气固逆流下行床流动结构的实验研究
文献类型：学位论文
| 作者 | 李正杰 |
| 学位类别 | 硕士 |
| 答辩日期 | 2012-05-21 |
| 授予单位 | 中国科学院研究生院 |
| 导师 | 林伟刚 ; 宋文立 |
| 关键词 | 气固逆流 下行床 流动结构 团聚物 |
| 其他题名 | Investigation on Gas-solid Flow Structure in a Countercurrent Downer |
| 学位专业 | 化学工程 |
| 中文摘要 | 气固逆流下行床概念的提出，最早源于郭慕孙院士在20世纪60年代提出的广义流态化理论。广义流态化包括三种操作状态：颗粒流体同向向上，颗粒流体同向向下，颗粒向下、流体向上。目前对于广义流态化的气固流化床研究主要集中在提升管（气固并流上行）和下行床（气固并流下行）两方面。提升管具有较高的气固接触效率，气固通量大等优点，但其径向流动结构具有严重的非均匀性并存在较大的轴向返混。气固并流下行床气固返混较小，气固接触时间较短，径向流动结构的均匀性较高，但是存在颗粒浓度低的缺点。在下行床中使气固逆流接触，可以强化气固两相间的传质与传热，气固的逆流接触可以提高床层的颗粒浓度。同时，气固逆流下行床气体阻力低，节省能耗，并且用于煤热解等反应时可以降低尾气温度。目前关于气固逆流下行床的相关研究还很少。本文设计、搭建并调试了一套高7m，内径90mm的气固逆流下行床冷态装置。颗粒进料方式采用螺旋给料，可以实现给料速率的稳定和颗粒循环速率Gs的精确控制。在逆流测试段下端布置气体分布器，气体从下端给入，经过料腿的预分布作用达到一定的均匀程度再逆重力向上流动。本文采用光纤浓度和速度测量仪研究了气固逆流下行床中不同截面高度上颗粒浓度和颗粒速度的径向分布情况。考察了截面平均颗粒浓度和截面平均颗粒速度沿轴向的变化和操作条件对其影响。研究结果表明：气固逆流下行床中，颗粒的流动在轴向上分为起始段和完全发展段，起始段中颗粒浓度和速度径向分布随轴向不断变化，完全发展段颗粒浓度和速度径向分布趋于稳定。研究发现，气固逆流下行床中存在颗粒团聚现象，随着气固流动轴向的发展，颗粒聚团显著的区域由中心转移至边壁。在操作气速范围内，颗粒平均停留时间和完全发展段截面平均颗粒浓度随着逆流表观气速的增大均近似呈线性增加。局部颗粒浓度的径向分布是不均匀的。在操作范围内提高表观逆流气速Ug和增大颗粒循环速率Gs可以增加床层颗粒浓度。受颗粒团聚物的影响，截面平均颗粒浓度在颗粒入口附近呈现先升高后下降的趋势。受单管入料的影响，在颗粒入口附近中心处颗粒浓度较高，边壁处颗粒浓度较低，在完全发展段，稳定后的颗粒浓度径向分布为中心（r/R<0.6）稀且分布较平坦，靠近边壁（r/R>0.6）颗粒浓度升高，且在边壁处颗粒浓度最高。通过实验数据拟合，得到了预测完全发展段局部颗粒浓度的经验公式。在完全发展区，颗粒速度径向分布呈中心和边壁低，约在r/R=0.85附近颗粒速度最大。不同径向位置的局部颗粒速度沿轴向的变化趋势不同，边壁区域（r/R>0.622）颗粒速度沿轴向单调递增，而中心区域（0 |
| 英文摘要 | Countercurrent downer was first proposed by Mooson Kwauk in 1960s. It is a branch of the theory of generalized fluidization, which includes gas–solids co-current upflow systems (riser), gas–solids cocurrent downflow systems (cocurrent downer) and gas upward–solids downward countercurrent systems. There is already considerable research on riser and cocurrent downer, but limited studies on countercurrent downer have been conducted in the past few years. Although riser has many advantages such as high contact efficiency between gas and solid and high gas-solids flux, it suffers from severe axial solids backmixing and macrosegregations of gas and solids due to the nonuniform gas and solids flow structure in the radial and axial directions. Cocurrent downer has many advantages such as less gas and solids backmixing, short contact time and uniform residence time distribution compared with riser, but it suffers a serious shortcoming: very low volumetric concentration of solids in the bed. It is possible to enhance the mass transfer and heat transfer between gas and solid in fluidized bed by solids moving against the gas flow. Gas upflow and solids downflow countercurrent fluidized bed is also a way to overcome the limitation of low gas–solids suspension density in concurrent downer. Air resistance is relatively low in gas-solid countercurrent downer, reducing energy consumption. When used for coal pyrolysis, exhaust air temperature could be lowered by the entering of relatively low temperature coal. A 0.09m i.d., 7m high gas upflow–solids downflow fluidized bed was designed, constructed and tested in this work. The solids feeder system consists of a screw feeder, which enables a steady feed rate, thus an accurate control of solids circulation rate is possible to reach by calibration before the experiment. Air feed which was introduced at the bottom of the test column flows upward after pre-distribution. Radial profiles of solid concentration and particle velocity at different heights were measured using fiber optical probe. Flow structure in countercurrent downer was analyzed under different operating conditions. The results show that: Axial flow structure of the countercurrent downer consists of an initial section and fully developed section. Radial profiles of solid concentration and particle velocity change with axial position in initial section and become steady in fully developed section. It was found that clusters exist in countercurrent downer. The phenomenon of significant clustering shift from the central area near the solid entrance to the area near the wall in developed region. Within the range of operation conditions, increasing gas velocity at a given solids flux leads to a nearly linear increase in mean residence time of particles and cross-sectional average solid concentration in developed region. Radial profiles of solid concentration is nonuniform in countercurrent downer. In the solid entrance region, solid concentration is higher in the center and lower in the wall region. In the developed region, solid concentration is relatively uniform in a core region of r/R<0.6 but increases monotonically with the radius in a annular region of r/R>0.6 with the densest peak right at the wall. Cross-sectional average solid concentration increases with increasing gas velocity and solids circulation rate. Cross-sectional average solid concentration increases and then decreases near the solid entrance. Particle velocity is lower both in the center and near the wall in the fully developed region. And a maximum particle velocity exists at around r/R=0.85. Local particle velocity in the region near the bed wall (r/R>0.622) increases along the column. While local particle velocity in the central region (0 |
| 语种 | 中文 |
| 公开日期 | 2013-09-25 |
| 源URL | [http://ir.ipe.ac.cn/handle/122111/1855]  |
| 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 | 李正杰. 气固逆流下行床流动结构的实验研究[D]. 中国科学院研究生院. 2012. |
入库方式： OAI收割
来源：过程工程研究所
浏览0
下载0
收藏0
其他版本
除非特别说明，本系统中所有内容都受版权保护，并保留所有权利。