Mechanism of Cohesive Forces of Cyclopentane Hydrates with and without Thermodynamic Inhibitors

Gas hydrates are commonly formed in oil and gas pipelines. One approach in their prevention is the injection of thermodynamic inhibitors (e.g., methanol, ethanol, monoethylene glycol (MEG)), so that the hydrate stability phase equilibrium can be moved into the fluid stable region. In this study, we directly measure cohesive forces of cyclopentane hydrates with thermodynamic inhibitors (2 wt % MEG, methanol, ethanol, and 1 and 3.5 wt % sodium chloride) to understand the effects of thermodynamic inhibitors (THIs) on the cohesive forces of hydrates. The cohesive forces are measured as a function of annealing time and temperature and determined from pull-off measurements based on the principle of Hooke’s law (<i>F</i> = <i>K</i>_spring × Δ<i>D</i>, where <i>K</i>_spring is the spring constant of the cantilever fiber, and Δ<i>D</i> is the displacement of the cantilever). A mechanism for the cohesive force of partially and fully converted hydrate particles is inferred and partially demonstrated by differential scanning calorimetry (DSC) measurements. These experiments and results are essential to quantify the impact of THIs on hydrate particle interactions, with implications on the usage of these chemicals, particularly in under-inhibited conditions

Design Principles for Nanoparticles Enveloped by a Polymer-Tethered Lipid Membrane

We propose the design for a nanoparticle carrier that combines three existing motifs into a single construct: a liposome is stabilized by anchoring it to an enclosed solid core <i>via</i> extended polymeric tethers that are chemically grafted to the core and physisorb into the surrounding lipid membrane. Such a design would exhibit several enticing properties, among them: (i) the anchoring stabilizes the liposome against a variety of external stresses, while preserving an aqueous compartment between core and membrane; (ii) the interplay of design parameters such as polymer length or grafting density enforces strong constraints on nanoparticle size and hence ensures a high degree of uniformity; and (iii) the physical and chemical characteristics of the individual constituents equip the construct with numerous functionalities that can be exploited in many ways. However, navigating the large parameter space requires a sound prior understanding for how various design features work together, and how this impacts potential pathways for synthesizing and assembling these nanoparticles. In this paper, we examine these connections in detail, using both soft matter theory and computer simulations at all levels of resolution. We thereby derive strong constraints on the experimentally relevant parameter space, and also propose potential equilibrium and nonequilibrium pathways for nanoparticle assembly

Dendritic Amphiphiles Strongly Affect the Biophysical Properties of DPPC Bilayer Membranes

Molecular dynamics (MD) simulations were used to gain insight on the molecular interactions in a model biological membrane comprised of a bilayer with DPPC (dipalmitoylphosphotidylcholine) and antimicrobial dendritic amphiphile molecules [RCONHC­(CH₂CH₂COOH)₃, where <i>R</i> is the saturated hydrocarbon tail (<i>R</i> = <i>n</i>-C_<i>n</i>H_2<i>n</i>+1), to be abbreviated as 3CAmn]. This study analyzes different biophysical properties of the equilibrated mixed bilayers, at 300 and 325 K, to determine how the presence of the 3CAmn, in varying concentrations and tail lengths, affects the lipid bilayer. Lipid tail order parameter data, bilayer thickness trends, and qualitative lipid tail tilt observations suggest that a molar ratio of 0.2 3CAm19/DPPC is sufficient to induce a phase transition in the bilayer from gel to liquid crystalline at 300 K. These results also imply that the phase transition temperature of the mixed bilayer decreases upon incorporation of higher concentrations of 3CAm19. Hydrogen bonding takes place between the 3CAmn and DPPC at specific sites, as evidenced by the radial distribution function. Increased hydrogen bonding and the smaller headgroup size of the 3CAmn molecule result in a decrease in the total lateral area with higher concentrations of 3CAm19. Diffusion constants of 3CAmn varied with concentration and tail length; diffusion constants of DPPC and 3CAm19 increased with increasing 3CAm19 concentration at 300 K and shorter 3CAmn tails had higher diffusion constants at both temperatures. These computational studies provide a comprehensive understanding of the biophysical changes to model biological membranes by the association of 3CAmn

Droplet Size Scaling of Water-in-Oil Emulsions under Turbulent Flow

The size of droplets in emulsions is important in many industrial, biological, and environmental systems, as it determines the stability, rheology, and area available in the emulsion for physical or chemical processes that occur at the interface. While the balance of fluid inertia and surface tension in determining droplet size under turbulent mixing in the inertial subrange has been well established, the classical scaling prediction by Shinnar half a century ago of the dependence of droplet size on the viscosity of the continuous phase in the viscous subrange has not been clearly validated in experiment. By employing extremely stable suspensions of highly viscous oils as the continuous phase and using a particle video microscope (PVM) probe and a focused beam reflectance method (FBRM) probe, we report measurements spanning 2 orders of magnitude in the continuous phase viscosity for the size of droplets in water-in-oil emulsions. The wide range in measurements allowed identification of a scaling regime of droplet size proportional to the inverse square root of the viscosity, consistent with the viscous subrange theory of Shinnar. A single curve for droplet size based on the Reynolds and Weber numbers is shown to accurately predict droplet size for a range of shear rates, mixing geometries, interfacial tensions, and viscosities. Viscous subrange control of droplet size is shown to be important for high viscous shear stresses, i.e., very high shear rates, as is desirable or found in many industrial or natural processes, or very high viscosities, as is the case in the present study

Voronoi Tessellation Analysis of Clathrate Hydrates

Molecular simulation of clathrate hydrate has provided significant advancements in our understanding of hydrate properties and formation. In this work, we report the application of Voronoi tessellation to characterize the structuring of water and guest molecules forming hydrates. Tessellation of perfect sI and sII hydrate reveals positions of Voronoi vertices similar to the oxygen atoms of enclathrating water molecules. Applying tessellation to a simulation trajectory of hydrate formation, and using a further selection criteria based on polyhedra volume and coordination number, we identify numbers and types of cagelike polyhedra. Voronoi analysis of this type results in similar numbers of identified cages but with differing topologies. However, once nearest neighbor methanes are also enclathrated, the topologies of the Voronoi polyhedra approach that of the actual water cages. Since only methane coordinates are required, Voronoi tessellation is a fast and simple tool that can be used as an order parameter to identify the structuring of molecules when studying hydrates in simulations

Gas Hydrate Sloughing as Observed and Quantified from Multiphase Flow Conditions

Sloughing of gas hydrates from deposits formed on the pipe wall is a phenomenon that can cause hydrate accumulation and blockage of the flow in oil/gas pipelines. While hydrate sloughing has been recognized as an important mechanism leading to hydrate blockage, its observation and measurements have not been reported. Experiments performed in a visual rocking cell to emulate multiphase flow conditions with a methane–ethane gas mixture, fresh water, and non-emulsifying oil or condensate as hydrocarbon liquid demonstrated that hydrate sloughing occurs at a wide range of subcooling and temperature gradient conditions. However, sloughing was not detected in a narrow operational window defined by both subcooling lower than 4 °C and temperature gradient in the cell lower than 1 °C. The potential existence of an operational window for conditions without sloughing might be valuable for the development of hydrate management strategies for blockage-free production

Gas Hydrates Phase Equilibria and Formation from High Concentration NaCl Brines up to 200 MPa

Gas hydrate phase equilibrium and kinetics at high NaCl concentrations (near and at saturation in solution) and very high pressures (up to ∼200 MPa) are investigated to study the interplay of hydrate formation and salt precipitation. Limited experimental data above 80 MPa exist for hydrate phase equilibrium in high salinity systems. This study reveals the unusual formation of gas hydrates under these extreme conditions of high salinity and very high pressure. In particular, the results demonstrate that hydrates can form from saturated salt solutions, and the formation of hydrates and salt precipitation are competing effects. It is determined that hydrates will remain in equilibrium with a saturated salt solution, with the amount of salt precipitation determined by the amount of hydrates formed. These data are essential fundamental data for gas hydrates applications in the oil and gas production flow assurance and seawater desalination

Model Water-in-Oil Emulsions for Gas Hydrate Studies in Oil Continuous Systems

Stable water-in-oil emulsions with water volume fraction ranging from 10 to 70 vol % have been developed with mineral oil 70T, Span 80, sodium di-2-ethylhexylsulfosuccinate (AOT), and water. The mean size of the water droplets ranges from 2 to 3 μm. Tests conducted show that all emulsions are stable against coalescence for at least 1 week at 2 °C and room temperature. Furthermore, it was observed that the viscosity of the emulsion increases with increasing water volume fraction, with shear thinning behavior observed above certain water volume fraction emulsions (30 vol % at room temperature and 20 vol % at 1 °C). Viscosity tests performed at different times after emulsion preparation confirm that the emulsions are stable for 1 week. Differential scanning calorimetry performed on the emulsions shows that, for low water volume fraction emulsions (<50 vol %), the emulsions are stable upon ice and hydrate formation. Micromechanical force (MMF) measurements show that the presence of the surfactant mixture has little to no effect on the cohesion force between cyclopentane hydrate particles, although a change in the morphology of the particle was observed when the surfactant mixture was added into the system. High-pressure autoclave experiments conducted on the model emulsion resulted in a loose hydrate slurry when the surfactant mixture was present in the system. Tests performed in this study show that the proposed model emulsion is stable, having similar characteristics to those observed in crude oil emulsions, and may be suitable for other hydrate studies

An Examination of the Prediction of Hydrate Formation Conditions in the Presence of Thermodynamic Inhibitors

<div><p>Abstract Gas hydrates are crystalline compounds, solid structures where water traps small guest molecules, typically light gases, in cages formed by hydrogen bonds. They are notorious for causing problems in oil and gas production, transportation and processing. Gas hydrates may form at pressures and temperatures commonly found in natural gas and oil production pipelines, thus causing partial or complete pipe blockages. In order to inhibit hydrate formation, chemicals such as alcohols (e.g., ethanol, methanol, mono-ethylene glycol) and salts (sodium, magnesium or potassium chloride) are injected into the produced stream. The purpose of this work is to briefly review the literature on hydrate formation in mixtures containing light gases (hydrocarbons and carbon dioxide) and water in the presence of thermodynamic inhibitors. Four calculation methods to predict hydrate formation in those systems were examined and compared. Three commercial packages (Multiflash®, PVTSim® and CSMGem) and a hydrate prediction routine in Fortran90 using the van der Waals and Platteeuw theory and the Peng-Robinson equation of state were tested. Predictions given by the four methods were compared to independent experimental data from the literature. In general, the four methods were found to be reasonably accurate. CSMGem and Multiflash® showed the best results.</p></div

Adhesion Force between Cyclopentane Hydrate and Mineral Surfaces

Clathrate hydrate adhesion forces play a critical role in describing aggregation and deposition behavior in conventional energy production and transportation. This manuscript uses a unique micromechanical force apparatus to measure the adhesion force between cyclopentane hydrate and heterogeneous quartz and calcite substrates. The latter substrates represent models for coproduced sand and scale often present during conventional energy production and transportation. Micromechanical adhesion force data indicate that clathrate hydrate adhesive forces are 5–10× larger for calcite and quartz minerals than stainless steel. Adhesive forces further increased by 3–15× when increasing surface contact time from 10 to 30 s. In some cases, liquid water from within the hydrate shell contacted the mineral surface and rapidly converted to clathrate hydrate. Further measurements on mineral surfaces with physical control of surface roughness showed a nonlinear dependence of water wetting angle on surface roughness. Existing adhesive force theory correctly predicted the dependence of clathrate hydrate adhesive force on calcite wettability, but did not accurately capture the dependence on quartz wettability. This comparison suggests that the substrate surface may not be inert, and may contribute positively to the strength of the capillary bridge formed between hydrate particles and solid surfaces