(53b) Novel Pathways for Fluidized Bed Coating with the Aid of Non Conventional Fluidizing Media: Experiments and Models

Non traditional fluidization media such as supercritical fluids offer novel operational avenues and schemes such as temperature swings for nanolayer deposition while avoiding environmentally questionable solvents. Experimental results from select pharmecautical and energy applications will be introduced and coupled with modeling. A robust model involving dynamic stochastic mass and energy balances for a fluidized bed with or without coating solution streams, where balance equations partitions the fluidized particle bed into well mixed control volumes in the axial direction, treating the fluidization fluid as plug flow and the particle bed as a CSTR is developed. There are two types of control volumes dependent on the penetration depth of the atomized coating fluid (if any), coating control volumes and non-coating control volumes – each with its own set of differential equations. The results from the dynamic mass and energy balances include: temperatures for (fluidization fluid, particles, and fluidized bed wall), solvent content (fluidization media and particle), and the coating mass balance.

The coating growth kinetics model developed for this work is based on the fluidized bed hydrodynamic properties (bubble diameter, velocity, and void fraction, fluidized bed void fraction, and particle circulation rate) calculations from the aforementioned results. The hydrodynamic properties are then extended to establish probabilities for being in the spray area and being coated in the spray area. In the absence of data, particle size distributions and droplet size distributions are generated from appropriate relationships (e.g. triangular distribution, Rosin-Rammler distribution). Mass balances for the droplet collisions with the particles in the spray area are tabulated to determine how much coating material is applied during each successful droplet-particle collision. The particle size distribution and coating thickness distribution are then produced as a function of time.

The model is used for a portfolio of applications and fluidization using supercritical carbon dioxide with or without additives in different fluidized beds such as wurster or circulating. Global sensitivity analysis is used for identification key parameters and their interaction. Coupled with pilot plant several encapsulated products were modeled to optimize encapsulant characteristics and development of operational policies.