【作者】 张新波；

【导师】 刘庆林；

【作者基本信息】 厦门大学 ， 化学工程， 2007， 硕士

【摘要】 选择固定点电荷的柔性模型和合适的势能函数，优化得到了水分子势能函数的参数。与其它模型相比较，本模型增加了氢原子的L-J作用和考虑了水的键长和键角变化，得到的结果比较理想。然后采用NVT-GEMC和CBMC方法在很宽的温度范围内计算了水的气-液相平衡、饱和压力、第二维里系数、结构特征、自扩散系数和在超临界区域的压力，并分别与文献计算的数据进行了广泛的比较，说明本模型在预测水的各种性质时比较准确。本模型对水的饱和液相密度计算的准确性有较大提高，并且得到了合理的第二维里系数和水的径向分布函数。计算得到的水的临界性质比现有的柔性模型都有很大的改进。在计算超临界条件下水的压力-密度时，不论压力和温度多高，本模型计算结果都很准确，体现了其优异性；而在计算自扩散系数时，当压力不太高时计算的结果也比较理想。最后还讨论了水分子在气态、液态和超临界条件下扩散系数不同的原因。本模型可以运用到化学工程的分离方面。采用全柔性的二氧化碳分子模型计算得到二氧化碳的相图及其它热力学性质。该模型和已经存在的模型有较大的区别，主要是本模型考虑了原子电荷的影响，键长和键角改变的作用。在选择了适合二氧化碳体系的势能函数及优化获得参数后，采用NVT-GEMC计算了二氧化碳的相图，计算得到的饱和液体和气体的密度很理想，饱和压力的计算也有较大的改进，预测到的临界性质基本与实验数据一致。并且采用NVT-MD模拟了二氧化碳分子在超临界条件下的结构特征，得到了二氧化碳分子在超临界下的非线型结构，即T-二聚体、凹型二聚体和平行二聚体，解释了二氧化碳分子在超临界状态下具有偶极距的原因。最后还计算了二氧化碳在恒温条件下不同压力时的自扩散系数，得到在压力13 MPa左右时计算的结果很准确，但随着压力的升高误差也随之增加。最后讨论了二氧化碳、甲烷、氦、丙烷等分子的扩散行为，从分子角度解释了相同温度下气态时扩散快，液体时扩散慢的原因。这些性质对化工分离和传质很有用途，可以为化工设计提供一定的依据。采用NPT-GEMC法计算了CO₂／H₂O体系的气-液相平衡热力学性质和二氧化碳富集相处于超临界状态的高压下的相平衡性质。在恒温条件下，得到了从较低压力到高压时的热力学性质。这为设计CO₂／H₂O体系的萃取剂提供了数据，并且为更高压力下研究该体系打下了基础。更多还原

【Abstract】 An improved fully flexible fixed-point charges model for water has been developed to predict the vapor-liquid coexistence properties using the NVT-Gibbs ensemble Monte Carlo technique （GEMC） and the pressure in supercritical region. The average deviation between our simulation and experimental data for saturated liquid densities is 2.75% over temperature range of 314 to 609K. Comparing with experimental data for T_c, P_c andρ_c （P_c=220.064bar, T_c=647.096K, andρ_c=0.322g/cm³ for the experimental data）, our calculated results （P_c=213bar, T_c=644.3K, andρ_c=0.325g/cm³ for our simulations） are acceptable and are better than those by the SPC-E and TIP4P models. The saturated pressure is calculated by evaluating the pressure of vapor from NPT-MD simulation at the coexistence vapor densities at the nominal temperature. The agreement of our simulated pressures of supercritical water at any density and temperature with the experimental values is excellent. The second virial coefficient and radial distribution function in ambient and supercritical conditions are also estimated. The radial distributions consist with experimental data very well.A folly flexible alterable -point charges model for carbon dioxide has been developed to predict the vapor-liquid coexistence properties using the NVT-Gibbs ensemble Monte Carlo technique （GEMC）. The average deviation between our simulation and the literature experimental data for saturated liquid densities is 0.3%. Comparing with the experimental data for T_c, P_c andρ_c （P_c=7.3773MPa, T_c=304.13K, andρ_c=0.4676g/cm³ for the experimental data）, our calculated results （P_c=7.39MPa, T_c=304.365 K, andρ_c=0.46673g/cm3 for our simulations） are good and are better than those by the EPM2 model. The agreement of our simulated saturated pressure and the pressures of supercritical water at any density and temperature with the experimental values are excellent. The radial distribution function in supercritical conditions is also estimated, which give that the carbon dioxide is a nonline molecule with the C=O bond length to be 1.17 A and the O=C=O bond angle to be 176.4°.The radial distributions consist with Car-Parrinello molecular-dynamics（CPMD） very well, but the EPM2 model shows large deviation.Finally, in this thesis phase coexistence properties of H₂O/CO₂ systems of interest for chemical industries were investigated using the NPT-Gibbs ensemble Monte Carlo （MC） simulations. Simulation results deviated from experimental data for the pressure-composition diagrams of the binary systems. And vapor density was compared to the density of pure CO₂.更多还原