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1.
Chaves R. Kuzmanov G. Sousa L. Vassiliadis S. 《Very Large Scale Integration (VLSI) Systems, IEEE Transactions on》2008,16(8):999-1008
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The catalytic reforming of methane by steam is an important industrial process that produces H2, CO and CO2, thus chemically transforming natural gas, coal gas and light hydrocarbon feedstocks to synthesis gas or hydrogen fuel. Methane-steam reforming may consist of a number of reactions depending on the reforming catalyst, operating conditions and feedstock composition, The typical industrially desirable reactions are the reverse of methanation (CH4 + H2O = CO + 3H2) and the water-gas shift (CO + H2O = CO2 + H2). Both reactions are equilibrium limited and the composition of the mixture that exits the reformer is in accordance with the one calculated thermodynarmically. Removal of reaction products at the reactor exit by means of selective membrane permeation can offer improved CH4 conversions and CO2 and H2 yields, assuming the subsequent utilization of the reject streams by a second methane-steam reformer. We numerically investigated the feasibility of a system of two tubular methane-steam reformers, in series with an intermediate permselective polyimide membrane permeator, as means of improving the overall CH4 conversion and the H2, CO2 yields over conventional methane-steam reforming equilibrium reaction-separation schemes that are currently in industrial practice. The unique feature of the permselective polyimide separator is the simultaneous removal of H2 and CO2 versus CH4 and CO from the reformed streams. The utilized 6FDA-3,3', 5,5'-TMB aromatic polyimide was reportedly characterized [10] and found to exhibit superior permselective properties compared with other polyimides of the same or different dianhydride sequence. Conversion and yield of the designed reactor-membrane permeator reforming system can be maximized by optimizing the permselective properties of the membrane material and the design variables of the reactors and the permeator. Product recovery and purity in the permeate stream need to be compromised to overall enhance methane conversion and product yield. The operating variables that were varied to investigate their effect on the magnitude of conversion and yield included the inlet pressure of the first reformer, the temperature of both reformers, and the permeator dimensionless Pe' number (variation of the first two variables results to a drastic change in the composition of the reformed stream that enters into the permeator). The numerical results show that the new reformer-membrane permeator cascade process can be more effective (it can offer increased CH4 conversions and H2, CO2 yields) than conventional equilibrium methane-steam reforming reaction-separation processes currently in practice. 相似文献
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A single-channel alternative detection system for magnetic resonance imaging and spectroscopy experiments is presented. After the phase error identification, which is realized through a simple method, one can correct the reconstructed image and discriminate between positive and negative frequencies without any loss of the S/N ratio. Since there is only one channel, that is a real signal, it is possible to use the Hartley instead of the Fourier transform. The former is faster and requires less storage memory. 相似文献
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This paper presents findings on the use of Web sites as a political marketing tool in the context of the 2007 Greek national elections. The questions guiding this research are how popular were online campaigns in terms of candidate use of Web sites and which trust and credibility-building Web site design cues were leveraged by candidates. This study also explored differences in online campaign practices both between Greek political parties and between the 2007 national Greek elections and the 2002 federal US elections. 相似文献
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Panos Vassiliadis Zografoula Vagena Spiros Skiadopoulos Nikos Karayannidis Timos Sellis 《Information Systems》2001,26(8)
Extraction-Transformation-loading (ETL) tools are pieces of software responsible for the extraction of data from several sources, their cleansing, customization and insertion into a data warehouse. Literature and personal experience have guided us to conclude that the problems concerning the ETL tools are primarily problems of complexity, usability and price. To deal with these problems we provide a uniform metamodel for ETL processes, covering the aspects of data warehouse architecture, activity modeling, contingency treatment and quality management. The ETL tool we have developed, namely
, is capable of modeling and executing practical ETL scenarios by providing explicit primitives for the capturing of common tasks.
provides three ways to describe an ETL scenario: a graphical point-and-click front end and two declarative languages: XADL (an XML variant), which is more verbose and easy to read and SADL (an SQL-like language) which has a quite compact syntax and is, thus, easier for authoring. 相似文献
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This paper aims to contribute to the growing body of research on online political marketing by investigating the use of websites as a marketing tool in the 2007 and 2009 general elections in Greece. The two main research objectives are firstly to establish the key factors affecting voters’ trust when it comes to using politicians’ websites and secondly to examine whether and to what extent the influence of trust-building factors changes over-time, as the online environment matures. The key findings of this research are that users tend to visit the websites of favoured candidates, which reinforces their positive perceptions of them, while as far as the websites themselves and their content and the services provided are concerned there is still room for improvement. 相似文献
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Vassiliadis S. Triantafyllos G. Kobrosly W. 《Knowledge and Data Engineering, IEEE Transactions on》1997,9(4):646-652
We address issues related to the definition of faults, errors and failures and their separability, and attribution to the different development processes of computing systems. In particular, we deal with historical databases, which presumably contain certain data (i.e., test failure data) and describe the methodology that can be used to analyze the database and obtain the pertinent information. The validation method may be of particular importance, especially when information from the database needs to be extrapolated for a purpose other than the one for which the database was developed. Our methodology was used to evaluate the historical data collected during the development of the IBM 4381 and 9370 family of computers, and to extrapolate the faults found during the function testing 相似文献
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