Development of High-Quality Molecular and Engineering Models for Hydrogen Fluoride and its Mixtures

摘要CTS-9720705 David A Kofke/SUNY氟化氢(HF)在许多行业中起着关键作用，但由于其剧毒和腐蚀性，其实验研究受到了极大的阻碍。因此，几乎没有已发表的实验数据可以作为设计使用HF工艺的基础。这项工作的目的是提高我们对HF分子水平行为的理解，从而开发出工程模型，从现有的少数实验数据中可靠地预测HF及其混合物的性质。该方法综合了计算化学、分子模拟和最先进的工程建模。特别值得关注的热力学性质包括汽液相平衡、热效应和表面张力。块状氟化氢是一种复杂的物质，实际上被描述为“迄今为止研究过的最不完美的气体”。HF非理想性的起源在于它具有很强的缔合性，形成了HF分子链。链的形成表现为异常大的汽相热容、不寻常的蒸气压力曲线和非常小的表面张力。实验没有提供关于这些链的性质(即链长度的分布，以及它们形成环和分支结构的程度)的确凿信息，这种无知阻碍了可以在广泛条件下预测HF行为的工程模型的形成。尽管整体氢氟酸是一种复杂的物质，但氢氟酸分子本身是一种非常简单的物质(不像聚合物分子，聚合物分子也形成复杂的物质)，并且通过复杂的理论手段对其进行了非常深入的研究。这些研究几乎是我们对HF缔合的所有详细了解的来源；然而，这一基本进展尚未应用于工程模型的制定，而且，结果主要与汽相有关，因此它们不能为理解汽液平衡或表面张力提供合适的基础。我们的初步研究发现，现有的分子模型中没有一个是适当健壮的，足以保证它们作为构建工程模型的基础。利用分子的简单性来系统地开发复杂体相的准确的分子和工程模型是这项工作的中心组成部分。这一努力是及时的，因为几个最新的发展汇聚在一起：对HF分子的良好理解的形成；计算硬件的极先进状态；帮助工程模型开发的复杂分子模拟算法的开发；以及用于缔合流体的适当的、理论上合理的工程方法的形成，这些方法可以用来构建HF模型。氢氟烃是生产氢氟烃的关键成分，氢氟烃被视为环境友好的发泡剂、溶剂、制冷剂，以及氟聚合物制造中的成分；根据国际协议，整个行业现在都在从消耗臭氧的氯氟烃转向氢氟碳化物技术。氢氟化氢对钢铁、石油、玻璃、电子、铝和能源工业也很重要，也是不断增长的氟化学领域的核心。安全是HF加工中的一个主要问题，因为生产设施的意外泄漏可能会产生灾难性的后果。尤其麻烦的是，高频云团在长途旅行时能够持续存在，而不会被分散、稀释，从而变得无害。这种持久性与氢氟酸异常低的表面张力以及形成小液滴或气溶胶的能力有关。提高对HF表面张力的了解将有助于制定促进危险的HF云扩散的添加剂。在这项研究中，计算化学、分子模拟和工程建模的完整定量综合以前从未完成过。HF是这一尝试的合乎逻辑的起点，因为它在整体规模上是复杂的，但在分子水平上相对简单。这一努力的成功将指导其他复杂材料的第一性原理模型的开发。

ABSTRACT CTS-9720705 David A Kofke /SUNY Hydrogen fluoride (HF) plays a key role in many industries, but the study of HF via experiment is greatly hampered by its highly toxic and corrosive nature. Consequently there exists very little published experimental data on which to base the design of processes that use HF. The objective of this work is to improve our understanding of the molecular-level behavior of HF, and thereby develop engineering models that reliably predict the properties of HF and its mixtures from the few experimental data that are available. The approach synthesizes computational chemistry, molecular simulation, and state-of-the-art engineering modeling. Thermodynamic properties of particular interest include vapor-liquid phase equilibria, heat effects, and surface tension. Bulk hydrogen fluoride is a complex substance, and in fact has been described as "the most imperfect gas so far studied". The origin of HF nonideality is in its very strong nature to associate, forming chains of HF molecules. Chain formation manifests itself in (among other things) anomalously large vapor-phase heat capacity, an unusual vapor-pressure curve, and very small surface tension. Experiment has not provided conclusive information about the nature of these chains (i.e., the distribution of chain lengths, and the degree to which they form rings and branched structures), and this ignorance has hampered the formulation of engineering models that can predict HF behavior over a wide range of conditions. Although bulk HF is a complicated substance, the HF molecule itself is a very simple one (unlike, say, polymer molecules, which also form complex substances), and it has been studied very intensively by sophisticated theoretical means. These studies are the source of almost all of our detailed understanding of HF association; however this fundamental progress has not yet been applied in the formulation of engineering models and, moreover, the results pertain mostly to the vapor phase , so they do not provide a suitable basis for understanding vapor-liquid equilibria or surface tension. Our preliminary studies have found none of the existing molecular models to be suitably robust to warrant their use as a basis for constructing engineering models. Exploiting the molecular simplicity to systematically develop accurate molecular and engineering models of the complex bulk phase is the central component of this work. This effort is timely because of the convergence of several recent developments: the formulation of a good understanding of the HF molecule; the greatly advanced state of computation hardware; the development of sophisticated molecular simulation algorithms to aid the engineering-model development; and the formulation of appropriate, theoretically sound engineering methods for associating fluids, methods with which an HF model may be constructed. HF is a key ingredient in the production of hydrofluorocarbons (HFCs), which see application as environmentally benign blowing agents, solvents, refrigerants, and as ingredients in fluoropolymer manufacture; by international agreement entire industries are now switching from ozone-depleting chlorofluorocarbon to HFC technologies. HF is also important to the steel, petroleum, glass, electronic, aluminum and energy industries, and it is central to the continually growing field of fluorine chemistry. Safety is a major concern in HF processing, as accidental releases from production facilities can have disastrous consequences. Particularly troublesome is the ability of HF clouds to persist while traveling over long distances, without being dispersed, diluted, and thereby rendered harmless. This persistence is related to the anomalously low surface tension of HF, and its ability to form small droplets, or aerosols. Improved understanding of HF surface tension would enable the formulation of additives that promote dispersion of dangerous HF clouds. The complete quantitative synthesis of computational chemistry, molecular simulation, and engineering modeling has never before been completed to the extent in this study. HF is a logical starting point for this attempt because it is complex at the bulk scale but relatively simple at the molecular level. Success in this effort would guide the development of first-principles models for other complex materials.