High pressure vapour liquid equilibrium of alkanolamine ... · PDF fileHigh pressure vapour...

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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

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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

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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

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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

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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

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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

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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

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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.

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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


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.

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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


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.

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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 - DMMEA 38wt% 40C

The physical solubility of CO2 is higher in mixtures of DMMEA-water than in MDEA-water at the same molar concentration.

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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



The physical solubility of CO2 is higher in mixtures of DMMEA-water than in MDEA-water at the same molar concentration.

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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


Jou (1982) - MDEA 50wt 40C

Kuranov (1996) MDEA 32wt% 40C

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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


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

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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


The increased pressure causes the equilibrium curve to shift to the left.

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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.

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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




Increasing the total pressure by adding methane.

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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





DMMEA does not seem to be as sensitive towards pressure as MDEA

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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

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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


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


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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

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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

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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.

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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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