| 题名 | 多相环流反应器的传递和反应性能数值模拟 |
| 作者 | 黄青山 |
| 学位类别 | 博士 |
| 答辩日期 | 2008-07-14 |
| 授予单位 | 中国科学院过程工程研究所 |
| 授予地点 | 过程工程研究所 |
| 导师 | 毛在砂 |
| 关键词 | 环流反应器 鼓泡流 计算流体力学 两流体模型 煤液化 |
| 其他题名 | Numerical Simulation of Multiphase Transport and Reaction in Loop Reactors |
| 学位专业 | 化学工程 |
| 中文摘要 | 环流反应器作为一种新型的气升式反应器，近年来已经广泛应用于许多工业过程。煤直接液化新工艺将内环流反应器应用于高温、高压的加氢过程，从而大大减少投资和操作费用。由于该型反应器在煤直接液化领域还未见有成功应用的先例，再加上影响环流反应器动力学的参数较多，设计和放大比较困难。本文用计算流体力学的方法研究环流反应器内的流动、传质和传热，建立环流反应器设计和放大的整体数学模型，为煤直接液化的工业化提供坚实的理论基础。 论文的主要研究内容及成果如下： 1）改进了出口边界条件和中心边界条件，应用循环三对角矩阵迭代促进收敛，改进多相流SIMPLE (Semi-Implicit Method for Pressure Linked Equations)算法和开发出了部分消除相间速度耦合的压力校正方法，成功将模拟流体力学行为的稳态拟均相k-ε模型和雷诺平均两流体模型运用于整个均匀鼓泡流，模拟结果与实验吻合较好。对稳态雷诺平均两流体模型的模型参数进行了灵敏性分析，比较了二维和三维模拟结果的差异，研究了曳力及曳力系数的选择、湍流对曳力系数的作用、出口滑移速度以及气泡直径的选择对模拟结果的影响。研究结果表明，气泡直径和曳力系数对模拟结果影响较大，其它因素包括稳态模拟出口滑移速度的选择对模拟结果影响较小，得到了适合模拟均匀鼓泡流流体动力学的稳态、雷诺平均两流体模型。 2）建立了适合模拟伴随有化学反应、气液两相流动的Favre平均两流体模型，考虑了气泡导致的粘度和壁面力及升力影响，成功模拟无扩大分离区、气含率较高的内环流反应器流体动力学，模拟结果与实验吻合较好。比较了环流反应器内考虑旋流与否对模拟结果的影响，通过定性和定量比较，表明考虑旋流影响得到的气液两相流流场更合理。研究表明，气泡导致的湍动在相含率较高时对模拟结果有重要影响，本研究采取的模型在一定程度上解决了气泡导致湍动的影响。模拟发现，气泡导致的湍动在某些区域远大于剪切导致的湍动。研究发现考虑气泡之间相互作用对模拟结果影响较小，反而增加了模拟收敛的难度。 3）建立了气液传质模型，比较了不同气液传质系数模型的差别并得到了适用范围较广的模型，模拟结果与实验吻合较好。开发出了适合稳态模拟气液连续进料的出口边界条件及流量校正方法。研究表明，环流反应器是一种较好的全混流反应器；考虑旋流影响与否对传质系数的影响不大；表观气速不变的条件下，增大表观液速使反应器内平均气含率和传质系数减小。 4）在获得流场及浓度分布的基础上，建立了气液传热模型，得到了煤直接液化反应器内的温度分布。由于环流反应器具有较强的换热能力，反应器内温度分布比较均匀。对于放热量较大且对温度比较敏感的反应体系，热量传递的数值模拟是非常必要的。 5）环流反应器完全能够满足煤直接液化反应的工艺要求，是煤直接液化一种比较理想的反应器。可以通过调整煤直接液化反应的气液比和及时移出热量，实现高温、高压反应器的稳定、最优操作。 |
| 英文摘要 | Airlift Loop Reactors (ALRs), which are newly developed airlift reactors in industries, have been widely used in many engineering processes. Internal airlift loop reactors (IALRs) have been designed to reduce the operating cost of coal hydrogenation in the new coal liquefaction technology of China. Because there is no application in the field of coal liquefaction and the performance of the reactor is affected by many parameters, it is very difficult for the design and scaling-up of airlift loop reactors. Therefore, it is important to study the hydrodynamic, mass transfer and heat transfer behaviors in such reactors. The performance of the IALRs is studied using the method of computational fluid dynamics (CFD) in this work and the mathematical models of fluid flow, mass and heat transfer are built up for providing some reliable basis for the design and scaling-up of reactors for industrial application in coal liquefaction. The boundary conditions at the cylindrical column axis and the outlet boundary for the steady bubbly flow are developed in this study. The cyclic tridiagonal matrix (CTDMA) iterative algorithm is used for accelerating the rate of convergence. The semi-implicit method for pressure linked equations (SIMPLE) algorithm for the multiphase flow is developed and a new method of correction pressure considering the partial elimination of inter-phase coupling is deduced. For modeling the hydrodynamics of an IALR with an enlarged separating section, the Reynolds averaging two-fluid model is used and the turbulence is resolved by the standard k-ε model for bubbly flow. In all tested cases, the predicted average gas holdup and average liquid velocity agree well with experimental data and it shows that the models in the present research are suitable for modeling the bubbly flow in IALRs. The influences of drag, drag coefficient, turbulence to drag coefficient, slip velocity of outflow boundary and estimated bubble diameter on the simulation results are studied. It demonstrates that the choice of bubble diameter and drag coefficient has strong influence on the predicted results, however, the outlet slip velocity and other factors have little impact on it. The steady Reynolds averaging two-fluid model for bubbly flow is built and can be used for modeling the hydrodynamics of IALRs. For modeling the dynamics of bubbly flow in IALRs with reaction, a Favre averaging two-fluid model is set up for avoiding a large source term of diffusion in the Reynolds averaging two-fluid model. Because of high gas holdup in IALRs without enlarged separating sections, the bubble induced turbulent viscosity is considered and validated in a large range of superficial gas velocities for bubbly flow. The predicted results agree well with experimental data in all simulation cases. In IALRs, the swirl of flow is obvious, especially in the top and bottom part of the reactor. However, the RNG k-ε turbulence model with swirl flow considered is demonstrated to be more rational than the standard k-ε turbulence model by qualitative (flow direction) and quantitative comparisons, though both models give almost the same results. Numerical simulations show that the bubble induced turbulence is much larger than the shear induced ones for the simulation of bubbly flow when the gas holdup is high. Compared to single bubble drag coefficient, using the multi-bubble one has little influence on the simulation results and makes it much more difficult to get a convergent solution. Mass transfer in airlift reactors is very important and there are many models describing its mechanism. Comparison of different models for mass transfer between gas and liquid is made and a suitable one is chosen. In all the tested cases, the predicted mass transfer coefficients agree very well with experimental data for a wide range of superficial gas velocity. A numerical technique of correcting the flow for overall mass conservation is developed for the case with continuous gas and liquid feeds. The IALR behaves more like a well mixed reactor due to the high capacity circulation. However, the swirl model has less improvement to the mass transfer coefficient of fluids. When the superficial liquid velocity increases, the gas holdup and mass transfer coefficients decrease while the superficial gas velocity keeps constant. On the basis of the resolution of hydrodynamics of bubbly flow and the distributions of concentrations of different components, a heat transfer model that describes the exchange of heat in gas and liquid phases is set up. The distribution of temperature for different fluids in an IALR with the reaction of hydrogenation for coal liquefaction is predicted using the heat transfer model. The temperature is found to be quite uniform in the reactor due to the capacity of heat exchange. It is found necessary for modelling the heat transfer in the system when dealing with temperature sensitive species and/or reaction with great heat discharge. IALR is totally fitted for the requirements of the new process of coal liquefaction and the optimal operation under high temperature and high pressure can be realized by controlling the flow rate of gas and liquid and by removing the heat in time as suggested by the numerical simulation conducted in the thesis. |
| 语种 | 中文 |
| 公开日期 | 2013-09-13 |
| 页码 | 224 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.ipe.ac.cn/handle/122111/1098]  |
| 专题 | 过程工程研究所_研究所（批量导入） |
| 推荐引用方式 GB/T 7714 | 黄青山. 多相环流反应器的传递和反应性能数值模拟[D]. 过程工程研究所. 中国科学院过程工程研究所. 2008. |
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