Hydrocarbon Processing - December 2022 - 22

Catalysts
in each section. The unit operation was
subsequently changed to a single-stage
recycle mode using the existing lineups
for startup circulation to improve middle
distillate selectivity.
The UCO from the hydrocracker is
used as feed to the FCCU. At first, the
plan was to use a UCO flow corresponding
to 50 vol% conversion of the maximum
hydrocracker feed capacity. This
amount of UCO proved to be challenging
to process in the FCCU because of
the low coke production, so the demand
for UCO dropped. This made it possible
to use other catalysts in the hydrocracking
unit and change the unit operation
from single-stage once-through (SSOT)
to SSREC to increase the conversion. By
only changing the catalysts provided by
ART, the unit increased conversion rates
and no other investment was required.
Before the 2020 pandemic, the unit
ran with high throughput at a typical
conversion of about 63 vol%. During the
pandemic, demand for fuels dropped, so
the unit was run at minimum throughput
with about 85 vol% conversion to meet
the contractual volumes of diesel. In this
period, the FCCU was shut down due to
economics. To reduce costs even further,
there was a focus on minimizing the consumption
of VGO. Together with ART,
Preemraff optimized the operation so
that conversion could stay continuously
close to the 85 vol%-90 vol% range.
Another challenge during the last
couple of years has been the new IMO
legislation for sulfur content in bunker
fuels that has required refineries to
change the crude slate from high-sulfur
crudes (mainly Urals) to mainly lowsulfur
crudes (Johan Sverdrup, Gullfaks
and Oseberg). For the hydrocracker, this
meant a reduction in sulfur content in the
VGO from about 1.5 wt% to 0.5 wt% S,
which was made possible through close
cooperation between ART/CLG technical
services and Preemraff operations.
An improved catalyst system for
Preemraff. Starting from the cogelbased
catalystsa
in the 1960s, scientists at
Chevron's Richmond Technology Center
(RTC), its global headquarters for innovation,
have developed and continue to
expand the family of catalystsa
. This effort
FIG. 3. Mode of operation pre-pandemic and pandemic period limiting fresh feed intake to the
bare minimum during the COVID-19 pandemic.
has been in response to the changing demand
of refiners-including Chevron-
to be able to process more difficult feeds,
produce more and better quality desired
liquid products, operate longer on each
charge of catalysts, and deliver superior
heavy polynuclear aromatics (HPNAs)
management by chemical means.
Chevron scientists have utilized varying
hydrogenation metals (from base
metal to noble metals), and a combination
of amorphous silica-alumina (ASA)
and crystalline silica-alumina (i.e. zeolites)
to customize the catalystsa
for each
application.1-4
Optimization of the hydrocracking
system, cycle-over-cycle,
unit catalyst
FIG. 4. Hydrocracking conversion levels, pandemic and pre-pandemic.
22 DECEMBER 2022 | HydrocarbonProcessing.com
helped Preemraff increase flexibility further
during the pandemic and maximize
conversion. The previous cycle showed a
surplus of activity to increase the mid-distillate
yield cycle-over-cycle. As a result,
a more selective system was designed for
the current cycle, which enabled the client
to use the extra flexibility to increase
conversion further without jeopardizing
the yields during the pandemic.

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Hydrocarbon Processing - December 2022

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