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81.
A. Haunold E. Rosenberg M. Grasserbauer 《International journal of environmental analytical chemistry》2013,93(1-4):157-172
Abstract Reliable and comprehensive sampling methods are required to obtain accurate data for VOC concentrations in air samples. The major drawback of the adsorption tube sampling method, widely employed in environmental studies, is the fact that C2 compounds are usually not trapped quantitatively. The focus of this work was thus to improve sampling based on adsorption tubes packed with Molsieve and Carbosieve. To improve the sampling efficiency for the C2 compounds, a cooling device, based on Peltier cooling was constructed, which could be operated at a temperature down to ?30°C. Experiments under laboratory and field conditions were carried out to study the influence of the sampling temperatuie on the recovery of ethane and ethene as the most volatile VOCs. The results clearly demonstrate the need for a cooled sampling device for the analysis of C2 compounds in air. Under the investigated conditions, the recoveries with ambient temperature sampling were only in the range of 38–46% for ethane and 33–59% for ethene respectively, in comparison to the cooled sampling device. These findings are only valid for the described conditions and can change significantly with temperature and concentration. A generalisation of the recovery is thus very difficult to give. The use of the sampling device for a field study is reported, where samples were collected simultaneously at three different altitudes in a diurnal profile on the slope of the Schulterberg mountain in Tyrol (Austria). 相似文献
82.
In this paper,an axial dispersion mathematical model is developed to simulate a three-phase slurry bubble column reactor for direct synthesis of dimethyl ether(DME) from syngas.This large-scale reactor is modeled using mass and energy balances,catalyst sedimentation andsingle-bubble as well as two-bubbles class flow hydrodynamics.A comparison between the two hydrodynamic models through pilot plantexperimental data from the literature shows that heterogeneous two-bubbles flow model is in better agreement with the experimental data thanhomogeneous single-bubble gas flow model.Also,by investigating the heterogeneous gas flow and axial dispersion model for small bubblesas well as the large bubbles and slurry(i.e.including paraffins and the catalyst) phase,the temperature profile along the reactor is obtained.Acomparison between isothermal and non-isothermal reactors reveals no obvious performance difference between them.The optimum values ofreactor diameter and height were obtained at 7 m and 50 m,respectively.The effects of operating variables on the axial catalyst distribution,DME productivity and CO conversion are also investigated in this research. 相似文献
83.
Gloria Papi 《Numerical Functional Analysis & Optimization》2013,34(3-4):327-357
The aim of this paper is the shape restoration of a plane object from measurements of its diffracted field at a discrete and finite set of points. The plane sampling lattice is supposed: i) rectangular; ii)periodic. The problem is approached as an interpolation one. A numerical algorithm for practical reconstructions is presented. A-priori limitations on the perimeter of the object and conditions on the samples lead to a-priori bounds able to estimate the precision of the reconstruction. 相似文献
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In [3], Poonen and Slavov recently developed a novel approach to Bertini irreducibility theorems over an arbitrary field, based on random hyperplane slicing. In this paper, we extend their work by proving an analogous bound for the dimension of the exceptional locus in the setting of linear subspaces of higher codimensions. 相似文献
87.
Abstract A comparison and evaluation is made of recent proposals for multivariate matched sampling in observational studies, where the following three questions are answered: (1) Algorithms: In current statistical practice, matched samples are formed using “nearest available” matching, a greedy algorithm. Greedy matching does not minimize the total distance within matched pairs, though good algorithms exist for optimal matching that do minimize the total distance. How much better is optimal matching than greedy matching? We find that optimal matching is sometimes noticeably better than greedy matching in the sense of producing closely matched pairs, sometimes only marginally better, but it is no better than greedy matching in the sense of producing balanced matched samples. (2) Structures: In common practice, treated units are matched to one control, called pair matching or 1–1 matching, or treated units are matched to two controls, called 1–2 matching, and so on. It is known, however, that the optimal structure is a full matching in which a treated unit may have one or more controls or a control may have one or more treated units. Optimal 1 — k matching is compared to optimal full matching, finding that optimal full matching is often much better. (3) Distances: Matching involves defining a distance between covariate vectors, and several such distances exist. Three recent proposals are compared. Practical advice is summarized in a final section. 相似文献
88.
S. Huzurbazar Ronald W. Butler 《Journal of computational and graphical statistics》2013,22(3):342-355
Abstract An importance sampling procedure is developed to approximate the distribution of an arbitrary function of the eigenvalues for a matrix beta random matrix or a Wishart random matrix. The procedure is easily implemented and provides confidence intervals for the p-values of many of the commonly used test statistics in multivariate analysis. An adaptive procedure allows for the control of either absolute error or relative error in this p-value estimation through the choice of importance sample size. 相似文献
89.
Abstract Algorithms are developed for constructing random variable generators for families of densities. The generators depend on the concavity structure of a transformation of the density. The resulting algorithms are rejection algorithms and the methods of this article are concerned with constructing good rejection algorithms for general densities. 相似文献
90.