Pressure-dependent Equilibrium Research Articles

Pressure-dependent equilibrium molecular simulation of shale gas and its distribution and motion characteristics in organic-rich nano-slit

As an unconventional natural gas resource, shale gas plays a significant role in satisfying increasing global energy demands. Because of the less costs and better repeatability of molecular simulation for nano-scale research, it’s becoming a nice way to study the distribution and motion characteristics of shale gas, which is the key to get insights into the nano-scale effects in shale matrix. In this paper, pressure-dependent equilibrium molecular simulation of shale gas in organic-rich nano-slit is conducted, where the gas is divided into clearly defined layers and the structural and motion characteristics of different parts are detailedly analyzed, making systematic observations and descriptions of nano-scale effects happen. By virtue of this, the variation law of shale gas in the nano-slit and the mechanical theory behind it are proposed. Results also reveal the main reason for the roundabout ascension variation of Knudsen layer (KL) thickness, the role of density gullies as “lubricant layers” to enhance the diffusion in adsorption layers, the evidence of the role of gas-gas interactions in controlling molecular motion in addition to gas-wall interactions, nano-scale effects on gas movability with pressure from the viewpoint of gas development, etc. The physical phenomena caused by nano-scale effects and their variations with pressure in organic-rich nano-slit have been explicitly clarified, which is expected to be a useful reference for shale gas evaluation and exploitation.

Conformational Mobility in Cytochrome P450 3A4 Explored by Pressure-Perturbation EPR Spectroscopy

We used high hydrostatic pressure as a tool for exploring the conformational landscape of human cytochrome P450 3A4 (CYP3A4) by electron paramagnetic resonance and fluorescence spectroscopy. Site-directed incorporation of a luminescence resonance energy transfer donor-acceptor pair allowed us to identify a pressure-dependent equilibrium between two states of the enzyme, where an increase in pressure increased the spatial separation between the two distantly located fluorophores. This transition is characterized by volume change (ΔV°) and P1/2 values of −36.8 ± 5.0 mL/mol and 1.45 ± 0.33 kbar, respectively, which corresponds to a Keq° of 0.13 ± 0.06, so that only 15% of the enzyme adopts the pressure-promoted conformation at ambient pressure. This pressure-promoted displacement of the equilibrium is eliminated by the addition of testosterone, an allosteric activator. Using site-directed spin labeling, we demonstrated that the pressure- and testosterone-sensitive transition is also revealed by pressure-induced changes in the electron paramagnetic resonance spectra of a nitroxide side chain placed at position 85 or 409 of the enzyme. Furthermore, we observed a pressure-induced displacement of the emission maxima of a solvatochromic fluorophore (7-diethylamino-3-((((2-maleimidyl)ethyl)amino)carbonyl) coumarin) placed at the same positions, which suggests a relocation to a more polar environment. Taken together, the results reveal an effector-dependent conformational equilibrium between open and closed states of CYP3A4 that involves a pronounced change at the interface between the region of α-helices A/A′ and the meander loop of the enzyme, where residues 85 and 409 are located. Our study demonstrates the high potential of pressure-perturbation strategies for studying protein conformational landscapes.

Investigation of temperature and pressure dependent equilibrium and transport properties of liquid acetone by molecular dynamics simulation

In this paper, equilibrium thermodynamic and transport properties of liquid acetone were studied by classical molecular dynamics simulation at P=1atm and T=280–330K, using OPLS force field. The simulated densities are quite in agreement with experiment (%AAD=2.0). Simulated viscosities based on Stokes–Einstein and Green–Kubo methods are agree with experiment with %AAD=13 and 4.1, respectively, which is reasonably good for OPLS force field. Also, the pressure dependent density and viscosity was simulated at low (320K) and high (398K) temperature in the range P=7.53–34.95MPa. The simulated density are comparable with experiment (%AAD=2.9 and 0.8 at 320K and 398K, respectively). Simulated viscosities by Green–Kubo formulism with respect to experiment (%AAD=2.9 and 4.8 at 320K and 398K, respectively) are much more accurate than by Stokes–Einstein method (%AAD=5.9 and 9.6 at 320K and 398K, respectively). Simulated viscosity versus pressure at 398K increases rather smoothly, however, at 323K it increases nonmonotonically with a marked inflection point characteristic of acetone and astonishingly maps an exact trend, mimicking the trend of experimental data accurately. These anomalous behaviors of the viscosities are a first report of such rend.

Adsorption of CO2 and CH4 on a magnesium-based metal organic framework

A magnesium-based metal organic framework (MOF), also known as Mg-MOF-74, was successfully synthesized, characterized, and evaluated for adsorption equilibria and kinetics of CO(2) and CH(4). The Mg-MOF-74 crystals were characterized with scanning electron microscopy for crystal structure, powder X-ray diffraction for phase structure, and nitrogen adsorption for pore textural properties. Adsorption equilibrium and kinetics of CO(2) and CH(4) on the Mg-MOF-74 adsorbent were measured in a volumetric adsorption unit at 278, 298, and 318 K and pressures up to 1 bar. It was found that the Mg-MOF-74 adsorbent prepared in this work has a median pore width of 10.2 Å, a BET specific surface area of 1174 m(2)/g, CO(2) and CH(4) adsorption capacities of 8.61 mmol g(-1) (37.8 wt.%) and 1.05 mmol g(-1) (1.7 wt.%), respectively, at 298 K and 1 bar. Both CO(2) and CH(4) adsorption capacities are significantly higher than those of zeolite 13X under similar conditions. The pressure-dependent equilibrium selectivity of CO(2) over CH(4) (q(CO2)/q(CH4)) in the Mg-MOF-74 adsorbent showed a trend similar to that of zeolite 13X and the intrinsic selectivity of Mg-MOF-74 at zero adsorption loading is 283 at 298 K. The initial heats of adsorption of CO(2) and CH(4) on the Mg-MOF-74 adsorbent were found to be 73.0 and 18.5 kJ mol(-1), respectively. The adsorption kinetic measurements suggest that the diffusivities of CO(2) and CH(4) on Mg-MOF-74 were comparable to those on zeolite 13X. CH(4) showed relatively faster adsorption kinetics than CO(2) in both adsorbents. The diffusion time constants of CO(2) and CH(4) in the Mg-MOF-74 adsorbent at 298 K were estimated to be 8.11 × 10(-3) and 4.05 × 10(-2) s(-1), respectively, showing a modest kinetic selectivity of about 5 for the separation CH(4) from CO(2).

Allosteric Mechanisms in Cytochrome P450 3A4 Studied by High-Pressure Spectroscopy: Pivotal Role of Substrate-Induced Changes in the Accessibility and Degree of Hydration of the Heme Pocket

Allosteric mechanisms in human cytochrome P450 3A4 (CYP3A4) in oligomers in solution or monomeric enzyme incorporated into Nanodiscs (CYP3A4ND) were studied by high-pressure spectroscopy. The allosteric substrates 1-pyrenebutanol (1-PB) and testosterone were compared with bromocriptine (BCT), which shows no cooperativity. In both CYP3A4 in solution and CYP3A4ND, we observed a complete pressure-induced high-to-low spin shift at pressures of <3 kbar either in the substrate-free enzyme or in the presence of BCT. In addition, both substrate-free and BCT-bound enzyme revealed a pressure-dependent equilibrium between two states with different barotropic parameters designated R for relaxed and P for pressure-promoted conformations. This pressure-induced conformational transition was also observed in the studies with 1-PB and testosterone. In CYP3A4 oligomers, the transition was accompanied by an important increase in homotropic cooperativity with both substrates. Surprisingly, at high concentrations of allosteric substrates, the amplitude of the spin shift in both CYP3A4 in solution and Nanodiscs was very low, demonstrating that hydrostatic pressure induces neither substrate dissociation nor an increase in the heme pocket hydration in the complexes of the pressure-promoted conformation of CYP3A4 with 1-PB or testosterone. These findings suggest that the mechanisms of interactions of CYP3A4 with 1-PB and testosterone involve an effector-induced transition that displaces a system of conformational equilibria in the enzyme toward the state(s) with decreased solvent accessibility of the active site so that the flux of water into the heme pocket is impeded and the high-spin state of the heme iron is stabilized.

Formation of carbamic acid in organic solvents and in supercritical carbon dioxide

The behavior of amines in various CO 2-rich solvents is described. 1H and 13C NMR spectra in DMSO, chloroform, and benzene show that the amines are completely converted into carbamic acid by just bubbling atmospheric CO 2 through the solution. However, the chemical shifts of several protons and the carbonyl 13C in the newly formed compounds strongly depend on the organic solvent used. FT-IR measurements confirm the observation that carbamic acid formation occurs. Changing the solvent to supercritical CO 2 shows a temperature- and pressure-dependent equilibrium between the amine and the carbamic acid; higher pressures shift the equilibrium towards the carbamic acid, whereas higher temperatures favor the free amine.

Carbon in Mid-Atlantic Ridge basalt glasses from 28°N to 63°N: Evidence for a carbon-enriched Azores mantle plume

We report on 91 analyses of CO 2 in fresh, ‘macrovesicle-free’ Mid-Atlantic Ridge (MAR) basalt glasses from 48 dredge stations located between 28°N and Iceland. A stepped heating gas chromatographic technique was used for the analyses. Concentrations of carbon as CO 2 in the glasses range from 30 to 1014 ppm and always exceed the experimentally determined pressure-dependent equilibrium CO 2 solubility curve for tholeiitic basalts at 1200°C determined by Stolper and Holloway [1]. The measured CO 2 content mostly represents oversaturation of CO 2 in the magma and, to a lesser extent, CO 2 trapped in microvesicles during the latest stage of magma emplacement and quenching on the seafloor. The degree of ‘oversaturation’ varies from 1 to 9 times the equilibrium solubility values at any given depth. It is highest over the deeper part of the MAR profile, along the so-called normal ridge segments, and lowest close to Iceland and over the center of the Azores platform, where the oceanic crust is thicker and the mantle hotter than usual. The C/ 3 He ratio ranges from 1.4 × 10 9 to 4.3 × 10 9, as compared to a mean of 2 ± 1 × 10 9 previously reported for normal MOR segments. The C/ 3 ratio in MAR glasses over the Azores province correlates positively with 87Sr/ 86Sr ratio and reaches a maximum over the Azores platform. Based on this correlation, binary mixing modeling indicates minimum enrichments of carbon in the Azores plume ranging from 3.3 to 4.6, using 87Sr/ 86Sr ratios, and 4.9-;8.8, using 143Nd/ 144Nd ratios, depending on the enrichment of Sr and Nd estimated in the plume relative to the depleted asthenosphere. For comparison, the H 2O content in the Azores plume is enriched by a factor of 2–4. The carbon enrichment over the Azores platform (plume) is in agreement with an independent predictive model which assumes that the likelihood of CO 2 outgassing and the percentage fusion along the ridge are proportional to crustal thickness or excess ridge elevation.

A Numerical Simulation Model for Thermal Recovery Processes

Abstract This paper describes a model for numerically simulating thermal recovery processes. The primary locus is on the simulation of in-situ combustion, but the formulation also represents fire-and-water flooding, steamflooding, hot water flooding, steam stimulation, and spontaneous ignition as well. The simulator describes the flow of water, oil, and gas, and includes gravity and capillary effects. Heat transfer by conduction, convection, and vaporization-condensation of both water and hydrocarbons are included. The rigorous but general nature of the simulator is obtained by employing conservation balance equations for oxygen, inert gases, a light hydrocarbon pseudocomponent, a heavy hydrocarbon pseudocomponent, water, coke, and energy. pseudocomponent, water, coke, and energy. Vaporization-condensation is governed by vaporliquid equilibrium using temperature and pressure-dependent equilibrium coefficients. Four pressure-dependent equilibrium coefficients. Four chemical reactions are accounted for: formation of coke from the heavy hydrocarbon component and the oxidation of coke and both heavy and light hydrocarbon components. Formulation details, numerical solution procedures, and computational results are presented. procedures, and computational results are presented. The computational results include both one- and two-dimensional cross-sectional studies. The simulator represents a major improvement in the ability to simulate thermal recovery processes under complex conditions. Introduction Considerable progress has been made in numerically simulating thermally enhanced oil-recovery processes during the last few years. This is particularly true for-processes involving steam, where we have seen a continual improvement of our ability to treat the problem. The most recent contributions provide an analysis capability for steam displacement and steam stimulation recovery methods, accounting for all the important physical mechanisms of these processes. Progress in simulating the performance of in-situ combustion processes is not so advanced. Initial simulation attempts were concerned primarily with the heat-transfer aspects of combustion. The most sophisticated heat-transfer model was developed by Chu. His numerical model considers the energy effects of vaporization and condensation on the temperature distribution, but neglects the accompanying phase changes by assuming constant fluid saturations. More recent heat transfer or heat-wave models for the in-situ combustion process were proposed by Kuo in 1969 and by Smith and Farouq-Ali in 1971. Kuo's model allows two temperature fronts-one at the combustion zone and one at a heat front. The heat-front position is predicted by gas flow that is allowed to have a velocity different from the velocity of the combustion front. The simulator proposed by Smith and Farouq-Ali is designed for proposed by Smith and Farouq-Ali is designed for predicting sweep efficiencies in confined well predicting sweep efficiencies in confined well patterns. Their numerical model accounts for heat patterns. Their numerical model accounts for heat generation by a combustion zone (assuming fixed fuel content all through the reservoir), heat transfer by conduction and convection (single-phase gas flow) in the reservoir, heat losses by conduction to adjacent formations, and different permeability-to-gas (air) flow on either side of the combustion zone. Special cases of the in-situ combustion process were studied by Gottfried and Khelil. These authors examine the heat transfer and oxygen use in reservoirs composed of an oil-bearing layer and an overlying "clean" porous zone containing only gas. These models were designed primarily to investigate the various transport mechanisms present when combustion is initiated in a reservoir present when combustion is initiated in a reservoir containing a gas cap. Because of the many assumptions invoked and the specialized geometry to which they apply, they do not satisfy the need for a general purpose simulator. SPEJ P. 37

The effect of porous barriers on the molecular composition and total flux of a reactive gas mixture

When a gas mixture that is governed by a pressure dependent equilibrium is passed through a porous barrier, the exit molecular fluxes are functions of the equilibrium constant and the atomic concentrations at the exit face of the barrier rather than of the molecular weights and impingement pressures as has been tacitly assumed for Knudsen flow conditions. It is shown that whether or not molecules of the gas react with each other, the ratio of the total atomic escape flux of each elemental constituent to the total atomic impingement flux of that element for Knudsen flow should be a/l, where l is the barrier thickness and a is the same constant, of the order of the average diameter of barrier pores for each element in the gas mixture. The equilibrium prediction is confirmed for the sodium chloride monomer/dimer equilibrium mixture passed through alumina barriers with pores of the order of 1–10 μ diameter and various thicknesses. Measured dimer fluxes are reduced by the barriers to less than 10−3 times the dimer fluxes in conventional effusion; yet the measured fluxes agree to within about 30% with predictions from the measured monomer fluxes and the monomer/dimer equilibrium constant. The prediction that total atomic flux reductions for different vapors have the same dependence on a/l is confirmed by measurement with sodium chloride vapor and zinc vapor. The influence of porous barriers on the apparent heats of vaporization of monomers and dimers and the possibility of significant influence of surface diffusion on the experimental measurements are discussed.