1,721,089 research outputs found
The effect of ultra-low temperature on the flammability limits of a methane/air/diluent mixtures
Full text linkNatural gas represents an attractive fuel for industrialized and developing countries seeking an alternative to petroleum. Due to economic and safety considerations, liquefied natural gas (LNG) at cryogenic conditions is preferred for storage and transportation. The main drawback is the poor understanding of the physical and chemical phenomena that occur at the storage conditions of liquid methane, i.e. at ultra-low temperatures around 110 K and, if released, at temperatures below ambient. In this work, a procedure to evaluate the laminar burning velocity, the flammability limit (FL) and the limiting oxygen concentration (LOC) of methane-air-diluent mixtures based on detailed kinetic mechanism at ultra-low temperatures is proposed. The estimation of the FL was obtained with the limiting burning velocity theory. The effects of inert content (extinguishing) and agent (N2, H2O and CO2) on FL were evaluated and compared with data retrieved from the literature. The agreement between experimental observation and model results from 200 K–300 K incentivizes the adoption of the new procedure for further studies of fuel reactivity and safety parameters. Moreover, the proposed procedure may be suitable for the estimation of the safety parameters of complex fuel mixtures whose composition is closer to the actual values of LNG
Laminar burning velocity of multi-component gaseous mixtures
Full text linkThe laminar burning velocity is the essential parameters for the safe design of equipment and process. Indeed, the knowledge of this parameter allows for the definition of flammability limits, minimum oxygen concentration and the gas deflagration index, Kg. Recently, the interest in the laminar burning velocity has raised because of the increased use of complex gaseous mixtures derived from biological (biogas), or pyrolysis and gasification (syngas) processes. Due to the large number of components of these gases, simplified correlations for the definition of the additivity of the burning velocity are questionable. Furthermore, the presence of carbon monoxide, hydrogen sulphide, ammonia and hydrogen, or other non-hydrocarbon substances, may strongly affect the correlation results. Le Chatelier's formula e.g. may produce an error of over 25% with respect to the experimental data for simple mixtures based on two hydrocarbon fuels. In this work, a review of the main additivity rules for the definition of the laminar burning velocity for multicomponent mixtures (≥ 3 fuels) is given, starting from the pioneering correlations and analysis of Spalding. The equations have been compared and validated with respect to experimental data. A comparison with the results obtained by using more recent advanced kinetic mechanism, which can be adopted for the prediction of mixture reactivity, is also given
Laminar burning velocity of methane, hydrogen, and their mixtures at extremely low-temperature conditions
No full textMethane consumption is strongly increasing as a result of its abundance from natural gas, thus leading to reduced costs, and as a result of the reduced environmental impact in the case of combustion, with a lower carbon content with respect to traditional fuels. These advantages encourage its utilization in several industrial applications, such as methane steam reforming for the production of hydrogen and automotive applications. The necessity of transporting the gases and large-scale distribution systems is however one of the main issues. Innovative processes, such as cryogenic storage, cryo-compression, and liquefaction, require detailed information on the thermal and chemical properties of the methane-hydrogen mixture at low and ultralow temperatures. In this framework, detailed kinetic models for the total and partial oxidation of methane, hydrogen, and methane-hydrogen mixtures in air at low (273 > T > 200 K) and ultralow (T < 200 K) temperatures must be developed and validated. In this work, the laminar burning velocity of these gases has been simulated and compared to the few available experimental data retrieved from the literature. Hence, simplified correlations for the burning velocity with respect to the initial composition and temperature have been adopted and further developed. The simplified approach proposed in this work reduces the number of degrees of freedom required for the application of the modified Gulder equation. Moreover, it is suitable for the description of the combined effect of the initial temperature and gaseous composition. The performed analysis of the concentration and temperature profiles with respect to burner head distance indicates, as a possible explanation for the methane-dominated regime, the presence of a limitation in the hydrogen concentration hindering its production. A sensitivity analysis was performed to evaluate the effect of hydrogen addition and initial temperature on the methane kinetic mechanism in the presence of air. The results show that, although the hydrogen production rate does not change, the reaction mechanism is strongly affected by the studied parameters
Gas-phase thermal explosions in catalytic direct oxidation of alkenes
No full textThe metal-based catalytic oxidation of alkenes to the corresponding epoxides is playing a significant role in the modern chemical industry. Nevertheless, these key processes are still lacking proper understanding with respect to the gas-phase runaway behaviour (thermal explosion) and to the hot spot formation on the catalytic surface, under the typical process conditions. This work aims to enlighten these aspects by considering either the catalytic or the gas-phase chemistry for the development of reactor operative diagrams, in order to define the best-operating conditions with respect to the selectivity, the productivity, and the process safety aspects. The proposed methodology has been applied to the oxidation of ethylene and propylene for the direct oxidation process by pure oxygen, considering a detailed kinetic model accounting for the homogeneous reactions, coupled with the heterogeneous catalytic mechanisms. Sensitivity and reaction path analyses were performed to individuate the ruling species and reactions determining the transition to runaway conditions
Evaluation of safety parameters of light alkenes by means of detailed kinetic models
No full textThe oxidation of light alkenes is the core of the modern chemical industry and a pivotal point for several environmental and safety considerations. However, few works have dealt with the prediction of fire and explosion parameters for the definition of process conditions, for the design and safe handling of such reactive substances. In this work, flammability parameters for light alkenes (ethylene, propylene and the three butylene isomers) have been calculated by adopting the detailed kinetic mechanisms of the University of California, San Diego, integrated with C4 reactions by the Lawrence Livermore National Laboratory mechanism. The new model has been adopted for the definition of flammability limits, adiabatic flame pressure and temperature, maximum rate of pressure rise, gas deflagration index (KG), auto-ignition temperature and minimum oxygen concentration. Flammability regions for nitrogen and carbon dioxide dilution have also been reported. © 2018 Institution of Chemical Engineer
Flammability limits of methane (LNG) and hydrogen (LH2) at extreme conditions
No full textIn the recent years the growing interest for cleaner and low carbon content energy sources has addressed the development of several industrial and civil applications based on methane, hydrogen, and their mixtures. The use of these gases rises, however, several technological issues for the storage and the transportation systems. Among others, cryogenic liquefaction (as liquified natural gas, LNG and liquified hydrogen, LH2) and cryo-compressed gases are considered among the most promising potential solution. On the other hand, when low and ultra-low temperature are considered, several questions regarding the safe use of such gases are raised, including the behaviour of the cold vapour phase in air after the release of liquid or cryo-compressed gas from containment system. In this work, the flammability limits of hydrogen, methane and their blends at low and ultra-low temperatures were estimated by using the laminar burning velocity obtained by means of detailed kinetic mechanism. Numerical results were compared with experimental data and empirical correlations commonly adopted for this purpose. The data agreement demonstrates the applicability of the developed procedure for the estimation of safety parameters at low and ultra-low temperature and for future technological applications, even at cryogenic conditions. Copyright © 2019, AIDIC Servizi S.r.l
Flammability parameters of liquified natural gas
No full textThe use of liquefied natural gas (LNG) is constantly growing. However, safety issues regarding cryogenic storage and transportation systems are still to be fully resolved. In particular, the evaluation of the efficiency of inerting systems for low-temperature LNG vapour is essential. In this work, the variation of the flammability range (in terms of lower and upper flammability limits and minimum oxygen concentration) obtained by adding nitrogen to pure air for some representative LNG mixtures has been evaluated at ambient temperature and at temperatures below 0 °C by using a detailed kinetic model entitled KIBO, which has been proved to be reliable for the description of C0–C4 reactions in oxidative conditions, and by the limiting burning velocity theory. Strong differences are reported among pure methane and natural gas mixtures for all the investigated temperatures. The effect of composition is therefore relevant. Furthermore, the obtained results suggest that the lower flammability limit is determined by thermal aspects at high temperature only, whereas at low temperature, kinetic limitations are more relevant. © 2018 Elsevier Lt
Implementation of gas-phase kinetic model for the optimization of the ethylene oxide production
No full textThe direct epoxidation of light olefins is a key process of the chemical industry. However, several concerns regarding industrial and safety aspects, such as the occurrence of runaway reactions and the relevance of side reactions reducing process selectivity, are still under investigation. To this aim, a reactor operation diagram was obtained under process relevant conditions, allowing for the identification of runaway, hot spots and pseudo adiabatic operation regions by using several criteria and kinetic models. Indeed, catalytic only or complete (catalytic + non-catalytic) kinetic mechanisms were adopted to this aim. The selection of different runaway criteria was found to be negligible on the region boundaries. On the contrary, significant discrepancies were observed for hot spot region boundaries and between catalytic and complete models. An in-depth analysis, based on thermodynamic and kinetic models, was performed to individuate the optimized operative conditions. Flammability limits were estimated by applying the limiting laminar burning velocity theory, in case of different inert composition and initial temperature. The results indicate that a decrease in the operative temperature has the potential to either reduce the capital costs or increase process safety. Furthermore, the proposed approach can be intended as a supporting procedure for the selection of process alternatives and reactor design
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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