Continuous Mass Crystallization of Vitamin C in l(+)-Ascorbic Acid−Ethanol−Water System: Size-Independent Growth Kinetic Model Approach

Abstract

Experimental results concerning continuous isohydrical drowning-out mass crystallization of vitamin C in an l(+)-ascorbic acid−ethanol−water system are presented. The process environment was created in a laboratory-scale draft tube, mixed suspension mixed product removal (DT MSMPR) crystallizer with internal circulation of suspension. Assuming a constant feed concentration of ethanol (20 mass %), the feed concentration of vitamin C was changed within the 30−50 mass % range. The mean residence time of suspension in working volume of a crystallizer was varied from 900 to 3600 s. Combinations of the presented above input process parameters resulted in a productivity 244−1692 kg/(m3 h) of crystal product, of mean size within 0.20−0.24 mm and of CV within the 50−60% range. The supersaturation level in mother solution reached relatively high values (up to ca. 6.5 mass %), in particular at a short mean residence time of suspension (900 s). With this time elongation to 3600 s, the supersaturation level decreased, however, by ca. 40% (to 3.9 mass %). The simplest size-independent growth (SIG) kinetic model was adopted for nucleation ((3.3−27.8) × 107 1/(s m3)) and crystal growth ((1.6−6.9) × 10−8 m/s) rates estimation. Kinetic relations and feedback between nucleation and crystals growth were identified and analyzed in detail.