576 research outputs found

    Efficient carbon utilization to dimethyl ether by steam adsorption enhancement

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    Computationsl modelling of dimethyl ether separation and steam reforming in fluidized bed reactors

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    This study presents a computational fluid dynamic (CFD) study of Dimethyl Ether (DME) gas adsorptive separation and steam reforming (DME-SR) in a large scale Circulating Fluidized Bed (CFB) reactor. The CFD model is based on Eulerian-Eulerian dispersed flow and solved using commercial software (ANSYS FLUENT). Hydrogen is currently receiving increasing interest as an alternative source of clean energy and has high potential applications, including the transportation sector and power generation. Computational fluid dynamic (CFD) modelling has attracted considerable recognition in the engineering sector consequently leading to using it as a tool for process design and optimisation in many industrial processes. In most cases, these processes are difficult or expensive to conduct in lab scale experiments. The CFD provides a cost effective methodology to gain detailed information up to the microscopic level. The main objectives in this project are to: (i) develop a predictive model using ANSYS FLUENT (CFD) commercial code to simulate the flow hydrodynamics, mass transfer, reactions and heat transfer in a large scale dual fluidized bed system for combined gas separation and steam reforming processes (ii) implement a suitable adsorption models in the CFD code, through a user defined function, to predict selective separation of a gas from a mixture (iii) develop a model for dimethyl ether steam reforming (DME-SR) to predict hydrogen production (iv) carry out detailed parametric analysis in order to establish ideal operating conditions for future industrial application. The project has originated from a real industrial case problem in collaboration with the industrial partner Dow Corning (UK) and jointly funded by the Engineering and Physical Research Council (UK) and Dow Corning. The research examined gas separation by adsorption in a bubbling bed, as part of a dual fluidized bed system. The adsorption process was simulated based on the kinetics derived from the experimental data produced as part of a separate PhD project completed under the same fund. The kinetic model was incorporated in FLUENT CFD tool as a pseudo-first order rate equation; some of the parameters for the pseudo-first order kinetics were obtained using MATLAB. The modelling of the DME adsorption in the designed bubbling bed was performed for the first time in this project and highlights the novelty in the investigations. The simulation results were analysed to provide understanding of the flow hydrodynamic, reactor design and optimum operating condition for efficient separation. Bubbling bed validation by estimation of bed expansion and the solid and gas distribution from simulation agreed well with trends seen in the literatures. Parametric analysis on the adsorption process demonstrated that increasing fluidizing velocity reduced adsorption of DME. This is as a result of reduction in the gas residence time which appears to have much effect compared to the solid residence time. The removal efficiency of DME from the bed was found to be more than 88%. Simulation of the DME-SR in FLUENT CFD was conducted using selected kinetics from literature and implemented in the model using an in-house developed user defined function. The validation of the kinetics was achieved by simulating a case to replicate an experimental study of a laboratory scale bubbling bed by Vicente et al [1]. Good agreement was achieved for the validation of the models, which was then applied in the DME-SR in the large scale riser section of the dual fluidized bed system. This is the first study to use the selected DME-SR kinetics in a circulating fluidized bed (CFB) system and for the geometry size proposed for the project. As a result, the simulation produced the first detailed data on the spatial variation and final gas product in such an industrial scale fluidized bed system. The simulation results provided insight in the flow hydrodynamic, reactor design and optimum operating condition. The solid and gas distribution in the CFB was observed to show good agreement with literatures. The parametric analysis showed that the increase in temperature and steam to DME molar ratio increased the production of hydrogen due to the increased DME conversions, whereas the increase in the space velocity has been found to have an adverse effect. Increasing temperature between 200 oC to 350 oC increased DME conversion from 47% to 99% while hydrogen yield increased substantially from 11% to 100%. The CO2 selectivity decreased from 100% to 91% due to the water gas shift reaction favouring CO at higher temperatures. The higher conversions observed as the temperature increased was reflected on the quantity of unreacted DME and methanol concentrations in the product gas, where both decreased to very low values of 0.27 mol% and 0.46 mol% respectively at 350 °C. Increasing the steam to DME molar ratio from 4 to 7.68 increased the DME conversion from 69% to 87%, while the hydrogen yield increased from 40% to 59%. The CO2 selectivity decreased from 100% to 97%. The decrease in the space velocity from 37104 ml/g/h to 15394 ml/g/h increased the DME conversion from 87% to 100% while increasing the hydrogen yield from 59% to 87%. The parametric analysis suggests an operating condition for maximum hydrogen yield is in the region of 300 oC temperatures and Steam/DME molar ratio of 5. The analysis of the industrial sponsor’s case for the given flow and composition of the gas to be treated suggests that 88% of DME can be adsorbed from the bubbling and consequently producing 224.4t/y of hydrogen in the riser section of the dual fluidized bed system. The process also produces 1458.4t/y of CO2 and 127.9t/y of CO as part of the product gas. The developed models and parametric analysis carried out in this study provided essential guideline for future design of DME-SR at industrial level and in particular this work has been of tremendous importance for the industrial collaborator in order to draw conclusions and plan for future potential implementation of the process at an industrial scale

    Process development for fine chemicals (Acetaldehyde Dimethylacetal) synthesis

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    Tese de doutoramento. Engenharia QuĂ­mica. Faculdade de Engenharia. Universidade do Porto. 200

    Sorption enhanced steam methane reforming process for continuous production of hydrogen in pressure swing adsorptive reactors

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    Tese de doutoramento. Engenharia QuĂ­mica e BiolĂłgica. Faculdade de Engenharia. Universidade do Porto. 200

    Soil carbon stabilization pathways as reflected by the pyrolytic signature of humic acid in agricultural volcanic soils

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    Molecular assessment of the origin and transformation processes of soil organic matter (SOM) was carried out based on information obtained from 13C NMR and analytical pyrolysis of humic acids (HAs) in soils from wine-growing regions in Tenerife (Canary Islands, Spain). Principal component analysis, using as variables pyrolysis products, shows different soil groups defined by the molecular assemblages released from the corresponding HAs, characterized by the predominance of: i) plant biomacromolecules (lignin) in soils on pumice substrate, ii) heterocyclic N-compounds and methoxyl-lacking aromatic structures, iii) a substantial domain of alkyl compounds in cultivated soils with active C turnover and finally, iv) polysaccharide and protein-derived compounds in soils developed on amorphous gels. The proportions of the pyrolytic compounds from soil HAs were represented by an upgraded graphical-statistical method (3D Van Krevelen plot) that was used to compare the major SOM structural domains in the different soils. The above results coincide with those suggested by the 13C NMR analysis, and were associated to two groups of local land management practices, in terms of their intensity respectively favoring either the transformation of plant-inherited macromolecular precursors from vascular plants, or the humification of aliphatic precursors in the presence of specific mineralogical substrates controlling microbial degradation and humification processes.This research has been funded by the Spanish CICyT under grant CGL2013-43845-P . The authors wish to thank to three anonymous referees by their constructive comments that contributed to improving the final version of the paper.info:eu-repo/semantics/publishedVersio

    Engineering Dopant Position in Structure-Controlled CeO2-ZrO2 Catalysts

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    CeO2-ZrO2 (CZO) nanoparticles (NPs) have application in many catalytic reactions, such as methane reformation, due to their oxygen cycling ability. Ni doping has been shown to improve the catalytic activity and acts as an active site for the decomposition of methane. In this work, Ni:CZO NPs were synthesized via a two-step co-precipitation/molten salt synthesis to compare Ni distribution, oxygen vacancy concentration, and catalytic activity relative to a reference state-of-the-art catalyst. To better understand the effects of Ni position and dispersion, and oxygen vacancy formation in these materials, the Ni concentration, reaction time, and deposition methods were varied. X-ray diffraction (XRD) measurements show a cubic phase with little to no segregation of Ni/NiO. Catalytic activity measurements displayed similar activity per surface area with an order of magnitude decrease in the coking rate for the particles synthesized by the molten salt method compared to a traditional insipient wetness impregnation synthesis. Additionally, this new approach resulted in an order of magnitude increase in oxygen vacancies which is attributed to the high dispersion of Ni2+ ions in the NP core. Tailoring active sites position and concentration on the catalyst surface has been shown to effect activity and stability of a catalyst. After an active Ni:CZO core has been finalized, a shell layer was subsequently deposited to active site concentration and dispersion. The robust structure of the core of the catalyst that is synthesized helps achieve better dispersion of active sites on the surface. Better dispersion of active sites along with availability of oxygen vacancies from the core resulted in a five-fold increase in catalytic activity per surface area and an order of magnitude decrease in coking. In this work, the role of Ni position on catalytic activity is probed to develop a two-step synthesis process which allows for spatially controlled dopant distribution for improved catalytic activity
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