高濕稻穀乾燥初期分析

Abstract

台灣高溫多雨的氣候衍生濕穀的問題。由於豪雨嚴重影響稻穀的收成，故農民會在雨季前搶收稻穀。然而，高濕稻穀的乾燥效率低，增加了國內乾燥中心的乾燥能源成本，延長乾燥機使用時間，導致乾燥機使用壅塞。此外，高濕稻穀在乾燥初期有一段快速乾燥的現象。故因應台灣特有的乾燥作業需求，探討高濕稻穀乾燥初期的現象可作為乾燥機設計參考。 本研究目的為探討高濕稻穀之乾燥初期現象，使用循環式乾燥模擬機進行乾燥試驗，並藉由調控通風溫度、乾燥時間與通風量的參數，探討高濕稻穀在進入恆率乾燥前的現象。乾燥試驗使用反應曲面法，並使用Box Behnken Design (BBD)設計模型，建立一個三階層之因子設計，通風溫度為55 °C、65 °C、75 °C，乾燥時間為10分鐘、15分鐘、20分鐘，通風量為0.2 CMM、0.3 CMM、0.4 CMM。並繪製反應曲面，藉以展現不同乾燥條件下之含水率變化、乾燥速率、穀溫、熱能效率等關係。 在高通風溫度、長乾燥時間、高通風量的情況下，發生乾燥的穀層愈厚。乾燥穀層厚度可達到6公分，且含水率已低於臨界含水率。超過此穀層厚度之稻穀沒有乾燥現象發生，且稻穀溫度接近當時的濕球溫度，出口的空氣亦接近飽和。穀層厚度為3公分以內的稻穀乾燥速率接近薄層乾燥的乾燥速率，故在此厚度範圍內，高濕稻穀乾燥可視為薄層乾燥。乾燥試驗中，乾空氣給稻穀的能量並非完全用於乾燥蒸發水分，其乾燥熱效率介於35 %至55 %，而剩下來的熱量大部分適用於稻穀升溫，此結果可由穀溫記錄驗證之。以上結果均可作為設計高濕稻穀預乾燥設計參考使用。

Hot and humid weather in Taiwan leads to the trouble of high moisture paddy. Peasants in Taiwan will harvest in emergency in rainy season because of the severe impact of heavy rain to harvest. However, the low efficiency of drying of high moisture paddy increase the drying cost of drying center, prolongs the drying time, and even congest the operating schedule. Moreover, there is a phenomenon of rapid drying in the prime stage of drying of high moisture content paddy. Hence, in order to fit the specific demand of drying operation in Taiwan, the research to the phenomenon in the prime stage of paddy drying is a reference to design of drier. The purpose his study is to investigate the phenomenon of high moisture content paddy in prime stage of drying. The drying operation test were conducted in the simulator, and research the phenomenon before the constant rate period of drying controlling of parameters of heating temperature, drying time, and air flowrate. Variouse parameters were chosen and model of Box Behnken Design (BBD) was adopted under three levels of factors. Levels of heating temperature are 55 °C, 65 °C, 75 °C; levels of drying time are 10 minutes, 15 minutes, and 20 minutes; the levels of air flowrate is 0.2 CMM, 0.3 CMM, 0.4 CMM. Experiment results showed that in the condition of higher level of heating temperature, drying time, and air flowrate, the more thichness of grains is dried, and the thickness is 6 cm, and the moisture content of paddy dropped below the critical moisture content. The drying rate is near to that of thin-layer drying, so the drying can be considered a thin-layer drying. The heat efficiency is 35 – 55 %, and the rest of energy was consumed by heating the paddy. The results above are suggested for further research of pre-drying.