Hydrocarbon Processing - December 2022 - 53

Process Engineering
lowest possible cost (FIG. 3), treating the
combined thermal oxidizer flue gas from
both SRUs. Caustic-based absorption
systems are a low-capital-cost option to
eliminate SO2
emissions in a simple configuration.
This option was best suited for
this case for the following reasons:
* Caustic consumption would be
limited due to the high recovery
efficiency (Note: at 99.5% recovery
efficiency, caustic consumption
to achieve full SO2
removal is
eight times lower at 97%).
* The inevitable high-salt content
effluent, containing all absorbed
SO2
in the form of dissolved
sodium sulfate salt, could be
disposed of without major
problems because this refinery
was in a coastal location.
* Both Units A and B were equipped
with forced-draft thermal oxidizers,
facilitating the insertion of the
caustic-based absorption system
downstream of the oxidizers.
The cost of the described revamping
measures was substantially lower than the
alternative of adding a new SRU train to
meet the required capacity and operational
flexibility.
CASE 2: RESIDUE
HYDROCRACKER WITH
TWO OPERATING CASES
The second case study presented here
is regarding a refinery revamp, which included
the addition of an ebullated bed hydrocracker
to convert the vacuum residue
to light and middle distillates. The project's
unique feature was that the refinery was
designed for two extremely heavy crude diets:
Crude A was very high in sulfur impurities,
and Crude B was much sweeter but
had much more nitrogenous impurities.
This unique combination of crude diet
generated two very different sulfur and nitrogen
balances for the refinery. For Crude
A, the sulfur load to the sulfur block was
highest, while for Crude B, the sulfur load
was considerably lower but the NH3
load
was much higher due to the high nitrogen
content. The NH3
high compared to the total H2
load for Crude B was so
S content in
the amine acid gas and SWS gas that it was
becoming challenging to achieve a high
enough furnace temperature for complete
NH3
destruction in the Claus furnace.
NH3 destruction in the Claus furnace
is ensured when the furnace temperature
FIG. 3. Emissions management with caustic absorption.
Hydrocarbon Processing | DECEMBER 2022 53
is kept high enough, ideally > 1,250°C at
all operating conditions. The following
options are available for increasing the
furnace temperatures:
* Maximizing the preheat: Preheat
all possible feed streams to the
Claus furnace (amine acid gas, SWS
gas and air). This is the simplest
approach but is limited by the extent
of preheating that may be achieved.
Typically, the high-pressure
steam generated from the waste
heat boilers is used as the heating
medium. The steam generated
from the waste heat boilers is
around 45 barg, so the maximum
preheat temperatures that may
be achieved for the feed streams
would be limited to 240°C or less.
* Oxygen enrichment: Using
oxygen enrichment significantly
improves the furnace temperatures,
as the inert nitrogen component
of air is partially eliminated. The
temperatures from the modified
Claus reaction (the sulfur forming
reaction) achieves a higher
furnace temperature with a richer
(oxygen) airflow. Low-level oxygen
enrichment (up to 28 vol% of
oxygen in the air/oxygen mixed
feed to the furnace) may easily be
applied with no major changes in
furnace design, while mid-level
oxygen enrichment (up to 45 vol%
oxygen) required design updates on
the Claus furnace and burner, as the
burner would need an independent
oxygen lance. Further oxygen
enrichment may also be applied
for extreme feed gas compositions
(very lean concerning H2
* Fuel gas co-firing: Another
method to increase the furnace
temperature is to inject small
amounts of fuel gas in the Claus
furnace; the energy from burning
the fuel gas would help increase the
furnace temperatures. Typically, fuel
gas co-firing is not recommended
continuously and should be limited
to intermittent usages for upset or
abnormal feed gas compositions.
Natural gas or equivalent quality
gas should be fired in the Claus
furnace, limiting the ingress of
heavy hydrocarbons. However, the
probability of soot formation while
burning the hydrocarbons in the fuel
gas can never be ruled out, as the
furnace firing conditions are reduced
at all normal operating conditions.
* Two-zone furnace design:
Another unique way of managing
the furnace temperatures for NH3
destruction is to design the Claus
furnace as a two-zone furnace,
where some portion of the H2
S-rich
amine acid gas is bypassed and sent
to the back end or the second zone
of the furnace. The front zone thus
burns with additional air, which
helps it achieve a higher flame
temperature. When the gas mixture
reaches the second zone, the final
temperature would be defined by
the thermodynamic equilibrium
for the modified Claus reaction.
Another alternative, also implemented
in refineries to manage NH3
S-rich gas and NH3
in the SRU
-rich gas. The
for
feed, is to utilize a two-stage SWS to separate
the H2
NH3 gas can then be combusted directly in
the incinerator or even used as NH3
other uses, while the H2
S-rich gas stream is
S content).
fed into the Claus section for sulfur recovery.
For the case study scenario, since the
variation of the sulfur and nitrogen content
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