2.4. Coating with the RESS-WTS
Compressed air that had been heated to 35 °C with an in-line heater (CAST X 500, Watson Marlow, UK) was introduced to the bottom of the fluidization apparatus that had been charged with 30 g of MCC particles. The air flow rate was just above that required for incipient fluidization (Umf) which was obtained by a test using only MCC particles and hot air in the Wurster coater. Valve V1 was opened to let pure CO2 join the air to fluidize the MCC. After fluidization had been initiated, valve V1 was closed and V2 was opened simultaneously to allow introduction of the solute-laden CO2 solution (produced in Section 2.3) into the RESS-WTS apparatus. The solution was jetted upward in the same direction as the air flow via a 102 μm diameter sapphire nozzle into the bed of excipient. The nozzle was located in the centre of the air distributor at the bottom of the Wurster coater where the solution sprayed into the bed as in a standard Wurster coater. The CO2 mass flow rate depends on its pre-nozzle pressure and temperature, nozzle size and nozzle conditions. The nozzle diameter was kept constant at 102 μm. Under some conditions, solid precipitated in and around the nozzle caused partial blocking. The CO2 flow rate measured in this work ranged from 12 to 311 g/min. Particles of the active precipitated from the RESS jet were brought into contact with the fluidized MCC carrier particles, where they formed a coat. Simultaneously, CO2 pressure dropped to ambient and the CO2 changed from supercritical states to gas and mixed with the air to fluidize the MCC particles. After a designated coating time, from 5 to 118 min, the CO2 + active solution pipeline was purged with pure CO2, while the particles remained fluidized for 5–10 min; their surfaces were unaffected during this time, as indicated by SEM images (not included here). The coated MCC particles were collected and kept in air-tight sample bottles for further characterization and analysis (see Section 2.6).