Study On The Crystallization Process Of Paracetamol

Crystallization is one of the most typical multiphase flow system,which involves mass and heat transfer processes between multiple phases.Generally,traditional crystallization research relies on experiment and experience.However,with the development of computer science,the application of computer simulation can present the parameters that cannot be monitored during the experiment,which is helpful to simulate and predict the crystallization process,understand the crystallization mechanism,and complete the process optimization and control.In this paper,paracetamol is used as a model substance,combined with the particle number balance equation and computational fluid dynamics,to simulate and optimize the crystallization process of its elution,combined with the discrete element method to study the behavior of crystal particles.First,the intermittent dynamic method is used to experimentally determine the crystallization kinetics of paracetamol.Based on the analysis of the nucleation and growth characteristics of paracetamol,a nucleation rate equation and a size-independent growth rate equation were adopted to fit the experimental data by the method of moment.The results showed that crystal growth is controlled by surface reaction,and lower supersaturation and suspension density,lower stirring intensity and higher temperature within a certain range under the premise of full mixing of the system are more conducive to crystal growth.Then,on the basis of the above research,a CFD-PBE coupling model of paracetamol dissolution and crystallization was established,and the particle number balance equation was solved using the high-resolution finite volume method,in which a porous impinging jet crystallizer with distributed feed was proposed.The mixing effect,crystal nucleation,growth rate and final crystal size distribution under different operating conditions were simulated and compared.It was found that the ratio of jet and cross flow velocities plays a key role in turbulent mixing.When the velocity ratio r=4,the mixing and average crystal size can be significantly improved,but too high velocity ratio may cause back mixing.Multiple feed points can improve the mixing performance of the crystallizer to a certain extent,which helps to keep the nucleation rate at a low level and promote crystal growth to produce crystals with larger particle size and uniform distribution.Finally,a CFD-DEM coupling model based on the Eulerian-Lagrangian framework was established to simulate the particle behavior of paracetamol crystals in a 5L stirred tank.The distribution of particles and fluid in the stirred tank was obtained,and the influence of the rotating speed of the stirring paddle on the particle distribution,the average slip velocity of the particles and the number of collisions of a single particle were also studied.The results showed that the six-blade disc stirring blade can lead to a large radial driving force and effectively promote the mixing in the tank,but the circulation flow mixing center is easy to form a flow "dead zone".The addition of a baffle can form a disturbance turbulence and reduce the flow to shrink the "Dead zone",but it will further reduce the particle circulation and cause particle deposition behind the baffle and the bottom of the tank.With the increase of the stirring speed,the particle dispersion becomes uniform,but when the rotation speed is higher than 300 rpm,the influence of the rising rotation speed on the dispersion of the particles is weakened.The average particle slip velocity and the number of collisions increase with the rise of the rotation speed,and the increase speed of the collision number is higher than the slip velocity.The study showed that,too high or too low rotation speed is not conducive to crystal growth and reducing the secondary influence caused by breakage.