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神东煤惰质组结构特征及其与CH_4、CO_2和H_2O相互作用的分子模拟
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摘要
煤层气的能源效应使其吸附的研究成为热点之一,但是绝大多数研究都是采用传统的实验方法,很难从微观上认识煤对气体及水分的吸附机理。本文结合实验和计算机理论模拟两种方法,构建了神东煤惰质组(SI)大分子结构模型,系统研究了SI对CH4、CO2和H2O的吸附,以从分子水平上认识煤对气体及水分吸附的机理。
     本文的研究方法及其主要结论如下:
     1.基于核磁共振(13C NMR)分析所得煤结构参数构建了SI平面模型,通过化学位移计算软件进行修改,得到了实验谱图与计算谱图相一致的平面构型,并应用分子力学和分子动力学方法进行空间构型优化后,得出SI的密度是1.14 g/cm3,与实验数据也基本一致,证明了SI空间构型是合理的。
     2.应用蒙特卡洛(GCMC)方法模拟了SI对CH4、CO2和H2O单组分的饱和吸附,得出如下结论:
     1)在相同条件下,SI对单组分CH4、CO2和H2O的饱和吸附量大小是H2O>CO2>CH4。
     2)通过观察SI对单组分CH4、CO2和H2O的饱和吸附构型发现,吸附质分子都是优先吸附于SI大分子边缘,而后才吸附于孔隙内。吸附质分子局部集中吸附,如甲烷两两集中吸附,其空间排列与乙烷的空间构型类似;二氧化碳分子出现两两交叉的排列形式。
     3.应用分子动力学方法,在温度为298.15 K,压力为0.001 GPa的条件下,模拟了SI对单组分CH4、CO2和H2O的吸附,得到以下主要结论:
     1)在同温同压下,SI对单组分CH4、CO2和H2O的吸附量大小是:CH4     2)观察各单组分的饱和吸附构型可以发现,CH4在吸附剂中的吸附排列是呈现两两交叉,类似于乙烷的分子构型。CO2分子在SI中的吸附排列呈平行或交叉,甚至垂直的特点。H2O分子更多的呈现平行排列。相比较而言,CH4和H2O分子在SI中的吸附排列比较单一,CH4呈交叉排列,H2O分子呈平行排列,而CO2分子的排列比较杂乱,既有平行的、交叉的,又有垂直排列的。这与应用蒙特卡洛(GCMC)方法得到的结论是相一致的。
     4.用巨正则蒙特卡洛(GCMC)方法模拟了SI对CH4、CO2和H2O单组分的等温吸附,得出如下结论:
     1)相同温度压力下,SI对单组分CH4、CO2和H2O的模拟等温吸附量大小是:H2O>CO2>CH4。
     2)在相同压力条件下,温度与吸附量成负相关关系,高温不利于煤对单组分CO2、CH4和H2O的吸附。
     3)压力对单组分CO2、CH4和H2O吸附的影响有一个临界值,这一临界值大约为8 MPa。当压力小于8 MPa时,随着压力的增大,各单组分的吸附量跟着增加,当压力大于8 MPa时,随着压力的增大,各单组分的吸附量不再增加,而有减小的趋势。而且,在相同温度压力条件下,压力的变化对H2O吸附量的影响比CO2和CH4大。
     4)通过对等量吸附热的分析,得出SI对单组分CO2、CH4和H2O的吸附属于物理吸附。
     5.应用蒙特卡洛(GCMC)方法模拟了SI对二元混和体系CH4/CO2、CH4/H2O和CO2/H2O等温吸附,得出如下结论:
     1)通过吸附选择性系数的判断,得出吸附质中竞争吸附的优势大小是H2O>CO2>CH4。
     2)在相同温度压力条件下,吸附量的大小顺序是CH4CO2>CH4。
     3)压力的变化对竞争吸附中处于弱势的吸附质的吸附量影响不大,而对竞争中处于优势的吸附质的吸附量影响很大,呈正相关关系。
     6.应用蒙特卡洛(GCMC)方法模拟了SI对三元体系CH4/CO2/H2O的等温吸附,得出如下结论:
     1)在相同压力条件下,吸附量大小顺序是CH4     2)压力的变化对竞争吸附中处于弱势的吸附质的吸附量影响不大,即对CO2和CH4吸附量的影响不大,而对竞争中处于优势的吸附质的吸附量影响很大,呈正相关关系,这一结论与二元组分等温吸附所到的结论相一致。通过对三元组分CH4/CO2/H2O竞争吸附的研究,发现压力对竞争吸附中处于优势吸附质的吸附量影响有一个临界值,大约为6 MPa,当压力小于6 MPa时,随着压力的增大,吸附量增加的很快,当压力大于6 MPa时,随着压力的增大,吸附量的增加相对较慢。
The study of adsorption of coalbed methane by energy efficiency is a hot spots, but most experimental methods are traditional ideas, and they are difficult to recognize the adsorption mechanism of gases on coal from microscopic scale. In this paper, experimental methods were combined with models to build macromolecular structure model of Shendong inertinite (SI), and analyse the adsotption of the CH4,CO2,and H2O, then the adsorption mechanism in molecular level could be known. The following results were obtained:
     1.The spatial crossing macromolecular structure model of SI was obtained after the optimization of the plain model by Molecular Mechanics and Molecular Dynamic, which based on the structure parameters from the analysis of the experiment of 13C NMR. And the consistence of simulated IR spectrum and density (1.14 g/cm3) with the experimental results showed that SI was reasonable.
     2.The simulation of saturated adsorption of pure CH4, CO2, H2O on SI by Grand Canonical Monte Carlo (GCMC), the following results were obtained:
     1) In the same condition,the size order of saturated adsorption was: H2O> CO2> CH4.
     2) Through the simulation of saturated adsorption of pure CH4, CO2, H2O on SI, it is found that adsorbate molecules were first adsorbed on the edge of SI molecules, and after that in the pores.Adsorbate molecules were both sides of the molecular chain in the SI, and some of the local concentration of adsorbate molecules, such as the adsorption of methane, and the spatial arrangement was similar to the ethane; carbon dioxide molecule was arranged in Cross-form.
     3.Through the simulation of adsorption of pure CH4, CO2, H2O on SI applying the molecular dynamics when the tempreture is 298.15 K and the pressure is 0.001 GPa, the following results were obtained:
     1) The size order of simulated adsorption isotherm for pure CH4, CO2 and H2O in the same tempreture and fugacity was: CH4     2) From the single-component adsorption configuration, we can find that, similar to ethane, CH4 in the arrangement is in Cross-form,.and the CO2 molecules in the adsorption arranged was in parallel or cross, or even vertical, more H2O molecule was parallel. In comparison, CH4 and H2O molecule adsorption in relatively simple arrangement, while the CO2 molecules are more messy, which and the GCMC was consistent.
     4. The simulation of the adsorption isotherm of pure CH4, CO2, H2O by Grand Canonical Monte Carlo (GCMC) on SI, the following results were obtained:
     1) The size order of simulated adsorption isotherm for pure CH4, CO2 and H2O in the same tempreture and fugacity was: H2O> CO2> CH4.
     2) The tempreture and the adsorption was linear negative in the same fugacity, and it was difficult that the adsorption of CH4, CO2 and H2O on coal in the high-temperature region.
     3) A threshold about 8 MPa was appeared while the single-component CO2, CH4 and H2O adsorption when the pressure was less than 8 MPa, as the pressure increases, the single-component adsorption was increased, but when the pressure was greater than 8 MPa, with pressure increasing, the adsorption was decrease moreover, at the same temperature and pressure conditions, the impact on H2O adsorption by pressure was greater than CO2 and CH4 .
     4) Through the analysis of isosteric heat ,the adsorption of pure CH4, CO2 and H2O on SI was physical adsorption
     5.Through the simulation of the adsorption isotherm of binary CH4/CO2, CH4/H2O and CO2/ H2O on SI, the following results were obtained:
     1) The size order of advantage of sorbates in competitive adsorption was: H2O> CO2> CH4.
     2) In the same tempreture and fugacity ,The size order of advantage of sorbates in competitive adsorption was: CH4     3) The change of pressure had great impact on the adsoption of the advantage sorbate in the competitive adsorption,they were linear positive, and the adsoption of the disadvantage sorbate had not this Phenomenon.
     6.Through the simulation of the adsorption isotherm of Ternary CH4/CO2/H2O on SI, the following results were obtained:
     1) The size order of simulated adsorption was CH4     2) The change on the pressure had a weak effect on the disadvantage sorbate, such as CO2 and CH4 ,while the competition dominant adsorbate have a great influence on the adsorption, this conclusion and binary component adsorption isotherms are consistent. Through the simulation of the adsorption isotherm of ternary CH4/CO2/H2O,it was found that the pressure on the competition dominant adsorbate had threshold of about 6 MPa,When the pressure was less than 6 MPa. As the pressure increases,the adsorption capacity increases rapidly,while the pressure was more than 6 MPa,the result was the opposite .
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