Study On Heat And Mass Transfer Mechanism Of Peanut Pod Drying Process

Peanut pods as a kind of cash crops and oil crops,sown area,wide range,planting area in China increased year by year,second only to wheat and corn,and low planting costs,high yield,while the sharp increase in output also brought many problems.Fresh peanut pods are rich in oil and moisture,if not dried in time to safe moisture,prone to mildew,affecting the quality of peanut pods.In order to solve the problem of drying peanut pods,it is an effective way to study its heat transfer mechanism.In this paper,taking peanut pods as the test object,the equipment of hot air drying method was used to dry the peanut pods,peanut kernels and peanut shells in a thin layer,the effect of hot air temperature on the drying characteristics of each component of peanuts was analyzed,and the drying kinetic model was constructed by fitting.Low-field NMR and electron microscopy scanning technology were used to study the distribution and migration of water states under different temperature conditions during the drying of peanut shells and peanut kernels,and to predict the moisture distribution of peanut pods during the drying process.Comsol Multiphysics software was used to simulate the temperature gradient and moisture gradient of pods in the hot air drying process,and to explore the effects of hot air drying on peanut heat and mass transfer,in order to provide scientific theoretical guidance for peanut hot air drying.The main contents are as follows:Using hot air drying equipment,the drying characteristics of peanut pods,peanut kernels and peanut shells were studied under different hot air temperature conditions.The results show that:(1)When the drying temperatures of the pods are35℃,40℃,45℃,50℃,55℃,drying to less than 10%of the moisture content,the required time is 20 h,16 h,13 h,12 h,10 h,respectively.When the drying temperature was increased from 35℃to 55℃,the drying time was accelerated by50%.Increasing the hot air temperature can significantly increase the drying rate,indicating that the temperature increase is conducive to increasing the drying rate.(2)The drying process of peanut pods is composed of two stages:acceleration stage and deceleration stage.Due to the protective mechanism of peanut shells,the internal moisture diffusion becomes the main factor in the reduction of the rate of drying in the later stage,and the different tissue structure of peanut shells and peanut kernels constitutes the unique drying characteristics of peanut pods.(3)The results of peanut pod drying test were analyzed and fitted with the commonly used thin-layer drying model,and the data were solved by linear regression,and it was concluded that in the Logarithmic model,the RMSE and SSE values were the smallest and the R~2values were the largest,so the model could well describe the hot air drying process of peanut pods.Using low-field nuclear magnetic resonance(LF-NMR)technology,the internal moisture distribution and migration law of peanut pods during hot air drying were studied,and the effects of different hot air temperatures on the moisture of peanut shells and peanut kernels were compared and analyzed,and the microstructure of peanut kernels after drying was observed by scanning electron microscopy.The results show:(1)There are 3 different states of water in peanuts:free water,bound water,and weak bound water.In the process of hot air drying,the hot air temperature of peanuts is increased to accelerate the transformation and migration between bound water,weakly bound water and free water.The LF-NMR transverse relaxation map shows that the area of free water peak and weakly bound water peak decreased significantly during drying,and the bound water peak area was basically unchanged.(2)From the MRI image and SEM results,the water evaporation process in peanut kernels is that the surface water evaporates at the same time,and the inner layer of water gradually migrates outward,but the migration rate of the inner layer of water is much smaller than the evaporation rate of surface water.The higher the temperature,the greater the degree of damage to the structure of the peanut kernel,and the faster the water migration.(3)Regression analysis of the total peak area of the water state in the peanut kernel and the moisture content of the dry base was carried out,and the R~2obtained was greater than 0.986,and the regression linear equation had a high degree of fit,which can be used to predict the relationship between the moisture state and the moisture content of the dry base in the hot air dried peanut kernel.Comsol Muiphysicos simulation software was used to construct and mesh peanut pods in 2D axisymmetric models.The temperature distribution and moisture distribution of peanut pods in the hot air drying process are numerically simulated.The main conclusions are as follows:(1)In the process of hot air drying,the overall temperature distribution of peanut pods shows a diffusion trend from outward to inward,which is consistent with the characteristics of hot air drying.During the drying process,the external temperature of peanut pods is preferentially increased and a temperature gradient is formed,prompting the outward migration of water.(2)The moisture distribution inside the peanut pod shows a trend from the inside to the outside,the external temperature rises,the surface water begins to evaporate,forming a moisture gradient,with the drying progression,the internal temperature rises,the absorbed energy increases,and the moisture diffuses outward,thereby reducing the internal and external moisture gradient.(3)When the drying temperature rises,the overall surface temperature rise rate is also accelerated,the internal temperature is also higher,and the temperature gradient and moisture gradient are increased,the heat transfer and mass transfer power is accelerated,and the moisture migration rate is improved.The water migration law of peanut pods is that the internal water diffuses outward while the surface water evaporates,but the inner diffusion rate is smaller than the speed of surface water evaporation,so the water dispersion rate in the same area is different,and the closer to the inner area,the slower the drying rate.