Hydrocarbon Processing - January 2022 - 34

Biofuels, Alternative/Renewable Fuels
rator equipment, where the CO and CO₂
are removed from the system. The concentration
of CO and CO₂ can be minimized
through the appropriate design and
operation of the hydrotreating section,5,6
and materials selection for this section
of the unit can further mitigate potential
corrosion. Additional corrosion concerns
include the potential for chloride stress
cracking and high-temperature hydrogen
attack, although these are also of concern
for conventional hydroprocessing units.
Heat
release and hydrogen consumption.
Hydroprocessing of renewable
feedstocks generates higher heat release
and consumes more hydrogen than
traditional diesel processing. Renewable
feedstocks from vegetable oils, animal
fats and used cooking oils are made up
of compounds known as triglycerides.5,6
These compounds have three fatty acid
chains connected by a propane backbone.
Different feedstocks have variations in
both the length of the chains and the
number of unsaturated bonds. In conventional
hydrotreating, the desired chemistry
is hydrodesulfurization, hydrodenitrification
and aromatic saturation. In the
hydrotreating of renewable feedstocks,
the desired reactions are saturation of the
double bonds on the fatty acid chains and
hydrodeoxygenation to produce n-paraffins,
water and propane.5,6
Both chemistries result in extremely
high heat release and hydrogen consumption.
Catalyst selection and design of the
catalyst stacking arrangement, quench capabilities,
liquid recycle and process controls
are critical to ensuring safe operation
of the unit, preventing runaway reactions,
O
O
O
CH3
O
O
O
16H2
6H2
O + C3
H3
3C18
H38
Hydrodeoxygenation
HDO
10H2
3H2O + 3CO + C3
H8
3C17
H36
Decarbonylation
DCN
CH3
7H2
3CO2
+ C3
H8
CH3
and avoiding premature coking and deactivation
of the catalyst.
Competing reactions to hydrodeoxygenation
are decarbonylation, which produces
CO and water as side products, and
decarboxylation, which produces CO₂ as a
side product. Both reactions lead to yield
reduction, since carbon is lost as CO and
CO₂, rather than retained on the paraffin
chain. CO and CO₂ can react with hydrogen
to form methane and water, generating
additional heat that can lead to coking.
The catalyst selection and stacking arrangement,
as well as process conditions
(such as pressure and treat gas availability),
impact the selectivity between hydrodeoxygenation,
decarbonylation and decarboxylation
and, therefore, are important for
controlling the overall desired yield.
Temperature is another factor for selectivity,
and, as the unit ages, the selectivity
will shift for these three reactions.
Hydrodeoxygenation maintains the highest
yield of the carbon number of the
feedstock; however, this reaction also
consumes the highest amount of hydrogen
(FIG. 1).5,6
Therefore, renewable diesel
processes require high treat gas availability
and high makeup rates to ensure that
sufficient hydrogen is present for the desired
dominant reaction.
Managing treat gas quality. CO, CO₂
and water generated during hydrotreating
must be removed before the hydrotreated
stream passes onto the dewaxing reactor
to ensure high dewaxing yields. Proper design
and operation of the separator equipment
and gas treating system are needed
to manage the concentration of these
contaminants. Specifically, a purge stream
Cold
flow improvement. Hydrodewaxing
of renewable feedstocks is key for
meeting cold flow properties. The composition
of hydrotreated conventional
diesel is a mixture of n-paraffins, aromatics
and naphthenes-whereas, the composition
of hydrotreated renewable diesel
is mainly n-paraffins. N-paraffins have
extremely poor cold flow properties, and
must be either isomerized or cracked to
meet diesel cold flow specifications.
For example, the cloud point of a con3C17
H36
Decarboxylation
DCN
FIG.
1. Hydrodeoxygenation maintains the highest yield of the carbon number of the feedstock;
however, this reaction also consumes the highest amount of hydrogen.
34 JANUARY 2022 | HydrocarbonProcessing.com
ventional hydrotreated diesel may be approximately
5°C to -5°C, while the cloud
point of a renewable hydrotreated diesel is
typically closer to 25°C or higher. Therefore,
to produce a -15°C diesel product,
the conventional sample only requires
10°C-20°C of cloud point improvement,
while the renewable sample requires 40°C
of cloud point improvement. The higher
dewaxing requirement is not a function
of the design and operation of the hydrotreating
section, but is rather due to
the composition of the renewable feedstock.
Although cracking reactions can
improve cold flow properties, they can
also lead to yield loss.
Therefore,
isomerization
chemistry,
specifically that which adds multiple
branches to the paraffin rather than a single
branch, is preferred to maintain yield
is needed to control the concentration of
contaminants like CO, but it is important
to minimize the flow of this stream to
manage the purity of the treat gas and the
required amount of makeup hydrogen.
Additionally, sulfur from dimethyl
sulfide, hydrogen sulfide (H₂S) or another
source is required to maintain the
sulfide state of the hydrotreating catalyst.
The gas treating system is responsible for
balancing the level of H₂S to ensure that
enough is present for the catalyst while
avoiding it cycling up over time. Similarly,
the gas treating and purge systems
should remove sufficient concentrations
of CO and CO₂ to ensure that these do
not cycle up and lead to inhibition of hydrotreating
catalyst activity.
Other components to consider are the
concentration of propane, which is the
backbone of the triglyceride molecules
and which is generated during hydrotreating,
and methane formed from methanation
reactions. Although propane and
methane will not inhibit catalyst activity,
they can dilute the treat gas purity.
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