No description available.

No description available.

The confined impinging jet reactor gives efficient mixing performance as required for fast reactions. In this work the mixing performance of CIJR is characterized through three measures: estimates of the energy dissipation, micromixing efficiency based on the yield of a homogeneous (iodide-iodate) reaction and particle size resulting from a heterogeneous (iron oxide) precipitation reaction. Whereas product yield and energy dissipation are used to test operational robustness of CIJR, iron oxide model system is used to study the effect of feed flow rate (mixing) and reactant concentration on precipitate agglomerate size. Mixing and concentration effects on nucleation, particle growth and particle agglomeration are tracked to understand the agglomeration process. Various types of stabilizers and additive concentrations to limit particle agglomeration are also tested. Effects of in situ and post-reaction sonication on agglomerate size are also investigated. Efforts are made to determine variations in mixing efficiency the operational robustness of the scale-up (2X and 4X) geometries. Also efforts are made to identify scaling parameters and the limit on geometric scale-up for good mixing performance. Energy dissipation is found to vary between 20 W/kg and 6800 W/kg in CIJR and decreases on scale-up at constant Reynolds number. The operation of the CIJR and the scale-up geometries is robust to changes in flow rate, exhibiting stable performance up to 30% difference in inlet flow rates. Reliable mixing performance is obtained until 2X scale-up, while at low flow rates, the jets fail to impinge in 4X scale-up, and sometimes failing to fill the reactor volume. Iron oxide primary and agglomerate particles are seen to vary with flow rate and reactant concentrations. Largest primary particles (and smallest agglomerates) are obtained at high flow rates and high reactant concentrations, which indicate to size dependent agglomerative tendency of the primary particles. Stabilizers added in situ see limited success. Post-reaction sonication is helpful in dispersing soft agglomerates, but in situ sonication shows no significant reduction in agglomerate size with or without stabilizer. Primary particles are understood to agglomerate due to collisions induced by Brownian motion, simple shear and velocity fluctuations in turbulent flows. These collision mechanisms operate at different length scales in the fluid mass. / Chemical Engineering

[pt] A presente tese tem como objetivo estudar as reações descontínuas que ocorreram nos contornos de grão (CG) sob condições de envelhecimento a altas temperaturas e identificar as fases que precipitam em função do tempo em uma liga de alto teor de Cr e Ni – Liga 33, tendo em vista o efeito destas transformações nas potenciais aplicações em indústrias de alto desempenho. Amostras na condição como recebida foram submetidas a tratamentos isotérmicos de envelhecimento a 700 graus C, 800 graus C e 900 graus C. Ênfase foi dada ao estudo do envelhecimento a 800 graus C em intervalos de tempo entre 10 minutos e 100 horas com o objetivo de promover fenômenos de precipitação nos modos convencional e descontínua. A caracterização microestrutural foi realizada por microscopia óptica (MO), microscopia eletrônica de varredura (MEV) analítica por espectroscopia por dispersão de energia de raios-X (XEDS), difração de elétrons retroespalhados (EBSD) e microscopia eletrônica de transmissão (MET) no modo convencional, utilizando contraste de difração, e no modo analítico (STEM/XEDS). Resultados obtidos por STEM/XEDS e difração de elétrons mostraram que a transformação descontínua, pela partição de solutos, gera colônias de precipitação descontínua (PD) concomitante à migração do CG e resulta na precipitação de cinco diferentes fases dentro de uma mesma colônia de PD: (1) carbeto-M(23)C(6) rico em Cr com estrutura CFC; (2) fase-eta enriquecida em Si com estrutura cúbica-diamante; (3) fase alfa rica em Cr com estrutura CCC; (4) fase intermetálica sigma com estrutura tetragonal e (5) nitreto-M(2)N rico em Cr com estrutura hexagonal. Em geral, as colônias de PD na Liga 33 evoluem no envelhecimento seguindo a sequência, a saber: nos estágios iniciais do processo ocorre precipitação intergranular (carbeto-M(23)C(6) e fase-eta) no CG original; com o aumento no tempo de envelhecimento, os contornos migraram alimentados pela difusão de soluto num processo conhecido como DIGM (diffusion-induced grain boundary migration), enquanto precipitados da fase alfa-Cr nuclearam adjacente ao CG e cresceram com morfologia lamelar acompanhando a migração do contorno, desenvolvendo, assim, as colônias de PD. Eventualmente, a fase-eta também precipita no interior da colônia e na frente de reação. Nos estágios finais do processo de crescimento das colônias de PD ocorre a formação da fase-sigma na frente de reação da PD e, posteriormente, a formação do nitreto-M(2)N na frente de reação e dentro da colônia. Foi verificado que a reação de PD é controlada, primeiramente, pela difusão de CG do Cr e, com o tempo de reação pela difusão de volume do Cr, o que resultou em um crescimento no estado não-estacionário da reação. Além da ocorrência de cinco fases precipitadas dentro da mesma colônia, outra característica marcante da reação de PD na Liga 33 refere-se à consistente evidência que a fase inicialmente precipitada nos CG (carbeto-M(23)C(6) é diferente da fase precipitado com morfologia lamelar (fase alfa-Cr) dentro da colônia. Tal observação constitui a primeira evidência para o fenômeno de PD envolvendo colônias multi-fases em materiais multicomponentes estruturais. / [en] This thesis aims at study the discontinuous reactions taking place at grain boundaries (GB) under high-temperature aging conditions and to identify the precipitated phases as a function of the time in a Cr-Fe-Ni alloy - Alloy 33, in view of the deleterious effect of these transformations on potential applications of this alloy in high-performance industries. Samples in the as-received condition were submitted to isothermal aging treatments at 700 C degrees, 800 C degrees and 900 C degrees. Emphasis was given to the study of aging at 800 C degrees in time intervals between 10 minutes and 100 hours in order to promote precipitation phenomena in the conventional and discontinuous modes. The microstructural characterization was carried out by light optical microscopy (LOM), analytical scanning electron microscopy (SEM) by X-ray energy dispersive spectroscopy (XEDS), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) under conventional mode using diffraction contrast and analytical mode (STEM/XEDS). Results obtained by STEM/XEDS and electron diffraction revealed that the discontinuous transformation, by solutes partitioning, generates discontinuous precipitation (DP) colonies concomitant with GB migration and results in the precipitation of five different phases within a single DP colony: (1) Cr-rich M(23)C(6)-carbide with FCC structure, (2) Si-enriched eta-phase with diamond-cubic structure, (3) Cr-rich alpha-phase with BCC structure, (4) intermetallic sigma-phase with tetragonal structure, and (5) Cr-rich M(2)N-nitride with hexagonal structure. In summary, DP colonies in Alloy 33 upon aging at 800 C degrees evolve according the following the sequence: in the initial stages of the process intergranular precipitation (M(23)C(6)-carbide and n-phase) occurs at original GB; with the increase in aging time, the boundaries migrated fed by solute atoms in a process known as diffusion-induced grain boundary migration (DIGM), whereas alpha-Cr phase precipitates have nucleated adjacent to the GB and grew with lamellar morphology accompanying the migration of the boundary thereby developing the DP colonies. Eventually, the n-phase also precipitates both within the colony and at the DP reaction-front. Over the final stages of the DP colonies growth process occurs the nucleation and growth of the sigma-phase at the GB reaction-front and, later, M(2)N-nitride precipitates also at the reaction-front and within the DP colony. It was verified that the DP reaction growth is controlled, initially by GB diffusion of Cr and, with the progress of reaction time by the volume diffusion of the Cr, which resulted in a non-steady state growth process. In addition to the occurrence of five precipitated phases within the same colony, another striking feature of the DP reaction in Alloy 33 refers to the consistent evidence that the phase initially precipitated at original GB position (M(23)C(6)-carbide) is different from the precipitated phase with lamellar morphology (alpha-Cr phase) within the colony. This observation constitutes the first evidence for the DP phenomenon resulting in multi-phase DP colonies in multicomponent structural materials.

Three mathematical models, two of primary alkaline battery cathode discharge, and one of primary alkaline battery discharge, are developed, presented, solved and investigated in this thesis. The primary aim of this work is to improve our understanding of the complex, interrelated and nonlinear processes that occur within primary alkaline batteries during discharge. We use perturbation techniques and Laplace transforms to analyse and simplify an existing model of primary alkaline battery cathode under galvanostatic discharge. The process highlights key phenomena, and removes those phenomena that have very little effect on discharge from the model. We find that electrolyte variation within Electrolytic Manganese Dioxide (EMD) particles is negligible, but proton diffusion within EMD crystals is important. The simplification process results in a significant reduction in the number of model equations, and greatly decreases the computational overhead of the numerical simulation software. In addition, the model results based on this simplified framework compare well with available experimental data. The second model of the primary alkaline battery cathode discharge simulates step potential electrochemical spectroscopy discharges, and is used to improve our understanding of the multi-reaction nature of the reduction of EMD. We find that a single-reaction framework is able to simulate multi-reaction behaviour through the use of a nonlinear ion-ion interaction term. The third model simulates the full primary alkaline battery system, and accounts for the precipitation of zinc oxide within the separator (and other regions), and subsequent internal short circuit through this phase. It was found that an internal short circuit is created at the beginning of discharge, and this self-discharge may be exacerbated by discharging the cell intermittently. We find that using a thicker separator paper is a very effective way of minimising self-discharge behaviour. The equations describing the three models are solved numerically in MATLABR, using three pieces of numerical simulation software. They provide a flexible and powerful set of primary alkaline battery discharge prediction tools, that leverage the simplified model framework, allowing them to be easily run on a desktop PC.