Taylor vortex effect on flocculation of hairy crystals of calcium lactate in anti-solvent crystallization

Abstract

A Taylor vortex flow was applied to inhibit the crystal flocculation of calcium lactate in anti-solvent crystallization. When using a conventional MSMPR crystallizer, hairy crystals of calcium lactate were formed and flocculated in the crystallizer. The whole suspension in the crystallizer then gelated and the solution trapped in the flocculated crystals was hardly removable from the gelated suspension. Thus, no purification of calcium lactate was achievable when using anti-solvent crystallization in the MSMPR crystallizer, regardless of a batch or continuous operating mode. In contrast, when using a Couette–Taylor (CT) crystallizer, short needle crystals (about 40μm) were produced and their flocculation/entanglement was completely prevented. Due to the effective mixing of the Taylor vortex, a high supersaturation was induced in the inlet region of the CT crystallizer, thereby nucleating a high number of needle crystals. This then restricted any one-dimensional overgrowth of crystals, preventing the formation of hairy crystals. According to this mechanism, the mean crystal size was reduced when increasing the rotation speed of the CT crystallizer, the feed concentration, and flow rate. Moreover, the recovery ratio of calcium lactate crystals in the CT crystallizer was always greater than 83% and depended most significantly on the feed flow rate.