Effects of a Quaternary Ammonium Salt on the Growth, Wettability, and Agglomeration of Structure II Hydrate Crystals

This work studied the effects of a water-soluble quaternary ammonium salt (called DA 50) on the growth, wettability, and agglomeration of cyclopentane (CP) hydrate crystals and methane (CH4)/propane (C3H8) hydrate crystals. The impact on these properties of adding 4 wt % NaCl to the DA 50 solution was also investigated. The hydrates were formed from water/CP, water/(CP + n-octane (n-C8)), and water/(CP + n-dodecane (n-C12)) mixtures at atmospheric pressure and from a water/n-C8/(CH4 + C3H8) mixture under pressure (about 67 bar). Experiments were performed at a subcooling of 6 °C in the case of the CP hydrates and 9–10 °C in the case of the CH4/C3H8 hydrates. In both hydrate systems, adding NaCl to the surfactant solution of 0.1 or 1 wt % DA 50 led to the formation of individual oil-wettable pyramidal crystals. Without salt, the hydrate formed a water-wettable shell that covered the water/oil interface just as the system without surfactant did. The antiagglomeration performance of the 1 wt % DA 50 solution was evaluated by performing torque measurements in an agitated batch reactor at a water cut of 30 vol %. Without NaCl, torque increased with the amount of CP hydrates. The system formed a nonflowable jelly-like phase, with water as the continuous phase, until a phase inversion occurred. From there on torque significantly decreased and the system became a flowable dispersion of large hydrate particles (∼700 μm) in the CP phase. With 4 wt % NaCl, the system consisted of small (∼70 μm) hydrate particles dispersed in the CP phase and the torque signal remained constant throughout the hydrate crystallization process. The torque profiles obtained at concentrations of 0 or 4 wt % NaCl for the CP hydrates and the CH4/C3H8 hydrates were similar, suggesting analogous states for both systems. For both hydrate systems, adding NaCl to the DA 50 solution led to the formation of oil-wettable hydrates and drastically improved the antiagglomeration performance of the surfactant molecules, revealing a correlation between the formation of individual crystals and the antiagglomeration performance of the surfactant. The similarity between the growth patterns and shapes of the CP–hydrate crystals and the CH4/C3H8–hydrate crystals confirmed that CP hydrates are an interesting model for evaluating the antiagglomeration performance of surfactants

Effect of a Hydrophilic Cationic Surfactant on Cyclopentane Hydrate Crystal Growth at the Water/Cyclopentane Interface

The effects of a water-soluble quaternary ammonium surfactant (called DA 50) on cyclopentane (CP) hydrate growth were studied at the water/CP interface. Microscopic observations were made without and with DA 50 at concentrations of 0.01, 0.1, and 1 wt % (in the aqueous phase). In addition, the effect of NaCl at different concentrations (0 to 4 wt %) was also investigated, in the presence and absence of the surfactant. Systems with 0, 0.1, or 1 wt % DA 50 and 0 or 2 wt % NaCl, as well as those with 0.01 wt % DA 50 and 2 or 3 wt % NaCl, all led to the formation of a hydrate layer, composed of an assembly of smooth and/or striated plates, at the water/CP interface. With 0.01 wt % DA 50 and without NaCl, hydrate needles formed at the interface before aggregating into a thick unconsolidated layer in the aqueous phase. For systems without DA 50 and with 3 or 4 wt % NaCl, and for the one with 0.01 wt % DA50 and 4 wt % NaCl, a few hexagonal, triangular, and needle-like crystals grew very slowly at the interface, and most of the interface remained free of hydrate crystals for several hours after the onset of crystallization. Spectacular changes in the hydrate growth pattern and morphology were observed for the systems with 3 or 4 wt % NaCl and 0.1 or 1 wt % DA 50, where small individual crystals, in the shape of step pyramids with their vertex pointing to the CP phase, formed at the interface. Results of interfacial tension measurements showed that the adsorption kinetics of the surfactant molecules and the amount of surfactant adsorbed on the water/CP interface increased significantly with NaCl concentration. A formation mechanism of the pyramidal hydrate crystals is proposed

Anti-agglomerant performance of surfactants evaluated in cyclopentane hydrate and CH4/C3H8 gas hydrate systems

The main objective of this work was to evaluate and compare the AA performance of surfactants of different affinity in cyclopentane (CP) hydrate and gas (methane – propane mixture) hydrate systems. The study was performed with two surfactants: Noramium® DA 50 and Inipol® AH 81, which are respectively water-soluble and dispersible in water. Their AA performance was evaluated and compared without or with 4 wt% NaCl. A comparative study of the effect of the surfactants on the water/CP interfacial activity was carried out by surface pressure measurements. The impact of the surfactants on hydrate formation and morphology was observed by microscopic observations at the water/CP interface. Lastly, the AA performance of DA 50 and AH 81, was evaluated at the macroscopic scale in a batch reactor under agitation. The experiments in reactor were performed in oil-dominated systems (70 vol%) with CP as the oil phase in the CP hydrate system, and n-octane in the gas hydrate one

Determination of thermophysical properties of cyclopentane hydrate using a stirred calorimetric cell

The cyclopentane hydrate, formed by combination of cyclopentane (CP) and water, is frequently used as a model system for clathrate hydrate compounds as it can form at atmospheric pressure and at temperatures below about 280 K. However, due to the immiscibility of CP and water, the dissociation enthalpy is challenging to obtain experimentally because total conversion of water to hydrate is difficult to achieve in quiescent conditions. Only two dissociation enthalpy values are available in literature, and a difference of 25 kJ.mol−1 between them clearly indicates a discrepancy. In this study, a stirring calorimetric cell was used with a Tian-Calvet heat-flow calorimeter, to measure phase change properties. The technical system made it possible to form pure CP-hydrate with complete conversion of water to hydrate. The dissociation temperature and dissociation enthalpy of the CP-hydrate (with max 5 wt% of residual liquid CP) were measured at 280.2 ± 0.5 K and 115,400 ± 7600 J.mol−1 of CP (377 ± 27 J.g−1 of water; 307 ± 21 J.g−1 of hydrate), respectively. This high enthalpy value opens new ways for using CP-hydrates in cold storage and refrigeration applications

Mechanically agitated calorimetric cells working under pressure at macro and micro scale: application to gas hydrates

Originally applied to fields related to oil and gas production and flow assurance, high pressure differential scanning calorimetry (HP-DSC) has now been involved in several new studies such as carbon dioxide sequestration by CO2/CH4 exchange in naturally occurring gas hydrates or CO2 hydrate reversible formation/dissociation for refrigeration loops. However, the technique still has some limitations, which are linked to the fact that the gas hydrate formation occurs at the gas/liquid interface, and because the hydrate nucleation can be rather difficult in small volumes especially in quiescent conditions. It leads to several problems such as inefficient gas dissolution, long induction times, formation of a hydrate crust covering the gas/liquid interface, low hydrate to water conversion, etc. As a result, it is very difficult to determine accurately the heat capacities and the kinetics of formation/dissociation of several systems involving gas hydrates. This study presents two prototypes of calorimetric cells equipped with an in-situ mechanical agitation system, which allow performing experiments under pressure (150 bar maximum for the cells used in this work). The first system presented, called MIXCEL®, was developed for macro-calorimetry analysis (experiments carried out with a BT 2.15 Calvet Calorimeter from SETARAM Instrumentation). Very recently, we have developed a novel prototype of micro-calorimetric agitated cell (called MICROMIXCEL®) for microDSC analyses (experiments carried out using a microDSC7 evo from SETARAM Instrumentation). Both technical aspects, and results obtained at macro and micro scales with gas hydrate systems are presented and discussed

Thermophysical properties of gas hydrates with stirred, high pressure calorimetric cells

High pressure differential scanning calorimetry (HP-DSC) is of importance in several fields involving gas hydrates, such as oil and gas production, flow assurance, carbon dioxide capture and storage, CO2 hydrates reversible formation/dissociation for refrigeration loops. However, the technique suffered for some limitations linked to the fact that the gas hydrate formation in the calorimetric cell occurs at the gas-liquids interface, leading to problems such as inefficient gas dissolution, formation of a hydrate crust covering the gas/liquid interface, low hydrate to water conversion, and difficulties to crystallize these compounds even at low temperature. It is for example rather difficult to determine accurately the heat capacities and the enthalpies of formation/dissociation of several systems involving gas hydrates. To overcome such limitations, we present two prototypes of calorimetric cells equipped with an in-situ mechanical agitation system, which allow performing experiments under pressure (150 bar max). The first one is called MIXCEL®, and was developed for macro-calorimetry analysis (experiments carried out with a BT 2.15 Calvet Calorimeter from SETARAM Instrumentation). The second one, called MICROMIXCEL®, was developed for micro-calorimetry analysis (experiments carried out using a microDSC7 evo from Setaram Instrumentation). In this study, technical details of the two cells and results obtained both at macro and micro scales will be presented, and compared to the case with no agitation. Thermophysical properties of the cyclopentane hydrate (phase change enthalpy, and specific heat) will be given and commented. The use of such novel calorimetric cells opens a wide range of possibilities for complex systems, such as gas hydrates, which must be analysed in both pressurized and agitated conditions

Mechanically agitated calorimetric cells working under pressure: technical aspects and results obtained at macro and micro scale

This study presents two prototypes of calorimetric cells equipped with an in-situ mechanical agitation system, which allow performing experiments under pressure. The first system presented, called MIXCEL®, was developed for macro-calorimetry analysis (experiments carried out with a BT 2.15 Calvet Calorimeter from Setaram Instrumentation) and already patented in 2012 1. Concerning the micro-calorimetric agitated cell (called MICROMIXCEL®), it is the first time that such a system will be presented. This novel prototype was developed for micro-DSC analysis (experiments carried out with a DSC VII from Setaram Instrumentation). Both technical aspects, and results obtained at macro and micro scale with polyphasic complex systems (gas hydrates or ice slurries), are presented and discussed. The use of such calorimetric cells opens a wide range of possibilities for systems which must be analyzed in both pressurized and agitated conditions

Effects of ionic surfactants on the interfaces and the gas hydrates agglomeration.

Lors de la production d’hydrocarbures, les conditions de pression et température dans les conduites peuvent être favorables à la formation d’hydrates de gaz (composés cristallins formés par l’association de molécules d’eau et de gaz). Leur formation peut entraîner le bouchage des conduites et mener à l’arrêt de la production, entraînant d’importantes pertes économiques. Pour remédier au risque « hydrate », les pétroliers disposent de diverses méthodes dont l’utilisation d’additifs antiagglomérants. Les antiagglomérants sont des tensioactifs capables de s’adsorber à la surface des cristaux d’hydrate et de les maintenir dispersés dans la phase hydrocarbonée, qui est généralement majoritaire. L’objectif de cette thèse est de progresser dans la compréhension des mécanismes d’action de tensioactifs ioniques pour la prévention de l’agglomération d’hydrates de gaz. Plusieurs tensioactifs cationiques ont été étudiés sur un hydrate de cyclopentane (CP) (qui se forme à pression atmosphérique) et sur un hydrate de méthane/propane (qui se forme sous pression).Pour les deux hydrates, l’effet des tensioactifs sur la morphologie des cristaux et sur leur mouillabilité a été étudié, et leur performance antiagglomérante (AA) a été évaluée en réacteur agité pour différentes conditions et compositions des systèmes. Les tensioactifs conduisant à la formation de cristaux individuels présentent les meilleures performances AA. Les observations montrent qu’il n’est pas indispensable que les tensioactifs rendent les cristaux mouillables à l’huile pour qu’ils procurent une bonne protection contre l’agglomération dans un système agité où l’huile est la phase majoritaire. Nous avons vu que la modification (par ajout de sel par exemple) de l’environnement physicochimique des molécules tensioactives peut jouer un rôle déterminant sur leurs propriétés AA. De même, la modification de la structure des molécules (nature du contre-ion, longueur des chaînes hydrocarbonées) impacte leur adsorption sur l’hydrate, la morphologie et la mouillabilité des cristaux, et par suite leur performance AA. Les principaux facteurs identifiés pour la bonne performance d’une molécule tensioactive sont sa capacité à se fixer efficacement et en quantité suffisante à la surface de l’hydrate, et à rendre les cristaux d’hydrate hydrophobes, ou dans le cas où il les rend hydrophiles d’abaisser fortement la tension interfaciale entre les phases aqueuse et huileuse de manière à réduire l’intensité des forces capillaires entre les particules. Enfin, nous avons pu établir une corrélation entre les observations faites à l’échelle microscopique et la performance AA des tensioactifs évaluée à l’échelle macroscopique. Ce travail confirme que l’hydrate de CP est globalement un bon modèle pour des évaluations simples de la performance de molécules tensioactives. L’utilisation de l’hydrate de CP présente néanmoins des limitations pour mener des études à forts sous-refroidissements et avec de grandes fractions volumiques d’eau.Pressure and temperature conditions encountered in the pipelines of hydrocarbons production may be favorable to the formation of gas hydrates (crystalline compounds formed by the association of molecules of gas and water). Their agglomeration in pipelines may form plugs and lead to production shutdowns and cause significant economic losses. To prevent it, oil and gas companies use various methods and more particularly anti-agglomerant additives. Anti-agglomerants are surfactants that can adsorb at the hydrate crystals surface and keep them dispersed in a hydrocarbon phase. The objective of this thesis is to progress in the understanding of mechanisms of action of ionic surfactant to prevent the gas hydrates agglomeration. Several cationic surfactants were studied on a cyclopentane (CP) hydrate (formed at atmospheric pressure) and on a methane/propane hydrate (formed under pressure). For both hydrates, the effect of surfactants on the crystals morphology and on their wettability was investigated, and their anti-agglomerant (AA) performance was evaluated in an agitated reactor for systems at different conditions and compositions. The surfactants leading to the formation of individual crystals had the best AA performances. In order to have a good protection against the agglomeration, it is not necessary that the surfactants make the crystals oil wettable in a system where the oil phase is in excess. We showed that the modification (by the addition of salt for example) of the physicochemical environment of surfactant molecules plays an important role on their AA properties. Similarly, the modification of the structure of molecules (counter-ion nature, length of the hydrocarbon chains) affects their adsorption on the hydrate, the morphology and wettability of crystals and consequently their AA performance. The main factors identified for a good performance of a surfactant molecule are its capacity to be efficiently fixed and in a sufficient amount on the hydrate surface in order to make the hydrate crystals hydrophobic. In the case where it makes the hydrate hydrophilic, the surfactant has to strongly reduce the interfacial tension between the aqueous and oil phases and then reduce the intensity of capillary forces between hydrate particles. Lastly, we set a correlation between the observations done at the microscopic scale and the AA performance of surfactants evaluated at the macroscopic scale. This work confirms that the CP-hydrate is overall a good model for a simple evaluation of the surfactant molecules performance. However, the use of the CP-hydrate has some limitations to conduct studies at high subcooling and watercut

Etude microscopique et macroscopiques de la propriété anti-agglomérante de tensioactifs sur l’hydrate de cyclopentane

Les journées thématiques « Hydrates / Clathrates » organisées en janvier 2015 et 2016 par la SFT et le séminaire « Hydrates de gaz » à l’Ifremer - Brest fin mars 2016 ont permis de faire se rencontrer les différents acteurs de la recherche sur les hydrates de gaz en France qui ont ainsi pu échanger sur de nombreux projets en cours. Ces rencontres ont également fait émerger une volonté commune de rassembler compétences scientifiques et technologiques dans une fédération nationale pour répondre à des problématiques scientifiques et applicatives liées aux hydrates. Le choix s’est porté sur la création d’un Groupement de Recherche (GdR), qui sera proposée au CNRS à l’automne ou au printemps 2017. Parmi les thèmes d’intérêt, l’étude de la dynamique des hydrates sédimentaires est apparue comme un sujet fédérateur car elle nécessite une approche multidisciplinaire, mêlant expérimentations en laboratoire, modélisation et confrontation des résultats avec des données de terrain. Le projet européen COST-MIGRATE, sur les hydrates dans les fonds marins, crée un contexte favorable au développement de travaux dans ce domaine. Les autres thèmes d’intérêt, plus proches du génie des procédés, incluent l’utilisation des hydrates pour la séparation et la capture des gaz, comme matériau à changement de phase pour la réfrigération secondaire, ou encore pour le traitement et la purification des eaux. En plus du projet européen COST-MIGRATE, trois projets ANR sont en cours sur ces sujets, rassemblant déjà une dizaine de laboratoires ou institutions de recherche français sur la thématique des hydrates de gaz et de leurs applications

Évaluation de la performance d'un nouveau AA-LHDI biodégradable sur des hydrates de cyclopentane et des hydrates de méthane/propane

International audienceThe main objective of this work is to evaluate the performance of a readily biodegradable anti-agglomerant, called AA-LDHI, not harmful for the environment following the European legislation. It was tested in cyclopentane (CP) hydrate and methane (CH 4)/propane (C 3 H 8) hydrate systems. The performance of AA-LDHI was first tested in a batch reactor by torque measurements with the two hydrate systems in oil-dominated conditions (70 vol%). The experiments were performed for a subcooling of 6 °C for the CP-hydrate and up to 17 °C for the gas hydrate. The impact of AA-LDHI on hydrate growth pattern and on hydrate crystal morphology were investigated by microscopic observations at the water/CP interface for the CP-hydrate system, and at a water/(n-octane + CH 4 /C 3 H 8) interface for the gas hydrate system. Then, AA-LDHI was evaluated in a semi-industrial flow loop. Without surfactant, the hydrate formed a polycrystalline shell at the water/oil interface. With AA-LDHI, large conical hydrate crystals with the vertex pointing to the aqueous phase grew at the interface before sinking in the aqueous phase. The results obtained with both CP and gas hydrates are consistent amongst themselves. In the batch reactor experiments without surfactant, hydrate crystallization led to a significant increase in the torque value and finally to the blockage of the agitator. With AA-LDHI, the torque remained almost constant at the baseline showing that this AA presented good anti-agglomeration performance. The microscopic observation in a CP phase of CP-hydrate particles formed in the reactor showed large water-wettable particles (about 400 μm) gathered in clusters but not agglomerated. AA performance of AA-LDHI was also evaluated in a semi-industrial flow loop (1-inch diameter & 35.6 m total length) in similar conditions with CH 4 /C 3 H 8 mix gas but using real condensate. This experiment proved that the AA-LHDI is fully efficient in transporting safely hydrates.Un nouveau AA-LHDI biodégradable est testé sur des hydrates de cyclopentane et des hydrates de méthane/propan