初始非饱和液体物料冷冻干燥二维数值验证

为提高过程经济性,对初始非饱和液体物料冷冻干燥这一思想进行二维数值验证。建立含湿多孔介质冷冻干燥二维柱坐标数学模型,使用有限体积法对控制方程组进行数值求解。模拟结果显示,初始非饱和冷冻物料确实能够有效减少干燥时间,达到强化冷冻干燥过程的目的,干燥时间随着初始孔隙率的增大而减小。对模拟得到的不同时刻物料内部饱和度和温度的侧形进行分析。     

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。     

具有初始孔隙的多孔物料冷冻干燥

实验研究了具有一定孔隙的非饱和多孔物料对液体物料冷冻干燥过程的影响。以甘露醇为主要溶质的待干料液采用“液氮制冰激凌法”制备非饱和物料进行冷冻干燥,并与常规饱和的冷冻物料相比较。结果表明,非饱和冷冻物料确实能够显著地强化液体物料的冷冻干燥过程。干燥产品SEM形貌分析显示,初始非饱和冷冻物料具有连续均匀的固体骨架和孔隙,固体基质更加纤细,孔隙空间更大,可以大大减小传质阻力。考察物料内部各点的温度变化发现,初始非饱和物料内部冰晶确实发生整体升华,但仍然存在主要升华区域;非饱和多孔物料的冷冻干燥过程主要是传热控制,而常规饱和物料冷冻干燥主要是传质控制。操作压力对过程的影响可以忽略。采用辐射/导热组合加热方式可改善初始非饱和多孔物料冷冻干燥过程的传热,进一步缩短干燥时间。     

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。     

径向热质传递对瓶装物料冷冻干燥的影响

基于经验证了的二维冻干模型对瓶装物料的冷冻干燥过程进行了数值计算 ,利用计算结果对径向热质传递的影响进行了分析 .结果表明 :瓶侧面的径向传热影响显著 ,但忽略水蒸气在径向上的传递作用不会给模拟计算结果带来明显误差 .     

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...     

真空冷冻干燥非稳态热质传递模型的应用

真空冷冻干燥非稳态热质传递模型的应用齐锡龄１杜小泽２李惟毅１（１天津大学热能工程系，天津３０００７２）（２清华大学热能工程系，北京１０００８４）关键词冷冻干燥热质传递升华压力１引言通过理论计算确定真空冷冻干燥速率或干燥时间对指导生产实践很有意义。但是...     

物料干燥过程—传质传热最优化方法的研究

论述了多孔介质干燥过程中传质传热的数学模型及干燥过程最优化准则的建立；并且动用控制理论产物料干燥过程传热的动态优化方法－－准则约束法，且得到了最优解。     

初始饱和度对微波冷冻干燥过程传热传质的影响

以非饱和含湿牛肉为例,进行了微波冷冻干燥实验研究。获得了干燥时间与物料初始饱和度近似成正比的结论。与升华面模型的计算结果比较表明,升华冷凝模型更符合实验规律,证实了非饱和含湿多孔介质微波冷冻干燥时升华冷凝区的存在。     

冷冻干燥过程中重要参数塌陷温度的研究

用冻干显微镜和差示扫描量热仪测定了氯化钠盐溶液、蔗糖水溶液、海藻糖水溶液和甘露醇水溶液的塌陷温度和玻璃化转变温度,研究两者之间的关系。实验表明,塌陷温度一般都比相应的玻璃化转变温度高5℃左右;质量浓度越大,降温速率越小,则塌陷温度越大;不同物质,分子量越大,则塌陷温度越大。塌陷温度是在真正的冷冻干燥过程中测得的,更能真实反映产品的特性,较玻璃化转变温度和共晶熔融温度对冷冻干燥更具指导意义。所以,以塌陷温度作为临界温度,可以提高升华干燥速率,缩短冻干时间。     

水溶液的冷冻干燥问题(英文)

Freeze drying or lyophilization of aqueous solutions is widely used in pharmaceutical industry.The increased importance of the process is gaining a worldwide interest of research.A growing body of literature has demonstrated that the scientific approach can result in improved product quality with minimum trial and error empiricism.Formulation and process development need a systematical understanding of the physical chemistry of freezing and freeze drying,material science and mechanisms of heat and mass transfer.This paper presents an overview on freeze drying of aqueous solutions based on publications in the past few decades.The important issues of the process are analyzed.     

On the Influence of Glass Transition on Shrinkage in Convective Drying of Fruits: A Case Study of Banana Drying

The ideal shrinkage model assumes that the extent of shrinkage is equal to the volume of liquid water removed from the dried medium. Generally if a material undergoes glass transition during the drying process, shrinkage will no longer be ideal. The aim of this study was to observe how the glass transition temperature influences the shrinkage kinetics. Cylindrical banana samples were dried. Shrinkage extent was significant for all drying conditions (temperature: 30–50°C, relative humidity: 0–80%). Deviation from linearity was found to be affected not only by drying air temperature but also by its relative humidity.     

On the Influence of Glass Transition on Shrinkage in Convective Drying of Fruits: A Case Study of Banana Drying

The ideal shrinkage model assumes that the extent of shrinkage is equal to the volume of liquid water removed from the dried medium. Generally if a material undergoes glass transition during the drying process, shrinkage will no longer be ideal. The aim of this study was to observe how the glass transition temperature influences the shrinkage kinetics. Cylindrical banana samples were dried. Shrinkage extent was significant for all drying conditions (temperature: 30-50°C, relative humidity: 0-80%). Deviation from linearity was found to be affected not only by drying air temperature but also by its relative humidity.     

Analysis of Heat and Mass Transfer in Freeze Drying

The main objective of this work is to build a mathematical model that describes heat and mass transfer in freeze-drying when both plate heating and radiation heating are applied and also to provide further understanding of the mechanism of the drying process. The model, unlike other models, may be used for situations in which sublimation occurs within a temperature range, i.e., the non-existence of a sharp interface and also for cases in which more than one interface may form. The developed model has been tested against experimental measurements of freeze-drying of milk under different operating conditions. Measurements were done using Virtis BT3.3ES freeze dryer with vertical manifolds. The milk was contained in a glassware, specially designed for this project. Four thermocouples were fixed at different positions to track the drying progress. The experimental measurements show no significant shrinkage in the frozen milk when dried, leaving the milk highly porous in structure. In this experimental work, the low thermal conductivity of the dried layer was found to control the process without any significant mass transfer resistance. This includes plate heating where drying was found to progress from the heating surface similar to radiation heating. This is unlike what has been reported in some of literature that drying starts always from the top surface. The model, which was based on heat transfer control, showed a reasonable agreement with the experimental measurements of both plate heating and radiation heating.     

Mathematical Modeling of the Fluidized Bed Drying of Cubic Materials

The unsteady‐state simultaneous heat and mass transfer between gas and potato cubes during the drying process in a batch fluidized bed was described by a mathematical model. Mass transfer was considered to occur in three dimensions whereas heat transfer between the gas and dried material was assumed to be lumped. It was found that the model could describe the drying process with acceptable accuracy. The moisture profile inside the material at any cross‐section and at any time can be predicted by the model.     

A Review of Drying Models Including Shrinkage Effects

ABSTRACT

This article presents a review of the different strategies used in the literature to model drying processes in which material shrinkage occurs. An analysis is provided of the different drying theories, the different approaches to deal with the shrinkage phenomenon, and the numerical methods used. It is common practice to directly apply the transfer models applicable for non-shrinking situations to the shrinking cases and to adopt a separate procedure to account for shrinkage. In addition, the use of effective transfer coefficients also abounds in the literature. Thus, coupling between the transfer processes and mechanical interactions is rarely considered.     

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.     

A Review of Drying Models Including Shrinkage Effects

This article presents a review of the different strategies used in the literature to model drying processes in which material shrinkage occurs. An analysis is provided of the different drying theories, the different approaches to deal with the shrinkage phenomenon, and the numerical methods used. It is common practice to directly apply the transfer models applicable for non-shrinking situations to the shrinking cases and to adopt a separate procedure to account for shrinkage. In addition, the use of effective transfer coefficients also abounds in the literature. Thus, coupling between the transfer processes and mechanical interactions is rarely considered.     

Characterization of the Surface Stickiness of Fructose-Maltodextrin Solutions During Drying

A probe tack test has been used for the in situ characterization of the surface stickiness of hemispherical drops with an initial radius of 3.5 mm while drying. Surface stickiness of drops of fructose and maltodextrin solutions dried at 63°C and 95°C was determined. The effect of addition of maltodextrin on fructose solution was studied with fructose/maltodextrin solid mass ratios of 4:1, 1:1, and 1:4. Pure fructose solutions remained completely sticky and failed cohesively even when their moisture approached zero. Shortly after the start of drying, the surface of the maltodextrin drops formed a skin, which rapidly grew in thickness. Subsequently the drop surface became completely nonsticky probably due to transformation of outer layers into a glassy material. Addition of maltodextrin significantly altered the surface stickiness of drops of fructose solutions, demonstrating its use as an effective drying aid.     

Convective Drying Kinetics and Darkening of Okara

Okara pellets were dried in a pneumatic tube from 78% of moisture content (w.b.) to 64% and then in a rotational drum to 3%. Time, temperature, and drum rotation were correlated to the okara darkening. The temperatures used were 130, 150, and 170°C in the pneumatic tube and 50, 60, and 70°C in the rotational dryer. The rotations used for the drum were 27 and 47 rpm. When okara was dried only in the pneumatic tube it became dark; however, when dried in both the tube and the dryer the darkening level decreased significantly. The results showed that the first drying level temperature does not influence the drying time of the combined process.