1,330 research outputs found
Physics-based large-signal sensitivity analysis of microwave circuits using technological parametric sensitivity from multidimensional semiconductor device models
No full textThe authors present an efficient approach to evaluate the large-signal (LS) parametric sensitivity of active semiconductor devices under quasi-periodic operation through accurate, multidimensional physics-based models. The proposed technique exploits efficient intermediate mathematical models to perform the link between physics-based analysis and circuit-oriented simulations, and only requires the evaluation of dc and ac small-signal (dc charge) sensitivities under general quasi-static conditions. To illustrate the technique, the authors discuss examples of sensitivity evaluation, statistical analysis, and doping profile optimization of an implanted MESFET to minimize intermodulation which makes use of LS parametric sensitivities under two-tone excitatio
A Measurement System for On-line Estimation of Weed Coverage
No full textThis paper describes two different solutions for the estimation of weed coverage. Both measuring systems discriminate the weed from the ground by means of the color difference between the weed and ground and can be used to on-line control tractor sprayers in order to reduce weedkiller use. The solutions differ with respect to the sensor type: one solution is based on a digital camera and a computer that analyzes the images and determines the weed amount, while the other simpler solution makes use of two photo detectors and an analog processing system. The camera-based solution provides an uncertainty of a few percentage, while the photo detector-based one, though extremely cheap, has an uncertainty of about 5% and suffers from changes in light conditions, which can alter the estimation
Generalized mixed-mode S-parameters
No full textThis paper presents an innovative approach to extend the S-parameter definition to multiport networks having conventional single-ended and differential ports, as is the case for operational amplifiers, transformers and baluns. To give maximum generality to this technique, for example, allowing for complex -parameter reference impedances, the mathematical derivation will be carried out with the most general definition of the -parameters. The presented approach gives the same results already published for circuits with differential ports only when the required simplifications are applied
Behavioral modeling of GaN-based power amplifiers: impact of electrothermal feedback on the model accuracy and identification
No full textIn this article, we discuss the accuracy of behavioral models in simulating the intermodulation distortion (IMD) of microwave GaN-based high-power amplifiers in the presence of strong electrothermal (ET) feedback. Exploiting an accurate self-consistent ET model derived from measurements and thermal finite-element method simulations, we show that behavioral models are able to yield accurate results, provided that the model identification is carried out with signals with wide bandwidth and large dynamics
In-flow synthesis of advanced precursors of Chiral APIs
No full textIn the field of APIs (Active Pharmaceutical Ingredients), the industry is gradually progressing towards enantiopure formulations. In this context, chiral amines are unanimously considered a class of paramount importance, due to their widespread diffusion in a plethora of compounds.1 The diastereoselective and metal free, trichlorosilane-mediate reduction of imines, bearing different and removable chiral auxiliaries, in combination either with achiral bases or catalytic amounts of chiral Lewis bases, represents a feasible approach to afford immediate precursors of chiral APIs.2
In the present work, the carbon-nitrogen double bond reduction was successfully performed in batch and in flow mode, in high yields and almost complete stereocontrol.3 By this metal-free approach, the formal synthesis of Rasagiline and Tamsulosin was successfully accomplished in micro(meso) flow reactors, under continuous flow conditions. Chiral primary amines were obtained, either by a continuous flow hydrogenolysis, or directly out from the flow reactor after an in-line aqueous work up, depending on the chiral auxiliary group at the amine nitrogen atom.
References
1. Chiral Amine Synthesis: Methods, Developments and Applications (Ed.: T. C. Nugent), Wiley-VCH, Weinheim, 2010.
2. D. Brenna, M. Benaglia, R. Porta, S. D. Fernandes, A. J. Burke, Eur. J. Org. Chem, 2016, doi:10.1002/ejoc.201601268.
3. D. Brenna, M. Pirola, L. Raimondi, A.J. Burke, M. Benaglia, Bioorg. & Med. Chem., 2017, doi: http://dx.doi.org/10.1016/j.bmc.2017.01.02
M come Mani
No full textLe mani umane sono spesso state ispirazione per il mondo del design e delle arti. Le loro giunzioni, la loto conformazione, la loro tipologia, le meraviglie che possono compiere, sono un riferimento dall'artigianato alle arti contemporanee fino all'architettura
Sonochemical Techniques for the Transformation of Bio-based oils into Fuel
No full textBiodiesel – (FAME – fatty acids methyl esters) is currently produced through the esterification of free fatty acids and the transesterification of triglycerides in multi-stage reactors that are limited by mass transfer. The search for highly efficient transformation methods is therefore a key issue in context of minimizing reactor sizes and accelerating rates, in particular when processing non-edible and raw oils1,2.
We evaluated the esterification rate of FFA to FAME over heterogeneous catalysts subject to either ultrasound (US) or microwaves (MW). Both US and MW remarkably accelerate the esterification reaction rates for temperatures below 313 K. In particular, at 293 K the FFA conversion triples when using US instead of conventionally stirred reactor. The use of MW is beneficial when associated with heterogeneous catalysts3 and contributes to increasing the oil-methanol solubility4.
Process intensification of biodiesel production by transesterification with US horns in batch reactors (traditional vessels and Rosett cell reactors) and continuous flow reactors was studied. Complete conversion within 30 minutes was achieved for most of the US-assisted methods, whereas the mechanically stirred transesterification required two steps and lasted over 150 minutes, in between which the separation of the products was necessary. After only 5 minutes, biodiesel yields higher than 90 % were measured in the Rosett cell reactor, which combines hydrodynamic and acoustic cavitation, when US pulses were applied. The most significant result of this work was achieving biodiesel yields higher than 90% after just one passage of the reagents in the continuous flow reactor in the presence of pulsed ultrasound, corresponding to a reaction time of 18 seconds. In this case the reaction rate of the ultrasound-assisted process accelerated the reaction rate by 300 times versus the conventional synthesis methodology
All the US horns used for the transesterification experiments were provided by Synetude (Chambery – France), including the continuous reactor Sonitube®.
References
1Bianchi. C.L.; Pirola, C.; Boffito, D.C.; Di Fronzo, A.; Carvoli, G.; Barnabè, D.; Rispoli. A.; Bucchi, R. Non edible oils: raw materials for sustainable biodiesel. In Biodiesel Feedstocks and Processing Technologies; Stoytcheva, M. and Montero G., Eds.; Intech, 2011; p. 3.
2Boffito, D.C.; Pirola, C.; Galli, F.; Di Michele, A.; Bianchi, C.L. Free Fatty Acids Esterification of Waste Cooking Oil and its mixtures with Rapeseed Oil and Diesel. Fuel 2012, 10.1016/j.fuel.2012.10.069.
3Chemat, F.; Poux, M.; Galema, S.A. Esterification of stearic acid by isomeric forms of butanol in a microwave oven under homogeneous and heterogeneous reaction conditions. J. Chem. Soc., Perkin Trans. 1997, 2, 2371-2374.
4Patil, P.D.; Gude, V.G.; Mannarswamy, A.; Cooke, P.; Munson-McGee, S.; Nirmalakhandan, N.; Lammers, P.; Deng, S.G. Optimization of microwave-assisted transesterification of dry algal biomass using response surface methodology. Biores. Technol. 2011, 102, 1399-1405
CO2 photoreduction at high pressure to both gas and liquid products over titanium dioxide: the effect of unconventional reaction conditions
No full textThe photoreduction of CO2 is an intriguing process, which allows the synthesis of fuels and chemicals. One of the limitations for CO2 photoreduction in the liquid phase is its low solubility in water. Several studies have been proposed during the last years in order to enhance the photocatalysts performance and improve the phototreactors for this purpose (1, 2). This point has been here addressed by designing a fully innovative concept of pressurized photoreactor, allowing operation up to 20 bar and applied for the first time to improve the productivity of this very challenging process (3-5). The photoreduction of CO2 in the liquid phase was performed using the commercial TiO2 (Evonink P25) in the presence of Na2SO3 as a hole scavenger. The different reaction parameters (temperature, working pressure, pH) and various catalysts have been considered for investigation of productivity and selectivity in the gas and liquid phase.
The expected and formed products in liquid phase in the constant pressure and temperature and in the course of reaction time were formic acid and formaldehyde, respectively. Moreover, for longer reaction time, gas phase products formed (H2 and CO with no trace of methanol or methane) after accumulation of significant amount of organic compounds in the liquid phase. The formation of gas products takes place within two parallel reaction pathway: i) CO2 photoreduction into formic acid which may further photoreduce to formaldehyde and finally evolve into CO/CO2+H2 (photoreforming), ii) enhancing the CO2 dissolution in the water by addition of a base with formation of carbonates (pH= 12-14) resulted in the reduction of carbonates to formaldehyde and consequently formed CO/CO2+H2 in the gas phase through photoreforming.
In order to improve visible light absorption and increase the lifetime of the photogenerated charges, Au was loaded on TiO2 (0.1-0.5 wt%) by a deposition-precipitation method. Methanol and methane were the main products in liquid and gas phase, respectively, demonstrating the higher reactivity of catalyst in the present of Au. Increasing the Au loading from 0.1wt% to 0.2 wt% improved the productivity toward methanol and methane in liquid and gas phase, respectively. However, further increasing in metal loading negatively affected the Au dispersion and catalyst surface area and resulted in lower H2 productivity. Furthermore, testing parameters, such as temperature and pressure directly affected the products formation. Increasing the pressure favored the liquid products accumulation was detrimental for H2/CH4 productivity. On the other hand, increasing the temperature, decreased the CO2 solubility in the water, but enhanced the kinetics and mass transfer leading to the formation of H2/CH4.
Acknowledgements: Fondazione Cariplo (grant 2016-0858 “UP – Unconventional Photoreactors”) is gratefully acknowledged.
References:
(1) Yamashita H., Fujii Y., Ichihashi Y., Zhang S.G., Ikeue K., Park D.R., Koyano K., Tatsumi T., Anpo M., 1998, Catalysis Today, 45, 221.
(2) Anpo M., Yamashita H., Ichihashi Y., EharaS., 1995, J. Electroanal. Chem, 396, 21.
(3) Rossetti I., Villa A., Pirola C., Pratia L., Ramis G., 2014, RSC Adv, 4, 28883.
(4) Rossetti I., Villa A., Compagnoni M., Prati L., Ramis G., Pirola C., et al., 2015, Catal. Sci. Technol, 5, 4481.
(5) Galli F., Compagnoni M., Vitali D., Pirola C., Bianchi C.L., Villa A., Prati L., Rossetti I., 2016, App Catal B: Environmental, 200, 386
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