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题名:
重质油体系的介观模拟研究
作者: 张胜飞
学位类别: 博士
答辩日期: 2012-05-04
授予单位: 中国科学院研究生院
导师: 温浩
关键词: 重质油 ; 耗散粒子动力学 ; 稳定性 ; 聚集动力学 ; 剪切流变性
其他题名: Meso-scale study on heavy crude oil system
学位专业: 应用化学
中文摘要: 本文改进标准DPD方法,引入刚性结构片段来表征稠合芳香环系,选择合理模拟参数和模型化合物,从介观尺度上研究了以沥青质为核心的重质油胶体复杂体系。主要探索了重质油的稳定性、扩散性能、剪切流变性等。建立了从微观聚集结构到体系的宏观性质研究的桥梁,主要的研究内容和结果如下: (1) 开发了适用于非保守力场的刚体定点自由转动算法,用于支持含稠合芳香环系化合物的微观运动模拟研究。从能量收敛、动量守恒、刚性维持等多方面的表现出发,选择了性能更优的四元组方法。将该方法置入标准DPD方法中,构建了重质油体系的介观模拟平台。针对微观结构研究的需要,提出了以3个CH2为一个DPD粒子的粗粒化标准,由此得到了DPD研究中的时间、空间尺度等参数与实际物理量的换算关系,用于模拟结果的解释。以Blends混合能计算为基础,提出了DPD粒子间的作用力参数,即介观力场。提出了选择重质油各组分的平均模型分子的标准。针对广泛存在的关于沥青质分子结构争议,构建了大陆型和海岛型结构,发现后者为主的沥青质分子更容易形成有序聚集结构,且能以分子间和分子内两种方式实现,在某些场合能更好的与实验观察吻合。 (2) 由沥青质、胶质、芳香分、饱和分、其他轻质组分等的DPD模型分子构成了重质油模拟体系,从微观层面上研究了其聚集分散结构特征。模拟得到的超分子结构层数、层间距等参数均与文献报道相近。模拟中发现了普遍存在的三种典型的稠合芳香环系间相互作用而形成的几何构型,而早期类似的结果得到了MD及其他领域的研究结果的确认。模拟结果确认重质油体系介观聚集结构具有明显的胶体分散体系特征,其中芳香度最高的沥青质为胶核,芳香度略低的胶质分子附着于胶核外层构成中间层,聚集体的最外层为芳香分。进一步的分析表明,重质油的聚集结构与沥青质分子的烷基侧链长度和芳香环稠合程度密切相关。将微观聚集结构用于重质油体系的稳定性预测,在大量模拟工作的基础上提出了判断标准,并同shell的实验结果对比,确认了该方法的可行性。此外,我们发现cross-plot方法的弊端,预测重质油体系的稳定性需要四组分完整信息。初步研究了油水乳化体系。确认了重质组分,尤其是沥青质有表面活性剂特性,倾向于分布在油水界面上。其浓度高低会影响形成的界面膜结构和厚度,从而最终决定界面膜机械强度和乳状液稳定性。 (3) 以稀释的沥青质溶液为重点,考察了重质油各组分的扩散性能和沥青质的聚集动力学。在较低浓度的沥青质-甲苯体系内,模拟计算获得的沥青质和甲苯的扩散系数跟文献数据有很好的吻合。确认了扩散系数随浓度负相关的关系。扩散系数和聚集形貌表明此时沥青质单体和聚集体共存。在极低浓度条件下,沥青质主要以单体形式存在,对该体系的探索可以更加准确的找到符合油品特性的模型分子结构和模型参数。计算了沥青质单体和聚集体的大小。在模拟体系中的单体小于1.76nm,而所形成的最大聚集体含有4个以上的沥青质分子,直径大于2.6nm。随模拟体系的增加和模拟时间的延长,有可能发现更大的聚集体。所得模拟数据均与相关实验数据可比。将聚集体大小带入Stokes-Einstein方程,关联计算的扩散系数同MSD统计结果自洽。通过观察体系中最大聚团随时间的演化过程,研究了沥青质的聚集动力学行为。确认了沥青质聚集体会经历聚集,分裂,再聚集的动态过程。借助沥青质聚集动力学过程,解释了在MD和DPD模拟中发现的沥青质MSD曲线波动现象。对重质油体系的模拟还揭示了各组分的扩散性能。发现沥青质和胶质的扩散性能接近,这和平衡态下的聚集体结构相吻合。该现象表明,沥青质跟胶质并没有本质的清晰界限,按照传统溶解度概念定义的做法引入了混乱。另外扩散性能跟分子大小负相关的特性也得到了印证。 (4) 在标准DPD的基础上构建了三种施加剪切的模拟方案,选择了RNEMD方法。该方法具有实现简单,没有剪切致热的优点。本文基于该方法研究了重质油及其相关溶液体系的流变特性。对该方法的考查表明,施加剪切未造成明显的粒子密度分布不均。在不同体系的流变性模拟中发现,重质油及其乳状液均能表现出剪切变稀的特性,而轻质原油则表现为牛顿流体特性。从模拟中观察到沥青质在对二甲苯及软沥青中的相对剪切粘度都符合多次方规律。表明沥青质的微观聚集结构对粘度的重要影响。该结论得到了实验结果的支持。在油水乳化体系研究中,大量加入表面活性剂,会提高乳状液体系的剪切粘度,增加输送成本。模拟和实验结果也表明,油相含量越高,流体粘度越高,非牛顿流体特性也越明显。为了达到降低输送成本的目的,需要在流体粘度和含油量之间做权衡。
英文摘要: Taking the fused aromatic rings in hydrocarbon as rigid body and deriving the proper parameters and model molecules for simulation, we study the heavy crude oil system and asphaltene related issues, based on the modified DPD method. This work mainly explores the properties of heavy crude oil like stability, diffusion behavior and rheology. It builds the bridge between the microstructural observation and macroscopic properties on heavy crude oil system. The primary works and consequences are as follows, First of all, we developed the rotational algorithm for rigid body in non –conserved force field and employ it in modeling the motion of hydrocarbons having fused aromatic rings. In light of the performance on energy convergence, moment conservation and rigidity maintenance, we take the quaternion method and integrate it into standard DPD method. All these efforts contribute to the newly developed simulation platform at meso-scale. To observe the microstructure formed by aggregating asphaltene and resin, we reduced to the choice of 3 CH2 as coarse-graining level. The relationship between the DPD parameters and their physical meaning is determined corresponding to this coarse-graining standard. Then the force field, the repulsive parameters, is obtained on the basis of blending energy calculation. After that, the way to select appropriate model molecules for each fractions in heavy crude oil come into our consideration and a serial of criteria are proposed. We find that the lower and upper limitation for the amount of fused rings in model molecule for asphaltene is 3 and 5 correspondingly. This finding could be supported by the molecular weight analysis too. The length and distribution of side chains in hydrocarbons could affect the solubility in light fractions, which is useful for screening side chains for model molecules. The regulation is proven truth by the structural parameters from experimental analysis. Concerning on the structural feature of asphaltene, both continental and archipelagic model are built. Our work finds that the latter one is prone to form more ordered structure by stack of fused aromatic rings, both inner and among molecules. Combining the model molecules for asphaltene, resin, aromatic and saturate, the heavy crude oil system is investigated from micro-scale. The macrostructural feature and the interlayer distance are close to the literature value. The three commonly existing geometries among those aggregating asphaltene are reappeared in our simulation, which strongly indicates that the parameters used make sense. The aggregating structure reflects the properties of colloidal systems and it largely depends on the length of side chains and the amount of fused aromatic rings. The simulation also reveals the peptization function of resin. Then the analysis of microstructures is employed to predict the stability of heavy crude oil system. The criteria for instability are proposed and the comparison among the simulation and experimental result (Shell) confirm its rationality. Furthermore, our work demonstrates that the cross-plot method adopted in shell’s work of stability prediction is not necessarily sufficient. More information for the four fractions required. The preliminary work on water-oil emulsion system finds the surfactant like feature of asphaltene, i.e., they are apt to stay on the interface between water and oil. More simulations illustrate that the high concentration of asphaltene can lead to increasement of the thickness of the membrane and improved mechanical performance and stability. Focused on the diluted solution of asphaltene, the diffusion capability of numerous compounds in heavy crude oil is studied. Good match between experimental observation and simulation on diffusion coefficient of asphaltene and toluene is obtained. The simulations show the negatively correlated relation between the diffuison coefficient and concentration. The equilibrium configuration illustrates the coexistence of monomer and aggregates. The measurements point out that the diameter of monomer is about 1.76 nm and that of aggregate is about 2.6 nm. Combining the diameter of particles and the classic Stokes-Einstein equation, the derived diffusion coefficient agrees with the one from MSD calculation. What’s more, the aggregating dynamics of asphaltene is investigated by observing the change of the biggest cluster. It shows that the cluster will experience aggregating, breaking up and reforming. However, the traditional opinion insist that the cluster could not break up once it forms by the interaction of π-π and hydrogen bond. This finding also offers a good explanation for the fluctuation of the MSD curve of asphaltene both in MD and DPD simulation. Three ways to exert shear effection on the fluid are discussed and the most excellent one, RNEMD method is taken in our work. It avoids the heating effection caused by inner friction and also owns the merit of ease to develop. The testing confirms that this method don’t cause uneven distribution of particle density. It also shows the approach is valid under limited shear ratio. Once the system is over sheared, the temperature will rise unacceptably. The testing of mono water molecule find the viscosity of water is 0.87, which is in the reasonable range and supported by other DPD works. Upon applying the shear to different fluid system, the non-Newtonian fluid like feature of heavy crude oil and emulsion is found, as well as the Newtonian fluid like feature of crude oil. When comes to the asphaltene-xylene and asphaltene-maltene system, we realized that the viscosity of the system could be the function of the concentration of asphaltene, which means that the rheology of heavy crude oil largely depends on the aggregating clusters in the fluid. We also come to the conclusion that the higher the weight percent of water, the lower the viscosity of the water-oil system. With the addition of surfactant, we study the emulsion system and find that the high concentration of surfactant can lead to increasement of viscosity of the emulsion and cost on transportation. There is a balance between the oil content and the viscosity of the fluid when considering the cost on transportation.
语种: 中文
内容类型: 学位论文
URI标识: http://ir.ipe.ac.cn/handle/122111/1871
Appears in Collections:研究所(批量导入)_学位论文

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张胜飞. 重质油体系的介观模拟研究[D]. 中国科学院研究生院. 2012.
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