The Study Of Characteristics And Process Optimization Of Rice On Hot Air-vacuum Combined Drying Technology

The rice harvested in rainy season with high temperature in southern China has high water content,which is prone to mildew and rot and causes great economic loss.Drying and sorting in time can ensure the drying quality of the rice harvesting.Hot air and vacuum drying are two common drying methods,but the hot air and vacuum single drying methods have their own inevitable limitations.The combined drying method can better dry the rice production by combing various drying methods.In this paper,the drying characteristics,drying model,process optimization and energy efficiency evaluation of rice are studied in the hot air,vacuum and hot air-vacuum combined drying methods.The main work is summarized as follows:（1）The hot air and vacuum single factor drying test is designed to analyze the influence degree of each factor on the drying characteristic curve.The Weibulll function is used to fit the water content ratio curve and calculate theαandβparameters.The Weibulll function moisture content ratio prediction model is established and verified its reliability.The water diffusion coefficients D_eff,D_calal and the geometric parameter R_g values are calculated.The results show that the hot air temperature and vacuum temperature have a great influence on the instantaneous drying rate and the total drying time.The hot wind velocity and vacuum have little effect on the instantaneous drying rate and total drying time.The fitting coefficient of the Weibulll function in each drying mode is above 0.96,and the RMSE value and theχ2 value are all less than 0.1,indicating that the Weibulll function has a better fitting effect on the water content ratio curve.The prediction model of rice hot air and vacuum Weibulll function is verified by experiments.It is found that the relative error between the predicted value and the experimental value of the model is less than 10%,which indicates that the prediction accuracy of the rice hot air and vacuum drying Weibulll function prediction model is high.（2）Designing hot air,vacuum regression universal rotation test program.The single-index prediction mathematical model of different drying methods is built and its significance test,variance and response surface analysis are carried out.The significance of each factor to the indicator is study.Through the normalization process and synthetic weighted,the 7 sets of comprehensive index values under different weight ratios are obtained and the process parameters are optimized.The results show that the effect of hot air temperature on the average drying rate and the bursting rate is significant,and the effect on drying energy consumption is generally significant?the wind velocity has a significant effect on the average drying rate and drying energy consumption,and has no significant effect on the bursting rate.The effect of vacuum temperature on all indicators is very significant?the effect of vacuum on drying rate and drying energy consumption is very significant,and the impact on the bursting rate is generally significant.All single-objective models R²,Adj R² and Pred R² above 0.9,and the coefficient of variation C.V.%is less than 10%,indicating that the model has good fitting effect and high credibility.The optimal parameters of the hot air drying method are:temperature 35°C,wind velocity 0.6 m/s,the average drying rate at this time is0.000685g/（g?min）,bursting rate is 7%,drying energy consumption 6552kJ/kg.The optimal parameters for vacuum drying are:temperature 44°C,vacuum 0.05Mpa.The average drying rate at this time was 0.000363g/（g?min）,bursting rate of 7.3%,and drying energy consumption of 2988 kJ/kg.（3）Based on the previous chapter,the Box-Behnken tentative protocol for hot air-vacuum combined drying is designed.The single-index prediction mathematical model of joint drying method is established and its significance test,variance and response surface analysis are carried out.Through the normalization process and weighted scoring,the 11 sets of comprehensive index values under different weight ratios are obtained and the process parameters are optimized.The advantage characteristics of joint drying and single drying method are compared and analyzed.The results show that the effect of hot air temperature on the average drying rate and the bursting rate is very significant,and the effect on drying energy consumption is not significant.The influence of wind velocity on the drying rate is not significant,and the impact on the bursting rate and drying energy consumption is very significant.The conversion moisture content and vacuum temperature have a significant effect on the single index.The R²,Adj R² and Pred R² of all single-index models are all above 0.9,and the coefficient of variation C.V.%is less than 10%,indicating that the model has good fitting effect and high credibility.The optimal process parameter is Y（3:6:1）:T0=40℃v=0.7m/s M=20.7%T1=38.1℃（V=0.05Mpa）,and the average drying rate is0.000483g/（g?min）,the bursting rate is 6.3%,and the drying energy consumption is2612kJ/kg.By comparing the single target value of the optimal parameter combination of the combined drying method with the single target value of the single drying mode optimal parameter,it is found that the combined drying method has obvious advantages over the single drying method,and should be researched and promoted.（4）The characteristics of moisture content ratio curves are analyzed by natural drying,natural ventilation drying and hot air drying experiment.The study on the transfer of hot air drying exergy is carried out to investigate the influence of temperature and wind speed on the efficiency of exergy and flow exergy.The energy efficiency of the hot air drying system is evaluated by analyzing the energy utilization efficiency and energy consumption structure of the rice hot air drying system.The results show that with the increase of temperature,the average heat exergy efficiency decreases and the average flow exergy efficiency increases.The average exergy efficiency and average turbulent efficiency decrease with the increase of wind velocity.The average of exergy and average of flowing exergy efficiency of different temperatures are the highest under the condition of wind speed 0.5m/s,and the average of exergy corresponding to 40,50and 60°C are:0.683,0.679,0.602,and the average of flow exergy efficiency corresponding to 40,50 and 60°C are:0.275,0.397,0.466.Comparing the average of exergy efficiency and the average of exergy efficiency,the results show that overall exergy efficiency is the highest at 50°C and 0.5 m/s.From the energy utilization efficiency and the energy consumption structure analysis of the drying system,the optimal energy efficiency conditions are 50°C,0.5 m/s,and the drying energy consumption under this condition is 0.27kW?h,which is consistent with the exergy efficiency analysis results.