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Alternative TitleGas Back-Mixing in Micro Fluidized Beds
Thesis Advisor许光文
Degree Grantor中国科学院研究生院
Place of Conferral北京
Degree Discipline化学工程
Keyword微型流化床   气体返混   气体停留时间分布   微反应器   数值模拟
Abstract微型流化床基础和应用在近几年受到越来越多的关注,其具有可在高温高压条件下安全工作,费用低,污染少等优点,可用于催化剂的高通量筛选,定量化学合成等,还被用于流固相反应分析,形成了微型流化床反应分析仪(MFBRA),以克服传统测量手段的不足,用于测量快速等温反应动力学参数。然而,针对微型流化床的相关研究仍局限于最小流化气速、流化区域的考察上,本文研究微型流化床中的气体返混特性,不仅对微型流化床反应器的设计具有重要意义,且优化建立气体接近平推流的微型流化床结构和操作条件可确保MFBRA动力学参数的准确测量。 本文采用脉冲示踪法,首先考察了10-35 mm内径的单层分布板微型流化床中的气体返混,表明:随管径和初始床高的减小,床料粒径的增大,气体返混程度减小。鉴于MFBRA现用20 mm内径双分布板微型流化床,对比了10 mm与20 mm内径的单层至四层分布板床中在总颗粒量一定时的气体返混,表明:分布板数目越多,床中气体返混越小,这是由于多层分布板降低了每层的颗粒床高度、抑制了气泡的形成、从而减少床内颗粒循环所致。 然后定量研究了10 mm 与21 mm内径的双分布板微型流化床,通过扣除气体在毛细取样管中的停留时间与测量仪器响应时间得到了准确的停留时间分布曲线起点,求算了返混参数。发现气体返混变化趋势与单分布板床中相同,且返混程度随气速增加而减小。在所有条件中,使用粒径约270 μm粗颗粒的两种床径的浅层床中的彼克列数均在27以上,证明了床内气体流动接近平推流,且流化均匀,气泡小,从而为利用微型流化床最小化气体返混对反应测试的影响、获得近本征反应动力学参数提供了流动特性的保障。 最后使用CFD软件FLUENT开展了微型流化床流动模拟以验证实验结果。发现层流模型比k-ε湍流模型更适宜用做粘性模型,而边界层加入与网格加密对模拟结果影响微小,且使用三维模型模拟可得到比二维更接近实验的结果。因此不加入边界层,使用1 mm边长六边形网格划分的层流三维模型进行模拟,模拟结果与实验值较为接近,且气体返混变化规律与实验规律一致,证明了实验结果的可靠性。
Other AbstractMore attention is being paid to the hydrodynamics and applications of micro fluidized beds (MFBs) in recent years because this kind of gas-solid micro reactor is potentially more suitable for applications under high temperatures and pressures and is considered to have low cost and pollution. For example, the MFB reactors can be used for high-throughput screening of catalyst, on-demand chemical synthesis and gas-solid reaction analysis. For the latest application the so-called Micro Fluidized Bed Reaction Analyzer (MFBRA) was for the first time developed in Institute of Process Engineering, Chinese Academy of Sciences. The MFBRA was proven to be able to overcome the limitations in diffusional inhibition and sample heating rate encountered by the traditional measurement methods such as in TG and could provide rapid heating and minimized gas diffusion inhibition to allow isothermal differential analysis of reaction kinetics. The reported studies on micro fluidized beds are still limited to fluidization characterization. Thus, this study aims to understand the gas back-mixing in micro fluidized beds, which is needed not only for reactor design but also for the accurate measurement of kinetic parameters in MFBRA by making the gas flow in the bed close to the plug flow. Using the gas impulse tracer approach, this work first investigated the gas back-mixing characteristics in single-distributor micro fluidized beds of 10-35 mm in diameter. The gas back-mixing extent increased with increasing the inner bed diameter and static particle bed height but decreased with increasing the bed material size. The test was also tested for the micro fluidized beds with 2-4 gas distributors in sizes of 10 and 20 mm, revealing that the gas back-mixing extent decreased slightly with increasing the number of distributors because with the more distributors the particle bed height becomes lower and the bubbles in each layer are in turn smaller. The gas back-mixing extents in two dual-stage micro fluidized beds of 10 mm and 21 mm in diameters were further investigated by estimating the Peclet number from the RTD curve with its starting time determined by removing the residence time of gas in capillary sampling tube and instrument response time from the MS-measured raw data. This study shows further that the gas back-mixing extent decreased with increasing the superficial gas velocity. For both the tested beds using the coarse particles of 270 μm in diameter as the bed material, the estimated Peclet numbers were all over 27 under the tested conditions which simulated the conditions applied to the MFBRA. Thus, the gas flow in the beds was very close to the plug flow. In plus the relatively uniform fluidization and few bubbles in the shallow particle bed for each layer, one can believe that the use of micro fluidized bed for gas-solid reaction analysis can minimize the effect of gas back-mixing on the measurement so that it is possible to get the near-intrinsic reaction kinetics. Finally, numerical simulation was conducted with the CFD software FLUENT to verify the experimental results. It was found that the laminar model was more suitable than the k-ε turbulence model for simulating the two-phase flow in MFBs, whereas the inclusion of a boundary layer and the grid compression little affected the simulation results. The better simulation results were obtained from using a 3D model than a 2D model. Case studies were thus conducted via simulations using the 1-mm hexagonal 3D model and laminar viscous model but without wall boundary layer. The results clarified that the simulation results complied well with the experimental measurements, and the gas back-mixing variations from the simulation agreed well with the experimental observations. While proving the experimental results, these show also the possibility for understanding the hydrodynamics in MFBs by CFD simulation.
Document Type学位论文
Recommended Citation
GB/T 7714
耿爽. 微型流化床内气体返混研究[D]. 北京. 中国科学院研究生院,2012.
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