高濕度稻穀乾燥模擬機的設計與分析

Abstract

國內濕穀問題產生來自雨災的搶收。高含水率稻穀的乾燥效率低，增加了乾燥中心的乾燥能源成本，延長乾燥機使用時間，造成乾燥機使用上的壅塞。為因應此特有的乾燥作業需求，必須進行高含水率稻穀的乾燥理論研究與設備開發。循環式乾燥機是一個成熟的商品，但由於機械構造不同，操作方式的差異，造成乾燥效率以及所需花費的能源迥異。本研究目的為設計出具循環式乾燥機原理的乾燥高濕稻穀設備，稱之為高濕度稻穀乾燥模擬機，並藉由穀層厚度、風量、乾燥時間等變數的調控，對高含水率稻穀在達到臨界含水率之前的作業進行試驗。乾燥試驗設計使用反應曲面法，Box Behnken Design(BBD)設計模型，建立一個三階層之因子設計，風量為0.3CMM/kg、0.5CMM/kg、0.7CMM/kg，穀層厚度為6公分、9公分、12公分，乾燥時間為10分鐘、20分鐘、30分鐘。再繪製出反應曲面，藉此以展現不同乾燥條件下之能量消耗關係。在實驗上穀層內部的變化不易量測，利用能量、質量平衡方程式建立稻穀乾燥過程溫度與水分變化的平衡，搭配有限元素法軟體(Comsol Multiphysics 4.3a)，可以計算出穀層內部的變化。藉由數學模型的建立與驗證可正確推估穀層內部的變化。在高濕稻穀初期乾燥作業中消耗的能量主要為加熱脫濕的熱能，佔總體乾燥所需能量的98%，其餘為乾燥通風鼓風機所消耗的能量。以一次乾燥循環將高濕稻穀乾燥至臨界含水率最能減省乾燥能量。本研究變數的範圍內消耗能量較為經濟的風量與乾燥時間的組合分別為0.3CMM/kg-30分鐘；0.5CMM/kg-20分鐘；0.7CMM/kg-20分鐘。而此三組試驗乾燥速率隨風量增大變快。因風量變數設定為單位稻穀重量的使用風量，因此乾燥層厚度的影響並不顯著。但從模擬結果發現厚穀層乾燥將造成在稻穀含水率上下層的不均勻，此現象可能影響後續乾燥作業，建議再做進一步探討。

High moisture content paddy emergency harvested in rainy season is a serious problem in Taiwan. It might prolong drying time to congest operating schedule of paddy drying centers and also increase drying costs. Currently, commercial circulating dryers are mature commodities. However, various designs and operations might lead to different drying efficiency and energy costs. The purpose of this study is to device a drying simulator based on circulating drying principles to investigate dryer designing and operation for high moisture content paddy. Pre-drying operation tests were conducted in the simulator to dry high moisture content paddy to its critical moisture content. Various parameters were chosen and model of Box Behnken Design (BBD) was adopted under three levels of factors. Levels of paddy layer thickness are 6cm, 9cm, and 12cm; levels of specific air flow rate are 0.3CMM/kg, 0.5CMM/kg, and 0.7CMM/kg; levels of drying time are 10min, 20min, and 30min. It is difficult to measure internal changes in the paddy layer during drying, so heat and mass transfer model was built and simulated by FEM software (Comsol Multiphysics 4.3a) to predict internal changes in the paddy layer. Experimental results showed 98% total energy consumed in drying process being used in heating and desorption of water in grain and the rest being used in air flowing to carry mass and energy. The most efficient way in pre-drying was to dry wet grain to the critical moisture content in one pass. Various passes drying might consume more energy in heating up grain to drying temperature in each pass; besides, the benefit of tempering was insignificant in pre-drying stage. In the range of operations, combinations of specific air flow rate and drying time in 0.3CMM/kg-30min, 0.5CMM/kg-20min, and 0.7CMM/kg-20min with high energy efficiency and high air flow rate, also resulted in high drying rate. Because specific air flow rate was used in experimental design the influence of the paddy layer thickness was not significant. However, simulation results showed that thick layer drying might lead to high moisture content variation along the air flow direction. This phenomenon may affect subsequent drying operation, so it is suggested for further research.