Sublimation Temperature

The vapor pressure versus temperature relationship for ice, shown in Figure 2, fixes PB in terms of Ts. In lightly loaded freeze dryers with wide vapor-transfer spaces, P0 equals the vapor pressure of the ice on the condenser. Figure 2 also depicts equation 2 solved to determine Ts and Ps for the freeze drying of a slab of coffee extract for which

1,200 r

1,200 r

Figure 2. Vapor pressure of ice versus temperature and pressure and temperature conditions at the outer surface (O) and sublimation interface (S) of a slab of food undergoing freeze drying in a chamber where the condenser temperature is — 40°C.

Jz=o kt(Ta

(ApX)ZdZ

Figure 2. Vapor pressure of ice versus temperature and pressure and temperature conditions at the outer surface (O) and sublimation interface (S) of a slab of food undergoing freeze drying in a chamber where the condenser temperature is — 40°C.

kt = 0.035 W • m_1 • °C_1, iT = 3.5 x 10"9 kg • m"1 • s"1 • (urn Hg)"1, and T0 = 30°C. The condensed ice temperature is — 40°C. Therefore, as shown by the dotted lines, P0 = 97 jLim Hg. Ta and Pa define point O. A line, drawn through O with slope —kt/(K ■ A), intersects the ice vapor pressure curve at S, providing Ps and Ta, which respectively are 301 /urn Hg and -29.4°C.

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