Developments in Petroleum Refining - Spent Hydroprocessing Catalysts - page 10

2.2 Revamped Conventional Refinery
Figures 2.1 and 2.2 [24] indicate the increased consumption of middle distillates relative to
that of gasoline and fuel oil as well as the growing imbalance between the availability of
vacuum residues and residual fuel demand, respectively. At the same time, the emissions
specifications for diesel oil, the main product among middle distillates, have been becoming
more stringent [28] . For refiners, these developments mean a higher consumption of H 2 and
catalyst inventory in the case that existing upgrading units are employed. This translates into a
significant increase in operating costs. The costs can be offset by a more efficient upgrading of
middle distillates to produce diesel fuels. In this regard, improvements in the performance of
hydroprocessing can by achieved by developing more active catalysts. However, refining
experience has shown that revamping of existing reactors and other units can improve the
efficiency of H 2 and catalyst utilization as well.
Recent developments in petroleum refining industry indicate on significant efforts aiming at
improvement of middle distillates upgrading achieved by revamping of catalytic reactors [29] .
For example, installation of Shell internals resulted in the improved flow distribution in the
reactor as evidenced by a lower temperature profile in catalyst bed. With these changes, the
resistance to catalyst fouling was increased. Key features of reactor internals developed by
Albermarle include a highly efficient mixing of process streams and almost an ideal liquid
distribution [30] . Similarly, the UltraMix internals developed by the Universal Oil Products
(UOP) prevent channeling, maldistribution, hot spots and bypassing. Bypassing is a
phenomenon in which a portion of process streams passes through the unit without significant
reaction [31] . Thus, in production of the ultra low sulfur diesel fuel, only 0.1% feed bypassing
could jeopardize the ability to meet product specification for sulfur. Patel et al. [32] reported
that the tray levelness must be carefully considered to avoid bypassing. This is shown in
Fig. 2.5 comparing three different distributors. It is evident that among three trays tested, only
vapor lift distributor ensured uniform flow profile. It was further observed that perforated plate
or sieve tray were less suitable because perforations could become easily plugged. Therefore,
bypassing phenomenon must receive adequate attention, particularly when sulfur
specifications of diesel fuel are the target. This is confirmed in Fig. 2.6 [32] correlating the
amount of feed bypassing and the content of sulfur in products. In this regard, a number of
patents describing various designs of distributors should be noted
[33-37] .
The efficiency of refining operation may be increased by maximizing the utilization of reactor
volume. This can be achieved by specially designed reactor internals as confirmed by Swain
et al. [38] . This is demonstrated on the example shown in Fig. 2.7 . Thus, by combination of
three catalyst beds in the pre-revamped reactor into one, bed capacity of the original reactor
shell was increased by about 30%. This was accomplished by installing a high dispersion tray
in the revamped reactor. The features of several other dispersion trays, described in the study