Anne-Marie Wilhelm

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.

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.

Chemical reactions under ultrasound : discrimination of chemical and physical effects

Abstract Liquid—solid mass transfer measurements under ultrasound are presented and compared to reaction conversions. These comparisons have been achieved with two emission conditions: a transducer located at the bottom of a cylindrical reactor, or a horn dipping into a vessel. Mass transfer coefficients were proved to be very different throughout the reactor: axially they vary according to a standing wave, and radially the high values are concentrated above the transducer. The power input has an influence up to a plateau. These trends have also been found for a homogeneous reaction (the oxidation of potassium iodide) carried out in the whole reactor or locally in microreactors. A solid—liquid reaction (Michael addition) exhibited the same power dependence; measurement of particle diameter together with the reaction yield proved ultrasound to prevent agglomeration of the solid. Nevertheless, acceleration is not only due to mass transfer effects, a chemical effect must exist; the latter effect was demonstrated by addition of a radical scavenger which reduced the reaction rate.

Influence of ultrasound on mixing on the molecular scale for water and viscous liquids.

Micromixing has a decisive action on the yield of fast reactions such as combustions, polymerizations, neutralizations and precipitations. The aim of this study was to test the possible effect of ultrasound on micromixing, through the phenomenon of acoustic cavitation. To evaluate the local state of micromixing, we used a system of parallel competing reactions involving the Dushman reaction between iodide and iodate, coupled with a neutralization. At first, we studied the effects of the acoustic frequency on micromixing (20-540-1000 kHz). It was found that micromixing through acoustic cavitation and acoustic streaming was more important at 20 kHz than at 540 kHz or 1 MHz. At high and low frequency, it was shown that the injection must be located near the ultrasonic emitter. The influence of the acoustic intensity proved to be predominant mostly for low intensities; for an acoustic intensity of 10 W cm(-2), a characteristic micromixing time of about 0.015 s has been obtained. Viscous media have been studied and experiments showed that micromixing is more difficult to achieve than in aqueous media, but that ultrasound may be as effective as classic stirring.

Effects of ultrasound emitter type and power on a heterogeneous reaction

Abstract In the course of our current interest in designing appropriate reactors for sonochemistry, we have investigated a heterogeneous reaction which is highly improved by ultrasound, with the aim of trying to understand the mechanism of the reaction rate enhancement and the influence of the sonication parameters. The chosen model reaction is a Michael reaction: the addition of ethyl malonate to chalcone in toluene under solid-liquid phase transfer conditions. This reaction is very sensitive to sonic irradiation: the initial reaction rate is increased by a factor of 10 under sonication. In order to separate the physical and chemical effects of ultrasound, we have studied the disruption of the solid catalyst (KOH) in toluene and compared the particle sizes with the values obtained during the reaction with and without ultrasound. It can be seen that sonication prevents particle agglomeration as the reaction proceeds. The effect of the generator power has been studied using a cuphorn. The reaction yield is dramatically increased near the cavitation power threshold and then remains nearly constant. Finally, different sonication systems have been compared, the cleaning bath giving the poorest results.

Ultrasound in gas-liquid systems : Effects on solubility and mass transfer

The effect of ultrasound on the pseudo-solubility of nitrogen in water and on gas-liquid mass transfer kinetics has been investigated in an autoclave reactor equipped with a gas induced impeller. In order to use organic liquids and to investigate the effect of pressure, gas-liquid mass transfer coefficient was calculated from the evolution of autoclave pressure during gas absorption to avoid any side-effects of ultrasound on the concentrations measurements. Ultrasound effect on the apparent solubility is very low (below 12%). Conversely ultrasound greatly improves gas-liquid mass transfer, especially below gas induction speed, this improvement being boosted by pressure. In typical conditions of organic synthesis: 323 K, 1100 rpm, 10 bar, k(L).a is multiplied by 11 with ultrasound (20 kHz/62.6 W). The impact of sonication is much higher on gassing out than on gassing in. In the same conditions, this enhancement is at least five times higher for degassing.

Degradation of paracetamol by catalytic wet air oxidation and sequential adsorption - Catalytic wet air oxidation on activated carbons

The concern about the fate of pharmaceutical products has raised owing to the increasing contamination of rivers, lakes and groundwater. The aim of this paper is to evaluate two different processes for paracetamol removal. The catalytic wet air oxidation (CWAO) of paracetamol on activated carbon was investigated both as a water treatment technique using an autoclave reactor and as a regenerative treatment of the carbon after adsorption in a sequential fixed bed process. Three activated carbons (ACs) from different source materials were used as catalysts: two microporous basic ACs (S23 and C1) and a meso- and micro-porous acidic one (L27). During the first CWAO experiment the adsorption capacity and catalytic performance of fresh S23 and C1 were higher than those of fresh L27 despite its higher surface area. This situation changed after AC reuse, as finally L27 gave the best results after five CWAO cycles. Respirometry tests with activated sludge revealed that in the studied conditions the use of CWAO enhanced the aerobic biodegradability of the effluent. In the ADOX process L27 also showed better oxidation performances and regeneration efficiency. This different ageing was examined through AC physico-chemical properties.

Competitive adsorption of phenolic compounds from aqueous solution using sludge‐based activated carbon

Preparation of activated carbon from sewage sludge is a promising approach to produce cheap and efficient adsorbent for pollutants removal as well as to dispose of sewage sludge. The first objective of this study was to investigate the physical and chemical properties (BET surface area, ash and elemental content, surface functional groups by Boehm titration and weight loss by thermogravimetric analysis) of the sludge‐based activated carbon (SBAC) so as to give a basic understanding of its structure and to compare to those of two commercial activated carbons, PICA S23 and F22. The second and main objective was to evaluate the performance of SBAC for single and competitive adsorption of four substituted phenols (p‐nitrophenol, p‐chlorophenol, p‐hydroxy benzoic acid and phenol) from their aqueous solutions. The results indicated that, despite moderate micropore and mesopore surface areas, SBAC had remarkable adsorption capacity for phenols, though less than PICA carbons. Uptake of the phenolic compound was found to be dependent on both the porosity and surface chemistry of the carbons. Furthermore, the electronegativity and the hydrophobicity of the adsorbate have significant influence on the adsorption capacity. The Langmuir and Freundlich models were used for the mathematical description of the adsorption equilibrium for single‐solute isotherms. Moreover, the Langmuir–Freundlich model gave satisfactory results for describing multicomponent system isotherms. The capacity of the studied activated carbons to adsorb phenols from a multi‐solute system was in the following order: p‐nitrophenol > p‐chlorophenol > PHBA > phenol.

Application of sludge-based carbonaceous materials in a hybrid water treatment process based on adsorption and catalytic wet air oxidation

This paper describes a preliminary evaluation of the performance of carbonaceous materials prepared from sewage sludges (SBCMs) in a hybrid water treatment process based on adsorption and catalytic wet air oxidation; phenol was used as the model pollutant. Three different sewage sludges were treated by either carbonisation or steam activation, and the physico-chemical properties of the resultant carbonaceous materials (e.g. hardness, BET surface area, ash and elemental content, surface chemistry) were evaluated and compared with a commercial reference activated carbon (PICA F22). The adsorption capacity for phenol of the SBCMs was greater than suggested by their BET surface area, but less than F22; a steam activated, dewatered raw sludge (SA_DRAW) had the greatest adsorption capacity of the SBCMs in the investigated range of concentrations (<0.05 mol L(-1)). In batch oxidation tests, the SBCMs demonstrated catalytic behaviour arising from their substrate adsorptivity and metal content. Recycling of SA_DRAW in successive oxidations led to significant structural attrition and a hardened SA_DRAW was evaluated, but found to be unsatisfactory during the oxidation step. In a combined adsorption-oxidation sequence, both the PICA carbon and a selected SBCM showed deterioration in phenol adsorption after oxidative regeneration, but a steady state performance was reached after 2 or 3 cycles.