醋酸体系汽液相平衡的分子模拟研究

The association between the gas phase molecules was caused by the strong hydrogen bonding interaction between acetic acid molecules, leading to the whole system seriously deviates from the ideal-gas behavior, which make the thermodynamic behavior of system extremely complex. At present, there is no one thermodynamic model which can accurately describe acetic acid and such strong association systems. In this project, the mechanism of action for hydrogen bond’s associating between acetic acid molecular was employed by investigating the micro configurations of acetic acid with molecular dynamics. And the Van der Waals parameters with association characteristics obtained by quantum chemistry calculation were used to build the new force field for acetic acid. Then, Gibbs ensemble Monte Carlo simulation was employed to study the vapor-liquid equilibrium for acetic acid. The project will not only help solve the practical engineering problems that the lack of related thermodynamic data in the process of chemical processing with acetic acid as raw materials, but also provide a new method for the establishment of accurate vapor-liquid equilibrium model for associate system.

醋酸分子间的强氢键作用可引起气相分子间的缔合，导致整个物系严重偏离理想行为，使得体系的热力学行为复杂。目前还未有描述醋酸这类强缔合体系准确可用的热力学模型。本项目通过分子动力学模拟对醋酸分子微观构型进行考察，探究醋酸分子间的氢键对缔合体形成的作用机制，借助于量子化学将其缔合特性引入力场中，选取适宜的势能函数对其进行描述，构建出适用于醋酸分子的特定力场，并依赖于吉布斯系综蒙特卡洛模拟将其用于含醋酸体系宏观热力学性质的研究中，获得此类物系的汽液相平衡数据。本项目不仅有助于解决以醋酸为原料的化学加工过程中相关热力学数据缺乏的实际工程问题，同时也能为研究缔合体系的汽液相平衡准确模型的建立提供一种新方法。

本项目采用分子动力学方法研究了醋酸在气相、液相及醋酸-丁烯醛体系中不同浓度下的缔合行为。通过径向分布函数考察了温度、压力及浓度对醋酸缔合比例的影响。研究结果表明，醋酸在气相中主要以二聚体的形式存在，液相中主要以链状片段的形式存在。以TraPPE-UA力场为基础，采用量子化学方法对醋酸的力场参数进行修正，并通过力场和输入文件来构建醋酸双分子模型。采用原始TraPPE-UA力场及修正后TraPPE-UA-DOUBLE力场参数模拟了醋酸纯组分及醋酸-乙烯，醋酸-乙醛和醋酸-丁烯醛二元体系汽液相平衡数据，并与实验数据进行比较。结果显示，醋酸纯组分的气相密度误差范围由1.1~13.5%降低到1.3~6.2%，液相密度误差范围由0.3~8.3%降低到0.1~5.9%；醋酸二元体系的计算结果有较高的模拟精度。以此为基础，采用获得的二元体系的TXY数据，拟合出NRTL、NRTL-HOC热力学模型的交互作用参数，并预测了醋酸-乙醛-丁烯醛三元体系的相平衡性质。本项目开发了一种能准确预测醋酸体系相平衡性质的新方法，并为以醋酸为原料进行下游产品的工艺设计提供基础数据。