Couette-Taylor crystallizer: Effective control of crystal size distribution and recovery of l-lysine in cooling crystallization

Abstract

A Couette-Taylor crystallizer is developed to enhance the l-Lysine crystal size distribution and recovery in the case of continuous cooling crystallization. When using the proposed Couette-Taylor (CT) crystallizer, the size distribution and crystal product recovery were much narrower and higher, respectively, than those from a conventional stirred tank (ST) crystallizer. Here, the coefficient of the size distribution for the crystal product from the CT crystallizer was only 0.45, while it was 0.78 in the case of the conventional ST crystallizer at an agitation speed of 700rpm, mean residence time of 20min, and feed concentration of 900(g/L). Furthermore, when using the CT crystallizer, the crystal product recovery was remarkably enhanced up to 100%wt with a mean residence time of only 20min, while it required a mean residence time of at least 60min when using the conventional ST crystallizer. This result indicates that the CT crystallizer was much more effective than the conventional ST crystallizer in terms of controlling a narrower size distribution and achieving a 100%wt l-lysine crystal product recovery from continuous cooling crystallization. The advantage of the CT crystallizer over the conventional ST crystallizer was explained based on the higher energy dissipation of the Taylor vortex flow and larger surface area for heat transfer of the CT crystallizer. Here, the energy dissipation of the Taylor vortex flow in the CT crystallizer was 13.6 times higher than that of the random fluid motion in the conventional ST crystallizer, while the surface area per unit volume for heat transfer of the CT crystallizer was 8.0 times higher than that of the conventional ST crystallizer. As a result, the mixing condition and heat transfer of the CT crystallizer were much more effective than those of the conventional ST crystallizer for the cooling crystallization of l-lysine, thereby enhancing the l-lysine crystal size distribution and product recovery.