Title:

Ultrasonic effects of crystallization processes

This thesis reports a study of the effects of ultrasound on crystal nuclcation from solution and particle breakage. Firstly, the effect of ultrasound on reactive crystallization when nuclcation is consistent with a predominantly homogeneous model is studied by measuring the induction time. Barium sulphate is used as the working substance precipitated by mixing aqueous BaCl2 and Na:S04 solutions. The relationship between amplitude of ultrasonic processor and power input was determined. The experiments were carried with various ultrasonic power inputs. It is observed that increasing ultrasound has a significant effect on reducing the induction time. At a given supersaturation level, the induction time decreases with increasing ultrasonic energy. The relationship between power input (E) and the activation energy of nucleation (E ) is discussed. And the equation between total power input (E) and the nucleation coefficient (A. ) is obtained. A cluster coagulation model, which brings together the current nucleation models and the theories describing the behavior of colloidal suspensions, was applied to estimate the induction time under various power inputs. A comparison between the predictions of the model and the results of experiments shows that the number of monomers in dominating clusters ( ) in the solution remains constant with increasing power input. The mechanism of the ultrasonic effect on homogeneous nucleation is analyzed. It is found that when temperature is kept constant, the main effect of ultrasound is to increase the diffusion coefficient (D,m) Other parameters change only slightly or remain constant within the ultrasonic field. This suggests that diffusion acceleration is the main reason for the reduction of the induction time. The relationship of DAn and power input is also discussed. A mathematical model of power input and diffusion coefficient is presented, from which it is predicted that diffusivity enhancement (DAB DAbo) has an exponential relationship with power input. The above relationship is consistent with the results of the experiments. Induction times have also been measured with and without ultrasound to investigate its effect on induction time of spectinomycin hydrochloride, roxithromycin and sucrose during antisolvent crystallization. The relationship between ultrasonic power input and diffusion coefficient is also investigated. It is found that at the same power input, the ultrasonic effect on DAH decreases with increased solution viscosity. The effect of ultrasound on BaSO.* nucleation is also studied by measuring the induction time when nucleation is consistent with a predominantly heterogeneous model. In these experiments, barium sulphate is used as the working substance precipitated by mixing aqueous BaCb and Na2SC > 4 solutions with relatively low concentrations compared to the experiments of homogeneous nucleation. It is observed that at a given supersaturation level, the induction time significantly decreases with increasing ultrasonic energy during heterogeneous nucleation. The mechanism of the ultrasonic effect on heterogeneous nucleation is analyzed. The effect of ultrasound is to increase the diffusion coefficient (DAb) while reducing the contact angle {0) and geometric factor if). This gives rise to an increase in the heterogeneous nucleation rate and shorter induction time. The ultrasonic effect on critical supersaturation ratio (Sfnf) is also investigated to determine the mechanism of ultrasonic effect on / and 0. It is proposed that both the force from the cavitational bubble implosion and the nucleation process occurring at bubble/solution/foreign solid interface are the predominant reasons why contact angle ( 0) decreases with the increase of power input. In addition, the effects of ultrasound on nucleation order (n) and the nucleation coefficient (ku) are also investigated, and compared with the corresponding effects during homogeneous nucleation. A cluster coagulation model is used to calculate the induction time at different levels of supersaturation when nucleation is consistent with a predominantly heterogeneous model. It is found that the dominant cluster size increases with increasing supersaturation and increasing ultrasonic energy. The effect of ultrasound on nucleation of BaS04 is studied by a Kodak Ektapro Hs motion analyzer. Firstly, the dispersion processes of ink with ultrasound and stirring are used to compare difference of the mixing processes and hydraulic flows. The vessel is divided into several sections to investigate the local mixing process by recording the movement of small silicon carbide crystals. Velocity variance is used to indicate the difference of turbulence when ultrasound is applied and when stirring is applied respectively. The cavitation distribution in different sections is also obtained. BaSC>4 is precipitated by mixing aqueous solutions of BaCb and Na2S04. The nucleation processes are observed by Kodak motion analyzer in different sections. The mechanism of nucleation by ultrasound is analyzed. Then, the induction times at different temperatures are measured with different power inputs at a given supersaturation level. The relationship between the cavitation distribution and temperature is established.
