Phase Behaviour and Structural Aspects of Ternary Clathrate Hydrate Systems. The Role of Additives

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Phase Behaviour and Structural Aspects

of Ternary Clathrate Hydrate Systems

The Role of Additives

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus Prof.dr.ir. J.T. Fokkema, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op dinsdag 13 januari 2004 om 10:30 uur

door

Miranda Mariëlle MOOIJER – VAN DEN HEUVEL scheikundig ingenieur

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Dit proefschrift is goedgekeurd door de promotor: Prof.dr.ir. J. de Swaan Arons

Toegevoegd promotor: Dr.ir. C.J. Peters

Samenstelling promotiecommissie:

Rector Magnificus voorzitter

Prof.dr.ir. J. de Swaan Arons Technische Universiteit Delft, promotor

Dr.Ir. C.J. Peters Technische Universiteit Delft, toegevoegd promotor Prof.dr. M.O. Coppens Technische Universiteit Delft

Prof.dr. G.J. Witkamp Technische Universiteit Delft Prof.dr. J.A. Schouten Universiteit van Amsterdam

Dr. J.A. Ripmeester National Research Council of Canada (NRC), Ottawa Carleton Chemistry Institute Canada, Ottawa, Canada Dr. U.C. Klomp Shell Global Solutions B.V., Amsterdam

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Dankwoord iii

Dankwoord

Bij het tot stand komen van dit proefschrift waren vele mensen direct of indirect betrokken, die ik allen op deze plaats wil bedanken voor hun bijdrage.

Mijn promotor Prof.dr.ir. J. de Swaan Arons en co-promotor Dr.ir. C.J. Peters hebben er voor gezorgd dat mijn interesse voor een promotie-onderzoek werd gewekt. De steun die ik kreeg bij het uitvoeren van dit onderzoek van jullie beiden is heel waardevol en heeft geresulteerd in dit proefschrift. Mijn co-promotor Dr.ir. C.J. Peters zorgde voor onder-steuning bij de dagelijkse gang van zaken en schiep de mogelijkheden mijn ideëen uit te werken en op diverse conferenties te presenteren. De getoonde interesse van m’n pro-motor Prof.dr.ir. J. de Swaan Arons was motiverend en zijn adviezen over het inzichtelijk opschrijven van resultaten zullen altijd van pas blijven komen.

Ook de overige leden van de vakgroep hebben zeker hun steentje bijgedragen. Hoewel Dr.ir. Th.W. de Loos niet direct bij mijn project betrokken was heb ik veel van hem geleerd, zowel op het gebied van de thermodynamica als in vele discussies over uiteen-lopende zaken. Wim Poot maakte me wegwijs op het gasrek en met de Cailletet appa-ratuur en zorgde voor de huiselijke sfeer op het lab. Louw Florusse en Eugene Straver stonden altijd voor me klaar als ik hulp nodig had.

De groep promovendi groeide in de tijd dat ik bij de vakgroep was gestaag van 2 naar 7, deze groep werd gecomplementeerd met post-docs, uitwisselings studenten en afstudeer-ders. Promovendus-collega van het eerste uur, Gerard wil ik bedanken voor de goede samenwerking die we op veel gebieden hebben gehad en je betrouwbare vriendschap door al die jaren heen. I want to thank Sona and Ali for being wonderful office mates and friends, and sharing your thoughts and experiences openly and with enthusiasm. Diana Nanu took care of the morning-coffee meetings and has been a very dear colleague for 3 years. Aleidus, José, Pei Jun and the other guests staying for longer or shorter terms brought their experience to the group and opened my view on the world. Enkelen van de gasten of studenten leverden een directe bijdrage aan het hier gepresen-teerde werk: Ricardo Witteman deed metingen aan het systeem H2O + CO2 + tetrahy-dropyran, Khashayar Nasrifar performed measurements on VLE in systems with gas and cyclic organic components and hydrate equilibria of gas mixtures containing ethane and propane, Nancy Sawirjo’s afstudeerwerk stond in het teken van menghydraten met flu-oroalkanen, and Rita Duarte made a nice contribution with the measurements of VLE in systems with gases and cyclobutanone.

I highly appreciated the opportunities that were given during my stay at the Centre for Hydrate Research of the Colorado School of Mines (CSM) and the National Research Council of Canada (NRC). The work I did during these periods are an important contri-bution to the work presented and my general appreciation of clathrate hydrates.

Prof.dr. E.D. Sloan was happy to have me spending a summer in Colorado and using the Raman spectroscopy for investigating the mixed clathrate hydrate structures. The

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‘hy-iv Dankwoord

drate busters’ at CSM made sure I felt at home and it was a joy to share my experiences with all of you, a few of them I want to thank personally: Adam Ballard for the good discussions on modelling, Marc Jager for introducing me to Raman spectroscopy and all the help and friendship during my time there and afterwards, and last but not least Keith Hester for practical help and bringing sun-shine in the dark lab.

I spent a snowy and for dutch standards cold winter in Ottawa, Canada, in the group of Prof.dr. J. Ripmeester. He and the experienced and highly-skilled people of his group revealed most important aspects of the crystallography of clathrate hydrates. Regent Dutrisac helped me preparing the samples and introduced me in most practical issues, Igor Moudrakovski was a nice room-mate in a room with a view on Parliament Hill, Kostia Udachin and Lee Wilson were always there to discuss issues and help sorting these out, and finally Jamy who made sure I kept my physical shape with the lunch-runs through snow, rain and sunshine.

Mijn verblijf in Canada werd financieel ondersteund door NWO (dossierno. R 74-55) en een gift van de Stichting Universiteitsfonds Delft (SUFD #38.02).

Maar ook diverse mensen buiten de universitaire wereld hebben hun steentje bijgedragen. Allereerst de commissie-leden die tijd vrijmaakten om kennis te nemen van dit proef-schrift en bij de verdediging aanwezig te willen zijn. Ik ben zeker ook dank verschuldigd aan Ulfert Klomp en de afdeling OGUF van Shell Global Solutions voor de financiële bijdrage.

Mijn huidige collega’s bij Shell, zowel in OGUF als CPD/2, wil ik bedanken voor de interesse in m’n promotiewerk en de steun tijdens de laatste maanden om het proefschrift goed af te ronden.

Het thuisfront verdient hier zeker ook een grote pluim. Mijn vrienden, kennissen en familieleden die altijd belangstelling hadden voor m’n werk, of ik nu dichtbij was of ver weg. Mijn schoonouders voor de getoonde interesse en de slaapplaats tijdens eerdere perioden van m’n studie. Oom Hans voor het aanwakkeren van de interesse voor chemie en chemische technologie toen ik nog op de middelbare school zat.

Mijn broer en zus hebben zich altijd oprecht geïnteresseerd voor mijn handel en wandel en dat waardeer ik zeer. Mijn ouders waren van de middelbare school en HTS tot en met het laatste stuk van mijn promotie altijd op de achtergrond als steun en toeverlaat aanwezig.

Een laatste woord van dank is voor Jacco, die meereisde over de figuurlijke en letterlijke toppen maar ook door de dalen die het studie en promotie traject met zich meebrachten. Altijd een luisterend oor of ik nu ver weg was of dichtbij, en in dat laatste geval ook een begrijpende blik. De fascinatie voor ijs(achtige) structuren zal nu weer langzaam tot normale proporties reduceren om er samen van te genieten.

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Table of Contents v

Chapter 1 Introduction

The fascinating character of clathrate hydrates initiates numerous research activities around the globe and in a variety of fields. Additionally, these substances are relevant for a very diverse range of technical applications. In this chapter, insight into what clathrate hydrates are and the general fields of interest for these compounds will be described, prior to a description of the objective and motivation of this Ph.D. study. Finally, a summary of the contents of this thesis will be given.

1.1 What are Clathrate Hydrates?

Clathrate hydrates pertain to the class of clathrates, which is a certain type of inclusion compounds. The term clathrate was first used by Powell [Powell, 1948], after the Latin word “clathratus”, which means “to encage”. Clathrates are characterised by formation of a regular crystal lattice, in which spaces are created that are occupied by guest molecules [Powell, 1984]. Examples of other inclusion compounds are zeolites, carbon nanotubes, urea adducts, clathrasils, and calixarenes.

In case of clathrate hydrates, the lattice is formed by water molecules that are hydrogen-bonded to each other. The lattice is referred to as β-lattice. The spatial configuration of the water molecules in the β-lattice is different from that of ice, though in both compounds each water molecule is bond to four other water molecules. The configuration of the water molecules in the clathrate hydrate lattice creates cavities that are substantially larger than those found in ice structures. The relatively large cavities imply that the empty β-lattice of clathrate hydrates is less stable than the crystal structure of ice. Stability of the β-lattice of clathrate hydrates is obtained from guest molecules that occupy the cavities. Interactive forces between guest molecules and the water molecules in the β-lattice are of the order of magnitude of van der Waals interactions, and contribute to the potential energy of the clathrate hydrates. The stability of the clathrate hydrates, which have an ice-like appearance, is so substantial that they can exist at temperatures appreciably higher than the temperature of the triple point of H2O (

2 tr , H O

T = 273.16 K). The principle of clathrate hydrate formation versus the formation of ice, and the interactive forces playing a role, are represented schematically in Figure 1.1.

In case of clathrate hydrates, there is a variety of molecules that can occupy the cavities. Size, shape and nature of the guest molecules determine whether a stable clathrate hydrate is formed. Well-known and most intensely studied are clathrate hydrates of natural gas constituents, e.g., CH4, C2H6, C3H8, H2S and CO2, which are all in the gaseous state at ambient conditions. Therefore, the clathrate hydrates containing H2O + gas, or H2O + mixture of gases, are commonly referred to as gas hydrates. Extensive reviews on

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2 Introduction

clathrate hydrates are available in literature [Davidson, 1973, Berecz and Balla-Achs, 1983, Holder et al., 1988, Englezos, 1993a, Makogon, 1997, Sloan, 1998].

Figure 1.1

The basic crystal structure (not on scale) of ice Ih (a) and a clathrate hydrate cavity with a guest molecule (b). The thickness of the arrows represents qualitatively the interactive forces that are present in ice and clathrate hydrates: (↔) for hydrogen bonding and (⇔) for van der Waals type of interactions. The arrows and guest molecule are added to the original pictures of the ice structure and cavity [Sloan, 1998].

Besides gaseous guest components, also components that are liquid at ambient conditions can form clathrate hydrates with H2O. The liquid components are typically of cyclic organic nature, because they fit into the cavities rather well [Davidson, 1973]. Clathrate hydrates of liquid components are sometimes referred to as liquid hydrates, equivalent to the reference of gas hydrates for gaseous components. Similar to gas hydrates these liquid hydrates are of solid appearance. A notable group of cyclic organic components that can form clathrates are cyclic ethers [Von Stackelberg and Meuthen, 1958]. The majority of the systems considered in this study comprises gaseous and liquid components that can be encaged in the β-lattice of hydrogen bonded H2O molecules. Therefore, the term clathrate hydrates will be used in the remainder of this thesis.

The characterisation of clathrate hydrates is concentrated in three fields of fundamental research:

• Phase behaviour: Investigation of the conditions at which phase equilibria, comprising a clathrate hydrate phase, are stable.

• Structure: Study of the types of structures formed and the occupancy of the structure by the (different) guest molecules.

• Kinetics: Research of the kinetics, both the rate as well as the mechanism of formation and dissociation of clathrate hydrates.

Fundamental knowledge of the various fields, and the way they influence each other, is required to evaluate the opportunities and problems for various applications where clathrate hydrates may be encountered.

a b H H C H H a b H H C H H H H C H H

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Introduction 3

1.2 General Interest for Clathrate Hydrates

In many respects, clathrate hydrates are fascinating compounds. For instance, they are stable at temperatures substantially higher thanTtr , H O₂ , while they contain a large amount

of H2O and have an ice-like appearance. If applications are considered where clathrate hydrates occur or may occur, these compounds exhibit a paradoxical nature. They are of scientific interest and, depending on the application, clathrate hydrates are regarded as a

nuisance, a blessing or an opportunity. The intriguing nature of clathrate hydrates is apparent

from the diversity of research-interests for these compounds, which is reflected in the following overview:

• Scientific curiosity

• Plug formation in gas- and oil transportation and processing facilities • Natural occurrence of clathrate hydrate fields filled with natural gas • Technical application, e.g., in storage and separation

1.2.1 Scientific Curiosity

Interest in clathrate hydrates started as a curiosity in laboratories and dates back to the beginning of the 19th_{century. The discovery of clathrate hydrates is attributed to Davy} [Davy, 1811]. Although the term clathrate hydrates was not used at that time, the phenomenon was described. Formation of solid ice-like crystals was observed in a saturated aqueous solution of Cl2 at a temperature as high as 282 K, which is substantially higher than

2 tr , H O

T . The stability of these crystals was ascribed to the presence of Cl2 in the liquid H2O and in the crystals. The research of Davy was followed by an impressive number of studies regarding components that could form solid crystals with H2O like those observed by Davy. Initially, the number of data points for the phase equilibria were scarce. The data points close to the triple point of H2O were given most attention. Bakhuis Roozeboom generated p,T-diagrams with series of data for several gases, showing univariant curves and quadruple points. These quadruple points are located at conditions where the ice phase starts to form in coexistence with a clathrate hydrate-, liquid water- and vapour phase, or at the point where components from the vapour phase start to condense, if applicable [Bakhuis Roozeboom, 1884 and 1885]. The research on the boundary conditions for stability of the clathrate hydrate phase was often combined with the determination of the ratio of H2O and the gaseous component.

1.2.2 Plugging of Pipelines

Hammerschmidt discovered that plugging of gas- and oil transportation pipelines was due to formation of clathrate hydrates rather than formation of ice [Hammerschmidt, 1934]. Plugging of these transportation pipelines was a serious safety issue, and the clathrate hydrates causing them were regarded as a nuisance.

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4 Introduction

This discovery initiated an increase in the experimental investigation of the boundary conditions of the clathrate hydrate stability region. Study of the clathrate hydrate structure, modelling and kinetics of formation evolved later. The research concentrated on the clathrate hydrates of natural gas and oil constituents

Knowledge of the boundary conditions of the region where clathrate hydrates are stable is essential to justify whether plugs of clathrate hydrate may be formed at processing conditions or not. Hydrocarbons are prominently present in natural gas and oil streams. Consequently, the phase behaviour in binary systems of hydrocarbon gases and H2O has been studied elaborately. The phase boundaries in the binary systems H2O + gas, for CH4, C2H6, C3H8 are depicted in Figure 1.2.

Figure 1.2

Phase boundaries confining the region of stable clathrate hydrate in a p,T-diagram for the binary systems: H2O + CH4 (---), H2O + C2H6 (―), and H2O + C3H8 (-.-.-). The specific equilibria are as

indicated; also the quadruple points are shown ().

Depending on the oil- or natural gas source, also other gaseous components are present in traces or larger amounts, which are able to form clathrate hydrates. Illustrative examples are components like CO2, N2, and H2S, for which the phase boundaries are visualised in Figure 1.3.

Experimental data for the phase behaviour of systems with mixtures of two or more gases are limited. However, these data are essential for a good understanding of the phenomenon of clathrate hydrate plug formation in gas transportation facilities. For ternary and multi-component systems, the phase boundary conditions are dependent on the overall-composition of the system. This causes an increase in complexity of the phase diagrams for these systems, compared to those of binary systems. Additionally, it is hard to test the consistency of various sets of data for multi-component systems. Considering the fact that there is a large variance in the composition of natural gases worldwide, there

260 270 280 290 Temperature [K] 0 2 4 6 Pressure [M Pa] I-H-V H-Lw-V H-L_w-V H-Lw-V I-H-V I-H-V H-L_w-L H-Lw-L 260 270 280 290 Temperature [K] 0 2 4 6 Pressure [M Pa] I-H-V H-Lw-V H-L_w-V H-Lw-V I-H-V I-H-V H-L_w-L H-Lw-L

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Introduction 5

are not many data sets on the phase behaviour available in literature for this kind of systems. Data from an extensive number of sources for the phase behaviour of binary, ternary and quaternary systems and systems containing natural gases are collected and assembled in an overview by Sloan [Sloan, 1998].

Figure 1.3

Phase boundaries confining the region of stable clathrate hydrate in a p,T-diagram for the binary systems: H2O + N2 (---), H2O + CO2 (―), and H2O + H2S (-.-.-). The specific equilibria are as

indicated, also the quadruple points are shown ().

Hammerschmidt’s discovery did not only encourage the study of phase behaviour, but also the investigation of the ratio between H2O and guest molecules and the actual structure of clathrate hydrates. With the definition of the clathrate hydrate structures, the stoichiometric ratio of the molecules is confirmed simultaneously. The work in the 1940ies and 1950ies with X-ray diffraction techniques produced a significant amount of information. It was established that two clathrate hydrate structures are formed, i.e., structure I (sI) and structure II (sII) [Von Stackelberg and Müller, 1951, Müller and Von Stackelberg, 1952, Claussen, 1951a, 1951b, Pauling and Marsh, 1952]. Formation of either sI or sII was suggested to depend on the size of the guest molecule. In time, more physical techniques for analysis of crystal structures became available and were applied to characterize the structures for a large variety of guest components. Examples of these techniques are:

• Nuclear Magnetic Resonance (NMR) spectroscopy • Neutron diffraction • Raman spectroscopy 260 270 280 290 300 310 Temperature [K] 0 5 10 15 20 25 Press ure [M Pa] H-Lw-V H-Lw-V H-Lw-V H-Lw-L H-Lw-L I-H-V 260 270 280 290 300 310 Temperature [K] 0 5 10 15 20 25 Press ure [M Pa] H-Lw-V H-Lw-V H-Lw-V H-Lw-L H-Lw-L I-H-V

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6 Introduction

Major contributions came from Jeffrey and co-workers [Jeffrey and McMullan, 1967, Jeffrey, 1984], and Davidson and co-workers [Davidson, 1973]. The latter group performed pioneering work in applying of dielectric methods and NMR spectroscopy to clathrate hydrates [Davidson and Ripmeester, 1984]. These efforts resulted in description of a third structure; i.e., structure H (sH). This structure is stable when two types of guest components are present, of which one has a relatively large molecular size. [Ripmeester et al, 1987, Ripmeester and Ratcliffe, 1990]. Later, a structure has been defined, which is a poly-type crystal of sII and sH [Udachin and Ripmeester, 1999].

Raman spectroscopy has been used to characterize the clathrate hydrate structure [Sum et al., 1997, Nakano et al., 1998, 1999] and to measure phase boundary conditions [Subramanian et al., 2000, Jager and Sloan, 2002]. These physical analysis techniques have also been applied to obtain insight into the fractional occupancy of the hydrate by their guests [Tulk et al., 2000 Morita et al., 2000, Wilson et al., 2002, Kini et al., 2002].

The clathrate hydrate structure is a microscopic property and the phase behaviour a macroscopic property. A bridge between the microscopic and macroscopic properties is provided by a statistical thermodynamic model, which is called the Van der Waals-Platteeuw model, after its founders [van der Waals and Waals-Platteeuw, 1959]. The basic idea of this model is to describe the distribution of the guest molecules over the distinguishable cavities with statistics. The chemical potential of water in the clathrate hydrate phase can be calculated from a grand-partition function. The criterion of phase equilibrium dictates that the chemical potential of water in the clathrate hydrate phase should be equal to that of water in the other phases present. The interaction between the guest molecule and H2O molecules in the cavity wall is described by a potential function. For calculation of the fractional occupancy of the various cavities in a clathrate hydrate structure, the analogy of clathrate hydrate formation with adsorption has been suggested.

The kinetics of clathrate hydrate formation is a relatively new field of fundamental research, compared to the fields of phase behaviour and structure. However, this field of research is certainly related to plugging of transportation pipelines and to the question how clathrate hydrates are formed, in particular. Makogon and co-workers initiated experimental work on kinetics of formation in bulk phases and porous media in the 1960ies [Makogon, 1965 and 1997]. Work on clathrate hydrate kinetics intensified twenty years later with experimental and modelling work. Today the application of physical techniques makes an entry in studying kinetics, and their application elucidates the time-dependence of the formation of clathrate hydrate.

Intensification of kinetic studies on clathrate hydrate formation and dissociation demonstrates the close connection between the various fields of fundamental research of clathrate hydrates, i.e., kinetics, phase behaviour, and structure. This implies that proper analysis, and methods for prevention, of plugging of pipelines require insight into, at least, the general behaviour of the system in all three fundamental research fields.

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Introduction 7

Hammerschmidt’s discovery strongly suggested that clathrate hydrates are a nuisance for gas and oil industry. The industry was eager to learn more about clathrate hydrates and methods to prevent their formation. An opportunity for scientific research emerged to investigate the phenomenon of clathrate hydrates extensively.

1.2.3 Natural Occurrence

Clathrate hydrates1_{of natural gas can also be formed in a natural environment, provided} that H2O and gases are present at pressure- and temperature conditions suitable for clathrate hydrate formation and stability. The first major clathrate hydrate deposit of natural gas was discovered in the permafrost of the Soviet Union in 1967. In 1969, natural gas was produced from the gas hydrates in Messoyakha field [Makogon, 1997]. In 1972, well drilling in Prudhoe Bay (Alaska) [Collett, 1983] and MacKenzie Delta (Canada) [Bily and Dick, 1974] provided cores of natural gas hydrate. Later it was recognised that natural gas hydrates also occur in shallow deep-water sediment of oceanic regions at outer continental margins [Claypool and Kaplan, 1974]. It is generally accepted that natural gas hydrates occur worldwide.

Knowledge of the amount of carbon that is stored in naturally occurring gas hydrate fields is surrounded by uncertainties and speculative assumptions. The latter are related to the uncertainties on porosity of sediments and the actual percentage of sediment that is occupied by the natural gas hydrate [Kvenvolden, 1998]. Rather realistic estimates are that approximately 2 to 4·1016_m3_{of CH4 gas is stored in natural gas hydrate deposits} [Kvenvolden and Claypool, 1988, MacDonald, 1990]. If these estimates are valid, the amount of carbon stored as natural gas hydrates is roughly twice as much as organic carbon that is present as other fossil fuels, i.e., coal, oil and conventional natural gas. The large quantities of natural gas hydrate and their wide geographic distribution make them attractive as a resource of energy, and in that respect clathrate hydrates, may be a blessing for future generations.

Naturally occurring gas hydrate fields are not recognised as a blessing only. From some viewpoints, they may be regarded as a nuisance. The stability of clathrate hydrates in natural environments depends on the pressure and temperature conditions of that environment. Slides or slumps of land or changes in climate may cause decomposition of large amounts of natural gas hydrate [Bell, 1982, Revelle, 1983, MacDonald, 1990, Henriet and Mienert, 1998]. The presence of natural gas hydrate fields close to oil and gas wells might be a safety issue. When drilling through those gas hydrate layers is required, or local decomposition of natural gas hydrates results in land slides, the mechanical stability of oil and gas processing or transportation facilities might be affected. Knowledge of geochemistry and geophysics is essential in this respect, as well as understanding of phase behaviour, structure and kinetics of clathrate hydrates.

1_{The clathrate hydrates in this section are all natural gas clathrate hydrates, and will be referred to as} natural gas hydrates for the remainder of this section.

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8 Introduction 1.2.4 Practical Applications

The high regularity and compact character of clathrate hydrates may provide an

opportunity for development of technologies that use these compounds. Possible

applications are:

• Storage and transportation of gases • Separation

The technical and economical feasibility of clathrate hydrate applications depend on the storage capacity and process conditions. The principles for the application of clathrate hydrates for storage and separation purposes are discussed.

The storage capacity of clathrate hydrates, with complete occupation of the various cavities, is substantially larger than for compressed gases. However, it varies for the different clathrate hydrate structures. An overview of the storage capacity per unity of volume is given in Table 1.1.

The storage capacity of liquefied natural gas (LNG) is larger than for the various clathrate hydrate structures, albeit that the order of magnitude is comparable. Technologies for compressing and liquefying gas are well developed and applied frequently. However, the conditions that are required for liquefying natural gas require substantial amounts of energy. Relatively low temperatures and high pressures are needed, e.g., temperatures of 113 K and elevated pressures. Compression of natural gas for storage and transportation is performed at pressures from 60 to 80 bar at ambient temperatures. Clathrate hydrates have been suggested as storage and transportation medium from the late 1940ies onwards [Miller and Strong, 1946].

Table 1.1

Storage capacity, m3_{gas per m}3_{clathrate hydrate, for different clathrate hydrate structures with}

complete occupancy of either the small type of cavities or all cavities.

Structure _θ_small2

[m3_{gas * m}-3_] _[mθ3_{gas * m}small+large-3_]

sI 56.02 169

sII 154.08 231

sH 200.93 241

compressed 1 1

liquefied natural gas 600 600

Interest in storage with the aid of clathrate hydrates fluctuated in time, following trends in oil price and the assumed amount of fossil fuels present as resource for future generations. In the last decade the interest in using clathrate hydrates as storage or transportation medium revived, for both natural gas and CO2. Regarding natural gas,

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Introduction 9

transportation as clathrate hydrates might be attractive for natural gas from stranded or marginal gas fields offshore [Gudmundsson et al., 2000, 2002, Iwasaki et al., 2002, Levik et al., 2002] and as an alternative for transportation of LNG [Shirota et al., 2002, Takaoki et al., 2002, Nakajima et al., 2002, Ota et al., 2002, Kimura et al., 2002, Miyata et al., 2002]. For the latter application the interest mainly comes from Japan, which has no access to own resources of conventional fossil fuels. Consequently, import of fossil fuels as LNG is required, which is costly.

The storage of CO2 in clathrate hydrates is related to the reduction of CO2 emission and its sequestration. The pressures and temperatures that occur at ocean depths, where these technologies would be performed, are suitable for clathrate hydrate formation [Aya, 1995, Brewer et al., 1999, Warzinski et al., 2000]. Additionally, ideas have emerged to replace the natural gas in naturally occurring clathrate hydrates fields by CO2 [Komai et al., 2000, Haneda et al., 2002].

The storage capacity is large for gaseous components and other components with a relatively small molecular size. The clathrate hydrates are selective for components that will be encaged. Moreover, the highly regular and compact clathrate hydrates contain mainly H2O, which builds the β-lattice. On the one hand, the separation is based on the containment of H2O as host in clathrate hydrates. On the other hand, the components that are encaged as guests may be a basis for separation technology. Three types of separations could benefit from application of clathrate hydrates:

• Reduction of H2O content of water-rich streams or media. • Production of potable water from seawater.

• Fractionation of mixtures of gases and/or liquids.

Concentration of water-rich streams might be a good alternative for freeze-crystallisation processes, which require relatively low temperatures [Huang et al., 1965 & 1966, Vaessen et al., 2000]. On the other hand, application of clathrate hydrates might also be a good alternative for processes that use evaporative techniques, which require higher temperatures, typically higher than the atmospheric boiling point of H2O [Werezak, 1969, Englezos, 1993b, Gaarder and Englezos, 1995].

Development of clathrate hydrate applications for concentration processes is not only relevant with respect to the reduction of energy consumption. Contents of the stream that should be concentrated may be sensitive to high or low temperatures, and could degrade at the conditions applied with freeze crystallisation or evaporative processes. This holds especially for certain streams in the food industry.

For the afore-mentioned processes, the solutes other than H2O in the process stream are the main purpose of separation. For regions in the world that have a shortage of potable water, but are relatively close to seawater, production of desalinated seawater maybe the main objective for applying clathrate hydrates in separation processes. The salts from the seawater are not enclosed into the clathrate hydrate structures, while the water is.

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10 Introduction

Retrieval of potable water from seawater is currently carried out by reverse osmosis or by evaporative processes [de Graauw and Bruinsma, 1996], which are energy demanding.

Figure 1.4

Schematic representation of a desalination process for seawater that uses clathrate hydrate formation.

Fractionation of mixtures of gases and/or liquids is based on the selectivity of the clathrate hydrates to encage certain molecules, while others are excluded. Hammerschmidt reported that there was a difference between the overall-compositions of the initial vapour phase and the vapour phase obtained from the dissociated clathrate hydrate [Hammerschmidt, 1934]. This finding implies that fractionation of streams by clathrate hydrate formation is possible, and actually is expected to occur commonly. In fact, two main methods of fractionation with clathrate hydrates can be considered:

• Enrichment of a vapour or liquid phase with one of the components [Davidson et al., 1983, Maekawa and Imai, 2000, Ballard and Sloan, 2002, Okano et al., 2002, Chen et al., 2002, Levik et al., 2002]. For these applications, one of the components should be encaged more than the other.

• Fractionation of, e.g., the complete vapour phase from the liquid phase by controlled formation of clathrate hydrates [Østergaard et al., 2000].

The main purpose for applying clathrate hydrates as a means of separation is driven by reduction of energy consumption, because of costs and environmental issues. Control of phase behaviour, structure and kinetics of formation and dissociation is essential for these applications.

1.3 Objectives and Motivation

This study focuses on the phase behaviour in some ternary systems, in which a clathrate hydrate can be formed. The ternary systems contain H2O, gas and an additive, which can

sea water CO2or air brine gas recycle Clathrate hydrate formation Separation potable H₂O Clathrate hydrate dissociation sea water CO2or air brine gas recycle Clathrate hydrate formation Separation potable H₂O Clathrate hydrate dissociation

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Introduction 11

be a liquid cyclic organic component or a second gaseous component. Earlier work indicated that these additives might reduce the equilibrium pressure [Saito et al., 1996, Jager et al., 1999, de Deugd et al., 2001]. However, the mechanisms contributing to these reductions were uncertain.

The experimental phase behaviour is determined with Cailletet equipment. The investigated range of pressure and temperature is 0.2 to 15 MPa and 274 to 305 K, respectively. The gaseous components considered are CH4, CO2, and C3H8 and the cyclic organic components are:

• Tetrahydropyran • Cyclobutanone • Cyclohexane • Methylcyclohexane

All the gases present in ternary systems with two gaseous components form clathrate hydrates in binary systems with H2O. Consequently, for the ternary systems it is expected that both gaseous components are encaged. The ternary systems studied are:

• H2O + CH4 + CF4 • H2O + CH4 + CHF3 • H2O + CO2 + CHF3 • H2O + C2H6 + C3H8

The molecular structures of the additives are given in Figure 1.5. The gaseous and liquid additives have been compared on how they influence the total phase behaviour. The phase diagrams of ternary systems are compared to those of the systems H2O + gas.

Figure 1.5

Schematic representation of molecular structures of the additives used: tetrahydropyran (a), cyclobutanone (b), cyclohexane (c), methylcyclohexane (d), CF4 (e), and CHF3 (f).

For a proper interpretation of the phase behaviour, knowledge of the formed clathrate hydrate structure and its occupancy is essential. The application of various physical characterisation techniques to analyse the mixed clathrate hydrates of gaseous and cyclic organic components has been studied and will be discussed.

Comparison of the size of the guest molecule with the size of the clathrate hydrate cavities may give a first impression of which cavity can be expected to encage a guest molecule. However, this method does not always give satisfactory results and, in

O CH3 O a b c C F F F F d e f C H F F F O O CHCH33 O O a b c C F F F F C F F F F d e f C H F F F C H F F F

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12 Introduction

addition, analysis with physical methods is required. More specifically the experimental methods applied are:

• Raman spectroscopy • Powder X-ray diffraction

• Nuclear magnetic resonance (NMR) spectroscopy

The phase behaviour and its details have been described with a thermodynamic model. In this work, the van der Waals – Platteeuw model has been used to represent the clathrate hydrate phase in these systems. The resulting model has been tested for its ability to predict phase equilibrium conditions in ternary systems.

The phase behaviour of the fluid phases is modelled with a cubic equation of state and a Henry coefficient model is applied to calculate the solubility of the gaseous components into the liquid H2O phase. The aqueous phase is described with a chemical potential model developed for modelling clathrate hydrate equilibria [Holder et al., 1980].

Additionally, the phase behaviour of these systems has been evaluated for possible practical applications of clathrate hydrate compounds. The extent to which equilibrium conditions can be shifted and the implications for the storage capacity of encaged gases are important factors to be considered for possible applications. Finally, our studies have revealed that phase equilibria, structural properties and kinetic factors together decide on the behaviour of these compounds in practical situations.

These practical situations may be encountered in the oil and gas industry, whether plugging of pipelines or the naturally occurring clathrate hydrate fields as natural gas reserves are concerned. Prerequisite is understanding of the phase diagrams and structures of binary and ternary systems, which can be used as a starting point to consider multi-component systems. Influence of other components than the main constituents of natural gas on clathrate hydrate formation has hardly been investigated. The knowledge of phase behaviour is required to answer the question whether clathrate hydrates may be formed at process or transportation conditions. In addition, the change of conditions in the environment might affect the stability of natural gas hydrate fields and the possibility to win natural gas from these fields. The structure formed, along with its occupancy, is an important aspect to assess the amount of gas that is stored in the plugs and natural clathrate hydrate fields in sub-sea sediments or permafrost.

For potential applications of clathrate hydrates in storage, transportation or separation, the process conditions are important, and therefore essential information. The energy demand of any process is currently a major issue, regarding costs and environmental impact. Pressure is an important variable and easily contributes to energy requirement. So, if a cyclic organic component is known to reduce the pressure of formation for

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Introduction 13

clathrate hydrates, this may be an important factor to study within the total energy picture of the process.

1.4 The Organisation of this Thesis

In Chapter 2 and 3, the theoretical aspects of clathrate hydrates are described. Theory of phase behaviour is clarified, specifically for binary and ternary systems, in which clathrate hydrates are formed. The phase diagrams given in Chapter 2 are schematic and are a basis for interpretation of the experimental results on the phase behaviour. In Chapter 3 the theory of clathrate hydrate structures will be dealt with. The fundamentals of application of Raman spectroscopy, powder X-ray diffraction (PXRD), and Nuclear Magnetic Resonance (NMR) spectroscopy to the analysis of clathrate hydrates will be specified.

In Chapter 4 and 5 the experimental results are reported. Chapter 4 is dedicated to the experimental data on the phase behaviour. The experimental equipment and procedures are described and results are given for binary systems containing H2O + gas, ternary systems containing H2O + gas + cyclic organic component and H2O + mixture of two gases. The experimental results from applying the various physical techniques to elucidate the clathrate hydrate structure in systems with H2O + gas + cyclic organic component, are given in Chapter 5.

The interpretation of the integrated experimental results for phase behaviour and structure is given in Chapter 6. The experimental results for the structure appear to be vital for a complete insight into the phase diagrams.

The relation between phase behaviour and structure is incorporated in the modelling of the phase behaviour of the clathrate hydrates in the ternary systems. The procedures and results are presented in Chapter 7, where also results for the optimised parameters are given. The implications of the modelling results for interpretation of the experimental results for phase behaviour and structure, together with their relation to kinetics, are presented.

Chapter 8 summarises conclusions and recommendations and Chapter 9 gives a perspective on future clathrate hydrate research.

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Introduction 17

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18 Introduction

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Phase Behaviour of Clathrate Hydrates 19

Chapter 2 Phase Behaviour of Clathrate Hydrates

Clathrate hydrates and their possible applications require fundamental knowledge of these compounds. The characteristics of the phase behaviour in systems forming clathrate hydrates will be elucidated in this chapter. The phase diagram of H2O is a starting point for constructing the phase diagrams for these systems, and described in Section 2.1. The increase of the number of components to two or three results in an increase in complexity of the phase behaviour. The schematic phase behaviour for binary systems, containing H2O and one guest component, is described in Section 2.2. The phase behaviour for ternary systems, is discussed in Section 2.3. Finally, in Section 2.4 the effect of structural transitions on the phase diagrams is clarified. For the ternary systems a classification of the phase diagrams, based on the behaviour of the two guest components, is presented.

2.1 The Unary System of the Host Water

Equilibrium in a system is described by the variables p, T, and the independent composition variables of each phase, for which normally mole fractions are used. If there are N components present in a system, the knowledge of (N-1) mole fractions xi is

required. The number of variables that is required to describe a system with Π phases is

(

)

+ ⋅ −

2 Π N 1 . At equilibrium, the chemical potential for a certain component is equal in each phase present, see Eqn. [2.1]:

= = =K

i i i

α β π

µ µ µ [2.1]

where µ_iα is the chemical potential of component i in phase α. This equilibrium equation is valid for every component present in the system, resulting in N⋅

(

Π −1

)

of these equations. The number of degrees of freedom (F) for a system is the difference between the number of variables 2+Π⋅

(

N 1−

)

and the number of equilibrium conditions

(

)

⋅ −

N Π 1 :

= − +

F N Π 2 [2.2]

Additional relations between the variables are possible, which are accounted for by the introduction of φ, as presented in Eqn. [2.3]:

= − + −

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20 Phase Behaviour of Clathrate Hydrates

This is the well-known Gibbs’ phase rule [Gibbs, 1961]. These additional relations occur, for example, in case of a binary azeotrope (then: N = 2, Π = 2, yi = xi , so φ = 1, which

results in F = 1). Bearing in mind that the minimum for Π is 1, the maximum of F for a

system is defined by:

max

F = N 1+ [2.4]

In Eqn. [2.4] the variable Fmax reflects the dimension of space required to represent the

complete phase behaviour of a system. [de Loos, 1994]

The unary system of H2O, with N = 1, is used to demonstrate Gibbs’ phase rule as an

example. This system is essential for interpretation of clathrate hydrate phase behaviour, because H2O is always present in the systems where clathrate hydrates occur. The phases that can occur in a unary system of H2O are ice (I), liquid (Lw) and vapour (V). It should be mentioned that various modifications of the ice phase, which can all be assigned as a distinct stable phase, are possible [Franks, 1972]. However, they will not be considered here. An overview of examples of applying Gibbs’ phase rule to the unary system of H2O is given in Table 2.1. The regions in the p,T-diagram for H2O, where one phase is stable, are bound by the equilibrium lines for two stable phases, i.e., I-Lw, Lw-V and I-V. These equilibrium curves intersect at the triple point I-Lw-V, where the three phases coexist stable. The Lw-V equilibrium terminates in the critical point where Lw=V, and all properties are equal for both the phases. Above the critical point the V and Lw phase are indistinguishable, forming a fluïdum. The phase behaviour of H2O is presented schematically in Figure 2.1.

Figure 2.1

Schematic p,T-diagram with the phase behaviour of H2O.

Temperature Press ure I V Lw L_w-V I-L_w I-V Lw=V I-Lw-V Temperature Press ure I V Lw L_w-V I-L_w I-V Lw=V I-Lw-V

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Phase Behaviour of Clathrate Hydrates 21 Table 2.1

Examples of applying Gibbs’ phase rule to the unary system of H2O.

Π φ F Example Presentation in p,T plane

1 0 2 Lw region, surface

2 0 1 I-Lw curve

1 2 0 Lw=V point

3 0 0 I-Lw-V point

2.2 Binary Systems of Water and One Guest Molecule

The complexity of the phase behaviour, and simultaneously phase diagrams representing this behaviour, is evident when binary systems are compared with unary systems. The number of coexisting phases increases, and also the types of phases that can be formed, e.g., solid phase of guest component (S), liquid phase other than Lw (L), and typically for the binary systems considered here: clathrate hydrate phase (H). The presence of a distinct L phase depends on the mutual solubility between H2O and the other component. The distinct L phase, in the systems considered, is either La, where ‘a’ is for cyclic organic component, or Lg, where ‘g’ refers to the liquefied gaseous component. In certain regions of temperature, pressure, or overall-composition, immiscibility between liquid phases might occur. In Table 2.2 some examples are given for applying Gibbs’ phase rule to a binary system (N = 2), containing H2O and a gaseous component, which can be encaged in the clathrate hydrate.

Table 2.2

Some examples of applying Gibbs’ phase rule to the binary system of H2O + clathrate hydrate

forming component.

Π φ F Examples Presentation in p,T,x-space

2 0 2 Lw+V, H+Lw, H+I region, surface

1 2 1 Lg=V curve

3 0 1 H-Lw-V, H-Lw-Lg, I-H-V curve

2 2 0 H+Lw=V point

4 0 0 I-H-Lw-V, H-Lw-Lg-V point

The points where four phases coexist are called quadruple points (Q), and the pressure and temperature cannot be freely chosen. This is similar to a triple point in a unary system. Equivalent to three two-phase curves starting from a triple point in a unary system, four three-phase equilibrium curves will intersect in a quadruple point in a phase diagram of a binary system.

Relation 2.4 shows that for a binary system Fmax = 3, which implies representation of the

phase behaviour in a three-dimensional diagram. A projection of the phase diagram on the p,T-plane is useful to obtain insight into the equilibrium conditions for coexisting phases. Furthermore, cross-sections at constant temperature (p,x-cross-section) and pressure (T,x-cross-section) are used. These cross-sections visualize the dependence on the composition of equilibrium conditions. In this section schematic p,T-projections and p,x- or T,x-cross-sections are shown to give insight into the phase behaviour of binary