The principle of sublimation is explained on the right using the phase diagram for water. In practice, the process is nearly always used for aqueous systems, but in recent years the popularity of freeze-drying special solvent/water mixtures has increased.
In a phase diagram of a pure material, the three states of vapor, liquid, and solid are each bounded by a pressure and temperature function curve. Phase transitions occur at each of these curves. The phase diagram for water is explained in more detail in this example. The vapor pressure curve (I) describes the phase transition of boiling/condensing. For example, it describes the boiling of water at 100 °C under atmospheric pressure. Lower pressure shifts the boiling point downwards (the principle of vacuum distillation), while higher pressure raises the boiling point (which is the reason that cooking times are faster in a pressure cooker).
The sublimation pressure curve (II) describes the phase transition of sublimating, or desublimating/ resublimating. The vapor transitions directly to the sold phase (ice). One example of this is the sublimation of CO2 under atmospheric conditions. The melt pressure curve (III) describes the transition between melting and freezing.
If the pressure is greater than 6.11 mbar, then H2O passes through all three states (solid, liquid, vapor) when the temperature increases or decreases. Below this point, that is, if the pressure is less than 6.11 mbar, however, then H2O passes directly from the solid to the vapor state. At precisely 6.11 mbar, at a temperature of 0.01 °C, the melt pressure curve, vapor pressure curve, and sublimation curve all meet at one point, the triple point. At this point, all three states occur simultaneously.
Above what is known as the critical point, at 373,95 °C and 220.64 bar, there is no clearly defined phase transition from the vapor phase to liquid.
All of the phase transitions are relevant for freeze-drying. The sublimation pressure curve is especially important here, as the sublimation from ice to vapor, as desired for freeze-drying, is possible only below the triple point. In practice, freeze-drying processes typically take place at temperatures from –20 °C to –40 °C.
The table on the right includes an excerpt of the sublimation pressure curve for water.
The units of measure typically used for conversion are:
Find more information about fundamentals, process management, and applications of freeze-drying here.
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