Neural Net Based Hybrid Modeling of the Methanol Synthesis Process

A Hybrid modeling approach, combining an analytical model with a radial basis function neural network is introduced in this paper. The modeling procedure is combined with genetic algorithm based feature selection designed to select informative variables from the set of available measurements. By only using informative inputs, the model's generalization ability can be enhanced. The approach proposed is applied to modeling of the liquid–phase methanol synthesis. It is shown that a hybrid modeling approach exploiting available a priori knowledge and experimental data can considerably outperform a purely analytical approach.     

Stereoselective hybrid catalysts: new opportunities

The combination of biotransformations and metal‐assisted catalysis within hybrid systems represents a promising strategy to extend the repertoire of reactions enabled in a biomolecular environment. Both covalent catalytic entities and supramolecular assemblies have been created recently to mediate transformations otherwise difficult to perform in ‘classical’ biocatalysis. In this context, various biopolymers can serve as ligand spheres simultaneously acting as chiral inducers. In particular, modular approaches embedding novel metal‐based catalytic centers via an affinity interaction are highly appealing, as this technology may serve as a versatile platform to incorporate diverse reactivities in a biological matrix in a stereoselective fashion. Copyright © 2007 Society of Chemical Industry     

Determination of the mechanical thermostat electrical contacts switching quality with sound and vibration analysis

A mechanical thermostat is a device that switches heating or cooling appliances on or off based on temperature. For this kind of use, electronic or mechanical switching concepts are applied. During the production of electrical contacts, several irregularities can occur leading to improper switching events of the thermostat electrical contacts. This paper presents a non-obstructive method based on the fact that when the switching event occurs it can be heard and felt by human senses. We performed several laboratory tests with two different methods. The first method includes thermostat switch sound signal analysis during the switching event. The second method is based on sampling of the accelerometer signal during the switching event. The results show that the sound analysis approach has great potential. The approach enables an accurate determination of the switching event even if the sampled signal carries also the switching event of the neighbour thermostat.     

Coke formation in pyrolysis furnaces in the petrochemical industry

In the petrochemical industry, an industrial pyrolysis furnace is a complicated piece of equipment that functions as both a reactor and high-pressure steam generator. During service, hard deposits of carbon (coke) build up on the inner wall of the tube, reducing heat transfer and restricting the flow of the hydrocarbon feedstocks. In this paper, an overview of coke formation, the impact of coke formation, and the methods by which coke formation can be prevented, as well as methods that can be successful in decoking of pyrolysis furnaces, is presented.     

Electrical properties and chemical compatibility of PZT thick film on Ni substrates

The characteristics of Pb(Zr,Ti)O₃(PZT) thick films that were printed and fired on Ni substrates were studied. The dielectric characteristics of samples sintered at 850°C on Ni substrates could not be measured due to the formation of a NiO layer at the Ni/PZT interface. The scanning electron microscope and energy dispersive X-ray analysis of cross-sections of the PZT thick films on Ni substrates, and of a mixture of PZT and NiO powders, fired at 850°C, did not indicate the formation of secondary phases. However, the transmission electron microscopy showed around 8% solid solubility of NiO in the PZT. A new structure with a prefired, Au thick-film layer was designed in order to prevent the diffusion of the NiO to the PZT layer during sintering. The dielectric properties of the PZT layers printed and fired on the Ni substrates with the prefired Au electrode were significantly better than those of the layers on the uncoated substrate, the dielectric losses decreased from 0.23 to 0.05.     

High mobility GaAs-AlxGa1?xAs single period modulation-doped heterojunctions

Single period modulation-doped structures composed of an AlxGa1?xAs layer, part of which is doped with Si, on top of an undoped GaAs layer have been grown by molecular beam epitaxy. The films were characterised using Hall effect measurements carried out at temperatures between 10 and 300 K. With 50?75 ? thick undoped (Al, Ga)As layers near the interface, mobilities in excess of 115000 cm2/Vs at 10 K and 7450 cm2/Vs at 300 K have been achieved for an average doping concentration of ?5×1016 cm?3. These are some of the highest mobilities as yet obtained from modulation-doped structures, and represent an increase in mobility over equivalently doped GaAs by about a factor of 20 at 10 K and by a factor of 2 at 300 K.     

Implication of PSA uncertainties on risk-informed decision making

Nuclear power plants risk-informed policy is introduced in order to improve safety decision making and regulatory efficiency. The corresponding regulatory guides define the acceptable risk measures and their changes resulting from the modifications in the licensed design of the nuclear power plant. The risk measures used in the acceptance guidelines are the core damage frequency and large early release frequency.The risk measures and their corresponding changes are assessed by the Probabilistic Safety Assessment. The uncertainties of Probabilistic Safety Assessment should be appropriately addressed in the context of the decision making, considering their implication on the obtained results. The Probabilistic Safety Assessment uncertainties include epistemic uncertainties resulting from parameter, model, and completeness uncertainties.The paper presents the obtained results from the uncertainty analysis of the Probabilistic Safety Assessment of the reference nuclear power plant and their implication on risk-informed decision making. The paper focuses particularly on parameter and model uncertainties. The analysed modification is extension of the test interval of the emergency diesel generators. The core damage frequency is the used risk measure in the analysis.The need for the appropriate consideration of the uncertainties in the Probabilistic Safety Assessment in order to adequately support the risk-informed decision making is identified. The deficiency of usage of percentile measures is identified and acknowledged. The need for the adaptation of the risk-informed decision-making principles considering new nuclear power plants is recognized.     

Natural radionuclides in milk from the vicinity of a former uranium mine

Natural radionuclides in milk from farms in the vicinity of a former uranium mine ?irovski vrh in Slovenia, from a farm on the reference location, in powdered milk purchased from Ljubljana shops and in milk intended for infant food, were determined. In the milk samples, ²³⁸U, ²³⁴U, ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po were determined. After sample preparation, radiochemical separation and preparation of counting sources, ²³⁸U, ²³⁴U, ²²⁶Ra and ²¹⁰Po were measured by alpha spectrometric system equipped with passivated implanted planar silicon detectors (PIPS) and ²¹⁰Pb by a gas-flow proportional counter. Activity concentrations varied from 0.009 to 0.354 Bq/kg dry weight for ²³⁸U, 0.019-0.177 Bq/kg dry weight for ²³⁴U, 0.041-0.110 Bq/kg dry weight for ²²⁶Ra, 0.290-0.652 Bq/kg dry weight for ²¹⁰Pb and 0.055-0.611 Bq/kg dry weight for ²¹⁰Po. Assessed combined annual effective doses per unit of intake were from 8.7 to 13.0 μSv/year for adults and from 195 to 648 μSv/year for infants.     

The price of risk reduction: Optimization of test and maintenance integrating risk and cost

Testing and maintenance (T&M) improve the reliability of safety systems and components in nuclear power plants, which is of special importance for standby systems. Early optimizations of single component test intervals were based on minimizing the risk, e.g. the time-average unavailability, without cost considerations. However, the appropriate development of T&M strategy depends not only on the T&M intervals but also on the resources (human and material) available to implement such strategies. Since these testing and maintenance activities are associated with substantial cost, they present an important domain, where risk reduction and costs can be balanced.The objective of this paper focuses on assessing how costs and component ageing may affect the T&M optimization in terms of minimal system risk. The costs are expressed as a function of the selected risk measure. The time-averaged function of the selected risk measure is obtained from probabilistic safety assessment, i.e. the fault tree analysis at the system level, extended with inclusion of time parameters related to T&M activities. Additionally, component ageing is taken into account while developing the system reliability model presented in this paper. The testing strategy is also addressed. Sequential and staggered testing strategies are compared. The developed approach is applied on a standard test system and the obtained results are presented. The results show that the risk-informed surveillance requirements differ from existing ones in technical specifications, which are deterministically based. The presented approach achieves a significant reduction in system unavailability accompanied with relatively small changes in total T&M costs.