factors influencing Particle breakage characteristics in the presence of ultrasound
Acoustic cavitation induced through ultrasound (US), sound of frequency greater than 20 kHz, has several multi-disciplinary applications. While the focus has been on intensifying rates and altering chemical reaction pathways through ultrasound, relative less focus has been given to its physical effects. While ultrasound has been routinely used in industries for cleaning purposes, opportunities exist for extending its application to several applications involving surface area creation from particle breakage and transport through the interface. Ultrasound, though a feasible option, induces complex phenomena, which collectively contribute to faster transport rates. These involve microjets, shock waves, interparticle collisions and localized turbulence at the vicinity of the liquid-solid interface (microstreaming).
2. SCOPE AND OBJECTIVES OF THIS WORK
Studies involving the effect of power ultrasound on simultaneous size reduction and dissolution of particles are scarce in open literature. Effect of ultrasound on various aspects of mass transfer involving both dispersed particle systems and whole solid objects such as cylinder and disk has been reported by Kannan and Pathan (2004), Sandilya and Kannan (2010, 2011). The scope of this presentation is to understand the process of ultrasonic particle size-reduction in solid–liquid systems where the solid is sparingly soluble and dispersed as particles in the liquid. The effect of power ultrasound (20 kHz) on the particle breakage dynamics of sparingly soluble benzoic acid was studied by dispersing it as particles (variable surface area) in aqueous solvents viz. distilled water and aqueous glycerol. The objectives of this work are to
- Determine the particle size distribution of benzoic acid particles in different solvents at different power levels of ultrasound
- Quantify the particle size distributions in form of statistical distributions
- Understand the mechanism of particle breakage through close-up photographs of the benzoic acid particles subject to ultrasound
- Illustrate the particle motion and breakage in the ultrasonic field through high speed photography
3. EXPERIMENTAL METHODS
Experiments pertaining particle breakage were carried out in a jacketed cylindrical vessel. A cryostat (Ultra Cryostat Circulator, Scientific Instruments, Chennai) was used to maintain constant temperature of the process vessel contents. An ultrasonic probe (VCX-500, Sonics and Materials Inc., USA), rated at 500 W with a resonating frequency of 20 kHz and a tip diameter of 13 mm was used. Also, the particle size distributions are measured using a laser particle size analyzer (S3500, Microtrac Inc., USA), which has a measuring range from 0.025 to 1408 ƒÝm. An optical microscope (Axioskop MAT2, Carl Zeiss Inc., Germany) was used to acquire close-up images of the benzoic acid particles treated under ultrasonic irradiation.
Particle breakage runs
Ultrasound–assisted particle breakage experiments were carried out both in the presence and absence of solute transfer. In the runs with mass transfer the particle sizes were influenced by simultaneous breakage and dissolution. To delineate the effect of ultrasound on the particle size reduction through breakage alone, a separate set of experiments was carried out with solutions of distilled water or 24% (w/w) aqueous glycerol pre–saturated with benzoic acid so that only particle breakage is allowed to take place. These were termed as non–mass transfer runs. The particle size distributions at regular time intervals were obtained under various experimental conditions using the laser particle size analyzer.
4. RESULTS AND DISCUSSION
4.1.Ultrasound assisted particle dissolution and particle breakage characteristics
The cumulative particle size distribution data were analyzed for the lognormal distribution trends using the procedure outlined by Allen (2003). Data pertaining to every experimental condition was plotted on a log-probability graph and the modality of the distribution (i.e. unimodal or bimodal or trimodal) were identified. The lognormal distribution was fitted to the experimental data and the distribution parameters were obtained by using the nlinsq toolbox of MATLAB® (The Mathworks Inc., 2007). The multi-modal distribution was fitted using a linear combination of lognormal distributions (Meier et al., 2009). The parameters of the model viz. blend factor , standard deviation and mean were thus estimated for each experiment. They were then used to predict the overall distribution of the particles. The lognormal distributions were found to fit the experimental data satisfactorily and R2 value better than 0.97 in few cases and better than 0.98 in the remaining were obtained.
The particle size distributions and their statistical parameters were considerably influenced by the presence or absence of solute transfer. The mean particle size, the spread and number of peaks were affected by the presence of solute transfer. Even though the mass transfer effects only occurred until saturation of the solute in the solvent, it was sufficient to create a considerable difference in the particle size distributions and their associated parameters.
4.2. Effect of ultrasound on the solid morphological characteristics
A qualitative treatment of the effect of sonication on the solid particles has been carried out using an optical microscope. The pictures indicates several indentations on the particle surface confirming the pitting action created by microjects impinging on the solid surface. The size of the indentation on the particle surface varied between 59 µm and 167 µm. The nature of the indentations reveal that multiple impingements on the same particle by microjets may have occurred. The orientation of the particle at the time of impingement may also influence the diameter and shape of the pits. Some indentations show a spread along the major axis of the particle which may have been due to the merging of the adjacent pits. It was clear from the images that the ultrasound microjects were active along the frontal surface of the particles and were not that effective along the lateral edges. This may be attributed to the frontal surface having higher surface area than the lateral surface. In addition to pits, cracks were also visible along the particle surface.
In addition to particles having pits and cracks, the images revealed that a few particles had ridges and valleys on the frontal surface which may be attributed to the shear effects caused by shockwaves. The valleys created on the surface of the particles may reduce their strength and hence become sites for particle breakage. A microjet impingement or collision with another particle may lead to its fragmentation. The curved edges observed on the particle images may be attributed to microstreaming phenomena.
4.3 Effect of ultrasound on particle motion and breakage
Video footages were obtained using a high speed camera (X-PRI, AOS Technologies AG, Switzerland) at a frame rate of 500 frames per second. Usually the particles were found to be fragmented as they came under the ultrasound probe.
Microstreaming, microjets and inter–particle collisions hold a complex interplay in the size reduction process of the particles in both the mass transfer and non mass transfer solid–liquid systems investigated. Linear combination of lognormal distributions fitted the experimental particle size distribution data satisfactorily. These ultrasound induced effects also influenced the shape of the particles. Video footages showed that the particles were primarily broken when they came under the ultrasonic jet created by ultrasound probe tip.
Allen, T., “Powder Sampling and Particle Size Distribution”, Elsevier, Amsterdam (2003).
Kannan, A. and S. K. Pathan, “Enhancement of solid dissolution process”, Chemical Engineering Journal 102, 45-49 (2004).
Meier, M., E. John, D. Wieckhusen, W. Wirth and W. Peukert, “Generally applicable breakage functions derived from single particle comminution data”, Powder Technology 194, 33-41 (2009).
Krishna Sandilya, D., A. Kannan, “Effect of ultrasound on the solubility limit of a sparingly soluble solid”, Ultrasonics Sonochemistry, Vol 17, pp. 427-434, 2010.
Krishna Sandilya, D., A. Kannan, “Intensification of the dissolution of a sparingly soluble solid from a spinning disk in the presence of power ultrasound”, Industrial & Engineering Chemistry Research (accepted, 2011)