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1
High pressure vapour liquid equilibrium of alkanolamine, water, carbon dioxide and methane
systems Øystein Jonassen1, Inna Kim2, Hallvard F. Svendsen1
1Norwegian University of Science and Technology 2SINTEF Materials and Chemistry
3rd Trondheim Gas Technology Conference, June 04th-05th
Trondheim, Norway
2
Outline
• Introduction
– ”A Green Sea” –project
• Dimethylmonoethanolamine, DMMEA
• High pressure vapor-liquid equilibrium
• Conclusions
3
Background - ”A green sea” -project
• 43% of the remaining gas resources contain CO2 and H2S (world energy outlook, 2008)
4
Background - ”A green sea”-project
• Overall objective is to identify, mature and evaluate technologies and concepts for acid gas removal
– Absorption, Adsorption, Membranes, Cryogenic methods
• Focus on offshore sweetening of natural gas
– High pressure
– Small ”footprint”
– Captured CO2 can be used for increased oil recovery (IOR)
– Pipe line specification (2.5% CO2) and LNG specification (approx. 50ppm CO2)
• Identify new solvent systems for natural gas sweetening that are environmentally friendly.
• Project partners
– Statoil, Gassco, Petrobras and the Research Council of Norway
5
Classification categories and criteria of chemicals used offshore as stated by the Norwegian Activities Regulation (PSA 2010)
6
Dimethylmonoethanolamine - DMMEA
• DMMEA is a tertiary amine
• DMMEA is a stronger base than MDEA (pKa of 9.2 vs 8.5 for MDEA)
• It is a smaller molecule than MDEA (DMMEA: 89.14 g/mol, MDEA: 119.16 g/mol)
• DMMEA is classified as a yellow chemical, (Eide-Haugmo, PhD thesis 2011) – Readily biodegradeable
– Low bioaccumulation potential
– Not toxic
• It showed lower thermal degradation after 5 weeks at 135°C with a loading of 0.5 mol CO2/mol amine than MDEA (Eide-Haugmo, PhD thesis 2011)
7
Solvent screening - method
TI
MFC
MFC
LabView Data
Acquisition System
Purge
N2
Absorber/Desorber
IR CO2
Analyzer
Flow
Meter
CO2
Cold Trap
Heating
Water In
Heating
Water Out
Magnetic Stirrer
Series Condensers
T2
Rapid screening apparatus
•Absorption at 40°C
•Desorption at 80°C
•Atmospheric pressure
8
Solvent screening – systems tested
System
Component 1 Component 2
Name C, mol/kg-solution Name C, mol/kg-solution
1 MDEA 4.2 - -
2 DMMEA 5.6 - -
3 DMMEA 4.2 - -
4 MDEA 4.2 MEA 0.35
5 DMMEA 4.2 MEA 0.35
6 MDEA 4.2 Piperazine 0.35
7 DMMEA 4.2 Piperazine 0.35
9
Solvent screening - results
0
5
10
15
20
25
30
0,00 0,10 0,20 0,30 0,40 0,50 0,60
CO2 loading, mol-CO2/mol-Am
Abso
rptio
n ra
te, m
ol m
-3 m
in-1
5M MEA (June 2011)
4.2m' (50 wt %) MDEA
5.6m' (50 wt %) DMMEA
4.2m' DMMEA
4.2m' MDEA+0.35m' MEA
4.2m' DMMEA+0.35m'MEA
4.2m' MDEA+0.35m' Pz
4.2m' DMMEA+0.35m' Pz
10
Solvent screening - results
0
5
10
15
20
25
30
35
40
45
50
0,00 0,10 0,20 0,30 0,40 0,50 0,60
CO2 loading, mol-CO2/mol-Am
Deso
rptio
n ra
te, m
ol m
-3 m
in-1
5M MEA (June 2011)
4.2m' (50 wt %) MDEA
5.6m' (50 wt %) DMMEA
4.2m' DMMEA
4.2m' MDEA+0.35m' MEA
4.2m' DMMEA+0.35m' MEA
4.2m' MDEA+0.35m' Pz
4.2m' DMMEA+0.35m' Pz
11
High pressure solvent screening - results
Absorption curves from the high pressure screening experiments. Absorption temperature 40oC, 50 % CO2 – 50 % CH4 gas mixture
System Loading PCO2, kPa
50% MDEA 0.257 29.086
50% DMMEA 0.203 5.065
4.2m' DMMEA+0.35m' Pz 0.229 12.923
2m' Ktau_1 0.116 8.707
2m' Ktau_2 0.280 241.16
12
High pressure VLE – method • Pressure region
– 0 – 20 MPa
• Temperature region
– (-20) - 180°C
• Equilibrium cell
– Sapphire tube equilibrium cell (approx. 32cm3)
– Possibility for adding other impurities (mercaptans)
• Sample analysis
– Online gas chromotograph (GC)
• Vapor phase and gas phase
13
High pressure VLE
Liquid
Vapor
CO2 CH4
CO2 CH4 H2O
H2O
Mole fractions determined using gas chromatography. Partial pressures calculated from Daltons law.
Difficult to measure the water content in the vapor phase using GC. It is therefore neglected.
Mole fractions determined using gas chromatography. The water/amine ratio is assumed to be constant. Moles of amine in the sample is calculated from the GC measurement of the water content in the sample. Amine
14
Experimental procedure
Adding CO2 to the cell
At equilibrium, both phases are sampled
Pressure is further increased with methane
At equilibrium, both phases are sampled
Filling the cell with solvent and increasing temperature
Pressure = 45 bar
Pressure = 150bar
15
Methane solubility in water – verifying the experimental method
• Validity of the experimental set-up
• Slightly lower solubility of methane from this work
• Methane solubility results sensitive towards the GC calibration curve of water
16
Equilibrium CO2 partial pressures
0
10
20
30
40
50
60
70
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Part
ial
Pre
ssu
re C
O2, [b
ar]
Loading CO2
This work - MDEA 50wt% 40C
Jou (1982) - MDEA 50wt 40C
Kuranov (1996) MDEA 32wt% 40C
Results from this work follow the same trend as seen in the data from Kuranov et al.
17
Equilibrium CO2 partial pressures
0
10
20
30
40
50
60
70
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Part
ial
pre
ssu
re C
O2 [
bar]
Loading CO2
This work - MDEA 50wt% 80C
Kuranov (1996) - MDEA 32wt% 100c
Jou (1982) - MDEA 50wt% 70C
Data at 80C are located between data from Jou at 70C and Kuranov at 100C.
18
Equilibrium CO2 partial pressures
0
10
20
30
40
50
60
70
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Part
ial
Pre
ssu
re C
O2, [b
ar]
Loading CO2
This work - MDEA 50wt% 120C
Kuranov (1996) - MDEA 32wt% 120C
At higher temperatures the effects of concentration becomes less pronounced as seen when our data is compared with data from Kuranov et al.
19
Equilibrium CO2 partial pressures – MDEA vs DMMEA
0
10
20
30
40
50
60
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Part
ial
Pre
ssu
re C
O2, [b
ar]
Loading CO2
This work - MDEA 50wt% 40C
This work - DMMEA 38wt% 40C
The physical solubility of CO2 is higher in mixtures of DMMEA-water than in MDEA-water at the same molar concentration.
20
Equilibrium CO2 partial pressures – MDEA vs DMMEA
0
5
10
15
20
25
30
35
40
45
50
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Part
ial
Pre
ssu
re C
O2, [b
ar]
Loading CO2
This work - MDEA 50wt% 80C
This work - DMMEA 38wt% 80C
The physical solubility of CO2 is higher in mixtures of DMMEA-water than in MDEA-water at the same molar concentration.
21
How does the addition of methane affect the CO2 liquid loading?
0
10
20
30
40
50
60
70
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Part
ial
Pre
ssu
re C
O2, [b
ar]
Loading CO2
This work - MDEA 50wt% 40C
Jou (1982) - MDEA 50wt 40C
Kuranov (1996) MDEA 32wt% 40C
22
How does the addition of methane affect the CO2 liquid loading?
0
10
20
30
40
50
60
1.1 1.2 1.2 1.3 1.3 1.4 1.4
Part
ial
pre
ssu
re C
O2 (
bar)
Loading CO2
This work - MDEA 50wt% 40C
Methane is added to the cell. Increasing the total pressure and causing methane to dissolve in the liquid phase as the partial pressure of methane increases
23
How does the addition of methane affect the CO2 liquid loading?
0
10
20
30
40
50
60
70
0.0 0.5 1.0 1.5
Part
ial
Pre
ssu
re C
O2, [b
ar]
Loading CO2
This work - MDEA 50wt% 40C, P(CH4)30bar
This work - MDEA 50wt% 40C
The increased pressure causes the equilibrium curve to shift to the left.
24
Is this behaviour visible in other experiments?
0
10
20
30
40
50
60
1.0 1.2 1.4 1.6
Part
ial
Pre
ssu
re C
O2, [b
ar]
Loading CO2
This work - DMMEA18wt% 40C
0
5
10
15
20
25
30
35
40
45
50
0.0 0.2 0.4 0.6 0.8
Part
ial
Pre
ssu
re C
O2, [b
ar]
Loading CO2
This work - MDEA50wt% 120C
It is visible in all tests in various degrees.
25
Is this behaviour visible in other experiments?
0
20
40
60
80
100
120
140
160
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
To
tal P
ressu
re,
[bar]
Loading CO2
This work - MDEA 50wt% 120C
This work - MDEA 50wt% 80C
This work - MDEA 50wt% 40C
Increasing the total pressure by adding methane.
26
Is this behaviour visible in other experiments?
0
20
40
60
80
100
120
140
160
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
To
tal P
ressu
re,
[bar]
Loading CO2
This work - DMMEA 38wt% 80C
This work - DMMEA 38wt% 40C
This work - DMMEA 18wt% 80C
This work - DMMEA 18wt% 40C
DMMEA does not seem to be as sensitive towards pressure as MDEA
27
What is causing the effect? Presence of Methane in the liquid phase?
Solubility of methane in pure MDEA (Jou,
2006)
The solubility of methane in water and pure MDEA is low
28
What is causing the effect? Presence of Methane in the liquid?
Dissolved hydrocarbons in the liquid phase will influence the dielectric constant (measure of polarity) of the amine. The mole fraction of methane in the liquid phase is low.
0
20
40
60
80
100
120
0.0000 0.0005 0.0010 0.0015
Part
ial
Pre
ssu
re C
H4,
[bar]
Mole fraction of CH4 in liquid phase
This work - DMMEA38wt% 80C
0
20
40
60
80
100
120
0.0000 0.0005 0.0010 0.0015
Part
ial
Pre
ssu
re C
H4,
[bar]
Mole fraction of CO2 in the liquid phase
This work - MDEA50wt% 80C
0
10
20
30
40
50
60
70
80
0.0000 0.0010 0.0020 0.0030
Part
ial
Pre
ssu
re C
H4,
[bar]
Mole fraction of CH4 in the liquid phase
This work - MDEA50wt% 120C
29
What is causing the effect? The increased pressure?
The Poynting correction The vapor-liquid equilibrium of CO2 can be calculated by equalizing the fugacity values of both phases
2 2
v L
CO COf f
Rewritten
2 2
2 2 2 22
exp
s
CO CO
COCO COCOCO
vPPyP mH
RT
30
What is causing the effect? The increased pressure?
The Poynting correction
2 2
2 2 2 22
exp
s
CO CO
COCO COCOCO
vPPyP mH
RT
Poynting correction (molar volume of CO2 in the liquid)
2
2
2
2
2
fugacity coefficient
vapor mole fraction
total pressure
activity coefficient
Henry constant
molality
CO
CO
CO
CO
CO
y
P
H
m
Increases with pressure
Constant
Reduction in liquid CO2 concentration
31
Conclusion
• DMMEA seems to be a faster solvent than MDEA
• The solubility of methane in the amine solvents is low.
• There is a reduction in the liquid phase CO2 at higher pressures for MDEA and to some degree DMMEA.
• The systems will be modeled with the electrolyte-NRTL equation.
32
Acknowledgements
This publication is based on the results from the research project “A Green Sea”, performed under the Petromaks program. The author(s) acknowledge the partners Statoil, Gassco, Petrobras, and the Research Council of Norway (200455/S60) for their support.
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