Regeneration - Spent Hydroprocessing Catalysts - page 123

Figure 6.1: Burn-off profiles for removal of carbon and sulfur during regeneration of spent
catalyst [From refs 367 and 368 . Reprinted with permission].
greater amount of the deposited coke. A lower reactivity of coke, because of more severe
conditions applied during hydroprocessing of VGO, may be a contributing factor as well.
It was indicated that there may be some exceptions where regeneration of the catalyst used for
hydroprocessing of the metals and asphaltenes containing feeds can be justified, i.e., the
catalysts from the third and fourth reactors either of the ARDS process or HYVAHL process
[12] . It is believed that in some cases, even the second reactor catalyst may be at least partly
utilized. For example, the catalyst unloaded from the bottom of the bed can be (after
regeneration) cascaded to the top bed of the same bed for the next utilization cycle.
For both light and heavy feeds, arsenic may accumulate on the catalyst surface during the
operation. It has been established that under hydroprocessing conditions organo-arsenic
compounds in the feed readily decompose and deposit on the catalyst surface [7,307] . Because
of the severe poisoning of the HDS active sites, the oxidative regeneration of catalysts may not
be a suitable method when the arsenic content approached about 0.4 wt.%. Such spent catalyst
must be classified as non-regenerable.