Sergio E. Quiñones-Cisneros

Phase and critical behavior in type III phase diagrams

Using a cubic equation of state, the phase and critical behavior in type III phase diagrams showing gas-gas immiscibility of the second kind has been studied. The analysis of the pressure-composition diagrams and the corresponding pressure-molar density diagrams has shown new criteria for delimiting the regions of liquid-vapor and liquid-liquid equilibria. Basic to these new criteria was the finding of a molar isopycnic curve on a temperature-pressure projection. This curve follows from a bifurcation of the liquid-vapor critical curve coming from the less volatile component. This bifurcation point provides a criterion for definition of the point of transition from the liquid-vapor critical curve into a liquid-liquid critical curve. In understanding complex phase behavior, the importance of studying molar density phase diagrams alongside the corresponding compositional phase diagrams is emphasized throughout this work.

Correlation for the Viscosity of Sulfur Hexafluoride (SF6) from the Triple Point to 1000 K and Pressures to 50 MPa

A wide-ranging correlation for the viscosity surface of sulfur hexafluoride (SF6) has been developed that incorporates generalized friction theory (GFT). The approach requires, as the core thermodynamic model, a reference-quality equation of state (EoS). Here the EoS of Guder and Wagner has been selected for that purpose. All available experimental data, to the extent of our knowledge, were considered in the development of the model. The correlation performs best in the low-pressure (less than 0.33 MPa) region from 300 K to 700 K where the estimated uncertainty (considered to be combined expanded uncertainty with a coverage factor of two) is 0.3%. In the region from 300 K to 425 K for pressures less than 20 MPa, the estimated uncertainty is less than 1%. Where there were data available for validation at temperatures from 230 K to 575 K for pressures up to 50 MPa, the estimated uncertainty is 2%. The correlation extrapolates in a physically reasonable manner and may be used at pressures to 100 MPa and temp...

An analytical equation of state for the hard core Yukawa fluid; the electroneutral mixture

An explicit analytical equation of state is presented for an electroneutral fluid mixture of equal-sized particles which interact via hard core Yukawa potentials. Based on a perturbative scheme, analytical expressions for the coefficients of an inverse temperature expansion of internal energy in the mean-spherical approximation (MSA) are given. Extending an approximate procedure used recently for the one-component hard core Yukawa fluid, closed analytical mathematical expressions for the thermodynamics and equation of state of the electroneutral mixture are obtained. Thermodynamic properties and phase diagrams are studied for the symmetrical binary mixture. Results calculated with the analytical formulas presented here are compared with MSA results and shows excellent agreement when the interaction potential is short range. Two different binary systems are represented with the model studied in this work. In the first the interaction between particles of one species is given by repulsive Yukawa tails and t...

New Model for the Correlation of the Surface Tension Based on Friction Theory

A new correlation method for the surface tension of fluids is proposed, which is based on friction theory applied to the interface of a two-phase system. The substance properties enter the model by a regular equation of state. Here we derive the method and test it with the Lennard-Jones 12-6 fluid as the reference system using molecular dynamics simulations of the vapor-liquid interface in combination with a new Lennard-Jones 12-6 equation of state. Further correlations of experimental surface tension data based on the Peng-Robinson and the PC-SAFT equations of state are presented. As a result, we find that the method allows an accurate correlation of the surface tension of pure fluids.

Chemical potentials and phase equilibria of Lennard-Jones chain fluids

Computer simulations (molecular dynamics) were performed for ensembles of flexible tangent Lennard-Jones chains consisting of n sites (n = 1, 2, 4, 8, and 16). From these simulations, the orthobaric liquid and vapour densities were calculated not only with the traditional method of simulating a liquid film in coexistence with vapour, but also using the rigorous thermodynamic condition of satisfying the chemical potential equality between the phases in equilibrium. The agreement with literature data, as far as such exist, is excellent.