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Microprocessing on a Large Scale

Use of microchannel reactors for the chemical process industries gathers momentum

by Michael Freemantle
October 11, 2004 | A version of this story appeared in Volume 82, Issue 41

Credit: Velocys
Velocys' facility in Plain City, Ohio, has several automated enclosures for testing microchannel devices.
Credit: Velocys
Velocys' facility in Plain City, Ohio, has several automated enclosures for testing microchannel devices.

Microchannel process technology is being hailed as the next big thing for the process industries, according to John Brophy, former general manager of corporate research at BP Chemicals, Sunbury-on-Thames, England.

"To date, industry has been skeptical, adopting a 'show us' approach to such a radical change in plant technology," he says. "Progress has been limited to university labs and a handful of small companies selling miniature construction sets for operation on lab benches. But now the pace is heating up with several companies developing and scaling up this brand-new technology."

Microchannel reactors consist of stacks of closely spaced thin plates designed to form microchannels with specific dimensions, regularity, and connectivity in the spaces between the plates. Process fluids pass through the channels, which may be coated internally with a catalyst. The microscale dimensions of the channels increase the surface area per unit volume--and thus increase the overall productivity of the process per unit volume. An added advantage is the short residence time of the fluids in the microchannels.

The geometry and size of individual channels in microchannel reactors remain the same as process capacity is increased from lab scale to commercial scale, observes Wayne Simmons, chief executive officer of Velocys, a company headquartered in Plain City, Ohio. The company was founded in 2001 by Battelle Memorial Institute to develop and commercialize Batelle's patented microchannel-based technology for the chemical and energy industries.

"The scale-up process for microchannels is 'numbering up'--merely duplicating the single channel many times--rather than conventional scale-up, which increases the size of reactor vessels as scale increases," Simmons adds. By connecting multiple microchannel reactors in parallel, it is then possible to achieve any desired plant capacity.

Brophy, who is a technical adviser for Velocys, points out that microchannel reactor technology grew out of attempts in the U.S. to develop tiny chemical separation systems for placement within underground nuclear storage facilities at, for example, the Hanford Site in Washington state.

"THE IDEA was to treat nuclear waste in situ, thus eliminating transport and the exposure of personnel," he explains. "While this application has not advanced beyond early concepts, the scientists and engineers developing the technology began exploring its potential advantages in other applications in the chemicals and petroleum industry."

Credit: UHDE
Pilot-scale microchannel reactor at Wolfgang Industrial Park, in Germany, allows safe operation of heterogeneous gas-phase syntheses in explosive regimes.
Credit: UHDE
Pilot-scale microchannel reactor at Wolfgang Industrial Park, in Germany, allows safe operation of heterogeneous gas-phase syntheses in explosive regimes.

Brophy makes a distinction between microchannel process technology and microreactor technology (C&EN, June 16, 2003, page 36; July 5, page 18). "Microreactor technology concerns microscale reactors that are typically used to screen catalysts or used for synthetic chemistry," he explains. "Their small size means that they are convenient in the lab but not for commercial production of significant quantities of chemicals. Microchannel process technology, on the other hand, is more general and is aimed at commercial production. It includes, for example, reactors, mixers, and heat exchangers, each of which uses microchannels to contain the process streams and increase performance. The dimensions of the channels--height or width--range from tens to hundreds of micrometers with channel lengths up to several meters. The important difference is that these microchannel components are integrated into systems containing tens to thousands of channels."

Reducing the size of conventional production equipment by replacing it with more compact equipment such as microchannel devices is known as process intensification. The benefits include lower investment costs and more efficient energy use.

Microchannel reactors are now a necessary option for intensified chemical plant design, comments Rocky C. Costello, president of R. C. Costello & Associates, an engineering firm based in Redondo Beach, Calif., that specializes in process intensification and process safety. Earlier this year, the firm reached a strategic alliance to work with International Mezzo Technologies, Baton Rouge, La., to bring their microchannel devices directly to the process industries. Mezzo specializes in the design and fabrication of microchannel heat exchangers.

"Microchannel devices can be mass-produced to produce cost-effective systems that are both compact and efficient," Costello says. "They can achieve temperature control to within 0.1 oC, which means that runaway reactions can be prevented, and their inherent design can prevent hot spots. Old catalytic microchannel reactors will be considered disposable when the catalytic coating is no longer active."

In April, the Department of Energy announced that Velocys, Dow Chemical, and Pacific Northwest National Laboratory (PNNL) in Richland, Wash., will carry out a project to apply the technology to the production of ethylene and other olefins.

"Process intensification enabled by microchannel reactors and other technologies holds the promise to greatly reduce the energy consumption and environmental footprint of the chemical industry," observes DOE's lead technology manager Dickson Ozokwelu at the DOE Office of Industrial Technologies.

PNNL Technical Group Manager Evan Jones points out that microchannel technology intensifies chemical processes by significantly improving heat- and mass-transfer operations. "The technology uses small parallel channels that can rapidly exchange heat and chemicals," he explains. "It can significantly reduce the size of process systems, frequently up to an order of magnitude; improve selectivity and yield; and lower costs through mass manufacturing of identical units."

The three-year $3.2 million DOE microchannel process technology project will focus on the high-intensity production of olefins by oxidation, such as the conversion of ethane to ethylene. The project will include reactor design, catalyst development, and process economic analyses.

The first phase of the project is focusing on the design, construction, and operation of a bench-scale reactor with channel dimensions identical to those of a potential commercial-scale unit. The second phase will establish the groundwork for increasing the process volume to commercial levels by demonstrating the use of reactors containing numerous channels. As part of the second phase, PNNL will spearhead work on catalyst optimization.

"Microchannel reactors can control process conditions far more precisely than conventional processing equipment, allowing the use of 'hotter' catalyst formulations," explains Yong Wang, technical leader at PNNL.

The final phase of the project will be an assessment of the economic and energy advantages of the new process based on measured performance and projected reactor costs.

Dow corporate R&D leader Jon Siddall notes that olefins are the world's largest volume chemicals. "Microchannel technology may allow us to reduce costs or otherwise improve the process for making an essential commodity of world commerce," he says. "The application is far removed from the more typical applications of the technology, which are for fine chemicals. The irony of making large-volume chemicals in small-volume reactors is unmistakable."

Velocys is developing microchannel process technology for a range of other applications including hydrogen production by steam reformation of natural gas (methane). The reaction produces synthesis gas (syngas)--a mixture of carbon monoxide and hydrogen. The production of hydrogen can be maximized by the reaction of the CO with steam at lower temperatures in the so-called water gas shift reaction.

The steam-reforming process is endothermic, however, so heat must be added. To do this, the Velocys technology uses the heat generated by the combustion in air of methane and excess hydrogen formed in the steam-reforming process. The endothermic and exothermic processes are therefore coupled. Each steam-reforming microchannel in the reactor is adjacent to another microchannel through which the hot combustion gases pass.

The short residence times in the reactor allow the reaction temperature of the steam-reforming process and cooling of the products to be achieved rapidly. Rapid heating and cooling are necessary to avoid carbon formation.

"These closely coupled steam-reforming and combustion processes rapidly approach equilibrium at high temperature and pressure," Brophy says. "They demonstrate the rapid heat flux and kinetics achievable in microchannel process reactors. Residence times are less than 10 milliseconds compared with up to 10 seconds in conventional reactors. As a result, overall system volumes can be reduced 10-fold or more, opening up applications for chemicals manufacturing, refining, and clean energy production."

VELOCYS HAS collaborated with Fortune 100 energy companies to develop and commercialize a low-cost, modular hydrogen production process for refineries based on the technology. "This project began in 2001," Velocys Business Development Manager Tad Dritz says. "We plan to begin constructing a pilot plant by the end of this year, and it should start up in early 2006."

Credit: Velocys
Credit: Velocys
Credit: Michael Freemantle
Credit: Michael Freemantle

According to Simmons, hydrogen generation using the Velocys technology can improve capital costs by 20%. "But this is just the beginning," he says. "Our development pipeline is full of programs that will change how, where, and at what cost chemicals are produced."

The programs include developing microchannel process technology to produce methanol from syngas. Velocys notes that the process is highly exothermic and that conventional reactors for synthesizing methanol from syngas are severely limited by their ability to control temperature. The Velocys technology can increase the conversion of CO every time it passes over a catalyst bed to more than 80%, compared with the current level of 22 to 32% obtained in conventional commercial reactors.

"Higher conversion of carbon monoxide can significantly reduce both capital and operating costs by eliminating the need for a recycle stream," according to Velocys. "Capital costs could be lowered by 8% by cutting the costs of the recycle compressor and associated heat exchangers. Operating costs could be decreased by 10% by eliminating the utility costs to run the recycle compressor."

Velocys is also developing microchannel process technology to convert natural gas to hydrocarbon liquids that can be used as clean diesel fuels or can be blended with crude oil. The technology involves feeding syngas from the steam-reforming process into a microchannel Fischer-Tropsch reactor, where the CO and hydrogen react to form hydrocarbons and water.

"The Fischer-Tropsch reaction is highly exothermic, so a great deal of heat must be removed from the process stream," Brophy explains. "If the heat is not removed effectively, high temperatures are generated that can deactivate the catalyst and increase the formation of useless products, particularly methane. Heat transfer is therefore critical to increasing reactor productivity and catalyst life.

"Velocys' Fischer-Tropsch reactors overcome these limitations by closely coupling the exothermic reaction with an endothermic process such as steam generation," he continues. "Very high process intensities can be achieved without producing large thermal gradients. Mass-transport limitations are reduced due to the high gas- flow rates that thin the liquid coating on the catalyst. Tests have shown that a microchannel Fischer-Tropsch reactor can process almost 40 times as much syngas per unit volume as a conventional system."

In a separate venture, Stevens Institute of Technology, in Hoboken, N.J., and FMC Corp., one of the world's largest producers of hydrogen peroxide, are engaged in a DOE-funded research project to develop a microchannel reactor system for the on-demand production of H2O2 at end-user sites. The principal investigators are Stevens' Adeniyi Lawal and FMC's Emmanuel Dada.

H2O2 is produced commercially by autoxidation of compounds such as 2-ethylanthrahydroquinone. However, the process is economically viable only when the H2O2 is produced at 70% concentration by weight using an energy-intensive distillation step. Most commercial applications require H2O2 at less than 15% concentration. End users therefore dilute H2O2 onsite before storage and use.

The direct combination of hydrogen and oxygen using a catalyst in aqueous solution for large-scale H2O2 production has been explored. However, mixtures of the two gases are flammable and even explosive when hydrogen concentration exceeds 5%. At such low concentrations, high pressures are required, making the process energy inefficient and economically unattractive.

"End users have increasingly become interested in the concept of on-site on-demand H2O2 generation to reduce transportation, storage, and 'concentration-dilution' costs," Dada says. "Our microchannel reactor approach provides a unique opportunity for directly combining hydrogen and oxygen in an explosive concentration regime while ensuring low-pressure, energy-efficient, and safe reactor operation. For highly exothermic reactions, the enhanced heat transfer of the reactors enables rapid wall quenching of free radicals, which in conventional-size reactors leads to thermal runaway conditions and concomitant explosions."

The microchannel hydrogen peroxide process employs nanostructured thin-film catalysts and avoids the use of the solvent processing, gas recycling and treatment, and product purification steps that are required in the conventional autoxidation process.

The FMC-Stevens project started in September 2002. It has two phases. The initial goal is to design, fabricate, evaluate, and optimize a laboratory-scale microchannel and heat exchanger system for H2O2 production.

"The aim of this phase is to demonstrate that we can produce 1% by weight of hydrogen peroxide in a microreactor setup," Dada says. "If the first phase is completed successfully, a demonstration unit will be built and tested for commercially viable on-site H2O2 production at one of our partner's sites on or before September 2007. The demo unit, if successful, will then lead to commercialization of the technology."


IN GERMANY, Degussa has been active for several years in screening microchannel process technology for various gas, liquid, and plasma processes. "At Degussa, we believe in the potential of microchannel process technology both as a development tool in the lab and for production technology," Degussa Senior Process Engineer Georg Markowz says.

He points out that, in 2000, the German Federal Ministry of Education & Research called for demonstrable proof of the operational suitability and commercial viability of microsystems for chemical processing in industry. In response, Degussa launched its Demonstration Project for the Evaluation of Microreaction Technology in Industrial Systems (DEMiS) three years ago. Project partners include engineering contractor Uhde and the University of Erlangen-Nuremberg, both in Dortmund; the Technical Universities of Chemnitz and of Darmstadt; and Max Planck Institute for Coal Research, Mülheim/Ruhr. Markowz and Uhde Senior Process Engineer Steffen Schirrmeister manage the project.

The team selected the gas-phase epoxidation of propene using H2O2 as a model reaction for the project.

"Propene oxide is commercially significant." Schirrmeister notes. "Global production amounts to some 5 million metric tons per year." Gas-phase propene epoxidation, he says, takes advantage of microeffects, such as rapid heat and mass transfer, because the reaction is highly exothermic. The process also saves on solvents and avoids by-products."

In May of last year, the team commissioned a pilot-scale microchannel reactor for propene oxide production at the Wolfgang Industrial Park, in Hanau. "The reactor concept incorporates a modular structure with respect to unit operations and capacity," Schirrmeister explains. "It can be applied to other syntheses and allows ... safe operation in explosive regimes."

Markowz remarks that initial results from trial operations are encouraging. "The reactor's operation is very stable and reproducible," he says. "All the participants are confident that the DEMiS project and the commercial implementation of the results in other gas-phase applications will be successful. That would be a breakthrough for microchannel process engineering."


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