Study On Freezing Performance Of Experimental Spray Freeze Drying Equipment

Spray freeze-drying(SFD)method refers to the technology of atomizing raw materials with an atomizer,and the raw marterials are quickly freezing into micro-particles with cold media,a nd then the producing frozen particles are drying by sublimation in a low-pressure environment.The technology has the advantages of freeze-drying and spray drying,and is widely used in the preparation of nano materials,powdered foods and medicines.Spray freezing process is one of the key factors that affect the quality of spray freeze-drying.Preliminary studies have shown that freezing rate has a great influence on the particle size of freeze-dried nano powder.The particle size of freeze-dried powders can be controlled by controlling freezing rate,but the existing freeze drying equipment is difficult to control the freezing rate accurately.Therefore,the freezing performance of the experimental liquid nitrogen spray freeze drying equipment is studied in this paper,and the main influencing factors and the response rules affecting the spray freezing rate are discussed,providing a theoretical basis for accurately controlling spray freezing rate.In order to investigate the influence of process parameters on freezing rate in spray freezing process,this paper studied the spray freezing process on the homemade spray freeze-drying equipment with water as the research object and orthogonal experiment as research method.Three factors and three levels of spray freezing experiments were carried out with nitrogen flow rate,material flow rate and nitrogen temperature as the orthogonal factors.The final freezing temperature and freezing rate of the materials under different experimental conditions were obtained.Through the variance and range analysis of the experimental results,the influence law and order of each factor on the final freezing temperature are obtained.Using homemade spray freeze dryer as prototype,the original single-layer cavity structure was transformed into a double-layer cavity structure to further optimize the heat transfer during spray freezing process Computational fluid dynamics(CFD)method was used to optimize the key structural parameters of the improved freeze-drying chamber.The optimized structure is as follows:the outer chamber size is(?)260 X 420 mm,the inner chamber size is(?)180× 380 mm,the distance between the inner and outer chambers is 20 mm,the number of through holes in the inner chamber wall is 8 rows and 8 columns,the diameter is 20 mm,and two 10 mm nitrogen inlets are symmetrically distributed on the top of the freeze-drying chamber.A mathematical model of droplet displacement,single droplet freezing and energy transmission in spray freezing process was established based on the lumped heat capacity method for the spray freezing process of water in the freeze-drying chamber of the above experimental spray freeze drying equipment.The effects of droplet size and initial temperature of nitrogen on the freezing time of atomized droplets and the effects of nitrogen flow rate and droplet flow rate on the final material freezing temperature and freezing rate of atomized droplets were obtained by solving the model.In order to veriify the accuracy of the above mathematical models,single droplet freezing process and spray freezing process were simulated respectively with water as the object,and finally the effects of nitrogen flow rate and material flow rate on the final freezing temperature and freezing rate were obtained.Comparing the simulation results with the results of the mathematical model,it is found that they have a high degree of coincidence.The maximum difference of the final freezing temperature is 10K,and the error rate is 4%;the maximum difference of the freezing rate is 20K/s,and the error rate is 7.1%.The results of the experimental verification on the improved equipment show that the experimental results of the influence of nitrogen flow rate on the final freezing temperature are basically consistent with the calculation results of the mathematical model,and the maximum difference is 8K,and the error rate is 5.2%.