Transient studies in fast-fluidized beds.

摘要

An important characteristic of fast-fluidized beds (FFBs) is the tendency of the solid particles to aggregate into clusters. Such clusters can strongly affect operational characteristics such as particle holdup, pressure drop, wall heat transfer, and axial mixing. Though the phenomenon has been known to occur in fluidized beds for more than two decades, very little cluster characteristic data is available. Therein lies the motivation for the current work. The primary research goals of this thesis are to (a) gather transient solid concentration data in both upflow and downflow fast-fluidized beds, hence determining cluster properties as functions of operating conditions; and (b) model the dynamics of cluster formation and disintegration using a simple discrete particle model. Needle capacitance probes were used to determine cluster characteristics in a 15-cm I.D. upflow FFB (riser) at Lehigh University, and in a 15-cm I.D. downflow FFB (downer) at Universitat Erlangen-Nürnberg. Experiments were conducted using two particle sizes (70, 120-μ m) at gas velocities warying from 4.0 to 6.6-m/ s. Solid flax was varied from 50 to 120-kg/ m2/s.; Results are presented that indicate the parametric effects of particle size, superficial gas velocity, and solid flux on cluster characteristics, such as solid volume fraction in clusters, cluster duration time, time-fraction of cluster existence, local cluster velocity, cluster length, and cluster flux, in both the riser and downer. For identical flow conditions, average solid density (and solid density in clusters) in the riser was higher than that in the downer. Number-averaged solid volume fraction in clusters was found to be 2.38 times the time-averaged local solid volume fraction. Interestingly, time fraction of cluster existence was found to be independent of flow conditions. Knowledge of clusters and dispersed phase density and velocity allowed us to calculate the relative contribution of solid-flux through cluster and dispersed phase. Both of these generalizations point toward a self-similarity in flow characteristics of fast fluidized beds. It was found that the ratio of local time averaged cluster flux to local total time-averaged solid flux was in the range of 0.3–0.6.; A discrete particle model was developed to examine the effect of particle-particle collision on clustering tendencies in particle systems. The model answers a fundamental question: Can inelastic particle-particle collisions alone give rise to particle clusters? One-dimensional results showed that systems with elastic collisions never induced clusters, while inelastic collisions always led to clustering.