Environmental Engineering Reference
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deactivation is caused by coke [80] , whereas the overwhelming information confirmed metals
as the main cause of deactivation [10,145-147] . However, these statements and/or information
tend to oversimplify the actual events, particularly in the case of fixed-bed reactors. Evidently,
for a high metal content feed, front of catalyst bed will be deactivated by metal deposits. The
contribution of metals to deactivation will then decrease and that of coke increase towards the
outlet of the catalyst bed. However, the front zone of the metals deactivated catalyst bed will
gradually move towards the outlet until the entire bed is deactivated. Before this point, HDM
reactions occurring near the end of catalyst bed are affected by the coke deposited during the
initial stages of the operation. It is therefore obvious that the relative contribution of coke and
metals to deactivation will vary between the inlet and outlet of catalyst bed.
The above discussion suggests that relative contribution of coke and metals to catalyst
deactivation will also vary with time on stream. Thus, during very early contacts of catalyst
with a heavy feed (start of run region in Fig. 4.1 ), the coke deposition may dominate catalyst
deactivation. At this point, little contribution of metals to the overall loss of catalyst activity
may be evident. General trends ( Fig. 4.3 ) show that coke deposition reaches a steady state,
while the contribution of metals increases almost linearly with time on stream.
For the graded systems comprising either several layers of the different catalysts in one
fixed-bed or several fixed-bed reactors containing different catalyst each connected in a series,
e.g., HDM, HDM/HDS and HDS in the first, second and third reactor, respectively, the
contribution of metals to deactivation will decrease from the first towards the third reactor
[12,148] . At the same time, the contribution of coke to deactivation will increase. It should be
noted that N-compounds in the feed are gradually converted to the hydrogenated N-containing
intermediates. The basicity of the latter is greater than that of the N-compounds originally
present in the feed [144] . This indicates an increased contribution to catalyst poisoning with
Figure 4.3: Deposition of carbon and metals on Mo/Al 2 O 3 catalyst in atmospheric residue
desulfurization (ARDS) process versus time on stream.
 
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