Figure 3 Electron probe microanalysis of sulphur poisoning of the catalyst pellets (progression of time from left to right)
However, due to a combination of the kinetics and thermodynamics of sulphur removal reactions, as well as how the HDS and sulphur removal units are operated, it is inevitable that some sulphur species slip through. These species will progressively poison the pre-reforming catalyst through dissociative chemisorption on nickel, and the catalyst bed therefore deactivates from the inlet to the exit. Concentrating poisons at the inlet prevents the catalyst further down the bed from being affected. The amount of sulphur a catalyst can adsorb under pre-reforming conditions is directly proportional to the nickel surface area under operating conditions. The rate at which an individual pellet adsorbs sulphur before becoming saturated also mitigates the impact of poisoning. As seen in Figure 3 , sulphur poisoning initially occurs at the outer surface and gradually moves towards the core. If the absorption proceeds quickly, it helps to protect catalyst pellets further down the bed from sulphur poisoning, which ensures that the overall pre-reformer catalyst charge performs satisfactorily for as long as possible. To enhance the rate of sulphur adsorption, the catalyst requires high porosity, which ensures that sulphur species can rapidly diffuse through the sulphided shell of a partially poisoned catalyst to the unpoisoned nickel core. The choice of appropriate pellet porosity requires an appropriate understanding of the potential trade- off between diffusion and the active surface area. Thermal sintering Depending on the feedstock and the overall temperature profile, different parts of a
pre-reforming catalyst bed can be exposed to high temperatures. Exposure of the catalyst to high temperatures leads to a gradual decline in
activity through thermal sintering. Sintering is the process of particle
agglomeration, which occurs below the melting point of the compound. When sintering occurs in pre-reforming, the catalyst gradually loses activity as the nickel surface area is reduced. The driving force for sintering is usually the minimisation of surface energy. A given mass of large particles has a lower surface energy than “ The extent of nickel sintering is mainly affected by process temperature, whereas the gas composition, particularly steam partial pressure, will determine the rate of sintering ” the same mass composed of small particles. In pre-reforming catalysts, sintering causes a reduction in the nickel surface area of the catalyst, which consequently causes a decrease in both catalytic activity and resistance to sulphur poisoning. The extent of nickel sintering is mainly affected by process temperature, whereas the gas composition, particularly steam partial pressure, will determine the rate of sintering. The surface chemistry and morphology of the catalyst also affect the rate of sintering to some degree. The full potential of catalyst life may not always be achieved when regularly switching
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