Hydrocarbon Processing - September 2021 - 18

Refining Technology
highlights the key areas of concern as HPNAs
build up in the unit.
To address the fouling concerns, almost
all high-conversion hydrocrackers are now
designed with a hot high-pressure separator
(HHPS) at a point between the last
cracking reactor outlet and the cold highpressure
separator. An HHPS typically
operates in a temperature range of 260°C
(500°F)-315°C (600°F), which is sufficiently
high to keep the HPNAs in liquid
stream that bypasses the colder sections of
effluent heat exchange and prevent solid
precipitation. While the hot separator design
element prevents exchanger fouling,
HPNAs are still present in the UCO and
will build up in the recycle stream, causing
significant catalyst deactivation.
It is still necessary to physically remove
the HPNAs from the hydrocracker;
this is usually accomplished by removing
a small portion of the UCO as a
purge stream. The bleed stream from the
hydrocracker can range anywhere from
3%-10%. If a refinery does not have a
fuel oil system or a fluid catalytic cracking
unit (FCCU), this option can lead
to indirect recycling of HPNAs, further
compounding the issues. It also offers
limited flexibility to withstand changes in
feed quality and catalyst aging with time.
Often, HPNA formation increases exponentially
and refiners are forced to increase
the UCO bleed rate to sustain the
unit operation. The resultant decrease in
conversion and middle distillate (MD)
yields can cost a refiner with an average
size HCU (50,000 bpd or 8,000 m3
/d)
more than $10 MM/yr of lost product
opportunity. FIG. 2 shows a typical conversion
vs. HPNA trend for units without any
active HPNA management.
Over the years, the authors' company
has developed several approaches to effectively
manage and limit the HPNA
formation in hydrocracker units. These
include both improvements to catalyst
systems for better HPNA conversion, as
well as new equipment innovations to aid
in HPNA removal. Catalytic solutions
mainly focus on saturating the starting
blocks of HPNA, i.e., feed refractory species
or PNAs to prevent sedimentation in
both reactor and product sections. This is
particularly a concern in high-conversion
recycle units processing VGO with a high
end point (> 600°C) and feed blends comprised
of thermally cracked gasoil streams
from coker, visbreaker, and FCC units and
other residue derived feedstocks. The inclusion
of proprietary high-activity and
deep hydrogenation catalystsa,b,c
can help
mitigate associated processing challenges
with these difficult feedstocks.
Catalytic solutions. The authors' company
brings a unified approach to cater to
diverse hydroprocessing configurations
and technologies offering a vast choice of
solutions for both hydrotreating and hydrocracking
catalysts. The company's hydroprocessing
catalyst portfoliod
comprises
of large variety of catalysts specifically
designed to meet diverse and unique refiner
needs around the globe. A wide range of
commercially proven catalysts and technical
support throughout the catalyst cycle
help operators to continuously optimize
unit performance, improve reliability and
meet challenging market needs. For units
struggling with HPNA issues, an ideal
combination is usually a high-activity pretreat
catalyst with a high-saturation cracking
catalyst.
The use of advanced analytical techniques
like a Titan 80-300 keV Super-X
aberration-corrected electron microscope
allows the company's scientists to better
understand interactions between the active
metal and the catalyst support. These
and other characterization techniques
have proved to be a game-changer and
helped the authors' company to swiftly
commercialize catalystse,f
specifically designed
to deliver improved hydrogenation.
The company's catalysts can be used alone
or stacked together depending on the unit
configuration, operating conditions and
cycle length. These catalysts offer excellent
hydrogenation that can either be utilized
for converting or saturating HPNAs
or for boosting UCO VI for lube oil production.
Catalyst activity is ideally suited
for maximum distillate production in both
single- and two-stage units.
Similarly,
proprietary
treating catalystsg,h
FIG. 1. Effects of HPNA buildup.
with enhanced metal
formulation and support have proven
experience in handling heavy refractory
feeds. These catalysts offer high start-ofrun
(SOR) activity that is suited to saturate
PNA, and excellent stability that delays
the approach to aromatic saturation
equilibrium and allows consistent operation
at low nitrogen slips, all of which drastically
reduce the HPNA make.
A novel unsupported catalystc
delivFIG.
2. Typical unit conversion trend to limit HPNA buildup.
18 SEPTEMBER 2021 | HydrocarbonProcessing.com
ers step out performance especially for
refiners struggling with catalyst bed volume
limitations. An advanced characterization
of feedstocks and products is
applied to elucidate the catalysis kinetics
and determine the most suitable location
for the catalyst layer(s)c
in the right quanhigh-activity
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