Numerical Simulation of Gas-Solid Dynamics in a Circulating Fluidized-Bed Riser with Geldart Group B Particles

Circulating Fluidized-Bed (CFB) Risers with Geldart Group B Particles Have Found Significant Application in Combustion Reactions. the Present Work Attempts to Study the Solids Flow Dynamics in a CFB Riser that is Operated with Group B Particles, using Computational Fluid Dynamics (CFD) Techniques. the Key Feature in the Present Study is that the Various Closure Schemes in the CFD Model Have Been Evaluated Against Data from Non-Invasive Experimental Techniques: Computer Automated Radioactive Particle Tracking (CARPT) for Solids Velocity Field and Computed Tomography (CT) for Solids Holdup. Since Solids Flow in a Riser is Multiscale in Character, in Addition to the Measured Averaged Solids Velocity Profiles and Solids\u27 Fraction Profiles in the Experimental Section, Mean Granular Temperature Profiles Have Also Been Compared. Two Flow Regimes (Viz., Fast Fluidization and Dilute Phase Transport) Have Been Considered in This Study. © 2007 American Chemical Society

A Comparison of Alternating Minimization and Expectation Maximization Algorithms for Single Source Gamma Ray Tomography

Lange and Carson (1984 J. Comput. Assist. Tomogr. 8 306-16) Defined Image Reconstruction for Transmission Tomography as a Maximum Likelihood Estimation Problem and Derived an Expectation Maximization (EM) Algorithm to Obtain the Maximum Likelihood Image Estimate. However, in the Maximization Step or M-Step of the EM Algorithm, an Approximation is Made in the Solution Which Can Affect the Image Quality, particularly in the Case of Domains with High Attenuating Material. O\u27Sullivan and Benac (2007 IEEE Trans. Med. Imaging 26 283-97) Reformulated the Maximum Likelihood Problem as a Double Minimization of an I-Divergence to Obtain a Family of Image Reconstruction Algorithms, Called the Alternating Minimization (AM) Algorithm. the AM Algorithm Increases the Log-Likelihood Function While Minimizing the I-Divergence. in This Work, We Implement the AM Algorithm for Image Reconstruction in Gamma Ray Tomography for Industrial Applications. Experimental Gamma Ray Transmission Data Obtained with a Fan Beam Geometry Gamma Ray Scanner, and Simulated Transmission Data based on a Synthetic Phantom, with Two Phases (Water and Air) Were Considered in This Study. Image Reconstruction Was Carried Out with These Data using the AM and the EM Algorithms to Determine and Quantitatively Compare the Holdup Distribution Images of the Two Phases in the Phantoms. When Compared to the EM Algorithm, the AM Algorithm Shows Qualitative and Quantitative Improvement in the Holdup Distribution Images of the Two Phases for Both the Experimental and the Simulated Gamma Ray Transmission Data. © 2008 IOP Publishing Ltd

Flow Mapping of Gas-Solid Riser

Gas-Solid Risers Are Extensively Used in Many Industrial Processes for Gas-Solid Reactions (Coal Combustion and Gasification, Etc.) and for Catalytic Gas Phase Reactions (Fluid Catalytic Cracking, Butane Oxidation to Maleic Anhydride, Etc.). Risers Pose Considerable Challenges Because It is Not Yet Possible to Produce Ab Initio Predictions of their Complex Fluid Dynamics, Which Impairs Reactor Modeling. in the Chemical Reaction Engineering Laboratory (CREL) at Washington University, Computer Automated Radioactive Particle Tracking (CARPT) Has Been Developed and Extensively Utilized for Measuring in a Non-Invasive Manner the Velocities, Turbulent Parameters and 3D Flow Mapping in Multiphase Flows. in Tis Study, Solids Flow Dynamics Has Been Investigated in Cold-Flow Circulating Fluidized Bed Risers using Non-Invasive Flow Techniques - CARPT and Computed Tomography (CT). Comparative and Symbiotic Analyses of the Results Obtained from CARPT and CT Have Been Used to Develop a Coherent Picture of the Solids Flow Field

Measurement of overall Solids Mass Flux in a Gas-Solid Circulating Fluidized Bed

It is Important to Know the overall Solids Flux and Solids Cycle Time Distribution (CTD) to Properly Characterize Circulating Fluidized Bed (CFB) Systems. Although Conceptually Simple, Estimating the Solids Mass Flux in These Closed Loop Systems, Either Via Experiments or by using a Model, is a Non-Trivial Problem. in the Present Work, a Non-Invasive in Situ Technique is Applied, Which is Sensitive, and Can Be Used on a Large-Scale Circulating Flow Systems. the Technique is based on Estimating the Solids Velocity and Volume Fraction Distribution in a Section (Downcomer) of the CFB Loop. Solids Velocity is Obtained by Tracking a Single Radioactive Particle as Tracer, using a Two Detector Setup, and the Volume Fraction Distribution is Obtained by Γ-Ray Densitometry Measurements. in the Downcomer Section of a 2-In.-Diameter Column, the Solids Flow Was Found to Be Close to a Moving Packed Bed Condition, with Minimal Random Fluctuations in the Solids Volume Fraction. Hence, Only a Negligible Error is Incurred by using the Cross-Sectionally Averaged Solids Velocity and Holdup Values for the Estimation of the Solids Mass Flow. a Calibration Curve Was Established for the overall Solids Mass Flux as a Function of Superficial Gas Velocity, and the Mass Flux Values Compared with Those Obtained from a Timing and Weighing Method Were within 4%. © 2004 Elsevier B.V. All Rights Reserved

Effect of inlet boundary conditions on computational fluid dynamics (cfd) simulations of gas-solid flows in risers

The effect of type of inlet conditions on the predictions of EulerianEulerian simulations of a circulating fluidized bed riser has been investigated in both 2D and 3D domains. The 2D simulations were conducted using 3 different inlet configurations: (A) solids entering in radial direction from a two-sided inlet and gas entering axially from the bottom inlet, (B) solid entering axially from a two-sided bottom inlet near the wall and gas entering axially from a bottom inlet at the center, and (C) gas phase entering axially from a two-sided bottom inlet near the wall and solids entering axially from a bottom inlet at the center. In 2D simulations, it was found that both time-averaged axial velocity and solids volume fraction radial profiles were functions of the inlet kinetic energy as well as gas solidmixingpatterns at the inlet. Whereas 2D simulations using boundary conditions A and C showed significant deviations from experimental profiles, the boundary condition B as well as full-scale 3D simulations gave reasonable agreements with experimental observations

Local time-averaged gas holdup in fluidized bed reactor using gamma ray computed tomography technique (CT)

Many Invasive and Non-Invasive Techniques Have Been Used to Analyze the Hydrodynamics of Fluidized Beds. in This Study, the Effect of Superficial Gas Velocity and Bed Particle Density on the Hydrodynamics of Gas–solid Fluidized Beds Was Investigated by using a Cylindrical Plexiglas Fluidized Bed Column, 14 Cm in Diameter. Air at Room Temperature Was Used as the Fluidizing Gas and Two Different Geldart Type-B Particles Were Used: Glass Beads and Copper Particles with Material Densities of 2.5 and 5.3 G/cm3, Respectively, with the Same Size Particle, 210 µm. to Measure the Time-Averaged Cross-Sectional Gas and Solid Holdup Distribution, Gamma Ray Computed Tomography Was Used for the First Time as a Non-Invasive Technique Instead of using X-Rays (Due to the Height Attenuation of the Copper Particles). the Results Show that Gas Holdup Increases by Increasing the Superficial Gas Velocity, and Decreasing the Particle Density Increases the Gas Holdup in the Bed