1,227 research outputs found
Hydrodynamic study of heat transfer in a fluidized bed by discrete particle simulations
Full text linkIn recent years, gas-solid fluidized beds have been extensively used in the process industries. Fluid catalytic cracking (FCC) is one of the most important conversion processes used in the biobased feedstock refinery. The FCC process vaporizes and breaks the long-chain molecules of the high-boiling hydrocarbon liquids into much shorter molecules by contacting the feedstock at high temperature, with fluidized powdered catalyst. Gas-particle heat transfer is a crucial element of the process. To numerically study three dimensional fluidized beds is still a challenge, due to the high computational cost, whereas flow visualization and measurements are difficult to perform in 3D fluidized beds.
In this work, the CFD–DEM approach, a computational fluid dynamics (CFD) model for gas-phase flow combined with a discrete element method (DEM) for particle motion (see review articles of Deen et al (1); van der Hoef et al (2)), was used to study the influence on fluidized bed thermal behavior of particles depending on the particle size and the superficial gas velocity. CFD-DEM simulations are performed on a pseudo 2D fluidized bed (shown in Figure 1) with spherical particles (dp = 1 mm, ρp = 667 kg/m3). The thermal energy equation of the particles contains a source term to mimic heat production due to exothermic chemical reactions. Instantaneous snapshots of the voidage in the fluidized bed as shown in Figure 2 shows the effect of inlet gas velocity. The simulations are carried out with an open-source package, OpenFOAM-CFDEM-LIGGGHTS, and will be compared to results obtained with an in-house CFD-DEM code.
REFERENCES Deen, N.G., Annaland, M.V., van der Hoef, M.A., Kuipers, J.A.M., Review of discrete particle modeling of fluidized beds. Chemical Engineering Science, 62(1-2): 28-44, 2007. Van der Hoef, M.A., Annaland, M.V., Deen, N.G., Kuipers, J.A.M., Numerical simulation of dense gas–solid fluidized beds: a multiscale modeling strategy. Annual Review of Fluid Mechanics, 40: 47-70, 2008.
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Wall-to-bed heat transfer in gas-solid bubbling fluidized bed
No full textThe wall-to-bed heat transfer in gas-solid fluidized beds is mainly determined by phenomena prevailing in a thermal boundary layer with a thickness in the order of magnitude of the size of a single particle. In this thermal boundary layer the temperature gradients are very steep and the local porosity profile near the wall strongly influences the heat-transfer rate. A two-fluid continuum model based on conservation laws for mass, momentum and thermal energy has been developed accounting for the porosity distribution near the wall. To validate the model, local instantaneous wall-to-bed heat-transfer coefficients were measured along a heated wall kept at constant temperature. The incorporation of a porosity profile by effective conductivities has remarkably improved the prediction of the heat-transfer coefficients compared against that of previous studies. The predicted local instantaneous heat-transfer coefficients are in good agreement with the experimental data for different jet velocities as well as for different particle sizes, provided that the near-wall porosity profile is accounted for. Two sets of closure equations for the solids-phase rheology (that is, the solids-phase stress tensor) have been considered: the constant viscosity model (CVM) and the kinetic Theory of granular flow (KTGF). For lower bed heights both models give good predictions of the wall-to-bed heat-transfer coefficients, but the KTGF predictions at higher bed heights agree better with the experimental data than the predictions by the CVM as the result of a better description of the passage of the bubble along the wall. The influence of an additional kinetic contribution to the effective solids-phase thermal conductivity arising from the fluctuating solids velocity was studied with the KTGF closures. Because of a large overprediction of the kinetic solids-phase thermal conductivity the wall-to-bed heat-transfer coefficients are largely overestimated when accounting for this additional contribution to the effective solids-phase thermal conductivity
Experimental validation of packed bed chemical-looping combustion
Full text linkChemical-looping combustion has emerged as a promising alternative technology, intrinsically integrating CO2 capture in power production. A novel reactor concept based on dynamically operated packed beds has been proposed [Noorman, S., van Sint Annaland, M., Kuipers, J.A.M., 2007. Packed bed reactor technology for chemical-looping combustion. Ind. Eng. Chem. Res. 46, 4212–4220] and in this work, packed bed chemical-looping combustion was investigated experimentally to provide an experimental proof-of-principle. Using information obtained from both the reduction and oxidation cycles, the measured maximum temperature rise and front velocities in the packed bed during the oxidation cycle corresponded very well with analytical expressions describing the system, especially when the contribution of the formation of carbon during the reduction cycle was taken into account
Redevoeringen dies natalis : 46ste dies natalis, 30 november 2007
Full text linkBevat: De millenniumdoelstellingen door Prof.dr. W.H.M. Zijm en In het brandpunt van energie door Prof. dr. ir. J.A.M. Kuiper
Fluid-particle interaction from lattice Boltzmann simulations for flow through polydisperse random arrays of spheres
Full text linkFluid–solid drag force correlations, such as the Ergun relation, are widely used in many areas of chemical engineering. In many practical applications, the solid phase consist of an assembly of spheres which are, more often than not, polydisperse. In this paper we report on a study of the fluid–particle interaction by fully resolved DNS-type simulations (lattice Boltzmann) of flow through polydisperse random arrays of spheres, both for log-normal and Gaussian size distributions. In a recent paper [Van der Hoef, M.A., Beetstra, R., Kuipers, J.A.M., 2005. Lattice Boltzmann simulations of low Reynolds number flow past mono- and bidisperse arrays of spheres: results for the permeability and drag force. J. Fluid Mech. 528, 233] we have shown that a correction factor should be applied to the monodisperse drag force relations, when used for bidisperse systems. On the basis of the data reported in this paper, we conclude that the correction factor also applies to general polydisperse systems
Direct numerical simulation of fluid flow accompanied by coupled mass and heat transfer in dense fluid-particle systems
No full textIn this paper we report the extension of an earlier reported DNS method (Deen et al., 2012 and Deen and Kuipers, 2013) based on a novel Immersed Boundary Method (IBM) which incorporates the fluid–solid coupling at the level of the discrete field equations. The extended method is used to study coupled mass and heat transport in dense fluid–particle systems where the coupling arises as a consequence of an exothermal chemical reaction proceeding at the exterior surface of the particles. Following a detailed verification (using an independent numerical technique) and validation (using established empirical correlations) we apply our DNS technique to study coupled mass and heat transfer in a dense fluid–particle system. In addition a comparison is made with results obtained from a simple one-dimensional (1D) heterogeneous reactor model which uses empirical closures for the fluid–particle mass and heat transfer coefficients. The main features of the complex transient temperature profiles obtained from our DNS agree quite well with the corresponding profiles obtained from the 1D heterogeneous reactor model
Fluidised bed membrane reactor for ultrapure hydrogen production via methane steam reforming: Experimental demonstration and model validation
Full text linkHydrogen is emerging as a future alternative for mobile and stationary energy carriers in addition to its use in chemical and petrochemical applications. A novel multifunctional reactor concept has been developed for the production of ultrapure hydrogen (<10 ppm CO) from light hydrocarbons such as methane for online use in downstream polymer electrolyte membrane fuel cells. A high degree of process intensification can be achieved by integrating perm-selective hydrogen membranes for selective hydrogen removal to shift the methane steam reforming and water-gas-shift equilibriums in the favourable direction and perm-selective oxygen membranes for selective oxygen addition to supply the required reaction energy via partial oxidation of part of the methane feed and enable pure CO2 capture without costly post-treatment. This can be achieved in a proposed novel multifunctional bi-membrane bi-section fluidised bed reactor [Patil, C.S., van Sint Annaland, M., Kuipers, J.A.M., 2005. Design of a novel autothermal membrane assisted fluidized bed reactor for the production of ultrapure hydrogen from methane. Industrial and Engineering Chemistry Research 44, 9502-9512]. In this paper, an experimental proof of principle for the steam reforming/water-gas-shift section of the proposed novel fluidised bed membrane reactor is presented. A fluidised bed membrane reactor for steam reforming of methane/water-gas-shift on a commercial noble metal-based catalyst has been designed and constructed using 10 H2 perm-selective Pd membranes for a fuel cell power output in the range of 50-100 W. It has been experimentally demonstrated that by the insertion of the membranes in the fluidised bed, the thermodynamic equilibrium constraints can indeed be overcome, i.e., increased CH4 conversion, decreased CO selectivity and higher product yield (H2 produced/CH4 reacted). Experiments at different superficial gas velocities and also at different temperatures and pressures (carried out in the regime without kinetic limitations) revealed enhanced reactor performance at higher temperatures (650 {ring operator} C) and pressures (3-4 bar). With a phenomenological two-phase reactor model for the fluidised bed membrane reactor, incorporating a separately developed lumped flux expression for the H2 permeation rate through the used Pd-based membranes, the measured data from the fluidised bed membrane reactor could be well described, provided that axial gas back-mixing in the membrane-assisted fluidised bed reactor is negligible. This indicates that the membrane reactor behaviour approached that of an ideal isothermal plug flow reactor with maximum H2 permeation. © 2007 Elsevier Ltd. All rights reserved
Multi-level modeling of dense gas-solid two-phase flows
Full text linkThe most widespread industrial application of dense gas-solid flows is encountered in the domain of gas-fluidized beds. If operated with well-matched gas and particle parameters, fluidized beds can provide many advantages such as uniform temperature distribution, high mass transfer rates, continuous operation, and relative simplicity in geometric configuration. For this reason, gas-fluidized beds have been widely applied to petroleum, metallurgical, chemical, energy, environmental, and food industries in the past decade
Euler-lagrange simulation of flow structure formation and evolution in dense gas-solid flows
Full text linkEulerian modeling of reactive gas-liquid flow in a bubble column
Full text linkDespite the widespread application of bubble columns and intensive research efforts devoted to understand their complex behavior, detailed knowledge on the fluid flow, mass transfer and chemical reactions as well as their interactions is currently very limited. Gas-liquid flow in bubble column reactors is characterized by a combination of inherently unsteady complex processes with widely varying spatial and temporal scales. The complicated interactions between the gas and the liquid phases comprising hydrodynamics, mass transfer and chemical reaction cause many challenging modeling problems to be solved. The Euler–Euler model is adopted throughout this thesis to investigate gas-liquid flow in bubble columns. In this study, efforts have been focused on the assessment of suitable closure laws for interfacial forces and for turbulence in the continuous phase. Furthermore, gas-liquid heterogeneous flow and reactive gas-liquid flows have been studied. All the numerical simulations were carried out with the commercial CFD package CFX-4.4 and all simulation results were compared with the available experimental PIV data of Deen (2001)
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