Freeze drying sublimation

Lyophilization Freezing Sublimation Freeze drying Full stoppering... 

The three states of water are so common because they all are stable under ordinary conditions. Carbon dioxide, on the other hand, is familiar as a gas and a solid (dry ice), but liquid CO2 occurs only at external pressures greater than 5 atm. At ordinary conditions, solid CO2 becomes a gas without first becoming a liquid. This process is called sublimation. Freeze-dried foods are prepared by sublimation. The opposite process, changing from a gas directly into a solid, is called deposition—you may have seen ice crystals form on a cold window from the deposition of water vapor. The heat of sublimation (Aff°ubi) is the enthalpy change when 1 mol of a substance sublimes. From Hess s law (Section 6.5), it equals the sum of the heats of fusion and vaporization ... 

Meryman, H.T., Sublimation freeze drying without vacuum, Science, 1959, 130, 628. 

Suppose a solid is warmed at a pressure below the pressure at the triple point. In a phase diagram, this corresponds to moving along a horizontal line below the triple point. You can see from Figure 11.11 that such a line will intersect curve AD, the vapor-pressure curve for the solid. Thus, the solid will pass directly into the gas that is, the solid will sublime. Freeze-drying of a food (or brewed coffee) is accomplished by placing the frozen food in a vacuum (below 0.00603 atm) so that the ice in it sublimes. Because the food can be dried at a lower temperature than if heat-dried, it retains more flavor and can often be reconstituted by simply adding water. 

Ereeze casting (59) is a hybrid of sHp casting, gel casting, and freeze drying in which a slurry is poured into a rigid mbber mold, frozen, and the frozen Hquid is removed by sublimation, ie, by freeze drying (see Cryogenics). 

Supercritical and Freeze Drying. To eliminate surface tension related drying stresses in fine pore materials such as gels, ware can be heated in an autoclave until the Hquid becomes a supercritical fluid, after which drying can be accompHshed by isothermal depressurization to remove the fluid (45,69,72) (see Supercritical fluid). In materials that are heat sensitive, the ware can be frozen and the frozen Hquid can be removed by sublimation (45,69). 

Sublimation of ice crystals to water vapor under a very high vacuum, about 67 Pa (0.5 mm Hg) or lower, removes the majority of the moisture from the granulated frozen extract particles. Heat input is controlled to assure a maximum product end point temperature below 49°C. Freeze drying takes significantly longer than spray drying and requires a greater capital investment. 

Drying is an operation in which volatile Hquids are separated by vaporization from soHds, slurries, and solutions to yield soHd products. In dehydration, vegetable and animal materials are dried to less than their natural moisture contents, or water of crystallization is removed from hydrates. In freeze drying (lyophilization), wet material is cooled to freeze the Hquid vaporization occurs by sublimation. Gas drying is the separation of condensable vapors from noncondensable gases by cooling, adsorption (qv), or absorption (qv) (see also Adsorption, gas separation). Evaporation (qv) differs from drying in that feed and product are both pumpable fluids. 

Freeze-drying or dehydrating equipment for sublimation drying of ... 

Shelf Devices Equipment having heated and/or cooled shelves is available but is httle used for divided-sohds heat processing. Most extensive use of stationaiy shelves is freezing of packaged solids for food industries and for freeze drying by sublimation (see Sec. 22). 

Freeze-drying, like all drying processes, is a method to separate liquid water from a wet solid product or from a solution or dispersion of given concentration. However, the main difference is that the liquid water is separated by solidification (i.e., the formation of ice crystals) and subsequent vacuum sublimation instead of evaporation. This allows a drying at subzero temperatures which can be advantageous in case of heat-sensitive products. There are two general applications... 

FREEZE-DRYING OR DEHYDRATING EOUIPMENT FOR SUBLIMATION DRYING OF ... 

In addition to the effects of formulation factors on freeze-drying behavior, it is important for the pharmaceutical scientist to understand basic principles of heat and mass transfer in freeze-drying [29,30]. Because of the high heat input required for sublimation (670 cal/g), transfer of heat from the heated shelf to the sublimation front is often the rate-limiting step in the coupled heat... 

Mass transfer in freeze-drying refers to the transfer of water vapor from the sublimation front through open channels in the partially dried layer, created by prior sublimation of ice, through the headspace of the vial, past the lyostopper, through the chamber, to the condenser. 

M. J. Pikal, S. Shah, D. Senior, and J. E. Lang, Physical chemistry of freeze drying measurement of sublimation rates of frozen aqueous solutions by a microbalance technique, J. Pharm. Sci., 72, 635-650 (1983). 

Barriers to heat transfer produce corresponding temperature differences in a freeze-drying system, the actual temperature profile depending upon the rate of sublimation, the chamber pressure, and the container system as well as the characteristics of the freeze dryer employed. An experimental temperature profile is shown in Figure 5 for a system where vials were placed in an aluminum tray with a flat 5 mm thick bottom and a tray lid containing open channels for escape of water vapor. Here, heat transfer is determined by four barriers ... 

Figure 8 An example of the decreasing heat requirement during primary drying at a chamber pressure of 0.15 torr. 5% mannitol maintained at -20°C during primary drying. Results obtained by computer simulation of freeze drying (see Ref. 3). Heavy curve Shelf Fluid. Light curve Shelf surface. Lightweight dashed curve Product Bottom. Heavy dashed curve Sublimation.

---