Hydroprocessing of Petroleum - Spent Hydroprocessing Catalysts - page 41

feeds, a small particle size may be needed to achieve a desirable level of catalyst utilization.
Then, the shape of the catalyst particles must be chosen to obtain the fixed bed with a sufficient
level of voidage. For example, this may be achieved by sock loading of the ring and lobe
particles giving 35 and 10% higher voidage, respectively, compared with the cylinders [90] .
Refinery experience indicates that the heavy feeds containing less than 120 ppm of V+Ni can
be successfully hydroprocessed using several fixed bed reactors in a series [127] . Under
optimized conditions, a high activity and the relatively low metal tolerance catalyst may be
suitable for heavy feeds containing less than 25 ppm. A dual catalyst system may be required
for feeds containing between 25 and 50 ppm of metals. In this case, the first stage catalyst
should possess a high metal tolerance, whereas the second stage, a high catalyst activity for
HDN and HDS. For heavy feeds containing between 50 and 100 ppm of metals, at least a
three-stage system employing fixed bed reactors may be necessary. In this case, the catalyst in
the first reactor should possess a high HDM activity and a high metal storage capacity to
ensure the long life of catalysts in the subsequent reactors. It is believed that heavy feeds
containing more than 150 ppm of metals can still be hydroprocessed in fixed bed reactor
systems providing that some modifications were undertaken. This may include the use of two
guard reactors, one in operation and the other on stand-by. Such guard reactors are part of the
HYVAHL process [120] . The sizing of these guard reactors, i.e., the total metal storage
capacity would need to be matched with the content of metals in the heavy feed. An
uninterrupted operation could be ensured by switching to the guard reactor with the fresh
catalyst as soon as the total metal storage capacity of the reactor on stream was approached
[120] . The addition of another reactor downstream may also be considered as an option.
However, such a step may drive costs of the operation to an unacceptable level.
Commercial processes employing fixed bed reactors have similar features, although they are
licensed by different process developers. The number of stages and/or reactors included in the
process is determined by the content of asphaltenes and metals in heavy feeds, the projected
daily throughput of the heavy feed and the anticipated quality of liquid products. It is unlikely
that for heavier feeds, a desirable level of hydroprocessing can be achieved in one stage. Thus,
even VGO may require a graded system, e.g., either multilayer bed or multisections reactor,
particularly when the objective is to produce the feed for FCC or to increase the yield of
middle distillates in the products.
Entirely different configurations of the fixed bed reactors and systems may be necessary when
the lube base oil is the targeted product. In this case, catalytic dewaxing reactor may be part of
the overall hydroprocessing of VGO and DAO followed by hydrofinishing step performed
under milder conditions as usually applied during hydroprocessing. It should be noted that the
catalyst formulations required for dewaxing and hydrofinishing may differ from those of the
conventional hydroprocessing catalysts.