The kinetics of sucrose crystallization

This study has been undertaken for two main purposes:- (1)To obtain factual information concerning the rate of growth of sucrose crystals; and (2)To secure some understanding of the operating mechanism of factors influencing crystal growth. Chapter 3 records details of the experimental method used and data gathered for actual growth rates. Associated with these in chapter 2 is set down information believed to have a significant bearing on the mechanism study. For the mechanism study growth has been looked upon as a physical rate process bearing certain analogies to chemical reactions and mathematical treatment has been carried out with this in mind, it being effectively a kinetic study in an essentially physical system. Chemical kinetics has to do with the study of rates of chemical reactions and of the influence of conditions upon these rates. The most important experimental conditions are concentrations (or pressures) of reacting substances, temperature, presence of a catalyst and radiation. In the case of sucrose crystallization the chemical reaction theory is transferred to the physical rate process. The kinetics of sucrose crystallization thus concerns the rate of crystallization of sucrose and the influence of conditions such as concentrations, temperature and impurities. Crystallization is a process in which the solute molecules move from the solution to the surface of the growing crystal and incorporate themselves into the crystal lattice. The relative influence of the crystal surface and mother liquor may then be examined in the light of rate process kinetics. The Arrhenius equation, which is the basis of kinetic studies, for chemical reactions may be stated, k = A e-E/RT where k is the velocity constant in the chemical reaction. In the current studies k is used in this equation as the velocity of crystallization of sucrose and is essentially the physical rate constant of concern in this system. A plot of log k v. 1/T is linear and the activation energy of crystallization (E) may be calculated from the slope of the line. The constant A in the Arrhenius relationship is essentially a frequency factor in chemically reacting systems and its study has been suitably incorporated in the activated complex theory of reaction rates. While some possible analogies are recognised, the full implications are not insisted upon in the present studies. The crystallization of sucrose consists of two consecutive stages, firstly, nuclei formation; and secondly, crystal growth. These current studies are essentially concerned with the latter growth process only. When studying chemically reacting systems, the chemical potential or driving force is the difference between the concentration of reacting material on the one hand and the concentration of the product on the other. In the system of crystallizing sucrose, the driving force is the difference between the concentration of the oversaturated solution and that of a saturated solution. In this thesis supersaturation (S) is defined as the ratio of the concentration of solution used in experiment expressed in g sucrose / g water to the concentration of a saturated solution at the same temperature and in similar units. In these current studies some experiments concerning rates of solution have also been carried out, and the rate of solution has been regarded as a negative rate of crystallization. Viscosity has a vital bearing on the study of the aqueous sucrose crystallizing system, and although the viscosity characteristics of this system are well documented for non-saturated conditions, there does not appear to be very much material concerning saturated and oversaturated concentrations. * Information in this region and which is relevant to the present studies has been sought experimentally, and is recorded and discussed in chapter 2. A study has also been made of the viscosity characteristics of the sucrose - ethanol - water system with the object of observing changes in viscosity activation energies and discussing their bearing on crystallization mechanisms. These are likewise recorded in chapter 2. JeLast ICUMSA reports (Copenhagen) contain viscosity data up to 86% solutions and 80 degrees.