气固并流下行循环流化床反应与流动特性试验研究
文献类型：学位论文
| 作者 | 范垂钢 |
| 学位类别 | 博士 |
| 答辩日期 | 2008-12-01 |
| 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 | 林伟刚 |
| 关键词 | 下行床 臭氧分解 传质 反应特性 流动结构 团聚物 流态化 |
| 其他题名 | Study of the Reaction and Hydrodynamic Characteristics of Gas-Solid Cocurrent Downflow Circulating Fluidized Beds |
| 学位专业 | 化学工程 |
| 中文摘要 | 气固并流下行循环流化床作为一种新型反应器，由于其气固接触效率高、气固轴向返混小以及接触时间短，近来受到学术界和工业界的广泛关注，被誉为“二 十一世纪取代提升管的一种高效、新型反应器”。反应器设计由流体动力学行为、传质、传热及反应机理等因素决定。前人对下行床的流动特性进行了系统的研究，但传质和化学反应方面的报道（特别是关于反应特性方面的基础研究）很少。本文针对实验室规模的下行床反应器（总高度15 米，下行段有效高度8.50 米，内径90 毫米），首次以臭氧分解这一简单化学反应作为模型反应，并结合考察相应流体力学行为，研究了设备的反应特性。 在多尺度思想框架下对反应过程进行定量考察。为下行床的运行、设计、放大和工业化提供理论与实验依据。 本文主要包括流体力学试验、臭氧催化分解试验和反应物扩散行为计算三部分内容。其中下行床内臭氧分解试验是全文核心，而流动结构研究则为观察到的， 与反应有关的实验现象提供相应的力学解释。 各部分主要内容和结果为： 1）考察了下行床流体力学特性，与前人不同之处在于并非针对流动结构进行专项系统考察，而是与床内进行的化学反应紧密结合，互为对照。试验发现， 在操作气速和颗粒流率取值较低时下行床仍然保持着均匀的流动结构，在较高气速和颗粒流率实验下观察到的有关流动的大多现象在当前条件下仍较明显，但也有一些与已有报道不同的现象，如： 低颗粒流率下径向颗粒浓度分布产生了一定变化，径向方向上的“稀相核/浓相环/稀相环”典型结构此时不再明显存在甚至消失。 床内轴向压力分布在低气速下呈线性升高，没有体现出第一加速段的存在。 低颗粒流率和低表观气速条件下，下行床内的气固滑移速度随操作气速的升高而升高（这一结论与部分高颗粒流率和气速下的结果不同）。 本文结合现有理论对以上现象给出了合理解释。此外还考察了低颗粒流率时床内的颗粒团聚现象。通过对颗粒浓度瞬时信号进行频域和时域分析发现即使在 低浓度下行床内，颗粒团聚仍明显存在，且团聚物较提升管更加不稳定。 2) 研究了下行床中臭氧催化分解的过程和特点。测定了常温常压下的臭氧分解反应速率常数，并发现在下行床内臭氧催化分解为动力学控制过程,且床内臭氧浓度分布明显比提升管内均匀。 恒速段臭氧浓度径向分布主要决定于相应的催化剂颗粒浓度分布：臭氧浓度在中心区分布均匀，近壁区有明显下降；加速段臭氧浓度径向分布随颗粒流率提 高，逐渐呈现出中心低，边壁高的趋势。 臭氧浓度轴向分布在边壁处轴线上呈明显上浓下稀,而沿中心区轴线上的浓度变化不大。随着颗粒运动由加速段过渡到恒速段，下行床边壁区臭氧分解程度 沿轴向逐渐加深。整体来看加速段对总转化率的贡献很大。经对比发现，臭氧浓度轴向分布与提升管亦有所差异。 改变颗粒流率或操作气速都会对床内的臭氧分解产生影响。对于动力学控制 过程，且在床内颗粒聚团不能稳定存在的前提下，只要操作条件的变化有利于颗粒浓度的提高，下行床内臭氧分解率就总体呈升高趋势；试验表明操作气速对转 化率的影响比颗粒流率更加复杂。此外，虽然就整体而言反应受动力学控制，但在边壁浓相区范围内，微观流动结构带来的传质方面的作用对转化率也有一定的 影响，但其程度并不深。 本文还综合对比了下行床内的传质、传热、流动和反应有关参数的空间分布， 认为下行床中传递现象的有关参数分布具有较强的相似性。 3） 定量讨论了单个催化剂颗粒表面的臭氧分解，并在此基础上建立二维非均相模型用于回归计算全床的气体径向扩散系数并拟合臭氧浓度场：计算发现气 体径向扩散系数随操作气速的提高而明显变大，但受颗粒流率的影响不大；计算结果对气体的轴向扩散系数不敏感；通过与拟均相模型计算结果进行对比可推断物理模型中引入不均匀流动结构会影响下行床内气相反应物扩散的计算结果，使扩散系数计算值变小。理论分析同时指出上述模型需选择较合适的颗粒浓度分布表达式方可较为简便地进行解析求解。 回归预测了加速段内臭氧分解。虽然具有一定的局限性，但因具体计算过程采用关于操作条件的多项式回归，这种方法可以得到较为简洁而灵活的结果。 全文最后对下一步工作从反应器尺度、反应器主体段设计和多种模型反应选择等角度提出了一些建议。 |
| 英文摘要 | Owing to the characteristics, such as improved gas/solid contact, narrow residence time distribution, uniform gas-solid flow structure with reduced back-mixing of gas and solids, gas-solid cocurrent downflow circulating fluidized bed (downer), which is a new type of chemical reactor, has received wide attention in the recent years. It is named as “a new high efficient reactor for potentially substituting the riser reactor in the 21st century ”. Understanding of hydrodynamics mass/heat transfer and reaction characteristics is of essential importance for design of such type of reactors. However, there is still a significant gap between the requirement and knowledge on the reactors, i.e. the studies on the latter part (especially the reaction characteristics) for a downer are not sufficient. In this study, catalytic decomposition of ozone is used as the model reaction for investigating the reactor characteristics of a downer (15m in total height, 8.5m in effective height, and 90 mm I.D.). The study for reaction is also coupled with corresponding hydrodynamic experiments. The multiscale approach is applied for quantitative analysis of the factors influencing the reaction characteristics in the downer. Mathematical models are developed to describe the gas dispersion performance and other reaction characteristics of the unit. This work consists of 3 main parts, i.e., ozone decomposition in the downer, corresponding hydrodynamic study, and modeling works. 1. Macroscopic characteristics of the flow in a downer are studied, which are operated at lower flow rates for both gas and solid than the previous studies for matching the chemical reaction in the reactor studied. It is found in the study that most of the flow characteristics of the downer at such conditions are similar to the previous observations. However, a few new phenomena are observed, which is summarized as follows: The typical “core/annular/annular” radial profiles in a downer is not very obvious (or even disappear) when the solids flux is very low. z Under very low superficial gas velocity, axial profiles of pressure in the downer is linearly increased with the lengthening of axial distance from the inlet part, hence, the first acceleration is not embodied from the pressure profiles. z When solids flux or superficial gas velocity is very low, the slip velocity between gas and solids is increased with the increase of superficial gas velocity which is different from part of previous research. And these differences are quantitatively or qualitatively explained. At last, Microscopic properties such as clustering in the downer are also investigated. Frequency and time domain analysis of transient signals of solids concentration reveal that even at very low solids concentration, particles clustering is still very significant in a downer, and the clusters are more unstable than those in the riser. 2. Characteristics of catalytic ozone decomposition in a downer are studied in this part: The reaction rate constant is btained from a test in a fixed bed. And the reaction in the downer is found to be a kinetics controlled process. The profiles of ozone in the downer appears to be much more uniform than that in a riser. Radial profiles of ozone concentration is well matched with the corresponding radial profiles of solids holdup in the fully developed region of a downer, which is very uniform in the core region and falls quickly close to the reactor wall. While in the acceleration region of the downer, with the increase of solids flux, ozone concentration in centre is more and more lower than that in the wall region. Along the axis near the wall, ozone concentration decreases significantly, but there is little change in the ozone concentration along the axis near the centerline of the downer column. With the particles traveling from the accelerated region to the fully developed region, the ozone near the wall is decomposed more quickly and thoroughly. Acceleration region makes great contributions to the ozone conversion in the whole reactor. By comparison, it is also found that the axial profiles of ozone in the downer is to some extent different from that in the riser. Changing the solids flux or gas velocity can both influence ozone decomposition in the downer. But for a kinetics-controlled process with reduced particle-clustering, as long as the change is good for the increase of the catalyst particle holdup, the overall conversion of ozone in the downer will be increased. In addition, thesuperficial gas velocity has a more complex influence on the average conversion than solids flux during observation. Though ozone decomposition in a downer is a kinetics-controlled process, within the wall region where the clustering is significant， mass transfer can also bring the conversion of the reaction some effects, but the degree is not very much. The profiles of parameters of mass/heat transfer, hydrodynamics and chemical reaction are also compared at the end of this part. The similarity is obvious which is good for the further work on analogous study of transport phenomena in downer reactor. 3. A 2D heterogeneous model is built to fit the gas dispersion coefficients and ozone concentration field based on an analysis of ozone decomposition on a single particle. Radial gas dispersion coefficients is increased with the increase of superficial gas velocity, but has a little relationship with the variation of solids flux during the calculation. Calculation results is not very sensitive to axial gas dispersion coefficients. Comparing the results with the calculation from another 2D pseudo-homogeneous model reveals that integrating the heterogeneous flow structure into the calculation will introduce obvious differences to the results of gas dispersion coefficients in the downer. Theoretical analysis points out that the model can be solved analytically when a proper expression of particle concentration distribution is proposed. Data of ozone decomposition in the acceleration zone is fitted using polynomial regression method, and this is a flexible and concise method which expresses the conversion as a polynomial function of the combination of product of operating conditions. At last, some suggestions on the scale-up of reactor, the design of the downer column part and the choice of model reactions for the future work are put forward. |
| 语种 | 中文 |
| 公开日期 | 2013-09-13 |
| 页码 | 215 |
| 源URL | [http://ir.ipe.ac.cn/handle/122111/1100]  |
| 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 | 范垂钢. 气固并流下行循环流化床反应与流动特性试验研究[D]. 过程工程研究所. 中国科学院过程工程研究所. 2008. |
入库方式： OAI收割
来源：过程工程研究所
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