3rd Trondheim Gas Technology Conference, June 04th-05th
Trondheim, Norway
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
1
Outline
• Introduction
– ”A Green Sea” –project
• Dimethylmonoethanolamine, DMMEA
• High pressure vapor-liquid equilibrium
• Conclusions
2
Background - ”A green sea” -project
• 43% of the remaining gas resources contain CO2 and H2S (world energy
outlook, 2008)
3
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
4
Classification categories and criteria of chemicals used
offshore as stated by the Norwegian Activities Regulation
(PSA 2010)
5
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)
–
–
–
•
6
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)
Solvent screening - method
Rapid screening apparatus
•Absorption at 40°C
LabView Data
Acquisition System
Purge
MFC
Series Condensers
N2
IR CO2
Analyzer
•Desorption at 80°C
CO2
MFC
Flow
Meter
TI
T2
•Atmospheric pressure
Heating
Water Out
Cold Trap
Absorber/Desorber
Heating
Water In
Magnetic Stirrer
7
Solvent screening – systems tested
Component 1
Component 2
System
8
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
Solvent screening - results
30
5M MEA (June 2011)
25
-3
Absorption rate, mol m min
-1
4.2m' (50 wt %) MDEA
20
5.6m' (50 wt %) DMMEA
4.2m' DMMEA
15
4.2m' MDEA+0.35m' MEA
4.2m' DMMEA+0.35m'
MEA
10
4.2m' MDEA+0.35m' Pz
5
4.2m' DMMEA+0.35m' Pz
0
0,00
0,10
0,20
0,30
0,40
CO2 loading, mol-CO2 /mol-Am
9
0,50
0,60
Solvent screening - results
50
45
5M MEA (June 2011)
-3
Desorption rate, mol m min
-1
40
4.2m' (50 wt %) MDEA
35
5.6m' (50 wt %) DMMEA
30
4.2m' DMMEA
25
4.2m' MDEA+0.35m' MEA
20
4.2m' DMMEA+0.35m' MEA
15
4.2m' MDEA+0.35m' Pz
10
4.2m' DMMEA+0.35m' Pz
5
0
0,00
0,10
0,20
0,30
0,40
CO2 loading, mol-CO2 /mol-Am
10
0,50
0,60
High pressure solvent screening - results
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
Absorption curves from the high pressure screening experiments. Absorption
temperature 40oC, 50 % CO2 – 50 % CH4 gas mixture
11
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
12
High pressure VLE
Vapor
CO2
CH4
H2O
Amine
Liquid
13
Difficult to measure the water content in the
vapor phase using GC. It is therefore neglected.
H2O
CO2
Mole fractions determined using gas
chromatography. Partial pressures calculated
from Daltons law.
CH4
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.
Experimental procedure
Filling the cell with solvent and increasing temperature
Adding CO2 to the cell
Pressure
= 45 bar
At equilibrium, both phases are sampled
Pressure is further increased with methane
Pressure
= 150bar
At equilibrium, both phases are sampled
14
Methane solubility in water – verifying
the experimental method
15
•
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
Equilibrium CO2 partial pressures
70
Results from this work follow the
same trend as seen in the data
from Kuranov et al.
Partial Pressure CO2, [bar]
60
50
40
This work - MDEA 50wt% 40C
Jou (1982) - MDEA 50wt 40C
30
Kuranov (1996) MDEA 32wt% 40C
20
10
0
0.0
16
0.2
0.4
0.6
0.8
1.0
Loading CO2
1.2
1.4
1.6
1.8
Equilibrium CO2 partial pressures
70
Data at 80C are located between
data from Jou at 70C and Kuranov
at 100C.
Partial pressure CO2 [bar]
60
50
40
This work - MDEA 50wt% 80C
Kuranov (1996) - MDEA 32wt% 100c
30
Jou (1982) - MDEA 50wt% 70C
20
10
0
0.0
17
0.2
0.4
0.6
0.8
1.0
Loading CO2
1.2
1.4
1.6
Equilibrium CO2 partial pressures
70
At higher temperatures the
effects of concentration becomes
less pronounced as seen when
our data is compared with data
from Kuranov et al.
Partial Pressure CO2, [bar]
60
50
40
This work - MDEA 50wt% 120C
30
Kuranov (1996) - MDEA 32wt% 120C
20
10
0
0.0
18
0.2
0.4
0.6
0.8
1.0
Loading CO2
1.2
1.4
1.6
1.8
Equilibrium CO2 partial pressures –
MDEA vs DMMEA
60
The physical solubility of CO2 is
higher in mixtures of DMMEAwater than in MDEA-water at the
same molar concentration.
Partial Pressure CO2, [bar]
50
40
30
This work - MDEA 50wt% 40C
This work - DMMEA 38wt% 40C
20
10
0
0.0
19
0.2
0.4
0.6
0.8
1.0
Loading CO2
1.2
1.4
1.6
Equilibrium CO2 partial pressures –
MDEA vs DMMEA
50
The physical solubility of CO2 is
higher in mixtures of DMMEAwater than in MDEA-water at the
same molar concentration.
45
Partial Pressure CO2, [bar]
40
35
30
25
This work - MDEA 50wt% 80C
This work - DMMEA 38wt% 80C
20
15
10
5
0
0.0
20
0.2
0.4
0.6
0.8
Loading CO2
1.0
1.2
1.4
How does the addition of methane
affect the CO2 liquid loading?
70
Partial Pressure CO2, [bar]
60
50
40
This work - MDEA 50wt% 40C
Jou (1982) - MDEA 50wt 40C
30
Kuranov (1996) MDEA 32wt% 40C
20
10
0
0.0
21
0.2
0.4
0.6
0.8
1.0
Loading CO2
1.2
1.4
1.6
1.8
How does the addition of methane
affect the CO2 liquid loading?
60
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
Partial pressure CO2 (bar)
50
40
30
This work - MDEA 50wt% 40C
20
10
0
1.1
22
1.2
1.2
1.3
Loading CO2
1.3
1.4
1.4
How does the addition of methane
affect the CO2 liquid loading?
70
The increased pressure causes the
equilibrium curve to shift to the
left.
Partial Pressure CO2, [bar]
60
50
40
This work - MDEA 50wt% 40C, P(CH4)
30bar
30
This work - MDEA 50wt% 40C
20
10
0
0.0
23
0.5
1.0
Loading CO2
1.5
Is this behaviour visible in other
experiments?
It is visible in all tests in
various degrees.
50
40
30
This work - DMMEA
18wt% 40C
20
10
0
1.0
1.2
1.4
Loading CO2
1.6
50
Partial Pressure CO2, [bar]
Partial Pressure CO2, [bar]
60
45
40
35
30
25
This work - MDEA
50wt% 120C
20
15
10
5
0
0.0
24
0.2
0.4
0.6
Loading CO2
0.8
Is this behaviour visible in other
experiments?
160
Increasing the total pressure by
adding methane.
140
Total Pressure, [bar]
120
100
This work - MDEA 50wt% 120C
80
This work - MDEA 50wt% 80C
This work - MDEA 50wt% 40C
60
40
20
0
0.0
25
0.2
0.4
0.6
0.8
Loading CO2
1.0
1.2
1.4
Is this behaviour visible in other
experiments?
160
DMMEA does not seem to be
as sensitive towards pressure
as MDEA
140
Total Pressure, [bar]
120
100
This work - DMMEA 38wt% 80C
80
This work - DMMEA 38wt% 40C
This work - DMMEA 18wt% 80C
60
This work - DMMEA 18wt% 40C
40
20
0
0.0
26
0.2
0.4
0.6
0.8
1.0
Loading CO2
1.2
1.4
1.6
1.8
What is causing the effect?
Presence of Methane in the liquid phase?
The solubility of methane in water and
pure MDEA is low
Solubility of methane in pure MDEA (Jou,
2006)
27
100
80
60
40
This work - DMMEA
38wt% 80C
20
0
0.0000
0.0005
0.0010
0.0015
Mole fraction of CH4 in liquid phase
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.
28
Partial Pressure CH4,
[bar]
120
120
100
Partial Pressure CH4,
[bar]
Partial Pressure CH4,
[bar]
What is causing the effect?
Presence of Methane in the liquid?
80
60
40
This work - MDEA
50wt% 80C
20
0
0.0000
0.0005
0.0010
0.0015
Mole fraction of CO2 in the liquid phase
80
70
60
50
40
30
20
10
0
0.0000
0.0010
0.0020
0.0030
Mole fraction of CH4 in the liquid phase
This work - MDEA
50wt% 120C
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
v
L
fC

f
O
C
O
2
2
Rewritten



s


v
P

P
C
O
C
O
2
2



yP


m
H
e
x
p
C
O
C
O
C
O
OC
O
2
2 C
2
2
2
R
T 



29
What is causing the effect?
The increased pressure?
The Poynting correction



s


v
P

P
C
O
C
O
2
2



yP


m
H
e
x
p
C
O
C
O
C
O
OC
O
2
2 C
2
2
2
R
T 



Constant
Poynting correction
(molar volume of CO2 in the liquid)
 CO 2  fugacity coefficient
y CO2  vapor mole fraction
P  total pressure
 CO2  activity coefficient
H CO2  Henry constant
m CO2  molality
30
Reduction in liquid CO2
concentration
Increases with pressure
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 electrolyteNRTL equation.
31
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.
32