Heterogeneous structure and phase change analysis of different kinds of supercritical fluids

doi:10.11949/0438-1157.20201463

Abstract

Abstract:

In order to reveal the characteristics of different supercritical fluids (SCFs) from a micro perspective, molecular dynamics study on heterogeneous structure and phase change of supercritical argon and water under different pressures and temperatures were performed. The influence of temperature and pressure on the fluctuation of local density time series, physical clusters and the proportion of different density regions were analyzed. According to the simulation results, the physical parameters of different SCFs are strongly consistent. Firstly, the temperature corresponding to the maximum root mean square errors of local density time series always deviates from the pseudo-critical temperature. The “ridge” gradually weakens and even disappears with pressure increasing. At specified pressure, the radial distribution function (RDF) shows the characteristics of liquid-like transiting to gas-like with temperature increasing, where the liquid-like region is characterized by “short-range order and long range disorder”. While the gas-like region being “short and long range disorder”.And the number of physical clusters increases and the proportion of atoms in the largest cluster atoms decreases. With the pressure increasing, the opposite trend is shown at specified temperature. SCFs are continuous condensed medium in the liquid-like region, and a continuous network of molecules is broken by holes of different sizes. While the gas-like region seems to be vacuum-like clusters, and the system is filled with different sizes and isomer clusters. The number of physical clusters and the proportion of atoms in the largest clusters strongly depend on the system density. With the temperature increasing, the proportion of homogeneous region decreases first and then increases under different pressures. The overall uniformity gradually increases with the increase of pressure. Secondly, the start and termination temperature for two-phase region can be determined theoretically. Then, it can be found that phase transition enthalpy corresponding to liquid-like transiting to gas-like increases with the increase of pressure, and is a linear function of pressure. Finally, the effect of entropy on the order of SCFs is discussed according to the relationship between entropy and temperature. It is considered to be an important mechanism that entropy drives the supercritical phase transition. The results can provide theoretical support for engineering application of SCFs.

Key words: supercritical fluids, molecular simulation, heterogeneous structure, phase change

摘要：

为了从微观角度揭示不同种类超临界流体性质，对超临界流体氩和水进行分子动力学模拟。分析了温度和压力对不同种类超临界流体局部密度时序曲线波动，物理团簇及不同密度区占比的影响。模拟结果表明，两种超临界流体物理参数的变化具有较强的一致性。首先，密度时序曲线的均方根误差的最大值所对应的温度均偏离拟临界点，随着压力增大，近临界压力出现的“脊”也逐渐减弱或消失。定压工况，径向分布函数随温度的升高，峰值和谷值均减弱，呈现出从类液状态向类气状态过渡的规律，物理团簇的个数随着温度的升高逐渐增大，最大团簇原子数的占比随着温度升高逐渐减小。定温时，随着压力的增大，表现出相反的变化趋势，物理团簇和最大团簇的占比均与系统密度有较强的依赖关系。不同压力下，系统内平均密度区占比随温度的升高，呈现出先减小后增加的变化规律，且随压力的增大，整体均匀性增强。其次通过理论方法确定两种超临界流体两相区的起止温度，发现从类液状态向类气状态转变的相变焓随压力的增加逐渐增大，是压力的线性函数。最后根据系统熵和温度的关系，阐述熵对超临界流体有序性的影响，指出熵是驱动超临界流体相变的重要作用机制。

关键词: 超临界流体, 分子模拟, 异质结构, 相变

CLC Number:

O 469

WANG Yan, XU Jinliang, LI Wen. Heterogeneous structure and phase change analysis of different kinds of supercritical fluids[J]. CIESC Journal, 2021, 72(4): 1906-1919.

王艳, 徐进良, 李文. 不同种类超临界流体异质结构及相变分析[J]. 化工学报, 2021, 72(4): 1906-1919.

Figures/Tables 9

Fig.1 Physical model of simulation system (a); Simulation point on Pr-Tr phase diagram (b); FCC structure for SCAr and SPC/E structure for SCW(c); Variation of system temperature, potential energy, and pressure during relaxation and equilibrium stage (d)

Fig.2 Time evolution of specific heat capacity and the root mean square error of local density time series of SCAr and SCW at Pr=1.5, 2.5 and 3.5

Fig.3 The radial distribution functions of SCAr and SCW with different temperatures at Pr=1.5

Fig.4 The number of SCAr physical clusters varies with temperature (a); Proportion of SCAr atoms in physical cluster of the largest size under different temperature (b); The number of SCW physical clusters varies with temperature (c); Proportion of SCW molecules in physical cluster of the largest size under different temperature (d)

Fig.5 Coordination number

Fig.6 System density proportion of high density region (), average density region () and low density region (?) varies with temperature at Pr=1.5，2.5 and 3.5

Fig.7 The PMF distribution over the xy plane at Pr=1.5

Fig.8 The start (Ts) and end temperature (Te) of two-phase region are determined by theoretical method (Pr=1.5, SCAr) (a); Variation of enthalpy with various temperature and the determination of phase change enthalpy under different pressures for SCAr and SCW[(b),(c)]; The change of phase change enthalpy of different pressure for SCAr and SCW(d)

Fig.9 Entropy at various temperatures and pressures

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