Control of Crystal Size Distribution using Non-Isothermal Taylor Vortex Flow

Abstract

Using a Couette-Taylor (CT) crystallizer, a non-isothermal technique was developed for effective control of the crystal size distribution (CSD) of the suspension. The proposed technique is based on the internal heating–cooling cycle in a non-isothermal CT crystallizer, consisting of a hot cylinder (Th) and cold cylinder (Tc). Thus, an internal loop of fines destruction of the suspension in the heating boundary layer of the hot cylinder and recrystallization in the cooling boundary layer of the cold cylinder is formed by the periodic circulating flow of the Taylor vortex in the non-isothermal CT crystallizer. The efficiency of the heating–cooling cycle for improving the CSD depends on the non-isothermal mode and non-isothermal parameters. When the inner cylinder temperature is hot and the outer cylinder temperature is cold (Mode-I), this is more efficient for improving the mean crystal size and dispersity of the CSD than when the cylinder temperatures are reversed (Mode-II). In addition, the efficiency of the heating–cooling cycle is optimized using the temperature difference between hot and cold cylinders (ΔT = Th – Tc) and saturated bulk temperature. The Taylor vortex fluid motion is always found to enhance the internal cycle efficiency. Thus, the initially small crystal size and broad CSD of the seed suspension (230 μm of mean crystal size and 81% of coefficient of variation) are improved to a large crystal size and narrow CSD of the product suspension (1020 μm of mean crystal size and 31% of coefficient of variation) at a non-isothermality of 8.7 ± 0.1 °C, saturated bulk temperature of 24.0 °C, and rotation speed of 800 rpm. The variation of the cycle efficiency is explained in terms of the driving forces for heating dissolution and cooling recrystallization.