受限流体分子热力学及其状态方程

Confinement effect, which is caused by the strong interaction between the solid wall of the pores and the fluid molecules, is seen in many physical processes where fluids are confined in a porous medium. It introduces significant differences of the physical properties and phase equilibrium of the confined fluids from the bulk phase, and hence exhibits huge effects on many chemical engineering processes. Therefore, suitable thermodynamic models for confinement effect are urgently needed to conduct the data association and theoretical prediction with the aim of the detection of the fluid mechanism, the reproduction of experimental data and the theoretical supports for the practical applications. In addition, the thermodynamic model is scientifically meaningful and practically valuable in understanding the interfacial and capillary phenomenon, heterogeneous transfer, absorption behavior and so on. In this project, the confinement effect of fluids is investigated. Based on the equation of state of square-well chain fluid with variable range (SWCF-VR), modern molecular thermodynamic methods incorporating with the equation of Young-Laplace and the theory of adsorption film are applied to establish molecular thermodynamic model in order to describe the physical properties and phase equilibrium of confined fluids. Further, confined-Helmholtz function is introduced by employing the scaled particle theory to establish the confined-SWCF-VR. The qualitative and quantitative rules between the properties of the confined fluids and the model parameters are set up by the combination of theoretical guidance and experiment data verification. Finally, the established thermodynamic model could be used in the oil and gas industry, carbon dioxide sequestration, adsorption process, capillary condensation and so on.

当流体处在受限空间时，孔壁与流体分子之间的相互作用会引起流体的受限效应，该效应的存在使得流体的性质与体相性质之间存在较大差异，对化工中的许多过程产生巨大影响。因此，需要有合适的热力学模型进行数据关联和理论预测，这不仅可洞察流体的作用机制、再现实验现象、为实际工业应用提供理论支撑，而且对理解界面与毛细现象、多相传递和吸附过程等均具有重要的科学意义和实际参考价值。本项目将针对流体的受限效应，基于变阱宽方阱链流体状态方程，结合Young-Laplace 方程和吸附膜理论，采用现代分子热力学研究方法构建能准确描述受限流体状态和相平衡的热力学模型。之后，基于定标粒子理论，引入受限剩余亥氏函数贡献项，建立受限-变阱宽方阱链流体状态方程。通过理论构建与实验数据验证的结合，建立流体分子结构与模型参数的定性定量规律，以期发挥分子热力学模型在油气工业、二氧化碳封存、吸附和毛细管冷凝等过程中的重要作用。

受限效应会在一定程度上改变流体的物性，如流体的临界性质、密度、表面张力、流体粘度和饱和蒸气压等，因此，若仍然从体相角度去研究受限流体，必然导致计算结果与实验值有较大偏差，甚至无法重现受限流体的物性。因此，对受限流体热力学性质的研究具有重要意义。随着研究的深入，化工过程“卡脖子”问题往往涉及受限流体，因此，需要有合适的热力学模型进行数据关联和理论预测，这不仅可洞察流体的作用机制、再现实验现象、为实际工业应用提供理论支撑，而且对理解界面与毛细现象、多相传递和吸附过程等均具有重要的科学意义。本项目针对流体受限效应，基于变阱宽方阱链流体状态方程，（1）建立了SWCF-VR状态方程+Young-Laplace方程联合模型。结果表明，随着压力的降低，受限效应越来越明显。以甲烷为例，当液相压力低于2.5MPa时，受限引起的毛细管压力已经超过了液相压力。（2）基于联合模型的纯物质受限效应计算，随着孔径的减小，受限流体液相饱和压力迅速降低，毛细管压力迅速增加，受限效应明显增强。（3）基于联合模型的混合物受限效应计算，提出固定接触角的概念，计算误差不超过0.2%，得到了非常满意的结果。（4）联合模型用于计算受限流体气液相平衡，采用固定接触角来反映壁面的影响，计算值的精度远远高于PR方程，同时简化了计算过程和收敛难度，可准确描述二氧化碳在MCM-41材料（孔径2nm）中的受限相平衡实验数据。（5）结合吸附膜理论，提出改进联合模型。结果表明，当孔径低于10纳米时，甲烷-戊烷受限混合物的受限效应非常明显，其对相平衡的影响是决定性的。压力对毛细管压力的影响比温度更明显，两种组分之间的差异越大，纳米孔半径对气液相平衡常数的影响越大。（6）结合定标粒子理论，构建受限-变阱宽方阱链流体状态方程，引入了受限剩余亥氏函数贡献项和受限压缩因子贡献项，建立了受限-变阱宽方阱链流体状态方程，该模型能直接描述流体在受限条件下的性质和状态，但准确度有待进一步提高。