Hydrocarbon Processing - April 2021 - 63

Process Optimization
the TGTU leading to sulfur slippages, vent gas from
the degassing section and high-H2S-containing
vent gases from the storage area.
* Case 4: TGTU bypass case-Offgases from the
Claus section, no offgas from the TGTU, vent gas
from the degassing section and high-H2S-containing
vent gases from the storage area.
* Case 5: Maximum possible H2S-Offgases from
the Claus section, no offgas from the TGTU, a nonoperational degassing section and high-H2S-containing
vent gases from the storage area. This is a very abnormal
condition for the SRU, where both the TGTU and the
degassing sections are shut down when the incinerator
trips, with only the Claus section running normally, and
associated degassing to occur from the sulfur storage
area. This was only considered mainly for the purpose to
assess the maximum possible H2S concentration in the
combined vent gas stream to the incinerator.
The analysis of the data provided in TABLE 1 shows that
there are no concerns with respect to fire hazards in the incinerator due to the presence of H2S in the vent gas streams
vs. with the H2S flammability limit being 1.6% [considered at
40% of the lower explosive limit (LEL) for H2S], even with the
very conservative case (Case 5).
The second component of interest is hydrogen (H2) from
the TGTU section-or from the Claus section with the
TGTU being bypassed for some operational reasons. The assessment was made to calculate the H2 content in the overall
stream to the incinerator. TABLE 2 provides data for the various
scenarios. The cases reported in TABLE 2 also correspond to
the definitions already provided in TABLE 1. The H2 data is reported only for three cases, as no H2 is present in the vent gas
streams from the degassing area and the sulfur storage area.
The data provided in TABLE 2 shows that the H2 concentration is of concern, with respect to fire hazard, only for Case
5. The amount of H2 content in the incinerator is above the
threshold limit; the threshold limit is typically defined as 40%
of LEL (4% for H2, so 1.6 vol% H2). All other cases are well
within the threshold limit.
Therefore, under normal SRU operating conditions (Cases
1, 2 and 3), the vent gases to the incinerator pose no fire hazard concern. The only point of concern is during the TGTU
bypass scenario with respect to the concentration of H2 in the
offgas streams. A safety measure was implemented in the logic
to not allow for a startup of the incinerator in case, simultaneously, the TGTU was also not operational. Health check feedback from the interlock for the TGTU section was provided as
a startup permissive to the incinerator. Therefore, the incinerator could start up even with the vent gas streams from the
TGTU, the degassing section and the storage section flowing
into the firebox of the incinerator, since they were determinable and safe. This configuration of the vent gas streams being
equivalent to the normal operating condition meant that the
overall availability of the SRU was increased. For every incinerator section trip, the entire SRU typically does not need to
shut down (with each case dependent on given safety concerns); therefore, improving SRU availability and avoiding
significant emissions from the acid gas flare and, hence, from
the entire refinery.

Takeaway. NFPA 86 mandates that all streams to any incinerator must be isolated before a safe and successful purge of the
furnace is allowed, as the gas compositions of the vent streams
are " indeterminable " in many furnaces. For an SRU incinerator,
the composition of each of the individual offgas streams can
be determined, to an acceptable degree of accuracy and confidence, with a conservative approach-consequently, these
compositions are no longer " indeterminable. " Analysis reveals
that the vent gas streams will not pose any specific fire hazard
threat, and, based on the results of the analysis, highlighted design modifications on the startup logic of the incinerator may
be implemented to allow for a safe startup of the incinerator
without tripping the entire SRU. This design flexibility improves the overall availability of the SRU, thereby leading to a
lesser amount of acid gas flaring-thus providing a greener, but
still safe, SRU design.
DEBOPAM CHAUDHURI is a Process Engineer with Fluor Daniel
India in New Delhi. He has 19 yr of experience in petroleum
refining, petrochemical complexes and upstream projects. Mr.
Chaudhuri earned BTech degrees in chemistry and chemical
engineering from the University of Calcutta.

HANS VAN DE RUIT is a Technical Manager of Process
Technology with Fluor in Amsterdam. He has 33 yr of experience
in refining and petrochemical projects, and earned an MS
degree in chemical engineering from the Delft University of
Technology, Netherlands.

Hydrocarbon Processing | APRIL 2021 63

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Hydrocarbon Processing - April 2021

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