate costly design since only one
engineering cycle would be needed. By scaling-out a process, (adding
microreactor units instead of scaling-up) failed units can be replaced easily
without interfering with other equipment, which is both a maintenance and
economic advantage. Although miniaturized reaction techniques offer many
advantages in the chemical engineering, pharmacy, medical, and biotechnology
industries, further research is necessary for a better understanding of these
compact reaction vessels.
Figure 1.
Microreactors about the size of a penny (Courtesy of www.abb.com).
Figure 2.
Five parallel connected microreactors have a aqueous solution throughput
of 300,000 tones per year. The transferable heat power is 1,000 kilowatt,
that is one megawatt. (Courtesy of hikww4.fzk.de).
Figure 3.
IMM Microreactors. A basic component for microreaction systems is
photosensitive glass (Foturan). (Courtesy of www.imm-mainz.de) Reactions Some common
reactions that have been carried out in microreactors include: methanol
oxidation, fluorination of toluene, ammonia oxidation, nitration of benzene,
ethane epoxidation, and dehydration of methanol to form formaldehyde. The
University of Twente and the Technical University of Eindhoven are in a joint
project to study the feasibility of silicon based microreactors for the
oxidation of ammonia and the partial oxidation of methane. At the Prague
Institute of Chemical Technology, the chemical engineering department is
studying the possibility of enzymatic separations of DNA fragments in
microscale chemical devises. Students at the University of Washington are
researching explosive mixtures of H2/O2 for catalytic combustion in cross
current microreactors. There are many more types of reactions to be explored
now that microreactors allow such elevated conditions, not permissible in
conventional reactors. Manufacturing of Microreactors Microreactors
are made of thin substrates or sheets, which are stacked into integrated
reaction systems. This development builds on MicroElectroMechanical Systems (MEMS),
a fabrication technique developed for microelectronics. Now, MEMS enables
fabrication of microchannel reaction systems. The eventual aim of
microreactors is to have production units that fit on a table or in a car and
allow distributed manufacturing and point-of-use production of harmful
chemicals. Conventional plants can be replaced with banks or stacks of
microreactors. Then simply plugging in as many stacks as needed could increase
the capacity of a plant. Banks could be switched in and out of service when
necessary, where as processes embedded in plants today are as flexible. The
disadvantage of producing microreactors is that fabrication plants are
expensive, and the design of microreactors is highly dependent on the
application.
Figure 4.
Interdigital micromixers made of glass with different outlet geometries were developed in a collaboration between IMM and mgt mikroglas technik AG, Mainz. The figure shows a triangular-shaped interdigital mixer. The mixers made of glass allow the visual investigation of the mixing process and the generation of emulsions. (Courtesy of www.imm-mainz.de)
Fluid Flow/Pumps/Micromixers Fluid
flow at the small scale of microreactors is laminar. For liquid phase
reactions there are many characteristics that affect the flow of the fluid.
The surface characteristics of channel walls will affect the way a liquid
moves in the microreactor and hydrophilic surfaces can aid in the flow.
Surface roughness should also be minimized to decrease friction. The size of
the channel can also affect the liquid viscosity. Sometimes, viscous fluids
and particle-laden flows can cause blockage and operational difficulties in
pumps and valves. This is why the type of pumps in microreactors is important.
Conventional pumps are difficult to interface with microstructures and
micropumps have poor backpressure that sometimes results in leakage. Because
both conventional and micropumps are ineffective, an alternate method was
developed. Electro-osmotic flow (EOF) moves fluids on a submilimeter scale
using surfactants with redox-active groups that electrochemically switch
between surface active and surface inactive states. This controls the
concentration of the surface-active species in the solution. The
Figure 5. Sectional view of the mixing chamber (Courtesy of www.mikroglas.com). concentration
gradient causes changes in the surface tension, which results in controlled
motion. When speaking of fluid flow, mixing is also an issue in microreactors.
There are different types of mixers than can be used in a microchannel system.
A simple type of mixing involves injecting one fluid into the flow of another,
perpendicular to the direction of flow, like in a macroscopic jet mixer. A
“mobius-type static mixer” contains layers of immiscible fluids separated
perpendicular to the boundary layer. Each new stream is then twisted through a
90o angle and then reunited. This is repeated several times until
mixing is complete. The last type of mixing is called “multi-lamination”.
This involves splitting feed streams into multiple smaller streams that are
then brought together inter-digitally. The two fluids then flow upwards into a
slit where the mixing takes place.
Thermal Properties/Heat Exchangers
Microreactors
have high heat fluxes (100 W/cm2) and high heat transfer
coefficients. Due to this, heat removal and input is quick and exothermic
reactions can be carried out without risk of runaway. Microreactors provide
safe operation at high temperatures and heat transfer in endothermic reactions
in efficient. These aggressive reactions are feasible due to rapid heat
removal and flame trap dimensions of channels. Theses systems also quench
rapidly so reactions can be halted quickly at the end of the reaction
improving selectivity, purity, and production. Heat transfer properties of
microreactors increase selectivity from 85% to 96%. Although microreactors can
withstand these elevated conditions, temperature gradients are important and
the microstructures are very sensitive to them. Any sudden or prolonged
heating at elevated temperatures may cause buckling of the structure. To
provide the heating and cooling for reactions, heat exchangers are integrated
into microreactors. Walls of microchannels have high heat transfer
coefficients, usually one magnitude greater than conventional heat exchangers.
Because of the small amount of quality material needed in a microchannel, they
are more expensive than traditional ones. A common type of heat exchanger
integrated into a microreactor is one with stacked sheets of machined channels
where hot and cold pass through alternating layers. Overall heat transfer
coefficients for this type of system have been reported to be ~ 55 kW/m2L
at water flow of 370 kg/hr. A commercialized microheater has been developed
that weighs less than 0.2 kg and supplies 30 W of heat per cm2 at
an efficiency of 80-85%.
Figure 6
.
Heat exchanger. Heat exchanger based on the tube in tube heat exchanger.
Liquid 1: 700 µm x 1000 µm Liquid 2: 400 µm x 1000 µm. Thickness of heat
transferring layer: 200 µm. Can be realized in glass or ceramic. (Courtesy of www.imm-mainz.de)
Figure 7..
Cross-current catalytic combustion microreactor. (Courtsey of W. Ehrfeld, H. Hessel, L. Lowe)
Types
of Microreactors
The
most basic of microreactors is the T-reactor. Reactants flow in at one end,
react in the middle, and then flow out the other end. This type of reactor is
being used at Massachusetts Institute of Technology for ammonia oxidation. At
EPFL, the dehydrogenation of methanol to formaldehyde is researched in a
double-sided heated reactor where the reactants and catalyst are injected from
the top and bottom. The mixture is then heated to 700-900oC, before
mixing in a central channel. Another model might have the reaction channel on
the topside of a stainless steel block and heat exchange channels on the
bottom for good temperature control. For the biochemistry application of
direct fluorination of aromatic hydrocarbons, a falling film microreactor is
used. These reactors have improved space-time yields over conventional packed
columns and have minimal hazard due to the small amounts of elemental fluorine
required.
Figure 8. Microreactor.
Enables to mix two liquids and to remove or add the process heat. 20 channels
in parallel, length of reaction channel: 30 mm, product channel: 700 µm x 200
µm, cooling channel: 700 µm x 700 µm (Courtesy of www.mikroglas.com)
Many
different types of microreactors are being used in industry for a variety of
different applications. A collaboration of DuPont and MIT has taken advantage
of microreactors for gas phase oxidation reactions whose kinetics are poorly
understood. IMM has applied microreactors to synthesizing small volumes of a
large number of compounds and for high-throughput assays. Dow Chemical has
invested into two start-up firms, which will commercialize the microreactor
assay systems. Merck has micromachined a mixer with a conventional process
rather than an integrated microreactor. DuPont is also working with the
Pentagon’s Defense Advanced Research Projects Agency to develop control
systems for parallel operation of 10-30 of the 40,000 – 50,000 lbs./year
microreactors, a $6-million project. As you can see, microreactors are already
being explored and taken advantage of for many different applications. There
are still many uncovered uses of these miniature reaction systems.
Figure 9. Multiphase packed bed reactor with active catalyst (Coursesy
of M. Losey, MIT).
Figure 10.
Schematic diagram of the flat plate reactor with separated feed.
Further Research
Although there are many applications of microreactors
already in use, there is still much research to continue. Development of
packaging and design of microreactors needs to be investigated, as well as
connection to external equipment and integration of control systems into
reactor modules. Microreactors small volumes make it hard to find measurable
quantities. A solution to this is computer modeling. The computer has to solve
complex differential equations of heat transfer, mass diffusivity, and fluid
flow. With the development of better modeling, it will allow for a better
understanding of the reaction. Microreactors have already proven to be an
integral part in reaction research, but to be able to move beyond the lab into
chemical production, microreactors must be integrated with sensors and
actuators either on the same chip of through hybrid integration schemes.
References
Angeli, P., Gobby, D., Gavriilidis, A.,
Ball, Ken,
Erhfeld, W., Hessel, V., Lowe, H., Microreactors. Wiley-VCH, Weinhein. 2000.
Fairley, Peter, “Microreactors: R&D Tool or
Gavriilidis, Asterios, University College London.
Gavriilidis, A., D. Gobby, P. Angeli, “Mixing
Harre,
K, http://www.mpip-mainz.mpg.de/documents/projects98/F6.htm,
Jensen, Klavs F., “Microchemical Systems:
Marek, M., J. Kosek, D. Snita, H. Sevcikova,
Schmidt, Martin, http://world.std.com/~fhapgood/nsgdir/98-11-03 , Visited March 27, 2001 (website no longer available)
http://utep.el.utwente.nl/tt/projects/FORSiM/
,
http://www.imm-mainz.de/english/developm/products/reactors.html, “Microreactors,” Published February 1, 1998. Visited March 27, 2001. (website no longer available)
http://students.washington.edu/annikan/micro.html , “Heat Exchange and Exothermic Reactions in Microreactors,” Visited March 27, 2001. (website no longer available)
http://www.ims.fhg.de/englisch/index.htm
,
http://www.mikroglas.com/reactor_2.htm
,
1.
Fabrication technique
4.
______Fogler
6.
Microreactors have__________ times less than one second
9.
Addition of microreactor units
12.
Outer
16.
Microreactor used for fluorination of aromatic hydrocarbons
18.
Basic microreactor
20.
Pace at which microreactors are cooled
21.
Stacking of integrated reaction schemes
23.
See 14 down
25.
Reactants
26.
Microreactors give higher________ than conventional reactors
27.Test
to receive highschool diploma
28.
Most important microreactor advantage
29.
“Stacks” of microreactors
30.
Undergraduate engineering degree
31.
Merck has included this with a conventional process
1.
Scale of microreactors
2.
Typical material of construction
3.
2000 OEP Project
5.
ChemE 344: abbr.
7.
Reaction that releases heat
8.
Change in concentration through reactor channel
10.
Flow regime
11.
Typical reaction carried out in microreactors
13.
Program for modeling chemical reactions
14.
Integration of these is required for microreactors to be used in chemical
plants
15.
Alternate method to pump fluid through channels
17.
Multi-___________ involves splitting of feed streams
19.
Liquid and ____________systems can be used in microreactors
21.
Affected by channel surface characteristics
22.
Agua
24.
High _______-to-volume ratios
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