Assessment of kinetics of dry reforming reaction occurring in a Single Pellet String Reactor

In industrial practice, catalyst pellets inserted into the reactor are usually pulverized to create, on a laboratory scale, a catalytic bed with a particle-to-reactor diameter ratio similar to that of industrial setups, and to minimize internal diffusion resistances. However, this practice has raised questions about the performance differences between whole and pulverized pellets, leading to the development of the Single Pellet Reactor, that directly uses a single (or a series of) intact pellet(s). Previous studies have shown that, for reactions not limited by mass transfer, the performance of the whole and the pulverized pellets is nearly identical. In this thesis, dry reforming reaction, which is characterized by carbon formation and rapid catalyst poisoning, was used as model reaction. The goal was to analyse the performance differences between whole and crushed pellets in terms of conversion, carbon formation, and deactivation, under both steady-state conditions and temperature ramps. Methane and carbon dioxide were fed in a 1:1 molar ratio, without dilution. In the absence of a catalyst, negligible conversion was observed up to 900°C. Activity tests were carried out using a commercial catalyst (Ni/Al2O3 5% w/w), originally in pellets of 17 mm, to compare the bed temperature gradients, the amount of active phase, and hydrogen absorption, showing minimal differences between whole and crushed (1000 μm) pellets. Steady state and transient measurements were performed, obtaining different performances. Reactive tests at 900°C and 800°C with two GHSVs (4185 h⁻¹ and 8330 h⁻¹) showed higher conversion with the crushed pellet, but also more significant pressure drops due to carbon formation, which could lead to excessive overpressures. The whole pellet, thanks to its channels and grooves, allowed an easier gas flow, keeping pressure drop limited. During temperature ramp tests, the conversion for the crushed pellet, initially higher, progressively decreased with the number of ramps, while it increased for the whole pellet, along with a significant reduction in carbon formation. This behaviour was attributed to the greater initial exposure of active sites in the pulverized pellet, which caused faster deactivation. In general, equilibrium conversion could be reached only with the crushed pellet, and at high temperatures. This too can be explained with a higher exposure of active sites in the crushed pellet, in fact at high temperature equilibrium is regulated by the methane dry reforming reaction, which takes place with equal reactant moles: given the higher availability of active sites, different species diffusion plays a less important role, and the reactants can reach them stoichiometrically. Finally, FTIR analysis quantified carbon formation, revealing a 50% higher carbon yield for the crushed pellet compared to the whole pellet.