(555f) Amino Acid Solvents for CO2 Absorption

Amino Acid Solvents for CO₂ Absorption

Le Li^a, Gary T. Rochelle^a*

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

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. 

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 (k_g').  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].

4.
References

[1] Eide-Haugmo
I, et al. Environmental impact of amines. Proceedings of the 9th International
Conference on Greenhouse Gas Control Technologies.16?20 Nov 2008, Washington
DC, USA

[2] Hook RJ. An
investigation of some sterically hindered amines as
potential carbon dioxide scrubbing compounds. Industrial & Engineering Chemistry Research. 1997;36:1779?1790.

[3] Kumar PS, Hogendoorn
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
Research. 2003;42:2832?2840.

[5] Majchrowicz ME, Brilman DWF, Groeneveld MJ.
Precipitation regime for selected amino acid salts for CO₂ capture
from flue gases. Energy Procedia. 2009;1:979?984.

[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
2009; 1(1): 1163-1169.