Melting curves, freeze-drying

Figure 1 Schematic evolution of the freeze-drying process. Temperatures (upper curve) and water content (lower curve) are indicated versus time. In the temperature diagram cs = maximum temperature of complete solidification 7 = minimum temperature of incipient melting = absolute limit for fast process = maximum allowed temperature for the dry product RMF, final requested residual moisture.

C. To improve, once more, the low-temperature stability of the system, it is cooled back a third from — 18°C to — 50°C and then rewarmed for analysis. 1417 RW 3 shows that, in the course of the third rewarming, all accidents have disappeared on the DTA curve, which only indicates progressive melting from — 8°C upward. Moreover, the stability of the frozen structure has been substantially improved since now Z sin cp remains almost constant during the whole period and only start to drop at — 19° C instead of — 32°C without thermal treatment. It is then easy to freeze-dry the solution at — 20°C or slightly below. 

The case of carbon dioxide, shown in Figure 12-28, differs from that of iodine in one important respect—the pressure at the triple point O is greater than 1 atm. A line at P = 1 atm intersects the sublimation curve, not the vapor pressure curve. If solid CO2 is heated in an open container, it sublimes away at a constant temperature of -78.5 °C. It does not melt at atmospheric pressure (and so is called "dry ice"). Because it maintains a low temperature and does not produce a liquid by melting, dry ice is widely used in freezing and preserving foods. 

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