1. Impact of Impurities on Formation of CO2 Hydrates
Prakash Kumar Nair & Dr Nejat Rahmanian
University of Bradford, Uk

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Contents
Introduction
Objective of Study
Methodology
Results and Discussion
Conclusions
Acknowledgments
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Introduction
What is a hydrate?
It is a crystalline compound, where a gas molecule is encaged in a water molecule cavitiy
Formed at high pressures and low temperatures
Source: MIDAS
Source: Adewumi (2016)
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Introduction
Why consider impurities?
The presence of impurities can causes the following issues:
CO2 hydrate formation
Impact on the vapour-liquid equilibrium (VLE) phase envelope
Corrosion
Carbon Dioxide hydrates?
These will be observed in the transport of high-CO2 streams, e.g. Carbon Capture and Storage (CCS)
Knowledge of how impurities affect high CO2 streams is essential for the combat against rising CO2 emissions
Source: Li et al. (2011)
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5. Objective of this Study
This has created a pathway for a study with the following objectives:
To investigate the impact of impurities on the formation of CO2 hydrates through binary and tertiary analysis
To determine the impurity/impurities effect on the vapour-liquid equilibrium (VLE) phase envelope
To investigate two real-life case studies
An aspect which has not been conducted before in literature is conducting this study through the eyes of an operator
From conducting an in-depth literature survey, the major limitations discovered:
Each study only considered certain impurities
They fail to consider changing compositions to fully appreciate the impurities affect
Little research has been conducted on how the impurities affect hydrate formation
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Methodology
Binary & Tertiary Analysis
The uneven mole fractions for each impurity is problematic – 5% mole fraction was used
Water will be included in each binary and tertiary system
Source: Munkejord et al. (2016) N2 O2 Ar
SO2
H2S/COS
NOx CO H2
CH4
H2O
Amines
NH3
𝒙 𝒎𝒊𝒏 (%)
0.02
0.04
0.005
<10-3
0.01
<0.002
0.06
0.7
𝒙 𝒎𝒂𝒙 (%) 10 5
3.5
1.5
0.3
0.2 4
6.5 3
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Methodology
Case Studies
– Cortez pipeline (2) – Sheep Mountain Pipeline
Sheep Mountain Pipeline
Cortez Pipeline
808 km, 30” diameter, pipeline which transports CO2 from McElmo Dome natural source to the Wasson oil field located in Texas
656 km pipeline which begins at Sheep Mountain and ends in Seminole
Cortez and Sheep Mountain pipeline accounted for 66% and 4%, respectively, of the total CO2 supply for enhanced oil recovery (EOR) in 2009 (Global CCS 2016)
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Methodology
Which specialist softwares were used?
(1) – Aspen HYSYS® V (2) – HydraFLASH® V3.3
This is a gas hydrate and thermodynamic prediction software designed to predict phase equilibria and physical properties (Hydrafact 2017)
Sophisticated workflow-orientated process simulation software used for various scenarios in the gas processing industries (Aspen Technology 2017)
Which equation of state (EoS) to use?
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Methodology
Model Validation
Peng-Robinson EoS is in excellent agreement to the literature data
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10. Results and Discussion
Binary Analysis
Binary Mixture
Hydrate Length (km)
Difference in Hydrate Length (km)
Hydrate Structure
(1) 95% 𝐶𝑂 2 −5% 𝑁 2 58 23
Type I
(2) 95% 𝐶𝑂 2 −5% 𝑂 2
100 65
(3) 95% 𝐶𝑂 2 −5%𝐴𝑟
(4) 95% 𝐶𝑂 2 −5% 𝑆𝑂 2 16
-19
(5) 95% 𝐶𝑂 2 −5% 𝐻 2 𝑆
(6) 95% 𝐶𝑂 2 −5%𝐶𝑂𝑆 15
-20
(7) 95% 𝐶𝑂 2 −5% 𝑁𝑂 2 17
-18
(8) 95% 𝐶𝑂 2 −5%𝐶𝑂
(9) 95% 𝐶𝑂 2 −5% 𝐻 2 18
-17
(10) 95% 𝐶𝑂 2 −5% 𝐶𝐻 4
(11) 95% 𝐶𝑂 2 −5% 𝑁𝐻 3
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11. Regulation for Single-Phase Flow
95% CO2 & 5%H2S
95% CO2 & 5%N2
Impurity
% Increase
Nitrogen
3.16
Oxygen
8.98
Argon
12.44
Hydrogen Sulfide
53.26
Methane
14.25
95% CO2 & 5% CH4
Impurity
Regulation for Single-Phase Flow
Temperature (⁰C)
Pressure (kPa)
Nitrogen
10-27
Oxygen
11-27
Argon
12-27
Hydrogen Sulfide
18-31
Methane
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12. Regulation for Single-Phase Flow
95% CO2 & 5%SO2
95% CO2 & 5%COS
95% CO2 & 5%NH3
95% CO2 & 5%H2
Impurity
% Decrease
Sulfur Dioxide
4.98
Carbonyl Sulfide
5.79
Nitrogen Dioxide
4.24
Carbon Monoxide
4.63
Hydrogen
1.23
Ammonia
5.22
Impurity
Regulation for Single-Phase Flow
Temperature (⁰C)
Pressure (kPa)
Sulfur Dioxide
10-40
Carbonyl Sulfide
11-35
Nitrogen Dioxide
10-43
Carbon Monoxide
12-27
Hydrogen 17
12,000
Ammonia
15-36
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13. Results and Discussion
Tertiary Analysis
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14. Regulation for Single-Phase Flow
95% CO2 & 2.5% H2S & 2.5%O2
95% CO2 & 2.5% H2S & 2.5%Ar
Impurity Mixture
% Increase
Hydrogen Sulfide + Oxygen
36.21
Hydrogen Sulfide + Argon
37.21
Hydrogen Sulfide + Methane
38.75
95% CO2 & 2.5% H2S & 2.5%CH4
Impurity Mixture
Regulation for Single-Phase Flow
Temperature (⁰C)
Pressure (kPa)
Hydrogen Sulfide + Oxygen
12-29
Hydrogen Sulfide + Argon
18-29
Hydrogen Sulfide + Methane
17-29
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15. Results and Discussion
Case Studies
Cortez Pipeline
Sheep Mountain Pipeline
Temperature (⁰C)
43.35 20
Pressure (MPa)
18.6 83
Capacity (MT/yr) 19
(1) – 6.3 (2) – 9.2
Pipe Length (km)
808
(1) – 296 (2) – 360
Pipe Diameter (inch) 30
(1) – 20 (2) – 24
CO2 Content
95% (minimum)
95-98%
Source: Oosterkamp and Ramsen (2008)
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16. Impurity (Hydrate Zone Dampener)
(1) – Cortez Pipeline
To recap – COS, SO2 and NH3 were the ‘Hydrate Zone Dampeners’ which caused the most significant reduction in hydrate stability zone
Oosterkamp and Ramsen (2008) stated a H2S limit of mol% – no hydrates formed
When 0.2 mol% H2S was incorporated within – hydrates were formed
Impurity (Hydrate Zone Dampener)
% Decrease
Sulfur Dioxide
4.98
Carbonyl Sulfide
5.79
Nitrogen Dioxide
4.24
Carbon Monoxide
4.63
Hydrogen
1.23
Ammonia
5.22
Therefore, addition of any of these impurities could counteract H2S
Base case simulation produced no hydrates.
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Case 1
Case 2
Case 3
Inlet
Outlet
Cortez Pipeline
2% COS
2% COS
2% COS
2% SO2
2% SO2
2% NH3
COS
COS
SO2
COS
SO2
NH3
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Each system significantly increased the hydrate formation pressure thus counteracting H2S
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19. Regulation for Single-Phase Flow
2% COS
2% COS & SO2
2% COS & SO2 & NH3
Impurity Mixture
Regulation for Single-Phase Flow
Temperature (⁰C)
Pressure (kPa)
2% COS
12-30
2% COS + SO2
10-33
2% COS + SO2 + NH3
12-35
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(2) – Sheep Mountain Pipeline
As it’s been proven ‘Hydrate Zone Dampeners’ can counteract H2S, purpose of this simulation is to discover which impurity is the most dominant out of COS, SO2 and NH3
No H2S limit was provided, so incorporating 1 mol% H2S
Hydrate formation pressure significantly reduced to produce hydrates
Base case simulation produced no hydrates
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Case 1
Case 2
Case 3
Inlet
Outlet
Sheep Mountain Pipeline
2% NH3
2% COS
2% SO2
NH3
COS
SO2
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NH3 increases the hydrate formation pressure the furthest
5 mol% of each impurity is recommended to ensure no hydrate formation
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23. Regulation for Single-Phase Flow
5% NH3
5% COS
Impurity Mixture
Regulation for Single-Phase Flow
Temperature (⁰C)
Pressure (kPa)
5% NH3
11-33
5% COS
11-32
5% SO2
10-38
5% SO2
If operator does not want to add impurities into pipeline, a H2S limit of 0.5mol% is recommended
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Conclusions
The key findings from the present study can be summarised as follows:
The presence of H2S causes a quicker onset of hydrate formation by expanding the hydrate stability zone (HSZ) and reducing the hydrate formation pressure
The presence of COS, SO2, NH3 causes the opposite where they reduce the HSZ and increase the hydrate formation pressure
H2 and N2 causes a significant expansion in the VLE phase envelope
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Acknowledgements
I would like to express my deepest gratitude towards Dr Nejat Rahmanian and the Chemical Engineering Division at the University of Bradford for their support during the course of this project.
Special thanks go to everyone involved at the Petroleum Engineering 2017 conference for allowing me to present my work.
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26. Thank you for listening! Any Questions?
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