1,720,977 research outputs found
The growing problem of waste bioplastics disposal, and a way to tackle it
No full textBiodegradable plastics will grow significantly in the coming years thanks to their potential for renewability and circularity, particularly in certain European countries. Despite being a niche, their disposal is already emerging as a challenge. Insufficient public awareness is a partial cause, with people being unaware or confused on how to deal with them. Other problems are inherent to the way organic waste treatment plants work. There is indeed a stark contrast between standard biodegradability tests and actual conditions at treatment facilities, often dismissed or ascribed to the negligence of plant operators. If bioplastics are really to be deployed on a large scale, a joint technological effort is essential to properly manage their end-of-life. This contribution briefly summarises the scenario of bioplastic use in the world, the European legislation on the matter (or lack thereof), and the difficulties that waste management plants face with these materials, with a focus on a reference case - Italy - and with an international perspective. Finally, we explore how hydrothermal treatments are emerging in the scientific literature as a possible solution to some of these problems, as they can solubilise most bioplastics and facilitate their treatment in the most updated organic waste treatment plants, which couple anaerobic digestion and composting
Mixing Due to Natural Convection in a Hydrothermal Reactor: A CFD Study
No full textThe successful design of biomass hydrothermal reactors could be substantially enhanced by computational fluid dynamics (CFD) simulations. However, their performance is still limited by the complexity of the involved physicochemical phenomena and the scarcity of experimental data. This study employs such simulations to analyze the thermo-fluid dynamics of an unstirred batch hydrothermal reactor. By assessing the reactor’s response to a lateral heat source, we show that correctly including various phenomena, such as phase transfers and temperature-dependent properties, enables accurate reproduction of natural convection, providing a more solid base for CFD studies on hydrothermal units. The results provide valuable insights into the reactor’s inner mixing, which for pure water appears adequate despite the absence of stirring. Additionally, we explore two approaches to include a biomass fixed bed or a non-Newtonian sludge. Discussing future efforts needed to make the simulation of such media more established, we show that their presence may indeed hinder mixing, with implications for the products’ quality and safety and for hydrothermal processing control and design
A CFD-DEM study of monocomponent and same size binary-solid beds at incipient fluidization
No full textThis work analyses in detail the transition process from the fixed to the fluidized state for homogenous and binary mixtures of particles differing in density, with different initial configurations. The phenomenon is studied in cylindrical columns fed with water, with the aid of CFD-DEM numerical simulations. With regard to the homogenous case, the well-established knowledge is substantially confirmed. The transition to the fluidized state strongly depends on the initial solid configuration in the case of same density binary-solid mixture. For the segregated case, the experimental behaviour reported by Di Felice and Scapinello in 2010 is closely reproduced, whereas for the initially perfectly mixed case, the contemporary presence of a fixed and a fluidized region is observed. A simple model that predicts the extent of these two regions as a function of the fluid velocity is proposed
On the influence of contact models on friction forces in discrete element method simulations
No full textThe discrete element method (DEM) is an important tool to simulate granular systems with high accuracy. Depending on the application, it is often unclear which model is more appropriate for the calculation of collision forces: the Hertzian approach is generally considered more accurate, but it makes the simulation significantly slower than the Hookean one. In this work, these two approaches are compared in two different situations: the stress distribution of static particles in a cylindrical column (DEM) and the onset of water fluidisation for a completely segregated mixture (CFD-DEM). In both cases, particle contact forces are of great relevance to determine the output. It is found that in the first case the Hookean approach does not produce the expected asymptotical stress trend and does not even respond satisfyingly to friction mobilisation. Conversely, in the fluidisation simulations the results are virtually identical, pointing out that the more complex Hertzian approach may be unnecessary in that case
A discrete element method study of solids stress in cylindrical columns using mfix
No full textFriction phenomena play a key role in discrete element method (DEM) modeling. To an-alyze this aspect, we employed the open-source program MFiX to perform DEM simulations of cylindrical vertical columns filled with solid particles. These are still associated with and described by the pioneering model by the German engineer H.A. Janssen. By adapting the program’s code, we were able to gather numerous insights on the stress distribution within the solids. The column was filled with different amounts of solids and, after the system had stabilized, we assessed the pressure in the vertical and radial directions and the distribution of the friction force for all particles. An analysis of the bottom pressure for varying particle loads allowed us to infer that the program can correctly predict the expected asymptotical behavior. After a detailed assessment of the behavior of a single system, we performed a sensitivity analysis taking into account several of the variables employed in the simulations. The friction coefficient and filling rate seem to affect the final behavior the most. The program appears suitable to describe friction phenomena in such a static system
Current status of energy production from solid biomass in North-West Italy
No full textData on the thermochemical plants fed by solid biomass in the north-west area of Italy (Liguria, Lombardy, Piedmont and Aosta Valley) have been organised, analysed and discussed. Moreover, the biomass availability and potential has been evaluated. A total of 28,167 plants have been categorised according to their typology and output: thermo-electric power plants for electricity production, thermal plants for heat production, cogeneration plants for combined heat and electricity production and district heating installations for local heating purposes. In general, separate observations for the different provinces may be drawn. Liguria stands out as the most evident case of under-exploited biomass potential, followed by Aosta Valley, which, however, is rich in hydroelectricity. Lombardy and Piedmont are more virtuous and have several plants in their territory. The construction of new plants and the upgrade of existing ones may bring noteworthy benefits, as well as the use of added value sub-products to foster circular economy approaches
Hydrothermal Carbonization as a Process to Facilitate the Disposal of Bioplastics
No full textBioplastics are steadily replacing fossil-based plastics due to their renewable origin and biodegradability. However, their end-of-life can be problematic: they are often collected with the organic fraction of municipal solid waste (OFMSW) but can be not satisfactorily biodegradable in plants that treat it, leading to their rejection at the entrance. This work focuses on five different commercial bioplastics employed in the eyewear industry: two based on cellulose acetate, one on galalithe, one on corn starch and one on polyamide. The aim was to assess their treatability via hydrothermal carbonization (HTC), which was never reported in the literature for these materials. Under HTC at 180 and 220°C for 1 h, four of the tested bioplastics show significant degrees of degradation, leading to the formation of different solid and liquid products, which were respectively characterised according to their elemental composition and pH. The interesting different behaviours may be ascribed to the different compositions and structures of the materials. HTC appears as a viable route to facilitate the degradation of these recalcitrant materials and may be followed by a material recovery or an energetic valorisation through anaerobic digestion or thermochemical pathways, depending on the purity of the waste stream
A CFD-DEM sensitivity analysis: The case of a pseudo-2D spouted bed
No full textThe coupling of computational fluid dynamics (CFD) and the discrete element method (DEM) permits to simulate complex multiphase flows, and has been widely used for research and industrial purposes. However, this approach relies on many sub-models and parameters, whose influence is not always clear. In this work, we performed an extensive sensitivity analysis, modifying one sub-model or parameter at a time and assessing the effect on the results. We run our simulations through the commercial program Ansys Fluent 19.1. We considered experimental data reported in literature for a pseudo bi-dimensional spouted bed, containing 2.033 mm glass particles. Spouted beds are suitable to process coarse, poli-dispersed and irregular particles, and are hence a popular research subject, often simulated through the CFD-DEM approach. Our results slightly overestimate the highest position reached by the particles, possibly because of the limits in the mesh refinement. The sensitivity analysis shows that the drag model, restitution coefficient, friction coefficient and Magnus lift model have the strongest influence on the particles' trajectories. It is important to include turbulence, the Magnus lift effect and to consider the rotation of particles. These results clarify the influence of the considered parameters and models in CFD-DEM, and can be considered for the optimisation of CFD-DEM simulations. Thanks to the conclusions of the sensitivity campaign, we enhanced the accuracy of our model
CFD-DEM simulations of a continuous square-based spouted bed and evaluation of the solids residence time distribution
No full textCFD-DEM is an established approach to simulate spouted beds, but researchers usually focus on batch units, without commenting on the residence time distribution of the particles. In this work, we employed our validated CFD-DEM approach to simulate a continuously-fed square-based spouted bed, in the framework of the development of a scaled-up multiple unit. After analysing several flow patterns of the spouted bed, we propose three methods that permit evaluating the residence time distribution curve of the particles on the basis of the results of the simulations. These methods allow exploiting CFD-DEM's great reliability without reproducing the actual tracer experiment, which would be too time-consuming. Each method has advantages and disadvantages, but all of them were able to provide a good match of the experimental E(t) curve and may be applied to other devices as well
Comparison of different drag models in CFD-DEM simulations of spouted beds
No full textSpouted beds are commonly simulated through the Computational Fluid Dynamics – Discrete Element Method approach. The choice of the drag model is still a matter of debate, as they feature peculiar operative conditions. In this work, we simulated two spouted beds containing Geldart-D particles. We tested seven drag models: three are classic models, while four are developed through advanced computational techniques. The results indicate that the key variable is the ratio between the operative and the minimum spouting gas velocity (u/ums). At u = ums only the Gidaspow model can always predict fluidisation, but at low u/ums values the Beetstra model is the best compromise. For higher values, the Rong and Di Felice models behave better, while the others overestimate the particles' velocity. These results can be useful to identify the best performing model and show there is a need for more appropriate models for spouted beds
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