Load-flexible fixed-bed reactors by multi-period design optimization

Abstract

Many research activities focus on load-flexible fixed-bed reactors in the context of Power-to-X concepts. One of the main issues is the occurrence of hazardous temperature excursions in steady state and during dynamic load changes. The dilution of the catalytically active fixed-bed with inert particles and the use of catalyst particles with active core and inert shell (so-called core–shell catalyst particles) are proven means to prevent insufficient thermal management. This work aims at comparing both concepts with respect to the reactor’s load-flexibility, exemplified for carbon dioxide methanation. In extension to our previous work of Zimmermann et al. (2020), a multi-period design optimization approach is performed for both concepts, considering one, two, and infinitely many axial fixed-bed segments. This approach simultaneously determines the optimal reactor design and operating parameters, which is inevitable for a sound technological comparison of the two concepts. Additionally, step responses are simulated as worst-case load change policy to switch from one optimized steady state to another. The results show that with core–shell particles shorter tubes can be used than with diluted fixed-beds, if one or two fixed-bed segments are considered. This results in lower pressure loss and higher space–time yield. Additionally, faster load changes can be realized with core–shell catalyst particles. In the case of infinitely many axial fixed-bed segments, both concepts converge to similar space–time yields, but show excessive temperature excursion during load changes.