The freeze-drying performance under pressures from 1.325 to 101.325 kPa

The low drying rate is the main limitation of atmospheric freeze drying. The drying rate depends on the balance between heat and mass transfer. Among the process variables, air pressure changes have the opposite effects on heat and mass transfer simultaneously. The increase in pressure favors heat transfer but hinders mass transfer. Therefore, a conjugate model was proposed and validated to quantitatively investigate the heat and mass transport characteristics of lamellar products during freeze-drying in a tunnel under different pressures (1.325–101.325 kPa). To describe the effects of air pressure, the Navier-Stokes equations of the external fluid were coupled with the adsorption-sublimation model. Results show the regulation of transfer resistance values and drying rate with moisture content during the drying process, indicating that freeze-drying is the external heat transfer control at first, then the internal mass transfer control, and eventually, the heat transfer promotion can accelerate the process. The optimal constant pressure value was found to be 5.325 kPa, with the shortest drying time (17.3 h). It has been demonstrated that freeze drying employing a specific laddering-controlled strategy, offers the benefit of reducing drying time in comparison to the use of optimal constant pressure; however, the impact was not remarkable.