High-pressure Phase Behavior Research Articles

STUDY OF SOLUBILITY IN SUPERCRITICAL FLUIDS: THERMODYNAMIC CONCEPTS AND MEASUREMENT METHODS - A REVIEW

Abstract Due to the importance of supercritical fluid technology (SFT) in different industries, it has been the subject of intense research in recent decades. Solubility is a key concept in SFT. In fact, obtaining knowledge about the theoretical concepts of solubility and related experimental measurement methods can be useful in developing and improving the quality of research in this field. This study reviews the fundamental knowledge of solubility in supercritical fluids and investigates the significant topics in this field, including high-pressure phase behavior, experimental measurement methods, modeling, and molecular simulation of solubility.

Pressure-Induced Polymorphism of Caprolactam: A Neutron Diffraction Study.

Caprolactam, a precursor to nylon-6 has been investigated as part of our studies into the polymerization of materials at high pressure. Single-crystal X-ray and neutron powder diffraction data have been used to explore the high-pressure phase behavior of caprolactam; two new high pressure solid forms were observed. The transition between each of the forms requires a substantial rearrangement of the molecules and we observe that the kinetic barrier to the conversion can aid retention of phases beyond their region of stability. Form II of caprolactam shows a small pressure region of stability between 0.5 GPa and 0.9 GPa with Form III being stable from 0.9 GPa to 5.4 GPa. The two high-pressure forms have a catemeric hydrogen-bonding pattern compared with the dimer interaction observed in ambient pressure Form I. The interaction between the chains has a marked effect on the directions of maximal compressibility in the structure. Neither of the high-pressure forms can be recovered to ambient pressure and there is no evidence of any polymerization occurring.

High pressure phase behaviour for the CO2 + n-dodecane + 3,7-dimethyl-1-octanol system

In this work, the solute-solute interaction which exist in the CO2 + n-dodecane+ 3,7-dimethyl-1-octanol system was investigated. Solubility data were measured for three mixtures containing solvent free n-dodecane fractions of 0.50, 0.75 and 0.85, between 308 K and 348 K and pressures up to 14.9 MPa, with solvent mass fractions ranging from 0.375 to 0.984, using a static synthetic method. The data revealed significant solute-solute interactions, which resulted in the occurrence of co-solvency in the two n-dodecane rich mixtures. The phase behaviour data of the system was modelled, using the RK-ASPEN model. The modelling results indicated that, with the inclusion of a solute-solute interaction parameter, the fitted model was capable of correlating fairly accurate solubility data, but it can only approximate vapour-liquid-equilibrium data.

High-pressure phase equilibrium data for carbon dioxide + dichloromethane + acetone and carbon dioxide + dichloromethane + acetone + N-acetylcysteine (NAC)

The N-Acetylcysteine (NAC) is a thiol compound with a strong antioxidant effect and demonstrated preventive action as protector in chronic kidney disease, cancer, pulmonary insufficiency, in the treatment of AIDS and other systemic diseases. Experimental phase equilibrium data provide fundamental information for the micronization supercritical fluid SEDS technique, which is characterized by the reduction of the average particle size leads to changes in physical structure of the compounds processed. There are numerous advantages in particle size reduction and make narrower particle size distribution, the main one being the resulting increased bioavailability. Therefore, the aim of this work was to study the high pressure phase behavior of the ternary system {carbon dioxide (1) + dichloromethane (2) + acetone (3)} and for the quaternary system {carbon dioxide (1) + dichloromethane (2) + acetone (3) + NAC (4)} in order to assist the micronization process in supercritical media. The experiments were performed using a variable volume cell over the temperature range from 308 to 328 K, pressure from 4.7 to 9.87 MPa, and mass ratio dichlorometane/acetone of 1.5:1 for the ternary system. For the quaternary system, the same mass ratio of dichlorometane/acetone was adopted and an amount of 0.001 and 0.004 g cm−3 of NAC was added to solution. Phase transitions of vapor-liquid type (bubble or dew points), in the presence or absence of solid phase were observed. The experimental results of the ternary system were modeled using the Peng-Robinson (PR) equation of state with the Wong-Sandler (PR-WS) mixing rule, providing a good representation of the experimental phase equilibrium data.

High-pressure phase behavior and equations of state of ThO2 polymorphs

ThO_2 is an important material for understanding the heat budget of Earth's mantle, as well as the stability of nuclear fuels at extreme conditions. We measured the in situ high-pressure, high-temperature phase behavior of ThO_2 to ∼60 GPa and ∼2500 K. It undergoes a transition from the cubic fluorite-type structure (thorianite) to the orthorhombic α-PbCl_2 cotunnite-type structure between 20 and 30 GPa at room temperature. Prior to the transition at room temperature, an increase in unit-cell volume is observed, which we interpret as anion sub-lattice disorder or pre-transformation “melting” (Boulfelfel et al. 2006). The thermal equation of state parameters for both thorianite [V_0 = 26.379(7), K_0 = 204(2), αK_T = 0.0035(3)] and the high-pressure cotunnite-type phase [V_0 = 24.75(6), K_0 = 190(3), αK_T = 0.0037(4)] are reported, holding K_0′ fixed at 4. The similarity of these parameters suggests that the two phases behave similarly within the deep Earth. The lattice parameter ratios for the cotunnite-type phase change significantly with pressure, suggesting a different structure is stable at higher pressure.

High-pressure polymorphs of LiPN2: A first-principles study

In this work, high-pressure phase behavior of LiPN2 within 0–300 GPa was studied by using an unbiased structure searching method in combination with first-principles calculations. Three pressureinduced phase transitions were predicted, as tI16 → hR4 → cF64 → oP8 at 44, 136, and 259 GPa, respectively. The six-fold coordination environments were found for all high-pressure polymorphs, which are substantially different from the four-fold coordination environments observed in the tI16 structure. The hR4 and cF64 structures consist of close-packed PN6 and LiN6 octahedra connected by edge-sharing, whereas the oP8 structure is built up from edge- and face-sharing PN6 and LiN6 octahedra with N lying in the center of the trigonal prisms. The electronic structure analysis reveals that LiPN2 is a semiconductor within the pressure range studied and P-N and Li-N bonds are covalent and ionic, respectively. The results obtained are expected to provide insight and guidance for future experiments on LiPN2 and other alkali metal nitridophosphates.

High Pressure Phase Behavior of the Homologous Series CO2 + 1-Alcohols

The phase behavior of the supercritical CO2 + 1-alcohol homologous series for 1-octanol and higher alcohols is investigated. A literature survey revealed that sufficient data are available lower alcohols (1-decanol and lower) but data are limited for higher 1-alcohols. Data for CO2 + 1-dodecanol, CO2 + 1-tetradecanol, and CO2+1-hexadecanol were thus measured using a visual static synthetic method for T = 313–353 K. Measured and literature data indicated that the phase transition pressures are very high (>28 MPa) for CO2 + 1-decanol and higher alcohols in the mixture critical region. Additionally, a temperature inversion (increasing temperature leading to decreasing phase transition pressure) is present in the mixture critical region and is more pronounced in systems containing higher 1-alcohols. The temperature inversion is postulated to result from the formation of alcohol multimers due to hydrogen bonding between alcohol molecules. From the phase behavior data as well as published upper critical end poi...

High pressure phase behaviour of carbon dioxide and two ionic liquids based on a benzyl functionalized cation

The high pressure phase behaviour of a CO2 and 1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [BzMeIM][NTf2], binary system was experimentally measured at high pressure up to about 13.5 MPa and at a temperatures range of 293.2–323.2 K. The solubilities were determined by measuring the bubble point pressure of the binary mixture comprising CO2 and an ionic liquid using a phase equilibrium apparatus including a high pressure variable-view cell. The effect of a benzyl functional group in different cations, imidazolium and pyridinium, on the high pressure phase behaviour of the CO2 and 1-benzylpyridinium bis(trifluoromethylsulfonyl)imide, [BzPY][NTf2], binary system was also investigated within similar temperature and CO2 mole fractions ranges. The experimental data correlated well with the Peng-Robinson equation of state. In general, higher CO2 solubilities were obtained in [BzPY][NTf2] than those in [BzMeIM][NTf2] at similar temperature and pressure. This may be attributed to the difference in aromaticity in pyridinium and imidazolium cations.

High Pressure Phase Equilibria of the CO2/Saturated Ethyl Esters Homologous Series

A systematic study on the phase behavior of saturated ethyl esters with supercritical CO2 is presented. High pressure phase behavior measurements for the systems CO2/ethyl decanoate, CO2/ethyl dodecanoate, CO2/ethyl tetradecanoate, and CO2/ethyl hexadecanoate were conducted in a static synthetic view cell in the temperature range 308–358 K. Phase transition pressures were measured in the range 5.86–23.01 MPa for ethyl ester mass fractions in the range 0.0174–0.657 and complement existing literature data. Throughout the temperature and compositional range measured, which encompassed the liquid phase, mixture critical region, and vapor phase, an increase in temperature leads to an increase in phase transition pressure with no temperature inversions or three phase regions observed. Additionally, an increase in hydrocarbon backbone length leads to an ever-increasing phase-transition pressure, suggesting fractionation of ethyl esters according to molecular mass with supercritical CO2 is possible. Finally, the measured and previously published CO2/ethyl ester data were successfully correlated with a modified Chrastil models and the MT model. While these models cannot easily be used predictively and depend on the data sets used for correlation, these are simple and easy methods that can be used to interpolate data.

Predicting the Fluid-Phase Behavior of Aqueous Solutions of ELP (VPGVG) Sequences Using SAFT-VR.

The statistical associating fluid theory for potentials of variable range (SAFT-VR) is used to predict the fluid phase behavior of elastin-like polypeptide (ELP) sequences in aqueous solution with special focus on the loci of lower critical solution temperatures (LCSTs). A SAFT-VR model for these solutions is developed following a coarse-graining approach combining information from atomistic simulations and from previous SAFT models for previously reported relevant systems. Constant-pressure temperature-composition phase diagrams are determined for solutions of (VPGVG)n sequences + water with n = 1 to 300. The SAFT-VR equation of state lends itself to the straightforward calculation of phase boundaries so that complete fluid-phase equilibria can be calculated efficiently. A broad range of thermodynamic conditions of temperature and pressure are considered, and regions of vapor-liquid and liquid-liquid coexistence, including LCSTs, are found. The calculated phase boundaries at low concentrations match those measured experimentally. The temperature-composition phase diagrams of the aqueous ELP solutions at low pressure (0.1 MPa) are similar to those of types V and VI phase behavior in the classification of Scott and van Konynenburg. An analysis of the high-pressure phase behavior confirms, however, that a closed-loop liquid-liquid immiscibility region, separate from the gas-liquid envelope, is present for aqueous solutions of (VPGVG)30; such a phase diagram is typical of type VI phase behavior. ELPs with shorter lengths exhibit both liquid-liquid and gas-liquid regions, both of which become less extensive as the chain length of the ELP is decreased. The strength of the hydrogen-bonding interaction is also found to affect the phase diagram of the (VPGVG)30 system in that the liquid-liquid and gas-liquid regions expand as the hydrogen-bonding strength is decreased and shrink as it is increased. The LCSTs of the mixtures are seen to decrease as the ELP chain length is increased.

Structure and bulk modulus of Ln-doped UO2 (Ln = La, Nd) at high pressure

The structure of lanthanide-doped uranium dioxide, LnxU1-xO2-0.5x+y (Ln = La, Nd), was investigated at pressures up to ∼50–55 GPa. Samples were synthesized with different lanthanides at different concentrations (x ∼ 0.2 and 0.5), and all were slightly hyperstoichiometric (y ∼ 0.25–0.4). In situ high-pressure synchrotron X-ray diffraction was used to investigate their high-pressure phase behavior and determine their bulk moduli. All samples underwent a fluorite-to-cotunnite phase transformation with increasing pressure. The pressure of the phase transformation increased with increasing hyperstoichiometry, which is consistent with results from previous computational simulations. Bulk moduli are inversely proportional to both the ionic radius of the lanthanide and its concentration, as quantified using a weighted cationic radius ratio. This trend was found to be consistent with the behavior of other elastic properties measured for Ln-doped UO2, such as Young's modulus.

Super- and near-critical fluid phase behavior and phenomena of the ternary system CO2 + 1-decanol + n-tetradecane

High-pressure phase behavior of six 1-decanol+n-tetradecane (solute+solute) mixtures in the presence of near- and supercritical CO2 (solvent) were experimentally studied between T=300K and T=358K using a visual static synthetic method. CO2 free or reduced n-tetradecane mass fractions (wcred) of 0.2405, 0.5000, 0.6399, 0.7698, 0.8162 and 0.9200g.g−1 were selected for the mixtures and the solute (i.e. combined 1-decanol+n-tetradecane) mass fractions (ws) were varied between 0.015g.g−1 and 0.65g.g−1. The ternary system displayed temperature inversions at T=308K and T=318K for the mixture wcred=0.2405g.g−1. For all other wcred mixtures, an increase in temperature leads to an increase in pressure. A liquid-like (bubble-point) isotherm and a vapor-like (dew-point) isotherm were used to graphically define the size and range of isothermal cosolvency effects in the ternary system. The data were used to construct ternary phase diagrams from which cosolvency effects were observed for each set temperature. The large occurrence of cosolvency lead to the penetration of the liquid-liquid-gas (l1l2g) three-phase surface, forming a two-phase liquid-gas (l-g) hole between wcred=0.7698g.g−1 and wcred=0.9200g.g−1. Closed isobaric miscibility windows were inferred for wcred=0.8162g.g−1 to classify the ternary system as Class T-IV phase behavior.

High-pressure phase behavior of turmeric waste and extracts in the presence of carbon dioxide, ethanol and dimethylsulfoxide

This work reports the validation of a phase equilibrium apparatus followed by the presentation of new high-pressure phase equilibrium data on systems containing turmeric waste and extracts, derived from processes that employs supercritical fluids and pressurized liquids, in the presence of carbon dioxide, ethanol and dimethylsulfoxide. In order to provide reliable information for further turmeric processing, experimental phase transition data were obtained using ten levels of temperature (303–348K) and pressures up to 23MPa. Results indicate the existence of complex phase behavior for the systems containing turmeric extracts and waste, with the occurrence of solid-vapor-liquid, solid-vapor-liquid-liquid, vapor–liquid, liquid–liquid and vapor–liquid–liquid phase transitions.

High-pressure phase behavior of SrCO3: an experimental and computational Raman scattering study

The high-pressure phase behavior of strontianite (SrCO3) was both experimentally and theoretically investigated by Raman spectroscopy up to 78 GPa in a diamond anvil cell and density functional theory-based calculations. Our study shows a phase transition between 23.7 and 26.8 GPa during compression from space group Pmcn to post-aragonite SrCO3, which is accompanied by significant changes in the vibrational spectrum. The excellent agreement between the observed and computed Raman frequencies and intensities implies that the high-pressure polymorph has space group Pmmn and contributes to resolving an existing disagreement concerning the correct space group symmetry of this high-pressure polymorph. It is shown that the transition pressure from the aragonite to a post-aragonite phase increases linearly with decreasing cation radius for (Ca, Sr, Ba, Pb) carbonates.

Partition Coefficients of Organics between Water and Carbon Dioxide Revisited: Correlation with Solute Molecular Descriptors and Solvent Cohesive Properties.

High-pressure phase behavior of systems containing water, carbon dioxide and organics has been important in several environment- and energy-related fields including carbon capture and storage, CO2 sequestration and CO2-assisted enhanced oil recovery. Here, partition coefficients (K-factors) of organic solutes between water and supercritical carbon dioxide have been correlated with extended linear solvation energy relationships (LSERs). In addition to the Abraham molecular descriptors of the solutes, the explanatory variables also include the logarithm of solute vapor pressure, the solubility parameters of carbon dioxide and water, and the internal pressure of water. This is the first attempt to include also the properties of water as explanatory variables in LSER correlations of K-factor data in CO2-water-organic systems. Increasing values of the solute hydrogen bond acidity, the solute hydrogen bond basicity, the solute dipolarity/polarizability, the internal pressure of water and the solubility parameter of water all tend to reduce the K-factor, that is, to favor the solute partitioning to the water-rich phase. On the contrary, increasing values of the solute characteristic volume, the solute vapor pressure and the solubility parameter of CO2 tend to raise the K-factor, that is, to favor the solute partitioning to the CO2-rich phase.

Use of real crude oil fractions to describe the high pressure phase behavior of crude oil in carbon dioxide

The knowledge about crude oil phase behavior at high-pressures is a challenge for scientists due to inherent complexity to this systems. Some works employ model systems attempting to predict crude oil behavior, but the results are normally poor. In this sense, the use of real crude oil fractions can be an alternative to improve the accuracy of the models used in these simulations. In this work, a light oil sample has been fractionated to generate four representative fractions. An approach to estimate the critical properties of these distilled fractions using only density and vapor pressure experimental data is presented. In sequence, phase behavior of the pseudo binary (carbon dioxide (CO2)+real crude oil fractions), multicomponent (mixture of fractions+CO2) and original crude oil+CO2 systems were determined using static synthetic method and static synthetic+NIR, with CO2 molar fraction range of 0.367-0.986, temperature range of 20⿿80°C and pressure up to 300bar. Binary interaction parameters (BIP) were estimated for each pair (CO2+distilled fraction) and employed to describe the phase behavior of a CO2+crude oil systems with Peng-Robinson equation of state (PR-EOS) and quadratic mixing rule. The results suggest that the use of real fractions can be a safer strategy for predicting the phase behavior of petroleum in carbon dioxide. Besides, NIR spectroscopy showed to be a good alternative to study phase behavior of dark/opaque systems.

Phase behaviour of the two binary systems formed by CO2 and the silane precursors N-[3-(trimethoxysilyl)propyl]aniline or (3-mercaptopropyl)trimethoxysilane

High-pressure phase behaviour data are reported for the binary systems CO2+N-[3-(trimethoxysilyl)propyl]aniline (TMSPA) and CO2+(3-mercaptopropyl)trimethoxysilane (MPTS). The organosilanes TMSPA and MPTS are employed to functionalise with the amino and mercapto groups, respectively, the surface of inorganic materials using supercritical CO2 as a solvent and reaction medium. The measurements were conducted in a high-pressure variable volume view cell at T=313.2, 323.2, and 333.2K and pressures up to p=25.0MPa. The CO2 mole fraction in the binary mixture was varied from 0.40 to 1.0. For both silanes, the immiscibility region enlarges as the temperature rises. For a given pressure and temperature MPTS is considerably more soluble in CO2 than TMSPA. For instance, if a temperature of 313.2K is chosen a pressure of 8.7MPa will suffice to assure the solubilisation of MPTS in the whole mole fraction range whereas a pressure of 21.0MPa will be necessary for the solubilisation of TMSPA. The Peng-Robinson equation of state was used to correlate the phase equilibrium data.

Modelling fluid phase equilibria in the binary system trifluoromethane + 1-phenylpropane

The paper presents the results on modelling high pressure phase behaviour of the systems consisting of refrigerant, trifluoromethane (R23) and 1-phenylpropane. There were used cubic equations of state (GEOS, SRK and PR) coupled with van der Waals mixing rules in a semi-predictive approach (SPA). Based on the experimental VLE isothermal data in the range 300–330 K, binary interaction parameters (BIPs) were optimized, through regression of bubble pressure type. Unique sets of interaction parameters were estimated for each EoS, and used in the SPA to calculate the critical, subcritical and supercritical behaviour of the system.The SPA calculations are comparatively discussed with the available experimental data in the temperature range (250–400) K and pressures up to 12 MPa. The calculations of critical line and of vapour-liquid, liquid-liquid, vapour-liquid-liquid phase equilibria, and of critical endpoints indicate a good modelling capacity of the tested EoSs and an accurate representation of the complex critical and subcritical behaviour of the investigated system.

Quantitative Raman Spectroscopic Measurements of CO2 Solubility in NaCl Solution from (273.15 to 473.15) K at p = (10.0, 20.0, 30.0, and 40.0) MPa

The knowledge of high pressure phase behavior of the CO2–H2O–NaCl system in a wide T–P–mNaCl range is of great interest in the injection of CO2 to deep reservoir for storage or enhancement of oil recovery (EOR). The calculation of CO2 solubility in brine is very important to predict the CO2 storage capacity in saline aquifers. However, CO2 solubility data at high salinity and high pressure are limited, and few thermodynamic models can accurately predict CO2 solubility when salinity is higher than 4.5 mol/kg. In this study, a noninvasive technique, quantitative Raman spectroscopy, was used to investigate the high pressure equilibria of the CO2–H2O–NaCl system. A total of 180 solubility data points were obtained for carbon dioxide in (1, 3, and 5) mol/kg NaCl solutions from (273.15 to 473.15) K up to 40 MPa. New parameters were derived to improve the Duan-type solubility model, thus it can be applied in CO2 sequestration and EOR to accurately calculate the solubility of CO2 in NaCl aqueous solution up to 6 ...

High pressure solubility of CH4, N2O and N2 in 1-butyl-3-methylimidazolium dicyanamide: Solubilities, selectivities and soft-SAFT modeling

The society and industry commitment to progressively reduce Green House Gases (GHGs) emissions forged important challenges that conventional gas separation processes are unable to overcome. Ionic liquids (ILs) have been attracting an outstanding attention during the last decade as a promising class of viable solvents to capture pollutants and for gas separation processes. Being the IL-based membranes gas separation controlled by the gas solubility in the IL rather than by its diffusivity, the solubility of gases in ILs stands as highly relevant input for their application in liquid membranes. As part of a continuing effort to develop an IL based process for high pressure capture of GHGs, the phase equilibria of nitrous oxide (N2O), methane (CH4) and nitrogen (N2) were investigated in this work. Experimental gas–liquid equilibrium data for N2O, CH4 and N2 in [C4C1im][N(CN)2] were determined in the (293 to 363) K temperature range, for pressures up to 70MPa and gas mole fractions up to 35 %.Unfavorable interactions towards the studied gases, with positive deviations to ideality, were observed for all the studied gases, placing the studied IL among those with the lowest selectivities reported. The observed behavior highlights that a delicate balance between the solvent polarity and its molar volume must be ascertained when a highly selective solvent for N2 or CH4 separation is envisaged. The soft-SAFT EoS successfully described the high pressure phase behavior data using the molecular model and parameters sets reported in previous works. A good description of the binary systems studied, including the small CH4 temperature dependency and the N2 reverse temperature dependency on the solubility were achieved using just one binary interaction parameter. This reinforces the use of soft-SAFT as an accurate model to describe the behavior of gases in ILs for different applications.