Browsing by Author "Ribeiro, Leonardo"
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- Assessment of an Innovative Way to Store Hydrogen in VehiclesPublication . Baptista, Andresa; Pinho, Carlos; Pinto, Gustavo Filipe; Ribeiro, Leonardo; Monteiro, Joaquim; Santos, TiagoThe use of hydrogen as an alternative to fossil fuels for vehicle propulsion is already a reality. However, due to its physical characteristics, storage is still a challenge. There is an innovative way, presented in this study, to store hydrogen in conventional vehicles propelled by spark-ignition reciprocating engines and fuel cells, using hydrogen as fuel; the storage of hydrogen will be at high pressure within small spheres randomly packed in a tank, like the conventional tank of fuel used nowadays in current vehicles. Therefore, the main purpose of the present study is to assess the performance of this storage system and compare it to others already applied by car manufacturers in their cars. In order to evaluate the performance of this storage system, some parameters were taken into account: The energy stored by volume and stored by weight, hydrogen leakage, and compliance with current standards. This system is safer than conventional storage systems since hydrogen is stored inside small spheres containing small amounts of hydrogen. Besides, its gravimetric energy density (GED) is threefold and the volumetric energy density (VED) is about half when compared with homologous values for conventional systems, and both exceed the targets set by the U.S. Department of Energy. Regarding the leakage of hydrogen, it complies with the European Standards, provided a suitable choice of materials and dimensions is made.
- Assessment of the Use of Solar Desalination Distillers to Produce Fresh Water in Arid AreasPublication . Monteiro, Joaquim; Baptista, Andresa; Pinto, Gustavo Filipe; Ribeiro, Leonardo; Mariano, HélderWater is an important resource for human beings, yet there are inhabited places tormented by the scarcity of it. The present study is concerned with places where, seemingly, the best way to get water is through solar distillers. These places should have, typically, high values of solar irradiation and a lack of human and economic resources to build and operate complex equipment. A set of sites scattered around the world was chosen, and then the presumed productivity and thermal efficiency that solar distillers would have if they were installed at these places was calculated. The mathematical model used with this purpose assumes steady-state operation; the values of mass of water distilled and distiller efficiency were calculated for every hour, but the results presented are annual averages. Then, an economic study was made based on local costs of construction materials for the distillers, the workforce, and the prices of water to predict the payback time of solar distillers. Finally, a study on environmental impact, particularly in terms of greenhouse gas (GHG) emissions, was made to compare reverse osmosis (RO) with solar distillation. For the sites studied, typical values of annual water output are in the range of 414 dm3/m2, for Évora, up to 696 dm3/m2, for Faya Largeau; the minimum efficiency was found for Évora, as 11.5%, and the maximum efficiency was found for Tessalit, as 15.2%. Payback times are very high, regardless of the areas of the globe where solar distillers are implanted. Regarding GHG emissions, solar distillation is preferable to RO.
- Biodiesel Production Systems: Operation, Process Control and TroubleshootingPublication . Caetano, Nídia; Ribeiro, Vera; Ribeiro, Leonardo; Baptista, Andresa; Monteiro, JoaquimBiodiesel is a renewable fuel, produced from waste cooking oils, animal fats, vegetable and algae oils. Its use is intended to replace diesel in conventional diesel engines, causing lower polluting emissions. To produce biodiesel, certain details must be carefully considered, namely feedstock composition, reaction parameters, process conditions, process equipment, purification processes, analysis of biodiesel properties, troubleshooting and storage. In what concerns feedstock composition, parameters such as acidity, insolubles, moisture, phospholipids, sulphur, polymerized triglycerides, impurities, etc., must be determined to decide about the pretreatment steps (washing, degumming, filtration, bleaching, deodorization, among others) to be implemented, and the need for esterification prior to transesterification. In what concerns the selection of process equipment some questions arise, namely the materials, heating methods and thermal insulation to use, alternatives to enhance the reaction, need for neutralization and process control system. The purification process includes biodiesel purification, methanol recovery and glycerine valorisation. The excess methanol must be recovered from biodiesel and glycerine by distillation and reused in the process while glycerine can be further purified and sold for application from the chemical to the pharmaceutical industry. The quality of biodiesel must be certified by the analyses performed according to the standards (e.g. EN 14214, ASTM D6751). Troubleshooting is needed in biodiesel production during start-up and under steady production of a facility; problems may arise regarding quality and appearance of biodiesel, reaction conditions, methanol removal, stirring in reactors, glycerine and biodiesel separation, as well as excess of water and other feedstock impurities. Biodiesel can be stored for up to 6 months; its storage poses challenges concerning degradation by contact with air and light, which cause oxidation. Some additives could extend the lifespan of biodiesel by increasing oxidation stability; other technique is the fractionation to remove the undesired fatty acid methyl ester (FAME).
- A simple approach to numerical modelling of propane combustion in fluidized bedsPublication . Ribeiro, Leonardo; Pinho, CarlosA mathematical model is proposed for the evolution of temperature, chemical composition, and energy release in bubbles, clouds, and emulsion phase during combustion of gaseous premixtures of air and propane in a bubbling fluidized bed. The analysis begins as the bubbles are formed at the orifices of the distributor, until they explode inside the bed or emerge at the free surface of the bed. The model also considers the freeboard region of the fluidized bed until the propane is thoroughly burned. It is essentially built upon the quasi-global mechanism of Hautman et al. (1981) and the mass and heat transfer equations from the two-phase model of Davidson and Harrison (1963). The focus is not on a new modeling approach, but on combining the classical models of the kinetics and other diffusional aspects to obtain a better insight into the events occurring inside a fluidized bed reactor. Experimental data are obtained to validate the model by testing the combustion of commercial propane, in a laboratory-scale fluidized bed, using four sand particle sizes: 400–500, 315–400, 250–315, and 200–250 µm. The mole fractions of CO2, CO, and O2 in the flue gases and the temperature of the fluidized bed are measured and compared with the numerical results.