The Effect Of Inert Particles On The Drying Kinetics Of Spray Freeze-drying Process

Spray Freeze Drying(SFD)is a new and advanced technology for the preparation of powders and granules,usually used for the preparation of high value-added food products and powders with heat sensitivity and biological activity.It reduces the impact of temperature on the product and the loss of flavor.The process is divided into two parts,the first part is the atomization and freezing process,firstly the material is atomized through the nozzles and secondly,the atomized material is frozen into ice particles through full contact with the cold medium.The second part is the freezing and vacuum drying process,in which the frozen material is dried in a high vacuum and low-temperature environment to obtain a dry powder.The powder produced by SFD has a uniform particle size distribution,a rich pore structure,high sphericity,and good solubility and rehydration.As a result of these advantages,powders are increasingly being used in the preparation of food,pharmaceutical,and biological products.However,the current SFD technology is not yet mature and has many problems.Due to the high vacuum and low temperature required during the drying process,the heat and mass transfer driving force of the material is limited,which affects the drying efficiency and the long drying time,resulting in high energy consumption and high costs.In addition,it is impossible to judge the drying status of the material in real-time during the drying process,and ending the drying too early or too late will affect the quality of the material,which seriously affects the development and application of SFD technology.Therefore,it is important to optimize the SFD technology,determine the drying status of the material in realtime,determine the boundary point of the drying phase,determine the endpoint of drying,reduce energy consumption,to increase the drying rate and to study the influence of drying kinetics in the SFD process.In this thesis,a freeze dryer equipment that can determine the critical point between primary and secondary drying in real-time was designed and developed to study the determination of the dividing and ending points of drying stages when preparing powders in this SFD process,as well as the influence of inert particles in each drying stage,and a spray freeze drying(IP-SFD)experiment with the introduction of inert particles was carried out using milk powder as the material,the specific steps were: dissolve the milk powder The material is then atomized and sprayed in liquid nitrogen,the material is frozen after the liquid nitrogen has evaporated,the frozen ice particles are evenly spread on the surface of the pre-cooled inert particles and then freeze-dried under vacuum in the freeze-dryer,and finally the inert particles are separated from the powder to produce dried milk powder particles.A selfdesigned and developed laboratory lyophilized was used as the experimental device to carry out conventional spray freeze drying(SFD)experiments and inert pellet spray freeze drying(IP-SFD)experiments respectively.The drying kinetics of spray freeze drying with the addition of inert pellets were investigated and discussed based on the changes in the relative moisture content,drying rate,drying time,temperature,and microstructure of the material.It was found that the drying time of the IP-SFD experiment was significantly shorter and more efficient than the conventional SFD experiment and that the larger the size of the inert particles of the same type and different sizes the shorter the drying time,and the relative moisture content decrease rate and drying rate were increased.In particular,the drying time was reduced by 35% for 5mm stainless steel inert pellets and by nearly 30% for 5mm glass inert pellets in the primary drying stage.In the secondary drying for heating,the shortest drying time was achieved by adding 5mm stainless steel inert particles to the SFD experiment,which reduced the drying time by 39.5% compared to the SFD.This is due to the poor thermal conductivity of the glass and the fact that the inert particles at the bottom of the material block the heat transfer from the bottom to the top,which in turn affects the drying rate.The drying kinetics of the spray freeze drying experiments with the addition of inert particles were explored and investigated by temperature and mass changes of the material during the drying process,including changes in drying time,temperature,relative moisture content,and drying rate of the material,respectively.Finally,the SEM images of the powders produced by spray freeze drying experiments with different types and sizes of inert particles were observed.It was found that the effect of adding inert particles on the pore structure of the material was less obvious when the SFD was not heated,and the surface of the particles all had a smooth shell structure with slight pore channels.The pore structure of the powder particles after heating in the secondary drying stage was richer,with many channels for water vapor to escape.The final comparison between the SFD and 5 mm stainless steel inert particle SFD powders showed that the addition of inert particles during the secondary drying stage produced powder particles with a richer surface pore structure.The above experimental results will play a vital role in the development and scale-up of spray freeze drying technology.