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发表于 2016-9-17 20:06:47
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 Figure 3. The product experiences several stages during drying. Elucidating these stages can help identify the major resistances to drying and enable better drying optimization.
Predicting the Drying CycleAn accurate prediction of commercial drying cycle time can be valuable for estimating manufacturing costs. The first step in making such a prediction is to determine the major resistances to drying. This involves performing a simple vacuum-oven drying study by monitoring the solvent content of the cake over the course of drying. For completeness, the test should be performed on several samples of varying cake thickness. For the test to be most meaningful, the sample used must be representative of the final process material.
Several types of drying may occur during a drying cycle. As the conditions in the dryer approach the boiling point of the solvent, the first solvent to be removed is the free, unbound solvent wetting the surface of the solids. This is removed at a fairly constant rate, limited primarily by the heat transfer rate, until the surface is no longer saturated. Once surface solvent is removed, then solvent that is trapped in interstitial spaces and micro-capillaries in the crystals is removed. Evaporation of this solvent is slower because additional energy is required to overcome capillary-attractive forces. Next, solvent that is completely trapped in vacuoles may be removed, but the rate of removal is very slow and limited by diffusion. It is best to assume that this solvent will not be completely removed, nor will solvent that is part of the molecular crystal lattice.
The drying study mentioned above can give a good indication of where the major resistances to drying occur. The solvent content vs. time data can be plotted directly, but it is more useful plotted as drying rate vs. time, or as drying rate vs. solvent content (figure 3). The period where the surface moisture is removed is called the constant-rate period, up to the point where there is no longer sufficient solvent to make a continuous layer over the surface (called the critical moisture content). This is followed by the falling-rate period, characterized by an ever-changing drying rate as, first, the solvent from the unsaturated surface is removed, followed, in turn, by the various components of the internal solvent. Often the falling-rate period dominates the drying cycle. It is a case of diminishing returns, which is why a reasonable drying specification is so important.
Many actual drying curves may not appear to fit this model well. Some product wet cakes may come out of the filter already below the critical moisture content, and then the entire drying cycle will consist of falling-rate drying. This is one reason why it is valuable to know the critical moisture content. In other cases, a short period of rapid solvent removal may occur, followed by a settling into the falling-rate period. This is most likely to occur if a product cake is placed in a preheated dryer and then the vacuum applied.
The results of laboratory drying studies can provide a great deal of useful information. For example, if the constant-rate period predominates in the bench test, then it will likely predominate at scale. Agitated dryers may then offer the shortest drying times by increasing the effective surface area of the cake exposed for heating and drying. Tray dryers may not be as advantageous because increased cake depth in larger-scale units means decreased heating surface area per unit mass.
If the falling-rate period dominates, it means that the process is diffusion limited, and this period will likely dominate at scale. In such cases, agitated dryers such as orbiting screw cones or combination filter dryers may offer advantages by increasing the surface area for diffusion by particle attrition.
More rigorous treatment of the theoretical aspects of drying and calculations of energy requirements, etc., can be found in standard chemical engineering references such as Unit Operations (McCabe & Smith) or Perry’s Chemical Engineering Handbook.
大家实际在实验室阶段是否会去做simple vacuum oven test呢？？？将时间，含湿量作图找到critical moisture content???然后根据这些来预估drying cycle???
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发表于 2016-9-17 19:23:47
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Figure 1. A number of dryer types are available for pharmaceutical applications, most of which are conductive (or contact) dryers, in which the product directly contacts the heated dryer surface.
