Hydrocarbon Processing - January 2022 - 33

Biofuels, Alternative/
Renewable Fuels
S. GREEN, ExxonMobil Chemical Company,
Houston, Texas
Key considerations for the design and operation
of a renewable diesel unit
Designs for the process configuration
and operation of the unit are fundamental
aspects in the production of renewable
diesel. Renewable diesel feedstocks
include vegetable oils, animal fats and
used cooking oils.
A key advantage of the renewable diesel
product is that it is fungible with conventional
diesel and can be blended with
no limitations.1
However, the process for
making renewable diesel has some specific
challenges that differentiate it from traditional
hydroprocessing. The feedstock
contains impurities and introduces corrosion
concerns not present with traditional
crudes, and the hydrotreating chemistry
produces side products that are atypical
for conventional operations.
The severity of the operation also
presents differences in heat release, hydrogen
consumption and dewaxing requirements.
Proper unit design and operation
are key for producing a high yield
of renewable diesel, while also addressing
these challenges and enabling a safe and
efficient operation.
Renewable feedstock contaminants.
Renewable diesel feedstocks contain
atypical contaminants, such as phospholipids
and free fatty acids, as well as
traditional contaminants like metals and
chlorides.2,3
Most of these contaminants must be
removed via pretreatment steps before
proceeding to hydroprocessing. Phospholipids
consist of fatty acid chains, glycerol
and a phosphate group, and have the tendency
to polymerize at high temperatures,
leading to fouling in both the feed preheat
train and the hydrotreater. These compounds
are removed via a process called
degumming, which solubilizes the phospholipids
by using water, acid or enzymes.3
This process also removes metals-
such as phosphorus, alkali metals and alkaline
earth metals-that are associated with
the phospholipid. Free fatty acids are those
in which one of the chains on the triglyceride
has broken off the propane backbone to
form a carboxylic acid. These compounds
also have the tendency to polymerize, but,
beyond that, they contribute to acidification
of the feed. A high concentration
of free fatty acids increases the total acid
number (TAN) and leads to corrosion of
the feed delivery system. Once the renewable
feedstock is mixed with hydrogen and
is in the presence of the hydrotreating catalyst,
these free fatty acids are hydrotreated
and the concern is eliminated. The presence
of metals-especially phosphorus-
is a concern for active catalyst deactivation
and reactor fouling, which leads to high
pressure drop. Free fatty acids, metals and
phosphorus are removed via chemical or
physical refining processes and bleaching/
adsorption steps.2
Beyond the contaminants,
certain feedstocks present unique
challenges, such as the presence of polyethylene
(PE) found in animal fats and
used cooking oils.4
At high concentrations, PE can cause
fouling and catalyst deactivation and must
be removed along with the other contaminants.
A renewable diesel producer
has the option of purchasing previously
pretreated feedstock or further investing
in the producer's own pretreat system.
While renewable feed pretreatment is
outside the scope of the hydroprocessing
unit, it is critical for maintaining the effectiveness
of the hydroprocessing operation.
Careful control and monitoring of
the contaminant levels enable operators
to mitigate fouling and catalyst deactivation
and to preserve cycle length.
Once the renewable feed is introduced
to the hydroprocessing unit, the remaining
mitigation is the inclusion of demetallation
catalysts and grading materials to
remove residual contaminants before they
reach the active catalysts. Proper selection
of demet and grading materials and their
stacking arrangement, as well as control
of the operating conditions to maximize
metals uptake and minimize fouling, are
key for protecting downstream catalyst.
Corrosion mitigation. Renewable feedstocks
tend to decompose at high temperatures,
increasing the concentration of
free fatty acids and TAN.
A renewable diesel process must take
into consideration the proper process
configuration to ensure that the feed
streams are sufficiently heated prior to
the inlet of the hydroprocessing reactors,
while avoiding high acidity that can
lead to corrosion. This concern increases
throughout the cycle length as reactor inlet
temperatures rise, requiring more feed
preheating. Appropriate materials selection
for the metallurgy of the feed preheat
section is critical for avoiding corrosion if
high acid concentrations are encountered.
Once the feed has entered the hydrotreating
reactors, the free fatty acids
are reacted and are no longer a corrosion
concern. However, hydrotreating renewable
feedstocks results in the formation of
carbon monoxide (CO) and carbon dioxide
(CO₂), which can lead to the presence
of carbonic acid. This could contribute to
corrosion of equipment between the outlet
of the hydrotreating reactors and sepaHydrocarbon
Processing | JANUARY 2022 33
Hydrocarbon Processing - January 2022

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