Investigations in the physical mechanism of sonocrystallization

Abstract

This paper addresses the issue of mechanistic aspects of sonocrystallization with approach of coupling experiments with simulations of bubble dynamics. The major experimental result of our study is that, as compared to a mechanically agitated crystallization system, the dominant crystal size (or median) of the crystal size distribution (CSD) of sonocrystallization systems is smaller, but span of CSD is larger. The CSD is influenced by nucleation rate and growth rate. The nature of convection in the medium is found to be the crucial factor. In a mechanically agitated system, uniform velocity field prevails in crystallization volume, due to which both dominant crystal size and span of CSD reduce. The convection in a sonicated system is of a different kind. This convection has two components, viz. microturbulence (or micro-convection), which is continuous oscillatory motion of liquid induced by radial motion of cavitation bubble, and shock waves, which are discrete, high pressure amplitude waves emitted by the bubble. These components have different impact on crystallization process due to their nature. Shock waves increase the nucleation rate and microtubulence governs growth of the nuclei. However, the effect of shock waves is more marked than microturbulence (or micro-convection). Nucleation rate shows an order of magnitude rise with sonication, while growth rate (and hence the dominant crystal size) reduces with sonication as compared to the mechanically agitated system.

Ultrasound Assisted Particle Size Control by Continuous Seed Generation and Batch Growth

“…The three most commonly reported hypotheses are the bubble as the nucleation center, the segregation theory, and the flow induced nucleation. The latter assumes that ultrasound generates flow streams in the solution which enhance the collisions of molecules and therefore increases the creation of clusters of molecules [40,41]. As a result, more nuclei are formed and the nucleation rate is increased.…”

Ultrasound for Improved Crystallisation in Food Processing

“…This group has also pointed towards the possible role of high-pressure pulses due to cavitation in the reduction of crystallisation temperature. Recently, Nalajala and Moholkar [54] revealed that shock waves originating from ultrasound increase the nucleation rate and that microturbulence governs the growth of the nuclei. However, the effect of shock waves has been reported to be more pronounced than microturbulence (or microconvection).…”

The Effect of Ultrasound on the Crystallisation of Paracetamol in the Presence of Structurally Similar Impurities

“…Ultrasound has been widely reported to influence the primary nucleation process accelerating nucleation kinetics, this is typically expressed in terms of reducing the induction time and MSZW [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Ultrasound can also increase the rate of secondary nucleation, this is manifested as a reduction on the product crystal size distribution [20,22,24,31,32,[34][35][36][37][38][39][40][41][42][43]. Ultrasound can also influence crystal growth [20,25,26,36,40,[44][45][46] although the effect on crystal growth is not as dramatic as on nucleation and arises largely from enhanced mass transfer [46] and can influence crystal morphology.…”