Fischer-Tropsch synthesis (FTS), developed in the early 1900's, is defined as the catalytic conversion of H2 and CO to hydrocarbons and oxygenates with the production of H2O and CO2. Accurate microkinetic modeling can in principle provide insights into catalyst design and the role of promoters. This work focused on gaining an understanding of the chemistry of the kinetically relevant steps in FTS on Fe catalyst and developing a microkinetic model that describes FTS reaction kinetics. Stable Al2O3-supported/promoted (20% Fe, 1% K, 1% Pt) and unsupported Fe (99% Fe, 1% Al2O3) catalysts were prepared and characterized. Transient experiments including temperature programmed desorption (TPD), temperature programmed hydrogenation (TPH), and isothermal hydrogenation (ITH) provided insights into the chemistry and energetics of the early elementary reactions in FTS on Fe catalyst. Microkinetic models of CO TPD, ITH, and FTS were developed for Fe catalyst by combining transition state theory and UBI-QEP formalism. These models support the conclusion that hydrocarbon formation occurs on Fe via a dual mechanism involving surface carbide and formyl intermediates; nevertheless, hydrocarbon formation is more favorable via the carbide mechanism. Carbon hydrogenation was found to be the rate determining step in the carbide mechanism. CO heat of adsorption on polycrystalline Fe at zero coverage was estimated to be -91.6 kJ/mol and -64.8 kJ/mol from ITH and FTS models respectively, while a mean value of -50.0 kJ/mol was estimated from the TPD model. Statistically designed steady-state kinetic experiments at conditions similar to industrial operating conditions were used to obtain rate data. The rate data were used to develop a microkinetic model of FTS. FTS and ITH appear to follow similar reaction pathways, although the energetics are slightly different. In both cases, hydrocarbon formation via the carbide mechanism was more favorable than via a formyl intermediate while carbon hydrogenation was the rate determining step. Promotion of Fe with K does not alter Fischer-Tropsch synthesis reaction pathways but it does alter the energetics for the steps leading to the formation of CO2. This phenomenon accounts for the CO2 selectivity of 0.3 observed for K-promoted Fe against 0.17 observed for un-promoted Fe. A Langmuir Hinshelwood rate expression derived from the microkinetic model was put into a fixed bed FTS reactor design code; calculated reactor sizes, throughput, temperature profiles and conversion are similar to those of pilot and demonstration FTS reactors with similar feed rates and compositions.

Submitted by Deyse Queiroz (deysequeirozz@hotmail.com) on 2018-06-14T14:16:39Z No. of bitstreams: 1 JOAQUIM PEREIRA RODRIGUES - DISSERTAÇÃO PPGEA 2003..pdf: 14449699 bytes, checksum: 7cf16212a6472107d11953d967cd33fa (MD5) / Made available in DSpace on 2018-06-14T14:16:39Z (GMT). No. of bitstreams: 1 JOAQUIM PEREIRA RODRIGUES - DISSERTAÇÃO PPGEA 2003..pdf: 14449699 bytes, checksum: 7cf16212a6472107d11953d967cd33fa (MD5) Previous issue date: 2003-07 / As espécies de Eucalyptus e Pinus brasileiros apresentam um alto nível de resistência aos incêndios florestais, sendo consideradas, por alguns, como plantas pirófitas ativas. E, por isto se pergunta: O comportamento das sementes dessas espécies evoluíram exclusivamente em relação às perturbações produzidas pelo fogo ou, pelo contrário, outras causas influenciaram nesse processo e ainda se essas espécies desenvolveram respostas adaptativas similares? Para responder estas perguntas, sementes de cinco espécies de Eucalyptus e três de Pinus foram submetidas a diferentes intensidades térmicas e tempo de exposição, para simular resposta aos diferentes regimes de fogo, avaliando os efeitos provocados pelas elevadas temperaturas a que foram submetidas às sementes, assim como o efeito das cinzas, tido como elemento mais notável do micro-ambiente em que há de se desenvolver posteriormente os propágulos. O material vegetal empregado foi produzido e adquirido pelo IPEF (Instituto de Pesquisas e Estudos Florestais) em Piracicaba, SP. Para avaliar a resposta das sementes ao efeito das temperaturas, fez-se uso do delineamento estatístico inteiramente casualisado com arranjo fatorial. Na ausência de tratamentos térmicos, as sementes de Eucalyptus e Pinus registraram altas taxas de germinação e forte redução já a partir de temperaturas igual ou superiores a 70 °C sugerindo que realmente não são plantas pirófitas ativas. Os valores da germinação obtidos com os tratamentos de temperatura igual a 50 °C diferiram estatisticamente da testemunha. No entanto, a redução da germinação, em termo médios, foi de 5% (87 para 92% da testemunha), sendo muito similares para todas as espécies de Eucalyptus e Pinus estudadas. Quando os valores da temperatura são iguais ou superiores a 110 °C se observa uma importante redução da germinação das sementes em todos os tratamentos. A avaliação simultânea do calor e das cinzas mostrou a existência de interação entre ambos os fatores, com inibição total da germinação em alguns tratamentos. / The Brazilians species of Eucalyptus and Pinus present a high resistance levei to the forest fires, being considered, for some, as plants active pyrofits. And, for this reason it wonders: Did the behavior of the seeds of those species develop exclusively in relationship the disturbances produced by the fire or for the thwart, do other causes influence in that process and if, those species developed answers similar adapted? To answer these questions, seeds of five species of Eucalyptuses and three of Pinus were submitted at different thermal intensities and time of exhibition, for similar answer to the different fire regimes, evaluating the effects provoked by high temperatures were submitted the one , as well as the effect of the ashes, had as the element more notable of the environment in that must grow the plants later. The material employed vegetable was produced and acquired by 1PEF (Researches and Forest Studies Institute) in Piracicaba; SP. To evaluate the answer of the seeds to the effect of the temperatures, it was made use of the statistical lineation entirely casualty with factorial arrangement. In the absence of thermal treatments, the of Eucalyptus and Pinus seeds already registered discharges germination rates and strong reduction starting from temperatures equal or superior to 70 °C suggesting that are not really plants active pyrofits. The values of the germination obtained with the temperature treatments same to 50 °C they differed statisticiality of control, however the reduction of the germination, in médium terms, was of 5% (87 for 92% of control), being very similar for ali the species of Eucalyptus and Pinus seeds studied. When the values of the temperature are same or superior to 110 °C an important reduction of the germination of the seeds of ali the treatments it is observed. The simultaneous evaluation of the heat and of the ashes, it showed the interaction existence between both the factors, with total inhibition of the germination in some treatments.