In recent years, methane pyrolysis has increasingly attracted attention because it allows production of hydrogen without releasing CO₂. But simultaneously, another product generated that is just as interesting, says Hans Kuipers, Professor of Multiscale Modelling of Multi-phase Flows at Eindhoven University of Technology. ‘By performing pyrolysis in a controlled way, you can produce high-quality functional carbon materials, such as carbon nanotubes and carbon fibres.’ These are high value materials that are used in many different applications. Being both lightweight and very strong, carbon fibres are, for instance, used in manufacturing electrodes or in composites. Kuipers: ‘Pyrolysis of methane can be an efficient way to produce them. That there is also hydrogen released is more of a bonus.’

‘That there is also hydrogen released is more of a bonus’

Hans Kuipers

From atom to process

In a multilateral ARC CBBC project that also includes, among others, the groups of Petra de Jongh at Utrecht University, Thijs Vlugt at Delft University of Technology Kuipers and several industrial partners, Kuipers is working on optimising the methane pyrolsysis process. What makes this project unique is that the research is being carried out at both the atomic and process scale. Kuipers: ‘We are approaching this project as a chain of knowledge; an integrated approach across the different scales. All aspects are covered, from catalysts to process and reactor science, and from reprocessing to techno-economic aspects. A total of five PhD students are working on the project.’

‘On some particles we can see the carbon growing as nanotubes, on others as fibres’

Petra de Jongh

Pyrolysis takes place in a fluidised bed reactor. In contrast to a fixed bed reactor, the catalyst particles can move freely. The particles are in suspension and behave like a liquid. According to Kuipers, this is familiar terrain for the chemical industry. ‘Almost every chemical plant operates this type of reactor.’ But so far, it has not been applied for methane pyrolysis. In the process, methane is fed into the reactor as a gas; the resulting hydrogen is also gaseous. The carbon precipitates as a solid on the catalyst particles. The big advantage of using a fluidised bed reactor is that the amount of carbon that can grow on the catalyst particles is significant. Kuipers: ‘The catalyst particles have a diameter of about 100 to 200 microns and the carbon layer grows to several millimetres. This would not work in a fixed bed reactor. The system would get clogged, leading to problems with the flow.’

Granular liquid

In the Eindhoven laboratories, PhD students focus on scaling up the process. Kuipers: ‘Scaling up poses a challenge due to the hydrodynamics in the fluidised bed reactor. The laboratory setup, which has a diameter of 10 centimetres, behaves very differently from a large-scale reactor with a height of ten to twenty metres. We use computer models to make predictions and then validate them on a laboratory scale.’ The researchers are looking at the effects of reactor conditions, with temperatures of 500 to 600 degrees Celsius and an elevated pressure of 20 bar. ‘The temperature of catalysed pyrolysis is lower than that at which the process normally takes place, which is around 1,000 degrees. The elevated pressure is used to discharge the hydrogen at the right pressure. The high pressure has a major effect on the behaviour of the fluidised bed. A fluidised bed consists of a “granular fluid”. At higher pressure, more bubbles of smaller diameter form in the flow. This also affects the formation of carbon.’ The flow also affects the discharge of the formed particles. If the gas is introduced at a certain speed and in a certain way, segregation occurs. The heavier particles, on which most of the carbon has formed, go down and can be discharged. At present, it is still a batch process, but it will become a continuous process as the project progresses.

‘Through this multidisciplinary collaboration, we are providing young talent with a relevant education’

Hans Kuipers

Watching it grow

Exactly how the particles grow on the catalyst is another key question in this project. Polymers such as polyolefins are produced on an industrial scale in a similar way in fluidised bed reactors. These particles grow as spheres. But catalysts not only lower the reaction temperature by hundreds of degrees, they also drive the formation of specific carbon nanostructures. PhD students in the group of Petra de Jongh, Professor of Catalysis and Materials for Sustainable Energy at Utrecht University, are investigating exactly how this works at the atomic level. Using advanced electron microscopy, they can study catalyst particles one by one. This enables them to monitor carbon growth not only on a larger scale, but also in a ‘mini-reactor’ in the electron microscope, under real reaction conditions.

‘We have to look for an optimum between all the variables’

Petra de Jongh

The beauty of the process is that this reaction is very easy to monitor. The feedstock that goes into the reactor, methane, is a gas that is not visible in an electron microscope. The hydrogen that is produced is also an invisible gas. However, the carbon nanomaterials are solid and visible, allowing you to watch the formation of carbon structures on hundreds of different catalyst nanoparticles and study how the catalyst affects the growth. The catalyst particles differ in shape and that is reflected in the way the carbon structures grow. ‘On some particles we can see the carbon growing as nanotubes, on others as fibres’, says De Jongh. ‘We can also see on which particles carbon grows quickly, but also how long they keep it up. There is a lot of variation between individual metal nanoparticles. If we understand what is happening on that level, we can make improvements there.’ De Jongh explains that the amount of product formed can be increased thanks to this approach. ‘We can really understand the performance of individual nanometallic particles and use that knowledge to develop better catalysts for the next round. This will allow us to take faster steps to achieve better results.’ This is needed to increase the scale of production, which is what the researchers hope to achieve during the five-year multilateral project. ‘We can now produce about a hundred grams of product with one gram of catalyst. We want to increase this by a factor of 10,000. To do this, we need a more active catalyst,’ says Kuipers, referring to the research being carried out at Utrecht University.

A delicate balance

The paradox between reactor-level optimisation and atomic-scale optimisation is a recurring theme. Matching the two completely different length scales requires a delicate balancing act between the various steps and variables involved in producing crystalline carbon. For example, very high temperatures enable fast reactions, but also the very short lifetime of the catalyst. ‘We have to look for an optimum between all the variables’, says De Jongh. This is why both Kuipers and De Jongh emphasize the importance of this collaboration between different universities and industrial partners. In projects like these, PhD students at the three universities learn a lot from each other. Kuipers: ‘We study the entire chain of knowledge that underlies this process. Through this multidisciplinary collaboration, we are providing young talent with a relevant education. The interaction with industry is also important. This makes our research relevant while at the same time, we are maintaining our academic freedom.’