Drying Processes without Vacuum

From time to time, drying at low temperatures at atmospheric pressure has been discussed and tried experimentally, because vacuum installations are high-cost investments and expensive to operate. There are three basic problems which must be solved by such a low temperature drying process  

1 kg of ice, when sublimated at 0.6 mbar, has a volume of approx. 2000 m3. Since the atmospheric pressure is approx. 1700 times larger, approx. 3.4 106 m3 of air must be transported to carry the water vapor (the vapor content is 1 per thousand). 

If only the diffusion of vapor in resting air is used to transport the vapor from the sublimation front to the condenser (or vapor absorber), only 4 10 2 g/m2 h can be transported over a distance of 100 cm. Even if the condenser could be positioned at a distance of 1 cm the result is only 4 g/m2 h. Transport of vapor by diffusion cannot be used practically. 

By mixing an absorbing granulate or powder with the product to be dried, the distances the diffusion can become very small, or the water molecules may move by surface diffusion. In both cases, the problem is the same First to find an acceptable drying agent (absorber) and then to separate it quantitatively from the dried product. 

In the last years, several publications have tackled these problems Kahn-Wyler [ 1.791 lists four reasons which prove, that fluidized-bed drying (solving problem 2 above) is not suitable  

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In this process, a vacuum extrusion step (melt treatment) is followed by a continuous SSP (solid treatment). The process starts with PET flakes which are introduced without pre-drying into a ring extruder. The flakes are dried, melted and degassed inside the extruder. A gear pump builds the necessary pressure for melt filtration, where solid particles are removed. After granulation the pellets are fed to a continuous three-step SSP unit [10]. 

Subsequent to freeze stabilization, wet books can be thawed and air dried with or without interleaving or they can be vacuum dried in heated chambers (5). The moisture in frozen books can be sublimed by freeze drying (10) or removed by a vacuum/thaw/outgassing process (8), with microwave energy (6, 14), with dielectric energy (6), or by solvent extraction with or without vacuum assistance (5). 

Protein denaturation. Protein denaturation is normally observed as an increase in viscosity and a decrease in wettability. It is temperature-sensitive, generally occurring between 40 and 80°C. A common drying process scheme is to dry thermally and under wet-bulb drying conditions without overheating and then vacuum, heat-pump, or freeze-dry to the target moisture. 

Decomposition or side reactions of the sample must be considered during drying. Material unstable under conditions of heat can be dried by setting it in a desiccator using a vacuum desiccator will hasten the drying process. If the sample is weighed without drying, the results will be on an as is basis and should be reported as such. 

In this method, the aqueous, concentrated solution is also atomised into fine droplets, but they are rapidly frozen by blowing them into a low-temperature bath, such as ice-acetone, liquid C6H14 or liquid Nj. The droplets are then dried in vacuum, by sublimation of the ice without melting. This means that the temperature must remain below the eutectic in the salt-H20 system. The anhydrous salts (nitrates, sulphates, chlorides, etc.) are calcined to produce powders which are 0.1 pm in size. A schematic freeze-drying process is shown in Fig. 3.5. 

An essential oil (EO) is internationally defined as the product obtained by hydro-, steam-, or dry-distillation of a plant or of some of its parts, or by a suitable mechanical process without heating, as in the case of Citrus fruits (AFNOR, 1998 Council of Europe, 2010). Vacuum distUladon solvent extraction combined offline with distillation simultaneous distillation extraction supercritical fluid extraction microwave-assisted extraction and hydro-distiUation and static, dynamic, and high concentration capacity headspace sampling are other techniques used for extracting the volatile fraction from aromatic plants, although the products of these processes cannot be termed EOs (Faleiro and Miguel, 2013). 

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