Hydrocarbon Processing - December 2022 - 42

Process Optimization
physical absorption, which is predominantly
a function of pressure, temperature,
residence time and solubility.
A good review of the available prior
work on the removal of COS and lighter
mercaptans in amine systems can be
found in literature by Kohl and Nielsen.2
Reports on removal rates are given by
other authors, but these are drawn from
a very limited number of data points-in
many cases, the results are from a single
installation. These numbers are summarized
in TABLE 1. To the authors' knowledge,
there is little information on heavier
mercaptans (C4
SH and higher), MES,
DMS or DMDS in open literature.
The following pathways for organic
sulfur species removal in an amine system
are possible:
1. Physical absorption
2. Chemical absorption (acid-based
reaction)
3. Direct chemical reaction
with amine
4. Direct chemical reaction with
water (hydrolysis) for some
species (COS and CS2
)
5. Condensation or absorption into a
separate liquid hydrocarbon phase
6. Conversion via reaction with
other species in the gas or
amine solvent solution.
With multiple organic sulfur species
and multiple pathways in an already-complex
system, many gaps exist in industry
knowledge with respect to how, and how
much of, an organic sulfur species will be
removed in a particular amine system.
Physical absorption is driven by the
difference in each species' partial pressure
in the hydrocarbon phase being treated
and in the aqueous amine phase. Absorption
will depend on temperature, pressure,
contact time and solubility. Organic sulfur
species are normally only present at ppm
levels and pressure is thus not a significant
driving force for their removal.
In addition to physical absorption,
mercaptan sulfur species will also undergo
chemical absorption, reacting with
amine to form mercaptide salts. The resulting
system is quite complex to model
mathematically and has previously been
described for R1
SH-R4
SH mercaptan
sulfur by Jones, et al.7
The other organic
S and
sulfur species are not known to dissociate.
Mercaptan competes with H2
carbonic acid (H2
CO3) for chemical absorption
by amine molecules. Protonated
amine bound to sulfide or carbonate is not
available for reaction with mercaptans and
reduces the driving force for their removal.
Until large amounts of H+
ions are no
S and
longer being formed (once all the H2
CO2 are removed), the mercaptan removal
by chemical absorption will be poor.
In the case of COS, it is believed by
many in the industry that a hydrolysis reaction
is the primary mechanism for its
removal.2
COS + H2
O → CO2
+ H2
This reaction is shown in Eq. 1:
S (1)
A zwitterion reaction mechanism
for COS with primary and secondary
amines was proposed by Little, et al.9
in
Eqs. 2 and 3:
COS + R2
NH } R2NH+COS-
R2NH+COS- + (Base) }
R2NCOS- + (Base) H+
(2)
(3)
This two-step reaction mechanism is
by amines. It is comprised of a first
comparable to that of the absorption of
CO2
step, where a COS-amine zwitterion is
formed, and a second step, where the
COS-amine zwitterion is deprotonated in
the presence of a base (unreacted amine
present in the solution).
For tertiary amines, Little, et al.10
proposed
the following (Eqs. 4 and 5):
COS + MDEA + H2
HCO2
MDEAH+ + HCO2
S-
MDEAH+ + HCO3
O }
S-
+ MDEA + H2
O →
- + HS-
(4)
(5)
The first reaction of COS with methylFIG.
1. Pseudo-first order rate constants for reaction of amine solvents with COS.11
TABLE 1. Percentage removal of various organic sulfur species by amines as reported
in literature
Amine
MEA/DEA
DGA
DGA
DGA
MDEA*
MDEA
R1
SH
45-55
14
R2
SH
20-25
12
7
98
90
8
R3
SH
0-10
-
COS
70-80
90
-
-
10
33
*45 wt% methyldiethanolamine (MDEA) with at least 0.07 mol/mol H2
42 DECEMBER 2022 | HydrocarbonProcessing.com
S loading
Source
Butwell, et al.3
Huval and van de Venne4
Harbison and Dingman5
Kenney, et al.6
Jones, et al.7
Seagraves8
diethanolamine (MDEA) to form protonated
MDEA and monothiocarbamate is
reversible and an order of magnitude faster
than the irreversible second reaction. The
overall reaction is very similar to hydrolysis.
A comparison of the reaction rates of
various amines with COS, adapted from
the thesis of Little11
, is shown in FIG. 1.
al.12
While prior research by Rahman, et
found no evidence for the direct reaction
of methyl mercaptan with monoethanolamine
(MEA), diethanolamine
(DEA), diglycolamine (DGA), diisopropanolamine
(DIPA) or MDEA, COS can
react directly with MEA, DEA, DGA and
possibly DIPA to form degradation products.
The rate of degradation with COS
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