(601d) Effects of Particle Size and Particle Size Distribution On 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 and computational simulations of fluidized bed drying 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 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.

Particle size and particle size distribution are important parameters for pharmaceutical materials. These may greatly affect the heat and mass transfer, drying rate and drying time of the fluidized bed drying process. In this work, moist pharmaceutical powders and granules, such as lactose, avicel and dibasic calcium phosphate anhydrous (DCPA) with different particle size distribution, have been dried using a Glatt GPCG-5 spray fluidized bed dryer/granulator and a Mini- Glatt spray fluidized bed dryer/granulator. The pressure drop across the bed, the inlet and outlet air temperatures and the product temperature are measured as a function of time. The moisture content in the product during drying is measured using two approaches: 1) Taking samples from the bed every 3 minutes, drying the samples in an oven and calculate 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 on-line measurement and the reference samples (dried in the oven), and a good match can be obtained. The NIR method can be used to determine the end point of the drying process. We varied the initial moisture content inside the pharmaceutical materials from 5% to 20% because in pharmaceutical manufacturing fluidized bed drying is generally carried out after the wet granulation step and the moisture content in the wet granules is normally in this range. We have classified the powders/granules into fines, medium, coarse, and examined the flow hydrodynamics, the drying rate and the agglomeration behaviors for each size cut as well as mixtures in the bed. This provides us useful information for the drying mechanisms given some specific powder/granule materials.

To better understand the drying process, we have carried out simulations for drying of a single particle with different sizes and geometries.  This serves to provide physical insight into the fundamentals of drying of a porous particle. The long term goal is to characterize performance of fluidized bed dryer systems and to develop a predictive model for drying of granulations with different size distributions and different moisture contents.