Water vapor transport

Hanna, L. M. (1983). Modeling of heat and water vapor transport in the human respiratory tract. Ph.D. Dissertation, University of Pennsylvania, Philadelphia. 

To insure an undisturbed water vapor transport (see Section 1.2.4) the leak rate of a freeze drying plant must allows BTM with sufficient accuracy. This applies for vapor pressure in the ice temperature range between -45 °C to -10 °C, corresponding to 0.07 mbar to 2.5 mbar. 

The water vapor transport in a freeze drying plant can be described schematically with the aid of Fig. 1.86 The ice (1) is transformed into vapor and has to flow out of the container (2) into the chamber (4). Between the chamber wall or any other limitation an area (3, FI) is necessary. The vapor flows then through the area (F2) into the condenser (7), having surface of (F3) on which the water vapor will mostly condenses. A mixture of remaining water vapor and permanent gas is pumped through (8), (9) and (10) by a vacuum pump (11). 

Fig. 1.86. Schema for the estimation of the water vapor transport in a freeze drying plant.

Fig. 1.87. Influence of different forms of stoppers on the water vapor transport out of the vials into the chamber. At a pressure of 1 mbar in the vial the following relative amounts of water vapor are transported into the chamber in 3 h ...

For production plants, the loading of the plant with the required amount of ice (which should correspond to a full charge) is time consuming and several tests should be avoided. For a sufficient estimation of the water vapor transport and the bottle necks of it, one test can be carried out as described here ... 

A plant having a shelf area of = 30 m2 has been loaded with 300 kg of water in trays and frozen on the shelves. Water vapor transport and condenser temperatures have been measured in this case between 0.4 and 0.6 mbar, which is approx, two to three times higher than the normally expected operation pressure of the plant (to get an measurable quantity of ice sublimed in a reasonable test time). The data of the test are shown in Fig. 2.19. Three Pt 100 (resistance thermometers) have been frozen in the ice. One CA each have been... 

Approx. 50 min after the start of heating and evacuation the equilibrium conditions start to become visible, and after approx. 90 min they are effective. The pressure difference between 7 ice (converted into pressure) and the chamber pressure depends on the absolute pressure, which corresponds to the amount of water vapor transported per time, but the difference (picc -pcb) shelf temperature has started to rise from the -30 °C seen at the start. After 2 h and 15 min the shelf temperature has been lowered to pass the pressure range of 0.3 mbar a second time in order to avoid a possible distortion by the non equilibrium conditions in the beginning. 

The following conclusions can be drawn from this experiment for the water vapor transport and the working of the condenser ... 

The conditions of water vapor transport between the chamber and condenser, the condenser surface, the capacity of the refrigerant compressors and the vacuum pumping capacity, must all be the same (pro rata) for the product dried in the laboratory as in the plant to which the process will be transferred. 

Water vapor transport from chamber to condenser6 Water vapor g/h at pch 0.06 and 0.3 mbar, measured by CA Water vapor g/h at pL.h 0.2 and 1.0 mbar measured by CA... 

Authors note The measured independence of the sublimation rate from the layer thickness shows, that the sublimation rate under the conditions of the experiment depends only from the heat transfer to the product and not from the water vapor transportation through the dried product.)... 

Fig. 21. Diagram showing the water vapor transport for two gas flow rates of CO 2 through the reaction chamber (furnace of the thermobalance) as a function of temperature under standard conditions ...

Term D, also linear with d, shows the influence of the water vapor transport from the sublimation surface through the dried product into the vial or tray on tMD. blp (kg/m h mbar)... 

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