18 research outputs found
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><sub>spring</sub> × Δ<i>D</i>, where <i>K</i><sub>spring</sub> 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
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(CH<sub>2</sub>CH<sub>2</sub>COOH)<sub>3</sub>, where <i>R</i> is the saturated hydrocarbon
tail (<i>R</i> = <i>n</i>-C<sub><i>n</i></sub>H<sub>2<i>n</i>+1</sub>), 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
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
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