(427d) Optimizing Crystal Growth In An Airlift Crystallizer

The crystal growth can be isolated in an airlift crystallizer as an alternative to conventional crystallizers especially for systems in which attrition has to be minimized. The process condition should be optimized in order to avoid primary nucleation and attrition.

Experiments were performed with ammonium sulphate – water solution in 2 l and in 18 l crystallizers and the hydrodynamic and the growth behaviors were study for different airflow rates, different sparger types and different seeds loads. The temperature gradients and thus the supersaturation gradients were also studied for different cooling rates. The hydrodynamic profiles of the clear saturated solution were measured in the 2 l crystallizer by liquid crystals particle tracking using a high speed digital camera. The time-resolved digital particle image velocimetry tool for MATLAB (PIVlab) was used to process the images and to calculate the velocities profiles. A hydrodynamic model which incorporates the properties of the liquid and gas, the geometry of the sparger and of the riser and down-comer was developed and validated based on the experimental measurements. The temperature in the crystallizer was experimentally measured by PIT using liquid crystals which change their colors in the range of temperature of 24 – 30 ⁰C. It was proved that the crystallizer is approaching ideal mixing with very small temperature grading for airflow rates higher 50 – 100 l/h even for high cooling or heating rates of 2 – 3 ⁰C per minute. The cooling crystallization experiments done in the 18 l crystallizer showed that ideal crystal growth is approached even for very low seed loads as 0.4% (just 7g of seeds for a product mass of 1750 g).

An industrial case in which struvite (magnesium ammonium phosphate hexahydrate) is precipitated in an airlift crystallizer was also studied in order to gain insights regarding the scale up of such a crystallizer. In this industrial crystallizer big struvite crystals were obtained (even 2 mm long).

This proves that crystal growth can be isolated and controlled in the airlift crystallizer and that this crystallizer can be applied in a task based design procedure.