1,720,992 research outputs found
High-pressure methane jet: Analysis of the jet-obstacle interaction
Full text linkThe study of unplanned high-pressure gas releases is of paramount importance in the industrial safety framework because of the possible large consequences, both in case of flammable and toxic substances leakage. In addition, if an obstacle is involved in the release, it is known that the main effect on the jet behavior is the enhancement of the risk area. Pointing out the importance to consider the obstacle presence, among the various available numerical approaches, the sole reliable tool able to correctly model the scenario of a jet interacting with an obstacle seems to be the Computational Fluid Dynamics (CFD). This work lies in the context outlined through the examination of a realistic unignited high-pressure methane jet interacting with a realistic obstacle placed along its axis via CFD simulations: a stationary 65-bara unignited methane jet outflowing from a one-inch diameter hole and a medium size horizontal cylindrical tank are the building blocks of the realistic scenario. The aim is to deeply investigate how the distance between obstacle and jet orifice modifies the jet behavior. In particular, the final purposes are: i) to establish when the obstacle most influences the jet cloud extent and, ii) to assess when the obstacle influence expires. Moreover, a sensitivity analysis on the obstacle shape and size is conducted for comparison purposes
Comparison through a CFD approach of static mixers in an emulsification process
Full text linkEmulsions, characterized by droplet dispersions of immiscible liquids in a continuous phase, are often found in many diverse fields of the chemical industry. The energy needed to break these droplets can be sourced from mechanical agitators in stirred tanks, active mixers like pulse-flow mixers, and ultrasonic mixers. However, static mixers serve as a convenient alternative due to their typically lower maintenance costs and increased resistance to failures. Static mixers are available in diverse shapes and dimensions, with two widely used types being the Kenics Static Mixers and the Sulzer Static Mixers. While these two static mixers have been extensively studied in the literature in terms of pressure drop and mixing efficiency, to the best of the authors’ knowledge there is no study comparing the two in emulsification processes. This work aims to compare the performance of these static mixers in the production of emulsions by employing a Computational Fluid Dynamics approach. Results showed that the Sulzer Static Mixers allow to obtain higher values of turbulent energy dissipation, which in turn leads to smaller droplet diameters
Statistical analysis of modelling approaches for CFD simulations of high-pressure natural gas releases
Full text linkThe use of computational fluid dynamics (CFD) in process safety to estimate the risk of a given incidental scenario has become ever more present in common industry practice. The simulation of high-pressure, compressible natural gas jets is often performed by modelling its source with a simpler notional diameter approach, such that the highly computationally expensive nearfield zone need not to be simulated; this is particularly determining when simulating a gas release in complex scenario like liquid natural gas (LNG) regasification plants. In this study, we analysed the structure of compressible and incompressible jets, using Birch 1984 (B84) and Birch 1987 (B87) models. In this work, a study on the positioning of the notional diameter with respect to the real orifice of the released gas is performed, along with a statistical analysis to assess the limits of the simpler model approaches. It was found that no spacing is needed between the virtual and real sources, as the potential core generated by the simpler model is as large as the fully simulated nearfield zone by the compressible model. Additionally, an end-of-transition zone position correlation is reported. The incompressible models can be used instead of the fully compressible model for a wide range of release conditions, with both models providing accurate predictions of axisymmetrical mole fraction, temperature, and velocity profiles between 2.5 and 130 bar of storage pressure at a 1-inch orifice diameter. However, as the diameter increases, B84 is not a viable model for a “full bore” (10-inch diameter size) release at 65 bar. While B84 is reliable, B87 is the superior model for its ability to account for the compressible effects of the expansion. Therefore, B87 should be used when simulating cases where temperature is of particular interest to the user
LNG Risk Mitigation: a Comparison Between Active and Passive Barriers
Full text linkIn the last years, there has been a rapid increase in the proposals for regasification terminals to import Liquefied Natural Gas (LNG) mainly due to the global uncertainties of the energy market. Therefore, there has been a fast increase in the interest in the risk assessment of LNG regasification terminals. LNG is not poisonous; instead, its rapid evaporation together with the vapour phase flammability presents a non-negligible risk. The concentration range in which the gas-air mixture at ambient conditions is flammable is about 4.4%v/v (Lower Flammability Limit - LFL) to 15% v/v (Upper Flammability Limit - UFL). One of the major accidental scenarios, involved in an LNG regasification terminal, is the breakage of a pipeline carrying natural gas in the liquid phase. This would result in the release of large amounts of LNG leading to a fast-evaporating pool and, consequently, to a large flammable cloud and possibly to fires and explosions. Therefore, mitigation measures must be provided to reduce the risk up to an acceptable value; among the various mitigation measures, a protective barrier able to limit the hazardous distance related to a given accidental scenario (and therefore to protect sensible population living close to the regasification terminal) can be used. In their simplest configuration, passive mitigation barriers are high walls acting as obstacles on the cloud path, therefore enhancing the flammable cloud-air mixing. Unfortunately, to be effective passive barriers often must be quite high, possibly preventing their practical implementation. As an alternative, active barriers can be used where the flammable cloud-air mixing is enhanced not only thanks to the wake effect of the wall but also to the direct entrainment into the flammable cloud. This entrainment can be induced (for instance) either by high-velocity jets or by fans. Therefore, the main aim of this paper is to provide a comparison among the pros and contras of using passive vs. active barriers to reduce the hazardous distance related to an accidental scenario in an LNG regasification terminal. In particular, the various barrier configurations were investigated through Computational Fluid Dynamic (CFD) simulations using the Ansys Fluent 2023R2 suite of programs
Kinetics-free process intensification: From semi-batch to series of continuous chemical reactors
Full text linkA kinetics-free procedure is developed to transform a reaction recipe carried out in an isothermal discontinuous semi-batch reactor into one based on a series of continuous tubular or tank reactors. Using a suitable number of reactors and a correct feeding policy, it is possible to reproduce any discontinuous recipe in flow reactors without knowing the kinetics of the system. The developed procedure allows to determine the number of reactors in series (either tubular or stirred tank) able to reproduce the performance of a semi-batch recipe in terms of selectivity, keeping the same productivity of the original semi-batch process. It was found that using 15 reactors in series allows to keep unchanged the performance of a large number of semi-batch processes once a correct policy of intermediate reactant feeding is implemented. This large number of reactors in series with intermediate feeding can be easily implemented using tubular reactors instead of stirred tank ones. Finally, the synthesis of an amine/epoxy resin was investigated as a case study, fully supporting the reliability of the proposed procedure
Modelling of indoor air pollutants dispersion: New tools
Full text linkVentilation systems are used for create a thermally comfortable environment and good indoor air quality. It is therefore essential to have adequate tools for predicting the performance of these systems. Among the various approachs, the computational fluid dynamics could be a useful tool for the design of the ventilation system. When dealing with pollutants dispersion problems, a steady state averaged simulation can be misleading because it is not able to properly predict and model peak concentrations, which can be relevant even if temporary. An interesting approach is the use of LES (Large Eddy Simulations) simulations to obtain a better description of concentrations oscillations. In this framework, the aim of this work is the validation of simulation carried out using the FDS (Fire Dynamic Simulator) software with an actual case study, already studied with a mock-up. Secondly, two new configurations of the ventilation system are proposed, in order to stress the capacity of the software to describe complex and different features, classical of HVAC (Heating, Ventilation and Air Conditioning) systems. Interesting conclusions about efficiency are drawn from the comparison, highlighting the potentiality of the software
Unignited High-Pressure Methane Jet Impinging a Pipe Rack: Practical Tools for Risk Assessment
Full text linkAlthough the diffusion of its storage and transport under liquefied conditions, nowadays it is common to have methane in gaseous form in several industrial applications. This leads to safety implications to be considered: hazards are linked to both the high-pressure at which the gas is kept and to its flammability. Scenarios where flammable jets impact an obstacle are of paramount importance because of their possible occurrence. Following a numerical approach, literature shows up that their assessment can be reliably performed by means of only Computational Fluid Dynamics tools. However, despite the improvements of computing power, Computational Fluid Dynamics costs still limit its use in daily risk analysts’ activities. Therefore, considering an accidental jet-obstacle scenario of industrial interest, the present work investigates how a pipe rack can influence the development of a high-pressure methane jet. Based on a Computational Fluid Dynamics analysis, main achievements of this work are a simple criterion able to identify the situations where the pipe rack does not influence the high-pressure methane jet behavior, therefore allowing to identify the scenarios where simpler models can be used (i.e., analytical correlations known for the free jet situation), and, if present, a simple analytical relationship that roughly predicts the influence of the pipe rack without the need of performing complex Computational Fluid Dynamics simulations
Intensification of Processes for the Production of Active Pharmaceutical Ingredients: Increase in Safety and Sustainability
Full text linkFine chemical compounds and so-called "specialties" are generally synthesized through batch or semi-continuous processes. This is largely because such syntheses often involve complex and highly exothermic reactions, to be performed in semi-batch reactors for safety and/or selectivity reasons. An effective way to reduce costs and improve the reproducibility of such batch processes is to transform them into their continuous counterparts to reduce volumes and investment costs, while increasing the inherent safety of the process thanks to fewer hold-ups. The “shift to continuous” allows to reduce both the overall process times, with a general decrease in operating costs, and the content of solvents used as thermal flywheels, thanks to the greater efficiency of the heat exchange systems. All these aspects are defined as process intensification. In this work, the intensification of the production process of N-(4-nitro, 2-phenoxyphenyl) methanesulfonamide (NIM) by nitration in glacial acetic acid of N-(2-phenoxyphenyl) methanesulfonamide (FAM) will be proposed. Starting from the original semi-batch recipe two different continuous configurations will be proposed: a series of tubular reactors and a series of continuous reactors with complete mixing, in both cases with intermediate injections. The solvent content (glacial acetic acid) has been drastically reduced (from 82.5% to 50% by weight) to increase the levels of environmental sustainability of the synthesis. The high exothermicity of the process and the extremely rapid reaction kinetics were two fundamental aspects which had to be considered in the transition to the continuous process of the new formulation with reduced solvent content. For this reason, an ad hoc procedure was developed which allows the semi-batch recipe to be transformed into a corresponding one conducted in a tubular reactor with continuous lateral injections; this reactor was then discretized in the two reactor configurations mentioned above. The results obtained have shown how it is possible to obtain the desired product with practically unitary conversions using: a) a series of 4 isoperibolic tubular reactors, each with 4 discrete lateral feeds; b) a series of 5 mixed reactors with discrete side feeds. In both cases, the correct distribution of both the flow rate fed between the reactors in series and the temperatures of the cooling fluid (defined on the basis of the procedure developed for the passage of the process from discontinuous to continuous) was decisive for obtaining the desired performance. The series of tubular reactors was found to be optimal from the point of view of thermal control of the process, confirming that a series of tubular reactors is to be preferred in terms of safety compared to its counterpart with mixed reactors
Ground influence on high-pressure methane jets: Different concentration clouds scenarios
No full textBecause of their relevant consequences (in particular, associated with domino effect), accidental highpressure flammable gas releases are one of the major hazards in the industrial safety framework. It is likely that the accidental loss of containment can involve obstacles that, as a matter of fact, are present in any process facility. As obstacle, flat surfaces (e.g., walls, ground, etc.), equipment (e.g., tanks, pipes, etc.) or structures can be counted. Focusing on the scenario of an accidental high-pressure unignited methane jet interacting with an obstacle, this work investigates how the proximity to the ground influences the jet cloud extent when considering different concentrations of methane in air. Varying the height above the ground of the source term, the effect of the ground was systematically studied through an extensive Computational Fluid Dynamics analysis. Thanks to the sensitivity analysis performed, the main achievement is the demonstration that methane releases observed at different concentrations in air, from sources at different pressures and outflowing from accidental holes of different sizes are similarly influenced by the ground presence. The conclusion of the present work is that, the assessment of the hazardous area extent of the flammable release at any concentration of interest can be evaluated exploit an analytical model specifically derived, providing a useful alternative of practical precision to more expensive CFD computations. This way, for this specific accidental scenario, delineating the area involved within the flammability limits become easier and faster
Cuboid obstacle influence on high-pressure jet dispersion: A CFD study
Full text linkIn the context of the process industry safety, one of the main accidental scenarios is the release of high-pressure gaseous material. Since natural gas is highly flammable, the likelihood of ignition increases as the jet develops, with a maximum area of effect related to its lower flammability limit (LFL). This work aims at simulating and evaluating the interaction between high-pressure natural gas jets and cuboid obstacles, which were selected due to their prevalence in the process industry as storage units or buildings present in industrial parks. The maximum extent of the cloud at the LFL of natural gas is often influenced by the jet-obstacle interactions, necessitating complex numerical methods like computational fluid dynamics (CFD) for accurate estimation. Therefore, this study provides pivotal insights that challenge traditional modelling approaches, like integral ones, offering cost-effective alternatives where needed without compromising on safety. The findings indicate that using a CFD approach is not always necessary, as it largely depends on the storage pressure, diameter size, and the release height of the jet. At storage pressures of 65–130 bar with an orifice diameter of 2.54 cm, and a release height above 2.75 m, simpler methods like integral models are applicable without any substantial reliability loss. This is especially true when the cuboid obstacle is farther away from the release source. At lower release heights, especially if coupled with a larger orifice diameter, the CFD approach should be utilised as jet-cuboid interactions become highly relevant to the development of the jet
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