Hydrocarbon Processing - December 2021 - 20

Corrosion A. GAJDACSI
Emerson Automation Solutions, London, England
Maintaining profitability with opportunity crudes
while mitigating naphthenic acid corrosion
Refiners are asked to produce products within ever tighter
margins in a changing industry. Operators that strive to increase
their profitability often turn to cheaper but more corrosive opportunity
crudes; however, that is not without risks.
These risks most often manifest as unplanned shutdowns
caused by the detection of unacceptably high corrosion activity
in a certain area of the plant. If unnoticed and unmitigated, this
excessive corrosion could result in a hydrocarbon leak and in the
worst case, an explosion or fire. Such an event may lead to human
tragedy, extended business interruption, loss of custom, the cost
of extensive rebuilding of equipment, negative impact to the company's
brand reputation and future enhanced regulatory scrutiny.
A recent example of the danger of undetected corrosion was
the 2019 explosion at a Philadelphia hydrocarbon facility caused
by an elbow joint that had been installed in 1973 and had worn
down to the point of failing. This disaster released dangerous
chemicals in the air and ultimately bankrupted the company.
One of the biggest challenges-and corrosion culprits-
when using cheap opportunity crudes is naphthenic acid corrosion,
which is a high-temperature corrosion mechanism that
commonly occurs in distillation columns. Since naphthenic acid
corrosion starts occurring at temperatures greater than 250°C
(482°F), it is often difficult to assess in real time and poses a
unique challenge to the corrosion mitigation plans at a refinery.
What is naphthenic acid and why is it so corrosive? High
total acid number crudes are known to bring high levels of naphthenic
acid with them. Naphthenic acid corrosion is a particularly
aggressive and often localized corrosion characterized by
an 'orange peel' effect within pipes. Naphthenic acid is naturally
occurring in certain types of crude brought up in oil exploration.
It only becomes a problem in areas of the crude and vacuum
distillation units at temperatures above 200°C (392°F). Starting
at the hot end of the preheat train, through the charge heater
coils and affecting the lower section of the crude distillation
tower, light and heavy gasoil side draw and residue product run
down until they are cooled back below this critical temperature
threshold (FIG. 1). The danger areas for naphthenic acid corrosion
are the same in the vacuum unit.
Generally, naphthenic acid is most aggressive to carbon steel.
Stainless steel has shown a greater resistance but would be prohibitively
expensive to use, which is why most operators continue
to use carbon steel.
The naphthenic acid molecules can be relatively inert until
they are 'activated' by high shear velocities-such as those
found at flow discontinuities caused by reducers and expanders,
by elbows and bends and by tee sections, or downstream of any
flow disturbances, such as a pump discharge, injection nozzles
or intrusive probes. This makes these areas most susceptible to
naphthenic acid corrosion.
Managing naphthenic acid corrosion. Remaining vigilant
to the dangers of naphthenic acid corrosion in a facility demands
having a plan in place to monitor on an ongoing basis.
Traditionally, the most common types of monitoring used are
intrusive corrosion probes or manual ultrasound. Both methods
pose challenges. The corrosion probe provides an indirect
measurement as the tips corrode off over time, thereby not providing
an accurate picture. Manual ultrasound requires personnel
to put themselves in potentially dangerous positions.
If the intrusive probe has a tip left, it will provide valuable
and instant data about the pipe conditions. They can quickly
alert an operator to a risk developing in a pipe. Still, it should be
noted that the installation and replacement of such probes can
introduce different types of safety risks.
Manual ultrasound presents additional problems in that the
results are not reproducible. It is highly unlikely that the technician
sent out to apply the manual ultrasound device to take
a reading will take readings at the exact same spot on the pipe
each time. This can lead to missed corrosion and an incomplete
map of where corrosion might be developing. In addition, manual
measurements are not practical to carry out frequently, so a
quickly developing corrosion risk may go unnoticed.
In areas of the process that undergo extreme temperatures,
FIG. 1. Example of high-risk locations for naphthenic acid attack
in the crude and vacuum distillation towers.
20 DECEMBER 2021 | HydrocarbonProcessing.com
corrosion monitoring becomes even more difficult, as instrumentation
is not able to cope with the temperature and the extreme
heat poses a safety risk to personnel.
http://www.HydrocarbonProcessing.com
Hydrocarbon Processing - December 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - December 2021
