Hydrocarbon Processing - January 2022 - 65

Bio-Based Processing
to 30% biofeedstock in its DHT as a summer-only
fuel.4
However, this was done
using a feedstock known as raw tall diesel,
which is a processed bioproduct of pulp
mills. This product is not equivalent to
vegetable oil and is only available in sufficient
quantities in some geographical locations.5
proportions
required some hydrotreater
modifications and solutions to challenges
related to hydrogen consumption, exotherm,
corrosion and catalyst selection.
Injection into a standalone green
diesel unit. The second process route
is to produce hydrotreated vegetable oil
(HVO) in a purpose-built standalone
vegetable oil hydrotreater designed specifically
for HVO production. The resulting
HVO-known as hydrotreated esters
and fatty acids (HEFA)-is then blended
with the hydrotreated petroleum diesel.
Certain processes that have successfully
employed this approach are Honeywell
UOP's Green Diesel and Neste Oil's
NEXBTL processes. While not technically
coprocessing, up to 30% HVO may be
blended with conventional transport diesel
to produce what is referred to as green
diesel. This approach allows for a greater
bio-content due to the extra processing in
the standalone HVO hydrotreater.
As previously stated, one issue with directly
injecting biofeed into the petroleum
hydrotreater is the high cloud point of the
resulting biodiesel. However, the HVOspecific
hydrotreater is a two-stage process.
In the first reaction stage, the fats are
hydrotreated with hydrogen to saturate
the double bonds and deoxygenate the
molecules. The second hydrocracking/
isomerization stage reduces the length
and increases the branching of the alkane
molecules. This second stage is also
called dewaxing, as this process reduces
the cloud point of the resulting HVO so
that it is consistent with petroleum fuel
specifications and has acceptable cold
flow properties. The investment for an
HVO standalone hydrotreater is greater
than in the first processing approach and
uses more hydrogen. However, the HVO
product is closer to petroleum diesel and
provides greater blending flexibility. The
mass yield of HVO liquids from raw lipid
material is approximately 80%, with the
balance being composed of mostly propane,
methane and oxygenated gases like
CO2
and CO. This approach produces
Note: These larger coprocessing
FIG. 2. Three routes for green diesel/biodiesel production.
significantly more propane than the first
approach, as the hydrogenation reaction
breaks the lipid esters from the propane
backbone. A more detailed background is
publicly available in the report by Kampman
et al. for the EU commission.6
Injection into the FCCU. The third
process route for coprocessing and blending
is to co-feed a biofuel with the crude
oil to the FCCUs, which operate at approximately
500°С and at low pressures
(1 barg-2 barg). The purpose of the
FCCU is to crack heavy hydrocarbon
molecules into gasoline and some middle
and heavy distillate fractions. This point
in the refinery is a good candidate for coprocessing
because the FCCU does not
require expensive hydrogen for deoxygenating
the feed. The catalyst is also more
tolerant to contaminants than hydrotreating
catalyst and can be regenerated onsite.
Common problems faced in FCCU
processing biofeeds are low yield, catalyst
deactivation and reactor plugging.
The vegetable oil cracking mechanism
in the FCCU is complex. Unlike in the
hydrotreating process, where hydrogen
is used to remove oxygenated hydrocarbons
to produce water, the FCCU process
rejects the oxygen atoms, which results
in the production of H2
other oxygenated hydrocarbons3
O, CO, CO2 and
such as
short-chain carboxylic acids, aldehydes,
ketones and phenols.7
The concentration
of these products depends on the biobased
oil type, the FCCU catalyst type,
the operating parameters of the FCCU
and other factors. TABLE 2 summarizes the
impacts of the various processing methods
on the quantity and quality of the important
liquid and gas streams.
Impacts on amine treating. In conventional
refining, hydrogen, fuel gas and
LPG streams are contacted with an amine
solvent to remove H2
vent will also remove CO2
S. The amine soland,
in some
instances, other sulfur species. A simple
process schematic of an amine system is
shown in FIG. 3. The greatest impact of biodiesel
production on amine treating is that
the feed gas has more CO2
and less H2
S.
Depending on the amine solvent used
in the refinery, the extra CO2
generated
by processing the biofeed can reduce the
quality of the acid gas feeding the SRU,
especially at greater co-feed proportions.
With primary and secondary amines, the
extra CO2
the amine solvent to ppm levels in the
treated gas or liquid, and there will be a
Hydrocarbon Processing | JANUARY 2022 65
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