1,801 research outputs found
Ultrasound to Enhance a LiquidâLiquid Reaction
Liquidâliquid mass transfer with ultrasound was investigated experimentally during the hydrolysis of n-amyl acetate. Power ultrasound is supposed to improve the yield and kinetics of such multiphase chemical reactions thanks to the mechanical effects of cavitation. Indeed, implosion of micro-bubbles at the vicinity of the liquidâ liquid interface generates disruption of this surface, and enhances mixing in the liquid around the inclusion, thus improving mass transfer between the two phases. This effect has been demonstrated here on the hydrolysis of n-amyl acetate by sodium hydroxide, a rather slow reaction but influenced by mass transfer; the reaction is carried out in a glass jacketed reactor, 500 mL of volume, equipped with a Rushton turbine and a 20 kHz sonotrode dipping in the solution. The ester is initially pure in the organic dispersed phase, and sodium hydroxide has an initial concentration of 300 mol/m3; one of the products, pentanol partitions between the two phases and the sodium salt stays in the aqueous phase. The initial apparent reaction rate is measured from the record of the conductivity giving the concentration of alkali versus time. The reaction rate was always found to increase when ultrasound is superimposed to mechanical stirring (at 600 rpm), with a positive influence of input power (20 and 50 W). When varying initial concentration (300 and 600 mol/m3), temperature (36 and 45°C) and ultrasound emitter (sonotrode or cuphorn), the benefit of ultrasound over mechanical agitation was systematic. The only case of a weak influence of ultrasound was the sonication of a dense medium, containing 23% of organic phase and impeding the propagation of ultrasound
Stability in multi-skill workforce scheduling
This paper analyzes stability in multi-skill workforce schedules. In our stability analysis, we extend the notion of blocking pairs as stated in the Marriage model of Gale-Shapley. It is shown that finding stable schedules is NP-Hard. In some special cases stable schedules can be constructed in polynomial time. An integer linear programming model is proposed to decrease the instability of schedules
A modern Fizeau experiment for education and outreach purposes
On the occasion of the laser's 50th anniversary, we performed a modern Fizeau
experiment, measuring the speed of light with a laser beam passing over the
city centre of Marseille. For a round trip distance of almost five kilometers,
the measurement has reached an uncertainty of about 10, mainly due to
atmospheric fluctuations. We present the experimental and pedagogical
challenges of this brilliant outreach experiment.Comment: accepted by Eur J Phys in november 201
The temperature dependence of the isothermal bulk modulus at 1 bar pressure
It is well established that the product of the volume coefficient of thermal
expansion and the bulk modulus is nearly constant at temperatures higher than
the Debye temperature. Using this approximation allows predicting the values of
the bulk modulus. The derived analytical solution for the temperature
dependence of the isothermal bulk modulus has been applied to ten substances.
The good correlations to the experiments indicate that the expression may be
useful for substances for which bulk modulus data are lacking
Inter-vehicle gap statistics on signal-controlled crossroads
We investigate a microscopical structure in a chain of cars waiting at a red
signal on signal-controlled crossroads. Presented is an one-dimensional
space-continuous thermodynamical model leading to an excellent agreement with
the data measured.Moreover, we demonstrate that an inter-vehicle spacing
distribution disclosed in relevant traffic data agrees with the thermal-balance
distribution of particles in the thermodynamical traffic gas (discussed in [1])
with a high inverse temperature (corresponding to a strong traffic congestion).
Therefore, as we affirm, such a system of stationary cars can be understood as
a specific state of the traffic sample operating inside a congested traffic
stream.Comment: 6 pages, 4 figures, accepted for publication in J. Phys. A: Math.
Theo
Geometric and Bayesian models for safe navigation in dynamic environments
Autonomous navigation in open and dynamic environments is an important challenge, requiring to solve several difficult research problems located on the cutting edge of the state of the art. Basically, these problems may be classified into three main categories: (a) SLAM in dynamic environments; (b) detection, characterization, and behavior prediction of the potential moving obstacles; and (c) online motion planning and safe navigation decision based on world state predictions. This paper addresses some aspects of these problems and presents our latest approaches and results. The solutions we have implemented are mainly based on the followings paradigms: multiscale world representation of static obstacles based on the wavelet occupancy grid; adaptative clustering for moving obstacle detection inspired on Kohonen networks and the growing neural gas algorithm; and characterization and motion prediction of the observed moving entities using Hidden Markov Models coupled with a novel algorithm for structure and parameter learnin
Experience with the LEP vacuum system at energies above 90 GeV and future expectations
The LEP storage ring has been operated at energies above 90 GeV for more than 1000 hours during 1997. Because of the rapid increase with the beam energy of the power radiated as synchrotron light, the vacuum system has been submitted to very stringent conditions as far as power evacuation and photon stimulated gas desorption are concerned. The operational experience acquired under these, up to now unexplored, conditions will be reviewed together with an outline of the limitations which were experienced at these high levels of radiation in the use of the available vacuum instrumentation. Based o n the available data detailed predictions concerning the beam lifetime, gas desorption and beam cleaning of the vacuum system under the impact of photons with a critical energy approaching 1 MeV will be formulated
- âŠ