Experimental Investigation And Numerical Simulation Of Crystallization Intensified By Fast Jet Mixing

The crystal size and crystal size distribution of energetic materials are important factors which dominate their application performance.The preparation of micron or submicron CL-20 and HMX with uniform crystal size distribution has an important influence on effectively reducing the burning rate of high energy solid propellants,improving the loading density and combustion stability.Crystallization is a common unit operation to control crystal particles.The crystallization method has a critical influence on the crystal size and its distribution.The process is usually divided into batch operation and continuous operation.The batch process has the problems of low production efficiency and difficulty in crystal size control.Whereas the continuous process has advantages of high-efficiency and controllable crystal products with specific crystal sizes,and has been widely researched and applied.In this study,we focus on the investigation of continuous jet crystallization process experimentally and numerically.Firstly,the mixing intensification by adding swirling flow in the jet mixer was studied.The influence of the jet configuration,operation conditions and the amplification of the mixer on the mixing performance were investigated.It was found that the time-averaged concentration distribution obtained by the large eddy simulation and the laser induced fluorescence(LIF)experiment are in good agreement,which verifies the accuracy of the simulation results.Inducing a fully developed swirling turbulence flow into one of the fluids to be mixed or the both can significantly reduce the mixing time and intensify the mixing process.When the mixer configuration is fixed,increase in the flow rates of two flows proportionally will lead to a proportional decrease in the mixing time with a similar flow pattern of the mixing process.When the size of the mixer was enlarged and the flow rates are kept constant,the dimensionless mixing distance is almost unchanged,but the absolute mixing time increases linearly.Secondly,in order to validate the enhancement effect of jet mixer on the antisolvent crystallization and provide guidance for the jet crystallization method to control the particle size and distribution of energetic materials or other crystal products,the antisolvent crystallization process of sodium chloride was chosen as the model system for the study.The water was taken as the solvent and the ethanol was taken as the antisolvent.The control rules of the crystal size and its distribution of Na Cl crystals by continuous jet crystallization and batch crystallization in stirred vessel was investigated experimentally.Basied on the experimental results,the plug flow model coupled with population balance equation(PFM-PBE)and computational fluid dynamics coupled with population balance equation(CFD-PBE)were developed to simulate the crystallization process and establish the corresponding method of crystallization kinetics investigation.Compared with the batch crystallization in the stirred tank,the jet crystallization method has a significant advantage in preparing crystals with small size and narrow crystal size distribution.The antisolvent process can be further intensified by adding swirling flow into jet mixer,and the crystal products with smaller crystal size and narrower crystal size distribution can be prepared.For the Na Cl crystallization process,crystal products with a crystal size distribution of 2 ?m ? 5 ?m were successfully prepared.Through the PFM-PBE numerical simulation method and the experimental results,the nucleation rate and growth rate of the crystallization process were fitted.In the simulation,a definition based on the solubility change of the solute in the pure solvent is proposed to define the supersaturation.Compared with the traditional definitions,it is more accurate to calculate the supersaturation level in the antisolvent crystallization process.Through the CFD-PBE simulation method,the effect of the macro-mixing and micro-mixing on the crystallization process was systematically investigated,and the kinetics of the crystallization process were further verified by the CFD simulations and the experiments.Finally,a new rapid cooling crystallization method was proposed by coupling the jet crystallization with the batch crystallization in the stirred tank.The crystallization process of dihydroxyglyoxime(DHG)was studied by using the proposed method.Comparing with the traditional cooling crystallization method,the rapid cooling crystallization method has the advantage of preparing crystal product with small size and narrow crystal size distribution.When the stirring rate is higher than 60r/min,the crystals can be fully dispersed in the solution.When increasing the stirring rate,the mean crystal size becomes smaller and the crystal size distribution becomes narrower.The faster cooling rate and higher supersaturation level,the smaller crystal size and the narrower crystal size distribution.Adding ultrasound after fully mixing of the fluids can intensify the nucleation process and prepare smaller crystals.The initial mixing concentration is also an important factor to determine the crystal size and its size distribution.Higher initinal concentration will produce more crystal nucleus when a fixed amount of the material was consumed,leading to smaller mean crystal size with narrower crystal size distribution.The mathematical model of the crystallization process was established.The crystallization kinetic parameters were obtained through nonlinear fitting of with the experimental data.The predicted mean crystal size and crystal size distribution using the fitted crystallization kinetic parameters are in good agreement with the experimental results.