7 research outputs found
Hydrate-Based Desalination Using Cyclopentane Hydrates at Atmospheric Pressure
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 efficiency of 81% (the salinity decreased
from 3.5 to 0.67 wt %) was achieved by using a three-step separation
method, including gravitational separation, filtration, and a washing
step. Washing the hydrate sample using filtered water decreased the
salinity from 1.5 wt % in the solid hydrates before washing to 1.05
wt % after washing
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
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
High-Pressure Rheology of Hydrate Slurries Formed from Water-in-Oil Emulsions
A unique high-pressure rheology apparatus is used to
study the <i>in situ</i> formation and flow properties of
gas hydrates from
a water-in-crude oil emulsion. Viscosity and pressure of the hydrate
slurry are measured during hydrate formation, growth, aggregation,
and dissociation. The rheology of the hydrate slurries varies with
time, shear rate (1–500 s<sup>–1</sup>), water content
(0–50%), and temperature (0–6 °C). Hydrate slurry
viscosity increases rapidly with time when hydrates form and then
decays after going through a maximum as hydrate aggregates breakup
or rearrange. Yield stress increases with annealing time up to 8 h
and then remains constant. Hydrate slurry viscosity decreases with
an increasing shear rate (i.e., they are shear thinning). Viscosity
and yield stress both increase with an increasing water content. During
dissociation, the viscosity increases just before the hydrate equilibrium
temperature. Finally, transient viscosity measurements at varying
temperatures suggest that mechanisms, such as cohesion forces and
shear forces, competitively affect hydrate slurry viscosity
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
Synthesis and Characterization of sI Clathrate Hydrates Containing Hydrogen
Previously, large cage occupancy of H<sub>2</sub> has
only been
confirmed in the structure II (sII) hydrate. Utilizing a hydrate synthesis
pathway involving pressurizing preformed structure I (sI) hydrates,
we now show H<sub>2</sub> occupancy in both the small and the large
cages of sI, as evidenced by powder X-ray diffraction and Raman spectroscopic
measurements. The new H<sub>2</sub> environments were determined to
be singly and doubly occupied 5<sup>12</sup>6<sup>2</sup> cages occurring
at 4125–4131 and 4143–4149 cm<sup>–1</sup>, respectively.
This work serves as proof-of-concept that, by altering the conventional
hydrate synthesis procedure to incorporate preformed hydrates, it
may be possible to promote the occupancy of H<sub>2</sub> or possibly
other guests in a desired structure through a “templating”
effect by simply changing the initial hydrate structure
Investigating the Thermodynamic Stabilities of Hydrogen and Methane Binary Gas Hydrates
When hydrogen (H<sub>2</sub>) is
mixed with small amounts of methane
(CH<sub>4</sub>), the conditions required for clathrate hydrate formation
can be significantly reduced when compared to that of simple H<sub>2</sub> hydrate. With growing demand for CH<sub>4</sub> as a commercially
viable source of energy, H<sub>2 </sub>+ CH<sub>4</sub> binary
hydrates may be more appealing than extensively studied H<sub>2</sub> + tetrahydrofuran (THF) hydrates from an energy density standpoint.
Using Raman spectroscopic and powder X-ray diffraction measurements,
we show that hydrate structure and storage capacities of H<sub>2</sub> + CH<sub>4</sub> mixed hydrates are largely dependent on the composition
of the initial gas mixture, total system pressure, and formation period.
In some cases, H<sub>2</sub> + CH<sub>4</sub> hydrate kinetically
forms structure I first, even though the thermodynamically stable
phase is structure II