Main drying time

In this example of the granulate, the main drying time (term A is assumed constant) depends largely on term C, while the vapor transport has virtually no influence. 

The possible drying times for the main drying are estimated in Section 1.2.1 and complemented by examples. The decisive qualities are the heat transfer coefficient from the shelf to the sublimation front of the ice (Ktot). The heat conductivity in the product does normally not play an important part (see Fig. 1.67), except that a granulated product is dried from the surface to the center (see Fig. 1.68). The shortest possible main drying time can be estimated with 5 or 10 % error, if the dimensions of the product and the maximum tolerable TKe (e. g. -10 °C) are given (Eq. (12), (12 a-c) in Section 1.2.1). 

Summarizing the selection of vials could influence the main drying time theoretically by a factor of 10 [see Eq. (12)], but the difference can easily be a factor of 2, and sometimes a factor of 4. 

No. of vials Average Tice (°C) Saturation vapor pressure ps (mbar) Standard deviation °fT-icerq Main drying time MD ( )... 

M = maximum amount of product (L) tMD = main drying time (h)... 

Table maximum water vapor flow-rates given by the hydrodynamic data of the plant. The maximum sublimation rates depend also on the heat transfer from the brine to the sublimation front of the ice [see Eq. (12)]. The main drying time will generally be governed above 0.06 mbar by the heat transfer and below 0.06 mbar by the water vapor transport, as shown in the example below ... 

The specifications of the plant are given in the caption of Figure 2.58. As can be seen from the tables in the caption, the main drying time depends on the heat transfer and the vapor flow rate. To optimize a freeze-drying cycle it is helpful to calculate a table as shown in Figure 2.58 for mns 1-7, which can be commented upon as follows ... 

During main drying the temperature of the sublimation front is the decisive data for the collapse and for the temperature gradient for energy transportation. Therefore, the main drying time depends on these data. 

Main drying time calculated from heat transfer data (h) 11 9... 

Solution a will be the most economical. But one must accept that the vapor transport may be somewhat smaller in the plants than the estimated 0.7 g/ hcm or 3g/hcm. If the measured transport data are, say, 0.6, resp. 2.5 g/ hcm (15% smaller), the main drying time has to be prolonged to 13 h, resp. 11 h. In view of the total freeze-drying cycle time (without cleaning and sterilization) this prolongation seems acceptable. Solution b will require two valves of almost the same diameter (for PA one should not consider two valves of different diameter). The two-valve solution can be preferable if future products are expected to need even lower and lower p. In view of the trends observed today Tjce down to —50°C may be required. 

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