Development of Acoustically Enhanced Heat and Mass Transfer in Microgravity Fluidized Particle Reactors.

Stephen C. Bates

Thoughtventions Unlimited LLC

40 Nutmeg Lane

Glastonbury, CT 06033


Professor Savas Yavuzkurt, Penn State University

Executive Summary

Microgravity fluidized bed technology is being developed using high intensity sound to enhance the mass and heat transfer to and from the particles in the bed. The environment of space eliminates gravity-induced particle/flow, particle/particle separation and accumulation that limits uniform solid/gas processing on earth in many applications. Resonant acoustical excitation can provide a low-power means for shrinking the size, increasing the processing capacity, or developing novel and high value applications of fluidized beds. Experimental results will be compared with theoretical prediction methods developed during this research and models will be supplied for future design and development purposes. Practical applications are being explored. Small-scale experiments can readily be performed within the constraints of International Space Station ISS capabilities. Experiments on earth are far more difficult; testing over extended periods is appropriate, so that even drop tower simulations have limited value. The research will be of value for many commercial chemical processing and filtration applications, for a variety of space station systems, and for potential development of commercial applications in space.

The basic goals of the effort are: 1) Develop a basic model of the heat/mass transfer process, improving the model using the experimental data developed in the program; develop empirical correlations for a microgravity fluidized bed, 2) design an experiment to demonstrate the characteristics and working principals of acoustically enhanced microgravity fluidized beds, 3) Develop, test, and perform acoustically enhanced microgravity fluidized bed experiments on the International Space Station, 4) Develop applications of the system and process.

Modeling. Detailed simulation modeling will be performed to capture the important characteristics of the effects of acoustic enhancement of fluidized bed reactions. Considering the complexity of creating microgravity fluidized bed-like conditions, fluidized bed mixing, and applied acoustic fields, the simulations will be carried out in several steps. Initially it will be assumed that particles are not entrained by the acoustic field, and a single particle will be assumed to be exposed to an oscillating acoustic field with and without a steady relative velocity component. Fluidized bed correlations for heat and mass transfer for fluidized beds in the earth's gravity field will be modified to create equivalent correlations under microgravity. Correlations will be improved with the help of the ground and space experimental data, together with the relevant important parameters of the new fluidized bed that is being designed. A simplified model of the fluidized bed under microgravity with acoustic fields will be developed using all the information gained from first theoretical simulations and fluidized bed simulations.

Reactor Design. An experimental reactor will be designed and an apparatus module prepared that can be used to test the behavior of an acoustic fluidized bed. The parameter space will be explored and the operating goals will be refined. Modeling will guide reactor and experiment design. The fluidized bed experiments proposed here are designed to test the behavior of µG particle/flow systems in general. Experiments will center on flows with a low volume density of particles. Such systems have demonstrated advantages of high solid surface area exposed inside a volume of flow, as well as low flow losses and low pressure drop. In a standard fluidized bed the particles are held in place by a balance between flow and gravity forces. In a µG environment another technique must be used to maintain the location of the particle field in the device. Consideration of this problem by Dr. Bates led to the concept of a periodically reversed flow, where the particles would be moved alternately in opposite directions, maintaining the same average location. With gas flows this is relatively easy to do, although one must be careful about the end-slugs of gas, which will undergo different conditions than the slugs that have flowed though the field and out of the device. Other options for keeping the particle field centered will also be considered. The particles may be able to be kept in place using a traveling acoustic wave. Magnetic suspension is also a possibility. This has been attempted and is awkward, but may be applicable here since the forces on the particles are so small, as would be the required magnetic fields.

Reactor Module Simulation. Space experiments will be simulated on earth to test experimental design and performance. Three basic types of experiments will be attempted: heat transfer, high mass transfer, and impurity level mass transfer, all of which have important applications. Heat Transfer Experiments: The gas flow drawn from a high pressure reservoir will undergo expansion cooling as it flows into the bed, and will be warmed by passing through the particle field. The entrance and exit temperature of the gas can be measured to determine the heat transfer from the ambient temperature particles. High Mass Transfer: Naphthalene spheres will be exposed to acoustic field in the reactor. Mass transfer from naphthalene spheres is due to sublimation. The average mass transfer rate will be be calculated form the mass change of these particles over time. From this information an average mass transfer coefficient will be obtained with and without acoustic fields and results could be compared. After the flow experiments are complete the spheres can be collected on the flow distributor plate by exhausting the flow for an extended period in that direction. The mass loss of the spheres can be measured by moving the distributor plate and deriving the total inertia of the plate plus particles. Impurity Mass Transfer: If the supply tanks are seeded with a vapor and absorbent particles are used, the vapor concentration after passing through the particle field can be measured very accurately by optical absorbance measurements in the dump tank. Earth Testing Modifications: This experiment is one that is much easier to perform in a µG environment than it is on earth. The ground-based simulation experiments will rely on conventional fluidized bed/DHX technology to experimentally create the particle field and perform the heat/mass transfer experiments. Research will be performed during testing to define the differences expected in µG testing and their effects, emphasizing key aspects such as those of particle size distribution spatial changes and particle field uniformity that are expected to be the major advantages of µG operation.

ISS µG Reactor Experiments and Analysis. G fluidized bed reactor experiments will be performed. Results will be analyzed and used to iterate on experiments. Space applications will be developed. Experiments will be done using a standard ISS experimental module. This module will be built and space tested at TvU. Experiments will be done for a range of residence times, gas flow rates, and particle types. The overall gas flow rate will control residence time using different gas feed rates. Each case will be done with and without high-intensity sound waves to test the change in heat/mass transfer.