Hydrocarbon Processing - November 2022 - 63

Plant Safety and Environment
Process fluid release. Process fluid releases
of concern from an SRU typically
fall into the following three categories:
1. H2
S gas [National Institute
for Occupational Safety and
Health " immediately dangerous
to life or health " (NIOSH
IDLH) value of 300 ppmv]:
° Present in high concentrations
everywhere in the sulfur plant
° Also contained in produced
liquid sulfur and released
at all stages of the liquid
sulfur collection, storage
and transportation
2. Sulfur dioxide (SO2
) gas
(NIOSH IDLH of 100 ppmv):
° Present in approximately a
2:1 H2
° All sulfur species present in
the sulfur plant tail gases are
normally oxidized to SO2
form in some tail gas
S:SO2 ratio in all streams
between the reaction furnace
and the TGU reactor
prior
to release to atmosphere; it can
be very high during upsets
° Can be present in nearly pure
SO2
technologies
° Generated in large
concentrations during sulfur fires
3. Liquid sulfur [typically more
than 150°C (302°F) and can
33%
cause severe burns when in
contact with skin]:
° Present in all SRU condensers
and other low-temperature areas
° Removed from the condensers
through a combination of sealing
devices and rundown lines
° Stored onsite in sulfur pits
and storage tanks
° Often transported from site
in trucks, rail cars, barges or
liquid-sulfur pipelines
° Can ignite, causing even greater
temperatures and increased
risk of contact.
tions of H2
The presence and typical concentraS,
SO2 and liquid sulfur within
the SRU are presented in FIG. 5. As shown,
virtually everywhere in the SRU, the process
concentrations of H2
S and SO2
are
above the air concentration values that
are considered immediately dangerous
to life or health. In most cases, the only
allowable/purposeful release of process
fluid to the environment is from the back
end of the TGU-released either directly
to atmosphere or via an incinerator/thermal
oxidizer. Even with these low concentrations,
the release is usually done
at elevation and at high temperatures to
minimize safety risks. Process upsets can
still increase the concentrations to levels
that are dangerous to nearby personnel.
Releases of unacceptable levels of
H2S, SO2 or liquid sulfur can, and do, occur
in many ways. In the least concerning
cases, these emissions may only be
an odor or a visual pollution problem.
However, in the worst cases, these emissions
can cause serious safety concerns,
such as shelter-in-place requirements for
the plant or surrounding neighborhoods,
injuries, hospitalizations or deaths. Some
of the ways that these unwanted releases
can occur include the following:
* Process gas escaping through
process lines (e.g., backflow of raw
acid gas through process air lines)
* Highly increased stack emissions
during serious upsets: SO2
S between 1% and 5%
* Personnel exposure to process
fluids during routine sulfur
rundown inspections:
° Most plant designs from
1950-2000 included open-air
look boxes to confirm liquid
sulfur flow from condensers.
° Operators are often not
properly aware of H2
S and
SO2 concentrations in these
look boxes.
° Seal legs upstream of these look
boxes can be over-pressured and
can release process gases, as well.
levels at
5% to more than 10%, and uncombusted
H2
50 to
90+%
50+%
6%
3%
2%
1%
1%
0.5%
5000ppm
2500 ppm
10,000+ ppm
Sulfur
10,000+ ppm
Sulfur
100s to
10,000s+ ppm
100s-
10,000s+ ppm
5,000+ ppm
Sulfur
1,000+ ppm
Sulfur
<250 ppm
0 to 1000s+ ppm
100s-10,000s+ ppm
500+ ppm
Sulfur
Legend
Red = H2S gas
Blue = SO2 gas
Green = liquid sulfur
FIG. 5. Process fluid concentrations in the SRU.
Hydrocarbon Processing | NOVEMBER 2022 63
Hydrocarbon Processing - November 2022

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