Despite their modest size, microreactors can outperform traditional reactors that are hundreds or thousands of times larger
PRESSED BY margin shrinkage and environmental considerations to streamline costs and reduce environmental damage, many of the biggest names in fine chemicals, specialty chemicals, pharmaceuticals and consumer products are developing applications for microreactors. Some companies are already using them for commercial production.
Microreactors are even being developed for the production of bulk chemicals such as ethylene, methanol, styrene, vinyl acetate monomer (VAM), formaldehyde and ethylene oxide (EO).
It might seem counterintuitive that something called a microreactor could produce 10,000 tonnes/year of a specialty chemical, much less contribute meaningfully to a commodity market, but microreactors are not a batch technology. Because they employ flow chemistry – a continuous process – the principles governing their operation and economics are fundamentally different.
Like petrochemical plants – the chemical industry’s most intensive users of flow processing – microreactors do chemistry in tubes. But while a typical hydrocracking reactor might have a diameter of 4m, the tubes in microreactors can range between 10 microns and 5mm.
Because reactions in these narrow spaces tend to be extremely fast, the channels, as they are called, are also quite short. Issues related to mixing – heat transfer, mass transfer and hydrodynamics – which slow transformations and allow side reactions in large reactors, are effectively eliminated in microreactors, where mixing is essentially instantaneous, hence the speed. Reaction kinetics become the only factor limiting progress, and rates can increase dramatically.
A growing number of chemical manufacturers are trying to exploit the resulting advantages. They include Switzerland-based Clariant, Sigma Aldrich subsidiary SAFC, Germany’s BASF and Evonik Industries, Netherlands-based DSM, US-based DuPont, and pharmaceutical companies Schering-Plough, Sanofi Aventis, Roche, GlaxoSmithKline, Novartis and Astra Zeneca. Even consumer products giant Procter & Gamble has worked with them.
Dutch fine chemical producer DSM, which has used a microreactor developed by US-based Corning to produce more than 25 tonnes of a nitration product under cGMP conditions in four weeks, will receive a Corning reactor capable of over 100 tonnes of the product annually later this year.
Siegfried, Schering-Plough, PCAS, Isochem and Lonza have also tested technology from Corning, which is developing reactors for “several other major customers,” as well, according to Gary Calabrese, vice president, science and technology at Corning.
Other developers of microreactor technology include Micronit Microfluidics, Future Chemistry, Ehrfeld Mikrotechnik BTS, Uniqsis, Syrris, the Institut fur Mikrotechnik Mainz and Velocys.
The devices supplied by these companies differ in various respects. For example, Corning’s microreactors feature relatively large channels in the millimeter range, which can increase throughput and reduce clogging. They could actually be called millireactors, and indeed the company prefers the term “Advanced Flow” reactor technology.
However, certain advantages are common to all microreactors: improved selectivity and reliability; safety; cost savings; and greater speed to market.
More selective and reliable chemistry results from greater process control. Flow rate, channel length and extremely efficient heat transfer can all be adjusted in micro-reactors to optimize reaction time and temperature. One benefit is less waste, which in turn lowers costs.
“Testing has shown over and over again that energy, processing chemicals and waste are all reduced when using these reactors,” says Calabrese. “In some cases, companies can even eliminate costly purification steps because the products coming out of the Corning reactor are of much higher purity.”
Costs also benefit from greater safety, the result not only of greater process control, but also of small reactor volume, which minimizes the impact of mishaps.
When expensive cryogenic reactors or explosion-proof facilities can be replaced with microreactors, costs are reduced, notes Andreas Weiler, global business director at US-based SAFC, which uses microreactors to produce about 50 commercial products of up to multi-kilogram quantities.
Costs may be further reduced by the smaller footprint, he adds. “Although a sizeable collection vessel will still be required and further work-up and purification procedures are often needed, the space and equipment requirements for the reactors themselves are lower.”
Microreactors also simplify scale up, shortening time to market. Whereas batch processes must often be altered considerably in the transfer to large-scale production, microreactor processes can proceed largely unmodified. Instead, they can be run using multiple microreactors in parallel, a practise called “numbering up.”
“Usually in scale up, modeling is not too predictive,” explains Volker Hessel, head of the department of chemical process technology at the Institut fur Mikrotechnik Mainz (IMM), in Germany. “Here you have very regular flows. You can predict some of the behavior, and you use the same units throughout the development chain – ideally. In practise, this is somewhat different, but it is still a major improvement. You don’t have to reinvent the wheel, as is done now.”
Microreactors often provide a better way to achieve established transformations, but the greater process control and safety also facilitate more difficult or novel chemistries.
“At SAFC, we are finding that continuous microreactors allow us to carry out a number of useful reactions that simply would not be possible in a batch reactor,” says Weiler. “Depending on the process and batch size, they can also be more cost-effective for reactions that already run well in a batch reactor.”
Corning’s Calabrese agrees. “The unique integration of heat control with the reaction layers enables new chemical paths not approachable with traditional batch technology, and the continuous process provides for effective management of unstable intermediates,” he observes.
The IMM’s Hessel, a widely acknowledged authority on microreactor technology, is methodically exploring the expanded range of chemistries possible in microreactors. He employs harsh conditions such as high temperature and pressure to access “novel process windows” in which reaction rates are much higher, even by orders of magnitude.
“We go to the limits to release the true chemical potential,” Hessel explains.
Most applications being developed for microreactors involve specialty chemicals, fine chemicals and pharmaceuticals, but Velocys, a US-based company recently acquired by UK-based Oxford Catalysts, is focused on bulk chemicals. Among its partners are US-based Dow Chemical Company and Japan-based Toyo Engineering.
“Most microreactor companies offer small-scale microreactors without catalysts that process liquid phase flow,” explains Laura J Silva, director, IP and licensing, at Velocys and Oxford Catalysts. “In contrast, Velocys microchannel reactors can incorporate heterogeneous catalysts or sorbents, and can process large amounts of material at a wide range of temperatures, including high temperatures, and a wide range of pressures, including high pressures.”
Applications of Velocys’ technology include ethylene via the oxidative dehydrogenation of ethane; dimethylether (DME) directly from syngas; the Fischer-Tropsch (FT) process; steam methane reforming; hydrocracking; higher alcohols from syngas; and the production of methanol, styrene, vinyl acetate monomer, formaldehyde and ethylene oxide.
Although microreactors will not replace the large petrochemical complexes that are the chemical industry’s most intensive users of flow chemistry, there are circumstances where the size and portability of microreactors are more beneficial than economies of scale. For example, it is not cost-effective to transport biomass long distances for conversion to fuel, hence Velocys’ biomass-to-liquids (BTL) project.
Last year, Velocys demonstrated a two-gallon-per-day BTL microreactor using a new, highly active FT catalyst developed by Oxford Catalysts. The device operated for more than 3,000 hours and achieved productivities of over 1,500 kg/m3/h. Standard fixed bed FT reactors typically operate at productivities of around 100 kg/m3/h, Silva notes, and slurry bed FT reactors at around 200 kg/m3/h. A larger-scale demonstration of the BTL process will take place later this year at the Wright-Patterson Air Force base in Dayton, Ohio, US.
The same principle lies behind the company’s offshore gas-to-liquids project, which targets stranded gas. In the case of materials such as ethylene oxide, on the other hand, the problem is the transportation of the product, and the solution is in situ generation at small scale.
Challenges do remain on the path to widespread adoption of microreactor technology, says Harmen Lelivelt, sales engineer at Netherlands-based Micronit Microfluidics. They include the integration of online control, the integration of separation steps, solids handling and the cost of large microreactors. Whereas a complete lab unit might cost from €10,000-70,000 ($13,000-90,000), a dedicated production plant runs from €100,000-500,000, he says.
However, microreactor technology clearly has momentum. IMM’s Hessel says it may be another 10 years before microreactors are considered a regular tool, but he expects a fifth of processors to employ them. Corning’s Calabrese believes that as much as 30% of the fine chemicals and drugs currently in production could be made more efficiently using microreactors.
Lelivelt anticipates growing interest among R&D groups and gradual acceptance and implementation by industry. “It will be an important [tool],” he says, “one that will certainly claim its place in chemical technology.”