Fluid bed drying

Drying is usually understood to be removal of moisture or solvent. Drying involves heat transfer and mass transfer. Heat is transferred to the product to evaporate liquid, and mass is transferred as a vapor in the surrounding gas hence, these two phenomena are interdependent. The drying rate is 

The evaporation rate of liquid film surrounding the granule being dried is related to the rate of heat transfer by the equation 

Schaefer and Worts (47) have shown that the higher the temperature differences between incoming air and the product, the faster the drying rate. 

Therefore, product temperature should be monitored closely to control the fluidized bed drying process. During fluid-bed drying, the product passes through three distinct temperature phases (Fig. 21). At the beginning of the drying process, the material heats up from the ambient temperature to approximately the wet-bulb temperature of the air in the dryer. This temperature is maintained until the granule moisture content is reduced to the critical level. At this point, the material holds no free surface water, and the temperature starts to rise further. 

The drying capacity of the air depends upon the relative humidity (RH) of the incoming air. At 100% RH, the air is holding the maximum amount of water possible at a given temperature, but if the temperature of the air is raised, the relative humidity drops and the air can hold more moisture. If air is saturated with water vapor at a given temperature, a drop in temperature will force the air mass to relinquish some of its moisture 

Drying a wet product in a fluid bed is a separate topic, but during the granulation process it becomes an integral part of the process hence, understanding fluid bed drying is important before we review the agglomeration process. 

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Clark, W.E., 1967. Fluid bed drying. Chemical Engineering Albany, 74(6), 177. 

Masters, K., Industrial Fluid Bed Drying Trends andDc c opments,"I luidization VII, (O. E. Potter, and D. J. Nicklin, eds.) Proceedings of the Engineering Foundation Conference, pp. 59-72, Brisbane, Australia (1992)... 

VanECek, V., Markvart, M. and Drbohlav, R. Fluid Bed Drying (Leonard Hill, 1966). 

Applications in the pharmaceutical industry included determining the end points of high-shear and fluid bed granulation [128-130], fluid bed drying, powder mixing [131], crystallisation [132] and obtaining information from other processes involving particulates [133]. 

White, A., Batch fluid bed drying. Food Processing, 52 (3) (1983) 37-39. 

Thus, based on the positive results obtained with the thesis work of Schade (15) and Dorr (17), a new plant for quasi-continuous wet granulation and multiple-chambered fluid-bed drying was developed by Glatt AG CH-Pratteln in cooperation with F. Hoffmann-La Roche Ltd. Basel and the Institute of Pharmaceutical Technology of the University of Basel. 

Figure 23 On-line moisture measurement during fluid bed drying vs. at-line measurement using Computrac . Source. From http //www.brimrose.com/pharmal. html anhor02.

Table 2 gives a comparison of the moisture contents following the drying and tumbling steps. The sizable increase in mean moisture content of the fluid bed-dried batches deserves further study. To determine whether or not all batches were uniformly affected, the mean moisture content was plotted in the order in which the batches were produced. Whereas the plot for the tray-dried batches is unremarkable, the fluid bed process chart (Fig. 4) depicts an unnatural pattern. Further investigation discloses that heating, ventilation, and air condition (HVAC) problems were experienced by the area in which a number of these batches were blended. 

Equipment capability/capacity The load that can be efficiently dried within the unit needs to be known. A larger load will require more moisture to be removed on drying and will affect the drying time. In the case of fluid bed drying, a maximum dryer load is that load above which the dryer will not fluidize the material. 

Morris, K.R. Stowell, J.G. Byrn, S.R. etal., Accelerated fluid bed drying using NIR monitoring and phenomenological modeling Drug Dev. Ind. Pharm. 2000, 26, 985-988. 

While a rotary dryer is shown, commonly used for grains and minerals, this control system has been successfully applied to fluid-bed drying of plastic pellets, air-lift drying of wood fibers, and spray drying of milk solids. The air may be steam-heated as shown or heated by direct combustion of fuel, provided that a representative measurement of inlet air temperature can be made. If it cannot, then evaporative load can be inferred from a measurement of fuel flow, which then would replace AT in the set-point calculation. 

Fluid bed drying Inlet air volume, temperature, dew point Granule size and distribution... 

Florin and Bartesch described fluid-bed drying of soybeans (23). Unlike steam-jacketed vessels that transfer heat from the vessels wall into the seeds as the seeds tumble through the vessel, a fluid-bed conditioner uses air to suspend and move the seed through the conditioning chamber. If the air is heated, it can transfer heat... 

Fig. 1 Application of Eq. (2) to (A) direct heating of a static solids bed (B) indirect heating of a static solids bed and (C) fluid bed-drying of a spherical particle.

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