Degradation of amines in CO2 Capture
Gary T. Rochelle, Stephanie Freeman, Alex Voice, Fred Closmann
Luminant Carbon Management ProgramThe University of Texas at Austin
Presented at TCCS-6
June, 2011
Messages• Stripper energy use is constrained by the max
T permitted by thermal degradation– TMEA < TMDEA <TAMP< TPZ
• As amines become more resistant, oxidation shifts from the absorber to the heat exchanger– MEA > Tertiary > Piperazine
• Amine degradation must be minimized to manage secondary environmental impact.– Volatile Products can leave with flue gas– Nonvolatile products make up reclaimer waste
Where is degradation most likely to occur?
Flue Gas10% CO25-10% O2
Purified Gas1% CO2
30% MEAα = 0.4-0.51 mM Fe+3
CO2H2O(O2)
30% MEAα = 0.3-0.4
Reboiler
Absorber40 -70 oC
1 atm
Stripper120 oC1 atm
CrossExchanger
Oxidative Degradation
Thermal Degradation
Thermal Degradation
180
240
300
360
5 10 15 20 25 30 35
Wto
tal(
kW
h/
ton
ne
CO
2)
(∆HCO2-∆HH20)∆(1/T) (kJ/gmol-K)
∆HCO2=60 kJ/mole
70
80
Thermal Degradation limits stripper performance because
Greater Tstrip & ∆HCO2 reduce Weq
MEA 120 C
PZ
Single stage flash at 90-150°CCompression to 150 barLean PCO2 = 0.5 kPa at 40°C
90 C 150 C
5 Mechanisms for Thermal Degradation
• 1. Carbamate Polymerization - MEA
• 2. Cyclic Urea - Ethylenediamine
• 3. Arm Switching/Elimination - Tertiary Amine
• 4. SN2 Ring Opening – Piperazine
• 5. Blend Synergism – Piperazine/MEA
Carbamate Polymerization
• ↔
MEA Carbamate Oxazolidone
→
MEA HEEDA
NHOH CO2- NHO
O
+ O- H
NHO
O
OHNH2 + OH
NHNH2 +
O
O
Primary & Secondary Alkanolamines Deg TAmine k1 = 2.91 × 10-8 s-1 Structure T (oC)
2-methyl-aminoethanol 103
Monoethanolamine 120
3-amino-propanol 126
2-piperidine ethanol 127
Diglycolamine® 133
2-methyl-2-amino-propanol 137
Cyclic urea
NH2
NH2 + O O
O
NHNH
1o & 2o Diamines = cyclic ureas Deg TAmine Structure T (oC)
Dimethylethylenediamine 100
Diethylenetriamine 105
Methylaminopropanolamine 114
Hydroxyethylethylenediamine 114
Ethylenediamine 121
Hexamethylenediamine 156
CH3
NHNH
CH3
2 Tertiary ↔ Quaternary + Secondary
+
CH3
OHN
OH
CH3
OHNH
+
OH
+ OHNH
OHCH3
CH3
OHN
+
OH
Tertiary1 + Secondary2 ↔ Tertiary2+ Secondary1
Tertiary1 + Quaternary2 ↔ Tertiary2+ Quaternary1
Elimination
CH3
CH3
OHN
+
OH + +CH3
CH3
OHNH
+
OHOH
OH2
3o amines→2o amines + other 3o aminesAmine Structure T (oC)
Dimethylmonoethanolamine 122
Tetramethylethylenediamine 125
Methyldiethanolamine 128
N-(2-Hydroxyethyl)PZ 132
N,N’-Dimethylpiperazine 139
1-methyl-piperazine 148
CH3 N N CH3
CH3CH3
N
CH3
CH3N
Ring Opening
NH NH2+
NH
NH
N NH3+
NH NH +
NH2O
OH NH O + OH2
Ring Closing
Cyclic ↔ LinearAmine Structure T (oC)
Diglycolamine® 133
Homopiperazine 133
Pyrrolidine 135
2-Methyl-Piperazine 152
Hexamethylenediamine 156
Piperazine 162
Morpholine 169
CH3
NH
NH
NH NH
Interactive Blends• Carbamate Polymerization
NHNHNH
O O+ NNH
NH
OH
O
+
+ OHNH2
+
OH
NHNH NH+
OH OH
NNH
Secondary2 + Tertiary1 ↔ Tertiary2+ Secondary1
Total Amine Loss in BlendsAmine (m) Structure T (oC)
MEA/PZ 104
MEA/AMP 123
4 AMP/6 PZ 135
7 MDEA/2 PZ 138
4 PZ/4 2MPZ 155
3.9 PZ/3.9 1MPZ/0.2 14DMPZ
160
Oxidation
O2 solubility & Mass Transfer
0,E+0
2,E-5
4,E-5
6,E-5
2,E-04 2,E-03 2,E-02
Am
ine
Oxi
dati
on (m
ol/m
olCO
2)
Oxygen Rate Constant (s-1)
Total
Absorber
ExchangerSump
PZMEAMDEA
Degradation Products and Environmental Impact
7 m MDEA/2 m PZ Oxidized at 120oC
Products (CO2 carrying)C-Loss
(%)
Diethanolamine/Methylaminoethanol 40
1-methyl PZ 8.4
1,4-Dimethyl PZ 0.9
Aminoethyl PZ 3.5
N-formyl PZ (amide) 8.3
Formate & other acids 2.5
Bicine 5.3
Hydroxyethyl sarcosine 10.5
~79.5
Message on Thermal Degradation
• Stripper energy is constrained by the max T permitted by Degradation
– Linear alkanolamines and diamines degrade by polymerization & urea formation at 100-130oC
– Tertiary amines degrade by arm switching &elimination at 120-140oC
– Piperazine and related cyclic amines degrade by ring opening at 150-165oC.
•
Message on Oxidation
• As amines become more resistant, oxidation shifts from the absorber to the heat exchanger
– MEA & alkanolamines readily oxidize in the absorber unless inhibited by radical or peroxide scavengers
– Tertiary amines inhibit self oxidation, probably by scavenging peroxides
– Piperazine oxidizes only at the higher T of the heat exchanger exit
Message on Environmental Impact
• Amine degradation must be minimized to manage secondary environmental impact.– Volatile Products can leave with flue gas
• Aldehydes, formate, ammonia, volatile amines, amides
– Nonvolatile products make up reclaimerwaste• Polyamines, Cyclic urea, amino acids
A review of previous Work
• University of Texas– Thermal: Austgen, Freeman, Closmann
– Oxidation: Goff, Sexton, Voice
• IFP – Thermal, Oxidation– Lepaumier, Carrette, et al.
• NTNU – Thermal, oxidation– Lepaumier, Eide-Haugm, et al.
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