超临界水介入的非常规相态强化重质油加氢热裂化

Free radical based hydrocracking, which is vital to the hydrotreatment of heavy oil, is seriously restricted by the mass transfer resistance embedded in current multi-phase operation modes. By the consideration that a pseudo-single phase structure can be formed for the mixture of heavy oil, hydrogen, and supercritical water (SCW), it is suggested that the hydrotreamnt of heavy oil may be applied in the presence of SCW. By changing the origianal multi-phase operation mode to the pseudo-single phase one, the mass transfer resistence between the gas phase and the oil phase can be effectively eliminated. Meanwhile, the free radical mechanism based reaction network of hydrotreamnt can be optimized with the critical effect. Through the phase equilibrium calculation, MD molecular simulation, and reaction kinetics measurement, the evolution of the phase structure of heavy oil/H2/SCW, the diffusivity of complex hydrocarbon molecules and radicals in SCW, the effect of critical solvation on the reaction kinetics of characteristic free radical reactions, and the topological structure of hydrotreatment of heavy oil under the pseudo-single phase structure will be extensively investigated. Hopefully, the performance of the hydrotreatment of heavy oil in SCW with abundant critcal phenomena can be clarified. Furthermore, the accomplishment of this project may provide theoretical supports for the intensification of the available hydrotreatment techniques.

自由基机理的加氢热裂化构成重质油加氢的核心反应，然而现有多相催化模式所固有的传质瓶颈严重制约了加氢效率。鉴于重质油/H2/超临界水体系能够形成以超临界水为连续介质的拟均相相态，本项目拟将超临界水引入重质油加氢。通过相态调控使常规条件下处于多流体相操作域的反应迁移至非常规拟均相操作域进行，在消除气液传质阻力的同时利用临界效应优化烃自由基加氢热裂化动力学。研究结合相平衡计算、分子动力学模拟、悬浮床加氢热裂化动力学测定、和过程模型化等手段，就超临界水介入后重质油加氢体系的相态结构演变、超临界水中复杂烃分子和自由基的扩散性质、临界溶剂化对特征烃自由基反应动力学的影响机制、和拟均相条件下重质油加氢热裂化的网络结构等进行探索。通过研究阐明在临界效应丰富的超临界水介质中烃自由基反应的动力学特性以及重质油加氢热裂化网络结构的发展规律。本项目的成功将为以过程强化为目标的重质油高效加氢提供理论基础。

项目采用分子动力学模拟、量子化学计算和集总反应动力学分析等手段就超临界水（SCW）介入的重油加氢热裂化进行了研究。高水密度、高水油比和剧烈搅拌的介入能够使得重油/SCW体系从水油部分互溶两相结构转化为以SCW作为连续相的拟均相结构。集总反应动力学分析表明，无论是在传统油相还是SCW相中进行的重油裂化均可用考虑了拟平衡和自催化修正的反应动力学模型进行描述。沥青质缩合对自催化的依赖程度远高于软沥青缩合，且随着反应温度的提高缩合对自催化的依赖程度下降。在SCW介质中，缩合摆脱自催化影响的温度远低于在油相中反应的相应值。SCW中重油裂化所涉及的芳烃脱烷基及芳烃缩合均得到加速。这种加速作用一方面来自于SCW提供的优越扩散条件，另一方面来自于SCW的溶剂化效应。沥青质在SCW介质中存在自发类焦自组装现象，组装速率和规模取决于SCW的热力学性质。类焦自组装的驱动力来自于SCW与沥青质之间的范德华排斥以及沥青质之间的π-π吸引。稠环芳烃的类焦自组装对于沥青质在SCW中的缩合具有关键性的加速作用。根据重油所含Ni和V富集于最重稠环芳烃以及稠环芳烃在SCW中具有优先缩合结焦的特性，可以形成SCW介入的重油高效脱金属工艺。SCW能够提供水合质子用于活化芳烃烷基侧链端的烯烃基团并形成碳正离子，随后碳正离子通过β-断裂方式进行解离并与烷基侧链自由基机理的β-断裂具有类似的反应动力学特征。由于水合质子的浓度与SCW的热力学状态相关，碳正离子机理芳烃脱烷基的介入在水的临界点附近最为明显。在重油/SCW裂化体系进一步引入非均相加氢后发现，加氢的效果并不如人意。主要原因在于两点：1）为了形成拟均相结构采用了高水密度，而在此条件下芳烃的缩合因为类焦聚集而被加速，这直接导致加氢速率无法匹配缩合速率；2）加氢催化剂的最终形式为固体，一旦焦在体系中形成将覆盖催化剂活性位导致催化剂失活。