Urea is an important nitrogen fertilizer for plant nutrition, but is very susceptible to moisture sorption and caking even at low moisture contents, e.g. 0.25% w/w. When urea particles adsorb moisture followed by drying, crystal bridges form between urea particles. For particles in a bed, this process is called caking. Cakes in stored urea cause a degradation of its quality and value. Investigations of the moisture absorption in beds of manufactured urea particles and adsorption on the external and internal surfaces of urea particles are a necessary step if engineers are to recommend procedures to reduce caking and control inventories. Research on moisture adsorption and cake strength of urea fertilizer has not been sufficiently explored. Only recently have researchers started to devise tests to investigate the crystal bonding between two urea particles. Prior to this research, investigations of the moisture interactions in beds of urea were nearly non-existent. This thesis presents experimental, theoretical and numerical methods to investigate the coupled heat and moisture transfer processes in a bed of urea particles while the bed is exposed to ambient air with changing temperature and humidity.<p> Urea particles are nearly spherical with uniform particle size distribution. The particle size, its internal pore structure and rough crystalline external surface depend on the manufacturing process. In this thesis, two types of urea products are investigated, i.e. prill Georgia urea and granular 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.<p> 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.<p> 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.<p> 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.<p> 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.

Urea is an important nitrogen fertilizer for plant nutrition, but is very susceptible to moisture sorption and caking even at low moisture contents, e.g. 0.25% w/w. When urea particles adsorb moisture followed by drying, crystal bridges form between urea particles. For particles in a bed, this process is called caking. Cakes in stored urea cause a degradation of its quality and value. Investigations of the moisture absorption in beds of manufactured urea particles and adsorption on the external and internal surfaces of urea particles are a necessary step if engineers are to recommend procedures to reduce caking and control inventories. Research on moisture adsorption and cake strength of urea fertilizer has not been sufficiently explored. Only recently have researchers started to devise tests to investigate the crystal bonding between two urea particles. Prior to this research, investigations of the moisture interactions in beds of urea were nearly non-existent. This thesis presents experimental, theoretical and numerical methods to investigate the coupled heat and moisture transfer processes in a bed of urea particles while the bed is exposed to ambient air with changing temperature and humidity.<p> Urea particles are nearly spherical with uniform particle size distribution. The particle size, its internal pore structure and rough crystalline external surface depend on the manufacturing process. In this thesis, two types of urea products are investigated, i.e. prill Georgia urea and granular 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.<p> 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.<p> 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.<p> 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.<p> 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.

<p> Brønsted acid zeolites can be synthesized in a wide range of topologies, each characterized by diverse void sizes, shapes, and micropore connectivity. The location of Brønsted acid sites (H+-sites) within microporous voids of different size and shape, and the relative proximity of H+-sites influences their reactivity. Additionally, the diffusion of reactant and product molecules through a given zeolite topology depends on micropore size, tortuosity, and connectivity. The coupled influences of reaction kinetics and intrazeolite reactant and product diffusion govern rates and selectivity for a plethora of zeolite-catalyzed reactions and underlie the well-established effects of “shape-selectivity”. The independent effects of reaction and diffusion on rates and selectivity for a given reaction are often obfuscated by concomitant changes in the zeolite properties governing diffusion (e.g., crystallite size) and reactivity (e.g., H+-site density or proximity) in zeolite materials synthesized with conventional methods. Herein, we develop synthetic methods to decouple H+-site density, proximity and crystallite size in medium-pore, 10-membered ring (10-MR) zeolites, and evaluate the independent effects of these material properties on the kinetic and transport phenomena that govern propene oligomerization catalysis. </p> <p>Among synthetic methods to influence H+-site proximity in zeolites, varying the charge-density and ratio of structure directing agent (SDA) cations that compensate anionic charges in frameworks at Al centers has been reported to influence H+-site proximity in MFI and CHA zeolites of fixed H+-site density. Changes in H+-site proximity can be evaluated using Co2+ cations to selectively titrate and quantify subsets of proximal H+-sites (H+-site pairs); conditions to perform such titrations were identified for MEL zeolites. The fraction of paired H+-sites changed concurrently with changes in framework Al content in MEL zeolites synthesized using a single organic SDA (OSDA), tetrabutylammonium hydroxide (TBA+). Synthesis of MEL with mixtures of TBA+ and Na+ as an inorganic SDA (ISDA), at fixed total SDA and Al content, allowed the fraction of paired H+-sites to be systematically varied in MEL zeolites of fixed H+-site density, reflecting changes in the location and quantity of charge-balancing SDAs with Na+/TBA+ ratio. The energetic favorability of SDA occlusion in MEL was also evaluated with density functional theory (DFT). In contrast to MEL, occluded SDA content in TON zeolites crystallized with varied OSDA (1,6-diaminohexane, or 1,8-diamooctane) and K+ content, at fixed total SDA content, was invariant with K+/OSDA ratio, reflecting a different mechanism of SDA occlusion in TON. These findings provide an approach to influence H+-site pairs in 10-MR zeolites of fixed H+-site density and demonstrate the dependence of SDA occlusion on zeolite topology.</p> <p>The independent influences of H+-site and crystallite properties on rates and selectivity of propene oligomerization to heavier alkenes in a representative medium-pore zeolite topology (MFI) were explored by interrogating suites of samples crystallized with independently varied H+-site density (0.3–5.7 H+/u.c.), proximity, and crystallite size (0.03–2.65 μm) over a wide range of reaction conditions (483–523 K, 7–615 kPa C3H6). Dimerization rates (per H+) decreased with increasing crystallite size among MFI materials synthesized with fixed H+-site density (0.3 or 1.3 H+/u.c.), revealing the strong and ubiquitous influence of intrazeolite diffusion limitations on measured dimerization rates. Weisz-Prater criterion analyses, in conjunction with dimerization rate transients upon step-changes in reaction conditions, indicate that these intrazeolite diffusion limitations arise from a product-derived organic phase occluded within zeolitic micropores during propene oligomerization catalysis, which restricts intrazeolite diffusion by lowering the effective diffusivities of propene and product alkenes. This occluded organic phase becomes heavier in composition at higher propene pressures and lower reaction temperatures, which favor chain growth over β-scission, resulting in more severe intrazeolite diffusional constraints. The composition of the occluded organic phase was also found to depend on H+-site density in MFI zeolites. Rate constants (per H+) of dimerization and trimerization were higher on MFI samples of dilute H+-site density, resulting in faster growth of heavier oligomer products and consequently lower effective diffusivities compared to MFI samples of higher H+-site density. The convoluted influences of reaction and diffusion on measured propene oligomerization rates result in apparent reaction orders that deviate from the first-order dependence of rates on propene pressure expected in the limit of strict kinetic control. Accounting for the coupled influences of reaction and diffusion on propene oligomerization rates and the influence of H+-site density on intrazeolite diffusion, rationalizes contradictory conclusions among prior reports about the dependence of oligomerization rates on H+-site density, proximity, and crystallite size, which did not identify or consider the influences of intrazeolite diffusion in their interpretations of rate data. </p> <p>Finally, we explore the consequences of zeolite pore size and connectivity for reactivity and intrazeolite diffusion during propene oligomerization by interrogating H-zeolites of different topologies. Intrazeolite diffusional constraints are imposed by an occluded organic phase and influence dimerization rates among medium-pore zeolite topologies (MFI, MEL, TON), but such constraints are alleviated on large-pore zeolite topologies (FAU, MOR, *BEA), reflecting the slower growth and faster diffusion of heavy oligomer products in large-pore zeolites. Among medium-pore zeolites, the composition of the occluded organic phase, and consequently the effective diffusivities of propene and product alkenes, is influenced by void size. Analysis of product selectivity on zeolites of different pore size and connectivity (TON, MOR, MFI) reveals that TON restricts the growth of heavier oligomer products, resulting in effective diffusivities that are higher on TON compared to MFI, and are relatively invariant with propene pressure and H+-site density. Together, the findings herein demonstrate the ability of slow-diffusing products to impose intrazeolite diffusional constraints on other products during alkene oligomerization catalysis, and reveal the critical influence of reaction conditions, H+-site density, and micropore size on the composition of this occluded organic phase, and consequently intrazeolite diffusional constraints. Ultimately, this work demonstrates how kinetic studies performed on well-defined zeolite materials can reveal important changes in reaction and diffusion phenomena, which are otherwise inextricably convoluted, and provides a framework through which such effects can be assessed for other zeolite-catalyzed molecular chain-growth reactions. </p>