1,721,075 research outputs found
A kinetic-free approach to the sensitivity analysis of axial flow continuous reactors: A general experimental method
No full textThe intensification of fast and exothermic reaction processes is an important development trend of the chemical industry, because of productivity, safety and product quality reasons. Whenever the reaction can be performed in tubular rather than in stirred tank reactors, a number of additional advantages can be achieved, in terms of a higher conversion as well as of an improved heat transfer efficiency per unit volume. In these cases, however, the reactor must be designed and operated so as to prevent its pseudo-adiabatic behavior, which makes the system sensitive to even normal fluctuations of the process variables as well as to the design parameters, with unpredictable effects on the process performances. In this work a general experimental procedure is presented, through which the sensitive behavior of a tubular reactor can be detected through local temperature and flowrate measurements, even in the absence of a detailed kinetic characterization of the system. The developed approach has a higher responsiveness compared with the existing criteria and can be easily adopted for finetuning the operating conditions of a running reactor as well as for the design of a new one. The proposed method has been validated through experimental data, both in the non-sensitive and sensitive regimes, of a well-known catalytic process of the chemical industry
Large dust grains in the inner region of circumstellar disks
No full textContext. Simple geometrical ring models account well for near-infrared interferometric observations of dusty disks surrounding pre-main sequence stars of intermediate mass. Such models demonstrate that the dust distribution in these disks has an inner hole and puffed-up inner edge consistent with theoretical expectations.
Aims. In this paper, we reanalyze the available interferometric observations of six intermediate mass pre-main sequence stars (CQ Tau, VV Ser, MWC 480, MWC 758, V1295 Aql and AB Aur) in the framework of a more detailed physical model of the inner region of the dusty disk. Our aim is to verify whether the model will allow us to constrain the disk and dust properties.
Methods. Observed visibilities from the literature are compared with theoretical visibilities from our model. With the assumption that silicates are the most refractory dust species, our model computes self-consistently the shape and emission of the inner edge of the dusty disk (and hence its visibilities for given interferometer configurations). The only free parameters in our model are the inner disk orientation and the size of the dust grains.
Results.In all objects with the exception of AB Aur, our self-consistent models reproduce both the interferometric results and the near-infrared spectral energy distribution. In four cases, grains larger than ~1.2 μm, and possibly much larger are either required by or consistent with the observations. The inclination of the inner disk is found to be always larger than ~30°, and in at least two objects much larger
GHG Emissions by (Petro)Chemical Processes and Decarbonization Priorities—A Review
Full text linkGlobal warming is becoming an increasing issue, and greenhouse gas (GHG) emissions represent the engine of such a phenomenon. This review aims to identify the origin of GHG emissions and focus in detail on the ones related to (petro) chemical industries. The industrial sector is the primary GHG emitter among all the other anthropogenic sources. The chemical industry is the first in charge of that (having accounted for about 6.5% of the global GHG emissions in 2018). Thought-provoking data such as yearly productivities and emission factors related to the predominant chemicals prompt the reader to acquire a sense of the critical activities responsible for carbon-intensive emissions, which should be the first to be decarbonized. Specifically, ammonia synthesis and steam cracking resulted in the most polluting processes of the chemical industry, being responsible for the release of about 440 and 228 Mt-CO2,eq/y, respectively, in 2020. The same approach also applies to oil refining. Due to the massive amounts of oil barrels produced daily, oil refining is a key player in industrial GHG emissions (about 3% of the global emissions in 2018). Indeed, in 2020, refineries emitted nearly 1313 Mt-CO2,eq/y
Towards the decarbonization of ammonia synthesis – A techno-economic assessment of hybrid-green process alternatives
Full text linkThis paper analyzes the retrofitting of a conventional, "gray" ammonia plant for its partial decarbonization. Specifically, the retrofitting involves the hybridization of the process by providing a certain amount of green hydrogen (i.e. by water electrolysis) for the ammonia synthesis to reduce the demand for the original gray hydrogen route (i.e. by fossil-based steam reforming) and consequently reduce CO2 emissions. As such green hydrogen is produced through an electrolyzer that consumes renewable electric energy, two design configurations are considered: (Option I) the supply of clean electricity from renewable sources; (Option II) the generation of clean electricity by an in -house photovoltaic power plant. Once having analyzed the advantages and limitations of the hybrid plant layouts, the ammonia production costs achieved by the three simulated plants (i.e. the reference "gray" plant and the two hybrid alternatives) are estimated by considering the United States (USA) and European Union (EU) market quotations in 2022 and previsions for 2030 and 2050. The main outcomes are that (i) hybrid -green ammonia proves to be a feasible, environmentally friendly retrofitting option for the ammonia production industry, showing production costs that are no more than 36 % higher than gray ammonia; and (ii) Option I achieves better cost-effectiveness than Option II in the USA, while the opposite happens in the EU. Specifically, the resulting 10 %-green-on-average hybrid -green ammonia production costs range between +0.3 % (EU -2022) and +30.8 % (USA -2022) for Option I, and between -1.3 % (EU -2022) and +35.3 % (USA -2022) for Option II, compared to the reference gray ammonia plant. (iii) Lastly, the projected (i.e. in 2030 and 2050) CO2 avoidance costs associated with hybrid -green ammonia configurations exceed 400 USD/tCO2 in the USA and 500 USD/tCO2 in the EU
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
Millimeter Imaging of MWC 758: Probing the Disk Structure and Kinematics
No full textWe investigate the structure and kinematics of the circumstellar disk around the Herbig Ae star MWC 758 using high-resolution observations of the 12CO (3-2) and dust continuum emission at the wavelengths of 0.87 and 3.3 mm. We find that the dust emission peaks at an orbital radius of about 100 AU, while the CO intensity has a central peak coincident with the position of the star. The CO emission is in agreement with a disk in Keplerian rotation around a 2.0 M sun star, confirming that MWC 758 is indeed an intermediate-mass star. By comparing the observation with theoretical disk models, we derive that the disk surface density Σ(r) steeply increases from 40 to 100 AU and decreases exponentially outward. Within 40 AU, the disk has to be optically thin in the continuum emission at millimeter wavelengths to explain the observed dust morphology, though our observations lack the angular resolution and sensitivity required to constrain the surface density on these spatial scales. The surface density distribution in MWC 758 disk is similar to that of "transition" disks, though no disk clearing has been previously inferred from the analysis of the spectral energy distribution (SED). Moreover, the asymmetries observed in the dust and CO emission suggest that the disk may be gravitationally perturbed by a low-mass companion orbiting within a radius of 30 AU. Our results emphasize that SEDs alone do not provide a complete picture of disk structure and that high-resolution millimeter-wave images are essential to reveal the structure of the cool disk mid-plane
Variations on the Author
Full text link“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
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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