Using microparticles to enhance micromixing in a high frequency continuous flow sonoreactor

In this paper, an experimental study was conducted to investigate the effect of high frequency ultrasound wave on micromixing efficiency in the presence of polymeric microparticles in a tubular sonoreactor. The size and volume fraction of polymeric microparticles were varied and the micromixing efficiency was studied by adopting the Dushman reaction coupled with a neutralization reaction. In addition, the effect of flow rate and liquid viscosity on the segregation index was studied. The experimental results showed that the movement and dispersion of the polymeric microparticles by ultrasound wave could improve the micromixing efficiency. The results showed that the size of the polymeric microparticles has great effect on mixing. Moreover, it was found that in the presence of these microparticles, segregation index decreases significantly and their effect reduces with increase in the solution viscosity. The presented results show that using high frequency ultrasound waves in the range of MHz and in the presence of microparticles can promise to reach an efficient micromixing in tubular sonoreactors.     

Experimental investigation on proper use of dual high-low frequency ultrasound waves—Advantage and disadvantage

An experimental study was conducted to investigate the proper use of dual frequency ultrasound irradiation in sonoreactors. For this purpose, two waves with frequencies of 1.7 MHz and 24 kHz were combined. A dual frequency rig was equipped with a high frequency 1.7 MHz ultrasound transducer and a low frequency one generating 24 kHz waves. Dushman competitive reactions were used to study micromixing, Weissler reaction for evaluating cavitation activity and ammonia degradation experiment to investigate the superposition effect of two coupling waves on decomposition strength. The results revealed that combining these waves has a positive effect on number of generating cavitation bubbles and micromixing performance. However, this combination reduced the strength of the low frequency wave and diminished the ammonia degradation process efficiency. The micromixing enhancement in terms of the segregation index in dual frequency irradiation mode is approximately 46.9% and 40.1% more than single low and high frequency irradiation, respectively. On the other hand, the ammonia removal efficiency is reduced by 10.3% in dual frequency irradiation compared to single low frequency layout.     

CFD Modeling of Micromixing and Velocity Distribution in a 1.7‐MHz Tubular Sonoreactor

The effect of high‐frequency (1.7 MHz) ultrasound waves on the mixing rate in a new continuous tubular sonoreactor was investigated by CFD modeling. Modeling of piezoelectric transducer (PZT) vibrations was done based on the dynamic mesh model. Results indicate that the acoustic streams were in the direction of wave propagation and their maximum velocity near the PZT surface agreed well with experimental measurements. The micromixing efficiency of the sonoreactor was studied by adopting the Villermaux/Dushman reaction in the modeling. Comparison of the calculated relative segregation index from modeling results with experimental data revealed reasonable accordance.     

Effect of high frequency ultrasound on micromixing efficiency in microchannels

In the present work, an investigation on the effect of high frequency ultrasound wave on micromixing in the studied microchannels was carried out. Three types of microchannels with different shapes are examined. A 1.7 MHz piezoelectric transducer (PZT) was employed to induce the vibration in these microchannels through an indirect contact. A method based on the Villermaux–Dushman reaction was employed to study the micromixing in these microchannels. The segregation intensity was determined for layouts with and in the absence of ultrasound irradiation. Further, the effect of ultrasound waves, in various flow rates and initial concentrations of acid, on the segregation index (X_S) and micromixing time (t_m) was investigated. The experimental results showed that the ultrasound waves have a significant influence on product distribution and segregation index at various flow rate ratios. The data obtained in all cases showed that the segregation index was reduced when the flow rate ratios were increased. Also the results demonstrate that in spite of a low energy consumption of PZT, the relative segregation index improved up to 18–36% at various flow rate ratios.     

Initial studies using ultrasonic spectroscopy for monitoring changes in residua with pyrolysis

Ultrasonic spectroscopy was explored for monitoring microstructure changes during pyrolysis of petroleum residua. Attenuation and sound velocity measurements were obtained for original and pyrolyzed residua using ultrasonic transducers with frequencies ranging from 1 to 10 MHz. Transducers at 1.0, 2.2, and 3.5 MHz provide signals that can be measured at room temperature. Little useful signal passes the solid samples at room temperature above 4 MHz. Symmetrical amplitude vs. time pulse signals are observed through water, which exhibits essentially no attenuation of ultrasound. When the ultrasound signal passes through a solid residuum sample, however, the raw pulse signal in the time domain shows significant asymmetry. By obtaining fast Fourier transform (FFT) of the time domain waveforms, amplitude vs. frequency spectra are obtained. The FFT spectrum for water is symmetrical. For FFT spectra of solid residua, the higher frequencies are attenuated more than the lower frequencies. Maximum intensities are near 1.1 MHz, regardless of the frequency of the transducer. For the 2.2 and 3.5 MHz transducers, the FFT spectra for all of the residua studied exhibit a shoulder above 1 MHz. The shoulder diminishes early during pyrolysis as the solvation shell structures associated with the suspended particles are destroyed, and the nature of the particles changes. The FFT shoulder begins to grow again as pre-coke particles form.     

CFD modeling of mixing intensification assisted with ultrasound wave in a T-type microreactor

This paper aims to demonstrate the effect of ultrasound wave on mixing in a T-type microreactor. In order to create vibration in this microreactor, a low frequency (42 kHz) piezoelectric transducer was used. A well-known parallel-competitive reaction (Villermaux–Dushman reaction) was employed to study the mixing in the microreactor and the segregation index values were found for layouts with and without sonication. Results show that the ultrasound waves have a significant favorable influence on product distribution and the segregation index at various total flow rates. In all cases, the segregation index decreased with increase in total flow rate. The results reveal that the segregation index improved up to 10–20% by consuming a low energy (2.45 W Kg⁻¹) by the piezoelectric transducer. Finally, the computational fluid dynamics (CFD) modeling was carried out to explain the observed experimental results.     

CFD simulation and experimental validation of fluid flow and particle transport in a model of alveolated airways

Accurate modeling of air flow and aerosol transport in the alveolated airways is essential for quantitative predictions of pulmonary aerosol deposition. However, experimental validation of such modeling studies has been scarce. The objective of this study is to validate computational fluid dynamics (CFD) predictions of flow field and particle trajectory with experiments within a scaled-up model of alveolated airways. Steady flow (Re=0.13) of silicone oil was captured by particle image velocimetry (PIV), and the trajectories of 0.5 and 1.2 mm spherical iron beads (representing 0.7–14.6 μm aerosol in vivo) were obtained by particle tracking velocimetry (PTV). At 12 selected cross sections, the velocity profiles obtained by CFD matched well with those by PIV (within 1.7% on average). The CFD predicted trajectories also matched well with PTV experiments. These results showed that air flow and aerosol transport in models of human alveolated airways can be simulated by CFD techniques with reasonable accuracy.     

New insight into micromixing in supercritical CO2 using a chemical method

Competing reactions have been studied in order to characterize micromixing efficiency in supercritical medium. A new chemical reaction test has been developed because the current systems involving ionic species are not suitable for a supercritical medium (CO₂). The system is based on two competitive reactions: the esterification of phenylacetic acid by ethyl alcohol using paratoluenesulfonic acid monohydrate as a catalyst and the neutralization of paratoluenesulfonic acid monohydrate by tributylamin.The neutralization reaction is faster than the esterification reaction. The test procedure consists in adding, in stoechiometric defect, paratoluenesulfonic acid monohydrate to a mixture of ethyl alcohol, phenylacetic acid, tributylamin and carbon dioxide under 17 MPa and 50 °C. The ester yield is directly linked to the micromixing efficiency.Preliminary studies such as miscibility or analytical method (GPC) studies have been carried out in order to test the reactions’ system in batch reactor. The sensitivity to micromixing effects has been investigated in a 0.5 L stirred reactor. Final composition in the vessel is dependent on the stirring speed. This new system is efficient to characterize micromixing effects in a supercritical medium, which are particularly important in the Supercritical AntiSolvent precipitation processes but also for chemical reactions.     

New crosslinked-porous poly-ammonium microparticles as CO2 adsorbents

A new porous polymeric microparticle, poly[4-vinylbenzyltriethylammonium chloride] (P[VBTEA][Cl]), is prepared from the crosslinking polymerization of 4-vinylbenzyltriethylammonium chloride with N,N-methylenebisacrylamide via inverse suspension polymerization using cyclohexane as continuous phase, Span 80-Tween 80 as dispersant, and PEG600 as porogen. Two other microparticles, P[VBTEA][BF₄] and P[VBTEA][PF₆] are further obtained through ion-exchange. FT-IR, TGA, SEM, and EDS analyzes indicate that these microparticles have good porosity (apparent porosities are 55.0%, 69.7% and 64.3%, respectively), high thermal stability, and large specific surface area, which make them potentially applicable as adsorbent agents. Their CO₂ adsorption–desorption performance is investigated. It is observed that such porous polymeric microparticles present high CO₂ sorption capability; typically, the CO₂ sorption of P[VBTEA][PF₆] is 1.38 wt% at 25 °C and 1 bar. Such porous polymeric microparticles are good candidates for CO₂ adsorption.     

A review of ultrasonic Coda Wave Interferometry in concrete

Concrete is a multicomposite material with heterogeneities ranging in size from micrometers to centimeters. At low frequency (below 50 kHz), ultrasound propagates through concrete without suffering scattering and absorption. On the contrary, above ≈ 100 kHz, the waves strongly interact with all the heterogeneities and enter a multiple scattering regime. This regime induces 1) substantial attenuation of coherent waves (direct waves and ballistic echoes), a feature that disables most conventional imaging techniques; and 2) the onset of late arrival signals that form the ultrasonic coda. An important feature of coda waves is their very high sensitivity to weak changes in the medium. Over the last years Coda Wave Interferometry (CWI) applied to ultrasound in concrete has been widely adopted in the non-destructive testing community. This article reviews several applications based on the precise processing of ultrasonic coda, in the case of thermal and/or stress and/or damage changes.     

Macro- and micromixing in a novel sonochemical reactor using high frequency ultrasound

This paper deals with the influence of ultrasound on macro- and micromixing in a new developed sonochemical reactor. Unprecedented piezoelectric transducer arrangement with a high frequency of 1.7 MHz has been used in this novel reactor. Macromixing quality has been investigated visually and the Dushman reaction (iodide-iodate) coupled with a neutralization reaction have been examined in order to characterize micromixing quality. In addition, the effect of liquid viscosity on the segregation index has been studied. The results show that this new developed reactor can establish reasonable macro- and micromixing inside the reactor. Moreover, the performance of this reactor has been compared with a stirred tank reactor equipped with a Rushton turbine impeller. It is found that with the same input electrical power, the obtained segregation index for stirred tank reactor is approximately 10% more than proposed new ultrasound reactor, which means the sonoreactor works more efficiently.     

Encapsulation of perfluorocarbon gases into lipid-based carrier by PGSS

For the first time, gas-filled microparticles were successfully prepared using a supercritical fluid based technology. Low molecular weight perfluorcarbon (PFC) gases, C₃F₈ or C₄F₈, have been encapsulated into Gelucire^® 50/13 (lipid-based carrier: polyethylene glycol glycerides), using PGSS^® (Particles from Gas Saturated Solution) technique. Particles were produced from the fast expansion of the melted lipid carrier saturated with a mixture of (CO₂ + PFC). The presence of the gas into the produced microparticles was verified by Nuclear Magnetic Resonance (NMR) analysis of fluorine atom. The effect of carrier to PFC mass ratio and PFC structure on the entrapment efficiency of the PFC gas into the particles was evaluated at fixed at 8.5 MPa and 353 K. These parameters were fixed in a preliminary study according to the morphology, size and flowability of the particles. The stability of encapsulated C₄F₈ in microparticles showed to be higher than C₃F₈; it was determined to be 2 h, at room conditions at the optimized carrier: PFC mass ratio of 30:1.     

Biodiesel process intensification in a very simple microchannel device

Biodiesel is normally obtained by transesterification of triglycerides with methanol in the presence of an alkaline catalyst. The reaction, performed in stirred tank reactors, requires 1–2 h of reaction time. As the reactants are immiscible, the reaction rate can be affected by mass transfer limitation. We have recently shown, that all methods favoring local micromixing can give place to high performances. At this purpose, we have recently developed a very simple laboratory device for testing the behavior of the mentioned reaction in microchannels of different sizes. This device is simply a tubular reactor filled with stainless steel spheres of different diameters. By opportunely changing the spheres’ diameters it is possible to obtain microchannels in a range of 300–1000 μm. However, in these reactors the void portion of the reactor is low and the productivity per volume is low, too. It is possible to obtain better results in terms of productivity by filling the tubular reactor with stainless steel wool, being in this case the void fraction about 0.9. In this paper, the performances obtained with this type of reactor are reported together with a discussion on the reaction mechanism in view of the future development of a kinetic biphasic model.     

Production of silica aerogel microparticles loaded with ammonia borane by batch and semicontinuous supercritical drying techniques

Silica aerogel microparticles were prepared by supercritical drying and used as support for hydrogen-storing ammonia borane (AB). The formation of aerogel microparticles was done using two different processes: batch supercritical fluid extraction and a semicontinuous drying process. Silica aerogel microparticles with a surface area ranging from 400 to 800 m²/g, a volume of pores of 1 cm³/g, and a mean particle diameter ranging from 12 to 27 μm were produced using the two drying techniques. The particle size distribution (PSD) of the microparticles was influenced by shear rate, amount of catalyst, hydrophilic–hydrophobic solvent ratio and hydrophobic surface modification. In particular, irregular aerogel particles were obtained from hydrophilic gels, while regular, spherical particles with smooth surfaces were obtained from hydrophobic gels. AB was loaded into silica aerogel microparticles in concentrations ranging from 1% till 5% wt. Hydrogen release kinetics from the hydride-loaded aerogel was analyzed with a volumetric cell at 80 °C. By stabilization of AB into the silica aerogel microparticles, an improvement of the release rate of hydrogen from AB was observed.     

Dispersion and stability of 8 mol.% yttria stabilized zirconia suspensions for dip-coating filtration membranes

The dispersion and the stability of the suspensions from commercially available 8 mol.% yttria stabilized zirconia (8YSZ) powder have been systematically investigated as a function of the system factors such as pH value, temperature, solid loadings, molecular weight and amounts of polyacrylic acid (PAA) dispersant. The interaction mechanisms between YSZ particles and organic additives in suspensions were analyzed and discussed in detail. The crack-free tubular membranes derived from the fully dispersed and well stabilized YSZ suspensions with appropriate viscosity exhibited an average pore diameter of 0.31 μm and membranes thickness of 10 μm. The pure water permeability of the membrane was up to 1.7 m³ h⁻¹ bar⁻¹ m⁻².     

Production and characterization of chitosan microparticles containing papain for controlled release applications

The production and modification of chitosan microparticles using crosslinking agents and papain were evaluated for controlled release applications. Chitosan microparticles were produced and crosslinked with sodium tripolyphosphate (TPP) 10% (w/v) solution or glutaraldehyde (GLU) 0.75% (w/w), with subsequent papain sorption. Microparticles were characterized by Fourier transformed infrared spectroscopy (FTIR) for chemical modifications, scanning electron microscopy (SEM) for morphology and X-ray diffraction (XRD) for crystallographic analysis. Chemical composition and the thermal stability of the material were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). It was observed that the presence of TPP and papain resulted in a decrease of the stability of the chitosan matrix. Papain release rates from the microparticles were also conducted in vitro. The amount of released papain in phosphate buffer (pH = 7.4) was analyzed with UV-spectroscopy, showing release profiles of enzymatic activity ranging from 0.006 to 0.011 μmol.min^? 1. The results indicate that both chitosan–TPP–papain and chitosan–GLU–papain microparticles can successfully be used for systems that aim at a controlled release of papain with potential use in the biomedical area.     

Power Ultrasound Mass Transfer Enhancement in Food Drying

Power ultrasound application could constitute a way to enhance food drying in order to improve not only mass transfer but also product quality, since it does not significantly heat the material. The main aim of this work was to assess the influence of power ultrasound on the mass transfer process during drying of different products, carrot, persimmon and lemon peel.Convective drying kinetics were carried out with ultrasound (US experiments 21.8 kHz, 75 W), or without ultrasound application (AIR experiments) at air velocities ranging between 0.5–12 m s⁻¹. Different geometries were used for each of the products: cubes in carrots (2 L = 8.5 mm), cylinders in persimmon (2 L = 30 mm and 2 R = 13 mm) and slabs in lemon peel (L = 10 mm). Drying kinetics were modelled by considering different diffusion models according to the geometry.The results show that air velocity and raw material characteristics play a role in convective drying kinetics assisted by power ultrasound. Power ultrasound increased effective moisture diffusivity at low air velocities for all the products. However, in the case of lemon peel, ultrasound also improved the drying rate at high air velocities. This behaviour may be explained by the disruption of the acoustic field at high air flow rates and the different level of intensity required due to the structure of the products. Therefore, the raw material constitutes an important variable to establish the influence of power ultrasound on convective drying.     

Microtubes made of carbon nanotubes

Microtubes made of multi-walled carbon nanotubes were prepared via infiltration of CNT-suspension through a microfiltration hollow fiber membrane. Shrinking of the entangled CNT network during the drying allows withdrawal of CNT-microtubes from the hollow fiber. Currently, microtubes have a length of ∼50 cm, outer diameter of ∼1.7 mm and scalable inner diameter by varying the infiltration time resulting in wall thicknesses of 130–320 μm. The BET surface area is 200 m²/g with a porosity of 48–67% and an electrical conductivity ∼20 S/cm. We propose to use such novel CNT-microtubes for the fabrication of tubular electrochemical cells and membrane filtration processes.     

Mechanochemical characterisation of silica-based coatings on Nitinol substrates

Commercial Nitinol substrates have been coated with mesoporous and microporous silica-based materials. Different surface pre-treatments have been carried out in order to study the influence of the Nitinol external surface on the deposition of the coating materials. Hydrophilic mesoporous amorphous spherical silica microparticles (∼840 nm) and hydrophobic silicalite-1 (an aluminium-free zeolite) crystals (0.5–1.7 μm long) were the materials used to coat the Nitinol substrates. The influence of those coatings on the mechanical performance of the coated Nitinol substrates was analysed by means of flexion studies.     

Impedance and modulus spectroscopy characterization of lead free barium titanate ferroelectric ceramics

In this work, dense BaTiO₃ ceramics were successfully prepared by the solid state reaction process. X-ray diffraction technique showed single-phase polycrystalline sample with perovskite structure. Dielectric behavior and the impedance relaxation were investigated in a wide range of temperature (room temperature (RT) – 40 °C) and frequency (1 kHz–1 MHz). A broad dielectric constant peak was observed over a wide temperature range around the phase transition temperature. The complex impedance plot exhibited two impedance semicircles identified over the frequency range of 1 kHz–1 MHz, which is explained by the grain and grain boundary effects. The presence of non-Debye type of relaxation has been confirmed by the complex impedance analysis. The centers of the impedance semicircles lie below the real axis, which indicates that the impedance response is a Cole–Cole type relaxation.