1,720,970 research outputs found
The transition from the fixed to the fluidized state of well‐mixed binary‐solid mixtures in a liquid upflow
No full textIn this work, an investigation of the transition from fixed to fluidized state of initially homogeneous binary-solid mixtures due to an upward liquid flux was carried out. Like for analogous gas systems, it was confirmed that the full transition occurs over a range of fluid velocities, from the initial to the final fluidization velocity. Based on fluid dynamic equilibrium considerations, we proposed a simple model that quantitatively predicts the different bed configurations as a function of fluid velocity. Thanks to the model, all the mixtures were categorized into four regions, which determined their behavior during the transition process. Extensive experimental investigations supported model predictions, offering insights that had not been reported in previous studies
Cohesive particle–fluid systems: An overview of their CFD simulation
No full textSolid particles may experience different kinds of cohesive forces, which cause them to form agglomerates and affect their flow in multiphase systems. When such systems are simulated through computational fluid dynamics (CFD) programs, appropriate modelling tools must be included to reproduce this feature. In this review, these strategies are addressed for various systems and scales. After an introduction of the different forces (van der Waals, electrostatic, liquid bridge forces, etc.), the modelling approaches are categorized under three methodologies. For diluted slurries of very fine particles, many researchers succeeded with pseudo-single phase approaches, employing a model for the non-Newtonian rheology. This was especially popular for sludges in anaerobic digestions or certain types of soils. In other cases, continuum-based approaches seem to be more adequate, including cohesiveness in the kinetic theory of granular flows or the restitution coefficient. Geldart-A particles experiencing van der Waals forces are the primary focus of such studies. Finally, when each particle is modelled as a discrete element, the cohesive force can be directly specified; this is especially widespread for the wet fluidization case. For each of these approaches, a general overview of the main strategies, achievements, and limits is provided
An experimental assessment of fluid–solid drag models based on the pressure drop in bidisperse fixed beds
No full textExperimental data on binary fixed beds were gathered from the literature and produced by us through experiments in a vertical column fed with liquid water. Then, we used the fluid pressure drop data to compare the performance of the existing models that can predict the fluid-solid drag force for polydisperse systems. The results show that the models behave quite similarly and do not offer significant advantages compared to the Ergun equation, aside from being able to yield the force values for the different particle types. The accuracy of the predictions does not appear to be influenced by the Reynolds number and the particle diameter ratio but seems generally better for equivolumetric mixtures. This work highlights that more research is needed in this field and provides additional experimental data that were lacking for certain operational conditions and may be useful for future works
A Comparison of Ansys Fluent and MFiX in Performing CFD-DEM Simulations of a Spouted Bed
No full textThe CFD-DEM methodology is a popular tool for the study of fluid–particle systems, and there are several programs that permit using it. In this study, we employed it to simulate a pseudo-2D spouted bed, comparing the performance of the programs Ansys Fluent and MFiX. The results are analysed and commented on in terms of both accuracy and computational efforts. Despite the similarity of the setup, MFiX seems to perform significantly better. The similarities and differences between the two programs are discussed in detail, offering useful insights to researchers regarding the selection of one over the other, depending on the application. The better suitability of the Di Felice drag model is confirmed for the device, while it is shown that the effect of the Magnus lift force may be more limited than was shown in a previous study
A CFD–DEM study of the behaviour of single-solid and binary mixtures in a pyramidal spouted bed
No full textWe simulated a lab-scale cold-flow spouted bed through computational fluid dynamics (CFD), coupled with the discrete element method (DEM) for the solid phase, using a commercial CFD program, ANSYS FLUENT 18.0. To limit the computational expense, we tested both a simplified pseudo-2D geometry and a complete 3D geometry. We found that the Haider and Levenspiel drag model is suitable for the pseudo-2D geometry; however, this model does not correctly predict fluidisation in the 3D geometry. Conversely, the Gidaspow drag model behaves accurately in the 3D geometry but overestimates the motion of particles in the pseudo-2D geometry. We studied several single-solid and binary mixtures to assess the reproducibility of segregation phenomena. The pseudo-2D model was able to predict the onset and minimum spouting flow rates of all mixtures with good accuracy. An analysis of the volume fraction contours of the binary mixtures permitted us to confirm that segregation phenomena were correctly predicted at low gas velocities. We showed that segregation decreased as the inlet gas flow rate was increased. Calculations performed in the complete 3D geometry were preliminarily assessed as more reliable but required almost four times as much computational time as those for the pseudo 2D geometry
On the Origin of Sand Injectites in Bahrah Area, Northern Kuwait Bay: A Fluid Dynamic Numerical Modeling
No full textCurrent Status of Energy Production from Solid Biomass in Southern Italy
No full textThis work analyses and discusses data on thermochemical plants in Southern Italy that are fed with solid biomass. The analysis takes into account the biomass availability and potential together with the cost-benefit analysis using technology development and economic indicators (LCOE). A total of 63,762 units have been categorised according to the employed technology and produced energy: power plants for electricity production or cogeneration plants for combined heat and electricity production (53 plants) and thermal units for heat production (63,709 units). The eight regions of the area have noteworthy differences. In terms of electricity generated from solid biomass Calabria is by far the largest producer, followed by Apulia. Sicily, Sardinia and Molise provide lower amounts while Abruzzo, Basilicata and Campania generate almost negligible amounts. Regarding thermal production, Campania and Calabria are the largest producers, but Basilicata, Molise and Abruzzo generate the highest amount per capita. The area is far from fully exploiting its biomass potential, and there are also no district heating grids. Bioenergy can be remarkably competitive, provided that capital costs are relatively low and low-cost biomass is available, as it is the case of Italy. New applications and markets for sub-products (i.e., char, ash) would help in lowering the still not competitive economic indicators (LCOE)
Modelling of Spouted and Spout-Fluid Beds: Key for Their Successful Scale Up
No full textThe development of robust mathematical models could provide the necessary tools for a more rapid, efficient, and reliable spouted bed technology development. Computer simulations can be very useful to aid this design and scale-up process: firstly, they can contribute to obtain a fundamental insight into their complex dynamic behavior by understanding the elementary physical principles such as drag, friction, dissipation etc.; secondly, the simulations can be used as a design tool where the ultimate goal is to have a numerical model with predictive capabilities for gas-particle flows at engineering scale. Clearly, one single simulation method will not be able to achieve this goal, but a hierarchy of methods modelling phenomena on different length and time scales can achieve this. The most fruitful approach will be when they are simultaneously followed, so that they can mutually benefit from each other. In this sense, this paper presents a review of the current state of the art of modelling on spouted and spout-fluid beds through an analysis of recent literature following a multiscale approach (molecular and particle, lab, plant and industrial scale). The main features of the different scales together with their current limits are discussed and specific topics are highlighted as paths that still need to be explored. In summary, the paper aims to define the theoretical setline and the basis of improvement that would lead to a robust multiscale model with solid links between micro and macroscopic phenomena. If done with the correct balance between accuracy and computational costs it will gear SB towards their reliable and successful implementation
Adsorption of H2S on residual biomass gasification char
No full textChar is the solid by-product of biomass gasification. Plant owners usually dispose of it at a cost, but its high carbon content and surface area could make it suitable for further applications. In this work, we studied its potential to adsorb hydrogen sulphide (H2S), a common pollutant, e.g. present in the producer gas of gasification, as well as in biogas from anaerobic digestion. We tested different samples of char collected from commercial gasification plants in South Tyrol (Italy). The adsorption was reproduced in a lab-scale fixed bed reactor. The results highlights that all samples could capture hydrogen sulphide, showing different adsorption performance. The surface area of the char and the ash amount seem to affect the removal capacity, although other properties of the materials are probably important. The inlet concentration of hydrogen sulphide does not influence the adsorption capacity significantly
Characterisation of the char obtained from biomass gasification in a spouted bed reactor
No full textThere is a need to find ways of employing the solid residue of biomass gasification, char, to enhance the economic and environmental advantages of the process. The possible ways depend on the properties of char, which in turn depend on the conditions in which it is produced. For this study, gasification experiments were performed in a spouted bed gasification pilot plant, using wood pellets and apple pruning residues as feedstock. The char was then collected and characterised, in order to find its composition, ash content, heating value and specific surface area. The results highlight strong differences in the properties of the obtained char, depending on which material is obtained from and in which operating conditions; these properties are not dissimilar from what other studies report. The specific surface areas do not suggest the direct use of this char as activated carbon, but a further activation step or its use as a catalyst support might be possible ways for its valorisation
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