Dynamically structured fluidization: Oscillating the gas flow and other opportunities to intensify gas-solid fluidized bed operation

Various approaches to structure gas-solid fluidized beds are reviewed, followed by detailed discussion on the use of gas pulsation to induce dynamic structuring. Granular media are dissipative systems, which develop complex spatiotemporal patterns when excited by an oscillating energy source. Here, we discuss how such perturbations initiate surface patterns and how these could propagate into a macroscopically organized flow. We call this dynamically structured fluidization. Vibrated shallow granular layers form ordered surface waves. The hydrodynamics of pulsed gas-fluidized layers are related, but more complex: Under appropriate conditions, surface waves transition into a three-dimensionally ordered bubbling flow. This occurs in much deeper granular beds than under vibration, indicating distinct physics. In this dynamically structured state, bubbles organize into a scalable sub-harmonic, triangular lattice that is highly predictable and responsive to changes in oscillation parameters, allowing for an unprecedented level of control. Structured bubbling is observed only under sufficiently dense conditions; thus, a dynamically structured fluidized bed sits between fixed and fluidized beds, offering opportunities for process intensification, due to less macromixing than traditional fluidization, but a higher level of control through micromixing. This informs new intensified designs for processes that are highly exothermic, involve particle formation, thermally sensitive or high-value materials

Confined two-dimensional fermions at finite density

We introduce the chemical potential in a system of two-dimensional massless fermions, confined to a finite region, by imposing twisted boundary conditions in the Euclidean time direction. We explore in this simple model the application of functional techniques which could be used in more complicated situations.Comment: 15 pages, LaTe

Finite density and temperature in hybrid bag models

We introduce the chemical potential in a system of two-flavored massless fermions in a chiral bag by imposing boundary conditions in the Euclidean time direction. We express the fermionic mean number in terms of a functional trace involving the Green function of the boundary value problem, which is studied analytically. Numerical evaluations for the fermionic number are presented.Comment: 19 pages, 4 figure

On the role of energy dissipation in a dynamically structured fluidized bed

This work explores the effect of interparticle friction on the stability of a structured bubble flow in gas–solid fluidized beds. We provide a detailed quantification of the evolution of bubble properties at varying frequency, comparing experiments with CFD-DEM (computational fluid dynamics – discrete element modeling) simulations. Friction plays a key role. It creates intermittent solid-like regions that restrict the mobility of solids and endow the flow with enough memory to correlate consecutive nucleation events. As friction decreases, solid-like regions widen, allowing the circulation of solids; simultaneously, bubbles grow, move apart and ultimately break up the structure. CFD-DEM reproduces this phenomenon well in a small bed, but shows qualitative differences in bubble shape and acceleration. These deviations propagate into substantial errors at higher frequency or larger domains displaying multiple bubble rows, which stresses the need for further research to understand the effects of other particle properties, polydispersity and the domain size

Agglomeration during spray drying: Air-borne clusters or breakage at the walls?

Particle agglomeration, wall deposition and resuspension are inherent to many industries and natural processes, and often inter-connected. This work looks into their relation in a confined particle laden swirling flow. It investigates how the size of detergent powder spray dried in a swirl counter-current tower responds to changes in the air flow. Four sets of sprays are investigated under varying combinations of air temperature and velocity that cause the same evaporation. The use of high air velocities accumulates more of the droplets and dry powder in the chamber swirling faster, but it leads to creation of a finer product. Particle-particle and particle-wall contacts are made more frequent and energetic but in turn the swirl troughs the solids to the wall where deposits constantly form and break. Past PIV and tracer studies revealed that the rates of deposition and resuspension are balanced; the data discussed here indicate that the dynamic nature of the deposits is a major contributor to particle formation. In contrast with the usual assumption, the product size seems driven not by inter-particle contacts in airborne state but the ability of the solids to gain kinetic energy and break up a collection of clusters layering on the wall. As a result, the dryer performance becomes driven by the dynamic of deposition and resuspension. This paper studies the efficiency of limiting operation strategies and shows that a low temperature design concept is better suited to control fouling phenomena and improve capacity and energy consumption

Agglomeration in counter-current spray drying towers. Part B: Interaction between multiple spraying levels

A new experimental method is developed here to investigate agglomeration in spray drying towers operating with multiple nozzles. It allows studying independently the contribution of each spray to the product and obtaining a valuable insight into the agglomeration processes. The paper studies a two level swirl counter-current dryer of detergent in a full-scale production system. It shows that operation with two nozzle levels increases the energy efficiency compared to the use of single sprays, but in turn promotes both agglomeration and elutriation of powder from the top of the dryer. The product size distribution becomes bi-modal and the composition and porosity of the product more heterogeneous due to the different thermal histories experienced by droplets from each spray. The method described here controls the air temperature and humidity nearby the nozzles to quantify the agglomerates resulting from particle contacts within each individual spray or from their interaction. Particle agglomeration is shown to be suppressed at the bottom of the dryer where the heat transfer rates are highest and promoted at the top spray, which originates a second coarse mode in the size distribution. Both levels do not operate independently; the powder elutriated upwards from the bottom nozzle is captured entirely by the top spray when it is centrally located. By isolating the independent impact of each nozzle in a dryer, the method provides powerful data to correlate the agglomeration behaviour with local process conditions, and so facilitate the development and validation of spray dryer models

A colour atlas of diseases of the vulva and perigenital area

This international congress will put a lot of work into female health. We therefore de\uaccided to present our clinical experience about an issue that is rarely discussed in der\uacmatological congresses, i.e. the diseases of the vulva and perigenital area. The diseases will be divided into different chapters: hereditary diseases (Darier disease); infectious diseases (herpes genitalis, genital warts, candidiasis, impetigo, erysipelas, syphilis); inflammatory diseases (allergic contact dermatitis, fixed drug eruption, psoriasis, lichen planus, lichen sclerosus, pemphigus) and neoplastic dis\uaceases (bowenoid papulosis, basal cell carcinoma, Bowen's disease, squamous cell carcinoma, extra-mammary Paget's disease, melanoma, Kaposi's sarcoma, T-cell lym\uacphoma and Langerhans' cell histiocytosis). All these diseases will be discussed ac\uaccording to clinical, histopathological and therapeutical points of view

Tensor gauge fields in arbitrary representations of GL(D,R): II. Quadratic actions

Quadratic, second-order, non-local actions for tensor gauge fields transforming in arbitrary irreducible representations of the general linear group in D-dimensional Minkowski space are explicitly written in a compact form by making use of Levi-Civita tensors. The field equations derived from these actions ensure the propagation of the correct massless physical degrees of freedom and are shown to be equivalent to non-Lagrangian local field equations proposed previously. Moreover, these actions allow a frame-like reformulation a la MacDowell-Mansouri, without any trace constraint in the tangent indices.Comment: LaTeX, 53 pages, no figure. Accepted for publication in Communications in Mathematical Physics. Local Fierz-Pauli programme achieved by completing the analysis of Labastid

Solids lateral mixing and compartmentalization in dynamically structured gas–solid fluidized beds

An adequate use of gas pulsation can create an ordered, dynamically structured bubble flow in a bed of Geldart B particles. A structured bed is more homogeneous, responds to external control and is scalable. While earlier studies have focused on describing the self-organization of the gas bubbles, the solid mixing and gas–solid contact patterns have remained unclear. In this work, the solids circulation and mixing behavior in structured and unstructured beds at various pulsation frequencies are compared with a traditional fluidized bed operation. The degree of lateral mixing is hereby quantified through an effective lateral dispersion coefficient extracted from CFD-DEM (discrete element modelling) simulations in a thin fluidized bed system. Mixing shows major quantitative and qualitative differences amongst the investigated cases. The coordinated motion of the gas bubbles wraps the solid flow into a series of compartments with minimal interaction, whereby effective lateral dispersion coefficients are an order of magnitude lower than in an unstructured operation. More importantly, unlike a traditional bed, dispersion in a structured bed is driven by advection and is no longer a diffusive process. Compartmentalization decouples the time scales of micro- and macromixing. Every pulse, the compartments rearrange dynamically, causing a level of local axial mixing that is scale-independent. While further work is necessary to fully understand the compartmentalization at a larger scale, the circulation described here indicates that a dynamically structured bed can provide a tight control of mixing at low gas velocities and a narrower distribution of stresses in the solid phase compared to traditional devices

Gravitational diffraction radiation

We show that if the visible universe is a membrane embedded in a higher-dimensional space, particles in uniform motion radiate gravitational waves because of spacetime lumpiness. This phenomenon is analogous to the electromagnetic diffraction radiation of a charge moving near to a metallic grating. In the gravitational case, the role of the metallic grating is played by the inhomogeneities of the extra-dimensional space, such as a hidden brane. We derive a general formula for gravitational diffraction radiation and apply it to a higher-dimensional scenario with flat compact extra dimensions. Gravitational diffraction radiation may carry away a significant portion of the particle's initial energy. This allows to set stringent limits on the scale of brane perturbations. Physical effects of gravitational diffraction radiation are briefly discussed.Comment: 5 pages, 2 figures, RevTeX4. v2: References added. Version to appear in Phys. Rev.