Main or Sublimation Drying

The main drying process (MD) has been photographed in a cryomicroscope by Kochs et al. as shown in Figure 6. The ice crystals grow extremely uniformly using a special freezing method. The ice sublimes and the remaining solids show their original structure after freezing. During sublimation the temperature of the ice at the sublimation front (Tice) has to be kept well below the collapse temperature Tc.

As can be seen in Figure 6 Tice cannot be measured by a sensor because the ice front travels. If the valve between chamber and condenser (8 in Figure 11) is closed for a short time (< 3 s) the water vapour pressure in the chamber rises until the saturation pressure (pg) of the ice front is reached. The rising pressure is measured 100 times per second and the change in the slope (after 2.14 s), if saturation is reached, is determined as 0.286 mbar, corresponding to — 32.7°C as shown by Haseley and Oetjen in Figure 7. This procedure is called barometric temperature measurement (BTM). It permits checking Tice during MD (e.g. every 10 min). To estimate the main drying time (tMD) the following equation

Figure 6 Course of main drying observed using a cryomicroscope, in which freeze-drying is carried out. The hydroxyethyl starch solution is optimally frozen. The dark lines show the form of the sublimated ice crystals. (Reproduced with permission from Kochs etal, 1991.)

Figure 7 Pressure rise as a function of time. 1, Pressure rise in the chamber after the valve is closed; 2, first derivative of 1. The maximum of 2 is reached at 2.14 s; the related equilibrium vapour pressure ps = 0.286 mbar, corresponding to Tice = - 32.7°C. (Reproduced with permission from Haseley and Oetjen, 1998.)

Figure 7 Pressure rise as a function of time. 1, Pressure rise in the chamber after the valve is closed; 2, first derivative of 1. The maximum of 2 is reached at 2.14 s; the related equilibrium vapour pressure ps = 0.286 mbar, corresponding to Tice = - 32.7°C. (Reproduced with permission from Haseley and Oetjen, 1998.)

expect values between 60 and 120 kJ m~2 h^C-1. Ktot is only slightly dependent on the operation pressure up to 0.1 mbar; then it increases up to 1 mbar by a factor of two. Xs is, in most cases, the figure for pure ice. Am has to be determined for each product by methods described above. b/p = 1.3 x 10-2 (kg m-1 h-1 mbar-1) is an average which is often found in practice, it can vary by a factor of two, but

l/d= 5.0 2.5 1.6 1

3 Jet

10° 1.5 2 3 5 101 20 40 60 102 Density of water vapour flow (g h_1cm~2)

10"

10° 1.5 2 3 5 101 20 40 60 102 Density of water vapour flow (g h_1cm~2)

Figure 8 Density of water vapour flow (g cm~2 fr1) as function of pch with jet flow and different //das parameter. (Reproduced with permission from Oetjen, 1999.)

the term with b/p in most freeze-drying processes only has an influence of a few per cent on iMD. The standard deviation of Tice, measurements during MD in the range of —15 to —45°C should be < 0.5°C, if measured automatically. iMD is in most cases governed by the value of Ttot and the term (1/Ktot). The term (d/2 ■ Ag) is, for d values below 10 mm, of the order of 10% or less of l/Ktot, growing to approximately 50% at d = 35 mm. Tice is the result of a ther-modynamic equilibrium between heat transfer to the sublimation front and energy consumption for sublimation. Both depend on several factors, but the heat supply and vapour transport to the condenser are most important during MD. Therefore the operation pressure is a very effective tool to control Tice, if the shelf temperature is kept constant and the condenser temperature is always below a maximum, which depends on the water vapour pressure in the chamber and the design of the plant. By changing the operation pressure, e.g. from 0.1 mbar to 0.8 mbar, Tice can be controlled between — 30°C and —20°C. For another product, a different product thickness or a different plant, the pressure range and its controlled range are different, but the dependence is reproducible. Since Tice depends also on the structure of the frozen product it can be used to prove that the structure of products in different runs is homogeneous and sufficiently identical. If the structure contains unfrozen water, Tice data will from time to time jump by 1°C or more (when the water evaporates) and the data will be different for a product frozen at different freezing rates.

At the end of MD the ice is mostly sublimed and the measured Tice decreases below the standard deviation of Tice during MD. This effect can be used to change automatically from main to secondary drying (SD), e.g. if the measured Tice becomes 2-3°C smaller than the average during MD. Other criteria are often suggested, such as an increase of product temperature, a decrease in operating pressure or a decrease in partial pressure of water vapour, but it is more difficult to use these other methods quantitatively.

Besides heat transfer, the water vapour transport from the chamber to the condenser is often critical in a freeze-drying process. The length (l) and diameter (d) of the connection between chamber and condenser in a freeze-drying plant as shown in Figure 11 determine the vapour flow, assuming that the other flow resistances in the chamber are relatively small by comparison. The vapour flow can be estimated using the Giinther-Jaeckel-Oetjen equation. Figure 8 shows that: the vapour flow density decreases in a nonlinear manner with the chamber pressure; the relation of l/d is of increasing importance with decreasing pressure; for example, at 4x 10-2 mbar the vapour flow density at l/d = 5 is only 30% that at l/d = 1. Right-angle bends contribute to the length not only by their physical dimensions but, depending on the design, by a factor of four or more of the measured length. For operation pressures below about 10-1 mbar, l/d > 2.5 should be avoided.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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