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

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 (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].

Amino acid (m)	CO2 Capacity (mol CO2/kg Solution)		kg'avg  (@40◦C) (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

[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.