Molecular Dynamics Simulation of Structural Relaxation of Asphaltene Aggregates

Abstract

For asphaltene obtained from vacuum residue of Khafji crude oil, the energy-minimum conformation calculated by molecular mechanics–dynamics simulations showed that aggregated structures of asphaltene molecules through noncovalent interactions are more stable. Changes induced in aggregated structures by pretreatment with solvents were investigated using molecular dynamics calculations. The simulation showed that in quinolin at 573 K, some staking interactions could be disrupted, while, in 1-methylnaphthalene it was not observed. Autoclave experiments showed that the coke yield after pyrolysis at 713 K was decreased when the asphaltene was pretreated with quinoline at 573 K for 1 h, compared to the yield without the pretreatment. While, in the case of pretreatment in 1-methylnaphthalene, the coke yield did not change significantly. The simulation's results above can be related to the difference in coke yield between two solvents; in quinoline some aromatic–aromatic stacking interactions could be disrupted and mobility of molecules was increased. This resulted in prevention of the asphaltenes from polymerizing, as in condensation reactions among aromatic rings. Consequently, the coke yield after the pretreatment with quinoline was decreased.     

沥青质分子聚集体的聚集内因

运用量子力学、分子动力学等方法,研究沥青质分子聚集体形成过程的分子构型变化、能量变化以及电子分布情况。结果表明：沥青质分子形成聚集体过程中形变能很小,沥青质分子发生形变不是沥青质聚集体形成的决定因素,但为沥青质分子聚集进而形成聚集体提供基础,沥青质分子具有很强的本征聚集活性;沥青质分子间相互作用能很大,是沥青质分子聚集体形成的决定因素,其中,属于分子间固有属性的范德华力及泡利排斥作用之和相对较小,与沥青质分子结构特征相关的π π相互作用及氢键作用之和很大,表明由沥青质分子的大芳环体系和多杂原子的结构特征引起的π π相互作用及氢键作用是导致形成沥青质分子聚集体的主要原因;在所研究体系中,金属卟啉分子与含吡啶氮的沥青质分子通过π π相互作用而非轴向配位作用形成聚集体。     

沥青质在石英表面吸附行为的分子动力学模拟

为考察沥青质分子在不同溶剂环境中在岩石表面的吸附情况,选用代表性的沥青质分子结构,采用分子动力学模拟的方法研究了正庚烷、甲苯和吡啶3种溶剂中沥青质分子在羟基化石英表面的吸附机理。沥青质分子的平衡吸附构型显示,在正庚烷中,沥青质分子以较强的弯曲构型吸附在石英表面上;在甲苯和吡啶中,沥青质分子自身构型变化减弱,向石英表面吸附的趋势也明显下降。沥青质分子在石英表面吸附能及其与溶剂相互作用能的计算结果表明,正庚烷中沥青质在石英表面的吸附强度最大,而在甲苯和吡啶中其在石英表面的吸附则较弱;库仑相互作用能是沥青质在石英表面吸附过程中的决定因素,而范德华相互作用能则在沥青质与溶剂相互作用中占主导地位。因此,分子动力学方法可对沥青质分子吸附的动力学过程进行有效模拟。     

沥青质分子聚集体中π-π相互作用的研究

采用量子力学与分子动力学相结合的方法对形成沥青质分子聚集体的分子间π-π相互作用进行了研究。结果表明：沥青质分子间的π-π相互作用能随着分子芳香环数的增加而增大;分子中含有的杂原子显著增加了沥青质分子间的π-π相互作用能;沥青质分子的支链长度及类型能够影响π-π相互作用能的大小;当沥青质分子聚集体中含有多个π-π相互作用时,其分子间的聚集作用力会大大增加;沥青质分子间形成π-π相互作用的过程中,分子间有少量的电子转移;色散作用是π-π相互作用中的主要作用。     

沥青质分子界面行为的耗散粒子动力学

运用耗散粒子动力学(D PD)方法模拟了"孤岛型"沥青质分子和其纳米聚集体在油-水界面的取向性、聚集行为,分析了富氧支链对沥青质分子取向性的影响.结果表明:存在油-水界面时,沥青质分子从油、水相中脱离,吸附在油-水界面处,稠芳环核与油-水界面平行,烷烃支链伸入油相;随着沥青质浓度升高,空间位阻作用使沥青质分子彼此分离,...     

沥青质分子聚集体的解离对策

针对沥青质分子大芳环体系和多杂原子结构特征引起的π-π相互作用及氢键作用,运用量子力学、分子动力学等方法,研究沥青质分子聚集体的解离对策。对于π-π相互作用,从降低沥青质分子间π电子云重叠和减少沥青质分子的π电子数目两个方向研究解离措施;对于氢键作用,从降低沥青质分子间S—H、N—H、O—H间轨道叠加电子转移效应和减少沥青质分子的S、N、O数目两个方向研究解离措施。结果表明：引入π电子云分散剂可有效降低沥青质分子间π电子云重叠程度,对沥青质分子的稠合芳环进行局部加氢饱和可以减少其π电子数目,两条途径的分子模拟结果均能实现沥青质分子聚集体的解离;削弱沥青质分子间π-π相互作用对减弱氢键作用具有明显的促进作用;针对π-π相互作用的解离思路和措施也适应于金属卟啉分子与沥青质分子形成的聚集体,镍卟啉分子与沥青质分子形成的聚集体的解离难度比钒卟啉的大;提高温度加剧分子热运动及脱除杂原子可削弱或消除氢键作用,但在沥青质分子的其他芳环体系未改变的前提下,消除氢键作用不能实现对沥青质分子聚集体的完全解离。     

沥青质分子结构与沥青材料性能关系的分子模拟

沥青是一种分子组成多样且介观相态构成复杂的黏弹性材料,其骨架结构为沥青质组成的高贮能模量相,常温环境在外力作用下以弹性响应为主.为了研究分子结构对沥青材料性能的影响,利用分子动力学模拟方法研究不同结构的沥青质在沥青工作温度(250～353 K)区间内的相态,并近似地用剪切模量和杨氏模量分别对各相的切向与正向应力-应变关...     

Preliminary Study of the Effect of Pressure on Asphaltene Disassociation by Molecular Dynamics

Abstract

Preliminary studies of the effect of pressure on a system composed of 32 asphaltene molecules immersed in a solvent (pentane or toluene) were carried out. In this case, this system was randomly distributed into a cubic box of 49430.9 Å³ of volume. The NPT simulations showed spontaneous asphaltene disassociation when an asphaltene aggregate was immersed in toluene as a function of the pressure. Among the main configurations found, offset π-stacked geometry was the most frequently observed stacking form. Calculation of the radial distribution function on the system also revealed that the nearest asphaltene molecules have an average separation distance around 3.8 Å. This is a value in agreement with the classical model for asphaltene aggregates developed almost 40 years ago. Some possible asphaltene micelle formation and phase transitions will be described.     

The Growth and Development of Asphaltene Aggregates in Toluene Solution

The effect of asphaltene concentrations from 10 to 140 mg/L on particle diameter of aggregates is studied by multiple angle dynamic and static light scattering. The particle diameter of aggregates is increasing with the asphaltene concentration rising. The observed concentration for massive self-aggregation of asphaltene is about 80 mg/L. And the process of growth and development of asphaltene aggregates is studied by measurements of particle video microscopy (PVM) and focused beam reflectance measurement (FBRM) on asphaltene in toluene solution simultaneously. PVM and FBRM provide dynamic images and real-time statistical data of asphaltene aggregates in toluene solution, which can show that asphaltene aggregation is stronger and stronger with the asphaltene concentration increasing. Statistical data demonstrate that small particles are first massively formed in the range of less than 5 μm and 5–10 μm. Then small particles can interact to result in large particles which are in the range of 10–50 μm and 50–80 μm with asphaltene concentration increasing. The number of particles in the range of less than 50 μm is dominant in solution despite asphaltene concentration increasing.     

通过分子动力学方法研究沥青质聚集体应变下势能变化

利用分子动力学构建不同稠合芳环结构沥青质构成的聚集体在不同温度下的模型,并对不同应变状态的模型进行模拟,统计各种势能的相对变化值。沥青质聚集体模型在拉伸和压缩应变的情况下势能均上升,且势能基本随应变绝对值增加而增加。但势能上升贡献的来源不同,压缩时势能增加的主要原因是分子形变势能上升,而拉伸时则是由于分子间相互作用和分子形变势能变化共同影响。沥青质稠合芳环结构会显著影响聚集体模型在应变下的势能变化种类和趋势,渺位缩合沥青质的形变势能随应变的变化较大,且主要由分子的扭矩变化造成;而迫位缩合沥青质形变势能变化较小,且主要是由分子键伸缩造成。随着应变增加,所有沥青质分子均能更好维持平面构型,从而使偏离平面附加势能降低。     

沥青质聚集体的解聚分离方法综述和探析

沥青质在原油、渣油体系中常以分子聚集体的形式存在,要研究沥青质的分子组成和结构以及含沥青质重油的加工,基础是要实现沥青质分子聚集体的解聚、分离。基于此,针对沥青质分子聚集体中存在的分子间相互作用（力）,从良溶剂稀释、消除聚集作用位点、适度热作用、超声波作用及碰撞诱导解离等五个方面阐述了沥青质分子聚集体解离方法及进展,进一步介绍了基于沥青质分子极性、分子尺寸、酸碱性以及反应性等方面开展沥青质分子水平分离的方法及进展。     

萘热解生焦过程的反应分子动力学模拟

采用分子模拟的方法深入研究萘在热解生成针状焦过程的反应分子动力学。运用反应力场ReaxFF模拟该反应过程,并进行量子化学分析,推断得到萘分子在热解生焦过程中的主要反应历程。萘分子在自由基夺氢的情况下生成自由基•C₁₀H₇, •C₁₀H₇两两结合生成1,1′-联萘,1,1′-联萘再脱氢缩合生成苝,苝再进行脱氢缩合反应,芳核也随之增大。反应分子动力学模拟结果表明,在萘生焦过程中,反应生成的小分子和自由基会促进萘热解反应。在针状焦生成前期,应当控制热解反应的温度和压力,适当抑制气体挥发速率,以保证小分子和自由基能够参与芳核的生长反应。     

有机蒸气/氮气混合物透过炭膜的非平衡分子动力学模拟

基于考虑渗透过程中外加力场影响的非平衡分子动力学方法,建立了计算有机蒸气/氮气混合物透过多孔C膜的渗透系数和分离因子的方案;并对甲烷/氮气(CH4/N2)和丙酮/氮气(CH3COCH3/N2)两种混合物透过炭膜的渗透过程进行计算。主要讨论了计算模型的建立和计算方法的选择。计算出的渗透系数和分离因子与文献中的计算结果或实验结果具有可比性,CH4/N2混合物透过0.75nm炭膜的分离机理为偏离Knudsen扩散、伴有表面扩散产生,分离因子为1.836;丙酮/N2混合物透过4.42nm炭膜的分离机理为毛细管冷凝,分离因子为44.46。炭膜可有效地用于回收丙酮。     

低硫柴油润滑性的分子动力学模拟

针对低硫柴油存在的润滑性问题,采用分子动力学(MD)模拟方法研究了低硫柴油(以正十六烷为模拟组分)在Fe (001)表面间的吸附润滑机理。结果表明,剪切速率低于20 m/s时,正十六烷润滑膜处于稳定工作状态,润滑膜的剪切应力以及润滑膜在铁表面间的吸附能和膜的内聚能都不受剪切速率的影响,整个润滑膜的温度也可维持在相对较低的水平。在剪切速率高于20 m/s时,随着剪切速率的增加,润滑膜的剪切应力线性增加,吸附能和内聚能都降低,同时润滑膜内部的温度急剧上升,开始出现温度失稳现象。由于吸附作用,正十六烷润滑膜在铁表面间呈对称波动分布的层状结构;按质量分数分布可分为类固态区和液态区,对应的速率分布则为扰动区和静流区。当剪切速率达到100 m/s时,正十六烷润滑膜发生了非常强烈的层间滑移,此时润滑膜处于润滑失效的状态。这些结论有助于从微观上理解低硫柴油的润滑性问题。     

表面活性剂界面吸附行为的分子动力学模拟

 采用分子动力学(MD,Molecular dynamics)方法模拟了油、水两相分离过程及表面活性剂十二烷基苯磺酸钠在油-水界面的吸附行为,考察了十二烷基苯磺酸钠分子支化程度、在油-水体系中的浓度和不同油相对油、水两相分离过程的影响及作用。结果表明,对于油-水体系,油、水两相在短时间内可达到分离平衡,形成一个明显的油-水界面;在烷烃-水体系中,以十二烷作为油相时,十二烷基苯磺酸钠在界面处浓度最大,吸附趋势最强;随着体系中十二烷基苯磺酸钠浓度的增大,模拟得到的吸附峰值浓度先增加然后略降,与实验结果相符。研究还提出,表面活性剂的界面接触面积(ASA,accessible surface area)可以作为衡量表面活性剂的油-水界面吸附能力及电解质降低油-水界面张力效果的指标。MD给出的分子水平的微观信息可以为三次采油技术中表面活性剂的筛选及有效应用提供指导。     

腰果酚聚氧乙烯醚磺酸钠在癸烷-水体系的分子动力学模拟

为揭示阴-非离子表面活性剂在油水界面的聚集行为,为三次采油中驱油用表面活性剂的选择以及有效应用提供理论指导,在癸烷-水体系中采用分子动力学方法模拟研究了腰果酚聚氧乙烯醚磺酸钠(CPES)的浓度、温度、盐浓度和种类对CPES界面活性的影响。研究结果表明,CPES的界面活性良好,饱和状态下的界面张力仅为1.83 mN/m;CPES分子结构中磺酸基的亲水性高于聚氧乙烯基,远大于醚基;CPES的耐温性较好,温度由298 K增至373 K时,界面张力最大值为16.74 mN/m;CPES的抗盐性良好,Ca~(2+)浓度由0.10增至0.35 mol/L时,CPES与水之间形成的氢键数目波动不大,界面张力范围为12.605±1.745 mN/m,其抗盐性顺序为Na~+Ca~(2+)Mg~(2+)。CPES具有优良的界面活性以及较强的耐温抗盐性,可望用作驱油用表面活性剂。     

Molecular Weight Measurement of UG8 Asphaltene Using APCI Mass Spectroscopy

Asphaltene molecular weight has been a controversial issue in the past several decades and continues on nowadays. From industrial application point of view, asphaltene molecular weight is important for setting up a heavy oil refining strategy so that the process is efficient and economically viable. If the measured average molecular weight of asphaltene is high and is the true molecular weight, then substantial amount of energy will be needed, in order to break the molecule into light products during refining process. This is likely not an economical option. On the other, if the measured high molecular weight is due to self-association and the true molecular weight is low (e.g., less than 1500 Da), it will be energetically attractive to refiners to develop heavy oil cracking technology. Vapor pressure osmometry (VPO) has been routinely used for measuring molecular weight. However, it measures the apparent molecular weight and is likely not the true molecular weight. In order to unambiguously measure the molecular weight, it is necessary to develop a convincing technology and a reliable experimental procedure that allows one to measure the molecular weight accurately and consistently. We chose the Atmospheric Pressure Chemical Ionization (APCI) technique and Atmospheric Pressure Photo Ionization (APPI) to measure UG8 asphaltene. Both APCI and APPI have mild ionization processes and have been applied to many unstable drug compounds such as proteins and peptides with reliable outcomes. In addition, we measured the sample on two APPI instruments to compare the results. We also demonstrated how one can choose wrong set of operating parameters and lead to erroneous results. The relevant parameters for APCI and APPI are temperature, voltage, and sample concentration. We chose 0.01 mg/mL as the concentration, much below any known critical aggregation concentration. As for temperature and ionization voltage, we varied systematically varied (T = 300-600°C; V = 30-150 V) in order to demonstrate the consistency of the methods and how one can easily make mistake. Through these measurements, an average molecular weight of 400 to 900 Da was obtained for UG8 asphaltene.     

水合物动力学抑制剂合成及其作用机理的分子动力学模拟

针对现有动力学抑制剂耐受过冷度较低、对水合物生成的诱导时间短的问题,采用 N-乙烯基吡咯烷酮 （NVP）、N-乙烯基己内酰胺（NVCL）、甲基丙烯酸二甲氨乙酯（DMAEMA）3种单体,制备了水合物动力学抑制剂 P（NVP-g-NVCL-g-DMAEMA）;以模拟天然气水合物抑制过程中的过冷度和诱导时间为指标,优化了合成条件; 用红外光谱仪表征了产物结构,利用分子动力学模拟软件模拟抑制过程,揭示水合物动力学抑制剂的作用机 理。结果表明,NVP、NVCL、DMAEMA单体质量比为8∶20∶1,引发剂（过硫酸铵、亚硫酸氢钠质量比为1∶1）为单 体总质量的0.5%,反应温度65 ℃,反应时间6 h为最佳反应条件且产物为目标产物。当该条件下合成的抑制剂 加量为1.0%时,常压条件下可将水合物形成的过冷度从2.6 ℃提高到9.6 ℃,诱导时间从20 min延长至945 min。 该抑制剂的作用机理主要是通过分子链上五元环及七元环上的双键氧和酯基上的双键氧与水分子形成氢键吸 附从而抑制水合物的生成,其次分子链上的氮原子也可以形成氢键而吸附;另外,抑制剂的空间位阻也会阻碍水 合物分子的聚集进而抑制水合物的结晶。该抑制剂不仅带有可形成氢键的活性基团,增强抑制剂对水合物笼的 吸附性,还存在能影响甲烷分子运动及分布状态的烷基链,进一步提高过冷度并延长诱导时间。     

Molecular Weight Measurement of UG8 Asphaltene Using APCI Mass Spectroscopy

Abstract

Asphaltene molecular weight has been a controversial issue in the past several decades and continues on nowadays. From industrial application point of view, asphaltene molecular weight is important for setting up a heavy oil refining strategy so that the process is efficient and economically viable. If the measured average molecular weight of asphaltene is high and is the true molecular weight, then substantial amount of energy will be needed, in order to break the molecule into light products during refining process. This is likely not an economical option. On the other, if the measured high molecular weight is due to self-association and the true molecular weight is low (e.g., less than 1500 Da), it will be energetically attractive to refiners to develop heavy oil cracking technology. Vapor pressure osmometry (VPO) has been routinely used for measuring molecular weight. However, it measures the apparent molecular weight and is likely not the true molecular weight. In order to unambiguously measure the molecular weight, it is necessary to develop a convincing technology and a reliable experimental procedure that allows one to measure the molecular weight accurately and consistently. We chose the Atmospheric Pressure Chemical Ionization (APCI) technique and Atmospheric Pressure Photo Ionization (APPI) to measure UG8 asphaltene. Both APCI and APPI have mild ionization processes and have been applied to many unstable drug compounds such as proteins and peptides with reliable outcomes. In addition, we measured the sample on two APPI instruments to compare the results. We also demonstrated how one can choose wrong set of operating parameters and lead to erroneous results. The relevant parameters for APCI and APPI are temperature, voltage, and sample concentration. We chose 0.01 mg/mL as the concentration, much below any known critical aggregation concentration. As for temperature and ionization voltage, we varied systematically varied (T = 300–600°C; V = 30–150 V) in order to demonstrate the consistency of the methods and how one can easily make mistake. Through these measurements, an average molecular weight of 400 to 900 Da was obtained for UG8 asphaltene.