Fouad Wael A, Vega Lourdes F
Gas Research Center and Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi 2533, United Arab Emirates.
Phys Chem Chem Phys. 2017 Mar 29;19(13):8977-8988. doi: 10.1039/c6cp08031f.
The use of hydrofluorocarbons (HFCs) as alternative non-ozone depleting refrigerants for chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) has grown during the last couple of decades. Owing to their considerable global warming potential, a global deal has been reached recently to limit the production and consumption of HFCs. For rational design of new refrigerants that are environmentally friendlier, the thermodynamics of current ones need to be well understood first. In this work, we examine the phase behavior of azeotropic refrigerants obtained by mixing HFCs with normal alkanes. The vapor-liquid equilibria (VLE) of these binary systems exhibit positive deviation from Raoult's law in the bulk, and a negative deviation from surface ideality (aneotrope) at the interface. The phase equilibria, second order thermodynamic derivative properties and interfacial properties of these complex systems were studied here using a modified version of the Statistical Associating Fluid Theory (SAFT) combined with Density Gradient Theory (DGT). The model was able to accurately capture the azeotropic nature of the phase equilibria and predict their composition and pressure at temperatures where experimental data are limited. In addition, accurate descriptions of the interfacial tensions were also obtained when compared with available experimental data, predicting the minimum found in surface tension as a function of composition. The molecular-based theory allowed the calculation of interfacial properties for which there is no experimental data available yet. Predictions show that the aneotrope occurs at a lower HFC composition for R-152a and R-134a systems in comparison to R-143a and R-125 systems. According to the calculated density profiles, HFC molecules appear to be preferentially adsorbed at the interface causing the surface tension of the n-alkane rich phase to decrease at low HFC concentrations. At high HFC concentrations, the phenomenon is inverted and n-alkane molecules are preferentially adsorbed causing the surface tension of the HFC rich phase to decrease. Consequently, the aneotrope point can be defined as the state at which the surface activity of both molecules is identical, or the relative adsorption of one component versus the other at the interface becomes zero.
在过去几十年中，氢氟烃（HFCs）作为氯氟烃（CFCs）和含氢氯氟烃（HCFCs）的非消耗臭氧层替代制冷剂的使用量不断增加。由于其巨大的全球变暖潜能值，最近达成了一项全球协议来限制HFCs的生产和消费。为了合理设计更环保的新型制冷剂，首先需要深入了解现有制冷剂的热力学性质。在这项工作中，我们研究了HFCs与正构烷烃混合得到的共沸制冷剂的相行为。这些二元体系的气液平衡（VLE）在本体中表现出对拉乌尔定律的正偏差，而在界面处表现出对表面理想性的负偏差（非均相共沸物）。本文使用统计缔合流体理论（SAFT）的改进版本结合密度梯度理论（DGT）研究了这些复杂体系的相平衡、二阶热力学导数性质和界面性质。该模型能够准确捕捉相平衡的共沸性质，并在实验数据有限的温度下预测其组成和压力。此外，与现有实验数据相比，还获得了对界面张力的准确描述，预测了表面张力随组成变化的最小值。基于分子的理论允许计算尚无实验数据的界面性质。预测结果表明，与R - 143a和R - 125体系相比，R - 152a和R - 134a体系的非均相共沸物出现在较低的HFC组成下。根据计算得到的密度分布，HFC分子似乎优先吸附在界面处，导致在低HFC浓度下富含正构烷烃相的表面张力降低。在高HFC浓度下，现象相反，正构烷烃分子优先吸附，导致富含HFC相的表面张力降低。因此，非均相共沸点可以定义为两种分子的表面活性相同的状态，或者一种组分相对于另一种组分在界面处的相对吸附变为零的状态。
