Henri Delmas

Emulsification by ultrasound: drop size distribution and stability.

The aim of this work is to compare the oil-in-water emulsions produced by mechanical agitation (Ultra-Turrax, 10,000 rpm, P = 170 W) or power ultrasound (ultrasound horn, 20 kHz, 130 W) using the same model system: water/kerosene/polyethoxylated (20 EO) sorbitan monostearate. The following parameters were varied: emulsification time, surfactant concentration, consumed power and volume fraction of oil. With ultrasound, the drop size (Sauter diameter, d32) is much smaller than that given by mechanical agitation under the same conditions, which makes insonated emulsions more stable. For a given drop size (d32), less surfactant is required.

Power measurement in sonochemistry

Abstract This paper is devoted to power measurements at different locations of an ultrasonic device: the power displayed at the frequency generator, the electrical power input at the transducer (by measuring the applied voltage and intensity) and the acoustic power dissipated in the liquid medium (determined by a calorimetric method). These different measurements were recorded and compared for different media and different liquid heights in the reactor. When a chemical reaction takes place, the chemical yield was plotted against the different power measurements. The dissipated acoustic power per unit volume, although less accurate, appears to be the best measurement to establish a correlation with the influence of ultrasound on a chemical reaction.

Ultrasound Emulsification—An Overview

ABSTRACT Fundamentals and applications of ultrasound emulsification are reviewed. The importance of cavitation is stressed, as also is power input to the multiphase fluid. The influence of surfactants, polymeric stabilizers, temperature, pressure, and ultrasonic parameters such as frequency, residence time, acoustic intensity, and energy density are described. The effects of other physicochemical parameters such as emulsifier concentration, disperse phase volume fraction, and viscosity are discussed. Applications to both water-in-oil and oil-in-water emulsions are discussed.

Mass transfer enhancement of gas absorption in oil-in-water systems: a review

A review is presented on the gas–liquid mass transfer enhancement due to the presence of a second dispersed liquid phase. An attempt has been made to describe the mass transfer characteristics in a gas–liquid–liquid system. The ability of an immiscible oil phase to influence the possible pathway for gas transfer from the gas phase to the aqueous phase and to affect the gas–liquid interface and the volumetric mass transfer coefficient kLa is considered. Though the mass transfer in series looks the most logical explanation, there are many gaps and contradictions in the reported results of kLa, preventing any definite conclusion being reached. An enhancement factor (E), which quantifies the effect of the oil addition on the gas–liquid mass transfer, is defined. Experimental enhancement factors are reported and compared to the theoretical maximum attainable enhancement factor (Emax). Possible mechanisms (“bubble covering”, “shuttle effect” and “permeability effect”) involved in mass transfer enhancement are assessed in detail. The commonly used “shuttle effect” mechanism, whose model proposes a direct usable expression of the enhancement factor, underestimates the reported experimental enhancement factors by about 20%. However, to date, it is not possible to satisfactorily propose a unique theory explaining the influence of the presence of an immiscible oil on mass transfer enhancement. Moreover, the development of sophisticated models has not yet reached satisfactory levels. Recommendations have been made for future research.

Determination of velocity, size and concentration of ultrasonic cavitation bubbles by the phase-Doppler technique

Abstract An experimental study about the velocity, size and concentration of ultrasonic cavitation bubbles is presented. Ultrasound is provided from a 20 kHz horn dipping vertically into a liquid container. Using the laser phase-Doppler technique, cavitation bubble velocity and size distributions are obtained at various locations inside the ultrasound field. Axial and radial profiles of average bubble velocity, mean diameters and volumetric flow rate are presented. The influence of ultrasound power input is also investigated: it increases bubble velocity quasi-linearly and has more complex effect on bubbles diameter. The system investigated possesses similar hydrodynamic properties with typical turbulent circular jet flows.

Crystallization of potash alum: effect of power ultrasound.

The influence of power ultrasound on the crystallization of potash alum was investigated. Experiments have been carried out in a batch stirred vessel. It was found that ultrasonic waves decrease the supersaturation limits and modify the morphology of the crystals produced. The average crystal size decreases with an increase of ultrasonic power. To investigate also the action of ultrasound on already existing crystals, crystals produced in silent conditions were suspended in saturated potash alum solution at various ultrasonic powers. The results show that ultrasound has also an abrasive effect on potash alum crystals for high power inputs.

Gas-liquid-liquid reaction engineering: hydroformylation of 1-octene using a water soluble rhodium complex catalyst

Abstract Hydroformylation of 1-octene was studied in a two-phase system using [RhCl(1,5-COD)] 2 complex catalyst and the trisodium salt of tri( m -sulfophenyl) phosphine (TPPTS) as a water soluble ligand. The reaction was carried out in a batch reactor at pressures between 1.5 to 2.5×10 3 kPa and temperatures of 333 and 343 K. Ethanol was added as a cosolvent to enhance the octene solubility in the aqueous phase and a buffer solution of sodium carbonate and bicarbonate was used to eliminate the formation of acetals. The hydroformylation products were n-nonanal and 2-methyl nonanal with a selectivity to linear aldehyde of about 80%. The reaction is first order with respect to octene and catalyst concentrations. The dependence on the carbon monoxide pressure was found to be complex with an enhancement at low partial pressures and an inhibition rate at high pressures. The reaction rate increased with the hydrogen partial pressure. A thermodynamic analysis concerning the solubilities of octene and gases in the reaction medium was studied and a semiempirical kinetic model was used to describe the rate of reaction.

Characterisation of the acoustic cavitation cloud by two laser techniques

An experimental investigation of the size and volumetric concentration of acoustic cavitation bubbles is presented. The cavitation bubble cloud is generated at 20 kHz by an immersed horn in a rectangular glass vessel containing bi-distilled water. Two laser techniques, laser diffraction and phase Doppler interferometry, are implemented and compared. These two techniques are based on different measuring principles. The laser diffraction technique analyses the light pattern scattered by the bubbles along a line-of-sight of the experimental vessel (spatial average). The phase Doppler technique is based on the analysis of the light scattered from single bubbles passing through a set of interference fringes formed by the intersection of two laser beams: bubble size and velocity distributions are extracted from a great number of single-bubble events (local and temporal average) but only size distributions are discussed here. Difficulties arising in the application of the laser diffraction technique are discussed: in particular, the fact that the acoustic wave disturbs the light scattering patterns even when there are no cavitation bubbles along the measurement volume. As a consequence, a procedure has been developed to correct the raw data in order to get a significant bubble size distribution. After this data treatment has been applied the results from the two measurement techniques show good agreement. Under the emitter surface, the Sauter mean diameter D(3, 2) is approximately 10 microm by phase Doppler measurement and 7.5 microm by laser diffraction measurement at 179 W. Note that the mean measured diameter is much smaller than the resonance diameter predicted by the linear theory (about 280 microm). The influence of the acoustic power is investigated. Axial and radial profiles of mean bubble diameters and void fraction are also presented.

Oxidation of phenol in wastewater by sonoelectrochemistry

Abstract Mass transfer was investigated in ultrasonic reactors by means of the electrochemical method (feri-ferro-cyanide couple, under diffusion limitations). This method allowed for the determination of the active zones in the reactor. The oxidation of a model pollutant, phenol, has been carried out in the previously screened reactor, with ultrasound alone and with ultrasound associated with electrolysis. With a 20 kHz sonication, the electrochemical oxidation of phenol in NaCl media allows the conversion of 75% of initial phenol within 10 minutes of treatment. However a toxic intermediate, p.quinone was formed. At 500 kHz a conversion of 95% of the initial phenol was obtained within the same treatment time, and final products of degradation were acetic and chloroacrylic acids.

Low frequency thermo-ultrasonication of Saccharomyces cerevisiae suspensions: Effect of temperature and of ultrasonic power

Abstract The combined effect of low frequency ultrasound (20 kHz) with temperature on the survival of a strain of Saccharomyces cerevisiae suspended in water was studied. The treatment temperatures tested were 45 °C, 50 °C and 55 °C; the actual ultrasonic powers tested were 50 W, 100 W and 180 W. Application of ultrasonic waves at a non-lethal temperature did not display a deactivating action; a higher effect of ultrasound at higher temperatures was observed, and the synergy between ultrasound and temperature was confirmed. These results proved that the ultrasonic waves do not destroy the yeasts cells; they damage them, thus increasing their sensitivity to heat. The existence of an optimal ultrasonic power for a maximal deactivating effect was shown, and was found to have a value of around 100 W (actual power).