The role of transport processes in crystallization kinetics of ammonium pentaborate

The growth rate of ammonium pentaborate octahydrate has been measured as a function of supersaturation for various particle sizes, at different temperature range of 20-50°C in a laboratory-scale fluidized bed crystallizer. The values of mass transfer coefficient K, reaction rate constant, k_r, and reaction order, r, were determined from the general mass transfer equation without assumption using an improved computer program which gives a high accuracy. The relative standard deviation between the overall growth rate data obtained from the general mass transfer equation by using kinetic parameters (k_r, K and r) found in an improved program and those experimentally obtained do not exceed 0.8%. It was found that increasing the temperature and particle size cause an increase in the values of the kinetic parameters (K_g, k_r, K). Overall growth rate of ammonium pentaborate octahydrate is controlled by both diffusion and reaction steps at temperatures lower than 40°C, whereas it is completely controlled by diffusion at temperatures higher than 40°C.