Phase Behavior Of Fluid Mixtures Research Articles

Phase separation in fluids with many interacting components

Fluids in natural systems, like the cytoplasm of a cell, often contain thousands of molecular species that are organized into multiple coexisting phases that enable diverse and specific functions. How interactions between numerous molecular species encode for various emergent phases is not well understood. Here, we leverage approaches from random-matrix theory and statistical physics to describe the emergent phase behavior of fluid mixtures with many species whose interactions are drawn randomly from an underlying distribution. Through numerical simulation and stability analyses, we show that these mixtures exhibit staged phase-separation kinetics and are characterized by multiple coexisting phases at steady state with distinct compositions. Random-matrix theory predicts the number of coexisting phases, validated by simulations with diverse component numbers and interaction parameters. Surprisingly, this model predicts an upper bound on the number of phases, derived from dynamical considerations, that is much lower than the limit from the Gibbs phase rule, which is obtained from equilibrium thermodynamic constraints. We design ensembles that encode either linear or nonmonotonic scaling relationships between the number of components and coexisting phases, which we validate through simulation and theory. Finally, inspired by parallels in biological systems, we show that including nonequilibrium turnover of components through chemical reactions can tunably modulate the number of coexisting phases at steady state without changing overall fluid composition. Together, our study provides a model framework that describes the emergent dynamical and steady-state phase behavior of liquid-like mixtures with many interacting constituents.

More Accurate Determination of Isothermal Compressibility and Isobaric Thermal Expansivity Using Volume-Translated Equations of State

Isothermal compressibility, κT, and isobaric thermal expansivity, αP, can describe the volumetric behavior of a given fluid in response to the change in pressure and temperature, respectively. They are two important parameters that are used in the simulation and modeling of many petroleum and chemical processes. Isothermal compressibility and isobaric thermal expansivity can be calculated with empirical correlations or thermodynamic models, but these methods are not sufficiently accurate. Cubic equations of state (CEOSs) are widely used in the petroleum and chemical industry to describe the phase behavior of fluid mixtures. It is recognized that one of the deficiencies of CEOSs is their inaccuracy in predicting liquid-phase volumes. The volume translation (VT) strategy was proposed to overcome this deficiency, which could significantly improve the performance of CEOSs in predicting volumetric properties. With recent development in the volume-translated equations of state (VT-EOSs), κT and αP could be predicted more accurately by VT-EOSs. However, the prediction accuracy of κT and αP by different VT-EOSs has not been comprehensively investigated. In this work, κT and αP of two pure example fluids (CH4 and CO2) are predicted by seven different VT models: one constant VT model, two linear temperature-dependent VT models, two exponential temperature-dependent VT models, and two temperature-pressure-dependent VT models. The accuracy of each model is evaluated by comparing the predictions with the NIST pseudo-experimental data for the liquid phase, the vapor phase, and the supercritical phase. The predicted results show that the distance-function-based temperature-pressure-dependent VT models exhibit relatively better performance in predicting κT and αP than the temperature-dependent and the constant VT models. This is because the distance function is a function of both temperature and pressure, which allows the distance-function-based VTs to provide more accurate volume corrections for most of the phase regions and thus can lead to better accuracy in predicting κT and αP. According to the overall prediction accuracy, the VT-PR EOS model proposed by Abudour et al. (2012) provides the most accurate predictions of κT, while the VT-SRK EOS model proposed by Chen and Li (2020) provides the most accurate predictions of αP. The findings of this study can help to predict κT and αP of pure substances at different temperature and pressure conditions by using the most accurate VT-EOS and provide insights for further improvement of volume translation strategies.

Thermodynamic Analysis of Phase Behavior at High Capillary Pressure

Summary High capillary pressure has a significant effect on the phase behavior of fluid mixtures. The capillary pressure is high in unconventional reservoirs because of the small pores in the rock, so understanding the effect of capillary pressure on phase behavior is necessary for reliable modeling of unconventional shale-gas and tight-oil reservoirs. As the main finding of this paper, first we show that the tangent-plane-distance method cannot be used to determine phase stability and present a rigorous thermodynamic analysis of the problem of phase stability with capillary pressure. Second, we demonstrate that there is a maximum capillary pressure (Pcmax) where calculation of capillary equilibrium using bulk-phase thermodynamics is possible and derive the necessary equations to obtain this maximum capillary pressure. We also briefly discuss the implementation of the capillary equilibrium in a general-purpose compositional reservoir simulator. Two simulation case studies for synthetic gas condensate reservoirs were performed to illustrate the influence of capillary pressure on production behavior for the fluids studied.

Capillary pressure effect on phase behavior of CO2/hydrocarbons in unconventional reservoirs

Small-pore sizes on the order of nanometers in unconventional reservoirs lead to a large capillary pressure, which significantly affects the phase behavior of fluid mixture and the fluid transport process. Although there are some efforts on studying the capillary pressure effect on phase behavior, it has not been clearly understood until recently in the field application considering multiple components in tight oil reservoirs. In this study, we present a methodology to investigate the phase behavior of carbon dioxide (CO2)/hydrocarbon systems with and without the capillary pressure effect. We modify the Peng-Robinson equation of state considering inequalities of hydrocarbon liquid and vapor pressures. Rachford-Rice flash calculation, criterion of Gibbs free energy minimization, and Newton-Raphson method are utilized together to calculate the phase equilibrium between vapor and liquid. In addition, the Young-Laplace equation is applied to evaluate the capillary pressure in the presence of pore size distribution. We validate the methodology against one experimental measurement with the capillary pressure effect. Subsequently, we perform two case studies including a two-component system and a typical fluid from Bakken tight oil reservoirs. The results indicate that the reduction in the nano-pore size causes noticeable difference in the two-phase envelope. Furthermore, for the real fluid, the capillary pressure effect cannot be neglected when the pore size is less than 50nm. Additionally, the bubble-point pressure of Bakken oil reduces nearby 500psi when the nano-pore size reduces to 10nm. The MMP of CO2 injection is also evaluated, which experiences a decreasing trend. The developed methodology provides a better understanding of the phase behavior of CO2/hydrocarbons system, especially in the case of CO2 injection into tight oil reservoirs.

Measurement and modeling of the phase behavior of the (carbon dioxide + water) mixture at temperatures from 298.15 K to 448.15 K

An analytical apparatus has been designed to study the phase behavior of fluid mixtures of relevance to CO2-enhanced oil recovery and carbon dioxide storage in deep aquifers or depleted oil fields. The fluid phases are circulated by means of a dual-channel magnetically-coupled pump and aliquots may be withdrawn from the re-circulation loops, by means of high-pressure sampling valves, for analysis by gas chromatography. The high-pressure cell is fitted with a special probe that may be rotated in order to draw liquid into the re-circulation loop from different heights within the cell, thereby permitting the study of three-phase vapor–liquid–liquid equilibria. The working temperature range of the apparatus is from (298 to 448)K and the maximum working pressure is 50MPa.In this work, measurements have been made on the binary system (CO2+H2O) at temperatures from (298.15 to 448.15)K and pressure from (1.5 to 18.0)MPa, and the results are compared with the available literature data. Vapor–liquid–liquid and liquid–liquid equilibrium points were also measured at T=298.15K. Standard uncertainties were 0.04K for temperature, 0.04% of reading for pressure, and typically 3×10−4 and 8×10−4 for the mole fractions in liquid and vapor phases respectively. The results have been correlated by means of an asymmetric approach based on the Peng–Robinson equation of state, for the vapor phase, and an extended form of Henry's law incorporating the NRTL solution model, for the aqueous liquid-phase. The ability of the Krichevsky–Kasarnovsky (KK) approach to correlate the data has also been evaluated.

Investigation of the global phase behavior of polymer mixtures in the shield region

This paper is a contribution of our systematic investigation of the global phase behaviors of the chain molecules mixtures, i.e., polymer mixture solutions. The phase behavior of fluid mixtures is understood by the critical lines in fluid-gas diagrams. The critical lines of binary fluid system may, under circumstances, exhibit closed loops in the critical lines. A distinction is made between free critical loops, as described by type VI in the Scott and van Konynenburg classification, and "rooted" critical loops, as found in the shield region. We define rooted loops as closed critical lines that are attached to the critical line structure by means of unstable critical line. We obtain the rooted loops in the global phase diagrams of the polymer mixture solutions within the framework of a model that combines the lattice gas model of Schouten, ten Seldam and Trappeniers with the Flory-Huggins theory, and we present the influence of the chain length of long molecules on the rooted critical loops. We present the results in the density-density and the temperature (T)-pressure (P) planes in detail.

Systematic investigation of the global phase behavior of polymer–solvent systems in the density–density plane

The phase behavior of fluid mixtures is understood by the critical lines in fluid–gas diagrams. We investigated the critical lines of polymer–solvent systems at the mathematical double point, where two critical lines meet and exchange branches, and its environment within the framework of a model that combines the lattice gas model of Schouten, ten Seldam and Trappeniers with the Flory–Huggins theory. The critical lines are expressed as a function of x 1 and x 2, the density of type 1 polymer molecules and the density of type 2 polymer molecules, respectively; in this way global phase diagrams are presented and discussed in the density–density plane. Density–density plots are preferable when studying the differences in behavior of different classes, since they enable us to follow the connectivities in a systematic way. In this study the connectivity of critical lines at the mathematical double point and its around is investigated in detail. We also discuss the topology of the critical lines according to the Sadus classification scheme for ternary mixtures.

Simulation of Phase Transitions in Highly Asymmetric Fluid Mixtures

We present a novel method for the accurate numerical determination of the phase behavior of fluid mixtures having large particle-size asymmetries. By incorporating the recently developed geometric cluster algorithm within a restricted Gibbs ensemble, we are able to probe directly the density and concentration fluctuations that drive phase transitions, but that are inaccessible to conventional simulation algorithms. We develop a finite-size scaling theory that relates these density fluctuations to those of the grand-canonical ensemble, thereby enabling accurate location of critical points and coexistence curves of multicomponent fluids. Several illustrative examples are presented.

Measurement of physical properties of refrigerant mixtures. Determination of phase diagrams

Abstract Accurate knowledge of high pressure phase equilibrium is of utmost interest in designing chemical processes and separation units operating at high pressures. Moreover, these data are essential to develop optimized models in order to provide continuous representation of high pressure phase equilibrium. The best models are required to permit process simulators to be the most accurate as possible. There are many experimental ways to obtain information about the phase behaviour of fluid mixtures. They can be classified into two main categories: open and closed circuit methods. Closed circuit methods are composed of two main categories: the synthetic methods where compositions are known a priori and analytic methods where samplers are needed. The laboratory has published several data sets concerning refrigerant systems (binaries and ternaries). The more recent are: – binary and ternary systems with 1,1,1,2,3,3,3-heptafluoropropane (R227ea), difluoromethane (R32) (two hydrofluorocarbons) and propane (R290), – binaries of R227ea or R32 with CO 2 (R744) and an old refrigerant SO 2 (R764), – ternary system composed of dimethyl ether (DME), pentafluoroethane (R125) and 1,1,1,2-tetrafluoroethane (R134a), – binary systems involving DME with R134a and R32. In this paper we have focused our attention on two types of models: – the first one is a thermodynamic model implying equations of state and several mixing rules, – the second one is a neural network model. It is not a “thermodynamic” model, but it can be used to calculate thermodynamics properties like enthalpy, entropy from density values. It has been used to represent density data of the R134a + R125 + DME ternary system.

Demixing in hard ellipsoid rod-plate mixtures

The phase behavior of fluid mixtures of hard uniaxial ellipsoids with elongations e and 1/e, and equal molecular volume, has been studied using constant-pressure Gibbs ensemble Monte Carlo simulations for e=15 and e=20. Four distinct phases are observed: isotropic (I), uniaxial nematic (N+ and N−) and biaxial nematic (B). The region of stability of the biaxial phase is found to be limited severely by demixing into two coexisting uniaxial phases. This is in agreement with recent theoretical predictions. The theory, however, does not account for the surprising asymmetry of the phase diagram that we find in our simulations.

Near‐infrared spectroscopic investigations on phase behaviour and hydrogen‐bonding of polar substances in supercritical chlorotrifluoromethane. Part I — phase behaviour

AbstractThe phase behaviour of fluid mixtures of polar substances in supercritical chlorotrifluoromethane CClF3 was investigated at pressures up to 100 MPa and temperatures from 323 K to 398 K. The measurements were performed by high pressure near‐infrared spectroscopy using the analytical method. An optical high pressure cell equipped with sapphire windows and fibre optics was used. The total substance concentration was obtained from the first overtone region of C‐H stretching vibration (1600–1800nm), as well as from the second overtone region (1150–1300 nm) and its combination with other vibrational molecular modes (2100–2350 nm). Solubilities of dodecanoic acid, hexadecanoic acid, their corresponding methyl esters, δ‐valerolactam and δ‐caprolactam in the supercritical gas are presented. The influence of molecular properties on the phase behaviour is discussed.

Rapid method for the determination of phase behavior of fluid mixtures employed in supercritical fluid experiments

Rapid method for the determination of phase behavior of fluid mixtures employed in supercritical fluid experiments

ChemInform Abstract: High Pressure Fluid Phase Equilibria: Experimental Methods and Systems Investigated (1978‐1987)

Abstract The phase behaviour of fluid mixtures at high pressure has received great attention over the past decade. This upsurge of interest has been occasioned by the importance that high pressure fluid phase equilibria has now assumed in some industrial areas. This work reviews and classifies recently developed experimental methods for investigating high pressure fluid phase equilibria. A review of the systems studied in the last 10 years is also given.

A diamond anvil cell for the study of fluid mixtures at high temperatures

Abstract A new diamond anvil cell has been developed to study the phase behavior of fluid mixtures at high temperature. Special care has been taken to achieve good temperature stability and small temperature gradients. Preliminar experiments show that the cell performs well.

Fluid mixtures at high pressures II. Phase separation and critical phenomena of (ethane + an n-alkanol) and of (ethene + methanol) and (propane + methanol)

To study various effects on the critical phase behaviour of fluid mixtures, the critical p( T) curves and liquid-liquid-gas (llg) three-phase lines of the six mixtures (ethane + an n-alkanol) with methanol, ethanol, n-propanol, n-butanol, n-pentanol, and n-octanol, and of (ethene + methanol) and (propane + methanol) have been investigated. All mixtures apart from (propane + methanol) exhibit an interrupted critical curve and show ll phase separation. The two branches of the critical curve end at an upper critical end point (UCEP) and at a lower critical end point (LCEP). These are connected by an llg three-phase line. Among the mixtures exhibiting an interrupted critical curve, (ethane + methanol) is the only one which does not have an LCEP. Quasi-binary mixtures of ethane with three different { xCH 3OH + (1 − x)C 2H 5OH} mixtures, where x is 0.50, 0.75, and 0.90, are used to demonstrate the transition from a mixture having only a UCEP, such as (ethane + methanol), to a mixture possessing a UCEP and an LCEP, such as (ethane + ethanol).