油菜籽流化床干燥传热传质特性与布风板结构参数优化的研究
发布时间：2019-06-14 18:57
【摘要】：菜籽油是我国主要的食用植物油之一,是世界上第三大食用植物油,仅次于大豆油和棕榈油,并在全球的经济发展上具有非常重要的作用和地位,从而受到全世界各国的普遍重视。然而油菜籽在高温高湿环境下特别容易生长霉菌,导致高水分油菜籽霉变,需尽可能在短时间内将其干燥至安全储藏水分的范围内。流化床干燥(热空气)可满足油菜籽安全储藏的要求,也能提高干燥效率和干燥品质,是油菜籽较理想的一种干燥技术。因此,本文结合大量的实验和理论分析,深入系统地研究流化床干燥特性和气-固两相传热传质规律,确定适合于油菜籽流化床干燥的干燥动力学模型,优化干燥工艺条件。并在此基础上,通过理论分析、试验研究和数值模拟,优化设计布风板的开孔率、布孔方式和结构尺寸,以确定最佳布风板的具体结构,进而揭示最佳布风板的气-固传热传质规律,为新干燥设备的开发,以及现有设备性能的改进提供理论依据。主要的研究内容如下：(1)通过实验深入研究了油菜籽初始含水率、热空气温度和热空气流速3个影响因素对油菜籽流化床干燥过程的影响,研究结果表明：油菜籽初始含水率越大、热空气温度越高和热空气流速越大,油菜籽流化床干燥速率、干燥热效率、对流传热传质系数以及水分有效扩散系数越大：3个影响因素对干燥速率和平均对流传热传质系数的敏感性顺序依次为：油菜籽初始水率热空气温度热空气流速；对水分有效扩散系数的敏感性顺序依次为：热空气温度热空气流速油菜籽初始含水率。(2)采用目标函数法对油菜籽流化床干燥速率进行优化,利用MATLAB7.0软件的非线性规划求解进行优化计算,优化结果发现3个影响因素的最优水平组合为：最佳优化工艺条件是油菜籽初始含水率为23.44%、热空气流速为2.25m/s和热空气温度为65℃。(3)通过4种常见薄层干燥动力学模型对试验数据进行非线性拟合,比较相关系数R2、卡方值X2和均方根误差RMSE 3个评价指标,发现Page模型是描述油菜籽流化床干燥失水规律的最优模型(R2=0.9993)。在此基础上,通过试验数据的拟合和响应曲面分析,分别建立了模型参数n和k的二次多项回归方程,从而进一步建立了油菜籽流化床干燥动力学模型,该模型能够较好地预测油菜籽流化床干燥过程的失水规律,其试验值与模型预测值的符合程度高,最大相对误差仅为3.2%。(4)通过Design-Expert 8.0.6软件对试验数据进行响应面分析,分别建立了平均对流传热传质系数和水分有效扩散系数的标准回归模型,通过试验验证,采用直接误差分析和成对双样本均值分析的t-检验分析试验值与模型预测值的差异性,结果表明：平均对流传热传质系数以及水分有效扩散系数的试验值与模型预测值的最大直接相对误差仅为4.4%、3.3%和2.8%,且无差异概率分别为0.9238、0.8633和0.9126,都大于给定显著水平0.05,表明这3个模型的预测值与试验值没有显著性差异,即平均对流传热传质系数和水分有效扩散系数的标准回归模型拟合较好,可靠性较高。(5)通过响应曲面分析方法,对任何2个因素的交互效应对平均对流传热传质系数和水分有效扩散系数的敏感性进行分析与评价,结果表明：初始含水率与热空气流速的交互作用对平均对流传热传质系数的影响最大,对水分有效扩散系数的影响最小;油菜籽初始含水率与热空气温度的交互作用的影响次之;热空气流速与热空气温度的交互作用对平均对流传热传质系数的影响最小,对水分有效扩散系数的影响最大：其中与平均对流传质系数的影响相比,各影响因素的交互作用对平均对流传热系数的影响稍小。(6)根据菲克第二定律,对试验数据进行线性回归拟合计算水分有效扩散系数和平均活化能,结果表明：水分有效扩散系数数量级为10-10～10-9 m2/s,分布在脱水食品的正常范围内(10-12～10-8m2/s)。油菜籽初始含水率在14.41%-29.72%范围内所对应的水分有效扩散系数为6.485x10-10-10.133×10-10m2/s：热空气流速在1.75-2.25m/s范围内所对应的水分有效扩散系数为7.296×10-10～9.525×10-10m2/s；热空气温度在45～65℃范围内所对应的水分有效扩散系数为5.269x 10-10～8.917x10-10 m2/s。油菜籽流化床干燥的平均活化能为22.84kJ/mol。(7)针对开孔率和布孔方式2个方面,优化设计了5种布风板(12.27%开孔率正三角形布孔、14.10%开孔率正三角形布孔、15.84%开孔率正三角形布孔和15.84%开孔率正三角形不均匀布孔),通过试验结果和数值模拟的结果进行对比分析,结果表明：与其他4种布风板相比,15.84%开孔率圆形不均匀布孔的布风板是油菜籽流化床干燥最佳的布风板,其不仅增强了热空气的流通性,减少了局部区域的热空气聚集,实现了气固两相流动的正常流态化,达到了流化床内油菜籽颗粒均匀分布,无沟流和死区的目的。同时也提高了流化床层内热空气温度沿轴向和径向分布的均匀性,扩大了径向高温区域的范围,增加了热空气与油菜籽颗粒以及油菜籽颗粒间的传热传质效率,增大了恒速干燥对流传热传质系数,从而提高了油菜籽流化床的干燥速率和干燥热效率,缩短了油菜籽流化床的干燥时间,减小了所需的单位能耗。(8)针对最佳布风板(15.84%开孔率正三角形不均匀布孔),根据欧拉(Euler)方法的双流体模型和标准k-ε湍流模型,采用计算流体力学(CFD)分析软件Ansys Fluent 15.0进行数值模拟分析了流化床层气-固传热传质规律,结果表明：在整个热空气流化床干燥油菜籽颗粒的过程中,热空气温度发生了明显的变化,其先缓慢地下降,而后迅速地降低,最后在整个流化床上部的热空气温度趋于稳定。在布风板区域附近,热空气温度、热空气速度和油菜籽温度显著地增大并达到最大,远离布风板后都急速下降,最后趋于稳定,而表压强则沿着流化床高度的增加而急剧下降最后趋于零。在整个流化床层内气-固热量传递的主要推动力是气-固流速差、温度差和油菜籽颗粒体积分数,其中在布风板附近起主要控制作用的影响因素是气-固温度差,在远离近布风板位置,油菜籽颗粒体积分数和气.固流速差是主要的影响因素；气,固质量传递的主要推动力是油菜籽颗粒体积分数、气一固流速差和压强差,其中在布风板区域内起主要控制作用的影响因素是气一固速度差,在远离近布风板位置时,油菜籽颗粒体积分数和压强差是质量传递的主要控制因素。(9)结合数值模拟的结果和传热传质系数的经验公式,对最佳布风板传热传质系数的变化规律进行了分析,结果表明：沿着流化床高度(轴向),气,固传热传质系数都呈现出先升高后逐渐降低再略有上升的趋势：沿着流化床径向,气,固传热传质系数均呈现出中心处气,固传热传质系数较高,且变化平坦,基本保持不变,而近流化床壁面的两侧,都逐渐递减。
[Abstract]:Rapeseed oil is one of the main edible vegetable oils in China, which is the third largest edible vegetable oil in the world, second only to the soybean oil and palm oil, and has a very important role and status in the global economic development, thus being widely valued by all the countries around the world. However, in that high-temperature and high-humidity environment, the rapeseed is particularly susceptible to the growth of mold, resulting in the mildew of high-moisture rapeseed, and it is necessary to dry it to a safe storage moisture in a short time as much as possible. The fluidized bed drying (hot air) can meet the requirements of the safe storage of the rapeseed, and also can improve the drying efficiency and the drying quality, and is an ideal drying technology for the rapeseed. In this paper, a large number of experiments and theoretical analysis are combined to study the drying characteristics of the fluidized bed and the heat and mass transfer law of gas-solid two-generation. The drying kinetic model suitable for the drying of the fluidized bed of rapeseed is determined, and the conditions of the drying process are optimized. On the basis of this, through the theoretical analysis, the test research and the numerical simulation, the opening ratio, the hole distribution method and the structure size of the air distribution plate are optimized, so as to determine the specific structure of the optimal air distribution plate, and then the air-solid heat transfer and mass transfer law of the optimal air distribution plate is disclosed, and the development of the new drying equipment is provided. And provides a theoretical basis for improving the performance of the existing equipment. The main contents of the research are as follows: (1) The effect of the initial water content, hot air temperature and hot air flow rate of the rapeseed on the drying process of the rapeseed fluidized bed is studied in detail. The results show that the higher the initial water content of rapeseed, The higher the hot air temperature and the higher the hot air flow rate, the higher the drying rate and the drying efficiency of the rapeseed, the greater the heat and mass transfer coefficient and the effective diffusion coefficient of the moisture. The more sensitive order of the three influencing factors to the drying rate and the average convective heat transfer coefficient is as follows: The hot air temperature and the hot air flow rate of the initial water of the rapeseed; and the sensitivity sequence for the effective diffusion coefficient of the moisture is the initial water content of the hot air temperature hot air flow rate rapeseed. and (2) optimizing the drying rate of the rapeseed fluidized bed by using a target function method, carrying out optimization calculation by using a non-linear programming solution of the MATLAB7.0 software, and finding that the optimal level combination of three influencing factors is as follows: the optimal optimization process condition is that the initial water content of the rapeseed is 23.44 percent, The hot air flow rate was 2.25 m/ s and the hot air temperature was 65 鈩，
本文编号：2499607
[Abstract]:Rapeseed oil is one of the main edible vegetable oils in China, which is the third largest edible vegetable oil in the world, second only to the soybean oil and palm oil, and has a very important role and status in the global economic development, thus being widely valued by all the countries around the world. However, in that high-temperature and high-humidity environment, the rapeseed is particularly susceptible to the growth of mold, resulting in the mildew of high-moisture rapeseed, and it is necessary to dry it to a safe storage moisture in a short time as much as possible. The fluidized bed drying (hot air) can meet the requirements of the safe storage of the rapeseed, and also can improve the drying efficiency and the drying quality, and is an ideal drying technology for the rapeseed. In this paper, a large number of experiments and theoretical analysis are combined to study the drying characteristics of the fluidized bed and the heat and mass transfer law of gas-solid two-generation. The drying kinetic model suitable for the drying of the fluidized bed of rapeseed is determined, and the conditions of the drying process are optimized. On the basis of this, through the theoretical analysis, the test research and the numerical simulation, the opening ratio, the hole distribution method and the structure size of the air distribution plate are optimized, so as to determine the specific structure of the optimal air distribution plate, and then the air-solid heat transfer and mass transfer law of the optimal air distribution plate is disclosed, and the development of the new drying equipment is provided. And provides a theoretical basis for improving the performance of the existing equipment. The main contents of the research are as follows: (1) The effect of the initial water content, hot air temperature and hot air flow rate of the rapeseed on the drying process of the rapeseed fluidized bed is studied in detail. The results show that the higher the initial water content of rapeseed, The higher the hot air temperature and the higher the hot air flow rate, the higher the drying rate and the drying efficiency of the rapeseed, the greater the heat and mass transfer coefficient and the effective diffusion coefficient of the moisture. The more sensitive order of the three influencing factors to the drying rate and the average convective heat transfer coefficient is as follows: The hot air temperature and the hot air flow rate of the initial water of the rapeseed; and the sensitivity sequence for the effective diffusion coefficient of the moisture is the initial water content of the hot air temperature hot air flow rate rapeseed. and (2) optimizing the drying rate of the rapeseed fluidized bed by using a target function method, carrying out optimization calculation by using a non-linear programming solution of the MATLAB7.0 software, and finding that the optimal level combination of three influencing factors is as follows: the optimal optimization process condition is that the initial water content of the rapeseed is 23.44 percent, The hot air flow rate was 2.25 m/ s and the hot air temperature was 65 鈩，
本文编号：2499607
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