Excess gibbs free energy of mixing

Experiment and model investigation of D-sorbitol as a thermodynamic hydrate inhibitor for methane and carbon dioxide hydrates

Abstract: Pipe plugging and cracking caused by gas hydrate blockage is the major problem in oil/gas flow assurance, especially in deep subsea pipelines. Thermodynamic inhibitors(THIs) are one of the effective methods to inhibit the formation of gas hydrate. This work evaluates the thermodynamic inhibition effect of D-sorbitol, a polyhydroxy compound, for both methane ( C H 4 ) and carbon dioxide ( C O 2 ) hydrates. The gas-liquid-hydrate equilibrium (GLHE) conditions for C H 4 and C O 2 were measured by the isothermal pressure search method in the presence of aqueous D-sorbitol solutions (1.00, 2.00, 3.00 mol%). The equilibrium pressure change ( Δ P ) and hydrate dissociation enthalpies ( Δ H d i s s ) were used to reflect the thermodynamic effects of D-sorbitol on C H 4 and C O 2 hydrates. The Δ P data show that D-sorbitol has an obvious inhibition impact on C H 4 and C O 2 hydrates formation. Compared with deionized water system, C H 4 hydrate GLHE pressure increased by 11.6, 25.0, 41.4%, while C O 2 hydrate GLHE pressure increased by 10.1, 18.6, 32.7% in 1.00, 2.00, 3.00 mol% D-sorbitol solutions, respectively. The Δ H d i s s data demonstrate that D-sorbitol does not participate in the formation of the hydrate crystal structure. Chen-Guo hydrate thermodynamic model and Wilson activity model were applied to predict the GLHE conditions of both C H 4 and C O 2 hydrates in the presence of D-sorbitol. The calculated results can well match the experimental values. The average relative deviations (ARD) between calculated and experimental data for C H 4 and C O 2 hydrate are less than 2.39% and 3.76%, respectively. D-sorbitol is a non-toxic and environment-friendly compound. The results in this work indicate that D-sorbitol can be applied to flow assurance strategies for hydrate blockage prevention.

Numerical modelling of free energy for methanol and water mixtures for biodiesel production

Abstract: As the demand for energy is ever increasing, biodiesel has become a major alternative to fossil fuels, primarily due to the latter’s depletion rate In methanol-water rectification, a vital part of the biodiesel production process, distillation is the preferred technique to recover methanol from water This study aims to investigate the excess Gibbs free energy (ΔG) behavior on methanol in a water mixture, for the binary system The study was conducted using the GROningen MAchine for Chemical Simulations (GROMACS©) The effect of several important parameters, namely, sub-atmospheric pressure, temperature and methanol concentration, were investigated on the excess Gibbs free energy of methanol in water The simulation results were partially validated, since the experimental data was limited The results demonstrate that the excess Gibbs free energy of methanol in a water mixture positively increases with an increase in methanol concentration, until it reaches a threshold concentration of 05 The positive excess Gibbs free energy of the mixture system indicates that the process is not spontaneous and requires additional energy to occur Increasing the temperature directly increases the excess Gibbs free energy of the system Increasing the pressure does not have a significant increase on the excess Gibbs free energy The simulation results are comparable with the experimental results; as validated by the Pearson correlation coefficient (ρ) = +09982 This study is beneficial to predict the thermodynamic behavior of methanol in a water system, in order to contribute to biodiesel production process optimization

Equation of State for Nonattracting Rigid Spheres

Abstract: A new equation of state for rigid spheres has been developed from an analysis of the reduced virial series. Comparisons with existing equations show that the new formula possesses superior ability to describe rigid‐sphere behavior.

Role of Repulsive Forces in Determining the Equilibrium Structure of Simple Liquids

Abstract: The different roles the attractive and repulsive forces play in forming the equilibrium structure of a Lennard‐Jones liquid are discussed. It is found that the effects of these forces are most easily separated by considering the structure factor (or equivalently, the Fourier transform of the pair‐correlation function) rather than the pair‐correlation function itself. At intermediate and large wave vectors, the repulsive forces dominate the quantitative behavior of the liquid structure factor. The attractions are manifested primarily in the small wave vector part of the structure factor; but this effect decreases as the density increases and is almost negligible at reduced densities higher than 0.65. These conclusions are established by considering the structure factor of a hypothetical reference system in which the intermolecular forces are entirely repulsive and identical to the repulsive forces in a Lennard‐Jones fluid. This reference system structure factor is calculated with the aid of a simple but accurate approximation described herein. The conclusions lead to a very simple prescription for calculating the radial distribution function of dense liquids which is more accurate than that obtained by any previously reported theory. The thermodynamic ramifications of the conclusions are presented in the form of calculations of the free energy, the internal energy (from the energy equation), and the pressure (from the virial equation). The implications of our conclusions to perturbation theories for liquids and to the interpretation of x‐ray scattering experiments are discussed.

Equilibrium Thermodynamic Properties of the Mixture of Hard Spheres

Abstract: An equation of state is proposed for the mixture of hard spheres based on an averaging process over the two results of the solution of the Percus–Yevick integral equation for the mixture of hard spheres. Compressibility and other equilibrium properties of the binary mixtures of hard spheres are calculated and they are compared with the related machine‐calculated (Monte Carlo and molecular dynamics) data. The comparison shows excellent agreement between the proposed equation of state and the machine‐calculated data.

Equilibrium Theory of Simple Liquids

Abstract: The perturbation theory of liquids developed recently by Weeks, Chandler, and Andersen (WCA) is examined in detail: Each assumption introduced by these authors is tested by comparison with "exact" computer results. It is shown that the basic assumptions of WCA are justified. An improved expression for the radial distribution function of the hard-sphere gas enables us to correct for some further inconsistent assumptions of the WCA theory. We then succeed in giving simple analytical expressions for the thermodynamic functions of the Lennard-Jones fluid shown to be quite good at high density. We show that the remainder of the perturbation series, which converges slowly at lower density, can be evaluated with the help of the Percus-Yevick equation. We therefore possess a unified theory of liquids which is especially simple at high density. Finally we reexpress the original WCA theory in an analytical form.

Perturbation theory for mixtures of simple liquids

Abstract: The perturbation approach developed by Weeks, Chandler, and Andersen (WCA) and by Verlet and Weis (VW) for pure systems is here generalized to the case of mixtures. We study binary mixtures of molecules interacting with the 12–6 Lennard-Jones potential, for which Monte Carlo simulations are available for comparison. The work is divided into two parts: The first part presents results of Monte Carlo calculations on mixtures of hard spheres of 864 and 1000 particles. The radial distribution functions generated are used to test the VW representation for the correlation functions of hard-sphere mixtures. This representation is found to work satisfactorily within the expected error limits. The second part deals with the two-step perturbation procedure for calculating the thermodynamic quantities of the Lennard-Jones system. The Lennard-Jones potential is divided into a reference potential, which is strictly repulsive, and an attractive part. The system of the reference potential is represented by a system of ha...