【作者】 彭润玲；

【导师】 徐成海；

【作者基本信息】 东北大学 ， 流体机械及工程， 2007， 博士

【摘要】 生物材料微尺度超常传热传质特性的研究是当前工程热物理领域传热传质学科研究的前沿课题之一。真空冷冻干燥(简称冻干)过程是低温低压两个特殊物理条件下的热质耦合传递过程。控制好生物材料冻干过程中的热质传递,可以使其生命特征和营养成分保持不变。本文以实验为依据,选择有独特传热传质特性的冻干过程为研究环境,选用具有植物特性单列细胞组成的螺旋藻、具有动物特性且组织结构复杂的海参和具有生物活性的纳豆激酶三种有高级营养保健功能的生物材料为主要研究对象,在以往研究宏观工艺参数对冻干过程传热传质影响的基础上,从微尺度出发,研究生物材料冻干过程传热传质特性,不仅可以为研究生物细胞超常传热传质理论打下基础,而且可以为生命科学、航天、医学和医疗事业的发展提供一些理论依据。生物材料的热物性参数是研究生物材料冻干过程热质传递本质规律和计算的重要数据。因此,本研究首先测量了三种物料的热物性参数,螺旋藻共晶点温度为-16℃,共熔点温度为-1.5℃,凝固潜热260J/g,熔化潜热-268J/g；纳豆激酶共晶点温度为-23℃,共熔点温度为-15℃,鲜海参肉共晶点温度为-35℃,海参打浆液共晶点温度为-25℃,海参浓缩液共晶点温度为-30℃。根据螺旋藻已干层的显微照片利用计算机图像处理软件得到了螺旋藻已干层分形多孔介质的分形维数和谱维数分别为1.722和1.384,从而建立起了联系生物材料冻干过程中其微观静态结构和动态传热传质特性的一个中间桥梁。预冻是冻干的前提,由于抽真空自冻结可以节能,降低冻干成本,有利于保证产品的质量,因此,本文重点研究了抽真空自冻结,通过大量实验得出了不同物料抽真空自冻结的条件和抽真空自冻结过程中传热传质的特性。抽真空自冻结过程是传质引起传热的一种高效自冻结过程,不同于搁板和冰箱冻结,传质是原动力,影响冻结速率和冻结温度的因素主要是物料本身的性质、尺寸和真空室压力。抽真空自冻结的优点在于不需要冷源,冻结速率快,冻结后生物材料细胞的体积收缩小,冻结过程同时又是除去物料中一部分水的过程,有利于进一步干燥。不同生物材料由于其显微组织结构不同,冻干过程的传热传质特性不同。纳豆激酶微观组织结构简单,没有完整的细胞结构,物料结构单一均匀,容易通过控制宏观参数控制冻干过程物料内部的热质传递,冻干后的活力为152.2U(10mg/ml)。螺旋藻由不分支的单列细胞组成,由于细胞膜的存在,物料内部的热质传递比较复杂,无法通过宏观参数控制物料内部的热质传递,在冻干过程中要保证细胞结构完整性和活性需加保护剂,平衡冷冻和干燥过程细胞内外液之间的热质传递。当采用稀培养液或10%牛奶为保护剂时,冻干后螺旋藻细胞收缩小,完整性好,几乎与新鲜螺旋藻一样,有利于保存其细胞活性。海参由于细胞组织结构复杂,在冻干过程中很难控制内部细胞间的热质传递,冻干后的收缩率一般都较大。整个海参的冻干当采用辐射加热时,可增加换热面积和水蒸气的逸出通道,有利于冻干过程热质传递,提高冻干效率。要保存海参细胞的活性,即使是加保护剂也很困难,因为海参不同部位的细胞所需的保护剂可能都不同,另外,考虑到其实际应用价值是其营养价值,因此,本文主要研究了冻干过程对海参营养成分的影响。冻干海参浓缩液可回收煮参过程流失的水溶性营养成分。将海参打浆后冻干,不仅完整的保留了海参的营养成分,且可做成粒径几微米到几百纳米的超细粉末,便于各种人群食用消化吸收,还可控制装料厚度,以便控制冻干过程的热质传递,提高冻干效率。本文在考虑细胞和冰界面之间的耦合传热传质、膜的传输特性和凝固界面的移动过程的情况下,建立了螺旋藻细胞冷冻过程冰界面与细胞之间热质传递的微尺度模型,借助Matlab软件计算了螺旋藻细胞被冰界面包围过程中细胞内外的温度场和浓度场,研究了螺旋藻细胞被冰界面包围过程中细胞体积的收缩情况及影响因素。螺旋藻当以5℃/min冷却速率冷冻时,不在-25℃左右停留,以10℃/min冷却速率冷冻时,不在-45℃左右停留,细胞体积收缩率较小,对细胞结构造成的损伤较小,有利于保持螺旋藻细胞的活性。不同生物材料的冻干过程,都包括低温低压下已干层多孔介质中的热质传递过程。实际的多孔介质一般具有分形的特点,因此可将分形理论应用到研究生物材料冻干过程传热传质特性的研究中来,用分形理论解释生物材料冻干过程热质传递的复杂性,在宏观参数相同的情况下,不同的生物材料,其微观组织结构一般都不一样,冻干时形成的已干层的空隙分形维数和谱维数就不同,从而扩散特性不同,热质传递特性不同。本文建立了生物材料冻干过程热质传递的分形模型,并以螺旋藻为例,模拟了螺旋藻的冻干过程,计算结果直观可靠,物理意义明确,且与实验结果相一致,证明了分形理论模型是合理的。更多还原

【Abstract】 Research on microscale supernormal heat and mass transfer of biological materials is one of the top academic studies in current engineering thermophysics. Vacuum freeze drying (lyophilization) process is coupling heat and mass transfer process under two special physical conditions of lower temperature and lower pressure. The biological cell would survive and the active nutritional ingredient would be preserved after freeze drying process if heat and mass transfer could be controlled properly during the process. This paper is based on experiments, using freeze drying as the research environment, which has unique heat and mass transfer characteristics.Three types of biological materials which have exclusive nutrition and health functions are chosen as the main research objects, i.e. spirulina platensis, a plant constructed by monostichous cells, stichopus japonicus, which is an animal constructed by the complex structure, and nattokinase which has biological activity. This study researches heat and mass transfer properties during the freeze drying process starting from microscale structure of biological material, based on the previous studies by controlling macroparameters. This study will not only form the theoretical basis of supernormal heat and mass transfer study on the biological cells, but also contribute to the theoretical development of life sciences, aerospace, medicine and health undertakings.The thermophysical parameters of biological materials are important data to study the essential heat and mass transfer laws of biological materials during freeze drying process. Therefore, thermophysical parameters of the three materials were measured beforehand. Spirulina platensis’s eutectic and melting point temperature are-16℃and-1.5℃, respectively, and its solidification and melting latent heat are 260J/g and 268J/g, respectively. Nattokinase solution’s eutectic temperature and melting point temperature are-23℃and-15℃,respectively. The eutectic temperature of fresh stichopus japonicus, stichopus japonicus pulp and concentrated liquid of stichopus japonicus boiling water are-35℃,-25℃and-30℃,respectively. The dried layer formed during the freeze drying process of spirulina platensis is fractal porous medium. It can be concluded that the fractal dimension and spectrum are 1.722 and 1.384, respectively by analysing the microscopic photograph of the dried layer through computer image processing software. Thus, an intermediate bridge was established to link the static microscopic structure and dynamic heat and mass transfer characteristics of the biological materials during freeze drying. Pre-freezing is a prerequisite to accomplishing freeze drying. Because self-freezing in increasing vacuum can save energy, reduce cost of the freeze drying, and guarantee product quality, this paper focused on self-freezing in increasing vacuum. Through a large number of experiments, we obtained self-freezing conditions and heat and mass transfer characteristics of different materials in increasing vacuum.As a highly efficient self-freezing process, heat transfer is generated by mass transfer. Self-freezing in increasing vacuum is different from the shelf and refrigerator freezing in that its driving force is mass transfer, the influence factors of the freezing rate and temperature are the characteristic and size of materials, and vacuum chamber pressure.Self-freezing in increasing vacuum is superior to other freezing, because it does not require cold source, has a faster freeze rate, and gets the frozen biological material cells with smaller shrinking. In the meantime part of water in the material is removed during self-freezing in increasing vacuum process, which will benefit further drying.Since microscopic structures of different biological materials are diverse, their heat and mass transfer characteristics during freeze drying process are different. Nattokinase has a simple microstructure, an incomplete cell structure and a single homogeneous structure in material, so the internal heat and mass transfer of material can be controlled by manipulating the macro-parameters of the freeze drying process, with the vitality of freeze-dried Nattokinase being 152.2U(10mg/ml).Spirulina platensis is constructed by monostichous cells. Because of the existence of pericellular membrane, the internal heat and mass transfer in Spirulina materials is more complicated and can’t be controlled only by macro-parameters of the freeze drying process. Protective agent is needed to balance the heat and mass transfer between the intracellular and extracellular solution in order to ensure the structural integrity and activity of spirulina cell during freeze drying process. Using diluted culture solution or 10% milk as the protective agent during the freeze drying process, the freeze dried spirulina cell shrinkage is small and the structure is almost the same as fresh spirulina, which is conducive to preserving spirulina cell activity.Stichopus japonicus has complicated organizational structure, whose internal heat and mass transfer during freeze drying is more difficult to control, resulting in greater shrinkage. Using radiation heating during the freeze drying of stichopus japonicus, heat transfer area and the escape channel of water vapor can be increased, so the freeze drying efficiency can be improved. It is very difficult to preserve stichopus japonicus cell activity even if the protective agents are added, because different parts of the stichopus japonicus differ from each other, so the protective agents should be different correspondingly, In addition, taking into account of its practical nutritional value, we only study the effect of freeze drying process on the nutrient composition. Freeze drying concentrated liquid of stichopus japonicus boiling water can recover water-soluble nutrient composition lost during the boiling process. If stichopus japonicus is freeze-dried after beaten to pulp, not only its integrity nutritional content can be retained, but it can be made to ultrafine powders whose particle diameter ranges from several micrometers to several hundred nanometers, so it can facilitate digestion and absorption by all types of consumers. And the loading of thickness can be changed, making it convenient for controlling heat and mass transfer during freeze drying process and improving freeze drying efficiency.The microscale numerical model about interaction between ice and spirulina cells in the freezing process was presented, considering coupled heat and mass transfer, transmission characteristics of membrane and translocation of solidification interface. The concentration and temperature fields both inside and outside cell during the freezing process were computed by Matlab software.Spirulina cell volume contraction and influencing factors were studied during the process of cells being surrounded by the ice interface. When the cells are frozen by 5℃/min cooling rate, its temperature should not stay around-25℃,and they are frozen by 10℃/min cooling rate, its temperature should not stay around-45℃,cell shrinkage is small and cell structural damage is less, which are conducive to maintaining spirulina cell activity.Freeze drying process of different biological materials is essentially heat and mass transfer in dried layer porous medium at lower temperature and lower pressure.Actual porous medium generally has fractal characteristics, so fractal theory can be applied to researching heat and mass transfer characteristics of biological materials during the freeze drying process, using fractal theory to explain the complexity of heat and mass transfer characteristics of the biological materials during freeze drying process. The fractal dimension and spectral dimension of the fractal porous medium in the dried layer formed during freeze drying process of biological materials are different, because different biological materials have different microstructure, even in the same macro parameters, so the heat and mass transfer characteristics are diverse. A fractal numerical simulation model of heat and mass transfer during the freeze drying process of biological materials was presented. The freeze drying process of spirulina was simulated by fractal model, and an example is given. Fluent and Matlab software are adapted for the heat and mass transfer calculation of spirilina, and the calculated results are intuitively reliable, and consistent with the experimental results, so the fractal theory model is proved to be valid.更多还原