High speed imaging of particle flow fields in CFB risers

Abstract

Particle flows of high particle concentration are important in many fields, including chemical processing, pharmaceutical processing, energy conversion and powder transport. Circulating fluidized beds (CFB) are widely employed in industry because they enhance reaction rates and heat transfer through rapid mixing of particles at high particle concentrations and high particle flow rates. However, despite decades of research and industrial application, the real time behavior of particle flow fields in CFB's is still not well understood. One of the reasons is that experimental data is difficult to acquire in such harsh, opaque environments. In this study, a new high speed particle imaging velocimetry (high speed PIV) technology, developed by the USDOE National Energy Technology Laboratory (NETL), is applied to observe and measure the real time behavior of individual particle motion inside the risers of CFB's. High speed PIV data acquired in three pilot scale CFB units at two laboratories: two CFB's with 0.305 m diameter risers and one CFB with a 0.2 m diameter riser. The high speed PIV system records high speed videos of particle motion with excellent spatial and temporal clarity. The high speed videos are analyzed to measure the concentration and the two-dimensional motion (velocity and trajectory) of individual particles. Data sample rates for velocity vectors are in the range of 0.1 to 3 million vectors per second thereby providing full resolution of the temporal domain of particle velocity. To see and measure particle motion inside the CFB risers at high particle concentrations, a custom borescope was inserted into the risers. The CFB risers were operated over a wide range of industrially relevant conditions: superficial gas velocities from 6.5 to 18.3 m/s with solid fluxes from 20 to 400 kg/m2/s. The particles used in the CFBs included fluid cracking catalyst (FCC) with a mean diameter of 70 μm, high density polyethylene (HDPE) with a mean diameter of 750 μm, and glass beads with mean diameters of 170 and 650 μm. High speed videos and high speed PIV data enabled careful study of the real time behavior of gas-particle flow fields in CFB risers. In all of the CFBs of this study, one or more “jets” of high speed gas were observed at any time in the CFB risers. The jets move around the riser and appear to wander from one location against the riser wall to another. The jets have width range of 1/10 to 1/2 of the riser diameter. When a jet moves away from an area, the void is immediately filled with large clusters of particles. The clusters have sizes up to several riser diameters and contain significant percentages of the total particle flow. Clusters reduce mixing and interaction of particles with the transport gas, and therefore may inhibit reaction rates. Shearing of the clusters by high speed jets gives rise to cluster shapes that are either undulating or in the form of long, thin vertical strands which are often called streamers. The well known core-annulus concentration profile does not exist in real time, but rather is a long time averaged phenomenon. The data and insight from this work should be valuable for design and operation of risers, and for development of computational fluid dynamic (CFD) models of riser flow fields.