Catalyst Deactivation - Spent Hydroprocessing Catalysts - page 76

Figure 4.21: Effect of time on stream on structure of TIS and THFIS of coke on catalyst
(Mo/Al 2 O 3 , atmospheric residue [AR], 653 K, 12MPa) [From refs 238 and 239 . Reprinted
with permission].
The H/C ratio of both TIS-coke and THFI-coke was greater than one even after 240 h on
stream, although the degree of alkyl substitution decreased significantly. These observations
suggest that some of these changes could not occur without the direct involvement of catalyst
surface. For example, a strong interaction with catalyst surface could be one reason for a low
solubility of the THFIS-coke. Figure 4.21 shows the structure of coke on the same catalyst
used for the HDM of the same feed in the industrial ARDS process after 6500 h. In this case, a
high degree of the aromaticity of cokes, particularly that of the THFI-coke, was quite evident.
The formation of such structure may be considered as the beginning of the coke graphitization
on the catalyst surface. A conclusion can be drawn from Figs. 4.20 and 4.21 . Thus, in the
course of operation, the HYD and deHYD of coke on catalyst may occur in parallel together
with some HCR, isomerization and condensation reactions.
The additional evaluation of spent catalysts from the study of Hauser et al. [238,239] was
undertaken by Matsushita et al. [240] using several complementary techniques. During
temperature programmed oxidation (TPO), they observed two maxima of the CO 2 formation,
i.e., one at 573 K and the other at 698 K, which were formed presumably from the oxidation of
a “soft” coke and a “hard” coke, respectively. In its structure, the “soft” coke may approach the