Axial dispersion of solids in spiral fluidised beds
TL;DR: In this article, the residence time distribution of solids in spiral fluidized beds was experimentally investigated, covering as variables the flow rates of the phases, the particle characteristics and the column and plate geometries.
Abstract: The residence time distribution (RTD) of solids in spiral fluidised beds was experimentally investigated, covering as variables the flow rates of the phases, the particle characteristics and the column and plate geometries. The RTD was adequately described by the Fickian diffusion model, indicating that particle dispersion can be predicted from a knowledge of particle diffusivity. Correlations for the particle mean residence times and the particle diffusivity are presented.
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TL;DR: In this article, the authors review modification techniques for fluidized beds from the perspective of the principles of process intensification (PI), that is, effective bubbling suppression and elutriation control.
Abstract: Fluidized beds enable good solids mixing, high rates of heat and mass transfer, and large throughputs, but there remain issues related to fluidization quality and scale-up. In this work I review modification techniques for fluidized beds from the perspective of the principles of process intensification (PI), that is, effective bubbling suppression and elutriation control. These techniques are further refined into (1) design factors, e.g. modifying the bed configuration, or the application of internal and external forces, and (2) operational factors, including altering the particle properties (e.g. size, density, surface area) and fluidizing gas properties (e.g. density, viscosity, or velocity). As far as two proposed PI principles are concerned, our review suggests that it ought to be possible to gain improvements of between 2 and 4 times over conventional fluidized bed designs by the application of these techniques.
61 citations
TL;DR: The utility of the VRT concept for cardiovascular studies, particularly if a subpopulation of all particles is to be tracked, is demonstrated, and it is concluded that the volumetric residence time is a useful tool.
Abstract: The quantification of particle (platelet) residence times in arterial geometries is relevant to the pathogenesis of several arterial diseases. In this manuscript, the concept of "volumetric residence time" (VRT) is introduced. The VRT takes into account where particles accumulate and how long they remain there, and is well-suited to characterizing particle distributions in the complex geometries typical of the cardiovascular system. A technique for the calculation of volumetric residence time is described, which assumes that platelets are neutrally buoyant passive tracer particles, and which tracks small Lagrangian fluid elements containing a uniform concentration of platelets. This approach is used to quantify particle (platelet) residence times in the region of a modeled stenosis with a 45 percent area reduction. Residence time distributions are computed for a representative population of platelets, and for a subpopulation assumed to be "activated" by exposure to shear stresses above a threshold value. For activated platelets, high particle residence times were observed just distal to the apex of the stenosis throat, which can be explained by the presence of high shear stresses and low velocities in the throat immediately adjacent to the vessel wall. Interestingly, the separation zone distal to the stenosis showed only modestly elevated residence times, due to its highly mobile and transient nature. This calculation demonstrates the utility of the VRT concept for cardiovascular studies, particularly if a subpopulation of all particles is to be tracked. We conclude that the volumetric residence time is a useful tool.
50 citations
TL;DR: In this paper, the authors interpreted the residence time distribution of coffee berries on a vibrated tray dryer with recycle by means of the dispersion coefficients and the Peclet number.
Abstract: This paper interprets the experimentally measured residence time distribution of coffee berries on a vibrated tray dryer with recycle by means of the dispersion coefficients and the Peclet number Drying was carried out on a vibrated tray dryer operating with recycle consisting of four parts: a vertical drying tunnel, vibration system, warm air supply to the drying tunnel and recycle system of coffee berries Using the stimulus–response method the flow behavior of the coffee berries was examined The dispersion coefficients were calculated by the Taylor Dispersion Model and Free Dispersion Model The differences in prediction of the dispersion coefficient between the two models were appreciable, but the more reliable values for the dispersion coefficients E z were those obtained by the Free Dispersion Model The dispersion coefficient (Free Dispersion) ranged from 232 × 10 −4 to 7681 × 10 −4 m 2 /s The average Peclet number, Pe was approximately equal to 65, despite greatest variation of the E z Therefore, flow velocity variation of coffee berries was the same magnitude of the dispersion coefficient variation
8 citations
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...Pydisetty, Krishnaiah, and Varma (1989) indicated a correlation for the dispersion coefficient of copper particles processing in fluidized bed by using nickel as a tracer, where Ez was in the range of 0.35 · 10 � 4 to 2.36 · 10� 4 m2/s, with particle diameter of 4 · 10� 4 m, operating with...
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TL;DR: In this article, a simple theory for the packing of irregular particles and mixtures considered as a perturbation of a packing of monodisperse spheres is presented. But this theory assumes that the irregular particles are of different shapes and sizes.
Abstract: We present a simple theory for the packing of irregular particles and mixtures considered as a perturbation of a packing of monodisperse spheres. Using the perturbation theory we extract the different geometrical effects which effect the packing density, and discuss why packings of irregular particles are generally less dense than packings of monodisperse spheres, whereas packings of mixtures of particles of different sizes are generally denser than monodisperse sphere packings.
7 citations
TL;DR: In this paper, a system of hard smooth, nearly elastic spherical particles is considered as a model of granular material flowing under dynamic conditions, i.e., when particle inertia is responsible for the transfer of momentum in the material.
Abstract: A system of hard smooth, nearly elastic (coefficient of restitution R → 1) spherical particles is considered as a model of granular material flowing under dynamic conditions, i.e . when particle inertia is responsible for the transfer of momentum in the material. On the basis of the elaborated theory (a generalization of the Enskog kinetic theory of dense gases), two different problems are considered: rapid shear flow of granular material and vibrofluidization of powder. Calculations for rapid granular material flow are in accordance with results previously obtained by other authors. For vibrofluidized beds a new microrheological model is elaborated, which is in agreement with the results of computer simulations and experimental data.
3 citations
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TL;DR: In this article, the influence of operating variables on the mean and variance of the residence time distribution for material transport in a continuous rotary drum is described and an approximate form of the exact solution to the axial dispersion model for an impulse input is provided.
Abstract: Experimental results for the influence of operating variables on the mean and variance of the residence time distribution for material transport in a continuous rotary drum are described. The variables considered are drum rotational speed, drum inclination, feed rate of material and particle size of material. A stimulus-response technique based on an impulse of a colored tracer, otherwise identical to the bulk material, was used. The validity of the axial dispersion model for the description of the behavior of this non-segregating transport system is demonstrated. This is accomplished with an approximate form of the exact solution to the axial dispersion model for an impulse input; it is valid for the relatively large values of the Peclet number characteristic of the experimental system.
57 citations
TL;DR: A critical survey of the different models proposed for solids transport in gas-fluidized beds is given in this paper, where it is shown that many of the models can fit the residence time distribution curve (the F t -t curve) fairly well despite the fact that the physical behaviour of the bed is not recognized in these models.
Abstract: A critical survey is given of the different models proposed for solids transport in gas-fluidized beds. It is shown that many of the models can fit the residence time distribution curve—the F t -t curve—fairly well despite the fact that the physical behaviour of the bed is not recognized in these models. The intensity curve is recommended as a good tool for comparison of the models with the experiments.
16 citations
TL;DR: In this paper, the residence time distribution of the solids in multistage fluidisation, wherein downcomers for transferring solids from stage to stage are provided to the horizontal perforated plates, is experimentally investigated covering a wide range in process variables using flat, baffle and spiral plates developed in the study.
Abstract: The residence time distribution of the solids in multistage fluidisation, wherein downcomers for transferring the solids from stage to stage are provided to the horizontal perforated plates, is experimentally investigated covering a wide range in process variables using flat, baffle and spiral plates developed in the study. The RTD data is modelled using (i) the multiple parameter model, (ii) the diffusional mixing model and (iii) the fractional tank extension model when it is noted that over certain conditions of operation both spiral and baffle plates indicate near piston-flow for the solids without hampering the fluidisation characteristics.
15 citations