Browsing by Author "Silva, Valter"
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- CFD Models Applied to Gasification Improvement and OptimizationPublication . Silva, Valter
- Comparative scaling analysis of two different sized pilot-scale fluidized bed reactors operating with biomass substratesPublication . Cardoso, João; Silva, Valter; Eusébio, Daniela; Brito, Paulo; Hall, M.J.; Tarelho, LuísThis paper presents a comparative scaling analysis of two different sized pilot-scale fluidized bed reactors operating with biomass substrates. A multiphase Eulerian-Eulerian 2-D mathematical model was implemented, coupled with in-house user-defined functions (UDF) built to enhance hydrodynamics and heat transfer phenomena. The model validation was attained by comparison to experimental data gathered from both reactors. A grid refinement study was carried out for both geometries to achieve an appropriate computational domain. Hydrodynamics was deeply studied for both reactors concerning the scale-up effect. Mixing and segregation phenomena, solid particle distribution and biomass velocity were matters of great concern. Results showed that UDF implementation successfully minimized deviations and increased the model’s predictability. The largest deviations measured between experimental and numerical results for syngas composition were of about 20%. Solids mixing and segregation was found to be directly affected by the particles size, density, and superficial gas velocity, with the larger reactor revealing improved mixing ability. Improved mixing occurred for smaller particles size ratio (dbiomass ¼ 3 mm), smaller particles density ratio (rbiomass ¼ 950 kg/m3), and higher dimensionless superficial gas velocities (U0=Umf¼3.5). The larger unit showed an increase in near-wall velocity, lateral dispersion, and bubble size. As for the smaller reactor, higher velocities were obtained at the center region due to a more pronounced wall boundary layer. Similarities were found between the two reactors regarding the bubble distribution, dimensionless average bed pressure drop and biomass velocity vector profiles when dimensionless parameters were employed.
- Hydrodynamic Modelling of Municipal Solid Waste Residues in a Pilot Scale Fluidized Bed ReactorPublication . Cardoso, João; Silva, Valter; Eusébio, Daniela; Brito, PauloThe present study investigates the hydrodynamics and heat transfer behavior of municipal solid waste (MSW) gasification in a pilot scale bubbling fluidized bed reactor. A multiphase 2-D numerical model following an Eulerian-Eulerian approach within the FLUENT framework was implemented. User defined functions (UDFs) were coupled to improve hydrodynamics and heat transfer phenomena, and to minimize deviations between the experimental and numerical results. A grid independence study was accomplished through comparison of the bed volume fraction profiles and by reasoning the grid accuracy and computational cost. The standard deviation concept was used to determine the mixing quality indexes. Simulated results showed that UDFs improvements increased the accuracy of the mathematical model. Smaller size ratio of the MSW-dolomite mixture revealed a more uniform mixing, and larger ratios enhanced segregation. Also, increased superficial gas velocity promoted the solid particles mixing. Heat transfer within the fluidized bed showed strong dependence on the MSW solid particles sizes, with smaller particles revealing a more effective process.
- Improved numerical approaches to predict hydrodynamics in a pilot-scale bubbling fluidized bed biomass reactor: A numerical study with experimental validationPublication . Cardoso, João; Silva, Valter; Eusébio, Daniela; Brito, Paulo; Tarelho, LuísA computational 2-D Eulerian-Eulerian approach was developed to simulate the hydrodynamics and heat transfer of a biomass gasification process in a pilot-scale bubbling fluidized bed reactor. The mathematical model was validated under experimental results collected from fluidization curves gathered at different temperatures in a pilot-scale reactor (75 kWth). Own user defined functions (UDFs) were developed in C programming and included to improve drag and heat transfer phenomena, as well to minimize deviations between experimental and numerical data found in previous works. Mesh selection was achieved by comparing solid fraction and pressure drop contours with grids comprised of different number of cells. A comparative study for particle diameter and inlet gas velocity was conducted for three different biomass feedstocks’ and their impact in the mixing and segregation index was studied. Mixing and segregation index were measured by implementing the standard deviation concept. Results indicated that UDFs significantly improved the mathematical model predictions on the reactor’s fluidization curves. Biomass and sand particles size and density showed direct influence on the solids distribution along the bed height. Smaller biomass particles revealed faster heat conduction and improved mixing properties.
- Introductory Chapter: How to Use Design of Experiments Methodology to Get Most from Chemical ProcessesPublication . Silva, Valter; Eusébio, Daniela; Cardoso, JoãoEconomic pressure and the need to target more competitive levels drive organizations to invest in efficient methodologies to get solutions able to provide clear advantages in a very demanding market. In this scenario, statistical approaches emerge as valuable tools to be used in the chemical process industry. Indeed, the chemical industry uses a wide set of statistical methodologies, ranging from descriptive approaches to complex optimization topics such as Design of Experiments (DoE), always targeting safer, more repeatable and profitable solutions.