Heat and Moisture Migration within a Porous Urea Particle Bed

In this thesis, two types of urea products are investigated, i.e. Georgia urea and Terico urea. The rough external surface and internal pore structure of each particle makes the total surface area exposed to water much larger than similar smooth and solid spherical particles. Although Georgia urea has higher external surface area than Terico urea, the latter type has larger total surface area and internal pore volume. For both Terico urea and Georgia, the internal surface area dominates the water sorption process but the external moisture sorption of Georgia urea is more important than that of Terico urea.;All the water vapor interaction experiments were carried out with air flow through a test bed because it shortens the duration of each experiment to a few hours in most cases. A series of experiments with step changes in inlet air temperature and humidity for air flow through a urea bed indicated that the measured outlet air temperature and humidity responses, each at a specific air flow rate, reveals a typical exponential or transient time change that can be characterized by a time constant. After formulating the theoretical problem for step changes in the inlet properties, the analytical solutions showed that the time constants of outlet response to whether a temperature step change or a humidity step change are functions of the convection coefficient and air velocity. The predicted outlet air temperature is determined by only one time constant for a temperature step change while it is determined by these two time constants for a humidity step change.;A new test cell with sampling test ports was developed to measure the transient moisture uptake of a urea particle bed and its distribution at any time without any interruption of the experiment. A novel particle sampling device, modified from a syringe and pistons, was designed to minimize the particle exposure to ambient air during the moisture content determination using a Karl Fischer titrator. Data from two continuous cyclic step changes in the inlet flow with relative humidities between 4% and 70% at room temperature showed a hysteresis in the isothermal moisture content for only the first cycle. After the second sorption-desorption cycle, the hysteresis disappeared. This implies that the internal pore and particle surface geometry changes are very slow after the first cycle.;A new theoretical porous media model was developed for a coupled heat and moisture transport process when humid air flowed uniformly through a large test bed in two coupled computational domains: internal domain (i.e., the particle phase) and the external domain (i.e., the interstitial air space). The moisture migration in two computational domains included: water vapor diffusion inside each particle, and water vapor convection and diffusion in the interstitial air space in the urea particle bed. For energy transport, the temperature was assumed to be uniform inside each particle, but heat convection and conduction between the urea particles and the interstitial air outside particles occurred throughout the bed. Both heat transfer and mass transfer in internal domain and external domain were coupled by the heat and mass convection at the gas-particle interface. The numerical simulation was compared with the data of moisture uptake and showed good agreement implying that the internal moisture diffusion that dominates the moisture uptake process is a very slow process.;These above experimental, theoretical and numerical research studies provide a set of information on how urea particles adsorb or desorb moisture from or to ambient air on the external and internal pore surface, which offers a useful suggestion for urea caking prevention and is also a first and necessary step to the study of further caking formation and strength. (Abstract shortened by UMI.).