Hydrocarbon Processing - June 2021 - GP-37

BACK TO BASICS
Sulfur recovery for gas processing plants
L. MICUCCI, Siirtec Nigi, Milan, Italy
Natural gas produced in gas fields is a
S) and organic sulfur
; and mercaptans, RSH),
mixture of hydrocarbons containing hydrogen
sulfide (H2
species (carbonyl sulfide, COS; carbon
disulfide, CS2
along with other impurities. The content
of the sulfur compounds spans 0.1 vol%-
25 vol%, depending on field location and
well age. These gases must be removed
due to their toxic properties and adverse
impacts on the environment. Therefore,
gas processing facilities originate gas effluents
rich in sulfur species, notably H2
Since the sulfur species cannot be disS.
charged
to the atmosphere, they must be
converted into elemental sulfur and recovered
in liquid or solid form by means
of a sulfur recovery unit (SRU). Eventually,
the residual sulfur species contained
in the offgases produced by SRUs are
thermally oxidized and released to the
atmosphere as SO2
and H2
O.
Increased public environmental awareemissions.
In several parts of the
of SO2
at the stack tip, is required.
ness has led to more severe restrictions
on SO2
world, a sulfur recovery efficiency as high
as 99.9+%, which corresponds to 50 mg/
Nm3
Moreover, restrictions are also required
on the yearly quantity of SO2
that
can be discharged to the atmosphere.
This constrains the number of startups
and shutdowns a central processing facility
(CPF) is allowed in a year. Consequently,
SRUs have become vital for the
operation of CPFs.
The regulatory framework, the variations
of plant throughput required by
well production schedules, and the possible
presence of BTEX (benzene, toluene,
ethylbenzene and xylene) in the SRU
feed make the design and operation of an
SRU a challenging task for a gas processing
facility. A number of variables must
be considered including gas pressure,
temperature and composition; the quantity
of sulfur species (H2
S, COS, CS2 and
RSH); the economic framework and the
location of the gas field.
An SRU encompasses a set of processing
units including an acid gas enrichment
unit, a Claus unit, a tail gas treatment
unit (TGTU), a sulfur degassing
unit (DGS) and an oxidizing unit. FIG. 1
shows a block diagram of the sulfur recovery
island of an inland CPF processing
natural gas from Mediterranean offshore
wells.
Acid gas enrichment. A Claus unit must
be fed with acid gas rich enough in H2
S for
the flame of the thermal reactor to be stable.
When the acid gas from the upstream
sweetening unit contains more than 35
vol% of H2
S, it can be handled directly in
the Claus reactor furnace without issue.
However, if the acid gas is lean (as
often is the case in gas field development
projects), then the H2
S concentration
must be increased above 35 vol% by
means of an acid gas enrichment unit.
When the acid gas contains 15 vol%-20
vol% of H2
S, the enrichment can be attained
through a regenerative absorption
on MDEA aqueous solutions. (Note:
The absorption process on alkanolWaste
gas
Lean/semi-lean MDEA solution
Acid gas
enrichment
Acid gas (H2S,CO2)
from gas sweetening unit
Claus unit
TGTU
Oxidizer
Stack
Sulfur degassing
Sulfur to storage
Offgas from
offshore facilities
FIG. 1. Sulfur recovery island in a CPF processing offshore Mediterranean gas.
Gas Processing & LNG | MAY/JUNE 2021 37
SO2
H2S
CO
O2
amines was described in the fifth article
of the " Back to Basics " series, published
in the September/October 2020 issue.)
Below 15 vol%, formulated alkanolamines
or hindered amines can be used to
attain an enrichment ratio (the ratio of the
H2
unit and the H2
S content of the acid gas to the Claus
S content of the acid gas
from the sweetening unit) greater than 3,
while maximizing the CO2
the CO2 remaining in the offgas).
Alternatively, the desired enrichment
can be pursued by recycling to the absorber
part of the acid gas extracted from
the regeneration tower. In this process
setup, the H2
S in the feed can be controlled
by adjusting the recycle flowrate,
as shown in the dotted line of FIG. 2.
For large-scale gas plants, this configuration
may be expensive due to large
volumes of acid gas flows, which translate
into large-diameter pipe and large valves.
Claus unit. The function of the Claus
unit is to convert H2
S with SO2
S into elemental sul,
according to the
+ 2H2
O
fur and water. This is accomplished by
reacting H2
reversible reaction shown in Eq. 1:
2H2
S + SO2 } 3/x Sx
Superheated
high-pressure steam
to CPF
(1)
Offgas
AI
rejection (i.e.,
Hydrocarbon Processing - June 2021

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