Fluid temperature, primary-drying

The chamber pressure during the sublimation step (i.e. the primary drying process) has been found to be related to the product and shelf-surface temperatures [8] however, determining the shelf temperature required is more difficult as it depends on the nature of the heat transfer fluid used to control the shelf temperature and also on the particular design of the freeze-dryer. 

Fig. 4.6 Example of results obtained during a freeze-drying cycle using the special balance with the embedded wireless temperature measurement in order to monitor the primary drying stage of a mannitol-dextran solution (6-14% by weight) in a pilot-scale freeze-dryer. The freezing stage was run at 223 K for about 5 h, while the main drying was carried out setting the fluid temperature at 293 l< and the chamber pressure at 10 Pa (Nv = 98 on tray, dv= 14.2 X 10 m, Lp = 7.2 x 10 m).

The primary structure of the fluids that are bound to undergo freeze-drying is, of course, of great interest. In that wake, the structure of water itself is a determinant item. This is why two chapters in this book relate directly to that topic (Watts and Bellissent-Funel/Teixeira). Accordingly, we will not deal with it. However, we would like to share with our readers and colleagues some ideas that evolved from recent experiments that we have carried out on the low-temperature thermoluminescence of different systems and, in particular, on water itself. 

The operating conditions of a fluid bed are to a high degree dictated by the properties of the material to be dried, as already indicated. One parameter can be chosen regardless of the flnidization process, namely, the fluidization air temperature. For most products, however, the temperature is of primary importance, since the fln-idized state results in very high heat-transfer rates so that heat sensitivity may restrict temperature and thereby prolong process time. 

The primary function of cryogenic expansion equipment is the reduction of the temperature of the gas being expanded to provide needed refrigeration. The expansion of a fluid to produce refrigeration may be carried out in two distinct ways (l)in a Joule-Thomson valve where no work is produced, and (2) in an expander where mechanical work is produced. Although both Joule-Thomson valves and expanders are discussed, turboexpanders are presently preferred for most dry expansion applications. 

---