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91.
Vilas H Rane Sopan T Chaudhari Vasant R Choudhary 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》2006,81(2):208-215
Rare earth (viz. La, Ce, Sm, Nd and Yb) promoted CaO catalysts have been investigated, comparing their surface properties (viz. surface area and basicity/base strength distribution) and catalytic activity/selectivity in the oxidative coupling of methane at different reaction conditions (temperatures, 650–800 °C, CH4/O2 ratios, 2.0–8.0 and space velocity, 51 360 cm3 g?1 h?1). The surface properties and catalytic activity/selectivity are strongly influenced by the rare earth promoter and its concentration. Apart from the Sm‐promoted CaO catalyst, both the total and strong basic sites (measured in terms of CO2 chemisorbed at 50° and 500 °C respectively) are decreased due to the promotion of CaO by rare earth metals (viz. La, Ce, Nd and Yb). The catalytic activity/selectivity is strongly influenced by the temperature, particularly below ?700 °C, whereas at higher temperature no further effect is seen. The La2O3? CaO, Nd2O3? CaO and Yb2O3? CaO catalysts showed high activity and selectivity, and also their results are comparable. Among the catalysts, Nd‐promoted CaO (with Nd/Ca = 0.05) showed the best performance (19.5% CH4 conversion with 70.8% C2+ selectivity) in the oxidative coupling of methane. A close relationship between the surface density of total and strong basic sites (measured in terms of CO2 chemisorbed at 50° and 500 °C respectively) and the C2+ selectivity and/or C2+ yield has been observed. Copyright © 2005 Society of Chemical Industry 相似文献
92.
This paper presents a discretization error estimator for displacement-based finite element analysis applicable to multi-material bodies such as composites. The proposed method applies a specific stress continuity requirement across the intermaterial boundary consistent with physical principles. This approach estimates the discretization error by comparing the discontinuous finite element effective stress function with a smoothed (C0 continuous) effective stress function for non-intermaterial boundary elements with a smoothed pseudo-effective stress function for elements which lie on the intermaterial boundary. Examples are presented which illustrate the effectiveness of the multi-material error estimator. The pointwise pseudo-effective stress and the L2 norm of the estimated stress error are seen to converge with mesh refinement, while Zienkiewicz and Zhu's error estimator failed to converge for elements on the intermaterial boundary due to the physically admissible stress discontinuities that exist on the intermaterial boundary. 相似文献
93.
94.
Meenakshi Choudhary V. N. Mishra R. Dwivedi 《Journal of Materials Science: Materials in Electronics》2014,25(3):1331-1340
In the present work, solid-state reaction and sol–gel route derived pure tin oxide (SnO2) powders have been used to develop the palladium (Pd)-doped SnO2 thick film sensors for detection of liquefied petroleum gas (LPG). Efforts have been made to study the gas sensing characteristics i.e., sensor response, response/recovery time and repeatability of the thick film sensors. The response of the sensors has been investigated at different operating temperatures from 200 to 350 °C in order to optimise the operating temperature which yields the maximum response upon exposure to fixed concentration of LPG. The optimum temperature is kept constant to facilitate the gas sensing characteristics as a function of the various concentration (0.25–5 vol%) of LPG. The structural and microstructural properties of Pd-doped SnO2 powder and developed sensors have been studied by performing X-ray diffraction and field emission electron microscopy measurements. The improvement in the response along with better response and recovery time have been correlated to the reduction in crystallite size of SnO2 powder and morphology of printed sensor in thick film form. It is found that the thick film sensor developed by using sol–gel route derived SnO2 powder with an optimum doping of 1 wt% Pd is extremely sensitive (86 %) to LPG at 350 °C. 相似文献
95.
M.K. Choudhary K.C. Karki S.V. Patankar 《International Journal of Heat and Mass Transfer》2004,47(26):5629-5638
Porous insulation used on pipes carrying cold fluids suffers thermal degradation due to condensation of water vapor and the build up of water in the insulation. Recently, it has been suggested that the thermal degradation can be significantly reduced by wrapping a hydrophilic wick fabric on the cold pipe. The capillary action of the fabric, aided by gravity, allows the condensed moisture to move to the outer surface of the insulation, from where, if ambient conditions are right, it evaporates. This paper presents the details of a mathematical model for condensation in the insulation in the presence of the wick fabric. The model is based on the volume-averaged equations for unsteady transport of heat, water vapor, and liquid water in a porous medium. The wick is modeled as an anisotropic porous medium. The model also allows for the presence of a vapor retarder jacket that is used to reduce the ingress of water vapor into the insulation. The model has been applied to an insulation layer around a horizontal pipe. The presence of the wick is shown to significantly reduce the amount of liquid water in the insulation. The results of the model have been verified using laboratory experiments and field tests. 相似文献
96.
Electromagnetic interference shielding behavior of poly(trimethylene terephthalate)/multi-walled carbon nanotube composites 总被引:1,自引:0,他引:1
Anju GuptaVeena Choudhary 《Composites Science and Technology》2011,71(13):1563-1568
Poly(trimethylene terephthalate) [PTT]/multiwalled carbon nanotube [MWCNT] composites having varying amounts of MWCNTs were fabricated with an aim to investigate the potential of such composites as an effective light weight electromagnetic interference (EMI) shielding material in the frequency range of 12.4-18 GHz (Ku-band). PTT/MWCNT composite with shielding effectiveness (SE) of 36-42 dB was obtained at 10% (w/w) MWCNT loading. Shielding mechanism was studied by resolving the total SE into absorption (SEA) and reflection loss (SER). PTT/MWCNT composite showed absorption dominated shielding; thus it can be used as microwave, radar absorbing and stealth material. The effect of MWCNT loadings on electrical conductivity (σ) and dielectric properties of PTT and the correlation among conductivity, tan δ, absorption loss and reflection loss were also studied. 相似文献
97.
Oxidative conversion of propane to propylene and ethylene over a V2O5/CeO2/SA5205 (V:Ce=1:1) catalyst, with or without steam and limited O2, has been studied at different temperatures (700–850 °C), C3H8/O2 ratio (4.0), H2O/C3H8 ratio (0.5) and space velocity (3000 cm3 g−1 h−1). The propane conversion, selectivity for propylene and net heat of reaction (ΔHr) are strongly influenced by the reaction temperature and presence of steam in the reactant feed. In the presence of steam and limited O2, the process involves a coupling of endothermic thermal cracking and exothermic oxidative conversion reactions of propane which occur simultaneously. Because of the coupling of exothermic and endothermic reactions, the process operates in an energy-efficient and safe manner. The net heat of reaction can be controlled by the reaction temperature and concentration of O2. The process exothermicity is found to be reduced drastically with increasing temperature. Due to the addition of steam in the feed, no coke formation was observed in the process. 相似文献
98.
Ensemble of surrogates 总被引:5,自引:3,他引:2
Tushar Goel Raphael T. Haftka Wei Shyy Nestor V. Queipo 《Structural and Multidisciplinary Optimization》2007,33(3):199-216
The custom in surrogate-based modeling of complex engineering problems is to fit one or more surrogate models and select the
one surrogate model that performs best. In this paper, we extend the utility of an ensemble of surrogates to (1) identify
regions of possible high errors at locations where predictions of surrogates widely differ, and (2) provide a more robust
approximation approach. We explore the possibility of using the best surrogate or a weighted average surrogate model instead
of individual surrogate models. The weights associated with each surrogate model are determined based on the errors in surrogates.
We demonstrate the advantages of an ensemble of surrogates using analytical problems and one engineering problem. We show
that for a single problem the choice of test surrogate can depend on the design of experiments. 相似文献
99.
100.
Ajay Arora Divya Choudhary Gaurav Agarwal Virender Pal Singh 《International Journal of Food Science & Technology》2008,43(11):1913-1921
Banana is one of the most important tropical fruits, and India ranks first in its world production. Banana cultivars rich in provitamin A carotenoids may offer a potential food source for alleviating vitamin A deficiency, particularly in developing countries. With a view to exploit banana peels as a source of valuable components, in the present study, proximate composition, carotenoids, beta‐carotene and some anti‐oxidative enzymes as well as carbohydrate content of selected Indian banana varieties were determined. Karpooravalli cultivar of banana showed the maximum accumulation of carotenoid content in the non‐edible (68 μg g?1 d.w.) portion of banana, while being the second highest in beta‐carotene content (143.12 μg per 100 g). However, Red Banana ranked highest in total carotenoid contents for pulp (4 μg g?1 d.w.) and beta‐carotene was estimated to be the highest in the case of peels (241.91 μg per 100 g) and in pulp (117.2 μg per 100 g). Karpooravalli cultivar of banana is also rich in carbohydrate content in terms of total starch (1786.0 μg g?1 d.w. in peels and 544.85 μg g?1 d.w. in pulp) and sugars (53.53 μg g?1 d.w. in peels and 39.05 μg g?1 d.w. in pulp). The catalase enzyme activity in these peels ranged from 5.66 to 35.57 nmol min?1 mg?1 proteins and was found at a higher level in cultivar Poovan, while the ascorbate peroxidase showed the range of 2.25 to 6.22 μmol min?1 mg?1 proteins. The peels of cultivars Red Banana and Karpooravalli are rich source of bioactive compounds, such as carotenoids (beta‐carotene), anti‐oxidative enzymes and carbohydrate contents. 相似文献