Chemical Reactions Engineering Weekly News

April 9, 2001       Volume 1       University of Michigan        Ann Arbor, MI         Jennifer Brakel

 

Microreactors:  An Invention of the Future

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., “Modelling of Gas-Liquid Catalytic Reactions in Microchannels,” Department of Chemical Engineering, Univesity College London, Torrington Place, London  WC1E7JE.

Ball, Ken, http://www.rimbach.com/home/penpage/articles/June%202000/6-0-a8.htm, “Microreactor Technology Gaining Momentum,” Published 1999. Visited March 27, 2001. (website no longer available)

Erhfeld, W., Hessel, V., Lowe, H., Microreactors. Wiley-VCH, Weinhein. 2000.

Fairley, Peter, “Microreactors: R&D Tool or Production Platform?,” Chemical Week, December 16, 1988.

Gavriilidis, Asterios, University College London. April 6, 2001

Gavriilidis, A., D. Gobby, P. Angeli, “Mixing Characteristics of T-type Microfluidic Mixers,” Journal of Micromechanics and Microengineering, 11, 126 (2001).

Harre, K, http://www.mpip-mainz.mpg.de/documents/projects98/F6.htm, “Microreactors,” Visited March 27,2001. (website no longer available)

Jensen, Klavs F., “Microchemical Systems: Status, Challenges, and Opportunities,” AIChE Journal, 45, 2051 (1999).

Marek, M., J. Kosek, D. Snita, H. Sevcikova, “Electric Field Driven Processses in Microreactors,” Prague Institute of Chemical Technology, November 22, 1999.

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/ , “Fast Oxidation Reaction in Si-technology based Microreactors,” Visited March 27, 2001.

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 , “Silicon Microreactors for Analytical Applications,” Visited March 27, 2001.

http://www.mikroglas.com/reactor_2.htm , “Microreactors,” Visited March 27, 2001.

 

 

Microreactor Crossword
 

 

Across

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

 

 

Down

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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