The Study Of Thermodynamic Properties And Mechanism Of Rapeseed Based On Vacuum Drying Technology

Rapeseed is a kind of oil crops with high oil content and high protein content,which has high comprehensive development and utilization value.It is extremely susceptible to environmental temperature and relative humidity in the shelf period which may result in mildew,decay and rancidity,and cause huge economic losses because of the high moisture content of newly harvested rapeseed?15%-30%w.b.?.Drying and storage are two important processing steps after harvest,which play the important roles in the industrialized production and processing of rapeseed.Because rapeseed is a heat-sensitive material,the quality of rapeseed will be seriously affected loss,serious loss of nutrients,reducing its economic benefits if the dryness is improperly handled.This paper focus on the rapeseed drying process kinetics,the quality of the mechanism and the safety of storage characteristics of the study.The main conclusions are as follows:?1?The kinetics of vacuum drying for rapeseed was analyzed by Weibull distribution function model.It could be concluded that?was negatively correlated with the temperature and the degree of vacuum,and positively correlated with the initial moisture content.It was inferred that the drying temperature can be properly increased in the early stage of drying to maintain the higher drying rate through the analysis of the scale parameter?.The value of shape parameter?was less than 1,which mean that there was no obvious stage of speed-up during rapeseed vacuum drying process and belonged to typical slowdown drying process,and the vacuum drying process was mainly controlled by the moisture diffusion inside the rapeseed.The activation energy E_a of rapeseed in the vacuum drying process was calculated and the value was22.369Kj/mol which showed the rapeseed vacuum drying is easy.?2?Preliminary study on the mechanism of rapeseed quality under different drying conditions showed temperature had a significant effect on each index,the temperature and the average drying rate r was positively correlated,and the germination rate g and bud length l was negatively correlated.The temperature should be in the range of50-70?in order to get the comprehensive optimization results of each indicator.The degree of vacuum V was only positively correlated with the drying rate r.The indexes g,l and e achieve the optimal value when the degree of vacuum was 0.05Mpa.So the vacuum degree should be around 0.05Mpa when the vacuum drying process of rapeseed was optimized.The initial moisture content M was positively correlated with unit energy consumption e,and negatively correlated with germination rate g,bud length l and average drying rate r.?3?Optimization of rapeseed vacuum drying process and establishment of mathematical model.By comparing the significance test coefficients F and p values of each model,it was found that the single factor T,V and M had influence on the indexes?g,l,r,e?,the order of influence was:germination rate model:T?p<0.0001?>M?p=0.0013?>V?p=0.0169?;Bud length model:T?p<0.0001?>M?p=0.0151?>V?p=0.0249?;Drying rate model:T?F=3677.28?>V?F=1360.74?>M?F=447.3?;unit energy consumption model:T?F=200.1?>V?F=78.82?>M?F=10.02?.The optimal combination of process parameters were:temperature 57.78?,vacuum 0.06Mpa,moisture content of 16.84%Under these conditions,the corresponding indexes were as follows:the drying rate 0.407279g/min,the germination rate86.6301%,the bud length 3.87514cm and the unit energy consumption 101.366Kj/kg,the credibility was 0.866.Using the optimal combination of parameters for experimental verification and the results showed that the relative error between the experimental value and the predicted value of each index was 6.21%,0.52%,1.21%and 3.88%respectively,all indexes were less than 10%.The results showed that the model of vacuum drying of rapeseed obtained better fitting model,which could provide reference for vacuum drying technology of rapeseed.?4?Study on isothermal and thermodynamic properties of rapeseed.The rapeseed adsorption and desorption isotherms were typical Type?curves,which were similar to the most porous media materials.Under certain temperature conditions,the equilibrium moisture content of rapeseed was positively correlated with water activity.Under a certain water activity,the equilibrium moisture content of rapeseed was negatively correlated with temperature.Therefore,a reasonable increase in storage temperature or reduce the ambient humidity can effectively extend the shelf life of rapeseed.?5?Thermodynamic stability of rapeseed and optimization of storage conditions.Through the analysis of the relationship between the net isobaric adsorption heat and the equilibrium moisture content of rapeseed,the q_stt value at the later stage of drying was larger,the energy required to release the water molecules from the adsorption site was higher,and the drying rate was decreased.During the process of rapeseed adsorption and desorption,the increase of water content and the decrease of temperature will increase the excess free energy on the surface of rapeseed,and reduce the affinity of water molecules to the adsorption sites,making it easier for the surface water molecules to detach from the adsorption sites point,macroscopically,the rate of drying increases in the early stage of drying.The higher water expansion coefficient???can increase the drying rate,while the lower value of?during storage can reduce the water activity,prolong the shelf life of the material and maintain the quality of the rapeseed.By analyzing the relationship between net enthalpy and equilibrium moisture content,the net integral enthalpy of desorption process is higher than that of adsorption process when the equilibrium moisture content is constant,q_inn and M_e are negative for both processes correlation,the higher the equilibrium moisture content,the lower net integral enthalpy is.By analyzing the relation between the net integral entropy(?S_in)and the equilibrium moisture content,the lowest value(?S_in)of the most closely related to the combination of the water molecules of rapeseed and the solid state is obtained,at the same time,the most stable storage conditions of rapeseed were obtained as follows:the equilibrium moisture content at 4.5%,4.32%and 4.23%at 20,30 and40?,respectively.