Freeze-drying is a low-pressure, low-temperature condensation pumping process widely used in manufacture of bio/pharmaceutical products. Because of the need to avoid the aggressive evaporation drying, typical operating pressures in freeze-drying are below 100 Pa. Understanding relevant rarefied flow physics can help improve freeze-drying systems and processes. The paper presents various rarefied gas dynamics aspects of freeze-drying including thermal creep and conduction, pressure and concentration-gradient driven flows and sonic expansion flows with icing. In particular, flow through the duct where the Knudsen number is small enough is modeled using conventional CFD techniques. The effect of the hardware on flow in the duct is discussed in detail. Furthermore, flow and icing in the freeze-drying condenser is modeled using the direct simulation Monte Carlo (DSMC) technique. The effect of duct length on the uniformity of mass flux and ice accretion rates are discussed. It is found that by increasing the duct length, there is a trade-off between increased residual pressure and improved uniformity. The simulations show that by augmenting the DSMC method with conventional CFD technique in appropriate regimes, the gas dynamics in the entire freeze-dryer system can be modeled numerically for given sublimation rate and geometry. (c) 2012 Elsevier Ltd. All rights reserved.
