Radial Non-uniformity Index Research on High-density,High-flux CFB Riser with Stratified Injection

A high-density, high-flux circulating fluidized bed (CFB) riser (100 mm in ID and 10.614 m in height) was applied in a wide range of operating conditions (with solid fluxes up to 400 kg/m2s and superficial gas velocities up to 12 m/s) to examine its radial non-uniformity dynamics. The solids holdup was determined through the use of a fiber-optic probe at 11 axial levels. The results indicated that under all operating conditions, the high superficial gas velocity and low solid fluxes maintained a low radial non-uniformity index (RNI). The high-density/flux CFB riser had several unique characteristics, so that the peak of the radial solids holdup profile occurred at a position with r/R=0.8. The RNI and solids holdup at the cross-sectional position had a good logarithmic relationship at the low-density condition (with a mean solids holdup of <0.2), and the RNI decreased when the mean solids holdup exceeded 0.2. Investigation of the dynamics of stratified injection revealed that the feed ratio had an important effect on Gs and on solids holdup distribution. A novel "<" shaped axial solids holdup profile was found. Gs decreased sharply when the up-flow feed ratio exceeded 0.5, and RNI was lowest when the up-flow feed ratio was 1.

Analysis of Flow Dynamics in the High-Flux Gas-Solid Riser Using Trajectory Distances across Attractors Reconstructed from Solid Concentration Signals

The study of the entrance and wall dynamics of a high-flux gas-solid riser was conducted using trajectory distances of the reconstructed attractors from solid concentration signals collected from a 76 mm internal diameters and 10 m high riser of a circulating fluidized bed (CFB) system. The riser was operated at 4.0 to 10.0 m/s gas velocity and 50 to 550 kg/m2s solids flux. Spent fluid catalytic cracking (FCC) catalyst particles with 67 μm mean diameter and density of 1500 kg/m3 together with 70% to 80% humid air was used. Solid concentration data were analyzed using codes prepared in FORTRAN 2008 to get trajectories of the reconstructed attractors and their distances apart. Trajectory distances were found to increase from the centre towards the wall indicating the expansion of the attractor. The probability density function (PDF) of the trajectory distances changes from single peak at the centre to multiple peaked profiles in the wall region. Multiple peaked profiles indicate multifractal flow behaviours. Cumulative distribution functions (CDF) of the trajectory distances changes from single S-shaped at the centre to multiple S-shaped profiles in some locations of the wall region indicating multifractal flow behaviours. The PDF distribution of these distances at the entrance section and in the wall region forms different types of statistical distributions showing differences in gas-solid flow structures in various spatial locations of the wall region and the entrance sections. Most of the distributions at the centre fall under the Gumbel max distribution for all flow development sections of the riser, especially at air velocities of 5.5 m/s and 8 m/s showing uniform flow structures. Further, it was found that increase of the number of the phase space reconstruction embedding dimension increases the trajectory distances between the state vectors leading to the expansion of the attractor.

Gas-solid flow in a high-density circulating fluidized bed riser with Geldart group B particles

We carried out experiments to explore and characterize the gas-solid flow dynamics of Geldart group B particles in a dense circulating fluidized bed riser. By reducing the pressure drop across the solid control valve and increasing the solid inventory in the storage tank, a high solid circulation rate and a solid holdup above 0.075 throughout the riser were simultaneously achieved. At a solid-to-gas mass flux ratio of approximately 105, flow transitioned from fast fluidization to a dense suspension upflow. In the axial direction of the riser, solid holdup had an exponential profile, increasing with increasing solid circulation rate and Jot decreasing superficial gas velocity. From the riser＇s center to its wall, the solid holdup increased markedly, exhibiting a steep parabolic profile. Increasing the solid circulation rate increased the radial non-uniformity of the solid concentration, while increasing the superficial gas velocity had the opposite effect, In our dense circulating fluidized bed riser, Geldart group B particles had similar slip characteristics to Geldart group A particles,

18m高密度循环流化床提升管反应器内气-固流动轴向分布特性

以18m提升管反应器为研究对象,基于压力数据研究了提升管内的气-固流动轴向分布特性。该18m实验装置操作弹性较大,当表观气速为5~9m/s时,颗粒循环速率可以在150~550kg/(m^2·s)之间控制操作。在表观气速为5m/s,颗粒循环速率达到400kg/(m^2·s)以上时,提升管反应器所有轴向高度的平均固含率均在0.1以上,表明整个提升管反应器达到了高密度操作状态。提升管内固含率的轴向分布呈指数型分布特点,与多段式分布特性存在一定的差异。此外,系统研究了操作条件对固含率轴向分布的影响。结果表明,颗粒循环速率的增加或表观气速的降低,均有助于提高提升管内各截面的固含率。

高颗粒通量循环流化床的构型作用

In order to achieve high solids circulation rate(Gs),an idea of coupling a moving bed to the bottom section of the riser of a circulating fluidized bed(CFB) was proposed and tested.The results from the preliminary study demonstrated that the solids circulation rate in the new-structure bed approached 370 kg·m-2·s-1 at superficial gas velocities around 10.5 m·s-1 for sand particles with an average Sauter mean size of 378 μm.This study was devoted to further justifying the effects of the coupled moving bed by performing comparative studies in two CFBs with conventional configurations.It was shown that the pressure at the riser bottom and the realized solid circulation rate were only about 15 kPa and 230 kg·m-2·s-1 in the two conventionally configured CFBs,obviously lower than 25 kPa and 370 kg·m-2·s-1 in the moving bed coupled CFB.These verified that the coupled moving bed increased the force driving particles form the particle recycling side into the riser.The study further tested the effect of a few specially designed riser exit configurations,revealing that a smooth riser exit could facilitate solids circulation to increase the solids circulation rate.