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首页> 外文期刊>Fluid Phase Equilibria >Evaluating perturbation contributions in SAFT models by comparing to molecular simulation of n-alkanes
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Evaluating perturbation contributions in SAFT models by comparing to molecular simulation of n-alkanes

机译：通过与正构烷烃的分子模拟进行比较来评估SAFT模型中的扰动贡献

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SAFT models are generally written as a perturbation series of the Helmholtz energy with reciprocal temperature as the argument. The perturbation coefficients are then functions of density and molecular size. The variation of the perturbation coefficients with molecular size is given primarily by Wertheim's theory [6-9], but there may be additional variations as in the PC-SAFT model. In the present work, we compare the characterization of perturbation coefficients inferred from PC-SAFT to those derived from molecular simulations.The molecular simulations are based on Discontinuous Molecular Dynamics (DMD) and second order Thermodynamic Perturbation Theory (TPT). DMD simulation is applied to the repulsive part of the potential model with molecular details like fused hard spheres for the interaction sites and 110° bond angles. The thermodynamic effects of disperse attractions are treated by rigorous application of TPT. The present work re-examines the related work of Elliott and Gray [35] in the low density and critical regions, focusing on n-alkanes with carbon numbers ranging from 3 to 80.We find that SAFT theory overestimates the repulsive contribution (A_0) and underestimates the first order contribution (A_1) of Helmholtz energy relative to simulation. Nevertheless, the correlations are qualitatively reasonable. Significant inconsistencies arise when considering the second order contribution (A_2). For example, the PC-SAFT characterization of A_2 becomes larger than A_1 in the low density, long chain limit, raising concerns about the convergence of the series. Furthermore, fluctuations are underestimated in the critical region and overestimated in the liquid region. In each case, we can suggest improved characterizations. Altogether, these results suggest ways to modify the SAFT formalism to achieve greater consistency between atomistic and coarse-grained models.
机译：SAFT模型通常写为亥姆霍兹能量的摄动级数，并以倒数为温度。然后，摄动系数是密度和分子大小的函数。微扰系数随分子大小的变化主要是由Wertheim理论[6-9]给出的，但可能与PC-SAFT模型中的变化一样。在本文中，我们将PC-SAFT推导的扰动系数的特征与分子模拟推导的特征进行了比较。分子模拟基于不连续分子动力学（DMD）和二阶热力学扰动理论（TPT）。 DMD模拟应用于势模型的排斥部分，其中包含分子详细信息，例如用于相互作用部位和110°键角的熔融硬球。严格应用TPT处理分散吸引力的热力学效应。目前的工作重新审视了Elliott和Gray [35]在低密度和临界区的相关工作，重点是碳数为3至80的正构烷烃。我们发现SAFT理论高估了排斥作用（A_0）并低估了亥姆霍兹能量相对于模拟的一阶贡献（A_1）。然而，相关性在质量上是合理的。考虑二阶贡献（A_2）时会出现明显的不一致。例如，在低密度，长链极限下，A_2的PC-SAFT表征变得大于A_1，这引发了对系列收敛性的担忧。此外，在临界区域中波动被低估，而在液体区域中波动被高估。在每种情况下，我们都可以建议改进特性。总而言之，这些结果提出了修改SAFT形式主义以在原子模型和粗粒度模型之间实现更大一致性的方法。

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