Astragalus Hot Air Drying Mechanism And Energy Efficiency Evaluation Analysis

Astragalus,as the abundant medicinal and edible value plant,has a variety of effects that include nhancing immunity,lowering blood sugar,anti-fatigue,antibacterial and so on.It will causes economic losses if it is not processed in time due to the freshly harvested astragalus will mildew and rot with moisture content of 40%-60%(w.b.),strong heat sensitivity and hydrophilicity.We can enhance the quality of astragalus that get better market prospects through drying it in time.In this paper,the traditional hot air drying method is used to study the drying kinetics characteristics,heat and mass transfer mechanism,process parameter optimization,and energy efficiency evaluation.The main conclusions are as follows:(1)Based on the single factor test,it was determined that the temperature of hot air and the thickness of the slice had a significant effect on the drying time,and the effect of hot air was generally significant.The Weibull distribution function is used to analyze the dynamic characteristics of astragalus hot air drying.The results show that the scale parameter?values is negatively correlated with temperature and wind vecolity,and positively correlated with slice thickness.The drying temperature can be appropriately increased at the initial stage of drying to maintain a higher drying rate.The shape parameter?values are all lower than 1,indicating that there is a typical slow-down drying process.And the drying process is mainly controlled by the moisture diffusion in astragalus.The activation energy coefficient Ea of the astragalus hot air drying process was calculated that value was 47.174 kj/mol,which indicates that the hot air drying of astragalus is generally easy.The application of hot air technology is conducive to popularizing in astragalus drying.(2)Two mathematical models were established for the average convective heat transfer coefficient and mass transfer coefficient as a function of hot air temperature,hot air velocity,and slice thickness.The results show that the determination coefficients R~2 of the two models are 0.9984 and 0.9973,the prediction decision coefficient Pre R~2 is 0.9782 and 0.9941,the coefficients of variation of the two models are:1.70%and 2.03%;indicating that the models have only 1.70%and 2.03%sample data cannot be interpreted,indicating that the two models can better predict heat and mass transfer change phenomenon during the drying process of astragalus(3)Establishment of evaluation index models and comprehensive optimization of process plans.The results show that hot air temperature,hot air speed,and slice thickness all have significant effects on the color difference value?E~*,rehydration ratio R,and unit energy consumption H,and the sensitivity is ranked as follows:color difference value model:temperature(F=699.40)>thickness(F=233.64)>wind vecolity(F=13.74),rehydration ratio model:thickness(F=665.86)>temperature(F=130.98)>wind vecolity(F=56.74),unit energy consumption model:temperature(F=3549.39)>thickness(F=1318.31)>wind vecolity(F=619.37);The optimal process parameter combination obtained through comprehensive optimization is:hot air temperature 41.369?,wind vecolity 0.625m/s,slice thickness 7.891mm,color difference value,rehydration ratio,unit energy consumption are 1.214,2.443,21.129Kj/kg,reliability all are higher than 0.9 and close to the target orientation value,indicating that the best combination of process parameters obtained has practical reference significance.(4)Based on the analysis of the energy utilization efficiency of astragalus hot air drying and the energy consumption structure of the drying system,the energy efficiency evaluation of the hot air drying system was performed.The results show that the hot air temperature has a positive correlation with the average thermal efficiency and a negative correlation with the average flow exergy efficiency;the hot blast wind speed has a negative correlation with the average thermal efficiency and the average flow exergy efficiency;the astragalus slice thickness has a positive correlation with the average thermal efficiency and the average flow exergy efficiency;the three groups of drying conditions with the highest comprehensive exergy efficiency:(1)50?,0.8m/s,3mm;(2)60?,0.8m/s,6mm;(3)45?,0.8m/s,6mm,flowing the corresponding average thermal efficiency and flow efficiency:0.6808,0.4742,0.6105,0.5439,0.5681,0.4092,the highest average thermal efficiency and average flow exergy efficiency are corresponding to the experimental conditions of50?,0.8m/s,3mm;the optimal conditions for energy efficiency evaluation basedon energy utilization efficiency and the analysis of the energy consumption structure of the hot air drying system are 50?,0.8m/s,and 3mm.Under this condition,the energy consumption is 1.18kW·h match the reasult of the exergy efficiency analysis.