Kinetics of melt crystallization of organic eutectic forming binary mixtures in non-flow systems

Abstract

The kinetics of melt crystallization of binary eutectic forming organic mixtures in non-flow systems with cooling from below for the preferential crystallization of one component alone have been studied experimentally. The effect of subcooling, superheating and the initial concentration of the melt on the crystallization kinetics have been studied. Based on the experimental data, a correlation for the instantaneous height of the crystal (volume fraction of the solidified mass) has been proposed. The experimental results were also analyzed based on a model from the literature, which considered simultaneous heat and mass transport. The analysis shows that in the present experiments, the melt crystal interface was very close to the initial melt composition, i.e. concentration equilibrium was maintained. This is attributed to the very low cooling rates used, even though the thermal diffusivity is much greater than the mass diffusivity of the melt.

Introduction

Melt crystallization is a process for the separation of a binary mixture of organic compounds. It is the process where a molten mixture is cooled to a little below its freezing point when some of the material solidifies. In binary mixtures, that form a eutectic, this will be a pure component. The remaining melt, called the residue, will contain some of the unsolidified pure (desired) component. The purified product is recovered by separating the solid from the residue and remelting it. It is an attractive method for separation of binary organic mixtures where the boiling points of the two components are very close to each other and distillation is not so easy. This is distinct from the solid solution systems where the phase diagram is similar to the vapor liquid equilibrium diagram of ideal systems. Unlike the eutectic forming systems, the crystal phase growing in equilibrium with the liquid of a given composition is not a pure substance and hence multistage operations are required to attain high purities.

Wynn [1] has classified melt crystallization processes into two categories — suspension processes and progressive freezing. In suspension processes the crystals are suspended in the melt and the density difference between crystals and melt causes a relative counter-current motion between the two. Progressive freezing, a technique unique to melt crystallization, involves the growth of the crystal layer on a cold surface immersed in the melt. The rather limited literature on melt crystallization has been reviewed by Ulrich [2] and Rittner and Steiner [3].

The kinetics of melt crystallization (rate of crystal growth) is a basic characteristic of the process. The present study focuses on the separation of binary organic eutectic mixtures by melt crystallization in non-flow or stagnant systems. The experimental data obtained in the present study is used to examine the effect of subcooling, superheating and the initial concentration of the melt on the crystal growth rates. The experimental data was also used to develop empirical correlations for estimating the melt crystallization kinetics. The experimental results are also examined using a model from the literature based on heat and mass transfer theory.

Fig. 1 shows the phase change diagram of binary organic eutectic forming mixtures. If the initial composition of the melt is below the eutectic point, as it cools, it will reach the liquidus line when pure naphthalene (in the benzene–naphthalene system shown in Fig. 1a) will crystallize out. As further cooling proceeds the composition will move along the liquidus line, while pure naphthalene continues to crystallize out, until the eutectic point is reached. At this point no more separation is possible and the solid forming will be a mixture of the binary at the eutectic composition. Similar behavior is observed with other two systems.