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甲烷化反应是煤制天然气工艺中重要的一环，但是其快速强放热的特性会对经济性生产构成威胁。为了解决这一问题，中国科学院过程工程研究所利用流化床反应器传热、传质效率高等优点，提出了“输送床-固定床两段甲烷化工艺”。其中输送床甲烷化工艺的一个优势是使用热容较高的催化剂颗粒作为主要的换热介质快速移除反应器内大量的反应热。催化剂表面产生的反应热，会同时向流体和催化剂颗粒传递，但二者之间的平衡机制尚不清楚，即当反应热形成瞬间，热量是更容易传递给催化剂颗粒还是产品气。所以本论文针对这一问题，通过实验与数学模型结合的方式，揭示了微米级催化剂颗粒上热质传递特性以及机制，主要研究内容如下：(1) 常压下毫米级单催化剂颗粒上甲烷化反应与温度变化动态特性研究。通过实验和二维动态单颗粒催化剂甲烷化反应数学模型，对 4-6 mm级催化剂颗粒上的反应和传热现象进行研究。实验结果与模拟结果吻合度高，证明了本论文所建立的数学模型有效。研究结果表明：1）催化剂颗粒内存在反应与扩散传递的竞争关系，当反应温度足够高，使得动力学速率足够快时，反应主要在催化剂颗粒表面区域进行，而当扩散速率较快时，主反应区域会向催化剂颗粒内部延伸。2）在系统稳态形成前的动态阶段，反应区域形成的反应热双向传递，即同时向催化剂颗粒和周围流体传递，使得催化剂颗粒的温度逐渐升高。3）颗粒内反应热的生成与传递之间的竞争平衡使稳态的温度分布为：直到颗粒内部具有甲烷化反应发生的位置（反应物能扩散进入的位置），温度由此向外逐渐降低，而由此向内温度几乎稳定在相同值，颗粒整体温度明显高于环境气体流温度，表明甲烷化反应明显升高了颗粒温度，使得催化剂颗粒表面区域形成的大量反应热向外传递进入流体相，而颗粒内部生成的较少反应热维持了由颗粒内部向表面逐渐降低的稳态温度分布。4）通过对毕渥数（Bip）的计算，确定了气-固相之间的热阻与催化剂颗粒内部的热阻的相对大小关系，表明气-固相之间热阻较大使反应热在初期动态阶段更多地被催化剂颗粒吸收，使催化剂颗粒温度升高。5）操作条件的变化，会对传热过程造成影响。增大催化剂颗粒的粒径会使气-固换热系数减小，而升高温度或增大气速则会使该值增大。(2) 输送床操作条件下微米级催化剂颗粒上的热质传递行为与反应机制。应用经实验验证的数学模型对输送床操作条件下微米级单催化剂颗粒上的热量传递、质量传递以及反应机制进行预测，结果表明：1）输送床反应器采用微米级催化剂颗粒，其热传递及反应效率高，送入床层的单个催化剂颗粒在0.1 s左右就能达到热平衡。2）由于催化颗粒微小，稳态下颗粒表面和内部的温度差极少，但仍然验证了温度由中心向表面逐渐降低的稳定分布特性，表明颗粒中心发生了甲烷化反应，是形成这种中心温度高的本质原因。3）稳态下催化剂颗粒内部的反应速率分布受两方面因素影响，一方面，催化剂颗粒的温度从表面向中心逐渐升高，使反应在中心位置具有最高的动力学速率；另一方面，反应物（合成气）向催化剂颗粒内部扩散的过程中存在阻力，导致反应物浓度在催化剂中心位置处较低，从而使得中心反应速率更低。二者相互协同竞争，在加压和较高气速反应条件下，催化剂颗粒内的传质速率快，颗粒内的甲烷化反应受动力学控制，导致反应速率由颗粒中心向表面逐步减低；反之，常压与低气速条件下，甲烷化反应受气体扩散控制，反应速率自催化剂表面向中心逐渐降低。;Methanation is an important unit in the coal-to-SNG (Synthetic natural gas) process. As a highly exothermic reaction, one of the major challenges for methanation technology is the efficient removal of the excessive reaction heat. Taking the full advantages of fluidized bed, mainly the high heat and mass transfer efficiency, a novel methanation process combined transport bed reactor with clean-up fixed bed reactor has been proposed by institute of Process Engineering, Chinese Academy of Science,. This process utilizes the catalyst particles rather than flowing gas as the main heat transfer medium. As methanation occurs over a catalyst particle, the generated reaction heat could transfer both inside of the catalyst itself and also the gas around the catalyst particles. However, there is deficient knowledge about how the balance between these two transfer pathways is achieved, i.e. whether the reaction heat generated will prefer transferring to particle center or directly to the surrounding gas. Aimed to solve these questions, the following research contents were covered in this thesis.(1) Dynamic characteristics of temperature for methanation over a single millimeter-scale catalyst particle under atmospheric pressure. Both experiments and 2D time dependent numerical simulations have been performed on catalyst particles with diameters ranging from 4-6 mm to investigate their reaction and heat transfer behavior under laboratory conditions. The experimental and calculated results fit well to each other verifying the applicability of the mathematical model. The results clarified that: 1) The competition between diffusion and reaction inside the catalyst particle. Under high operating temperature, the reactions mainly take place on the surface of the catalyst particle since the reaction rate is quick enough, on the contrary the reaction region would extend further inside the catalyst particle. 2) The bi-directional transfer behavior of the reaction heat. In the unsteady state, the generated reaction heat would transfer both outwards to the atmosphere and inwards to the inside of the catalyst particle, this effect increases the temperature inside the catalyst particle. 3) The competition between reaction heat generation and its transport. Both experimental and simulated results show that the temperature inside the catalyst particle decreased gradually from center to surface when the steady state is achieved. In particular, the location of reaction divided the temperature distribution profile into two parts, temperature inner than which remains nearly constant while temperature in the outer part decreases sharply. In steady state, the reaction heat generated in the near-surface region inside the catalyst particle would transfer more into the surrounding gas owing the intensive effect of convection, while that formed in the core region transfer less in this direction, thus leading the temperature distribution that the temperature decreased gradually from center to surface. 4) The influence of heat transfer resistance. Through the calculation of Biot number which serves as the criterion of the heat transfer resistances on the surface and at the inside of the body, it is suggested that interphase heat transfer resistance is relatively larger than its intra-phase counterpart so that the reaction heat tends to transfer more to the inside of catalyst itself. 5) The effects of operating conditions on heat transfer behaviors. Specifically, the gas-solid heat transfer coefficient would decrease when catalyst particle grows larger, and it would increase as the temperature or gas flowrate increases. 6) Heat transfer behavior when Biot number is slightly larger than 10. The external gas flow poses apparent influence on the temperature distribution on the catalyst particle.(2) The reaction mechanism and dynamic heat transfer over a single micrometer scale catalyst particle for transport bed nethanation. The heat and mass transfer behaviors as methanation occurs under the conditions in transport bed was predicted by the numerical simulations verified in the first part. The results revealed that: 1) The dynamic period over a single catalyst particle is short, on 100 μm catalyst particle it takes only 0.1s to get through this period, and the temperature between gas and catalyst particle is quite little as well (0.02 K), this ensures the superiority of transport bed reactor which is rapid and isothermal. 2) The temperature profile still characterizes decreasing gradually from center to surface in spite of the little temperature difference between particle center and surface, suggesting that methanation reactions occur inside the catalyst particle. 3) The reaction distribution inside the catalyst particle is affected by two opposite factors. On one hand, as mentioned previously, the temperature increased from surface to center and it accelerates reaction at the center. On the other hand, however, the diffusion of reactant inside the catalyst particle would be hindered by the internal diffusion resistance thus resulting in lower reactant concentration in core area, which is unfavorable for reaction. Therefore, under conditions in which the reactants diffuse quick enough, methanation inside the particle is controlled by the kinetics, and the reaction rate would decrease from particle center to its surface. On the contrast, the reactions rate distribution would be reversely and the reaction is subject to mass transfer.
|程永刚. 微米级甲烷化催化剂颗粒热质传递行为与反应机制的研究[D]. 中国科学院大学,2019.|
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