Freeze-drying energy requirement

It is no news that, compared with the solid state reaction, the glycine-nitrate route, sol-gel and freeze-drying techniques require lower calcination temperatures to yield pnire crystals of perovskite phases, with the resulting energy saving (very important nowadays). The disadvantage is, however, the fact that they are more time consuming and require more controlled synthesis conditions. In the same way, it is clearly observed that the ceramic... 

The energy requirements for doing have been studied rather intensively in recent years because the cost of a product can be markedly affected by this energy intense operation. Some researchers have observed that drying costs are much lower for air drying and drum drying than for freeze-drying. 

The weak point of freeze-drying is its low productivity (expressed in number of cycles per year) and its cost. Pharmaceutical industry constraints, operation under vacuum, and the implementation of the sublimation phenomenon indeed require great investments, large energy consumption, and production delays far above those required with more classical dehydration techniques industrial... 

Table 1 Basic energy requirements for the freeze-drying of 1 kg of an aqueous solution containing 5% solids (sucrose), based on the following assumptions ice sublimation is performed at —40°C, with a condenser temperature of 60°C (units kJ kg )...

According to Nijhuis et al. (1996), freeze drying (known as a suitable dehydration process for pharmaceutical and food products) is not suitable for the production of homogeneous films because the films obtained are generally very spongy. Also, a freeze-dried product tends to be porous and the problem of rapid rehydration may arise once the product is exposed to a more humid environment. Moreover freeze drying is very energy intensive. The equipment is also more expensive than atmospheric pressure dryers. It is best suited for heat-sensitive materials, or when solvent recovery is required, or if there are risks of fire and/or explosion. 

Characteristically, the relatively high amount of removed water per unit of input energy at elevated temperatures decreases dramatically with temperature drop, mostly because of the low COP of the refrigerator. Therefore, higher drying temperatures should be used unless temperatures below the freezing point are required for quality reasons. 

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