Prediction of the liquid–vapor equilibrium pressure using the quasi‐Gaussian entropy theory

Abstract The molecular dynamics simulations of the liquid-vapor equilibrium of water including both water phases — liquid and vapor — in one simulation are presented. Such approach is preferred if equilibrium curve data are to be collected instead of the two distinct simulations for each phase separately. Then the liquid phase is not restricted, e.g. by insufficient volume resulting in too high pressures, and can spread into its natural volume ruled by chosen force field and by the contact with vapor phase as vaporized molecules are colliding with phase interface. Averaged strongly fluctuating virial pressure values gave untrustworthy or even unreal results, so need for an alternative method arisen. The idea was inspired with the presence of vapor phase and by previous experiences in gaseous phase simulations with small fluctuations of pressure, almost matching the ideal gas value. In presented simulations, the first idea how to calculate pressure only from the vapor phase part of simulation box were applied. This resulted into very simple method based only on averaging molecules count in the vapor phase subspace of known volume. Such simple approach provided more reliable pressure estimation than statistical output of the simulation program. Contrary, also drawbacks are present in longer initial thermostatization time or more laborious estimation of the vaporization heat. What more, such heat of vaporization suffers with border effect inaccuracy slowly decreasing with the thickness of liquid phase. For more efficient and more accurate vaporization heat estimation the two distinct simulations for each phase separately should be preferred.

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Liquid-vapor equilibrium in the system helium-methane

AIChE Journal ◽

10.1002/aic.690130335 ◽

1967 ◽

Vol 13 (3) ◽

pp. 593-599 ◽

Cited By ~ 21

Author(s):

C. K. Heck ◽

M. J. Hiza

Keyword(s):

Vapor Equilibrium ◽

Liquid Vapor

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EQUATIONS OF STATE FOR FLUIDS: THE LIQUID-VAPOR EQUILIBRIUM (LVE)

International Journal of Modern Physics B ◽

10.1142/s021797920804942x ◽

2008 ◽

Vol 22 (30) ◽

pp. 5335-5347 ◽

Cited By ~ 3

Author(s):

JIANXIANG TIAN ◽

YUANXING GUI

Keyword(s):

Experimental Data ◽

Equations Of State ◽

Packing Fraction ◽

Optimal Choice ◽

Critical Conditions ◽

Mechanics Model ◽

Vapor Equilibrium ◽

Pure Substances ◽

Compressibility Factors ◽

Liquid Vapor

Historically, the development of equations of state for fluids has almost invariably followed the lead of the van der Waals (vdW) equation which includes an attraction term and a repulsion term. In this paper, using a simple statistical mechanics model, we introduce a parameter σ as both the power and a coefficient of the packing fraction y which locates at the numerator of the vdW attraction term. Then nine equations of state are constructed to solve the critical conditions and the main thermodynamic properties of pure substances at liquid-vapor equilibrium. As a result, the correct critical compressibility factors of Nitrogen, Argon, Carbon dioxide, Ethene, Methane, Oxygen, Propene, Water and Hydrogen, are obtained with an optimal choice of parameter σ. Good predictions of these equations to the liquid-vapor equilibrium properties below critical temperature are reported and compared with experimental data.

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