Hydrocarbon Processing - April 2021 - 25

Special Focus

Clean Fuels
E. CHAN, Burns & McDonnell, Houston, Texas

Converting a petroleum diesel refinery
for renewable diesel
As refiners consider renewable, low-carbon alternatives,
renewable diesel-refined from agricultural products using
petroleum refinery processes-is gaining traction. Rather
than constructing new grassroots renewable diesel production
units, refineries with existing hydroprocessing units may be
able to increase their speed to market with conversion projects.
The use of fossil hydrocarbons has fostered growth and
prosperity more than any other fuel resource in modern times.
Today's stronger focus on environmental, social and governance issues have driven interest in more sustainable alternatives. Renewable diesel is on the rise among those who seek
renewable and sustainable transportation fuels.
Renewable diesel is refined from agricultural products,
particularly vegetable oils, waste cooking oils and animal fats
that are sustainable and available. Because it uses the same hydrotreating and separation processes used for petroleum diesel, it employs the same basic infrastructure and equipment.
Renewable diesel does not contain oxygen, eliminating the
freezing, storage and blending challenges associated with other
renewable fuels, such as biodiesel. Because renewable diesel
has the same chemical structure as petroleum diesel, it can be
used in engines designed to run on conventional diesel fuel-a
" drop-in " diesel substitute with no blending limit.
Making the grassroots vs. conversion decision. The
question refiners interested in renewable diesel are asking is
whether to build a new renewable diesel plant or convert an
existing hydrotreater unit.
For many, the answer may seem simple. Refiners often realize multiple benefits by converting an existing refinery hydrotreater unit rather than constructing a grassroots plant. Perhaps the most significant is time savings. A typical renewable
diesel conversion project can be completed in approximately
two years, or roughly half the time needed to design and build
a new grassroots unit.
Because the power, water, waste, utility and flare systems
needed to support a hydrotreater for renewable diesel are already present in a refinery, a conversion project will cost less
than a grassroots project. A renewable diesel plant on a greenfield site will require the addition of this new infrastructure.
Even more than initial construction cost savings, the greater
financial benefit of a conversion project is the ability it gives a
refiner to get renewable diesel products to market more quickly.
The largest market for renewable diesel fuel in the U.S. is
California, where credits from the federal Renewable Fuel

Standards program in combination with California's Low Carbon Fuel Standards (LCFS) help make it cost competitive. At
least 18 other states and Washington D.C. have legislation in
place for transportation fuel standards comparable to California. The refiners who are first-to-market in these states will
be the biggest beneficiaries of fuel credits. Once the market is
saturated, credit availability will likely decline.
However, time-to-market is not the only factor to consider
when choosing whether to convert an existing unit or build a
new one. To determine if an existing unit is a good fit for a conversion, it is important to evaluate the condition and usability
of its existing equipment and ancillary systems. Process simulations and other analysis will likely be needed to demonstrate
the viability of a conversion project. Some factors to consider
include:
* High reaction exotherm-Whether using agricultural
waste or crude oil, a hydrotreater's reaction releases
heat when breaking chemical bonds in the feedstock.
However, renewable diesel reactions are significantly
more exothermic than petroleum diesel desulfurization
reactions. Therefore, it is important for these units to
be equipped with high liquid product recycle capacity
that can be used to absorb this heat. It is also necessary
to recalibrate production expectations based on the high
product recycle through the unit. A hydrotreater that
operates at 50,000 bpd for petroleum diesel production
may only be able to accommodate 5,000 bpd of fresh
feed when converted to renewable diesel.
* Emergency depressurization systems-Because
of the high heat release associated with renewable
diesel reactions, hydrotreaters require emergency
depressurization systems to manage the reaction safely
in the event the recycle and quench systems fail. These
systems quickly depressurize the reactor to a flare,
stopping the reaction.
* Hydrogen consumption-Renewable diesel reactions
consume a significant amount of hydrogen. Therefore,
refineries with excess hydrogen capacity are particularly
good candidates for conversion projects. Refineries with
limited hydrogen availability may need to budget for the
construction of an additional hydrogen plant.
* Feed train considerations-Depending on the quality
of the renewable diesel feedstock, it may be necessary
to upgrade the metallurgy in the unit's feed train system.
For example, feedstock that is high in free fatty acids
Hydrocarbon Processing | APRIL 2021

Hydrocarbon Processing - April 2021

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