Juray De Wilde

The effects of abrupt T-outlets in a riser: 3D simulation using the kinetic theory of granular flow

Abstract Gas–solid flow in circulating fluidized beds is calculated using the Eulerian–Eulerian approach with the kinetic theory of granular flow. The usefulness of this approach and the necessity of performing 3D calculations are illustrated by calculating the exit effects of single abrupt outlet configurations of different outlet surface area.

Reformulating and quantifying the generalized added mass in filtered gas-solid flow models

To account for mesoscale phenomena in coarse grid simulations, Reynolds stress terms appearing in the filtered gas-solid flow equations have to be modeled. A generalized added mass approach previously proposed by Zhang and VanderHeyden [Int. J. Multiphase Flow 28, 805 (2002)] to model the acoustic gas-solid interaction Reynolds stress term is analyzed. Theoretically, it is shown that a generalized added mass term appears directly from the filtered acoustic gas-solid interaction term and that it corresponds to a redistribution of the filtered gas phase pressure gradient over the phases. This direct contribution scales according to the mean square of the solid volume fraction fluctuations. Two-dimensional dynamic mesoscale simulations over a broad solid volume fraction range and for two domain sizes and two grid resolutions are carried out to calculate the magnitude of the generalized added mass effect. Calculated values of the mean square of the solid volume fraction fluctuations are qualitatively in agreement with the experimental observations of Zenit and Hunt [Int. J. Multiphase Flow 26, 763 (2000)]. A second, indirect contribution to the generalized added mass term from the filtered acoustic gas-solid interaction term is shown to be statistically significant, but one order of magnitude smaller than the direct contribution. A further quantification of the maximum generalized added mass effect as a function of the filter frequency is obtained from a mixture speed of sound test. The results show that a large generalized added mass coefficient, as previously reported by Zhang and VanderHeyden [Int. J. Multiphase Flow 28, 805 (2002)], is justified only in case the filter frequency is low (< 20 Hz), i.e., if the grid is, either spatially or temporally, sufficiently coarse. (c) 2005 American Institute of Physics.

An extension of the preconditioned advection upstream splitting method for 3D two-phase flow calculations in circulating fluidized beds

Abstract For the calculation of gas–solid flow in circulating fluidized beds, a Eulerian–Eulerian approach is taken. An integration scheme based on dual time stepping and a finite volume technique is developed and implemented in 3D. The inviscid part of the equations is treated following an extension of the preconditioned advection upstream splitting method (AUSMP) to two-phase flows. Calculations on an industrial size straight riser are performed. The influence of the inelasticity of particle–particle collisions on the stability of the flow is investigated. Further, the effects of a double abrupt side outlet configuration are shown.

Drying of Biomass Particles: Experimental Study and Comparison of the Performance of a Conventional Fluidized Bed and a Rotating Fluidized Bed in a Static Geometry

The performance of a conventional fluidized bed and a rotating fluidized bed in a static geometry (RFB-SG) for the drying of woody biomass is experimentally studied. The performance with respect to the specific biomass drying rate, product uniformity, and air utilization is evaluated. RFB-SGs are shown to intensify the drying process by easily one order of magnitude, due to a combination of increased gas–solid slip velocities and improved particle bed density and uniformity. The cost in terms of increased and less efficient air utilization can be minimized by optimizing the RFB-SG drying chamber design.

The generalized added mass revised

The reformulation of the generalized or apparent added mass presented by De Wilde [Phys. Fluids 17, 113304 (2005)] neglects the presence of a drag-type force in the gas and solid phase momentum equations. Reformulating the generalized added mass accounting for the presence of a drag-type force, an apparent drag force appears next to the apparent distribution of the filtered gas phase pressure gradient over the phases already found by De Wilde in the above-cited reference. The reformulation of the generalized added mass and the evaluation of a linear wave propagation speed test then suggest a generalized added mass type closure approach to completely describe filtered gas-solid momentum transfer, that is, including both the filtered drag force and the correlation between the solid volume fraction and the gas phase pressure gradient

Low-Temperature Pyrolysis and Gasification of Biomass: Numerical Evaluation of the Process Intensification Potential of Rotating- and Circulating Rotating Fluidized Beds in a Static Fluidization Chamber

The process intensification potential of rotating- and circulating rotating fluidized beds in a static fluidization chamber when used for the low-temperature pyrolysis and gasification of biomass is numerically evaluated. The species continuity equations and energy balance equations are based on complete mixing for the particles within given zones of the particle bed and plug flow for the gas. The reaction mechanism accounts for pyrolysis of biomass and tar, gas-phase combustion, the water gas shift reaction, and combustion and gasification of char. A comparison with current circulating fluidized bed riser technology is made. The circulation of inert solid with a high heat capacity and the separation of the flue gas from the production gas are also studied.

Kinetic modeling of the thermal degradation of methacrylate copolymers by thermogravimetric methods

The thermal degradation kinetics of a random copolymer of isobutyl methacrylate / lauryl methacrylate, produced by free-radical solution polymerization is investigated over a temperature range of 350 to 750 K, using dynamic thermogravimetric experiments. Heat treatment of the copolymer affects the main polymer backbone and side chains. The thermal degradation of the copolymer proceeds in three distinct steps of weight loss: the first and easiest step is initiated by scissions of head-to-head linkages representing one type of defect in the polymer backbone; the second and more difficult step is initiated by scissions at the vinylidene chain ends; the third and most energetic step is initiated by random scissions within the polymer chain.The time evolution of molecular weight distribution (MWD) is measured by gel permeation chromatography (GPC). The most pronounced changes in the trend of the average molecular weight are observed during the transition from each degradation step to the subsequent one. A continuous distribution kinetic model based on a population balance is developed to describe the observed degradation behaviour of the copolymer. This comprehensive model conforms to the special mechanisms for random chain-scission and chain-end depolymerization. The pseudo-kinetic rate parameters for each degradation step are estimated to be respectively equal to 1.1 10-8, 5.6 10-8 and 1.08 10-7 mol g-1 min-1. The average calculated activation energies are respectively 89.2, 116.4 and 134.8 kJ/mol.Global kinetic parameters of degradation are also determined using dynamic thermogravimetric (TGA/DTGA) data. The model-fitting and model-free isoconversional methods are used to retrieve the kinetic parameters of the degradation process. The model-free isoconversional method can satisfactorily describe the dependence of the activation energy on the conversion and is recommended over the model-fitting methods for obtaining the reliable and consistent kinetic parameters of polymer degradation.

Atmospheric-pressure plasma enhanced chemical deposition: role of the reactor flow dynamics

An atmospheric- instead of low-pressure plasma deposition reactor for the inline production and processing of thin films is investigated. Process optimization and scale-up require a fundamental understanding of the reactor behaviour. The influence of the reactor design and the operating conditions on the reactor hydrodynamics and the film deposition rate are studied experimentally and by means of detailed computational fluid dynamics simulations. The influence of the process gas and its flow rate to the inlet channels, the precursor injection velocity and different geometric reactor design parameters is focused on.

Experimental study of paddy drying in a vortex chamber

ABSTRACT The intensification of interfacial mass, heat, and momentum transfer makes vortex chambers potentially interesting for the efficient drying of paddy, allowing shorter drying times and/or more compact equipment. The presence of a shell introduces particular challenges. Intraparticle diffusion limitations are strong and may reduce the advantage from intensified interfacial mass and heat transfer and the efficiency of air usage. Furthermore, high shear and normal stresses in the fast rotating particle bed may cause damage to the paddy shell, posing problems for transport and storage. With these specific aspects in mind, the use of vortex chambers for paddy drying is experimentally evaluated.

Experimental and numerical study of rotating fluidized beds in a static geometry

The new concept of a rotating fluidized bed in a static geometry was numerically and experimentally studied. The particle bed can be both tangentially and radially fluidized by injecting the fluidization gas tangentially in the static fluidization chamber via multiple gas inlet slots located in its outer cylindrical wall. The tangential fluidization of the particles induces a rotating motion of the particle bed. As a result of the particle bed rotational motion, the solids experience a radially outwards centrifugal force. A radially inwards gas-solid drag force and radial fluidization of the particle bed can be introduced by forcing the fluidization gas to leave the fluidization chamber via a chimney with one or multiple gas outlet slots, positioned at the axis of the fluidization chamber. The solids can be continuously fed and removed in and out of the fluidization chamber via solids inlet and outlet holes in the front or back ends of the fluidization chamber.The fluidization patterns of low-density polymer particles with a large diameter and of high-density salt particles with a small diameter were experimentally studied in a 24-cm diameter, 13.5-cm long non-optimized static fluidization chamber at different solids loadings. Scale-up to a 36-cm diameter fluidization chamber was also studied. With both types of particles, a rotating fluidized bed and an acceptable gas-solid separation was obtained provided that the solids loading was sufficiently high. Slugging and channeling and a non-uniform distribution of the gas over the gas inlet slots to the fluidization chamber may occur at low solids loadings and can be detected via well-chosen pressure measurements. The fluidization patterns observed in the same fluidization chamber were completely different with the polymer particles and with the salt particles. The polymer particles tend to form a dense and uniform bed, its behavior being mainly characterized by tangential fluidization. The salt particles tend to form a less dense, bubbling fluidized bed that is both tangentially and radially fluidized.Computational fluid dynamics simulations give an improved insight in the gas and solid phase flow pattern.