Freeze shelf temperature

M. J. Pikal, S. Shah, M. L. Roy, and R. Putman, The secondary drying stage of freeze drying drying kinetics as a function of shelf temperature and chamber pressure, Int. J. Pharm., 60, 203-217 (1990). 

At shelf temperatures of 100 °C, approx. 2000— fOOO kJ/h m2 are transmitted, depending on the product temperature. At lower shelf temperatures, as is usual in freeze-drying plants for pharmaceutical products, q values between 500 and 1500 kJ/m2 can be expected. However for e = 0.12, these data are reduced by a factor of 0.12. 

Fig. 1.80.1. Enlarged freezing phase of the two complete processes shown in Fig. 1.80.2 and 1.80.3. Top Vials with mannitol solution are cooled with 0.6 °C/min, while the shelf temperature is lowered from room temperature to -50 °C.

Fig. 3.22. Shelf temperature (Tsh) as a function of drying time during the freeze drying of bone corticalis and spongiosa and the related residual moisture content (RM).

Stoppers may also slide into the vials and cause their virtual closure. This can happen during freezing of the product, especially if the shelf temperatures are very low. The dimensions of the stoppers need to be tested not only for the pressure to close them after drying, but also for shrinking at low temperatures. 

The outer vials are influenced (if the shelf temperature is uniform) by a different temperature of the walls and door of the chamber. If the chamber walls and the door are not kept at shelf temperature, the outer vials must be shielded or they may be too warm during freezing (e. g. freezing differently) or too cold during secondary drying (see Fig. 1.68), and this may lead to a different residual moisture content, from that in inner vials. 

Gieseler et al. utilized tunable diode laser absorption spectroscopy to detect water vapor concentrations, gas velocities and mass flow during freeze-drying of pure water at different pressure and shelf temperature settings and of a 5%w/w mannitol solution. The analyzer was interfaced to the spool that connected the dryer chamber to the condenser. The reported method was advantageous in that primary and secondary drying end-point control based upon mass flow rate was independent of freeze-dryer size and configuration. ... 

The qualily of a frozen food may be determined more by the temperature at which it is stored than by the method or rate of freezing. Storage temperatures may fluctuate as products move from manufacturing through distribution channels lo the consumer s home freezer. The useful shelf life or a frozen food may be severely limited by exposure to storage temperatures above - I8 C. even for a few hours. 

To establish the shelf temperature necessary to completely solidify the product during freezing, the required temperature necessary to achieve complete solidification is determined by thermal analysis early in product development. In addition, if the formulation undergoes crystallization, such behavior during freezing and the optimal processing parameters used for cooling the product are critical and need to be well defined. 

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. 

Figure 2 Representative plot of shelf temperature and product temperatures during freeze-drying.

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