Hidratos de gás são importantes em questões que vão desde o bloqueio de tubulações na indústria de produção de óleo e gás, até o sequestro de dióxido de carbono, transporte de gás natural, dessalinização de água salgada e purificação de água contaminada. A fim de investigar estes campos e aplicações, necessita-se determinar inicialmente a temperatura e pressão nas quais os hidratos se formam. Isto pode ser feito por meio de experimentos de formação e/ou dissociação de hidratos, bem como utilizando-se de modelos termodinâmicos, correlações empíricas e métodos computacionais. Estudou-se nesse trabalho o desempenho da PC-SAFT na modelagem da fase fluida no equilíbrio da formação de hidratos, comparando seus resultados com a equação de Peng-Robinson e o modelo proposto por Klauda e Sandler. Para a molécula de água, em especial, avaliaram-se as configurações possíveis para o esquema de associação. Para a fase hidrato, avaliou-se a equação de van der Waals e Platteuw (vdWP), e levou-se em conta o efeito das cascas adicionais na estimativa da constante de Langmuir. Por fim, foram propostas duas equações com termos ajustáveis para a fugacidade da água na fase hidrato em função da temperatura, uma linear e outra quadrática, para utilização conjunta com a equação PC-SAFT. Para comparação, quando possível os sistemas foram avaliados com auxílio do software CSMGem. A utilização da equação PC-SAFT em conjunto com as expressões propostas resultou na melhor predição da pressão de equilíbrio em temperaturas determinadas. / Gas hydrates are important for issues that include flow assurance in oil and gas industries, carbon sequestration, natural gas transport, seawater desalinization, and purification of contaminated water. To investigate those fields and applications, temperature and pressure in which hydrates are formed must be determined. This can be done through hydrate formation and/or dissociation experiments, as well as through thermodynamic models, empirical correlations, and computational methods. In this work, the performance of the PC-SAFT in modeling the fluid phase in hydrate phase equilibrium was investigated. Its results were compared to the Peng-Robinson equation of state and to the model proposed by Klauda and Sandler. For the water molecule, different association schemes were considered. For the hydrate phase, the van der Waals and Platteuw (vdWP) equation was used. The effect of additional shells in the estimation of the Langmuir constant was assessed. Finally, equations for water fugacity in hydrate phase, as a function of temperature, were proposed. These equations were intended to be used with the PC-SAFT equation of state, and both linear or quadratic equations were considered. For comparison, when possible the software CSMGem was used to compute the equilibrium conditions. The use of the PC-SAFT EOS, along with the proposed equations, resulted in a better prediction of the equilibrium pressure as a function of temperature.

When ice is compressed alongside a gas, crystalline 'host - guest' inclusion compounds known as gas clathrate hydrates form. These compounds are of interest not only for their environmental and possible technological impact as gas storage and separation materials, but also for their ability to probe networks not readily adopted by the pure `host' water molecules, and to study the interactions between water and gas molecules. Despite the pressure dependent crystal structures being fully determined for a large variety of `guest' gas species there is still relatively little known about the crystal structures in small guest gas systems such as H2 hydrate. The majority of structural studies have been done with x-ray diffraction and report a number of conflicting structures or hydrogen contents for the four known stable phases (sII, C0, C1 and C2). As this is a very hydrogen rich system the most ideal method to study the structure is neutron diffraction, which is able to fully determine the location of the hydrogen atoms within the structure and would allow a direct measurement of any hydrogen ordering within the host structure and the H2 content. In this work the phase diagram of the deuterated analogue of the H2-H2O system is explored at low pressures (below 0.3 GPa) with neutron diffraction. In the pressure/temperature region where the sII phase is known to be stable, two metastable phases were observed between the formation of sII from ice Ih and that this transition sequence occurred in line with Ostwald's Rule of Stages. One of these metastable phases was the C0 phase known to be stable in the H2-H2O system above 0.5 GPa, and the other is a new structure not previously observed in this system and is dubbed in this work as C-1 . Prior to this work the C0 phase has been reported with various structures that were determined with x-ray diffraction, and here the crystal structure and H2 content at low pressure are determined with neutron diffraction. The C0 phase was found to form a similar host structure to those of the previous studies with spiral guest sites but is best described with highly mobile H2 guests and a higher symmetry space group which make it the same structure as the spiral hydrate structure (s-Sp) recently observed in the CO2 hydrate system. In addition to this structure being determined at pressure a sample of C0 was also recovered to ambient pressure at low temperature and its structure/H2 content is presented as it was warmed to decomposition. The crystal structure of the C-1 phase was determined to be similar to ice Ih and a sample was recovered to ambient pressure to study its decomposition behaviour. Evidence for a similar structure in the helium hydrate system at low pressure is also reported here. This work was then extended to higher pressures with the recent developments of a hydrogen-compatible gas loader and large-volume diamond anvil cells. Several test experiments on gas-loaded Paris-Edinburgh presses are described on systems that are similar to hydrogen-water like urea-hydrogen and neon-water. And a further preliminary high pressure study on the deuterated analogue of the H2- H2O system in a diamond anvil cell between 3.6 and 28 GPa shows decomposition behaviour as pressure was increased.

Salt hydrates have the potential to be used in heat storage as both phase-change materials (PCMs) and thermochemical materials (TCMs). These materials offer advantages over traditional heat storage methods due to their high energy densities. However, both domestic and industrial applications require thousands of thermal cycles and there are often many issues that need to be overcome before these materials can be used reliably for heat storage. One of the major issues with using salt hydrates as PCMs is incongruency - the formation of anhydrous phases during melting. In this research, the mechanisms of the action of polymers to prevent incongruency in sodium acetate trihydrate have been investigated. A new polymorph of anhydrous sodium acetate, Form IV, was obtained in the presence of the polymer. This polymorph crystallises as long, blade-shaped crystals, thereby increasing the surface area to volume ratio. Indexing of the crystal faces revealed that every face had Na+ or the oxygen atoms of the acetate ion near or on the surface, as opposed to hydrophobic methyl groups found on the faces of the anhydrous salt grown without polymer. These two factors are believed to significantly increase the dissolution kinetics. This technique has the potential to be used for screening polymers to reformulate other salt hydrates that display incongruent behaviour. Eutectic compositions of NaCl and KCl with strontium hydroxide octahydrate were investigated as a potential means to prevent the incongruency of this PCM. However, degradation was observed with thermal cycling. Variable temperature PXRD studies discovered a new Sr(OH)2 hydrate when heating above 75 °C - Sr(OH)2. ⅓H2O. The recrystallisation of the octahydrate from the new phase was slow with incomplete conversion, explaining the degradation with continuous cycling. The effect of addition of NaCl and KCl to congruent barium hydroxide octahydrate was also investigated. On heating, a phase transition was observed, but the samples remained solid. Variable temperature PXRD investigations discovered that this was due to the formation of the salt hydrate, Ba(OH)Cl.2H2O. This hydrate melted at 110 °C, showing its potential as a high temperature PCM. The dehydration pathways of magnesium sulfate heptahydrate were investigated. In-situ PXRD studies showed that changing the heating rate changed the intermediates present during the dehydration. The fast dehydration rate saw both the known phases of trihydrate and 2.5 hydrate form as the dehydration product of the tetrahydrate. These both then dehydrated to the known dihydrate. This differed when the slower heating rate was used, as the trihydrate was the only product of dehydration from the tetrahydrate. The trihydrate then proceeded to dehydrate to a new phase. This was found to be a new polymorph of the dihydrate, β-MgSO4.2H2O. Dehydration of MgSO4.7H2O with 50 mol% NaCl was also performed. Loeweite, Na12Mg7(SO4)13.15H2O, a dication sulfate hydrate, was formed as the major intermediate. This mixture showed advantages over the pure MgSO4.7H2O as dehydration to the monohydrate took less time and occurred at a lower temperature. There were also three fewer intermediate phases before dehydration to the monohydrate. Suspension and encapsulation materials were used in order to overcome the major issue of agglomeration with magnesium sulfate. Liquid water was ruled out as a viable hydration medium. Apparatus was developed to test humidity cycling, which allowed the effects of dehydration time and temperature to be investigated, as well testing a range of different formulations.

Les fluides frigoporteurs diphasiques sont des suspensions de cristaux solides. Exploitant l'énergie de changement de phase solide/liquide, ils sont intéressants d'un point de vue énergétique pour le transport du froid. De plus, leur utilisation est un moyen de réduire les quantités de réfrigérants classiques utilisés. Des coulis de glace sont actuellement mis en place dans le domaine de la réfrigération. Cette technologie peut être étendue vers d'autres domaines d'application, à d'autres domaines de températures : la congélation et la climatisation. <br />Pour une application en climatisation, nous étudions une solution de TBAB (Bromure de Tetra-ButylAmmonium). A pression atmosphérique et pour des températures positives, cette solution cristallise pour former des composés similaires à la glace : des hydrates. Les propriétés physiques de la solution initiale et sa capacité à cristalliser ont tout d'abord été étudiées. Nous avons mis en évidence la possibilité de former deux types de cristaux de natures différentes.<br />Un dispositif expérimental permettant la caractérisation rhéologique des fluides frigoporteurs diphasiques a été élaboré. Les suspensions sont formées dans un échangeur à surface brossée et caractérisées par des mesures de débit et pertes de charge en conduites cylindriques. <br />Les coulis d'hydrates de TBAB peuvent être décrits par un modèle de type « fluide de Bingham ». Nous montrons que les paramètres rhéologiques (viscosité apparente et contrainte seuil) dépendent non seulement de la teneur en particules mais aussi du type d'hydrate en suspension et de la concentration initiale de la solution. La contrainte seuil est interprétée par les forces d'interactions entre particules de type Van der Waals. <br />Enfin, d'éventuels effets de stratification sont modélisés par une méthode de différences finies. Le principe est de calculer les profils de vitesse et de concentration en particules au cours de l'écoulement d'une suspension de cristaux en conduite. Les calculs sont validés par les profils de vitesse obtenus expérimentalement par P. Reghem (2002). Ce modèle permet d'évaluer l'influence du profil de concentration en particules sur les pertes de charge.

La capture et la séquestration du CO2 en sortie des usines d'incinération, des centrales thermiques ou des cimenteries est devenu un enjeu mondial. La capture de ce gaz par voie hydrate est une alternative prometteuse. L'objet de cette thèse est l'étude de la stabilité des systèmes d'hydrates mixtes contenant du CO2 et un autre gaz (N2, CH4 et H2) avec l'eau pure, ou encore avec un additif permettant l'abaissement des pressions de formation : le tetrabutylamonium bromure (TBAB), dans une perspective de séparation de gaz. La technique expérimentale que nous avons utilisée est la calorimétrie différentielle programmée (DSC). Elle nous a permis de mesurer les températures et les enthalpies de dissociation des différents systèmes d'hydrates avec l'eau pure : N2, CH4, N2+ CO2, CH4+CO2, H2+CO2 ; mais aussi des systèmes semi-clathrates: CO2+CH4 et CO2+N2 à différents pourcentages massiques de TBAB (10, 20, 30 et 40). La dernière partie de cette thèse concerne la modélisation thermodynamique des semi-clathrates, où nous avons développé le cas particulier du système d'hydrate: CH4+TBAB.

Les hydrates de gaz sont des composés solides formés à partir de molécules de gaz emprisonnées dans des structures cristallines formées par des molécules d'eau reliées par liaisons hydrogène. Ils sont stables sous des conditions de haute pression et de basse température. Dans les conduites pétrolières, la formation d'hydrate de gaz peut être responsable du colmatage des conduites et du blocage des vannes. Pour éviter leur cristallisation, il existe plusieurs solutions : l'isolation ou le réchauffage de la conduite pétrolière ainsi que l'injection d'additifs cinétiques ou thermodynamiques. Une autre solution envisagée est l'utilisation d'additifs anti-agglomérants. Il s'agit d'agents tensio-actifs qui favorisent d'abord la formation d'une émulsion eau dans huile et ensuite limitent l'agglomération entre les cristaux une fois formés. De cette façon, la taille des particules d'hydrates serait limitée par la taille des gouttelettes d'eau dans l'émulsion. Cette méthode a été utilisée dans les travaux de Camargo (2001) à l'IFP et puis de Fidel-Dufour (2004) à l'ENSM de Saint-Etienne. Cette thèse est une étude consacrée à la caractérisation du couplage entre la cristallisation des hydrates et la rhéologie des écoulements pétroliers diphasiques (émulsion eau dans huile) laminaires ou turbulents dont l'objectif final est la production d'un modèle de cristallisation en écoulement. Elle s'appuie sur les mesures réalisées à l'aide de la sonde FBRM (Focused Beam Reflectance Measurement) qui permet des mesures en longueurs de cordes (CLD) in-situ lors de la formation d'hydrates en systèmes dispersés. La formation des hydrates de gaz en écoulement est étudiée de manière expérimentale sur deux dispositifs : une boucle de circulation Archimède située à l'ENSM-SE et une boucle de circulation Lyre à l'IFP Lyon. Les résultats obtenus avec les deux dispositifs sont comparés. La plus grande partie de ce travail a porté sur l'interprétation des mesures de longueurs de corde de la FBRM. Une série d'algorithmes permettant de générer des agrégats aléatoires ont été élaborés, suivis du calcul de leurs CLD. Ces CLD sont comparées avec les CLD obtenues expérimentalement permettant ainsi le suivi de l'agglomération pendant la cristallisation en écoulement.

L'exploitation de gisements pétroliers off shore doit souvent faire face à des problèmes de colmatage de conduites, notamment dus à la cristallisation d'hydrates de gaz. Actuellement, les opérateurs ont recours à des additifs antigels, dont l'efficacité est limitée par des conditions d'exploitation et des normes anti-pollution de plus en plus sévères. Aussi les recherches s'orientent-elles vers une nouvelle classe d'inhibiteurs dits à faible dose. Afin de comprendre l'influence de tels additifs, nous avons réalisé un réacteur haute pression muni d'un dispositif d'injection de liquide et d'un capteur turbidimétrique <i>in situ</i>. L'accès à la granulométrie de la suspension aux premiers stades de la cristallisation et à la consommation de gaz permet de caractériser la cinétique de formation de l'hydrate de méthane. Nous avons développé un protocole opératoire original qui autorise une maîtrise accrue de la germination des cristaux, grâce à un ensemencement initial de la solution. Le temps de latence devient alors un paramètre représentatif de l'efficacité des inhibiteurs. Nous avons alors évalué l'influence des conditions de pression et d'agitation sur l'évolution de la population de cristaux en l'absence d'additif. Puis nous avons déterminé l'effet inhibiteur d'un additif cinétique modèle, la polyvinylpyrrolidone (PVP). Mis en solution avant la cristallisation, il allonge la période de latence, diminue la vitesse de consommation du gaz et ralentit la création de nouvelles particules durant plusieurs heures. Par contre, lorsque ce polymère est injecté dans le milieu en cours de formation, il n'affecte plus la cinétique de la réaction. Nous donnons enfin les bases d'un modèle relevant des processus élémentaires de cristallisation : germination, croissance et agglomération des particules. Confrontée aux données expérimentales, une étude paramétrique nous a permis d'émettre des hypothèses quant à l'effet des inhibiteurs cinétiques sur la formation des hydrates de gaz.

The first project in this work used classical molecular dynamics to study the ice recrystallization inhibition potential of a series of carbohydrates and alcochols, using the hydration index, partial molar volumes and isothermal compressibilities as parameters for measuring their cryogenic efficacy. Unfortunately, after 8 months of testing, this work demonstrates that the accuracy and precision of the density extracted from simulations is not sufficient in providing accurate partial molar volumes. As a result, this work clearly demonstrates that current classical molecular dynamics technology cannot probe the volumetric properties of interest with sufficient accuracy to aid in the research and development of novel cryoprotectants.The second project in this work used molecular dynamics simulations to evaluate the Gibbs free energy change of substituting CO2 in sI clathrate hydrates by N2,CH4, SO2 and H2S flue gas impurities under conditions proposed for CO2 capture (273 K, 10 bar). Our results demonstrate that CO2 substitutions by N2 in the small sI cages were thermodynamically favored. This substitution is problematic in terms of efficient CO2 capture, since the small cages make up 25% of the sI clathrate cages, therefore a significant amount of energy could be spent on removing N2 from the flue gas rather than CO2. The thermodynamics of CO2 substitution by CH4, SO2 and H2S in sI clathrate hydrates was also examined. The substitution of CO2 by these gases in both the small and large cages were determined to be favorable. This suggests that these gases may also disrupt the CO2 capture by sI clathrate hydrates if they are present in large concentrations in the combustion flue stream. Similar substitution thermodynamics at 200 K and 10 bar were also studied. With one exception, we found that the substitution free energies do not significantly change and do not alter the sign of thermodynamics. Thus, using a lower capture temperature does not significantly change the substitution free energies and their implications for CO2 capture by sI clathrate hydrates.

Considerable research has been done to improve hydrate formation rate. One of the ideas is to introduce mechanical mixing which later tend to complicate the design and operation of the hydrate formation processes. Another approach is to add surfactant (promoter) that will improve the hydrate formation rate and also its storage capacity to be closer to the maximum hydrate storage capacity. Surfactant is widely known as a substance that can lower the surface or interfacial tension of the water when it is dissolved in it. Surfactants are known to increase gas hydrate formation rate, increase storage capacity of hydrates and also decrease induction time. However, the role that surfactant plays in hydrate crystal formation is not well understood. Therefore, understanding of the mechanism through morphology studies is one of the important aspects to be studied so that optimal industrial processes can be designed. In the present study the effect of three commercially available anionic surfactants which differ in its alkyl chain length on the formation/dissociation of hydrate from a gas mixture of 90.5 % methane – 9.5% propane mixture was investigated. The surfactants used were sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate (STS), and sodium hexadecyl sulfate (SHS). Memory water was used and the experiments for SDS were carried out at three different degrees of under-cooling and three different surfactant concentrations. In addition, the effect of the surfactant on storage capacity of gas into hydrate was assessed. The morphology of the growing crystals and the gas consumption were observed during the experiments. The results show that branches of porous fibre-like crystals are formed instead of dendritic crystals in the absence of any additive. In addition, extensive hydrate crystal growth on the crystallizer walls is observed. Also a “mushy” hydrate instead of a thin crystal film appears at the gas/water interface. Finally, the addition of SDS with concentration range between 242ppm – 2200ppm (ΔT =13.10C) was found to increase the mole consumption for hydrate formation by 14.3 – 18.7 times. This increase is related to the change in hydrate morphology whereby a more porous hydrate forms with enhanced water/gas contacts.

Authigenic carbonate rock (ACR) is derived from microbial oxidation of methane, biodegradation of crude oil, and oxidation of sedimentary organic matter. The precipitation of ACR was characterized petrographically, mineralogically, and geochemically. ACR collected from the seafloor in the Gulf of Mexico (GOM) and ACR recovered from drilled cores in the Krishna-Godawari (KG) basin offshore India were used. All study sites are associated with hydrocarbon gas venting, seepage, free gas, or gas hydrate. ACR from the GOM is densely cemented and extremely irregular in shape, whereas ACR from offshore India is generally an oval-shaped smooth nodule and also densely cemented. The dominant mineral in ACR is authigenic calcite. ACR contains carbon derived from sedimentary organic carbon oxidation that geologically sequesters much fossil carbon. Bulk carbon and oxygen isotopes of ACR were measured. ACR from the GOM is strongly depleted in 13C with ?13C of ?42.5? and enriched in 18O with ?18O of 4.67?. The ?13C of hydrocarbon is typically more depleted in 13C than in the associated ACR. The reason is that authigenic carbonate cements from hydrocarbon oxidation generally enclose skeletal material characterized by normal marine carbonate. Three groups that represent different hydrocarbon sources to ACR were classified in this study: primary carbon sources to ACR from (1) methane plus biodegraded oil, (2) methane, or (3) biodegraded oil. Wide ranges in ?13C (?49.12 to 14.06?) and ?18O ( 1.27 to 14.06?) were observed in ACR from offshore India. In sediments, the ?13C may be affected by differences in the rate of organic carbon oxidation, which generate varying ?13C with depth during methanogenesis. Based on the wide range in ?13C, ACR from offshore India was classified: (1) ?13C may reflect high rates of organic carbon oxidation, (2) ACR may be derived primarily from methane oxidation, and (3) ?13C may reflect low rates of organic carbon oxidation. ?18O values are heavier than those of normal marine carbonates. The ?18O may be caused by reaction with deep-sourced water that was isotopically heavier than ambient seawater. Some samples may reflect heavy ?18O from gas hydrate decomposition, but it would not cause significant heavy oxygen isotopes.