1,720,993 research outputs found
Application of method of lines in chemical engineering problems
Get PDFIn this work, two problems in chemical engineering are studied and
solved. Estimation of an important parameter of dust explosions, the
deflagration index kST , and a study of unsteady state with axial diffusion Plug Flow Reactors are presented. Both problems are approached by
characterizing the physical phenomena involved with suitable transport
equations. Such equations have been developed with the synergy of both
consolidated theoretical models and ad hoc assumptions and semi-empiric
approaches, according to the specific problem analyzed. The final equation systems result in a system of non-linear Partial Differential Equations.
The numerical solution of such equations has been performed by implementing the Method of Lines, a numerical method based on the discretization of spatial derivative operators, transforming a system of PDEs into a
system of ODEs or DAEs. The resulting ODEs/DAEs systems have been
implemented and solved inside MAT LABTMenvironment. The Method
of Lines is presented for uniform and non-uniform grids, generalized with
the use of spatial derivatives discretization stencils of several orders of
accuracy.
For the estimation of kST , we validated the model with 8 organic dust:
Aspirin, Cork, Corn starch, Niacin, Polyethylene, Polystyrene, Sugar and
Wheat flour. Results showed an interesting match between experimental
and simulated data: predictions for the deflagration index were good,
while the evolution of process variables (such as the temperature of the
gas phase), still leaves room for improvements.
For the PFR study, we propose 1-D models, taking in account the
reactor start-up, thermal and material axial diffusion, and the presence
of a heating/cooling system. In order to judge the quality of the results,
we took as case study a reaction well studied in the literature over the
years: the oxidation of Naphthalene. We developed the so-called Runaway Boundaries for the reaction considered. Our results found good
matches with the available literature data and analysis. We also noticed
a shifting of the Runaway Boundaries when considering a more realistic
heating/cooling system
Safe intensification of potentially runaway reactions: From semibatch to continuous processes
No full textFast and highly exothermic reactions are commonly carried out in semibatch reactors (SBRs) in order to better control the heat evolution by the feeding rate. In fact, for such processes, a phenomenon known as "thermal runaway", that is an uncontrolled reactor temperature increase, may be triggered whenever the rate of heat removal becomes lower than the rate of heat release. This dangerous temperature increase, occurring in practically adiabatic conditions, can trigger secondary undesired exothermic reactions or, in some cases, the decomposition of the whole reacting mixture with consequent reactor pressurization and, eventually, explosion followed by the release of high amounts of hazardous products. As a consequence, several studies on the detection of the so called "runaway boundaries" have been performed during years. However, from a practical perspective, the desired goal of whatever enterprise is to attain the maximum productivity maintaining safe conditions. Such a goal can be achieved using a series of continuous stirred tank reactors (CSTRs) operated in the isothermal temperature control mode; but a possible change of the reactor type, from discontinuous (e.g. batch or SB) to continuous (series of CSTRs), with the aim of increasing the productivity cannot be performed so easily when a potentially runaway process is involved. The main aim of this work has been to compute the number of CSTRs in a series that, guaranteeing the requested productivity and reactants conversion under safe operating conditions (runaway phenomena cannot be triggered), minimizes the volume of each reactor of the series. Such a number results to be a function of the employed kinetic scheme and the dosing policy of the co-reactants. In this work, two different dosing policies (1-co-reactant dosed into the first reactor of the series; 2) co-reactant dosed into the first NR-1 reactors of the series) will be analyzed for the relevant case study of the synthesis of N-(4-nitro, 2-phenoxyphenyl) methane sulphonamide. The obtained results have shown that it is possible to increase the overall productivity of the process, simply shifting from discontinuous to continuous operating mode, also achieving a safe intensification, that is, having lower reacting volumes at the full plant
A Mathematical Model for the Prediction of the Kst for Metallic Dusts as a Function of the Particle Size Distribution
Get PDFFor several years, dust explosions have been one of the major causes of industrial accidents, spanning from metalworking to pharmaceuticals sectors. In accordance with the latest Chemical Safety Board (CSB) investigations, three out of four dust explosions in the United States involved metallic dusts (iron, titanium, zirconium and aluminum). Many chemical processes involve metal powders for their exceptional mechanical, optical and catalytic properties, such as the production of plastics, rubber, paints, coatings, inks, pesticides, detergents and even drugs. The severity of a dust explosion can be defined using experimental parameters such as the maximum explosion pressure (pmax), the maximum rate of pressure rise ((dp/dt)max) and the deflagration index (Kst), which are employed to predict the consequences of a dust explosion for a given scenario. Among these parameters, the deflagration index plays a fundamental role, as it is used for the design of deflagration nozzles aimed to protect industrial equipment and silos from internal dust explosions. The purpose of this work is to develop a mathematical model able to predict the Kst value of metal powders as a function of chemical-physical data and the particle size distribution (DD50 was used as global information). The model structure is based on the writing and resolution of the material and energy balance equations on the single dust particle, also estimating the contribution of oxygen diffusion which, in the case of metal powders, greatly depends on both tortuosity and porosity. The results well agreed with experimental data, providing the basis for the development of more detailed models
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Application of an enhanced version of Recursive Operability Analysis for combustible dusts risk assessment
No full textOrganic dust explosions were and are still today a critical issue in the food, pharmaceutical, and fine chemical industry. Materials such as flour, corn starch, sugar and APIs represent a cause of severe accidents. In this framework, we investigated a modified version of Recursive Operability Analysis−Incidental Sequence Diagrams (ROA–ISD), called ROA Plus−ISD, specifically tailored to describe industrial processes involving organic combustible dusts. Compared to more classical techniques such as Hazard and Operability (HazOp), ROA−ISD allows for a direct generation of fault trees, providing a useful tool to connect Qualitative with Quantitative Risk Analysis (QRA). ROA Plus−ISD is very similar to ROA−Cause Consequence Diagrams (CCD), which has already proven to be an effective tool to perform both risk assessment on existing plants and reconstructing already occurred accidents, given its logical structure and width of the application fields. In this work, we modified specific parts of the standard ROA−CCD method: (1) the Failure Mode and Operability Analysis (FMEA) database has been structured in order to retrieve the well-known explosion pentagon (for dusts) and all the instruments, devices, apparatuses and controllers typical of industries which process organic dusts; (2) a new comprehensive list of process variables has been compiled. In this way, it is possible to tailor the information required for the generation of the fault trees concerning top events involving mainly dust explosions and fires. This
method has been implemented in order to reconstruct the dynamics of the February 2008 Imperial Sugar refinery plant accident (Port Wentworth, GA, USA). Results demonstrated the applicability of the enhanced method by highlighting the criticalities of the process already showed by a previously detailed reconstruction performed by the Chemical Safety Board
Runaway problems in unsteady state tubular reactors
Get PDFIn the specific literature of the last seventy years, the problem of identifying the operating conditions (temperatures, pressures, concentrations, residence times, etc.) in correspondence of which, for different types of reactor and operating modes (e.g., isothermal and isoperibolic), the thermal control of a reacting system can be lost, has been widely analyzed. For some industries, the conversion of chemical reactants is carried out using Plug Flow Reactors (PFRs), because continuous production is required or strongly advised. Throughout the scientific literature, the thermal behavior of these reactors have been always characterized referring to steady state operating conditions neglecting axial diffusivities (some models took into account radial diffusivities). The reason has been found in the extremely rapid dynamics that characterizes these systems and, consequently, leads them to rapidly approach steady state conditions. But in some cases, i.e., a change in operating conditions, originated by unexpected failures or just by wrong operations, can shift the process course from steady to unsteady state. Another case could be the start-up procedure: according to many industrial accidents reports that involved fast and highly exothermic reactions, this is one of most susceptible moments. For all these cases, from the safety point of view, the system dynamical behavior could be very important and cannot be neglected. The aim of this work has been to compare runaway boundaries obtained for steady state conventional operations of PFRs with that obtained under unsteady state operating conditions considering axial diffusivities. Obtained results have shown that the unsteady state aspect is very important to be considered in a safety analysis: this is because, even setting operating parameters ranges that a conventional steady state model predicts to be safe, during a start-up (or simply unsteady) operations temperatures capable of causing a runaway phenomenon can be reached
Safe optimization of potentially runaway reactions: From fedbatch to continuous stirred tank type reactor
Get PDFA great variety of processes of both fine chemical and pharmaceutical industries are commonly carried out in semibatch reactors (SBRs). Most of such syntheses often involve strongly exothermic and very fast reactions where the control of the heat evolution is achieved thanks to the combined effect of a dedicated cooling system and the feeding of a “cold” (mostly, ambient temperature) co-reactant. For these types of process, the well known phenomenon of “thermal runaway” can take place. Accordingly to the ever increasing market requests, the desired goal of whatever enterprise is to increase the productivity of a certain product being sure of maintaining safe operating conditions during all the synthesis steps. This is a very hard task which can be performed by changing the reactor operating mode from semibatch to continuous. A continuous reactor can achieve the same productivity of a discontinuous reactor using significantly lower reaction volumes; this means that the intrinsic safety of the process is increased. In this work, the switch from a semibatch reactor to a series of continuous stirred tank reactors has been investigated using as a case study the nitration of N-(2-Phenoxyphenyl) methane sulphonamide. Particularly, after the determination of the most effective number of reactors in the series and their effective volumes, dynamical simulations have been carried out in order to guarantee that possible thermal instabilities are not generated during the synthesis in both the start-up and the normal exercise phases
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