fluid-bed processing

Blow air up through a layer of sand fast enough and the sand starts to churn and bubble like a boiling liquid — that is fluidisation. Fluid-bed processing uses the same trick on pharmaceutical powders: a stream of warm air lifts and suspends the particles so they tumble freely in the gas, each one bathed in air and surrounded by space. In that floating state you can dry, granulate or coat them very efficiently because every particle is exposed.

The same machine does several jobs depending on what you spray and where. With no spray it is a dryer (hot air evaporates moisture fast because of the huge particle surface in contact with air). Spray a binder solution down onto the bed and you get fluid-bed (top-spray) granulation, building granules as droplets glue particles together. Spray from below or tangentially and you get particle coating, laying an even film or sugar layer on each granule, pellet or small tablet.

Its strength — enormous gas-to-particle contact — is also its main risk. Fine powders fluidised in dry air can build static charge and, with organic solvents or fine combustible dust, create an explosion hazard, so the equipment needs earthing, explosion relief and often inert nitrogen. Process control matters too: air volume, temperature and spray rate must be balanced, or the bed can over-wet and collapse into lumps, or particles can be blown out and lost into the filters.