Mechanism of fluid cracking catalysts deactivation by Fe

It has been recently recognized that Fe can be an important factor causing FCC catalyst deactivation, most often in the form of lost activity and bottoms cracking. Using a combination of different techniques such as EPMA, SEM/EDS, Optical Microscopy, XPS and Sink/Float separation to study in-unit deactivated FCC catalysts, we have been able to determine that Fe deposits only on the exterior surface of catalyst particles forming Fe-rich rings. In these areas, Fe, Ca, and Na oxides mix with silica from the underlying catalyst giving the catalyst a characteristic texture with surface nodules and a "glassy" appearance. After Fe deposits, it is generally immobile. However, interparticle Fe transport is possible via a mechanism involving the movement of fine Fe-rich particulates from one catalyst particle to another. In combination with thermodynamic analysis, we have determined that low melting temperature phases containing Fe, Na, and Ca oxides as well as silica form on the surface of catalysts made with silica-based binding systems. These phases cause pore closing and accelerated sintering. The destruction of the surface pore structure in the areas covered by the Fe rings leaves the particle interior largely unaffected. However, the blockage of the surface pores that carry the heavy hydrocarbon molecules inside the catalyst particles for cracking, causes activity and bottoms cracking loss. Alumina does not mix with Fe to form such low melting temperature phases, and when it does the melting temperature remains very high. FCC catalysts made with alumina binding systems have most of the pores carrying the heavy hydrocarbon feed molecules in the alumina structure. Thus, although in these catalysts Fe-rich nodules can form, the surface pore structure is resistant to deactivation by Fe, and the catalysts maintain activity and bottoms cracking even at high levels of Fe contamination.