基于分层模型的溶液除湿谷物就仓干燥系统性能及能耗

    Performance and energy consumption of liquid desiccant dehumidification grain in-bin drying system via layered model

    • 摘要: 为实现安全、高效且节能的粮食干燥过程,发展就仓干燥新型技术,该研究提出一种基于分层模型的热泵驱动溶液除湿谷物就仓干燥系统,建立并验证系统中各部件数学模型,模拟了粮堆高度3m、初始水分0.2的稻谷采用通风量为120 m3/(h·t)在空气温度20~32 ℃,相对湿度55%~80%的不同室外天气参数下干燥致顶层稻谷达到安全水分的过程,对干燥完成时间、干燥后顶层稻谷干物质损失(Dry Matter Loss, DML)和白度值(Whiteness)、以及系统单位能耗和总耗能进行研究,并将系统与其他干燥方式进行性能对比分析。结果表明:完成干燥所需时间整体范围为194~358 h,以粮温20和25 ℃为例完成干燥时间均满足安全干燥期要求,确定了干燥时间规范上的可行性;干燥后的顶层稻谷干物质损失为0.33%~0.52%,大部分天气参数满足规范要求而稻谷不发生霉变,白度值为51.331~51.452,干燥后整体白度值降低较小;室外空气经除湿换热后以接近于室外空气温度、低含湿量状态进入粮仓,进而验证了热平衡模型以及薄层干燥方程选取的恰当性;系统单位能耗范围为2.09~3.25 kW·h/(%·t),总耗能为6 930~9 530 kW·h,室外空气温度较高的时更利于降低系统单位能耗和总能耗,提高干燥的效能;系统与热泵加热干燥相比速率快、能耗少、稻谷相关品质较优,具有优越性。系统在特定天气条件下干燥的时效、稻谷相关品质指标及能耗等方面表现良好,所建立模型适当且准确,可为实现安全、高效且节能的谷物就仓干燥提供一种选择。

       

      Abstract: Abstract: Wet grain drying is a significant processing step in grain production. Grain in-bin drying is one of the most widely-used and effective drying ways to integrate with drying and storage in grain drying areas. Therefore, this study aims to clarify the performance and energy consumption of liquid desiccant dehumidification grain in-bin drying using a layered model. A system of grain in-bin drying was also designed using liquid desiccant dehumidification driven by a heat pump. New technology was finally developed to achieve safe, efficient, and energy-saving grain drying and storage process for grain in-bin drying. The mathematical models were established to verify, including heat pump, dehumidifier/regenerator, and grain in-bin drying. A specific process was simulated, where the top layer of rice with a grain pile height of 3 m and initial moisture content of 0.2 (wet basis) was dried to reach the safe moisture content of 0.135 (wet basis) by the air flow rate of 120 m3/(h·t) under different outdoor weather parameters with the air temperature of 20-32 ℃ and relative humidity of 55%-80%. Some parameters were determined, including the drying time, the unit and total energy consumption, the dry matter loss (DML), and the whiteness of the top rice after drying. After that, a comparison was made on the performance of the system with various drying. The results showed that the range of drying completion time was 194-358 h. Taking the grain temperature of 20 ℃ and 25 ℃ as examples, the drying completion time was within 21 and 14 d of the safe drying period, respectively. Both met the requirements of a safe drying period, indicating the feasibility of drying time specification. Moreover, the DML top layer of rice after drying was 0.33%-0.52%, where most of the weather parameters met the specification requirement of DML < 0.5%. The initial whiteness value of the top layer of rice was 51.5, while the whiteness value after drying was 51.331-51.452 acceptable in the market (>45), indicating that the overall whiteness value decreased slightly after drying. The suitability of the heat balance model and thin-layer drying equation were verified, where the air entered the grain bin in a state close to the outdoor air temperature and low humidity after dehumidification and heat exchange. Furthermore, the unit energy consumption range was 2.09-3.25 kW·h/ (1% moisture·t), and the total energy consumption was 6 930-9 530 kW·h. It was more conducive to reduce the unit and total energy consumption of the system for high drying efficiency when the outdoor air temperature was higher than before. Nevertheless, the high temperature may lead to more DML of the top layer of rice, even to lower quality. It was found that each thin layer of rice met the dual specification requirements of DML (<0.5%), and the unit energy consumption of the system (<2.5 kW·h/(1%·t)) at the air temperature of 30 ℃. The drying rate and energy consumption under the summer parameters were better than those under the autumn, whereas, the DML and color of rice were inferior to the autumn parameters. The improved system presented a fast speed, less energy consumption, and better rice-related quality, compared with heat-pump drying. Consequently, the system performed well in terms of drying time, energy consumption, and rice-related quality indicators under specific weather conditions, indicating the appropriate and accurate model. The finding can provide a preferred choice for safe, efficient, and energy-saving grain in-bin drying.

       

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