Modeling and validation of heat and mass transfer characteristics within a packed bed: Water immersion and hot-air drying of propellants

Solvent removal is a critical unit operation in the industrial production of nitrocellulose (NC)-based propellants. This study investigates a coupled solvent-removal process comprising water immersion and hot-air drying using a representative elementary volume (REV)-scale model validated at the pilot scale, revealing the multiphase transport mechanisms within a propellant packed bed. Results demonstrate that during water immersion, the transient ethanol leaching rate exhibits a non-monotonic trend. The initial surge is driven by accelerated intra-particle diffusion stemming from a rapid temperature rise, whereas the subsequent decline is attributed to the diminishing interphase concentration gradient. Raising the operating temperature from 317.7 K to 327.8 K increases the peak ethanol leaching rate at the bed center by 84.1%. During the subsequent drying process, as hot air traverses the packed bed, axial moisture accumulation increases while the gas temperature declines. Initially, a distinct plateau appears in both the transient interphase mass transfer rate in the windward region and the propellant temperature profile in downstream regions. Elevating the drying temperature from 336.8 K to 346.8 K increases the peak heat and mass transfer rates in the windward region by 26.2% and 29.4%, respectively. Based on coupled multi-physics constraints, the recommended operating temperature range for water immersion is 313.15 K - 333.15 K, balancing ethanol leaching efficiency against additive losses and thermal energy consumption. For hot-air drying, the recommended range is 323.15 K - 342.15 K, ensuring rapid moisture removal while maintaining a strict safety margin exceeding 10.0 K below the self-accelerating decomposition temperature.