Optimal control of a freeze dryer—I Theoretical development and quasi steady state analysis

A mathematical model which describes the nonsteady state heat and mass transfer operations during the freeze drying process is presented. With the use of this model the problem of operating a freeze dryer to obtain a fixed final moisture content in minimum time is formulated as an optimal control problem. Radiator energy output and total pressure in the drying chamber are considered control variables. Constraints are placed on the system state variables by the scorch point of the dried product and the melting point of the frozen interface. A quasi steady state analysis is performed which provides general guidelines about the optimal control policy at the beginning of the drying process (when neither of the state constraints is active), as well as during operation when the process may be heat and/or mass transfer limited.