Heating shelf, temperature

Figure 9.24 Top graph Comparison of the size of the nanoparticles obtained from different suspensions containing sucrose and Cremophor EL (A 5% sucrose + 5% Cremophor EL B 5% sucrose + 2.5% Cremophor EL C 2.5% sucrose + 5% Cremophor EL D 2.5% sucrose + 2.5% Cremophor EL) before freeze-drying ( ) and after freeze-drying carried out at different heating shelf temperatures = -20°C, = -30°C). Acetone, = 5 mg/mL, v. = 1.7 m/s, W/S = 1, quench volumetric ratio = 0.33, = 5...

Fig. 4.15 Effect of chamber pressure and heating shelf temperature on the primary drying time for constant shelf temperature. The locus corresponding to the minimum of the primary drying time for the various shelftemperatures is also shown (dotted line). The dashed line...

Fig. 5.3 Values of the exergy loss for the prima7d7ing step in (a) the d ing chamber and (b) for vapor condensing as a function of chamber pressure and heating shelf temperature (solid line -30°C symbols 0°C).

However there is a major difference between the two forms of trays and heating systems. As shown in Fig. 1.57, approx. 1.3 kg ice/h m2 can be sublimated by radiation heat, if the shelves have a temperature of + 100 °C and the product temperature is -20 °C. The main difference is the method of heat transfer With a flat tray and mostly radiation energy, the density of the heat flow is limited, and it can be substantially larger with ribbed trays standing on the heated shelf. Using the temperatures as above and an average value Klol = 100 kJ/h m2 °C from Tables 1.9 and 1.10, approx. 4.3 kg ice/h m2 can be sublimated. 

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. 

Note for the last two points A too high or too low ice temperature at the sublimation front can be changed by two operation data By changing the shelf temperature or by changing the heat transfer from the shelf to the bottom of the vials or trays. As shown in Table 1.10 Klol can vary from =117 kJ/m2 h °C at = 0.5 mbar to = 62 kJ/m2 h° C at 0.36 mbar (all other data constant). The change of Tice with the pressure is rapid as shown in Fig. 2.37.1. Figure 2.37.2 shows, that the range of control is approx. 10 °C (-28 °C to -18 °C) under the conditions of these runs, in which all other data have been constant. 

The change of the shelf temperature changes Tice much more slowly, since the heat capacity of the shelves and the heating medium is large. It can take 1-2 h before an equilibrium is reached after a shelf temperature change. 

Shelf heating rate Shelf cooling rate Shelf temperature control Condenser cooling System evacuation rate Pressure control Leak test Sublimation rate Condenser capacity... 

As compared with a higher pressure and lower shelf temperature outlined in Table 7, drying rates with the reversed conditions of lower pressure and higher shelf temperature would be expected to be slower than the conditions at target shelf temperature and chamber pressure. Compared with those conditions, freezing would be expected to require more time. Primary drying rates would also be reduced because heat transfer rates would be less, product temperatures lower, and residual moisture higher. 

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