Temperature during main drying

If the product temperature approaches T%- from lower temperatures, the viscosity changes rapidly (within a few degrees) by several decimal powers. Since also the product temperature can be measured only with a certain accuracy (see Section 1.2.3), one has to account for both uncertainties. It is recommended that the maximum product temperature during main drying 3 to 5 °C below T. be chosen. 

Test run the results of this run are only shown in Figure 1.71 in curves 3. In this run the shelf with the tray was inclined in such a way that a uniform thickness of 7 mm was varied from 5 to 9 mm. Otherwise, the conditions were the same as those in the second test (Figure 1.64). The ice temperature during main drying was similar but the DR value of 5.5%/h at 9 h shows the variation of thickness of the layer. A DR of 0.5%/h was reached not in 10 h, but in 13 h. The test also showed (not in the figure) that the product temperature (T ) varied at 9 h from 0 to +22 °C. 

T tot = 7(h - rice, average temperature difference during main drying 43.88 °C... 

The most important data during main drying is the temperature at the moving sublimation front which cannot be measured by Ths or RTDs. In 1958, Neumann and Oetjen 11.651 showed that the barometric temperature measurement (BTM) measures exactly this data. In Fig. 1.77 this is schematically shown if the drying chamber is separated from the condenser by a valve for a short time the pressure in the chamber rises to the saturation vapor pressure (ps) corresponding to the temperature of the sublimation front. ps can be converted into the ice temperature by the water vapor- temperature diagram (e. g. 0.3 mbar = -30 °C). Data for accurate conversion are given in Table 1.11 the temperatures between -100 and -1 °C. 

The condenser design and surface can handle the vapor flow during main drying of this test. The possible low temperatures could be needed during secondary drying. 

Uniformity of temperature for all shelves, e. g. 1 °C during main drying. 

During main drying, the situation is very different the condition of a close contact with the product is only true at the beginning of MD thereafter, the measured temperature depends on circumstances which are difficult to analyze. The position of the sensor, on top, in the center or near the bottom contact with the vial wall, decides the measured data, as shown in Figure 1.76. If the filling volume of vials is small (a few millimeters layer thickness) or if the product is granular, it is especially difficult to ob-... 

The decisive data during main drying (MD) is the temperature of the ice at the sublimation front, Tice, which can only be measured by barometric temperature measurement (BTM). Figure 1.77 shows the principle the valve between the chamber and condenser is closed for less than 3 s, to fill the chamber with water vapor of saturation pressure p., corresponding to Tke as shown in Table 1.11. The two conditions for the use of BTM a leak rate of the chamber below a certain limit and enough ice subliming during the time the valve is closed, are described in detail in Section 1.2.1. 

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. 

Ice temperature is determined during main drying by the barometric temperature measurement. 

Heating of shelves to a preset temperature after a preset pressure in the chamber has been reached Pressure control during main drying... 

---