基于单颗粒示踪法内循环流化床颗粒混合的研究

发布时间：2018-11-09 08:00

【摘要】：内循环流化床(Internally circulating fluidized bed, ICFB)由两个相互连通、处于不同流化风速的流化床构成,两床被设置有循环口的隔板或中心管分开,床料通过隔板或中心管上的循环口以及床层上方区域在两床间形成规则的内循环。内循环流化床除了具备一般流化床传热传质快等优点,还能够有效组织和控制颗粒在床内的宏观运动以及不同区域的停留时间,从而能够对反应进行分级控制。内循环流化床因其内部特殊的流动结构以及突出的优点而得到广泛的工业应用。颗粒混合关系到床内热量的分布以及气固反应的效果,是流态化基础研究的重要课题之一。对于内循环流化床而言,颗粒混合包括两方面：某一侧床内存在横向和纵向的颗粒混合：两侧床之间还存在着宏观的颗粒混合,即内循环。本文采用荧光大颗粒示踪和图像处理法,跟踪床内示踪颗粒的运动,分析包括颗粒运动轨迹、速度分布场、速度统计分布以及停留时间和循环时间等运动特征,定性地揭示内循环床内颗粒混合特性。当高速床流化风速高于4倍初始流化风速时,低速床中存在明显的死区,主要在靠近隔板区域以及循环口以下区域。颗粒的速度分布概率受流化风速的影响较大,且颗粒横向和纵向速度分布概率的特征显著不同。颗粒速度在低速床中主要集中在0-0.2m/s,而高速床在0～0.8m/s区间分布。低速床中,颗粒纵向速度超过50%分布在负值区域,表现出向下运动的特征。颗粒内循环流率的大小直接影响着反应器内传热、传质以及反应效率,因而针对它的研究一直备受关注。本文直接跟踪和估算循环口处颗粒流率,为了便于测算,实验在循环口处设置40mmm导流方管。实验考察了循环口尺寸、床料粒径等设计参数以及低速床流化风速、高速床流化风速、静止床高等操作参数对于循环流率的影响,还提出了循环流率与各个参数的预测关联式。研究发现,循环口处压差是颗粒从低速床向高速床流动的主要驱动力。静止床高、流化风速以及孔口面积的增加都有利于颗粒内循环流率的增加,循环流率主要在10～60Kg·m-2·s-1区间变化。而床料粒径增加(A、B和D类颗粒),循环口处流动的阻力增加,不利于孔口处颗粒流动。另外,本文还在给定床体几何尺寸、床料特性等设计参数和床料量、两床的流化风速等操作参数的情况下,建立颗粒内循环流动流体动力学模型,计算内循环流率。与已有实验数据进行比较,验证模型有效性。最后,利用该模型研究各个参数变化对内循环的影响,给出了流化风速和床料量等参数的操作范围。
[Abstract]:The inner circulating fluidized bed (Internally circulating fluidized bed, ICFB) consists of two interconnected fluidized beds with different fluidized wind speeds. The two beds are separated by compartments or central tubes with circulating openings. The bed material forms regular internal circulation between the two beds through the circulation opening on the separator or central tube and the area above the bed. The inner circulating fluidized bed not only has the advantages of fast heat and mass transfer, but also can effectively organize and control the macroscopic movement of particles in the bed and the residence time in different regions, so that the reaction can be controlled by stages. Internal circulating fluidized bed has been widely used in industry because of its special flow structure and outstanding advantages. Particle mixing is one of the most important research topics in fluidized bed, which is related to the distribution of heat in bed and the effect of gas-solid reaction. For the inner circulating fluidized bed, particle mixing includes two aspects: transverse and longitudinal particle mixing in one side bed and macroscopic particle mixing between two sides bed, that is, internal circulation. In this paper, the movement of tracer particles in the bed is tracked by means of fluorescent large particle tracing and image processing. The motion characteristics of the particles are analyzed, including the movement trajectory, velocity distribution field, velocity statistical distribution, residence time and cycle time, etc. The mixing characteristics of particles in the inner circulating bed are revealed qualitatively. When the fluidization velocity of high speed bed is higher than 4 times of initial fluidization velocity, there is an obvious dead zone in low speed bed, mainly near the partition area and below the circulation opening. The velocity distribution probability of particles is greatly affected by fluidized wind speed, and the characteristics of velocity distribution probability of particle transverse and longitudinal are significantly different. The particle velocity in the low velocity bed is mainly 0-0.2 m / s, while the high velocity bed is distributed in the 0~0.8m/s region. In the low velocity bed, the longitudinal velocity of particles is more than 50% in the negative region, showing downward motion. The size of the circulating flow rate in the particle directly affects the heat transfer mass transfer and reaction efficiency in the reactor so the research on it has been paid more and more attention. In this paper, the particle flow rate at the circulation port is directly tracked and estimated. In order to be easy to measure, the 40mmm diversion tube is installed at the circulation port. The effects of the design parameters such as the size of circulation port, the particle size of bed material, the velocity of fluidization in low speed bed, the velocity of fluidization in high speed bed and the height of static bed on the circulating flow rate are investigated experimentally. The prediction correlation between circulating flow rate and each parameter is also put forward. It is found that the pressure difference is the main driving force of particle flow from low velocity bed to high speed bed. The increase of static bed height, fluidization wind speed and pore opening area is beneficial to the increase of circulating flow rate in particles, and the circulation rate mainly changes in the 10~60Kg m ~ (-2) s ~ (-1) region. However, with the increase of particle size of bed material (Agna B and D particles), the flow resistance at the circulation mouth increases, which is not conducive to the particle flow at the orifice. In addition, the hydrodynamic model of circulating flow in particles is established and the internal circulation flow rate is calculated under the condition of given the design parameters such as the geometric size of bed body, the characteristics of bed material, and the operating parameters such as fluidization velocity of two beds and so on. Compared with the existing experimental data, the validity of the model is verified. Finally, the influence of various parameters on internal circulation is studied by using the model, and the operating range of fluidized wind speed and bed material is given.
【学位授予单位】：东南大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ051.13

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