Supercritical and Near-critical Carbon Dioxide Assisted Low-Temperature Bubble Drying

If supercritical or near-critical carbon dioxide is mixed with an aqueous solution and the mixture is subsequently decompressed through a flow restrictor, a dense aerosol plume is formed1 that can be rapidly dried at 25−95 °C. This temperature range is lower than that typically needed in conventional spray-drying aqueous solutions. At 1100 psi and ambient temperature, up to ≈2 mol % of carbon dioxide can dissolve in aqueous solutions, and when the aqueous solution is ejected from a 5-cm-long flow restrictor, 50−130 μm in inner diameter, it is hypothesized that fine droplets and microbubbles are formed. This aerosol plume can be diluted with dry nitrogen (or air if there is no explosion hazard) at temperatures usually between 25 and 80 °C. In a drying chamber <1-m long, the bubbles and droplets become rapidly dried and the powder is collected on a filter. The particles are usually spherical or nearly spherical and residual moisture is typically 1% or less. For some substances, such as sodium chloride, mannitol, or tobramycin sulfate, hollow particles can be formed. For others, such as lactose and albuterol sulfate, the spherical particles are solid, with diameters mostly between 0.5 and 5 μm. Depending on the solute and conditions of drying, the particles are crystalline in some cases and amorphous in others.