介电材料辅助的微波冷冻干燥的数值模拟

通过数值求解一个考虑吸湿效应的带有移动升华界面的多孔介质热、质传递耦合模型,理论考察介电材料对微波加热冷冻干燥过程的影响.介电材料用烧结的碳化硅(SiC).甘露醇,一种典型的药物赋形剂被选为待干溶液中的溶质.模拟结果表明在微波冷冻干燥过程中使用介电材料可以加快冷冻干燥速率,特别是在待干溶液的固含量很低或者固体产品的介电损耗因子很小的情况下尤为有效.模型预测和实验测定的干燥曲线相比较显示了良好的一致性.通过考察冰饱和度和温度的分布侧形,研究分析了物料内部的质热传递机理,并讨论了干燥速率的控制因素.     

吸湿多孔介质冷冻干燥传质传热模型

本研究在作者提出的吸附—解吸平衡关系的基础上,建立了一个全新的考虑吸湿效应的多孔介质冷冻干燥数学模型。模型用有限差分法进行求解,并带有一个移动边界,以模拟介电材料辅助的微波冷冻干燥过程。介电材料选用碳化硅（SiC）,原料液为脱脂奶。模拟结果表明：介电材料能够有效强化微波冷冻干燥过程。在典型操作条件下,介电材料辅助的微波冷冻干燥所用的时间比普通微波冷冻干燥减少33.1%。当料液中固体含量较低或者固体产品的损耗因子较小时,介电材料对微波加热的效果不明显。基于冰饱和度、温度和水蒸气浓度的分布,本文分析了干燥过程中的传质传热机理,并对干燥速率控制因素进行了讨论。     

吸波材料辅助的液体物料微波冷冻干燥多物理场耦合模型

为了研究吸波材料辅助微波加热对传统冷冻干燥过程的强化作用,建立了多孔介质温度、浓度和电磁场耦合的多相传递模型;以烧结的碳化硅(SiC)为吸波材料、以甘露醇水溶液为待干料液进行了微波冷冻干燥实验,并测定了甘露醇固体的介电特性。模拟和实验结果均表明,吸波材料对初始非饱和多孔物料微波冷冻干燥具有显著的强化作用。初始非饱和样品微波冷冻干燥时间比传统冷冻干燥缩短了18%,比常规饱和样品传统冷冻干燥缩短了30%。模拟结果与实验数据吻合良好。这表明提出的新型干燥方法确实能够实现过程传热传质的同时强化。通过考察样品内部温度、饱和度和电场强度的实时分布,分析了微波冷冻干燥过程的传热传质和电磁波传播与耗散机理。在微波冷冻干燥过程中,初始非饱和样品累计吸收的辐射能和微波能的总和与传统冷冻干燥相当。这说明,该干燥方法只是提高了能量效率,从而大幅缩短了冷冻干燥时间。     

吸波材料辅助的液体物料微波冷冻干燥多物理场耦合模型

为了研究吸波材料辅助微波加热对传统冷冻干燥过程的强化作用,建立了多孔介质温度、浓度和电磁场耦合的多相传递模型;以烧结的碳化硅(SiC)为吸波材料、以甘露醇水溶液为待干料液进行了微波冷冻干燥实验,并测定了甘露醇固体的介电特性。模拟和实验结果均表明,吸波材料对初始非饱和多孔物料微波冷冻干燥具有显著的强化作用。初始非饱和样品微波冷冻干燥时间比传统冷冻干燥缩短了18%,比常规饱和样品传统冷冻干燥缩短了30%。模拟结果与实验数据吻合良好。这表明提出的新型干燥方法确实能够实现过程传热传质的同时强化。通过考察样品内部温度、饱和度和电场强度的实时分布,分析了微波冷冻干燥过程的传热传质和电磁波传播与耗散机理。在微波冷冻干燥过程中,初始非饱和样品累计吸收的辐射能和微波能的总和与传统冷冻干燥相当。这说明,该干燥方法只是提高了能量效率,从而大幅缩短了冷冻干燥时间。     

介电物质对微波冷冻干燥影响的理论研究（二）

用有限差分方法计算的数值求解一个微波冷冻干燥质、热传递数学模型，以考察介电物质对微波冷冻干燥的影响。被干燥物料水溶液的溶质选用乳糖(Lactose)——一种典型的药物赋形荆。介电物质为烧结的碳化硅(SiC)。数值计算结果表明，介电物质能够有效地强化微波冷冻干燥过程，干燥时间大为缩短。在典型操作条件下，干燥时间为179．1min，比普通微波冷冻干燥节省43.4％。通过考察温度、冰饱和度、蒸汽质量浓度和压力分布，分析传质传热机理，确定了干燥速率控制因素。     

介电物质对微波冷冻干燥影响的理论研究（一）

根据Luikov法建立了一个介电物质强化的微波冷冻干燥质、热传递数学模型，以考察介电物质对微波冷冻干燥的影响。该模型采用更具有实际意义的柱坐标系，考虑了非饱和多孔冰区内的升华一凝华现象。干燥过程存在两个移动边界。     

具有预制孔隙的维生素C水溶液微波冷冻干燥

设计和组装了一套实验室规模的多功能微波冷冻干燥装置,探究了具有初始孔隙的非饱和物料微波冷冻干燥过程。以维生素C为溶质,采用"软冰"冷冻技术制备了初始饱和与非饱和的冷冻样品。结果表明,软冰冷冻制备的样品能够避免崩塌。在35℃和20 Pa条件下,初始非饱和物料的干燥时间比饱和物料缩短了30.4%。SEM表征显示,非饱和物料具有疏松的球状孔隙结构、连通性好,有利于水蒸气的迁移。采用吸波材料碳化硅辅助的微波加热能够进一步强化冷冻干燥过程。在相同条件下,非饱和物料的微波冷冻干燥(5 W功率)时间比常规冷冻干燥(0 W功率)缩短了28.1%,比饱和物料的常规冷冻干燥缩短了50.0%。吸波材料辅助的初始非饱和物料微波冷冻干燥实现了传热传质的同时强化。     

具有预制孔隙的维生素C水溶液微波冷冻干燥

设计和组装了一套实验室规模的多功能微波冷冻干燥装置，探究了具有初始孔隙的非饱和物料微波冷冻干燥过程。以维生素C为溶质，采用“软冰”冷冻技术制备了初始饱和与非饱和的冷冻样品。结果表明，软冰冷冻制备的样品能够避免崩塌。在35℃和20 Pa条件下，初始非饱和物料的干燥时间比饱和物料缩短了30.4%。SEM表征显示，非饱和物料具有疏松的球状孔隙结构、连通性好，有利于水蒸气的迁移。采用吸波材料碳化硅辅助的微波加热能够进一步强化冷冻干燥过程。在相同条件下，非饱和物料的微波冷冻干燥(5 W功率)时间比常规冷冻干燥(0 W功率)缩短了28.1%，比饱和物料的常规冷冻干燥缩短了50.0%。吸波材料辅助的初始非饱和物料微波冷冻干燥实现了传热传质的同时强化。     

吸波材料辅助的初始非饱和物料微波冷冻干燥

为了强化冷冻干燥过程,制备咖啡溶液的初始饱和与非饱和2种冷冻样品,分别使用石英与碳化硅底盘进行常规和微波冷冻干燥实验,同时采用电阻法测定溶液的共晶温度.结果表明,在温度为25℃、压力为22 Pa时采用石英底盘,初始非饱和物料冷冻干燥时间比饱和物料缩短了14.9％,适当提高操作温度可以强化冷冻干燥过程.在25℃、22 P...     

煤泥间歇微波干燥模拟及介电性质

微波干燥是一种提高煤泥干燥效率的先进技术，其中间歇式在能耗和超温问题上具有更大的改进潜力。在微波干燥过程中，煤泥的介电性质会显著影响干燥性能，然而当前缺乏介电性质影响煤泥微波干燥的机理研究。本文首先建立了一种适用于间歇微波干燥的多相多孔介质模型，通过与实验数据对比，验证了模型的有效性。由于介电性质实际是温度和水分含量的函数，从机理上着重分析了温度、水分含量相关的介电性质（VP）对煤泥平均温度、水蒸气质量分数、气相压力及液态水饱和度的影响，通过与以往处理成常数的介电性质（CP）相对比，结果发现，CP平均温度明显低于VP，并且温升速率在干燥过程中没有显著变化，而VP的平均温度表现出先急剧升高后维持稳定的趋势。CP水蒸气质量分数和气相压力分布虽然与VP分布趋势相同，但是CP明显低于VP。VP能够模拟出间歇微波干燥期间的“泵送效应”，而CP不能有效显示此现象。因此在煤泥间歇微波干燥中VP比CP更加符合实际。     

Effect of Dielectric Material on Microwave Freeze Drying of Skim Milk

Abstract:

A simultaneous heat and mass transfer model of the dielectric material–assisted microwave freeze drying was derived in this study considering the vapor sublimation-desublimation in the frozen region. The mathematical model was solved numerically by using the finite-difference technique with two moving boundaries. Silicon carbide (SiC) was selected as the dielectric material, and the skim milk was used as the representative solid material in the aqueous solution to be freeze-dried. The results show that the dielectric material can significantly enhance the microwave freeze drying process. The drying time is greatly reduced compared to cases without the aid of the dielectric material. Profiles of the temperature, ice saturation, vapor concentration, and pressure during freeze drying were presented. Mechanisms of the heat and mass transfer inside the material sphere were analyzed. For an initially unsaturated frozen sample of 16 mm in diameter with a 4-mm-diameter dielectric material core, the drying time is 288.2 min, much shorter than 380.1 min of ordinary microwave freeze drying and 455.0 min of conventional vacuum freeze drying, respectively, under typical operating conditions.     

Studies on the Microwave Freeze Drying Technique and Sterilization Characteristics of Cabbage

The vacuum freeze-drying (FD) technique used in the food industry can yield a high-quality product, but it is very expensive and requires a long processing time. Besides, the quantity of microorganisms in FD products can often exceed the required standard. As a result, it will be important to develop a new freeze-drying technique. In this article, cabbage was used as a model material, and the microwave field was used as a heat source to supply sublimation heat so that the drying time was shortened greatly. The effect of the microwave sterilization during the drying process was evaluated. Effects of the pressure, thickness of material being dried, and the input microwave power on such indices as drying time and the microorganism number were studied. Compared with the method of ordinary freeze drying, microwave freeze drying (MFD) can greatly reduce the drying time and has a notable sterilization effect.     

Studies on the Microwave Freeze Drying Technique and Sterilization Characteristics of Cabbage

The vacuum freeze-drying (FD) technique used in the food industry can yield a high-quality product, but it is very expensive and requires a long processing time. Besides, the quantity of microorganisms in FD products can often exceed the required standard. As a result, it will be important to develop a new freeze-drying technique. In this article, cabbage was used as a model material, and the microwave field was used as a heat source to supply sublimation heat so that the drying time was shortened greatly. The effect of the microwave sterilization during the drying process was evaluated. Effects of the pressure, thickness of material being dried, and the input microwave power on such indices as drying time and the microorganism number were studied. Compared with the method of ordinary freeze drying, microwave freeze drying (MFD) can greatly reduce the drying time and has a notable sterilization effect.     

Microwave Freeze Drying of Apple Slices Based on the Dielectric Properties

Freeze drying (FD) yields the best quality of dried apple slices but requires a long drying time and is not cost-effective. To mitigate these problems, a microwave freeze drying (MFD) technique was developed to dry apple slices. The relationship between corona discharge and microwave power at various pressures and initial moisture content conditions was studied to avoid the possibility of corona discharge during MFD. It was found that with change of moisture content and temperature of samples during MFD, the dielectric property also changed, which resulted in dynamic microwave dissipation. Based on the dielectric property of samples, a changed microwave loading scheme can lead to perfect product quality and greatly reduce the drying time; thus, MFD can be used to replace the traditional FD technique.     

Microwave Freeze–Drying Characteristics and Sensory Quality of Instant Vegetable Soup

Instant vegetable soup mix was dehydrated in a microwave freeze dryer to study the drying characteristics and sensory properties of the dried product. The mix was dried at different microwave power levels, material thicknesses, and material loads. As expected, microwave power significantly influenced the total drying time and sensory quality of the final product. The total drying time increased with the increase of material thickness and load. A material layer that is too thin causes the product quality to deteriorate.     

Microwave Freeze Drying of Sea Cucumber Coated with Nanoscale Silver

To develop an improved dehydration method for sea cucumber, microwave freeze drying was tested as a potential method. According to our experimental results, microwave freeze drying can reduce drying time to about half of the traditional vacuum freeze-drying process. To ensure a high degree of sterilization, a novel nanoscale silver coating technique was combined with microwave freeze drying. Microwave freeze drying combined with nanoscale silver coating treatment leads to a much lower microorganism number with no significant effect on drying efficiency and sensory quality.     

Microwave Freeze Drying of Sea Cucumber Coated with Nanoscale Silver

To develop an improved dehydration method for sea cucumber, microwave freeze drying was tested as a potential method. According to our experimental results, microwave freeze drying can reduce drying time to about half of the traditional vacuum freeze-drying process. To ensure a high degree of sterilization, a novel nanoscale silver coating technique was combined with microwave freeze drying. Microwave freeze drying combined with nanoscale silver coating treatment leads to a much lower microorganism number with no significant effect on drying efficiency and sensory quality.     

Microwave Energy Absorption Behavior of Foamed Berry Puree Under Microwave Drying Conditions

The microwave energy absorption behavior of foamed berry puree was studied considering the variations in the physical properties of the material. The absorptions of microwave energy within foamed berry puree indicated fluctuating trends in decay as microwave drying progressed, which were attributed to the effects of temperature, moisture content, and density of the material. Mathematical models of the dielectric constant ?′ and dielectric loss factor ?″ as a function of temperature and moisture content were developed using second-order functions. During microwave drying of foamed berry puree, moisture content had the most important negative influence on the dielectric constant and dielectric loss factor, followed by temperature and porosity.     

构建微波冷冻干燥质扩散系数的一种新方法

针对现有微波冷冻干燥模型中必须依靠实验以确定扩散系数的问题．提出一种构建扩散系的新方法。根据毛细管低压气体输运理论，利用物料本身的细微结构、气体分子平均自由程、气体状态参数和气体物性参数来构建扩散系数，在此基础上建立微波冷冻干燥模型。并结合牛肉的微波冷冻干燥行模型可靠性分析。结果表明，新建模型能够很好地描述微波冷冻干燥过程温度场的变化及其特征．升过程干燥曲线的理论值与实验值之间的相对误差小于10％。     

具有电介质核圆柱多孔介质微波冷冻干燥过程的双升华界面模型

建立了具有电介质核多孔介质微波冷冻干燥过程的耦合传热传质的数学模型,应用变时间步长的有限体积法对各控制方程进行数值求解.计算结果表明：（1）多孔介质内部存在着两个升华界面;（2）同无核相比,合理选用电介质核可大大缩短干燥时间;（3）在初始饱和度较低时（S₀=0.2）,有、无电介质核两种情况下所需干燥时间相差较大,仍可在物料中加入电介质核来加速干燥.