Simulations of vapor-liquid

Smit B, Karaborni S and Siepmann J I 1995 Computer simulations of vapor-liquid phase equilibria of n-alkanesJ. Chem. Phys. 102 2126-40... 

Nath S K, Esoobedo F A and de Pablo J J 1998 On the simulation of vapor-liquid equilibria for alkanes J. Chem. Phys. 108 9905-11... 

Garzon B, Lago S, Vega C et al (1994) Computer simulation of vapor-liquid equilibria of linear quadrupolar fluids. Departures from the principle of corresponding states. J Chem Phys 101(5) 4166-4176... 

R. Yamamoto and K Nakanishi, Computer Simulation of Vapor-liquid Phase Separation in two- and three-dimensional Fluids Growth Law of Domain Size, Phys. Rev. B 49 (1994) 14958-14966 II. Domain Structure, Phys. Rev. B 51... 

S. T. Cui, P. T. Cummings, and H. D. Cochran, Fluid Phase Equilibria, 141, 45 (1997). Configurational Bias Gibbs Ensemble Monte Carlo Simulation of Vapor-Liquid Equilibria of Linear and Short-Branched Alkanes. 

Estimated Critical Properties from GEMC Simulations OF Vapor-Liquid Equilibria... 

Nath SK, de Pablo JJ (2000) Simulation of vapor-liquid equilibria for branched alkanes. Mol... 

Such an NVT Gibbs ensemble method has become a popular tool for investigating the pure component vapor-liquid equilibrium of simple molecules and poljuneric species. The technique has been used for simulating the vapor-liquid equilibria of simple gases such as nitrogen (160). The NPT variant of this technique has been widely used for the simulation of vapor-liquid and liquid-liqiud mixture equilibria. The applications of this technique can be found in studies of the vapor-liquid mixture eqiulibria of methane, ethane, and carbon dioxide (161) as well as of nitrogen and butane mixtures (160). 

Bukowski, R., Szalewicz, K. (2001). Complete ab initio three-body nonadditive potential in monte carlo simulations of vapor-liquid equilibria and pure phases of argon. Journal of Chemical Physics, 114, 9518. 

The structure formation in an ER fluid was simulated [99]. The characteristic parameter is the ratio of the Brownian force to the dipolar force. Over a wide range of this ratio there is rapid chain formation followed by aggregation of chains into thick columns with a body-centered tetragonal structure observed. Above a threshold of the intensity of an external ahgn-ing field, condensation of the particles happens [100]. This effect has also been studied for MR fluids [101]. The rheological behavior of ER fluids [102] depends on the structure formed chainlike, shear-string, or liquid. Coexistence in dipolar fluids in a field [103], for a Stockmayer fluid in an applied field [104], and the structure of soft-sphere dipolar fluids were investigated [105], and ferroelectric phases were found [106]. An island of vapor-liquid coexistence was found for dipolar hard spherocylinders [107]. It exists between a phase where the particles form chains of dipoles in a nose-to-tail... 

The most important aspect of the simulation is that the thermodynamic data of the chemicals be modeled correctly. It is necessary to decide what equation of state to use for the vapor phase (ideal gas, Redlich-Kwong-Soave, Peng-Robinson, etc.) and what model to use for liquid activity coefficients [ideal solutions, solubility parameters, Wilson equation, nonrandom two liquid (NRTL), UNIFAC, etc.]. See Sec. 4, Thermodynamics. It is necessary to consider mixtures of chemicals, and the interaction parameters must be predictable. The best case is to determine them from data, and the next-best case is to use correlations based on the molecular weight, structure, and normal boiling point. To validate the model, the computer results of vapor-liquid equilibria could be checked against experimental data to ensure their validity before the data are used in more complicated computer calculations. 

In Figure 1 we compare our numerical solutions with the molecular dynamics computer simulations of Thompson, et al. (7). In this comparison we use liquid and vapor densities obtained from the simulation studies. In the next section we obtain the required boundary values by approximate evaluation of vapor-liquid equilibrium for a small system. 

Thermodynamic properties (i.e., fugacities, entropies, and enthalpies) are required by this simulating program in the calculations of vapor/liquid phase equilibrium, compression/ expansion paths, and heat balances. Fugacities are required for the individual components of the existing vapor and liquid mixtures. Enthalpies and entropies are required for the vapor mixture or the liquid mixture. Also, mixture densities are required for both phases. 

The simulation of the liquid has given a vaporization energy underestimated by 7%, while pressure appears to be overcorrected with respect to MCY at 305 K (P =-1180 + 470atm). Experimental specific heat is well reproduced, as pair correlation functions, apart from the usual shift to larger distance of the first peak of the 0-0 g(r). The effective dipole moment is 2.8 D and diffusion coefficient (r> = 2.5 0.110" cmVs) compares well with the experimental data at 25 C, = 2.4-10" cm /s). Experimental is reproduced... 

Care is needed when modeling compressible gas flows, flows of vapor-liquid mixtures, slurry flows, and flows of non-Newtonian liquids. Some simulators use different pipe models for compressible flow. The prediction of pressure drop in multiphase flow is inexact at best and can be subject to very large errors if the extent of vaporization is unknown. In most of these cases, the simulation model should be replaced by a computational fluid dynamics (CFD) model of the important parts of the plant. 

In using simulation software, it is important to keep in mind that the quality of the results is highly dependent upon the quahty of the liquid-liquid equilibrium (LLE) model programmed into the simulation. In most cases, an experimentally vmidated model will be needed because UNIFAC and other estimation methods are not sufficiently accurate. It also is important to recognize, as mentioned in earlier discussions, that binary interaction parameters determined by regression of vapor-liquid equilibrium (VLE) data cannot be rehed upon to accurately model the LLE behavior for the same system. On the other hand, a set of binary interaction parameters that model LLE behavior properly often will provide a reasonable VLE fit for the same system—because pure-component vapor pressures often dominate the calculation of VLE. 

Stewart, E. Shilds, R.L. Taylor, R.S. Molecular dynamics simulations of the liquid/vapor interface of aqueous ethanol solutions as a function of concentration. J. Phys. Chem. B 2003,107 (10), 2333. 

The column is solved by computer simulation with vapor-liquid equilibrium calculations based on the van Laar liquid activity coefficient equation (Chapter 1). Initially the column is solved without a side draw to determine the composition profiles in the column. The column is solved at a reflux ratio of 1 and a bottoms rate of 800 kmol/h. The composition profiles with no side draw are shown in Figure 9.7. The trays are numbered from the top down, with the condenser as number one. A total of 20 trays are shown, which include the condenser and the reboiler. 

A straightforward, but tedious, route to obtain information of vapor-liquid and liquid-liquid coexistence lines for polymeric fluids is to perform multiple simulations in either the canonical or the isobaric-isothermal ensemble and to measure the chemical potential of all species. The simulation volumes or external pressures (and for multicomponent systems also the compositions) are then systematically changed to find the conditions that satisfy Gibbs phase coexistence rule. Since calculations of the chemical potentials are required, these techniques are often referred to as NVT- or NPT- methods. For the special case of polymeric fluids, these methods can be used very advantageously in combination with the incremental potential algorithm. Thus, phase equilibria can be obtained under conditions and for chain lengths where chemical potentials cannot be reliably obtained with unbiased or biased insertion methods, but can still be estimated using the incremental chemical potential ansatz [47-50]. 

In the example distillation system considered in Chapters 3 and 4, we studied the binary propane/isobutane separation in a single distillation column. This is a fairly ideal system from the standpoint of vapor-liquid equilibrium (VLE), and it has only two components, a single feed and two product streams. In this chapter, we will show that the steady-state simulation methods can be extended to multicomponent nonideal systems and to more complex column configurations. 

Schmit, C.E., Perkins, J., and Eldridge, R.B. (2004), Investigation of X-ray imaging of vapor-liquid contactors 2 - Experiments and simulations of flows in an air-water contactor, Chemical Engineering Science, 59 1267-1283. 

F. Y. Hansen, J. P. O Connell, and J. Abildskov, 2007c, State Conditions Transferability of Vapor-Liquid Equilibria via Fluctuation Solution Theory with Correlation Function Integrals from Molecular Dynamics Simulation, Fluid Phase Equilibria, 260,169. Reprinted with permission from Elsevier.)... 

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