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Freeze Drying of Aluminum Nanoflakes: Process Optimization and Modeling.

机译：铝纳米片的冷冻干燥：工艺优化和建模。

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摘要

Freeze drying is a process wherein a moisture-containing product is frozen and dehydrated while under vacuum. The lack of a liquid-vapor interface during drying means that capillary forces are not present. This is useful for the production of dry, monodipsersed nanoparticles as capillary forces are known to be a major driving force in formation of particle aggregates during drying. Freeze drying is an industrially viable process, although there is currently little published work related to the drying of nanoparticles.;The presented work focuses on optimizing and modeling the freeze drying of aluminum nanoflakes. Four processing parameters such as the choice of carrier were investigated to determine their effects on maximizing both the dispersibility of the dried aluminum flakes and the overall drying rate. A series of experimental drying runs were carried out varying the selected parameters with the dispersibility of the aluminum powders being approximated by measuring their mass extinction coefficients (MEC). Flakes dried from cyclohexane displayed a sizable increase in dispersibility (MEC of 6.0 m2/g) when compared to flakes dried via evaporation (MEC of 2.0 m2/g). Aluminum samples dried from cyclohexane also showed less aggregation than flakes dried from tertiary butanol (MEC of 5.0 m2/g) or water (MEC of 4.0 m2/g). This was significant as cyclohexane is not a solvent that is traditionally used in freeze drying. Additional experimental results showed that the optimum choices for the other processing parameters, aluminum concentration, freezing temperature, and shelf temperature, were influenced by the choice of carrier. For example, aluminum flakes dried from cyclohexane showed greater dispersibilities when low aluminum concentrations and low shelf temperatures were selected (MEC of 6.0 m2/g) than when high concentrations and high temperatures were employed (MEC of 3.5 m2/g). Tertiary butanol and water systems showed the opposite behavior and preferred the latter set of conditions (MEC of 5.0 m2 /g for butanol and 4.5 m2/g for water) to the former (MEC of 3.5 m2/g for butanol and 2.5 m2/g for water).;Drying rates were measured by placing a balance inside the drying chamber during the drying cycle. Experiments showed that factors which increased the equilibrium vapor pressure of the frozen solvent such as drying at higher temperatures led to increased drying rates. Maximum drying rates were much greater for cyclohexane based samples (67 mg/min) than for water based samples (10 mg/min). Observations of the drying rates also led to the conclusion that cyclohexane and tertiary butanol systems were mass transfer limited processes. This is not typical for most water based systems.;A one-dimensional mathematical model was developed to describe the freeze drying process in this study. Several assumptions were made to reduce the model to a steady state form so that an analytical solution could be obtained. Comparisons between the model and experimental results showed that the model was able to predict the sublimation interface velocity with relative accuracy (differing by no more than a factor of two) for the non-transient portions of the primary drying cycle. An analysis of the model parameters led to the conclusion that the shelf temperature and the vapor permeability of the porous region were the two major controlling factors in determining the rate of freeze drying for metallic nanoparticle systems.
机译：冷冻干燥是其中在真空下将含水产品冷冻并脱水的过程。在干燥过程中缺少液体-蒸汽界面意味着没有毛细作用力。这对于生产干燥的，单分散的纳米颗粒很有用，因为已知毛细作用力是干燥过程中形成颗粒聚集体的主要驱动力。冷冻干燥是工业上可行的过程，尽管目前很少有与纳米颗粒干燥有关的已发表工作。提出的工作集中在优化和建模铝纳米薄片的冷冻干燥上。研究了四个加工参数（例如载体的选择），以确定它们对最大化干燥铝片的分散性和总体干燥速率的影响。通过改变所选参数进行一系列实验干燥操作，通过测量铝粉的质量消光系数（MEC）估算铝粉的分散性。与通过蒸发干燥的薄片（MEC为2.0 m2 / g）相比，用环己烷干燥的薄片表现出可分散性的大幅提高（MEC为6.0 m2 / g）。从环己烷干燥的铝样品也显示出比从叔丁醇（MEC为5.0平方米/克）或水（MEC为4.0平方米/克）干燥的薄片更少的聚集。这很重要，因为环己烷不是传统上用于冷冻干燥的溶剂。其他实验结果表明，其他工艺参数（铝浓度，冷冻温度和搁板温度）的最佳选择受载体选择的影响。例如，与选择高浓度和高温（MEC为3.5 m2 / g）相比，选择低铝浓度和低贮存温度（MEC为6.0 m2 / g）时，用环己烷干燥的铝片显示出更大的分散性。叔丁醇和水系统表现出相反的行为，并且优先选择前者（丁醇的MEC为3.5平方米/克，水为2.5平方米/克，MEC为3.5平方米/克，水为4.5平方米/克）。干燥速率是通过在干燥周期内在干燥室内放置天平来测量的。实验表明，增加冷冻溶剂平衡蒸气压的因素（例如在较高温度下干燥）导致干燥速率增加。环己烷基样品的最大干燥速率（67 mg / min）比水基样品的最大干燥速率（10 mg / min）大得多。对干燥速率的观察还得出结论，即环己烷和叔丁醇系统是传质受限的过程。对于大多数水基系统而言，这不是典型的情况。；本研究开发了一维数学模型来描述冷冻干燥过程。进行了一些假设，以将模型简化为稳态形式，从而可以获得解析解。模型与实验结果之间的比较表明，该模型能够针对一次干燥循环的非瞬态部分以相对准确度（相差不超过两倍）预测升华界面速度。对模型参数的分析得出的结论是，架子温度和多孔区域的蒸汽渗透性是决定金属纳米粒子系统冷冻干燥速率的两个主要控制因素。

著录项

  • 作者

    Schaefer, Justen R.;

  • 作者单位

    Clarkson University.;

  • 授予单位 Clarkson University.;
  • 学科 Engineering Chemical.
  • 学位 Ph.D.
  • 年度 2012
  • 页码 220 p.
  • 总页数 220
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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