The efficacy of eutectic freeze crystallization (EFC) in crystallizers and in subsequent downstream washing or filtration steps is a direct function of size and morphology of the crystalline products. These product characteristics are influenced by supersaturation and residence time. To investigate the effect of residence time and supercooling on ice product characteristics in a continuous EFC process, crystallization experiments were performed in an EFC crystallizer with Na 2 SO 4 aqueous solution. Crystal size distribution and morphology during the crystallization process were analyzed by digital image processing. Longer residence times enhanced the mean crystal size and disk-shaped ice crystals were formed. Increasing supercooling resulted in a larger mean ice equivalent diameter.

Interactions between heat and mass diffusion determine growth mechanisms during ice crystallization. The effects of heat and mass transfer on ice growth in pure water and magnesium sulfate solution were investigated by studying the evolution of the gradient of the refractive index using color Schlieren deflectometry. For pure water, the gradient of the refractive index of water was used to calculate the temperature and therefore the local supersaturation. Its effect on the ice crystal growth rate and morphology was studied. It was found that, for local supersaturations greater than 2.8, the morphology was dendritic ice, with a growth rate 2 orders of magnitude higher than that for layered growth. During dendritic growth, 3–16% of the heat of crystallization diffused to the liquid side, which is counter to current understanding. At the transition (between the time of partial melting of the dendritic ice and the beginning of the layered ice growth), a higher supersaturation than that responsible for layered growth was observed. For ice growth from an aqueous salt solution, a mass and thermal diffusion boundary layer in front of the growing ice was created by diffusion of the solutes from the ice and by the release of heat of crystallization.

The presence of impurities in solution during crystallization can influence the crystallization kinetics of the desired product. Consequently, this affects the product quality parameters such as the crystal size distribution (CSD), purity, and morphology. This paper focuses on the application of eutectic freeze crystallization (EFC) to the purification of petrochemical brine rich in sulfate salts. The effect of phenol on the CSD, nucleation and growth rates, purity, and morphology of ice and sodium sulfate decahydrate crystals was evaluated using a batch EFC setup. Nucleation and growth rates were obtained from in situ image acquisition and analyses. In the presence of 0.2 wt % phenol, larger crystals were obtained for both ice and sodium sulfate decahydrate due to increased growth rates and reduced nucleation rates. The crystal habit of both ice and salt was not modified by the phenol impurity. Ice crystals presented elongated disk shapes and dendritic habit, and crystal sizes up to 1000 μm were obtained. Salt particles exhibited prismatic shape and monoclinic class as expected for Na 2 SO 4 •10H 2 O. Ice purity of about 97% was obtained before washing in the presence of phenol. Three washing cycles increased the ice purity to 100% implying that impurities were physically adhered on the ice surfaces and not incorporated into the crystal lattice, which deserves additional studies to verify this proposal. X-ray diffraction (XRD) analyses of salt crystals confirmed that Na 2 SO 4 •10H 2 O with the structure of thenardite was crystallized. Any other phase was detected.

Reverse osmosis (RO) plants are widely used for recovering potable water from industrial wastewater streams. The RO retentate stream is highly concentrated in salts, thereby limiting further treatment using RO due to increased scaling potential of the salts. Currently, the brine streams are disposed of in evaporation ponds in which pure salts cannot be recovered. This makes eutectic freeze crystallization (EFC) an attractive separation method for purification of such streams because it can recover both pure water and salt. However, antiscalants are present in RO and other industrial wastewater streams as impurities. These could have an effect on the thermodynamics and crystallization kinetics during EFC. This paper focuses on the effect of a phosphonate-based antiscalant on the crystallization kinetics of both ice and Na 2 SO 4 •10H 2 O in a continuous EFC process under subeutectic conditions. It was observed that the salt nucleation rate remained almost constant, while a general decrease in the ice nucleation rate occurred with an increase in antiscalant concentration. A general increase in the growth rates of both ice and salt was observed. A threshold antiscalant concentration in the range 350−500 mg/L was observed, beyond which the effect was reversed. The morphology of salt crystals from both the control and antiscalant-dosed solutions was monoclinic and prismatic. The morphology of ice could not be detected due to agglomeration. Product washing was very effective in removing impurities, suggesting that adsorption occurred by physisorption.

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