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基于多尺度CFD耦合PBM的甲醇制烯烃反应器模拟及放大效应研究
张景远
学位类型硕士
导师鲁波娜 ; 王维
2017-07
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业化学工程
关键词放大 流化床反应器 Cfd模拟 Emms 群平衡模型
摘要

新兴的甲醇制烯烃(MTO)工艺可实现甲醇到低碳烯烃的转化过程。因为甲醇可方便地从煤和天然气中获得,因此MTO工艺架起了煤化工和石油化工之间的桥梁,有望在不久的将来成为生成低碳烯烃的主要路线。目前,MTO工艺已实现商业化运行,而相关的化学反应工程的基础研究很少报道,特别是反应器的放大仍然依赖于经验和逐级放大实验,缺乏可靠的理论指导。传统的反应器放大准则主要关注放大过程中的流动参数的相似性,而忽略了流动和反应的双向耦合,因此无法有效指导实际放大过程。近年来,随着计算流体力学(CFD)技术和多相流理论的迅速发展,特别是介尺度理论的兴起,采用CFD模拟研究反应器的放大过程将有助于深刻理解反应器中流动和反应及其耦合行为随着反应器放大的变化规律,有望缩短传统的基于实验的反应器放大进程。然而,反应器的放大势必涉及操作流域的一系列改变(如从鼓泡流域到湍动流域的变化)以及随之带来的对停留时间、反应行为等的影响,这给CFD模拟带来新的挑战:1) 反应器放大过程中涉及流域转变,不同流域存在不同的流动结构,如鼓泡床中的“气泡”和快速床中的“颗粒团聚物”,在不同流域中选择合适的曳力本构关系是个非常关键的问题;2)受限于计算量,在CFD模拟中,一般采用集总模型描述反应动力学。而集总反应动力学模型通常基于小型流化床实验,过滤了外部流场变化带来的影响,它是否适用于操作在不同流域的大规模反应器的模拟需要深入探索和验证;3)工业MTO反应器尺寸大且颗粒停留时间长,Lu等[1]人提出的全混流(CSTR)模型用作CFD模拟的初值预测从而加速反应模拟的方式是否也适用于工业反应器的加速模拟,还需进一步研究。针对上述挑战,本文围绕MTO反应器的放大过程开展了一系列研究。论文第一章介绍MTO工艺的发展现状、工艺特点、各种模拟方法、以及已开展的模拟工作。在此基础上,引出本文的研究内容。论文第二章以大连化学物理研究所开发的MTO工艺(DMTO)的放大过程为切入点,开展不同尺度的DMTO反应器的放大模拟研究。其中,流动模型采用双流体模型(TFM),曳力模型根据不同反应器的操作状态(如鼓泡和湍动状态),分别采用了EMMS/bubbling模型和最新开发的二步法模型,反应动力学采用了平行反应的七集总模型。在此基础上,分析了流动和反应行为随着反应器放大的变化情况,进一步考察了反应动力学模型对模拟结果的影响。研究表明,上述模拟方法可准确预测不同尺度反应器内的流场分布和甲醇转化率,但对主要产物(乙烯和丙烯)的预测,则随着反应器的放大而逐渐偏离实验。考虑产物转化反应的交叉动力学模型代替平行反应模型,依然无法改善对反应产物的预测。由于产物的选择性与催化剂的焦炭含量密切相关,而基于TFM的模拟无法区分同一固相中的颗粒差异,从而不足以准确预测催化剂颗粒的焦炭含量分布。为在模拟中考虑焦炭含量分布的影响,论文第三章提出采用群体平衡模型(PBM)来描述由焦炭含量确定的催化剂颗粒分布情况。先由全混流反应器模型估算颗粒停留时间分布,结合焦炭的生成速率方程,计算得到焦炭含量分布,作为TFM耦合PBM模拟的初始值,采用离散法求解PBM中的焦炭含量的分布密度函数。在此基础上,对DMTO示范反应器进行了二维模拟。研究表明,上述方法能够有效地预测焦炭含量分布,显著提高模拟对主要产物选择性的预测。论文最后对全文进行了总结,并对未来如何完善流化床反应器的放大模拟工作进行了展望。

其他摘要

The newly proposed MTO (methanol-to-olefins) process is an economical route to produce light olefins such as ethylene and propylene. Since methanol can be easily obtained from coal and natural gas, the MTO process thus links the coal industry to petrochemical industry, and is expected to become the primary route for the production of light olefins in the near future. Nowadays, the MTO process has been commercialized since 2013, but the related fundamental research on chemical reaction engineering is seldom reported, especially the current reactor scale-ups which still rely upon the experience and step-by-step experiments. Traditional scaling theories focus on hydrodynamics similarity and neglect the coupling between reactions and hydrodynamics, therefore they are insufficient to comprehensively and exclusively scale up fluidized bed reactors. With the development of Computational Fluid Dynamics (CFD) and multiphase models, a “CFD-aided” scale-up approach is expected to speed up the “experiment-based” scale-up process with lower cost. As the reactor scale up is associated with flow regime transition and concomitant change of reaction behaviors, it poses a big challenge to CFD modeling: 1) since the flow regime transition always happens in the scale-up process, how to choose a drag model which considers the suitable meso-scale structures to characterize different flow regimes is recognized as a key factor to reliable CFD simulations; 2) for the reactive simulations of large reactors, the lumped-species kinetic model is widely used to describe the reaction kinetics. Since the lumped model is generally based on experiments over micro-scale reactors which filter the influence of dynamic flow structures, whether such a lumped model is suitable for larger-scale reactors running over different flow regime remains an open problem; 3) to simulate a large reactor economically, coarse-grid resolution with adequate accuracy is always necessary. However, even using coarse meshes, it is still formidable to afford when the investigated system needs a long period of time to evolve such as coke deposition in MTO reactions. A reactor model was proposed by Lu et al.[1] to provide the initial distribution of coke for the reactive simulation of a pilot-scale MTO reactor and found to help the simulation quickly reach the steady state. Whether this reactor model is still effective for harnessing the speedup of simulations of larger-scale MTO reactors needs to be further investigated. To investigate the above issues, this study focuses on the reactor scale-up effects through a series of simulations of different-sized MTO fluidized-bed reactors. The first Chapter introduces the development of the MTO process, the process features, simulation approaches as well as the previous work on simulations of MTO reactors. Then, the content of this study will be demonstrated. In the second Chapter, a series of simulations of four different-sized DMTO (MTO process developed by Dalian Institute of Chemical Physics) reactors are carried out. The two-fluid model (TFM) is used to describe the flow hydrodynamics. For the drag force which plays the predominant role over the other interaction forces, the EMMS/bubbling model and its newly developed two-step version are chosen to simulate the bubbling fluidized bed and turbulent fluidized bed reactors, respectively. The seven-lumped parallel kinetic model is adopted to describe the reaction kinetics. Then, the flow and reaction behaviors with scaling up of reactor are investigated. It is found that the simulation can successfully predict the typical flow structures and methanol conversion in the different-sized reactors, however, the prediction on the selectivity of light olefins (ethylene and propylene) gradually deviates from the experiment as the dimension of reactor increases. Changing the seven-lumped parallel model to the cross reaction model with considering product conversion, seems not helpful to improve the prediction of light olefins. The possible reason is the insufficient prediction on the distribution of coke content by using current TFM-based simulation, since the distribution of coke content which is closely related to the product selectivity is filtered or averaged by TFM.In order to consider the influence of coke content distribution in the simulation, the third Chapter proposes to use PBM to describe the distribution of catalyst particles with different coke contents. Firstly, a CSTR model is proposed to provide the distribution of residence time which is combined with coke formation rate to give the distribution of coke content. Secondly, the coke content distribution is taken as the initial value and described by PBM in the CFD simulation. The simulation is then performed on a two-dimensional demo-scale DMTO reactor to investigate the influence of coke distribution on the reaction behaviors. It is found that with assistance of initial distribution from the reactor model, the introduction of PBM into TFM simulation improves the prediction of product selectivity at limited computational cost.In Chapter 4, a summary and the future work on improving the reactor scale-up simulation are delivered. 

语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/24221
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张景远. 基于多尺度CFD耦合PBM的甲醇制烯烃反应器模拟及放大效应研究[D]. 北京. 中国科学院研究生院,2017.
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