Regeneration - Spent Hydroprocessing Catalysts - page 179

Figure 6.36: Schematic of gravity settling unit for catalyst separation [From ref. 462 . Reprinted
with permission].

applied to hydroprocessing catalysts of varying shapes and size particles as well as to the FCC
catalysts [464] .

The effectiveness of density grading is enhanced if the particles are of a uniform size. For
extrudate shape of particles, the length grading may be necessary prior to density grading. The
length-density grading according to vanadium content has been successfully applied [465] .
This was based on the observations that the compacted bulk density and surface area of spent
catalyst correlated with the level of vanadium contamination. The results of length grading of
the spent NiMo/Al 2 O 3 catalyst are shown in Table 6.11 , whereas the particle distribution of
spent catalyst as received and the fraction of 1.5-3.5mm in Fig. 6.37 [465] . The 1.5-3.5mm
fraction accounted for more than 70 wt.% of the spent catalyst. The fraction of particles shorter
than 1.5mm had to be rejected before regeneration. The remaining spent catalyst was
subjected to density grading to obtain four fractions, i.e., light, medium-light, medium-heavy,

Table 6.11: Yield and average length of particles after length grading [From ref. 465 . Reprinted
with permission].

Catalyst Yield (wt.%) Average length (mm)

Fresh 0 2 . 3

< 1.5 mm 16 1 , 2

1.5-3.5 71 2 . 3

> 3.5 13 3 . 6