Intensification of transfer fluxes and control of product properties in freeze-drying

In this review the unit operation ‘lyophilization’ is analysed under the point of view of ‘intensification’ and product-quality control; some original results are also presented, in order to illustrate some of the concepts discussed. Various methods for enhancing transfer fluxes are revised, highlighting advantages and disadvantages and applicability for different types of materials and pointing out their effect on the control of the product final quality. Modifications of the operative conditions and employment of different energy sources are considered, together with the use of equipment different from the usual tray chamber. Emphasis is placed on the discussion of possible process modifications or process alternatives: modification of one of the steps (pre-freezing, primary drying, secondary drying), addition of a solute and use of mixed solvents for intensifying mass transfer and controlling the microscopic structure of the product.