Phase Equilibrium in Amino Acid Salt
Systems for CO2 Capture
Louise With Sengeløv and Kaj Thomsen ([email protected])
Porsgrunn, February 3rd 2011
Outline
Introduction
Carbonate‐
Methionine system
Comparison of absorbents
• General introduction
• Experimental data for amino acid salt systems
• Methionine as a case study
• Equilibrium constants
• Vapor‐Liquid equilibrium data
• Freezing point depression data
• Thermodynamic modelling
• Absorber conditions
• Desorber conditions
• Heat of desorption
• Conclusion
2
Introduction
Advantages in using amino acid salts instead of alkanolamines:
• Might be less prone to degradation in oxygen rich environments
• Less volatile than alkanolamines due to their ionic nature
• Same affinity towards CO2 as alkanolamines
• Form carbamates
Amino acids participate in the transportation of CO2 in the blood by the formation of carbamate
Problems and drawbacks:
• More expensive
• Limited solubility of amino acids in water
• Limited amount of experimental data available in literature
3
Experimental data for amino acid salt solutions
•
CO2 Equilibrium data
–
–
–
–
–
•
Potassium taurate – Kumar et al. (2003) 38 data points, T=25 and 40°C
Potassium glycinate (Portugal et al. 2009) 103 data points, T= 20‐50°C
Potassium methionate (Kumelan et al. 2010) 65 data points, T= 80‐120°C
Potassium sarcosinate (Aronu et al. 2010) graph with 55 points, T=40‐120°C
Potassium prolinate (unpublished, DTU, UTwente)
Absorption kinetics
– Kumar et al. (2003) (potassium salt of taurine and glycine)
– Van Holst et al. 2009 (potassium salts of 7 different amino acids)
– Prakash et al. 2010 (potassium salts of taurine and glycine)
•
Equilibrium constants
– Sharma et al. 2003
– Hamborg et al. 2007
•
•
Heat capacity and other thermal properties ‐ none
Freezing point depression data
– Sengeløv (2010) (loaded and unloaded potassium salt of methionine) 20 data points ‐14 to 0°C
4
Methionine, C5H11NO2S
•
•
•
•
•
Molar mass 149.21 g/mol
Decomposition temperature 281°C
Solubility in water at 25°C: 0.38 molal
pKa 2.28 and 9.21
Found in cereal grains and in nuts
5
Iso‐electric point of methionine
1
Isoelectric point: 5,74
0,9
0,8
0,7
0,6
pKa=9,21
pKa=2,28
0,5
0,4
0,3
0,2
0,1
0
0
2
4
6
8
10
12
pH
6
Solubility of methionine in water O and in NaCl solution X
Hill and Robson, Biochemical
Journal, 28(1934)1008‐1013
7
Potassium salt of methionine –
Equilibrium constants
• Sharma VK; Zinger A; Millero FJ; De Stefano C; Dissociation constants of protonated methionine species in NaCl media, Biophysical Chemistry, 105(2003)79‐87
– Determined in temperature range 5 – 45°C
• Hamborg ES; Niederer JPM; Versteeg GF; Dissociation constants and thermodynamic properties of amino acids used in CO2 absorption from (293 to 353) K, J. Chem. Eng. Data, 52(2007)2491‐2502 – Determined in temperature range 20 – 80°C
• Equilibrium constant for the carbamate formation of methionine not available
• Equilibrium constant for the protonated form of methionine
not relevant at pH above 6
8
Potassium salt of methionine –
experimental data
• Kumelan J, Pérez‐Salado Kamps A, Maurer G, Solubility of CO2 in Aqueous Solutions of Methionine and in Aqueous Solutions of (K2CO3 + Methionine), Ind. & Eng. Chem. Res. 49(2010)3910‐3918
– VLE‐data for the system CO2‐H2O‐K2CO3‐Methionine at 80‐120°C
– It corresponds to loaded solutions of potassium salt of methionine
– Some precipitation in experiments at 80°C with 2.5 molal K2CO3
and 0.83 molal methionine
• Sengeløv L., Bachelor Thesis, Technical University of Denmark, 2010
– Freezing point depression data for potassium salt of methionine
and for loaded solutions of this salt
9
10
Kumelan et al. (2010), 353K
Extended UNIQUAC
Extended UNIQUAC, CO2 in water
9
8
(Eq. constant from Sharma et al.)
Total pressure in MPa
6
Without K2CO3
5
4
353.15 K
3
m(Met)=0.4826 mol/(kg H2O)
2
1
0
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
m(CO2) in mol/(kg H2O)
10
Kumelan et al. (2010), 353K
0.83 molal Methionine
2.5 molal K2CO3
9
Extended UNIQUAC 353.1 K
8
Total pressure in MPa
Results for the CO2‐H2O‐
K2CO3‐
Methionine system
7
7
Extended UNIQUAC 353.1 K without methionine
6
Kumelan et al. (2010), 373K
5
Extended UNIQUAC 373.1 K
4
3
Extended UNIQUAC 373.1 K without methionine
2
Kumelan et al. (2010), 393K
1
Extended UNIQUAC 393.1 K
0
1,5
1,7
1,9
2,1
2,3
2,5
m(CO2) in mol/(kg H2O)
2,7
2,9
Extended UNIQUAC 393.1 K 10
without methionine
Results for the CO2‐H2O‐K2CO3‐Methionine system (Equilibrium constant from Sharma et al.)
Extended UNIQUAC model
10
Kumelan et al. (2010), 353K
0.42 molal Methionine
1.27 molal K2CO3
9
Extended UNIQUAC 353 K
8
Extended UNIQUAC 353 K without methionine
Kumelan et al. (2010), 373K
7
Total pressure in MPa
Pitzer model,
Kumelan et al.
6
5
Extended UNIQUAC 373 K
4
Extended UNIQUAC 373 K without methionine
Kumelan et al. (2010), 393K
3
2
Extended UNIQUAC 393.1 K
1
0
0,8
1
1,2
1,4
1,6
1,8
Extended UNIQUAC 393.1 K without methionine
m(CO2) in mol/(kg H2O)
m(K2CO3)≈1.27 mol/(kg H2O) & m(Met) ≈0.42 mol/(kg H2O)
11
Measurements of freezing point depressions for mixtures of 1:1 (mole ratio) aqueous Met‐KOH and 1:3 (mole ratio) aqueous Met‐K2CO3
12
Freezing point depressions: Results and model correlation (Equilibrium constant from Sharma et al.)
1
mol ratio methionine:K2CO3 1:3
Freezing point in °C
‐1
‐3
‐5
‐7
‐9
Experimental
‐11
Extended UNIQUAC
‐13
‐15
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
m (Potassium salt of methionine) in mol/(kg H2O)
0
mol ratio methionine:KOH 1:1
Freezing point in °C
‐1
‐2
‐3
‐4
Experimental
‐5
Extended UNIQUAC
‐6
‐7
0
0,2
0,4
0,6
0,8
1
1,2
m(potassium salt of methionine) in mol/(kg H2O)
1,4
1,6
13
The potassium salt of methionine as an absorbent in post‐
combustion contra MEA, MDEA and K2CO3
MDEA
2,5
Potassiumcarbonate
2
Potassium salt of methionine
MEA
1,5
2 molal absorbent, 30 °C
6
Partial pressure of CO2 in kPa
Equilibrium constant from
Sharma et al.
Partial pressure of CO2 in kPa
Absorber
conditions
1 molal absorbent, 30 °C
3
1
0,5
0
MDEA
5
Potassiumcarbonate
4
Potassium salt of methionine
MEA
3
2
1
0
0
0,1
0,2
0,3
0,4
0,5
0
0,1
0,2
CO2 loading
Partial pressure of CO2 in kPa
Partial pressure of CO2 in kPa
Equilibrium
constant from
Hamborg et al.
Potassiumcarbonate
2
Potassium salt of Methionine
MEA
1,5
1
MDEA
5
Potassiumcarbonate
4
2
1
0
0
0,1
0,2
0,3
CO2 loading
0,4
0,5
Potassium salt of Methionine
MEA
3
0,5
0
0,5
2 molal absorbent, 30 °C
6
MDEA
2,5
0,4
CO2 loading
1 molal absorbent, 30 °C
3
0,3
0
0,1
0,2
0,3
0,4
0,5
CO2 loading
14
The potassium salt of methionine as an absorbent in post‐
combustion contra MEA, MDEA and K2CO3
Partial pressure of CO2 in kPa
1200
Potassium salt of methionine
1000
MDEA
800
MEA
600
Potassiumcarbonate
2 molal absorbent, 115 °C
9000
400
200
Potassium salt of methionine
MDEA
8000
7000
6000
MEA
5000
Potassiumcarbonate
4000
3000
2000
1000
0
0
0
0,1
0,2
0,3
0,4
0
0,5
0,1
Potassium salt of methionine
MEA
2830
2330
MDEA
1830
Potassiumcarbonate
1330
830
330
0
0,2
0,4
0,6
CO2 loading
0,8
1
Heat of reaction in kJ/(kg CO2 captured)
1 molal absorbent, 115 °C
3330
0,2
0,3
0,4
0,5
CO2 loading
CO2 loading
Heat of reaction in kJ/(kg CO2 captured) Equilibrium constant from
Sharma et al.
10000
1 molal absorbent, 115 °C
1400
Partial pressure of CO2 in kPa
Desorber
conditions
2 molal absorbent, 115 °C
4330
3330
Potassium salt of methionine
MEA
2830
MDEA
2330
Potassiumcarbonate
3830
1830
1330
830
330
0
0,2
0,4
CO2 loading
0,6
0,8
15
The potassium salt of methionine as an absorbent in post‐
combustion contra MEA, MDEA and K2CO3
4000
1 molal absorbent, 115 °C
900
600
Partial pressure of CO2 in kPa
700
MEA
500
Potassiumcarbonate
400
300
200
2500
Potassiumcarbonate
1500
1000
0
0
2100
0,2
0,3
CO2 loading
0,4
0,5
0
1 molal absorbent, 115 °C
1700
Potassium salt of methionine
MEA
1500
MDEA
1300
Potassiumcarbonate
1900
1100
900
700
500
MEA
2000
500
0,1
Potassium salt of methionine
MDEA
3000
100
0
2 molal absorbent, 115 °C
3500
Potassium salt of methionine
MDEA
800
Heat of reaction in kJ/(kg CO2 caputred)
Partial pressure of CO2 in kPa
Equilibrium constant from
Hamborg et al.
1000
Heat of reaction in kJ/(kg CO2 captured)
Desorber
conditions
0,1
0,2
0,3
CO2 loading
0,4
0,5
2 molal absorbent, 115 °C
2600
Potassium salt of methionine
MEA
2100
MDEA
Potassiumcarbonate
1600
1100
600
300
0
0,2
0,4
0,6
CO2 loading
0,8
1
0
0,2
0,4
CO2 loading
0,6
0,8
16
Conclusion I
•
The extreme difference between the properties of the potassium salt of methionine at absorber conditions and at desorber conditions indicates:
– A strong temperature dependence of the equilibrium constant of methionine
– Acidic properties of methionine strongest at desorber temperature
•
Portugal et al. found that the carbon dioxide solubility in potassium glycinate did not change much in the temperature range investigated (20‐
50°C)
17
Conclusion II
•
•
•
•
•
Due to the limited amount of experimental data available, it is possible to model CO2 solubility in amino acid salt solutions without taking carbamate
formation into account
The potassium salt of methionine seems to have very interesting properties for post‐combustion usage. However VLE‐data at absorber column conditions are needed to validate the results
The limited solubility and risk of precipitation provide challenges in the use of the potassium salt of methionine and must be investigated further
– The 2 molal solutions for which calculations were shown here might be precipitating!
More research is required to clarify the role of methionine in the absorption process: Especially regarding carbamate formation and kinetics
Much more research is required on amino acid salt solutions. They have very interesting properties and could be real alternatives to alkanolamines.
18