1,720,959 research outputs found
Comparing true countercurrent and Simulated Moving-Bed chromatographic reactors
No full textSimulated Moving Bed reactors (SMBR) combine chemical reaction and adsorptive separation within one single continuous and countercurrent unit. This integration promises substantial improvements in process performance, especially when applied to equilibrium-limited reactions involving such heat-sensitive products as fine chemicals and pharmaceuticals. In this work, the interplay among the relevant process design parameters (dimensionless ratios of the fluid and solid flow rates, and the Damkohler numbers for each section of the unit) is investigated. For this, an analytical solution of differential mass-balance equations for the corresponding true countercurrent process (TCC), using as a model system the reaction A reversible arrow B + C with each species exhibiting linear adsorption behavior, was developed. Based on this solution, criteria were derived for the optimum process design with respect to productivity and solvent consumption. Comparing these results with numerical simulations of an SMBR unit shows that the TCC model does not apply to SMBR units with a finite number of columns per section, that is, units of practical relevance, because the two units exhibit different residence time distributions and, hence, lead to different degrees of conversion
Design of Stationary Phase Properties for Optimal Performance of Reactive Simulated-Moving-Bed Chromatography
No full textSynthesis of methylacetate in a simulated moving-bed reactor: Experiments and modeling
No full textSimulated moving-bed reactors (SMBR) combine chemical reaction and adsorptive separation within one single unit. Toward the design of this rather complex unit, guidelines for the proper choice of the operating parameters, that is, the flow-rate ratios and the Damkohler numbers within the different sections, have recently been devised based on a mathematical analysis of a linear-model system. For nonlinear systems, though, the influence of the operating parameters on the unit performance is less well understood. In this work, the interplay between the operating conditions and unit performance is investigated through numerical simulations and experiments, focusing on the influence of the feed stream composition. As a model system, the Synthesis of methylacetate from methanol and acetic acid, catalyzed by a sulfonated ion-exchange resin, is considered. The experiments were carried out in an SMB reactor of miniplant scale. In addition, the reliability of the model predictions is evaluated by comparing the numerical simulation results with experimental data. It is shown through simulations that optimal performance is achieved where the feed to the SMBR is constituted of an equimolar mixture of the two reactants. Simulations and experimental results allow for an understanding of such a result
A new reaction-separation unit: The simulated moving bed reactor
Full text linkProcess intensification is one of the most promising tools for improving current technologies. This requires dealing in most cases with complex systems where many different and often interacting physicochemical phenomena take place simultaneously. Continuous reactive chromatography, implemented through the simulated moving bed reactor technology, represents a novel example of such an intensified process. In this case, only a detailed description of the selective polymer swelling and reaction kinetics enables the modeling and the understanding of the behavior of the chemical production process. The optimization of the unit reveals its major economic limitation to arise from the sorptive properties of the currently available stationary phase. Based on a proper understanding of the unit operation, it is possible to design new stationary phases, tailor-made for specific reacting systems, which allow the performance of the process, to be significantly improved
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
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
- …
