Hydrocarbon Processing - November 2021 - 78

Valves, Pumps and Turbomachinery
The pretreatment process requires a
wide variety of processing steps, with particular
steps depending on the raw materials
used as feedstock. The best control
valves for pretreatment will vary widely
(depending on the specific operations required),
and selecting the proper materials
of construction and alloy components
will play heavily in the design effort.
Once
pretreated,
the
green diesel
intermediate is fed into the same hydrotreating
process used in typical diesel
production (FIG. 4). Green diesel processing
usually produces an elevated level
of corrosion due to the oxides and acids
in the intermediates. Temperatures and
pressures can also run higher in a green
diesel hydroprocessing unit. This can create
challenges for instrumentation and
control valve selection.
The temperatures, pressures and corrosive
nature of green diesel production
pose unique challenges for operations.
Paraffin-like components also make the
process more " sticky, " causing control
problems over time. The reactor section
of the hydrotreater is the most critical unit
for maximizing profitability, with reliable
and tight control required for smooth operations.
Poor control performance can
shorten catalyst life and reduce yield, so
picking the right control valves is a crucial
step in the design process.
FIG. 2. A wide variety of renewable materials
can be converted to green diesel. Carbon
intensity values vary by product but
generally favor used cooking oil and tallow.
Source: Renewable Energy Group.
Separator letdown. One critical control
element is the separator letdown
control valve. This valve controls the liquid
level in the high-pressure separator
and passes hydrocarbons with entrained
gas and particulate. Temperatures can exceed
260°C (500°F), and the valve must
reduce an upstream pressure of 1,100 psi
to 900 psi. The high pressure drop and
very high temperature would be a challenge
for any valve, but the corrosive and
sticky nature of the material makes this a
particularly difficult application.
A typical letdown valve would employ
many very small internal passages
to stage the pressure drop and handle
the flashing conditions, but this type of
valve would quickly plug in this service.
Instead, the preferable valve design employs
a multistage letdown arrangement
with large flow passages to allow particulate
to move through the valve. Particular
proprietary high-pressure multi-stage
angle valvesa
(FIG. 5) can be an excellent
choice for this application.
These types of valves are typically
offered with a variety of corrosion-resistant
alloys, and they are designed to
handle the very high temperatures and
pressures encountered in this application.
These types of valves also have
large, multistage flow passages to handle
the entrained solids and offgassing that
are typical in this service.
The hot/high-pressure and cold/
high-pressure separator letdown valves
are similar applications, but within the
hydrotreater unit. The best valve design
in these services will often depend on the
expected ratio of outgassing of the material.
A very high gas volume ratio will
FIG. 3. Green diesel pretreatment involves different processing steps, depending on which raw material is used as a feedstock. Some combination of
polyethylene removal, degumming and bleaching is usually required to convert the feedstocks into a common intermediate for further processing.
78 NOVEMBER 2021 | HydrocarbonProcessing.com

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Hydrocarbon Processing - November 2021

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