Hydrocarbon Processing - June 2021 - GP-38

BACK TO BASICS
The kinetic of the Claus reaction
would be too slow; therefore, it requires
either catalysis or high temperature to occur.
The SO2
troduced into the chemical environment
by burning one-third of the fed H2
atmospheric air through a Claus burner,
as shown in Eq. 2:
H2S + 3/2O2
} SO2
+ H2
O
(2)
The oxidation reaction in Eq. 2, being
strongly exothermic, develops a temneeded
by the reaction is inS
with
perature higher than 1,000°C if the H2
S
concentration in the acid gas is greater
than 40 vol%.
An industrial Claus unit encompasses
a thermal front end and a catalytic end.
In the thermal stage, the acid gas and
air are admitted into the thermal reactor
(TR)-an internally refractory-lined
pressure vessel-through a Claus burner.
The enthalpy of the process gas exiting
the TR at 1,000°C is recovered in the
waste heat boiler (WHB) by rising medium-pressure
or high-pressure steam.
Acid gas
to Claus unit
Acid gas recycle
Air cooler
Offgas to
oxidizer
MDEA
absorber
CWS
Air cooler
MDEA
Semi-lean MDEA
from TGTU
Acid gas from
sweetening
unit
Surge
drum
regenerator
Reflux
drum
Most of the elemental sulfur formed in
the thermal stage is condensed and recovered
as liquid.
The process gas exiting the first sulfur
condenser contains unreacted H2
S, H2O and N2
S, SO2
,
. This stream is directed
into a " catalytic assembly " comprising a
reheater, an adiabatic fixed-bed catalytic
converter and a sulfur condenser. In the
catalytic converter, the Claus reaction
(Eq. 1) takes place at low temperature,
and further elemental sulfur is yielded.
To prevent the formed sulfur from condensing
inside the catalyst pores, effluents
from the previous condenser must
be reheated to above the sulfur dewpoint
in the reheater. The catalyst of a Claus
unit is alumina (Al2
O3). When the Claus
unit is not integrated with a TGTU based
on oxidized sulfur species reduction, and
the acid gas contains COS and CS2
er of titanium (TiO2
, a lay)
catalyst is placed
on the top of the alumina bed. The titanium
catalyst promotes organic sulfur
hydrolysis to CO2
and H2
S.
The recovery of sulfur in a two- or
three-stage conventional Claus plant
is thermodynamically limited. Typical
Claus plant recovery efficiencies
are 90%-96% for a two-stage converter
plant and 95%-98% for a three-stage reactor
plant. Most Claus units are fitted
with two catalytic converters (FIG. 3).
The key process variable of a Claus
unit is the " air demand " -i.e., the H2
SO2 ratio measured at the outlet of the
S/
Rich MDEA
pump
Lean MDEA
pump
FIG. 2. Acid gas enrichment process flowsheet.
Air
FIC
High-pressure steam to thermal oxidizer HTHR
Air demand
FFC
Reheater
Reheater
AIC
2 Claus
Converter
Preheater
FIC
1 Claus
Converter
Claus
burner
Acid gas
Preheater
FIG. 3. Claus unit process flow diagram.
38 MAY/JUNE 2021 | GasProcessingNews.com
Thermal
reactor
Waste heat
boiler
Liquid sulfur
Liquid sulfur
Tail gas
to TGTU
Tail gas to
knockout drum
Lean MDEA
pump
Reflux
pump
plant. This variable is used to fine-tune
the air flowrate to the burner in a way
that ensures the optimum operation of
the unit.
In a CPF, the acid gas flowrate and
its composition can change over time.
To provide the Claus unit with the operational
flexibility needed to tackle these
variations, in addition to the installation
of feed and air preheaters to support the
Claus operation, the thermal stage of the
Claus unit must be designed to accommodate
different operational modes, depending
on the feedstock characterization.
These modes can be grouped into
four categories: straight-through, splitfeed,
cofiring and fuel-support mode.
In the straight-through mode, the entirety
of the acid gas and airflow passes
through the Claus burner. This mode is
set for rich acid gases.
When the H2
S concentration falls below
35 vol%-40 vol%, the operation of
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