Fluidized bed coating at supercritical fluid conditions

As a model system, silica-particles and glass beads were encapsulated with a wax, which is common in technical coating applications. For this a homogeneous mixture of molten paraffin and supercritical carbon dioxide is prepared in an autoclave and injected into the fluidized bed from the bottom through a nozzle with an orifice diameter of 50 or 100 μm. Due to different conditions in the mixing-autoclave and the fluidized bed the paraffin precipitates in the vicinity of the nozzle and adheres to the solid particles. The coating experiments were carried out at fluid velocities up to 2.23 times the minimum fluidization velocity. The operating conditions for the coating process were determined by the investigation of the system paraffin–CO2 by means of solubility and differential scanning calorimetry measurements. An even distribution of the coating material within the fluidized bed was observed at fluid velocities higher than 1.2 times the minimum fluidization velocity. SEM-images of the achieved coatings show different morphologies according to the process parameters and solids used. In the case of glass beads with a diameter of 100–200 μm a uniform, thin but incomplete coating was produced.