Experimental Research On Freeze Drying Of Porous Frozen Material With Prefabricated Porosity

As the basic operation unit,drying plays an important role in chemical industry.Inavailable drying methods,freeze drying has the unique advantages,for example,low residual moisture and high product quality.So it is extensively applied in the filed of food and medicine especially in the drying of heat sensitive products.However,the problem of the high energy consumption caused by long drying time and low efficiency has existed in the freeze drying all the time.Therefore,how to strengthen the process of freeze drying and reduce energy consumption have become an hot research focus in the area of freeze drying.In the traditional freeze drying,liquid material is solidified under the condition of low temperature.Then ice is gotten rid by sublimation in subsequent drying process.Because of the operating conditions such as low temperature and low pressure,the products can reserve active ingredients and have higher quality than other products.However,the migration resistance of vapor is very high in the traditional drying process,because of the lack of initial pores in the material.So there are long drying time and low efficiency in drying process.Aiming at solving the technical question,a novel idea that making initially unsaturated material artificially is put forward by our team.And the idea is successfully achieved by means of using “liquid nitrogen ice-cream making method”.Then experiments are conducted in the micro-lyophilizer,which is built based on our past work.The research results reveal that,in the process of drying drug and coffee,the initially unsaturated sample can obviously decrease the freeze-drying time.There is no obvious benefit in the case of the pressure on the freeze drying process.On the contrary,improving air temperature can raise the efficiency largely;SEM morphology analysis displays that initially unsaturated material has spherical porous structure that are continuous and uniform than those conventional one.Specifically,in the process of drying drug,determination of internal temperature during drying shows that the ice in unsaturated materials is almost overall sublimation,and saturated material has obvious sublimation interface.What’s more,low freezing rate has a little positive effect on saturated material while annealing is more beneficial on the unsaturated one.Heating with thermal conductive rod,to a great extent,can make a significant effect on drying.In the process of drying drug,unsaturated material can be frozen more quickly than the saturated.The way of combined heating can further reduce the drying time both in thesaturated and unsaturated.However,saturated material will collapse when the temperature reaches a high degree of 55 o C.For conventional saturated one,freeze drying rate is a typical formal type,which including three phases: acceleration phase,stable phase and deceleration phase,while the unsaturated material is in the deceleration phase throughout the drying.