E ect of Droplet Size on the Macroscopic Morphology of ...
Transcript of E ect of Droplet Size on the Macroscopic Morphology of ...
Effect of Droplet Size on the Macroscopic
Morphology of Methane Hydrates
Marıa Alejandra Aguirre
Olga Juliana Mesa
Department of Chemistry and Chemical Engineering
Royal Military College of Canada, Kingston
June, 2012
A thesis submitted to Universidad de Los Andes in partial fulfillment of the
requirements of the degree of Ingeniero Quımico
c©Maria A. Aguirre, Olga J. Mesa 2012
Abstract
Methane clathrate formation on water films without previous hydrate formation his-
tory was studied to assess the effect of water droplet on the macroscopic morphology
of hydrates. Two volumes of water were evaluated under a constant subcooling of 2
K, and it was found that hydrates formed from a 20 µL water droplet presented a
coarse morphology that suggests bigger hydrate grains than the ones found in the
hydrates formed from a 60 µL droplet. It was also observed growth sites counting
with several sites in the 20 µL samples, while in the other volume a single growth
site appeared on the periphery of the water droplet. A third hydrate layer growing
outside of the original water boundary was also observed in both cases, with a clear
difference in morphology between volumes. A hydrate band that separated the halo
from the interior of the hydrate was observed in both volumes, suggesting a common
mechanism that governs hydrate growth. In addition, it was established comparable
local growth rates which may indicate the same transport limitations in both cases.
Resumen
La formacion de hidratos de metano a partir de pelıculas de agua sin historia
previa fue estudiada. Dos volumenes de agua se evaluaron manteniendo constante el
∆Tsub en 2 K, encontrando que los hidratos formados a partir de las gotas de 20 µL
presentan una morfologıa rugosa lo cual sugiere granos mas grandes en relacion a los
encontrados en los hidratos de 60 µL. Adicionalmente, se observo una diferencia en
el numero de sitios de crecimiento contando con la aparicion de diversos puntos en 20
µL, mientras que en el caso de 60 µL un unico sitio de crecimiento se evidencio en la
periferia de la gota de agua. En ambos casos se observo una tercera capa de hidrato
que crecio afuera de los lımites iniciales de la gota de agua, con una clara diferencia
en la morfologıa entre los volumenes evaluados. Otra caracterıstica observada en
los dos volumenes fue una banda que separo el interior del hidrato de la capa que
se extendio por fuera del volumen inicial de agua, esta observacion sugirio que
el crecimiento del hidrato fue gobernado por el mismo mecanismo . Finalmente,
se determino una razon de crecimiento local comparable lo cual puede indicar las
mismas limitaciones en cuanto a la formacion de los hidratos.
Acknowledgements
We would like to thank our thesis advisor, Dr. Juan G. Beltran, the members of
the hydrate research group at the Royal Military College of Canada (RMCC) and
the department of Chemical Engineering at RMCC and Los Andes University. We
also want to thank our families for the company and support during the develop-
ment of our work.
Agradecemos a nuestro asesor de proyecto de grado, Dr. Juan G. Beltran, los
miembros del grupo de investigacion en hidratos en la universidad Royal Military
College of Canada (RMCC) y al departamento de ingenierıa Quımica de RMCC y
de la Universidad de los Andes. Tambien queremos agradecer a nuestras familias las
cuales sirvieron de apoyo durante el desarrollo del presente trabajo.
Contents
1 Introduction 1
2 Background 4
2.1 Clathrate Hydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1 Thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.2 Hydrate Kinetics . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1.3 Location and impact on human activities . . . . . . . . . . . 11
2.2 Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3 Experimental apparatus 26
3.1 Experimental Apparatus . . . . . . . . . . . . . . . . . . . . . . . . 26
3.2 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4 Results 29
4.1 Hydrate Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.2 Growth Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5 Discussion 38
5.0.1 Growth Rate . . . . . . . . . . . . . . . . . . . . . . . . . . 43
6 Conclusions, Recommendations and Future Work 46
6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
6.1.1 Recommendations and Future Work . . . . . . . . . . . . . . 47
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CONTENTS ii
Bibliography 52
List of Figures
2.1 The three common hydrate unit crystal structures. . . . . . . . . . 6
2.2 Schematic of a pressure vs. temperature diagram for the system methane
+water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Region of interest from the partial phase diagram of the system
methane+water. Literature data as compiled by (Sloan and Koh,
2008) was used to give an estimate of temperature and pressure. . . 8
2.4 Map showing worldwide locations of known and inferred gas-hydrate
deposits around the world. . . . . . . . . . . . . . . . . . . . . . . . 11
2.5 Primary methane hydrate film formed in static (non stirred) on a free
gas-water surface.(Makogon et al., 2000). . . . . . . . . . . . . . . . 14
2.6 Methane hydrate whiskery crystals growth with seawater (Makogon
et al., 2000). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.7 Soft gel massive methane hydrate crystals formed in water phase
(Makogon et al., 2000). . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.8 Methane hydrate covering the surface of water droplets under high
driving force, 10 minutes after nucleation. (Servio and Englezos, 2003). 16
2.9 Methane hydrate covering two water droplets under low driving force
at three different times (Servio and Englezos, 2003). . . . . . . . . . 17
2.10 Formation of a polycrystalline hydrate shell in water presaturated
with R-141b (Ohmura and Mori, 1999). . . . . . . . . . . . . . . . . 18
2.11 Sequential graphs of the growth of dentritic hydrate crystals in liquid
water presaturated with CO2. . . . . . . . . . . . . . . . . . . . . . 19
iii
LIST OF FIGURES iv
2.12 Sequencial graphs of the growth of methane-hydrate crystals into
liquid water presaturated with methane (Ohmura et al., 2005). . . . 21
2.13 Methane hydrate formation experiment from dissolved methane with-
out any stirring in the cell (Subramanian and Sloan, 2000). . . . . . 22
2.14 Water films completely covered by hydrate at low (left) and high
(right) driving force. Regions appreciated: interior of the film, pe-
riphery, and clathrate extending outside the original water boundary
(Beltran and Servio, 2010). . . . . . . . . . . . . . . . . . . . . . . . 23
2.15 Detail of the hydrate film formed at low driving force (Beltran and
Servio, 2010). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.16 Sequence of images of methane hydrate films formed at the surface of
suspended water droplets at 273.35 K and 4.86 and 6.65 MPa (Zhong
et al., 2011). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.1 Experimental apparatus. . . . . . . . . . . . . . . . . . . . . . . . . 27
4.1 Hydrate formation sequence on a water droplet (20 µL) with no pre-
vious hydrate formation history. . . . . . . . . . . . . . . . . . . . . 31
4.2 Detail of the hydrate film formed from a 20 µL water droplet with
no previous hydrate formation history. . . . . . . . . . . . . . . . . 32
4.3 Hydrate formation sequence on a water droplet (60 µL) with no pre-
vious hydrate formation history. . . . . . . . . . . . . . . . . . . . . 33
4.4 Detail of a hydrate film formed from a 60 µL water droplet with no
previous hydrate formation history. . . . . . . . . . . . . . . . . . . 34
4.5 Crystal growth progression of a hydrate formed from a 20 µL water
droplet with no previous hydrate formation history. . . . . . . . . . 36
4.6 Crystal growth progression of a hydrate formed from a 60 µL water
droplet with no previous hydrate formation history. . . . . . . . . . 37
5.1 First clathrate growth site located on the periphery of the water droplet. 39
5.2 Water films completely covered by hydrate. (a) 20 µL sample and (b)
60 µL sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
LIST OF FIGURES v
5.3 Detail of a hydrate film (interior), 20 µL (left) and 60 µL (right) . . 41
5.4 Detail of a hydrate film (boundary), 20 µL (left) and 60 µL (right). 42
5.5 Detail of a hydrate film (halo), 20 µL (left) and 60 µL (right) . . . 42
5.6 Initial growth rates of hydrates formed from 20 and 60 µL water
droplets under the range of experimental temperatures. . . . . . . . 44
5.7 Initial growth rates of hydrates formed from 20 and 60 µL water
droplets under the range experimental pressures. . . . . . . . . . . . 45
List of Tables
4.1 Experimental conditions for methane hydrate formation on 20 µL
and 60 µL water droplets without previous hydrate formation history. 29
5.1 Crystal growth rates for methane hydrate without previous hydrate
formation history. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
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Chapter 1
Introduction
Aqueous clathrate hydrates, also known as gas hydrates are nonstoichiometric,
crystalline compounds consisting of hydrogen bonded water molecules that trap
small molecules at high pressures and low temperatures (Sloan, 2003). There are
more than a hundred species which can combine with water and form these struc-
tures. Examples of gaseous guest molecules that can be enclathrated are methane,
carbon dioxide, ethane, propane, argon and krypton, among others (Makogon,
2010).
Natural hydrate formations are restricted in location because their stability de-
pends on the pressure, temperature and composition of both the gas and liquid
phases. Therefore, gas hydrates can be found in the deep-ocean and in permafrost
regions where conditions are appropiate for hydrate stabilization (Smelik and King,
1997).
For gas hydrates applications include natural gas storage and transportation,
separation of gases and water, storage material for hydrogen, a mean of cool energy
storage, and desalination of seawater (Sloan and Koh, 2008). Nevertheless, the for-
mation of hydrate blockages in the oil and gas pipelines and the key role of CO2
in the greenhouse effect, among others, are examples of potential hazards and con-
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CHAPTER 1. INTRODUCTION 2
cerns around the gas hydrates.In addition, gas hydrates represent an engineering
challenge due to the importance they have in gas-dominated systems. There is the
need to understand the initial stages of formation and growth considering variables
such as guest molecules, size of particles, temperature, and pressure, among others.
As an engineering challenge different models have been developed to simulate the
hydrate deposition in pipelines (Jassim et al., 2010). In general, there is the need to
develop models considering the size of the particles since the formation of hydrate
plugs seems to be polydispersed since different sizes of particles are formed (Jassim
et al., 2010), and as a consequence, all models and simulations should consider the
effect of different droplet volumes.
In order to take advantage of hydrate applications numerous studies have been
performed to understand the hydrate crystallization process and which factors af-
fect the morphology. In recent years, substantive progress is seen in the study of the
morphology of hydrate crystals formed at the guest water interface, which is briefly
reviewed here.
First studies on morphology reported different geometries of hydrate crystals
such as threadlike and dendritic (Makogon, 1997). Factors like supercooling, the
operation pressure and the gaseous guest can affect the geometry of gas hydrates.
Maini and Bishnoi demonstrated that hydrates grow until the bubble was com-
pletely entrapped in a hydrate layer (Maini and Bishnoi, 1981). Hydrate crystals
with dendritic morphology grew in large numbers into the liquid-water phase from
the hydrate film when ∆Tsub ≥ 3K, whereas dendritic crystals were replaced by
skeletal or polyhedral crystal when ∆Tsub ≤ 2K (Ohmura et al., 2004). Ohmura,
Sakemoto and Tanaka observed variations on the morphology of the hydrate using
methane, ethane, and propane, concluding that the size of the individual hydrate
crystal increases with decreasing the subcooling (Tanaka et al., 2009). Servio and
Englezos working with CH4 and CO2 as gaseous guests found that the type of hy-
drate guest did not have an effect on the crystal morphology (Servio and Englezos,
CHAPTER 1. INTRODUCTION 3
2003). Beltran and Servio studied the influence of the driving force, the memory
effect, the aging and the reformation on the morphology of a methane hydrate; it
was found that higher driving forces produced smaller hydrate grains and smoother
surfaces than lower driving forces in hydrates without history (Beltran and Servio,
2010).
Although several studies on morphology have been performed, the influence of
the droplet size in the morphology of a hydrate has not been defined yet. The present
study conducted crystal morphology experiments with two different droplet sizes 20
µL and 60 µL using CH4 as the forming gas in an unstirred system, and to correlate
variations in morphology with the different volumes mentioned above trying to keep
all the other variables constant. The present work is organized as follows: Chapter 2
presents general information about hydrates and previous researches about morphol-
ogy. Chapter 3 presents the experimental procedure; Chapter 4 reveals the results
obtained for hydrate morphology and growth rate are later analyzed and discussed
in Chapter 5; Finally, Chapter 6 presents the overall conclusions with some recom-
mendations for future work.
Chapter 2
Background
2.1 Clathrate Hydrates
Gas hydrates are non-stoichiometric, inorganic crystalline substances. They are
inclusion compounds which generally consist of two molecular species that arrange
themselves in space so that one (host) physically entraps the other (guest) (Engle-
zos, 1993). Gas hydrates have crystal structure and are composed of approximately
85-mol% host molecule; therefore many of the hydrate mechanical properties re-
semble those of ice. Some exceptions to these properties are yield strength, thermal
expansivity, and thermal conductivity (Sloan and Koh, 2008).
Gas hydrates are metastable compounds that can form in gas and oil pipelines,
where temperature and pressure are favorable.There are more than 100 species
which can combine with water (Englezos, 1993), but the most widely observed
guest molecules in gas mixture are methane, ethane, propane, i-butane, n-butane,
nitrogen, carbon dioxide and hydrogen sulfide. However, among those, methane hy-
drates occurs the most naturally (Sloan and Koh, 2008).
All common natural gas hydrates belong to the three crystal structures: cubic
structure I (sI), cubic structure II (sII), and hexagonal structure (sH). The spe-
4
CHAPTER 2. BACKGROUND 5
cific equilibrium hydrate structure is mainly determined by the size of the guest.
Structures I and II are the most common hydrate crystal structures and are com-
posed mainly of light hydrocarbons (Sum et al., 2009). Structure I is formed with
guest molecules having diameters between 4.2 and 6 A, such as methane, ethane,
carbon dioxide, and hydrogen sulfide. Nitrogen and small molecules including hy-
drogen (d<4.2 A) form structure II as single guests. Larger single guest molecules
(6 A<d<7 A) such as propane or iso-butane can form sII. Even larger molecules
(typically 7A<d<9 A) such as iso-pentane or neohexane can form structure H when
accompanied by smaller molecules such as methane, hydrogen sulfide, or nitrogen
(Sloan, 2003).
Hydrate structures are composed of five polyhedral cages formed by hydrogen-
bonded water molecules. Each polyhedron can be described using Jeffrey’s nomen-
clature nimi, where ni is the number of edges in face type i, and mi is the number
of faces in with ni edges. Figure 2.4 shows the different structures for gas hydrates.
Hydrate crystal structure I is composed of pentagonal dodecahedrons 512 cavities
and 14-sided cavities with 12 pentagonal faces and 2 hexagonal faces 51262. Struc-
ture II presents guests such as propane, nitrogen and iso-butane among others, and
corresponds to the hexakaidecahedron denoted 51264 based on the four hexagonal
faces, and structure H includes the irregular dodecahedron and icosahedrons called
435663 and, 51268 respectively, presenting a mixture of small and large guests such
as methane+neohexane and methane+adamantine (Sloan and Koh, 2008).
Molecules that are trapped inside the cavities of water are unbound to them so
they are free to rotate and vibrate. Hydrate cavities are prevented from collapse by
the repulsive presence of guest molecules (the cavities are not stable by themselves),
either in the cavity itself or in a large percentage of the neighboring cavities; most
of the times hydrates are formed by introducing hydrophobic gas molecules such
as methane, so the cavity expansion is mainly maintained by the guest repulsion
instead of the attraction between hydrogen bonds in water (Sloan and Koh, 2008).
CHAPTER 2. BACKGROUND 6
Figure 2.1: The three common hydrate unit crystal structures. The numbers insquares indicate the number of cage types (Beltran, 2009).
2.1.1 Thermodynamics
Hydrate phase equilibria are normally determined in terms of four variables:
pressure, temperature, water-free hydrocarbon phase composition and the free wa-
ter composition. However, phase equilibrium is mostly discussed in terms of pressure
and temperature as they are the commonly measured variables in a process (Sloan
and Koh, 2008).
A pressure temperature diagram is used to describe the phase behavior of a sys-
tem. Figure 2.2 provides a partial phase diagram for the methane+water system,
the point Q1 represents the temperature and pressure at which ice, hydrate, water
CHAPTER 2. BACKGROUND 7
and the vapour phases coexist. Quadruple point (Q1) is also the starting point for
three-phase lines:
1. Liquid water (LW ), hydrate(H) and vapour (V)
2. Ice (I), hydrate (H) and vapour (V)
3. Ice (I), liquid water (LW ) and hydrate
4. Ice (I), liquid water (LW ) and vapour (V)
Figure 2.2: Schematic of a pressure vs. temperature diagram for the systemmethane+water. I = ice; L = liquid rich in water; V = vapour; H = hydrate;Two-phase and three-phase regions are represented by areas, and lines respectively.Q1 represents the I-H-L-V quadruple point (Sloan and Koh, 2008).
CHAPTER 2. BACKGROUND 8
The pressures and temperatures of the LW -H-V and the I-H-V lines mark the
limits to hydrate formation: an upper region where hydrates are stable and a lower
region where the hydrate phase does not exist. There is also a third line (I-LW -V)
that divides the two regions mentioned before as seen in Figure 2.2.
The present study is concerned in the region of the methane+water phase di-
agram above the freezing point of water (Figure 2.3). Above the equilibrium line
(H-LW -V) two phases can coexist (liquid water and hydrate).While below the equi-
librium line only liquid water and vapor phase can coexist.
320310300290280270
400
300
200
100
0
T/K
P/Mpa
Figure 2.3: Region of interest from the partial phase diagram of the systemmethane+water. Literature data as compiled by Sloan and Koh, (2008) was usedto give an estimate of temperature and pressure.
However, conditions of temperature and pressure within the stable zone do not
CHAPTER 2. BACKGROUND 9
ensure the formation of a hydrate due to metastability (the ability of a nonequilib-
rium state to persist for a long period of time) (Sloan and Koh, 2008).
2.1.2 Hydrate Kinetics
Hydrate formation is a crystallization process and as such it can be broken down
into two steps: nucleation and crystal growth. Later, other crystallization steps may
simultaneously occur such as agglomeration (Monfort et al., 2000).
Nucleation is a stochastic process during which small clusters of water and gas
grow and disperse in an attempt to achieve critical size for continued growth (Sloan,
2003).
A delay or induction time (metastability) from the moment the system is ther-
modynamically favorable to form hydrates, to the observed crystallization time is
characteristic of clathrate formation (Sum et al., 2009). Nucleation can only be
achieved when the solution is supersaturated (when the water (solvent) contains
more dissolved gas (solute) than can be ordinarily accommodated at a tempera-
ture). Supersaturation can represent the driving force for crystallization as it indi-
cates the deviation from equilibrium mole fractions (solubility). The driving force
is the deviation from the equilibrium conditions (Sloan and Koh, 2008).
It is thought that hydrate nucleation and growth will occur within the metastable
region at the water-hydrocarbon interface (Long, 1994). This hypothesis is based
on the fact that the interface lowers the Gibbs free energy of nucleation and hydro-
carbon species concentrations are normally higher at the interface
On the molecular level the mechanism of hydrate growth is considered a combi-
nation of (1) the kinetics of crystal growth at the hydrate surface Englezos-Bishnoi
model (Sloan and Koh, 2008), (2) mass transfer of components to the growing crys-
tal surface Skovborg-Rasmussen model (Sloan and Koh, 2008), and (3) heat transfer
CHAPTER 2. BACKGROUND 10
of the exothermic heat of hydrate formation away from the growing crystal surface
(Sloan and Koh, 2008). Nevertheless, the mechanism of growth on the water droplets
is not known, but is normally assumed that water is transferred through capillaries
within the porous hydrate layer and reacts with gas that is surrounding the droplet
(Servio and Englezos, 2003).
Makogon et al., (2000) established that kinetics can be affected because of sev-
eral factors such as (Makogon et al., 2000):
1. Supercooling.
2. Overpressurization.
3. Rate of cooling.
4. Stirring rate.
5. Previous temperature history of water available for hydrate formation.
6. Presence of the sites for hydrate nucleation, such as steel walls of the reactor
or pipeline.
7. Presaturation of water with hydrate forming gas.
8. Additives.
CHAPTER 2. BACKGROUND 11
2.1.3 Location and impact on human activities
Deposits of gas hydrates have been discovered around the world near the conti-
nental slopes in oceans, and at depths below the permafrost, these deposits contain
a huge amount of potential energy for the 21st century (Zhong et al., 2011). There
are different locations of gas hydrates but it is still hard to quantify how much
methane, ethane, propane, i-butane, or n-butane is trapped in this sources, and it
has not been done for all the known locations (Sloan and Koh, 2008). Nevertheless,
there are approximations that state that the amount of methane present in these
deposits is equivalent to at least twice the amount of energy of all other fossil fuels
combined, which in terms of volume there are some suggestions near 200000 tril-
lion cubic feet (TCF) at STP, or at least two order of magnitude greater than the
quantity found in conventional sources (Makogon, 2010).
Figure 2.4: Map showing worldwide locations of known and inferred gas-hydratedeposits around the world. Yellow circles represent the recovered gas hydrates, whilered circles show the inferred gas hydrate locations.
CHAPTER 2. BACKGROUND 12
The major difficulty in considering natural gas hydrates as energy sources is
the dispersed character of the locations were they are found. Hydrates are difficult
to access considering that they are located in deep oceans and permafrost regions
making harder to recover the gas from them than that from normal gas reservoirs.
Nevertheless, it is likely that in a near future mankind will need to tap that fuel
source to meet growing energy demands (Makogon, 2010).
In addition to the energy represented as gas hydrates, there are some prob-
lems associated to the formation of them in pipelines and wells. The importance of
pipeline blockage increased in the 70’s when plugging of even the largest diameter
pipelines from offshore, arctic fields or the wells from high-pressure underground
storage facilities were reported (Sloan and Koh, 2008).
Natural gas hydrates can be dangerous compounds not only during construc-
tion stages but also during operation stages of process facilities such as platforms,
pipelines and producing gas wells before the gas has been dehydrated. The pre-
vention of hydrates requires substantial investments in inhibitors up to millions
of dollars per year for oil companies (Kvenvolden, 2006). Besides the economical
impact, there are problems related to the production and transportation of gas hy-
drates. They are associated to operation safety problems when the formation of
plugs leads to the line rupture (Austvik et al., 2000).
The sizes of the droplets that may form hydrates are important in gas-dominated
flowlines. As an engineering challenge different models have been developed to sim-
ulate the hydrate deposition in pipelines (Jassim et al., 2010). Jassim et al. (2010)
proposed the concept of the particle deposition velocity as a function of the parti-
cles size; their model presents how small particles are influenced by the main fluid
velocity and how this effect diminishes for relatively large particles. Their analysis
also suggests a certain size of particles in which any growth of it has no significative
effect on the distance deposition. In general, there is the need to develop models
CHAPTER 2. BACKGROUND 13
considering the size of the particles since the formation of hydrate plugs seems to be
polydispersed since different sizes of particles are formed (Jassim et al., 2010), and
as a consequence, all models and simulations should consider the effect of different
droplet volumes.
2.2 Morphology
Several studies have been conducted in order to provide a physical picture of the
phenomena that occur upon hydrate crystallization on the water surface (Beltran
and Servio, 2010). Servio and Englezos (2003), and later Shi et al., have observed
that unconverted water entrapped inside a hydrate shell led to a collapse of the
hydrate layer (Servio and Englezos, 2003). They studied the natural gas hydrate
formation, growth, and the variations of gas consumption (Shi et al., 2011).
According to Makogon et al., (2000), there are three types of hydrate crystals:
massive, whiskery, and gel-like crystals. Under certain conditions all three types of
hydrate crystals can form and coexist (Makogon et al., 2000).
Massive crystals grow due to the adsorption of gas and water on the crystal
surface that is being formed. Although crystals may grow in the gas phase, they
are more likely to form on the crystal surface (Figure 2.5). Porosity of the massive
crystals can be up to 80-90%, depending on growth conditions. Whiskery crystals
(Figure 2.6) can grow because of the adsorption of gas and water. These crystals
are the strongest, have the highest density, and dissociate after the increment of
temperature and once all the other crystals are dissociated. Massive or gel crystals
do not grow on the surface of a whiskery crystal. The gel-like crystals (Figure 2.7)
normally grow in the liquid water phase during a pressure or temperature drop and
once a small amount of gas is dissolved in the water. Gel crystals are very soft and
their porosity is near 95-98% (Makogon et al., 2000).
CHAPTER 2. BACKGROUND 14
Figure 2.5: Primary methane hydrate film formed in static (non stirred) on a freegas-water surface. p= 56 bar and T= 279.4 K. (Makogon et al., 2000)
The morphology of methane and carbon dioxide hydrates formed from water
droplets, was studied by Servio and Englezos (2003) (Servio and Englezos, 2003).
The droplets were placed on a 316 stainless-steel cylinder covered with a layer of
Teflon and each experiment was performed with two droplets 5 mm and 2.5 mm in
diameter or three droplets with a diameter of 2.5 mm. It was observed that within
less than 5 s after the first growth evidence, the surface of the droplet quickly be-
came jagged and exhibited many fine needle-like crystals extruding away from the
gas-hydrate-water interface. That behavior was observed for all cases working un-
der high driving force, independent of the hydrate forming gas. Servio and Englezos
(2003) established that the thickness and length of the hydrate needles extruding
from the surface is related to the size of the droplet.
Servio and Englezos, (2003) found that independent of the driving force all the
droplets nucleated at the same time, although hydrate formation under high driv-
CHAPTER 2. BACKGROUND 15
Figure 2.6: Methane hydrate whiskery crystals growth with seawater. p= 81 barand T= 274 K (Makogon et al., 2000)
Figure 2.7: Soft gel massive methane hydrate crystals formed in water phase. p=93 bar and T= 280.6 K (Makogon et al., 2000).
ing force was observed to evolve in three phases that are divided as follows: (1) a
hydrate layer appeared around the water droplet along with the needle-like crystals
CHAPTER 2. BACKGROUND 16
which grew after time in size and thickness, (2) the crystal needles collapsed onto
the hydrate layer covering the water droplet, and (3) were depressions appeared in
the hydrate layer surrounding the water droplet. The authors suggested that the
collapse of the hydrate layer surrounding the droplets shows evidence that the water
is still being converted to hydrate. This can be appreciated in Figure 2.9.
(d)
(a) (b) (c)
Figure 2.8: Methane hydrate covering the surface of water droplets under high driv-ing force, 10 minutes after nucleation (Servio and Englezos, 2003).
On the other hand, for the experiments performed under low-pressure Servio and
Englezos (2003) observed the lack of any hydrate needles from the hydrate-covered
CHAPTER 2. BACKGROUND 17
layer. The texture was smooth and shiny which is opposite to the results obtained
for high driving force, in which the surface was rough and dull. This can be related
to the rate of growth which increases with the degree of supersaturation (Smelik
and King, 1997).
(b)
(a)
(c)
Figure 2.9: Methane hydrate covering two water droplets under low driving forceat three different times. (a) Water droplets at the beginning of the experiment,(b) hydrate covered the initial water droplet 10 h after the experiment began, (c)hydrate covered the initial water droplet 25 h after the experiment began (Servioand Englezos, 2003)
CHAPTER 2. BACKGROUND 18
Ohmura, (1999) and Uchida, (1999) also observed the appearance of crystals
growing radially around the hydrate covered surface (Uchida et al., 1999) . Ohmura
et al., (1999) described these crystals as platelike and standing upright on the outer
surface of the drop-enclosing hydrate shell formed as it is seen in Figure 2.10. These
crystals were not observed under low subcooling conditions of approximately 2 K
(Ohmura and Mori, 1999).
(a) 10 sec (b) 30 sec (c) 1 min
(d) 11 min (e) 45 min (f) 180 min
Figure 2.10: Formation of a polycrystalline hydrate shell in water presaturated withR-141b, (a)-(c), and subsequent formation of single-crystal plates growing into waterphase from the hydrate-shell surface, (d)-(f).Indicated below each picture is the lapseof time after artificial hydrate nucleation on the drop surface. (Ohmura and Mori,1999).
Visual observations of the variations in macroscopic morphology of hydrate crys-
tals growing in liquid water saturated with CO2 has been reported (Ohmura et al.,
2004). Water droplets (approximately 4 cm3) were poured into the test cell to form
a pool in the lower portion of the inner space of the test cell. After 10 hours the hy-
drates crystals were dissociated in order to ensure the presaturation of liquid water
with CO2 and hence, shorten the induction time for hydrate re-formation creating
a memory of the prior hydrate formation. The morphology of individual crystals is
CHAPTER 2. BACKGROUND 19
presumably feather-like or dendritic shape. As the subcooling is reduced, the authors
recognized dendritic crystals growing downward into liquid water which resulted in
thicker and less densely packed dendrites at the lowest subcoolings (Ohmura et al.,
2004).
(a) 15 s (b) 22 s
(c) 30 s (d) 55 s
Figure 2.11: Sequential graphs of the growth of dentritic hydrate crystals in liquidwater presaturated with CO2. p= 3.4 MPa, T=277.6 K, and ∆Tsub= 3.6 K. Thetime lapse after the formation of a hydrate film covering the CO2-water interfaceis indicated below each graph. Some of the dendritic crystals detached from thehydrate film are falling in liquid water in figures (b) and (d). (Ohmura et al., 2004).
Ohmura et al., (2005) showed distinct variations in the morphology of hydrate
crystals that grew in liquid water depending on the pressure. At pressures between
CHAPTER 2. BACKGROUND 20
6 and 8 MPa, hydrate crystals presented skeletal and columnar morphology, while
at the pressure of 10 MPa, hydrates showed dendritic crystals. Hydrate crystals
seem to appear first at the inner surface of the test cell in contact with liquid water
instead of the methane-water interface. Those crystals floated up to the methane-
water interface, where they became a polycrystalline hydrate film, and continued to
grow in the liquid phase (Ohmura et al., 2005).
The crystal growth of methane hydrate presented by Ohmura et al., (2005) was
believe to depend on a mechanism of mass transfer of dissolved methane to the
hydrate-crystal surfaces in contact with liquid water presaturated with methane.
They concluded that hydrate crystals first form a thin polycrystalline layer between
methane and water, and then hydrate crystals grew into the liquid-water phase from
the hydrate film.
Sugaya and Mori, (1996) found that the surface morphology of the hydrate layer
formed at the interface depends strongly on the degree of saturation of the water
with the guest component (Sugaya and Mori, 1996), while Ohmura et al., (1999) es-
tablished that as a general trend, it was observed that subcoolings ≥ 3 K produced
sword-like crystals while at smaller subcooling there was evidence of polygonal faces
and bigger size of latters (Ohmura and Mori, 1999) .
In the same way, visual observations were made by Subramanian et al., (2000)
during methane hydrate formation from dissolved methane in non stirred systems.
They show that hydrates first form as a film at the vapor-liquid interface. After
that, further hydrate growth occurred as fine needles that extended into the bulk
aqueous phase. New methane guest molecules supplied either by the vapor or the
aqueous phase, were needed in order to keep the growth into the bulk liquid. The
authors suggested it may occur by the slow diffusion of methane through open mi-
croscopic cracks in the hydrated V-L interface (Subramanian and Sloan, 2000).
There are two possible sources of the methane molecules that end up being en-
CHAPTER 2. BACKGROUND 21
(a) 8 min (b) 10 min
(c) 37 min (d) 300 min
Figure 2.12: Sequencial graphs of the growth of methane-hydrate crystals into liquidwater presaturated with methane. T= 273.7 K and p= 8.2 MPa. The time lapseafter the hydrate nucleation at the methane-water interface is indicated below eachgraph (Ohmura et al., 2005).
clathrated in the growing needles: (1) vapor phase by diffusing through the cracks in
the film to dissolve in the aqueous phase, (2) from the aqueous phase where it is al-
ready dissolved. Nevertheless, in the aqueous phase, water molecules tend to cluster
around the methane molecule in order to maximize the hydrogen bonding around
the hydrophobic solute methane. Figure 2.13 shows the hydrate needles growing
into the bulk aqueous phase.
Tanaka et al., (2009) reported the visual observations of the formation and
CHAPTER 2. BACKGROUND 22
0.1 cm
Figure 2.13: Methane hydrate formation experiment from dissolved methane withoutany stirring in the cell. There is evidence of the hydrated V-L interface and thehydrate needles growing into the bulk aqueous phae. The point at which the laseris focused is indicated by a star (Subramanian and Sloan, 2000).
growth of clathrate hydrate crystals on the surface of a water droplet exposed to
gaseous methane, ethane or propane (Tanaka et al., 2009). The growth first occurred
at a random point on the water droplet and then grew to form a polycrystalline
layer covering the surface. They observed the individual crystals that constitute the
polycrystalline hydrate layer and classified the morphology of the hydrate crystals
depending on the system subcooling (∆Tsub). They concluded as a general trend
that at ∆Tsub ≥ 3.0 K, the shape of the hydrate crystals is typically swordlike or
triangular, whereas at ∆Tsub from 2.0 to 3.0 K the shape changes to a polygon. It
was concluded that crystal morphology for methane, ethane or propane gas can be
classified using the ∆Tsub as the common criterion (Tanaka et al., 2009).
Beltran and Servio, (2010) studied the morphology of methane-hydrate films
formed on a glass surface and reported the growth of a hydrate layer outside the
original water boundary (Beltran and Servio, 2010). It was shown two different
morphologies on the water films covered with hydrate, one within the film and the
CHAPTER 2. BACKGROUND 23
other on the periphery of it. Differences in methane hydrate morphology were also
found to be dependant on driving force; The high driving force nucleation showed
a smooth surface while a striated pattern was observed at low driving force. It was
found for low driving force that the annulus part was smoother and narrower than
at high driving force, but as it was said, the contrary occurred for the inside of the
hydrate film where coarse grains were observed at low driving force and finer grains
at higher driving force (Figure 2.14). The authors identified hydrate history as a key
factor to determine the macroscopic hydrate growth and morphology in hydrates
growing on a water droplet deposited on a glass surface, and they concluded that
hydrate nucleation occurred on the periphery of a film first, followed by the nucle-
ation within the water film, and that the crystals formed had a completely different
morphology than the ones formed on the water edge.
Figure 2.14: Water films completely covered by hydrate at low (left) and high (right)driving force. Three different regions are appreciable: interior of the film, peripheryof it, and clathrate extending outside the original water boundary. (a) Nucleationand growth occurred at T= 275 K, p= 3.6 MPa. (b) T= 274 K, p= 8.2 MPa(Beltran and Servio, 2010).
Saito et al., (2010) reported detailed observations of the morphology of indi-
vidual hydrate crystals on the surface of a water droplet with methane, ethane, or
CHAPTER 2. BACKGROUND 24
Figure 2.15: Detail of the hydrate film formed at low driving force. Light gray:hydrate formed in the periphery. Darker gray: hydrate formed within the waterfilm. Hydrate formed at T= 275 K and p= 3.6 MPa (Beltran and Servio, 2010).
propane, as guest gases (Saito et al., 2010). They observed that the nucleation of
the hydrate first occurred at a random point on the water droplet and then floated
up to the apex of it along the surface. The hydrate grew down to form a polycrys-
talline layer covering the surface of the water droplet. The authors also indicate
that the hydrate crystal morphology has a significant dependence on the system
subcooling, reporting that the size of the individual hydrate crystals decreased with
the increasing ∆Tsub irrespective of the guest substances. According to their results,
pressure difference had no significant effect on the hydrate crystal morphology. The
authors established that hydrates presented a rougher surface at smaller ∆Tsub, and
that the time required for the complete coverage of the water-droplet surface by the
hydrate layer depended significantly on ∆Tsub, indicating that the lateral growth
rate of the hydrate film propagation increased with the increasing ∆Tsub.
Finally Zhong et al., (2011) observed the morphology of hydrate films formed at
the droplet surface and showed that the growth rate of methane hydrate is signifi-
cantly increased as the supersaturation is increased and the droplet size is reduced
CHAPTER 2. BACKGROUND 25
(Zhong et al., 2011).
a b c d
e f g h
i j k l
273.35 K,
4.86 MPa,
r0= 2.0 mm
273.35 K,
6.08 MPa,
r0= 2.0 mm
273.35 K,
6.65 MPa,
r0= 2.0 mm
Figure 2.16: Sequence of images of methane hydrate films formed at the surface ofsuspended water droplets at 273.35 K and 4.86 and 6.65 MPa. The time lapse afterthe hydrate formation is indicated below the pictures. (a) 0 s, (b) 10 s, (c) 30 s, (d)10 min, (e) 0 s, (f) 10 s, (g) 30 s, (h) 10 min, (i) 0 s, (j) 10 s, (k) 30 s, (l) 10 min(Zhong et al., 2011).
The sizes of the droplets that may form hydrates are important to simulate
the hydrate deposition in pipelines. In general, there is the need to develop models
considering the size of the particles since the formation of hydrate plugs seems to be
polydispersed since different sizes of particles are formed (Jassim et al., 2010), and
as a consequence, all models and simulations should consider the effect of different
droplet volumes. Jassim et al. (2010) proposed the concept of the particle deposition
velocity as a function of the particles size (Jassim et al., 2010).
Chapter 3
Experimental apparatus
3.1 Experimental Apparatus
Figure 3.1 shows a schematic of the experimental apparatus. Crystallization oc-
cured inside a 316 stainless steel cell, fitted with two sapphire windows on the top
and bottom. A Neslab RTE740, laboratory chiller provided the necessary cooling by
recirculating a mixture of ethylene glycol and water (50/50, V/V) through a cop-
per coil fitted around the pressure vessel’s body. The cell had several ports used as
follows: to feed gas, to purge gas out of the cell, to insert a thermocouple, and to con-
nect the interior of the vessel to a pressure transducer. Temperature was measured
with a type K mini thermocouple probe (±1 K) (Omega Engineering, QC, Canada).
Pressure was monitored with a Rosemount 3051S pressure transducer (Laurentide
Controls, QC, Canada) with an accuracy of ± 0.025% of the span.
A PCO.2000 camera (Optikon Corporation, ON, Canada) recorded high resolu-
tion images of the crystallization process through the top sapphire window. A Schott
KL 2500 (Optikon Corporation, On, Canada) cold light source fitted with an artic-
ulated light pipe was used to light the cell through the bottom sapphire window.
The video camera, the temperature signal and the pressure signal were connected
to a personal computer in order to acquire and analyze the data. Deionized water
26
CHAPTER 3. EXPERIMENTAL APPARATUS 27
Figure 3.1: Experimental apparatus
and methane gas, 99.99% purity (Air liquide, ON, Canada), were used.
3.2 Procedure
A precleaned, microscope, glass slide was cut to fit inside the high pressure cell
(Figure 3.1), and a water droplet was deposited on the slide. Experiments were per-
formed with 20 µL and 60 µL water droplets. Methane was fed to the reactor and
purged several times to minimize the presence of air inside the pressure vessel.
CHAPTER 3. EXPERIMENTAL APPARATUS 28
After purging, the pressure in the reactor was adjusted in order to maintain
a constant driving force (∆Tsub)∗ of approximately 2 K. A hydrate formation ex-
periment was terminated when it became apparent that no liquid water was left
in the microscope slide. After reducing the pressure, the remaining water droplet
was discarded, and a fresh water droplet was used for the next experiment. Three
replicates were performed for each droplet size adjusting the pressure.
∗. ∆Tsub (subcooling) is the difference between the system temperature and the equilibriumtemperature (on the hydrate phase boundary) at the system pressure. Subcooling can be consideredto represent the driving force for hydrate formation Tanaka et al. (2009).
Chapter 4
Results
4.1 Hydrate Morphology
The conditions for methane hydrate formation on 20 µL and 60 µL water droplets
are summarized in Table 4.1.
Table 4.1: Experimental conditions for methane hydrate formation on 20 µL and 60µL water droplets without previous hydrate formation history.
Experiment Droplet size/µL T/K p/MPa Tequilibrium/K a ∆Tsub/K1 20 275 3.8 277 2.02 20 275 4.0 277 2.13 20 276 5.1 279 2.14 60 280 6.6 282 2.05 60 278 5.6 280 2.36 60 280 6.0 281 1.7
a. From data compiled by Sloan and Koh, (2008).
Figure 4.1 presents a sequence of frames of hydrate formation and growth (T=
275 K, p= 4 MPa) on a 20 µL water film without previous hydrate formation his-
tory. Figure 4.1.(a) shows the intact water surface. The first growth site appeared
29
CHAPTER 4. RESULTS 30
on the periphery of the water film followed by several other growth sites (Figure
4.1.(b)). Hydrate propagated from each growth site until the newly formed crystals
covered the entire droplet surface in less than 20 s (Figure 4.1.(c,d)). Following the
complete coverage of the water surface by the hydrate, a thin hydrate film (halo)
extended beyond the original water boundary (Figure 4.1.(e,f)).
Close-up views of the hydrate formed from a 20 µL water droplet are shown in
Figure 4.2. A black strip, approximately 0.06 mm in thickness, separated the hy-
drate formed in the interior of the water droplet from the clathrate that extended
beyond the original water boundary (Figure 4.2.(b,c)). Both interior and exterior
hydrate exhibited a granular texture; however, the halo –or exterior hydrate– ap-
peared lighter and rougher (Figure 4.2.(d)) than the hydrate formed in the interior
(Figure 4.2.(a)).
Figure 4.3 shows the hydrate formed at T= 280 K and p= 6.6 MPa from a 60
µL water droplet. The intact water surface is shown in Figure 4.3.(a). A clathrate
growth site appeared on the periphery of the water film (Figure 4.3.(b)) and grew
into a polycrystalline layer that covered the water surface. After the appearance
of the first growth site, no new growth sites were observed (Figure 4.3.(c)). Figure
4.3.(d) shows the complete coverage of the water surface after 29 s. The hydrate ex-
tended outside the water boundary and covered the complete glass surface (Figure
4.3.(e,f)).
CHAPTER 4. RESULTS 31
2 m
m
(a)
(b)
(c)
(d)
(e)
(f)
Fig
ure
4.1:
Hydra
tefo
rmat
ion
sequen
ceon
aw
ater
dro
ple
t(2
0µ
L)
wit
hno
pre
vio
us
hydra
tefo
rmat
ion
his
tory
.T
=27
5K
,p=
4M
Pa,
∆Tsu
b=
2K
.(a
)W
ater
film
bef
ore
hydra
tefo
rmat
ion.
(b)t=
0s,
Hydra
tefo
rmat
ion
isob
serv
edat
the
upp
ersi
de
ofth
edro
ple
t,fo
llow
edby
the
app
eare
nce
ofnew
grow
thsi
tes,
(c)t=
4s,
the
spot
ted
hydra
teco
nti
nues
togr
ow.
(d)t=
18s,
the
wat
erfilm
isco
vere
dco
mple
tely
,(e
)ap
pea
rence
ofa
hydra
tela
yer
outs
ide
ofth
eor
igin
alw
ater
bou
ndar
y(h
alo)
,an
d(f
)t=
30s,
hal
oex
tends
onth
egl
ass
surf
ace.
CHAPTER 4. RESULTS 32
100 µm
(a) (b)
(c) (d)
Figure 4.2: Detail of the hydrate film formed from a 20 µL water droplet withno previous hydrate formation history and a ∆Tsub= 2 K. Hydrate formed at T=275 K, p= 4.0 MPa. (a) Hydrate in the interior of the water droplet, (b) and (c)boundary of the hydrate. (d) Halo
Figure 4.4 presents a detailed view of the different regions of the hydrate formed
from a 60 µL water droplet. A clearly distinct hydrate band separated the halo from
the hydrate in the interior of the film (Figure 4.4.(b,c)). The halo appeared smooth
and shiny (Figure 4.4.(d)) compared to the interior where dark spots were observed
(Figure 4.4.(a)).
CHAPTER 4. RESULTS 33
3 m
m
(a)
(b)
(c)
(d)
(e)
(f)
Fig
ure
4.3:
Hydra
tefo
rmat
ion
sequen
ceon
aw
ater
dro
ple
t(6
0µ
L)
wit
hno
pre
vio
us
hydra
tefo
rmat
ion
his
tory
.T
=28
0K
,p
=6.
6M
Pa,
∆Tsu
b=
2K
.(a
)W
ater
film
bef
ore
hydra
tefo
rmat
ion.
(b)t=
0s,
firs
tev
iden
ceof
hydra
tefo
rmat
ion
occ
urs
atth
eupp
erp
erip
her
yof
the
dro
ple
tan
dth
ehydra
tefilm
floa
tsto
the
cente
rof
the
film
.(c
)t=
18s,
grow
thof
the
hydra
tein
the
mid
dle
ofth
efilm
.(d
)t=
29s,
the
wat
erfilm
isco
mple
tely
cove
red.
(e)t=
40s,
hydra
teex
tends
outs
ide
ofth
eor
igin
alw
ater
bou
ndar
y.(f
)t=
300
s,hydra
teou
tsid
eth
eor
igin
alw
ater
bou
ndar
yco
vers
the
glas
ssl
ide
CHAPTER 4. RESULTS 34
100 µm (a) (b)
(c) (d)
Figure 4.4: Detail of a hydrate film formed from a 60 µL water droplet with noprevious hydrate formation history. Hydrate formed at T= 280 K, p= 6.6 MPa,∆Tsub= 2 K. (a) Hydrate in the interior of the water droplet, (b) and (c) boundaryof the hydrate, (d) halo
The first hydrate growth site appeared at the edge of the water film for both
the 20 µL and 60 µL samples (Figure 4.3.(b) and Figure 4.1.(b)). The 60 µL sam-
ples developed from this first growth site only (Figure 4.3); in contrast, the 20 µL
samples propagated from several new growth sites (Figure 4.1). Growth of a thin
hydrate layer outside of the original boundary was observed both for 20 µL (Figures
4.1.(e) and 4.1.(f), and Figures 4.2.(c) and 4.2.(d)) and 60 µL (Figures 4.3.(e) and
CHAPTER 4. RESULTS 35
4.3.(f), and Figures 4.4.(c) and 4.4.(d)) droplets, after complete hydrate coverage
of the water surface. This process was significantly slower than the initial growth
inside the original water droplet.
After complete coverage of the water film the hydrate on the 60 µL samples
seemed to cave into the water droplet (Figure 4.3.(f)). This collapse of the hydrate
layer was not observed on the 20 µL samples (Figure 4.1.(f)).
The 20 µL samples presented coarse grains and a dark surface (Figure 4.2). The
contrary seemed to be true for the surface of the 60 µL samples which exhibited a
lighter and smoother surface (Figure 4.4). Wavy patterns characterized the halo of
the 60 µL samples (Figure 4.4.(d)), whereas the same layered structure appeared to
be masked by the rough surface of the halo in the 20 µL samples (Figure 4.2.(d)).
4.2 Growth Rate
Hydrate growth rates for 20 µL and 60 µL samples were calculated by tracking
the position of the crystal interface as a function of time. Crystal growth rates var-
ied between samples and appeared to slow down after 10 s (Figures 4.5 and 4.6).
Initial growth rates seemed to be fairly linear throughout the samples, and a least
squares fit was used to compare growth rates for the first 8 s of growth (Table 5.1).
On average, the 20 µL samples were covered with hydrate in 23 s while clathrates
covered the 60 µL samples in 29 s.
CHAPTER 4. RESULTS 36
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20 25
Gro
wth
/cm
t/s
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20 25
Growth
/cm
t/s
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20 25
Growth
/cm
t/s
(a)
(b)
(c)
Figure 4.5: Crystal growth progression of a hydrate formed from a 20 µL waterdroplet with no previous hydrate formation history. ∆Tsub= 2 K. (a) Experiment 1,T= 275 K, p= 3.8 MPa. (b) Experiment 2, T= 275 K, p= 4.0 MPa. (c) Experiment3, T= 276 K, p= 5.1 MPa
CHAPTER 4. RESULTS 37
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20 25
Gro
wth
/cm
t/s
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20 25
Gro
wth
/cm
t/s
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20 25
Gro
wth
/cm
t/s
(a)
(b)
(c)
Figure 4.6: Crystal growth progression of a hydrate formed from a 60 µL waterdroplet with no previous hydrate formation history. ∆Tsub= 2 K. (a) Experiment 4,T= 280 K, p= 6.6 MPa. (b) Experiment 5, T= 278 K, p= 5.6 MPa. (c) Experiment6, T= 280 K, p= 6.0 MPa
Table 5.1: Crystal growth rates for methane hydrate without previous hydrate for-mation history.
20 µL 60 µLExperiment p/MPa Growth rate (mm s−1) Experiment p/MPa Growth rate (mm s−1)
1 3.8 0.14 5 5.6 0.342 4.0 0.30 6 6.0 0.223 5.1 0.22 4 6.6 0.15
Chapter 5
Discussion
Hydrate growth sites first appeared on the periphery of the droplet for both
the 20 µL and 60 µL samples (Figure 5.1.(a,b)). This observation agrees with the
location described by Beltran and Servio, (2010) for the first hydrate growth site.
However, the macroscopic morphology that we observed (Figure 4.1 and Figure 4.3)
did not exhibit the dual morphology observed by Beltran and Servio, (2010), nei-
ther was our morphology characterized by dendritic behavior. Our observation of a
specific location for the initial growth site contrasts with Tanaka et al., (2009) work
where initial growth sites were observed to appear at arbitrary locations.
Despite the difference between our low magnification results (Figure 4.1 and
Figure 4.3) and those of Servio’s group (2010), our detailed views (Figure 4.2 and
38
CHAPTER 5. DISCUSSION 39
2 mm (a) (b)
Figure 5.1: First clathrate growth site located on the periphery of the water droplet.∆Tsub= 2 K. (a) 20 µL sample, T= 275 K, p= 4.0 MPa. (b) 60 µL sample, T= 280K, p= 6.6 MPa.
Figure 4.4) revealed a close resemblance to those obtained at high magnification
by Beltran and Servio, (2010). In both our work (Figure 4.2 and Figure 4.4) and
Beltran and Servio’s (2010), a hydrate band separated the interior of the original
water droplet from the hydrate that grew outside of the original water boundary
(halo). We believe that this similitude is an indication of the common mechanism
that governs hydrate growth.
Our 20 µL samples exhibited several growth sites (Figure 4.1.(b) and 4.1.(c))
similar to the numerous growth sites observed by Beltran and Servio, (2010) for 35
CHAPTER 5. DISCUSSION 40
µL water droplets. Only one growth site was observed for our 60 µL samples (Figure
4.3.(b) and 4.3.(c)), which agrees with the observation of Tanaka et al., (2009) for
similar water droplet volumes.
Figure 5.2 compares the hydrate formed from a 20 µL water droplet to the
hydrate formed from a 60 µL droplet under the same magnification and lighting
conditions. The hydrate from the 60 µL sample appeared light gray (Figure 5.2.(b))
while the hydrate on the 20 µL sample appeared dark (Figure 5.2.(a)). Light passed
easily through the 60 µL sample, which may indicate that the hydrate layer in the
60 µL was thinner than in the 20 µL sample.
The higher light intensity transmitted through the 60 µL sample could also be
due to the rougher surface in the hydrate formed from a 20 µL water droplet (Figure
5.3). A plausible explanation for the difference in surface roughness is the number
of initial growth sites: single site in the 60 µL samples (Figure 4.3) and multiple
sites in 20 µL samples (Figure 4.1). Many crystal growth centers are known to favor
a rough surface (Servio and Englezos, 2003).
Figure 5.2.(b) shows what seemed to be a depression on the hydrate surface. Shi
et al., (2011) considered the collapse of the hydrate layer as evidence that uncon-
verted water trapped inside the hydrate shell (Shi et al., 2011).
At low magnifications we observed different hydrate morphologies in the inte-
rior of our 20 µL samples (Figure 5.2.(a)) and our 60 µL samples (Figure 5.2.(b)).
Figure 5.3 –obtained at higher magnification– revealed that this difference is due
to grain size. Furthermore, both the 60 µL and 20 µL samples presented the same
overall pattern (Figure 5.4): a hydrate band separating a granular interior from the
hydrate that grew outside of the original water boundary.
A undulating pattern was clearly observed throughout the halo of the 60 µL
CHAPTER 5. DISCUSSION 41
2 mm (a) (b)
Figure 5.2: Water films completely covered by hydrate. ∆Tsub= 2 K. (a) 20 µLsample, T= 275 K, p= 4.0 MPa. (b) 60 µL sample, T= 280 K, p= 6.6 MPa.
80 µm (a) (b)
Figure 5.3: Detail of the interior of a hydrate film. ∆Tsub= 2 K. (a) 20 µL sample,T= 275 K, p= 4.0 MPa. (b) 60 µL sample, T= 280 K, p= 6.6 MPa.
samples (Figure 5.5.(b)) while the waves were more difficult to recognize in the 20
CHAPTER 5. DISCUSSION 42
100 µm
(a) (b)
Figure 5.4: Detail of a hydrate film, 20 µL (left) and 60 µL (right). ∆Tsub= 2 K. (a)20 µL sample, T= 275 K, p= 4.0 MPa. (b) 60 µL sample, T= 280 K, p= 6.6 MPa.
(a) (b)
100 µm
Figure 5.5: Detail of a hydrate film, 20 µL (left) and 60 µL (right). ∆Tsub= 2 K. (a)20 µL sample, T= 275 K, p= 4.0 MPa. (b) 60 µL sample, T= 280 K, p= 6.6 MPa.
CHAPTER 5. DISCUSSION 43
µL samples (Figure 5.5.(a)). A plausible explanation for this wavy pattern is the
movement of water underneath the hydrate layer, drawn by capillarity toward the
bare glass (Beltran and Servio, 2010). The smooth appearance of the halo from the
60 µL samples compared to the 20 µL samples could be explained by the availability
of unconverted water beneath the hydrate layer covering the 60 µL water droplet.
This hypothesis seems to be confirmed by the fact that the halo from the 60 µL
droplet extended over a much wider area than the halo of the 20 µL droplets (Figure
5.2).
5.0.1 Growth Rate
Figures 4.5 and 4.6 showed how hydrate growth slowed down as time progressed.
This is expected, as the clathrate layer reduces the water-gas interface area and in-
creases resistance for the diffusive transfer of methane (Skovborg and Rasmussen,
1994).
Clathrate initial growth rates were measured at pressures from 3.8 to 5.1 MPa at
an average temperature of 275 K for 20 µL droplets and from 5.6 to 6.6 MPa at an
average temperature of 279 K for 60 µL droplets (Table 4.1 and 5.1). Although sub-
cooling was kept constant, varying experimental pressures and temperatures could
have affected the hydrate growth rates (Makogon et al., 2000). Our results show
that, at constant subcooling, initial growth rates were comparable for both droplet
sizes, irrespective of the experimental temperature and pressure (Figures 5.7 and
5.6)
The coarse texture of the hydrate formed from a 20 µL water droplet could
suggest faster growth rates (Servio and Englezos, 2003) than the ones observed on
our 60 µL samples 5.3. However, the observed, initial growth rates for both the 20
†. The coefficient of variation (CV) quantifies scatter points. It is defined as the standarddeviation of a group of samples divided by their mean (?).
CHAPTER 5. DISCUSSION 44
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
274 276 278 280 282
Gro
wth
ra
te/m
ms-1
T/K
20 µL samples
60 µL samples
Figure 5.6: Initial growth rates of hydrates formed from 20 and 60 µL water dropletsunder the range of experimental temperatures. ∆Tsub= 2 K. The CV † for 20 µLsamples was 0.36 while the 60 µL samples presented a value of 0.29. Blue diamondsrepresent data for the 20 µL samples, while red squares are the data for 60 µLsamples.
µL and 60 µL samples averaged approximately 0.2 mm s-1 (Figures 5.7 and 5.6).
In other words, at constant subcooling, the droplet volume did not affect the initial
growth rate. The latter is an indication that the same transport limitations operate
on both droplet sizes.
Considering that the 60 µL droplet had three times the volume of the 20 µL
droplet and that the coverage times were comparable in both cases it could be in-
ferred that the hydrate film formed on the 60 µL samples was thinner than the
one observed on 20 µL samples. This conjecture is supported by the images which
showed that light traversed the 60 µL hydrate with less difficulty than it went
through the 20 µL clathrate (Figures 5.2 to 5.4).
CHAPTER 5. DISCUSSION 45
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 2 4 6 8
Gro
wth
ra
te/m
m s
-1
p/MPa
20 µL samples
60 µL samples
Figure 5.7: Initial growth rates of hydrates formed from 20 and 60 µL water dropletsunder the range experimental pressures. ∆Tsub= 2 K. The CV for 20 µL sampleswas 0.36 while the 60 µL samples presented a value of 0.29. Blue diamonds representdata for the 20 µL samples, while red squares are the data for 60 µL samples.
Chapter 6
Conclusions, Recommendations
and Future Work
6.1 Conclusions
We performed a set of experiments for methane hydrate formation and growth
to determine the effect of the water volume on the macroscopic morphology of
methane hydrates. In most of the runs for both volumes, the hydrate growth oc-
curred at a specific point on the periphery of the water film. For methane hydrates
formed from 20 µL water droplets without previous hydrate formation history, sev-
eral growth sites appeared on the droplet surface seconds after the appearance of
the first growth site. Those hydrates also presented an apparent thicker hydrate
layer with a rougher surface that suggested coarse crystal grains.
On the other hand, the hydrates formed from 60 µL water droplets grew only
from one growth site and presented a smooth and shiny surface. Other regions of
the hydrate where also analyzed and compared, such as the halo and the boundary
(division between the halo and the interior of the hydrate). A wavy pattern was
observed on the halo surface for both volumes. However, the halo observed in the
hydrate formed from the 20 µL droplet was rougher and thicker. Both the 60 µL
46
CHAPTER 6. CONCLUSIONS, RECOMMENDATIONS AND FUTUREWORK47
and 20 µL samples presented the same overall pattern: a hydrate band separating a
granular interior from the hydrate that grew outside of the original water boundary.
A depression on the surface of the 60 µL sample was observed and explained as an
effect of the water migration theory. No visible collapse or transformation of the
surface of the hydrate formed from a 20 µL water droplet was appreciated.
In general, four main characteristics were observed on the methane hydrate mor-
phology for the two volumes of water evaluated. (1) A difference in the number of
growth sites, (2) a smoother surface was observed in the 60 µL hydrates which sug-
gest smaller grains, (3) a band that separated the interior of the hydrate from the
halo was present in 20 µL and 60 µL samples, and (4) the presence of a layered halo
was observed in both cases, although the roughness in the 20 µL samples masked
it, making harder to recognize the different layers on it.
In addition, it was established that both volumes share the same transport limi-
tations due to equal values of local growth rates. Based on the difference in coverage
times, it could be inferred that the hydrate film formed from a 60 µL water droplet
was thinner than the one observed on 20 µL samples.
Through the study of the initial stages of formation and growth considering
variables such as guest molecules, size os particles, temperature, and pressure, our
results provide a better understanding of the crystallization process, which may help
to develop models that would describe accurately hydrate behavior in pipelines and
wells, providing solutions to one of the most important problems in the oil industry.
6.1.1 Recommendations and Future Work
Based on the analysis of the data showed in the present work, we formulated
the following recommendations or future work:
1. Experiments were performed with water films with no previous hydrate for-
CHAPTER 6. CONCLUSIONS, RECOMMENDATIONS AND FUTUREWORK48
mation history. However, hydrate history is a key factor when determining the
morphology of a gas hydrate. Our recommendation is to carry experiments with
water films with previous hydrate formation history.
2. Perform more experiments with different water volumes to track the behavior
of the system and obtain a better idea of trends in the morphology and growth rate
of the system.
3. Study the dissociation mechanism to observe the main characteristics in the
decomposition of hydrates formed from different water volumes.
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