The determination of the optimal freeze-drying cycle is of utmost
importance as the freeze-drying process requires a large
amount of energy. If only drying time is taken into account, it appears
that the values of the operating conditions (Tfluid and Pc) that
should be selected are in the upper right portion of the design
space, i.e. high values of shelf temperature and low values of chamber
pressure: 5 Pa and 5 C appear to be the (near) optimal operating
conditions in case a simple cycle is desired (i.e. a cycle with
constant values of Tfluid and Pc).
Exergy analysis can be effective to account for the efficiency in
the use of energy. For the coffee freeze-drying process the exergy
losses are minimized if high values of chamber pressure and low
values of heating fluid temperature are considered: 30 Pa and
20 C are the (near) optimal operating conditions in case a simple
cycle is desired.
Therefore, in order to improve the sustainability of the freezedrying
process, i.e. to maximize the energy efficiency, the best
operating conditions are different from those resulting in the
minimum drying time. It has to be highlighted that this result is
affected by the thermal properties of the system, as well as by
the values of the heat and mass transfer coefficients and, thus,
different conclusions may be achieved in case the freeze-drying
of different products is investigated.