Environmental Engineering Reference
In-Depth Information
Deasphalting of atmospheric and vacuum residues is another option for producing additional
liquids. However, deasphalted oil (DAO) requires additional hydroprocessing to be suitable for
further utilization, e.g., as the feed for fluid catalytic cracking (FCC), lube base oil production,
etc. Compared with asphalt (vacuum residue), the asphalt from deasphalting has limited
applications because of undesirable mechanical properties.
An extreme case would be a conventional refinery processing sweet crude, i.e., Ekofisk and
Arab Light crudes ( Table 2.1 ) [27] . However, even for such crude, commercial fuels could not
be produced without some catalytic treatment even during the period when lax environmental
regulations were in effect. Thus, it is unlikely that straight run distillates could be used directly
as fuels. For example, for gasoline, a reforming step would be necessary to attain desirable
level of octane number unless significant amount of an additive (e.g., tetraethyl lead) were
added. Because of the noble metals containing catalysts employed, the feed for reforming has
to be subjected to hydroprocessing to remove sulfur and nitrogen. Otherwise, the life of
reforming catalyst would be affected. To ensure stability, the reformate may require additional
hydroprocessing to remove olefins.
In the case of diesel fuel and aviation fuels, desirable cold flow properties (e.g., pour point,
cloud point and freezing point) have to be attained. The values of these parameters specified by
performance standards can be attained by removing straight chain paraffins from the feed. For
this purpose, a catalyst selectively enhancing the hydrocracking (HCR) and
hydroisomerization (HIS) of n-paraffins is necessary. Again, another hydroprocessing step
may be required to ensure stability of the produced fuel. There is an option to conduct all these
steps in the same reactor vessel employing several layers of different catalysts. For example, a
front layer acidic catalyst would perform HCR and HIS functions, whereas the end-layer the
hydrogenation (HYD) function. In another arrangement, several sections with different
catalysts in the same reactor vessel may be employed.
Assuming the following range of the severity of hydroprocessing conditions, i.e.,
low < moderate < high < extra high, conventional refineries were dominated by catalytic
hydroprocessing units operating under low severity and moderate severity conditions. The
former were suitable for upgrading atmospheric distillates, whereas a moderate severity would
be necessary for the conversion of VGO and DAO to fuels. The upgrading units comprised
either stationary fixed-bed reactors or multilayer beds in the same reactor vessel. Such systems
fulfilled all requirements during the period when environmental regulations just began to
evolve. All spent hydroprocessing catalysts generated by conventional refineries were
regenerable. In fact, an acceptable level of activity recovery could be achieved after several
utilization-regeneration cycles before other utilization options for spent hydroprocessing
catalysts would have to be considered.
 
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