Browsing by Author "Merouani, Slimane"

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    A method for predicting the number of active bubbles in sonochemical reactors
    (Elsevier, 2015) Merouani, Slimane; Ferkous, Hamza; Hamdaoui, Oualid; Rezgui, Yacine
    Knowledge of the number of active bubbles in acoustic cavitation field is very important for the prediction of the performance of ultrasonic reactors toward most chemical processes induced by ultrasound. The literature in this field is scarce, probably due to the complicated nature of the phenomena. We introduce here a relatively simple semi-empirical method for predicting the number of active bubbles in an acoustic cavitation field. By coupling the bubble dynamics in an acoustical field with chemical kinetics occurring in the bubble during oscillation, the amount of the radical species _OH and HO_ 2 and molecular H2O2 released by a single bubble was estimated. Knowing that the H2O2 measured experimentally during sonication of water comes from the recombination of hydroxyl (_OH) and perhydroxyl (HO2 2) radicals in the liquid phase and assuming that in sonochemistry applications, the cavitation is transient and the bubble fragments at the first collapse, the number of bubbles formed per unit time per unit volume is then easily determined using material balances for H2O2, _OH and HO2 2 in the liquid phase. The effect of ultrasonic frequency on the number of active bubbles was examined. It was shown that increasing ultrasonic frequency leads to a substantial increase in the number of bubbles formed in the reactor
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    Comprehensive experimental and numerical investigations of the effect of frequency and acoustic intensity on the sonolytic degradation of naphthol blue black in water
    (Elsevier, 2015) Ferkous, Hamza; Merouani, Slimane; Hamdaoui, Oualid; Rezgui, Yacine; Guemini, Miloud
    In the present work, comprehensive experimental and numerical investigations of the effects of frequency and acoustic intensity on the sonochemical degradation of naphthol blue black (NBB) in water have been carried out. The experiments have been examined at three frequencies (585, 860 and 1140 kHz) and over a wide range of acoustic intensities. The observed experimental results have been discussed using a more realistic approach that combines the single bubble sonochemistry and the number of active bubbles. The single bubble yield has been predicted using a model that combines the bubble dynamics with chemical kinetics consisting of series of chemical reactions (73 reversible reactions) occurring inside an air bubble during the strong collapse. The experimental results showed that the sonochemical degradation rate of NBB increased substantially with increasing acoustic intensity and decreased with increasing ultrasound frequency. The numerical simulations revealed that NBB degraded mainly through the reaction with hydroxyl radical ( OH), which is the dominant oxidant detected in the bubble during collapse. The production rate of OH radical inside a single bubble followed the same trend as that of NBB degradation rate. It increased with increasing acoustic intensity and decreased with increasing frequency. The enhancing effect of acoustic intensity toward the degradation of NBB was attributed to the rise of both the individual chemical bubble yield and the number of active bubbles with increasing acoustic intensity. The reducing effect of frequency was attributed to the sharp decrease in the chemical bubble yield with increasing frequency, which would not compensated by the rise of the number of active bubbles with the increase in ultrasound frequency.
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    Computer simulation of chemical reactions occurring in collapsing acoustical bubble
    (Springer, 2013) Merouani, Slimane; Hamdaoui, Oualid; Rezgui, Yacine; Guemini, Miloud
    Acoustic cavitation is responsible for both sonochemistry and sonoluminescence. In this theoretical investigation, computer simulation of chemical reactions occurring in an isolated cavitation bubble oscillating in water irradiated by an ultrasonic wave has been performed for various acoustic amplitudes, different static pressures and diverse liquid temperatures to study the relationship between these three key parameters in sonochemistry and the oxidants created in the bubble. The results of the numerical simulations indicated that the main oxidants created in an O2 bubble are •OH radical and O atom. The amount of the oxidants formed in the bubble at the end of the bubble collapse increases as the acoustic amplitude increases from 1.5 to 3 atm. For each acoustic amplitude, there exists an optimal static pressure for the production of the oxidants, which shifts toward a higher value as the acoustic amplitude increases. Correspondingly, for each acoustic amplitude, an optimum of liquid temperature was observed at 20 C for •OH, HO2 • and H2O2. The simple model adopted in this work, after comparisons with the trends obtained with the literature experimental observations, seems to satisfactorily explain the experimental observations and should practically aid in optimization of operating conditions for sonochemical reactions. Results from this study were discussed and some recommendations were given
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    Effects of ultrasound frequency and acoustic amplitude on the size of sonochemically active bubbles – Theoretical study
    (Elsevier, 2013) Merouani, Slimane; Hamdaoui, Oualid; Rezgui, Yacine; Guemini, Miloud
    Numerical simulation of chemical reactions inside an isolated spherical bubble of oxygen has been performed for various ambient bubble radii at different frequencies and acousticamplitudes to study the effects of these two parameters on the range of ambient radius for an active bubble in sonochemical reactions. The employed model combines the dynamic of bubble collapse with the chemical kinetics of single cavitation bubble. Results fromthismodel were compared with some experimental results presented in the literature and good apparent trends between them were observed. The numerical calculations of this study showed that there always exists an optimal ambient bubble radius at which the production of oxidizing species at the end of the bubble collapse attained their upper limit. It was shown that the range of ambient radius for an active bubble increased with increasing acoustic amplitude and decreased with increasing ultrasound frequency. The optimal ambient radius decreased with increasing frequency. Analysis of curves showing optimal ambient radius versus acoustic amplitude for different ultrasonic frequencies indicated that for 200 and 300 kHz, the optimal ambient radius increased linearly with increasing acoustic amplitude up to 3 atm.However, slight minima of optimal radius were observed for the curves obtained at 500 and 1000 kHz.
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    Energy analysis during acoustic bubble oscillations
    (Elsevier, 2014) Merouani, Slimane; Hamdaoui, Oualid; Rezgui, Yacine; Guemini, Miloud
    In this work, energy analysis of an oscillating isolated spherical bubble in water irradiated by an ultrasonic wave has been theoretically studied for various conditions of acoustic amplitude, ultrasound frequency, static pressure and liquid temperature in order to explain the effects of these key parameters on both sonochemistry and sonoluminescence. The Keller–Miksis equation for the temporal variation of the bubble radius in compressible and viscous medium has been employed as a dynamics model. The numerical calculations showed that the rate of energy accumulation, dE/dt, increased linearly with increasing acoustic amplitude in the range of 1.5–3.0 atm and decreased sharply with increasing frequency in the range 200–1000 kHz. There exists an optimal static pressure at which the power w is highest. This optimum shifts toward a higher value as the acoustic amplitude increases. The energy of the bubble slightly increases with the increase in liquid temperature from 10 to 60 C. The results of this study should be a helpful means to explain a variety of experimental observations conducted in the field of sonochemistry and sonoluminescence concerning the effects of operational parameters
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    Experimental and numerical investigation of the effect of liquid temperature on the sonolytic degradation of some organic dyes in water
    (Elsevier, 2016) Merouani, Slimane; Hamdaoui, Oualid; Boutamine, Zineb; Rezgui, Yacine; Guemini, Miloud
    This paper presents a comprehensive experimental and numerical investigation of the effects of liquid temperature on the sonochemical degradation of three organic dyes, Rhodamine B (RhB), Acid orange 7 (AO7) and Malachite green (MG), largely used in the textile industry. The experiments have been carried out for an ultrasonic frequency of 300 kHz. The obtained experimental results were discussed using a new approach combining the results of single-bubble event and the number of active bubbles. The single-bubble event was predicted using a model that combines the bubble dynamics with chemical kinetics occurring inside a bubble during the strong collapse. The number of active bubbles was predicted using a method developed in our previous work. The experiments showed that the degradation rate of the three dyes increased significantly with increasing liquid temperature in the range 25–55 C. It was predicted that the main pathway of pollutants degradation is the attack by OH radicals. The simulations showed that there exists an optimum liquid temperature of about 35 C for the production of OH inside a bubble whereas the number of active bubbles increased sharply with the rise of the liquid temperature. It was predicted that the overall production rate of OH increased with increasing liquid temperature in the range 25–55 C. Finally, it was concluded that the effect of liquid temperature on the sonochemical degradation of the three dyes in aqueous phase was controlled by the number of active bubbles in the range 35–55 C and by both the number of bubbles and the single bubble yield in the range 25–35 C.
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    Mechanism of the sonochemical production of hydrogen
    (Elsevier, 2015) Merouani, Slimane; Hamdaoui, Oualid; Rezgui, Yacine; Guemini, Miloud
    It has been long recognized that propagation of an ultrasonic wave in water results in hydrogen production. The chemical effects of ultrasound (sonochemistry) originate from acoustic cavitation, that is, the formation, growth and implosive collapse of microscopic bubbles in liquid irradiated by ultrasound wave. Enormous temperatures and pressures are generated within the bubbles at the collapse, making each bubble as a microreactor within which typical flame reactions occur. The combustion in the cavitation bubbles yield species such as _OH, H_, O, HO2 _ and others. Although H2 is the most molecular product of water sonolysis, the mechanism of its production is until now not understood and the most reported suggestions are controversial. In this paper, a comprehensive numerical work was carried out, for the first time, to explain the mechanism of ultrasound induced generation of H2 in water. Computer simulations of chemical reactions occurring inside a bubble oscillating in water irradiated by an ultrasonic wave have been performed for different conditions. A kinetics mechanism of 25 reversible chemical reactions was proposed for studying the internal bubble-chemistry. The numerical simulations have evidenced the formation of H2 as well as other products such as O2, H2O2, _OH, H_, HO2 _ and O in the bubble during implosion. In all cases, H2 was the main product formed in the bubbles at appreciable amount. Basing on the simulation results and using material balance for hydrogen in the gas and liquid phases, the production rate of H2 in each phase has been quantified. The conclusion was that the main source of H2 production during water sonolysis is the gas phase of the bubbles through the reaction H_ ‏ _OH 4 H2 ‏ O. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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    New interpretation of the effects of argon-saturating gas toward sonochemical reactions
    (Elsevier, 2014) Merouani, Slimane; Ferkous, Hamza; Hamdaoui, Oualid; Rezgui, Yacine; Guemini, Miloud
    A number of literature reports showed that argon provides a more sonochemical activity than polyatomic gases because of its higher polytropic ratio; whereas several recent studies showed that polyatomic gases, such as O2, can compensate the lower bubble temperature by the self decomposition in the bubble. In this work, we show for the first time a numerical interpretation of these controversial reported effects. Computer simulations of chemical reactions inside a collapsing acoustic bubble in water saturated by different gases (Ar, O2, air and N2) have been performed for different frequencies (213–1100 kHz). In all cases, _OH radical is the main powerful oxidant created in the bubble. Unexpectedly, the order of saturating gases toward the production rate of _OH radical was strongly frequency dependent. The rate of production decreases in the order of Ar > O2 > air > N2 for frequencies above 515 kHz, and Ar starts to lose progressively its first order to the following gases with a gradually decreasing of frequency below 515 kHz up to a final order of O2 > air _ N2 > Ar at 213 kHz. The analysis of chemical kinetic results showed a surprising aspect: in some cases, there exists an optimum bubble temperature during collapse at which the chemical yield is much higher than that of the maximum bubble temperature achieved in the bubble. On the basis of this, we have concluded that the lower sonochemical activity induced by Ar for frequencies below 515 kHz is mainly due to the forte consumption of radicals inside a bubble prior the complete collapse being reached.
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    Sensitivity of free radicals production in acoustically driven bubble to the ultrasonic frequency and nature of dissolved gases
    (Elsevier, 2014) Merouani, Slimane; Hamdaoui, Oualid; Rezgui, Yacine; Guemini, Miloud
    Central events of ultrasonic action are the bubbles of cavitation that can be considered as powered microreactors within which high-energy chemistry occurs. This work presents the results of a comprehensive numerical assessment of frequency and saturating gases effects on single bubble sonochemistry. Computer simulations of chemical reactions occurring inside a bubble oscillating in liquid water irradiated by an ultrasonic wave have been performed for a wide range of ultrasonic frequencies (213–1100 kHz) under different saturating gases (O2, air, N2 and H2). For O2 and H2 bubbles, reactions mechanism consisting in 25 reversible chemical reactions were proposed for studying the internal bubble-chemistry whereas 73 reversible reactions were taken into account for air and N2 bubbles. The numerical simulations have indicated that radicals such as OH, H , HO2 and O are created in the bubble during the strong collapse. In all cases, hydroxyl radical ( OH) is the main oxidant created in the bubble. The production rate of the oxidants decreases as the driving ultrasonic frequency increases. The production rate of OH radical followed the order O2 > air > N2 > H2 and the order becomes more remarkable at higher ultrasonic frequencies. The effect of ultrasonic frequency on single bubble sonochemistry was attributed to its significant impact on the cavitation process whereas the effects of gases were attributed to the nature of the chemistry produced in the bubble at the strong collapse. It was concluded that, in addition to the gas solubility, the nature of the internal bubble chemistry is another parameter of a paramount importance that controls the overall sonochemical activity in aqueous solutions.
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    Theoretical estimation of the temperature and pressure within collapsing acoustical bubbles
    (Elsevier, 2014) Merouani, Slimane; Hamdaoui, Oualid; Rezgui, Yacine; Guemini, Miloud
    Formation of highly reactive species such as OH, H , HO2 and H2O2 due to transient collapse of cavitation bubbles is the primary mechanism of sonochemical reaction. The crucial parameters influencing the formation of radicals are the temperature and pressure achieved in the bubble during the strong collapse. Experimental determinations estimated a temperature of about 5000 K and pressure of several hundreds of MPa within the collapsing bubble. In this theoretical investigation, computer simulations of chemical reactions occurring in an O2-bubble oscillating in water irradiated by an ultrasonic wave have been performed for diverse combinations of various parameters such as ultrasound frequency (20–1000 kHz), acoustic amplitude (up to 0.3 MPa), static pressure (0.03–0.3 MPa) and liquid temperature (283– 333 K). The aim of this series of computations is to correlate the production of OH radicals to the temperature and pressure achieved in the bubble during the strong collapse. The employed model combines the dynamic of bubble collapse in acoustical field with the chemical kinetics of single bubble. The results of the numerical simulations revealed that the main oxidant created in an O2 bubble is OH radical. The computer simulations clearly showed the existence of an optimum bubble temperature of about 5200 ± 200 K and pressure of about 250 ± 20 MPa. The predicted value of the bubble temperature for the production of OH radicals is in excellent agreement with that furnished by the experiments. The existence of an optimum bubble temperature and pressure in collapsing bubbles results from the competitions between the reactions of production and those of consumption of OH radicals at high temperatures.