Liquid Critical Curve Research Articles

Fluid-phase behavior of the guaiacol + CO2 system at high pressures

The binary phase behavior of supercritical CO2 with guaiacol, a model lignin monomer that is a liquid at near-ambient conditions, has been determined. A variable-volume, high-pressure view cell was used to observe both liquid–liquid cloud points as well as bubble points along the liquid–liquid–vapor (LLV) three-phase line for this system. CO2+guaiacol was found to exhibit Type III phase behavior according to the classification system of Scott and Van Konynenburg, indicating that even at high pressures the mutual solubility of the two components is relatively low. However, the system does exhibit a pressure minimum in the liquid–liquid critical curve at 15°C and pressures just under 500bar. The upper critical end point U for the CO2-rich liquid phase extends to temperatures and pressures 2.1°C and 2.2bar above the critical point of CO2.

Miscibility Holes and Continuous Liquid−Liquid Miscibility Curves in Type III and IV Systems

Liquid−liquid miscibility in binary and ternary fluids has been studied to a great extent by Professor Schneider. One of the important results of his studies was the description of the so-called immiscibility holes or islands in type IV systems. In these holes, the liquid−liquid coexistence curve virtually disappears; i.e., it goes below the vapor pressure of the mixture where the liquid becomes metastable to the liquid−vapor phase transition. In the case of the ternary system, the critical end-points then form a closed loop which is the so-called immiscibility hole. Changing, for example, the composition in ternary systems or the molar mass in binary systems containing chain molecules, one can show that a type III-like liquid−liquid critical curve can emerge from this hole. Here we show that this type III-like critical curve already exists within the hole. As a consequence, the liquid−liquid critical curves of type III and IV systems are similar if metastable states are included. The possibility of indir...

High-Pressure Phase Behavior of the Binary Systems (Propane + Diamantane) and (Ethane + Diamantane)

(Solid + fluid) and (vapor + liquid) equilibria for the binary systems (propane + diamantane) and (ethane + diamantane) were determined according to the synthetic method in the temperature range 360−510 K and in the pressure ranges 0.3−14 and 0.3−200 MPa, respectively. Also, for both systems the high-temperature branch of the three-phase curve (solid diamantane + liquid + vapor) was determined. In both systems, this curve starts at the second critical endpoint where the three-phase curve intersects the vapor—liquid critical curve and runs to higher temperature with a negative slope dP/dT to the triple point of diamantane. Due to a solid—solid transition of pure diamantane, the three-phase curve (solid diamantane + liquid + vapor) of both systems shows an abrupt change in slope at the point where it intersects the solid—solid transition curve of diamantane. This quadruple point SISIILV is found for (propane + diamantane) at T = 442.64 K and P = 4.110 MPa and for (ethane + diamantane) at T = 445.49 K and P ...

Estimation of Liquid−Liquid−Vapor Equilibria in Binary Mixtures of n-Alkanes

This study tests the ability of the best presently available semipredictive models, namely, predictive Soave−Redlich−Kwong (PSRK), linear combination of Vidal mixing rules (LCVM), and the novel global phase diagram approach (GPDA), to estimate the three-phase liquid−liquid−vapor equilibrium (LLVE) in the homologous series methane−n-alkanes, ethane−n-alkanes, propane−n-alkanes, and n-butane−n-alkanes. It is demonstrated that at high pressures GE mixing rules predict the wrong shape of the liquid−liquid critical curves, which involves fictitious closed loops of liquid−liquid equilibrium. Such a behavior is responsible for predicting nonrealistic three-phase LLVE behavior along the homologous series under consideration. In addition, both PSRK and LCVM fail in estimating the lower critical end-point curve. The results presented here for the GE-based models under consideration are probably characteristic for them. In contrast, GPDA is consistent in the entire thermodynamic phase space and does not predict nonp...

Critical curves of liquid–liquid equilibria in ternary systems: Description by a regular-solution model

In this paper relations between the shapes of the liquid–liquid critical curves in ternary systems and the temperature characters of the heterogeneous regions are discussed. The regular-solution model with temperature-dependent parameters was employed to describe critical curves of liquid–liquid equilibria in ternary systems. Relations for the limiting values of the slopes of critical curves were derived. The corresponding type of the critical curve and consequently the temperature character of the liquid–liquid equilibrium can be deduced from these values. Model calculations were performed for ternary systems characterised by two binary subsystems with either an upper critical solution temperature or with a lower critical solution temperature, and the third binary subsystem which is homogeneous in the considered temperature range. In the calculations the following approach was used: the behaviour of two binary systems was fixed by the temperature dependencies of their parameters and ternary critical curves of LLE were then calculated for variants differing in non-ideality (deviations from the Raoult’s law) of the third homogeneous binary subsystem. It was found that the level of non-ideality of the homogeneous system can influence the shape of the critical curve significantly.

The effect of the addition of water, propane, or docosane on the vapour–liquid and solid–fluid equilibria in asymmetric binary n-alkane mixtures

In this paper experimental results on vapour–liquid and solid–fluid phase equilibria for three types of ternary mixtures are reported. The ternary systems studied were methane+water+hexadecane, methane+propane+tetracosane, and methane+docosane+tetracosane. The influence of an excess water phase on the vapour–liquid critical curve of the methane+hexadecane system is determined and found to be negligible in a temperature range from 290 to 350 K. The effect of the addition of propane on the phase behaviour of two mixtures of methane+tetracosane was studied. On addition of propane ( x C3=0.15) a decrease of the bubble- and dew-point pressures of up to 40 MPa was observed. Also a shift of the solid+fluid/fluid)boundary curve towards lower temperatures at constant pressure of approximately 6 K was found. For 20 mixtures of the ternary system methane+docosane+tetracosane, the solid–fluid and vapour–liquid equilibria were studied at temperatures up to 350 K and at pressures up to 200 MPa. The different isobaric solid-solubility curves show discontinuities at a relative solute mole fraction of tetracosane of 0.15 and 0.5. This most likely indicates the presence of different mixed crystal structures in the solid phases precipitating from the ternary mixtures at high pressures.

Phase Behavior of the Acetonitrile + Butane System at Temperatures from 311.40 K to 436.82 K

Visual observations of the co-existing phases over the temperature range from 311.40 K to 436.82 K and up to gas−liquid critical pressure for acetonitrile + butane mixtures have been made. Nine gas−liquid critical points, dew and bubble pressures, and densities at 20 K intervals from 333.15 K to 433.15 K and p−T−ρ−x relationships for three-phase liquid−liquid−vapor equilibrium were established. This system forms a minimum boiling azeotrope up to the gas−liquid critical point. On the p,T projection, the three-phase line terminates in a liquid−liquid upper critical end point, from which the liquid−liquid critical curve (of upper critical solution temperatures) starts vertically to high pressures.

On the gas–liquid critical curves of different size hard-spheres binary mixtures interacting via the Lennard-Jones potential

The critical behavior of binary mixtures represented by a potential model of different size hard spheres plus a Lennard-Jones interaction is studied by the application of thermodynamic perturbation theory. The Lennard-Jones potential function is the perturbing potential to the hard-sphere reference system. It is found that convexity of the critical curves of type I is sensitive to small changes of the unlike interaction parameter values. Results for Ar–Kr and CH4–Kr mixtures are in good agreement with experimental data.