News from AIChE: Big Opening Seen for Microchannels

N/Ag work in the United States and abroad is focusing on ways to make unit processes even smaller using microchannels and other miniaturized devices. This technology is being proposed for applications as diverse as aerosol sampling, pharmaceutical development, automobile fuel cells, nuclear waste cleanup, and a scheduled mission to Mars.

At the Second International Conference on Microreaction Technology co-sponsored by <%=company%> Institut für Mikrotechnik Mainz GmbH (IMM), Deutsche Gesellschaft für Chemisches Apparatewesen e.V. (DECHEMA), and Battelle Memorial Institute, which was held during the Spring AIChE Meeting in New Orleans last month, plenary speaker Robert Wegeng, an engineer with Pacific Northwest National Laboratory (PNNL), Richland, Wash., predicted that soon after the turn of the century microfabricated components will carry out the operations now performed in plants by pumps, valves, compressors, heat exchangers, reactors, and separation units.

Wegeng and his associates have been developing microcomponents at PNNL with funding from the U.S. Department of Defense's Advanced Research Projects Agency (DARPA) and the Department of Energy for the last seven years.

"In the foreseeable future," he says, "we can anticipate the assembly of compact chemical processing and energy conversion systems that range in size from smaller than a cubic centimeter to assemblies encompassing several cubic meters." (An article about process miniaturization appeared in Chemical Engineering Progress, April 1996, p. 12).

Aerosol sampler on the market
At least one company has begun producing such a device. MesoSystems Technology, Richland, Wash., a PNNL spin-off, is working with its former parent to develop MicroVIC, a microfabricated aerosol sampler and filter that uses a large array of sub-millimeter elements to filter or collect samples of micron-sized particles suspended in air.

According to MesoSystems' president, Dr. Charles Call, the technology can be used for characterizing gas/powder streams and, potentially, for size sorting particulates as small as 1 micron. "One important use of the technology is for trapping airborne biological organisms such as anthrax and other deadly pathogens. Existing aerosol sampling technology is large, expensive, and non-portable," Call says.

He adds that the company is focusing on particulate sizes in the 1-10 micron range, often referred to as "respirable aerosols." Flow rates are currently 30 to 300 L/min. The device also can be used for sampling or filtering pigments and pharmaceuticals because it can operate continuously without the need to replace filters.

"In this type of filter, the minor flow, which contains all the particulate greater than 10 microns, is discarded or exhausted. This flow is usually between 5 and 10 percent of the total flow," Call says, adding that this type of filter can be used for such applications as prefiltering air in a chemical process. The exhaust air is rejected to the ambient along will all the filtrate (>10 micron particulate). The main flow, which is >90 percent of the original air and is now filtered, is used in the process.

Promising role in processing
But, what might be of greater interest to chemical engineers is the impact that microreaction technology might have on chemical processing, Dr. Irven H. Rinard, a chemical engineering professor at the City University of New York and co-chair of the meeting program, says. "For example, there is a substantial improvement in the selectivity of cyclodecatriene to cyclododecene at high conversion rates when using a microchannel reactor compared to more conventional reactors."

He notes that the performance of microchannel heat exchangers is quite remarkable. For instance, an exchanger in the form of a cube measuring 3 cm on a side developed a heat-transfer rate of 200 kW between hot and cold water with throughputs in the range of 7,000 kg/h.

"The improvement came from both heat-transfer coefficients in the range of 2,500 kW/m², an order of magnitude higher than that of conventional heat exchangers, and from higher surface-per-unit- volume ratios as well." Rinard adds that these exchangers have extremely short residence times (a few milliseconds), and heat-up and cool-down rates on the order of 10,000 K/min.

But, not everyone is convinced that microtechnology, while intriguing, will take off quickly, Rinard adds. Research to date continues to "take place at the National Laboratories with government support. Unfortunately, scant interest has been shown by industry in the United States with few exceptions."

This has not been the case in Europe, however, where several projects are underway, many of them funded by government-industry coalitions.

Dr. Wolfgang Ehrfeld, a conference co-chair and director of (IMM), says he has worked on the concept of microtechnology for the past 20 years. Recently, his laboratory completed work on a microreactor for the creation of a vitamin precursor for BASF using microchannels technology. "With traditional technology, yield is usually between 80 and 85 percent but using microchannels, this can be increased to between 90 and 95 percent."

It is in the pharmaceutical industry, Ehrfeld believes, that microchannel technology will be developed first.

Researchers at University College, London, and pharmaceutical company Boehringer GmbH, Tutzing, Germany, for example, have created Microspot, a miniaturized ultra-sensitive chip technology for the simultaneous detection of antigens, antibodies, and nucleic acid targets in a single determination. According to University College's Dr. Roger Ekins, the ligand-binding substances are immobilized in microarrays and deposited by inkjet technology.

Achieving near-ideal kinetics
DECHEMA, the German chemical engineering society that works closely with industry, last year formed a working party on microreaction technology on Ehrfeld's initiative. Several studies and projects are underway.

Dr. J. P. Baselt, a member of the working party, noted at the New Orleans Meeting that "microchannel systems are well-suited for designing and deploying compact reactors with millisecond reaction times because of the rapid thermal quenching that results from integrated microchemical heat exchangers." Ehrfeld and his team of researchers found that the most-striking inherent safety feature of a microchannel reactor is its low reactant inventory, which eliminates the accumulation of flammable gases.

Baselt emphasized that microchannel reactors "offer the promise for reducing the size of the conventional chemical reactors because intrinsic kinetics are realized. Heat- and mass-transport limitations that reduce reaction kinetics are minimized in microchannel reactors by reducing the effective transport distance. A much-smaller boundary layer is found within each microchannel than in the bulk flow of a conventional reactor. As the boundary layer shrinks, the corresponding contribution of slow conduction and diffusion to the heat exchanger or catalyst surface is reduced. This overall reduction allows the achievement of intrinsic kinetics in a high throughput reactor."

In the DECHEMA study, a microchannel reactor was used to demonstrate non-equilibrium chemistry for methane partial oxidation. "This short-contact-time reaction is well-suited for the microchannel reactor architecture. Ultimately, this novel reactor concept will be instrumental for the deployment of small compact processing units that produce hydrogen on demand for fuel cell applications," contends Baselt.

Ehrfeld adds that "micromixing opens the way to stable emulsions of uniform distribution without using surfactants." Microreactor concepts for fluorination of aromatic compounds as well as for heterogeneously catalyzed gas-phase reactions at high temperature levels are currently under investigation at IMM.

He adds: "One major applications will be the screening of catalysts for gas-phase reactions in microreactors, for example, the formation of ethylene oxide. Microreactors also attract attention in the pharmaceutical industry and diagnostics as they find applications for high throughput synthesis and screening of biomolecules. But, the most important task is the production of devices and systems that combine technical innovation and sustainable development with profitability."

Fitted for fuel cells
Both DECHEMA and PNNL researchers are working on projects that involve hydrogen generation for Proton Exchange Membrane (PEM) fuel cells, which may be practical for transportation and stationary applications. Because they are compact and produce powerful electric current relative to their size, they can deliver higher power density, resulting in reduced weight, cost, and volume, Wegeng says.

Baselt notes ethane is a feedstock that fits well with the distributed nature of this technology for PEMs "because the existing natural gas pipeline infrastructure makes it readily available and accessible at any point along the distribution chain." Currently, hydrogen is produced from methane steam reforming in a conventional fixed-bed technology that is not well-suited for miniaturization.

Mobile power-generation sources
The PEM fuel cell under development at PNNL is a joint project with the U.S. Dept. of Energy and American automotive manufacturers as part of their Partnership for the Next Generation of Vehicles (PNGV). Wegeng says that work at PNNL includes the development of microchannel reactors to perform partial-oxidation reactions to produce syngas, water-gas-shift reactions to convert the energy content of carbon monoxide in syngas to hydrogen, and preferential oxidation reactions to reduce CO level to less than 10 ppm so that it will not poison fuel-cell catalysts.

"Effective thermal management is needed, especially where exothermic reactions are used, to ensure that waste energy is recycled within the process. Progress in the catalytic microchannels leads us to believe that an automotive fuel processing system, including all reactors and heat exchangers, would be less than 8 L. As the PNGV target for a fuel cell is more than 10 times this size, it appears likely that the microchannel fuel-processing system is a candidate for fuel-cell powered automobiles," Wegeng adds.

Compact cleanup units
Liquid/liquid separations based on solvent extraction in microchannels are possible, Wegeng says, but "we still have a lot of work to do before they could be used for our targeted applications. Several issues have to addressed before this type of technology could be considered, especially for the cleanup of radiochemical wastes in tanks such as those at the DOE Hanford, WA, site now.

In the interim, hardware development may well lead to compact separation systems that could provide downwell treatment of groundwater or remove metals from aqueous wastes such as possible on ships," he adds.

A space odyssey
Although it seems like the stuff of science fiction, Wegeng claims that the surface of Mars is one location "that soon may receive chemical processing plants."

NASA's Insitu Resource Utilization (ISRU) program is working with PNNL researchers to develop compact chemical systems for these applications. Wegeng says that for the Martian project, ISRU program leaders hope to react atmospheric gases - predominately CO₂ - with stored hydrogen to make propellants and oxygen.

Under consideration is a system consisting of adsorption units (for the acquisition of atmospheric CO₂), an electrochemical unit (for the reduction of CO₂ to separate O₂), a catalytic microchannel reactor (for methanol synthesis from CO and H₂), and a microchannel liquid/vapor separator (for separating methanol from unreacted CO and H₂). Embedded microchannel heat exchangers may be able to provide for thermal integration of the systems allowing, for example, the exothermic heat from one unit to provide the heat requirements of another.

Claudia M. Caruana,
associate editor, Chemical Engineering Progress

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