(555f) Amino Acid Solvents for CO2 Absorption

Authors: 
Li, L., University of Texas at Austin


Amino Acid Solvents for CO2 Absorption

Le Lia, Gary T. Rochellea*

aDepartment of Chemical Engineering,
The University of Texas at Austin, 1 University Station C0400, Austin, Texas
78712, USA

Abstract

Amino acid solvents
were tested for CO2 capture performance at optimized absorber
conditions.  The solvents are: 6 m
potassium glycine (GlyK),
6.5 m potassium β-alanine (β-AlaK), 3 m / 5 m potassium taurine
/ homotaurine (TauK/HtauK), 6 m potassium sarcosine (SarK), and 4.5 m sodium sarcosine
(SarNa). A Wetted Wall Column (WWC) was used to
measured the absorption/desorption rates and CO2 solubility of each
solvent at variable CO2 loadings and temperatures ( 40°C, 60°C,
80°C, 100°C).  Solvents are analyzed at
coal fired power plant flue gas conditions and gas turbine combined cycle
(GTCC) plant conditions.  The operation
lean/rich CO2 loading is assumed to correspond to CO2
equilibrium partial pressures of 500 Pa/ 5000 Pa for coal, and 100 Pa / 1000 Pa
for GTCC.  The absorption/desorption
rates, cyclic capacity, and heat of CO2 absorption are reported for
each solvent at both conditions and compared against 7 m monoethanolamine
(MEA).  All amino acid solvents have low
capacities at 0.2-0.3 mol CO2/mol alk,
which is 50% of 7 m MEA. The absorption rate of 6 m SarK
is competitive against 7 m MEA.  3 m / 5
m TauK /HtauK has an
attractive high heat of absorption at 80 kJ/mol. 

Key words:Amino
acid; Solvent screening; Natural gas; Absorption/desorption rates; Cyclic
capacity; Heat of CO2 absorption

1.
Introduction

Amino acid solvents are attractive for post
combustion CO2 absorption because of their low environmental impact,
with characteristics such as zero volatility, low ecotoxicity,
and high biodegradability [1]. To absorb CO2, amino acids must be
activated in water with the addition of an equi-molar
amount of base.  In the presence of added
base, the amino group on the amino acid reacts with CO2 like amines
[2,3]. Potassium (K+) was used as the base
in four tested solvents: 6 m SarK, 6.5
β-AlaK, 6 m GlyK, and
3 m/5 m TauK/HtauK;
sodium (Na+) was used for 4.5 m SarNa.  Many amino acid solvents precipitate with CO2
loadings [4,5]. As aqueous solvents, this physical
property limits solvent cyclic capacity and the potential for flexible
operation at rich loadings.

Typical coal fired
power plants generate flue gas with 12% CO2 and 5-8% O2.
GTCC plants with similar power capacity generate flue gas with 3% CO2,
15% O2, and twice the molar flow rate of coal flue gas. These
differences in flue gas properties results in changes in solvent performance.  When used for GTCC, desirable solvent
properties include: stability towards oxidative degradation, good absorption
performance at lean CO2 loadings, and low volatility.

2.
Experimental Method and Data Analysis


Experimental
data were collected using a WWC, the same as the apparatus and method used by
Chen [6] and Dugas [7].  The absorption/desorption rates are reported
using liquid film mass transfer coefficients (kg').  A semi-empirical VLE model (Equation 1) is
used to model experimental data and represent solvent CO2 solubility
(Figure 1).

  (1)

This model is used to calculate solvent capacity and
heat of CO2 absorption (Figure 2).


Figure 1: CO2 solubility in 6.5 m β-alaK. Filled points: measured data. Solid lines: model prediction (Eq.1). Dashed lines: 7 m MEA model, Ref [6]

Figure 2: Cyclic capacity and heat of CO2 absorption anlaysis for 6.5 m β-alaK

3. Results

Table 1: Summary of absorption properties of tested
amino acid solvents, compared against 7 m MEA [7].

Amino

acid (m)

CO2 Capacity

(mol CO2/kg Solution)

kg'avg  (@40C)

(x 10-7 mol CO2/s Pa m2)

Mid DHabs  (kJ/mol)

P*CO2

=1.5kPa

P*CO2

=0.5kPa

Coal

Gas

Coal

Gas

Coal

Gas

GlyK (3.55)

0.25

0.25

3

10.2

64

69

GlyK (6)

0.35*

0.35

0.2*

3.2

64*

SarK (6)

0.22

0.236

5

18.9

56.5

64

Tau/Htau (3/5)

0.195*

0.23

2.2*

10.3

74.5*

80

β AlaK (6.5)

0.25*

0.29

2*

7.4

64*

67

MEA (7)

0.47

0.55

4.3

11.7

82

83

4.
References

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DC, USA

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JA, Feron PHM, Versteeg GF.
Kinetics of the reaction of CO2 with aqueous
potassium salt of taurine and glycine.
American Institute of Chemical Engineers Journal. 2006;49(1):203-213

[4]
Kumar PS, Hogendoorn JA, Feron
PHM, Versteeg GF. Equilibrium solubility of CO2
in aqueous potassium taurate solutions: Part 1.
Crystallization in carbon dioxide loaded aqueous salt solutions of amino acids. Industrial & Engineering Chemistry
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[5] Majchrowicz ME, Brilman DWF, Groeneveld MJ.
Precipitation regime for selected amino acid salts for CO2 capture
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[6] Chen X, Closmann
F, Rochelle GT. Accurate screening of amines by the wetted wall column. Energy Procedia 2010.

[7] Dugas R, Rochelle G. Absorption and
desorption rates of carbon dioxide with monoethanolamine
and piperazine. Energy Procedia
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