| With the demand of the agricultural product market improved, there are a large number of fresh agricultural products to meet the market demand every year. The storage and keeping fresh of fresh agricultural products is the major two problems. Nearly a third of the world agriculture products wasted every year, due to the lack of appropriate and timely drying process. Drying is the oldest, simple and widely used method to save agricultural products.Variable temperature drying can reduce the drying time, energy consumption and improve quality. The basic principle is easy to operate. It can adopt different temperature to realize the material drying method according to the different drying stages of material. Variable temperature drying is better than traditional method. In numerous drying technology, variable temperature drying is not only simple, low cost, and quality of agricultural products has a huge improvement.Variable temperature drying is changing temperature in different drying stage with different drying medium temperature, according to the drying process of material resistance changes in the internal moisture migration and heat sensitive components to the requirement of temperature. It can not only save a lot of energy, but also can ensure the quality of dry products in the drying process. It can greatly improve the color, nutrition and physical properties of products. In order to solve the temperature lag, strong time-varying and the difficulty of linearity error in the drying process, a set of accurate temperature control and acquisition system is designed. Labview PID control, Fuzzy control and Fuzzy PID control are used to control the corresponding temperature programming respectively. Fuzzy-PID control in temperature control system can make the output error of the temperature control in a small range. The error is only 0.01. It has a strong adjustment and adaptability, the temperature curve is high robustness. Fuzzy- PID is the most outstanding to adjust the temperature control.Lotus seed segment variable temperature drying is a complicated heat and mass transfer process. In order to study the characteristics of lotus seed, make the drying effect more objective and draw a more optimized segmentation variable temperature drying process parameters in drying process, the experiment designed a set of lotus seed drying system. The system is mainly composed of hot air system, hot air heating system, hot blast temperature,flow measurement, control system and acquisition system, etc. The overall design, installation and debugging of the system has been successful completed based on segmentation variable temperature drying test requirements. The bench can control hot blast temperature and flow.The online test data is the most important characteristic.The hot air drying was conducted on lotus seed to shorten the drying time and improve the dried product quality. The Surface configuration, rehydration ratio, energy consumption and drying characteristics of lotus seed were investigated at 50~90℃and 60(2~4h)80℃. The effective diffusion coefficient and activation energy of lotus seed were calculated. The results show that the drying time of lotus seed decreased largely with the increase of air temperature from 50℃ to 90℃.The improvement of air temperature has a negative effect on product qualities such as color, shrinkage rate and rehydration ratio. The variable temperature drying process of lotus seed were conducted and their dried product qualities were compared. Under the optimum drying condition(60(3h)-80℃),the rehydration ratio of lotus seed is 169.41%.The unit energy consumption decreased from 14022 k J/g to 11989 k J/g compared with 60℃hot air drying. The Effective moisture diffusivity was calculated by using Fick’s second law.Lotus seed formed by hot air drying had effective diffusion coefficient variations of 1.79×10-9,2.14×10-9, 3.21×10-9, 4.00×10-9, 5.83×10-9, 2.97×10-9,2.52×10-9 and 2.44×10-9 m2/s.The linear relationship between the natural logarithm of the effective diffusion coefficient and reciprocal of temperature was built based on the Arrhenius empirical formula. The activation energy value of lotus seed is 28.33 k J/mol. A valuable reference for the parameter optimization and equipment design of lotus seed hot air drying was provided by the experimental results. |
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