甲烷在黏土矿物狭缝孔中吸附的分子模拟研究

Molecular simulation study on the adsorption behaviors of methane in slit-like clay mineral pore

利用巨正则蒙特卡罗模拟方法和分子动力学方法研究甲烷分子在4类黏土矿物(蒙脱石、高岭石、伊利石和绿泥石)中赋存微观结构和微观吸附机理,并研究不同孔径和不同压力对甲烷在4类黏土矿物中吸附行为的影响。研究结果表明:甲烷的平均等量吸附热随着孔径增大而下降,且小于42 k J/mol,证明甲烷在黏土矿物中的吸附属于物理吸附;甲烷分子受到黏土矿物孔壁面势能作用影响,在孔壁面附近区域聚集从而形成吸附层,其为吸附相,而远离孔壁区域,受到孔壁面势能较弱或未受到孔壁面势能作用影响,甲烷分子分散于孔中,其为游离相;甲烷分子在不同类型黏土矿物不同尺度的孔隙中赋存状态存在差异;黏土矿物微孔中,甲烷吸附量随着孔径增大而增大,而中孔中,甲烷吸附量随着孔径增大而减小;从微观角度来看相同孔径中,不同类型黏土矿物对甲烷近似有相同的吸附能力,但是宏观角度来看不同类型黏土矿物样品对甲烷的吸附能力差异较大,说明不同类型黏土矿物样品对甲烷吸附能力主要通过比表面积因素来影响;甲烷分子在孔中吸附气量所占比例随着压力增大或孔径增大而呈下降趋势。

In this article, the adsorption mechanism and microstructures of the methane in slit-like clay mineral pore （montmorillonite,illite, chlorite and kaolinte） have been investigated by using Grand Canonical Monte Carlo simula- tions and molecular dynamics simulation, and the influence of different pore sizes, different pressures on the methane adsorption on clay minerals have been discussed. The results show that the isosteric heat of methane decreased with the increase of pore size,which was less than 42 kJ/mol, suggesting that the adsorption of methane on clay minerals was physical adsorption. The methane molecules affected by the potential energy of pore walls gathered near pore walls to form adsorption phase. However, the area far away from hole walls were not affected by the potential energy of pore walls and methane molecules as free phase were dispersed in pores. The adsorption capacity of methane increased with the increase of pore size in micropore, whereas the adsorption capacity of methane decreased with the increase of pore size in mesopore. The proportion of the adsorbed gas amount of methane molecules in pores decreased as pressure or pore size increases. From the microcosmic point of view, the methane adsorption capacity on clay minerals was approxi-mately same in the same pore size, whereas from the macroscopic point of view, the methane adsorption capacity on clay minerals had a bigger difference, illustrating that the methane adsorption capacity on the different types of clay minerals was mainly affected by the factor of specific surface area. The proportion of the adsorbed gas amount of meth- ane molecules in pores showed a decreasing trend as pressure or pore size increases.