discovered_01_2015

FOCUS// THE HZDR RESEARCH MAGAZINE WWW.HZDR.DE 14 15 Hans-Ulrich Härting Hans-Ulrich Härting pursued his studies at the TU Dresden and the University of Sevilla (Spain). He graduated from TU Dresden with a degree in process engineering. The Leipzig-born 32-year-old is married and has a two-year-old daughter. reactions occur at the catalyst and the final goods leave the reactor at the bottom. But in practice, it is not all that simple. ‘With this typical trickling flow, you always get certain maldistributions in the reactor,’ Markus Schubert explains. ‘Many reactions also release heat that must be removed. But because the catalysts often conduct heat very poorly, and some areas are even not irrigated by the flow at all, hot spots develop, areas of high heat, which, in the worst case, can destroy the reactor.’ Another problem is undesired byproducts, which then have to be removed in a complicated additional separation step. ‘Maldistribution means that the catalyst packing is not used to its full potential,’ Hans-Ulrich Härting adds. This is an unnecessary cost factor for the facility operator - catalysts are often based on precious metals and hence ‘not exactly cheap’. Eliminating hot spots Markus Schubert had already worked with fixed-bed reactors in the context of his dissertation. He currently conducts his research with the help of a European Research Council ‘Starting Grant’. Based on the groundwork he laid in his doctorate, he has now developed and built an inclined rotating fixed-bed reactor together with doctoral candidate Hans- Ulrich Härting. The inclination of the reactor can be adjusted from upright to horizontal; its rotational speed can also be varied. ‘We can set it to different flow regimes by systematically combining the angle of the inclination and the rotational speed,’ Härting explains. The flow of liquids or gases can be sickle- or ring-shaped, dispersed or stratified. Since the intended reaction can only take place inside the reactor with the help of the catalyst, it is imperative that the gas and the liquid reach the catalyst in the first place. In the Dresden prototype, the first step is splitting the phases, which means that gas and liquid flow at separate places in the reactor. ‘We rotate the reactor tube, immersing the catalyst beads again and again, we basically dip the catalyst in the liquid and then let it run dry again. That way, the gas also has better access to the catalyst,’ says Hans-Ulrich Härting, explaining the ideal operation with what is called a stratified flow. Also, when the catalyst is dipped in the liquid, the heat that is generated by the reaction is transferred from the catalyst to the liquid and is discharged, which prevents the formation of dangerous hot spots. Since gas and liquid flow separately from one another, they cannot get in each other’s way; the pressure on pumps and compressors is lower – which increases energy efficiency. Industrial use still quite a way away ‘Our studies were able to show that there are operation points at which our reactor outperforms established reactors,’ Hans- Ulrich Härting emphasizes. The yields are significant. The output can be up to twice as high. A fact that should make industrial operators weep for joy. But it doesn’t. ‘Once they have built such a huge facility and have it running tolerably well, operators are reluctant to replace it,’ Härting knows. ‘So far, our work has been very fundamental, we have only demonstrated the performance enhancements in one model system,’ his boss explains and adds, ‘There are various processes and operating conditions with which our concept can increase reactor performance, but it is too early to draw a general conclusion.’ OPTIMIZED: Catalyst particles facilitate chemical reactions. Photo: Oliver Killig

Seitenübersicht