Spent catalyst, sulfur level

In the second phase, performed at a maximum temperature of about 370°C, the sulfur and a portion of the coke are removed by combustion. The rate and exothermicity are controlled by limiting the flow of combustion gas through the catalyst. Spent base metal catalysts may have sulfur levels of from 6 to 12 wt % in the form of metal sulfides. A high degree of sulfur removal must be achieved in these first two regeneration steps to avoid the formation of sulfate on the support during the final combustion step. Such a formation causes a loss of catalyst activity. 

A major problem in the catalytic hydrodesulfurization of residual oils is the deactivation of the catalyst by metal-containing asphaltenic species in the feed. As can be seen from the results of a typical desulfurization experiment presented in Fig. 1, the catalyst shows a rapid initial decline which is attended with a fast build-up of coke on the catalyst. At a relatively low catalyst age 0, as defined in Section IV, a stationary coke level is reached. In contrast, the deposition of the inorganic remnants of the hydro-cracked asphaltenes (mainly vanadium and nickel sulfides) continues and gradually clogs the pores in the outer zone of the catalyst particles, as confirmed by electron microprobe analyses of spent catalyst samples (see Fig. 2). This causes a slow further loss in desulfurization activity over a longer period of time. Ultimately, the catalyst becomes totally inactive for desulfurization because the - still active - inner core has become completely inaccessible to the sulfur-bearing molecules. 

---