含蜡原油储存中蜡晶微观行为与原油传热的耦合作用机制

During the storage of waxy crude oil, there is the microscopic behavior including the precipitation, development, coalescence, smash, movement and deposition of wax crystal. Moreover, these microscopic behaviors are coupled with the heat transfer course of oil in a complicated manner, which increases the complexity of heat transfer and component transportation behavior of waxy crude oil. In order to dissect the coupling mechanism intensively, based on the population balance model, the mathematical characterization for the granularity distribution and related microscopic behavior of the wax crystal is established. The discretization method is developed to disperse the wax crystal which has the successive granularity distribution into the multigroup waxy crystal each group of which has a certain granularity. A conceive that the waxy crude oil is regarded as the multiphase system when the temperature is under wax precipitation point is proposed. The effort is taken on seeking some methods, including how to represent the granularity distribution evolution of wax crystal by the mass transfer, how to associate the microscopic behavior of wax crystal and the macroscopical rheological property of oil by the acting force between different phases, and how to represent the release of latent heat by the heat transfer between different phases. Based on the coupling of PBM and Euler-Euler equation, the mathematic model representing the multiphase system of waxy crude oil is established. On this basis, the detailed microscopic behavior of wax crystal and heat transfer characteristic of oil is obtained by the numerical simulation and experiment, and the coupling mechanism is also illuminated by a further study. The expected research achievement is not only benefit for the intensive comprehension of the heat transfer characteristic of oil and microscopic behavior characteristic of wax crystal, but also a new idea can be supplied to the investigation of the heat transfer and flow characteristic of waxy crude oil.

含蜡原油储存过程中存在蜡的结晶析出、生长、聚并、破碎、迁移和沉降等微观行为，与原油传热过程形成复杂的耦合关系，增加了含蜡原油传热和组分传输行为的复杂性。为了深入剖析该耦合作用机制，在本项目中，基于群体平衡模型，构建对蜡晶粒度分布及与之相关的微观行为的数学表征，发展对群体平衡方程的离散方法，将连续粒度分布的蜡晶颗粒离散为多组具有特定粒度的颗粒。提出将析蜡点以下的含蜡原油看作是多相分散体系的构想，探求以相间质量传递表征蜡晶粒度分布的演化，以相间作用力关联蜡晶微观行为与原油的宏观流变性，以相间热量传递表征蜡结晶潜热释放的方法，基于欧拉-欧拉和群体平衡方程耦合的方法建立对含蜡原油多相体系的数学描述。在此基础上，运用实验和数值模拟获取含蜡原油传热及蜡晶微观行为细节，阐明两者的耦合作用机制。预期成果不仅有利于深化对蜡晶微观行为及原油传热规律的认识，还将为含蜡原油的传热及流动特性研究提供新思路。

含蜡原油在长期储存中存在流变性恶化、易于胶凝的问题，而为了保证其安全储存，又因加热带来了较大的能耗，因此有必要对含蜡原油储存中的传热特性和微观机理进行深入研究，为实际工程中含蜡原油的安全经济储存提供理论指导。该项目取得的主要研究成果包括：（1）研制完成两套用于模拟原油静态罐储、收发油和加热过程，并可对其温度场和流场演变规律进行可视化研究的实验装置和方法。（2）构建了能够实现对含蜡原油在经受剪切作用下，蜡晶微观动力学行为原位观测的实验方法，首次捕捉到蜡晶的旋转、翻滚运移以及蜡晶间的聚集和絮凝等微观动力学行为。发现由溶胶向凝胶转变过程中，蜡晶微观动力学行为的转变规律：①处于溶胶状态的含蜡原油，蜡晶具有多样的个体运动和与流场协同的运动表现；②蜡晶聚集体大量形成导致单体蜡晶数量减少、个体运动减弱；③蜡晶聚集体与液烃形成大尺寸絮凝体，极大限制了蜡晶的个体运动；④蜡晶在二维平面内形成网状絮凝结构，导致蜡晶与流场的协同运动成为唯一的运动形式；⑤蜡晶三维网状絮凝结构形成，使含蜡原油处于整体结构性胶凝状态。进一步建立了以碰撞频率、破碎频率、聚集效率和成核速率等定量表征蜡晶微观动力学行为的计算模型，构建了适用于蜡晶的群体平衡方程。（3）剖析了含蜡原油储存中蜡晶微观行为与原油传热的耦合作用机制：蜡晶的微观动力学行为会改变含蜡原油的物理属性，进而对其宏观传热特性产生影响。同时，原油传热特性不同导致其温度场分布、温降速率和自然对流强度发生改变，这些宏观物理场的变化会反过来对含蜡原油中蜡晶的微观形态、结构及其聚集和分散等微观动力学行为产生影响，最终构成了蜡晶微观行为与原油传热的耦合作用。（4）采用数值模拟方法对6种不同因素对含蜡原油储存过程传热特性的影响规律进行了系统分析。在此基础上，基于灰色关联分析方法对不同因素对表征含蜡原油传热和蜡晶微观表现的4个特征参数的影响程度进行了定量计算和显著性排序。