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The use of a hydrate-based technology in seawater desalination is an interesting potential hydrate application since salt ions would be excluded from the hydrate crystal lattice. In order to better understand the hydrate-based desalination process, experiments have been conducted using cyclopentane (CyC5, sII) hydrates, which can be formed at atmospheric pressure and temperatures below 7.7 °C. The hydrate formation experiments were performed at various subcoolings for aqueous solutions with different salinities in a bubble column. The hydrate formation times decreased and the hydrate conversion increased with increasing subcooling and agitation. Various hydrate-former injection methods were studied, with the most effective method involving spraying finely dispersed CyC5 droplets (around 5 μm in diameter) into the water-filled bubble column. The latter method resulted in a 2-fold increase in seawater conversion to hydrate crystals compared with injecting millimeter-scale CyC5 droplets. A desalination effic...

Clathrate hydrates of hydrogen are of specific interest due to their potential ability to store molecular hydrogen. In particular, structure H (sH) hydrate has a higher theoretical storage capacity in comparison with the other two more common hydrate structures (sI and sII). This paper investigates the effect of hydrogen (H2) concentration on the phase equilibria of sH hydrate in a quaternary system of water, methane, hydrogen, and methylcyclohexane. Phase equilibria and cage occupancies of the quaternary system were predicted using the van der Waals and Platteeuw (vdWP) model for different hydrogen/methane ratios ranging from 0 to 7. Model predictions for the quaternary systems were found to be in good agreement with measured experimental data. It was evident from the thermodynamic equilibrium conditions of the quaternary system (MCH + H2O + CH4 + H2) that as the H2 concentration increases (H2:CH4 ratio increased from 0 to 7), higher pressures are required to produce sH hydrates at the same temperature. It was also found that the fractional cage occupancy of CH4 in the small and medium cages of sH hydrate increases with pressure. Fil: Khan, M. Naveed. Petroleum Institute. Chemical Engineering Department; Emiratos Arabes Unidos. Colorado School of Mines. Chemical & Biological Department. Center for Hydrate Research; Estados Unidos Fil: Rovetto, Laura Jorgelina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Planta Piloto de Ingeniería Química (I). Grupo Vinculado al Plapiqui - Investigación y Desarrollo en Tecnología Química; Argentina. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina Fil: Peters, Cor J.. Petroleum Institute. Chemical Engineering Department; Emiratos Arabes Unidos Fil: Sloan, E. Dendy. Colorado School of Mines. Chemical & Biological Department. Center for Hydrate Research; Estados Unidos Fil: Sum, Amadeu K.. Colorado School of Mines. Chemical & Biological Department. Center for Hydrate Research; Estados Unidos Fil: Koh, Carolyn A.. Colorado School of Mines. Chemical & Biological Department. Center for Hydrate Research; Estados Unidos

Hydrogen sulfide is an exceptionally stable structure I (sI) gas hydrate forming guest molecule that is becoming increasingly prevalent in oil and gas production. However, phase equilibria data on pure hydrogen sulfide hydrate reported in the literature are relatively limited and inconsistent compared to other common hydrate formers such as methane or carbon dioxide. In this study, 61 hydrate phase equilibria measurements for sI hydrates containing hydrogen sulfide are reported in the temperature range from T = 273.68 K to 301.53 K and pressure range from p = 0.108 MPa to 1.960 MPa. Experimental data were measured using the isochoric pressure search (IPS) method which has been well established, as well as a modified IPS method, termed the phase boundary dissociation (PBD) method, which gives more efficient measurements of pure hydrate phase equilibria data. For example, it was shown in this work that using the new PBD method reduced the experimental run time to approximately 4.8 h per data point, compared...

Understanding gas hydrate formation on gas bubbles evolved from an oil/gas blowout and the stability conditions of the hydrates formed are key to controlling hydrates during a blowout and its containment. In this work, methane hydrate formation and dissociation conditions on suspended gas bubbles in water were studied. For the formation process, methane gas was gradually injected into a counter flowing water column until a full hydrate shell on suspended gas bubbles was observed. The hydrate shells were then dissociated by either depressurization or heating. The minimum methane concentration to form hydrate shells on suspended gas bubbles in water was determined for the pressure range of (7.0 to 17.3) MPa at 277 K. The dissociation pressures of hydrates are also reported for temperatures from (276 to 286) K. It is observed that the hydrate shells on gas bubbles formed and remained stable only when a minimum dissolved concentration (∼0.0013 mole fraction) of gas in water was reached. During dissociation, h...

The formation and accumulation of natural gas hydrates in deep subsea flow lines are one of the most challenging flow assurance problems. Typical high pressure/low temperature operation conditions in deep subsea facilities promote rapid formation of gas hydrates. Recent observations in flow loop experiments in gas/water systems suggested a relationship between hydrate volume and flow regime on pressure drop. In the current work, a simple hydrodynamic slug flow model, based on fundamental multiphase flow concepts, is coupled with a transient hydrate kinetics model to study the effect of hydrate formation on slug flow in gas/water systems.

As oil/gas subsea fields mature, the amount of water produced increases, which results in an increased risk of gas hydrate plug formation in the flowlines. It is important to understand the mechanism of gas hydrate plug formation in high water cut systems in order to manage gas hydrate risk. In this work, we performed an extensive series of gas hydrate formation and dissociation experiments in a 4-inch diameter flowloop at the ExxonMobil research facility at Friendswood, TX. The flowloop was instrumented for pressure, temperature, density, and differential pressure measurements. The effect of mixture velocity (1–2.5 m/s) and liquid loading (50–90 vol.%) on gas hydrate plug formation was studied for 100 vol.% water cut (no oil present) systems, with methane as the gas hydrate former. The pressure drop across the pump due to flow did not substantially increase until a certain concentration of gas hydrates, defined as ϕ transition , formed; this transition, as measured by the rapid increase in pressure drop, can be used as an indication for onset of hydrate plug formation. ϕ transition was found to be unaffected by liquid loading and the presence of salt (3.5 wt.% NaCl) in water, while it increased with increasing mixture velocity. A gas hydrate plugging mechanism for 100 vol.% water cut systems is proposed, which involves a transition from homogeneous to heterogeneous suspensions of gas hydrate particles in water. We hypothesize that the large pressure drop observed after ϕ transition results from the formation of a gas hydrate bed and wall deposit. A correlation between ϕ transition and the mixture velocity is presented, this correlation allows the prediction of an onset of hydrate plug formation based on the fluids mixture velocity.

One of the major issues in flow assurance includes pipeline plugging due to hydrate formation and deposition. A key uncertainty in gas pipelines is hydrate deposition on the pipe wall. This work demonstrates hydrate formation and deposition on a cold surface in water-saturated gas systems. Methane hydrate deposition can be achieved in a laboratory-scale apparatus by formation of hydrates from the gas phase on the outer surface of a cold surface. The deposit evolves from the initial formation to growth to hardening stages, observed to be initially a porous deposit that later anneals to a relatively nonporous deposit. The hydrate deposit thickness gradually reaches a limit as the hydrate surface approaches the hydrate equilibrium temperature. Performing the measurements at higher initial subcooling in the system results in a thicker hydrate deposit. The average calculated hydrate deposit porosity decreases during the experiment and reaches ∼5% during the annealing stage.

Fundamental understanding of gas hydrate formation and decomposition processes is critical in many energy and environmental areas and has special importance in flow assurance for the oil and gas industry. These areas represent the core of gas hydrate applications, which, albeit widely studied, are still developing as growing fields of research. Discovering the molecular pathways and chemical and physical concepts underlying gas hydrate formation potentially can lead us beyond flowline blockage prevention strategies toward advancing new technological solutions for fuel storage and transportation, safely producing a new energy resource from natural deposits of gas hydrates in oceanic and arctic sediments, and potentially facilitating effective desalination of seawater. The state of the art in gas hydrate research is leading us to new understanding of formation and dissociation phenomena that focuses on measurement and modeling of time-dependent properties of gas hydrates on the basis of their well-established thermodynamic properties.

High pressure differential scanning calorimetry was utilized to study the mass transfer rates across methane hydrate films grown at hydrocarbon−water interfaces in a quiescent system, as a function...

A review of surface chemistry concepts is presented, with the principal objective of identifying interfacial phenomena and surface chemistry interactions involved in gas hydrate formation and agglomeration in oil and gas pipelines. There are five types of interfaces where gas hydrates may form and aggregate: gas/liquid, liquid/liquid, gas/solid, liquid/solid, and solid/solid; where the gas is the hydrocarbon gas, liquid is either oil, water, or condensate, and solid is either gas hydrate or the pipe wall surface. A review of fundamental interfacial concepts can help create a better understanding of phenomena at these interfaces, and can help industry move from hydrate prevention to risk management. Two areas of surface chemistry have been selected to illustrate the concepts and mechanisms associated with these systems: surfactants and emulsions. Examples from the literature pertaining to gas hydrates are presented for each system.

Water droplet sizes in crude oil emulsions were measured using an in situ particle video microscope (PVM) probe and a focused beam reflectance measurement (FBRM) probe for a variety of oils spanning over two orders of magnitude in viscosity and for varying shear rates. The arithmetic or Sauter mean diameter was found to maintain the same constants of proportionality with the maximum (99th percentile) droplet size for different distributions, previously only shown for water-continuous emulsions. The FBRM values for the mean droplet size, while lower than the PVM values, could be related to the latter by an empirical quadratic relationship with an average error of less than 20%. The droplet size distribution was found to be represented well by a log-normal distribution with good agreement between correlated and measured mean droplet size. Following the agreement between the mean and maximum droplet sizes, the log-normal standard deviation was linearly related to the mean droplet size. The PVM probe was foun...

Significant factors controlling gas hydrate growth in water and water-in-oil dispersions have been tested. In particular, the influence of shear rate, presence of oil, and thermodynamic driving force (represented by pressure supersaturation) on hydrate growth rates is included. Formation rates in water show some discrepancy compared to previous work, which is likely caused by differences in the apparatus geometries. A model is proposed for growth of hydrate in oil, in which a hydrate shell forms on a water droplet, followed by additional conversion of the water core to hydrate.

Clathrate hydrates have steadily emerged as an important field in the areas of flow assurance, energy storage and resource, and environment. To better understand the role of hydrates in all of these areas, knowledge developed in laboratory experiments must be effectively transferred to address the challenges related to hydrate formation, dissociation, agglomeration, and stability. This paper highlights the recent hydrate literature focusing on the thermodynamics, kinetics, structural properties, particle properties, rheological properties, and molecular mechanisms of formation. The foundation for continued understanding and development of hydrates in engineering practice will rely on laboratory measurements utilizing traditional and innovative tools capable of probing time-dependent and time-independent properties.

Current models for hydrate formation in subsea pipelines require an arbitrary assignment of a subcooling criterion for nucleation. In reality hydrate nucleation times depend on both the degree of subcooling and the amount of time the fluid has been subcooled. In this work, differential scanning calorimetry was applied to study hydrate nucleation for gas phase hydrate formers. Temperature ramping and isothermal approaches were combined to explore the probability of hydrate nucleation for both methane and xenon. A system-dependent subcooling of around 30 K was necessary for hydrate nucleation from both guest molecules. In both systems, hydrate nucleation occurred over a narrow temperature range (2–3 K). The system pressure had a large effect on the hydrate nucleation temperature but the ice nucleation temperature was not affected over the range of pressures investigated (3–20 MPa). Cooling rates in the range of (0.5–3 K/min) did not have any statistically significant effect on the nucleation temperature for each pressure investigated. In the isothermal experiments, the time required for nucleation decreased with increased subcooling.

In this study we measure the effect of hydrate kinetic inhibition in emulsions. Because hydrate nucleation is stochastic, many experiments normally are needed to obtain accurate analysis of the effectiveness of kinetic inhibitors. Using differential scanning calorimetry (DSC), we show how emulsions can reduce the number of kinetic samples needed to obtain a statistical analysis of the effectiveness of polyvinylcaprolactam, PVCap, a common kinetic inhibitor. PVCap is shown to delay the average hydrate nucleation time and also causes hydrate nucleation to become more stochastic. This novel method uses less material and experimental time compared to traditional methods used to test kinetic inhibitors.

The accuracy of the focused beam reflectance measurement (FBRM) probe, which measures a chord length distribution, from Mettler–Toledo Lasentec ® has been explored. A particle video microscope (PVM) probe, which provides in situ digital images, was used as a direct visual method to test the reliability of the FBRM results. These probes can provide in situ particle characterization at high pressures. The FBRM has been used to study emulsions and ice and clathrate hydrate formation. The ability of the FBRM to accurately characterize unimodal and bimodal distributions of particles and droplets and to measure agglomeration events was investigated. It was found that while the FBRM can successfully identify system changes, certain inaccuracies exist in the chord length distributions. Particularly, the FBRM was found to oversize unimodal distributions of glass beads, but undersize droplets in an emulsion and was unable to measure full agglomerate sizes. The onset of ice and hydrate nucleation and growth were successfully detected by the FBRM, but quantitative analysis of the particle and agglomerate sizes required simultaneous PVM measurements to be performed.

Methane hydrate formation and dissociation studies from high water content (>60vol% water) – crude oil emulsions were performed. The hydrate and emulsion system was characterized using two particle size analyzers and conductivity measurements. It was observed that hydrate formation and dissociation from water-in-oil (W/O) emulsions destabilized the emulsion, with the final emulsion formulation favoring a water continuous state following re-emulsification. Hence, following dissociation, the W/O emulsion formed a multiple o/W/O emulsion (60 vol% water) or inverted at even higher water cuts, forming an oil-in-water (O/W) emulsion (68 vol% water). In contrast, hydrate formation and dissociation from O/W emulsions (⩾71vol% water) stabilized the O/W emulsion.

Clathrate hydrate film growth has been investigated at the hydrocarbon/water interface for cyclopentane and methane hydrate, using video microscopy combined with gas consumption measurements. Hydrate formation was characterized by the film thickness, propagation rate across the hydrocarbon/water interface, and gas consumption. The film formation processes of cyclopentane and methane hydrate were measured over the temperature range of 260–273 K and pressure range of atmospheric to 8.3 MPa. Hydrate formation was initiated by the propagation of a thin, porous film across the hydrocarbon/water interface. This thickening rate was strongly dependent on the hydrate former solubility in the aqueous phase, in the absence and presence of hydrate. The methane hydrate film thickness began at about 5μm and grew to a final thickness (20–100μm) which increased with subcooling. The cyclopentane hydrate film thickness began at about 12μm and grew to a final thickness (15–40μm) which again increased with subcooling. The hydrate film grew into the water phase. Gas consumption indicated that the aqueous phase supplied hydrate former during the initial hydrate growth, and the free gas supplied the hydrate former for film thickening.

Clathrate hydrates have long been known as non-stoichiometric compounds, but the relation between hydrate and overall system composition is often overlooked. Previous density measurements showed that the composition of ethylene oxide (EO) structure I (sI) hydrate changes when formed from different EO solutions. In our X-ray diffraction (XRD) experiments, it was found that at 263.15 K , the unit cell (11.987 A ) of EO hydrate formed from EO-lean ( 14%) solutions (12.020 A ) . Since all conditions except overall composition were constant, the hydrate lattice parameter could only be changed via hydrate composition. Raman spectroscopy measurements also showed that the hydrate composition changes when formed from different solutions. The hydrate composition change was predicted using the statistical mechanics model developed by van der Waals and Platteeuw. The predictions agreed well with the measured data. An EO–H 2 O phase diagram with a hydrate solid solution range is proposed based upon these measurements and calculations. It is probable that a solid solution range also exists in CH 4 and CO 2 hydrates. This may impact energy recovery from sea-floor methane hydrates and CO 2 sequestration in the deep ocean. Schematic phase diagrams are presented for CH 4 +H 2 O and CO 2 +H 2 O which account for the hydrate solid solution range.

Hydrogen storage from two independent sources of the same material represents a novel approach to the hydrogen storage problem, yielding storage capacities greater than either of the individual constituents. Here we report a novel hydrogen storage scheme in which recoverable hydrogen is stored molecularly within clathrate cavities as well as chemically in the clathrate host material. X-ray diffraction and Raman spectroscopic measurements confirm the formation of beta-hydroquinone (beta-HQ) clathrate with molecular hydrogen. Hydrogen within the beta-HQ clathrate vibrates at considerably lower frequency than hydrogen in the free gaseous phase and rotates nondegenerately with splitting comparable to the rotational constant. Compared with water-based clathrate hydrate phases, the beta-HQ+H2 clathrate shows remarkable stability over a range of p-T conditions. Subsequent to clathrate decomposition, the host HQ was used to directly power a PEM fuel cell. With one H2 molecule per cavity, 0.61 wt % hydrogen may be stored in the beta-HQ clathrate cavities. When this amount is combined with complete dehydrogenation of the host hydroxyl hydrogens, the maximum hydrogen storage capacity increases nearly 300% to 2.43 wt %.

Three-phase (ice + hydrate + vapor and liquid water + hydrate + vapor) equilibrium data are presented for xenon structure I (sI) hydrate in the temperature range 228−288 K. Four-phase (ice + hydrate + liquid hydrocarbon + vapor and liquid water + hydrate + liquid hydrocarbon + vapor) equilibrium data are reported for structure H (sH) hydrates of xenon + 2,2-dimethylbutane and methane + 2,2-dimethylbutane. The data for both of the latter systems indicated two invariant five-phase points (I + liquid water + sH + V + liquid hydrocarbon) at 273.15 K while the data for the water + xenon + 2,2-dimethylbutane system indicated an additional quintuple point (liquid water + sI + sH + vapor + liquid hydrocarbon) at 283.2 K.

Vapor-liquid equilibrium data for the binary system methane + carbon dioxide were measured at 230, 250, and 270 K. The ethane + carbon dioxide system was studied at 207, 210, 213, 230, 250, and 270 K, and the methane + ethane system was studied at 210, 230, 250, and 270 K. Ternary vapor-liquid equilibria for the methane + ethane + carbon dioxide system were measured at 230 K over the pressure range from 1.15 to 6.59 MPa. The Peng-Robinson equation of state was used to model the systems, and binary interaction coefficients are reported.

In order to understand the mechanism of gas hydrate kinetics and to explore the existance of other new cavities in the hydrate structure, we have used Molecular Dynamics (MD) simulation to study a system comprising two Lennard-Jones particles and 214 water molecules. Equilibrium structure and properties of twelve cases have been investigated. Our findings were as follows: • Apolar molecules promote spherical liquid water clusters in a hydrate-like labile cavity. • The size of the cavity and the coordination number is dependent upon the size of the apolar molecule. • The coordination number of water molecules is quantized in jumps of four. • Similarities are observed between the labile cavities and cavities in solid hydrates and in other chemical structures such as Buckminsterfullerene. • Such a simulation procedure suggests the possibility of other clusters which may exist in yet-to-be-found hydrates. A separate question involves whether such suggested cavities can be combined with other cavities...

This work presents equilibrium measurements for hydrate three-phase (vapor-hydrate-aqueous liquid) behavior for natural gas components with high concentrations of carbon dioxide, at temperatures between the ice point and the quadruple point of each mixture. Binary hydrate phase equilibria were measured for carbon dioxide with each of the following gases: methane, ethane, propane, isobutane, and normal butane. Data for three of those binary systems (carbon dioxide+ethane, 2-methylpropane (isobutane), and butane) do not exist in the literature

Hydrogen hydrates with tetrahydrofuran (THF) as a promoter molecule are investigated to probe critical unresolved observations regarding cage occupancy and storage capacity. We adopted a new preparation method, mixing solid powdered THF with ice and pressurizing with hydrogen at 70 MPa and 255 +/- 2 K (these formation conditions are insufficient to form pure hydrogen hydrates). All results from Raman microprobe spectroscopy, powder X-ray diffraction, and gas volumetric analysis show a strong dependence of hydrogen storage capacity on THF composition. Contrary to numerous recent reports that claim it is impossible to store H(2) in large cages with promoters, this work shows that, below a THF mole fraction of 0.01, H(2) molecules can occupy the large cages of the THF+H(2) structure II hydrate. As a result, by manipulating the promoter THF content, the hydrogen storage capacity was increased to approximately 3.4 wt % in the THF+H(2) hydrate system. This study shows the tuning effect may be used and developed for future science and practical applications.

This paper reports on three-phase equilibrium conditions of hydrates (water-rich liquid, hydrate, and vapor) of carbon dioxide and methane binary mixtures that were investigated in the temperature range of 273-288 K and the pressure range of 1.2-11.0 MPa. The vapor-phase concentrations were measured by a gas chromatograph. The results obtained were in good agreement with previous measurements done by researchers at the University of Michigan who had calculated the carbon dioxide concentration in the vapor phase. The data obtained in this work did not show unusual hydrate phenomena, as reported in a more recent publication by researchers in Hungary.

The flow assurance industry is progressively moving away from avoidance of hydrate formation towards risk management. Risk management allows hydrates to form but prevents hydrates from agglomerating and forming a plug, or delays hydrate formation within the timescale of the residence time of the water in the hydrate-prone section of the flow line. A key factor in risk management for an oil-dominated system is the stability of the emulsified water with gas hydrate formation. It is shown using Differential Scanning Calorimetry (DSC) that gas hydrate formation and dissociation has a destabilizing effect on W/O emulsions, and can even lead to a free water phase through agglomeration and coalescence of dissociated hydrate particles. Gas hydrate formation/dissociation has been shown to cause rapid hydrate agglomeration and emulsion destabilization. High asphaltene content crude oils are shown to resist hydrate destabilization of the emulsion.

High-resolution neutron diffraction measurements have been used to confirm the crystal structure of the binary clathrate hydrate of cyclohexanone and hydrogen, making cyclohexanone one of the largest known sII hydrate formers. The position, orientation, and occupancy of the guest molecules within the hydrate cavities have been determined. Because cyclohexanone falls into the class of large sII forming molecules that require a second guest for stability, the role of hydrogen in hydrate stability is further elucidated. From these results, we suggest that the stabilization behavior of hydrogen is analogous to that of other more common hydrate forming molecules and suggest that hydrogen has the potential to stabilize other clathrate structures with greater hydrogen storage capacity.

A hydration number of 20 has been measured for aqueous CH4 in the temperature range 273−298 K on the basis of the simultaneous observation of the 13C isotropic resonance lines of CH4 in both aqueous solution and the dodecahedral cage of CH4 structure I hydrate. Additional experimental evidence and analysis reported in this paper suggest that the dominant aqueous hydration number of methane is 20.

This work presents measurements on methane hydrate phase equilibrium in the three-phase region (ice + hydrate + vapor). The results complement partial data sets from other laboratories and provide new results in the region 193--260 K. These data indicated that methane hydrate of structure 1 does not undergo a phase transition to structure 2 as proposed in recent literature.

The first phase equilibrium data are reported for mixtures of heavier liquid hydrocarbons+methane forming structure H hydrates. Four-phase equilibrium conditions (water, liquid hydrocarbon, vapor, and structure H hydrate) were measured for three systems: methane+2,2-dimethylbutane, methane+2-methylbutane, and methane+methylcyclohexane. These hydrocarbons constitute a small fraction of crude of reservoirs where they coexist with various gases, including methane. Consequently this work suggests the possibility of the natural occurrence of structure H hydrates in such natural and artificial environments, and these initial phase equilibrium data have the potential to impact industrial perspectives about hydrates

An experimental method was generated and proven for the measurement of the water content of a liquid hydrocarbon which is in equilibrium only with hydrates. The method was used to determine the water content of both liquid propane and liquid ethane when hydrates were present, without the presence of a free water phase. The dielectric constant apparatus enabled the water concentration measurement of the hydrocarbon liquid without sample withdrawal. The temperature range was 246-276 K with pressures of 0.77 MPA (propane) and 3.5 MPa (ethane), and the water concentrations in the hydrocarbon were from 16 to 176 ppm (mol).

An experimental apparatus was designed for measuring the solubility of gases in liquids at low pressure and temperatures. Accurate data for the solubility of gaseous cyclopropane in aqueous potassium chloride are reported. The data were taken at 273, 276, and 278 K, at KCI concentrations of 0, 0.5, 1.1, 1.5, and 10 g/L and at pressures up to cyclopropane hydrate formation conditions. Gas solubility followed Henry’s law over the entire range of data. Reciprocal Henry’s constants were fitted to an emplrical relation In temperature and salt content and confidence limits are given for the relation.

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