Inline determination of supersaturation and metastable zone width of MgSO₄.12H₂O with conductivity and refractive index measurement techniques

Inline measurement of the supersaturation is important to support process control and for the evaluation of crystallization experiments. Especially, when different hydrates or polymorphs of the crystallized component occur, very precise measurement is required. For example MgSO₄.7H ₂O-MgSO₄.12H₂O lines are quite close, and when operating under eutectic freeze crystallization (EFC) or cooling crystallization both salts might be produced. Therefore conductivity and refractive index inline measurement methods are investigated and compared for MgSO₄ solution having concentration range between 16 and 22 wt% and temperature range between 10 and -5 °C. Conductivity measurements are correlated to concentrations and temperature with the empirical Casteel-Amis equation and refractivity index measurements are correlated to concentrations and temperature with a developed empirical model. The metastable lines for ice and salt of MgSO₄ system is drawn based on the measurements at the onset of crystallization upon cooling of several solution concentrations. In the working range of EFC, the relative supersaturations are calculated to be σicemax=0.2 and σsaltmax=0.23. The accuracy for conductivity measurement in relation to the metastable zone width is roughly calculated to be around 20% and for refractive index measurements, 3%.