A discrete particle model of a gas-fluidised bed has been developed and in this the twodimensional motion of the individual, spherical particles was directly calculated from the forces acting on them, accounting for the interaction between the particles and the interstitial gas phase. Our collision model is based on conservation laws for linear and angular momentum and requires, apart from geometrical factors, two empirical parameters: a restitution coefficient and a friction coefficient. A sequence of collisions is processed using techniques which find their application in hard-sphere simulations which are commonly encountered in the field of molecular dynamics. The hydrodynamic model of the gas phase is based on the volume-averaged Navier-Stokes equations. Simulations of bubble and slug formation in a small twodimensional bed (height 0.50 m, width 0.15 m) with 2400 particles (dp = 4 mm, material: aluminium, p = 2700 kg m-3) showed a strong dependency of the flow behaviour with respect to the restitution and friction coefficient. A preliminary experimental validation of our model was performed using a small scale "two-dimensional" gas-fluidised bed (height 0.30 m, width 0.15 m, depth 0.015 m) with 850/~m ballotini glass particles (p = 2930 kgm-3) as the bed material. Results compared fairly well with the results of a simulation which was performed with 40,000 particles using realistic values for the restitution and friction coefficients which were obtained from simple independent experiments.
The pyrolysis kinetics of low-density polyethylene, high-density polyethylene, polypropylene, and polystyrene has been studied at temperatures below 450°C. In addition, a literature review on the low-temperature pyrolysis of these polymers has been conducted and has revealed that the scatter in the reported kinetic data is significant, which is most probably due to the use of simple first-order kinetic models to interpret the experimental data. This model type is only applicable in a small conversion range, but was used by many authors over a much wider conversion range. In this investigation the pyrolysis kinetics of the forementioned polymers and a mixture of polymers has been studied at temperatures below 450°C by performing isothermal thermogravimetric analysis (TGA) experiments. The TGA experimental data was used to determine the kinetic parameters on the basis of a simple first-order model for high conversions (70-90%) and a model developed in the present study, termed the random chain dissociation (RCD) model, for the entire conversion range. The influence of important parameters, such as molecular weight, extent of branching and-scission on the pyrolysis kinetics was studied with the RCD model. This model was also used to calculate the primary product spectrum of the pyrolysis process. The effect of the extent of branching and the initial molecular weight on the pyrolysis process was also studied experimentally. The effect of the extent of branching was found to be quite significant, but the effect of the initial molecular weight was minor. These results were found to agree quite well with the predictions obtained from the RCD model. Finally, the behavior of mixtures of the aforementioned polymers was studied and it was found that the pyrolysis kinetics of the polymers in the mixture remains unaltered in comparison with the pyrolysis kinetics of the pure polymers.
In this paper a detailed hydrodynamic model for gas liquid two-phase flow will be presented. The model is based on a mixed Eulerian Lagrangian approach and describes the time-dependent two-dimensional motion of small, spherical gas bubbles in a bubble column operating in the homogeneous regime. The motion of these bubbles is calculated from a force balance tbr each individual bubble, accounting for all relevant forces acting on them. Contributions from liquid-phase pressure gradient, drag, virtual mass, liquid-phase vorticity and gravity are considered, whereas direct bubble-bubble interactions are accounted for via an interaction model resembling the collision model developed by Hoomans et a/. (1996) to model gasfluidized beds. The liquid-phase hydrodynamics are described using the volume-averaged, unsteady, Navier-Stokes equations. A preliminary model validation has been performed by comparing the computational results with experimental observations published previously in literature by various authors. The model is shown to predict correctly the motion of a bubble plume in a pseudo-two-dimensional bubble column operated at different superficial gas velocities, provided that a detailed description of the bubble dynamics is incorporated in thc model. The effect of bubble column aspect ratio on the hydrodynamic behaviour of the column has also been investigated. Our model predicts the effect of aspect ratio on the flow structure in the bubble column. The importance of the various forces acting on the bubbles will also be discussed and it will be shown that the added mass lbrce and the lift force cannot be neglected in bubble column simulation. Finally, the model has been used to study the start-up behaviour of a two-dimensional bubble column. It will be shown that the history of the gas-liquid two-phase flow significantly affects the flow structure ultimately obtained in a bubble column. This finding has, to our knowledge, not been reported before in literature..~, 1997 Elsevier Science Ltd. All rights reserved
A first-principles model for a gas-fluid&d bed based on the so-called "two-fluid model" (TFM) has been developed_ In the TFM approach, both phases are considered to be continuous and fully interpenetrating. The equations for mass, momentum and thermal energy conservation, supplemented with the necessary constitutive equations, have been sotved by a finite-difference technique on a mini-computer. The computer model calculates the porosity, the pressure, the fluid phase temperature, the solid phase temperature and the velocity fields of both phases in two-dimensional Cartesian or axi-symmetrical cylindrical coordinates. Contrary to previous modelling work, all important terms have been retained in the transport equations. As a test of the theoretical model, the phenomena associated with the formation, propagation and eruption of a single bubble in a cold-flow two-dimensional air-fluidized bed with one central orifice have been calculated theoretically. The calculation was done for mono-sized spherical solid particles with a diameter of 500 pm and a true density of 2640 kg/ma. Our preliminary calculations indicate strong leakage of bubble gas into the emulsion phase, especially during the initial stage of bubble formation. In its present state, the model does not correctly display all the details associated with the propagation of bubbles in gas-fluidized beds. The further development of the model, both from a physical (bed rheology) and mathematical (finite-difference approximations) point of view, Seems highly desirable.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.