Hydrocarbon Processing - November 2022 - 61

Plant Safety
and Environment
G. E. BOHME, Sulphur Experts Inc.,
Calgary, Alberta, Canada
Why sulfur plants fail: An in-depth study
of sulfur recovery unit failures-Part 2
Equipment malfunction or an unplanned
shutdown of a sulfur recovery
unit (SRU) can have a significant effect
on a production company's profitability,
along with an equally serious impact on
personnel safety and the environment.
The goal of this article is to establish the
highest-probability threats to the reliability
of sulfur recovery facilities and
to focus industry attention on this issue.
This article is a continuation of Part 1,
which was featured in the October issue
of Hydrocarbon Processing.
Thermal excursions. Thermal excursion
(FIG. 4) is a general term that encompasses
both sulfur fires and unacceptably high
process temperatures. Thermal excursions
in the sulfur recovery industry typically
fall into the following five categories:
* Auto-ignition of sulfur vapor/
liquid [auto-ignition temperature
of 232°C (450°F)]:
° Auto-ignition of sulfur vapor is
common when air is purposefully
or accidentally introduced into a
hot SRU during a trip or a restart.
° Some catalyst beds and
condenser inlets contain sulfur
vapor or liquid at or above the
auto-ignition temperature.
* Pyrophoric ignition of sulfur
at any temperature:
° Iron sulfide (FeS) produced
from wet sulfur contact
corrosion is pyrophoric.
° Exposure of the pyrophoric
material provides the ignition
source to ignite sulfur at
any temperature.
° Pyrophoric ignition is relatively
common in sulfur storage vessels.
° It can occur anywhere in the SRU
where sulfur has been allowed
to freeze on metal surfaces.
* Self-heating tail gas unit (TGU)
catalyst [at temperatures over
93°C (200°F)]:a
° The active form of TGU catalyst
(cobalt and molybdenum
sulfides) is self-heating/
pyrophoric and will auto-ignite
in the presence of oxygen.
* Overheating of burner chambers
on fuel gas firing [refractory
damage possible above
1,537°C (2,800°F)]:
° Stoichiometric natural gas
firings during shutdowns,
startups and standbys generate
a flame temperature of
1,982°C (3,600°F).
° Overheating of burner chambers
can damage refractory unless
sufficient cooling gas is added.
* Overheating of burner chambers
on acid gas firing [refractory
damage above 1,537°C (2,800°F)]:
° Pure H2
S at one-third burn
stoichiometry generates a peak
flame temperature of 1,454°C
(2,650°F); this does not result
in damage.
° Temperatures can be increased
by oxygen enrichment,
hydrocarbon addition, acid
gas bypass operation and
stoichiometries greater than onethird,
among others. Worst-case
temperatures are over 4,704°C
(8,500°F) (100% combustion
of H2
S with pure oxygen).
Thermal excursions can result in rapid
catalyst damage at temperatures of more
FIG. 4. Examples of SRU thermal excursion.
than 500°C (932°F). Even more significantly,
thermal excursions can result in
the softening of carbon steel at temperatures
over 380°C (716°F) and in the
complete melting of carbon steel at temperatures
over 1,425°C (2,597°F). The
melting of carbon steel will result in the
additional failure route of an immediate
process fluid release.
The author's company's case studies
include hundreds of thermal excursion
incident investigations. The auto-ignition
of sulfur, the pyrophoric ignition
of sulfur, and the overheating of burner
chambers while burning fuel gas all occur
relatively frequently and in roughly
equal proportions (around 5-6 cases/yr
each), indicating that the SRU industry
still suffers from numerous design and
procedural flaws. Thermal excursions
due to oxygen contacting active TGU
catalyst are less common (around one
case/yr), likely due to a combination
of increased awareness around this issue
(i.e., catalyst vendors are good at informing
of the self-heating nature of this
catalyst), as well as the more limited ability
for air to enter this part of the SRU.
The final category of thermal excursions
from acid gas firing is the least common;
it is usually associated with problems occurring
during oxygen enrichment. The
author's company has only encountered
Hydrocarbon Processing | NOVEMBER 2022 61
Hydrocarbon Processing - November 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - November 2022
