The incorporation of impurities into sucrose crystals during the crystallisation process.

Abstract

The main objective of this work is to propose a mechanism for the transfer of impurities into the sucrose crystal. To this end the transfer of impurities into the sucrose crystal was investigated, under crystallisation conditions similar to those found industrially. Most of the impurities, namely, colour bodies, potassium, calcium and starch, were selected on the basis of their industrial importance, but some exotic species, namely lithium and nickel, were chosen to represent other mono- and di-valent ions respectively, and dyes, such as methylene blue, which have been used in work with single crystal sucrose crystallisation, were included to make the results more general. A parameter to measure the rate at which impurities are transferred into the sucrose crystal was proposed. Experiments, carried out in a pilot plant evaporative crystalliser, were performed to establish the effect of selected factors on both the concentrations of impurities found in the sucrose crystal, and on the rate at which these impurities are incorporated into the crystal. All the factors selected, namely the rate of crystallisation, the temperature, the concentration and type of impurity, the diffusivity of the impurity in concentrated sucrose solutions, and the crystal dimensions, are shown to influence the rate of impurity transfer. Only the concentration in the feed and type of impurity, however, affect the final concentration of the impurity in the crystal. Concepts involving partition coefficients and adsorption isotherms were also investigated. The experimental data did not fit the adsorption isotherm models well, but the values obtained for the partition coefficients were similar to those quoted in the literature when exchange types of reactions are operative. Activation energies have been measured, both for the rate of crystallisation of sucrose, and for the rates of impurity transfer. The values obtained, particularly for the rate of impurity transfer, indicate that a transport mechanism is effective. The experimental results have been used to investigate the relevance of two models, one involving a two-step approach and the other an interfacial process, for the incorporation of the impurity into the sucrose crystal. The results obtained indicate that the interfacial breakdown model describes the transfer of all the impurities studied here, except for starch.