Numerical simulation of the flow and mass transport during the growth of ADP crystals by additional stirrer

Abstract A stirrer is added to the traditional rotating crystal growth system of ammonium dihydrogen phosphate (NH4H2PO4, ADP), aiming at effectively utilizing the convection to improve the mass transfer and the distribution homogeneity of the supersaturation on the crystal face. 3D numerical simulation on flow and surface supersaturation distribution under different conditions is performed employing the finite-volume method, and the effect of flow on the surface supersaturation distribution is analyzed in detail. Compared with the traditional rotating-crystal system, the introduction of a stirrer can produce stronger axial flow and accordingly improve the magnitude and distribution homogeneity of the surface supersaturation, which is favorable to grow high quality crystals. As the stirrer rotation rate increases, the magnitude of the axial flow velocity increases, resulting in a reduction of the thickness of the solute boundary layer and an increase of the supersaturation on the crystal faces. Meanwhile, the homogeneity of the surface supersaturation is also improved. The crystal rotation rate strongly influences the magnitude and distribution of the prismatic surface supersaturation. As the crystal size increases from 25 cm to 35 cm, the distribution homogeneity of the prismatic surface supersaturation improves significantly, which is beneficial to crystal growth. Additionally, the importance of natural and forced convections in mass transport are discussed. Results show that the effects of natural convection adjacent to crystal faces can be ignored when stirrer rotation rate is greater than 400 rpm.