Effect of antiscalants during eutectic freeze crystallization of a reverse osmosis retentate

Master Thesis


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University of Cape Town

Eutectic Freeze Crystallization (EFC) is a separation technique which involves simultaneous crystallization of water and solute under eutectic conditions. It can be applied to treatment of various industrial aqueous streams containing dissolved organic and inorganic contaminants, such as reverse osmosis (RO) retentate brine streams. Since antiscalants are dosed in RO feed streams, these become concentrated in the retentate brine stream and could have an undesirable effect on crystallization kinetics of both ice and salt in EFC. In this study, the impact of a phosphonate antiscalant on the kinetic processes of nucleation and growth in EFC was investigated. Firstly, the effect of an antiscalant on the thermodynamic phase equilibria of a binary Na2SO4 aqueous solution was experimentally determined. The effect of the antiscalant on the nucleation and growth rates of both ice and salt in a continuous EFC process was then established for concentrations of 200, 350 and 500 mg/L of antiscalant. Product quality parameters such as the Crystal Size Distribution (CSD), morphology and purity of crystals were also measured since they are directly affected by the kinetic rate processes investigated.

Includes bibliography.