Nanostructures improve the next generation of chemical reactors

Paul Boughton

Novel nanostructures, improved support materials and advances in process technology are driving advances in the next generation of chemical reactors. Sean Ottewell outlines the latest developments.

Novel nanostructures that respond to stimuli, such as pH, heat and light will pave the way for safer, greener and more efficient chemical reactors, according to UK engineers.

Being developed by a consortium of UK universities, the nanostructures can regulate reactions, momentum, and heat and mass transfer inside chemical reactors - and will provide a step change in reactor technology for the chemical, pharmaceutical and agrochemical industries.

Professor Yulong Ding of the Institute of Particle Science and Engineering at the University of Leeds said: "This research programme is an important step towards producing the next generation of smart 'small footprint', greener reactors. The responsive reaction systems we are investigating could make the measurement systems currently used in reactors redundant."

The technique is being developed through a collaborative research programme initiated by Ding together with Alexei Lapkin at the University of Bath, and professor Lee Cronin at the University of Glasgow.

The programme involves designing and producing molecular metal oxides and polymers as building blocks, and engineering those blocks to form nanoscale structures, which are responsive to internal and/or external stimuli such as pH, heat or light. The structures can be dispersed in fluid, or coated on the reactor walls.

As conditions inside the reactor change, the nanostructured particles will respond by changing their size, shape, or structure. These changes could in turn alter transport properties such as thermal conductivity and viscosity, and catalyst activity - and hence regulate the reactions.

Ding also believes that these systems also have the potential to eliminate the risk of 'runaway', where a chemical reaction goes out of control.

Meanwhile, scientists at the Netherlands Organisation for Scientific Research have developed a foam reactor that is ten times more efficient than other similar technologies.

In this project, Charl Stemmet investigated a new, structured support for catalysts for use in gas-liquid reactors. He used a highly porous solid foam as the support material, having up to 97 per cent open space available and a very large surface area per reactor volume. This large surface area is important for mass-transfer-limited, gas-liquid reactions; the larger the surface area, the greater the production per unit reactor volume.

To make a good reactor design with this new catalyst support, Stemmet first of all examined the flow behaviour of gas and liquid, and experimentally determined the design equations. He then compared the foam reactor with the current standard for gas-liquid reactions using a solid catalyst: a so-called packed bed of stacked catalyst particles.

The foam reactor has a volume 1.5 times larger than that of the packed bed for the same gas and liquid flows and the same production rate. However, the energy efficiency of the foam reactor is ten times higher than that of the packed bed.

The results will be used by the industrial partners involved in this project: BASF Nederland, DSM Research, Ecoceramics, Lummus Technology, Recemat and Shell Global Solutions International.

Towards green chemicals?

Following independent paths of investigation, two US-based research teams announced in October that they had successfully converted sugar-potentially derived from agricultural waste and non-food plants-into gasoline, diesel, jet fuel and a range of other valuable chemicals.

Chemical engineer Randy Cortright and his colleagues at Virent Energy Systems of Madison, Wisconsin, a National Science Foundation (NSF) Small Business Innovation Research awardee, and researchers led by NSF-supported chemical engineer James Dumesic of the University of Wisconsin at Madison announced that sugars and carbohydrates can be processed like petroleum into the full suite of products that drive the fuel, pharmaceutical and chemical industries.

The process Virent discovered in early 2006, and announced at the Growing the Bioeconomy conference sponsored by Iowa State University on 9 September this year is the subject of patent applications published in October.

That announcement was followed by the publication of a separate discovery of the same process in the Dumesic laboratory. Dumesic and his colleagues announced their findings in the 18 September online ScienceExpress, which was followed in print in the 18th October issue of Science.

The key to the breakthrough is a process developed by both Dumesic and Cortright called aqueous phase reforming. In passing a watery slurry of plant-derived sugar and carbohydrates over a series of catalysts-materials that speed up reactions without sacrificing themselves in the process-carbon-rich organic molecules split apart into component elements that recombine to form many of the chemicals that are extracted from non-renewable petroleum.

According to Dumesic, a key feature of the approach is that between the sugar or starch starter materials and the hydrocarbon end products, the chemicals go through an intermediate stage as an organic liquid composed of functional compounds.

"The intermediate compounds retain 95 per cent of the energy of the biomass but only about 40 per cent of the mass, and can be upgraded into different types of transportation fuels, such as gasoline, jet and diesel fuels," said Dumesic. "Importantly, the formation of this functional intermediate oil does not require the need for an external source of hydrogen," he added, since hydrogen comes from the slurry itself.

As part of a suite of second generation biofuel alternatives, green gasoline approaches like aqueous phase reforming are generating interest across the academic and industrial communities because they yield a product that is compatible with existing infrastructure, closer than many other alternatives in their net energy yield, and most importantly, can be crafted from plants grown in marginal soils, like switchgrass, or from agricultural waste.

While several years of further development will be needed to refine the process and scale it for production, the promise of gasoline and other petrochemicals from renewable plants has led to broad industrial interest.

Virent's process, called BioForming, is allowing the company to address one of the key goals of NSF's SBIR programme, commercialisation, and a broader NSF target, American competitiveness. A recent alliance with one of the world's largest energy companies aims to bring these alternative fuels to market, and investment from major automotive and agricultural companies from around the world are broadening the company's impact (Fig.1).

"The early support of NSF helped lay the groundwork for our technical, and subsequent industrial, successes," said Cortright, chief technology officer at Virent. "Our scientists now have years of expertise with our BioForming process and are rapidly moving the technology to commercial scale. We are quickly working to put our renewable, green gasoline and other hydrocarbon biofuels in fuel tanks all over the world."

Added Rose Wesson, the NSF programme officer who oversaw Virent's grant, "The technology developed by Virent is extremely promising, and has been refined over the last six years. The aqueous phase reforming process used by both research is an innovative approach that may yield an important, positive impact on the energy demands of the US and worldwide."