(344e) Parametric Analysis of Fluidized Bed Drying of Pharmaceutical Materials

Fluidized beds are extensively used in the pharmaceutical and other chemical industries either as a batch or continuous process for drying moist powders and granular solids because of good mixing of solids and intensive heat and mass transfer between the solid and hot gas phases in the system. Generally, high energy input is required during this process to provide the latent heat of water evaporation, and thus on-line measurements become important to determine the optimal operation conditions in order to minimize energy usage. In the pharmaceutical industry, the drying step in many pharmaceutical production lines is often a bottleneck because it is difficult to determine the time required to dry the materials as well as analytically verify that a predetermined end point has been reached. If the active pharmaceutical ingredient is heat sensitive or has a low melting point, controlling the drying process becomes more critical due to the sensitivity of the formulation to the temperature and the risk of compromising product quality and performance during drying.

In this work, two dibasic calcium phosphate anhydrous (DCPA1 and DCPA2) species with different particle sizes and porosities, have been examined using a Glatt GPCG-5 spray fluidized bed dryer/granulator and a Mini- Glatt spray fluidized bed dryer/granulator. The variations of the pressure drop across the bed, the product temperature and the humidity in the dryer air are measured as a function of time. The moisture content of the product during drying is measured using two approaches: 1) LOD - Taking samples from the bed every 5 to 10 minutes, drying the samples in an oven and calculating the sample moisture based on the mass loss before and after drying. 2) Using NIR to on-line measure the sample moisture. We have compared the NIR predictions and the LOD measurements, and a good match can be obtained. This indicates that NIR is a reliable approach to determine the end point of the drying process.

Parametric analysis is carried out based on two-factorial tests. We have examined the impact of drying air temperature, drying air flow rate, material loading, and initial moisture content on the end point of the drying time. From the statistical analysis we found that the four variables used in the factorial tests are independent and the most significant variables are drying air flow rate and initial moisture content. If the initial water volume is above the total pore volume of the particulate materials, drying can be divided into three stages. At the beginning of drying, water in the bed voids is evaporated (Stage I). Once all the water in the bed voids is removed, convection drives the water inside the particle pores to flow towards the particle surface, and then water evaporation occurs at the surface (Stage II). With further drying, the effect of convection reduces due to the loss of water and the drying front starts to penetrate inside the pores (Stage III). In stage I and II, drying is dominated by a constant drying rate stage with a constant bed temperature. In stage III, the drying process transits from a constant rate stage to a falling rate stage, and the bed temperature goes up. Drying is mainly dominated by the constant drying rate stage with zero order kinetics if drying is carried out at a relatively slow drying regime which is required by many pharmaceutical productions. A comparison of these two Glatt dryers and an evaluation of energy efficiency are also carried out in this work. Our work serves to characterize performance of fluidized bed dryer systems and provide physical insight into the fundamentals of drying of porous pharmaceutical materials.