Evaluation of the performance of a newly developed eutectic freeze crystallizer: Scraped cooled wall crystallizer

Eutectic freeze crystallization (EFC) separates aqueous inorganic solutions into pure water and pure salt. By operating at the eutectic point, ice and salt can be formed simultaneously as two separate phases. The scraped cooled wall crystallizer (SCWC), specially developed for eutectic operation, is introduced. It features cooling through an outer wall combined with an internal cooling cylinder. Experiments were executed in a 1151 model SCWC using a ternary aqueous system of KNO₃-HNO₃. The SCWC is equipped with temperature sensors measuring bulk temperature and the temperatures of the coolant entering and exiting the cooling walls. The total heat flux from coolant to bulk equals 490-585 W m^-2 K^-1 at a temperature difference between cooling wall and bulk of 5-6K. At this heat flux the production capacity equals 3.8 × 10^-4 kg m^-2 K^-1 s^-1 for ice and 3.1 × 10^-5 kg m^-2 K^-1 s^-1 for salt. The KNO₃ crystals produced are reasonably well faceted with average size of 80-230 μm. Both ice and salt crystals are easily filtered. Impurities in the ice crystals drop below 50 ppm K⁺-ions after three washings. Ice solid content measurements indicated that 0.01 wt% of the ice slurry consisted of entrained salt crystals. Washed salt samples contained less than 5 ppm of ionic impurities while impurity levels in the feed ranged from 50 to 500 ppm.