qatar university graduate studies college of engineering
TRANSCRIPT
QATARUNIVERSITY
GraduateStudies
CollegeofEngineering
AVOIDING GAS HYDRATE PROBLEMS IN QATAR OIL AND
GAS INDUSTRY: ENVIRONMENTALLY FRIENDLY
SOLVENTS FOR GAS HYDRATE INHIBITION
AThesisin
EnvironmentalEngineering
By
NabilaAdamMohamed
SubmittedtoQatarUniversityinpartialfulfillmentofthe
requirementsforthedegreeof
MASTEROFSCIENCE
November2014
i
Copyright2014NabilaAdamMohamed
ii
“To the best of my knowledge, the thesis contains no material previously published
or written by another person or institution, except where due reference is made in
the text of the thesis. The thesis contains no material which has been accepted for
the award of any other degree in any university or other institution.”
Signature ______________________________________ Date__________
“To the best of my knowledge, the thesis conforms the requirements of Qatar
University, and I endorse this thesis for examination.”
Name __________________________________________
Signature _______________________________________ Date__________
iii
ABSTRACT
Gas Hydrate is an area of concern for oil and gas industry and one of the top issues in
the offshore operations. Due to the vast development in deep waters activities
carried out in many locations all over the world including Qatar, flow assurance has
become one of the major and critically important challenges in overcoming hydrates
problem. Qatar is considered the world’s largest exporter of liquefied natural gas
(LNG) and has the third largest proven reserves of natural gas at 885 trillion cubic
feet. Since, almost all of the natural gas production in Qatar comes from the North
Field, it’s essential to provide sustainable offshore operations both technically and
economically. The formation of stable gas hydrates in the production and
transmission pipelines in oil and gas industries can cause operational and safety
problems, as well as production and massive economical loss.
Traditionally, industries avoid hydrate formation by injecting thermodynamic
inhibitors (THIs), commonly methanol and mono‐ethylene glycol that maintain
unfavorable conditions for hydrate formation. Yet, due to the health, safety, and
environmental concerns of THIs, low dosage hydrate inhibitors (LDHIs) have been
developed to delay the onset and/or the growth of hydrates. A new class of novel
environmentally friendly inhibitors, so called ionic liquids (ILs) requires further
research studies for their implementation as a promising solution for hydrates
problem.
This work presents experimental investigation of the gas hydrate formation
conditions and characteristics of synthetic multi‐component gas mixture that
resembles Qatari type natural gas. Moreover, various types of chemical inhibitors
iv
were investigated including classical THIs as well as the novel ILs. Three types of ILs
(choline chloride (ChCl), choline bistriflimide (Ch)(NTF2), and choline acetate
(ChOAc)) were tested on both pure methane and Qatari natural gas (QNG‐S1)
mixture. The experimental tests were conducted through two apparatuses, Gas
Hydrate Autoclave (GHA) and Rocking Cell (RC5). HydraFLASH® commercial
simulation software was used to acquire initial prediction of natural gas hydrate
formation characteristics and conditions of Qatari type gas. Each IL was tested at two
concentrations, i.e. 1wt% and 5wt%. The obtained results illustrate that all the ILs
used had a clear inhibition performance for both CH4 and QNG‐S1; however, the
degree of inhibition and shifting the hydrate equilibrium curve varied with each
system. For methane system, the three ILs had very close inhibition effect at 1wt%;
however at 5wt% ChOAc and ChCl performed better than (Ch)(NTF2). Nonetheless,
ChCl showed the best performance at higher pressures above 94bar with maximum
inhibition of 1 shift. On the other hand, the behavior of ILs in QNG‐S1 system was
totally different presenting that ChOAc has the best hydrate inhibition performance
at both 1wt% and 5wt% with 0.6 to 1.2 and 0.9 to 2 respectively.
v
DEDICATION
This thesis is dedicated to my beloved parents, family, and friends for their endless
support, encouragement, and love.
vi
ACKNOWLEDGEMENT
I would like to express my sincere gratitude to all the individuals who supported and
encouraged me during my master study and research, and have made this
achievement possible.
First and foremost, I am grateful to my supervisor, Dr. Mert Atilhan for his guidance
and valuable feedbacks. I am also thankful to Dr. Mohammad Tariq for his
continuous assistance and technical support in laboratory.
I would like to take this opportunity to thank all the academic and administrative
staff members in the Department of Chemical Engineering at Qatar University for
helping and motivating me during my graduate study. I also want to extend my
appreciation to my colleagues who made my graduate experience one that I will
cherish forever.
Finally, I owe my deepest gratitude to my beloved parents, family, and friends to
whom this thesis is dedicated to, for being a constant source of love, patience, and
strength all these years.
vii
TABLEOFCONTENTS
ABSTRACT .................................................................................................................................. iii
ACKNOWLEDGEMENT ............................................................................................................... vi
TABLE OF CONTENTS ................................................................................................................ vii
LIST OF FIGURES ........................................................................................................................ ix
LIST OF TABLES .......................................................................................................................... xi
Chapter 1: Introduction1, 2 ................................................................................................... 1
1.1 Background ................................................................................................................ 1
1.1.1 Safety considerations ........................................................................................ 4
1.1.2 Hydrates prediction software programs ........................................................... 5
1.1.3 Solving hydrate problem in industry ................................................................. 6
1.2 Motivation and scope of work .................................................................................. 7
1.3 Thesis structure ....................................................................................................... 10
Chapter 2: Literature Review about Gas Hydrates2 .......................................................... 11
2.1 Gas hydrate ............................................................................................................. 11
2.2 Historical perspective .............................................................................................. 12
2.3 Hydrate structures and characteristics ................................................................... 14
2.3.1 The crystal structures and cavities .................................................................. 14
2.4 Hydrates formation mechanism .............................................................................. 18
2.4.1 Hydrates nucleation ........................................................................................ 18
2.4.2 Hydrate growth ............................................................................................... 21
2.5 Hydrate formation and dissociation process .......................................................... 22
2.6 Formation conditions of gas hydrate in industry .................................................... 23
2.7 Investigation of hydrate formation in laboratory ................................................... 26
2.7.1 Laboratory experimental conditions and methods ......................................... 29
2.8 Gas hydrate prevention techniques ........................................................................ 31
2.8.1 Conventional methods for gas hydrate remediation ...................................... 32
2.8.1.1 Low dosage hydrate inhibitors (LDHI) ......................................................... 32
2.8.1.2 Thermodynamic hydrate inhibitors (THIs)................................................... 36
2.8.2 Benefits and challenges of conventional inhibitors ........................................ 38
2.8.3 Ionic liquids ...................................................................................................... 41
Chapter 3: Methodology ................................................................................................... 47
viii
3.1 Experimental setup and material used ................................................................... 47
3.1.1 Gas hydrate autoclave (GHA) .......................................................................... 47
3.1.2 Rocking cell‐ RC5.............................................................................................. 50
3.1.3 HydraFLASH® software ..................................................................................... 52
3.1.4 Material used ................................................................................................... 55
3.2 Experimental procedure .......................................................................................... 57
3.2.1 High pressure autoclave .................................................................................. 58
3.2.2 Rocking cell (RC5) ............................................................................................ 61
Chapter 4: Results and Discussion ..................................................................................... 66
4.1 Mechanism of obtaining hydrate equilibrium curve ............................................... 67
4.2 Methane hydrates ................................................................................................... 70
4.2.1 Pure methane hydrate equilibrium curve ....................................................... 70
4.2.2 Methane hydrates inhibition by ionic liquids .................................................. 72
4.3 Natural gas mixture (QNG‐S1) hydrates .................................................................. 76
4.3.1 QNG‐S1 hydrate equilibrium curve ................................................................. 77
4.3.2 QNG‐S1 hydrates inhibition by ionic liquids .................................................... 78
4.4 Gas hydrate autoclave measurements .................................................................... 83
4.5 Effectiveness of ionic liquids as a promising hydrate inhibitors ............................. 84
Chapter 5: Conclusion and Future Work ........................................................................... 86
5.1 Conclusion ............................................................................................................... 86
5.2 Future work ............................................................................................................. 88
REFERENCES ............................................................................................................................ 89
ix
LISTOFFIGURES
Figure 1‐1.Different offshore pipelines applications. (Guo et al., 2005) ................................... 2
Figure 1‐2.Increasing the depth and of extraction platforms in the North Sear.2 (Giavarini and
Hester, 2011)2 ............................................................................................................................ 3
Figure 1‐3. Oil and gas pipelines from offshore facilities in Qatar. (EIA, 2014) ........................ 8
Figure 2‐1. Hydrate crystal unit structures: (a) sI , (b) sII, and (c) sH. (Momma et al., 2011) . 15
Figure 2‐2. Three cavities in gas hydrate: (a): Pentagonal dodecahedron (512), (b):
tetrakaidecahedron (51262), (c): hexakaidecahedron (51264), (d): irregular dodecahedron (43 56
63) and (e): icosahedron (512 68). (Sloan and Koh, 2008) ......................................................... 16
Figure 2‐3. Gas hydrate formation in pipelines with multiphase flow. (Sum et al., 2009) ..... 18
Figure 2‐4. Model representing the labile cluster growth. (Sloan and Koh, 2008) ................. 19
Figure 2‐5. Nucleation at the gas‐water interface by adsorption of gas onto labile cages.
(Sloan and Koh, 2008) ............................................................................................................. 20
Figure 2‐6. P‐T plot for methane hydrate formation‐ dissociation loop ................................. 23
Figure 2‐7.Hydrates formation zones in offshore operations. (Giavarini and Hester, 2011) .. 25
Figure 2‐8.Schematic representation of hydrate sedimentary deposits at continental margins
and below the permafrost. (Krey et al., 2009) ........................................................................ 25
Figure 2‐9. Sapphire cell high‐pressure test equipment. (Del Villano and Kelland, 2011) ...... 27
Figure 2‐10. The flow loop used for experimental research on hydrates installed at the
research center of the French institute of petroleum in Lyon. (Giavarini and Hester, 2011) . 29
Figure 2‐11. Graphical representation of anti‐agglomerants effect. (Giavarini and Hester,
2011) ........................................................................................................................................ 33
Figure 2‐12. Pressure‐temperature plot for a typical natural gas hydrate. (Kelland, 2006) ... 34
Figure 2‐13. Graphical representation of KHIs absorbed into hydrate surface. (Giavarini and
Hester, 2011) ........................................................................................................................... 35
Figure 2‐14. The effect of some THIs on methane hydrate stability.(Giavarini and Hester,
2011) ........................................................................................................................................ 37
Figure 2‐15. Typical PT diagram illustrating the areas where the LDHIs and THIs can be
applicable. (Giavarini and Hester, 2011) ................................................................................. 40
Figure 3‐1. Gas hydrate autoclave experimental setup. (1): Autoclave cell, (2): gas cylinder,
(3): High pressure generator, (4): Thermostat, (5): Control‐PC .............................................. 48
Figure 3‐2. High‐pressure autoclave cell ................................................................................. 49
Figure 3‐3. Rocking cell experimental setup (1): Base unit of RC5, (2): Thermostat, (3):
Control‐PC. .............................................................................................................................. 50
Figure 3‐4. Left: the five test cells immersed in the RC5 bath. Right: the main components of
the individual rocking cell ........................................................................................................ 51
Figure 3‐5.Main window of HydraFLASH® software showing the required input data to
predict HLVE curve for QNG‐S1 ............................................................................................... 54
Figure 3‐6. Output of HLVE calculation for QNG‐S1 ................................................................ 54
Figure 3‐7. Predicted HLVE curve for QNG‐S1 ......................................................................... 55
Figure 3‐8. Experimental script prepared through hydrate V4 software ................................ 60
Figure 3‐9. Opening mechanism of the rocking cells .............................................................. 61
x
Figure 3‐10. Experimental script prepared through RC5 software ......................................... 63
Figure 3‐11. Initialization step in RC to check leak and maintain the initial conditions .......... 63
Figure 3‐12. Mixing mechanism in RC5 ................................................................................... 64
Figure 3‐13 RC5 software window showing end of the experiment ....................................... 65
Figure 4‐1.Hydrate formation‐ dissociation loop for methane ............................................... 68
Figure 4‐2.Mechanism of obtaining HLVE point ...................................................................... 69
Figure 4‐3. Methane Hydrate Equilibrium Curve .................................................................... 69
Figure 4‐4. Comparison of methane hydrate equilibrium data .............................................. 71
Figure 4‐5.Inhibition effect of 1wt% of different ILs on methane hydrates ........................... 73
Figure 4‐6.Inhibition effect of 5wt% of different ILs on methane hydrates ........................... 74
Figure 4‐7.Comparison between the performance of ChOAc and THIs by HydraFLASH® in
methane system ...................................................................................................................... 76
Figure 4‐8.Autoclave and HydraFLASH® QNG‐S1 hydrate equilibrium data ........................... 78
Figure 4‐9.Inhibition effect of 1wt% of different ILs on QNG‐S1 hydrates ............................. 80
Figure 4‐10.Inhibition effect of 5wt% of different ILs on QNG‐S1 hydrates ........................... 81
Figure 4‐11. Comparison between the performance of ChOAc and THIs by HydraFLASH® in
QNG‐S1 system ........................................................................................................................ 82
Figure 4‐12. QNG‐S1 with 5wt% methanol process. 1: Starting of the experimental run, 2‐3:
Hydrates start forming, 4: Completion of hydrate formation, 5: Hydrate dissociation, 6:
Hydrate completely dissociated .............................................................................................. 83
Figure 4‐13. Methane with 5wt% (Ch)(NTF2) process. 1: Starting of the experimental run, 2‐
3: Hydrates start forming, 4: Completion of hydrate formation, 5: Hydrate dissociation, 6:
Hydrate completely dissociated .............................................................................................. 83
xi
LISTOFTABLES
Table 2‐1. Geometry of cages in three hydrates crystal structures. (Sloan, 2003a) ............... 17
Table 2‐2. Summary of the advantages and challenges of conventional hydrate inhibitors .. 39
Table 2‐3. List of ILs used in published studies ...................................................................... 44
Table 2‐4. Continuation of the list of ILs used in published studies ........................................ 45
Table 2‐5. Continuation of the list of ILs used in published studies ........................................ 46
Table 3‐1. Gas hydrate autoclave specifications ..................................................................... 49
Table 3‐2. Rocking Cell RC5 specifications .............................................................................. 52
Table 3‐3. Compositions of Qatar Gas Mixture (QNG‐S1) ....................................................... 56
Table 3‐4. Common name, chemical formula and structure of the ILs studied in this work .. 57
Table 4‐1. Methane hydrates measurements ......................................................................... 70
Table 4‐2. Methane hydrate equilibrium conditions .............................................................. 71
Table 4‐3. Experimental HLVE points for 1wt% ILs inhibition in methane hydrate system .... 73
Table 4‐4.Experimental HLVE points for 5wt% ILs inhibition in methane hydrate system ..... 75
Table 4‐5.QNG‐S1 hydrate measurements ............................................................................. 77
Table 4‐6.QNG‐S1 hydrate equilibrium conditions ................................................................. 78
Table 4‐7.Experimental HLVE points for 1wt% ILs inhibition in QNG‐S1 hydrate system ....... 79
Table 4‐8.Experimental HLVE points for 5wt% ILs inhibition in QNG‐S1 hydrate system ....... 81
1
1 This thesis follows Harvard style
2 Reprinted figures with kind permission of Springer Science+ Business Media.
Chapter1: Introduction1,2
1.1 Background
The global energy demand continues to grow with vast development in oil and gas
sectors. The necessity for petroleum products increased the requirements to search
for oil in the offshore regions of different locations in the world. In early 1897, the
offshore operations for oil exploration and production have been started from the
Summerland, California where the first offshore pipeline was initiated in southeast
of Santa Barbara (Guo et al., 2005). After their first implementation, the offshore
pipelines have become one of the unique techniques in transporting offshore oil and
gas. Recently, deep‐water offshore operations have been established and operated
all over the world such as the West Africa, Gulf of Mexico, and the North Sea. The
classification of offshore pipelines can be based on the specific mission of using
certain pipeline or based on the location of the pipelines and their main connections.
Figure 1‐1 shows the different types of pipelines including (Guo et al., 2005):
‐ Pipelines from satellite subsea wells to subsea manifolds.
‐ Pipelines from subsea manifolds to production facility platforms.
‐ Infield pipelines between production facilities platforms.
‐ Export pipelines from production facility platforms to shore.
‐ Pipelines connections from production facility platforms, through subsea
injection manifolds, to injection wellheads.
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2
Typical operation that maintains these pipelines to function under consistent
conditions for the fluids to flow from the reservoir to the sales location is the flow
assurance. In oil and gas industries, preserving a good and efficient process and
production facilities is essential which add special attention when it comes to flow
assurance (Guo et al., 2005). The incidence of gas hydrates is considered one of the
main problems that threaten the production and transportation of oil and gas in
subsea pipelines. Gas Hydrates are undesirable compounds that usually form during
drilling and production processes, specifically when natural gas and water coexist in
a suitable temperature and pressure conditions (Sloan and Koh, 2008). The
Figure 1‐1.Different offshore pipelines applications. (Guo et al., 2005)
3
formation of gas hydrate in the offshore facilities can block the pipelines, wellheads,
and damage the valves which leads to many economical and production loss
accompanied by safety and environmental concerns (Kelland, 2006).
Gas hydrates have become a special topic after Deep Water Horizon rig explosion in
2010 in the Gulf of Mexico (Giavarini and Hester, 2011). As deep‐water activities and
development moved to deeper operations; i.e. more than 6000ft depth (Figure 1‐2),
the temperature and pressure at seafloor become more favorable for hydrate
formation and deposition. Moreover, this post additional risks and challenges in
avoiding and preventing gas hydrates (Giavarini and Hester, 2011, Koh and Sloan,
2007).
Figure 1‐2.Increasing the depth and of extraction platforms in the North Sear.2 (Giavarini and Hester, 2011)2
4
In oil and gas wells, the production of undesired excess water is very common and
the coexistence of water and hydrocarbons fluids in the pipelines provides
appropriate habitat for hydrate formation (Sloan, 2003b). Nevertheless, hydrates
accumulation and blockage are not easily formed under normal pipelines operations
(Koh and Sloan, 2007). In other words, the hydrate plugs in the pipelines form if
some anomalous operations take place such as the following:
‐ When excess of water is produced or the failure in water removal step that
can be by injecting inhibitor or failure in dehydration process.
‐ During process startup and shut‐in.
‐ The expansion of the gas through valves and orifice plates causing Joule‐
Thompson (JT) effect that lowers the temperature and hence condensate
water.
1.1.1 Safetyconsiderations
Gas hydrate formation doesn’t only affect the economic situation of the oil and gas
production but also adds some safety and environmental concerns. For the best
hydrate remediation method to be selected, it’s critical to know the specification of
the operating system, the length of the pipelines and the distance of the blockage
from the platform, and the time requires for the blockage dissociation.
Hydrate has high specific gravity (0.9) comparing to fluid hydrocarbon, which is
typically 0.8 or less. This leads to form high dense hydrate plugs that detach at the
pipe walls immediately after dissociation. Consequently, any practice in this region
5
with a pressure difference will cause the dense plug to travel to downstream with
very high velocity (approximately 300 km/hr), hence, compressing the downstream
gas. This may result in eruption through pipeline bends damaging downstream
facilities and threaten personnel lives. Secondly, one of the hydrate properties is the
ability to hold large volumes of hydrate formers (gas). Thus, the local heating of
hydrate as a technique to dissociate the plugs can post another safety issue. This
technique can develop a high pressure from the escaped gas after hydrate
dissociation causing pipeline to burst (Sloan, 2003b). For these reasons, it’s
important to make sure that the released gas from the hydrates is not trapped.
1.1.2 Hydratespredictionsoftwareprograms
Recently, hydrates stability and thermodynamics conditions are predictable through
software programs which are based on the Gibbs energy extension of the van der
Waals and Platteeuw method (Sloan, 2003b). These programs are used both in
industrial scale and laboratory purposes and can simulate ranges of hydrate formers
including single and complex gas mixtures. Moreover, it provides the ability to check
experimental accuracy as well as the certainty of the used apparatus. The available
and most commonly used programs in industry include MultiFlash, PVTSim, and
CSMGem. In addition, HydraFlash®, EQUI‐Hydrate, and Prosim are also available
software packages that are used to predict hydrate phase equilibrium (Giavarini and
Hester, 2011). In this work, HydraFlash® was used as a tool to predict the hydrate
formation conditions for both pure methane gas and multi component gas mixture
(QNG).
6
1.1.3 Solvinghydrateprobleminindustry
Oil and gas industries rely on a specific strategy to overcome hydrate formation
issue, which requires a logical approach for mitigation and remediation. The
following approach is applied usually to solve possible production and safety
problems cause by hydrates (Sinquin et al., 2004):
1. Prediction through software packages as discussed in the previous section.
2. Prevention. This step is accomplished by keeping the temperature and
pressure conditions outside the hydrate stability ranges by insulating or
heating pipelines hence reduce the heat loss from the hot fluids and the cold
environment. Moreover, prevention method can be done through the
injection of chemical inhibitors such as thermodynamics (THIs) and low
dosage inhibitors (LDHIs) including kinetic inhibitors (KHIs) and anti‐
agglomerant (AAs).
3. Remediation. So far, one of the proven applied methods is two‐sided
depressurization; however this technique may not be practical when many
hydrate blockages are formed along the line. Thus, a one‐sided
depressurization is another alternative for hydrate remediation. Mostly,
remediation techniques are still considered hazardous and it has major
safety, concerns which shift the focus on developing the prevention methods.
In many countries, the research studies and efforts have been shifted to investigate
the energy potential of gas hydrates. Since large amounts of hydrates are
accumulated in nature deposits, some studies emphasis on understanding the
7
natural phenomenon of gas hydrates as non‐conventional energy sources (Bai et al.,
2012, Fink, 2012, Lee et al., 2003, Koh et al., 2012, Seo et al., 2009). Besides this
study, gas hydrates are also considered as a possible technique for the
transportation and storage of gases. This system is one of the latest interesting
topics in hydrate studies; offering other alternative than LNG to transport standard
natural gas (Giavarini and Hester, 2011). Moreover, gas hydrates studies covered the
carbon dioxide capture and storage. As carbon dioxide can be used as hydrate
former, the implementation of hydrates in the separation process for integrated
gasification combined cycle has been studied recently (Cha et al., 2013, Seo and
Kang, 2010, Kang et al., 2013).
Having mentioned all these current research interests of gas hydrates, however for
many years until now, the studies related to flow assurance driven by the serious
concerns in oil and gas industry are still grabbing the attention (Giavarini and Hester,
2011).
1.2 Motivationandscopeofwork
Qatar’s economy counts on the energy sector, which is the cornerstone for the
country’s development. In 2012, the oil and gas sectors accounted for 57.8% of
Qatar's gross domestic product and according to the U.S. Energy Information
Administration (EIA), the country grossed USD 55 billion from oil exports. Since
2006, Qatar is considered the world’s largest exporter of liquefied natural gas (LNG)
and recently has the third largest proven reserves of natural gas at 885 trillion cubic
feet. The production of crude oil and lease condensate in Qatar is ranked as the 19th
8
in the world and most of this production is exported. The natural gas resources have
been developed to meet the country’s energy demand, mainly in the North Field
(Figure 1‐3). According to dry natural gas production in 2012, Qatar was ranked as
the second largest producer in the Middle East and the forth country in the world
after United States, Russia, and Iran. In 2013, the production of liquid fuels including
crude oil, condensates, natural gas plant liquids, gas‐to‐liquids, and other liquids
reached approximately 1.6 million barrels per day (bbl/d).
Figure 1‐3. Oil and gas pipelines from offshore facilities in Qatar. (EIA, 2014)
9
Since, almost all of the natural gas exploration and production in Qatar comes from
the North Field by offshore facilities, it’s essential to provide sustainable offshore
operations both technically and economically. The formation of stable gas hydrates
in the production and transmission pipelines in oil and gas industries can cause
operational and safety problems, as well as production and massive economical loss
for the country. In the period between 2011 and 2013, Qatar oil and gas industry
were threaten by gas hydrate formation causing operational problems that lead to
production shutdown. The shutdown period ranged from couple of days to one
month. Qatar oil and gas companies used unknown quantities of chemical inhibitors
in order to overcome this issue and maintain the flow assurance. However, hydrates
problem cost average of USD 10 million/day as a result of the production loss and
the additional cost of the applied amounts of inhibitors (Atilhan, 2012).
All these concerns emphasize the importance of studying the characteristics of
Qatari natural gas under typical operational conditions. Moreover, it’s quite
necessary to establish a literature base for this particular type of gas; providing the
hydrate equilibrium curves and the performance of the conventional
thermodynamics inhibitors which are already used in industry. This work explores a
clear knowledge of hydrate formation conditions of Qatari natural gas mixture (QNG)
and provides a new horizon for the possibility of using ILs as hydrate inhibitors.
10
1.3 Thesisstructure
With the overview about gas hydrates provided by the current chapter, the
remaining chapters focus on presenting the main stages that were accomplished to
reach the objectives of this work. A brief description of the upcoming chapters is
given as follows:
‐ Chapter 2 provides a literature review about the gas hydrate and research
related efforts done to address this issue; providing the mechanism and
fundamentals of hydrate formation as well as inhibition and prevention
techniques.
‐ Chapter 3 addresses some of the experimental methods used recently to
study gas hydrate and a detailed description of the experimental apparatuses
used in this work with a description of the followed procedure. For this
project, Gas hydrate Autoclave (GHA) and Rocking Cell (RC5) were used to
test hydrate formation conditions and the performance of different ILs
hydrate inhibitors.
‐ Chapter 4 presents the obtained data from the experimental tests together
with data analysis and discussion.
‐ Chapter 5 provides the conclusion and future work of this project.
11
2 Reprinted figures with kind permission of Springer Science+ Business Media.
Chapter2: LiteratureReviewaboutGasHydrates2
2.1 Gashydrate
Gas hydrates, are crystalline solids consisting of gas molecules trapped and
enclathered by a cage of water molecules. The water molecules is the host which is
connected through hydrogen bonds forming a gate like a crystal structure and the
gas molecules are the guests which are trapped in the water cavities(Koh et al.,
2002). The water and gas molecular interaction is controlled by physical dispersion
or van der Waals forces. Hence, gas hydrates are also known by clathrate hydrates
(Oellrich, 2004). The formation of gas hydrates requires special conditions of
temperature and pressure, generally when the gas and water are combined at low
temperature and high pressure. Although hydrates look similar to ice, however, it’s
distinguished by their formation at temperatures well above 0 in pressurized
systems (Lorimer and Ellison, 2000). Moreover, unlike ice which is a pure
component, hydrates won’t form without containing a gas or a suitable sized guest
(Sloan and Koh, 2008). Common natural gas molecules comprise methane, ethane,
propane, carbon dioxide and hydrogen sulphide (Koh et al., 2002). Gas hydrates has
several crystal structures, yet, the most common natural gas hydrates are the cubic
structure I (sI), cubic structure II (sII), and hexagonal structure H (s) going to be
discussed in the upcoming subsections (Sloan and Koh, 2008).
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12
2.2 Historicalperspective
Historically, the discovery of gas hydrates was around 200 years ago (Oellrich, 2004).
The discovery and research efforts on natural gas hydrates can be categorized into
three periods including its first discovery until recent studies. The first period
includes gas hydrates initiation and their first discovery in 1810 and also the curiosity
of knowing the nature of water and gas transformation into solid gas hydrates (Sloan
and Koh, 2008). The literature refers the gas hydrates discovery to Humphrey Davy
in 1811(Oellrich, 2004). Davy was inspecting the chlorine‐water system when he
realized the stability of crystalline solids at temperature well above 0 . These solids
crystals were chlorine hydrates.
Makogon and Gordejev stated that clathrate hydrates might be discovered more
than 30 years before Davy (Sloan and Koh, 2008). They related the discovery to
Joseph Priestley in 1778 when he was investigating sulphur dioxide in his laboratory
(Makogon, 2010). However, Priestley didn’t describe them with the name of gas
hydrates. Moreover, the temperature of gas mixture in his experiment was well
below the freezing point of water (‐10 ), which can’t be an explicit evidence on
hydrate existence. Besides, there is no documentation for the experiments that were
done by Priestley (Sloan and Koh, 2008). Accordingly, in spite of the first creation of
hydrates was in Priestley’s laboratory, Davy’s discovery of chlorine hydrate is
frequently considered and credited as the first observance (Sloan and Koh, 2008).
The second period of the history of gas hydrates started in 1934 when
Hammerschmidt studied the blockages of pipes and published the results of the
13
monitoring the U.S. gas pipelines (Hammerschmidt, 1934). The United States started
the transportation of natural gas in pipelines and operation on larger scale in the late
20ies and early 30ies of the last century (Oellrich, 2004). The high pressure
operational conditions made gas hydrates more favorable to form. Since then, gas
hydrates became a source of many operational problems and pipelines plugging that
may cause safety hazards and significant economic risks. The first explanation of this
phenomenon was related to ice formation in the pipeline as a result of liquid water
and condensed water freezing. However, Hammerschmidt investigation approved
that the solid plugs weren’t just ice but gas hydrates formed from transported gas
(Makogon, 2010). Afterwards, gas hydrates became not just a mere curiosity but a
door for new industrial challenges that require additional cost of operation,
maintenance, and affect the production capacity. Accordingly, it became essential to
study the gas hydrates with more systematic research activities to investigate their
formation conditions including pressure and temperature as well as gas
compositions (Oellrich, 2004).
The history of gas hydrates in the third period of their discovery mainly related to
the natural gas hydrates since it’s considered as an unconventional source of energy
for the coming decades. In 1960s, the natural existence of gas hydrates was proven
and their important role in the planet formation, atmosphere, and hydrosphere was
addressed in literature. Over the past forty years, more than 9000 research papers
on natural gas hydrates have been published and the research efforts are
dramatically increasing (Makogon, 2010).
14
2.3 Hydratestructuresandcharacteristics
Clathrate hydrates are inclusion compounds where the water molecules (host) are
connected through hydrogen bonds forming gates. These hydrogen bonds are strong
enough to separate the water molecules rigidly affecting the solid (ice) density to be
less than liquid water density (Sloan and Koh, 2008). The density decreases on
freezing i.e. in ice; water molecules occupy 34% of the volume where the liquid
water occupies 37% (Sloan and Koh, 2008). Comparing to covalent bond, hydrogen
bond is considerably weaker. The hydrogen bonds are the only bonds accounted
between the adjoining molecules during the gas hydrates formation and dissociation
without a requirement to break the chemical bonds between oxygen and hydrogen
within a water molecule (Sloan and Koh, 2008). However, Van der Waals forces are
there, but not very strong relative to hydrogen bonds and they mainly control the
molecular interaction between the water and gas (Oellrich, 2004). Hydrogen bonds
are also responsible of the formation of hydrate crystal structures as discussed in the
following subsection.
2.3.1 Thecrystalstructuresandcavities
Sizing the water lattice cages enclosing the guest molecules is controlled by the
guest (gas) repulsion force, forming three hydrate unit crystals (I, II, H) (figure 2‐1).
These hydrates structures are composed of irregular spherical cages of hydrogen‐
bonded water molecules.
15
Figure 2‐1. Hydrate crystal unit structures: (a) sI , (b) sII, and (c) sH. (Momma et al., 2011)
On molecular scale, each water cage normally involves one guest (gas) molecule
grasped inside the cage by dispersion forces (Sloan and Koh, 2008). For these
irregular spherical cages, Jeffrey suggested a general description ( ) where is
the number of edges in face type and is the number of faces with edges
(Jeffrey and Atwood, 1984). For example, the pentagonal dodecahedron (Figure 2‐2)
is considered a 12‐sided cavity and denoted as 5 ( 5, 12 which means
that this cavity has 12 pentagonal faces with equal edge angles and lengths. On the
other hand, tetrakaidecahedron is a 14‐sided cavity which has 12 pentagonal and 2
hexagonal faces and described as5 6 . The so called hexakaidecahedral cavity, 16‐
hedron, has 12 pentagonal faces with 4 hexagonal faces and is signified as 51264.
Additionally, the irregular dodecahedron cavity consists of three square faces and six
pentagonal faces which donated as 435663. The largest cavity is called icosahedron
and comprises of 12 pentagonal faces with 6 hexagonal faces and a hexagonal face
each at the top and bottom of the cavity (51268) (Sloan and Koh, 2008).
16
Figure 2‐2. Three cavities in gas hydrate: (a): Pentagonal dodecahedron (512), (b): tetrakaidecahedron (51262), (c): hexakaidecahedron (51264), (d): irregular dodecahedron (43
56 63) and (e): icosahedron (512 68). (Sloan and Koh, 2008)
Gas hydrates cavities are expanded compared to ice. Without the guest’s molecules,
hydrates cavities are not stable thermodynamically (Oellrich, 2004). This means that
the repulsion forces, either in the cavity itself or in a large percentage of the
adjoining cavities of the guest’s molecules, prevent the cavities from collapsing
(Sloan and Koh, 2008).
The number and the sizes of the lattice cages or cavities are used to distinguish
between the different structures of gas hydrates. The different structures of hydrate
crystals depend on the guest molecule (Giavarini and Hester, 2011). Structure I (si) is
a body‐centred cubic structure forms with small guest molecules (i.e. smaller than
propane) that have diameters of 0.4‐0.55 nm (Sloan, 2003a). This structure is mostly
found in the natural environments, particularly in deep oceans with biogenic gases
mostly methane, carbon dioxide and hydrogen sulphide (Sloan, 1998).
17
The second structure is structure II(sll) which is a diamond lattice within a cubic
structure forms by having guests molecules larger than ethane but smaller than
pentane having diameters of 0.6– 0.7nm (Sloan, 2003a). In contrast to the previous
structure, the hydrates having structure II are frequently occur in man‐made
environments, such as gas and oil processing facilities (Sloan, 1998). However, the
third structure, the hexagonal structure H, may occur in both natural and man‐made
environments. The guest’s molecules in this structure are mixtures of both small or
large molecules having diameters between 0.8‐0.9nm (Sloan, 2003a). Small
molecules can be methane, nitrogen or carbon dioxide with the larger molecules
such as iso‐pentane and neohexane (Sloan and Koh, 2008). Table 2‐1 shows the
three unit crystal properties (Sloan, 2003a).
Table 2‐1. Geometry of cages in three hydrates crystal structures. (Sloan, 2003a)
Hydrate Crystal
Structure I II H
Cavity Small Large Small Large Small Medium Large
Description 512 512 62 512 512 64 43 56 63 512 512 68
Number of cavities/unit cell
2 6 16 8 2 3 1
Average cavity Radius (nm)
0.395 0.433 0.391 0.473 0.38 0.385 5.2
Variation in radiusa (%)
3.4 14.4 5.5 1.73 Not available
Coordination number b
20 24 20 28 20 20 36
Number of waters/unit cell
46 136 34
a Variation in distance of oxygen atoms from center of cage b Number of oxygen atoms at the periphery of each cavity
18
Figure 2‐3. Gas hydrate formation in pipelines with multiphase flow. (Sum et al., 2009)
2.4 Hydratesformationmechanism
Gas hydrate formation and growth kinetics is vital for industry and research studies.
The best description for hydrate’s time‐dependent phenomena can be though
crystallization theories involving hydrate nucleation, growth, agglomeration, and
dissociation process (Figure 2‐3) (Sum et al., 2009). However, it’s believed that
hydrate form in two steps process: nucleation and growth (Fink, 2012, Sloan and
Koh, 2008, Kashchiev and Firoozabadi, 2002, Englezos et al., 1987).
2.4.1 Hydratesnucleation
Nucleation is basically a microscopic stochastic process in which gas‐water molecules
form clusters that grow and accumulate until stable hydrate nuclei is formed. This
nucleation step is followed by hydrate nuclei growth into solid hydrates. However,
nucleation may take place without the existence of impurities, which called in this
19
case homogeneous nucleation, or it might occur within some impurities
(heterogeneous nucleation).
In fact, there are two hypotheses for the early stages of hydrate formation;
presenting two different mechanisms of nucleation and growth as (i) labile cluster
nucleation and (ii) nucleation at interface (Fink, 2012, Sloan and Koh, 2008). The first
hypothesis, i.e. labile cluster nucleation suggests that hydrate formation occur in
four main stages: molecular species, labile clusters, agglomerates and stable hydrate
nuclei (Figure 2‐4) (Fink, 2012, Sloan and Koh, 2008). As explained by (Christiansen
and Sloan, 1994), the hydrogen‐ bonded water molecules gather and surround the
dissolved gas molecules in the aqueous phase forming labile clusters. The number of
the water molecules forming the clusters differs according to the size of the guest
gas. The labile clusters are then joined and agglomerate by sharing farces forming
metastable agglomerates. When these agglomerates reach critical size, hydrate
nucleus is formed and the growth begins.
Figure 2‐4. Model representing the labile cluster growth. (Sloan and Koh, 2008)
20
Figure 2‐5. Nucleation at the gas‐water interface by adsorption of gas onto labile cages. (Sloan and Koh, 2008)
One the other hand, the second hypothesis of hydrate formation is nucleation at the
interface. A model developed by (Rodger et al., 1996) clarify this hypothesis which
suggest that gas molecules are adsorbed on the surface of water or ice causing the
water molecule to form cages around the gas (Fink, 2012, Sloan and Koh, 2008). The
surface diffusion makes the gas migrates into appropriate location in the formed
cages; where partial cages are first formed then followed by further growth until the
adsorbed gas become fully covered by the complete cages ( Figure 2‐5) (Sloan and
Koh, 2008). The formed clusters are then joined and grow on the vapor side of the
surface causing another clathrate cage to form and eventually create stable nuclei
(Sloan and Koh, 2008).
21
The difference between the two discussed hypotheses for hydrate nucleation is that
the theory by (Christiansen and Sloan, 1994) refer the early stages for hydrate
formation to the solute (gas) in the aqueous solution and relates the nucleation to
the liquid side of the gas‐liquid interface. On the other hand, theory by (Rodger et
al., 1996) refers the nucleation to the local surface of the water (ordered water‐
guest structure) and the vapor side on the interface (Fink, 2012, Sloan and Koh,
2008). Actually, experimental verification is essential to figure out which nucleation
theory has the accurate description of nucleation mechanism. Yet, the experimental
test to verify this process is hard to reproduce due to the stochastic nature of this
process and time dependency that may last for days before observing macroscopic
signs of hydrate formation. Thus, stochastic models and molecular simulation can be
of support to study the nucleation and formation mechanism (Oellrich, 2004).
2.4.2 Hydrategrowth
Hydrate growth after nucleation depends on many parameters, which control the
growth process and growth rate. Despite the importance of gas composition,
equilibrium conditions, agitation, and surface area; the growth process is mostly
dominated by mass and heat transfer. This can be related to the fact that hydrates
contains up to 15 mol% gas and the exothermic nature of hydrate formation process.
It’s believed that the hydrate growth in molecular level consists of three main
correlations: intrinsic crystal growth kinetics at hydrate surface, mass transfer to the
surface of the boosting crystal, and the transfer of the exothermic heat from the
surface of the growing crystal.
22
2.5 Hydrateformationanddissociationprocess
The previously discussed process of hydrate formation can be explained through
Pressure‐ Temperature schematic (P‐T trace) obtained from experimental test.
Considering methane hydrates as an example, hydrate formation and dissociation
process is similar to the loop shown in Figure 2‐6. This test was carried out in Rocking
Cell (RC5) using isochoric‐pressure search method. According to the P‐T plot, the
region from point A to B shows a slight decrease of the pressure as a result of
cooling. However, hydrates don’t form immediately in this region due to the meta‐
stability of the system and only short‐lived cages start to form. The period from A to
B is known as the induction period or induction time where the onset of hydrates
formation is at point B. The pressure remains decreasing until sudden drop occur to
point C; indicating the catastrophic growth of hydrates due to the amount of the
guest gas being incorporated and trapped in the hydrates structures.
Afterwards, hydrate dissociation started from point C driven by heating the system
where the pressure starts to increase slowly for a certain time then increases rapidly
to the point D. Hydrates become completely dissociated at point D which is known
as the hydrate liquid vapor equilibrium point (HLVE) indicating hydrate dissociation
temperature and pressure. This complete loop in Figure 2‐6 demonstrates the whole
process of hydrate formation and dissociation including cooling and heating region.
It’s worth mentioning that in experimental studies; the dissociation region of the
loop must be implemented carefully with slow heating rate (about 0.12 K/h) to
assure the system to reach equilibrium accurately (Rovetto et al., 2006, Tohidi et al.,
23
40
45
50
55
60
65
0 5 10 15 20
P (bar)
T (oC)
A
DB
C
Heating
ΔP
2000). Moreover, the driving force for hydrate formation is indicated in the
subcooling area, i.e. the difference between the equilibrium or dissociation
temperature (point D) and point B is known as the subcooling temperature (Sloan
and Koh, 2008, Kelland, 2006).
Figure 2‐6. P‐T plot for methane hydrate formation‐ dissociation loop
2.6 Formationconditionsofgashydrateinindustry
Hydrates formation in industry require special conditions, mostly suitable
temperature and pressure conditions in the presence of water and gas molecules.
Generally, low temperature and high pressure are responsible to form hydrates
when appropriate hydrate former coexist with water. The hydrocarbons produced in
oil and gas production are the hydrate formers and during the production operations
water is usually present. In offshore operations, the amount of excess water tends to
24
increase with time in the well. The depth or hydrostatic head pressure and the
operating pressure in the pipelines are the reasons of having high pressure in
offshore processes. On the other hand, the environmental conditions outside the
pipeline such as in seabed and the Joule‐Thompson effect, as discussed previously,
are the main factors of having low temperature in the process. Thus, in deep oceans
, i.e. at depth lower than 500‐1000 m, the temperature is round 3 and stable
natural gas hydrate can be formed with pressure less than 0.7MPa (7bar) which is
much less than operating pressures in pipeline (Gbaruko et al., 2007, Giavarini and
Hester, 2011). In offshore facilities, the incident of hydrates before the well is
banned by the high temperature of the reservoir, yet, the region between the well
inlet and the platform is typically threatened by hydrates (Figure 2‐7) (Giavarini and
Hester, 2011). Moreover, hydrate stability regions in offshore pipelines are close to
the P/T conditions available to form hydrate deposits in ocean sediments (Figure 2‐8)
(Krey et al., 2009).
25
Figure 2‐7.Hydrates formation zones in offshore operations. (Giavarini and Hester, 2011)
Figure 2‐8.Schematic representation of hydrate sedimentary deposits at continental margins and below the permafrost. (Krey et al., 2009)
26
2.7 Investigationofhydrateformationinlaboratory
To study the hydrate formation process and investigate the appropriate T/P
conditions for different hydrate formers, laboratory scale reactors are used. These
reactors are usually selected based on their ability to simulate the required industrial
conditions and the purpose of conducting the test. Hydrates reactors are available in
different sizes and types such as continuous and batch modes. Usually batch reactors
are used for investigating the thermodynamics behavior and kinetics of hydrate
formation. The following sections describe the most common laboratory techniques
used for gas hydrate research studies.
1) High pressure autoclave
Recently, autoclave‐type of reactors and cells with cooling jackets or temperature
control bath are commonly used to (i) investigate hydrate formation/ dissociation
process (ii) study the effectiveness of chemical inhibitors. Some of the cells are
equipped with sapphire window allowing good observation of hydrate growth
(Giavarini and Hester, 2011). Autoclave cells are available in different sizes and
materials, such as stainless steel, titanium, and sapphire (Lone and Kelland, 2013).
Figure 2‐9 shows the experimental setup for sapphire cell located in a water bath,
where the sapphire tube is held by two stainless steel holders. One of the main
features in autoclave cells is the ability to conduct long experimental runs.
27
2) Rocking cell
Rocking cell is a high pressure cells that are usually made of steel or sapphire (RC‐S)
(transparent cells for visual observation). This equipment consists of several cells,
depending on the design, and they commonly have volume of 10‐50 ml. Each cell of
the rocking cell contains a steel or glass ball used to create agitation and turbulence
of the fluid inside the cell. Moreover, these cells move back and forth with a certain
rocking rate and rocking angle that can be specified to simulate the typical
conditions in production and transmission pipelines. Rocking cell has some
important features that make it effective equipment for hydrate experiments.
Figure 2‐9. Sapphire cell high‐pressure test equipment. (Del Villano and Kelland, 2011)
28
Despite autoclave cell that has its own cooling bath, however in rocking cell, multiple
cells are mounted on a movable axis and are placed in a single water bath. This
provides the ability to run several experiments in series, which saves time and
increases the productivity. This equipment is very effective in testing KHIs and AAs.
3) Flow loop
Flow loop is a complex pipe‐wheel or loop‐wheel used for the purpose of assessing
hydrate risks and testing the fluid flow under real operation conditions (Figure 2‐10).
Recently, high pressure loops are used for natural gas, oil, condensate and aqueous
phases. Flow loops are available in different scales range from ¼ in to 4in diameter,
such as mini‐loop and full scale pilot loop (Kelland, 2006). Flow loops are available
technique for both laboratory and industrial scales. For instance, micro loop is small
scale equipment with proven capability that can reach the performance of the
conventional loops. Micro loop has some unique features such as the simplicity to
operate and the ease in cleaning and maintenance. Besides, it requires few samples
and has the ability to run more tests, up to six times more than the conventional
loops (Tian et al., 2011). Generally, the flow loops are also useful for testing the
performance of hydrate inhibitors. Nevertheless, hydrates may get crushed by the
pump in some cases, which is considered one of the drawbacks of this equipment
that add restrictions and difficulty in obtaining and interpreting experimental data
(Kelland, 2006).
29
Figure 2‐10. The flow loop used for experimental research on hydrates installed at the research center of the French institute
of petroleum in Lyon. (Giavarini and Hester, 2011)
Moreover, some other techniques for hydrate investigation includes: X‐ray
diffraction used for studying hydrate crystals structures, nuclear magnetic resonance
spectroscopy (NMR) which is considered very effective in studying chemical and
physical properties of hydrates, and differential scanning calorimetry (DSC) which is
a thermal analyzer to check the variations in enthalpy (Giavarini and Hester, 2011).
2.7.1 Laboratoryexperimentalconditionsandmethods
The conditions of hydrates formation depends on some factors like the cooling rate,
water history, degree of subcooling, and the presence of impurities. Three different
conditions and methods are used for hydrate measurements in the presence of
aqueous phase (i) isothermal, (ii) isochoric and (iii) isobaric (Tohidi et al., 2000). In
the isothermal method, the pressure of the hydrate cell is the variable parameter
30
where the hydrates are typically formed by increasing the cell pressure and
dissociated by reducing the pressure to obtain the HLVE point. One the other hand,
to measure hydrate formation/dissociation using isochoric or isobaric methods, the
temperature of the cell is the variable parameter that can be decreased to form
hydrate and increased to dissociate it (Tohidi et al., 2000). In this work, isochoric
pressure search method was implemented in all the experiments where
temperature was the only variable parameter during the experimental runs. The
hydrate equilibrium point (dissociation point) was obtained by pressure versus
temperature plot.
To assess the performance of hydrate inhibitors, especially KHIS, several methods
are available. Some of widely used methods are briefly explained as follows:
1) Constant temperature or isothermal method. To form hydrates using this
method, the fluids are cooled down to a specific temperature and then held
at this point giving adequate time for hydrate to start forming (Del Villano
and Kelland, 2011). This procedure can be applied with or without providing
agitation and mixing. This method is frequently used to indicate the induction
time which is the time it takes to observe pressure drop in the system.
2) Constant cooling. This method is carried out by cooling the fluids to a high
subcooling (very low temperature) with providing agitation in the system.
However, the difficulty in determining the induction time especially for high
cooling rate is one of the drawbacks of this method (Del Villano and Kelland,
2011).
31
3) Ramping method. Ramping is a stepwise procedure that’s applied in a
repetitive manner. In this method, fluids are cooled with mixing to a specific
subcooling, and then kept for few hours. Then, another cooling starts rapidly
to a higher subcooling and kept again for a while. These steps are repeated
until hydrates form. This method requires long time due to its stepwise
manner. Yet, it’s considered one of the effective methods in obtaining
induction time since the pressure remains constant for sufficient time during
the rapid cooling steps (Del Villano and Kelland, 2011).
2.8 Gashydratepreventiontechniques
The research efforts done to address the issue of gas hydrates are currently
increasing with a growing interest in obtaining a better understanding of the
available techniques in mitigating and preventing the hydrate formation in pipelines.
There are traditional methods for preventing the gas hydrate formation (Gbaruko et
al., 2007):
1) System heating (Thermal Control); by maintaining the system operates at a
temperature above the hydrate formation threshold. This will prevent the
system from providing the suitable conditions for hydrate formation by
insulation, hot oil, hot water circulation, or electrical heating.
2) Depressurizing; by keeping the operating pressure below the hydrate formation
pressure. However, this method is impractical especially at high production rates
which require high pressures close or exceeds hydrate formation conditions.
Moreover, some depressurizing techniques are life‐threatening and have major
32
safety concerns such as one‐side depressurization which can lead to move the
plug as projectile due to pressure difference. However, two‐sided
depressurization usually reduces this concern (Koh and Sloan, 2007).
3) Water removal or dehydration; by drying the gas is such a way no condensate
can be formed. This can be achieved by glycol dehydration tower.
4) Adding chemical inhibitors:
a. Low dosage inhibitors (LDHI) such as anti‐agglomerants (AAs) and Kinetic
inhibitors (KHI).
b. Thermodynamic inhibitors (THI).
Not all the above mentioned hydrate prevention techniques are suitable to be
applied in the industries, mainly due to operating conditions limitations and safety
concerns. Yet, some other conventional methods are widely used in industries as
described in details in section 2.8.1.
2.8.1 Conventionalmethodsforgashydrateremediation
Among the above mentioned methods, adding inhibitors is considered the most
feasible method in preventing gas hydrate (Xiao et al., 2010). These inhibitors are
classified into two categories, Low dosage hydrate inhibitors (LDHI) and
thermodynamic inhibitors (THI). Each category performs differently to inhibit gas
hydrate.
2.8.1.1 Lowdosagehydrateinhibitors(LDHI)
The low dosage hydrate inhibitors are generally effective at low concentration, i.e
less than 1wt% (Yang and Tohidi, 2011). LDHI have been used and developed to
33
overcome the high cost of thermodynamic inhibitors (THIs) and the amount required
for gas hydrates inhibition. This category of chemical additives consists of two types,
kinetic hydrate inhibitors (KHI) and anti‐agglomerants (AAs).
1) Anti‐ agglomerants (AAs)
The addition of anti‐agglomerants, such as quaternary ammonium salts prevents
hydrates from agglomerating and accumulating into large masses. AAs allow the
fluid flow freely by keeping the formed hydrate in a transportable suspended slurry
state dispersed in the hydrocarbon fluid (Figure 2‐11) (Koh and Sloan, 2007, Kelland,
2006).
Figure 2‐11. Graphical representation of anti‐agglomerants effect. (Giavarini and Hester, 2011)
34
Although this method doesn’t inhibit the hydrate formation, however the best AAs
achieve at higher subcoolings (∆ ) than the kinetic inhibitors (Yang and Tohidi, 2011,
Kelland, 2006). The difference between the hydrate equilibrium temperature and
the operating temperature at a particular pressure is the driving force for hydrate
formation known as the subcooling (∆ ) (Figure 2‐12).
2) Kinetic hydrate inhibitor (KHIs)
Kinetic inhibitors (KHIs) are polymer based chemicals (polymer chain with attached
polar groups) that delay the formation of hydrates by changing the formation
geometry and influencing the growth rates (Richard and Adidharma, 2013). For
example for gas exploration in deep sea, kinetic inhibitors are used to delays hydrate
formation to have a longer time than the time required for hydrate formation in the
Figure 2‐12. Pressure‐temperature plot for a typical natural gas hydrate. (Kelland, 2006)
35
hydrate prone section of the pipeline. Yet, in some cases KHIs are not very effective
in long distance pipelines and requires the addition of other inhibitors, such as
thermodynamic inhibitors (THIs). The inhibition mechanism of KHIs is still not
completely understood (Kelland, 2006). Thus, it’s expected that the polar groups in
these inhibitors interact with the partly formed cages on the hydrate surface, where
the polymer chain expands over the hydrate surface preventing more growth (Figure
2‐13) (Giavarini and Hester, 2011). Nonetheless, the performance of KHIs can be
affected by the hydrate crystal structure (Giavarini and Hester, 2011).
KHIs are usually effective at low dosage (i.e. less than 1%) which makes them
economically favorable (Xiao and Adidharma, 2009). These chemicals involve homo‐
and co‐polymers of the N‐vinyl pyrrolidone and N‐vinyl caprolactam (Cha et al.,
2013). Poly(N‐vinylpyrrolidone) (PVP), poly(N‐ vinylcaprolactam) (PVCap) are
examples of some common KHIs that have been studied (Xiao and Adidharma,
Figure 2‐13. Graphical representation of KHIs absorbed into hydrate surface. (Giavarini and Hester, 2011)
36
2009). The effectiveness of the KHIs is usually evaluated by two main criteria,
subcooling (as illustrated previously) and the induction time which is the time
required to form stable hydrate nuclei (Natarajan et al., 1994, Yang and Tohidi,
2011). For the conventional KHIs, the typical upper limit of subcooling degree with
good induction time is around 12 . On the other hand, higher range of subcooling
can be achieved by AAs, i.e. > 15‐20 which is usually the subcooling required in
deep water operations (Zheng et al., 2011, Giavarini and Hester, 2011).
2.8.1.2 Thermodynamichydrateinhibitors(THIs)
Besides KHIs, thermodynamic inhibitors (THIs) have been the most applicable
conventional technique for long‐distance subsea production transport pipelines.
Unlike the LDHIs, THIs are dosed at much higher amounts, i.e. 20‐50 wt% (Kelland,
2006). The main function of thermodynamic inhibitors (THIs) is shifting the hydrate
dissociation curve to higher pressures and lower temperatures maintaining
unfavorable conditions for hydrate formation. These chemicals are strongly polar
molecules or ions which hence disturb the hydrogen bonded network of water
molecules (Lafond et al., 2012). Alcohols, glycols, and inorganic salts are types of
thermodynamics inhibitors which were studied widely. However, for the
thermodynamic inhibitors to have a strong inhibition effect, they must have strong
electrostatic charges or bonding to water molecules through hydrogen bonds (Li et
al., 2011). The output of various studies demonstrated the effect of applying several
thermodynamic inhibitors specially methanol (CH3OH) and monoethylene glycol
(MEG) as the most common THIs being used. The inhibition performance of some
37
THIs as function of concentration is shown in Figure 2‐14. The estimated worldwide
cost of methanol reached US$740,000 daily (Joshi et al., 2013). Currently, gas
industries favor the usage of MEG over methanol due to the health, safety, and
environmental concerns. Still, most of the conventional THIs require higher injection
rates and larger storage as the production pipelines moves into colder and deeper
areas (Giavarini and Hester, 2011).
Figure 2‐14. The effect of some THIs on methane hydrate stability.(Giavarini and Hester, 2011)
38
2.8.2 Benefitsandchallengesofconventionalinhibitors
As mentioned earlier, the available methods to prevent gas hydrates such so
heating, depressurization, and dehydration are frequently unfeasible under many
circumstances. This made the addition and injection of chemical inhibitors the only
applicable and the best selection to avoid hydrate problem in gas and oil industries
(Li et al., 2011). However, the selection of the suitable hydrate inhibitor is still an
issue and in many cases the choice of injecting a certain inhibitor or the combination
of more than one (hybrid inhibitors) depends on the following(Gbaruko et al., 2007):
a. The system structure and configuration.
b. Compatibility with operating conditions, i.e. expected temperature/ pressure
ranges over the operating life.
c. Relative volumes of gas
d. Cost considerations, capital expenditure (CAPEX) and operational expenditure
(OPEX).
e. Water and hydrocarbon liquids involved.
Summary of the benefits and limitations of conventional inhibitors are illustrated in
Table 2‐2.
39
Table 2‐2. Summary of the advantages and challenges of conventional hydrate inhibitors
Inhibitor Advantages Challenges and imitations
Thermodynamic Hydrate
inhibitors (THIs)
‐ Reliable and well understood inhibition mechanism
‐ Effective for long distance gas‐condensate tie‐back systems.
‐ Cost considerations ‐ High injection rate (10‐60
wt%) and large storage requirement
‐ Flammability and volatility ‐ Toxic and does not easily
biodegraded. ‐ Some THIs are corrosive
Anti‐agglomerants
(AAS)
‐ Economically effective‐ Operative at low dosage
(<1 wt%) ‐ Wide range of subcooling
(>15‐20 ). ‐ Less toxicity than THIs
‐ Compatibility with some conditions in the field (temperature, pressure, and salinity)
‐ Not effective at high water cuts.
‐ Not effective during shutdowns.
Kinetic Hydrate inhibitors (KHIs)
‐ Economically effective ‐ Operative at low dosage
(<1 wt%) ‐ Less toxicity than THIs
‐ Limited to small subcooling range (12 ) and residence time.
‐ Compatibility with some conditions in the field (temperature, pressure, and salinity)
‐ Lack of biodegradability features of some KHIs.
‐ Not effective during shutdowns
The implementation of the software tools, usually based on experimental tests,
enables the companies to predict the thermodynamics conditions for hydrate
formation (Giavarini and Hester, 2011). This provides the key solution and guidance
to select of the appropriate inhibitor and the required dosage according to the
available conditions. The suggestion of the suitable inhibitor can be represented as a
40
schematic plot such as PT diagrams (Figure 2‐15). The plot illustrates the areas
where the LDHIs and THIs can be applicable where the hydrate formation occurs on
the left of the equilibrium curve. As it’s labeled in the plot, the central area
containing the equilibrium curve is expected to be suitable for LDHIs, particularly
AAs. On the other hand, THIs can be effective moving the left of the central area.
Figure 2‐15. Typical PT diagram illustrating the areas where the LDHIs and THIs can be applicable. (Giavarini and Hester, 2011)
41
2.8.3 Ionicliquids
Xiao and Adidharma introduced ionic liquids (ILs), a new class of hydrate inhibitors
that have a dual function which perform as both thermodynamic and kinetic
inhibitors for gas hydrate (Xiao and Adidharma, 2009). These type of inhibitors
known by ILs are basically organic salts with low melting point, hence are liquid at
relatively low or at room temperature (Xiao and Adidharma, 2009, Jiang and
Adidharma, 2013). ILs comprises massive organic cations with alkyl chain
substituents. Imidazolium and pyridinium ions are some common cations, on the
other hand tetrafluoroborate (BF4−), dicyanamide (N(CN)2−), nitrate, chloride, and
bromide are some common anions (Xiao and Adidharma, 2009).
The effectiveness of salt solutions as thermodynamic inhibitors have been widely
investigated which shows the possibility of ILs to perform as these conventional
inhibitors (Richard and Adidharma, 2013). From studies done by (Xiao and
Adidharma, 2009), it was shown that some ILs inhibitors have strong electrostatic
charges and capability to form hydrogen bonds with water. Hence, ILs can slow
down the hydrate nucleation rate and delay their formation, which is the main
function of kinetic inhibitors besides performing as thermodynamic inhibitors. Some
types of ILs performed as good kinetic inhibitors such as tetrafluoroborate anion
containing ILs, showing at the same time some thermodynamic inhibition behavior
(Xiao et al., 2010).
The first ILs used by (Xiao and Adidharma, 2009) were imidazolium cation based
ones. Some experiments were then implemented by (Del Villano and Kelland, 2010)
42
on this type of ILs at typical subsea temperatures and subcooling. The experiments
showed that these ILs are very weak KHIs at 5000–10,000 ppm concentrations.
Further studies of six other dialkylimidazolium halide ILs were continued by (Xiao et
al., 2010) using concentrations about 10 wt% to investigate the thermodynamic
effects and 1 wt% for kinetics effect. The outputs of this study showed a
temperature decrease in the dissociation temperature of methane hydrate (i.e. 0.2–
1.2 K). It was found that the most effective thermodynamic inhibitor was achieved
by 1‐ethyl‐3‐ methyl‐imidazolium chloride ([EMIM]‐Cl). On the other hand, 1‐butyl‐3‐
methyl‐ imidazolium iodide ([BMIM]‐I) was the best kinetic inhibitor (Xiao et al.,
2010, Partoon et al., 2013).
ILs usage as hydrate inhibitors gained attention not only for methane hydrates but
also for other hydrate formers such as carbon dioxide (Richard and Adidharma,
2013). Some criteria were assumed by (Kim et al., 2011) for the appropriate IL to be
used as hydrate inhibitor. These criteria require the ILs to have hydrophilic
properties and functional groups that are able to create intermolecular hydrogen
bonding with water molecules (Partoon et al., 2013). Accordingly, N‐(2‐
hydroxyethyl)‐N‐methylpyrrolidinium tetra‐ fluoroborate ([HEMP]‐BF4) and N‐butyl‐
N‐methylpyrrolidinium tetrafluoroborate ([BMP]‐BF4) were selected for synthesis
and analysis their inhibition effects (Partoon et al., 2013). The results show that the
thermodynamic inhibitions performance of these ILs is good at 10 wt%
concentration and the selection of anion and cation groups could lead to good low
dosage hydrate inhibitors (LDHIs).
43
In fact, the interest in ILs, besides having dual function for inhibition, is due to their
properties like having extremely low vapor pressures and high thermal stability (Xiao
and Adidharma, 2009, Li et al., 2011, Peng et al., 2010). Moreover, ILs have wide
liquid ranges and can solubilize many compounds (Peng et al., 2010). The ILs can be
tailored from uncountable possible combination of the cations and anions to reach
higher and effective inhibition of gas hydrates (Xiao and Adidharma, 2009, Xiao et
al., 2010, Jiang and Adidharma, 2013). Some studies showed that ILs are not
effective as conventional thermodynamic inhibitors. Yet, their tunable structure of
the cations and anions, i.e. forming hydrogen bonds with water; encourage
investigating the possibility for ILs to function as hydrate inhibitors (Li et al., 2011) .
Actually, the previously mentioned properties of ILs are not the only reasons that
attracted the attention, however, some ILs are also considered biodegradable and
environmentally friendly solvents for organic reactions due to their stability (Xiao
and Adidharma, 2009, Jiang and Adidharma, 2013, Del Villano and Kelland, 2010).
Some studies suggested that ILs with shorter alkyl substituents of the cations are
likely to perform better than longer chains of alkyl substituents. Furthermore, the
results of a study done by (Li et al., 2011) illustrated that the inhibition performance
can be, but not necessarily enhanced by the hydroxyl group in the hydroxyl‐
functionalized cations in the ILs. Summary of the ILs found in literature for gas
hydrate inhibition is provided in Tables 2‐3, 2‐4, and 2‐5.
44
Table 2‐3. List of ILs used in published studies
IL‐Symbol Chemical name Purity Chemical Structure Reference
EMIM‐Cl 1‐ethyl‐3‐methylimidazolium chloride
≥97%
(Richard and Adidharma, 2013, Xiao et al., 2010)
EMIM‐Br 1‐ethyl‐3‐methylimidazolium bromide
≥98%
(Richard and Adidharma, 2013, Xiao et al., 2010)
[OH‐C2MIM]‐Cl
1‐Hydroxylethyl‐3‐methyl‐imidazolium chloride
99.8 %
(Partoon et al., 2013)
[MMM]‐I 1,3‐dimethyl‐imidazolium iodide
≥99.4%
(Li et al., 2011)
[EMIM]‐I 1‐ethyl‐3‐methyl‐imidazolium iodide
≥99.4%
(Li et al., 2011)
[OH‐C2MIM]‐Cl
1‐hydroxyethyl‐3‐methyl‐imidazolium chloride
≥99.4%
(Li et al., 2011)
[N1,1,1,1]‐Cl Tetramethyl‐ammonium chloride
≥99.4%
(Li et al., 2011)
[N1,1,1,eOH]‐Cl Hydroxyethyl‐trimethyl‐ammonium chloride
≥99.4%
(Li et al., 2011)
45
Table 2‐4. Continuation of the list of ILs used in published studies
IL‐Symbol Chemical name Chemical Structure Reference
EMIM–BF4 1‐ethyl‐3‐methylimidazolium tetrafluoroborate
(Del Villano and Kelland, 2010, Xiao and Adidharma, 2009)
BMIM–BF4 1‐eutyl‐3‐methylimidazolium tetrafluoroborate
(Del Villano and Kelland, 2010, Xiao and Adidharma, 2009)
EMIM–N(CN)2 1‐ethyl‐3‐methylimidazolium dicyanamide
(Xiao and Adidharma, 2009)
EMIM–CF3SO3 1‐ethyl‐3‐methylimidazolium trifluoromethanesulfonate
(Xiao and Adidharma, 2009)
EMIM–EtSO4 1‐ethyl‐3‐methylimidazolium ethylsulfate
(Xiao and Adidharma, 2009)
PMIM‐I 1‐propyl‐3‐methylimidazolium iodide
(Xiao et al., 2010)
BMIM‐Cl 1‐butyl‐3‐methylimidazolium chloride
(Xiao et al., 2010)
BMIM‐Br 1‐butyl‐3‐methylimidazolium bromide
(Xiao et al., 2010)
BMIM‐I 1‐butyl‐3‐methylimidazolium iodide
(Xiao et al., 2010)
46
Table 2‐5. Continuation of the list of ILs used in published studies
IL‐Symbol Chemical name Reference
[Hmim][Cl] 1‐Hexyl‐3‐methylimidazolium chloride (Peng et al., 2010)
[Omim][Cl] 1‐Octyl‐3‐methylimidazolium chloride (Peng et al., 2010)
[Bmim][BF4] 1‐Butyl‐3‐methylimidazolium tetrafluoroborate
(Peng et al., 2010)
[Hmim][BF4] 1‐Hexyl‐3‐methylimidazolium tetrafluoroborate
(Peng et al., 2010)
[Bmim][TA] 1‐Butyl‐3‐methylimidazolium trifluoroacetate
(Peng et al., 2010)
[Bmim][PF6] 1‐Butyl‐3‐methylimidazolium hexafluorophosphate
(Peng et al., 2010)
47
Chapter3: Methodology
3.1 Experimentalsetupandmaterialused
3.1.1 Gashydrateautoclave(GHA)
Gas hydrate autoclave is a well‐known apparatus used for gas hydrate experiments
that provides the ability to control and monitor the formation of gas hydrate in a
fully automated manner. GHA can be used not only to study the formation
conditions of gas hydrate but also for testing kinetics and thermodynamics hydrate
inhibitors as well. The equipment was supplied by PSL Systemtechnik GmbH,
Germany and it consists of the following components (Figure 3‐1):
‐ Autoclave (High pressure cell) with integrated stirrer.
‐ Pressure sensor, temperature sensor, and camera.
‐ Light source
‐ Thermostat
‐ Control‐PC with Hydrate V4 software.
The body and lid of the GHA are made of stainless steel. The cell has pressure
resistant sapphire‐glass window assists in photo capturing and video recording of the
hydrate formation process inside the cell via borescope‐camera. Autoclave has a
total volume of 450ml and it’s equipped with an integrated magnetic stirrer to
provide deep‐sea conditions for gas and oil transport. The temperature and pressure
sensors are installed to the autoclave lid measuring the T/P conditions inside the cell
(Figure 3‐2). The temperature sensor is connected to the sensor socket of the
48
2
3
5
1
4
thermostat so that the latter can regulate the temperature according to the required
set point by the control‐PC. On the other hand, the cable of the pressure sensor is
connected directly to the control‐PC. The specifications of the GHA are summarized
in Table 3‐1.
Figure 3‐1. Gas hydrate autoclave experimental setup. (1): Autoclave cell, (2): gas cylinder, (3): High pressure generator, (4): Thermostat, (5): Control‐PC
49
Table 3‐1. Gas hydrate autoclave specifications
Name of the apparatus Gas hydrate autoclave (GHA)
Supply company PSL Systemtechnik GmbH, Germany
Material of the cells Stainless steel
Size of the cells 450ml
Maximum working pressure 200 bar
Pressure accuracy 0.5%
Temperature range ‐10 to 60
Temperature accuracy 0.1
Figure 3‐2. High‐pressure autoclave cell
Boroscope‐camera
Gas inlet valve
Thermocouple
Gas outlet
Pressure
transducer
50
1
2
3
3.1.2 Rockingcell‐RC5
Most of the experiments in this work were conducted in Rocking Cell (RC5) supplied
by PSL Systemtechnik GmbH, Germany (Figure 3‐3). The main tasks of RC5 are to
investigate and detect the gas hydrate formation process and conditions in the
pipelines and to test the chemical additives that affect the flow characteristics. The
equipment consists of five stainless steel test cells with a volume of 40ml each
capable in providing deep‐sea conditions to test and simulate natural gas and crude
oil transport (Figure 3‐4).
RC5 has some characteristics that make it very effective and widely used laboratory
device in testing and analyzing gas hydrates with chemical inhibitors:
‐ The possibility to run different experiments simultaneously by charging each cell
with different pressure and filling with different test mixture.
Figure 3‐3. Rocking cell experimental setup (1): Base unit of RC5, (2): Thermostat, (3): Control‐PC.
51
Test cell
Temperature
sensor
Screw lid
Mixing ball
‐ The mixing method in the RC5 can provide turbulent deep‐sea pipeline
conditions for natural gas and oil transport.
‐ The small volume of the cells makes it cost effective in using the chemical
inhibitors.
The rocking cells can be charged with the test gas through gas supply tubes up to
200bar working pressure. To provide agitation and mixing of the loaded solution
inside the cells (water with the chemical additives), stainless steel ball,
approximately 10mm diameter, is placed inside each cell. The cells can be adjusted
with rocking movement back and forth with an angle of ‐45° to +45° and the desired
rocking rate providing turbulence flow that simulates the pipelines conditions. The
cells are mounted on a movable axis placed in the integrated bath (RC5 bath) which
is controlled by an integrated stepper‐motor. The RC5 bath is filled with coolant
Figure 3‐4. Left: the five test cells immersed in the RC5 bath. Right: the main components of the individual rocking cell
52
mixture of water and glycol and it is connected to a thermostat that monitors the
temperature in the bath in the range ‐10 to 60 . The base unit of RC5 is
connected to control‐PC with instilled WinRCS software. This software assists in
specifying the experiment’s conditions such as cooling/ heating process, rocking rate
and angle, and duration of the experiment. Summary of the Rocking Cell RC5
specifications are provided in Table 3‐2.
Table 3‐2. Rocking Cell RC5 specifications
Name of the apparatus Rocking Cell (RC5)
Supply company PSL Systemtechnik GmbH, Germany
Material of the cells and mixing balls Stainless steel
Size of the cells 40 ml
Diameter of mixing balls 10 mm
Maximum working pressure 200 bar
Temperature range ‐10 to 60
Rocking rate 1 to 20 / min
Rocking angle range ‐45° to +45°
3.1.3 HydraFLASH®software
Software programs are widely used in major oil and gas industries for the purpose of
predicting hydrate phase equilibrium such as MultiFlash, PVTSim, CSMGem, and
CSMHYD. For this study, HydraFlash® software (version 2.2) was used to estimate
hydrate formation P/T conditions with and without the presence of hydrate
inhibitors. Historically, HydraFlash® has been developed since 1986 and showed its
functionality as a thermodynamic model capable to predict the phase equilibrium
53
data of hydrate formation. This program has the ability to simulate different
operational conditions such as the presence of heavy hydrate formers, methanol,
and salt deposition. In addition, the thermodynamic model of this program is
equipped with several equations of state including the Valderrama‐Patel‐Teja
Equation of State (VPT EoS), Soave‐Redlich‐Kwong (SRK), cubic plus association
(CPA), and Peng‐Robinson (PR). However, for modeling fluid phase equilibria in such
systems as hydrates containing hydrogen bonds forming components (i.e. water,
methanol, and ethanol) and hydrocarbons, cubic plus association (CPA) model is
used. Moreover, the software uses the solid solution theory van der Waals and
Platteeuw (VDW‐Platteeuw EOS) for the solid‐gas phase and solid‐liquid phase
predictions. The main window of HydraFLASH® software and the required inputs to
run the hydrate liquid vapor equilibrium (HLVE) calculation are illustrated in Figure 3‐
5. All the predicted data using HydraFLASH® were based on 30% aqueous mole
fraction with is the same fraction used for the experimental runs. Example of HLVE
data and curve from for QNG‐S1 mixture is shown in Figure 3‐6 and Figure 3‐7.
54
Figure 3‐5.Main window of HydraFLASH® software showing the required input data to predict HLVE curve for QNG‐S1
Figure 3‐6. Output of HLVE calculation for QNG‐S1
55
3.1.4 Materialused
Material used for hydrates experiments are basically gas cylinders and ILs. The
details of these materials are the following:
1. Gas Cylinders used:
‐ Pure methane with 99.99% purity purchased from Buzwair.
‐ Gravimetrically prepared synthetic multi‐component gas mixture that resembles
Qatari type natural gas purchased from National Industrial Gas Plants (NIGP) with
maximum pressure of 68bar. Compositions of the gas mixture are illustrated in
mole basis (Table 3‐3).
Figure 3‐7. Predicted HLVE curve for QNG‐S1
56
Table 3‐3. Compositions of Qatar Gas Mixture (QNG‐S1)
Component Composition
Methane 0.84990
Ethane 0.05529
Propane 0.02008
Iso Butane 0.00401
N Butane 0.00585
Ison Pentane 0.00169
N Pentane 0.00147
N Octane 0.00152
Toluene 0.00090
Methyl Cyclopentane 0.00102
Nitrogen 0.03496
Carbon Dioxide 0.02331
* Relative uncertainty for samples: CH4 0.2%, C2 to C4 2.0%, C5 plus higher 5%, N2 and CO2 2%.
2‐ Three ILs were used as hydrate inhibitors in this work, all supplied by IoLiTec
(Table 3‐4). These ILs are choline based which mean they have the same cation
N,N,N‐trimethylhydroxyethylammonium, [N1112OH]+. However, each tested IL has
different anion and their combination with choline cation produces the following
ILs:
‐ (2‐Hydroxyethyl)trimethylammonium chloride. This IL is commonly named as
choline chloride (ChCl).
‐ (2‐Hydroxyethyl)trimethylammonium bis(trifluoromethylsulfonyl)imide, also
known as choline bistriflimide (Ch)(NTF2).
57
‐ (2‐Hydroxyethyl)trimethylammonium acetate, commonly known as Choline
acetate (ChOAc).
Table 3‐4. Common name, chemical formula and structure of the ILs studied in this work
IL IL
abbreviation Empirical Formula
Molecular Weight (g/mol)
Chemical Structure
Choline Chloride
ChCl C5H14ClNO 139.62
Choline Bistriflimide
(Ch)(NTF2) C7H14F6N2O5S2 384.34
Choline Acetate
ChOAc C7H17NO3 163.21
3.2 Experimentalprocedure
This section will describe the procedure of using the two apparatuses discussed
earlier, i.e. high‐pressure autoclave cell and rocking cell. Although there are some
common steps followed in preparing both cells, yet each method was clarified
separately. Equipment preparation is a critical step in hydrates experiments and
requires special attention in cleaning, cell tightening, pressurizing the cell and leak
test. The following subsections will illustrate the procedure utilized for each
experimental apparatus.
58
3.2.1 Highpressureautoclave
Isochoric pressure‐search method was implemented in all the experiments in the
present work. The initial conditions (pressure and temperature), the type of the
tested gas, and the chemical additives as hydrate inhibitors were selected depending
on the aim of each experiment. The main experimental procedure includes the
following steps:
1. Starting with cleaning the cell, all parts of the autoclave were disassembled
and rinsed with deionized water and ethanol. The cleaning part is important
to remove all the impurities from the hydrate system especially the magnet
housing (cell bottom), and the temperature and pressure sensors with
borescope‐camera connected to autoclave lid.
2. The cell was filled up to 30% of the total cell volume (i.e. 135ml) with
deionized water at room temperature. In the case of testing some chemicals
such as hydrate inhibitors, the water with the chemicals to be tested were
placed inside the cell.
3. The o‐ring sealing for top lid was greased with lubricant and the cell was
appropriately closed.
4. To remove the air content, autoclave was flushed with the gas to be tested
up to 60‐80bar which is the maximum pressure inside the available gas
cylinders. The cell is purged twice with the gas to ensure that the cell is free
of air.
59
5. The cell was charged with the testing gas up to the desired initial pressure.
However, since the gas cylinder of QNG‐ S1 has a maximum pressure of
60bar, high pressure generator (manually operated piston screw pump) was
used to compress the gas and develop higher pressure.
6. After obtaining the desired conditions inside the cell that can be read from
the autoclave software main window, the whole experimental script was
designed based on Isochoric pressure‐search method (Figure 3‐8). The script
consist of three main parts:
Maintaining the desired initial conditions (pressure and temperature),
which lie in the hydrate free zoon. The initial temperature was
adjusted at 20 for all the experiments with high stirring rate, i.e.
500RPM to provide saturated liquid‐gas mixture. The system was left
to stabilize for one hour ensuring that no leak is occurring in the
system.
The second step in the experimental script was the cooling procedure
with stirring rate of 150RPM. The hydrate system was cooled rapidly
from 20 to 2 with a cooling rate of 1.8 /h. The temperature of
the system was fixed at 2 for two days (48 hr) after onset of hydrate
was formed giving it enough time for hydrate formation and growth
inside the cell.
60
When the hydrate completely formed, the experiment was
terminated by heating step to dissociate the hydrates. The system
was heated back to the initial temperature with very slow heating
rate of 0.01 /h obtaining the hydrate equilibrium point.
Figure 3‐8. Experimental script prepared through hydrate V4 software
61
Assembling aid Jaw wrench
Opening mechanism
3.2.2 Rockingcell(RC5)
Five rocking cells were used to conduct most of the experiments in this work using
Isochoric pressure‐search method, the same method used in autoclave. As discussed
before, rocking cells can shorten the experimental duration by delivering the
required hydrate curve within 8days of running. Rocking cells were mainly used to
inspect the behavior of ILs as hydrate inhibitors. The experimental procedure
consists of the following steps:
- To kill the memory effect, the existence of some crystals from the previous
experiment, the cell must be heated up to 30 for one hour then cooled
down in one hour to 20 .
- In order to make the 5 cells ready for a new experiment, the cells were
depressurized and disconnected from the gas suppliers and temperature
sensors.
- The cells were opened using jaw wrench to disconnect the screw lid (Figure
3‐9). Then each cell and the mixing balls were washed with deionized water
and ethanol leaving them to dry for few minutes.
Figure 3‐9. Opening mechanism of the rocking cells
62
- The testing solution was prepared using deionized water according to the
required concentration of the IL (i.e. 1wt% and 5wt %). Each cell was filled
with 15ml of the aqueous solution that is approximately 30% of the cell
volume (40ml).
- The cells were closed and sealed using the same manner for opening, making
sure that each cell is closed properly to prevent any possible leak or over
tightening. After returning the cells to their places on the movable axis of
RC5, the temperature sensors and gas suppliers were connected to each cell
making them ready to be pressurized.
- To supply the test gas, it’s important to make sure that the cells are occupied
by the test gas only with no air. Thus, the same procedure used to pressurize
the autoclave, illustrated is step 4 and 5, was conducted.
- After fixing the required initial pressure for each cell, the experimental script
was designed to run the whole experiments based on isothermal cooling
method (Figure 3‐10). The RC5 script has the same main steps as autoclave in
terms of initialization step as shown in Figure 3‐11 and cooling/ heating
steps. However, the mixing method to provide agitation and the hold time
needed for hydrate growth are different than that used in autoclave.
63
Figure 3‐10. Experimental script prepared through RC5 software
Figure 3‐11. Initialization step in RC to check leak and maintain the initial conditions
64
- The holding time at constant temperature after cooling is necessary to
ensure that hydrates are completely formed. Thus, since the volume of each
cell is 40ml, the hold time used was 24 hours at 2 which is less than the
hold time used for autoclave mainly due to the smaller volume of the rocking
cells
- Mixing in RC5: the turbulence flow was provided with mixing balls which
were agitated with rocking rate of 10 rocks/ min and rocking angle of 30°
(Figure 3‐12).
To check if the experiment is running as programmed in the script, the pressure and
temperature behavior in hydrate formation dissociation process can be tracked
through the RC5 software window Figure 3‐13.
Figure 3‐12. Mixing mechanism in RC5
65
Figure 3‐13 RC5 software window showing end of the experiment
66
Chapter4: ResultsandDiscussion
This chapter aims to present the main findings of investigating the thermodynamic
behavior of gas hydrate using the two hydrates forming gases, methane and type I
Qatari natural gas (QNG‐S1). Further, the inhibition performance of the novel ILs was
tested experimentally on both hydrate formers. Besides the experimental tests,
HydraFLASH® software was used to determine the thermodynamics of hydrate
formation and provide the initial estimation of hydrate liquid vapor equilibrium
(HLVE) points. The measurements were conducted in two commercial pre‐calibrated
and fully automated hydrate cells which can stand high pressures: Gas Hydrate
Autoclave (GHA) and Rocking Cell (RC5). The work conducted in this thesis mainly
highlights the following three sections:
a. Obtaining hydrate equilibrium curves including growth/dissociation
conditions for both methane and QNG‐S1.
b. Measuring the inhibition performance of three different ILs: choline chloride
(ChCl), choline bistriflimide (Ch)(NTF2), and choline acetate (ChOAc) on both
pure methane and QNG‐S1.
c. Comparing the experimental results with the ones obtained by HydraFLASH®
simulation with and without the presence of classical thermodynamic
inhibitors and ILs.
For the best illustration of the data and results achieved in this work, this chapter
will first discuss the procedure and mechanism of obtaining hydrate equilibrium
67
curve. Subsequently, the results of the two hydrate promoters (methane and QNG‐
S1) will be discussed including the behavior of ILs and classical THIs.
4.1 Mechanismofobtaininghydrateequilibriumcurve
As mentioned earlier in the experimental procedure in chapter 3, the
thermodynamics data, i.e. hydrate liquid vapor equilibrium (HLVE) data were based
on isochoric pressure‐search method that was implemented in all the experiments in
this work. This method is usually implemented for high‐pressure hydrate systems
and provides a visual observation of hydrate formation process. The output data of
isochoric pressure‐search method is commonly demonstrated by pressure–
temperature trace or P‐T plots of hydrate formation‐dissociation loop (Figure 4‐1
illustrate P‐T plot for methane hydrate). The temperature of the high‐pressure cell is
lowered rapidly from the initial temperature (20 ) to a temperature just above the
water freezing point to ensure hydrate formation. This isochoric cooling of the
mixture of gas‐water inside the cell leads to slight decrease of the pressure (point A
to B). The formation of hydrates doesn’t occur immediately due to the meta‐stability
limit of the system; however, the onset of hydrates formation is observed at point B.
Hydrate formation is usually deducted by the remarkable pressure drop that occur
suddenly as a result of the amount of the guest gas being incorporated and trapped
in the hydrates structures. After keeping the system at constant temperature for 24
hours to provide enough time for hydrate agglomeration and growth, the system is
then heated slowly to dissociate the hydrates. The dissociation region of the loop
68
40
45
50
55
60
65
0 5 10 15 20
P (bar)
T (oC)
A
DB
C
Hyd
rate
form
ation
Hydrates start to dissociate
HLVE
must be implemented carefully with slow heating rate (about 0.12 K/h) to assure the
system to reach equilibrium accurately.
Hydrates become completely dissociated at point D which is known as the hydrate
liquid vapor equilibrium point (HLVE) providing hydrate dissociation temperature
and pressure.
It’s worth mentioning that HLVE points are critically important data in this study and
must be collected carefully after obtaining the hydrate formation‐dissociation loop.
HLVE point is basically the point of intersection of the heating (dissociation) trace
and the initial cooling trace started at the beginning of the experiment. To obtain the
exact value of HLVE point, each trace was split into segment line and fitted with a
trend‐line using a linear regression approximately equal or close to 1. Having the
algebraic equations of the two lines assist in calculating them simultaneously and
Figure 4‐1.Hydrate formation‐ dissociation loop for methane
69
y = 5.7949x + 15.653R² = 0.9869
y = 0.3337x + 77.322R² = 0.7807
74
75
76
77
78
79
80
81
82
83
10 10.5 11 11.5 12 12.5 13 13.5
P (bar)
T (oC)
30
50
70
90
110
130
0 5 10 15 20
P (bar)
T (oC)
119bar
84bar
61bar
Equilibrium Curve
find the intersection point which is HLVE point. Figure 4‐2 shows the mechanism of
obtaining the HLVE point for methane hydrate loop in Figure 4‐1.
The same procedure was followed for each hydrate formation‐dissociation loop;
producing one HLVE point for a particular experimental run with a specific initial
pressure. Consequently, to develop HLVE curve, minimum of three HLVE points with
different initial pressures are usually required (Figure 4‐3).
Figure 4‐2.Mechanism of obtaining HLVE point
Figure 4‐3. Methane Hydrate Equilibrium Curve
70
4.2 Methanehydrates
Methane gas (CH4) was used as a hydrate promoter to study the hydrate formation
conditions and the inhibition performance of the three chosen ILs. Before moving to
the multicomponent system of natural gas (QNG‐S1), it’s essential to study the
conditions of hydrates and the inhibition effect of ILs in a much simpler and single
component system (CH4). This will lead to a better understanding and judgment of
the obtained results. Methane tests were all carried out using rocking cell (RC5).
Summary of the conducted experiments using methane gas as a hydrate former is
illustrated in Table 4‐1.
Table 4‐1. Methane hydrates measurements
4.2.1 Puremethanehydrateequilibriumcurve
Following the procedure discussed in chapter 3, methane hydrates equilibrium curve
was obtained from three HLVE points at different pressures (Figure 4‐3, Table 4‐2).
Methane measurement
99.99% pure Methane
Hydrate inhibition study
Amount used Ionic liquids name
1% (wt)
ChCl
(Ch)(NTF2)
ChOAc
5% (wt)
ChCl
(Ch)(NTF2)
ChOAc
71
50
60
70
80
90
100
110
120
7 8 9 10 11 12 13 14 15
P (bar)
T (oC)
Experimental Using RC5
HydraFLASH
(Cheng et al., 2013)
(Gayet et al., 2005)
Table 4‐2. Methane hydrate equilibrium conditions
Initial pressure at 20
Equilibrium temperature (
Equilibrium pressure (bar)
% deviation from (Gayet et
al., 2005)
119 14.41 114.66 1.36
84 11.29 81.08 0.15
61 8.31 58.70 0.51
This equilibrium curve was compared with the predicted curve calculated using
HydraFLASH® software and the published data in the literature conducted through
experimental tests under high pressure isochoric conditions (Figure 4‐4) (Gayet et
al., 2005, Cheng et al., 2013). It’s clear that the data collected in this work is very
close to the ones obtained from literature. Yet, the small deviation of the equilibrium
data can be referred to the system accuracy. On the other hand, methane hydrate
equilibrium data collected from HydraFLASH® software deviate slightly from the
experimental data with less than one degree difference.
Figure 4‐4. Comparison of methane hydrate equilibrium data
72
4.2.2 Methanehydratesinhibitionbyionicliquids
A high‐pressure pre‐calibrated rocking cell (RC5) was used in this work for the
purpose of testing hydrate inhibition by ILs. As mentioned earlier, three different
types of ILs: choline chloride (ChCl), choline bistriflimide (Ch) (NTF2), and choline
acetate (ChOAc) were studied at two mass amounts of 1wt% and 5wt% with
different initial pressures (Table 4‐3 and 4‐4). The inhibition performance of 1wt% of
ILs was almost the same for the three types showing very week effect for hydrate
inhibition (Figure 4‐5). Both (Ch) (NTF2) and ChOAc show similar behavior at low
pressure (below 82bar). However at high pressure i.e. above 104 bar, ChOAc have
better inhibition than the other ILs used and below this pressure to 82 bar it has
strange behavior as hydrate promoter which lies under the pure methane curve.
Thus, between 104 and 82 bar, (Ch)(NTF2) showed inhibition performance which is
better than the other ILs. On the other hand, 1wt% of ChCl performed as hydrate
promoter; however, at higher pressure (above 110bar) it seems that the
performance will change. The reason behind the unexpected reverse performance of
ChCl as a hydrate promoter may refer to global experimental uncertainties and need
to be addressed in the future. The uncertainty of the measurements was calculated
considering the uncertainty of pressure transducer ( 0.25 bar), the temperature
sensor ( 0.1 K), and the uncertainty of ILs compositions ( 0.01). The overall
uncertainty in methane measurements was 27%.
73
50
60
70
80
90
100
110
120
7 9 11 13 15
P (bar)
T (oC)
Pure methane
1wt% (Ch)( NTF2)
1wt% ChOAc
1wt% ChCl
Table 4‐3. Experimental HLVE points for 1wt% ILs inhibition in methane hydrate system
Initial pressure at 20 (bar)
Equilibrium temperature ( )
Equilibrium pressure (bar)
1wt% ChCl
123.02 14.61 118.84
99.11 12.71 93.94
80.22 10.92 76.53
59.68 8.14 56.57
1wt% (Ch)(NTF2)
120.36 14.51 116.30
101.21 12.77 97.55
80.70 10.91 77.62
62.15 8.30 58.99
1wt% ChOAc
121.25 14.13 116.95
98.58 12.73 94.64
79.30 10.64 75.84
59.76 7.92 56.59
40.08 3.97 37.46
Figure 4‐5.Inhibition effect of 1wt% of different ILs on methane hydrates
74
50
60
70
80
90
100
110
120
7 8 9 10 11 12 13 14 15
P (bar)
T (oC)
Pure methane
5wt% (Ch)(NTF2)
5wt% ChOAc
5wt% ChCl
1
0.8
0.2
The inhibition effect of ILs on methane hydrates is clearer at higher concentration
(5wt %). All the ILs used behaved as hydrate inhibitors and showed inhibition effect
but with different performances (Figure 4‐6). It's clear that (Ch)(NTF2) has the lowest
inhibition effect with 0.2 degrees shift in methane equilibrium curve. On the other
hand, ChCl and ChOAc have close inhibition performance where the inhibition by
ChOAc is constant along the pressure variation with approximately 0.8 degrees.
Nonetheless, at high pressure i.e. above 94 bar, ChCl showed better results and
reached 1 degrees shift at 114 bar. Yet below 94bar, ChOAc performed slightly
better with 0.1 degrees shift more than inhibition reached by ChCl.
Figure 4‐6.Inhibition effect of 5wt% of different ILs on methane hydrates
75
Table 4‐4.Experimental HLVE points for 5wt% ILs inhibition in methane hydrate system
Since ILs are unpredictable by HydraFLASH® program, the obtained experimental
results were compared with some common classical THIs estimated by
HydraFLASH®. This step will allow judging on the performance of ILs as a proposed
solution for hydrate problem. The inhibition effect of the tested ILs on methane
hydrates was clear at 5wt% and since ChOAc showed the maximum inhibition, thus,
it was compared with the estimated THIs (Figure 4‐7). Methanol (MeOH) and
Ethylene Glycol (EG) are widely used as an effective THIs inhibitors. The inhibition
reached by MeOH and EG was more than ChOAc’s performance by approximately
2 and 1 , respectively.
Initial pressure at 20 (bar)
Equilibrium temperature ( )
Equilibrium pressure (bar)
5wt% ChCl
118.17 13.34 113.80
96.49 11.59 92.37
78.52 9.85 75.37
63.65 7.83 60.29
5wt% (Ch)(NTF2)
120.15 14.27 116.34
100.57 12.58 96.80
80.29 10.57 76.71
61.68 8.01 58.57
5wt% ChOAc
120.84 13.69 116.11
106.36 12.51 102.04
79.54 9.78 75.67
59.40 7.15 56.15
76
50
60
70
80
90
100
110
120
7 8 9 10 11 12 13 14 15
P (bar)
T (oC)
Pure methane
5wt% ChOAc
5wt% MeOH‐ HydraFLASH
5wt% EG‐ HydraFLASH
1
2
4.3 Naturalgasmixture(QNG‐S1)hydrates
After conducting hydrate experimental tests on pure methane and test the behavior
of the three selected ILs, the gas mixture QNG‐S1 was investigated as a second
hydrate promoter. The initial work on QNG‐S1 has been started in late 2013 using
gas hydrate autoclave cell. This hydrate cell allows visualizing the whole hydrate
process including formation and dissociation stages. Thus, according to the limitation of
predicting the behavior of QNG‐S1, the initial study of this gas mixture was started using
autoclave cell. HydraFLASH® software was also of help in predicting the hydrate
structure type which is structure (sII) for QNG‐S1. In addition, for the purpose of
studying the inhibition effect of ILs on QNG‐S1, rocking cell (RC5) was used. Table 4‐5
shows summary of all measurements conducted for QNG‐S1 using both autoclave and
rocking cell.
Figure 4‐7.Comparison between the performance of ChOAc and THIs by HydraFLASH® in methane system
77
Table 4‐5.QNG‐S1 hydrate measurements
4.3.1 QNG‐S1hydrateequilibriumcurve
For hydrate experimental tests with complex mixtures, it’s critical to check the
accuracy of the equilibrium points in order to make a clear judgment on hydrate
formation conditions. Yet, unlike the single component system, it’s challenging to
find the same compositions of a particular gas mixture reported through published
researches. Thus, QNG‐S1 was simulated using HydraFLASH® software, which
provides an estimation of the equilibrium curve for structure (sII) hydrate. Table 4‐6
shows the experimental HLVE data for QNG‐S1 collected using gas hydrate
autoclave. The comparison between the hydrate equilibrium curve obtained by the
experimental test through autoclave and HydraFLASH® prediction is illustrated in
Figure 4‐8 showing a deviation up to 2.6 from HydraFLASH® prediction. This
deviation can be related to the composition uncertainty, as stated earlier (Table 3.3),
QNG‐S1 measurements
Hydrate cell used Experiment aim
Gas Hydrate Autoclave QNG‐S1 equilibrium curve measurement
Rocking Cell (RC5)
Hydrate inhibition study
Amount used Ionic liquids name
1% (wt)
ChCl
(Ch)(NTF2)
ChOAc
5% (wt)
ChCl
(Ch)(NTF2)
ChOAc
78
40
50
60
70
80
90
100
110
10 11 12 13 14 15 16 17 18 19 20 21
P (bar)
T( oC)
QNG‐S1
HydraFlash
or the software capability to predict the equilibrium data for such complex mixture.
The overall uncertainty of QNG‐S1 measurements was 38%.
Table 4‐6.QNG‐S1 hydrate equilibrium conditions
Initial pressure at 20
Equilibrium temperature (
Equilibrium pressure (bar)
90.78 19.97 89.82
71.10 18.77 70.26
58.48 17.78 57.23
50.34 16.58 47.81
4.3.2 QNG‐S1hydratesinhibitionbyionicliquids
To study the inhibition performance of ILs on QNG‐S1, rocking cell (RC5) was also
used with the same three ILs tested on pure methane gas and the same mass
amounts (1wt% and 5wt %). Since QNG‐S1 is a complex mixture consists of different
components, the inhibition behavior of ILs deviates from the effect previously
observed on methane. In the case of 1wt% of ILs, the inhibition effect on QNG‐S1
Figure 4‐8.Autoclave and HydraFLASH® QNG‐S1 hydrate equilibrium data
79
was slightly better than methane. Each type of ILs shifts the equilibrium curve with
different degrees (Table 4‐7). In the case of ChCl, the maximum shift of QNG‐S1
equilibrium curve was 0.6 degrees from 50 to 60bar. On the other hand, the
inhibition effect of (Ch)(NTF2) reached 1 degree at 60bar. However, the maximum
performance as hydrate inhibitor for 1wt% of the tested ILs was reached by ChOAc
with 1.2 degrees shift at 55bar (Figure 4‐9).
Table 4‐7.Experimental HLVE points for 1wt% ILs inhibition in QNG‐S1 hydrate system
Initial pressure at 20 (bar)
Equilibrium temperature ( )
Equilibrium pressure (bar)
Inhibition effect ( )‐ Shifted Degrees
1wt% ChCl
79.55 19 78.88
0.3‐0.6 66.86 18 65.97
58.83 17 55.84
49.25 16 48.32
1wt% (Ch)(NTF2)
91.50 19.90 91.42
0.2‐1 75.91 18.41 75.10
67.95 17.57 67.03
54.74 16.47 53.72
1wt% ChOAc
89.02 19.85 88.43
0.6‐1.2 78.33 18.59 77.72
55.78 16.23 54.77
39.32 13.52 38.23
80
30
40
50
60
70
80
90
100
13 14 15 16 17 18 19 20 21
P (bar)
T (oC)
QNG‐S1
1wt%ChCl
1wt% (Ch)(NTF2)
1wt% ChOAc
More inhibition effect was reached by increasing the mass amount of the tested ILs
to 5wt%. One may expect to obtain the same thermodynamic behavior of ILs with
more shift of the inhibited HLVE curve to lower temperatures and higher pressures.
To prove this point, however, wide ranges of pressures must be studied to know the
exact trend of each type of ILs. For QNG‐S1 with 5wt% of ILs, the same trend as
1wt% was observed for all the experimented ILs providing more inhibition effect
(Figure 4‐10). ChCl and (Ch)(NTF2) performed better with inhibition up to 0.7‐1.5
and 0.9‐1.4 respectively (Table 4‐8). The maximum inhibition was reached by
ChOAc shifting the HLVE curve by 2 at 48bar.
Figure 4‐9.Inhibition effect of 1wt% of different ILs on QNG‐S1 hydrates
81
40
50
60
70
80
90
100
110
14 15 16 17 18 19 20 21
P (bar)
T(oC)
QNG‐S1
5wt%ChCl
5wt% (Ch)(NTF2)
5wt% ChOAc
Table 4‐8.Experimental HLVE points for 5wt% ILs inhibition in QNG‐S1 hydrate system
Initial pressure at 20 (bar)
Equilibrium temperature
( )
Equilibrium pressure (bar)
Inhibition effect( )‐ Shifted Degrees
5wt% ChCl
94.39 19.6 93.76
0.7‐1.5 78.67 18.51 77.73
60.58 16.51 59.62
5wt% (Ch)(NTF2)
103.69 20.28 103.45
0.9‐1.4 82.00 18.32 81.17
67.86 17.57 67.30
56.88 16.29 55.87
5wt% ChOAc
94.37 19.64 93.93
0.9‐2 80.35 17.66 79.23
63.11 16.58 62.15
46.28 14.12 45.048
Figure 4‐10.Inhibition effect of 5wt% of different ILs on QNG‐S1 hydrates
82
40
50
60
70
80
90
100
110
120
14 15 16 17 18 19 20 21
P (bar)
T (oC)
QNG‐S1
5wt% ChOAc5wt% MeOH‐ HydraFLASH
5wt% EG‐ HydraFLASH
2.4
The obtained results from the experimental tests were compared with the
equilibrium data for QNG‐S1 in the presence of the common THIs predicted by
HydraFLASH®. For a complex mixture such as QNG‐S1, the behavior of methanol
(MeOH) and ethylene glycol (EG) differ from the inhibition reached in the case of
pure methane. To make a clear judgment, however, ChOAc with 5wt% was also
compared with MeOH and EG data obtained from HydraFLSAH. The comparison
showed more inhibition effect performed by the classical THIs than that reached by
ChOAc with 3.4 and 2.4 maximum difference reached by MeOH and EG,
respectively (Figure 4‐11).
Figure 4‐11. Comparison between the performance of ChOAc and THIs by HydraFLASH® in QNG‐S1 system
3.4
83
1 2 6543
4.4 Gashydrateautoclavemeasurements
Gas hydrate autoclave was also used in this study for both methane and QNG‐S1
hydrates. This cell was used to visualize the whole process of hydrate formation and
dissociation via borescope‐camera. The captured pictures can be used as a
supplementary and supporting material providing the mechanism and the conditions
where gas hydrates are most likely to form. For this study, the main focus was to
visualize the inhibited hydrate system by both classical THIs and ILs. Due to time
limitations, the process of only one inhibitor was captured for each gas system
(methane and QNG‐S1). QNG‐S1 hydrate process was captured in the presence of
5wt% of methanol and the process of hydrate formation/ dissociation is illustrated in
Figure 4‐12. On the other hand, for pure methane gas, 5wt% of (Ch)(NTF2) was
tested (Figure 4‐13).
Figure 4‐12. QNG‐S1 with 5wt% methanol process. 1: Starting of the experimental run, 2‐3: Hydrates start forming, 4: Completion of hydrate formation, 5: Hydrate dissociation, 6: Hydrate completely
1 2 3 4 5 6
Figure 4‐13. Methane with 5wt% (Ch)(NTF2) process. 1: Starting of the experimental run, 2‐3: Hydrates start forming, 4: Completion of hydrate formation, 5: Hydrate dissociation, 6: Hydrate completely
84
4.5 Effectivenessofionicliquidsasapromisinghydrate
inhibitors
The industrial solutions for the hydrates problem are widely known and their
effectiveness in preventing the formation of hydrates has been extensively studied.
The classical thermodynamic inhibitors (THIs) and kinetics inhibitors (KHIs) are
constructively used by gas and oil industry. The classical THIs are industry favorites,
especially methanol (MeOH) and mono‐ethyleneglycol (MEG). Though, toxicity,
corrosively, and the high cost of the huge amounts of THIs required for hydrate
preventing are of major concerns. In addition, the toxicity of THIs requires
separation and recovery units which adds more cost for their implementation. These
serious operational problems particularly the corrosive nature may also reduce the
life time of the flow‐lines and process equipment. All these environmental and
economic concerns coincided with the classical inhibitors posed the intense need for
pursuing novel hydrate inhibitors research.
Recently, the world‐class hydrates inhibition researches are focusing on ILs and
seeking for more environmentally friendly, biodegradable green solvents with good
inhibition performance. ILs proved to have all these desired characteristics with the
ability to be designed from widespread ranges of cations and anions providing tuning
properties. Despite the better performance reached by the HydraFLASH® estimation
for MeOH and EG than the three tested ILs in this work yet, ILs showed a decent
inhibition effect.
85
ILs are still new candidates for gas hydrate inhibition and are still under
development. However, their tunable chemical and physical properties increase
their applications and might produce effective hydrate inhibitors. The myriad range
of ILs that could be designed for hydrate inhibition requires more fundamental and
systematic study for their thermodynamic and kinetic behavior. Moreover, the
molecular size and the length of the alkyl substituents chain must be taken into
consideration as it has been proved to have a remarkable effect on inhibition
performance. Each cation and anion group performs differently which make it hard
to be studies in a single research project. Thus, the selected ILs in this study were
choline based with different anions and were tested both in single and multi‐
component systems to provide a new horizon of the current knowledge on ILs.
The applicability of IL as hydrate inhibitors for Qatar natural gas mixture requires
more experimental runs. Yet, the time consuming nature and instability of the
hydrates experimental runs require molecular modeling simulation studies.
HydraFLASH® is an excellent tool for equilibrium data prediction that can be
compared with the empirical data; however it might not be applicable for complex
mixtures. Thus, it’s necessary to validate the equilibrium curves of QNG‐S1 obtained
by HydraFLASH® software. This can be achieved by optimizing or developing models
for complex mixtures such as QNG‐S1 through experimental runs for binary and
ternary systems before moving into complex mixtures.
86
Chapter5: ConclusionandFutureWork
5.1 Conclusion
This work presents the initial findings of investigating natural gas hydrate formation
characteristics of Qatari type gas in both experimental and molecular simulation
methods. Both single and multi‐component systems have been considered in this
thesis. The empirical data were collected from the two fully automated apparatuses:
gas hydrate autoclave and rocking cell. Hydrate born regions and conditions were
investigated for methane gas and QNG‐S1 mixture and hydrate equilibrium curves
were obtained with and without the presence of hydrate inhibitors. ILs were
designed and used for testing their inhibition performance on both methane and
QNG‐S1. Following are the main outputs of this thesis:
Hydrate formation/ dissociation process for methane and QNG‐S1 were studied
experimentally and their HLVE curves were compared with the HydraFLASH®
prediction. For pure methane, the predicted equilibrium data obtained by
HydraFLASH® were somehow close to that obtained from experimental runs done in
this work and collected from literature. Yet, for a complex mixture such as QNG‐S1;
the empirical data deviates by 2.6 from the estimated data. This suggests that
HydraFLASH® is a good tool for getting initial estimation of T/P conditions to start
hydrate experiments. However, its applicability for complex mixtures must be
studied and improved by enhancing mathematical models.
87
The selected ILs for this study showed inhibition effect with different performances
in each gas system and weight percentage. For methane system with 1wt% of ILs,
the inhibition effect of the three tested ILs was almost negligible. The performance
with 5wt%, however, showed a moderate inhibition effect where the maximum
inhibition reached by ChCl at higher pressures above 94bar shifting the HLVE curve
by 1 . On the other hand, the behavior of ILs diverges for QNG‐S1 system. For 1wt%
of ILs, ChCl shifted the HLVE curve up to 0.6 and 1 by (Ch)(NTF2) in the pressure
range of 50 to 60 bars. Yet, the best performance was reached by ChOAc with 1.2
shift of HLVE curve. The selected ILs achieved even better inhibition by increasing
the mass amount to 5wt% showing more shift in HLVE curve up to 0.7‐1.5 for ChCl
and 0.9‐1.4 for (Ch)(NTF2). The best inhibition was reached by ChOAc shifting the
HLVE curve by 2 .
ILs tend to not have the same effectiveness as the common classical THIs. This was
validated in the two gas systems by comparing the HLVE curve inhibited by 5wt% of
ChOAc with the HydraFLASH® estimated data for 5wt% of methanol and ethylene
glycol. For methane system, the comparison showed more inhibition effect reached
by Methanol and Ethylene Glycol with 2 and 1 , respectively, more than that
reached by ChOAc. Alternatively, for QNG‐S1 system, more inhibition effect
performed by the classical THIs than that reached by ChOAc with 3.4 and 2.4
maximum difference reached by methanol and ethylene glycol, respectively. The
overall uncertainty for each system was 27% for methane system and 38% for QNG‐
S1. Despite the moderate performance of the tested ILs, however, the purpose of
88
this work wasn’t seeking for a direct utilization of ILs in the industry and gaining
instant practical applications. Yet, the intention was to provide a new horizon of the
current available knowledge about ILs and its applicability for Qatar natural gas.
5.2 Futurework
For better understanding of IL behavior as gas hydrate inhibitors and to improve
their applicability for multi‐component systems, the following points are suggested
for the future work:
d. Study different groups of ILs with same anion to investigate the effect of
cations on hydrate formation especially for QNG‐S1 mixture.
e. Conduct molecular modeling simulation studies to develop or optimize
models for complex mixtures such as QNG‐S1 through experimental runs for
binary and ternary systems before moving into complex mixtures.
f. Investigate the ability of ILs to perform as kinetic inhibitors (KHIs) by
inspecting the induction time and memory effect.
89
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