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

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.