Batch Fluid Bed Dryers

In the category of fluid bed dryers, there are two types of processes commonly used to suspend the material—pneumatic and mechanical fluidization. 

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Figure 12-75 shows a sketch of a typical batch fluid-bed dryer as used in the food and pharmaceutical industries. The process chamber is conic in order to create a freeboard velocity in the upper part of the chamber that is lower than the flnidizing velocity just above the plate. 

Due to the tacky nature of the product, one should begin the dryer selection process with a mechanically agitated style and proceed to test various types from there. If the material were free-flowing, a batch fluid bed dryer with an operating volume of 6.25 fl would be ideal. 

FIGU RE 3.20 Temporal changes of solid moisture content and temperature and exit air humidity and temperature in a sample batch fluid bed dryer. Bed diameter 0.6 m, bed height 1.2 m, particle diameter 3 mm, particle density 1200 kg/m, air temperature 150°C, and humidity 1 g/kg. 

High-quality standards and high price of the final product demand high reliability and justify more sophisticated construction of pharmaceutical dryers than dryers used in most other industries. Traditional dryers of the pharmaceutical industry, such as shelf or batch fluid bed dryers, are continuously supplemented by dryers first used in other fields of application. Increasing quantities of products may call for continuous dryers. Increased quality control and larger... 

FIGURE 33.3 Batch fluid bed dryer (1, fluidizing chamber 2, gas distributor 3, plenum chamber 4, blower 5, heater 6, filters 7, bag dust collector). 

FIGURE 33.23 Closed-cycle batch fluid bed dryer (1, drying chamber 2, bag filter 3, blower 4, heater 5, condenser 6, pump 7, product collector). 

Sundaramoorthy and Rao designed and implemented a DDC scheme on a batch fluid-bed dryer, which was drying wet sawdust [32]. The system was described by a state-space model with parameters that were estimated from experimental data. The performance of the designed controller was checked by closed-loop simulation and then the scheme was implemented online using the heater power supply as the final control element to regulate the inlet-air temperature. 

Alden et al. extended the method by estimating the equilibrium relative humidity based on the wet-bulb temperature (TJ, the partial pressure of water at the solid s temperature, and the saturated vapor pressure of water at 7 [46]. Thus, the moisture content of a given material can be inferred from the estimated temperature difference (AT) and the equilibrium relative humidity. This method has been successfully implemented to control the endpoint in batch fluid-bed dryer for aqueous and aqueous-alcohol granules. An pirical method was used to determine the desired AT value by frequent sampling of the dried material and moisture content determination. The realtime AT was estimated using a computer program based on the temperature measurements this value was then compared with the desired value and used as the drying-endpoint indicator. 

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