【作者】 郭树国；

【导师】 李成华；

【作者基本信息】 沈阳农业大学 ， 农业机械化工程， 2012， 博士

【摘要】 真空冷冻干燥技术是集真空科学、低温工程、流体技术、控制工程、传热传质和动力工程于一体的综合技术工艺，在食品、药品、生物制品等领域得到了广泛应用。由于真空冷冻干燥过程是在低温和真空条件下进行的传热传质过程，因此干燥过程时间长，能耗大，干燥速率低，生产成本高。本论文在分析真空冷冻干燥机理的基础上，以人参为研究对象，探讨真空冷冻干燥过程中工艺参数对干燥特性的影响，为采用真空冷冻干燥技术对人参进行干燥加工提供指导。在对已有研究成果进行总结和对真空冷冻干燥特点深入分析的基础上，建立了考虑因素较为全面的新二维真空冷冻干燥模型，使模型的理论计算更接近实际真空冷冻干燥过程，为真空冷冻干燥的机理分析和热控制提供了理论参考。采用美国产Q200DSC差示扫描量热仪，在升温速率和降温速率为10℃/min的条件下测得常压状态下人参的共晶点温度和共熔点温度分别为-16℃和-0.05℃。利用电阻测定法测量了真空状态下人参共晶点温度为-15℃，与差示扫描量热法测得的共晶点温度吻合。测量结果为真空冷冻干燥过程工艺参数的选择提供了参考依据。利用FLUENT软件模拟分析了干燥室加热板与物料上表面之间的空间温度场分布，物料离上加热板越近，获得的热能就越多，干燥所需的时间就越少。物料边缘的温度梯度较中心部位大，所以物料的边缘部分获得的能量相对于中心部分来说更多，因此更容易干燥。利用ANSYS软件模拟分析了人参切片在预冻过程中温度的变化，结果表明温度场模拟数据与试验实测数据基本相符，为准确判断冻结结束时间提供了参考。研究了平行平板间辐射传热的过程及状态，推导辐射传热过程中有效能和最大有效能热传输效率时的最佳接收温度的计算公式。结果表明在稳态传热时接收板温度最佳值与材料表面性质无关，两平行板间空间的温度场分布不均匀，接收板中心点的温度高于边缘处的温度，温度梯度随两平行板辐射距离的减小而增加。因此，为提高物料受热均匀性，应增加冻干物料与加热板间的距离。应用ANSYS软件模拟分析了在分段函数加热曲线、抛物线型加热函数、指数型加热函数、有理函数加热曲线四种不同加热方式下人参切片内部各点温度随时间的变化规律，结果表明分段型加热函数使人参切片升温最快。分析结论为加速干燥速率以达到节能目的提供了依据。应用二次回归通用旋转组合试验建立了人参切片厚度、加热温度和干燥室压强与干燥速率、生产率和面积收缩率之间关系的回归方程，利用非线性优化方法，对三个回归方程分别进行优化求解，得出优化工艺参数。当人参切片厚度取值为4.8mm、加热板温度取值为50.4℃、干燥室压强取值为48.7Pa时，干燥速率具有最大值1.56h^-1。当人参切片厚度取值为3mm、加热板温度取值为60℃、干燥室压强取值为80Pa时，单位面积干燥生产率具有最大值111.7g/h·m²。当人参切片厚度取值为12mm、加热板温度取值为40℃、干燥室压强取值为40Pa时，切片面积收缩率具有最小值9﹪。利用BP神经网络理论对真空冷冻干燥过程进行了模拟研究，结果表明，BP神经网络能较精确的模拟真空冷冻干燥过程。采用人参切片干燥正交试验结果对BP神经网络进行训练后，对真空冷冻干燥工艺条件进行了预测和优化，预测值与试验实测值的相对误差较小，表明用BP神经网络理论模拟真空冷冻干燥过程具有较高的准确性。更多还原

【Abstract】 Vacuum freeze-drying is a comprehensive technology integrated with vacuum science, cryogenicsengineering, fluid technique, control engineering, heat and mass transfer and dynamic engineering, whichhas been applied widely in food engineering, medicine engineering and biological product engineering. Thefreeze-drying is a procedure of heat and mass transfer under lower temperature and high vacuum, thedrying process needs long time and much energy because of lower drying velocity, so the production costsare relative high. The objectives of the study were to investigate the impact of vacuum freeze-dryingprocess, and to provide practical guidance for application of vacuum freeze-drying for ginseng processing.On the basis of summary of research results and analysis of vacuum freeze-drying, a comprehensivenew tow-dimension model had been established, and the theoretical values of the model was close identicalwith experiment data, which provided theoretical references for thermal control of vacuum freeze-drying.The eutectic point temperature and melting point temperature of ginseng were measured respectivelyusing differential scanning calorimetry （DSC）. The DSC analysis indicated that the eutectic temperatureand melting point temperature were-16℃and-0.05℃respectively at heating and cooling rates of10℃/min. The eutectic point temperature of ginseng was-15℃measured by electric resistivity method,which was equal to DSC method. The measured results could be used for determination of technologicalparameters for vacuum freeze-drying procedure.Using FLUENT software the temperature field of the space between heating plate and the materialupper surface was simulated in the drying chamber, and the material could obtain much heating energy as itwas nearer to the heating plate, which led to a shorter drying time. As the temperature gradient of the fringeis higher than that of the center, so the material in the fringe section can get more energy and be dried morequickly.The procedure and state of thermal radiation between two parallel plates was studied, and calculationformula for effective and maximum effective transmission efficiency of heat receiving by plates duringradiation procedure was developed. The results showed that optimum temperature of receiving plate wasnot affected by material surface properties during the steady heat transfer process, and the temperaturebetween two parallel plates was nonconformity, the temperature of the receiver center was higher than that of the edge, the temperature gradient increased with the decrease of the radiation distance. Therefore, inorder to improve the uniformity of temperature within the material, the distance of the two plates should beenlarged.The change of temperature in ginseng in the frozen process was analyzed with ANSYS software, andthe results showed that simulation temperature field consistent with experimental data which provided ofaccurate judgment for frozen ending time.The temperature change in each point in Ginseng slice with time was analyzed in four differentheating modes by means of the ANSYS software; the heating modes were separable heating function,parabolic heating function, and exponential heating function, power heating function. The results showedthat separable heating function made the ginseng slice warming fastest. The result provided theoreticalreferences for increasing drying velocity and saving energy for freeze-drying process.Using quadratic regression experiment with general rotary combination design, regression equationsdescribing relations of criteria with ginseng slice, heating temperature and drying cabinet pressure, dryingvelocity, productivity and the area contraction ratio were developed. Based on regression equations andusing the method of nonlinear optimization, three optimal drying parameters were obtained as follows: Thehighest drying velocity was1.56h^-1with ginseng slice thickness4.8mm, heating temperature50.4℃anddrying cabinet pressure48.7Pa; the maximum drying productivity was111.7g/h·m²with ginseng slicethickness3mm, heating temperature60℃and drying cabinet pressure80Pa; the minimum area contractionratio was9﹪with ginseng slice thickness12mm, heating temperature40℃and drying cabinet pressure40Pa.Using BP neural network, vacuum freeze-drying process was simulated. The result showed that the BPneural network could precisely simulate vacuum freeze-drying process. After training of BP neural networkwith results of ginseng slice in orthogonal experiment, drying process conditions was predicted andoptimized, and the simulation results fitted well with experiment data. The results showed that the BPneural network had higher accuracy for prediction of drying procedure.更多还原