Graduation Year

2013

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Chemical Engineering

Major Professor

Aydin K. Sunol

Abstract

Fluidized beds represent a cost effective, scalable technology invaluable to the chemical processing community as part of the global scheme of particle design. However, despite wide spread use and multiple approaches (empirical and mechanistic) to modeling a fluidized bed coating process, operations must be done at different scales due to changes within the balance of phenomenological forces (e.g. gravity, buoyancy, drag, surface tension, viscosity, kinetic energy).

Ammonium nitrate is synthetic compound that has multiple applications such as fertilizer, rocket fuel, and self-cooling applications. Ammonium nitrate has five temperature dependent crystal structures that are accompanied by a change in density. The density fluctuations can result in altered solid properties including cracking and agglomeration (also called caking). The presence of a coating on the ammonium nitrate surface can serve multiple functions including: acting as a barrier between moisture in the air and the ammonium nitrate - thus preventing deliquescence, preventing unwanted agglomeration between ammonium nitrate particles, and potentially providing a means of controlled release.

Polyethylene glycol (PEG) with a molecular weight of 3400 was used as the coating material for this work. PEG 3400 is a biodegradable water soluble polymer that is part of a class of polymers used in cosmetics, pharmaceuticals, soaps, and phase change materials (PCM's). PEG 3400 also has a partially amorphous structure which also makes it an attractive candidate as a coating for ammonium nitrate. The solvent used for PEG 3400 was water.

In this work, a stochastic modeling approach is used to determine the following outputs for both the top spray and Wurster orientation fluidized beds: the coating efficiency, the final particle size distribution, and the coating thickness distribution. Coating efficiencies were determined with isothermal calorimetry and UV/Vis absorbance. With the aid of two tunable parameters, which are functions of the fluidized bed geometry and the fluidization air flow rate, the coating growth kinetics model matches the experimental coating efficiency for both orientations to about ±1%. A sensitivity analysis done for the coating growth model shows that the particle porosity, liquid-solid contact angle, and the simulation control volume height have the most significant impact on the calculated average coating thickness. SEM and AFM analysis proved the ammonium nitrate particle morphology changes from a smooth to rough texture following the coating operation.

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