Phase Carbon Dioxide Research Articles

Molecular dynamics study of calcite, hydrate and the temperature effect on CO2 transport and adsorption stability in geological formations

Molecular dynamics (MD) simulations at several different temperatures were run to investigate the transport, adsorption, and stability of carbon dioxide (CO2) and water phases in contact with a calcite surface. All simulated systems showed evidence of CO2 transport and interface stability heavily affected by the presence of calcite and the simulation temperature. The number of CO2 molecules that successfully traversed the water layer and adsorbed on the calcite surface increased with temperature, while the adsorption stability (indicated by the adsorption energy) decreased. It was found that the short-range potential has a significant impact on the preferred CO2 orientation and adsorption selectivity. Carbon dioxide tended to fill partial hydrate cavities at the water–hydrate interface, potentially promoting the formation of new hydrate. These findings indicate the need to consider the implications that CO2 injection will have for reservoirs with pre-existing clathrate hydrates.

Diastereomer salt formation of ibuprofen in supercritical carbon dioxide

The first in situ diastereomeric salt formation reaction in supercritical carbon dioxide is reported in this paper. The reaction studied was the salt formation between racemic ibuprofen and (R)-(1)-phenylethylamine, and was combined with supercritical fluid extraction in order to separate the unreacted enantiomers from the formed salts. The effects of pressure, temperature and reaction time were studied in detail. With increasing pressure, the reaction rate and the resolution efficiency at equilibrium increase. With increasing temperature, the optical purity of ibuprofen in the carbon dioxide phase increases, as does the reaction rate.

Asymmetrical Flow Field-Flow Fractionation for Characterization of Cyclotrimethylene Trinitramine (RDX) Particles Prepared by Supercritical Anti-Solvent Recrystallization

In this work, asymmetrical flow field-flow fractionation (AsFlFFF) was employed to determine the mean diameters and size distributions of cyclotrimethylene trinitramine (RDX) particles. The micro-particles of RDX were prepared using a supercritical anti-solvent recrystallization process. The influence of the spraying method, i.e., the pressure drop generated during the spraying of the RDX solution into the supercritical carbon dioxide (CO2) phase, on the size and shape of the recrystallized RDX particles, was investigated. It was found that the mean diameter of the recrystallized RDX particles decreased as the pressure drop increased, mainly due to the reduction in the population of larger particles. Also, the size distribution tended to become narrower with increasing pressure drop for the same reason. Near-spherical RDX particles with a relatively narrow size distribution were obtained at a pressure drop of 11 MPa. The mean diameters of the RDX particles determined by AsFlFFF were in reasonable agreement with those obtained by optical microscopy (OM). The AsFlFFF seems to be a useful tool for the separation and characterization of RDX particles.

SIMULATION AND PROCESS OPTIMIZATION OF A MEMBRANE-BASED DENSE GAS EXTRACTION USING HOLLOW FIBER CONTACTORS

Supercritical fluid and membrane technology coupling is a relatively new concept applicable to solvent separation and solute extraction. In these processes a hydrophobic or hydrophilic macroporous membrane is used as a two-different-nature solutions contactor. This methodology is an alternative to conventional liquid solution supercritical fluid extraction processes, which are associated with high investment costs. In the present work, a membrane-based supercritical fluid extraction module is modeled, simulated, and optimized as an independent industrial-scale operational unit. UniSim design suite R390 software from Honeywell was used as the platform for the simulation. Acetone and ethanol literature extraction results and methanol experimental extraction results (27.6% to 14.5% with a 10 wt.% aqueous solution; 7.1% to 5.9% with a 500 ppm aqueous solution) were used for validation of the model and definition of the semi-empirical equation parameters. The generated industrial-scale system optimization, which used a modular membrane arrangement, was strongly dependent on thermodynamic, economic, and energetic variables (higher mass transfer resistance in the carbon dioxide phase increased the number of membranes needed; process feasibility was affected by the number of membrane units, carbon dioxide flow rate, and product added value; compression energy requirements affected the optimization result). The modeled system proved to be an important aid in the design, scaling, and optimization of systems that use membranes as phase contactors in liquid solution supercritical carbon dioxide extraction.

The oxygen isotope exchange between carbon dioxide and water in the Larderello geothermal field (Italy) during fluid reinjection

The oxygen isotope compositions of CO 2 and water vapor samples collected from Larderello geothermal wells after the start of the fluid reinjection program suggest that if the oxygen isotope exchange in the vapor phase does, in fact, exist, it is a very slow process when compared with the residence time of the fluids in the geothermal reservoir. This is because carbon dioxide and water vapor phases could not have equilibrated significantly in the vapor-dominated reservoir. This conclusion implies that the oxygen isotope composition of carbon dioxide may possibly be used as a tool in geothermal exploration for revealing the presence of liquid water in deep geothermal systems. Based on the interpretation of the oxygen isotope data of the CO 2, I propose that the origin of the low oxygen isotope ratios of carbon dioxide at Larderello is the high-temperature exchange with liquid water in the lower reservoir. In Larderello, the liquid water–rock interaction in the lower reservoir may have increased the 18O/ 16O ratio of the recharge meteoric component. By contrast, lack of high-temperature liquid water in the upper reservoir suggests that the large “δ 18O shift” described for the upper-reservoir steam during the last decades reflects varying degrees of dilution of the lower-reservoir fluid by the low- 18O vaporized liquid water of meteoric origin that recharges the field at shallow depth, with local contribution from still deeper high- 18O water vapor of magmatic origin. The low oxygen isotope composition of the Mesozoic carbonaceous rocks that form the upper reservoir, consequently, likely represents a “fossil” record of the past hot-water geothermal stage.

Development of a Microfluidic Device for Measurement of Distribution Behavior between Supercritical Carbon Dioxide and Water

A measuring method for the distribution behavior between supercritical carbon dioxide and water by a microchip was developed. A surface modification of the microchannel by dichlorodimethylsilane induced a spontaneous phase separation of the supercritical carbon dioxide and aqueous phases in the microchip. The maximum contact time of the aqueous phase to the supercritical carbon dioxide phase was obtained as 0.58 s. This device withstood pressure up to 12.8 MPa. The distribution of tris(acetylacetonato)cobalt(III) (Co(acac)(3)) from the supercritical carbon dioxide phase to the aqueous phase in a microchannel could be measured. The concentration of Co(acac)(3) distributed into the aqueous phase was increased by lengthening the contact time of both phases. These demonstrations showed that the method developed in this study could be used to measure the distribution behavior between supercritical carbon dioxide and water.

Growth of Structure I Carbon Dioxide Hydrate from Molecular Dynamics Simulations

The growth of carbon dioxide hydrates and the mechanism are studied using molecular dynamics simulations. The melting temperatures at different pressures are determined using a three-phase molecular model consisting of a liquid carbon dioxide phase, a liquid water phase, and a solid hydrate phase. The growth simulations are then performed at a temperature (260 K) below the melting point. It is found that the growth rate of CO2 hydrates slightly decrease with pressure, from 0.132 m/s at 3 MPa to 0.096 m/s at 100 MPa under a supercooling of 25 K. The growth mechanism of CO2 hydrates is found to be largely similar to that of methane hydrates. One remarkable difference is the occurrence of the unprecedented transient 4151062 water cages (consisting of hydrogen bonding network of one rectangle, ten pentagons, and two hexagons) at the growing interface. Such cages may stay at the interface for as long as 15 ns and are transformed to the typical 512 and 51262 cages of structure I as the crystallization proceeds....

Near-Critical Extraction of the Fermentation Products by Membrane Contactors: A Mass Transfer Simulation

A 2D mathematical model was developed to study liquid extraction by membrane contactors. The extraction system included a hollow-fiber membrane extractor, an aqueous solution of ethanol and acetone as the feed, and a near-critical phase of carbon dioxide as the solvent. Axial and radial diffusions of the ethanol and acetone within the tube side, through the membrane, and within the shell side of the contactor were considered in the model. The equations of the model were solved by a numerical method based on the finite element method (FEM). The predicted results of the extraction for ethanol and acetone show an average difference of 15.66% and 2.55% with the experimental data, respectively. Simulation results for extraction of acetone are more accurate than those for ethanol mostly due to a better estimation of the transport properties within the aqueous phase, which controls extraction of this solute. The developed mass transfer model is more accurate than the other few provided models in the literature and represents a general approach which can be applied for similar membrane separation systems.

Uranium dioxide in ionic liquid with a tri-n-butylphosphate–HNO3 complex—dissolution and coordination environment

Uranium dioxide can be dissolved directly in an imidazolium-based ionic liquid (IL) at room temperature with a tri-n-butylphosphate(TBP)-HNO(3) complex. The dissolution process follows pseudo first-order kinetics initially. Raman spectroscopic studies show the dissolved uranyl ions are coordinated with TBP in the IL phase with a molar ratio of (UO(2))(2+) : TBP = 1 : 2. The dissolved uranyl species can be effectively transferred to a supercritical fluid carbon dioxide (sc-CO(2)) phase. No aqueous phase is formed in either the IL dissolution or the supercritical fluid extraction process. Absorption spectra of the extracted uranyl species in the sc-CO(2) phase suggests the presence of a UO(2)(TBP)(2)(NO(3))(2) and HNO(3) adduct probably of the form UO(2)(TBP)(2)(NO(3))(2)·HNO(3). The adduct dissociates in a water-dodecane trap solution during pressure reduction resulting in UO(2)(TBP)(2)(NO(3))(2) collected in the dodecane phase.

Measurement and thermodynamic modeling of partition coefficients in N,N-dimethylacetamide–water–carbon dioxide system

The partition coefficients of N,N-dimethylacetamide (N,N-DMA) between the water and the supercritical and near-critical carbon dioxide (CO 2) phases were measured in the temperature range of 298.15–328.15 K and the pressure range of 8.3–24.1 MPa. The measurements were carried out in a 56 ml vessel by contacting the carbon dioxide and the aqueous phases. The partition coefficients of N,N-DMA increased from 0.05 to 0.150 with increasing pressure at a constant temperature and increased with temperature at a constant density. The bubble point pressures of N,N-DMA–CO 2 mixtures were measured at 298.15 K, 308.15 K and 318.15 K and were found to increase with increasing mole fraction of CO 2. The partition coefficients were modeled using the Peng-Robinson Equation of State (PREOS) combined with modified van der Waals mixing rule. The binary interaction parameters for the CO 2–H 2O pair were taken from the literature and were regressed for CO 2–N,N-DMA and H 2O–N,N-DMA pairs by fitting partition coefficients data. The binary interaction parameter for CO 2–N,N-DMA pair was found to depend linearly on temperature. The bubble point pressures of N,N-DMA and CO 2 were also measured and could be predicted fairly well using the regressed binary interaction parameters.

Equation of state and anharmonicity of carbon dioxide phase I up to 12 GPa and 800 K

We present an extended investigation of phase I of carbon dioxide by x-ray diffraction and spectroscopic techniques at simultaneous high pressure and high temperature, up to 12 GPa and 800 K. Based on the present and literature data, we show that a Mie-Grüneisen-Debye model reproduces within experimental uncertainties the equation of state of CO(2) over the entire range of stability of phase I. Using infrared and Raman spectroscopy, we have determined the frequencies of the zone-center lattice modes as a function of pressure and temperature. We have then extracted the volume and temperature dependencies of the optical lattice mode frequencies and their respective Grüneisen parameters. We find a large difference between the thermodynamic Grüneisen parameter obtained from the P-V-T data and those associated with the optical lattice modes. This suggests, within the quasiharmonic approximation, that acoustic modes have a dominant contribution to the anharmonicity of the system.

Interfacial tension measurements and modelling of (carbon dioxide + n-alkane) and (carbon dioxide + water) binary mixtures at elevated pressures and temperatures

Supercritical carbon dioxide (CO 2) is often used as a process fluid for enhanced oil recovery. The storage of carbon dioxide in underground formations is a potential way of mitigating climate change during a transition period to more sustainable energy sources. Combining injection with subsequent trapping of the non-wetting supercritical carbon dioxide phase in the pores of a depleted reservoir is a promising scheme for allowing the continued use of fossil fuels with minimal environmental consequences. The design of such processes is ultimately linked to the confined behaviour of the fluids in question at reservoir conditions, which is largely controlled by interfacial forces. Measurements of the relevant interfacial tensions for systems containing alkanes, carbon dioxide and water are currently limited and inconsistent while models usually fail to capture the pressure dependence of the interfacial tension. In this work, a density functional theory based on the SAFT-VR equation of state was used to predict the interfacial tension of (H 2O + CO 2 + n-alkane) binary systems over wide ranges of temperature and pressure. The comparison with a new set of reported experimental data of three ( n-alkane + CO 2) systems at pressures up to the critical points, as well as with the (H 2O + CO 2) system at pressures up to 60 MPa, for a temperature range of (298–443) K, is discussed.

Coesite-likeCO2: An analog toSiO2

We report recent findings of coesite-like carbon dioxide phases: coesite I phase $(cI{\text{-CO}}_{2})$ formed by laser-heating pseudo-six-fold phase VI above 50 GPa, a high-pressure form of coesite $(cII{\text{-CO}}_{2})$ by further compressing $cI$ phase above 70 GPa, and an amorphous form above 100 GPa. The existence of coesite-like carbon dioxide, an important linkage between fourfold quartz and sixfold stishovite in ${\text{SiO}}_{2}$, underscores the periodic analogy between ${\text{SiO}}_{2}$ and ${\text{CO}}_{2}$, which has been in a recent controversy. Its pressure-temperature stability field signifies the possibility of volatile ${\text{CO}}_{2}$ incorporated in silicate minerals in the Earth's lower mantle.

Nanosilica/PMMA composites obtained by the modification of silica nanoparticles in a supercritical carbon dioxide–ethanol mixture

Nanosilica/poly(methyl methacrylate) (PMMA) composites are used to improve the mechanical properties of neat PMMA polymer. In order to obtain superior mechanical properties, it is essential to achieve good bonding between the SiO2 nanoparticles and the PMMA matrix, which is typically achieved by coating silica nanoparticles with silane coupling agents. In this study, conventional and supercritical coating methods were investigated together with their influence on the mechanical properties of the obtained nanosilica/PMMA composites. The results indicate advantageous properties of nanosilica modified in the supercritical phase of carbon dioxide and ethanol in terms of particle size distribution, amount of coated silane, and dispersion in the PMMA matrix. Careful dispersion of the starting silica nanoparticles in ethanol at low temperatures in order to obtain a nanosilica sol plays an important role in deagglomeration, dispersion, and the coating process. The resulting nanosilica/PMMA composite containing nanoparticles obtained by supercritical processing of the nanosilica sol showed an increase in hardness by 44.6% and elastic modulus by 25.7% relative to neat PMMA, as determined using the nanoindentation technique. The dynamic mechanical analysis reveals that addition of nanoparticles as nanosilica sol and nanosilica gel enhances composite storage modulus by about 54.3 and 46.5% at 40 °C. At the same temperature, incorporation of modified silica nanoparticles with conventional method leads to an increase of 15.9% for the storage modulus, probably due to a large silica particle size and lower silane content in this sample.

Structure of Carbon Dioxide Phase IV: Breakdown of the Intermediate Bonding State Scenario

The existence of "intermediate bonding states" in solid CO2, separating the low-pressure molecular phases from the high-pressure polymeric forms, has been the matter of a long-standing debate. Here we determine the structure of CO2-IV using x-ray diffraction of single crystals grown inside a diamond anvil cell at 11.7 GPa and 830 K. It is rhombohedral, space group R3[over ]c, and is composed of individual, linear CO2 molecules with bond lengths of 1.155(2) A at 15 GPa. This shows that CO2 remains a purely molecular solid in this P-T range, and thus invalidates the intermediate bonding state scenario. First-principles calculations confirm the stability of the proposed structure and match very well observations, including the Raman and IR spectra. Furthermore, these results evidence a striking similarity between the high-pressure polymorphs of solid CO2 and N2.

Electronic absorption spectra and nonlinear optical properties of CO2 molecular aggregates: A quantum chemical study

We have investigated the structural aspects of several carbon dioxide molecular aggregates and their spectroscopic and nonlinear optical properties within the quantum chemical theory framework. We find that, although the single carbon dioxide molecule prefers to be in a linear geometry, the puckering of angles occur in oligomers because of the intermolecular interactions. The resulting dipole moments reflect in the electronic excitation spectra of the molecular assemblies. The observation of significant nonlinear optical properties suggests the potential application of the dense carbon dioxide phases in opto-electronic devices.

Density and Volume Fraction of Supercritical CO2 in Pores of Native and Oxidized Aerogels

The density and volume fraction of an adsorbed phase of carbon dioxide (CO2) in aerogels was investigated using a formalism based on independent measurements of neutron transmission and small-angle neutron scattering from fluid-saturated absorbers (Rother et al. J. Phys. Chem. C 111, 15736 (2007)). The range of excess fluid pressures (0 < P < 8 MPa) and temperatures (T = 35°C and 80°C) corresponded to the supercritical regime above the critical temperature TC = 31.1°C and critical density ρC = 0.468 g · cm−3 of the bulk fluid. The results demonstrate that a porous aerogel matrix works to create an adsorbed phase with liquid-like fluid densities reaching ~1.1 g · cm−3 and ~0.8 g · cm−3 at T = 35°C and 80°C, respectively. Thus, despite the fact that the density and volume fraction of the adsorbed fluid both decrease with temperature, the dense adsorbed phase is still present in the aerogel at temperatures far exceeding the TC. Heat treatment (“oxidation”) of the aerogel at 500°C for 2 h, which removes a significant fraction of the alkyl groups from the aerogel surface, has little effect on the adsorption properties. The observed reduction of the density and volume fraction of the adsorbed CO2 with temperature and its minor dependence on the surface modification are consistent with predictions of the pore-filling model.

Partition Coefficients of Methyl Methacrylate and Acetone between Polymer and Compressed Carbon Dioxide Phases by an in situ Infrared Spectroscopy Method

Partition coefficients of methyl methacrylate and acetone between some polymer and compressed gaseous carbon dioxide phases were determined by an in situ Fourier transform infrared (FTIR) spectroscopy method with a special designed high pressure IR cell at 318.2 K and 2.0−7.0 MPa. At the same temperature and pressure, the partition coefficient for methyl methacrylate was higher than that for acetone between the same polymer and subcritical carbon dioxide. For most solute/polymer/CO2 systems, the partition coefficient of solute between the polymer and compressed gaseous carbon dioxide decreased with the pressure increase. Only for a methyl methacrylate/poly(methyl methacrylate)/CO2 system, the partiton coefficient was not sensitive to the pressure change at pressures lower than 3.9 MPa. A linear solvation free energy relationship was used to estimate the main tendency of the pressure dependency. With the pressure dependent polarizability−dipolarity parameter of carbon dioxide, the calculated results were in accord with some experimental results.

Comparison of Two-Equation Turbulence Models in Simulation of a Non-Swirl Coal Flame in a Pilot-Scale Furnace

The capability of six two-equation Reynolds-averaged Navier-Stokes (RANS) models for simulation of a non-swirl coal flame in a pilot-scale furnace has been investigated. These turbulence models—the standard k-ϵ model, re-normalization group (RNG) k-ϵ model, modified k-ϵ model, Wilcox k-ω model, Menter k-ω model (also called BSL model), and Shear-stress transport (SST) model—are assessed with the use of measured gas phase velocity, temperature, oxygen, and carbon dioxide volume fraction data from the literature. Predictions of the standard k-ϵ model, RNG k-ϵ model, BSL, and SST model are generally in good agreement with the experimental data. The Wilcox k-ω model generally overpredicts O2 volume fraction and underpredicts CO2 volume fraction. The modified k-ϵ model yields results that have large discrepancies from measurements.

High-pressure polymeric phases of carbon dioxide.

Understanding the structural transformations of solid CO(2) from a molecular solid characterized by weak intermolecular bonding to a 3-dimensional network solid at high pressure has challenged researchers for the past decade. We employ the recently developed metadynamics method combined with ab initio calculations to provide fundamental insight into recent experimental reports on carbon dioxide in the 60-80 GPa pressure region. Pressure-induced polymeric phases and their transformation mechanisms are found. Metadynamics simulations starting from the CO(2)-II (P4(2)/mnm) at 60 GPa and 600 K proceed via an intermediate, partially polymerized phase, and finally yield a fully tetrahedral, layered structure (P-4m2). Based on the agreement between calculated and experimental Raman and X-ray patterns, the recently identified phase VI [Iota V, et al. (2007) Sixfold coordinated carbon dioxide VI. Nature Mat 6:34-38], assumed to be disordered stishovite-like, is instead interpreted as the result of an incomplete transformation of the molecular phase into a final layered structure. In addition, an alpha-cristobalite-like structure (P4(1)2(1)2), is predicted to be formed from CO(2)-III (Cmca) via an intermediate Pbca structure at 80 GPa and low temperatures (<300 K). Defects in the crystals are frequently observed in the calculations at 300 K whereas at 500 to 700 K, CO(2)-III transforms to an amorphous form, consistent with experiment [Santoro M, et al. (2006) Amorphous silica-like carbon dioxide. Nature 441:857-860], but the simulation yields additional structural details for this disordered solid.