Hydrocarbon Processing - September 2021 - 17

Special Focus Refining Technology
S. SINGH and I. CLARKE, Honeywell UOP, Bracknell, UK;
and E. R. ÖNDER, TÜPRAŞ, Izmir, Turkey
HPNA woes-A thing of the past
2020 can be viewed as one the most
challenging periods in the history of the
oil and gas sector. Unprecedented quarantines
and lockdowns imposed due to the
COVID-19 pandemic had a dwindling
effect on fuel demand and oil prices. Refineries
across the world were either shut
down or had to operate at their turndown
capacities. These unforeseen developments
exerted significant pressure on refining
volumes and margins, forcing refiners
to improvise continuously to survive in
this challenging market.
Hydrocracking units play an important
role in converting the bottom of
the barrel, waste streams and renewable
feedstocks into high-value fuels, lubricants
and chemicals to boost the value of
refinery product slate. However, without
adequate means of managing heavy poly
nuclear aromatics (HPNAs), most operators
would struggle to process these challenging
refractory feedstocks, especially
in high-conversion units.
Adapting to cyclical market demands,
new product specifications and increasingly
stringent environment regulations
are nothing new for refiners, but they
now need to act swiftly and conduct
thorough evaluations of their assets to
remain competitive.
Refiners are increasingly processing
heavier feeds, such as heavy coker gasoil,
heavy vacuum gasoil and deasphalted oil, in
their hydrocracker units. This is due to the
industry trend for increased fuel oil conversion,
particularly since the rollout of the
latest International Maritime Organization
(IMO) regulations. However, these factors
can cause a host of operational issues, such
as fouling of both equipment and catalysts
that can curtail the cycle length.
The authors' company has been successfully
tackling these reliability issues
and offers proven catalytic and equipment
solutions that ensure uninterrupted cycle
runs. Each solution is tailored around the
unique features of unit configuration to
harness its full potential. This proven approach
reduces both the revamp scope
and duration that, in most cases, ensures
a swift budgetary approval and unit turnaround
to implement revamp modifications.
Timely commissioning of revamp is
critical to the success of these projects and
puts refiners in a unique position to make
the most from evolving market needs.
HPNA formation,
challenges and
mitigation options. Straight-run vacuum
gasoil (VGO) feeds naturally contain
2-6 ringed polynuclear aromatic (PNA)
compounds. The concentration of these
PNAs-and, therefore, their tendency
to form HPNAs-increases with the increase
in the VGO end point and type. For
example, the HPNA formation potential
of straight-run HVGO with an end point
of 600°C is significantly lower compared
to heavy coker gasoil or any other residue
processed stream with the same end point.
As the feed passes through the hydrocracking
reactors, a combination of condensation
and side-chain cyclization reactions
form HPNAs. These compounds,
formed by undesired side reactions, are
stable and virtually impossible to crack.
HPNAs are fused polycyclic aromatic
compounds with more than seven rings
[e.g., coronenes (C24
(C32
H12) and ovalenes
H14)]. The rate of HPNA formation
increases at low hydrogen partial pressures,
high conversion and at high temperatures,
with the bulk of these compounds ending
up in the recycle oil due to their high boiling
points. HPNA build-up affects both
the unit performance and reliability. Some
of the key detrimental effects include:
* Hydrogen-deficient, highmolecular
weight species increase
the rate of coke formation,
which deactivates the catalyst
and shortens the cycle length.
HPNA formation will depend
on several factors (hydrogen
partial pressure, catalyst type,
etc.) but experience suggests that
the rate of deactivation increases
exponentially as coronenes in
recycle oil reaches 1,000 ppm.
* As concentration of HPNAs and
other difficult-to-crack compounds
build up in recycle oil, reactor
temperatures are raised further to
achieve the target conversion. The
increase in reactor temperatures
often shifts the selectivity towards
lighter products, leading to a
decline in valuable product yields.
* HPNA buildup increases the risk of
fouling in heat exchangers, leading
to increased heater duty and high
pressure drop across the reactor
circuit. As the reactor effluent cools,
HPNAs drop out of the solution
due to their limited solubility and
deposit in the colder exchangers
in the reactor effluent train-
particularly in the air cooler and
cold separators-causing plugging
of tubes and coalescer pads. As their
concentration increases, they will
also begin to deposit in warmer
upstream exchangers. In some
instances, plugging in unconverted
oil (UCO) rundown circuits has
also been reported, either due to
excessive cooling or high HPNA
concentration. Once this happens,
it becomes virtually impossible to
convert or remove these HPNA
deposits online, eventually leading
to unplanned unit shutdown.
A change in UCO color is usually one
of the first signs of HPNA build-up. The
color changes progressively from white
to yellow, then to orange and red as the
HPNA concentration increases. FIG. 1
Hydrocarbon Processing | SEPTEMBER 2021 17
Hydrocarbon Processing - September 2021

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