SⅡ型R134a水合物稳定性的分子动力学模拟
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摘要
SⅡ型R134a制冷剂笼状水合物是理想的蓄冷介质之一。基于分子动力学(MD)方法,建立晶体结构模型,在NPT系综下运算,通过观察最终构象,分析计算体系中粒子的均方位移(MSD)和径向分布函数(RDF),从微观上揭示了小分子和温度对SⅡ型水合物晶体结构稳定性的影响。结果表明:小分子对小晶穴(512)占有率从0.25变化到1.00,占有率为0.25和0.50时,水合物晶体结构分解严重,体系中分子混乱,晶体结构难以稳定存在,占有率在0.75以上时,水合物晶体结构清晰有序,能够稳定存在;当体系温度由260K增大到290K时,水合物结构的稳定性逐渐降低,在280K时晶体结构出现扭曲变形,此时有部分水合物开始分解。
R134a refrigerant SⅡ clathrate hydrate is an ideal medium for accumulation of cold. Based on the molecular dynamics( MD) method,a crystal structure model was built and operated under the NPT ensemble. By observing the final conformation,the mean square displacement( MSD) and radial distribution function( RDF) of particles in the system were analysed,the affect of small molecules and temperature on the stability of the hydrate crystalline structure was revealed from the microscopic mechanism. Results indicate that the occupancy ratio of the small molecule to small crystal cavity( 512) varies from 0. 25 to 1. 00.When the occupancy ratio is 0. 25 and 0. 50,the hydrate crystal structure decomposes gravely and the molecules in the system move chaotically,so the crystal structure is unstable. If the occupancy ratio is above 0. 75,the hydrate crystal structure is so clear and orderly,the structure exists steadily. The hydrate structure stability reduces gradually when the temperature increases from 260 K to 290 K,the crystal structure at 280 K distorts and hydrate starts to decompose partially.
R134a refrigerant SⅡ clathrate hydrate is an ideal medium for accumulation of cold. Based on the molecular dynamics( MD) method,a crystal structure model was built and operated under the NPT ensemble. By observing the final conformation,the mean square displacement( MSD) and radial distribution function( RDF) of particles in the system were analysed,the affect of small molecules and temperature on the stability of the hydrate crystalline structure was revealed from the microscopic mechanism. Results indicate that the occupancy ratio of the small molecule to small crystal cavity( 512) varies from 0. 25 to 1. 00.When the occupancy ratio is 0. 25 and 0. 50,the hydrate crystal structure decomposes gravely and the molecules in the system move chaotically,so the crystal structure is unstable. If the occupancy ratio is above 0. 75,the hydrate crystal structure is so clear and orderly,the structure exists steadily. The hydrate structure stability reduces gradually when the temperature increases from 260 K to 290 K,the crystal structure at 280 K distorts and hydrate starts to decompose partially.
引文
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[4]Hiratsuka M,Ohmura R,Sum A K,et al.Vibrational spectra of deuterated methane and water molecules in structure I clathrate hydrate from,ab initio MD simulation[J].Molecular Simulation,2014,41(10-12):1-5.
[5]Torres T A,Kroon M C,Peters C J,et al.Inter-cage dynamics in structure I,II,and H fluoromethane hydrates as studied by NMR and molecular dynamics simulations.[J].Journal of Chemical Physics,2014,140(21):1-11.
[6]Erfan-Niya H,Modarress H,Zaminpayma E.Molecular dynamics study on the structure I clathrate-hydrate of methane+ethane mixture[J].Energy Conversion&Management,2011,52(1):523-531.
[7]Yuhara D,Barnes B C,Suh D,et al.Nucleation rate analysis of methane hydrate from molecular dynamics simulations.[J].Faraday Discussions,2015,179:463-474.
[8]Wilson D T,Barnes B C,Wu D T,et al.Molecular dynamics simulations of the formation of ethane clathrate hydrates[J].Fluid Phase Equilibria,2015,413:229-234.
[9]丁丽颖,耿春宇,赵月红,等.Ⅰ型甲烷水合物晶体稳定性的分子动力学模拟[J].计算机与应用化学,2007,24(5):569-574.
[10]祁影霞,汤成伟,刘道平,等.Ⅰ型甲烷水合物稳定性的分子动力学模拟[J].广州化工,2010,38(2):63-67.
[11]马鸿凯,孙志高,焦丽君,等.添加剂对静态条件下HCFC-141b水合物生成的促进作用[J].制冷学报,2016(1).
[12]陈美园,沈辉,舒碧芬.表面剂对HCFC141b的静态水合反应的作用[J].制冷学报,2009,30(1):14-18.
[13]李娜,郭云飞,覃小焕,等.HCFC141b乳液及微乳液体系水合物生成过程实验研究[J].西安交通大学学报,2010,44(2):119-124.
[14]Hashemi H,Babaee S,Mohammadi A H,et al.Experimental measurements and thermodynamic modeling of refrigerant hydrates dissociation conditions[J].Journal of Chemical Thermodynamics,2015,80(11):30-40.
[15]Ngema P T,Petticrew C,Naidoo P,et al.Experimental measurements and thermodynamic modeling of the dissociation conditions of clathrate hydrates for(refrigerant+nacl+water)systems[J].Journal of Chemical&Engineering Data,2014,59(2).
[16]Yi L,Liang D,Zhou X,et al.Molecular dynamics simulations for the growth of CH4-CO2mixed hydrate[J].Journal of Energy Chemistry,2014,23(6):747-754.
[17]Geng C Y,Wen H,Zhou H.Molecular simulation of the potential of methane reoccupation during the replacement of methane hydrate by CO2[J].Journal of Physical Chemistry A,2009,113(18):5463-9.
[18]Takeuchi F,Hiratsuka M,Ohmura R,et al.Water proton configurations in structures I,II,and H clathrate hydrate unit cells[J].Journal of Chemical Physics,2013,138(12):309-309.