Research On Batch And Continuous Crystallization Processes Of Sodium Phosphate

Crystal size distribution has great impact on product quality.In order to dealing with the practical problems of unstable product quality and small particle size distribution in the prosess of sodium phosphate crystallization,in this paper,mathematical analysis and experimental investigation was used to optimize the batch and continuous crystallization of sodium phosphate.The paper is intended as a reference for the industrialization of the sodium phosphate crystallization process.Various basic data required for the design of the sodium phosphate crystallization process were measured,including solubility of sodium phosphate solution（Quimociac analysis）,boiling point elevation of sodium phosphate solution at different temperatures,viscosity of sodium phosphate solution（Rotor viscometer）,specific heat capacity of sodium phosphate solution（Calorimeter）.To overcome the problem that crystal size is difficult to control during the batch crystallization of sodium phosphate,a methodology that combined experiments and models was proposed to optimize the crystallization process.The methodology was based on the modified Mullin’s model.The key parameter g_max in the model function was determined experimentally.The optimized cooling curve and critical seed loading chart was calculated simultaneously by utilizing g_max.The better product quality could be achieved according to the optimized operating strategy.Achieving steady-state is the basic requirement for continuous crystallization.In this paper,the steady-state condition was analyzed in terms of mathematics and mechanism.A general steady-state condition was proposed by extending the prior mathematics model.By analyzing the mechanism of unstable behavior,the influence of concentration on the stability of crystallization process was deduced theoretically,the steady operation concentration range of continuous crystallization process was determined,and the stability of sodium phosphate continuous crystallization was examined experimentally.Two-dimensional crystallization kinetics of sodium phosphate with breakage was determined by continuous crystallization method.By utilizing the characteristic of the steady-state particles distribution of continuous crystallization,an appropriate growth rate model was selected.Quadrature method of moment was used to solve the population balance model.Quadrature abscissas and weights were calculated using the particle swarm algorithm.By analyzing the effect of breakage on the particles distribution,the fitting step was divided into several steps,and the crystal growth kinetics and nucleation kinetics equations are determined respectively.The characteristic of sodium phosphate continuous crystallization was analysed mathematically.A simplified design strategy for continuous sodium crystallization was proposed.The effects of fines-dissolving operation and crystallizer configuration on the products quality of continuous crystallization were experimentally determined.Industrialization of continuous crystallization of sodium phosphate was achieved.