Hydrocarbon Processing - April 2021 - 62

Process Optimization
the design engineer will allow certain liberties to be taken with
respect to the startup purge sequence of the incinerator, without
making any compromise on the safety aspect of the
operation. The following case study will provide a
few typical data samples to show how the composiAn SRU incinerator is unique,
tion of the feed gas streams to the incinerator may be
determined and how that allows some flexibility in
with respect to the applicability of
the design and operation of the SRU, with a benefithe NFPA 86 standard in furnace design.
cial effect on safety and plant emissions.

isolation of contaminated air sources during pre-ignition purge.
In complex systems involving multiple sources where it is not al-

ways possible to shut down all indeterminate sources, providing
a fresh air source and positive isolation from all contaminated
sources is necessary to ensure proper pre-ignition purging.
This is an extremely important statement related to the
safe operation of the furnace. Before furnace startup, the atmosphere inside the firebox must be inert with respect to any
fire hazards. To ascertain this inertness, and to avoid the possibility of an explosion during the startup, the firebox must be
purged with air or any other inert medium. During this purging, all other sources or feed streams to the incinerator need to
be stopped. This is required due to the uncertainty of the actual
composition of the gas mixture inside the firebox. For an incinerator, which typically burns off waste streams from various
sources, the composition of the source gases may vary within a
wide range-hence, the composition of the gas mixture inside
the firebox can be indeterminate. To mitigate this unknown
factor of the gas composition, the startup purge sequence is
completed by shutting down all feed streams to the furnace to
ensure the safe startup of the incinerator.
The SRU incinerator is unique since the feed gas streams to
the incinerator are all coming from controlled process systems
that allow the gas compositions to be limited within a specific
range. In a way, the SRU incinerator feed streams are not " indeterminable " or unknown. Proper assessment of the system by
TABLE 1. H2S concentrations in vent gas streams
Stream no.

Stream
description

Claus
offgas

Offgas
from
TGTU

Degassing
section
vent gas

Tank
vent
gas

Total
vent
gas

Case 1

N/A

20 ppmv

0.02%

0.05%

140 ppm

Case 2

N/A

20 ppmv

0.02%

0.5%

0.02%

Case 3

N/A

200 ppmv

0.02%

0.5%

0.03%

Case 4

0.58%

N/A

0.02%

0.5%

0.31%

Case 5

0.58%

N/A

0.5%

0.58%

TABLE 2. H2 concentrations in vent gas streams
Stream no.

Stream
description

Claus
offgas

Offgas
from
TGTU

Degassing
section
vent gas

Tank
vent
gas

Total
vent
gas

N/A

2.5%

0.85%

Case 1
Case 4

2.9%

N/A

1.26%

Case 5

2.9%

N/A

2.85%

APRIL 2021 | HydrocarbonProcessing.com

Case study data. This case study had all of the possible four vent gas streams feeding into the incinerator (FIG. 1): the
Claus offgas or the TGTU offgas, the vent gases from the degassing section and the tank vent gases. Two unique design aspects
exist in this SRU incinerator. First, the vent gases from the degassing section are always routed to the incinerator, with no option
for them being sent to the upstream Claus section. The unit was
able to achieve the targeted sulfur recovery efficiency even without this recycle of the vent gases to the Claus section. Secondly,
the liquid sulfur storage system-normally expected to hold
pure degassed sulfur-had the option to have undegassed sulfur
containing high H2S. Therefore, the venting system on the sulfur
storage system was designed to handle high-H2S-containing vent
gases from the sulfur storage section. Another constraint in the
SRU design in the case study refinery was that, although the SRU
was a multi-train design, there was hardly any operating flexibility
or spare capacity available in the total sulfur handling capability.
This meant that, if one SRU train shut down, there would be a
significant amount of acid gas flaring-hence, SOx emissions.
Typically, whenever the SRU incinerator section would trip,
the upstream Claus section, TGTU section and degassing section would also need to shut down, leading to a total SRU shutdown and consequent acid gas flaring from the upstream amine
regeneration unit and sour water stripper. To improve the availability of the SRU, a flexibility on the SRU logic was considered
as a design improvement-this being a 1-hr delay in the tripping
of the entire SRU once the incinerator shuts down. This would
allow the SRU to continue running for 1 hr, even without the
incinerator-thus providing the operator a chance to bring the
incinerator back into operation without shutting down the SRU.
One major hindrance in the successful application of this
logic was the NFPA clause, which mandates that the feed
streams to the incinerator must be isolated due to their gas
compositions being indeterminable. Therefore, a detailed analysis of the streams was performed to determine the gas compositions. The initial focus of this study was directed to assess the
amount of H2S in the vent gas streams.
TABLE 1 provides the H2S composition data for the offgas
streams for the various cases covering typical SRU operations:
* Case 1: Normal operation of the entire SRU-
No gas from the Claus section, on-spec operation
of the TGTU, vent gas from the degassing section
and normal vent gases from the storage area.
* Case 2: Degassing in the storage area-No gas
from the Claus section, on-spec operation of the
TGTU, vent gas from the degassing section and
high-H2S-containing vent gases from the storage area.
* Case 3: Process fluctuation in the TGTU unit-
No gas from the Claus section, operational upset in

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Hydrocarbon Processing - April 2021

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