(584a) Prototyping Egg-Yolk and Egg-White Catalyst Pellets for Exothermic Reactions

Exothermic reactions are often conducted in catalytic fixed-bed reactors. A reliable heat management is necessary to achieve specified product qualities, while staying inside feasible temperature bounds along the whole length of the reactor. It was shown by computational studies, that reactors, which are filled with tailored catalyst pellets, show favorable properties. These pellets consist of a catalytically active core, which is surrounded by an inert shell. At high temperatures, the diffusion of the reactants through the inert shell becomes rate-determining (Fig. 1). In this case, the inert shell effectively substitutes the reaction kinetics by diffusion kinetics, which significantly reduces the parametric sensitivity of the fixed-bed reactor, as the apparent activation energy diminishes. Based on these computational studies, our aim was to prepare and validate core-shell pellets for the methanation of carbon dioxide, with carbon monoxide as side product.

For this purpose, spherical ‘egg-yolk’ catalyst pellets and cylindrical ‘egg-white’ catalyst pellets with differing coating thickness were prepared by fluidized-bed coating of methanation catalyst pellets at pilot-plant scale. The resulting catalyst pellets were analyzed via nitrogen sorption, mercury intrusion, helium pycnometry, and electron microscopy. Furthermore, the catalytic activity of the catalyst pellets was tested in a lab-scale quartz glass reactor and compared to the catalyst pellets without coating as well as to crushed catalyst pellets.

The results show that ‘egg-yolk’ catalyst spheres and ‘egg-white’ catalyst cylinders were successfully prepared by fluidized-bed coating. The inert shell exhibits a porosity of about 50 % and a mean pore diameter of 5 nm. Furthermore, the inert shell of the catalyst pellets is crack-free, which is essential for the diffusion of the reactants to become rate-determining at high temperatures. This is also confirmed by activity measurements. Compared to catalyst pellets without inert shell, the activation energy of the coated catalyst pellets decreases significantly with increasing temperature.