Deactivation from silica poisoning

These tests show that composition of the additive has little effect on controlling deactivation from silica poisoning. On an equal initial activity basis, four of the five additives tested showed very similar deactivation. Alumina, however, had a much higher susceptibility. 

From a scientific standpoint, there is an interesting relationship between the fouling and poisoning mechanisms that are typical of silica deactivation, and the poisoning mechanism that more accurately describes phosphorous deactivation. It is of interest to carefully review the dependence and interplay of these processes as a function of the reactant, catalyst support, pore structure, metal loading, and oxidation conditions. 

SOx emissions from FCCU s can be reduced by the use of SOx catalysts, especially SOx additives which can be added to the FCCU independently of the cracking catalyst. The effectiveness of these catalysts is favored by lower regenerator temperatures, the presence of combustion promoter, and higher oxygen concentrations. Deactivation of these catalysts occurs by loss of surface area and poisoning by silica. We believe that SOx additives will eventually be used by most refiners to control SOx emissions from FCCU s, either on a spot or continuous basis. 

Sodium on fluid cracking catalyst, FCC, comes from the raw materials used in the catalyst manufacturing process as well as salt contamination in the feedstock. Sodium can deactivate cracking catalysts by poisoning the acid sites on the matrix and zeolite and by promoting sintering of silica-alumina (1). Sodium can act synergistically with vanadium to accelerate the destruction of zeolite (2). 

The influence of titania can be isolated by selectively poisoning the titania-associated sites. When the Cr(VI)/silica-titania is treated with CO at 100-150 °C, the more reactive, low-MW producing sites are preferentially reduced first (i.e., those associated with titania). If the catalyst is cooled in CO, those same reduced sites adsorb CO and are deactivated, while other sites are unaffected (Scheme 15). By analysis of the polymer from such partially poisoned catalysts, the influence of titania becomes more apparent. 

Vanadia supported on silica(with titania) promoted by Fe and CTu oxides was studied for SCR of NOx by Bjorklund et al.28 Both Fe and Cu (as oxides) enhanced the activity however, the resistance to deactivation by SO2 differed as the Fe-promoted catalyst became slightly more active with time on stream and the Cu-promoted catalyst decreased in activity to less than half the initial activity with time on stream. The activity for SCR was related to the concentration of as inferred from the solid electrical conductivity. In this case different promoters were shown to change dramatically the ability of a potential poison to deactivate the SCR catalyst Nikolov, Klissurski, and Hadjiivanov29 also studied the deactivation of vanadia/titania. A combination of ESCA, XRD, and IR were employed to characterize the surface and bulk compositions. They concluded that deactivation involved the transformation of the active anatase titania to inactive rutile. Further, there was a concomitant decrease in total surface area and a loss of phosphate promoters for the selective oxidation of xylenes to phthalic anhydride. 

---