Solid flux in travelling fluidized bed operating in square-nosed slugging regime

The performance of gas-fluidized bed reactors depends significantly on their hydrodynamics. Among the important properties that dictate the characteristics of a gas-fluidized bed, local solid flux plays a significant role, influencing vital parameters such as bed-to-surface heat exchange and solid circulation rate. Developing techniques that can provide accurate measurements of solid flux is extremely important for: 1) assessing the accuracy of other measurement techniques applicable to industrial units, and 2) validation of CFD models. Comparison of different measurement techniques that provide similar hydrodynamic information is helpful in assessing the errors associated with each methodology. Most measurement techniques for obtaining solid flux in gas-fluidized beds are based on intrusive probes that can simultaneously measure solid velocity and voidage. Previously (1), the novel travelling fluidized bed (TFB) was operated to determine particle velocity from radioactive particle tracking (RPT), positron emission particle tracking (PEPT) and borescopy with silica sand particles of mean diameter 292 μm at superficial gas velocities from 0.4 to 0.6 m/s. In this study, the TFB, operated under identical conditions, was deployed to compare RPT and PEPT for the investigation of solid flux in square-nosed slugging. Both techniques provided solid flux data of the same order, but there were significant quantitative differences. Differing physical properties of tracer particles and the bed material, and differences in the tracer localization techniques are among the factors that contributed to the observed discrepancies. The results provide useful insights on the merits and challenges associated with advanced techniques for measuring solids flux in gas-fluidized beds. REFERENCES S. Tebianian, K. Dubrawski, N. Ellis, R. A. Cocco, R. Hays, S.B.R. Karri, T. W. Leadbeater, D.J. Parker, J. Chaouki, R. Jafari, P. Garcia-Trinanes, J.P.K. Seville, J.R. Grace. Comparison of Particle Velocity Measurement Techniques in a Fluidized Bed Operating in the Square-Nosed Slugging Flow Regime. Powder Technol., 2015. doi:http://dx.doi.org/10.1016/j.powtec.2015.08.040

Comparison of alternative advanced experimental techniques for measurement of hydrodynamic characteristics of gas-fluidized beds

A novel travelling fluidized bed, designed to facilitate deployment at different research centres, was used to compare advanced measurement techniques for the study of key hydrodynamic properties of gas-fluidized beds. Fast X-ray imaging was employed to visualize the internal flow structures of the bubbling and turbulent fluidization flow regimes. Transition between flow regimes based on X-ray system images were compared with results from pressure fluctuations. Average Shannon entropy reached a maximum plateau at superficial gas velocities close to Uc derived from pressure fluctuations, whereas average kurtosis and skewness leveled off at lower Ug’s. The degree of interference of a 4-mm intrusive probe inserted in the fluidized bed was found to be small by comparing the time-average voidage in a region with and without the probe present. Voidage data obtained by different measurement techniques in a previous study were extended by new data based on fast X-ray imaging and borescopy. Fair, but imperfect agreement among voidage results from alternate techniques was observed and quantified in terms of deviations from the overall average results of all measurement techniques at each gas velocity. Radial profiles of time-average particle velocity in FCC (a Geldart A powder) and sand (a Geldart B powder) fluidized beds at different operating conditions, obtained by radioactive particle tracking (RPT – non-invasive, Ecole Polytechnique), positron emission particle tracking (PEPT – non-invasive, University of Birmingham), optical fibre probe (invasive, UBC) and borescopic high-speed particle image velocimetry (invasive, PSRI) were directly compared. For FCC, each of these techniques provided similar trends with respect to profiles of time-average particle velocity, but with significant differences in some cases. For sand, there were significant quantitative differences among the profiles in many cases. The reasons for the discrepancies included lack of matching of tracer particles, probe intrusiveness, unmatched sensitivities to the direction of motion and different analysis procedures. The RPT, PEPT and borescopy data were further analyzed to obtain solid mass and momentum flux for identical operating conditions. All three techniques provided broadly similar time-average flux profiles. The experimental results obtained in this study provide a unique hydrodynamic benchmark database for validation of CFD codes and other models.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat

Flow structures and mixing patterns in the freeboard of gas-fluidized bed reactors

Flow structures and gas mixing patterns establishing in the freeboard of a bubbling gas-fluidized bed have been investigated by means of a planar laser light scattering (PLLS) technique. Nondiffusive tracing of the flow structures has been accomplished by injection of an aerosol of micrometer-sized scattering particles at a preset axial position in the splash zone of the reactor. A digital image acquisition and analysis technique has been developed and used to characterize the macroscopic and microscopic features of the flow structures. Experiments have been carried out in the freely bubbling regime at gas superficial velocities ranging from incipient fluidization to U/Umf = 2.6. Bed solids consist of glass beads of two different size ranges belonging to group B of the Geldart classification of powders. Results are analyzed with the aim of determining effective gas dispersion coefficients and characteristic time-scales for gas micromixing in the splash zone as a function of the excess gas superficial velocity

Biodiesel and electrical power production through vegetable oil extraction and byproducts gasification: Modeling of the system

Aim of this work is to introduce an alternative to the standard biodiesel production chain, presenting an innovative in situ system. It is based on the chemical conversion of vegetable oil from oleaginous crops in synergy with the gasification of the protein cake disposed by the seed press. The syngas from the gasifier is here used to produce electrical power while part of it is converted into methanol. The methanol is finally used to transform the vegetable oil into biodiesel. Through a coupled use of ASPEN PLUS (TM) and MATLAB (TM) codes, a rapeseed, soy and sunflower rotation, with a duration of three year, was simulated considering 15 ha of soil. This surface resulted sufficient to feed a 7 kW(el) power plant. Simulation outputs proven the system to be self-sustainable. In addition, economical NPV of the investment is presented. Finally the environmental, economical and social advantages related to this approach are discussed

Characterization of a pressurized feeder for biomass injection into gas-solid systems

Injection of biomass powders into multiphase reactors is a challenging task that may impact the process yield and limit the operation due to the feeder’s blockage or unstable flow. A horizontal pressurized gas-solid feeder is characterized in this study to inject a batch of biomass powders of different diameters into cold-flow model operating under single phase, and multiphase fixed and fluidized regimes. The gas-solid hydrodynamics in the injection line is investigated for different operating conditions, using a fast camera with acquisition rate of 3000 images/s. Stable injections are performed in a wide range of solids fluxes, and under different solid concentration flow conditions. The operation map for the feeder is produced, and the phenomena that produce difficulty of stable injection are discussed. Ultimately, this type of solids injector is useful for feeding powders into lab-scale reactors for mixing and kinetic studies and might be also scaled-up for treating waste biomass powders into pilot-to-industrial scale reactors under batch or semi-continuous operations

CFD simulation for characterization and scale-up of pulsed biomass transport

Stable feeding of biomass powders into reactors represents a technical operational challenge for renewable energy generation. The injection of sawdust powders has been recently published with a horizontal pressurized gas injector under several experimental conditions. Valid numerical models based on computational fluid dynamics (CFD) are useful for hydrodynamics characterization of different equipment scales. In this study, the applicability of the CFD multiphase particle-in-cell approach (MP-PIC) for the considered injector is investigated. The model is tested under different operating conditions, showing relative deviations lower than 14% and 2% respectively for solids flux and flow concentration experimental data. The effect of model inputs (mesh refinement, drag model, particle-to-wall interaction) on the system’s hydrodynamics is discussed. The gas–solid flow hydrodynamics is obtained from the simulations providing additional insight on pulsed biomass transport. Ultimately, the model is used to investigate different injector diameters and to propose a feeder operation map as a function of dimensionless parameters