Seite - 200 - in Advanced Chemical Kinetics

without demanding exorbitant expenses. To sort out that drawback, ultrasound methods emergeasa tool capableofassisting in thebiodieselmanufacturingprocess. 3.Ultrasoundasa tool for liquidcharacterization Ultrasound is amechanical wave that propagates in fluids or solidmaterials at frequencies greater than20kHz, i.e., outof theaudible range forhealthyhumans [19–25]. An often used ultrasonicmeasurementmethod consists on a pulse/echo arrangement. Basi- cally, it consistson EMISSION!PROPAGATION!REFLECTION!PROPAGATION!RECEPTION Two important quantities are easily assessed from the pulse/echo ultrasonic measurement method: timeof flight andpulse (or signal) amplitude. Both aremeasuredafter the reception of the ultrasoundwave.Whenever an acoustic impedance mismatch occurs, the ultrasonic wave is partially reflected in the discontinuity boundary. The amount of reflection depends on the acoustic impedance difference between the two media, due to what is called the reflection coefficient of the interface. In a typical pulse/echo experimental set-up in sonochemistry, thepropagationmedium is fluid and the reflection takesplace in an interface withasolidobject, genericallydenominatedreflecting target. Similarly, the liquid-air interface isa reflecting target, aswell. Throughout the propagation, other physical phenomena diminish the ultrasonic amplitude due to differentmechanisms.Mainly, scattering and absorption are in charge for ultrasonic attenuation, mitigating the capability of free propagation. All those phenomena are natural andunavoidable.Nevertheless, aproperexperimentalultrasonic set-upwill either concernon itsquantification,orwilldealwithotherquantities thatarenotundesirablyaffected. Thespeedofsoundisaquantity that isnot relatedtoattenuationphenomena,orat least isnot the case in a linear range of frequencies and in infinite-like three-dimensional propagation medium, even if there is a constraint in one dimension. In the linear range of ultrasonic propagation, thesoundvelocity inanydeterminedmediumormaterialvariesasa functionof the temperature, densityandviscosity.Asamatter of fact, thosequantities arenot absolutely correlated to eachother,whatmakes the establishmentof amathematical functionavirtually unrealisable task for complexmixturesof fluids. Formonophasic simple liquids, suchaspure waterorhydrocarbonates, it iseasier todefineafunctionrelatingthosequantities,but it isnot thecase fora transesterificationprocess. To assess the speed of sound, the typical approach is to measure the time of flight of an ultrasonic pulsewithin avesselwith apre-determineddistance from the surface to the emit- ting ultrasonic transducer and aproperly designed reflecting target. It is the so-called pulse/ echo experimental method. Materials in different macrophysics states transmit ultrasonic waves with different velocities. In general, but not in a universal way, the more rigid is a Advanced Chemical Kinetics200