1,721,111 research outputs found
On the energy efficiency of hydrogen production processes via steam reforming using membrane reactors
No full textThe novel paradigm of distributed energy production foresees the production of hydrogen from methane and biomasses in small plants, which may take advantage from membrane-based processes. By means of a modeling approach, this paper compares the energy efficiency of two membrane-based processes to produce H2 from methane steam reforming. The two-step process (TS) envisages a high temperature classical reactor and a following WGS stage in a membrane reactor, while the alternative process uses a simple packed-bed membrane reactor (MR). Both processes show a general increase of H2 production and energy efficiency with the pressure and a maximum energy efficiency for S/C of 4, while the increase of the space velocity reduces the performances of the MR. The results show that the TS process performs better than the studied MR and that the maximum energy efficiency of both processes is between 30 and 40%. A comparison with the literature shows that the TS process may achieve similar performances respect to an intensified MR. © 2018 Hydrogen Energy Publications LL
Sensitivity Analysis and Dimensioning of Reactor-Scale Pd/Ag Permeators for the Tritium Extraction and Removal System of the EU-HCPB Blanket
No full textOne of the reference technologies for the fuel cycle of fusion machines is Pd/Ag membranes. This technology is proposed to be implemented in tritium recovery systems because of their exclusive selectivity toward molecular hydrogen isotopes (Q = H, D, T). To perform scaling-up studies for the Tritium Extraction and Removal System of the European DEMOnstration fusion power reactor (DEMO) with a solid blanket, a one-dimensional simulation code was recently developed and successfully validated with experiments. This code relies on different operational (e.g., feed pressure and temperature), geometrical (e.g., permeator length), and membrane-intrinsic (e.g., Q2 permeability) parameters given as input. The main outcome is the Q2 permeation efficiency, defined as the Q2 permeate–to–feed flow ratio. Because of the low concentrations of Q2 expected at the He stream purging the solid blanket, the surface effects are expected to be important, decreasing the separation efficiency of the Pd/Ag permeators. In this paper the role of surface effects on the permeation efficiency is studied for a DEMO-relevant scenario (feeding mixture: HT/H2/He). Moreover, a sensitivity study is also given demonstrating the high impact of the permeation area, temperature, and feed pressure on the permeation efficiency of HT
Testing of ceramic porous membranes for separation of binary mixtures of Plasma Enhancement Gases
No full textPlasma Enhancement Gas (PEG), namely inert gases (nitrogen, neon, argon, etc.), are used in order to mitigate the heat load over plasma facing components. In particular, PEG separation is part of the Plasma Exhaust Processing System of DEMO. Previous studies have been dedicated to study the PEG separation via ceramic porous membranes by modeling the mass transfer mechanisms and performing preliminary tests with pure gases. This manuscript reports the results of gas permeability tests carried out on two commercial ceramic porous membranes with binary mixtures of H2 with Ar, N2, He, and DT. The experiments have been carried out at room temperature and feed pressure 105–150 kPa by collecting the permeate stream at atmospheric pressure. Since preliminary binary tests have shown that the experimental apparatus adopted was not capable to realize any detectable gas separation between the permeate and retentate streams, two following tests with the retentate valve closed have been carried out. In the first test only hydrogen was fed through the membrane, while in the second test a mixture of hydrogen and a PEG gas was sent to the membrane. The flow rate of PEG gas needed to achieve the same pressure drop adopted in the test with only hydrogen has been used to assess the gas selectivity. The equation used to calculate selectivity values is valid only when both: pressure drop across the membrane is equal for both tests and composition of gas mixture on both side of the membrane is assumed to be equal for the second test. Experiments with binary mixtures exhibited selectivity values lower than those assessed theoretically in previous works where single gases have been tested. Through the values of measured selectivity, a new preliminary design of a membrane system consisting of a cascade of ceramic porous membranes followed by a Pd-permeator has been carried out
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
Numerical study and experimental verification of protium permeation through Pd/Ag membranes for fusion applications
No full textPd/Ag membranes are one of the reference technologies for the fuel cycle of deuterium-tritium fusion machines. This technology is proposed to be implemented in tritium recovery systems, due to their exclusive selectivity towards molecular hydrogen isotopes. For instance, these membranes are proposed to process and separate Q 2 (Q = H, D, T) species from impurities (e.g., inert gases) coming from the plasma exhaust of ITER and also foreseen for DEMO. In view of up-scaling this technology to a DEMO-relevant case, a one-dimensional simulation code was developed to first predict H 2 permeation efficiencies. In this contribution, a numerical code which computes the permeation efficiency of protium through Pd/Ag membranes at different operating conditions and tubular geometries is presented. A good agreement between the numerical outcomes and actual experimental results obtained at ENEA is highlighted and 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
Recent achievements of the Pd-Ag membrane technologies in tritium extraction system applications
No full textThe main expertise of the Membrane Laboratory of ENEA Frascati is related to the study and development of Pd-based membrane technologies (both permeators and membrane reactors), which are one of the reference processes in the fuel cycle of nuclear fusion reactors. Principal characteristics of Pd-based membranes are infinite hydrogen selectivity, high hydrogen permeability, continuous operation and modularity. In the last few years, the ENEA-Frascati laboratory has erected two experimental facilities for the study of decontamination performance of single and multi-tube Pd-Ag membrane modules. Several experimental and simulation activities have been carried out to investigate the possible application of these technologies in the tritium extraction system of the solid breeder blanket concept. Firstly, the paper describes the proposal for the back-up solution of a tritium extraction system for HCPB based on membrane technologies. Then, the most significant results obtained for the decontamination of water and H2 permeation using two different facilities are presented and discussed. The main difference among the two facilities is the number of Pd-Ag tubes installed. Water decontamination experiments have been performed to investigate the effect of different catalysts. The results obtained with the water gas shift reactions are discussed in terms of reactor performance (i.e., decontamination factor) and methane production in the lumen side. During the He-H2 permeation experiments several operating conditions (e.g., membrane's temperature, lumen pressure, etc.) have been investigated. The effect of the He-H2 feed ratio (which impacts the driving force across the membrane) gives an interesting and unique view of the membrane module performance in terms of permeation efficiency
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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