1,720,979 research outputs found
Methodology for risk assessment of COVID-19 pandemic propagation
Full text linkThis paper proposes a methodology to perform risk analysis of the virus spread. It is based on the coupling between CFD modelling of bioaerosol dispersion to the calculation of probability of contact events. CFD model of near-field sneeze droplets dispersion is developed to build the SARS-CoV-2 effect zones and to adequately capture the safe distance. The most shared classification of droplets size distribution of sneezes was used. Droplets were modeled through additive heating/evaporation/boiling laws and their impact on the continuous phase was examined. Larger droplets move behind the droplet nuclei front and exhibit greater vertical drop due to the effect of gravity. CFD simulations provided the iso-risk curves extension (i.e., the maximum distance as well as the angle) enclosed by the incident outcome effect zone. To calculate the risk indexes, a fault tree was developed and the probability of transmission assuming as of the top event “COVID-19 infection” was calculated starting from the virus spread curve, as main base case. Four phases of virus spread evolution were identified: initiation, propagation, generalised propagation and termination. For each phase, the maximum allowable close contact was computed, being fixed the values of the acceptable risk index. In particular, it was found that during the propagation case, the maximum allowable close contacts is two, suggesting that at this point lockdown should be activated. The here developed methodology could drive policy containment design to curb spread COVID-19 infection
Risk Assessment of the Large-Scale Hydrogen Storage in Salt Caverns
Full text linkSalt caverns are accepted as an ideal solution for high-pressure hydrogen storage. As well as considering the numerous benefits of the realization of underground hydrogen storage (UHS), such as high energy densities, low leakage rates and big storage volumes, risk analysis of UHS is a required step for assessing the suitability of this technology. In this work, a preliminary quantitative risk assessment (QRA) was performed by starting from the worst-case scenario: rupture at the ground of the riser pipe from the salt cavern to the ground. The influence of hydrogen contamination by bacterial metabolism was studied, considering the composition of the gas contained in the salt caverns as time variable. A bow-tie analysis was used to highlight all the possible causes (basic events) as well as the outcomes (jet fire, unconfined vapor cloud explosion (UVCE), toxic chemical release), and then, consequence and risk analyses were performed. The results showed that a UVCE is the most frequent outcome, but its effect zone decreases with time due to the hydrogen contamination and the higher contents of methane and hydrogen sulfide
Effect of turbulence spatial distribution on the deflagration index: Comparison between 20 L and 1 m3 vessels
Full text linkIn this work, the effect of spatial distribution and values of the turbulent kinetic energy on the pressure-time history and then on the explosion parameters (deflagration index and maximum pressure) was quantified in both the standard vessels (20 L and 1 m3). The turbulent kinetic energy maps were computed in both 20 L and 1 m3 vessels by means of CFD simulations with validated models. Starting from these maps, the turbulent flame propagation of cornstarch was calculated, by means of the software CHEMKIN. Then, the pressure-time history was evaluated and from this, the explosion parameters. Calculations were performed for three cases: not uniform turbulence level as computed from CFD simulations, uniform turbulence level and equal to the maximum value, uniform profile and equal to the minimum value. It was found that the cornstarch in the 20 L vessel get variable classes (St-1, St-2, St-3) with respect to the 1 m3 (St-1). However, simulations performed on increasing the ignition delay time, shown that the same results can be attained only using 260 ms as ignition delay time in the 20 L vessel
CFD Simulation of the Dispersion of Binary Dust Mixtures in the 20 L Vessel
No full textThere are at least two main requirements for repeatable and reliable measurements of flammability and explosibility parameters of dusts: a uniform dispersion of solid particles inside the test vessel, and a homogeneous degree of turbulence. In several literature works, it has been shown that, in the standard 20 L sphere, the dust injection system generates a non-uniform dust cloud, while high gradients characterize the turbulent flow field. In this work, the dust dispersion inside the 20 L sphere was simulated for nicotinic acid/anthraquinone mixtures (with different pure dust ratios, while keeping the total dust concentration constant) with a validated three-dimensional CFD model. Numerical results show that the fields of dust concentration, flow velocity and turbulence are strongly affected by both diameter and density of the pure dusts. These different dust properties lead to segregation phenomena with the formation of zones richer in one component and leaner in the other one and vice versa, and also result in preferential paths for the solid particles inside the sphere. Overall, the obtained results highlight the need for developing a dust injection system able to overcome the shortcomings of the actual one even when testing dust mixtures
Towards H2-free shipboard storage: Energetic and risk analysis of oxidative methanol steam reforming in integrated fuel cell systems
No full textThis work investigates the energy self-sufficiency of a methanol-based hydrogen production system through oxidative steam reforming integrated with HT-PEM fuel cells, explores process conditions for autothermal systems, and assesses accident scenarios during refueling/unloading and accidental releases. Comparisons with conventional marine gasoil highlighted differences in risk indices, pool fire zones, and toxicity. Results show that the thermal self-sufficiency can be achieved under stoichiometric conditions, but also in more water-rich conditions (water/MeOH 0.5), eventually heat recovery from products cooling. The individual risk index is always below 10−5 years−1, the pool fire impact zone for the methanol-based system is always smaller than that of marine gasoil (up to 14 m diameter for the considered maritime case study); in contrast, the issue of toxicity is more relevant (even at distances >100 m). In the absence of the blanketing system, only partial use of the liquid is permitted under safe conditions, which in the case of pure methanol and methanol-water mixture at stoichiometric composition corresponds to 25 % of the total volume. Thanks to the effect of water, the adoption of methanol-water mixtures with H2O/MeOH = 3 is intrinsically safe
Risk analysis of sodium hypochlorite production process
Full text linkSodium hypochlorite poses explosive hazards associated with its complex reactive chemistry. The production process of sodium hypochlorite consists of a first block where the chlorine, caustic soda and hydrogen are produced in an electrolytic cell from brine and a second block where chlorination of caustic soda to form hypochlorite is carried out. This process is characterized by several hazards such as chlorine gas toxicity, explosive hazards due to the presence of hydrogen and chlorine and corrosive hazards. Loss of control of such substances has the potential to cause high-consequence low-probability events. Thus, specific safety measures have to be designed to mitigate risk. In the present work, the risk assessment of the first block of the process is performed, focusing on hydrogen risks. To this end, HAZOP analysis was performed to identify the top events. For each top event, based on properly developed fault trees, the frequency analysis was performed. Eventually, the consequence analysis was carried out by the simulation of phenomena leading to dispersion and consequent ignition of the cloud as function of the distance from the source. Simulations were performed by means of the software PHAST
CFD simulation of turbulent fluid flow and dust dispersion in the 1 m3 explosion vessel equipped with the rebound nozzle
Full text linkAgainst dust explosions, all the flammability and explosibility parameters must be evaluated following standard procedures using the 20 L and/or the 1 m3 vessel. Previous results comparing the dust dispersion in the 20 L sphere equipped with rebound or perforated annular nozzle showed that the initial turbulence level, the dust concentration, and the feeding are affected by the type of nozzle used. In this work, a similar investigation was performed on the 1 m3 vessel, simulating the fluid flow evolution which is obtained with the rebound nozzle. Results showed that the 1 m3 vessel equipped with rebound nozzle presents a less uniform degree of turbulence and a higher amount of dust fed, compared to the case of perforated annular nozzle. However, the greatest effect on the initial level of turbulence and turbulent combustion regime is determined by the size of the vessel and not by the type of nozzle used
Dust particle sedimentation in the 20 L standard vessel for dust explosion tests
Full text linkAccording to the current international standards, to perform the correct evaluation of the explosion and flammability parameters, a uniform distribution of the dust particles should be achieved inside the 20 L and/or 1 m3 standard vessels. CFD simulations have shown that in both standard test vessels (20 L and 1 m3), the dust particles are not uniformly dispersed, being mostly concentrated at the edge of the macro-vortices generated by the injection of the fluid and particle through the nozzle. In addition, only a partial fed of the particles is obtained, and dust particles sedimentation phenomena can occur. As a result, the dust participating to the reactive process may be much lower than the expected nominal concentration in the vessel due to sedimentation and incomplete feeding. Consequently, misleading values of the flammability/explosion parameters could be measured. Particle sedimentation and incomplete feeding depends both on the Stokes number and on the Reynolds number, whereas the concentration distribution depends on the turbulence level, the fluid flow maps, and the number of particles which enter into the vessel through the nozzle. The aim of this work is to evaluate the key parameters (particle size, particle density, and fluid velocity) affecting sedimentation and incomplete feeding in 20 L vessel. To this end, CFD simulations of dust dispersion are performed at varying the particle density and size. Operating maps, in terms of the key parameters and/or their dimensionless combinations, are developed and a correlation for correction of the data is proposed
Ammonia as a Hydrogen Carrier: Energetic Assessment of Processes Integrated with Fuel Cells for Power Generation
No full text: In the context of the near-future hydrogen economy, ammonia is regarded as one of the most promising hydrogen carriers in the short-to-medium term. As part of the broader transition to a new energy paradigm, the well-established and extensive ammonia infrastructure can serve as a platform for green hydrogen transportation, storage, and utilization. This study analyzes various process configurations integrated with different types of fuel cells for ammonia utilization through Aspen Plus simulations. The evaluation focuses on overall energetic efficiency (ranging from 31.20 to 51.50% depending on the adopted configuration), autothermality (42.00 to 100.00%, based on the adopted process configuration), and emissions from external heat sources (0.03-0.07 kgCO2/kWh). Assessments are conducted parametrically across different fuel cell efficiencies (50.0-65.0%). Results suggest that high-temperature PEMFC and direct ammonia solid oxide fuel cells (SOFCs) offer a balance between overall efficiency (40.2-51.5 and 35.00-52.0%, respectively) and feasibility of achieving autothermal operations under nitrogen dilution (up to 25.0%). Considering technological maturity and operational lifespan, high-temperature PEMFCs and SOFCs emerge as a promising component for such integrated systems
CFD simulations of dust dispersion in the 1 m3 explosion vessel
Full text linkAccording to standard procedures, flammability and explosion parameters for dusts and dust mixtures are evaluated in 20 L and/or 1 m3 vessels, with equivalent results provided a correct ignition delay time (60 ms in the 20 L vessel; 600 ms in the 1 m3 vessel). In this work, CFD simulations of flow field and dust concentration distribution in the 1 m3 spherical vessel are performed, and the results compared to the data previously obtained for the 20 L. It has been found that in the 1 m3 vessel, the spatial distribution of the turbulent kinetic energy is lower and much more uniform. Concerning the dust distribution, as in the case of the 20 L, dust is mainly concentrated at the outer zones of the vortices generated inside the vessel. Furthermore, an incomplete feeding is attained, with most of the dust trapped in the perforated annular nozzle. Starting from the maps of dust concentration and turbulent kinetic energy, the deflagration index KSt is calculated in both vessels. In the conditions of the present work, the KSt is found to be 2.4 times higher in the 20 L than in the 1 m3 vessel
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