(344f) Magnetic Particle Tracking in Spouting and Bubbling Fluidized Beds

 J.S. Halow* (Separation Design Group, Waynesburg, PA 15370),

Stuart Daw (Oak Ridge National Laboratory, Knoxville, TN 37932)

*halow@windstream.net Introduction

Fluidized beds are extensively used for many types of fluid-solid contacting and reaction applications.  Spouted beds are  used where uniform inter-phase contacting is important in applications such as drying and coating.  Bubbling beds are often used where uniform temperature and good contacting are important.   Fluidized beds have been extensively studied for applications which use a single type of bed material,  but the basic hydrodynamics and mixing patterns in systems with disparate types of bed particles are not well characterized.  Some biomass gasification processes, for example, use bubbling beds with a fine sand bed into which larger biomass particles are fed.  Biomass material usually comprises a small fraction of the bed so the dynamics involve a few coarser lighter particles circulating in a finer denser fluidized bed.  Spouted bed applications can sometimes involve mixed particle sizes and densities.   Both types of systems can lead to segregation and sometimes defluidization.. We have used a magnetic particle tracking to measure particle motion and segregation behavior in both spouting and bubbling beds. 

Experimental Materials and Methods

We previously reported (1,2) on a new magnetic particle tracking method in 39 and 55 mm three-dimensional bed fluidized with air at ambient conditions.  Safe inexpensive magnetic tracers and detectors are used thus avoiding the issues associated with past particle tracking methods.  In this study, we used neodymium magnets embedded in plastic and wooden spherical particles as flow tracers and externally positioned magnetic field detectors to continuously locate the position a tracer particle.  The method takes advantage of the tendency of the tracer particles to align their magnetic axes with the earth’s magnetic field like a compass needle.  Proper positioning of the probes can take advantage of this tendency, which simplifies the data analysis.  We also collected high-definition slow-motion videos of the bed surface to observe top segregated tracer motion.

In the spouting bed size studies, binary mixtures of 0.7 to 3 mm glass beads were used.  For the density difference tests we used mixtures of glass bead and zirconium oxide (density ratio of ~2).  In both studies, various ratios of materials were studied over a range of velocities.  A bead with an imbedded magnet having a composite density equivalent to the glass bed material was used to track bed motion along with visual observation of the bed surface.  In the bubbling bed tests 200 micron glass beads were studied over a range of velocities with magnetic tracers with densities ranging from 0.53 to 1.2 g/cc.  Single tracer particles were dropped into the beds and their positions determined over a 5 minute run time.  Tracer positions were calculated using an algorithm we previously reported on.  Statistical analysis on the time series position data was used to characterize the tracer behavior.  Autocorrelation analysis was particularly useful to determine periodic patterns of solids flow.

Results and Discussion

In the spouted bed tests, segregation occurred when the size ratio was 3/1 or greater.  This segregation exhibited a hysteresis pattern with velocity.  With a density ratio of 2.2 (zircon and glass) little segregation occur but segregation did occur with higher density ratio mixtures. 

We previously reported on the position and segregation tendencies of light and heaver tracer particles in the fine glass bead . In this report we present our recent times series analysis of the bubbling bed tests data.  Autocorrelation analysis of vertical position provides characteristic time scales for solids circulation and bubble growth rated.  Auto correlation of radial and angular positions reveal other characteristic times and point out the lack of angular symmetry in bubbling beds and the need to use 3-dimensional modeling to achieve realistic results.

1. Patterson et.al., An Innovative Method Using Magnetic Particle Tracking to measure Solids Circulation in a Fluidized Bed, Ind. Eng. Chem. Res. 2010, 49, 5037-5043.
2. Halow et. Al. Observed Mixing Behavior of Single Particles in a Bubbling Fluidized bed,  submitted

Abstract submitted for presentation at the 2012 Annual A.I.Ch.E. meeting in Pittsburgh, PA