Hydrocarbon Processing - June 2021 - GP-40

BACK TO BASICS
referred to as an incinerator). The H2
S
captured by the solvent is stripped in the
regeneration tower of the acid gas enrichment
section of the SRU and then sent
back to the Claus unit, thereby achieving
an overall sulfur recovery efficiency
greater than 99.9%.
When only 99%-99.5% sulfur recovery
efficiency is allowed, processes
based on selective oxidation of H2
S to
elemental sulfur can be used. The selective
oxidation of H2
of the
,
elemental sulfur formed via the reverse
reaction of (1, FIG. 3). Other compounds
in the process gas, such as COS and CS2
are neither oxidized nor hydrolyzed.
Well-known selective oxidation processes
include SUPERCLAUS and EUROCLAUS
(both licensed by Worley/
Comprimo and Jacobs).
Thermal oxidizer (incinerator). The
oxidizer/incinerator is the interface between
the CPF and the environment. It
receives offgases from both the enrichment
section and the TGTU. It also receives
the sweep air from the sulfur pit.
All these streams still contain " traces "
that must be conof
H2
S, COS and CS2
verted into less noxious compounds,
such as SO2
, CO2 and H2
O. Generally,
these pollutants are destroyed by means
of an oxidation process carried out at
temperatures between 450°C and 650°C
(up to 940°C when CO oxidation is required
by local legislation) before being
discharged to the atmosphere.
FIG. 5 shows an example of a thermal
oxidizer installed in a large CPF. It includes
several elements:
* A combustion chamber, where a
fuel gas is burned with excess air
to produce a flue gas hot enough
to bring the offgas temperature
to the desired level
* An oxidation chamber,
where effluents are mixed
and thermally oxidized
* A high-temperature heat recovery
(HTHR) section, including a
waste heat boiler and one or
more superheating coils for
rising high-pressure steam.
For small-scale CFP, the quantity
of high-pressure steam that can be produced
may not justify the investment in
40 MAY/JUNE 2021 | GasProcessingNews.com
S is achieved by
means of a catalyst that does not promote
the Claus equilibrium reaction
and prevents oxidation to SO2
HTHR. In these cases, the hot effluent is
routed toward the atmosphere through
the stack after having been quenched
with secondary air in the quench chamber
of the oxidizer.
Liquid sulfur degassing. Liquid sulfur
is formed in the condensers and collected
in a pit by gravity. In the condensers,
liquid sulfur is in equilibrium with
its vapor; therefore, it dissolves a small
quantity of H2
not only to supply the Claus captive users
(feed preheaters, the reheaters and
the plant steam tracing), but also to supply
energy to the reducing catalyst of
the TGTU through a condensing steam
heat exchanger with significant cost advantages
over the traditional gas-gas heat
exchanger or inline burners.
Moreover, the net valuable high-presS.
Under the condenser
S
operating conditions, the dissolved H2
Sx, with x > 1). These
S. Liquid sulfur, if not
S.
H2Sx slowly and naturally decompose to
form sulfur and H2
degassed, would be risky due to the release
of flammable, toxic, corrosive H2
In a sulfur degassing system, hot air
Sx decomposition and as a stripping
S. The H2
S concentration in
(sweeping air) can be used as a catalyst for
the H2
agent for H2
the degassed liquid sulfur is specified at
less than 10 ppmw, which is generally accepted
as a safe concentration.
Several commercial liquid degassing
systems exist, one of which is the Shell
Degassing process. In this system, hot
air is bubbled below an open box submerged
in the liquid sulfur to create a
natural circulation flow that promotes
H2
Sx decomposition and H2
S stripping.
Other processes use hot air as the
Sx decomposition. These
stripping agent and the promoter/catalyst
for the H2
processes accomplish the degassing
operation in an out-of-pit pressure vessel
fitted with mass transfer equipment.
Among these are D'GAAS (licensed by
Fluor) and SN Degassing (licensed by
Siirtec Nigi).
Material and energy integration. An
SRU encompasses several intertwined
process units, with each one achieving
a specific function involving heat exchange
and material transformation. The
ways in which these specific functions
are interwoven affects the service factor
of the materials stream and the energy
efficiency, which ultimately impacts the
economics of a CPF.
In general, the Claus unit produces
approximately 2,300 kg-3,000 kg of
steam per ton of recovered liquid sulfur.
When the waste heat boiler is designed
to produce high-pressure steam at 45
barg and greater, this steam can be used
reacts with elemental sulfur to form
polysulfides (H2
sure steam produced in the SRU block
can be used by other steam users of the
CPF in exchange for the less-valuable
steam at 3.5 barg that is needed by the
SRU's MDEA regenerator.
The acid gas load of the MDEA withdrawn
from the absorber bottom of the
TGTU is in the range of 0.12 mol/mol-
0.15 mol/mol; significant pickup capability
for a lean MDEA solution remains
(note: the load of an MDEA solution is
generally 0.45 mol/mol-0.5 mol/mol).
This residual pickup capacity can be
exploited to reduce the overall MDEA
circulation flowrate in the SRU, which
entails a further reduction of CAPEX
and OPEX due to the reduced duties
of the heat exchangers and the electric
power requirement of the pumps. This
material integration is achieved by feeding
the above stream to an intermediate
tray of the enrichment absorption tower
(FIG. 4), where it is transformed into
rich MDEA.
The thermal and material integrations
of the processes inside the SRU
maximize the thermodynamic efficiency
and streams utilization, with economic
benefits both in terms of CAPEX and
OPEX. GP
NOTE
a Variation introduced by Siirtec Nigi
LORENZO MICUCCI is a Senior
Director at Siirtec Nigi SpA.
He has more than 30 yr of
experience in the engineering
and contracting industry, most
of which have been spent in the
natural gas sector. In 2001, he
joined Siirtec Nigi in Milan, where he directed the
process design and operations department and the
research and development department. During his
time as R&D head, three patents have been granted
to Siirtec Nigi, two of which have been implemented
on an industrial scale. At present, he is the Senior
Director of the technology and marketing
departments. Mr. Micucci also worked for Saipem
(Snamprogetti) as a Plant Designer for integrated
gasification combined cycle and gas-to-liquids
plants. He holds an MS degree in chemical
engineering from the University of Bologna in
Italy and is enrolled as a Qualified Engineer
in the Register of Milan Order of Engineers.
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Hydrocarbon Processing - June 2021

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