Environmental Engineering Reference
In-Depth Information
CHAPTER 4
Catalyst Deactivation
Deactivation is evidenced by decline in the rate of hydroprocessing reactions with time on
stream. In refinery practice, the activity decline is offset by increasing temperature, as it is
shown in Fig. 4.1 . According to Fig. 4.1 , three regions of the activity decline are generally
observed. The efforts have been made to develop more active and stable catalysts with the aim
to minimize the loss of activity. Then, the catalyst consumption and generation of spent
catalysts would be decreased as well. The extent of deactivation depends on several
parameters, e.g., properties of feeds, operating conditions, structure of catalysts, etc. For
example, Fig. 4.2 [143] shows the effect of temperature and H 2 pressure on carbon and
vanadium deposited on catalyst as well as on surface area. The increase in V deposition with
increasing temperature may be attributed to enhance hydrodemetallization (HDM) via
non-catalytic route [27] . However, the H 2 pressure must be maintained to ensure an optimal
H 2 S/H 2 ratio. Thus, importance of the H 2 S/H 2 ratio for controlling the coke deposition on
catalyst during hydroprocessing operations has been confirmed [27,49,53,96] . Obviously, a
significant difference between the catalyst deactivation during hydroprocessing of heavy feeds
and that of light feeds can be anticipated.
For atmospheric distillates, the catalyst deactivation is dominated by the poisoning involving
N-compounds and coke deposition. The N-bases, which are always present in every petroleum
feed, contribute to the catalyst deactivation by preferentially adsorbing on active sites and as
such slow down the hydrogen activation process [49,55,56,144] . General trends indicate that
during the operation, nitrogen accumulates in coke as indicated by its increasing content with
time on stream as coke becomes more refractory. It has been established that the relative
contribution of N-bases to the overall loss of catalyst activity increased from residues towards
vacuum gas oil (VGO)/heavy gas oil (HGO) feeds and atmospheric distillates. In other words,
poisoning effect increases with decreasing molecular weight of N-compounds in the feed.
Furthermore, it was indicated that the conversion of N-compounds to hydrocarbon products
might be influenced by the H 2 S/H 2 ratio [10] . There may be the optimal H 2 S/H 2 ratio for
which the conversion (e.g., hydrodesulfurization [HDS] and hydrodenitrogenation [HDN])
reached a maximum [40] . The optimal ratio may exhibit a continuous change with time on
stream because of the catalyst deactivation. This further contributes to the complexity of
deactivation mechanism. This ratio may vary from feed to feed and from catalyst to catalyst. In
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