Diffusivities of an Equimolar Methane–Propane Mixture Across the Two-Phase Region by Dynamic Light Scattering

Abstract

The present contribution examines the accessibility of diffusivities across the twophase region of an equimolar methane-propane mixture for dynamic light scattering (DLS) experiments. Heterodyne DLS experiments and theoretical calculations of the Rayleigh ratio were performed at 125 different thermodynamic states including the gas, liquid, supercritical, and the two-phase region. The present measurements document that two diffusivities can be determined simultaneously in the liquid state and saturated liquid phase for temperatures and pressures which correspond to densities larger than 1.15 times the critical density. Based on a rigorous assignment of the signals detected in this work, the slow and fast diffusivities could be associated with the Fick and thermal diffusivities. For all other thermodynamic states, a single hydrodynamic mode or signal was obtained experimentally. With the help of theoretical Rayleigh ratios as well as from the general behavior of the diffusivities as a function of temperature and pressure, the signals were identified to be related to the Fick diffusivity in the supercritical state and to a mixed diffusivity in the gas state and the saturated vapor phase. The results are discussed in connection with the behavior of the diffusivities along certain paths in the pressure-temperature projection of the phase diagram of the mixture.

Diffusivities in Binary Mixtures of [AMIM][NTf2] Ionic Liquids with the Dissolved Gases H2, He, N2, CO, CO2, or Kr Close to Infinite Dilution

“…In the case of a binary mixture, a heterodyne detection scheme, and neglecting pressure fluctuations, the normalized intensity CF is represented as a function of the correlation time τ by where the constants b 0 , b t , and b c are determined by the characteristics of the experimental setup as well as by the thermodynamic state. For the present investigations, a coupling between heat and mass transfer is negligible, and thus, the mean lifetimes of the fluctuations in temperature and concentrations τ C,t and τ C,c are associated with the thermal diffusivity a and the Fick diffusion coefficient D 11 by Assuming quasi-elastic light scattering, the modulus of the wave vector q is given by Despite the presence of anions and cations in ILs, only one Fick diffusion coefficient characterizes the diffusive mass transfer in binary mixtures of ILs with dissolved gases approaching the electroneutrality condition for ILs . In eq , n fluid is the refractive index of the fluid at the laser wavelength in vacuo λ 0 , and Θ s is the scattering angle.…”

Diffusivities in Binary Mixtures of n-Hexane or 1-Hexanol with Dissolved CH4, Ne, Kr, R143a, SF6, or R236fa Close to Infinite Dilution

“…Dynamic light scattering (DLS) has been proven to be an excellent tool for the simultaneous and accurate determination of a and D 11 in liquids with dissolved gases under various thermodynamic conditions. − In contrast to conventional techniques, DLS does not require calibration and is applied at the macroscopic thermodynamic equilibrium. Therefore, no macroscopic gradient, which may lead to a convective/advective mass transport besides the molecular one, must be applied.…”

Viscosity and Interfacial Tension of Binary Mixtures of n-Hexadecane with Dissolved Gases Using Surface Light Scattering and Equilibrium Molecular Dynamics Simulations

“…In combination with experimental determination, molecular dynamics simulations have been shown to be a valuable tool for the determination of multiple thermophysical properties. ,,− Here, the advantages lie in the applicability to thermodynamic states that are hardly accessible by experiments and in the insight into the fluid structure like the enrichment of solute molecules at the interface, which helps to establish structure–property relationships . Equilibrium molecular dynamics (EMD) simulations, which are used in this work, allow the determination of equilibrium and transport properties by analyzing the molecular motion under equilibrium conditions similar to those in SLS experiments.…”