Environmental Engineering Reference
In-Depth Information
Besides refining issues associated with decreasing quality of crude, significant modifications
in refining strategy were necessary in response to stringent environmental regulations. For
example, as it was indicated above, the ultra deep HDS and even deep HDS could not be
achieved using conventional refining methods. Using the unmodified reactors, a deep HDS
could only be achieved with the significantly lowered feed/catalyst ratio and increased H
2
consumption. It became soon evident that HDS can be enhanced by modifying the reactor
design to improve hydrodynamics in the fixed-bed of catalyst. Of course, new types of
catalysts with the enhanced activity and selectivity have been developed and/or are in various
stages of development. The improved properties of the catalysts ensured several
utilization-regeneration cycles before other options have to be found.
Distillation residues have been receiving much attention as the source of additional liquids. In
this regard, new upgrading concepts involved multistage catalytic systems comprising several
fixed-bed reactors in a series. Compared with the distillate feeds, for residues, the feed/catalyst
ratio significantly decreases. The regenerability of catalysts in such systems increased from the
first reactor contacting the feed towards the last reactor contacting a much upgraded feed.
Moving-bed and ebullated-bed reactors were introduced with the aim to accommodate more
problematic feeds, i.e., those with content of metals approaching 300 ppm of V+Ni. The
regenerability of spent catalysts from such systems is rather low however; other methods of the
catalyst reactivation (e.g., rejuvenation) have been developed. It should be noted that the
management of spent catalysts has been an integral part of all advanced
refineries.
Non-catalytic, carbon-rejecting processes were introduced to deal with the most problematic
feeds, e.g., those containing more than 300 ppm. The primary liquids from coking processes
require hydroprocessing steps to attain specifications of commercial fuels. Generally, the
content of sulfur, nitrogen and aromatics in such liquids is much greater than that in the
conventional distillates of a similar boiling range. Therefore, a successful upgrading of such
liquids may not be accomplished with fixed-bed reactors used for hydroprocessing
conventional distillates. In this regard, modifications may include multireactor systems and/or
a single fixed-bed comprising several layers of different catalysts. Systems comprising several
sections with different catalyst in the same reactor vessel is another alternative. The presence
of more refractory compounds (e.g., S-, N-containing heterorings and aromatics) in coking
distillates requires more severe hydroprocessing conditions compared with conventional
distillates. This can only be accomplished by incorporating advanced catalytic reactors into
refining schemes, as it is shown in
Fig. 2.10 [41]
. Otherwise, a coking process or deasphalting
process has to be employed. Nevertheless, spent catalysts from upgrading coking liquids
should still exhibit good regenerability, although the number of utilization-regeneration cycles
may be lower than that for spent catalysts from upgrading distillates having a similar boiling
range, but of a conventional origin.
