Hydrocarbon Processing - October 2021 - 48

Sustainability
K2CO3) is a weak solvent, which means that it is easy to desorb
CO2, although absorption is a challenge. The reactions in the
regenerator are considered at chemical equilibrium with masstransfer
limitations, while absorption in the absorber is considered
to be a rate-controlled process, which governs the overall
performance of the system. The low absorption rate of the K2
CO3
solution requires larger equipment areas to meet the design target,
resulting in higher capital costs. To overcome the low absorption
rate, promoters are added to increase the reaction rate.
Eickmeyer7
lysts or promoters, the solution activity of K2
1,000
100
10
1
0.1
0.01
0.001
0.0001
0.01
0.1
FIG. 2. CO2 absorption in MDEA, MEA, and K2
100
10
0.001
0.01
0.1
1
50
55
MDEA
TEA
K2CO3
AMP
NH3
PZ
MEA
60
65
70
75
Heat of solution, kJ/mol
FIG. 3. Heat of solution of various solvents for CO2 absorption.11
0.02
0.018
0.016
0.014
0.012
0.01
0.008
0.006
0.004
0.002
5
10
Bicarbonate reaction factor
FIG. 4. Effect of lean loading and kinetic reaction rate on CO2
absorption.
48 OCTOBER 2021 | HydrocarbonProcessing.com
15
20
80
85
DGA
90
95
α CO2, moles CO2/mol solvent
CO3
at 40°C.
1
8 M MDEA#
7 M MEA+
7 M K2CO3*
3 M K2CO3*
7 M K2CO3-Calc
showed that with the addition of 5%-10% cataCO3
is enhanced by
a factor of 2-4, and this has been demonstrated in commercial
operations. Various promoters, including many amines, have
been studied since the HPC process was first commercialized.
Ayittey et al.8
studied the boric acid-promoted HPC process using
the rate-based simulation model. They validated their model
with literature data and showed that with up to a 6% addition of
boric acid, the CO2
Cullinane and Rochelle9
tential of the K2
capture efficiency can be increased by 1.2%.
proved that the CO2
proved with the addition of piperazine. Borhani et al.10
absorption poCO3
aqueous solution can be significantly imcompared
DEA,
monoethanolamine (MEA), diglycolamine (DGA), diisopropanolamine
(DIPA) and methyldiethanolamine (MDEA) as
promoters for the HPC process using rate-based simulations.
They concluded that except for MDEA, which is too weak, all
the other amines are good enough to be used as promoters in the
HPC process, with MEA and DEA as the most cost-effective.
The rate-based model will incorporate the rigorous kinetic
models, which account for the enhanced absorption rate with
promoters. Different promoters show different levels in rate enhancement,
and the model will use the reported enhancement
data for model setup. The rigorous kinetic model enables better
understanding of the process, adding confidence in proposed
design improvements.
Characteristics of K2CO3 solvent. Mathias et al.11
have summarized
the desired characteristics of solvents based on one
important solvent characteristic: the interplay between CO2
artificial solvents using the proprietary softwarea
capacity
and heat of regeneration. A systematic analytical method
was proposed to guide the selection of desired solvents based
on systematic variation in CO2
In general, the desired solvent should have high absorption
capacity but low heat of absorption. Potassium carbonate
aqueous solution is a low-cost solvent with high stability and
low heat of absorption. However, its solvent strength is low
compared to most of the amines. As shown in FIG. 2, when the
loading factor α < 0.5 (mol CO2
a stronger solubility of CO2
/mol solvent), it demonstrates
than that of MDEA, but a weaker
solubility than that of MEA. The predicted CO2
sures for 7 M K2
= 0.39
= 0.38
= 0.35
= 0.34
= 0.3
ducing absorption effectiveness.
FIG. 3 shows11
partial presCO3
in FIG. 2 are based on a validated thermodynamic
model. The appropriate industry application range is
0.1 < α < 0.5. Leaner loading (α < 0.1) needs significantly more
solvent. High CO2
absorbent strength and heat of absorption with CO2
ger the solvent, the higher the heat of solution. The CO2
al potential of K2
: the stronremovCO3
is close to MDEA, which has the lowest
heat of solution among the seven commonly used solvents.
Chemistry of the process. The chemistry of CO2
tions (Eqs. 1-3):
Equilibrium: 2H2
Kinetic:
Equilibrium: H2
CO2
O ←→ OH-
+ OH-
O + HCO3
→ HCO3
+ H3O+
-
- ←→ CO3
-2 + H3O+
absorption
in K2CO3 can be expressed by the following three reac(1)
(2)
(3)
Reactions
1 and 3 are fast at Benfield or CATACARB process
conditions and, therefore, are assumed to be at chemical equiloading
will saturate the solvent quickly, rea
general trend in the relationship between the
capacity and heat of solution of
.
y (CO2) in absorber exit
CO2 PP (kPa) at α = 0.15
CO2 Patrial pressure, kPa
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