Development and Application of Simulation Methods for Clathrate Hydrates Phase Equilibria

碳酸盐水合物是一种非化学计量固体(通常是甲烷的水合物)，具有有趣的分子结构，其分子结构由包裹着小的非极性分子的水腔组成。准确和可靠的水合物相平衡预测需要水合物组成的知识，这一数量不容易获得，但鉴于天然气水合物在能源资源格局中日益重要，这是非常必要的。到目前为止，关于水合物相平衡的许多知识都来自van der Waals和Platteeuw基于固溶体理论的预测，尽管这种预测是实用的，但也有几个根本的缺陷。这一建议进行了一项基础性研究，从分子组成的角度研究笼状水合物的分子热力学。这个项目的具体目标是开发一种严格的计算方法来确定水合物腔中客体的组成和分布，然后将其应用于笼状水合物的三相(LW-H-V)平衡的预测。为了达到规定的目标，这项研究将开发和实施能够确定固相平衡的计算方法。为此，两项主要任务将构成研究的核心：i)开发和实现一种新的算法，用于计算具有螺旋边界条件的固相Gibbs系综蒙特卡罗，以严格捕捉固相平衡，最初应用于模型体系和水/冰，最终应用于笼状水合物；以及ii)进行GEMC和经典热力学积分(TI)模拟，同时努力确定笼状水合物中一个或多个客体的组成和分布，包括沿三相(LW-H-V)平衡的条件。虽然费时费力，但与热力学积分法的并行工作将验证和扩展该方法的适用性。所提出的工作具有创新性和变革性，因为：i)该方法克服了迄今为止禁止使用Gibbs系综蒙特卡罗进行直接固相模拟的严重采样效率低下的问题，以及ii)它将首次通过模拟或实验直接评估水合物笼中客体的绝对平衡组成和分布。如果成功，这项研究将极大地将实际模拟的范围扩大到许多涉及固相平衡的领域，并显著推进笼状水合物相平衡的基础科学，这些应用支撑着它们作为一种重要能源的日益重要的作用。这项研究将开发一个框架，以实现更准确和可靠的甲烷水合物相平衡预测，这在工业上是经常需要的，从而为如何解释水合物结构中的客体组成提供更好的知识。水合物相平衡在当前水合物技术感兴趣的所有领域中都发挥着核心作用。此外，PIs和研究生将参与由CSM的Barbara Moskal教授协调的外联活动。这项计划为从小学到高中的学生和教师提供支持，在课堂之外提供与PI的直接联系，以准备和提供课程和教学材料。目前的CSM GK-12项目服务于丹佛地区的三所公立学校系统，其中有大量少数族裔学生。在与Moskal教授的合作下，PI将开发专注于笼状水合物科学的材料，它们在能源技术中的潜在重要性，以及对分子模拟的介绍。PI还与CSM的水合物研究中心和NSF可再生能源MRSEC密切合作。笼状水合物是这些中心研究和教育活动的中心，这项工作的影响可能会通过利用PI和学生之间的相互互动和合作而扩展到中心的其他及时项目

0933856SumClathrate hydrates are non-stoichiometric solids (generally hydrates of methane) with intriguing molecular structures consisting of water cavities enclosing small non-polar molecules. Accurate and reliable predictions of hydrate phase equilibria require knowledge of the hydrate composition, a quantity that is not easily accessible, but is greatly needed given the increasing importance of gas hydrates in the energy resource landscape. To date, much of the knowledge on hydrate phase equilibria derives from predictions based on the solid-solution theory by van der Waals and Platteeuw, which albeit practical, has several fundamental shortcomings. This proposal undertakes a fundamental study to investigate the molecular thermodynamics of clathrate hydrates in terms of their composition. The specific objectives for this project are to develop a rigorous computational methodology to determine the composition and distribution of guests in hydrate cavities and to then apply it in the prediction of three-phase (Lw-H-V) equilibria of clathrate hydrates.Intellectual Merit. To achieve the stated objectives, this study will develop and implement computational methods capable of determining solid phase equilibria. To accomplish this, two main tasks will form the core of the study: i) Develop and implement a novel algorithm for solid-phase Gibbs ensemble Monte Carlo with helical boundary conditions to rigorously capture solid phase equilibria, initially applied to model systems and water/ice, for the eventual application to clathrate hydrates, and ii) Perform GEMC and classical thermodynamic integration (TI) simulations, in a parallel effort to determine the composition and distribution of one or more guests in clathrate hydrates, including at conditions along the three-phase (Lw-H-V) equilibrium. Although laborious, the parallel effort with the thermodynamic integration method will validate and extend the applicability of the method. The proposed work is novel and transformative in that: i) the methodology overcomes severe sampling inefficiency that heretofore prohibited direct solid phase simulations using Gibbs ensemble Monte Carlo, and ii) it will be the first to directly evaluate, either through simulation or experiment, the absolute equilibrium composition and distribution of guests in the hydrate cages. If successful, this study will greatly expand the scope of practical simulations to the many areas that involve solid phase equilibria, and significantly advance the underlying science of clathrate hydrates phase equilibria underpinning applications such as their increasing role as a prominent energy resource.Broader Impact. This study will develop a framework toward more accurate and reliable methane hydrate phase equilibria predictions, which are routinely needed in industry, and consequently provide better knowledge on how to account for the composition of guests in the hydrate structure. Hydrate phase equilibria plays a central role in all current areas of technological interest on hydrates. In addition, the PIs and graduate students will be involved in the outreach activities coordinated by Prof. Barbara Moskal at CSM. This program supports students and teachers from elementary to high school by providing direct contact with the PIs in an out of the classroom to prepare and deliver lessons and instruction materials. The current CSM GK-12 program serves three public schools systems in the Denver area with large representations of minority students. In collaboration with Prof. Moskal, the PIs will develop materials focused on the science of clathrate hydrates, their potential importance in energy technologies, as well as an introduction to molecular simulation. The PIs are also closely involved with the Center for Hydrate Research and the NSF Renewable Energy MRSEC at CSM. Clathrate hydrates is central in the research and educational activities of these centers, and the impact of this work will potentially extend to other timely projects in the centers by leveraging mutual interaction and collaboration among PIs and students