In order to improve the application value of natural microcrystalline graphite with carbon content of 49.5%, high-purity microcrystalline graphite was prepared by emulsifying kerosene flotation firstly, and then purifying hydrofluoric acid and hexafluorosilicic acid. Then the purified microcrystalline graphite was prepared for the lithium-ion battery anode material, its microstruture and electrochemical properties were analyzed, the purification mechanism and lithium storage mechanism were discussed. The research results show that carbon content of microcrystalline graphite after emulsified kerosene flotation and mixed acid purification are 93.5% and 99.0% respectively. After pickling, high-purity microcrystalline graphite shows the largest layer spacing, which is 0.351 5 nm and is 0.001 4 nm higher than that of natural microcrystalline graphite. Size disparity of acid washing sample is larger, layered structure is more obvious, cycle performance and magnification performance are better than those of floation sample. The pickled sample has the highest initial reversible specific capacity of 477.4 mAh/g, and the first Coulomb efficiency is 61.3%. Charge transfer impedance, interface impedance and SEI membrane impedance, and lithium ion diffusion impedance in electrode material are significantly lower than those of microcrystalline graphite after flotation.
The solid–liquid–gas
equation of state (SLV-EOS)
is based on the initial cubic equation of state, the van der Waals
equation. Since the van der Waals equation is not accurate enough
to predict gas–liquid properties, SLV-EOS cannot better predict
the gas–liquid properties of hydrocarbons in actual gas reservoirs.
Therefore, a modified solid–liquid–gas unified equation
of state was constructed inthis paper, which was developed using the
material’s actual critical compressibility factor
Z
c
. The minimum liquid-phase volume at the triple point
is also introduced to limit the value of
c
in the
equation, which effectively avoids the solution of Maxwell’s
equal-area rule in the solid–liquid transformation process.
The model extends the classical Peng–Robinson equation of state
for fluid-only (liquid and vapor) states. The predicted
p
-
T
and
p
-ρ phase transition
diagrams are reported in this paper for methane, ethane, propane,
carbon dioxide, hydrogen sulfide, and sulfur, and they are in good
agreement with the experimental data. This methodology is suitable
for any substance for which the density of the solid phase is higher
than that of the liquid phase. Additionally, the modified SLV equation
can be used to estimate the solubility of solid sulfur in the absence
of relevant experimental data.
The determination of a reasonable drainage rate of coalbed methane (CBM) in vertical wells in the single-phase flow stage can provide maximise the transmission of water pressure over distance. Based on the principle of effective stress and Darcy’s law, a mathematical model for dynamic changes of the permeability in the single-phase flow stage was established; on this basis, the relationship between permeability and threshold pressure gradient was experimentally attained; according to the linkage of changes of the transmission distance of water pressure, permeability, and pressure drop in the wellbore in the drainage process, a mathematical model for a reasonable reduction rate of the working fluid level in the single-phase flow stage taking the change of the permeability into account was established. The accuracy of the mathematical model was verified according to practical drainage data from CBM wells in Daning Block in Qinshui Basin, Shanxi Province, China. The results show that the rate of pressure drop decreases in a negative exponential manner with the increase of the drainage time. Different rates of pressure drop were required in coal reservoirs with different permeabilities; when keeping other conditions constant, the larger the permeability of coal reservoirs, the lower the threshold pressure gradient and the lower the rate of daily pressure drop. The research results provide a theoretical basis and reference for the reasonable drainage system in the single-phase flow stage.
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