Sonochemical activity in ultrasonic reactors under heterogeneous conditions

Due to its physical and chemical effects, ultrasound is widely used for industrial purposes, especially in heterogeneous medium. Nevertheless, this heterogeneity can influence the ultrasonic activity. In this study, the effect of the addition of inert glass beads on the sonochemical activity inside an ultrasonic reactor is investigated by monitoring the formation rate of triiodide, and the ultrasonic power is measured by calorimetry and by acoustic radiation. It was found that the sonochemical activity strongly depends on the surface area of the glass beads in the medium: it decreases above a critical area value (around 10^-2 m²), partly due to wave scattering and attenuation. This result is confirmed for a large range of frequencies (from 20 to 1135 kHz) and glass beads diameters (from 8-12 µm to 6 mm). It was also demonstrated that above a given threshold of the surface area, only part of the supplied ultrasonic power is devoted to chemical effects of ultrasound. Finally, the acoustic radiation power appears to describe the influence of solids on sonochemical activity, contrary to the calorimetric power.

Intensification of heat and mass transfer by ultrasound: application to heat exchangers and membrane separation processes

This paper aims to illustrate the interest of ultrasound technology as an efficient technique for both heat and mass transfer intensification. It is demonstrated that the use of ultrasound results in an increase of heat exchanger performances and in a possible fouling monitoring in heat exchangers. Mass transfer intensification was observed in the case of cross-flow ultrafiltration. It is shown that the enhancement of the membrane separation process strongly depends on the physico-chemical properties of the filtered suspensions.

A new way to apply ultrasound in cross-flow ultrafiltration: application to colloidal suspensions

A new coupling of ultrasound device with membrane process has been developed in order to enhance cross-flow ultrafiltration of colloidal suspensions usually involved in several industrial applications included bio and agro industries, water and sludge treatment. In order to reduce mass transfer resistances induced by fouling and concentration polarization, which both are main limitations in membrane separation process continuous ultrasound is applied with the help of a vibrating blade (20 kHz) located in the feed channel all over the membrane surface (8mm between membrane surface and the blade). Hydrodynamic aspects were also taking into account by the control of the rectangular geometry of the feed channel. Three colloidal suspensions with different kinds of colloidal interaction (attractive, repulsive) were chosen to evaluate the effect of their physico-chemical properties on the filtration. For a 90 W power (20.5 W cm(-2)) and a continuous flow rate, permeation fluxes are increased for each studied colloidal suspension, without damaging the membrane. The results show that the flux increase depends on the initial structural properties of filtered dispersion in terms of colloidal interaction and spatial organizations. For instance, a Montmorillonite Wyoming-Na clay suspension was filtered at 1.5 × 10(5)Pa transmembrane pressure. Its permeation flux is increased by a factor 7.1, from 13.6 L m(-2)h(-1) without ultrasound to 97 L m(-2)h(-1) with ultrasound.

Improvement of heat transfer by means of ultrasound: Application to a double-tube heat exchanger

A new kind of ultrasonically-assisted heat exchanger has been designed, built and studied. It can be seen as a vibrating heat exchanger. A comprehensive description of the overall experimental set-up is provided, i.e. of the test rig and the acquisition system. Data acquisition and processing are explained step-by-step with a detailed example of graph obtained and how, from these experimental data, energy balance is calculated on the heat exchanger. It is demonstrated that ultrasound can be used efficiently as a heat transfer enhancement technique, even in such complex systems as heat exchangers.

Ultrasonic depolymerization of an exopolysaccharide produced by a bacterium isolated from a deep-sea hydrothermal vent polychaete annelid

Low frequency ultrasound was used to depolymerize a high-molecular-weight exopolysaccharide (EPS) produced by a deep-sea hydrothermal bacterium Alteromonas macleodii subsp. fijiensis biovar deepsane. The influence of several parameters was examined including the duration of ultrasonic irradiation, EPS concentration, reaction temperature and volume of the sonicated solution. With the aim of optimizing the depolymerization, the native EPS was simultaneously treated with hydrogen peroxide and ultrasound. This study identified the sonication conditions that produce low-molecular-weight derivatives from the native EPS (>10(6)Da) with good reproducibility.

Enhancement of the knowledge on the ultrasonic reactor behaviour by an interdisciplinary approach

Our work is a step to a better understanding of high frequency ultrasonic reactors behaviour. Using finite elements calculations, it was demonstrated that localization of chemical and physical effects can be well correlated with mechanical behaviour of ultrasound emitter. This complementary approach enables us to propose a full interpretation of the sonochemical reactor behaviour. A major reason of scientific interest on ultrasound is the well-known enhancement of chemical or physical phenomena. This is so important that "Enhancement" is probably the most used word in the title of related publications. To fully understand experimental results, present work demonstrates that ultrasound needs also to significantly enhance a very difficult knowledge transfer operation that might be named interdisciplinary co-working. Hence, ultrasound is now used and studied in many different fields of science such as acoustic, chemistry, medical imaging, disease treatment (lithotripsy), non-destructive testing. Each one has his own vocabulary, approach, and method to describe the phenomenon. In this work four different methodologies were involved to study of the same effect but using a chemical, chemical engineering, physical and mechanical approach respectively. All these viewpoints were then brought together in order to explain new original results.

A comparative study between classical stirred and ultrasonically-assisted dead-end ultrafiltration

The aim of this work was to determine mass transfer coefficients in the cases of ultrasonically-assisted and classical stirred dead-end ultrafiltration. A comparative study was then performed, and mass transfer coefficients obtained under ultrasonic conditions are described by an empirical model. This correlation results from an analogy with what is observed using a stirred cell and involves the ultrasonic power as the main parameter. The hydrodynamics are assumed to depend on the intensity of the ultrasound effects illustrated by the agitation arising within the cell. This agitation is due to convective currents as well as physical effects due to cavitation. The concentration polarization phenomenon is therefore affected by this action of ultrasonic waves.

Degradation of pentachlorophenol aqueous solutions using a continuous flow ultrasonic reactor: experimental performance and modelling

The degradation of aqueous solutions of pentachlorophenol (PCP) in a three-stage sonochemical reactor operating in the continuous flow mode has been investigated. The experimental reactor may be considered as a series of three high-frequency ultrasonic units. The influence of several parameters such as ultrasonic power, reactor volume and volumetric feed flow rate on the reactor performance is reported. Application of classical basic chemical engineering principles leads to a model that enables us to predict the PCP concentration within the reactor. In steady state, experimental conversion rates are shown to be in good agreement with model predictions.

Experimental study of the hydrodynamic behaviour of a high frequency ultrasonic reactor

In relation to design and modeling of sonochemical reactors, the hydrodynamic behaviour of a high-frequency ultrasonic reactor has been investigated. Residence time distribution (RTD) measurements have been performed by means of a tracer method. The influence of ultrasound on the response to an inlet pulse was evidenced. It was shown that the reactor behaves like a completely stirred tank reactor (CSTR) as soon as ultrasonic irradiation operates. Preliminary observations on acoustic streaming occurring within the reactor will also be presented.