(29a) An Innovative Method Using Magnetic Particle Tracking to Measure Solids Circulation Rates in a Spouted Bed | AIChE

(29a) An Innovative Method Using Magnetic Particle Tracking to Measure Solids Circulation Rates in a Spouted Bed

Authors 

Patterson, E. E. - Presenter, Waynesburg University
Daw, C. S. - Presenter, Oak Ridge National Laboratory
Finney, C. E. A. - Presenter, Oak Ridge National Laboratory


Introduction

Spouted fluidized beds are used extensively for many types of solid-fluid reaction applications where uniform inter-phase contacting is important. While various commercial applications have been developed, the basic hydrodynamics and mixing patterns are not well characterized. Most past studies of spouted bed dynamics have relied on visual observation and pressure measurements to determine hydrodynamic parameters such as minimum spouting velocity and spout height. Studies of bulk solids circulation, however, have been very limited because of the difficulty of directly observing solids motion in these beds. In three-dimensional beds, it is usually impossible to track the motion of individual particles due to shielding by surrounding neighbors. For this reason, two-dimensional transparent beds and beds with semicircular cross-sections (with a flat viewing window) have been used to enhance optical access. While providing some information, visual methods typically introduce wall effects and anomalous flow patterns that do not match the behavior in the three-dimensional beds. Other non-visual techniques have been used to measure solids motion, including piezoelectric probes and computer aided radioactive particle tracking (CARPT). These techniques, however, also have limitations. In-bed probes can disturb solids and gas flows from their normal condition leading to uncertainty in the measurements. Radioactive particle tracking is less intrusive but requires expensive specialized equipment and access to radioactive isotopes that involve significant safety issues.

Experimental Materials and Methods

The particle tracking method we describe here is suited for use in three-dimensional beds, does not involve the introduction of flow-disturbing probes, and uses safe, inexpensive magnetic tracers thus avoiding the issues associated with past methods. Our particle tracking method utilizes neodymium magnets embedded in plastic spherical particles and externally positioned Hall-effect magnetic field detectors to continuously locate the position of a tracer particle over time. 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 studied the solids flow behavior of various spoutable materials in a 39-mm-diameter spouted bed with a 45-degree conical bottom. Four externally mounted magnetic field detectors located at the level of the bed surface tracked our tracer particles. We have developed semi-empirical relationships to relate tracer particle position with the magnetic field measurements. The vertical position of the tracer versus time is calculated and circulation frequencies and solids circulation rates determined from this parameter. A range of particle properties, bed depths, and superficial velocities were studied in our 39-mm bed.

Results and Discussion

Our experiments show that magnetic particle tracking in spouted beds is indeed feasible and can provide quantitative measurements of bulk particle circulation rates. We have correlated circulation rates for the limited range of parameters studied so far. With further testing and improvements in the technique, we expect to develop a more general correlation of solids circulation in spouted beds and provide useful data to develop and validate spouted bed models.