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.