Crystal growth phenomena of paraxylene crystals

Abstract

A study on the morphology of paraxylene crystals growing from the melt and from different solvents revealed that two distinct morphologies develop under all circumstances: (i) crystals with {001} as the predominant faces and (ii) crystals with {010} as the predominant faces. The occurence of type (i) crystals can be explained on the basis of the crystal structure, but the existence of type (ii) can only be ascribed to the actual supersaturation allowing for the exposure of the {010} face or to the action of an unidentified impurity allowing for the growth of the {010} faces. Experiments on crystal growth kinetics from the melt showed that paraxylene crystals show stable growth up to an undercooling of 2°C. The order of the surface integration step suggests that growth proceeds by the propagation of cooperating growth spirals. At higher supersaturations, the formation of dendrites and needles occurs. Growth rate measurements from solutions containing 70% paraxylene revealed that (i) the growth rate is length-independent in the measured size range, and (ii) growth and dissolution kinetics do not depend on the nature of the solvent within the accuracy of our experiments. Measurements from 30% solutions showed much lower growth rates. Comparison between the kinetic parameters assessed under the different conditions suggests that under the conditions prevailing in the process industry (-45°C), rather low growth rates are to be expected because volume diffusion becomes more rate-determining at lower concentrations.