Prediction of Phase Behaviors of Associating Fluids with Consideration of Hydrogen Bonding Characters

• Main Authors: Wei-Lin Chen, 陳威霖
• Other Authors: Shiang-Tai Lin
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/xq4vmf

博士 === 國立臺灣大學 === 化學工程學研究所 === 105 === The activity coefficient of a chemical in a mixture is important to understand the thermodynamic properties and the phase behaviors of the mixture. The COSMO-SAC model based on the result of quantum mechanical implicit solvation calculations has been shown to provide reliable predictions of activity coefficients for mixture fluids. However, it is found that the prediction accuracy is in general inferior for associating fluids. This work aims to improve the prediction of phase behaviors of associating fluids through consideration of hydrogen bond direction and the formation of local fluid structures. First existing COSMO-SAC methods for describing the hydrogen-bonding interaction consider the strength of interaction based only on the polarity of the screening charges, neglecting the fact that the formation of hydrogen bonds require specific orientation between the donor and acceptor pairs. We develop a new approach that takes into account the spatial orientational constraints in hydrogen bonds. Based on the Valence Shell Electron Pair Repulsion (VSEPR) theory, the molecular surfaces associated with the formation of hydrogen bonds are limited to those in the projection of the lone pair electrons of hydrogen bond acceptors, in additional to the polarity of the surface screening charges. Our results show that the directional hydrogen bond approach, denoted as the COSMO-SAC(DHB) model, requires fewer number of universal parameters and is significantly more accurate and reliable compared to previous models for a variety of properties, including vapor-liquid equilibria (VLE), infinite dilution activity coefficient (IDAC) and water-octanol partition coefficient (Kow). With the increasing strength of interactions between hydrogen bonded molecules, the formation of local fluid structure, such as the dimers and/or hydrogen bonded clusters, can strongly affect the phase behavior of fluids. For acetic acid, we identify four local fluids structures, including the acetic acid monomer, cyclic dimer and chain fragment and the cross-associating structure with water (or alcohol), and develop a novel approach to explicitly consider these local fluid structures in the predictive thermodynamic model, PR+COSMOSAC equation of state. The transition of acetic acid in these local fluid structures are considered via chemical reaction. The results show that the phase behaviors of pure acetic acid and its mixtures with various chemicals can be described only when all the significant local fluid structures are included. In addition to the efforts on the development of a better description for associating fluids, the utilization of different quantum calculation methods for COSMO solvation calculation is carefully examined for different COSMO-SAC models. The model parameters are reoptimized for each quantum calculation method, and the performance is evaluated using a large set of experimental databases covering VLE, liquid-liquid equilibrium (LLE), IDAC and Kow (containing more than 22,000 data points). The results show that the modification introduced in either COSMO-SAC 2010 or COSMO-SAC(DHB) is applicable for all quantum calculation methods and the performance of COSMO-SAC(DHB) model is generally better than others. Besides, the COSMO-SAC(DHB) model is sensitive to the quantum chemical method used. The use of a basis set that allows for higher molecular polarity, such as b3lyp/6-31+G(d,p), often results in a better prediction accuracy. The finding implies apart from the refinement in COSMO-based method itself, the improvement in the COSMO solvation calculation is also important for the development of COSMO-based model.