6 research outputs found
Dissociation and Nucleation of Tetra‑<i>n</i>‑butyl Ammonium Bromide Semi-Clathrate Hydrates at High Pressures
The equilibrium pressure–temperature
relations of the tetra-<i>n</i>-butyl ammonium bromide (TBAB)
semiclathrate hydrate were
measured at pressures of up to 80 MPa by high-pressure differential
scanning calorimetry. As a pressurizing medium, tetrafluoromethane
(CF<sub>4</sub>), which cannot occupy any hydrate cages in the TBAB
semiclathrate hydrate at the present experimental pressures, was used.
The dissociation temperature of tetragonal TBAB semiclathrate hydrate
(TBAB·26H<sub>2</sub>O) increases with the increase in pressure,
whereas the dissociation enthalpy is (192 ± 3) J·g<sup>–1</sup> and almost constant at pressures of up to 80 MPa. The temperature
difference between formation and dissociation at the same pressure,
that is, the maximum allowable degree of supercooling, is (17.7 ±
0.7) K and independent of the pressure
Structure‑H Methane + 1,1,2,2,3,3,4-Heptafluorocyclopentane Mixed Hydrate at Pressures up to 373 MPa
Thermodynamic
stability boundary of structure-H hydrates with large
guest species and methane (CH<sub>4</sub>) at extremely high pressures
has been almost unclear. In the present study, the four-phase equilibrium
relations in the structure-H CH<sub>4</sub> + 1,1,2,2,3,3,4-heptafluorocyclopentane
(1,1,2,2,3,3,4-HFCP) mixed hydrate system were investigated in a temperature
range of (281.05 to 330.12) K and a pressure range up to 373 MPa.
The difference between equilibrium pressures in the structure-H CH<sub>4</sub> + 1,1,2,2,3,3,4-HFCP mixed hydrate system and the structure-I
simple CH<sub>4</sub> hydrate system gets larger with increase in
temperature. The structure-H CH<sub>4</sub> + 1,1,2,2,3,3,4-HFCP mixed
hydrate survives even at 330 K and 373 MPa without any structural
phase transition. The maximum temperature where the structure-H CH<sub>4</sub> + 1,1,2,2,3,3,4-HFCP mixed hydrate is thermodynamically stable
is likely to be beyond that of the structure-H simple CH<sub>4</sub> hydrate
High-Pressure Phase Equilibria of Tertiary-Butylamine Hydrates with and without Hydrogen
Thermodynamic
stability boundaries of the simple tertiary-butylamine
(<i>t</i>-BA) hydrate and <i>t</i>-BA+hydrogen
(H<sub>2</sub>) mixed hydrate were investigated at a pressure up to
approximately 100 MPa. All experimental results from the phase equilibrium
measurement, in situ Raman spectroscopy, and powder X-ray diffraction
analysis arrive at the single conclusion that the <i>t</i>-BA hydrates, under pressurization with H<sub>2</sub>, are transformed
from the structure VI simple <i>t</i>-BA hydrate into the
structure II <i>t</i>-BA+H<sub>2</sub> mixed hydrate. The
phase transition point on the hydrate stability boundary in the mother
aqueous solutions with the <i>t</i>-BA mole fractions (<i>x</i><sub><i>t</i>‑BA</sub>) of 0.056 and 0.093
is
located at (2.35 MPa, 267.39 K) and (25.3 MPa, 274.19 K), respectively.
On the other hand, in the case of the pressurization by decreasing
the sample volume instead of supplying H<sub>2</sub>, the simple <i>t</i>-BA hydrate retains the structure VI at pressures up to
112 MPa on the thermodynamic stability boundary
Effects of Halide Anions on the Electrical Conductivity in Single-Crystalline Tetra‑<i>n</i>‑butylammonium Salt Semiclathrate Hydrates
Semiclathrate hydrates are electroconductive materials
for possible
solid electrolytes and are also useful for monitoring their formation
and dissociation processes. In the present study, the electrical conductivities
and electrical relaxation times in the single-crystalline tetra-n-butylammonium (TBA) chloride and fluoride semiclathrate
hydrates were measured and compared with those of the TBA-bromide
semiclathrate hydrate. In the descending order of the electrical conductivity,
the largest was the TBA-bromide semiclathrate hydrate, followed by
TBA-chloride and TBA-fluoride semiclathrate hydrates. On the other
hand, 2H NMR spin–lattice relaxation times in their
deuterates were similar. Although the reorientation motion of water
molecules should be a significant factor to govern the electrical
conductivity in these semiclathrate hydrates, the present results
reveal that the difference between the electrical conductivities in
three TBA-halide semiclathrate hydrates would be caused by the concentration
of the proton, a conduction carrier, rather than the diffusion processes.
Additionally, electrical conductivity in the single-crystalline TBA-hydroxide
semiclathrate hydrate was measured. The electrical conductivity even
in single crystals was much higher than those in the TBA-halide semiclathrate
hydrates
High-Pressure Phase Equilibrium and Raman Spectroscopic Studies on the 1,1-Difluoroethane (HFC-152a) Hydrate System
High-pressure phase equilibrium relations of the 1,1-difluoroethane (HFC-152a) + water binary system were investigated in a temperature range of (275.03 to 319.30) K and a pressure range up to 370 MPa. Four three-phase coexisting curves of hydrate + aqueous + gas phases, hydrate + HFC-152a-rich liquid + gas phases, hydrate + aqueous + HFC-152a-rich liquid phases, and aqueous + HFC-152a-rich liquid + gas phases originate from the quadruple point of hydrate + aqueous + HFC-152a-rich liquid HFC-152a + gas phases located at (288.05 ± 0.15) K and (0.44 ± 0.01) MPa. The structure of HFC-152a hydrate remains structure I (s-I) in the pressure range up to 370 MPa. Raman spectra of the HFC-152a molecule in the HFC-152a hydrate indicate that the HFC-152a molecules occupy only large cages of s-I HFC-152a hydrate in the presence of completely vacant small cages at a pressure up to 370 MPa
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>2Â </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